Spectroscopic Detection and Theoretical Description of Spin-Crossover Effect in Co(II)-terpyridine System

Abstract

Spin-crossover (SCO) effect on octahedral Co(II) centers is much rarer and less known than in the case of Fe(II) complexes, although it is no less promising for the development of fundamental research, sophisticated memories, and sensors. The representative compound [Co(terpy)₂]I₂ ⋅ 2H₂O (terpy=2,2′:6′,2′′-terpyridine) was synthesized, and its physicochemical properties were studied both experimentally and theoretically. Magnetic experiments revealed a very gradual SCO between a low-spin state (S_Co(II)-LS=1/2) and a high-spin state (S_Co(II)-HS=3/2) for the pristine sample and an abrupt SCO with a narrow thermal hysteresis around 130 K for the desolvated sample. Moreover, temperature-dependent vibrational and electronic spectroscopy were employed to track the evolution of spectroscopic features related to SCO and desolvation processes. Additionally, quantum chemical calculations were utilized to determine the electronic structure, molecular orbitals, and vibrations for different spin states at various temperatures, offering a more comprehensive understanding of the compound's spectroscopic and magnetic behaviors. These extensive investigations provide valuable insights into Co(II)-based SCO materials and their theoretical underpinnings, paving the way for the application of their thermometric properties.