Zhang Huaiyu, Cao Zexing, Wu Wei, Mo Yirong
Institute of Computational Quantum Chemistry, College of Chemistry and Material Science, Hebei Normal University, Shijiazhuang, China.
Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry and State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, iChEM, Xiamen University, Xiamen, 361005, China.
Angew Chem Int Ed Engl. 2018 Oct 1;57(40):13076-13081. doi: 10.1002/anie.201805952. Epub 2018 Aug 31.
It is a current trend to explore multi-bonded and unsaturated main group compounds that can interact with small molecules, in order to find non-metal catalysts. Notably, Braunschweig et al. found that diboryne stabilized by N-heterocyclic carbenes (NHCs) can bind and activate CO. Here we explore the bonding nature of B (NHC) and its activation mechanism for CO from a novel theoretical perspective. While the ground state of B is of a single bond, the approach of NHCs excites B to its third excited state of a triple bond with two significant σ-holes at the two ends. The subsequent electrostatic attraction drives the formation of B (NHC) . However, only one of the two π bonds (HOMOs) of B (NHC) fits to one of the degenerate LUMOs of CO. Interestingly, the strong steric repulsion between CO and B (NHC) leads to the HOMO-LUMO swap in the latter. Subsequently, both HOMO and HOMO-1 of B (NHC) can effectively interact with the two π* anti-bonding orbitals (LUMO and LUMO+1) of CO, resulting in substantial electron back-donation and the ultimate activation of CO.
探索能够与小分子相互作用的多键和不饱和主族化合物以寻找非金属催化剂是当前的一个趋势。值得注意的是，布伦瑞克等人发现，由氮杂环卡宾（NHCs）稳定的二硼炔能够结合并活化CO。在此，我们从一个新颖的理论视角探索B（NHC）的键合本质及其对CO的活化机制。虽然B的基态是单键，但NHCs的靠近将B激发到其三键的第三激发态，在两端有两个显著的σ空穴。随后的静电吸引驱动了B（NHC）的形成。然而，B（NHC）的两个π键（最高占据分子轨道）中只有一个与CO的简并最低未占分子轨道之一匹配。有趣的是，CO与B（NHC）之间强烈的空间排斥导致了后者的最高占据分子轨道 - 最低未占分子轨道交换。随后，B（NHC）的最高占据分子轨道和最高占据分子轨道 - 1都能与CO的两个π*反键轨道（最低未占分子轨道和最低未占分子轨道 + 1）有效相互作用，导致大量电子回授并最终活化CO。
