Mechanisms of thermal decomposition of trans- and cis-dialkylbis-(tertiary phosphine) palladium(II). Reductive elimination and trans to cis isomerization.
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概要
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Series of <I>trans</I>- and <I>cis</I>-dialkylpalladium(II) complexes having tertiary phosphine ligands (L) of various basicities and bulkiness have been prepared and their thermolysis and isomerization mechanisms in solution have been studied. Examination of the cause of selective formation of <I>cis</I>-dialkyl isomers by using alkyllithium revealed a new type of <I>trans</I> to <I>cis</I> isomerization promoted by the alkyllithium. A process involving the formation of a trialkyl-palladate intermediate is proposed as a mechanism for the <I>trans</I> to <I>cis</I> isomerization. Evidence to support the mechanism has been obtained by experiments using LiCD<SUB>3</SUB>. Thermolysis of <I>cis</I>-PdR<SUB>2</SUB>L<SUB>2</SUB> has been demonstrated to proceed through a unimolecular process initiated by a rate-determining dissociation of L to produce a three-coordinate "<I>cis</I>-PdR<SUB>2</SUB>L" which reductively eliminates the R groups. Addition of free ligand to the system containing <I>cis</I>-PdMe<SUB>2</SUB>L<SUB>2</SUB> effectively blocks the reductive elimination pathway thus forcing the complex to be thermolyzed by a route involving liberation of methane. The second, novel type of <I>trans</I> to <I>cis</I> isomerization reaction proceeding <I>via</I> an intermolecular methyl transfer process has been discovered. As the crucial intermediate in the process a methyl-bridged complex formed between the partly dissociated three-coordinate species and undissociated complex has been postulated. Thermolysis of <I>trans</I>-PdMe<SUB>2</SUB>L<SUB>2</SUB> has been found to proceed <I>via</I> initial isomerization to the <I>cis</I> form followed by reductive elimination. The <I>trans</I>-<I>cis</I> isomerization equilibrium greatly favors the <I>cis</I> form for complexes having phenyl-substituted phosphines. For the PEt<SUB>3</SUB>-coordinated palladium dimethyl, however, an equilibrium <I>trans</I>/<I>cis</I> ratio of 1.2 is reached at 39 °C. Factors influencing the stability of the palladium alkyls having the tertiary phosphine ligand are discussed on the basis of the present results as well as comparison of the thermolysis behavior of <I>trans</I>-PdEt<SUB>2</SUB>L<SUB>2</SUB> and other transition metal alkyls. The presence of an energy barrier between the dissociated T-shaped intermediates <I>trans</I>-PdMe<SUB>2</SUB>L and <I>cis</I>-PdMe<SUB>2</SUB>L has been assumed. A unimolecular reductive elimination pathway proceeding from the T-shaped <I>cis</I>-PdMe<SUB>2</SUB>L intermediate through a Y-shaped transition state consistently accounts for the thermolysis as well as isomerization behavior of the <I>trans</I>-and <I>cis</I>-PdMe<SUB>2</SUB>L<SUB>2</SUB>.
- 公益社団法人 日本化学会の論文
著者
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Nakamura Yoshiyuki
Research Laboratory Of Resources Utilization Tokyo Institute Of Technology
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Ito Takashi
Research Institute for Nanodevice and Bio Systems, Hiroshima University, Higashihiroshima, Hiroshima 739-8527, Japan
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Yamamoto Akio
Research Laboratory of Resources Utilization, Tokyo Institute of Technology
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Ozawa Fumiyuki
Research Laboratory of Resources Utilization, Tokyo Institute of Technology
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