Transition-metal complexes of pyrrole pigments. I. Electronic and vibrational spectra of cobalt(II), nickel(II) and copper(II) complexes of some dipyrromethenes

Abstract

The cobalt, nickel(II), and copper(II) complexes of 3,4,5-trimethyl-, 3,3′,5,5′-tetramethyl- and 3,3′,4,4′,5,5′-hexamethyldipyrromethene have been synthesized in this work and investigated by visible, near-infrared, infrared and far-infrared spectroscopy. The ligand-field bands for these metal complexes have provided valuable information on their geometry around the central metal atoms. Namely, the copper complexes assume the tetragonally distorted tetrahedral configuration while the nickel and cobalt complexes are approximately regular tetrahedral. Moreover, the removal of the 5′-methyl group of a dipyrromethene moiety in the copper complexes results in a significant structural alteration to the further flattened tetrahedron. In the far-infrared spectra, the prominent absorption peaks which can be associated with the metal-ligand stretching modes have been observed in the 400–350 cm −1 range. In addition, strong bands appearing at ∼1600 cm −1 have been assigned to the pyrroles-skeletal stretching mode. After examination of these infrared spectral data, the strength of the coordinate bonds has been noticed to increase in the order: Cu<Ni≳Co. The lowered strength of coordinate bonds for the copper complexes and the nature of coordinate bonds involved in the metal complexes of this work were discussed in connection with their stereochemistry.