Reductive electrochemistry of rhodium porphyrins. Disproportionation of intermediary oxidation states

The reduction of rhodium(III) porphyrins in polar aprotic solvents is a two-electron irreversible reaction yielding directly the Rh(I) complex. The cause of this irreversibility is not the metal-metal dimerization of the initially formed Rh(II) complex as believed earlier but rather deligation which generates a secondary Rh(IT) species easier to reduce than the starting Rh(III) porphyrin. This is confirmed by the fact that sterically encumbered porphyrins, such as those bearing cross-trans basket-handle superstructures which forbid the approach of two molecules at bonding distance, exhibit the same behavior as simple rhodium porphyrins. The occurrence of such an ECE-disproportionation process, seldom observed in the redox chemistry of metallo-porphyrins or similar complexes, is probably related to the tendency of the rhodium atom to shift out of the porphyrin plane, particularly at the Rh(I) oxidation state. It is remarkable that strong and soft ligands, e.g., tertiary phosphines, annihilate the disproportionation of the rhodium(II) complex.