Extended one- and two-dimensional copper(II) complexes with bridging (N–N) diazineligands: structural and magnetic studies

Abstract

One- and two-dimensional copper(II) complexes of the polydentate diazine ligands pahap and pypz are reported, in which two copper(II) centers are bridged by a rotationally flexible single N–N bridge in a basic dinuclear subunit, with these subunits linked by halogen atom bridges to provide extended chains and layers. Crystal structures are reported for [Cu₂(pypz)Cl₄]·H₂O (1), [Cu₂(pypz)Br₄]·H₂O (2) and [Cu₂(pahap)Cl₄] (3). In 1 and 2 the dinuclear subunits are internally bridged by halogen atoms to form tetranuclear dimers, which then link to form a two-dimensional chicken wire lattice. The dinuclear subunits exhibit intra-molecular ferromagnetic coupling, associated with acute angles of rotation of the copper magnetic planes around the N–N single bond bridge, in complete agreement with previous magneto-structural results. However, low temperature phase transitions (<15 K) indicate the presence of significant antiferromagnetic components, which are associated with spin pairing between adjacent ferromagnetic layers. In addition long range, weak ferromagnetic ordering is observed below 5 K, and confirmed by magnetic hysteresis measurements. The complicated magnetic data have been interpreted in terms of a model involving one-dimensional ferromagnetic chains arranged in two-dimensional layers, with the layers linked antiferromagnetically, leading to metamagnetic behaviour, with additional interchain ferromagnetic coupling appearing at low temperatures. Compound 3 has a much larger rotational angle of the copper magnetic planes around the N–N bond (120.3°), and a different extended structure involving a linear chain of chlorine atom bridged tetranuclear subunits. The magnetic properties of 3 are interpreted using an alternating chain model, and are dominated by intramolecular antiferromagnetic exchange between copper(II) centers within each dinuclear subunit.