Basic Reactivity Pattern of a Cyclic Disilylated Germylene

In order to estimate the reactivity of disilylated germylene phosphine adducts, a cyclic version of this compound class was reacted with a number of different reagents. Reactions with the chalcogens sulfur, selenium, and tellurium led to dimers of the heavy ketone analogues. Reactions with water and ethyl bromide proceeded to give the respective oxidized germanol and germyl bromide. Two different reactions with alkynes were observed which led either to a germacyclopropene, by addition of tolane to the germylene, or to a silagermacyclobutene, likely formed by addition of the alkyne across a silagermene. Reaction via the silagermene was also observed in the reaction with benzophenone. Reaction of a germylene phosphine adduct with GeCl2·(dioxane) provided insertion of the silylated germylene into a Ge–Cl bond, leading to a germylated chlorogermylene phosphine adduct.


■ INTRODUCTION
The chemistry of higher carbene analogues (tetrylenes) is a very active field of research. 1−5 Most of the studied compounds bear amino substituents, which stabilize the coordinative unsaturation of the divalent group 14 element by π-donation of the nitrogen lone pairs into its empty p orbital. Compounds with σ-donating alkyl or aryl substituents, which lack this particular stabilization, are intrinsically more reactive and require bulky substituents to prevent di-or oligomerization. 5 In the course of the last few years we have investigated the chemistry of tetrylenes with even more σ-donating oligosilanyl and germyl substituents. 6−14 ■ RESULTS AND DISCUSSION Recently, we have described the synthesis of a cyclic germylene by reaction of an oligosilanyl-1,3-diide with GeCl 2 ·(dioxane). 12 The formed germylene was found to be unstable and dimerizes to an exocyclic digermene, which subsequently undergoes a rearrangement to form an endocyclic digermene. 12 While this reaction was found to be remarkably clean, attempts to carry out analogous reactions with oligosilanyl-1,4-diides 2a,b turned out to be rather messy (Scheme 1). The only compounds that could be isolated from these reactions were 3a,b, which contain extra trimethylsilyl groups in comparison to the starting material. While the reason for this is unclear, it should be mentioned that in the recent attempted synthesis of a related disilylated silylene by reducing the respective dichlorosilane, 15 also a compound with an additional trimethylsilyl group was obtained and it thus seems not unlikely that 5a,b were formed in the course of the reactions but are likely too reactive to be detected or isolated (Scheme 1).
This unfortunate situation was resolved carrying out the reaction of 1,4-silanediide 1a with GeBr 2 ·(dioxane) in the presence of either PEt 3 10 or PMe 3 . This way germylene 5a is trapped as a base adduct (4a,b) (Scheme 2) and cannot undergo a 1,2-silyl shift to yield a silagermene. 10 In order to study the principal reactivity of silylated germylene phosphine adducts, a number of addition reactions were carried out. Oxidation of divalent germanium compounds with elemental chalcogens formally causes the formation of heavy ketone analogues. However, due to the limited stability of double bonds between germanium and the heavier chalcogens, usually products of dimerization via head-to-tail [2 + 2] cycloadditions are observed. This was reported for cyclic 16−20 and acyclic 21−23 germylenes with sulfur, [16][17][18][19][20][21][22]24 selenium, 16,17,21−23 and tellurium. 16,21−23 For very bulky germylenes Tokitoh and co-workers have shown that stable thianones and selenones can be isolated. 25,26 For the case of phosphine adduct 4b, in the reactions with 2 mol equiv of either sulfur or selenium and with 1 equiv of tellurium clean conversion was observed to yield compounds 7a−c, in addition to the respective oxidized phosphines SPEt 3 and SePEt 3 for the S and Se cases (Scheme 3). The formation of the dimeric compounds clearly shows that the steric bulk exercised by the trimethylsilyl groups at the silicon atoms in a position α to the germylene is by no means sufficient to inhibit head-to-tail dimerization processes of the presumably initially formed heavy ketone analogues 6a−c (Scheme 3).
Also of interest were addition reactions of the germylene with polar reagents and unsaturated organic compounds. The first group of reagents is expected to test for nucleophilic reactivity, whereas the second group is more likely to indicate electrophilic reactivity of the tetrylene.
The situation for reactions of germylenes with alkyl halides is somewhat similar. Again only a few clear-cut examples have been reported. Baines and co-workers found that NHC adducts of several germylenes with alkyl iodides led mainly to the alkylation of the germylene lone pair and upon further reaction with CDCl 3 provided the oxidation product Mes 2 Ge(Cl)R. 32 Conversely, reaction of Lappert's Ge[CH(SiMe 3 ) 2 ] 2 with methyl iodide gave Me(I)Ge[CH(SiMe 3 ) 2 ] 2 . 33 It is likely that such reactions involve the transient formation of germyl radicals. Banaszak Holl and co-workers have been able to exploit this reactivity for facile C−H activation chemistry. 34 In order to probe the reactivity of 4b against alkyl halides, a reaction with ethyl bromide was carried out, leading cleanly to the ethylated germyl bromide 9, which is the product of oxidative addition of 5a (Scheme 4).
Addition of an alkyne to disilylated germylenes was achieved by Sekiguchi and co-workers, who reacted disilylated dichlorogermanes with molten potassium, in the presence of bis(trimethylsilyl)acetylene without a solvent, to obtain the corresponding germacyclopropenes after 19 h at reflux. 38 The related reaction of 4b with tolane occurred under less drastic conditions and provided germacyclopropene 11 in a yield of 86% (Scheme 4). As the reaction of 4b with tolane can be considered to be analogous to the addition of bis-(trimethylsilyl)acetylene to titanocene 39 or zirconocene, 40 we wondered whether the reaction of 4b with 1,4-bis-(trimethylsilyl)butadiyne would also mimic the respective reaction with titanocene. 41 However, instead of addition to the germylene to give either a cumulene or an alkynylsubstituted germacyclopropene, the diyne underwent [2 + 2] cycloaddition to the silagermene isomer of 5a, 10 yielding a mixture of the two regioisomers 12a,b (Scheme 4). Addition of phenylacetylene across silagermenes was observed previously in a similar way by Baines 42 and Sekiguchi. 43, 44 Scheschkewitz's [2 + 2] cycloaddition of phenylacetylene to a NHC-stabilized silagermenylidene constitutes another related precedent. 45 The formation of 12a,b seems to indicate that in solution 4b is in equilibrium with 5a (Scheme 1) and its silagermene isomer. Although we have found that the silagermene can undergo dimerization via a head-to-tail [2 + 2] cycloaddition, 10 the addition of an alkyne to the Ge−Si double bond seems to be energetically much more facile. The presence of a silagermene isomer was further substantiated by the reaction of 4b with benzophenone. 46 Again, addition across the Si−Ge double bond was observed in a regioselective manner to form Si−O and Ge−C bonds in compound 13 (Scheme 4). This behavior is consistent with earlier studies by Baines and coworkers. 47−49 Reaction of 4b with GeCl 2 ·(dioxane) provides another interesting insertion reaction where the silylated germylene inserts into a Ge−Cl bond. In this way the germylated chlorogermylene adduct 15 forms. The dioxane which served as the Lewis base in GeCl 2 ·(dioxane) is replaced by PEt 3 . The reaction is not completely selective, and in addition to compound 15 also a similar amount of dichlorogermane 14 forms (Scheme 4). Repeating the reaction at different

Organometallics
Article temperatures and concentrations did not have an influence on the amount of formation of 14.
Recently, there have been reports of reactions of carbenes 50,51 and other low-valent main-group compounds with hydrogen 52,53 and carbon monoxide. 54 We have therefore attempted reactions of these two small molecules with 4b, but no reaction was observed. This lack of reactivity may be explained by the fact that despite it being very electrophilic, germylene 5a released from 4b is not nucleophilic enough to participate in carbonylation or hydrogenation reactions.
NMR Spectroscopy. The NMR spectroscopic characterization of the magnesium germanide 2b is not as straightforward as it is for 2a, where the typical upfield-shifted 29 Si resonances of metalated silicon atoms around −170 ppm are very characteristic. 55 The 29 Si NMR spectrum of 2b displays only the SiMe 3 and SiMe 2 signals at −3.4 and −23.7 ppm, respectively, which are slightly shifted toward low field; this is common for germanium-connected silyl groups. The same is observed in the 29 Si NMR spectrum of 3a, where the trimethylsilyl group attached to germanium is found at 0.6 ppm. The other 29 Si signals of 3a are two SiMe 3 resonances for the two different ring sides at −3.4 and −7.8 ppm and the SiMe 2 (−24.3 ppm) and Si(SiMe 3 ) (−105.2 ppm) resonances with rather ordinary chemical shifts.
The reactions of 4b can be conveniently monitored using 29 Si NMR spectroscopy. The chemical shifts of the 1 H (and to some extent also the 13 C) resonances of the formed products are very similar and do not cover a vast spectral region. Thus, these signals are not very indicative. Nevertheless, the number and intensity of observed 1 H and 13 C resonances provide straightforward insight into the molecular symmetry. Compounds 4a,b (Table 1) are interesting compounds, as 1 H, 13 C, and 29 Si spectra taken at ambient temperature show different rates for the inversion of the coordination sphere around the low-valent germanium center. For 4a two different signals for the trimethylsilyl groups were observed in the 1 H, 13 C, and 29 Si spectra (Table 1). Although this means that the inversion process at germanium is slow on the NMR time scale, the observed signals are broad, indicating that the coalescence temperature is close to ambient temperature. 2 J Si−P and 3 J Si−P couplings to the germylated silicon atom and the SiMe 2 unit can be observed with values of 16 and 10 Hz, respectively. For 4b the slightly diminished degree of interaction between the germylene and the phosphine leads to an increased rate of inversion of germanium. The signals for the trimethylsilyl groups at the two different sides of the ring therefore coalesce at room temperature and are only visible as broad singlets. The signals for the Si(SiMe 3 ) 2 (−127.1 ppm) and SiMe 2 (−22.7 ppm) atoms are not affected by the inversion process and appear as sharp doublets due to the coupling to 31 P with 2 J Si−P and 3 J Si−P coupling constants very similar to those observed for 4a. 10 The reactions of 4b with elemental chalcogens to give the structurally related compounds 7a−c result, as expected, in rather similar 29 Si spectra. The signals of the trimethylsilyl groups and of the germylated silicon atoms resonate at unusually low field at −5.4, −90.8 (7a), −4.7, −92.6 (7b), and −3.2, −99.4 (7c) ppm ( Table 1). As the resonances of the SiMe 2 groups around −30 ppm are rather conventional, the downfield shift can likely be assigned to the proximity of the chalcogen atoms. In this context it should be noted that both selenium and tellurium possess spin 1/2 nuclei with abundances of 7.58% for 77 Se and 6.99% for 125 Te. Despite the fact that 3 J P−Si coupling was clearly visible for 4b, no satellite signals were observed for 3 J Se−Si or 3 J Te−Si coupling in 7b or 7c, respectively. The structure of the dichlorogermane 14 is actually rather similar to those of 7a−c, and accordingly the 29 Si NMR spectrum of 14 is similar to the signals of the SiMe 3 groups and of the germylated silicon atoms at −4.3 and −96.7 ppm (Table 1).
While the spectra of germirene 11 are unremarkable, compound 15 is more interesting. As observed for 7a−c and 14, the attachment of heavy atoms to germanium causes the aforementioned downfield shift of the SiMe 3 and SiGe resonances to −3.9/−4.3 and −102.5 ppm for 15 ( Table 1). The presence of the PEt 3 is visible by a 3 J Si−P coupling of 6.0 Hz to the germylated silicon atoms. The spectroscopic appearance of compound 15 suggests C s symmetry. This leads to the conclusion that the germylene atom cannot be configuratively stable and at ambient temperature the inversion process must be fast on the NMR time scale. The 31 P resonance at 10.5 ppm indicates a diminished degree of interaction in comparison to that in 4b, where the 31 P resonance of PEt 3 was observed at 14.8 ppm. The 29 Si NMR spectra of compound 10 are similar to those of 14 without the strong deshielding effect on the SiMe 3 and SiGe groups. Spectra of compounds 8 and 9 are similar to those of 15, indicating side differentiation of the fivemembered ring. 29 Si NMR chemical shifts of 8 and 9 are comparable and completely lie within the range of expected values (Table 1). Like compound 14, 10 also has C 2v symmetry, giving similar spectra. However, the two oxygen atoms attached to germanium in 10 do not cause the same downfield shift behavior as observed for 7a−c, 14, and 15 ( Table 1). The C 1symmetric silagermene addition products 12a,b and 13 are more interesting with respect to their 29 Si NMR spectra.
Isomeric compounds 12a,b were formed in a 1:1 ratio and could not be separated. Therefore, a 29 Si NMR spectrum with each silicon atom giving rise to two signals was observed. Nevertheless, a tentative assignment of resonances is possible. The trimethylsilyl groups at germanium appear around −3.5 ppm, and four trimethylsilyl resonances were found between −7.4 and −12.6 ppm which can be assigned to the three silylsubstituted and the vinylated SiMe 3 groups. Two signals between −22.1 and −28.4 ppm can be associated with SiMe 2 units, and that around −37 ppm corresponds to the silicon atom which is part of the silagermabutene ring with a strong downfield shift, as expected for four-membered rings. The signal at −112.5 ppm is in the typical region for tetrasilylated silicon atoms. 56 The 29 Si NMR spectrum of 13 is much like that of 12a,b but is less complicated. The oxygenate silicon incorporated into the four-membered ring resonates at 40.3 ppm. Again the trimethylsilyl group at germanium is shifted downfield (−2.1 ppm), and the other SiMe 3 groups are found at −7.9, −8.5, and −14.0 ppm. The signals for the two SiMe 2 groups at −25.3 and −37.2 (SiMe 2 ) and the tetrasilylated silicon atom at −104.7 ppm complete the spectrum.
X-ray Crystallography. Compounds 2b (Figure 1), 3b ( Figure Figure 8) reported in this study were subjected to singlecrystal X-ray diffraction analysis (Table 2 and Tables S1−S3 in the Supporting Information). The cyclic 1,4-digermyl magne-Organometallics Article sium compound 2b (Figure 1) crystallizes in the orthorhombic space group P2 1 2 1 2 1 , isotypic with the previously characterized 1,4-disilyl magnesium compound 2a. 55 While the structural features of 2a,b are essentially the same, it is interesting to note that the Ge−Mg distances of 2.637(2) Å in 2b are slightly shorter than the analogous interactions in 2a. 55 The structure of 2-bromo-1,2,3-trigerma-1,1,2,3,3pentakis(trimethylsilyl)tetramethylcyclopentasilane 3b ( Figure  S1), which crystallizes in the triclinic space group P1̅ , is somewhat unusual. The five-membered ring exhibits a rather flat envelope conformation with the trimethylsilyl group attached to germanium in an equatorial position with the bromine atom in the sterically less demanding axial position. Ge−Ge The solid-state structure of germylene phosphine adduct 4a (Figure 2) is quite similar to those of 4b and a related digermylated PEt 3 adduct. 10 The ring exhibits an envelope conformation with one of the Si(SiMe 3 ) 2 units out of plane. The PMe 3 coordinates in an orthogonal fashion to the germylene atom (d P−Si = 2.3484(6) Å) with the angle between the SiGeSi plane and the phosphine of 112.3°similar to the 113.6°of 4b. As found for other disilylated germylene base adducts, the Si−Ge distances of 4a are elongated to 2.4589(5) and 2.4533(6) Å. Si−Ge distances of compounds with an analogous ring skeleton but tetravalent germanium atoms are typically close to 2.40 Å (see below).
Compounds 7a (Figure 3), 7b ( Figure S2 in the Supporting Information), and 7c ( Figure S3 in the Supporting Information) are structurally much alike, and it was therefore expected that these compounds would exhibit similar structures in the solid state. However, 7a was found to crystallize in the monoclinic space group C2/c together with a disordered pentane molecule, whereas 7b,c crystallized in the triclinic space group P1̅ . Nevertheless, the molecular structures of 7a−c in the crystal form are analogous. In all molecules planar Ge− E−Ge−E (E = S, Se, Te) units are found.
T h e 1 -g e r m a -2 , 2 , 5 , 5 -t e t r a k i s ( t r i m e t h y l s i l y l )tetramethylcyclopentasilane units of 7a−c, which over the germanium atoms have a spirocyclic connection to the Ge−E− Ge−E rings, all show the five-membered rings in envelope    All Si−Si distances and angles of these five-membered rings are rather similar ( Table 2). The two rings of the spirocyclic arrangement are situated almost orthogonally to each other (angles between planes between 87.8 and 89.5°). The arguably most unusual structural property of compounds 7a−c is the Ge−Si distance. In previous studies we have noticed that this particular bond is sensitive to the germanium oxidation state. The Si−Ge distances of PR 3 adducts of disilylated germylene are typically around 2.47 Å, whereas Si−Ge distances of tetravalent germanes are close to 2.40 Å. 14 The respective distances in 7a−c are between 2.44 and 2.48 Å (Table 2), indicating at least some degree of unsaturation at germanium. The number of dimeric structures of higher germanones which have been subjected to structural analysis in the solid state is small.
The CCDC database 57 lists 26 examples comparable to compound 7a featuring four-membered rings with alternating Ge and S atoms. The Ge−S distances are in a narrow range between 2.209 and 2.258 Å with a mean value of 2.235 Å. The corresponding Ge−S distance in compound 7a is 2.291(1) Å and thus is considerably longer. This is most likely caused by a combination of the bulky silanylene substituents of the germylene unit and its stronger electron-donating character. CCDC examples 57 of four-membered rings with alternating Ge and Se atoms comparable to 7b exhibit Ge−Se distances in a range between 2.350 and 2.433 Å, with most compounds exhibiting Ge−Se distances at the lower limit. The compound with the longest distance ((TbtMesGeSe) 2 ) 25 features very large substituents at germanium and a folded asymmetric ring. All other compounds contain perfectly planar SeGeSeGe rings with two equivalent Ge−Se distances. Due to its lower     Figure S5 in the Supporting Information) and 10 ( Figure 5) feature spirocyclic structures. The germacyclopentasilane rings of the latter also engage in envelope conformations which, however, are substantially flatter than those of 7a−c.
In addition, the Ge−Si distances of 11 (2.38 Å) and 10 (2.40 Å) ( Table 2)  The Si−Si bond lengths are within the expected ranges, and the Si−Ge distances remain below or at least very close to 2.40 Å. Although the number of structurally characterized germacyclopropenes 14,38,58−61 is rather small, the range of Ge−C bond lengths is surprisingly broad, ranging from 1.915 60 to 2.005 Å. 38 The value found for compound 11 is 1.958(2) Å, which is a rather typical Ge−C distance and is comparable to that of the related bis[tris(trimethylsilyl)silyl]germirene reported recently. 14 The Ge−Cl distance of dichlorogermane 14 (2.1961(9) Å), the Ge−OH distance of germanol 8 (1.788(4) Å), the Ge-OC distance of 10 (1.834(4) Å), and the Ge−Br distance of 9 (2.3672(9) Å) are all somewhat shorter than the values typically found but are still completely within the ranges of what is expected.
The germylene adduct 15 (Figure 8), however, exhibits a rather long Ge−Ge distance of 2.5290(9) Å, which is quite long but is likely a consequence of the fact that one of the two Ge atoms involved is in the divalent state. This is further reflected by the Ge−Cl distance in compound 15. The bond to the tetravalent Ge atom is, at 2.283(1) Å, shorter than the respective bond to the divalent Ge atom (2.319(2) Å). In addition to 7a−c compound 15 is also the only other structure displaying significantly elongated Si−Ge bond lengths (2.422(1) and 2.431(2) Å).
The structures of 12a ( Figure 6) and 13 (Figure 7) do not possess a 1-germa-2,2,5,5-tetrakis(trimethylsilyl)-tetramethylcyclopentasilane substructure but a 1-germa-1,2,5,5-tetrakis(trimethylsilyl)tetramethylcyclopentasilane unit where one of the trimethylsilyl groups is shifted to germanium. This is caused by the fact that both compounds are formed by a [2 + 2] cycloaddition across the silagermene formed by a 1,2trimethylsilyl shift of the germylene derived from 4b. The compounds thus both possess a bicyclo[3.2.0]heptane structure with the tetrahedral coordination environment of the bridgehead atoms being severely distorted. Apart from a slightly elongated Ge−Si bond length of 2.4306(9) in 13, the Si−Si and Si−Ge distances in 12a and 13 are within the expected range.

■ CONCLUSION
Recently reports of small-molecule activation of carbenes and other low-valent main-group compounds have attracted considerable attention. The main reasons these compounds exhibit such interesting reactivity are the coordinative unsaturation, small singlet−triplet gaps, and fitting orbital energies. In order to investigate the question of whether the recently introduced class of silylated germylene phosphine adducts is also capable of small-molecule activation, we studied the reactions of a cyclic disilylated germylene phosphine adduct with a number of small molecules including elemental chalcogens, water, alkyl halides, alkynes, ketones, and αdiketones. All these reagents reacted in a defined way with the germylene or its silagermene isomer in a manner that can be considered as oxidation of the germylene or the silagermene isomer. Further attempted reactions with carbon monoxide and hydrogen, however, did not proceed.
It has been pointed out conclusively that the range of the small singlet−triplet gap energy (ΔE ST ) of a low-valent maingroup compound can be considered as an inverse measure of its electrophilicity and further that the orbital energy of the HOMO (lone pair) is proportional to its nucleophilicity. 5,50 A qualitative comparison of the values for ΔE ST and HOMO of 5a 12 reveals a small ΔE ST of 86 kJ/mol, 12 consistent with it having highly electrophilic behavior, whereas the HOMO energy of −6.46 eV, 11,62 indicating low basicity, explains the lacking nucleophilic properties. This picture is fully consistent with the facile addition chemistry and the reactions with polar reagents, and it also clarifies the reluctant behavior in the attempted hydrogenation and carbonylation reactions.

■ EXPERIMENTAL SECTION
General Remarks. All reactions involving air-sensitive compounds were carried out under an atmosphere of dry nitrogen or argon using either Schlenk techniques or a glovebox. All solvents were dried using a column-based solvent purification system. 63 Chemicals were  29 Si) and 85% H 3 PO 4 ( 31 P). If not noted otherwise, the solvent of the samples was C 6 D 6 . To compensate for the low isotopic abundance of 29 Si, the INEPT pulse sequence was used for the amplification of the signal. 69,70 Elementary analysis was carried out using a Heraeus VARIO ELEMENTAR instrument. For a number of compounds no good elemental analysis values could be obtained, which is a typical problem for these compounds caused by silicon carbide formation during the combustion process. High-resolution (HR) electrospray ionization mass spectrometry (ESI-MS) spectra were recorded on a Thermo Scientific Orbitrap LTQ XL mass spectrometer and the raw data evaluated using the Xcalibur computer program.

Organometallics
Article