Hydroquinone-bridged dinuclear CuII complexes and single-crystalline CuII coordination polymers

Abstract

Cationic dinuclear Cu^II complexes 3 and 4 have been prepared using the novel hydroquinone-based imine chelators 2,5-(ⁱPr₂NCH₂CH₂NCH)₂-1,4-(OH)₂–C₆H₂ (1) and 2,5-(pyCH₂CH₂NCH)₂-1,4-(OH)₂–C₆H₂ (2), respectively (py = 2-pyridyl). X-Ray quality crystals of both complexes were grown from their DMF solutions. The sterically more encumbered compound 3 crystallizes in the form of discrete dinuclear entities with Cu^II centres in a distorted square-planar ligand environment (one coordination site is occupied by a DMF molecule). The pyridyl derivative 4 features dinuclear hydroquinone-bridged subunits similar to 3. However, the Cu^II ions are now six-coordinate with two DMF molecules at an axial and an equatorial position of a Jahn–Teller-distorted octahedron. Moreover, the dinuclear subunits are no longer isolated but linked with each other via bridging hydroquinone oxygen atoms which occupy the second apical position of each octahedron. The structure suggests that the magnetic properties of the resulting coordination polymer of 4 could be described by a model valid for dimerized spin chains. As a result of this analysis the antiferromagnetic coupling constants J₁/k_B = 9.9 K (intradimer) and J₂/k_B = 0.9 K (interdimer) are obtained. Both in 3 and in 4, the hydroquinone → semiquinone transition of the central bridging unit (E°′ = + 0.57 V, 3; E°′ = + 0.51 V, 4; DMF; vs. SCE) displays features of chemical reversibility. In the case of 4, reduction of Cu^II centres requires a peak potential of E_p = − 0.42 V.