Fe–N ₂ /CO complexes that model a possible role for the interstitial C atom of FeMo-cofactor (FeMoco)

Abstract

We report here a series of four-and five-coordinate Fe model complexes that feature an axial tri(silyl)methyl ligand positioned trans to a substrate-binding site. This arrangement is used to crudely model a single-belt Fe site of the FeMo-cofactor that might bind N 2 at a position trans to the interstitial C atom. Reduction of a trigonal pyramidal Fe(I) complex leads to uptake of N 2 and subsequent functionalization furnishes an open-shell Fe-diazenido complex. A related series of five-coordinate Fe-CO complexes stable across three redox states is also described. Spectroscopic, crystallographic, and Density Functional Theory (DFT) studies of these complexes suggest that a decrease in the covalency of the Fe-C alkyl interaction occurs upon reduction and substrate binding. This leads to unusually long Fe-C alkyl bond distances that reflect an ionic Fe-C bond. The data presented are contextualized in support of a hypothesis wherein modulation of a belt Fe-C interaction in the FeMo-cofactor facilitates substrate binding and reduction.nitrogenase | nitrogen fixation | Fe-N 2 complexes | small molecule activation | ammonia production

Nitric Oxide Activation by Distal Redox Modulation in Tetranuclear Iron Nitrosyl Complexes

“…Furthermore, the importance of axial ligand flexibility has also been invoked in mononuclear iron models of the FeMoco cluster of nitrogenase, where the degree of elongation between the axial atom (Si, B, C) of tripodal supporting ligands was speculated to affect the degree of N2 and CO activation. 37 …”

Ligand binding to the FeMo-cofactor: Structures of CO-bound and reactivated nitrogenase

“…Additionally, Fe1, the interstitial carbon and molybdenum are no longer aligned, creating an asymmetry in the resulting [7Fe:8S:C:Mo] cluster (Figure 1C). The modest adjustments of the remaining scaffold upon CO binding are suggestive of an important role for the interstitial carbon in stabilizing the cofactor during rearrangements and substitutions to the coordination environment of the irons (34, 35). …”

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.…”