Three's company: Co-crystallization of a self-assembled S4 metallacyclophane with two diastereomeric metallacycle intermediates

Article abstract of DOI:10.1039/b927034e

Three discrete supramolecular self-assembled arsenic(iii) complexes including an unusual S₄-symmetric tetranuclear [As₄L ₂Cl₄] metallacyclophane and two diastereomeric cis/trans-[As₂LCl₂] metallacycle intermediates co-crystallize within a single crystal lattice.

Full text of DOI:10.1039/b927034e

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Three’s company: co-crystallization of a self-assembled S4
metallacyclophane with two diastereomeric metallacycle intermediatesw
Nathan R. Lindquist, Timothy G. Carter, Virginia M. Cangelosi, Lev N. Zakharov and
Darren W. Johnson*
Received (in Austin, TX, USA) 6th January 2010, Accepted 22nd February 2010
First published as an Advance Article on the web 9th March 2010
DOI: 10.1039/b927034e
Three discrete supramolecular self-assembled arsenic(III)
complexes including an unusual S4-symmetric tetranuclear
[As4L2Cl4] metallacyclophane and two diastereomeric cis/
trans-[As2LCl2] metallacycle intermediates co-crystallize within
a single crystal lattice.
Increasing interest in incorporating main-group elements into
supramolecular assemblies has led to numerous new and
fascinating coordination compounds previously unattainable
using the more familiar geometries and binding modes
afforded by transition metals.1,2 While common coordination
geometries for transition metals include octahedral, tetrahedral,
and square planar, integrating main-group elements in supra-
molecular self-assembly applications allows for markedly
Scheme 1 Synthesis of the [As2L1Cl2] and [As4L12Cl4] assemblies.
different structural motifs. For example, trivalent pnictogens
such as arsenic, antimony, and bismuth all prefer a trigonal
Herein we report the synthesis of a novel tetranuclear
arsenic(III) [As4L1 Cl4] metallacyclophane with idealized S4
pyramidal coordination geometry when bonded to thiolate
ligands.3 In this structure design, arsenic(III) maintains a stereo-
2
symmetry using the tetradentate rigid bridging ligand H4L1
chemically active lone pair of electrons—an important design
(1,2,4,5-tetrakis(mercaptomethyl)benzene,13 Scheme 1). In
consideration for supramolecular self-assembly applications. Past
1,1,2,2-tetrachloroethane (TCE) the reaction between H4L1
work in our laboratory has incorporated this structurally
and AsCl3 yields crystals of the [As4L1 Cl4]ꢀC2H2Cl4 complex
2
distinct feature into the synthesis and characterization of a
series of dinuclear arsenic cryptands [As2L3]4,5 and macrocycles
(Fig. 1a and b) after three days of slow evaporation.14
[As2L2Cl2]6,7 utilizing rigid dithiolates as bridging ligands
Unexpectedly, when crystals are grown in dichloromethane,
the [As4L1 Cl4] metallacyclophane co-crystallizes with two
(e.g., H2L = 1,4-bismercaptomethylbenzene).8
2
diastereomeric cis/trans-[As2L1Cl2] metallacycle reaction
Our efforts in supramolecular arsenic coordination chemistry
intermediates (trans Fig. 1c, cis Fig. 1d).15 These results prove
have exploited the sufficient lability of the As–S(thiolate)
bond9 to synthesize a variety of new metalla and supramolecular
intriguing for a number of reasons: (1) the [As4L1 Cl4]
2
structure, to the best of our knowledge, is the first reported
cyclophanes. Although still a relatively young field, recent
example of a self-assembled tetranuclear arsenic-containing
work has provided many inorganic analogs of traditional cyclo-
phanes in the form of metallacyclophanes and metallacycles.10,11
Unlike their hydrocarbon counterparts, these distinct compounds
form via coordination-directed self-assembly routes, greatly
simplifying overall synthesis and increasing accessibility of
novel structure types. In addition, metallacyclophanes provide
a new approach to heteroatom bond formation, intra-
molecular interactions, and host–guest interactions.12
Department of Chemistry and Material Science Institute,
University of Oregon, and the Oregon Nanoscience and
Microtechnologies Institute (ONAMI), Eugene, OR 97403-1253,
USA. E-mail: dwj@uoregon.edu; Fax: +1-541-346-0487;
Tel: +1-541-346-1695
w Electronic supplementary information (ESI) available: Experimental
Fig. 1 Crystallographic data: (a) stick representation of [As4L12Cl4],
procedures, 1H NMR, and MALDI and X-ray data. CCDC 743840
top view; ORTEP representations (ellipsoids set at 50% probability,
hydrogen atoms and solvent molecules are omitted for clarity) of
(b) [As4L12Cl4], side view, (c) trans-[As2L1Cl2], and (d) cis-[As2L1Cl2].
and 743841. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/b927034e
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structure, (2) S4 symmetry is an unusual point group seldom
encountered in self-assembling systems,16 (3) a remarkable
four metal–p interactions per benzene ring stabilize the tetra-
nuclear assembly, and (4) we observe the co-crystallization of
three discrete supramolecular assemblies in the same single
crystal by simply varying solvent conditions.
To prepare [As4L1 Cl4], two equivalents of AsCl3 were
2
added to a suspension of H4L1 in 1,1,2,2-tetrachloroethane
and the mixture was left overnight at room temperature, after
which undissolved solids, most likely starting material and
coordination polymer, were removed by filtration. Crystals
suitable for X-ray diffraction were first observed after allowing
the filtrate to slowly evaporate at room temperature for three
days. Further crystallization in this manner yields the
Fig. 2 Fragment of the cis/trans-[As2L1Cl2] and [As4L12Cl4] crystal
packing (hydrogen atoms and solvent removed for clarity). Inter-
molecular contacts include AsꢀꢀꢀS (red), SꢀꢀꢀS (green), and Asꢀꢀꢀp (blue).
[As4L1 Cl4]ꢀC2H2Cl4 complex in 14% isolated yield; however,
2
1H NMR spectroscopy shows nearly complete conversion to
the tetrameric species.
In the [As4L1 Cl4] metallacyclophane, an endo configuration
crystal packing. This suggests a possible emerging interaction
motif in crystal engineering.21
2
is observed for all four metalloid centers with the As(III)
lone pairs pointing toward the cavity.17 This endohedral
functionality is consistent with other trigonal thiolate
arsenic(III) complexes.4,18 Asꢀ ꢀ ꢀCaryl distances in this structure
range between 3.235 and 3.449 A, within the expected range
for metal–arene contacts and shorter than the sum of the van
Essential to the formation of the [As4L1 Cl4] metalla-
2
cyclophane is the notion of dynamic covalent chemistry22
between the self-assembling components, made possible in this
system by the reversibility and lability of the As–thiolate
bond.5,9 1H and gCOSY NMR spectroscopy studies reveal
constant relative ratios of the complexes throughout the
der Waals radii for As and C (3.7 A).1,19 The [As4L1 Cl4]
2
crystallization process. When single crystals of [As4L1 Cl4]ꢀ
assembly is unusual in that each aryl ring has four intra-
molecular As(III)ꢀ ꢀ ꢀp close-contacts. We hypothesize that
these interactions serve as the weak, reversible guiding forces
necessary for supramolecular self-assembly.
2
C2H2Cl4 are redissolved in TCE-d2, dissociation into
[As2L1Cl2] is not observed in the 1H NMR spectrum, even
after heating at 80 1C. The [As4L1 Cl4] complex maintains
2
S4 symmetry in solution, as evidenced by the four doublets
observed for its sixteen total methylene protons (Fig. 3). We
attribute the increased thermodynamic stability of the
Similar to the above procedure, slow evaporation of a 1 : 2
mixture of H4L1 and AsCl3 in dichloromethane yields crystals
suitable for X-ray diffraction after two weeks at ambient
temperature. X-Ray diffraction of these crystals reveals the
co-crystallization of three discrete supramolecular structures:
[As4L1 Cl4] assembly to the high number of As(III)ꢀ ꢀ ꢀp
2
interactions—four per benzene ring—as compared to the
cis/trans-[As2L1Cl2] metallacycles, which lack any such intra-
molecular As(III)ꢀ ꢀ ꢀp close-contacts in the solid state.
the previously described [As4L1 Cl4] structure as well as two
2
structurally discrete cis/trans-[As2L1Cl2] diastereomers. These
dinuclear [As2L1Cl2] metallacycles crystallize as cis and trans
structural isomers, as defined by the orientation of the stereo-
chemically active As(III) lone pairs in relation to one another.
As expected, each metalloid center maintains its predictable
trigonal pyramidal coordination geometry and stereochemically
active lone pair. The seven-membered rings each exhibit the
same basic chair conformation in the solid state.
The inorganic arsenic cryptands [As2L3] and macrocycles
[As2L2Cl2] previously synthesized in our laboratory bear
strong resemblance to the corresponding organic p-prismands
and bicyclophanes.5,23 In many respects, the [As4L1 Cl4]
2
complex presented here is analogous to another spectacular
example from the organic cyclophane literature. The
(1,2,4,5)(1,5,4,2) linkage found in our [As4L1 Cl4] assembly
2
closely parallels that of tetrathia[3.3.3.3]cyclophane, first
There are a number of intermolecular interactions observed
in the crystal packing that may give hints as to how three
distinct supramolecular structures have co-crystallized
together within a single crystal (Fig. 2). Intermolecular Asꢀ ꢀ ꢀp
interactions are present in the crystal, with Asꢀ ꢀ ꢀCaryl
close-contact distances of 3.535 A. The crystal structure also
reveals other intermolecular short range interactions including
Asꢀ ꢀ ꢀS contacts ranging from 3.548 to 3.621 A and Sꢀ ꢀ ꢀS
interactions of just 3.334 A.20 Previous work by Atwood and
co-workers includes the synthesis and structural characterization
synthesized and characterized by Klieser and Vogtle
¨
(Fig. 4).24 To date, we have not isolated the [As4L1 Cl4]
2
assembly with corresponding (1,2,4,5)(1,2,4,5) connectivity,
of 3-chloro-4H,7H-5,6-benz-1,3,2-dithiarsepine,
a
seven-
membered arsenic(III) dithiolate metallacycle in the chair
conformation in the solid state.18 These crystal structures also
reveal intermolecular Asꢀ ꢀ ꢀp close-contacts (3.546 A in this
case), providing further insight into the potential role of these
non-trivial secondary bonding interactions as templates for
Fig.
3
1H NMR spectrum of methylene region from isolated
[As4L12Cl4] single crystals.
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9 V. M. Cangelosi, T. G. Carter, L. N. Zakharov and D. W. Johnson,
Chem. Commun., 2009, 5606–5608.
10 For reviews: M. Fujita and K. Ogura, Coord. Chem. Rev., 1996,
148, 249–264; P. J. Stang and B. Olenyuk, Acc. Chem. Res., 1997,
30, 502–518; M. Fujita, Chem. Soc. Rev., 1998, 27, 417–425;
G. F. Swiegers and T. J. Malefetse, Chem. Rev., 2000, 100,
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K. P. Ho, J. Am. Chem. Soc., 2004, 126, 15852–15869;
I. Fernandez, R. Ruiz, J. Faus, M. Julve, F. Lloret, J. Cano,
´
Fig.
4
(a) [As4L12Cl4] metallacyclophane with two structural
isomers of tetrathia[3.3.3.3]cyclophane: (b) (1,2,4,5)(1,5,4,2) and
(c) (1,2,4,5)(1,2,4,5).
X. Ottenwaelder, Y. Journaux and M. C. Munoz, Angew. Chem.,
Int. Ed., 2001, 40, 3039–3042; J. Hua and W. Lin, Org. Lett., 2004,
6, 861–864.
nor do we see NMR evidence of its existence. Future studies of
the dynamic covalent nature of the arsenic(III)–thiolate bond
may allow access to the inorganic analogs of other important
examples from the cyclophane literature through supra-
12 F. Vogtle, G. Pawlitzki and U. Hahn, in Modern Cyclophane
¨
Chemistry, ed. R. Gleiter and H. Hopf, Wiley-VCH, Weinheim,
2004, pp. 41–80.
13 H. Kawai, T. Umehara, K. Fujiwara, T. Tsuji and T. Suzuki,
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Soc. Jpn., 1989, 62, 164–170.
molecular self-assembly. The [As4L1 Cl4] metallacyclophane
2
has a cavity volume of 28 A3, so further development of this
cage—either by transmetallation or through reactions involving
larger pnictogens—could increase potential for host–guest
interactions.25
14 Crystallographic data for [As4L12Cl4]ꢀC2H2Cl4: C24H24As4Cl12S8,
M = 1293.99, monoclinic, a = 17.9104(11) A, b = 25.7564(16) A,
c = 18.9947(12) A, b = 93.086(1)1, V = 8749.7(9) A3, T = 173 K,
space group P21/c (no. 14), Z = 8, Z0 = 2, 97 922 reflections,
19 054 independent reflections [Rint = 0.0341], R1 = 0.0476, wR2 =
0.1486 and GOF = 1.286 for 15 435 reflections (757 parameters)
with I 4 2s(I); R1 = 0.0579, wR2 = 0.1554, GOF = 1.286 for all
reflections.
In summary, we report the synthesis of three discrete
self-assembled supramolecular structures utilizing the trigonal
pyramidal coordination geometry of trivalent arsenic,
co-crystallized in a single crystal structure. Of particular
15 Crystallographic data for [As4L12Cl4]ꢀcis-[As2L1Cl2]ꢀtrans-
[As2L1Cl2]ꢀCH2Cl2: C37H39As7Cl11S14, M = 1846.91, triclinic,
a = 14.3168(10) A, b = 14.5379(10) A, c = 16.8865(11) A, a =
80.564(1)1, b = 80.511(1)1, g = 61.527(1)1, V = 3032.5(4) A3, T =
interest, the [As4L1 Cl4] structure exhibits S4 symmetry, a
2
point group rare in nature and relatively uncommon in
synthetic chemistry. Importantly, the synthesis and characteri-
zation of these novel [As4L1 Cl4] and [As2L1Cl2] structures
ꢀ
173(2) K, space group P1 (no. 2), Z = 2, 34 380 reflections, 13 158
2
independent reflections [Rint = 0.0280], R1 = 0.0313, wR2
=
0.0721 and GOF = 1.028 for 11 084 reflections (649 parameters)
with I 4 2s(I); R1 = 0.0406, wR2 = 0.0766, GOF = 1.029 for all
reflections.
support the claim that main group elements, with their distinct
coordination geometries, can provide access to new and
interesting supramolecular structure types and designs.
We gratefully acknowledge funding from the National
Science Foundation (CAREER award CHE-0545206).
D.W.J. is a Cottrell Scholar of Research Corporation. This
material is based upon work supported by an NSF Integrative
Graduate Education and Research Traineeship No. DGE-
0549503 (T.G.C.) and the U.S. Department of Education
under Award No. P200A070436 (V.M.C.). The purchase of
the MALDI Mass Spectrometer was made possible by a grant
from the NSF (CHE-0639170).
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25 The cavity volume was measured to be 28 A3 using solvent surfaces
(radius = 1.4 A) in WebLab ViewerPro 4.0. This value was
estimated to be the difference in volume of a fully filled [As4L12Cl4]
cavity and the empty [As4L12Cl4] from the crystal structure. NMR
spectroscopic studies showed no evidence of guest binding or
protonation in solution upon treatment with methane or TFA,
respectively.
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This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 3505–3507 | 3507