Influence of Ligand Substituent Conformation on the Spin State of an Iron(II)/Di(pyrazol-1-yl)pyridine Complex

The temperature of the solution-phase spin-crossover equilibrium in iron(II) complexes of 4-alkylsulfanyl-2,6-di{pyrazol-1-yl}pyridine (bppSR) complexes depends strongly on the alkylsulfanyl substituent. DFT calculations imply this reflects the conformation of the alkylsulfanyl groups, which lie perpendicular to the heterocyclic ligand donors in [Fe(bppStBu)2] 2+ but are oriented coplanar with the ligand core for smaller SR substituents.