Halogen Substitution Effects on N2O Schiff Base Ligands in Unprecedented Abrupt FeII 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)salicylal-dimines)) 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 degrees 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 p-p and P4AE interactions found in [Fe-II(qsal-X)(2)] compounds, are believed to be accountable for the spin transitions occurring at ambient temperatures.