From B(C₆F₅)₃ to B(OC₆F₅)₃:  Synthesis of (C₆F₅)₂BOC₆F₅ and C₆F₅B(OC₆F₅)₂ and Their Relative Lewis Acidity

The pentafluorophenyl esters of bis(pentafluorophenyl)borinic acid (C6F5)2BOC6F5 (2) and pentafluorophenylboronic acid C6F5B(OC6F5)2 (3) have been prepared and characterized by multinuclear NMR and X-ray analysis. VT NMR studies have shown that restricted rotation around the B−O bond in 2 occurs below 193 K, corresponding to ΔG⧧ = 35 kJ/mol for this process. This low barrier and the random torsion angles around the B−O bonds observed in the solid state structures of compounds 2, 3, and B(OC6F5)3 (4) suggest that these torsion angles are not related to pπ−pπ interactions between boron and oxygen, but more likely a consequence of the extensive intermolecular F−π interactions seen in the solid state structures. The Lewis acidity of 2, 3, and 4 has been compared with B(C6F5)3 (1), using various Lewis bases. All compounds 1−4 appear to be strong Lewis acids, whereby 4 interacts more strongly with hard bases whereas 1 binds more strongly to softer bases.