Manipulating the spin crossover behaviour in a series of cyanide-bridged {FeIII2FeII2} 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 {FeIII2FeII2} square complexes {[(Tp*)Fe III (CN) 3 ] 2 [Fe II (TPPO)] 2 [NCE] 2 }·Sol (E = S, Sol = 2CH 3 OH·6H 2 O, 1; E = Se, Sol = 2MeCN·2CH 2 Cl 2 ·2H 2 O, 2; E = BH 3 , Sol = 4CH 3 OH·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 {FeIII,LS2FeII,HS2} and {FeIII,LS2FeII,LS2} electronic states. As the ligand field increases from NCS - to NCSe - to NCBH 3 - , 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.