Tetra- and dinuclear nickel(II)-vanadium(IV/V) heterometal complexes of a phenol-based N2O2 ligand: synthesis, structures, and magnetic and redox properties

The tetra- and binuclear heterometallic complexes of nickel(II)-vanadium(IV/V) combinations involving a phenol-based primary ligand, viz., N,N'-dimethyl-N,N'-bis(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine (H2L1), are reported in this work. Carboxylates and beta-diketonates have been used as ancillary ligands to obtain the tetranuclear complexes Ni(II)(2)V(V)(2)(RCOO)(2)(L(1))(2)O(4)] (R = Ph, 1; R = Me(3)C, 2) and the binuclear types (beta-diket)Ni(II)L(1)V(IV)O(beta-diket)] (3 and 4), respectively. X-ray crystallography shows that the tetranuclear complexes are constructed about an unprecedented heterometallic eight-membered Ni(2)V(2)O(4) core in which the (L(1))(2)- ligands are bound to the Ni center in a N(2)O(2) mode and simultaneously bridge a V atom via the phenoxide O atoms. The cis-N(2)O(4) coordination geometry for Ni is completed by an O atom derived from the bridging carboxylate ligand and an oxo O atom. The latter two atoms, along with a terminal oxide group, complete the O5 square-pyramidal coordination geometry for V. Each of the dinuclear compounds, (acac)Ni(II)L(1)V(IV)O(acac)] (3) and (dbm)Ni(II)L(1)V(IV)O(dbm)] (4) Hdbm = dibenzoylmethane], also features a tetradentate (L(1))(2)- ligand, Ni in an octahedral cis-N(2)O(4) coordination geometry, and V in an O(5) square-pyramidal geometry. In 3 and 4, the bridges between the Ni and V atoms are provided by the (L(1))(2)- ligand. The Ni...V separations in the structures lie in the narrow range of 2.9222(4) A (3) to 2.9637(5) A (4). The paramagnetic Ni centers (S = 1) in 1 and 2 are widely separated (Ni...Ni separations are 5.423 and 5.403 A) by the double V(V)O(4) bridge that leads to weak antiferromagnetic interactions (J = -3.6 and -3.9 cm-1) and thus an ST = 0 ground state for these systems. In 3 and 4, the interactions between paramagnetic centers (Ni(II) and V(IV)) are also antiferromagnetic (J = -8.9 and -10.0 cm-1), leading to an S(T) = 1/2 ground state. Compound 4 undergoes two one-electron redox processes at E(1/2) = +0.66 and -1.34 V vs Ag/AgCl reference due to a V(IV/V) oxidation and a Ni(II)/I reduction, respectively, as indicated by cyclic and differential pulse voltammetry.