Molybdenum-Catalyzed Ammonia Formation Using Simple Monodentate and Bidentate Phosphines as Auxiliary Ligands

Abstract

We have found molybdenum-catalyzed ammonia formation using simple and commercially available monodentate and bidentate phosphines as auxiliary ligands with a simple and convenient procedure. Molybdenum complexes generated in situ from [MoI3(THF)3] and the corresponding phosphines such as PMePh2 and 1,5-bis­(diphenylphosphino)­pentane worked effectively toward ammonia formation.

A Chatt‐Type Catalyst with One Coordination Site for Dinitrogen Reduction to Ammonia

“…On the other hand, Nishibayashi et al. recently showed that Chatt complexes with mono‐ and bidentate ligands indeed catalyse the generation of ammonia from N 2 if SmI 2 /H 2 O (or SmI 2 /alcohol) is used as reductant and proton source [20] . Using cis , mer ‐[Mo(NNH 2 )(OTf) 2 (PMePh 2 ) 3 ] as example for a NNH 2 intermediate also led to catalytic amounts of NH 3 .…”

“…To determine the N−H BDFE of the NNH‐intermediate for a given N 2 ‐reduction catalyst, DFT calculations may be employed [20] . Transfer of one electron and one proton to the Mo 0 ‐dinitrogen complex leads to the neutral Mo I ‐diazenido(−) intermediate.…”

“…recently showed that Chatt complexes with mono-and bidentate ligands indeed catalyset he generation of ammonia from N 2 if SmI 2 /H 2 O( or SmI 2 /alcohol) is used as reductant and protons ource. [20] Using cis,mer-[Mo(N-NH 2 )(OTf) 2 (PMePh 2 ) 3 ]a se xample for aN NH 2 intermediate also led to catalytic amounts of NH 3 .F rom this observation it was inferred that the SmI 2 -mediatedr eductionp athway of Chatttype complexes probably follows the Chatt cycle.…”

“…[32] To determine the NÀHB DFE of the NNH-intermediate for a given N 2 -reduction catalyst, DFT calculations may be employed. [20] Transfer of one electron and one proton to the Mo 0dinitrogen complex leads to the neutral Mo I -diazenido(À)i ntermediate. An estimate of the corresponding energetics was obtainedb yD FT,s imulating the reaction of [Mo(N 2 )(pentaPod)] with TEMPO-H, aH -atom transfer reagent having aw ell-defined OÀHB DFE of 65.2 kcal mol À1 in benzene, [33] to give the [Mo(NNH)(pentaPod)] complex.S ubtraction of the reaction TEMPO-H!TEMPOC + HC leads to aN ÀHB DFE of 19.2 kcal mol À1 for the Mo I -diazenido(À)c omplex (D r,theo G 298 (solv,b enzene), cf.…”

A Practical Synthesis of Ammonia from Nitrogen Gas, Samarium Diiodide and Water Catalyzed by a Molybdenum–PCP Pincer Complex

“…[3][4][5][6] Quite recently, we developed a procedure for the catalytic formation of ammonia using a molybdenum trichloride complex bearing an N-heterocyclic carbene-and phosphine-based PCP-type pincer ligand [MoCl 3 (PCP)] 1[PCP = 1,3-bis(di-tert-butylphosphinomethyl)benzimidazol-2-ylidene], samarium diiodide (SmI 2 ) as a reductant and water as a proton source (Scheme 1). 7,8 In this reaction system, the amount of generated ammonia reached 4350 equivalents based on the molybdenum atom of the catalyst and the turnover frequency reached 113 equivalents of ammonia per minute based on the molybdenum atom of the catalyst. This effective and rapid reaction system for ammonia synthesis prompted us to investigate the reaction of atmospheric pressure of nitrogen gas with SmI 2 and water in the presence of 1 as the catalyst on a large scale.…”