Primary studies on variation in position of trifluoromethyl groups in several aromatic group-14 derivatives by 19F-NMR spectroscopy

Article abstract of DOI:10.1002/hlca.201200389

Variation in the position of CF₃ groups in several aromatic Group-14 compounds was studied by ¹⁹F-NMR spectroscopy. In these compounds R_nECl_4-n (n=1 or 2; E=Si, Ge, or Sn; R=2,4,6-(CF₃)₃C

Full text of DOI:10.1002/hlca.201200389

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Helvetica Chimica Acta – Vol. 96 (2013)
Primary Studies on Variation in Position of Trifluoromethyl Groups in Several
Aromatic Group-14 Derivatives by 19F-NMR Spectroscopy
by Bao Yu Xue*a) and Keith B. Dillonb)
a) Institute of Chemistry, Henan Academy of Sciences, Zhengzhou 450002, P. R. China
(phone: þ 86-371-65511511; fax: þ 86-371-65511998; e-mail: byxue@hotmail.com)
b) Chemistry Department, Durham University, South Road, Durham DH13LE, UK
Variation in the position of CF3 groups in several aromatic Group-14 compounds was studied by 19F-
NMR spectroscopy. In these compounds RnECl4ꢀn (n ¼ 1 or 2; E ¼ Si, Ge, or Sn; R ¼ 2,4,6-(CF3)3C6H2
(¼Ar), 2,6-(CF3)2C6H3 (¼Ar’), or 2,4-(CF3)2C6H3 (¼Ar’’)), Ar, Ar’, and Ar’’ are all bulky, strongly
electron-withdrawing ligands. The 19F-NMR studies of the variation in position of the CF3 substituents in
these compounds as revealed by chemical shifts could be correlated with the electronegativities of the
central elements E, and with intramolecular E – F interactions derived from single-crystal X-ray
diffraction data. These interactions are considered to play an important role in the stabilization of these
compounds.
Introduction. – The ꢀfluoromesꢁ ligand 2,4,6-(CF3)3C6H2 (¼Ar, henceforth) is well
known for its stabilizing influence in the compounds of transition metals [1][2] and
main-group elements [3 – 10]. This effect is due to the high electronegativity, combined
with some M ! C p back-donation and steric demands, of the two ortho-positioned
trifluoromethyl groups to stabilize low-valent, low-coordinate main group compounds.
The ꢀfluoroxylꢁ 2,6-(CF3)2C6H3 (¼Ar’) group, which possesses similar advantages, is
used much less frequently, partly because lithiation of its precursor 1,3-bis(trifluoro-
methyl)benzene (Ar’H) can proceed at two different positions, leading to a mixture of
Ar’ and 2,4-(CF3)2C6H3 (¼Ar’’) derivatives [11 – 15]. In Group 14, ArLi reacts with
ECl4 (E ¼ Ge or Sn) in a 2 :1 ratio to yield both ArECl3 and Ar2ECl2 [14]. However, it
has been found that the mixture of Ar’Li and 2,4-(CF3)2C6H3Li (Ar’’Li) reacts with
ECl4 (E ¼ Si, Ge, or Sn) to produce either Ar’nECl4ꢀn or Ar’’nECl4ꢀn (n ¼ 1 or 2); the
reaction does not produce the mixed-ligand compound Ar’Ar’’ECl2. There is a further
complication in the Si derivatives, since F/Cl exchange takes place readily, particularly
for Ar [1g][8][14].
Few papers report on derivatives of Group 14 (e.g., Si, Ge, and Sn) with Ar, Ar’, or
Ar’’, especially with respect to the variation in the position of the CF3 substituents. The
present paper is the first to study the CF3 positions in these compounds by 19F-NMR
spectroscopy.
Experimental. – General. All manipulations, including NMR sample preparation, were carried out
either under dry N2 or in vacuo, by means of standard Schlenk procedures or a glovebox. Chemicals of the
best available commercial grades were used, without further purification. 19F-NMR Spectra: Varian-
Mercury-200 spectrometer; at 188.18 MHz; chemical shifts d(F) in ppm relative to external CFCl3.
ꢂ 2013 Verlag Helvetica Chimica Acta AG, Zꢃrich

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Lithiation. By the methods of Goodwin [9] and Roden [15], ArH was directly treated with 2.5m BuLi
in Et2O at ꢀ 788 (acetone/dry ice bath) to produce ArLi. Ar’H reacted with BuLi under the same
conditions to produce a mixture Ar’Li/Ar’’Li in a ca. 1:1 ratio [15]. WARNING: It is important in these
reactions to keep a slight excess of the hydrocarbon (ArH or Ar’H) with respect to BuLi at all times, to
avoid any attack at a CF3 group and the possible explosive formation of LiF (Scheme).
Scheme. Synthetic Reactions (all performed at ꢀ 788)
Chloro Derivatives 1 – 10. All of the chloro derivatives were prepared by reacting the corresponding
Group-14 tetrachloride ECl4 with 2 equiv. of ArLi or a mixture Ar’Li/Ar’’Li (to a total of 2 equiv.). The
new compounds synthesized were: Ar’2SiCl2 1, Ar’’2SiCl2 2, ArGeCl3 3, Ar2GeCl2 4, Ar’2GeCl2 5,
Ar’’2GeCl2 6, ArSnCl3 7, Ar2SnCl2 8, Ar’2SnCl2 9, and Ar’’2SnCl2 10 (Fig. 1)1). All products were dissolved
in Et2O and the solns. placed in the refrigerator. Six of these produced well-formed colorless crystals,
which were suitable for single-crystal X-ray diffraction [14]. All of these compounds were now further
analyzed by 19F-NMR spectroscopy. As found by previous authors [1g][8][14], attempted synthesis of
Ar2SiCl2 led to rapid F/Cl exchange, and the only product which could be isolated was Ar2SiF2. Slower
exchange occurred for Ar’- and Ar’’-substituted chlorosilanes [14], as shown in the Results and
Discussion section.
Results and Discussion. – The results of the 19F-NMR study of 1 – 10 are listed in
Table 1, while Fig. 2 shows some typical 19F-NMR spectra in the CF3 region. The
spectra of ArGeCl3 3 and Ar2SnCl2 8 (Fig. 2,a and b, resp.) as the main species present
exhibited the expected F-atom 2 :1 intensity ratio of the o-CF3/p-CF3 groups. The
solution produced by reaction of SnCl4 with the Ar’Li/Ar’’Li mixture showed a ca. 1.8 : 1
ratio of Ar’2SnCl2 9 to Ar’’2SnCl2 10 (Fig. 2,c). Compound 9 yielded a single resonance,
having o-CF3 groups only, while 10 gave two signals in a 1:1 intensity ratio for the o-
and p-CF3 groups. Fig. 2,d, showed that the main component Ar’’2SiCl2 2 had
undergone partial Cl/F exchange to Ar’’2SiF2, in the Cl/F ratio of ca. 4 :1 (the o-CF3
5
signal of Ar’’2SiF2 was a t due to J(F,F) coupling, and there was also a sept. signal at
d(F) ꢀ133.0 (not shown in Fig. 2,d) for the F-atoms bound to Si, with 5J(F,F) 12.4 Hz
[14]). The p-CF3 signals of 2 and Ar’’2SiF2 overlapped, hence the 1.25 relative intensity
integral included both signals. The spectrum of Ar’’2GeCl2 6 (Fig. 2,e) again gave the
expected 1:1 intensity ratio for the o- and p-CF3 groups.
Roden has shown that Ar’H reacts with BuLi to produce an Ar’Li/Ar’’Li 1:1
mixture, which reacts with PCl3 to generate Ar’Ar’’PCl and Ar’’2PCl; these have both
1
)
Systematic names of 1 – 10: 1,1’-(dichlorosilylene)bis[2,6-bis(trifluoromethyl)benzene] (1), 1,1’-
(dichlorosilylene)bis[2,4-bis(trifluoromethyl)benzene] (2), trichloro[2,4,6-tris(trifluoromethyl)phe-
nyl]germane (3), dichlorobis[2,4,6-tris(trifluoromethyl)phenyl]germane (4), bis[2,6-bis(trifluoro-
methyl)phenyl]dichlorogermane (5), bis[2,4-bis(trifluoromethyl)phenyl]dichlorogermane (6),
trichloro[2,4,6-tris(trifluoromethyl)phenyl]stannane (7), dichlorobis[2,4,6-tris(trifluoromethyl)phe-
nyl]stannane (8), bis[2,6-bis(trifluoromethyl)phenyl]dichlorostannane (9), and bis[2,4-bis(trifluor-
omethyl)phenyl]dichlorostannane (10).

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Fig. 1. Structures of the Group-14 compounds studied
Table 1. 19F-NMR Chemical Shifts of Si, Ge, and Sn Compounds. d in ppm.
Compound
d(F) (o-CF3)
4J(Sn,F) [Hz]
d(F) (p-CF3)
1
2
3
4
5
6
7
8
9
Ar’2SiCl2
Ar’’2SiCl2
ArGeCl3
Ar2GeCl2
Ar’2GeCl2
Ar’’2GeCl2
ArSnCl3
Ar2SnCl2
Ar’2SnCl2
Ar’’2SnCl2
ꢀ 58.9 (s, 12 F)
ꢀ 57.9 (s, 6 F)
ꢀ 54.9 (s, 6 F)
ꢀ 54.4 (s, 12 F)
ꢀ 53.8 (s, 12 F)
ꢀ 58.8 (s, 6 F)
ꢀ 55.9 (6 F)a)
ꢀ 57.1 (12 F)a)
ꢀ 57.0 (12 F)a)
ꢀ 59.1 (6 F)b)
ꢀ 64.2 (s, 6 F)
ꢀ 64.2 (s, 3 F)
ꢀ 64.1 (s, 6 F)
ꢀ 64.2 (s, 6 F)
ꢀ 63.0 (s, 3 F)
ꢀ 64.2 (s, 6 F)
19
10
10
10
ꢀ 64.0 (s, 6 F)
a) s with Sn satellites. b) Sn satellites not resolved.

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Fig. 2. 19F-NMR Spectra of several Group-14 compounds
been characterized by single-crystal X-ray diffraction [10][12][15]. Initially, it was
thought that the structure of the predominant isomer of the diaryl-substituted germane
and stannane derivatives would be Ar’Ar’’ECl2 (E ¼ Ge or Sn), for steric reasons. It
was found, however, that a solution of Ar’Li and Ar’’Li reacting with ECl4 (E ¼ Ge or
Sn) produced a mixture Ar’2ECl2/Ar’’2ECl2 (cf. Fig. 2,c) and not Ar’Ar’’ECl2. This
result is rather surprising from a steric viewpoint.
Table 2 shows the van der Waals radii and electronegativities of the central element
E [16], together with the shortest distances between F-atoms of the o-CF3 groups and E
for compounds where the crystal and molecular structures have been determined [14].
In all cases, these distances were considerably shorter than the sums of the van der

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Table 2. Bond Distance EꢀF of Group-14 Compounds
Compound
van der Waals
radius of E [ꢄ]
Electronegativity
of E
Sum of van der
Waals radii [ꢄ]
Shortest EꢀF
distance [ꢄ]
2 Ar’’2SiCl2
3 ArGeCl3
4 Ar2GeCl2
6 Ar’’2GeCl2
8 Ar2SnCl2
9 Ar’2SnCl2
2.08 (Si)
2.12 (Ge)
2.12 (Ge)
2.12 (Ge)
2.18 (Sn)
2.18 (Sn)
1.89 (Si)
2.07 (Ge)
2.07 (Ge)
2.07 (Ge)
1.86 (Sn)
1.86 (Sn)
1.47 þ 2.08 ¼ 3.55
1.47 þ 2.12 ¼ 3.59
1.47 þ 2.12 ¼ 3.59
1.47 þ 2.12 ¼ 3.59
1.47 þ 2.18 ¼ 3.65
1.47 þ 2.18 ¼ 3.65
2.882(2)
2.909(2)
2.757(2)
2.848(2)
2.720(4)
2.686(2)
Waals radii, as shown. These results clearly indicate an additional weak (ꢀsecondaryꢁ)
coordination or attractive electrostatic interactions, which can play an important role in
the stabilization of these compounds (Table 2).
The 19F-NMR chemical shifts for p-CF3 substituents in the Si, Ge, and Sn chloro
compounds with Ar, Ar’, and Ar’’ were between d(F) ꢀ 64.0 and ꢀ 64.2, except for
ArSnCl3 7 with a d(F) of ꢀ 63.0 (Table 1). These results were explained by the fact that
p-CF3 groups are remote from the main-group atoms E; thus, the corresponding F-
atoms did not interact with E, and their d(F) were very similar.
A much larger shift range was found for the o-CF3 groups, from d(F) ꢀ 53.8 to
ꢀ 59.1 (Table 1), due at least in part to the intramolecular FꢀE interactions mentioned
above. The atoms E also differ in covalent radius and electronegativity (Table 2), which
can affect the chemical shifts. As shown in Table 1, the d(F) value was always more
negative for the o-CF3 groups where E ¼ Sn than for the analogous compound where
E ¼ Ge, consistent with their relative electronegativities [16]. Fewer values for Si
compounds could be compared because of the rapid F/Cl exchange found in Ar
systems. Hence, Ar2SiCl2 was not observed, although it may well be formed as a
reactive intermediate [1g][8][14]. The F-atoms of the o-CF3 groups of Ar’2SiCl2 1 had a
more negative shift than those of either Ar’2GeCl2 5 or Ar’2SnCl2 9. Ge is generally
regarded as being more electronegative than Sn or Si, which have comparable
electronegativities (Table 2); so the less negative shifts in Ge compounds probably
reflected this, while the results for Si and Sn compounds were very similar where data
were available. The exception to this generally good correlation appeared to be
Ar’’2GeCl2 6, which had a more negative d(F) for the o-CF3 groups than any of the
other germanium compounds. Both ArGeCl3 3 and Ar’’2GeCl2 6 have only six F-atoms
of the o-CF3 groups; however Ar2GeCl2 4 and Ar’2GeCl2 5 have twelve such F-atoms;
so, in ArGeCl3 3 and Ar’’2GeCl2 6, there are fewer possible weak interactions of the
type discussed previously. In a similar way, Ar’’2SnCl2 10 had a more negative shift than
Ar’2SnCl2 9, although the reverse pattern appeared to hold for Si compounds 1 and 2,
4
where the values were not very different (Table 1). Small J(Sn,F) couplings were
observed in compounds 7, 8, and 9 (Table 1), though it was not possible to distinguish
19
19
the separate 117Snꢀ F and 119Snꢀ F satellites. These values were entirely compatible
with the number of bonds involved.
19F-NMR Spectroscopy is clearly valuable as a means of identifying the products
formed in these reactions, particularly those from the Ar’Li/Ar’’Li mixture. In the
disubstituted compounds R1R2ECl2, e.g., if R1 ¼ R2 ¼ Ar’, only a single signal from

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twelve F-atoms of o-CF3 groups should be seen. If R1 ¼ R2 ¼ Ar’’, then two equally
intense signals should be apparent, one from six F-atoms of o-CF3 groups at higher
frequency and one from six F-atoms of p-CF3 groups at lower frequency. In the event of
formation of the mixed species Ar’Ar’’ECl2, there should be two separate signals for
the F-atoms of o-CF3 groups in a 2 :1 intensity ratio, and one signal of intensity 1 for the
F-atoms of the p-CF3 group. As stated above, such a species was not detected in the
present work. More complex mixtures of possible products can be similarly analyzed.
Conclusions. – The variation in 19F-NMR chemical shifts for some Group-14 aryl-
substituted chloro derivatives RnECl4ꢀn (R ¼ 2,4,6-(CF3)3C6H2 (¼Ar), 2,6-(CF3)2C6H3
(¼Ar’), or 2,4-(CF3)2C6H3 (¼Ar’’); E ¼ Si, Ge, or Sn; n ¼ 1 or 2) was studied for the
first time. While the d(F) for p-CF3 groups remained essentially constant, those for the
o-CF3 groups showed interesting variations, which could be correlated with the number
of weak EꢀF interactions found in the solid state, and with the electronegativities of the
Group-14 elements. 19F-NMR spectroscopy was particularly valuable for identifying
the components of mixtures of species in solution. We hope to extend this approach to
other groups of the Periodic Table in the near future.
We thank the Committee of Vice Chancellors and Principals of the Universities of the United
Kingdom (CVCP) for an ORS award, the Graduate Society of Durham University for a graduate-student
award for B. Y. Xue, A. M. Kenwright, C. F. Heffernan, and I. H. McKeag for assistance in recording some
of the NMR spectra, and Y. G. Fan and J. H. Zhao for help drawing figures.
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Received July 23, 2012