Structure and magnetism of novel tetranuclear µ4-oxo-bridged copper(II) complexes

Abstract

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 x_p= 1.0(1)% paramagnetic impurity for 1 and g= 2.10(2), J₁₂=–261(1) cm^–1, J₁₃=–21.6(5) cm^–1 and x_p= 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.