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 (T1/2↓ = 218 K; T1/2↑ = 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.