Structure and magnetism of novel tetranuclear µ 4 -oxo-bridged copper( II ) complexes

Reaction of CuCl2·2H2O 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 F2 by full-matrix least squares. They consist of a tetrahedron of copper atoms centred by a µ4-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), J12=–275(1) cm–1, J13=–27.8(5) cm–1 and xp= 1.0(1)% paramagnetic impurity for 1 and g= 2.10(2), J12=–261(1) cm–1, J13=–21.6(5) cm–1 and xp= 1.2(1)% for 2. By comparison to similar µ4-oxo-bridged copper(II) complexes, a linear relationship between the magnetic intradimer coupling constant and the mean Cu–O–Cu angle is obtained.