Synthesis and Magnetic Properties of Mononuclear Cobalt(II) Spin Crossover Complexes from Complementary Terpyridine Ligand Pairing

The cobalt.. complexes containing terpyridine (terpy) and its derivatives compose a large family of Co(II) SCO-active (SCO=spin-crossover) compounds and the reported cases are mainly built from homoleptic type terpy ligands. Herein we report the SCO properties in three mononuclear cobalt(II) complexes constructed from complementary terpy ligand pairing. Their SCO behaviors are largely affected by the substituents of terpy at the 4-position. The archetypical complex 1 and its CF3-substituted one 3 showed a gradual and incomplete spin transition from the low spin state of S=1/2 to the high spin state of S=3/2. The fluorine-substituted complex 2 exhibited a solvent-dependent spin transition phenomenon. The solvated form which contains three lattice water molecules showed a similar gradually incomplete spin transition. Whereas the entire removal of water molecules resulted in a repeatable thermal hysteresis loop with a width of ca. 50 K. Impressively, the adsorption and desorption of water molecules are reversible in structure and magnetism. In addition, absorption spectroscopy and cyclic voltammetry show that the substituent on the ligands can regulate the electronic structures of the central cobalt ion. CCDC: 2162742, 1 (150 K); 2162743, 2 (120 K); 2162744, 2 (299 K); 2162745, 3 (120 K).