Electron Spin Lattice Relaxation of V(IV) Complexes in Glassy Solutions between 15 and 70 K

Electron spin lattice relaxation rates for four vanadium(IV) complexes: bis(acetylacetonate)oxovanadium(IV), (VO(acac)2), 1; bis(maltolato)oxovanadium(IV), (VO(maltol)(2)), 2; Cesium N,N'-ethylenebis(salicylideneiminato-5'-sulfonato)oxovanadium(IV), Cs-2[VO(salen-SO3)(H2O)], 3; and bis(N-hydroxyiminodiacetato)oxovanadium(IV), (Ca[V(hida)(2)]), 4; in 1:1 water:glycerol glasses were measured by long-pulse saturation recovery at X-band. Although these complexes have coordination spheres that vary from O-5 to N4O and N2O6, the relaxation rates differ by factors of only about 2 at 15 K and 4 at 70 K. Relaxation rates for 4, which does not contain an oxo group, are very similar to those for oxo-containing complexes. At 70 K relaxation rates decrease in the order aquo VO2+ > VO(acac)(2) similar to VO(maltol)(2) > [V(hida)(2)](2-) > [VO(salen-SO3) (H2O)](2-) > vanadyl porphyrin. This order correlates with decreasing flexibility of the ligands and coordination sphere. The temperature dependence of spin lattice relaxation rates was analyzed in terms of contributions from the direct process, the Raman process, and local modes.