Mechanistic Information from Low-Temperature Rapid-Scan and NMR Measurements on the Protonation and Subsequent Reductive Elimination Reaction of a (Diimine)platinum(II) Dimethyl Complex

Abstract

A detailed kinetic study of the protonation and subsequent reductive elimination reaction of a (diimine)platinum(II) dimethyl complex was undertaken in dichloromethane over the temperature range of -90 to +10 degrees C by stopped-flow techniques. Time-resolved UV-vis monitoring of the reaction allowed the assessment of the effects of acid concentration, coordinating solvent (MeCN) concentration, temperature, and pressure. The second-order rate constant for the protonation step was determined to be 15200 +/- 400 M(-1) s(-1) at -78 degrees C, and the corresponding activation parameters are DeltaH = 15.2 +/- 0.6 kJ mol(-1) and DeltaS = -85 +/- 3 J mol(-1) K(-1), which are in agreement with the addition of a proton that results in the formation of the platinum(IV) hydrido complex. The kinetics of the second, methane-releasing reaction step do not show an acid dependence, and the MeCN concentration also does not significantly affect the reaction rate. The activation parameters for the second reaction step were found to be DeltaH = 75 +/- 1 kJ mol(-1), DeltaS = +38 +/- 5 J mol(-1) K(-1), and DeltaV = +18 +/- 1 cm(3) mol(-1), strongly suggesting a dissociative character of the rate-determining step for the reductive elimination reaction. The spectroscopic and kinetic observations were correlated with NMR data and assisted the elucidation of the underlying reaction mechanism.

Combined Low Temperature Rapid Scan and1H NMR Mechanistic Study of the Protonation and Subsequent Benzene Elimination from a (Diimine)platinum(II) Diphenyl Complex Relevant to Arene C−H Activation

“…This is in accordance with the associative nature of the reaction. These data may be compared with our previously published data 80 on the protonation of (N-N)- Figure 4 and is characterized by an increase in absorbance in the selected wavelength region with time.…”

“…One important finding has been that low-temperature protonation of (diimine)Pt(II) dialkyl and Pt(II) diaryl complexes lead to observable, but thermally sensitive, Pt(IV) hydridoalkyl and hydridoaryl complexes that eliminate the respective hydrocarbons upon heating. 60,[76][77][78][79][80][81] Scheme 9 summarizes the mechanistic picture that has emerged for these reactions at (diimine)Pt(II) systems 60,63,74,[82][83][84][85] and at related Pt species with bidentate ligands. Substitution of π-benzene for an aqua (or TFE) ligand occurs as a solvent assisted process for which there is controversy over an associative mechanism.…”

“…80 Protonation in acetonitrile solvent mixture led to the observation of (diimine)PtMe 2 H(NCMe) + at low temperature. The intermediate suggested but didn't prove that the metal was the kinetic site of protonation.…”

“…When heated, the compounds undergo benzene elimination to furnish (N- has been reported, 80,157 when examining the reductive elimination at (dppe)PtMe 3 Py, leading to ethane and (dppe)PtMePy for which an initial pyridine dissociation followed by rate determining reductive elimination of ethane was determined. In our case, where an inhibition of the kinetics is characterized at high acetonitrile concentration, the expression of the rate law k obs for benzene reductive elimination can be determined using the steady state approximation by (1), in reference to Scheme 8.…”

“…Et 2 O to provide the Pt(IV) complex (Ar-DAB)PtMe 2 H(NCMe) + and the subsequent elimination of methane from this Pt(IV) complex. 80 The latter reaction was found to proceed by initial and rate-limiting MeCN dissociation. In this chapter, we present the details of kinetic studies of protonation reactions and benzene-producing reactions at Pt (II) and Pt(IV) species that are derived from (Ar-DAB)PtPh 2 1a.…”