Generation of an organotellurium(II) cation

Article abstract of DOI:10.1002/ejic.201101313

We expected that the chemical trapping products of low-coordinated tellurenyl cation species can also be kinetically stabilized by the Tbt or Bbt group. Recently, we have reported the halogenation reactions of Bbt-substituted ditelluride, BbtTeTeBbt, lead

Full text of DOI:10.1002/ejic.201101313

SHORT COMMUNICATION
DOI: 10.1002/ejic.201101313
Generation of an Organotellurium(II) Cation
Koh Sugamata,[a] Takahiro Sasamori,*[a] and Norihiro Tokitoh*[a]
Keywords: Tellurium / Cations / Steric protection / Structure elucidation
We expected that the chemical trapping products of low-
coordinated tellurenyl cation species can also be kinetically
stabilized by the Tbt or Bbt group. Recently, we have re-
ported the halogenation reactions of Bbt-substituted ditellur-
ide, BbtTeTeBbt, leading to the formation of the TeII–TeIV
mixed-valent tellurenyl fluoride, BbtTeTe(F)2Bbt, and tellur-
enyl halides, BbtTeX (X = Cl, Br, I). During the dehalogen-
ation reactions of these tellurium halides, it is rational to pos-
tulate the formation of a tellurenyl cation species as an inter-
mediate. In this paper, we report the successful trapping of
tellurenyl cation species with butadienes or triphenylphos-
phane, and the regeneration of the tellurenyl cation species
by thermal retro [1+4] cycloaddition of the diene adducts.
ing advantage of a bulky aryl substituent, Tbt, or Bbt
group.[9,10] Accordingly, we expected that the chemical trap-
ping products of low-coordinated tellurium(II) cation spe-
cies can be kinetically stabilized by the Tbt or Bbt group.
Recently, we have reported the halogenation reactions of
Bbt-substituted ditelluride, BbtTeTeBbt, leading to the for-
mation of the mixed-valent ditelluride difluoride, BbtTe-
Te(F)2Bbt (2) and tellurium(II) halides, BbtTeX (3, X = Br,
4, X = I).[11] It is rational to postulate the formation of
tellurium(II) cation species 1 as an intermediate during the
dehalogenation reactions of 2–4. In this paper, we present
the successful trapping of tellurium(II) cation species 1+
with butadienes or triphenylphosphane and the regenera-
tion of 1+ by thermal retro [1+4] cycloaddition of the diene
adducts.
Introduction
A number of studies on sulfur(II) cations, RS+, have
been reported, and they have raised interest in many re-
search fields.[1] Although analogous species of selenium(II)
and tellurium(II) cations have also attracted much atten-
tion, only a few examples are known, because they are diffi-
cult to isolate as a result of their intrinsic high reactivity.[2]
Thus, only a few chalcogen(II) cationic species have been
structurally characterized to date.[3] The tellurium(II) cat-
–
ion, [2,6-(Me2NCH2)2C6H3Te]+ PF6 , was stabilized by in-
tramolecular coordination of the N atom, though no struc-
tural details were disclosed.[4] Recently, by applying the
(N,C,N) “pincer” ligand, Silvestru and co-workers have suc-
ceeded in the first isolation of tellurium(II) cation species
as stable crystals.[5]
On the other hand, methyl or 4-fluorophenyl-substituted
ditelluride was reported to be oxidized by nitrosonium salts,
giving the corresponding transient tellurium(II) cation spe-
cies.[6] The generation of the tellurium(II) cationic species
was confirmed only by the isolation of the corresponding
phosphane adduct. In recent studies, there are two remark-
able reports on the trapping reaction of tellurium(II) cation
species, where synthesis of aryltellurium(II) cations stabi-
lized by the coordination of NHC (Scheme 4)[7] and the
[1+2] adducts of tellurium(II) cations with alkynes[8] were
reported by the Beckmann and Seppelt groups, respectively.
Meanwhile, we have succeeded in the first chemical trap-
ping of a stibinidene (R–Sb:), which exhibits a structure
that is isoelectronic to that of a tellurium(II) cation, by tak-
Results and Discussion
BbtTeTe(F)2Bbt (2) was treated with 2 equiv. of TMSOTf
or TMSNTf2 in the presence of isoprene or 2,3-dimethyl-
1,3-butadiene in hexane at –40 °C for 1 h. The color
changed from dark-red to colorless, and then a colorless
solid precipitated. The crude product was recrystallized
from CH2Cl2/hexane. As a result, the corresponding diene
adducts 5, 6, and 8 were obtained in yields of 70, 88, and
77%, respectively. On the other hand, the reaction of
BbtTeI (4) with AgBF4 in the presence of 2,3-dimethyl-1,3-
butadiene afforded diene adduct 7 (71%, Scheme 1). The
debromination reaction of 3 with AgOTf in the presence of
2,3-dimethyl-1,3-butadiene (Scheme 2) also afforded cyclo-
adduct 6 (66%), which is identical with the product ob-
tained from 2. Thus, the formation of [1+4] adducts 5–8
is most likely to have proceeded through an intermediate
tellurium(II) cation species generated by the dehalogenation
reactions of 2 and 3.
[a] Institute for Chemical Research, Kyoto University
Gokasho, Uji, Kyoto 611-0011, Japan
Fax: +81-774-38-3209
E-mail: sasamori@boc.kuicr.kyoto-u.ac.jp
tokitoh@boc.kuicr.kyoto-u.ac.jp
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201101313.
Eur. J. Inorg. Chem. 2012, 775–778
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
775

K. Sugamata, T. Sasamori, N. Tokitoh
SHORT COMMUNICATION
similar to those of PPh3Te(Ph)I.[14] There are weak interac-
tions between Te+ and counterion. It was found that the
interactions increase in the order TfO– Ͼ BF4– Ͼ NTf2– on
the basis of the observed Te+···counterion distances
(Table 1), and there seems to be almost negligible interac-
tion between NTf– and Te+ in 11. The Te+···anion distances
of carbene adducts 12 and 13 (Scheme 4) are 3.270(4)
(Te···Cl) and 3.651(6) Å (Te···I), which are shorter than the
sums of the van der Waals radii, 3.81 and 4.04 Å, respec-
tively. In contrast, the Te···O distance in 11 is 3.605(3) Å,
which is longer than the sum of the van der Waals radii
(3.60 Å). However, the phosphane adduct PhTe(PPh3)I con-
tains a rather covalent Te–I bond [3.0930(9) Å].
Scheme 1. Synthesis of diene adducts 5–8.
Scheme 2.
On the other hand, the reaction of 2 with TMSOTf or
TMSNTf2 in the presence of triphenylphosphane gave the
corresponding phosphane adducts 9 and 11 as yellow solids
in yields of 67% and 54%, respectively. The reaction of
BbtTeBr (3) with AgBF4 in the presence of triphenylphos-
phane afforded phosphane adduct 10 (67%, Scheme 3).
Since the blank experiments of the treatment of tel-
lurium(II) halides 2 and 3 with dienes and phosphanes in
the absence of TMSOTf or Ag+ salt resulted in no reac-
tions, the formation of phosphane adducts 9, 10, and 11
from 2 mentioned above strongly suggest the generation of
1+ species.
Figure 1. Molecular structure of 6 with thermal ellipsoids set at
50% probability. Hydrogen atoms are omitted for clarity.
Figure 2. Molecular structure of 9 with thermal ellipsoids set at
50% probability. Hydrogen atoms are omitted for clarity.
Scheme 3. Synthesis of phosphane adducts 9–11.
The molecular structures of 5–11 were unambiguously
The 125Te NMR chemical shifts of telluronium com-
determined by X-ray crystallographic analysis.[12] The mo- pounds 5–11 in CDCl3 are shown in Table 1. The signals
lecular structures of 6 and 9 are shown in Figures 1 and 2. for the diene adducts, excluding compound 5, appeared at
All cycloadducts exhibit pseudo-trigonal-bipyramidal ge- around 600 ppm, while those of the phosphane adducts
ometry as hypervalent species.[13] The phosphane adducts were observed at around 465 ppm.[15] No reason can be of-
9–11 exhibit T-shaped structures, slightly distorted for steric fered for the observed signal of compound 5 at 713 ppm.
reasons [as can be seen in compound 9; C–Te–P 103.25(8)°, These results indicate that the central TeIV moieties of diene
Te–P–O 76.91(5)°]. The P–Te–O angle in 9 is almost linear adducts 6–8 would be slightly affected by the counteranion
[174.95(9)°]. These remarkable structural parameters are depending on the distance, and the electronic effects of the
Table 1. Some structural parameters and 125Te NMR chemical shifts for compounds 5–11.
5
6
7
8
9
10
11
Te–anion distance [Å]
2.916(9)
2.894(5)
2.955(8)
2.988(5)
3.072(2)
3.189(3)
3.605(3)
Sum of the van der Waals radii[16] [Å]
125Te NMR chemical shifts [ppm]
Te–O: 3.60 Te–O: 3.60 Te–F: 3.24 Te–O: 3.60 Te–O: 3.60 Te–F: 3.24 Te–O: 3.60
713 613 607 593 464 463 465
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Eur. J. Inorg. Chem. 2012, 775–778

Generation of an Organotellurium(II) Cation
benzene (5 mL), and the mixture was stirred for 1 h at 0 °C. The
mixture was then warmed up to room temperature. The resulting
white precipitate was filtered and recrystallized from toluene at –40
C to afford a colorless solid. Yield: 71% (74.5 mg, 80.9 μmol).
Compounds 9 and 11: To a hexane (10 mL) solution of BbtTeTe-
(F2)Bbt (2) (50 mg, 32.4 μmol) in the presence of triphenylphos-
phane (15.0 mg, 48.6 μmol, 1.5 equiv.) was added trimethylsilyl tri-
fluoromethansulfonate (11.7 μL, 64.8 μmol for 9) or N-trimethyl-
silyl bis(trifluoromethanesufonyl)imide (14.9 μL, 64.8 μmol for 11),
and the mixture was stirred for 1 h at –78 °C. The mixture was then
warmed up to room temperature. Filtration of the reaction mixture
followed by purification with gel permeation liquid chromatog-
raphy afforded the phosphane adducts. Analytically pure samples
were obtained by crystallization from CH2Cl2/hexane for 9, toluene
for 11. Yields: 67% (50.5 mg, 43.4 μmol) for 9 and 54% (45.5 mg,
0.0351 mmol) for 11.
Scheme 4. Reported carbene adducts of aryltellurium(II) cations 12
and 13.
counteranion toward those of phosphane adducts 9–11
would be negligible.
Thermolysis of 5 in [D6]benzene in the presence of an
excess amount of 2,3-dimethyl-1,3-butadiene (120 °C for 1 h
in a sealed tube) afforded the corresponding diene-ex-
changed product 6 quantitatively. Moreover, 6 was heated
in the presence of 10 equiv. of triphenylphosphane in [D6]-
benzene to give phosphane adduct 9 quantitatively. These
results can be most likely interpreted in terms of the inter-
mediacy of the tellurium(II) cations (1+ [TfO]–) in the ther-
mal retrocycloaddition of the diene adduct 6 (Scheme 5).
Thermolysis of 6 in the absence of a trapping reagent under
the same conditions resulted in the formation of a red solu-
tion containing a sole product, compound X. Although
compound X would be the expected species 1+[OTf]–, it has
not been identified yet because it was difficult to perform
detailed spectroscopic analysis of the product due to its ex-
treme instability even in an inert atmosphere, giving BbtTe-
TeBbt.
Compound 10: To a benzene (10 mL) solution of BbtTeBr (50 mg,
60.0 μmol) in the presence of PPh3 (23.6 mg, 90.0 μmol) was added
AgBF4 (11.7 mg, 60.0 μmol) in benzene (5 mL), and the mixture
was stirred for 1 h at 0 °C. The mixture was then warmed up to
room temperature. Filtration of the reaction mixture followed by
purification with gel permeation liquid chromatography afforded
the crude product. An analytically pure sample of 10 was obtained
by crystallization from CH2Cl2/hexane. Yield: 67% (44.2 mg,
40.1 μmol).
Supporting Information (see footnote on the first page of this arti-
cle): General procedures, analytical data, and X-ray diffraction
studies for compounds 5–11.
Acknowledgments
This work was partially supported by the Ministry of Education
Culture, Sports, Science and Technology, Japan (through Grants-
in-Aid for Scientific Research (B) No. 22350017, Young Scientist
(A) No. 23685010, and the Global COE Program B09). The syn-
chrotron X-ray crystallography experiment was performed at the
BL38B1 in the Spring-8 with the approval of the Japan Synchro-
tron Radiation Research Institute, JASRI, (proposal number
2011A1409). We thank S. Baba, N. Mizuno, and K. Miura (JASRI)
for their support of the synchrotron X-ray measurements.
Scheme 5. Thermolysis of diene adduct 5.
Further investigation on the synthesis and isolation of
tellurium(II) cations with a new bulky ligand is currently in
progress.
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Experimental Section
Compounds 5, 6, and 8: To a hexane solution (10 mL for 5, 5 mL
for 6, 8) of BbtTeTe(F)2Bbt (2) (100 mg, 64.8 μmol for 5, 50.0 mg,
32.4 μmol for 6, 8) in the presence of an excess amount of 2,3-
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6) or N-trimethylsilyl bis(trifluoromethanesufonyl)imide (14.9 μL,
64.8 μmol for 8), and the reaction mixture was stirred for 1 h at
–78 °C. The mixture was then warmed up to room temperature.
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vacuo. Analytically pure samples were obtained by crystallization
from CH2Cl2/hexane. Yields: 70% (87.7 mg, 90.5 μmol) for 5, 88%
(56.0 mg, 57.0 μmol) for 6, and 77% (55.7 mg, 50.0 μmol) for 8.
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1,3-buadiene (0.5 mL) was added AgBF4 (22.1 mg, 11.4 μmol) in
Eur. J. Inorg. Chem. 2012, 775–778
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K. Sugamata, T. Sasamori, N. Tokitoh
SHORT COMMUNICATION
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Received: November 24, 2011
Published Online: January 11, 2012
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