The Influence Of Nce- (E = S, Se, Bh3) Ligands On The Temperature Of Spin Crossover In A Family Of Iron(Ii) Mononuclear Complexes

A precise regulation of the temperature of spin crossover (SCO) is an essential task to envisage practical applications of switchable materials. Here, we present three isostructural complexes with the formula [Fe(L-Cl)(NCE)(2)], where L-Cl is the tetradentate ligand N,N-bis[(4-chloro-2-pyridyl)methyl]ethane-1,2-diamine and E = S (1-S), Se (1-Se), and BH3 (1-BH3). Single-crystal X-ray diffraction, magnetic measurements, Mossbauer spectroscopy and DSC measurements have been carried out to study their SCO properties. 1-S remains in a high spin state over all temperatures due to the weak ligand field strength imposed by NCS- ligands. The replacement of NCS- by NCSe- allows the occurrence of SCO around 177 K. Using NCBH3- instead allows a further increase of the conversion temperature (T-1/2) by ca. 30 K. A literature survey of complexes containing NCE- ligands was carried out to examine their influence on the SCO properties. A 50-90 K difference in T-1/2 was found between NCS- and NCSe- derivatives and a 30-50 K difference was found between NCSe- and NCBH3- derivatives.