A structure and reactivity analysis of monomeric Ni(ii)–hydroxo complexes prepared from water

Abstract

The nickel(II) chemistry with the tridentate ligands bis[(N′-R-ureido)-N-ethyl]-N-methylamine (H₄1^R, R = isopropyl, tert-butyl) is described. The Ni(II)–OH complexes, [Ni^IIH₂1^R(OH)]⁻ were generated using water as the source of the hydroxo ligand. These complexes are pseudo-square planar, in which the primary coordination sphere contains three nitrogen donors from [H₂1^R]²⁻ and the oxygen atom from the hydroxide (Ni–O(H), 1.857(1) Å). The Ni(II)–OH unit also is involved in two intramolecular hydrogen bonds between the urea groups of the [H₂1^R]²⁻ and the hydroxo oxygen atom. Attempts to deprotonate the Ni(II)–OH unit to produce Ni(II)–oxo complexes were unsuccessful. A variety of bases with pK_a of less than 15 (in DMSO) were unable to deprotonate the hydroxo ligand. Treating the Ni(II)–OH complexes with KOBu^t (pK_a∼ 29) afforded the ligand substitution product, [Ni^IIH₂1^R(OBu^t)]⁻. Ni(II)–siloxide complexes were isolated when the [Ni^IIH₂1^R(OH)]⁻ complexes were allowed to react with K[N(TMS)₂].