Aqua, Alcohol, and Acetonitrile Adducts of Tris(perfluorophenyl)borane:  Evaluation of Brønsted Acidity and Ligand Lability with Experimental and Computational Methods

Abstract

Equilibrium studies have been performed to determine the Brønsted acidity of [(C 6 F 5 ) 3 B(OH 2 )]‚ H 2 O, the aqua species that exists in acetonitrile solutions of B(C 6 F 5 ) 3 in the presence of water. NMR spectroscopic analysis of the deprotonation of [(C 6 F 5 ) 3 B(OH 2 )]‚H 2 O with 2,6-Bu t 2 C 5 H 3 N in acetonitrile allows a pK value of 8.6 to be determined for the equilibrium [(C 6 F 5 ) 3 B(OH 2 )]‚H 2 O a [(C 6 F 5 ) 3 B(OH)] -+ [H 3 O] + . On the basis of a calculated value for the hydrogen bond interaction in [(C 6 F 5 ) 3 B(OH 2 )]‚H 2 O, the pK a for (C 6 F 5 ) 3 B(OH 2 ) is estimated to be 8.4 in acetonitrile. Such a value indicates that (C 6 F 5 ) 3 B(OH 2 ) must be regarded as a strong acid, with a strength comparable to that of HCl in acetonitrile. Dynamic NMR spectroscopic studies indicate that the aqua and acetonitrile ligands in (C 6 F 5 ) 3 B(OH 2 ) and (C 6 F 5 ) 3 B(NCMe) are labile, with dissociation of H 2 O being substantially more facile than that of MeCN, by a factor of ca. 200 in rate constant at 300 K. Ab initio calculations were performed in the gas phase and with a dielectric solvent model to determine the strength of B-L bonds (L ) H 2 O, ROH, MeCN) and hydrogen bonds involving B-OH 2 and B-O(H)R derivatives.

Direct observation of a borane–silane complex involved in frustrated Lewis-pair-mediated hydrosilylations

“…8,28 This cumulative body of evidence has resulted in a high degree of acceptance for this mechanism in the literature, but the specific nature of the adducts I, or encounter complexes of type II, remain experimentally opaque. 22 29 Si NMR spectroscopy at 233 K. Noting, however, that their procedures did not appear to involve careful drying of the borane before use, it seemed likely that the peak assigned to this adduct is due in fact to Et3SiOSiEt3, formed rapidly when wet borane 35,36 is treated with silane via rapid dehydrogenative silation of water. 37,38 We confirmed this postulate by recording the 29 Si NMR spectrum of an authentic sample of Et3SiOSiEt3.…”

Non‐Metal‐Mediated Homogeneous Hydrogenation of CO2 to CH3OH

“…of C 6 F 5 H and CH 3 OB(C 6 F 5 ) 2 , whilst that of the former was slower, proceeding to the boroxin [OB(C 6 F 5 )] 3 [18] (presumably via HOB(C 6 F 5 ) 2 ); all of these are identified as products in the completed reaction using 1 H, 19 Since the only labile source of protons in the decomposition reactions of 3 and 4 is the TMPH cation, this suggests that recombination of the ion pairs to form TMP and ROH·B(C 6 F 5 ) 3 (R = CH 3 , H) must occur (Scheme 4); corroborating this hypothesis is the detection of H 2 O·B(C 6 F 5 ) 3 H-bonded with various anions in the MS(ES). [19] Whilst studies have shown that such adducts can dissociate to give ROH and free B(C 6 F 5 ) 3 , [20] at these temperatures protonation of the ipso-C on the C 6 F 5 rings appears to be faster (yielding ROB(C 6 F 5 ) 2 and C 6 F 5 H), which preclude any catalytic turnover. Finally, addition of one equivalent of CO 2 to a 1:1 mixture of TMP/B(C 6 F 5 ) 3 (4 eq., excess, under H 2 atmosphere) in C 7 D 8 demonstrated quantitative conversion to CH 3 OB(C 6 F 5 ) 2 via 2, after 6 days at 160°C.…”

Photo Lewis acid generators: photorelease of B(C6F5)3 and applications to catalysis

“…[26][27][28] Light-induced catalysis is of particular importance in the area of photolithography, where photoacid generators (PAGs) eject protons upon irradiation, 29,30 which mediate the deprotection of photoresist polymers and allow for patterning to sub-20 nm resolution. Given the importance of B(C 6 F 5 ) 3 and other perfluoroaryl boranes as Lewis acid catalysts for siloxane synthesis and modification, we became interested in devising molecules that, when irradiated, would liberate this borane. In other words, we sought photo Lewis acid generators, or "PhLAGs".…”

“…33 Therefore, PhLAG molecules that are thermally robust and produce a well-defined Lewis acid efficiently are of potential value in materials synthesis. In a preliminary communication, 34 we reported preparation of the first PhLAG molecule capable of the photorelease of B(C 6 F 5 ) 3 and applications of this design to photoinduced hydrosilylation and Piers-Rubinsztajn catalysis. The compound was a triphenylsulfonium-carbamatoborate ion pair, 1-Ph 3 S (Chart 1), which decomposes under irradiation with 254 nm light to release one equivalent of B(C 6 F 5 ) 3 , along with other products that were innocuous in terms of desired hydrosilation catalysis.…”

“…Tris-( pentafluorophenyl)borane, B(C 6 F 5 ) 3 , is a widely employed borane Lewis acid for a host of catalytic applications.…”