Gradual or Hysteretic Transition: Anion Effects on Cobalt(II) Spin Crossover Complexes

To better understand the impact of different anions on the structures and SCO properties of the CoII SCO complexes, six new complexes [Co(terpy-CH2OH)2]A2<middle dot>sol (terpy-CH2OH = 4 '-(hydroxymethyl)-2,2 ';6 ',2 ''-terpyridine, A = Br- (1, sol = 1.5H(2)O), I- (2), N3- (3, sol = 2H2O), H3BCN- (4), OTf- (5), and TsO- (6, sol = 4H2O<middle dot>(CHCN)-C-3), have been synthesized and characterized. All six compounds consist of mononuclear [Co(terpy-CH2OH)(2)](2+) cations and charge-balancing anions that differ in size, shape, and hydrogen bonding capacity. Complexes 1, 2, 3, and 6 displayed incomplete gradual SCO transitions, whereas 4 and 5 exhibited abrupt hysteretic spin transitions with loops of 12 and 16 K (250.0-262.0 K for 4, and 370.0-386.0 K for 5, respectively), closely resembling our previously reported complexes with SCN- and SeCN- anions. The occurrence of the order-disorder transition of the CH2OH groups and their transition temperatures are determined by the size and hydrogen bonding capability of the anions. Remarkably, the transition temperatures of complexes with H3BCN-, SCN-, OTf-, and SeCN- anions exhibit an upward trend as the size and mass of the anions increase, as confirmed through detailed single crystal structure analyses conducted in both high-spin and low-spin states for all four complexes.