Conformational Equilibria of Bridled Chiroporphyrins in Solution Investigated by Vibrational Circular Dichroism

Abstract

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, αβαβ, αααβ, or ααββ) depending on the position of the meso groups relative to the macrocycle (α when above and β 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 αβαβ 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.

Chirality detection of two enantiomorphic 3D lanthanide coordination polymers by vibrational circular dichroism spectra

“…13 The VCD signals at 1590, 1580, 1560 and 1540 cm −1 are attributable to the stretching vibrations of the C-C in the L 3− . 14 The distinct VCD signals indicated that the crystals of 1 exists in two kinds of enantiomorphism isomers, R for P4 1 32 and S for P4 3 32. The temperature dependence of magnetic susceptibilities of 1 was measured from 2 to 300 K with 1000 Oe.…”

Vibrational Circular Dichroism: Recent Advances for the Assignment of the Absolute Configuration of Natural Products

“…VCD has allowed the re-examination of the AC of some natural products like ()-brevianamide [6] and (+)-schizandrin [7], and it has eventually aided the re-examination and reassignment of the molecular structure of klaivanolide [8]. The VCD superiority over other spectroscopic methods has allowed the structural analysis of complex molecules, like the atropisomer distribution of bridled chiroporphyrins in solution [9], or the planar configuration of nonaand dodecamethoxycryptophanes [10]. It also has been used to determine the secondary structure of NH-indazoles such as (4S,7R)campho [2,3-c]pyrazole [11].…”

Reversible transformation between chiral and achiral Dy6Mo4clusters through a symmetric operation

“…24 The VCD couplets at 1560, 1540, 1520, 1510 and 1490 cm À1 are assigned to the antisymmetrical stretching of the CQN bond and skeleton vibrations of the aromatic rings in the ligand of L 3À in III, 25 while the VCD couplet at about 1450 cm À1 is ascribed to the bending vibrations of the methylene (dCH 2 ) or methyl (dCH 3 ) groups in III. 26 It was mentioned that the distinct VCD signals observed in III indicate that VCD spectra are a remarkable complementarity that can be used as an advantage when a particular system is investigated by the selection of chiroptical procedures.…”