Two-dimensional spin-crossover coordination polymers based on the 1,1,2,2-tetra(pyridin-4-yl)ethene ligand

A series of two-dimensional (2D) spin-crossover coordination polymers (SCO-CPs) [Fe^II(TPE)(NCX)₂]·solv (1: X = BH₃, solv = H₂O·2CH₃OH·DMF; 2: X = Se, solv = H₂O·2CH₃OH·0.5DMF; 3: X = S, solv = H₂O·2CH₃OH·0.5DMF) were synthesized by employing 1,1,2,2-tetra(pyridin-4-yl)ethene (TPE) and pseudohalide (NCX^-) coligands. Magnetic measurements indicated that complexes 1-3 exhibited SCO behaviors with diminishing thermal hysteresis (7/4/0 K) upon decreasing the ligand-field strength. The critical temperatures (T_c) during spin transition were found to be inversely proportional to the coordination ability parameters (a™) with a linear correlation. The guest effect was also investigated in the solvent-exchanged phases 1-SE/2-SE/3-SE wherein the DMF molecules were replaced by methanol molecules. Compared with 1-3, 1-SE/2-SE/3-SE displayed more abrupt and complete single-step SCO behaviors but narrower thermal hysteretic loops. The results reported here demonstrate that the T_c values of these two families were dominated by the ligand-field strength of the NCX^- anions (NCBH₃ > NCSe > NCS), whereas the guest effect only modulated the kinetic factor of the SCO nature in this system.