The Equilibrium Between the Octahedral and Square Pyramidal Form and the Influence of an Axial Ligand on the Molecular Properties of V^IVO Complexes: A Spectroscopic and DFT Study

Abstract

The previously unreported equilibrium in aqueous solution between the V(IV)O 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 pentacoordinated species and values of K approximately 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 (51)V 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(IV)O 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 tau) and spectroscopic (|A(z)|, |A(iso)| and nu(V=O)) properties as a function of the axial V-OH(2) distance (R) are also presented. Finally, the electronic structures of the penta- and hexacoordinated complexes are discussed.

Coordinating Properties of Pyrone and Pyridinone Derivatives, Tropolone and Catechol toward the VO2+ Ion: An Experimental and Computational Approach

“…Moreover, the "additivity" rule [32] used with the contribution for O ket proposed by Hamstra et al [33] Finally, the value of A z for the mono-chelated species with or without a water molecule in the axial position was simulated; as has been demonstrated in the literature recently, the presence of a water ligand trans to the V=O group can significantly lower A z . [34] In this case, however, the results of the simulation did not allow us to demonstrate unambiguously which of the two structures, [ 3 ]. [35] The electronic absorption spectra of cis-[VO(ma) 2 -, the low-energy transition shifts to higher wavelength, whereas the second band shifts to lower wavelength.…”

“…EPR spectra were simulated with the computer program Bruker WinEPR SimFonia. [57] The coordination mode of the species formed has been attributed with the comparison of the 51 V A z experimental values with those obtained through DFT simulations and the application of the "additivity" rule [Equation (7)]; [32] unfortunately, the additivity rule does not take into account the influence of an axial donor, recently demonstrated, [34] but usually the predicted values fall in the range of around 3 ϫ 10 -4 cm -1 with respect to the experimental ones. Electronic absorption spectra on the aqueous solution were recorded with a Jasco Uvidec 610 spectrophotometer in the same concentration range as used for potentiometry.…”

Vanadium polypyridyl compounds as potential antiparasitic and antitumoral agents: New achievements

“…Once a particular binding mode is assumed, the values of A can be estimated (A || est ) using the additivity relationship proposed by Würthrich [40] and Chasteen [41], with estimated accuracy of ±3 × 10 −4 cm −1 . In this work we will not take into account the influence of axially-bound donor groups as recently suggested [42]. However, for some of the donor groups under consideration their predicted contributions to the parallel hyperfine coupling constant are not straightforward, namely the contributions of N semicarbazone (=N smc ), of O CO and of N phenanthroline (=N phen ).…”

Application of DFT methods to the study of the coordination environment of the VO2+ ion in V proteins

“…5. The possible structures of site B with an axially bound water can be ruled out: it has recently been demonstrated that such an axial interaction results in a lowering of |A z | exptl [85]. Concerning site A, the coordination of a water molecule rather than of an OH -ion has to be preferred.…”