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

Comprehensive Summary To better understand the impact of different anions on the structures and SCO properties of the Co II SCO complexes, six new complexes [Co(terpy‐CH 2 OH) 2 ]A 2 ·sol (terpy‐CH 2 OH = 4′‐(hydroxymethyl)‐2,2′;6′,2″‐terpyridine, A = Br – ( 1 , sol = 1.5H 2 O), I – ( 2 ), N 3 – ( 3 , sol = 2H 2 O), H 3 BCN – ( 4 ), OTf – ( 5 ), and TsO – ( 6 , sol = 4H 2 O·CH 3 CN), have been synthesized and characterized. All six compounds consist of mononuclear [Co(terpy‐CH 2 OH) 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 CH 2 OH groups and their transition temperatures are determined by the size and hydrogen bonding capability of the anions. Remarkably, the transition temperatures of complexes with H 3 BCN – , 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.