STRUCTURE AND MAGNETISM OF NOVEL TETRANUCLEAR MU(4)-OXO-BRIDGED COPPER(II) COMPLEXES

Reaction of CuCl2 . 2H(2)O or CuBr2 with the tridentate ligand 4-methyl-2,6-bis(pyrrolidin-1-yl-methyl)phenol (Hmbpp) in methanol yielded two copper(II) complexes [Cu4OCl4(mbpp)(2)]. 2MeOH 1 and [Cu4OBr4(mbpp)(2)] 2. They were characterised by single-crystal X-ray diffraction analyses. Both structures were solved using direct methods and refined on F-2 by full-matrixleast squares. They consist of a tetrahedron of copper atoms centred by a mu(4)-bridging oxygen atom. Magnetic susceptibility measurements in the temperature range 7.0-487.3 K indicated significant antiferromagnetic coupling between the copper(ii) centres of both complexes. The experimental data were fitted to a modified Heisenberg model by analysing the tetranuclear complexes as two dinuclear Cu2O2 subunits. The best least-squares fit parameters were g = 2.12(2). J(12) = -275(1) cm(-1). J(13) = -27.8(5) cm(-1) and x(p) = 1.0(1)% paramagnetic impurity for 1 and g = 2.10(2), J(12) = -261(1) cm(-1), J(13) = -21.6(5) cm(-1) and x(p) = 1.2(1)% for 2. By comparison to similar mu(4)-oxo-bridged copper (II) complexes, a linear relationship between the magnetic intradimer coupling constant and the mean Cu-O-Cu angle is obtained.