The synthesis and characterisation of some Group 14 compounds containing the 2,4,6-(CF3)3C6H2, 2,6-(CF 3)2C6H3 or 2,4-(CF3) 2C6H3

Article abstract of DOI:10.1039/b302544f

New (aryl)₂ECl₂ and (aryl)ECl₃ compounds [E = Si, Ge or Sn; aryl = 2,4,6-(CF₃)₃C₆H ₂ (Ar), 2,6-(CF₃)₂C₆H₃ (Ar′) and/or 2,4-(CF₃

Full text of DOI:10.1039/b302544f

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The synthesis and characterisation of some Group 14 compounds
containing the 2,4,6-(CF3)3C6H2, 2,6-(CF3)2C6H3 or 2,4-(CF3)2C6H3
ligands
Andrei S. Batsanov, Stéphanie M. Cornet, Keith B. Dillon,* Andrés E. Goeta,
Amber L. Thompson and Bao Yu Xue
Chemistry Department, University of Durham, South Road, Durham, UK DH1 3LE
Received 5th March 2003, Accepted 15th April 2003
First published as an Advance Article on the web 9th May 2003
New (aryl)2ECl2 and (aryl)ECl3 compounds [E = Si, Ge or Sn; aryl = 2,4,6-(CF3)3C6H2 (Ar), 2,6-(CF3)2C6H3 (ArЈ)
and/or 2,4-(CF3)3C6H3 (ArЉ)] were prepared by reactions of ECl4 with 2 equivalents of ArLi or of a ArЈLi/ArЉLi
mixture. The latter gives predominantly the less sterically hindered product ArЉ2ECl2 for E = Si or Ge, but ArЈ2SnCl2
for the larger central atom. The products were characterised by elemental analysis, 19F and (where appropriate)
119Sn NMR spectroscopy, and single-crystal X-ray diffraction for ArЉ2SiCl2, ArGeCl3, Ar2GeCl2, ArЉ2GeCl2,
Ar2SnCl2 and ArЈ2SnCl2. For E = Si the synthesis is complicated by Cl/F exchange: besides ArЈ2SiCl2 and ArЉ2SiCl2,
19F NMR spectroscopy identified in solution ArЉ2SiF2 and ArЈ2SiF2. The latter was isolated and its X-ray structure
determined. In all compounds, the E atom has a strongly distorted tetrahedral coordination, supplemented by
short intramolecular E ؒ ؒ ؒ F contacts (secondary coordination) with o-CF3 group(s).
Introduction
Results and discussion
All the chloro-derivatives were prepared by reaction of the
corresponding Group 14 tetrachloride ECl4, with 2 equivalents
of ArLi (from ArH), or with a mixture of ArЈLi and ArЉLi
(to the total of 2 equivalents), obtained by lithiation of ArЈH
(Scheme 1). In agreement with earlier reports,9,10 the synthesis
of Ar2SiCl2 (1a) was frustrated by chlorine/fluorine exchange
and the only product isolated was Ar2SiF2 (2). For ArЈ and ArЉ
derivatives, the Cl/F exchange was slower, and the compounds
ArЈ2SiCl2 (3a), ArЉ2SiCl2 (4a), ArЈ2SiF2 (5) and ArЉ2SiF2 (6)
were all detected in solution by means of 19F NMR spectro-
scopy (see Table 1). Of the two isomeric chlorides, the less
sterically hindered 4a was present in larger amount than 3a, but
interestingly, the opposite was observed for the fluorides: 5 was
more abundant than 6. Probably, 3a undergoes faster Cl/F
exchange than 4a, with the overall order of exchange rates
The ‘fluoromes’ ligand 2,4,6-(CF3)3C6H2 (henceforth, Ar) is
known for its stabilising influence in the compounds of trans-
ition metals1,2 and main group3 elements, including phos-
phorus4 and arsenic.5 This is due to the high electronegativity
(compared with most aryl ligands) of this group, combined
with some ability for M C π back donation and the steric
demands of the two ortho trifluoromethyl groups, which can
hinder rotation of the ligands as well as favour low co-
ordination numbers by protecting vacant coordination sites.
The ‘fluoroxyl’ 2,6-(CF3)2C6H3 group (ArЈ), possessing similar
advantages, is used much less,4b,f,6–8 partly because lithiation
of its precursor 1,3-bis(trifluoromethyl)benzene (ArЈH) can
proceed in two different positions, leading to a mixture of ArЈ
and 2,4-(CF3)2C6H3 (ArЉ) derivatives.4f,7,8 Recently we under-
took a systematic study of a series of Group 15 compounds
with Ar, ArЈ and/or ArЉ ligands.8 The corresponding derivatives
of tetravalent Group 14 elements (Si, Ge and Sn), remain
comparatively unexplored, particularly simple halides and/or
hydrides. Attempts to prepare Ar2SiCl2 from reaction of ArLi
with SiCl4, were frustrated by fluorine/chlorine exchange,
yielding only Ar2SiF2.9 Similarly, Ar2SiHF was obtained from
reaction between HSiCl3 and ArLi.10 Ar2GeH2 was synthesised
from the germanium() precursor Ar2Ge,11 while ArSnPh3 was
similarly prepared from ArLi and Ph3SnCl.12 Various other
derivatives containing Ar groups, often produced by reaction
between an E() precursor Ar2E (E = Ge or Sn) and oxidising
agents, have been structurally characterised,4a,13–16 but none
containing ArЈ or ArЉ groups, although ArЈSnMe3,7b ArЉ-
SnMe3 7b and ArЈ2Sn17 have been prepared. X-Ray structures of
these compounds consistently reveal intramolecular E ؒ ؒ ؒ F
separations shorter than the sums of the van der Waals radii,
indicative of additional weak (“secondary”) coordination,
or attractive electrostatic interactions, which can play an
important role in stabilisation of these molecules.
In the present work we have synthesised a series of the Ar,
ArЈ and ArЉ derivatives of Group 14 elements (i.e. Si, Ge and
Sn), which have been characterised by elemental analysis,
19F and (where appropriate) 119Sn NMR solution-state spectro-
scopy. X-Ray crystal structures of seven products have been
determined at low temperatures.
Scheme 1 Synthetic reactions (all performed at Ϫ78 ЊC).
D a l t o n T r a n s . , 2 0 0 3 , 2 4 9 6 – 2 5 0 2
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
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Table 1 19F NMR spectra for Si() compounds
o-CF3
δ/ppm
Si–F
p-CF3
δ/ppm
5JF–F/Hz
δ/ppm
5JF–F/Hz
2
Ϫ57.3 t (12F)
Ϫ58.9 s (12F)
Ϫ57.9 s (6F)
Ϫ57.5 t (12F)
Ϫ59.2 t (6F)
12.8
Ϫ64.2 s (6F)
Ϫ64.2 s (6F)
Ϫ64.1 s (6F)
Ϫ124.5 m (2F)
12.8
3a
4a
5
12.3
12.4
Ϫ125.5 m (2F)
Ϫ133.0 septet (2F)
12.5
12.3
6
Table 2 19F and 119Sn NMR spectra (δ/ppm) for Ge() and Sn()
decreasing in the sequence Ar2SiCl2 > ArЈ2SiCl2 > ArЉ2SiCl2. A
possible mechanism for this exchange is presented in Scheme 2.
Products 4a and 5 were isolated as colourless crystalline solids,
and their X-ray structures were ascertained.
compounds
o-CF3
4JSn–F/Hz
p-CF3
119Sn
1b
3b
4b
7b
1c
3c
4c
7c
Ϫ54.4 s (12F)
Ϫ53.8 s (12F)a
Ϫ58.7 s (6F)
Ϫ52.9 s (6F)
Ϫ56.9 s (12F)b
Ϫ56.7 s (12F)b
Ϫ58.9 s (6F)c
Ϫ55.9 s (6F)b
Ϫ64.1 s (6F)
Ϫ64.1 s (6F)
Ϫ63.5 s (3F)
Ϫ63.9 s (6F)
10.0
10.0
Ϫ146.7
Ϫ141.1
Ϫ97.4
Ϫ63.8 s (6F)
Ϫ63.0 s (3F)
19.2
Ϫ140.7
a See Text. b (singlet) with Sn satellites. c Weak signal, Sn satellites
unobserved.
Table 3 Selected bond distances (Å) and angles (Њ) in 7b
Ge–C(1)
1.981(2)
2.1277(4)
2.1117(8)
2.909(2)
C(1)–Ge–Cl(1)
C(1)–Ge–Cl(2)
Cl(1)–Ge–Cl(2)
Cl(1)–Ge–Cl(1Ј)
Cl(1Ј)–Ge ؒ ؒ ؒ F(1)
113.72(4)
111.89(6)
108.46(2)
99.82(3)
168.1(1)
Ge–Cl(1)
Ge–Cl(2)
Ge ؒ ؒ ؒ F(1)
Scheme 2 Possible mechanism of Cl/F exchange in Si derivatives.
sterically hindered disubstituted product ArЈ2SnCl2 (3c), which
has also been characterised crystallographically. The less
hindered isomer ArЉ2SnCl2 (4c) was identified in solution from
its 19F and 119Sn NMR spectra. The larger size of the Sn atom
relative to Si and Ge must reduce the steric hindrance between
ligands in these ψ-tetrahedral structures, which probably
explains the reversal in isomeric ratio between 3 and 4. 19F and
119Sn NMR data for new Ge and Sn compounds are listed in
Table 2.
No F/Cl exchange was observed for the germanium and tin
derivatives. This difference possibly arises because the bond
energy terms are more favourable for exchange in the case of
silicon. The sum of a C–Cl and an Si–F bond energy term
(taken from values in tetrahalides18) is 912 kJ molϪ1, whereas
the sum of a C–F and an Si–Cl term is 887 kJ molϪ1. Thus the
exchange should give a net energy gain of 25 kJ molϪ1. The
corresponding sums for germanium are 792 and 827 kJ molϪ1
and for tin, 730 and 803 kJ molϪ1, respectively, giving in both
cases a negative balance, Ϫ35 kJ molϪ1 for Ge and Ϫ73 kJ
molϪ1 for Sn. Although the actual bond energies can be some-
what different in the present compounds because of different
ligand environment, it is evident from the general trend that the
Cl/F exchange is energetically more profitable, the only source
of fluorine in each instance being the dissociation of a C–F
bond.
Molecular structures studied by single-crystal X-ray crystal-
lography are shown in Figs. 1–4, while selected bond distances
and angles are compared in Tables 3 and 4. It is noteworthy
The reaction of ArLi with GeCl4 yielded a mixture of
Ar2GeCl2 (1b) and ArGeCl3 (7b) in a ca. 2 : 1 ratio. Both
products were isolated and characterised by X-ray crystallo-
graphy. The ArЈLi/ArЉLi mixture reacted similarly with GeCl4
to give a solution containing predominantly ArЉ2GeCl2 (4b),
according to the 19F NMR spectra. This compound, too, was
recrystallised and characterised by X-ray crystallography.
A single 19F resonance at Ϫ53.8 ppm was tentatively assigned
to the symmetrical disubstituted isomer ArЈ2GeCl2 (3b),
particularly in view of the similarity of its shift to those of the
fluorines in the o-CF3 groups of 1b (Ϫ54.4 ppm) and 7b (Ϫ52.9
ppm). There were other small impurity peaks present, however,
and the possibility that the signal at Ϫ53.8 ppm could arise
from the monosubstituted precursor ArЈGeCl3, which should
also give a single 19F resonance, cannot be entirely discounted.
Reaction of ArLi with SnCl4 in a 2 : 1 molar ratio led to the
isolation of mainly Ar2SnCl2 1c, together with a small quantity
of ArSnCl3 7c. Similar treatment of the ArЈLi/ArЉLi mixture
with SnCl4 yielded a solution containing mainly the more
Fig. 1 Molecular structures of ArЉ2SiCl2 (4a) and ArЉ2GeCl2 (4b).
Henceforth atomic displacement ellipsoids are drawn at 50%
probability level.
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Table 4 Selected bond distances (Å) and angles (Њ)
Compound
E
4a
Si
5
Si
1b
Ge
4b
Ge
1c
Sn
3c
Sn
E–C(11)
E–C(21)
E–Cl(1)
E–Cl(2)
1.884(2)
1.884(2)
2.050(1)
2.048(1)
1.899(2)
1.895(2)
1.579(1)a
1.569(1)a
1.997(3)
2.017(3)
2.1513(9)
2.1174(9)
1.958(2)
1.957(2)
2.1484(7)
2.1496(7)
2.183(6)
2.195(6)
2.326(2)
2.298(2)
2.177(2)
2.183(2)
2.3266(7)
2.3372(7)
C(11)–E–C(21)
C(11)–E–Cl(1)
C(11)–E–Cl(2)
C(21)–E–Cl(1)
C(21)–E–Cl(2)
Cl(1)–E–Cl(2)
117.47(8)
108.88(6)
108.22(7)
108.00(7)
109.29(6)
104.17(5)
115.53(8)
113.17(8)a
105.27(7)a
104.65(7)a
113.76(7)a
104.06(6)a
120.07(12)
113.46(9)
103.34(9)
96.65(9)
118.17(9)
104.33(4)
119.95(10)
107.77(7)
108.65(7)
108.26(7)
108.21(7)
102.64(3)
120.3(2)
119.0(2)
96.1(2)
103.8(2)
113.9(2)
102.58(7)
115.73(7)
120.66(6)
98.18(6)
100.96(6)
121.53(6)
100.11(3)
E ؒ ؒ ؒ F(11)
E ؒ ؒ ؒ F(21)
E ؒ ؒ ؒ F(14)
E ؒ ؒ ؒ F(15)
E ؒ ؒ ؒ F(25)
2.901(2)
2.882(2)
–
–
–
2.793(1)
2.745(1)
–
3.054(1)
3.073(1)
2.757(2)
2.809(2)
3.379(2)
3.399(2)
3.010(2)
2.860(2)
2.848(2)
–
–
–
2.720(4)
2.799(4)
3.344(4)
3.382(4)
2.979(3)
2.686(2)
2.768(1)
–
3.203(2)
3.002(2)
Cl(1)–E ؒ ؒ ؒ F(11)
Cl(2)–E ؒ ؒ ؒ F(21)
C(11)–E ؒ ؒ ؒ F(25)
C(21)–E ؒ ؒ ؒ F(15)
174.30(4)
176.92(4)
–
–
172.60(6)
173.14(5)
160.3(1)
160.5(1)
167.77(5)
169.32(5)
164.5(1)
152.6(1)
172.45(5)
177.20(4)
–
–
170.6(1)
170.3(1)
164.6(2)
154.1(2)
168.26(5)
169.33(3)
160.69(6)
153.91(6)
a F ligands instead of Cl.
Fig. 4 Molecular structure of ArGeCl3 (7b), showing the disorder of
the p-CF3 group. Atoms generated by the mirror plane are primed.
such disorder has been frequently observed for both Group
1410,14c and Group 153c,d,8 derivatives of Ar. In 3c, one ortho-
CF3 group is rotationally disordered.
Fig.
2
Molecular structures of ArЈ2SiF2 (5) and ArЈ2SnCl2 (3c),
showing the disorder of one o-CF3 group in 3c.
Molecules of all the diaryl-dihalo compounds have no
crystallographic symmetry and show a distorted tetrahedral
coordination of the central atom (E), with the C–E–C bond
angle the widest and the Cl–E–Cl angle (or F–E–F in 5) the
smallest. This distortion can be explained by the steric
repulsion between bulky aryl groups. However, the difference
between these two angles is higher in 4b than in 4a, and in 1c
than in 1b, i.e. the distortion increases with the increase of the
E atom size, which should apparently relieve the steric over-
crowding. It is also noteworthy that in ArЉ derivatives 4a and
4b all four Cl–E–C angles are similar, while in Ar and ArЈ deriv-
atives 1b,c and 3c two Cl–E–C angles are much wider than the
other two. The F–E–C angles in 5 show a similar, but more
regular, distortion. These distortions are obviously due to the
fact that both Ar and ArЈ ligands have two CF3 groups in ortho
positions to E and thus cause more steric overcrowding than
ArЉ, which has only one ortho-CF3. Similar asymmetry has been
observed earlier for Ar2EX(Y) compounds, where E = Si or Sn,
X = F or Cl, and Y is a unidentate ligand.9,10,14 Thus, in Ar2SiF2
the F–Si–C angles vary from 102.8(2) to 112.8(2)Њ,9 and in
Ar2SiHF from 105.9(1) to 112.5(1)Њ.10 Even larger variations
occur in Ar2Sn(Cl)(µ2O)Sn(Cl)Ar2 (Cl–Sn–C angles 99.1(1)–
119.7(1)Њ)14a and in Ar2Sn(F)L, where L = N-(1-adamantyl)-
[(pentafluoro-2-propenyl)thio]amine, C13H15F5NS (C–Sn–F
angles 90.1(2)–107.7(2)Њ).14c
Fig. 3 Molecular structures of Ar2GeCl2 (1b) and Ar2SnCl2 (1c),
showing the disorder of one p-CF3 group.
that compound 4a is crystallographically isostructural (iso-
morphous) with 4b, and 1b with 1c. In molecules 1b and 1c, the
para-CF3 group of one Ar ligand is rotationally disordered;
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Molecule 7b lies on a crystallographic mirror plane normal to
the benzene ring and passing through the Ge, Cl(2), C(1) and
C(4) atoms. Here the Ge atom also has distorted tetrahedral
coordination and the CF3 group in the para-position to the
Ge is disordered between two orientations (related in this case
by the mirror plane).
carrying opposite charges) and/or weak (“secondary”) co-
ordination, i.e. donation of lone electron pairs of F into the
outer-shell orbitals of E. The latter interpretation agrees with
relatively high chemical stability of the compounds. It is also
noteworthy that the Ge ؒ ؒ ؒ F distances in 4b are shorter by 0.04
Å than Si ؒ ؒ ؒ F in the isostructural 4a, which contradicts the
simple repulsive model (the Ge atom is larger than Si), but can
be explained by a weakly-bonding model (the outer orbitals of
Ge are more diffuse and hence more suitable for interaction
with F).
The E–F and E–Cl bonds in diaryldihalogenides are 0.03–
0.05 Å longer than in the corresponding Group 14 tetra-
halogenides, viz. SiF4 (1.540(1) Å),19a SiCl4 (2.008(1) Å),19b
GeCl4 (2.096(2) Å in the crystal,20 2.113(3) Å in the gas phase21)
and SnCl4 (2.279(3) Å).22 The lengthening is obviously due to
the replacement of the halogen by a less electronegative aryl
ligand, and correspondingly the effect is smaller in the
monoaryltrichloride 7b (ca. 0.02 Å) than in the diaryl analogue
1b, and it increases with decreasing number of electron-with-
drawing CF3 groups (compare 1b with 4b, and 1c with 3c).
Structural studies of Group 14 compounds containing only
aryl and halogen ligands, are scarce, especially for Si and Ge.
The Si–C bonds in Ph3SiCl (1.862 Å)23 are slightly shorter than
in 4a (1.884(2) Å), while still shorter Si–C bonds (ca. 1.84 Å)
were found in two compounds where Si atoms are incorporated
into fused-ring systems, viz. 9,9,10,10-tetrachloro-9,10-disila-
9,10-dihydroanthracene24 and 9,9-dichloro-9-sila-9-hydro-
fluorene.25 The mean Si–C bond distance in 5 (1.897(2) Å) can
be compared with those in 2 (1.901(5) Å),9 Ar2SiHF (1.906(5)
Å),10 and o-Tol3SiF (1.861 Å).26 Finally, comparison can be
In the ArЉ derivatives 4a and 4b, the coordination of Si and
Ge is complemented to (4 ϩ 2) by the F(11) and F(21) atoms
(belonging to different ArЉ ligands), approximately in trans
positions to the chloro ligands; there is no other E ؒ ؒ ؒ F
contact within 3.7 Å. A similar pair of short E ؒ ؒ ؒ F contacts
exists in each of the bis-Ar and bis-ArЈ derivatives also (which
can be regarded as evidence of specific character of these inter-
actions), but the presence of two more o-CF3 groups gives rise
to additional, somewhat longer, E ؒ ؒ ؒ F contacts. In the ArЈ
derivatives 5 and 3c, these “additional” o-CF3 groups con-
tribute one contact each, viz. E ؒ ؒ ؒ F(15) and E ؒ ؒ ؒ F(25),
both approximately in trans positions to E–C bonds. The result-
ing (4 ϩ 4) coordination of E can be described as a tetrahedron
capped on each face. The same description was applied pre-
9
viously to the structures of Ar2SiF2 and Ar2SiHF.10 In 5, the
E ؒ ؒ ؒ F(15) and E ؒ ؒ ؒ F(25) distances are almost equal, while
in 3c they differ by 0.2 Å. This non-equivalence is obviously
connected with strongly asymmetric distortions of bond angles
(between covalent bonds) at the Sn atom, the causes of which
are unclear. In 1b and 1c, one of the o-CF3 groups adopts a
different orientation: instead of one F atom pointing roughly
towards the E atom, there are two longer contacts, E ؒ ؒ ؒ F(14)
and E ؒ ؒ ؒ F(15), the E atom lying close to the bisectral plane of
the F(14)C(18)F(15) angle.
Molecule 7b contains two (symmetrically related) Ge ؒ ؒ ؒ F(1)
contacts in trans positions to Cl(1) and its equivalent. It is
noteworthy that the Ge–Cl(1) bond is 0.016 Å longer than
Ge–Cl(2), which suggests a certain (if small) covalent character
of the Ge ؒ ؒ ؒ F(1) interaction.
27
28
made with tetraaryl derivatives, e.g. SiPh4 and Si(p-Tol)4
with Si–C distances 1.877 and 1.873(3) Å, respectively. Thus
halogeno ligands have no definite effect on the Si–C(aryl)
bonds, while CF3 substituents in the aryl ligands tend to weaken
them, probably by diminishing the electron density on the
benzene ring and hence the π back-donation.
No compound with Ge–C(aryl) and Ge–Cl bonds has been
structurally characterised before, except 10,10-dichloro-10-
germa-9-oxa-9,10-dihydroanthracene,29 where the Ge–C bonds
of 1.890 Å are incorporated into a fused-ring system. The
Ge–C bonds in Ar derivative 1b are 0.05 Å longer than in its ArЉ
analogue 4b, and in the latter nearly the same as in tetra-aryl
30
28
compounds GePh4 (1.957(4) Å) and Ge(p-Tol)4 (1.948(5)
Å). On the other hand, the Sn–C(Ar) bonds in 1c are less than
0.01 Å longer than Sn–C(ArЈ) in 3c, and in both cases are
substantially weaker than in Ph2SnCl2 (2.113(5) Å),31 or
(mes)2SnCl2, a non-fluorinated analogue of 1c (2.117 Å).32 Thus
a CF3 group in an ortho position affects an E–C bond much
more than one in a para position, which can be attributed to
higher steric overcrowding and direct CF3 ؒ ؒ ؒ E interactions
(see below), rather than to mere electron withdrawing by
this group. Indeed, bulkier ortho-substituents cause similar
Sn–C(aryl) bond lengths even in the absence of fluorination,
e.g. in (2,4,6-Pri3C6H2)2SnCl2 (2.147(4) Å)33 and (2,4,6-But3C6-
H2)2SnCl2 (2.198(4) Å).34
Experimental
All manipulations, including NMR sample preparation, were
carried out either under an inert atmosphere of dry nitrogen or
in vacuo, using standard Schlenk procedures or a glovebox.
Chemicals of the best available commercial grades were used, in
general without further purification. 19F NMR spectra were
recorded on a Varian Mercury 200, Varian VXR 400, or Varian
Inova 500 Fourier-transform spectrometer at 188.18, 376.35,
and 470.26 MHz, respectively. 119Sn NMR spectra were
recorded on the Varian Inova 500 spectrometer at 186.37 MHz.
Chemical shifts were measured relative to external CFCl3 (19F)
or Me4Sn (119Sn), with the higher frequency direction taken as
positive. Microanalyses were performed by the microanalytical
services of the Department of Chemistry, University of
Durham.
Indeed, a salient feature of all the compounds studied herein
is short intramolecular E ؒ ؒ ؒ F contacts with o-CF3 groups of
the aryl ligands. Such contacts have been observed earlier in
numerous Group 14 derivatives,3a,9–16 as well as in some Group
15 compounds.8 Although the van der Waals radii of Group 14
elements are difficult to determine directly (because these atoms
are seldom exposed sufficiently to participate in intramolecular
contacts), a variety of indirect techniques gives consistent
values of 2.1 Å for Si and Ge and 2.25 Å for Sn.35 Thus (assum-
ing a radius of 1.5 Å for F), each molecule contains 2 to 5
E ؒ ؒ ؒ F contacts well below the sum of the van der Waals radii,
which are listed in Tables 3 and 4. These contacts can be
compared also with the sums of “equilibrium” radii, the sums
of which correspond to the minimum of the atom–atom poten-
tial curve and hence the point of zero van der Waals force,36 viz.
2.26 Å (Si), 2.32 Å (Ge), 2.46 Å (Sn) and 1.65 Å (F).35 Thus,
insofar as van der Waals forces are concerned, the E ؒ ؒ ؒ F
interactions should be substantially repulsive. They, however,
can be counterbalanced by electrostatic attraction (E and F
Syntheses
Lithiation reactions were carried out as described pre-
viously.4,8,37 WARNING: It is important in these reactions
to keep a slight excess of the hydrocarbon (ArH or ArЈH) to n-
butyllithium at all times, to avoid any attack on a CF3 group
and the possible explosive formation of LiF.
Ar2SiF2 (2). A solution of ArLi (100 ml, 30 mmol) in diethyl
ether was added dropwise to a solution of SiCl4 (2.5 g, 1.72 ml,
15 mmol) in hexanes at Ϫ78 ЊC. The solution was allowed to
warm to room temperature and stirred for 5 h. A precipate
formed. The solution was filtered and solvents were removed
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under vacuum, leaving a yellow oil. This oil was distilled under
reduced pressure (0.01 Torr), giving a yellow oil, bp 85 ЊC. Yield
1.8 g (19% based on ArH). Anal. Calc. for C18H4F20Si: C 34.41,
H 0.64. Found: C 32.9, H 0.75%.
ArЉ2SiCl2 (4a). An ArЈLi/ArЉLi (50 ml, 20 mmol) solution in
diethyl ether was added dropwise to a solution of SiCl4 (1.7 g,
10 mmol) in pentane at Ϫ78 ЊC. The solution was allowed
to warm to room temperature and stirred for 3 h. The pre-
cipitated LiCl was filtered off and the solvents and excess SiCl4
were removed under vacuum, leaving a yellow sticky oil which
was distilled under reduced pressure (0.01 Torr). The fraction
collected at 120 ЊC was recrystallised from pentane, yielding
1.8 g (32.4%) of 4a. Anal. Calc. for C16H6Cl2F12Si: C 36.6,
H 1.15. Found: C 36.8, H 1.24%.
ArЈ2SiF2 (5). An ArЈLi/ArЉLi (50 ml, 40 mmol) solution in
diethyl ether was added dropwise to a solution of SiCl4 (3.39 g,
2.3 ml, 20 mmol) in hexanes at Ϫ78 ЊC. The solution was
allowed to warm to room temperature and stirred for 3 h. The
precipitated LiCl was filtered off, and the solvents and excess
SiCl4 were removed under vacuum, leaving a yellow oil (4a) and
a white solid. The solid was washed three times with hexanes
and purified by sublimation under vacuum, giving white
crystals of 5. Yield: 2.5 g (12.7%). Anal. Calc. for C16H6F14Si:
C 39.04, H 1.23. Found: C 38.3, H 1.24%.
Ar2GeCl2 (1b) and ArGeCl3 (7b). An ArLi (50 ml, 30 mmol)
solution in diethyl ether was added dropwise to a GeCl4
solution (3.2 g, 1.71 ml, 15 mmol) in hexanes at Ϫ78 ЊC. The
solution was allowed to warm to room temperature and stirred
for 4 h. A white precipitate of LiCl appeared and was filtered
off. The solvents and excess GeCl4 were removed under
vacuum, leaving a yellow oil and a white solid. The oil was
filtered and then distilled under reduced pressure (0.01 Torr),
giving a colourless oil, bp 85 ЊC. Analysis showed that this was
impure but contained mainly 7b. Yield: 2.6 g (19%). After one
month, fine crystals of 7b formed. There was insufficient
material for further analysis but a single-crystal X-ray structure
determination was carried out. The filtered-off solid was
washed three times with hexanes, yielding 3.17 g (30%) of 1b.
Crystals were grown from dichloromethane. Anal. Calc. for
C18H4Cl2F18Ge: C 30.64, H 0.57. Found C: 30.59, H 0.58%
ArЉ2GeCl2 (4b). A solution of ArЈLi/ArЉLi (60 ml, 40 mmol)
in diethyl ether was added dropwise to a solution of GeCl4 (4.29
g, 2.6 ml, 20 mmol) in diethyl ether at Ϫ78 ЊC. The solution was
allowed to warm to room temperature and stirred for 2 h.
A white precipitate of LiCl formed. The solution was filtered
and the solvents were removed under vacuum, leaving a black
oil. The oil was distilled under reduced pressure (0.01 Torr),
and a fraction was collected at 80–90 ЊC. Yield: 5.8 g (51%).
After one week, small crystals formed. Anal. Calc. for
C16H6Cl2F12Ge: C 33.7, H 1.06, Cl 12.45. Found: C 32.4, H
1.53, Cl 12.8%.
Ar2SnCl2 (1c) and ArSnCl3 (7c). An ArLi (50 ml, 30 mmol)
solution in diethyl ether was added slowly to a solution of SnCl4
(3.90 g, 2.75 ml, 15 mmol) in hexanes. The solution was then
allowed to warm to room temperature and stirred for 5 h. A
white precipitate of LiCl appeared. The solution was filtered
and the solvents were removed under vacuum, leaving a brown
oil and a solid. The oil was filtered and distilled under reduced
pressure, giving a yellow oil of 7c (bp 85 ЊC) in a small quantity.
The solid (1c) was washed three times with hexanes, dried
under vacuum and recrystallised from diethyl ether. Yield 3.8 g
(51%). Anal. Calc. for C18H4Cl2F18Sn: C 28.76, H 0.54. Found:
C 28.60, H 0.78%.
ArЈ2SnCl2 (3c) and ArЉ2SnCl2 (4c). An ArЈLi/ArЉLi (250 ml,
94 mmol) solution in diethyl ether was added dropwise to
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Macdonald, D. J. LeBlanc and T. S. Cameron, Organometallics,
2000, 19, 152.
a solution of SnCl4 (12.24 g, 8.63 ml, 47 mmol) at room tem-
perature. The solution was stirred for 4 h. A white precipitate of
LiCl appeared. The brown solution was filtered and solvents
and excess SnCl4 were removed under vacuum, leaving a brown
sticky oil and a brown solid. The oil (4c) was filtered, the solid
washed with pentane and dichloromethane and dried in vacuo,
giving a beige solid (3c), which was recrystallised from pentane
and diethyl ether. Yield (3c) 3.48 g (57%). Anal. Calc. for
C16H6Cl2F12Sn: C 31.21, H 0.98. Found C 29.7, H 1.26%.
4 (a) M. Scholz, H. W. Roesky, D. Stalke, K. Keller and F. T.
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8 A. S. Batsanov, S. M. Cornet, K. B. Dillon, A. E. Goeta,
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9 J.-K. Buijink, M. Noltemeyer and F. T. Edelmann, J. Fluorine
Chem., 1993, 61, 51.
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X-Ray crystallography
Single crystal X-ray diffraction experiments were carried out at
low temperature, 120 or 150 K, using graphite-monochromated
¯
Mo-Kα radiation (λ = 0.71073 Å) on a Bruker SMART (CCD
1 K area detector) diffractometer equipped with a Cryostream
N2 open-flow cooling device.38 Series of narrow ω-scans (0.3Њ)
were performed at several ꢀ-settings in such a way as to cover
a sphere of reciprocal space to a maximum resolution between
0.70 and 0.77 Å. Cell parameters were determined and refined
using the SMART software,39 and raw frame data were inte-
grated using the SAINT program.40 The structures were solved
by direct methods and refined by full-matrix least squares on F 2
using SHELXTL software.41 Crystal data and experimental
details are listed in Table 5. For structure 4b, the reflection
intensities were corrected by numerical integration based on
measurements and indexing of the crystal faces (using
SHELXTL software).41 For the remaining structures, the
absorption corrections were carried out by the multi-scan
method, based on multiple scans of identical and Laue
equivalent reflections (using the SADABS software).42 Non-
hydrogen atoms were refined anisotropically, except for the
disordered component of structure 1b. For structures 4a,b and
7b the hydrogen atoms were found in difference Fourier maps.
For structures, 1b,c, 3c and 5 the hydrogen atoms were
positioned geometrically and refined using a riding model.
CCDC reference numbers 205552–205558.
See http://www.rsc.org/suppdata/dt/b3/b302544f/ for crystal-
lographic data in CIF or other electronic format.
16 K. W. Klinkhammer, T. F. Fassler and H. Grützmacher, Angew.
Chem., Int. Ed., 1998, 37, 124.
17 M. P. Bigwood, P. J. Corvan and J. J. Zuckerman, J. Am. Chem. Soc.,
1981, 103, 7643.
Acknowledgements
We thank the EPSRC for the award of studentships (to S. M. C.
and A. L. T.), the CVCP for an ORS award (to B. Y. X.), and
A. M. Kenwright, C. F. Heffernan and I. H. McKeag for
assistance in recording some of the NMR spectra.
18 (a) Thermodynamical Properties of Individual Substances, ed.
L. V. Gurvich, I. V. Veits and S. B. Alcock, Hemisphere Publ. Corp.,
Washington DC, 4th edn., 1991, vol. 2, pt. 1; (b) Handbook of
Chemistry and Physics, ed. D. R. Lide, CRC Press, Boca Raton, FL,
USA, 76th edn., 1995–1996; (c) S. S. Batsanov, Strukturnaya
Khimiya, Dialog MGU, Moscow, 2000 (in Russian).
19 (a) D. Mootz and L. Kortle, Z. Naturforsch., Teil B, 1984, 39, 1295;
(b) L. N. Zakharov, M. Yu. Antipin, Yu. T. Struchkov, A. V. Gusev,
A. M. Gubin and N. V. Zhernenkov, Kristallografiya, 1986, 31, 171.
20 K. Merz and M. Driess, Acta Crystallogr., Sect. C, 2002, 58, i101.
21 Y. Morino, Y. Nakamura and T. Iijima, Chem. Phys., 1960, 95, 643.
22 H. Reuter and R. Pawlak, Z. Anorg. Allg. Chem., 1999, 626, 925.
23 E. B. Lobkovskii, V. N. Fokin and K. N. Semenenko, Zh. Strukt.
Khim., 1981, 22(4), 152.
24 V. A. Sharapov, A. I. Gusev, Yu. T. Struchkov, N. G. Komalenkova,
L. N. Shamshin and E. A. Chernyshev, Zh. Strukt. Khim., 1980,
21(6), 114.
25 Y. Liu, T. C. Stringfellow, D. Ballweg, I. A. Guzei and R. West,
J. Am. Chem. Soc., 2002, 124, 49.
26 S. Dell, D. M. Ho and R. A. Pascal, Jr., J. Org. Chem., 1999, 64,
5626.
27 V. Gruhnert, A. Kirfel, G. Will, F. Wallrafen and K. Recker, Z.
Kristallogr., 1983, 163, 53.
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