From molecular germanates to microporous Ge@C via twin polymerization.

Four molecular germanates based on salicyl alcoholates, bis(dimethylammonium) tris[2-(oxidomethyl)phenolate(2-)]germanate (1), bis(dimethylammonium) tris[4-methyl-2-(oxidomethyl)phenolate(2-)]germanate (2), bis(dimethylammonium) tris[4-bromo-2-(oxidomethyl)phenolate(2-)]germanate (3) and dimethylammonium bis[2-tert-butyl-4-methyl-6-(oxidomethyl)phenolate(2-)][2-tert-butyl-4-methyl-6-(hydroxymethyl)phenolate(1-)]germanate (4), were synthesized and characterized including single crystal X-ray diffraction analysis. In the solid state, compounds 1 and 2 exhibit one-dimensional hydrogen bonded networks, whereas compound 4 forms separate ion pairs, which are connected by hydrogen bonds between the dimethylammonium and the germanate moieties. The potential of these compounds for thermally induced twin polymerization (TP) was studied. Germanate 1 was converted by TP to give a hybrid material (HM-1) composed of phenolic resin and germanium dioxide. Subsequent reduction with hydrogen provided a microporous composite containing crystalline germanium and carbon (Ge@C -C-1, germanium content ∼20%). Studies on C-1 as an anode material for Li-ion batteries revealed reversible capacities of ∼370 mA h gGe@C(-1) at a current density up to 1384 mA g(-1) without apparent fading for 500 cycles.


Introduction
The concept of twin polymerization (TP), which is defined as a concerted formation of two polymers in one synthetic step starting from a single monomer, 1 provides a convenient approach for the synthesis of organic-inorganic hybrid materials (HM) such as polyfurfuryl alcohol/MO x (MO 2 = Si, 2,3 Sn, 4 Ti, 5,6 Zr and Hf, 7 MO 3 = WO 3   8   ), poly(thiophene-2-methanol)/MO 2 (M = Sn, 4 and Ti 5 ), phenolic resin/MO x (MO 2 = Si, 9,10 Sn, 4 Zr and Hf, 7 MO 3 = WO 3 , 8 MO x = MO 2 /SiO 2 with M = Sn, 11 Zr and Hf 7 ), since it was reported in 2007 for the first time. 2With regard to homo-geneity of the materials, salicylic alcoholates of silicon were shown to be ideal precursors to provide nanostructured interpenetrating networks of the organic and inorganic components. 9,10Such hybrid materials can be converted into highly porous carbon and/or metal oxide based materials depending on the reaction conditions.For instance, reduction of a tin-containing HM with hydrogen gave a porous Sn@C/SiO 2 material 11 accounting for the suitability of twin polymerization for the synthesis of metal containing porous carbon materials.Moreover, microporous carbon exhibiting BET surface areas up to 1260 m 2 g −1 are accessible by reduction of a material as obtained by polymerization of spirocyclic salicyl alcoholates of silicon and subsequent removal of SiO 2 . 9,10With this in mind, we anticipated that extension of the concept of TP to spirocyclic salicyl alcoholates of germanium may result in hybrid materials that can be converted into highly porous germanium-containing carbon materials (Ge@C).6][17] However, our attempts to synthesize spirocyclic salicyl alcoholates of germanium starting from germanium alkoxides and GeCl 4 failed. 18Similarly, reactions of Ge(NMe 2 ) 4 with salicyl alcohols did not result in the germanium alcoholates, but gave the germanates 1-4 (Scheme 1).
These dimethylammonium germanates are the first representatives of anionic molecular precursors, which are potentially suitable for twin polymerization.Hence, in addition to their characterization we studied the reactivity of these compounds.Exemplarily, the as-prepared hybrid material as obtained from 1 was converted into microporous Ge@C following the synthetic concept as given in Scheme 2. In order to provide a proof of principle, the Ge@C composite was tested as anode material for rechargeable LIBs.

Syntheses and characterization
The germanates 1-4 were synthesized starting from Ge(NMe 2 ) 4 and the respective salicyl alcohol with yields in the range of 64%-91% (Scheme 1).Good solubility in polar organic solvents was observed for the germanates 1, 2 and 4. Compound 4 is additionally soluble in nonpolar solvents such as n-hexane, whereas 3 is hardly soluble in polar organic solvents such as THF.The compounds were fully characterized including single crystal X-ray diffraction analysis for 1, 2 and 4. 1 H NMR, 1 H- 13 C 19 HSQC and 13 C{ 1 H} NMR spectroscopic analyses of the germanates 1 and 2 in CDCl 3 gave two sets of resonance signals, respectively, at ambient temperature (Fig. S1 and S2 †) that are assigned to the compounds 1 and 2 and species possessing a pentacoordinated anion.Temperaturedependent 1 H NMR spectroscopy exemplarily carried out for bis(dimethylammonium) tris [2-(oxidomethyl)phenolate(2-)] germanate (1) revealed an equilibrium between the two species with 1 being favored at lower temperatures (Fig. S3 †).If the amount of HNMe 2 is varied these equilibriums between the hexacoordinated dianion and the pentacoordinated anion of the germanates will be shifted accordingly as was shown by 1 H NMR spectroscopy experiments exemplarily carried out for germanate 1 in CDCl 3 solution (Fig. S4 †).Thus, the hexacoordinated compounds 1 and 2 show a dynamic behavior in solution to form an equilibrium with pentacoordinated species, respectively, in solution as illustrated in eqn (1).We propose that the pentacoordinated anions feature a similar structure (compounds 1a and 2a) to that of compound 4 (Scheme 1) in solution and in the solid state.However, isomers that exhibit bonding via the phenolic oxygen atom rather than via the benzylic oxygen atom cannot be completely ruled out on the basis of the NMR data.Notably, addition of a proton source to 1 and 2 induces decomposition rather than abstraction of a salicyl alcohol to give the neutral germanium alkoxide.
The 1 H NMR, 1 H-13 C{ 1 H} HSQC and 13 C{ 1 H} NMR spectra of compound 3 were recorded in [D 8 ]THF solution due to its poor solubility in other solvents.Two sets of resonance signals that are assigned to compound 3 and a pentacoordinated species (compound 3a) were observed in the 1 H NMR spectrum at ambient temperature (Fig. S5 †).In contrast to 1 and 2, compound 3a is assumed to be the dominant species in THF solution of 3 as indicated by the intensities of the resonance signals (Fig. S5 †).One set of resonance signals was observed in the 1 H NMR, 1 H-13 C{ 1 H} HSQC and 13 C{ 1 H} NMR spectra, respectively, of germanate 4 (Fig. S6 †).The multiplicity and the integral ratios of the resonance signals are in agreement with the structural motif of a pentacoordinated species as depicted in Scheme 1.
In the solid state, broad absorption bands assigned to ν N-H (2718 and 2462 cm −1 for 1, 2732, 2462 and 2361 cm −1 for 2, 2736 and 2448 cm −1 for 3 and 2460 cm −1 for 4) vibrational modes were determined by attenuated total reflection (ATR) FT-IR spectroscopy for all germanates, whereas compound 4 gave an additional absorption band maximum at 3268 cm −1 , which was assigned to a ν O-H vibration (Fig. S7 †).
Single crystals suitable for X-ray diffraction analysis were obtained from saturated CH 2 Cl 2 (1 and 2) and n-hexane (4) solutions by slow evaporation of the volatile solvents at ambient temperature.The compounds 1 and 2 show similar molecular structures of their dianions and their hydrogen bonded networks interconnecting the germanate dianions by dimethylammonium cations, whereas 4 exhibits a different structural motif of its anion in the solid state.Therefore, only the molecular structures of the germanates 1 and 4 are discussed.Details of the crystal structure of compound 2 (Fig. S8 and S9) are given in the ESI † and a summary of crystallographic data are presented in Table 1.Compound 1 possesses two crystallographically independent germanate dianions exhibiting Δ-{Δ-[Ge1(Sal) 3 ] 2− with Sal = salicyl alcoholate(2-)} (Fig. 1) or Λ-configuration {Λ-[Ge2(Sal) 3 ] 2− } in the solid state (Fig. S10 †).Selected bond lengths and bond angles of the compounds 1 and 2 are given in Tables S1-S3.† The germanium atoms of the dianions of 1 are hexacoordinate by the oxygen atoms of the three salicyl alcoholate moieties with benzylic and phenolic oxygen atoms being opposite to each other to give a slightly distorted octahedral coordination geometry in both cases.The germanium oxygen bond lengths vary in the range from 1.879( 2 22 and the germanium enterobactin 23 Four dimethylammonium cations bind by hydrogen bonds of moderate strength 24 to the germanate dianion to give a one-dimensional, infinite hydrogen bonded network (Fig. S10 and Table S2 †).
In contrast to the germanates 1 and 2, a molecular ion pair of dimethylammonium cations and germanate anions was determined for 4 in the solid state due to presence of the sterically demanding tert-butyl group in ortho position of the salicyl alcoholate moieties.The molecular structure of 4 is depicted in Fig. 2 and selected bond lengths and bond angles are presented in Table S1.† The germanium atom of compound 4 is pentacoordinated showing a distorted trigonal bipyramidal coordination sphere     (green) 20 hydrogen bonding motifs are depicted as dashed lines.
hydrate and meso-[1,4-piperaziniumdiylbis(methylene)]bis{bis- 25,26 The dimethylammonium cation and the germanate anion are bridged by hydrogen bonding [N1-O1 2.809(2) Å with N1-H1C-O1 being 159°and N1-O5 2.7718(18) Å with N1-H1D-O5 being 176°] of moderate strength 24 showing the R 2 2 (6) motif. 20Moreover, an intramolecular hydrogen bond [O4-O6 2.826(2) Å with O4-H6-O6 being 163°] bridges the phenolic hydroxido group (O6) and the phenolic oxygen atom (O4) at axial position.Note that a phenol group rather than a benzyl alcohol group of the single coordinating salicyl alcoholate moiety is present in germanate 4, which is remarkable, because the much higher acidity of phenol groups ( pK s ≈ 10) in comparison with aliphatic hydroxyl groups ( pK s ≈ 15) makes the phenol more reactive. 27owever, an intramolecular hydrogen bridge of the phenol group is formed, which stabilizes the observed isomer.As a result of this hydrogen bond bridge and the steric hindrance that is caused by the tert-butyl group, the determined structure is altogether energetically favored over the formation of the alternative isomer with the phenol group deprotonated and coordinated to the germanium atom, and exhibiting a hydrogen bond bridge of an aliphatic hydroxyl group.

Twin polymerization of germanate 1
Thermally induced TP of germanate 1 results in the formation of a hybrid material (HM) composed of phenolic resin/GeO 2 as illustrated in eqn (2) given for an idealized conversion of the starting material.
Bulk phase TP of compound 1 was carried out under inert atmosphere at 200 °C and gave an amorphous phenolic resin/ GeO 2 hybrid material (HM-1) with a yield of 67% after the work-up procedure.The polymerization temperature was chosen according to the results of the differential scanning calorimetry (DSC) measurements (Fig. S11 †).The germanates 1-3 show complex thermochemical behaviors upon heating exhibiting dominant exothermic processes with onset temperatures of 173 °C (1), 174 °C (2) and 180 °C (3), which are assigned to their polymerization.The onset temperatures are similar to those onset temperatures that were reported to initiate TP for the structurally related spirocyclic silicon salicyl alcoholates such as 4H,4′H-2,2′,spiro[benzo[d][1,3,2]dioxasiline] (196 °C). 9 Contrastingly to the latter class of silicon compounds, the germanates 1-3 do not melt.It is noteworthy that 2-[(dimethylamino)methyl]phenol is formed as a volatile byproduct of the TP process of 1 being in accordance with the experimentally determined higher weight loss of ca.33% (bulk phase experiment: 33% and thermogravimetric analysis (TGA): 34% -Fig.S12 †) as compared to the theoretically maximum weight loss of 20.7% due to the formation of HNMe 2 and water as indicated by the idealized eqn (2).HM-1 was characterized by 13 C{ 1 H} cross polarization magic angle spinning (CP-MAS) NMR and ATR-FT-IR spectroscopy, powder X-ray diffraction (PXRD), CHN analysis and energy-dispersive X-ray (EDX) spectroscopy.The 13 C{ 1 H} CP-MAS NMR spectrum of HM-1 is depicted in Fig. 3.
All expected resonance signals for a phenolic resin with typical chemical shifts as reported for hybrid materials obtained by thermally induced TP e.g., of spirocyclic silicon salicyl alcoholates 9 were observed for HM-1.The intensive signals with chemical shifts centered at δ = 36 ppm, δ = 130 ppm and δ = 153 ppm show relatively small widths at half height.The latter and the chemical shifts that were determined for the bridging methylene groups i.e., centered at δ = 36 ppm (ortho/ortho′ connectivity centered at δ = 30 ppm and ortho/para′ connectivity centered at δ = 35 ppm [28][29][30] ), indicate that the phenolic resin possesses a prevailing ortho/para′ connectivity of the bridging methylene groups.However, the signal of lower intensity centered at δ = 120 ppm (unsubstituted carbon atoms in para position centered at δ = 120 ppm [28][29][30] ) most likely originates from the presence of a minor secondary ortho/ortho′ connectivity pattern of the bridging methylene groups in HM-1.The broad signal of low intensity centered at δ = 43 ppm is assigned to a small portion of terminating CH 2 NMe 2 groups that presumably result from incorporation of 2-[(dimethylamino)methyl]phenol, which is formed during the polymerization process as a byproduct.
contents of 53.8%, 4.9% and 2.6%, respectively, that differ from the expected values (C, 59.6% and H, 4.3%) as calculated for the idealized composition based on eqn (2).However, the latter is in agreement with the formation of 2-[(dimethylamino)methyl]phenol as byproduct and its partial incorporation into the HM during the polymerization process.This is further supported by EDX analysis of HM-1 [N, (5.0 ± 1.7)%].

Synthesis and characterization of the porous materials
Conversion of HM-1 under oxidative conditions gave crystalline hexagonal GeO 2 exhibiting an adsorption isotherm assigned to type II (Fig. S14 †) with a BET surface area of 27 m 2 g −1 , which is indicative for the formation of non-porous GeO 2 .A microporous material consisting of germanium and carbon (Ge@C material C-1) was obtained by conversion of HM-1 under reducing conditions (Ar/H 2 95/5) at 800 °C (Scheme 2).Analysis of the adsorption isotherm, which is assigned to a type I isotherm (Fig. S14 †) using a QSDFT model for slit and cylindrical pores, revealed a micropore content of 61% with a BET surface area of 470 m 2 g −1 .A CHN analysis revealed a carbon content of 59.6%, which is slightly increased compared to the carbon contents of the pristine compound 1 (56.5%) and HM-1 (53.8%).A germanium content of (20.5 ± 1.0)% was detected for C-1 by EDX analysis.Crystalline germanium particles with average primary particle sizes of (27 ± 1) nm were determined based on PXRD analysis (Fig. S15 †).Raman spectroscopy using a confocal micro Raman system revealed areas for C-1 that exhibit high germanium but low carbon contents and areas of high carbon but low germanium contents (Fig. S16 †).The latter is indicative that both germanium-enriched and carbonenriched domains ranging up to the μm-scale were formed during the formation of C-1 due to sintering processes of the germanium particles at higher temperatures (>600 °C).

Electrochemical measurements
In order to provide a proof of principle, C-1 was tested as potential anode material for rechargeable Li-ion batteries.Electrodes with carbon black (CB) as conductive additive and carboxymethylcellulose (CMC) as binder were prepared with a ratio of 80 : 10 : 10.The electrode was tested in Li-ion half cells with 1 M LiPF 6 in a mixture (1 : 1 by weight) of ethylene carbonate (EC) and dimethylcarbonate (DMC) as electrolyte.Further, fluoroethylene carbonate (FEC) was added to the electrolyte to improve cycling stability. 31The electrochemical tests using C-1 as electrode material are shown in Fig. 4. Currents and capacities are given related to the mass of C-1 (Ge@C).
At both current densities of 346 mA g Ge@C −1 and 1384 mA g Ge@C −1 (corresponding to approximately 1C and 4C given the actual capacity of the material) no fading is observed for 100 and 500 cycles, respectively, with capacities of ∼370 mA h g Ge@C −1 .Due to the large surface area and therefore high irreversible charge loss caused by the solid electrolyte interface (SEI) formation the coulombic efficiency in all cases is only ∼25% in the first cycle, but increases to ≥99% during subsequent cycling.The irreversible charge loss during the first discharge is further apparent by the peak at 0.4 V vs. Li + /Li observed in the cyclic voltammogram (Fig. 4d).EDX analysis gave a germanium content of (20.5 ± 1.0)% for C-1.In the light of the low germanium content, the porous Ge@C material as-prepared starting from a TP process holds considerable potential as high-performance anode material for rechargeable LIBs with respect to its excellent cycling stability and rate capability.

Conclusions
The  4) in the solid state.NMR spectroscopic analyses revealed that the germanates 1-3 are in an equilibrium in solution between their dianionic structure as determined in the solid state and species possessing pentacoordinated anions that feature a similar structure as the anion of 4. Germanate 4 does not form such an equilibrium in solution most presumably due to the steric hindrance of its tert-butyl groups in ortho position at the salicyl alcoholate moieties.Single crystal X-ray diffraction analysis revealed hydrogen bonds between the dimethylammonium cations and the germanates resulting in one-dimensional, infinite hydrogen bridged networks within chains formed by the dimethylammonium cations and germanate dianions for the racemic crystals of the compounds 1 and 2. Molecular ion pairs of the dimethylammonium cation and the germanate anion were observed in case of the racemate 4 in the solid state.Thermally induced twin polymerization of compound 1 gave a hybrid material, which is composed of a phenolic resin and GeO 2 .The latter was converted into either microporous Ge@C under reducing conditions or crystalline hexagonal GeO 2 under oxidative conditions.An excellent cycling stability and rate capability were observed upon first tests of the as-obtained Ge@C composite as anode material in LIBs.Although the effective capacity of ∼370 mA h g Ge@C −1 is only moderate, which is attributed to the low germanium content of ∼20% within the Ge@C material, the results concerning cycling stability are promising.We demonstrated for the first time that the approach of twin polymerization can be applied to anionic molecular precursors and without the need for any catalyst.
The reduction process offers a convenient way towards microporous Ge@C composites.However, increasing the germanium content and/or graphitizing the carbon matrix will be a prerequisite for further developments of this approach with regard to high performance anode materials and is currently under investigation.

Experimental section
All reactions were performed under argon using Schlenk techniques or in a glovebox.Solvents were purified and dried by applying standard techniques.The reactions were carried out with freshly distilled, anhydrous solvents. 1H, 13 C{ 1 H} and 1 H-13 C{ 1 H} HSQC NMR spectra were recorded with a Bruker Avance III 500 spectrometer.Solid state NMR spectra were collected at 9.4 T with a Bruker Avance 400 spectrometer equipped with double-tuned probes capable of magic angle spinning (MAS). 13C{ 1 H} CP MAS NMR spectra were measured at 100.6 MHz in 3.2 mm standard zirconium oxide rotors (BRUKER) spinning at 20 kHz.Cross polarization (CP) with a contact time of 3 ms was used to enhance sensitivity.The recycle delay was 5 s.The spectrum was referenced externally to tetramethylsilane (TMS) as well as to adamantane as secondary standard (38.48 ppm for 13 C).All spectra were collected with 1 H decoupling using a two-pulse phase modulation sequence.ATR-FT-IR spectra were recorded with a BioRad FTS-165 spectrometer.Raman spectra were collected on a LabRam HR800 confocal micro Raman system equipped with a Helium-Neon-laser (λ = 632.8nm, P = 3.08 mW) without usage of any filter (D0) at fiftyfold magnification.Energydispersive X-ray spectroscopy (EDX) was performed using a Nova-Nano SEM from FEI with the following parameters: pressure (∼10 −5 mbar), work distance (5 to 7 mm) and acceleration voltage (18 kV) using a Si Drift Detector XFlash 3001 from Bruker AXS.Melting points were determined with a Melting Point B-540 apparatus from Büchi.CHN analyses were determined using a FlashEA 1112 NC Analyzer from Thermo Fisher Scientific.DSC experiments were determined with a Mettler Toledo DSC 30 using 40 μL aluminum crucibles.The measurements were performed up to 300 °C with a heating rate of 10 K min −1 in N 2 atmosphere and a volume flow of 50 mL min −1 .TGA experiments were determined with a Mettler Toledo TGA/ DSC1 1600 system with an MX1 balance.The measurement was performed from 40 to 800 °C with a heating rate of 10 K min −1 in Ar atmosphere and a volume flow of 60 mL min −1 .Nitrogen physisorption isotherms were obtained at −196 °C using an Autosorb IQ2 apparatus from Quantachrome.All samples were activated in vacuum at 150 °C for 3 h prior to the measurements.Specific surface areas were calculated applying the BET equation ( p/p 0 = 0.150 ± 0.002).3][34][35][36][37][38][39] The specific micropore and total pore volume were also calculated by the above mentioned DFT models.Powder X-ray diffraction (PXRD) patterns were collected using a STOE STADI P diffractometer from STOE with Cu-K α radiation (40 kV, 40 mA) and a Ge(111) monochromator.The crystallite size was estimated using the Scherrer equation: τ = Kλ/β cos θ, where τ is the volume weighted crystallite size, K is the Scherrer constant here taken as 1.0, λ is the X-ray wavelength, θ is the Bragg angle in °and β is the full width of the diffraction line at half of the maximum intensity (FWHM, background subtracted).The FWHM is corrected for instrumental broadening using a LaB 6 standard (SRM 660) purchased from NIST (National Institute of Standards and Technology).The value of β was corrected from (β measured 2 and β instrument 2 are the FWHMs of measured and standard profiles): Germanium(IV) chloride was purchased from ABCR GmbH & Co KG. 2.5 M n-butyllithium, 2-tert-butyl-4-methylphenol and 5-bromo-2-hydroxybenzaldehyde were purchased from Merck Schuchardt OHG (Hohenbrunn).2-Hydroxybenzyl alcohol, 4-methylphenol and dimethylamine (2 M solution in THF) were purchased from Alfa Aesar GmbH & Co KG (Karlsruhe).3-tert-butyl-2-hydroxy-5-methylbenzyl alcohol, 9 LiNMe 2 , 40 Ge(NMe 2 ) 4 , 41 2-hydroxy-5-methylbenzaldehyde, 42 2-hydroxy-5-methylbenzyl alcohol 9 and 5-bromo-2-hydroxybenzyl alcohol 9 were synthesized according to literature procedures.2-Hydroxybenzyl alcohol was purified by column chromatography (on silica gel using an ethyl acetate/n-hexane (ratio 20/80, v/v) mixture as eluent) before usage.

Fig. 2
Fig. 2 Molecular structure of germanate 4 in the solid state.Thermal ellipsoids are drawn with 50% probability.All hydrogen atoms (ball and stick style) that are bonded to carbon atoms have been omitted for clarity.The intramolecular S (orange) and the intermolecular R 2 2 (6)

Fig. 4 1 . 1 .
Fig. 4 Electrochemical performance of C-1 as anode material for Li-ion batteries.Capacities and currents are either related to the whole C-1 (Ge@C) composite or only to the fraction of germanium excluding carbon.(a) Cycling stability at a current of 346 mA g Ge@C −1 .(b) Cycling stability at a current of 1384 mA g Ge@C −1 .(c) Galvanostatic charge and discharge curves of at a current of 346 mA g Ge@C −1 .(d) Cyclic voltammogram of C-1 at a scan rate of 0.1 mV s −1 .All cells were cycled in the potential range 0.005-1.0V.