Mechanisms of C-H Bond Activation by Platinum(II)

Abstract

The rates of C–H bond activation for various alkanes by [(N–N)PtII(CH₃)(TFE-d₃)]⁺ (N–N = Ar–N=C(CH₃)–C(CH₃)=N–Ar; Ar = 3,5-di-tert-butylphenyl; TFE-d₃ = CF₃CD₂OD) were studied. Both linear and cyclic alkanes give the corresponding [(N–N)PtII(H)(alkene)]⁺ cation. Second-order rate constants for cycloalkane activation (CnH2n) are proportional to the size of the ring (k ~ n). For cyclohexane, the deuterium kinetic isotope effect (kH/kD) of 1.28(5) is consistent with the proposed rate determining alkane coordination to form a C–H σ complex. Comparing the relative rates of cyclic and linear alkanes indicates that the platinum center is relatively unselective with respect to different C–H bonds: the rate constants (per C–H bond) for the substrates examined all fall into a narrow range, and there does not appear to be any significant preference for either primary or secondary C–H bonds. The protonolysis of platinum(II) methyl complexes was investigated by both experiment and computation. Experimental results showed that protonolysis of (COD)PtII(CH₃)₂ (COD = 1,5 – cyclooctadiene) by trifluoroacetic acid gave abnormally large (greater than 10) kinetic isotope effects (kH/kD) at room temperature and higher. The temperature dependence of kH/kD for the protonolysis of (COD)PtII(CH₃)₂ gave Arrhenius parameters outside semi-classical limits. On the other hand, protonolysis of (tmeda)PtII(CH₃)Cl (tmeda = N,N,N’,N’–tetramethyl–ethylenediamine) by trifluoroacetic acid gave normal kinetic isotope effects and classical Arrhenius parameters. Density functional theory (DFT) was used to examine the mechanism of protonolysis of these two systems, and the results were found to be consistent with experimental observations. Based on our experimental and computational work, we propose that protonolysis of methylplatinium(II) complexes can occur through either a concerted or stepwise pathway that is highly ligand dependent; more electron-rich ligands will favor the stepwise mechanism, while electron-deficient ligands with stronger trans influence will favor the concerted mechanism. Finally, we propose that the presence of abnormally large KIEs is an indication for a concerted pathway, and that there is a connection between the magnitude and temperature dependence of the KIE and mechanism. In aqueous solution, [PtII(glycinato)Cl₂]⁻ catalyzes oxidation by [PtIVCl₆]²⁻ of the methyl group of p-toluenesulfonate to the corresponding alcohol and aldehyde, with no further oxidation to the carboxylic acid. Both rate and selectivity are improved in comparison to the original Shilov system that employs [PtIICln(H₂O)4-n]2-n as the catalyst.