Synthesis and coordination ability of a partially silicon based crown ether

Article abstract of DOI:10.1039/c6cc07520g

The first hybrid crown ether with two adjacent disilane fragments was synthesized through reaction of O(Si₂Me₄Cl)₂ (3) with O(C₂H₄OH)₂. By means of DFT calculations, the complexation ability of 1,2,4,5-tetrasila[12]crown-4 (7) towards Li⁺ was determined to be considerably higher compared to [12]crown-4.

Full text of DOI:10.1039/c6cc07520g

ChemComm
View Article Online
View Journal
COMMUNICATION
Synthesis and coordination ability of a partially
silicon based crown ether†
Cite this:DOI: 10.1039/c6cc07520g
a
b
c
c
Kirsten Reuter, Gunther Thiele, Thomas Hafner, Frank Uhlig and
¨
a
Received 14th September 2016,
Accepted 14th October 2016
Carsten von Hanisch*
¨
DOI: 10.1039/c6cc07520g
www.rsc.org/chemcomm
The first hybrid crown ether with two adjacent disilane fragments was
synthesized through reaction of O(Si Me Cl) (3) with O(C OH)
2
4
2
2
H
4
2
.
By means of DFT calculations, the complexation ability of 1,2,4,5-
+
tetrasila[12]crown-4 (7) towards Li was determined to be considerably
higher compared to [12]crown-4.
+
6 2
Scheme 1 Binding modes of Li in D (D = Me SiO) (left), [12]crown-4
Compared to ethers, siloxanes show low coordination ability (centre) and 1,2-disila[12]crown-4 (right) with increasing complex stability
1
towards Lewis acids. This has been attributed to both electronic from left to right.
and structural effects: the electron density of the O atom is less
available as a result of the interaction between the lone pairs of the
O atom and the s*-orbitals of the Si–C bonds. This negative
2
observed for ring contracted crown ethers like [17]crown-6 and
sila[17]crown-6. However, all hitherto described disila-crown
8
hyperconjugation is then not only competing with the coordination
3
ethers incorporate fully C or half C and half Si affected donor
of a cation, it also leads to a widening of the Si–O bond angle. On
7
atoms. In this work, we report on the synthesis and the complex
the other hand, the ionic character of the Si–O bond increases with
4
stability of a novel crown ether bearing organic, hybrid and fully
inorganic substituted O atoms.
A suitable synthesis path for the required inorganic fragment,
2 4 2
O(Si Me Cl) (3), starts with the asymmetric chlorination of 1,1,2,2-
larger Si–O angles. The high anionicity of the O atom would rather
suggest an increased coordination ability. This is especially not the
case for linear Si–O linkages, which can be related to the delocalized
5
and hardly polarizable electron density at the O atom. Additionally,
tetramethyldisilane by the use of trichloroisocyanuric acid (TCCA)
recent studies revealed that the positively polarized Si atoms in
9
6
(Scheme 2). The chlorination of the two Si atoms appears not to
siloxanes show a considerable repulsion towards cations.
happen statistically. The distribution of the three resulting disilane
species can be estimated from comparison of the intensities of the
Lately, we reported on hybrid crown ethers which partially
7
incorporate a disilane and an ethylene framework (Scheme 1). The
29
respective similar substituted Si atoms in the Si NMR spectrum
Fig. 1). 10% of H Si Me (I), 70% of HSi Me Cl (1) and 20% of
Cl Si Me (II) can thusly be detected in the reaction solution. Since
2
presence of Si -units leads to a structural equivalence of organic
(
2
2
4
2
4
and inorganic fragments with respect to the number of atoms
between the donor atom. By DFT calculations and dynamic proton
NMR studies, the stability of these hybrid disila-crown ethers
complexes was determined to be in the range of organic crown
ether complexes. Unexpectedly, the insertion of disilane units does
not reduce the coordination ability of the ligand as has been
2
2
4
a small excess of TCCA has been used, the proportion of I and II is
not precisely equal. After distillative purification, 64% of the
asymmetrically chlorinated species 1 was obtained.
a
Fachbereich Chemie and Wissenschaftliches Zentrum f u¨ r Materialwissenschaften
(
WZMW), Philipps-Universit a¨ t Marburg, Hans-Meerwein-Straße 4, 35043 Marburg,
Germany. E-mail: haenisch@chemie.uni-marburg.de; Fax: +49-6421-2825653
Department of Chemistry, University of California, 210 Lewis Hall, Berkeley,
California 94720, USA
b
c
Institut f u¨ r Anorganische Chemie, TU Graz, 6330 Graz, Austria
Electronic supplementary information (ESI) available: Experimental details, compu-
†
tational and crystallographic data. CCDC 1499542 (7) and 1499541 (8). For ESI and
Scheme 2 Synthesis path for the inorganic building block of a partially Si
crystallographic data in CIF or other electronic format see DOI: 10.1039/c6cc07520g
based crown ether.
This journal is ©The Royal Society of Chemistry 2016
Chem. Commun.

View Article Online
Communication
ChemComm
29
Fig. 1
Si{H} NMR spectrum (D
2
O-capillary, 59.60 MHz, DEPT) of the
formation of HSi
2
Me Cl after removal of the cyanuric acid.
4
Fig. 2 Molecular structure of 7 in the crystal. Thermal ellipsoids represent
the 50% probability level.
Through treatment with water, compound 1 hydrolysed and
condensed immediately in a clean reaction to O(Si Me H) (2).
2
4
2
In complex 7, the lithium cation is coordinated by the four
In case of impurities, 2 was purified via distillation (b.p. 86 1C,
0 mmHg). In the next step, the remaining H atoms at the
crown ether O atoms and one O atom of the trifluoromethane-
1
À
sulfonate anion (OTf ), giving a coordination number of five
terminal Si atoms were again substituted by the use of TCCA. In
this reaction, the purity of the educt 2 is crucial, since 3 cannot
be purified by distillation as a result of its low decomposition
temperature. Simultaneously, another synthesis path for 3 was
investigated. However, the abstraction of the phenyl groups of
O(Si Me Ph) (5) by trifluoromethanesulfonic acid was not
(
Fig. 2). The three different types of O atoms in the ligand show
+
strong deviations in their bond lengths to Li . While the completely
C and the hybrid substituted O atoms O4, O1 and O3 establish
similar bond lengths of 209.4(6) pm and 201.4(6) pm respectively,
the fully Si substituted O atom O2 exhibits a considerably long
bonding (231.0(6) pm, Table 1). These results, based on X-ray
diffraction, differ from the calculated structure of [Li(1,2,4,5-
2
4
2
1
0
successful so that this route was finally abandoned.
In high dilution, the inorganic building block 3 reacts selec-
tively with diethylene glycol and triethylamine (NEt ) to the hybrid
crown ether 1,2,4,5-tetrasila[12]crown-4 (6) (Scheme 3). Compound
is a highly viscous oil which is well miscible in DCM, THF and
+
tetrasila[12]crown-4)] , wherein the anion has not been considered.
3
+
As can be deducted from the bond lengths between Li and the
three types of O atoms, the cation is more centrally located,
with the hybrid substituted O atoms showing the shortest and
the C substituted O atom the longest bonding (Table 1). These
deviations may be a result of the hydrogen bonding between
6
n-pentane, but only poorly miscible in water. Differently to the
before described hybrid crown ethers like 1,2-disila[12]crown-4,
no decomposition of 6 was observed in water. Ligand 6 incorpo-
rates three different types of O atoms, C, C/Si and Si substituted
ones, which generates ideal conditions for comparing the respec-
tive coordination ability.
À
the anion F CSO and the H atoms of the methyl groups. The
3
3
distance between one of the H and one F atom (260.0(2) pm) is
beneath the sum of the van-der-Waals-radii. The orientation of
À
F
3
CSO
3
towards the methyl groups may push the cation in the
Lithium triflate, which is only poorly soluble in DCM, was the salt
of choice. The complexation by compound 6 in DCM was success-
fully completed after 4 h, resulting in a clear solution. Removal of
the solvent led to a colourless solid, which gave colourless rods after
recrystallization from DCM/cyclopentane. [Li(1,2,4,5-tetrasila[12]-
crown-4)OTf] (7) crystallizes in the orthorhombic space group Pbca
with sixteen molecules per unit cell. Different to the free ligand 6,
complex 7 is highly sensitive towards traces of water as has also been
direction of the carbon bonded O atom (O4).
As expected, the presence of two disilane units leads to an
increased diameter of the crown ether. The ring diameter affects –
beside the donor ability of the used anion – the position of the cation
+
in the crown ether. For example in [Li([12]crown-4)OTf] (8),‡ Li is
located 83.6(9) pm above the plane generated by the ring systems’ O
+
atoms. In the hybrid crown ether complex 7, the Li cation is located
7
only 56.8(4) pm above the plane of the ligands’ O atoms, indicating a
better matching with respect to the ring diameter. Moreover, the
adjacent disilane units lead to an asymmetric structure of the ligand
as a result of the mean Si–Si bond length of 233.8(1) pm and the
comparatively smaller mean C–C bonding of 150.1(1) pm (Fig. 3). As
reported before, C–O–C bond angles are generally smaller compared
observed for other disila-crown ether complexes.
1
1
to Si–O–Si angles. In complex 7, the bond angles widen in the
series C–O–C o C–O–Si o Si–O–Si (Table 1).
As observed in the reported disila-crown ether complexes,
the interaction p(O) - s*(Si–C) can be observed with respect to
Scheme 3 Reaction path for ligand 6.
Chem. Commun.
This journal is ©The Royal Society of Chemistry 2016

View Article Online
ChemComm
Communication
+
Table 1 Selected bond lengths and angles of compound 7, the calculated cationic complex [Li@6] and ligand 6
Selected bond lengths [pm]
Selected angles [1]
C–O–C C–O–Si
231.0(6) 167.2(2) 166.7(1) 185.8(5) 113.6(2) 123.4(2) 134.3(1)
a
b
c
a
C
c
Compound
Method Li–O
C
Li–OC/Si
Li–OSi
Si–O
Si–OSi
Si–C
Si–O–Si
d
7
XRDe
DFT
DFT
209.4(6) 201.4(6)
+
[
Li(1,2,4,5-tetrasila[12]crown-4)]
207.1
—
197.1
—
201.6
—
171.3
168.2
170.8
165.2
187.3
188.4
114.8
114.7
123.6
124.2
129.1
149.2
e
6
a
d
b
c
The subscript C denotes complete carbon-substituted O atoms. O atoms bonded to both carbon and silicon. Completely Si bonded O atoms.
e
Structures determined by X-ray diffraction. BP86 functional at the def2-TZVP level of theory.
7
the bond lengths changes between the calculated free ligand isomer 1,2,7,8-tetrasila[12]crown-4, the complexation ability of 6
8
structure and the calculated complex. Compound 6 incorpo- is considerably high. Hybrid disila-crown ethers generally require
rates two types of Si bonded O atoms: completely Si (Si–OSi) or an increased amount of energy for changing from the free ligand
Si and C bonded (Si–O ) oxygen atoms. As expected, the impact structure to the structure found within the complex compared to
C
of the negative hyperconjugation is stronger in the Si–OSi bond, organic crown ethers. This can be related to the adoption of the
which shows a considerable increase of the calculated bond ecliptic arrangement of the methyl groups in the complex, which
6
length by complexation from 165.2 pm in the free ligand to has also been observed in cyclosiloxane complexes. The methyl
1
70.8 pm in the lithium complex. The impact is smaller for the groups in complex 7 also adopt an approximately eclipsed con-
Si–O bonds, which increase from 168.2 pm to 171.3 pm (Table 1). formation with dihedral angles between 4.2(1)1–13.5(1)1. This is
Simultaneously, the eight Si–C bonds length show a minor not only the case between directly bonded Si atoms, but also
C
decrease as a result of the complexation.
consistently between the O bridged Si atoms Si2 and Si3.
2
9
In the Si{H} NMR spectrum, the two signals show a con-
Calculation of the energy differences DEgeom between the free
siderable downfield shift from d = 0.9/10.9 ppm in the free ligand and the complex geometries of 1,2,4,5-tetrasila[12]crown-4 (6) reveals
À1
À1
to 9.4/15.9 ppm in the complex, indicating a strong electron a value of 69.89 kJ mol (Scheme 5), which is 9.48 kJ mol higher
+
7
withdrawing effect through the complexation of Li . The signal compared to the constitution isomer 1,2,7,8-tetrasila[12]crown-4.
at 0.9 ppm in the free ligand corresponds to the equivalent The increased amount of energy refers to the steric hindrance of
Si atoms Si2 and Si3 (Fig. 3), which undergo a downfield shift the ecliptic Me groups between the O bridged disilane fragments.
of D(d) = 8.5 ppm, while Si1 and Si4 show a smaller shift of Although DEgeom is high, ligand 6 shows a considerably coordination
D(d) = 5.0 ppm. This can be attributed to the stronger polarisation ability (Scheme 4), which implies a significant electrostatic attraction
+
of the completely Si bonded O atom O2 in the solution, in between the completely Si substituted O atom and Li . With respect
7
comparison to the Si/C bonded O atoms O1 and O3.
to the before described complex stabilities of disila-crown ethers,
The relative binding affinity of 6 was studied by means of DFT, the basicity of the C H and/or Si Me substituted O atoms increases
2
4
2
4
1
2
13
implemented in Turbomole V7.0.1, using the BP86 functional
and the def2-TZVP basis set with inclusion of dispersion
in the series O
C
o OSi/C o OSi
.
In conclusion, the synthesis of O bridged disilane fragments
14
+
interactions. As presented in Scheme 4, the exchange of Li from like O(Si
12]crown-4 to the hybrid crown ether 6 releases 29.51 kJ mol
2
Me
4
Cl)
2
(3) provides access to Si based crown ethers. For
this purpose, TCCA serves as versatile agent for the asymmetric
À1
[
.
Particularly with respect to the before described constitution chlorination of tetramethyldisilane. The hybrid crown ether 6 was
obtained through reaction of 3 with diethylene glycol and incor-
porates three types of O atoms: completely C, half C and Si and
completely Si substituted ones. Complexation of lithium triflate
yielded single crystals of [Li(1,2,4,5-tetrasila[12]crown-4)OTf] (7),
which were characterized by X-ray diffraction. DFT calculations
(BP86 functional and def2-TZVP basis set) revealed that the
presence of the completely Si substituted O atom leads to a
considerable increase of the coordination ability compared to
7
[12]crown-4 as well as the hitherto known disila-crown ethers.
The strong interaction between the lithium cation and the
completely Si bonded O2 atom has also been confirmed by
2
9
Si NMR spectroscopy by reference to the significantly high
downfield shift of the equivalent Si atoms Si2 and Si3.
Fig. 3 A top-view of the molecular structure in the crystal of [Li(1,2,4,5-
tetrasila[12]crown-4)] . The anion is not displayed.
Scheme 4 Relative energy for the lithium exchange from [12]crown-4 to
1,2,4,5-tetrasila[12]crown-4 (6) as obtained from DFT calculations.
+
This journal is ©The Royal Society of Chemistry 2016
Chem. Commun.

View Article Online
Communication
ChemComm
83, 761; (c) B. D. Shepherd, J. Am. Chem. Soc., 1991, 113, 5581;
(
d) C. von H ¨a nisch, O. Hampe, F. Weigend and S. Stahl, Angew.
Chem., Int. Ed., 2007, 46, 4775; (e) U. Dittmar, H. C. Marsmann and
E. Rikowski, in Organosilicon Chemistry Set: From Molecules
to Materials, ed. N. Auner and J. Weis, Wiley-VCH Verlag GmbH,
Weinheim, Germany, 2005, ch. 64b.
2
(a) A. Decken, J. Passmore and X. Wang, Angew. Chem., Int. Ed., 2006,
1
4
18, 2839; (b) J. S. Ritch and T. Chivers, Angew. Chem., Int. Ed., 2007,
6, 4610.
3
4
F. Weinhold and R. West, J. Am. Chem. Soc., 2013, 135, 5762.
(a) R. J. Gillespie and S. A. Johnson, Inorg. Chem., 1997, 36, 3031;
Scheme 5 Illustration of the transformation from the free ligand structure
to the molecular structure found within the complex and the resulting
energetic effort.
(
b) J. Passmore and J. M. Rautiainen, Eur. J. Inorg. Chem., 2012, 6002.
5 S. Grabowsky, M. F. Hesse, C. Paulmann, P. Luger and J. Beckmann,
Inorg. Chem., 2009, 48, 4384.
6
(a) A. Decken, F. A. LeBlanc, J. Passmore and X. Wang, Eur. J. Inorg.
Chem., 2006, 4033; (b) T. S. Cameron, A. Decken, I. Krossing,
J. Passmore, J. M. Rautiainen, X. Wang and X. Zeng, Inorg. Chem.,
These results challenge the so far accepted theories concerning
the low coordination ability of siloxanes. Negative hyperconjugation –
although present – doesn’t significantly reduce the basicity of
disila-crown ethers. Through insertion of disilane instead
of monosilane fragments, the distance between the Si atoms and
the metal cation is high enough to prevent electrostatic repulsion –
2
013, 52, 3113.
7 K. Reuter, M. R. Buchner, G. Thiele and C. von H ¨a nisch, Inorg.
Chem., 2016, 55, 4441.
8
M. Ouchi, Y. Inoue, T. Kanzaki and T. Hakushi, Bull. Chem. Soc. Jpn.,
984, 57, 887.
1
9 (a) S. Varaprath and D. H. Stutts, J. Organomet. Chem., 2007,
692, 1892; (b) T. Hafner, Master thesis, TU Graz, 2013.
0 Experimental details on the synthesis of PhSi
O(Si Me Ph) (5) can be found in the ESI†.
1 S. Shambayati, J. F. Blake, S. G. Wierschke, W. L. Jorgensen and
S. L. Schreiber, J. Am. Chem. Soc., 1990, 112, 697.
2 Turbomole Version 7.0.1, Turbomole GmbH 2016. Turbomole is a
development of University of Karlsruhe and Forschungszentrum
Karlsruhe 1989–2007, Turbomole GmbH since 2007.
6
a major factor for the low coordination ability of cyclosiloxanes.
1
1
1
2 4
Me Cl (4) and
The synthesis and complexation of completely disilane based
crown ethers will give further insights into the coordination ability
of Si bonded O atoms, which is still a matter of ongoing research.
This work was financially supported by the Deutsche For-
schungsgemeinschaft (DFG). G. T. thanks the Leopoldina Nationale
Akademie der Wissenschaften for a postdoctoral scholarship.
2
4
2
13 (a) F. Weigend and R. Ahlrichs, Phys. Chem. Chem. Phys., 2005,
, 3297; (b) F. Weigend, Phys. Chem. Chem. Phys., 2006, 8, 1057;
c) M. Dolg, H. Stoll, A. Savin and H. Preuss, Theor. Chim. Acta, 1989,
7
(
7
2
Notes and references
5, 173; (d) H. Stoll, B. Metz and M. Dolg, J. Comput. Chem., 2002,
3, 767.
‡
For details concerning compound 8 see ESI.†
1
4 (a) S. Grimme, J. Antony, S. Ehrlich and H. Krieg, J. Chem. Phys.,
2010, 132, 154104; (b) S. Grimme, S. Ehrlich and L. Goerigk,
J. Comput. Chem., 2011, 32, 1456.
1
(a) H. J. Emel ´e us and M. Onyszchuk, J. Chem. Soc., 1958, 604;
(
b) R. West, L. S. Whatley and K. J. Lake, J. Am. Chem. Soc., 1961,
Chem. Commun.
This journal is ©The Royal Society of Chemistry 2016