Manipulating the spin crossover behavior in a series of {Fe III2 FeII} complexes

Three cyanide-bridged {FeFe} complexes of formula {[(Tp)Fe(CN)][Fe(bnbpen)]}·S (Tp= Tp for1·S, Tpfor2·S, and Tp* for3·S, respectively; bnbpen =N,N'-bis-(2-naphthylmethyl)-N,N'-bis(2-picolyl)-ethylenediamine) have been prepared and characterized here. Single-crystal X-ray diffraction analysis revealed that all compounds feature right angled trinuclear structures with two [(Tp)Fe(CN)]units at the ends and one [Fe(bnbpen)]at the centre. Besides the rich hydrogen bonds, remarkable p-p interactions are evidenced in all compounds between the intermolecular naphthyl and pyrazolyl rings. As a result, compounds1·Sand2·Sexhibit irreversible two-step and one-step spin-transitions, respectively, during the process of removing solvents, withTat 314 K and 376 K for1·Sand 350 K for2·S, while compound3·Sshows a reversible one-step spin transition at relatively lower temperatures withT= 250 K, and what's more, an irreversible small magnetic change accompanied by the loss of lattice solvents was observed. The solvent-free compounds show SCO properties at lower temperatures than the corresponding solvated ones; however, the solvent-free compound1maintains an unexpected high-spin state and does not follow the trend that their transition temperatures decrease with the enhancement of the electron donor properties of the Tpligands (Tp* < Tp< Tp), likely due to the more compact p-p stacking mode in comparison with those for compounds2·Sand3·S.