Macrocyclic dimeric vanadium(IV) and heterodinuclear vanadium(IV)-nickel(II) complexes. Structure, magnetic, electronic and redox properties

Abstract

The oxovanadium(IV) complexes [{VO(H₂L1)}₂(µ-SO₄)][NO₃]₂, [(VO)L1Ni(H₂O)₂(SO₄)]·H₂O and [(VO)L1Ni(µ-SO₄)(H₂O)]·2H₂O have been synthesized from the dinucleating tetraaminodiphenol macrocycle H₂L¹. The crystal structure of the first, in which two unco-ordinated amino nitrogens are protonated, has been determined. The two distorted-octahedral vanadium centres in the complex, separated by 6.741(4)Å, are bridged by sulfate and have an anti-oxo configuration. In the solid state the magnetic moment of this complex per vanadium decreases from 1.71 to 1.64 µB on lowering the temperature from 299 to 5 K, indicating very weak intra-/or inter-molecular exchange interactions. Its ESR spectra in fluid solution and glass, however, showed the absence of VO ··· VO electron-spin interaction. This complex undergoes stepwise oxidation to produce VO^IVVO^V and VO^VVO^V species with E_1/2(1)= 0.445 V and E_½(2)= 0.60 V vs. Ag-AgCl electrode; the comproportionation constant Kc= 4.2 × 102. The other two complexes have identical composition but differ in their IR and UV/VIS spectra and electrochemistry. In the first the oxovanadium(IV) and nickel(II) centres are revesibly oxidized at 0.436 and 0.756 V, respectively, while in the second irreversible oxidation of both metal centres takes 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.