Quantitative Assessment of the Lewis Acidity of Silylium Ions

Article abstract of DOI:10.1021/acs.organomet.5b00556

The Lewis acidity of several aryl-substituted tetrylium ions was classified experimentally by applying the Gutmann-Beckett method and computationally by calculation of fluoride ion affinities (FIA) (tetrel elements = Si, Ge). According to these measures,

Full text of DOI:10.1021/acs.organomet.5b00556

Article
pubs.acs.org/Organometallics
Quantitative Assessment of the Lewis Acidity of Silylium Ions
Henning Großekappenberg, Matti Reißmann, Marc Schmidtmann, and Thomas Muller*
̈
Institute of Chemistry, Carl von Ossietzky Universitat Oldenburg, Carl von Ossietzky-Straße 9-11, D-26129 Oldenburg, Federal
̈
Republic of Germany
S
* Supporting Information
ABSTRACT: The Lewis acidity of several aryl-substituted
tetrylium ions was classified experimentally by applying the
Gutmann−Beckett method and computationally by calculation
of fluoride ion affinities (FIA) (tetrel elements = Si, Ge).
According to these measures, tetrylium ions are significantly
more Lewis acidic than boranes, and aryl-substituted silylium
borates are among the strongest isolable Lewis acids. A fine-
tuning of the Lewis acidity of silylium ions is possible by taking
advantage of electronic and/or steric substituent effects.
silyl cations was studied computationally using fluoride anion
affinities (FIA).19 Here we report that for a well-defined subset
of aryl-substituted silylium ions the Gutmann−Beckett method
allows an assessment of their Lewis acidity and that correlation
to Lewis acid scales based on theoretical derived negative ion
affinities exists.20−25
INTRODUCTION
■
Current interest in the synthesis and properties of strong Lewis
acids is very high. The recent development and success of the
concept of frustrated Lewis pairs (FLPs)1,2 and the application
of strong Lewis acids in bond activation reactions and catalysis
are important driving forces for the further development of new
main group based Lewis acids.3−9 Recently, we have found a
straightforward preparative access to a series of aryl-substituted
silylium and germylium ions, the congeners of tricoordinated
carbenium ions.10 The high reactivity of this class of cationic
species, previously the major obstacle for their isolation and
characterization, is now successfully applied in bond activation
chemistry, catalysis, and rearrangement reactions, to name only
a few examples.11,12 Clearly, the origin of this reactivity is the
Lewis acidity of tetrylium ions (tetrel elements: silicon and
germanium), and the notion of “strong Lewis acids” is
omnipresent in reports describing the chemistry of silylium
salts and related compounds, although a quantitative
classification was in no case satisfactorily provided. For neutral
silicon Lewis acids several methods are described to assess and
quantify experimentally their Lewis acidity in comparison with
other Lewis acids.13 In this respect, methods based on the
change in NMR chemical shifts of a probe Lewis base upon
coordination to a series of Lewis acids found widespread
application. For example, in the Gutmann−Beckett method the
31P NMR chemical shift of the probe base OPEt3 is
monitored,14,15 and in Child’s method the base is crotonalde-
RESULTS AND DISCUSSION
■
The cationic Lewis acids used in our study were prepared
according to reported literature procedures.10 Solutions of
triarylsilylium borates 1[B(C6F5)4]−5[B(C6F5)4] in benzene
were obtained by reaction of 1.5 equiv of the corresponding
diarylmethylsilane with 1 equiv of trityl tetrakispentafluoro-
phenyl borate, [Ph3C][B(C6F5)4] (Scheme 1, eq a). Benzene
solutions of diarylsilylium borate 6[B(C6F5)4], diarylgermylium
borate 7[B(C6F5)4], and triethylsilylarenium borate 8[B-
(C6F5)4] were synthesized by the standard hydride transfer
reaction (Scheme 1, eqs b,c).10,11 All attempts to adapt Childs
method for quantifying the Lewis acidity of the prepared
Scheme 1. Synthesis of Cationic Silicon- and Germanium-
Based Lewis Acids
16
1
hyde and the H NMR resonance of the γ-proton is probed.
2
Hilt and co-workers previously used the H NMR chemical
shift of the para deuterium in pyridine-d5 adducts of silyl
triflates to quantify their Lewis acidity.17 Recently, Hilt,
Oestreich, and co-workers probed the Lewis acidity of a family
of ferrocene-stabilized silicon cations by Lewis pair formation
with OPEt3 and pyridine-d5 by NMR spectroscopic methods
and found that the Lewis acidity of silicon cations cannot be
correlated with NMR resonances of the obtained adducts.18
Subsequently, the Lewis acidity of these ferrocenyl-substituted
Received: June 25, 2015
© XXXX American Chemical Society
A
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triarylsilylium ions failed due to extensive side reactions of the
crotonaldehyde with the applied silylium ions or follow-up
reactions of the silylium/aldehyde complex. In contrast, the
reactions of silylium and germylium ions with phosphane
oxides, R3PO, are much cleaner, and therefore we concentrated
our experimental investigation on the Gutmann−Beckett
method. The siloxyphosphonium borates [1(OPR3)]−
[6(OPR3)][B(C6F5)4] and germyloxyphosphonium borate
[7(OPR3)][B(C6F5)4] (R = Et, Ph) were obtained as glassy
amorphous solids by addition of a benzene solution of the
phosphane oxide to the solution of the corresponding borate at
room temperature (Scheme 2, eqs a,b). After washing with
identified in the 31P NMR spectra of the formed complexes by
detection of the corresponding satellites (see Figure 2).
Additional 1H and 13C NMR spectra further confirm the
quantitative formation of the phosphonium ions [1(OPR3)]+−
[7(OPR3)]+ and [9(OPR3)]+.
Scheme 2. Synthesis of Siloxy- and Germyloxyphosphonium
Borates from the Corresponding Silylium and Germylium
Salts
Figure 2. 202.35 MHz 31P{H} NMR spectrum (C6D6, 305 K) of
[1(OPEt3)][B(C6F5)4].
Structural evidence for the Lewis acid/base adduct formation
was obtained in the case of [1(OPPh3)]+. Crystals suitable for
X-ray diffraction analysis (XRD) were grown from a 1,2-
difluorobenzene solution of the aluminate [1(OPPh3)][Al-
(OC(CF3)3)4].28,29 The compound crystallizes in the trigonal
space group R3. The quality of the structure solution suffered
from significant structural disorder of the aluminate anion; it
allows however a short discussion of the main structural
features of the siloxyphosphonium cation. Noteworthy is the
linear arrangement of the Si−O−P linkage with a relatively long
Si−O bond (d(SiO) = 172.7 pm in [1(OPPh3)]+ vs 161.6 pm
in Ph3Si−O−SiPh3) (Figure 3). In addition, the P−O bond is
clearly elongated compared to that in triphenylphosphane
(d(PO) = 153.1 pm in [1(OPPh3)]+ vs 146 pm in Ph3PO), but
it is still markedly shorter than a standard P−O single bond
benzene to remove byproducts and excess phosphane oxide,
the residues were dissolved in benzene-d6 and investigated by
NMR spectroscopy to determine the 31P NMR chemical shift
difference (Δδ31P) between the free phosphane oxide and the
formed complex with the cationic Lewis acid. Silylium ions 3
and 4 undergo slow decomposition at ambient conditions with
the formation of protonated arenes. To obtain also for these
cases meaningful results, addition of 1 equiv of the proton
sponge 2,6-di-tert-butyl-4-methylpyridine prior to the phos-
phane oxide was necessary.26 For comparison the triethylsiloxy-
phosphonium borates [9(OPR3)][B(C6F5)4] and betaine
10(OPEt3)27 were synthesized and investigated by NMR
spectroscopy (Scheme 2, eqs c,d).
In the case of the silyl cationic Lewis acids, the formation of
the donor−acceptor complex is indicated in each case by a
significant high-field shift of the 29Si NMR resonance and by a
2
doublet splitting of the 29Si NMR signal due to the J(SiP)
coupling (see Figure 1 for an example). This coupling was also
Figure 3. Ellipsoid presentation of the molecular structure of
[1(OPPh3)]+ in the crystal (the aluminate anion is not shown for
clarity reasons): (a) view perpendicular to the SiOP vector; (b) view
along the SiOP vector (H atoms omitted for clarity; thermal ellipsoids
at 50% probability). Color code: gray, carbon; purple, silicon; red,
oxygen; blue, phosphorus. Pertinent bond lengths [pm] and bond and
dihedral angles [deg]: Si−Cipso: 188.89(28), Si−O: 172.73(33); P−O:
153.14(33); P−Cipso: 178.33(33); P−O−Si: 180.00; Cipso−Si−P−Cipso
13.596(136).
Figure 1. 99.32 MHz 29Si{H} NMR spectra (C6D6, 305 K): (a)
[1(OPEt3)][B(C6F5)4], (b) 1[B(C6F5)4].
B
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Table 1. Selected NMR Parameters of Silylium and Germylium/Phosphane Oxide Complexes and Δδ 31P Values Derived
a
Therefrom and Their Calculated FIA Values
f
Lewis acid/phosphanoxide
complex
δ31P
(R = Et)
Δδ31P
δ29Si (2J(SiP))
(R = Et) [Hz]
δ31P
(R = Ph)
Δδ31P
δ29Si (2J(SiP))
(R = Ph) [Hz]
FIA
(R = Et)
(R = Ph)
[kJ mol−1
]
b
OPR3
46.2
85.4
25.1
54.7
[(Me5C6)3SiOPR3]+
39.2
−5.1 (20.3)
29.6
−2.6 (21.7)
381
[1(OPR3)]+
[Dur3SiOPR3]+ [2(OPR3)]+
[Mes3SiOPR3]+ [3(OPR3)]+
[Xylyl3SiOPR3]+ [4(OPR3)]+
[Tipp3SiOPR3]+ [5(OPR3)]+
86.5
87.4
88.5
91.1
91.3
40.3
41.2
42.3
44.9
45.1
−5.1 (20.2)
−5.3 (20.1)
−6.3 (20.3)
6.8 (14.5)
55.4
56.5
57.2
56.3
30.3
31.4
32.1
31.2
−2.8 (21.7)
−3.9 (21.5)
−5.1 (21.3)
10.2 (16.0)
397
407
420
358
401
[Tipp2EtSiOPR3]+
11.1 (14.7)
[6(OPR3)]+
[Tipp2MeGeOPR3]+
86.6
40.4
11.1 (14.7)
35.4 (17.7)
352
[7(OPR3)]+
[Et3SiOPR3]+ [9(OPR3)]
88.6
42.4
52.7
27.6
38.5 (16.7)
c
c
d
d
(C6F5)3BOPR3 [10(OPR3)]
76.8
30.6
45.8
20.3
160
153
d
(C6F5O)3BOPR3
80.9
34.5
46.6
21.1
[14(OPR3)]
e
[CatBOPR3]+ [13(OPR3)]
106.9
60.7
35.9
40.4
770
g
[11(OPR3)]
h
[12(OPR3)]
91.1
331
a
b
For comparison literature data from boron- and phosphorus-based Lewis acids are provided. Slightly different 31P NMR chemical shifts for the
c
d
phosphane oxides in C6D6 are reported in ref 4: δ31P(OPEt3) = 46.4; δ31P(OPPh3) = 25.5. Ref 4 reports δ31P = 76.6 and Δδ31P = 30.2. Data from
ref 4. Data from ref 5a. Calculated at PCM/M05-2X/6-31G(d) (see the SI for details). Data from ref 33. Data from ref 8.
e
f
g
h
(d(PO) = 174 pm) (Figure 3).30,31 Not too surprising, from a
Scheme 3. Selected Main Group Lewis Acids and Their
structural point of view the cation [1(OPPh3)]+ takes an
intermediate position between the covalently bound tetracoor-
dinated silanes and tricoordinated silylium ions. For example,
the Si−Cipso bond lengths in [1(OPPh3)]+ are intermediate
between that found for silylium ion 1 and for silane 1(H)
(d(SiCipso) = 188.9 pm ([1(OPPh3)]+); 184.7 pm (1); 194.6
pm (1(H)).10 Similarly, the sum of the bond angles around the
silicon atom in [1(OPPh3)]+ is ∑α(Si) = 348.3°, which
deviates significantly from ∑α(Si) = 360.0° found for silylium
ion 1 and from ∑α(Si) = 333.2° detected for 1(H).10
Δδ31P Values Obtained with OPEt3 as a Reference Lewis
Base4,5,8,33
The obtained 31P NMR chemical shift differences, Δδ31P, are
the key quantities to determine the Gutmann−Beckett acceptor
numbers (AN) for Lewis acids. The transformation to a specific
AN is done applying a constant scaling factor (2.348), which is
used in the original paper to span a scale from hexane (AN = 0)
to SbCl5 (AN = 100).14 As the 31P NMR chemical shift of the
used phosphane oxides and their complexes with Lewis acids
are solvent dependent and our measurements were done in
benzene-d6 in contrast to most literature data, which refer to
values obtained in dichloromethane-d2, we refrain from giving
acceptor numbers but use in our analysis the NMR chemical
shift difference Δδ31P in benzene.32 The Lewis acidity of the
investigated silylium ions is characterized by Δδ31P values
between 39 and 45 when Et3PO is used as the reference base
(Table 1). Therefore, triarylsilylium and diarylalkylsilylium ions
are significant stronger Lewis acids than neutral boron
compounds such as B(C6F5)3 (Δδ31P = 30.6) and B(OC6F5)3
(Δδ31P = 34.5)4 or than the neutral silicon Lewis acid
Si(CatF)2, 11 (Δδ31P = 35.9), recently reported by Liberman-
Martin et al. (Scheme 3).33 Their Lewis acidity is in the same
range as that determined for electrophilic phosphorus(V)
cations such as [(F5C6)3PF]+, 12 (Δδ31P = 40.4).8 Silylium ions
are outperformed with respect to Lewis acidity only by cationic
borenium compounds such as 13. Ingleson and co-workers
reported that cation 13 is formed as a transient species and is
isolated in the form of its complex [13(OPEt3)], which is
characterized by a 31P NMR chemical shift of δ31P = 106.9 and
the corresponding Δδ31P value of 61.5 The Lewis acidity of
triarylsilylium ions varies only little with the substitution at the
aryl group. The data summarized in Table 1 indicate however a
clear and significant trend, despite the small absolute NMR
chemical shift differences. The Lewis acidity of triarylsilylium
ions is a function of the π/3p(Si) conjugation between the aryl
substituents and the central silicon atom. In a simple FMO
picture this interaction raises the LUMO energy and weakens
the Lewis acidity of the cation. This is verified in the series of
methyl-substituted triarylsilylium ions, in which the Lewis
acidity increases slightly with decreasing number of methyl
groups at the aryl substituent. To understand the increased
Lewis acidity of the tri-iso-propylphenyl (Tipp)-substituted
silylium ion 5, two opposing effects have to be considered. (i)
Larger alkyl groups in the ortho-position of the arylsubstituents
disfavor π/3p(Si) conjugation; consequently the Tipp sub-
stituent increases the Lewis acidity of cation 5 compared to that
of the mesityl-substituted cation 3. (ii) The large substituents in
C
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the ortho-position disfavor the formation of the tetracoordi-
nated Lewis acid/base complex for steric reasons. Therefore, a
decreased Lewis acidity for cation 5 might be expected.34 The
experimental data (Table 1) clearly show that in contrast to the
situation for isoelectronic boron compounds the steric
argument (ii) is of only minor importance and that the Lewis
acidity of triarylsilylium ions is mainly determined by electronic
conjugation effects. In line with these arguments also the
replacement of one aryl substituent by an alkyl group (cation 5
versus cation 6) slightly increases the Lewis acidity of silylium
ions. Only one germylium ion, cation 7, was tested, and the
comparison with the related silylium ion 6 suggests that
germylium ions are weaker Lewis acids compared to silylium
ions (Table 1). The experimental data obtained for the OPPh3
complexes provide a general picture that is completely in
accordance with the conclusions drawn from the results when
OPEt3 is used as a reference Lewis base (Table 1).
Lewis acid scales based on calculated anion affinities, such as
fluoride ion affinities (FIA) and hydride anion affinities (HIA),
have the advantage of being applicable to a wide range of
possible Lewis acids and are therefore very popular. Recently
several computational approaches toward a systematic evalua-
tion of the Lewis acidity of chemically very diverse Lewis acids
have been published.19−25 In the context of our study the
recent computational study by Muether et al. on FIAs of
internally stabilized silyl cations is of interest.19 This study
revealed a significantly higher Lewis acidity for substituted silyl
cations than for equally substituted carbocations. We used in
our work the quasi-isodesmic reactions shown in Scheme 4 to
Figure 4. Plot of the fluoride ion affinities of Lewis acids calculated
according to eq 4a vs Δδ31P determined using OPEt3 as a reference
Lewis base (red circles). Plot of the LUMO energies of Lewis acids vs
Δδ31P (blue circles).
determined 31P NMR chemical shift differences for both cations
indicate a higher Lewis acidity than the theoretical methods
suggest. In the case of these Tipp-substituted silylium ions
NMR spectroscopic evidence that was supported by the results
of quantum mechanical calculations indicated an interaction
between the ortho-iso-propyl methine groups and the central
silicon atom (Figure 5).10b This C−H···Si+ interaction lowers
Scheme 4. Reactions Used to Determine Fluoride and
Hydride Affinity of Main Group Lewis Acids (LA)
Figure 5. Sketch of the C−H···Si+ interaction that is operative in
triisopropylphenyl-substituted cations such as silylium ion 5.10b
assess the Lewis acidity also computationally.35−37 In principle,
these two equations provide the fluoride ion (4a) and the
hydride ion affinity (4b) of the investigated Lewis acids and use
triethyl borane as an anchor point for both scales. Both affinity
scales are used frequently for the thermodynamic quantification
of Lewis acidity, and their simultaneous use allows evaluating
the influence of the hardness/softness of the applied Lewis base
on the relative Lewis acidity of the investigated species.24,25 As
both calculated anion affinities gave for silylium ions nearly
identical results, we will limit our discussion on the fluoride ion
affinities. All relevant data for both anion affinities and
additional plots are given in the SI. An independent measure
of Lewis acidity that is accessible by quantum mechanical
calculations is the energy level of the LUMO.24a Both
theoretically assessed criteria, the calculated FIA and the
LUMO energy level, show for the series of investigated boron-,
germanium-, and silicon-based Lewis acids a rough correlation
with the determined Δδ31P values using Et3PO as a reference
base. That is, the FIA increases with increasing Δδ31P value and
the LUMO energy decreases with increasing Δδ31P value (see
Figure 4). Particularly good is the correlation between Δδ31P
values and FIA/LUMO energies in the series of methyl-
substituted triarylsilylium ions. In both cases almost linear
correlations exist between the FIA/LUMO energies computed
for the silylium ions 1−4 and the corresponding Δδ31P
values.38 More interesting is the clear deviation from these
correlations for the Tipp-substituted silylium ions 5 and 6. The
the Lewis acidity of the Tipp-substituted silylium ions 5 and 6
compared to that of methyl-substituted triarylsilylium ions such
as trimesitylsilylium 3. The calculated FIA clearly accounts for
this decreased Lewis acidity in cations 5 and 6 since the
calculated structure and energy of the cations directly influence
the result of reaction 4a. On the other hand, the basis for the
Gutmann−Beckett method is the interaction of the phosphine
oxide with the silylium ion in the investigated complex, and this
strong interaction cancels the more subtle C−H···Si+
interaction in the free cation. Consequently, the Gutmann−-
Beckett method is not well suited for determining the Lewis
acidity for either intra- or intermolecular stabilized Lewis acids
such as the Tipp-substituted silylium ions 5 and 6.39 Similar
problems of the Gutmann−Beckett method were recently
encountered for intramolecular ferrocenyl-stabilized silyl
cations18 and have to be expected for every Lewis acid
stabilized by donors.
CONCLUSION
■
The Gutmann−Beckett method was applied for the determi-
nation of the Lewis acidity of aryl-substituted silylium and
germylium ions. The Δδ31P values determined for these
tricoordinated cationic Lewis acids indicate a significantly
higher Lewis acidity than for neutral boranes. In the case of
D
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Preparation of Silyloxyphosphonium Aluminate
[(Me5C6)3SiOPPh3][Al(OC(CF3)3)4]. A Schlenk tube was oven-dried,
charged with 252 mg (0.74 mmol) of (Me5C6)2SiMeH, and evacuated
for at least 1 h. The flask was flushed with dry argon, and 500 mg of
[Ph3C][Al(OC(CF3)3)4] (0.41 mmol) was added. The mixture was
dissolved in 3 mL of dry benzene. The resulting biphasic mixture was
stirred for 30 min. Then, the phases were allowed to separate. The
polar phase was washed twice with portions of 2 mL of dry benzene.
The solution of the obtained silylium aluminate [(Me5C6)3Si][Al-
(OC(CF3)3)4] was treated with a solution of 114 mg (0.41 mmol) of
Ph3PO in 2 mL of dry C6H6 and stirred for 10 min. The dark-colored
polar phase lightened up quickly. The polar phase was then washed
twice with portions of 2 mL of dry benzene and washed once with 2
mL of CH2Cl2. The solvent was removed under vacuum, replaced by 1
mL of dry 1,2-difluorobenzene, and stored overnight at −20 °C to
obtain colorless crystals suitable for XRD analysis.
triarylsilylium ions, which show no indication of intra- or
intermolecular interaction, the Δδ31P values correlate with
theoretically obtained measures of Lewis acidity such as anion
affinities and LUMO energy level. This correlation is mainly
dominated by the electronic factors that determine the
energetic position of the LUMO. In contrast with this finding,
deviations from this correlation indicate inter- or intramolecular
interactions that influence the Lewis acidity of silylium ions.
This was shown previously of ferrocenyl-substituted silyl
cations18 and is extended here to even very subtle interactions,
for example, C−H···Si+ agostic interaction. Finally, from a
practical point of view, it is worth mentioning that the
investigated arylsilylium salts are among the strongest isolable
Lewis acids and that a fine-tuning of the Lewis acidity is
possible by taking advantage of electronic and/or steric
substituent effects.
Characterization of Tetryloxyphosphonium Borates [1−7,
9(OPEt3)][B(C6F5)4]. (Signals of anion omitted for clarity.) [1-
(OPEt3)]+: 1H NMR (499.87 MHz, 305.0 K, C6D6): δ 0.44 (dt,
2
9H, 3JP,H = 19.8 Hz, 3JH,H = 7.7 Hz, PCH2CH3), 1.38 (dq, 6H, JP,H
=
EXPERIMENTAL SECTION
3
■
12.1 Hz, JH,H = 7.7 Hz, PCH2CH3), 2.05−2.13 (m, 45H, CH3).
13C{1H} NMR (125.71 MHz, 305.0 K, C6D6): δ 4.7 (d, 2JC,P = 5.0 Hz,
PCH2CH3), 16.1 (s, m-CH3), 16.8 (s, p-CH3), 18.4 (d, 1JC,P = 64.3 Hz,
PCH2CH3), 23.1 (s, o-CH3), 134.3 (b, 2 × Cq), 135.2 (Cq), 138.9
(Cq). 31P{1H} NMR (202.35 MHz, 305.2 K, C6D6): δ 85.4 (s).
General Procedures. Benzene, [D6]benzene, [D8]toluene, and n-
hexane were distilled from sodium. The [Ph3C][B(C6F5)4] and
[Ph3C][Al(OC(CF3)3)4] salts were synthesized as described pre-
viously.40,41
All reactions were carried out under inert conditions using standard
Schlenk and glovebox techniques, unless stated otherwise. Argon
99.999% was used as inert gas and was dried over Si-capent prior to
use. NMR spectra were recorded on Bruker Avance 500 and Avance
29Si{1H} NMR (99.32 MHz, 305.0 K, C6D6): δ −5.1 (d, JSi,P = 20.3
2
Hz).
[2(OPEt3)]+: H NMR (499.87 MHz, 305.1 K, C6D6): δ 0.44 (dt,
1
2
9H, 3JP,H = 19.7 Hz, 3JH,H = 7.7 Hz, PCH2CH3), 1.38 (dq, 6H, JP,H
=
1
III 500 spectrometers at 305 K, if not stated otherwise. H NMR
3
12.0 Hz, JH,H = 7.7 Hz, PCH2CH3), 1.95 (s, 18H, m-CH3), 2.04 (s,
spectra were calibrated using residual protio signals of the solvent
(δ1H(CHCl3) = 7.24, δ1H(C6D5H) = 7.20, δ1H(C6D5(CD2H)) =
2.08). 13C NMR spectra were calibrated using the solvent signals
(δ13C(CDCl3) = 77.0, δ13C(C6D6) = 128.0, δ13C(C7D8) = 20.4
(CD3)). 29Si NMR spectra were calibrated using external Me2HSiCl
(δ29Si = 11.1 vs TMS), 31P NMR spectra against external (MeO)3P
(δ31P = 141 vs 85% H3PO4(aq)), 19F NMR spectra against external
CFCl3 (δ19F(CFCl3) = 0.0), and 11B NMR spectra against BF3 OEt2
(δ11B(BF3 OEt2) = 0.0). X-ray diffraction analyses were performed on
a Bruker Apex II. Structure solution and refinement was done using
the SHELXS97 and SHELXL97 software.42
General Procedure for the Preparation of Tetrylium Borates
[1−7][B(C6F5)4]. A Schlenk tube was oven-dried and then charged
with 400 mg of [Ph3C][B(C6F5)4] (0.43 mmol) and the
corresponding diarylmethylsilane or -germane (0.69 mmol). The
mixture was evacuated for at least 1 h. After that period the flask was
flushed with dry argon and the mixture was dissolved in 4 mL of dry
benzene. The resulting biphasic mixture (an upper, unpolar phase and
a lower, polar phase; typical for salts of [B(C6F5)4]− in aromatic
hydrocarbons) was stirred for 1 h. Then, the phases were allowed to
separate. The polar phase was washed twice with 4 mL of dry benzene.
The solutions of the obtained silylium, 1−6, or germylium, 7, borates
were used in further experiments as obtained.
General Procedure for the Preparation of Tetryloxyphos-
phonium Borates [1−7, 9(OPEt3)][B(C6F5)4]. A 0.53 mL portion of
a Et3PO solution (0.81 M in C6D6) was transferred to the solution of
the corresponding tetrylium borate in C6H6 via a PTFE cannula. The
dark-colored polar phase lightened up appreciably upon stirring the
mixture for 1 h. The polar phase was then washed two times with
portions of 4 mL of dry benzene. The solvent was removed under
vacuum and replaced by 1 mL of dry C6D6.
General Procedure for the Preparation of Tetryloxyphos-
phonium Borates [1−5, 9(OPPh3)][B(C6F5)4]. A Schlenk tube was
oven-dried and then charged with 123 mg (0.44 mmol) of Ph3PO and
evacuated for 1 h. A 2 mL amount of C6H6 was transferred via a PTFE
cannula to Ph3PO. The colorless solution of Ph3PO in C6H6 was
transferred to the corresponding tetrylium borate in C6H6 via a PTFE
cannula. The dark-colored polar phase lightened up appreciably upon
stirring the mixture for 1 h. The polar phase was then washed two
times with portions of 4 mL of dry benzene. The solvent was removed
under vacuum and replaced by 1 mL of dry C6D6.
18H, o-CH3), 6.96 (s, 3H, p-CH3). 13C{1H} NMR (125.71 MHz,
2
1
305.0 K, C6D6): δ 4.7 (d, JC,P = 4.9 Hz, PCH2CH3), 18.4 (d, JC,P
=
64.0 Hz, PCH2CH3), 19.8 (s, o-CH3), 21.6 (s, m-CH3), 135.2 (CH),
135.7 (2 × Cq), 136.8 (Cq). 31P{1H} NMR (202.35 MHz, 305.3 K,
C6D6): δ 86.5 (s). 29Si{1H} NMR (99.32 MHz, 305.0 K, C6D6): δ
2
−5.1 (d, JSi,P = 20.2 Hz).
1
[3(OPEt3)]+: H NMR (499.87 MHz, 305.0 K, C6D6): δ 0.45 (dt,
3
3
2
9H, JP,H = 19.8 Hz, JH,H = 7.7 Hz PCH2CH3), 1.42 (dq, 6H, JP,H
=
3
11.9 Hz, JH,H = 7.7 Hz, PCH2CH3), 1.98 (s, 18H, o-CH3), 2.16 (s, 9
H, p-CH3), 6.74 (s, 6H, m-CH). 13C{1H} NMR (125.71 MHz, 305.0
K, C6D6): δ 5.1 (d, 2JC,P = 5.3 Hz, PCH2CH3), 18.4 (d, 1JC,P = 63.2 Hz,
PCH2CH3), 20.8 (s, p-CH3), 24.2 (s, o-CH3), 130.6 (CH), 130.9
(Cq), 142.2 (Cq), 144.1 (Cq). 31P{1H} NMR (202.35 MHz, 305.2 K,
C6D6): δ 87.4 (s). 29Si{1H} NMR (99.32 MHz, 305.0 K, C6D6): δ
2
−5.3 (d, JSi,P = 20.1 Hz).
[4(OPEt3)]+: H NMR (499.87 MHz, 305.0 K, C6D6): δ 0.42 (dt,
1
9H, 3JP,H = 19.8 Hz, 3JH,H = 7.7 Hz, PCH2CH3), 1.38 (dq, 6H, JP,H
=
2
11.8 Hz, 3JH,H = 7.7 Hz, PCH2CH3), 1.94 (s, 18H, o-CH3), 6.71−6.75
(m, 6H), 6.80−6.86 (m, 6H), 7.24−7.26 (m, 3H). 13C{1H} NMR
2
(125.71 MHz, 305.0 K, C6D6): δ 5.0 (d, JC,P = 4.7 Hz, PCH2CH3),
1
18.4 (d, JC,P = 63.5 Hz, PCH2CH3), 24.3 (s, o-CH3), 129.6 (CH),
134.0 (Cq), 142.1 (CH), 144.0 (Cq). 31P{1H} NMR (202.35 MHz,
305.2 K, C6D6): δ 88.5 (s). 29Si{1H} NMR (99.32 MHz, 305.0 K,
2
C6D6): δ −6.3 (d, JSi,P = 20.3 Hz).
[5(OPEt3)]+: H NMR (499.87 MHz, 297 K, C6D6): δ 0.57 (dt,
1
9H, 3JP,H = 19.7 Hz, 3JH,H = 7.7 Hz, PCH2CH3), 1.19 (d, 14H, 3JH,H
=
6.9 Hz, o-CH(CH3)2), 1.27 (d, 7H, 3JH,H = 7.0 Hz, p-CH(CH3)2), 1.43
2
3
(dq, 6H, JP,H = 11.9 Hz, JH,H = 7.0 Hz, PCH2CH3), 2.74 (sep, 6H,
3JH,H = 6.9 Hz, o-CH(CH3)2), 2.84 (sep, 3H, JH,H = 6.9 Hz, p-
3
CH(CH3)2), 7.00 (s, 2H, m-CH). 13C{1H} NMR (125.71 MHz, 297.0
K, C6D6): δ 4.38 (d, 2JC,P = 5.4 Hz, PCH2CH3), 17.53 (d, 1JC,P = 62.2
Hz, PCH2CH3), 23.6 (p-CH(CH3)2), 24.4 (o-CH(CH3)2), 34.5 (p-
CH(CH3)2), 34.8 (o-CH(CH3)2), 122.4 (CH), 149.1 (Cq), 153.6
(broad, 2 × Cq). 31P{1H} NMR (202.35 MHz, 297.1 K, C6D6): δ 91.1
(s). 29Si{1H} NMR (99.32 MHz, 296.9 K, C6D6): δ 6.8 (d, 2JSi,P = 14.5
Hz).
1
[6(OPEt3)]+: H NMR (499.87 MHz, 305.1 K, C6D6): δ 0.58 (dt,
9H, 3JP,H = 19.9 Hz, 3JH,H = 7.7 Hz, PCH2CH3), 0.8 (t, 3H, 3JH,H = 7.7
3
Hz, SiCH2CH3), 0.91−0.97 (m, 2H), 1.00 (d, 3H, JH,H = 6.5 Hz),
1.03 (d, 3H, 3JH,H = 6.3 Hz), 1.16−1.22 (m, 18H), 1.24 (d, 6H, 3JH,H
=
E
DOI: 10.1021/acs.organomet.5b00556
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
3
1
6.8 Hz), 1.29 (d, 4H, JH,H = 6.8 Hz), 1.33−1.40 (m, 2H), 1.47 (dq,
[5(OPPh3)]+: H NMR (499.87 MHz, 295.1 K, C6D6): δ 0.86−
0.91 (m, 36H), 1.22 (d, 9H, 3JH,H = 7 Hz, p-CH(CH3)2), 1.23 (d, 9H,
2
3
6H, JP,H = 12.3 Hz, JH,H = 7.7 Hz, PCH2CH3), 2.54−2.59 (m, 1H),
2.71−2.79 (m, 3H), 3.02−3.08 (m, 1H), 3.24−3.30 (m, 1H), 6.88 (s,
1H), 6.94 (s, 1H), 7.02−7.06 (m, 1H), 7.34−7.37 (m, 1H). 13C{1H}
3JH,H = 7 Hz, p-CH(CH3)2), 2.77 (sep, 3H, 6H, JH,H = 6.9 Hz, o-
3
CH(CH3)2), 3.10−3.24 (m, 6H, o-CH(CH3)2, 7.06−7.10 (m, 12H),
7.22−7.28 (m). 13C{1H} NMR (125.71 MHz, 297.8 K, C6D6): δ 23.8
(p-CH(CH3)2), 24.2 (o-CH(CH3)2), 33.1 (p-CH(CH3)2), 34.5 (o-
CH(CH3)2), 121.3 (d, 1JC,P = 111.2 Hz, i-CP), 122.9 (CH), 130.1 (d,
2
NMR (125.71 MHz, 305.0 K, C6D6): δ 4.5 (d, JC,P = 4.3 Hz,
PCH2CH3), 8.6 (SiCH2CH3), 15.9 (SiCH2CH3), 17.7 (d, 1JC,P = 62.5
Hz, PCH2CH3), 22.0, 22.9, 23.7, 23.9, 24.7, 25.7, 31.1, 33.2, 35.1,
122.5 (CH), 123.4 (CH), 152.5 (Cq), 153.4 (Cq), 153.7 (Cq), 153.8
(Cq), 154.1 (Cq), 155.5 (Cq). 31P NMR (202.35 MHz, 305.2 K,
C6D6): δ 91.3 (s). 29Si NMR (99.32 MHz, 305.0 K, C6D6): δ 11.1 (d,
2JSi,P = 14.7 Hz).
2JC,P = 13.8 Hz, o-CH), 133.3 (d, JC,P = 12.2 Hz, m-CH), 136.1 (d,
3
2JC,P = 2.3 Hz, p-CH), 153.0 (Cq), 154.0 (Cq). 31P NMR (202.35
MHz, 305.3 K, C6D6): δ 56.3 (s). 29Si NMR (99.32 MHz, 305.0 K,
2
C6D6): δ 10.2 (d, JSi,P = 16.0 Hz).
[7(OPEt3)]+: H NMR (499.87 MHz, 305.1 K, C6D6): δ 0.58 (dt,
[9(OPPh3)]+: H NMR (499.87 MHz, 298.1 K, C6D6): δ 0.36 (d,
1
1
3
3
9H, 3JP,H = 19.2 Hz, 3JH,H = 7.6 Hz, PCH2CH3), 1.05 (s, 3H, GeCH3),
6H, JH,H = 8.1 Hz, SiCH2CH3), 0.65 (t, 9H, JH,H = 8.1 Hz
SiCH2CH3), 7.11−7.18 (m, 12H), 7.30−7.33 (m, 3H). 13C{1H} NMR
(125.71 MHz, 298.1 K, C6D6): δ 5.4 (SiCH2CH3), 5.7 (SiCH2CH3),
122.0 (d, JC,P = 111.1 Hz, i-CP), 130.2 (d, JC,P = 13.9 Hz, o-CH),
132.3 (d, 3JC,P = 12.5 Hz, m-CH). 31P{1H} NMR (202.35 MHz, 297.9
K, C6D6): δ 52.7 (s). 29Si{1H} NMR (99.32 MHz, 305.0 K, C6D6): δ
2
1.17−1.20 (m, 22H), 1.25−1.27 (m, 10H), 1.34 (dq, 6H, JP,H = 11.9
3
Hz, JH,H = 7.6 Hz, PCH2CH3), 2.72−2.87 (m, 6H, o-CH(CH3)2, p-
1
2
CH(CH3)2), 7.00 (s, 1H), 7.05 (s, 5H). 13C{1H} NMR (125.71 MHz,
2
305.0 K, C6D6): δ 4.5 (d, JC,P = 5.4 Hz, PCH2CH3), 12.6 (s, CH3),
18.2 (d, JC,P = 63.4 Hz, PCH2CH3), 23.6 (CH3), 24.3 (CH3), 34.5
1
2
38.5 (d, JSi,P = 16.7 Hz).
(CH3), 34.9 (CH3), 38.1 (CH3), 122.4 (CH3), 123.3 (CH3), 132.1
(Cq), 147.63 (Cq), 149.1 (Cq), 152.9 (Cq), 153.3 (Cq). 31P{1H}
NMR (202.35 MHz, 305.2 K, C6D6): δ 86.6 (s).
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.5b00556.
■
[9(OPEt3)]+: H NMR (499.87 MHz, 305.1 K, C6D6): δ 0.32 (q,
1
S
6H, 3JH,H = 7.9 Hz, SiCH2CH3), 0.54 (dt, 9H, 3JP,H = 19.6 Hz, 3JH,H
=
7.8 Hz, PCH2CH3), 0.65−0.72 (m, 27H, 2 × *PCH2CH3,
SiCH2CH3), 1.05−1.16 (m, 18H, 2 × *PCH2CH3, PCH2CH3);
*signals from the starting material Et3PO. 13C{1H} NMR (125.71
2
MHz, 305.0 K, C6D6): δ 4.1 (d, JC,P = 5.05 Hz, PCH2CH3), 5.3
Experimental, computational, and analytical details and
data (PDF)
Crystallographic data (CIF)
1
(SiCH2CH3), 5.7 (SiCH2CH3), 17.0 (d, JC,P = 63.5 Hz, PCH2CH3).
31P{1H} NMR (202.35 MHz, 305.2 K, C6D6): δ 88.6 (s). 29Si{1H}
2
NMR (99.32 MHz, 305.0 K, C6D6): δ 35.4 (d, JSi,P = 17.7 Hz).
Chemical structure (XYZ)
Characterization of Tetryloxyphosphonium Borates [1−5,
9(OPPh3)][B(C6F5)4]. (Signals of anion omitted for clarity.) [1-
(OPPh3)]+: 1H NMR (499.87 MHz, 305.0 K, C6D6): δ 1.68 (s, 18H),
2.04 (s, 9H), 2.20 (s, 9H), 2.30 (s, 9H), 6.88−6.93 (m, 6H), 6.95−
6.99 (m, 6H), 7.28−7.31 (m, 3H). 13C{1H} NMR (125.71 MHz,
305.0 K, C6D6): δ 16.0 (CH3), 16.3 (CH3), 16.8 (CH3), 23.0 (CH3),
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: thomas.mueller@uni-oldenburg.de.
1
2
23.5 (CH3), 123.0 (d, JC,P = 113.1 Hz, i-CP), 129.6 (d, JC,P = 14.0
Hz, o-CH), 133.0 (d, 3JC,P = 12.2 Hz, m-CH), 134.5 (Cq), 135.1 (Cq),
135.5 (d, 4JC,P = 2.6 Hz, p-CH), 138.6 (Cq), 138.7 (Cq), 140.5 (Cq).
31P{1H} NMR (202.35 MHz, 305.2 K, C6D6): δ 54.7 (s). 29Si{1H}
Notes
The authors declare no competing financial interest.
2
ACKNOWLEDGMENTS
NMR (99.32 MHz, 305.0 K, C6D6): δ −2.6 (d, JSi,P = 21.7 Hz).
■
[2(OPPh3)]+: H NMR (499.87 MHz, 305.1 K, C6D6): δ 1.61 (s,
1
This study was supported by the CvO University Oldenburg
and by the DFG (Mu-1440/7-1). The High End Computing
Resource Oldenburg (HERO) at the CvO University is
thanked for computer time.
9H), 1.72 (s, 9H), 2.07 (s, 9H), 2.14 (s, 9H), 6.87−6.92 (m, 6H),
6.95−6.98 (m, 6H), 7.26−7.29 (m, 3H). 13C{1H} NMR (125.71
MHz, 305.0 K, C6D6): δ 19.6 (CH3), 20.1 (CH3), 21.7 (CH3), 21.8
(CH3), 122.8 (d, 1JC,P = 112.7 Hz, i-CP), 129.7 (d, 2JC,P = 14.1 Hz, o-
CH), 132.3 (d, JC,P = 12.0 Hz, m-CH), 133.2 (d, JC,P = 12.0 Hz, m-
CH), 135.0 (CH), 135.3 (Cq), 135.6 (d, 4JC,P = 2.6 Hz, p-CH), 136.7
(Cq), 139.5 (Cq), 141.1 (Cq). 31P{1H} NMR (202.35 MHz, 305.3 K,
C6D6): δ 55.4 (s). 29Si{1H} NMR (99.32 MHz, 305.0 K, C6D6): δ
3
3
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■
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1
9H), 2.06 (s, 9H), 6.71−6.86 (m, 9H). 13C{1H} NMR (125.71 MHz,
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2
i-CP), 129.2 (CH), 129.8 (d, JC,P = 14.3 Hz, o-CH), 131.8 (CH),
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3
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134.1 (Cq), 135.9 (d, JC,P = 2.0 Hz, p-CH). 31P{1H} NMR (202.35
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2
K, C6D6): δ −5.1 (d, JSi,P = 21.3 Hz).
F
DOI: 10.1021/acs.organomet.5b00556
Organometallics XXXX, XXX, XXX−XXX

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(27) Morgan, M. M.; Marwitz, A. J. V.; Piers, W. E.; Parvez, M.
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(29) Recently, the solid-state structure of a related siloxyphospho-
́
nium ion was reported; see: Stahl, T.; Hrobarik, P.; Konigs, C. D. F.;
̈
Ohki, Y.; Tatsumi, K.; Kemper, S.; Kaupp, M.; Klare, H. F. T.;
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(32) For example, for B(C6F5) and OPEt3 as a reference base the
following 31P NMR chemical shifts are reported: δ31P = 77.8 (CD2Cl2,
ref 24) and δ31P = 76.6 (C6D6, ref 4).
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G
DOI: 10.1021/acs.organomet.5b00556
Organometallics XXXX, XXX, XXX−XXX