Topological control in coordination polymers by non-covalent forces

Article abstract of DOI:10.1039/a903163d

Reaction of Zn²⁺ salts with the terephthalate dianion results in a herringbone motif coordination polymer; the orientation of the terephthalate spacer ligands and the coordination geometry about the Zn²⁺ ion is crucially dependent on hydrogen bonding to ancillary ligands; replacement of coordinated water with ethylenediamine results in marked changes to the polymer orientation without disruption of the fundamental features of the material.

Full text of DOI:10.1039/a903163d

Topological control in coordination polymers by non-covalent forces
Gemma Guilera and Jonathan W. Steed*
Department of Chemistry, King’s College London, Strand, London, UK WC2R 2LS. E-mail: jon.steed@kcl.ac.uk
Received (in Columbia, MO, USA) 20th April 1999, Accepted 23rd June 1999
Reaction of Zn2+ salts with the terephthalate dianion results
in a herringbone motif coordination polymer; the orientation
of the terephthalate spacer ligands and the coordination
geometry about the Zn2+ ion is crucially dependent on
hydrogen bonding to ancillary ligands; replacement of
coordinated water with ethylenediamine results in marked
Fig. 1 XSeed21 view of the coordination polymer [Zn(H2O)2(m-O,OA-tph)]∞
changes to the polymer orientation without disruption of the
2. Selected bond lengths (Å): Zn(1)–O(1) 2.0049(15), Zn(1)–O(2)
fundamental features of the material.
2.5500(16), Zn(1)–O(3) 1.9855(18) Å.
A key objective in the emerging field of crystal engineering is
Zn2+ centres linked by a herringbone pattern of tph22 spacers
the control and manipulation of weak interactions in order to
tune the properties of the bulk material.1–6 Potential applica-
tions of such studies include the preparation of new non-linear
optical materials, design of porous solids with novel inclusion
or reactivity properties (e.g. for use in separation science and
heterogeneous catalysis, particularly in stereo- or enantio-
specific synthesis) and new sensor materials and coatings.7–11
The vast majority of current work centres around the controlled
assembly of donor and acceptor building blocks (particularly
involving hydrogen bond acid–base pairs, e.g. nucleobases12) in
order to generate an entirely ‘supramolecular polymer’, i.e. a
material held together solely by non-covalent interactions.
While such systems can be remarkably robust,12 such an
approach is fundamentally limited by the intrinsic strength of
the constituent interactions, which are markedly weaker than
covalent or ionic bonds, although our recent work has focussed
on the importance of such interactions.13–16 With this in mind,
we have begun a programme of research aimed at the
construction of molecular coordination polymers of low
dimensionality in which mechanical strength is imparted by
strong coordinate bonds, while admitting scope for weaker,
lateral interactions which might enable the material to be tuned
in the same way as in more conventional crystal engineered
solids. We now report the results of simple, proof of principle
studies, which demonstrate how an inherently robust coordina-
tion polymeric framework may be manipulated by weak
interactions.
aligned approximately along the crystallographic a/c diagonal.
The tph ligands are essentially monodentate with Zn–O(1)
2.0049(15) Å, however the Zn–O(2) distance of 2.5500(16) Å
suggests a non-negligible interaction with the uncoordinated
oxygen, which may be described as a semi-chelating coordina-
tion mode.19 The two water molecules within the coordination
sphere of the metal cation engage in two distinct hydrogen
bonded interactions, which, together, are responsible for the
conformation of the polymer. In the crystallographic b direc-
tion, perpendicular to the direction of chain propagation, the
metal cations stack precisely on top of one another and are
linked in an eight membered hydrogen bonded ring [R22 (8) in
graph set notation20] involving the water hydrogen atom H31
attached to O3 and the coordinated oxygen atoms of the tph
ligand, Fig 2(a). In addition, lateral hydrogen bonds involving
the second unique water hydrogen atom H32 and the uncoordi-
nated terephthalate oxygen atoms link one chain to those
adjacent in R22 (12) hydrogen bonded sets, Fig. 2(b). The aryl
In attempting to prepare coordination polymers we were
struck by the utility of avoiding the need for counter anions
which must inter alia disrupt the solid state packing and give
rise to additional cation–anion interactions which might be
difficult to control.17 Accordingly we have chosen to examine
the reactions of divalent metal cations with the terephthalate
dianion, (tph22, 1) in the expectation of generating a neutral
polymeric material. We chose initially relatively labile Zn2+ (as
nitrate or acetate salts) in the hope of generating reaction
products under thermodynamic control. The structure of zinc
acetate itself, Zn(O2CMe)2·2H2O, consists of two essentially
bidentate acetate ligands and two water molecules engaging in
intra- and intermolecular hydrogen bonding interactions.18 We
reasoned that replacement of acetate with terephthalate would
result in an analogous structure consisting of both a polymer
chain and ancillary aqua ligands which would exert additional
control over the polymer geometry by means of strong
hydrogen bonding interactions. Consistent with expectation, an
excellent yield of crystalline material of stoichiometry
[Zn(tph)]·2H2O 2 was deposited within 24 h. This material was
characterised by low temperature X-ray crystallography† (100
K) and proved to consist of the desired coordination polymer,
Fig. 1. The compound consists of approximately tetrahedral
Fig. 2 (a) Stacking of [Zn(H2O)2(m-O,OA-tph)]∞ 2 in the b direction via R22
(8) hydrogen bonded sets,20 O…O 2.776(2) Å, (b) lateral R22 (12) motifs
which link one polymeric chain to those adjacent, O…O 2.699(3) Å.
Chem. Commun., 1999, 1563–1564
1563

chain and its ability to tolerate other ancillary ligands. In order
to explore the phase space of the Zn2+–tph22 system a large
variety of reactions between these two components were set up
with mole ratios of 1+1 up to 1+4 in a range of solvents and with
a range of Zn2+ salts. Compound 2 was formed in excellent yield
in every case (as determined by solid state IR spectroscopy and
measurement of unit cell dimensions) except for one occasion in
which the reaction of hydrated zinc acetate with Na2(tph) in
water in a 1+1 ratio gave a mixture of 2 and a further product of
formula [Zn(H2O)(tph)]∞ 4. Compound 4 is a highly cross-
linked polymer involving coordination of all four tph oxygen
atoms, each to a different trigonal bipyramidal Zn2+ centre
(aqua ligand equatorial). The structure is based on a ladder
arrangement of puckered eight-membered Zn2O4C2 rings.
Thermogravimetric analysis of 2 indicates stepwise loss of the
two aqua ligands at 168 and 192 °C, respectively, suggesting the
possible conversion of 2 into 4. Full details of this system will
be reported fully in a separate paper.
Fig. 3 Crystal packing in 2.
rings of the tph ligands stack in an offset fashion perpendicular
to the b direction, ca. 3.7 Å apart. The overall crystal packing is
shown in Fig. 3.
We thank the EPSRC and King’s College London for funding
of the diffractometer system and the Nuffield Foundation for the
provision of computing equipment. Grateful acknowledgement
is also given to the ULIRS Thermochemistry service at UCL.
Clearly the presence of two labile water molecules within the
Zn2+ coordination sphere, playing only an indirect part in the
polymeric chain, suggests that they might be replaced by other
ligands with different hydrogen bonding characteristics. This
should influence, via hydrogen bonding, the geometry of the
coordination polymer. Hence we carried out the reaction of Zn2+
salts with Na2tph in the presence of 1 equivalent of ethylenedia-
mine (en). This resulted in the formation of an exactly
analogous coordination polymer, [Zn(en)(m-O,O’-tph)]∞ 3, in
which the two water molecules have been replaced by a single,
chelating en ligand. Fascinatingly, the X-ray crystal structure of
this material† reveals the same essential features of the
coordination geometry and herringbone packing of the polymer
observed in 2 despite the greatly different characteristics of the
en ancillary ligand. As in 2, the Zn2+ centres stack precisely
along the crystallographic b direction. However, in 3 there is no
possibility of the propagating R22 (8) hydrogen bonded links
because of the steric bulk of the ethylene spacer and this is
reflected in the dramatic expansion of the crystallographic b
axis from 5.00 to 7.25 Å. One of the amine protons on each
donor atom is thus unable to take part in any hydrogen bonding.
The NH2 functions are, however, capable of engaging in lateral
interactions from one chain to another in the same fashion as 2
to give twelve-membered hydrogen bonded rings, Fig. 4. The
most obvious consequence of the reduction in inter-chain
connectivity is the dramatic reorientation of the aryl groups of
the tph ligands such that in 3 they are approximately parallel
with the crystallographic b direction, as opposed to perpendicu-
lar, as in 2. This has the curious consequence both of disrupting
the p-stacking interactions seen for 2 and changing the
coordination mode of the tph ligands from ‘semi-chelating’, to
entirely unidentate, with a non-bonded Zn–O(2) distance of
2.931(3) Å, compared to 2.5500(16) Å in 2 and a shortening of
the Zn–O(1) bond length to 1.947(3) Å. Since it is the non-
coordinated tph oxygen atom O(2) which is involved as a
hydrogen bond acceptor in both cases it is clear that the
accommodation of the lateral hydrogen bonds plays a much
more significant role in determining the overall structure than
factors such as weak interactions of O(2) with the zinc centre
and p-stacking.
Notes and references
† Crystal data: 2: C8H8O6Zn, M = 265.51, monoclinic, space group C2/c,
a = 14.9503(8), b = 5.0031(4), c = 12.1617(11) Å, b = 103.647(6)°, U
= 883.99(12) Å3, Dc = 1.995 Mg m23, Z = 4, m = 27.84 cm21, T =
100(2) K, Reflections measured: 8316, unique data: 1010 (Rint = 0.069),
parameters: 78, R1 [F2 > 2s(F2)] = 0.0310, wR2 (all data) = 0.0802.
3: C10H12N2O4Zn, M = 289.59, monoclinic, space group P2/n, a =
5.6508(11), b = 7.2496(14), c = 13.179(3) Å, b = 100.468(2)°. U =
530.90(18) Å3, Dc = 1.812 Mg m23, Z = 2, m = 23.17 cm21, T = 100(2)
K, Reflections measured: 2189, unique data: 1212 (Rint
= 0.078),
parameters: 87, R1 [F2 > 2s(F2)] = 0.0511, wR2 (all data) = 0.1297.
CCDC 182/1299. See http://www.rsc.org/suppdata/cc/1999/1563/ for
crystallographic data in .cif format.
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Fig. 4 Lateral R22 (12) motifs in [Zn(en)(m-O,OA-tph)]∞ 3, N···O 2.984(5) Å
(CH hydrogen atoms omitted for clarity). Note the second NH proton does
not take part in any hydrogen bonds.
Communication 9/03163D
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Chem. Commun., 1999, 1563–1564