Lewis Acid Properties of Tris(pentafluorophenyl)borane. Structure and Bonding in L−B(C₆F₅)₃ Complexes

Abstract

A variety of donor adducts of tris(pentafluorophenyl)borane were experimentally generated by reaction of a Lewis base with an excess of B(C6F5)3 in pentane. In this way, nitrile complexes (C6F5)3B·NCR (R = CH3 1a, p-CH3−C6H4 1b, p-NO2−C6H4 1c), isonitrile complexes (C6F5)3B·CNR (R = C(CH3)3 3a, C(CH3)2CH2C(CH3)3 3b, 2,6-(CH3)2−C6H3 3c), and the phosphine adduct (C6F5)3B·P(C6H5)3 (6) could be prepared. The compounds were characterized by IR and NMR spectroscopy and by X-ray structure analyses (1a, 1c, 3a, 3b, and 6). Coordination of the nitriles as well as the isonitriles to the neutral Lewis acid leads to a substantial increase in the C⋮N bond strength. This is evident from a marked shift of the ν̃C ⋮ N IR band to higher wavenumbers, and this interpretation is supported by the small but experimentally significant decrease of the C⋮N bond length observed by X-ray diffraction. The experimental work is complemented by a density functional study on the model complexes (C6F5)3B·L, L = CNCH3, NCCH3, PH3, CO. A detailed analysis revealed that the bonding in (C6F5)3B·L complexes is mainly dominated by electrostatic interaction, which in turn is responsible for the observed structural and spectroscopic changes. In the context of this work, the bonding of the neutral B(C6F5)3 Lewis acid is compared to the positively charged organometallic d0-Cp3M+ system (M = Zr, Hf). It was found that electrostatic effects are more pronounced for B(C6F5)3 than for the transition metal fragments. The question as to the existence of a nonclassical main group carbonyl complex, (C6F5)3B·CO, is addressed.

Reversible Heterolytic Si–H Bond Activation by an Intramolecular Frustrated Lewis Pair

“…The 11 B NMR spectrum of 8a shows a signal at δ = −20 ppm, which is in the typical [HBR(C 6 F 5 ) 2 ] − borate range, and the 31 P NMR spectrum shows a resonance at δ = −9.9 ppm. The 29 Si NMR signal occurs at δ = −30.0 ppm, significantly downfield shifted compared to PhSiH 3 (δ = −60.2 ppm, C 6 D 6 , 299 K), with a coupling constant of 1 J PSi = 59.5 Hz.…”

“…2). The hydrogen atoms of the P-bound SiH 2 unit are diastereotopic un- der these conditions, resulting in the observed doublet of doublets pattern with 29 Si satellites ( 1 J SiH ≈ 240 Hz) and 2 J PH coupling constants of 20.1 and 22.3 Hz, respectively (Fig. 2).…”

“…Most significantly, this results in the observation of a singlet with 29 Si satellites of the P-SiH 2 unit and a single set of signals of the pair of mesityl groups at the phosphorus atom in the 1 H NMR spectrum. Concurrently, only three 19 F NMR resonances of the now symmetry-equivalent C 6 F 5 groups at the boron atom are observed.…”

“…A remarkable exception in this context is the strong Lewis acid tris(pentafluorophenyl)borane, B(C 6 F 5 ) 3 [5 -7] which serves as a potent catalyst [8 -12], e. g., in the hydrosilylation of ketones and imines [12 -27]. Piers et al had shown that the role of B(C 6 F 5 ) 3 is not to activate the carbonyl function, as one might have thought [28,29], but that it activates the silane by an abstractive coordination of the hydridic Si-H bond to the Lewis acidic boron atom [30]. Concomitant silyl transfer to the carbonyl group via an S N 2-Si mechanism [31] followed by hydride transfer from the borohydride to the activated former carbonyl carbon atom then closes the catalytic cycle to give the respective hydrosilylation product.…”

12-(N-Methylnitrilium)monocarba-closo-dodecaborate Ylide

“…Only a few known nitrilium ylides carry a non-conjugating negatively charged group on the CN carbon 12−16 and in these, the IR stretch occurs near 2300 cm −1 and is thus similar to that in free nitrilium cations 31 and the related ylides of the type R 3 BC=NMe, which are best described as Lewis acid-base adducts of RB 3 with an isonitrile. 32,33 The new ylide 3 carries a negative charge adjacent to the CN carbon atom and thus belongs to the rarer category. Its CN stretch occurs at 2347 cm −1 , in a region characteristic of free nitrilium cations.…”