Manipulating the spin crossover behaviour in a series of cyanide-bridged {Fe III2Fe II2} molecular squares through NCE− co-ligands

Abstract

Manipulating the transition temperature (T_1/2) of spin-crossover (SCO) complexes capable of fulfilling practical criteria through different synthetic strategies is one of the main focuses in the field of molecular magnetism. The reaction of the tricyanometallate precursor [(Tp*)Fe^III(CN)₃]⁻ and Fe(II) salt with the “facially” tridentate ligand tris(2-pyridyl)phosphine oxide (TPPO) and NCE⁻ anions afforded three isostructural {Fe^III₂Fe^II₂} square complexes {[(Tp*)Fe^III(CN)₃]₂[Fe^II(TPPO)]₂[NCE]₂}·Sol (E = S, Sol = 2CH₃OH·6H₂O, 1; E = Se, Sol = 2MeCN·2CH₂Cl₂·2H₂O, 2; E = BH₃, Sol = 4CH₃OH·2MeCN, 3). Detailed structural analysis, variable-temperature IR analysis, magnetic susceptibility measurements and DFT calculations revealed that all compounds exhibit complete and one-step SCO behaviour between the {Fe^III,LS₂Fe^II,HS₂} and {Fe^III,LS₂Fe^II,LS₂} electronic states. As the ligand field increases from NCS⁻ to NCSe⁻ to NCBH₃⁻, T_1/2 shifts dramatically from 214 to 250 to 288 K for 1, 2 and 3, respectively, demonstrating another effective way to tune the SCO properties of the [Fe^III–CN–Fe^II] systems through the introduction of NCE⁻ co-ligands.