A structure and reactivity analysis of monomeric Ni(II)-hydroxo complexes prepared from water

The nickel(II) chemistry with the tridentate ligands bis[(N'-R-ureido)-N-ethyl]-N-methylamine (H(4)1(R), R = isopropyl, tert-butyl) is described. The Ni(II)-OH complexes, [Ni(II)H(2)1(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 [H(2)1(R)](2-) and the oxygen atom from the hydroxide (Ni-O(H), 1.857(1) angstrom). The Ni(II)-OH unit also is involved in two intramolecular hydrogen bonds between the urea groups of the [H(2)1(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 KOBut (pK(a) similar to 29) afforded the ligand substitution product, [Ni(II)H(2)1(R)(OBut)](-). Ni(II)-siloxide complexes were isolated when the [Ni(II)H(2)1(R)(OH)](-) complexes were allowed to react with K[N(TMS)(2)].