Synthesis and characterization of a hypervalent tellurium cation, Ph5Te+: A stable nonclassical onium compound [13]

Full text of DOI:10.1021/ja992828v

10852
J. Am. Chem. Soc. 1999, 121, 10852-10853
Scheme 1
Synthesis and Characterization of a Hypervalent
Tellurium Cation, Ph5Te+: A Stable Nonclassical
Onium Compound
Mao Minoura, Takahiro Mukuda, Takao Sagami, and
Kin-ya Akiba*
compound, pentaphenyltelluronium, Ph5Te+(C6F5)4B-, 1, and its
structure and reactivity.
Department of Chemistry, Graduate School of Science
Hiroshima UniVersity, 1-3-1 Kagamiyama
Higashi-Hiroshima 739-8526, Japan
The synthesis of 1 was achieved by chlorine abstraction of Ph5-
TeCl, 2, which was readily prepared by chlorination of Ph6Te, 3,
with Cl2 in chloroform. Reaction of 2 with silver triflate followed
by treatment with LiB(C6F5)4 in dichloromethane at -78 °C gave
1 (Scheme 1). After removal of the silver chloride and lithium
triflate by filtration under argon, the filtrate was concentrated in
vacuo and recrystallized from hexane-dichloromethane to give
pure 1 quantitatively as yellow crystals.9
Surprisingly, this cation is thermally stable and not moisture-
sensitive both in solution and in the solid state. The molecular
structure of 1 was determined by X-ray crystallographic analysis
(see Figures 1 and 2).10 In the crystal, the cationic tellurium atom
of 1 has five phenyl rings and is not coordinated by the counter
borate anion. The distance from tellurium to the boron is well
separated, 8.439(5) Å, and that to the nearest fluorine atom in
the anion is 3.584(3) Å, which is longer than sum of the van der
Waals radii (3.55 Å). These values are larger than those of the
corresponding classical onium salt Ph3Te+(C6F5)4B-, 4, (Te-B:
5.622(3), Te-F: 3.363(2) Å). Accordingly, it is clear that the
cation in 1 is “free” in the solid state. When the Pauling’s bond
index equation is applied for those distances, the result indicated
almost no cation-anion interaction.11
ReceiVed August 9, 1999
As represented by carbocations, positively charged onium
species have played important roles in both fundamental and
applied chemistry.1 During the last several years, rapid develop-
ment of the synthesis and characterization of “free” group 14
element cationic compounds such as tricoodinate silyl and germyl
cations in the condensed phase has occurred as the complement
to carbocations.2 On the other hand, a large number of onium
species derived from heavier group 15-17 elements such as
Ph4P+, Ph3Te+, and Ph2I+ have been known for a long time and
used to prepare many neutral hypervalent compounds such as
Ph5P, Ph4Te, and Ph3I.3 The chemistry of hypervalent organic
compounds has stimulated interdisciplinary interest due to their
intriguing bonding nature and reactivity.4 Nonetheless, very little
is known for chalcogen compounds with valence VI compared
to those with valence IV, and no hexaarylated chalcogen had been
described until we recently reported the synthesis of hexaaryl-
telluriums as neutral compounds comprising TeVI.5,6 Moreover,
the pentaorganochalcogenonium ion, hypervalent onium species,
has been a missing class of onium ions against the background
of well-studied classical triorganochalcogenonium salts and
pentaarylpnictogens which are isoelectronic to the hypervalent
oniums.7 Consequently, there is a great deal of interest to explore
pentasubstituted hypervalent cations, which should be essentially
different from pentacoordinate carbocations, i.e., hypercarbons.8
Here, we present the first synthesis of a hypervalent onium
It is noteworthy that the tellurium environment of 1 is a square-
pyramidal (SP) geometry which has one apical phenyl group and
four propeller-like basal phenyl groups. The structure is analogous
to that of Ph5Sb.12 Bond angles around the Te indicated that the
structure has high SP character (91%) which was calculated by
(8) Olah, G. A.; Prakash, G. K. S.; Williams, R. E.; Field, L. D.; Wade, K.
Hypercarbon Chemistry; John Wiley & Sons: New York, 1987; Olah, G. A.;
Rasul, G. Acc. Chem. Res. 1997, 30, 245-250; White, E. T.; Tang, J.; Oka,
T. Science 1999, 284, 135-137.
(1) For reviews, see: Olah, G. A.; Laali, K. K.; Wang, Q.; Prakash, G. K.
S. Onium Ions; John Wiley & Sons: New York, 1998; Olah, G. A. Angew.
Chem., Int. Ed. Engl. 1995, 34, 1393-1405.
(2) Lambert, J. B.; Zhang, S.; Stern, C. L.; Huffman, J. C. Science 1993,
260, 1917-1918; Lambert, J. B.; Zhang, S.; Ciro, S. M. Organometallics
1994, 13, 2430-2443; Lambert, J. B.; Kania, L.; Zhang, S. Chem. ReV. 1995,
95, 1191-1201; Lambert, J. B.; Zhao, Y. Angew. Chem., Int. Ed. Engl. 1997,
36, 400-401; Reed, C. A. Acc. Chem. Res. 1998, 31, 325-332; Sekiguchi,
A.; Tsukamoto, M.; Ichinohe, M. Science 1997, 275, 60-61; Ichinohe, M.;
Fukaya, N.; Sekiguchi, A. Chem. Lett. 1998, 1045-1046.
(3) Wittig, G.; Rieber, M. Justus Liebigs Ann. Chem. 1949, 562, 187-
192; Wittig, G.; Fritz, H. Justus Liebigs Ann. Chem. 1952, 577, 39-46; Wittig,
G.; Clauss, K. Justus Liebigs Ann. Chem. 1952, 577, 136-146.
(4) Musher, J. I. Angew. Chem., Int. Ed. Engl. 1969, 8, 54-68; Kutzelnigg,
W. Angew. Chem., Int. Ed. Engl. 1984, 23, 272-296; Chemistry of Hyper-
Valent Compounds; Akiba, K.-y., Ed.; Wiley-VCH: New York, 1999.
(5) Minoura, M.; Sagami, T.; Akiba, K.-y.; Modrakowski, C.; Sudau, A.;
Seppelt, K.; Wallenhauer, S. Angew. Chem., Int. Ed. Engl. 1996, 35, 2660-
2662; Minoura, M.; Sagami, T.; Miyasato, M.; Akiba, K.-y. Tetrahedron 1997,
53, 12195-12202.
1
(9) Characterization of 1: mp 139-140 °C; H NMR (400 MHz, CDCl3)
δ 7.34 (d, J ) 7.8 Hz, 10H, ortho), 7.47 (t, J ) 7.8 Hz, 10H, meta), 7.61 (t,
J ) 7.8 Hz, 5H, para); 13C NMR (100 MHz, CDCl3) δ 124 (B-ipso), 133.0-
1
(Te-para), 132.3(Te-ortho), 130.7(Te-meta), 136.2 (d, JCF ) 244.5 Hz,
1
B-meta), 138.1 (d, JCF ) 246.4 Hz, B-para), 140.4 (Te-ipso), 148.2 (d,
1JCF ) 240.8 Hz, B-ortho);125Te NMR (126 MHz, CDCl3) δ 659.0; 19F NMR
(376 MHz, CDCl3) δ -133.1 (ortho), -163.7 (para), -167.3 (meta);11B NMR
(128 MHz, CDCl3) δ -16.7; UV-visible spectra (CH2Cl2) λmax (ꢀ) 235 (36,
000), 275 (7, 500), 315 (2, 100).
(10) Crystal data for 1: C54H25BF20Te, monoclinic, P21/n, a ) 16.9860(4)
Å, b ) 16.0790(6) Å, c ) 17.2840(6) Å, â ) 91.565(2)°, V ) 4718.8(2) Å3,
Z ) 4, Dcalcd ) 1.678 g cm-3. Data were collected at 190 K on a MAC Science
DIP2030 imaging plate with graphite-monochromated Mo KR radiation (λ )
0.71073 Å). The structure was solved using the teXsan (Rigaku) system, and
all non-hydrogen atoms were refined anisotropically. The hydrogen atoms
were included at calculated positions but not refined. The final R ) 0.064
(Rw ) 0.105) and GOF ) 1.14 for 6442 observed reflections (685 parameters)
with I > 3.00σ(I).
(6) For hypervalent organochalcogen reviews, see: Lunazzi, L.; Martin, J.
C. In Organic Sulfur Chemistry; Bernardi, F., Csizmadia, I. G., Mangini, A.,
Eds.; Elsevier: Amsterdam, 1985; pp 408-483; Bergman, J.; Engman, L.;
Side´n, L. In The Chemistry of Organic Selenium and Tellurium Compounds;
Patai, S., Rappoport, Z., Eds.; John Wiley & Sons: New York, 1986; Vol. 1,
pp 517-558; Detty, M. R.; O’Regan, M. B. In Tellurium-Containing
Heterocycles; Taylor, E. C., Ed.; John Wiley & Sons: New York, 1994; pp
425-490; Furukawa, N.; Sato, S. In Rodd’s Chemistry of Carbon Compounds;
Sainsbury, M., Ed.; Elsevier: Amsterdam, 1996; Vol. IIIA, pp 469-520.
(7) The term of hyper- or peronium ions concerning positively charged
hepta- or pentacoordinate chalcogen species have been suggested by Musher.
However, those species have never been isolated or characterized. [Musher,
J. I. Ann. N.Y. Acad. Sci. 1972, 192, 52-59; Hellwinkel, D. Ann. N.Y. Acad.
Sci. 1972, 192, 158-166.] According to IUPAC rules, R5Te+ is named λ6-
tellanylium, and we call this hypervalent telluronium in this report.
(11) Pauling, L. Science 1994, 263, 983; Pauling, L. The Nature of the
Chemical Bond, 3rd ed.; Cornell University Press: New York, 1960. For the
Te with the nearest atom in the borate, the Pauling bond number n (0.002)
was obtained which is calculated from the equation 0.60 log n ) D(1) -
D(n). The value of D(1), covalent bond length of Te-F (1.971 Å), was taken
from the length of Ph4TeF2 in ref 5.
(12) Gillespie, R. J.; Hargittai, I. The VSEPR Model of Molecular Geometry;
Allyn and Bacon: Boston, 1991. For some examples of SP structures see:
Wheatley, P. J. J. Chem. Soc. 1964, 3718-3723; Beauchamp, A. L.; Bennett,
M. J.; Cotton, F. A. J. Am. Chem. Soc. 1968, 90, 6675-6680; Schmuck, A.;
Buschmann, J.; Fuchs, J.; Seppelt, K. Angew. Chem., Int. Ed. Engl. 1987, 26,
1180-1182; Schmuck, A.; Leopold, D.; Wallenhauer, S.; Seppelt, K. Chem.
Ber. 1990, 123, 761-766; Pulham, C.; Haaland, A.; Hammel, A.; Rypdal,
K.; Verne, H. P.; Volden, H. V. Angew. Chem., Int. Ed. Engl. 1992, 31, 1464-
1467.
10.1021/ja992828v CCC: $18.00 © 1999 American Chemical Society
Published on Web 11/06/1999

Communications to the Editor
J. Am. Chem. Soc., Vol. 121, No. 46, 1999 10853
Scheme 2
In solution, the 125Te NMR spectrum of 1 exhibits upfield shift
(659.0 ppm) in comparison with that of Ph3Te+ (4, 753.0 ppm).
This is consistent with the trend in chemical shift for these series
Ph2Te (688 ppm), Ph4Te (509 ppm),14 and Ph6Te (3, 493 ppm),
which shows that the shift to higher fields is induced by the
increase of oxidation state and the decrease in the number of lone-
1
pair electrons on the tellurium. In the H and 13C NMR spectra
of 1, only one set of phenyl groups could be observed even at
-70 °C in CDCl3. This is due to rapid Berry pseudorotation with
a small energy difference between SP and TBP structures, as is
usually the case for pentaorgano-element compounds.15 Therefore
this suggests the possibility that Ph5Te+ would have an SP
structure in the solid-state induced by lattice control.
Hypervalent compounds, such as Ph5Sb and Ph4Te, are known
to decompose thermally by ligand-coupling reactions to give the
corresponding reductively eliminated products, i.e., Ph3Sb and
Ph2Te, respectively.16 Although Ph6Te, 3, and Ph3Te+, 4, are quite
stable up to the melting points, thermolysis of 1 proceeds at 150
°C in the solid state to give 4 quantitatively along with biphenyl
(∼80%) and a small amount of benzene. The reaction of 1 with
phenyllithium or methyllithium proceeded at the cationic tellurium
center and afforded the corresponding neutral hexaorganotellu-
rium, 3, or Ph5TeMe, 5, quantitatively (Scheme 2). Thus, 1 can
be regarded as a useful source of hexavalent organotellurium
compounds.
Figure 1. Molecular structure of 1 (thermal ellipsoids with 30%
probability). Selected bond lengths (Å) and angles (deg): Te1-C1, 2.101-
(5); Te1-C7, 2.195(5); Te1-C13, 2.208(5); Te1-C19, 2.196(5); Te1-
C25, 2.205(5); C1-Te1-C7, 99.2(2); C1-Te1-C13, 103.5(2); C1-
Te1-C19, 99.1(2); C1-Te1-C25, 101.8(2); C7-Te1-C13, 88.4(2);
C7-Te1-C19, 161.7(2); C7-Te1-C25, 87.8(2); C13-Te1-C19, 86.8-
(2); C13-Te1-C25, 154.6(2); C19-Te1-C25, 88.9(2).
In view of the long-known existence and widespread use of
onium compounds, the easy synthesis and great stabilities of the
hitherto unknown hypervalent onium species will provide new
chemistry.
Acknowledgment. This work was partially supported by a Grant-
in-Aid for Scientific Research (Nos. 09440218, 10740296, and 11304044)
from the Ministry of Education, Science, Sports, and Culture, Japan.
Supporting Information Available: Characterization and X-ray
crystallographic details with positional parameters, bond lengths and
angles, and ORTEP drawings for 1, 2, 4, and 5 (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
Figure 2. Space-filling representation of the unit cell of 1 down the
crystallographic c axis. Color code: Te, red; C, gray; H, white; B, blue;
F, green.
JA992828V
the dihedral angle method.13 The tellurium-apical carbon bond
length (2.101(5) Å) is significantly shorter than the four basal
carbon bond lengths (2.195(5)-2.208(5) Å). All of the C-Te
bond lengths in 1 are shorter than those of neutral 3 (2.228 Å)5
and this may be explained by cationic attractive interaction
between the tellurium and five carbon ligands.
(14) Jones, C. H. W., Sharma, R. D. J. Organomet. Chem. 1987, 332, 115-
121; Smith, C. S.; Lee, J.-S.; Titus, D. D.; Ziolo, R. F. Organometallics 1982,
1, 350-354.
(15) Berry, R. S. J. Chem. Phys. 1960, 32, 933-938; Gillespie, P.; Hoffman,
P.; Klusacek, H.; Marquarding, D.; Pfohl, S.; Ramirez, F.; Tsolis, E. A.; Ugi,
I. Angew. Chem., Int. Ed. Engl. 1971, 10, 687-715.
(16) Barton, D. H. R.; Glover, S. A.; Ley, S. V. J. Chem. Soc., Chem.
Commun. 1977, 266-267; Glover, S. A. J. Chem. Soc., Perkin Trans. 1 1980,
1338-1344; Akiba, K.-y. Pure Appl. Chem. 1996, 68, 837-842; Finet, J.-P.
Ligand Coupling Reactions with Heteroatomic Compounds; Pergamon:
Oxford, 1998.
(13) Hoffmann, R.; Howell, J. M.; Muetterties, E. L. J. Am. Chem. Soc.
1972, 94, 3047-3058; Holmes, R. R. Prog. Inorg. Chem., 1984, 32, 119-
235.