Synthesis and structure of zwitterionic silylborates and silylzincates with pendant polydonor arms

Article abstract of DOI:10.1021/om300652n

The synthesis and structure of well-defined zinc- and potassium-containing heterobimetallic zwitterions that have pendant polydonor arms is reported. The zwitterionic complexes [Me₂ZnSi(SiMe₂OCH ₂CH₂OMe)₃Li] (Li-2), [Cl ₂ZnSi(SiMe₂OCH₂CH₂OMe)₃K] (K-3), and [I₂ZnSi(SiMe₂OCH₂CH ₂OMe)₃K] (K-4) were prepared from reactions of [Si(SiMe₂OCH₂CH₂OMe)₃M] (M = Li, K) (M-1) with ZnCl₂, ZnI₂, and ZnMe₂, respectively. Reaction of K-1 with B(C₆F₅)₃ gave the potassium silyl borate [(C₆F₅) ₃BSi(SiMe₂OCH₂CH₂OMe)₃K] (K-5) in 90% yield. Treatment of 2 equiv of K-1 with ZnCl₂ afforded the nonzwitterionic zinc silanide Zn(Si(SiMe₂OCH₂CH ₂OMe)₃)₂ (6), as a liquid, which upon treatment with 1 equiv of ZnMe₂ and 4 equiv of B(C₆F ₅)₃ could be converted into the zwitterionic zinc silyl borate [(C₆F₅)₃BSi(SiMe₂OCH ₂CH₂OMe)₃Zn][MeB(C₆F ₅)₃] (7). The structures of the zwitterionic complexes K-4, K-5, and 7 were determined by X-ray crystallography.

Full text of DOI:10.1021/om300652n

Article
pubs.acs.org/Organometallics
Synthesis and Structure of Zwitterionic Silylborates and Silylzincates
with Pendant Polydonor Arms
Hui Li, Fernando Hung-Low, and Clemens Krempner*
Department of Chemistry and Biochemistry, Texas Tech University, Box 41061, Lubbock, Texas 79409-1061, United States
S
* Supporting Information
ABSTRACT: The synthesis and structure of well-defined
zinc- and potassium-containing heterobimetallic zwitterions
that have pendant polydonor arms is reported. The
zwitterionic complexes [Me2ZnSi(SiMe2OCH2CH2OMe)3Li]
(Li-2), [Cl2ZnSi(SiMe2OCH2CH2OMe)3K] (K-3), and
[I2ZnSi(SiMe2OCH2CH2OMe)3K] (K-4) were prepared
from reactions of [Si(SiMe2OCH2CH2OMe)3M] (M = Li,
K) (M-1) with ZnCl2, ZnI2, and ZnMe2, respectively. Reaction
of K-1 with B(C6F5)3 gave the potassium silyl borate
[(C6F5)3BSi(SiMe2OCH2CH2OMe)3K] (K-5) in 90% yield.
Treatment of 2 equiv of K-1 with ZnCl2 afforded the nonzwitterionic zinc silanide Zn(Si(SiMe2OCH2CH2OMe)3)2 (6), as a
liquid, which upon treatment with 1 equiv of ZnMe2 and 4 equiv of B(C6F5)3 could be converted into the zwitterionic zinc silyl
borate [(C6F5)3BSi(SiMe2OCH2CH2OMe)3Zn][MeB(C6F5)3] (7). The structures of the zwitterionic complexes K-4, K-5, and 7
were determined by X-ray crystallography.
zwitterionic alkaline metal silanides with pendant polydonor
arms.
INTRODUCTION
■
Zwitterionic silanides are a relatively new class of silyl anions1
with dual functionalities that have the potential of being useful
as spectator ligands for main group and transition metals.2−7
Interest in these zwitterionic silanide ligands primarily arises
from the “naked” silyl anion that is rigidly locked and insulated
from the metal cation by internal donor bridges. In striking
contrast to tetracoordinate borate-based zwitterions,8 zwitter-
ionic silanides are tricoordinated and bear a stereochemically
active electron pair localized at the central silyl anion, which
behaves as a Lewis base and can bind to electrophilic transition
and main group metal centers. In fact, we have recently
demonstrated that zwitterionic alkaline and alkaline earth metal
silanides can efficiently be generated in high yields and used as
precursors for the convenient synthesis of novel heterobime-
tallic zwitterions6 (Chart 1). Herein, we report the synthesis
and structural characterization of well-defined zinc-containing
monometallic and heterobimetallic zwitterions derived from
RESULTS AND DISCUSSION
■
We first investigated the reaction of dimethylzinc with lithium
silanide Li-16 and isolated from the reaction mixture the
zwitterionic Lewis acid−base adduct Li-2 as the only product
(Scheme 1). Li-2, which was characterized by 1H, 13C, 29Si, and
7Li NMR spectroscopy, can be viewed as a heterobimetallic
silylzincate in which the lithium cation is coordinated by the six
neutral oxygen donors, while the zinc is being coordinated by
the silyl anion to form a zincate structure. Li-2 is thermally
unstable and decomposes slowly upon standing for longer
periods of time at room temperature. It is soluble in most
organic solvents (benzene, ether, toluene, THF), except
hexanes and pentane, where it is insoluble.
When potassium silanide K-1 was reacted with ZnCl2 in
THF as solvent, a colorless microcrystalline material insoluble
in pentanes and hexanes was isolated in 64% yield and
identified by 1H, 13C, and 29Si NMR data and elemental
Chart 1
analyses as the zwitterionic ZnCl2 adduct K-3. The 1H and 13
C
NMR spectra showed singlets for the SiMe2 and OMe groups
and triplets for the two chemically nonequivalent CH2 groups,
which is consistent with a structure of nearly C3 symmetry in
which the central potassium cation is equally coordinated by
the three pendant OCH2CH2OMe donor arms. The reaction of
K-1 with ZnI2 in diethyl ether behaved similarly to that of K-1
with ZnCl2 as the zwitterionic ZnI2 adduct K-4 precipitated
Received: July 11, 2012
Published: October 5, 2012
© 2012 American Chemical Society
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NMR data and elemental analyses; the spectral features were
similar to those of K-3.
Scheme 1. Formation of the Zwitterionic Lewis Acid−Base
Adducts Li-2, K-3, and K-4
In addition, the molecular structure of K-4 was determined
by X-ray crystallography (see Figure 1). K-4 forms in the solid
state a dimer composed of a square-planar Zn2I2 core. The zinc
centers exhibit a distorted tetrahedral geometry with angles
ranging from 96° to 119°. As expected, the Zn−Ibridging
distances ranging from 2.741 to 2.768 Å are significantly larger
than the Zn−Iterminal distances [Zn2−I4 2.592(1), Zn1−I3
2.614(1) Å]. The Si−Zn bond lengths [2.389 and 2.395 Å] are
only slightly larger than those of the nonzwitterionic zinc
silanides (Me3 Si)3 SiZnCl·TMEDA [2.367 Å],9
[ ( M e 3 S i ) 3 S i Z n C l · T H F ] 2 [ 2 . 3 5 4 Å ] , 1 0 [ P h -
(Me3Si)2SiZnCl·THF]2 [2.365 Å],10 [(Me3Si)3Si]2Zn [2.355
Å],11 (But3Si)2Zn [2.384 Å],12 and [(But3Si)2ZnBr]4 [2.376
Å].12 In the dimer the two potassium cations are each
coordinated by the three OCH2CH2OMe donor arms, resulting
in a distorted octahedral coordination environment for both
potassium cations with O−K−O angles and K−O distances
ranging from 61.7° to 141.7° and 2.63−2.78 Å, respectively.
Notably, both adducts K-3 and K-4 are stable at room
temperature and do not spontaneously undergo elimination of
KX (X = Cl, I) to generate salt-free zinc silanides. To the best
of our knowledge, K-3 and K-4 represent the first stable ZnX2-
silanide adducts.13,14 In fact, these adducts can be considered
intermediates in the formation of zinc silanides of the general
formula R3Si−Zn−SiR3 via an addition−elimination mecha-
nism at the zinc center. The relative stability of K-3 and K-4
toward elimination of KX at room temperature can reasonably
be explained by the potassium cation being sequestered within
the cavity of the ligand with its three pendant donor arms
resulting in a stable octahedral coordination environment for
potassium.
In an attempt to generate zwitterionic zinc silanides, where
the zinc dication would be exclusively coordinated octahedrally
by the OCH2CH2OMe donor arms and charge separated from
the anion, K-16 was reacted with the strong Lewis acid
from solution as a colorless crystalline material in 90% yield. Its
1
composition was established by means of H, 13C, and 29Si
Figure 1. Solid-state structure of dimeric K-4 (the disordered MeOCH2CH2O group was omitted for clarity). Selected distances [Å] and angles
[deg]: Zn1−Si1 2.389(2), Zn1−I3 2.614(1), Zn1−I1 2.741(1), Zn1−I2 2.756(1), Zn2−Si5 2.395(2), Zn2−I4 2.592(1), Zn2−I1 2.754(1), Zn2−I2
2.768(1), Si1−Zn1−I3 118.84(5), Si1−Zn1−I1 116.17(5), I3−Zn1−I1 103.55(3), Si1−Zn1−I2 111.84(5), I3−Zn1−I2 108.07(3), I1−Zn1−I2
95.60(3), Si5−Zn2−I4 118.71(5), Si5−Zn2−I1 112.78(5), I4−Zn2−I1 105.19(3), Si5−Zn2−I2 113.29(5), I4−Zn2−I2 109.04(3), I1−Zn2−I2
95.02(2), Zn1−I1−Zn2 84.89(2), Zn1−I2−Zn2 84.36(2), Si4−Si1−Zn1 108.95(7), Si3−Si1−Zn1 104.33(8), Si2−Si1−Zn1 111.45(7), Si6−Si5−
Zn2 103.37(8), Si8−Si5−Zn2 109.98(7), Si7−Si5−Zn2 111.57(8).
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B(C6F5)3 and subsequently treated with zinc halides. However,
all attempts to generate such a zwitterionic zinc silylborate via
salt metathesis (Scheme 2) failed primarily due to the very low
K-5, which represents one of the very few examples of silyl-
substituted tris(pentafluorophenyl)borates,13,14 forms in the
solid state a polymeric network held together by intermolecular
H···F contacts. Close intramolecular H···F contacts are seen in
the monomeric subunits: between the ortho- (F4, F6, and F11)
and meta-fluorine (F5, F7, and F12) atoms and the next
neighboring hydrogen atoms of the SiMe2 groups ranging from
2.52 to 2.87 Å. The silicon boron distance [2.167 Å] of K-5 is
somewhat larger than of the structurally related silylene borate
complexes L→SiCl2→B(C6F5)3 [2.106 Å]15 and [Ph(NBut)-
CNBut]SiCl→B(C6F5)3 [2.108 Å]16 reported by Roesky and
Stalke. The larger Si−B distance of K-5 perhaps is the result of
significant steric repulsion occurring between the metal silanide
and B(C6F5)3 units, because the Si−B distance in the sterically
less demanding potassium silyl borate [(Me3Si)3SiBH3K·THF]2
[1.991 Å]17 is significantly shorter. Steric repulsion in K-5 is
also reflected by a lengthening of the Si−Si bonds to about 2.38
Å; zwitterionic K-1 has an average Si−Si distance of ca. 2.32 Å.
Similar to K-4, the potassium cation of K-5 is coordinated by
three OCH2CH2OMe donor arms, which gives rise to a
distorted octahedral coordination environment for potassium
with O−K−O angles and K−O distances ranging from 62.5° to
146.7° and 2.67−2.74 Å, respectively.
With the aim of synthesizing a zwitterionic disilyl zinc
complex, reactions of various zinc salts with two equivalents of
zwitterionic metal silanide, M-1 (M = Li, Na, K), were
examined. In most cases mixtures of liquid products were
obtained, which could not be separated. When two equivalents
of K-16 were treated at low temperatures with ZnCl2 in THF, a
mixture of two major products was obtained. Upon heating the
mixture in hexanes, one of the two major products converted
into the other one with elimination of KCl. By multinuclear
NMR spectroscopy, the isolated liquid product was identified as
the homoleptic zinc silanide of formula Zn-
[Si(SiMe2OCH2CH2OMe)3]2 (6) (Scheme 3).
Scheme 2. Attempted Synthesis of a Zwitterionic Zinc
Silylborate
solubility of K-5 in most organic solvents. Reactions of K-5 with
ZnI2 in THF gave several products, which could not be
separated from each other due to their low solubilities in
organic solvents. Despite its poor solubility, K-5 was isolated
from the reaction of K-1 and B(C6F5)3 in toluene as a
1
microcrystalline material, and its structure determined by H,
13C, 19F, 11B, and 29Si NMR data, elemental analyses, and X-ray
crystallography (Figure 2).
Homoleptic zinc silanide 6 appears to be nonzwitterionic as
judged by its good solubility in aliphatic hydrocarbons, its liquid
nature, and the 29Si NMR chemical shift of its formally anionic
silicon (−146.3 ppm), a value that is within the range of
structurally related and nonzwitterionic zinc silanides (see
Table 1). Compared to the 29Si NMR chemical shift of the
silicon nuclei of the dicoordinated homoleptic zinc silanides
[(Me3Si)3Si]2Zn (123.9 ppm) and [(Me3SiMe2Si)3Si]2Zn
(−116.4 ppm), however, the central silicon nucleus of 6
resonates at significantly higher field (−146.3 pm), more
consistent with a tetracoordinated zinc center. In fact, the
central silicon nuclei of tetracoordinated zinc silanides resonate
within the relatively narrow range −150 to −157 ppm. Thus, it
might be that the zinc center in compound 6 is additionally
coordinated by one or more of the intramolecular
OCH2CH2OMe donor arms.
Owing to its liquid nature, 6 could not be further purified,
and consequently the results of combustion analysis were
nonsatisfactory. However, 6 was sufficiently pure to perform
reactivity studies. Although 6 was nonreactive in the presence
of excess ZnMe2 or BEt3, it reacted readily with one equivalent
of ZnMe2 after addition of four equivalents of the strong Lewis
acid B(C6F5)3 to form a colorless microcrystalline precipitate
from toluene solutions. Multinuclear NMR spectroscopic
studies, the results of elemental analysis, and single-crystal X-
ray crystallography revealed the isolated compound to be the
zwitterionic zinc silylborate salt 7 (Scheme 3). Complex 7 is
thermally stable, soluble in THF, poorly soluble in CD2Cl2, and
Figure 2. Solid-state structure of K-5. Selected distances [Å] and
angles [deg]: Si4−B1 2.167(4), K1−O2 2.666(3), K1−O4 2.679(2),
K1−O6 2.680(3), K1−O5 2.704(2), K1−O3 2.720(2), K1−O1
2.736(2), O1−Si1 1.675(2), O3−Si2 1.671(2), O5−Si3 1.683(2),
Si1−Si4 2.3821(12), Si2−Si4 2.3770(13), Si3−Si4 2.3813(12), Si1−
O1−K1 123.36(10), Si2−O3−K1 126.78(11), Si3−O5−K1
125.57(11), O1−Si1−Si4 104.42(8), O3−Si2−Si4 104.35(9), O5−
Si3−Si4 106.44(8), B1−Si4−Si2 115.68(10), B1−Si4−Si3 114.48(10),
Si2−Si4−Si3 103.06(5), B1−Si4−Si1 115.59(10), Si2−Si4−Si1
102.81(4), Si3−Si4−Si1 103.51(4).
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Scheme 3. Synthesis of Homoleptic Zinc Silanide 6 and Zwitterionic Zinc Silylborate 7
Table 1. 29Si NMR Chemical Shifts [ppm] of the Silyl Anion
(shown in bold) of Selected Zinc Silanides
borate units, as it shows six signals in a 2:2:1:1:2:2 ratio due to
the ortho-, para-, and meta-fluorine nuclei. Interestingly, the
ortho-fluorine of the SiB(C6F5)3− unit appears as a broad signal
at room temperature (−130.3 ppm), which upon cooling to
−50 °C splits into two distinct but still broad signals at −121.8
and −135.9 ppm, indicative of a hindered rotation along the
central B−C bonds in the SiB(C6F5)3 unit with a calculated
rotational barrier of ca. 9.6 kcal/mol (Figure 3). Also the signal
of the meta-fluorine nuclei of the SiB(C6F5)3 unit, which at
room temperature appears as a quintet, changes into a broad
featureless signal upon cooling to −50 °C, whereas the signal of
compound
solvent Si NMR δ
ref
a
(Me3Si)3SiZnCl·TMEDA
(Me3Si)3SiZnBr·2THF
[(Me3Si)3Si]2Zn
THF
THF
−156.8
−149.9
−123.9
−150.8
−116.4
−180.2
9
a
9
C6D6
C6D6
C6H6
11
11
18
[(Me3Si)3Si]2Zn·2,2-bipyridine
[(Me3SiMe2Si)3Si]2Zn
b
Me2ZnSi(SiMe2OCH2CH2OMe)3Li
(Li-2)
THF-D8
this work
I2ZnSi(SiMe2OCH2CH2OMe)3K (K-4) THF-D8
−149.9
−156.9
this work
this work
1
the SiMe2 group in the H NMR spectrum splits into two
Cl2ZnSi(SiMe2OCH2CH2OMe)3K (K-
3)
C6D6
distinct singlets at −50 °C (see also Supporting Information).
In the solid state, zinc salt 7 forms a polymeric network held
together by intermolecular H···F contacts similar to what is
seen in K-5 (Figure 4). Again, close intramolecular H···F
contacts are seen in the monomeric subunits between the
ortho- (F5, F10, and F11) and meta-fluorine (F4 and F9)
atoms and the next neighboring hydrogen atoms of the SiMe2
groups, ranging from 2.59 to 2.90 Å. The observation of close
Zn[Si(SiMe2OCH2CH2OMe)3]2 (6)
C6D6
−146.3
this work
a
b
Measured with D2O capillary. Measured with DMSO-D6 capillary.
insoluble in aromatic and aliphatic hydrocarbons. The 11B{1H}
NMR spectrum of 7 shows two distinct signals: one at −14.8
assigned to the MeB(C6−F5)3− borate and a second one at −20.8
due to the SiB(C6F5)3 borate. The 19F{1H} NMR of 7 is
consistent with the presence of two tris(pentafluorophenyl)-
1
H···F contacts is consistent with variable-temperature H and
Figure 3. VT 19F NMR spectra of 7 in THF-D8 as solvent.
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Although the mechanism of formation of 7 remains unclear,
the function of the added Lewis acid B(C6F5)3 in this rather
unusual multistep reaction appears to be twofold: (1)
Replacement and release of the central zinc dication in 6,
followed by its coordination by the OCH2CH2OMe pendant
donor arms, to generate zwitterionic cation [(C6F5)3BSi-
(SiMe2OCH2CH2OMe)3Zn]+ (A) and anion [(C6F5)3BSi-
(SiMe2OCH2CH2OMe)3]− (B) and (2) abstraction of the
methyl groups from the added ZnMe2 to produce two
equivalents of anion [MeB(C6F5)3]− and a zinc dication; the
latter is picked up by anion A to generate a second equivalent
of cation B (Scheme 4).
Scheme 4. Proposed Mechanism of the Reaction of 6 with
ZnMe2 and B(C6F5)3
Figure 4. Solid-state structure of 7. Selected distances [Å] and angles
[deg]: Zn1−O5 2.0720(15), Zn1−O1 2.0794(15), Zn1−O3
2.0945(15), Zn1−O4 2.1137(16), Zn1−O2 2.1317(16), Zn1−O6
2.1326(16), O1−Si1 1.7260(16), O3−Si2 1.7198(16), O5−Si3
1.7230(16), Si1−Si4 2.3505(10), Si2−Si4 2.3466(10), Si3−Si4
2.3630(10), Si4−B1 2.139(3), O5−Zn1−O1 95.93(6), O5−Zn1−
O3 94.11(6), O1−Zn1−O3 91.95(6), O5−Zn1−O4 100.38(6), O1−
Zn1−O4 161.00(6), O3−Zn1−O4 77.29(6), O5−Zn1−O2
164.16(6), O1−Zn1−O2 78.43(6), O3−Zn1−O2 100.82(6), O4−
Zn1−O2 88.14(6), O5−Zn1−O6 77.99(6), O1−Zn1−O6 101.61(6),
O3−Zn1−O6 164.87(6), O4−Zn1−O6 91.33(6), O2−Zn1−O6
88.57(6), Si1−O1−Zn1 125.35(8), Si2−O3−Zn1 128.54(8), Si3−
O5−Zn1 126.66(8), O1−Si1−Si4 102.89(6), O3−Si2−Si4 101.82(6),
O5−Si3−Si4 101.97(6), B1−Si4−Si2 119.26(8), B1−Si4−Si1
116.91(7), B1−Si4−Si3 116.77(8), Si2−Si4−Si1 99.96(3), Si2−Si4−
Si3 99.44(3), Si1−Si4−Si3 101.20(3).
19F NMR studies in solution, which show at room temperature
broad signals of the ortho-fluorine nuclei and the SiMe2
protons due to a hindered rotation along the C−B bonds.
The silicon−boron distance of 7 [2.139 Å] is slightly shorter
than that of K-5 presumably due to the smaller size of the zinc
dication forcing the central B−Si−Si angles to be larger and the
Si−Si−Si angle to be smaller, which would reduce steric
repulsion in the molecule. Similar to the potassium cations in
K-4 and K-5, the zinc dication is octahedrally coordinated by
three OCH2CH2OMe donor arms, with O−Zn−O angles and
Zn−O distances ranging from 77.3° to 164.9° and 2.07−2.13 Å,
respectively. Interestingly, the average Si−O distance [1.72 Å]
is significantly larger than in K-1, K-4, and K-5 (see also Table
2 for comparison). Clearly, the elongation of the Si−O bonds
in 7 is the result of an O→M donor−acceptor interaction,
which is stronger for the small zinc dication (M = Zn2+) than
for the significantly larger potassium monocation (M = K+).
With the synthesized zwitterionic zinc salt 7 in hand we
investigated its reactivity toward Lewis bases such as THF,
pyridine, and dimethylaminopyridine (DMAP) and 2,2-
bipyridine. We were particularly interested in whether these
donor molecules would be capable of competing with the
pendant donor arms for the coordination sphere of the zinc
dication. The addition of THF to a CD2Cl2 suspension of 7 had
only a minor effect on the 1H and 19F NMR chemical shifts due
to slight changes in solvent polarity. Upon addition of 2.5
equivalents of 2,2-bipyridine to a CD2Cl2 suspension of 7,
however, a complete set of new signals in the 1H NMR
spectrum (Figure 5) and three new fluorine signals in the 19F
NMR spectrum appeared. Comparison of the integrals of the
aromatic 2,2-bipyridine signals with those of the silanide signals
Table 2. Selected Average Distances [Å] and Angles [deg]
for K-1, K-4, K-5, and 7
1
in the H NMR spectrum revealed three molecules of 2,2-
K-16
K-4
K-5
7
bipyridine to coordinate to zinc, suggesting the formation of the
hexacoordinate zinc species [Zn(2,2-bipyridine)3]2+ along with
the anionic species [(C6F5)3BSi(SiMe2OCH2CH2OMe)3]− (A)
and [MeB(C6F5)3]− (Figure 5). As more than three equivalents
of 2,2-bipyridine were added, the signals for 7 completely
disappeared and signals arising from the aromatic protons of
free 2,2-bipyridine appeared, indicating no exchange of free
with coordinated 2,2-bipyridine. Similar results were found for
Si−Si
Si−O
2.32
1.68
2.74
2.74
100
113
125
2.32
1.67
2.68
2.73
111
109
120
2.38
1.68
2.72
2.68
103
105
125
2.35
1.72
2.08
2.13
100
102
127
O−M(SiOC)
O−M(COC)
Si−Si−Si
Si−Si−O
Si−O−M
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1
Figure 5. H NMR spectra of the NMR tube reactions of 7 with 2,2-bipyridine (CD2Cl2) taken after adding various amounts of 2,2-bipyridine.
the addition of DMAP and pyridine to 7. After adding 4.5
equivalents of DMAP, clean formation of a single species that
contains four molecules of coordinated DMAP consistent with
the tetracoordinate zinc species [Zn(DMAP)4]2+ was observed
according to the NMR data. In contrast, addition of 10
equivalents of the weaker Lewis base pyridine to 7 gave a
complex mixture with compound 7 as the major species in
solution. In the presence of 30 equivalents of pyridine, however,
CONCLUSION
■
In conclusion, we have reported the synthesis and structure of
the first zwitterionic silylborate silylzincates of potassium and
zinc. These zwitterions form discrete structures with the central
metal cations being octahedrally surrounded by the pendant
polydonor groups. The ZnX2 adducts K-3 and K-4 are stable at
room temperature and do not spontaneously undergo
elimination of KX (X = Cl, I) due to the stable octahedral
coordination environment of the potassium cation provided by
the polydentate silylborate ligand. The first stable zwitterionic
zinc silylborate (7) could conveniently be made in good overall
yield via a two-step synthetic process involving the initial
formation of the nonzwitterionic zinc silanide 6, derived from
the reaction of K-1 with ZnCl2 followed by the addition of
ZnMe2 and B(C6F5)3. Further studies regarding the synthesis
and structural characterization of transition metal-containing
zwitterionic silanides and silylborates are currently ongoing.
1
signals of 7 in the H and 19F NMR completely disappeared,
indicating the formation of [Zn(pyridine)4]2+ as the major
cationic species along with the anions [(C6F5)3BSi-
(SiMe2OCH2CH2OMe)3]− (A) and [MeB(C6F5)3]− (see
Supporting Information for a detailed analysis of the NMR
data).
From these preliminary NMR data it can be concluded that
the silylborate ligand [(C6F5)3BSi(SiMe2OCH2CH2OMe)3]−
(A) with its three OCH2CH2OMe donor arms has a relatively
high affinity to bind to metal dications and that only strong
nitrogen-containing Lewis bases are capable of replacing
[(C6F5)3BSi(SiMe2OCH2CH2OMe)3]− (A) as supporting
ligand for these dications.
EXPERIMENTAL SECTION
■
The manipulation of air-sensitive compounds involved standard
Schlenk line and glovebox techniques. THF, THF-D8, toluene, and
n-hexane were distilled under nitrogen from alkali metals and stored
over molecular sieves prior to use. C6D6 and CD2Cl2 were dried and
stored over molecular sieves prior to use. The compounds
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Organometallics
Article
Si(SiMe2OCH2CH2OMe)3K (K-1),6 Si(SiMe2OCH2CH2OMe)3Li
31.6 (SiMe2O), −149.9 (SiSi3). Anal. Calcd for C15H39O6Si4ZnKI2
(786.1): C, 22.92; H, 5.00; Found: C, 22.77; H, 5.08.
(Li-1),6 and B(C6F5)3 were prepared as described in the literature.
19
All other chemicals were purchased from commercial sources and used
(C6F5)3BSi(SiMe2OCH2CH2OMe)3K (K-5). In a glovebox, a
precooled toluene solution (ca. −20 °C) of B(C6F5)3 (92 mg, 0.18
mmol) was added to a precooled toluene solution (ca. −20 °C) of K-1
(86 mg, 0.18 mmol). A precipitate was formed immediately, which
after stirring for 30 min was centrifuged, washed several times with
hexanes, and dried under vacuum to give the title compound as pale
yellow powder in 90% yield (160 mg). 1H NMR (CD2Cl2, 300 MHz):
1
7
without further purification. The H, Li, 13C, 19F, 11B, and 29Si NMR
spectra were obtained from a Varian Unity Inova 500 and JEOL ECS
400. All measurements, unless noted otherwise, were carried out at
7
298 K, and NMR chemical shifts are given in ppm. The Li NMR
spectra were referenced to a 0.1 M solution of LiCl in D2O (δ = 0),
the 11B NMR spectra to H3BO3 in D2O (δ = 36 ppm), the 19F NMR
spectra to C6F6 in C6D6 (δ = −164.9 ppm), and the 29Si NMR spectra
to TMS (δ = 0 ppm). 29Si NMR spectra were obtained by using the
3
δ −0.01 (s, SiMe2, 18 H), 3.35 (s, OCH3, 9 H), 3.46 (t, JH−H = 3.6
Hz, SiOCH2, 6 H), 3.64 (t, 3JH−H = 3.6 Hz, SiOCH2, 6 H). 13C NMR
(CD2Cl2, 125.7 MHz): δ 1.9 (SiMe2), 59.0 (OMe), 61.6 (OCH2), 74.7
(CH2OMe), 125.6 (br, ipso-C), 137.1 (br d, JC−F = 241.3 Hz, meta-
C), 138.5 (br d, JC−F = 245.0 Hz, para-C), 148.4 (br d, JC−F = 237.5
Hz, ortho-C). 29Si NMR (CD2Cl2, 99.3 MHz): δ 33.1 (SiMe2O),
(SiSi3 not found). 11B NMR (CD2Cl2, 160.3 MHz): δ −21.1. 19F
1
INEPT pulse sequence. The H NMR spectra were referenced to the
1
residual protonated solvent for H, and the 13C NMR spectra were
referenced to the deuterated solvent peaks. The following abbrevia-
tions were used to describe peak multiplicities in the reported NMR
spectroscopic data: “m” for complex multiplet and “br” for broadened
resonances. Elemental analyses were performed by Atlantic Microlab
Inc. (Norcross, GA, USA).
NMR (CD2Cl2, 376 MHz): δ −127.5 (br s, ortho-F), −163.2 (t, JF−F
=
19.1 Hz, para-F), −166.9 (m, JF−F = 19.1 Hz, meta-F). Anal. Calcd for
C33H39BF15KO6Si4 (978.9): C, 40.49; H, 4.02. Found: C, 40.43; H,
3.90.
Single crystals of K-4, K-5, and 7 suitable for X-ray diffraction were
coated with polyisobutylene oil in a drybox and were quickly
transferred to the goniometer head of a Bruker Apex II detector
system equipped with a molybdenum X-ray tube (λ = 0.71073 Å).
Data were collected at 150 K. Preliminary data revealed the crystal
system. A hemisphere routine was used for data collection and
determination of lattice constants. The space group was identified, and
the data were processed using the Bruker AXS SHELXTL software
(version 6.14) and corrected for absorption using SADABS. The
structures were solved using direct methods (SHELXS) completed by
subsequent Fourier transformation and refinement by full-matrix least-
squares procedures. The structure of K-4 contained a disordered
MeOCH2CH2O group, which was modeled over two positions and
refined isotropically.″
Zn[Si(SiMe2OCH2CH2OMe)3]2 (6). In a glovebox a precooled
THF solution (ca. −20 °C) of ZnCl2 (31.3 mg, 0.23 mmol) was added
to a precooled THF solution (ca. −20 °C) of K-1 (214 mg, 0.46
mmol) with stirring. The resulting suspension was stirred for 1 h, the
solvent was removed under vacuum, and the oily residue was extracted
twice with hexanes. The combined hexanes solution was heated at 55
°C for 4 h, upon which more precipitate was formed. After filtration of
the suspension, solvent and other volatiles were removed under
vacuum to leave 180 mg (85%) of the title compound as a colorless
liquid that contained minor amounts of unidentified impurities.
1
Therefore the results of elemental analysis were nonsatisfactory. H
NMR (C6D6, 300 MHz): δ 0.61 (s, SiMe2, 18 H), 3.19 (s, OMe, 9 H),
3
3
Me2ZnSi(SiMe2OCH2CH2OMe)3Li (Li-2). In a glovebox, a 1.2 M
toluene solution of ZnMe2 (0.15 mL, 0.18 mmol) was added to Li-1
(70 mg, 0.16 mmol) dissolved in 1 mL of toluene and allowed to stay
for 4 h. Then the solvent was removed under vacuum, and the residue
was washed with hexanes and dried under vacuum to give the title
compound as a colorless solid (yield 70%). 1H NMR (C6D6, 300
MHz): δ 0.23 (s, ZnMe2, 6 H), 0.53 (s, SiMe2, 18 H), 2.84 (s, OMe, 9
3.43 (t, JH−H = 4.9 Hz, CH2OMe, 6 H), 3.81 (t, JH−H = 4.9 Hz,
SiOCH2, 6 H). 13C NMR (C6D6, 125.7 MHz): δ 5.6 (SiMe2), 58.9
(OMe), 63.0 (OCH2), 74.9 (CH2OMe). 29Si NMR (C6D6, 99.3
MHz): δ 27.2 (SiMe2O), −146.3 (SiSi3).
[(C6F5)3BSi(SiMe2OCH2CH2OMe)3Zn][MeB(C6F5)3] (7). In a
glovebox a solution of B(C6F5)3 (220 mg, 0.45 mmol) in toluene
was added at room temperature to a solution of 6 (100 mg, 0.11
mmol) in 15 mL of toluene, and the mixture was stirred for 5 min.
Then, a 1.2 M toluene solution of ZnMe2 (90 μL, 0.11 mmol) was
added, and the resulting suspension was stirred overnight. A white
precipitate was formed, which was filtered, suspended in CH2Cl2, and
stirred rapidly for 2 h. The solid was collected by centrifugation and
dried under vacuum to give the title compound as a microcrystalline
3
3
H), 2.84 (t, JH−H = 4.8 Hz, CH2OMe, 6 H), 3.22 (t, JH−H = 4.8 Hz,
SiOCH2, 6 H). 13C NMR (C6D6, 125.7 MHz): δ −2.4 (ZnMe2), 4.3
(SiMe2), 58.5 (OMe), 61.0 (OCH2), 73.2 (CH2OMe). 29Si NMR
(C6D6, 99.3 MHz): δ 39.9 (SiMe2O), −180.2 (SiSi3). 7Li NMR (C6D6,
194.2 MHz): δ −1.5. Due to the thermal instability of the title
compound at room temperature, attempts to obtain satisfactory
elemental analysis data failed.
1
material in 61% yield. H NMR (THF-D8, 300 MHz, 298 K): δ 0.31
Cl2ZnSi(SiMe2OCH2CH2OMe)3K (K-3). In a glovebox a THF
solution of ZnCl2 (24 mg, 0.18 mmol) was added to K-1 (86 mg, 0.18
mmol) dissolved in 1 mL of THF, and the mixture was allowed to stir
for 4 h. THF was removed under vacuum, and the product was
extracted twice with toluene. After removal of solvent under vacuum,
the residue was washed twice with hexanes and dried under vacuum to
give 70 mg (64%) of the title compound as a white powder. 1H NMR
(C6D6, 300 MHz): δ 0.66 (s, SiMe2, 18 H), 3.12 (s, OMe, 9 H), 3.19
(br, CH2OMe, 6 H), 3.57 (br, SiOCH2, 6 H). 13C NMR (C6D6, 125.7
MHz): δ 4.4 (SiMe2), 58.5 (OMe), 61.9 (SiOCH2), 74.3 (CH2OMe).
29Si NMR (C6D6, 99.3 MHz): δ 30.1 (SiMe2O), −156.9 (SiSi3). Anal.
Calcd for C15H39O6Si4ZnKCl2 (603.2): C, 29.87; H, 6.52; Cl, 11.76.
Found: C, 29.60; H, 6.39; Cl, 11.48.
(br, SiMe2, 18 H), 0.49 (s, BMe, 3 H), 3.78 (s, OMe, 9 H), 3.97 (br,
CH2OMe, 6 H), 4.12 (br, SiOCH2, 6 H); (THF-D8, 500 MHz, 233
K): δ 0.03, 0.52 (2s, SiMe2, 2 × 9 H), 0.47 (s, BMe, 3 H), 3.79 (s,
OMe, 9 H), 3.93 (br d, CH2OMe, 3 H), 4.22 (br d, CH2OMe, 3 H),
4.03 (br quint, SiOCH2, 6 H). 13C NMR (THF-D8, 125.7 MHz, 298
K): δ 2.3 (SiMe2), 10.5 (BCH3), 62.2 (OMe), 63.2 (OCH2), 72.5
(CH2OMe), 123.8, 130.6 (br, ipso-C), 137.1 (br d, JC−F = 243.8 Hz,
C), 137.7 (br d, JC−F = 237.5 Hz, C), 138.3 (br d, JC−F = 242.5 Hz, C),
139.6 (br d, JC−F = 235.0 Hz, C), 148.8 (br d, JC−F = 236.6 Hz, ortho-
C), 149.3 (br d, JC−F = 238.7 Hz, ortho-C). 29Si NMR (THF-D8, 99.3
MHz): δ 26.2 (SiMe2O), −159.1 SiSi3). 11B NMR (THF-D8, 160.3
MHz, 298 K): δ −14.8 (BCH3), −20.8 (BSi). 19F NMR (THF-D8, 376
MHz, 298 K): δ −130.3 (br, ortho-F SiB(C6F5)3), −134.7 (d, JF−F
=
I2ZnSi(SiMe2OCH2CH2OMe)3K (K-4). In a glovebox, ZnI2 (60 mg,
0.18 mmol) was added to K-1 (86 mg, 0.18 mmol) dissolved in 1 mL
of ether, upon which a white precipitate was formed. After stirring the
resulting suspension for 4 h at room temperature, the precipitate was
filtered, washed once with ether, and dried under vacuum to give 130
19.1 Hz ortho-F, CH3B(C6F5)3), −163.8 (t, JF−F = 20.8 Hz, para-F,
SiB(C6F5)3), −168.0 (m, JF−F = 20.8 Hz, meta-F, SiB(C6F5)3), −168.5
(t, JF−F = 20.8 Hz, para-F, CH3B(C6F5)3), −170.7 (m, JF−F = 20.8 Hz,
meta-F, CH3B(C6F5)3); (THF-D8, 376 MHz, 233 K): δ −121.7, (br,
ortho-F, SiB(C6F5)3), −135.6 (br, ortho-F, SiB(C6F5)3), −132.9 (d,
JF−F = 19.1 Hz ortho-F, CH3B(C6F5)3), −161.6 (t, JF−F = 20.8 Hz,
para-F, SiB(C6F5)3), −165.8 (br, meta-F, SiB(C6F5)3), −166.1 (t, JF−F
= 20.8 Hz, para-F, CH3B(C6F5)3), −168.3 (t, JF−F = 20.8 Hz, meta-F,
CH3B(C6F5)3). Anal. Calcd for C52H42B2F30O6Si4Zn (1532.2): C,
40.76; H, 2.76. Found: C, 40.18; H, 2.78.
1
mg (90%) of the title compound as a white powder. H NMR (THF-
D8, 300 MHz): δ 0.40 (s, SiMe2, 18 H), 3.36 (s, OMe, 9 H), 3.47 (t,
3
3JH−H = 4.6 Hz, CH2OMe, 6 H), 3.74 (t, JH−H = 4.6 Hz, SiOCH2, 6
H). 13C NMR (THF-D8, 125.7 MHz): δ 4.4 (SiMe2), 59.0 (OMe),
62.6 (CH2OSi), 75.2 (CH2OMe). 29Si NMR (THF-D8, 99.3 MHz): δ
7123
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Organometallics
Article
(16) Jana, A.; Azhakar, R.; Sarish, S. P.; Samuel, P. P.; Roesky, H. W.;
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9270.
(18) Dobrovetsky, R.; Kratish, Y.; Tumanskii, B.; Botoshansky, M.;
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(19) Chernega, A. N.; Graham, A. J.; Green, M. L. H.; Haggitt, J.;
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ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic data for K-4 (CCDC 885796), K-5 (CCDC
885794), and 7 (CCDC 885795) including CIF files. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: clemens.krempner@ttu.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The support of our work by Texas Tech University is greatly
acknowledged. David Purkiss is thanked for carrying out some
of the NMR experiments.
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■
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