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

Abstract

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 ₂OH) ₂]A ₂·sol (terpy-CH ₂OH = 4′-(hydroxymethyl)-2, 2′;6′, 2″-terpyridine, A = Br ^– ( 1, sol = 1.5H ₂O), I ^– ( 2), N ₃ ^– ( 3, sol = 2H ₂O), H ₃BCN ^– ( 4), OTf ^– ( 5), and TsO ^– ( 6, sol = 4H ₂O·CH ₃CN), have been synthesized and characterized. All six compounds consist of mononuclear [Co(terpy-CH ₂OH) 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 ₂OH 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 ₃BCN ^–, 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.

Keywords

Cobalt / Solid-state structures / Magnetic properties / Spin crossover / Anion effect

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