Spin crossover in mononuclear Fe(II) complexes based on a tetradentate ligand.

ABSTRACT

Three isostructural complexes with the formula [Fe(L5Me)(NCE)2] L5Me = N,N'-bis(5-methyl-2-pyridylmethyl)ethane-1,2-diamine and E = S (1-S), E = Se (1-Se), E = BH3 (1-BH 3 ) have been synthesized and characterized by single-crystal x-ray diffraction, magnetic susceptibility and DSC studies. All the three derivatives are spin crossover (SCO) active, showing complete one-step spin conversion. The SCO midpoint temperatures (T 1/2) are 193 K for 1-S, 226 K for 1-Se, and 330 K for 1-BH 3 , which are among the highest values for the homologous Fe(II)-NCE complexes with comparable tetradentate ligands. The almost linear Fe-N ≡ C(E) angles are consistent with the strong ligand field (LF) strength imposed by these NCE- co-ligands. Strong hydrogen-like bonding N-H…E was observed to connect the molecules into 2D supramolecular sheets parallel to the bc plane. However, such supramolecular interaction is not sufficient enough to transmit strong cooperativity. A discussion on the factors governing the LF strength and the cooperativity has been made, based on the comparison of analogous complexes and also based on UV-vis spectroscopy studies of the Ni(II) complexes.