Diaryl tellurium dihalides: From Te-tetracoordinated compounds to Te-tricoordinated molecular complexes

Article abstract of DOI:10.1080/10426500590906256

X-ray structural studies demonstrate that diaryl tellurium diiodides can be constrained by steric factors (ortho-alkyl substituents) to adopt a trigonal pyramidal structure in preference to the expected bisphenoidal structure. DFT calculations are shown to reproduce successfully structural details of the title compounds; they allow one to predict which coordination number should be energetically favored. All compounds are chiral, but the barrier to stereomutation by intra- and intermolecular pathways (60-85 kJmol^-1) is rather low. Copyright Taylor & Francis Inc.

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Diaryl Tellurium Dihalides:
From Te-Tetracoordinated
Compounds to Te-
Tricoordinated Molecular
Complexes
Peter H. Laur a , Seyedeh M. Saberi-Niaki a , Michael
Scheiter a , Chunhua Hu a , Ulli Englert a , Yuekui
Wang b & Jörg Fleischhauer b
a Institute of Inorganic Chemistry, RWTH Aachen
University of Technology , Aachen, Germany
b Institute of Organic Chemistry (Theoretical Chem.
Section), RWTH Aachen University of Technology ,
Aachen, Germany
Published online: 01 Feb 2007.
To cite this article: Peter H. Laur , Seyedeh M. Saberi-Niaki , Michael Scheiter ,
Chunhua Hu , Ulli Englert , Yuekui Wang & Jörg Fleischhauer (2005) Diaryl Tellurium
Dihalides: From Te-Tetracoordinated Compounds to Te-Tricoordinated Molecular
Complexes, Phosphorus, Sulfur, and Silicon and the Related Elements, 180:3-4,
1035-1044, DOI: 10.1080/10426500590906256
To link to this article: http://dx.doi.org/10.1080/10426500590906256
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Phosphorus, Sulfur, and Silicon, 180:1035–1044, 2005
Copyright © Taylor & Francis Inc.
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500590906256
Diaryl Tellurium Dihalides: From Te-Tetracoordinated
Compounds to Te-Tricoordinated Molecular Complexes
Peter H. Laur
Seyedeh M. Saberi-Niaki
Michael Scheiter
Chunhua Hu
Ulli Englert
Institute of Inorganic Chemistry, RWTH Aachen University
of Technology, Aachen, Germany
Yuekui Wang
¨
Jorg Fleischhauer
Institute of Organic Chemistry (Theoretical Chem. Section),
RWTH Aachen University of Technology, Aachen, Germany
X-ray structural studies demonstrate that diaryl tellurium diiodides can be con-
strained by steric factors (ortho-alkyl substituents) to adopt a trigonal pyramidal
structure in preference to the expected bisphenoidal structure. DFT calculations are
shown to reproduce successfully structural details of the title compounds; they allow
one to predict which coordination number should be energetically favored. All com-
pounds are chiral, but the barrier to stereomutation by intra- and intermolecular
pathways (60–85 kJmol−1) is rather low.
Keywords DFT calculations; diaryl chalcogen dihalides; enantiomerization; molecular
complexes; stereochemistry; X-ray structural data
INTRODUCTION
The steric disposition of the compounds RR’EHal2 (E = S, Se, Te) in the
solid state is known to be either bisphenoidal (ψ-trigonal bipyramidal,
Received January 29, 2004; accepted October 18, 2004.
Dedicated to Professor Kurt Mislow, Princeton University, Princeton, N.J., on account
of his 80th birthday.
Financial support by the Deutsche Forschungsgemeinschaft and the Fonds der
Chemischen Industrie is gratefully acknowledged.
Address correspondence to Peter H. Laur, Institute of Inorganic Chemistry, Ruth
Aachen University of Technology, Aachen, Templer-Graben 52056, Germany. E-mail:
peter.laur@ac.ruth-aachen.de
1035

1036
P. H. Laur et al.
SCHEME 1
distorted tetrahedral), A (cn = 4, coordination number at the chalco-
gen), or trigonal pyramidal, B (cn = 3), (Scheme 1) depending on the
relative electronegativity of the chalcogen and the halogen atoms.1
Whereas class A compounds are covalent molecules, B constitute
molecular complexes RR’E(Hal2) or ionic species RR’EHal+Hal−. All
diorganyl tellurium dihalides, including, e.g., Ph2TeI2, are expected
to be of type A, which has been amply substantiated by structure
determinations.2 We have observed, however, that the properties of
sterically hindered diaryl tellurium diiodides (and also of related di-
aryl selenium dibromides) differ in typical ways from those of the un-
hindered molecules, as described below; these compounds show rather
a close similarity to diorganylselenium diiodides that are unquestion-
ably of type B. We have, therefore, carried out some investigations in
order to test whether steric hindrance could cause a change-over from
the more space-demanding cn = 4 to cn = 3.
EXPERIMENTAL
We have prepared and studied various, mostly new, multiply
o-alkyl substituted diaryl tellurium dihalides, including (2,4,6-
R3C6H2)2TeHal2 1–3 (1, R Me: Mes; 2, Et: Tep; 3, iPr: Tip; a, Hal
F; b, Cl;, c, Br; d, I), and some derivatives thereof, starting from the cor-
responding diaryl tellurides. The latter were obtained either (a) by the
reaction of aryllithiums with TeCl4, or (b) by detelluration of the ditel-
lurides [thermal or Cu-catalyzed or by use of the reagent [(Me2N)3P]. In
contrast, attempts at similar detellurations of the sterically even more
biased bis(supermesityl) ditelluride led only to rearranged or cyclized
products, probably by radical reactions. However, the desired (2,4,6-
tBu3C6H2)2Te could be isolated from the complex product mixture of the
ArLi/TeCl4 reaction (see Schemes 2 and 3; only main products shown).
The dihalides easily were prepared by standard methods (reaction of

Diaryl Tellurium Dihalides
1037
SCHEME 2 ArLi/TeCl4—reaction.
SCHEME 3 Detelluration reactions. (a): ꢀ > 2000◦C (melt); (b): Cu, toluene
(reflux); (c): (Me2N)3P, r.t. (CHCl3). (analogous reactions have been observed in
the Se series.)

1038
P. H. Laur et al.
TABLE I (2,4,6-R3C6H2)2TeHal2; Mes: R = CH3; Tep: R = C2H5; Tip =
(CH3)2CH
125Te NMR
mp [◦C]
δ [ppm]
ꢀ1/2[Hz]
Mes2Te3
Tep2Te
Tip2Te
Mes2TeBr2 (1c)
Tep2TeBr2 (2c)
Tip2TeBr2 (3c)
Mes2TeI2 (1d)
Tep2TeI2 (2d)
Tip2TeI2 (3d)
128
34
80
195
172
148
95
275
209
171
744
722
823
389
234
297
(2)
(10)
(10)
(6)
(10)
3
(20)
3
(200)
(200)
(200)
70
153
Ar2Te with elemental halogen or XeF2; halogen exchange of Ar2TeBr2
with AgF), except for the supermesityl system: Here, halogenation af-
forded derivatives of the rearranged products exclusively.
The diaryl tellurium difluorides/dichlorides/dibromides are stable
compounds, but the diiodides tend to disintegrate, both in solid and
in solution, with the liberation of iodine. This instability increases
with increasing steric hindrance and is paralleled by broadening of
the 125Te NMR signals: ꢀ1/2 Ph2TeI2 40, Mes2TeI2 200, (p-Tol)MesTeI2
even 1000 Hz) (see Table I). In solution, free iodine can be monitored by
spectroscopy (UV, MCD), and its concentration increases on dilution.
The dibromides tend to dissociate only upon high dilution in polar sol-
vents, but rather into Ar2TeBr+ + Br−. The corresponding dichlorides
and difluorides do not decompose under similar conditions.
RESULTS
In order to ascertain unequivocally, the structures of the halogen
adducts we have resorted to single crystal X-ray structure determi-
nation. In Tables II and III the results are listed as to the compounds
1c–3c and 1d–3d, respectively.
All dibromides, including the most hindered compound studied so
far, viz., 3c, belong to class A, as expected. Their structures are strictly
similar to one another, including the conformation. However, steric con-
gestion leads to a breakdown of the extensive intermolecular bonding
network otherwise typical of Te(IV) centers.2 No significant intermolec-
ular contacts that involve either tellurium or bromine exist at distances
shorter than the sum of the van der Waals-radii. Whereas all bond

Diaryl Tellurium Dihalides
1039
TABLE II Mes2TeBr2, Tep2TeBr2, Tip2TeBr2 (C2 Structure/type A)
(X-ray Data). RotAr: Rotation of the Aryl Rings from the Plane of the
C Te C Bonds; Σrcov:5 Te C 213, Te Br 250 pm
Te
C
Te Br
C
Te
C
C
Te Br Br Te Br
RotAr
Mes TeBr 213.8 (3) pm 269.14 (9)
113.50 (16)◦ 86.96 (8)◦ 179.98 (2)◦ 31.87 (16)◦
2
2
(1c)
213.8 (3)
269.14 (9)
266.71 (10) 112.3 (3)
270.93 (10)
93.03 (8)
89.00 (19) 177.23 (4)
91.8 (2)
31.87 (16)
26.8 (4)
28.8 (4)
Tep TeBr 213.8 (7)
(2c)
2
2
214.3 (7)
90.27 (19)
91.0 (2)
Tip TeBr
2
(3c)
217.7 (4)
217.7 (4)
267.39 (8)
267.74 (8)
122.63 (17)
96.3 1.2
87.49 (11) 176.58 (2)
93.34 (11)
88.09 (11)
28.1 (2)
31.9 (2)
2
94.36 (11)
4
2
Ph TeBr
214
3
268.2 0.3
178.0 0.2 56
2
distances and most bond and dihedral angles are unexceptional, the
molecules obviously alleviate steric strain by opening of the C Te C
angle from typically 100◦ (unhindered Ar2TeBr2) to 113◦ in 1c and 2c,
and even 123◦ in 3c.
On the other hand, the diiodides 1d–3d are novel compounds of class
B, in contrast to the Ar2TeI2 compounds previously investigated. Also
here the molecular packing in the crystal does not indicate any in-
termolecular association. Clearly, both the Te I and (less so) the I I
bonds are considerably longer than the sum of the van der Waals-radii,
but the iodine molecular unit is still distinctly recognizable, allowing
the compounds to be described as molecular complexes. The most con-
spicuous structural feature results from the fact that the Te I I unit,
bisects the C Te C angle asymmetrically giving rise to two different
C Te I angles of approximately 100◦ and 120◦. The Te I I unit it-
self is slightly bent at ca. 172◦, probably also indicating some steric
influence.
In order to understand better the observed structural variations,
we have performed a series of density functional theory calculations
(DFT/TZVP/B3LYP). The results match almost exactly the experimen-
tally obtained structural data, as is apparent from a comparison of the
observed and calculated molecular structures of, e.g., 1c and 1d as de-
picted in Figures 1 and 2. This correspondence supports our contention
that the molecules discussed are gas-phase like also in the crystal. As-
suming that the title compounds could in principle adopt both structural
types A and B, we have extended our calculations to include also the

1040

Diaryl Tellurium Dihalides
1041
FIGURE 1 Experimental (left) and calculated (right) Solid State Structure of
Mes2TeBr2 (1c).
(so far not realized) alternative isomers, like 1c of type B or 1d of type
A. For these hypothetical molecules, reasonable geometric parameters
are found that agree well with those observed in the related real com-
pounds. It is striking, however, that the calculated parameters of type
B molecules lacking steric strain neither exhibit the C Te I nor the
Te I I angle peculiarities denoted above for 1d–3d (e.g., Ph2TeI2 of C1
structure/type B, calc.: C Te I 99.5, 101.4◦; Te I I 179.6◦).
The calculations also yield data on the relative ground-state energies
of the related isomers of type A and B; the results are displayed in
Table IV.
Clearly, even in sterically burdened Ar2TeBr2 molecules the C2 struc-
ture/type A should be much preferred, as found by experiment, whereas
in Ar2TeI2 the preference for C2 is only slight and can be reversed easily
by steric factors (Sterically hindered Ar2SeBr2 and Ar2SCl2 molecules
should also adopt a C1 rather than the common C2 structure, as indi-
cated in Table IV). We therefore propose to modify the structure matrix
for RR’EHal2,1 as suggested in Scheme 4, with the molecular complexes
FIGURE 2 Experimental (left) and calculated (right) Solid State Structure of
Mes2TeI2 (1d).

1042
P. H. Laur et al.
TABLE IV DFT/TZVP/B3LYP Calculations of some
Ar2ChalcHal2 (Relative Ground State Energies [kcal/mol])
Ar2Chalc(Hal)(Hal)
Ar2Chalc(Hal2)
(C2-Structure/Type A
(C1-Structure/Type B)
Ph2TeBr2
Mes2TeBr2
Ph2TeI2
Mes2TeI2
Mes2SeBr2
Mes2SCl2
0
0
0
7.6
6.8
8.3
17.1
8.5
2.8
0
0
0
B in sterically biased Te/I, Se/Br, and S/Cl systems competing with the
otherwise preferred type A molecules.
SCHEME 4 Solid state structure matrix of diorganyl chalcogen dihalides.
DISCUSSION
All compounds A are chiral as a consequence of the contra-rotation of
the aromatic rings; the highest possible symmetry is C2. If enantiomer-
ization by rotation and/or Berry pseudorotation should be hindered in
such o-substituted diaryl chalcogen dihalides, anisochrony of various
groupings is expected to be evident in the NMR spectra. For example,
the Me/Et/iPr substituents in the o and oꢀ positions on each aryl ring of
1A–3A should give rise to different signals, as is in fact always observed
at room temperature. Similar effects can also be seen in related com-
pounds with bulky substituents on one ligand only, like PhMesTeBr2,
but a differentiation of the two ortho or meta positions of unsubstituted
phenyl rings cannot be discerned. At elevated temperatures coalescence
is observed, while the Te/F coupling is maintained in, e.g., 1a–3a. This

Diaryl Tellurium Dihalides
1043
latter phenomenon indicates that the coalescence results from an in-
tramolecular process. The general similarity of the physical and chem-
ical character of the dichlorides/dibromides with that of the difluorides
suggests an intramolecular reorganization to cause the coalescence also
in their respective cases. The barriers to stereomutation deduced from
the NMR spectra lie in the range of 60–85 kJmol−1 (e.g., 1a, 69; 1b, 64;
1c, 85 kJmol−1). Such barriers are too low as to enable the existence of
optically stable enantiomers at room temperature. We have, therefore,
not tried to resolve the compounds.
The NMR features described should also pertain to the diiodides of
type B, viz., 1d–3d. However, none of the anisochrony expected for a
pyramidal structure of the sterically biased diiodides can be observed
by solution NMR, contrary to the case of the corresponding Te-oxides.
Although this finding could be rationalized by invoking a low barrier to
pyramidal inversion in the diiodides, it is not clear why such a barrier
should be much lower than in the oxides. The presence of free iodine
in the solution rather suggests a different mechanism, viz., dissocia-
tion/association.
Since all the compounds 1–3 discussed are chiral, there in principle
could exist the possibility of obtaining optically active samples—albeit
of fleeting optical stability—by spontaneous resolution. We have, there-
fore, checked the space group symmetry of the crystals of the dibromides
1c–3c and the diiodides 1d–3d, as well as of some related diaryl tel-
lurium Te-oxides like Tep2TeO: In all cases, the crystals themselves are
achiral, i.e., they contain an equal number of R and S molecules (true
racemates) and cannot give rise to optical activity on solution.
In this context, earlier reports of optical activity in solutions of, e.g.,
Ph2TeBr2 and Ph(p-Tol)TeI2 should be mentioned.6 Although the re-
ported experimental details are not convincing and the very small and
variable rotations vanished within minutes, and although the stereo-
chemical theory7 prompting the investigations (predicting D2 symme-
try for R4Te) is now obsolete, occasional observations of optical activ-
ity in solutions of such compounds might well be trustworthy, as both
Ph2TeBr4 and α-Ph2TeI2 crystallize in chiral crystals consisting of ho-
8
mochiral2molecules (conglomerates), and should be resolvable by crystal
picking.
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[1] N. C. Baenziger, R. E. Buckles, R. J. Maner, and T. D. Simpson, J. Am. Chem. Soc.,
91, 5749 (1969).
[2] J. Zukerman-Schpektor and I. Haiduc, Phosph., Sulf., Silicon, 171, 73 (2001).
[3] K. Lederer, Ber. Dtsch. Chem. Ges., 49, 345 (1916).

1044
P. H. Laur et al.
[4] G. D. Christofferson and J. D. McCullough, Acta Cryst., 11, 249 (1958).
[5] C. Glidewell, Inorganica Chimica Acta, 20, 113 (1976).
[6] M. G. Ter Horst, Rec. Trav. Chim. Pays-Bas, 55, 697 (1936).
[7] F. M. Jaeger, Spatial Arrangements of Atomic Systems and Optical Activity (Cornell
Lectures, McGraw-Hill, New York, 1930).
[8] N. W. Alcock and W. D. Harrison, J. Chem. Soc., 869 (1984).