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Multielectron Atom Transfer Reactions of Perchlorate and Other Substrates Catalyzed by Rhenium Oxazoline and Thiazoline Complexes:  Reaction Kinetics, Mechanisms, and Density Functional Theory Calculations

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posted on 28.06.2004, 00:00 by Lee D. McPherson, Markus Drees, Saeed I. Khan, Thomas Strassner, Mahdi M. Abu-Omar
The title complexes, the Re(O)L2(Solv)+ complexes (L = hoz, 2-(2‘-hydroxyphenyl)-2-oxazoline(−) or thoz, 2-(2‘-hydroxyphenyl)-2-thiazoline(−); Solv = H2O or CH3CN), are effective catalysts for the following fundamental oxo transfer reaction between closed shell molecules:  XO + Y → X + YO. Among suitable oxygen acceptors (Y's) are organic thioethers and phosphines, and among suitable oxo donors (XO's) are pyridine N-oxide (PyO), t-BuOOH, and inorganic oxyanions. One of the remarkable features of these catalysts is their high kinetic competency in effecting perchlorate reduction by pure atom transfer. Oxo transfer to rhenium(V) proceeds cleanly to afford the cationic dioxorhenium(VII) complex Re(O)2L2+ in a two-step mechanism, rapid substrate (XO) coordination to give the precursor adduct cis-ReV(O)(OX)L2+ followed by oxygen atom transfer (OAT) as the rate determining step. Electronic variations with PyO derivatives demonstrated that electron-withdrawing substituents accelerate the rate of ReVII(O)2L2+ formation from the precursor adduct cis-ReV(O)(OX)L2+. The activation parameters for OAT with picoline N-oxide and chlorate have been measured; the entropic barrier to oxo transfer is essentially zero. The potential energy surface for the reaction of Re(O)(hoz)2(OH2)+ with PyO was defined, and all pertinent intermediates and transition states along the reaction pathway were located by density functional theory (DFT) calculations (B3LYP/6-31G*). In the second half of the catalytic cycle, Re(O)2L2+ reacts with oxygen acceptors (Y's) in second-order reactions with associative transition states. The rate of OAT to substrates spans a remarkable range of 0.1−106 L mol-1 s-1, and the substrate reactivity order is Ph3P > dialkyl sulfides > alkyl aryl sulfides > Ph2S ∼ DMSO, which demonstrates electrophilic oxo transfer. Competing deactivation and inhibitory pathways as well as their relevant kinetics are also reported.