Chalcogen Capture by an Al/P-Based Frustrated Lewis Pair: Formation of Al-E-P Bridges and Intermolecular Tellurium–Tellurium Interactions

Aluminum in Frustrated Lewis Pair Chemistry

This review article describes the development of the use of aluminum compounds in the chemistry of frustrated Lewis pairs (FLPs) over the last 14 years. It also discusses the synthesis, reactivity and catalytic applications of intermolecular, intramolecular and so-called hidden FLPs with phosphorus, nitrogen and carbon Lewis bases. The intrinsically higher acidity of aluminum compounds compared to their boron analogs opens up different reaction pathways. The results are presented in a more or less chronological order. It is shown that Al FLPs react with a variety of polar and non-polar substrates and form both stable adducts and reversibly activate bonds. Consequently, some catalytic applications of the title compounds were presented such as dimerization of alkynes, hydrogenation of tert-butyl ethylene and imines, C-F bond activation, reduction of CO2, dehydrogenation of amine borane and transfer of ammonia. In addition, various Al FLPs were used as initiators in polymerization reactions.

Formation of N,N,N',N'-tetramethylethylenediamine via coupling of the two charge reversed C-N bonds of Me3NO in the presence of an Eu(II) bis(trimethylsilyl)amide complex

An N,N,N',N'-tetramethylethylenediamine (TMEDA) complex of Eu(II) was synthesized via an unprecedented self-aminomethylation of Me3NO in the presence of Eu[N(SiMe3)2]2(THF)2. The neutral TMEDA ligand is formed via the coupling of two charge reversed carbon centres from the same precursor Me3NO. In contrast, the reactions of LnII[N(SiMe3)2]2(THF)2 (Ln = Yb, Sm) with Me3NO led to the oxygen-bridged dinuclear complexes of Ln(III).

Frustrated Lewis Pair Chelation in the p-Block

Frustrated Lewis pairs (FLPs) have been the subject of considerable study since the field's inception. While much of the research into FLPs has centered around small molecule activation for diverse stoichiometric and catalytic transformations, intramolecular FLPs also show promise as chelating ligands. The cooperative action of Lewis basic and acidic moieties enables intramolecular FLPs to stabilize low oxidation state centers and (consequently) reactive molecular fragments through a donor-acceptor approach, making them an attractive ligand class in main group element chemistry. This review outlines the state of FLP chelation to date throughout the p-block, encompassing primarily groups 13-16.

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.

An Adduct of Sulfur Monoxide to a Frustrated Sn/P Lewis Pair

The geminal frustrated Lewis pair (F5 C2 )3 SnCH2 P(tBu)2 (1) reacted with N-sulfinylaniline PhNSO to afford the first sulfur monoxide adduct of a main group metal, (F5 C2 )3 SnCH2 P(tBu)2 ⋅SO (2), which contains a SnCPSO ring. The second product is a phenylnitrene adduct of 1. The surprising stability of 2 was compared with the stabilities of the so far inaccessible O2 and S2 adducts of 1. Attempts to prepare these from 1 and the elemental chalcogens (O2 , S8 , Se∞ , Te∞ ) led to four-membered SnCPE ring systems. Quantum-chemical investigations of 2 demonstrate the bond polarity of the SO unit to stabilize 2.

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.

Chalcogen Bonding: An Overview

In the last few decades, "unusual" noncovalent interactions like anion-π and halogen bonding have emerged as interesting alternatives to the ubiquitous hydrogen bonding in many research areas. This is also true, to a somewhat lesser extent, for chalcogen bonding, the noncovalent interaction involving Lewis acidic chalcogen centers. Herein, we aim to provide an overview on the use of chalcogen bonding in crystal engineering and in solution, with a focus on the recent developments concerning intermolecular chalcogen bonding in solution-phase applications. In the solid phase, chalcogen bonding has been used for the construction of nano-sized structures and the self-assembly of sophisticated self-complementary arrays. In solution, until very recently applications mostly focused on intramolecular interactions which stabilized the conformation of intermediates or reagents. In the last few years, intermolecular chalcogen bonding has increasingly also been exploited in solution, most notably in anion recognition and transport as well as in organic synthesis and organocatalysis.

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.

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.

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.

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.