Crystal structure of (2,4-dimethoxybenzyl)triphenylphosphonium trifluoroacetate — trifluoroacetic acid (1/1), C31H27F6O6P

Smit, J.; Marais, Charlene; Bezuidenhout, Barend C.B.

doi:10.1515/ncrs-2018-0573

“…We propose that the role of additional acid is to favor the formation of the five-coordinate intermediate by stabilizing the TFA – anion in solution and possibly assisting its dissociation (Scheme ). − …”

Section: Resultsmentioning

confidence: 99%

Variable Kinetic Isotope Effect Reveals a Multistep Pathway for Protonolysis of a Pt–Me Bond

Rettig

¹

,

Xu

²

,

Knight

³

et al. 2022

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The reaction of (cod)PtMe2 (cod = 1,5-cyclooctadiene) with trifluoroacetic acid to release methane is an important system because it represents the microscopic reverse of desirable methane activation and because it has an unusually large kinetic isotope effect (KIE) that has been tentatively attributed to proton tunneling. A detailed kinetic and mechanistic investigation of this system was conducted using stopped-flow and traditional time-dependent UV–vis spectroscopy, supported by NMR and density functional theory studies. Consistently large KIE values (∼14) in line with previous reports were obtained over a large range of reactant concentrations (0.1–1.6 mM (cod)PtMe2 and 3.2 mM to 6.0 M acid). At lower concentrations of acid, the KIE decreased significantly (KIE = ∼6 for 0.1 mM (cod)PtMe2 and 0.2 mM acid). This concentration-dependent KIE suggests a multistep reaction mechanism, eliminating the need to invoke proton tunneling. The reaction exhibits first-order dependence on (cod)PtMe2 and approximately second-order dependence on acid, with at least 2 equiv of acid required for complete conversion. Overall, the kinetic data indicate a multimolecular, multistep reaction mechanism for the protonolysis of (cod)PtMe2, thus ruling out the previously accepted bimolecular single-step mechanism. A mechanistic alternative consistent with the kinetic data is proposed, in which sequential oxidative addition and reductive elimination occur, and the second equivalent of acid serves to stabilize the trifluoroacetate anion in solution.

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“…We propose that the role of additional acid is to favor the formation of the 5-coordinate intermediate by stabilizing the TFAanion in solution, and possibly assisting its dissociation (Scheme 3). [72][73][74][75][76][77][78][79][80][81][82]…”

Section: Table 2 Values For K(obs)h K(obs)d and Kie From Timeresolved...mentioning

confidence: 99%

Variable Kinetic Isotope Effect Reveals Multi-step Pathway for Protonolysis of a Pt–Me Bond

Rettig

¹

,

Xu

²

,

Knight

³

et al. 2022

Preprint

0

View full text Add to dashboard Cite

The reaction of (cod)PtMe2 (cod = 1,5-cyclooctadiene) with trifluoroacetic acid (TFAH) to release methane is an important system because it represents the microscopic reverse of desirable methane activation, and because it has an unusually large kinetic isotope effect (KIE) that has been tentatively attributed to proton tunneling. A detailed kinetic and mechanistic inves-tigation of this system was conducted using stopped-flow and traditional time-dependent UV-vis spectroscopy, supported by NMR and DFT studies. Consistently large KIE values (~14) in line with previous reports were obtained over a large range of reactant concentrations (0.1 – 1.6 mM (cod)PtMe2 and 3.2 mM to 6.0 M acid). At lower concentrations of acid, the KIE decreased significantly (KIE = ~6 for 0.1 mM (cod)PtMe2 and 0.2 mM acid). This concentration-dependent KIE suggests a multi-step reaction mechanism, eliminating the need to invoke proton tunneling. The reaction exhibits first-order dependence on (cod)PtMe2 and approximately second-order dependence on acid, with at least 2 equivalents of acid required for complete conversion. Overall, the kinetic data indicate a multi-molecular, multi-step reaction mechanism for the protonolysis of (cod)PtMe2, thus ruling out the previously accepted bimolecular single-step mechanism. A mechanistic alternative consistent with the kinetic data is proposed, in which sequential oxidative addition and reductive elimination occur, and the second equivalent of acid serves to stabilize trifluoroacetate anion in solution.

show abstract

“…We propose that the role of additional acid is to favor the formation of the 5coordinate intermediate by stabilizing the TFAanion in solution, and possibly assisting its dissociation (Scheme 3). [75][76][77][78][79][80][81][82][83][84][85]…”

Section: Table 2 Values For K(obs)h K(obs)d and Kie From Time-resolve...mentioning

confidence: 99%

Variable Kinetic Isotope Effect Reveals Multi-step Pathway for Protonolysis of a Pt–Me Bond

Rettig

¹

,

Xu

²

,

Knight

³

et al. 2022

Preprint

0

View full text Add to dashboard Cite

The reaction of (cod)PtMe2 (cod = 1,5-cyclooctadiene) with trifluoroacetic acid (TFAH) to release methane is an important system because it represents the microscopic reverse of desirable methane activation, and because it has an unusually large kinetic isotope effect (KIE) that has been tentatively attributed to proton tunneling. A detailed kinetic and mechanistic inves-tigation of this system was conducted using stopped-flow and traditional time-dependent UV-vis spectroscopy, supported by NMR and DFT studies. Consistently large KIE values (~14) in line with previous reports were obtained over a large range of reactant concentrations (0.1 – 1.6 mM (cod)PtMe2 and 3.2 mM to 6.0 M acid). At lower concentrations of acid, the KIE decreased significantly (KIE = ~6 for 0.1 mM (cod)PtMe2 and 0.2 mM acid). This concentration-dependent KIE suggests a multi-step reaction mechanism, eliminating the need to invoke proton tunneling. The reaction exhibits first-order dependence on (cod)PtMe2 and approximately second-order dependence on acid, with at least 2 equivalents of acid required for complete conversion. Overall, the kinetic data indicate a multi-molecular, multi-step reaction mechanism for the protonolysis of (cod)PtMe2, thus ruling out the previously accepted bimolecular single-step mechanism. A mechanistic alternative consistent with the kinetic data is proposed, in which sequential oxidative addition and reductive elimination occur, and the second equivalent of acid serves to stabilize trifluoroacetate anion in solution.

show abstract

Crystal structure of (2,4-dimethoxybenzyl)triphenylphosphonium trifluoroacetate — trifluoroacetic acid (1/1), C₃₁H₂₇F₆O₆P

Abstract: C31H27F6O6P, triclinic, P1̄ (no. 2), a = 8.0017(2) Å, b = 11.1702(3) Å, c = 17.8353(4) Å, α = 104.9000(10)°, β = 96.5360(10)°, γ = 97.7380(10)°, V = 1508.05(7) Å3, Z = 2, Rgt(F) = 0.0354, wRref(F2) = 0.0896, T = 150(2) K.

Cited by 3 publications

References 8 publications

Variable Kinetic Isotope Effect Reveals a Multistep Pathway for Protonolysis of a Pt–Me Bond

Variable Kinetic Isotope Effect Reveals a Multistep Pathway for Protonolysis of a Pt–Me Bond

Variable Kinetic Isotope Effect Reveals Multi-step Pathway for Protonolysis of a Pt–Me Bond

Variable Kinetic Isotope Effect Reveals Multi-step Pathway for Protonolysis of a Pt–Me Bond

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