An Analysis of Porphyrin Molecular FlexibilityUse of Porphyrin Diacids

The crystal structures of three porphyrin diacid species, [H 4 OEP](ClO 4 ) 2, [H 4 TPP](ClO 4 ) 2, and [H 4 TMP](ClO 4 ) 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 . [H 4 OEP](ClO 4 ) 2 (|C b | = 0.46 Å), [H 4 TMP](ClO 4 ) 2 (|C b | = 0.67 Å, molecule 1 ), and [H 4 TPP](ClO 4 ) 2 (|C b | = 0.93 Å) show increasingly saddled core conformations with effective D 2 d symmetry. The mean porphyrin−aryl group dihedral angles in [H 4 TPP](ClO 4 ) 2 and [H 4 TMP](ClO 4 ) 2 (molecule 1 ) are 27(2)° and 63(13)°, respectively. The steric bulk of the mesityl substituents in [H 4 TMP] 2+ limits the range of observed porphyrin−aryl group dihedral angles to >50° and, consequently, the magnitude of the core distortion. [H 4 TMP] 2+ is therefore less flexible than [H 4 TPP] 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: [H 4 TPyP] 2+ ≈ [H 4 TPP] 2+ > [H 4 T-2,6-(OH) 2 PP] 2+ ≈ [H 4 T-2,6-F 2 PP] 2+ > [H 4 T-2,6-Cl 2 PP] 2+ ≈ [H 4 TMP] 2+ . Grid searches of conformational space for [H 4 porphine] 2+, [H 4 OEP] 2+, [H 4 TPP] 2+, and [H 4 T-2,6-Cl 2 PP] 2+ located pairs of inversion-related minima with D 2 d -saddled and C 2 h -stepped core conformations. The in vacuo strain energy barrier to inversion of the lowest-energy D 2 d -saddled conformation increases from 0.45 kcal/mol in [H 4 porphine] 2+ to 1.9 kcal/mol in [H 4 T-2,6-Cl 2 PP] 2+ . The calculations indicate that the relative stability and magnitude of distortion of the D 2 d isomer increases as the steric bulk of the peripheral substituents increases; [H 4 OEP] 2+ is therefore calculated to be less distorted than [H 4 TPP] 2+, in agreement with the X-ray structures of these species.