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)2·6H2O (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·- anion-radicals, disordered σ- and π-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 σ- and π-dimerization in the disordered (TCNQ)2 unit are shifted in favor of σ-dimerization; moreover, variation of the weaker C–C σ-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 σ- and π-dimerization in the disordered (TCNQ)2 unit was studied by quantum chemical calculations which showed smallest energy difference for σ and π-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.