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

[Me2Si(C5Me4)(2)]MoH2 reacts with furan and benzofuran to yield products resulting from C-H bond cleavage, namely [Me2Si(C5Me4)(2)]Mo(eta(1)-C-C4H3O)H and [Me2Si(C5Me4)(2)]Mo(eta(2)-C-C8H5O)H, whereas the corresponding reactions of selenophene and benzoselenophene yield products resulting from C-Se bond cleavage, namely [Me2Si(C5Me4)(2)]Mo(eta(2)-C,Se-SeC4H4) and [Me2Si(C5Me4)(2)]Mo(eta(2)-C,Se-SeC8H6). 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 [Me2Si(C5Me4)(2)]MoH2 and C-H bond cleavage by Cp2MoH2 is dictated by thermodynamic factors. Specifically, the Me2Si ansa bridge thermodynamically favors [Me2Si(C5Me4)(2)]Mo(eta(2)-C,S-SC4H4) over [Me2Si(C5Me4)(2)]Mo(eta(1)-C-SC4H3)H because the bridge promotes a shift in the coordination of the cyclopentadienyl rings from eta(5)-coordination towards eta(3)-coordination and this thermodynamically unfavorable modification is stabilized by sulfur-to-metal pi-donation within [Me2Si(C5Me4)(2)]Mo(eta(2)-C,S-SC4H4). (c) 2005 Elsevier Ltd. All rights reserved.