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-L₁ and SS-L₁ = (RR- or SS-L₁ = 1R,2R or 1S,2S)-N¹,N²-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-L₂)(NCX)₂] and [Fe(SS-R-L₂)(NCX)₂] (X = S or Se) are formed by the new asymmetrical ligand L₂. In L₂, 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 L₁, 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, L₂ 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.