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+ ([Mn 2 (O) 2 ] 4+ ), [(phen)2MnIV(μ-O)2MnIII(phen)2]3+ ([Mn 2 (O) 2 ] 3+ ), and [(phen)2MnIII(μ-O)(μ-OH)MnIII(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)2MnIII(μ-OH)2MnII(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.