Synthesis, Structure, and Reactivity of Dinuclear Nickel Amino-Thiophenolate Complexes Bearing Bridging VO₂(OH)₂^– and VO₂(OR)₂^– Coligands

A series of novel mixed ligand dinickel complexes of the type [Ni^II₂L­(μ-L′)]⁺, where L′ is a tetrahedral oxo-alkoxo vanadate (L′ = [O₂V^V(OR)₂]⁻, R = H or alkyl) and L a macrocyclic N₆S₂ supporting ligand, have been prepared, and their esterification reactivity has been studied. The orthovanadate complex [Ni₂L­(μ-O₂V­(OH)₂)]⁺ (2), prepared by reaction between [Ni₂L­(μ-Cl)]­ClO₄ with Na₃VO₄ and a phase transfer reagent in CH₃CN, reacts smoothly with MeOH and EtOH forming the vanadate diesters [Ni₂L­(μ-O₂V­(OMe)₂)]⁺ (3) and [Ni₂L­(μ-O₂V­(OEt)₂)]⁺ (4). The dialkyl orthovanadate esters in 3 and 4 are readily transesterified with mono- and difunctional alcohols. Complex 3 can also be generated from 4 by transesterification with MeOH. Complexes 3 and 4 react with diols (ethylene glycol, propylene glycol and diethylene glycol) as well to afford the complexes [Ni₂L­(μ-O₂V­(OH)­(OCH₂CH₂OH))]⁺ (5), [Ni₂L­(μ-O₂V­(OCH₂)₂CH₂)]⁺ (6), and [Ni₂L­(μ-O₂V­(OCH₂CH₂)₂O)] (7). The crystal structures of the tetraphenylborate salts of complexes 3–7 reveal in each case four-coordinate O₂V^V(OR)₂^– groups bonded in a μ_1,3-bridging mode to generate trinuclear complexes with a central N₃Ni­(μ-S)₂(μ_1,3-O₂V­(OR)₂)­NiN₃ core. The stabilization of the four-coordinate V^VO₂(OR)₂^– moieties is a consequence of both the two-point coordinative fixation to and the steric protection of the bowl-shape binding pocket of the [Ni₂L]²⁺ fragment. Cyclic voltammetry experiments reveal that the encapsulated vanadate esters are not reduced in a potential window of −2.0 to +2.5 V vs SCE. The spins of the nickel­(II) (S_i = 1 ions) in 3 are weakly ferromagnetically coupled (J = +23 cm^–1, (H = −2JS₁S₂)) to produce an S = 2 ground state.