Heterodinuclear M^IINi^II (M = Co, Ni, Cu, Zn) complexes of a macrocyclic compartmental ligand. Anomalous EPR of Cu^IINi^II complex by coordination of 1-methylimidazole

A phenol-based macrocyclic compartmental ligand, (L^2;2)^2-, having N(amine)₂2and N(imine)₂O₂ metal binding sites sharing two phenolic oxygen atoms, has formed the following heterodinuclear M^IINi^II complexes: [CoNi (L^2;2)(AcO)]PF₆·CH₃CN (1), [NiNi(L^2;2)(AcO)]PF₆ (2), [CuNi(L^2;2)(AcO)]PF₆ (3) and [ZnNi(L^2;2)(AcO)]PF₆·CH₃CN (4). Furthermore, [CuNi(L^2:2)(CH₃CN)](PF₆)₂ ·CH₃CN (3′) was obtained when 3 was recrystallized from acetonitrile. X-ray crystallographic studies for 1, 2, 3′ and 4 demonstrate that the Ma resides in the N(amine)₂O₂ site and the Ni^II in the N(imine)₂O₂ site. The Ni^II in the N(imine)₂O₂ site has a planar geometry and is diamagnetic in all the complexes. The Co^II of 1 and the Ni^II of 2 in the N(amine)₂O₂have a six-coordinate geometry together with a bidentate acetate group. The Cu^II in 3′ has a five-coordinate geometry together with one acetonitrile molecule, and the Zn^II 4 has a square-pyramidal geometry together with a unidentate acetate group. Complex 3′ in DMF combines 1-methylimidazole (MeIm) at the axial site of the Ni^II and the Melm adduct in frozen DMF solution at 77.4 K shows EPR signals of g_∥ = 2.06 and g⊥ = 2.26 with a multi-line structure superimposed on the g_∥ component (A_∥ = 15.8 × 10^-4 cm^-1). The EPR signals are ascribed to the spin-doublet ground state (S_T = 1/2) of antiferromagnetically coupled Cu^II(S = 1/2) - Ni^II(S = 1), and the multi-line structure is explained by considering hyperfine interaction with Cu nucleus (A_Cu = 50.0 × 10^-4 cm^-1) and superhyperfine interaction with N nucleus of Melm (A_N = 15.0 × 10^-4 cm^-1).