Manipulating the spin crossover behaviour in a series of cyanide-bridged {FeFe} 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 {Fe III 2 Fe II 2 } 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 {Fe III,LS 2 Fe II,HS 2 } and {Fe III,LS 2 Fe II,LS 2 } 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. Three molecular squares with tunable transition temperature ( T 1/2 ) through the substitution of NCE − (E = S, Se, BH 3 ) anions were prepared, demonstrating another effective way to tune the SCO properties of the [Fe III -CN-Fe II ] systems.