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 = 3C(2)H(5)OH.2.5H(2)O for complex 4, X = S and Y = 2C(2)H(5)OH for 5, X = Se and Y = 2C(2)H(5)OH.H2O for 6, X = Se and Y = 0.67CH(2)Cl(2).1.33C(2)H(5)OH.0.67H(2)O for 7, X = BH3 and Y = 3C(2)H(5)OH.2H(2)O for 8, X = BH3 and Y = 2CH(2)Cl(2).2C(2)H(5)OH 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 Schlafli 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