Co-ligand and solvent effects on the spin-crossover behaviors of PtS-type porous coordination polymers

In our previous work (Chen, X.-Y.; Chem. Commun. 2013, 49, 10977-10979), we have reported the crystal structure and spin-crossover properties of a compound [Fe(NCS)₂(tppm)]·S [1·S, tppm = 4,4,4″, 4‴-tetrakis(4-pyridylethen-2-yl)tetraphenylmethane, S = 5CH ₃OH·2CH₂Cl₂]. Here, its analogues [Fe(X)₂(tppm)]·S [X = NCSe^-, NCBH₃ ^-, and N(CN)₂^- for compounds 2·S, 3·S, and 4·S, respectively] have been synthesized and characterized by variable-temperature X-ray diffraction and magnetic measurements. The crystal structure analyses of 2·S and 3·S reveal that both compounds possess the same topologic framework (PtS-type) building from the tetrahedral ligand tppm and planar unit FeX₂; the framework is two-fold self-interpenetrated to achieve one-dimensional open channels occupied by solvent molecules. Powder X-ray diffraction study indicates the same crystal structure for 4. The average values of Fe-N distances observed, respectively, at 100, 155, and 220 K for the Fe1/Fe2 centers are 1.969/2.011, 1.970/2.052, and 2.098/2.136 Å for 2·S, whereas those at 110, 175, and 220 K are 1.972/2.013, 1.974/2.056, and 2.100/2.150 Å for 3·S, indicating the presence of a two-step spin crossover in both compounds. Temperature-dependent magnetic susceptibilities (X_MT) confirm the two-step spin-crossover behavior at 124 and 200 K in 2·S, 151 and 225 K in 3·S, and 51 and 126 K in 4·S, respectively. The frameworks of 2-4 are reproducible upon solvent exchange and thereafter undergo solvent-dependent spin-crossover behaviors.