Atom transfer reactions of high-valent chromium and manganese porphyrins
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The terminally bound nitrido ligand of manganese(V) porphyrin complexes serves as a good bridging species in electron transfer processes. Thus, the nitrogen atom of a porphyrinatomanganese nitrido complex was reversibly transferred to porphyrinatomanganese(II), in a formal three-electron redox process. Evidence is presented that supports an inner-sphere mechanism for this process. The coordinating solvent, tetrahydrofuran, decreased the rate constant for this reaction by three orders of magnitude compared to that observed in the noncoordinating solvent, toluene. The number of redox equivalents exchanged in nitrogen atom transfer processes can be varied from three to two by choosing the appropriate reductant such as a porphyrinatomanganese(III) chloride complex. Evidence is presented to support a mechanism for the reaction between porphyrinatomanganese nitride and porphyrinatomanganese chloride which involves prior dissociation of the chloride ion before the formation of a [mu]-nitrido species. Thus when given a choice of potential bridging ligands (eg. N vs Cl or pivalate) the multi-electron process studied here preferentially selects the nitrido ligand;The terminally bound oxo ligand of chromium porphyrin complexes has also been shown to also serve as a good bridging ligand in electron transfer processes. It was demonstrated that a one-electron transfer involving complete atom transfer was possible by choosing the correct redox partner, a porphyrinatochromium(III) complex. The mechanism for this reaction has been shown to proceed through a [mu]-oxo species. Unlike the mechanism for porphyrinatomanganese chloride, the porphyrinatochromium chloride complex does not require prior dissociation of the chloride ion. The rate constant was not decreased by addition of a ten fold excess of chloride ion.