Mechanisms of Thermal Decomposition of trans -Chloroneopentylbis (tricyclopentylphosphine) platinum (II)

The most probable mechanism for the thermal decomposition of trans-chloroneopentylbis(tricyclopentylphosphine)platinum(II) (L₂PtNpCl, 1) to trans-chlorohydridobis(tricyclopentylphosphine)platinum(II) (L₂PtHCl, 5) and 1,1-dimethylcyclopropane (DMC) in cyclohexane solution involves initial equilibrium dissociation of tricyclopentylphosphine (L) from 1, reversible intramolecular oxidative addition of a y-C-H bond of the neopentyl moiety to platinum and formation of an intermediate platinacyclobutane, 3, and rate-limiting reductive elimination of 1,1-dimethylcyclopropane (Scheme I). Reassociation of L yields 5. In the absence of added L, the reaction is half-order in 1; with added L (0.068-0.54 M), the reaction is first-order in 1 and inverse first-order in L. Arrhenius activation parameters A₁and E_a,1 for the first-order decomposition of 1 to 5 and dimethylcyclopropane were determined in cyclohexane containing L ([L]₀= 0.39 M): E_a,1= 49 ± 1 kcal/mol; log A₁ = 21 ± 2; ΔH^‡ (156 °C) = 48 ± 1 kcal/mol; AS* (156 °C) = 34 ± 1 eu. Substitution of deuterium for hydrogen in the neopentyl group of 1 results in a deuterium kinetic isotope effect of k_H/k_D = 2.9. The dependence of the rate of reaction on the concentration of 1 allows estimation of the dissociation constant K_eqof L from 1; K_eq(156 °C) = 0.017 ± 0.002 M. From the Arrhenius parameters in the presence and absence of L, the temperature dependence of K_eq can be determined: ΔS_eq= 30 ± 5 eu and ΔH_eq= 16 ± 3 kcal/mol. A side reaction (3-20%) in the decomposition of 1 produces neopentane and appears to proceed through free neopentyl radicals. trans-Chloro(l-norbomylmethyl)-bis(tricyclopentylphosphine)platinum(II) (L₂Pt(CH₂Nb)Cl, 6) decomposes thermally at 156 °C and gives 1-methylnorbornane, L₂PtHCl (5), and a product, 7, derived from cyclometalation of the phosphine L.