Unique behaviour of dinitrogen-bridged dimolybdenum complexes bearing pincer ligand towards catalytic formation of ammonia

Abstract

It is vital to design effective nitrogen fixation systems that operate under mild conditions, and to this end we recently reported an example of the catalytic formation of ammonia using a dinitrogen-bridged dimolybdenum complex bearing a pincer ligand, where up to twenty three equivalents of ammonia were produced based on the catalyst. Here we study the origin of the catalytic behaviour of the dinitrogen-bridged dimolybdenum complex bearing the pincer ligand with density functional theory calculations, based on stoichiometric and catalytic formation of ammonia from molecular dinitrogen under ambient conditions. Comparison of di- and mono-molybdenum systems shows that the dinitrogen-bridged dimolybdenum core structure plays a critical role in the protonation of the coordinated molecular dinitrogen in the catalytic cycle.

Catalytic Nitrogen Fixation via Direct Cleavage of Nitrogen–Nitrogen Triple Bond of Molecular Dinitrogen under Ambient Reaction Conditions

“…One is diminution of reaction steps of protonation and reduction in the catalytic cycle. In sharp contrast to the previous reaction pathway, 6,12 where six pairs of stepwise protonation and reduction are necessary, only three pairs to the molybdenumnitride complex promote the formation of ammonia from dinitrogen molecule. The other is the inhibition of dihydrogen formation.…”

“…12,18,19 After an extensive study, we have now found that similar molybdenum nitride complexes can be generated from dinitrogen-bridged dimolybdenumiodide complexes via cleavage of the nitrogen nitrogen triple bond of the bridging dinitrogen in the MoNŅ Mo core and they have a high catalytic activity toward nitrogen fixation using molecular catalysts, where up to 830 equiv are formed based on the dinitrogen-bridged dimolybdenum complex (415 equiv of ammonia based on the molybdenum atom). We are sure that this high catalytic activity is realized by a novel reaction pathway via direct cleavage of the nitrogennitrogen triple bond of nitrogen gas under ambient reaction conditions.…”

“…12 Finally, the ligand exchange of C with another dinitrogen molecule affords the starting complex A together with ammonia.…”

Remarkable catalytic activity of dinitrogen-bridged dimolybdenum complexes bearing NHC-based PCP-pincer ligands toward nitrogen fixation

“…In 2003, Yandulov and Schrock29303132 reported the first successful example of the catalytic conversion of dinitrogen gas into ammonia using a molybdenum–dinitrogen complex as a catalyst and in 2013 Peters and co-workers33343536373839 reported the iron-catalysed transformation using an iron–dinitrogen complex as a catalyst. We also found that several dinitrogen-bridged dimolybdenum complexes such as [{Mo(N 2 ) 2 (PNP)} 2 (μ-N 2 )] ( 2 ; PNP=2,6-bis(di- tert -butylphosphinomethyl)pyridine)404142 and molybdenum–nitride complexes bearing PNP-type pincer ligands43 or mer -tridentate triphosphine44 worked as more effective catalysts towards ammonia formation under ambient reaction conditions, where up to 63 equiv. of ammonia were produced based on the molybdenum atom of the catalyst.…”

Nitrogen Fixation via a Terminal Fe(IV) Nitride

“…3 Such a scenario would be similar to that originally proposed by Chatt for molybdenum-mediated N 2 -to-NH 3 conversion 4 and thought to be operative in the catalytic cycles of known Mo catalysts for this reaction. 5 Whereas terminal Fe≡N complexes have been shown to liberate NH 3 via reductive protonation, 3a,6 such Fe≡N species have to-date been generated by N atom transfer reactions from azide (N 3 − ) or alternative reagents, not from N 2 . 3a,6,7 Hence, their potential role in synthetic or biological N 2 -to-NH 3 conversion catalysis has remained unclear.…”