Polymorphism and “reverse” spin transition in the spin crossover system [Co(4-terpyridone)2](CF3SO3)2·1H2O

Abstract

Compound [Co(4-terpyridone)₂](CF₃SO₃)₂·1H₂O, where 4-terpyridone is 2,6-bis(2-pyridyl)-4(1H)-pyridone, forms two polymorphs. Polymorph 1 displays a continuous spin conversion in the temperature region 300–120 K while polymorph 2 shows, on cooling, the onset of a continuous high-spin (HS) to low-spin (LS) conversion interrupted by an abrupt “reverse” spin transition in the temperature region 217–203 K. The formed unstable HS intermediate phase (IP) undergoes a strong cooperative “normal” spin transition characterised by a hysteresis loop 33 K wide. The structural data give support for a crystallographic phase transition, which takes place concomitantly with the “reverse” spin state transition.