New molecular architectures by aggregation of tailored zinc(ii) Schiff-base complexes

Article abstract of DOI:10.1039/c1nj20618d

This contribution reports the synthesis, characterization, ¹H NMR, optical absorption and fluorescence properties of a bis(salicylaldiminato) Zinc(ii) Schiff base complex, 1, having dipodal, alkyl ammonium bromide derivatized side chains in the salicylidene rings. The comparison of ¹H NMR and optical spectroscopic properties of 1 with those of the closely related amphiphilic alkyl derivative allowed getting insights into the nature of species in solutions. Present data suggest the existence of Zn...Br interactions, thus avoiding intermolecular Zn...O interactions, as usually occur in the absence of other coordinating species. SEM images of samples obtained by drop-casting from dichloromethane solutions of 1 reveal the existence of branched nanostructures, consistent with intermolecular Zn...Br interactions. The present approach thus suggests a new method to unique supramolecular architectures for coordination metal complexes.

Full text of DOI:10.1039/c1nj20618d

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PAPER
New molecular architectures by aggregation of tailored zinc(II)
Schiff-base complexesw
Ivan Pietro Oliveri,a Salvatore Failla,b Graziella Malandrinoa and Santo Di Bella*a
Received (in Victoria, Australia) 14th July 2011, Accepted 14th September 2011
DOI: 10.1039/c1nj20618d
1
This contribution reports the synthesis, characterization, H NMR, optical absorption and
fluorescence properties of a bis(salicylaldiminato)Zinc(II) Schiff base complex, 1, having dipodal,
alkyl ammonium bromide derivatized side chains in the salicylidene rings. The comparison of
1H NMR and optical spectroscopic properties of 1 with those of the closely related amphiphilic
alkyl derivative allowed getting insights into the nature of species in solutions. Present data
suggest the existence of ZnÁ Á ÁBr interactions, thus avoiding intermolecular ZnÁ Á ÁO interactions, as
usually occur in the absence of other coordinating species. SEM images of samples obtained by
drop-casting from dichloromethane solutions of 1 reveal the existence of branched nanostructures,
consistent with intermolecular ZnÁ Á ÁBr interactions. The present approach thus suggests a new
method to unique supramolecular architectures for coordination metal complexes.
Introduction
Tetracoordinated ZnII complexes, in particular Schiff-base
derivatives, are Lewis acidic species1 which saturate their
coordination sphere by coordinating a large variety of neutral
substrates such as alcohols, carbonyls, and nitrogen based
donors Lewis bases,1–3 including various anions,4 or in their
absence, can be stabilized through intermolecular ZnÁ Á ÁO axial
coordination involving the phenolic oxygen atoms of the ligand
framework.5 Thus, a variety of molecular architectures,6
supramolecular assemblies,7 columnar mesomorphic,8 and
nanostructures,9 have been found. Schiff-base ZnII complexes have
Chart 1
salicylidene rings, bearing an alkyl ammonium bromide
1
(Chart 1), and studied its H NMR and optical spectroscopy
recently been investigated also for their variegated fluorescent
features, related to the structure of the salen template10 and the
axial coordination,2–4,10,11 and second-order nonlinear optical
properties.12
properties in solution. Actually, the bromide ion is expected to
be a suitable Lewis base to coordinate the ZnII ion, competing
with the phenolic oxygen donor atoms of salicylaldehyde.
Thus, a focus of the present study will be to investigate the
aggregation properties of this derivatized alkyl-ammonium
bromide complex in comparison to our previous studies on
the analogous simpler amphiphilic species, 2.2
The demand for the axial binding to the ZnII ion to saturate its
coordination sphere in these complexes allows for the design of
new molecular architectures, for example, using interconnecting
ditopic Lewis donor7c or anionic7a species. Alternatively, the
appropriate design of ligands possessing flexible Lewis donor
atoms as side substituents, suitable to axially coordinate the ZnII
atom of another molecular unit, offers the advantage to achieve
new tailored ZnII supramolecular architectures.
Results and discussion
With these ideas in mind, we have synthesized a ZnII Schiff
base complex, 1, having dipodal alkyl side chains in the
We have recently demonstrated that the closely related
amphiphilic 4-(undec-10-enyloxy)-derivatized ZnII Schiff base
complex, 2, forms aggregates in solutions of non-coordinating
solvents, e.g., dichloromethane (DCM). The switching to the
monomer can be driven by addition of a coordinating species
and involves substantial optical variations and a dramatic
enhancement of the fluorescence emission. As this process
occurs because of the axial coordination to the ZnII ion, it
has been found that it is selective and sensitive depending upon
a Dipartimento di Scienze Chimiche, Universita` di Catania,
I-95125 Catania, Italy. E-mail: sdibella@unict.it
b Dipartimento di Ingegneria Industriale e Meccanica,
`
Universita di Catania, I-95125 Catania, Italy
w Electronic supplementary information (ESI) available: Additional
1H NMR data, UV/vis absorption and fluorescence titrations, SEM
images. See DOI: 10.1039/c1nj20618d
c
2826 New J. Chem., 2011, 35, 2826–2831
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Fig. 1 UV/vis absorption and fluorescence (lexc = 564 nm) spectra of
1 in DMSO (5.0 Â 10À6 M) (
), DCM (TT) and THF (
). Absorption
Scheme 1 Reagents and conditions: (i) Br(CH2)12Br, K2CO3, CH3CN,
reflux; (ii) N(CH3)3, EtOH, 50 1C; (iii) diaminomaleonitrile,
Zn(OAc)2Á2H2O, EtOH, reflux.
spectra in DCM and THF were recorded with an absorbance of the longer
wavelength band equal to that recorded in DMSO.
coordination sphere of the ZnII ion and avoiding intermolecular
ZnÁ Á ÁO interactions, as usually occur in the absence of other
coordinating species.5
the Lewis basicity of the coordinating species. On the other
1
hand, H NMR and optical spectroscopy properties of 2 in a
solution of coordinating solvents suggest the existence of
monomeric species having the solvent axially coordinated.
These properties are superimposable to those achieved in
non-coordinating solvents upon axial coordination with a
Lewis base.2
To verify this hypothesis, we performed a series of experiments
using DCM solutions of 2, as reference species, by adding DCM
solutions of a bromide, and compared the results with those
achieved for the complex 1.
The synthesis of the dipodal alkyl ammonium-derivatized
complex 1 was accomplished in a three step approach
(Scheme 1).
As previously found for treatment of DCM solutions of
2 with neutral Lewis bases,2 the addition of a stoichiometric
amount of tetrabutylammonium (TBA) bromide to a DCM-d2
solution of 2 results in a sizable down-field shift of aromatic H3
and the OCH2– signals, in agreement with the deaggregation
process and formation of the 2ÁBrTBA adduct (Fig. 2).
1H NMR spectra of 1 in a solution of DMSO-d6 show the
presence of sharp signals with the expected multiplicity,
according to its molecular structure and consistent with the
existence of monomeric species, having the axially coordinated
solvent. In fact, 1H NMR signals related to the aromatic,
OCH2– and CHQN hydrogens are identical to those found in
DMSO-d6 solutions of 2 (Fig. S1, ESIw).2
1
Moreover, the resulting H NMR spectrum is similar to that
found for the 2Ápyridine adduct in DCM-d2 solutions (Fig. S2,
ESIw), thus suggesting an analogous axially coordination
mode of the bromide to the ZnII ion.
1
With the exception of DMSO and DMF, compound 1 is very
low soluble in most common coordinating (e.g., acetonitrile,
THF, methanol) or low-polarity non-coordinating (e.g., DCM)
solvents, while is almost insoluble in non-polar solvents. The
substantial higher solubility in DMSO and DMF indicates the
existence of monomeric species of 1 in these solvent media, as
The H NMR spectrum of 1 in DCM-d2 solution is almost
identical, in terms of chemical shift and multiplicity of the
1
suggested by H NMR analysis (vide supra).
Surprisingly, in strict contrast to our previous studies on 2,2
optical absorption spectral features of 1 are independent from
the nature of the solvent. In fact, either in coordinating (THF,
DMSO) or non-coordinating (DCM) solvents optical spectra
are identical to each other, except for a red-shift of the longer
wavelength band in the case of the more polar DMSO solvent
(Fig. 1). The optical absorption features of 1 resemble those of
the parent complex 2 in coordinating solvents or in DCM
upon axial coordination of a Lewis base.2 In other words,
optical absorption properties of 1 in solution, independently of
the presence or not of a Lewis base, indicate the existence of an
axially coordinated species to the ZnII ion. Moreover, complex
1 exhibits a moderate fluorescence emission (F E 0.08), with
the presence of an unstructured band, lmax between 602 and
620 nm, independent from the excitation wavelength.
Fig. 2 1H NMR spectra in DCM-d2 of 2 (a) and 2ÁBrTBA adduct
(obtained upon addition of an equimolar amount of TBA bromide to
a DCM-d2 solution of 2) (b). The 1H NMR spectrum of 1 in DCM-d2
is reported for comparison (c).
Therefore, present data suggest the existence of ZnÁ Á ÁBr
interactions in 1 in a solution of DCM, thus satisfying the
c
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signals, to that found for the above 2ÁBrTBA adduct (Fig. 2,
Table S1, ESIw). Moreover, the evidence of a single signal for
H1–H5 protons of the two salicylidene moieties indicates that
in both species a local C2 axis passing through the central ZnII
ion and perpendicular to the chelate ring is present. These data
suggest in both cases an analogous coordination mode of the
alkyl-ammonium bromide to the ZnII ion, ruling-out the
existence of intramolecular ZnÁ Á ÁBr interactions in 1.
In order to study the optical properties of the 2ÁBrTBA
adduct, spectrophotometric and spectrofluorimetric titrations
of 10 mM DCM solutions of 2 were performed using DCM
solutions of TBA bromide as titrant (Fig. S3, ESIw). The
resulting optical absorption spectra of the 2ÁBrTBA adduct
upon reaching the saturation point, except for a slightly
red-shift (B6 nm) of the longer wavelength band, are equivalent
to those of 1 in DCM (Fig. 3). Moreover, Job’s plot analysis13
clearly indicates the formation of 1 : 1 adducts (Fig. S4, ESIw).
Analogously, a comparison of fluorescence spectra for the
2ÁBrTBA adduct and 1 indicates an identical emission, in
terms of intensity and fluorescence maxima. Note however
that, while optical absorption spectra of the 2ÁBrTBA adduct
do not significantly differ from those obtained for 1 : 1 adducts
with neutral Lewis bases,2 the fluorescence enhancement upon
formation of the 2ÁBrTBA adduct is lower. Actually, starting
from DCM solutions of 2, the formation of 1 : 1 adducts with
neutral Lewis bases, e.g., pyridine, involves a fluorescence
enhancement of almost one order of magnitude.2 Conversely,
in the case of the 2ÁBrTBA adduct, about a two-fold increase
of the fluorescence intensity is observed (Fig. S3, ESIw). This
lower fluorescence enhancement for the 2ÁBrTBA adduct can
be related to a quenching effect due to the heavy bromide
ion.14 As expected, analogous titrations of DCM solutions of
1 with TBA bromide or using non-coordinating anion species,
e.g., TBA hexafluorophosphate, do not involve appreciable
optical absorption or fluorescence variations.
Scheme 2 A possible aggregation mode of 1 in DCM solution.
interactions or a combination of them are possible, however,
spectroscopic data and the very low solubility of 1 in DCM in
comparison to that of the 2ÁBrTBA adduct15 suggest the
occurrence of intermolecular ZnÁ Á ÁBr interactions and, hence,
aggregate structures, e.g., dimeric (Scheme 2) or oligomeric
species, in solution.16,17
These species are very stable in DCM and do not exhibit
bromide displacement even with the addition of stoichiometric
amounts of strong Lewis bases, e.g., pyridine. To observe an
appreciable 1Ápyridine adduct formation it needs adding
2000-fold mole excess of pyridine, as can be monitored by a
significant increase of the fluorescence intensity. A complete
bromide displacement is achieved by addition of ca. 105-fold
mole excess of pyridine (Fig. S5, ESIw). Conversely, the
addition of ca. 103-fold mole excess of pyridine is required
for a complete bromide displacement in the case of the
2ÁBrTBA adduct, in agreement with its simpler monomer
nature.
Scanning electron microscopy (SEM) images of samples
obtained by drop-casting from dilute DCM solutions of
1 indicate the existence of coral-like nanostructures (Fig. 4)
whose structural motif may be related to an aggregate structure
In summary, the comparison of optical absorption and
fluorescent properties of 1 in DCM solution with those for
the 2ÁBrTBA adduct, analogously to 1H NMR studies,
indicates the existence of ZnÁ Á ÁBr interactions. Although on
statistical ground both intra- or intermolecular ZnÁ Á ÁBr
Fig. 3 UV/vis absorption and fluorescence (lexc = 501 nm) spectra in
DCM of 1 (TT) and the 2ÁBrTBA adduct (– –) (1.0 Â 10À5 M). The
absorption spectrum of 1 was recorded with an absorbance of the
longer wavelength band equal to that recorded for the 2ÁBrTBA
adduct.
Fig. 4 Low (top) and high (bottom) magnification SEM images of
1 deposited by casting from a dilute DCM solution.
c
2828 New J. Chem., 2011, 35, 2826–2831
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nanostructures, whose structural motif is consistent with the
existence of intermolecular ZnÁ Á ÁBr interactions. These results
represent an unprecedented example of a coordination
complex exhibiting a branched nanostructure.
Experimental section
Materials and general procedures
Zinc acetate dihydrate, 2,4-dihydroxybenzaldehyde, 1,12-
dibromododecane, trimethylamine, and TBA bromide (Aldrich)
were used without purification. Diaminomaleonitrile (Aldrich)
was purified by recrystallization from an ethanol solution.
Dichloromethane (Aldrich) stabilized with amylene was used.
DCM-d2 was dried over anhydrous sodium sulfate and calcinated
with zinc oxide overnight before using.
Measurements
Elemental analyses were performed on a Carlo Erba 1106
elemental analyzer. Solution NMR experiments were carried
out on a Varian Unity Inova 500 (499.88 MHz for 1H)
spectrometer using an inverse detection tunable triple
resonance pfg 5 mm 1H13C{X} probe capable of generating
60 G cmÀ1 field strengths. Optical absorption spectra were
recorded at room temperature with a Varian Cary 500 UV-Vis-NIR
spectrophotometer. Fluorescence spectra were recorded at
room temperature with a Fluorolog-3 (Jobin Yvon Horiba)
spectrofluorimeter. The fluorescence quantum yield was
obtained using fluorescein (FF = 0.925) in 0.1 M NaOH as
standard. The absorbance value of the samples at and above
the excitation wavelength was lower than 0.1 for 1 cm
pathlength cuvettes. ESI mass spectra were recorded with a
Finnigan LCQ-Duo ion trap electrospray mass spectrometer
(Thermo). MALDI TOF mass spectra were recorded in linear
Scheme 3 A potential aggregation motif of 1 in the coral-like
nanostructures.
as sketched in Scheme 3. In fact, considering intermolecular
ZnÁ Á ÁBr interactions and given the presence of the flexible side
alkyl chains, a branched nanostructure can be envisaged.
Although branched nanostructures are rather common among
inorganic materials,18 there are only a few molecular materials
showing such a structure.19 In this view, the present investigation
represents a unique example of a metal–organic coordination
complex exhibiting a branched nanostructure. On the other
hand, SEM images of samples achieved by drop-casting from
dilute DMSO solutions of 1 (Fig. S6, ESIw) indicate a very
different, nanorod structure, in which 1 is presumably axially
coordinated to the DMSO, thus avoiding any intermolecular
branched aggregation.
mode with
a Voyager-DE PRO (Perseptive Biosystem)
mass spectrometer instrument, equipped with a nitrogen laser
emitting at 337 nm, with a 3 ns pulse width and working in
positive ion mode. The sample dissolved in DCM solvent was
mixed with malononitrile/DMF, used as a matrix. The film
surface morphology was examined by field emission scanning
electron microscopy (FE-SEM) using a ZEISS SUPRA VP
55 microscope. Samples for SEM analysis were obtained by
drop-casting dilute DMSO or DCM solutions (B1 Â 10À5 M)
onto cleaned Si(100) substrates. Samples were sputtered with
gold to avoid charging effects.
Conclusions
The comparison of 1H NMR and optical spectroscopic
properties of the tailored bis(salicylaldiminato)Zinc(II) Schiff
base complex, 1, having dipodal, alkyl ammonium bromide
derivatized chains in the salicylidene rings, with those of the
reference complex 2, through the study of the 1 : 1 2ÁBrTBA
adduct, allowed getting insights into the nature of species in
solutions.
In strict contrast to our previous studies on 2, 1H NMR and
optical absorption spectral features of 1 are independent from
the coordinating or non-coordinating nature of the solvent.
With the exception of DMSO and DMF, in which 1 : 1 adducts
with the solvent are likely to occur, present data suggest the
existence of ZnÁ Á ÁBr interactions in 1 in solution, thus satisfying
its coordination sphere. This avoids intermolecular ZnÁ Á ÁO
interactions, as usually occur in the absence of other coordinating
species.
Syntheses
4-(12-Bromododecyloxy)-2-hydroxybenzaldehyde. A solution
of 2,4 dihydroxybenzaldehyde (0.691 g, 5.00 mmol), 1,12-
dibromododecane (1.641 g, 5.00 mmol) and potassium carbo-
nate (0.346 g, 2.50 mmol) in acetonitrile (200 mL) was heated
at reflux with stirring for 4 h under a nitrogen atmosphere. The
reaction was followed by TLC (silica gel; eluent: cyclohexane/
ethyl acetate 90 : 10 (v/v)). The solution was concentrated and
the product was purified by column chromatography (silica
gel; eluent: cyclohexane/ethyl acetate from 98 : 2 to 95 : 5 (v/v))
to obtain a white solid product (0.756 g, 39%). 1H NMR
(500 MHz, DMSO-d6, TMS): d = 1.26–1.39 (m, 16H; CH2),
1.69–1.80 (m, 4H; CH2), 3.51 (t, 3JHH = 6.5 Hz, 2H; CH2–Br),
SEM analysis of samples obtained by drop-casting from
DCM solutions of 1 reveals the existence of branched
c
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3
4.01 (t, JHH = 6.5 Hz, 2H; OCH2), 6.46 (d, JHH = 2.0 Hz,
4
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1H; ArH), 6.55 (dd, JHH = 8.5 Hz, JHH = 2.0 Hz, 1H;
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´
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saturated with trimethylamine. The yellow-brine solution was
heated at 50 1C with stirring for 12 h and then concentrated to
obtain a yellow solid product (0.542 g, quantitative yield).
1H NMR (500 MHz, DMSO-d6, TMS): d = 1.39–1.42
(m, 16H; CH2), 1.64–1.72 (m, 4H; CH2), 3.03 (s, 9H;
N(CH3)3+), 3.03–3.27 (m, 2H; CH2–N(CH3)3+), 4.02
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4
(t, JHH = 6.5 Hz, 2H; OCH2), 6.46 (d, JHH = 2.0 Hz, 1H;
3
4
ArH), 6.54 (dd, JHH = 8.5 Hz, JHH = 2.0 Hz, 1H; ArH),
3
7.61 (d, JHH = 8.5 Hz, 1H; ArH), 9.99 (s, 1H; CHO).
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[2,3-Bis[[2-hydroxy-4-(1-N,N,N,N-trimethyldodecyl-12-oxy-
ammonium bromide)benzylidene] amino]-2-butenedinitrilato]
ZnII (1). To a solution of 12-(4-formyl-3-hydroxyphenoxy)-
N,N,N-trimethyldodecan-1-ammonium bromide (0.542 g,
1.22 mmol) in ethanol (30.0 mL), diaminomaleonitrile
(0.0660 g, 0.610 mmol) was added with stirring. The mixture
was heated at reflux with stirring for 1 h, under a nitrogen
atmosphere. To the red solution, zinc acetate dihydrate
(0.134 g, 0.610 mmol) was added and the mixture was heated
at reflux with stirring for 2 h, under a nitrogen atmosphere.
After cooling, a purple precipitate product was collected by
filtration, washed with ethanol, and dried in vacuo (0.324 g,
60%). C48H74Br2N6O4Zn (1024.35): calcd C, 56.28; H, 7.28;
N, 8.20%; found C, 56.73; H, 7.34; N, 8.30%. ESI: m/z =
1024 [M]+. MALDI-TOF: m/z = 1968 [(M)2 À Br]+.
1H NMR (500 MHz, DMSO-d6, TMS): d = 1.27–1.40
(m, 32H; CH2), 1.64–1.73 (m, 8H; CH2), 3.02 (s, 18H;
N(CH3)3+), 3.23–3.26 (m, 4H; CH2–N(CH3)3+), 3.99
3
4
(t, JHH = 6.5 Hz, 4H; OCH2), 6.17 (d, JHH = 2.0 Hz, 2H;
3
ArH), 6.21 (dd, JHH = 8.5 Hz, JHH = 2.0 Hz, 2H; ArH),
4
3
7.33 (d, JHH = 8.5 Hz, 2H; ArH), 8.36 (s, 2H; CH = N).
Acknowledgements
This research was supported by the MIUR and PRA (Progetti
di Ricerca di Ateneo).
Notes and references
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16 Note that, quaternary ammonium compounds when dissolved in
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ZnÁ Á ÁBr interactions necessarily implies the formation of aggregate
structures.
c
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c
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