Molecular Graphics and Computation Facility, College of Chemistry, University of California, Berkeley, California 94720, USA.
Inorg Chem. 2011 Dec 19;50(24):12836-43. doi: 10.1021/ic2020176. Epub 2011 Nov 21.
The hydrogenation of ketones catalyzed by 2,6-bis(diisopropylphosphinomethyl)pyridine (PNP)-ligated iron pincer complexes was studied using the range-separated and dispersion-corrected ωB97X-D functional in conjunction with the all-electron 6-31++G(d,p) basis set. A validated structural model in which the experimental isopropyl groups were replaced with methyl groups was employed for the computational study. Using this simplified model, the calculated total free energy barrier of a previously postulated mechanism with the insertion of ketone into the Fe-H bond is far too high to account for the observed catalytic reaction. Calculation results reveal that the solvent alcohol is not only a stabilizer of the dearomatized intermediate but also more importantly an assistant catalyst for the formation of trans-(PNP)Fe(H)(2)(CO), the actual catalyst for hydrogenation of ketones. A direct reduction mechanism, which features the solvent-assisted formation of a trans dihydride complex trans-(PNP)Fe(H)(2)(CO), direct transfer of hydride to acetophenone from trans-(PNP)Fe(H)(2)(CO) for the formation of a hydrido alkoxo complex, and direct H(2) cleavage by hydrido alkoxo without the participation of the pincer ligand for the regeneration of trans-(PNP)Fe(H)(2)(CO), was predicted.
用范围分离和色散校正的 ωB97X-D 函数与全电子 6-31++G(d,p)基组研究了 2,6-双（二异丙基膦基甲基）吡啶（PNP）配体铁钳状配合物催化的酮加氢反应。为了进行计算研究，采用了一个经过验证的结构模型，其中实验中的异丙基基团被甲基基团取代。使用这个简化的模型，计算出的先前假定的机制的总自由能垒，其中酮插入 Fe-H 键，高得离谱，无法解释观察到的催化反应。计算结果表明，溶剂醇不仅是去芳构化中间体的稳定剂，而且更重要的是，它是形成反式（PNP）Fe（H）（2）（CO）的助催化剂，反式（PNP）Fe（H）（2）（CO）是酮加氢的实际催化剂。预测了一种直接还原机制，其特征是溶剂辅助形成反式二氢化物配合物反式（PNP）Fe（H）（2）（CO），直接将氢化物从反式（PNP）Fe（H）（2）（CO）转移到苯乙酮，形成氢氧络合物，以及直接通过氢氧络合物裂解 H2，而不涉及钳状配体，从而再生反式（PNP）Fe（H）（2）（CO）。
