Potentiometric, Spectroscopic and DFT Study of the (VO)-O-IV Complexes Formed by Di(pyridin-2-yl) Ligands

The coordinating properties of a series of di(pyridin-2-yl) derivatives towards the (VO2+)-O-IV ion have been studied through the combined application of potentiometric and spectroscopic (electronic absorption and EPR spectroscopies) methods. In particular, di(pyridin-2-yl)amine (DPA), di(pyridin-2-yl)methane (DPM), di(pyridin-2-yl) ketone (DPK), di(pyridin-2-yl)methanol (DPMO), 2-acetylpyridine (2-AP) and 2-hydroxymethylpyridine (2-HMP), and two amino acid derivatives of di(pyridin-2-yl)methylamine (DPMA), namely N-glycyl-DPMA and N-histidyl-DPMA, have been examined. The stability constants of proton and (VO)-O-IV complexes (log beta) were measured at 25 degrees C and at a constant ionic strength of 0.2 m (KCl). The results show that the simple di(pyridin-2-yl) derivatives having only pyridine nitrogen atoms form mono-chelated species with the (N-pyr, N-pyr) donor set, whereas those provided with carbonyl or hydroxy groups form mono- and bis-chelated complexes with (N-pyr, CO) or (N-pyr, O-) coordination. The square pyramidal species with 2x(N-pyr, O-) coordination exists in equilibrium with the octahedral isomer, which has an axially bound water molecule and is characterized by an anomalously low value of the V-51 hyperfine coupling constant along the z-axis (A(z)). DFT calculations were used to optimize the geometry of the complexes and to predict the A(z) values measured in the EPR spectra for the penta- and hexacoordinate complexes formed by DPK, DPMO, 2-AP and 2-HMP and for the [VOLH-1](+) and [VOLH-2] species formed by the amino acid derivatives of DPMA. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)