Nickel(II)-induced activation of a carbon–nitrogen single bond(s) of a co-ordinated ligand by hydroxylated substrates: generation of mono- and bi-nuclear complexes with modified ligand frameworks†

Abstract

In the presence of some hydroxylated compounds as substrates, complexations of nickel(II) with the pentadentate N₄S ligand methyl 2-{2-bis[(3,5-dimethylpyrazol-1-ylmethyl)amino]propylamino}cyclopent-1-ene-1-carbodithioate (Hmmpcd) having a pair of flexible pyrazolyl arms proceeded through unusual reaction paths involving C–N bond cleavage process. For example, the product 1 obtained in methanol as substrate is a square planar compound with a modified ligand structure that includes a methoxy group provided by the solvent. With an acidic substrate viz. phenol the product is also a square planar compound 2 with one of the pendant arms of Hmmpcd being replaced by a hydrogen atom. On the other hand, in the presence of a basic substrate viz. ethanolamine the ligand undergoes more extensive breakdown to generate a binuclear complex 3 with a bridging pyrazolyl group. These complexes 1–3 were characterised by X-ray crystallography and ¹H NMR spectroscopy. Compound 1 displayed a reversible one-electron Ni^II → Ni^I reduction at E_½ ≈ –1.0 V vs. Ag–AgCl reference as established by cyclic voltammetry and combined coulometry/EPR experiments. Such reversibility of electron transfer is unusual for a square-planar nickel(II) compound in an acyclic ligand environment. Stabilisation of the reduced nickel(I) species by methoxy group co-ordination provides a probable explanation for this reversible electron transfer.