Synthesis and NMR-Study of the 2,3,4,5-Tetraethylsilole Dianion [SiC₄Et₄]²⁻•2[Li]⁺

Abstract

The previously unknown silole dianion [SiC₄Et₄]²⁻•2[Li]⁺ (3) was prepared by the sonication of 1,1-dichloro-2,3,4,5-tetraethyl-1-silacyclopentadiene [Cl₂SiC₄Et₄, 2] with more than four equivalent of lithium in THF. ¹H-, ¹³C-, and ²⁹Si-NMR data of 3 are compared with those of the reported silole dianion [SiC₄Ph₄]²⁻. Trapping of 3 with trimethylchlorosilane gave 1,1-bis(trimethylsilyl)-2,3,4,5-tetraethyl-1-silacyclopentadiene [(Me₃Si)₂SiC₄Et₄, 4] in high yield. The silole of 2 was synthesized in high yield in three steps by a modified procedure using Cp₂ZrCl₂ via Cp₂ZrC₄Et₄ and 1,4-dibromo-1,2,3,4-tetraethyl-1,3-butadiene.

1. Introduction

Since the first silole dianion, 2,3,4,5-tetraphenyl-1-silacyclopentadienide dianion, was prepared in 1990 by Joo and Hong [1], the aromaticity of the silole dianion [2] and germole dianion [3] was suggested by NMR study and it was confirmed by X-ray crystallography of the structures [4,5,6,7,8] and by theoretical study [9,10]. The chemistry of group 14 metallole dianions has been developed enormously [11,12], and recently the stannole dianion [SnC₄Ph₄]²⁻ was also reported [13,14,15,16].

In contrast, only two silole dianions have been reported so far; [SiC₄Ph₄]²⁻ (I) [1,2,4], [SiC₄Me₄]²⁻ (II) [5] with the silafluorenyl dianion [SiC₄(CH₂)₈]²⁻2[M]⁺ (III) [17,18] and the silaindenyl dianion [(CH₂)₄C₂SiC₂PhBu]²⁻•2[M]⁺ (IV) [19] since the available 1,1-dihalosiloles are limited (Figure 1).

Figure 1. Silole dianions, silafluorenyl dianions, and silaindenyl dianions.

Figure 1. Silole dianions, silafluorenyl dianions, and silaindenyl dianions.

Only two silole dianions are reported since the synthetic methods for 1,1-dihalosiloles are limited to 1,1-dichloro-2,3,4,5-tetraphenyl-1-silacyclopentadiene, 1,1-dibromo-2,3,4,5-tetramethyl-1-silacyclopentadiene, and 1,1-dichloro-2,3,4,5-tetrametyl-1-silacyclopentadiene. The former is readily prepared from SiCl₄ and 1,4-dilithio-2,3,4,5-tetraphenyl-1,3-butadiene, which is easily produced from diphenylacetylene and lithium, however, it is unable to exchange the phenyl groups with other groups [1]. 1,1-Dibromo-2,3,4,5-tetramethyl-1-silacyclopentadiene is synthesized from Cp₂ZrC₄Me₄ and SiBr₄ in very low yield [20]. 1,1-Dichloro-2,3,4,5-tetrametyl-1-silacyclopentadiene is synthesized from 1,4-diiodo-1,2,3,4-tetramethyl-1,3-butadiene via Cp₂ZrC₄Me₄ [21]. Here we report the synthesis of 1,1-dichloro-2,3,4,5-tetraethyl-1-silacyclopentadiene [Cl₂SiC₄Et₄] and an NMR study of the silole dianion [SiC₄Et₄]²⁻•2[Li]⁺.

2. Results and Discussion

We have prepared 1,4-dibromo-1,2,3,4-tetraethyl-1,3-butadiene (1) by a modified procedure using Cp₂ZrCl₂ [22] and bromine (Scheme 1).

Scheme 1. Synthesis of 1,1-dichloro-2,3,4,5-tetraethyl-1-silacyclopentadiene (2).

Scheme 1. Synthesis of 1,1-dichloro-2,3,4,5-tetraethyl-1-silacyclopentadiene (2).

Addition of SiCl₄ to 1,4-dilithio-1,2,3,4-tetraethyl-1,3-butadiene, which is obtained by the metallation of 1 by t-BuLi, gives a 75% yield of pure 1,1-dichloro-2,3,4,5-tetraethyl-1-silacyclopentadiene [Cl₂SiC₄Et₄, 2]. It has been previously reported that 1,4-diioodo-1,2,3,4-tetraethyl-1,3-butadiene and 2 have been isolated only as impure materials [8]. Sonication of 2 with more than four equivalents of lithium in THF produces a dark red solution. Trapping of 3 with trimethylchlorosilane provides [(Me₃Si)₂SiC₄Et₄, 4] in 95% yield (Scheme 2).

Scheme 2. Synthesis of 1,1-bis(trimethylsilyl)-2,3,4,5-tetraethyl-1-silacyclopentadiene (4).

Scheme 2. Synthesis of 1,1-bis(trimethylsilyl)-2,3,4,5-tetraethyl-1-silacyclopentadiene (4).

The NMR study of the red solution in THF-d₈ shows the only one species, which is assigned to the structure 3. The ¹³C-NMR spectrum of 3 presents six peaks, consistent with C₂ symmetry, and the ²⁹Si spectrum of 3 shows only one resonance. Upon lithiation of 2 to 3, the ²⁹Si resonance is shifted downfield (Δδ = 16.66 ppm, 8.30 ppm for 2 and 24.96 ppm for 3, Table 1) and the ¹³C resonances of C_α and C_β are shifted upfield compared with 2 {Δδ(C_α) = −4.84 and Δδ(C_β) = −3.55)} [23] (Table 2).

Table 1. ²⁹Si Chemical shifts.

2 a	3 b	Ia b	Ib b	IIa c	III b	IVa b	IVb b
8.30	24.96	68.54	92.79	29.77	29.00	29.19	30.44

^a In CDCl₃, reference; external TMS as standard; ^b In THF-d₈, reference = 25.30 ppm; ^c In THF-d₈, reference; external TMS as standard.

Table 1. ²⁹Si Chemical shifts.

2 a	3 b	Ia b	Ib b	IIa c	III b	IVa b	IVb b
8.30	24.96	68.54	92.79	29.77	29.00	29.19	30.44

^a In CDCl₃, reference; external TMS as standard; ^b In THF-d₈, reference = 25.30 ppm; ^c In THF-d₈, reference; external TMS as standard.

Table 2. ¹³C-NMR chemical shifts.

	2 a		3 b		4 a			3-2
Cα	155.48		150.64		155.03		Δδ(Cα)	−4.84
Cβ	130.59		127.04		139.36		Δδ(Cβ)	−3.55
T	572.17		555.36		588.78		ΔT	−16.78
	α Et	β Et	α Et	β Et	α Et	β Et		α Et	β Et
C1	20.82	20.56	22.24	26.28	21.38	22.83	ΔC1	1.42	5.72
C2	14.37	14.06	19.59	21.89	15.24	16.88	ΔC2	5.22	7.83

^a In CDCl₃, reference; external TMS as standard; ^b In THF-d₈, reference = 25.30 ppm.

Table 2. ¹³C-NMR chemical shifts.

	2 a		3 b		4 a			3-2
Cα	155.48		150.64		155.03		Δδ(Cα)	−4.84
Cβ	130.59		127.04		139.36		Δδ(Cβ)	−3.55
T	572.17		555.36		588.78		ΔT	−16.78
	α Et	β Et	α Et	β Et	α Et	β Et		α Et	β Et
C1	20.82	20.56	22.24	26.28	21.38	22.83	ΔC1	1.42	5.72
C2	14.37	14.06	19.59	21.89	15.24	16.88	ΔC2	5.22	7.83

^a In CDCl₃, reference; external TMS as standard; ^b In THF-d₈, reference = 25.30 ppm.

These chemical shifts of ²⁹Si and ¹³C resonances are consistent with delocalization of the negative charge into the silole ring, which is supported by the calculated negative NICS value of dilithiumsilole dianion [9,10]. In addition the signals of the ethyl groups in the ¹H- and ¹³C-NMR spectra of 3 shift downfield due to the anisotropic effect of the ring current from the delocalization {Δδ(¹³C of CH₂CH₃) = 1.42–7.83 ppm and Δδ(¹H of CH₂) = 0.16–0.19 ppm)} (Table 3).

Table 3. ¹H-NMR chemical shifts.

	2 a		3 b		4 a			3-2
	α Et	β Et	α Et	β Et	α Et	β Et		α Et	β Et
CH2	2.31	2.34	2.50	2.50	2.34	2.36	ΔC1	0.19	0.16
CH3	1.03	1.18	1.13	1.13	0.99	1.05	ΔC2	0.10	−0.05

^a In CDCl₃, reference; external TMS as standard; ^b In THF-d₈, reference = 1.73 ppm.

Table 3. ¹H-NMR chemical shifts.

	2 a		3 b		4 a			3-2
	α Et	β Et	α Et	β Et	α Et	β Et		α Et	β Et
CH2	2.31	2.34	2.50	2.50	2.34	2.36	ΔC1	0.19	0.16
CH3	1.03	1.18	1.13	1.13	0.99	1.05	ΔC2	0.10	−0.05

^a In CDCl₃, reference; external TMS as standard; ^b In THF-d₈, reference = 1.73 ppm.

Surprisingly, the chemical shift of ²⁹Si resonance in 3 is similar to those of IIa, III, and IV, even though III and IV have conjugated benzene rings on the silole rings (Table 1). In addition the chemical shifts of C_α and C_β (150.64 and 127.04 ppm) in 3 are very close to those of the reported tetraphenyl substituted silole dianions {Δδ(C_α) = 0.58 and Δδ(C_β) = 2.71 ppm for [SiC₄Ph₄]²⁻•2[Li]⁺(Ia) and Δδ(C_α) = 3.10 ppm and Δδ(C_β) = 3.88 ppm for [SiC₄Ph₄]²⁻•2[Na]⁺(Ib)} even if 3 has four ethyl groups on the ring. However, the chemical shifts of C_α and C_β (150.64 and 127.04 ppm) in 3 are quite different from those of C_α and C_β (138.97 ppm and 119.97 ppm) in IIa [21]. These data unambiguously indicate that four phenyl groups on the ring have no conjugation with the butadiene ring as shown by X-ray crystallography [4] and instead, the π-polarization of the phenyl groups on the ring is observed in I due to the increased electron density on the ring [2] (Table 4).

Table 4. ¹³C-NMR chemical shifts.

	[Cl2SiC4Ph4] a[1]		Ia b[2]		Ib b[1]
Cα	154.74		151.22		153.74
Cβ	132.28		129.71		130.92
T	574.04		561.86		569.32
	α Ph	β Ph	α Ph	β Ph	α Ph	β Ph
Ci	136.67	135.37	151.67	145.83	151.29	146.71
Co	139.48	129.27	129.97	133.43	129.48	133.16
Cm	127.84	128.24	126.38	126.38	126.55	126.72
Cp	127.37	127.10	119.48	121.83	118.25	121.42
Ci-Cp	7.00	8.27	32.19	24.00	33.04	25.29
Sum (Ci-Cp)	17.57		56.19		58.33

^a In CDCl₃, reference; external TMS as standard; ^b In THF-d₈, reference = 25.30 ppm.

Table 4. ¹³C-NMR chemical shifts.

	[Cl2SiC4Ph4] a[1]		Ia b[2]		Ib b[1]
Cα	154.74		151.22		153.74
Cβ	132.28		129.71		130.92
T	574.04		561.86		569.32
	α Ph	β Ph	α Ph	β Ph	α Ph	β Ph
Ci	136.67	135.37	151.67	145.83	151.29	146.71
Co	139.48	129.27	129.97	133.43	129.48	133.16
Cm	127.84	128.24	126.38	126.38	126.55	126.72
Cp	127.37	127.10	119.48	121.83	118.25	121.42
Ci-Cp	7.00	8.27	32.19	24.00	33.04	25.29
Sum (Ci-Cp)	17.57		56.19		58.33

^a In CDCl₃, reference; external TMS as standard; ^b In THF-d₈, reference = 25.30 ppm.

The ²⁹Si chemical shift for 3 at 24.96 ppm shifts downfield comparing to 8.30 ppm for 2, however the chemical shift is more downfield than those of the tetraphenyl substituted silole dianion [SiC₄Ph₄]²⁻•2[M]⁺ (68.54 ppm for M = Li, Ia and 92.79 ppm for M = Na, Ib).

In ³¹P-NMR of the phosphoryl anion, which is isoelectronic with the silole dianion, the same downfield chemical shifts are observed [24]. The large downfield shifts of the phosphoryl anions have been ascribed to the conjugation effect of p-π orbital electrons and to the presence of the in-plane lone pair weakly coupled to the ring [25,26]. This paramagnetic shift depends on the narrow energy gap between HOMO and LUMO. The smaller gap is between HOMO and LUMO, the more paramagnetic shielding is assigned to the NMR chemical shifts [27]. If the in-plane nonbonding orbital is the HOMO, the energy level of the HOMO is less affected by the substituents of the butadiene moiety relatively. However LUMO greatly depends on the substituents of the butadiene moiety since the LUMO is one of the anti-bonding MOs of the 5-membered ring. Therefore the LUMO of [SiC₄Ph₄]²⁻•2[Li]⁺ should be stabilized relatively compared to that of [SiC₄Ph₄]²⁻•2[Li]⁺ by the effect of the substituents on the butadiene moiety or vice versa. This rationale is reinforced by the comparison of the electronegativities between the phenyl and the ethyl groups (the phenyl group has higher electronegativity than the ethyl group, 2.717 and 2.481, respectively [28]. The difference between ²⁹Si chemical shifts of [SiC₄Ph₄]²⁻ and [SiC₄Et₄]²⁻ might be due to the paramagnetic shielding effect of the substituents on the silole ring.

3. Experimental

General Procedures

All reactions were performed under an inert nitrogen atmosphere using standard Schlenk techniques. Air sensitive reagents were transferred in a nitrogen-filled glovebox. THF and ether were distilled from sodium benzophenone ketyl under nitrogen. Hexane and pentane were stirred over concentrated H₂SO₄ and distilled from CaH₂. NMR spectra were recorded on JEOL GSX270 and GSX400 spectrometers. GC-MS and solid sample MS data were obtained on a Hewlett-Packard 5988A GC-MS system equipped with a methyl silicon capillary column. Elemental analyses were done by Desert Analytics (Tucson, AZ, USA).

1,4-Dibromo-1,2,3,4-tetraethyl-1,3-butadiene (1). The synthetic procedures for the preparation of Cp₂ZrC₄Et₄ are modified from the known procedures [20]. A mixture of Mg (7.78 g, 320 mmol) and HgCl₂ (8.69 g, 32 mmol) in THF (100 mL) was stirred for 1 h, to this was added a solution of Cp₂ZrCl₂ (23.4 g, 80 mmol) and 3-hexyne (18.14 mL, 160 mmol) in THF (250 mL) with stirring at room temperature. Stirring overnight gave a dark red solution. The solvent was removed under reduced pressure, and the red-orange residue was extracted with hexane. Removal of the hexane yielded a red-orange solid of pure Cp₂ZrC₄Et₄ (27.6 g, yield 90%). Bromine (7.40 mL, 143 mmol) was slowly added to Cp₂ZrC₄Et₄ (27.6 g, 71.5 mmol) in ether (300 mL) at −78 °C with stirring. After it was stirred for 1 h, the mixture was warmed up to room temperature. The reaction mixture was filtered and the filtrate was treated with the saturated aqueous Na₂S₂O₃ solution. The organic layer was separated, dried with Na₂SO₄, filtered and distilled do give 1,4-dibromo-1,2,3,4-tetraethyl-1,3-butadiene. Yield, 16.2 g (70%, purity; 99% by GC), bp 110–125 °C/0.1 mmHg; ¹H-NMR (CDCl₃, ref; ext. TMS = 0.00 ppm), 1.08 (t, Me, 6H, J = 7.33 Hz), 1.16 (t, 6H, Me, J = 7.33 Hz), 2.05–2.22 (m, 2H, CH₂), 2.30–2.42 (m, 2H, CH₂), 2.42–2.65 (m, 4H, CH₂); ¹³C-NMR (CDCI₃, ref; solvent = 77.00 ppm), 140.13 (C1), 126.47 (C2), 30.70 (CH₂ of C1), 25.60 (CH₂ of C2), 13.18 (Me of C1), 12.89 (Me of C2); MS(M⁺, relative abundance), 266 (M⁺+4, 5), 265(M⁺+3, 4), 264 (M⁺+2, 18), 263 (M⁺+1, 6), 262 (M⁺, 27), 235 (M⁺-27, 16), 233 (M⁺−29, 21), 164 (C₄Et₄⁺, 100), 149 (C₄Et₄⁺−15, 55), 135 (95), 107(42).

[Cl₂SiC₄Et₄] (2). To 1,4-dibromo-1,2,3,4-tetraethyl-1,3-butadiene (11.8 g, 36.41 mmol) in ether (200 mL) was added t-BuLi in hexane (64 mL, 1.7 M, 109.2 mmol) at −78 °C. After it was stirred for 2 h, to it was added SiCl₄ (11.51 mL, 109.2 mmol) with stirring at −78 °C. The mixture was warmed up to room temperature, and stirred overnight to produce a clear yellow solution. After the solvent was removed under reduced pressure, the remaining solid was extracted with pentane. Distillation of pentane under reduced pressure gave a colorless liquid. Yield, 7.0 g (74%, purity; 99% by GC), bp 140–160 °C under aspirator pressure; MS (M⁺, relative abundance), 327 (M⁺+5, 1), 326 (M⁺+4, 9), 325 ((M⁺+3, 2), 324 (M⁺+2, 20), 323 (M⁺+1, 1), 322 (M⁺, 10), 245 (M⁺−81, 68), 243 (M⁺−79, 68), 164 (C₄Et₄⁺, 45), 163 (C₄Et₄⁺−1, 45), 149 (C₄Et₄⁺−15, 38), 135 (61), 107 (100). Anal Calcd. for C₁₂H₂₀SiCl₂: C, 54.74; H, 7.66, Found: C, 54.50; H, 7.69.

[(Me₃Si)₂SiC₄Et₄] (4). Sonication of 2 (1.26 g, 4.79 mmol) and lithium (0.15 g, 21.43 mmol) for 12 h gave a dark red solution. After filtration, it was added to an excess of trimethylchlorosilane (2.0 mL, 15.80 mmol). Stirring for 2 h at room temperature produced a pale brown solution immediately. All volatiles were removed under reduced pressure, and the residue was extracted with hexane. Evaporation of the hexane gave a colorless oil. It had been previously reported [8] that the product 4 was obtained in 45% yield. Yield; 95% by ¹H-NMR integration; ¹H-NMR (CDCl₃, ref; ext. TMS = 0.00 ppm), 0.14 (s, SiMe₃, 18H), 0.99 (t, 6H, Me, J = 7.33 Hz), 1.05 (t, 6H, J = 7.33 Hz), 2.34 (q, 4H, CH₂, J = 7.33 Hz), 2.36 (q, 4H, CH₂, J = 7.33 Hz);¹³C-NMR (CDCI₃, ref; solvent = 77.00 ppm), −0.08 (SiMe); ²⁹Si-NMR (THF-d₈, ref; ext. TMS = 0.00 ppm), −14.99 (ring Si), −38.04 (SiMe₃); MS (M⁺, relative abundance), 340 (M⁺+2, 2), 339 (M⁺+1, 5), 338 (M⁺, 13), 309 (M⁺−29, 2), 267 (M⁺+2−73, 4), 266 (M⁺+1−73, 11), 265 (M⁺−73, 38), 235 (3), 73 (100), 59 (36); Anal Calcd for C₁₈H₃₈Si₃: C, 63.82, H, 11.31, Found: C, 63.73; H, 11.54.

[SiC₄Et₄]²⁻•2[Li]⁺; Sonication of 2 (0.12 g, 0456 mmmol) and lithium (0.020 g, 2.857 mmmol) in 1.5 mL of THF-d₈ for 6 h gave a dark red solution.

[SiC₄Ph₄]²⁻•2[M]⁺ (M = Li, Na); It was prepared according to the known procedure [1,2].

3. Conclusions

1,1-Dichloro-2,3,4,5-tetraethyl-1-silacyclopentadiene (2) is prepared from SiCl₄ and 1,4-dilithio-1,2,3,4-tetraethyl-1,3-butadiene, the precursor of which, 1,4-dibromo-1,2,3,4-tetraethyl-1,3-butadiene (1), is synthesized from 3-hexyne and Cp₂ZrCl₂. Sonication of the silole 2 with an excess of lithium in THF produces the silole dianion [SiC₄Et₄]²⁻•2[Li]⁺(3), the treatment of which with trimethylchlorosilane gives 1,1-bis(trimethylsilyl)-2,3,4,5-tetraethyl-1-silacyclopentadiene (4). The NMR study of 3 shows that the ²⁹Si resonance in 3 shifts downfield and the ¹³C resonances of C_α and C_β in 3 shift upfield compared with 2. In particular both chemical shifts of C_α and C_β in 3 are very close to those of the reported tetraphenyl-substituted silole dianions, [SiC₄Ph₄]²⁻•2[Li]⁺ and [SiC₄Ph₄]²⁻•2[Na]⁺.