Supramolecular chirogenesis in zinc porphyrins: Complexation with enantiopure thiourea derivatives, binding studies and chirality transfer mechanism

Complexation of three (1R,2R)-2-aminocyclohexyl arylthioureas (aryl = 3,5-bis(trifluoromethyl)phenyl; phenyl; 3,5-dimethylphenyl) as the guests to ZnTPP and ZnOFP hosts has been studied by means of UV-vis, CD spectroscopies and computational simulation. The complexation proceeds via coordination of the primary amino group to the zinc ion of the metalloporphyrin and induces a moderate circular dichroism signal in the Soret band of the porphyrins. The association constants increase with larger electron-withdrawing properties of the porphyrin host and larger electron-donating ability of the guest, indicating predominantly electrostatic (Lewis acid-base) character of the complexation. Computational study of the (1R,2R)-2-aminocyclohexyl-(3,5-bis(trifluoromethyl)phenyl)thiourea and ZnTPP complex revealed slight asymmetric distortion of the porphyrin plane caused by the chiral guest and additional it-ir interactions between the host and guest molecules. The calculated CD spectrum for the same system reproduces the experimentally observed one.