Reductive Elimination/Oxidative Addition of Carbon−Hydrogen Bonds at Pt(IV)/Pt(II) Centers:  Mechanistic Studies of the Solution Thermolyses of Tp^Me₂Pt(CH₃)₂H

Abstract

Reductive elimination of methane occurs upon solution thermolysis of kappa(3)-Tp(Me)2Pt(IV)(CH(3))(2)H (1, Tp(Me)2 = hydridotris(3,5-dimethylpyrazolyl)borate). The platinum product of this reaction is determined by the solvent. C-D bond activation occurs after methane elimination in benzene-d(6), to yield kappa(3)-Tp(Me)2Pt(IV)(CH(3))(C(6)D(5))D (2-d(6)), which undergoes a second reductive elimination/oxidative addition reaction to yield isotopically labeled methane and kappa(3)-Tp(Me)2Pt(IV)(C(6)D(5))(2)D (3-d(11)). In contrast, kappa(2)-Tp(Me)2Pt(II)(CH(3))(NCCD(3)) (4) was obtained in the presence of acetonitrile-d(3), after elimination of methane from 1. Reductive elimination of methane from these Pt(IV) complexes follows first-order kinetics, and the observed reaction rates are nearly independent of solvent. Virtually identical activation parameters (DeltaH(++)(obs) = 35.0 +/- 1.1 kcal/mol, DeltaS(++)(obs) = 13 +/- 3 eu) were measured for the reductive elimination of methane from 1 in both benzene-d(6) and toluene-d(8). A lower energy process (DeltaH(++)(scr) = 26 +/- 1 kcal/mol, DeltaS(++)(scr) = 1 +/- 4 eu) scrambles hydrogen atoms of 1 between the methyl and hydride positions, as confirmed by monitoring the equilibration of kappa(3)-Tp(Me)()2Pt(IV)(CH(3))(2)D (1-d(1)()) with its scrambled isotopomer, kappa(3)-Tp(Me)2Pt(IV)(CH(3))(CH(2)D)H (1-d(1'). The sigma-methane complex kappa(2)-Tp(Me)2Pt(II)(CH(3))(CH(4)) is proposed as a common intermediate in both the scrambling and reductive elimination processes. Kinetic results are consistent with rate-determining dissociative loss of methane from this intermediate to produce the coordinatively unsaturated intermediate [Tp(Me)2Pt(II)(CH(3))], which reacts rapidly with solvent. The difference in activation enthalpies for the H/D scrambling and C-H reductive elimination provides a lower limit for the binding enthalpy of methane to [Tp(Me)2Pt(II)(CH(3))] of 9 +/- 2 kcal/mol.

Hydrocarbon C—H Bond Activation with Tp'Pt Complexes

“…Thermolysis of complex 1 in benzene (or benzene-ife) leads to complete conversion to Tp'PtPhzH (5) {or Tp'Pt(C 6 D6)2D (5-rf)} in a few hours (38). Similar results were observed in seconds at room temperature with the addition of substoichiometric amounts of B(C6Fs)3 as a catalyst.…”

“…Similar σ-alkane intermediates have been proposed to explain isotopic scrambling in late metal alkyl hydride complexes (20,28,(39)(40)(41)(42)(43)(44)(45)(46)(47)(48)(49)(50)(51). To further support the presence of a σ-methane intermediate, thermolysis of Tp'PtMe2D (1-rf) in toluene-tf* at temperatures below those required for methane loss was monitored for several hours (38). As expected, deuterium incorporation to form the stronger C-D bond and appearance of a Pt-H *H NMR signal was observed.…”

“…Thermolysis of the dimethyl hydride complex 1 in QDÔ induces reductive elimination of methane and activation of solvent to give Tp'Pt(CH3)(C6D 5 )D, which then reductively eliminates CH 3 D and activates a second solvent molecule to give the final Tp'PtiQDs^D (5-rf) product (38). The presence of methane isotopomers (CH3D, CH2D2, and CHD3) was evident in the *H NMR spectrum, and these were attributed to isotopic scrambling through a putative σ-bound methane intermediate, Pt n (CH 3 -H).…”

The σ‐CAM Mechanism: σ Complexes as the Basis of σ‐Bond Metathesis at Late‐Transition‐Metal Centers

“…Early examples are summarized by Crabtree, Hall, and Perutz as well as by Shilov and Shulpin. [3,4,14] + (tacn = triazacyclononane), [78] [Tp*RhH-(CH 2 R) L] (Tp* = tris(3,5-dimethylpyrazolyl)borate), [12,79] [{H 2 Si(C 5 H 4 ) 2 }WH(Me)], [80] [Tp*PtH(Me) 2 ], [81] and related platinum complexes. [13,40] Two classes of Pt-catalyzed alkane-activation reactions have received particular attention.…”

The Gas‐Phase Chemistry of cis‐Diammineplatinum(II) Complexes: A Joint Experimental and Theoretical Study

“…Jensen et al [29] theoretically examined CH activation for the example of alkane activation involving Pt II and Pt…”