Dependence of the rotational barrier of the Ar-group in RArTeX2 on the R-group [Ar=2,6-(MeO)2C6H3; R=Me, Et, i-Pr; X=Cl, Br, I]

Article abstract of DOI:10.1039/b210033a

Alkyl(2,6-dimethoxyphenyl)tellurium dihalides, RArTeX2 [Ar=2,6-(MeO)2C6H3; X=Cl 2a-c, Br 3a-c, I 4a-c; R=Me a, Et b, i-Pr c] were prepared by the reactions of alkyl 2,6-dimethoxyphenyl telluride, RArTe 1, with SOCl2, Br2 or I2, respectively. The rotationa

Full text of DOI:10.1039/b210033a

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Dependence of the rotational barrier of the Ar-group in RArTeX2
on the R-group [Ar ؍ 2,6-(MeO)2C6H3; R ؍ Me, Et, i-Pr;
X ؍ Cl, Br, I]
Masahiro Asahara,* Shoichiro Taomoto, Masahito Tanaka, Tatsuo Erabi and Masanori Wada
Department of Materials Science, Faculty of Engineering, Tottori University, Koyama, Tottori,
680-8552, Japan
Received 11th October 2002, Accepted 8th January 2003
First published as an Advance Article on the web 30th January 2003
Alkyl(2,6-dimethoxyphenyl)tellurium dihalides, RArTeX2 [Ar = 2,6-(MeO)2C6H3; X = Cl 2a–c, Br 3a–c, I 4a–c;
R = Me a, Et b, i-Pr c] were prepared by the reactions of alkyl 2,6-dimethoxyphenyl telluride, RArTe 1, with SOCl2,
Br2 or I2, respectively. The rotational barrier ∆G‡ of the Ar-group around the Te–C bond in 2a–c, 3a–c and 4a–c
estimated by variable temperature 1H NMR spectra was dependent on the alkyl (R) group as well as on the halogen
atoms. It decreased in the order R = Me > Et > i-Pr as well as X = Cl > Br > I. The 125Te resonances of 1 were
observed at higher magnetic fields than those of RPhTe, and those of 1a–c, 2a–c, 3a–c and 4a–c shifted to lower
magnetic field in the order R = Me > Et > i-Pr. The X-ray crystallographic analyses of 2a–c, 3a, 3b and 4a showed
that the geometry around tellurium was pseudo-trigonal bipyramidal with the alkyl group, the Ar group and a lone
pair of electrons in the equatorial positions and with two halogen atoms in the apical positions. Whereas each of the
Te–C(Ar) bond distances were very similar [2.10 0.01 Å], the Te–C(R) bonds of 2a–c were longer than Te–C(Ar)
and increased in length in the order R = Me < Et < i-Pr. The C(Ar)–Te–C(R) bond angles also increased in the order
R = Me < Et < i-Pr. These molecules were bridged by intermolecular Te ؒ ؒ ؒ X bonding to form dimers or polymers.
Based on these results and VSEPR theory, the dependence of the rotational barrier ∆G‡ of the Ar-group in
RArTeX2 on the R-group is discussed.
Valence shell electron-pair repulsion theory (VSEPR theory)
predicts that the Te atom in diphenyltellurium dihalides,
Ph2TeX2, in solutions is in a pseudo-trigonal bipyramidal co-
ordination with two Te–C bonds and a lone pair of electrons
occupying the equatorial sites and with two halogen atoms
occupying the apical sites.1 The actual shape of the molecule is
like a seesaw form. The prediction has been supported by crys-
tal structure analyses.2–5 When the phenyl groups have substi-
tuents at 2,6-positions, the rotation of the phenyl group around
the Te–C(Ar) bonds is restricted due to the barriers between the
spectroscopy and X-ray crystal structure analyses. We also
discuss the influence of an alkyl group at an equatorial position
on the rotational barrier of the Te–C(Ar) bonds. During the
course of our present investigation, we noticed fundamental
errors in calculations of the rotational barrier∆G‡ in the previ-
ous paper.7 It was calculated by applying equation (1),16 but
∆G‡/(RT c) = 22.96 ϩ loge(T c/δν)
(1)
with an incorrect unit for δν (ppm instead of Hz). Thus, we
correct ∆G‡ values for Ar2TeX2 2d–5d [Ar = 2,6-(MeO)2C6H3;
X = Cl 2, Br 3, I 4, SCN 5] as shown in Table 4 (later).
1
2,6-substituents and the halogen atoms, as observed in the H
NMR studies of bis(2,6-dimethylphenyl)tellurium dihalides,6
bis(2,6-dimethoxyphenyl)tellurium dihalides,7 and bis(2,6-
difluorophenyl)tellurium dihalides.8 In these studies, the influ-
ence of the interaction between the two aryl groups on the
rotational barrier is unknown. In addition, while crystal struc-
Experimental
General
1H NMR spectra were recorded for solutions using a JEOL
model JNM-GX270 spectrometer. 1H chemical shifts were
referenced to internal TMS (δ 0.00) in CDCl3 or DMSO-d6
(δ 2.49). 13C and 125Te NMR spectra were recorded for solutions
in CDCl3 using a JEOL model JNM-ECP500 spectrometer.
13C NMR chemical shifts were referenced to internal CDCl3
(δ 77.00), and 125Te NMR chemical shifts were referenced
ture analyses of a variety of diaryltellurium dihalides have been
2–15
reported,
few are known for alkyltellurium derivatives. In
the present paper, we report the systematic investigation of
alkyl(2,6-dimethoxyphenyl)tellurium dihalides, RArTeX2 2a–c,
3a–c and 4a–c (see Scheme 1) by 1H, 13C and 125Te NMR
1
to external diphenylditelluride (δ 450). The H, 13C, and 125Te
NMR spectral data are summarized in Tables 1 and 2. The
preparations of ArTeTeAr, MeArTe 1a, EtArTe 1b and
Ar2TeX2 2d–5d have been reported elsewhere.7,17
Preparation of isopropyl 2,6-dimethoxyphenyl telluride 1c
To a suspension of ArTeTeAr (2.6 g, 5 mmol) in ethanol
(100 cm3) was added NaBH4 (0.63 g, 15 mmol) with stirring,
followed by addition of 2-bromopropane (0.94 cm3, 10 mmol).
After stirring for 3 h, the reaction mixture was concentrated
in vacuo, and water (100 cm3) and chloroform (100 cm3) were
added for extraction. The chloroform layer was dried over
anhydrous magnesium sulfate and was concentrated in vacuo to
give a pale-brown liquid of 1c in 86% yield. It was characterized
Scheme 1 Reagents and conditions: (i) LiCl/THF, 0 ЊC∼rt, 20 h;
(ii) ϩO2; (iii) ϩNaBH4/EtOH; (iv) ϩRX, rt, 3 h; (v) ϩSOCl2/toluene,
ϩBr2/EtOH, or ϩI2/EtOH, rt, 0.5 h.
1
1
by H and 13C NMR spectra. H NMR (270 MHz, CDCl3):
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
D a l t o n T r a n s . , 2 0 0 3 , 9 7 3 – 9 7 9
973

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Table 1 1H NMR spectral data for RArTeX2 in CDCl3 at 25 ЊCa
Compound
4-Hb
3,5-Hc
2,6-MeO-Hd
-CH3
Others
2a
2b
2c
3a
3b
3c
4a
4b
4c
7.43
7.42
7.42
7.44
7.43
7.43
7.46
7.45
7.45
6.66
6.66, 6.65
6.66
3.95, 3.99
3.93, 3.98
3.93, 3.96e
3.95, 4.00
3.95, 3.99
3.95e
3.98
3.96
3.96
3.48d (2JTe–H= 33 Hz)
1.86b (3JTe–H = 66 Hz)
1.90c (3JTe–H = 48 Hz)
3.61d (2JTe–H= 35 Hz)
1.83b (3JTe–H = 65 Hz)
1.93c (3JTe–H = 45 Hz)
3.61d (2JTe–H= 33 Hz)
1.73b (3JTe–H = 64 Hz)
1.96c (3JTe–H = 40 Hz)
4.12f
4.73g
6.63
6.64e
6.64
4.23f
4.81g
6.58
6.59
6.59
4.18f
4.70g
f
a 270.17 MHz, δ/ppm, All JH–H are 8 Hz. b A triplet. c A doublet. d A singlet. e Broad. A quartet, -CH2 . g A septet, ≡CH.
-
Table 2 13C NMRa and 125Te NMRb spectral data for RArTeX2 in CDCl3 at 25 ЊC
2,6-C
4-C
3,5-C
1-C
-OCH3
TeC
TeCC
125Te
2a
2b
2c
3a
3b
3c
4a
4b
4c
161.3, 159.3
160.6, 159.3
160.3, 159.9
161.7, 159.4
161.1, 159.4
160.7, 160.0
162.2, 159.3
161.5, 159.5
160.7
134.7
134.6
134.6
134.7
134.7
134.7
134.5
134.5
134.6
106.0, 104.6
105.7, 104.6
105.6, 104.9
106.0, 104.7
105.8, 104.7
105.6, 105.0
106.1, 104.8
105.9, 105.0
105.3
112.8
114.0
114.0
108.5
109.9
109.7
101.6
103.3
103.0
57.2, 56.2
57.1, 56.2
57.0, 56.0
57.2, 56.3
57.1, 56.3
57.0, 56.0
57.1, 56.2
57.0, 56.1
56.3
26.8
42.3
53.9
25.5
41.7
52.4
23.2
39.8
49.1
794 (33)d
882 (66)e
990 (48)f
717 (35)d
816 (65)e
955 (45)f
602 (33)d
714 (64)e
901 (40)f
10.9
21.2 (43)c
11.0
21.8 (37)c
11.0
22.9 (33)c
c 3
a 125.8 MHz, δ/ppm. b 157.9 MHz, δ/ppm.
J
/Hz. d A quartet, 2JTeH/Hz. e A quartet, 3JTeH/Hz. f A septet, 3JTeH/Hz.
TeC
δ 7.25 (t, JHH = 8 Hz, 1H; 4-H ), 6.53 (d, JHH = 8 Hz, 2H; 3,5-H ),
3.85 (s, 6H; 2,6-CH3O), 4.03 (septet, JHH = 8 Hz, 1H; (CH3)2-
CH-), 1.53 (dd, JHH = 8 Hz, 3JTeH = 34 Hz, 6H; CH3-); 13C NMR
(125.8 MHz, CDCl3): δ 161.3, 129.9, 103.4, 93.5, 55.9, 26.1
(JTeC = 36 Hz), 14.6 (JTeC = 141 Hz); 125Te NMR (157.9 MHz,
CDCl3): δ 455.
i-PrArTeBr2 3c. Using 1c as above, i-PrArTeBr2 3c was pre-
pared in 85% yield as pale yellow crystals; mp (decomp.) 135 ЊC
(Found: C, 28.27; H, 3.25%. C11H16Br2O2Te1 requires C, 28.25;
H, 3.45%).
Preparations of alkyl(2,6-dimethoxyphenyl)tellurium diiodides
4a–c
Preparations of alkyl(2,6-dimethoxyphenyl)tellurium dichlorides
2a–c
MeArTeI2 4a. To a solution of 1a (0.28 g, 1.0 mmol) in eth-
anol (6 cm3) was added iodine (0.31 g, 1.2 mmol), and the mix-
ture was stirred vigorously for 0.5 h. The resultant precipitates
were filtered to give orange crystals of methyl(2,6-dimethoxy-
phenyl)tellurium diiodide, MeArTeI2 4a in 83% yield; mp
(decomp.) 131–140 ЊC (Found: C, 19.97; H, 2.08%. C9H12I2O2-
Te1 requires C, 20.26; H, 2.27%).
MeArTeCl2 2a. To a solution of 1a (0.70 g, 2.5 mmol) in dry
toluene (25 cm3) was added SO2Cl2 (0.2 cm3, 2.5 mmol) under
an argon atmosphere, and the mixture was stirred vigorously
for 0.5 h. The resultant precipitates were recrystallized from
toluene/hexane to give white crystals of methyl(2,6-dimethoxy-
phenyl)tellurium dichloride, MeArTeCl2 2a in 84% yield; mp
(decomp.) 190–192 ЊC (Found: C, 30.75; H, 3.12%. C9H12Cl2-
O2Te1 requires C, 30.82; H, 3.45%).
EtArTeI2 4b. Using 1b as above, EtArTeI2 4b was prepared in
90% yield as orange crystals; mp (decomp.) 121 ЊC (Found: C,
21.87; H, 2.63%. C10H14I2O2Te1 requires C, 21.93; H, 2.58%).
EtArTeCl2 2b. Using 1b as above, EtArTeCl2 2b was prepared
in 80% yield as white crystals; mp 171–173 ЊC (Found: C, 32.88;
H, 3.74%. C10H14Cl2O2Te1 requires C, 32.93; H, 3.87%).
i-PrArTeI2 4c. Using 1c as above, i-PrArTeI2 4c was pre-
pared in 90% yield as orange crystals; mp (decomp.) 115 ЊC
(Found: C, 23.47; H, 2.70%. C11H16I2O2Te1 requires C, 23.52;
H, 2.87%).
i-PrArTeCl2 2c. Using 1c as above, i-PrArTeCl2 2c was pre-
pared in 88% yield as white crystals; mp (decomp.) 189–192 ЊC
(Found: C, 34.86; H, 4.11%. C11H16Cl2O2Te1 requires C, 34.88;
H, 4.26%).
X-Ray crystallography
Single crystals of 2a–c, 3a,b and 4a suitable for X-ray crystal
structure analysis were obtained by recrystallization from
nitromethane. The intensity data were collected at 173 K on a
Rigaku RAXIS Rapid-S imaging plate area detector with
graphite-monochromated MoKα (λ = 0.71070 Å) radiation.
The data were corrected at a temperature of Ϫ100 1 ЊC and
for Lorentz and polarization effects. Their structures were
solved by direct methods (SIR92)18 and expanded using Fourier
techniques (DIRDIF99).19 The non-hydrogen atoms were
refined anisotropically. Hydrogen atoms were included at calcu-
lated positions but not refined. All calculations were performed
using the CrystalStructure crystallographic software pack-
age.20,21 Their crystal data and experimental details are listed in
Table 3.
Preparations of alkyl(2,6-dimethoxyphenyl)tellurium dibromides
3a–c
MeArTeBr2 3a. To a solution of 1a (0.28 g, 1.0 mmol) in
ethanol (6 cm3) was added bromine (0.064 cm3, 1.2 mmol). The
mixture was stirred vigorously for 0.5 h to give pale yellow
crystals of methyl(2,6-dimethoxyphenyl)tellurium dibromide,
MeArTeBr2 3a in 86% yield; mp (decomp.) 164 ЊC (Found: C,
24.54; H, 2.56%. C9H12Br2O2Te1 requires C, 24.59; H, 2.75%).
EtArTeBr2 3b. Using 1b as above, EtArTeBr2 3b was prepared
in 70% yield as pale yellow crystals; mp (decomp.) 156 ЊC
(Found: C, 26.39; H, 2.89%. C10H14Br2O2Te1 requires C, 26.48;
H, 3.11%).
CCDC reference numbers: 195252–195257.
D a l t o n T r a n s . , 2 0 0 3 , 9 7 3 – 9 7 9
974

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Table 3 Crystal data and structure refinements for 2a–c, 3a,b and 4a
2a
2b
2c
3a
3b
4a
Empirical formula
Formula weight
Crystal size/mm3
Crystal system
Lattice parameters:
a/Å
C9H12O2Cl2Te
350.70
C10H14O2Cl2Te
364.73
C11H16O2Cl2Te
378.75
C9H12O2Br2Te
439.60
C10H14O2Br2Te
453.63
C9H12O2I2Te
533.60
0.30 × 0.20 × 0.20 0.20 × 0.20 × 0.15 0.30 × 0.30 × 0.20 0.40 × 0.30 × 0.20 0.40 × 0.30 × 0.30 0.20 × 0.20 × 0.10
orthorhombic
orthorhombic
orthorhombic
orthorhombic
orthorhombic
monoclinic
8.3486(1)
13.6876(2)
20.6906(3)
6.7317(2)
15.2528(3)
25.9327(5)
7.0124(4)
15.1295(5)
26.187(1)
8.5520(1)
13.6294(2)
21.2777(3)
6.8905(2)
15.5279(8)
26.0281(7)
7.6021(4)
11.5143(3)
15.5013(5)
92.204(2)
1355.87(8)
P21/a (No. 14)
4
2.614
67.27
54.9
b/Å
c/Å
β/Њ
V/Å3
2364.36(5)
Pbca (No. 61)
8
1.970
29.41
2662.70(9)
Pbca (No. 61)
8
1.819
26.15
2778.3(2)
Pbca (No. 61)
8
1.811
25.10
2480.10(5)
Pbca (No. 61)
8
2.354
88.37
2784.9(1)
Pbca (No. 61)
8
2.164
78.74
Space group
Z
Dcalcd/g cmϪ3
µ(MoKα)/cmϪ1
2θmax/Њ
54.9
54.9
54.4
54.9
54.9
Reflections total
Observations
(I > Ϫ10σ(I ))
No. of variables
R1, wR2
21625
2697
23374
2542
22671
3078
20934
2816
23541
3133
11584
3069
139
0.023, 0.087
1.03
150
0.020, 0.071
1.00
161
0.024, 0.108
1.02
139
0.023, 0.040
1.04
150
0.034, 0.132
1.02
140
0.028, 0.139
1.02
GOF
See http://www.rsc.org/suppdata/dt/b2/b210033a/ for crystal-
lographic data in CIF or other electronic format.
Results
Preparation of alkyl(2,6-dimethoxyphenyl)tellurium dihalides,
RArTeX2
Alkyl 2,6-dimethoxyphenyl tellurides, RArTe 1a–c were
readily transformed into alkyl(2,6-dimethoxyphenyl)tellurium
dihalides, RArTeX2 2a–c, 3a–c and 4a–c by the reaction with
SOCl2, Br2 or I2, respectively, in 70–90% yields, as shown in
Scheme 1. They were white (2), yellow (3) or orange (4) crystals
with definite melting points at higher temperatures than 100 ЊC.
1H, 13C and 125Te NMR spectra
1
The H, 13C and 125Te NMR spectral data of 2–4 are summar-
ized in Tables 1 (1H) and 2 (13C and 125Te), respectively. As
observed for Ar2TeX2 2d–5d,7 the 1H NMR spectra of 2a–c, 3a–
c and 4a–c were temperature-dependent (Fig. 1), indicating that
the rotation of Te–C(Ar) bonds is restricted in solution. The
coalescence temperatures, T c, of 2a–c in CDCl3 were too high
to be observed, but they could be observed for DMSO-d6 solu-
tions. Those of 3a–c and 4a–c could be observed for CDCl3
solutions but not for DMSO-d6 solutions due to the solidifi-
cation at such low temperatures. We also failed to record the
spectra of 2a–c, 3a–c and 4a–c in D2O due to their poor solu-
bilities. From these T c and the chemical shift difference δν, the
rotational barriers ∆G‡ of the Ar-group were calculated as
summarized in Table 4, in which the corrected ∆G‡ values of
2d–5d are also given.
As observed for 2d–5d, the rotational barriers ∆G‡ of 2a–c,
3a–c and 4a–c were also influenced by the halogen X, and
decreased in the order X = Cl > Br > I. Although we could not
calculate ∆G‡ values for 3a–c in CDCl3, they must be larger in
CDCl3 than in DMSO-d6.7 A very interesting result obtained in
the present study is that the ∆G‡ values in 2a–c, 3a–c and 4a–c
were influenced also by the equatorial substituent R, and
decreased in the order R = Me > Et > i-Pr, the reverse order of
bulkiness. In order to obtain more information, the X-ray crys-
tal structure analyses for some of the present compounds were
performed (see below).
1
Fig. 1 Temperature-dependent H NMR spectra of (a) 2a and (b) 2c
in DMSO-d6.
ipso-carbon resonance was quite sensitive to the changes of X,
and shifted to higher magnetic field in the order X = Cl < Br < I.
The Te–C(R) carbon resonance was observed at higher mag-
netic fields in the order Me > Et > i-Pr, and it also shifted to
higher magnetic fields in the order X = Cl < Br < I. These 13C
resonances of 2a–c, 3a–c and 4a–c were observed at lower
magnetic fields than those of 1a–c, respectively.
The 13C NMR spectra of 2a–c, 3a–c and 4a–c, except for
4c, measured at 25 ЊC in CDCl3 were consistent with the
asymmetry of the 2,6-dimethoxyphenyl group, showing six
nonequivalent resonances for the phenyl carbons and two
nonequivalent resonances for the methoxy carbons. The
The 125Te NMR spectra of RPhTe (R = Me, Et, i-Pr) in
CDCl3 have been reported, and the 125Te resonance was
D a l t o n T r a n s . , 2 0 0 3 , 9 7 3 – 9 7 9
975

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Table 4 Temperature-dependent 1H NMRa data of RArTeX2 2–5
R
X
T c/K
δν/Hz
∆G‡/kJ molϪ1
2ab
2bb
2cb
3a
Me
Et
i-Pr
Me
Et
i-Pr
Me
Et
i-Pr
Ar
Cl
Cl
Cl
Br
Br
Br
I
I
I
Cl
Br
I
353
343
320
328
316
285
298
276
235
360
324
244
≥333
10.0
11.9
6.8
13.2
13.8
7.8
15.4
14.6
3.0
96.7
95.1
88.1
157.8
78
75
71
71
69
63
64
61
53
3b
3c
4a
4b
4c
2db
3db
4d
73c
65c
49c
≥66c
Ar
Ar
Ar
5d
SCN
a 270.17 MHz in CDCl3. b DMSO-d6. c The corrected data for ref. 7 based on their δν value.
Fig. 2 X-Ray crystal structures of RArTeX2 2a–c, 3a,b and 4a drawn at 30% probability level. All hydrogen atoms are omitted for clarity.
2,4,5
observed at lower magnetic fields in the order R = Me (δ ϩ349)
> Et (δ ϩ552) > i-Pr (δ ϩ720).22 The 125Te resonances of 1 were
observed at higher magnetic fields [R = Me (δ ϩ127), Et
(δ ϩ307), i-Pr (δ ϩ455)] than those of RPhTe, indicating the
possibility of an interaction of the 2,6-methoxy oxygen atoms
with tellurium. The 125Te resonances of 2a–c, 3a–c and 4a–c
were observed also at lower magnetic field in the order R= Me >
Et > i-Pr, as observed for R2TeCl2.23,24 The resonances were
sensitive also to changes of X, and they shifted to the lower
magnetic field in the order X = Cl < Br < I, as observed for
Me2TeX2.7,23,24 As reported for R2TeBr2 (R = Me, Et, i-Pr),23
of reported compounds such as Me2TeX2,25–27 Ph2TeX2
and Ar2TeX2 2d–4d7 are given in Table 6.
Whereas each of the Te–C(Ar) bond distances are almost
equal [2.091–2.109 Å] to those found for 2d–4d,7 the Te–C(R)
bonds of 2a–c, 3a, 3b, and 4a were longer than the Te–C(Ar)
bonds, and they increased remarkably in the order R = Me
[2.111–2.123 Å] < Et [2.145, 2.149 Å] < i-Pr [2.178 Å].
The C–Te–C bond angles [101.9–108.0Њ] of 2a–c, 3a,b and 4a
are much smaller, as expected from VSEPR theory, than 120Њ,
the ideal angle of trigonal bipyramidal geometry. The angles of
the methyl derivatives 2a, 3a, 4a [102.7, 101.9, 102.2Њ, respect-
ively] are wider than those reported for Me2TeX2 [91–98Њ]25–27
and Ph2TeX2 [94–99Њ]2,4,5 but narrower than those reported for
2d–4d [106.2–107.6Њ]. The C–Te–C bond angle of 2a–c increases
in the order R = Me < Et < i-Pr [102.7, 106.1, 108.0Њ, respect-
ively], which is attributed tentatively to the steric effects of
R-groups.
The two Te–X bond lengths in 2a–c, 3a,b and 4a are not
equivalent due probably to the intermolecular secondary bond-
ing interaction (Fig. 3). The bond lengths [Te–Cl 2.483–2.559
Å, Te–Br 2.641–2.722 Å, Te–I 2.898, 2.935 Å] are in the range
or somewhat longer than those reported for Me2TeX2, Ph2TeX2
and Ar2TeX2 [Te–Cl 2.480–2.541 Å, Te–Br 2.622–2.707 Å, Te–I
2.854–2.994 Å]. These molecules are bridged by intermolecular
Te ؒ ؒ ؒ X bonds to form dimers (2a–c, 3a,b) or a polymer (4a).
The intermolecular Te ؒ ؒ ؒ X bonds in the dimers are located
opposite to the Ar–Te bond. The intermolecular bond lengths
2
no coupling constant JTe–H could be observed for R = Et and
3
i-Pr derivatives of 2a–c, 3a–c and 4a–c, while JTe–H could be
observed.
X-Ray crystal structures of some alkyl(2,6-dimethoxyphenyl)-
tellurium dihalides
The molecular structures of 2a–c, 3a,b and 4a are shown in
Fig. 2, and their intermolecular relationships are shown in
Fig. 3. The crystal data are listed in Table 3, and selected inter-
atomic distances and angles are given in Table 5. The geometry
around the tellurium atom of each compound is essentially
pseudo-trigonal bipyramidal with one alkyl group (Me, Et or
i-Pr), one Ar-group and a lone pair of electrons occupying the
equatorial sites and with two halogen atoms occupying the
apical sites. For comparison, selected bond distances and angles
D a l t o n T r a n s . , 2 0 0 3 , 9 7 3 – 9 7 9
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Table 5 Selected interatomic distances (Å) and angles (Њ) for 2a–c, 3a,b and 4a
2a (X = Cl)
2b (X = Cl)
2c (X = Cl)
3a (X = Br)
3b (X = Br)
4a (X = I)
Bond distances/Å
Te–C1(Ar)
Te–C9(R)
Te–X1
2.104(2)
2.111(3)
2.559(1)
2.483(1)
2.098(2)
2.145(3)
2.519(1)
2.508(1)
2.100(3)
2.178(3)
2.488(1)
2.532(1)
2.109(3)
2.123(3)
2.641
2.091(4)
2.149(5)
2.669(1)
2.677(1)
2.109(3)
2.115(4)
2.935
Te–X2
2.722
2.898
Bond angles/Њ
C1–Te–C9
X1–Te–X2
X1–Te–C1
X1–Te–C9
X2–Te–C1
X2–Te–C9
102.7(1)
172.29(2)
87.48(7)
85.06(8)
88.32(7)
89.56(8)
106.1(1)
172.12(2)
88.34(6)
88.49(7)
87.31(6)
86.41(7)
108.0(1)
174.95(3)
88.00(8)
90.81(8)
88.25(8)
87.12(8)
101.9(1)
174.23(1)
89.42(8)
90.9(1)
87.54(8)
85.0(1)
106.2(2)
173.35(2)
87.8(1)
87.6(1)
89.0(1)
87.7(1)
102.2(2)
178.77(1)
89.2(1)
90.4(1)
90.0(1)
88.8(1)
Interatomic distances/Å
Te ؒ ؒ ؒ O1
Te ؒ ؒ ؒ O2
Te ؒ ؒ ؒ X*
Te* ؒ ؒ ؒ X
Te* ؒ ؒ ؒ I**
Te** ؒ ؒ ؒ I*
3.272(2)
3.223(2)
2.909(2)
3.617
3.225(2)
2.912(2)
3.823
3.271(2)
2.886(2)
3.558
3.240(3)
2.898(3)
3.716(1)
3.716(1)
3.318(3)
2.853(3)
4.018
4.03
4.018
4.03
2.899(2)
3.469(1)
3.995(1)
3.617
3.823
4.114
Fig. 3 Oligomeric structures of RArTeX2 2a–c, 3a,b, and 4a.
are close to the sums of the van der Waals radii of Te and X:
Te ؒ ؒ ؒ Cl = 3.81 Å, Te ؒ ؒ ؒ Br = 3.91 Å, and Te ؒ ؒ ؒ I = 4.04 Å,
respectively.28 The presence of such intermolecular Te ؒ ؒ ؒ X
bonding has been observed for a variety of compounds of type
R2TeX2.2–6,8–13,25–27
The X–Te–X bond angles in 2a–c, 3a and 4a are very close to
but slightly narrower than 180Њ as observed for Me2TeX2 and
Ph2TeX2. The X–Te–X bonds are both bent slightly with the
halogen atoms located closer between the alkyl and Ar groups,
as expected from VSEPR theory.4
Discussion
A very interesting result obtained in the present study is that the
rotational barrier ∆G‡ of the Ar-group around the Te–C(Ar)
bond in 2a–c, 3a–c and 4a–c was influenced not only by the
axial halogen atoms, X, but also by the equatorial substituent
R, and it decreased in the orders X = Cl > Br > I and R = Me >
Et > i-Pr, both the reverse orders of bulkiness. It seems neces-
sary, however, to reconsider the precise causes of the rotational
barrier ∆G‡.
D a l t o n T r a n s . , 2 0 0 3 , 9 7 3 – 9 7 9
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Table 6 Selected bond distances (Å) and angles (Њ) for R2TeX2
Ar2TeCl2 2d7
Ar2TeBr2 3d7
Ar2TeI2 4d7
C(Ar)–Te
Te–X
2.100(2)
2.513(1)
2.092(3)
2.6746(3)
2.111(5)
2.9362(4)
C(Ar)–Te–C(Ar)
X–Te–X
107.6(2)
177.57(3)
107.5(2)
179.36(1)
106.2(3)
177.60(2)
25
27
26
Me2TeCl2
Me2TeBr2
Me2TeI2
C(Me)–Te
Te–X
2.08(3), 2.10(3)
2.480(10), 2.541(10)
2.10(2), 2.22(2)
2.622(3), 2.707(3)
2.10(3)–2.16(3)
2.854(3)–2.994(3)
C(Me)–Te–C(Me)
X–Te–X
98.2(11)
172.3(3)
97.4(8)
173.9(1)
91(2)–97(2)
177.3(2)–178.3(6)
4
2
5
Ph2TeCl2
Ph2TeBr2
Ph2TeI2
C(Ph)–Te
Te–X
2.102(7), 2.111(7)
2.482(2), 2.529(3)
2.18(3)
2.682(3)
2.122(15)–2.153(8)
2.883(1)–2.959(1)
C(Ph)–Te–C(Ph)
X–Te–X
99.01(29)
175.54(7)
93.9(12)
177.9(2)
94.2(4)–96.7(5)
174.23(4)–175.53(5)
The rotational barrier ∆G‡ is the difference of the energies
between the ground state and the transition state. The most
probable conformations of these states are shown in Fig. 4,
of which the transition state resembles an initial stage of the
familiar Berry pseudo-rotation mechanism used to explain the
exchange process between the equatorial halogens and the axial
halogens in trigonal bipyramidal PX5 molecules (X = F, Cl).29 It
is assumed for 2a–c, 3a–c and 4a–c that, at the transition state,
the X–Te–X sequence is bent with the halogen atoms located
closer to both the lone pair electrons and the alkyl group to
avoid the steric interaction between the Ar group and the X
atoms. If the free rotation of the Ar-group in 2a–c, 3a–c and
4a–c follows this process, the rotational barrier ∆G‡ must be
related to the difference of the total VSEPR energies between
the ground state and such a transition state. At the transition
state, the repulsion energy between the Ar–Te bonding electrons
and the Te–X bonding electrons must decrease because of the
increased Ar–Te–X angles, while the repulsion energies between
the Te–X bonding electrons and both the lone pair electrons
and the Te–C(R) bonding electrons must increase because of
the decreased R–Te–X angles. The repulsion energy of 2a–c at
the transition state must be largest for 2a because it has the
shortest Te–C(R) bond among the three compounds, and the
repulsion energy of 2c must be the smallest because it has the
longest Te–C(R) bond, as observed from the crystal structure
analyses. Essentially in an analogous manner, the order of
halogen influence may be explained, where the Te–X bonding
electrons are located apart from the other electron pairs in the
order X = Cl < Br < I.
In spite of the bulkiness of the Ar-group, the rotational
barrier ∆G‡ of 2d was smaller than those of 2a and 2b, and that
of 4d was the smallest among the four iodides 4a–d (Table 4).
The Te–C(Ar) bonds were shorter than the Te–C(R) bonds.
These results apparently are inconsistent with the explanation
mentioned above. The true cause for the smaller ∆G‡ of 2d–4d
than most of 2a–c, 3a–c and 4a–c is unknown at present. It is
worth noting here that the δν values of 2d–4d used for the calcu-
lation by equation (1) were much larger than those of 2a–c,
3a–c and 4a–c (Table 4).
References
1 R. J. Gillespie and R. S. Nyholm, Quart. Rev. Chem. Soc., 1957, 11,
339; R. J. Gillespie, J. Am. Chem. Soc., 1960, 82, 5978; R. J. Gillespie,
Chem. Soc. Rev., 1992, 21, 59.
2 G. D. Christofferson and J. D. McCullough, Acta Crystallogr., 1958,
11, 249.
3 F. J. Berry and A. J. Edwards, J. Chem. Soc., Dalton Trans., 1980,
2306.
4 N. W. Alcock and W. D. Harrison, J. Chem. Soc., Dalton Trans.,
1982, 251.
5 N. W. Alcock and W. D. Harrison, J. Chem. Soc., Dalton Trans.,
1984, 869.
6 Y. Takaguchi, H. Fujihara and N. Furukawa, J. Organomet. Chem.,
1995, 498, 49.
7 M. Asahara, M. Tanaka, T. Erabi and M. Wada, J. Chem. Soc.,
Dalton Trans., 2000, 3493.
8 T. M. Klapötke, B. Krumm, P. Mayer, K. Polborn and O. P. Ruscitti,
Inorg. Chem., 2001, 40, 5169.
9 G. Y. Chao and J. D. McCullough, Acta Crystallogr., 1962, 15,
887.
10 R. K. Chadha, J. E. Drake and M. A. Khan, Acta Crystallogr.,
Sect. C, 1983, 39, 45.
11 R. H. Jones and T. A. Hamor, J. Organomet. Chem., 1984, 262,
151.
12 R. K. Chadha and J. E. Drake, Acta Crystallogr., Sect. C, 1984, 40,
1349.
13 M. de Matheus, L. Torres, J. F. Piniella, J. L. Briansó and
C. Miravitlles, Acta Crystallogr., Sect. C, 1991, 47, 703.
14 Y. Wu, K. Ding, Y. Wang, Y. Zhu and L. Yang, J. Organomet. Chem.,
1994, 468, 13.
15 E. S. Lang, R. M. Fernandes Jr., E. T. Silveira, U. Abram
and E. M. Vázquez-López, Z. Anorg. Allg. Chem., 1999, 625,
1401.
16 R. J. Abraham, J. Fisher and P. Loftus, Introduction to NMR
Spectroscopy, John Wiley & Sons, Chichester, Sussex, England,
1988.
Fig. 4 Rotational barrier energy diagram.
D a l t o n T r a n s . , 2 0 0 3 , 9 7 3 – 9 7 9
978

View Online
17 M. Asahara, T. Morikawa, S.-i. Nobuki, T. Erabi and M. Wada,
J. Chem. Soc., Perkin Trans. 2, 2001, 1899.
18 SIR92: A. Altomare, M. C. Burla, M. Camalli, M. Cascarano,
C. Giacovazzo, A. Guagliardi and G. Polidori, J. Appl. Crystallogr.,
1994, 27, 435.
19 DIRDIF99: P. T. Beurskens, G. Admiraal, G. Beurskens, W. P.
Bosman, R. de Gelder, R. Israel and J. M. M. Smits, The DIRDIF-
99 program system, Technical Report of Crystallography
Laboratory, University of Nijmegen, The Netherlands, 1999.
20 CrystalStructure 2.00: Crystal Structure Analysis Package, Rigaku
and MSC, Tokyo, Japan, 2001.
21 CRYSTALS Issue 10: D. J. Watkin, C. K. Prout, J. R. Carruthers
and P. W. Betteridge, Chemical Crystallography Laboratory, Oxford,
UK, 1996.
22 D. H. O’Brien, N. Dereu, C.-K. Huang, K. J. Irgolic and F. F. Knapp
Jr., Organometallics, 1983, 2, 305.
23 H. C. E. McFarlane and W. McFarlane, J. Chem. Soc.,
Dalton Trans., 1973, 2416.
24 J. H. E. Bailey, J. E. Drake, L. N. Khasrou and J. Yang, Inorg. Chem.,
1995, 34, 124.
25 G. D. Christofferson, R. A. Sparks and J. D. McCullough,
Acta Crystallogr., 1958, 11, 782.
26 L. Y. Y. Chan and F. W. B. Einstein, J. Chem. Soc., Dalton Trans.,
1972, 316.
27 J. E. Drake, L. N. Khasrou, A. G. Mislankar and R. Ratnani,
Inorg. Chem., 1999, 38, 3994.
28 A. Bondi, J. Phys. Chem., 1964, 68, 441.
29 R. S. Berry, J. Chem. Phys., 1960, 32, 933.
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