The influence of NCE− (E = S, Se, BH3) ligands on the temperature of spin crossover in a family of iron(ii) mononuclear complexes

Abstract

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, Mössbauer 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.