Ammonia formation using chemical reagents

Beyond the Haber Bosch (HB) process, renewed interest in alternative pathways for ammonia formation raised around the 1960’s, mainly because of increasing knowledge around nitrogenase. Inorganic compounds capable of N2-chemistry could function as bio-mimicking models to thrive the understanding of the biological mechanism of dinitrogen activation. Yet, most breakthrough results first came in the current century and an overview of some highlight findings both in the (electro)chemical homogeneous as heterogeneous field are given. An extended overview of all efforts on molecular ammonia formation is recently covered by Peters.⁸⁶

A pioneering example for stoichiometric N-H bond formation comes from Chatt in 1975:

protonation of cis-[W(N2)2(PMe2Ph)4] with a strong acid releases ammonium in slightly less then quantitative yields (see Scheme 18). This is accompanied by some hydrazine formation and the release of 1 eq. of N2, indicating ligand dissociation.^87,88 It is acknowledged that in absence of external reductant, the metal centre is oxidised and likely W(VI)oxo species are formed.

Encouraged by the subsequent isolation of several intermediates, a potential catalytic cycle was formulated for ammonia formation, cycling through Mo^0/III states, known as the ‘Chatt cycle’.

This is also a working hypothesis model for nitrogenase. Compared to the dissociative direct N2 -splitting in the HB process, this mechanism is associative, indicating that the N-N bond is not broken until the first ammonia is released. The functionalisation is proposed to occur via a distal pathway.^89,90 It is worth mentioning that very recently, a catalytic response was realised for the related cis-Mo analogue using the potent SmI2/ethylene glycol reagent system (vide infra).⁹¹

Another iconic example is the hydrogenation of side-on N2-bridging [{Zr(Cp’)2(N2)2}2(μ-η²:η² -N2)] (Cp’ = C5Me4H⁻), which splits H2 to form [{Zr(Cp’)2(N2)2}2(μ-η²:η²-N2H2)] complex, see Scheme 18.³⁹ At the time (2004), this was the first characterised example of N2 hydrogenation to hydrazine on a transition metal. Further hydrogenation at high temperatures releases NH3 in sub stoichiometric yields. A remarkable aspect of this example is that the closely related yet end-on bridging [{Zr(Cp*)2(N2)2}2(μ-η¹:η¹-N2)] with per-methylated Cp*-rings dissociates its N2 -ligands upon reaction with H2. These subtle ligand differences and their implications for its N2 -reactivity is remarkable. A recent example by Walter also released sub-stoichiometric ammonia from H2, but from reaction with N2-derived nitrides via a dissociative pathway. Upon reduction of Fe^II [{FeCp”}2(μ-η²-I)2] (Cp” = 1,3,5-(tBu)3-C5H2) using KC8 under N2, a tri-iron compound is obtained with (μ-η³-N) nitride linkages. Protonation of this compound releases ammonium in circa 75 % yield. More interestingly, the compound also reacts with (high pressure) H2, which was examined both in the solid state and in solution. In the latter, ammonia is formed in low yield (3-7 %), accompanied by the formation of a di-iron nitride species and a bis(imido) species.

This latter compound is formed quantitatively from solid-state reactivity with H2. One-pot reduction of the starting compound in presence of N2 and H2 unfortunately only formed iron-hydrides, hampering catalytic formation of ammonia.⁹²

Scheme 18. Selected examples of stoichiometric ammonia release from N2 by Chatt and Chirik.^39,87

Between the initial work of Chatt and the first example of catalytic ammonia on a well-characterised molecular system, some less-defined examples appeared. Shilov combined MoCl5 -salts, a large excess of Na/Hg and both phosphine and long-chain phosphites ligands, that generates hydrazine and ammonia catalytically. The pre-catalyst is believed to be a Mo(III)phosphine complex, which is not further characterised. In comparison to Chatt, the use of external reducing equivalents was already identified to re-generate a catalytically active species.⁹³ A well-defined molecular system was developed in 2003 by Schrock, using [Mo((N(HIPT)CH2CH2N3)(N2)] (HIPT = 3,5-(2,4,6-iPr3C6H2)2)) as catalyst, Figure 8.⁹⁴ Compared to the earlier N2-fixation work (see Figure 7), the instable Namide-Si bonds in the backbone were exchanged for more stable Namide-C bonds with extensive steric bulk to shield the metal centre. Reacting this Mo(III) species in presence of excess luthidinium BAr^F24 as acid and

Cr(Cp*)2 (E1/2 = −1.47 V vs. Fc^+/0 (THF))⁵⁹ in benzene unfortunately resulted almost exclusively in hydrogen evolution (HER). HER represents the main competing reaction when ammonia formation is attempted from separate proton and electron sources, since the redox potentials for both reactions are close. This was recently discussed and graphically visualised in a Pourbaix diagram for reactions in MeCN.⁹⁵ To overcome this competing reaction, the solvent was changed to heptane where the acid is only limited soluble, and the reductant was slowly added via a syringe pump. Satisfactory formation of circa 8 equivalents of ammonium per molybdenum was obtained. Labelled ammonia was formed when performing the reaction under ¹⁵N2, confirming atmospheric dinitrogen uptake. Several catalytic active intermediates were isolated or separately prepared (i.e. Mo-NNH, Mo-NNH2+, Mo=N, and M-NH3+), which allowed to propose a catalytic cycle. In analogy to the ‘Chatt cycle’, this ‘Schrock cycle’ proposes a distal pathway via a Mo nitride, yet cycling through Mo^III/VI oxidation states.⁹⁴

Figure 8. Selected examples of catalytic ammonia generation from molecular complexes using chemical reductants by Schrock, Nishibayashi and Peters. Ammonia eq. are reported normalised per metal centre.52,60,94,96

In 2010, Nishibayashi developed a Mo^o system capable of catalytic ammonia formation. Slow addition of Co(Cp)2 to a solution of dinuclear end-on bridging [{Mo(N2)2(PNPyP)}2(μ-N2)] in toluene with luthidinium triflate yields approx. 12 eq. ammonia per Mo-centre (49 % yield), (Figure 8, top right). In addition to ammonia, H2 is formed in circa 37 % yield.⁶⁰ Initially, an exclusively monomeric mechanism was proposed. Later on, this was re-considered into a distal mechanism where the dinuclear framework is retained for the first ammonia release from the terminal nitrogen. Subsequently a mononuclear nitride complex reacts to form the second equivalent of ammonia, upon which the dinuclear dinitrogen coordination structure is re-formed.⁸²

Catalytic ammonia formation was also established from the same system that shows N2-splitting into terminal nitrides (Section I.1.4), for both the precursor [MoX3(PNPyP)] as the resulting nitride [MoNX(PNPyP)] complexes with X = Cl, Br, and I (Figure 8, bottom right). From this series, a clear preference for catalytic activity moving to the heavier halide congeners was found.

The authors claim that this is because the heavier congeners are easier reduced, as apparently the reduction potential of the nitrides follows the same order.⁵²

By now, catalytic ammonia formation is also reported for many transition metals besides Mo.

One example using iron is discussed from the group of Peters, using their archetypical tetrapodal triphosphine borane ligand. The complex [Fe(P3B)]⁺ (P3B = B((2-PiPr2)C6H4)3) successfully catalyses dinitrogen to ammonia using KC8 (circa −2.6 V (NMP))⁹⁷ and HBAr^F24·(Et2O)2. Both reagents are strong and to prevent competing HER, the reaction proceeds at low temperatures (−78 ˚C) in Et2O.⁹⁶ Yet, better results were obtained by combining Co(Cp*)2 and (Ph2PH2)OTf (Figure 8, bottom right). This is attributed to the formation of protonated [Co(Cp*)(η⁴ -C5Me5H))]OTf that acts as PCET reagent. By calculations, this C-H bond strength is estimated to be approx. 31 kcal mol^-1. The resulting diazenido from the first (and most difficult) H-atom transfer to the [Fe-NN] complex is calculated to have a N-H bond strength of 35 kcal mol^-1, indicating a feasible initial reaction step. This behaviour is extended by calculations to other Cp-bearing reducing agents, as used in the previous discussed examples. Notably, such a protonated species could be spectroscopically characterised upon reaction between an acid and Co(Cp*)2 at low temperatures.⁹⁸ The exciting finding of such a potent PCET strategy can off course be used for other transformations beyond N2-activation.⁹⁹ In general, the many intermediates that were characterised for this system (among which Fe-NNH, Fe-NNH2, Fe=N⁺, Fe-NH2NH2+) suggest a hybrid mechanism between a distal and alternating pathway, mainly because of the isolation of the latter intermediate.⁸⁶

Scheme 19. Left: catalytic ammonia formation using SmI2/water by Nishibayashi. Right: A Sm(III)-resting state.⁵⁷

Recently, a very efficient PCET reagent was established for catalytic ammonia formation that represents a substantial improvement within this field. Nishibayashi found that the combination of SmI2 and alcohols (ethylene glycol) or even water proved an efficient HAT donor for nitrogen reduction on Mo-complexes (Scheme 19). In a large run experiment using [MoCl3(PCP)] (PCP = 1,3-bis((di-tert-butylphosphino)methyl)benzamidazol-2-ylidine) as catalyst, > 4000 eq. of NH3

and only 150 eq. of H2 were found, which correspond to 91 and 2 % yield respectively, see Scheme 19. Also the turn-over frequency is circa 1-2 order of magnitudes larger compared to previous established systems.⁵⁷ The O-H bond strength of a SmI2/H2O mixture was recently estimated to be approx. 26 kcal mol^-1,¹⁰⁰ which is far below the calculated N-H bond of several diazenido M-NNH species, explaining its efficiency.^46,101 Notably, catalytic ammonia formation from [MoX3(PNPyP)] (X = Cl, Br, I) was re-considered using the SmI2/ethylene glycol system where the catalytic activity was found in the order Cl ≈ Br > I. This is clearly opposite of what was found using Co(Cp*)2/collidinium triflate (see Figure 8), yet is not further commented. After catalysis, multinuclear O-linkage Sm(III) compounds were found, see Scheme 19. In a separate study, electrochemical reduction of SmI3 as model compound was established in high yields, thereby potentially recycling the reducing agent for this reaction.¹⁰² The discovery of this potent reagent in the field of ammonia formation has already inspired established and new systems to explore these promising conditions.^63,84,91