Synthesis of Intramolecular P/Al‐Based Frustrated Lewis Pairs via Aluminum‐Tin‐Exchange and their Reactivity toward CO 2

Frustrated Lewis pairs (FLPs) composed of acidic alane and basic phosphane functions, separated by a xanthene linker, can be prepared through the corresponding Me₃Sn derivative and methyl aluminum compounds with elimination of Me₄Sn. This way MeClAl‐, Cl₂Al‐ and (C₆F₅)₂Al‐ moieties could be introduced and the resulting FLPs are stabilized by a further equivalent of the alane precursors. In contact with the FLPs CO₂ is bound via the C atom at the phosphane functions and the two O atoms at the Al centers. The residues at the latter determine the binding strength. Hence, in case of MeClAl CO₂ capture occurs at higher pressure and under ambient conditions CO₂ is released again, while for Cl₂Al and (C₆F₅)₂Al CO₂ binding becomes irreversible. The results of DFT calculations rationalize these findings by the high thermodynamic stabilization in case of more electronegative residues, which concomitantly lead to higher barriers, and in case of (C₆F₅)₂Al further stabilization is achieved through a low reorganization energy.

Solid-State Nuclear Magnetic Resonance Techniques for the Structural Characterization of Geminal Alane-Phosphane Frustrated Lewis Pairs and Secondary Adducts

The first comprehensive solid-state nuclear magnetic resonance (NMR) characterization of geminal alane-phosphane frustrated Lewis pairs (Al/P FLPs) is reported. Their relevant NMR parameters (isotropic chemical shifts, direct and indirect 27 Al-31 P spin-spin coupling constants, and 27 Al nuclear electric quadrupole coupling tensor components) have been determined by numerical analysis of the experimental NMR line shapes and compared with values computed from the known crystal structures by using density functional theory (DFT) methods. Our work demonstrates that the 31 P NMR chemical shifts for the studied Al/P FLPs are very sensitive to slight structural inequivalences. The 27 Al NMR central transition signals are spread out over a broad frequency range (>200 kHz), owing to the presence of strong nuclear electric quadrupolar interactions that can be well-reproduced by the static 27 Al wideband uniform rate smooth truncation (WURST) Carr-Purcell-Meiboom-Gill (WCPMG) NMR experiment. 27 Al chemical shifts and quadrupole tensor components offer a facile and clear distinction between three- and four-coordinate aluminum environments. For measuring internuclear Al⋅⋅⋅P distances a new resonance-echo saturation-pulse double-resonance (RESPDOR) experiment was developed by using efficient saturation via frequency-swept WURST pulses. The successful implementation of this widely applicable technique indicates that internuclear Al⋅⋅⋅P distances in these compounds can be measured within a precision of ±0.1 Å.

New Reactivity Patterns in 3H‐Phosphaallene Chemistry [Aryl‐P=C=C(H)‐ t Bu]: Hydroboration of the C=C Bond, Deprotonation and Trimerisation

3H‐Phosphaallenes, R−P=C=C(H)C−R’ (3), are accessible in a multigram scale on a new and facile route and show a fascinating chemical reactivity. BH₃(SMe₂) and 3 a (R=Mes*, R’=tBu) afforded by hydroboration of the C=C bonds of two phosphaallene molecules an unprecedented borane (7) with the B atom bound to two P=C double bonds. This compound represents a new FLP based on a B and two P atoms. The increased Lewis acidity of the B atom led to a different reaction course upon treatment of 3 a with H₂B‐C₆F₅(SMe₂). Hydroboration of a C=C bond of a first phosphaallene is followed in a typical FLP reaction by the coordination of a second phosphaallene molecule via B−C and P−B bond formation to yield a BP₂C₂ heterocycle (8). Its B−P bond is short and the B‐bound P atom has a planar surrounding. Treatment of 3 a with tBuLi resulted in deprotonation of the β‐C atom of the phosphaallene (9). The Li atom is bound to the P atom as demonstrated by crystal structure determination, quantum chemical calculations and reactions with HCl, Cl‐SiMe₃ or Cl‐PtBu₂. The thermally unstable phosphaallene Ph−P=C=C(H)‐tBu gave a unique trimeric secondary product by P−P, P−C and C−C bond formation. It contains a P₂C₄ heterocycle and was isolated as a W(CO)₄ complex with two P atoms coordinated to W (15).

Aspects of Phosphaallene Chemistry: Heat-Induced Formation of 1,2-Dihydrophosphetes by Intramolecular Nucleophilic Aromatic Substitution and Photochemical Generation of Tricyclic Phosphiranes

3H-Phosphaallenes are accessible on a new and facile route and show a fascinating chemical behavior. The thermally induced rearrangement of Mes*P═C═C(H)R' (R' = tBu, Ad) afforded by C-H activation, isobutene elimination, and C-C and P-H bond formation bicyclic 1-benzo-dihydrophosphetes (2) with PC₃ heterocycles. DFT calculations suggest a mechanism with intramolecular nucleophilic aromatic substitution and replacement of an alkyl group by the nucleophilic α-C atom of the phosphaallene. These bicycles formed W(CO)₅ complexes (3) or afforded 1,2-dihydrophosphetes with P-bound alkenyl groups by catalyst-free hydrophosphination of alkynes (4 and 5). The resulting bulky phosphines formed complexes with IrCp*Cl₂, RuCl₂, AuCl, or CuO₃SCF₃. The Ru atom is coordinated by the P atom and a phenyl group. Irradiation of TripP═C═C(H)tBu led by the insertion of the central C atom of the P═C═C group into the α-C-H bond of an iPr substituent and by C-C and P-C bond formation to a new isomer of phosphaallenes, 10, which features a strained PC₂ heterocycle. It formed adducts with M(CO)₅ (M = Cr, Mo, W) and AuCl and reacted with SO₂Cl₂ by cleavage of one of the phosphirane P-C bonds to yield PC₄ or PC₅ heterocycles. Hydrolysis yielded a PC₅ compound with a P(O)Cl group.

Al/P- and Ga/P-Based Frustrated Lewis Pairs and Electronically Unsaturated Substrates: Ring Cleavage and Ring Closure, C-C and C-N Bond Formation

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.

3H-Phosphaallenes Revisited: Facile Synthesis by Hydroalumination of Alkynylphosphines and β-Elimination, Stability and Trapping of Transient Species by Coordination to Transition Metal Atoms

A facile method for the efficient synthesis of 3H-phosphaallenes, R-P=C=C(H)-R', is presented, which comprises treatment of dialkynylphosphines with dialkylaluminium hydrides (hydroalumination) and elimination of aluminium alkynides from intermediate alkenyl-alkynylphosphines. The stability of the phosphaallenes depends on steric shielding by the substituents at phosphorus (aryl or CH(SiMe₃ )₂ groups). Only supermesityl compounds are persistent at room temperature in solution. This simple method starting with easily accessible dialkynylphosphines and commercially available aluminium hydrides (HAlEt₂ , HAliBu₂ ) allows the generation of transient species, which were trapped by coordination to transition metals. The η¹ -coordination via a P-W bond was observed for tungsten, while the side-on coordination via the P=C bond resulted with platinum. Decomposition of the mesityl derivative yielded an unprecedented product, which may be formed by 1,3-H shift to the P atom, hydrophosphination of the P=C bond of a second phosphaallene and formation of a P-P bond.

Structure and reactivity of an Al/P-based frustrated Lewis pair bearing relatively small substituents at aluminium

Reaction of Mes₂P─C≡C─Ph (Mes = mesityl) with dineopentylaluminium hydride afforded by hydroalumination a geminal Al/P-based frustrated Lewis pair (FLP; 4). 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 4 yielded adducts with Me₃C─NCO or benzaldehyde via the formation of Al-O and P-C bonds. Trimethylsilyl azide reacted with 4 under surprisingly mild conditions to afford a nitrene complex by spontaneous N₂ 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 4 with a bifunctional methylene-bridged phenylene isocyanate precursor.This article is part of the themed issue 'Frustrated Lewis pair chemistry'.

Hydroalumination of alkynyl-aminophosphines as a promising tool for the synthesis of unusual phosphines: P–N bond activation, a transient phosphaallene, a zwitterionic AlP2C2heterocycle and a masked Al/P-based frustrated Lewis pair

Hydroalumination of alkynyl-aminophosphines afforded an AlP₂C₂heterocycleviaP–N bond activation and a transient phosphaallene.