[Fe(TPT)2/3{MI(CN)2}2]•nSolv (MI=Ag, Au): New Bimetallic Porous Coordination Polymers with Spin-Crossover Properties

Two new heterobimetallic porous coordination polymers with the formula [Fe(TPT)2/3{MI(CN)2}2]nSolv (TPT=[(2,4,6-tris(4-pyridyl)-1,3,5-triazine]; MI=Ag (nSolv=0, 1MeOH, 2CH2Cl2), Au (nSolv=0, 2CH2Cl2)) have been synthesized and their crystal structures were determined at 120K and 293K by single-crystal X-ray analysis. These structures crystallized in the trigonal R-3m space group. The FeII ion resides at an inversion centre that defines a [FeN6] coordination core. Four dicyanometallate groups coordinate at the equatorial positions, whilst the axial positions are occupied by the TPT ligand. Each TPT ligand is centred in a ternary axis and bridges three crystallographically equivalent FeII ions, whilst each dicyanometallate group bridges two crystallographically equivalent FeII ions that define a 3D network with the topology of NbO. There are two such networks, which interpenetrate each other, thereby giving rise to large spaces in which very labile solvent molecules are included (CH2Cl2 or MeOH). Crystallographic analysis confirmed the reversible structural changes that were associated with the occurrence of spin-crossover behaviour at the FeII ions, the most significant structural variation being the change in unit-cell volume (about 59 angstrom 3 per FeII ion). The spin-crossover behaviour has been monitored by means of thermal dependence of the magnetic properties, Mossbauer spectroscopy, and calorimetry.