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

Abstract

A family of halogen-substituted Schiff base iron(II) complexes, [Fe (qsal-X) ], (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 values obtained, to date, for Fe N/O species. We have recently reported subtle symmetry breaking in [Fe (qsal-Cl) ] 2 with two spin transition steps occurring at 308 and 316 K. A photomagnetic study reveals almost full HS conversion of [Fe (qsal-I) ] 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 (qsal-X) ] 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 (qsal-X) ] , the two extra π-π and P4AE interactions found in [Fe (qsal-X) ] compounds, are believed to be accountable for the spin transitions occurring at ambient temperatures.

Iron(II) and Iron(III) Complexes of Tridentate NNO Schiff Base‐like Ligands – X‐ray Structures and Magnetic Properties

“…In the case of 10a and 15a this interaction is not observed, the packing is less dense and a gradual spin transition takes place. Please note that there are several examples in literature, where P4AE interactions are believed to be responsible for abrupt SCO – quite in contrast to our results presented herein.…”

“…Again, it is of interest to compare the results with examples from literature. For the system with qsal‐ X ( X = Cl, Br, I) as negatively charged three‐dentate N 2 O ligand, spin crossover is observed in the case of iron(II), whereas the corresponding iron(III) complexes are high spin . The differences could be due to packing effects that are known for their strong impact on SCO properties.…”

“…This development is accelerated by reports on iron(III) systems with wide hysteresis , . In the case of iron(II) the most frequently observed donor atom set is N 6 , and although there are examples with an N 4 O 2 , or N 4 S 2 donor atom set, they are comparatively rare. In the case of iron(III) complexes, spin crossover is more frequently observed for systems with an N 4 O 2 coordination sphere, which in their vast majority use Schiff base ligands , .…”

“…In the case of iron(III) complexes, spin crossover is more frequently observed for systems with an N 4 O 2 coordination sphere, which in their vast majority use Schiff base ligands , . So far, only few examples are reported where both, the iron(II) and iron(III) with the same ligand set are synthesized , …”

Halogen Substituent Effect on the Spin-Transition Temperature in Spin-Crossover Fe(III) Compounds Bearing Salicylaldehyde 2-Pyridyl Hydrazone-Type Ligands and Dicarboxylic Acids

“…In terms of practical applications, it is preferable for the spin-transition temperature to be near room temperature. Therefore, the effects of replacing anion and solvent molecules and the introduction of substituents into ligands on the spin-transition temperature have been actively studied for many types of SCO complexes [4][5][6][7][8][9][10][11][12][13][14][15][16].…”