A tripodal trisilanol ligand and its complexation behavior towards CuI, CuII, and ZnII

Article abstract of DOI:10.1002/ejic.201400088

A three-step synthetic route for a new tripodal branched trisilanol ligand PhSi(OSiPh₂OH)₃ (LH₃) was developed. X-ray diffraction analysis revealed that the trisilanol crystallizes as a dimer with a cyclic hydrogen-bonding network. The reaction of LH₃ with three equivalents of Cu_nMes_n (Mes = mesityl) led to a hexanuclear compound [L₂Cu₆] (1), which was characterized by single-crystal X-ray diffraction analysis as well as by solution NMR spectroscopy. The crystal structure of 1 revealed that the compound features a hexagonal planar Cu^I₆ ring, which is the first of its kind in an oxygen environment. Deprotonation of the ligand with n-butyllithium and subsequent reaction with CuBr₂ resulted in the dinuclear Cu^II complex [L′₂Cu₂][Li(THF₂)]₂ (2, THF = tetrahydrofuran), which contains a new siloxide ligand formed from L^3- by the elimination of a SiOPh₂ unit, as evidenced by X-ray diffraction analysis. To check if the "Ph₂SiO" elimination is a general behavior of this trisilanol, the reaction with ZnBr₂ was investigated under analogous conditions. However, this led to the isolation of [L₂Zn₂][Li(OEt)]₂ (3) without any rearrangement of the siloxide ligand. A tripodal branched trisilanol ligand PhSi(OSiPh₂OH)₃ (LH₃) was synthesized. Its reaction with three equivalents of Cu_nMes_n (Mes = mesityl) led to a hexanuclear compound [L₂Cu₆] with a planar Cu^I₆ ring. Deprotonation of LH₃ with n-butyllithium and subsequent reaction with either CuBr₂ or ZnBr₂ resulted in dinuclear complexes. In the case of CuBr₂, the original ligand eliminates a "Ph₂SiO" unit.

Full text of DOI:10.1002/ejic.201400088

FULL PAPER
DOI:10.1002/ejic.201400088
A Tripodal Trisilanol Ligand and Its Complexation
Behavior towards CuI, CuII, and ZnII
Fabian Schax,[a] Beatrice Braun,[a] and Christian Limberg*[a]
Keywords: Copper / Zinc / Metal aromaticity / Rearrangement / Siloxides / Silanols
A three-step synthetic route for a new tripodal branched tri-
silanol ligand PhSi(OSiPh2OH)3 (LH3) was developed. X-ray
diffraction analysis revealed that the trisilanol crystallizes as
a dimer with a cyclic hydrogen-bonding network. The reac-
tion of LH3 with three equivalents of CunMesn (Mes = mes-
ityl) led to a hexanuclear compound [L2Cu6] (1), which was
characterized by single-crystal X-ray diffraction analysis as
well as by solution NMR spectroscopy. The crystal structure
of 1 revealed that the compound features a hexagonal planar
CuI6 ring, which is the first of its kind in an oxygen environ-
ment. Deprotonation of the ligand with n-butyllithium and
subsequent reaction with CuBr2 resulted in the dinuclear
CuII complex [LЈ2Cu2][Li(THF2)]2 (2, THF = tetrahydrofuran),
which contains a new siloxide ligand formed from L3– by the
elimination of a SiOPh2 unit, as evidenced by X-ray diffrac-
tion analysis. To check if the “Ph2SiO” elimination is a gene-
ral behavior of this trisilanol, the reaction with ZnBr2 was
investigated under analogous conditions. However, this led
to the isolation of [L2Zn2][Li(OEt)]2 (3) without any rearrange-
ment of the siloxide ligand.
tural and electronic similarities.[12] In turn, their reactions
with transition metal precursors resemble the grafting pro-
cesses applied for the preparation of heterogeneous cata-
lysts. Accordingly, the chemistry of these siloxide precursors
and of the corresponding metallasilsesquioxanes is well de-
veloped.[13–21] However, a comprehensive investigation of
transition metal siloxide chemistry requires considerations
of siloxide functions in different orientations; thus, new
polysilanols with differing connectivities are needed. Re-
cently, we described our results obtained upon the introduc-
tion of disilanols/disiloxides into copper(I) chemistry.[3] We
report here the synthesis of a new trisilanol ligand precur-
sor, which is a promising starting material for transition
metal complexation, as evidenced by the first results ob-
tained on its employment in copper chemistry.
Introduction
Metal siloxides have found widespread use as precursors
for new materials, homogeneous catalysts, and hetero-
geneous catalysts. Moreover, they have raised interest as
structural and functional models for metal-derivatized silica
and zeolite surfaces: to obtain molecular insights into the
surface chemistry of such materials, the investigation of sol-
uble metal siloxides can provide valuable information.[1,2]
We recently became interested in copper(I) siloxide com-
pounds in particular,[3] because they may function as model
compounds for copper(I)-grafted zeolites.
A few years ago, it was reported that copper(II) ions
grafted on ZSM-5 underwent reduction to CuI species upon
heating, which in turn could activate O2 or N2O for the
oxygenation of methane to methanol; [Cu–O–Cu]2+ surface
species were identified as the active sites.[4–6]
With this background, explorations on the siloxide chem-
istry of copper(I) at the molecular level appear worthwhile.
With the exception of a special compound featuring a
Si=EǞCu–O–Si (E = S, Se) unit[7] and one prepared from
R–Si(OH)3 [R = (2,6-iPr2C6H3)N(SiMe3)],[8] the copper(I)
siloxide compounds reported so far are derived either from
simple silanols[9,10] or from incompletely condensed silses-
quioxane cages.[11] The latter are polysilanols and probably
represent the most realistic low-molecular-weight analogues
for hydroxylated silica surfaces, as they possess both struc-
Results and Discussion
Veith et al. reported the synthesis of the first tripodal
organotrisilanol that was not derived from a silsesquioxane,
tBuSi(OSiMe2OH)3 (L*H3). It contains a tert-butyl group
at the silicon atom in the backbone and methyl groups at
the silicon atoms in the branches.[22]
Owing to poor crystal quality Veith et al. were unable to
refine the crystal structure of the newly synthesized com-
pound properly, and only a structural model was proposed.
Therefore, we set out to substitute the tert-butyl and methyl
groups at the silicon atoms with phenyl groups, as phenyl
siloxanes typically exhibit good crystallization behavior,
which extends to the corresponding silanols as well as to
siloxides in coordination chemistry.[1,23,24]
[a] Humboldt-Universität zu Berlin, Institut für Chemie,
Brook-Taylor-Str. 2, 12489 Berlin, Germany
E-mail: christian.limberg@chemie.hu-berlin.de
http://www.chemie.hu-berlin.de/aglimberg/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201400088.
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Owing to the volatility of the precursors used by Veith et
al., their synthetic procedure utilized purification by distil-
lation.[22] Naturally, this route cannot be applied for the
preparation of trisilanols based on less-volatile organosil-
icon precursors, because they tend to decompose at the tem-
peratures required for distillation. Therefore, we have devel-
oped a new synthetic strategy that can be applied generally
for the synthesis of branched silanols. The basic idea was
to synthesize a trisilane first that can be oxidized cleanly to
the corresponding trisilanol by dimethyldioxirane
(Scheme 1).[25]
Figure 1. Molecular structure of LH3. Silicon atoms are grey, hy-
drogen atoms are white, and oxygen atoms are red. All phenyl hy-
drogen atoms are omitted for clarity.
For the reasons outlined above, we employed the new
tripodal organotrisilanol (LH3) as a ligand precursor to in-
vestigate its coordination behavior towards a copper(I)
source. Compound LH3 was dissolved in THF and treated
with CunMesn (Mes = mesityl) in a molar ratio of 1:3
(Scheme 2). A synthetic advantage of CunMesn is that de-
protonation of the silanol and concomitant complexation
with copper(I) can be realized in one step; thus, side prod-
ucts other than mesitylene, which can be removed under
high vacuum, are avoided. The reaction mixture was stirred
at room temperature and changed from light yellow to col-
orless within 30 min; stirring was continued overnight. Af-
ter workup, a colorless air-sensitive powder was obtained.
NMR experiments revealed two 29Si resonances at δ = –81.8
and –36.4 ppm in a ratio of 1:3, which indicates that the
ligand is part of a highly symmetric CuI complex [L2Cu6], 1.
Scheme 1. Synthetic pathway for LH3.
Phenyltrimethoxysilane (a) was hydrolyzed to the corre-
sponding phenylsilanetriol (b) under acidic conditions by
following established procedures.[26] The treatment of b at
low temperature with three equivalents of diphenylchloro-
silane in the presence of triethylamine led to the formation
of a colorless oil in good yield; this compound could be
identified as the branched silane PhSi(OSiPh2H)3 (c) by
multinuclear NMR spectroscopy. The hydrogen chloride
formed in the course of the reaction precipitates as
Et3NHCl, which can easily be removed by filtration. This
silane could then be oxidized cleanly and almost quantita-
tively to the corresponding colorless crystalline trisilanol
PhSi(OSiPh2OH)3 (LH3) with an excess of dimethyldi-
oxirane solution. The identity of the product was mani-
fested by a broad SiO–H vibration in the IR spectrum at ν
˜
= 3135 cm–1, typical for ν(O–H) stretching vibrations in-
volved in hydrogen bonds,[22,27] as well as by multinuclear
NMR spectroscopy. The NMR experiments revealed two
29Si resonances at δ = –78.9 and –39.2 ppm in a ratio of 1:3
1
and two H resonances at δ = 7.60 and 7.55 ppm for the
ortho-phenyl protons in a ratio of 1:6.
As expected, this compound showed good crystallization
behavior, and the resulting crystals obtained from a concen-
trated tetrahydrofuran/n-hexane (THF/n-hexane) solution
were investigated by X-ray diffraction analysis. Compound
Scheme 2. Synthesis of 1.
The molecular structure of 1·2THF was elucidated by X-
LH3 crystallizes in the space group Pbca as a dimer (Fig- ray diffraction analysis. Suitable crystals of 1·2THF could
ure 1). Dimerization occurs through a cyclic hydrogen-bond be obtained by layering a hot concentrated toluene solution
system, which is similar to the one reported by Pietschnig of 1·2THF with n-hexane at room temperature. Complex
et al.[28] The crystal structure shows extensive orientational 1·2THF crystallizes in the centrosymmetric space group
disorder: all atoms except for the backbone silicon atoms C2/c.
are statically disordered. For the main disordered part, the
The solid-state structure of 1·2THF (Figure 2) reveals
hydroxylic H atoms could be detected in the Fourier map that the product contains two deprotonated siloxide ligands
and were freely refined: they form a H network between (L3–), which coordinate from above and below to form a
one trisilanol unit and its symmetry equivalent.
perfectly hexagonal planar CuI6 entity.
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As the structural parameters of our CuI6–siloxide com-
plex [Cu–Cu distances, Cu–X (X = O or N)] are in good
agreement with those of the reported CuI6 complex sup-
ported by guanidinates,[31] we believe that the strength of
the Cu···Cu interactions and the overall electronic structure
will not be much different.
Having investigated the coordination chemistry of the
new ligand precursor towards CuI ions, we were also inter-
ested in its behavior towards CuII ions.
The ligand precursor was dissolved in THF, and the
SiOH groups were deprontonated at low temperature with
three equivalents of n-butyllithium; afterwards, the depro-
tonated ligand was treated with CuBr2 to yield
2
(Scheme 3). The color changed immediately to intense blue
and stirring was continued overnight. After workup, the
product was recrystallized from a THF/n-hexane solution,
and blue block-shaped crystals were isolated in good yield
and investigated by X-ray diffraction. Complex 2 crys-
tallizes in the centrosymmetric space group P21/c.
Figure 2. Molecular structure of 1·2THF (left) and its core motif
(right). Silicon atoms are grey. Phenyl hydrogen atoms and cocrys-
tallized solvent molecules are omitted for clarity. Selected bond
lengths [Å] and angles [°]: Cu1–Cu3 2.6237(16), Cu1–Cu2
2.7260(17), Cu2–Cu3Ј 2.6369(16), Cu1–O1 1.835(8), Cu1–O4Ј
1.822(7), Cu2–O1 1.848(6), Cu2–O6Ј 1.849(6), Cu3–O6 1.841(7),
Cu3–O4Ј 1.835(7), O1–Cu1–O4Ј 177.5(3), O1–Cu2–O6Ј 175.6(3),
O4Ј–Cu3–O6 173.6(3), Cu3–Cu1–Cu2 123.38(5), Cu1–Cu3–Cu2Ј
115.09(6), Cu1–Cu2–Cu3Ј 121.52(5).
In 1·2THF, all of the Cu atoms are almost linearly coor-
dinated by two siloxide units with Cu–O bond lengths of
1.822(7)–1.841(7) Å and O–Cu–O angles of 173.6(3)–
177.5(3)°, which are comparable to those displayed by pre-
viously reported copper(I) siloxide complexes.[8–11] Three
Cu atoms and their symmetry equivalents form a hexagonal
Cu6 motif, and the Cu–Cu distances of 2.6237(16)–
2.7260(17) Å and the Cu–Cu–Cu angles of 115.09(6)–
123.38(5)° suggest that there are metallophilic interac-
tions.[3,29,30]
Scheme 3. Synthesis of 2.
The product contains two trisiloxane ligands; however,
they do not correspond to the original L3–: they are derived
from L3– by elimination of one “Ph2SiO” group [to
form LЈ3–, PhSiO–(OSiPh2O–)2], which was clearly induced
by contact with the copper(II) ions. The newly formed li-
gands span two CuII centers in almost square-planar coor-
dination spheres (Figure 3). Two lithium ions coordinate to
the siloxide ligands as well as to THF molecules to compen-
sate the remaining negative charges.
Previously reported CuI siloxide structures all had a
planar Cu4O4 structural fragment as a recurring mo-
tif,[3,9–11] except for a recently published globular 56-mem-
bered copper(I) siloxide cluster.[8] Overall, nearly planar
hexagonal CuI6 rings are quite rare and have only been
structurally characterized for nitrogen[31] and sulfur[32] do-
nor spheres. They are of high interest because computa-
tional studies by Tsipis et al. have suggested that planar
cyclic copper hydride rings CunHn (n = 3–6) possess a de-
gree of cyclic electron conjugation, which leads to further
stabilization of planar CuI rings and to the equivalence of
Cu–Cu bonds.[33] Further theoretical investigations on
planar CunHn systems by Wang and Boldyrev confirmed
the aromatic character but revealed that it is due to delocal-
ization of σ bonds composed of H 1s atomic orbitals (AOs)
and Cu 4s AOs and not to delocalized σ, π, or δ orbitals
composed of 3d AOs of Cu.[34]
Having structurally characterized a complex with a
planar [Cu(μ-TAG)]6 (TAG = 1,1,3,3-tetraalkylguanidinate)
core, Bunge et al. undertook DFT calculations to quantify
the differences in electron cyclic conjugation in their com-
plex and the Cu6H6 system. These calculations suggested
that the extent of Cu···Cu interactions in Cu6X6 are modest
and less than those within Cu6H6.[31]
Figure 3. Molecular structure of 2. Phenyl hydrogen atoms are
omitted for clarity. Selected bond lengths [Å] and angles [°]: Cu1–
Cu1Ј 2.9263(6), Cu1–O3 1.9283(18), Cu1Ј–O3 1.9477(17), Cu1–O1
1.9088(18), Cu1–O5Ј 1.8949(18), O5Ј–Cu1–O1 88.18(8), O5–Cu1–
O3Ј 168.39(8), O1–Cu1–O3Ј 168.58(8), O1–Cu1–O3 97.64(7), O5–
Cu1Ј–O3 94.32(7).
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The Cu–O bond lengths are 1.9283(18) and 1.9477(17) Å
for the bridging O atoms and 1.9088(18) and 1.8949(18) Å
for the terminal O atoms. The Cu–Cu separation is
2.9263(6) Å, and the compound is to some extend distorted
from a perfectly planar coordination. The O–Cu–O cis che-
late angles are 97.64(7) and 94.32(7)°, and the trans O–Cu–
O angles are 168.39(8) and 168.58(8)°.
Very recently, a similar structural motif, also obtained
by rearrangement of copper siloxides, was reported.[35] The
reaction of sodium phenylsiloxide with a poly(copper silses-
quioxane) [(PhSiO1.5)2(CuO)]n gave a new dinuclear cage-
like copper silsesquioxane [(PhSiO1.5)10(CuO)2(NaO0.5)2·
4EtOH]. The silsesquioxane cage is composed of one four-
membered and two five-membered condensed siloxane
rings that cap two almost square-planar CuII ions.
Scheme 4. Synthesis of 3.
The magnetic properties of 2 were analyzed by the Evans
method. The μeff value at room temperature of 2.38 μB is
slightly lower than the spin-only value expected for two
weakly coupled S = 1/2 centers (2.45 μB).
To shed light on the fate of the Ph2SiO moiety, the fil-
trate of the crystallization experiment was evaporated and
dried under vacuum. Subsequently, the light green solid was
analyzed by solution NMR spectroscopy. Even though it
still contained some paramagnetic compounds, it could be
shown that several Si-containing species formed in the
course of the reaction. The dominant side product is octa-
phenylcyclotetrasiloxane, which could be identified by com-
1
parison of the H and 29Si NMR spectra of the bulk mate-
rial with those of the independently synthesized tetrasilox-
ane (see Supporting Information).
We observed the elimination of R2SiO moieties from or-
ganosiloxides in the presence of transition metal ions al-
ready previously for the abovementioned Veith ligand
Figure 4. Molecular structure of 3·1.25Et2O. Phenyl hydrogen
(L*H3).[36] The combination of the deprotonated trisilanol atoms and cocrystallized solvent molecules are omitted for clarity.
Selected bond lengths [Å] and angles [°]: Zn1–Zn1Ј 2.9065(8), Zn1–
O7Ј 1.932(2), Zn1–O4 1.941(2), Zn1Ј–O1 1.978(2), Zn1–O1
1.996(2), O4–Zn1–O1 118.25(10), O7Ј–Zn1–O1 124.71(10), O7–
Zn1Ј–O1 115.79(10), O4–Zn1–O1Ј 124.87(10), Zn1–O1–Zn1Ј
and Fe(OTf)2 (OTf = trifluoromethanesulfonate) resulted in
the loss of Me2SiO units and the formation of a rare almost
square-planar dinuclear FeII complex [L*Ј2Fe2][Na(OEt2)2]2
[L*Ј3– = tBuSiO–(OSiMe2O–)2]. In this work, the ligand
93.99(9), O1–Zn1–O1Ј 86.01(9).
had also been reacted with Zn(OTf)2 under analogous con-
ditions to those for the Fe(OTf)2 reaction to check if the
“Me2SiO” elimination is a general behavior of such trisilan-
The asymmetric unit contains two independent mo-
ols in the presence of transition metals in the +II oxidation lecules of 3, which show similar bond lengths and angles
state. This led to a dinuclear zinc complex [L*2Zn2][Na- and, therefore, only one molecule will be discussed. The O–
(OEt2)2]2 containing intact L*3– ligands in excellent yield.
Zn–O angles vary between 115.79(10) and 124.87(10)°, as
To assess what difference the phenyl residues in LH3 expected for a distorted tetrahedron. The Zn–O bond
make, we also followed the same protocol for our newly lengths are 1.978(2) and 1.996(2) Å for the bridging O
developed ligand precursor and reacted it with ZnBr2 under atoms and 1.932(2) and 1.941(2) Å for the terminal O
conditions analogous to those for the synthesis of the CuII atoms. The Zn–Zn separation is 2.9065(8) Å.
siloxide complex (Scheme 4).
The metric data of the zinc core motif are very similar
After workup, the product was recrystallized from a di- to those of our previously described complex [L*2Zn2][Na-
ethyl ether/n-hexane solution, and colorless block-shaped (OEt2)2]2.[36] Only a few other dinuclear zinc complexes
crystals were isolated. Their X-ray diffraction analysis re- with a tetrahedral siloxide ligation have been reported;
vealed that a dinuclear zinc complex [L2Zn2][Li(OEt2)]2 (3) these are based on simple siloxide ligands, namely, trimeth-
had formed; 3 contains two intact L3– ligands that coordi- ylsiloxide[37] and tris(tert-butoxy)siloxide.[38]
nate the two zinc ions in a distorted tetrahedral fashion, as
Multinuclear NMR spectroscopy experiments showed
expected for a ZnII d10 electron configuration (Figure 4). that the solid-state structure of 3 is also retained in solution,
Complex 3·1.25Et2O crystallizes in the centrosymmetric as evidenced by three 29Si resonances at δ = –75.0, –42.5,
1
¯
space group P1.
and –36.1 ppm in a ratio a 2:4:2 and four H resonances
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Materials: Solvents were purified, dried, degassed, and stored over
molecular sieves prior to use. CunMesn,[39] PhSi(OH)3,[26] and di-
methyldioxirane[40] were prepared by literature procedures. A com-
mercially available n-butyllithium solution (2.6 m in toluene) was
used. PhSiCl2H, ZnBr2, and CuBr2 were purchased and used with-
out further purification.
corresponding to the ortho-phenyl protons at δ = 8.23, 7.85,
7.68, and 7.48 ppm in a ratio of 8:8:8:4.
The formation of 3 shows that the contact with a Lewis
acidic metal ion is not sufficient to induce the reaction
L
3– ǞLЈ3– + “Ph2SiO”. Furthermore, the elimination of
R2SiO appears to be independent of the base employed and
the countercation {NaOMe for [L*Ј2Fe2][Na(OEt2)2]2 and
n-butyllithium solution for [LЈ2Cu2][Li(THF2)]2}. The elec-
tronic and steric properties do not seem to play a decisive
role either.
Crystal Structure Determination: Data collections were performed
at 100 K with a STOE IPDS 2T diffractometer for LH3, 2, and
3·1.25Et2O and with a Bruker D8 Venture area detector for
1·2THF. The crystals were mounted on glass fibers and then trans-
ferred into the cold nitrogen gas stream of the diffractometer. In
all cases, Mo-Kα radiation (λ = 0.71073 Å) was used; the radiation
Clearly, the preferential coordination sphere of the tran-
sition metal ion is the driving force for the cleavage of the source was a sealed tube generator with a graphite monochromator
for LH3, 2, and 3·1.25Et2O and an Incoatec microsource with
multilayer optics for 1·2THF. Numerical absorption corrections for
2, empirical absorption corrections for 1·2THF, and multiscan ab-
sorption corrections[41] for 3·1.25Et2O and LH3 were applied to the
data. The structures were solved by direct methods (SHELXS-
97)[42] and refined by full-matrix least-squares procedures based on
F2 with all measured reflections (SHELXL-2013; see Supporting
Information, Table S1).[43] All non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were introduced at idealized posi-
tions and refined with riding models, except for H4, H5, and H6
Si–O bonds. Hence, the overall reaction including the cleav-
age of the Si–O bonds, the formation of 2, and the forma-
tion of the octaphenylcyclotetrasiloxane ring should be exo-
thermic. Indeed the complexes appear thermodynamically
quite stable in their dinuclear form, as various ligand-to-
metal ratios in the procedure described above for the syn-
thesis of CuII and ZnII complexes always resulted in 2 and
3, respectively (with unfavorable ratios, further unidentified
side products are formed). Variation of the ligand-to-metal
ratio for the CuI complex is not reasonable, as the employed in LH3, which were found on the electron density map and freely
refined. The crystallographic details are summarized in the Sup-
porting Information.
precursor CunMesn acts as both the base and the copper
source, so that the employed ratio of CunMesn to LH3 is
inherent.
Compound c: Diphenylchlorosilane (5.00 g, 22.86 mmol) was dis-
solved in dry THF (20 mL); the solution was cooled to –80 °C, and
dry triethylamine (2.31 g, 22.86 mmol) was added. Phenylsilanetriol
(1.15 g, 7.37 mmol) was dissolved in THF (150 mL) and added in
small portions to the cooled solution. The resulting suspension was
slowly warmed to room temperature and stirred overnight. The
white precipitate was removed by filtration, and the resulting color-
less solution was evaporated. After drying under high vacuum over-
Conclusions
The oxidation of a branched silane with dimethyldi-
oxirane led to a new branched trisilanol. This new com-
pound is a good starting material for the synthesis of transi-
tion metal siloxide complexes. The treatment of the ligand
with CunMesn gave a unique complex (1) with a hexagonal
planar CuI6 ring. Complexes of this type are rather rare,
and 1 is the first one with an oxygen donor environment at
the copper centers. After deprotonation of the ligand pre-
cursor with n-butyllithium and subsequent treatment with
CuBr2 or ZnBr2, dinuclear complexes were obtained. How-
ever, upon contact with CuII ions, one “Ph2SiO” unit per
ligand was eliminated so that a square-planar coordination
sphere could be realized. As ZnII ions prefer a tetrahedral
coordination, the original ligand remained intact in this
case.
night, the product was obtained as
a colorless oil (5.11 g,
7.26 mmol, 98%), which was used without further purification. 1H
3
NMR (400.1 MHz, CDCl3, 20 °C): δ = 7.42 (dd, JH,H = 8.0 Hz,
4JH,H = 1.3 Hz, 2 H, o-CH), 7.41 (dd, JH,H = 8.0 Hz, JH,H
=
3
4
1.3 Hz, 12 H, o-CH), 7.35 (m, 6 H, p-CH), 7.33 (m, 1 H, p-CH),
7.22 (m, 12 H, m-CH), 7.19 (m, 2 H, m-CH), 5.43 (br s, 3 H, SiH)
ppm. 1H–29Si HMBC NMR (CDCl3, 20 °C): δ = –75.2 (1 Si), –21.2
(3 Si) ppm. MS (EI–, MeCN): m/z = 702 [M]–.
Compound LH3: The silane c (1.20 g, 1.71 mol) was dissolved in
CHCl3 (150 mL) and treated with an excess of a cold (–30 °C) di-
methyldioxirane solution in acetone. The reaction mixture was
stirred overnight and then evaporated. The crude product was
recrystallized from THF/n-hexane (1:5) to yield colorless crystals of
LH3, which were dried overnight under high vacuum. The product
(0.90 mg, 1.19 mmol, 70%) was isolated as a colorless solid.
C42H38O6Si4 (751.10): calcd. C 67.17, H 5.10; found C 67.35, H
5.12. 1H NMR (400.1 MHz, [D6]acetone, 20 °C): δ = 7.60 (dd,
Experimental Section
4
3
3JH,H = 8.3 Hz, JH,H = 1.4 Hz, 2 H, o-CH), 7.55 (dd, JH,H
=
8.3 Hz, 4JH,H = 1.4 Hz, 12 H, o-CH), 7.32 (m, 6 H, p-CH), 7.27 (m,
1 H, p-CH), 7.18 (m, 12 H, m-CH), 7.12 (m, 2 H, m-CH), 6.33 (br
General Considerations: All manipulations with air-sensitive com-
pounds, namely, 1, 2, and 3, were performed in a glovebox or by
means of Schlenk-type techniques involving the use of a dry argon
atmosphere. The 1H and 1H–29Si HMBC NMR spectra were re-
corded with a Bruker AV 400 NMR spectrometer with solvents as
s, 3 H, SiOH) ppm. H–29Si HMBC NMR ([D6]acetone, 20 °C): δ
1
= –78.9 (1 Si), –39.2 (3 Si) ppm. IR (KBr): ν = 3135 (s), 3070 (m),
˜
3050 (m), 1965 (w), 1894 (w), 1827 (w), 1776 (w), 1591 (w), 1487
(w), 1429 (m), 1263 (w), 1132 (s), 1119 (s), 1075 (s), 1056 (s), 1026
(m), 997 (w), 899 (w), 870 (s), 741 (s), 717 (s), 670 (s), 511 (m), 489
(m), 462 (s) cm–1. HRMS (ESI–, MeCN): calcd. for C42H35O5Si4
[M – H3O+]– 731.1538; found 731.1562.
1
stated at 20 °C. The H NMR spectra were calibrated against the
residual proton signals (C6D6: δ = 7.16 ppm, CDCl3 δ = 7.26 ppm,
[D6]acetone δ = 2.50 ppm). Microanalyses were performed with a
Leco CHNS-932 elemental analyzer. Infrared (IR) spectra were re-
corded with samples prepared as KBr pellets with a Shimadzu
8400S FTIR spectrometer.
Compound 1: Compound LH3 (200 mg, 266 μmol) was dissolved in
THF (20 mL) and CunMesn (144 mg, 787 μmol) was added to this
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solution. Within 30 min, the stirred solution turned from light yel-
low to colorless. Stirring of the solution was continued overnight
at room temperature. Subsequently, all volatiles were removed from
the reaction mixture, and the remaining residue was dried overnight
under high vacuum. The crude product was washed with small
amounts of n-hexane, and the resulting colorless residue was dried
to yield 1 (217 mg, 116 μmol, 87%). Colorless, block-shaped crys-
tals of 1 could be obtained within 1 d by layering a hot concen-
trated toluene solution of 1 with n-hexane. C84H70Cu6O12Si8·
(C9H12)0.48: calcd. C 54.82, H 3.94; found C 54.94, H 3.76. p-
Xylene could not be completely removed under high vacuum. The
Supporting Information (see footnote on the first page of this arti-
cle): Additional NMR spectroscopic data, selected structural pa-
rameters for LH3, and crystallographic data.
Acknowledgments
The authors are grateful to the Humboldt-Universität zu Berlin.
Valuable discussions within the Cluster of Excellence “Unifying
Concepts in Catalysis” are highly appreciated.
1
1
content of p-xylene was determined by H NMR spectroscopy. H
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Compound 3: Compound LH3 (50 mg, 67 μmol) was dissolved in
THF (15 mL), the solution was cooled to –78 °C, and n-butyllith-
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stirred at low temperature for 3 h, and subsequently ZnBr2 (15 mg,
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3
3
(d, JH,H = 7.3 Hz, 8 H, o-CH), 7.68 (d, JH,H = 7.5 Hz, 4 H, o-
3
CH), 7.48 (d, JH,H = 7.3 Hz, 8 H, o-CH), 7.45–7.36 (m, 12 H),
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˜
1896 (w), 1828 (w), 1775 (w), 1608 (m), 1590 (m), 1485 (w), 1428
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743 (m), 713 (s), 700 (s), 547 (m), 519 (s), 486 (m) cm–1. HRMS
(ESI–, MeCN): calcd. for C84H71O12Si8Zn2 [L2Zn2H]– 1627.1614;
found 1627.1684.
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