Tetracationic and Tetraanionic Manganese Porphyrins: Electrochemical and Spectroelectrochemical Characterization

The electrochemistry and spectroelectrochemistry of four tetrapositively charged and two tetranegatively charged porphyrins were characterized in two nonaqueous solvents (dimethyl sulfoxide and N,N-dimethylformamide) containing 0.1 M tetra-n-butylammonium perchlorate. The tetrapositively charged compounds are represented by the tetrapyridylporphyrins [TRPyPM]⁴⁺(X^-)₄, where R is a methyl or [2-[2-(2-methoxy)ethoxy]ethoxy]ethyl group, M = Mn^IIII, Mn^IIICl, Cu^II, or Pd^II, and X = I^- or Cl^-. The tetranegatively charged porphyrins are represented by the tetrasulfonato derivatives [TPPSMn(OAc)]^4-(NH₄⁺)₄ and [TArPSMn(OAc)]^4-(NH₄⁺)₄, where Ar = 4-O-[2-[2-(2-methoxy)ethoxy]ethoxy]ethylphenyl. Up to seven electrons can be added to the tetrapyridyl porphyrins in three to five reversible reductions, while up to four electrons can be added to the tetrasulfonato derivatives in four reversible processes. Three types of electrochemical behaviors are observed for reduction of the pyridinium groups on the tetrapyridyl porphyrins. One is for the manganese(II) complexes where the four equivalent pyridinium groups are reduced in a single overlapping four-electron-transfer step. Another is for the free-base porphyrin, where four well-separated one-electron reductions occur, while the third is for copper(II) and palladium(II) derivatives, where reduction of the four pyridinium groups proceeds in two well-separated two-electron-transfer steps. The electrochemical and spectroelectrochemical properties were also characterized for a 1:1 mixture of the tetrapositively and tetranegatively charged manganese porphyrins to investigate possible interactions between these two species. An interaction between the two porphyrins was indeed observed in both solvents after electroreduction of the four pyridinium groups, which led to a substantial change in the mechanism for reduction of the pyridinium groups from an initial single overlapping four-electron-reduction process to two well-separated two-electron-transfer processes.