Chiral spin-crossover complexes based on an enantiopure Schiff base ligand with three chiral carbon centers.

The preparation of Fe(II) complexes combining monodentate NCX - (X = S or Se) and the tetradentate Schiff base chiral ligands RR -L 1 and SS -L 1 = ( RR - or SS -L 1 = 1 R ,2 R or 1 S ,2 S )- N 1 , N 2 -bis(pyridin-2-ylmethylen)cyclohexane-1,2-diamine in acetone results in an unexpected reaction. Thus, four enantiomerically pure compounds of formulas [Fe( RR-S -L 2 )(NCX) 2 ] and [Fe( SS-R -L 2 )(NCX) 2 ] (X = S or Se) are formed by the new asymmetrical ligand L 2 . In L 2 , one acetone solvent molecule is incorporated into the ligand forming a bond with the C atom of one of the two CN imine groups of L 1 , which is transformed into an amine (Mannich reaction). This reaction is diastereoselective as the incorporation of acetone leads to an asymmetric C adjacent to the NH group with opposite chirality S - or R - to that of the cyclohexane carbons ( RR - or SS -, respectively). Therefore, L 2 contains three C chiral centers. Structural and magnetic characterization of these compounds demonstrates that they show in the bulk a gradual spin-crossover behavior and LIESST effect. Interestingly, the presence of an intramolecular hydrogen bond between the integrated acetone molecule and the NH group can trigger a secondary stimuli-responsive behavior in the system. Therefore, by changing the solvent polarity, the color of the complex in solution can be easily tuned.