Borane-Induced Dehydration of Silica and the Ensuing Water-Catalyzed Grafting of B(C₆F₅)₃ To Give a Supported, Single-Site Lewis Acid, ≡SiOB(C₆F₅)₂

Abstract

A supported, single-site Lewis acid, ≡SiOB(C(6)F(5))(2), was prepared by water-catalyzed grafting of B(C(6)F(5))(3) onto the surface of amorphous silica, and its subsequent use as a cocatalyst for heterogeneous olefin polymerization was explored. Although B(C(6)F(5))(3) has been reported to be unreactive toward silica in the absence of a Brønsted base, we find that it can be grafted even at room temperature, albeit slowly. The mechanism was investigated by (1)H and (19)F NMR, in both the solution and solid states. In the presence of a trace amount of H(2)O, either added intentionally or formed in situ by borane-induced dehydration of silanol pairs, the adduct (C(6)F(5))(3)B·OH(2) hydrolyzes to afford C(6)F(5)H and (C(6)F(5))(2)BOH. The latter reacts with the surface hydroxyl groups of silica to yield ≡SiOB(C(6)F(5))(2) sites and regenerate H(2)O. When B(C(6)F(5))(3) is present in excess, the resulting grafted boranes appear to be completely dry, due to the eventual formation of [(C(6)F(5))(2)B](2)O. The immobilized, tri-coordinate Lewis acid sites were characterized by solid-state (11)B and (19)F NMR, IR, elemental analysis, and C(5)H(5)N-TPD. Their ability to activate two molecular C(2)H(4) polymerization catalysts, Cp(2)ZrMe(2) and an (α-iminocarboxamidato)nickel(II) complex, was explored.

Development of silica-supported frustrated Lewis pairs: highly active transition metal-free catalysts for the Z-selective reduction of alkynes

“…The DRIFT spectrum of 3a (Figure 1c) is moderately altered in the 2800-3200 cm -1 range but displays new bands at 1645, 1515, and 1472 cm -1 , characteristic of ν(C=C) vibrations from the pentafluorophenyl rings. 51,52,72 Importantly, a band at 2405 cm -1 appeared. This frequency is in line with ν(BH) of HB(C 6 F 5 ) 2 coordinated to a Lewis base, such as THF.…”

“…Grafting methods of Lewis acids on silica, such as BArF or Piers' reagent (HB(C 6 F 5 ) 2 ) have already been reported. 51,52 However, the resulting surface species undergo interactions with nearby siloxane ligands due to their high Lewis acidity. Similar interactions between oxophilic elements or Lewis acids on silica surface have been described in the literature as observed by EXAFS and 17 O NMR.…”

Highly Sensitive NH3 Detection Based on Organic Field-Effect Transistors with Tris(pentafluorophenyl)borane as Receptor

“…15 TPFB is frequently used as a strongly Lewis acidic co-catalyst in numerous reactions, such as dehydration, 16 Friedel-Crafts reactions, 17 ring-opening reactions, 18 and syndiospecific living polymerization. 19 Here, TPFB was chosen as the NH3 receptor additive to the OFET semiconductors due to the strong interaction between boron atoms and nitrogen atoms 20 and the known vacuum sublimability of TPFB.…”

Activation of Water at the Active‐Site Cavity of Zinc Phthalocyanine with Tris(pentafluorophenyl)borane

“…First, we checked the reactivity of water bound to 1 with the addition of tris(pentafluorophenyl)borane (TPFB). TPFB has been used as a strong Lewis acidic cocatalyst in various reactions . Here, TPFB was chosen as the additional receptor for electron pairing with O atom in the coordinated water.…”