Tílvez Elkin, Cárdenas-Jirón Gloria I, Menéndez María I, López Ramón
Departamento de Química Física y Analítica, Facultad de Química, Universidad de Oviedo , C/Julián Clavería 8, 33006 Oviedo, Principado de Asturias, Spain.
Inorg Chem. 2015 Feb 16;54(4):1223-31. doi: 10.1021/ic501416u. Epub 2015 Jan 29.
A thoroughly mechanistic investigation on the Cp2Mo(OH)(OH2)-catalyzed hydrolysis of ethyl acetate has been performed using density functional theory methodology together with continuum and discrete-continuum solvation models. The use of explicit water molecules in the PCM-B3LYP/aug-cc-pVTZ (aug-cc-pVTZ-PP for Mo)//PCM-B3LYP/aug-cc-pVDZ (aug-cc-pVDZ-PP for Mo) computations is crucial to show that the intramolecular hydroxo ligand attack is the preferred mechanism in agreement with experimental suggestions. Besides, the most stable intermediate located along this mechanism is analogous to that experimentally reported for the norbornenyl acetate hydrolysis catalyzed by molybdocenes. The three most relevant steps are the formation and cleavage of the tetrahedral intermediate immediately formed after the hydroxo ligand attack and the acetic acid formation, with the second one being the rate-determining step with a Gibbs energy barrier of 36.7 kcal/mol. Among several functionals checked, B3LYP-D3 and M06 give the best agreement with experiment as the rate-determining Gibbs energy barrier obtained only differs 0.2 and 0.7 kcal/mol, respectively, from that derived from the experimental kinetic constant measured at 296.15 K. In both cases, the acetic acid elimination becomes now the rate-determining step of the overall process as it is 0.4 kcal/mol less stable than the tetrahedral intermediate cleavage. Apart from clarifying the identity of the cyclic intermediate and discarding the tetrahedral intermediate formation as the rate-determining step for the mechanism of the acetyl acetate hydrolysis catalyzed by molybdocenes, the small difference in the Gibbs energy barrier found between the acetic acid formation and the tetrahedral intermediate cleavage also uncovers that the rate-determining step could change when studying the reactivity of carboxylic esters other than ethyl acetate substrate specific toward molybdocenes or other transition metal complexes. Therefore, in general, the information reported here could be of interest in designing new catalysts and understanding the reaction mechanism of these and other metal-catalyzed hydrolysis reactions.
采用密度泛函理论方法以及连续介质和离散 - 连续介质溶剂化模型，对[Cp2Mo(OH)(OH2)]⁺催化乙酸乙酯水解反应进行了全面的机理研究。在PCM - B3LYP/aug - cc - pVTZ（Mo采用aug - cc - pVTZ - PP）//PCM - B3LYP/aug - cc - pVDZ（Mo采用aug - cc - pVDZ - PP）计算中使用明确的水分子对于表明分子内羟基配体攻击是与实验建议一致的首选机制至关重要。此外，沿着该机制定位的最稳定中间体类似于实验报道的钼茂催化乙酸降冰片烯酯水解的中间体。三个最相关的步骤是羟基配体攻击后立即形成的四面体中间体的形成和裂解以及乙酸的形成，其中第二个步骤是速率决定步骤，吉布斯能垒为36.7 kcal/mol。在检查的几种泛函中，B3LYP - D3和M06与实验结果的一致性最好，因为获得的速率决定吉布斯能垒分别仅比在296.15 K下测量的实验动力学常数得出的能垒相差0.2和0.7 kcal/mol。在这两种情况下，乙酸消除现在成为整个过程的速率决定步骤，因为它比四面体中间体裂解的稳定性低0.4 kcal/mol。除了阐明环状中间体的身份并排除四面体中间体形成作为钼茂催化乙酸乙酯水解机制的速率决定步骤外，乙酸形成和四面体中间体裂解之间发现的吉布斯能垒的微小差异还揭示了在研究除乙酸乙酯底物之外对钼茂或其他过渡金属配合物具有特异性的羧酸酯的反应性时，速率决定步骤可能会发生变化。因此，一般来说，这里报道的信息对于设计新催化剂以及理解这些和其他金属催化水解反应的反应机制可能是有意义的。
