Electron paramagnetic resonance spectra of some oxovanadium(IV) chelates

Abstract

X-Band e.p.r. spectra of magnetically dilute glasses at 77 K show that ethanol and pyridine form weak complexes with vanadyl chelates and that vanadium(iv) in solutions of these chelates is readily oxidised to vanadium (v).A method which enables spin Hamiltonian parameters to be extracted from systems which have CeV symmetry is described, and the magnetic and optical properties of some five-and six-membered ring chelates of the V 0 2 + ion, and of their complexes with ethanol and with pyridine, are listed. The principal values of the hyperfine tensor components and the isotropic contribution to hyperfine coupling can be used to distinguish five-from six-membered ring chelates. Spin Hamiltonian parameters are equated to the atomic orbital coefficients in some of the molecular orbitals involved in bonding in these chelates. The weak CZv component of the ligand field mixes the vanadium 3dz*-,$. 3dza. and 4s orbitals and the magnetic resonance data is used to estimate the extent of this mixing. Mixing 3dz* character into the orbital containing the unpaired electron accounts for ' in-plane anisotropy ' in the g-tensor components. Mixing 4s character accounts for characteristic differences between hyperfine tensor components observed for five-and six-membered ring chelates and for the changes which occur in the magnetic properties of these compounds when additional complexing with solvent molecules takes place.THE oxonietallate ion, oxovanadium(Iv), has been extensively studied : theoretical m e t h ~d s , l -~ spectrophotometric techniques,l96-8 X-ray crystallographic analy~es,~-ll and electron paramagnetic resonance methods 2-53 12-15 have all been employed in characterising it. In its chelates the ligand field has dominant C,, symmetry but this is reduced to C,, or even to C, by bidentate ligands.5~g9-11~16~17 Although several e.p.r. studies 5916*15 of doped single-crystals containing the V02. ion report this reduction in symmetry, to our knowledge there has only been one report of the effects of these lower symmetries on the e.p.r. spectra of vanadyl chelates dispersed in magnetically dilute glasses, although numerous reports of analyses of such systems with apparent C,, symmetry have been made.12-15 Several computational techniques have been developed to enable spin Hamiltonian parameters to be extracted from spectra of V02+ species with C,, symmetry12*18-21 but so far none have been reported for species with C,, symmetry. Some e.p.r. spectra of vanadyl chelates which have this lower symmetry are therefore reported below and a method which can be used to quickly analyse spectra obtained from their dilute solutions in glasses is described. E Y PER I 31 E N T ALThe vanadyl chelates (I)-(VI) were prepared by mixing, in 2 : 1 molar ratios, the appropriate enols, or their sodium C.

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