Spin crossover behaviour in one-dimensional FeII compounds based on the [M(CN)4]2− (M = Pd, Pt) units

Abstract

Four one-dimensional heterobimetallic coordination polymers {Fe(pic)₂[M(CN)₄]}_n (M = Pd^II (1) and Pt^II (2), pic = 2-picolylamine), and {Fe(pypz)₂[M(CN)₄]}_n (M = Pd^II (3) and Pt^II (4), pypz = 2-(1H-pyrazol-3-yl)pyridine) have been synthesized and characterized by infrared spectroscopy, X-ray diffraction, magnetic measurements and differential scanning calorimetry (DSC). Single-crystal X-ray analyses show that all the compounds are 1D neutral zigzag chain structures in which the planar [M(CN)₄]²⁻ anion acts as a μ₂-bridging ligand, and the two pic/pypz molecules as chelating coligands. Examination of the intermolecular contacts in compounds 1–4 reveals the existence of the hydrogen bonding interactions involving the hydrogen donor groups of the pic and pypz ligands and the nitrogen atoms of the non-bridging cyanide groups of the [M(CN)₄]²⁻ anions. Weak π–π interactions were also found to be important for the formation of the 3D structures of compounds 3 and 4. The SCO properties of all compounds were confirmed by the detailed structural analyses of the coordination environments of the Fe^II centres, DSC analyses, and magnetic susceptibility measurements. Compounds 1 and 2 exhibit complete SCO behaviour with very narrow thermal hysteresis loops centred near the room temperature (T_1/2↓ = 270 K and T_1/2↑ = 272 K for 1 and T_1/2↓ = 272 K and T_1/2↑ = 274 K for 2), whereas 3 and 4 exhibit abrupt SCO at 186 and 180 K, respectively. Compared to the mononuclear species of the pic and pypz ligands, the SCO temperatures are adjusted by the different ligand field strength of the [M(CN)₄)]²⁻ units. The cooperativity from both the coordination bonds and supramolecular interaction leads to the observation of the hysteresis loops in the Fe–pic systems and the abrupt SCO transition in the Fe–pypz systems. Furthermore, the light-induced excited-spin-state trapping (LIESST) effect was observed for 2.