X-Ray structures and magnetism of azido-bridged mixed-ligand binuclear copper(II) complexes. Model complexes of the type III copper protein centre

Abstract

The structures of the binuclear copper(II) complexes [CuL(N₃)Cu(bipy)][ClO₄]₂(1) and [CuL(N₃)Cu(pmdt)][ClO₄]₂(2)[L = a Schiff base derived from 2,3-butanedione 2-oxime and 2-(2-aminoethyl)pyridine, bipy = 2,2′-bipyridine, pmdt =N,N,N′,N″,N″-pentamethyldiethylenetriamine)] have been determined by X-ray analysis. The geometry around Cu(2) in (2) is a square pyramid with N (4) at the apical position, while the other copper environment is essentially square planar though each Cu^II is semi-co-ordinated by ClO₄^– above and/or below a co-ordination plane. For both complexes two Cu^II ions antiferromagnetically couple with a singlet–triplet separation of 2J=–520 cm^–1 for (1) and –296 cm^–1 for (2), where J denotes the exchange integral defined by the Hamiltonian ℋ=–2JS₁. S₂. Complexes (1) and (2) are e.s.r.-silent in the solid state at room temperature. In (1) the two co-ordination planes are nearly coplanar and the two Cu^II couple through N₃^– and the oxime portion (NO^–) of the ligand L. Geometrical considerations indicate that in (2) the magnetic interaction through the N(4) atom is negligibly small. The observed coupling is mainly through the N–O^– bond of L. It is concluded that in the native type III copper protein two magnetic orbitals on two Cu^II are nearly coplanar and two unpaired electrons exchange extensively to give diamagnetic behaviour.