Dinuclear and tetranuclear copper(II) complexes with bridging (N–N) diazineligands: variable magnetic exchange topologies

Abstract

Dinuclear and tetranuclear copper(II) complexes of three polydentate diazine ligands (pahap, pmhap, bdpah), based on potentially bridging N–N single bond fragments, are reported. The 2∶2 complexes [Cu₂(pahap)₂(NO₃)(H₂O)₂][NO₃]₃·H₂O (1) and [Cu₂(pmhap)₂(NO₃)₂][NO₃]₂·3H₂O (2) involve dinuclear centres bridged by two N–N single bonds, and are essentially uncoupled, as a result of strict orbital orthogonality or an acute twisting of the copper magnetic planes relative to the N–N bridge. A mixed, double bridged (pyridazine/N–N) complex [Cu₂(pahap)(dppn)(NO₃)(H₂O)][NO₃]₃ (3) has a larger angle between the magnetic planes, resulting in weak antiferromagnetic behaviour (2J = −32.5 cm⁻¹). The rotationally more flexible 1∶1 complexes [Cu₂(pahap)(C₂O₄)₂]·0.5H₂O (4) (2J = −4.4 cm⁻¹) and [Cu₂(pahap)(acac)₂(H₂O)₂][NO₃]₂·H₂O (5) (2J = −69.7 cm⁻¹) involve single N–N bridges, and the angle between the copper magnetic planes depends on steric factors and hydrogen bonding interactions respectively. A tetranuclear complex [Cu₂(pahap − H)(dpa)]₂[NO₃]₄·4H₂O (6) results from the linking of two {Cu₂(N–N)} subunits with a secondary ligand strap, 1,3-diamino-2-propanol, and leads to a trans Cu₂(N–N) bridging arrangement and strong antiferromagnetic coupling (2J = −186.4 cm⁻¹). The complex [Cu₂(bdpah − H)(NO₃)₂][NO₃] (7) has a locked conformation with a large Cu–N–N–Cu torsional angle (130.3°), leading to moderately strong antiferromagnetic coupling (2J = −112.0 cm⁻¹). The magnetic results are entirely consistent with previous magnetostructural correlations relating the twist of the copper magnetic planes around the N–N bond to the exchange integral.