Synthesis of a α-Chlorosilyl Functionalized Donor-Stabilized Chlorogermylene

Peripherally functionalized low-valent main group species allow for the introduction/interconversion of functional groups without increasing the formal oxidation state of the main group center. Herein, we report a straightforward method for the incorporation of a α-chlorosilyl moiety adjacent to the NHC-coordinated germanium(II) center.


Introduction
In recent years, the chemistry of the heavier analogues of carbenes (tetrylenes) has been expanded beyond mere synthetic curiosity [1][2][3] towards application in synthesis.Heavier carbene analogues are applied as a donor ligands in low-valent main group species [4,5] as well as in transition metal complexes [6,7].An increasing number of examples show competitive catalytic activity in different organic transformations [8].As the complexity of the tetrylenes increases with more intricate ligand architectures [9], functionalization protocols in the presence of uncompromised low-valent Group 14 centers conveniently allow for a comparatively straightforward diversification in the final stages of ligand synthesis.While the interconversion of functional groups is just beginning to emerge in the case of heavier multiple bonds [10], numerous examples have been reported for the heavier tetrylenes [11][12][13][14][15]. Recently, Scheschkewitz et al. have taken a similar approach with the synthesis of the multiply functional NHC-coordinated silagermenylidenes, I and II (Scheme 1) [16,17], which serve as precursors for cyclic NHC-coordinated germylenes of type III, IV, and V under consumption of the Si=Ge bond, but retention of the low-valent germanium center (Scheme 1) [16,18,19].The leaving group characteristics of the peripheral chloro functionality of II can be exploited for the incorporation of different organic substituents by treatment with organolithium reagents in order to fine-tune the steric requirements of the ligand scaffold of cyclic germylenes of type IV [20,21].Directly chloro-functionalized silylenes and germylenes are readily converted to a variety of novel low-valent group 14 compounds by functional group interconversion at the low-valent tetrel center [22].An additional leaving group adjacent to a chloro-functionalized heavier carbene center would in principle provide a precursor for the synthesis of further examples of heavier vinylidene such as I and II.The synthesis of an NHC-coordinated (chlorogermyl)chlorogermylene, IV, from NHC-coordinated diaryl germylene and NHC-or 1,4-dioxane coordinated dichlorogermylene has been reported by the groups of Baines and Tobitah [23,24].Herein, we now report the synthesis of NHC-stabilized (chlorosilyl)chlorogermylene, 1.

Results and Discussions
We anticipated that West's N-heterocyclic silylene, 2 [25] would insert into the Ge-Cl bond of the NHC-germanium(II)dichloride adduct 3 [26] as it is well known for the oxidative addition of different types of bonds e.g., C-Cl [27] and Ge-N [28].Indeed, the reaction of 2 and 3 in a 1:1 ratio in toluene at room temperature afforded the NHC-stabilized (chlorosilyl)chlorogermylene 1 which was isolated as a crystalline compound (Scheme 2).We did not obtain any indication for the formation of donor-acceptor adducts between 2 and 3 or rearrangement products as often described for reactions of silylenes and germylenes [29,30].Compound 1 was characterized in solution state by NMR spectroscopy as well as in solid state by single crystal X-ray molecular structure determination.The insertion of the silylene into the Ge-Cl bond turns the germanium atom into a center of chirality.As a result the two diastereotopic C-H protons of C2N2Si-moiety give rise to two doublets at δ = 5.87 and 6.03 ppm ( 1 J(H, H) = 3.92 Hz) in the 1 H NMR. Similarly, the two tBu groups show 1 H NMR resonances at δ = 1.29 and 1.65 ppm.Hindered rotation can be excluded as the explanation for the doubling of these resonances.Despite the increased congestion about the germanium center, the NHC retains the local rotational C2-symmetry in solution: the 1 H NMR shows only a single septet for the two CH moieties of the isopropyl groups.In contrast, there are again two signals for the adjacent diastereotopic methyl groups of the N-isopropyl moiety.In 13 C{ 1 H} NMR, the carbenic carbon shows a resonance at δ = 171.01ppm, which is similar to the chemical shifts observed for other NHC-coordinated Ge(II) An additional leaving group adjacent to a chloro-functionalized heavier carbene center would in principle provide a precursor for the synthesis of further examples of heavier vinylidene such as I and II.The synthesis of an NHC-coordinated (chlorogermyl)chlorogermylene, IV, from NHC-coordinated diaryl germylene and NHC-or 1,4-dioxane coordinated dichlorogermylene has been reported by the groups of Baines and Tobitah [23,24].Herein, we now report the synthesis of NHC-stabilized (chlorosilyl)chlorogermylene, 1.

Results and Discussions
We anticipated that West's N-heterocyclic silylene, 2 [25] would insert into the Ge-Cl bond of the NHC-germanium(II)dichloride adduct 3 [26] as it is well known for the oxidative addition of different types of bonds e.g., C-Cl [27] and Ge-N [28].Indeed, the reaction of 2 and 3 in a 1:1 ratio in toluene at room temperature afforded the NHC-stabilized (chlorosilyl)chlorogermylene 1 which was isolated as a crystalline compound (Scheme 2).We did not obtain any indication for the formation of donor-acceptor adducts between 2 and 3 or rearrangement products as often described for reactions of silylenes and germylenes [29,30].An additional leaving group adjacent to a chloro-functionalized heavier carbene center would in principle provide a precursor for the synthesis of further examples of heavier vinylidene such as I and II.The synthesis of an NHC-coordinated (chlorogermyl)chlorogermylene, IV, from NHC-coordinated diaryl germylene and NHC-or 1,4-dioxane coordinated dichlorogermylene has been reported by the groups of Baines and Tobitah [23,24].Herein, we now report the synthesis of NHC-stabilized (chlorosilyl)chlorogermylene, 1.

Results and Discussions
We anticipated that West's N-heterocyclic silylene, 2 [25] would insert into the Ge-Cl bond of the NHC-germanium(II)dichloride adduct 3 [26] as it is well known for the oxidative addition of different types of bonds e.g., C-Cl [27] and Ge-N [28].Indeed, the reaction of 2 and 3 in a 1:1 ratio in toluene at room temperature afforded the NHC-stabilized (chlorosilyl)chlorogermylene 1 which was isolated as a crystalline compound (Scheme 2).We did not obtain any indication for the formation of donor-acceptor adducts between 2 and 3 or rearrangement products as often described for reactions of silylenes and germylenes [29,30].Compound 1 was characterized in solution state by NMR spectroscopy as well as in solid state by single crystal X-ray molecular structure determination.The insertion of the silylene into the Ge-Cl bond turns the germanium atom into a center of chirality.As a result the two diastereotopic C-H protons of C2N2Si-moiety give rise to two doublets at δ = 5.87 and 6.03 ppm ( 1 J(H, H) = 3.92 Hz) in the 1 H NMR. Similarly, the two tBu groups show 1 H NMR resonances at δ = 1.29 and 1.65 ppm.Hindered rotation can be excluded as the explanation for the doubling of these resonances.Despite the increased congestion about the germanium center, the NHC retains the local rotational C2-symmetry in solution: the 1 H NMR shows only a single septet for the two CH moieties of the isopropyl groups.In contrast, there are again two signals for the adjacent diastereotopic methyl groups of the N-isopropyl moiety.In 13 C{ 1 H} NMR, the carbenic carbon shows a resonance at δ = 171.01ppm, which is similar to the chemical shifts observed for other NHC-coordinated Ge(II) Scheme 2. Synthesis of 1.
Compound 1 was characterized in solution state by NMR spectroscopy as well as in solid state by single crystal X-ray molecular structure determination.The insertion of the silylene into the Ge-Cl bond turns the germanium atom into a center of chirality.As a result the two diastereotopic C-H protons of C 2 N 2 Si-moiety give rise to two doublets at δ = 5.87 and 6.03 ppm ( 1 J (H, H) = 3.92 Hz) in the 1 H NMR. Similarly, the two tBu groups show 1 H NMR resonances at δ = 1.29 and 1.65 ppm.Hindered rotation can be excluded as the explanation for the doubling of these resonances.Despite the increased congestion about the germanium center, the NHC retains the local rotational C 2 -symmetry in solution: the 1 H NMR shows only a single septet for the two CH moieties of the isopropyl groups.In contrast, there are again two signals for the adjacent diastereotopic methyl groups of the N-isopropyl moiety.In 13 C{ 1 H} NMR, the carbenic carbon shows a resonance at δ = 171.01ppm, which is similar to the chemical shifts observed for other NHC-coordinated Ge(II) compounds [18,19].In 29 Si{ 1 H} NMR, the singlet at δ = −3.39ppm is strongly highfield shifted compared with that of the free N-heterocyclic silylene (δ = +78.3ppm) [25].Notably, even repeated crystallization of 1 did not yield NMR spectra uncontaminated by residual 2, which led us to speculate about the reversibility of the oxidative Ge-Cl addition to the silylene.NMR at variable temperatures, however, did not show any temperature dependence of the sample composition.
Nonetheless, single crystals of 1 suitable for a X-ray diffraction study were obtained from saturated toluene solution at −20 • C after one day.Compound 1 crystallizes in the monoclinic P2 1 /c space group.However in the obtained single crystal X-ray diffraction data we did not see any residual electron density for the cocrystalization of 2 along with 1. Analysis of molecular structure determination revels the presence of α-chlorosilyl moiety adjacent to the NHC-coordinated germanium(II) center; which was anticipated from the solution state structure (Figure 1).The Si-Ge bond length is 2.4969 (7) Å which is close to reported Si(IV)-Ge(II) bond length [19].The distance between carbenic carbon and germanium(II) center is 2.081(2) Å, which is slightly shorter than that of the corresponding NHC-coordinated germanium(II)dichloride (2.106(3) Å) [26].
Inorganics 2018, 6, 6 3 of 6 compounds [18,19].In 29 Si{ 1 H} NMR, the singlet at δ = −3.39ppm is strongly highfield shifted compared with that of the free N-heterocyclic silylene (δ = +78.3ppm) [25].Notably, even repeated crystallization of 1 did not yield NMR spectra uncontaminated by residual 2, which led us to speculate about the reversibility of the oxidative Ge-Cl addition to the silylene.NMR at variable temperatures, however, did not show any temperature dependence of the sample composition.Nonetheless, single crystals of 1 suitable for a X-ray diffraction study were obtained from saturated toluene solution at −20 °C after one day.Compound 1 crystallizes in the monoclinic P21/c space group.However in the obtained single crystal X-ray diffraction data we did not see any residual electron density for the cocrystalization of 2 along with 1. Analysis of molecular structure determination revels the presence of α-chlorosilyl moiety adjacent to the NHC-coordinated germanium(II) center; which was anticipated from the solution state structure (Figure 1).The Si-Ge bond length is 2.4969 (7) Å which is close to reported Si(IV)-Ge(II) bond length [19].The distance between carbenic carbon and germanium(II) center is 2.081(2) Å, which is slightly shorter than that of the corresponding NHC-coordinated germanium(II)dichloride (2.106(3) [26].

General Information
All manipulation were carried out under an argon atmosphere using either a Schlenk line technique or inside a GloveBox.All solvents were dried by Innovative Technology solvent purification system.Compounds 2 [25] and 3 [26] were prepared according to literature procedures.Benzene-d6 was dried and distilled over potassium under argon.NMR spectra were recorded on a Bruker Avance III 300 MHz NanoBay NMR spectrometer (Bruker, Switzerland). 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{ 1 H}). 29Si{ 1 H} NMR spectra were referenced to external SiMe4.

General Information
All manipulation were carried out under an argon atmosphere using either a Schlenk line technique or inside a GloveBox.All solvents were dried by Innovative Technology solvent purification system.Compounds 2 [25] and 3 [26] were prepared according to literature procedures.Benzene-d6 was dried and distilled over potassium under argon.NMR spectra were recorded on a Bruker Avance III 300 MHz NanoBay NMR spectrometer (Bruker, Switzerland). 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{ 1 H}). 29Si{ 1 H} NMR spectra were referenced to external SiMe 4 .

X-ray Crystallographic Analysis
Single crystals of 1 were obtained from saturated toluene solution at −20 • C. Intensity data were collected on a Bruker SMART APEX CCD diffractometer (Bruker, Germany with a Mo Kα radiation (λ = 0.71073 Å) at T = 182(2) K.The structures were solved by a direct method (SHELXS [31]) and refined by a full-matrix least square method on F 2 for all reflections (SHELXL-2014 [32]).All hydrogen atoms were placed using AFIX instructions, while all other atoms were refined anisotropically.Crystallographic data (Supplementary Materials) were deposited at the Cambridge Crystallographic Data Center (CCDC; under reference number: CCDC-1587144) and can be obtained free of charge via https://www.ccdc.cam.ac.uk/structures/.X-ray crystallographic data for 1: M = 520.15,monoclinic,

Conclusions
We have demonstrated a proof of principle study for the straightforward incorporation of a α-chlorosilyl moiety adjacent to the donor-stabilized germanium(II) center.The resulting product features a 1,2-dicholoro functionality and should therefore in principle be suitable as precursor for the targeted synthesis of NHC-coordinated silagermenylidenes, the heavier analogues of vinylidenes.So far, attempts to eliminate the two chloro substituents reductively were not met with success.