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

Article abstract of DOI:10.1515/znb-2018-0254

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 diffrac

Full text of DOI:10.1515/znb-2018-0254

Z. Naturforsch. 2019; 74(2)b: 153–158
Edith Alig, Isabelle Georg, Inge Sänger, Lothar Fink, Matthias Wagner
aSnyd Hnatnsh-Weoslfriasm Laernnedr* structure of the donor-free
potassium silanide K[SiPh3]
https://doi.org/10.1515/znb-2018-0254
Kleeberg and Okuda have synthesized a variety of donor-
Received December 5, 2018; accepted December 15, 2018
supported triphenylsilanides such as K(18-crown-6)
(thf)n[SiPh3] (nꢀ=ꢀ0, 1) by Si–B or Si–Si bond cleavage reac-
tions with K[OtBu] (Scheme 1) [15–17]. Up to now unsup-
ported triphenylsilanides M[SiPh3] (Mꢀ=ꢀLi, Na, K, Rb,
Cs) have not been structurally characterized as far as we
know.
Abstract: The donor-free potassium silanide K[SiPh3] was
prepared by the reaction of hexaphenyldisilane, Ph3Si–
SiPh3, with potassium metal in benzene at room temper-
ature. The solid-state structure, determined by powder
X-ray diffraction consists of {K[SiPh3]}2 units, which inter-
act with adjacent dimers to form an infinite chain along the
crystallographic c axis (orthorhombic, space group Cmc21,
Zꢀ=ꢀ4). The structure features short contacts between the
π system of the phenyl rings and the potassium atoms of
neighbouring K[SiPh3] units.
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 con-
nectivity pattern as well as the geometry of the structure of
Keywords: potassium; silanide; silyl anion; X-ray struc- K[SiPh3] arise from strong interactions between the arene
ture analysis.
π systems and the potassium atoms.
1 Introduction
2 Results and discussion
Silyl compounds of the alkali metals, also known as
alkali metal silanides, have attracted considerable inter-
est in the last decades [1–6]. 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 regard-
ing 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] (Mꢀ=ꢀ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
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 potas-
sium to Ph3Si–SiPh3 in benzene and stirring the reaction
mixture under argon for 7 days at room temperature quan-
titatively led to the formation of the silanide K[SiPh3].
During the reaction the product precipitates from the solu-
tion. Generally, unsupported silanides such as K[SiPh3]
are poorly soluble in hydrocarbons. The product was ana-
lyzed by powder X-ray diffraction and NMR spectroscopy.
The corresponding adduct K(thf)n[SiPh3] was quantita-
tively 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.
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].
*Corresponding author: Hans-Wolfram Lerner, Institut für
Anorganische Chemie, Goethe-Universität Frankfurt am Main, Max-
von-Laue-Straße 7, 60438 Frankfurt am Main, Germany,
Fax: +49-69-79876329151, E-mail: lerner@chemie.uni-frankfurt.de
Edith Alig, Isabelle Georg, Inge Sänger, Lothar Fink and Matthias
Wagner: Institut für Anorganische Chemie, Goethe-Universität
Frankfurt am Main, 60438 Frankfurt am Main, Germany
Brought to you by | Tulane University
Authenticated
Download Date | 2/14/19 10:13 AM

ꢀꢀꢀꢁ
154ꢀ ꢀE. Alig et al.: Synthesis and structure of the donor-free potassium silanide K[SiPh3]
Scheme 1:ꢀSynthesis of the donor-supported triphenylsilanides K(18-crown-6)(thf)n[SiPh3] (nꢁ=ꢁ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 (pinꢁ=ꢁpinacol) [15, 16]; (ii) –
((iPrN)2(C2H4))BOtBu [16]; (iii) – Me3SiOtBu [17].
The structure determination of unsupported K[SiPh3] [SiMe(SitBu3)2] [21]. By contrast, unsolvated Na[SiPhtBu2]
was performed by powder X-ray diffraction at two differ- forms chains in the solid state [22].
ent temperatures. The molecular structure of the silanide
The unit cell of K[SiPh3] contains four formula units. As
is shown in Figs. 1–3. The compound crystallizes in the alluded above, the crystal structure consists of two K[SiPh3]
orthorhombic space group Cmc21 with Zꢀ=ꢀ4. The molecule units, which interact with adjacent dimers {K[SiPh3]}2, to
is located on a crystallographic mirror plane. Selected form an infinite chain along the crystallographic c axis
bond lengths and angles are listed in the caption of Fig. 1 (Fig. 3). The K atom is coordinated to one silicon atom and
and in Table 1. In the temperature range from −120 to in η6 fashion to three phenyl rings of two neighbouring
+80°C no changes in constitution and symmetry or space silyl groups, as is shown in Fig. 2. Therefore, the K atoms
group were observed. Crystal data and refinement details possess the coordination number 19. The most characteris-
are given in Table 2.
tic features in the crystal structure of K[SiPh3] are interac-
The general structural motif of the silanide K[SiPh3] tions between the metal atoms and the π systems of the
features a ring formed by two silanide molecules (Fig. 2). phenyl rings. No agostic interactions have been observed.
This motif was also established in the crystal structures of
In contrast to the polymeric structure of K[SiPh3],
Na(thf)[SiPhtBu2], K(thf)n[SiPhtBu2] (nꢀ=ꢀ1, 2), and K(C6H6) the central framework of the solvent-free supersilanides
Scheme 2:ꢀSynthesis of the donor-free potassium silanide K[SiPh3].
Fig. 2:ꢀArrangement and connectivity of the dimers {K[SiPh3]}2
Fig. 1:ꢀMolecular structure of the donor-free potassium silanide
K[SiPh3] measured at Tꢁ=ꢁ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 1ꢁ−ꢁx, y, z.
(Tꢁ=ꢁ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; COGꢁ=ꢁ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: 1ꢁ−ꢁx, y, z.
Brought to you by | Tulane University
Authenticated
Download Date | 2/14/19 10:13 AM

ꢁꢀꢀꢀ
E. Alig et al.: Synthesis and structure of the donor-free potassium silanide K[SiPh3]ꢂ
ꢂ155
Fig. 3:ꢀPacking diagram of K[SiPh3] (Tꢁ=ꢁ298 K).
M[SitBu3] (Mꢀ=ꢀLi, Na) [23] and solvent-free hypersilanides than the crown-ether-thf adduct K(18-cr-6)(thf)[SiPh3].
M[Si(SiMe3)3] (Mꢀ=ꢀLi, Na, K) [24] forms a four-membered Likewise both adducts K(18-cr-6)(thf)n[SiPh3] (nꢀ=ꢀ0, 1)
ring. Due to intermolecular CH–Na interactions, the very possess longer Si–K bonds than the unsolvated potassium
bulky silanide Na[SiMe(SitBu3)2] [9] displays a polymeric silanide K[SiPh3] (cf. Table 1). Due to the bulky tert-butyl
arrangement in the solid state.
groups, the sum of the C–Si–C angles for K(C6D6)3[SitBu3]
The lengths of alkali metal-silicon bonds give evi- (Σ C–Si–C: 317.9(1)°) [23] is significantly greater than those
dence of the charge separation in silanides (Table 1). Gen- determined for the structures of the potassium triphenyl-
erally, strong donors on the alkali metal center lengthen silanides (cf. Table 1).
the Si–M bond and lead to more negative charge on the Si
In summary, the synthesis of unsupported K[SiPh3] was
center. As is shown in Table 1, the crown-ether-supported achieved by the reaction of Ph3Si–SiPh3 with potassium in
silanide K(18-cr-6)[SiPh3] possesses a shorter K–Si bond 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
Table 1:ꢀGeometric parameters of potassium triphenylsilanides.
Ph3Si–SiPh3 gives a convenient access to unsupported tri-
phenylsilanides. 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 crystal-
lographic c axis. In detail, the structure reveals short con-
tacts between the π systems of the phenyl rings and the
potassium atoms of neighbouring K[SiPh3] units.
Si–K (Å) Si–C (Å)a ΣC–Si–C (deg) Reference
K[SiPh3]
3.458(5) 1.994(6)
3.540(1) 1.929(1)
295.6(2) This work
297.0(6) [11]
306.8(2) [12]
K(18-cr-6)[SiPh3]
K(18-cr-6)(thf)[SiPh3] 3.635(1) 1.935(4)
aAverage values.
Brought to you by | Tulane University
Authenticated
Download Date | 2/14/19 10:13 AM

ꢀꢀꢀꢁ
156ꢀ ꢀE. Alig et al.: Synthesis and structure of the donor-free potassium silanide K[SiPh3]
Table 2:ꢀCrystal and structure refinement data of K[SiPh3] at
Tꢁ=ꢁ298 K and 153 K.
K[SiPh3]
K[SiPh3]
Temperature, K
298
153
Formula
C18H15SiK
298.15
Powder
Orthorhombic
Cmc21
C18H15SiK
298.15
Powder
Orthorhombic
Cmc21
Mr
Crystal size, mm3
Crystal system
Space group
a, Å
b, Å
c, Å
V, Å3
Z
16.5948(5)
11.6568(3)
8.32999(16)
1611.37(7)
4
16.3678(9)
11.5455(6)
8.2963(3)
1567.79(13)
4
D
calcd., g cm−3
1.23
1.27
μ(CuKα), mm−1
F(000), e
3.4
624.0
13, 9, 6
0.417
514
286
0.0155
97
0.0256/0.0354
0.0340
1.043
3.5
624.0
13, 9, 6
0.417
498
277
0.0168
97
0.0457/0.0637
0.0406
1.569
Fig. 4:ꢀRietveld plot of the donor-free potassium silanide K[SiPh3]
hkl range
measured at Tꢁ=ꢁ153 K. Color code: Black circlesꢁ=ꢁIobs; red lineꢁ=ꢁIcalc
grey lineꢁ=ꢁIobsꢁ−ꢁIcalc; blue ticksꢁ=ꢁreflection positions. Excluded
regions at 2θꢁ=ꢁ27.2 and 31.4° are due to traces of impurity and at
22.7° due to icing of the capillary.
;
((sinθ)/λ)max, Å−1
Refl. measured
Refl. unique
Rint
Param. refined
a
RP/RwP
a
Rexp
GoFa
aR values and GoF according to Topas [20]: RPꢁ=ꢁΣꢁ|ꢁYoꢁ−ꢁYcꢁ|ꢁ/ΣYo;
RwPꢁ=ꢁ{Σ[w(Yoꢁ−ꢁYc)2/{Σ wYo2)}0.5, Rexpꢁ=ꢁ{Σ Mꢁ−ꢁP/Σ wYo2)}0.5
;
Rintꢁ=ꢁΣꢁ|ꢁIoꢁ−ꢁIcꢁ|ꢁ/Σ Io; GoFꢁ=ꢁ{Σ[w(Yoꢁ−ꢁYc)2]/(Mꢁ−ꢁP)}0.5; wꢁ=ꢁ1/σ(Yo)2 with M
the number of data points, P the number of parameters.
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 pro-
cedure [25]. All other starting materials were purchased
from commercial sources and used without further puri-
fication. The NMR spectra were recorded on Bruker
Avance 300, Avance 400, and Avance 500 spectrometers.
NMR chemical shifts are reported in ppm. Abbreviations:
sꢀ=ꢀsinglet; mꢀ=ꢀmultiplet. Elemental analyses were carried
out by the Microanalytical Laboratory of the Goethe Uni-
versity Frankfurt.
Fig. 5:ꢀRietveld plot of the donor-free potassium silanide K[SiPh3]
measured at Tꢁ=ꢁ298 K. Color code: Black circlesꢁ=ꢁIobs; red lineꢁ=ꢁIcalc
grey lineꢁ=ꢁIobs – Icalc; blue ticksꢁ=ꢁreflection positions. Excluded
regions at 2θꢁ=ꢁ10.1 and 20.2° are due to traces of the starting
material.
;
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 dif-
fraction (cf. Fig. 4) and by NMR spectroscopy after dis-
solution 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 prob-
ably due to carbide formation (the analysis was done
without additives).
3.1 Synthesis of potassium triphenylsilanide
K[SiPh3]
(a) 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
Brought to you by | Tulane University
Authenticated
Download Date | 2/14/19 10:13 AM

ꢁꢀꢀꢀ
E. Alig et al.: Synthesis and structure of the donor-free potassium silanide K[SiPh3]ꢂ
ꢂ157
(b) A mixture of Ph3Si–SiPh3 [25] (10.26 g, 19.78 mmol) Topas [20]. First, a Pawley refinement was carried out to
and potassium pieces (1.50 g, 36.36 mmol) in benzene refine background, zero point error, unit cell parameters,
(200 mL) was stirred at room temperature under argon peak width, and peak asymmetry parameters. In the Riet-
for 1 week. The formed precipitate was separated by veld refinements, the phenyl rings including H atoms were
filtering. The colorless solid was washed with ben- restrained to be flat; the bond lengths and bond angles
zene and dried in vacuo. The product was analyzed in the rings were restrained to meaningful values. The
by powder X-ray diffraction (cf. Fig. 5). Peaks of the values of the isotropic displacement parameters of the
starting material Ph3Si–SiPh3 were also detected. The hydrogen atoms were constrained to be 1.2 times of those
analysis of the Rietveld plot indicates 96% of K[SiPh3] of the atoms to which they are attached. K and Si were
with 4% of Ph3Si–SiPh3 as impurity.
not restrained. Two regions at 2θꢀ=ꢀ10.1 and 20.2° were
(c) To K[SiPh3] (0.03 g, 0.1 mmol) 1 mL thf was added excluded due to traces of the starting material.
1
at room temperature. The H/13C/29Si NMR spectra
CCDC 1883105 (K[SiPh3] at Tꢀ=ꢀ298 K) and 1883106
exclusively revealed the signals of the thf-supported (K[SiPh3] at Tꢀ=ꢀ153 K) contain the supplementary crystal-
1
silanide K(thf)n[SiPh3]. – H NMR (300 MHz, thf- lographic data for this paper. These data can be obtained
d8): δꢀ=ꢀ7.38–7.34 (m, 6 H, o-CHAr), 6.90–6.85 (m, free of charge from The Cambridge Crystallographic Data
6 H, m-CHAr), 6.77–6.71 (m, 3 H, p-CHAr), 3.54 (m, Centre via www.ccdc.cam.ac.uk/data_request/cif.
OCH2), 1.72 (m, CH2). 13C{1H} NMR (126 MHz, thf-d8):
δꢀ=ꢀ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
References
(59.6 MHz, thf-d8): δꢀ=ꢀ−7.1 (s).
(d) A solution of K[SiPh3] (0.05 mmol) in 0.5 mL thf-d8 was
[1] P. D. Lickiss, C. Smith, Coord. Chem. Rev. 1995, 145, 75.
[2] K. Tamao, A. Kawachi, Adv. Organomet. Chem. 1995, 38, 1.
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] N. Wiberg, Coord. Chem. Rev. 1997, 163, 217.
[4] J. Belzner, U. Dehnert, The Chemistry of Organic Silicon
Compounds, Vol. 2, (Eds.: Z. Rappoport, Y. Apeloig), Wiley,
Chichester, 1998, p. 779.
[5] H.-W. Lerner, Coord. Chem. Rev. 2005, 249, 781.
[6] C. Marschner, Silicon-Centered Anions in Organosilicon Com-
pounds. Theory and Experiment (Synthesis), Vol. 1, (Ed.: V. Y.
Lee), Academic Press, Oxford, 2017, chapter 7, pp. 295–360.
[7] M. Nakamoto, T. Fukawa, V. Y. Lee, A. Sekiguchi, J. Am. Chem.
Soc. 2002, 124, 15160.
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 (λꢀ=ꢀ1.54056 Å). The speci-
men 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 measure-
ments were carried out in a sealed 1.0 mm glass capillary
under nitrogen.
[8] N. Wiberg, W. Niedermayer, H. Nöth, M. Warchhold, J. Orga-
nomet. Chem. 2001, 628, 46.
[9] I. Sänger, M. Gärtner, M. Bolte, M. Wagner, H.-W. Lerner, Z.
Anorg. Allg. Chem. 2018, 644, 925.
[10] H. Gilman, T. C. Wu, H. A. Hartzfeld, G. A. Guter, A. G. Smith, J.
J. Goodman, S. H. Eidt, J. Am. Chem. Soc. 1952, 74, 561.
[11] H. Gilman, T. C. Wu, J. Am. Chem. Soc. 1951, 73, 4031.
[12] R. A. Benkeser, R. G. Severson, J. Am. Chem. Soc. 1951, 73,
1424.
[13] M. V. George, D. J. Peterson, H. Gilman, J. Am. Chem. Soc. 1960,
82, 403 and ref. cited therein.
[14] E. Wiberg, O. Stecher, H. J. Andraschek, L. Kreuzbichler, E.
Staude, Angew. Chem. Int. Ed. Engl. 1963, 2, 507.
[15] C. Kleeberg, C. Borner, Eur. J. Inorg. Chem. 2013, 2799.
[16] C. Kleeberg, Dalton Trans. 2013, 42, 8276.
[17] V. Leich, K. Lamberts, T. P. Spaniol, J. Okuda, Dalton Trans.
2014, 43, 14315.
[18] M. J. McGeary, K. Folting, W. E. Streib, J. C. Huffman, K. G.
Caulton, Polyhedron 1991, 10, 2699.
[19] A. Budanow, H. Hashemi Haeri, I. Sänger, F. Schödel, M. Bolte,
T. Prisner, M. Wagner, H.-W. Lerner, Chem. Eur. J. 2014, 20,
10236.
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 anneal-
ing in Dash. Rietveld refinements were performed with
[20] A. Coelho, Topas Acad., User Man. Tech. Ref. Brisbane (Aus-
tralia) 2009.
Brought to you by | Tulane University
Authenticated
Download Date | 2/14/19 10:13 AM

ꢀꢀꢀꢁ
158ꢀ ꢀE. Alig et al.: Synthesis and structure of the donor-free potassium silanide K[SiPh3]
[21] H.-W. Lerner, S. Scholz, M. Bolte, M. Wagner, Z. Anorg. Allg.
Chem. 2004, 630, 443.
[25] T. Bernert, B. Winkler, Y. Krysiak, L. Fink, M. Berger, E. Alig, L.
Bayarjargal, V. Milman, L. Ehm, P. W. Stephens, N. Auner, H.-W.
Lerner, Cryst. Growth Des. 2014, 14, 2937.
[22] H.-W. Lerner, S. Scholz, M. Bolte, Z. Anorg. Allg. Chem. 2001,
627, 1638.
[26] WinXPOW 210, STOE & Cie GmbH, Darmstadt (Germany)
2004.
[23] N. Wiberg, K. Amelunxen, H.-W. Lerner, H. Schuster, H. Nöth,
I. Krossing, M. Schmidt-Amelunxen, T. Seifert, J. Organomet.
Chem. 1997, 542, 1.
[27] A. Boultif, D. Louër, J. Appl. Crystallogr. 1991, 24, 987.
[28] W. I. F. David, K. Shankland, J. van de Streek, E. Pidcock, W. D.
S. Motherwell, J. C. Cole, J. Appl. Crystallogr. 2006, 39, 910.
[24] K. W. Klinkhammer, Chem. Eur. J. 1997, 3, 1418.
Brought to you by | Tulane University
Authenticated
Download Date | 2/14/19 10:13 AM