Spectral, Electrochemical, and ESR Characterization of Manganese Tetraarylporphyrins Containing Four β,β′-Pyrrole Fused Butano and Benzo Groups in Nonaqueous Media

Abstract

Two series of β,β′-pyrrole butano- and benzo-substituted mangenese­(III) tetraarylporphyrins were synthesized and characterized with regard to their spectral and electrochemical properties. The investigated compounds have the general formula butano­(Ar)4PorMnCl and benzo­(Ar)4PorMnCl, where Por is the dianion of the porphyrin and Ar is a p-CH3Ph, Ph or p-ClPh group on each of the four meso-positions of the macrocycle. Each manganese­(III) butano- or benzoporphyrin was examined in CH2Cl2 and/or pyridine containing 0.1 M tetra-n-butylammonium perchlorate and the data then were compared to that of the parent tetraarylporphyrins having the same meso-substituents. Up to four reductions are observed for each compound, the first being metal-centered to generate a Mn­(II) porphyrin, and the second and third being porphyrin ring-centered to give a Mn­(II) porphyrin π-anion radical and dianion, respectively. The one-electron reduced manganese porphyrins have an ESR spectrum with signals at g ⊥= 5.6–5.8 and g // = 2.0, indicating a mixture of the four- and five-coordinated Mn­(II) complexes in a high-spin state (3d5, S = 5/2, I = 5/2). Data from cyclic voltammetry and spectroelectrochemistry both suggest that formation of the porphyrin dianion is followed by a chemical reaction at the electrode surface to give an electroactive phlorin anion. The effects of solvent and porphyrin substituents on ultraviolet–visible light (UV-vis) spectra, redox potentials, and electron transfer mechanisms are discussed.

Solvent and Anion Effects on the Electrochemistry of Manganese Dipyrrin-Bisphenols

“…The quasireversible (slow) electron transfer for the Mn III/II reduction process in CH 2 Cl 2 and C 2 H 4 Cl 2 (as indicated by a Δ E p ≫ 60 mV and the lack of a well-defined reoxidation) and the faster electron transfer for the first oxidation under the same solution conditions (Δ E p = 80 to 100 mV) is consistent with not only the different sites of electron transfer (i.e., a ligand-centered oxidation and a metal-centered reduction) as assigned above but also the possible presence of a coupled chemical reaction on reduction. The larger Δ E p value associated with a slow metal-centered reduction is also consistent for what has been reported for the Mn III/II process of many porphyrins − and also other Mn III macrocycles. − …”

“…A summary of the measured oxidation and reduction potentials for (Ph)DPPMn in these solvents containing 0.1 M TBAPF6 is given in Table 2 and examples of cyclic voltammograms are shown in Figure 5, where three types of current-voltage curves are seen for reduction. The first is in CH2Cl2 and C2H4Cl2 where the reduction peak is broad and no clear reoxidation peak is observed, perhaps due to the formation of dimers in solution as seen in the solid state 10 and also in the gas phase as seen by the ESI-MS results in Figures S4 and S5 Mn III/II process of many porphyrins [38][39][40] and also other Mn III macrocycles. [41][42][43] It is also worth noting that, with the exception of CH2Cl2 and C2H4Cl2, the cathodic peak currents for the first reduction (ipc) are essentially the same as the anodic peak currents for the first oxidation (ipa), consistent with the same number of electrons transferred in each step.…”

“…It should also be noted that a dimerization detectable during reduction but not oxidation can be rationalized by the different sites of electron transfer; i.e., an electron is added to the manganese center during reduction, while electron abstraction takes place on the (Ar)­DPP ring during oxidation. It should also be noted that ring-centered redox processes are typically rapid (reversible) electron transfers, while Mn III -centered reductions would be slower as is often observed for Mn III reductions at the metal center. − ,− …”

“…It should also be noted that ring centered redox processes are typically rapid (reversible) electron 21 transfers while Mn III -centered reductions would be slower as is often observed for Mn III reductions at the metal center. [16][17][18][38][39][40][41][42][43] Figure 12. Cyclic voltammograms of (a) (Ph)DPPMn and (b) (Mes)DPPMn at 10 -3 M in CH2Cl2 containing 0.1 M TBAPF6 before and after the addition of 2.0 eq TFA.…”

Facile Synthesis of β‐Brominated Manganese Porphyrins and their Catalytic Potentials for Haloperoxidases‐Like Activity

“…[53] The second reduction in the range À 1.12 to À 1.60 V corresponds to the ring centred reduction which lead to the formation of Mn II porphyrin-anion radical (for 1-3) and Mn III porphyrin-anion radical (for 4). [54,55] The first oxidation in the range 1.11 to 1.44 V corresponds to the ring centred oxidation which lead to the formation of Mn III porphyrin-cation radical (for 1-3) and Mn IV porphyrin-cation radical (for 4), while the second in the range 1.52 to 1.66 V corresponds to the another ring centred oxidation leading to the formation of Mn III porphyrin-dication (for 1-3) and Mn IV porphyrin-dication (for 4), respectively. Complex 2 exhibits a huge anodic shift of 200 mV in both first ring oxidation and reduction potentials compared to complex 1 due to addition of four bromines at β-positions and nonplanar conformation.…”