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)

Abstract

A new family of electron-deficient tris(aryl)boranes, B(C(6)F(5))(3-n)(C(6)Cl(5))(n) (n = 1-3), has been synthesized, permitting an investigation into the steric and electronic effects resulting from the gradual replacement of C(6)F(5) with C(6)Cl(5) ligands. B(C(6)F(5))(2)(C(6)Cl(5)) (3) is accessed via C(6)Cl(5)BBr(2), itself prepared from donor-free Zn(C(6)Cl(5))(2) and BBr(3). Reaction of C(6)Cl(5)Li with BCl(3) in a Et(2)O/hexane slurry selectively produced B(C(6)Cl(5))(2)Cl, which undergoes B-Cl exchange with CuC(6)F(5) to afford B(C(6)F(5))(C(6)Cl(5))(2) (5). While 3 forms a complex with H(2)O, which can be rapidly removed under vacuum or in the presence of molecular sieves, B(C(6)Cl(5))(3) (6) is completely stable to refluxing toluene/H(2)O 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-C(6)Cl(5) bonds; each exhibits a trigonal planar geometry about B, despite having different ligand sets. The spectroscopic characterization using (11)B, (19)F, and (13)C NMR indicates that the boron center becomes more electron-deficient as n increases. Optimized structures of B(C(6)F(5))(3-n)(C(6)Cl(5))(n) (n = 0-3) using density functional theory (B3LYP/TZVP) are all fully consistent with the experimental structural data. Computed (11)B 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 C(6)F(5) for C(6)Cl(5). The direct solution cyclic voltammetry of B(C(6)F(5))(3) has been obtained for the first time and electrochemical measurements upon the entire series B(C(6)F(5))(3-n)(C(6)Cl(5))(n) (n = 0-3) corroborate the spectroscopic data, revealing C(6)Cl(5) to be a more electron-withdrawing group than C(6)F(5), with a ca. +200 mV shift observed in the reduction potential per C(6)F(5) group replaced. Conversely, use of the Guttmann-Beckett and Childs' methods to determine Lewis acidity on B(C(6)F(5))(3), 3, and 5 showed this property to diminish with increasing C(6)Cl(5) content, which is attributed to the steric effects of the bulky C(6)Cl(5) 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((11)B) 10G. Measurements of the exponential decay of the EPR signal (298 K) reveal [6](•-) to be considerably more stable than its perfluoro analogue.

A Combined “Electrochemical–Frustrated Lewis Pair” Approach to Hydrogen Activation: Surface Catalytic Effects at Platinum Electrodes

“…This is consistent with the parent borane, 2 , being more electrophilic than 1 . [35] However, in the case of [ 2 -H] − , no current cross-over was observed at any scan rate, indicating that the steric bulk around the B–H bond in [ 2 -H] − is sufficient to prevent the formation of any Pt–H surface species, and thus any electrocatalysis by the electrode surface, and lends further support for our proposed mechanism for the electrocatalytic oxidation of [ 1 -H] − in Scheme 2. Instead a new, quasi-reversible redox wave was observed at a mid-peak potential of +0.98 mV versus Cp 2 Fe 0/+ .…”

“…[29–33] Aside from our recent studies using carbon electrodes, the non-aqueous redox chemistry of bulkier borohydrides, such as [ 1 -H] − ,[27, 34] and their parent electron-deficient, Lewis acidic boranes,[35, 36] remain largely unexplored. Therefore, to better understand the oxidation mechanism of important intermediates, such as [ 1 -H] − in combined electrochemical–FLP systems, herein, we seek to explore and understand the electrochemical behaviour of [ 1 -H] − at Pt electrode surfaces.…”

Versatile Catalytic Hydrogenation Using A Simple Tin(IV) Lewis Acid

“…The high chemical shift is consistent with significant stannylium ion character, although it is considerably upfield of the value reported for [ n Bu 3 Sn][CB 11 Me 12 ] ( δ =454 ppm), which displays the least coordinated trialkylstannylium core to date 20. Gutmann–Beckett Lewis acidity measurements support this conclusion,21 indicating increased electrophilicity in comparison with n Bu 3 SnOTf, although still lower than B(C 6 F 5 ) 3 22 {AN=64.2 n Bu 3 SnOTf; 68.0 [ 1 ]OTf; 78.1 B(C 6 F 5 ) 3 }. [ 1 ]OTf has also been characterized by 1 H, 13 C and 19 F NMR spectroscopy, MS and elemental analysis (see the Supporting Information (SI)).…”

Friedel–Crafts Acylation with Amides

“…For example, perfluorinated aryl groups are known to exert powerful electron withdrawing properties. 10 As such, this should weaken the amide resonance and lead to acyl transfer. When amide 1a is reacted with triflic acid in CHCl 3 (25°C), a fair yield of 1-indanone ( 2 ) is obtained.…”