Chiroptical and Computational Studies of a Bridled Chiroporphyrin and of Its Nickel(II), Copper(II), and Zinc(II) Complexes

Circular dichroism (CD) spectra and density functional theory (DFT) calculations are reported for a series of conformationally bistable chiroporphyrins with 8-methylene bridles MBCP-8, which can display either an alpha alpha alpha alpha or an alpha beta alpha beta orientation of their meso substituents. From DFT geometry optimizations, the most stable form of ZnBCP-8 is found to be the alpha alpha alpha alpha conformer. By passing to NiBCP-8, there is a strong stabilization of the alpha beta alpha beta conformation with respect to the alpha alpha alpha alpha conformation, consistent with the X-ray structures of alpha alpha alpha alpha-ZnBCP-8 and alpha beta alpha beta- NiBCP-8. A correlation between the sign of the CD signal in the Soret region and the conformation of the BCP-8 compounds is reported: the alpha alpha alpha alpha conformers H2BCP-8 and ZnBCP-8 show a positive CD signal, whereas the alpha beta alpha beta conformers NiBCP-8 and CuBCP-8 exhibit a negative signal. The possible contributions to the rotational strengths of alpha beta alpha beta-NiBCP-8 and alpha alpha alpha-ZnBCP8, calculated on the basis of their crystal structures, have been analyzed. The CD signals are found to result from a combination of both the inherent chirality of the porphyrin and of extrinsic contributions due to the chiral bridles. These results may have a broad significance for understanding the chiroptical properties of chiral porphyrins and hemoproteins and for monitoring stimuli-responsive, conformationally bistable chiroporphyrin compounds.