An Analysis of Porphyrin Molecular FlexibilityUse of Porphyrin Diacids

Abstract

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 2d 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 mesotetraaryl porphyrin diacids decreases as the steric bulk of the peripheral substituentsrelated minima with D 2d -saddled and C 2h -stepped core conformations. The in Vacuo strain energy barrier to inversion of the lowest-energy D 2d -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 2d 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.

meso-Aryl-substituted free-base porphyrins: formation, structure and photostability of diprotonated species

“…1) undergoes protonation at one or both of the pyrrole nitrogen atoms, which results in formation of either a monocation or dication, respectively [6][7][8][9][10][11][12][13][14][15][16][17][18]. Although the existence of the mono-protonated cation was reported elsewhere, the diprotonated porphyrin form was practically the only stable product found in acidic media [9][10][11][12][13][14][15][16][17].…”

Conformações distorcida e planar do anel porfirínico em complexos e hemoproteínas: propriedades físico-químicas e implicações espectroscópicas

“…Além da influência dos ligantes axiais, os grupos volumosos, seja no anel porfirínico ou no próprio ligante axial, também devem ser considerados, pois a inserção de influências estéricas consiste em um dos principais causadores de distorção do macrociclo 28 , proporcionando aquilo que se convencionou chamar de "cavidades", cujas áreas dependem do grau de distorção 29,30 . É fundamental ressaltar que a distorção constringe todo o sistema, diminuindo a velocidade de rotação dos ligantes axiais 31 . Em alguns casos a distorção chega a fixar a orientação destes ligantes, fenômeno este que ocorre com os grupos hemes nas proteínas, sendo que essa orientação definida está correlacionada com as altas freqüências das linhas espectrais encontradas para os sistemas distorcidos em ressonância magnética nuclear 29 .…”

Spectral-Luminescent Properties of meso-Tetraarylporphyrins Revisited: The Role of Aryl Type, Substitution Pattern and Macrocycle Core Protonation

“…[2,[31][32][33] In this framework, essential progress has been achieved in the elucidation of the molecular conformations of diprotonated porphyrins and their relation with the peripheral substitution pattern. [31][32][33][34][35] For meso-aryl substituted porphyrins, it was shown that the degree of saddle-type macrocycle distortion upon formation of the diprotonated form and the decrease in the value of the dihedral angle q are correlated with electronic factors. [35] Saddle-type distortion allows the steric hindrance to be overcome and favors the coplanar conformation of the macrocycle and meso-aryl substituents.…”

“…in order to relieve the steric repulsion of the inner hydrogen atoms in the macrocyclic core of the diprotonated form, a pyrrole tilting angle (saddling angle) j of 10−15° is sufficient, [35] but the synergism of the pyrrole ring tilting and aryl group rotation in meso-aryl substituted porphyrins allows the formation of highly distorted conformations with a saddling angle as high as 28-33°. [10,32] Upon protonation of meso-arylporphyrins with bulky substituents at the orthopositions, both the saddling angle and dihedral angle were found to decrease due to the imposed steric hindrance. [32] 5,10,15,20-Tetraphenylporphyrin (Ph 4 ) and 5,10,15,20-tetramesitylporphyrin (Mes 4 ) are the prototype representatives of the two extreme cases described above, unhindered and hindered meso-aryl substituted porphyrins.…”

“…[10,32] Upon protonation of meso-arylporphyrins with bulky substituents at the orthopositions, both the saddling angle and dihedral angle were found to decrease due to the imposed steric hindrance. [32] 5,10,15,20-Tetraphenylporphyrin (Ph 4 ) and 5,10,15,20-tetramesitylporphyrin (Mes 4 ) are the prototype representatives of the two extreme cases described above, unhindered and hindered meso-aryl substituted porphyrins. Being almost indistinguishable in their free base (FB) forms, no similarity remains upon protonation of these porphyrins.…”