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 ox^2– 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)Å, 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)Å, and the Ni–O distances are 2.086(4) and 2.111(5)Å for oxalate and 2.120(5)Å 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, T_N 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 ox^2– 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.