When might silylenes behave more like carbenes? : Sihli, a triplet silylene

doi:10.1016/S0040-4020(01)96383-5

Tetrahedron

Volume 41, Issue 8, 1985, Pages 1429-1434

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Abstract

Qualitative arguments and preliminary theoretical studies by Harrison suggest that lithiosilylene (SiHLi) may have a triplet electronic ground state. This possibility has been confirmed in the present detailed ab initio quantum mechanical study. Using double-zeta and double-zeta plus polarization basis sets, the different low-lying electronic states of SiHLi have been investigated using self-consistent-field and configuration interaction methods. The triplet ground state potential surface is very flat, with two nearly degenerate minima at θ (HSiLi) values of 137° and 48°, respectively. The lowest singlet state lies ~ 7 kcal higher in energy and is predicted to have an equilibrium bond angle of ~93°, much like the parent silylene SiH₂. Vibrational frequencies are predicted for all stationary points.

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Molecular structures and energies of low-lying Li<inf>x</inf> Si<inf>x</inf> (x = 1 s(-) 4) clusters: Comparison with Li<inf>x</inf> C<inf>x</inf> (x = 1, 2, 4) clusters
2008, Chemical Physics Letters
The structures and energies of ${Li}_{x} {Si}_{x} (x = 1 – 4)$ were investigated by the DFT and MP2 methods by using a variety of basis sets of 6-31G(d), 6-311+G(2df), and cc-pVXZ (X = T, Q, 5). Two singlet low-lying doubly-bridged bent ( $C_{2 v}$ ) and planar ( $D_{2 h}$ ) structures are found to be the global minimum and a transition state on the potential energy surface of ${Li}_{2} {Si}_{2}$ , respectively. The preference of polylithium compounds ${Li}_{x} {Si}_{x} (x = 2 – 4)$ for nonclassical structures can be taken as an indication that the extensive bonding prevails presumably owing to ionic contribution.
From halo-azasilylenes to halo-phosphasilylenes (X-CNSi vs. X-CPSi) at ab initio and DFT levels
2006, Journal of Organometallic Chemistry
The higher the electropositivity of the substituents has been associated with the higher possibility of encountering a triplet ground state silylene [P.P. Gaspar, M. Xiao, D. Ho Pae, D.J. Berger, T. Haile, T. Chen, D. Lei, W.R. Winchester, P. Jiang, J. Organomet. Chem. 646 (2002) 68]. Here, going from the reported halo-azasilylenes X-CNSi [M.Z. Kassaee, S.M. Musavi, H. Hamadi, M. Ghambarian, S.E. Hosseini, J. Mol. Struct. (Theochem) 730 (2005) 33] to the analogues more electropositively substituted halo-phosphasilylenes X-CPSi has reduced the chances of encountering triplet ground state silylenes (X = H, F, Cl and Br). Even though all singlet isomers seem to be more stable than their corresponding triplets, singlet–triplet cross over diagrams may help the future design of new acyclic triplet state silylenes.
Ab initio and DFT energetics of silylenic X-CNSi (X=H, F, Cl, and Br)
2005, Journal of Molecular Structure: THEOCHEM
Singlet–triplet energy splitting for 24 silylenic reactive intermediates, X–CNSi (where X=H, F, Cl and Br), are compared and contrasted at 11 levels of theory: B1LYP/6-31++G**, B3LYP/6-31++G**, B1LYP/6-311++G**, B3LYP/6-311++G**, MP3/6-31G*, MP3/6-311++G**, MP2/6-31+G**, MP2/6-311++G**, MP4 (SDTQ)/6-311++G**, QCISD(T)/6-311++G** and CCSD(T)/6-311++G**. Each X-substituted silylenic species may either be singlet (s) or triplet (t), with one of the following three structures: 3-X-2-aza-1-silacyclopropenylidene (1_s-X, 1_t-X); [(X-imino)methylene]silylene (2_s-X, 2_t-X); and X-cyanosilylene (3_s-X, 3_t-X). For all X–CNSi species studied, orders of singlet–triplet energy separations (ΔE_s-t,X), appear as a function of electro-negativity (F>Cl>Br>H). For the six H–CNSi isomers (X=H), stability order is: 3_s-H>1_s-H>2_t-H>3_t-H>2_s-H>1_t-H. Likewise, stability order for the six isomers with X=F, is: 3_s-F>3_t-F>1_s-F>1_t-F>2_s-F>2_t-F. For X=Cl, the order of stability is: 3_s-Cl>1_s-Cl>3_t-Cl>2_t-Cl>1_t-Cl>2_t-Cl. Finally, the order of stability for six isomers of Br–CNSi is: 3_s-Br>3_t-Br>1_s-Br>2_s-Br>2_t-Br>1_t-Br. The lowest energy minimum, among all 24 species scrutinized, appears to be the singlet acyclic 3_s-X. Triplet silylene 2_t-H is suggested to be more stable than its corresponding 2_s-H at MP3, MP2 and DFT levels of theory. Comparisons between relative stabilities; multiplicities and geometrical parameters of 1–3 are discussed.
Substituent effect of second row elements on silyl centers
1994, Journal of Molecular Structure: THEOCHEM
The entire set of single- and double-bonded second row element-substituted silicon compounds, silyl radicals and silylenes, along with their parent silicon hydrides, SiH_n (n = 2–4), have been investigated by using ab initio methods. All structural parameters were optimized by use of the 6–31 G^∗ basis sets at the HF and MP2 levels of theory. The main structural features of the low-valent silicon species do not differ significantly from those of the first row element-substituted counterparts. Shortening of the SiR (R = substituent) bond (relative to the unsaturated derivatives) is observable with substituents from the left side of the periodic table if there is an odd electron on the silyl center (silyl radical or triplet silylene), while substituents from the right side of the periodic table cause shortening of the otherwise longer (cf. SiH₂ and SiH₄) Siz.sbnd;R bonds in the case of singlet silylenes.
Stabilization energies (relative to the saturated silanes) due to the substitution were evaluated by using isodesmic reactions. It has been shown that, while silyl radicals and triplet silylenes are stabilized by substituents having empty p orbitals (being capable of delocalizing the odd electron on the silicon center), singlet silylenes are stabilized by substituents having unshared electron pairs (being capable of delocalizing to the empty p orbital of the silylene unit). Rotational barriers about the Siz.sbnd;R bonds give similar va the isodesmic stabilization energies. Stabilization caused by the second row elements is not less than that of first row substituents,depends instead on the electron donor/acceptor properties of the substituting group. In certain cases (H₂AlSiH₂, HSiPH₂) the delocalization stabilization energy is significantly reduced (compared to the expectations based on bond shortening and electronegativity of the substituting group), as the inversion barriers (at the silyl and phosphine centers respectively) should be surmounted by the delocalization stabilization. The strong electron-acceptor character of singlet silylene can be rationalized by the reduction of the phosphine inversion barrier (about 35 kcal mol⁻¹ in PH₃) to 0.2 kcal mol⁻¹ at the MP2/6–31G^∗//MP2/6–31G^∗ level of theory (0.35 and 0.4 kcal mol⁻¹ at the MP4/6–31G^∗//MP2/6–31G^∗ and MP2/6–31G^∗∗//MP2/6–311G^∗∗ levels of theory respectively). This reduction of the inversion barrier is even larger than that reported for BH₂ substituent.
Singlet-triplet splittings and electron affinities of some substituted silylenes
1992, Journal of Molecular Structure: THEOCHEM
The lowest electronic states of methyl-, silyl-, and lithium-substituted silylene have been investigated. CH₃ acts as an electronegative substituent and increases the singlet-triplet splitting, SiH₃ behaves as an electropositive substituent and decreases this energy gap, and the lithium-substituted silylenes even have triplet electronic ground states. It is found that the effects of the electropositive substituents prevail against those of the electronegative ones. Silyl substitution increases the electron affinity significantly, whereas methyl substituents strongly decrease it, or even make it vanish. Despite the highly electropositive lithium substituent, HSiLi and SiLi₂ both exhibit unexpectedly large electron affinities, because Li facilitates the delocalization of the extra electron.
<sup>1</sup>H CIDNP Study of the Addition Reaction of Me<inf>2</inf>E (E = Si,Ge) to a Carbon≡Carbon Triple Bond
1991, Mendeleev Communications

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¹: “Aspirant” of the Belgian National Fund for Scientific Research.

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When might silylenes behave more like carbenes? : Sihli, a triplet silylene

Abstract

J. Molec. Spectrosc.

Chem. Phys. Lett.

J. Am. Chem. Soc.

J. Chem. Phys.

J. Am. Chem. Soc.

J. Chem. Phys.

J. Am. Chem. Soc.

J. Am. Chem. Soc.

J. Am. Chem. Soc.

J. Am. Chem. Soc.

J. Am. Chem. Soc.

J. Chem. Phys.

Chem. Phys. Lett.