Direct versus Water-Mediated Protodecarboxylation of Acetic Acid Catalyzed by Group 10 Carboxylates, [(phen)M(O2CCH3)]+

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.

本研究结合离子阱质谱仪中的多级质谱实验、密度泛函理论（DFT, Density Functional Theory）计算与理论动力学建模，对第10族金属配合物催化的乙酸气相质子脱羧反应进行了考察。本研究考察了两个共享两种共同中间体的相关催化循环。两个循环的起始物种均为乙酸金属配合物[(phen)­M­(O₂CCH₃)]⁺（其中phen代表1,10-邻菲啰啉），该配合物通过对水溶液中的配合物[(phen)­M­(O₂CCH₃)₂]进行直接电喷雾电离制得。两个循环的步骤1均为在碰撞诱导解离（CID, Collision-Induced Dissociation）条件下，[(phen)­M­(O₂CCH₃)]⁺发生脱羧反应，生成有机金属物种[(phen)­M­(CH₃)]⁺。通过能量分辨CID实验测得的脱羧反应难易顺序为Pd > Pt > Ni，该结果与DFT计算得到的脱羧活化能估算值一致。循环1的步骤2为[(phen)­M­(CH₃)]⁺与乙酸发生离子-分子反应，通过再生乙酸金属配合物[(phen)­M­(O₂CCH₃)]⁺完成催化循环。DFT计算结果表明，酸碱乙酰解机制相较于经由M(IV)中间体[(phen)­M­(CH₃)­(H)­(O₂CCH₃)]⁺的氧化加成/还原消除机制更具优势。与之相反，循环2的步骤2为[(phen)­M­(CH₃)]⁺与水反应生成氢氧化物配合物[(phen)­M­(OH)]⁺；随后在步骤3中，该氢氧化物配合物与乙酸反应重新生成[(phen)­M­(O₂CCH₃)]⁺与水，从而完成整个催化循环。实验与理论研究结果均表明，循环2仅适用于M=Ni的体系。