How Does Nishibayashi’s Molybdenum Complex Catalyze Dinitrogen Reduction to Ammonia?

Abstract

Recently, Nishibayashi et al. reported a dimolybdenum-dinitrogen complex that is catalytic for complete reduction of dinitrogen to ammonia. This catalyst is different from the Schrock molybdenum catalyst in two fundamental aspects: it contains two metal centers, and the oxidation state is Mo(0) instead of Mo(III). We show that a remarkable feature of the bimetallic complex is the bond-mediated delocalized electronic states, resulting from the two metal centers bridged by a dinitrogen ligand. Using first-principles calculations, we found that this property makes the bimetallic complex the effective catalyst, as opposed to the originally postulated monometallic fragment. A favorable reaction pathway is identified, and the nature of the intermediates is examined. Furthermore, studies of the intermediate states led us to propose possible deactivation processes of the catalyst. The finding that the central bimetallic unit (Mo-N2-Mo) is relevant for catalytic activity may provide a guideline for the development of more efficient dinitrogen-reducing catalysts.

Synthesis and Ligand Modification Chemistry of a Molybdenum Dinitrogen Complex: Redox and Chemical Activity of a Bis(imino)pyridine Ligand

“…11 While the performance of the catalyst is highly sensitive to ligand substitution,11, 12 these findings have inspired the study of other pincer‐ligated molybdenum complexes in the context of N 2 functionalization chemistry and to understand the mechanism of action of the catalytic system 13. 14…”

Catalytic Dinitrogen Fixation to Form Ammonia at Ambient Reaction Conditions Using Transition Metal-Dinitrogen Complexes

“…Both experimental and theoretical studies indicated that the existence of the dinitrogen‐bridged dimolybdenum core should play an important role during the catalytic reaction. Based on the new findings of experimental and theoretical studies, a revised novel reaction pathway can be proposed, where the maintenance of the dinuclear structure of {( t Bu PNP )Mo(μ‐N 2 )Mo( t Bu PNP )} plays a crucial role in exhibiting catalytic activity for catalytic ammonia formation (Figure ) 136, 139. A synergy seems to exist between two molybdenum moieties linked with a bridging dinitrogen ligand at the protonation step of the coordinated dinitrogen ligand in the starting dinitrogen‐bridged dimolybdenum‐dinitrogen complex [{Mo(N 2 ) 2 ( t Bu PNP )} 2 (μ‐N 2 )], where one molybdenum moiety acts as a ligand for the other molybdenum moiety at the active site by donating an electron to the other via the bridging dinitrogen ligand, furnishing terminally‐coordinated dinitrogen ligands with reductive activation to receive a proton.…”

“…Based on the new findings of experimental and theoretical studies, a revised novel reaction pathway can be proposed, where the maintenance of the dinuclear structure of {( t BuPNP)Mo(l-N 2 )Mo( t BuPNP)} plays a crucial role in exhibiting catalytic activity for catalytic ammonia formation ( Figure 14). [136,139] A synergy seems to exist between two molybdenum moieties linked with a bridging dinitrogen ligand at the protonation step of the coordinated dinitrogen ligand in the starting dinitrogen-bridged dimolybdenum-dinitrogen complex [{Mo(N 2 ) 2 ( t BuPNP)} 2 (l-N 2 )], where one molybdenum moiety acts as a ligand for the other molybdenum moiety at the active site by donating an electron to the other via the bridging dinitrogen ligand, furnishing terminally-coordinated dinitrogen ligands with {Mo(N 2 ) 2 ( t BuPNP)}], were observed by mass spectrometry from the catalytic and stoichiometric reactions of [{Mo(N 2 ) 2 ( t BuPNP)} 2 (l-N 2 )], [136] supporting the in situ formation of intermediary dinuclear nitrido and ammonia complexes [{( t BuPNP)Mo(N)(OTf)}(l-N 2 ){Mo(N 2 ) 2 ( t BuPNP)}] and [{( t BuPNP)Mo(NH 3 )(N 2 )}(l-N 2 ){Mo(N 2 ) 2 ( t BuPNP)}], proposed by the DFT calculations, as shown in Figure 14. This proposal is in sharp contrast to the common role of the dinitrogen-bridged dinuclear metal complexes bearing pincer ligands, for it is well-known that dinitrogen-bridged dinuclear transition-metal complexes bearing pincer ligands are apt to work only as precursors of mononuclear reactive metal species.…”

Synthesis and Reactivity of Molybdenum‐Dinitrogen Complexes Bearing PNN‐Type Pincer Ligand

“…In the reaction system described herein, some related mono‐ and dinuclear molybdenum‐dinitrogen complexes did not work as catalysts, where only a stoichiometric amount of ammonia was produced based on the catalyst 7,8. Quite recently, the exact role of the dinitrogen‐bridged dimolybdenum core bearing the PNP‐type pincer ligands during the catalytic conversion, based on the experimental results and the DFT calculations, where the dinuclear structure of the dinitrogen‐bridged dimolybdenum‐dinitrogen complex plays a decisive role in exhibiting catalytic ability for the transformation of molecular dinitrogen into ammonia has been elucidated 9,10…”