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 alphaalphaalphaalpha or an alphabetaalphabeta orientation of their meso substituents. From DFT geometry optimizations, the most stable form of ZnBCP-8 is found to be the alphaalphaalphaalpha conformer. By passing to NiBCP-8, there is a strong stabilization of the alphabetaalphabeta conformation with respect to the alphaalphaalphaalpha conformation, consistent with the X-ray structures of alphaalphaalphaalpha-ZnBCP-8 and alphabetaalphabeta-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 alphaalphaalphaalpha conformers H2BCP-8 and ZnBCP-8 show a positive CD signal, whereas the alphabetaalphabeta conformers NiBCP-8 and CuBCP-8 exhibit a negative signal. The possible contributions to the rotational strengths of alphabetaalphabeta-NiBCP-8 and alphaalphaalphaalpha-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.

Transition metal complexes of chiroporphyrins, in which two adjacent meso substituents are linked by a strap of eight methylene groups, [M(BCP8)], can exist as either an RRRR or RβRβ atropisomer depending on the nature of the coordinated metal cation. This remarkable conformational versatility was investigated by density-functional theory calculations for the d 5 chloroiron(III) complex in the low-spin and high-spin states and for the d 4 high-spin chloromanganese(III) complex. The lowest-lying electronic state of all of the conformers of the chloroiron(III) bridled chiroporphyrin is found to be the high-spin state. For the chloroiron(III) complex in the low-spin or the high-spin state and for the high-spin chloromanganese(III) complex, the most stable form is predicted to be the RRRR conformer in which the chloride axial ligand is located within the cavity provided by the bridles. The predicted stereochemistries are compared with those similarly obtained (i) for the chloroiron(III) and chloromanganese(III) complexes of the tetramethylchiroporphyrin, which is devoid of straps, and (ii) for the d 10 zinc(II) and low-spin d 8 nickel(II) BCP8 complexes, on the basis of the effects tied to the occupancy of the stereochemically active d x 2 -y 2-type antibonding orbital level, to the restraints imposed by the straps, and to the presence of the axial chloride ligand.