Opening of Bent Bi−C Bonds by Silanols To Give Stable Cp₂Mo(μ-BiOR)₂MoCp₂ Complexes

First, the reactivity of molybdocene dihydride toward bismuth siloxides was studied. Depending on the stoichiometry, compounds of the type [Cp2Mo{Bi(OR)2}2] (R = SiMe2tBu, 1, R = SiPh3, 3) or [Cp2Mo(μ-BiOR)2MoCp2] (R = SiMe2tBu, 2t, R = SiPh3, 4t) were obtained. Apart from 4 all of these compounds were characterized via single-crystal X-ray diffraction analyses, which revealed for the compound 2t a trans orientation of the two silanolate groups, similar to that found in case of the corresponding tert-butoxide derivatives. In contrast to those, however, the silanolates 2t and 4t proved to be stable in solution with respect to subsequent intramolecular silanol eliminations, which allowed for detailed NMR spectroscopic investigation. These studies revealed that on dissolution of 2t and 4t, they slowly enter into an equilibrium with isomers containing the two silanolate ligands in a cis configuration, 2c and 4c. However, the cis isomers cannot be isolated from the isomeric mixtures, as the trans isomer always precipitates first. Due to the thermodynamic stability of 2 and 4, these complexes can also be obtained starting from [(η5-Cp)Mo(μ2-η5:η1-C5H4)Bi]2, which contains bent Bi−C bonds and is formed from the tert-butoxide derivative of 2 and 4, [Cp2Mo{Bi(OtBu)}2MoCp2], IIH, by further alcohol elimination as the thermodynamically most favored product in that system: Reacting this compound with the silanols HOSiMe2tBu and HOSiPh3 provides 2t and 4t, respectively, which underlines that these complexes represent the thermodynamic holes within these corresponding systems.