Large Thermal Hysteresis for Iron(II) Spin Crossover Complexes with <i>N</i>‑(Pyrid-4-yl)isonicotinamide

A new series of iron­(II) 1D coordination polymers with the general formula [FeL1­(pina)]·<i>x</i>solvent with L1 being a tetradentate N₂O₂^2– coordinating Schiff-base-like ligand [([3,3′]-[1,2-phenylenebis­(iminomethylidyne)]­bis­(2,4-pentanedionato)­(2-)-<i>N</i>,<i>N</i>′,<i>O</i>²,<i>O</i>²′], and pina being a bridging axial ligand <i>N</i>-(pyrid-4-yl)­isonicotinamide, are discussed. The X-ray crystal structure of [FeL1­(pina)]·2MeOH was solved for the low-spin state. The compound crystallizes in the monoclinic space group <i>P</i>2₁<i>/c</i>, and the analysis of the crystal packing reveals the formation of a hydrogen bond network where additional methanol molecules are included. Different magnetic properties are observed for the seven samples analyzed, depending on the nature of the included solvent molecules. The widest hysteresis loop is observed for a fine crystalline sample of composition [FeL1­(pina)]·<i>x</i>H₂O/MeOH. The 88 K wide thermal hysteresis loop (<i>T</i>_1/2↑ = 328 K and <i>T</i>_1/2↓ = 240 K) is centered around room temperature and can be repeated without of a loss of the spin transition properties. For the single crystals of [FeL1­(pina)]·2MeOH, a 51 K wide hysteresis loop is observed (<i>T</i>_1/2↑ = 296 K and <i>T</i>_1/2↓ = 245 K) that is also stable for several cycles. For a powder sample of [FeL1­(pina)]·0.5H₂O·0.5MeOH a cooperative spin transition with a 46 K wide hysteresis loop around room temperature is observed (<i>T</i>_1/2↑ = 321 K and <i>T</i>_1/2↓ = 275 K). This compound was further investigated using Mössbauer spectroscopy and DSC. Both methods reveal that, in the cooling mode, the spin transition is accompanied by a phase transition while in the heating mode a loss of the included methanol is observed that leads to a loss of the spin transition properties. These results show that the pina ligand was used successfully in a crystal-engineering-like approach to generate 1D coordination polymers and improve their spin crossover properties