Small Molecule Activation by Two-Coordinate Acyclic Silylenes

Isolable T-Shaped Planar Silyl Anion

Silyl anions have garnered significant attention due to their synthetic abilities. However, previously reported silyl anions have been limited to either trigonal-pyramidal or trigonal-planar geometries, which confine them primarily as nucleophiles in substitution reactions. Herein, we report the isolation of the unprecedented T-shaped planar silyl anion salt 2 by employment of a geometrically constrained triamido pincer ligand. Theoretical calculations disclosed that the silicon centre in 2 possesses both a lone pair of electrons and an empty 3pz orbital. In addition to nucleophilic substitution reactions with Ph3PAuCl and W(CO)6, 2 readily undergoes oxidative additions with CO2 and 2,6-dimethylphenylisonitrile at room temperature. Furthermore, under mild conditions, compound 2 cleaves Csp2-H, Csp2-H, and H-H bonds in 1,2,4,5-tetrafluorobenzene, an intramolecular iPr group, and dihydrogen, representing the first examples of C-H and H-H activations mediated by a silyl anion, respectively. This work unveils new reactivity of silyl anions owing to the non-classical geometry and electronic structure.

Recent progress in transition metal complexes featuring silylene as ligands

Silylenes, divalent silicon(II) compounds, once considered highly reactive and transient species, are now widely employed as stable synthons in main-group and coordination chemistry for myriad applications. The synthesis of stable silylenes represents a major breakthrough, which led to extensive exploration of silylenes in stabilizing low-valent main-group elements and as versatile ligands in coordination chemistry and catalysis. In recent years, the exploration of transition metal complexes stabilized with silylene ligands has captivated significant research attention. This is due to their robust σ-donor characteristics and capacity to stabilize transition metals in low valent states. It has also been demonstrated that the transition metal complexes of silylenes are effective catalysts for hydroboration, hydrosilylation, hydrogenation, hydrogen isotope exchange reactions, and small molecule activation chemistry. This review article focuses on the recent progress in the synthesis and catalytic application of transition metal complexes of silylenes.

Preparation of a high-coordinated-silicon-centered spiro-cyclic compound

Silicon compounds containing silicon-silicon bond with a variety of unusual oxidation states are quite important, because their high reactivity leads to the formation of a variety of silicon compounds. The isolation of such compounds with unusual oxidation states requires a resilient synthetic strategy. Herein, we report the synthesis of a silicon based spirocyclic compound containing a hyper-valent silicon atom and a silicon-silicon bond. The computational calculations employing natural bond orbital (NBO) analysis and energy decomposition analysis-natural orbitals for chemical valence (EDA-NOCV) reveal that the nature of bonding between the silicon atoms is of an electron sharing nature.

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.

Alkyl backbone variations in common β-diketiminate ligands and applications to N-heterocyclic silylene chemistry

We report the extension of the common β-diketimine proligand class, RArnacnacH (HC(RCNAr)2H), where R is an alkyl group such as Et or iPr, plus Ph, and Ar is a sterically demanding aryl substituent such as Dip = 2,6-diispropylphenyl, Dep = 2,6-diethylphenyl, Mes = 2,4,6-trimethylphenyl or mesityl, Xyl = 2,6-dimethylphenyl, via one-pot condensation procedures. When a condensation reaction is carried out using the chemical dehydrating agent PPSE (polyphosphoric acid trimethylsilylester), β-diketiminate phosphorus(V) products such as (iPrMesnacnac)PO2 can also be obtained, which can be converted to the respective proligand iPrMesnacnacH via alkaline hydrolysis. The RArnacnacH proligands can be converted to their alkali metal complexes with common methods and we have found that deprotonation of iPrDipnacnacH is significantly more sluggish than that of related β-diketimines with smaller backbone alkyl groups. The basicity of the RArnacnac- anions can play a role in the success of their salt metathesis chemistry and we have prepared and structurally characterised the EtDipnacnac-derived silicon(II) compounds (EtDipnacnac)SiBr and (EtDipnacnac')Si, where EtDipnacnac' is the deprotonated variant MeCHC(NDip)CHC(NDip)Et.

Synthesis, Reactivity, and Complexation with Fe(0) of a Tight-bite Bis(N-heterocyclic silylene)

The synthesis, reactivity, and complexation with Fe(0) precursor of a tight-bite bis(N-heterocyclic silylene) (bis(NHSi)) ligand 1 are reported. The reaction of 1 with p-toluidine led to the activation of both N-H bonds across Si(II) atoms to afford a four-membered heterocyclic cyclodisilazane 2, with hydride substituents attached to five-coordinate Si atoms. A 1 : 2 reaction of 1 with Fe(CO)5 led to an intriguing dinuclear complex 3 featuring a five-membered (N-Si-Fe-Fe-Si) ring with a Fe-Fe bond distance of 2.6892(13) Å. All compounds (1-3) were thoroughly characterized by various spectroscopic methods and X-ray diffraction studies conclusively established their molecular structures. DFT calculations were carried out to shed light on bonding and energetic aspects in 1-3.

Unusual nucleophilic reactivity of a dithiolene-based N-heterocyclic silane

While the dithiolene-based N-heterocyclic silane (4) reacts with two equivalents of BX3 (X = Br, I) to give zwitterionic Lewis adducts 5 and 8, respectively, the parallel reaction of 4 with BCl3 results in 10, a dithiolene-substituted N-heterocyclic silane, via the Si-S bond cleavage. Unlike 5, the labile 8 may be readily converted to 9via BI3-mediated cleavage of the Si-N bond. The formation of 5 and 8 confirms that 4 uniquely possesses dual nucleophilic sites: (a) the terminal sulphur atom of the dithiolene moiety; and (b) the backbone carbon of the N-heterocyclic silane unit.

Polar X-H Bond (X=O, S, N) Activation at a Cage Silanide

Low-valent silicon compounds such as neutral silylenes display versatile reactivity for the activation of small molecules. In contrast, their anionic congeners silanides ([R3 Si- ]) have primarily been investigated for their nucleophilic reactivity. Here we show that incorporating a silanide center in a bicyclic cage structure allows for formal oxidative addition of polar element-hydrogen bonds (RX-H, R=aromatic residue, X=O, S, NH). The resulting hydrosilicates were isolated and characterized structurally and spectroscopically. Density Functional Theory (DFT) calculations and experimental observations support an ionic mechanism for RX-H bond activation. Finally, the reactivity of the RS-H bond adduct was further investigated, revealing that it behaves as a Lewis pair upon facile heterolytic cleavage of the Si-S bond.

Preparation of a Compound with a SiII -SiIV -SiII Bonding Arrangement

Herein, we report the synthesis of a rare bis-silylene, 1, in which two SiII atoms are bridged by a SiIV atom. Compound 1 contains an unusual SiII -SiIV -SiII bonding arrangement with SiII -SiIV bond distances of 2.4212(8) and 2.4157(7) Å. Treatment of 1 with Fe(CO)5 afforded a dinuclear Fe0 complex 2 with two unusually long Si-Si bonds (2.4515(8) and 2.4488(10) Å). We have also carried out a detailed computational study to understand the nature of the Si-Si bonds in these compounds. Natural bond orbital (NBO) and energy decomposition analysis-natural orbital for chemical valence (EDA-NOCV) analyses reveal that the Si-Si bonds in 1 and 2 are of an electron-sharing nature.

Amidinatotetrylenes Donor Functionalized on Both N Atoms: Structures and Coordination Chemistry

E(hmds)(bqfam) (E = Ge (1a), Sn (1b); hmds = N(SiMe3)2, bqfam = N,N'-bis(quinol-8-yl)formamidinate), which are amidinatotetrylenes equipped with quinol-8-yl fragments on the amidinate N atoms, have been synthesized from the formamidine Hbqfam and Ge(hmds)2 or SnCl(hmds). Both 1a and 1b are fluxional in solution at room temperature, as the E atom oscillates from being attached to the two amidinate N atoms to being chelated by an amidinate N atom and its closest quinolyl N atom (both situations are similarly stable according to density functional theory calculations). The hmds group of 1a and 1b is still reactive and the deprotonation of another equivalent of Hbqfam can be achieved, allowing the formation of the homoleptic derivatives E(bqfam)2 (E = Ge, Sn). The reactions of 1a and 1b with [AuCl(tht)] (tht = tetrahydrothiophene), [PdCl2(MeCN)2], [PtCl2(cod)] (cod = cycloocta-1,5-diene), [Ru3(CO)12] and [Co2(CO)8] have been investigated. The gold(I) complexes [AuCl{κE-E(hmds)(bqfam)}] (E = Ge, Sn) have a monodentate κE-tetrylene ligand and display fluxional behavior in solution the same as that of 1a and 1b. However, the palladium(II) and platinum(II) complexes [MCl{κ3E,N,N'-ECl(hmds)(bqfam)}] (M = Pd, Pt; E = Ge, Sn) contain a κ3E,N,N'-chloridotetryl ligand that arises from the insertion of the tetrylene E atom into an M-Cl bond and the coordination of an amidinate N atom and its closest quinolyl N atom to the metal center. Finally, the binuclear ruthenium(0) and cobalt(0) complexes [Ru2{μE-κ3E,N,N'-E(hmds)(bqfam)}(CO)6] and [Co2{μE-κ3E,N,N'-E(hmds)(bqfam)}(μ-CO)(CO)4] (E = Ge, Sn) have a related κ3E,N,N'-tetrylene ligand that bridges two metal atoms through the E atom. For the κ3E,N,N'-metal complexes, the quinolyl fragment not attached to the metal is pendant in all the germanium compounds but, for the tin derivatives, is attached to (in the Pd and Pt complexes) or may interact with (in the Ru2 and Co2 complexes) the tin atom.

A bromo(boryloxy) silylene and its heavier analogues

A N-heterocylic boryloxy ligand equipped with bulky diazaboryl N-substituents is used to synthesize divalent and dicoordinate group 14 compounds which represent the first examples of acyclic halo(boryloxy) tetrylenes. The bromo(boryloxy) silylene reacts swiftly with benzophenone to a siloxindane.

Cooperative small molecule activation by apolar and weakly polar bonds through the lens of a suitable computational protocol

Small molecule activation processes are central in chemical research and cooperativity is a valuable tool for the fine-tuning of the efficiency of these reactions. In this contribution, we discuss recent and remarkable examples in which activation processes are mediated by bimetallic compounds featuring apolar or weakly polar metal-metal bonds. Relevant experimental breakthroughs are thoroughly analyzed from a computational perspective. We highlight how the rational and non-trivial application of selected computational approaches not only allows rationalization of the observed reactivities but also inferring of general principles applicable to activation processes, such as the breakdown of the structure-reactivity relationship in carbon dioxide activation in a cooperative framework. We finally provide a simple yet unbiased computational protocol to study these reactions, which can support experimental advances aimed at expanding the range of applications of apolar and weakly polar bonds as catalysts for small molecule activation.

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).

Multiple ethylene activation by heteroleptic L(Cl)Ga-substituted germylenes

Ethylene insertion into the Ga-Ge bond of the L(Cl)Ga-substituted germylene LGa(μ-Cl)GeDMP 1 (L = HC(C(Me)NAr)2, Ar = 2,6-iPr2C6H3; DMP = 2,6-Mes2C6H3, Mes = 2,4,6-Me3C6H2) at ambient temperature is followed by dimerization of the as-formed germylene to give the digermene 3, which further reacted with ethylene in a [2 + 2] cycloaddition to give the 1,2-digermacyclobutane 4. In marked contrast, the amino-substituted germylene L(Cl)GaGeN(SiMe3)Ar 2 reacted directly to the 1,2-digermacyclobutane 5. Quantum chemical calculations confirmed the assumed reaction mechanism, hence demonstrating the crucial role of the substituent on the reaction mechanism.

π-Electron Fluctuation-Induced P+ /C- Ambiphilic Interaction for Intramolecular CAr -H Bond Activation

Herein, we describe how computational mechanistic understanding has led directly to the discovery of new 2H-phosphindole for C-CAr bond activation and dearomatization reaction. We uncover an unexpected intramolecular C-H bond activation with a 2H-phosphindole derivative. This new intriguing experimental observation and further theoretical studies led to an extension of the reaction mechanism with 2H-phosphindole. Through DFT calculations, we confirm that within a five-membered ring, the polarizable PC3 unit orchestrates the formation of an electrophilic phosphorus atom (P+ ) and a nucleophilic carbon atom (C- ). This kinetically accessible ambiphilic phosphorus/carbon couple is spatially separated by geometric constraints, and their reactivity is modulated through structural resonance.

Fast and scalable solvent-free access to Lappert's heavier tetrylenes E{N(SiMe3)2}2 (E = Ge, Sn, Pb) and ECl{N(SiMe3)2} (E = Ge, Sn)

Iconic Lappert's heavier tetrylenes E{N(SiMe3)2}2 (E = Ge (1), Sn (2), Pb (3)) have been efficiently prepared from GeCl2·(1,4-dioxane), SnCl2 or PbCl2 and Li{N(SiMe3)2} via a completely solvent-free one-pot mechanochemical route followed by sublimation. This fast, high-yielding and scalable approach (2 has been prepared in a 100 mmol scale), which involves a small environmental footprint, represents a remarkable improvement over any synthetic route reported over the last five decades, being a so far rare example of the use of mechanochemistry in the realm of main group chemistry. This solventless route has been successfully extended to the preparation of other heavier tetrylenes, such as ECl{N(SiMe3)2} (E = Ge (4), Sn (5)).

Activation of the GaI Cation for Bond Activation: from Oxidative Additions into C-Cl and H-P Bonds to Reversible Insertion into P4

Although the discovery of the GaI complex salt [Ga(PhF)2-3 ][Al(ORF )4 ] (RF =C(CF3 )3 , PhF=C6 H5 F) invoked the preparation of a diverse library of cationic Ga(I) coordination complexes and clusters, studies on small molecule activation with low-valent GaI cations are scarce. Herein, a first experimental study on the reactivity of a monomeric Ga(I) cation activated with a pyridine-diimine pincer ligand (in [Ga(PDIdipp )][Al(ORF )4 ]) towards small-molecules is reported. First controlled oxidative additions of the GaI cation into C-Cl, H-P and P-P bonds are presented. Moreover, the [4+1]cycloaddition to butadienes was achieved. Intriguingly, the isolated, blue insertion product into the P-P bond of P4 allows for the quantitative release of the P4 molecule upon reaction with AlEt3 and butadienes. Reversible P4 insertion of main-group metals has previously been reported for Ge and Sn, respectively. The experimental study is supported by high-level computational analysis of the in-part reversible oxidative additions at the DLPNO-CCSD(T)/def2-TZVPP//PBEh-3c/def2-mSVP level of theory with COSMO-RS solvation in 1,2-difluorobenzene.

A complete series of N-heterocyclic tetrylenes (Si-Pb) with a 1,1'-ferrocenediyl backbone enabled by 1,3,2-diazaborolyl N-substituents

The use of bulky 1,3,2-diazaborolyl N-substituents has allowed the synthesis of the complete series of ferrocene-based N-heterocyclic tetrylenes fc[(N{B})2E] (fc = 1,1'-ferrocenediyl, {B} = (HCNC6H3-2,6-iPr2)2B, E = Si-Pb). The silylene fc[(N{B})2Si] is inert towards NH3, CO2 or N2O under ambient conditions and thus significantly less reactive than the N-aryl homologue fc[(NC6H3-2,6-iPr2)2Si]. In accord with its higher reactivity, computational results indicate a more pronounced ambiphilicity of fc[(NC6H3-2,6-iPr2)2Si]. Our computational investigation of the model compound fc[(NBMe2)2Si] suggests that silylenes of this type may be superior to fc[(NC6H3-2,6-iPr2)2Si] in terms of ambiphilicity.

Germanium Complexes with ONO Tridentate Ligands: O-H Bond Activation Control According to DFT Calculations

Polydentate ligands are used for thermodynamic stabilization of tetrylenes-low-valent derivatives of Group 14 elements (E = Si, Ge, Sn, Pb). This work shows by DFT calculations how the structure (the presence or absence of substituents) and type (alcoholic, Alk, or phenolic, Ar) of tridentate ligands 2,6-pyridinobis(1,2-ethanols) [^AlkONO^R]H₂ and 2,6-pyridinobis(1,2-phenols) [^ArONO^R]H₂ (R = H, Me) may affect the reactivity or stabilization of tetrylene, indicating the unprecedented behavior of Main Group elements. This enables the unique control of the type of the occurring reaction. We found that unhindered [ONO^H]H₂ ligands predominantly led to hypercoordinated bis-liganded {[ONO^H]}₂Ge complexes, where an E(+2) intermediate was inserted into the ArO-H bond with subsequent H₂ evolution. In contrast, substituted [ONO^Me]H₂ ligands gave [ONO^Me]Ge: germylenes, which may be regarded as kinetic stabilized products; their transformation into E(+4) species is also thermodynamically favorable. The latter reaction is more probable for phenolic [^ArONO]H₂ ligands than for alcoholic [^AlkONO]H₂. The thermodynamics and possible intermediates of the reactions were also investigated.

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.

Frustrated Lewis pair-ligated tetrelenes

The reactivity of [PB{SiX₂}] (X = Cl, Br; PB = 1,2-ⁱPr₂(C₆H₄)BCy₂; E = Si, Ge) adducts is described, with an initial focus on reduction attempts to access [PB{E}]_x species; however, in all cases only free PB ligand was formed as the soluble product. Moreover, computations were performed to evaluate the energy penalty associated with EX₂ dissociation from the PB chelates. Moving up the periodic table, the formal methylene adduct [PB{CH₂}] was isolated and its reactivity was compared with its heavier element congeners of [PB{EH₂}]. We also introduce new phosphine-borane frustrated Lewis pair (FLP) chelates and explore preliminary coordination chemistry with these ligands.

Room-Temperature-Observable Interconversion Between Si(IV) and Si(II) via Reversible Intramolecular Insertion Into an Aromatic C-C Bond

An easily isolable silacycloheptatriene (silepin) 1 b was synthesized from the reaction of a N-heterocyclic imino (IPrN) substituted tribromosilane IPrNSiBr₃ with the sterically congested bis(trimethylsilyl)triisopropylsilyl silanide KSi(TMS)₂ Si(ⁱ Pr)₃ (BTTPS). In solution, the Si(IV) silepin 1 b is in a thermodynamic equilibrium with the acyclic Si(II) silylene 1 a. The relative concentration of the Si(II) or Si(IV) isomers can be controlled by temperature variation and observed by variable temperature NMR and UV/Vis spectroscopy. DFT calculations show a small reaction barrier for the Si(II)⇌Si(IV) interconversion and a small energy gap between the Si(II) and Si(IV) species. The reactivity of 1 a/b is demonstrated on a variety of small molecules.

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 [L^Ph SiCl] (L^Ph =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.

Unravelling the Potential of Ylides in Stabilizing Low-Valent Group 13 Compounds: Theoretical Predictions of Stable, Five-Membered Group 13 (Aluminum and Gallium) Carbenoids Capable of Small-Molecule Activation

Computational investigations provide evidence toward the remarkable ability of strongly electron-donating ylidic functionalities in stabilizing singlet group 13 carbenoids with promising ligand properties. All of the proposed carbenoids are found to be considerably nucleophilic and possess significant singlet-triplet energy separation values. The calculated activation barriers and reaction free energies obtained for the cleavage of different enthalpically strong bonds by these carbenoids are found to be either comparable to or lower than those of the experimentally evaluated aluminum and gallium carbenoids, thereby indicating their potential in small-molecule activation.

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.

First-Row Transition Metal Complexes of a Phosphine-Silylene-Based Hybrid Ligand

We have prepared two new silylene-phosphine-based hybrid ligands Si{N(R)C₆H₄(PPh₂)}{PhC(N^tBu)₂} [R = TMS {trimethylsilyl} (1) and TBDMS {tert-butyldimethylsilyl} (2)], which possess two donor sites. Furthermore, the treatment of the bidentate ligand 1 with base metal halides {FeBr₂, CoBr₂, NiCl₂·dme [nickel chloride(II) ethylene glycol dimethyl ether]} and 2 with NiBr₂·dme [nickel bromide(II) ethylene glycol dimethyl ether] afforded four-coordinate six-membered metal complexes 3-6, respectively, which feature coordination from both Si(II) and P(III) sites. Subsequently, complexes 3 [(FeBr₂)Si{N(SiMe₃)C₆H₄(PPh₂)}{PhC(N^tBu)₂}], 4 [(CoBr₂)Si{N(SiMe₃)C₆H₄(PPh₂)}{PhC(N^tBu)₂}], 5 [(NiCl₂)Si{N(SiMe₃)C₆H₄(PPh₂)}{PhC(N^tBu)₂}], and 6 [(NiBr₂)Si{N(Si^tBuMe₂)C₆H₄(PPh₂)}{PhC(N^tBu)₂}] are studied for their redox and magnetic properties with the help of UV-vis spectroscopy, cyclic voltammetry, SQUID magnetometry, and theoretical calculations. Complexes 3-6 were found to display a paramagnetic behavior. All the compounds are well established by single-crystal X-ray diffraction studies.

Selective [2+1+1] Fragmentation of P 4 by heteroleptic Metallasilylenes

Small‐molecule activation by low‐valent main‐group element compounds is of general interest. We here report the synthesis and characterization (¹H, ¹³C, ²⁹Si NMR, IR, sc‐XRD) of heteroleptic metallasilylenes L¹(Cl)MSiL² (M=Al 1, Ga 2, L¹=HC[C(Me)NDipp]₂, Dipp=2,6‐ⁱPr₂C₆H₃; L²=PhC(N^tBu)₂). Their electronic nature was analyzed by quantum chemical computations, while their promising potential in small‐molecule activation was demonstrated in reactions with P₄, which occurred with unprecedented [2+1+1] fragmentation of the P₄ tetrahedron and formation of L¹(Cl)MPSi(L²)PPSi(L²)PM(Cl)L¹ (M=Al 3, Ga 4).

Synthesis of a Triphenylphosphinimide-Substituted Silirane as a "Masked" Acyclic Silylene

Phosphinimides are long known as useful ligands in transition metal chemistry, but examples of these in low-valent silicon chemistry are rather rare. Hence, in this work, we report on the implementation of a triphenylphosphinimide moiety as a ligand of a novel silylene that is trapped as a silirane with cyclohexene. By performing activation reactions with B(p-Tol)₃, HSiEt₃, N₂O, and NH₃, we demonstrate that the silirane exhibits a silylene-like behavior, making it a "masked" silylene. Furthermore, we treated the silirane with ethylene, propylene, and trans-butene, which led to an olefin exchange. In the case of ethylene and propylene, an additional insertion of the olefin into the silicon-silicon bonds of the respective siliranes could be achieved. As the insertion of trans-butene was not feasible, we surmise that the scope of this reactivity is restricted by the steric demand of the olefin.

Facile activation of inert small molecules using a 1,2-disilylene

Reactions of the known amidinate stabilised 1,2-disilylene, [{ArC(NDip)₂}Si]₂1 (Dip = 2,6-diisopropylphenyl, Ar = 4-C₆H₄Bu^t) with a series of inert, unsaturated small molecule substrates have been carried out. Compound 1 reduces Bu^tNC: to give the singlet biradicaloid 1,3-disilacyclobutanediyl [{ArC(NDip)₂}Si(μ-CNBu^t)]₂3, which can be oxidised by 1,2-dibromoethane to give [{ArC(NDip)₂}(Br)Si(μ-CNBu^t)]₂4. Disilylene 1 reduces two molecules of ethylene to give an unprecedented disilabicyclo[2.2.0]hexane, [{ArC(NDip)₂}Si(μ-C₂H₄)]₂5. In contrast, only one molecule of ethylene inserts in the Ge-Ge bond of the digermylene analogue of 1, viz. [{ArC(NDip)₂}Ge]₂6, leading to the formation of the bis(germylene), [{ArC(NDip)₂}Ge]₂(μ-C₂H₄) 7. Compound 1 reduces CO₂, generating CO, and the oxo/carbonate-bridged disilicon(IV) system, {ArC(NDip)₂}Si(μ-CO₃)₂(μ-O)Si{(NDip)₂CAr} 10, while its reaction with N₂O proceeds via generation of N₂, and a hydrogen abstraction process, to give the oxo/hydroxy disilicon(IV) species, [{ArC(NDip)₂}(HO)Si(μ-O)]₂11. This study highlights new small molecule activation chemistry for 1,2-disilylenes, which could lead to further adoption of compound 1 as a potent reducing reagent for the transformation of inert unsaturated molecules into value added products.

Mechanistic Studies of the Catalytic Reduction of CO2 to CO: Efficient CO-Releasing Si- and Ge-Based Catalysts

Besides its significant challenges, efficient catalytic conversion of CO₂ to value-added chemicals is highly desired. Herein, we report efficient silicon- and germanium-based catalysts for CO₂ activation and its reduction to CO studied using B3LYP-GD3/6-31++G(d,p)/tetrahydrofuran (THF) and M06-2X/6-311++g(d,p)/THF density functional theory methods. The catalysts were systematically designed based on the previously reported silicon- and germanium-based compounds. The germanium-based catalysts are reported for the first time in this study. The calculated transition state energy barriers (5.7-15.8 kcal/mol) indicate that all the catalysts can easily activate CO₂. Among all the B3LYP-GD3-calculated transition-state energy barriers, the highest energy barrier found (27.2-28.3 kcal/mol) is for the protonation of the carboxylic acid group of the silacarboxylic and germacarboxylic acids. Once the silacarboxylic and germacarboxylic acids are protonated, the water molecule can easily dehydrate and leave the catalysts with CO. The electrochemical reduction of the M-CO (M = Si and Ge) complexes further enhances the complexes to easily release CO, with all transition state energy barriers being lower than 10 kcal/mol. The results show that both CO₂ activation and its reduction to CO using the studied catalysts are thermodynamically and kinetically favorable. This work provides an important insight for CO₂ activation and its reduction to CO using earth-abundant and nontoxic main group element-based catalysts.

Stable N-heterocyclic borylenes with promising ligand properties: a contribution from theory

DFT calculations reveal the power of ylides in stabilizing neutral singlet cyclic borylenes that are found to be capable of activating a variety of small molecules having enthalpically strong bonds.

Molecular Main Group Metal Hydrides

This review serves to document advances in the synthesis, versatile bonding, and reactivity of molecular main group metal hydrides within Groups 1, 2, and 12-16. Particular attention will be given to the emerging use of said hydrides in the rapidly expanding field of Main Group element-mediated catalysis. While this review is comprehensive in nature, focus will be given to research appearing in the open literature since 2001.

Single‐Site and Cooperative Bond Activation Reactions with Ylide‐Functionalized Tetrylenes: A Computational Study

Due to their transition metal‐like behavior divalent group 14 compounds bear huge potential for their application in bond activation reactions and catalysis. Here we report on detailed computational studies on the use of ylide‐substituted tetrylenes in the activation of dihydrogen and phenol. A series of acyclic and cyclic ylidyltetrylenes featuring various α‐substituents with different σ‐ and π‐donating capabilities have been investigated which demonstrate that particularly π‐accepting boryl groups lead to beneficial properties and low barriers for single‐site activation reactions, above all in the case of silylenes. In contrast, for the thermodynamically more stable germylenes and stannylenes an alternative mechanism involving the active participation of the ylide ligand in the E−H bond (E=H or PhO) activation process by addition across the element carbon linkage was found to be energetically favored. Furthermore, the boryl substituted tetrylenes allowed for a further activation pathway involving the active participation of the boron element bond. These cooperative mechanisms are especially attractive for the heavier cyclic ylidyltetrylenes in which the loss of the protonated ylide group is prevented due to the cyclic framework. Overall, the present studies suggest that cyclic ylide‐substituted germylenes and stannylenes bear huge potential for cooperative bond activations at mild conditions which should be experimentally addressed in the future.