Metalloprotein models, location of the magnetic axes in low-symmetry complexes. Single crystal electron paramagnetic resonance, magnetic susceptibility anisotropy, and angular overlap ligand field calculations on a complex containing the distorted tetrahedral Co IIN 2O 2 coordination unit, bis(N-isopropylsalicyclaldiminato)cobalt(II)

Abstract

Single-crystal and powder electron paramagnetic resonance (epr) spectra of bis(N-isopropylsalicylaldiminato)cobalt(II), Co(iPr-sal) 2, doped into the isomorphous zinc complexes were obtained at liquid helium temperatures. Principal molecular g-values of g x, 7.09; g y 0.57; and g z, <0.3, were obtained. The orientation of the principal magnetic axes with respect to the distorted tetrahedral Co IIN 2O 2 coordination unit of approximate C 2 symmetry was determined from the angular variation of the single-crystal epr signals using the method of Schonland. The x-axis approximately bisects the NCoO angles of the individual bidentate ligands, the y-axis is the approximate bisector of the NCoO angles formed between different ligands, while the z-axis is taken as the approximate molecular C 2 axis. Ligand field calculations employing the angular overlap model (AOM) predict principal molecular magnetic axis directions in excellent agreement with experiment. Principal crystal magnetic susceptibility anisotropies were measured by the critical torque method. The derived molecular susceptibilities were used to evaluate dipolar nuclear magnetic resonance shifts of ligand nuclei. When dipolar contributions to the observed shifts are accounted for, the unpaired electron spin delocalization patterns of the Co(iPr-sal) 2 and Ni(iPr-sal) 2 are quite similar. The relevance of these results to the problem of achieving a detailed understanding of the properties of cobalt(II)-substituted zinc metalloproteins is stressed.