Silylone

Silylones are a class of zero-valent monatomic silicon complexes, 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 highly reactive Si(0) 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). They are capable of reactions with a variety of substrates, including chalcogens and carbon dioxide.

Theoretical predictions

The structure of carbene-stabilized silylones were first predicted using theoretical calculations by Gernot Frenking and coworkers in 2009. Their theoretical study of silylones was inspired from the development and synthesis of carbones: an analogous structure containing carbon(0) stabilized by two donor-acceptor ligand interactions. It was also inspired by previous reports of trisilaallene: a silylene complex featuring a bent geometry about the Si-Si-Si center. The unexpected bent trisilaallene bond angle was dissimilar that of carbon allenes (C=C=C) yet like that of NHC-stabilized carbones. Towards the goal of rationalizing this structure and to investigate silylones in general, the authors analyzed the energetics of di-coordinated model complexes of silicon(0) (L2Si) and carbon(0) (L2C). More specifically, the analysis was conducted using boron trihydride (BH3) binding analysis and proton affinity analysis at the BP86/TZVPP level of theory. Compared to analogous model complexes of carbon, the silicon complexes displayed very different characteristics. For example, for two of the examined models, the structure of L2C(BH3)2 could not be energetically minimized whereas it could be for L2Si(BH3)2. Both the silicon and carbon model complexes contained two lone pair orbitals: one with σ-orbital character and one with π-orbital character. However, bonding of one of the model complexes with a single BH3 occurred at the π-lone pair for the silicon complex and at the σ-lone pair for the carbon complex. As a consequence, the bonding geometries of the resultant complexes differed. 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 of di-coordinated BH3 complexes (e.g. 26.2, 47.8, 48.1, and 36.6 kcal/mol) were also found. In conjunction with frontier orbital analysis, the high electron density found at the silicon centers suggested that the ligands bonded as donors than covalently. Therefore, the authors claimed that the model complexes were better described as silylones than silylenes. Since high values of the proton affinity were also found for trisilallene, the previously reported complex was also suggested to be a silylone rather than as a silylene. As a result of this analysis, the authors encouraged their exploration by experimentalists.

cAAC stabilized silylones Cyclic alkyl amino carbene (cAAC) ligands, which generally contain sterically bulky and highly electron-donating ligands, have been utilized to synthesize silylone structures.

Synthesis

The first cAAC stabilized silylone was first reported by Mondal et al. in 2013 (where cAAC is ligand used was :C(CH2)(CMe2)2N-2,6-iPr2C6H3). The complex was synthesized by reduction of (cAAC)2SiCl2, a stable biradical precursor species, with two equivalents of potassium graphite (KC8) reducing agent in tetrahydrofuran (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 product was 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 The identity of Mondal et al.'s cAAC-stabilized silylone was confirmed using a combination of spectroscopic, crystallographic, and computational analysis techniques. Silicon-29 nuclear magnetic resonance spectroscopy (29Si-NMR) revealed a signal at 𝛿Si = 66.71 ppm, while ultraviolet-visible spectroscopy (UV-Vis) showed six absorption bands (λ = 270, 327, 392, 570, and 611 nm). Crystal structure analysis revealed a C-Si-C bond angle of 117.70(8)°, distinguishing it from the previously reported trisilaallene . 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 electron paramagnetic resonance (EPR) signal, in agreement with electronic structure calculations. 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 areas, 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 highest-occupied molecular orbital (HOMO) showed that only one is formally non-bonding while the other participates in 3-centered 2-electron π-bonding. The resultant Si-C π-interaction agreed with calculated bond lengths, as well as general understanding of the cAAC ligands as good π-acceptors relative to NHC ligands. From this, the central silicon atom was proposed to act as a π-electron donor, while the carbene carbons acted as σ-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 SiII 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.

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