Two novel lineal d10 metal-organic frameworks with a ditopic flexible linker: Structures, blue luminescence and thermal stability

Article abstract of DOI:10.1016/j.inoche.2011.03.022

Two novel supramolecular Ag(I) complexes, {[AgL]NO₃}_n (1), and {[AgL]CF₃SO₃}_n (2), (L) = ethane-1,2-diyl bis(pyridine-3-carboxylate) have been prepared by self-assembly of Ag(I) salts with ethane-1,2-diyl bis(pyridine-3-carboxylate) (L) in THF/H₂O system. The IR, TGA and elemental analysis have been recorded and both complexes were structurally characterized by X-ray crystallography confirming that complexes (1) and (2) are one-dimensional coordination polymers with lineal and helical chain motifs respectively. Solid emission spectra at room temperature of (1) and (2) show interesting blue phosphorescence with maximum intensity at 414 and 411 nm respectively, which are assigned to (LLCT) transition.

Full text of DOI:10.1016/j.inoche.2011.03.022

Inorganic Chemistry Communications 14 (2011) 897–901
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Two novel lineal d10 metal-organic frameworks with a ditopic flexible
linker: Structures, blue luminescence and thermal stability
a
b
c
Iván Brito a, , Javier Vallejos , Alejandro Cárdenas , Matías López-Rodríguez ,
⁎
Michael Bolte d, Jaime Llanos e
Departamento de Química, Universidad de Antofagasta, Av. Angamos 601, Antofagasta, Chile
Departamento de Física, Universidad de Antofagasta, Av. Angamos 601, Antofagasta, Chile
Instituto de Bio-Orgánica ‘Antonio González', Universidad de La Laguna, Astrofísico Francisco Sánchez No. 2, La Laguna, Tenerife, Spain
a
b
c
d
Institut für Anorganische Chemie der Goethe-Universität Frankfurt, Max-von-Laue-Strasse 7, D-60438 Frankfurt am Main, Germany
Departamento de Química, Universidad Católica del Norte, Av. Angamos 0610, Antofagasta, Chile
e
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel supramolecular Ag(I) complexes, {[AgL]NO3}n (1), and {[AgL]CF3SO3}n (2), (L)=ethane-1,2-diyl
bis(pyridine-3-carboxylate) have been prepared by self-assembly of Ag(I) salts with ethane-1,2-diyl bis
(pyridine-3-carboxylate) (L) in THF/H2O system. The IR, TGA and elemental analysis have been recorded and
both complexes were structurally characterized by X-ray crystallography confirming that complexes (1) and
(2) are one-dimensional coordination polymers with lineal and helical chain motifs respectively. Solid
emission spectra at room temperature of (1) and (2) show interesting blue phosphorescence with maximum
intensity at 414 and 411 nm respectively, which are assigned to (LLCT) transition.
Received 13 October 2010
Accepted 8 March 2011
Available online 17 March 2011
Keywords:
Coordination polymers
X-ray diffraction
© 2011 Elsevier B.V. All rights reserved.
Thermal analysis
Phosphorescence emission
Luminescent organic and Metal–Organic Frameworks (MOFs)
have been an active research area for decades because of their various
potential applications in materials sciences. Recently, the applications
of novel blue fluorescent materials in blue light emitting devices
(LEDs) remain an ongoing interest in materials science [1]. Lumines-
cent metal complexes with pyridine-containing ligand have attracted
much attention due to high luminescent efficiency. The d10 metal
complexes with nitrogen-containing ligand have been synthesized
and their luminescent behavior studied [2–5]. Ligand-based emission,
where closed-shell metal ions act to stabilize an intrinsically
luminescent organic, is less common. A few hybrid structures showing
this type of ligand-centered luminescence have been described [5].
Other factors related to luminescent properties, are the closed-shell
metal ion interactions which have received much attention, due to the
existence of photoluminescence in linearly coordinated complexes of
monovalent ions such silver and gold [6–8].
To achieve full-color electroluminescent display, three color
components, i.e., green, blue, and red, must be available. Stable blue
luminescent compounds that are useful in electroluminescent devices
are still scarce and very challenging to prepare [9].
Compared to the pure organic materials used in LED and
electroluminescent (EL) devices, the major advantages of the MOF
compounds are that the properties of this class of compounds, such as
volatility, stability and luminescent properties can be modified readily
by manipulating the coordination environment around the central
atom as well as counter-ion and solvent [10].
We herein describe a full account on the preparation and structural
characterization of two Ag(I) complexes containing flexible ditopic
ligand ethane-1,2-diyl bis (pyridine-3-carboxylate) which were found
to exhibit blue photoluminescence.
The reaction of the flexible ligand, ethane-1,2-diyl-bis-(pyridyl-3-
carboxylate), (L) and AgNO3, and AgCF3SO3 salts, under same exper-
imental conditions leads to the formation of two coordination polymers
with different motifs: {[Ag(L)(NO3)]}n (1) and {[Ag(L)(CF3SO3)]}n (2).
As shown in Figs. 1 and 2 each AgI center is coordinated by two N
atoms from (L) symmetry-related in a slightly distorted linear fashion
to form linear chains for (1) and helical chains for (2). Ag―N bond
distances are in the range from 2.157(3) to 2.204(3)Å, typical values
for AgI―Npy coordination distances [11–13]. The bond angles N―Ag―
N 158.02(15) and 168.95(11)° for (1) and (2) respectively are indicative
of presence of a distortion from linearity in both compounds. Ag⋯Ag
separations across (L) are 14.917(8) and 11.795(9)Å for (1) and (2).
The helical chain is connected to another one, (symmetry code: 2−x,
1−y, 1−z) through Ag―Ag interactions, with Ag―Ag=3.1937(4)Å,
which is shorter than the van der Waals radii of two silver atoms
(3.44 Å) comparable with similar distances reported [13]. Consequent-
ly, H-shaped dimeric units are generated. The pyridine rings on the
side of the Ag―Ag bond are almost co-planar, forming a dihedral
angle of 14.7(2)°, Fig. 3. In both compounds, there are also weak
Ag⋯C interactions with Ag⋯C distances in the range from 3.071(3) to
⁎
Corresponding author.
E-mail address: ivanbritob@yahoo.com (I. Brito).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.03.022

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I. Brito et al. / Inorganic Chemistry Communications 14 (2011) 897–901
Fig. 1. Linear chain structure of (1).
Fig. 2. Parallel arrangement of helical chains structure of (2). Triflate anion and H atoms are omitted for charity.
3.141(5)Å and fall in the secondary bonding range (the sum of the van
der Waals radii of Ag and C is 3.42 Å) [14]. Some polymeric silver (I)
compounds have been reported with similar Ag⋯C interactions, ranging
2.80–3.34 Å [12,15]. Therefore, these interactions are very important
for the packing in the solid state, for the compounds under study.
In (1), AgI cation is six coordinated in a distorted trigonal prismatic
square geometric by two N atoms from symmetry-related, two O
atom from symmetry-related and two O atom from a single nitrate
anion, Fig. 4. The bond angles around AgI are ranging from 53.76(11)
to 162.82(10)°. This non-lineal angle at the silver cations is due to
the asymmetric coordination of the nitrate anions. The crystal struc-
ture consists of one-dimensional AgI–L chains, which are further
extended by μ2–κ2 O:O nitrate anions into a two-dimensional (4,4)
sheet, Fig. 5. The Ag―O bond lengths are comparable to those in
related compounds [16,17].
In (2), AgI cation is four-coordinated and adopts a saw-horse
geometry to two N atoms from symmetry-related and two O atom
from a triflate anion, Fig. 6. The N―Ag―N is 168.95(10) and the
O―Ag―O is 111.27(8)°. The N―Ag―O ranges from 85.28(7) to
104.31(8)°. The crystal structure consists of one-dimensional AgI–L
helical chains, which are further extended by triflate anions into a
two-dimensional sheet, Fig. 7. The Ag―O bond lengths are compa-
rable to those in related compounds [16–18]. In both compounds, the
ligand, (L) acts in a μ2-N:N′-bidentate fashion to link AgI cations to
form a linear and helical chain motif along [010] for (1) and [111] for
(2). The ethylene moiety of (L) retains the gauche conformation [19].
The dihedral angles between the two pyridyl rings are 22.4(2) and
78.9(2)° for (1) and (2) respectively.
Metal–organic frameworks constructed from d10 metal centers
and conjugated organic linkers are promising candidates for hybrid
photo-active materials with potential applications such a light-emitting
diodes (LEDs) [20]. Since Ag(I) d10 configuration prevents stabiliza-
tion of excited states via the ligand field, LMCT and LLCT transitions
dominate the photophysics of these compounds. In majority of cases,
Fig. 3. A view of the H-shaped dimeric units.

I. Brito et al. / Inorganic Chemistry Communications 14 (2011) 897–901
899
Fig. 4. Ag(I) coordination environment in compound (1) (symmetry codes: (O31 and O33 (d) 3/2-x, 1/2+y, 1/2-z; x, y+1, z).
Fig. 5. Coordination of NO3- with Ag(I) in (1) forming two-dimensional (4,4) sheet.
Fig. 6. Ag(I) coordination environment in compound (2) (symmetry codes: O13d and N43: 2-x, 1-y, 1-z; 3/2-x, -1/2+y, 1/2-z).

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I. Brito et al. / Inorganic Chemistry Communications 14 (2011) 897–901
Fig. 8. Emission and excitation spectra for (L), (1) and (2) compounds.
transition. Meanwhile the lifetime of (2) can be fitted by a double-
exponential function with τ1 and τ2 at 0.68 and 0.80 ms, respectively;
therefore the emission band may be also assigned to LLCT, admix with
MC transition, belonging to the Ag(I)―Ag(I) interaction with the
presence of the ndσ*→(n+1)pσ transitions [25,26]. The long lifetime
of emissive state in the ms range is probably suggestive of triplet origin
and comparable to those in related compounds [28,29]. Therefore, the
red-shift of (1) and (2) compared to (L) is due to the stabilization of
(3LLCT) state, that shows to significantly affect the energy gap of the
frontier orbitals [26]. Furthermore, emission intensity of compound (1)
is a little weaker than that of the free ligand, which can be attributed to
theheavy atom effect[27]. The enhance emission intensity of compound
(2) can be originated to Ag(I)―Ag(I) interaction, that effectively
increases the structural rigidity of metal–organic framework and
reduces the loss of energy via vibration motion [10].
The thermal stability of both polymers studied using thermal
gravimetric analysis (TGA), from 25 to 600 °C under flowing Argon at a
heating rate of 10 °C/min (Fig. 9-1 and 9-2) for compounds (1) and (2)
is thermally stable up to 194 °C and 212 °C respectively. Furthermore
the TGA curves show that there are no chemical decomposition up to
230 °C for compound (1) and 500 °C for compound (2).
The weight loss for compound (1) (observed: 63.3%, calculated:
65.5%) occurs from 194 to 221 °C, which is attributable to the
complete decomposition of the sample. The residue is AgNO2. For
compound (2) (observed: 79%, calculated: 79.6%) occurs from 212 to
512 °C, which is attributable to the complete decomposition of the
sample. The residue is metallic silver.
Two novel Ag(I) coordination polymers were prepared by a
diffusion method between AgNO3 and AgCF3SO3 with (L)=ethane-
1,2-diyl bis(pyridine-3-carboxylate), and these structures were
characterized by X-ray crystallographic, TGA and IR techniques.
The luminescent properties of the ligand free (L) were modified,
preparing coordination polymers for diffusion methods between (L)
and silver salts. The nature of the used anion allows to obtain co-
ordination polymers with different motifs, thermal stabilities and
luminescent properties. This should presumably lead to new tuneable
materials.
Fig. 7. Part of the crystal structure for (2) consists of one-dimensional Ag(I)-L helical
chains, which are further extended by triflate anions into a two-dimensional sheet.
emission originates from excited 3LMCT or 3LLCT states (triplet
emitters), due to strong spin-orbit coupling induced by heavy atom,
consequently, between the HOMO–LUMO states that are lowered,
results in a red-shift in the emission wavelength. However, as it is
known Ag(I) may emit weak photoluminiscence at low temperature,
but room temperature examples are scarce [21–24]. Because single
crystals of (1) and (2) are air-stable and insoluble in common solvents,
solid-state emission properties of luminescent complexes (1) and (2) as
well as free ligands (L) were investigated at room temperature. The
results indicate that the emission properties of (L) were affected by their
incorporation of metal-containing coordination compounds because
there is a red shift displacement in emission maximum of (1) and (2)
with respect to (L). Fig. 8 shows emission and excitation spectra for (L)
with one maximum at 394 nm upon excitation at 352 nm. Under same
experimental conditions the emission and excitation spectra for
compounds (1) and (2) have been studied, compound (1) exhibits
one emission band at 414 nm while compound (2) exhibits one
emission maximum at 411 nm upon excitation at 368 and 346 nm
respectively. It is hard to propose a correct mechanistic conclusion for
their luminescence based only onemission spectra, although theexcited
state lifetime (τ) measure may be helpful in verifying the photo-
luminescence mechanism [25]. The lifetime of (1) can be well fitted by a
single-exponential function with τ at 0.78 ms, which is similar to free
ligand (L) lifetime (τ=0.74 ms), in accordancewith thesimilarityof the
lifetime, the emission band of (1) may be attributed mainly to LLCT
The ligand free was synthesized according to the reported method
[19].
Compounds (1) and (2) were obtained by diffusion method
between AgNO3 and AgCF3SO3 with (L) [30]. Single-crystal X-ray
diffraction analysis reveals that both compounds crystallize in space
group P21/n and consist of slightly distorted lineal [Ag(L)]n chains for
(1) and helical chains for (2) [31].

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Acknowledgements
[30] Compound (1) A solution of AgNO3 (16.9 mg, 0.1 mmol) in water was slowly
added to a solution of L (27.2 mg, 0.1 mmol) in THF (4 ml). Colorless single
crystals suitable for X-ray were obtained after a few days were obtained in 64%.
Anal. Calc. for C14H12N3O7Ag (441.03): C, 38.1; H, 2.72; N, 9.52% Found: C, 38.8,
H, 2.81, N, 9.32%. The IR (KBr, cm−1): 1736 s, 1609 m, 1455 m, 1240 s and 635 m.
Compound (2): A solution of AgCF3SO3 (25.7 mg, 0.1 mmol) in water was slowly
added to a solution of L (27.2 mg, 0.1 mmol) in THF (4 ml). Colorless single
crystals suitable for X-ray were obtained after a few days were obtained in 75%
yield. Anal. Calc. for C15H12N2O7SF3Ag (528.04): C, 34.09; H, 2.27; N, 5.30%.
Found: C, 35.3, H, 2.41, N, 5.23%. The IR (KBr, cm−1): 1727 s, 1605 m, 1424 m,
1278 s and 632 s.
The authors thank the Spanish Research Council (CSIC) for the
provision of a free-of-charge license to the Cambridge Structural
Database. J.V. thanks Universidad de Antofagasta for a PhD fellowship.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.inoche.2011.03.022.
[31] Single crystals analysis were performed at 173(2) K with a STOE IPDS II two-
circle-diffractometer with Mo Kα radiation (λ=0.71073 Å) by the ω-2θ scan
technique. All data were collected for absorption by multi-scan method using
[31a]. The program X-area [31b] was applied for integration of the diffraction
profiles. The structures were solved with direct method using SHELXS-97
followed by structure refinement on F2 with program SHELXL-97 [31c]. All
nonhydrogen atoms were refined anisotropically. The hydrogen atoms were
positioned geometrically with isotropic thermal parameters set to 1.2Ueq of
the parent atom. Crystal data for (1): C14H12AgN3O7, Mr=442.14, Monoclinic
P21/n, a=13.0008(8) Å, b=7.8631(4) Å, c=14.9166(7) Å, β=98.301(4)°,
V=1508.90(14) Å3, Z=4, Dc=1.946 g cm-3, μ=1.382 mm-1, F(000)=880,
21810 reflections measured with 2900 unique reflections, GOF=0.902. The
final [IN2σ(I) R=0.0344, wR2=0.0754]. The CCDC reference number is 795281.
Crystal data for (2): C16H14AgF3N2O7S, Mr=543.22, Monoclinic P21/n,
a = 7.9745(6) Å, b = 10.9040(6) Å, c = 21.8561(13) Å, β = 94.865(5)°,
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