Enhanced Spin-Crossover Behavior Mediated by Supramolecular Cooperative Interactions

Three one-dimensional (ID) hetereobimetallic coordination polymers [Fe-II(L)(2)(AgCN)(2)].Solv (L = bpt(-), 1; L = Mebpt(-), Solv = 1.75EtOH, 2; L = bpzt(-), 3) with in situ generated AgCN species were synthesized by solvothermal reactions of Fell salt, K[Ag(CN)(2)], and the corresponding ligands [bptH = 3,5-bis(pyridin-2-yl)-1,2,4-triazole, MebptH = 3(3-methyl-2-pyridyl)-5-(2-pyridyl)-1,2,4-triazole, and bpztH = 3,5-bis(pyrazin-2-yl)-1,2,4-triazole]. They were further characterized by X-ray crystallography, magnetic and photomagnetic measurements, and differential scanning calorimetry. Single-crystal X-ray analyses show that they are isostructural with ID zigzag chain structures with rhombus {Fe2Ag2} units, in which the substituted bpt- ligand connects the Fe-II ion and AgCN species in a cis bridging mode. Then the zigzag chains are packed into three-dimensional supramolecular structures by pi center dot center dot center dot pi it interactions. Most importantly, weak Ag center dot center dot center dot N interactions (2.750 angstrom at 150 K) between the it-stacked neighboring chains present in complex 3. Magnetic susceptibility measurements exhibit that complex 1 displays characteristic paramagnetic behavior in the temperature range investigated. Complex 2 undergoes a gradual spin-crossover (SCO) with critical temperatures T-1/2 down arrow, = 232 K and T-1/2 up arrow = 235 K, whereas 3 exhibits an abrupt SCO with critical temperatures T-1/2 down arrow = 286 K and T-1/2 up arrow = 292 K. The magnetostructural relationships suggest that the magnetic behaviors can be modulated from paramagnetic behavior to abrupt and hysteretic SCO near room temperature through adjustment of the electronic substituent effect and intermolecular interactions.