Nhc-coordinated Silagermenylidene Functionalized in Allylic Position and Its Behaviour as a Ligand †

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 function-alized silagermenylidene stabilized by an N-heterocyclic carbene (NHC). Silagermenylidene (Tip 2 Cl)Si


Introduction
The chemistry of low-coordinate germanium has received considerable attention in recent years. 1 Important bonding motifs experimentally realized include two-coordinate germylenes 2 and digermynes, 3 as well as three-coordinate digermenes, 4 silagermenes, 5 and germachalcogenones 6 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. 8In germanium chemistry, germylene-type compounds (e.g.dihalogermylenes, 9 digermanium(0) Ib 10 ), and inherently polar/polarizable multiple bonds (e.g.germachalcogenones, 11 digermynes 12 ) 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, Tip 2 SivGe•NHC iPr2Me2 II (NHC iPr 2 Me 2 = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene, Scheme 1). 13With the SivGe 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 SivGe bond. 13In 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. 15In the case of heavier analogues of carbenes, transition metal coordination compounds are known. 16ur recent isolation of a stable NHC iPr2Me2 -stabilized aryl (disilenyl)silylene III 17 encouraged us to target the corresponding disilenyl-substituted chlorogermylene 3. We thus reacted disilenide 1 18 and NHC-coordinated germanium(II) chloride, NHC iPr2Me2 •GeCl 2 2 9c (Scheme 2).Monosubstituted NHC-coordinated chlorogermylenes IV (Scheme 1) have been prepared via similar approaches. 19

Results and discussion
Surprisingly, instead of the targeted 3 the 1 : 1 reaction of 1 18 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 29 Si 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). 13The 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. 13rystals 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 ( t Bu 3 Si) 2 SivGeMes 2 (2.2769(8) Å; Mes = 2,4,6-Me 3 C 6 H 2 ).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 GeCl 2 precursor 2 (2.106(3) Å). 9c Interestingly, in solution, 4-E slowly converts to a new compound with 29 Si 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-d 6 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 . 20he calculated 29 Si 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-i Pr 2 C 6 H 3 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. 17ills et al. had obtained the first structurally characterized transition metal complexes of diphenylvinylidene from diphenylketene and Fe(CO) 5 . 21The reaction of 4-E/Z with Fe 2 (CO) 9 in Scheme 2 Synthesis of 4 (R = Tip = 2,4,6-i Pr 3 C 6 H 2 ).
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 SiTip 2 Cl 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 29 Si NMR, both resonances of silicon appear at 113.7 (SiTip) and 91.5 (SiTip 2 ) ppm, which hints at the absence of saturated silicon atoms, such as in the chlorosilyl side chain of 5-Z/E.

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-d 6 , toluene-d 8 , and THF-d 8 were dried and distilled over potassium under argon. 1 H and 13 C{ 1 H} NMR spectra were referenced to the peaks of residual protons of the deuterated solvent ( 1 H) or the deuterated solvent itself ( 13 C). 29Si{ 1 H} NMR spectra were referenced to external SiMe 4 .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.

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.