Crystal structure and magnetic properties of a tetramer with a step-like Cu 4N 4O 2 core containing asymmetric bridged dimeric subunits related to the hemocyanin copper active site

Abstract

The Cu(II)-derivate of the macrocycle L derived from 2-hydroxy-5-methylisophthalaldehyde and 3- dimethylamino-1-propylamine forms tetranuclear {[LCu 2(N 3) 3](ClO 4) 2} 2 units. The crystal structure determined by X-ray crystallography is based on a step-like Cu 4N 4O 2-core with interdimeric μ(1)-azido bridging. The tetramer is formed from two dimeric subunits which are connected by a center of symmetry. The subunits are asymmetric bridged dkneric complexes with a phenolate and μ(1)-azido bridge. The coordination polyhedra around the two copper atoms are 4 + 2 and 4 + 1 including one coordinating perchlorate ion. The magnetic susceptibility of the complex has been measured in the temperature range 4.2 K to 315.6 K. The theoretical analysis using models for tetrameric and dimeric interactions revealed only a small interdimeric interaction. The magnetic exchange coupling of the asymmetric bridged dimeric unit (2 J = −528 cm −1) is comparable to that of known di-oxygen bridged dimers. The results are related to the magnetic behaviour of the asymmetric bridges found in the binuclear copper sites in hemocyanin. Infrared measurements have been taken and there is a discussion of the asymmetric vibrations of the three different azido-groups.