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FULL PAPER
Homo- and Heteronuclear Compounds with a Symmetrical
Bis-Hydrazone Ligand: Synthesis, Structural Studies and
Luminescent Properties
Sabina Rodríguez-Hermida,[a],[b] Ana B. Lago,[a] Rosa Carballo,[a] Oscar Fabelo[c] and Ezequiel M.
Vázquez-López*[a]
Abstract: Nine new coordination compounds have been
synthesized by the reaction of salts of bivalent metal ions (a = ZnII, b
= CuII, c = NiII, d = CoII) with the bis(benzoylhydrazone) of 4,6diacetylresorcinol (H4L). Three kinds of complexes have been
obtained: homodinuclear compounds [M2(H2L)2].nH2O (1a, 1b, 1c
and 1d), homotetranuclear compounds [M4(L)2].n(solv) (2a and 2c)
and heterotetranuclear compounds [Zn2M2(L)2].n(solv) (2ab, 2ac and
2ad). The structures of the free ligand H4L·2DMSO and its
complexes [Zn2(H2L)2(DMSO)2] (1a*), [Zn4(L)2(DMSO)6] (2a*) and
[Zn0.45Cu3.55(L)2(DMSO)6]·2DMSO (2ab*) were elucidated by singlecrystal X-ray diffraction. The ligand shows luminescence properties
and its fluorimetric behavior towards MII metals (M = Zn, Cu, Ni and
Co) has been studied. Furthermore, the solid-state luminescence
properties of the ligand and compounds have been determined at
room temperature. 1H-NMR monitoring of the reaction of H4L with
ZnII showed the deprotonation sequence of the OH/NH groups upon
metal coordination. Heteronuclear reactions have also been
monitored by ESI-MS and spectrofluorimetric techniques.
is often disturbed by intermolecular contacts. In this context, it is
necessary to overcome this concentration quenching in order to
obtain efficient solid-state emissive compounds.[4]
Control of the coordination architecture is a crucial factor in
determining the chemical and physical properties of the resulting
materials. Appropriate synthetic routes are required to construct
compounds with the desired topologies and these depend on the
appropriate choice of metal ions, in terms of their coordination
number and geometry, and bridging ligands with a suitable
shape, size, flexibility and denticity. The metal ions play both a
structural role (directing and sustaining the solid-state
architecture) and a functional one (through magnetic, optical or
redox properties).[5]
Introduction
The preparation of solids with useful functionalities is one of the
main goals of crystal engineering. Fluorescent chemosensors
are of interest in many research areas, such as environmental
control,[1] medical diagnostics[2] and biological studies.[3]
Nevertheless, many of the fluorescent materials prepared to
date cannot fluoresce efficiently in the aggregated or solid state
because the intrinsic radiative decay pathway of the fluorophore
[a]
[b]
[c]
Dr. S. Rodríguez-Hermida, Dr. A.B. Lago, Prof. R. Carballo, Prof.
E.M. Vázquez-López
Instituto de Investigación Biomédica - Universidade de Vigo
Departamento de Química Inorgánica
Facultade de Química,
E-36310 Vigo,
Galicia, Spain
E-mail: [email protected]
Current address:
ICN2-Institut Catala de Nanociencia i Nanotecnologia,
Campus UAB
08193 Bellaterra, Barcelona (Spain)
Dr. O. Fabelo
Institut Laue Langevin
Diffraction Group
71 avenue des Martyrs
Grenoble, France
Supporting information for this article is given via a link at the end of
the document.
Scheme 1. Schematic representation of H4L showing the intramolecular O–
H…N interactions and the two tridentate binding pockets.
Schiff base ligands are commonly used as fluorescence
probes[6] due to their ease of functionalization, convenient metalcoordination and, therefore, the possibility of modifying their
optical properties. The use of tridentate Schiff bases to
modulate the oxidation state of the metal ion has recently been
reported[7a] but, although their properties in the formation of
dynamic metallosupramolecular polymers[7b] and constitutional
libraries were reported by the Lehn group[7c] almost a decade
ago, tridentate poly-hydrazone ligands have been less widely
studied.[7d,e] In the latter ligands, the presence of two or more
reactive sites (which may provide additional properties to the
molecule, such as fluorescence) provides an additional option to
tune the molecular response towards various chemical
species.[8] In this way, heterometallic transition metal complexes
can be synthesized with polydentate ligands since one
approach to isolate heteronuclear compounds is the use of
metalloligands that incorporate different and separate binding
sites in the organic skeleton or through a bridging-coordinative
mode of the donor groups.[9]
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In this study we selected H4L (Scheme 1) which is a multi-site
coordinating ligand that contains six donor atoms separated by
an aromatic ring to define two symmetrical coordination pockets.
The intramolecular hydrogen bonds associated with the salicyl
and the hydrazone groups preorganize the pockets towards a
cisoid conformation (see for example the different conformation
observed in the non-hydroxylated and in the mono-hydroxylated
ligands[10]) while the methyl group at both C=N hydrazone
groups leads the metal attack only by the same side of the
ligand molecule. Preorganization of the hydrazone groups may
play a relevant role in the stability[7c] and reactivity of the
hydrazone complexes.[9d] An additional property associated with
the presence of salicylaldimine groups is luminescence based
on the existence of an excited-state intramolecular proton
transfer mechanism (ESIPT).[11] In the work described here we
explored the coordinative behavior and luminescent response of
this ditopic hydrazone ligand[12] (H4L) towards different first-row
transition metal ions. The presence of two clearly defined
coordinative sites allowed us to explore the effect of the gradual
coordination of different ions on the luminiscence properties.
Simultaneously, the ability of the hydroxyl group to establish
bridges[13] between two metal centers allowed us to isolate
complexes with different nuclearity (Scheme 2).
<< Scheme 2 here >>
Results and Discussion
Synthesis and Characterization
H4L was synthesized by a modification of the method previously
published by Kotali[12] but using ethanol in an acidic medium as
the solvent. The symmetrical, ditopic and potentially tridentate
double-Schiff base ligand was obtained in good yield. The ligand
is stable in air and is soluble in DMF and DMSO. The ligand was
characterized by 1H-NMR, IR, EA and MS (ESI+), and its
structure was confirmed by X-ray diffraction analysis. The optical
properties (UV-Vis absorption and emission) were measured in
solution and in the solid state.
Three different types of complex based on the M/H4L (M = ZnII,
CuII, NiII and CoII) system were obtained by adjusting the
synthetic conditions (Scheme 2), although the complexes were
isolated from reactions carried out with an equimolar metal
acetate/ligand ratio (1:1:1 in heteronuclear compounds) using
methanol as solvent.
The different levels of deprotonation were achieved without
additional base under conventional (complexes of H 2L2–, i.e.,
compounds 1a, 1b, 1c and 1d) or solvothermal (complexes of
L4–, i.e., compounds 2a and 2c) conditions. Solvothermal
reactions with CuII and CoII yielded unidentified products. All of
the compounds were characterized by elemental analysis, mass
spectrometry (ESI+), infrared, diffuse reflectance, and 1H-NMR
spectroscopy in the case of the diamagnetic compounds.
Attempts to obtain single crystals of all compounds from the
mother liquor were unsuccessful, although single crystals of Zn II
complexes 1a*, 2a* and the heteronuclear complex ZnII/CuII
2ab* were obtained from DMSO solutions. The results of
spectroscopic studies (vide infra) suggest that the replacement
of the water present in the as-synthesized compounds by DMSO
in the single crystals did not lead to significant modifications in
the coordinative characteristics of the metal-ligand core.
The IR spectra of the complexes provide information about the
involvement of one or two ligand pockets in the coordination to
the corresponding metal ion. The structurally significant IR
bands for the free ligand and its complexes are listed in Table
S1. The infrared spectrum of the free ligand contains bands
centered at 1641, 1513 and 1278 cm–1, which are attributed to ν
(C=O), ν (C=N) and ν (C–N), respectively. In the IR spectrum of
the homodinuclear compounds 1a–d, two bands corresponding
to amide I (ν (C=O)) are observed. This behavior has been
related to the co-existence of coordinated and uncoordinated
carbonyl groups in other ditopic dihydrazones.[10b] By contrast,
the IR spectra of 2a and 2c each contain a single red-shifted
vibration attributed to amide I at 1627 and 1623 cm –1,
respectively, indicating that both binding pockets are
coordinated to metal centers. The diffuse reflectance spectra
contain bands attributed to d-d transitions of metal atoms (Table
S2) in 1b–d and 2c in addition to bands with CT (charge
transfer) or IL (intra-ligand) character that are common to all of
the complexes.
The thermal stability of compounds 1a–d, 2a and 2c was studied
by thermogravimetric analysis (Figures S6–S8). The compounds
are stable up to the range 250–300 ºC, at which point
decomposition takes place. The thermal behavior seems to be
independent of the di-/tetra-nuclear metallic nature. The thermal
analysis results helped us to assign the different number of
trapped solvent molecules and the conclusions were consistent
with the elemental analysis data. It was observed that all of the
compounds seem to be stabilized with different numbers of
water or methanol molecules and this is also evident in the
isolated single crystals, in which DMSO molecules are present in
all cases. However, the results do not allow the unequivocal
determination of the true role of solvents in the structure.
Consequently, the compounds were formulated as solvate
complexes although the coordination of water, for example in
compounds 1a and 2a, is very plausible.
NMR studies (on the free ligand and its zinc complexes) and
electrospray ionization (ESI) mass spectrometry were performed
on DMSO-d6 solutions due to the low solubility of the
compounds in other solvents. The 1H-NMR signals due to the –
OH and –NH groups were observed at 13.99 and 11.30 ppm,
respectively, for H4L (the atomic numbering schemes used in
this paper are shown in Scheme 4, see experimental section). In
1a these signals were shifted to slightly higher field and the
integration was half, which is consistent with the simultaneous
deprotonation of the –NH and –OH of one binding pocket. In the
spectrum of freshly prepared solutions of 2a, the absence of
these signals in the set of main signals is consistent with the
tetra-anionic character of the ligand and the coordination of two
metal centers by the two pockets. Nevertheless, very weak
signals due to traces of 1a could be detected and this suggests
an equilibrium in solution between 2a and 1a (vide infra).
Peaks in the ESI mass spectrum at 985, 983 and 974 (m/z) in 1a,
1b and 1d, respectively, can be assigned to the molecular ion
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[M2(H2L)2 – H]+, thus suggesting the robustness of the dinuclear
species even in DMSO solutions.
Bearing in mind the frequent occurrence of the di-µ-hydroxybridge core motif (M2O2) in the complexes of Schiff base ligands
derived from salicylaldehyde[13] (Scheme 2, vide infra), we
believe that the same structural motif is present in all of the
isolated structures and is a key factor in the stability of the
compounds. As a consequence, the behavior of compounds 1 is
very different to that of the asymmetric ligand recently reported
by us (only an –OH group is present in the aromatic spacer)
because coordination polymers were exclusively isolated for a
1:1 (MII/L) stoichiometry.[10b] On the other hand, the isolation of
homotetranuclear compounds (2a, 2c) under solvothermal
conditions and their reluctance to retain their discrete nature,
encouraged us to study the {Zn2(H2L)2} system as metalloligand
for the synthesis of new heterometallic complexes. This
hypothesis was reinforced by the ESI-TOF characterization of
the reaction solutions (DMSO) between 1a (formed in situ) and
equimolar amounts of MII acetates (Table S13). The results of
spectrofluorimetric studies (vide infra) on these reaction media
are consistent with the spectrometry results.
The mononuclear and homodinuclear species [M(H3L)2] and
[M2(H2L)2] (M = Zn, Co, Ni) were detected in the mass spectra
(Figure S5, Table S13) along with the heterodinuclear species
[ZnCo(H2L)2] and [ZnNi(H2L)2], although the ZnII cation seems to
be replaced by CuII and only mono- and di-copper species
[Cu(H3L)2] and [Cu2(H2L)2] were observed.
Finally, two heterotetranuclear compounds [Zn2M2(L)2].n(solv)
(M = CuII, 2ab and NiII, 2ac) were isolated from the preformed
dimeric compound [Zn2(H2L)2]. In addition, the resulting solids
were characterized by ICP-OES (inductively coupled plasma
optical emission spectrometry) and EDS (energy-dispersive Xray spectroscopy) analysis. Both techniques indicated an
equimolar metal-to-metal ratio within the bulk materials.
Interestingly, the resulting materials of the reaction with Co II
(described as 2ad in the experimental section) are quite stable
in solution since species such as |ZnCo(H2L)2+H|+ was detected
in the ESI-MS. Nevertheless, the lack agreement in the ZnII:CoII
molar ratio found by EDS (1:0.6) with ICP-OES (1:0.5) suggests
a non-homogeneous nature of the materials.
Moreover, a single shifted amide I band was observed in the IR
spectra (Table S1) and this suggests a coordination mode
involving both binding pockets. The diffuse reflectance spectra
of the heterotetranuclear complexes confirm the presence of the
two metal centers, with broad bands that are characteristic of dd transitions[14] of CuII (2ab), NiII (2ac) and CoII (2ad) (Table S2).
The bimetallic nature of this kind of compound could also be
studied by X-ray diffraction of the single crystals of 2ab*,
obtained by slow evaporation of a DMSO solution of 2ab. This
system can be modulated by exchange of the metal ion, as
observed in the crystal structure of 2ab* where replacement of
the zinc atom by copper was detected. This type of modularity is
helpful in the design of new materials because it allows the
physical properties to be tuned without changing the structure of
the material.[15]
Structural Studies
Structure of the ligand H4L·2DMSO
The molecular structure of H4L·2DMSO is represented in Figure
1 together with the atomic numbering scheme. Selected bond
distances and angles are listed in Table S4. The ligand
crystallizes with two crystallographically independent and
distorted DMSO molecules in the monoclinic P21 space group.
The molecule is almost planar: the angles between the planes
defined by the three rings are 21.99(12) ° and 27.73(13) °, with
the aromatic rings clearly located in planar segments with rms
values ranging from 0.0039 to 0.0067. The C(6)–N(6) and C(6)–
O(6) bond distances are typical of the amide resonance form
(Table S4).
Figure 1. Up: molecular structure of H4L·2DMSO showing the atomic
numbering. Down: supramolecular arrangement of H4L·2DMSO showing
details of the N–H…O, C–H…O interactions and the O–H…N intramolecular
interaction (bottom).
An interesting parameter that highlights differences between the
free ligand and its complexes is the length of the intramolecular
O–H···N hydrogen bond. The structural parameters in the
interaction O(3)–H···N(5) are indicative of an interaction
between a strong and moderate hydrogen bond[16] (Table S9) to
generate six-membered rings [rms = 0.0338 (N5a–C5a–C4a–
C3a–O3a–H3a) and 0.0498 (N5b–C5b–C4b–C3b–O3b–H3b)].
These intramolecular H-bonds are of particular interest for a
number of reasons: (i) they are probably responsible for the
planar arrangement of the molecular structure of the free
ligand;[10] (ii) they are responsible for the luminescence of H4L
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(vide infra) –a property that should be affected by the metal
coordination.
The molecular association in the crystal structure is achieved by
means of weak H-bonds involving aromatic C–H groups and
hydroxyl groups (Table S9, Figure 1) and the DMSO solvent
molecules, which act as a ‘glue’ between the ligand molecules:
as H-donors with the ligand oxygen atoms O(3) and O(6) and as
acceptors with the N(6)–H groups. These interactions are
reinforced by C(12)–H…O(s) and C(13)–H…O(s) interactions
(Table S9) in a similar way to that observed in a previously
published asymmetrical dihydrazone.[10b] In addition, C(11)–
H…O(6) and C–H…π interactions are also observed in the
supramolecular arrangement.
Structures of complexes 1a*, 2a* and 2ab*
Crystal data and selected bond distances and angles for 1a*, 2a*
and 2ab* are listed in Tables S3 and S5–S7, respectively.
Compound 2a* crystallizes in the triclinic P-1 space group and
1a* and 2ab* in the monoclinic P21/c and P21/n space groups,
respectively. Compound 1a* is a homodinuclear dimer with an
uncoordinated pocket. Compounds 2a* and 2ab*, both of which
do not have uncoordinated pockets, are homotetra- and
heterotetranuclear compounds, respectively.
The structures correspond to neutral discrete compounds in
which DMSO solvent molecules complete the coordination
sphere and also contribute to the supramolecular association by
establishing different interactions (vide infra) (Scheme 2).
All MII cations are {NO4} pentacoordinated through O(3), N(5)
and O(6) atoms of one ligand molecule to generate 5- and 6membered chelate rings and by an oxygen atom of a DMSO
molecule. The fifth position is occupied by one O(3) atom
belonging to a neighboring ligand (coordination sphere A, Figure
2) [in 1a* the Zn(1) atom of 2a* and in M(1) and M(3) in 2ab*] or
by another DMSO molecule [Zn(2) in 2a* and Cu(2) and Cu(4) in
2ab*] (Coordination sphere B).
The geometry around the metallic centers can be described as a
square pyramid since the Addison parameter (τ) [17] values are
within the range 0.008–0.355 (Table S8). The main distortion is
found in the outer zinc metal atoms coordinated to two DMSO
molecules in compound 2a*. The apical position is occupied by a
DMSO molecule in the three structures with M–O distances in
the range 2.016–2.461 Å for M = Zn and 2.045–2.236 Å for M =
Cu.
In the homodinuclear compound 1a* the ligand is bideprotonated
to form a dianionic binding site in one pocket, which leads to a
dimeric structure in which two metal-ligand units are linked by a
µ-phenolate-bridged Zn2O2 core. In 2a*and 2ab* the ligand is
tetradeprotonated and adopts the µ-κ3O,N,O´:κO´ and κ3O,N,O´
coordination modes. The bridging coordinative behavior of the
O(3) atom allows the formation of M2O2 cores. This core is a
common structural motif in all of the structures. The Zn···M (M =
ZnII or CuII) distance is 3.102 Å, 3.184 Å and 3.058 Å in 1a*, 2a*
and 2ab*, respectively.[13] In all cases this core is coplanar with
the planar ligand binding pocket. This structural feature has
been observed previously in aroylhydrazone complexes with
M···M distances in a similar range (3.058–3.184 Å).[13b,18] This
unit is always composed of zinc atoms except in 2ab*, where the
structural disorder in the central core was interpreted as a partial
replacement of zinc atoms (using EADP Shelxl instructions [19])
from the initial [Zn2(H2L)2] precursor by copper atoms to
generate ((Cu/Zn)2-O2) units, whereas the two external pockets
are exclusively occupied by copper atoms.
Figure 2. Schematic representation and main structural differences in
complexes 1a*, 2a* and 2ab*.
The ligand moiety retains its planarity after coordination in 2ab*
(29.18°, 5.04°, 6.38° and 30.34°), whereas in 1a* and 2a* the
terminal phenyl rings are rotated and this planarity is broken
(14.98°, 27.66°, 54.41° and 2.02° for 1a* and 14.45° and 57.59°
for 2a*). In 1a* and 2a*, the ligand moieties are arranged in a
parallel manner (the angles between the planes formed by the
central aromatic ring of both ligand molecules is 9.09° and 0°,
respectively). By contrast, in 2ab* the ligands form an angle of
50.35°, which is similar to the H-bond angle N(6b)–H–O(6b)
established in 1a*, vide infra.
Coordination prevents the formation of the O–H…N
intramolecular H-bonds observed in the free ligand and in the
uncoordinated pocket of 1a*. Moreover, the deprotonation of NH
groups upon coordination implies the presence of the enol form
in the carboxamide fragments in the coordinated pockets, as
deduced from the bond distances (Tables S5–S7).
The protonated nature of the –NH group in the uncoordinated
pocket of 1a* allows the interaction with the oxygen atom of a
DMSO molecule of a neighboring unit. The –NH and –CH
groups of the ligand act as donors and the oxygen atom of the
DMSO molecule acts as an acceptor in this interaction (Figure 3).
This situation leads to the formation of the stable synthon
observed in the free ligand and in a previous study with a related
asymmetrical ligand (Table S10).[10b] Zig-zag chains are formed
by C(10b)–H…O(6d) interactions along the crystallographic b
axis. These chains are interconnected in a double strand by the
supramolecular N(6b)–H…O(6b) synthon and this arrangement
is reinforced by C–H…π interactions (Figure 3, Table S10).
These double-strand chains are interconnected by means of
N(6d)–H…O(2s) interactions with the DMSO molecules.
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Figure 3. Supramolecular association in 1a* showing details of the
supramolecular synthon.
Similar hydrogen bonding interactions, based on a donor N–H
group, are not possible due to the deprotonated character of the
ligand in 2a* and 2ab*. Indeed, in these complexes nitrogen
atoms act as acceptors and establish interactions with the C–H
groups [C(11b)–H and C(43s)–H] in 2a*. Moreover, C(9b)–
H…O(2s) interactions established with DMSO solvent molecules
are observed (Figure S1, Table S11). In 2ab*, DMSO molecules
(coordinated and uncoordinated) are responsible for the
supramolecular
association
through
DMSO···DMSO,
DMSO···O(6) and C(9)–H···DMSO interactions (Figure S1,
Table S12).
Fluorescence studies
The two intramolecular O–H…N hydrogen bonds associated
with the ketimine groups of the ligand H4L encouraged us to
explore the luminescence properties, since this intramolecular
hydrogen bond allows triggered emission through an excited
state intramolecular proton transfer mechanism (ESIPT) (Figure
S2).[11,16,20] The ESIPT process involves the transfer of the
hydroxyl proton to an imine nitrogen located less than 2 Å away
through a pre-existing six-membered ring hydrogen bonding
configuration.[21] ESIPT dyes generally have large Stokes shifts
and are ideal candidates for use as fluorescence labels.[22]
The photophysical properties of H4L were studied at room
temperature in DMSO solution, due to the lack of solubility in
other solvents, and in the solid state (Figure 4) in order to
confirm the structural integrity in solution.
Three absorption bands were observed in the spectrum of H4L in
DMSO solution: two intense bands at 295 and 350 nm and a
weak absorption at 420 nm. The solid diffuse reflectance
spectrum of H4L exhibits intense absorptions at λ max 260 and
335 nm. The highest energy band is probably due to a π→π*
transition of aromatic rings while the intense band above 300 nm
is assigned to the n→π* transition of the hydrazone
(azomethine) chromophore. The latter band (which is usually
assigned to π-π* absorptions associated with the C=N
azomethine bond[7a]) in the solid state has a tail at around 400
nm while there is a clear band in the solution spectrum with a
maximum at 420 nm. This bathochromic shift is interesting
because this band is associated with the emission: upon
excitation at 430 nm (and 350 nm), the emission bands in solid
state and in solution are centered at λ max 535 and 500 nm,
respectively.
The differences between the two states are probably due to
different intermolecular interactions involving the solvent
(DMSO) in the ground and excited states. However, it is not
easy to establish correlations between the two states because
the intrinsic radiative decay pathway is often disturbed by
intermolecular contacts in the aggregate or the solid state.[23]
Gradual addition of NaOH to the H4L solution led to an increase
in the intensity of the absorption band around 430 nm (Figure
S3A). More interestingly, although the intensity of the emission
(along with splitting of the band) was initially observed, after
reaching the maximum intensity at a 1:1 (H4L:NaOH) ratio the
emission was finally quenched when a 1:2 molar ratio was
reached (Figure S3B). Although the first process can be
attributed to the improved protonation transfer in the H 3L–
species as a result of resonance stabilization induced by the
deprotonated pocket similarly to the effects were observed by
Xu and Pang in bis(benzoxazole) derivatives,[24] the small value
of the Stokes shift makes doubt of the true origin of this emission
in H3L-. The final quenching at a 1:2 (H4L:NaOH) ratio suggests
the formation of H2L2– by deprotonation of both phenolic OH
groups – a situation that is consistent with the higher acidity
observed for this group in the salicylaldimine hydrazones.
The addition of M(AcO)2 (M = Zn, Co, Cu and Ni) to H4L in
DMSO at r.t. also caused changes in the absorption spectra
(Figures 5 and 6). The addition of the MII cation led to a
progressive increase in the intensity of the band at 420 nm and
this change was accompanied by a decrease in the absorption
bands at 295 and 350 nm along with the emergence of a new
band at around 320 nm. The presence of an isosbestic point in
all experiments (at 375 nm) suggests the formation of new
species during the additions.
Figure 4. Absorption (left) and emission (right) spectrum at r.t. of H4L in the
solid state (solid line) and in DMSO solution (dashed line; [H4L] = 1·10–5 M).
λexc = 430 nm (black lines); λexc = 350 nm (grey lines).
It was possible to distinguish two kinds of fluorescence response
of H4L depending on the electronic nature of the M II cations. The
addition of increasing amounts of CoII, NiII and CuII led to a
quenching in the emission intensity, which was total when a 1:1
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molar ratio was reached (Figure 5). This feature can be related
to the paramagnetic nature of the metal atoms, that plays a
relevant role in the quenching process.[25] On the other hand,
this quenching occurs at the equimolar point; therefore, the
formation of a coordination polymer of the type [M(H2L)]n.[10b]
cannot be ruled out.
The behavior was completely different when ZnII was used.
Fortunately, the diamagnetic nature of this cation allowed the
reaction in DMSO-d6 to be monitored by 1H-NMR spectroscopy
(Figure S4). In this experiment signals were observed for the O–
H and N–H groups (13.49 and 11.12 ppm, respectively) at a 1:1
(H4L:ZnII) molar ratio and only when a 1:2 was reached were
these signals absent.
Spectrofluorometric monitoring of the reaction showed a marked
increase in the emission intensity, i.e., around a 60-fold
enhancement, upon addition of one equivalent of zinc acetate
(Figure 6). The ESI-TOF mass spectra of the reaction mixtures
at this molar ratio contained signals due to [Zn 2(H2L)2]+ and
[Zn(H2L)]+, which suggests the predominance of species based
on 1a. The solid state fluorescence spectrum of 1a displays a
broad band centered at 475 nm and a shoulder at 535 nm
(Figure 6) and this matches reasonably well with the spectra of
the ZnII/H4L solutions in a 1:1 molar ratio. Once again,
deprotonation, and probably coordination in the present case, of
one of the pockets seems to increase the basic character of the
nitrogen atom in the other pocket, thus improving the
emission.[26]
<< Figure 5 here >>
The addition of more zinc acetate caused changes in the
absorption and emission spectra, which is consistent with the
interaction of the second ZnII with the free pocket of the species
1a formed in solution. An interesting aspect of this reaction is
that although the fluorescence intensity decreases when more
ZnII is added, it is still higher compared with that of the free
ligand [25 fold enhancement when a 1:2 (H4L:ZnII) molar ratio is
reached]. Although the NMR spectrum of this medium showed
only very small signals for the –OH proton, the better sensitivity
of fluorescence spectroscopy provided evidence for the
existence of a protonation-chelation equilibrium in solution on
the second pocket (Scheme 3).
The formation of heterotetranuclear [Zn2M2(L)2] complexes from
[Zn2(H2L)2] was also monitored by spectrofluorimetric techniques.
In these studies, zinc acetate was added to H4L until a 1:1 molar
ratio was reached and increasing amounts of the other M II
acetate (M = Cu, Ni, Co) were then added (Figure 7).
The initial increase in the emission intensity is consistent with
the formation of [Zn2(H2L)2] at a 1:1 molar ratio and this was
followed by a gradual decrease in fluorescence on addition of M II
until the molar ratio was 1:1:1 (L:Zn:M). When copper or cobalt
acetates were added a plateau appeared in the spectrum and
fluorescence quenching was observed. It is noteworthy that the
point at which the fluorescence is switched off was only reached
at a 1:1:3 ratio when nickel acetate was added.
Figure 6. Spectrophotometric and fluorimetric monitoring of the reaction of
H4L with ZnII (ldown) in DMSO at r.t. ([H4L] = 1·10–5 M, [ZnII] = 1·10–2 M).
Fluorimetric measurement of H4L (black), 1a (gray) and 2a (gray dashed line)
at r.t. in the solid state (up). λexc = 430 nm.
FULL PAPER
Scheme 3. Representation of the ESIPT mechanism of 1a and the equilibrium
between 1a and 2a in solution.
The switch-off in fluorescence emission can be attributed to the
replacement of the Zn by Cu or Co (with behavior analogous to
that observed in the direct reaction of H4L and CuII or CoII) or to
the formation of 1:1:1 (L:Zn:M) complexes, where the
paramagnetic nature of M may influences in final quenching. In
agreement with the latter hypothesis, signals attributable to
heterodinuclear species were observed in the ESI-TOF spectra
of the H4L/Zn/Co reaction medium but they were absent in the
H4L/Zn/Cu system. Consequently, a different behavior of
[Zn2(H2L)2] in conjunction with CoII or CuII cannot be ruled out.
Nevertheless, the case of NiII warrants further consideration
because: (i) the ESI-TOF spectra of the reaction media display
signals due to Ni/Zn species (Figure S5); (ii) in the absence of
Zn, the reaction with the free ligand led to quenching of the
fluorescence (vide supra); (iii) in the presence of [Zn 2(H2L)2]
additional overconcentration is necessary to quench the
fluorescence. In fact, the behavior in this case was similar to that
observed in the reaction of [Zn2(H2L)2] with ZnII.
<< Figure 7 here >>
Conclusions
The nature of the symmetrical and multi-site coordinating
hydrazone ligand H4L allowed the isolation, from different
reactions with a variety of divalent metal atoms, of three different
kinds of complexes: homodinuclear, homotetranuclear and
heterotetranuclear compounds. The structural parameters of
these three types of compounds in the solid state were analyzed
by X-ray diffraction. It was demonstrated that the fluorescence
behavior of H4L depends on the coordination of the {ONO}
binding pockets because this emission is based on an excited
state intramolecular proton transfer (ESIPT) mechanism.
Deprotonation of one of the pockets on addition of a base or by
coordination of ZnII gives rise to an extraordinary enhancement
in the emission intensity and only if deprotonation of the second
pocket is achieved is the emission finally quenched. The results
of ESI-TOF mass spectrometry studies on the zinc-containing
reaction media suggest the presence of dimeric species and the
X-ray
structures
of
[Zn2(H2L)2(DMSO)2]
(1a*)
and
[Zn4(L)2(DMSO)6] (2a*) confirm this hypothesis. In both cases,
the association of the two units is based on the diamond-shaped
Zn2O2 core formed by the bridging phenolate groups.
The fluorimetric response of the ligand H4L towards CoII, NiII and
CuII is completely different because a gradual decrease in the
emission intensity was observed on addition of the metal.
Although the formation of polymeric compounds in which both
binding pockets are used in the metal coordination cannot be
ruled out, ESI-TOF studies on the reaction media suggest the
formation of compounds based on dimeric [M2(H2L)2] units.
The ability of the fragment {Zn2(H2L)2} to act as a metalloligand
was also tested against MII and several heterotetranuclear
compounds were isolated. The reactions were monitored by
ESI-TOF MS, with peaks due to heterodinuclear species
observed, and by analysis of the spectrophotometric and
fluorimetric response. The fluorimetric responses of {Zn 2(H2L)2}
(which is different to that of H4L) towards the different MII cations
were not identical. The fluorescence was completely quenched
at 1:1:1 (H4L:Zn:M) ratios when M = CoII and CuII, whereas an
excess of NiII was necessary to achieve quenching. Although
these results suggest that the reacting metal cations interact
with the free pocket of the metalloligand, the isolation of single
crystals of [Zn0.45Cu3.55(L)2(DMSO)6]·2DMSO (2ab*) proves that
replacement of the metal in the original Zn2O2 diamond is also
possible. The bimetallic nature of these materials could also be
analyzed in the solid state by EDS and OIC-PC techniques.
Experimental Section
Materials and physical measurements
Metallic salts and solvents were obtained commercially and were used as
supplied. Elemental analyses (C, H, N) were carried out on a Fisons EA1108 microanalyser. IR spectra were recorded from KBr discs (4000–400
cm–1) on a Bruker Vector 22 spectrophotometer. 1H NMR spectra were
obtained with a Bruker AMX 400 spectrometer. Mass spectra were
recorded on a Hewlett-Packard 5989A spectrometer. TGA was
performed on a SETSYS Evolution Setaram thermogravimetric analyser
in a flow of N2 with a heating rate of 10 ºC min–1. UV–Vis absorption and
emission spectra were obtained on HP 8453 and Horiba-Jobin–Yvon
Fluoromax-3 TCSPC spectrophotometers in DMSO at r.t.; excitation and
emission slits of 2.0 or 5.0 nm were used for the measurements.
Spectrofluorimetric measurements in the solid state were collected on a
Jasco-FP-8300 spectrofluorimeter with excitation and emission
bandwidths of 5 nm. Determination of metals [Co(II), Ni(II), Cu(II), Zn(II)]
in heterometallic compounds was carried out with a Perkin-Elmer Optima
4300 DV Optical Inductively Coupled Plasma Spectrometer (OIC-PC).
The heterometallic nature of the compounds was also confirmed with a
JEOL JSM6700F Field Emission Scanning Electron Microscope (FE-
FULL PAPER
SEM) equipped with an Oxford Inca Energy SEM 300 X-ray detector
(EDS).
Synthesis of the compounds
Synthesis of H4L. A warm colorless solution of 4,6-diacetylresorcinol
(0.1 g, 0.51 mmol) in EtOH (20 mL) was added dropwise to a solution of
benzoic hydrazide (0.208 g, 1.53 mmol) in EtOH (20 mL). Immediately, 1
drop of HCl(c) was added. The colorless solution was heated under reflux
for 2 h and the resulting white solid was filtered off and vacuum dried
over CaCl2/NaOH. Suitable crystals of H4L·2DMSO were obtained after
storing a DMSO solution at r.t. for 20 days.
Data for H4L: Yield: 0.190 g (86%). 1H-NMR (400 MHz, [D6]DMSO,
25ºC): δ= 13.99 (s, 2H, O3-H), 11.30 (s, 2H, N6-H), 7.94 (d, J = 7.2 Hz,
4H, C8-H + C12-H), 7.83 (s, 1H, C2-H), 7.63 (t, J = 7.3 Hz, 2H, C10-H),
7.53 (t, J = 7.4 Hz, 4H, C9-H + C11-H), 6.37 (s, 1H, C1-H), 2.55 ppm (s,
6H, C13-H). IR (KBr): ν = 3434 + 3224m (OH-NH), 1641vs (C=O), 1513m
(C=N), 1278s cm−1 (C–N). MS-ESI: m/z (%): 431 (100) [H5L+]. Elemental
analysis calcd (%) for C24H22N4O4: C, 66.97; H, 5.15; N, 13.02; found: C,
66.65; H, 5.17; N, 12.97.
Data for 1b, [Cu2(H2L)2].4(H2O) (Green): Yield: 0.109 g (45%). IR (KBr):
ν = 3438vs (OH–NH), 1646m + 1618vs (C=O), 1517vs (C=N), 1267s
cm−1 (C–N). MS-ESI: m/z (%): 431 (6) [H5L]+, 492 (7) [Cu(H3L)]+, 570
(100) [Cu(H3L)(DMSO)]+, 631 (27) [Cu2(HL)(DMSO)]+, 922 (27) [Cu(H3L)2
+ H]+, 983 (11) [Cu2(H2L)2 + H]+. Elemental analysis calcd (%) for
C48H40N8O8Cu2.4H2O: C, 54.59; H, 4.58; N, 10.61; found: C, 54.64; H,
4.23; N, 10.48.
Data for 1c, [Ni2(H2L)2].5(H2O) (Dark orange): Yield: 0.108 g (42%). IR
(KBr): ν= 3401m (OH–NH), 1622sh + 1592s (C=O), 1524vs (C=N),
1275s cm−1 (C–N). MS-ESI: m/z (%): 431 (29) [H5L]+, 565 (96)
[Ni(H3L)(DMSO)]+, 917 (100) [Ni(H3L)2 + H]|+. Elemental analysis calcd
(%) for C48H40N8O8Ni2.5H2O: C, 54.17; H, 4.74; N, 10.53; found: C,
54.55; H, 4.30; N, 10.33.
Data for 1d, [Co2(H2L)2].8(H2O) (Dark brown): Yield: 0.110 g (43%). IR
(KBr): ν= 3417m (OH–NH), 1625m + 1577vs (C=O), 1521vs (C=N),
1276m cm−1 (C–N). MS-ESI: m/z (%): 431 (7) [H5L]+, 487 (7) [Co(H3L)]+,
566 (27) [Co(H3L)(DMSO)]+, 916 (6) [Co(H3L)2 + H]+, 974 (11) [Co2(H2L)2
+ H]+. Elemental analysis calcd (%) for C48H40N8O8Co2.8H2O requires: C,
51.53; H, 5.04; N, 10.01; C, 51.22; H, 4.46; N, 9.60.
Synthesis of homotetranuclear complexes of general formula
[M4(L)2].n(solv): 2a (ZnII) and 2c (NiII)
The reaction in solvothermal conditions (heating at 110 ºC for 24 h
followed by a slow cooling to r.t. at a rate of 0.60 ºC/h) of H4L (0.1 g, 0.23
mmol) and M(AcO)2·nH2O (0.22 mmol) in MeOH (40 mL) afforded
homogeneous precipitates in both syntheses. The solids were filtered off
and vacuum dried over CaCl2/NaOH.
Scheme 4. Schematic representation of the ligand and its complexes
showing the atomic labeling used.
Synthesis of homodinuclear complexes of general
[M2(H2L)2].nH2O: 1a (ZnII), 1b (CuII), 1c (NiII) and 1d (CoII)
Data for 2a, [Zn4(L)2].2(H2O) (Yellow): Yield: 0.058 g (21%). 1H-NMR
(400 MHz, [D6]DMSO, 25ºC): δ= 8.20–7.38 (m, 12H, C1-H + C2-H, C8-H
+ C9-H + C10-H + C11-H + C12-H), 2.79–2.67 ppm (m, 6H, C13-H). IR
(KBr): ν= 3430s (OH), 1627s (C=O), 1539s (C=N), 1259m cm−1 (C–N).
MS-ESI: m/z(%): 431 (9) [H5L]+, 493 (11) [Zn(H3L)]+, 571 (14)
[Zn(H3L)(DMSO)]+, 730 (11) [Zn2(HL)(H2O)(DMSO)2]+, 923 (15) [Zn(H3L)2
+ H]+. Elemental analysis calcd (%) for C48H36N8O8Zn4.2H2O: C, 50.11; H,
3.50; N, 9.74; found: C, 50.39; H, 3.87; N, 9.61.
formula
2a*, [Zn4(L)2(DMSO)6]
A solution of M(AcO)2·nH2O (0.22 mmol) in MeOH (10 mL) was added to
a suspension of H4L (0.1 g, 0.23 mmol) in MeOH (20 mL). The
suspension was heated under reflux for 2 h and the resulting solid was
filtered off and vacuum dried over CaCl2/NaOH.
1H-
Data for 1a, [Zn2(H2L)2].2(H2O) (Dark yellow): Yield: 0.102 g (43%).
NMR (400 MHz, [D6]DMSO, 25ºC): δ= 13.43 (s, 1H, O3a-H), 11.09 (s, 1H,
N6a-H), 8.09 (s, 2H, C8a-H + C12a-H), 7.94 (s, 2H, C8b-H + C12b-H),
7.71 (s, 1H, C2-H), 7.61–7.40 (m, 6H, C9a-H + C9b-H + C10a-H + C10bH + C11a-H + C911-H), 6.03 (s, 1H, C1-H), 2.73 ppm (s, 6H, C13a-H +
C13b-H). IR (KBr): ν = 3442, 3226m (OH–NH), 1641vs + 1625vs (C=O),
1508vs (C=N), 1276s cm−1 (C–N). MS-ESI: m/z (%) = 431 (29) [H5L]+,
571 (10) [Zn(H3L)(DMSO)]+, 923 (30) [Zn(H3L)2 + H]+, 985 (17) [Zn2(H2L)2
+ H]+. Elemental analysis calcd (%) for C48H40N8O8Zn2.2H2O requires: C,
56.32; H, 4.33; N, 10.95; found: C, 56.29; H, 4.68; N, 10.88.
1a*, [Zn2(H2L)2(DMSO)2]
The DMSO solvate of this complex was obtained as single crystals by
slow evaporation (1 month) at r.t. of a DMSO solution of 1a.
The DMSO solvate of this complex was obtained as single crystals by
slow evaporation (1 month) at r.t. of a DMSO solution of 2a.
Data for 2c, [Ni4(L)2].2(MeOH).4(H2O) (Brown): Yield: 0.056 g (23%). IR
(KBr): ν= 3413m (OH), 1623s (C=O), 1536m (C=N), 1260m cm−1 (C–N).
MS-ESI: m/z(%): 431 (4) [H5L]+, 543 (44) [Ni2(HL)]+, 917 (4) [Ni(H3L)2 +
H]+. Elemental analysis calcd (%) for C48H36N8O8Ni4.2MeOH.4H2O: C,
49.07; H, 4.28; N, 9.16; found: C, 49.02; H, 4.51; N, 9.51.
Synthesis of heterotetranuclear complexes of general formula
[Zn2M2(L)2].n(solv): 2ab (M = CuII), 2ac (M = NiII) and 2ad (M = CoII)
A solution of Zn(AcO)2.2H2O (0.056 g, 0.26 mmol) in MeOH (10 mL) was
added to a suspension of H4L (0.114 g, 0.26 mmol) in the same solvent
(20 mL). The yellow suspension was heated under reflux for 2 h. The
suspension was cooled down to r.t. and then a solution of 0.26 mmol of
M(AcO)2.nH2O (0.057 g of Co(AcO)2.4H2O, 0.045 g of Cu(AcO)2.H2O or
0.064 g of Ni(AcO)2.4H2O) in MeOH (10 mL) was added. The mixture
was heated under reflux for 2 h. The resulting solids were filtered off and
dried under vacuum over CaCl2/NaOH.
FULL PAPER
Data for 2ab, [Zn2Cu2(L)2].2(H2O) (Pale green): Yield: 0.132 g (41%). IR
(KBr): ν = 3446m (OH), 1590s (C=O), 1564s (C=N), 1266m cm−1 (C–N).
MS-ESI: m/z(%): 431 (5) [H5L]+, 570 (69) [Cu(H3L)(DMSO)]+, 631 (9)
[Cu2(HL)(DMSO)]+, 922 (100) [Cu(H3L)2 + H]+. Elemental analysis calcd
(%) for C48H36N8O8Zn2Cu2·2H2O: C, 50.27; H, 3.52; N, 9.77; Cu, 10.47;
Zn, 10.63; found: C, 50.16; H, 3.91; N, 9.65; Cu, 11.08; Zn, 11.40.
2ab*, [Zn0.45Cu3.55(L)2(DMSO)6]·2DMSO
The DMSO solvate of this complex was obtained as single crystals by
storing a DMSO solution of 2ab at r.t. for 1 month. Insufficient crystals
were obtained for elemental analysis or a meaningful estimation of yield.
Data for 2ac, [Zn2Ni2(L)2].4(H2O).2(MeOH) (Dark yellow): Yield: 0.122 g
(38%). IR (KBr): ν= 3411m (OH), 1594s (C=O), 1565s (C=N), 1281m
cm−1 (C–N). MS-ESI: m/z(%): 431 (24) [H4L]+, 565 (83) [Ni(H3L)(DMSO)]+,
571 (16) [Zn(H3L)(DMSO)]+, 917 (70) [Ni(H3L)2 + H]+, 923 (19) [Zn(H3L)2
+ H]+. Elemental analysis calcd (%) for C48H36N8O8Zn2Ni2.4H2O.2MeOH:
C, 48.54; H, 4.24; N, 9.06; Ni, 9.49; Zn, 10.57; found: C, 48.68; H, 4.00;
N, 8.76; Ni, 9.41; Zn, 9.05.
Data for 2ad, [Zn2Co2(L)2].(H2O).(MeOH) (Brown): Yield: 0.087 g (27%).
IR (KBr): ν= 3422m (NH), 1585s (C=O), 1518s (C=N), 1273m cm−1 (C–N).
MS-ESI: m/z(%): 431 (4) [H5L]+, 493 (11) [Zn(H3L)]+, 918 (92) [Co(H3L)2 +
H]+, 923 (100) [Zn(H3L)2 + H]+, 980 (10) [ZnCo(H2L)2 + H]+, 985 (7)
[Zn2(H2L)2 + H]+.
Acknowledgements
Financial support from the Spanish Ministry of Education and
Science
(project
CTQ2010-19386/BQU)
is
gratefully
acknowledged. We acknowledge the Spanish BM16 beamline
(Dr. A. Labrador) facility at the ESRF (Grenoble, France) for
provision of synchrotron radiation beam time under project 1601-747. A. B. L. thanks the Xunta de Galicia for a postdoctoral
contract under the “ÁngelesAlvariño” program.
Keywords: Hydrazone • luminescence • copper • zinc • nickel
[1]
[2]
[3]
[4]
[5]
[6]
Crystallography
Crystallographic data for H4L·2DMSO, 1a* and 2a* were collected at 100
K and with a λ value of 0.7513 Å in the ESRF synchrotron Spanish
BM16-CRG beamline (Grenoble, France). Data were indexed, integrated
and scaled using the HKL2000 program.[27] Absorption corrections were
not applied. Data for 2ab* were collected at 100 K on a microfocal Bruker
Smart 6000 CCD diffractometer using graphite monochromated Cu-Kα
radiation (λ = 1.54178 Å) and were corrected for Lorentz and polarization
effects. The frames were integrated with the Bruker SAINT[28] software
package and absorption correction was applied using the program
SADABS.[29]. The max/min transmission factors were 0.753/0.557.
[7]
[8]
[9]
The structures were solved by direct methods using the program
SHELXS97.[30] All non-hydrogen atoms were refined with anisotropic
thermal parameters by full-matrix least-squares calculations on F2 using
the program SHELXL2013/2.[19]
[10]
Hydrogen atoms were inserted at calculated positions and constrained
with isotropic thermal parameters, except for the hydrogen atoms of the –
NH and –OH groups, which were located from difference Fourier maps in
H4L·2DMSO and 1a*.
In 2ab*, the AEDP command was used to refine the metal center
positions, giving rise to 90% and 65% occupancy for Cu(1) and Cu(3),
and 10% and 35% for Zn(1) and Zn(3), respectively. Drawings were
produced with MERCURY[31] and special computations for the crystal
structure discussions were carried out with PLATON.[32]
The structural data have been deposited with the Cambridge
Crystallographic Data Centre (CCDC) with the reference numbers
included in Table S3. Selected bond lengths and angles and hydrogen
bond distances for the crystal structures are listed in Tables S4–S12.
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Scheme 2. Schematic representation of the compounds isolated and those characterized by X-ray diffraction (bottom). Some of the DMSO molecules are omitted
for clarity.
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Figure 5. Spectrophotometric (top) and fluorimetric (bottom) monitoring of the reactions of H4L with CoII (left), NiII (middle) and CuII (right) cations in DMSO at r.t.
[H4L] = 1·10–5 M, [MII] = 1·10–2 M. λexc= 430 nm.
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Figure 7. Top: schematic representation of the coordination sequence. Bottom: spectrophotometric (top) and fluorometric (bottom) monitoring of the reactions of
H4L (black lines) with ZnII (gray solid lines) to a 1:1 molar ratio and then with CoII (A), NiII (B), CuII (C) cations (gray dashed lines) in DMSO at r.t. ([H4L] = 1·10–5 M,
[MII] = 1·10–2 M, λexc = 430 nm).
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Reactions of bis(benzoylhydrazone)
of 4,6-diacetylresorcinol with M2+ (M =
Zn, Cu, Ni, Co) yield three kinds of
complexes: homodinuclear [M2(H2L)2],
homotetranuclear
[M4(L)2]
and,
heterotetranuclear
compounds
[Zn2M2(L)2]. The ligand luminescence
properties
and
its
fluorimetric
behavior towards MII metals (M = Zn,
Cu, Ni and Co) has been studied.
Sabina Rodríguez-Hermida, Ana B.
Lago, Rosa Carballo, Oscar Fabelo and
Ezequiel M. Vázquez-López*
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Homo- and Heteronuclear
Compounds with a Symmetrical BisHydrazone Ligand: Synthesis,
Structural Studies and Luminescent
Properties
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Author(s), Corresponding Author(s)*
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Title
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