Self-assembled antimony-thiolate Sb2L3 and Sb2L2Cl2 complexes
W. JAKE VICKARYOUS, LEV N. ZAKHAROV, AND DARREN W. JOHNSON*
Department of Chemistry, University of Oregon and the
Oregon Nanoscience and Microtechnologies Institute (ONAMI)
Department of Chemistry
1253 University of Oregon
Eugene, Oregon 97403, U.S.A
Phone: (+1) 541-346-1695
Fax: (+1) 541-346-0487
E-mail: dwj@uoregon.edu
WWW: http://www.uoregon.edu/~chem/dwjohnson.html
1
Abstract: The synthesis and single crystal X-ray structures of two new self-assembled
antimony(III)-thiolate complexes are described: Sb2L3 (1) and “anti”-Sb2L2Cl2 (2), where
H2L is α,α'-dimercaptoxylene. The Sb(III) ions in both complexes exhibit tripodal
coordination geometries with stereochemically-active lone pairs. The extended structure
of 1 in the crystalline state reveals numerous C-H···S interactions, while the packing in 2
is dominated by numerous antimony-sulfur and antimony-chlorine secondary bonding
interactions. These structures demonstrate that a supramolecular design strategy can be
developed to prepare self-assembled structures based on the coordination geometry of
Sb(III) and the reversible formation of antimony-thiolate bonds in solution. This strategy
offers promise in the specific chelation of toxic ions such as antimony, a contaminant of
increasing environmental concern.
Keywords: antimony-arene interactions, antimony compounds, coordination chemistry,
pnictogens, supramolecular chemistry, thiolate ligands
1. Introduction
The use of reversible metal-ligand interactions as a directing force for selfassembly reactions has provided a wide variety of discrete supramolecular
architectures.[1-15] The symmetry and overall structure of these multimetallic complexes
depends markedly on the coordination geometry of the metal and the binding geometry of
the ligand. The metal-ligand interaction also plays an essential role in driving the selfassembly reaction and influences whether the favored product of a reaction is discrete or
2
polymeric. The majority of supramolecular coordination complexes use transition metal
elements for this purpose — main-group elements are only rarely utilized as design
components in supramolecular chemistry.[16-22] This may in part result from the ability of
main group elements to blur the distinction between metals and nonmetals, making their
coordination chemistry less predictable.
This paper describes the use of the main group metalloid antimony in
coordination driven self-assembly involving organothiolate ligands. An organic bidentate
thiolate ligand (H2L) assembles with an Sb(III) source (SbCl3) to form dinuclear
complexes. Both a triple-stranded helical Sb2L3 complex and a double-stranded
macrocyclic Sb2L2Cl2 complex have been isolated and structurally characterized by
single crystal x-ray diffraction. Both dinuclear complexes exhibit close contacts between
antimony and arene rings in the solid state characteristic of antimony-π interactions.[23]
Thus, antimony-arene interactions provide a novel additional stabilizing factor favoring
formation of these discrete supramolecular structures.[24,25]
2. Results and Discussion
2.1 Sb2L3 Helicate
The addition of SbCl3 to a mixed CH2Cl2/methanol solution of deprotonated α,α'dimercapto-p-xylene (H2L) leads to the formation of an insoluble precipitate. The 1H
NMR spectrum of the crude filtrate is very simple: a singlet at δ 7.21 in the aromatic
region and a singlet at δ 3.89 in the methylene region are observed. This is consistent
3
with a single symmetric species remaining in solution following the reaction. Slow
diffusion of pentane into a CHCl3 solution of the product yields single crystals of Sb2L3
suitable for X-ray diffraction. The discrete Sb2L3 helicate crystallizes in space group
P21/c with four molecules per unit cell (Figure 1).
[Insert Figure 1]
Each antimony atom has a trigonal pyramidal coordination geometry with the
stereoactive lone pair directed inside the cavity. Multiple antimony-arene interactions are
evident between each Sb(III) atom and the three phenyl rings. Each antimony atom
makes close contacts (defined as less than the sum of the van der Walls radii) with at
least six carbon atoms of the three phenyl rings. The antimony lone pairs are tilted
significantly with respect to the phenyl rings. This is likely a more favorable geometry
for a donor-acceptor interaction than a direct perpendicular angle between the antimony
lone pair and the arene ring. Many known complexes of arsenic, antimony, and bismuth
with arenes have the pnictogen lone-pairs at a significant angle with respect to the arene
ring reminiscent of an η2 or η3 interaction; this tilted geometry facilitates this type of
bonding mode.[26]
These close contacts along with the appropriate geometric orientations provide
suitable complementary evidence indicating multiple intramolecular antimony-arene
interactions exist in the solid state. The antimony-arene interactions may also stabilize
the dinuclear complex in solution. There is no steric reason why the complex could not
form with the antimony lone pairs pointing outside rather than inside. Given that the
4
barrier to inversion in stibines is quite high, it is surprising that a mixture of isomers
(consisting of dinuclear complexes with two, one and zero antimony lone pairs pointing
inside) is not observed in solution by NMR spectroscopy. This suggests the antimonyarene interaction occurs during an early point in the assembly process, perhaps
preorganizing a precursor for formation of the discrete species with lone pairs directed
endohedrally.
The helical structure of the dinuclear complexes can be seen by looking down the
antimony-antimony axis (Figure 2a). Although each helicate is chiral, they crystallize as
a racemic mixture in space group P21/c. Each unit cell contains two left-handed helices
and two right-handed helices. Although the methylene protons of each helical
enantiomer are formally diastereotopic, only a single methylene signal is observed by 1H
NMR. This suggests rapid interconversion between enantiomers in solution. The
mechanism for this interconversion most likely involves flipping the twisted orientation
of each antimony-sulfur-carbon bond angle. A similar low-energy interconversion
process has been observed with the arsenic analogue As2L3.[24]
[Insert Figure 2]
It has been observed that when antimony thiolates are sterically blocked from
forming Sb···S interactions, C-H···S interactions invariably dominate the crystal
packing.[27,28] This generalization is again demonstrated in the packing of the Sb2L3
helicate (Figure 2b). The right- and left-handed helicates are segregated into separate
columns that stack along the b axis. C-H···S interactions between sulfur and phenyl
5
hydrogens link helices of the same handedness, and C-H···S interactions between sulfur
and methylene protons link columns of different helicity. C-H···S interactions are the
main packing interaction in this structure; S···S interactions are entirely absent (all
intermolecular S-S distances are over 4 Å). This is in contrast to many examples of
sulfur-containing macrocycles where S···S interactions dominate the packing in the solid
state.[29]
2.2 Sb2L2Cl2 Macrocycle
When SbCl3 and α,α'-dimercapto-p-xylene (H2L) are combined in the absence of
base, no immediate precipitation is observed. Crystallization from the reaction mixture
by slow diffusion of pentane yields abundant crystals allowing full structure
determination by single crystal X-ray diffraction. Two ligands bridge two antimony
atoms forming an eighteen-membered macrocycle (Figure 3). The antimony atoms each
remain coordinated to one chlorine with the Sb-Cl bonds oriented on opposite sides of the
ring in an anti conformation.
[Insert Figure 3]
The antimony lone pairs point inward toward the center of the macrocycle, and
the geometry and close contact distances with regard to the phenyl rings indicate
intramolecular stabilization through antimony-arene interactions. Each antimony atom
makes close contacts with three carbons of one phenyl ring and two carbons of the other
6
phenyl ring. This asymmetric coordination of antimony by arenes has firm precedent in
the Menshutkin complexes of antimony halides where η3 and η2 hapticity is often
observed.[30] This preference is manifested in an unusual manner in the present dinuclear
macrocycle, where a “slipped sandwich” geometry reduces the hapticity and provides
coordination by two arenes.
The antimony-arene distances range from 3.20 to 3.61 Å , which is somewhat
longer than expected based on the short distances observed in the analogous antiAs2L2Cl2 macrocycle (ranging from 3.16-3.58 Å)[25] and the increased Lewis acidity of
Sb(III) compared to As(III). This most likely stems from the longer Sb-S bonds, as
antimony has a larger covalent radius compared to arsenic. The macrocycles pack in
layers along the a axis. The sulfur atoms in adjacent layers are oriented towards the
interlayer region so that there are several intermolecular sulfur-sulfur distances shorter
than 3.6 Å, the sum of the van der Waals radii of two sulfur atoms.[31] However, before
describing a close contact as a secondary interaction, one should also assess whether the
orientation of local electron density is appropriate for a weak bonding interaction. In the
case of secondary S···S interactions, the optimum interaction geometry involves donation
from a sulfur lone pair into a σ* orbital 180° opposite a sulfur bonding orbital.[32,33]
Contrary to the model and many examples of intermolecular S···S interactions in
molecular crystals,[29] the Sb2L2Cl2 macrocycles pack such that the C-S-Sb planes are
nearly coplanar (Figure 4).
[Insert Figure 4]
7
The crystal packing of the macrocycles is likely rather the result of antimonysulfur and antimony-chlorine interactions. Between layers of macrocycles along the a
axis, there is a seam of cocrystallized SbCl3 molecules which participate in secondary
bonding interactions. A chlorine atom of each SbCl3 molecule is 3.81 Å from an
antimony atom in a macrocycle (Figure 4). This distance is just over the sum of the van
der Waals radii (3.80 Å) according to a well-established tabulation of values.[31]
However, there is, as yet, no clear consensus for the van der Waals radii of antimony.[28]
If one considers the orientation of the available orbitals, the contact is highly consistent
with the known coordination behavior. Sb(III) is very often four-coordinate with a
disphenoidal (seesaw) coordination geometry, and the contact in the solid state may be
regarded as remote coordination of a chloride to each antimony atom in the macrocycle.
Similar, though shorter, antimony-chlorine secondary interactions have been found to
organize a number of molecular crystals.[34]
The cocrystallized antimony trichloride molecules also exhibit another close
contact between an antimony atom (SbCl3) and a sulfide linkage in the macrocycle
(Figure 4). The antimony-sulfur contact distance (3.67 Å) is well within the sum of the
van der Waals radii (3.85 Å),[31] especially when one considers that some surveys of
antimony-sulfur secondary interactions have included all Sb-S distances less than 4.0
Å.[28] The geometry of the secondary interaction is also consistent with a donor-acceptor
interaction between a Lewis basic sulfur and a Lewis acidic antimony.[33]
3. Conclusions
8
Antimony is ubiquitous in the environment albeit in low concentrations. The
concentration of antimony in nonpolluted waters is usually less than 1 ppb.[35] However,
antimony is of increasing environmental concern due to its widespread use in plastics and
flame retardants. The replacement of asbestos with Sb2S3 in brake pads has led to high
concentrations of antimony in urban dusts.[36] Elevated antimony concentrations have
also been found in bottled water due to leaching from the polyethylene terephthalate
(PET) containers.[37]
Antimony and arsenic share some similarities in their toxicological profiles[38] and
techniques used in managing arsenic warrant investigation with antimony. Thiolate
ligands have an extensive history of use as effective chelators for As(III), and arsenic and
antimony often occur together as environmental co-contaminants.[39] Therefore, an
understanding of antimony-thiolate coordination chemistry is relevant due to the hazards
of antimony alone and also because the presence of antimony may modulate the
effectiveness of arsenic remediation techniques.
The present report demonstrates that a supramolecular design strategy does allow
the preparation of multinuclear Sb(III) complexes. Multiple types of secondary bonding
interactions are found to stabilize the packing in the solid state. The complexes are futher
stabilized by intramolecular antimony-arene interactions. Arene complexes of pnictogens
(arsenic, antimony, and bismuth) are known to adopt a variety of sandwich, halfsandwich, and inverse sandwich structures with arenes. The present dinuclear complexes
represent novel slipped sandwich structures, as they exhibit reduced hapticity with
multiple benzene rings.
9
4. Experimental Section
All NMR specta were recorded using a Varian Inova-300 spectrometer operating
at 299.93 and 75.43 MHz for 1H and 13C nuclides, respectively. Spectra were referenced
using the residual solvent resonances as an internal standard (7.26 and 77.0 ppm for 1H
and 13C signals of residual CHCl3). Single crystal X-ray diffraction studies were
performed on a Bruker SMART APEX diffractometer. SbCl3 was used as received from
Strem Chemicals and stored in a vacuum desiccator. H2L (α, α'-dimercapto-p-xylene)
was prepared as described in the literature.[40]
Sb2L3 (1). H2L (202 mg, 1.19 mmol) was added to a degassed solution consisting
of 32 mL CH2Cl2 and 24 mL of a 0.0998 M solution of KOH (2.40 mmol) in MeOH.
SbCl3 (210 mg, 0.921 mmol) was sublimed before use and rapidly added to this clear
colorless solution. The reaction mixture was stirred under N2 at 30 °C for 3h 50min. The
solution was decanted to yield a yellowish-white solid. The yellow solid was extracted
with hot CHCl3 (50 °C), and the resulting extract was filtered through glass wool to yield
a clear colorless solution. The solvent was evaporated to yield a white solid (276 mg,
0.369 mmol, 93%). 1H NMR (CDCl3): δ 7.21 (s, 12H, C6H4), 3.89 (s, 12H, CH2);
C{1H} NMR (CDCl3): δ 139.77, 128.89, 33.33. Single crystals suitable for X-ray
13
diffraction were grown by dissolving this solid in a small amount of CHCl3, filtering the
solution through celite and then slowly diffusing pentane into this solution at room
temperature over the course of one week.
Single Crystal Growth of Sb2L2Cl2 (2). H2L (26 mg, 0.153 mmol) was added to
a solution of SbCl3 (136 mg, 0.596 mmol) dissolved in 5 mL CHCl3. An aliquot of the
10
resulting clear, colorless solution was transferred to a vial, suspended in pentane and the
system was capped and stored at room temperature. Slow diffusion of pentane into the
CHCl3 solution yielded crystalline plates of 2•SbCl3 after one week.
X-ray Structure Details. Table 1 provides the structural data for compounds 1
and 2, and Table 2 lists relevant bond distances and angles. CCDC numbers 610468 (1)
and 610467 (2) contain the supplementary crystallographic data and experimental details
for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif, by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033.
Acknowledgments
This research was supported by the University of Oregon and an NSF-CAREER award
(CHE-0545206). D.W.J is a Cottrell Scholar of Research Corporation. W.J.V. gratefully
acknowledges the National Science Foundation (NSF) for an Integrative Graduate
Education and Research Traineeship. The purchase of the X-ray diffractometer was
made possible by a grant from the NSF (CHE-0234965) to the University of Oregon.
References
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16
Table 1. Crystallographic Data and Refinement Parameters
empirical
formula
formula weight
temperature (K)
wavelength (Å)
crystal system
space group
a (Å)
b (Å)
c (Å)
α
β(deg)
γ
volume (Å3)
Z, Z'
absorption coefficient
F(000)
crystal size
theta range for data
collection
reflections collected
independent reflections
completeness to theta =
28.26°
absorption correction
max. and min.
transmission
refinement method
data / restraints /
parameters
goodness-of-fit on F2
final R indices
[I>2sigma(I)]
R indices (all data)
Sb2L3 (1)
Sb2L2Cl2•SbCl3 (2•SbCl3)
C24H24S6Sb2
C8H8Cl4S2Sb2
748.29
153(2)
0.71073
monoclinic
P2(1)/c
13.2086(10)
10.5558(8)
19.6778(14)
90
92.4780(10)
90
2741.1(4)
4, 1
2.440 mm-1
1464
0.20 x 0.15 x 0.15 mm3
1.54 to 28.26°
553.568
293(2)
0.71073
monoclinic
P2(1)/c
10.5321(7)
17.2414(11)
8.7666(6)
90
99.9710(10)
90
1567.87(18)
4, 1
4.366 mm-1
1032
0.12 x 0.08 x 0.02 mm3
1.96 to 28.20°
23179
6450 [R(int) = 0.0196]
95.0 %
9433
3613 [R(int) = 0.0180]
93.6 %
Semi-empirical from
equivalents
1.000 and 0.764
Semi-empirical from
equivalents
1.000 and 0.633
Full-matrix least-squares
on F2
6450 / 0 / 385
Full-matrix least-squares on
F2
3613 / 0 / 145
1.020
1.019
R1 = 0.0212, wR2 = 0.0489 R1 = 0.0296, wR2 = 0.0663
R1 = 0.0256, wR2 = 0.0508 R1 = 0.0433, wR2 = 0.0722
17
Table 2. Selected Bond Distances (Å) and Angles (°).
Sb2L3 (1)
Sb(1)-S(3)
Sb(1)-S(2)
Sb(1)-S(1)
Sb(2)-S(6)
Sb(2)-S(5)
Sb(2)-S(4)
2.4322(5)
2.4322(6)
2.4331(6)
2.4214(6)
2.4369(6)
2.4405(6)
S(3)-Sb(1)-S(2)
S(3)-Sb(1)-S(1)
S(2)-Sb(1)-S(1)
S(6)-Sb(2)-S(5)
S(6)-Sb(2)-S(4)
S(5)-Sb(2)-S(4)
C(1)-S(1)-Sb(1)
C(17)-S(2)-Sb(1)
C(9)-S(3)-Sb(1)
C(16)-S(4)-Sb(2)
C(8)-S(5)-Sb(2)
C(24)-S(6)-Sb(2)
91.03(2)
90.64(2)
94.19(2)
94.41(2)
89.70(2)
92.26(2)
102.46(9)
100.69(8)
101.90(8)
101.25(9)
100.09(9)
104.20(8)
Sb2L2Cl2•SbCl3 (2•SbCl3)
Sb(1)-Cl(1)
Sb(1)-S(2)
Sb(1)-S(1)
S(1)-C(1)
S(2)-C(5)
2.3766(14)
2.3942(11)
2.4143(10)
1.822(5)
1.828(5)
Cl(1)-Sb(1)-S(2)
Cl(1)-Sb(1)-S(1)
S(2)-Sb(1)-S(1)
C(1)-S(1)-Sb(1)
C(5)-S(2)-Sb(1)
C(2)-C(1)-S(1)
C(6)-C(5)-S(2)
98.10(5)
95.36(6)
82.76(4)
103.17(16)
102.34(14)
115.1(3)
113.5(3)
18
Figure 1
S5
S4
C8
S6
C16
C5
C13
C24
Sb2
C6
C22
C21
C15C23
C20
C14
C12
C4
C7
C11
C3
C19
C10
C2
Sb1
C18
C9
C1
S3
S1
C17
S2
19
Figure 2
20
Figure 3
Cl1
S2
S1
C5
Sb1
C7
C6
C8
C4
C3
C2
C1
21
Figure 4
22
Figure Captions
Figure 1. Crystal structure of the Sb2L3 helicate. Only the right-handed enantiomer is
shown. Antimony is shown in cyan, sulfur yellow, carbon grey and hydrogen white. (a)
ORTEP diagram, (b) wire-frame representation, (c) and space-filling representation.
Figure 2. (a) View down the Sb-Sb axis in the right-handed enantiomer of the Sb2L3
helicate showing the helical structure of the complex. (b) View looking down the b axis
showing the C-H···S interactions guiding the packing in the solid state.
Figure 3. Crystal structure of the Sb2L2Cl2 macrocycle. The cocrystallized SbCl3
molecule is omitted for clarity. Antimony is shown in cyan, sulfur yellow, chlorine
green, carbon grey and hydrogen white. (a) ORTEP diagram, (b) wire-frame
representation, (c) and space filling representation.
Figure 4. Multiple intermolecular close contacts are present in the packing of the
Sb2L2Cl2 macrocycle. (a) View of close S···S contacts. (b) View antimony-chlorine and
antimony-sulfur secondary interactions.
23