Complexes of Dichlorogermylene with Phosphine/Sulfoxide-Supported Carbone as Ligand †

Due to their remarkable electronic features, recent years have witnessed the emergence of carbones L2C, which consist in two donating L ligands coordinating a central carbon atom bearing two lone pairs. In this context, the phosphine/sulfoxide-supported carbone 4 exhibits a strong nucleophilic character, and here, we describe its ability to coordinate dichlorogermylene. Two original stable coordination complexes were obtained and fully characterized in solution and in the solid state by NMR spectroscopy and X-ray diffraction analysis, respectively. At 60 °C, in the presence of 4, the Ge(II)-complex 5 undergoes a slow isomerization that transforms the bis-ylide ligand into an yldiide.


Introduction
The rapid development of homogeneous catalysis in the last decades is highly related to the intensive research that was accomplished toward ligand design. Due to their lone pair(s) and their related nucleophilic character, oxygen-, nitrogen-, sulfur-and phosphoruscontaining ligands have been dominating in the field for several decades [1]. In the late 1980s, the discovery of the first stable carbenes represented the milestone leading to the emergence of carbon-based ligands (I, Figure 1) [2,3]. Indeed, the development of these molecules, containing a divalent C(II) atom bearing a vacant orbital and a lone pair, exhibiting a high σ-donation and a strong binding ability toward transition metals, render them essential tools for catalysis. The corresponding organometallic complexes have been proven to be particularly robust and efficient and offering in numerous catalytic processes a larger scope of reaction [4][5][6][7]. The related carbon(0) species (II), also named carbones, bearing two lone pairs on the central carbon atom are a new emerging class of η 1 -carbon ligands. Even though carbodiphosphoranes (II, L, L = PPh 3 ) were discovered in the 1960s by Ramirez [8], these species were at first regarded as two cumulated phosphorus ylide functions on a central carbon atom. It was only in 2006, after the theoretical investigations by Frenking et al. [9][10][11][12], that these molecules were considered as a carbon atom in the zerooxidation state stabilized by two L-phosphine ligands, in agreement with the description initially used by Kaska in 1973 [13]. Since then, this family of ligands has considerably grown, leading to a large structural diversity of carbones II and a better understanding of their behavior [14][15][16][17][18][19][20][21][22][23]. Naturally, owing to the existence of two lone pairs, they are strong σand π-donors (two-or four-electron donating ability); they have been used as original ligands for the preparation of organometallic complexes with interesting applications in catalysis [24][25][26][27][28][29]. The strong donor ability of carbones II also enables the synthesis and isolation of novel reactive species. For example, Alcarazo et al. took advantage of the two available lone pairs of carbodiphosphoranes to stabilize reactive molecules such as dihydrido borenium cation III [30]. In the same vein, several groups have used carbones or strong σ-donating ligands to prepare dichlorogermylene adducts giving access to germyliumylidenes (IV) or germylones [31][32][33][34][35][36][37][38]. In this context, we report here the coordination ability of a phosphine/sulfoxide-supported carbone 4 [39] towards dichlorogermylene.

Synthesis
For the preparation of the phosphine/sulfoxide-supported carbone 4, we followed the previously described synthesis [39] but several practical aspects were improved. Indeed, after the complete oxidation of methyldiphenylsulfonium, acidification and filtration of the precipitates (carboxylic acids), the expected methyldiphenylsulfoxonium salt 1 was extracted from the aqueous solution by liquid/liquid extraction using dichloromethane as a solvent (Scheme 1). This extraction avoids the possible thermal degradation of the sulfoxonium salt during the evaporation of water under reduced pressure (if prolonged heating above 50 • C is performed). The yield of this step was improved to 53%, after two successive recrystallizations. The coupling reaction between sulfoxonium salt 1 and chlorophosphonium 2 in the presence of two equivalents of lithium diisopropylamide (LDA) was also improved in terms of reaction time (Scheme 1). It was found that heating the reaction mixture up to 60 • C considerably sped up the reaction since a full conversion was reached in 24 h instead of 96 h at room temperature. Protonated precursor 3 was obtained as a white powder upon concentration (70% yield). The final deprotonation was performed in THF solution at RT with potassium hydride (KH) leading to the selective formation of 4, which was isolated in 69% yield.

Dichlorogermylene Coordination
Previous experimental results and DFT calculations have already established that phosphine/sulfoxide-supported carbone 4 exhibits a strong nucleophilic character [39]. The potential usefulness of 4 as a ligand towards transition metals was demonstrated by selective reactions with several organometallic complexes [Au(I), Rh(I)] [39]. Therefore, its coordinating ability toward GeCl 2 /dioxane should be an interesting approach to access original low-valent germanium derivatives.
Ligand 4 reacts immediately with one equivalent of GeCl 2 dioxane leading to the selective formation of complex 5 which has been isolated as colorless crystals from a saturated solution of CH 2 Cl 2 /pentane (yield 80%, Scheme 2). In comparison with the hexaphenylcarbodiphosphorane analogue [33], complex 5 exhibits a good solubility in common organic solvent and could be fully characterized by NMR spectroscopy. The 31 P NMR spectrum displays a signal at lower field (δ = 49.5 ppm) compared to that of free ligand 4 (δ = 29.0 ppm), reminiscent of the protonated precursor 3 (δ = 45.0 ppm). In the 13 C NMR spectrum, the central carbon atom appears as a doublet at δ = 47.0 ppm (J PC = 77.3 Hz). The addition of a second equivalent of GeCl 2 ·dioxane to 5, or the direct use of two equivalents of GeCl 2 ·dioxane with 4, leads to the quantitative formation of bisgermylene complex 6, which has been isolated in the crystalline form from C 6 D 6 solution (yield 75%). It has been fully characterized by NMR spectroscopy, and particularly, the 31 P NMR spectrum indicates a signal at δ = 51.2 ppm and the central carbon atom appears in 13 C NMR spectrum as a doublet at δ = 46.6 ppm (J PC = 79.1 Hz).

Scheme 2.
Coordination of phosphine/sulfoxide-supported carbone 4 with one or two equivalents of germylene dichloride. Isolated in the crystalline form, the molecular structures of complexes 5 and 6 were confirmed by X-ray diffraction analysis ( Figure 2, see Supplementary Materials). The selected geometrical parameters for experimental structures can be found in Table 1.
Contrary to the carbodiphosphorane analogues, chloride abstraction from 5 using AlCl 3 or KB(C 6 F 5 ) 4 in order to prepare germyliumylidene derivatives only led to complex mixtures. Nevertheless, when 5 is treated in the presence of one equivalent of 4 at 60 • C for 60 h, we observed the gradual consumption of 5 with the concomitant formation of a new compound that exhibits a signal at δ = 56.3 ppm in 31 P NMR (this reaction does not occur in the absence of 4) (Scheme 3). In fact, the process is base-catalyzed, but with 10 mol % of 4, the reaction time is slower and needs 90 h [Note: catalytic amounts (15 mol %) of alternative Lewis bases such as DMAP or Et 3 N can be used but the reactions are less selective, see Supplementary Materials for more details]. The structure of the new product 7, determined by X-ray diffraction analysis (Figure 3), involves the 1,3-migration of a phenyl group from the sulfur to the germanium atom. Considering the ligand moiety, this isomerization transforms a bis-ylide (carbone) to an original yldiide [44]. Unfortunately, because of the presence of 4 in the media, product 7 could not be isolated in pure form for complete characterization despite several attempts.   (4), O1-S1-C1 112.64(15), O1-S1-C16 105.28 (16), C1-S1-C16 100.04(16).

Conclusions
In summary, the excellent coordination ability of the phosphine/sulfoxide-supported carbone ligand 4 allows the preparation of Ge(II)-complex 5. The strong donation of 4 results in an enriched germylene 5, which becomes sufficiently nucleophilic to coordinate a second GeCl 2 unit. The two original and stable coordination complexes 5 and 6 were fully characterized by NMR spectroscopy and X-ray diffraction analysis. Interestingly, the Ge(II)-complex 5 shows an original isomerization in the presence of 4 that transforms the bis-ylide ligand into an yldiide, thanks to a phenyl migration. Efforts are currently underway to extend the diversity of organometallic complexes that can be obtained with 4 to consider its application in catalysis.

General Comments
All manipulations were performed under an inert atmosphere of argon by using standard Schlenk techniques or high-pressure NMR tube techniques. Dry and oxygen-free solvents were used. 1 H, 13 C, 19 F and 31 P NMR spectra were recorded on Brucker Avance II 300 MHz, Avance III HD 400 MHz and Avance I and II 500 MHz spectrometers (Brucker, Karlsruhe, Germany). Chemical shifts are expressed in parts per million with residual solvent signals as internal reference ( 1 H and 13 C{ 1 H}). 19 F and 31 P NMR chemical shifts were reported in ppm relative to CFCl 3 and 85% H 3 PO 4 , respectively. The following abbreviations and their combinations are used: br, broad; s, singlet; d, doublet; t, triplet; q, quartet; hept, heptuplet; m, multiplet. 1 H and 13 C resonance signals were attributed by means of 2D COSY, HSQC and HMBC experiments. Mass spectra were recorded on a Hewlett Packard 5989A spectrometer (Hewlett-Packard, Palo Alto, CA, USA). All commercially available reagents were used without further purification otherwise noted. Preparation of diphenylsulfonium, 2 and 4 were prepared following previously reported procedures [39].

X-ray Data
The data of the structures for 5, 6 and 7 were collected at 193 K on a Bruker-AXS APEX II CCD Quazar diffractometer (7) equipped with a 30 W air-cooled microfocus source or on a Brucker-AXS D8-Venture diffractometer (5 and 6) equipped with a CMOS area detector with MoKα radiation (wavelength = 0.71073 Å) by using phi-and omega-scans. The data were integrated with SAINT, and an empirical absorption correction with SADABS was applied [45]. The structures were solved using an intrinsic phasing method (ShelXT) [46] and refined using the least-squares method on F 2 (ShelXL-2014) [47]. All non-H atoms were treated anisotropically. All H atoms attached to C atoms were fixed geometrically and treated as riding on their parent atoms with C-H = 0.95 Å (aromatic), 0.98 Å (CH 3 ), 0.99 Å (CH 2 ) or 1.0 Å (CH) with U iso (H) = 1.2U eq (CH, CH 2 ) or U iso (H) = 1.5U eq (CH 3 ).