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 mu-oxo and mu-hydroxo dimers [(phen)(2)Mn-IV(mu-O)(2)Mn-IV(phen)(2)](4+) ([Mn-2(O)(2)](4+)), [(phen)(2)Mn-IV(mu-O)(2)Mn-III(phen)(2)](3+) ([Mn-2(O)(2)](3+)), and [(phen)(2)M-III(mu-O)(mu-OH)Mn-III(phen)(2)](3)+ ([Mn-2(O)(OH)](3+)). Dihydroanthracene, xanthene, and fluorene are oxidized by [Mn-2(O)(2)](3+) to give anthracene, bixanthenyl, and bifluorenyl, respectively. The manganese product is the bis(hydroxide) dimer, [(phen)(2)Mn-III (mu-OH)(2)Mn-II(phen)(2)](3)+ ([Mn-2(OH)(2)](3+)). Global analysis of the UV/vis spectral kinetic data shows a consecutive reaction with buildup and decay of [Mn-2(O)(OH)](3+) as an intermediate. The kinetics and products indicate a mechanism of hydrogen atom transfers from the substrates to oxo groups of [Mn-2(O)(2)](3+) and [Mn-2(O)(OH)](3+). [Mn-2(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 [Mn-2(O)(2)](3+). The oxidation of toluene by [Mn-2(O)(2)](4+), however, is not inhibited by [Mn-2(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 [Mn-2(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 pK(a) 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 [Mn-2(O)(2)](3+) and [Mn-2(O)(OH)](3+) are consistent with their 79 and 75 kcal mol(-1) affinities for H+. In the oxidation of p-methoxytoluene by [Mn-2(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.