Dinuclear Nickel(II) Complexes of an Unsymmetric “End-Off” Compartmental Ligand:  Conversion of Urea into Cyanate at a Dinuclear Nickel Core

Abstract

A phenol-based “end-off” compartmental ligand, 2-[ N, N -di(2-pyridylmethyl)aminomethyl]-6-{ N -[2-(dimethylamino)ethyl]iminomethyl}-4-methylphenol (HL), having an iminic bidentate and an aminic tridentate chelating arms on the 2- and 6-positions of the phenolic ring, respectively, forms dinuclear nickel complexes [Ni 2 (L)(AcO)(NCS) 2 ] ( 1 ), [Ni 2 (L)(AcO) 2 (MeOH)]PF 6 ( 2 ), and [{Ni 2 (L)(OH)(MeOH)} 2 (CO 3 )](PF 6 ) 2 ( 3 ). Complex 1 crystallizes in the monoclinic space group P 2 1 / c, a = 14.165(5) Å, b =15.198(4) Å, c = 17.395(8) Å, β = 100.62(4)°, V = 3680(2) Å 3, and Z = 4. The pair of Ni ions present are bridged by the phenolic oxygen of L - and an acetate group in syn − syn mode (Ni−Ni: 3.373(3) Å). An isothiocyanate nitrogen atom coordinates to each Ni providing an asymmetric dinuclear core with a mixed {5/6} coordination number set. Complex 2 crystallizes in the monoclinic space group P 2 1 / c, a = 13.505(5) Å, b = 12.028(4) Å, c = 22.774(9) Å, β = 103.78(3)°, V = 3592(2) Å 3, and Z = 4. It has a dinuclear core bridged by the phenolic oxygen of L - and two acetate groups in syn − syn mode, providing a μ-phenoxo-bis(μ-carboxylato)dinickel(II) core (Ni−Ni: 3.396(6) Å). A methanol molecule coordinates to the Ni bound to the bidentate arm, forming a dinuclear core having a {6/6} coordination number set and an asymmetric donor atom environment. Complex 3 crystallizes in the orthorhombic space group Pbcn, a = 19.056(5) Å, b = 18.997(4) Å, c = 19.919(6) Å, α = β = γ = 90°, V = 7211(5) Å 3, and Z = 8. In each dinuclear unit a pair of Ni ions are bridged by the phenolic oxygen of L - and a hydroxo oxygen (Ni−Ni: 3.087(2) Å). A carbonate further bridges two of the dinuclear units to present a composite dimer. The Ni bound to the bidentate arm attains six-coordinate geometry by further interaction with two oxygens of the bridging carbonato group. The Ni bound to the tridentate arm assumes six-coordinate geometry by further coordination of a methanol oxygen. Complexes 1 − 3 react with urea in ethanol to form the isocyanate complexes [Ni 2 (L)(AcO)(NCS)(NCO)] ( 1 ‘ ), [Ni 2 (L)(AcO)(NCO)(EtOH)]PF 6 ( 2 ‘ ), and [{Ni 2 (L)(NCO)(EtOH)} 2 (CO 3 )](PF 6 ) 2 ( 3 ‘ ), respectively. Complex 3 ‘ crystallizes in the triclinic space group P 1̄, a = 20.072(7) Å, b = 21.145(6) Å, c = 18.688(6) Å, α = 106.20(2)°, β = 90.01(3)°, γ = 88.73(3)°, V = 7614(4) Å 3, and Z = 4. It has a dimeric structure very similar to that of 3, except for the replacement of the hydroxy bridge and the methanol ligand in 3 by isocyanate bridge and ethanol ligand, respectively, in 3 ‘.