Jensen Michael P, Wick Douglas D, Reinartz Stefan, White Peter S, Templeton Joseph L, Goldberg Karen I
Department of Chemistry, Box 351700, University of Washington, Seattle, Washington 98195-1700, USA.
J Am Chem Soc. 2003 Jul 16;125(28):8614-24. doi: 10.1021/ja028477t.
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.
在κ(3)-Tp(Me)2Pt(IV)(CH(3))(2)H（1，Tp(Me)2 = 氢三(3,5 - 二甲基吡唑基)硼酸酯）的溶液热解过程中发生甲烷的还原消除反应。该反应的铂产物由溶剂决定。在苯-d(6)中甲烷消除后发生C-D键活化，生成κ(3)-Tp(Me)2Pt(IV)(CH(3))(C(6)D(5))D（2-d(6)），其经历第二次还原消除/氧化加成反应生成同位素标记的甲烷和κ(3)-Tp(Me)2Pt(IV)(C(6)D(5))(2)D（3-d(11)）。相比之下，在氘代乙腈存在下，由1消除甲烷后得到κ(2)-Tp(Me)2Pt(II)(CH(3))(NCCD(3))（4）。这些Pt(IV)配合物的甲烷还原消除遵循一级动力学，且观察到的反应速率几乎与溶剂无关。在苯-d(6)和甲苯-d(8)中对1的甲烷还原消除测量得到几乎相同的活化参数（ΔH(++) (obs) = 35.0 ± 1.1 kcal/mol，ΔS(++) (obs) = 13 ± 3 eu）。一个能量较低的过程（ΔH(++) (scr) = 26 ± 1 kcal/mol，ΔS(++) (scr) = 1 ± 4 eu）使1的氢原子在甲基和氢化物位置之间发生交换，通过监测κ(3)-Tp(Me)()2Pt(IV)(CH(3))(2)D（1-d(1)()）与其交换后的同位素异构体κ(3)-Tp(Me)2Pt(IV)(CH(3))(CH(2)D)H（1-d(1')）的平衡得以证实。σ-甲烷配合物κ(2)-Tp(Me)2Pt(II)(CH(3))(CH(4))被认为是交换和还原消除过程中的共同中间体。动力学结果与该中间体甲烷的速率决定解离损失一致，从而产生配位不饱和中间体[Tp(Me)2Pt(II)(CH(3))]，其与溶剂快速反应。H/D交换和C-H还原消除的活化焓之差为甲烷与[Tp(Me)2Pt(II)(CH(3))]的结合焓提供了9 ± 2 kcal/mol的下限。
