N-Heterocyclic Carbene Ligands as Mimics of Imidazoles/Histidine for the Stabilization of Di- and Trinitrosyl Iron Complexes

N-heterocyclic carbenes (NHCs) are shown to be reasonable mimics of imidazole ligands in dinitrosyl iron complexes determined through the synthesis and characterization of a series of {Fe(NO)2}10 and {Fe(NO)2}9 (Enemark–Feltham notation) complexes. Monocarbene complexes (NHC-iPr)(CO)Fe(NO)2 (1) and (NHC-Me)(CO)Fe(NO)2 (2) (NHC-iPr = 1,3-diisopropylimidazol-2-ylidene and NHC-Me = 1,3-dimethylimidazol-2-ylidene) are formed from CO/L exchange with Fe(CO)2(NO)2. An additional equivalent of NHC results in the bis-carbene complexes (NHC-iPr)2Fe(NO)2 (3) and (NHC-Me)2Fe(NO)2 (4), which can be oxidized to form the {Fe(NO)2}9 bis-carbene complexes 3+ and 4+. Treatment of complexes 1 and 2 with [NO]BF4 results in the formation of uncommon trinitrosyl iron complexes, (NHC-iPr)Fe(NO)3+ (5+) and (NHC-Me)Fe(NO)3+ (6+), respectively. Cleavage of the Roussin’s Red “ester” (μ-SPh)2[Fe(NO)2]2 with either NHC or imidazole results in the formation of (NHC-iPr)(PhS)Fe(NO)2 (7) and (Imid-iPr)(PhS)Fe(NO)2 (10) (Imid-iPr = 2-isopropylimidazole). The solid-state molecular structures of complexes 1, 2, 3, 4, 5+, and 7 show that they all have pseudotetrahedral geometry. Infrared spectroscopic data suggest that NHCs are slightly better electron donors than imidazoles; electrochemical data are also consistent with what is expected for typical donor/acceptor abilities of the spectator ligands bound to the Fe(NO)2 unit. Although the monoimidazole complex (Imid-iPr)(CO)Fe(NO)2 (8) was observed via IR spectroscopy, attempts to isolate this complex resulted in the formation of a tetrameric {Fe(NO)2}9 species, [(Imid-iPr)Fe(NO)2]4 (9), a molecular square analogous to the unsubstituted imidazole reported by Li and Wang et al. Preliminary NO-transfer studies demonstrate that the {Fe(NO)2}9 bis-carbene complexes can serve as a source of NO to a target complex, whereas the {Fe(NO)2}10 bis-carbenes are unreactive in the presence of a NO-trapping agent.