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, CpFe(L(R))Cl (2a; L(Mes) = 1,3-dimesityl-imidazol-2-ylidene, 2b; L(iPr) = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene, Cp = eta(5)-C5Me5), have been synthesized by the reaction of CpFe{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 CpFe(L(Mes))R (3a; R = Me, 3b; R = Ph). These complexes, in turn, undergo cyclometalation at elevated temperatures, and CpFe{kappa2-(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, CpFe{kappa2-(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 [CpFe{kappa2-(C,C)-L'(iPr)}]2(mu-eta(1):eta(1)-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 CpFe(L(iPr))(2-C4H3E) (8; E = S, 9; E = O) and CpFe(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 CpFe(L(iPr))(H2Bcat) (14).