Reactions of a Ga/P‐Based Frustrated Lewis Pair with HX(X= F – I), HeterocumulenesR–NCY(Y= O, S) and Chalcogens–Adduct Formation and Surprising Stability towards Protolysis

Abstract

Treatment of the geminal Ga/P‐based frustrated Lewis pair (FLP) Mes2P–C(GatBu2)=C(H)–Ph (1) with HX (X = F, Cl, Br, I) afforded the corresponding adducts 2 with the protons bound to the P and the halide anions coordinated to the Ga atoms. Short intramolecular contacts may indicate P–H···X hydrogen bonding interactions. The Br and I compounds (2c, 2d) were accessible in moderate yields even when aqueous solutions of the acids were employed. These unexpected reactions confirm the surprising stability of FLP 1 towards protolysis. Heterocumulenes R–N=C=Y (Y = O, S) and 1 yielded adducts with two different structural motifs. The N=C=Y groups were coordinated to the FLP either via the C=Y (Y = S; Ph–N=C=O) or the C=N bonds (Ph–N=C=O, Et–N=C=O). For phenyl isocyanate the C=O bonded isomer was observed in the solid state, while both isomeric forms were detected in solution. Steric shielding and the hardness of the atoms may influence the formation of the respective isomer. Cleavage of the C=S bonds of isothiocyanates was observed for the first time and afforded a sulfur adduct 9a, in which an S atom (electron sextet) was bound to the lone pair of electrons at phosphorus and to the Lewis acidic Ga atom. Four‐membered PCGaY heterocycles resulted, which were also synthesized in high yields by the direct reaction of 1 with propylene sulfide or selenium.

Supramolecular Chemistry Based on Frustrated Lewis Pairs – Reactions of an Al/P FLP with Potassium Formate and Cesium Fluoride

“…The formation of the adducts was readily apparent from signals in the downfield region of the 1 H NMR spectra at about δ = 10 ppm with characteristic large coupling constants of 1 J PH >500 Hz, which indicate four‐coordinate P atoms with a directly bound hydrogen atom , . The relatively large 3 J PH coupling constants of about 49 Hz ( 1 : 3 J PH = 17.7 Hz) confirm this conclusion and are consistent with a cis arrangement of P and vinylic H atoms . The 1 H NMR resonances of the phosphorus bound hydrogen atoms show strong low field shifts, which may indicate weak P–H ··· X hydrogen bonding.…”

“…They had protons bound to phosphorus and halide anions attached to aluminum and were stable in the solid state and in solution at room temperature. A similar reactivity was observed for the Ga analogue of 1 , which was treated even with aqueous solutions of HBr and HI to obtain the respective H X adducts …”

“…When solutions of FLP 1 in toluene were treated with aqueous solutions of HBr or HI at room temperature the corresponding adducts Mes 2 (H)PC(Al Xt Bu 2 )=CHPh [ X = Br ( 4 ), I ( 5 )] were obtained after recrystallization in 66 % and 78 % yield (Scheme ). The synthesis of both compounds is frequently accompanied by the formation of the protolysis product trans ‐Mes 2 P–C(H)=C(H)–Ph, which was identified by its characteristic 31 P NMR shift ( δ = –23.5 ppm) and can be separated by washing with hydrocarbon solvents and recrystallization of the crude product from polar solvents such as CH 2 Cl 2 or 1,2‐F 2 C 6 H 4 . Once isolated, 4 and 5 are stable in the solid state or in solution in the absence of moisture and air.…”

“…Only recently, we became interested in the reactivity of FLPs based on Al or Ga and P atoms towards Brønstedt acids , . In view of the extreme sensitivity of Al–C or Ga–C bonds towards protolysis such reactions are by no means trivial and unselective reactions are to be expected.…”

Reactivity of a P–H Functionalized Al/P‐Based Frustrated Lewis Pair – Hydrophosphination versus Classic Adduct Formation

“…Complete decomposition required heating to 100 °C for 16 h. cis ‐Mes(H)P–C(H)=C(H)– t Bu was identified by NMR spectroscopy ( 31 P, δ = –96.8 ppm; 1 J PH = 219.1, 3 J PH = 23.5 Hz) as the main product, which results from protolytic cleavage of the Al–C(vinyl) bond of the FLP backbone. An analogous phosphaalkene, trans ‐Mes 2 P–C(H)=C(H)–Ph, was observed as a frequent side product in decomposition reactions of FLP 1 . (Me 3 Si) 2 CH 2 was detected as another by‐product upon thermal degradation of 5 [ 1 H NMR: δ = 0.06 ppm (SiMe 3 ), –0.37 ppm (CH 2 )].…”