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

Abstract

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)Mn(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 C(6)H(5)CH(3) and C(6)H(5)CD(3) 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.

A synthetic precedent for the [Fe IV 2 (μ-O) 2 ] diamond core proposed for methane monooxygenase intermediate Q

“…8-10). In all three cases, DHA was oxidized and kinetic isotope effects (KIEs) Ͼ 9 were observed with DHA-d 4 as substrate ( Table 2), indicating that hydrogen atom abstraction is the rate-determining step for the oxidations (32,33). Analysis of the oxidation products showed that anthracene was produced in Ϸ50% yield for 1b and 4b, as deduced from its characteristic UV bands (SI Figs.…”

Proton-Coupled Electron Transfer in Organic Synthesis: Fundamentals, Applications, and Opportunities

“…The measured hydride affinity of the complex is provided to be 122 kcal mol −1 by the authors. The mechanism of activation by these complexes is highly substrate dependent and may proceed by initial HAT, electron transfer, or hydride transfer [79]. On the other hand, prior reduction of this species to the Mn IV -Mn III allows for HAT reactivity: (phen) 2 Mn(μ-O) 2 Mn(phen) 2 ] 3+ may undergo two HAT events to form the bridged hydroxo species (phen) 2 Mn(μ-OH) 2 Mn(phen) 2 ] 3+ .…”

The roles of the first and second coordination spheres in the design of molecular catalysts for H2production and oxidation

“…15 The sum of reactions (2)- (4) is the heterolytic cleavage of the M-H bond to form H À (reaction (5)) and the associated metal fragment, and the free energy associated with this heterolytic bond cleavage reaction, DG1 H À, is the sum of the free energies for reactions (2)- (4 Using thermodynamic cycles of this type, the hydride donor/acceptor abilities of a large range of compounds have been determined using acetonitrile as the solvent. These include a diverse array of compounds such as NADH analogues (a biological hydride donor), [16][17][18] hydroquinone derivatives, 19 transition metal hydrides of various types, 16,[20][21][22] bridging hydroxides in bimetallic complexes, 23 and aromatic hydrocarbons. 24,25 Of these different classes of compounds, those containing transition metals were of interest because of their ability to bind and activate H 2 to form either dihydrogen or dihydride complexes.…”