COMMUNICATION pubs.acs.org/Organometallics
Synthesis and Striking Reactivity of an Isolable Tetrasilyl-Substituted Trisilaallene Hiroaki Tanaka,† Shigeyoshi Inoue,‡ Masaaki Ichinohe,† Matthias Driess,‡ and Akira Sekiguchi*,† † ‡
Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan Institute of Chemistry, Metalorganics and Inorganic Materials, Technische Universit€at Berlin, Strasse des 17. Juni 135, Sekr. C2 D-10623 Berlin, Germany
bS Supporting Information ABSTRACT: The first silyl-substituted Si3-allene, namely 1,1,3,3tetrakis(di-tert-butylmethylsilyl)trisilaallene (4), was prepared as an air- and moisture-sensitive red solid by the reaction of the dilithiosilane (tBu2MeSi)2SiLi2 with the dichlorosilylene NHC complex :SiCl2rNHC (NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) in benzene at room temperature. Remarkably, the reaction of 4 with methanol proceeds regioselectively to form only the 3,3-dimethoxypentasilane derivative 5, in contrast to the case for a previously reported trisilaallene. In addition, 4 undergoes unprecedented thermal isomerization to tetrakis(di-tert-butylmethylsilyl)cyclotrisilene (7).
T
he chemistry of compounds containing unsaturated silicon has developed rapidly in the past 30 years, since the first synthesis and isolation of a disilene with a siliconsilicon double bond1 and of a silene with a siliconcarbon double bond in 1981.2 In contrast to the chemistry of heavier alkenes >E14dE14E14dE14dE14< has been scarcely explored.35 The first heavier group 14 allene analogue, (tBu3Si)2SndSndSn(SitBu3)2, was synthesized by Wiberg and co-workers in 1999.6 In contrast to the linear CdCdC subunit in allenes, the SnSnSn bending angle in Wiberg’s tristannaallene is 155.9° and the terminal tin atoms are pyramidalized. The first silicon congener of allene 1a was synthesized by Kira and co-workers in 2003.7 Trisilaallene 1a has also a flexible SidSidSi skeleton, in which the bending angle is 136.5°. They also reported the synthesis and isolation of trigermaallene 1b,8a 1,3-digermasilaallene 1c,8a and 1,3-disilagermaallene 1d.8b All of them are strongly bent at the central atom of the allene skeleton (EEE bond angles of 122.6, 125.7, and 132.4°, respectively).8 The origin of the bent and fluxional skeleton of heavy allenes is explained by JahnTeller distortion associated with the effective mixing of the π and σ* orbitals.8d However, to date no other synthesis of an isolable trisilaallene derivative has been reported (Chart 1). Apeloig and co-workers performed a computational quantummechanical study of trisilaallene and reported the properties of the parent H2SidSidSiH2 and its relationship to other Si3H4 isomers9 as well as the approach to the linear allenic type of trisilaallenes and the substituent effect.10 In addition, Vespremi and co-workers studied the potential energy surface of Si3H4 and its silylene characteristics.11 Very recently, Frenking and coworkers suggested that trisilaallene has a divalent silicon(0) character, such as Lf:Si:rL (L = cyclic silylene).12 Because their theoretical studies revealed that both structural and electronic r 2011 American Chemical Society
Chart 1. Heavier Group 14 Allene Analogues
properties of trisilaallene greatly depend on the substituent effect, the synthesis and investigation of new substituted trisilaallenes may help to understand their basic properties and bonding characteristics. Herein, we report the first synthesis of the isolable tetrasilyl-substituted trisilaallene 4 by the reaction of dilithiosilane (tBu2MeSi)2SiLi2 (1) with the :SiCl2rNHC complex 3 (NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), showing that 4 has distinctive electronic properties and reactivity toward alcohols compared with those of 1a, as well as demonstrated by its thermal isomerization to tetrakis(di-tert-butylmethylsilyl)cyclotrisilene 7. Dilithiosilane (tBu2MeSi)2SiLi2 (2)13 readily reacted with 314 in benzene at ambient temperature. The yellow suspensions of mixtures of 2 and 3 immediately turned to an intensely red solution because of the formation of the tetrasilyl-substituted trisilaallene derivative 4.15 Accordingly, 1H NMR analysis of the reaction mixture confirmed the formation of 4 (4, ∼80%; (tBu2MeSi)2SiH2, ∼20%) along with “free” NHC. Trisilaallene 4 Received: May 18, 2011 Published: June 14, 2011 3475
dx.doi.org/10.1021/om200405e | Organometallics 2011, 30, 3475–3478
Organometallics
COMMUNICATION
Scheme 1. Synthesis of Trisilaallene 4 by the Reaction of Dilithiosilane 2 with :SiCl2rNHC Complex 3
was isolated as an air- and moisture-sensitive red solid in 40% yield by silica gel column chromatography with hexane in a glovebox under argon (Scheme 1). It remains unchanged in benzene solution at room temperature. The choice of the reaction solvent is crucial, and the product yield decreased when polar solvents such as THF and ether were used. The reaction mechanism is not clear at this moment; however, the reaction proceeded cleanly when the molar ratio of 2 to 3 was 1:1. The structure of trisilaallene 4 was established by HRMS and 1 H, 13C, and 29Si NMR spectroscopic data, as well as product analysis of the reaction with methanol and methanol-d, as shown below. In the 1H NMR spectrum of 4, only two signals, which are assignable to the tBu and Me groups of tBu2MeSi substituents, are observed. Trisilaallene 4 shows three 29Si NMR signals at δ 22.4, 44.6, and 418.5 ppm that are assignable to the substituent silicon atoms (tBu2MeSi), terminal silicon atoms (SidSidSi), and central silicon atom (SidSidSi), respectively, on the basis of comparison with the calculated 29Si NMR chemical shifts of the optimized structure of 4, (tBu2MeSi)2SidSidSi(SiMetBu2)2 (see below); the chemical shifts of the terminal silicon atoms (72.6 and 93.1 ppm) are shifted far more upfield than that of the central silicon atom (471.3 ppm). The fact that the signal of the central silicon atom was observed at a lower field than that of the terminal silicon atoms contrasts with the 29Si NMR chemical shifts of 1a (central Si, 157.0 ppm; terminal Si, 196.9 ppm);6 however, this trend is similar to that observed for the 13C NMR chemical shifts of allenes.16 The UVvis absorption spectrum of 4 in hexane exhibits an absorption band at 400 nm (ε 3400 M1cm1) assignable to a ππ* transition (see the Supporting Information). Hitherto we have not succeeded in an X-ray diffraction analysis of 4.17 We performed theoretical calculations of 4 to gain insights into its geometric and electronic structure. So far, theoretical studies on the parent trisilaallene (H2SidSidSiH2)810 and its methyl-substituted derivative (Me2SidSidSiMe2)7,8d,10 and H3Si-substituted derivative ((H3Si)2SidSidSi(SiH3)2)10 have been reported. Quantum-mechanical calculations show that the linear structure is not a minimum on the potential energy surface; instead, trisilaallene prefers an unusual highly bent SidSidSi structure (69.4° for H2SidSidSiH2 and 74.2° for Me2SidSidSiMe2).10 Apeloig et al. predicted that the central bend angle would be highly affected on electronic effects by the substituents: silyl substitution has a more profound effect on the central bend angle (126.7° for (H3Si)2SidSidSi(SiH3)2).10 In addition to the electronic effect by the silyl substituents, the steric effect by the four bulky tBu2MeSi groups at the terminal silicon atoms would lead to a structure of 4 more linear than that of the model H3Si-substituted trisilaallene derivative. The calculation of the real compound 4 was performed at the B3LYP/6-31G(d) level; the optimized structure is shown in Figure 1. As expected,
Figure 1. Optimized structure of the trisilaallene 4 at the B3LYP/6-31G(d) level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å): Si1Si2 = 2.1792, Si2Si3 = 2.1742, Si1Si4 = 2.4117, Si1Si5 = 2.4086, Si3Si6 = 2.4000, Si3Si7 = 2.4094. Selected bond angles (deg): Si1Si2Si3 = 164.3, Si2Si1Si4 = 117.0, Si2Si1Si5 = 119.2, Si4Si1Si5 = 120.6, Si2Si3Si6 = 116.1, Si2Si3Si7 = 119.5, Si6Si3Si7 = 122.3.
the central bend angle (164.3°) is larger than that of (H3Si)2SidSidSi(SiH3)2 (126.7°). The SidSi bond lengths are 2.1792 Å for Si1Si2 and 2.1742 Å for Si2Si3. The two terminal silicon atoms (Si1 and Si3) are slightly pyramidalized (356.8° for the sum of the bond angles around the Si1 atom and 357.9° for the sum of the bond angles around the Si3 atom). Furthermore, the planes defined by the terminal tBu2MeSi groups are almost perpendicular to each other (86.6°), as in the case of allenes.16 Thus, the HOMO and LUMO of 4 are almost degenerate (see the Supporting Information) as in the case of allene, and they have the shapes of π and π* orbitals of SidSi double bonds. These MOs of 4 are quite different from those of 1a, in which the HOMO1, HOMO, LUMO, and LUMOþ1 are all twisted bonding π and antibonding π* orbitals and neither of the two π and π* orbitals is degenerate.7 The reactivity of 4 toward alcohols is also different from that of 1a. Trisilaallene 4 reacts with MeOH to form the 3,3-dimethoxypentasilane derivative 5 as a single product, isolated by gel permeation chromatography as colorless crystals in 42% yield (Scheme 2). When methanol-d was used for the quenching reaction of 4, 2,4-dideuterio-3,3-dimethoxypentasilane was obtained in 42% yield (Scheme 2). No 2,4-dimethoxypentasilane derivatives were found by NMR analysis of the reaction mixture. In contrast, it is reported that 1a reacts with water7 and MeOH18 to give the 1,3-dihydroxy and 1,3-dimethoxy 3476
dx.doi.org/10.1021/om200405e |Organometallics 2011, 30, 3475–3478
Organometallics
COMMUNICATION
Scheme 2. Reaction of Trisilaallene 4 with Methanol and Methanol-d, Giving 3,3-Dimethoxypentasilane Derivatives 5
Scheme 3. Reaction of Trisilaallene 1a with Water and Methanol, Giving 1,3-Dioxytrisilanes
Scheme 4. Thermal Isomerization of Trisilaallene 4 to Cyclotrisilene 7
compounds, respectively, having hydroxy and methoxy groups on both terminal silicon atoms (Scheme 3). The formation of 5 can be understood as a result of the initial addition of MeOH across an SidSi double bond in 4 to form the methoxydisilene intermediate 6 having a methoxy group on the central silicon atom because of the charge distribution of 4, >SiδdSiδþdSiδ
