Well-defined stibonic and tellurinic acids

Article abstract of DOI:10.1002/anie.200803997

Upon going from the 4th to the 5th period a qualitative change in the molecular structures of pnictogenic and chalcogenic acids takes place. Unlike the lighter monomeric Group 15 and 16 congeners, the depicted stibonic and tellurinic acids adopt μ₂-oxo-bridged dinuclear structures featuring Sb and Te atoms with trigonal-bipyramidal coordination. DFT calculations on the model compounds give dissociation energies of the dimers. (Figure Presented).

Full text of DOI:10.1002/anie.200803997

Communications
DOI: 10.1002/anie.200803997
Main-Group Chemistry
Well-Defined Stibonic and Tellurinic Acids**
Jens Beckmann,* Pamela Finke, Malte Hesse, and Burkhard Wettig
[1]
The first stibonic acids RSb(O)(OH)2 and tellurinic acids
RTe(O)(OH)[2] (R = aryl, alkyl) were extensively investigated
more than 90 years ago in the context of pharmacological
studies on arsonic acids RAs(O)(OH)2 and the closely related
remedies atoxyl and salvarsan, marking the beginning of
modern chemotherapy.[3] Unlike their lighter Group 15 and
16 congeners, all hitherto described stibonic and tellurinic
acids are ill-defined, amorphous, high-melting compounds
that are poorly soluble in most organic solvents. Molecular
weight determinations[4] and 121Sb Mꢀssbauer spectroscopic
studies[5] confirm a high degree of aggregation and a trigonal-
bipyramidal structure for PhSb(O)(OH)2. By contrast, all
phosphonic and arsonic acids RE(O)(OH)2 (E = P, As), as
well as sulfinic and seleninic acids RE(O)(OH) (E = S, Se),
are well-defined molecular compounds with tetrahedrally
Mes2C6H3SbCl4 (1; Mes = 2,4,6-Me3C6H2) provided the m2-
oxo-bridged
Mes2C6H3Sb(O)(OH)Cl]2
Mes2C6H3Sb(O)(OH)2]2 (3) in high yields (Scheme 1). Our
dinuclear
products
and
[2,6-
[2,6-
(2)
=
coordinated central atoms E, polar (formal) E O double
À
bonds and E OH groups that are usually involved in
intermolecular hydrogen bonding in the solid state.
Scheme 1. Synthesis of 2, 3, 5, and 6. R=2,6-Mes2C6H3.
Aggregation was also observed for related triarylanti-
mony oxides and diaryltellurium oxides, which exist in two
distinctively different structures, namely as asymmetric
recent attempts at preparing 3 by the basic hydrolysis of 2,6-
Mes2C6H3SbCl2 under aerobic conditions gave rise to the
kinetically controlled formation of (2,6-Mes2C6H3SbIIICl)2O
and the mixed-valent antimony oxo clusters (2,6-
dimers, for example (Ph3SbO)2 and (Ph2TeO)2 ,[7] and as
[6]
one-dimensional polymers, for example, (Ph3SbO)n[8] and (p-
Ans2TeO)n (Ans = MeOC6H4).[9]
Mes2C6H3SbV)2(ClSbIII)4O8 and (2,6-Mes2C6H3SbV)4(ClSbIII)4-
III
À
(HOSb )2O14, which evolved from partial cleavage of Sb C
bonds.[10] Given that reaction conditions were similar in both
V
À
studies, it appears that the Sb C bonds are more resistant
towards hydrolysis than the SbIII C bonds of the same m-
À
terphenyl substituent.
The kinetically controlled hydrolysis of 2,6-
Mes2C6H3TeCl3 (4) in a two-layer system of toluene and
0.5m aqueous sodium hydroxide solution affords the m2-oxo-
bridged dinuclear products [2,6-Mes2C6H3Te(O)Cl]2 (5) and
[2,6-Mes2C6H3Te(O)(OH)]2 (6) in high yields (Scheme 1).
The molecular structures of 3 and 6 comprise asymmetric
four-membered Sb2O2 and Te2O2 ring structures with one or
two exocyclic OH groups (Figures 1 and 2).[11] Taking into
account the lone pair at the tellurium center, the spatial
arrangement around the Sb and Te atoms is best described as
distorted trigonal-bipyramidal with the expected occupancies
of the ligand atoms. The inorganic cores of 3 and 6 are
effectively shielded by the bulky m-terphenyl substituents,
which prevent further aggregation, while in 6 two mesityl
groups in ortho position are engaged in Menshutkin type
interactions with the lone pairs of the Te atoms (centroid
(C20–C25)···Te1 3.399(1) ꢁ).[12] In 3 the equatorial and axial
These observations prompted us to study the structures of
archetypal stibonic and tellurinic acids that are kinetically
stabilized by a bulky m-terphenyl substituent. The kinetically
controlled hydrolysis under basic conditions (in a two-layer
system of toluene and 0.1m aqueous sodium hydroxide) of 2,6-
[*] Prof. Dr. J. Beckmann, Dipl.-Chem. P. Finke, Dipl.-Chem. M. Hesse,
B.Sc. B. Wettig
Institut fꢀr Chemie und Biochemie, Anorganische und Analytische
Chemie, Freie Universitꢁt Berlin
Fabeckstrasse 34–36, 14195 Berlin (Germany)
E-mail: beckmann@chemie.fu-berlin.de
À
endocyclic Sb O bond lengths (1.913(2) and 2.035(2) ꢁ) vary
[**] We are grateful to Irene Brꢀdgam (Freie Universitꢁt Berlin) for the
measurement of X-ray crystallography data and to the Deutschen
Forschungsgemeinschaft for financial support.
only marginally (by 0.122(2) ꢁ) and compare well with those
of (Ph3SbO)2 (1.928(3) and 2.071(4) ꢁ).[6] By contrast, in 6 the
À
equatorial and axial endocyclic Te O bond lengths (1.897(5)
and 2.143(5) ꢁ) differ by 0.246(5) ꢁ.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200803997.
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Chemie
respectively, which is consistent with the dissociation energies
ED calculated below.
In an effort to estimate the dissociation energies ED of 3
and 6, DFT calculations[13] were performed at the B3PW91/
TZ level of theory for the m2-oxo bridged model compounds
[PhSb(O)(OH)2]2 (3a) and [PhTe(O)OH]2 (6a) and com-
pared to the hypothetical forms of lighter phenylpnictogenic
acids [PhE(O)(OH)2]2 (E = As, P) and phenylchalcogenic
acids [PhE(O)OH]2 (E = Se, S) with similar dinuclear struc-
tures (Figures 3 and 4). As expected, the dissociation energy
ED of the phenylchalcogenic acids [PhE(O)OH]2 increases
when going from E = S (À0.6 kJmolÀ1) to E = Se
(16.1 kJmolÀ1) to E = Te (106.2 kJmolÀ1). While no energy
minimum was found for the m2-oxo bridged phenylphosphonic
acids [PhP(O)(OH)2]2, the dissociation energies ED of the
heavier phenylpnictogenic acids [PhE(O)(OH)2]2 also
increase when going from E = As (47.0 kJmolÀ1) to E = Sb
(223.6 kJmolÀ1). Despite the fact that [PhAs(O)(OH)2]2 and
[PhSe(O)OH]2 show significant ED values, all reported
arylarsonic acids and arylseleninic acids show no evidence
for the formation of m2-oxo bridged dimeric structures. The
lighter phenylpnictogenic acids [PhE(O)(OH)2]2 (E = As, P)
and phenylchalcogenic acids [PhE(O)OH]2 (E = Se, S) are
usually involved in hydrogen-bonded networks in the solid
state and in solution. Therefore, the dissociation energies ED
Figure 1. Molecular structure of 3; thermal ellipsoids are set at the
30% probability level. Selected bond parameters [ꢂ,8]: Sb1–O1
1.913(2), Sb1–O1a 2.035(2), Sb1–O2 1.940(3), Sb1–O3 1.917(3), Sb1–
C10 2.136(3); O1-Sb1-O3 118.4(1), O1-Sb1-O2 91.9(1), O3-Sb1-O2
87.8(1), O1-Sb1-O1a 78.1(1), O2-Sb1-O1a 165.7(1), O3-Sb1-O1a
88.1(1), O1-Sb1-C10 125.6(1), O1a-Sb1-C10 94.9(1), O2-Sb1-C10
99.2(1), O3-Sb1-C10 115.1(1), Sb1-O1-Sb1a 102.0(1).
Figure 2. Molecular structure of 6; thermal ellipsoids are set at the
30% probability level. Selected bond parameters [ꢂ,8]: Te1–O1
1.897(5), Te1–O1a 2.143(5), Te1–O2 2.232(4), Te1–C10 2.151(6); O1-
Te1-O1a 76.5(2), O1-Te1-C10 108.4(2), O1a-Te1-C10 89.3(2), O1-Te1-
O2 86.0(2), O1a-Te1-O2 161.2(2), C10-Te1-O2 89.68(2), Te1-O1-Te1a
103.5(2).
Figure 3. Dissociation energies ED of dimeric phenylpnictogenic acids
and triphenylantimony oxide.
In the related compound (Ph2TeO)2, the difference
À
between the equatorial and axial Te O bonds (av 1.89(1)
and 2.55(1) ꢁ) of 0.66(1) ꢁ is even more pronounced. This
compound can be viewed as two Ph2TeO monomers that are
only weakly associated by secondary interactions.[7] From the
À
relative asymmetry of the endocyclic E O bond, it can be
assumed that the degree of association within these four-
membered-ring structures increases in the order (Ph2TeO)2 !
6 ! (Ph3SbO)2 < 3.
Unlike previously described stibonic[1] and tellurinic
acids,[2] compounds 3 and 6 readily dissolve in organic
solvents, such as toluene, CH2Cl2, and ethanol, but are
insoluble in hexane or aqueous NaOH. Osmometric molec-
ular weight determinations of 3 and 6 (2.5–5.0 gkgÀ1 toluene)
at 608C reveal degrees of aggregation of 2.0 and 1.6,
Figure 4. Dissociation energies ED of dimeric phenylchalcogenic acids
and diphenyltellurium oxide.
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Communications
6 (recrystallized from EtOH): Yield: 441 mg, 0.465 mmol; 93%
of two phenylpnictogenic acid molecules and two phenyl-
3
1H NMR (400 MHz, C6D6): d = 7.12 (t, J(H,H) = 7.5 Hz, 1H), 6.98
chalcogenic acid molecules associated by hydrogen bonding
were also calculated; these energies vary between 54.4 and
86.7 kJmolÀ1. The two types of aggregation for these acids
(d, 3J(H,H) = 7.5 Hz, 2H), 6.87 (s, 4H), 2.22 (s, 6H), 2.14 ppm (s,
12H); 13C NMR (100 MHz, C6D6): d = 147.6, 140.4, 136.8, 135.5,
127.9, 127.1, 126.6, 126.2, 20.2, 19.9 ppm; 125Te NMR (126 MHz,
C6D6): d = 1403 ppm; IR (KBr): n˜OH = 3590 cmÀ1; elemental analysis
(%) calcd for C48H52O4Te2 (948.10): C 60.81, H 5.53; found: C 60.62,
H 5.14.
=
might be interpreted in terms of competition between E···O
=
E and EOH···O E donor–acceptor interactions. From the
comparison of these ED values it can be concluded that the m2-
oxo-bridged dimers are energetically more favored for the
heavier pnictogenic acids and chalcogenic acids of the 5th
period, while for the lighter congeners of the 3rd and 4th
periods the hydrogen-bonded dimers are more stable.
Attempts to optimize the geometry of a hydrogen-bonded
complex for [PhSb(O)(OH)2]2 gave rise to formation of a
second m2-oxo-bridged dimer 3b, in which the phenyl groups
occupy the axial positions (Figure 3). Consistent with the
Bent rule, the dissociation energy ED of 3b (209.8 kJmolÀ1) is
only slightly lower than that of 3a (223.6 kJmolÀ1). In
comparison, the dissociation energy ED of (Ph3SbO)2
(151.9 kJmolÀ1) is about 50 kJmolÀ1 lower than that of 3a
and 3b. Besides 6a, for [PhTe(O)(OH)]2 a second m2-oxo-
bridged dimer 6b was also calculated, in which the phenyl
groups are situated in the axial positions. Interestingly, the
dissociation energies ED of 6a (106.2 kJmolÀ1) and 6b
(58.3 kJmolÀ1) differ substantially, which might be attributed
to the different trans effect imposed by the endocyclic axial
ligands. The value for 6b compares well with the dissociation
energy ED of (Ph2TeO)2 (61.6 kJmolÀ1).
Received: August 13, 2008
Published online: November 12, 2008
Keywords: antimony · hypervalent compounds · oxides ·
.
tellurium
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108, 4206.
The calculated dissociation energies ED of the model
compounds increase in the order (Ph2TeO)2 ! 6a !
(Ph3SbO)2 < 3a, reflecting the degree of association of the
experimental four-membered-ring structures (see above).
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Experimental Section
A solution of 1 (577 mg, 1.00 mmol) or 4 (547 mg, 1.00 mmol) in
toluene (50 mL) was hydrolyzed by addition of aqueous NaOH
(50 mL, 0.1m for 2 and 3; 10 mL, 0.5m for 5 and 6). The mixture was
vigorously stirred for 1 h (for 2 and 5) or 24 h (for 3 and 6) before the
layers were separated. The organic layer was dried over Na2SO4 and
the solvent removed under reduced pressure to give a colorless solid.
[11] a) Crystal data for 3 (C48H54O6Sb2·C4H8O): Mc = 1042.52, mono-
clinic space group P21/n, a = 11.005(2), b = 9.6807(17), c =
20.432(4) ꢁ, b = 95.673(4)8, V= 2166.1(7) ꢁ3, Z = 2, 1calcd
=
2
(recrystallized from CH2Cl2/hexane): Yield 482 mg,
1.544 gcmÀ3, crystal dimensions 0.38 ꢄ 0.25 ꢄ 0.17 mm3. 19846
collected and 6520 unique reflections. Final residuals R1 =
0.0325, wR2 = 0.0802 (I > 2s(I)); R1 = 0.0454, wR2 = 0.0869 (all
data). GooF = 1.093, 257 parameters; b) crystal data for 6
(C48H52O4Te2): Mc = 948.12, monoclinic space group P21/n, a =
8.1999(14), b = 16.639(3), c = 15.696(3) ꢁ, b = 101.132(4)8, V=
0.478 mmol; 96%. 1H NMR (400 MHz, CDCl3): d = 7.57 (t, 3J-
(H,H) = 7.5 Hz, 1H), 7.20 (d, 3J(H,H) = 7.4 Hz, 2H), 6.95 (s, 4H),
2.26 (s, 6H), 2.21 ppm (s, 12H); 13C NMR (100 MHz, CDCl3): d =
140.3, 140.1, 138.4, 133.7, 132.4, 131.1, 129.0, 21.5, 21.1 ppm; IR (KBr):
n˜OH = 3521 cmÀ1; elemental analysis (%) calcd for C48H52Cl2O4Sb2
(1007.30): C 57.23, H 5.20; found: C 58.02, H 5.22.
2101.1(6) ꢁ3, Z = 2, 1calcd = 1.499 gcmÀ3
, crystal dimensions
3 (recrystallized from THF and dried in vacuum): Yield 425 mg,
0.438 mmol; 88%. 1H NMR (400 MHz, C6D6): d = 7.07 (t, 3J(H,H) =
7.6 Hz, 1H), 6.85 (s, 4H), 6.80 (d, 3J(H,H) = 7.5 Hz, 2H), 2.17 (s, 6H),
2.15 (s, 12H), 0.92 ppm (s, 2H); 13C NMR (100 MHz, C6D6): d =
146.7, 138.2, 138.0, 137.4, 129.6, 128.4, 128.3, 127.8, 21.5, 21.2 ppm;
0.36 ꢄ 0.10 ꢄ 0.09 mm3. 11491 collected and 4098 unique reflec-
tions. Final residuals R1 = 0.0602, wR2 = 0.1432 (I > 2s(I)); R1 =
0.0661, wR2 = 0.1450 (all data). GooF = 1.424, 239 parameters.
CCDC-702787 (3) and 702790 (6) contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
IR (KBr): n˜OH = 3653 cmÀ1
C48H54O6Sb2 (970.46): C 59.41, H 5.61; found: C 59.39, H 5.57.
; elemental analysis (%) calcd for
5 (recrystallized from THF): Yield 420 mg, 0.426 mmol; 85%.
1H NMR (400 MHz, C6D6): d = 7.02 (t, J(H,H) = 7.5 Hz, 1H), 6.80
3
[12] H. Schmidbaur, A. Schier, Organometallics 2008, 27, 2361.
[13] The calculations at the DFT/B3PW91 level of theory use the
large-core correlation-consistent SDB-cc-pVTZ basis sets with
the appropriate relativistic electron core potential for the heavy
atoms Te and Sb and the split-valence 6-311 + G(2df,p) basis set
for all other atoms. See the Supporting Information for details.
(d, 3J(H,H) = 7.6 Hz, 2H), 6.77 (s, 4H), 2.27 (s, 6H), 2.04 ppm (s,
12H); 13C NMR (100 MHz, C6D6): d = 148.0, 140.8, 136.4, 135.9,
128.5, 127.9, 127.6, 127.2, 20.8, 20.7 ppm; 125Te NMR (126 MHz,
C6D6): d = 1372 ppm; elemental analysis (%) calcd for
C48H50Cl2O2Te2 (985.02): C 58.53, H 5.12; found: C 58.24, H 4.75.
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Angew. Chem. Int. Ed. 2008, 47, 9982 –9984