Design, synthesis and characterization of self-assembled As 2L3 and Sb2L3 cryptands

Article abstract of DOI:10.1039/c1dt10817d

The syntheses and X-ray crystal structures of six new self-assembled supramolecular As and Sb-containing cryptands are described. Analysis in the context of previously reported As₂L₃ and Sb ₂L₃ cryptands reveals that small differences in ligand geometries result in significant differences in the helicity of the complexes and the stereochemistry of the metal coordination within the assembled complexes. Additionally, a new synthetic route is described which involves exposure of reactants to vacuum to help facilitate self-assembly.

Full text of DOI:10.1039/c1dt10817d

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Structural and metallo selectivity in the assembly of [2 × 2] grid-type metallosupramolecular
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PAPER
Design, synthesis and characterization of self-assembled As2L3 and Sb2L3
cryptands†
Sean A. Fontenot,‡ Virginia M. Cangelosi,‡ Melanie A. W. Pitt, Aaron C. Sather, Lev N. Zakharov,
Orion B. Berryman and Darren W. Johnson*
Received 3rd May 2011, Accepted 25th May 2011
DOI: 10.1039/c1dt10817d
The syntheses and X-ray crystal structures of six new self-assembled supramolecular As and
Sb-containing cryptands are described. Analysis in the context of previously reported As2L3 and Sb2L3
cryptands reveals that small differences in ligand geometries result in significant differences in the
helicity of the complexes and the stereochemistry of the metal coordination within the assembled
complexes. Additionally, a new synthetic route is described which involves exposure of reactants to
vacuum to help facilitate self-assembly.
This report provides full synthetic details and characterization
of new As2L3 and Sb2L3 cryptands (Table 1). This structural report
Fabrication of complex structures from simple components has
will discuss in detail how small changes to the ligand can greatly
Introduction
been a research topic of great interest in recent years.1 Supramolec-
affect the helicity of the assemblies and the stereochemistry of
ular self-assembly processes and dynamic covalent chemistry offer
the pnictogen ions. Additionally, a new route that facilitates the
a powerful set of tools for the bottom-up synthesis of complex
supramolecular syntheses is described, in which brief exposure of
structures with new properties and emergent functionality.2,3
A
the reaction mixture to vacuum is sufficient to complete the self-
assembly of the cryptands from an equilibrating mixture of lower
order structures.
defining feature of these synthetic strategies is that information
contained within relatively simple components determines the
formation of much more complex structures. Chemists have
compiled an enormous library of self-assembled supramolecular
complexes utilizing the directing properties of the transition
metals. In comparison, there are relatively few examples of
assemblies that were designed to use the main group elements as
directing components, showing that main group supramolecular
chemistry is still developing the tools for the predictable formation
of well-defined structures.4 The main group elements generally
prefer unusual “coordination” geometries compared to the tran-
sition metals, making them attractive targets when seeking novel
properties, new topologies and alternative functionalities in self-
assembled molecules and materials. However, there exists perhaps
a misconception that bonds to main group elements are not labile
enough for self-assembly.5 Contradictory to this belief, our lab has
shown that thiolate bonds to the Group 15 elements phosphorus,
arsenic, antimony and bismuth are sufficiently reversible and can
be used to drive the formation of supramolecular assemblies.6–8
Results and discussion
Cryptands overview
Our lab has previously shown that E2L3 (E = As, Sb) cryptands
form in self-assembly reactions between ECl3, base, and rigid
dithiol ligands H21, H22, or H23 (Scheme 1, Chart 1).9–10 Charac-
terization of these dynamic assemblies in solution and in the solid
state revealed several structural similarities despite the differences
in the rigid spacers between the mercaptomethyl groups of the
ligands (ligand backbones). Consistently, each complex features
short E ◊ ◊ ◊ p distances, a consequence of the favorable pnictogen-p
interactions, typically described as donation of electron density
from the p-system of the ligands into the s* orbitals of the E
atoms.11 The different backbones of ligands H21, H22, or H23 cause
subtle variations in the angles and distances between the pnictogen
atoms and phenyl rings (Table 2). In order to further explore
this interaction, a series of E2L3 cryptands were assembled using
dithiol ligands with newly designed backbones (H24–H28, Chart
1). Within each ligand, the mercaptomethyl arms were preserved
but the size and rigidity of the backbone spacer was varied.
Department of Chemistry, Materials Science Institute, and the Oregon
Nanoscience and Microtechnologies Institute (ONAMI), University of Ore-
gon, Eugene, OR, 97403-1253, USA. E-mail: dwj@uoregon.edu; Tel: 541-
346-1695
† Electronic supplementary information (ESI) available: Experimental
details for ligand and complex syntheses, full details on X-ray crystal
structure determinations and refinements. CCDC reference numbers
824398–824403. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c1dt10817d
Synthesis of cryptands
As2L3 (L = 1–8) and Sb2L3 (L = 1, 2, 8) cryptands were prepared
‡ These two authors contributed equally to this work.
in self-assembly reactions from ECl3 and dithiolate ligands. The
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Dalton Trans., 2011, 40, 12125–12131 | 12125
©

Table 1 Crystallographic data and refinement parameters
As243
As253
As263
As273
As283
Sb283
Formula
C30 H36 As2 S6
C54 H42 As2 S6
C48 H48 As2 S6
C48 H36 As2 S6
C36.33 H30.33
As2 Cl S6
844.59
C37.25 H31.38 Cl3
N0.13 S6 Sb2
1022.96
Formula weight
738.79
1033.08
967.06
954.97
T/K
173(2)
0.71073
173(2)
0.71073
173(2)
0.71073
173(2)
0.71073
173(2)
0.71073
173(2)
0.71073
˚
Wavelength/A
Crystal system
Monoclinic
P21/n
Monoclinic
C2/c
Monoclinic
C2/c
Monoclinic
C2/c
Hexagonal
R3c
21.3256(7)
21.3256(7)
41.358(3)
90
90
120
16288.8(13)
18
1.550
Rhombohedral
R3
21.0255(7)
21.0255(7)
21.0255(7)
69.14
69.14
69.14
7831.4(5)
8
1.735
¯
¯
Space group
˚
a/A
14.4540(6)
12.6952(5)
17.6319(7)
90
90.1500(10)
90
3235.4(2)
4
1.517
26.660(7)
10.399(3)
20.779(6)
90
125.192(7)
90
4708(2)
4
1.458
1.723
30.468(3)
13.6420(12)
11.0855(10)
90
106.3970(10)
90
4420.2(7)
4
1.453
31.093(2)
13.5902(10)
10.6951(8)
90
107.090(1)
90
4319.7(6)
4
1.468
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
3
˚
V/A
Z
Dc/g cm-3
m/cm-1
2.473
1512
1.829
1992
1.871
1944
2.293
7692
1.932
4038
F(000)
2112
Crystal size/mm
Index ranges
0.32 ¥ 0.06 ¥ 0.04
-17< = h< = 17
-15< = k< = 15
-20< = l< = 20
30492
0.07 ¥ 0.06 ¥ 0.05
-34< = h< = 17
-13< = k< = 11
-24< = l< = 26
9193
0.37 ¥ 0.14 ¥ 0.04
-38< = h< = 38
-17< = k< = 17
-14< = l< = 14
24133
0.18 ¥ 0.16 ¥ 0.12
-39< = h< = 39
-17< = k< = 17
-13< = l< = 13
19714
0.36 ¥ 0.28 ¥ 0.18
-27< = h< = 27
-27< = k< = 27
-52< = l< = 52
58788
0.16 ¥ 0.16 ¥ 0.08
-26< = h< = 26
-26< = k< = 26
-26< = l< = 26
88616
Reflections collected
Independent
5689 [0.0546]
4993 [0.0856]
4837 [0.0417]
4719 [0.0416]
3969 [0.0307]
11413 [0.0556]
Reflections [Rint
]
Data/restraints/
Parameters
5689/0/343
4993/0/280
4837/0/253
4719/0/325
3969/0/199
11413/0/577
Goodness-of-fit on F2 1.066
1.004
1.088
1.091
1.075
1.37
R1, wR2 [I > 2s(I)]
R1, wR2 (all data)
0.0456/0.1025
0.0651/0.1123
0.0756/0.1014
0.1626/0.1318
0.752/-0.453
0.0468/0.1210
0.0670/0.1416
1.064/-0.452
0.0418/0.0884
0.0616/0.1004
0.609/-0.233
0.0385/0.1041
0.0447/0.1081
1.145/-0.498
0.0633/0.1789
0.0810/0.1886
1.900/-1.630
Largest diff. peak and 0.571/-0.289
-3
˚
hole/e A
Table 2 Stereochemical outcomes and selected geometric information for E2L3 assemblies
E ◊ ◊ ◊ p
Stereochemical
Configurationb
E ◊ ◊ ◊ E
Twist angle
(◦)c
Degree of◦
Helicityd( )
Interior
Volume/A
S–E–S (◦)
Reference
a
˚
3e
˚
˚
˚
Complex
(A)
(A)
E–S (A)
As213
3.30
3.30
3.35
3.31
3.33
3.30
3.25
3.29
3.31
3.34
3.31
3.36
D,K
5.03
5.11
9.19
5.09
9.52
11.71
12.28
7.55
4.30
4.83
6.44
7.01
36.6
35.7
39.1
34.7
33.3
27.3
23.5
29.2
43.0
33.7
40.0
29.1
—
6.0
—
38
44
55
38
90
67
64
72
36
46
63
74
2.25
2.25
2.24
2.25
2.25
2.24
2.24
2.24
2.43
2.42
2.42
2.44
94.6
94.2
97.3
94.1
94.8
96.1
97.5
94.7
92.0
91.5
91.1
91.3
9
6
As223
D,K,P
D,K,M+Pf
D,K
D,D,P
D,D,M
D,D,M
D,D,P
D,D,P
D,K
As233
10
—
—
—
—
—
12
6
As243
—
As253
29.5
54.3
65.8
7.7
12.7
—
As263
As273
As283
Sb213
Sb223
Sb283 (1)g
Sb283 (2)
D,D,M
D,D,M
39.9
9.1
—
—
a Measured as the average distance between E center and centroid of close contacts with neighboring arene rings. b See Fig. 3–5 for examples of helical
domain designations. To simplify discussion, for homoconfigurational cryptands only the D,D isomers are shown. c Twist of the octahedral coordination
environment - estimated as the torsion angle between a sulfur, a centroid defined using the three sulfurs, the E center, and the closest aryl contact.
d Reported as an average of the three –CH2–As–As–CH2– torsion angles. e Cavities were measured using solvent (0.7 A) surfaces in WebLab ViewerPro
˚
4.0. Cavity volumes were estimated as the difference between the volume of the E2L3 cryptand filled with chloride atoms and the E2L3 cryptand with a
void cavity. f (D,K,M)- and (D,K,P)-As2L2 form as a racemic mixture. g There are two different polymorphs observed in the same crystal, Sb283(1) and
3
Sb283(2).
conditions previously reported for the synthesis of cryptands
As213 (Fig. 1a), Sb213 (Fig. 1b), Sb223 (Fig. 1d), and As233
(Fig. 1e), involving deprotonation of H2L with base followed by the
addition of ECl3 (E = As, Sb), also allowed for the preparation of
cryptands As243 (Fig. 2), As223 (Fig. 1c), As253 (Fig. 3), and As263
(Fig. 4).6,9,10,12 Crystals were grown of each assembly (Table 2),
allowing the direct comparison of these structures by single crystal
X-ray diffraction.
Interestingly, base was not necessary for the synthesis of As273
(Fig. 5) which assembled under conditions which normally result
in As2L2Cl2 macrocycle. Furthermore, As283 would not assemble
even with base present. These two surprising results prompted us to
12126 | Dalton Trans., 2011, 40, 12125–12131
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©

Scheme 1 Self-assembly of As2L3 cryptands.
Fig. 2 Stick representations of the X-ray crystal structures of the As243
cryptand: (a) side-view showing the ligand domain and (b) top-view down
the As-As axis showing one of the As(III) coordination domains.
Fig. 3 Stick representations of the X-ray crystal structures of D,D,P
isomer of the As253 cryptand: (a) side-view showing the P configuration
in the ligand domain and (b) top-view showing D configuration of one of
the As(III) coordination domains.
Chart 1 Ligands used in synthesis of cryptands.
consider the role of base in this reaction. Was deprotonation of the
ligand with base prior to reaction with arsenic necessary or was the
role of the base simply to remove the HCl byproduct of As–S bond
formation from the reaction? We previously established that when
a 1 : 1 ratio of dithiol ligand and AsCl3 are combined in the absence
of base, two thiol functional groups substitute onto each AsCl3
molecule, resulting in the formation of As2L2Cl2 macrocycles.13
These results show that base is not necessary for the addition of
these first two ligands. However, even if this reaction is carried out
in the presence of excess ligand, substitution of a third thiol ligand
onto As does not occur before As2L2Cl2 precipitates from solution
after several days (except in the case of As273). Each substitution
of thiolate onto AsCl3 results in the formation of an equivalent of
HCl and one theory to explain our observations is that equilibrium
with HCl causes the reaction to stop after two substitutions have
been made at each arsenic center. In order to test this hypothesis, a
chloroform solution of H21 and 2/3 AsCl3 was heated and exposed
to high vacuum at intervals over the course of the reaction (under
continuous vacuum all solvent would have evaporated). Under
these conditions As213 formed, suggesting that the presence of
the HCl byproduct is what limits the addition of ligands onto
arsenic. Further support for this theory lies in our previous report
Fig. 1 Stick representations of the X-ray crystal structures of previously reported E2L3 cryptands: (a) As213,9 (b) Sb213,12 (c) As223,6 (d) Sb223,6 and (e)
As233.10 Arsenic is shown in purple, antimony in teal, sulfur in yellow, carbon in black and hydrogen in white.
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Dalton Trans., 2011, 40, 12125–12131 | 12127
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Fig. 4 Stick representations of the X-ray crystal structures of the D,D,M
isomer of the As263 cryptand: (a) side-view showing the M configuration
in the ligand domain and (b) top-view showing D configuration of one of
the As(III) coordination domains.
Fig. 5 Stick representations of the X-ray crystal structures of the D,D,M
isomer of the As273 cryptand: (a) side-view showing the M configuration
in the ligand domain and (b) top-view showing D configuration of one of
the As(III) coordination domains.
Fig. 6 Stick representations of the X-ray crystal structures of the E283
cryptands: (a) As283, (b) Sb283(1), and (c) Sb283(2). (b) and (c) are isomeric
forms of Sb283 which cocrystallize.
that heating a mixture of dissolved As2L2Cl2 crystals with excess
H2L allows for the formation As2L3.13a Because we started from
crystalline As2L2Cl2 in that case, only two equivalents of HCl were
formed while making As2L3, instead of the six equivalents formed
when cryptand was prepared directly from AsCl3 and H2L.
The removal of HCl by vacuum allows for cryptand formation
while avoiding the use of potentially harsh and complicating
bases. To test the scope of this new vacuum-enabled synthetic
technique, H28 was mixed with AsCl3 in chloroform at 50 ◦C and
the reaction vessel was exposed to high vacuum for a few seconds
Discussion of cryptand size and shape
Single crystals of each cryptand were grown and the structures
were obtained using X-ray diffraction (Fig. 1–6). Comparison
reveals several structural features that are general for this class
of E2L3 cryptand. In each case, the E ◊ ◊ ◊ p interaction causes the
arsenic or antimony atoms to be positioned within the cryptand
cavities. Consequently, their lone pairs of electrons point toward
one another making the cavities uniquely Lewis basic. As expected,
the distance between the pnictogen centers in each complex
depends on the distance between the thiolate functionalities on
the ligands (Table 2). In the arsenic structures, As ◊ ◊ ◊ As distances
1
every 1.5 h. Within a few hours H NMR spectroscopy revealed
no free H28 and that As242Cl2 and a new highly symmetric species
were both present in solution. After one more day of heating, the
new high-symmetry species was the dominant product. Crystals
were grown by layering this chloroform solution with hexanes and
the structure revealed that the main product of the reaction was
the As283 cryptand (Fig. 6a). Similarly, Sb283 was prepared using
vacuum-enabled synthesis (Fig. 6b–c). Both As283 and Sb283 could
not be prepared by other routes, suggesting that this relatively
gentle synthetic route may allow for the preparation of new, larger-
order assemblies. Additionally, this further supports that As2L2Cl2
macrocycles are intermediates in cryptand formation.7,8
˚
˚
range from 5.03 A for As213, the shortest cryptand, to 12.28 A
for As273, the longest cryptand (Table 2). While this result is not
surprising for the rigid cryptands, (As213, As223, As243, As253,
As273, and As283) some flexibility is expected for As233 and As263.
Indeed, As233 exhibits this flexibility as the ligands are folded
into the cavity, destroying the C3 symmetry of the complex and
˚
giving a shorter than expected As ◊ ◊ ◊ As distance of 9.19 A. All
other cryptands have apparent C3 symmetry. Despite the inherent
ligand flexibility within As263, in the solid state the ligand is fully
extended and the C3 symmetry of the complex is maintained. This
12128 | Dalton Trans., 2011, 40, 12125–12131
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©

bonding arrangement allows for the expected anti conformation
for the phenyl rings on the ethylene spacer.
each cryptand, short As ◊ ◊ ◊ p contacts are observed (Table 2).
The apparent C3 symmetry of all of the structures except As233
results in approximately equal E ◊ ◊ ◊ p contacts between each
metal center and each ligand. The cryptand with the shortest
Similarly, Sb ◊ ◊ ◊ Sb distances trend with ligand length (Table 2).
Compared to As ◊ ◊ ◊ p, the relatively stronger Sb ◊ ◊ ◊ p interaction
can provide for shorter E ◊ ◊ ◊ p contacts although this is not
observed here due to the constraints imposed by the rigid ligand
framework and the relatively longer Sb–S bonds.7,14 The stronger
Sb ◊ ◊ ◊ p interaction may cause twisting of the ligands in Sb213
to allow for shorter contacts than would be present without
twisting. Due to the naphthalene group on H22, twisting within
E233 assemblies is limited sterically: while the S ◊ ◊ ◊ S distances in
H21 and H22 are the same, the Sb ◊ ◊ ◊ Sb distances for Sb213 (4.30
˚
average E ◊ ◊ ◊ p contacts (3.25 A) is As273, while As253 had the
˚
longest average contacts (3.33 A). This very narrow range of
contact distances does not correlate to the E ◊ ◊ ◊ E distance or the
length of the ligand spacer. Additionally, the hapticity observed
2
for each structure (h ) was not affected by the length of the
ligand.
Analysis of chirality in E2L3 cryptands
˚
˚
˚
A) and Sb223 (4.83 A) differ by > 0.5 A. This is remarkable
because the As ◊ ◊ ◊ As distances in As213 and As223 vary by <
0.1 A. This larger difference in distances within the antimony
cryptands is manifested by the tight helical twist found in the Sb213
complex.
As expected, cavity volumes trend roughly with ligand length:
the volume increases in concert with the S ◊ ◊ ◊ S distance (Table 2).
If considered a chelate, the pnictogen-p contact completes a five-
membered chelate-type ring that contains arsenic or antimony,
the thiolate sulfur, the methylene carbon, and the two aryl
˚
2
carbons (ipso and ortho) involved in the h -E ◊ ◊ ◊ p interaction.
The bidentate nature of the primary/secondary coordination
spheres creates a distorted octahedral coordination environment
around the pnictogen center. In the solid state, the coordination
environments of all observed E2L3 cryptands possess a con-
formational chirality which can be described with established
stereochemical conventions. The absolute configuration of the
pnictogen coordination domain is defined by the direction of
torsional twist observed along the C3 axis. A clockwise twist is
designated as D and a counter-clockwise twist is designated as K
(Fig. 3–5).18,19
However, the ligands in As233 are folded into the cavity, resulting
3
˚
in a smaller than expected volume in the solid state (51 A ). The
cavities in As263 and As273 are also smaller than expected, a result
of the relatively narrow ligands not completely enclosing a well-
defined cavity. The volumes of the corresponding As- and Sb-
containing cryptands are approximately equal.
The S–E–S bond angles and E–S bond distances were measured
and the averages for each complex are listed in Table 2. The E–
S bond distances do not vary much from complex to complex,
As213, As243, and Sb223 cryptands are “mesocates” - one
pnictogen center is D and the other is K and the ligand domain is
not significantly twisted - there is very little, if any, degree of helicity
(Fig. 7).20a On the other hand, Sb213, Sb283, As283 and As253–As273
are helicates in which helicity is observed in the ligand domain
and both pnictogen centers have identical stereochemistry. The
ligands of As233 adopt an asymmetric conformation in the solid
state, resulting in the crystallization of a racemic pair of helices:
D,KM-As233 and D,KP-As233. Here, M designates a left-handed
twist in the ligand domain and P designates a right-handed twist
(Fig. 3–5).10 All helical structures crystallize as racemic mixtures
of enantiomers (D,D and K,K). For the sake of simplicity, only D,D
isomers are shown and discussed. These complexes are examples of
chiral structures originating from coordination assemblies lacking
chiral components.20,21
˚
˚
averaging 2.24 A for As–S bonds and 2.44 A for Sb–S bonds. The
S–E–S angles vary slightly more. In the arsenic complexes, the
individual (not average) angles range from 90.6◦ to 103.0◦ with an
average of 95.4◦. In the antimony complexes, these angles range
only from 89.7 to 94.4◦ with an average angle of 92.6◦. As233 has
the most diverse set of observed angles which is not surprising
given the lack of symmetry in the complex.
The degree of helicity in the cryptands was measured as the
offset of the methylene carbons on each ligand (Table 2, Fig. 7). It
was found to range from 6◦ (nearly untwisted) for As223 to 65◦ for
As273. In general, greater E ◊ ◊ ◊ E distances have larger degrees of
helicity, but this can vary greatly depending on ligand orientation.
This is evidenced by the structures of co-crystallized Sb283(1) and
Sb283(2) in which the methylene carbons are offset by pitches 39.9◦
and 9.1◦, respectively, but only differ in Sb ◊ ◊ ◊ Sb distances by
˚
0.05 A.
Discussion of pnictogen-p interactions
Lewis acid/base adducts formed by interactions between pnicto-
gens and arene rings have been rigorously studied by Schmidbaur
and co-workers, although only recently have they been used in
a supramolecular context as a structural motif in the design of
larger, higher order assemblies.9–14,16 Pnictogen-p interactions are
defined as short contacts (less than the sum of the respective
van der Waals radii) between the trivalent metalloid center and
arene carbon atoms; the attractive interaction is measured between
Fig. 7 Illustrations of degree of helicity and twist angle of pnictogen
coordination domains. (a) Degree of helicity was measured as an average
of the three –CH2–E–E–CH2- torsion angles; -CH2- carbon atoms
highlighted green; (b) twist angle was estimated as the torsion angle
between a sulfur, a centroid defined using the three sulfurs, the E center,
and the closest aryl contact.
2
the pnictogen-center and the centroid of these close contacts. h -
coordination is typically observed, though in some cases hapticity
6
can be as high as h when sterically demanding ring substituents
center the metal over the ring.11,17 In the crystal structure of
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The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 12125–12131 | 12129
©

In general, there are three origins of helicity in metallo-
supramolecular architectures: 1) helical induction by intramolecu-
lar effects, 2) axial chirality in the ligands, and 3) helical induction
by intermolecular interactions.18,19,22 Presumably, cryptands adopt
the degree of helicity necessary to reach a conformation in which
interactions that promote twisting of the ligand domain are
balanced with interactions that oppose twisting. Interactions that
promote twisting in the ligand domain include favorable ligand-
ligand interactions and E ◊ ◊ ◊ p interactions. An E ◊ ◊ ◊ p distance
Experimental
General
Complete schemes, experimentals and citations for the ligand
syntheses and techniques used for single crystal growth are
provided in the ESI. Caution: arsenic compounds are highly toxic
and should be handled with care!
X-ray crystallography
˚
of about 3.3 A is remarkably preserved across all observed
Diffraction intensities were collected at 173(2) K on a Bruker
cryptands. This is evidence that the E ◊ ◊ ◊ p interaction is significant
to the conformation and, thus, the stereochemistry of these
cryptands. Steric interactions between individual components of
an assembly could be considered interactions which would oppose
helical twisting. For instance, in the smaller, non-helical cryptands
such as As213, twisting is limited by the unfavorable steric
interactions which would result from short As ◊ ◊ ◊ As distances.
The bulky ligands of the non-helical As223 and As243 cryptands
exacerbate the effects of the interactions opposing twisting due
to unfavorable steric clash between neighboring ligands in the
cryptand. For As253, As263, and As273, these cryptands are large
enough to allow twisting up to the point that steric interactions
predominate.
With the exception of the highly asymmetrical As233, the
observed helical structures have E centers of identical chirality
(i.e. D,D or K,K) while the non-helical structures possess D,K
configurations. Since D and K configurations of the pnictogen
coordination environments are energetically equal, it is reasonable
to suspect that the helicity in the ligand domain (or lack thereof)
gives rise to the relative configurations (pairing or non-pairing) of
chirality of the E centers and not the other way around.
Apex CCD diffractometer using Mo-Ka radiation l = 0.71073
A. Absorption corrections were applied by SADABS.23 Structures
˚
were solved by direct methods and Fourier techniques and refined
on F2 using full matrix least-squares procedures. All non-H atoms
were refined with anisotropic thermal parameters. H atoms were
refined in calculated positions in a rigid group model. Solvent
molecules, CHCl3 in As283 and CH3CN in Sb283, are disordered
around -3 axis and were treated by SQUEEZE.24 Corrections of the
X-ray data by SQUEEZE (343 and 23 electron/cell, respectively
for As283, Sb283) are close to the required values of 348 and 22
electron/cell for six and one molecules, respectively, in the full unit
cells. In As263 elongations of thermal ellipsoids for some carbon
atoms in ligands perpendicular to C–C bonds were found. Such
elongations seem to be related to a disorder for these carbon atoms.
Some structural features in Sb283 also indicate that the molecules in
this structure are flexible or could be disordered as well. As results
some of C–C bond distances in Sb283, As253 and As263 were not
able to be measured precisely. All calculations were performed by
the Bruker SHELXTL (v. 6.10) package.25
Conclusions
Since helicity results in the coordination environments of
the E centers being offset from one another (i.e., the sulfur
atoms are not eclipsed), in order for the two E centers in
helical cryptands to have equivalent environments, they must
have identical stereochemical configurations. We observe that a
small degree of helicity may allow for D,K configuration of the
E coordination domains. For example, As223 has a small (6◦)
degree of helicity yet maintains D,K configuration. A unique
coupling relationship exists between the stereochemical domains.
In related dinuclear assemblies (e.g., double, triple, and quadruple-
stranded metallohelicates) the helicity is coupled to the absolute
stereochemical configuration of each metal center. For instance, in
the bis-chatecolate structures studied by Raymond and coworkers,
the metal coordination domains of P-helices all have D,D config-
urations as a result of strong mechanically coupling between the
metal centers.22 In our structures, the absolute stereochemistry of
the E coordination domains is decoupled from that of the helical
domain. However, the coupling of the relative stereochemistry
of the two E coordination domains to the ligand domain is
still intact. In cryptands with significant helical twist, the E
coordination domains have the same stereochemistry, while in
non-helical cryptands, the E coordination domains have opposite
stereochemistry (the cryptands are “meso”). The unrestricted
rotation around the sulfur-methylene bonds seems to be sufficient
to decouple the absolute stereochemistry of the ligand domain
from that of the As and Sb coordination domains. In related
metallohelicates, the ligands are much more rigid and no similar
rotation is possible.21,22
In this report, the X-ray crystal structures of six new As2L3 and
Sb2L3 supramolecular cryptands were analyzed in the context
of previously reported As- and Sb-cryptands. A new vacuum-
enabled synthesis allowed for the formation of two of these
new assemblies which could not be prepared by other routes.
Comparing the stereochemistries of these assemblies revealed that
in cryptands with significant helicity, the As and Sb coordination
environments showed identical stereochemistry (i.e., D,D or K,K),
whereas all non-helical cryptands were also mesocates (D,K-
configuration) as expected. Surprisingly, the mechanical coupling
in these cryptands is not sufficient to dictate helicity: P helicates
can give rise to either D,D or K,K metal center configurations
showing the stereochemical domains are decoupled, which is
unusual in such metallohelicates. The degree of helicity deter-
mines the stereochemical relationship between the As and Sb
coordination environments. The pnictogen-p interaction is an
anchoring point for a chelate ring and influences the degree of
helicity and overall stereochemistry in the observed cryptands.
This reinforces the conclusion that pnictogen-p interactions are
important supramolecular forces in directing the conformations
of main group element-containing structures and, thus, are
important to the design of these complexes.
Acknowledgements
We gratefully acknowledge funding from the National Science
Foundation (CAREER award CHE-0545206). D.W.J. is a Cottrell
12130 | Dalton Trans., 2011, 40, 12125–12131
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The Royal Society of Chemistry 2011
©

Scholar of Research Corporation for Science Advancement. This
material is based upon work supported by the U.S. Department
of Education under Award No. P200A070436 (V.M.C.) and an
NSF Integrative Graduate Education and Research Traineeship
(M.A.P., O.B.B., and S.A.F., DGE-0549503).
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©