Direct Transformation of Molecular Dinitrogen into Ammonia Catalyzed by Cobalt Dinitrogen Complexes Bearing Anionic PNP Pincer Ligands

Abstract

The direct formation of ammonia from molecular dinitrogen under mild reaction conditions was achieved by using new cobalt dinitrogen complexes bearing an anionic PNP-type pincer ligand. Up to 15.9 equivalents of ammonia were produced based on the amount of catalyst together with 1.0 equivalent of hydrazine (17.9 equiv of fixed nitrogen atoms).

N–H Bond Dissociation Enthalpies and Facile H Atom Transfers for Early Intermediates of Fe–N2 and Fe–CN Reductions

“…12 These species are characteristic of a “distal” or “Chatt-type” pathway for substrate reduction that is commonly thought to operate in certain N 2 reduction systems. 3,4,7 Recent mechanistic studies of one of our reported Fe systems (P 3 Si Fe) also hints at competing downstream pathways following the generation of the doubly protonated distal intermediate P 3 Si Fe=N–NH 2 . These Fe=N–NH 2 (and related Fe≡C–NH 2 ) species display a high degree of instability in comparison to analogous Mo=N–NH 2 species; for instance, they decay rapidly at ambient and even lower temperatures in solution and furnish mixtures of H 2 and NH 3 .…”

“…2 Free N 2 exhibits a low electron affinity (−1.9 eV) and proton affinity (118 kcal/mol), 3 and activation by a suitable catalyst is hence required. While growing in number, 4 few well-defined molecular systems mediate catalytic N 2 -to-NH 3 conversion; those that do use a combination of inorganic reductant and acid rather than H 2 . Nitrogenase enzymes represent the most efficient and well-studied non-Haber–Bosch catalysts and are collectively responsible for the generation of ∼50% of the global fixed nitrogen pool.…”

“…4 An early hallmark of the Mo model systems, noteworthy owing to the presence of Mo in the FeMo-cofactor of MoFe-nitrogenase, has been the ability to independently prepare many of the intermediate Mo–N x H y species thought to be relevant to N 2 -to-NH 3 conversion. 7 …”

“…Very recently, Nishibayashi and co-workers disclosed a phosphine–pyrrole Fe–N 2 system that is a competent catalyst for NH 3 generation (along with some N 2 H 4 ), 4a and Ashley and co-workers reported a low-valent phosphine-only Fe–N 2 system that mediates catalytic N 2 -to-N 2 H 4 conversion. 4b Fe-mediated, catalytic N 2 activation in these Fe systems is thought to involve an initial end-on binding of N 2 to a single Fe atom, followed (in our P 3 E Fe-systems at least (E = B, C, Si)) by reductive protonation at the terminal N atom to afford Fe=N–NH 2 species (Scheme 1). 13 Fe-mediated reduction of P 3 Si Fe–CN is thought to proceed via an isostructural Fe≡C–NH 2 species.…”

Catalytic Nitrogen-to-Ammonia Conversion by Osmium and Ruthenium Complexes

“…3 In contrast, synthetic models based on metals other than Fe or Mo that display efficacy for catalytic N 2 RR are to date limited to two cobalt systems, both of which use a combination of strong acid and reductant in the form of [H(OEt 2 ) 2 ][BAr F 4 ] (HBAr F 4 , BAr F 4 = tetrakis (3,5-bis(trifluorome thyl)phenyl)borate) and KC 8 . 4 …”