Tuning of spin crossover properties in a series of mononuclear cobalt(II) complexes based on a macrocyclic tetradentate ligand and pseudohalide coligands

The three mononuclear cobalt(II) complexes, (Co(L)(NCX)(2)] (L = N,N'-di-tert-butyl-2,11-diaza[3,3](2,6)pyridinophane, and X = S (1), Se (2), and [C(CN)(2)] (3)), have been synthesized and characterized using variable temperature single-crystal X-ray crystallography, and spectroscopic, magnetic, and electrochemical studies. All three complexes have a similar distorted octahedral CoN6 coordination geometry produced by the macrocyclic tetradentate ligand L and two NCX - co-ligands in the cis position. Complexes 1 and 2 crystallized in the monoclinic P2(1)/n (Z = 4) space group, while complex 3 was isolated in the monoclinic P2(1)/c (Z = 4) space group. Interestingly, complexes 1 and 2 exhibit a reversible and gradual spin-crossover behavior between the high-spin (S = 3/2) and low-spin = 1/2) states at ca. 168 K and 255 K, respectively. However, for complex 3, a low-spin configuration persists up to 260 K and it exhibits an incomplete and reversible spin crossover even at 400 K. In addition, complex 1 displays a reversible redox behavior indicating the paramagnetic cobalt(II) to diamagnetic cobalt(III) conversion.

Automated summary

The synthesis and characterization of three mononuclear cobalt(II) complexes were investigated using various analytical techniques. Two complexes exhibited reversible spin-crossover behavior between high-spin and low-spin states, while one complex maintained a low-spin configuration even at higher temperatures and displayed incomplete and reversible spin-crossover behavior. Additionally, one complex showed reversible redox behavior.