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).

Enhancement of Ion Pairing of Sr(II) and Ba(II) Salts by a Tritopic Ion-Pair Receptor in Solution

Tritopic ion-pair receptors can bind bivalent salts in solution; yet, these salts have a tendency to form ion-pairs even in the absence of receptors. The extent to which such receptors can enhance ion pairing has however remained elusive. Here, we study ion pairing of M2+ (Ba2+ , Sr2+ ) and X- (I- , ClO4- ) in acetonitrile with and without a dichlorooxacalix[2]arene[2]triazine-related receptor containing a pentaethylene-glycol moiety. We find marked ion association already in receptor-free solutions. When present, most of the MX+ ion-pairs are bound to the receptor and the overall degree of ion association is enhanced due to coordinative, hydrogen-bonding, and anion-π interactions. The receptor shows higher selectivity for iodides but also stabilizes perchlorates, despite the latter are often considered as weakly coordinating anions. Our results show that ion-pair binding is strongly correlated to ion pairing in these solutions, thereby highlighting the importance of taking ion association in organic solvents into account.

Additive-Free Conversion of Internal Alkynes by Phosphanylalumanes: Production of Phosphorus/Aluminum Frustrated Lewis Pairs

Internal and terminal alkynes react with phosphanylalumanes, i. e., P-Al single-bond species, through heating but without any additional additives. The reactions with internal alkynes afforded the corresponding adducts with cis-1,2-form in moderate yields (54-80 %). In addition, alkyne adducts thus obtained function as P/Al-based C₂ -vicinal FLPs, and the FLP addition reactions with benzaldehyde and CO₂ were demonstrated. Furthermore, in the reaction of 2,4,6-trimethylphenyl-substituted alkyne adducts with dimethyl acetylenedicarboxylate, an unexpected C_Ar -C_Me bond cleavage characteristic of this system was demonstrated.

Using the Secondary PH/BH Functional Groups of an Active Geminal Frustrated Lewis Pair for Carbon Monoxide Reduction and Reactions with Nitriles and Isonitriles

Reaction of the secondary alkynyl(Mes*)PH phosphane 2 with (Fmes)BH₂ ⋅SMe₂ gives the geminal PH/BH frustrated Lewis pair (FLP) 3. The PH and the BH functions are jointly used in the reduction of carbon monoxide under mild reaction conditions to give the [P]-CH₂ -O-[B] product. A subsequent cycloaddition sequence results in the liberation of formaldehyde. The FLP 3 reacts with benzonitrile to give a P-benzamidine, and it couples two isonitriles at the FLP framework.

Triple Frustrated Lewis Pair-Type Reactivity on a Single Rare-Earth Metal Center

Rare-earth metal cations have been used rarely as Lewis-acidic components in the chemistry of frustrated Lewis pairs (FLPs). Herein, we report the first cerium/phosphorus system (2) employing a heptadentate N₄ P₃ ligand, which exhibits triple FLP-type reactivity towards a series of organic substrates, including isocyanates, isothiocyanates, diazomethane, and azides on a single rare-earth Lewis acidic Ce center. This result shows that the Ce center and three P atoms in 2 could simultaneously activate three equivalents of small molecules under mild conditions. This study broadens the diversity of FLPs and demonstrates that rare earth based FLP exhibit unique properties compared with other FLP systems.

Phosphanylalanes and Phosphanylgallanes Stabilized only by a Lewis Base

The synthesis and characterization of the first parent phosphanylalane and phosphanylgallane stabilized only by a Lewis base (LB) are reported. The corresponding substituted compounds, such as IDipp⋅GaH2 PCy2 (1) (IDipp=1,3-bis(2,6-diisopropylphenyl)-imidazolin-2-ylidene) were obtained by the reaction of LiPCy2 with IDipp⋅GaH2 Cl. However, the LB-stabilized parent compounds IDipp⋅GaH2 PH2 (3) and IDipp⋅AlH2 PH2 (4) were prepared via a salt metathesis of LiPH2 ⋅DME with IDipp⋅E'H2 Cl (E'=Ga, Al) or by H2 -elimination reactions of IDipp⋅E'H3 (E'=Ga, Al) and PH3 , respectively. The compounds could be isolated as crystalline solids and completely characterized. Supporting DFT computations gave insight into the reaction pathways as well as into the stability of these compounds with respect to their decomposition behavior.

Synthesis and Characterization of Pheox- and Phebox-Aluminum Complexes: Application as Tunable Lewis Acid Catalysts in Organic Reactions

Pheox- and Phebox-aluminum complexes were synthesized and subsequently characterized by spectroscopic analysis. These complexes acted as Lewis acid catalysts, and their catalytic activities were controlled by using the Pheox, Phebox, and heteroatom ligands. The Pheox-aluminum complex exhibited an opposite substrate selectivity to AlCl₃ in a competitive hetero-Diels-Alder reaction between electron-rich and electron-deficient aldehydes.

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.

LiAlH4 : From Stoichiometric Reduction to Imine Hydrogenation Catalysis

Imine-to-amine conversion with catalytic instead of stoichiometric quantities of LiAlH₄ is demonstrated (85 °C, catalyst loading≥2.5 mol %, pressure≥1 bar). The effects of temperature, pressure, solvent, and catalyst modifications, as well as the substrate scope are discussed. Experimental investigations and preliminary DFT calculations suggest that the catalytically active species is generated in situ: LiAlH₄ +Ph(H)C=NtBu→LiAlH₂ [N(tBu)CH₂ Ph]₂ . A cooperative mechanism in which Li and Al both play a prominent role is proposed.

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.

Activation of Dihydrogen by Masked Doubly Bonded Aluminum Species

Activation of dihydrogen by masked dialumenes (Al=Al doubly bonded species) is reported. Reactions of barrelene-type dialumanes, which have the reactivity as masked equivalents of 1,2-diaryldialumenes ArAl=AlAr, with H2 afforded dihydroalumanes ArAlH2 at room temperature (Ar: bulky aryl groups). These dihydroalumanes form hydrogen-bridged dimers [ArHAl(μ-H)]2 in the crystalline state, while a monomer-dimer equilibrium was suggested in solution. The 1,2-diaryldialumenes generated from the barrelene-type dialumanes are the putative active species in the cleavage of H2 .

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).

Frustrated Lewis pair chemistry: development and perspectives

Frustrated Lewis pairs (FLPs) are combinations of Lewis acids and Lewis bases in solution that are deterred from strong adduct formation by steric and/or electronic factors. This opens pathways to novel cooperative reactions with added substrates. Small-molecule binding and activation by FLPs has led to the discovery of a variety of new reactions through unprecedented pathways. Hydrogen activation and subsequent manipulation in metal-free catalytic hydrogenations is a frequently observed feature of many FLPs. The current state of this young but rapidly expanding field is outlined in this Review and the future directions for its broadening sphere of impact are considered.

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.

Hydrometallation of amino-trialkynylsilanes--intramolecular M-N interactions (M = Al, Ga) and potential activation of Si-N bonds

Hydrometallation of a pyrrolidyl functionalised trialkynylsilane, H8C4N-Si(C≡C-CMe3)3, with equimolar quantities of H-M(CMe3)2 (M = Al, Ga) resulted in the formation of mixed alkenyl-dialkynylsilanes (3a, 3b) which have a Lewis-acidic Al or Ga atom in the geminal position to the Si atom and form four-membered M-N-Si-C heterocycles by a strong interaction of the amine N atoms with the Lewis-acidic metal atoms. This interaction results in a concomitant lengthening and weakening of the Si-N bonds. Dual hydrometallation afforded alkynyl-dialkenylsilanes (4a, 4b) with two Lewis-acidic metal atoms. Al-N and Ga-N interactions to one of the Lewis-acidic centres led again to the formation of M-N-Si-C heterocycles. The second Al atom of 4a interacted with C-H bonds of the vinylic tert-butyl group, while the Ga atom of 4b was coordinated to the α-C atom of the remaining alkynyl substituent. Dual hydrometallation of the corresponding pyrrolyl-trialkynylsilane resulted in compounds with different structures (5a, 5b) due to the delocalisation of the lone pair of electrons at nitrogen in the aromatic ring. One metal atom is coordinated to the α-C atom of the alkynyl group, and the other has close contact to a C-H bond of the pyrrole ring. The synthesis of 4b gave an unprecedented bicyclic by-product (6) which has a Ga-H-Ga 3c-2e bond. It was formally formed by hydrogallation of the trialkyne with the sesquihydride [H2Ga-CMe3]2[H-Ga(CMe3)2]2 and was also obtained by the selective reaction with this starting material.

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)

The Al/P-based frustrated Lewis pair (FLP) Mes2P-C(═CH-Ph)-Al(CMe3)2 (1) reacted with boron halides BX3 (X = F, Cl, Br, I) as an ambiphilic ligand to form complexes (2-5) in which the boron atoms were coordinated to phosphorus and one of the halogen atoms to aluminum. Nonplanar five-membered heterocycles resulted that had five different ring atoms (AlCPBX). The distance of the bridging halogen atoms to the AlCPB plane increased steadily with the radius of the halogen atoms. Only the BF3 adduct showed a dynamic behavior in solution at room temperature with equivalent tert-butyl or mesityl groups in the NMR spectra, while in other cases, the rigid conformation led to the magnetic inequivalence of the substituents at Al and P with well-resolved signals for each group. The BBr3 and BI3 complexes underwent in solution at room temperature a spontaneous stereoselective rearrangement with the concomitant release of isobutene. The obtained products, Mes2P-(μ-C═CH-Ph)(μ-HBX2)-AlX(CMe3) (6 and 7) may be viewed as unique adducts of a modified new Al/P-based FLP, Mes2P-C(═CH-Ph)-AlX(CMe3) (X = Br, I), with dihalogenboranes, HBX2. The trapped boranes are either completely unknown (X = I) or unstable in the free form. Quantum-chemical calculations suggest an ionic rearrangement mechanism via the formation of a borenium cation, β-hydride elimination, and hydride transfer. The bromine migration from boron to aluminum corresponds to a formal suprafacial 1,3-sigmatropic rearrangement.