Reactivity of Dimeric P/Al-Based Lewis Pairs toward Carbon Dioxide and tert-Butyl Isocyanate

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.

Understanding the coupling of non-metallic heteroatoms to CO2 from a Conceptual DFT perspective

CONTEXT

A Conceptual DFT (CDFT) study has been carry out to analyse the coupling reactions of the simplest amine (CH3NH2), alcohol (CH3OH), and thiol (CH3SH) compounds with CO2 to form the corresponding adducts CH3NHCO2H, CH3OCO2H, and CH3SCO2H. The reaction mechanism takes place in a single step comprising two chemical events: nucleophilic attack of the non-metallic heteroatoms to CO2 followed by hydrogen atom transfer (HAT). According to our calculations, the participation of an additional nucleophilic molecule as HAT assistant entails important decreases in activation electronic energies. In such cases, the formation of a six-membered ring in the transition state (TS) reduces the angular stress with respect to the non-assisted paths, characterised by four-membered ring TSs. Through the analysis of the energy and reaction force profiles along the intrinsic reaction coordinate (IRC), the ratio of structural reorganisation and electronic rearrangement for both activation and relaxation energies has been computed. In addition, the analysis of the electronic chemical potential and reaction electronic flux profiles confirms that the highest electronic activity as well as their changes take place in the TS region. Finally, the distortion/interaction model using an energy decomposition scheme based on the electron density along the reaction coordinate has been carried out and the relative energy gradient (REG) method has been applied to identify the most important components associated to the barriers.

METHODS

The theoretical calculation were performed with Gaussian-16 scientific program. The B3LYP-D3(BJ)/aug-cc-pVDZ level was used for optimization of the minima and TSs. IRC calculations has also been carried out connecting the TS with the associated minima. Conceptual-DFT (CDFT) calculations have been carried out with the Eyringpy program and in-house code. The distortion/interaction model along the reaction coordinate have used the decomposition scheme of Mandado et al. and the analysis of the importance of each components have been done with the relative energy gradient (REG) method.

Quest for Active Species in Al/B-Catalyzed CO2 Hydrosilylation

We demonstrate the catalytic role of aluminum and boron centers in aluminum borohydride [(2-Me2CH2C6H4)(C6H5)Al(μ-H)2B(C6H5)2] (6) during carbon dioxide (CO2) hydrosilylation. Preliminary investigations into CO2 reduction using [(2-Me2NCH2C6H4)(H)Al(μ-H)]2 (1) and [Ph3C][B(3,5-C6H3Cl2)4] (2) in the presence of Et3SiH and PhSiH3 resulted in CH2(OSiR3)2 and CH3OSiR3, which serve as formaldehyde and methanol surrogates, respectively. In pursuit of identifying the active catalytic species, three compounds, B(3,5-C6H3Cl2)3 (3), [(2-Me2NCH2C6H4)(3,5-C6H3Cl2)Al(μ-H)2B(3,5-C6H3Cl2)2] (4), and [(2-Me2NCH2C6H4)2Al(THF)][B(3,5-C6H3Cl2)4] (5), were isolated. Among compounds 2-5, the highest catalytic conversion was achieved by 4. Further, 4 and 6 were prepared in a straightforward method by treating 1 with 3 and BPh3, respectively. 6 was found to be in equilibrium with 1 and BPh3, thus making the catalytic process of 6 more efficient than that of 4. Computational investigations inferred that CO2 reduction occurs across the Al-H bond, while Si-H activation occurs through a concerted mechanism involving an in situ generated aluminum formate species and BPh3.

Donor-Acceptor Activation of Carbon Dioxide

The activation and functionalization of carbon dioxide entails great interest related to its abundance, low toxicity and associated environmental problems. However, the inertness of CO2 has posed a challenge towards its efficient conversion to added-value products. In this review we discuss one of the strategies that have been widely used to capture and activate carbon dioxide, namely the use of donor-acceptor interactions by partnering a Lewis acidic and a Lewis basic fragment. This type of CO2 activation resembles that found in metalloenzymes, whose outstanding performance in catalytically transforming carbon dioxide encourages further bioinspired research. We have divided this review into three general sections based on the nature of the active sites: metal-free examples (mainly formed by frustrated Lewis pairs), main group-transition metal combinations, and transition metal heterobimetallic complexes. Overall, we discuss one hundred compounds that cooperatively activate carbon dioxide by donor-acceptor interactions, revealing a wide range of structural motifs.

Differences in the Reactivity of Geminal Si-O-P and Al-O-P Frustrated Lewis Pairs

The new oxygen-bridged geminal Si/P Frustrated Lewis Pair (FLP) tBu2 P-O-Si(C2 F5 )3 (2) is able to reversibly bind carbon dioxide at ambient temperature. We compared its reactivity towards benzil, but-3-en-2-one, nitriles and phenylacetylene to that of the Al/P FLP tBu2 P-O-AlBis2 (Bis=-CH(SiMe3 )2 ) (1). When reacted with benzil, both, 1 and 2, form the 1,2-addition product, but in the Si/P FLP 2, the second carbonyl function additionally binds to the silicon atom. With but-3-en-2-one 2 forms the 1,2-addition product, while 1 binds in 1,4-position. The reaction with acetonitrile yielded an unexpected etheneimine adduct for both systems, while only 1 reacted with tert-butylnitrile. With benzonitrile and acrylonitrile, 2 showed reversible addition to the C≡N bond and 1 forms a stable adduct with benzonitrile. Solely 1 shows reactivity towards phenylacetylene affording a mixture of the CH deprotonation adduct tBu2 P(H)-O-AlBis2 (CCPh) and the FLP -C≡C 1,2-addition adduct under ring formation. All compounds were characterized by multinuclear NMR spectroscopy, XRD and elemental analysis.

A strained intramolecular P/Al-FLP and its reactivity toward allene

FLPs featuring aluminum-phosphane interactions, spring-loaded by a rigid biphenylene linker, have been accessed through a route where trimethyltin units at phosphane-functionalized organic backbones are exchanged by an AlCl₂ moiety. Upon contact with substrates like CO₂ these are readily bound by the Al/P site with release of strain. The system could also be utilized for a unique reactivity, namely the activation of allene.

Use of 5,10-Disubstituted Dibenzoazaborines and Dibenzophosphaborines as Cyclic Supports of Frustrated Lewis Pairs for the Capture of CO2

The reactivity of 5,10-disubstituted dibenzoazaborines and dibenzophosphaborines towards carbon dioxide was studied at the DFT, M06-2X/def2-TZVP, computational level. The profile of this reaction comprises of three stationary points: the pre-reactive complex and adduct minima and the transition state(TS) linking both minima. Initial results show that dibenzoazaborines derivatives are less suitable to form adducts with CO2 than dibenzophosphaborine systems. The influence of the basicity on the P atom and the acidity on the B center of the dibenzophosphaborine in the reaction with CO2 was also explored. Thus, an equation was developed relating the properties (acidity, basicity and boron hybridization) of the isolated dibenzophosphaborine derivatives with the adduct energy. We found that modulation of the boron acidity allows to obtain more stable adducts than the pre-reactive complexes and isolated monomers.

Intramolecular frustrated Lewis pair mediated approach to the CO bond activation and cleavage of carbon dioxide

A thermally stable FLP-CO₂ adduct of pronounced nucleophilic properties that forms a range of Lewis acid-base adducts with strong Lewis acids is reported. Upon addition of Tf₂O, it generates a cationic triflate, which undergoes C-O bond cleavage to give the formal FLP adduct of the elusive dication C₂O₃²⁺.

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.

CO2 Capture by 2-(Methylamino)pyridine Ligated Aluminum Alkyl Complexes

A set of novel, easily synthesized aluminum complexes, Al[κ2-N,N-2-(methylamino)pyridine]2R (R = Et, iBu) are reported. When subjected to 1 atm of CO2 pressure, each hemilabile pyridine arm dissociates and facilitates cooperative activation of the CO2 substrate reminiscent of a Frustrated Lewis Pair. This reaction has limited precedent for Al/N based Lewis Pair systems, and this is the first system readily shown to sequester multiple equivalents of CO2 per aluminum center. The ethyl variant then reacts further, inserting a third equivalent of CO2 into the aluminum alkyl to generate an aluminum carboxylate. Examples of this type of reactivity are rare under thermal conditions. A joint experimental/computational study supports the proposed reaction mechanism.

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.

A synthetic equivalent for unknown 1,3-zwitterions? – A K/OR phosphinidenoid complex with an additional Si–Cl function

Although the chemistry of frustrated Lewis pairs (FLPs) has seen tremendous developments, investigations on anionic, mono-molecular FLPs are still scarce and 1,3-zwitterions are unknown, but a functional K/OR phosphinindenoid complex can be used instead.

New Insights in Frustrated Lewis Pair Chemistry with Azides

The geminal frustrated Lewis pair (FLP) tBu2 PCH2 BPh2 (1) reacts with phenyl-, mesityl-, and tert-butyl azide affording, respectively, six, five, and four-membered rings as isolable products. DFT calculations revealed that the formation of all products proceeds via the six-membered ring structure, which is thermally stable with an N-phenyl group, but rearranges when sterically more encumbered Mes-N3 and tBu-N3 are used. The reaction of 1 with Me3 Si-N3 is believed to follow the same course, yet subsequent N2 elimination occurs to afford a four-membered heterocycle (5), which can be considered as a formal FLP-trimethylsilylnitrene adduct. Compound 5 reacts with hydrochloric acid or tetramethylammonium fluoride and showed frustrated Lewis pair reactivity towards phenylisocyanate.

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.

Non-Innocent Methylene Linker in Bridged Lewis Pair Initiators

Deprotonation usually occurs as an unwanted side reaction in the Lewis pair polymerization of Michael acceptors, for which the conjugated addition of the Lewis base to the acid-activated monomer is the commonly accepted initiation mechanism. This has also been reported for B-P-based bridged Lewis pairs (BLPs) that form macrocyclic addition products. We now show that the formerly unwanted deprotonation is the likely initiation pathway in the case of Al-P-based BLPs. In a detailed study of a series of Al-P-based BLPs, using a combination of single-crystal diffraction experiments (X-ray and neutron) and mechanistic investigations (experimental and computational), an active role of the methylene bridge was revealed, acting as a base towards the α-acidic monomers. Additionally, the polymerization studies proved a living behavior combined with significantly high activities, narrow molecular mass distributions, and the possibility of copolymerization.

Aggregation Behavior of a Six-Membered Cyclic Frustrated Phosphane/Borane Lewis Pair: Formation of a Supramolecular Cyclooctameric Macrocyclic Ring System

A new six-membered cyclic frustrated phosphane/borane Lewis pair was liberated from its HB(C₆ F₅ )₂ adduct by treatment with vinylcyclohexane. The system is an active frustrated Lewis pair that undergoes cycloaddition reactions with suitable π reagents and it splits dihydrogen. At room temperature in solution the new compound is a monomer, however, in the crystal and in solution at low temperature it aggregates to a thermodynamically favoured supramolecular macrocyclic cyclooctamer.

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.

A hydroboration route to geminal P/B frustrated Lewis pairs with a bulky secondary phosphane component and their reaction with carbon dioxide

The secondary aryl-P(H) phosphanyl substituted tert-butylacetylenes 7a,b (aryl: Mes or Mes*) undergo hydroboration with [HB(C₆F₅)₂] to give the geminal vinylidene-bridged P/B Lewis pairs 8a,b. The treatment of 8a,b with benzonitrile, N-sulfinylaniline, and phenyl isothiocyanate, respectively, gives the addition products 12a,b, 13a,b, and 14 with proton transfer from the phosphorus to the more basic nitrogen site. The reaction of the FLPs 8a,b with carbon dioxide yields a doubly boron bonded addition product. The reaction of 8b with a conjugated ynone formally proceeded by trans-1,2-hydrophosphination of the alkyne at the geminal FLP framework to give the seven-membered heterocycle 21.

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.

N-Geminal P/Al Lewis Pair-Alkyne Dipolar Cycloaddition to the Zwitterionic C2PNAl-Heterocyclopentene

The N-geminal P/Al Lewis pair [Ph₂PN(2,6-iPr₂C₆H₃)AlEt₂]₂ (1) has been prepared and studied for reaction with a series of alkynes. The reaction of 1 with RC≡CR yielded zwitterionic C₂PNAl-heterocyclopentene [Ph₂ ^⌜PN(2,6-iPr₂C₆H₃)AlEt₂](CR=^⌝CR) (R = Me (2), Ph (3)); with PhC≡CEt produced two isomers, [Ph₂ ^⌜PN(2,6-iPr₂C₆H₃)AlEt₂](CPh=^⌝CEt) (4a) and [Ph₂ ^⌜PN(2,6-iPr₂C₆H₃)AlEt₂](CEt=^⌝CPh) (4b); and with other alkynes generated [Ph₂ ^⌜PN(2,6-iPr₂C₆H₃)AlEt₂](CR¹=^⌝CR²) (R¹, R² = CO₂Et, Ph (5); SiMe₃, Ph (6); PPh₂, Ph (7); SiMe₃,H (8); H, EtO (9)). Natural bond orbital analysis of the charge separation of the C≡C bond of alkynes was carried out, and then, the electronic matching interaction mode between the combined Lewis acid (AlEt₂) and base (PPh₂) groups of 1 and the C≡C bond of such alkynes was discussed. Reactions of 1 with alkene, nitrile, and carbodiimide molecules were also carried out, and cycloaddition compounds 10-12 were produced.

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.

"Masked" Lewis-acidity of an aluminum α-phosphinoamide complex

The reaction of Ph₂P(DIPP)NH with AlMe₃ cleanly gives an aluminum amide complex that crystallizes as a centrosymmetric dimer with a six-membered Al-N-P-Al-N-P ring. In aromatic solvents the dimer remains intact but the Al-P bond is readily broken upon addition of THF to form Ph₂P(DIPP)NAlMe₂·THF. Efforts to use [Ph₂P(DIPP)NAlMe₂]₂ as a "masked" Lewis acidic activator for olefin polymerization catalysts were unsuccessful but the complex showed a Frustrated Lewis pair reactivity instead. The P/Al complex reacts with isocyanates to give the C[double bond, length as m-dash]O inserted product that crystallizes as a five-membered ring system Al-O-C(NR)-P-N. The reaction of [Ph₂P(DIPP)NAlMe₂]₂ with CO₂, however, gave an insertion in the N-Al bond and the dimeric product [Ph₂P(DIPP)NCO₂AlMe₂]₂ was isolated. The dimer [Ph₂P(DIPP)NAlMe₂]₂ is one of the few Al/P FLPs that can activate C[double bond, length as m-dash]C double bonds irreversibly. A reaction with allyl methyl sulfide and 1-hexene led to the clean formation of the structurally similar activated alkene products [(DIPP)N-Ph₂P-CH(CH₂SMe)CH₂]AlMe₂ and [(DIPP)N-Ph₂P-CH(C₄H₉)CH₂]AlMe₂.

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.

Reactivity of the geminal phosphinoborane tBu2PCH2BPh2 towards alkynes, nitriles, and nitrilium triflates

The reactivity of the geminal phosphinoborane tBu₂PCH₂BPh₂ towards terminal alkynes, nitriles and nitrilium salts is investigated.