FERROMAGNETIC COUPLING BETWEEN SEMIQUINONE TYPE TRIDENTATE RADICAL LIGANDS MEDIATED BY METAL-IONS

Reactions between 3,5-di-tert-butylcatechol, aqueous ammonia and titanium, vanadium, germanium, and tin salts yield neutral bis- complexes of the resulting tridentate Schiff base biquinone ligand. On the basis of their magnetic, electrochemical, and spectral properties all the isolated compounds can be formulated as metal(IV) derivatives of the dinegative radical ligand Cat-N-SQ, M(Cat-N-SQ)(2). This suggestion is supported by the structural characterization of the titanium and vanadium derivatives. Both complexes crystallize in the triclinic space group P1, with Z = 2 in unit cells of the following dimensions: Ti(C28H40NO2)(2) a = 11.711(4) Angstrom, b = 12.246(5) Angstrom, c = 20.944(10) Angstrom, alpha = 89.19(4)degrees, beta = 74.15(4)degrees, gamma = 75.27(4)degrees; V(C28H40NO2)(2) a = 11.570(3), b = 12.282(8), c = 20.767(13) Angstrom; alpha = 89.65(5), beta = 74.58(4), gamma = 75.98(4)degrees. The structural features of the ligands, when compared with those reported earlier for other 3d metal complexes, are in agreement with the proposed charge distribution. This conclusion is also supported by the comparison of the rR spectra of all the 3d metal complexes formed by this Schiff base biquinone ligand. Magnetic and EPR data show that the titanium, germanium, and tin complexes are characterized by triplet ground state arising from the intramolecular ferromagnetic interaction between the two S = 1/2 radical ligands. This peculiar magnetic behavior may be rationalized on the basis of the structural properties of these compounds. The role of the metal orbitals in determining the exchange mechanism is discussed. The vanadium complex is characterized by a doublet ground state as a result of the spin cc,coupling between the two radical ligands and the S = 1/2 metal ion.