NHC Supersilyl Silver Complex [Ag(IPr)SitBu3] as a Promising Agent for Substitution Reactions

Article abstract of DOI:10.1002/zaac.201900304

The NHC supersilyl silver complex [Ag(IPr)SitBu₃] (IPr = NHC_IPr) was prepared by treatment of Ag(IPr)Cl with Na(thf)₂[SitBu₃] in benzene/thf at room temperature. X-ray quality crystals of the NHC supersilyl silver complex [Ag(IPr)SitBu₃] (monoclinic, space group P2₁/m) were grown from heptane at room temperature. The ²⁹Si NMR spectrum of a solution of [Ag(IPr)SitBu₃] in C₆D₆ revealed two doublets caused by coupling to ¹⁰⁷Ag and ¹⁰⁹Ag nuclei. We further investigated the possibility of a conversion of triel halides EX₃ by treatment with [Ag(IPr)SitBu₃]. At ambient temperature the reaction of [Ag(IPr)SitBu₃] with an excess of EX₃ yielded tBu₃SiEX₂ (E = B, Al; X = Cl, Br; E = Ga; X = Cl) and IPr·EX₃ (EX₃ = BCl₃, BBr₃, AlCl₃, AlBr₃, GaCl₃). The identity of tBu₃SiEX₂ and IPr·EX₃ was confirmed by comparison with authentic samples.

Full text of DOI:10.1002/zaac.201900304

Journal of Inorganic and General Chemistry
DOI: 10.1002/zaac.201900304 SHORT COMMUNICATION
Zeitschrift für anorganische und allgemeine Chemie
NHC Supersilyl Silver Complex [Ag(IPr)SitBu3] as a Promising
Agent for Substitution Reactions
Frauke Schödel,[a] Michael Bolte,[a] Matthias Wagner,[a] and Hans-Wolfram Lerner*[a]
Abstract. The NHC supersilyl silver complex [Ag(IPr)SitBu3] (IPr =
gated the possibility of a conversion of triel halides EX3 by treatment
NHCIPr) was prepared by treatment of Ag(IPr)Cl with Na(thf)2[SitBu3] with [Ag(IPr)SitBu3]. At ambient temperature the reaction of
in benzene/thf at room temperature. X-ray quality crystals of the NHC [Ag(IPr)SitBu3] with an excess of EX3 yielded tBu3SiEX2 (E = B, Al;
supersilyl silver complex [Ag(IPr)SitBu3] (monoclinic, space group X = Cl, Br; E = Ga; X = Cl) and IPr·EX3 (EX3 = BCl3, BBr3, AlCl3,
P21/m) were grown from heptane at room temperature. The 29Si NMR AlBr3, GaCl3). The identity of tBu3SiEX2 and IPr·EX3 was confirmed
spectrum of a solution of [Ag(IPr)SitBu3] in C6D6 revealed two dou-
blets caused by coupling to 107Ag and 109Ag nuclei. We further investi-
by comparison with authentic samples.
Introduction
To gain supersilyl silver complexes there is a need for com-
plexation of Ag+ to lower the redox potential. In pursuit of a
stable silver supersilanide we decided to treat NHC-stabilized
Ag+ cations with supersilylsodium.
Over the past decades silanides [SiR3]– (R = organyl) have
been widely investigated in basic academic research.[1–5] Both
supported and unsupported silanides are potent nucleophiles
and it has been shown that their donor strength correlates to
their reduction behavior.[6] For instance K[SitBu3] as a very
strong Brønsted base already deprotonates benzene at room
temperature.[6] It is therefore not surprising, that the supersilyl
anion [SitBu3]– can also easily release one electron to form the
related supersilyl radical ·SitBu3. As shown in Figure 1, one
can consider four possible isomers of the supersilyl radical
·SitBu3.[7]
Disupersilylated trielanes of the type (tBu3Si)2EX (E = B,
Al, Ga, In, Tl; X = F, Cl, Br)[9–12] can be prepared from EX3
and Na[SitBu3].[4,6] However, when EX3 in pentane was
treated with one or two molar equivalents of Na[SitBu3][4,6] in
both cases, reaction of EX3 with Na[SitBu3][4,6] exclusively
yielded the disupersilylated compound (tBu3Si)2EX. Thereby
no monosupersilylated compound tBu3SiEX2 was formed.[10]
In these reactions the second substitution step must be much
faster than the first one. We also suggested that in solution an
equilibrium of (tBu3Si)2EX and EX3 with the ion pair
[tBu3Si-E-SitBu3][EX4] exists.[10] The previously published
preparation protocols revealed quite low yields for tBu3SiEX2.
As an example, tBu3SiAlBr2 was obtained in 34% yield by
treatment of [Zn(SitBu3)2] with AlBr3.[11] The purpose of this
paper is to establish a direct conversion of triel halides EX3
by supersilyl silver with forming monosupersilylated trielanes
tBu3SiEX2.
Figure 1. Isomers of the supersilyl radical ·SitBu3 (A).
DFT calculations have shown that the silicon-centered
radical ·SitBu3 (A) is not the lowest-energy structure. Instead
the carbon-centered radical B, where a methyl group has mi-
grated from one tBu group to the silicon atom, is significantly
lower in energy than A. These calculations also revealed that
structures C and D formed by 1,2 H-shifts are more disfavored
than B.[7,8]
Results and Discussion
The NHC-supported silver complex [Ag(IPr)SitBu3] (1) was
prepared
by
the reaction
of
[Ag(IPr)Cl]
with
In many cases Na[SitBu3][4,6] is able to reduce transition
metal cations to the respective elements.[4] For example by the
reaction of Ag+ with supersilylsodium elemental silver along
with the supersilyl radical dimer tBu3SiSitBu3 was formed.[6]
Na(thf)2[SitBu3][4,6] in a mixture of benzene and thf at room
temperature, as shown in Scheme 1.
* Dr. H.-W. Lerner
E-Mail: lerner@chemie.uni-frankfurt.de
[a] Institut für Anorganische Chemie
Goethe-Universität
Max-von-Laue-Str. 7
60438 Frankfurt am Main, Germany
Scheme 1. Preparation route for the NHC supersilyl silver complex
[Ag(IPr)SitBu3] (1). (i) –NaCl; benzene/thf, room temperature.
© 2020 The Authors. Published by Wiley-VCH Verlag GmbH &
Co. KGaA. · This is an open access article under the terms of the
Creative Commons Attribution License, which permits use, distri-
bution and reproduction in any medium, provided the original work
is properly cited.
After removing all volatiles, the residue was extracted into
heptane. The NMR spectra of the reaction mixture solely
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Journal of Inorganic and General Chemistry
SHORT COMMUNICATION
Zeitschrift für anorganische und allgemeine Chemie
showed resonances due to the silver complex 1. Single crystals
of 1 were grown from filtered heptane solution at room tem-
perature. The NHC supersilyl silver complex 1 could be char-
acterized by X-ray crystallography (monoclinic, space group
P21/m).
The molecular structure of 1 is shown in Figure 2 (selected
bond lengths and angles are in the Figure caption). The com-
plex 1 is located on a crystallographic mirror plane with only
half a molecule in the asymmetric unit. Besides two
other silyl complexes [Ag(IPr)SiEt(SiMe3)2][13] and
[Ag(IPr)Si(SiMe3)3],[13] the supersilyl silver complex 1 is the
third IPr silyl silver complex which could be characterized by
X-ray crystallography. Due to the bulky supersilyl group the
Si–Ag distance in 1 [Si–Ag 2.4244(9) Å] is somewhat
longer than the appropriate Si–Ag bonds found in
[Ag(IPr)SiEt(SiMe3)2] [Si–Ag bond lengths: 2.4006(6) Å][13]
and
[Ag(IPr)Si(SiMe3)3]
[Si–Ag
bond
lengths:
2.3936(5) Å],[13] respectively. One of the tBu groups in 1 is
disordered over two position with a site occupation factor of
0.521(9) for the major occupied site. Bond lengths and angles
in the two disordered sites were restrained to be equal. The
molecules of 1 are arranged in an alternate fashion, as shown
in Figure 3.
Figure 3. Packing diagram of the supersilyl silver NHC complex
[Ag(IPr)SitBu3] (1).
Table 1. Structural parameters /Å,° of group 11 and 12 supersilanides.
a)
a)
M–Si
Si–C
C–Si–C
a)
Na(thf)2[Cu(SitBu3)2] [14] 2.361(1)
Na(thf)4[Cu(SitBu3)2] [14] 2.307(2)
1.978(5)
1.972(6)
1.971(3)
1.943(4)
1.943(4)
1.937(8)
108.7(2)
108.5(2)
109.2(3)
111.8(2)
111.9(2)
112.2(4)
[Ag(IPr)SitBu3] (1)
[Zn(SitBu3)2] [15]
[Cd(SitBu3)2] [15]
[Hg(SitBu3)2] [15]
2.424(1)
2.384(1)
2.524(2)
2.495(2)
a) Average.
The 29Si{1H} NMR spectrum of a solution of 1 in C6D6
revealed two doublets caused by coupling to 107Ag and 109Ag
nuclei, as shown in Figure 4. When considering the 29Si spec-
tra of the group 11 and 12 supersilanides certain trends can be
observed. As is to be expected, the signals of the silicon nuclei
of the group 11 as well as the group 12 element supersilanides
are shifted upfield as the element is changed from up to down
in the periodic table (Table 2).
Figure 2. Structure and parameters of the supersilyl silver NHC com-
plex [Ag(IPr)SitBu3] (1) (monoclinic, space group P21/m). Selected
bond lengths /Å and angles /°: Ag(1)–C(1) 2.155(3), Ag(1)–Si(1)
2.4244(9), Si(1)–C(3) 1.966(4), Si(1)–C(4) 1.974(3), N(1)–C(1)
1.358(2), N(1)–C(2) 1.391(3), N(1)–C(11) 1.442(3), C(1)–N(1)#1
1.358(2), C(2)–C(2)#1 1.358(5), C(1)–Ag(1)–Si(1) 176.54(8), C(3)–
Si(1)–C(4) 108.32(13), C(4)#1–Si(1)–C(4) 111.0(3), C(3)–Si(1)–Ag(1)
110.79(13),
111.94(19),
125.63(19),
C(4)–Si(1)–Ag(1)
C(1)–N(1)–C(11)
N(1)–C(1)–N(1)#1
109.21(12),
122.34(17),
103.7(2),
C(1)–N(1)–C(2)
C(2)–N(1)–C(11)
N(1)–C(1)–Ag(1)
128.15(12). Symmetry transformation used to generate equivalent
atoms: #1 x, –y + 1/2, z.
As shown in Table 1, the structure of 1 features C–Si–C
angles smaller than 110°, indicating a negatively polarized Si
atom comparable to those of Na(thf)2[Cu(SitBu3)2][14] and
Na(thf)4[Cu(SitBu3)2].[14] Whereas the C–Si–C values of
Figure 4. 29Si{1H} NMR spectrum (59.6 MHz) of [Ag(IPr)SitBu3] (1)
in C6D6: δ = 49.0 ppm.
We further investigated the possibility of a conversion of
group 12 supersilanides are in the “normal” range for neutral triel halides EX3 by treatment with 1. As shown in Scheme 2,
supersilyl compounds.
we found 100% conversion of the silver silanide 1 in all these
Z. Anorg. Allg. Chem. 2020, 264–267
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Journal of Inorganic and General Chemistry
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Table 2. 29Si NMR resonances of group 11 and 12 supersilanides in
C6D6.
ined by NMR spectroscopy. The resonances of tBu3SiEX2 (E = B, Al;
X = Cl, Br; E = Ga; X = Cl) and IPr·EX3 (IPr·EX3 = IPr·BCl3,[16]
IPr·BBr3,[17] IPr·AlCl3, IPr·AlBr3, IPr·GaCl3[18]) were observable in
the NMR spectra of these solutions.
δ(29SitBu3) /ppm
Na(thf)x[Cu(SitBu3)2] [14]
[Ag(IPr)SitBu3] (1)
[Zn(SitBu3)2] [15]
[Cd(SitBu3)2] [15]
[Hg(SitBu3)2] [15]
23.2
49.0
25.9
47.6
88.3
29Si NMR resonances of tBu3SiEX2 recorded by 1H/29Si HETCOR
NMR spectroscopy in C6D12: δ = –1.6 (E = B, X = Cl); 0.3 (E = B,
X = Br); 14.3 (E = Al, X = Cl); 15.1 (E = Al, X = Br); 13.5 ppm
(E = Ga; X = Cl).
Note: Treatment of a mixture of AlX3 (AlCl3: 20 mg, 0.15 mmol,
AlBr3: 44 mg, 0.16 mmol) in 6 mL heptane with IPr (IPr for AlCl3:
50 mg, 0.13 mmol, IPr for AlBr3: 50 mg, 0.13 mmol) yielded quantita-
tively IPr·AlX3 according to the NMR spectra.
reactions. Thereby the related monosupersilylated trielanes
tBu3SiEX2 and the NHC complexes IPr·EX3 were formed
(E = B, Al; X = Cl, Br; E = Ga; X = Cl). The identity of
tBu3SiEX2 as well as IPr·EX3 was confirmed by comparison
with authentic samples.
3
IPr·AlCl3: 1H NMR (300.1 MHz, C6D6): δ = 0.94 [d, JH,H = 6.9 Hz;
3
12 H; CH(CH3)], 1.43 [d, JH,H = 6.9 Hz; 12 H; CH(CH3)], 2.71 [m,
4 H, CH(CH3)], 6.40 (s, 2 H, =CH), 6.94–7.24 ppm (m, ArH). 13C{1H}
NMR (75.5 MHz, C6D6): δ = 22.3 [CH(CH3)], 25.5 [CH(CH3)], 28.8
[CH(CH3)], 124.1 (=CH), 125.1, 131.2, 133.5, 145.3 ppm (ArC).
IPr·AlBr3: 1H NMR (300.1 MHz, C6D6): δ = 0.94 [d, 3JH,H = 6.9 Hz;
3
12 H; CH(CH3)], 1.43 [d, JH,H = 6.9 Hz; 12 H; CH(CH3)], 2.67 [m,
4 H, CH(CH3)], 6.41 (s, 2 H, =CH), 6.95–7.22 ppm (m, ArH). 13C{1H}
NMR (75.5 MHz, C6D6): δ = 23.7 [CH(CH3)], 25.3 [CH(CH3)], 28.7
[CH(CH3)], 124.2 (=CH), 125.7, 131.5, 133.6, 145.5 ppm (ArC).
Scheme 2. Reaction of of triel halides EX3 (E = B, Al; X = Cl, Br; E
= Ga; X = Cl) by treatment with the NHC-supported silver complex
[Ag(IPr)SitBu3] (1). i) -AgX, cyclohexane, room temperature.
X-ray Crystallography of 1: Data were collected on a STOE IPDS
II two-circle diffractometer with a Genix Microfocus tube with mirror
optics using Mo-Kα radiation (λ = 0.71073 Å) and were scaled using
frame scaling procedure in the X-AREA program system.[19] An empiri-
cal absorption correction with the program PLATON was performed
for both structures. The structure[20] was solved by direct methods
using the program SHELXS and refined with full-matrix least-squares
on F2 using the program SHELXL.[21]
Experimental Section
All experiments were carried out in a dry argon or nitrogen atmosphere
using standard Schlenk and glove box techniques. Alkane solvents
were dried with sodium and freshly distilled prior to use. Benzene and
thf were distilled from sodium/benzophenone. Na(thf)2[SitBu3][6] was
prepared according to the published procedure. All other starting mate-
rials were purchased from commercial sources and used without fur-
ther purification.
Crystallographic data (excluding structure factors) for the structure in
this paper have been deposited with the Cambridge Crystallographic
Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies
of the data can be obtained free of charge on quoting the depository
number CCDC-1941530 (1) (Fax: +44-1223-336-033; E-Mail:
deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk)
Synthesis of [Ag(IPr)SitBu3] (1). A mixture of [Ag(IPr)Cl] (99 mg,
0.186 mmol) in 4 mL benzene and 0.4 m solution of Na(thf)x[SitBu3][6]
(0.2 mmol) in 0.5 mL thf was stirred for 1 d at room temperature in
an Ar-atmosphere. The NMR spectra of the reaction mixture showed
resonances due to the silver complex 1. After removing all volatiles,
the residue was extracted into 10 mL heptane. Colorless crystals of 1
were grown from filtered heptane solution at room temperature. Yield:
Acknowledgements
Generous donations of lithium organyls by Albemarle GmbH are
gratefully acknowledged.
1
3
110 mg (85%). H NMR (300.1 MHz, C6D6): δ = 1.07 [d, J(H,H) =
3
6.9 Hz; 12 H; CH(CH3)], 1.25 [s, 27 H, C(CH3)3], 1.43 [d, JH,H
=
Keywords: Silanide; Silicon; Silver; Triel; X-ray diffraction
6.9 Hz; 12 H; CH(CH3)], 2.58 [m, 4 H, CH(CH3)], 6.35 (s, 2 H, =CH),
7.05–7.23 ppm (m, ArH). 13C{1H} NMR (75.5 MHz, C6D6): δ = 23.7
3
[CH(CH3)], 24.0 [d, J13C,107/109Ag
= 9.8 Hz; C(CH3)3], 24.6
4
[CH(CH3)], 28.6 [CH(CH3)], 33.4 [d, J13C,107/109Ag
= 3.8 Hz;
References
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Received: November 20, 2019
Published Online: February 28, 2020
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