Carbon–hydrogen versus carbon–chalcogen bond cleavage of furan, thiophene and selenophene by ansa molybdenocene complexes

Abstract

[Me 2Si(C 5Me 4) 2]MoH 2 reacts with furan and benzofuran to yield products resulting from C–H bond cleavage, namely [Me 2Si(C 5Me 4) 2]Mo(η 1- C-C 4H 3O)H and [Me 2Si(C 5Me 4) 2]Mo(η 1- C-C 8H 5O)H, whereas the corresponding reactions of selenophene and benzoselenophene yield products resulting from C–Se bond cleavage, namely [Me 2Si(C 5Me 4) 2]Mo(η 2- C, Se-SeC 4H 4) and [Me 2Si(C 5Me 4) 2]Mo(η 2- C, Se-SeC 8H 6). On this basis, the reactivity of the selenophene derivatives is analogous to that of previously reported thiophene derivatives, while the reactivity of the furan derivatives is unique. DFT calculations indicate that C–E (E = O, S, Se) bond cleavage is thermodynamically more favored than C–H bond cleavage for each of the chalcogen derivatives. As such, the calculations provide evidence that C–O bond cleavage of the furan derivatives is not observed because of kinetic factors. DFT calculations also demonstrate that the observation of C–S bond cleavage of thiophene by the ansa metallocene [Me 2Si(C 5Me 4) 2]MoH 2 and C–H bond cleavage by Cp 2MoH 2 is dictated by thermodynamic factors. Specifically, the Me 2Si ansa bridge thermodynamically favors [Me 2Si(C 5Me 4) 2]Mo(η 2- C, S-SC 4H 4) over [Me 2Si(C 5Me 4) 2]Mo(η 1- C-SC 4H 3)H because the bridge promotes a shift in the coordination of the cyclopentadienyl rings from η 5-coordination towards η 3-coordination and this thermodynamically unfavorable modification is stabilized by sulfur-to-metal π-donation within [Me 2Si(C 5Me 4) 2]Mo(η 2- C, S-SC 4H 4).