Gold(I)-mediated silicon-silicon bond metathesis at room temperature

Article abstract of DOI:10.1002/anie.200905950

(Figure Presented) Digging for gold: Treatment of K[Au{Si(SiMe ₃)₃}₂] with Me₃SiCl at room temperature leads to redox processes and Si-Si bond metathesis reactions. Numerous very unusual gold complexes are forme

Full text of DOI:10.1002/anie.200905950

Angewandte
Chemie
DOI: 10.1002/anie.200905950
Si–Si Metathesis
Gold(I)-Mediated Silicon–Silicon Bond Metathesis at Room
Temperature
Marion Wilfling and Karl W. Klinkhammer*
Dedicated to Professor Gerd Becker on the occasion of his 70th birthday
Whereas silyl complexes of copper have been known for
many decades and have been employed as valuable reagents
for organic syntheses,[1] only very few homologous derivatives
of the heavier coinage metals had been synthesized and
almost nothing is known about their reactivity. In particular
for gold complexes, unique properties may be expected that
are related to the existence of the two competing oxidation
states + I and + III. The majority of the few gold complexes
bonds in the few neutral phosphane-stabilized silyl gold
complexes (2.290–2.363 ꢀ) characterized to date.[2]
In contrast to the copper salts, a change in the initial ratio
of the reactants from 1:2 to 1:1.5 or 1:1 does not lead to
multinuclear anionic or neutral products.[7] We expected,
however, that neutral and putatively oligonuclear gold
complexes should be accessible through desilylation of the
aurate 2 with chlorotrimethylsilane, because treatment of the
homologous cuprate 3 with that reagent furnishes the neutral
complex CuHyp in high yields (Scheme 1).[5,8,9]
À
that had been synthesized to date have the composition R’3P
AuSiR3. Most of these were reported by Schubert et al. to be
obtained in low yields (about 20–30%) by metathesis
reactions of triorganylphosphane complexes of gold halides
with the appropriate lithium silanides.[2] The parent homo-
leptic neutral derivatives AuSiR3 are however still unknown.
Herein we present a route to anionic hypersilyl complexes of
gold and present some very unexpected reactions arising from
attempts to use them in the synthesis of the neutral compound
hypersilylgold, [AuSi(SiMe3)3] (1).
Scheme 1. Desilylation of KCuHyp2 with Me3SiCl.
Therefore we attempted to desilylate complex 2 by
treatment with an excess of Me3SiCl in diethyl ether at
À208C to yield the parent neutral hypersilylgold compound
AuHyp (1). Indeed, after a few hours the formation of
Si(SiMe3)4 and of one new gold complex is detected by NMR
spectroscopy, and after one day at À208C the aurate 2 had
been totally consumed. The workup furnishes colorless
crystals that contain the dinuclear aurate [(toluene)K]-
[Au2Hyp3] (4), the product of an incomplete desilylation of
2. The potassium cation is coordinated by a toluene molecule
and located on top of the Au2Si3 scaffold of the anion
(Figure 1).[6] The structure of the [Au2Hyp3] anion resembles
that of the homologous dicuprate anion in the salt [Li7-
We are currently investigating a novel route to phos-
phane-free silyl complexes of the coinage metals; that is, the
metathesis of coinage-metal halides MX (M = Cu, Ag, Au;
X = Cl, Br, I) and potassium silanides in liquid ammonia. As
both reactants generally show good solubility in ammonia,
such reactions can be performed under homogenous con-
ditions and usually deliver the products in high yields.[3] If
freshly prepared gold(I) iodide[4] is thus reacted with potas-
sium hypersilanide KHyp (Hyp = Si(SiMe3)3) in liquid ammo-
nia at À408C in a stoichiometric ratio 1:2, the homoleptic
aurate KAuHyp2 2 is the only product detected by NMR
spectroscopy, and it may be isolated as colorless crystalline
toluene solvate in excellent yields. The molecular structure of
2 greatly resembles the structure of its known copper
analogue KCuHyp2 (3).[5,6] Complex 2 consists of an aurate
anion with an almost linear AuSi2 dumbell and is intimately
coordinated by the potassium countercation. As expected
(OtBu)6][Cu2Hyp3], with relatively short terminal and long
[10]
À
internal Au Si bonds. All the NMR spectra obtained for
[D6]benzene solutions of 2 reveal a fast exchange of terminal
and bridging Hyp ligands on the NMR timescale (1H NMR,
400 MHz, 298 K).
À
from the difference in atomic radii, the observed Au Si bond
lengths (average 2.388 ꢀ) are significantly longer than the
To accomplish further desilylation, we carried out the
reaction with Me3SiCl in Et2O at room temperature. At first,
the dinuclear complex 4 is again detected, but on prolonged
stirring for three days, the total consumption of 4, a color
change to dark green, and precipitation of KCl was observed.
NMR spectroscopy experiments on the reaction mixture
revealed that Si(SiMe3)4 is the main component in solution,
À
À
Cu Si bonds in 3 (2.297 ꢀ), and even longer than the Au Si
[*] Dr. M. Wilfling, Prof. Dr. K. W. Klinkhammer
Institut fꢀr Anorganische und Analytische Chemie
Johannes-Gutenberg-Universitꢁt Mainz
Duesbergweg 10–14, 55128 Mainz (Germany)
Fax : (+49)6131-392-7223
1
although H and 29Si NMR spectra indicate that appreciable
amounts of hexamethyldisilane are also present in the
reaction mixture. As neither HypH nor Hyp2 nor elemental
gold are detected in significant amounts, and the solutions are
EPR-silent down to 120 K, we preclude single-electron
transfer processes as the main source for the hexamethyldi-
E-mail: klink@uni-mainz.de
Homepage: aragorn.chemie.uni-mainz.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200905950.
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Figure 1. Molecular structure of diaurate 4 with hydrogen atoms
omitted for clarity. Selected bond lengths [ꢂ] and angles [8]: Au1–Si1
2.390(2), Au1–Si3 2.579(2), Au2–Si2 2.384(2), Au2–Si3 2.557(2),
Au1···Au2 2.7524(4); Au1-Si3-Au2 64.82(4), Si1-Au1-Si3 158.29(6), Si2-
Au2-Si3 157.99(7).
Scheme 2. Reaction of dihypersilylaurate 2 with Me3SiCl.
silane. Instead, we believe that it mostly stems from partial
fragmentation of the hypersilyl substituents by Si–Si bond
metathesis with neutral hypersilylgold (1) acting as a possible
intermediate. This hypothesis is corroborated by the consti-
tution of the gold complexes that we could finally isolate and
structurally characterize (see below).[6,11]
After evaporating the reaction mixture to dryness, a dark
green oily residue was obtained; after dissolving it in a few
milliliters of n-pentane and storing at À608C for four weeks, a
significant amount of solid material precipitates, which
consists of at least four different crystalline compounds.
Apart from a large amount of colorless crystals, a significant
amount of burgundy red crystals, fewer yellow specimens, and
only two dark blue crystals could be isolated from a typical
run. The colorless crystals were identified by NMR spectros-
copy and X-ray diffraction to be Si(SiMe3)4, whereas the
colored specimens proved to be some very unexpected
anionic gold complexes that are obviously products from
redox and/or Si–Si bond metathesis processes (Scheme 2).
The burgundy red crystals consist of the subvalent gold cluster
compound [K(toluene)]2[Au4Hyp4] (5). The tetrahedral Au4
scaffold bears axial hypersilyl groups and two potassium
countercations, which cap two of the four faces of the
tetrahedron (Figure 2). However, in C6D6 solution, all of the
hypersilyl groups in 5 are magnetic equivalent at room
temperature (1H NMR, 400 MHz, 298 K). The potassium
cations are either mobile on the cluster surface or solvent-
Figure 2. Molecular structure of the gold cluster 5 with hydrogen
atoms omitted for clarity. Selected bond lengths [ꢂ]: Au1–Au2
2.7549(3), Au1–Au3 2.7885(3), Au1–Au4 2.7458(3), Au2–Au3
2.7563(3), Au2–Au4 2.7657(3), Au3–Au4 2.7521(3).
long Au···Au contacts are found (2.726–2.736 and 3.663–
3.708 ꢀ, respectively). There are also two groups of Au–Si
bonds: short (2.372–2.384 ꢀ) to the four-coordinate central
silicon atoms of the Dis substituents, and markedly longer
(2.542–2.610 ꢀ) to the five-coordinate silicon atoms of the
hypersilyl groups, thus indicating higher or lower bond orders,
respectively. This assumption is corroborated by NBO
analysis of DFT densities derived from calculations using
the experimental structural parameters.[14] The potassium
countercations are located above and below the Au4Si4 plane;
intermolecular agostic K···Me interactions lead to one-dimen-
sional coordination polymers.[15] Although compound 6 is
obtained in only very small amounts, it could be fully
characterized by NMR spectroscopy. Whereas the 29Si NMR
signal for the central silicon atom of the hypersilyl groups (d =
À65.9 ppm) lies in the region found for the other hyper-
silylgold complexes, the central silicon atom of the dianionic
À
separated ion pairs may have formed in solution. All Au Au
bonds have similar lengths (ca. 2.76 ꢀ) and are a few
picometers longer than in closely related subvalent cationic
phosphane-stabilized gold clusters.[12,13]
The two other crystalline compounds and the volatile
hexamethyldisilane appear to result from Si–Si bond meta-
thesis processes. The yellow crystals contain the novel
molecular tetraaurate(I) complex K2[Au4Hyp2Dis2]
6
(Figure 3), comprising two different kinds of silyl ligands:
two monoanionic hypersilyl (Hyp) and two dianionic bis-
(trimethylsilyl)silanediyl (Dis) groups. The [Au4Hyp2Dis2]
dianion has an almost planar Au4Si4 skeleton; within the
rectangular arrangement of gold atoms, two short and two
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Figure 3. Molecular structure of the aurate 6 with hydrogen atoms
omitted for clarity. Selected bond lengths [ꢂ]: Au1–Si1’ 2.378(2), Au1–
Si2 2.559(2), Au2–Si1 2.376(2), Au2–Si2 2.579(2), Au1···Au2 2.7518(5),
Au1···Au2’ 3.7017(6), K1···Au1 3.353(2), K1···Au2 3.350(2).
Figure 4. Molecular structure of disilene 7 with hydrogen atoms
omitted for clarity. The almost planar Au2Si6 scaffold is highlighted by
=
Dis group is shifted significantly upfield and has a resonance
at d = À134.4 ppm.
open ( ) bonds. Selected bond lengths [ꢂ] and angles [8]: Si1–Si1’
2.337(4), Au1–Si1 2.387(2), Au1–Si3 2.396(2), Au2–Si2 2.401(2), Au2–
Si1 2.524(2), Au2–Si1’ 2.527(2), Si11-Si1-Si1’ 129.12(7), Si11-Si1-Au1
116.29(5), Au1-Si1-Si1’ 114.24(9).
The blue crystals indeed contain the desired neutral
hypersilylgold 1. It is, however, not present as a pure
compound, but as adduct to the p system of a much-
unexpected dianionic disilene. The planar disilene unit of
the potassium salt 7 bears two cis-oriented trimethylsilyl
substituents and two formally negatively charged hypersilyl-
aurato groups (Figure 4).
1H NMR spectroscopy. However, after storing the concen-
trated filtrate for 1 month at À608C, small dark green crystals
could be isolated.
=
The length of the formal Si Si double bond (2.341 ꢀ) is in
X-ray diffraction reveals the formation of the novel
pentaaurate KAu5[Si(SiMe3)2]6 (8) comprising an almost
planar Au5Si6 scaffold (Figure 5). Pentaaurate 8 contains no
hypersilyl group at all. Again, Si–Si bond metathesis must
have taken place. According to NBO analyses and by
comparing the structural parameters with other silyl gold
complexes, the best interpretation of its constitution[14] is that
the complex is a 2:1 adduct of two neutral dinuclear
hexasilanediyldigold moieties Au2Si2(SiMe3)4 to the mono-
the typical single-bond range, however, and only slightly
shorter than the single bond Si1 Si11 (2.350 ꢀ) to the SiMe3
À
group. This elongation may be partially attributed to electro-
static repulsion of the two anionic silylene fragments, but also
to a significant weakening of the p bond by coordination to
two Lewis acidic neutral hypersilylgold moieties. The NAO–
Wiberg bond index (WBI) from DFT calculations clearly
À
indicate appreciable covalent Au Si bonding (WBI: 0.27)
between the disilene fragment and the coordinating AuHyp
À
moieties and also significant weakening of the Si Si double
bond (WBI: 1.20).[14] The calculated bond indices for Au Si
À
bonds within the disilene scaffold are about 0.5, similar bond
À
indices are calculated for the Au Si bonds of similar length
(ca. 2.38 ꢀ) within the simple aurate 2. In accordance with
À
calculated bond ndices of about 0.25, the coordinative Au Si
interactions in complex 7 to the neutral AuHyp moieties are
about 0.15 ꢀ longer. The potassium countercations are
located above and below the gold atoms of the disilene
moieties such that similar distances to three gold atoms are
accomplished. Apart from intermolecular agostic interactions
to methyl groups of the hypersilyl ligands, no additional
solvation of the cations is observed. Unfortunately, we have
not yet been able to obtain spectroscopic data for 7.
A further very unexpected silylgold complex is obtained
when aurate 2 is reacted with Me3SiCl in acetonitrile instead
of toluene. After four days, a green suspension is obtained and
again some expected products, such as Si(SiMe3)4 and
hexamethyldisilane, are detected in larger amounts by
Figure 5. Molecular structure of aurate 8 with hydrogen atoms omitted
for clarity. Selected bond lengths [ꢂ] and angles [8]: Au1–Si1 2.438(2),
Au2–Si2 2.399(2), Au2–Si1 2.544(2), Au3–Si1 2.516(2), Au3–Si3
2.388(2), K1···Au1 3.288(3), K1···Au2 4.124(2), K1···Au3 4.044(2), Si11-
Si1-Si12 111.19(12), Au1-Si1-Si11 114.64(10), Au1-Si1-Si12 134.16(10).
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nuclear aurate K[Au{Si(SiMe3)2}2]. The anionic part
is formally composed of a gold(I) cation and two
bis(trimethylsilyl)silylene anion radicals.[16] The
potassium countercation is located above the center
of the Au5Si6 scaffold (K1···Au1 3.288(3) A), and
gives rise to the formation of one-dimensional
coordination polymer along the crystallographic
c axis by agostic K···CH3 interactions to neighboring
À
aurate anions. The Au Si bonds within the hexasila-
nediylgold fragment (2.39–2.40 ꢀ) are of similar
lengths to those found for aurate 2, whereas the
À
Au Si bonds within the central aurate fragment is
Scheme 3. Calculated reaction path for Si–Si bond metathesis in silyl complexes
of gold. The numbers given are relative electronic energies (including zero-point
energy correction) [kJmolÀ1] calculated at the B2PLYP level for local minimum
and transition-state structures with R=R’=R’’=Me.[20] Further data and compu-
tational details are given in the Supporting Information.
À
about 0.05 ꢀ longer. The Au Si distances between
the fragments resemble those found for the coordi-
native interactions between the AuHyp moieties and
the disilene unit of 7.
In our attempt to synthesize neutral hypersilyl-
gold 1 by desilylation of the aurate 2 with the
nucleophile Me3SiCl in the absence of stabilizing soft neutral
bases, such as phosphanes, we found that redox reactions and
a Si–Si bond metathesis took place at room temperature
(Scheme 2). Although 1 is a probable intermediate, there is
are no experimental data available that give hints to the
mechanism that is involved. The NMR data gained from the
reaction mixtures indicate the presence of further gold
containing compounds, but none of them could be isolated
to date. As we never observed similar reactions for related
silyl complexes of copper or silver, neither in the formation of
subvalent cluster compounds nor bond metathesis, the
unusual reactivity of the aurates must correlate to the
peculiar electronic structure of gold, namely the presence of
relativistically stabilized s and p orbitals and destabilized
d orbitals.[17] Filled and empty frontier orbitals of similar
energies are available for oxidative addition reactions, and
gold(III) species seemed to be very probable intermediates
for metathesis reactions.
hope, however, that by further theoretical and experimental
work, and especially by varying the employed electrophile for
the desilylation of KAuHyp2 (2) and analyzing the resulting
products, we hope to be able to gain deeper insight into the
mechanisms that are operating.
Received: October 22, 2009
Revised: December 22, 2009
Published online: March 26, 2010
Keywords: bond metathesis · cluster compoundss · gold ·
.
silicon
[1] Overview: a) I. Fleming in Organocopper Reagents: A Practical
Approach (Ed.: R. J. K. Taylor), Oxford University Press,
Oxford, 1994, chap. 12, p. 257; b) R. K. Dieter in Modern
Organocopper Chemistry (Ed.: N. Krause), Wiley-VCH, Wein-
heim, 2002, chap. 3, p. 79.
[2] a) M. C. Baird, J. Inorg. Nucl. Chem. 1967, 29, 367; b) J. Meyer, J.
Willnecker, U. Schubert, Chem. Ber. 1989, 122, 223; c) J. Meyer,
H. Piana, H. Wagner, U. Schubert, Chem. Ber. 1990, 123, 791;
d) H. Piana, H. Wagner, U. Schubert, Chem. Ber. 1991, 124, 63;
e) M. M. Oroz, A. Schier, H. Schmidbaur, Z. Naturforsch. B
1999, 54, 26; f) M. Theil, P. Jutzi, B. Neumann, A. Stammler, H.-
G. Stammler, J. Organomet. Chem. 2002, 662, 34.
[3] In nonpolar or weakly polar aprotic solvents, such as hydro-
carbons and ethers, which are suitable solvents for alkali metal
silanides, the coinage metal halides are almost insoluble. Under
such heterogenous conditions (and in the absence of phos-
phanes), we typically observed the formation of dark slurries
that mainly contained metal powder and metal-free silanes
rather than the formation of well-defined silyl complexes.
Similar observations have also been made by others; see, for
example: A. H. Cowley, T. M. Elkins, R. A. Jones, C. M. Nunn,
Angew. Chem. 1988, 100, 1396; Angew. Chem. Int. Ed. Engl.
1988, 27, 1349.
[4] AuI is obtained by oxidation of gold powder with an aqueous KI3
solution under ambient conditions; it crystallizes from this
solution as a bright yellow crystalline material. The structure
derived from a single crystal could be confirmed with previously
obtained powder diffraction data. Further details of the crystal
structure determination are given with the Supporting Informa-
tion.
Indeed, DFT calculations on model systems indicated that
a neutral monomeric silyl gold complex R3SiAu may insert
À
into Si Si bonds of oligosilanes via a low-lying transition state
ts1 (Scheme 3).[18] However, the resulting complex
[R3SiAu(R’3Si-SiR’’3)] (min2) does not exhibit the expected
T shape for an AuIII complex, but instead it adopts a structure
very similar to the experimentally and theoretically inves-
tigated Y-shaped dihydrogen complex [HAu(H2)].[19] The Si
À
Si bond of the attacked oligosilane is however markedly
stretched and the corresponding bond order is diminished.
Further calculations showed that a reaction path from min2 to
the isomer min3 with a low-lying transition state (ts2) exists.
The transition state ts2 has a structure expected for the
initially proposed gold(III) species. Dissociation of min3 via
transition state ts3 finally leads to the products (min4) of a
formal Si–Si bond metathesis. Such reactions seem to be
unique among the coinage metals, as inspection of the
hypersurfaces of the corresponding copper and silver systems
did not reveal analogous local minima or similar reaction
paths. Owingb to very low barriers for the isomerization to the
stable states min1 and min4, the Y-shaped intermediates min2
and min3 should has very short lifetimes and are not expected
to be detectable by conventional spectroscopic methods. We
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[5] K. W. Klinkhammer, J. Klett, Y. Xiong, S. Yao, Eur. J. Inorg.
Chem. 2003, 3417.
d) D. Zhang, J. Dou, D. Li, D. Wang, J. Coord. Chem. 2007, 60,
825.
[6] CCDC 751847–751854 contain the supplementary crystallo-
graphic 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.
[7] This is in sharp contrast to analogous reactions with copper or
silver halides, where under similar conditions trihypersilyldime-
tallates K[M2Hyp3] and neutral oligonuclear complexes
{MHyp}n (M = Cu, Ag; n = 3, 4) are formed: M. Wilfling, K. W.
Klinkhammer, unpublished results.
[8] All known homoleptic neutral molecular M(I) hydrocarbyl
complexes are cyclic oligomers. For typical early examples, see:
a) J. A. J. Jarvis, R. Pearce, M. F. Lappert, J. Chem. Soc. Dalton
Trans. 1977, 999; b) S. Gambarotta, C. Floriani, A. Chiesi-Villa,
C. Guastini, J. Chem. Soc. Chem. Commun. 1983, 1087, S.
Gambarotta, C. Floriani, A. Chiesi-Villa, C. Guastini, J. Chem.
Soc. Chem. Commun. 1983, 1156, S. Gambarotta, C. Floriani, A.
Chiesi-Villa, C. Guastini, J. Chem. Soc. Chem. Commun. 1983,
1304.
[13] Overviews on structures and bonding in ligand-stabilized gold
clusters: a) P. J. Jones, Gold Bull. 1983, 16, 115; b) D. M. P.
Mingos, Gold Bull. 1984, 17, 5.
[14] Bond indices were calculated on experimental structures using
the NBO 3.0 module implemented in Gaussian03 RevisionD.01,
M. J. Frisch, et al. Complete citations can be found in the
Supporting Information.
[15] The tetraaurate 6 had also been obtained as diethyl ether and
toluene solvates; the potassium cations are coordinated by a
solvent molecule, thus preventing further agostic interactions.
Details are given in the Supporting Information.
[16] For silylene anion radicals, see: a) D. Bravo-Zhivotovskii, I.
Ruderfer, S. Melamed, M. Botoshansky, B. Tumanskii, Y.
Apeloig, Angew. Chem. 2005, 117, 749; Angew. Chem. Int. Ed.
2005, 44, 739; b) S. Inoue, M. Ichinohe, A. Sekiguchi, J. Am.
Chem. Soc. 2007, 129, 6096.
[17] a) P. Pyykkꢂ, J. Desclaux, Acc. Chem. Res. 2002, 12, 276; b) P.
Pyykkꢂ, Chem. Rev. 1988, 88, 563.
[9] J. Klett, K. W. Klinkhammer, M. Niemeyer, Chem. Eur. J. 1999,
5, 2531.
[18] A closer inspection of the hypersurface revealed a further very
shallow minimum in between min1 and min2 that corresponds to
loosely bonded van der Waals complexes of the starting
compounds. Further details are given in the Supporting Infor-
mation.
[19] X. Wang, L. Andrews, J. Am. Chem. Soc. 2001, 123, 12899.
[20] Augmenting the basis sets of silicon and gold with diffuse and
polarization functions leads to even smaller barriers for the
metathesis. For R = R’ = R’’ = Me, the formation of the adduct
min2 is then calculated to be even slightly exergonic. Detailed
information is given in the Supporting Information.
[10] K. W. Klinkhammer, Z. Anorg. Allg. Chem. 2000, 626, 1217.
[11] The mild oxidating agent pyridine-N-oxide undergoes no redox
reaction when heated for 24 h with aurate 2 in boiling toluene
(1118C). Instead, a crystalline pyridine-N-oxide solvate of 2 is
obtained from this experiment in high yields.
[12] Other Au4 clusters: a) E. Zeller, H. Beruda, H. Schmidbaur,
Inorg. Chem. 1993, 32, 3203; b) Y. Yang, P. R. Sharp, J. Am.
Chem. Soc. 1994, 116, 6983; c) M. J. Calhorda, O. Crespo, M. C.
Gimeno, P. G. Jones, A. Laguna, J. M. Lꢁpez de Luzuriaga, J. L.
Perez, M. A. Ramꢁn, L. F. Veiros, Inorg. Chem. 2000, 39, 4280;
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