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

Nitrous oxide as diazo transfer reagent

Nitrous oxide, commonly known as "laughing gas", is formed as a by-product in several industrial processes. It is also readily available by thermal decomposition of ammonium nitrate. Traditionally, the chemical valorization of N2O is achieved via oxidation chemistry, where N2O acts as a selective oxygen atom transfer reagent. Recent results have shown that N2O can also function as an efficient diazo transfer reagent. Synthetically useful methods for synthesizing triazenes, N-heterocycles, and azo- or diazo compounds were developed. This review article summarizes significant advancements in this emerging field.

A Bis(silylene)silole - synthesis, properties and reactivity

A 1,1-bis(silylene)silole has been synthesised by a double salt-metathesis reaction from potassium silacyclopentadienediide, K2[1], and an amidinato-stabilized silylene chloride in a 1 : 2 ratio. The red colour of the title compound is due to the lp(Si)/π*(silole) transition. This band is bathochromically shifted compared to that of other 1,1-bissilylsiloles suggesting enhanced conjugation between the silole π-system and the newly formed Si(II)-Si(IV)-Si(II) group. The bissilylene is easily oxidised by the elemental chalcogens S, Se, and Te and forms a bissilaimide by reaction with an arylazide.

Ferrocene-Based N-Heterocyclic Silylenes: Monomeric Silanechalcogenones, Silanimines, Silirenes, and Insertion Products with P4

The stable ferrocene-based N-heterocyclic silylenes fc[(N{B})2Si] (A; fc=1,1'-ferrocenylene, {B}=(HCNDipp)2B, Dipp=2,6-diisopropylphenyl) and fc[(NDipp)2Si] (B) are compared in a study focussing on their reactivity towards a range of small to moderately sized molecular substrates, viz. P4, S8, Se8, MesN3 (Mes=mesityl), RC≡CH, and RC≡CR (R=Ph, SiMe3). The Dipp-substituted congener B exhibits a more pronounced ambiphilicity and is sterically less congested than its 1,3,2-diazaborolyl-substituted relative A, in line with the higher reactivity of the former. The difference in reactivity is obviously due more to electronic than to steric reasons, as is illustrated by the fact that both A and B react with the comparatively bulky substrate MesN3 under mild conditions to afford the corresponding silanimine fc[(N{B})2Si=NMes] and fc[(NDipp)2Si=NMes], respectively. The heavier ketone analogues fc[(N{B})2Si=E] (E=S, Se, Te) are readily available from A and the corresponding chalcogen. In contrast, the reaction of the more reactive silylene B with elemental sulfur or selenium is unspecific, affording product mixtures. However, fc[(NDipp)2Si=Se] is selectively prepared from B and (Et2N)3PSe; the Te analogue is also accessible, but crystallises as head-to-tail dimer.

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.

An Air-Stable Storage Compound for White Phosphorus: Reversible Addition to a Stannylene and Chemical Release of P4

Controlled insertion into a single P-P bond of white phosphorus (P4) was achieved by employing a diaryl stabilized stannylene, Ar*2Sn (Ar*=2,6-bis(benzhydryl)-4-iPr-phenyl). Conversions of the stannylene with P4 gave a non-pyrophoric, air-stable storage compound, which releases P4 quantitively upon irradiation with light (354 or 455 nm). Alternatively, the phosphorus cage is detached by reacting the storage compound with PhChChPh (Ch=Se, Te). Despite the recent advances in the directed conversion of P4 using main group element compounds, Ar*2Sn constitutes only the second structurally characterized example of a stannylene capable of performing controlled, reversible addition and release of white phosphorus.

Synthesis and Reactivity of Base-Stabilized and Base-Free Silaimidoyl Bromides

The reactivity of the base-free bromosilylene dtbpCbzSiBr (dtbpCbz = 1,8-bis(3,5-di-tert-butylphenyl)-3,6-di-tert-butylcarbazolyl) toward carbodiimides and azides was studied in order to generate base-stabilized and base-free silaimidoyl bromides, respectively. The steric bulk of carbodiimides and azides allows control over the reactivity. While with small substituents such as tert-butyl or adamantyl, the reactions cannot be stopped at the Si═N stage, with large substituents, they lead to C-H activation in the product. The Dipp substituent (Dipp = 2,6-diisopropylphenyl) allowed the isolation of the silaimidoyl bromide dtbpCbzSi(Br)NDipp and its CNDipp-coordinated analogue. The reactivity of the Si═N double bond species was studied with respect to cycloaddition and donor exchange reactions.

Reactivity of NHI-Stabilized Heavier Tetrylenes towards CO2 and N2 O

A heteroleptic amino(imino)stannylene (TMS2 N)(It BuN)Sn: (TMS=trimethylsilyl, It Bu=C[(N-t Bu)CH]2 ) as well as two homoleptic NHI-stabilized tetrylenes, (It BuN)2 E: (NHI=N-heterocyclic imine, E=Ge, Sn) are presented. VT-NMR investigations of (It BuN)2 Sn: (2) reveal an equilibrium between the monomeric stannylene at room temperature and the dimeric form at -80 °C as well as in the solid state. Upon reaction of the homoleptic tetrylenes with CO2 , both compounds insert two equivalents of CO2 , however differing bonding modes can be observed. (It BuN)2 Sn: (2) inserts one equivalent of CO2 into each Sn-N bond, giving carbamato groups coordinated κ2 O,O' to the metal center. With (It BuN)2 Ge: (3), the Ge-N bonds stay intact upon activation, being bridged by one molecule of CO2 respectively, forming 4-membered rings. Furthermore, the reactivity of 2 towards N2 O was investigated, resulting in partial oxidation to form stannylene dimer [((It BuN)3 SnO)(It BuN)Sn:]2 (6).

Boryl-substituted low-valent heavy group 14 compounds

Low valent group 14 compounds exhibit diverse structures and reactivities. The employment of diazaborolyl anions (NHB anions), isoelectronic analogues to N-heterocyclic carbenes (NHCs), in group 14 chemistry leads to the exceptional structures and reactivity. The unique combination of σ-electron donation and pronounced steric hindrance impart distinct structural characteristics to the NHB-substituted low valent group 14 compounds. Notably, the modulation of the HOMO-LUMO gap in these compounds with the diazaborolyl substituents results in novel reaction patterns in the activation of small molecules and inert chemical bonds. This review mainly summarizes the recent advances in NHB-substituted low-valent heavy Group 14 compounds, emphasizing their synthesis, structural characteristics and application to small molecule activation.

Direct synthesis of oxalic acid via oxidative CO coupling mediated by a dinuclear hydroxycarbonylcobalt(III) complex

Oxidative coupling of CO is a straightforward and economic benign synthetic route for value-added α-diketone moiety containing C₂ or higher carbon compounds in both laboratory and industry, but is still undeveloped to date. In this work, a rare coplanar dinuclear hydroxycarbonylcobalt(III) complex, bearing a Schiff-base macrocyclic equatorial ligand and a μ-κ¹(O):κ¹(O')-acetate bridging axial ligand, is synthesized and characterized. The Co(III)-COOH bonds in this complex can be feasibly photocleaved, leading to the formation of oxalic acid. Moreover, the light-promoted catalytic direct production of oxalic acid from CO and H₂O using O₂ as the oxidant with good selectivity (> 95%) and atom economy at ambient temperature and gas pressure based on this dicobalt(III) complex have been achieved, with a turnover number of 38.5. The ¹³C-labelling and ¹⁸O-labelling experiments confirm that CO and H₂O act as the sources of the -COOH groups in the dinuclear hydroxycarbonylcobalt(III) complex and the oxalic acid product.

Synthesis of Homo-Metallic Heavier Analogues of Cyclobutene and the Cyclobutadiene Dianion

The reduction of the boryl-substituted SnII bromide {(HCDippN)2 B}Sn(IPrMe)Br with 1.5 equivalents of potassium graphite leads to the generation of the cyclic tetratin tetraboryl system K2 [Sn4 {B(NDippCH)2 }4 ], a homo-metallic heavier analogue of the cyclobutadiene dianion. This system is non-aromatic as determined by Nucleus Independent Chemical Shift Calculations (NICS(0)=-0.28, NICS(1)=-3.17), with the primary contributing resonance structures shown by Natural Resonance Theory (NRT) to involve a Sn=Sn double bond and 1,2-localized negative charges. Abstraction of the K+ cations or oxidation leads to contraction or cleavage of the Sn4 unit, respectively, while protonation generates the neutral dihydride 1,2-Sn4 {B(NDippCH)2 }4 H2 (a heavier homologue of cyclobutene) in a manner consistent with the predicted charge distribution in the [Sn4 {B(NDippCH)2 }4 ]2- dianion.

Isolation of C1 through C4 derivatives from CO using heteroleptic uranium(iii) metallocene aryloxide complexes

The conversion of C1 feedstock molecules such as CO into commodity chemicals is a desirable, but challenging, endeavour. When the U(iii) complex, [(C5Me5)2U(O-2,6- t Bu2-4-MeC6H2)], is exposed to 1 atm of CO, only coordination is observed by IR spectroscopy as well as X-ray crystallography, unveiling a rare structurally characterized f element carbonyl. However, using [(C5Me5)2(MesO)U (THF)], Mes = 2,4,6-Me3C6H2, reaction with CO forms the bridging ethynediolate species, [{(C5Me5)2(MesO)U}2(μ2-OCCO)]. While ethynediolate complexes are known, their reactivity has not been reported in much detail to afford further functionalization. For example, addition of more CO to the ethynediolate complex with heating forms a ketene carboxylate, [{(C5Me5)2(MesO)U}2(μ 2:κ 2:η 1-C3O3)], which can be further reacted with CO2 to yield a ketene dicarboxylate complex, [{(C5Me5)2(MesO)U}2(μ 2:κ 2:κ 2-C4O5)]. Since the ethynediolate showed reactivity with more CO, we explored its reactivity further. A [2 + 2] cycloaddition is observed with diphenylketene to yield [{(C5Me5)2U}2(OC(CPh2)C([double bond, length as m-dash]O)CO)] with concomitant formation of [(C5Me5)2U(OMes)2]. Surprisingly, reaction with SO2 shows rare S-O bond cleavage to yield the unusual [(O2CC(O)(SO)]2- bridging ligand between two U(iv) centres. All complexes have been characterized using spectroscopic and structural methods, and the reaction of the ethynediolate with CO to form the ketene carboxylate has been investigated computationally as well as the reaction with SO2.

Disproportionation and Ligand Lability in Low Oxidation State Boryl-Tin Chemistry

Boryltin compounds featuring the metal in the+1 or 0 oxidation states can be synthesized from the carbene-stabilized tin(II) bromide (boryl)Sn(NHC)Br (boryl={B(NDippCH)2 }; NHC=C{(Ni PrCMe)2 }) by the use of strong reducing agents. The formation of the mono-carbene stabilized distannyne and donor-free distannide systems (boryl)SnSn(IPrMe)(boryl) (2) and K2 [Sn2 (boryl)2 ] (3), using Mg(I) and K reducing agents mirrors related germanium chemistry. In contrast to their lighter congeners, however, systems of the type [Sn(boryl)]n are unstable with respect to disproportionation. Carbene abstraction from 2 using BPh3 , and two-electron oxidation of 3 both result in the formation of a 2 : 1 mixture of the Sn(II) compound Sn(boryl)2 , and the hexatin cluster, Sn6 (boryl)4 (4). A viable mechanism for this rearrangement is shown by quantum chemical studies to involve a vinylidene intermediate (analogous to the isolable germanium compound, (boryl)2 Ge=Ge), which undergoes facile atom transfer to generate Sn(boryl)2 and trinuclear [Sn3 (boryl)2 ]. The latter then dimerizes to give the observed hexametallic product 4, with independent studies showing that similar trigermanium species aggregate in analogous fashion.

Anionic Magnesium and Calcium Hydrides: Transforming CO into Unsaturated Disilyl Ethers

The synthesis, characterisation and reactivity of two isostructural anionic magnesium and calcium complexes is reported. By X-ray and neutron diffraction techniques, the anionic hydrides are shown to exist as dimers, held together by a range of interactions between the two anions and two bridging potassium cations. Unlike the vast proportion of previously reported dimeric group 2 hydrides, which have hydrides that bridge two group 2 centres, here the hydrides are shown to be "terminal", but stabilised by interactions with the potassium cations. Both anionic hydrides were found to insert and couple CO under mild reaction conditions to give the corresponding group 2 cis-ethenediolate complexes. These cis-ethenediolate complexes were found to undergo salt elimination reactions with silyl chlorides, allowing access to small unsaturated disilyl ethers with a high percentage of their mass originating from the C1 source CO.

Transition metal-free ketene formation from carbon monoxide through isolable ketenyl anions

The capacity of transition metals to bind and transform carbon monoxide (CO) is critical to its use in many chemical processes as a sustainable, inexpensive C1 building block. By contrast, only few s- and p-block element compounds bind and activate CO, and conversion of CO into useful carbonyl-containing organic compounds in such cases remains elusive. We report that metalated phosphorus ylides provide facile access to ketenyl anions ([RC=C=O]^-) by phosphine displacement with CO. These anions are very stable and storable reagents with a distinctive electronic structure between that of the prototypical ketene (H₂C=C=O) and that of ethynol (HC≡C-OH). Nonetheless, the ketenyl anions selectively react with a range of electrophiles at the carbon atom, thus offering high-yielding and versatile access to ketenes and related compounds.

Neutral Homoaromatic Diboradisilacyclobutene: Synthesis, Structure, and Reactivity

A formally neutral homoaromatic diboradisilacyclobutene was prepared and structurally characterized via the reduction of borylaminobromosilane with potassium graphite, which represents the first neutral boron and silicon mixed analogue of the homocyclopropenylium ion. Single-crystal X-ray analyses revealed a folded four-membered B₂Si₂ ring (26.2°) and a remarkably short transannular B-Si distance of 2.306(3) Å. DFT calculations showed that two π-electrons delocalize over the BSi₂ moiety. Both structural feature and DFT calculations show the significant 2π-homoaromatic nature of the diboradisilacyclobutene. Furthermore, diboradisilacyclobutene reacted readily with CuCl or HCl to afford the diboradisilacyclobutanes.

Stereoselective Activation of Small Molecules by a Stable Chiral Silene

The reaction of the dilithium salt of the enantiopure (S)-BINOL (1,1'-bi-2-naphthol) with two equivalents of the amidinate-stabilized chlorosilylene [LPh SiCl] (LPh =PhC(NtBu)2 ) led to the formation of the first example of a chiral cyclic silene species comprising an (S)-BINOL ligand. The reactivity of the Si=C bond was investigated by reaction with elemental sulfur, CO2 and HCl. The reaction with S8 led to a Si=C bond cleavage and concomitantly to a ring-opened product with imine and silanethione functional groups. The reaction with CO2 resulted in the cleavage of the CO2 molecule into a carbonyl group and an isolated O atom, while a new stereocenter is formed in a highly selective manner. According to DFT calculations, the [2+2] cycloaddition product is the key intermediate. Further reactivity studies of the chiral cyclic silene with HCl resulted in a stereoselective addition to the Si=C bond, while the fully selective formation of two stereocenters was achieved. The quantitative stereoselective addition of CO2 and HCl to a Si=C bond is unprecedented.

Oxidative addition or Werner coordination complex? Reactivity of β-diketiminate supported main group and first-row transition metal complexes towards ammonia

A series of neutral LM (L = [HC{(H₃C)C(Dipp)N}₂], Dipp = 2,6-ⁱPr₂C₆H₃, M = group 13: B-In, TM: Fe, Co, Ni, Cu) and L'M (L' = [HC{(CCH₂)(CCH₃)(DippN)₂}], M = group 14: C-Pb) compounds including a main group 13/14 and first-row transition metal complexes were studied computationally by density functional theory (DFT). The optimised complexes were assessed in terms of structural parameters and electronic structures to find trends and characteristics that could be used to predict their reactivity towards ammonia. In addition, the differences in oxidative addition and Werner coordination complex formation depending on the identity of the central element were investigated and the Werner complexes were evaluated by QTAIM and EDA-NOCV approaches. The computational results complement the earlier experimental studies and shed light on the feasibility of isolating novel main group Werner complexes or transition metal oxidative addition products.

Coordination and Homologation of CO at Al(I): Mechanism and Chain Growth, Branching, Isomerization, and Reduction

Homologation of carbon monoxide is central to the heterogeneous Fischer-Tropsch process for the production of hydrocarbon fuels. C-C bond formation has been modeled by homogeneous systems, with [CnOn]2- fragments (n = 2-6) formed by two-electron reduction being commonly encountered. Here, we show that four- or six-electron reduction of CO can be accomplished by the use of anionic aluminum(I) ("aluminyl") compounds to give both topologically linear and branched C4/C6 chains. We show that the mechanism for homologation relies on the highly electron-rich nature of the aluminyl reagent and on an unusual mode of interaction of the CO molecule, which behaves primarily as a Z-type ligand in initial adduct formation. The formation of [C6O6]4- from [C4O4]4- shows for the first time a solution-phase CO homologation process that brings about chain branching via complete C-O bond cleavage, while a comparison of the linear [C4O4]4- system with the [C4O4]6- congener formed under more reducing conditions models the net conversion of C-O bonds to C-C bonds in the presence of additional reductants.

CO reductive oligomerization by a divalent thulium complex and CO2-induced functionalization

The divalent thulium complex [Tm(Cpttt)2] (Cpttt = 1,2,4-tris(tert-butyl)cyclopentadienyl) reacts with CO to afford selective CO reductive dimerization and trimerization into ethynediolate (C2) and ketenecarboxylate (C3) complexes, respectively. DFT calculations were performed to shed light on the elementary steps of CO homologation and support a stepwise chain growth. The attempted decoordination of the ethynediolate fragment by treatment with Me3SiI led to dimerization and rearrangement into a 3,4-dihydroxyfuran-2-one complex. Investigation of the reactivity of the C2 and C3 complexes towards other electrophiles led to unusual functionalization reactions: while the reaction of the ketenecarboxylate C3 complex with electrophiles yielded new multicarbon oxygenated complexes, the addition of CO2 to the ethynediolate C2 complex resulted in the formation of a very reactive intermediate, allowing C-H activation of aromatic solvents. This original intermolecular reactivity corresponds to an unprecedented functionalization of CO-derived ligands, which is induced by CO2.

Activation of CO Using a 1,2-Disilylene: Facile Synthesis of an Abnormal N-Heterocyclic Silylene

Reaction of the 1,2-disilylene, [{ArC(NDip)₂ }Si]₂ 1 (Dip=2,6-diisopropylphenyl, Ar=4-C₆ H₄ Bu^t ), with CO proceeds via insertion of CO into one Si-N bond, and Si-Si bond cleavage, to cleanly give the bis(silylene), {ArC(NDip)₂ }Si(:)O C S i ( : ) ( N D i p )​ 2 C ‾ Ar 2, under ambient conditions. The reaction can be partially reversed when solutions of 2 are subjected to UV irradiation. The five-membered heterocyclic fragment of 2 represents the first silicon analogue of an "abnormal" N-heterocyclic carbene (aNHC), a view which is substantiated by a computational analysis of the compound. Reaction of 2 with [Mo(CO)₆ ] under UV light affords the chelate complex, [Mo(CO)₄ (κ² -Si,Si-2)] 3, while reaction with [Fe(CO)₅ ] gives the unusual silyleneyl bridged complex, [{Fe₂ (CO)₆ }{μ-Si[(NDip)₂ CAr]}₂ ] 4. The same coordination complexes can be accessed by reaction of 1 with [Mo(CO)₆ ] or [Fe(CO)₅ ] under UV light. As is the case for aNHCs, d-block metal complexes of bis(silylene) 2 could prove useful as bespoke catalysts for organic transformations.

Molecular Catalysts for the Reductive Homocoupling of CO2 towards C2+ Compounds

The conversion of CO2 into multicarbon (C2+ ) compounds by reductive homocoupling offers the possibility to transform renewable energy into chemical energy carriers and thereby create "carbon-neutral" fuels or other valuable products. Most available studies have employed heterogeneous metallic catalysts, but the use of molecular catalysts is still underexplored. However, several studies have already demonstrated the great potential of the molecular approach, namely, the possibility to gain a deep mechanistic understanding and a more precise control of the product selectivity. This Minireview summarizes recent progress in both the thermo- and electrochemical reductive homocoupling of CO2 toward C2+ products mediated by molecular catalysts. In addition, reductive CO homocoupling is discussed as a model for the further conversion of intermediates obtained from CO2 reduction, which may serve as a source of inspiration for developing novel molecular catalysts in the future.

Tetracyclic silaheterocycle formed through a pericyclic reaction cascade including a two-fold intramolecular C–C bond activation

Reductive debromination of the tribromoamidosilane 2 gave the tetracyclic silaheterocycle 3 through a unique reaction cascade involving unprecedented two-fold intramolecular cycloaddition by transient silylenes. Experimental and computational analyses of the reaction mechanism allowed the identification of the key intermediates that lead to the silaheterocycle 3 or, alternatively, to the cyclotrisilene 19.

Reductive debromination of the tribromoamidosilane 2 gave the tetracyclic silaheterocycle 3 through a unique reaction cascade involving unprecedented two-fold intramolecular cycloaddition by transient silylenes.

Carbon monoxide bond cleavage mediated by an intramolecular frustrated Lewis pair: access to new B/N heterocycles via selective incorporation of single carbon atoms

Utilizing an intramolecular frustrated Lewis pair (FLP) decorated with a strongly donating guanidino moiety enabled the formation of a thermally remarkably stable FLP-CO adduct, which at 120 °C underwent CO migration to form an acyl borane. Both compounds underwent rapid CO cleavage in the presence of strong electrophiles leading to the selective formation of a range of new 1,2- and 1,3-benzazaboroles in good yields under mild conditions.

Reactions of a Dilithiomethane with CO and N2 O: An Avenue to an Anionic Ketene and a Hexafunctionalized Benzene

Synthesis of value-added products from simple C1 feedstocks is an attractive alternative avenue to traditional fossil fuels. Hexa-substituted benzene derivatives are highly useful molecules but are often challenging to prepare. Herein, we report that the lithium complex [(Ph₂ P(S))₂ CLi₂ (THF)]₂ 1 reacts with CO lead to C-C bond formation and migration of a Ph₂ P(S)-fragment affording 2. Subsequent reaction with N₂ O results in oxidative cleavage of a P-C bond affording [Ph₂ P(S)OLi(THF)₂ ]₂ 4 and the anionic ketene-derivative Ph₂ P(S)CCOLi(THF)₂ 5. Heating 5 prompts cyclotrimerization giving the hexa-substituted benzene derivative [Ph₂ P(S)CCOLi(THF)₂ ]₃ 6 regioselectively. This transition metal-free protocol to a hexa-substituted benzene is viable on a gram scale and permits the incorporation of ¹³ C labels. The mechanisms of these reactions are detailed via extensive DFT computations.

Reactions of aluminium(i) with transition metal carbonyls: scope, mechanism and selectivity of CO homologation

Over the past few decades, numerous model systems have been discovered that create carbon–carbon bonds from CO. These reactions are of potential relevance to the Fischer–Tropsch process, a technology that converts syngas (H₂/CO) into mixtures of hydrocarbons. In this paper, a homogeneous model system that constructs carbon chains from CO is reported. The system exploits the cooperative effect of a transition metal complex and main group reductant. An entire reaction sequence from C₁ → C₂ → C₃ → C₄ has been synthetically verified. The scope of reactivity is broad and includes a variety of transition metals (M = Cr, Mo, W, Mn, Re, Co), including those found in industrial heterogeneous Fischer–Tropsch catalysts. Variation of the transition metal fragment impacts the relative rate of the steps of chain growth, allowing isolation and structural characterisation of a rare C₂ intermediate. The selectivity of carbon chain growth is also impacted by this variable; two distinct isomers of the C₃ carbon chain were observed to form in different ratios with different transition metal reagents. Based on a combination of experiments (isotope labelling studies, study of intermediates) and calculations (DFT, NBO, ETS-NOCV) we propose a complete mechanism for chain growth that involves defined reactivity at both transition metal and main group centres.

A homogeneous model system that constructs carbon chains from CO is reported. The system exploits the cooperative effect of a transition metal complex and main group reductant. An entire reaction sequence from C₁ → C₂ → C₃ → C₄ has been synthetically verified.

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.

Reductive Dimerization of CO by a Na/Mg(I) Diamide

Sodium reduction of [{SiNDipp}Mg] [{SiNDipp} = {CH2SiMe2N(Dipp)}2; Dipp = 2,6-i-Pr2C6H3] provides the Mg(I) species, [{SiNDipp}MgNa]2, in which the long Mg-Mg bond (>3.2 Å) is augmented by persistent Na-aryl interactions. Computational assessment indicates that this molecule is best considered to comprise a contiguous tetrametallic core, a viewpoint borne out by its reaction with CO, which results in ethynediolate formation mediated by the dissimilar metal centers.

Carbon Monoxide Coupling Reactions via a Frustrated Lewis Pair-Derived η2-Formyl Borane

The η²-formyl borane system 3 is readily available by carbonylation of the vicinal P/BH frustrated Lewis pair (FLP) 1. It serves as a frustrated C/B Lewis pair toward carbon dioxide or phenylisocyanate. In the presence of B(C₆F₅)₃, it forms the coupling product between two CO-derived units. The resulting compound 13 rearranged to a doubly O-borylated endiolate, with both of the central carbon atoms originating from carbon monoxide. The subsequent treatment with a silane gave a rare macrocyclic silicon endiolate.

Reactivity of an N-heterocyclic silylene with a 1,1'-ferrocenediyl backbone towards carbonyl compounds, including carbon suboxide

Reactions of a silylene with a ketene and with carbon suboxide are reported, respectively leading to the first silaallene oxide and to a silylketene, whose reaction with water affords the first structurally characterised stable methyleneketene and constitutes a unique type of single-crystal-to-single-crystal transformation of a molecular solid by a stoichiometric gas-solid chemical reaction.

Systematic reductive oligomerization of isocyanides with a vanadium(ii) complex

Using a V(ii) complex as a reducing reagent, we demonstrate controlled reduction of isocyanides, resulting in decyanation of alkyl isocyanides to form a V(iii) cyanide complex or oligomerization of aryl isocyanides to form trimers and a tetramer. The pathways leading to the trimers involve a divanadium ynediamido intermediate, which further reacts with the third isocyanide molecule to selectively produce a tri(imino)deltate or an indolenine complex, by altering the temperature and stoichiometry.

Chemical Bonding in Silicon Carbonyl Complexes

Although silylene-carbonyl complexes are known for decades, only recently isolable examples have been accomplished. In this work, the bonding situation is re-evaluated to explain the origins of their remarkable stability within the Kohn-Sham molecular orbital theory framework. It is shown that the chemical bond can be understood as CO interaction with the silylene via a donor-acceptor interaction: a σ-donation from the σCO into the empty p-orbital of silicon, and a π-back donation from the sp2 lone pair of silicon into the π*CO antibonding orbitals. Notably, it was established that the driving force behind the surprisingly stable Si-CO compounds, however, is another π-back donation from a perpendicular bonding R-Si σ-orbital into the π*CO antibonding orbitals. Consequently, the pyramidalization of the central silicon atom cannot be associated with the strength of the π-back donation, in sharp contrast to the established chemical bonding model. Considering this additional bonding interaction not only shed light on the bonding situation, but is also an indispensable key for broadening the scope of silylene-carbonyl chemistry.

Formation of silaimines from a sterically demanding iminophosphonamido chlorosilylene via intramolecular N-P bond cleavage

The sterically demanding iminophosphonamido chlorosilylene [Ph2P(DipN)2]SiCl (Dip = 2,6-diisopropylphenyl) was synthesized and fully characterized using NMR spectroscopy and X-ray crystallography. Substitution reactions of [Ph2P(DipN)2]SiCl with N- and Fe-nucleophiles led to the unexpected formation of the corresponding silaimine derivatives. This process involves the ring-opening rearrangement of three-coordinated silylene intermediates that proceeds via intramolecular N-P bond cleavage.

Reactions of a diborylstannylene with CO2 and N2O: diboration of carbon dioxide by a main group bis(boryl) complex

The reactions of the boryl-substituted stannylene Sn{B(NDippCH)2}2 (1) with carbon dioxide have been investigated and shown to proceed via pathways involving insertion into the Sn-B bond(s). In the first instance this leads to formation of the (boryl)tin(ii) borylcarboxylate complex Sn{B(NDippCH)2}{O2CB(NDippCH)2} (2), which has been structurally characterized and shown to feature a κ2 mode of coordination of the [(HCDippN)2BCO2]- ligand at the metal centre. 2 undergoes B-O reductive elimination in hexane solution (in the absence of further CO2) to give the boryl(borylcarboxylate)ester {(HCDippN)2B}O2C{B(NDippCH)2} (3) i.e. the product of formal diboration of carbon dioxide. Alternatively, 2 can assimilate a second equivalent of CO2 to give the homoleptic bis(borylcarboxylate) Sn{O2CB(NDippCH)2}2 (4), which can be prepared via an alternative route from SnBr2 and the potassium salt of [(HCDippN)2BCO2]-, and structurally characterized as its DMAP (N,N-dimethylaminopyridine) adduct. Structural and reactivity studies also point to the possibility for extrusion of CO from the [(HCDippN)2BCO2]- fragment to generate the boryloxy system [(HCDippN)2BO]-, a ligand which can be generated directly from 1via reaction with N2O. The initially formed unsymmetrical species Sn{B(NDippCH)2}{OB(NDippCH)2} has been shown to be amenable to crystallographic study in the solid state, but to undergo ligand redistribution in solution to generate a mixture of 1 and the bis(boryloxy) complex Sn{OB(NDippCH)2}2.

Mechanistic Insights into Activation of Carbon Monoxide, Carbon Dioxide, and Nitrous Oxide by Acyclic Silylene

Owing to an empty p orbital and a lone pair of electrons on the Si center, silylene exhibits reactivity similar to a transition-metal system capable of activating H₂/C-H bonds and small molecules. In this work, with the aid of density functional theory calculations, we systematically investigated the reactions of an acyclic silylene with CO, CO₂, and N₂O. The detailed mechanisms obtained lead to an in-depth understanding of the silylene single-site ambiphilic reactivity.

Oxidative addition of cyanogen bromide to C,N-chelated and Lappert's stannylenes

Stannylenes of L2Sn type bearing either C,N-chelating (1, L = LCN = 2-(N,N-dimethylaminomethyl)phenyl) or bulky amido (2, L = LN = N(SiMe3)2) ligands react with cyanogen bromide (Br-C[triple bond, length as m-dash]N) via an oxidative-addition reaction to give monomeric six-coordinate (LCN)2Sn(Br)CN (1a) and four-coordinate (LN)2Sn(Br)CN (2a) stannanes in moderate yields. In solution, both 1a and 2a undergo instantaneous bromido-cyanido ligand redistribution reactions, leading to mixtures containing 1a, (LCN)2SnBr2 (1b) and (LCN)2Sn(CN)2 (1c) or 2a, (LN)2SnBr2 (2b) and (LN)2Sn(CN)2 (2c), respectively. The prepared species were characterised by multinuclear NMR spectroscopy in solution (1a-c and 2a-c) and in the solid state (1a-c). The crystal structures of 1a/b/c, 2a/b/c and sole 2b were determined by XRD analyses. DFT calculations and QTAIM analysis were also carried out to corroborate the experimental results.

A Stable N-Heterocyclic Silylene with a 1,1'-Ferrocenediyl Backbone

The N-heterocyclic silylene [{Fe(η5 -C5 H4 -NDipp)2 }Si] (1DippSi, Dipp=2,6-diisopropylphenyl) shows an excellent combination of pronounced thermal stability and high reactivity towards small molecules. It reacts readily with CO2 and N2 O, respectively affording (1DippSiO2 )2 C and (1DippSiO)2 as follow-up products of the silanone 1DippSiO. Its reactions with H2 O, NH3 , and FcPH2 (Fc=ferrocenyl) furnish the respective oxidative addition products 1DippSi(H)X (X=OH, NH2 , PHFc). Its reaction with H3 BNH3 unexpectedly results in B-H, instead of N-H, bond activation, affording 1DippSi(H)(BH2 NH3 ). DFT results suggest that dramatically different mechanisms are operative for these H-X insertions.

Bis(silanetellurone) with C-H···Te Interaction

Herein, we report the synthesis of a series of bis(silanechalcogenones) [Ch = Te (2), S (3), or Se (4)] using an N-heterocyclic silylene-based SiCSi pincer ligand (1). 2 is the first example of a bis(silanetellurone) derivative. The bonding patterns of 2-4 were extensively studied by natural bond orbital, quantum theory of atoms in molecules, and noncovalent interaction index analyses, and these exhibit weak C-H···Ch interaction. The analogous reaction of 1 with trimethyl N-oxide produced a novel bis(cyclosiloxane) derivative (5). All of the complexes are duly characterized by single-crystal X-ray diffraction studies, multinuclear nuclear magnetic resonance (¹H, ¹³C, and ²⁹Si) spectroscopy, and high-resolution mass spectrometry.

Synthesis and Reactivity of Bis(silylene)-Coordinated Calcium and Divalent Lanthanide Complexes

Divalent lanthanide complexes of Eu (1) and Yb (2) coordinated by a chelating pyridine-based bis(silylene) ligand were isolated and fully characterized. Compared to the EuII complex 1, the YbII complex 2 presents a lower thermal stability, resulting in the activation of one SiII -N bond and formation of an YbIII complex (3), which features a unique silylene-pyridyl-amido ligand. The different thermal stability of 1 and 2 points towards reduction-induced cleavage of one SiII -N bond of the bis(silylene) ligand. Successful isolation of the corresponding redox-inert bis(silylene) CaII complex (5) was achieved at low temperature and thermal decomposition into a CaII complex (4) bearing the same silylene-pyridyl-amido ligand was identified. In this case, the thermolysis reaction proceeds through another, non-redox induced, mechanism. An alternative higher yielding route to 4 was developed through an in situ generated silylene-pyridyl-amine proligand.