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 T1/2 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 T1/2 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, T1/2(NCS–) T1/2(NCSe–) T1/2(NCBH3–). These results further facilitate the design and synthesis of spin-crossover complexes with spin-state conversion.