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General structure of a silylone.

Silylones are a class of zero-valent monatomic silicon complexes within the group 14 metallyone family. They are characterized as having two lone pairs and two donor-acceptor ligand interactions stabilizing a silicon(0) center. Synthesis of silylones generally involves the use of sterically bulky carbenes to stabilize the highly reactive Si centers. For this reason, silylones are sometimes referred to siladicarbenes. To date, silylones have been synthesized with cyclic alkyl amino carbenes (cAAC) and bidentate N-heterocyclic carbenes (bis-NHC).

Theoretical predictions

Schematic of theoretically studied silylones by Takagi et al.

The structure carbene-stabilized silylones were first predicted from theoretical calculations by the work of Gernot Frenking and coworkers in 2009. Their study was inspired from the development and synthesis of carbones: an analogous structure containing carbon(0) stabilized by two donor-acceptor ligand interactions. Towards this goal of examining other group 14 analogues of carbones, the bonding of previously reported trisilaallene was reexamined under the consideration that it may be better described as a silylone. Using the BP86/TZVPP level of theory, the authors compared energetic differences between di-coordinated model complexes of silicon and carbon using BH₃ binding and proton affinity analysis. Compared to analogous model complexes of carbon, the silicon complexes displayed very different characteristics. For example, for two of the examined complexes, the structure of L₂C(BH₃)₂ could not be energetically minimized whereas it could be for L₂Si(BH₃)₂. Furthermore, bonding with a single BH₃ molecule occurred at different Si orbitals with differing sigma and pi character. The resulting different bonding geometries thus differed between analoguous complexes. High values of the second proton affinity (e.g. PA = 142.9, 129.3, 166.8, and 123.9 kcal/mol) and of the bond dissociation energy (BDE) of the second BH₃ ligand between each of the studied model complexes (e.g. 26.2, 47.8, 48.1, and 36.6 kcal/mol) were found. In conjunction with frontier orbital analysis, the authors concluded that the complexes are best described as silylones as opposed to silylenes and should be experimentally realizable. The authors also concluded that trisilaallene is better described as a silylone.

cAAC stabilized silylones

Synthesis

Schematic of cAAC-stabilized silylone synthesis.

Cyclic alkyl amino carbene stabilized silylones were first reported by Mondal et al. in 2013. The complex can be synthesized by reduction of (cAAC)₂SiCl₂, a stable biradical precursor species, with two equivalents of KC₈ reducing agent in THF solution. Under this preparation, 95% yield of product was achieved and formed a dark blue solution in hexane with rod-shaped crystals. The crystallized silylones were found to be stable under inert atmosphere and unreactive towards hydrogen gas, carbon dioxide, and ammonia. Furthermore, they were found to melt at 195 °C and decompose at 220 °C.

Structure

Identity of the complex was confirmed using a combination of spectroscopic, crystallographic, and computational analysis techniques. ²⁹Si-NMR revealed a signal at 𝛿_Si = 66.71 ppm, while UV-Vis showed six absorption bands (λ = 270, 327, 392, 570, and 611 nm). Crystal structure analysis revealed bonding between the two cAAC carbene carbons to the central Si atom with a bond angle of 117.70(8)°, distinguishing it from previously reported trisilylallene structures. The Si-C bond lengths were determined to be 1.8411 and 1.8417 Å: similar to Si-C single bonds yet much larger than Si=C double bonds. The authors suggested a singlet ground state for the molecule based on the lack of an observed EPR signal, in agreement with electronic structure calculations using the M05-2X/SVP level of theory.

Further electronic structure calculations supported the assignment of the structure as a silylone. Analysis of the valence shell charge concentrations (VSCCs) in the non-bonding region of the complex revealed two distinct regions, indicating the presence of two lone pairs on the central Si. Though the structure contains two lone pairs, Natural bond orbital (NBO) analysis of the HOMO showed that only one is formally non-bonding while the other participates in 3-centered 2-electron pi-bonding. The resultant pi-interactions between Si and C agreed with calculated bond lengths, as well as general understanding of the cAAC ligands as good pi-acceptors relative to NHC ligands. From this, the Si was proposed to act as a pi-electron donor, while the carbene carbons act as sigma-electron donors. Calculation of the first and second proton affinities (PA(1) = 272.2 kcal/mol, PA(2) = 186.7 kcal/mol) of the complex supported the identity of the structure as a Si⁰ silylone than a Si^II silylene, particularly due to the large value of the second proton affinity. Finally, Bader charge analysis of the complex agreed with those predicted from NBO analysis.

Reactivity

Schematic of cAAC-stabilized silylone conversion to cyclic silylene, with the activated hydrogen atom highlighted in red. The cAAC ligand shown is Cy = :C(CH₂)(CMe₂) (C₆H₁₀)N-2,6-iPr₂C₆H₃.

In 2014, Roy et al. reported the intermolecular cyclization of the complex with potassium metal reductant in THF through tertiary C-H bond activation. Cyclic voltammetric analysis of the complex showed a quasi-reversible reduction at E_1/2 = –1.55 V vs. Fc/Fc⁺. From this data, and in consideration of the pi-accepting character of the Si-C bond, the authors speculated that one electron reduction occurred at the carbene carbon. The quasi-reverisible nature of the CV suggested that the complex then underwent further chemical rearrangement following reduction at the electrode. Reduction using metallic potassium in THF as a chemical reductant produced a solution that changed color from dark blue to greenish-yellow over the course of the reaction. The yellow solid product was then isolated with 80% yield.

The product was determined to be a three-coordinate six-membered cyclic silyene: an isomer of the parent silylone. Mass spectroscopy (m/z (100%): 678.4), ²⁹Si-NMR (𝛿_Si = 55.98 ppm), and ¹³C-NMR (𝛿_CH = 69.4 ppm and 𝛿_C: = 173.5 ppm) of the complex supported its description as an silylene isomer. X-ray single-crystal analysis revealed a trigonal pyramidal geometry with respect to the central silicon and three carbon atoms, which by bond length analysis revealed six-membered ring formation. Density functional theory (DFT) calculations suggested a closed-shell singlet ground state of the complex, while also suggesting that the remaining Si-C bond to the unreacted cAAC ligand is best described as a covalent double bond rather than as a donor-acceptor interaction. In total consideration of the data, the mechanism reported suggests an initial one-electron reduction of the carbene carbon, followed by radical activation of the ligand's Me₂(Ar)C−H bond and subsequent Si-C bond formation.

bis-NHC stabilized silylones

Synthesis

Schematic of bis-NHC stabilized silylone synthesis.

With inspiration from cAAC-stabilized silylones and Robinson's seminal NHC-stabilized disilicon(0) complex, the synthesis of bis-NHC stabilized silylones were first reported by Xiong et al. in 2013. The chlorosilyliumylidene precursor complex is first prepared by ligand exchange of ^DNHC->SiCl₂ with bis-NHC in equimolar amounts in THF. The precursor can then be extracted using acetonitrile in 57% yield. The structure of the precursor was characterized by ²⁹Si NMR, revealing a large upfield signal of 𝛿_Si = –58.4 ppm in CD₃CN and 𝛿_Si = –57.8 ppm in solid-state CP/MAS. The largely shielded signals were similarly calculated to be 𝛿_Si = –66.3 ppm using GIAO/B3LYP/6-311(d) and 6-311G(3d) level of theory. The upfield-shifted signals were attributed to large electron donation character from the NHC ligands: a known property of the structure. Crystallographic analysis revealed a trigonal pyramidal coordination geometry, with a Cl^- bond length of 2.139(2) Å and average Si->C dative bond lengths of 1.961(4) Å and 1.985(4) Å. Orbital analysis via DFT revealed HOMO-LUMO similarities to a chlorogermyliumylidene precursor analogue, which was previously successful for forming the analogous bis-NHC stabilized germylone complex.

This precursor was thenfurther treated with sodium naphthalide reductant in a 2:1 molar ratio in THF at –60°C to form the final Si(0) complex with 68% yield. A dark red color change was observed over the course of the reaction, which retained its color when converted into a powder.

Structure

HOMO and HOMO-1 orbitals of bis-NHC stabilized silylone.

In contrast to the cAAC stabilized silylone, the bis-NHC stabilized silylone was found to have a more electron-rich Si center. ²⁹Si-NMR analysis of the complex revealed a highly shielded signal at 𝛿 = -80.1 ppm in C₆D₆. The increased shielding was speculated to be due to the higher sigma donation and weaker pi-accepting character of the NHC ligands, as well as the acute 89.1(1)° bond angle between the silicon center and carbene carbons. Further calculations of the ²⁹Si shift and NBO charges of the complex supported the interpretation of the NHC ligands as strong sigma donors.

XRD analysis confirmed the di-coordinate structure of the complex, while also providing insight into its overall geometry. The C₃N₂Si ring was found to exist in a puckered conformation, though the bonding of the carbene carbons was trigonal planar with respect to the central silicon. The asymmetrical Si-C bond lengths of 1.864 Å and 1.874 Å were found to be shorter than that of the precursor complex. The shortened bonds were also verified from DFT calculations at the B3LYP/6-31G(d) level, wherein the HOMO shows an additional Si pi-interaction with the carbene carbons. UV-Vis analysis of the complex in toluene shows four absorption maxima, the λ = 547 nm (ɛ = 7.5 x 10³) maxima of which was assigned to be the HOMO-LUMO transition via TD-DFT calculations.

High values of the proton affinity of the complex (281.7 kcal/mol and 189.4 kcal/mol) suggested that the ligands interact with the silicon center datively rather covalently. which was consistent with its description as a silylone. Furthermore, it was found to have even stronger donor-acceptor interactions than the cAAC-stabilized analogue. Yet, similar to the cAAC stablized complex, the orbital character of the lone pairs was found to be asymmetric with one lone pair residing in an orbital with predominant s-type character and the other with predominant p-type character.

Reactivity

Lewis acids

Reaction schematic of bis-NHC stabilized silylone with Lewis acids to form adducts.

bis-NHC stabilized silylones were found to react in a one- and two-fold fashion with Lewis acids. Xiong et al. reported the formation of the monomeric complex (bis-NHC)Si(GaCl₃) from addition of the Lewis acidic GaCl₃ in THF, which appeared as a yellow solid isolatable in 58% yield. X-ray diffraction analysis revealed a C-Si-C bond angle (88.59(9)°) similar to that of the reactant silylone, though ²⁹Si{¹H}-NMR revealed a large upfield signal (𝛿 = –119.0 ppm). Yao et al. also reported the reactivity of the silylone with two equivalents of ZnCl₂ in THF to form colorless crystals of (bis-NHC)Si(ZnCl₂)₂. The coordination environment of the complex was observed to be tetrahedral around the silicon, as determined by XRD analysis. However, the coordination was shown to be asymmetric between the two Zn centers, where one is trigonal planar and the other is tetrahedral (as a result of additional coordination with a molecule of THF).

Schematic of silylone-activated reduction of silicon and germanium complexes.

These species were also found to act as a reducing agents, as demonstrated by their ability to reduce GeCl₂(dioxane) to Ge⁰ and NHC->SiCl₂ to form Si⁰ and dinuclear silicon.

Structures of other chalcogenide-ligated silicon complexes.

Chalcogenides

Schematic of the synthesis of chalcogenide-ligated silicon complexes from bis-NHC-stabilized silylones.

The bis-NHC stabilized silylones have also been found to react with chalcogens to form silicon(II) monochalcogenides and silicon(IV) dichalcogenides. For example, reaction of the complex with elemental sulfur resulted in the formation of a colorless powder in 89% yield. Its structure was characterized using high-resolution electrospray ioniziation mass spectrometry (HR-ESI-MS, m/z = 5.6125220) and solid state CP/MAS ²⁹Si{¹H} NMR (𝛿 = –32.5 ppm). The complex was found to retain Lewis basic properties despite not being Si(0). For example, the disulfur complex was found to form an adduct with GaCl₃ with 91% yield. The resulting structure was analyzed using X-ray diffraction, revealing asymmetric Si-S bond lengths of 2.106(2) Å and 2.006(2) Å and a S-Si-S bond angle of 115.03(8)°. The structure and identity of this complex was also further supported by HR-ESI-MS and CP/MAS ²⁹Si{¹H} NMR analysis. Natural resonance theory (NRT) analysis of the disulfide complex revealed symmetric Si-S single bonds that are semi-polar in character. Minor resonance contributions show asymmetric Si-S bonds in which one Si-S contains no bonding while the other double bonding resulting from pi-interactions with the Si center. The weight of the no-bond/double-boning resonance was enhanced under addition of the GaCl₃ adduct into the model, in expectation with one of the sulfides acting as an electron donor and verified the aforementioned asymmetry in bond length.

Other chalcogenide structures have also been synthesized. Reaction of (bis-NHC)Si(GaCl₃) with selenium can produce the monochalcogenide (bis-NHC)SiSe(GaCl₃). Dichalcogenide analogues with Se and Te can also be synthesized, whose structures were confirmed using ²⁹Si{¹H}-NMR, infrared spectroscopy (IR), MS, and single-crystal X-ray diffraction. Though different in terms of the weighting compared to the disulfide structures, NRT analysis of such complexes reveal a similar predominance of the S-X (X = Se, Te) semi-polar single bond resonance forms. All complexes under this class have been isolated and characterized as monomeric species, in comparison to many previously synthesized SiX₂ complexes which were polymeric.

Carbon dioxide

Activation of carbon dioxide by bis-NHC stabilized silylones was first reported by Burchert et al. in 2017. The reaction was achieved by exposing a cooled solution of the silylone at –30°C in THF to carbon dioxide, from which colorless crystals formed over the course of four days. The crystals were then isolated as a white solid in 75% yield. The dicarboxylated reaction product (bis-NHC)Si(CO₃)₂ was isolated and characterized using IR (ν_CO = 1746 cm^-1) and ²⁹Si{¹H}-NMR (𝛿_Si = 55.98 ppm). Crystallographic analysis of the complex revealed a Si^IV center coordinated by two carbonate ligands in a distorted octahedral geometry. Carbon monoxide was also verified as a reaction product by means of carbon-13 analysis using ¹³CO₂ as a reactant. Using DFT analysis, the authors proposed a reaction mechanism involving the formation two successive silicon-oxo bonds, which was calculated to be a favorable reaction pathway under their reaction conditions.

Schematic of CO2 activation using bis-NHC-stabilized silylone.