Addition of excess of NN-dimethylbenzylamine to a chloroform solution of [ Pd,(O,CMe),] leads to instantaneous depolymerisation of the trimer giving the monomer trans-[Pd(O,CMe),( PhCH,NMe,),].Reversible dissociation of the amine from the latter affords a pseudothree-co-ordinate 14-electron intermediate [ Pd(O,CMe),( PhCH,NMe,)] which undergoes subsequent rate-limiting ortho-palladation to form the cyclopalladated acetato-bridged dimerThe rate-limiting step is electrophilic in character; the slope of the corresponding Hammett plot for differently ring-substituted NN-dimethylbenzylamines is -1.6.The kinetic isotope effect, k,/k,, for PhCH,NMe, is 2.2 f 0.2. The activation entropy for the ratelimiting step is very negative, ca. -250 J K-' mol-' for PhCH,NMe,, suggesting a highly ordered tight transition state. This dissociative path is a factor of ca. 100 faster than a parallel one, involving the 16-electron monomer trans-[Pd(O,CMe),( PhCH,NMe,),] but without loss of the amine. Ortho-palladation is not rate-limiting in acetic acid solvent, where slow cleavage of acetate bridges in polynuclear palladium species occurs, affording a vacant co-ordination site for subsequent rapid ortho-palladation. A comparison of intra-and inter-molecular activations of carbon-hydrogen bonds in arenes by palladium(ii) in terms of 'effective molarities' shows that the ratio klntra/klnter is not less than 3.6 x 1 O2 mot drn-,.
The pseudo-first-order rate constants k obs for the reaction between the ring-substituted orthoruthenated N,N-dimethylbenzylamines [RuII(η6-C6H6)(o-C6H3RCH2NMe2)Cl] (R = 4,5-(MeO)2, 5-Me, H, 5-F), and alkynes R‘C⋮CR‘‘ (R‘/R‘‘ = Ph/Ph, Ph/C6H4CF3-3, Ph/C6H4NO2-4, Et/Et, CO2Me/CO2Me) to afford the isoquinolinium cations coordinated to the [Ru0(η6-C6H6)] moiety are a linear function of [R‘C⋮CR‘‘] in methanol at 20.0−40.0 °C in accord with the rate-determining insertion of alkyne into the Ru−C bond. The latter was confirmed by the observation that the electron-donating groups R at the dimethylbenzylamine ruthenocycle favor the insertion and the slope of the corresponding Hammett plot equals −1.6 against σ+. The electron-poor alkynes react slower than the electron-rich ones. An X-ray structural investigation of the product of the MeO2CC⋮CCO2Me reaction with the related ruthenocycle [(3-MeC5H3N-2-C6H4)Ru(μ-I)(CO)2]2 demonstrates that the alkyne inserts into the Ru−C bond. Addition of LiCl retards the insertion markedly indicative of the importance of the ionization of the coordinated chloride. The major reaction pathway involves the solvento species [RuII(η6-C6H6)(o-C6H3RCH2NMe2)(MeOH)]+, the existence of which was confirmed by the spectrophotometric study of the starting compound in the presence of LiCl. All the observations reported show that the d6 system under study is very mechanistically similar to the insertion of alkynes into the dinuclear d8 PdII complexes [Pd(o-C6H3RCH2NMe2)(μ-Cl)]2, the key difference being the ways of creation of a coordinative site readily accessible by alkyne. These are the ionization of the Ru−Cl bond and the cleavage of the [Pd(μ-Cl)2Pd] bridge in the ruthenium and palladium cases, respectively.
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