Conformational changes of an oxovanadium complex probed by ENDOR spectroscopy and DFT calculations

An angular selective ENDOR study, in combination with DFT calculations, was used to investigate the structure and conformational changes of the bis(acetylacetonato)-oxovanadium(IV) complex ([V-IV=O(acac)(2)]) in frozen solution. In the presence of a non-coordinating solvent ( dichloromethane), both ENDOR analysis and DFT calculations confirm the expected square pyramidal structure of the complex. However, when a strongly coordinating solvent (d(5)-pyridine) was used, different structural isomers of the resulting ([V-IV=O(acac)(2) (C5D5N)]) adduct were found to co-exist in the frozen solution. ENDOR simulations indicate that 60% of the adduct is present as the cis-isomer, while 40% is present as the trans-isomer. The ligand coordinates of both isomers, as determined by ENDOR spectroscopy, were in excellent agreement with the optimised structure based on DFT calculations. In addition, the DFT calculations showed that the total energy of the two isomers for the pyridine case differ by only 3 kJ mol(-1), in agreement with the experimentally observed equilibrium mixture containing both isomers.