| Autor: |
Kim RS; Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States., Wegener EC; Davidson School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive, West Lafayette, Indiana 47907, United States., Yang MC; Department of Chemistry and Biochemistry, University of Oregon, Eugene, Oregon 97403, United States., O'Reilly ME; Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States., Oh S; Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States., Hendon CH; Department of Chemistry and Biochemistry, University of Oregon, Eugene, Oregon 97403, United States., Miller JT; Davidson School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive, West Lafayette, Indiana 47907, United States., Surendranath Y; Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States. |
| Abstrakt: |
High-valent Pd complexes are potent agents for the oxidative functionalization of inert C-H bonds, and it was previously shown that rapid electrocatalytic methane monofunctionalization could be achieved by electro-oxidation of Pd II to a critical dinuclear Pd III intermediate in concentrated or fuming sulfuric acid. However, the structure of this highly reactive, unisolable intermediate, as well as the structural basis for its mechanism of electrochemical formation, remained elusive. Herein, we use X-ray absorption and Raman spectroscopies to assemble a structural model of the potent methane-activating intermediate as a Pd III dimer with a Pd-Pd bond and a 5-fold O atom coordination by H x SO 4 (x-2) ligands at each Pd center. We further use EPR spectroscopy to identify a mixed-valent M-M bonded Pd 2 II,III species as a key intermediate during the Pd II -to-Pd III 2 oxidation. Combining EPR and electrochemical data, we quantify the free energy of Pd dimerization as <-4.5 kcal/mol for Pd 2 II,III and <-9.1 kcal/mol for Pd III 2 . The structural and thermochemical data suggest that the aggregate effect of metal-metal and axial metal-ligand bond formation drives the critical Pd dimerization reaction in between electrochemical oxidation steps. This work establishes a structural basis for the facile electrochemical oxidation of Pd II to a M-M bonded Pd III dimer and provides a foundation for understanding its rapid methane functionalization reactivity. |
