Aniline-zinc porphyrin complexes

Hunter (60) reported a self-assembled open polymer formed by a zinc porphyrin bearing one para-aniline substituent at the meso position. The ortho- and mela-analogs discussed above form closed dimers, but the geometry of the para-derivative precludes this, and polymerization is the only alternative (76, Fig. 31). Although the dilution experiments could be fitted to a non-cooperative polymerization model with a pairwise association constant (K = 190 M 1) practically identical to that found for simple aniline-zinc porphyrin complexes (K = 130 M 1), broadening of the 4H NMR spectrum at high concentrations is characteristic of oligomerization. 

The aniline-zinc porphyrin interaction has also been exploited to form dimers. Hunter (60) reported the dimerization of porphyrins functionalized at one meso position with ortho or meta aniline groups (47, 48, Fig. 15). Both compounds showed concentration-dependent H NMR spectra with large upfield shifts for the aniline protons. The dimerization constants are 160 and 1080 M-1 respectively for 47 and 48, and these values are an order of magnitude higher than the association constants of simple reference complexes (K — 10 and 130 M 1 respectively), which is indicative of cooperative self-assembly. The complexa-tion-induced changes in chemical shift were used to obtain three-dimensional structures of the dimers. 

Furthermore, porphyrins react like extremely electron-rich aromatic compounds, comparable to phenol, aniline, or pyrrole. Central metal ions, in particular zinc(II) and copper(II), further activate them in electrophilic substitution reactions, l pical examples of the di- and tetrachlorination, mono- and dinitration, and most important Vilsmeier mono me o-formylation are given (Scheme 6.4.3) (Bonnett and Stephenson, 1965). Vilsmeier formylation of tetraphenylpor-phyrins gives mono (3-formyl products, which have been used to attach side chains to an imidazole end. Five coordinated metal complexes thus become accessible (not shown). 

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