Coligand and solvent effects on the architectures and spin-crossover properties of (4,4)-connected iron(II) coordination polymers

The self-assemblies of 1,4-bis(pyrid-4-yl)benzene (bpb) and Fe(NCX)₂ (X = S, Se, BH₃) afforded six coordination polymers with the general formula of [Fe(bpb)₂(NCX)₂]·Y (X = S and Y = 3C₂H₅OH·2.5H₂O for complex 4, X = S and Y = 2C₂H₅OH for 5, X = Se and Y = 2C₂H₅OH·H₂O for 6, X = Se and Y = 0.67CH₂Cl₂·1.33C₂H₅OH·0.67H₂O for 7, X = BH₃ and Y = 3C₂H₅OH·2H₂O for 8, X = BH₃ and Y = 2CH₂Cl₂·2C₂H₅OH for 9). The frameworks of complexes 4 and 5 with the NCS^- anion as coligand are supramolecular isomers, of which complex 4 features a threefold self-interpenetrated three-dimensional (3D) CdSO₄-type topological structure with a Schläfli symbol of 6⁵·8, and complex 5 is a two-dimensional (2D) 4⁴ rhombic grid network. These two complexes are purely high-spin systems. Complexes 6 and 7 with the NCSe^- anion as coligand, both having the 3D 6⁵·8 CdSO₄-type framework, show gradual and incomplete spin-crossover behaviors with transition temperature T_1/2 being equal to 86 and 96 K, respectively. The usage of NCBH₃^- anion as coligand leads to the formation of 2D 4⁴ rhombic grid networks for both complexes 8 and 9, which undergo relatively abrupt, complete spin crossover with T_1/2 being equal to 247 and 189 K, respectively. The structural divergences are attributed to the coligands NCX^- (X = S, Se, BH₃) and solvent molecules. Meanwhile, a significant coligand effect is observed on the spin-crossover behaviors of these complexes, and the completeness and transition temperature of spin-state conversion depends on the nature of the coligand, that is, T_1/2(NCS^-) < T_1/2(NCSe^-) < T_1/2(NCBH₃^-). These results further facilitate the design and synthesis of spin-crossover complexes with spin-state conversion.