Bonding properties of Cu(II)-O chromophores: electronic and molecular structure of bis(imidazole)bis( N-methylacetamide)bis(perchlorato)copper(II)

Abstract

The crystal and molecular structure of bis(imidazole)bis( N-methylacetamide)bis(perchlorato)copper(II) has been determined using three-dimensional X-ray diffraction data. The complex crystallizes in monoclinic P2 1/ c with Z = 2, a = 7.720(2), b = 16.169(3), c = 8.742(2) Å and β=96.08(1)°. Least-squares refinement of the structure yielded a final R value of 6.62% and a R w of 7.55% for 1388 independent reflections at 294 K. The copper(II) ion is coordinated by six ligands to form a centrosymmetrically elongated octahedron with CuN(imidazole) 1.969(6), CuO( N-methylacetamide) 1.982(5) and CuO(ClO 4 −) 2.760(10) Å. The molecules are held together with hydrogen bonding between imidazole and perchlorate and between N-methylacetamide and perchlorate. The imidazole ligands, having donating nitrogens with mainly ‘pyrrole’ character as evident by the nearly equal bond lengths of the CN(H) and CN(Cu) bonds of the rings, form strong σ-bonds with the central copper ions. The N-methylacetamide ligand, having a very short CO (1.159(12) Å) and a long NC(O)(1.330(13) Å) bond, suggests a canonical structure with restricted π-delocalization. The deconvoluted Gaussian component dd bands of the title complex and the pyridine N-oxide analogues are analyzed and assigned with reference to their molecular structures. The bonding properties of Cu(II)O are deduced: in elongated octahedral copper(II) complexes, pyridine N-oxide is a moderately strong σ-donor and a strong π-donor, whereas N-methylacetamide is merely a moderately strong σ-donor.