Synthesis and properties of thallium(III) porphyrin chelates

Abstract

Treatment of porphyrins with thallium(III) acetate or trifluoroacetate affords the corresponding thallium(III) chelates, which suffer ligand exchange during chromatography on alumina to furnish the aquoporphinatothallium-(III) hydroxides [e.g.(2c)]; other examples of ligand exchange are presented. The aquo-hydroxides are obtained directly when porphyrins are metallated with thallium(III) nitrate trihydrate. Demetallation is readily accomplished with acids, by reduction (Tl^III→ Tl^I), or by a combination of both of these methods.

Information on metal–ligand interactions is obtained from the ¹H n.m.r. spectra; thallium–proton couplings are reported and are shown to occur via a predominantly π-bond mechanism for the meso-protons, but through a σ-bond mechanism for the peripheral protons. The visible absorption spectra indicate no major distortion of the porphyrin ring by the metal atom, and together with n.m.r. evidence, this suggests strongly that the metal atom is situated out of the plane of the porphyrin ring. The mass spectra of thallium(III) porphyrins show several anomalous features, the most notable being extrusion of the metal atom to give the free porphyrin ion, which is often the base peak in the spectra.