Calcium-Ligand Cooperation Promoted Activation of N2O, Amine, and H2 as well as Catalytic Hydrogenation of Imines, Quinoline, and Alkenes

Bond activation and catalysis using s-block metals are of great significance. Herein, a series of calcium pincer complexes with deprotonated side arms have been prepared using pyridine-based PNP and PNN ligands. The complexes were characterized by NMR and X-ray crystal diffraction. Utilizing the obtained calcium complexes, unprecedented N2O activation by metal-ligand cooperation (MLC) involving dearomatization-aromatization of the pyridine ligand was achieved, generating aromatized calcium diazotate complexes as products. Additionally, the dearomatized calcium complexes were able to activate the N-H bond as well as reversibly activate H2, offering an opportunity for the catalytic hydrogenation of various unsaturated molecules. DFT calculations were applied to analyze the electronic structures of the synthesized complexes and explore possible reaction mechanisms. This study is an important complement to the area of MLC and main-group metal chemistry.

Recent advances in the chemistry of isolable carbene analogues with group 13-15 elements

Carbenes (R2C:), compounds with a divalent carbon atom containing only six valence shell electrons, have evolved into a broader class with the replacement of the carbene carbon or the RC moiety with main group elements, leading to the creation of main group carbene analogues. These analogues, mirroring the electronic structure of carbenes (a lone pair of electrons and an empty orbital), demonstrate unique reactivity. Over the last three decades, this area has seen substantial advancements, paralleling the innovations in carbene chemistry. Recent studies have revealed a spectrum of unique carbene analogues, such as monocoordinate aluminylenes, nitrenes, and bismuthinidenes, notable for their extraordinary properties and diverse reactivity, offering promising applications in small molecule activation. This review delves into the isolable main group carbene analogues that are in the forefront from 2010 and beyond, spanning elements from group 13 (B, Al, Ga, In, and Tl), group 14 (Si, Ge, Sn, and Pb) and group 15 (N, P, As, Sb, and Bi). Specifically, this review focuses on the potential amphiphilic species that possess both lone pairs of electrons and vacant orbitals. We detail their comprehensive synthesis and stabilization strategies, outlining the reactivity arising from their distinct structural characteristics.

Diversity of transformation of heteroallenes on acenaphthene-1,2-diimine aluminum oxide

The reactions of oxide [(dpp-bian)Al(μ2-O)2Al(dpp-bian)] (1) (dpp-bian = 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene) with phenyl- or cyclohexylisocyanates result in the formation of carbonimidate derivatives [(dpp-bian)Al(μ-O)(μ-RNCO2)Al(dpp-bian)] (R = Ph, 2; Cy, 3). Addition of N,N'-dicyclohexylcarbodiimide to compound 1 leads to the formation of ureate complex [(dpp-bian)Al(μ-O)(μ-(CyN)2CO)Al(dpp-bian)] (4). The reactions of the oxide 1 with pinacolborane and catecholborane afford oxo-bridged hydride [{(dpp-bian)Al(H)}(μ-O){Al(OBpin)(dpp-bian)}] (5) and compound [{(dpp-bian)Al(OBCat)}2(μ-O)] (7), respectively. Insertion of cyclohexylisocyanate into the Al-H bond of compound 5 gives CO insertion product [{(dpp-bian)Al(OC(H)NCy)}(μ-O){Al(OBpin)(dpp-bian)}] (6). New compounds have been characterized by ESR and IR spectroscopy; their molecular structures have been established by single-crystal X-ray analysis. The oxide 1 serves as a catalyst for the hydroboration of heteroallenes (isocyanates, carbodiimides) with pinacolborane.

Reductive Coupling of a Diazoalkane Derivative Promoted by a Potassium Aluminyl and Elimination of Dinitrogen to Generate a Reactive Aluminium Ketimide

The reaction of 9-diazo-9H-fluorene (fluN2 ) with the potassium aluminyl K[Al(NON)] ([NON]2- =[O(SiMe2 NDipp)2 ]2- , Dipp=2,6-iPr2 C6 H3 ) affords K[Al(NON)(κN1 ,N3 -{(fluN2 )2 })] (1). Structural analysis shows a near planar 1,4-di(9H-fluoren-9-ylidene)tetraazadiide ligand that chelates to the aluminium. The thermally induced elimination of dinitrogen from 1 affords the neutral aluminium ketimide complex, Al(NON)(N=flu)(THF) (2) and the 1,2-di(9H-fluoren-9-yl)diazene dianion as the potassium salt, [K2 (THF)3 ][fluN=Nflu] (3). The reaction of 2 with N,N'-diisopropylcarbodiimide (iPrN=C=NiPr) affords the aluminium guanidinate complex, Al(NON){N(iPr)C(N=CMe2 )N(CHflu)} (4), showing a rare example of reactivity at a metal ketimide ligand. Density functional theory (DFT) calculations have been used to examine the bonding in the newly formed [(fluN2 )2 ]2- ligand in 1 and the ketimide bonding in 2. The mechanism leading to the formation of 4 has also been studied using this technique.

Aluminium and Gallium Silylimides as Nitride Sources

Terminal aluminium and gallium imides of the type K[(NON)M(NR)], bearing heteroatom substituents at R, have been synthesised via reactions of anionic aluminium(I) and gallium(I) reagents with silyl and boryl azides (NON=4,5-bis(2,6-diisopropyl-anilido)-2,7-di-tert-butyl-9,9-dimethyl-xanthene). These systems vary significantly in their lability in solution: the N(Sii Pr3 ) and N(Boryl) complexes are very labile, on account of the high basicity at nitrogen. Phenylsilylimido derivatives provide greater stabilization through the π-acceptor capabilities of the SiR3 group. K[(NON)AlN(Sit BuPh2 )] offers a workable compromise between stability and solubility, and has been completely characterized by spectroscopic, analytical and crystallographic methods. The silylimide species examined feature minimal π-bonding between the imide ligand and aluminium/gallium, with the HOMO and HOMO-1 orbitals effectively comprising orthogonal lone pairs centred at N. Reactivity-wise, both aluminium and gallium silylimides can act as viable sources of nitride, [N]3- , with systems derived from either metal reacting with CO to afford cyanide complexes. By contrast, only the gallium system K[(NON)Ga{N(SiPh3 )}] is capable of effecting a similar transformation with N2 O to yield azide, N3 - , via formal oxide/nitride metathesis. The aluminium systems instead generate RN3 via transfer of the imide fragment [RN]2- .

Infrared Spectroscopic and Theoretical Investigations of Group 13 Oxyfluorides OMF2 and OMF (M=B, Al, Ga, In)

Group 13 oxyfluorides OMF2 were produced by the reactions of laser-ablated group 13 atoms M (M=B, Al, Ga and In) with OF2 and isolated in excess neon or argon matrices at 5 K. These molecules were characterized by matrix-isolation infrared spectroscopy and isotopic substitution experiments in conjunction with quantum-chemical calculations. The calculations indicate that the OMF2 molecules have a 2 B2 ground state with C2v symmetry. The computed molecular orbitals and spin densities show that the unpaired electron is mainly located at the terminal oxygen atom. Oxo monofluorides OMF were only observed in solid argon matrices and exhibit a linear structure in the singlet ground state. The M-O bonding in the OMF molecules can be rationalized as highly polar multiple bonds based on the calculated bond lengths and natural resonance theory (NRT) analyses. In particular, the molecular orbitals of OBF exhibit the character of a triple bond B-O resulting from two degenerate electron-sharing π bonds and an O→B dative σ bond formed by the oxygen 2p lone pair which donates electron density to the boron empty 2p orbital.

Crystalline Neutral Aluminum Selenide/Telluride: Isoelectronic Aluminum Analogues of Carbonyls

Neutral aluminum chalcogenides (R-Al(L)═Ch; L = ligand, Ch = chalcogen), stabilized by a Lewis base ligand, represent isoelectronic counterparts to carbonyl compounds and have long been pursued for isolation. Herein, we present the synthesis of an aluminum selenide, [N]-Al(ⁱPr₂-bimy)═Se, and an aluminum telluride, [N]-Al(ⁱPr₂-bimy)═Te, under ambient conditions ([N] = 1,8-bis(3,5-di-tert-butylphenyl)-3,6-di-tert-butylcarbazolyl; ⁱPr₂-bimy = 1,3-diisoproplylbenzimidazole-2-ylidene). These compounds arise from the oxidation reaction of [N]-Al(ⁱPr₂-bimy) with Se and (ⁿBu)₃P═Te, respectively. One notable characteristic of the Al and Ch interaction is the presence of an Al-Ch σ bond, strengthened by the electrostatic attraction between the Al⁺ and Ch^- centers as well as the donation of lone pairs from Ch into vacant orbitals at Al. This results in an Al-Ch multiple bond with an ambiphilic nature. Preliminary investigations into their reactivity unveil their remarkable propensity for facile (cyclo)addition reactions with diverse substrates, including PhCCH, PhCN, AdN₃, MeI, PhSiH₃, and C₆F₆, leading to the formation of unprecedented main group heterocycles and alumachalcogenides.

Controlled reductive C-C coupling of isocyanides promoted by an aluminyl anion

We report the reaction of the potassium aluminyl, K[Al(NON)] ([NON]^2- = [O(SiMe₂NDipp)₂]^2-, Dipp = 2,6-iPr₂C₆H₃) with a series of isocyanide substrates (R-NC). In the case of tBu-NC, degradation of the isocyanide was observed generating an isomeric mixture of the corresponding aluminium cyanido-κC and -κN compounds, K[Al(NON)(H)(CN)]/K[Al(NON)(H)(NC)]. The reaction with 2,6-dimethylphenyl isocyanide (Dmp-NC), gave a C₃-homologation product, which in addition to C-C bond formation showed dearomatisation of one of the aromatic substituents. In contrast, using adamantyl isocyanide Ad-NC allowed both the C₂- and C₃-homologation products to be isolated, allowing a degree of control to be exercised over the chain growth process. These data also show that the reaction proceeds through a stepwise addition, supported in this study by the synthesis of the mixed [(Ad-NC)₂(Dmp-NC)]^2- product. Computational analysis of the bonding within the homologised products confirm a high degree of multiple bond character in the exocyclic ketenimine units of the C₂- and C₃-products. In addition, the mechanism of chain growth was investigated, identifying different possible pathways leading to the observed products, and highlighting the importance of the potassium cation in formation of the initial C₂-chain.

An Aluminum Telluride with a Terminal Al=Te Bond and its Conversion to an Aluminum Tellurocarbonate by CO2 Reduction

Facile access to dimeric heavier aluminum chalcogenides [(NHC)Al(Tipp)-μ-Ch]₂ (NHC=IiPr (1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene, IMe₄ (1,3,4,5-tetramethylimidazol-2-ylidene); Tipp=2,4,6-iPr₃ C₆ H₂ ; Ch=Se, Te) by treatment of NHC-stabilized aluminum dihydrides with elemental Se and Te is reported. The higher affinity of IMe₄ in comparison with IiPr toward the Al center in [(NHC)Al(Tipp)-μ-Ch]₂ can be used for ligand exchange. Additionally, the presence of excess IMe₄ allows for cleavage of the dimers to form a rare example of a neutral multiply bonded heavier aluminum chalcogenide in the form of a tetracoordinate aluminum complex, (IMe₄ )₂ (Tipp)Al=Te. This species reacts with three equivalents of CO₂ across two Al-C^NHC and the Al=Te bond affording a pentacoordinate aluminum complex containing a dianionic tellurocarbonate ligand [CO₂ Te]^2- , which is the first example of tellurium analogue of a carbonate [CO₃ ]^2- .

Reduction chemistry yields stable and soluble divalent lanthanide tris(pyrazolyl)borate complexes

Reduction of the heteroleptic Ln(III) precursors [Ln(Tp)₂(OTf)] (Tp = hydrotris(1-pyrazolyl)borate; OTf = triflate) with either an aluminyl(I) anion or KC₈ yielded the adduct-free homoleptic Ln(II) complexes dimeric 1-Eu [{Eu(Tp)(μ-κ¹:η⁵-Tp)}₂] and monomeric 1-Yb [Yb(Tp)₂]. Complexes 1-Ln have good solubility and stability in both non-coordinating and coordinating solvents. Reaction of 1-Ln with 2 Ph₃PO yielded 1-Ln(OPPh3)2. All complexes are intensely coloured and 1-Eu is photoluminescent. The electronic absorption data show the 4f-5d electronic transitions in Ln(II). Single-crystal X-ray diffraction data reveal first μ-κ¹:η⁵-coordination mode of the unsubstituted Tp ligand to lanthanides in 1-Eu.

The emerging chemistry of the aluminyl anion

The chemistry of low valent p-block metal complexes continues to elicit interest in the research community, demonstrating reactivity that replicates and in some cases exceeds that of their more widely studied d-block metal counterparts. The introduction of the first aluminyl anion, a complex containing a formally anionic Al(I) centre charge balanced by an alkali metal (AM) cation, has established a platform for a new area of chemical research. The chemistry displayed by aluminyl compounds is expanding rapidly, with examples of reactivity towards a diverse range of small molecules and functional groups now reported in the literature. Herein we present an account of the structure and reactivity of the growing family of aluminyl compounds. In this context we examine the structural relationships between the aluminyl anion and the AM cations, which now include examples of AM = Li, Na, K, Rb and Cs. We report on the ability of these compounds to engage in bond-breaking and bond-forming reactions, which is leading towards their application as useful reagents in chemical synthesis. Furthermore we discuss the chemistry of bimetallic complexes containing direct Al-M bonds (M = Li, Na, K, Mg, Ca, Cu, Ag, Au, Zn) and compounds with Al-E multiple bonds (E = NR, CR₂, O, S, Se, Te), where both classes of compound are derived directly from aluminyl anions.

An Aluminium Imide as a Transfer Agent for the [NR]2- Function via Metathesis Chemistry

The reactions of a terminal aluminium imide with a range of oxygen-containing substrates have been probed with a view to developing its use as a novel main group transfer agent for the [NR]^2- fragment. We demonstrate transfer of the imide moiety to [N₂ ], [CO] and [Ph(H)C] units driven thermodynamically by Al-O bond formation. N₂ O reacts rapidly to generate the organoazide DippN₃ (Dipp=2,6-ⁱ Pr₂ C₆ H₃ ), while CO₂ (under dilute reaction conditions) yields the corresponding isocyanate, DippNCO. Mechanistic studies, using both experimental and quantum chemical techniques, identify a carbamate complex K₂ [(NON)Al-{κ² -(N,O)-N(Dipp)CO₂ }]₂ (formed via [2+2] cycloaddition) as an intermediate in the formation of DippNCO, and also in an alternative reaction leading to the generation of the amino-dicarboxylate complex K₂ [(NON)Al{κ² -(O,O')-(O₂ C)₂ N-(Dipp)}] (via the take-up of a second equivalent of CO₂ ). In the case of benzaldehyde, a similar [2+2] cycloaddition process generates the metallacyclic hemi-aminal complex, K_n [(NON)Al{κ² -(N,O)-(N(Dipp)C(Ph)(H)O}]_n . Extrusion of the imine, PhC(H)NDipp, via cyclo-reversion is disfavoured thermally, due to the high energy of the putative aluminium oxide co-product, K₂ [(NON)Al(O)]₂ . However, addition of CO₂ allows the imine to be released, driven by the formation of the thermodynamically more stable aluminium carbonate co-product, K₂ [(NON)Al(κ² -(O,O')-CO₃ )]₂ .

Synthesis, Characterization, and Structural Analysis of [Al(NONDipp)(H)(SiH2Ph)] ( = Li, Na, K, Rb, Cs) Compounds, Made Via Oxidative Addition of Phenylsilane to Alkali Metal Aluminyls

We report the oxidative addition of phenylsilane to the complete series of alkali metal (AM) aluminyls [AM{Al(NON^Dipp)}]₂ (AM = Li, Na, K, Rb, and Cs). Crystalline products (1-AM) have been isolated as ether or THF adducts, [AM(L)_n][Al(NON^Dipp)(H)(SiH₂Ph)] (AM = Li, Na, K, Rb, L = Et₂O, n = 1; AM = Cs, L = THF, n = 2). Further to this series, the novel rubidium rubidiate, [{Rb(THF)₄}₂(Rb{Al(NON^Dipp)(H)(SiH₂Ph)}₂)]⁺ [Rb{Al(NON^Dipp)(H)(SiH₂Ph)}₂]^-, was isolated during an attempted recrystallization of Rb[Al(NON^Dipp)(H)(SiH₂Ph)] from a hexane/THF mixture. Structural and spectroscopic characterizations of the series 1-AM confirm the presence of μ-hydrides that bridge the aluminum and alkali metals (AM), with multiple stabilizing AM···π(arene) interactions to either the Dipp- or Ph-substituents. These products form a complete series of soluble, alkali metal (hydrido) aluminates that present a platform for further reactivity studies.

Hydroboration and Deoxygenation of CO2 Mediated by a Gallium(I) Cation

Hydroboration of CO₂ to formoxy borane occurs under ambient conditions in acetonitrile using pinacolborane HBpin in the presence of gallium(I) cation [(Me₄TACD)Ga][BAr₄] (1; Me₄TACD = N,N',N″,N'''-tetramethyl-1,4,7,10-tetraazacyclododecane; Ar = C₆H₃-3,5-Me₂). Slow turnover was accompanied by side reactions including ligand scrambling of HBpin to give BH₃(CH₃CN) and crystalline B₂pin₃. When 1 was reacted with CO₂ alone, the formation of the gallium(III) carbonato complex [(Me₄TACD)Ga(κ₂-O₂CO)][BAr₄] (3) along with CO was observed. This complex was assumed to form via the unstable oxido cation [(Me₄TACD)Ga=O]⁺ (4). Reaction of 1 with N₂O in the presence of BPh₃ confirmed the formation of the oxido cation, which was spectroscopically characterized as a triphenylborane adduct [(Me₄TACD)Ga=O(BPh₃)][BAr₄] (4·BPh₃). CO was also detected when CO₂ was reacted with 1 in the presence of HBpin, suggesting that compound 3 may also be formed in initial stages of catalysis. Compound 3 reacts with HBpin to give formoxy borane, borane redistribution products, and an unidentified Me₄TACD-containing species 5, which was also observed in "catalytic" runs starting from 1, HBpin, and CO₂. Hydroboration of CO₂ using HBpin with slow turnover and competitive ligand scrambling was also observed in the presence of gallium(III) hydride dication [(Me₄TACD)GaH][BAr₄]₂ (2), which is unreactive toward CO₂ in the absence of HBpin.

Deoxygenating Reduction of CO2 by [Cp*Al]4 to Form a (Al3O2C)2 Cluster Featuring Two Ketene Moieties

Herein we report that the reaction of the low-valent aluminum(I) species [Cp*Al]₄ (Cp* = pentamethylcyclopentadienyl) with CO₂ exhibits complete cleavages of the C═O bonds. The deoxygenating reduction reaction of [Cp*Al]₄ with CO₂ at 120 °C afforded [(Cp*)₃Al₃O₂C(CO)]₂ (1), which featured two stacked (Al₃O₂C)₂ units and two C═C═O ketene moieties. Moreover, the isoelectronic analogues of diimine and isothiocyanate with CO₂ were also investigated, and the reactions of [Cp*Al]₄ with Dipp*-N═C═N-Dipp* and Dipp-C═N═S [Dipp* = 2,6-bis(diphenylmethyl)-4-tert-butylphenyl; Dipp = 2,6-diisopropylphenyl] afforded dialuminylimine (2) and tetrameric [Cp*AlS]₄ (3), respectively.

Isolating elusive 'Al(μ-O)M' intermediates in CO2 reduction by bimetallic Al-M complexes (M = Zn, Mg)

The reaction of compounds containing Al-Mg and Al-Zn bonds with N₂O enabled isolation of the corresponding Al(μ-O)M complexes. Electronic structure analysis identified largely ionic Al-O and O-M bonds, featuring an anionic μ-oxo centre. Reaction with CO₂ confirmed that these species correspond to the proposed intermediates in the formation of μ-carbonate compounds.

Diverse reactivity of an Al(I)-centred anion towards ketones

The reactivity of a seven-membered cyclic potassium diamidoalumanyl toward a variety of ketone small molecules has been assessed. Whilst acetophenone generates an aluminium pinacolate derivative, reductive C-C coupling is induced between the ketyl and ortho-carbon centres of two equivalents of benzophenone. In contrast, whereas oxidative addition of an enolisable proton is observed with 2,4-dimethyl-3-pentanone, 2,2,4,4-tetramethyl-3-pentanone undergoes an unprecedented hydroalumination process, where the reducing hydride may be traced to intramolecular oxidative addition of a (sp³)C-H bond.

Facile C-H bond activation on a transient gallium imide

Reaction of NacNacGa with azide N₃SiMe₃ results in the generation of a transient imide NacNacGa(NSiMe₃) that can cleave unactivated sp³ C-H and sp² C-H bonds of different substrates, affording gallium amides. Pyridine, cyclohexanone, ethyl acetate, DMSO, and triethylphosphine oxide were activated in this process producing corresponding gallium amides. All new compounds were characterised by multinuclear NMR and X-ray diffraction.

Carbon-chalcogen bond formation initiated by [Al(NONDipp)(E)]- anions containing Al-E{16} (E{16} = S, Se) multiple bonds

Multiply-bonded main group metal compounds are of interest as a new class of reactive species able to activate and functionalize a wide range of substrates. The aluminium sulfido compound K[Al(NONDipp)(S)] (NONDipp = [O(SiMe2NDipp)2]2-, Dipp = 2,6-iPr2C6H3), completing the series of [Al(NONDipp)(E)]- anions containing Al-E{16} multiple bonds (E{16} = O, S, Se, Te), was accessed via desulfurisation of K[Al(NONDipp)(S4)] using triphenylphosphane. The crystal structure showed a tetrameric aggregate joined by multiple K⋯S and K⋯π(arene) interactions that were disrupted by the addition of 2.2.2-cryptand to form the separated ion pair, [K(2.2.2-crypt)][Al(NONDipp)(S)]. Analysis of the anion using density functional theory (DFT) confirmed multiple-bond character in the Al-S group. The reaction of the sulfido and selenido anions K[Al(NONDipp)(E)] (E = S, Se) with CO2 afforded K[Al(NONDipp)(κ2 E,O-EC{O}O)] containing the thio- and seleno-carbonate groups respectively, consistent with a [2 + 2]-cycloaddition reaction and C-E bond formation. An analogous cycloaddition reaction took place with benzophenone affording compounds containing the diphenylsulfido- and diphenylselenido-methanolate ligands, [κ2 E,O-EC{O}Ph2]2-. In contrast, when K[Al(NONDipp)(E)] (E = S, Se) was reacted with benzaldehyde, two equivalents of substrate were incorporated into the product accompanied by formation of a second C-E bond and complete cleavage of the Al-E{16} bonds. The products contained the hitherto unknown κ2 O,O-thio- and κ2 O,O-seleno-bis(phenylmethanolate) ligands, which were exclusively isolated as the cis-stereoisomers. The mechanisms of these cycloaddition reactions were investigated using DFT methods.

Sigma/pi Bonding Preferences of Solvated Alkali-Metal Cations to Ditopic Arylmethyl Anions

Arylmethyl anions allow alkali-metals to bind in a σ-fashion to the lateral carbanionic centre or a π-fashion to the aryl ring or in between these extremities, with the trend towards π bonding increasing on descending group 1. Here we review known alkali metal structures of diphenylmethane, fluorene, 2-benzylpyridine and 4-benzylpyridine. Next, we synthesise Li, Na, K monomers of these diarylmethyls using polydentate donors PMDETA or Me6 TREN to remove competing oligomerizing interactions, studying the effect that two aromatic rings has on negative charge (de)localisation via NMR, X-ray crystallographic and DFT studies. Diphenylmethyl and fluorenyl anions maintain C(H)-M interactions regardless of alkali-metal, although the adjacent arene carbons engage in interactions with larger alkali-metals. Introducing a nitrogen atom into the ring (at the 2- or 4-position) encourages relocalisation of negative charge away from the deprotonated carbon and onto nitrogen. Phenyl(2-pyridyl)methyl moves from an enamide formation at one extremity (lithium) to an aza-allyl formation at the other extremity (potassium), while C- or N-coordination modes become energetically viable for Na and K phenyl(4-pyridyl)methyl complexes.

Cycloaddition of isoselenocyanates to sodium and magnesium metallacycles

Heterocumulenes SeCNR (R = C₆H₄OMe-2, C₆H₄Me-2) undergo facile cycloaddition to [(H-dpp-bian)Na(Et₂O)₂] (1) (H-dpp-bian = N-protonated 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene) resulting in cycloadducts [(H-dpp-bian)Na(SeCNR)(DME)] (2, 3), which are the first cycloadducts derived from a sodium metallacycle reported so far. A comparative reaction of [(dpp-bian)Mg(THF)₃] (10) with SeCNR gives magnesium cycloadducts [(dpp-bian)Mg(SeCNR)(Solv)₂] (11, 12), which undergo fast decomposition at room temperature. New compounds are characterized by NMR, EPR, and IR spectroscopy, and elemental and X-ray diffraction analysis. Their electronic structures and reaction pathways were probed using DFT calculations.

A Tetraorganyl-Alumaborane with An Al-B σ-Bond and Two Adjacent Lewis-Acidic Centers

A tetraorganyl-alumaborane (3) that contains an Al-B bond and twisted Al and B planes was synthesized and structurally characterized. UV-vis absorption spectroscopy, electrochemical measurement, and DFT calculations were employed to reveal the electronic properties of 3. The reactivity of 3 toward DMSO and CO was studied to demonstrate its deoxygenating abilities. On the basis of the results of the DFT calculations, a detailed reaction mechanism was developed, which highlighted the important role of the distinct Lewis acidity of the group-13 elements Al and B in 3.

Bismuth Redox Catalysis: An Emerging Main-Group Platform for Organic Synthesis

Bismuth has recently been shown to be able to maneuver between different oxidation states, enabling access to unique redox cycles that can be harnessed in the context of organic synthesis. Indeed, various catalytic Bi redox platforms have been discovered and revealed emerging opportunities in the field of main group redox catalysis. The goal of this perspective is to provide an overview of the synthetic methodologies that have been developed to date, which capitalize on the Bi redox cycling. Recent catalytic methods via low-valent Bi(II)/Bi(III), Bi(I)/Bi(III), and high-valent Bi(III)/Bi(V) redox couples are covered as well as their underlying mechanisms and key intermediates. In addition, we illustrate different design strategies stabilizing low-valent and high-valent bismuth species, and highlight the characteristic reactivity of bismuth complexes, compared to the lighter p-block and d-block elements. Although it is not redox catalysis in nature, we also discuss a recent example of non-Lewis acid, redox-neutral Bi(III) catalysis proceeding through catalytic organometallic steps. We close by discussing opportunities and future directions in this emerging field of catalysis. We hope that this Perspective will provide synthetic chemists with guiding principles for the future development of catalytic transformations employing bismuth.

Extending Chain Growth Beyond C1 → C4 in CO Homologation: Aluminyl Promoted Formation of the [C5O5]5– Ligand

The TMEDA supported potassium aluminyl (NONDipp)Al–K(TMEDA)2 (NONDipp = [O(SiMe2NDipp)2]2–, Dipp = 2,6-iPr2C6H3), containing a Al–K bond, activates and reductively couples cabon monoxide gas to give a new aluminium species contianing...

A Highly Strained Al-Al σ-Bond in Dianionic Aluminum Analog of Oxirane for Molecule Activation

Since aluminum is the most electropositive element among the p-block elements, the construction of molecules bearing a dianionic Al-Al σ-bond is inherently highly challenging. Herein, we report the first synthesis of a dianionic dialane(6) 2 based on the Al₂O three-membered ring scaffold, namely, an aluminum analog of oxirane. The structure of 2 has been unambiguously ascertained by spectroscopic analysis as well as X-ray crystallography, and computational studies revealed that 2 bears a highly strained Al-Al σ-bond. 2 readily reacts with the unsaturated substrates such as isocyanide, ethylene, and ketone, concomitant with the cleavage of the Al-Al σ-bond under mild conditions, leading to the four- and five-membered heterocycles 3-5. Furthermore, the reaction of 2 with two molecules of benzonitrile (PhCN) furnishes a seven-membered heterocycle 6, resulting from the C-C coupling reaction of PhCN. We further delineate that 2 selectively activates an arene ring C-C bond of biphenylene, rendering a di-Al-substituted benzo[8]annulene derivative 7. Preliminary computational studies propose that the stepwise reaction mechanism involves the Al-Al σ-bond cleavage, dearomative Al-C bond formation, subsequent sigmatropic [1,3]shifts, and a pericyclic reaction.

Nitrous oxide as a diazo transfer reagent: the synthesis of triazolopyridines

Nitrous oxide is a potential diazo transfer reagent, but its applications in organic chemistry are scarce. Here, we show that triazolopyridines and triazoloquinolines are formed in the reactions of metallated 2-alkylpyridines or 2-alkylquinolines with N₂O. The reactions can be performed under mild conditions and give synthetically interesting triazoles in moderate to good yields.

Probing the Extremes of Covalency in M-Al bonds: Lithium and Zinc Aluminyl Compounds

Synthetic routes to lithium, magnesium, and zinc aluminyl complexes are reported, allowing for the first structural characterization of an unsupported lithium-aluminium bond. Crystallographic and quantum-chemical studies are consistent with the presence of a highly polar Li-Al interaction, characterized by a low bond order and relatively little charge transfer from Al to Li. Comparison with magnesium and zinc aluminyl systems reveals changes to both the M-Al bond and the (NON)Al fragment (where NON=4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene), consistent with a more covalent character, with the latter complex being shown to react with CO₂ via a pathway that implies that the zinc centre acts as the nucleophilic partner.

Synthesis and Reactivity of Acyclic Germanimines: Silyl Rearrangement and Cycloadditions

We report the synthesis of aromatic germanimines [(HMDS)₂Ge═NAr] (Ar = Ph, Mes, Dipp; Mes = 2,4,6-Me₃C₆H₂, Dipp = 2,6-iPr₂C₆H₃) and an investigation into their associated reactivity. [(HMDS)₂Ge═NPh] decomposes above -30 °C, while [(HMDS)₂Ge═NDipp] engages in an intramolecular reaction at 60 °C. [(HMDS)₂Ge═NMes] was shown to rearrange via a 1,3-silyl migration to give [(HMDS){(SiMe₃)(Mes)N}Ge(NSiMe₃)] in a 1:7 equilibrium mixture at room temperature. These latter germanimines react with unsaturated polar substrates such as CO₂, ketones, and arylisocyanate via a [2 + 2] cycloaddition pathway.

Acid-Base Free Main Group Carbonyl Analogues

Main group carbonyl analogues (R2 E=O) derived from p-block elements (E=groups 13 to 15) have long been considered as elusive species. Previously, employment of chemical tricks such as acid- and base-stabilization protocols granted access to these transient species in their masked forms. However, electronic and steric effects inevitably perturb their chemical reactivity and distinguish them from classical carbonyl compounds. A new era was marked by the recent isolation of acid-base free main group carbonyl analogues, ranging from a lighter boracarbonyl to the heavier silacarbonyls, phosphacarbonyls and a germacarbonyl. Most importantly, their unperturbed nature elicits exciting new chemistry, spanning the vista from classical organic carbonyl-type reactions to transition metal-like oxide ion transfer chemistry. In this Review, we survey the strategies used for the isolation of such systems and document their emerging reactivity profiles, with a view to providing fundamental comparisons both with carbon and transition metal oxo species. This highlights the emerging opportunities for exciting "crossover" reactivity offered by these derivatives of the p-block elements.

Strongly Polarized Iridiumδ--Aluminumδ+ Pairs: Unconventional Reactivity Patterns Including CO2 Cooperative Reductive Cleavage

The iridium tetrahydride complex Cp*IrH₄ reacts with a range of isobutylaluminum derivatives of general formula Al(iBu)_x(OAr)_3-x (x = 1, 2) to give the unusual iridium aluminum species [Cp*IrH₃Al(ⁱBu)(OAr)] (1) via a reductive elimination route. The Lewis acidity of the Al atom in complex 1 is confirmed by the coordination of pyridine, leading to the adduct [Cp*IrH₃Al(ⁱBu)(OAr)(Py)] (2). Spectroscopic, crystallographic, and computational data support the description of these heterobimetallic complexes 1 and 2 as featuring strongly polarized Al(III)^δ+-Ir(III)^δ- interactions. Reactivity studies demonstrate that the binding of a Lewis base to Al does not quench the reactivity of the Ir-Al motif and that both species 1 and 2 promote the cooperative reductive cleavage of a range of heteroallenes. Specifically, complex 2 promotes the decarbonylation of CO₂ and AdNCO, leading to CO (trapped as Cp*IrH₂(CO)) and the alkylaluminum oxo ([(iBu)(OAr)Al(Py)]₂(μ-O) (3)) and ureate ({Al(OAr)(ⁱBu)[κ²-(N,O)AdNC(O)NHAd]} (4)) species, respectively. The bridged amidinate species Cp*IrH₂(μ-CyNC(H)NCy)Al(ⁱBu)(OAr) (5) is formed in the reaction of 2 with dicyclohexylcarbodiimine. Mechanistic investigations via DFT support cooperative heterobimetallic bond activation processes.

Oxidative Addition of Hydridic, Protic, and Nonpolar E-H Bonds (E = Si, P, N, or O) to an Aluminyl Anion

The aluminyl anion K[Al(NON^Dipp)] {NON^Dipp = [O(SiMe₂NDipp)₂]^2-; Dipp = 2,6-iPr₂C₆H₃} engages in oxidative additions with the E-H (E = Si, P, N, or O) bonds of phenylsilane (PhSiH₃), mesityl phosphane (MesPH₂; Mes = 2,4,6-Me₃C₆H₂), 2,6-di-iso-propylaniline (DippNH₂), and 2,6-di-tert-butyl-4-methylphenol (ArOH). The resulting (hydrido)aluminate salts are formed regardless of the E-H bond polarity. All of the products were characterized by nuclear magnetic resonance and infrared spectroscopic techniques and single-crystal X-ray diffraction. This study highlights the versatility of aluminyl anions to activate hydridic, acidic, and (essentially) nonpolar E-H bonds.

A Xanthene-Based Mono-Anionic PON Ligand: Exploiting a Bulky, Electronically Unsymmetrical Donor in Main Group Chemistry

The synthesis of a novel mono-anionic phosphino-amide ligand based on a xanthene backbone is reported, togetherr with the corresponding GaI complex, (PON)Ga (PON = 4-(di(2,4,6-trimethylphenyl)phosphino)-5-(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene). The solid-state structure of (PON)Ga (obtained from X-ray crystallography) reveals very weak O⋅⋅⋅Ga and P⋅⋅⋅Ga interactions, consistent with a R2 NGa fragment which closely resembles those found in one-coordinate amidogallium systems. Strong N-to-Ga π donation from the amido substituent is reflected in a very short N-Ga distance (1.961(2) Å), while the P⋅⋅⋅Ga contact (3.076(1) Å) is well outside the sum of the respective covalent radii. While the donor properties of the PON ligand towards GaI are highly unsymmetrical, oxidation to GaIII leads to much stronger coordination of the pendant phosphine as shown by P-Ga distances which are up to 20 % shorter. From a steric perspective, the PON ligand is shown to be significantly bulkier than related β-diketiminate systems, a finding consistent with reactions of (PON)Ga towards O-atom sources that proceed without oligomerization. Despite this, the enhanced P-donor properties brought about by oxidation at gallium are not sufficient to quench the reactivity of the highly polar Ga-O unit. Instead, intramolecular benzylic C-H activation is observed across the Ga-O bond of a transient gallanone intermediate.

Main Group Multiple Bonds for Bond Activations and Catalysis

Since the discovery that the so-called "double-bond" rule could be broken, the field of molecular main group multiple bonds has expanded rapidly. With the majority of homodiatomic double and triple bonds realised within the p-block, along with many heterodiatomic combinations, this Minireview examines the reactivity of these compounds with a particular emphasis on small molecule activation. Furthermore, whilst their ability to act as transition metal mimics has been explored, their catalytic behaviour is somewhat limited. This Minireview aims to highlight the potential of these complexes towards catalytic application and their role as synthons in further functionalisations making them a versatile tool for the modern synthetic chemist.

Double insertion of CO2 into an Al–Te multiple bond

Two equivalents of CO₂ react with a terminal Al–Te bond to form the tellurodicarbonate ligand.

Pincer-Type Ligand-Assisted Catalysis and Small-Molecule Activation by non-VSEPR Main-Group Compounds

In 2005, a facile dihydrogen activation was reported by the Power group using an alkyne analog of germanium [ArGe≡GeAr; Ar=2,6-Trip₂ -C₆ H₃ (Trip=2,4,6-ⁱ Pr₃ -C₆ H₂ )]. After that, a significant progress has been made in the activation of various small molecules by main-group compounds, and a variety of stoichiometric and catalytic processes have been formulated using the p-block elements. In this regard, compounds containing low-valent main-group elements with a frontier orbitals of relatively small energy gaps or compounds forming frustrated Lewis pair (FLP) became quite successful. In spite of these promising stoichiometric and catalytic transformations, redox-cycling catalysts based on main-group elements remain extremely rare. Recently, it has been observed that pincer type ligands supported geometry constrained main-group compounds are capable of acting as redox catalysts similar to those of the transition metals. In this review, we focus on the synthesis and the structural aspects of the geometry constrained main-group compounds using pincer ligands. Emphasis has been placed on their applications on catalytic activity and small molecules activation.

A meta-Selective C-H Alumination of Mono-Substituted Benzene by Using An Alkyl-Substituted Al Anion through Hydride-Eliminating SN Ar Reaction

Reaction of an Al-centered anion with toluene proceeded to form C-H cleaved product with a perfect meta-selectivity and a relatively small kinetic isotope effect (KIE, k_H /k_D =1.51). DFT calculations suggested a two-step reaction mechanism and electronically controlled meta-selectivity arising from the electron-donating methyl group. The reaction with other mono-substituted arenes was also investigated.

Recent advances in low oxidation state aluminium chemistry

The synthesis and isolation of novel low oxidation state aluminium (Al) compounds has seen relatively slow progress over the 30 years since such species were first isolated. This is largely due to the significant challenges in isolating these thermodynamically unstable compounds. Despite challenges with isolation, their reactivity has been widely explored and they have been utilized in a wide range of processes including the activation of strong chemicals bonds, as ligands to transition metals and in the formation of heterobimetallic M-M compounds. As such, attempts to isolate novel low oxidation state Al compounds have continued in earnest and in the last few years huge advances have been made. In this review we highlight the remarkable recent developments in the low oxidation state chemistry of aluminium and discuss the variety of new reactions these compounds have made possible.

Carbon-Carbon Bond Forming Reactions Promoted by Aluminyl and Alumoxane Anions: Introducing the Ethenetetraolate Ligand

[K{Al(NON^Dipp )}]₂ (NON^Dipp =[O(SiMe₂ NDipp)₂ ]^2- , Dipp=2,6-iPr₂ C₆ H₃ ) reacts with CS₂ to afford the trithiocarbonate species [K(OEt₂ )][Al(NON^Dipp )(CS₃ )] 1 or the ethenetetrathiolate complex, [K{Al(NON^Dipp )(S₂ C)}]₂ [3]₂ . The dimeric alumoxane [K{Al(NON^Dipp )(O)}]₂ reacts with carbon monoxide to afford the oxygen analogue of 3, [K{Al(NON^Dipp )(O₂ C)}]₂ [4]₂ containing the hitherto unknown ethenetetraolate ligand, [C₂ O₄ ]^4- .