Magnetic Exchange in Imidazolate-Bridged Copper(II) Complexes

Abstract

Variable-temperature (4.2–300 K) magnetic susceptibility studies of a variety of imidazolate and substituted imidazolate-bridged dicopper(II) complexes reveal that the imidazolate (im) group can mediate moderate antiferromagnetic interactions. The observed coupling constants (J) for a series of tridentate amine copper(II) complexes in which the bridging ligands are imidazolate, 2-methylimidazolate, benzimidazolate, and 2-methylbenzimidazolate, −26.90 (2), −38.1 (3), −16.99 (1), and −29.82 (4) cm−1, respectively, parallel the first pKa values of the respective free imidazoles. This behavior is best accounted for by a σ-exchange pathway. Two coupling constants, −87.64 (4) and −35.0 (4) cm−1, are required to fit the data for [Cu2(bpim)(im)]24+, a cation having two geometrically distinct imidazolate-bridged dicopper(II) units. Assignment of the coupling constants to their respective bridging imidazolate rings in this tetranuclear cation is achieved by comparison to the J values obtained for Cu2bpim3+ and [Cu(pip)]2(im)3+ (bpim is 4,5-bis[((2-(2-pyridyl)ethyl)imino)methyl]imidazolate and pip is 2-[((2-(2-pyridyl)ethyl)imino)methyl]pyridine). The large negative J value observed for Cu2bpim3+ is rationalized on the basis of the near collinearity of the Cu—N(imidazolate) bonds which increases the through-bond interaction of the nitrogen lone pair orbitals with each other and leads to increased antiferromagnetic coupling. These studies verify the existence of an imidazolate-bridged dicopper(II) center in a form of bovine erythrocyte superoxide dismutase in which Cu(II) replaces Zn(II) and provide a foundation for the identification of imidazolate (histidine) bridged centers in other metallobiomolecules. © 1982, American Chemical Society. All rights reserved.