Mechanisms of Thermal Decomposition of trans- and cis-Dialkylbis(tertiary phosphine)palladium(II). Reductive Elimination and trans to cis Isomerization

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Abstract
Series of trans- and cis-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 cis-dialkyl isomers by using alkyllithium revealed a new type of trans to cis isomerization promoted by the alkyllithium. A process involving the formation of a trialkyl-palladate intermediate is proposed as a mechanism for the trans to cis isomerization. Evidence to support the mechanism has been obtained by experiments using LiCD3. Thermolysis of cis-PdR2L2 has been demonstrated to proceed through a unimolecular process initiated by a rate-determining dissociation of L to produce a three-coordinate “cis-PdR2L” which reductively eliminates the R groups. Addition of free ligand to the system containing cis-PdMe2L2 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 trans to cis isomerization reaction proceeding via 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 trans-PdMe2L2 has been found to proceed via initial isomerization to the cis form followed by reductive elimination. The trans-cis isomerization equilibrium greatly favors the cis form for complexes having phenyl-substituted phosphines. For the PEt3-coordinated palladium dimethyl, however, an equilibrium trans/cis 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 trans-PdEt2L2 and other transition metal alkyls. The presence of an energy barrier between the dissociated T-shaped intermediates trans-PdMe2L and cis-PdMe2L has been assumed. A unimolecular reductive elimination pathway proceeding from the T-shaped cis-PdMe2L intermediate through a Y-shaped transition state consistently accounts for the thermolysis as well as isomerization behavior of the trans-and cis-PdMe2L2.

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