NHC-coordinated silagermenylidene functionalized in allylic position and its behaviour as a ligand

Abstract

Vinylidenes are common in transition metal chemistry with catalytic applications in alkene and alkyne metathesis. We report here the isolation of a heavier analogue of vinylidene, an α-chlorosilyl functionalized silagermenylidene stabilized by an N-heterocyclic carbene (NHC). Silagermenylidene (Tip₂Cl)Si-(Tip)Si=Ge·NHC^iPr₂Me₂ (4-E/Z; Tip = 2,4,6-ⁱPr₃C₆H₂; NHC^iPr₂Me₂ = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) is available as an E/Z-equilibrium mixture from Tip₂Si=Si(Tip)Li and NHC^iPr₂Me₂·GeCl₂. Reaction of 4-E/Z with Fe₂(CO)₉ affords a silagermenylidene Fe(CO)₄ complex, which slowly isomerizes to its E-isomer at 25 °C. A rearranged Fe(CO)₃ complex with an allylic SiGeSi ligand is obtained as a side product at 65 °C.

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Introduction

The chemistry of low-coordinate germanium has received considerable attention in recent years.¹ Important bonding motifs experimentally realized include two-coordinate germylenes² and digermynes,³ as well as three-coordinate digermenes,⁴ silagermenes,⁵ and germachalcogenones⁶ on the other hand. Since Robinson et al. reported the NHC-stabilized disilicon(0) species Ia,⁷ the use of strong donors for the isolation of highly reactive low-valent species by raising the coordination number has drastically increased.⁸ In germanium chemistry, germylene-type compounds (e.g. dihalogermylenes,⁹ digermanium(0) Ib¹⁰), and inherently polar/polarizable multiple bonds (e.g. germachalcogenones,¹¹ digermynes¹²) are prominent examples that are stabilized by base-coordination under retention of remarkable reactivity. Very recently, we reported on a N-heterocyclic carbene stabilized silagermenylidene, Tip₂Si=Ge·NHC^iPr₂Me₂II (NHC^iPr₂Me₂ = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene, Scheme 1).¹³ With the Si=Ge bond, the lone pair of electrons and the coordination site of the NHC, compound II offers various potential sites for further manipulation. Initially, we demonstrated the clean [2 + 2] cycloaddition of an alkyne to the Si=Ge bond.¹³ In view of the prominent role of carbon-based vinylidene complexes in catalysis,¹⁴ an open question remains the coordination behavior of isolable heavier vinylidenes towards transition metals.¹⁵ In the case of heavier analogues of carbenes, transition metal coordination compounds are known.¹⁶

Scheme 1 Chemical structures of Ia, Ib, II, III, and IV (Dip = 2,6-ⁱPr₂C₆H₃, R = Tip = 2,4,6-ⁱPr₃C₆H₂, Mes = 2,4,6-Me₃C₆H₂).

Our recent isolation of a stable NHC^iPr₂Me₂-stabilized aryl(disilenyl)silylene III¹⁷ encouraged us to target the corresponding disilenyl-substituted chlorogermylene 3. We thus reacted disilenide 1¹⁸ and NHC-coordinated germanium(II)chloride, NHC^iPr₂Me₂·GeCl₂2^9c (Scheme 2). Monosubstituted NHC-coordinated chlorogermylenes IV (Scheme 1) have been prepared via similar approaches.¹⁹

Scheme 2 Synthesis of 4 (R = Tip = 2,4,6-ⁱPr₃C₆H₂).

Results and discussion

Surprisingly, instead of the targeted 3 the 1 : 1 reaction of 1¹⁸ and 2^9c in toluene at −78 °C affords the NHC-coordinated silagermenylidene 4 in 62% yield (mp. 126–128 °C) (Scheme 2) with an additional peripheral Si–Cl functionality (see the Experimental section). The reaction plausibly proceeds through the NHC-stabilized chloro(disilenyl)germylene 3 as a transient followed by subsequent 1,3-migration of chlorine from germanium to the β-silicon. In solution, the ²⁹Si resonances of 4 at 162.5 and 7.3 ppm served as the first indication of the formation of a silagermenylidene due to the close similarity to the low-field resonance of II (158.9 ppm).¹³ The red color of 4 is due to the longest wavelength absorption in the UV/vis spectrum at λ_max = 451 nm (Table 1, ε = 9220 L mol⁻¹ cm⁻¹), which almost matches with that of compound II (λ_max = 455 nm). In contrast to II, however, the second absorption of 4 appears as a shoulder (4: λ_max = 389 nm, II: λ_max = 365 nm). To gain more information about the origins of the UV/vis absorptions, we performed TD-DFT calculations of the silagermenylidene II on the basis of the experimentally determined molecular structure in the solid state. Solvent effects were approximated using the Tomasi's polarized continuum model (PCM) at the B3LYP/6-31G(d,p) level of theory.† The calculated lowest-energy excitation of II at 439 nm is predominantly associated with the π–π* transition (HOMO → LUMO), in very good agreement with the experimental value of 455 nm. The second experimental absorption band at λ_max = 365 nm is due to various excitations, but does contain a significant component originating from the n–π* transition (HOMO−1 → LUMO) as suspected in our previous communication.¹³

Table 1 ²⁹Si{¹H}^a, ¹³C{¹H} NMR, UV/vis data of 4-E, 4-Z, 5-Z, 5-E, and 6

	4-E^b	4-Z^b	5-Z^c	5-E^c	6
a Calculated values in parentheses. b In d6-benzene. c In d8-toluene. d In THF.
δ^29Si (SiTip)	162.5 (159.5)	134.0 (90.4)	100.7	98.1	113.7 (138.3)
δ^29Si (SiTip2)	7.3 (1.7)	−0.2 (−9.8)	3.0	−3.9	91.5 (110.7)
δ^13C (NCN)	178.4 (168.4)	178.3(165.1)	165.8	167.0	—
δ^13C Fe(CO)4	—		217.6	216.8	219.0, 216.0
λ max [nm]	451, 389	503	512; 427	368^d

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Crystals of 4 suitable for X-ray diffraction analysis were obtained from pentane at 25 °C. The structure in the solid state (Fig. 1) confirmed the constitution of 4 as the sterically most favorable E-stereoisomer. The Ge1–Si1 bond length is by 2.2757(10) Å slightly longer than in II (2.2521(5) Å),¹³ whereas it is almost identical with that of the bulkily substituted silagermene (^tBu₃Si)₂Si=GeMes₂ (2.2769(8) Å; Mes = 2,4,6-Me₃C₆H₂).^5c As in II, the NHC coordinates to germanium in a near-orthogonal manner with respect to the Si1–Ge1 bond vector (C46–Ge1–Si1 101.90(10)°). The Ge1–C46 distance in 4-E (2.061(4) Å) is between that of the simple silagermenylidene II (2.0474(18) Å) and the GeCl₂ precursor 2 (2.106(3) Å).^9c

Fig. 1 Structure of 4-E in the solid state (thermal ellipsoids at 30%). Hydrogen atoms are omitted. Selected bond lengths [Å]: Si1–Ge1 = 2.2757(10), Ge1–C46 = 2.061(4), Si1–Si2 = 2.3776(13), Si2–Cl1 = 2.1179(13).

Interestingly, in solution, 4-E slowly converts to a new compound with ²⁹Si NMR resonances at 134.0 and −0.2 ppm, which we assign to stereoisomer 4-Z (Scheme 3). Equilibrium is reached after approximately 4 h in benzene-d₆ at an E/Z ratio of 0.85 : 0.15, essentially unaffected by temperature (+70 to −60 °C) or the presence of excess NHC^iPr₂Me₂.²⁰

Scheme 3 Equilibrium of 4-E to 4-Z and synthesis of transition metal complexes 5-Z, 5-E and 6 (a: THF, Fe₂(CO)₉; R = Tip = 2,4,6-ⁱPr₃C₆H₂; NHC^iPr₂Me₂ = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene).

The calculated ²⁹Si shifts [GIAO/B3LYP/6-31G(d,p) for H, C, N, 6-311+G(2d,p) for Si, Ge, Cl] of the truncated model systems for both isomers 4Dip-E and 4Dip-Z (R = Dip = 2,6-ⁱPr₂C₆H₃ instead of Tip) are 159.5, 1.7 and 90.4, −9.8 ppm, respectively.† Although the experimental trend is reproduced, the absolute agreement of the calculated and the experimental values is better for the major isomer 4-E. The deviations presumably arise from the neglect of dispersive forces that should affect the sterically unfavorable isomer 4-Z considerably more than 4-E.¹⁷

Mills et al. had obtained the first structurally characterized transition metal complexes of diphenylvinylidene from diphenylketene and Fe(CO)₅.²¹ The reaction of 4-E/Z with Fe₂(CO)₉ in THF at room temperature initially affords only the Z-stereoisomer of the silagermenylidene complex 5, which corresponds to the relative orientation of the chlorosilyl group and the NHC^iPr₂Me₂ ligand in 4-E (Scheme 3) (see the Experimental section). The iron germenylidene complex 5-Z was isolated as a red-brown solid (mp. 140–142 °C) in 63% yield. In the case of germylenes, similar complexes have been reported.^16b In the ¹³C NMR of 5-Z, the two downfield resonances at 216.8 and 167.0 ppm were assigned to the carbonyl ligands at the Fe-center and coordinated NHC^iPr₂Me₂, respectively.²² The ²⁹Si NMR exhibits signals at 100.7 and 3.0 ppm; the formal sp²-Si is substantially upfield shifted compared to that of the free ligand 4-E (Table 1). As shown by the new ²⁹Si signals at 98.1 and −3.9 ppm in a 1 : 1 ratio, 5-Z slowly – but in this case irreversibly – rearranges to the stereoisomer, 5-E (mp. 158–160 °C) in solution. This is in contrast to the steric preferences of the NHC^iPr₂Me₂ ligand in 4-E/Z, which, however, can readily be explained by the comparatively larger Fe(CO)₄ moiety (Scheme 3) (see the Experimental section). In the IR, the most intense carbonyl stretching bands of 5-Z and 5-E are observed at 2018, 2010, 1923, 1899 cm⁻¹ and 2011, 2008, 1917, 1897 cm⁻¹, respectively. Apparently, silagermenylidenes 4-E/Z are somewhat stronger σ-donors compared to, for instance, the N-heterocyclic carbene, NHC^Dip (NHC^DipFe(CO)₄:^23aν = 2035, 1947, 1928, 1919 cm⁻¹) (NHC^Dip = C{N(Ar)CH}₂, Ar = 2,6-diisopropylphenyl) and intramolecularly base-coordinated germylene LGeOH^23b (CO stretching frequencies of LGe(OH)Fe(CO)₄: ν = 2039, 1956, 1942 cm⁻¹) (L = CH{(CMe)₂(2,6-ⁱPr₂C₆H₃N)₂}). Incidentally, the carbonyl stretching frequencies of 5-Z and 5-E are very similar to those of the carbon-based vinylidene complex H(CHO)C=C=Fe(CO)₂(P(OMe)₃)₂ (ν = 2015, 2007 cm⁻¹).^23c

The iron complex 5-E crystallizes from concentrated pentane solution (Fig. 2). The structural model confirmed that the Si=Ge bond is retained upon coordination (Ge1–Si1 2.2480(10) Å). Both formally sp²-hybridized heavier atoms, Si and Ge, are pyramidalized (∑ of angles: Ge1 354.74°; Si1 351.25°). The Si=Ge bond adopts a strongly trans-bent geometry (Si1: 30.27 (1)°, Ge1: 21.07 (8)°). Another noteworthy feature of 5-E is the twisting angle of 17.09(1)° between the planes of C1–Si–Si2 and Fe1–Ge1–C16. The bond length of Ge1–C16 is by 2.020(3) Å significantly shorter than in the free ligand (4-E: 2.061(4) Å). The Ge1–Fe1 bond length (2.3780(6) Å) is slightly longer than in germylene-coordinated iron(0)tetracarbonyl complexes (LGeOH Å 2.330(1) Å,^23b LGeF 2.3262(7) Å,^23d L = CH{(CMe)₂(2,6-ⁱPr₂C₆H₃N)₂}). The C16–Ge1–Si1 bond angle is by 115.59(10)° much wider than that in silagermenylidenes (II,¹³ C31–Ge–Si 98.90(5)° and 4-E, C(46)–Ge(1)–Si(1) 101.90(10)°). These structural parameters are reminiscent of the η¹-vinyl coordination mode in Tip₂Si=SiTip-(Cl)ZrCp₂.²⁴ In the light of the current discussion on the use of arrows in the context of donor–acceptor interactions²⁵ it should be noted that obviously the formulation of 5-Z as zwitterionic complex 5′-Z is equally valid.

Fig. 2 Structure of 5-E in the solid state (thermal ellipsoids at 30%). Hydrogen atoms are omitted. Selected bond lengths [Å]: Si1–Ge1 = 2.2480(10), Ge1–C16 = 2.020(3), Ge1–Fe1 = 2.3780(6), Fe1–C59 = 1.772(4), C59–O3 = 1.162(5), Si1–Si2 = 2.3767(12), Si2–Cl1 = 2.1118(11).

When the isomerization process of 5-Z was carried out at 65 °C, an additional product 6 is formed in 14% yield (Scheme 3) along with the major product 5-E (56%) (see the Experimental section). Notably, 6 cannot be obtained by heating an isolated sample of 5-E. Spatial proximity between the Fe(CO)₄ and SiTip₂Cl moieties seems to be required for the isomerization under loss of one CO ligand. By fractional crystallization, we isolated 6 as yellow blocks (mp. 197–199 °C). In ²⁹Si NMR, both resonances of silicon appear at 113.7 (SiTip) and 91.5 (SiTip₂) ppm, which hints at the absence of saturated silicon atoms, such as in the chlorosilyl side chain of 5-Z/E.

An X-ray diffraction study on single crystals of 6 revealed a bicyclo[1.1.0]butane-like butterfly structure with the Fe1 and Ge1 in the bridgehead positions (Fig. 3). Apparently, a chlorine migration from the SiTip₂ moiety to the SiTip moiety took place during conversion from 5-Z to 6. The ²⁹Si NMR shifts of 6 are close to those of Ogino's alkoxy- and amido-bridged bis(silylene)iron complexes 7 (Scheme 4).²⁶ In the present case, however, the bridging unit is the NHC^iPr₂Me₂-stabilized germylene moiety so that an analogous electronic description (6′, Scheme 4) probably contributes less than a zwitterionic resonance form of allylic type (6′′).

Fig. 3 Structure of 6 in the solid state (thermal ellipsoids at 30%). Hydrogen atoms are omitted. Selected bond lengths [Å]: Ge1–C46 = 2.053(2), Ge1–Si1 = 2.3870(7), Ge1–Si2 = 2.3249(7), Fe1–Si1 = 2.3520(7), Fe1–Si2 = 2.3166(8), Ge1–Fe1 = 2.6875(4).

Scheme 4 Chemical structures of 7 and canonical forms of 6 (R = Tip = 2,4,6-ⁱPr₃C₆H₂).

This assertion finds support in the pertinent structural features of 6. The averaged distance between Fe1 and Si1/Si2 in 6 is 2.3343(7) Å, shorter than that in the tetracarbonyl iron complexes of a Z-1,2-dichlorodisilene (2.4358(6) Å, average distance),²⁷ but longer than the Si–Fe distances in silylene–iron complexes ((CO)₄Fe=Si(Me)₂·HMPA,^28a 2.280(1) and 2.294(1) Å for two crystallographic independent molecules; (CO)₄Fe=Si(S^tBu)₂·HMPA^28b (2.278(1) Å) (HMPA = (NMe₂)₃PO/hexamethylphosphoric triamide). The average distance between silicon and germanium in 6 is 2.3560(7) Å, considerably longer than that of the reported 2-germadisilaallene (2.2370(7) Å, average distance).²⁹ The mechanism for formation of 6 remains obscure. However, the intramolecular activation of a silicon–silicon bond in oligosilyl iron complexes has been reported³⁰ and recently Marschner et al. demonstrated the Lewis acids catalyzed shuttling of germanium atoms into branched polysilanes.³¹

Experimental section

General remarks

All experiments were carried out under a protective atmosphere of argon applying standard Schlenk techniques or in a glove box. All the solvents were refluxed over sodium/benzophenone, distilled and stored under argon. Benzene-d₆, toluene-d₈, and THF-d₈ were dried and distilled over potassium under argon. ¹H and ¹³C{¹H} NMR spectra were referenced to the peaks of residual protons of the deuterated solvent (¹H) or the deuterated solvent itself (¹³C). ²⁹Si{¹H} NMR spectra were referenced to external SiMe₄. UV/vis spectra were acquired using a Perkin-Elmer Lambda 35 spectrometer using quartz cells with a path length of 0.1 cm. IR spectra were recorded using a Varian 2000 FT-IR FTS 2000 spectrometer. Melting points were determined under argon in closed NMR tubes and are uncorrected. Elemental analyses were performed using a Leco CHN-900 analyzer.

Synthesis of 4-E: A precooled (–78 °C) solution of 1 (2.70 g, 3.17 mmol, in 30 mL of toluene) was transferred by cannula to a suspension of 2 (1.02 g, 3.17 mmol, in 15 mL of toluene) at −78 °C. The reaction mixture was allowed to warm slowly to room temperature and stirred overnight. All the volatiles were removed under vacuum and the solid residue dissolved in 30 mL of hexane. After filtration, the solution was concentrated to about 10 mL and kept overnight at room temperature. The red precipitate was separated from the supernatant solution, washed with 5 mL of pentane and dried under vacuum to yield 1.90 g (62%) of 4-E. Single crystals suitable for X-ray diffraction were obtained from a saturated pentane solution after keeping for 2 days at room temperature. Mp: 126–128 °C. ¹H NMR (300 MHz, benzene-d₆, TMS): δ 7.08 (s, 4H, TipH), 7.03 (s, 2H, TipH), 5.38 (2H, sept, NⁱPr–CH), 4.47–3.96 (m and br, altogether 6H, o-ⁱPr–CH), 2.83–2.65 (m, 3H, p-ⁱPr–CH), 1.52 (s, 6H, CH₃C=C), 1.33–1.11 (br and m, altogether 48H, ⁱPr–CH₃), 1.01 (d, 6H, ⁱPr–CH₃), 0.85 (d, 12 H, NⁱPr–CH₃) ppm. ¹³C NMR (75.4 MHz, benzene-d₆, TMS): δ 178.45 (NCN), 154.41, 153.96, 149.51, 149.17, 142.48, 138.44 (TipC_quart), 126.43 (NCCN), 122.51, 121.64 (TipCH), 53.92 (NⁱPr–CH), 35.74, 34.79, 34.53, 34.42 (ⁱPr–CH), 25.64, 25.25, 24.95, 24.29, 24.09, 24.05 (ⁱPr–CH₃), 20.69 (N ⁱPr–CH₃), 10.05 (CH₃C=C) ppm. ²⁹Si NMR (59.5 MHz, benzene-d₆, TMS): δ 162.50 (SiTip), 7.3 (SiTip₂) ppm. UV/vis (hexane): λ_max(ε) = 451 nm (9220 L mol⁻¹ cm⁻¹), 389 nm (sh). Anal. Calcd for C₅₆H₈₉ClGeN₂Si₂ (954.58): C, 70.46; H, 9.40; N, 2.93. Found: C, 70.47; H, 9.47; N, 2.93. Crystallographic data: C₅₆H₈₉ClGeN₂Si₂, M_r = 954.51, monoclinic, space group P2(1)/c, a = 20.3816(11), b = 10.9027(6), c = 25.5288(13) Å, β = 90.840(3)°, V = 5672.3(5) ų; Z = 4, ρ_c = 1.118 g cm⁻³, T = 133(2) K, λ = 0.71073 Å, 48 361 reflections, 14 017 independent (R_int = 0.1264), R₁ = 0.0619 (I > 2σ(I)) and wR₂(all data) = 0.1332, GooF = 0.970, max/min residual electron density: 0.827/−0.923 e Å⁻³.

Equilibrium between 4-E and 4-Z and NMR data of 4-Z: In solution 4-E isomerizes to 4-Z reaching equilibrium after about 4 h. The ratio of the two isomers was approximately 0.84 : 0.16 (4-E/4-Z). 4-Z: ¹H NMR (300 MHz, benzene-d₆, TMS): δ 7.24 (br, 2H, TipH), 7.21 (br, 1H, TipH), 6.97 (br, 1H, TipH), 6.77 (br, 1H, TipH), 5.67 (2H, sept, NⁱPr–CH), 4.75–4.54 (m and br, altogether 2H, o-ⁱPr–CH), 3.76–3.64 (m, 2H, o-ⁱPr–CH), 1.80 (d, 3H, ⁱPr–CH₃), 1.76–1.69 (m, 6H, ⁱPr–CH₃), 1.61 (s, 6H, CH₃C=C), 0.61 (d, 3H, ⁱPr–CH₃), 0.47 (d, 3H, ⁱPr–CH₃), 0.40 (d, 3H, ⁱPr–CH₃), 0.30 (d, 3H, ⁱPr–CH₃) ppm. ¹³C NMR (75.4 MHz, [D₆]benzene, TMS): δ 178.33 (NCN) ppm. ²⁹Si NMR (59.5 MHz, [D₆]benzene, TMS): δ 134.02 (SiTip), −0.20 (SiTip₂) ppm (minor isomer).

Synthesis of 5-Z: Dry and degassed THF (30 mL) was added to a Schlenk flask containing compound 4-E (1.90 g, 1.99 mmol) and Fe₂(CO)₉ (0.80 g, 2.19 mmol) at room temperature. The color of the reaction mixture changed immediately to deep red. The reaction mixture was stirred for another 4 h and all the volatiles were removed under vacuum. The solid residue was extracted with 80 mL of hexane and the resulting solution was filtered to remove insoluble impurities. The hexane was distilled off under vacuum. After addition of 20 mL of pentane, 1.40 g (63%) of 5-Z were isolated as a dark-red solid. Mp: 140–142 °C. ¹H NMR (300 MHz, toluene-d₈, TMS): δ 7.26, 7.18, 6.86, 6.81, 6.65 (d, each having 1H, TipH), another Tip-H signal is masked by residual proton signals of toluene-d₈, 5.83 (sept, 1H, ⁱPr–CH), 5.09 (sept, 1H, ⁱPr–CH), 5.00 (sept., 1H, ⁱPr–CH), 4.55 (sept, 1H, ⁱPr–CH), 4.07 (sept, 1H, ⁱPr–CH), 3.80–3.62 (m, 2H, ⁱPr–CH), 2.78 (sept, 1H, ⁱPr–CH), 2.70–2.53 (m, 2H, ⁱPr–CH), 2.48 (sept, 1H, ⁱPr–CH), 1.73 (d, 3H, ⁱPr–CH₃), 1.67–1.51 (s and m, altogether 15H, ⁱPr–CH₃ and CH₃C=C), 1.45–1.39 (s and m, altogether 6H, ⁱPr–CH₃ and CH₃C=C), 1.34–1.25 (m, 9H, ⁱPr–CH₃), 1.22 (d, 6H, ⁱPr–CH₃), 1.17–1.16 (m, 9H, ⁱPr–CH₃), 1.09–1.03 (m, 6H, ⁱPr–CH₃), 0.48 (d, 3H, ⁱPr–CH₃), 0.43 (d, 3H, ⁱPr–CH₃), 0.37 (d, 3H, ⁱPr–CH₃), 0.20–0.11 (m, 9H, ⁱPr–CH₃) ppm. ¹³C NMR (75.4 MHz, toluene-d₈, TMS): δ 217.56 (CO), 165.81 (NCN) ppm. (We were unable to assign other signals correctly, because they overlap with its isomer 5-E.) ²⁹Si NMR (59.5 MHz, toluene-d₈, TMS): δ 100.67 (SiTip), 2.98 (SiTip₂) ppm. Mp.: 140–142 °C. UV/vis (hexane): λ_max(ε) = 503 nm (8260 Lmol⁻¹ cm⁻¹). IR (KBr, cm⁻¹): ν = 2018 (s), 2010 (s), 1923 (s), 1899 (s). Anal. Calcd for C₆₀H₈₉ClFeGeN₂O₄Si₂ (1122.47): C, 64.20; H, 7.99; N, 2.50. Found: C, 64.10; H, 7.74; N, 2.63.

Synthesis of 5-E: A solution of 5-Z (1.00 g, 0.89 mmol) in toluene (60 mL) was stirred for five days at room temperature. All volatiles were removed under vacuum and the solid residue extracted with 70 mL of hexane. The solution was concentrated to about 20 mL and after keeping at −20 °C for a week afforded brown-red crystals of 5-E (0.77 g, 76%). Mp: 158–160 °C. ¹H NMR (300 MHz, toluene-d₈, TMS): δ 7.27–7.23 (m, 2H, TipH), 7.21 (d, 1H, TipH), 7.05 (1H, TipH, masked by toluene-d₈), 6.96 (d, 1H, TipH), 6.73 (d, 1H, TipH), 5.72 (sept, 1H, NⁱPr–CH), 5.51 (sept, 1H, NⁱPr–CH), 4.62 (sept, 1H, ⁱPr–CH), 4.54 (sept, 1H, ⁱPr–CH), 4.30 (sept, 1H, ⁱPr–CH), 3.67 (sept, 1H, ⁱPr–CH), 3.57 (sept, 1H, ⁱPr–CH), 2.88–2.62 (m, 4H, ⁱPr–CH), 2.00 (d, 3H, ⁱPr–CH₃), 1.79 (d, 3H, ⁱPr–CH₃), 1.68–1.46 (s and m, altogether 30H, ⁱPr–CH₃ and CH₃C=C), 1.27–1.21 (m, 12H, ⁱPr–CH₃), 1.19–1.15 (m, 12H, ⁱPr–CH₃), 0.56 (d, 3H, ⁱPr–CH₃), 0.43 (d, 3H, ⁱPr–CH₃), 0.34 (d, 3H, ⁱPr–CH₃), 0.24 (d, 3H, ⁱPr–CH₃) ppm. ¹³C NMR (75.4 MHz, toluene-d₈, TMS): δ 216.85 (CO), 166.99 (NCN), 156.72, 155.76, 155.69, 154.05, 153.23, 152.37, 151.32, 150.41, 137.67, 133.07, 130.55, 127.35, 127.00 (TipC_quart and NCCN), 124.07, 123.37, 122.53, 122.40, 122.12, 120.96 (TipCH), 55.53, 54.84 (NⁱPr–CH), 38.48, 38.11, 37.20, 35.24, 34.81, 34.62, 34.47, 33.97, 31.04 (ⁱPr–CH), 32.00, 30.66, 27.80, 27.30, 26.00, 25.36, 25.16, 24.83, 24.30, 24.16, 24.02, 23.97, 23.92, 23.87, 23.05, 22.50, 22.43, 22.40, 21.87, 20.83, 14.29 (ⁱPr–CH₃) 10.17, 9.92 (CH₃C=C) ppm. ²⁹Si NMR (59.5 MHz, toluene-d₈, TMS): δ 98.14 (SiTip), −3.89 (SiTip₂) ppm. UV/vis (hexane): λ_max(ε) = 512 nm (7050 Lmol⁻¹ cm⁻¹), 427 nm (6670) nm (L mol⁻¹ cm⁻¹). IR (KBr, cm⁻¹): ν = 2011 (s), 2008 (s), 1917 (s) 1897 (s). Anal. Calcd for C₆₀H₈₉ClFeGeN₂O₄Si₂ (1122.47): C, 64.20; H, 7.99; N, 2.50. Found: C, 64.15; H, 7.90; N, 2.26. Crystallographic data: C₆₀H₈₉ClFeGeN₂O₄Si₂·0.25C₅H₁₂, M_r = 1140.44, triclinic, space group P1̄, a = 13.2910(4), b = 19.9842(5), c = 24.8636(7) Å, α = 89.1430(10), β = 95.311(2), γ = 76.122(2)°, V = 6407.6(3) ų; Z = 4, ρ_c = 1.182 g cm⁻³, T = 123(2) K, λ = 0.71073 Å, 105 034 reflections, 28 114 independent (R_int = 0.0436), R₁ = 0.0619 (I > 2σ(I)) and wR₂ (all data) = 0.1744, GooF = 1.427, max/min residual electron density: 2.142/−0.789 e Å⁻³.

Synthesis of 6: A solution of 5-Z (0.50 g, 0.44 mmol) in toluene (30 mL) was stirred in a sealed Schlenk flask overnight at 65 °C. All the volatiles were removed under vacuum and the solid residue extracted with 40 mL of hexane. The solution was concentrated to about 20 mL and after keeping at room temperature for two days afforded yellow blocks of 6 (0.075 g, 14%). Keeping the mother liquor at −20 °C for a week afforded brown-red crystals of 5-E (0.28 g, 56%). 6: Mp: 197–199 °C. ¹H NMR (300 MHz, toluene-d₈, TMS): δ 7.23 (d, 1H, TipH), 7.13 (1H, TipH, masked by toluene-d8), 7.02 (sept, 1H, NⁱPr–CH), 7.01 (1H, TipH, masked by toluene-d₈), 6.98–6.95 (m, 2H, TipH), 6.87 (d, 1H, TipH), 6.41 (sept, 1H, NⁱPr–CH), 5.25 (sept, 1H, ⁱPr–CH), 4.86 (sept, 1H, ⁱPr–CH), 4.35 (sept, 1H, ⁱPr–CH), 4.22 (sept, 1H, ⁱPr–CH), 3.61 (sept, 1H, ⁱPr–CH), 3.44 (sept, 1H, ⁱPr–CH), 2.79 (sept, 1H, ⁱPr–CH), 2.77 (sept, 1H, ⁱPr–CH), 2.65 (sept, 1H, ⁱPr–CH), 1.87–1.78 (m, 6H, ⁱPr–CH₃), 1.68 (s, 3H, CH₃C=C), 1.66 (s, 3H, CH₃C=C), 1.56–1.37 (m, altogether 18H, ⁱPr–CH₃), 1.31–1.04 (m, altogether 30H, ⁱPr–CH₃), 0.72 (d, 3H, ⁱPr–CH₃), 0.48 (d, 3H, ⁱPr–CH₃), 0.45 (d, 3H, ⁱPr–CH₃), 0.19 (d, 3H, ⁱPr–CH₃) ppm. ¹H NMR (300 MHz, thf-d₈, TMS): δ 7.03 (d, 1H, TipH), 6.96 (d, 1H, TipH), 6.95 (sept, 1H, NⁱPr–CH), 6.80 (d, 1H, TipH), 6.76 (d, 1H, TipH), 6.71 (d, 1H, TipH), 6.67 (d, 1H, TipH), 6.19 (sept, 1H, NⁱPr–CH), 4.82 (sept, 1H, ⁱPr–CH), 4.61 (sept, 1H, ⁱPr–CH), 4.15 (sept, 1H, ⁱPr–CH), 3.82 (sept, 1H, ⁱPr–CH), 3.34–3.18 (m, 2H, ⁱPr–CH), 2.87–2.68 (m, 2H, ⁱPr–CH), 2.63 (sept, 1H, ⁱPr–CH), 2.41 (s, 3H, CH₃C=C), 2.35 (s, 3H, CH₃C=C), 1.85 (d, 3H, ⁱPr–CH₃), 1.62 (d, 3H, ⁱPr–CH₃), 1.58–1.49 (m, altogether 9H, ⁱPr–CH₃), 1.44 (d, 3H, ⁱPr–CH₃), 1.24 (d, 3H, ⁱPr–CH₃), 1.20–1.11 (m, altogether 21H, ⁱPr–CH₃), 1.08–1.03 (m, altogether 9H, ⁱPr–CH₃), 0.88 (d, 3H, ⁱPr–CH₃), 0.45 (d, 3H, ⁱPr–CH₃), 0.18–0.14 (m, altogether 6H, ⁱPr–CH₃), −0.20 (d, 3H, ⁱPr–CH₃) ppm. ¹³C NMR (75.4 MHz, thf-d₈, TMS): δ 218.99, 215.98 (CO), 157.19, 156.89, 155.09, 154.75, 154.08, 152.28, 150.90, 150.71, 149.48, 143.26, 142.18, 136.52 ((TipC_quart), 129.52, 129.26 (NCCN), 123.68, 122.72, 122.66, 122.06, 121.88, 121.43 (TipCH), 56.24, 54.08 (NⁱPr–CH), 38.20, 36.65, 36.46, 35.29, 35.39, 34.98, 34.83, 34.16, 30.24 (ⁱPr–CH), 28.21, 26.76, 26.28, 25.97, 25.65, 25.48, 24.91, 24.38, 24.36, 24.28, 24.18, 23.99, 23.90, 23.84, 23.40, 22.88, 22.23, 21.98 (ⁱPr–CH₃), 11.21, 10.73 (CH₃C=C) ppm (we did not observe the carbenic carbon resonance). ²⁹Si NMR (59.5 MHz, toluene-d₈, TMS): δ 113.66 (SiTip), 91.46 (SiTip₂) ppm. ²⁹Si NMR (59.5 MHz, [D₈]THF, TMS): δ 111.36 (SiTip), 88.27 (SiTip₂) ppm. UV/vis (THF): λ_max(ε) = 368 nm (6820 Lmol⁻¹ cm⁻¹). IR (KBr, cm⁻¹): ν = 1982 (s), 1920 (s), 1916 (s). Anal. Calcd for C₅₉H₈₉ClFeGeN₂O₃Si₂ (1094.46): C, 64.75; H, 8.20; N, 2.56. Found: C, 65.35; H, 8.08; N, 2.38. Crystallographic data: C₅₉H₈₉ClFeGeN₂O₃Si₂, M_r = 1094.39, orthorhombic, space group P_bca, a = 19.6288(5), b = 24.6607(7), c = 24.6974(7) Å, V = 11 955.0(6) ų; Z = 8, ρ_c = 1.216 g cm⁻³, T = 132(2) K, λ = 0.71073 Å, 104 743 reflections, 14 312 independent (R_int = 0.0577), R₁ = 0.0442 (I > 2σ(I)) and wR₂ (all data) = 0.1131, GooF = 1.022, max/min residual electron density: 1.184/−0.536 e Å⁻³.

Conclusions

We have shown an efficient method for the synthesis of side chain-functionalized silagermenylidene stabilized by coordination of an N-heterocyclic carbene. Its suitability as a ligand for transition metal complexes was demonstrated by coordination to the Fe(CO)₄ fragment. Moreover, the resulting silagermenylidene iron complex thermally rearranges to an apparently more stable complex of unprecedented allylic structure, which is undoubtedly a consequence of the ease of migration of the residual chlorine functionality.

Acknowledgements

Support for this study was provided by the EPSRC (EP/H048804/1), the Alfried Krupp von Bohlen und Halbach Foundation. We thank Dr Carsten Präsang for assistance with the IR measurement.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: NMR and UV/vis spectra of all new compounds, X-ray crystallographic data (CIF) for 4-E, 5-E, and 6, and computational details. CCDC 953520–953522. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt00094c

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