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

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.