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 = 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)(2)Cl, 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 B-11, F-19, and C-13 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 B-11 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 = 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](center dot-); the EPR signal of Na+[6](center dot-) is centered at g = 2.002 with a(B-11) 10G. Measurements of the exponential decay of the EPR signal (298 K) reveal [6](center dot-) to be considerably more stable than its perfluoro analogu.