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

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-Pr2Me2 = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) are reported. While the reaction of 1a with the sterically more demanding Im-Pr2Me2 led to the formation of the mono-NHC adduct of arylbromosilylene, (Im-Pr2Me2)→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-Pr2Me2)2→Si(Eind)][Br] (3a′), and 4a have been determined by X-ray crystallography.

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.
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.

Reactions of (Eind)SiHBr 2 (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)SiHBr 2 (4a), with NHCs (Scheme 2).The precursor (4a) was prepared as pale brown crystals by the dibromination of the Eind-based trihydrosilane, (Eind)SiH 3 [37,38], with allyl bromide in the presence of a catalytic amount of PdCl 2 [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 CuBr 2 in the presence of CuI [40].The formation of 4a was deduced on the basis of the spectroscopic data (Figures S1-S4).In the 1 H NMR spectrum, the Si-H signal was found at δ = 6.89 ppm with satellite signals, due to the 29 Si nuclei [ 1 J( 29 Si-1 H) = 288 Hz].The 29 Si NMR signal appeared at δ = −28.7 ppm, similar to that of (Bbt)SiHBr 2 (δ = −28.47ppm) [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 SiHBr 2 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)PCl 2 [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).
Inorganics 2018, 6, 5 of 11 deduced on the basis of the spectroscopic data (Figures S1-S4).In the 1 H NMR spectrum, the Si-H signal was found at δ = 6.89 ppm with satellite signals, due to the 29 Si nuclei [ 1 J( 29 Si-1 H) = 288 Hz].The 29 Si NMR signal appeared at δ = −28.7 ppm, similar to that of (Bbt)SiHBr2 (δ = −28.47ppm) [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 Imi Pr2Me2 proceeded more smoothly at room temperature in comparison to the reaction of 1a with Imi Pr2Me2 (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  S4).In the 1 H NMR spectrum, the Si-H signal was found at δ = 6.89 ppm with satellite signals, due to the 29 Si nuclei [ 1 J( 29 Si-1 H) = 288 Hz].The 29 Si NMR signal appeared at δ = −28.7 ppm, similar to that of (Bbt)SiHBr2 (δ = −28.47ppm) [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 Imi Pr2Me2 proceeded more smoothly at room temperature in comparison to the reaction of 1a with Imi Pr2Me2 (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 As shown in Scheme 2, the reaction of 4a with two equivalents of Im-i Pr 2 Me 2 proceeded more smoothly at room temperature in comparison to the reaction of 1a with Im-i Pr 2 Me 2 (Scheme 1), producing the mono-NHC adduct 2a in 59% crude yield.The reaction of 4a with three equivalents of Im-Me 4 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-Me 4 )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)SiH 3 ; thus the bromination of (Eind)SiH 3 with N-bromosuccinimide (NBS) first affords the tribromosilane, (Eind)SiBr 3 , then the reduction of (Eind)SiBr 3 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.
Nuclear magnetic resonance (NMR) measurements were carried out using a JEOL ECS-400 spectrometer (399.8MHz for 1 H, 100.5 MHz for 13 C, and 79.4 MHz for 29 Si) or JEOL JNM AL-300 spectrometer (300 MHz for 1 H, 75 MHz for 13 C, and 59 MHz for 29 Si) (JEOL Ltd., Tokyo, Japan).Chemical shifts (δ) are given by definition as dimensionless numbers and relative to 1 H chemical shifts of the solvents for 1 H (residual C 6 D 5 H in C 6 D 6 , 1 H(δ) = 7.15, residual CD 2 HCN in CD 3 CN, 1 H(δ) = 1.94), and 13 C chemical shifts of the solvent for 13 C (C 6 D 6 : 13 C(δ) = 128.06and CD 3 CN: 13 C(δ) = 118.26).The signal of tetramethylsilane ( 29 Si(δ) = 0.0) was used as an external standard in the 29 Si 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).

Synthesis of (Eind)SiHBr 2 (4a)
To a solution of (Eind)SiH 3 (4.09g, 9.91 mmol) in toluene (30 mL) was added PdCl 2 (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).A mixture of 1a (158 mg, 0.16 mmol) and Im-i Pr 2 Me 2 (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-i Pr 2 Me 2 and unidentified compounds (Figure S5).A mixture of 4a (476 mg, 0.97 mmol) and Im-i Pr 2 Me 2 (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).A mixture of 1a (102 mg, 0.11 mmol) and Im-Me 4 (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-Me 4 and unidentified compounds (Figure S8).A mixture of 4a (70.3 mg, 0.12 mmol) and Im-Me 4 (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 1 H NMR spectrum indicated the formation of a mixture of 4a and [(Im-Me 4 )H] + [Br − ].

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 F 2 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: deposit@ccdc.cam.ac.uk).The final R w (F 2 ) value was 0.0803 (all data).The goodness-of-fit on F 2 was 1.016.

Figure 1 .
Figure 1.Examples of coordination-stabilized arylhalosilylenes and their derivatives.Each one of the possible canonical forms is depicted.

Figure 1 .
Figure 1.Examples of coordination-stabilized arylhalosilylenes and their derivatives.Each one of the possible canonical forms is depicted.

Figure 2 .
Figure 2. Molecular structure of 3a′.The thermal ellipsoids are shown at the 50% probability level.All hydrogen atoms and benzene molecule are omitted for clarity.

Figure 2 .
Figure 2. Molecular structure of 3a .The thermal ellipsoids are shown at the 50% probability level.All hydrogen atoms and benzene molecule are omitted for clarity.

Figure 3 .
Figure 3. Molecular structure of 4a: Side view (left), front view (right).The thermal ellipsoids are shown at the 50% probability level.The hydrogen atoms, except for the Si-H group, are omitted for clarity.

Figure 3 .
Figure 3. Molecular structure of 4a: Side view (left), front view (right).The thermal ellipsoids are shown at the 50% probability level.The hydrogen atoms, except for the Si-H group, are omitted for clarity.

Figure 3 .
Figure 3. Molecular structure of 4a: Side view (left), front view (right).The thermal ellipsoids are shown at the 50% probability level.The hydrogen atoms, except for the Si-H group, are omitted for clarity.