Two new "onium" fluorosilicates, the products of interaction of fluorosilicic acid with 12-membered macrocycles: Structures and spectroscopic properties

Article abstract of DOI:10.1039/b703645k

Two novel compounds, (L¹H)₂[SiF₆] ·2H₂O (1) and (L²H)₂[SiF ₅(H₂O)]₂·3H₂O (2), resulting from the reactions of H₂SiF₆ with 4′-aminobenzo-12- crown-4 (L¹) and monoaza-12-crown-4 (L²), respectively, were studied by X-ray diffraction and characterised by IR and ¹⁹F NMR spectroscopic methods. Both complexes have ionic structures due to the proton transfer from the fluorosilicic acid to the primary amine group in L¹ and secondary amine group incorporated into the macrocycle L². The structure of 1 is composed of [SiF₆]^2- centrosymmetric anions, N-protonated cations (L¹H)⁺, and two water molecules, all components being bound in the layer through a system of NH...F, NH...O and OH...F hydrogen bonds. The [SiF ₆]^2- anions and water molecules are assembled into inorganic negatively-charged layers via OH...F hydrogen bonds. The structure of 2 is a rare example of stabilisation of the complex anion [SiF ₅(H₂O)]^-, the labile product of hydrolytic transformations of the [SiF₆]^2- anion in an aqueous solution. The components of 2, i.e., [SiF₅(H₂O)] ^-, (L²H)⁺, and water molecules, are linked by a system of NH...F, NH...O, OH...F, OH...O hydrogen bonds. In a way similar to 1, the [SiF₅(H₂O)]^- anions and water molecules in 2 are combined into an inorganic negatively-charged layer through OH...F and OH...O interactions. The Royal Society of Chemistry.

Full text of DOI:10.1039/b703645k

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PAPER
www.rsc.org/dalton | Dalton Transactions
Two new “onium” fluorosilicates, the products of interaction of fluorosilicic
acid with 12-membered macrocycles: structures and spectroscopic properties
Vladimir O. Gelmboldt,a Eduard V. Ganin,b Marina S. Fonari,*c Yurii A. Simonov,c Larisa V. Koroeva,a
Alim A. Ennan,a Stepan S. Basok,d Sergiu Shova,c Hanspeter Ka¨hlig,e Vladimir B. Arion f and
Bernhard K. Keppler f
Received 12th March 2007, Accepted 8th May 2007
First published as an Advance Article on the web 9th May 2007
DOI: 10.1039/b703645k
Two novel compounds, (L1H)2[SiF6]·2H2O (1) and (L2H)2[SiF5(H2O)]2·3H2O (2), resulting from the
reactions of H2SiF6 with 4ꢀ-aminobenzo-12-crown-4 (L1) and monoaza-12-crown-4 (L2), respectively,
were studied by X-ray diffraction and characterised by IR and 19F NMR spectroscopic methods. Both
complexes have ionic structures due to the proton transfer from the fluorosilicic acid to the primary
amine group in L1 and secondary amine group incorporated into the macrocycle L2. The structure of 1
is composed of [SiF6]2− centrosymmetric anions, N-protonated cations (L1H)+, and two water
molecules, all components being bound in the layer through a system of NH · · · F, NH · · · O and
OH · · · F hydrogen bonds. The [SiF6]2− anions and water molecules are assembled into inorganic
negatively-charged layers via OH · · · F hydrogen bonds. The structure of 2 is a rare example of
stabilisation of the complex anion [SiF5(H2O)]−, the labile product of hydrolytic transformations of the
[SiF6]2− anion in an aqueous solution. The components of 2, i.e., [SiF5(H2O)]−, (L2H)+, and water
molecules, are linked by a system of NH · · · F, NH · · · O, OH · · · F, OH · · · O hydrogen bonds. In a way
similar to 1, the [SiF5(H2O)]− anions and water molecules in 2 are combined into an inorganic
negatively-charged layer through OH · · · F and OH · · · O interactions.
state and structurally described as host–guest complexes with aza-
and 1,10-diaza-18-crown-6,13 cis,syn,cis-dicyclohexyl-18-crown-
Introduction
614 and 18-crown-6,15,16 respectively. It is noteworthy that the
smaller 1,7-diaza-15-crown-517 and azamacrocycles18 exclusively
form the anhydrous or hydrated ammonium hexafluorosilicates.
It should, however, be noted that the complexing ability of
other macrocycles, e.g. 12-crown-4 and its substituted derivatives
in the reaction with FSA remains practically unexplored. The
present study is carried out in continuation of our previous
systematic investigations11,19 and is devoted to the specific features
of FSA complex formation with the macrocyclic ligands. We report
herein the synthesis, spectroscopic characterisation and structural
organisation of compounds resulting from the reaction of FSA
with two 12-membered macrocycles, 4ꢀ-aminobenzo-12-crown-4
(L1) and monoaza-12-crown-4 (L2) (Scheme 1).
The products of reaction of fluorosilicic acid (FSA) with organic
bases, the fluorosilicates of the relevant “onium” cations, find ap-
plication as inorganic–organic hybrid functional materials,1 ionic
liquids2 and reagents for modified zeolite catalytic agents,3–5 and in
the synthesis of silicon tetrafluoride complexes,6,7 in the production
of fungicides,8 and in the preparation of herbicide-containing
mixtures.9 In addition, these compounds are of interest for the
stabilisation of various fluorido complexes of silicon, the products
of the hydrolytic transformations of FSA.10,11 The equilibria in
the FSA solution, and number and composition of the co-existing
silicon fluorido complexes, still remain a subject of debate.10–12
The most effective reagents for the selective stabilisation of certain
silicon fluorido complexes still remain 18-membered crown- and
azacrown-ethers. In particular, the labile products of hydrolytic
transformations of the [SiF6]2− anion, viz. complexes [SiF5(H2O)]−,
[SiF5]− and trans-[SiF4(H2O)2], have been stabilised in the solid
aPhysico-Chemical Institute of Environment and Human Protection, Min-
istry of Education and Science of Ukraine and National Academy of
Sciences of Ukraine, Preobrazhenskaya str., 3, 65026, Odessa, Ukraine.
E-mail: eksvar@ukr.net
bOdessa State Environmental University, Lvovskaya str., 15, 65016, Odessa,
Ukraine
Scheme 1 12-Membered crown ethers used in this study.
cInstitute of Applied Physics, Academy of Sciences of Moldova, Academy
str. 5, MD2028, Chisinau, Moldova. E-mail: fonari.xray@phys.asm.md
dA. V. Bogatsky Physico-Chemical Institute, National Academy of Sciences
of Ukraine, Lustdorfskaya doroga 86, 65080, Odessa, Ukraine
Results and discussion
eInstitute of Organic Chemistry of the University of Vienna, Wa¨hringerstr.
38, A-1090, Vienna, Austria
Synthesis
4ꢀ-Aminobenzo-12-crown-4 (L1) was prepared in 3 steps from
pyrocatechol following the modified Pedersen procedure.20
f
Institute of Inorganic Chemistry of the University of Vienna, Wa¨hringerstr.
42, A-1090, Vienna, Austria
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Aza-12-crown-4 (L2) was obtained following the protocol reported
in the literature.21 Compounds 1 and 2 were synthesised by mixing
a methanol solution of the corresponding crown ether and an
aqueous solution of fluorosilicic acid in a 1 : 3 molar ratio. Crystals
suitable for X-ray study were obtained by slow solvent evaporation
at room temperature.
The components are bound by a network of charge-assisted
hydrogen bonds of NH · · · F, NH · · · O and OH · · · F types (Fig. 1).
The [SiF6]2− anion has an usual distorted octahedral geometry with
˚
the Si–F distances in the range 1.6588(15)–1.6719(11) A, F(cis)–
Si–F(cis) bond angles being close to 90◦ and F(trans)–Si–F(trans)
angles equal to 180◦. Each of the [SiF6]2− octahedra is involved in
four bifurcated NH · · · F hydrogen bonds with ammonium groups
of four macrocycles and in four OH · · · F hydrogen bonds with four
water molecules as multi-center acceptors via F(2) and F(3) atoms
(Fig. 1, Table 1). The water molecules act as H-acceptors towards
the ammonium group via a single NH · · · O hydrogen bond, while
as H-donors they bridge two symmetry-related [SiF6]2− anions via
one single and one bifurcated OH · · · F hydrogen bond. These
latter interactions result in the negatively-charged layer (Fig. 2)
that runs perpendicular to the [100] direction in the crystal.
Crystal structures
Compound (L1H)2[SiF6]·2H2O (1) crystallises in the monoclinic
space group P21/c. The asymmetric unit of 1 consists of one half
of the hexafluorosilicate anion in partial position on an inversion
center, one ammonium-benzo-12-crown-4 cation and one water
molecule, both occupying the general positions (Fig. 1). The
FSA molecule protonates the primary amine group of the crown
ether molecule. Complex 1 is composed of the [SiF6]2− anion, two
macrocyclic cations and two water molecules.
Fig. 2 Fragment of the inorganic negatively-charged layer in 1.
The macrocyclic ammonium cations are joined via NH · · · F and
NH · · · O hydrogen bonds in a centrosymmetric mode to the both
sides of this inorganic sheet, forming its two hydrophobic surfaces
(Fig. 3).
Fig. 1 ORTEP view of 1 with the atom numbering scheme. Displacement
ellipsoids are drawn at the 50% probability level. Dashed lines represent
the hydrogen bonds.
The host–guest complex (L2H)2[SiF5(H2O)]2·3H2O (2) is a quite
rare example of stabilisation of the complex anion [SiF5(H2O)]−,
Table 1 Hydrogen bonding geometry for 1 and 2
d(DHA)/◦
Symmetry transformation for acceptor
˚
˚
˚
D–H · · · A
d(D–H)/A
d(H · · · A)/A
d(D · · · A)/A
Complex 1
1
2
N(1)–H(1A) · · · F(3)
N(1)–H(1A) · · · F(2)
N(1)–H(1B) · · · F(2)
0.95(3)
0.95(3)
0.86(3)
0.86(3)
0.97(2)
0.90(4)
0.90(4)
0.80(4)
0.80(4)
1.86(3)
2.48(3)
2.10(3)
2.20(3)
1.74(2)
1.91(4)
2.60(4)
2.24(4)
2.26(4)
2.731(2)
3.235(2)
2.903(2)
2.883(2)
2.701(3)
2.790(3)
3.280(3)
2.915(3)
2.949(3)
151(3)
136(2)
156(2)
137(2)
174(2)
166(3)
133(3)
143(3)
145(3)
x, −y + 12 , z +
−x, y − 12 , −z +
3
2
x, y, z
N(1)–H(1B) · · · F(3)
x, y, z
N(1)–H(1C) · · · O(1W)
O(1W)–H(1W1) · · · F(1)
O(1W)–H(1W1) · · · F(2)
O(1W)–H(2W1) · · · F(1)
O(1W)–H(2W1) · · · F(2)
Complex 2
x, y, z
x, y, z + 1
−x, −y + 1, −z + 2
1
x, −y + 32 , z +
2
1
2
x, −y + 32 , z +
N(1)–H(1N1) · · · F(4)
N(1)–H(1N1) · · · F(3)
N(1)–H(2N1) · · · O(2)
N(1)–H(2N1) · · · O(1)
N(2)–H(1N2) · · · O(5)
N(2)–H(1N2) · · · O(6)
N(2)–H(2N2) · · · F(8)
N(2)–H(2N2) · · · F(7)
O(1W)–H(1W1) · · · F(10)
O(1W)–H(2W1) · · · O(3W)
O(2W)–H(1W2) · · · O(4W)
O(2W)–H(2W2) · · · F(3)
O(3W)–H(1W3) · · · F(7)
O(3W)–H(2W3) · · · F(9)
O(4W)–H(1W4) · · · F(5)
O(4W)–H(2W4) · · · F(1)
0.83(3)
0.83(3)
0.83(3)
0.83(3)
0.86(3)
0.86(3)
0.86(3)
0.86(3)
0.85(4)
0.88(2)
0.87(2)
0.86(3)
0.83(3)
0.79(3)
0.84(4)
0.79(3)
2.02(3)
2.46(2)
2.09(3)
2.38(3)
2.04(3)
2.38(3)
1.94(3)
2.47(3)
1.78(4)
1.72(2)
1.71(2)
1.77(3)
1.97(3)
1.96(4)
1.90(4)
1.98(3)
2.829(2)
2.982(2)
2.811(2)
2.785(2)
2.786(2)
2.799(2)
2.786(2)
3.079(2)
2.629(2)
2.588(2)
2.569(2)
2.626(2)
2.794(2)
2.733(2)
2.744(2)
2.754(2)
164(2)
121(2)
146(2)
111(2)
145(2)
111(2)
169(2)
128(2)
175(3)
171(3)
169(3)
172(3)
172(3)
168(3)
173(3)
167(3)
x, y, z
x, y, z
x, y, z
x, y, z
x, y, z
x, y, z
x, y, z
x, y, z
1
2
x, −y − 12 , z +
x, y, z
x, y, z
x, −y + 12 , z −
1
2
x, −y + 12 , z +
1
2
−x + 1, y + 12 , −z +
1
2
x, −y − 12 , z −
1
2
−x, y − 12 , −z +
1
2
2916 | Dalton Trans., 2007, 2915–2924
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Fig. 4 ORTEP view of two (L2H)2[SiF5(H2O)]2·3H2O units in 2 with the
atom numbering scheme. Displacement ellipsoids are drawn at the 50%
probability level. Dashed lines represent the hydrogen bonds.
Fig. 3 Packing of the layers in 1.
the labile product of hydrolytic transformations of [SiF6]2−. The
asymmetric unit of 2 consists of two 12-membered macrocyclic
cations, two [SiF5(H2O)]− anions and three water molecules, one of
which, O(5W), has been found to show 25% occupancy. In the crys-
tal each of two crystallographically independent FSA molecules
protonates the NH-binding site of the independent macrocycle and
is linked with the latter via a bifurcated NH · · · F hydrogen bond
(Fig. 4, Table 1) with formation of two neutral units. The second
hydrogen of the crown ether ammonium group is directed inside
the cavity and is involved in the bifurcated NH · · · O hydrogen
bond responsible for the folded conformation of both cations.
The [SiF5(H2O)]− anion has a distorted octahedral geometry with
coordinated to the silicon atoms’ water molecules, are involved
in the intermolecular interactions, and the layer is built up of
two alternative ribbons. The first one represents the alteration
of five H-bonded unique rings with the following graph set
notations:22 R4 (12), R3 (10), R3 (12) [taking into account non-
localised H-atoms of the O(5W) water molecule, the corresponding
O(5W) · · · F(9) and O(5W) · · · F(9)i (i = 1 − x, −1 − y, −z)
4
3
5
5
3
˚
separations being 2.664(8) and 2.856(8) A], R4 (12), and R3 (10).
The next ribbon with the edges shared with the previous one is
6
3
4
built up of four unique alternating rings, R6 (16), R3 (10), R4 (12)
3
and R3 (10).
˚
the Si–F distances in the range 1.6424(12)–1.6979(11) A. The
As is evident from Fig. 5, the two water molecules coordinated
to two silicon atoms [O(1W) and O(2W)] and the two lattice
[O(3W) and O(4W)] water molecules use the maximal number
of their proton-donor capabilities within the layer: all water
molecules act as double donors and all but O(5W) act as single
acceptors (via coordination or hydrogen bond) within this layer.
As we have already noted, this is only the third example of
crystal structures that include [SiF5(H2O)]− anions and proto-
nated N,O-containing macrocycles [those previously reported were
complexes (L3H·H2O)[SiF5(H2O)]·H2O and (L4H2)[SiF5(H2O)]2,
where L3 = aza-18-crown-6 and L4 = 1,10-diaza-18-crown-6].13
The supramolecular architecture of these related compounds
reveals an essential difference: the {[SiF5(H2O)]−}2 units are joined
˚
distances Si–O(H2O) adopt the values 1.8416(15) and 1.8675(14) A
in two independent anions, the F(cis)–Si–F(cis) bond angles being
87.32(6)–93.94(6)◦, F(trans)–Si–F(trans) angles being in the range
173.21(6)–176.65(7)◦, H2O(cis)–Si–F(cis) angles being 85.99(6)–
89.10(6)◦ and H2O(trans)–Si–F(trans) angles adopting the values
178.31(7) and 177.89(7)◦ in two independent anions, respectively
(Fig. 4). The geometry of the [SiF5(H2O)]− anion is in agreement
with the reported data.13,19
As in 1, the inorganic anions [SiF5(H2O)]− and water molecules
in 2 form the negatively-charged layers via OH · · · F and OH · · · O
interactions (Fig. 5). The fluoride ligands of both [SiF5(H2O)]−
anions, which are situated in approximately the same plane as those
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the hexafluorosilicate anion may be a suitable component for ionic
liquids when paired with an appropriate linear or planar base. The
family of hexafluorosilicates of aromatic bases is restricted to a
few examples26–29 with different networks of hydrogen bonding in
these compounds resulting in different final supramolecular ar-
chitectures. Thus, in complexes [o-CH3O(O)CC6H4NH3]2[SiF6]26
and [o-C6H4(NH3)2]2[SiF6]29 all fluoride ligands of the [SiF6]2−
anions are involved in hydrogen-bonding. In the case of [o-
CH3O(O)CC6H4NH3]2[SiF6] the components form chains, and
the chains are involved in stacking interactions via benzene
rings. The general topology of the structure [o-C6H4(NH3)2]2SiF6
corresponds to a 2D network formed by [o-C6H4(NH3)2]2+ cations
and [SiF6]2− anions. The hydrates of p-bromoanilinium hexaflu-
orosilicate and p-toluidinium hexafluorosilicate28 both also form
2D structures, where the hexagonal H-bonded network built up of
[SiF6]2− anions, ammonium cations and bridging water molecules
is surrounded in a centrosymmetric manner by the aromatic rings
of the cations arranged perpendicular to the inorganic network
and forming two hydrophobic layer surfaces.
The use of 4-aminobenzenesulfonamide or the 4-aminobenzoic
acid in the reaction with fluorosilicic acid results in the correspond-
ing ammonium hexafluorosilicates29 with a complex system of
hydrogen bonding that combines the components into a 3D grid.
The presence of a tetrahedral ammonium group in L1 provides the
only 2D supramolecular architecture in 1. In the crystal structure
there are no interactions between H-donor groups and oxygen
atoms of the polyether ring, indicating that 1 is not a host–guest
complex. Instead, it can be categorised as the salt-like adduct
of functionalised arylammonium cation. The starting thermolysis
temperature, t0, for compound 1 and for a number of hexafluo-
rosilicates of ortho-substituted anilinium hexafluorophosphates26
(Table 3) has been determined, showing a correlation between the
t0 values and pKa values of the corresponding arylamines.
Fig. 5 Fragment of the inorganic negatively-charged layer in 2.
into centrosymmetric dimers or in chains due to incorporation of
lattice water molecules, and the hybrid organic–inorganic layer
combines these inorganic motifs and organic cations. Compound
2 is the first example of layer organisation of the negatively-charged
species by incorporation of additional lattice water molecules.
This leads to different types of anion–anion associations with
water molecules mediating the interaction between neighbouring
[SiF5(H2O)]− anions.
In a manner similar to 1, the macrocycles are attached to this
inorganic layer using the axial fluoride ligands that are not involved
in the intermolecular interactions within the inorganic layer, Fig. 6.
It is worth noting that none of the products resulting from
reactions of crown ethers or their aza-analogues with FSA are
inclusion complexes.11,19 The interaction between the inorganic
“guest” and macrocycle occurs exclusively via a net of hydrogen
bonds. Therefore the influence of the macrocycle structure on the
identity of the extracted from aqueous solution of FSA inorganic
species should be considered from the point of view of their ability
to form hydrogen bonds. Starting from different macrocycles, we
succeeded to stabilise (along with [SiF6]2−) the labile products
of its hydrolytic transformations, viz., [SiF5(H2O)]−, [SiF5]− and
[SiF4(H2O)2]. In particular, the homoleptic compounds with
equivalently sterically-shielded macrocycle sides, viz., 18-crown-
6, 15-crown-5 and 12-crown-4, stabilise selectively the trans-
[SiF4(H2O)2] species, whereas the cis,syn,cis-dicyclohexane-18-
crown-6, with non-equivalently shielded macrocycle sides, forms
an ionic complex with [SiF5]− anion. The observed relationship
between the crown-ether structure and the composition of the
bound fluoro-containing inorganic complex (molecular or ionic)
has also been documented for related “host–guest” systems based
on neutral BF3·H2O and anionic [BF4]−.11 Stabilisation of the
[SiF5(H2O)]− anion has been observed when azacrown-ethers with
the N-to-O ratio in favour of oxygen have been used as “host”
molecules and organic bases.
Fig. 6 Packing of the layers in 2.
It is well known that FSA does not exist as an individual
anhydrous compound H2SiF6.23 It crystallises as low-stability
hydrates of the general formula H2SiF6·nH2O (n = 4, 6, 9.5),
which represent the oxonium salts of composition (H5O2)2SiF6,
(H5O2)2[SiF6]·2H2O and (H5O2)2[SiF6]·4.5H2O.24 Recently, the
relatively stable (mp ≥60 ◦C) “simplest hydrate” H2SiF6·2H2O
was obtained, and its structure was described as an oxonium
hexafluorosilicate, (H3O)2[SiF6].25 Despite its high symmetry (Oh),
Obviously, in each particular case upon interaction of FSA or
similar equilibrium system with the corresponding macrocycle,
the species is sequestered from aqueous solution, for which the
2918 | Dalton Trans., 2007, 2915–2924
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Table 2 Crystal data and structure refinement parameters for 1 and 2
Compound
1
2
Chemical formula
Formula weight
Temperature/K
C24H40F6N2O10Si
658.67
100(2)
C16H46F10N2O11Si2
688.73
100(2)
˚
Wavelength/A
0.71073
0.71073
Crystal system
Space group
Monoclinic
P21/c
21.8548(6)
8.0635(2)
8.5300(2)
98.599(2)
1486.31(7)
2, 1.472
Monoclinic
P21/c
14.8889(7)
9.3635(5)
20.3291(9)
93.900(3)
2827.6(2)
4, 1.618
0.247
˚
a/A
˚
b/A
˚
c/A
b/◦
3
˚
V/A
Z, qcalc/g cm−3
l/mm−1
0.173
F(000)
692
1448
h range/◦
1.88–30.82
−31 ≤ h ≤ 31
−11 ≤ k ≤ 11
−12 ≤ l ≤ 12
33020/4660 [R(int) = 0.0296]
1.111
R1 = 0.0493,
wR2 = 0.1370
R1 = 0.0752,
wR2 = 0.1515
0.380/−0.335
2.35–27.50
−19 ≤ h ≤ 19
−12 ≤ k ≤ 12
−26 ≤ l ≤ 26
88034/6465 [R(int) = 0.0409]
1.063
R1 = 0.0410,
wR2 = 0.1148
R1 = 0.0512,
wR2 = 0.1199
0.594/−0.380
Limiting indices
Reflections collected/unique
Goodness-of-fit on F2
Final R indices [I > 2r(I)]
R indices (all data)
−3
˚
Largest diff. peak and hole, e A
at 1130 cm−1 with the shoulder at 1100 cm−1 is found for 1, while
the band at 1090 cm−1 changes its intensity but not the position.
In addition, the vibrations ms(COC) appear in the spectrum as an
intense singlet at ca. 1040 cm−1, instead of two bands of medium
and weak intensities at 1060 and 1045 cm−1 in the spectrum of L1.
We note that the assignment of the skeleton vibrations of the
macrocycle in the spectra of L1 and 1 (Table 4) was made taking
into account the data for the related monobenzo-12-crown-4.30
The IR spectrum of 2 (Table 4) shows the m(NH) vibrations of the
protonated secondary amine group and the m(OH) vibrations of
coordinated and lattice water molecules as broad bands of medium
intensity with weakly pronounced maxima between 3590 and
3110 cm−1. The position of higher frequency components at 3590
and 3550 cm−1 is in line with the X-ray data, indicating the presence
of outer-sphere water molecules in 2. The complex band with a
maximum at 1585 cm−1 and shoulders around 1725, 1655 and
1625 cm−1 was assigned to deformation vibrations d(H2O), d(NH2)
and d(CNH). Intraligand vibrations of macrocycle attributed to
the fragments NCH2CH2O and OCH2CH2O [mas(CNC), mas(COC)
in the range 1170–1050 cm−1, and ms(COC), ms(CNC) and m(CC),
m(CN) in the range 1050–940 cm−1] were also observed. A marked
difference between the IR spectra for L2 and 2 in the range of the
mas,s(CNC) and mas,s(COC) vibrations, as well as in the range typical
of valence-deformation vibrations of the ligand (1000–800 cm−1)
indicates the change of conformation and macrocycle symmetry
upon complex formation.
Table 3 Relationship between the pKa of arylamines and thermolysis
temperatures t0 of the corresponding arylammonium hexafluorosilicates26
Amine
pKa
4.63a
4.49
4.39
2.64
2.60
2.23
2.11
t0/◦C
L1
190b
175
135
125
125
115
140
100
o-CH3OC6H4NH2
o-CH3C6H4NH2
o-ClC6H4NH2
o-BrC6H4NH2
o-CH3O(O)CC6H4NH2
o-HO(O)CC6H4NH2
o-O2NC6H4NH2
−0.29
a Calculated value. b For the anhydrous form.
most effective stabilisation of the host–guest complex by a net
of hydrogen bonding interactions is achieved. In addition, lattice
water molecules often act as bridging species between “inorganic”
and “organic” fragments with formation of supramolecular archi-
tecture of the host–guest complex.
Infrared spectra
Broad bands of medium intensity between 3650 and 3030 cm−1 in
the IR spectrum of 1 (Table 4) have been assigned to the stretching
+
vibrations of the NH3 groups and water molecules involved
in the hydrogen bonding network. The deformation vibrations
d(NH3) and d(H2O) were registered at 1655 and 1620 cm−1,
correspondingly. Absorptions at 705, 465 and 440 cm−1 were
attributed to the m(SiF) and d(SiF2) vibrations of the [SiF6]2−
anion, respectively. The character of absorption changes in
the range 1145–1095 cm−1, where the bands assigned to the
conformationally-sensitive mas(COC) vibrations of macrocyclic
fragment were registered: instead of the observed for L1 doublet
band of medium intensity at 1145 and 1115 cm−1, an intense band
In accordance with the X-ray data, the interpretation of the
vibrations of the SiF5O polyhedron was carried out using an
approximation of point group symmetry C4v. Thus, the most
intense component (760 cm−1) of the complex absorption band
in the range 800–700 cm−1 includes the m(SiFe) (A1) vibration,
and a clearly pronounced shoulder around 720 cm−1 – vibrations
m(SiFa) (A1). The band of doubly degenerate vibrations m(SiF) (E)
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Table 4 IR spectral data for 1 and 2 and free organic bases
L1
1
L2
2
3560 m
3455 m
3415 m w
3480 m w
3590 m w
3550 m w
3400 m w
3430 m
3345 m w
3170 m w
3300 m w
3090 m w
3130 m w
3110 sh
3230 m w
3110 sh
3045 sh
1620 m
3030 sh
1655 m
1620 m
1655 sh
1635 m
1620 m
1725 sh
1655 sh
1625 sh
1585 s
1585 sh
1595 s
1555 m
1510 s
1585 s
1570 sh
1505 s
1360 sh
1320 m
1295 m
1275 m
1240 m
1220 sh
1340 sh
1320 m
1360 sh
1340 sh
1275 m
1240 m
1220 sh
1280 s
1240 s
1220 sh
1285 m
1240 sh
1220 s
1180 m
1165 m
1145 m
1115 m
1090 sh
1060 m
1045 w
1030 s
1160 sh
1130 s
1100 sh
1090 m
1170 sh
1140 s
1130 s
1100 s
1070 s
1055 sh
1020 m
1070 m
1050 sh
1020 m
1040 m
1025 sh
965 m
915 m
900 m
885 sh
870 sh
830 m
815 sh
965 m
920 s
905 sh
890 w
855 m
930 s
900 s
940 m
920 sh
900 sh
850 m
820 sh
840 w
820 m
800 sh
765 m
750 sh
725 sh
770 sh
760 sh
705 s
780 m
755 m
720 sh
760 s
720 sh
680 sh
620 w
565 w
530 w
610 sh
565 w
530 w
620 sh
510 m
610 sh
565 sh
510 m
465 m
440 sh
465 m
420 m
Abbreviations: w: weak, m: medium, s: strong, sh: shoulder.
is revealed as a shoulder around 680 cm−1. The plane vibrations
d(SiF4) (E) were assigned to the band around 465 cm−1; out-of-
plane vibrations d(SiF2) (A1) were registered as a shoulder around
420 cm−1.
the spectrum measured at the lowest temperature of 243 K was
used for the assignment of the different fluoride complexes (Fig. 8
and Table 5).
The intense singlet at d −127.86 ppm and a heteronuclear
coupling constant J(29Si,19F) = 108 Hz corresponds to the [SiF6]2−
anion.31 Only at 243 K can the silicon satellites be observed, as
at higher temperatures the line width exceeds the J coupling. Two
signals of equal intensity with chemical shift values of −123.81 and
−136.05 ppm can be attributed to cis-SiF4·2H2O (Fig. 8). They
appear as poorly resolved triplets due to a rather small fluorine
coupling constant J(19F,19F) = 1.3 Hz at 243 K, which increases
19F NMR study on 2
19F NMR spectra for 2 in acetonitrile solution show several signals,
some of them being temperature-dependent, typical for systems
with fluorine exchange (Fig. 7). Their line widths decrease with
decreasing temperature due to the decreased exchange rate. Thus,
2920 | Dalton Trans., 2007, 2915–2924
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Fig. 7 19F NMR spectra for 2 at different temperatures (vertical scaling optimised for the minor signals, with the main signal in the inset).
Fig. 8 19F NMR spectrum of 2 at 243 K.
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Table 5 19F NMR data for complex 2 in CD3CN at 243 K
stabilise and describe the crystal structure of the trans-[SiF4(H2O)2]
isomer isolated in the solid state as a host–guest complex of com-
position (trans-SiF4·2H2O)·18-crown-6·2H2O. The possibility for
SiF4·2H2O to exist as cis- andtrans-isomers has been predicted37 on
the basis of gaseous-phase non-empirical calculations for the prod-
ucts of hydrolysis of silicon tetrafluoride. The theoretical study
revealed higher thermodynamic stability (by 5.0 kcal mol−1) for
cis-SiF4·2H2O (symmetry C2v) compared to the trans-SiF4·2H2O
(symmetry Ci) for the idealised geometries in the gaseous state.
The evident influence of the medium (solvation and other factors)
on the relative stability of the isomeric forms of SiF4·2H2O in
liquid phase has also been discussed.37
Complex
d(F)/ppm a
J/Hz
[SiF6]2−
−127.86
(29Si,19F) 108
(19F,19F) 1.3
trans-SiF4·2H2O
cis-SiF4·2H2O
HF
−133.0 (Fc)
−123.81b (Fc); −136.05b(Ft)
−137.81
−
BF4
−151.27; −152.32
a Fc, Ft: fluorine atoms cis and trans to H2O, respectively. b Incompletely
resolved triplet.
slightly to 2 Hz upon elevating the temperature to 301 K. In
addition, as the only fluorine–silicon species, these signals exhibit
the same sharp line over the whole measured temperature range.
The signal with the chemical shift d(F) around 131.96 ppm
can presumably be referred to the trans-isomer. The assignment
has been made taking into account the closeness of the chemical
shifts of cis- and trans-SiF4·2H2O with those of isomeric complexes
SiF4·2DMSO with an analogous O-donor.32 The cis-isomers of
SiF4 with the molecular N-donors show 19F signals for the vector
N–Si–F shifted to higher fields.33,34 The broad signal with the
chemical shift at −137.8 ppm is probably due to HF or exchange
Conclusions
A novel 12-membered crown ether, 4ꢀ-aminobenzo-12-crown-4
(L1), has been synthesised using the modified Pedersen procedure.
This macrocycle, containing a primary amine group, together with
another as-yet poorly exploited 12-membered crown ether, aza-
12-crown-4 (L2), with a secondary amine group incorporated into
the macrocyclic framework, were used as ligands in the reaction
with fluorosilicic acid. Two novel complexes, (L1H)2[SiF6]·2H2O
and (L2H)2[SiF5·2(H2O)]2·3H2O, were obtained and studied by
IR and 19F NMR spectroscopy and X-ray diffraction meth-
ods. In complex 1, the ligand does not use its macrocyclic
function, and the final product belongs to the family of the
aromatic ammonium hexafluorosilicates sustained by the plethora
interaction between HF, F− and HF2
,
formed as a result of
−
31
hydrolytic transformations of fluorido complexes. The strong field
signals at −151.27 and −151.32 ppm with relative intensities of
(resonance 10BF4 and 11BF4
−
35
−
−
1 : 4 refer to the anion BF4
correspondingly, with a chemical shift difference between the two
signals of about 20 Hz as the sequence of the isotopic shift 10B–
11B), which is probably formed as a by-product by reaction of HF
with the borosilicate glass of the NMR tube.34
The results obtained may be interpreted as follows: the labile
anions [SiF5(H2O)]− formed from the weak Lewis acid (SiF5
and weak Lewis base (H2O) as a result of the first step of the
hydrolysis of SiF62− anions in solutions of FSA10 can be stabilised
in the solid state as host–guest complexes with the protonated
forms of azacrown ethers. The system of charge-assisted hydrogen
bonds serves as a stabilising basis for complex 2 and related
compounds reported previously, e.g. (L3H·H2O)[SiF5(H2O)]·H2O
and (L4H2)[SiF5(H2O)]2 (L3 = aza-18-crown-6, L4 = 1,10-diaza-18-
crown-6). Upon dissolution in the absence of that stabilising factor
the anions [SiF5(H2O)]− disproportionate according to eqn (1):
of charge-assisted NH(NH3 ) · · · F− and OH(H2O) · · · F− hydro-
+
gen bonds. The majority of the bases that belong to vari-
ous classes of compounds, e.g., arylamines,26–29 alkylamines,38,39
propargylamine,40 nitrogen-containing heterocycles of the piperi-
dine or quinoline type,39 derivatives of guanine,41,42 hydrazides,
thiosemicarbazides and tetraazamacrocycles,11 form complexes
exclusively with the highly symmetric hexafluorosilicate anion,
[SiF6]2−. At the same time, only two examples of stabilisation
of the labile complex [SiF5(H2O)]− (present in solutions of
FSA) by azacrown ethers have been documented so far.13 The
complex (L2H)2[SiF5(H2O)]2·3H2O is only the third example of
[SiF5(H2O)]− anion stabilised by complex formation with a pro-
tonated N-containing crown ether. The major role in stabilisation
of the less symmetric and labile species [SiF5(H2O)]− is played
by the multiple H-bonding interactions between “organic” and
“inorganic” components of the host–guest complex.
−
)
2[SiF5(H2O)]− → [SiF6]2− + [SiF4(H2O)2]
(1)
with the formation of a more stable [SiF6]2− anion and [SiF4(H2O)2]
complex. The last species may undergo further hydrolysis with the
participation of the outer-sphere water molecules according to
eqn (2):
Experimental
General methods
All chemicals were purchased from commercial suppliers and used
as received. Elemental analyses were carried out at the Physico-
Chemical Institute of Environment and Human Protection,
Odessa, Ukraine. EI mass spectra were recorded on an MX-1321
instrument (direct sample injection into the ion source, ionisation
voltage 70 eV). Infrared spectra were recorded on a Specord 75IR
spectrophotometer (4000–400 cm−1, samples in Nujol mulls, KRS-
5 glasses). Thermogravimetric analysis (TGA) and differential
thermal analysis were performed simultan◦eously with an OD-102
Paulik–Paulik–Erdey apparatus, with a 10 C min−1 heating rate in
air. All NMR experiments were recorded on Avance spectrometers
[SiF4(H2O)2] + H2O → SiO2·H2O + 4HF
(2)
This, together with the volatility of the diaqua-complex, proba-
bly explains the rather low content of cis- and trans-SiF4·2H2O in
acetonitrile solution. We note here that the complex SiF4·2H2O,
which is a strong Brønsted acid,36 is relatively stable only in highly
acidic solutions of FSA; the decrease of solution acidity results in
the hydrolytic decomposition of the complex.
It should be also noted that 19F NMR spectroscopy data
indicate that [SiF4(H2O)2] is present in acetonitrile solution in
the form of two geometric isomers. Previously,15,16 we succeeded to
2922 | Dalton Trans., 2007, 2915–2924
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1
(Bruker Biospin GmbH, Rheinstetten, Germany), i.e. H NMR
extracted by boiling n-heptane (3 × 200 ml) for 20–25 min. The
combined extracts were evaporated to one fourth of the initial
volume and cooled to 5–10 ◦C, producing an oil that crystallised
on standing for 12–24 h. The product was thoroughly ground and
dried in vacuo. Yield: 9.9 g (83%), mp 86–88 ◦C. 1H NMR (CDCl3)
d 3.6 (s, broad, 2H, NH2), 3.80 (s, 4H, OCH2CH2O), 3.81–3.94 (m,
4H, ArOCH2CH2O), 4.00–4.16 (m, 4H, ArOCH2), 6.24 (dd, 1H,
spectra on a DPX 400 instrument (UltraShieldTM Magnet) at a
resonance frequency of 400.13 MHz, and 19F NMR spectra on
a DRX 400 WB instrument equipped with an ATM-BBO probe
(outer coil double tunable for 1H or 19F) at a resonance frequency
of 376.5 MHz. Chemical shifts were referenced internally to the
1
residual, non-deuterated solvent signal for H, or externally to
CFCl3 (d = 0 ppm) for 19F.
H6, J2 3 = 8 Hz, J3 5 = 3 Hz), 6.34 (d, 1H, H5 ), 6.72 (d, 1H, H2 ).
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
The synthesis of L1 was performed by catalytic reduction of 4ꢀ-
nitrobenzo-12-crown-4, resulting from the nitration of the initial
benzo-12-crown-4 by 50% nitric acid.
Aza-12-crown-4 (L2). Obtained following the literature
protocol.21
(L1H)2[SiF6]·2H2O (1). To a solution of L1 (0.24 g, 1.0 mmol)
in methanol (10 ml) was added 45% H2SiF6 (molar ratio 1 : 3). The
resulting solution was allowed to stand at room temperature. Slow
evaporation of methanol resulted in almost quantitative formation
of a colourless crystalline product, mp ≥180 ◦C (decomp.). Anal.
Calc. for C24H40F6N2O10Si: C 43.76, H 6.12, F 17.31, N 4.25, Si
4.26%; Found: C 43.71, H 6.16, F 17.38, N 4.33, Si 4.30%. Mass
Benzo-12-crown-4 [2,3-benzo-1,4,7,10-tetraoxacyclododeca-2-
ene]. The compound was synthesised using the modified
Pedersen procedure.19 To a solution of pyrocatechol (22.0 g,
0.20 mol) in n-butanol (300 ml) stirred under inert atmosphere
a solution of LiBr·2H2O (61.5 g, 0.50 mol) and LiOH·H2O
(17.8 g, 0.42 mol) in water (45 ml) was added. The reaction
mixture was brought to reflux and then within 5–10 min 1,8-
dichloro-3,6-dioxaoctane (38.5 ml, 0.20 mol) was added. The
mixture was allowed to react at this temperature for 28 h. After
cooling to room temperature the reaction mixture was acidified by
concentrated HCl to pH 3–4, filtered and the filtrate evaporated
to dryness. The residue was dissolved in chloroform and washed
successively with water, saturated Na2CO3 and NaCl solutions.
After chloroform removal the resulting product was extracted
by boiling heptane (2 × 100 ml). After cooling the combined
fractions yielded an oil. The supernatant was decanted. The
addition of diethyl ether to the remaining oil produced crystals of
dibenzo-24-crown-8, which were filtered off, washed ◦with diethyl
ether and dried in air. Yield: 1.2 g (27%), mp 112–113 C (lit.20 mp
113–114 ◦C). The filtrate after evaporation of diethyl ether yielded
benzo-12-crown-4, which was purified by distillation in vacuo, bp
140–145 ◦C, mp 43–44 ◦C. An analytical-grade sample was
obtained by re-crystallisation from hexane, mp 44–45 ◦C, in
accordance with the literature data19 (mp 44–45.5 ◦C). Yield:
11.7 g (26%).
spectrum: [ML1]+ (m/z = 239, I = 100%), SiF3 (m/z = 85, I =
+
45%).
(L2H)2[SiF5(H2O)]2·3H2O (2). L2 (0.18 g, 1.0 mmol) in
methanol (5 ml) was mixed with 45% H2SiF6 in a 1 : 3 molar
ratio. The resulting solution was allowed to stand at room
temperature. Slow evaporation of methanol resulted in almost
quantitative formation of a colourless crystalline product, mp
≥110 ◦C (decomp.); Anal. Calc. for C16H46F10N2O11Si2: C 27.90, H
6.73, F 27.59, N 4.07, Si 8.16%; Found: C 27.96, H 6.77, F 27.64,
N 4.14, Si 8.23%. Mass spectrum: [ML2]+ (m/z = 175, I = 4%),
[ML2 − CH2OH]+ (m/z = 144, I = 8%), SiF3 (m/z = 85, I =
+
58%).
X-Ray diffraction measurements
Diffraction intensities for two complexes were collected at 100 K
on a X8APEX II CCD diffractometer with MoKa radiation
˚
(k = 0.71073 A) in u and x scan modes. The structures were
solved by direct methods and refined by full-matrix least-squares
method on F2 using SHELXS97 and SHELXL97 programs,
respectively.44 Non-hydrogen atoms were refined anisotropically.
C-bound hydrogen atoms were placed in geometrically calculated
positions, N- and O-(water) bound hydrogen atoms were found
from difference Fourier maps and refined isotropically. The
hydrogen bonding geometries for complexes 1 and 2 are listed
in Table 1, and crystallographic data in Table 2. Crystallographic
data (CIF files) for the structural analysis of complexes 1 and 2
have been deposited with the Cambridge Crystallographic Data
Centre, CCDC reference numbers 635343 and 635344, respectively.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b703645k).
4ꢀ-Nitrobenzo-12-crown-4 [2,3-(4ꢀ-nitrobenzo)-1,4,7-tetraoxa-
cyclododeca-2-ene]. To a boiling solution of benzo-12-crown-4
(11.2 g, 0.05 mol) in acetonitrile (15 ml), 56% nitric acid (6.0 ml,
0.075 mol) was carefully added. The reaction mixture was allowed
to react at this temperature for 30 min and then poured into
the ice–water. The precipitate formed was filtered off, washed
with water until neutral pH, dried in air and re-crystallised
◦
from ethanol. Yield: 12.4 g (92%), mp 108–109 C (lit.,43 mp
105–108 ◦C).
4ꢀ-Aminobenzo-12-crown-4 [2,3-(4ꢀ-aminobenzo)-1,4,7,10-tetra-
oxacyclododeca-2-ene]. In a hydrogenation flask were placed
10% Pd/C (1.0 g) and anhydrous methanol (100 ml). The catalyst
was activated by passing a stream of hydrogen through the
suspension for 1 h. Then a solution of 4ꢀ-nitrobenzo-12-crown-4 in
methanol (100 ml) was added and the mixture was hydrogenated
for 8–10 h to complete the conversion of the substrate [TLC
control, DC-Alufolien Kieselgel 60 F254 “Merck”, eluent hexane–
acetone (1 : 1) or GC control, DIP, glass column (3 × 1200 mm), 5%
Acknowledgements
We are thankful to Mr A. Roller for the collection of X-ray
diffraction data.
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