Bis-<i>N</i>-heterocyclic Carbene Palladium(IV) Tetrachloride Complexes: Synthesis, Reactivity, and Mechanisms of Direct Chlorinations and Oxidations of Organic Substrates

Abstract

This Article describes the preparation and isolation of novel octahedral CH2-bridged bis-(N-heterocyclic carbene)palladium(IV) tetrachlorides of the general formula LPdIVCl4 [L = (NHC)CH2(NHC)] from LPdIICl2 and Cl2. In intermolecular, nonchelation-controlled transformations LPdIVCl4 reacted with alkenes and alkynes to 1,2-dichlorination adducts. Aromatic, benzylic, and aliphatic C−H bonds were converted into C−Cl bonds. Detailed mechanistic investigations in the dichlorinations of alkenes were conducted on the 18VE PdIV complex. Positive solvent effects as well as kinetic measurements probing the impact of cyclohexene and chloride concentrations on the rate of alkene chlorination support a PdIV−Cl ionization in the first step. Product stereochemistry and product distributions from various alkenes also support Cl+-transfer from the pentacoordinated PdIV-intermediate LPdIVCl3+ to olefins. 1-Hexene/3-hexene competition experiments rule out both the formation of π-complexes along the reaction coordinate as well as in situ generated Cl2 from a reductive elimination process. Instead, a ligand-mediated direct Cl+-transfer from LPdIVCl3+ to the π-system is likely to occur. Similarly, C−H bond chlorinations proceed via an electrophilic process with in situ formed LPdIVCl3+. The presence of a large excess of added Cl− slows cyclohexene chlorination while the presence of stoichiometric amounts of chloride accelerates both PdIV−Cl ionization and Cl+-transfer from LPdIVCl3+. 1H NMR titrations, T1 relaxation time measurements, binding isotherms, and Job plot analysis point to the formation of a trifurcated Cl−···H−C bond in the NHC-ligand periphery as a supramolecular cause for the accelerated chemical events involving the metal center