Centripetal molecules as multifunctional building blocks for coordination networks

Article abstract of DOI:10.1039/b709942h

Centripetally shaped molecules are crystallized with AgSbF₆ to yield coordination networks featuring novel coordination modes, network connectivity and chiral/helical structures. The Royal Society of Chemistry.

Full text of DOI:10.1039/b709942h

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Centripetal molecules as multifunctional building blocks for
coordination networks{
Yan-Qiong Sun,a Jun He,a Zhengtao Xu,*a Guo Huang,a Xiao-Ping Zhou,a Matthias Zellerb and
Allen D. Hunterb
Received (in Cambridge, UK) 2nd July 2007, Accepted 31st August 2007
First published as an Advance Article on the web 10th September 2007
DOI: 10.1039/b709942h
and is thus primed to be interlinked by metal ions to generate
potentially interesting topological and helical features in the
resultant network.
Centripetally shaped molecules are crystallized with AgSbF6 to
yield coordination networks featuring novel coordination
modes, network connectivity and chiral/helical structures.
Even though molecules with rigid back-bended side chains
reminiscent of the above centripetal geometry have been reported,3
the purposeful integration of the centripetal geometry into
crystalline networks appears to be unprecedented. As a preliminary
exploration in this direction, we here report two crystal structures
based on Ag(I) salt and the above L1 and L2 ligands. In particular,
we wish to highlight how the modification of the G2 sites impacts
the network formation, and the potential noncentrosymmetric and
helical features in the solid state.
The development of functional organic ligands constitutes an
essentially important exercise in the field of coordination networks
(or metal–organic frameworks),1 as one major advantage of these
networks hinges upon the much richer chemical functionalities in
comparison to the inorganic crystalline materials. Particularly
attractive are systems of building blocks that provide for
convenient and systematic functional modifications in both
molecular and solid state structure.2 Most reported metal–organic
frameworks, however, are based on relatively simple organic
ligands—additional functional groups therefore tend to substan-
tially alter the coordinating characteristics, and consequently
complicate the control of the solid state structure.
The complex 2L1?4AgSbF6?8C6H6 (1) was obtained from
reacting a toluene solution of AgSbF6 with a benzene solution
of L1, and the complex L2?AgSbF6 (2) from the toluene solution
of AgSbF6 with a THF solution of L2.{ X-Ray single-crystal
analysis§ reveals that complex 1 crystallizes in the noncentrosym-
metric space group P3121 and consists of 1D coordination chains
based on the linear coordination of the –CN groups to the Ag(I)
ions (Fig. 1) The L1 molecules all adopt similar conformations
with a C2 symmetry, and display the same atropisomeric chirality
(Fig. S4, ESI{). Overall homochiral features are thus preserved in
the coordination chains and throughout the crystal structure.
The coordination chains are aligned in a parallel fashion to form
a distinct layer perpendicular to the c axis. The layers are packed
such that the coordination chains are successively rotated by 60u in
the same direction proceeding across the neighboring layers along
the c axis (Fig. 2). The packing thus creates equilateral, triangular
channels along the c axis, which are filled with a second set of
Ag(I) ions bound by three benzene molecules (the Ag+?3C6H6
complex) and the SbF62 anions (see Fig. 1, S5 and S6, ESI{ for the
complete coordination environments of the three distinct Ag ions).
The Ag+?3C6H6 complex here represents an entrapped version of
the previously reported complex AgBF4?3C6H6,4 and features a
We here introduce a class of branchy molecules with back-
folded, centripetal geometry as functional building blocks for
coordination networks. A relatively simple system is shown in
Chart 1. Therein, a diacetylene-based linear rod forms the main
branch (i.e., G1 branch, G standing for generation), and the
corresponding two terminal sites (G1 sites) are equipped with the
cyano group. From the G1 branch outgrow four phenylacetylene
side chains (i.e., the G2 branches) terminated by the methoxy
groups.
A number of advantages can be readily identified from these
molecules. Thus the spatially well-removed G1 and G2 sites can be
modified systematically without compromising the coordinating
ability of each other. For example, the methoxy groups in L1 can
be replaced by the thiomethoxy group to probe the impact on the
formation of the coordination network. In addition, the centripetal
shape provides for the G2 branches to restrict the rotation of the
diacetylene unit of the G1 branch, and thus creating a potentially
chiral molecular scaffold similar to the atropisomers in general
organic chemistry. As will be shown in this paper, the conforma-
tion can be captured in an enantiomeric form in the crystal
structure, lending potential use to the construction of noncen-
trosymmetric solid state materials. Also, the centripetal orientation
of the G2 sites of thiomethoxy composes an inchoate spiral shape,
aDepartment of Biology and Chemistry, City University of Hong Kong,
83 Tat Chee Avenue, Kowloon, Hong Kong, P. R. China.
E-mail: zhengtao@cityu.edu.hk; Fax: +852-27844679
bDepartment of Chemistry, Youngstown State University, One
University Plaza, Youngstown, Ohio, 44555, USA
{ Electronic supplementary information (ESI) available: Experimental
section, additional crystallography figures and spectra, and further
crystallographic data. See DOI: 10.1039/b709942h
Chart 1
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ligands adopting the same chiral atropisomeric configuration
throughout the crystal structure (Fig. 3). However, unlike the free-
standing methoxy groups in 1, the thiomethoxy (MeS–) groups on
the crossing G2 branches of L2 provide additional coordination
˚
bonds to the Ag(I) centers [Ag–S distances: 2.378(5)–2.582(2) A],
and bridge the parallel-aligned chains into a 2D coordination
network (Fig. 3). Thus the Ag(I) ion interacts with two pairs of
sulfur atoms at notably different distances (the S atoms from one
˚
˚
pair are both at 2.378 A and the other at 2.702 A), forming a
highly distorted octahedral geometry together with the two –CN
groups. Note also that each pair of S atoms emanate from the
same molecule which thus chelates the Ag(I) center in a macrocylic,
tweezer-like motif encompassing two crossing side arms (G2
branches) and the diacetylene backbone (Fig. S4, ESI{).
The tweezer-like chelation motif take on an inchoate spiral
shape, and, as a result, two types of double-stranded helix
structures can be identified within the 2D coordination layer
(Fig. 3). The first type runs perpendicular to the Ag–CN
coordination chain (along the b axis). The constituent strand
therein follows two meta-positioned phenylacetylene (G2) side
arms and the associated Ag(I) ions as the repeating unit, with each
complete turn involving four side arms (from two L2 molecules)
Fig. 1 Parallel alignments of the coordination chains in 1: (a) chains of
Ag1 ions; (b) chains of Ag2 ions. Red spheres, Ag; blue, N; orange, O;
grey, C.
˚
and two Ag(I) ions [pitch: 42.8084(8) A (Fig. 3(a), S8, ESI{)]. The
two strands are of the same chirality (e.g., both right-handed in the
structure of Fig. 3), and they intersect periodically at the Ag(I)
ions, and are interlinked by the diacetylene units of the L2
molecules involved.
The second type of double-stranded helix runs parallel to the
Ag–CN coordination chains, and occupies the space between every
two neighboring Ag–CN coordination chains (Fig. 4). The
repeating unit consists of one benzonitrile group, one of its two
G2 side arms, and the associated Ag(I) center. Each complete turn
˚
contains two such repeating units, with a pitch of 21.4042(8) A
Fig. 2 Schematic presentation of the packing arrangement for the
coordination chains (shown as individual rods) of 1. The Ag+?3C6H6
complexes in the channels are also shown.
(Fig. 3(c), S9, ESI{). The benzonitrile groups point in the same
direction within each single strand, and thus create a net overall
dipole. The polarities of the two strands are, however, opposite,
resulting in no net polarization for the double helix (reminiscent of
the opposing directions of the two strands in the DNA double
rare case of a discrete complex included in a coordination host
network.
The packing motif of the coordination chains as described
above suggests a 61 screw element along the c axis. The actual 31
symmetry in the crystal structure is due to the two crystal-
lographically inequivalent layers that alternate with each other
along the c axis. Specifically, one layer contains the Ag1 ions in the
constituent coordination chains, while the other contains Ag2. The
Ag1 ion is coordinated to two crystallographically related benzene
molecules that are incorporated as guest molecules (Fig. S5, ESI{),
while Ag2 is bonded to two of the above-mentioned Ag+?3C6H6
complexes via one of the three benzene molecules (Fig. S6, ESI{).
Although the Ag atoms and the benzene guests are disordered over
…
two sets of positions, the shorter Ag C distances can be deduced
˚
as being about 2.40 A and above, which compares well with the
5,6
…
Ag C distances in other arene–Ag(I) complexes.
The structure of 2 was refined in the noncentrosymmetric space
¯
group I42d, with one half L2 molecule and one AgSbF6 unit
Fig. 3 (a) The double-stranded helix structure along the b axis in 2. The
blue and grey colors highlight the two individual strands. (b) The 2D
coordination network of 2. A linear Ag–CN chain is marked black on top
for clarity. Color code: Ag, red; S, green; C, grey; N, blue. (c) The tubular
structure based on two helical strands (marked orange and grey,
respectively) along the a axis.
contained in the asymmetric portion of the unit cell. Like in 1, the
cyano groups of the L2 ligands coordinate to the Ag(I) ions in a
linear fashion to establish a chain of alternating L2 molecules and
˚
Ag(I) ions [Ag–N distances: 2.244(4) and 2.274(5) A], with the L2
4780 | Chem. Commun., 2007, 4779–4781
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This work is supported by a grant from the Research Grants
Council of the Hong Kong Special Administrative Region, China
[Project No. 9041109 (CityU 102406)]. Y. S. is supported by a
postdoctoral grant from the Research Scholarship Enhancement
Scheme of City University of Hong Kong. The diffractometer was
funded by NSF grant 0087210, by the Ohio Board of Regents
grant CAP-491, and by YSU.
Notes and references
Fig. 4 Packing of the 2D coordination nets in 2, highlighting the
perpendicular orientation of the neighboring nets. Color code: Ag, red; S,
green; C, grey; N, blue.
{ Syntheses of 1 and 2: A benzene solution of L1 (2.3 mg, 3.0 mM, 0.5 mL)
or a THF solution of L2 (2.6 mg, 3.0 mM, 0.5 mL) was loaded into a glass
tube and carefully layered with a toluene solution of AgSbF6 (2.4 mg,
7.0 mM, 0.5 mL) without disturbing the interface. The tube was then sealed
and kept in the dark. Needle-like, yellow crystals of 1 and orange diamond
crystals of 2 suitable for X-ray studies were obtained after one week. Yield:
83 and 34% for 1 and 2 (based on L1 or L2, respectively). Chemical
analysis of 1 yields the following: found: C 52.01, H 3.04, N 1.76%. This
result closely matches the formula C150H106Ag4F24N4O8Sb4 (i.e.,
2L1?4AgSbF6?7C6H6): calc.: C 51.97, H 3.08, N 1.62%, suggesting partial
loss of the benzene molecules during the sample preparation and
measurement process. For comparison, formula C156H112Ag4F24N4O8Sb4
(i.e., 2L1?4AgSbF6?8C6H6) gives the following calculated composition: C
52.85, H 3.18, N 1.58%. Anal. Calc. for C54H32AgF6N2S4Sb 2: C 54.93, H
2.73, N 2.37. Found: C 54.00, H 3.26, N 2.88%.
helix). The two strands intersect at the Ag(I) centers, and are
chemically linked through the diacetylene units between the
benzonitrile groups (note that the two crossing G2 arms from
the L2 molecule in Fig. 3(c) belong to the two different strands).
Unlike the first type of helix, where the intersecting Ag(I) ions and
the interlinking diacetylene units are located around the center of
the helix, the Ag(I) ions and the diacetylene units are located along
the wall region of the helix, thus imparting a tubular feature to the
helix. The end-on view reveals a rather squashed shape of the
tubes, mainly due to the flattened conformation of the constituent
L2 molecules. No space is thus available for inclusion of
counterions or guests.
§ Crystal data for 1: C156H112Ag4F24N4O8Sb4, Mr = 3545.02, trigonal,
˚
3
˚
space group P3121, a = b = 21.368(1), c = 27.279(3) A, V = 10786(1) A ,
Z = 3, Dc = 1.637 g cm23, m = 1.362 mm21, F(000) = 5244, GOF = 1.233,
A total of 92898 reflections were collected and 14532 are unique (Rint
=
0.0395). R1 (wR2) = 0.0654 (0.1626) for 1071 parameters and 14532
reflections [I . 2s(I)]. For 2: C54H32AgF6N2S4Sb, Mr = 1180.68,
The solid sample of 1 is unstable in air, and quickly loses the
benzene guests and decomposes into an amorphous black solid.
TG analysis of an as-synthesized solid sample of 1 was able to
capture part of the initial step of weight loss of 4.46% at around
100 uC (Fig. S3, ESI{), which corresponds to the removal of two
benzene molecules per formula unit (calc. 4.41%). The sample
instability was also reflected in a series of fluorescence spectra
taken over time, which featured steady decrease of the emission
intensity (Fig. S10, ESI{).
¯
˚
tetragonal, space group I42d, a = b = 21.4042(8), c = 21.408(2) A, V =
9807.9(9) A , Z = 8, Dc = 1.599 g cm23, m = 1.182 mm21, F(000) = 4688,
3
˚
GOF = 1.065. A total of 36308 reflections were collected and 6110 are
unique (Rint = 0.0605). R1 (wR2) = 0.0497 (0.1116) for 330 parameters and
6110 reflections [I . 2s(I)]. The intensity data were collected on a Bruker
AXS SMART APEX CCD diffractometer with graphite-monochromated
˚
Mo-Ka radiation (l = 0.71073 A) at 100 K. All absorption corrections
were performed using the SADABS program. The structures were solved
by direct methods and refined by full-matrix least squares on F2 using the
SHELXTL 6.14 program package. CCDC 646836 (1) and 646837 (2). For
crystallographic data in CIF or other electronic format see DOI: 10.1039/
b709942h
Fluorescence measurements of L1 and L2 show that the overall
emission features in THF (lmax-L1 = 492 nm, lmax-L2 = 540 nm) are
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similar to those in the solid state (lmax-L1 = 500 nm, lmax-L2
=
532 nm), although the respective reasons for the red shift in the L1
case (i.e., from 492 to 500 nm) and the blue shift for L2 (from 540
to 532 nm) are unclear at this point. By comparison, the emission
peaks of complexes 1 and 2 both shift, in a larger degree, to the red
(lmax-1 = 546 nm, lmax-2 = 565 nm, Fig. S11 and S12, ESI{).
Theoretical study of the electronic structures of the ground and
excited states of these centripetal molecules might help account for
the impact of the Ag(I) species on the fluorescence properties.
In a wider perspective, the centripetal shape can be installed in a
more complex molecular scaffold. For example, the G1 branches
can take on a trigonal or tetragonal shape, as shown in molecules
M1 and M2 in the ESI.{ Or, one can equip the G2 branches with
G3 branches in a similarly centripetal form (see M3 in ESI{), and
thus begin to approach a structural hierarchy comparable to that
of dendrimers. The syntheses of these novel self-similar molecules
are underway, and their use in constructing functional coordina-
tion networks and other supramolecular aggregates will be
explored in due course.
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Chem. Commun., 2007, 4779–4781 | 4781