Isolable Geminal Bisgermenolates: A New Synthon in Organometallic Chemistry

Abstract We have synthesized the first isolable geminal bisenolates L2K2Ge[(CO)R]2 (R=2,4,6‐trimethylphenyl (2 a,b), L=THF for (2 a) or [18]‐crown‐6 for (2 b)), a new synthon for the synthesis of organometallic reagents. The formation of these derivatives was confirmed by NMR spectroscopy and X‐ray crystallographic analysis. The UV/Vis spectra of these anions show three distinct bands, which were assigned by DFT calculations. The efficiency of 2 a,b to serve as new building block in macromolecular chemistry is demonstrated by the reactions with two different types of electrophiles (acid chlorides and alkyl halides). In all cases the salt metathesis reaction gave rise to novel Ge‐based photoinitiators in good yields.


Reaction of 1 with K o and [18]-crown-6 in benzene (Method C)
A flask was charged with 2.00 g potassium-tris(2,4,6-trimethylbenzoyl)germanide*0.5 DME (3.34 mmol; 1.00 eq.), 1.77 g [18]-crown-6 (6.68 mmol; 2.00 eq.) and 0.26 g potassium (6.68 mmol; 2.00 eq.). Subsequently 12 ml benzene were added. The suspension was stirred for 48 hours. The violet crystalline product was filtered off and washed with cold n-pentane and benzene. Yield: 1,61 g (1.65 mmol; 50 %) of analytically pure 2b as violet powder. Reaction of 2a with MeI 0.22 ml iodomethane (3.51 mmol; 2.10 eq.) was added to a solution of 2a at -70°C [prepared from 1.00 g (1.67 mmol; 1.00 eq.) 1 with 0,13 g (3.34 mmol; 2.00 eq.) potassium in 12ml THF]. Afterwards, the reaction mixture was brought to room temperature and stirred for another hour. The solution was added to 200 ml of saturated NH4Cl solution and ice. After phase separation, three-fold washing of the aqueous phase with 100 ml of Et2O, drying of the combined organic layers with Na2SO4 and evaporation of the solvents in vacuum, the product was purified via flash column chromatography (n-pentane/diethylether 3:1) to result a pale yellow crystalline solid. Yield: 0.42 g (1.06 mmol; 63 %) of analytically pure 7a. Reaction of 2a with EtBr 0.23 ml bromoethane (3.05 mmol; 2.10 eq.) was added to a solution of 2a at -70°C [prepared from 0.87 g (1.45 mmol; 1.00 eq.) 1 with 0,11 g (2.90 mmol; 2.00 eq.) potassium in 12ml THF]. Afterwards, the reaction mixture was brought to room temperature and stirred for another hour. The solution was added to 200 ml of saturated NH4Cl solution and ice. After phase separation, three-fold washing of the aqueous phase with 100 ml of Et2O, drying of the combined organic layers with Na2SO4 and evaporation of the solvents in vacuum, the product was recrystallized from acetone and isolated. Yield: 0.37 g (0.87 mmol; 60 %) of analytically pure 7b as pale yellow crystalline solid.

Synthesis of 7c
2.94 ml benzoyl chloride (25.05 mmol; 5.00 eq.) was added to a solution of 2a at -70°C [prepared from 3.00 g (5.01 mmol; 1.00 eq.) 1 with 0.39 g (10.01 mmol; 2.00 eq.) potassium in 15ml THF]. Afterwards, the reaction mixture was brought to room temperature and stirred for another hour. The solution was added to 200 ml of saturated NH4Cl solution and ice. After phase separation, three-fold washing of the aqueous phase with 100 ml of Et2O, drying of the combined organic layers with Na2SO4 and evaporation of the solvents in vacuum, the product was purified via flash column chromatography (toluene) to result a yellow oil. Yield: 1. Synthesis of 7d 0.74 ml 2-methylbenzoyl chloride (5.65 mmol; 2.10 eq.) was added to a solution of 2a at -70°C [prepared from 1.61 g (2.69 mmol; 1.00 eq.) 1 with 0.21 g (5.37 mmol; 2.00 eq.) potassium in 12ml THF]. Afterwards, the reaction mixture was brought to room temperature and stirred for another hour. The solution was added to 200 ml of saturated NH4Cl solution and ice. After phase separation, three-fold washing of the aqueous phase with 100 ml of Et2O, drying of the combined organic layers with Na2SO4 and evaporation of the solvents in vacuum, the product was purified via flash column chromatography (toluene/n-pentane 2:1) to result a yellow oil. Synthesis of 7e 1.47 g 1-adamantanecarbonyl chloride (7.01 mmol; 2.10 eq.) was added to a solution of 2a at -70°C [prepared from 2.00 g (3.34 mmol; 1.00 eq.) 1 with 0.26 g (6.68 mmol; 2.00 eq.) potassium in 12ml THF]. Afterwards, the reaction mixture was brought to room temperature and stirred for another hour. The solution was added to 200 ml of saturated NH4Cl solution and ice. After phase separation, three-fold washing of the aqueous phase with 100 ml of Et2O, drying of the combined organic layers with Na2SO4 and evaporation of the solvents in vacuum, the product was purified via flash column chromatography (toluene/pentane 2:1) to result a yellow crystalline solid. Yield: 0.97 g (1,40 mmol; 42 %) of analytically pure 7e. Synthesis of 7f 0.88 ml pivaloyl chloride (7.01 mmol; 2.10 eq.) was added to a solution of 2a at -70°C [prepared from 2.00 g (3.34 mmol; 1.00 eq.) 1 with 0.26 g (6.68 mmol; 2.00 eq.) potassium in 12ml THF]. Afterwards, the reaction mixture was brought to room temperature and stirred for another hour. The solution was added to 200 ml of saturated NH4Cl solution and ice. After phase separation, three-fold washing of the aqueous phase with 100 ml of Et2O, drying of the combined organic layers with Na2SO4 and evaporation of the solvents in vacuum, the product was purified via flash column chromatography (toluene/pentane 2:1) to result a yellow crystalline solid. Yield Synthesis of 7g 0.40 ml 1,4-dibromobutane (3.34 mmol; 1.00 eq.) was added to a solution of 2a at -70°C [prepared from 2.00 g (3.34 mmol; 1.00 eq.) 1 with 0.26 g (6.68 mmol; 2.00 eq.) potassium in 12ml THF]. Afterwards, the reaction mixture was brought to room temperature and stirred for another hour. The solution was added to 200 ml of saturated NH4Cl solution and ice. After phase separation, three-fold washing of the aqueous phase with 100 ml of Et2O, drying of the combined organic layers with Na2SO4 and evaporation of the solvents in vacuum, the product was purified via flash column chromatography (toluene/pentane 2:1) to result a yellow crystalline solid. Yield: 0.51 g (1,21 mmol; 36 %) of analytically pure 7g.

DFT Calculations
The calculations of the optical absorption spectra are based on the crystal structure of 2b including the two crown ethers and two potassium counter ions. Density functional theory was applied using the B3LYP [4] functional supplemented by Grimme's dispersion correction with Becke-Johnson damping D3BJ [5] (B3LYP-D3BJ), and the def2-SVP [6] basis set was used. For the simulation of the absorption spectrum, 30 vertical excitations were calculated by timedependent DFT (TD-DFT) using the def2-TZVPP basis set. [7] The UV/Vis spectrum was simulated by a Lorentzian broadening with FWHH of 3000 cm -1 (the same value as the first peak in the experimental spectrum) using the orca_asa program. [8] The solvent THF was modelled by the conductor-like polarizable continuum model (CPCM). [9] The program ORCA4.2.1 was used for all simulations. Figure S1. Crystal structure RF1873.pdb (left) and optimized DFT structure of 2b (right). The violet dot represents the K + counter-ion. Figure S2. Relevant orbitals for the three most intense vertical excitations (S1, S9, S10). Contour values of 0.025 a.u. were chosen.

S7
DFT geometry optimization retends the X-ray structure with a 10 degree shift of the mesityl groups relative to the K + -Ge-K + axis ( Figure S1), see Table S1 for a comparison of geometry data. The calculated absorption spectrum agrees well with the experimental one (see Figure 2 in the manuscript). The relevant transitions are listed in Table S2. The orbitals of the HOMO and LUMO are shown in Figure 2 in the manuscript, the other relevant orbitals for the intense bands are depicted in Figure S2. Table S1. Relevant geometry parameters (distances d, angles α, dihedral angles γ) for compound 2b: DFT optimized geometry (see Table S3

Mechanistic Investigations
Cw-EPR measurements were performed with a Bruker EMX X-band EPR spectrometer (100 kHz field modulation) equipped with a variable-temperature unit (Eurotherm B-VT 2000). Typical conditions for the acquisition of the EPR spectra were 2 mW microwave power and 0.1 mT field modulation. The public domain program WinSim (in the versions 0.96 and 0.98) [10] was used to analyze and simulate the spectra.
A custom-made three-compartment EPR sample tube connected to a high-vacuum line was used for the preparation of a sample of compound 6. A potassium metal mirror was sublimated to the wall of the tube, and the investigated compound was dissolved in freshly condensed THF (ca. 0.4 mL; stored over Na/K alloy). Afterward, the sample was degassed by three freeze−pump−thaw cycles and sealed under high vacuum. The reduction was performed by bringing the THF solution of 6 in contact with the K metal mirror in the evacuated sample tube.
UV−Vis spectra were acquired on a UV−Vis spectrometer equipped with optical fibers and a 1024-pixel diode-array detector (J&M Analytik AG, Essingen, Germany). The spectra were measured through the EPR capillary of the sample tube, which was fixed in a 1x1 cm quartz cuvette using 3D printed holders. The cuvette was filled with deionized water to minimize reflections. As a reference a similar cell filled with THF was used. Figure S2. Left: UV-Vis spectra of 6 and reduction products in ultra-dry THF (inert conditions) in an EPR tube. Right: EPR spectra at the beginning (first contact; g = 2.0052 ± 0.0005) at the end of the experiment (prolonged contact; g = 2.0047 ± 0.0005) at T = 204 K.
Before performing the reductions, a UV-Vis spectrum of 6 in ultra-dry THF, which was prepared in the special three-compartment EPR cell, was acquired (black curve in Figure S2). Afterwards the solution was repeatedly brought in close contact with the potassium mirror. After each contact, UV-Vis and EPR spectra were acquired (see additional curves in Figure S2 and EPR spectra on the right side of the graph).
The UV-Vis spectra clearly show that at first a new peak is formed at about 425 nm (marked with an asterisk) indicating the formation of the germenolate 1. [11] At the same time, the EPR spectrum depicted in the right left corner is obtained. Further contact at the K metal mirror leads S12 to the formation of a new peak at about 550 nm, which is attributed to the dianion 2a (marked with I). During the experiment, the colour of the solution changed from yellowish (i.e. the colour of 6) over orange to reddish brown (germenolate 1) to violet (dianion 2a). The EPR spectrum of the solution also substantially changed upon prolonged contact.
In both cases, the EPR spectra are a superposition of at least two different species making an interpretation very tedious. A part of the first spectrum could be simulated with the parameters shown in Table S2. The magnitude of the hyperfine coupling constants are comparable to the ones obtained for the radical anion of mesitoyl-substituted phosphorus-based photoinitiator BAPO, where the spin density is delocalized over two the mesitoyl groups (see Table for hyperfine coupling constants). [12] Figure S3. EPR spectrum (black curve) after the first contact of a solution of 6 in THF with K metal mirror and simulation in red. Table S2. Hyperfine coupling constants for the EPR spectrum shown in Figure S3 and comparison to the BAPO radical anion.

│a H p│ / mT │a H o│/ mT │a H m│/ mT species 1
0.044 (6 H) 0.0315 (12 H) 0.009 (4 H) BAPO radical anion [12] 0.060 (6 H) 0.039 (12 H) 0.012 (4 H) Based on these observations, the EPR spectrum might be assigned to a radical anion in which the spin is evenly distributed among two mesitoyl units. This radical might be directly derived from the tetramesitoylgermane or might be the radical anion of the mesitoyl-substituted derivative of benzil (mesityl, ((MesC=O)2), which is expected to be formed as by-product of the germa-acyloin condensation. In Figure S4 the rather well resolved EPR spectrum of the reaction mixture containing 1 and potassium as reducing agent in presence of [18]-crown-6 is shown. Quite interestingly, an almost identical spectrum was also described by us in a previous publication [11] , where we tentatively attributed it to a follow-up product of 1 and the tert-butoxy radical derived from KOtBu. Since no KOtBu was added to reaction mixture, the radical must be derived from tetramesitoylgermane.

CIDNP Experiments
CIDNP (chemically induced dynamic nuclear polarization) NMR experiments were carried out on a 200 MHz Bruker AVANCE DPX spectrometer equipped by a custom-made CIDNP probe head. A Quantel Nd-YAG Brilliant B laser (355 nm, ∼50 mJ per pulse, pulse length 8-10 ns) operating at 20 Hz was employed as the light source. The pulse sequence of the experiment consists of a series of 180° radio-frequency (RF) pulses to suppress the NMR signals of the parent compounds, the laser flash, the 90° RF detection pulse and the acquisition of the free induction decay (FID). "Dummy" CIDNP spectra employing the same pulse sequence but without the laser pulse were always measured. Samples were prepared in toluene-d8 and deoxygenated by bubbling with nitrogen before the experiment. Chemical shifts (δ) are reported in ppm relative to tetramethylsilane (TMS) using the residual methyl signal of deuterated acetonitrile as an internal reference (δH = 1.94 ppm). If necessary, line broadening (1 Hz, exponential) was applied to the spectra.

Results:
Investigated compounds Figure S5. Chemical structures and abbreviations of the investigated compounds In the Figure S6, 1 H NMR spectrum together with CIDNP spectrum of 7g in the presence of butyl acrylate is shown in deuterated acetonitrile. The signal of mesitaldehyde (MH, δ = 10.54 ppm) is visible. Signals enclosed in the green rectangle can be assigned to the α-H proton of the radical reaction product 7g with coupling constants 2 J = 11.1 Hz and 3 J = 6.0 Hz. Signal corresponds to well resolved doublet of doublets. Situation is slightly different in toluene-d8 as a solvent ( Figure S7). In this case the second order effects influence the coupling of α-H proton (marked in green) shifting its apparent coupling pattern towards that of a triplet resulting in a coupling constant of 3 J = 9.1 Hz and 3 J = 6.0 Hz. The signal of MH is visible at δ = 10.37 ppm.
In both solvent, toluene-d8 and acetonitrile-d3, signals corresponding to the β-H protons of 7g product, were not assigned though to the substantial overlap with other signals in aliphatic region. S15 Figure S6. 1   In the Figure S8. and S9. 1 H NMR spectrum and CIDNP spectrum of 7e in the presence of butyl acrylate is shown using acetonitrile-d3 and toluene-d8 as solvents respectively. In both cases signal corresponding to MH is visible with shifts of δ = 10.53 ppm in case of acetonitrile-d3 and δ = 10.37 ppm in case of toluene-d8. Coupling of the α-H proton (marked in green, with the corresponding CIDNP signals in green rectangle) is presented in both solvents. Similar situation as in the case of the 7g derivative is observed. In acetonitrile-d3 as solvent doublet of doublets is observed with coupling constants 2 J = 16.2 Hz and 2 J = 6.0 Hz. Whereas in toluene-d8 corresponding signals shift towards the apparent triplet shape though the second order effects with coupling constant of 3 J = 7 Hz. Once again signals corresponding to the β-H protons of 7e product, could not be unambiguously assigned.

MAS NMR Experiments
NMR spectra under magic angle spinning (MAS) conditions were recorded with a 500 MHz Avance spectrometer (Bruker,11.7 T To increase the sensitivity for 13 C spectra and eliminate broadening from chemical shift anisotropy and dipolar coupling, [13] additionally to spinning the samples at 25 kHz in the magic angle (54.74° with respect to the main magnetic field), we performed polarization transfer experiments at 30°C. The contact time was set to 7 ms and we accumulated 8192 scans for the spectra with a recycle delay of 3 s. The 13 C spectra were referenced to the signal of adamanthane (upfield signal at 38.48 ppm from trimethylsilane). All C-atoms of compound 2a and 2b could be identified via MAS NMR. For the carbonyl-C the weak dipolar coupling to 1 H-nuclei and thereby hindered polarization transfer results in a small signal intensity.

X-ray Crystallography
For single crystal X-ray diffractometry suitable crystals were covered with a layer of silicone oil. Under a microscope a single crystal was selected, mounted on a glass rod on a copper pin, and placed in the cold N2 stream provided by an Oxford Cryosystems cryometer (T=100 K). XRD data collection was performed on a Bruker APEX II [14] diffractometer with use of Mo Kα radiation (λ= 0.71073 Å) from an IµS microsource and an APEX II CCD area detector. Data integration was carried out using SAINT. [14] Empirical absorption corrections were applied using SADABS. [15] The structure was solved by the dual space algorithm implemented in SHELXT. [16] Fourier analysis and refinement were performed by the full-matrix least-squares methods based on F 2 implemented in SHELXL [17] as implemented in SHELXLE. [18] The space group assignments and structural solutions were checked and evaluated using PLATON. [19] All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were placed in calculated positions corresponding to standard bond lengths and angles using riding models. CIF files were edited, validated and formatted with the programs encifer [20] and Olex2. [21] Structural plots and figures were generated with MERCURY. [22] Details on refinements of 2b, 7b, 7f and 7g: 2b cocrystallizes in the orthorhombic space group Pnma with two molecules of benzene per dianionic fragment. 7b crystallizes in the orthorhombic space group Pbca. 7f crystallizes in the monoclinic space group C2/c. 7g crystallizes in the triclinic space group P-1. Details concerning data collection and refinement are provided in Crystallographic Table. Although several samples were tested and recrystallization from other solvents was attempted, all crystals examined proved to be twinned. For integrations SAINT [14] was used and the data set was afterwards treated with TWINABS. [23] The structure was refined as a two-component twin against data in hkl5 format where the batch scale factor refined to a 51:49 ratio (twin law as determined by CELL_NOW [24] : 0.938 0.123 -0.047 / 0.976 -0.938 -0.025 / -0.05 0.04 -1.00)).