Protonolysis of Dialkyl- and Alkylarylplatinum(II) Complexes and Geometrical Isomerization of the Derived Monoorgano-Solvento Complexes: Clear-Cut Examples of Associative and Dissociative Pathways in
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- 作者：Raffaello Romeo ; Maria Rosaria Plutino ; and Lars I. Elding
- 刊名：Inorganic Chemistry
- 出版年：1997
- 出版时间：December 3, 1997
- 年：1997
- 卷：36
- 期：25
- 页码：5909 - 5916
- 全文大小：212K
- 年卷期：v.36,no.25(December 3, 1997)
- ISSN：1520-510X
文摘
Protonolysis of the complexescis-[PtR2(PEt3)2](R = Me, Et, Prn, Bun,CH2C(Me)3,CH2Si(Me)3) andcis-[Pt(R)(R')(PEt3)2] (R = Ph,2-MeC6H4,2,4,6-Me3C6H2; R' = Me) inmethanol selectively cleaves one alkyl group,yieldingcis-[Pt(R)(PEt3)2(MeOH)]+and alkanes. The reactions occur as single-stage conversions fromthe substrateto the product. There is no evidence by UV and by low-temperature1H and 31P NMR spectroscopy forthepresence of significant amounts of Pt(II) or Pt(IV)intermediate species. Reactions are first order with respecttocomplex and proton concentrations and are strongly retarded by stericcongestion at the Pt-C bond, varyingfrom k2 = (2.65 ± 0.08) × 105M-1 s-1 for R =R' = Et to k2 = 9.80 ± 0.44M-1s-1 for R =R'= CH2Si(Me)3.Low enthalpies of activation and largely negative volumes ofactivation are associated with the process. Themechanism involves a rate-determining proton transfer either to themetal-carbon 
bond (SE2 mechanism) or tothe metal center (SE(oxidative) mechanism), followedby fast extrusion of the alkane and simultaneous blockingof the vacant coordination site by the solvent to generatecis-[Pt(R)(PEt3)2(MeOH)]+species. The subsequentslower process, cis to trans isomerization ofcis-[Pt(R)(PEt3)2(MeOH)]+,is characterized by high values of enthalpiesof activation, positive entropies of activation, and largely positivevolumes of activation. The reaction is shownto proceed through the dissociative loss of the weakly bonded moleculeof solvent and the interconversion of twogeometrically distinct T-shaped 14-electron 3-coordinate intermediates.The presence of 
-hydrogens on theresidual alkyl chain produces a great acceleration of the rate (R =Me, ki = 0.0026 s-1;R = Et, ki = 44.9s-1)as a consequence of the stabilization of the 3-coordinate[Pt(R)(PEt3)2]+transition state through an incipientagostic interaction. The results of this work, together with thoseof a previous paper, give a rationale of the"elusive" nature of these compounds. The following factorsconcur: (i) electron release by the phosphine ligands,(ii) steric repulsion and distortion of the square-planarconfiguration, and (iii) interaction of the metal with-hydrogens.
bond (SE2 mechanism) or tothe metal center (SE(oxidative) mechanism), followedby fast extrusion of the alkane and simultaneous blockingof the vacant coordination site by the solvent to generatecis-[Pt(R)(PEt3)2(MeOH)]+species. The subsequentslower process, cis to trans isomerization ofcis-[Pt(R)(PEt3)2(MeOH)]+,is characterized by high values of enthalpiesof activation, positive entropies of activation, and largely positivevolumes of activation. The reaction is shownto proceed through the dissociative loss of the weakly bonded moleculeof solvent and the interconversion of twogeometrically distinct T-shaped 14-electron 3-coordinate intermediates.The presence of -hydrogens on theresidual alkyl chain produces a great acceleration of the rate (R =Me, ki = 0.0026 s-1;R = Et, ki = 44.9s-1)as a consequence of the stabilization of the 3-coordinate[Pt(R)(PEt3)2]+transition state through an incipientagostic interaction. The results of this work, together with thoseof a previous paper, give a rationale of the"elusive" nature of these compounds. The following factorsconcur: (i) electron release by the phosphine ligands,(ii) steric repulsion and distortion of the square-planarconfiguration, and (iii) interaction of the metal with-hydrogens.