Computational Studies Explain the Importance of Two Different Substituents on the Chelating Bis(amido) Ligand for Transfer Hydrogenation by Bifunctional Cp*Rh(III) Catalysts

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• 作者：Ainara Nova ; David J. Taylor ; A. John Blacker ; Simon B. Duckett ; Robin N. Perutz ; Odile Eisenstein
• 刊名：Organometallics
• 出版年：2014
• 出版时间：July 14, 2014
• 年：2014
• 卷：33
• 期：13
• 页码：3433-3442
• 全文大小：515K
• ISSN：1520-6041

文摘

A computational approach (DFT-B3PW91) is used to address previous experimental studies (Chem. Commun. <b>2009b>, 6801) that showed that transfer hydrogenation of a cyclic imine by Etb>3b>N路HCOb>2b>H in dichloromethane catalyzed by 16-electron bifunctional Cp*Rh^III(XNCb>6b>Hb>4b>NX鈥? is faster when XNCb>6b>Hb>4b>NX鈥?= TsNCb>6b>Hb>4b>NH than when XNCb>6b>Hb>4b>NX鈥?= HNCb>6b>Hb>4b>NH or TsNCb>6b>Hb>4b>NTs (Cp* = 畏⁵-Cb>5b>Meb>5b>, Ts = toluenesulfonyl). The computational study also considers the role of the formate complex observed experimentally at low temperature. Using a model of the experimental complex in which Cp* is replaced by Cp and Ts by benzenesulfonyl (Bs), the calculations for the systems in gas phase reveal that dehydrogenation of formic acid generates CpRh^IIIH(XNCb>6b>Hb>4b>NX鈥睭) via an outer-sphere mechanism. The 16-electron Rh complex + formic acid are shown to be at equilibrium with the formate complex, but the latter lies outside the pathway for dehydrogenation. The calculations reproduce the experimental observation that the transfer hydrogenation reaction is fastest for the nonsymmetrically substituted complex CpRh^III(XNCb>6b>Hb>4b>NX鈥? (X = Bs and X鈥?= H). The effect of the linker between the two N atoms on the pathway is also considered. The Gibbs energy barrier for dehydrogenation of formic acid is calculated to be much lower for CpRh^III(XNCHPhCHPhNX鈥? than for CpRh^III(XNCb>6b>Hb>4b>NX鈥? for all combinations of X and X鈥? The energy barrier for hydrogenation of the imine by the rhodium hydride complex is much higher than the barrier for hydride transfer to the corresponding iminium ion, in agreement with mechanisms proposed for related systems on the basis of experimental data. Interpretation of the results by MO and NBO analyses shows that the most reactive catalyst for dehydrogenation of formic acid contains a localized Rh鈥揘H 蟺-bond that is associated with the shortest Rh鈥揘 distance in the corresponding 16-electron complex. The asymmetric complex CpRh^III(BsNCb>6b>Hb>4b>NH) is shown to generate a good bifunctional catalyst for transfer hydrogenation because it combines an electrophilic metal center and a nucleophilic NH group.