Crystal structure and spin-doublet electron spin resonance of a magnetically coupled di(µ-phenoxo)-copper(II)nickel(II) complex

Abstract

The complex [CuNiL][ClO₄]₂·H₂O (S_Ni= 1) of a dinucleating macrocycle H₂L, derived from the [2: 1 : 1] condensation of 2,6-diformyl-4-methylphenol, 1,2-diaminoethane and 1,4-diaminobutane, has been obtained. The efflorescent dimethylformamide (dmf) adduct [CuNiL][ClO₄]₂·H₂O·2dmf crystallizes in the orthorhombic crystal system, space group P2₁2₁2₁ with Z= 4, a= 15.62(1), b= 16.49(1) and c= 14.75(1)Å. Crystallographic refinement converged with R= 0.060 and R′= 0.048 for 2958 reflections having I > 3.00σ(I). The structure comprises [CuNiL(dmf)(H₂O)]²⁺ where the two metal ions are bridged by the endogenous phenolic oxygens of the macrocycle and Cu ⋯ Ni is 3.038(2)Å. The Cu resides at a N₂O₂ site with an ethylene lateral chain and assumes an essentially planar geometry, the Ni at a site with a tetramethylene lateral chain and a pseudo-octahedral geometry having dmf and water molecules at the axial sites. The complex [CuNiL][ClO₄]₂·H₂O exhibits a strong antiferromagnetic spin exchange (J=–90 cm^–1 based on H=–2JS₁S₂) and shows an axial ESR feature with g_∥= 2.16, g_⊥= 2.27 and A_∥= 64 × 10^–4 cm^–1(in frozen dmf at 78 K), attributable to a spin-doublet ground state. The relations g_⊥ > g_∥ and A_∥=–⅓A_Cu{A_Cu is the hyperfine coupling constant of the analogous [CuZnL][ClO₄]₂·H₂O} demonstrate that one unpaired electron of the spin-coupled complex resides in the molecular orbital of d_z² character comprising d_z²(Cu) and d_z²(Ni).