Cooperative Spin Transition in the Two-Dimensional Coordination Polymer [Fe(4,4′-bipyridine)2(NCX)2]·4CHCl3 (X = S, Se)

Two new isostructural two-dimensional (2D) coordination polymers exhibiting spin crossover (SCO) behavior of formulation [Fe(4,4′-bipy)2(NCX)2]·4CHCl3 (4,4′-bipy = 4,4′-bipyridine; X = S [1·4CHCl3], Se [2·4CHCl3]) have been synthesized and characterized, and both undergo cooperative spin transitions (ST). For 1·4CHCl3 the ST takes place in two steps with critical temperatures of Tc1down = 143.1 K, Tc2down = 91.2 K, Tc1up = 150.7 K, and Tc2up = 112.2 K. 2·4CHCl3 displays half ST characterized by Tcdown = 161.7 K and Tcup = 168.3 K. The average enthalpy and entropy variations and cooperativity parameters associated with the ST have been estimated to be ΔH1av = 5.18 kJ mol–1, ΔS1av = 35 J K–1 mol–1, and Γ1 = 2.8 kJ mol–1 and ΔH2av = 3.55 kJ mol–1, ΔS2av = 35 J K–1 mol–1, and Γ2 = 2.6 kJ mol–1 for 1·4CHCl3, and ΔHav = 6.25 kJ mol–1, ΔSav = 38.1 J K–1 mol–1, and Γ = 3.2 kJ mol–1 for 2·4CHCl3. At T > [Tc1 (1·4CHCl3); Tc (2·4CHCl3)], both compounds are in the space group P2/c while at T Tc1 (1·4CHCl3); Tc (2·4CHCl3)] they change to the C2/c space group and display an ordered checkerboard-like arrangement of iron(II) sites where the high- and low-spin states coexist at 50%.