New Adamantane-like Silicophosphate Cage and Its Reactivity toward Tris(pentafluorophenyl)borane

Article abstract of DOI:10.1021/acs.inorgchem.7b01572

The condensation reaction between Ph₂Si(OC(O)CH₃)₂ and OP(OSiMe₃)₃ leads to elimination of CH₃C(O)OSiMe₃ and the formation of the new silicophosphate cage molecule Ph₁₂Si₆P₄O₁₆ (1) with an adamantane-like core possessing four terminal P=O moieties and six O-SiPh₂-O bridging groups. Compound 1 was further reacted with the Lewis acid B(C₆F₅)₃. We observed adduct formation by coordination through the P=O→B bonds and isolated and structurally characterized two new molecules. In the first of them, the adamantane-like cage is preserved and three phosphoryl oxygen atoms coordinate to boranes, forming Ph₁₂Si₆O₁₆P₄·3B(C₆F₅)₃ (2); the remaining P=O group is inverted toward the cage center pointing along a C₃ molecular axis. The molecule is chiral, and the compound 2 crystallizes as a conglomerate of homochiral crystals. Enantiomers 2M and 2P were both structurally characterized. The second adduct resulted from an unexpected reorganization of the Si-O-P linkages in the adamantane cage during the reaction of 1 with 4 equiv of B(C₆F₅)₃. The bis-adduct Ph₆Si₃O₈P₂·2B(C₆F₅)₃ (3) was formed with an inorganic core representing half of the parent molecule 1.

Full text of DOI:10.1021/acs.inorgchem.7b01572

Article
pubs.acs.org/IC
New Adamantane-like Silicophosphate Cage and Its Reactivity
toward Tris(pentafluorophenyl)borane
Ales Styskalik,†,‡ Michal Babiak,†,‡ Petr Machac,† Bohuslava Relichova,† and Jiri Pinkas*,†,‡
†Masaryk University, Department of Chemistry, Kotlarska 2, CZ-61137 Brno, Czech Republic
‡Masaryk University, CEITEC MU, Kamenice 5, CZ-62500 Brno, Czech Republic
S
* Supporting Information
ABSTRACT: The condensation reaction between Ph2Si(OC-
(O)CH3)2 and OP(OSiMe3)3 leads to elimination of CH3C-
(O)OSiMe3 and the formation of the new silicophosphate cage
molecule Ph12Si6P4O16 (1) with an adamantane-like core
possessing four terminal PO moieties and six O−SiPh2−O
bridging groups. Compound 1 was further reacted with the
Lewis acid B(C6F5)3. We observed adduct formation by
coordination through the PO→B bonds and isolated and
structurally characterized two new molecules. In the first of
them, the adamantane-like cage is preserved and three phosphoryl oxygen atoms coordinate to boranes, forming Ph12Si6O16P4·
3B(C6F5)3 (2); the remaining PO group is inverted toward the cage center pointing along a C3 molecular axis. The molecule is
chiral, and the compound 2 crystallizes as a conglomerate of homochiral crystals. Enantiomers 2M and 2P were both structurally
characterized. The second adduct resulted from an unexpected reorganization of the Si−O−P linkages in the adamantane cage
during the reaction of 1 with 4 equiv of B(C6F5)3. The bis-adduct Ph6Si3O8P2·2B(C6F5)3 (3) was formed with an inorganic core
representing half of the parent molecule 1.
(1)
(2)
(3)
(4)




Si−Cl + HO−P →  Si−O−P + HCl
Si−OH + Cl−P →  Si−O−P + HCl
Si−OLi + Cl−P →  Si−O−P + LiCl
Si−OR + Cl−P →  Si−O−P + RCl
Si−OC(O)CH3 + Me3SiO−P → ≡ Si−O−P
+ CH3C(O)OSiMe3
INTRODUCTION
■
Silicophosphates are compounds containing a network of
bridging Si−O−P bonds between silicate and phosphate
moieties. The hydrolytic instability of Si−O−P bridges limits
the available synthetic methods to non-aqueous or high-
temperature reactions.1 The heating of silicon and phosphorus
compounds (e.g., SiO2 and P4O10 or H3PO4) leads to
reorganization of Si−O−Si and P−O−P bonds, condensation
between Si−OH and P−OH groups, and water evaporation.
The syntheses at increased temperatures provide up to 10
crystalline phases of silicon phosphates.2−4 Their crystal
structures are very similar, which impairs the reproducibility
of the synthetic reactions and the purity of their products. The
phosphoryl groups in these compounds readily coordinate to
silicon atoms, which act as Lewis acids and thus become six-
coordinated.4−6
The situation becomes significantly different if an organic
group is attached to the silicon atom by a stable Si−C bond.
Such Si atoms do not acquire octahedral coordination, and
bulky organic groups act as steric hindrance to the formation of
3D cross-linked inorganic networks.7 For these reasons
molecular silicophosphates are obtained in the reactions of
organosilicon with organophosphorus compounds. Condensa-
tion of silicon and phosphorus precursors is performed in
nonaqueous solvents under strictly anhydrous conditions in
order to prevent the hydrolysis of the newly formed Si−O−P
bonds. Possible reaction pathways include elimination of
hydrochloric acid (eqs 1 and 2),8−10 precipitation of metal
halide (eq 3),11 and alkyl halide elimination (eq 4).12,13

(5)
We have already reported that the condensation of
acetoxysilanes and trimethylsilyl esters of phosphoric and
phosphonic acids accompanied by the elimination of
trimethylsilyl acetate is suitable for the nonhydrolytic syntheses
of a broad variety of silicophosphate xerogels (eq 5).7,14
Moreover, this reaction principle provided us with the
molecular silicophosphate [(Ph2Si{O2P(O)OSiMe3})2] as
only the second example of a central Si2P2O4 ring (A in
Chart 1),7 which is similar to the structure obtained by
Vaugeois et al. by precipitation of metal halide.11 The
hydrolytic instability of silicophosphates still presents a
synthetic challenge, and therefore examples of structurally
characterized molecular silicophosphates are limited only to
trimethylsilyl esters of phosphoric and phosphonic acids and a
few other examples, including [(Ph2Si{O2P(O)OSiMe3})2],7
[(tBu2Si{O2P(O)Ph})2],11 [RSi(OH){OP(O)(H)(OH)}]2O
Received: June 30, 2017
© XXXX American Chemical Society
A
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Article
a
Chart 1. Inorganic Cores of Molecular Silicophosphates
a
Phenyl substituents on Si are omitted for clarity.
0.034 mmol) was dissolved in CDCl3 (0.45 cm3), and then B(C6F5)3
(50 mg, 0.098 mmol) was added. It dissolved immediately and formed
a slightly yellowish reaction solution. The 31P NMR spectra were
acquired within 15 min of mixing.
(R = (2,6-iPr2C6H3)NSiMe3),10 and ionic [Et3NH]2[(Si-
(PO4)6(SiO4Et2)6)].9
Here we report for the first time the silicophosphate
compound 1 with an adamantane-like core (B in Chart 1). It
was obtained from diacetoxydiphenylsilane and tris-
(trimethylsilyl)phosphate (TTP). The condensation reaction
of these two reagents was performed under strictly anhydrous
conditions and was accompanied by the elimination of
trimethylsilyl acetate, similarly to our previous syntheses of
xerogels containing a network of bridging Si−O−P bonds.7,14
This cage compound 1 with an adamantane-like core was
employed in the coordination reactions with the Lewis acid
tris(pentafluorophenyl)borane in analogy to already known
phosphine oxide−borane adducts.15,16 Interestingly, the borane
not only showed the ability to form PO→B coordinate
bonds but was able to reorganize the P−O−Si bridges as well.
This behavior led to two new structures. In 2 the parent
adamantane-like cage was maintained, while in 3 a new
silicophosphate cage was formed (C in Chart 1).
31P NMR (1 + 3 B(C6F5)3, ppm): δ −29.6 (s, (SiO)3PO, 1P),
−37.2 (broad s, 0.7P), −40.9 (broad s, (SiO)3PO→B(C6F5)3
chemical exchange, 18P), −44.9 (broad s, 0.3P), −45.1 (s, 0.1P),
−47.1 (s, (SiO)3PO→B(C6F5)3, 0.1P), −54.0 (s, (SiO)3PO→
B(C6F5)3, 3P).
The 31P NMR spectrum after 72 h at room temperature did not
change significantlyseveral new signals of low intensity were
observed in the region between −30 and −55 ppm. The relative
intensity of the peak at −47.1 ppm increased from 0.1P to 1.5P and
continued to grow when we prolonged the reaction time to 2 weeks
(Figure 2).
The 31P NMR spectrum acquired at −60 °C showed splitting of the
original broad signal at −40.9 ppm to new, relatively sharp signals at
−29.8, −31.1, −48.7, and −50.4 ppm (Figure S2 in the Supporting
Information).
We performed the reactions between silicophosphate 1 and
tris(pentafluorophenyl)borane also in 1:2, 1:4, and 1:6 ratios and
followed their course by 31P NMR spectroscopy. All spectra showed
the three main signals similar to those for the reaction in a 1:3 ratio:
−29.6 ppm (s, (SiO)3PO, 1P), −40.9 (broad s, (SiO)3PO→
B(C6F5)3 chemical exchange, xP), −54.0 (s, (SiO)3PO→B(C6F5)3,
3P), where x decreased with an increasing amount of B(C6F5)3 in the
reaction mixture. The low-intensity sharp signal at −47.1 ppm grew in
all reactions after standing for a few days at room temperature.
Finally, we observed small colorless crystals on the walls of the
NMR tube with the reaction mixture of 1 and B(C6F5)3 (1:3) after a
few days at room temperature. The molecular structure of this product
was determined by a single-crystal X-ray diffraction analysis as
Ph12Si6O16P4·3B(C6F5)3·2CHCl3 (2·2CHCl3, Figure 3).
EXPERIMENTAL SECTION
General experimental methodology and characterization data are
described in the Supporting Information.
■
Synthesis of Molecular Silicophosphate Ph12Si6O16P4·1/
4CHCl3·9/8C6H6 (1·1/4CHCl3·9/8C6H6). TTP (3.607 g; 11.47
mmol) was added to a solution of diphenyldiacetoxysilane (5.065 g;
16.86 mmol) in toluene (20 cm3). The reaction mixture was kept at 90
°C for 24 h. Toluene and trimethylsilyl acetate were then removed in
vacuo, affording a white viscous liquid. It was gradually heated from
room temperature up to 250 °C in vacuo over a period of 8 h. The
resulting pale brown solid (mixture of condensation products) was
purified by refluxing in benzene. The undissolved white precipitate was
filtered off, and after it was dissolved in CDCl3, it featured one
resonance in the 31P NMR spectrum (product 1). It was dried in vacuo
and yielded desolvated 1 (3.074 g, 74.2%). A solution of 1 in CHCl3
was kept in a small open vial placed in a closed flask containing
benzene (4 cm3; Caution! carcinogen). It allowed free diffusion of
benzene into chloroform. Crystals of 1·1/4CHCl3·9/8C6H6 suitable
for an X-ray diffraction analysis were obtained after several days
(Figure 1).
Crystals of 2·2CHCl3 were decanted and redissolved in CDCl3.
Their 31P NMR spectrum was almost identical with what we observed
after mixing of silicophosphate cage 1 with tris(pentafluorophenyl)-
borane: −29.6 ppm (s, (SiO)3PO, 1P), −40.9 (broad s, (SiO)3P
O→B(C6F5)3 chemical exchange, 29P), −47.1 (s, (SiO)3PO→
B(C6F5)3, 12P), −54.0 (s, (SiO)3PO→B(C6F5)3, 3P).
IR and MS data of 2·2CHCl3 are given in the Supporting
Information.
Characterization and Analyses of 1. 1H NMR (1, CDCl3, ppm): δ
7.11−7.52 ((C6H5)2Si, multiplet, 60H). 13C{1H} NMR (1, CDCl3,
ppm): δ 128.1 ((C6H5)2Si, s, para), 128.5 ((C6H5)2Si, s, meta), 131.4
((C6H5)2Si, s, ortho), 134.5 ((C6H5)2Si, s, ipso). 29Si NMR (1, CDCl3,
ppm): δ −38.5 ((C6H5)2SiO2, s). 31P NMR (1, CDCl3, ppm): δ −32.5
(O3PO, s).
Synthesis of Silicophosphate Ph6Si3O8P2·2B(C6F5)3·2C6H6 (3·
2C6H6). Ph12Si6O16P4 (1; 250 mg, 0.170 mmol) was dissolved in
CHCl3 (2 cm3), and then B(C6F5)3 was added (347 mg, 0.678 mmol).
It dissolved immediately and formed a slightly yellowish reaction
solution. It was kept in a small open vial placed in a closed flask
containing benzene (4 cm3; Caution! carcinogen). This allowed free
diffusion of benzene into chloroform. Colorless crystals of 3·2C6H6
were obtained in the small vial after 1 week with a yield of 150 mg
(23.1%); the molecular structure of this product was determined by a
single-crystal X-ray diffraction analysis (Figure 4).
IR, MS, TG, elemental analyses, and melting points are given in the
Supporting Information.
NMR-Tube reaction between 1 and B(C6F5)3 in a 1:3 Molar
Ratio. Molecular Structure of 2·2CHCl3. Compound 1 (50 mg,
B
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Characterization and analyses of 3·2C6H6. 31P NMR (3·2C6H6,
CDCl3, immediately after dissolution, ppm): δ −46.7 (s, (SiO)3P
O→B(C6F5)3). The spectrum after standing overnight was almost
identical with what we observed in NMR reactions of silicophosphate
cage 1 and tris(pentafluorophenyl)borane. It showed the three main
signals and only a low-intensity resonance coming probably from the
original crystals: −29.6 ppm (s, 2; (SiO)3PO, 1P), −40.9 (broad s,
(SiO)3PO→B(C6F5)3 chemical exchange, 40P), −47.1 (s, 3;
(SiO)3PO→B(C6F5)3, 0.6P), −54.0 (s, 2; (SiO)3PO→B(C6F5)3,
3P).
spectroscopically characterized and its molecular structure
determined by a single-crystal X-ray diffraction analysis (Figure
1). Selected bond and angle parameters are gathered in Table 1.
IR, MS, TG, elemental analyses, and melting points are given in the
Supporting Information.
Crystallography. Single crystals suitable for diffraction experi-
ments were picked (using LithoLoops) from samples immersed in
silicone oil. For all structures (unless explicitly specified), all non-
hydrogen atoms were refined anisotropically. Hydrogen atoms were
placed in calculated positions, with their Uiso values set to 1.2 times the
Ueq value of the carrier atom and refined as riding. Selected
crystallographic data are gathered in Table S1 in the Supporting
Information.
The molecule 1 (Figure 1) crystallizes as a solvate with CHCl3 and
C6H6. The symmetrically independent part of the unit cell contains
one-third of the molecular fragment of 1 lying on a special position (3-
fold screw axis passing through P2_2) and another whole molecule 1
lying on a general position. Both fragments exhibit positional disorder.
The disorder was treated with appropriate geometry restraints and
atomic displacement parameter restraints with the sums of individual
corresponding parts set to 100%. Solvent that could not be modeled
reasonably was treated by the SQUEEZE17,18 procedure. Finally, an R1
index of slightly over 5% was reached. It is possible that the R1 index
could be further lowered by treating other currently ordered atoms as
disordered, but the ratios of site occupancy factors already reached
values as low as 14.7%/85.3%. The highest peak in the Fo − Fc
difference Fourier map is as low as 0.83 e Å−3; therefore, the
refinement was terminated at this state.
The molecule 2 (Figure 3) crystallizes as a solvate with 2 equiv of
CHCl3. The main fragment and both solvent molecules lie on special
positions (3-fold proper rotation axes passing through the P2−O6
bond and CDCl3 C−D bonds, respectively). The deuterium scattering
factor type is assigned to the trichloromethane hydrogens only. The
structure crystallizes in the noncentrosymmetric space group R3; the
Flack parameter values −0.027(71) and 0.001(28) confirm the correct
absolute structure assignment of both characterized crystals
(enantiomers M and P, respectively, Figure 4). The refinement was
straightforward.
Figure 1. Molecular structure of 1. Solvent molecules, all hydrogen
atoms, and another symmetrically independent molecule of 1 are
omitted for clarity. Atoms forming the silicophosphate cage are drawn
as temperature ellipsoids (50% probability); other (phenyl) moieties
are represented using stick models.
In this molecular product, condensation reaction proceeded
further beyond [(Ph2Si{O2P(O)OSiMe3})2]; all acetoxy and
trimethylsilyl groups reacted and were completely eliminated
(eq 6). The novel silicophosphate cage 1, which has not been
reported previously, was formed. It consists of four PO
phosphoryl groups connected through six O−SiPh2−O bridges,
forming an adamantane-like cage (B in Chart 1). The geometry
of two symmetrically independent molecules is approximately
tetrahedral, although (in agreement with lower site symmetry)
there is some distortion: e.g., P−P intramolecular distances are
not equal. Due to the disorder, it is not straightforward to
describe this simply by enumeration of P−P distances.
However, the molecule retains tetrahedral symmetry in
The molecule 3 (Figure 5) crystallizes as a solvate with 2 equiv of
C6H6. A small part of the low-angle reflections is missing from the data
set due to the beam stop setting and detector pixel overflows. The
refinement was straightforward.
1
solution, as confirmed by H, 13C, and 31P NMR spectra (see
RESULTS AND DISCUSSION
■
below). The inorganic Si6O16P4 core (B in Chart 1) is
surrounded by phenyl groups bound to Si atoms. Phosphoryl
groups are directed outside the cage and are available for the
formation of coordination bonds.
Molecular Silicophosphate Ph12Si6O16P4 (1). The
reaction of TTP and diphenyldiacetoxysilane (eq 6) produced
a mixture of molecular oligomeric products at 90 °C. One of
them, trans-[(Ph2Si{O2P(O)OSiMe3})2], crystallized out of the
reaction mixture and was isolated and characterized; its
molecular structure has already been reported.7 In this
molecular product, condensation stops when a centrosym-
metric Si2P2O4 core (A in Chart 1) is formed. The ring is in a
chair conformation, similar to what is observed in the related
[{tBu2Si{O2P(O)Ph})2].11 When the reaction mixture contain-
ing molecular products of condensation was further heated
without any solvent up to 250 °C, a pale brown solid was
obtained. It was recrystallized from hot benzene and dried in
vacuo, providing the white product 1 in good yield. Colorless
crystals suitable for an X-ray diffraction analysis were obtained
by crystallization from a CHCl3/C6H6 solution after standing at
room temperature for several days. This compound has been
6Ph2Si(OC(O)CH3)2 + 4OP(OSiMe3)3
→ Ph12Si6O16P + 12CH3C(O)OSiMe3
(6)
4
The silicon as well as the phosphorus centers are slightly
distorted from tetrahedral coordination; the inner ring O2−
Si1−O4 (102.51(9)°) and O4−P1−O3 (104.11(11)°) angles
deviate only slightly from the ideal values (Table 1). The angles
at the oxygen centers O2, O3, and O4 are large (Table 1,
131.27(12)−150.36(13)°), similarly to previously reported
cyclic molecular silicophosphates (142.02(8) and 144.60(8)°;7
146.1(2) and 148.8(2)°11). The Si1−O2 (1.6561(18) Å) and
terminal P1O1 bond lengths (1.455(2) Å), as well as the
other P1−O4 distances (1.5541(18) Å), are comparable to
C
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Table 1. Comparison of Selected Bond Distances (Å) and Bond Angles (deg) for Silicophosphates 1, 2P, and 3
Ph12Si6O16P4·1/4CHCl3·9/8C H6
a 6
b
(1·1/4CHCl3·9/8C6H6)
Ph12Si6O16P4·3B(C6F5)3·2CHCl3 (2·2CHCl3)
Ph6Si3O8P2·2B(C6F5)3·2C6H6 (3·2C6H6)
P1O1
1.455(2)
P1O1
1.480(2)
P1O1
P1−O2
P1−O3
P1−O4
1.4787(14)
P1−O2
P1−O3
P1−O4
1.5585(18)
1.551(2)
P1−O2
1.535(2)
1.5365(14)
1.5464(14)
1.5401(14)
P1−O3
P1−O4
1.538(2)
1.5541(18)
1.543(2)
P2O6
1.463(4)
P2−O5
1.564(2)
Si1−O2
Si1−O4
Si2−O3
1.6561(18)
1.6639(18)
1.633(2)
Si1−O2
Si1−O4
Si2−O3
1.663(2)
Si1−O2
Si1−O6
1.6715(14)
1.6743(14)
1.675(2)
1.676(2)
Si2−O5
1.655(2)
B1−O1
1.542(4)
B1−O1
1.566(3)
O1−P1−O4
O1−P1−O3
O4−P1−O3
113.92(12)
115.57(13)
104.11(11)
O1−P1−O2
O1−P1−O3
O2−P1−O4
O5−P2−O5
O6−P2−O5
O2−Si1−O4
O3−Si2−O5
B1−O1−P1
P1−O2−Si1
P2−O5−Si2
109.68(12)
111.35(13)
106.89(12)
104.29(10)
114.26(9)
105.66(11)
106.45(11)
159.0(2)
O1−P1−O2
O1−P1−O3
O2−P1−O4
110.69(8)
110.92(8)
109.03(8)
O2−Si1−O4
102.51(9)
O2−Si1−O6
103.29(7)
B1−O1−P1
P1−O2−Si1
P1−O3−Si2
142.01(13)
147.95(10)
139.38(9)
P1−O2−Si1
P1−O3−Si2
131.27(12)
150.36(13)
152.33(15)
132.90(15)
a
Bond lengths and angles for 1·1/4CHCl3·9/8C6H6 are listed for residue 2 (atom names with suffix “_2”). In cases where the structure contains
b
more chemically equivalent bond lengths, one representative value was chosen. A full listing of bond lengths can be found in the CIF files. Data
given in the table are for enantiomer 2P (CCDC 1555739); bond lengths and angles for 2M are virtually the same and can be found in the CIF file
(CCDC 1558817).
those found in other molecular silicophosphates and crystalline
silicon phosphates.4,11
Both 31P and 29Si NMR spectra reveal only one resonance
with the expected chemical shifts, and all four peaks of
monosubstituted phenyl rings (ortho, meta, para, ipso) are
observed in the 13C{1H} NMR spectrum. The 1H NMR
spectrum displays a multiplet in the aromatic region (7.11−7.52
ppm). All NMR spectra agree well with the discussed structure.
IR spectroscopic data19 and elemental analyses are consistent
with the results of X-ray diffraction analysis. In addition, the
APCI-MS spectrum of 1 shows the molecular peak of the
silicophosphate cage at 1474 m/z, while EI-MS contains only
signals of cage fragments (see the Supporting Information).
NMR-Tube Reaction between 1 and B(C6F5)3 in a 1:3
Molar Ratio. Molecular Structure of 2·2CHCl3. As already
Figure 2. 31P NMR spectra of the reaction of molecular
mentioned, the phosphoryl oxygen atoms of silicophosphate
silicophosphate 1 and tris(pentafluorophenyl)borane in a molar ratio
of 1:4.
cage 1 are directed outward and therefore we tried to use them
in coordination reactions with the very strong Lewis acid
tris(pentafluorophenyl)borane. We observed an immediate
probably to uncoordinated PO moieties, similar to the
starting cage 1, which shows a chemical shift of −32.5 ppm.
The other signals are shifted upfield from the original
resonance by 8−22 ppm, attesting to the coordination of
these phosphoryl oxygens to boron.16 Moreover, the broadness
of the signal at −40.9 ppm points to some dynamic exchange
processes present in the reaction mixture, which will be
discussed later.
Since the 1:3 intensity ratio of the −29.6 and −54.0 ppm
resonances was constant and independent of the reaction
stoichiometric ratios, reaction times etc., we assumed that a tris-
adduct of silicophosphate cage and B(C6F5)3 was formed (eq
7). Subsequently, we were able to obtain several crystals from
the walls of the NMR tube, which allowed us to determine
reaction between the cage and borane after mixing in the 31P
NMR spectra (Figure 2). The predicted coordination of
phosphoryl oxygen to boron was anticipated to occur quickly,
and this is in agreement with an instantaneous change in the
NMR spectra. It was noteworthy that the 31P NMR spectra
were very similar for all the reactions performed at different
1:B(C6F5)3 ratios (1:2, 1:3, 1:4, and 1:6). We always observed
two sharp resonances at −29.6 and −54.0 ppm, which held a
constant 1:3 intensity ratio, and a broad intense signal at −40.9
ppm. After standing for a few days at room temperature, a
fourth significant signal appeared in all reaction solutions at
−47.1 ppm (Figure 2). On the basis of the NMR shifts, the
phosphoryl groups featuring a peak at −29.6 ppm belong
D
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molecular structure of 2 by the single crystal X-ray diffraction
analysis.
Ph12Si6O16P + 3B(C6F5)3 → Ph12Si6O16P ·3B(C6F5)3
4
4
(7)
The molecule of compound 2 indeed was an adduct of the
parent adamantane-like silicophosphate cage 1 and borane in a
1:3 ratio (Figure 3), as predicted on the basis of the 31P NMR
Figure 4. Molecular cores of 2P (left) and 2M (right) representing
opposite enantiomers. Each crystal of 2 is built up solely from one of
these enantiomers. The 3-fold axes are perpendicular to the paper.
111.35(13)°). These changes were not observed at the unique
uncoordinated PO4 group (around the P2 atom)this
tetrahedron remained more distorted. The Si−O bond lengths
and the O−Si−O and P−O−Si angles remained virtually
unchanged. The O−B−C and C−B−C angles were close to the
ideal tetrahedral values ranging from 107.0(3) to 108.2(2)° and
from 105.7(3) to 114.5(3)°, respectively.
These results can be compared with adducts of B(C6F5)3
with triphenyl- and triethylphosphine oxides, which were fully
described by a single-crystal X-ray diffraction analysis.15,16 In
our case, the PO bond lengths are shorter than those in the
reported compounds (1.480(2) Å vs 1.497(2) and 1.4973(17)
Å for Ph3PO and Et3PO adducts, respectively) and B−O bond
lengths (1.542(4) Å) are comparable to those in Ph3PO and
Et3PO adducts (1.538(3) Å and 1.533(3) Å, respectively). The
P−O−B angle was almost linear (178.7(2)°) in the case of
Ph3PO·B(C6F5)3, and this was explained by strong π−π
stacking interactions between the phenyl rings, which were
present in this compound. This was not the case for Et3PO·
B(C6F5)3, with the corresponding angle of 161.04(16)°.15,16
This value was slightly lower in our case (159.0(2)°) and thus
very similar to what was observed in the Et3PO·B(C6F5)3
adduct.
Figure 3. Molecular structure of 2P. CDCl3 and hydrogens are
omitted for clarity. Atoms forming the silicophosphate cage including
boron atoms are drawn as temperature ellipsoids (50% probability);
other (phenyl and pentafluorophenyl) moieties are represented using
stick models.
spectra. Interestingly, the fourth phosphoryl group remained
uncoordinated and, unlike the case for the starting precursor 1,
it is inverted and points inside the silicophosphate cage. This
molecule crystallizes as a solvate with two molecules of CHCl3
in enantiomorphic space group R3 and resembles a propeller
with C3 symmetry. The P2O6 bond coincides with the 3-fold
axis, and the molecule is chiral. The crystals, which were
characterized by a single-crystal X-ray diffraction analysis, were
stereochemically pure on the basis of Flack parameter values
(−0.027(71) and 0.001(28), respectively). A comparison of the
molecular structures of the two characterized crystals (Figure
4) clearly shows that these were built up from opposite
enantiomers, which are denoted 2M and 2P, respectively,
according to IUPAC recommendations regarding axial
chirality.20 The bond lengths, angles, and other crystal structure
parameters are virtually equal for both enantiomers. From this
point on, we will discuss only the 2P enantiomer.
The bond lengths and angles of 2P changed only slightly in
comparison to the parent silicophosphate cage 1 after the
formation of new coordination bonds (Table 1). The
differences in distances between double and single bonds in
distorted PO4 tetrahedrons (around three equivalent P1 atoms)
leveled outthe double PO bonds are elongated from
1.455(2) to 1.480(2) Å, while the single P−O bonds are
shortened (1.535(2)−1.543(2) Å). The O−P−O angles moved
closer to the ideal tetrahedral values (106.89(12)−
NMR spectroscopy and EI-MS analyses (see the Supporting
Information) showed that the molecule 2 is quite unstable
similarly to 3 (see below). The IR spectra (see the Supporting
Information) display clearly the presence of a silicophosphate
cage as well as tris(pentafluorophenyl)borane.19,21−23
Molecular Silicophosphate 3. The most intense reso-
nance in all acquired 31P NMR spectra was the broad peak at
−40.9 ppm. It decreased in intensity after standing for several
days at room temperature, while the originally minor signal at
−47.1 ppm increased significantly (Figure 2). On the basis of
these facts we concluded that some slow reaction leads to a new
molecular product with only one resonance in the 31P NMR
spectrum. The existence of a coordinate PO→B bond was
assumed due to the shift to high field in comparison with the
precursor. In order to obtain a suitable crystal for the X-ray
diffraction analysis, we performed vapor diffusion of benzene
(antisolvent) into the mother liquor. The reaction mixture of 1
with B(C6F5)3 in a 1:4 stoichiometric ratio (featuring
resonances of 2 and some reaction intermediates in 31P NMR
spectramainly the broad signal at −40.9 ppm) deposited
colorless crystalline product after standing in a closed vial for
several days in a glovebox. It displayed one resonance at −47.1
ppm in the 31P NMR spectrum measured immediately after
dissolution; however, the four signals at −29.6, −40.9, −47.1,
and −54.0 ppm characteristic for NMR reactions between
parent silicophosphate cage 1 and tris(pentafluorophenyl)-
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DOI: 10.1021/acs.inorgchem.7b01572
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
borane were observed after standing in the NMR tube for
several hours. Thus, the product was not stable in solution and
a single-crystal X-ray diffraction analysis was the most effective
and powerful characterization method.
The molecular structure consists of a Ph6Si3O8P2 silicophos-
phate cage and two B(C6F5)3 moieties bound through the
coordination PO→B bonds (Figure 5). However, the cage in
single P−O bonds are in the range from 1.5365(14) to
1.5464(14) Å. The O−P−O angles were again close to the
ideal tetrahedral values (106.76(8)−112.34(8)°). The Si−O
bond lengths and the O−Si−O and P−O−Si angles remained
virtually unchanged. The B−O bonds were longer (1.566(3)
and 1.570(3) Å) in this case in comparison to those in adduct
2, Ph3PO·B(C6F5)3, and Et3PO·B(C6F5)3.15,16 The P−O−B
angle decreased significantly to 142.01(13)° (in comparison
with 158.94(13)° in the case of 2). The angles around B1 and
B2 atoms (O−B−C and C−B−C) were in the range from
102.48(16) to 115.42(17)° and from 104.42(16) to
115.62(17)°, respectively, and deviate more from the ideal
tetrahedral values in comparison to molecule 2. Together with
P2O5 bond shortening and elongation of both B−O bonds,
this is reasonable evidence for weakening of O→B coordination
bonds in this case. This fact is in broader agreement with the
low stability of this compound in solution, as revealed by NMR
and MS analyses (discussed below).
The spectroscopic data agree well with the described adduct
structure. In the 31P NMR spectrum we observed a shift from
−32.5 ppm for the parent cage 1 to −47.1 ppm for the new
molecule 3 upon coordination to the borane. This is consistent
with what is observed in the case of methyl, ethyl, and phenyl
esters of phosphoric acid.16 It is noteworthy that in the case of
phosphine oxides the 31P NMR shifts move in the opposite
direction and the extent of the shift is often used as a measure
of Lewis acidity.16 Molecule 3 provides only one signal in its
31P NMR spectrum because the molecular symmetry in
solution increases to the ideal D3h point group and equilibrates
the two phosphorus atom environments observed in the crystal
structure. Because of the instability of the compound in
solution (see the Experimental Section and discussion above)
we were not able to acquire 13C and 29Si NMR spectra with
sufficient quality. The IR spectra (see the Supporting
Information) were described with the help of cited
literature19,21−23 and display clearly the presence of both
parts of the moleculethe silicophosphate cage as well as
tris(pentafluorophenyl)borane. The shift of absorption band of
the PO stretch to lower wavenumbers upon coordination is
probably 45 cm−1, and it is comparable to phosphine oxide
adducts with B(C6F5)3.16 However, we were not able to
distinguish the absorption band of the PO valence vibration
in the parent cage unambiguously because it was overlapped
with the absorption band of the C−H bending vibration. Both
APCI-MS and EI-MS spectra confirm the low stability of this
adduct 3, as they both show a signal at 737 m/z which belongs
to the silicophosphate cage Ph6Si3P2O8, without coordinated
B(C6F5)3 molecules. Moreover, the EI-MS contains signals of
B(C6F5)3 and its fragments. The molecular peak of the adduct 3
was not observed by either method.
Figure 5. Molecular structure of 3. C6H6 and hydrogens are omitted
for clarity. Atoms forming the silicophosphate cage including boron
atoms are drawn as temperature ellipsoids (50% probability); other
(phenyl and pentafluorophenyl) moieties are represented using stick
models.
this case is only half of the cage 1 we used as a precursor and it
contains only two phosphorus atoms connected through three
O−SiPh2−O bridges. The Si atoms still bear phenyl groups in
Ph2Si moieties. The molecule crystallizes as a solvate with two
molecules of benzene (eq 8).
Ph12Si6O16P + 4B(C6F5)3 → 2Ph6Si3O8P2·2B(C6F5)3
(8)
4
Four P−O−Si bonds had to be broken and formed again in
order to divide the former silicophosphate cage 1 into two new
cages 3. This was effectively but slowly accomplished by the
strong Lewis acid B(C6F5)3. Very strong electron-withdrawing
properties of B(C6F5)324 probably weaken the P−O−Si bridges
and allow for their reorganization, which affords the product
with relatively low stability in solution.
The reorganization of the P−O−Si bonds and probably also
the instability of the PO→B coordination bonds lead to the
broad signal at −40.9 ppm in the 31P NMR spectrum, which
was only slowly transformed into the sharp peak of the new
borane adduct of silicophosphate cage 3. We assume that the
instability of the PO→B coordination bonds could be one of
the possible reasons for the signal broadening in the 31P NMR
spectra, because the broad signal splits at −60 °C into four
resonances at −29.8, −31.1, −48.7, and −50.4 ppm (Figure S2
in the Supporting Information), which belong to some reaction
intermediates. The two downfield signals correspond most
likely, according to their chemical shift, to uncoordinated
phosphoryl groups.
CONCLUSIONS
■
This work describes the synthesis and complete character-
ization of new silicophosphate cage molecules. An adamantane-
like structure (1) with four phosphorus atoms connected via six
O−SiPh2−O bridges was prepared by nonhydrolytic con-
densation of Ph2Si(OC(O)CH3)2 and OP(OSiMe3)3. This
reaction principle based on CH3C(O)OSiMe3 elimination
could become useful for the preparation of other molecular
silicophosphates and -phosphonates. The four phosphoryl
oxygen atoms in 1 are directed outside the cage and offered
the possibility for use in coordination chemistry. We reacted
this silicophosphate cage 1 with Lewis acidic B(C6F5)3 and
The bond lengths and angles in 3 differ again only slightly
from the previously described structures (Table 1). The double
P1O1 bond is only slightly elongated to 1.4787(14) Å, and it
is almost similar to the respective bond in silicophosphate−
borane adduct 2 (1.480(2) Å). However, the second double
P2O5 bond is significantly shortened to 1.4668(14) Å. The
F
DOI: 10.1021/acs.inorgchem.7b01572
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(3) Makart, H. Untersuchungen an Siliciumphosphaten. Helv. Chim.
Acta 1967, 50 (2), 399−405.
observed both coordination and reorganization of the P−O−Si
bonds. Two new products were obtained and structurally
characterized by the X-ray diffraction analysis. In the first of
them the adamantane-like structure is preserved, and it forms
the adduct of 2 with borane in a 1:3 ratio. One phosphoryl
oxygen remained free and points inside the cage. The molecule
can be described as a chiral C3 propeller. The compound
crystallizes as a conglomerate of homochiral crystals. Crystals of
both enantiomers 2P and 2M were obtained and structurally
characterized. In the second product we observed only half of
the parent adamantane-like cage forming the adduct 3 with
B(C6F5)3 in a 1:2 ratio. The four P−O−Si bonds had to be
broken and formed again during the reaction in order to build
this smaller silicophosphate cage. The presented molecules
could serve as models for structural building units present in
the silicophosphate xerogels. We will be investigating further
reactivity modes of 1 with other reagents.
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ASSOCIATED CONTENT
* Supporting Information
■
S
(10) Murugavel, R.; Prabusankar, G.; Walawalkar, M. G. Organic
Soluble Silicophosphonate [RSi(OH){OP(O)(H)(OH)}]2O (R =
(2,6-I-Pr2C6H3)NSiMe3): The First Silicophosphonate Containing
Free Si−OH and P−OH Groups. Inorg. Chem. 2001, 40 (6), 1084−
1085.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01572.
Characterization details and spectroscopic data (PDF)
(11) Vaugeois, Y.; De Jaeger, R.; Levalois-Mitjaville, J.; Mazzah, A.;
Accession Codes
Worle, M.; Grutzmacher, H. Synthesis and Structure of New Eight-
̈
̈
CCDC 1555739, 1555762−1555763, and 1558817 contain the
supplementary crystallographic 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.
Membered Si−O−λ5σ4−P Heterocycles. New J. Chem. 1998, 22 (8),
783−785.
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Silicophosphorsaure Und Ihre Derivate. Z. Anorg. Allg. Chem. 1963,
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325 (1−2), 26−42.
(13) Corriu, R. J. P.; Leclercq, D.; Mutin, P. H.; Sarlin, L.; Vioux, A.
Nonhydrolytic Sol-Gel Routes to Layered metal(IV) and Silicon
Phosphonates. J. Mater. Chem. 1998, 8 (8), 1827−1833.
(14) Styskalik, A.; Skoda, D.; Moravec, Z.; Abbott, J. G.; Barnes, C.
E.; Pinkas, J. Synthesis of Homogeneous Silicophosphate Xerogels by
Non-Hydrolytic Condensation Reactions. Microporous Mesoporous
Mater. 2014, 197, 204−212.
AUTHOR INFORMATION
Corresponding Author
*J.P: tel, +420549496493; fax, +420549492443; e-mail,
jpinkas@chemi.muni.cz.
■
(15) Beckett, M. A.; Brassington, D. S.; Coles, S. J.; Hursthouse, M.
B. Lewis Acidity of Tris(pentafluorophenyl)borane: Crystal and
Molecular Structure of B(C6F5)3·OPEt3. Inorg. Chem. Commun.
2000, 3 (10), 530−533.
(16) Beckett, M. A.; Brassington, D. S.; Light, M. E.; Hursthouse, M.
B. Organophosphoryl Adducts of Tris(pentafluorophenyl)borane;
Crystal and Molecular Structure of B(C6F5)3·Ph3PO. J. Chem. Soc.,
Dalton Trans. 2001, 1768−1772.
ORCID
Jiri Pinkas: 0000-0002-6234-850X
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
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silanetriols. II. Crystalline Phenylsilanetriol and Phenylsilanetriol-d3.
Appl. Spectrosc. 1978, 32 (5), 469−479.
The results of this research have been acquired within the
CEITEC 2020 (LQ1601) project with financial contribution
made by the MEYS CR within special support paid from the
National Program for Sustainability II funds. CIISB research
infrastructure project LM2015043 funded by MEYS CR is
gratefully acknowledged for the financial support of the
measurements at the CF X-ray Diffraction and Bio-SAXS and
the Josef Dadok National NMR Centre. The authors thank Dr.
M. Bittova for MS measurements, Dr. K. Novotny and L.
Simonikova for ICP-OES analyses, and Dr. Z. Moravec for TG-
DSC measurements.
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