Direct versus Water-Mediated Protodecarboxylation of Acetic Acid Catalyzed by Group 10 Carboxylates, [(phen)M(O<sub>2</sub>CCH<sub>3</sub>)]<sup>+</sup>
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The gas-phase protodecarboxylation of acetic acid catalyzed by group 10 metal complexes was examined using a combination of multistage mass spectrometry experiments in an ion trap mass spectrometer, DFT calculations, and theoretical kinetic modeling. Two related catalytic cycles sharing two common intermediates were examined. The entry points to both cycles are the metal acetate complexes [(phen)M(O2CCH3)]+ (where phen = 1,10-phenanthroline), which were formed via direct electrospray ionization of solutions of the complexes [(phen)M(O2CCH3)2] in water. Step 1 of both cycles involves decarboxylation of [(phen)M(O2CCH3)]+ under collision-induced dissociation (CID) conditions to form the organometallic species [(phen)M(CH3)]+. The ease of decarboxylation follows the order Pd > Pt > Ni as determined via energy-resolved CID experiments, which is in agreement with the activation energies for decarboxylation estimated from DFT calculations. Step 2 of cycle 1 involves an ion–molecule reaction between [(phen)M(CH3)]+ and acetic acid to close the cycle by regenerating the metal acetate complex [(phen)M(O2CCH3)]+. DFT calculations reveal that an acid–base acetolysis mechanism is favored over an oxidative addition/reductive elimination mechanism proceeding via the M(IV) intermediate [(phen)M(CH3)(H)(O2CCH3)]+. In contrast, step 2 of cycle 2 involves [(phen)M(CH3)]+ reacting with water to form the hydroxide [(phen)M(OH)]+, which subsequently reacts with acetic acid in step 3 to re-form [(phen)M(O2CCH3)]+ and water, thereby completing the catalytic cycle. Experiment and theory reveal that cycle 2 operates only for M = Ni.
