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

Abstract

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 beta-ethyl groups into the tetraphenylporphyrin parent macrocycle gave access to diethyltetraphenylporphyrin (H(2)DETPP), two regioisomers of tetraethyltetraphenylporphyrin (H(2)tTETPP, H(2)cTETPP), and hexaethyltetraphenylporphyrin (H(2)HETPP). These conformationally designed compounds bridge the gap between the well-known tetraphenylporphyrin (H(2)TPP) and the very nonplanar octaethyltetraphenylporphyrin (H(2)OETPP), 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 beta-ethyl substitution and the number and localization of potential beta-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 beta-pyrrole displacements reaching those described for dodecasubstituted porphyrins. Overall, higher displacements from planarity were found close to beta-ethyl-meso-phenyl groups whereas smaller displacements were observed in parts of the molecules bearing beta-hydrogen atoms. Nevertheless, a certain amount of redistribution of steric strain occurs as evidenced by significant displacements for pyrrole carbon atoms with beta-hydrogens. Synthesis and structural investigation of the respective metal complexes with M = Cu(II), Ni(II), and Zn(II) showed similar correlations between beta-ethyl substitution, bathochromic shift of absorption bands and nonplanarity as described for the free bases. The only exception was found for Ni(II)tTETPP, 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.