C−H Bond Activation of Heteroarenes Mediated by a Half-Sandwich Iron Complex of N-Heterocyclic Carbene

Half-sandwich iron complexes of N-heterocyclic carbenes, Cp*Fe(LR)Cl (2a; LMes = 1,3-dimesityl-imidazol-2-ylidene, 2b; LiPr = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene, Cp* = η5-C5Me5), have been synthesized by the reaction of Cp*Fe{N(SiMe3)2} (1) with the corresponding imidazolium salts. Treatment of 2a with either methyllithium or phenyllithium replaces the chloride with either a methyl or a phenyl group, generating Cp*Fe(LMes)R (3a; R = Me, 3b; R = Ph). These complexes, in turn, undergo cyclometalation at elevated temperatures, and Cp*Fe{κ2-(C,C)-L′Mes} (4; L′Mes = CH2C6H2-3,5-Me2-2-(3-mesityl-imidazol-2-ylidene-1-yl)) was isolated. On the other hand, methylation of 2b at room temperature leads directly to the formation of a cyclometalated complex, Cp*Fe{κ2-(C,C)-L′iPr} (6; L′iPr = CH2CH(CH3)(3-isopropyl-4,5- dimethylimidazol-2-ylidene-1-yl)). The Fe(II) center of 6 traps atmospheric dinitrogen reversibly to produce a dinuclear end-on N2 complex [Cp*Fe{κ2-(C,C)-L′iPr}]2(μ-η1:η1-N2) (7). Complex 6 also promotes C−H bond activation of thiophene, furan, benzothiophene, and benzofuran at room temperature. In these reactions, C−H bond cleavage occurred exclusively at the 2-position of the rings, generating Cp*Fe(LiPr)(2-C4H3E) (8; E = S, 9; E = O) and Cp*Fe(LiPr)(2-C8H5E) (10; E = S, 11; E = O), while C−H cleavage took place mainly at the 4-position in the case of pyridine. Coupling reactions between heteroarenes and catecholborane (HBcat) can be carried out by treatment of 6 with heteroarenes followed by the addition of excess HBcat, giving rise to 2-boryl-heteroarenes and the borohydride complex Cp*Fe(LiPr)(H2Bcat) (14).