The Metal Is the Kinetic Site of Protonation of (Diimine)Pt Dimethyl Complexes

Abstract

Protonolysis of (diimine)PtMe2 (1) complexes in CD2Cl2 containing CD3CN at -78 degrees C yields (diimine)PtMe2(H)(NCCD3)+ (4), (diimine)PtMe(NCCD3)+ (5), and methane. The relative yields of 5 and methane decrease with increasing concentrations of CD3CN. This is consistent with protonation of 1 occurring directly at the metal, rather than at a methyl group. The principle of microscopic reversibility then implies that the deprotonation in "Shilov-type C-H activation" occurs from a Pt(IV) hydridomethyl intermediate, rather than from a Pt sigma-methane complex.

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

“…Apparently, the requirements concerning relative rates were met during the protonation of (Ar-DAB)PtMe 2 species, 111 but not in the present case. An offset of one of the rates by an order of magnitude or so might be sufficient to prevent the observation of the sought-after trend.…”

“…Concentration, temperature, and pressure-dependent kinetic measurements allow us to evaluate the kinetics for most of the cascade of reactions that are depicted in Scheme 1. present was taken as evidence that the metal, rather than the methyl ligand, was the kinetically preferred site of protonation (see chapter 1). 76,79 The lack of a clear-cut trend in the present system renders this question unresolved for the time being concerning the protonation of (Ar-DAB)PtPh 2 . The presence of both products might indicate that two reaction channels are available.…”

“…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.…”

“…Such a σ-bond metathesis process appears to nicely explain H/D exchanges between two phenyl groups, via a reasonably looking calculated transition-state structure. 102 (It might be pointed out that the mechanism is not necessarily involved in previously reported, related exchange processes; it appears to us less likely to be involved in exchanges between two methyl groups, 77,111 or between phenyl and methyl groups 60,83 below that for the oxidative cleavage pathway. If these computational results give a true description of the mechanism, then one obvious implication will be that the numerical value that is used for k OC is an upper limit to the real value: The EXSY kinetics represent the energetics for reaching the transition-state for the direct exchange process; the transition state for the oxidative cleavage must necessarily be higher in energy and thence the corresponding rate must be even slower -which strengthens the arguments that led to the conclusion in the previous paragraph: Pt is the kinetically preferred site of protonation.…”

Catalysis and Organometallic Chemistry of Monometallic Species

“…254 The complex [PtMe(H) 2 Tp] has also been shown to form a σ-CH 4 complex that is kinetically resistant to methane liberation 255 and the metal has been shown to be the kinetic site of protonation in diimine platinum() dimethyl complexes. 256 The application of asymmetric activation to an sp 3 hybridised C-H bond using platinum() complexes for the total synthesis of (Ϫ)-rhazinilam has also been elegantly demonstrated. 257 Theoretical work includes a DFT study on the mechanism of C-X (X = Sn,Ge, Si, C, H) activation in [M(PH 3 ) 2 ] (M = Ni, Pd, Pt) investigating the orientation of the substrate with respect to the MP 2 plane prior to oxidative addition.…”