Multielectron transfer and field-induced slow magnetic relaxation in opto-electroactive spin crossover cobalt(ii) complexes: structure-function correlations

Designing and implementing multielectron transfer and single-molecule magnet properties in spin crossover compounds constitute a promising way to obtain a new class of multiresponsive and multifunctional materials. This contribution uses N-phenyl-substituted pyridine-2,6-diimine (PDI) ligands to explore a novel family of spin-crossover cobalt(ii) complexes, with general formula [CoL2](ClO4)(2)<middle dot>xH(2)O [L = 4-MePhPDI (1, x = 1), 4-MeOPhPDI (2, x = 0), 4-MeSPhPDI (3, x = 0), 4-Me2NPhPDI (4, x = 0.5), and 2,4-Me2PhPDI (5, x = 0)]. These mononuclear six-coordinate octahedral cobalt(ii) bis(chelating) complexes incorporate tridentate PDI derivatives with various electron-donating substituents at para (X-4 = Me, OMe, SMe, and Me2N) or ortho and para positions (X-2, X-4 = Me) of terminal phenyl rings. Our investigations reveal that these complexes exhibit both thermally-induced low-spin (LS)/high-spin (HS) transition and field-induced slow magnetic relaxation in the LS (S = 1/2) and HS (S = 3/2) states. Moreover, they display a dual multiredox behaviour featuring one-electron oxidation of the paramagnetic Co-II ion to the diamagnetic Co-III ion and stepwise two-electron reduction of each PDI ligand to the corresponding imine-type PDI center dot- pi-radical anion and diimine-type PDI2- dianion, which positions them as candidates for prototypes of spin quantum transistors and capacitors, offering potential applications in quantum information processing.