Synthesis and Characterization of Silylated Phosphonium [P(OSiMe3)4]+ and Phosphate [O2P(OSiMe3)2]- Salts

Article abstract of DOI:10.1021/acs.inorgchem.8b01323

Starting from an optimized synthesis of silylated phosphoric acid, OP(OSiMe₃)₃, a borate salt bearing the [P(OSiMe₃)₄]⁺ cation was generated in the reaction of OP(OSiMe₃)₃ with [Me₃Si-H-SiMe₃][B(C₆F₅)₄], isolated, and fully characterized. Analogously to the protonated species, phosphoric acid (H₃PO₄) reaction of OP(OSiMe₃)₃ with a base led to the formation of the unknown [O₂P(OSiMe₃)₂]^- anion, which could be crystallized as potassium salt and structurally characterized, too. Both [P(OSiMe₃)₄]⁺ and [O₂P(OSiMe₃)₂]^- can be regarded as the formal autoprotolysis products of OP(OSiMe₃)₃.

Full text of DOI:10.1021/acs.inorgchem.8b01323

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Synthesis and Characterization of Silylated Phosphonium
[P(OSiMe3)4]+ and Phosphate [O2P(OSiMe3)2]− Salts
†
‡
,‡,∥
and Alexander Villinger‡
́
Paul Felgenhauer, Rene Labbow, Axel Schulz,*
†Institut fu
̈
̈
r Chemie, Carl von Ossietzky Universitat Oldenburg, Carl-von-Ossietzky-Straße 9-11, 26129 Oldenburg, Germany
̈
‡Institut fu
r Chemie, Universitat Rostock, Albert-Einstein-Straße 3a, 18059 Rostock, Germany
̈
∥Leibniz-Institut fu
̈
r Katalyse an der Universitat Rostock, Albert-Einstein-Straße 29a, 18059 Rostock, Germany
̈
S
* Supporting Information
ABSTRACT: Starting from an optimized synthesis of silylated phosphoric acid,
OP(OSiMe3)3, a borate salt bearing the [P(OSiMe3)4]+ cation was generated in the
reaction of OP(OSiMe3)3 with [Me3Si−H−SiMe3][B(C6F5)4], isolated, and fully
characterized. Analogously to the protonated species, phosphoric acid (H3PO4)
reaction of OP(OSiMe3)3 with a base led to the formation of the unknown
[O2P(OSiMe3)2]− anion, which could be crystallized as potassium salt and
structurally characterized, too. Both [P(OSiMe3)4]+ and [O2P(OSiMe3)2]− can be
regarded as the formal autoprotolysis products of OP(OSiMe3)3.
and Lehmann et al. isolated [Me3Si][CHB11F11] featuring
INTRODUCTION
■
short cation−anion contacts.12 To get access to a truly
tricoordinate free silylium ion without stabilization, bulkier
substituents around the silicon center were needed as it is the
At the first glance, the chemistry of the trimethylsilylium ion
[Me3Si]+ seems to be quite different to that of a proton,
although [Me3Si]+ is often referred to as a big proton.1−6 For
example, the physical properties (e.g., melting (mp) or boiling
point (bp)) of the halides or pseudohalides are quite different
(Table S6). This term is more related to the fact that both
cations are very strong electrophiles, reacting with almost any
donor molecule (e.g., HX, X = halogen, pseudohalogen, or
even arenes)7 to give cations (Scheme 1), which led to a long
quest for salts bearing the solvent-free cations.8−10 By using
weakly coordinating anions (wca),7 Reed et al. succeeded in
preparing a solvent-free H[CHB11Cl11] salt,11 while Willner
14
case in [Mes3Si][CHB11Me5Br6]13 and [Pemp3Si]2[B12Cl12]2
(Mes = 2,4,6-trimethylphenyl, Pemp = pentamethylphenyl). As
for the proton, also for [Me3Si]+ homoleptic (pseudo)halogen,
(pseudo)chalcogen or pnictogen species are known as well as
their silylated cations (Scheme 1), which feature small
trimethylsilyl affinities (TMSA)2,3,15−17 similar to the small
proton affinities (PA)18 known for the analogous protonated
species. Hence, all these silylated cations represent highly
labile, strong [Me3Si]+ transfer reagents, which are only stable
in the presence of chemically robust and inert wca’s. Following
our interest in [Me3Si]+ chemistry, we studied the similarities
between ortho-phosphoric acid, OP(OH)3, and its silylated
congener OP(OSiMe3)3 (1). Especially, we were intrigued by
the idea to synthesize the formal autoprotolysis products of
2OP(OSiMe3)3 → [P(OSiMe3)4]+ + [O2P(OSiMe3)2]−. While
the protonated species, [P(OH)4]+, has been known since
1999, when Minkwitz et al. used super acidic systems such as
HF/MF5 (M = As, Sb) to protonate OP(OH)3, the existence
of a tetrakis(trimethylsilyl)phosphonium ion,19 [P-
(OSiMe3)4]+, was discussed in 1974 in the equilibrium of
the reaction OP(OSiMe3)3 with Me3Si-X (X = halogen, NO3,
ClO4, and SCN)20 but a full characterization (1H, 31P shifts,
Scheme 1. Known Homoleptic Multiply Substituted
Trimethylsilyl Substituted Cations2−5,12,15,21,22
a
a
[(Me3Si)3O]+ and [(Me3Si)3S]+ were only observed by NMR
spectroscopic experiments.23,24
Received: May 15, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b01323
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 2. Synthesis of Silylated Phosphorous Species 1, 2+, and 3−
−167.65 (m, m-CF, 1J(19F−13C) = 246 Hz), −163.84 (t, p-CF,
selected IR bands, conductivity in CH2Cl2, EA for P and I are
known) including structural data remained to be carried out.
1
1J(19F−13C) = 245 Hz), −133.08 (s, o-CF, J(19F−13C) = 241 Hz).
29Si INEPT NMR (25 °C, CD2Cl2, 59.63 MHz): δ = 35.61 (ddec,
2
2
POSiCH3, J(29Si−1H) = 6.9 Hz, J(29Si−31P) = 1.6 Hz). 31P{1H}
EXPERIMENTAL SECTION
■
NMR (25 °C, CD2Cl2, 121.49 MHz): δ = −35.92 (s). IR (ATR, 8
Synthesis of (Me3SiO)3PO 1. In a 250 mL three-neck flask with
dropping funnel and pressure equalized reflux condenser, 9 g (66.1
mmol) of potassium dihydrogen phosphate KH2PO4 was suspended
in 100 mL n-hexane. Trimethylsilyl chloride Me3SiCl (43 g, 396
mmol) was added within 30 min. The reaction mixture was refluxed
for 9 h. After checking the reaction mixture by 31P NMR
spectroscopy, the reaction mixture was filtered through a G4 frit,
and the solvent was thermally distilled off. The residue was condensed
into another flask in vacuo at 70 °C. This procedure yields in 19.9 g
(315 mmol, 96%) of a colorless liquid (Me3SiO)3PO.
scans, 25 °C, cm−1)*: 3668 (νH O), 2969 (w), 2910 (w), 1643 (m,
2
νC−Ctoluene), 1598 (w), 1556 (w), 1511 (m), 1459 (s), 1415 (w),
1382 (w), 1375 (w), 1367 (w), 1261 (m), 1116 (s), 1081 (s), 1029
(w), 975 (s), 908 (w), 852 (s), 827 (s), 765 (s), 756 (s), 725 (m),
707 (m), 682 (m), 659 (m), 609 (m), 603 (m), 572 (m). Raman
(743 nm, 43 mW, 60 s, 10 acc., 25 °C, cm−1): 2973 (1), 2907 (1),
1643 (1, νC−Ctoluene), 1418 (1), 1377 (1), 1263 (1), 1100 (1), 859
(1), 820 (2), 767 (1), 700 (1), 642 (1), 583 (10), 575 (2), 492 (6),
475 (6), 448 (7), 423 (6), 390 (4), 357 (2), 345 (1), 277 (1), 243
(3), 159 (5). ESI+ (Mcalc, (Mfound)): 387.14283 (387.144)
[(Me3SiO4)4P]+. ESI− (Mcalc, (Mfound)): 678.97737 (678.9831)
[B(C6F5)4]−.
1
C9H27O4PSi3 (314.54 g/mol). bp. 63 °C (0.9 mbar). H NMR
(25 °C, CD2Cl2, 300.13 MHz): δ = 0.20 (s, J(1H−13C) = 119 Hz,
1
2J(1H−29Si) = 7.0 Hz). 1H NMR (25 °C, DMSO-[d6], 300.13 MHz):
1
2
δ = 0.22 (s, J(1H−13C) = 119 Hz, J(1H−29Si) = 7.3 Hz). 13C{1H}
*Rapid hydrolysis during sample preparation. Prolonged exposition
to ambient temperatures and moisture lead to decomposition of
2[B(C6F5)4].
(25 °C, CD2Cl2, 75.47 MHz): δ = 0.93 (d, J(13C−29Si) = 60.4 Hz,
1
2J(13C−31P) = 1.5 Hz). 13C{1H} (25 °C, DMSO-[d6], 75.47 MHz): δ
= 0.24 (d, 1J(13C−29Si) = 60.2 Hz). 17O (25 °C, CD2Cl2, 67.80
MHz): δ = 83.1 (POSi, Δν1/2 = 315 Hz), 105.32 (d, PO, 1J(17O−31P)
= 150 Hz, Δν1/2 = 100 Hz). 29Si INEPT (25 °C, CD2Cl2, 59.63
MHz): δ = 20.39 (m). 29Si INEPT (25 °C, DMSO-[d6], 59.63 MHz):
δ = 20.40 (m). 31P{1H} NMR (25 °C, CD2Cl2, 121.51 MHz): δ =
−25.81 (s). 31P{1H} NMR (25 °C, DMSO-[d6], 121.51 MHz): δ =
−25.78 (s). IR (ATR, 8 scans, 25 °C, cm−1): 2962 (w), 2902 (w),
1457 (w), 1419 (w), 1276 (m), 1249 (s), 1004 (s), 835 (s), 757 (s),
696 (m), 607 (m). Raman (633 nm, 5 mW, 10 s, 20 acc., 25 °C,
cm−1): 3115 (1), 2967 (3), 2904 (10), 2493 (1), 1416 (1), 1279 (1),
1255 (1), 1075 (1), 850 (1), 762 (1), 697 (1), 652 (2), 615 (4), 592
(2), 452 (1), 349 (1), 259 (1), 245 (1), 216 (1), 186 (2), 170 (2).
Synthesis of 2[B(C6F5)4]. The trimethylsilane adduct of
trimethylsilylium tetrakis(pentafluorophenyl)borate [(Me3Si)2H][B-
(C6F5)4] (180 mg, 0.22 mmol) was suspended in 4 mL of toluene.
The suspension was degasified three times by a freeze−pump−thaw
procedure. Tristrimethylsilyl phosphate OP(OSiMe3)3 (69 mg, 0.22
mmol) was added to this suspension via microliter syringe. The
suspension was set in an ultrasonic bath for 1 h. Single crystals
suitable for X-ray structure elucidation were grown from this solution
overnight at −20 °C. The supernatant was removed by syringe, and
the colorless crystals were washed with a small amount of cold n-
pentane (at −40 °C) and dried in vacuo at −20 °C, yielding in 185 mg
(0.17 mmol, 83%) of tetrakis(trimethylsiloxy)phosphonium tetrakis-
(pentafluorophenyl)borate [(Me3SiO)4P][B(C6F5)4].
Synthesis of [K-(18-crown-6)]3. To a clear, yellowish stirred
solution of potassium tert-butoxide K[OC(CH3)3] (0.52 g, 4.63
mmol) and 1,4,7,10,13,16-hexaoxa-cyclo-octadecane (18-crown-6, 1.2
g, 4.63 mmol) in 20 mL DME, tris(trimethylsilyl) phosphate
OP(OSiMe3)3 (1.46 g, 4.63 mmol) was added via syringe. This
immediately led to a color change of the reaction solution to colorless.
The reaction mixture was stirred for another 10 min and was
afterward filtered through a G4 frit.
Single crystals suitable for X-ray structure elucidation were grown
overnight in the refrigerator (−20 °C). The crystals were washed with
2 × 5 mL of n-pentane. The remaining colorless crystals were dried in
vacuo at 60 °C for 15 min, yielding 2.1 g (3.9 mmol, 85%) of [K-18-
crown-6][O2P(OSiMe3)2].
C18H42KO10PSi2 (544.76 g/mol). mp. 121 °C (dec). EA calc.
(found), %: C, 39.69 (39.24); H, 7.77 (6.87). 1H NMR (25 °C,
CD2Cl2, 300.13 MHz): δ = 0.16 (s, 18H, SiCH3, 1J(1H−13C) = 116.2
Hz, 2J(1H−29Si) = 6.8 Hz), 3.62 (s, 24H, OCH2, 1J(1H−13C) = 141.4
Hz). 13C{1H} NMR (25 °C, CD2Cl2, 75.47 MHz): δ = 1.40 (s,
1
SiCH3, J(13C−29Si) = 60 Hz), 70.6 (s, OCH2). 17O NMR (25 °C,
CD2Cl2, 67.83 MHz): δ = (no signal could be observed). 29Si INEPT
NMR (25 °C, CD2Cl2, 59.62 MHz): δ = 10.28 (unresolved signal).
31P{1H} NMR (25 °C, CD2Cl2, 121.51 MHz): δ = −13.01 (s,
PO(Si)). IR (ATR, 16 scans, 25 °C, cm−1): 2954 (w), 2885 (m),
2829 (w), 1471 (w), 1454 (w), 1417 (w), 1349 (m), 1284 (w), 1243
(m), 1218 (m), 1103 (s), 1089 (s), 1010 (m), 962 (s), 939 (s), 865
(m), 835 (s), 756 (m), 686 (m), 599 (m), 532 (m). Raman (632 nm,
10 mW, 10 s, 20 acc., 25 °C, cm−1): 2961 (5), 2899 (10), 2846 (4),
2809 (2), 2732 (1), 2702 (1), 1477 (2), 1457 (1), 1412 (1), 1365
(1), 1289 (1), 1273 (2), 1246 (1), 1149 (2), 1141 (2), 1112 (1),
1094 (4), 1072 (1), 952 (1), 873 (5), 832 (2), 812 (1), 758 (1), 692
(1), 669 (1), 616 (9), 598 (3), 548 (1), 380 (1), 364 (1), 326 (1),
281 (3), 253 (1). 202 (1). MS (ESI−, m/z calc. (found)): 241.04867
(241.04905). (ESI+, m/z calc. (found)): 303.12045 (303.1197).
C36H36BF20O4PSi (1066.76 g/mol). mp. < 76 °C (dec.). EA calc.
(found), %: C, 40.53 (40.47); H, 3.40 (3.58). 1H NMR (25 °C,
1
CD2Cl2, 300.13 MHz): δ = 0.41 (s, SiCH3, J(1H−13C) = 120.5 Hz,
2J(1H−29Si) = 6.9 Hz). 11B NMR (25 °C, CD2Cl2, 96.29 MHz): δ =
−16.56 (s). 13C{1H} NMR (25 °C, CD2Cl2, 75.47 MHz): δ = 0.77
1
3
(d, SiCH3, J(13C−29Si) = 60.8 Hz, J(13C−31P) = 1.7 Hz), (ipso-CF
not observed), 136.81 (dm, m-CF, J(13C−19F) = 246 Hz), 138.59
1
1
1
(dm, p-CF, J(13C−19F) = 243 Hz), 148.74 (dm, o-CF, J(13C−19F) =
238 Hz). 17O NMR (25 °C, CD2Cl2, 67.80 MHz): δ = 78.4 (b, Δν1/2
= 160 Hz). 19F{1H} NMR (25 °C, CD2Cl2, 282.40 MHz): δ =
B
DOI: 10.1021/acs.inorgchem.8b01323
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
moisture and oxygen sensitive 2[B(C6F5)4] had to be stored at
−40 °C because it decomposed slowly at ambient temperature
even under an argon atmosphere but very fast at temperatures
above 76 °C.
RESULTS AND DISCUSSION
■
Synthesis. OP(OSiMe3)3, generated from hexamethyldisi-
loxane and phosphoric anhydride, was first mentioned in
1944.25 Ever since different synthetic approaches have been
published.26−30 An easy high-yielding synthetic process was
published in 2014 by Wessjohann et al., who treated
KH2[PO4] with an excess of Me3SiCl in formamide affording
pure 1 in high yields.31 We have applied a slightly modified
process by using n-hexane as solvent (yield 96%) as illustrated
in Scheme 2 (eq 1). A similar H+/[Me3Si]+ exchange
procedure was used to generate silylated pyrophosphoric acid
(see Supporting Information) starting from Na4[P2O7].31 For
the synthesis of a [P(OSiMe3)4]+ salt, we utilized a super Lewis
acidic medium (OP(OSiMe3)3/[Me3Si][wca])2 to further
silylate OP(OSiMe3)3 in analogy to the super acid medium
used by Minkwitz.19 Hence, with freshly prepared OP-
(OSiMe3)3 in hand, we treated 1 with the [Me3Si]+ transfer
reagent [Me3Si−H−SiMe3][(B(C6F5)4] affording the desired
silylated phosphonium salt [P(OSiMe3)4][(B(C6F5)4] (2[B-
(C6F5)4], Figure 1, Scheme 2 eq 2, yield 83%). Highly
Synthesis of salts containing bissilylated phosphate [O2P-
(OSiMe3)2]− (3−) was carried out again in analogy to the
protonated species (Scheme 2, eq 3), by adding one equivalent
of a base (KOCMe3) to “neutralize” exactly one [Me3Si]+ ion
by ether formation yielding colorless crystals of [K-(18-crown-
6)][O2P(OSiMe3)2] ([K-(18-crown-6)]3) suitable for struc-
ture elucidation after adding a crown ether (18-crown-6) at
−20 °C (yield 85%). Complexes, containing the anion
[O2P(OSiMe3)2]−, have been known in the literature since
the late 1990s.32−36 However, all these structures have in
common that the anionic phosphate is coordinated by two
metal centers (Al, Ti, Zn, and Ga). Herein, we report on a
metal complex containing an almost naked bissilylated anion
[O2P(OSiMe3)2]− weakly coordinated to only one potassium
ion (see below). Crystals of [K-(18-crown-6)]3 were also
moisture and oxygen sensitive but thermally considerably more
stable than salts of 2+ (121 °C vs. 75 °C). Furthermore, the
autoprotolysis equilibrium known for phosphoric acid (vide
supra) could also be verified by NMR experiments: The
reaction of exactly one equivalent 2+ with one equivalent of 3−
led to the formation of the formal [Me3Si]+ exchange product
1 ([P(OSiMe3)4]+ + [O2P(OSiMe3)2]− → 2 OP(OSiMe3)3).
Both isolated species 2+ and 3− were fully characterized by 1H,
13C, 17O, 29Si, and 31P NMR techniques in CD2Cl2 and DMSO
as well as IR/Raman spectroscopy (Table 1). We do want to
stress that the back reaction represents only a formal
autoprotolysis back reaction since by no means were the
autoprotolysis products ([P(OSiMe3)4]+ + [O2P(OSiMe3)2]−)
Table 1. Selected Experimental NMR (δ [ppm], Solvent =
CD2Cl2), Computed TMSA/PA Values (kcal·mol−1), and
NPA (Natural Population Analysis) Charges (e)
a
3−
0.16
1
Ppa
0.26
2+
0.41
1H
0.20
0.9
13C
29Si
31P
1.4
0.9
0.8
b
10.3
−13.0
20.4
24.2
35.6
−25.8
−31.5
82.4(325)
105.1
2.647
1.958
−1.165
−1.117
0.677
−1.913
163.5
308.0
−35.9
78.4(160)
c
17O POSi
n.s.
83.1(315)
105.3(150)
2.649
c
PO
q(P)
q(Si)
q(OSi)
q(Ofree
q(Me3Si)
q(PO4)
n.s.
d
2.580
1.975
2.755
1.945
−1.157
d
1.960
d
−1.205
−1.177
0.592
−2.185
276.9
−1.171
−1.133
0.665
−1.996
171.7
d
)
d
+0.718
−1.872
76.5
e
TMSA
Figure 1. Ball-and-stick representation of the molecular structure of
2+ (top) and 3− (bottom) in the crystal. Disorder not shown. Selected
structural data (bond length [Å], angles [deg]): 2[B(C6F5)4] P1−O1
1.519(2), P1−O2 1.534(2), P1−O3 1.509(2), P1−O4 1.521(2),
O1−Si1 1.713(2), O2−Si2 1.719(2), O3−Si3 1.707(2), O4−Si4
1.721(1); O3−P1−O1 110.36(8), O3−P1−O2 109.18(7), O3−P1−
O4 111.28(8), O4−P1−O2 107.94(7), P1−O1−Si1 139.89(9), P1−
O2−Si2 138.45(9), P1−O3−Si3 150.7(2), P1−O4−Si4 138.22(8);
[K-(18-crown-6)]3 K1−O7 3.016(2), K1−O8 2.669(2), P1−O7
1.483(2), P1−O8 1.481(2), P1−O9 1.588(2), P1−O10 1.6(2), Si1−
O9 1.64(2), Si2−O10 1.648(2); O7−P1−O9 111.30(8), O7−P1−
O10 110.56(8), O9−P1−O10 99.61(8), O8−P1−O7 118.17(8),
O8−P1−O9 107.85(9), O8−P1−O10 107.69(9), P1−O9−Si1
135.79(9), P1−O10−Si2 131.60(8).
f
PA
457.5
328.6
198.7
a
b
c
Ppa = pyrophosphoric acid = (Me3SiO)4P2O7. 29Si INEPT. No
signal because 17O is a quadrupolar nucleus with I = 5/2 affording
broad signals, often not to be observed. For more than one atom/
group the average value is given. Trimethylsilyl affinity (TMSA) of
d
e
A(g) is defined as the negative of the reaction enthalpy ΔH(g)
°
in
→
(298)
kcal/mol at 298.15 K for the reaction A(g) + [Me3Si]+
(g)
[A(Me3Si)]+(g),2 [A(Me3Si)](g) = 3−, 1, Ppa, 2+; hence, the TMSA
value given is for the conjugated acid−base pair A(g)/[A(Me3Si)]+
.
(g)
f
Proton affinity (PA) is defined analogously like the TMSA; the
corresponding [AH](g) species are [O2P(OH)2]−, OP(OH)3,
H2[P2O7], and [P(OH)4]+.
C
DOI: 10.1021/acs.inorgchem.8b01323
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
detected when OP(OSiMe3)3 was dissolved, e.g., in CH2Cl2;
however, reaction 2+ with one equivalent of 3− affords the
formation of 1 at once.
1.6(2) Å). This local symmetry decrease is also reflected by
two different O−P−O angles (∠(O9−P1−O10) = 99.61(8),
∠(O8−P1−O7) = 118.17(8)°).
Spectroscopic and Structural Details. The high-field 31
P
Charge and Thermodynamic Consideration. To get
some insight into the charge transfer upon silylation and
desilylation, respectively, we computed the partial net charges
of the elements and the [Me3Si] and [PO4] moieties within
species 1, 2+, and 3− (Table 1) at the pbe0/aug-cc-pwCVDZ
level of theory. Two interesting features can be derived from
these data: (i) the atomic charges within this series do not
change much upon increasing silylation degree. (ii) Hence,
only a moderate charge change is found for the [Me3Si] and
[PO4] moieties indicating that the charge difference of two
between 2+ and 3− is compensated by delocalization over the
entire species. However, in accord with our NMR data a small
increase of the charge at the [PO4] and a small decrease at the
[Me3Si] unit is found along the series 3− to 2+. Finally, the
trimethysilyl affinities (TMSA, Table 1) of 1, 2+, and 3− were
computed increasing along 76.5 (2+) < 171.7 (1) < 276.9 (3−)
< 584.9 kcal/mol ([O3P(OSiMe3]2−) (cf. 32.8 [Me3Si−H−
SiMe3]+, 94.7 [Me3Si][CHB11F11], and 109.8 kcal/mol
[Me3Si][CHB11H11]).42 Hence, 2+ represents the best
silylation species and most “naked” [Me3Si]+ species among
the considered species here. Nevertheless, within [Me3Si−H−
SiMe3]+, “[Me3Si]+” is less strongly bound and can be used to
generate 2+ salts as experimentally demonstrated here and in
accord with the computed TMSA value.
nuclear magnetic resonance shifts between −13 and −36 ppm
observed for species 1, 2+, and 3− in CD2Cl2 at ambient
temperatures are characteristic for phosphates (cf. δ[31P] =
−31 (Me3SiO)2P(O)O−P(O)(OSiMe3)2;31 34.4 [P-
(OSiMe3)4]I).20 Notably, the resonance of cation 2+ (δ[31P]
= −35.9) was shifted to higher field by 10 ppm relative to
compound 1 (−25.8), while the anion 3− was observed at
considerably lower field (δ[31P] = −13.0). As expected, the
29Si resonance of 2+ (δ[29Si] = 35.6) was shifted by 15 ppm to
lower field compared to 1 (δ[29Si] = 20.4), while a high-field
shift by 10 ppm was observed for 3− (δ[29Si] = 10.3, cf. 32
[Me-CN-SiMe3]+,3 computed 385.1 naked [Me3Si]+(g), see
Supporting Information, Table S7) indicating strong inter-
actions between the oxygen atom and the [Me3Si]+ moiety for
all species. Interestingly, 17O NMR shifts were less sensitive
with respect to the degree of silylation and charge. Only a small
low-field shift of 5 ppm was detected for 2+ (78 vs. 83 for 1; cf.
113 [PO4]3− and 77 ppm of OP(OMe)3).37−39 Due to the
large quadrupole moment (I = 5/2) of 17O, usually these
signals were rather broad (Δν1/2 between 325−100 Hz), which
was also the reason why a resonance was neither detected for
3− nor the 18-crown-6 compound. Despite these broad
1
resonances, a rarely detected J(17O−31P) coupling of 150
Hz was found in the 17O NMR spectrum of 1 in CD2Cl2
solution (Figures S20/21, cf. 160 Hz in OP(OMe)3).
CONCLUSION
■
In summary, highly labile salts bearing [P(OSiMe3)4]+ and
[O2P(OSiMe3)2]− ions were generated utilizing super Lewis
acidic media and bulky, chemical robust counterions similar to
the chemistry known for analogous protonated species.
Formally, [P(OSiMe3)4]+ and [O2P(OSiMe3)2]− ions might
be regarded as the formal product of the autoprotolysis of
OP(OSiMe3)3 and the big proton analogs of [P(OH)4]+
(protonated phosphoric acid) and [O2P(OH)2]− (dihydrogen
phosphate).
The molecular structure of 2[B(C6F5)4 and [K-(18-crown-
6)]3 was unequivocally proven by single crystal X-ray
diffraction (Figure 1). Both compounds crystallized in the
̅
triclinic space group P1. A toluene solvate 2[B(C6F5)4·2.5
toluene) was also crystallized (see Supporting Information).
Besides, a rather poor data set for crystals of silylated
pyrophosphoric acid (Ppa), establishing the connectivity, was
also obtained (Figure S1). As depicted in Figure 1 (top), there
are only three short contacts between 2+ and two fluorine
atoms of one C6F5 ring of the [B(C6F5)4]− anion (d(H···F)
2.6−2.8 Å, cf. ∑rvdW(H···F) = 2.67 Å).40 Therefore, the
molecular structure can be described as two weakly interacting
almost sphere-shaped ions with the positive charge mainly
localized at the phosphorus atom (q(P) = 2.75 e, Table 1) in
2+ and the negative charge delocalized over the 20 fluorine
atoms of the borate anion. In 2+, which is isoelectronic to
(Me3SiO)4Si and [(Me3SiO)4Al]−, the central phosphorus
atom is almost tetrahedrally surrounded by four [OSiMe3]
moieties with P(V)−O distances ranging between 1.509−1.534
Å in accord with structural data of H3PO4 (1.55 Å)41 and
[P(OH)4]+ (1.53 Å).19 A slightly different situation is found in
crystals of [K-(18-crown-6)]3, for which rather strong cation−
anion interactions can be assumed (Figure 1 bottom). Since
two oxygen atoms of the PO4 core in 3− remain unsilylated,
they are utilized for coordinating to the K+ center, which is
embedded in the 18-crown-6 ring system (d(K−O18‑crown‑6) =
2.82−2.97 Å). However, both K−OPO4 distances were found
to be significantly different (d(K1−O7) = 3.016(2), d(K1−
O8) = 2.669(2) Å). In contrast to 2+, 3− displays a strongly
distorted tetrahedral central P atom with approximately local
C2v point group symmetry of the PO4 core. Hence, two shorter
P−O distances, displaying partial double bond character
(d(P1−O7) = 1.483(2), d(P1−O8) = 1.481(2) Å), and two
longer are found (d(P1−O9) = 1.588(2), d(P1−O10) =
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b01323.
Experimental details; structure elucidation; starting and
reference materials; synthesis of compounds; NMR
spectra; IR and Raman spectra; computational details
(PDF)
Accession Codes
CCDC 1828618−1828620 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail: axel.schulz@uni-rostock.de.
■
ORCID
Axel Schulz: 0000-0001-9060-7065
D
DOI: 10.1021/acs.inorgchem.8b01323
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
Deutsche Forschungsgemeinschaft (DFG SCHU 1170/12-1)
is acknowledged for financial support. We thank L. A.
Wessjohann (Leibniz Institute Halle) and M. Dessoy
(University of Campinas) for helpful advice and Dr. D.
Michalik (University of Rostock) for the measurement of 17O
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