MACROCYCLIC DIMERIC VANADIUM(IV) AND HETERODINUCLEAR VANADIUM(IV)-NICKEL(II) COMPLEXES - STRUCTURE, MAGNETIC, ELECTRONIC AND REDOX PROPERTIES

The oxovanadium(IV) complexes [{VO(H(2)L(1))}(2)(mu-SO4)][NO3](2), [(V O)L(1)Ni(H2O)(2)(SO4)]. H2O and [(VO)L(1)Ni(mu-SO4)(H2O)]. 2H(2)O have been synthesized from the dinucleating tetraaminodiphenol macrocycle H(2)L(1). The crystal structure of the first, in which two unco-ordina ted amino nitrogens are protonated, has been determined. The two disto rted-octahedral vanadium centres in the complex, separated by 6.741(4) Angstrom, are bridged by sulfate arid have an anti-oxo configuration. In the solid state the magnetic moment of this complex per vanadium d ecreases from 1.71 to 1.64 mu(B) on lowering the temperature from 299 to 5 K, indicating very weak intra-/or inter-molecular exchange intera ctions. Its ESR spectra in fluid solution and glass, however, showed t he absence of VO ... VO electron-spin interaction. This complex underg oes stepwise oxidation to produce (VOVOV)-V-IV and (VOVOV)-V-V species with E(1/2)(1) = 0.445 V and E(1/2)(2) = 0.60 V vs. Ag-AgCl electrode ; the comproportionation constant K-c = 4.2 x 10(2). The other two com plexes have identical composition but differ in their IR and UV/VIS sp ectra and electrochemistry. In the first the oxovanadium(IV) and nicke l(II) centres are reversibly oxidized at 0.436 and 0.756 V, respective ly, while in the second irreversible oxidation of both metal centres t akes place at 1.05 and 1.25 V. Both behave ferromagnetically and their exchange coupling constants J are 10(1) and 6(1) cm(-1), respectively .