Hydrogen bond-induced abrupt spin crossover behaviour in 1-D cobalt(ii) complexes – the key role of solvate water molecules

Abstract

The magnetic properties and structural aspects of the 1-D cobalt(II) complexes, [Co(pyterpy)Cl₂]·2H₂O (1·2H₂O; pyterpy = 4′-(4′′′-pyridyl)-2,2′:6′,2′′-terpyridine) and [Co(pyethyterpy)Cl₂]·2H₂O (2·2H₂O; pyethyterpy = 4′-((4′′′-pyridyl)ethynyl)-2,2′:6′,2′′-terpyridine) are reported. In each complex the central cobalt(II) ion displays an octahedral coordination environment composed of three nitrogen donors from the terpyridine moiety, a nitrogen donor from a pyridyl group and two chloride ligands which occupy the axial sites. 1·2H₂O exhibits abrupt spin-crossover (SCO) behaviour (T_1/2↓ = 218 K; T_1/2↑ = 227 K) along with a thermal hysteresis loop, while 2·2H₂O and the dehydrated species 1 and 2 exhibit high-spin (HS) states at 2–300 K as well as field-induced single-molecule magnet (SMM) behaviour attributed to the presence of magnetic anisotropic HS cobalt(II) species (S = 3/2). 1·2H₂O exhibited reversible desorption/resorption of its two water molecules, revealing reversible switching between SCO and SMM behaviour triggered by the dehydration/rehydration processes. Single crystal X-ray structural analyses revealed that 1·2H₂O crystalizes in the orthorhombic space group Pcca while 2 and 2·2H₂O crystallize in the monoclinic space group P2/n. Each of the 1-D chains formed by 1·2H₂O in the solid state are bridged by hydrogen bonds between water molecules and chloride groups to form a 2-D layered structure. The water molecules bridging 1-D chains in 1·2H₂O interact with the chloride ligands occupying the axial positions, complementing the effect of Jahn–Teller distortion and contributing to the abrupt SCO behaviour and associated stabilization of the LS state.