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

Uhl W., Possart J., Hepp A., Layh M.

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.