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

Disilicon-Mediated Carbon Monoxide Activation: From a 1,2,3-Trisila- to 1,3-Disilacyclopentadienes with Hypercoordinate λ4Si-λ3C Double Bonds

The facile reaction of the SiPh2-bridged bis-silylene (LSi:)2SiPh2 (L=PhC(NBut)2) with diphenylacetylene affords the unprecedented 1,2,3-trisilacyclopentadiene (LSi)2(PhC)2SiPh2 1 with a hypercoordinate λ4Si-λ3Si double bond. Compound 1 is very oxophilic and consumes three molar equivalents of inert N2O to form the bicyclic oxygenation product 2 through O-atom insertion in the Si=Si and Si-Si bonds. Strikingly, 1 can completely split the C≡O bonds of carbon monoxide under ambient conditions (1 atm, room temperature), yielding the 1,3-disilacyclopentadiene 3, representing the first hypercoordinate example of a cyclosilene with a λ4Si-λ3C double bond. Likewise, reaction of Xyl-NC (Xyl=2,6-dimethylphenyl), an isocyanide isoelectronic with CO, with 1 furnishes the related 1,3-disilacyclopentadiene 4 but with an amidinato silylene pendent attached to the Si=C carbon ring atom.

A neutral crystalline imino-substituted silyl radical

The neutral three-coordinated imino(silyl)silyl radical was isolated from the reaction of N-heterocyclic iminosilicon tribromide (ItBuN-SiBr3) with NaSitBu2Me. The radical was fully characterized by X-ray crystallography and electron paramagnetic resonance spectroscopy and supported by quantum chemical calculations.

Dialumene-Mediated Production of Phosphines through P4 Reduction

The formation of phosphorus-rich alanes featuring butterfly-like geometries is achieved. The two-electron reduction products feature a unique P4 2- structure and can act as a source of P3-. The treatment of these phosphorus containing products with electrophiles under mild conditions results in the formation of different phosphines. This approach eliminates the need for high temperatures and/or high pressures, which are commonly required in industrial processes for the preparation of useful phosphines.The activation and further functionalization of white phosphorus (P4) by main group complexes has become an increasingly studied topic in recent times. Herein, we report the controlled formation of phosphorus-rich alanes featuring butterfly-like geometries from the selective reaction of P4 with dialumenes, ([L(IiPr)Al]2) (1: L=Tripp=2,4,6-iPr3C6H2; 2: L=tBu2MeSi; IiPr=[MeCN(iPr)]2C)). The two-electron-reduction product of P4 features a P4 2- structure and is shown to be able to act as a source of P3-. Treatments of different electrophiles (e.g., chlorotrimethylsilane (Me3SiCl), iodotrimethylsilane (Me3SiI), HCl, or acetyl chloride (CH3COCl)) with these alanes under mild conditions gave the corresponding phosphines (e.g., P(SiMe3)3, PH3, or P(COCH3)3).

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.

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).

Ring Strain Energy of Diheteropnictogeniranes El2 Pn (Pn=N, P, As, Sb)- Accurate versus Additive Approaches

Accurate ring strain energy (RSE) values for sixty-six parent pnictogeniranes having two other identical p-block elements, El2 Pn, have been reported. A decrease in RSE was observed to correlate with an increase in the p character of the AO used in endocyclic bonds, which is particularly remarkable on descending the groups 15 and 16. The latter also parallels higher -NICS(1) values, which seems not to be related with an increase in aromaticity, as pointed out by other NICS-related criteria, but to atom-centred diatropic currents mostly arising from the presence of lone pairs. Only in case of pnictogenaditrieliranes Tr2 Pn (Tr=B, Al, Ga), the decrease of -NICS(1) is related to a lower Hückel-type 2π-electron aromaticity on descending group 13. The use of an additive methodology based on atom-strain contributions enables estimation of RSEs for a large majority of all possible three-membered rings containing group 13-16 elements with modest accuracy (RMSE=4.371 kcal/mol), that could be remarkably improved by using bond-strain contributions (RMSE=1.183 kcal/mol) instead.

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 .

Room temperature stable E,Z-diphosphenes: their isomerization, coordination, and cycloaddition chemistry

E,Z-isomers display distinct physical properties and chemical reactivities. However, investigations on heavy main group elements remain limited. In this work, we present the isolation and X-ray crystallographic characterization of N-heterocyclic vinyl (NHV) substituted diphosphenes as both E- and Z-isomers (L[double bond, length as m-dash]CH-P[double bond, length as m-dash]P-CH[double bond, length as m-dash]L, E,Z-2b; L = N-heterocyclic carbene). E-2b is thermodynamically more stable and undergoes reversible photo-stimulated isomerization to Z-2b. The less stable Z-isomer Z-2b can be thermally reverted to E-2b. Theoretical studies support the view that this E ↔ Z isomerization proceeds via P[double bond, length as m-dash]P bond rotation, reminiscent of the isomerization observed in alkenes. Furthermore, both E,Z-2b coordinate to an AuCl fragment affording the complex [AuCl(η2-Z-2b)] with the diphosphene ligand in Z-conformation, exclusively. In contrast, E,Z-2b undergo [2 + 4] and [2 + 1] cycloadditions with dienes or diazo compounds, respectively, yielding identical cycloaddition products in which the phosphorus bound NHV groups are in trans-position to each other. DFT calculations provide insight into the E/Z-isomerisation and stereoselective formation of Au(i) complexes and cycloaddition products.

Reversible Isomerization Between Silacyclopropyl Cation and Cyclic (Alkyl)(Amino)Silylene

A phosphine-stabilized silacyclopropyl cation 2 has been synthesized and fully characterized. Of particular interest, 2 reversibly isomerizes into the corresponding seven-membered cyclic (alkyl)(amino)silylene 3 at room temperature via a formal migratory ethylene insertion into the Si-P bond. Although silylene 3 has not been spectroscopically detected, its transient formation has been evidenced by the isolation of the corresponding disilene dimer 5 as well as by trapping reactions.

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.

A strongly twisted SiSi bond with resemblance to a buckled dimer in an unexpected isomer of hexasilabenzene

The reductive debromination of {N(SiMe₃)Ph}SiBr₃1 with Rieke magnesium yields the six-vertex amido-substituted silicon cluster 2 with zwitterionic character that represents an unprecedented isomer of hexasilabenzene. The topology of Si1 and Si2 in 2 has bonding features of a highly twisted disilene and resembles that of a buckled dimer of Si(100)2 × 1 reconstructed surfaces. Cluster 2 forms the adducts 3 and 4 with NHC^Me4 and DMAP, respectively. The NHC adduct 4 additionally coordinates to BH₃ which affords the saturated cluster BH₃NHC^Me4Si₆{N(SiMe₃)Ph}₆ (5). Furthermore, 2 undergoes addition with MeI and iodine to form the halogenated silicon clusters 6 and 7, respectively.

Generation of a π-Bonded Isomer of [P4 ]4- by Aluminyl Reduction of White Phosphorus and its Ammonolysis to PH3

By employing the highly reducing aluminyl complex [K{(NON)Al}]₂ (NON=4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene), we demonstrate the controlled formation of P₄ ^2- and P₄ ^4- complexes from white phosphorus, and chemically reversible inter-conversion between them. The tetra-anion features a unique planar π-bonded structure, with the incorporation of the K⁺ cations implicit in the use of the anionic nucleophile offering additional stabilization of the unsaturated isomer of the P₄ ^4- fragment. This complex is extremely reactive, acting as a source of P^3- : exposure to ammonia leads to the release of phosphine (PH₃ ) under mild conditions (room temperature and pressure), which contrast with those necessitated for the direct combination of P₄ and NH₃ (>5 kbar and >250 °C).

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.