Mechanistic Investigation of Benzene C−H Activation at a Cationic Platinum(II) Center:  Direct Observation of a Platinum(II) Benzene Adduct

Abstract

The platinum(II) methyl cation [(N−N)Pt(CH3)(solv)]+BF4 - (N−N = ArNC(Me)C(Me)NAr, Ar = 2,6-(CH3)2C6H3, solv = H2O (2a) or TFE = CF3CH2OH (2b)) is prepared by treatment of (N−N)Pt(CH3)2 with 1 equiv of aqueous HBF4 in TFE. Reaction of a mixture of 2a and 2b with benzene in TFE/H2O solutions cleanly affords the platinum(II) phenyl cation [(N−N)Pt(C6H5)(solv)]+BF4 - (3). Investigations of the kinetics and isotopic labeling experiments indicate that reaction of 2 with benzene proceeds via benzene coordination, reversible oxidative addition of benzene C−H bonds, reversible formation of a methane C,H-σ complex, and final dissociation of methane. Under conditions where [(N−N)Pt(CH3)(H2O)]+BF4 - (2a) is the major starting complex, rate-determining benzene coordination to 2b is implicated by the observed kinetic rate law (inverse first order in [H2O] and first order in [C6H6] to 3.8 M) and the small kinetic deuterium isotope effect for C6H6 vs C6D6 (k H/k D = 1.06 ± 0.05 at 25 °C). When deuterated benzenes C6D6 and 1,3,5-C6H3D3 are used, almost full statistical scrambling of deuterium from one benzene into methane is achieved, indicating that the energetic barriers for dissociating benzene and methane are considerably higher than interconversions of intermediate hydrocarbon complexes and [(N−N)Pt(C6H5)(CH3)H]+. Protonation of (N−N)Pt(CH3)(C6H5) with HBF4 in TFE, which provides an independent route into the manifold of postulated intermediates, gives a mixture of 3 + CH4 (82%) and 2 + C6H6 (18%). Protonation of (N−N)Pt(CH3)(C6H5) with triflic acid in methylene chloride/diethyl ether mixtures at −69 °C allows direct low-temperature NMR observation of a fluxional π benzene complex, [(N−N)Pt(CH3)(C,C-η2-C6H6)]+.

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

“…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.…”

“…85,[95][96][97] Although not observed during C-H activation experiments, the π-benzene complex was independently generated and observed by NMR at low temperatures. 60 From then the mechanism postulated is a Pt(II)…”

Mechanistic Studies of Ethylene Hydrophenylation Catalyzed by Bipyridyl Pt(II) Complexes

“…32,34,46,47 The impact of the weakly coordinating Lewis base THF on the rate of benzene CÀD activation by 1 was investigated. The reaction rate decreases with increasing THF concentration ( Figure 6).…”

C–H Bond Activation ofMethane with Gaseous [(CH3)Pt(L)]+ Complexes (L = Pyridine, Bipyridine, and Phenanthroline)

“…Particularly intriguing in this respect is a recent debate about the relevance of gas phase studies for reactions occurring in condensed media [6,7], which dealt with the C-H bond activation of benzene by cationic [(CH 3 )-Pt(L)] + complexes [8,9], where L corresponds to a 0932-0776 / 07 / 0300-0309 $ 06.00 © 2007 Verlag der Zeitschrift für Naturforschung, Tübingen · http://znaturforsch.com bidentate nitrogen ligand. Here, we describe the reactions of methane with similar [(CH 3 )Pt(L)] + species in the gas phase which are generated directly from solution by means of electrospray mass spectrometry [10].…”