A cobalt(II)-terpyridine complex showing field-induced slow magnetic relaxation behavior via reverse spin-crossover

Abstract

Multistable molecular magnetic materials that exhibit different magnetic bistability are highly attractive, however, materials showing multistablilties are relatively rare. Herein, we present a rare example of the coexistence of thermally-induced spin-crossover (SCO) and field-supported single-ion magnet (SIM) observed in bromine-functionalized cobalt(II) complex [Co(Brphtpy)2](PF6)2 (Brphtpy = 4'-(4-Bromophenyl)-2,2':6',2''-terpyridine, 1). The temperature-dependent magnetic susceptibility and differential scanning calorimetry experiment shows a gradual SCO behavior in the temperature range of 400-125 K with the T1/2 = 325 K. Interestingly, reverse SCO behavior was observed below 125 K, which induces to a mixed high spin and low spin Co2+ ions in complex 1. At lower temperature, frequency- and temperature-dependent slow magnetic relaxation behaviors processed via Raman and direct relaxation were evidenced under an applied dc field of 1000 Oe with the estimated effective energy barrier being 26.2 K, possibly originating from the small fraction of high spin Co(II) ions or low spin Co(II) ions. This work provides not only a new bifunctional SCO-SIM complex but also a promising way for designing multi-stable molecular magnetic materials through reverse spin-crossover behavior.