Synthesis and Structural Characterization of Nonplanar Tetraphenylporphyrins and Their Metal Complexes with Graded Degrees of β-Ethyl Substitution

Different porphyrin conformations are believed to play a role in
controlling the cofactor properties in natural
tetrapyrrole−protein complexes. In order to study the
correlation between macrocycle nonplanarity and
physicochemical properties in detail, a series of six porphyrins with
graded degrees of macrocycle distortion
were synthesized via mixed condensation of pyrrole, diethylpyrrole, and
benzaldehyde. The formal introduction
of successively more β-ethyl groups into the tetraphenylporphyrin
parent macrocycle gave access to diethyltetraphenylporphyrin (H2DETPP), two regioisomers of
tetraethyltetraphenylporphyrin (H2tTETPP,
H2cTETPP), and
hexaethyltetraphenylporphyrin (H2HETPP). These
conformationally designed compounds bridge the gap between
the well-known tetraphenylporphyrin (H2TPP) and the
very nonplanar octaethyltetraphenylporphyrin
(H2OETPP),
which are also formed during the reaction. Crystallographic
studies showed that the macrocycle distortion in the
solid state increases gradually in the order TPP < DETPP < tTETPP <
cTETPP < HETPP < OETPP, i.e. with
increasing degree of β-ethyl substitution and the number and
localization of potential β-ethyl meso-phenyl
interactions. This correlates well with increasing bathochromic
shifts of the absorption bands in solution.
Depending on the substituent pattern, different saddle-shaped
macrocycle conformations were observed. While
the conformation of tTETPP was symmetric, DETPP, cTETPP, and HETPP
showed asymmetric distortion modes
with individual β-pyrrole displacements reaching those described for
dodecasubstituted porphyrins. Overall, higher
displacements from planarity were found close to
β-ethyl−meso-phenyl groups whereas smaller
displacements
were observed in parts of the molecules bearing β-hydrogen atoms.
Nevertheless, a certain amount of redistribution
of steric strain occurs as evidenced by significant displacements for
pyrrole carbon atoms with β-hydrogens.
Synthesis and structural investigation of the respective metal
complexes with M = Cu(II), Ni(II), and
Zn(II)
showed similar correlations between β-ethyl substitution,
bathochromic shift of absorption bands and nonplanarity
as described for the free bases. The only exception was found for
NiIItTETPP, which exhibited a highly
nonplanar
ruffled conformation. Additionally, the metal complexes allowed a
study of the conformational effects of different
metals at each level of macrocycle distortion. As observed for
symmetric, nonplanar porphyrins larger metals
led to a decrease in conformational distortion with associated changes
in bond lengths and bond angles.