Syntheses, Structures, and Properties of Imidazolate-Bridged Cu(II)−Cu(II) and Cu(II)−Zn(II) Dinuclear Complexes of a Single Macrocyclic Ligand with Two Hydroxyethyl Pendants

The imidazolate-bridged homodinuclear Cu(II)−Cu(II) complex, [(CuimCu)L]ClO4·0.5H2O (1), and heterodinuclear
Cu(II)−Zn(II) complex, [(CuimZnL-2H)(CuimZnL-H)](ClO4)3 (2), of a single macrocyclic ligand with two hydroxyethyl
pendants, L (L = 3,6,9,16,19,22-hexaaza-6,19-bis(2-hydroxyethyl)tricyclo[22,2,2,211,14]triaconta-1,11,13,24,27,29-hexaene), have been synthesized as possible models for copper−zinc superoxide dismutase (Cu2,Zn2-SOD). Their
crystal structures analyzed by X-ray diffraction methods have shown that the structures of the two complexes are
markedly different. Complex 1 crystallizes in the orthorhombic system, containing an imidazolate-bridged dicopper(II)
[Cu−im−Cu]3+ core, in which the two copper(II) ions are pentacoordinated by virtue of an N4O environment with
a Cu···Cu distance of 5.999(2) Å, adopting the geometry of distorted trigonal bipyramid and tetragonal pyramid,
respectively. Complex 2 crystallizes in the triclinic system, containing two similar Cu−im−Zn cores in the asymmetric
unit, in which both the Cu(II) and Zn(II) ions are pentacoordinated in a distorted trigonal bipyramid geometry, with
the Cu···Zn distance of 5.950(1)/5.939(1) Å, respectively. Interestingly, the macrocyclic ligand with two arms possesses
a chairlike (anti) conformation in complex 1, but a boatlike (syn) conformation in complex 2. Magnetic measurements
and ESR spectroscopy of complex 1 have revealed the presence of an antiferromagnetic exchange interaction
between the two Cu(II) ions. The ESR spectrum of the Cu(II)−Zn(II) heterodinuclear complex 2 displayed a typical
signal for mononuclear trigonal bipyramidal Cu(II) complexes. From pH-dependent ESR and electronic spectroscopic
studies, the imidazolate bridges in the two complexes have been found to be stable over broad pH ranges. The
cyclic voltammograms of the two complexes have been investigated. Both of the two complexes can catalyze the
dismutation of superoxide and show rather high activity.