Silylenes with a Small Chalcogenide Substituent: Tuning Frontier Orbital Energies from O to Te

: The general synthesis of heteroleptic acyclic silylenes with a bulky carbazolyl substituent ( dtbp Cbz) is detailed and a series of compounds with a chalcogenide substituent of the type [( dtbp Cbz)SiE 16 R] (E 16 R = O t Bu, SEt, SePh, TePh) is reported. With the bulky carbazolyl substituent present, the chalcogenide moiety can be very small, as is shown by incorporating groups as small as ethyl, phenyl or tert -butyl. For the first time, the electronic properties of the silylene can be tuned along a complete series of chalcogenide substituents. The effects are clearly visible in the NMR and UV/Vis spectra, and were rationalised by DFT computations. The reactivity of the heaviest chalcogenide-substituted silylenes was probed by reactions with trimethylphosphine selenide and the terphenyl azide TerN 3 (Ter = 2,6-dimesitylphen-yl).


Introduction
Divalent silicon compounds have attracted great interest since Jutzi's discovery of silicocene in 1986. [1]While this example features divalent silicon, it is not a typical silylene because of the unique properties of Cp ligands.Utilising the same strategy that had helped to stabilise carbenes before by Arduengo, [2] Denk et al. prepared the first genuine silylene in 1994 (Scheme 1, A). [3] The involvement of Nsubstituents contributed to the stabilisation, but also diminished the acidity of the silylene.Five years later, Kira obtained a cyclic dialkyl silylene with higher reactivity (B). [4]n 2006, Driess obtained cyclic dicoordinated silylenes derived from the β-diketiminate scaffold. [5,6]Reversible base-stabilisation of silylenes was discovered by Tokitoh and Okazaki who trapped a diarylsilylene with an isonitrile. [7,8][13][14][15] The concept of base-stabilisation was also employed successfully in Roesky's amidinatosilylene, [16] Kato's phosphine-amido silylenes [17,18] and Nakata's imidophosphonamidosilylene [19] which feature bidentate monoanionic ligands and enabled rich chemistry.Acyclic silylenes proved to be a synthetic challenge that was solved simultaneously by Power (C), Inoue (D) and Aldridge (E).While Power employed bulky terphenylthiolato substituents (C) in his work, [20] Inoue introduced an N-heterocyclic imino scaffold (NHI) to silylene chemistry and prepared (NHI)SiCp* (D). [21]Aldridge initially reported on the borylsilylene E and subsequently developed a modular access to borylsilylenes with a bulky silyl substituent. [22,23][26][27] These silylenes reversibly form silepins and the equilibrium was observed spectroscopically. [28]Rivard incorporated the related N-heterocyclic olefin substituents (NHO) in a bis-NHO silylene and NHO/silyl silylene (F) where he did not observe silepin formation. [29,30]Inoue recently demonstrated, that backbone methylation of NHI substituents also inhibits silepin formation in that case. [31]In another line of investigation, Aldridge introduced a boryloxy substituent, isoelectronic to NHI as well, and prepared the corresponding bissubstituted silylene. [32]In all these instances, two bulky substituents are required.
The reduction reaction can be followed straightforwardly by NMR spectroscopy.Characteristic resonances of the ethyl group of 1 in the 1 H NMR spectrum at 0.66 and 1.95 ppm are downfield-shifted to 0.78 and 2.14 ppm in 2. Similarly, the carbazol C 4,5 H signals are observed at lower field in 2 (1: 8.17 ppm, 2: 8.49 ppm).The most striking effect was observed in the 29 Si NMR spectrum, where the signal of 1 at À 23.9 ppm in the typical silane region disappears, and instead a resonance at + 205.6 ppm was found which is more shielded than that of Power's bisthiolatosilylene C (+ 285.5 ppm). [20]The only resonance that is upfield-shifted upon reduction is the 13 C NMR signal of the α-C atom of the ethyl group, which was found at 26.13 ppm in 1 and at 24.23 ppm in 2, while the β-C atom was observed at 15.83 ppm in 1 and at 18.73 ppm in 2.
For the alkoxides, it was not feasible to isolate monosubstituted tribromosilanes Br 3 SiOR (R = Et, Mes).However, t BuOSiBr 3 could be obtained by treatment of SiBr 4 with KO t Bu.This, in turn, did not allow access to [( dtbp Cbz)SiBr 2 O t Bu] (3) by metathesis with [( dtbp Cbz)K], as only decomposition was observed.We turned again to attempts of metathesis at dtbp CbzSiBr 3 which eventually succeeded when KO t Bu was employed (Scheme 3), affording [( dtbp Cbz)SiBr 2 O t Bu] (3).Treatment with [( Mes BDI)Mg] 2 enabled the formation of the corresponding silylene [( dtbp Cbz)SiO t Bu] (4) but required heating to 105 °C overnight for complete consumption of the reducing agent.
Upon reduction, the 29 Si NMR resonance of 3 at À 70.2 ppm disappeared and instead, 4 was observed at + 96.3 ppm, more downfield-shifted than that of Inoue's (NHI)SiOSi t Bu 3 (+ 58.9 ppm). [26]Also, in this instance, the α-C atom becomes more shielded after reduction  When an analogous protocol was applied targeting the tellurolatosilane [( dtbp Cbz)SiBr 2 TePh], instead the trisubstituted [( dtbp Cbz)Si(TePh) 3 ] (7, Scheme 4) was obtained.This could not be reduced productively, and again other routes had to be explored.Fortunately, in this instance, the direct metathesis of bromosilylene [( dtbp Cbz)SiBr] with NaTePh was possible and afforded the desired tellurolatosilylene [( dtbp Cbz)SiTePh] (8).Spectroscopically, the silane 7 features a resonance at À 48.0 ppm in the 29 Si NMR spectrum with satellites due to 1 J SiTe coupling of 521 Hz.The corresponding resonance in the 125 Te NMR spectrum is broadened and was observed at 275 ppm.In contrast, for the silylene 8, both are downfield-shifted, so that the resonances are observed at 246.7 ppm in the 29 Si NMR spectrum and at 806.With a complete series of chalcogenide-substituted acyclic silylenes available, a comparative computationally supported analysis can be undertaken.The calculations were carried out with Gaussian16 with the PBE0 functional and def2-TZVP basis sets.The structural parameters show SiÀ E bond lengths that deviate considerably from the sum of covalent radii for the lighter chalcogens (Table 1). [45]The polarisation of the SiÀ E bond decreases from O to Te as the difference in electronegativity decreases.The charge on Si decreases as well from + 1.35 for 4 (O t Bu) to + 0.79 for 8    The frontier orbitals of the four silylenes are depicted in Figure 5.The LUMO is generally dominated by the unoccupied p atomic orbital of the Si atom.Its energy progressively decreases from À 1.125 to À 1.617 eV the heavier the chalcogen is (see Table 2).The HOMO shows contributions from the carbazole π scaffold.The contribution of a σ(SiÀ E) bonding orbital as well as an s-type Si atomic orbital increases the heavier the chalcogen is (Figure 5).However, the change in HOMO energy is smaller and increases from À 5.396 eV for 4 (O t Bu) to À 5.233 eV for 8 (TePh).It should be noted that the transition from alkyl to aryl substituents along the series of silylenes also impacts the frontier orbital energies: For 6 and 8, the HOMO and LUMO energies are 0.07 and 0.02 eV higher than for their ethyl-substituted analogues.The combined effects on the frontier orbitals are detectable in the lowest-energy electronic transition, which is from 358 nm for the alkoxy-substituted silylene 4 to 484 nm for the tellurolatosilylene 8.
Spectroscopically, it is also interesting to note that upon formation of silylene from silane precursors, many characteristic resonances in the 1 H, 13 C, 29 Si, 77 Se and 125 Te NMR spectra are downfield-shifted, with the exception of the α-C of the chalcogenide substituent.This effect is reproduced for the alkoxy and thiolato substituents by DFT calculations (see Supporting Information 4.1).
NBO analysis was conducted to gain further insight into the electronic structure.For all four silylenes, there are stype natural valence orbitals at both silicon and the chalcogen which show a near-integer occupation > 1.9 e.In contrast, the p-type non-bonding orbital of the chalcogen shows decreased occupation between 1.86 for 4 (O t Bu) and 1.75 for 8 (TePh), indicating a degree of delocalisation towards the Si atom.This is reflected by occupation of the p-type natural orbital at silicon which increases from 0.24 with the O t Bu substituent to 0.36 for the TePh group.The nature of the SiÀ E bond changes as well with increasing atomic number of E. Corresponding with the electronegativity difference, natural charges and bond polarisation decrease, while the Wiberg bond index increases as the bond is more and more covalent.An AIM analysis [46,47] also shows drastic change of the SiÀ E bond from O to Te: For the alkoxysilylene, the bond critical point shows a depletion of electron density and a positive r 2 (ρ) which, in conjunction with a small ELF basin population N, [48] is indicative of charge-shift bonding. [49,50]These factors are diminished but still present for the heavier homologues, until Te, where the SiÀ Te bond resembles a conventional covalent bond with r 2 (ρ) > 0 and the occupation of the ELF basin close to 1.
Crystalline material of the compound was eventually obtained and the product was identified by SC-XRD experiments (Figure 6).It is noteworthy, that iminosilane 11 is stable, while silaimidoylbromide with Br instead of Ph was not obtained. [54]Structurally, the iminosilane 11 features a planarised Si atom (sum of angles 360°) and a short Si=N double bond of 1.553(2) Å, comparing well to other iminosilanes (1.549-1.573Å). [55,56]

Conclusion
We have shown that for acyclic silylenes with chalcogenide substituents, steric bulk is not required on that part of the molecule.The synthesis is not straightforward and follows a metathesis and reduction approach.For the thiolatosilylene, the introduction of the chalcogenide moiety was required prior to carbazolyl substitution, while for the alkoxide and selenolato substituent the chalcogenide was incorporated at the stage of carbazolylsilane.In contrast, the tellurolatosilylene could only be prepared by direct metathesis with the carbazolylbromosilylene.Thus, a complete series with small substituents spanning the whole series of chalcogens was investigated, allowing the fine tuning of electronic properties of the silylene.The bonding situation was elucidated by computational methods, revealing that the changes in bond polarity and covalency directly impact the silicon atom, which is then reflected in observable properties such as UV/ Vis and NMR spectra.The heavier the chalcogen is, the smaller is the HOMO-LUMO gap, and the more deshielded the 29 Si nucleus becomes.An initial reactivity test showed that in particular for the Te-based substituent, the heavy chalcogenide can be expulsed from the product, so that the substituent is transferred directly onto silicon, due to the inherent weakness of the SiÀ Te bond.Further studies to expand the scope of substituents on carbazolyl silylenes are in progress.

M. P. Müller, A. Hinz* e202405319
Silylenes with a Small Chalcogenide Substituent: Tuning Frontier Orbital Energies from O to Te Acyclic dicoordinated silylenes with small chalcogenide substituents were prepared and characterised, enabled by a bulky carbazolyl scaffold.The electronic properties change dramatically from O to Te, with decreasing HOMO-LUMO gaps and increasing chemical shift of the 29 Si NMR resonances.