Modeling of Spin Crossover in Iron(II) Complexes with N4S2 Coordination

• Published in: Journal of physical chemistry. C Vol. 123; no. 32; pp. 19984 - 19990
• Main Authors: Klokishner, S, Reu, O
• Format: Journal Article
• Language: English
• Published: American Chemical Society 15-08-2019
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.9b05064

Typical FeII spin crossover (SCO) complexes exhibit {N6} coordination, which offers a stronger ligand field than the {N4S2} ligand set. Recently, the synthesis, crystal structures, and magnetic and spectroscopic studies on the family of neutral mononuclear complexes, [Fe­(bpte)­(NCE)2] (where E = S, Se, or BH3) with mixed nitrogen–sulfur ligand surrounding of the iron­(II) ions have been reported. A priori, it is not clear what is the origin of SCO in these complexes, which contain in the nearest surrounding of the FeII ion four nitrogen ligands, creating a crystal field of intermediate strength, and two sulfur ligands, giving a much weaker field. The present paper looks at how on the basis of density functional theory calculations an explanation can be given of the SCO phenomenon in the iron­(II) complexes with mixed nitrogen–sulfur coordination environment. It is demonstrated that in compounds [Fe­(bpte)­(NCE)2] (E = S, BH3), the classical scenario of spin crossover with the participation of iron­(II) states with spin values S = 0 and 2 takes place. In the α-polymorph of the [Fe­(bpte)­(NCSe)2] compound, two types of FeII ions in the unit cell demonstrate different behavior: the ions of one type remain in the high-spin state in the whole temperature range, while the other type of ions manifest spin crossover. The spin transitions demonstrated by the two types of iron­(II) ions in the γ-polymorph of the [Fe­(bpte)­(NCSe)2] compound occur at different temperatures.