Solid state polymerization causing transition to a ferromagnetic state. Crystal structures and magnetic properties of [Cu2(dpp)(H2O)(dmso)Cl4]·dmso and [Cu2(dpp)Cl4]n (dpp = 2,3-bis(2-pyridyl)pyrazine)

Abstract

The preparation, crystal structures and variable-temperature magnetic susceptibility data for [Cu₂(dpp)(H₂O)(dmso)Cl₄]·dmso (1a) and [Cu₂(dpp)Cl₄]_n (2) (dpp = 2,3-bis(2-pyridyl)pyrazine, dmso = dimethyl sulfoxide) are reported. 1a consists of dinuclear dpp-bridged molecules with chlorine, water and dmso serving as terminal ligands. The two copper atoms are crystallographically independent, with distorted square pyramidal and trigonal bipyramidal coordination geometries, respectively. In a first approximation 2 may be described as a chain compound with out-of-plane mono-μ-chloro bridges (Cu–Cl–Cu–Cl). Only one of the crystallographically independent copper atoms participates in this chain formation. Neighbouring chains are, however, linked into zig-zag sheets through somewhat weaker, out-of-plane, di-μ-chloro interactions involving the other copper atom. Both copper atoms have square pyramidal coordination geometries, dpp coordinating in equatorial positions through both its pyridyl and pyrazine nitrogen atoms. Compound 1a is desolvated under heating in a dynamic dinitrogen atmosphere at 130 °C, resulting in compound 1b. Variable-temperature magnetic susceptibility measurements on 1a (solvated state) reveal a very weak antiferromagnetic interaction between the copper(II) ions across bridging dpp (J = −0.6(1) cm⁻¹). 1b exhibits an overall ferromagnetic behaviour which is identical to that of compound 2 (J = +6.8(1) cm⁻¹, J′ = +1.0(1) cm⁻¹, J and J′ being the exchange coupling through the single and the double chloro bridge, respectively). Powder diffraction measurements on 1b confirm that this compound is isostructural with 2, in agreement with the magnetic results.