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 β-diketonates have been used as ancillary ligands to obtain the tetranuclear complexes [NiII2VV2(RCOO)2(L1)2O4] (R = Ph, 1; R = Me3C, 2) and the binuclear types [(β-diket)NiIIL1VIVO(β-diket)] (3 and 4), respectively. X-ray crystallography shows that the tetranuclear complexes are constructed about an unprecedented heterometallic eight-membered Ni2V2O4 core in which the (L1)2- ligands are bound to the Ni center in a N2O2 mode and simultaneously bridge a V atom via the phenoxide O atoms. The cis-N2O4 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)NiIIL1VIVO(acac)] (3) and [(dbm)NiIIL1VIVO(dbm)] (4) [Hdbm = dibenzoylmethane], also features a tetradentate (L1)2- ligand, Ni in an octahedral cis-N2O4 coordination geometry, and V in an O5 square-pyramidal geometry. In 3 and 4, the bridges between the Ni and V atoms are provided by the (L1)2- ligand. The Ni···V separations in the structures lie in the narrow range of 2.9222(4) Å (3) to 2.9637(5) Å (4). The paramagnetic Ni centers (S = 1) in 1 and 2 are widely separated (Ni···Ni separations are 5.423 and 5.403 Å) by the double VVO4 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 (NiII and VIV) are also antiferromagnetic (J = −8.9 and −10.0 cm-1), leading to an ST = 1/2 ground state. Compound 4 undergoes two one-electron redox processes at E1/2 = +0.66 and −1.34 V vs Ag/AgCl reference due to a VIV/V oxidation and a NiII/I reduction, respectively, as indicated by cyclic and differential pulse voltammetry.