Novel mercury(II) complexes of porphyrins

Treatment of octa-alkylporphyrins (e.g. 1–3) with mercury(II) acetate in methylene chloride and tetrahydrofuran affords novel complexes (4) containing two porphyrin rings layered between three Hg atoms. A new type of stereoisomerism in regularly substituted porphyrins (e.g. 1a, 2a) is identified; it is observed by NMR spectroscopy and occurs because two forms of the complex (4) are possible, depending upon which faces of the porphyrin molecule are turned in towards the central Hg atom. The NMR spectra also indicate that in concentrated solution the double sandwich complexes (4) are stable towards disproportionation and recombination. On the basis of the inherent geometrical arrangements in the complex (4), an efficient NMR method for unambiguous identification of the four primary type isomers of the aetio- and copro-porphyrin series is described.

In very dilute solution, or in mixtures containing nucleophilic solvents such as pyridine, methanol, dioxan, or tetrahydrofuran (in the absence of excess mercury(II) acetate), the double sandwich complexes (4) are destroyed to afford normal 1:1 metalloporphyrins.