An Analysis of Porphyrin Molecular FlexibilityUse of Porphyrin Diacids

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.