Synthesis and characterization of imidazolate-bridged polynuclear copper complexes

Copper(II) complexes of N4-donor ligands containing imidazole moieties, 4-[bis(1-methylimidazole-2-yl-methyl)aminomethyl]imidazole (Him-im₂) and 4-[bis(1-methylimidazole-2-yl-methyl)aminoethyl]imidazole (Hhis-im₂), were prepared, and [Cu(Him-im₂)Cl]ClO₄ (1) and [Cu(Hhis-im₂)Cl]ClO₄ (2) were structurally characterized by the X-ray diffraction method. Complexes 1 and 2 have a mononuclear structure with a coordinated chloride ion. The geometry of the Cu(II) center in 1 was found to be 5-coordinate trigonal-bipyramidal, whereas that of 2 was square-pyramidal. Complexes 1 and 2 showed different absorption and EPR spectra in MeOH, indicating that these compounds in solution maintain the structures revealed in the solid state. On the other hand, the reaction of Him-im₂ with Cu(ClO₄)₂ · 6H₂O under basic conditions gave a tetranuclear Cu(II) complex, [Cu₄(im-im₂)₄](ClO₄)₄ (3), whereas using the ligand Hhis-im₂ gave two kinds of polynuclear complexes [Cu₄(his-im₂)₄](ClO₄)₄ (4) and [Cu₆(his-im₂)₆](ClO₄)₆ (5) exhibiting discretely different structures. X-ray crystal structure analysis of the polynuclear complexes revealed their cyclic structures bridged by the imidazolate moiety. The geometry difference of the Cu(II) centers between 1 and 2 is thus concluded to determine the structures of tetranuclear complexes 3 and 4, respectively. Temperature dependent magnetic susceptibility measurements of complexes 3, 4, and 5 have shown an antiferromagnetic exchange interaction with a coupling constant of J = -32.5, -27.1 and -22.8 cm^-1, respectively.