Isolation and Characterization of the Principal Kinetic Products in the Rothemund Synthesis of Sterically Hindered Tetraarylporphyrins. Crystal and Molecular Structures of [Tetrakis(2,6-dichlorophenyl)Porphinato]zinc(II) and Bis[(meso-2,6-dichlorophenyl)-5-(o,o′-dichlorobenzyl)dipyrromethene]zinc(II) Complexes

Abstract

The Rothemund condensation reaction, the most useful route to hindered 5,10,15,20-tetraarylporphyrins, has been examined in detail in context with the production of tetrakis(2,6-dichlorophenyl)porphyrin, TDCPP, a porphyrin used extensively at present in homogeneous oxygenation and biomimetic heme studies. The principal products from the reaction of pyrrole, 2,6-dichlorobenzaldehyde, and zinc acetate in refluxing 2,4,6-collidine are the bis(dipyrromethene) complex bis[(meso-2,6-dichlorophenyl)-5-(o,o′-dichlorobenzyl)dipyrromethene]zinc(II) (2), produced in ca. 40% yield, and the Zn porphyrin [tetrakis(2,6-dichlorophenyl) porphinato]zinc(II), ZnIITDCPP (1), produced in ca. 10% yield. In contrast to a previous report, there is no evidence that meso,5,5′-trisubstituted dipyrromethenes are formed. Furthermore, no zinc complexes of pyrromethenes nor dipyrromethanes build up to observable levels under the conditions of this aerobic high-temperature condensation reaction. The X-ray crystal structure of 2 coupled with 1H NMR, 13C NMR, infrared, UV-visible, and mass spectral data confirms the formulation of the compound as a Zn2+ ion coordinated by two planar bidentate disubstituted dipyrromethene ligands. Although the data indicate that carbon-carbon bond formation and cyclization in 2 to form the metalloporphyrin, 1, should not be a difficult process, 2 does not produce 1 in appreciable yield under the Rothemund conditions examined in this work. The metalloporphyrin, 1, isolated from the reaction, crystallizes as its five-coordinate N-methylpyrrolidinone (NMP) complex in triclinic space group P1: a = 11.470 (9) Å, b = 13.155 (7) Å, c = 19.894 (10) Å, a = 84.25 (4)°, β = 79.00 (5)°, γ = 75.59 (6)°; V= 2849.3 (3.3) Å3, Z = 2. The zwitterionic canonical form of the O-bound axial NMP is considerably enhanced upon ligation to the zinc, and the pocket depth defined by the eight phenyl o-chloro substituents in 1 is ca. 2.6 Å. Although the spectroscopic data indicate that free-radical bromination of 2 followed by treatment of the product under Rothemund condensation conditions yields some porphyrin, the principal products are brominated derivatives of 1 and 2. © 1988, American Chemical Society. All rights reserved.