Hydrocarbon Oxidation by Bis-μ-oxo Manganese Dimers: Electron Transfer, Hydride Transfer, and Hydrogen Atom Transfer Mechanisms

Described here are oxidations of alkylaromatic compounds by dimanganese μ-oxo and μ-hydroxo
dimers [(phen)2MnIV(μ-O)2MnIV(phen)2]4+ ([Mn2(O)2]4+), [(phen)2MnIV(μ-O)2MnIII(phen)2]3+ ([Mn2(O)2]3+), and
[(phen)2MnIII(μ-O)(μ-OH)MnIII(phen)2]3+ ([Mn2(O)(OH)]3+). Dihydroanthracene, xanthene, and fluorene are
oxidized by [Mn2(O)2]3+ to give anthracene, bixanthenyl, and bifluorenyl, respectively. The manganese
product is the bis(hydroxide) dimer, [(phen)2MnIII(μ-OH)2MnII(phen)2]3+ ([Mn2(OH)2]3+). Global analysis of
the UV/vis spectral kinetic data shows a consecutive reaction with buildup and decay of [Mn2(O)(OH)]3+
as an intermediate. The kinetics and products indicate a mechanism of hydrogen atom transfers from the
substrates to oxo groups of [Mn2(O)2]3+ and [Mn2(O)(OH)]3+. [Mn2(O)2]4+ is a much stronger oxidant,
converting toluene to tolyl-phenylmethanes and naphthalene to binaphthyl. Kinetic and mechanistic data
indicate a mechanism of initial preequilibrium electron transfer for p-methoxytoluene and naphthalenes
because, for instance, the reactions are inhibited by addition of [Mn2(O)2]3+. The oxidation of toluene by
[Mn2(O)2]4+, however, is not inhibited by [Mn2(O)2]3+. Oxidation of a mixture of C6H5CH3 and C6H5CD3
shows a kinetic isotope effect of 4.3 ± 0.8, consistent with C−H bond cleavage in the rate-determining
step. The data indicate a mechanism of initial hydride transfer from toluene to [Mn2(O)2]4+. Thus, oxidations
by manganese oxo dimers occur by three different mechanisms:  hydrogen atom transfer, electron transfer,
and hydride transfer. The thermodynamics of e-, H•, and H- transfers have been determined from redox
potential and pKa measurements. For a particular oxidant and a particular substrate, the choice of mechanism
is influenced both by the thermochemistry and by the intrinsic barriers. Rate constants for hydrogen atom
abstraction by [Mn2(O)2]3+ and [Mn2(O)(OH)]3+ are consistent with their 79 and 75 kcal mol-1 affinities for
H•. In the oxidation of p-methoxytoluene by [Mn2(O)2]4+, hydride transfer is thermochemically 24 kcal mol-1
more facile than electron transfer; yet the latter mechanism is preferred. Thus, electron transfer has a
substantially smaller intrinsic barrier than does hydride transfer in this system.