Reaction of MesP─C≡C─Ph (Mes = mesityl) with dineopentylaluminium hydride afforded by hydroalumination a geminal Al/P-based frustrated Lewis pair (FLP; ). Its steric shielding is relatively low, and its reactivity in various secondary reactions is less hindered by steric repulsion than observed for related compounds having bulkier groups attached to aluminium. FLP yielded adducts with MeC─NCO or benzaldehyde via the formation of Al-O and P-C bonds. Trimethylsilyl azide reacted with under surprisingly mild conditions to afford a nitrene complex by spontaneous N elimination below room temperature. A carbodiimide molecule was coordinated via one of the C=N bonds to form a five-membered AlCPNC heterocycle with an intact C=N bond in an exocyclic position. A very large molecule was obtained by the reaction of two equivalents of with a bifunctional methylene-bridged phenylene isocyanate precursor.This article is part of the themed issue 'Frustrated Lewis pair chemistry'.

Hydroalumination of R-P(H)-C[triple bond, length as m-dash]C-tBu with bulky H-Al[CH(SiMe3)2]2 afforded the new P-H functionalized Al/P-based frustrated Lewis pair R-P(H)-C[[double bond, length as m-dash]C(H)-tBu]-AlR2 [R = CH(SiMe3)2; FLP 7]. A weak adduct of 7 with benzonitrile (8) was detected by NMR spectroscopy, but could not be isolated. tert-Butyl isocyanide afforded a similar, but isolable adduct (9), in which the isocyanide C atom was coordinated to aluminium. The unique reactivity of 7 became evident from its reactions with the heteroatom substituted nitriles PhO-C[triple bond, length as m-dash]N, PhCH2S-C[triple bond, length as m-dash]N and H8C4N-C[triple bond, length as m-dash]N. Hydrophosphination of the C[triple bond, length as m-dash]N triple bonds afforded imines at room temperature which were coordinated to the FLP by Al-N and P-C bonds to yield AlCPCN heterocycles (10 to 12). These processes depend on substrate activation by the FLP. Diphenylcyclopropenone and its sulphur derivative reacted with 7 by addition of the P-H bond to a C-C bond of the strained C3 ring and ring opening to afford the fragment (Z)-Ph-C(H)[double bond, length as m-dash]C(Ph)-C-X-Al (X = O, S). The C-O or C-S groups were coordinated to the FLP to yield AlCPCX heterocycles (13 and 14). The thiocarbonyl derived compound 14 contains an internally stabilized phosphenium cation with a localized P[double bond, length as m-dash]C bond, a trigonal planar coordinated P atom and a short P[double bond, length as m-dash]C distance (168.9 pm). Insight into formation mechanisms, the structural and energetic properties of FLP 7 and compounds 13 and 14 was gained by quantum chemical DFT calculations.

The P−H functionalized FLPs R(H)PC(AlBis 2 )C(H)-CMe 3 (1a, R = Bis; 1b, R = Mes; Bis = CH(SiMe 3 ) 2 ) combine the typical FLP properties based on Lewis acidic Al and basic P atoms with the reactivity of a P−H bond. They allow the coordination of substrates followed by hydrophosphination with the activated P−H group. Reactions of 1a with R′−NCS (R′ = Ph, CMe 3 ) or of both FLPs with Et−N CCPh 2 afforded five-membered AlCPCS/N heterocycles (4 and 5) via coordination of CS or CN bonds to the FLP backbone. Isomerization by a 1,3-H shift from P to N or C (6 and 7) was only achieved in the presence of an auxiliary base (DABCO; 1−5 mol %). (Z)-1a coordinated CO 2 to yield the adduct 8, which features a five-membered AlCPCO heterocycle with an exocyclic CO bond. Addition of bases such as DABCO and DBU afforded by deprotonation of the P atom the compounds [Bis-PC(AlBis 2 )C(H)−CMe 3 (CO 2 )] − [HB] + (9a, B = DABCO; 9b, B = DBU) that displayed hydrogen bonding between the ammonium ions and the exocyclic O atom of the FLP adducts in solution and the solid state. The stronger base n-BuLi afforded the dimeric Li compound 10, in which the Li cation was coordinated in a chelating manner to the oxygen atoms of one FLP adduct and additionally to the exocyclic O atom of the second adduct, resulting in a fourmembered Li 2 O 2 heterocycle. The related reaction of (Z)-1a with DABCO(SO 2 ) 2 led in contrast to the elimination of BisH and formation of a SO 2 -bridged dimer that features a central (AlOSO) 2 heterocycle with Al−O and S−O single bonds. The resulting unusual structural motifs may be derived from those of dialkylcarbamic or dialkylamidosulfinic acids with the N atoms replaced by P atoms.

Al/P‐ and Ga/P‐based frustrated Lewis pairs (FLPs) reacted with an azirine under mild conditions under cleavage of the heterocycle on two different positions. Opening of the C−C bond yielded an unusual nitrile–ylide adduct in which a C−N moiety coordinated to the FLP backbone. Cleavage of a C−N bond afforded the thermodynamically favored enamine adduct with the N atom bound to P and Al or Ga atoms. Ring closure was observed upon treatment of an Al/P FLP with electronically unsaturated substrates (4‐(1‐cyclohexenyl)‐1‐aza‐but‐1‐en‐3‐ynes) and yielded by C−N bond formation hexahydroquinoline derivatives, which coordinated to the FLP through P−C and Al−C bonds. Diphenylcyclopropenone showed a diverse reactivity, which depending on steric shielding and the polarizing effect of Al or Ga atoms afforded different products. An AltBu2/P FLP yielded an adduct with the C=O group coordinated to P and Al. The dineopentyl derivative gave an equilibrium mixture consisting of a similar product and a simple adduct with O bound to Al and a three‐coordinate P atom. Both compounds co‐crystallize. The Ga/P FLP only formed the simple adduct with the same substrate. Rearrangement resulted in all cases in C3‐ring cleavage and migration of a mesityl group from P to a former ring C atom by C−C bond formation. Diphenylthiocyclopropenone (evidence for the presence of P=C bonds) and an imine derivative afforded similar products.

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.