[1,2,4]Triazolo[4,3-a]pyridines as bridging ligands--magnetism of azole-bridged dinuclear copper(II) complexes influenced by the trigonal distortion parameter τ.

The dinuclear doubly azole-bridged copper(II) complexes Cu(II)2(L)2(MeCN)44·3.73MeCN·0.80H2O and Cu(II)2(L)64·solvent (solvent = 2MeCN·H2O; 2MeCN·2H2O; 1.5MeOH·3.5H2O) were prepared [L = 3-(6-methyl-2-pyridyl)-[1,2,4]triazolo[4,3-a]pyridine]. Structural characterizations revealed very different local geometries about the copper(II) ions, being trigonal bipyramidal for the former (τ = 0.76) and square pyramidal for the latter (τ = 0.07, 0.15, 0.07) complex. Magnetic measurements of bulk material Cu(II)2(L)2(H2O)44 and Cu(II)2(L)64·2H2O revealed antiferromagnetic coupling in both complexes, however, of very different strengths. Electron paramagnetic resonance (EPR) spectroscopy was applied to investigate magnetic properties of the complexes in detail. These experimental findings were supported by broken-symmetry DFT calculations. Systematic magneto-structural correlations are discussed.