Insights into Molecular Beryllium–Silicon Bonds

Abstract

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.

1. 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(SiMe₃)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 = SiR₃, 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.

2. Results and Discussion

2.1. 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 ⁹Be 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 (¹H, ⁹Be, ¹³C, ²⁹Si; Figures S1–S8).

2.2. NMR-Spectroscopy

The ⁹Be 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 ⁹Be NMR shifts of several heteroleptic complexes of the general type RBeX depending on the coordination mode is given in

Table 1. Overview of typical ⁹Be NMR shifts depending on their complex geometry and their respective coordination modes.

Compound	9Be NMR Shifts (δ in ppm)	Solvent	Coordination Mode	Coordination Number	Literature
(Me3Si)3SiBeCl∙(thf)	2.45	thf-d8	tetrahedral	4	this work
(Me3Si)3SiBeI∙(thf)	−0.92	thf-d8	tetrahedral	4	this work
BeCl2(OEt2)2	2.6	C6D6	tetrahedral	4	[13]
BeBr2(OEt2)2	3.0	C6D6	tetrahedral	4	[13]
CpBeSiMe3	−27.70	C6D6	aromatic (η5)	6	[12]
CpBe(SiMe2SiMe3)	−27.20	C6D6	aromatic (η5)	6	[12]
TpBeF	4.54	thf-d8/C7D8 3:5	tetrahedral	4	[9]
TpBeCl	4.95	thf-d8	tetrahedral	4	[9]
TpBeBr	5.15	thf-d8/C6D6 1:3	tetrahedral	4	[9]
TpBeI	4.66	thf-d8	tetrahedral	4	[9]
TptBuBeCl	2.7	C6D6	tetrahedral	4	[15]
TptBuBeBr	2.4	C6D6	tetrahedral	4	[15]
TptBuBeI	1.3	C6D6	tetrahedral	4	[15]
Cp*BeCl	−14.88	C6D6	aromatic (η5)	6	[10]
Cp*BeBr	−14.81	C6D6	aromatic (η5)	6	[10]
Cp*BeI	−15.78	C6D6	aromatic (η5)	6	[10]
DDPBeCl	12.2	C6D6	trigonal planar	3	[16,17]
DDPBeI	13.4	C6D6	trigonal planar	3	[16,17]
Ph2P(NDipp)2BeCl	11.36	C6D6	trigonal planar	3	[18]
Ph2P(NDipp)2BeBr	11.94	C6D6	trigonal planar	3	[18]
Ph2P(NDipp)2BeI	11.53	C6D6	trigonal planar	3	[18]
TerphenylBeCl∙(Et2O)	12.8	C6D6	trigonal planar	3	[13]
TerphenylBeBr∙(Et2O)	13.4	C6D6	trigonal planar	3	[13]
[1,3-(SiMe3)2C3H3]2Be∙(Et2O)	18.2	C6D6	trigonal planar	3	[19]
Be[N(SiMe3)2]2	12.3	C6D6	linear	2	[4]
Be[N(SiMe3)2]2	9.6	thf-d8	linear	2	[4]
Be[N(SiMe3)2]2	12.4	tol-d8	linear	2	[4]

Cp = cyclopentadienyl (C₅H₅), Tp = 1-Tris(pyrazolyl)borate, Tp^tBu = Tris(3-tert-butylpyrazolyl)hydroborato, Cp* = Pentamethylcyclopentadienyl (C₅Me₅), DDP = 2-((2,6-diisopropylphenyl)amino)-4-((2,6-diisopropylphenyl)imino)-pent-2-enyl), Ph₂P(NDipp)₂ = N,N’-bis(2,6-diisopropylphenyl) aminoiminodi(phenyl)phosphorane, Terphenyl = 2,6-di(2,4,6-trimethylphenyl)phenyl.

, which also includes ether-coordinated 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.

2.3. 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.

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^tBu₃)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^tBu₃)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.

2.4. 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(SiMe₃)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. Overview of calculated and observed bond lengths [Å] and angles [°] for 2a and 1b–4b.

	1b 1	2a	2b 1	3b 1	4b 1
Be–X	1.614	2.101 1	2.112	2.292	2.531
Be–Si	2.249	2.239(5)	2.222	2.213	2.203
Be–O	1.670	1.654(5)	1.670	1.655	1.649
X–Be–X	90.60	96.5(2)	97.4	98.2	99.0
Be–X–Be	89.4	83.5(2)	82.6	81.8	81.0
Si–Be–X1	118.0	116.0(2)	114.6	113.6	112.0
Si–Be–X2	123.5	118.5(2)	118.9	118.0	116.8
Si–Be–O	113.4	116.4(3)	115.7	117.1	118.5
Be–X–X–Be	180.0	180.0(3)	179.6	180.0	180.0
X–X–Be–Si	129.1	126.2(4)	136.7	125.5	123.8

¹ Values are averaged.

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 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. Overview of calculated and observed NMR shifts for 1–4.

	1	2	3	4
a	–	2.45	–	−0.92
b	6.90	10.62	12.14	15.93
c	14.01	25.53	29.16	24.78

) 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.

3. 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-^tBu-benzene to the solution of the ligand in C₆D₆, followed by comparative integration of the signals. BeCl₂ and BeI₂ 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 (¹H), 42.4 MHz (⁹Be), 75.5 MHz (¹³C), and 59.6 MHz (²⁹Si). ¹H and ¹³C{¹H}–NMR spectra were referenced to internal C₆D₅H (δ(¹H) = 7.154 ppm; δ(¹³C) = 128.0 ppm), ⁹Be-NMR was referenced to external BeSO₄ in D₂O (δ(⁹Be) = 0 ppm) and ²⁹Si spectra were referenced to an external standard of neat Si(CH₃)₄ (δ(²⁹Si) = 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.

General Procedure for the Synthesis of HypSiBeX∙(thf) (X = Cl 2a, I 4a)

Equimolar amounts of HypSiLi∙(thf)_2.6 (0.221 g, 0.5 mmol) and BeX₂ (X = Cl 0.040 g, 0.5 mmol, I 0.131 g, 0.5 mmol) were dissolved in a mixture of toluene (20 mL) and thf (1 mL). The resulting solution was heated to 60 °C and stirred for 16 h. After that, all volatiles were removed in vacuum, and the residue was dried for an additional 2 h in an oil-pump vacuum. The residue was extracted with 2 mL of toluene and 1 mL of thf and filtrated by using a Teflon cannula, which was covered with a glass micro fiber filter (Whatman^®, ME14 2LE Maidenstone, Kent, UK). Compounds 2a and 4a were obtained after storage of the clear solutions at −28 °C for 12 h.

HypSiBeCl∙(thf) (2a): Yield: 140 mg (96%). ¹H NMR (300 MHz, thf-d₈, 25 °C): δ = 0.18 (s, 27H, Me), 1.72 (s (broad), 0.54H, thf), 3.58 (s (broad), 0.62H, thf). ⁹Be NMR (C₆D₆, 42.2 MHz, 25 °C): δ = 2.45 (s). ¹³C NMR (125 MHz, C₆D₆, 25 °C): δ = 1.68 (s, Me), 25.46 (thf), 67.58 (thf). ²⁹Si NMR (59.6 MHz, C₆D₆, 25 °C): δ = −82.05 (s, (Me₃Si)₃Si, −12.99 (s, (Me₃Si)₃Si).

HypSiBeI∙(thf) (4a): Yield: 157 mg (93%). ¹H NMR (300 MHz, thf-d₈, 25 °C): δ = 0.17 (s, 27H, Me), 1.74 (quint., 5.83H, thf), 3.58 (quint., 5.03H, thf). ⁹Be NMR (C₆D₆, 42.2 MHz, 25 °C): δ = −0.92 (s). ¹³C NMR (125 MHz, C₆D₆, 25 °C): δ = 1.87 (Me), 25.37 (thf), 67.46 (thf). ²⁹Si NMR (59.6 MHz, C₆D₆, 25 °C): δ = −83.94 (s, (Me₃Si)₃Si, −14.91 (s (Me₃Si)₃Si).

Single crystal X-ray diffraction. [C₂₆ H₇₀ Be₂ Cl₂ O₂ Si₈], M = 728.46, colorless crystal (0.07 mm × 0.05 mm × 0.05 mm); monoclinic, space group P2₁/n; a = 10.3667(7) Å, b = 18.8628(12) Å, c = 11.3870(7) Å; α = 90°, β = 98.150(4)°, γ = 90°, V = 2204.2(2) ų; Z = 2; μ = 0.386 mm⁻¹; ρ_calc = 1.098 g⋅cm⁻³; 17848 reflections (θ_max = 26.46°), 4491 unique (R_int = 0.1159); 190 parameters; largest max./min in the final difference Fourier synthesis 0.451 e⋅Å⁻³/−0.407 e⋅Å⁻³; max./min. transmission 0.75/0.67; R₁ = 0.0564 (I > 2σ(I)), wR₂ = 0.1242 (all data).

The crystal was mounted on a nylon loop in inert oil. Data were collected on a Bruker AXS D8 Kappa diffractometer with APEX2 detector (mono-chromated MoKα radiation, λ = 0.71073 Å). The structure was solved by Direct Methods (SHELXS-97) [40] and refined anisotropically by full-matrix least-squares on F² (SHELXL-2014) [41,42]. Absorption correction was performed semi-empirically from equivalent reflections on basis of multi-scans (Bruker AXS APEX2, Karlsruhe, Germany). Hydrogen atoms were refined using rigid methyl groups.

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.

4. Conclusions

We successfully synthesized compounds of the general type HypSiBeX∙(thf) (HypSi = Si(SiMe₃)₃, X = Cl 2a, I 4a) bearing a Be–Si–bond and structurally characterized (2a), the third example of such a compound. The bond lengths in 2a were compared with those obtained from quantum chemical calculations, which were expanded to the other halides as well as on the solvent-free complexes. Further analysis of the calculated data provided a deeper insight into so far not properly investigated Be–Si bond and demonstrated that the Be–Si–bond is mainly covalent (SEN: 1.4-1.3). The calculated ⁹Be NMR shifts shows that tetrahedral coordination mode is also present in solution, which fits well to the reported ⁹Be NMR shifts of comparable compounds.