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

Article abstract of DOI:10.1021/ja00332a027

Treatment of trans-Pd(CH3CN)2Cl2 with AgNO2 in acetonitrile gives cis-Pd(CH3CN)2ClNO2 (1) that partially dissociates in chloroform solution to give 2 (2).Alkenes react with 1 or 2 to give the alkene nitro complexes 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 (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 4g derived from the substituted norbornene dicyclopentadiene has been characterized by a single-crystal X-ray structure determination.Pd2Cl2(C10H12NO2)2*CH2Cl2 crystallizes in the space group Pbca with a=24.717(6) Angstroem, b=9.733(2) Angstroem, c=20.655(5) Angstroem, V=4969(1) Angstroem³, 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) Angstroem) are longer than those trans to nitrogen (2.331(4) and 2.328(4) Angstroem).The Pd-N distances (1.94(1) and 1.95(1) Angstroem) 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.