| Abstrakt: |
Small bimetallo biomimetic zinc-based compounds containing a bridging μ-OH functionality can be easily hydrated to yield complexes containing an intramolecular Zn–H3O2–Zn bridge. Because such bridges are intrinsically more nucleophilic than the corresponding μ-OH functionality, this effectively activates the complex. The Zn–H3O2–Zn unit is labile, and one can successively substitute the “water” and the “hydroxide” in the bridge for methanol and methoxide to form metal-bound HO–H–OMe and MeO–H–OMe functionalities. We have performed an extensive density functional investigation of this substitution mechanism for a series of bimetallo zinc complexes which have been synthesised and investigated by F. Meyer et al. There are two mechanisms for water/methanol exchange – an addition–elimination and a direct exchange mechanism. If the metal ions are coordinatively unsaturated, and their ligand sphere does not sterically hinder the approach of the methanol, both pathways compete with each other. For coordinatively saturated bimetallo complexes with larger ligand spheres, only the direct exchange mechanism is possible. Regardless of the mechanism operating, the exchange is facile and is controlled by the relative thermodynamic stability of the intermediates. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) |
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