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

This Article describes the preparation and isolation of novel octahedral CH2-bridged bis-(N-heterocyclic carbene)palladium(IV) tetrachlorides of the general formula (LPdCl4)-Cl-IV [L = (NHC)CH2(NHC)] from (LPdCl2)-Cl-II and Cl-2. In intermolecular, nonchelation-controlled transformations (LPdCl4)-Cl-IV 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 Pd-IV 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 Pd-IV-Cl ionization in the first step. Product stereochemistry and product distributions from various alkenes also support Cl+-transfer from the pentacoordinated Pd-IV-intermediate (LPdCl3+)-Cl-IV to olefins. 1-Hexene/3-hexene competition experiments rule out both the formation of pi-complexes along the reaction coordinate as well as in situ generated Cl-2 from a reductive elimination process. Instead, a ligand-mediated direct Cl+-transfer from (LPdCl3+)-Cl-IV to the pi-system is likely to occur. Similarly, C-H bond chlorinations proceed via an electrophilic process with in situ formed (LPdCl3+)-Cl-IV. The presence of a large excess of added Cl- slows cyclohexene chlorination while the presence of stoichiometric amounts of chloride accelerates both Pd-IV-Cl ionization and Cl+-transfer from (LPdCl3+)-Cl-IV. H-1 NMR titrations, T1 relaxation time measurements, binding isotherms, and Job plot analysis point to the formation of a trifurcated Cl-center dot center dot center dot H-C bond in the NHC-ligand periphery as a supramolecular cause for the accelerated chemical events involving the metal center.