Efforts to prepare and characterize terminal imido complexes of iron have aimed to elucidate their roles in nitrene group transfer catalysis and to model plausible intermediates in biological nitrogen fixation. In this latter regard, it is unclear in what ways the findings from mononuclear Fe–imido chemistry would translate to the chemistry of Fe–S clusters (such as the catalytic cofactors of nitrogenases). This is in part because mononuclear Fe–imido complexes are themselves electronically diverse and have been characterized across a wide range of oxidation and spin states. Moreover, the ability of Fe–S clusters to support Fe–N multiple bonding is not well-established, and it is not known how the quintessential electronic features of Fe–S clusters (weak ligand fields and significant Fe–Fe exchange coupling) would impact the Fe–N bonding in an Fe–imido group. To address these questions, we describe the synthesis and characterization of the first terminal imido complex of an Fe–S cluster, (IMes)3Fe4S4=NDipp (IMes = 1,3-dimesitylimidazol-2-ylidene, Dipp = 2,6-diisopropylphenyl), which is generated by oxidative group transfer from DippN3 to an all-ferrous cluster. This two-electron process is achieved by formal one-electron oxidation of the imido-bound Fe site and one-electron oxidation of two IMes-bound Fe sites. Structural, spectroscopic, and computational studies establish that the Fe–imido site is best described as a high-spin Fe3+ center, which is reflected in its long Fe–N(imido) distance of 1.763(2) Å. This cluster also abstracts hydrogen atoms from weak C–H bonds to yield the corresponding anilido complex. The interplay between H-atom abstraction reactivity, N-centered spin density, and the cluster’s highly plastic Fe–Fe, Fe–N, and Fe–S interactions will be a primary focus of the talk.

Synthesis and characterization of a terminal imido complex of an iron–sulfur cluster

ACS Fall 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.