Luminescent coordination polymer gels based on rigid terpyridyl phosphine and Ag(I)

Article abstract of DOI:10.1039/c2dt00035k

Rigid bridging terpyridyl phosphine and AgOTf form nanofibres to induce gelation of organic solvents, and the gel emits blue luminescence by suppressing π-π interactions between ligands. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2dt00035k

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Luminescent coordination polymer gels based on rigid terpyridyl phosphine
and Ag(I)†
Xin Tan, Xue Chen, Jianyong Zhang* and Cheng-Yong Su
Received 6th January 2012, Accepted 8th February 2012
DOI: 10.1039/c2dt00035k
photochemical and electronic properties.10 Only AgOTf has
Rigid bridging terpyridyl phosphine and AgOTf form
been found to induce gelation, other than other Ag(I) salts.
Synthesis of Py2Phos was accomplished through the reaction
sequence shown in Scheme 1. Condensation of a 2 : 1 molar
ratio of 4-acetylpyridine and 4-fluorobenzaldehyde with
ammonia afforded 4′-(4-fluorophenyl)-4,2′:6′,4′′-terpyridine
(Py2F). Py2Phos was synthesised from Py2F by reaction with
potassium diphenylphosphide in THF. The 31P NMR spectrum
of Py2Phos shows a single resonance at −4.5 ppm. Py2Phos is
a three-connecting ligand with two pyridyl N donors and one P
donor, as the nitrogen atom of the central pyridine ring is not
coordinated to metal centres as previously known.11
Transparent gels formed after 30 min upon simple mixing of a
CHCl3 solution of Py2Phos and a solution of AgOTf in CH3CN
at a molar ratio of 1 : 1 at RT (Fig. 1). The optically transparent
gels turned opaque after several days. The gel was only formed
at the Ag–L = 1 : 1 stoichiometry. Using more than one equival-
ent of Ag+, viscous solutions formed instead. On the other hand,
the use of less than one equivalent of Ag+ resulted in the for-
mation of solutions, from which a crystalline product (denoted
AgOTf, see below) could form after 30 min. Gels formed at
gelator concentrations between 0.009 and 0.020 mol L−1. NMR
studies of the Ag–L = 1 : 1 mixture in CHCl3–MeCN before and
after gelation revealed that the added materials (Py2Phos and
Ag+) were all incorporated in the NMR-silent aggregates. This is
consistent with previous observations that supramolecular gels
yield either almost unobservable or very broad resonance
signals.12
nanofibres to induce gelation of organic solvents, and the gel
emits blue luminescence by suppressing π–π interactions
between ligands.
Supramolecular gels are self-assembled through non-covalent
intermolecular interactions including van der Waals, π–π stack-
ing, hydrogen bonding, and/or hydrophobic effects. Discrete
metallomolecules or coordination polymers are found to be gela-
tors through non-covalent interactions including metal–ligand
coordination.1 Due to incorporation of metals, metallogels have
potential applications in catalysis, sensing, magnetic materials,
stimuli-responsive materials etc.1 Rules based on organogelator
have been usually applied to design metal–organic gels since the
relationship between molecular structure and gelation is largely
poorly understood. Auxiliary moieties such as cholesteryl, urea,
long hydrocarbon, ether chain etc. are required in these gels.2 To
rationally design metallogels a crystal engineering approach has
been developed by Dastidar and others.3 In this approach, the
gel fibres are extended via anisotropic interactions (e.g. self-
complementary hydrogen bonding, π–π interactions).3,4 On the
other hand, we and others recently observed a novel group of
metal–organic gels based on simple rigid bridging ligands
without any auxiliary moieties.5,6 These rigid-ligand based gels
show promising properties, e.g. in adsorption,5b,6c and
catalysis.6b
While a variety of metallogels are investigated, reports on
phosphine gels are rare despite phosphines being extremely
attractive in coordination chemistry7 and catalysis.8 Uozumi
et al. reported a Pd(II) gel synthesised from a triphosphine with
auxiliary hydrophobic alkyl-chain tether showing interesting cat-
alytic properties.2e We obtained viscous, thixotropic silver
coordination polymers in solution using a rigid bridging tripho-
sphine, 1,3,5-tris(diphenylphosphanyl)benzene.9 Herein we wish
to report that metal–organic gels have been successfully obtained
based on Ag(I) and rigid terpyridyl phosphine ligand, Py2Phos,
in which terpyridine groups would generate interesting
The formation of gels only occurred with AgOTf, while pre-
cipitates or solutions were obtained upon using Ag(I) salts of
SbF6−, ClO4−, NO3−, OTs−, or CF3COO−. No gel could form in
the simultaneous presence of OTf− and other anions, e.g.
−
−
OTf− : BF4 = 1 : 1 or OTf− : ClO4 = 1 : 1. Gels formed in
various binary solvent systems of CHCl3–MeCN, CHCl3–DMF,
THF–MeCN, or THF–DMF. The gels are thermo-reversible.
KLGHEI of Environment and Energy Chemistry, School of Chemistry
and Chemical Engineering, Sun Yat-Sen University, Guangzhou,
510275, China. E-mail: zhjyong@mail.sysu.edu.cn
†Electronic supplementary information (ESI) available: Experimental
section, NMR, XPS, IR spectra and SEM, structural figures. CCDC
857926–857927. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c2dt00035k
Scheme 1 Synthesis of Py2Phos.
3616 | Dalton Trans., 2012, 41, 3616–3619
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Fig. 1 Phase transition between the gel and the solution.
Fig. 3 X-Ray structures of AgOTf, (a) molecular structure showing
coordination geometry of Ag+ ion, and (b) (4,4) grid network showing
the size of the Ag4L4 rhombic rings, with OTf−, PPh2 phenyl rings and
uncoordinated pyridyl rings omitted for clarity.
The crystal structures of related complexes could be obtained.
From the solution mixture of Py2Phos and AgOTf or AgBF4
with a Ag–L ratio of 1 : 2, AgOTf (Ag(Py2Phos)2OTf·xH2O)
and AgBF4 (Ag(Py2Phos)2BF4·H2O) formed. X-Ray diffraction
analyses reveal that AgOTf and AgBF4 are isostructural with 2D
coordination polymeric structures, and only AgOTf is discussed
here in detail. The centre Ag+ ion adopts a tetrahedral geometry
and is tetracoordinate with two P donors and two pyridyl N
donors from four different ligands (Fig. 3a). Each ligand binds
two Ag+ ions with one P and one outer pyridyl N donor, leaving
the other outer and central pyridyl N donors uncoordinated.
Overall the Ag+ ions as 4-connected nodes are bridged by 2-con-
nected ligands to form 2D grid structures of (4,4) net topology
(Fig. 3b). The Ag⋯Ag distance separated by Py2Phos is
15.25 Å. The rhombus grid has a size of 28.82 × 10.02 Å,
with the angles being 38.4 and 141.6°, which is estimated by
two diagonal Ag⋯Ag distances and two vertex angles
∠Ag⋯Ag⋯Ag with the Ag4L4 grid. The OTf− counteranions
are located in the cavities formed in a single (4,4) network.
Fig. 2 TEM images of the xerogel.
Upon heating at 80 °C, gel-to-sol transition was observed, and
the solutions reverted to gels upon cooling (Fig. 1). The gel-sol
temperature (Tgel) was determined to be 70–80 °C for the
CHCl3–MeCN gel (0.018 mol L−1) by using the “dropping ball”
method in a closed vial. No single solvent was found to achieve
gelation. The gels were sensitive to pH. Upon addition of acid
(HOTf or HOTs), the coordinated pyridyl group of Py2Phos can
be protonated, resulting in decomposition of the coordinated
complex, which destroys the gel aggregation.
The morphology of xerogels was investigated by scanning
electron microscopy (SEM) and transmission electron
microscopy (TEM) (Fig. 2). The gels consist of elongated fibres
with diameters of ca. 20 nm, which created an entangled fibrillar
network. Such an extended network is typical of gels. The gel
nanofibres are amorphous according to the powder XRD pattern
of fresh gel, which exhibits only weak and very broad peaks.
FT-IR spectrum indicates the presence of OTf− by the charac-
teristic adsorption bands at 1260 and 1160 cm−1 corresponding
to the C–F and SvO vibrations, respectively. The surface ana-
lyses by XPS reveal the presence of Ag(I), as evidenced by
signals at 368.9 and 374.9 eV in the Ag 3d5/2 and 3d3/2 region,
respectively. Signals at 131.1 and 131.8 eV in the P 2P3/2 and
2P1/3 region, respectively, confirm attachment of the Ag(I) atoms
to the PPh2 groups. Thermogravimetric analysis of the xerogel
shows no weight loss until ca. 290 °C, above which the gel
network may be anticipated to collapse.
−
Instead BF4 and solvated water are located in the cavities for
AgBF4. Three pyridyl rings of the ligand are non-planar with
dihedral angles 17–30°. Intralayer π–π stacking interactions exist
between the parallel displaced bridging phenyl and central
pyridyl rings with interplanar distances ca. 3.35 and 3.61 Å,
respectively. The 2D networks do not interpenetrate and stack
along the c axis in an ABAB sequence with an interlayer separ-
ation of c/2 (ca. 10.04 Å) via offset-face-to-face π–π interactions
between the central and uncoordinated pyridyl rings and edge-
to-face π–π interactions between phenyl rings. The phase hom-
ogeneity of AgOTf and AgBF4 could be confirmed by powder
XRD, the pattern of which closely matches the simulated one
from the single-crystal analysis.
More information on the supramolecular organisation is pro-
vided by luminescent spectroscopy (Fig. 4). The free ligand dis-
plays a broad emission profile at the concentration of 10−4 mol
L−1 in CHCl3–MeCN (v : v = 1 : 1), in which two maximum
peaks can be identified at ca. 407 and 430 nm (λex = 370 nm).
For the Ag–L = 1 : 1 solutions at the concentration of 10−5 and
10−4 mol L−1 in CHCl3–MeCN (v : v = 1 : 1), neither wave-
length of maximum emission nor intensity of fluorescence
Ag(I)-based compounds are labile and readily manipulated by
anions.13 The gel was selectively responsive towards specific
anions. Addition of salts with weakly coordinating anions such
as [Bu4N][OTf], [Me4N][NO3], [Bu4N][PF6], [Bu4N][BF4] or
[Bu4N][ClO4] had no visible instant effect on the gel. In contrast,
addition of [Bu4N]Br, [Bu4N]Cl, or [Bu4N]OAc to the gel
resulted in immediate breakdown of the gel network to form a
solution, which may be attributed to change of the Ag+ coordi-
nation mode induced by the anions.
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solution. Upon formation of the Ag–L = 1 : 1 gel, the lumines-
cent spectrum of the gel is similar to the overall profile of the
ligand in dilute solution with comparable emission intensity,
suggesting interesting emission enhancement in the gel state.14
We would conclude that the Ag–L = 1 : 1 systems and gel for-
mation significantly reduce the degree of aggregation via π–π
interactions.
A possible formation mechanism of the gel nanofibres may be
thus proposed. Py2Phos with divergent phosphorus and pyridyl
donors may efficiently bridge the Ag+ ions to form oligomeric or
polymeric species as revealed by NMR. The Ag–L = 1 : 1 com-
position of the gel and the 1 : 2 composition of the crystal struc-
tures suggest that differences in coordination interactions may be
important in establishing whether gelation or crystallisation
occurs. Stronger Ag–P and Ag–N(pyridyl) coordination bonds
are the primary forces to direct the structure and form the nanofi-
bres, while π–π interactions between the ligands are remarkably
suppressed. Especially, anions (OTf−) play a key role in for-
mation and stabilisation of the nanofibres, which however is
undefined so far.
In summary, rigid terpyridine phosphine-containing lumines-
cent gels have been synthesised. The gel formation is dependent
on metal-to-ligand ratio, anion, solvent and concentration. Ag-
ligand coordination bond is crucial for the gelation by reducing
π–π interactions between terpyridine groups. The gel emits blue
luminescence, which emission intensity is comparable to that of
the ligand in dilute solution.
Acknowledgements
Fig. 4 (a) Photoluminescence spectra of AgOTf/Py2Phos = 1 : 1
systems in solution and gel states, (b) Ag/Py2Phos = 1 : 1 solutions
(0.0025 mol L−1) with different anions, and AgOTf/Py2Phos = 1 : 2 sol-
ution (c(Py2Phos) = 0.01 mol L−1), measured in CHCl3–MeCN (v : v =
1 : 1) at RT, and (c) solid-state emission spectra of Py2Phos (λex
360 nm) and AgOTf (λex 385 nm).
We acknowledge the NSFC (20903121 and U0934003), the
NSFGP (S2011010001307), the RFDP of Higher Education of
China, the Fundamental Research Funds for the Central Univer-
sities, and the 973 Program (2012CB821701) for support.
Notes and references
changes significantly. The emission is indicative of ligand-based
intramolecular charge transfer (ICT) character. A structured lumi-
nescent spectrum was obtained for the Ag–L = 1 : 1 solution at
higher concentration (10−2 mol L−1, prior to gelation). The stron-
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utions (ca. 407 and 430 nm excited at 375 nm) with decreased
emission. It is noticeable that the emission increases as the sol-
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emission characteristics upon excitation at 375 nm (Fig. 4a).
In comparison, the Ag–L = 1 : 2 solution shows a lower
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