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

Abstract

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.

Hexa-Fe(III) Carboxylate Complexes Facilitate Aerobic Hydrocarbon Oxidative Functionalization: Rh Catalyzed Oxidative Coupling of Benzene and Ethylene to Form Styrene

“…A potentially relevant reaction for the acidic proton of HOPiv to undergo is protonolysis of a Rh–Ph intermediate in a reverse of the benzene C–H activation step (Scheme ), which would likely be more rapid with HOPiv than with DOPiv. Since the undesired reverse C–H activation would be faster with HOPiv than with DOPiv, the rate of styrene production would consequently be slower with HOPiv than with DOPiv, which would likely result in an inverse KIE in the absence of other effects. , …”

“…Since the undesired reverse C–H activation would be faster with HOPiv than with DOPiv, the rate of styrene production would consequently be slower with HOPiv than with DOPiv, which would likely result in an inverse KIE in the absence of other effects. 88 , 89 …”

“…Since the undesired reverse C−H activation would be faster with HOPiv than with DOPiv, the rate of styrene production would consequently be slower with HOPiv than with DOPiv, which would likely result in an inverse KIE in the absence of other effects. 88,89 To determine if benzene C−H activation is reversible, deuterium or proton incorporation into the styrene product was studied using GC-MS for reactions using a combination of C 6 H 6 and DOPiv or C 6 D 6 and HOPiv. As shown in Figure 11a, the ratio of the m/Z = 105 to m/Z = 104 peak areas, which corresponds to monodeutero and per-protio styrene, respectively, does not undergo a statistically significant change over time when using a combination of C 6 H 6 and DOPiv.…”

Hydrogen Tunneling in Stoichiometric and Catalytic Reactions involving Transition Metals

“…The calculated KIEs agree perfectly with the experimental data and could indicate a tunneling effect. However, further detailed kinetic and mechanistic investigation of this reaction suggests that the high KIE values are the result of a multi‐step reaction rather than the involvement of hydrogen tunneling [50] …”

“…However, further detailed kinetic and mechanistic investigation of this reaction suggests that the high KIE values are the result of a multi-step reaction rather than the involvement of hydrogen tunneling. [50] The next selected example in which the possibility of hydrogen tunneling is present and the metallic center is directly involved in the carbon-to-osmium hydrogen atom reaction reported by Grills, Bullock and Fujita in 2014 (Figure 6b). [51] This reaction consists of the hydrogen atom transfer from xanthene to [(η 5 -i Pr 4 C 5 H)(CO) 2 Os] • radical, formed by photoinduced homolysis of the corresponding osmium dimers.…”