The equilibrium between the octahedral and square pyramidal form in VIVO complexes and the influence of an axial ligand on their molecular properties: a spectroscopic and DFT study

The previously unreported equilibrium in aqueous solution between the (VO)-O-IV 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-pdcH(2)) 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 penta-coordinated species and values of K approximate to 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 V-51 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 vertical bar A(z)vertical bar, in contrast to what was previously believed on the basis of the "additivity rule", and this explains the anomalous behaviour of the (VO)-O-IV complexes formed by N-{2- [(2-pyridylmethylene)amino]phenyl}-pyridine-2-carboxamide (Hcapca) and several amidrazone derivatives. The decrease in vertical bar A(z)vertical bar for the axial binding of a solvent molecule is mainly a result of the reduction of vertical bar A(iso)vertical bar 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 tau) and spectroscopic (vertical bar A(z)vertical bar, vertical bar A(iso)vertical bar and v(V=O)) properties as a function of the axial V-OH2 distance (R) are also presented. Finally, the electronic structures of the penta-and hexacoordinated complexes are discussed.