Synthesis, Characterization, and Equilibria of Palladium(II) Nitrile, Alkene, and Heterometallacyclopentane Complexes Involved in Metal Nitro Catalyzed Alkene Oxidation Reactions

Treatment of trans-Pd(CH₃CN)₂Cl₂with AgNO₂in acetonitrile gives cis-Pd(CH₃CN)₂ClNO₂(1) that partially dissociates in chloroform solution to give [Pd(CH₃CN)ClNO₂]₂(2). Alkenes react with 1 or 2 to give the alkene nitro complexes [Pd(alkene)ClNO₂]₂(3), the cis-cyclooctene member of which has been isolated and fully characterized. Complexes 3 subsequently undergo internal cyclization by nucleophilic attack of a nitro oxygen atom on the coordinated alkene to give heterometallacyclopentane (chelated alkyl nitrite) complexes [formula omitted] (4), in which the alkyl substituents (if any) are preferentially located on the carbon β to Pd. Representative examples of these metallacycles have been isolated, and complex derived from the substituted norbornene dicyclopentadiene has been characterized by a single-crystal X-ray structure determination. Pd₂Cl₂(C₁₀H₁₂NO₂)₂·CH₂Cl₂crystallizes in the space group Pbca with a = 24.717 (6) Å, b = 9.733 (2) Å, c = 20.655 (5) Å, V = 4969 (1) ų, and Z = 8 at 120 K. The final discrepancy indices were R₁(F_o) = 0.062 and R₂(F_o) = 0.077. The Pd-Cl bond distances trans to carbon (2.499 (4) and 2.491 (4) Å are longer than those trans to nitrogen (2.331 (4) and 2.328 (4) Å). The Pd-N distances (1.94 (1) and 1.95 (1) Å) are among the shortest reported to date. Complexes 1–4 are all in rapid (often NMR time scale) equilibrium with each other under the reaction conditions. (Similar behavior is observed for the previously known dichloro analogues of complexes 1–3.) Although quantitative data regarding these equilibria are not obtainable, qualitative kinetic and thermodynamic observations are presented that are relevant to the extensive Pd(II)-mediated chemistry of alkenes. Metallacycles 4 are also involved in bridge cleavage and possibly ring-opening equilibria.