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(L-R)CI (2a; L-Mes = 1,3-dimesitylimidazol-2-ylidene, 2b; L-iPr = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene, Cp* = eta(5)-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(L-Mes)R (3a; R = Me, 3b; R = Ph). These complexes, in turn, undergo cyclometalation at elevated temperatures, and Cp*Fe{k(2)-(C,C)-L'(Mes)} (4; L'(Mes) = CH2C6H2-3,5-Me-2-2-(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{k(2)-(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 N-2 complex [Cp*Fe{k(2)-(C,C)-L'(iPr)}]2(mu-eta(1):eta(1)-N-2) (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(L-iPr)(2-C4H3E) (8; E = S, 9; E = O) and Cp*Fe(L-iPr)(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(L-iPr)(H(2)Bcat) (14).