Structural characterization and electrochemical properties of novel salicylidene phosphonate derivatives

Article abstract of DOI:10.1016/j.saa.2010.05.011

In this study, three novel salicylidene phosphonate ligands, diethyl (4-{[(1E)-(2-hydroxyphenyl)methylidene]amino}benzyl)phosphonate (HL ¹), diethyl (4-{[(1E)-(2-hydroxy-3-methoxyphenyl)methylidene]amino} benzyl)phosphonate (HL²) and diethyl (4-{[(1E)-(2,4-dihydroxyphenyl) methylidene]amino}benzyl)phosphonate (HL³) were synthesized and characterized by the analytical and spectroscopic techniques. We obtained their single crystals from the ethanolic solution. There are intramolecular phenol-imine hydrogen bonds in all three compounds between O1 and N1 atoms. The ligand HL³ contains a second phenol group and this is makes an intermolecular hydrogen bond with the phosphine oxide of a neighbouring molecule O2-O3 (under symmetry operation -x, 0.5 + y, 0.5 - z). In order to investigate the redox behaviours of the salicylidene phosphonate ligands (HL ¹-HL³), we were studied electrochemical properties of the ligands at the different pH and scan rates.

Full text of DOI:10.1016/j.saa.2010.05.011

Spectrochimica Acta Part A 77 (2010) 219–225
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Structural characterization and electrochemical properties of novel salicylidene
phosphonate derivatives
Mustafa Dolaza,∗, Vickie McKeeb, Muhammet Köseb, Ays¸ egül Gölcüc, Mehmet Tümerc
a Kahramanmaras Sutcu Imam University, Faculty of Engineering and Architecture, Department of Environmental Engineering, K.Maras, Turkey
b Univ Loughborough, Dept Chem, Loughborough LE11 3TU, Leics, England, United Kingdom
c Kahramanmaras Sutcu Imam University, Chemistry Department, 46100, K.Maras, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, three novel salicylidene phosphonate ligands, diethyl (4-{[(1E)-(2-hydroxyphenyl)methy-
lidene]amino}benzyl)phosphonate (HL1), diethyl (4-{[(1E)-(2-hydroxy-3-methoxyphenyl)methyli-
dene]amino}benzyl)phosphonate (HL2) and diethyl (4-{[(1E)-(2,4-dihydroxyphenyl)methyli-
dene]amino}benzyl)phosphonate (HL3) were synthesized and characterized by the analytical and
spectroscopic techniques. We obtained their single crystals from the ethanolic solution. There are
intramolecular phenol-imine hydrogen bonds in all three compounds between O1 and N1 atoms. The
ligand HL3 contains a second phenol group and this is makes an intermolecular hydrogen bond with the
phosphine oxide of a neighbouring molecule O2–O3 (under symmetry operation −x, 0.5 + y, 0.5 − z). In
order to investigate the redox behaviours of the salicylidene phosphonate ligands (HL1–HL3), we were
studied electrochemical properties of the ligands at the different pH and scan rates.
Received 25 November 2009
Received in revised form 5 May 2010
Accepted 15 May 2010
Keywords:
Salicylidene
Phosphonate
Schiff base
X-ray
Electrochemical
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
in several biological redox-active system [7], a number of cop-
per proteins including enzymes have been reported and proteins
The Schiff bases, formed by the condensation of an amine with
an aldehyde [1], usually coordinate to a metal through the imine
nitrogen and another group, usually oxygen, nitrogen or sulfur
situated on the original aldehyde. Condensation of the carbonyl
function of a ␤-ketone with only one end of a diamine produces
terdentate ligand and this can be make to undergo further conden-
sation with another carbonyl function to produce unsymmetrical
tetradentate Schiff base ligands. The Schiff bases are very impor-
tant tools for the inorganic chemists as these are widely used to
design molecular ferromagnets, in catalysis, in biological modeling
applications, as liquid crystals and as heterogeneous catalysts [2].
Schiff base complexes are once again topical in connection with
self-assembling cluster complexes [3].
The transition metal complexes having oxygen and nitrogen
donor Schiff bases possess unusual configuration, structural lability
and are sensitive to molecular environment [4]. The environment
around the metal center ‘as coordination geometry, number of
coordinated ligands and their donor group’ is the key factor for
metalloprotein to carry out a specific physiological function [5].
About 20 Zinc enzymes are known in which Zinc is generally tetra-
hedrally four coordinate and bonded to hard donor atoms such
as nitrogen or oxygen [6]. Manganese plays an important role
containing iron participate in oxygen transport [8]. The prepara-
tion of model complexes having similar spectroscopic features is
perhaps the most important step to understand the structure and
behaviour of these biological systems. Schiff base metal complexes
attract considerable interest and occupy an important role in the
development of the chemistry of chelate systems [9,10] due to the
fact that especially these with N2O2 tetradentate ligands, such sys-
tems closely resemble metallo-proteins. Survey of the literature
reveals an excellent work devoted to synthesis and characteriza-
tion of many metal complexes of Schiff base [11,12], furthermore
the coordination compounds of phosphonic acid and their esters
with transition metal ions have been the subject of several studies
due to the fact that complexes of such compounds exhibit biological
activity [13].
In this study, we report synthesis and structural characterization
of three novel salicylidene phosphonate ligands (HL1–HL3). In addi-
tion, we also investigated the spectroscopic and electrochemical
properties of the ligands.
2. Experimental
2.1. General
Salicylaldehyde, 3-methoxy-2-hydroxybenzaldehyde, 2,4-
dihydroxybenzaldehyde, diethyl (4-aminobenzyl)phosphonate
(starting material), and organic solvents were obtained from Fluka
∗
Corresponding author. Tel.: +90 344 2191460; fax: +90 344 2191052.
E-mail address: mdolaz@ksu.edu.tr (M. Dolaz).
1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2010.05.011

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M. Dolaz et al. / Spectrochimica Acta Part A 77 (2010) 219–225
Table 1
and used as received, unless noted otherwise. Elemental analyses
(C, H, N) were performed using a LECO CHNS 932 elemental ana-
Crystallographic data of the ligands HL1–HL3.
lyzer. IR spectrum was obtained using KBr discs (4000–400 cm−1
)
Identification code
HL1
HL2
HL3
on a Shimadzu 8300 FT-IR spectrophotometer. The electronic
spectra in the 200–500 nm range were obtained on a Perkin-Elmer
Lambda 45 spectrophotometer. Molar conductance measurement
of the Schiff base ligand (L) was determined in methanol (∼10−3 M)
at room temperature using a Jenway Model 4070 conductivity
meter. Mass spectrum of the ligand was recorded on an LC/MS
APCI AGILENT 1100 MSD spectrophotometer. 1H NMR spectra of
the ligands were recorded on Bruker – 300 instrument. TMS was
used as internal standard and CDCl3 as solvent.
The voltammetric measurements at a glassy carbon electrode
was performed using a BAS 100 W (Bioanalytical System, USA) elec-
trochemical analyzer. A glassy carbon working electrode (BAS; ˚:
3 mm diameter), an Ag/AgCl reference electrode (BAS; 3 M KCl) and
platinum wire counter electrode and a standard one-compartment
three-electrode cell of 10 mL capacity were used in all experiments.
The glassy carbon electrode was polished manually with aqueous
slurry of alumina powder (˚: 0.01 ␮m) on a damp smooth polish-
ing cloth (BAS velvet polishing pad), before each measurement. All
measurements were realized at room temperature.
Empirical formula
Formula weight
Crystal system
Space group
Unit cell a (Å)
b (Å)
C18H22NO4P
347.34
Triclinic
¯
P1
8.8549(13)
10.4087(15)
10.8325(16)
84.960(2)
68.491(2)
73.155(2)
888.8(2)
C19H24NO5P
377.36
Triclinic
¯
P1
C18H22NO5P
363.34
Monoclinic
P21/c
12.588(2)
8.9001(16)
16.908(3)
90
105.895(3)
90
8.6927(9)
9.9463(11)
12.1329(13)
78.848(2)
79.636(2)
66.970(2)
940.85(17)
2
0.175
9461
4514 [0.0254]
0.0435, 0.1068
0.0566, 0.1159
c (Å)
˛ (◦)
ˇ (◦)
ꢂ (◦)
Volume (Å3)
Z
1821.9(6)
4
0.178
2
Abs. coeff. (mm−1
Refl. collected
)
0.176
7968
3626 [0.0398]
0.0461, 0.0939
0.0780, 0.1072
17953
Ind. Refl. [Rint
]
4517 [0434]
0.0441, 0.1040
0.0656, 0.1161
R1, wR2 [I > 2(I)]
R1, wR2 (all data)
(2H, d, –P–CH2–), 4.07 (2H, m, –CH2–), 6.96–7.33 (8H, m, aro-
matic protons), 8.41 (1H, s, –CH N–). UV–vis: (ꢀmax, nm; εmax
,
M
−1 cm−1), CH3OH as solvent: 443 (8.4 × 10−3), 345 (9.0 × 10−3),
308 (3.7 × 10−3), 246 (2.6 × 10−4), 220 (3.2 × 10−4). Infrared spec-
trum (cm−1, KBr disk): 2960, 2845 ꢁ(CH2), 1625 ꢁ(C N), 1250
ꢁ(P O).
The pH was measured using a pH meter Model 538 (WTW,
Austria) using a combined electrode (glass electrode–reference
electrode) with an accuracy of 0.05 pH.
2.2. Preparation of the novel salicylidene phosphonate ligands
(HL1–HL3)
2.3. X-ray structure solution and refinement for the Schiff base
ligands HL1–HL3
The salicylidene phosphonate ligands were prepared
according to the similar methods. A solution of diethyl (4-
aminobenzyl)phosphonate (0.243 g, 1 mmol) in ethanol (15 ml)
was added drop wise to an ethanolic solution (30 ml) of the
benzaldehyde derivatives (0.122 g for salicylaldehyde; 0.152 g
for 3-methoxy-2-hydroxybenzaldehyde and 0.138 g for 2,4-
dihydroxybenzaldehyde, 1 mmol) with stirring. The yellow
solution was stirred and refluxed for 1 h and the solvent was
evaporated in air. The yellow solid was separated by filtration
through G4 sintered bed and washed thoroughly with hexane and
water. Finally, the isolated compound was dried in vacuo over
P4O10. Single crystals of the ligands (HL1–HL3) were obtained by
recrystallization from C2H5OH solution.
X-ray diffraction data for these three compounds were col-
lected at 150(2)K on a Bruker Apex II CCD diffractometer using Mo
K␣ radiation (ꢀ = 0.71073 Å). The structures were solved by direct
methods and refined on F2 using all the reflections [14]. All the
non-hydrogen atoms were refined using anisotropic atomic dis-
placement parameters and hydrogen atoms bonded to carbon were
inserted at calculated positions using a riding model. Hydrogen
atoms bonded to oxygen were located from difference maps and
allowed to refine with temperature factors riding on those of the
carrier atoms. The crystal data and details of the structure solu-
tion and refinement are given in Table 1; selected bond lengths are
given in Table 2. Further experimental details have been deposited
as supplementary material at the Cambridge Crystallographic Data
Centre (CCDC 746866–746868).
HL1 – Yield: 72%, m.p. 95 ◦C. color: yellow. Analysis Calc. for
C
18H22NO4P: C, 62.24; H, 6.38; N, 4.03%. Found: C, 62.30; H, 6.35;
N, 4.02%. Mass spectrum (LC/MS APCI): m/z 303 [M+H–OCH2CH3]+.
1H NMR: (CDCl3 as solvent, ı in ppm): 1.26 (3H, t, –CH3), 3.15
(2H, d, –P–CH2–), 4.01 (2H, m, –CH2–), 6.93–7.42 (8H, m, aromatic
3. Results and discussion
In this study, we obtained three novel phosphonate
Schiff base compounds from the reaction of the sali-
protons), 8.64 (1H, s, –CH N–), 13.27 (H, s, –OH). UV–vis: (ꢀmax
,
nm; εmax, M−1 cm−1), CH3OH as solvent: 380 (2.5 × 10−3), 315
(7.1 × 10−3), 294 (5.5 × 10−3), 278 (1.0 × 10−4), 245 (6.2 × 10−4),
228 (8.8 × 10−4). Infrared spectrum (cm−1, KBr disk): 2960, 2845
ꢁ(CH2), 1625 ꢁ(C N), 1245 ꢁ(P O).
cylaldehyde
(HL1),
3-methoxy-2-hydroxybenzaldehyde
(HL2), 2,4-dihydroxybenzaldehyde (HL3) and diethyl (4-
aminobenzyl)phosphonate compounds in ethanol solution.
Obtained compounds were characterized by the analytical and
spectroscopic methods. The suitable crystals for the X-ray anal-
HL2 – Yield: 84%, m.p. 115 ◦C. color: light yellow. Analysis Calc.
for C19H24NO5P: C, 60.47; H, 6.41; N, 3.71%. Found: C, 60.40; H,
6.35; N, 3.75%. Mass spectrum (LC/MS APCI): m/z 378 [M+H]+. 1H
NMR: (CDCl3 as solvent, ı in ppm): 1.26 (3H, t, –CH3), 3.16 (2H,
d, –P–CH2–), 3.95 (3H, s, –OCH3), 4.01 (2H, m, –CH2–), 6.87–7.39
(8H, m, aromatic protons), 8.65 (1H, s, –CH N–), 13.70 (H, s,
–OH). UV–vis: (ꢀmax, nm; εmax, M−1 cm−1), CH3OH as solvent: 430
(4.9 × 10−3), 338 (7.3 × 10−3), 305 (6.9 × 10−3), 240 (8.3 × 10−4),
225 (9.2 × 10−4). Infrared spectrum (cm−1, KBr disk): 2960, 2845
ꢁ(CH2), 1625 ꢁ(C N), 1250 ꢁ(P O).
Table 2
Selected bond lengths(Å) of the ligands HL1–HL3.
Distance
HL1
HL2
HL3
C1–O1
C2–O2
C3–O3
P–O
1.349(3)
1.3521(19)
1.3694(19)
1.352(2)
1.355(2)
1.4655(15)
1.5720(15)
1.5847(15)
1.789(2)
1.282(3)
1.422(3)
1.4699(12)
1.5737(12)
1.5845(12)
1.7959(16)
1.290(2)
1.4754(12)
1.5690(12)
1.5755(12)
1.7885(18)
1.284(2)
P–OR
HL3 – Yield: 87%, m.p. 158 ◦C. color: orange. Analysis Calc.
for C18H22NO5P: C, 59.50; H, 6.10; N, 3.85%. Found: C, 59.47; H,
6.07; N, 3.90%. Mass spectrum (LC/MS APCI): m/z 364 [M+H]+.
1H NMR: (CDCl3 as solvent, ı in ppm): 1.32 (3H, t, –CH3), 3.19
P–C
N1–C7
N1–C8
1.4213(19)
1.421(2)

M. Dolaz et al. / Spectrochimica Acta Part A 77 (2010) 219–225
221
Table 3
Hyrogen bond geometry of the ligands HL1–HL3.
D–H. . .A
D–H
H. . .A
D. . .A
<(DHA)
HL1
O1–H1A. . .N1
O1–H1. . .N1
O1–H1. . .N1
O2–H2. . .O3a
0.88(3)
0.88(2)
0.90(2)
0.86(2)
1.79(3)
1.76(2)
1.74(2)
1.79(2)
2.585(2)
150(3)
153(2)
155(2)
167(2)
HL2
2.5737(17)
2.5824(19)
2.6420(19)
HL3
HL3 (intermolecular)
a
Symmetry operation −x, 0.5 + y, 0.5 − z.
ysis were obtained by recrystallization from C2H5OH solution.
The ligands soluble in common polar organic solvents such as
EtOH, MeOH, CHCl3 and CH2Cl2. But, they partially soluble in
apolar organic solvents such as benzene, hexane and toluene. The
compounds are very stable solids at room temperature without
decomposition. The ligands were obtained as coloured plate
crystals in the range 72–87% yield.
protons. The multiplets of the aromatic ring protons were seen in
the 6.87–7.42 ppm range. The azomethine proton of the ligands
were seen in the range 8.41–8.64 ppm range [16].
The electronic properties of the ligands HL1–HL3 were inves-
tigated in the methanol solution and spectral data were given in
Section 2. In the spectra of the ligands, the bands in the 443–305 nm
range can be attributed to the n–␲* transitions of the azomethine
and hydroxyl groups. The bands in the 294–225 nm range may be
assigned to the benzenoid ␲–␲* transitions [17].
In order to further information about the ligands, their 1H NMR
spectra were recorded using CDCl3 as a solvent and spectral data
were given in Section 2. The 1H NMR spectra of the ligands were
given in Supporting Information. In the spectra of the ligands, the
triplets in the 1.26–1.32 ppm range can be attributed to the CH3
hydrogen atoms. The dublets in the 3.15–3.19 ppm range may be
assigned to the –P–CH2– group protons [15]. In the spectrum of the
ligand HL2, the singlet at 3.95 ppm can be attributed to the –OCH3
The infrared spectral data were given in Section 2. The bands
observed in the 2960–2845 cm−1 range can be attributed to
the aliphatic ꢁ(C–H) vibrations. The azomethine ꢁ(CH N) vibra-
tions were observed in the 1625 cm−1 range [18]. The ꢁ(P O)
bands of the ligands were seen in the 1245–1250 cm−1 range
[19].
Fig. 1. ORTEP diagrams of the ligands (HL1–HL3) with thermal ellipsoids at 50% probability.

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M. Dolaz et al. / Spectrochimica Acta Part A 77 (2010) 219–225
Fig. 2. Intermolecular ␲–␲ interactions in the ligands HL1 (top) and HL2 (bottom).
The mass spectral studies of the salicylidene phosphonate
in the conformation about the phosphorus atom and the dihedral
angle between the two aromatic rings.
derivatives (HL1 and HL3) were done and obtained data are given in
Section 2. The mass spectra of the ligands were given in Supporting
Information. In the spectra of the ligands, the molecular ion peak
[M+H]+ of the ligand HL3 was seen at the m/z 364. The highest inten-
sity peak of the ligands HL1 and HL3 were seen at m/z 303 (100%)
due to the loosing –OCH2–CH3 group. In the spectrum of the lig-
and HL2, this peak also was seen at same region but as a very weak
intensity (5%).
Selected bond lengths (Å) of the ligands HL1–HL3 were given
in Table 2. Three ligands have also intramolecular phenol-imine
hydrogen bonds between the atoms O1–N1 (Table 3). The ligand
HL3 contains a second phenol group and this group makes an
intermolecular hydrogen bond with the phosphine oxide group of
a neighbouring molecule O2–O3 (under symmetry operation −x,
0.5 + y, 0.5 − z), linking the molecules into H-bonded chains (Fig. 1).
There are no similar interactions in the compounds HL1 and HL2. In
addition, these ligands show distinct the ␲–␲ interactions.
3.1. X-ray structure properties of the novel salicylidene
phosphonate derivatives
The dihedral angles between the two aromatic rings are
16.93(10), 24.40(8) and 8.48(12)◦, for HL1, HL2 and HL3,
respectively. This difference reflects the different intermolecular
interactions in the lattice. There is a evidence of ␲–␲ phenyl stack-
ing in both HL1 and HL2. In the ligand HL1, the phenol-imine
Perspective views of all three compounds are shown in Fig. 1.
All bond lengths of the compounds are in the normal ranges. The
molecular structures are broadly similar, but differing principally
Fig. 3. H-bonding and C–H–␲ interactions in the ligand HL3.

M. Dolaz et al. / Spectrochimica Acta Part A 77 (2010) 219–225
223
3.2. Electrochemical properties of the ligands
Electrochemical studies of the novel salicylidene phosphonate
derivatives (HL1–HL3) were run in CH3CN/C2H5OH (v/v, 1:1) mix-
ture at 293 K. The studies were done both reduction and oxidation
at pH 2–12 in Britton–Robinson buffer and in the (+2000) − (−2000)
potential range. When the peak current–pH relationship was inves-
tigated, the highest peak current value was obtained at pH 3.
HL1–HL3 appear to be an electroactive compound for both direc-
tions. Specifically, the compounds are capable to be both oxidable
and reducible. To demonstrate the usefulness of a solid electrode
for investigation of HL1–HL3, which may offer advantages for the
use of such electrodes as sensors, the electrochemical behaviour
of HL1–HL3 on a glassy carbon electrode was investigated in this
research. Therefore, several measurements with different electro-
chemical parameters were performed using various supporting
electrolytes (0.1 M H2SO4, 0.5 M H2SO4, 0.2 M phosphate buffer at
pH 2.0–9.0, 0.04 M Britton–Robinson buffer at pH 2.32–12.0 and
0.2 M acetate buffer at pH 3.5–5.5) and buffers in order to obtain
such information. Cyclic voltammograms of HL1–HL3 exhibited one
distinct and well-defined cathodic peak at ∼−0.5 V and ill-defined
anodic wave at ∼+0.5 at pH 3.0. The effects of potential scan rates (ꢁ)
between 5 and 1000 mV s−1 on the peak potential and peak current
of compounds were also evaluated. Scan rate studies were then car-
ried out to assess whether the processes on glassy carbon electrode
were under diffusion or adsorption control. A linear relationship on
the oxidation peak current (Ip) with the square root of the scan rate
(ꢁ1/2) showed the diffusion control process. The equation is noted
below in Britton–Robinson buffer at pH 3:
Fig. 4. Unit cell packing for HL3.
fragment of the ligand (C1–N1) is stacked with the same section
of an adjacent molecule having symmetry operation 2 − x, 1 − y, −z
with interplanar separation with 3.40 Å. The C6 numbered carbon
atoms of the ligands HL1 and HL2 are separated by with 3.397 Å
(Fig. 2). There is similar stacking in compound HL2 under symme-
try operation 2 − x, 1 − y, −z with interplanar separation of 3.40 Å.
The C3 and C7 carbon atoms are separated by 3.360 Å. The dis-
tance between two adjacent phenyl rings of the ligand HL1 is
longer according to the ligand HL2. There is no evidence of ␲–␲
stacking in HL3. Presumably, it is not compatible with the inter-
molecular hydrogen bond, however, there are some edge-to-face
C–H. . .␲ interactions (Fig. 3). The different intermolecular inter-
actions between the ligands are reflected in the unit cell packing
of the three molecules. Unit cell plots of the ligands HL1 and HL2
show ␲–␲ stacking dominates (Fig. 4). Whereas, the most strik-
ing feature of the ligand HL3 is the formation of H-bonded chains
running parallel to b (Fig. 5).
Ip (␮A) = 0.221 ꢁ1/2 (mV s−1) + 2.089
(r = 0.991, n = 10 for starting material)
Ip (␮A) = 0.363ꢁ1/2(mV s−1) + 0.021 (r = 0.990, n = 10, for HL1)
Ip (␮A) = 0.478 ꢁ1/2(mV s−1) + 2.956 (r = 0.991, n = 10, for HL2)
Ip (␮A) = 0.981ꢁ1/2(mV s−1) + 0.603 (r = 0.991, n = 10, for HL3)
The effect of scan rate on peak current was also examined under
the above conditions with a plot of logarithm of peak current (log i)
Fig. 5. Packing diagrams for HL1 and HL2.

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M. Dolaz et al. / Spectrochimica Acta Part A 77 (2010) 219–225
1/2
Fig. 6. (a) Typical Ip–␯ and log ip–log ꢁ curves for starting material. (b) Typical Ip–ꢁ1/2 and log ip–log ꢁ curves for HL2.
versus logarithm of scan rate (log ꢁ), giving a straight line within
the same scan rate range. This linear relationship was obtained as
follow:
log ip (␮A)=0.174 log ꢁ (mV s−1)+0.061 (r=0.995, n=10 for HL1)
log ip (␮A)=0.353 log ꢁ(mV s−1)+0.187 (r=0.985, n=10 for HL2)
log ip (␮A)=0.459 log ꢁ (mV s−1)+0.109(r=0.985, n=10 for HL3)
log ip (␮A) = 0.284 log ꢁ(mV s−1) + 0.084
(r = 0.991, n = 10 for starting material)
The slopes (between 0.174 and 0.459) of the relationship are
close to the theoretically expected (0.5) for an ideal reaction of
solution species [20,21], so in this case the process had a diffu-
sive component. Typical Ip–ꢁ1/2 and log ip–log ꢁ curves and cyclic
voltammograms of 1 × 10−4 M starting material and HL2 have been
given in Figs. 6 and 7 Fig. 6a and b and Fig. 7, respectively.
4. Supplementary material
Crystallographic data for structural analysis has been
deposited with the Cambridge Crystallographic Data Centre,
CCDC 746866–746868 for the reported ligands (HL1–HL3). Copies
of this information may be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: +44
1223 336033; deposit@ccdc.cam.ac.uk or www.ccdc.cam.ac.uk).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.saa.2010.05.011.
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