Hysteretic Spin Crossover with High Transition Temperatures in Two Cobalt(II) Complexes

Cooperative spin crossover transitions with thermal hysteresis loops are rarely observed in cobalt(II) complexes. Herein, two new mononuclear cobalt(II) complexes with hysteretic spin crossover at relatively high temperatures (from 320 to 400 K), namely, [Co(terpy-CH₂OH)₂]·X₂ (terpy-CH₂OH = 4'-(hydroxymethyl)-2,2';6',2″-terpyridine, X = SCN^-(1) and SeCN^- (2)), have been synthesized and characterized structurally and magnetically. Both compounds are mononuclear Co^II complexes with two chelating terpy-CH₂OH ligands. Magnetic measurements revealed the existence of the hysteretic SCO transitions for both complexes. For compound 1, a one-step transition with T_1/2↑= 334.5 K was observed upon heating, while a two-step transition is observed upon cooling with T_1/2↓(1) = 329.3 K and T_1/2↓(2) = 324.1 K (at a temperature sweep rate of 5 K/min). As for compound 2, a hysteresis loop with a width of 5 K (T_1/2↓ = 391.6 K and T_1/2↑ = 396.6 K, at a sweep rate of 5 K/min) can be observed. Thanks to the absence of the crystallized lattice solvents, their single crystals are stable enough at high temperatures for the structure determination at both spin states, which reveals that the hysteretic SCO transitions in both complexes originate from the crystallographic phase transitions involving a thermally induced order-disorder transition of the dangling -CH₂OH groups in the ligand. This work shows that the modification of the terpy ligand has an important effect on the magnetic properties of the resulting cobalt(II) complexes.