Synthesis, crystal structure, magnetic properties, and solution study of the complex µ-oxalato-bis[aqua(1,4,7-triazacyclononane)nickel(II)] nitrate dihydrate

Abstract

A new binuclear nickel(II) complex of formula [Ni2L12(H2O)2(ox)][NO3]2·2H2O, where L1 and ox2– refer to 1,4,7-triazacyclononane and oxalate anion respectively, has been synthesized and its crystal structure solved at room temperature. This compound crystallizes in the monoclinic system, space group P21/c, with a= 8.704(2), b= 7.344(5), c= 20.321(6)A, β= 98.38(2)°, and Z= 2. The structure was refined to R′= 0.037. It consists of centrosymmetrical binuclear cations [Ni2L12(H2O)2(ox)]2+ with water molecules of crystallization and nitrate counter ions. Each nickel atom is six-co-ordinated in an octahedral distorted fac-NiN3O3 arrangement: the Ni–N distances are in the range 2.059(5)–2.080(6)A, and the Ni–O distances are 2.086(4) and 2.111(5)A for oxalate and 2.120(5)A for co-ordinated water. The oxalate joins two adjacent nickel atoms acting as a bis(bidentate) ligand. The C–C bond is perpendicular to the Ni ⋯ Ni vector giving two five-membered chelate rings. Magnetic susceptibility measurements in the temperature range 4.2–300 K show that the two nickel(II) ions of the binuclear unit are antiferromagnetically coupled with J=–25.5 cm–1 and g= 2.10, TN being ca. 32 K. The electronic spectra, measured both in solution and in the solid state, allowed values of 977 and –135 cm–1 for the Racah parameter and the spin–orbit coupling constant, respectively, to be determined. The equilibrium constant relative to the addition of one ox2– to the complex [NiL1]2+ has been determined potentiometrically in aqueous solution, log K= 3.26(1)(298.15 K and 0.1 mol dm–3 KNO3). The enthalpy of this reaction has been measured by direct microcalorimetry. From the value obtained, ΔH⊖=–1.5(1) kcal mol–1, a value of TΔS⊖ of 2.9 kcal mol–1 can be calculated. These parameters are interpreted and analysed taking into account previous data on analogous reactions for some tetra-azamacrocyclic nickel(II) complexes.