Host-guest interactions in a series of self-assembled As2L 2Cl2 macrocycles

Article abstract of DOI:10.1039/b802178c

The reaction of AsCl₃ with H₂L (where L = a rigid dithiolate) results in the self-assembly of As₂L₂Cl ₂ supramolecular macrocycles. For ligands 4,4′- bis(mercaptomethyl)biphenyl (H₂1), 4,4′-bis(mercaptomethyl)- trans-stilbene (H₂2), and 1,4-dimethoxy-2,5-bis(mercaptomethyl) benzene (H₂3), the macrocyclic cavities of the resulting assemblies are large enough to host aromatic solvent molecules, as revealed by single crystal X-ray structures of the inclusion complexes. As₂L ₂Cl₂ macrocycles form in solution as a mixture of diastereomers, but the diastereomers can be selectively crystallized and separated. Crystallization of syn- or anti-As₂3₂Cl ₂ can be controlled using host-guest interactions by the prudent choice of crystallization solvents. anti-As₂3₂Cl ₂ crystallizes exclusively from chloroform and benzene, while a [(syn-As₂3₂Cl₂)₂?p-xylene] dimer crystallizes from p-xylene and a mixture of [(syn-As₂3 ₂Cl₂)(anti-As₂3₂Cl ₂)?toluene] and [(syn-As₂3₂Cl ₂)₂?toluene] dimers crystallize from toluene.

Full text of DOI:10.1039/b802178c

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PAPER
www.rsc.org/dalton | Dalton Transactions
Host–guest interactions in a series of self-assembled As2L2Cl2 macrocycles†
Virginia M. Cangelosi, Lev N. Zakharov, Sean A. Fontenot, Melanie A. Pitt and Darren W. Johnson*
Received 7th February 2008, Accepted 26th March 2008
First published as an Advance Article on the web 18th April 2008
DOI: 10.1039/b802178c
The reaction of AsCl3 with H2L (where L = a rigid dithiolate) results in the self-assembly of As2L2Cl2
supramolecular macrocycles. For ligands 4,4ꢀ-bis(mercaptomethyl)biphenyl (H21),
4,4ꢀ-bis(mercaptomethyl)-trans-stilbene (H22), and 1,4-dimethoxy-2,5-bis(mercaptomethyl)benzene
(H23), the macrocyclic cavities of the resulting assemblies are large enough to host aromatic solvent
molecules, as revealed by single crystal X-ray structures of the inclusion complexes. As2L2Cl2
macrocycles form in solution as a mixture of diastereomers, but the diastereomers can be selectively
crystallized and separated. Crystallization of syn- or anti-As232Cl2 can be controlled using host–guest
interactions by the prudent choice of crystallization solvents. anti-As232Cl2 crystallizes exclusively from
chloroform and benzene, while a [(syn-As232Cl2)2·p-xylene] dimer crystallizes from p-xylene and a
mixture of [(syn-As232Cl2)(anti-As232Cl2)·toluene] and [(syn-As232Cl2)2·toluene] dimers crystallize from
toluene.
While many of these systems require extensive synthesis, self-
Introduction
assembled arsenic-ligand supramolecular14 structures provide easy
The use of main-group elements as components in metal–ligand
access to inherently endohedrally-functionalized hosts: the As(III)
supramolecular assemblies is not common, and has led to new
lone-pairs point into the host’s cavity. We are exploring inclusion
structure types that are inaccessible using the more traditional
complexes of these hosts that are uniquely suited for unusual host–
transition metals.1 Self-assembled from metal ions and multi-
guest interactions.
dentate organic ligands, metal–ligand supramolecular assemblies
Arsenic(III) has an unusual, but predictable, trigonal pyramidal
are dynamic systems,2–5 often capable of encapsulating guest
coordination geometry that features a stereochemically active
molecules within their three-dimensional cavities. The walls of
lone-pair when coordinated by sulfur-based ligands. Our labo-
these cavities usually consist of phenyl rings causing the cavity
interiors to be highly hydrophobic, an environment that can
ratory has shown that this geometry can be targeted in the self-
assembly of As2L2Cl2 macrocycles (where L = a rigid dithiolate)
from AsCl3 and dithiol ligands.15,16 In the solid state, the arsenic
atoms in these macrocycles sit within the macrocyclic cavity,
partially due to the As–p interaction. This causes the lone-pair of
electrons on each arsenic atom to point directly into the cavity,
which, when combined with the electron-rich aromatic ligand
walls, results in a Lewis-basic interior for the macrocyclic cavity.
Unfortunately, all As2L2Cl2 macrocycles reported to date are too
small to host any guest molecules, and no interactions of potential
guests with the unusual Lewis-basic interiors of these cavities have
been observed. In this article, the single crystal X-ray structures
for three macrocycles with larger cavities are reported, and their
inclusion complexes with aromatic guests are described. In one
case, guest inclusion drives the crystallization of the sterically-
hindered isomer of a macrocycle into a dimeric “capsule” around
that guest.
differ significantly from that of the solvent outside the cavity.
It is difficult, but also of great interest, to spice up these bland
interiors by preparing cavities with endohedral functionality.6
Guest molecules can interact specifically with inward-directed
functional groups,7 potentially increasing the selectivity of the
cavity and the likelihood of reaction or catalysis within that cavity.
Unfortunately, endohedral functionalization is synthetically very
challenging. Gibb and coworkers have demonstrated this challenge
by showing that reactive sites on the exterior of a cavitand are
more likely to react than reactive sites within the cavitand.8,9 As
one elegant example of inward-directed functionality, Rebek and
coworkers have prepared bowl-shaped cavitands with functional
groups that dangle over the bowl-opening,10–12 allowing them to
trap reactive intermediates that are not normally observable on
the NMR timescale.13
Department of Chemistry and Materials Science Institute, University of
Oregon, Oregon Nanoscience and Microtechnologies Institute (ONAMI),
Eugene, OR, 97403-1253, USA. E-mail: dwj@uoregon.edu; Tel: 541-346-
1695
† Electronic supplementary information (ESI) available: Experimental
details for the ligands synthesized from modified literature procedures,
discussion of volume calculations using GRASP, discussion of the
measurement of the bend in the ligand and a figure showing the
disorder in As212Cl2, [(As232Cl2)2·p-xylene], and [(As232Cl2)2·toluene].
CCDC reference numbers 676181 ([anti-As222Cl2·benzene]), 676182
([As212Cl2·toluene]), 676183 (anti-As232Cl2), 676184 ([(syn-As232Cl2)2·p-
xylene]), and 676185 ([(syn-As232Cl2)2·toluene]). For crystallographic data
in CIF or other electronic format see DOI: 10.1039/b802178c
Results and discussion
AsCl3 anddithiolligands H21, H22, and H23 self-assemble into syn-
and anti-As2L2Cl2 macrocycles (Scheme 1). These macrocycles
were crystallized as inclusion complexes with a variety of aromatic
guests (Table 1). In each case it was found that guest inclusion
does not affect the As–p interaction, an attractive electrostatic
interaction between Lewis-acidic As(III) and Lewis-basic aromatic
rings.17–19 While the presence of this interaction results in arsenic
lone pairs within the macrocyclic cavities, the strong As–p
This journal is
The Royal Society of Chemistry 2008
Dalton Trans., 2008, 3447–3453 | 3447
©

Table 1 Crystallographic data and refinement parameters for As212Cl2, As222Cl2, and As232Cl2
[anti-
12 [(syn-
As232Cl2)2·toluene]
1 [(syn-As232Cl2)2·p-
2
1 [As212Cl2·toluene]
As222Cl2·benzene]
anti-As232Cl2
xylene]
2
Formula
Formula weight
Temperature/K
Wavelength/A
Crystal system
C21H20AsClS2
446.86
173(2)
0.71073
Triclinic
¯
P1
C38H34As2Cl2S4
839.63
173(2)
0.71073
Monoclinic
P21/c
15.404(7)
6.395(3)
19.576(8)
90
104.071(5)
90
C20H24As2Cl2O4S4
677.37
173(2)
0.71073
Triclinic
¯
P1
8.4805(11)
9.3290(12)
9.6719(12)
117.4210(10)
101.198(2)
98.034(2)
642.79(14)
1, 0.5
1.750
C23.50H28As2Cl2O4S4
723.44
173(2)
0.71073
Monoclinic
C24H29As2Cl2O4S4
730.45
173(2)
0.71073
Monoclinic
˚
Space group
P21/c
P21/c
˚
a/A
9.3876(7)
9.6322(8)
13.8813(13)
9.8378(9)
21.764(2)
90
90.202(2)
90
2972.1(5)
4, 2
1.617
13.9458(17)
9.8297(12)
21.814(3)
90
90.784(2)
90
2990.0(6)
4, 1
1.623
˚
b/A
˚
c/A
12.0300(9)
107.6460(10)
99.3000(10)
101.4170(10)
987.09(13)
2, 1
a/◦
b/◦
c /◦
3
˚
V/A
1870.5(14)
2, 0.5
1.491
Z, Zꢀ
D
calcd/mg m−3
1.503
0.2070
l/cm−1
0.2179
0. 3158
0.2738
0.2722
F(000)
Crystal size/mm
Index ranges
456
856
340
1460
1476
0.18 × 0.16 × 0.12
−12 ≤ h ≤ 12,
−12 ≤ k ≤ 12,
−15 ≤ l ≤ 15
11379
0.19 × 0.14 × 0.02
−18 ≤ h ≤ 18,
−7 ≤ k ≤ 7,
−23 ≤ l ≤ 23
12553
0.27 × 0.14 × 0.10
−10 ≤ h ≤ 10,
−11 ≤ k ≤ 11,
−12 ≤ l ≤ 12
5920
0.32 × 0.26 × 0.08
−17 ≤ h ≤ 17,
−12 ≤ k ≤ 12,
−27 ≤ l ≤ 27
33472
0.18 × 0.14 × 0.10
−13 ≤ h ≤ 17,
−8 ≤ k ≤ 12,
−27 ≤ l ≤ 26
14607
Reflections collected
Independent reflections [Rint
Data/restraints/parameters
Goodness-of-fit on F2
R1/wR2 [I > 2r(I)]
]
4453 [0.0211]
4438/0/318
1.103
0.0382/0.0876
0.0450/0.0910
0.926/−0.489
3268 [0.1021]
3268/0/208
1.122
0.0897/0.2091
0.1412/0.2356
1.294/−1.428
2713 [0.0131]
2713/0/193
1.031
0.0280/0.0692
0.0316/0.0713
0.661/−0.399
12933 [0.0186]
12933/11/643
1.045
0.0516/0.1469
0.0615/0.1550
1.155/−0.772
6483 [0.0309]
6483/4/371
1.134
0.0713/0.1340
0.0985/0.1440
1.037/−1.096
R1/wR2 (all data)
−3
˚
Largest diff. peak and hole/e A
by the slow diffusion of hexanes into a toluene solution of
As212Cl2.21 In the crystal structure, the observed As–Caryl distances,
˚
the shortest of which is 3.24 A, reveal intramolecular As–p
interactions (Fig. 1(a)). However, no interactions between the
arsenic atoms and the toluene guest molecule are observed (Fig
˚
1(b)). The distances (∼3.5 A) between the closest carbon atoms
in the biphenyl ligands and toluene indicate the presence of p–p
stacking between host and guest. The biphenyl ligands bow out
slightly, presumably to accommodate the toluene guest. A search
of the Cambridge Structural Database22 reveals that this degree
of bending is within the observed range for biphenyl moieties
in known structures.23 All S–As–S, S–As–Cl, and C–S–As angles
are within the typical range observed for As2L2Cl2 macrocycles
(Table 2).14,16
anti-As222Cl2
Scheme 1 Self-assembly of As2L2Cl2 macrocycles.20
4,4ꢀ-Bis(mercaptomethyl)-trans-stilbene (H22) is longer than H21
and was designed to form macrocycles with even taller cavities.
As222Cl2 was prepared by mixing H22 with AsCl3 in CH2Cl2.
X-Ray quality crystals of [anti-As222Cl2·benzene] were obtained
by the slow diffusion of benzene layered on top of a CH2Cl2
solution of macrocycle.24 The crystal structure reveals the presence
of intramolecular As–p interactions as evidenced by short As–Caryl
interactions shelter the lone-pairs from interacting with the guests.
However, guests do dictate their host’s structure, and in fact, host–
guest interactions can even be used to provide diastereocontrol in
the self-assembly reactions in some cases.
anti-As212Cl2
˚
contacts, the shortest of which is 3.23 A (Fig. 2(a)). A slight bowing
4,4ꢀ-Bis(mercaptomethyl)biphenyl (H21) was designed to form
macrocycles with cavities that are tall enough along the As–
As axis to host small guest molecules. As212Cl2 was prepared
by mixing H21 with AsCl3 in toluene. X-Ray quality crystals
of the [anti-As212Cl2·toluene] inclusion complex were obtained
of the stilbene ligands is observed, allowing guest inclusion. No
unusual bond angles are observed (Table 2) suggesting that there
is not much strain imposed on the macrocycle by the guest
molecules. One benzene molecule per macrocycle is present, but
is shared with a neighboring macrocycle, so that each macrocycle
3448 | Dalton Trans., 2008, 3447–3453
This journal is
The Royal Society of Chemistry 2008
©

◦
a
˚
Table 2 Selected bond lengths (A) and angles ( )
[As212Cl2·toluene]
[(syn-As232Cl2)2·toluene]
[(syn-As232Cl2)2·p-xylene]
As(1)–S(1)
As(1)–S(2)
As(1)–Cl(1)
As(1)–S(1A)
As(1)–S(2A)
As(1)–Cl(1A)
S(1)–As(1)–S(2)
S(1)–As(1)–Cl(1)
S(2)–As(1)–Cl(1)
C(1)–S(1)–As(1)
C(14)#1–S(2)–As(1)
S(1A)–As(1)–S(2A)
S(1A)–As(1)–Cl(1A)
S(2A)–As(1)–Cl(1A)
As(1)–S(1A)–C(1)
As(1)–S(2A)–C(14A)#1
2.1922(9)
2.2125(9)
2.2524(9)
2.185(6)
2.110(6)
2.464(8)
88.19(3)
101.63(4)
101.23(4)
103.37(1)
101.12(1)
92.4(3)
As(1)–S(1)
As(1)–S(4)
As(1)–Cl(1)
As(2)–S(2)
As(2)–S(3)
As(2)–Cl(2)
As(2A)–S(2A)
As(2A)–S(3A)
As(2A)–Cl(2A)
As(3)–S(5)
As(3)–S(8)
As(3)–Cl(3)
As(4)–S(7)
As(4)–S(6)
As(4)–Cl(4)
As(4A)–S(6A)
As(4A)–S(7A)
As(4A)–Cl(4A)
S(1)–As(1)–S(4)
S(1)–As(1)–Cl(1)
S(4)–As(1)–Cl(1)
C(1)–S(1)–As(1)
C(18)–S(4)–As(1)
S(2)–As(2)–S(3)
S(2)–As(2)–Cl(2)
S(3)–As(2)–Cl(2)
C(8)–S(2)–As(2)
C(11)–S(3)–As(2)
S(3A)–As(2A)–S(2A)
S(2A)–As(2A)–Cl(2A)
S(3A)–As(2A)–Cl(2A)
C(8)–S(2A)–As(2A)
C(11)–S(3A)–As(2A)
S(5)–As(3)–S(8)
S(5)–As(3)–Cl(3)
S(8)–As(3)–Cl(3)
C(21)–S(5)–As(3)
C(38)–S(8)–As(3)
S(7)–As(4)–S(6)
S(6)–As(4)–Cl(4)
S(7)–As(4)–Cl(4)
C(28)–S(6)–As(4)
C(31)–S(7)–As(4)
S(6A)–As(4A)–S(7A)
S(6A)–As(4A)–Cl(4A)
Cl(4A)–As(4A)–S(7A)
C(28)–S(6A)–As(4A)
C(31)–S(7A)–As(4A)
2.203(2)
2.214(2)
2.248(2)
2.220(3)
2.244(3)
2.232(3)
2.13(2)
As(1)–S(1)
As(1)–S(3)
As(1)–Cl(1)
As(2)–S(2)
As(2)–S(4)
As(2)–Cl(2)
As(2A)–S(2)
As(2A)–S(4)
As(2A)–Cl(2A)
S(3)–As(1)–S(1)
S(3)–As(1)–Cl(1)
S(1)–As(1)–Cl(1)
C(1)–S(1)–As(1)
C(11)–S(3)–As(1)
S(4)–As(2)–S(2)
Cl(2)–As(2)–S(2)
S(4)–As(2)–Cl(2)
C(8)–S(2)–As(2)
C(18)–S(4)–As(2)
S(2)–As(2A)–S(4)
S(2)–As(2A)–Cl(2A)
S(4)–As(2A)–Cl(2A)
C(8)–S(2)–As(2A)
C(18)–S(4)–As(2A)
2.2140(2)
2.199(2)
2.246(2)
2.245(5)
2.218(5)
2.235(7)
2.105(2)
2.177(1)
2.24(2)
87.80(8)
103.22(1)
100.27(8)
100.1(2)
102.4(3)
87.58(2)
101.8(2)
102.8(3)
103.0(3)
101.7(3)
92.3(7)
2.13(3)
2.336(1)
2.192(2)
2.212(2)
2.247(2)
2.201(3)
2.209(3)
2.248(3)
2.13(5)
94.9(3)
95.4(3)
104.5(3)
93.1(3)
2.32(3)
2.254(2)
87.79(8)
102.93(1)
100.74(9)
102.5(3)
99.5(2)
88.47(1)
100.14(1)
101.88(1)
100.0(2)
101.4(3)
93.0(8)
[anti–As222Cl2·benzene]
As(1)–S(1)
As(1)–S(2)
2.204(3)
2.213(3)
2.244(4)
87.52(1)
102.69(2)
98.77(2)
102.9(4)
99.5(4)
99.9(8)
86.7(2)
103.7(4)
105.6(6)
As(1)–Cl(1)
S(1)–As(1)–S(2)
S(1)–As(1)–Cl(1)
S(2)–As(1)–Cl(1)
C(1)–S(1)–As(1)
C(16)#2–S(2)–As(1)
anti–As232Cl2
As(1)–S(1)
As(1)–S(2)#3
As(1)–Cl(1)
S(2)#3–As(1)–S(1)
S(2)#3–As(1)–Cl(1)
S(1)–As(1)–Cl(1)
C(1)–S(1)–As(1)
C(10)–S(2)–As(1)#3
93.1(6)
88.4(8)
2.2182(7)
2.2018(8)
2.2586(8)
86.76(3)
101.50(4)
102.73(3)
102.92(9)
103.85(1)
100.7(3)
103.2(3)
88.47(9)
102.02(1)
99.47(9)
103.1(3)
100.3(2)
87.83(1)
100.64(1)
101.94(1)
103.7(3)
102.3(3)
86.8(1)
110.8(1)
105.9(1)
94.3(5)
99.4(5)
a Symmetry codes: #1 −x + 2, −y + 2, −z + 2; #2 −x, −y + 1, −z; #3 −x, −y, −z + 2. Disordered atoms in the second positions are indicated with the
symbol A.
partially hosts two guests and each guest is partially encapsulated
by two macrocycles (Fig. 2(c)). Alternating macrocycles and
benzene molecules form infinite chains of p–p stacked host–guest
assemblies (Fig. 2(b)).
and dictates whether anti-As232Cl2, [(syn-As232Cl2)2·guest], or
[(syn-As232Cl2)(anti-As232Cl2)·guest] crystallizes.25
[(As232Cl2)2·guest] dimers
If As232Cl2 is crystallized from p-xylene a [(syn-As232Cl2)2·p-
xylene] inclusion complex is observed (Fig. 3). In this structure, the
organic ligand backbones are at a slight angle (31.5◦) to each other
resulting in an opening of one side of the macrocycle and a closing
of the other. The chlorine ligands of syn-As232Cl2 are directed
toward the sterically hindered closed end of the macrocycle, which
defines the tapered end of a bowl-shaped structure. The open ends
of two adjacent macrocycles are sandwiched together to form
As232Cl2
1,4-Dimethoxy-2,5-bis(mercaptomethyl)benzene (H23) was de-
signed to be used in the preparation of macrocycles that are
wide but not tall. The methoxy groups on this ligand expand the
macrocyclic cavity. As232Cl2 was prepared by mixing H23 with
AsCl3 in chloroform, benzene, toluene, or p-xylene. The solvent
used for crystallization becomes the guest in the inclusion complex
3
26
˚
a cavity with an interior volume of ca. 216 A . Within each
This journal is
The Royal Society of Chemistry 2008
Dalton Trans., 2008, 3447–3453 | 3449
©

Fig. 2 ORTEP (30% probability ellipsoids) representations of the
single-crystal X-ray structure of (a) anti-As222Cl2 and (b) the pack-
ing diagram of [anti-As222Cl2·benzene] along the macrocyclic axis. (c)
Space-filling representation of the single-crystal X-ray structure of the
[anti-As222Cl2·benzene] inclusion complex.
Fig. 1 (a) ORTEP (30% probability ellipsoids) representation of the
single-crystal X-ray structure of anti-As212Cl2 and (b) space-filling rep-
resentation of the single-crystal X-ray structure of the [As212Cl2·toluene]
inclusion complex. Only one position for disordered groups is shown for
clarity.
(see below), showing that the macrocycle is sufficiently flexible to
accommodate guest molecules. The guest molecules are oriented
in the cavity to best fill the space with the methyl groups on
each guest pointing into the cavity of one or both components
of the dimer (Fig. 3(b), (d)). There are no obvious p–p or CH–p
interactions between the host and guest molecules—only van der
Waals interactions.
macrocycle, the two organic ligands can be eclipsed (methoxy
groups on both ligands aligned) or staggered, and both species
exist within the crystal structure (the disorder was modeled
showing 73% of the eclipsed structure is present). For ease of
viewing, only the eclipsed structures are shown.
If As232Cl2 is crystallized from toluene a mixture of homo-
and hetero-dimers forms: [(syn-As232Cl2)2·toluene] and [(syn-
As232Cl2)(anti-As232Cl2)·toluene]. The structure of these dimers
is very similar to that of the [(syn-As232Cl2)2·p-xylene] dimer
anti-As232Cl2
If appropriately-sized guest molecules are not present during the
crystallization process, anti-As232Cl2 crystallizes exclusively. anti-
As232Cl2 is observed when crystals are grown from chloroform or
benzene, both of which are apparently too small or of the wrong
shape to satisfactorily fill the cavity of the (As232Cl2)2 dimer. In
the anti-As232Cl2 solid-state structure (Fig. 4(a)), the two organic
ligands that make up the macrocycle are parallel and the methoxy
groups of each ligand are aligned. The chlorine ligands on each
arsenic atom are directed away from the sterically bulky methoxy
groups. The expanded cavity is filled by the methoxy groups of
neighboring macrocycles and face-to-face p–p stacking occurs
with a 29.9◦ angle between the organic ligand backbones and
3
an interior volume of ca. 184 A .27 With the decrease in symmetry
˚
in the guest molecules from p-xylene to toluene comes a decrease
in the symmetry of the dimer. While [(syn-As232Cl2)2·p-xylene]
is disordered only in the orientation of the methoxy groups
and the position of a As–Cl fragment, the toluene-containing
dimer contains a mixture of syn and anti macrocycles, disordered
methoxy groups, and a disordered toluene guest (see Experimental
section for details on the modelling of the disorder).
Guest inclusion in the dimer can be accomplished without
destroying the strong As–p interaction. The shortest As–Caryl
˚
between neighboring macrocycles (Caryl–Caryl distance of 3.56 A)
˚
˚
contact (3.26 A and 3.22 A for toluene and p-xylene guests
(Fig. 4(b), (c)).
respectively) is significantly less than the sum of the van der Waals
In this structure, the As–p interaction is completely unaffected
by guest molecules as none are present. The As–Caryl distance
˚
˚
radii for arsenic (2.0 A) and a phenyl carbon (1.7 A). The As–
˚
˚
As distances (4.83 and 4.86 A for toluene guest and 4.89 and
of 3.11 A for anti-As232Cl2 is the shortest contact for any
As2L2Cl2 macrocycle that has been reported to date.14,16 This
˚
4.74 A for p-xylene guest) within each macrocycle of the dimer are
significantly longer than the As–As distance in the anti-macrocycle
As–p interaction is shorter, and presumably stronger, than those
3450 | Dalton Trans., 2008, 3447–3453
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previously observed because the aryl rings in this system
are more electron rich due to the electron donating methoxy
substituents.
Structural trends in As2L2Cl2 macrocycles
Analysis of the five crystal structures obtained for As212Cl2,
As222Cl2, and As232Cl2 gives insight into the ability of these
macrocycles to host guest molecules. Each macrocycle is too small
to completely encapsulate a guest molecule, but is large enough to
partially host one while maintaining As–p interactions. As222Cl2
has a longer cavity than As212Cl2 and this difference in size is
reflected in the number of solvent molecules that each can host.
While As212Cl2 partially hosts one toluene molecule, each As222Cl2
macrocycle partially hosts two benzene molecules. In either case
guest inclusion results in a slight bowing of the organic ligands.
In the As232Cl2 macrocycle, the organic ligands do not deviate
from planarity to accommodate a guest molecule, but rather the
macrocycle walls tilt out of parallel allowing one end of the
macrocyclic cavity to open up and a guest molecule to fill the
void. In each case it is shown that guest inclusion dictates the
structure—and in some cases drives the diastereoselectivity in the
self-assembly—of the macrocycle.
Conclusions
In this paper, five new X-ray crystal structures were presented for
three new As2L2Cl2 macrocycles. In each example, appropriately
sized aromatic solvent molecules fill the macrocyclic cavity. In the
case of As232Cl2, the choice of guest molecule allows stereocontrol
over which diastereomer is crystallized. Larger guest molecules
(toluene, p-xylene) are encapsulated by an [(As232Cl2)2·guest]
inclusion complex, while smaller guest molecules (chloroform,
benzene) are too small to fill the dimer cavity forcing anti-
As232Cl2 to crystallize exclusively. When the guest is p-xylene,
the dimer consists of only anti macrocycles, and when the guest
is toluene, the structure contains and mixture of anti–anti and
syn–anti dimers. We have shown that larger dithiol ligands will
predictably form expanded As2L2Cl2 macrocycles with spacious
cavities; even larger ligands are needed for complete encapsulation
of a guest or the formation of kinetically stable inclusion complexes
in solution. We will continue these studies by exploring the
possibility of “threading” a guest molecule through the cavity
Fig. 3 ORTEP (30% probability ellipsoids) and space-filling represen-
tations of the single-crystal X-ray structure of (a, c) [(syn-As232Cl2)2·
p-xylene] and (b, d) [(syn-As232Cl2)2·toluene] inclusion complexes. Only
one position for disordered groups is shown for clarity.
Fig. 4 ORTEP (30% probability ellipsoids) representation of the single-crystal X-ray structure of (a) anti-As232Cl2. Stick representation showing the
packing of anti-As232Cl2 in the crystal structure from (b) top and (c) side views.
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©

of these macrocycle to make rotaxanes and synthetically linking
macrocycles to give larger assemblies.
As232Cl2. AsCl3 (15.3 lL, 0.179 mmol) was added to a solution
of H23 (41.2 mg, 0.179 mmol) in p-xylene (10 mL) to yield a
mixture of four macrocycle isomers. Colorless single crystals of
[(syn-As232Cl2)2·p-xylene] were grown by the slow diffusion of
pentane into a p-xylene solution of As232Cl2 after 6 d (5.19 mg,
3.55 lmol, 7.9%). dH(300 MHz; CDCl3; Me4Si) 6.80 (m, 2 H, CH),
4.52 (m, 4 H, CH2), 3.79 (m, 6 H, CH3). Replacing p-xylene with
toluene or chloroform as the solvent yields [(As232Cl2)2·toluene]
and anti-As232Cl2, respectively.
Experimental
General procedures
1H NMR spectra were measured using a Varian INOVA-300
spectrometer operating at 299.935 MHz. J values are given in
Hz. Commercially available reagents were used as received. H2128
and H2329 were prepared following modified literature procedures
(see ESI† for details on changes to the literature procedure).
Complete transformation (>99% yield) of ligand and AsCl3 to
macrocycles is revealed by 1H NMR spectroscopy. Isolated single-
crystal yields are reported. Caution: arsenic compounds are highly
toxic and should be handled with care!
Crystallography
All X-ray diffraction data were collected on a Bruker Smart
Apex diffractometer at 173 K using MoKa radiation (k =
32
˚
0.71073 A). Absorption corrections were applied by SADABS.
Crystal structures were solved by direct methods. The crystal
structure of [(syn-As232Cl2)2·p-xylene] was determined to be in the
monoclinic space group P21/c. Crystals of [(As232Cl2)2·toluene]
are also monoclinic with unit cells that are close to that of
[(syn-As232Cl2)2·p-xylene]. However, for the crystal structure of
[(As232Cl2)2·toluene] there are several hundred non-zero h0l-type
reflections which are inconsistent with a c glide plane. Therefore,
the structure was solved in space group P21. The collected data
suggested that the crystal of [(As232Cl2)2·toluene] was a racemic
twin with a 30 : 70 ratio of the two phases. The disorder presumably
arises from the mismatched symmetry of the macrocycle and
the solvent guest molecules in [(As232Cl2)2·toluene] and [(syn-
As232Cl2)2·p-xylene], the latter of which is centrosymmetric. Two
of the four S2AsCl fragments in [(As232Cl2)2·toluene] are also
disordered over two positions with opposite orientations of the
As–Cl bonds. Thus in the structure there are two types of molecules
with syn and anti configurations in 88 : 12 and 91 : 9 ratios
for the two independent molecules. Toluene guest molecules in
[(As232Cl2)2·toluene] are disordered over two positions in a 50 : 50
ratio. One of two AsCl fragments in each macrocycle in [(syn-
As232Cl2)2·p-xylene] is also disordered over two positions. In con-
trast to [(As232Cl2)2·toluene], all molecules in [(syn-As232Cl2)2·p-
xylene] have a syn configuration. The disorder of the AsCl group
in [(syn-As232Cl2)2·p-xylene] results from the two positions for
the As atom, one above and one below the average plane of
the connected –CH2S · · · SCH2– groups. The –OMe groups in
[(As232Cl2)2·toluene] and [(syn-As232Cl2)2·p-xylene] are disordered
over two positions corresponding to two opposite orientations
for the benzene rings of the ligand, resulting in macrocycles
that are eclipsed or staggered. The ratios of the occupancies
for these two positions are 73 : 27 in [(syn-As232Cl2)2·p-xylene]
and 84 : 16 and 62 : 38 for the two symmetrically independent
molecules in [(As232Cl2)2·toluene]. In [anti-As212Cl2·toluene] both
S2AsCl fragments are disordered over two positions with opposite
orientations of the As–Cl bonds in a ratio of 82 : 18. Non-H atoms
in all structures were refined with anisotropic thermal parameters.
H atoms in [(As232Cl2)2·toluene], [(syn-As232Cl2)2·p-xylene] and
[anti-As222Cl2·benzene] were refined in calculated positions in a
rigid group model. In [anti-As232Cl2] and [anti-As212Cl2·toluene]
H atoms were found on the residual electron density map and
refined with isotropic thermal parameters, except those at the
CH2 groups connected to the disordered fragments which were
treated in calculated positions. Disordered fragments were refined
with restrictions; standard distances of bonds were used in the
Syntheses
As212Cl2. AsCl3 (4.0 lL, 0.047 mmol) was added to H21
(12.7 mg, 0.0475 mmol) in toluene (4 mL). Gold-colored X-ray
quality crystals of the anti macrocycle were obtained after 5 d
by the diffusion of hexanes into this solution at 4 ◦C (1.5 mg,
4.1 lmol, 17%). Dissolving the crystals yields a solution of both
syn and anti macrocycles. dH(300 MHz; CDCl3; Me4Si) 7.13 (d, 8
H, CH, J 7.8), 7.04 (d, 8 H, CH, J 9.3), 4.24 (ABq, 4 H, CH2, J
13), 4.23 (ABq, 4 H, CH2, J 13).
4,4ꢀ-Bis(mercaptomethyl)-trans-stilbene30
(H22). 4,4ꢀ-Bis-
(bromomethyl)-trans-stilbene31 (903 mg, 2.69 mmol) was
dissolved in solution of absolute ethanol (20 mL) and acetone
(3 mL). Thiourea (613 mg, 8.06 mmol) was added and the
solution was heated to reflux for 4 h. The off-white precipitate was
filtered, washed with acetone, and dried under vacuum (1.07 g,
2.07 mmol, 77%). A 3-neck round bottom flask was charged
with this precipitate (200 mg, 386 mmol), equipped with a stir
bar and condenser, and placed under a N2 atmosphere. Degassed
2 M NaOH solution (30 mL) was added via a cannula, and
the resulting mixture was heated to 80 ◦C for 4 h. The cloudy
solution was cooled to room temperature, and degassed 4 M HCl
(20 mL) was added via a cannula. A precipitate formed as the
acid was added and pH paper was used to verify that the solution
was acidic (pH < 3). The reaction mixture was extracted with
CH2Cl2 (3 × 20 mL) and the combined organics were dried over
sodium sulfate and evaporated to dryness to yield a white powder
(43.0 mg, 158 mmol, 41%). mp 156.4–158.1 ◦C; dH(300 MHz;
CDCl3; Me4Si) 7.46 (d, 4 H, CH, J 8.1), 7.31 (d, 4 H, CH, J 8.1),
=
7.07 (s, 2 H, HC CH), 3.74 (d, 4 H, CH2, J 7.2), 1.76 (t, 2 H, SH,
J 7.2); dC(75 MHz; CDCl3) 140.7, 136.3, 128.6, 128.3, 127.0, 29.0;
m/z (EI) 272 (58%, M+), 239 (100, M − SH), and 206 (58, M −
2 × SH).
As222Cl2. AsCl3 (4.0 lL, 0.047 mmol) was added to a solution
of H22 (12.7 mg, 0.0475 mmol) in CH2Cl2 (4 mL). Pale yellow X-
ray quality crystals were obtained aft◦er 5 d by layering of benzene
on top of this CH2Cl2 solution at 4 C (1.0 mg, 2.5 lmol, 12%).
Dissolving the crystals yields a solution of both syn and anti
macrocycles. dH(300 MHz; CDCl3; Me4Si) 7.07 (d, 8 H, CH, J
=
7.5), 7.01 (d, 8 H, CH, J 7.2), 6.97 (s, 4 H, HC CH), 4.17 (ABq,
4 H, CH2, J 13), 4.16 (ABq, 4 H, CH2, J 13).
3452 | Dalton Trans., 2008, 3447–3453
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refinement as targets for corresponding chemical bonds and atoms
disordered in similar positions were refined with the same thermal
parameters. All calculations were performed using the Bruker
SHELXTL 6.10 package.33
18 H. Schmidbaur, R. Nowak, O. Steigelmann and G. Muller, Chem. Ber.,
1990, 123, 1221–1226.
19 W. J. Vickaryous, R. Herges and D. W. Johnson, Angew. Chem., Int.
Ed., 2004, 43, 5831–5833.
20 In the syn-As2L2Cl2 macrocycles, both chlorine atoms are on the same
side of the macrocyclic cavity while in the anti-As2L2Cl2 macrocycles,
the chlorines atoms are on opposite sides.
21 Crystals were obtained only when toluene or benzene were present,
although the crystals grown from benzene were not of X-ray quality.
The following solvents did not yield single crystals; presumably, they
were not appropriately sized to serve as guests: CH2Cl2, CHCl3, p-
xylene, mesitylene, and THF.
Acknowledgements
We gratefully acknowledge the National Science Foundation for
a CAREER award (CHE-0545206) and the University of Oregon
for generous financial support. D. W. J. is a Cottrell Scholar
of Research Corporation. This material is based upon work
supported by the U.S. Department of Education under Award No.
P200A070436 (V. M. C.). The purchase of the X-ray diffractometer
was made possible by a grant from the NSF (CHE-0234965) to
the University of Oregon.
22 Conquest Version 1.7, February 2005 Release.
23 A description of how this range was measured can be found in the ESI†.
24 Crystals were not obtained when an aromatic solvent molecule was
not present. The crystals grown from benzene were of X-ray quality,
but those obtained from toluene, mesitylene, and p-xylene were not.
The following solvents were not appropriately sized to serve as guests:
CH2Cl2, CHCl3, THF, DCE (1,2-dichloroethane) and TCE (1,1,2,2-
tetrachloroethane).
25 The following molecules were screened as guests, but were not
found to drive dimer formation: p-dimethoxybenzene, hydro-
quinone, p-benzenedimethanol, p-xylenediamine, phenol, bromoben-
zene, iodobenzene, mesitylene, 1,2,3-trimethylbenzene, cyclohexane,
1,4-cyanobenzene, and m-xylene. In each case, either the anti-As232Cl2
macrocycle or nothing crystallized from solution. Crystallization from
o-xylene resulted in an [(As232Cl2)2·o-xylene] dimer that was highly
disordered in the solid state.
Notes and references
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˚
26 Cavities were measured using solvent (1.4 A) surfaces in WebLab View-
erPro 4.0, December 1, 2000 release. The cavity volume was estimated
as the difference between the volume of the [(syn-As232Cl2)2·p-xylene]
host–guest complex and the volume of the p-xylene guest. By this
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6 K. Goto and R. Okazaki, Liebigs Ann. Chem., 1997, 2393–2407.
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˚
27 Cavities were measured using solvent (1.4 A) surfaces in WebLab
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8 Z. R. Laughrey and B. C. Gibb, J. Org. Chem., 2006, 71, 1289–1294.
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The Royal Society of Chemistry 2008
Dalton Trans., 2008, 3447–3453 | 3453
©