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• 作者：Andrew E. Ashley ; Thomas J. Herrington ; Gregory G. Wildgoose ; Hasna Zaher ; Amber L. Thompson ; Nicholas H. Rees ; Tobias Kr盲mer ; Dermot O鈥橦are
• 刊名：Journal of the American Chemical Society
• 出版年：2011
• 出版时间：September 21, 2011
• 年：2011
• 卷：133
• 期：37
• 页码：14727-14740
• 全文大小：1229K
• 年卷期：v.133,no.37(September 21, 2011)
• ISSN：1520-5126

文摘

A new family of electron-deficient tris(aryl)boranes, B(C₆F₅)_3鈥?i>n(C₆Cl₅)_n (n = 1鈥?), 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鈥揅l 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鈥揅₆Cl₅ 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鈥?i>n(C₆Cl₅)_n (n = 0鈥?) 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鈥?i>n(C₆Cl₅)_n (n = 0鈥?) 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鈥?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]^{鈥⑩€?/b>}; the EPR signal of Na⁺[6]^{鈥⑩€?/b>} is centered at g = 2.002 with a(¹¹B) 10G. Measurements of the exponential decay of the EPR signal (298 K) reveal [6]^{鈥⑩€?/b>} to be considerably more stable than its perfluoro analogue.