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.