A {Fe(NO)3}10 Trinitrosyliron Complex Stabilized by an N-Heterocyclic Carbene and the Cationic and Neutral {Fe(NO)2}9/10 Products of Its NO Release

In contrast to the instability of XFe(NO)(3) and [R3PFe(NO)(3)](+), the N-heterocyclic carbene (NHC)containing trinitrosyliron complex (TNIC) [(IMes)Fe(NO)(3)][BF4] (1) [IMes =1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene] can be readily isolated and manipulated in solution under ambient conditions. Nevertheless, in the presence of thiolates (SR-), this EPA-silent TNIC (denoted {Fe(NO)3}1 in the Enemark-Feltham notation) releases gaseous NO, affording in the case of SR- = SPh- the EPR-active, neutral dinitrosyliron complex (DNIC) (IMes)Fe(SPh)(NO)(2) (3, {Fe(NO)(2)}(9)). Carbon monoxide enforces a bimolecular reductive elimination of PhSSPh from 3, yielding (IMes)(CO)Fe(NO)(2) (2), a reduced {Fe(NO)2}1 DNIC. The NO released from TNIC 1 in the presence of SPh- could be taken up by the NO-trapping agent [(bme-dach)Fe](2) [bme-dach = N,N-bis(2-mercaptoethyl)-1,4-diazacycloheptane] to form the mononitrosyliron complex (MNIC) (bme-dach)Fe(NO). In the absence of SPh-, direct mixing of [bme-dach)Fe](2) with 1 releases both NO and the NHC with formation of a spin-coupled, diamagnetic {Fe(NO)}7 (Fe(NO)(2)}(9) complex, [(NO)Fe(bme-dach)Fe(NO)(2)][BF4] (4). In 4, the MNIC serves as a bidentate metallodithiolate ligand of Fe(NO)(2), forming a butterfly complex in which the Fe-Fe distance is 2.7857(8) A. Thus, 1 is found to be a reliable synthon for [{Fe(NO)(2)}(9)](+). The solid-state molecular structures of complexes 1-3 show that all three complexes have a tetrahedral geometry in which the bulky mesitylene substituents of the carbene ligand appear to umbrella the Fe(NO)(2)L [L = NO (1), CO (2), SPh (3)] motif.