Temperature-dependent Dimerization of TCNQ Anion-Radical in [ni(bpy)3]2(tcnq–tcnq)(tcnq)2·6h2o: Single-crystal Structure, Magnetic and Quantum Chemical Study

The crystal structure of [Ni(bpy)(3)](2)(TCNQ-TCNQ)(TCNQ)(2)center dot 6H(2)O (1) was studied by X-ray single-crystal structure analysis at 145 K and 100 K. The crystal structures of 1 at these two temperatures are essentially the same as the crystal structure studied previously at 200 K: the structure is built up of [Ni(bpy)(3)](2+) complex cations, two centrosymmetric crystallographically independent TCNQ(center dot-) anion-radicals, disordered sigma- and pi-dimerized (TCNQ)(2) units, and water molecules of crystallization. Lowering the temperature from 200 K, via 145 K-100 K has shown that at lower temperatures the proportions of sigma- and pi-dimerization in the disordered (TCNQ)(2) unit are shifted in favor of sigma-dimerization; moreover, variation of the weaker C-C sigma-bond formed upon dimerization was observed. In addition, lowering the temperature led to a shortening of the distance between the two crystallographically independent anion-radicals which are stacked along the b-axis with overlapped exo groups. The sigma- and pi-dimerization in the disordered (TCNQ)(2) unit was studied by quantum chemical calculations which showed smallest energy difference for sigma and pi-dimer at 200 K with respect to 145 K and 100 K in line with a lowest proportion of the dimerization observed experimentally. Temperature-dependent (1.8-270 K) magnetic study of 1 has shown the contribution of Ni(II) ions (S = 1) and the contribution of four S = 1/2 species carried by TCNQ radicals at higher temperatures, strongly coupled by antiferromagnetic (AFM) exchange interaction at 270 K while at low temperature a negligible contribution of TNCQ radical spins was observed.