Separating Electrophilicity and Lewis Acidity: The Synthesis, Characterization, and Electrochemistry of the Electron Deficient Tris(aryl)boranes B(C6F5)3–n(C6Cl5)n (n = 1–3)

A new family of electron-deficient tris(aryl)boranes, B(C6F5)3–n(C6Cl5)n (n = 1–3), has been synthesized, permitting an investigation into the steric and electronic effects resulting from the gradual replacement of C6F5 with C6Cl5 ligands. B(C6F5)2(C6Cl5) (3) is accessed via C6Cl5BBr2, itself prepared from donor-free Zn(C6Cl5)2 and BBr3. Reaction of C6Cl5Li with BCl3 in a Et2O/hexane slurry selectively produced B(C6Cl5)2Cl, which undergoes B–Cl exchange with CuC6F5 to afford B(C6F5)(C6Cl5)2 (5). While 3 forms a complex with H2O, which can be rapidly removed under vacuum or in the presence of molecular sieves, B(C6Cl5)3 (6) is completely stable to refluxing toluene/H2O for several days. Compounds 3, 5, and 6 have been structurally characterized using single crystal X-ray diffraction and represent the first structure determinations for compounds featuring B–C6Cl5 bonds; each exhibits a trigonal planar geometry about B, despite having different ligand sets. The spectroscopic characterization using 11B, 19F, and 13C NMR indicates that the boron center becomes more electron-deficient as n increases. Optimized structures of B(C6F5)3–n(C6Cl5)n (n = 0–3) using density functional theory (B3LYP/TZVP) are all fully consistent with the experimental structural data. Computed 11B shielding constants also replicate the experimental trend almost quantitatively, and the computed natural charges on the boron center increase in the order n = 0 (0.81) n = 1 (0.89) n = 2 (1.02) n = 3 (1.16), supporting the hypothesis that electrophilicity increases concomitantly with substitution of C6F5 for C6Cl5. The direct solution cyclic voltammetry of B(C6F5)3 has been obtained for the first time and electrochemical measurements upon the entire series B(C6F5)3–n(C6Cl5)n (n = 0–3) corroborate the spectroscopic data, revealing C6Cl5 to be a more electron-withdrawing group than C6F5, with a ca. +200 mV shift observed in the reduction potential per C6F5 group replaced. Conversely, use of the Guttmann-Beckett and Childs’ methods to determine Lewis acidity on B(C6F5)3, 3, and 5 showed this property to diminish with increasing C6Cl5 content, which is attributed to the steric effects of the bulky C6Cl5 substituents. This conflict is ascribed to the minimal structural reorganization in the radical anions upon reduction during cyclic voltammetric experiments. Reduction of 6 using Na(s) in THF results in a vivid blue paramagnetic solution of Na+ [6]•–; the EPR signal of Na+[6]•– is centered at g = 2.002 with a(11B) 10G. Measurements of the exponential decay of the EPR signal (298 K) reveal [6]•– to be considerably more stable than its perfluoro analogue.

本研究合成了一类新型缺电子三(芳基)硼烷（tris(aryl)boranes）衍生物，通式为B(C₆F₅)₃–ₙ(C₆Cl₅)ₙ（n=1~3），旨在探究以五氯苯基（C₆Cl₅）配体逐步取代五氟苯基（C₆F₅）配体所引发的空间位阻与电子效应变化。其中化合物B(C₆F₅)₂(C₆Cl₅)（3）可通过中间体C₆Cl₅BBr₂制备得到，而C₆Cl₅BBr₂本身则由无给体配体的Zn(C₆Cl₅)₂与三溴化硼（BBr₃）反应制得。将五氯苯基锂（C₆Cl₅Li）与三氯化硼（BCl₃）在二乙基醚/正己烷（Et₂O/hexane）浆液中反应，可选择性生成B(C₆Cl₅)₂Cl；该中间体可与CuC₆F₅发生B-Cl交换反应，得到B(C₆F₅)(C₆Cl₅)₂（5）。尽管化合物3可与水（H₂O）形成加合物，且该加合物可在真空条件下或分子筛存在下快速脱除水分子，但B(C₆Cl₅)₃（6）在回流的甲苯/水混合体系中连续数天仍可保持完全稳定。研究团队通过单晶X射线衍射（single crystal X-ray diffraction）对化合物3、5及6完成结构表征，这也是首次对含B–C₆Cl₅键的化合物进行结构解析；尽管三者的配体组合各不相同，但中心硼原子均呈现三角平面几何构型。通过¹¹B、¹⁹F及¹³C核磁共振（NMR）对上述化合物进行光谱表征，结果显示随着n值增大，硼中心的缺电子程度逐渐升高。采用密度泛函理论（density functional theory, B3LYP/TZVP）对B(C₆F₅)₃–ₙ(C₆Cl₅)ₙ（n=0~3）的优化结构均与实验测得的结构数据高度吻合。计算得到的¹¹B屏蔽常数也几乎定量地重现了实验趋势，硼中心的计算自然电荷按n=0（0.81）、n=1（0.89）、n=2（1.02）、n=3（1.16）的顺序递增，这一结果支持了“以五氯苯基取代五氟苯基可使硼中心的亲电性随之增强”的假说。本研究首次获得了B(C₆F₅)₃的直接溶液循环伏安法（cyclic voltammetry）测试数据，且对整系列B(C₆F₅)₃–ₙ(C₆Cl₅)ₙ（n=0~3）的电化学测试结果佐证了光谱表征结论：五氯苯基是比五氟苯基吸电子能力更强的基团，每替换一个五氟苯基，还原电位约向正方向移动200 mV。与之相反，采用古特曼-贝克特法（Guttmann-Beckett）与蔡尔兹法（Childs’ methods）对B(C₆F₅)₃、3及5的路易斯酸性进行测定，结果显示该酸性随五氯苯基取代比例升高而降低，这一现象可归因于体积庞大的五氯苯基取代基带来的空间位阻效应。上述矛盾可通过循环伏安测试过程中还原生成的自由基阴离子几乎不发生结构重排来解释。以金属钠（Na(s)）在四氢呋喃（THF）中还原化合物6，可得到呈鲜亮蓝色顺磁性（paramagnetic）溶液的Na⁺[6]•⁻；该物种的电子顺磁共振（EPR）信号中心位于g=2.002，耦合常数a(¹¹B)=10 G。对该EPR信号在298 K下的指数衰减过程进行测定，结果表明[6]•⁻的稳定性远高于其全氟代类似物。