WO2013036579A1 - Molecular catalysts for n2 conversions at lower temperatures and pressures - Google Patents

Abstract

The present invention is directed in various embodiments to catalysts and methods for activation of dinitrogen N₂, (e.g., nitrogen gas) such as for reduction of N₂ with a reductant in the presence of the catalyst to ammonia NH₃, or conversion to nitrogen-containing products such as urea and amines. In various embodiments, the invention provides methods of preparation of ammonia and other nitrogen-containing products from nitrogen gas in the presence of a reductant such as hydrogen gas, under conditions of relatively low temperature and low pressure compared with temperatures and pressures used in art methods of nitrogen fixation (reduction). Catalysts of the invention are molecular, i.e., not heterogeneous, catalysts, soluble in a solvent medium comprising a strong base and a liquid solvent.

Description

MOLECULAR CATALYSTS FOR N₂ CONVERSIONS AT LOWER TEMPERATURES AND PRESSURES

CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the priority of U.S. provisional application Serial

Number 61/532,747, filed September 9, 2011, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND

Approximately one third of the earth's population is fed through the generation of NH₃ from N₂, making this process one of the largest volume chemicals manufactured on the planet. Additionally, NH₃ is the ultimate precursor to essentially most synthetic nitrogen-containing chemicals. Consequently, it can be anticipated that NH₃ will continue to be an essential commodity chemical into the foreseeable future. See, for example, Skene, K. Contemp. Rev. 2010, 292, 1696; Hager, T. The Alchemy of Air. Harmony Books, New York, NY, 2008; Wolfe, D. W. Tales from the underground a natural history of subterranean life. Perseus Pub, Cambridge, Ma:. 2001 ; Smil, V. 2004. Enriching the Earth: Fritz Haber, Carl Bosch, and the

Transformation of World Food Production. MIT Press: Cambridge, MA, 2004; Max A. Ammonia. In Ullmann 's Encyclopedia of Industrial Chemistry. Wiley-VCH, Weinheim, Germany, 2006.

The current technologies for the production of NH₃ are all based on the over 100 years old Haber-Bosch process that utilizes supported Ru or Fe heterogeneous catalysts to facilitate reaction between N₂ and H₂. Although there has been an incremental improvement in these catalyst systems over time, the intrinsic catalytic activities remain low and reactions must be carried out at high temperatures (-450 °C) and pressures (-4000 psig), with multiple reactors and extensive gas recycle to achieve high yields. These characteristics lead to high capital (typically the largest cost for commodity chemicals) and operating costs for today's commercial processes. Additionally, the high energy demands of these processes lead to extensive use of fossil fuels and excessive CO₂ emissions. The indirect use of natural gas as a source of H₂ that is generated through high temperature syngas technologies compounds the capital and energy challenges associated with the production of NH3 from N₂.

Estimates are that the production of NH₃ consumes -5% of the world's natural gas production and -2% of the world's annual energy supply. As we face environmental and supply challenges into the 21^s century, it is important to develop more sustainable technologies for large scale processes such as NH₃.

SUMMARY

The present invention is directed in various embodiments to catalysts and methods for activation and conversion of molecular nitrogen (dinitrogen, N₂), (e.g., nitrogen gas), such as for reduction of N₂ with a reductant (reducing agent) in the presence of the catalyst to ammonia NH₃, or conversion in the presence of the catalyst to nitrogen-containing products such as urea and amines. In various embodiments, the invention provides methods of preparation of ammonia and other nitrogen- containing products from nitrogen gas in the presence of a reductant such as hydrogen gas, under conditions of relatively low temperature and low pressure compared with temperatures and pressures used in art industrial methods of nitrogen fixation (reduction), such as the Haber-Bosch process. Catalysts of the invention are molecular, i.e., not heterogeneous, catalysts, being single molecules rather than a mixture of various catalytic sites. The catalysts of the invention are soluble in a solvent medium comprising a strong base and a liquid solvent. These molecular catalysts can also be supported using supported liquid phase catalysis (US 6218326, Supported molten-metal catalysts; Datta, R., and Rinker, R. G., "Supported Liquid- Phase Catalysis. I. A Theoretical Model for Transport and Reaction, " J. Catal.,

95,181 -192, 1985) or by other means of attachment to solid supports.

In various embodiments, the invention provides a catalyst for reduction of N₂ to NH₃, comprising:

(a) a complex comprising a group IVA, VA, VIA, VIIA or electropositive group VIIIA metal M in a low oxidation state complexed by a unidentate, bidentate, tridentate, or tetradentate ligand L to provide a complex L-M, wherein at least one of the complexing atoms of the ligand L is a nitrogen atom, the complex optionally comprising additional ligands, wherein the complex L-M is soluble in a solvent medium;

(b) a solvent medium comprising a strong base.

In various embodiments, the invention provides a method of reducing N₂ to NH₃, comprising contacting the catalyst of the invention and N₂ and a reductant under conditions suitable to provide the NH3. The reductant can be hydrogen, or can be a reduced substance catalytic oxidation of which with N₂ produces a material wherein the reduced form can be regenerated therefrom with hydrogen. In various embodiments, the invention provides a method of reducing N₂ to urea or to an amine, comprising contacting the catalyst of the invention and N₂ and a reductant, and carbonate or a hydrocarbon respectively, under conditions suitable to provide the urea or the amine.

In various embodiments the invention provides a method of making the catalyst of the invention, comprising contacting an inorganic salt of metal M and ligand L and, optionally, one or more additional ligands, under conditions suitable to provide L-M, then dissolving L-M in the solvent medium comprising the strong base.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 shows the equilibrium constant as a function of temperature for the synthesis of NH₃ from N₂ and ¾

Figure 2 shows a hypothetical cycle of N₂ activation and functionalization to generate NH₃.

Figure 3 shows examples of formal activated H₂ species that could react with activated N₂.

Figure 4 shows hypothetical mechanism for the ionic hydrogenation of N₂ activated species.

Figure 5 shows conceptual orbital interaction diagram between a highly lying, occupied d-orbital with the anti-bonding orbitals of N≡N and CH bonds.

Figure 6 shows conceptual diagram and experimental data showing the effect of base on CH Activation with a strong π-donor transition metal complex with liquid ligands.

Figure 7 shows a proposed mechanism for the design of practical molecular catalysis for nitrogen reduction.

Figure 8 shows examples of predicted metal catalysts for the conversion of N₂ and H₂ to NH₃ in basic solvents.

Figure 9 shows a summary of reactions of the (bpb)Os motif that were examined to provide initial proof of principle for proposal catalysts designs. Solid Boxes indicate fully characterized molecules; Dashed boxes indicate molecules based on in situ characterization;

Unboxed molecules are postulated.

Figure 10 shows quantitative ¹³C NMR (400 MHz) of Os(BPB)(¹³CN)₃ after reduction with Zn in 2 M KOD/D₂0. Figure 11 shows 400 MHz ^XH NMR spectra of (a) Os(BPB)NCl₂ and (b) Os(BPB)Cl₃ using 35 eq of Zn in 2.85 M KOD at RT for 2 h.

Figure 12 shows 100 MHz ¹³C NMR spectra of (a) Os(BPB)NCl₂ and (b) Os(BPB)Cl₃ using 35 eq of Zn in 2.85 M KOD at RT for 2 h followed by treatment

13

with l5 eq of K CN at RT for 2 h followed by heating for several hours at 50 °C.

Figure 13 shows the base dependence on formation of the N₂ complex. The N₂ complex is represented by the doublet at about 7.7. Under 20% KOD the complex becomes partially insoluble.

Figure 14 shows the dependence of formation of the N₂ and "hydride" complex on H₂ pressure, in 10 % KOD / D₂0.

Figure 15 is a stack plot of various conditions to show formation of N₂ complex in 10 % KOD / D₂0.

Figure 16 shows a comparison of Os(BPB)NCl₂ with Os(BPB)Ci₃ after reduction.

DETAILED DESCRIPTION

Definitions

As used in the specification and the appended claims, the singular forms "a," "an" and "the" include plural referents unless the context clearly dictates otherwise.

The term "about" as used herein, when referring to a numerical value or range, allows for a degree of variability in the value or range, for example, within 10%, or within 5% of a stated value or of a stated limit of a range.

All percent compositions are given as weight-percentages, unless otherwise stated.

"Substantially" as the term is used herein means completely or almost completely; for example, a composition that is "substantially free" of a component either has none of the component or contains such a trace amount that any relevant functional property of the composition is unaffected by the presence of the trace amount, or a compound is "substantially pure" is there are only negligible traces of impurities present.

Phrases such as "under conditions suitable to provide" or "under conditions sufficient to yield" or the like, in the context of methods of synthesis, as used herein refers to reaction conditions, such as time, temperature, solvent, reactant

concentrations, and the like, that are within ordinary skill for an experimenter to vary, that provide a useful quantity or yield of a reaction product. It is not necessary that the desired reaction product be the only reaction product or that the starting materials be entirely consumed, provided the desired reaction product can be isolated or otherwise further used.

"Molecular catalysts" are defined as single molecules (attached to a surface or in solution) that unlike existing classical heterogeneous, supported metal catalysts, are all identical "active sites" that can be fully characterized (i.e. the composition and structure fully determined). The term "molecular catalyst" is more precise than the commonly discussed "homogeneous catalysts" which is typically used only to describe systems where the catalyst is soluble in some reaction solvent. Significantly, the reaction mechanism of molecular catalysts can be accurately determined by a combination of theoretical and experimental chemistry and the information can be utilized to generate structure-function relationships which can lead to rational predictions for catalyst improvement or new designs. Another key advantage over heterogeneous catalysts is that the large knowledge base of synthetic methodologies applicable to almost any molecular catalyst allows for generation of the new or modified catalysts guided by these predictions. These advantages over traditional heterogeneous catalysis can lead to much faster rates (and associated lower temperatures and pressures), and higher selectivities, as well as faster time to market and substantial reductions in product costs.

The term "catalyst" as used herein refers to a combination of a complex molecular entity comprising a metal and one or more ligands, in a solvent medium with strong base present. The catalyst can also include other ingredients, and can be disposed in a gas-liquid reactor, or on a solid support suitable for bringing about gas phase reactions, using techniques known in the art.

A group IVA, VA, VIA, VIIA or electropositive group VIIIA metal atom M, as the term is used herein, refers to a metal atom derived from one of the designated elements as shown on a periodic table. An electropositive group VIIIA element is an element in the left column of the three-column group VIIIA element group, i.e., Fe, Ru, Os, or in the middle column, Co, Rh, Ir, but not in the right column, Ni, Pd, Pt.

The term "metal M in a low oxidation state" refers to an atom of one of the designated elements that is either in a zerovalent state or is in an oxidized state, but not in the highest oxidized state available to the element under chemical conditions. For example, Os(II) and Os(III) are low oxidation states for osmium, whereas Os(VI) is a high oxidation state for osmium. Os(0) is also a low oxidation state for osmium. The term "complexed by a unidentate, bidentate, tridentate, or tetradentate ligand L" refers to the metal atom M being coordinated by ligands forming a metal atom - ligand complex such as is well known in the art, wherein the ligand complexing the metal atom in the complex is uni-, bi-, tri-, or tetradentate, i.e., has 1 , 2, 3, or 4 points of complexation of the metal per ligand molecule. The ligand can be a multicyclic heteroaryl species comprising at least one ring nitrogen atom.

The term "complex L-M" as used herein refers to the metal atom complexed with the unidentate, bidentate, tridentate, or tetradentate ligand L; it is understood that other ligands may be present in the species.

The term "solvent medium" as used herein refers to either a solution or a melt in which the molecular catalysts of the invention can be dissolved.

The term "strong base" as used herein refers to a material such as an alkali metal or alkaline earth hydroxide or amide, or a mixture thereof, or a molten salt composition comprising an alkali metal or alkaline earth hydroxide or amide, or a mixture thereof. Other alkaline materials capable of producing at least a low concentration of hydride ion from hydrogen, or a carbanion from a carbon-based molecule, are also included. Further examples or strong bases includes alkoxides and alkylamides. When solutions are present, the solvent can be, e.g., water, ammonia, alcohols, amines, and the like.

An alkylamine includes a mono-, di- or tri-alkylamine, or a

tetraalkylammonium species.

The expression that "cleavage of the nitrogen-nitrogen bond of the N2 occurs with oxidation of the metal atom" as used herein refers to a process wherein the metal atom center(s) of the L-M complex, optionally comprising additional ligands, and bonded to dinitrogen species (L-M-N2 or L-M-N2-M-L), undergoes oxidation to a higher metal oxidation state as the N≡N bond is cleaved or is reduced to a lower bond order, a reductive process. The process is believed to be an intramolecular oxidation (of the metal) / reduction (of the nitrogen-nitrogen bond) process. For example, although not wishing to be bound by theory, it is believed that an embodiment of the reaction can be described as the reaction: L-M(red)-N≡N-M(red)-L => 2 L-M(ox)≡N.

A "proton H abstractable by strong base" is a hydrogen atom bonded to, for example, an NH group of a heteroaryl moiety of a ligand, than can be removed by the strong base of the solvent medium. When a complex L-M contains such an abstractable proton, dissolving L-M in the strong base solution results in the deprotonation of complex L-M to form the complex L^" -M in the solvent medium. Accordingly, when a ligand with a relatively acidic proton (i.e., one that can be removed under the basic conditions used), the metal-ligand complex is of formula L^" - M, and the product of reaction with nitrogen can be L^" -M-N or L^" -M-N₂.

Detailed Description

As can be seen from Figure 1 and Equation 1 , the reaction of H₂ and N₂ to generate NH₃ is exoergic (favoring the products) at temperatures below ~200°C. Eq. 1 N₂ + 3H₂ -» 2NH₃ AG₁₈oc = -0.2 kcal/mol

Rapidly catalyzed reactions that can operate with practical volumetric productivities at these lower temperatures would lead to higher yields per-pass, decrease in pressure requirements, the number of reactor trains and extent of gas recycle. These improvements would lead to new processes that are both less capital intensive and more sustainable. The key to this and other new reactions with N₂ is the design of new, more active, stable and inexpensive catalysts for the selective reactions of N₂ at lower temperature and pressure, such as the catalysts of the present invention as disclosed and claimed herein.

The discovery of the catalyst of the invention, allowing the use of lower temperature and pressure conditions for the reduction of N₂ to NH3 can provide a basis for designing new routes for the direct conversion of N₂ to other N-containing chemicals. These new processes can be carried out in one plant that this is much less expensive and energy intensive than current processes used for nitrogen-containing products such as urea and aniline (see Eq. 2 and Eq. 3 respectively). Currently, conversion of natural gas to H₂ to be used for NH₃ production is carried out in separate reforming processes that require large amounts of energy that both increase costs and emissions. A less expensive and energy intensive route involves a one plant, two-step, heat integrated process that uses a lower temperature oxidative process to generate urea as shown in the reaction in Eq. 2. Similarly, the reaction between benzene, N₂ and H₂ to directly generate aniline, Eq. 3., is more sustainable and less expensive than the current process that requires conversion of N₂ to NH₃, oxidation to HNO₃, nitration of benzene, and reduction of nitrobenzene to aniline. Eq. 2 CH₄ + ½ 0₂ + N₂ -» H₂NC(0)NH₂ AG_180C = -37 kcal/mol Eq. 3 C₆H₆ + ½ N₂ + ½ H₂ -» C₆H₅NH₂ AG_180C = +6 kcal/mol

The interest and uses of molecular catalysts in the chemical industry continues to grow due to the potential for extraordinary increases in rates and selectivity relative to existing heterogeneous catalysts. Molecular catalysts are defined as single molecules (attached to a surface or in solution) that unlike existing classical heterogeneous, supported metal catalysts, are all identical "active sites" that can be fully characterized (i.e. the composition and structure fully determined). The term "molecular catalyst" is more precise than the commonly mentioned term

"homogeneous catalyst" which is typically used only to describe a system where the catalyst is soluble in some reaction solvent. Significantly, the reaction mechanism of molecular catalysts can be accurately determined by a combination of theoretical and experimental chemistry and the information can be utilized to generate structure- function relationships which can lead to "rational" predictions for catalyst improvement or new designs. Another key advantage over heterogeneous catalysts is that the large knowledge base of synthetic methodologies applicable to almost any molecular catalyst allows for generation of the new or modified catalysts guided by these predictions. These advantages over traditional heterogeneous catalysis can lead to much faster rates (and associated lower temperatures and pressures), higher selectivities, faster time to market, and substantial reductions in product costs. The key challenges that have traditionally limited the use of molecular catalysts are instability, high costs, and product/catalyst separation. However, with evolution of the field, methods of addressing these disadvantages have been developed and the number of commercial processes based on molecular catalysts continues to grow.

Given the enormous incentives, the design of more active, stable and inexpensive catalysts for the direct conversion of N₂ to chemicals has been a long standing industrial and academic research challenge (albeit with very little success even after over 100 years of research). This challenge is a result of the poorly reactive characteristics of N₂ that is apparent from parameters such as the bond dissociation energy (941 kJ ιηοΓ¹), ionization potential (15.58 eV), proton affinity

(5.12 eV) and HOMO-LUMO gap (-15.6 eV to +7.3 eV, respectively). These features are the primary reasons that current N₂ reduction catalysts require the high temperatures and pressures that are used in the commercial process. The many inherent limitations of current heterogeneous catalysts make it unlikely that this approach would lead to the substantial improvements required for lower temperature and pressure processes. However, it is important to note that these systems do catalyze the reaction of interest. This could suggest that the chemistry of new systems should be consistent with the general mechanism proposed for the commercial heterogeneous catalysts shown in Scheme 1. It is likely this combination of the chemistry of established and proposed new catalysts could lead more accurate models that could provide a stronger basis for design of more active catalysts.

Production of NH^ from Nj

Scheme 1. Proposed General Mechanism of Commercial Catalyst

1. N₂ (g)→ N₂ (adsorbed)

2. N₂ (adsorbed)→ 2 N (adsorbed)

3. H₂(g)→ H₂ (adsorbed)

4. H₂ (adsorbed)→ 2 H (adsorbed)

5. N (adsorbed) + 3 H(adsorbed)→ NH₃ (adsorbed)

6. NH₃ (adsorbed)→ NH₃ (g)

An important aspect of the current commercial Fe and Ru based

heterogeneous catalysts is the requirement for various salts of Cs, Na, K, Ba, etc. as promoters. Some studies show that these salts exist as oxides that interact with the Fe or Ru metal centers. Other studies suggest that these exist as the reduced forms. Various mechanisms have been proposed to explain the role of these promoters. One proposed role is to increase the number of active sites of the Fe or Ru catalysts. It has also been proposed that these promoters act by help to reduce the Fe and Ru centers. Yet another, is that the promoters help to react with the H₂. It has also been proposed that these promoters help to expel the NH₃.

While there are limitations to the reported molecular systems, these studies have shown that the general reactions identified in molecular systems correlate with models developed for the heterogeneous systems. Interestingly, however, to our knowledge no molecular systems have mimicked or utilized the role of the promoters in the commercial systems. These studies established modes of reactivity that can provide guidance to the "rational" design of new molecular catalysts. The two general steps that have been identified for molecular systems are N₂ coordination and cleavage (either by reduction in bond order of the coordinated N₂ or by complete cleavage of the nitrogen molecule) that we refer to in this proposal as "N₂ activation", followed by reaction with activated hydrogen sources to generate NH₃ which we refer to in this proposal as "Functionalization". It is also, possible that reaction could proceed by direct reaction of the H₂ molecule, but there is little precedent for this pathway in studies of molecular systems or heterogeneous catalysts. As can be seen in Figure 2, these general steps can combined into a hypothetical catalytic cycle for the overall reaction of H₂ with N₂ to generate NH₃. Importantly, as can be seen by comparison to the general features of the proposed mechanism for the heterogeneous catalysts, these steps are common to both molecular and heterogeneous models. This suggests that consideration of the common aspects between these models could lead to more accurate models to guide the design of new, more active catalysts.

Critically, any proposal for new practical molecular catalysts for the lower temperature and pressures conversion of N₂ and H₂ to NH₃ must address, or show the potential to address, the key deficiencies of the prior work. Therefore any new system should meet the following requirements: A) be capable of reaction with gaseous N₂, B) utilize ¾ as the overall reductant, C) utilize practical solvents, D) be based on steps in a proposed catalytic cycle that are mutually compatible, E) be stable under reaction conditions, F) not directly mimic the Nitrogenase enzymes, G) exhibit reaction rates fast enough to obtain practical volumetric productivities at levels of catalyst that are economical, and H) address practical issues such as catalyst life, selectivity, rate, product separation, solvents that are compatible with NH₃, and cost of catalyst system. Critical to success these requirements must be considered simultaneously in the catalyst design process. A more sequential approach to addressing these requirements, typical of most reported work, will likely fail as each requirement is dependent on the same composition, structure, solvent and operating conditions. As discussed above, a key guiding principle is that the general catalytic cycle should be based on N₂ activation and functionalization steps shown in Figure 2, and other considerations that are consistent with both the heterogeneous and well- defined molecular models.

Of the two steps, N₂ activation and functionalization, the N₂ activation is more developed with molecular catalysts and the observations are generally consistent with the observations and models of heterogeneous catalysts. On the basis of the heterogeneous and molecular observations, it is likely that the catalysis must be based on metals rather than "metal- free" systems based on classical acid/base catalysis. A key guiding principle to identifying the class of metals for focus is the observation that more electropositive transition metals such as Os, Re, Ru and Fe have a much higher propensity to react with gaseous N2 than the more commonly used catalyst used in the petrochemical or pharmaceutical industry based on the more

electronegative metals such as Pt and Pd. Importantly, these observations are parallel to those in heterogeneous catalysis, where it has been reported that Os is among the most active (but impractical due to cost and volatility) systems while Fe and Ru are used commercially. Another key observation is that molecular systems utilize these metals in the lower oxidation states (0, I, II), as opposed to the higher (III, IV, etc.) oxidation states for reaction with N₂. In heterogeneous catalysis, this is consistent with the general observation that the active supported metal catalyst are in reduced states and with the observation that catalysis can be rapidly initiated by pre-treatment of freshly prepared catalyst with H₂. These considerations would suggest that molecular complexes generated from these low oxidation state, electropositive metals, (e.g. Os, Re, Ru or Fe) could provide the requirements for N2 activation and ideal system for the basis of new molecular catalysts. However, as noted above there are no explicit parallels between the studies on heterogeneous and molecular catalysts related to the importance of promoters based on Ba, Cs, K, etc. on catalytic activity in the commercial catalysts.

Of the two general steps for catalyst that could convert N2 and H₂ to NH₃ there has been much less work on the functionalization step by ¾, with reported molecular catalysts, vide supra. Consequently, this must be a key consideration in the design of more practical molecular catalysts. As can be seen in Figure 3 , there are four formal possibilities for the reaction of H₂ that can lead to the functionalization of activated N₂. Both molecular and heterogeneous model systems show that activated forms of H₂ are required for the formal reduction of N₂. For example, the proton coupled electron transfer pathway, H⁺/e^", is utilized by the natural systems but leads to the use of reductants that cannot be generated from ¾. Another possible pathway for the functionalization of N2 could operate by reaction with two hydrogen free radicals (2 Η·) or by covalent metal hydride, L'-M'-H, that react by formal or actual homolytic cleavage. The free radical pathway is unlikely to be operational at lower temperatures given the high homolytic bond strength of ¾ (105 kcal/mol). The second pathway involving covalent metal hydrides, L'-M'-H, is more feasible and could have some precedent in the literature. However, while seemingly elegant, a key disadvantage of using this strategy is that another molecular complex (likely also a transition metal) must be present for reaction with H₂ or the same molecular motif that activates N2 also activates the H₂ to generate LM(N)(H) type species. While there is precedent for both of these possibilities, we consider this pathway to be very restrictive and would likely require very unique and complex systems. Such systems would be very limited, challenging to synthesize and likely unstable. The pathway based on the direct reaction of free, unactivated H₂ with an activated N2 species has no precedent in either molecular systems or studies of heterogeneous catalysts and is unlikely to be useful in the design of new systems.

The pathway we have focused on for incorporating H₂ into the catalytic cycle is via the ionic hydrogen pathway which formally involves addition of H^" and H⁺ species. While it may seem untenable to generate hydrides (or species with hydridic character) and protons (or species with protic character) in the same reaction system (required for practicality) there is strong precedent for this approach. The so-called ionic hydrogenation of ketones using well-defined metal hydride complexes under mild conditions has been developed as a new form of hydrogenation. This involves a transition metal hydride with hydridic character and a liquid ligand or media that can be made to react with unactivated ketones (uncoordinated or cleaved C=0 bond) via concerted transition states. However, we consider that pathways that involve two metal centers, one for H₂ activation and one for N₂ activation or one metal center for both are strategically undesirable.

Another demonstrated approach to this problem has been the activation of H₂ in a metal-free ionic hydrogenation that is based on the weakly acidic properties of H₂. The pka of H₂ is -36 and therefore can be reversibly deprotonated, Eq 4, by very strong bases (B:) to generate low concentrations of highly reactive, incipient H^" or discrete solvated hydrides, H^" _soi. Significantly, in these systems, the activation of the H₂ is carried out by the strong bases rather than transition metals complexes. Such a strategy for H₂ activation removes the requirement for more complex systems that require transitions metals for H₂ activation. As will be discussed below, this strategy also leads to compatibility with the N₂ activation step. Eq. 4 ^H2 ^{+ B : so1} "* ^H -^{1 + BH}

A practical approach for H₂ activation is to utilize strongly basic media such as concentrated aqueous (or anhydrous molten salts) alkali hydroxides or amides in the conjugate amine. These systems are known to react with H₂ to generate H^" as an incipient species or solvated species, H^" _soi . Evidence for this is that H/D exchange between H₂ and D₂0 solvent can be rapidly catalyzed by alkali OD^". Importantly, this rate of exchange exponentially increases from 1M to 18M [KOH]. While this could be explained by concerted mechanism via a cyclic transition state involving ¾, OD^" and D₂O and transfer of an incipient hydride, another more likely explanation is the reversible generation of low concentrations of solvated hydrides, H^" _soi, by the general acid base reaction shown in Eq 4. In this equation, B: is solvated hydroxide, OH^" _soi or other strong base. Consistent with the generation of solvated H^" _soi, these exchange reaction with H₂ and other weak acids such as hydrocarbons have been utilized to determine the H. scale (a measure of proton affinity in non-aqueous media) for solutions of ionic hydroxides. Importantly, the concentration and activity of H^" _soi in concentrated alkali hydroxide solvents is not linearly dependent on the concentration of OH^" since the activity of OH^" increases exponentially as the concentration increases. Studies show that 18M aqueous solution of KOH is -1000 more basic than a 1M solution and is the basis for the exponential increase in rates of H/D exchange between H₂/D₂O catalyzed by OH^". At higher concentrations, the basicity would be expected to be exponentially higher with a correspondingly higher concentration of H^" soi. As expected, these acid base reactions are even more facile in stronger bases such alkali amides, such as NaNH₂ in NH₃.

Whether generated as an incipient species or as discrete solvated species, the hydride would be expected to be a very reactive species since as it is both strongly reducing and is well known as a power nucleophile capable of reacting with unsaturated species. Consequently, it is plausible that such species could reduce the N₂ activated species while also generating N-H bonds required for functionalization through the hypothetical mechanisms shown in Figure 4. As can be seen in these mechanisms, both the formation of an N-H bond and the required reduction occurs at the metal center. Incorporating such a reaction into a catalytic cycle for ^conversion to NH3 could be seen as the "ionic hydrogenation" of N₂.

The attractive characteristics of using H₂ in strongly alkaline media as the overall reductant in N₂ conversion to NH₃ are: A) that it does not involve the use of another metal complex, or the same complex used for N₂ activation; B) that reactive hydrides as discrete solvated or incipient species could be expected to both reduce the active metal center and begin generation of N-H bonds, C) that aqueous alkali hydroxides or amides in the corresponding conjugate acid are potentially practical solvents given the low cost, thermal stability, and compatibility with NH₃ (e.g. no impractical protonation to NY ⁺ would occur as in acidic solvents).

An intriguing consideration that could provide an additional basis for the proposed use of basic solvents for designing new molecular catalysts is that such solvents could play the role of the basic promoters in the commercial heterogeneous systems. Indeed, it is possible that the Ca, Na, Ba or K salts used as promoters in the heterogeneous catalysts also lead to the generation ionic hydrides. Indeed,

KOH/NaOH as well as NaNH₂/KNH₂ form low melting eutectics that could provide a reaction "solvent" as molten salts on the surface of the catalyst support. As proposed in Figure 4, these surface hydrides could react with N₂ activated intermediates on the supported metal catalyst to generate NH₃, reduce the metal centers and release NH₃.

On initial consideration, it could seem untenable to design catalysts that involve N₂ activation (coordination and cleavage of the N≡N triple bond) in the strongly basic solvents required to facilitate the functionalization step with ¾ discussed above. This is because N₂ is generally considered to be a weakly coordinating ligand and strongly basic solvents based on alkali hydroxides or amides would be expected to be much more coordinating that N₂. This would prevent the coordination of N₂ to the metal center that is required for the N₂ activation step. Conceptual considerations that provide a basis for overcoming this potential dilemma can be obtained by noting, that in spite of the difference in bond orders, a N≡N and a CH bond share some common electronic characteristics. This can be seen by noting that the electron affinities of both N₂ and CH₄ are ~5 eV. This is consistent with presence of high lying, anti-bonding LUMOs as well as low lying HOMOs characteristic of strong bonds. Consistent with these properties, both molecules resist reaction with strong Bronsted acids and bases. However, it is interesting to note, that both CH bonds and N₂ readily react with electron-rich metal complexes with high lying, filled orbitals that have the same π-symmetry of the anti-bonding orbitals in the orientations shown in Figure 5, A and B, respectively.

These considerations parallel recent observations we have made in the area of CH activation that could be relevant to the design of N₂ conversion catalysts that operate in basic media. Thus, as shown in Figure 6, we have demonstrated that electron-rich metal complexes with liquid ligands can be designed to coordinate and activate CH bonds in strongly basic media. Furthermore, our studies show that the basic media, rather inhibiting or only providing a solvent media, can accelerate reaction. This counter intuitive possibility is consistent with the long known

"conjugate-base" mechanism that explains the observations that bases can accelerate ligand exchange of octahedral complexes but only if liquid ligands are present that can be deprotonated. The conceptual basis for the cleavage of the CH bond in strongly basic media is shown in Figure 6 A. As can be seen, we postulated that deprotonation of liquid ligands on low oxidation state, electron-rich, metal centers from Os, Ru, Re, etc., can increase interaction between the filled metal-based d- orbitals and the empty anti-bonding orbitals of strong bonds of similar symmetry in the orientation shown. Consistent with these possibilities, as can be seen on the right of Figure 6B, increasing basicity of the solvent in these systems leads to acceleration of H/D exchange via CH activation. Kinetic analysis shows that the rate constant for reaction by the Ruⁿ-OH catalysts are >100 times more active than that for KOH. This shows that the π-type interactions between the Ru¹¹ center and the CH bond is greater than the classical σ-type interaction with OH^" that leads to deprotonation of the CH bonds. While reactions could not be examined beyond 8 M aqueous KOH due to catalyst insolubility the data shown could be suggestive of the expected exponential increase in rate beyond 8 M if the reactions rate at the metal center correlated with the solvent basicity.

Given the electronic similarities between the N≡N and a CH bond and facile interaction of both with transition metals with high lying filled d-orbitals as noted above, it is plausible that metal complexes could be designed that could both coordinate as well as activate gaseous N2 in strongly basic media. This prediction is strengthened by noting that low oxidations state Ru(II) shown to be active for CH activation in basic media is also one of the electropositive metals in the group of more electropositive metals Os, Re, Ru, etc. that are known to be active for N2 activation in both reported molecular complexes and heterogeneous catalysts. Critically, the proposal that basic solvents would facilitate the activation of both N2 and ¾ could ensure the compatibility between these steps that would be require for catalysis.

A reaction mechanism that meets these requirements and embodies the design principles discussed herein is shown in Figure 7. This reaction mechanism will be a primary guide to this research program to develop more efficient molecular catalysts for N2 conversion to chemicals. On the basis of the conjugate-base mechanism, it is expected that coordination of N2 to the metal center will be facilitated by

deprotonation of liquid ligands. Additionally, as shown in the boxed insets, a key basis for N₂ activation is that this ligand deprotonation will also facilitate the interactions between filled, high energy orbitals of the metal that are available for donation into the π-acceptor, anti-bonding orbitals of N₂. This quantum mechanical model provides a plausible explanation for the lower oxidation of Fe and Ru that are required in the commercial systems.

This proposed mechanism meets the guiding principles discussed above: A) The reaction mechanism is fundamentally different from that of the natural enzymes and the reported systems that use impractical strong one-electron reductants, B) the system will utilize H₂ directly as solvated hydride (H^" _sol) and H₂0 rather than by a sequence of proton coupled electron transfers H⁺/e^" that is characteristic of the natural systems and the bulk of reported research, C) The catalysts will be based on low oxidation state metals such as Ru¹¹, Os¹¹, Re™, etc. that are characterized by high lying occupied orbitals that can interact with the anti-bonding orbitals of N₂, D) The boxed insets show the acid base mechanism that allows the direct use of H₂ as the overall reductant, E) the role of the basic solvent in facilitating both the N₂ activation and H₂ functionalization steps will ensure compatible, F) basic solvents will also facilitate the generation and release of NH₃, G)basic solvents such as aqueous NaOH or liquid NH₃ (below the critical temperature) can be practical solvents. Other practical considerations such as such as catalyst stability, cost, etc. can be addressed by the choice of various ligand-metal combinations.

These considerations would suggest that the key components for the design of an efficient molecular catalyst for the conversion of N₂ and H₂ to NH₃ are the use of:

1) Low oxidation state, electropositive, transition metals such Os, Re, Ru, Fe, etc.

2) Strongly basic solvents.

3) Polyliquid, ligands based on C, N, O, P, etc.

4) Thermally stable and soluble complexes.

A key challenge in using these metals in basic media such as aqueous NaOH is the loss of catalytic activity due to the formation of insoluble metal oxides. Thus addition of the metal salts, e.g. ruthenium chloride, to aqueous NaOH immediately forms insoluble ruthenium oxides. One method that is commonly used to minimize the formation of insoluble species is to use ligands. Thus, an important requirement is that the ligands be identified that can prevent the formation of these oxides or other insoluble materials. To address the issue of stability, the ligand designs are based on chelating ligands. This is based on the precedent that, due to entropic effects, polydentate, chelating ligands, i.e., bidentate, tridentate, or tetradentate, are much less labile than monodentate ligands. Deprotonation of the liquid ligands such as those noted below in Figure 8 lead to ionic species that are soluble in aqueous NaOH or other polar basic media such as liquid NH₃. It is also possible, that deprotonation of the ligands, in addition to activating the metal center, vide supra, could strengthen binding to the metal center and help to minimize formation of inactive, insoluble metal oxides. Liquid, N-, C-, 0-, and S-donor ligands are preferred. This is because these ligands are more donating that typical phosphines due to lower propensity for back bonding and the corresponding metal centers are more electron-rich. Additionally, many of the reported molecular complexes are based on N-donor ligands and there is strong precedent in CH activation studies that N-donor such as bipyrimdine can be stable above 200 °C for days or potentially even longer. Use of d⁶, 6-coordinate, pseudo octahedral geometries is preferred as this is the geometry of the majority of the low oxidation state complexes of the metals listed above and the electronic state most likely to generate electron-rich metal centers. Examples of metal complexes and reaction media are shown in Figure 8.

We examined these two key steps in the predicted reaction mechanism with one of the proposed complexes. Demonstrating feasibility for these steps would provide a basis for the efforts required to identify molecular complexes and reaction conditions that could carry out all three steps integrated into a reaction cycle required for catalysis. The two steps chosen for feasibility studies were the N2 coordination step and the functionalization step on the expected N2 activated species. Importantly, both of these reaction were examined with the same ligand metal motif and with the same basic media as the reaction solvent. As noted above, to our knowledge there is no precedent for systems that have shown this reactivity.

The liquid ligand metal motif, 2,6-dibenzimidazoylpyridine coordinated to Os, (bpb)Os, chosen for study is shown in Figure 9. The bpb ligand was utilized because the Ru complex with the related NNN-pincer ligand, 2,6-diimidizoylpyridine was found to be soluble, thermally stable and active for CH activation in basic aqueous KOH. The primary basis for the use of Os in this motif rather than the Ru, which is less expensive, is the precedent that complexes with Os are generally more stable than those with Ru. This increased stability, while likely not optimum for reactivity, is important at this feasibility stage where rigorous study of the reactivity is required to validate the proposed mechanism shown in Figure 7. Various models of the intermediates of this (bpb)Os motif that would result from catalytic conversion of N₂ and ¾ to NH₃ in basic media by the reaction mechanism proposed in Figure 7, were synthesized and fully characterized. Thus, the novel nitride (bpb)Os^VIN, is a model of N₂ activated species, [LM≡N]², in the proposed mechanism, Figure 7. The trichloride (bpb)Os^mCl₃, was also synthesized and fully characterized as precursors to various intermediates. The corresponding paramagnetic (bpb)Osⁿⁱ(CN)3 was synthesized and characterized for identification of products that could result from the trapping of various intermediates in the proposed catalytic cycle by treatment with CN^". Treatment of an aqueous KOH generated a diamagnetic species assigned as [(bpb)Osⁿ(CN)3]^~. This material could not be isolated and was characterized on the basis of qualitative and quantitative ^XH as well as ¹³C NMR. Consistent with the assignment, quantitative ¹³C NMR showed the expected two to one ratio of ¹³C signals resulting from the tris-cyano arrangement. These quantitative ¹³C NMR studies were carried out with samples generated from 100% isotopically enriched (bpb)Osⁿ(¹³CN)₃] . Reduction of (bpb)OsⁿⁱCl₃ with Zn in aqueous KOH, generated several new, diamagnetic species assigned as the aquo, hydoxo complexes

(bpb)Osⁿ(OH)_n(H₂0)3-n]^<2"n)+ where n could be 1 to 3. Evidence for this is provided by the observation that treatment with excess CN^" generated one diamagnetic species that was spectrochemically identical [(bpb)Osⁿ(CN)3]^~. The reactions of these various complexes that were examined are summarized in Figure 9. The complexes that were synthesized and fully characterized are shown in boxes with solid lines. Complexes characterized by in situ methods are designated by boxes with dashed lines. Plausible intermediates are shown without boxes.

As noted above, a key consideration in the design of practical molecular catalysts is the requirement that H₂ be utilized for the overall conversion of N₂ to NH₃. As can be seen in the proposed mechanism in Figure 7, H₂ is utilized to reduce the possible N₂ activated intermediates such as coordinated N₂, LM-N₂, or the nitrido species, LMN. To show feasibility for the proposed mechanism, key observations would be whether these reactions with gaseous ¾ generate NH₃ and the reduced metal center that would be required to bind and activate N₂. To examine the feasibility for these key requirements, solutions of the nitride complex, (bpb)Os^VIN, dissolved in 0.9 - 3.7 M aqueous KOH were treated with gaseous H₂. To determine whether the NH3 resulted from the nitride position of (bpb)Os N the reactions was carried using 100% enriched ¹⁵N in the nitride position. Importantly, these reactions were found to generate 100% ¹⁵N enriched ammonia, ¹⁵NH₃. Direct analyses of the crude solutions resulting from these experiments were complicated by the wide variety of various possible deprotonated species. To determine whether these were reduced the aquo, hydro xo species, (bpb)Os^II(OH)_n(H₂0)3-n] ^(2"n)+ where n could be 1 to 3, the crude reaction mixture after treatment with H₂ and generation of NH₃ was treated with excess CN^" to trap any intermediates as the stable cyanide derivatives.

Consistent with the formation of (bpb)Os^II(OH)_n(H₂0)3_-n]^(2"n)+, analysis by in situ *H and ¹³C NMR showed that a single diamagnetic species, [(bpb)Osⁿ(CN)₃] , was generated. This assignment was made by comparison to the ^XH and ¹³C NMR spectra of solutions of [(bpb)Osⁿ(CN)3]^~ that were generated from the independently generate (bpb)Osⁿ(OH)_n(H20)3-n] ^(2"n)+ n could be 1 to 3 as shown in Figure 9. These studies show that the reaction of the solutions of (bpb)Os^VIN in aqueous KOH with gaseous ¾ generated NH₃ as well as the reduced (bpb)Os¹¹ aquo/hydroxo complex. In addition to these studies the yield of NH₃ from reaction of of (bpb)Os^VIN with ¾ in aqueous KOH was found to increase with increasing concentration of the aqueous KOH solvent. This is consistent with the expectation that the functionalization step to generate NH₃ could be expected to show dependence on the concentration of base since the reaction requires the generation of solvated hydride, boxed inset in Figure 7. Further evidence for a hydride mechanism for the functionalization step was the observation that NaB¾ (which is stable in aqueous KOH) was also found to generate NH₃ on reaction with (bpb)Os^VIN. These initial results establish strong feasibility for the generation of NH₃ and the reduced form of the catalyst our proposed

functionalization methodology.

As can be seen in Figure 7, the expected reaction mechanism would require that the low oxidation state (bpb)Os¹¹ or (bpb)Os¹¹¹) motifs coordinate N₂. Evidence for this was shown by reduction of (bpb)OsⁱⁿCl₃ to the [(bpb)Osⁿ(OH)„(H₂0)3-n] ^{(2" n)+} complex in the presence of N₂. This generates a single diamagnetic complex in solution that we assign as the N₂ coordinated complex (bpb)Osⁿ(N₂)(OH)_n(H₂0)₂- j(n-2)+ _compi_{eX; w}here n = 1 or 2. Efforts at isolating these species were unsuccessful due to instability and water solubility. Attempts at detecting a coordinated N2 by use of ¹⁵N NMR was complicated by poor sensitivity. Strong evidence for assignment of this new species as the N₂ complex is its reversible formation from (bpb)Osⁿ(N2)(OH)_n(H20)2-n](^n"2)+ by alternate addition and removal of gaseous N₂. Thus, sweeping a solution of the presumed N₂ complex, (bpb)Osⁿ(N₂)(OH)_n(H₂0)₂- _n]C²⁾⁺ with Ar to remove N₂ resulted in the loss of the NMR signals associated with this species. Reintroduction of N₂ regenerated the signals due

(bpb)Osⁿ(N2)(OH)n(H20)2-n](^n"2)+. These observation provide evidence for the feasibility of N₂ coordination to the lower oxidation state of the (bpb)Os motif in strongly basic, aqueous KOH solutions.

The cleavage of the coordinate N₂ was not observed at room temperature most likely because the Os^m or Os¹¹ species utilized are not sufficiently reducing to undergo this type of stoichiometric reaction. However, we anticipate that such a reaction could be reversible at temperatures below 250 °C when complexes with sufficient thermal stability are designed. Significantly, treatment of the nitride, (bpb)Os^VIN with aqueous KOH leads to generation of gaseous N₂ along with minor amounts of NH₃. This reaction is the microscopic reverse of the N₂ cleavage reaction and the formation of N₂ from nitride coupling is consistent with the possibility for such a cleavage. To determine if N₂ did result from the coupling of two (bpb)Os^VIN molecules to generate the postulated intermediate, (bpb)Os(N₂)Os(bpb) followed by dissociation of N₂ as shown in Figure 9 the reaction was examined with a 1 :1 mixture of (bpb)Os¹⁵N and (bpb)OsN. Consistent with this coupling of the nitrides to generate N₂, analysis of the gas phase by GC/MS showed the expected a 1 :2:1 mixture of ¹⁴N₂, ¹⁵N¹⁴N and ¹⁵N₂. While this observation shows feasibility for the cleavage, it is not sufficient data to prove that the microscopic reverse, N₂ cleavage can occur. However, this is not the only expected pathway for activation of coordinated N₂ and other pathways involving direct attach of H^" or ¾0 on the coordinated N₂ could also be possible.

We believe the overall catalytic sequence for production of ammonia to be: HL-M-OH₂ + OH- -> [LM-OH₂]- + H20 This is the activation step of the catalyst by base. [LM-OH₂]- + N₂ -» ½ [LMN] + ½ H₂0 OR [LM-N2]- H₂0

Then LMN or LMN₂ reacts with H₂ and H₂0/OH- to regenerate the added catalyst [LM-OH₂]- while generating NH₃. The desired catalyst (or more accurately the catalyst precursor) is what we add; HL-M-H₂0.

All of these steps can be carried out in "one pot" and none of the species are isolated. In a working system, these sequential reactions proceed and repeat many times in a minute. The use of strongly pi-basic metals such as the one we use to coordinate or cleave N₂ is well documented. However, it is non-obvious that these metals can be made to coordinate and activate N₂ in basic media. This is because it would generally be considered that these metals would A) form insoluble oxides and B) tightly bind OH and not allow N₂ coordination. An embodiment of the invention is the use of basic solvents to facilitate: A) coordination and activation of N₂ by use of specific metals with liquid ligands and B) activation of H2 to facilitate reduction of the activated N₂. The activation can result by just coordination or by cleavage to the nitride.

The activation of ¾ by basic solvents is a long known, albeit somewhat forgotten, reaction. The key to using this reaction to facilitate the reduction of N₂ is to design catalysts that could allow coordination of N₂ in such basic media. This is where the use of liquid ligands and the judicious choice of metals comes into play. With these metal complexes, hydroxide does not prevent coordination of N2. In fact, it facilitates the coordination and possibly even the cleavage.

Reaction Products Other Than NH?

Products other than ammonia can be prepared using the catalysts of the invention. As shown in Scheme 2, urea is currently made from three separate plants that are likely not heat integrated. In plant 1 , ¾ is generated from C¾ in a separate, capital and energy intensive, high temperature reforming step. In plant 2, the ¾ is then utilized in a separate, capital and energy intensive, high temperature, high pressure, process to generate NH_3. In plant 3, NH₃ and CO₂ are reacted in two steps, at different temperatures to generate urea.

Scheme 2. Current Process for the production of Urea

Plant 1 : C¾ + 2 H₂0 -» C0₂ + 4 H₂ 900°C

Plant 2: N₂ + 3 H₂ -» 2NH₃ 450°C

Plant 3 : Step 1 : 2NH₃ + C0₂ -» H2NCO2 NH Low Temperature

Step 2: H2NCO2 NH -» H₂N(CO)NH₂

Carrying out the overall process in a one plant by a two step, heat integrated, lower temperature process could lead to lower capital and operating costs advantages as well as greater sustainability. As can be seen in the proposed process, Scheme 3, rather than a capital and energy intensive syngas process, ¾ can be generated by an oxidative, lower temperature, less energy intensive and capital process. This oxidative generation of ¾ would be carried out in a liquid phase basic solvent at

~180°C with the generation of C0₃ ^2" _(aq). The heat of reaction could be readily integrated into the second lower temperature step that utilizes the C0₃ ^2" _(aq) and H₂ generated in the first step to produce urea by direct reaction with N₂.

Scheme 3. Lower Temperature Process for the Overall Conversion of CFU. O? and

C0₃ ²-_(aq, + I¾_3q| ÷ 3H_2{aq¾ -^■> Η₂ ~αθ)~ Η₂ , + 2ΗΟ-₍ AG = -2 kcai/mol at 180C

CH ^!,4f₃q| 2 :iOH + ½ 0 3 (aq) + 3H AG = -49 kcai/mol at 180C

AH = -49 kcai/mol at 180C

CH₄ + ½ O, + N, H₂P -C(0)- H₂ AG - -37 kcai/mol at 180C

The process proposed in Scheme 3 could be developed by using plausible extensions of CH activation chemistry in basic media that is being developed at Scripps for the conversion of CH₄ to oxygenates in basic media. The second step could be based on the lower temperature conversion of N₂ H₂ to NH₃ discussed above and carried out in the presence of basic solutions containing CO₃ ^2" from the ¾ generation step. As can be seen there is no net generation of carbonate or

consumption of OH^" and the overall process is a one-plant, heat integrated, two step oxidative process for the direct conversion of N₂, O₂ and CH₄ to urea with 100% atom efficiency. This is a more complex project than the production of NH₃ from N₂ H₂ proposed above and could be examined after the molecular catalysts for the lower temperature production of NH₃ has been demonstrated.

Adapting the mechanism shown in Figure 7 to mixture of ¾ and

hydrocarbons, such as benzene, methane, etc., could lead processes to catalytically convert hydrocarbons and N₂ directly to ammines. The fundamental basis for this is the similarities in the weak acidity of H₂ and hydrocarbons. The pKa of H₂ is -36 and the pka of hydrocarbons such as benzene is -43. Consequently, it is possible that the ionic hydrogenation mechanism proposed in Figure 7 involving the deprotonation of H₂ by a strongly basic solvent to generate H^~ _soi, can be extended to the reversible deprotonation of hydrocarbon, RH, such as benzene, methane, etc., to generate the conjugate carbanions, Eq 5. Carbanions and hydrides are both strong bases but in general carbanions are considered to be more nucleophilic. Thus, by adjusting the ratio of ¾ to RH, these carbanions, as is proposed for H^" _soi, can react with the activated N₂ intermediates (LMN or LM-N₂ ) to generate C-N bonds that can lead to amines directly from N2 as shown in Scheme 4. There is precedent in the literature for this C-N bond forming reaction.

Eq 5 RH + :B -» R ^" + HB Scheme 4. Plausible Catalytic Reaction Mechanism for Generations of amines from hydrocarbons, RH, directly from N_?

RH + 20H^" -» 2R^" + 2H₂0

2 LMⁿOH + N₂ -» [LMⁿN₂ML]]²⁺ + 20H^"

[LMⁿN₂ML]]²⁺ -» 2[LM⁽ⁿ⁺³⁾N]⁺

2R^~ + 2[LM⁽ⁿ⁺³⁾N]⁺ ^ 2LM⁽ⁿ⁺¹⁾N-R

2LM⁽ⁿ⁺¹⁾NR + 4H₂0 -» 2LM⁽ⁿ⁺¹⁾(OH)₂ + 2RNH₂

2LM⁽ⁿ⁺¹⁾⁽OH)₂ + H₂ ^ 2 LMⁿOH + 2H₂0

NET: 2 RH + N₂ + H₂ -» 2 RNH₂ AG_180C = +6 kcal/molfor RH = C₆H₆

Another reaction pathway could be based on catalytic generation of incipient carbanions rather by deprotonation. As shown in Scheme 5, the incipient carbanions, LN-R°^", would be generated by base accelerated catalytic CH activation reactions of LN-OH with the hydrocarbons, RH, that we have demonstrated at Scripps.^Error! Bookmark not defⁱned^, j^jgg^ ^_{ata s}h_ows that these base accelerated CH activation reactions are orders of magnitude faster than acid/base deprotonation reactions with the basic solvent shown in Scheme 4. Significantly, as there is a basis for anticipating strong similarities between CH activation and N₂ activation in basic media, vide supra, is the possible that the same catalyst could carry out both N₂ and CH activation reactions with the relative rates adjusted by varying the concentrations of the N₂ , H₂ and RH reactants. Reaction of these LN-R°^" intermediates with the activated N₂ complexes could lead to amines as shown in Scheme 5.

Scheme 5. Catalytic Coupling of CH Activation with N? Activation to Generate RNH_Z

2RH + 2LN-OH -» 2LN-R + 2H₂0

2 LMⁿOH + N₂ -» [LMⁿN₂ML]]²⁺ + 20H^"

[LMⁿN₂ML]]²⁺ -» 2[LM⁽ⁿ⁺³⁾N]⁺

2LN-R + 2[LM⁽ⁿ⁺³⁾N]⁺ + 20H^" -» 2LM⁽ⁿ⁺¹⁾N-R + 2LN-OH

2LM⁽ⁿ⁺¹⁾NR + 4H₂0 -» 2LM⁽ⁿ⁺¹⁾(OH)₂ + 2RNH₂

2LM⁽ⁿ⁺¹⁾⁽OH)₂ + H₂ -» 2 LMⁿOH + 2H₂0

NET: 2 RH + N₂ + H₂ -» 2 RNH₂ AG₁₈oc = +6 kcal/molfor RH = C₆H₆ Embodiments of the Invention

The present invention is directed in various embodiments to catalysts and methods for activation of dinitrogen N₂, (e.g., nitrogen gas) such as for reduction of N₂ with a reductant in the presence of the catalyst to ammonia NH3, or conversion to nitrogen-containing products such as urea and amines. In various embodiments, the invention provides methods of preparation of ammonia and other nitrogen-containing products from nitrogen gas in the presence of a reductant such as hydrogen gas, under conditions of relatively low temperature and low pressure compared with

temperatures and pressures used in art methods of nitrogen fixation (reduction). Catalysts of the invention are molecular, i.e., not heterogeneous, catalysts, soluble in a solvent medium comprising a strong base and a liquid solvent.

In various embodiments, the invention provides a catalyst for a process comprising reduction of N₂ to a reduced nitrogen-containing product, comprising:

(a) a complex comprising a group IVA, VA, VIA, VIIA or electropositive group VIIIA metal M in a low oxidation state complexed by a unidentate, bidentate, tridentate, or tetradentate ligand L to provide a complex L-M, wherein at least one of the complexing atoms of the ligand L is a nitrogen atom, the complex optionally comprising additional ligands, wherein the complex L-M is soluble in a solvent medium comprising a strong base;

(b) the solvent medium comprising a strong base.

More specifically, the invention provides the catalyst wherein the ligand L comprises a proton H abstractable by strong base, and wherein the base deprotonates complex L-M to form the complex L^" -M in the solvent medium. For example, ligand L can be a polycyclic heteroaryl group or several linked heteroaryl groups, comprising at least one and optionally multiple nitrogen atoms, wherein one of more of the nitrogen atoms forms an NH group of, e.g., a pyrrole or indole or

benzimidazole heteroaryl system, wherein the NH group can be deprotonated by strong base to provide an anionic ligand. Thus, the anionic L^" -M complex is a deprotonated form of a L-M metal-ligand complex wherein the ligand comprises at least one abstractable (i.e., at least weakly acidic) proton bonded to a nitrogen atom.

In various embodiments, the metal atom is an ionic (cationic) form of a metal element selected from a group consisting of a IVA, VA, VIA, VIIA and an electropositive group VIIIA metal. As is evident from inspection of the periodic tables, such metals include Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Tc (radioactive), Re, Fe, Ru, and Os. By an electropositive group VIIIA metal is meant Fe, Ru, and Os, but not Ni, Pd, or Pt, which are considered electronegative group VIIIA metals. In other embodiments, the metal atom is a zerovalent form of the metal element.

More specifically, the metal complex is formed of a metal selected from the group consisting of Os, Re, Ru, Mn, and Fe. A low oxidation state of any of these metals is preferred for the catalyst of the invention. A low oxidation state can include a zerovalent oxidation state, or a cationic oxidation state wherein the oxidation state is not the highest available oxidation state under chemical conditions in solution.

A ligand L can be a unidentate, bidentate, tridentate, or tetradentate complexing agent of the metal atom selected. The ligand contains at least a single nitrogen atom, and can contain multiple nitrogen atoms, some but not all of which can be involved in the formation of the metal-ligand complex with the selected metal. For example, the ligand can include one or more pyrrolyl, pyrrazolyl, imidazolyl, pyridinyl, pyrimidinyl, pyrazinyl, indolyl, benzimidazolyl, or pyridinopyrrolyl ring systems, or the like. The rings systems can be bonded directly to each other, or can be bonded via a linker group such as a methylene, amino, carbonyl, oxy, sulfonyl group or the like. In various embodiments, two or more of the rings can be mutually conjugated. In other embodiments rings are electronically isolated from each other.

In various embodiments, the ligand can be any of:

wherein a dotted line indicates that a substructure can be present or absent.

In various embodiments, the invention provides a catalyst wherein the strong base in the solvent medium is an alkali metal or alkaline earth metal hydroxide in water or an alkali metal or alkaline earth metal amide in liquid ammonia, or a alkali metal or alkaline earth metal alkoxide in an alcohol, or an alkali metal or alkaline earth metal alkylamide in an alkylamine; or is a molten salt medium comprising one or more alkali metal or alkaline earth metal hydroxide or one or more alkali metal or alkaline earth metal amides or one or more alkali metal or alkaline earth metal alkoxide or at least one or more alkali metal or alkaline earth metal alkylamide, or any combination thereof. Examples include KOH or NaOH in water, Mg(OH)₂ or Ca(OH)₂ in water; KNH₂, NaNH₂, Mg(NH₂)₂, or Ca(NH₂)₂ in liquid NH₃; salt mixtures of NaOH/KOH or NaNH₂/KNH₂ in molten form, and the like. Accordingly, a solvent medium for dissolving the catalyst can be a protic medium such as water or liquid ammonia dissolving the respective strong base, or can be a molten salt medium wherein the base(s) are also the liquid.

More specifically, in various embodiments, for the catalyst the metal atom M ligand L can be

This ligand is of the type bearing an abstractable proton, such that in strong base the conjugate anion of one or both of the benzimidazole ring systems is produced in the catalyst of the invention. It is believed that formation of such an anionic ligand can both improve solubility of the metal-ligand complex in the solvent medium, and increase the electron density of the metal atom in the low oxidation state. As described above, increased electron density, such as in the d orbitals of the metal, can enhance catalytic properties for N≡N bond cleavage (or for C-H bond cleavage) by transfer of electron density from the metal d orbitals to antibonding orbitals of the N≡N (or C-H) bond, facilitating cleavage.

The availability of higher oxidation states of the metal can also be significant in the catalytic activity of the metal-ligand complex, in that reduction of bond order of the N≡N molecule can occur in conjunction with oxidation of the metal center(s) in various embodiments of the invention. The dinitrogen molecule can be complexed by one or two of the metal-ligand complex, leading to the formation of the cleaved species, believed to be a metal nitride species or a metal dinitrogen species, which can then be reduced with a reductant such as nitrogen to produce ammonia or other nitrogen-containing products. More specifically, the optional additional ligands can comprise ^" OH, H₂0, halide, or any combination thereof. More specifically, the metal atom can be Os and the oxidation state of the metal atom can be the Os¹¹ or Os^m oxidation state.

In various embodiments, the invention provides a method of reducing N₂ to NH3, comprising contacting the catalyst of the invention, and N₂ and a reductant in the presence of a solvent medium comprising strong base, under conditions suitable to provide the NH₃. For example, the reductant can be H₂, or can be a substance, the oxidation product of which can be regenerated to the reductant by hydrogen, optionally with use of a reduction catalyst.

An outstanding advantage of the invention is the use of temperature and pressure conditions less extreme than those required by art methods of nitrogen reduction. Specifically, in various embodiments, the conditions can comprise a temperature of less than 250°C and less than 1000 psig (6.67 mPa). In the formation of ammonia from nitrogen gas, preferably the catalyst is exposed to the N₂ and the H₂ concurrently, such as under the stated conditions in a sealed vessel, continuous flow gas-liquid reactor, or other pressure apparatuses such as are well known in the art. The catalyst can be disposed on a solid support with which gas contact can be made, as is well known in the art.

In various embodiments, it is believed that the reduction of N2 can occur via formation of an intermediate ligand-metal-nitride complex or a ligand-metal- dinitrogen complex or a ligand-metal-dinitrogen-metal-ligand complex, which can then subsequently undergo reduction with hydrogen and, optionally, a second reagent.

Thus catalyst the invention can be used for production of ammonia by contacting the respective complex with a reducing agent. In various embodiments, the catalyst can also be used for production of other nitrogen-containing products, such as urea, or amines. For example, when only a reductant such as H₂ is used, the product is ammonia. When an additional reagent is added, other nitrogen-containing products can be obtained. For example, when the reducing agent is H₂, and the second reactant is carbonate, the nitrogen-containing product can be urea

H₂NC(=0)NH₂. When the reducing agent is a hydrocarbon and, optionally, hydrogen, the nitrogen-containing product can be an amine. For instance, when the reducing agent is a mixture of benzene and hydrogen, the nitrogen-containing product can be aniline. When the reductant is hydrogen, it is believed that the hydrogen is activated by the presence of the strong base to produce a solvated hydride ion, at least in low concentrations, by abstraction of a proton from H₂. This hydride species is activated for functionalization of the reduced nitrogen species bound to the metal catalyst, e.g., the nitride L-M-N or reduced dinitrogen complex L-M-N2, as discussed above.

Analogously, carbon-based substances comprising a C-H bond can be activated by strong base through formation of a carbanion, which allows production of amines by functionalization of the reduced nitrogen species bound to the metal catalyst, e.g., the nitride L-M-N or reduced dinitrogen complex L-M-N₂,. Precedent for activation of a C-H bond in this manner is found in the literature; see, for example, T.J. Crevier and J.M. Mayer, "Direct Attack of Phenyl Anion at an Electrophilic Osmium-Nitrido Ligand", J. Am. Chem. Soc. (1998), 120, 5595-5596; and T.J.

Crevier, et al, "C-N Bond Formation on Attack of Aryl Carbanions to the

Electrophilic Nitrido Ligand in TpOsNCl₂", J. Am. Chem. Soc. (2001), 123, 1059- 1071.

Correspondingly, various embodiments of the invention provide a method of forming a metal nitride complex of formula L-M-N, or a metal-dinitrogen complex L- M-N₂ wherein the nitrogen- nitrogen bond is of reduced bond order compared to dinitrogen, from N₂, comprising contacting the N₂ and the catalyst of the invention in the presence of a strong base in a liquid solvent under conditions suitable for production of the metal nitride complex. Formation of the complex L-M-N or L-M- N₂ can comprise a temperature of less than 250°C and less than 1000 psig (6.67 mPa), i.e., substantially less extreme than the high temperatures and pressures employed in art processes of activating nitrogen for reduction/functionalization. Again, as described above the metal nitride complex can be of formula L^" -M-N.

As described for the catalyst instrumental in forming the L-M-N or L-M-N₂ complex from N₂, the invention provides in various embodiments a method further comprising converting the metal nitride complex of formula L-M-N or metal dinitrogen complex of formula L-M-N₂ to a nitrogen-containing product, comprising contacting the L-M-N or L-M-N₂, and a reducing agent and, optionally, a second reactant, in the solvent medium comprising the strong base, at a temperature of less than 250°C and less than 1000 psig (6.67 mPa). When the reducing agent is H₂ the nitrogen-containing product is NH₃. When the reducing agent is H₂, the second reactant is carbonate, the nitrogen-containing product is urea Fl₂NC(=0)NH₂. When the reducing agent is a hydrocarbon and, optionally, hydrogen, the nitrogen- containing product is an amine. When the reducing agent is a mixture of benzene and hydrogen, the nitrogen-containing product is aniline.

Accordingly, an embodiment of the invention provides a method for conversion of N₂ to NH3, comprising contacting the N₂ and the catalyst of the invention and, in the presence of the strong base and the liquid solvent, either concurrently or sequentially, contacting with H₂ to provide NH₃, carried out at a temperature of less than 250°C and less than 1000 psig (6.67 mPa). An embodiment provides a method of converting N₂ to urea H₂NC(=0)NH₂ comprising contacting the N₂ and the catalyst of the invention and, in the presence of the strong base and the liquid solvent, either concurrently or sequentially, contacting with ¾ and carbonate to provide the urea, carried out at a temperature of less than 250°C and less than 1000 psig (6.67 mPa). An embodiment provides a method of converting N₂ to an amine comprising contacting the N₂ and the catalyst of the invention and, in the presence of the strong base and the liquid solvent, either concurrently or sequentially, contacting with ¾ and a hydrocarbon to provide the amine, carried out at a temperature of less than 250°C and less than 1000 psig (6.67 mPa). For example, when the hydrocarbon is benzene the amine is aniline.

The invention provides in various embodiments ammonia, urea, and amines produced by methods of the invention employed catalysts of the invention.

In various embodiments the invention provides methods of preparing catalysts and catalysts of the invention, suitable for use in practice of methods of the invention. For example, the invention provides a method of making the catalyst of the invention, comprising contacting an inorganic salt of metal atom M and ligand L and, optionally, one or more additional ligands, under conditions suitable to provide L-M, then dissolving L-M in the solvent medium comprising the strong base. More specifically, the metal M and the ligand L can be contacted in the presence of halide ion to provide a metal-ligand -halide complex. For example, when the ligand L bears a proton on a nitrogen atom such that upon contact with the strong base in the solvent medium, the ligand L is deprotonated to provide the complex L^" -M. For example,

the metal can be Os, and the in the complex L-M the Os can be in the Os(III) oxidation state.

Further specifics are provided below, in the Examples section.

Examples Synthetic Procedures

General Considerations:

All air and water sensitive procedures were carried out either in an MBraun inert atmosphere glove box, or using standard Schlenk techniques under argon.

Anhydrous methanol was purchased from Alfa Aesar and used without further purification. All deuterated solvents (Cambridge Isotopes) and 40% KOD/D₂0 (Sigma-Aldrich) were used as received. The KOD/D₂0 solutions were prepared by diluting of 40% KOD/D₂0 with D₂0 to the desired concentration. OsC was purchase from Electron Microscopy Sciences, (NH₄)₂0sCl₆ was purchased from Alfa Aesar, and all other chemicals were purchase from Alfa Aesar, Sigma-Alrich, or Acros Chemical. Synthesis of 2,6-bis(benzimidazoyl)pyridine (BPB),

[Bu₄N][OsNCi4] and [Bu₄N] [Os¹⁵NCi4] was prepared using a previously published method (Cowman, C. D.; Trogler, W. C ; Mann, K. R. ; Poon, C. K. ; Gray, H. B. Inorg. Chem. 1976, 15, 1747). NMR spectra were obtained on a Bruker Digital Avance III 400 (400.132 MHz for ^lU and 100.623 MHz for ¹³C) spectrometer. All chemical shifts are reported in units of ppm and referenced to the residual protonated solvent. FTIR was performed on a PerkinElmer Spectrum One FT-IR Spectrometer equipped with a UATR with a ZnSe crystal top-plate. Gas measurements were analyzed with a Shimadzu GC-MS QP2010S equipped with an Agilent GasPro or HP- MoleSieve column. All high-resolution mass spectra were obtained by Mass

Spectrometry Laboratory at the University of Illinois at Urbana-Champaign on Q-Tof Ultima mass spectrometer. Elemental Analysis was performed by Columbia

Analytical Services of Tucson, Arizona.

Synthesis of Osftfc-BPB^CU (1 ):

To a 50 mL Schlenk flask equipped with a reflux condenser and vacuum adapter, (NH^OsCle (2.00 g, 4.56 mmol), 2,6-bis(benzimidazoyl)pyridine (1.42 g, 4.56 mmol, 1 eq.), LiCl (0.58 g, 13.67 mmol, 3 eq.), and anhydrous N,N- dimethylacetamide (20 mL) were added. The solution was heated to 140°C for 15 h. After cooling with an ice bath under Ar, the flask was opened to air and ether (70 mL) was added and stirred. An oil was formed and the ether layer was decanted. This process was repeated three times and MeOH (20 mL) was added to the solution. Upon addition of MeOH, a dark brown ppt was formed and ether (70 mL) was added to the solution. The resulting brown solid was filtered and washed with ether to ensure removal of residual DMA. The resulting solid was washed successively with methanol (3 x 40 mL), 1 N HC1 (3 x 10 mL), and diethyl ether (3 x 40 mL). The resulting brown solid was dried under high vacuum at 150 °C for 3 days to yield 1.68 g (61 % yield). HRMS (ESI): Calculated for C₁₉H₁₃CI₂N5OS (M-Cl) 573.0163, found 573.0159. Elemental Analysis: Calculated for C₁₉H₁5CI₃N5OOS (Os(BPB)Cl₃'H₂0): C, 36.46 ; H, 2.42; N, 11.19. Found: C, 36.62; H, 2.53 ; N, 11.12. FTIR (ZnSe) in cm^"1 = 3500-2600 (bw), 1608 (w), 1557 (w), 1465 (vs), 1362 (w), 1322 (m), 1233 (w), 1 144 (w), 1062 (w), 997 (w), 913 (w), 851 (w), 811 (m), 748 (vs) (where bw = broad weak, w = weak, m = medium, and vs = very sharp).

Synthesis of Os(H-BPB)NCl_z (2):

A previously published procedure (Williams, D. S.; Coia, G. M. ; Meyer, T. J. Inorg. Chem. 1995, 34, 586) was modified to obtain higher purity products. To a 8 dram amberized vial equipped with a stirbar, [B114N] [OsNCU] (2.37g, 4.01 mmol, 1.25 eq.), 2,6-bis(benzimidazoyl)pyridine (1.00 g, 3.21 mmol), NaHC0₃ (0.07 g, 0.8 mmol, 0.25 eq.) and anhydrous MeOH (25 mL) was added and stirred for 15 h. The solvent was removed under reduced pressure and redissolved in anhydrous THF (50 mL). The residue was washed with anhydrous THF (3 x 30 mL) and the solvent was removed under reduced pressure. To the resulting residue, anhydrous MeOH (75 mL) was added and the orange solid was filtered. The solid was washed successively with MeOH (3 x 25 mL) and ether (3 x 25 mL). The resulting orange solid was placed under high vacuum at 100 °C overnight to remove residual solvent to yield a deep red product (60-70% yield). ^lU NMR (400 MHz, d⁷-DMF, δ/ppm) 8.87 (t, 1H, ; = 7.8 Hz), 8.57 (dt, 2H, ; = 8.1 Hz, 0.8 Hz), 8.55 (d, 2H, 7.8 Hz), 7.93 (dt, 2H, ; = 8.1 Hz, 0.8 Hz), 7.50 (ddd, 2H, ; = 8.2 Hz, 7.1 Hz, 1.2 Hz), 7.38 (ddd, 2H, ; = 8.2 Hz, 7.1 Hz, 1.2 Hz). ^uC{ ^lR} NMR (400 MHz, d⁷-DMF, δ/ppm) 164.1 , 152.5, 147.9, 147.9, 147.0, 125.0, 123.1 , 121.3, 120.8, 115.8. HRMS (ESI): Calculated for Ci₉HnClN₆0s (M-Cl) 551.0427, found 551.0417. Elemental Analysis: Calculated for

Ci₉Hi₂Cl₂N₆OOs: C, 38.98; H, 2.07; N, 14.35. Found: C, 38.72; H, 2.33 ; N, 14.03. FTIR (ZnSe) in cm^"1 = v(Os≡¹⁵N) 1099 (vs).

Synthesis of Os(H-BPB)¹⁵NCl_z (2):

This was prepared by the same method starting with [Bu₄N] [Os^i:,NCi₄]. H

NMR (400 MHz, d⁷-DMF, δ/ppm) 8.85 (t, 1H, ; = 7.8 Hz), 8.57 (dt, 2H, ; = 8.1 Hz, 0.8 Hz), 8.55 (d, 2H, 7.8 Hz), 7.93 (dt, 2H, ; = 8.1 Hz, 0.8 Hz), 7.50 (ddd, 2H, ; = 8.2 Hz, 7.1 Hz, 1.2 Hz), 7.38 (ddd, 2H, ; = 8.2 Hz, 7.1 Hz, 1.2 Hz). ^l3C{ ^lU} NMR (400 MHz, d⁷-DMF, δ/ppm) 164.1 , 152.5, 148.0, 147.9, 147.0, 125.0, 123.1, 121.3, 120.9, 1 15.8. HRMS (ESI): Calculated for

(M-Cl) 552.0397, found

552.0392. Elemental Analysis: Calculated for

C, 38.91 ; H, 2.06; N, 14.50. Found: C, 39.22; H, 2.03; N, 14.31. FTIR (ZnSe) in cm^"1 = v(Os≡¹⁵N) 1063 (vs).

Synthesis of Qs(BPB)(CN)₂:

In an oven dried, 50 mL Schlenk flask, 400 mg (0.66 mmol) of Os(BPB)Cl₃ was dissolved in 20 mL of anhydrous DMA under argon. To this mixture, 10 eq of NaCN (6.58 mmol) was added to a stirring solution. The mixture was then heated in an oil bath at 100 °C for 3 days under argon. The reaction was then cooled to RT and the dark red solution was slowly added to 500 mL of stirring ethyl ether. The solution was stirred for 15 min and then stopped allowing the solid to settle to the bottom. A majority of the ether was decanted off and then the flask was refilled with 500 mL of ether a second time, stirred for 15 min and then the ether decanted off and repeated once more. The residue and remaining ether was dried on the rotovap. The residue was then dissolved in minimal DI water. With stirring, 3 M HCl was added drop wise until a solid precipitated from the solution (pH = 1 by litmus paper test), which was then filtered across a fine sintered glass frit. The frit was then washed with portions of 3 M HCl followed by 4 x lOOmL of ether. The sample was then dried overnight on a vac line. The dried sample was then dissolved in MeOH and washed across a frit containing a plug of celite, washed with MeOH repeatedly and then dried to give a 72% yield as a dark brown/black powder. NMRs were taken by dissolving a sample in 2 M KOD/D₂O and addition of excess Zn dust followed by filtration of the residual zinc over a celite pipette filter. HRMS (ESI): Calculated for C₂₂Hi₄N₈0s [M+H]⁺ 582.0956, found 582.0950. Elemental Analysis: Calculated for Ci₉Hi₂Cl₂N5¹⁵NOs: C, 38.91 ; H, 2.06; N, 14.50. Found: C, 39.22; H, 2.03; N, 14.31. FTIR (ZnSe) in cm^"1 v = 2618 (bw), 2076 (Os-¹²C≡N, m), 1594 (w), 1463 (vs), 1320 (s), 1233 (ws), 1145 (s), and 760 (vs) (where bw = broad/weak, b = broad, w = weak, m = medium, s = sharp, ws = weak/sharp, and vs = very sharp). NMRs are following reduction with Zn in KOD/D₂O. ^lU NMR (400 MHz, D₂0, δ/ppm) 7.95 (dd, 2H, ; = 5.8, 3.6 Hz), 7.87 (d, 2H, ; = 8.3 Hz), 7.69 (t, 1H, ; = 7.9 Hz), 7.56 (m, 2H), and 7.22 (dd, 4H, ; = 5.9, 3.2 Hz). "C^H} NMR (400 MHz, D₂0, δ/ppm) 163.2, 154.2 (t,; = 4.4 Hz), 153.8, 150.4 (d,; = 4.4 Hz), 147.3, 144.5, 121.5, 121.2, 117.7, 117.5, 117.2.

Synthesis of Qs(BPB)("CN½:

This was prepared by a similar procedure as Os(BPB)(CN)3 except K¹³CN (10 eq) was used instead. HRMS (ESI): Calculated for ¹²Ci₉ ¹³C₃Hi₄N₈Os [M+H]⁺ 585.1057, found 585.1062. FTIR (ZnSe) in cm^"1 v = 2618 (bw), 2020 (Os-¹³C≡N, m), 1593 (w), 1462 (vs), 1319 (s), 1233 (ws), 1145 (w), and 760 (vs) (where bw = broad/weak, b = broad, w = weak, m = medium, s = sharp, ws = weak/sharp, and vs = very sharp). NMR data for ^lH and ¹³C experiments were comparable to that of

Os BPB)(CN)₃.

Reduction of Qs(BPB)NCl_z and Os(BPB)C½ to Os(BPB)(CNK

In an Argon filled wet box, a 40 mM solution of either Os(BPB)Ci₃ or

Os(BPB)NCi₂ was prepared in 2 mL of 2.85 M KOD/D₂0 and stirred in the presence of 35 eq of Zn for 2 h at RT. A H NMR was taken. Then 10 eq of K¹³CN was added and stirred for 2 h at RT followed by warming to 50 °C for several hours. The samples were then analyzed by ¹³C NMR. The appearance of the doublet (lC)/triplet(2C) pattern was clearly seen following reduction of both complexes. The H NMR following reduction of both Os(BPB)Ci3 and Os(BPB)NCi₂ shows the same complex an Osⁿ(BPB) hydroxo/aquo complex. This owes to the high lability of the coordinated hydroxo or aquo ligands around the Os(II) center. Therefore, displacement of π- acceptors such as N₂ is plausible.

Reaction of Os(BPB)Ch with N? in aqueous base

General Protocol: 50 mg of Os(BPB)Ci₃ (and 100 mg of Zn when used as a reductant) was placed into each of 4, 10 mL high pressure stainless steel reactors equipped with Teflon liners and stirbars. The reactors were brought into an argon filled wetbox and 2 mL degassed 1-20 % KOD / D₂O was added to each reactor. The reactors were sealed (hand tight) and brought out of the box. The reactors were immediately tightened using the vice. The reactor headspace was sparged with N₂ or Ar and then pressurized and sparged again to ensure gas dissolving in the solution. The reactors were stirred at room temp for 2 hours at room temperature. Pressure was released and loosened on the vice, hand-tightened, and brought into the argon filled wetbox to prepare NMR samples.

Reduction of Qs(BPB)NCl₂ and Os(BPB)Ck with H₂

General Protocol: 50 mg of Os(BPB)Cl₃ and 48 mg of Os(BPB)NCl₂ (and 100 mg of Zn when used as a reductant) were placed into separate, 10 mL high pressure stainless steel reactors equipped with Teflon liners and stirbars. The reactors were brought into an argon filled wetbox and 2 mL degassed 10 % KOD / D₂0 was added to each reactor. The reactors were sealed (hand tight) and brought out of the box. The reactors were immediately tightened using the vice. The reactor headspace was sparged with H₂ or Ar and then the reactor was pressurized (300 psi) and sparged repeatedly (5 x) to ensure gas dissolving in the solution. The reactors were stirred were heated to 80 °C for 2 h and subsequently stirred at room temperature for an additional 2 h. Pressure was released and loosened on the vice, hand-tightened, and brought into the argon filled wetbox to prepare NMR samples.

Formation of Isotopologues of N? from Qs≡N in basic solvents.

Into 3 separate 8 mL vials, 48.0 mg of (BPB)Os(¹⁴N)Cl₂(#l), 50:50

(BPB)Os(¹⁴N)Cl₂:(BPB)Os(¹⁵N)Cl₂(#2), and (BPB)Os(¹⁵N)Cl₂ (#3) were massed. To each, 5 mL of 0.5% KOH was added. The reactions were sealed, degassed under argon and then stirred overnight (15 hrs) in an inert atmosphere glove box. The headspace was sampled via syringe, with the gas analyzed on a GC/MS mol sieve column.

The analysis indicated that the expected distributions of MW 29 and MW 30 N₂ gas present for a bimolecular reaction between two Os centers had occurred (#1 : minimal observed; #2: 2:1 ; #3: 0:1)

Reduction of Qs≡N to produce N¾ with H₂. NaBPL. and Zn.

A single stock 12.55 mM stock solution of (BPB)Os(¹⁵N)Cl₂ was made by dissolving 195.5 mg (0.314 mn) of (BPB)Os(¹⁵N)Cl₂ in 25 mL of 0.5% KOH. This solution was allowed to stir for 30 minutes at room temperature to ensure a homogeneous solution; and to allow for any C17OH exchange events to take place. It is important that all of the subsequent steps are performed quickly after this 30 minute stir period, as the complex has been shown to react to release N₂ gas over time in hydroxide solutions.

During this time, two 0.2 M stock solutions of NaBFL were prepared by dissolving 103.7 mg and 114.7 mg into 13.7 mL 10% KOH and 15.2 mL 35% KOH respectively. Attempts to dissolve NaB¾ into lower [KOH] solutions resulted in significant bubbling due to H₂ release.

2 mL of the Os stock solution (25.11 μιηοΐ) was measured into 7 teflon cups (Rxn #1- #7), and 2 glass vials (Rxn #8-9). To each, 2 mL of the following solutions/additives were added: #1 - 0.5% KOH; #2 - 10% KOH; #3 - 35% KOH; #4 - 0.5% KOH; #5 - 10% KOH; #6 - 35% KOH; #7 - 35% KOH and 100 mg Zn dust; #8 - 0.2 M

NaBH4/10% KOH; #9 - 0.2 M NaBH4/35% KOH.

A stock solution of 14.27 mM 1 ,3,5-trimethoxybenzene was made by dissolving 60.0 mg (0.357 mmol) in ife-DMSO. 1 mL of this solution was measured into 9 4 mL vials, and 2 drops of HCl(_COnc) were added to each, as to acidify the solvent to trap any evolved NH₃.

Reactions in the teflon cups (#1 -7) were placed into a high pressure mini reactor with a mini-cross stirbar, degassed and then pressurized with: #1-3: 500 psi Ar; #4-6: 500 psi H2; #7: 500 psi Ar. Reactions in the glass vials were sparged with Ar (5 minutes) before heating.

Reactions #1-7 were heated at 80°C for 2 hrs, cooled in an ice bath and then vented through the aforementioned Std/DMSO trapping solution. Reactions #1-7 were then heated at 80°C under a constant Ar sparge to remove any ¹⁵NH₃ still dissolved in solution. Reactions #8-9 were heated to 80°C for 2 hrs under a constant Ar sparge, with the outlet bubbling through the Std/DMSO trapping solution.

Analysis of the Std/DMSO solutions indicated <1% recovery of ¹⁵NH₃ when no reductant is present (#1-3); and increasing amounts of ¹⁵NH₃ when H₂ is present as [KOH] increases (<1 %, 3%, 34%; #4-6).

All patents and publications referred to herein are incorporated by reference herein to the same extent as if each individual publication was specifically and individually indicated to be incorporated by reference in its entirety.

The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention that in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the concepts herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention as defined by the appended claims.

Claims

CLAIMS What is claimed is:

1. A catalyst for a process comprising reduction of N2 to a reduced nitrogen- containing product, comprising:

(a) a complex comprising a group IVA, VA, VIA, VIIA or electropositive group VIIIA metal M in a low oxidation state complexed by a unidentate, bidentate, tridentate, or tetradentate ligand L to provide a complex L-M, wherein at least one of the complexing atoms of the ligand L is a nitrogen atom, the complex optionally comprising additional ligands, wherein the complex L-M is soluble in a solvent medium comprising a strong base;

(b) the solvent medium comprising a strong base.

2. The catalyst of claim 1 wherein the reduced nitrogen-containing product is NH₃, urea, or an alkylamine.

3. The catalyst of claim 1 wherein the ligand L comprises a proton H abstractable by strong base, and wherein the base deprotonates complex L-M to form the complex L^" -M in the solvent medium.

4. The catalyst of claim 1 wherein the metal atom is an Os, Re, Ru, Mn, or Fe metal atom.

5. The catalyst of claim 1 wherein the ligand is any of:

wherein a dotted line indicates that a substructure can be present or absent.

6. The catalyst of claim 1 wherein the strong base in the solvent medium is an alkali or alkaline earth metal hydroxide in water or an alkali or alkaline earth metal amide in liquid ammonia, or an alkoxide in alcohol, or an alkylamide in an alkylamine; or is a molten salt medium comprising a mixture of at least two alkali or alkaline earth metal hydroxides or at least two alkali or alkaline earth metal amides, alkoxides, or alkylamides.

7. The catalyst of claim 1 m M is Os and ligand L is

8. The catalyst of claim 1 wherein the optional additional ligands comprise OH, ¾0, halide, or any combination thereof.

9. The catalyst of claim 1 wherein the metal atom is Os and the oxidation state of the metal atom is the Os¹¹ or Os¹¹¹ oxidation state.

10. The catalyst of claim 1 wherein cleavage or a reduction of the bond order of a nitrogen-nitrogen bond of N2 during the process of reduction of the N2 is

accompanied by an increase in the oxidation state of the metal atom.

11. The catalyst of claim 1 wherein upon contacting with N₂, a reducing agent and, optionally, a second reactant, a nitrogen-containing product is provided.

12. The catalyst of claim 11 wherein the reducing agent is hydrogen and the nitrogen-containing product is NH₃.

13. The catalyst of claim 11 wherein the reducing agent is H₂, the second reactant is carbonate, and the nitrogen-containing product is urea H₂NC(=0)NH₂.

14. The catalyst of claim 11 wherein the reducing agent is a hydrocarbon and, optionally, hydrogen, and the nitrogen-containing product is an amine.

15. The catalyst of claim 14 wherein the reducing agent is a mixture of benzene and hydrogen, and the nitrogen-containing product is aniline.

16. A method of reducing N₂ to NH3, comprising contacting the catalyst of claim 1 and N₂ and a reductant in the presence of a strong base in a solvent medium under conditions suitable to provide the NH₃.

17. The method of claim 16 wherein the reductant is H₂.

18. The method of claim 16 wherein the conditions comprise a temperature of less than 250°C and less than 1000 psig (6.67 mPa).

19. The method of claim 16 wherein the ligand L comprises a proton H abstractable by strong base, and wherein the base deprotonates complex L-M to form the complex L^" -M in the solvent medium.

20. The method of claim 17 wherein the catalyst is exposed to the N₂ and the H₂ concurrently.

21. The method of claim 17 wherein the catalyst is exposed to the N₂ and the ¾ sequentially.

22. A method of converting N₂ to urea H₂NC(=0)N]¾ comprising contacting the N₂ and the catalyst of claim 1 and, in the presence of the strong base and the liquid solvent, either concurrently or sequentially, contacting with H₂ and carbonate to provide the urea, carried out at a temperature of less than 250°C and less than 1000 psig (6.67 mPa)..

23. A method of converting N₂ to an amine comprising contacting the N₂ and the catalyst of claim 1 and, in the presence of the strong base and the liquid solvent, either concurrently or sequentially, contacting with ¾ and a hydrocarbon to provide the amine, carried out at a temperature of less than 250°C and less than 1000 psig (6.67 mPa).. The method of claim 23 wherein the hydrocarbon is benzene and the amine is