Copper(II)–π interaction in cis-[Cu(orotato)(NH 3) 2] and the crystal structure of [Cu(orotato)(H 2O) 4] · H 2O: X-ray, vibrational spectroscopy and density functional study

Abstract

The crystal structure of cis-[Cu(orotato)(NH 3) 2] ( 1) has been reinvestigated, and the structure of [Cu(orotato)(H 2O) 4] · H 2O ( 2) has been determined by single crystal X-ray diffraction and vibrational spectroscopic methods. In both complexes, the copper(II) ion is chelated by the deprotonated nitrogen atom (N1) of the uracil ring and the carboxylate oxygen atom of the orotate ligand. The square-planar environment of Cu is completed by two ammonia N atoms in 1, and two water O atoms in 2. Evidence is presented that the coordination sphere in 1 should be described as a (4+1+1)-geometry, where one axial position is occupied by the carbonyl oxygen atom and the other apical coordination site is located at the C C double bond of the neighboring uracilate ring. The non-covalent Cu(II)–π binding force is very important in stabilizing the columnar, polymeric structure of 1. The molecular structure of 2 is an asymmetric elongated octahedron (4+2) with two long Cu–O (water) axial bonds. Theoretical studies have been performed using the density functional unrestricted UB3LYP method. The natural charges on the atoms are derived from natural bond orbital (NBO) analysis. It is suggested that the π-type interaction between the chelated Cu 2+ cation and the aromatic compounds can play a significant role in biological systems, e.g. in the active sites of copper enzymes.