Insights into Molecular Beryllium – Silicon Bonds

We present the synthesis of two silyl beryllium halides HypSiBeX·(thf) (HypSi = Si(SiMe3)3, X = Cl 2a, I 4a) and the molecular structure of 2a as determined by single crystal X-ray diffraction. Compounds 2a and 4a were characterized via multi-nuclear NMR spectroscopy (1H, 9Be, 13C, 29Si), and the bonding situation was further investigated using quantum chemical calculations (with the addition of further halides X = F 1b, Cl 2b, Br 3b, I 4b). The nature of the beryllium silicon bond in the context of these compounds is highlighted and discussed.


Introduction
The organometallic and coordination chemistry of beryllium has gained attention as several groups, including ours, started to focus their research on this rather unknown and hitherto neglected element [1].In the last five years, many groundbreaking discoveries, including the first compound with beryllium in the formal oxidation state 0 by the Braunschweig group [2], a beryllium-induced C-N bond activation in an N-heterocyclic carbene [3], the structural characterization of the long known Be(N(SiMe 3 ) 2 ) 2 [4], the synthesis of the first bis(diphenylphosphinimino)methanide and -methanediide beryllium complexes [5], and the synthesis of ether-free beryllium organyls [6], were reported.Even in the field of material chemistry and catalysis, groups around the world started to investigate the unique properties of beryllium compounds [7].The prejudices and reservations that people might have against beryllium due to its potential risks were discussed and elucidated in our recently reported review [8].
Part of our research focuses on the synthesis of heteroleptic beryllium compounds bearing at least one halide of the RBeX type (R = organic substituent, X = F, Cl, Br, I) and their structural characterization via single crystal X-ray diffraction, multi-nuclear NMR, and quantum mechanical calculations to achieve a deeper understanding of the bonding nature in beryllium compounds [4,9,10].This led to several publications, which highlight the subtle influences of the substituents on the chemical and electronic properties of the resulting compounds.We extended these studies now to silicon-containing substituents, the heavier congener of carbon.To the best of our knowledge, compounds of the type R'BeX (R = SiR 3 , X = F, Cl, Br, I) with a beryllium-silicon bond have to date rarely been studied [11,12].Our findings are presented in this work to blaze a trail for research on the chemistry of beryllium-silicon compounds and for a deeper understanding of the beryllium-silicon bond.

Synthesis
The reaction of LiSi(SiMe 3 )•(thf) 2.6 with BeX 2 (X = Cl, I) in a 1:1 mixture of dry toluene/thf at 60 • C led to the formation of 2a and 4a after stirring for 16 h (Scheme 1), which was confirmed by 9 Be NMR spectroscopy.All volatiles were removed under reduced pressure, and the residue was dried for 2 h in an oil-pump vacuum.The residue was then dissolved in a small amount of dry toluene and filtrated, and the compounds were crystallized at −28 • C (yield: 2a 83%, 4a 87%).2a and 4a were obtained as colorless crystalline solids, which dissolve easily in a number of typical coordinating (Et 2 O, THF) and non-coordinating solvents (hexane, cyclohexane, toluene) and were characterized by multi-nuclear NMR spectroscopy ( 1 H, 9 Be, 13 C, 29 Si; Figures S1-S8).

Synthesis
The reaction of LiSi(SiMe3)•(thf)2.6 with BeX2 (X = Cl, I) in a 1:1 mixture of dry toluene/thf at 60 °C led to the formation of 2a and 4a after stirring for 16 h (Scheme 1), which was confirmed by 9 Be NMR spectroscopy.All volatiles were removed under reduced pressure, and the residue was dried for 2 h in an oil-pump vacuum.The residue was then dissolved in a small amount of dry toluene and filtrated, and the compounds were crystallized at −28 °C (yield: 2a 83%, 4a 87%).2a and 4a were obtained as colorless crystalline solids, which dissolve easily in a number of typical coordinating (Et2O, THF) and non-coordinating solvents (hexane, cyclohexane, toluene) and were characterized by multi-nuclear NMR spectroscopy ( 1 H, 9 Be, 13 C, 29 Si; Figures S1-S8).

NMR-Spectroscopy
The 9 Be NMR shifts underline that the tetrahedral geometry of the complexes that is found in the solid state is also preserved in solution.The thf molecules are still coordinating with the beryllium.An overview of typical 9 Be NMR shifts of several heteroleptic complexes of the general type RBeX depending on the coordination mode is given in Table 1, which also includes ethercoordinated BeX2(OEt2)2 (X = Cl, Br), which adopt tetrahedral structures both in solution and in the solid state [13,14], silyl-substituted compounds CpBeSiR3 and Be[N(SiMe3)2]2, respectively.
Table 1.Overview of typical 9 Be NMR shifts depending on their complex geometry and their respective coordination modes.Scheme 1. Synthesis of 2a and 4a.

NMR-Spectroscopy
The 9 Be NMR shifts underline that the tetrahedral geometry of the complexes that is found in the solid state is also preserved in solution.The thf molecules are still coordinating with the beryllium.An overview of typical 9 Be NMR shifts of several heteroleptic complexes of the general type RBeX depending on the coordination mode is given in Table 1, which also includes ether-coordinated BeX 2 (OEt 2 ) 2 (X = Cl, Br), which adopt tetrahedral structures both in solution and in the solid state [13,14], silyl-substituted compounds CpBeSiR 3 and Be[N(SiMe 3 ) 2 ] 2 , respectively.
Table 1.Overview of typical 9 Be NMR shifts depending on their complex geometry and their respective coordination modes.

Solid State Structure
Compound 2a crystallizes in the monoclinic space group P2 1 /n with the molecule placed on the special position.Be is coordinated by Cl, its symmetry equivalent, a thf molecule, and a hypersilyl ligand in a slightly distorted tetrahedral fashion (Figure 1).Crystallographic data of 2a are given in Tables S1-S5.

Solid State Structure
Compound 2a crystallizes in the monoclinic space group P21/n with the molecule placed on the special position.Be is coordinated by Cl, its symmetry equivalent, a thf molecule, and a hypersilyl ligand in a slightly distorted tetrahedral fashion (Figure 1).Crystallographic data of 2a are given in Tables S1-S5.
(2a) The bond angles around the central Be atom range from 96.5(2)° to 118.5(2)°, and the largest ones involve the central Si of the hypersilyl group.This distortion most likely results from the vast steric demand of the hypersilyl group.The Cl atom bridges the Be1 and its symmetry equivalent in a slightly asymmetric manner, as the difference between both Be-Cl bonds is about 0.1 Å.The Be-Cl bond lengths in 2a (2.099(5), 2.112(5) Å) match well with the mean value of 2.02(7) Å for Be-Cl single bonds found in 48 structures with tetrahedrally coordinated Be atoms in the CSD [20].The Be-O bond length also agrees with the mean value of 1.62(5) Å (187 hits), but weakly bonded thf molecules show Be-O bond lengths of up to 1.737 Å [21].To the best of our knowledge, CpBeSiMe3 [12] and Be(Si t Bu3)2 [11] are the only compounds containing a Be-Si bond that to date have been structurally characterized.From these structure determinations, three independent values for the bond lengths are available (2.1930(10) Å, 2.2085(10) Å Be(Si t Bu3)2 [22], and 2.185(2) Å CpBeSiMe3 [12]).The fourth value available now from 2a is in the same order but slightly larger (2:239(5) Å), which may again be attributed to the high steric demand of the hypersilyl group.The Si-Si bond lengths and Si-Si-Si bond angles within the hypersilyl substituent are almost identical to those previously reported for compounds containing this specific substituent.The bond angles around the central Be atom range from 96.5(2) • to 118.5(2) • , and the largest ones involve the central Si of the hypersilyl group.This distortion most likely results from the vast steric demand of the hypersilyl group.The Cl atom bridges the Be1 and its symmetry equivalent in a slightly asymmetric manner, as the difference between both Be-Cl bonds is about 0.1 Å.The Be-Cl bond lengths in 2a (2.099(5), 2.112(5) Å) match well with the mean value of 2.02(7) Å for Be-Cl single bonds found in 48 structures with tetrahedrally coordinated Be atoms in the CSD [20].The Be-O bond length also agrees with the mean value of 1.62(5) Å (187 hits), but weakly bonded thf molecules show Be-O bond lengths of up to 1.737 Å [21].To the best of our knowledge, CpBeSiMe 3 [12] and Be(Si t Bu 3 ) 2 [11] are the only compounds containing a Be-Si bond that to date have been structurally characterized.From these structure determinations, three independent values for the bond lengths are available (2.1930(10) Å, 2.2085(10) Å Be(Si t Bu 3 ) 2 [22], and 2.185(2) Å CpBeSiMe 3 [12]).The fourth value available now from 2a is in the same order but slightly larger (2:239(5) Å), which may again be attributed to the high steric demand of the hypersilyl group.The Si-Si bond lengths and Si-Si-Si bond angles within the hypersilyl substituent are almost identical to those previously reported for compounds containing this specific substituent.

Quantum Chemical Calculations
Quantum chemical calculations were performed to gain a deeper understanding of the bonding situation in the heteroleptic complexes HypSiBeX•(thf) (HypSi = Si(SiMe 3 ) 3 , X = F 1b, Cl 2b, Br 3b, I 4b).The crystal structure of 2a was used as a starting point to model the gas phase structures of 1b-4b.Geometries of these model structures were then optimized using RI-DFT [23][24][25][26] methods with the B3-LYP functional [27,28] and TZVPP basis sets [29] and third generation Becke-Johnson-damped dispersion correction [30,31].To further investigate the influence of the coordination mode of Be on the NMR shift of this compound, additional model systems without the coordinated thf molecules were designed (1c-4c) and optimized with the same methods.NMR chemical shifts of 1b-4b and 1c-4c were calculated via GIAO methods [32].Shared electron number (SEN) [33] calculations, electron localization function (ELF) [34,35] plots and localized orbital locator (LOL) [36] plots were performed for 1b-4b.
The gas phase structures of 1b-4b (Figure 2) are very similar to the crystal structure of 2a.They each show dimeric structures, in which the Be atom adopts distorted tetrahedral coordination environments, and approximately show an inversion center between the two halide ligands (Figures S17-S20, Table S6).The two Be-X units approximately form a rhombus.Table 2 summarizes important bond lengths and angles of 2a in comparison to those calculated for 1b-4b.Geometries of these model structures were then optimized using RI-DFT [23][24][25][26] methods with the B3-LYP functional [27,28] and TZVPP basis sets [29] and third generation Becke-Johnson-damped dispersion correction [30,31].To further investigate the influence of the coordination mode of Be on the NMR shift of this compound, additional model systems without the coordinated thf molecules were designed (1c-4c) and optimized with the same methods.NMR chemical shifts of 1b-4b and 1c-4c were calculated via GIAO methods [32].Shared electron number (SEN) [33] calculations, electron localization function (ELF) [34,35] plots and localized orbital locator (LOL) [36] plots were performed for 1b-4b.
The gas phase structures of 1b-4b (Figure 2) are very similar to the crystal structure of 2a.They each show dimeric structures, in which the Be atom adopts distorted tetrahedral coordination environments, and approximately show an inversion center between the two halide ligands (Figures S17-S20, Table S6).The two Be-X units approximately form a rhombus.Table 2 summarizes important bond lengths and angles of 2a in comparison to those calculated for 1b-4b.The Be-X bond lengths increase with increasing atomic number from F to I, while the Be-Si and Be-O bond lengths marginally decrease, which could be due to slightly reduced steric stress.In accord with Bent's rule and the steric demand of the halogen atoms, the X-Be-X angle increase with decreasing electronegativity and increasing atomic radii of the halide ion (X) from fluoride to iodide and the Be-X-Be angles consequently decrease.Moreover, the other angles also vary slightly.The  The Be-X bond lengths increase with increasing atomic number from F to I, while the Be-Si and Be-O bond lengths marginally decrease, which could be due to slightly reduced steric stress.In accord with Bent's rule and the steric demand of the halogen atoms, the X-Be-X angle increase with decreasing electronegativity and increasing atomic radii of the halide ion (X) from fluoride to iodide and the Be-X-Be angles consequently decrease.Moreover, the other angles also vary slightly.The crystal structure of 2a is in good agreement with the calculated gas phase structure of 2b for most displayed values.Slight differences in the Cl-Cl-Be-Si dihedral angle can be attributed to packing effects.
The gas phase structures of 1c-4c, which have been calculated without any coordinating thf molecule, show trigonally coordinated Be atoms (Figures S21-S24).For 1c-3c, the two Be-X units approximately lie in a rhombus (Be-X-X-Be dihedral angle of 179.7 • to 178.5 • ), while that rhombus is significantly folded in 4c (Be-X-X-Be dihedral angle of 163.6 • ).Similarly, Be and the halide substituents lie approximately in the same plane for 1c-3c (X-X-Be-Si dihedral angle of 179.3 • to 178.7 • ), while 4c shows a significant deviation from planarity (X-X-Be-Si dihedral angle of 170.3 • ).
The calculated NMR chemical shifts (Table 3) of 1b-4b and 1c-4c are not very close to the measured signals, but fall in line with our calculations of other beryllium shifts, which tend to be higher than measured signals.Nonetheless, it can be clearly seen that there is a difference between the calculated chemical shifts of these substances for the thf-coordinated 1b-4b in comparison to the thf-free 1c-4c.Unfortunately, the shifts of these species overlap and one cannot clearly distinguish between thf-containing or thf-less species just by checking these theoretical values.Thus, a comparison with other measured Be-containing species is needed to evaluate or predict coordination modes.The Be-Si bonds were investigated by looking at their shared electron number (SEN) values.The values range from 1.42 for 1b and 1.36 for 2b and 3b, to 1.31 for 4b.These values are within the typical range for covalent bonds between Si-Si, Si-C and C-H in these molecules, which suggests that the Be-Si bond is quite covalent.This is exemplified further by looking at electron localization function (ELF) and localized orbital locator (LOL) plots (see also Figures S9-S16) of the Be-Si-Cl plane in Figure 3, where a Be-Si bond electron pair can clearly be seen around (x,y) = (6.58,4.52)bohr.approximately lie in a rhombus (Be-X-X-Be dihedral angle of 179.7° to 178.5°), while that rhombus is significantly folded in 4c (Be-X-X-Be dihedral angle of 163.6°).Similarly, Be and the halide substituents lie approximately in the same plane for 1c-3c (X-X-Be-Si dihedral angle of 179.3° to 178.7°), while 4c shows a significant deviation from planarity (X-X-Be-Si dihedral angle of 170.3°).
The calculated NMR chemical shifts (Table 3) of 1b-4b and 1c-4c are not very close to the measured signals, but fall in line with our calculations of other beryllium shifts, which tend to be higher than measured signals.Nonetheless, it can be clearly seen that there is a difference between the calculated chemical shifts of these substances for the thf-coordinated 1b-4b in comparison to the thf-free 1c-4c.Unfortunately, the shifts of these species overlap and one cannot clearly distinguish between thf-containing or thf-less species just by checking these theoretical values.Thus, a comparison with other measured Be-containing species is needed to evaluate or predict coordination modes.The Be-Si bonds were investigated by looking at their shared electron number (SEN) values.The values range from 1.42 for 1b and 1.36 for 2b and 3b, to 1.31 for 4b.These values are within the typical range for covalent bonds between Si-Si, Si-C and C-H in these molecules, which suggests that the Be-Si bond is quite covalent.This is exemplified further by looking at electron localization function (ELF) and localized orbital locator (LOL) plots (see also Figures S9-S16) of the Be-Si-Cl plane in Figure 3, where a Be-Si bond electron pair can clearly be seen around (x,y) = (6.58,4.52)bohr.

Materials and Methods
Beryllium and its compounds are regarded as highly toxic and carcinogenic and they also have an allergic potential if inhaled with the risk of causing chronic beryllium disease (CBD) [37].They should therefore be handled with appropriate safety precautions [8].All experiments described herein were performed in fume hoods, in gloveboxes, or with advanced Schlenk techniques under extremely dry and oxygen-free (Caution) Ar atmosphere.HypSiLi•(thf)2.6 was prepared in accordance

Materials and Methods
Beryllium and its compounds are regarded as highly toxic and carcinogenic and they also have an allergic potential if inhaled with the risk of causing chronic beryllium disease (CBD) [37].They should therefore be handled with appropriate safety precautions [8].All experiments described herein were performed in fume hoods, in gloveboxes, or with advanced Schlenk techniques under extremely dry and oxygen-free (Caution) Ar atmosphere.HypSiLi•(thf) 2.6 was prepared in accordance with a procedure found in the literature [38].The amount of coordinated thf in the HypSiLi•(thf) 2.6 was determined by adding a definite amount of 1,3,5-tri-t Bu-benzene to the solution of the ligand in C 6 D 6 , followed by comparative integration of the signals.BeCl 2 and BeI 2 were synthesized from the elements at elevated temperatures followed by high-temperature, high-vacuum fractional sublimation [39].NMR spectra were recorded on a Bruker Avance 300 spectrometer (Karlsruhe, Germany) at 25 • C at 300.1 MHz ( 1 H), 42.4 MHz ( 9 Be), 75.5 MHz ( 13 C), and 59.6 MHz ( 29 Si). 1 H and 13 C{ 1 H}-NMR spectra were referenced to internal C 6 D 5 H (δ( 1 H) = 7.154 ppm; δ( 13 C) = 128.0ppm), 9 Be-NMR was referenced to external BeSO 4 in D 2 O (δ( 9 Be) = 0 ppm) and 29 Si spectra were referenced to an external standard of neat Si(CH 3 ) 4 (δ( 29 Si) = 0 ppm).Elemental analyses were not determined because of the potential toxicity of the complexes.The purity of the compounds was verified by NMR spectroscopy.All reactions were performed under an argon atmosphere.
CCDC-1539267 contains the supplementary crystallographic data for this paper.These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Figure 1 .
Figure 1.Solid-state structures of 2a (pale colored part generated by inversion); thermal ellipsoids are shown at 50% probability levels.

Figure 1 .
Figure 1.Solid-state structures of 2a (pale colored part generated by inversion); thermal ellipsoids are shown at 50% probability levels.

Figure 2 .
Figure 2. Calculated gas phase structure of 2b (left) and 2c (right); X displayed in green, Be in orange; Si in ocher, C in grey, O in red and H in white.

Figure 2 .
Figure 2. Calculated gas phase structure of 2b (left) and 2c (right); X displayed in green, Be in orange; Si in ocher, C in grey, O in red and H in white.

Table 3 .
Overview of calculated and observed NMR shifts for 1-4.

Table 3 .
Overview of calculated and observed NMR shifts for 1-4.