The influence of reversible trianionic pincer OCO3−μ-oxo Criv dimer formation ([Criv]2(μ-O)) and donor ligands in oxygen-atom-transfer (OAT)

Abstract

The oxygen-atom-transfer (OAT) from [^tBuOCO]Cr^V(O)(THF) (2) (where ^tBuOCO = [2,6-C₆H₃(6-^tBuC₆H₃O)₂]³⁻, THF = tetrahydrofuran) to triphenylphosphine (PPh₃) in THF produces [^tBuOCO]Cr^III(THF)₃ (1) and triphenylphosphine oxide (OPPh₃) at a rate of 69.5(±1.9) M⁻¹s⁻¹ (22 °C). Identical rate constants were attained when acetonitrile (MeCN) and dichloromethane/THF (CH₂Cl₂/THF) were used as solvents. Electron paramagnetic resonance (EPR) data shows that the six-coordinate complex, [^tBuOCO]Cr^V(O)(THF)₂ (2a) forms upon addition of THF to 2, suggesting 2a as the active OAT species in THF. Similarly, addition of OPPh₃ has no influence on the rate of OAT, but the addition of triphenylphosphorus ylide (CH₂PPh₃) to form [^tBuOCO]Cr^V(O)(CH₂PPh₃) (4) prevents OAT to PPh₃. In CH₂Cl₂, a [Cr^IV]₂(μ-O) intermediate forms during the OAT from 2 to PPh₃. The OAT from {[^tBuOCO]Cr^IV(THF)}₂(μ-O) (3) to PPh₃ reveals a zero-order dependence in PPh₃ indicating the dimer must first dissociate prior to OAT. The decay of 3versus time does not follow first-order kinetics due to the formation of a [^tBuOCO]Cr^III(THF) species (5) that inhibits the dissociation of 3. The change in concentration of 3versus time during OAT was simulated to obtain approximate rate constants.