Halogen Substitution Effects on N₂O Schiff Base Ligands in Unprecedented Abrupt Fe^II Spin Crossover Complexes

A family of halogen-substituted Schiff base iron(II) complexes, [Fe^II(qsal-X)₂], (qsal-X=5-X-N-(8-quinolyl)salicylaldimines)) in which X=F (1), Cl (2), Br (3) or I (4) has been investigated in detail. Compound 1 shows a temperature invariant high spin state, whereas the others all show abrupt spin transitions, at or above room temperature, namely, 295 K (X=I) up to 342 K (X=Br), these being some of the highest T_1/2 values obtained, to date, for Fe^II N/O species. We have recently reported subtle symmetry breaking in [Fe^II(qsal-Cl)₂] 2 with two spin transition steps occurring at 308 and 316 K. A photomagnetic study reveals almost full HS conversion of [Fe^II(qsal-I)₂] 4 at low temperature (T(LIESST)=54 °K). The halogen substitution effects on the magnetic properties, as well as the crystal packing of the [Fe^II(qsal-X)₂] compounds and theoretical calculations, are discussed in depth, giving important knowledge for the design of new spin crossover materials. In comparison to the well known iron(III) analogues, [Fe^III(qsal-X)₂]⁺, the two extra π–π and P4AE interactions found in [Fe^II(qsal-X)₂] compounds, are believed to be accountable for the spin transitions occurring at ambient temperatures.