Polynuclear tantalum oxoalkoxides.: Crystal structures of [Ta8O10(OEt)20], [Ta7O9(OPri)17] and [Ta5O7(OBut)11]•C6H5Me

By controlled hydrolysis of the parent pentaalkoxides of tantalum [Ta(OR)(5), R=Et, Pr-i or Bu-t] in mixed alcohol-toluene solutions the following crystalline oxoalkoxides were obtained and their structures determined by single crystal X-ray diffraction: [Ta-8(mu(3)-O)(2)(mu-O)(8)(mu-OEt)(6)(OEt)(14)] 1, [Ta-7(mu(3)-O)(3)(mu-O)(6)(mu-OPri)(4)(OPri)(13)] 3 and [Ta-5(mu(3)-O)(4)(mu-O)(3)(mu-OBut)(OBut)(10)]. C6H5Me 4. Each compound contained all of its tantalum atoms in distorted octahedral co-ordination. Using O-17-enriched (10 atom%) water these compounds were obtained with O-17-enriched oxo atoms and their O-17 NMR spectra measured in toluene solution and related to their molecular structures. Structural aspects of these tantalum oxoalkoxides are discussed in terms of the steric effects of the alkoxo ligands and pi-electron donation of terminal alkoxo ligands.