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

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 CuII is semi-co-ordinated by ClO4- above and/or below a co-ordination plane. For both complexes two CuII 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 CuII 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 CuII are nearly coplanar and two unpaired electrons exchange extensively to give diamagnetic behaviour.