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Synthesis and structure of the donor-free potassium silanide K[SiPh3]

  • Edith Alig , Isabelle Georg , Inge Sänger , Lothar Fink , Matthias Wagner and Hans-Wolfram Lerner EMAIL logo
Published/Copyright: January 12, 2019
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Zeitschrift für Naturforschung B
From the journal Zeitschrift für Naturforschung B Volume 74 Issue 2

Abstract

The donor-free potassium silanide K[SiPh3] was prepared by the reaction of hexaphenyldisilane, Ph3Si–SiPh3, with potassium metal in benzene at room temperature. The solid-state structure, determined by powder X-ray diffraction consists of {K[SiPh3]}2 units, which interact with adjacent dimers to form an infinite chain along the crystallographic c axis (orthorhombic, space group Cmc21, Z=4). The structure features short contacts between the π system of the phenyl rings and the potassium atoms of neighbouring K[SiPh3] units.

Keywords: potassium; silanide; silyl anion; X-ray structure analysis

1 Introduction

Silyl compounds of the alkali metals, also known as alkali metal silanides, have attracted considerable interest in the last decades [1], [2], [3], [4], [5], [6]. Compounds with negatively charged silicon atoms are of fundamental importance in organometallic and inorganic synthesis. In particular, sterically encumbered organo-substituted silanides such as Li[Si(SiMetBu2)3] [7] or Na[SiMe(SitBu3)2] [8], [9] proved to be very useful σ-donating ligands. First papers regarding the synthesis and reactivity of silanides M[SiR3] were published by Gilman et al. in the 1950s and 1960s [10], [11]. Examples studied in detail were the triphenylsilanides M[SiPh3] (M=Li, Na, K, Rb, Cs) [12], [13]. Early syntheses used Ph3Si–SiPh3 as the starting material, which was treated with the respective alkali metal in liquid ammonia or in donor solvents [13], [14]. Very recently, the groups of Kleeberg and Okuda have synthesized a variety of donor-supported triphenylsilanides such as K(18-crown-6)(thf)n[SiPh3] (n=0, 1) by Si–B or Si–Si bond cleavage reactions with K[OtBu] (Scheme 1) [15], [16], [17]. Up to now unsupported triphenylsilanides M[SiPh3] (M=Li, Na, K, Rb, Cs) have not been structurally characterized as far as we know.

Scheme 1: Synthesis of the donor-supported triphenylsilanides K(18-crown-6)(thf)n[SiPh3] (n=0, 1) by Si–B or Si–Si bond cleavage reactions with K[OtBu] in the presence of 18-crown-6 (18-cr-6); +K[OtBu], +18-cr-6, in thf; (i) – pinBOtBu (pin=pinacol) [15], [16]; (ii) – ((iPrN)2(C2H4))BOtBu [16]; (iii) – Me3SiOtBu [17].
Scheme 1:

Synthesis of the donor-supported triphenylsilanides K(18-crown-6)(thf)n[SiPh3] (n=0, 1) by Si–B or Si–Si bond cleavage reactions with K[OtBu] in the presence of 18-crown-6 (18-cr-6); +K[OtBu], +18-cr-6, in thf; (i) – pinBOtBu (pin=pinacol) [15], [16]; (ii) – ((iPrN)2(C2H4))BOtBu [16]; (iii) – Me3SiOtBu [17].

Our approach to the unsupported triphenylsilanide K[SiPh3] was to cleave the Si–Si bond of Ph3Si–SiPh3 by treatment with potassium in apolar solvents. Here we report the synthesis and unusual structural features of the non-solvated potassium silanide K[SiPh3]. The connectivity pattern as well as the geometry of the structure of K[SiPh3] arise from strong interactions between the arene π systems and the potassium atoms.

2 Results and discussion

Given the interesting reactivity reported by Kleeberg and Okuda and our on-going interest in the chemistry of silanides, we have investigated the potassium-induced conversion of hexaphenyldisilane, Ph3Si–SiPh3. Our goal was to synthesize the donor-free potassium silanide K[SiPh3]. As is shown in Scheme 2, addition of potassium to Ph3Si–SiPh3 in benzene and stirring the reaction mixture under argon for 7 days at room temperature quantitatively led to the formation of the silanide K[SiPh3]. During the reaction the product precipitates from the solution. Generally, unsupported silanides such as K[SiPh3] are poorly soluble in hydrocarbons. The product was analyzed by powder X-ray diffraction and NMR spectroscopy. The corresponding adduct K(thf)n[SiPh3] was quantitatively formed when K[SiPh3] was treated with the donor solvent thf. In contrast to unsupported K[SiPh3], the thf-complexed silanides K(thf)n[SiPh3] are soluble in common organic solvents and therefore can be examined by NMR spectroscopy. The potassium silanide K[SiPh3] and its donor adducts are extremely sensitive to air and moisture.

Scheme 2: Synthesis of the donor-free potassium silanide K[SiPh3].
Scheme 2:

Synthesis of the donor-free potassium silanide K[SiPh3].

The siloxide K[OSiPh3] [18] is formed as a main product when K[SiPh3] is exposed to air. For a plausible mechanism of this process see Ref. [19].

The structure determination of unsupported K[SiPh3] was performed by powder X-ray diffraction at two different temperatures. The molecular structure of the silanide is shown in Figs. 1–3 . The compound crystallizes in the orthorhombic space group Cmc21 with Z=4. The molecule is located on a crystallographic mirror plane. Selected bond lengths and angles are listed in the caption of Fig. 1 and in Table 1. In the temperature range from −120 to +80°C no changes in constitution and symmetry or space group were observed. Crystal data and refinement details are given in Table 2.

Fig. 1: Molecular structure of the donor-free potassium silanide K[SiPh3] measured at T=298 K. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å), and bond angles (deg): Si–C(2) 2.020(5), Si–C(6) 1.966(6); C(2)–Si–C(6) 105.1(2), C(2)–Si–C(2′) 95.4(2). Symmetry-equivalent atoms (‘) were generated by applying the symmetry operator 1−x, y, z.
Fig. 1:

Molecular structure of the donor-free potassium silanide K[SiPh3] measured at T=298 K. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å), and bond angles (deg): Si–C(2) 2.020(5), Si–C(6) 1.966(6); C(2)–Si–C(6) 105.1(2), C(2)–Si–C(2′) 95.4(2). Symmetry-equivalent atoms (‘) were generated by applying the symmetry operator 1−x, y, z.

Fig. 2: Arrangement and connectivity of the dimers {K[SiPh3]}2 (T=298 K). View along the crystallographic a axis. Hydrogen atoms are omitted for clarity. For further atom numbering see Fig. 1. Intermolecular distances (Å): K(1)···COG1 3.028 Å, K(1)···COG2 2.918 Å; COG=center of gravity of the phenyl rings; COG1: C(2)C(4)C(5)C(7)C(9)C(12), COG2: C(6)C(11)C(15)C(18); symmetry operator of primed atoms: 1−x, y, z.
Fig. 2:

Arrangement and connectivity of the dimers {K[SiPh3]}2 (T=298 K). View along the crystallographic a axis. Hydrogen atoms are omitted for clarity. For further atom numbering see Fig. 1. Intermolecular distances (Å): K(1)···COG1 3.028 Å, K(1)···COG2 2.918 Å; COG=center of gravity of the phenyl rings; COG1: C(2)C(4)C(5)C(7)C(9)C(12), COG2: C(6)C(11)C(15)C(18); symmetry operator of primed atoms: 1−x, y, z.

Fig. 3: Packing diagram of K[SiPh3] (T=298 K).
Fig. 3:

Packing diagram of K[SiPh3] (T=298 K).

Table 1:

Geometric parameters of potassium triphenylsilanides.

Si–K (Å)Si–C (Å)aΣC–Si–C (deg)Reference
K[SiPh3]3.458(5)1.994(6)295.6(2)This work
K(18-cr-6)[SiPh3]3.540(1)1.929(1)297.0(6)[11]
K(18-cr-6)(thf)[SiPh3]3.635(1)1.935(4)306.8(2)[12]

  1. aAverage values.

Table 2:

Crystal and structure refinement data of K[SiPh3] at T=298 K and 153 K.

K[SiPh3]K[SiPh3]
Temperature, K298153
FormulaC18H15SiKC18H15SiK
Mr298.15298.15
Crystal size, mm3PowderPowder
Crystal systemOrthorhombicOrthorhombic
Space groupCmc21Cmc21
a, Å16.5948(5)16.3678(9)
b, Å11.6568(3)11.5455(6)
c, Å8.32999(16)8.2963(3)
V, Å31611.37(7)1567.79(13)
Z44
Dcalcd., g cm−31.231.27
μ(CuKα), mm−13.43.5
F(000), e624.0624.0
hkl range13, 9, 613, 9, 6
((sinθ)/λ)max, Å−10.4170.417
Refl. measured514498
Refl. unique286277
Rint0.01550.0168
Param. refined9797
RP/RwPa0.0256/0.03540.0457/0.0637
Rexpa0.03400.0406
GoFa1.0431.569

  1. aR values and GoF according to Topas [20]: RP=Σ|Yo−Yc|/ΣYo; RwP={Σ[w(Yo−Yc)2/{Σ wYo2)}0.5, Rexp={Σ M−P/Σ wYo2)}0.5; Rint=Σ|Io−Ic|/Σ Io; GoF={Σ[w(Yo−Yc)2]/(M−P)}0.5; w=1/σ(Yo)2 with M the number of data points, P the number of parameters.

The general structural motif of the silanide K[SiPh3] features a ring formed by two silanide molecules (Fig. 2). This motif was also established in the crystal structures of Na(thf)[SiPhtBu2], K(thf)n[SiPhtBu2] (n=1, 2), and K(C6H6)[SiMe(SitBu3)2] [21]. By contrast, unsolvated Na[SiPhtBu2] forms chains in the solid state [22].

The unit cell of K[SiPh3] contains four formula units. As alluded above, the crystal structure consists of two K[SiPh3] units, which interact with adjacent dimers {K[SiPh3]}2, to form an infinite chain along the crystallographic c axis (Fig. 3). The K atom is coordinated to one silicon atom and in η6 fashion to three phenyl rings of two neighbouring silyl groups, as is shown in Fig. 2. Therefore, the K atoms possess the coordination number 19. The most characteristic features in the crystal structure of K[SiPh3] are interactions between the metal atoms and the π systems of the phenyl rings. No agostic interactions have been observed.

In contrast to the polymeric structure of K[SiPh3], the central framework of the solvent-free supersilanides M[SitBu3] (M=Li, Na) [23] and solvent-free hypersilanides M[Si(SiMe3)3] (M=Li, Na, K) [24] forms a four-membered ring. Due to intermolecular CH–Na interactions, the very bulky silanide Na[SiMe(SitBu3)2] [9] displays a polymeric arrangement in the solid state.

The lengths of alkali metal-silicon bonds give evidence of the charge separation in silanides (Table 1). Generally, strong donors on the alkali metal center lengthen the Si–M bond and lead to more negative charge on the Si center. As is shown in Table 1, the crown-ether-supported silanide K(18-cr-6)[SiPh3] possesses a shorter K–Si bond than the crown-ether-thf adduct K(18-cr-6)(thf)[SiPh3]. Likewise both adducts K(18-cr-6)(thf)n[SiPh3] (n=0, 1) possess longer Si–K bonds than the unsolvated potassium silanide K[SiPh3] (cf. Table 1). Due to the bulky tert-butyl groups, the sum of the C–Si–C angles for K(C6D6)3[SitBu3] (Σ C–Si–C: 317.9(1)°) [23] is significantly greater than those determined for the structures of the potassium triphenylsilanides (cf. Table 1).

In summary, the synthesis of unsupported K[SiPh3] was achieved by the reaction of Ph3Si–SiPh3 with potassium in benzene at room temperature. The silanide K[SiPh3] could be isolated in good yield and high purity by this procedure. Therefore, alkali metal-induced Si–Si bond cleavage of Ph3Si–SiPh3 gives a convenient access to unsupported triphenylsilanides. The solid-state structure of unsupported K[SiPh3] consists of dimers {K[SiPh3]}2, which interact with adjacent dimers to form an infinite chain along the crystallographic c axis. In detail, the structure reveals short contacts between the π systems of the phenyl rings and the potassium atoms of neighbouring K[SiPh3] units.

3 Experimental section

The solvents thf, thf-d8, benzene, and C6D6 were stirred over sodium/benzophenone and distilled prior to use. Ph3Si–SiPh3 was prepared according to the published procedure [25]. All other starting materials were purchased from commercial sources and used without further purification. The NMR spectra were recorded on Bruker Avance 300, Avance 400, and Avance 500 spectrometers. NMR chemical shifts are reported in ppm. Abbreviations: s=singlet; m=multiplet. Elemental analyses were carried out by the Microanalytical Laboratory of the Goethe University Frankfurt.

3.1 Synthesis of potassium triphenylsilanide K[SiPh3]

  1. A mixture of Ph3Si–SiPh3 [25] (1.00 g, 1.93 mmol) and potassium pieces (0.29 g, 7.49 mmol) in benzene (11 mL) was stirred at room temperature under argon for 1 week. After removing the remaining K pieces, the formed precipitate was separated by filtering. The colorless solid was washed with benzene and dried in vacuo. The product was analyzed by powder X-ray diffraction (cf. Fig. 4

    Fig. 4: Rietveld plot of the donor-free potassium silanide K[SiPh3] measured at T=153 K. Color code: Black circles=Iobs; red line=Icalc; grey line=Iobs−Icalc; blue ticks=reflection positions. Excluded regions at 2θ=27.2 and 31.4° are due to traces of impurity and at 22.7° due to icing of the capillary.
    Fig. 4:

    Rietveld plot of the donor-free potassium silanide K[SiPh3] measured at T=153 K. Color code: Black circles=Iobs; red line=Icalc; grey line=Iobs−Icalc; blue ticks=reflection positions. Excluded regions at 2θ=27.2 and 31.4° are due to traces of impurity and at 22.7° due to icing of the capillary.

    ) and by NMR spectroscopy after dissolution in thf (below) (yield: 1.13 g, 98%). – Anal. for C18H15KSi (298.5) calcd. C 72.43, H 5.07; found C 68.20, H 5.11. The discrepancy in the carbon content is probably due to carbide formation (the analysis was done without additives).

  2. A mixture of Ph3Si–SiPh3 [25] (10.26 g, 19.78 mmol) and potassium pieces (1.50 g, 36.36 mmol) in benzene (200 mL) was stirred at room temperature under argon for 1 week. The formed precipitate was separated by filtering. The colorless solid was washed with benzene and dried in vacuo. The product was analyzed by powder X-ray diffraction (cf. Fig. 5

    Fig. 5: Rietveld plot of the donor-free potassium silanide K[SiPh3] measured at T=298 K. Color code: Black circles=Iobs; red line=Icalc; grey line=Iobs – Icalc; blue ticks=reflection positions. Excluded regions at 2θ=10.1 and 20.2° are due to traces of the starting material.
    Fig. 5:

    Rietveld plot of the donor-free potassium silanide K[SiPh3] measured at T=298 K. Color code: Black circles=Iobs; red line=Icalc; grey line=Iobs – Icalc; blue ticks=reflection positions. Excluded regions at 2θ=10.1 and 20.2° are due to traces of the starting material.

    ). Peaks of the starting material Ph3Si–SiPh3 were also detected. The analysis of the Rietveld plot indicates 96% of K[SiPh3] with 4% of Ph3Si–SiPh3 as impurity.

  3. To K[SiPh3] (0.03 g, 0.1 mmol) 1 mL thf was added at room temperature. The 1H/13C/29Si NMR spectra exclusively revealed the signals of the thf-supported silanide K(thf)n[SiPh3]. – 1H NMR (300 MHz, thf-d8): δ=7.38–7.34 (m, 6 H, o-CHAr), 6.90–6.85 (m, 6 H, m-CHAr), 6.77–6.71 (m, 3 H, p-CHAr), 3.54 (m, OCH2), 1.72 (m, CH2). 13C{1H} NMR (126 MHz, thf-d8): δ=160.8 (ipso-CAr), 131.1 (o-CHAr), 126.4 (m-CHAr), 123.0 (p-CHAr), 71.0 (OCH2), 25.4 (CH2). – 29Si{1H} NMR (59.6 MHz, thf-d8): δ=−7.1 (s).

  4. A solution of K[SiPh3] (0.05 mmol) in 0.5 mL thf-d8 was kept in air for 3 h at room temperature. The 1H/13C/29Si NMR spectra revealed signals which can be attributed to K[OSiPh3] [18]. Additionally, signals of the side-products, Ph3SiH and Ph3SiOH, were observed.

3.2 Crystal structure determinations

The X-ray powder diffraction experiments were performed on a STOE Stadi-P diffractometer equipped with a Ge(111) monochromator and a linear position-sensitive detector (PSD) using CuKα1 radiation (λ=1.54056 Å). The specimen was measured in a sealed borosilicate glass capillary with a diameter of 1.0 mm. The data was collected with the STOE WinXPOW software [26].

Temperature-dependent XRPD measurements were performed on an identical STOE Stadi-P diffractometer equipped with an Oxford Cryosystem 700. The measurements were carried out in a sealed 1.0 mm glass capillary under nitrogen.

3.3 Structure determinations from X-ray powder data

The powder patterns were indexed with Dicvol91 [27]. The space group determination was carried out in Dash [28]. The structures were solved with a starting molecular structural model in direct space using simulated annealing in Dash. Rietveld refinements were performed with Topas [20]. First, a Pawley refinement was carried out to refine background, zero point error, unit cell parameters, peak width, and peak asymmetry parameters. In the Rietveld refinements, the phenyl rings including H atoms were restrained to be flat; the bond lengths and bond angles in the rings were restrained to meaningful values. The values of the isotropic displacement parameters of the hydrogen atoms were constrained to be 1.2 times of those of the atoms to which they are attached. K and Si were not restrained. Two regions at 2θ=10.1 and 20.2° were excluded due to traces of the starting material.

CCDC 1883105 (K[SiPh3] at T=298 K) and 1883106 (K[SiPh3] at T=153 K) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.

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Received: 2018-12-05
Accepted: 2018-12-15
Published Online: 2019-01-12
Published in Print: 2019-02-25

©2019 Walter de Gruyter GmbH, Berlin/Boston

Articles in the same Issue

  1. Frontmatter
  2. In this Issue
  3. Synthesis and structure of the donor-free potassium silanide K[SiPh3]
  4. A series of Keggin- and Wells-Dawson-polyoxometalate-based compounds constructed from oxygen-functional imidazole derivatives
  5. Synthesis, crystal structure, photoluminescence and photochemistry of bis(triphenylphosphine)silver(I) flavonolate
  6. Facile synthesis of new pyrazolo[4′,3′:5,6]pyrano[2,3-d]pyrimidin-5(1H)-ones via the tandem intramolecular Pinner–Dimroth rearrangement and their antibacterial evaluation
  7. Synthesis of 2,6-dimethoxy-9-phenyl-1H-phenalen-1-one and structural revision of the benzoindenone from Eichhornia crassipes
  8. Addition of some 6-amino-4-aryl-2(1H)-pyridones to phenylisocyanate and related reactions
  9. Study on the chemical constituents of Dacrydium elatum and their cytotoxic activity
  10. RhSn3 and the Modifications of RhSn4 – Structure and 119Sn Mössbauer spectroscopic characterization
  11. Equiatomic iron-based tetrelides TFeSi and TFeGe (T = Zr, Nb, Hf, Ta) – A 57Fe Mössbauer-spectroscopic study
  12. The reaction of a particularly electrophilic acyclic diaminocarbene with carbon monoxide: formation of β- and γ-lactams
  13. Zinc-lead ordering in equiatomic rare earth plumbides REZnPb (RE=La–Nd and Sm–Tb)
  14. Ni(II) complexes with thioether-functionalized silylamide ligands. Synthesis and crystal structures of [Ni{Me2Si(N-C6H4-2-S-t-Bu)2}], [Ni{Ph2Si(N-C6H4-2-SMe)2}] and [Ni{Ph2Si(N-C6H4-2-SPh)2}]
https://doi.org/10.1515/znb-2018-0254
Keywords for this article
potassium; silanide; silyl anion; X-ray structure analysis

Articles in the same Issue

  1. Frontmatter
  2. In this Issue
  3. Synthesis and structure of the donor-free potassium silanide K[SiPh3]
  4. A series of Keggin- and Wells-Dawson-polyoxometalate-based compounds constructed from oxygen-functional imidazole derivatives
  5. Synthesis, crystal structure, photoluminescence and photochemistry of bis(triphenylphosphine)silver(I) flavonolate
  6. Facile synthesis of new pyrazolo[4′,3′:5,6]pyrano[2,3-d]pyrimidin-5(1H)-ones via the tandem intramolecular Pinner–Dimroth rearrangement and their antibacterial evaluation
  7. Synthesis of 2,6-dimethoxy-9-phenyl-1H-phenalen-1-one and structural revision of the benzoindenone from Eichhornia crassipes
  8. Addition of some 6-amino-4-aryl-2(1H)-pyridones to phenylisocyanate and related reactions
  9. Study on the chemical constituents of Dacrydium elatum and their cytotoxic activity
  10. RhSn3 and the Modifications of RhSn4 – Structure and 119Sn Mössbauer spectroscopic characterization
  11. Equiatomic iron-based tetrelides TFeSi and TFeGe (T = Zr, Nb, Hf, Ta) – A 57Fe Mössbauer-spectroscopic study
  12. The reaction of a particularly electrophilic acyclic diaminocarbene with carbon monoxide: formation of β- and γ-lactams
  13. Zinc-lead ordering in equiatomic rare earth plumbides REZnPb (RE=La–Nd and Sm–Tb)
  14. Ni(II) complexes with thioether-functionalized silylamide ligands. Synthesis and crystal structures of [Ni{Me2Si(N-C6H4-2-S-t-Bu)2}], [Ni{Ph2Si(N-C6H4-2-SMe)2}] and [Ni{Ph2Si(N-C6H4-2-SPh)2}]
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