Precise Activation of Ammonia and Carbon Dioxide by an Iminodisilene

Applications of low-valent compounds with heavy group-14 elements

Over the last two decades, the low-valent compounds of group-14 elements have received significant attention in several fields of chemistry owing to their unique electronic properties. The low-valent group-14 species include tetrylenes, tetryliumylidene, tetrylones, dimetallenes and dimetallynes. These low-valent group-14 species have shown applications in various areas such as organic transformations (hydroboration, cyanosilylation, N-functionalisation of amines, and hydroamination), small molecule activation (e.g. P4, As4, CO2, CO, H2, alkene, and alkyne) and materials. This review presents an in-depth discussion on low-valent group-14 species-catalyzed reactions, including polymerization of rac-lactide, L-lactide, DL-lactide, and caprolactone, followed by their photophysical properties (phosphorescence and fluorescence), thin film deposition (atomic layer deposition and vapor phase deposition), and medicinal applications. This review concisely summarizes current developments of low-valent heavier group-14 compounds, covering synthetic methodologies, structural aspects, and their applications in various fields of chemistry. Finally, their opportunities and challenges are examined and emphasized.

Stretched Central Double Bonds in Dialumene and Disilene by Amino Substituents: A Case of Lone Pair Repulsion

There have been remarkable advances in the syntheses and applications of groups 13 and 14 homonuclear ethene analogues. However, successes are largely limited to aryl- and/or silyl-substituted species. Analogues bearing two or more heteroatoms are still scarce. In this work, the block-localized wavefunction (BLW) method at the density functional theory (DFT) level was employed to study dialumene and disilene bearing two amino substituents whose optimal geometries exhibit significantly stretched central M=M (M=Al or Si) double bonds compared with aryl- and/or silyl-substituted species. Computational analyses showed that the repulsion between the lone electron pairs of amino substituents and M=M π bond plays a critical role in the elongation of the M=M bonds. Evidently, replacing the substituent groups -NH2 with -BH2 can enhance the planarity and shorten the central double bonds due to the absence of lone pair electrons in BH2 .

N-Heterocyclic imine-based bis-gallium(I) carbene analogs featuring a four-membered Ga2N2 ring

A combination of Ga(I) centers as important building blocks and scaffolds containing N-heterocyclic imines gives new insights into low-valent Ga chemistry. In this study, a mixture of LDipNLi (LDip = 1,3-bis(2,6-diisopropylphenyl)-imidazolin-2-ylidene), tBuOK, and Cp*Ga (Cp* = pentamethylcyclopentadienyl) in toluene afforded [LDipN-Ga]2 (1) via salt metathesis. X-ray structure analysis of 1 revealed a four-membered Ga2N2 ring, and DFT studies indicated the presence of a lone pair at each Ga center. In addition, compound 1 demonstrated diverse reactivities towards methyl trifluoromethanesulfonate, diphenyl disulfide, 9,10-phenanthrenequinone, and ECl2 (E = Ge or Sn).

Synthesis of Cobalt-Tin and -Lead Tetrylidynes-Reactivity Study of the Triple Bond

Tetrylidynes [TbbSn≡Co(PMe3 )3 ] (1 a) and [TbbPb≡Co(PMe3 )3 ] (2) (Tbb=2,6-[CH(SiMe3 )2 ]2 -4-(t-Bu)C6 H2 ) are accessed for the first time via a substitution reaction between [Na(OEt2 )][Co(PMe3 )4 ] and [Li(thf)2 ][TbbEBr2 ] (E=Sn, Pb). Following an alternative procedure the stannylidyne [Ar*Sn≡Co(PMe3 )3 ] (1 b) was synthesized by hydrogen atom abstraction using AIBN from the paramagnetic hydride complex [Ar*SnH=Co(PMe3 )3 ] (4) (AIBN=azobis(isobutyronitrile)). The stannylidyne 1 a adds two equivalents of water to yield the dihydroxide [TbbSn(OH)2 CoH2 (PMe3 )3 ] (5). In reaction of the stannylidyne 1 a with CO2 a product of a redox reaction [TbbSn(CO3 )Co(CO)(PMe3 )3 ] (6) was isolated. Protonation of the tetrylidynes occurs at the cobalt atom to give the metalla-stanna vinyl cation [TbbSn=CoH(PMe3 )3 ][BArF 4 ] (7 a) [ArF =C6 H3 -3,5-(CF3 )2 ]. The analogous germanium and tin cations [Ar*E=CoH(PMe3 )3 ][BArF 4 ] (E=Ge 9, Sn 7 b) (Ar*=C6 H3 (2,6-Trip)2 , Trip=2,4,6-C6 H2 iPr3 ) were also obtained by oxidation of the paramagnetic complexes [Ar*EH=Co(PMe3 )3 ] (E=Ge 3, Sn 4), which were synthesized by substitution of a PMe3 ligand of [Co(PMe3 )4 ] by a hydridoylene (Ar*EH) unit.

Cyclic alkyl(amino)iminates (CAAIs) as strong 2σ,4π-electron donor ligands for the stabilisation of boranes and diboranes(4): a synthetic and computational study

Singly and doubly cyclic alkyl(amino)iminate (CAAI)-substituted boranes and diboranes(4) were synthesised by halosilane elimination between a silylimine and halo(di)borane precursors. ¹¹B NMR-spectroscopic studies show that the CAAI ligand is a much stronger electron donor than amino ligands. X-ray crystallographic analyses reveal that the degree of B-N_CAAI double bonding increases with the electron-withdrawing capacity of the other substituents at boron. The C-N-B bond angle displays a great flexibility, ranging from 131° to near-linear 176°, the narrowest angles being observed for NMe₂-substituted derivatives and the widest angles for highly sterically demanding substituents. Density functional theory (DFT) calculations on the electronic structures of the anionic CAAI ligand compared to unsaturated and saturated N-heterocyclic iminate (NHI) ligands show that the former is the best σ donor of the three but less π-donating than the unsaturated NHI. Nevertheless, the linear (CAAI)BH₂ complex displays somewhat stronger C-N and N-B π bonding than the corresponding ((S)NHI)BH₂ complexes.

Widening the Landscape of Small Molecule Activation with Gold-Aluminyl Complexes: A Systematic Study of E-H (E=O, N) Bonds, SO2 and N2 O Activation

The electronic features of gold-aluminyl complexes have been thoroughly explored. Their similarity with Group 14 dimetallenes and other metal-aluminyl complexes suggests that their reactivity with small molecules beyond carbon dioxide could be accessed. In this work, the reactivity of the [^t Bu₃ PAuAl(NON)] (NON=4,5-bis(2,6 diisopropylanilido)-2,7-ditert-butyl-9,9-dimethylxanthene) complex towards water, ammonia, sulfur dioxide and nitrous oxide is computationally explored. The reaction mechanisms computed for each substrate strongly suggest that all activation processes are in principle experimentally feasible. Electronic structure analysis highlights that, in all cases, the reactivity is driven by the presence of the poorly polarized electron-sharing gold-aluminyl bond, which induces a radical-like reactivity of the complex towards all the substrates. A flat topology of the potential energy surface (PES) has been found for the reaction with N₂ O, where two almost isoenergetic transition states can be located along the same reaction coordinate with different geometries, suggesting that the N₂ O binding mode may not be a good indicator of the nature of N₂ O activation in a cooperative bimetallic reactivity. In addition, the catalytic potentialities of these complexes have been explored in the framework of nitrous oxide reduction. The study reveals that the [^t Bu₃ PAuAl(NON)] complex might be an efficient catalyst towards oxidation of phosphines (and boranes) via N₂ O reduction. These findings underline recurring trends in the novel chemistry of gold-aluminyl complexes and call for experimental feedbacks.

Room Temperature Intermolecular Dearomatization of Arenes by an Acyclic Iminosilylene

A novel nontransient acyclic iminosilylene (1), bearing a bulky super silyl group (-Si^tBu₃) and N-heterocyclic imine ligand with a methylated backbone, was prepared and isolated. The methylated backbone is the feature of 1 that distinguishes it from the previously reported nonisolable iminosilylenes, as it prevents the intramolecular silylene center insertion into an aromatic C-C bond of an aryl substituent. Instead, 1 exhibits an intermolecular Büchner-ring-expansion-type reactivity; the silylene is capable of dearomatization of benzene and its derivatives, giving the corresponding silicon analogs of cycloheptatrienes, i.e. silepins, featuring seven-membered SiC₆ rings with nearly planar geometry. The ring expansion reactions of 1 with benzene and 1,4-bis(trifluoromethyl)benzene are reversible. Similar reactions of 1 with N-heteroarenes (pyridine and DMAP) proceed more rapidly and irreversibly forming the corresponding azasilepins, also with nearly planar seven-membered SiNC₅ rings. DFT calculations reveal an ambiphilic nature of 1 that allows the intermolecular aromatic C-C bond insertion to occur. Additional computational studies, which elucidate the inherent reactivity of 1, the role of the substituent effect, and reaction mechanisms behind the ring expansion transformations, are presented.

Isolation and Reactivity of Tetrylene-Tetrylone-Iron Complexes Supported by Bis(N-Heterocyclic Imine) Ligands

The germanium iron carbonyl complex 3 was prepared by the reaction of dimeric chloro(imino)germylene [IPrNGeCl]₂ (IPrN=bis(2,6-diisopropylphenyl)imidazolin-2-iminato) with one equivalent of Collman's reagent (Na₂ Fe(CO)₄ ) at room temperature. Similarly, the reaction of chloro(imino)stannylene [IPrNSnCl]₂ with Na₂ Fe(CO)₄ (1 equiv) resulted in the Fe(CO)₄ -bridged bis(stannylene) complex 4. We observed reversible formation of bis(tetrylene) and tetrylene-tetrylone character in complexes 3 vs. 5 and 4 vs. 6, which was supported by DFT calculations. Moreover, the Li/Sn/Fe trimetallic complex 12 has been isolated from the reaction of [IPrNSnCl]₂ with cyclopentadienyl iron dicarbonyl anion. The computational analysis further rationalizes the reduction pathway from these chlorotetrylenes to the corresponding complexes.

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.

Facile Reduction of Phosphine Oxides and <i>H</i>-Phosphonates by Ditetrelenes

The addition of secondary phosphine oxides to tetramesityldisilene and -digermene results in the mild, partial reduction of the P(V) centre of the organophosphorus oxide to P(III) to yield disilyl and digermyl phosphinite derivatives and illustrates the potential of ditetrelenes to serve as mild reducing agents under ambient conditions. An analogous reaction happens with <i>H</i>-phosphonates. Mechanistic experiments, including deuterium-labelling, kinetic isotope effect (KIE) and variable time normalization analysis (VTNA) experiments, reveal that the 1,3-PH addition likely proceeds through a stepwise reaction pathway with the organophosphorus oxide acting as the nucleophile towards the ditetrelene. Furthermore, a facile exchange between the R₂PO- moiety on the digermylphosphinite with the R₂PO moiety of phosphine oxides was discovered and likely proceeds through a direct substitution mechanism.

Activation of Ammonia by a Carbene-Stabilized Dithiolene Zwitterion

A carbene-stabilized dithiolene zwitterion (3) activates ammonia, affording 4^• and 5, through both single-electron transfer (SET) and hydrogen atom transfer (HAT). Reaction products were characterized spectroscopically and by single-crystal X-ray diffraction. The mechanism of the formation of 4^• and 5 was probed by experimental and computational methods. This discovery is the first example of metal-free ammonia activation via HAT.

Selective 1,2 addition of polar X-H bonds to the Ga-P double bond of gallaphosphene L(Cl)GaPGaL

Gallaphosphene L(Cl)GaPGaL 1 (L = HC[C(Me)N(2,6-i-Pr₂-C₆H₃)]₂) reacts at ambient temperature with a series of polar X-H bonds, i.e. ammonia, primary amines, water, phenol, thiophenol, and selenophenol, selectively with 1,2 addition at the polar Ga-P double bond. The gallium atom serves as electrophile and the phosphorous atom is protonated in all reactions. The resulting complexes L(Cl)GaP(H)Ga(X)L (X = NH₂2, NHi-Pr 3, NHPh 4, OH 5, OXyl 6, SPh 7, SePh 8) were characterized by IR and heteronuclear (¹H, ¹³C{¹H}, ³¹P{¹H}) NMR spectroscopy, elemental analysis, and single-crystal X-ray diffraction.

NH bond activation of ammonia and amines by ditetrelenes: key insights into the stereochemistry of nucleophilic addition

The NH bond activation of ammonia, primary and secondary amines by tetramesityldisilene and -digermene was investigated. In each case, a disilyl- or digermylamine was formed as the only product of amine addition. The mechanism of the addition of ammonia to tetramesityldisilene was computed and revealed a three-step reaction pathway: formation of the anti-ammonia-disilene adduct, inversion at the β-silicon, and syn-transfer of the proton to give the syn-product, where each step follows a distinct stereochemical course. Examination of the reaction landscape also revealed several additional insights: (a) that, in the initial step, the formation of the anti-oriented zwitterionic intermediate is kinetically more preferable than formation of the syn-oriented zwitterionic intermediate, (b) that intermolecular transfer of a proton is not energetically feasible in non-polar solvents, and (c) that the bulk of the substituents can have a profound effect on the stereochemical course of the reaction. With this detailed understanding, nucleophilic additions to ditetrelenes can be exploited in the future.

Subvalent mixed SixGey oligomers: (Cl3Si)4Ge and Cl2(Me2EtN)SiGe(SiCl3)2

(Cl₃Si)₄Ge (1; 91%) is accessible from GeCl₄, the Si₂Cl₆/[nBu₄N]Cl silylation system, and excess SiCl₄. A key intermediate step involves Cl^- sequestration with AlCl₃ in the course of the reaction between the first-formed germanide [(Cl₃Si)₃Ge]^- and SiCl₄. The related adduct Cl₂(Me₂EtN)SiGe(SiCl₃)₂ (2; quantitative conversion) was prepared either by amine-induced cleavage of 1 or by a bottom-up synthesis starting from GeCl₄ and Si₂Cl₆.

Computational Exploration of Mechanistic Avenues in Metal-Free CO2 Reduction to CO by Disilyne Bisphosphine Adduct and Phosphonium Silaylide

Recent years have seen a growing interest in metal-free CO₂ activation by silylenes, silylones, and silanones. However, compared to mononuclear silicon species, CO₂ reduction mediated by dinuclear silicon compounds, especially disilynes, has been less explored. We have carried out extensive computational investigations to explore the mechanistic avenues in CO₂ reduction to CO by donor-stabilized disilyne bisphosphine adduct (R1^M ) and phosphonium silaylide (R2) using density functional theory calculations. Theoretical calculations suggest that R1^M exhibits donor-stabilized bis(silylene) bonding features with unusual Si-Si multiple bonding. Various modes of CO₂ coordination to R1^M have been investigated and the coordination of CO₂ by the carbon center to R1^M is found to be kinetically more facile than that by oxygen involving only one or both the silicon centers. Both the theoretically predicted reaction mechanisms of R1^M and R2-mediated CO₂ reduction reveal the crucial role of silicon-centered lone pairs in CO₂ activations and generation of key intermediates possessing enormous strain in the Si-C-O ring, which plays the pivotal role in CO extrusion.

Thermoneutral N-H Bond Activation of Ammonia by a Geometrically Constrained Phosphine

A geometrically constrained phosphine bearing a tridentate NNS pincer ligand is reported. The effect of the geometric constraint on the electronic structure was probed by theoretical calculations and derivatization reactions. Reactions with N−H bonds result in formation of cooperative addition products. The thermochemistry of these transformations is strongly dependent on the substrate, with ammonia activation being thermoneutral. This represents the first example of a molecular compound that reversibly activates ammonia via N−H bond scission in solution upon mild heating.

Imino(silyl)disilenes: application in versatile bond activation, reversible oxidation and thermal isomerization

Two novel disilenes of type ABSi[double bond, length as m-dash]SiAB bearing N-heterocyclic imino (A = NItBu) and trialkylsilyl (B = SitBu31, B = SitBu2Me 2) groups are reported. The reduced steric demand in 2 results in a highly stable, nonetheless flexible system, wherefore (E/Z) isomerization is observed from room temperature up to 90 °C. The proposed isomerization mechanism proceeds via monomeric silylenes in line with experimental results. Despite enhanced stability, disilene 2 retains high reactivity in the activation of small molecules, including H2. The rare example of a disilene radical cation 7 is isolated and shows reversible redox behavior. White phosphorous (P4) selectively reacts with 2 to give the unique cage-compound 8. Selective thermal rearrangement of 2 at higher temperatures yields the A2Si[double bond, length as m-dash]SiB2-type disilene 9 (A = NItBu, B = SitBu2Me), which bears characteristics of a zwitterionic and a dative central Si-Si bond. The proposed mechanism proceeds via an initial NHI migration followed by silyl migration.

Multi-Talented Gallaphosphene for Ga-P-Ga Heteroallyl Cation Generation, CO2 Storage, and C(sp3 )-H Bond Activation

Gallaphosphene L(Cl)GaPGaL (2; L=HC[C(Me)N(2,6-i-Pr2 C6 H3 )]2 ), which is synthesized by reaction of LGa(Cl)PCO (1) with LGa, reacts with [Na(OCP)(dioxane)2.5 ] to LGa(OCP)PGaL (3), whereas chloride abstraction with LiBArF 4 yields [LGaPGaL][BArF 4 ] (4; BArF 4 =B(C6 F5 )4 ). 4 represents a heteronuclear analog of the allyl cation according to quantum chemical calculations. Remarkably, 2 reversibly reacts with CO2 to yield L(Cl)Ga-P[μ-C(O)O]2 GaL (5), while reactions with acetophenone and acetone selectively give compounds 6 and 7 by C(sp3 )-H bond activation.

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.

Hydroamination of non-activated alkenes with ammonia: a holy grail in catalysis

This review covers the hydroamination of non-activated alkenes with simple amines, with a special focus on ammonia.

Recent advances of group 14 dimetallenes and dimetallynes in bond activation and catalysis

Since the first heavy alkene analogues of germanium and tin were isolated in 1976, followed by West's disilene in 1981, the chemistry of stable group 14 dimetallenes and dimetallynes has advanced immensely. Recent developments in this field veered the focus from the isolation of novel bonding motifs to mimicking transition metals in their ability to activate small molecules and perform catalysis. The potential of these homonuclear multiply bonded compounds has been demonstrated numerous times in the activation of H2, NH3, CO2 and other small molecules. Hereby, the strong relationship between structure and reactivity warrants close attention towards rational ligand design. This minireview provides an overview on recent developments in regard to bond activation with group 14 dimetallenes and dimetallynes with the perspective of potential catalytic applications of these compounds.

Cooperative N-H bond activation by amido-Ge(ii) cations

N-heterocyclic carbene (NHC) and tertiary phosphine-stabilized germylium-ylidene cations, [R(L)Ge:]+, featuring tethered amido substituents at R have been synthesized via halide abstraction. Characterization in the solid state by X-ray crystallography shows these systems to be monomeric, featuring a two-coordinate C,N- or P,N-ligated germanium atom. The presence of the strongly Lewis acidic cationic germanium centre and proximal amide function allows for facile cleavage of N-H bonds in 1,2-fashion: the products resulting from reactions with carbazole feature a tethered secondary amine donor bound to a three-coordinate carbazolyl-GeII centre. In each case, addition of the components of the N-H bond occurs to the same face of the germanium amide function, consistent with a coordination/proton migration mechanism. Such as sequence is compatible with the idea that substrate coordination via the pπ orbital at germanium reduces the extent of N-to-Ge π donation from the amide, thereby enhancing the basicity of the proximal N-group.

Oxidation reactions of a versatile, two-coordinate, acyclic iminosiloxysilylene

Due to their outstanding reactivity, acyclic silylenes have emerged as attractive organosilicon alternatives for transition metal complexes on the way to metal-free catalysis. However, exploration of their reactivity is still in its infancy, as only a few derivatives of this unique compound class have been isolated so far. Here, we present the results of an extensive reactivity investigation of the previously reported acyclic iminosiloxysilylene 1. Divalent silylene 1 proved to be a versatile building block for a plethora of novel organosilicon compounds. Thus, not only the activation of the rather challenging targets NH₃ and P₄ could be achieved, but also the conversion into a reactive donor-free silaimine, which itself turned out to be a useful reagent for small molecule activation. In addition, 1 served as an excellent precursor for gaining access to donor-stabilized heavier carbonyl compounds. Our results thus provide further insights into the chemistry of low-valent silicon at the interface between carbon and transition metals.

Dialumenes - aryl vs. silyl stabilisation for small molecule activation and catalysis

Main group multiple bonds have proven their ability to act as transition metal mimics in the last few decades. However, catalytic application of these species is still in its infancy. Herein we report the second neutral NHC-stabilised dialumene species by use of a supporting aryl ligand (3). Different to the trans-planar silyl-substituted dialumene (3Si), compound 3 features a trans-bent and twisted geometry. The differences between the two dialumenes are explored computationally (using B3LYP-D3/6-311G(d)) as well as experimentally. A high influence of the ligand's steric demand on the structural motif is revealed, giving rise to enhanced reactivity of 3 enabled by a higher flexibility in addition to different polarisation of the aluminium centres. As such, facile activation of dihydrogen is now achievable. The influence of ligand choice is further implicated in two different catalytic reactions; not only is the aryl-stabilised dialumene more catalytically active but the resulting product distributions also differ, thus indicating the likelihood of alternate mechanisms simply through a change of supporting ligand.

Ligand controlled reactivity: a trans-bent and twisted geometry enables dihydrogen activation and enhanced catalytic activity for NHC-stabilised dialumenes.

N–H cleavage vs. Werner complex formation: reactivity of cationic group 14 tetrelenes towards amines

Two-coordinate cations [(N-nacnac)E]⁺ (E = Si, Ge, Sn) react differently towards ammonia/amines: simple adduct formation (Ge,Sn) contrasts with N–H activation (Si).

CO2 Fixation and Catalytic Reduction by a Neutral Aluminum Double Bond

CO₂ fixation and reduction to value-added products is of utmost importance in the battle against rising CO₂ levels in the Earth's atmosphere. An organoaluminum complex containing a formal aluminum double bond (dialumene), and thus an alkene equivalent, was used for the fixation and reduction of CO₂ . The CO₂ fixation complex undergoes further reactivity in either the absence or presence of additional CO₂ , resulting in the first dialuminum carbonyl and carbonate complexes, respectively. Dialumene (1) can also be used in the catalytic reduction of CO₂ , providing selective formation of a formic acid equivalent via the dialuminum carbonate complex rather than a conventional aluminum-hydride-based cycle. Not only are the CO₂ reduction products of interest for C₁ added value products, but the organoaluminum complexes isolated represent a significant step forward in the isolation of reactive intermediates proposed in many industrially relevant catalytic processes.

An N-Heterocyclic Boryloxy Ligand Isoelectronic with N-Heterocyclic Imines: Access to an Acyclic Dioxysilylene and its Heavier Congeners

Introduced here is a new type of strongly donating N-heterocyclic boryloxy (NHBO) ligand, [(HCDippN)₂ BO]^- (Dipp=2,6-diisopropylphenyl), which is isoelectronic with the well-known N-heterocyclic iminato (NHI) donor class. This 1,3,2-diazaborole functionalized oxy ligand has been used to stabilize the first acyclic two-coordinate dioxysilylene and its Ge, Sn, and Pb congeners, thereby presenting the first complete series of heavier group 14 dioxycarbene analogues. All four compounds have been characterized by X-ray crystallography and density-functional theory, enabling analysis of periodic trends: the potential for the [(HCDippN)₂ BO]^- ligand to subtly vary its electronic-donor capabilities is revealed by snapshots showing the gradual evolution of arene π coordination on going from Si to Pb.

Reduction of Carbon Oxides by an Acyclic Silylene: Reductive Coupling of CO

Reactions of a boryl-substituted acyclic silylene with carbon dioxide and monoxide are reported. The former proceeds through oxygen atom abstraction, generating CO (with rearrangement of the putative silanone product through silyl-group transfer). The latter is characterized by reductive coupling of CO to give an ethynediolate fragment, which undergoes formal insertion into the Si-B bond. The net conversion of carbon dioxide with two equivalents of silylene offers a route for the three-electron reduction of CO₂ to [C₂ O₂ ]^2- .

Successive Protonation of an N-Heterocyclic Imine Derived Carbonyl: Superelectrophilic Dication Versus Masked Acylium Ion

Carbonyl cations are among the most commonly invoked reactive intermediates in organic synthesis. While Olah pioneered superacids to provide a "stable ion" environment for their study in situ, isolated examples are rare. Here, we disclose successive protonation of an N-heterocyclic imine (NHI) derived carbonyl compound (IDippN)₂ CO, 2, to the monocation [(IDippN)(IDippNH) CO]⁺ , [3]⁺ , and the doubly protonated dication [(IDippNH)₂ CO]²⁺ , [4]²⁺ . [3]⁺ represents a rare example of an N-protonated carbonyl cation and [4]²⁺ the first example of a superelectrophilic carbonyl dication. All three compounds have been characterized by X-ray crystallography and IR spectroscopy, revealing stepwise strengthening of the C=O bond on protonation. The unique stability of these systems is attributed to the enhanced basicity and steric profile provided by the NHI substituents. In addition, we report the related singly NHI-stabilized cation [IDippNCO]⁺ , [5]⁺ . Crystallographic and DFT analyses provide insight into the interaction between the carbonyl fragment and the NHI, which reveals that the [CNCO]⁺ unit (isoelectronic to CCCO) can be described as an acylium cation "masked" as a cumulene.