An Analysis of Porphyrin Molecular FlexibilityUse of Porphyrin Diacids

Abstract

The crystal structures of three porphyrin diacid species, [H4OEP](ClO4)2, [H4TPP](ClO4)2, and [H4TMP](ClO4)2, have been determined from low-temperature X-ray diffraction data to delineate how the peripheral substituents of the porphyrin affect the overall molecular flexibility. [H4OEP](ClO4)2 (|Cb| = 0.46 Å), [H4TMP](ClO4)2 (|Cb| = 0.67 Å, molecule 1), and [H4TPP](ClO4)2 (|Cb| = 0.93 Å) show increasingly saddled core conformations with effective D2d symmetry. The mean porphyrin−aryl group dihedral angles in [H4TPP](ClO4)2 and [H4TMP](ClO4)2 (molecule 1) are 27(2)° and 63(13)°, respectively. The steric bulk of the mesityl substituents in [H4TMP]2+ limits the range of observed porphyrin−aryl group dihedral angles to >50° and, consequently, the magnitude of the core distortion. [H4TMP]2+ is therefore less flexible than [H4TPP]2+. Molecular mechanics calculations, using a modified version of MM2(87) and a newly developed force field for porphyrin diacids, correctly predict that the flexibility of meso-tetraaryl porphyrin diacids decreases as the steric bulk of the peripheral substituents increases: [H4TPyP]2+ ≈ [H4TPP]2+ > [H4T-2,6-(OH)2 PP]2+ ≈ [H4T-2,6-F2 PP]2+ > [H4T-2,6-Cl2 PP]2+ ≈ [H4TMP]2+. Grid searches of conformational space for [H4porphine]2+, [H4OEP]2+, [H4TPP]2+, and [H4T-2,6-Cl2 PP]2+ located pairs of inversion-related minima with D2d -saddled and C2h-stepped core conformations. The in vacuo strain energy barrier to inversion of the lowest-energy D2d-saddled conformation increases from 0.45 kcal/mol in [H4porphine]2+ to 1.9 kcal/mol in [H4T-2,6-Cl2PP]2+. The calculations indicate that the relative stability and magnitude of distortion of the D2d isomer increases as the steric bulk of the peripheral substituents increases; [H4OEP]2+ is therefore calculated to be less distorted than [H4TPP]2+, in agreement with the X-ray structures of these species.