Activation of C–H, N–H, and O–H Bonds via Proton-Coupled Electron Transfer to a Mn(III) Complex of Redox-Noninnocent Octaazacyclotetradecadiene, a Catenated-Nitrogen Macrocyclic Ligand

Reaction
of 1,3-diazidopropane with an electron-rich Mn­(II) precursor
results in oxidation of the metal center to a Mn complex with concomitant
assembly of the macrocyclic ligand into the 1,2,3,4,­8,9,10,11-octa­aza­cyclo­tetra­deca-2,9-diene-1,4,8,11-tetraido
(OIM) ligand. Although describable as a Werner Mn­(V) complex, analysis
by X-ray diffraction, magnetic measurements, X-ray photoelectron spectroscopy,
cyclic voltammetry, and density functional theory calculations suggest
an electronic structure consisting of a Mn­(III) metal center with
a noninnocent OIM diradical ligand. The resulting complex, (OIM)­Mn­(NHtBu), reacts via proton-coupled electron transfer
(PCET) with phenols to form phenoxyl radicals, with dihydroanthracene
to form anthracene, and with (2,4-ditert-butyltetrazolium-5-yl)­amide
to extrude a tetrazyl radical. PCET from the latter generates the
isolable corresponding one-electron reduced compound with a neutral,
zwitterionic axial 2,4-ditert-butyltetrazolium-5-yl)­amido
ligand. Electron paramagnetic resonance and density functional theoretical
analyses suggest an electronic structure wherein the manganese atom
remains Mn­(III) and the OIM ligand has been reduced by one electron
to a monoradical noninnocent ligand. The result indicates PCET processes
whereby the proton is transferred to the axial ligand to extrude tBuNH2, the electron is transferred
to the equatorial ligand, and the central metal remains relatively
unperturbed.