An Al/P-based frustrated Lewis pair (FLP) reacted with PhMgCl by an unexpected transmetalation and formation of a phosphinylvinyl Grignard reagent. This compound is well suited for the transfer of the basic FLP component to other Lewis acidic metal atoms and allowed the generation of a Ga/P and an In/P FLP. The Ga FLP showed a behavior different to that of the corresponding Al FLP, the In FLP allowed the chelating coordination of an Au atom by Au-Cl bond activation.

The Lewis acids Ga(CF), In(CF) and Ga(CCl) are prepared and their Lewis acidity has been probed experimentally and computationally. The species Ga(CF) and In(CF) in conjunction with phosphine donors are shown to heterolytically split H and catalyse the hydrogenation of an imine. In addition, frustrated Lewis pairs (FLPs) derived from Ga(CF) and In(CF) and phosphines react with diphenyldisulfide to phosphoniumgallates or indates of the form [BuPSPh][PhSE(CF)] and [BuPSPh][(μ-SPh)(E(CF))] (E = Ga, In). The potential of the FLPs based on Ga(CF), In(CF) and Ga(CCl) and phosphines is also shown in reactions with phenylacetylene to give pure or mixtures of the products [BuPH][PhCCE(CX)] and RP(Ph)C=C(H)E(CX) A number of these species are crystallographically characterized. The implications for the use of these species in FLP chemistry are considered.This article is part of the themed issue 'Frustrated Lewis pair chemistry'.

The reactivity of the Ga/P-based frustrated Lewis pair (FLP) Mes 2 P− C[C(H)−Ph]−Ga t Bu 2 (3) is influenced by the relatively weak Lewis acidity of its Ga atom and differs significantly from that of the analogous Al compound 1. The adduct of 3 with CO 2 was only detectable at low temperature by NMR spectroscopy. Benzaldehyde was coordinated only via a Ga−O bond; the P atom was not involved. In contrast, a relatively persistent adduct was formed with soft CS 2 to yield a fivemembered GaCPCS heterocycle. Dehydrocoupling with H 3 B←NHMe 2 afforded the dimeric amidoborane (H 2 B−NMe 2 ) 2 , while an adduct with a GaCPBN heterocycle was isolated with the sterically less shielded ammonia−borane H 3 B←NH 3 . The latter product was unstable in solution and decomposed by H 2 elimination and formation of oligomeric BN compounds. Small quantities of 3 catalyzed hydrogen transfer from H 3 B←NH 3 to an imine. The Lewis acidities of the Al/P-and Ga/P-based FLPs were examined by experiments (Gutmann−Beckett method) and by calculation of the fluoride ion affinity (including the B and In analogues). The Al compound is the strongest Lewis acid; the Ga FLP is significantly weaker but is a stronger F − acceptor in comparison to the unknown analogues of B and In. These results reflect the different reactivities of these FLPs and may help to develop FLPs with finely adjusted properties.

The Al- and Ga-based frustrated Lewis pairs (FLPs) Mes2P–C(MR2)=CH-R′ (1, M=Al, R=tBu; 2, M=Al, R=CH2tBu; 3, M=Ga, R=tBu) and the unique P–H functionalized FLP Mes(H)P–CH(AlR2)=C(H)-tBu [4, R=CH(SiMe3)2] were treated with a variety of azides R′-N=N=N [R′=tBu, SiMe3, Ph, CH2Ph, C6H4(4-Cl), C6H4(4-CF3), C6H4(4-Me), CH2C6H4(4-Cl), CH2C6H4(4-tBu), C6H4(2-CH=CHPh)] in order to study systematically the influence of the substituents at nitrogen, phosphorus and the metal atoms on the reaction courses and the thermal stability of the products. Azide adducts (5–8) were isolated in which the terminal nitrogen atoms of the azides (Nγγ) were bound to the phosphorus and the respective metal atoms resulting in four-membered PCMN heterocycles as the sole structural motif despite the wide range of substituents and the variation in the metal atoms of the FLPs. Thermal activation of selected azide adducts led to the elimination of N2 and the formation of the nitrene adducts 9–11 in which formally a transient, highly reactive nitrene N–R with an electron sextet nitrogen atom is trapped by the FLPs. For the first time FLPs were treated with a diazomethane, Me3Si–C(H)=N=N. Reactions with 1 and 2 afforded the adducts [Mes2P–C(AlR2)=CH-Ph](μ-N2CH–SiMe3) 12 (R=tBu, CH2tBu) which had structures and spectroscopic properties similar to those of the corresponding azides. These compounds are thermally stable and do not eliminate dinitrogen upon warming or irradiation. Protonation of 12a with HCl in Et2O resulted in cleavage of the Al–N bond and formation of the zwitterionic phosphonium salt Mes2P[NH–N=C(H)–SiMe3]–C(AlCltBu2)=C(H)-Ph 13 with an intramolecular N–H···Cl hydrogen bond.

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.