Manipulating the spin crossover behavior in a series of {FeFe^II} complexes

Three cyanide-bridged {Fe₂Fe} complexes of formula {[(Tp^R)Fe(CN)₃]₂[Fe(bnbpen)]}·S (Tp^R = Tp for 1·S, Tp^3-Me for 2·S, and Tp* for 3·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^R)Fe(CN)₃]^- units at the ends and one [Fe(bnbpen)]²⁺ at the centre. Besides the rich hydrogen bonds, remarkable π-π interactions are evidenced in all compounds between the intermolecular naphthyl and pyrazolyl rings. As a result, compounds 1·S and 2·S exhibit irreversible two-step and one-step spin-transitions, respectively, during the process of removing solvents, with T_1/2 at 314 K and 376 K for 1·S and 350 K for 2·S, while compound 3·S shows a reversible one-step spin transition at relatively lower temperatures with T_1/2 = 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 compound 1 maintains 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 Tp^R ligands (Tp* < Tp^3-Me < Tp), likely due to the more compact π-π stacking mode in comparison with those for compounds 2·S and 3·S.