Manipulating the spin crossover behaviour in a series of cyanide-bridged {(Fe2Fe2II)-Fe-III} molecular squares through NCE- co-ligands

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 {(Fe2Fe2II)-Fe-III} square complexes {[(Tp*)Fe-III(CN)(3)](2)[Fe-II(TPPO)](2)[NCE](2)}center dot Sol (E = S, Sol = 2CH(3)OH center dot 6H(2)O, 1; E = Se, Sol = 2MeCN center dot 2CH(2)Cl(2)center dot 2H(2)O, 2; E = BH3, Sol = 4CH(3)OH center dot 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-2(III),Fe-LS(2)II,HS} and {Fe-2(III),Fe-LS(2)II,LS} 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.