An Al/P-based frustrated Lewis pair as an efficient ambiphilic ligand: coordination of boron trihalides, rearrangement, and formation of HBX₂ complexes (X = Br, I)

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.

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.

Reactions of an Aluminium/Phosphorus Frustrated Lewis Pair (FLP) with α,β-Unsaturated Carbonyl Compounds: FLPs as Efficient Two-Electron Reductants with the Formation of Enolates, a cis-Enediolate, and an Allene

The Al/P-based frustrated Lewis pair (FLP) Mes₂ P-C(AltBu)₂ =C(H)Ph (1; Mes=mesityl) reacted as an efficient two-electron reductant with benzil to afford a cis-enediolate that was coordinated to the FLP through P-O and Al-O bonds and the formation of a seven-membered heterocycle (2). The phosphorus atom is oxidised from +III to +V. Similar heterocycles (3 a to 3 f) were formed if 1 was treated with various enones (acrolein, acrylate, acrylamide). The resulting enolates are bound to the FLP through P-C and Al-O bonds. Cyclopropenone gave an adduct (4) with the C=O bond coordinated by P and Al. Ynones gave a fascinating variety of different structures. 1,3-Diphenylprop-2-yn-1-one afforded a remarkable allene-type moiety with two cumulated C=C bonds (5); 3-hexyn-2-one yielded a ligand with two conjugated C=C bonds by C-H bond activation at the carbonyl methyl group (7); and 4-(trimethylsilyl)-3-butyn-2-one reacted by C-H bond cleavage, formation of an enolate group with a terminal C=C bond, and shift of the proton to the P atom (8). The C≡C bond was not affected. Allene compound 5 rearranged at elevated temperature and in daylight through the formation of a tricyclic compound by C-H bond activation and C-C bond formation. DFT calculations on this unusual rearrangement suggest insertion of the central allene C atom into the C-H bond of a methyl group and the intermediate formation of a C₃ ring.

A P-H functionalized Al/P-based frustrated Lewis pair - hydrophosphination of nitriles, ring opening with cyclopropenones and evidence of P[double bond, length as m-dash]C double bond formation

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.

Synthesis of an oxygen-linked germinal frustrated Lewis pair and its application in small molecule activation

An oxygen-linked germinal frustrated Lewis pair was synthesized and its reactivities against a series of small molecules were investigated.

Reactions of Al/P, Ga/P and P–H functionalized frustrated Lewis pairs with azides and a diazomethane – formation of adducts and capture of nitrenes

The Al- and Ga-based frustrated Lewis pairs (FLPs) Mes2P–C(MR2)=CH-R′ (1, M=Al, R= t Bu; 2, M=Al, R=CH2 t Bu; 3, M=Ga, R= t Bu) and the unique P–H functionalized FLP Mes(H)P–CH(AlR2)=C(H)- t Bu [4, R=CH(SiMe3)2] were treated with a variety of azides R′-N=N=N [R′= t Bu, SiMe3, Ph, CH2Ph, C6H4(4-Cl), C6H4(4-CF3), C6H4(4-Me), CH2C6H4(4-Cl), CH2C6H4(4- t Bu), 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 (Nγ γ) 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= t Bu, CH2 t Bu) 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(AlCl t Bu2)=C(H)-Ph 13 with an intramolecular N–H···Cl hydrogen bond.

Cooperative Activation of Isocyanates by Al-N-Based Active Lewis Pairs and the Generation of a C5 Chain by Simultaneous Formation of Two C-C Bonds

The active Al/N Lewis pair, (Me₃ C)₂ Al-C(SiMe₃ )=C(H)-N(CHMe-CH₂ )₂ CH₂ (2), reacted with isocyanates to afford a fascinating variety of products. One equivalent of Ph-N=C=O yielded by the release of H-C≡C-SiMe₃ an urea-type ligand which coordinated the Al atom in a chelating manner (4). Dipp-N=C=O gave a similar product, but the bulky substituent hindered the approach of the N-aryl group to Al. A situation similar to that of frustrated Lewis pairs resulted in the coordination of the alkyne to the Al and N atoms (6) by C-H bond activation. Dual insertion was observed upon treatment of 2 with two equivalents of isocyanates (8 to 11). The preferred formation of cyclic oligomers is prevented by the specific cooperative properties of the Lewis pair. A metal-free dimeric isocyanate (13) was formed by hydrolysis. Replacement of the CMe₃ groups in 2 by less bulky isobutyl groups (7) afforded the insertion of two isocyanate molecules into the Al-vinyl bonds without alkyne elimination. The resulting highly functionalised compound had a chain formed by two isocyanates and the organic backbone of the Lewis pair. Me₃ C-N=C=O and 2 afforded a unique compound (14) in which an isocyanate ligand connects two molecules of 2 by the release of dimethylpiperidine. The combination of a C₁ building block and two C₂ groups gave an unsaturated branched C₅ moiety by the simultaneous formation of two C-C bonds. The molecular structure showed an interaction between an Al atom and a C-C π-bond.

Facile Modulation of FLP Properties: A Phosphinylvinyl Grignard Reagent and Ga/P- and In/P2 -Based Frustrated Lewis Pairs

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/P₂ 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.

Reaction of an Al/P-based frustrated Lewis pair with benzophenone: formation of adducts and aluminium alcoholates via β-hydride elimination

Treatment of the Al/P-based frustrated Lewis pair (FLP) Mes2P–C(Al t Bu2)=C(H)-Ph (1) with benzophenone afforded the simple 1:1 adduct (4) with a O=CPh2 molecule coordinated to the aluminum atom by an Al←O donor-acceptor bond. Steric repulsion may prevent an interaction between the electrophilic carbonyl carbon atom and the Lewis-basic phosphorus atom. 4 is unstable in solution at room temperature, the coordination to aluminium increases the polarisation of the carbonyl group and favours its reduction. As suggested by quantum chemical calculations, a C–H bond of a t Bu group approaches the electrophilic center and facilitates β-hydride elimination with the release of isobutene and the formation of an Al–OCHPh2 ligation. An intact O=CPh2 molecule completes the coordination sphere of the metal atom (5). The second t Bu group at aluminium reacts similarly by the selective formation of an Al(OCHPh2)2 moiety (6). The thermodynamics of adduct formation and the mechanism of the hydride shift have been evaluated by quantum chemical DFT calculations.

A Highly Reactive Geminal P/B Frustrated Lewis Pair: Expanding the Scope to C-X (X=Cl, Br) Bond Activation

The geminal frustrated Lewis pair tBu2 PCH2 B(Fxyl)2 (1; Fxyl=3,5-(CF3 )2 C6 H3 ) is accessible in 65 % yield from tBu2 PCH2 Li and (Fxyl)2 BF. According to NMR spectroscopy and X-ray crystallography, 1 is monomeric both in solution and in the solid state. The intramolecular P⋅⋅⋅B distance of 2.900(5) Å and the full planarity of the borane site exclude any significant P/B interaction. Compound 1 readily activates a broad variety of substrates including H2 , EtMe2 SiH, CO2 /CS2 , Ph2 CO, and H3 CCN. Terminal alkynes react with heterolysis of the C-H bond. Haloboranes give cyclic adducts with strong P-BX3 and weak R3 B-X bonds. Unprecedented transformations leading to zwitterionic XP/BCX3 adducts occur on treatment of 1 with CCl4 or CBr4 in Et2 O. In less polar solvents (C6 H6 , n-pentane), XP/BCX3 adduct formation is accompanied by the generation of significant amounts of XP/BX adducts. FLP 1 catalyzes the hydrogenation of PhCH=NtBu and the hydrosilylation of Ph2 CO with EtMe2 SiH.

Lewis acid–base adducts of group 13 elements: synthesis, structure and reactivity toward benzaldehyde

[LGa-M(C₆F₅)₃] (M = B 1, Al 2, Ga 3) reacts with benzaldehyde in different ways including the formation of the zwitterionic species [LGa(C₆F₅){CH(Ph)(OAl(C₆F₅)₂)}] (5).

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.

Alkynyl functionalized Al/P-based frustrated Lewis pairs – aluminium alkynide elimination and evidence for the formation of 3H-phosphaallenes [R-PCC(H)-tBu]

Hydroalumination of dialkynylphosphines by H-Al[CH(SiMe₃)₂]₂afforded alkenyl–alkynyl phosphines which eliminate dialkylaluminium alkynides spontaneously to form phosphaallenes, aryl-PCC(H)-^tBu.

Dative Au→Al interactions: crystallographic characterization and computational analysis

Hitherto unknown Au→Al interactions have been evidenced upon coordination of the geminal phosphorus-aluminum Lewis pair Mes2 PC(=CHPh)AltBu2 (Mes=2,4,6-trimethylphenyl). Four different gold(I) complexes featuring alkyl (Me), aryl (Ph, C6F5), and alkynyl (C≡CPh) co-ligands have been prepared. X-ray diffraction analyses show that P→Au→Al bridging coordination induces noticeable bending of the ligand (the PCAl bond angle shrinks by 13°). This new type of transition metal→Lewis acid interaction has been analyzed by DFT calculations.