Investigation of Solid and Solution Structures of N-Substituted Cu(II) Salicyldimines by X-Ray Absorption Spectroscopy

Abstract

A series of bis(N-alkylsalicylaldiminato)copper(II) complexes that serve as models for mononuclear, nontetragonal copper(II) sites in copper proteins are examined by X-ray absorption spectroscopy in order to investigate their structural characteristics in crystalline, room temperature solution, and frozen solution forms. Previous electron paramagnetic resonance (EPR) studies of frozen solution samples of bis(tert-butylsalicylaldiminato)copper(II) (Cu{sup II}(t-butSal){sub 2}) suggest solvent dependent variation in site symmetry. In addition, linear electric field effect (LEFE) investigations in frozen solution reveal a deviation from centrosymmetry greater than expected based on X-ray crystal structure. X-ray edge results indicate that a more tetrahedral coordination geometry is assumed by Cu{sup II}(t-butSal){sub 2} in solution preparations, both at room temperature or frozen, when compared with crystalline samples. Extended X-ray absorption fine structure (EXAFS) studies of the Cu{sup II}(t-butSal){sub 2} species show no difference in metal - ligand distances between crystalline or frozen solution samples. In contrast, no differences in X-ray edge spectra were detected between crystalline and solution forms of bis(methyl-) and bis-(propylsalicylaldiminato)copper(II) complexes nor for bis(salicylaldehyde)copper(II), which suggests that bulkiness of the N-alkyl group in salicyldimines mediates the geometric sensitivity to local environmental conditions.