METALLOPORPHYRIN PI-CATION RADICALS - MOLECULAR-STRUCTURE AND SPIN COUPLING IN A VANADYL OCTAETHYLPORPHYRINATE DERIVATIVE - AN UNEXPECTED SPIN COUPLING PATH

The preparation and characterization of a pi-cation radical derivative of a vanadyl porphyrinate is described. [VO(OH2)(OEP(.))]SbCl6 is pre pared by chemical oxidation of [VO(OEP)]; the formulation has been con firmed by a single-crystal X-ray structure determination. The coordina ted water molecule of the unusual six-coordinate vanadyl complex is de rived from solvent. Axial bond distances are the following: V-O = 1.57 8(4) Angstrom and V-O(H2O) = 2.473(8) Angstrom; the vanadyl ion is dis placed 0.46 Angstrom from the 24-atom mean plane. The average V-N-p di stance is 2.063 Angstrom. Crystal data: a = 15.530(2) Angstrom, b = 14 .586(4) Angstrom, c = 18.965(3) Angstrom, and beta = 106.20(1)degrees, monoclinic, space group P2(1)/n, V = 4125.4 Angstrom(3), Z = 4, [VO2N 4C36H45]SbCl6, R(1) = 0.053 and R(2) = 0.064 for 4707 observed data. E PR spectra have been measured for [VO(OH2)(OEP(.))]SbCl6 in the solid (powder) state, in a single crystal, and in fluid and frozen solution. The solution-state EPR spectra of [VO(OH2)(OEP(.))]SbCl6 are quite di stinct, with vanadium hyperfine lines and with differing spectra in fl uid and frozen solution. These solution spectra are identical to those reported earlier by others. In the solid state, the EPR spectrum of a polycrystalline sample consists of a single broad line with g = 1.99; single-crystal spectra also show vanadium hyperfine-splitting consist ent with V-V coupling. The solid-state EPR intensity increases with in creasing temperature. [VO(OH2)(OEP(.))]SbCl6 has also been characteriz ed by a detailed temperature-dependent (6-300 K) magnetic susceptibili ty study. Satisfactory fits of the temperature-dependent moments are o btained from a model in which the vanadyl electron is ferromagneticall y coupled to the porphyrin cation electron (J(v-r) = 63 cm(-1)) and tw o radical spins are antiferromagnetically coupled (J(r-r) = -139 cm(-1 )). This model also gives a satisfactory prediction of all solid-state EPR properties. The intramolecular ferromagnetic coupling is consiste nt with the principle of cospatial, orthogonal magnetic orbitals. The intermolecular antiferromagnetic coupling appears to arise from a nove l, steplike orientation of pairs of (OEP(.)) radicals.