Differences in the Reactivity of Geminal Si−O−P and Al−O−P Frustrated Lewis Pairs

Abstract

The new oxygen‐bridged geminal Si/P Frustrated Lewis Pair (FLP) tBu2P−O−Si(C2F5)3 (2) is able to reversibly bind carbon dioxide at ambient temperature. We compared its reactivity towards benzil, but‐3‐en‐2‐one, nitriles and phenylacetylene to that of the Al/P FLP tBu2P−O−AlBis2 (Bis=−CH(SiMe3)2) (1). When reacted with benzil, both, 1 and 2, form the 1,2‐addition product, but in the Si/P FLP 2, the second carbonyl function additionally binds to the silicon atom. With but‐3‐en‐2‐one 2 forms the 1,2‐addition product, while 1 binds in 1,4‐position. The reaction with acetonitrile yielded an unexpected etheneimine adduct for both systems, while only 1 reacted with tert‐butylnitrile. With benzonitrile and acrylonitrile, 2 showed reversible addition to the C≡N bond and 1 forms a stable adduct with benzonitrile. Solely 1 shows reactivity towards phenylacetylene affording a mixture of the CH deprotonation adduct tBu2P(H)−O−AlBis2(CCPh) and the FLP −C≡C 1,2‐addition adduct under ring formation. All compounds were characterized by multinuclear NMR spectroscopy, XRD and elemental analysis.

Reactivity of Oxygen‐Bridged Geminal Al/P and Si/P Frustrated Lewis Pairs towards Heterocumulenes

“…In contrast, 2 forms a stable and simple adduct upon treatment with SO 2 . Its signals in the 1 H and 13 C NMR spectra are extremely broad at room temperature, a phenomenon which has also been observed for the benzil adduct [26] . Significantly sharper signals were observed at 233 K. In the 1 H NMR spectrum, the t Bu groups give rise to two doublets at 1.61 and 1.49 ppm, the 31 P{ 1 H} NMR spectrum exhibits a signal at 92.0 ppm which is significantly low‐field shifted compared to the SO 2 adduct of the analogous methylene‐linked FLP t Bu 2 P−CH 2 −Si(C 2 F 5 ) 3 (71.8 ppm) [17] …”

“…However, this view does not take into account the Lewis acid strength, which might not be sufficient in the case of 2 to react with PhNCS and CS 2 . 2 was, in contrast to 1 and as shown earlier, not able to split dihydrogen and forms a much weaker adduct with CO 2 [25,26] . We furthermore calculated the free energies for the reactions of 1 and 2 with CS 2 and PhNCS with the r 2 SCAN‐3c composite method [33] (for details see the Supporting Information).…”

“…PhNSO (>98 %, fluorochem) was distilled, inertly filtered over silica gel and degassed prior to use. t Bu 2 POAlBis 2 ( 1 ) and t Bu 2 POSi(C 2 F 5 ) 3 ( 2 ) were prepared as reported previously [25,26] . NMR spectra were recorded using Bruker Avance III 500, Avance III 500 HD or Avance III 600 spectrometer at ambient temperature if not stated otherwise.…”

“…The Ge/P system, however, showed no reactivity towards CO 2 under comparable conditions. Recently, we reported the synthesis of the oxygen‐bridged Al/P and Si/P FLP t Bu 2 P−O−AlBis 2 (Bis=CH(SiMe 3 ) 2 , 1 ) and t Bu 2 P−O−Si(C 2 F 5 ) 3 ( 2 ) by hydrogen or salt elimination and described their reactivity towards a series of small molecules [25,26] . The P−O−Al FLP 1 forms a stable adduct with CO 2 and reversibly binds molecular hydrogen; the H 2 addition product of 1 stoichiometrically reduces CO 2 to the formate stage.…”

“…Recently, we reported the synthesis of the oxygen-bridged Al/P and Si/P FLP tBu 2 PÀ OÀ AlBis 2 (Bis = CH(SiMe 3 ) 2 , 1) and tBu 2 PÀ OÀ Si(C 2 F 5 ) 3 (2) by hydrogen or salt elimination and described their reactivity towards a series of small molecules. [25,26] The PÀ OÀ Al FLP 1 forms a stable adduct with CO 2 and reversibly binds molecular hydrogen; the H 2 addition product of 1 stoichiometrically reduces CO 2 to the formate stage. Compared to 1, PÀ OÀ Si FLP 2 reversibly binds CO 2 at room temperature and completes adduct formation below 0 °C.…”