Conformational equilibria of bridled chiroporphyrins in solution investigated by vibrational circular dichroism.

A series of bridled chiroporphyrins (BCP) and their metal complexes were prepared, in which two n-methylene straps connect adjacent meso substituents by ester linkages. These compounds can exist as four atropisomers (αααα, αßαß, αααß, or ααßß) depending on the position of the meso groups relative to the macrocycle (α when above and ß when below). We characterized the conformation of these chiral porphyrins and their metal (Zn, Ni, Mn) complexes by vibrational circular dichroism (VCD) associated with ab initio calculations. VCD spectra of the three metalloporphyrins were recorded in CDCl3 and benzene solutions and ab initio calculations of their four atropoisomers were performed at the Density Functional Theory (DFT) level. The bridled chiroporphyrin with the longer straps (9 CH2) and its nickel(II) complex can be isolated as the αßαß atropisomer in the solid state and were found with the same conformation in CDCl3 and benzene solutions. The bridled chiroporphyrin with the shortest straps (8 CH2) and its zinc(II) complex can be isolated as the αααα atropisomer in the solid state, but in solution they are subject to atropisomeric equilibria, resulting in atropisomer distributions that are strongly solvent-dependent. Comparison of the experimental VCD spectra with the predicted spectra of the four atropisomers allowed the quantification of these distributions. Finally, the manganese(III) complex also exhibits an atropisomeric equilibria in solution which is slightly solvent-dependent.

Density-functional theory study of the stereochemistry of chloroiron(III) and chloromanganese(III) complexes of a bridled chiroporphyrin.

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 alpha alpha alpha alpha or alpha beta alpha beta 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 alpha alpha alpha alpha 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.

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 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.