Porphyrins reversible oxidation

The picket fence Ni11 complex [Ni(TpivPP)] ((70) TpivPP meso-a-a-a-a-lclrakis-(o-pivalamidophenyl)porphyrin) is reversibly oxidized by three electrons in two steps and generates as final product [Ni(Tipw)]3+ which was characterized as a Nilv cation radical in solution by both UV-visible and EPR spectroscopy.289... 

A comparison of Schemes V and VI show that the electrochemistry of (0EP)Ge(C6H5)0H and (0EP)Si(C6H5)0H are similar. The first oxidation of (0EP)Si(C6H5)0H occurs at the hydroxyl ligand and generates (0EP)Si(C6H5)C10, as a finai product. This compound can be reversibly oxidized by up to two electrons at more positive potentials and gives a porphyrin it cation radical and dication(36). 

Electrochemistry. Compounds 15a [Fen(pz)pzEts] 15b [Fen(l-MeIm)2Ets] exhibited two reversible oxidations at 0.21 and -0.06 V (vs Ag/AgCl), which were attributed to the 2 —> 3 and 3 —> 4 metal-centered oxidations. These values are 400 mV more positive than those reported for the analogous Fe TPP, suggesting that the pz stabilizes the lower oxidation state, Fe(II) better than does the porphyrin (63). This stabilization of a basic oxidation state for the pz may be attributable to the lower energy n orbitals of the pz making it a better n acceptor than the porphyrin. 

The thallium complexes show somewhat different electrochemical behavior, and reversible oxidations are observed for both n-alkyl and n-aryl thallium porphyrins, indicating that the oxidized complexes have a more stable metal—carbon bond than the gallium or indium analogs. Spectroelectrochemistry revealed that the first oxidation is porphyrin ring-centered. The first reduction is reversible and ring... 

The electrochemistry of (TPPBrs)M, where M = Co, Ni, Cu, Zn, Pt, Pd, and Ag generally revealed the expected easier reduction for both metal-centered and ring-centered reactions as a result of the electron withdrawing bromo substituents. Quantitative work carried out on the (TPPBrjt)M and (TPPBr )MCl series " " yielded a wealth of information and allowed for a separation of geometric (steric) from electronic effects caused by the complete or partial yS-pyrrole bromination. As pointed out earlier, investigations performed on the (TPPBrjc)Zn series of compounds revealed two one-electron oxidations and two one-electron reductions, as expected for porphyrins bearing electroinactive metal ions. Since the first oxidation and first reduction both involved the porphyrin ring, it was possible to estimate the HOMO-LUMO gap (potential difference between the first reversible oxidation and first reversible reduction of the macrocycle) and compare the results with data from computational and spectral studies. As discussed earlier, the computational and experimental plots of... 

For example, <7-bonded porphyrins with Rh, P, As, or Sb central ions undergo reversible oxidations and reductions at the 7T-conjugated macrocycle and there is little effect of the cr-bonded axial ligand on the porphyrin electrochemical behavior [12, 21]. However, derivatives with Al, Ga, In or T1 central metals all undergo a rapid cleavage of the metal-carbon bond after a one-electron oxidation of the compound. In contrast, the Fe, Ru, and Co cr-bonded metalloporphyrins (see further sections of the review) will undergo a metal-centered... 

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