The effect of charged axial ligands on the EPR parameters in oxovanadium(IV) compounds: An unusual reduction of the A(z)(V-51) values

Two series of octahedral oxovanadium(IV) compounds, containing charged or n eutral axial ligands, with the tetradentate amidate molecules Hcapca and H( 2)capcah of the general formulae trans-[(VOX)-O-IV(capca)](0/4) (where X = Cl- (1 . CH2Cl2),SCN- (2), N-3 (3), CH3C- OO- (4), PhCOO (5), imidazole (6 . CH3NO2), and eta -nBuNH(2) (7)) and cis-[(VOX)-O-IV(Hcapcah)](0/4) where X= Cl- (8 . 0.5 CH2Cl2), SCN- (9), N-3(-) (10 . 2CH(3)OH), and imidazole (1 1)), were synthesized and characterized by X-ray crystallography (1 . CH3OH , 8 . CHCl3, 9 . 2 CH3CN, 10 . CH3CN and cis-[VO(imidazole)(Hcapcah)](+)) a nd continuous-wave electron paramagnetic resonance (cw EPR) spectroscopy. I n addition to the synthesis, crystallographic and EPR studies, the optical, infrared and magnetic properties (room temperature) of these compounds are reported. Ab initio calculations were also carried out on compound 8 . CHC l3 and revealed that this isomer is more stable than the trans isomer, in g ood agreement with the experimental data. The cw EPR studies of compounds 1 -5, that is, the (VO2+)-O-IV species containing monoanionic axial ligands, revealed a novel phenomenon of the reduction of their A, components by abou t 10% relative to the N-4 reference compounds ([(VO)-O-IV(imidazole)(4)](2) and [(VO)-O-IV(2,2-bi-pyririne)(2)](2+), see reference [46]). In marked c ontrast, such a reduction is not observed in compounds 6 CH3NO2-11, which c ontain neutral axial ligands Based on the spin-Hamiltonian formalism a theo retical explanation is put forward according to which the observed reductio n of A, is due to a reduction of the electron - nuclear dipolar coupling (P ). The present findings bear strong relevance to cw EPR studies of oxovanad ium(IV) in vanadoproteins, (VO2+)-O-IV-substituted proteins, and in (VO2+)- O-IV; model compounds, since the hyperfine coupling constant, A(2), has bee n extensive used as a benchmark for identification of equatorial-donor-atom sets in oxovanadium(IV) complexes.