Spin crossover properties of enantiomers, co-enantiomers, racemates, and co-racemates

Abstract

Through multi-component self-assembly of chiral phenylethylamine, 1-alkyl-2-imidazolecarboxaldehyde and iron(II) ions, two couples of enantiomeric iron(II) complexes 4R, 4S, 5R and 5S with the formula of fac-Λ or Δ-[Fe(L)₃]²⁺(L = R or S-1-phenyl-N-(1-alkyl-1H-imidazol-2-ylmethylene)ethanamine) have been designed and synthesized as building blocks. Further binary cocrystallization of the prefabricated enantiomers enabled us to construct spin crossover co-enantiomers 4R5R and 4S5S, racemates 4RS and 5RS, and co-racemate 4RS5RS. Compared with 4R in a high spin state and 5R with spin crossover at 291 K, the co-enantiomers 4R5R exhibited gradual spin crossover at a higher temperature of 301 K, and the racemic alloys showed hysteresis loops induced by desolvation above room temperature. It was demonstrated that molecular chirality could be used effectively for stereochemical engineering of spin crossover materials. In addition, crystal packing, intramolecular π–π stacking, intermolecular C–H⋯π interactions and solvent effects were elucidated to be responsible for the distinct spin crossover properties. This collective structural and magnetic study not only enriched the spin crossover library, but also provided a full comparison of optically pure, homochiral, and racemic materials with similar molecular structures.