Roy Lisa, Ghosh Boyli, Paul Ankan
Raman Centre for Atomic Molecular and Optical Sciences, Indian Association for the Cultivation of Science , 2A and 2B, Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India.
J Phys Chem A. 2017 Jul 13;121(27):5204-5216. doi: 10.1021/acs.jpca.7b03843. Epub 2017 Jun 29.
We employ quantum chemical calculations to study the hydrogenation of carbon dioxide by amine boranes, NMeBH (AB) and NHBH (AB) weakly bonded to a bulkier Lewis acid, Al(CF) (LA). Additionally, computations have also been conducted to elucidate the mechanism of hydrogenation of carbon dioxide by AB while captured between one Lewis base (P(o-tol), LB) and two Lewis acids, Al(CF). In agreement with the experiments, our computational study predicts that hydride transfer to conjugated HCO, generated in the reaction of AB-LA with CO, is not feasible. This is in contrast to the potential hydrogenation of bound HCOH, developed in the reduction of CO with AB-LA, to further reduced species like HC(OH). However, the FLP-trapped CO effortlessly undergoes three hydride (H) transfers from AB to produce a CHO derivative. DFT calculations reveal that the preference for a H abstraction by an intrinsically anionic formate moiety is specifically dependent on the electrophilicity of the 2 e reduced carbon center, which in particular is controlled by the electron-withdrawing capability of the associated substituents on the oxygen. These theoretical predictions are justified by frontier molecular orbitals and molecular electrostatic potential plots. The global electrophicility index, which is a balance of electron affinity and hardness, reveals that the electrophilicity of the formate species undergoing hydrogenation is twice the electrophilicity of the ones where hydrogenation is not feasible. The computed activation energies at M06-2X/6-31++G(d,p) closely predict the observed reactivity. In addition, the possibility of a dissociative channel of the frustrated Lewis pair trapped CO system has been ruled out on the basis of predominantly high endergonicity. Knowledge of the underlying principle of these reactions would be helpful in recruiting appropriate Lewis acids/amine boranes for effective reduction of CO and its hydrogenated forms in a catalytic fashion.
我们采用量子化学计算方法，研究了与体积更大的路易斯酸Al(CF)（LA）弱键合的胺硼烷NMeBH（AB）和NHBH（AB）对二氧化碳的氢化反应。此外，还进行了计算以阐明AB在被一种路易斯碱（P(o-tol)，LB）和两种路易斯酸Al(CF)捕获时对二氧化碳的氢化机理。与实验结果一致，我们的计算研究预测，氢化物向AB-LA与CO反应生成的共轭HCO转移是不可行的。这与AB-LA还原CO生成的结合态HCOH进一步氢化为HC(OH)等还原产物的潜在氢化反应形成对比。然而，FLP捕获的CO轻松地经历了来自AB的三次氢化物（H）转移，生成了一种CHO衍生物。DFT计算表明，本质上呈阴离子形式的甲酸根部分对H的夺取偏好特别取决于2e还原碳中心的亲电性，而这尤其由氧上相关取代基的吸电子能力控制。这些理论预测通过前线分子轨道和分子静电势图得到了验证。全局亲电性指数是电子亲和性和硬度的平衡，它表明发生氢化反应的甲酸根物种的亲电性是氢化不可行的甲酸根物种亲电性的两倍。在M06-2X/6-31++G(d,p)水平下计算得到的活化能准确地预测了观察到的反应活性。此外，基于主要的高吸能性，排除了受阻路易斯对捕获的CO体系发生解离通道的可能性。了解这些反应的潜在原理将有助于选择合适的路易斯酸/胺硼烷，以催化方式有效还原CO及其氢化形式。
