Synthesis and Characterization of N-Heterocyclic Carbene-Coordinated Silicon Compounds Bearing a Fused-Ring Bulky Eind Group

Abstract

The reactions of the fused-ring bulky Eind-substituted 1,2-dibromodisilene, (Eind)BrSi=SiBr(Eind) (1a) (Eind = 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl (a)), with N-heterocyclic carbenes (NHCs) (Im-Me₄ = 1,3,4,5-tetramethylimidazol-2-ylidene and Im-ⁱPr₂Me₂ = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) are reported. While the reaction of 1a with the sterically more demanding Im-ⁱPr₂Me₂ led to the formation of the mono-NHC adduct of arylbromosilylene, (Im-ⁱPr₂Me₂)→SiBr(Eind) (2a′), a similar reaction using the less bulky Im-Me₄ affords the bis-NHC adduct of formal arylsilyliumylidene cation, [(Im-Me₄)₂→Si(Eind)]⁺[Br⁻] (3a). The NHC adducts 2a′ and 3a can also be prepared by the dehydrobromination of Eind-substituted dibromohydrosilane, (Eind)SiHBr₂ (4a), with NHCs. The NHC-coordinated silicon compounds have been characterized by spectroscopic methods. The molecular structures of bis-NHC adduct, [(Im-ⁱPr₂Me₂)₂→Si(Eind)]⁺[Br⁻] (3a′), and 4a have been determined by X-ray crystallography.

1. Introduction

Over many years, a number of unsaturated silicon compounds have been successfully obtained by virtue of the complexation of metal ions and/or coordination of ligands (mainly Lewis bases) in addition to steric protection with bulky substituents [1,2,3,4,5,6,7,8,9,10,11]. Among them, the coordination chemistry of highly-reactive halosilylenes, i.e., halogen-substituted divalent Si(II) species, have attracted a lot of attention as potentially useful precursors for the construction of a wide range of silicon-containing compounds [12,13,14,15,16,17]. Figure 1 shows recent examples of coordination-stabilized arylhalosilylenes and their derivatives [18,19,20,21,22,23,24]. In 2010, Filippou’s group reported the first N-heterocyclic carbene (NHC) adducts of arylchlorosilylenes bearing sterically large m-terphenyl groups, (Im-Me₄)→SiCl(Ar) (Ic and Id) (Ar = 2,6-(Mes)₂C₆H₃ (Mes = 2,4,6-Me₃C₆H₂) (c) and 2,6-(Tip)₂C₆H₃ (Tip = 2,4,6-ⁱPr₃C₆H₂) (d)), which were prepared by the dehydrochlorination of the aryldichlorohydrosilanes, (Ar)SiHCl₂, with NHC (Im-Me₄ = 1,3,4,5-tetramethylimidazol-2-ylidene) along with the formation of imidazolium chloride, [(Im-Me₄)H]⁺[Cl⁻] [18]. The NHC adduct Id reacted with [Li⁺][CpMo(CO)₃]⁻ to afford the silylidene complex, Cp(CO)₂Mo=Si(Ar)(Im-Me₄) (IId), and the subsequent treatment with B(C₆H₄-4-Me)₃ produced the first silylidyne complex, Cp(CO)₂Mo≡Si(Ar) (IIId), featuring a metal-silicon triple bond [19,25].

In 2011, we reported on the 4-pyrrolidinopyridine (PPy) adducts of arylbromosilylenes with fused-ring bulky Rind groups, PPy→SiBr(Rind) (IVa and IVb) (Rind = 1,1,3,3,5,5,7,7-octa-R-substituted s-hydrindacen-4-yl; Eind (a: R¹ = R² = Et) and EMind (b: R¹ = Et, R² = Me)) [26,27], which were formed by the addition of PPy to 1,2-dibromodisilenes, (Rind)BrSi=SiBr(Rind) (1a and 1b) [20]. Also in 2011, Cui reported on the related NHC-coordinated aminochlorosilylene [28], and recently Driess’s group reported on the aminochlorosilylene-nickel complex [29]. In 2012, a platinum complex of arylbromosilylene, (Bbt)BrSi=Pt(PCy₃)₂ (Ve), was synthesized by the treatment of 1,2-dibromodisilene, (Bbt)BrSi=SiBr(Bbt) (1e), bearing the bulky Bbt groups (Bbt = 2,6-{CH(SiMe₃)₂}₂-4-C(SiMe₃)₃-C₆H₂ (e)) [30] with Pt(PCy₃)₂ [21,31]. In this context, some arylbromosilylidene and arylsilylidyne complexes of nickel and platinum were reported [32,33,34]. In 2013, Filippou’s group reported the unprecedented dicationic NHC complexes of silicon(II) and NHC adducts of iodesilyliumylidene cation SiI⁺ [35]. Subsequently, we reported the reaction of 1,2-dibromodisilenes (1b, 1e, and 1f) having EMind, Bbt, and Tbb groups (Tbb = 2,6-{CH(SiMe₃)₂}₂-4-^tBu-C₆H₂ (f)) with NHCs (Im-Me₄ and Im-ⁱPr₂Me₂) (Im-ⁱPr₂Me₂ = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) leading to the formation of mono-NHC adducts of arylbromosilylenes, NHC→SiBr(Ar) (VIb′ and VIe), and the bromide salts of the bis-NHC adducts of formal arylsilyliumylidene cations, [(NHC)₂→Si(Ar)]⁺[Br⁻] (VIIb, VIIb′, and VIIf) [22]. Inoue’s group also reported the synthesis of chloride salts of bis-NHC adducts of formal arylsilyliumylidene cations, [(Im-Me₄)₂→Si(Ar)]⁺[Cl⁻] (VIIIc and VIIIg) (Ar = Tip (g)) [23] and their unique conversion to silicon analogues of acylium ions, [(Im-Me₄)₂→Si(O)(Ar)]⁺[Cl⁻] (IXc and IXg) [24]. Recently, Inoue’s group reported on the chalcogen-atom transfer and exchange reactions of NHC-bound heavier silaacylium ions, [(Im-Me₄)₂→Si(E)(Ar)]⁺[Cl⁻] (E = S, Se, and Te) [36].

In this article, we describe the preparation and characterization of NHC-coordinated silicon compounds bearing the bulky Eind group, which have been obtained by two different synthetic procedures, i.e., the NHC-induced fragmentation of the Eind-based 1,2-dibromo-disilene and the dehydrobromination of the Eind-based dibromohydrosilane with NHCs.

2. Results and Discussions

2.1. Reactions of (Eind)BrSi=SiBr(Eind) (1a) with NHCs

We first performed an NMR tube scale reaction of Eind-based 1,2-dibromodisilene, (Eind)BrSi=SiBr(Eind) (1a) [20], in C₆D₆ with two equivalents of the sterically more bulky NHC, Im-ⁱPr₂Me₂, relative to Im-Me₄. The progress of the reaction was monitored by ¹H NMR spectroscopy, indicating the selective formation of the mono-NHC adduct of the arylbromosilylene, (Im-ⁱPr₂Me₂)→SiBr(Eind) (2a′), after overnight heating at 70 °C. In the ²⁹Si NMR spectrum, only one signal was observed at δ = 18.0 ppm, which is comparable to those of (Im-ⁱPr₂Me₂)→SiBr(EMind) (VIb′) (δ = 13.1 ppm) and (Im-Me₄)→SiBr(Bbt) (VIe) (δ = 10.9 ppm) [22]. The ¹³C signal at δ = 170.6 ppm for 2a′ is characteristic of a carbene carbon atom, similar to those for VIb′ (δ = 169.7 ppm) and VIe (δ = 167.5 ppm) [22]. Based on the NMR tube experiment, the mono-NHC adduct 2a′ was synthesized as an orange solid in 88% crude yield (Scheme 1).

We also examined the reaction of 1a with four equivalents of Im-ⁱPr₂Me₂. After 1-day heating at 70 °C in C₆D₆, the ²⁹Si NMR spectrum indicated the formation of a mixture containing the mono-NHC adduct 2a′ (δ = 18.0 ppm) as the major product and the bis-NHC adduct of the arylsilyliumylidene cation, [(Im-ⁱPr₂Me₂)₂→Si(Eind)]⁺[Br⁻] (3a′) (δ = −59.6 ppm), as the minor product. The latter ²⁹Si signal was shifted upfield compared to the former, and was similar to those of the formal arylsilyliumylidene cations, [(NHC)₂→Si(EMind)]⁺[Br⁻] (VIIb and VIIb′) (δ = −60.8 and −75.9 ppm) and [(Im-Me₄)₂→Si(Tbb)]⁺[Br⁻] (VIIf) (δ = −70.9 ppm) [22]. However, we found that the reaction was not completed even after prolonged heating (longer than 1 week), probably due to the severe steric repulsion between the Eind group and Im-ⁱPr₂Me₂ molecules. Thus, we were unable to isolate 3a′. Nevertheless, single red crystals of 3a′ could be obtained from the reaction mixture, whose structure was determined by X-ray crystallography (Figure 2).

Figure 2 shows a separated ion pair of 3a′ in the crystal. The closest Si⋯Br distance (7.6669(6) Å) is analogous to that of VIIb′ (7.732(3) Å) [22], thus being much longer than the sum of the van der Waals radii of Si and Br (3.95 Å). The Si atom is three-coordinate adopting a distorted pyramidal geometry, which can be explained by the presence of a lone pair of electrons. The sum of the surrounding angles around the Si atom (ΣSi = 327.3°) is almost the same as that of VIIb′ (ΣSi = 327.0°) [22]. The Si–C(Rind) bond length in 3a′ (Si1–C1 = 1.9482(19) Å) is similar to that in VIIb′ [1.927(8) Å] [22] and longer than typical Si–C bonds (ca. 1.88 Å), suggesting the high s-character of the lone pair of electrons on the Si atom and the high p-character of the Si–C(Rind) bond. The Si←C(NHC) coordination distances in 3a′ (Si1–C29 = 1.953(2) and Si1–C40 = 1.942(2) Å) are comparable to those observed in VIIb′ (1.955(9) and 1.979(8) Å) [22].

We next investigated the reaction of 1a with two equivalents of the less bulky NHC, Im-Me₄, and C₆D₆. After 1 day at room temperature, two signals mainly appeared at δ = 73.3 and −63.3 ppm in the ²⁹Si NMR spectrum, corresponding to the unreacted 1a and the bis-NHC adduct of the arylsilyliumylidene cation, [(Im-Me₄)₂→Si(Eind)]⁺[Br⁻] (3a). This indicated that the NHC-arylbromosilylene adduct, (Im-Me₄)→SiBr(Eind) (2a), which serves as a potential intermediate, is more reactive toward Im-Me₄ compared to 1a. When the dibromodisilene 1a was treated with four equivalents of Im-Me₄ in benzene, the bis-NHC adduct 3a was efficiently formed (Scheme 1). We obtained 3a as an orange powder in 54% crude yield. The upfield-shifted ²⁹Si resonance for 3a (δ = −63.3 ppm) suggests the contribution of the canonical form due to the bis(imidazolium) adduct of a silyl anion, whose electronic structure was previously supported by the theoretical calculations of VIIb′ and VIIf [22]. In the ¹³C NMR spectrum of 3a in CD₃CN, one NHC carbene signal was observed at δ = 162.0 ppm, comparable to those for VIIb (δ = 160.5 ppm), VIIb′ (δ = 162.4 ppm), and VIIf (δ = 160.4 ppm) [22].

2.2 Reactions of (Eind)SiHBr₂ (4a) with NHCs

We also examined another synthetic route for the NHC-coordinated silicon compounds, i.e., the dehydrobromination of the Eind-substituted dibromohydrosilane, (Eind)SiHBr₂ (4a), with NHCs (Scheme 2). The precursor (4a) was prepared as pale brown crystals by the dibromination of the Eind-based trihydrosilane, (Eind)SiH₃ [37,38], with allyl bromide in the presence of a catalytic amount of PdCl₂ [39]. We found that this reaction exclusively afforded 4a even using an excess amount of allyl bromide with prolonged heating (longer than 1 week), most likely due to the steric bulkiness of the Eind group. In this context, Kunai, Ohshita, and their co-workers previously reported the selective dibromination of trihydrosilanes with CuBr₂ in the presence of CuI [40]. The formation of 4a was deduced on the basis of the spectroscopic data (Figures S1–S4). In the ¹H NMR spectrum, the Si–H signal was found at δ = 6.89 ppm with satellite signals, due to the ²⁹Si nuclei [¹J(²⁹Si–¹H) = 288 Hz]. The ²⁹Si NMR signal appeared at δ = −28.7 ppm, similar to that of (Bbt)SiHBr₂ (δ = −28.47 ppm) [41]. The infrared spectrum exhibited a Si–H stretching band at 2317 cm^–1 in the KBr-pellet (Figure S4) and at 2298 cm^–1 in THF [42,43]. The molecular structure of 4a was determined by single-crystal X-ray diffraction analysis (Figure 3). The hydrogen atom on the silicon atom was located on difference Fourier maps and isotropically refined. In the crystal, the SiHBr₂ group is fixed in one conformation with respect to the rotamer around the Si–C bond. A similar conformation was also observed in the crystal of (Eind)PCl₂ [44]. The Si–C bond length for 4a (1.8746(18) Å) is comparable to those of typical Si–C bonds (ca. 1.88 Å).

As shown in Scheme 2, the reaction of 4a with two equivalents of Im-ⁱPr₂Me₂ proceeded more smoothly at room temperature in comparison to the reaction of 1a with Im-ⁱPr₂Me₂ (Scheme 1), producing the mono-NHC adduct 2a′ in 59% crude yield. The reaction of 4a with three equivalents of Im-Me₄ also afforded the bis-NHC adduct 3a on the basis of the NMR data. In these reactions, it is essential to remove the byproducts, imidazolium bromides, [(NHC)H]⁺[Br⁻], for the isolation procedure of the silicon products, which may be considered as a disadvantage when compared to the no-byproduct strategy of using 1a as a precursor (vide supra). Actually, the separation of 3a and [(Im-Me₄)H]⁺[Br⁻] was found to be difficult in our experiments. However, dibromodisilene 1a can only be obtained by a two-step synthesis from the trihydrosilane, (Eind)SiH₃; thus the bromination of (Eind)SiH₃ with N-bromosuccinimide (NBS) first affords the tribromosilane, (Eind)SiBr₃, then the reduction of (Eind)SiBr₃ with two equivalents of lithium naphthalenide (LiNaph) produces 1a [20]. Therefore, the dehydrobromination of 4a with NHCs can be considered as a convenient short-step synthesis for NHC-coordinated silylene derivatives.

3. Materials and Methods

3.1. General Procedures

All manipulations of the air- and/or moisture-sensitive compounds were performed either using standard Schlenk-line techniques or in a glove box under an inert atmosphere of argon. Anhydrous hexane, benzene, and toluene were dried by passage through columns of activated alumina and supported copper catalyst supplied by Nikko Hansen & Co., Ltd. (Osaka, Japan). Anhydrous pentane and acetonitrile were purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan), and used without further purification. Deuterated benzene (C₆D₆, benzene-d₆) was dried and degassed over a potassium mirror in vacuo prior to use. Deuterated acetonitrile (CD₃CN, acetonitrile-d₃) was dried and distilled over calcium hydride (CaH₂) prior to use. (Eind)SiH₃ [37,38], (Eind)BrSi=SiBr(Eind) (1a) [20], 1,3,4,5-tetramethylimidazol-2-ylidene (Im-Me₄) [45] and 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene(Im-ⁱPr₂Me₂) [45] were prepared by the literature procedures. All other chemicals and gases were used as received.

Nuclear magnetic resonance (NMR) measurements were carried out using a JEOL ECS-400 spectrometer (399.8 MHz for ¹H, 100.5 MHz for ¹³C, and 79.4 MHz for ²⁹Si) or JEOL JNM AL-300 spectrometer (300 MHz for ¹H, 75 MHz for ¹³C, and 59 MHz for ²⁹Si) (JEOL Ltd., Tokyo, Japan). Chemical shifts (δ) are given by definition as dimensionless numbers and relative to ¹H chemical shifts of the solvents for ¹H (residual C₆D₅H in C₆D₆, ¹H(δ) = 7.15, residual CD₂HCN in CD₃CN, ¹H(δ) = 1.94), and ¹³C chemical shifts of the solvent for ¹³C (C₆D₆: ¹³C(δ) = 128.06 and CD₃CN: ¹³C(δ) = 118.26). The signal of tetramethylsilane (²⁹Si(δ) = 0.0) was used as an external standard in the ²⁹Si NMR spectra. The absolute values of the coupling constants are given in Hertz (Hz) regardless of their signs. Multiplicities are abbreviated as singlet (s), doublet (d), triplet (t), quartet (q), multiplet (m), and broad (br). The mass spectra were recorded by a JEOL JMS-T100LC AccuTOF LC-plus 4G mass spectrometer (ESI-MS) with a DART source. The elemental analyses were performed in the Microanalytical Laboratory at the Institute for Chemical Research (Kyoto University, Uji, Japan). Melting points (m.p.) were determined by a Stanford Research Systems OptiMelt instrument. We were unable to obtain a satisfactory elemental analysis for 2a′ and 3a, probably due to their extremely high air- and moisture-sensitivity as well as a contamination of NHCs and unidentified compounds associated with some thermal decomposition (Figures S5–S10).

3.1.1. Synthesis of (Eind)SiHBr₂ (4a)

To a solution of (Eind)SiH₃ (4.09 g, 9.91 mmol) in toluene (30 mL) was added PdCl₂ (38.0 mg, 0.21 mmol) and allyl bromide (4.2 mL, 48.5 mmol). The reaction mixture was heated at 80 °C for 8 days. After the solvent was removed in vacuo, the residue was dissolved in hexane and the resulting mixture was centrifuged to remove the insoluble materials. The supernatant was concentrated to dryness and the resulting residue was recrystallized from pentane to afford 4a as pale brown crystals in 81% yield (4.58 g, 8.02 mmol).

¹H NMR (399.8 MHz, C₆D₆, 30 °C): δ = 0.78 (t, J = 7.3 Hz, 12 H, CH₂CH₃), 0.80 (br. s, 12 H, CH₂CH₃), 1.48–1.65 (m, 8 H, CH₂CH₃), 1.76 (s, 4 H, CH₂), 2.11 (br. s, 8 H, CH₂CH₃), 6.89 (s, 1 H, satellite, J_Si–H = 288 Hz, SiH), 7.01 (s, 1 H, ArH). ¹³C NMR (100.5 MHz, C₆D₆, 25 °C): δ = 9.3, 10.5 (br), 33.5, 34.2 (br), 42.6 (br ×1), 44.8 (br ×1), 47.9 (br ×2), 54.7 (×2), 123.6, 125.1, 150.8 (one aromatic peak is broadened at 155–158); ²⁹Si NMR (79.4 MHz, C₆D₆, 30 °C): δ = −28.7 (d, J_Si–H = 288 Hz). IR (KBr, cm⁻¹): ν = 2317 (Si−H); IR (THF, cm⁻¹): ν = 2298 (Si−H). DART-HRMS (positive-mode) Calcd. for C₂₈H₄₆Br₂Si + H: 569.1814. Found: 569.1820. Anal. Calcd. for C₂₈H₄₆Br₂Si: C, 58.94; H, 8.13. Found: C, 59.41; H, 8.19. Melting point (argon atmosphere in a sealed tube) 102–105 °C.

3.1.2. Synthesis of (Im-ⁱPr₂Me₂)→SiBr(Eind) (2a′)

(Method A) Reaction of (Eind)BrSi=SiBr(Eind) (1a) with Im-ⁱPr₂Me₂

A mixture of 1a (158 mg, 0.16 mmol) and Im-ⁱPr₂Me₂ (63.0 mg, 0.35 mmol) was dissolved in benzene (5 mL). The reaction mixture was heated overnight at 70 °C. After the solvent was removed in vacuo, the residue was washed with pentane to afford 2a′ as an orange solid in 88% crude yield (190 mg, 0.28 mmol). We were unable to isolate 2a′ in pure form, because 2a′ was not thermally stable in solution, gradually giving Im-ⁱPr₂Me₂ and unidentified compounds (Figure S5).

¹H NMR (399.8 MHz, C₆D₆, 60 °C): δ = 0.81–1.00 (m, 24 H, CH₂CH₃), 1.12 (br. s, 6 H, CH(CH₃)₂–(Im-ⁱPr₂Me₂)), 1.19 (d, J = 7.0 Hz, 6 H, CH(CH₃)₂–(Im-ⁱPr₂Me₂)), 1.61 (s, 6 H, CH₃–(Im-ⁱPr₂Me₂)), 1.62–1.95 (m, 20 H, CH₂ + CH₂CH₃), 5.00–5.27 (m, 2 H, CH(CH₃)₂–(Im-ⁱPr₂Me₂)), 6.71 (s, 1 H, ArH). ¹³C NMR (100.5 MHz, C₆D₆, 70 °C): δ = 9.3, 9.4, 9.9, 10.0, 10.5, 20.7, 21.4, 24.6, 33.7 (br, overlapped, Im-ⁱPr₂Me₂ and CH₂CH₃), 42.8, 48.4, 51.1 (Im-ⁱPr₂Me₂), 54.5, 119.9, 125.9 (Im-ⁱPr₂Me₂), 147.2, 148.5, 153.7, 170.6 (Im-ⁱPr₂Me₂); ²⁹Si NMR (79.4 MHz, C₆D₆, 25 °C): δ = 18.0. HRMS (ESI, positive) Calcd. for C₃₉H₆₅BrN₂Si + H: 669.4179. Found: 669.4211. Melting point (argon atmosphere in a sealed tube) 152–156 °C (dec.).

(Method B) Reaction of (Eind)SiHBr₂ (4a) with Im-ⁱPr₂Me₂

A mixture of 4a (476 mg, 0.97 mmol) and Im-ⁱPr₂Me₂ (352 mg, 1.95 mmol) was dissolved in benzene (7 mL). After stirring overnight at room temperature, the resulting orange suspension was filtered through a polytetrafluoroethylene (PTFE) syringe filter to remove the insoluble materials. The filtrate was concentrated to dryness and the resulting residue was washed with pentane to afford 2a′ as an orange solid in 59% crude yield (197 mg, 0.29 mmol).

3.1.3. Synthesis of [(Im-Me₄)₂→Si(Eind)]⁺[Br]^– (3a)

(Method A) Reaction of (Eind)BrSi=SiBr(Eind) (1a) with Im-Me₄

A mixture of 1a (102 mg, 0.11 mmol) and Im-Me₄ (54 mg, 0.43 mmol) was dissolved in benzene (6 mL). After stirring for 1 day at room temperature, the resulting orange solid was separated and washed with a mixture of hexane and benzene to afford 3a as an orange powder in 54% crude yield (85.2 mg, 0.12 mmol). We were unable to isolate 3a in pure form, because 3a was not thermally stable in solution leading to the contamination of Im-Me₄ and unidentified compounds (Figure S8).

¹H NMR (399.8 MHz, CD₃CN, 20 °C): δ = 0.61 (br. t, J = 6.3 Hz, 12 H, CH₂CH₃), 0.80 (t, J = 7.3 Hz, 12 H, CH₂CH₃), 1.50–1.70 (m, 8 H, CH₂CH₃), 1.75 (br. s, 4 H, CH₂), 1.93−1.97 (m, overlapped, 8 H, CH₂CH₃), 2.15 (s, 12 H, CH₃–(Im-Me₄)), 3.25 (br. s, 12 H, CH₃–(Im-Me₄)), 6.75 (s, 1 H, ArH). ¹³C NMR (100.5 MHz, CD₃CN, 19 °C): δ = 9.2, 9.4, 10.0 (br, Im-Me₄), 34.1 (br, overlapped, Im-Me₄ and CH₂CH₃), 42.8, 48.1, 54.2, 121.8, 128.5 (Im-Me₄), 136.5, 150.7, 162.0 (Im-Me₄) (one aromatic peak is overlapped). ²⁹Si NMR (79.4 MHz, CD₃CN, 25 °C): δ = −63.3. DART-HRMS (positive-mode) Calcd. for C₄₂H₆₉BrN₄Si + H: 737.4553. Found: 737.4562. Melting point (argon atmosphere in a sealed tube) 169–174 °C (dec.).

(Method B) Reaction of (Eind)SiHBr₂ (4a) with Im-Me₄

A mixture of 4a (70.3 mg, 0.12 mmol) and Im-Me₄ (50.6 mg, 0.41 mmol) was dissolved in benzene (6 mL). After stirring for 1 day at room temperature, an orange suspension was formed. An insoluble orange solid was collected by filtration, whose ¹H NMR spectrum indicated the formation of a mixture of 4a and [(Im-Me₄)H]⁺[Br⁻].

3.2. X-ray Crystallographic Studies of 3a′ and 4a

Single crystals suitable for X-ray diffraction measurements were obtained from benzene for 3a′ and from hexane for 4a. Intensity data were collected using a Rigaku XtaLAB P200 with a PILATUS 200K detector for 3a′ and a Rigaku AFC-8 with a Saturn 70 CCD detector for 4a (Rigaku Corporation, Tokyo, Japan). All measurements were carried out using Mo Kα radiation (λ = 0.71073 Å). The integration and scaling of the diffraction data were carried out using the programs CrysAlisPro [46] for 3a′ and CrystalClear [47] for 4a. Lorentz, polarization, and absorption corrections were also performed. The structures were solved by an iterative method with the program of SHELXT [48], and refined by a full-matrix least-squares method on F² for all the reflections using the program SHELXL-2017/1 [49]. The non-hydrogen atoms were refined by applying anisotropic temperature factors. Positions of all the hydrogen atoms were geometrically calculated, and refined as riding models. The Si–H hydrogen atom was located on difference Fourier maps and isotropically refined. Full details of the crystallographic analysis and accompanying CIF files can be obtained free of charge from the Cambridge Crystallographic Data Centre (CCDC numbers 1811699 and 1811700) via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44-1223-336033; E-mail: [email protected]).

3.2.1. [(Im-ⁱPr₂Me₂)₂→Si(Eind)]⁺[Br⁻] (3a′)

C₅₀H₈₅BrN₄Si·C₆H₆, M = 928.32, crystal size 0.36 × 0.15 × 0.12 mm, triclinic, space group P−1 (#2), a = 10.6548(3) Å, b = 12.0149(2) Å, c = 21.7366(5) Å, α = 80.5199(18)°, β = 82.343(2)°, γ = 72.816(2)°, V = 2611.54(11) ų, Z = 2, D_x = 1.181 g cm⁻³, μ(Mo Kα) = 0.849 mm⁻¹, 65594 reflections collected, 13761 unique reflections, and 579 refined parameters. The final R(F) value was 0.0491 [I > 2σ(I)]. The final R_w(F²) value was 0.1364 (all data). The goodness-of-fit on F² was 1.031.

3.2.2. (Eind)SiHBr₂ (4a)

C₂₈H₄₆Br₂Si, M = 570.56, crystal size 0.16 × 0.17 × 0.41 mm, triclinic, space group P−1 (#2), a = 7.972(3) Å, b = 11.070(4) Å, c = 16.621(5) Å, α = 89.972(4)°, β = 80.770(3)°, γ = 73.314(5)°, V = 1385.1(8) ų, Z = 2, D_x = 1.368 g cm⁻³, μ(Mo Kα) = 2.992 mm⁻¹, 22599 reflections collected, 6332 unique reflections, and 292 refined parameters. The final R(F) value was 0.0298 [I > 2σ(I)]. The final R_w(F²) value was 0.0803 (all data). The goodness-of-fit on F² was 1.016.

4. Conclusions

We have synthesized some new NHC-coordinated silicon species having the fused-ring bulky Eind group by two methods; one is via the reactions of the stable diaryldibromodisilene, (Eind)BrSi=SiBr(Eind) (1a), with NHCs, and the other is the dehydrobromination of the aryldibromohydrosilane, (Eind)SiHBr₂ (4a), with NHCs. In both synthetic pathways, we have mainly obtained the mono-NHC adduct of the arylbromosilylene, (Im-ⁱPr₂Me₂)→SiBr(Eind) (2a′), and the bis-NHC adduct of the formal arylsilyliumylidenecation, [(Im-Me₄)₂→Si(Eind)]⁺[Br⁻] (3a), depending on the steric bulk of the NHCs (Im-ⁱPr₂Me₂ vs. Im-Me₄). Further studies on the reactivities of the NHC-coordinated silicon compounds are now in progress.