• Chiroptical and Computational Studies of a Bridled Chiroporphyrin and of Its Nickel(II), Copper(II), and Zinc(II) Complexes
    G. Maheut, A. Castaings, J. PÚcaut, L.M. Lawson Daku, G. Pescitelli, L. Di Bari and J.-C. Marchon
    Journal of the American Chemical Society, 128 (19) (2006), p6347-6356
    DOI:10.1021/ja054926o | unige:3289 | Abstract | Article HTML | Article PDF
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 ╬▒╬▒╬▒╬▒ or an ╬▒╬▓╬▒╬▓ orientation of their meso substituents. From DFT geometry optimizations, the most stable form of ZnBCP-8 is found to be the ╬▒╬▒╬▒╬▒ conformer. By passing to NiBCP-8, there is a strong stabilization of the ╬▒╬▓╬▒╬▓ conformation with respect to the ╬▒╬▒╬▒╬▒ conformation, consistent with the X-ray structures of ╬▒╬▒╬▒╬▒-ZnBCP-8 and ╬▒╬▓╬▒╬▓-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 ╬▒╬▒╬▒╬▒ conformers H2BCP-8 and ZnBCP-8 show a positive CD signal, whereas the ╬▒╬▓╬▒╬▓ conformers NiBCP-8 and CuBCP-8 exhibit a negative signal. The possible contributions to the rotational strengths of ╬▒╬▓╬▒╬▓-NiBCP-8 and ╬▒╬▒╬▒╬▒-ZnBCP-8, 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.



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