An Inorganic Analogue of Borylcyclopentadienide

Cyclopentadienide (Cp–) is a ubiquitous species in organic and organometallic chemistry. Its derivatization has been investigated enthusiastically to impart an additional function to the molecules. For instance, replacement of the skeletal carbon atoms by boron and nitrogen atoms yields 1,2-diaza-3,5-diborolanide (CB2N2–), which has recently been focused on as an inorganic analogue of Cp–. In addition, the introduction of heteroatoms onto the skeletal carbon atom can be another strategy to vary the properties of Cp–. Among them, boryl-substituted Cp– (boryl-Cp–) has interesting properties originated by the π-accepting nature of the boryl group, such as the contribution of the borataalkene structure. In this communication, we prepared the first potassium salt of boryl-substituted CB2N2– (boryl-CB2N2–) 3. The solution- and solid-state structures of 3 were determined by NMR and single crystal X-ray diffraction analyses, respectively. In the crystal structure, the C–B bond lengths are lying in the range of C–B bond lengths for the reported borataalkene species. Computational studies suggested that the C–B­(O2C6H4) bond in 3 is likely a polarized single bond rather than a double bond. Compound 3 can serve as a C-centered anion, which was demonstrated by protonation. In addition, competitive reaction between compounds 3 and 1H revealed that boryl-CB2N2– 3 is less basic than hydrogen-substituted CB2N2– 1H.环戊二烯负离子（Cyclopentadienide, Cp–）是有机化学与有机金属化学中普遍存在的物种。其衍生化反应得到了广泛且深入的研究，旨在为分子赋予额外功能。例如，将骨架碳原子替换为硼与氮原子，可得到1,2-二氮杂-3,5-二硼杂环戊烯负离子（1,2-diaza-3,5-diborolanide, CB2N2–），该物种近期作为Cp–的无机类似物受到广泛关注。此外，在骨架碳原子上引入杂原子，亦是调控Cp–性质的另一有效策略。其中，硼基取代的环戊二烯负离子（boryl-substituted Cp–, boryl-Cp–）因硼基的π接受特性展现出独特性质，例如硼杂烯烃（borataalkene）结构的贡献。本研究首次合成了硼基取代的CB2N2–（boryl-CB2N2–）的钾盐3。通过核磁共振波谱（NMR）与单晶X射线衍射分析，分别确定了该化合物在溶液态与固态下的结构。晶体结构中，其C-B键长处于已报道的硼杂烯烃物种的C-B键长范围之内。计算化学研究表明，化合物3中的C-B(O2C6H4)键更倾向于极化单键而非双键。化合物3可作为碳中心负离子，这一点通过质子化反应得到验证。此外，通过化合物3与1H的竞争反应证实，硼基取代的CB2N2– 3的碱性弱于氢取代的CB2N2– 1H。