title: N-heterocyclic carbene ligands as mimics of imidazoles/histidine for the stabilization of di- and trinitrosyl iron complexes.
abstract: 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)(2)Fe(NO)(2) (3) and (NHC-Me)(2)Fe(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]BF(4) 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.
authors.affiliation: Department of Chemistry, Texas A&M University, College Station, Texas 77843, USA.
authors.forename: Jennifer L
authors.initials: JL
authors.lastname: Hess
authors.forename: Chung-Hung
authors.initials: CH
authors.lastname: Hsieh
authors.forename: Joseph H
authors.initials: JH
authors.lastname: Reibenspies
authors.forename: Marcetta Y
authors.initials: MY
authors.lastname: Darensbourg
mesh_terms: D018360:Crystallography, X-Ray; D005290:Ferric Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D006571:Heterocyclic Compounds / Q000737:chemistry*; D006639:Histidine / Q000737:chemistry*; D007093:Imidazoles / Q000737:chemistry*; D008024:Ligands; D008697:Methane / Q000031:analogs & derivatives* / Q000737:chemistry; D008958:Models, Molecular; D008968:Molecular Conformation; D009569:Nitric Oxide / Q000737:chemistry*; D013237:Stereoisomerism
chemical_list: D005290:Ferric Compounds; D006571:Heterocyclic Compounds; D007093:Imidazoles; D008024:Ligands; C030011:carbene; D009569:Nitric Oxide; D006639:Histidine; D008697:Methane