Spin crossover behavior of dinuclear Fe^II complexes with bis-tetradentate bridging-type ligands

The preparation and characterization of two new FeII dinuclear complexes with bis-tetradentate bridging-type ligands [{Fe(NCS)₂}₂ (L)]·DMF·2H₂O (1: L = L1{2,5-bis[N,Nbis( 2-pyridylmethyl)aminomethyl]pyrazine}, 2: L = L2{2,3- bis[N,N-bis(2- pyridylmethyl)aminomethyl]pyrazine}) are reported. Magnetic measurements reveal that complexes 1 and 2 exhibit different thermal spin-crossover behavior. Complex 1 shows a crystalline solvent effect on spin crossover (SCO). After desorption of the crystalline solvent by heating, an abrupt one-step transition with a hysteresis loop was observed for the desolvated form 1′. Conversely, a gradual twostep SCO was obtained for 2. The photomagnetic effect was also investigated in 1′ and 2, and both complexes clearly show light-induced excited spin state trapping. Interestingly, one-step and two-step thermal relaxation were observed for 1′ and 2, respectively. Such different SCO properties occur because of the cooperative effects owing to the structure of the ligand.