Synthesis, Characterization, and Application of Two Al(OR^F)₃ Lewis Superacids

Abstract

We report herein the synthesis and full characterization of the donor-free Lewis superacids Al(OR(F))(3) with OR(F) = OC(CF(3))(3) (1) and OC(C(5)F(10))C(6)F(5) (2), the stabilization of 1 as adducts with the very weak Lewis bases PhF, 1,2-F(2)C(6)H(4), and SO(2), as well as the internal C-F activation pathway of 1 leading to Al(2)(F)(OR(F))(5) (4) and trimeric [FAl(OR(F))(2)](3) (5, OR(F) = OC(CF(3))(3)). Insights have been gained from NMR studies, single-crystal structure determinations, and DFT calculations. The usefulness of these Lewis acids for halide abstractions has been demonstrated by reactions with trityl chloride (NMR; crystal structures). The trityl salts allow the introduction of new, heteroleptic weakly coordinating [Cl-Al(OR(F))(3)](-) anions, for example, by hydride or alkyl abstraction reactions.

The Al(ORF)3/H2O/Phosphane [RF = C(CF3)3] System – Protonation of Phosphanes and Absolute Brønsted Acidity

“…Compound 10 is isostructural to the fluorine analog, which is known from the decomposition of Al(OR F ) 3 . [16] At two of the three hydroxo groups, H bridging to the fluorine atoms of two PhF molecules can be observed and further supports our assignment. These interactions with O···F separations of 274.0 and 284.2 pm can be classified as rather strong hydrogen bonds (see Figure 6) and even coordinate the weak donor PhF.…”

“…[44] The 31 P NMR chemical shifts are in the same range as those observed for similar Al/B-P complexes [MePh 2 P-BBr 3 δ( 31 P) = -9.2, [43] MePh 2 P-AlMe 3 δ( 31 P) = -24.2, [45] Ph 3 P-AlMe 3 δ( 31 P) = -7.3 ppm]. [45] For 2, the distance between the two maxima in the 31 [16] Furthermore, the signals with 1 H chemical shifts of 7.62, 7.78, and 7.92 ppm (ortho, meta, para) are too sharp to result from an adduct (free AsPh 3 : one broad signal at 7.34 pm). Consistent with the increased central atom size (P vs. As) and the decreased calculated adduct stability, dissociation of 3 and a further reaction with the solvent CD 2 Cl 2 seems to be reasonable.…”

“…The 19 F NMR signals at δ = -75.4 and -74.3 ppm as well as the broad 27 Al NMR signals at δ = 43.9 and 41.4 ppm are typical for strongly bound Al(OR F ) 3 complexes. [16] The line-width of the 27 Al signal is too broad to show splitting by the P-Al coupling. The 31 P NMR spectra showed a broad signal at δ = -26.6 ppm for 1 (cf.…”

“…Thus, a stronger Lewis acid should further increase the proton acidity. The use of one of the strongest monomeric Lewis superacids, Al(OR F ) 3 , [16,17] should result in one of the highest available Brønsted acidities.…”

“…However, according to the higher donor strength of the phosphane and arsane ligands, the average Al-O distance increases by about 1.2 to 3.6 pm, if compared to the weaker donor adducts Do-Al(OR F ) 3 with Do = PhF, 1,2-F 2 C 6 H 4 , or SO 2 . [16] …”