Materials Research Center, Department of Chemical Sciences, Ariel University, Ariel 40700, Israel.
Molecules. 2021 Oct 25;26(21):6428. doi: 10.3390/molecules26216428.
The coordination of the B fragment by two σ-donor ligands L: could lead to a diboryne compound with a formal triple bond L:→B≡B←:L. σ-Type coordination L:→B leads to an excess of electrons around the B central fragment, whereas π-back-donation from the B≡B moiety to ligand L has a compensation effect. Coordination of the σ-donor and π-acceptor ligand is accompanied by the lowering of the BB bond order. Here, we propose a new approach to obtain the perfect triple BB bond through the incorporation of the BB unit into a rigid molecular capsule. The idea is the replacement of π-back-donation, as the principal stabilization factor in the linear NBBN structure, with the mechanical stabilization of the BB fragment in the inert molecular capsule, thus preserving the perfect B≡B triple bond. Quantum-chemical calculations show that the rigid molecular capsule provided a linear NBBN structure and an unusually short BB bond of 1.36 Å. Quantum-chemical calculations of the proposed diboryne adducts show a perfect triple bond B≡B without π-back-donation from the B unit to the host molecule. Two mechanisms were tested for the molecular design of a diboryne adduct with a perfect B≡B triple bond: the elimination of π-back-donation and the construction of a suitable molecular trap for the encapsulation of the B unit. The second factor that could lead to the strengthening or stretching of a selected chemical bond is molecular strain produced by the rigid molecular host capsule, as was shown for B≡B and for C≡C triple bonds. Different derivatives of icosane host molecules exhibited variation in BB bond length and the corresponding frequency of the BB stretch. On the other hand, this group of molecules shows a perfect triple BB bond character and they all possess a similar level of HOMO.
双 σ-供体配体 L 对 B 片段的配位：可能导致双硼烷化合物形成形式三键 L:→B≡B←:L。σ 型配位 L:→B 在 B 中心片段周围导致电子过剩，而 B≡B 部分向配体 L 的 π-反馈捐赠具有补偿作用。σ-供体和 π-受体配体的配位伴随着 BB 键序的降低。在这里，我们提出了一种通过将 BB 单元引入刚性分子胶囊中获得完美三 BB 键的新方法。其想法是用 BB 片段在惰性分子胶囊中的机械稳定取代 NBBN 结构中主要的稳定化因素——π-反馈捐赠，从而保持完美的 B≡B 三键。量子化学计算表明，刚性分子胶囊提供了线性 NBBN 结构和异常短的 BB 键长为 1.36 Å。所提出的双硼烷加合物的量子化学计算表明，没有来自 B 单元到主体分子的π-反馈捐赠，具有完美的三键 B≡B。为设计具有完美 B≡B 三键的双硼烷加合物，测试了两种机制：消除π-反馈捐赠和构建合适的分子陷阱以封装 B 单元。第二个可以导致所选化学键加强或拉伸的因素是刚性分子主体胶囊产生的分子应变，正如 B≡B 和 C≡C 三键所示。二十烷主体分子的不同衍生物表现出 BB 键长和相应的 BB 伸缩频率的变化。另一方面，这组分子表现出完美的三 BB 键特征，它们都具有相似的 HOMO 水平。
