Abstract

The previously unreported equilibrium in aqueous solution between the V^IVO square pyramidal and trans octahedral form with an axial water molecule for a number of bidentate ligands with (N,O) and (O,O) donor sets (6-methylpicolinic (6-mepicH) and 6-methyl-2,3-pyridinedicarboxylic (6-me-2,3-pdcH₂) acids, dipyridin-2-ylmethanol (Hdpmo) and 1,2-dimethyl-3-hydroxy-4(1H)-pyridinone (Hdhp)) has been demonstrated by the combined application of EPR spectroscopy and DFT methods. The EPR spectra suggest that, with increasing ionic strength, the equilibrium is shifted towards the formation of the pentacoordinated species and values of K≈4.0 and 7.0 for the systems containing 6-methyl-2,3-pyridinedicarboxylic acid and dipyridin-2-ylmethanol were measured. DFT calculations performed with Gaussian 03 and ORCA software predict the ⁵¹V anisotropic hyperfine coupling constant along the z axis (A_z), which can be used to demonstrate the presence of an axially bound ligand trans to the V=O bond. The results suggest that an axial donor (charged or not) can lower |A_z|, in contrast to what was previously believed on the basis of the “additivity rule”, and this explains the anomalous behaviour of the V^IVO complexes formed by N-{2-[(2-pyridylmethylene)amino]phenyl}-pyridine-2-carboxamide (Hcapca) and several amidrazone derivatives. The decrease in |A_z| for the axial binding of a solvent molecule is mainly a result of the reduction of |A_iso| and this was also observed when the solid [VO(6-methylpicolinato)₂] was dissolved in DMSO or DMF. The variations in the structural (V=O, V-O and V-N distances, O-V-O and N-V-N angles, and the trigonality index t) and spectroscopic (|A_z|, |A_iso| and v(V=O)) properties as a function of the axial VOH₂ distance (R) are also presented. Finally, the electronic structures of the pentaand hexacoordinated complexes are discussed.

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