2D/3D spin crossover porous coordination polymers based on isomeric tetrapyridyl benzene ligands

Abstract

By employing cyanoborohydride and isomeric tetrapyridyl benzene ligands, a new three-dimensional (3D) [Fe(3-tpb)(NCBH₃)₂] (1·2H₂O, 3-tpb = 1,2,4,5-tetra(pyridin-3-yl)benzene) and a two-dimensional (2D) [Fe(4-tpb)(NCBH₃)₂] (2·2-NapSMe and 2·H₂O·3DMF, 4-tpb = 1,2,4,5-tetra(pyridin-4-yl)benzene, 2-NapSMe = 2-(methylthio)naphthalene) porous coordination polymer (PCP) were synthesized, and exhibited the diamond (dia) and square lattice (sql) topologies, respectively. Complex 1·2H₂O shows gradual and incomplete spin crossover (SCO) behavior because of the rigid framework and larger octahedral distortion. In contrast, abrupt SCO and hysteretic two-step SCO behaviors are observed in 2·2-NapSMe and 2·H₂O·3DMF, respectively. This work proves that the modulation of host–guest interactions significantly influences the transition temperatures and SCO characteristics in a flexible PCP. More importantly, diverse multidentate ligands can be used to construct PCPs with novel topologies and tunable SCO properties.