Synthesis, characterization and molecular sensing behavior of [ZnCl2(η3-N,N,O-dpkbh)] (dpkbh = di-2-pyridyl ketone benzoyl hydrazone)

Article abstract of DOI:10.1016/j.molstruc.2007.03.001

The reaction between ZnCl₂ and di-2-pyridyl ketone benzoyl hydrazone (dpkbh) in acetonitrile under ultrasonic or reflux conditions gave [ZnCl₂(η³-N,N,O-dpkbh)] in good yield. The identity of the compound was established from the results of its elemental analysis and a number of spectroscopic measurements. Solid-state infrared spectra of [ZnCl₂(η³-N,N,O-dpkbh)] reveal the coordination of dpkbh, the presence of the amide proton and binding of the oxygen atom of dpkbh. ¹H NMR spectra of [ZnCl₂(η³-N,N,O-dpkbh)] show sensitivity to solvent and temperature variations and confirm the coordination of dpkbh. The electronic absorption spectra of [ZnCl₂(η³-N,N,O-dpkbh)] in non-aqueous media show high sensitivity to changes in their surroundings and divulge two interlocked intra-ligand-charge transfer (ILCT) transitions of the donor-acceptor type between 300 and 500 nm. Optical measurements show reversible inter-conversion between two forms of [ZnCl₂(η³-N,N,O-dpkbh)] that may be due to complex-substrate interactions and thermodynamic analysis gave changes in enthalpy (ΔH^θ) of -23.3 and +14.1 kJ mol^-1, entropy (ΔS^θ) of -51 and +31 J mol^-1K^-1, and free energy (ΔG^θ) of -8.16 and +4.54 kJ mol^-1 at 298.15 K in DMF and DMSO, respectively. Substrates in concentrations as low as 10^-5 M can be detected and determined using [ZnCl₂(η³-N,N,O-dpkbh)] in polar solvents. A provisional model for the binding of substrate to [ZnCl₂(η³-N,N,O-dpkbh)] is developed and a binding constant in the range 6000-8000 M^-1 is obtained in the case of ZnCl₂ in DMSO. Electrochemical measurements on [ZnCl₂(η³-N,N,O-dpkbh)] in DMF show irreversible metal and ligand-based redox processes, and electrochemical reactions of dpkbh with ZnCl₂ show facile coordination of ZnCl₂ and formation of [ZnCl₂(η³-N,N,O-dpkbh)]. X-ray structural analysis done on a crystal grown from a DMF solution of [ZnCl₂(η³-N,N,O-dpkbh)] confirmed the identity of [ZnCl₂(η³-N,N,O-dpkbh)] and shows an extensive network of non-covalent interactions that connects all molecules.

Full text of DOI:10.1016/j.molstruc.2007.03.001

Available online at www.sciencedirect.com
Journal of Molecular Structure 873 (2008) 17–28
www.elsevier.com/locate/molstruc
Synthesis, characterization and molecular sensing behavior
of [ZnCl2(g3-N,N,O-dpkbh)] (dpkbh = di-2-pyridyl ketone
benzoyl hydrazone)
*
Mohammed Bakir , Orville Green, Willem H. Mulder
Department of Chemistry, The University of the West Indies – Mona Campus, Kingston 7, Jamaica
Received 13 January 2007; accepted 1 March 2007
Available online 12 March 2007
Abstract
The reaction between ZnCl2 and di-2-pyridyl ketone benzoyl hydrazone (dpkbh) in acetonitrile under ultrasonic or reflux condi-
tions gave [ZnCl2(g3-N,N,O-dpkbh)] in good yield. The identity of the compound was established from the results of its elemental
analysis and a number of spectroscopic measurements. Solid-state infrared spectra of [ZnCl2(g3-N,N,O-dpkbh)] reveal the coordi-
nation of dpkbh, the presence of the amide proton and binding of the oxygen atom of dpkbh. 1H NMR spectra of [ZnCl2(g3-
N,N,O-dpkbh)] show sensitivity to solvent and temperature variations and confirm the coordination of dpkbh. The electronic
absorption spectra of [ZnCl2(g3-N,N,O-dpkbh)] in non-aqueous media show high sensitivity to changes in their surroundings
and divulge two interlocked intra-ligand-charge transfer (ILCT) transitions of the donor–acceptor type between 300 and 500 nm.
Optical measurements show reversible inter-conversion between two forms of [ZnCl2(g3-N,N,O-dpkbh)] that may be due to com-
plex–substrate interactions and thermodynamic analysis gave changes in enthalpy (DHh) of ꢀ23.3 and +14.1 kJ molꢀ1, entropy
(DSh) of ꢀ51 and +31 J molꢀ1Kꢀ1, and free energy (DGh) of ꢀ8.16 and +4.54 kJ molꢀ1 at 298.15 K in DMF and DMSO, respec-
tively. Substrates in concentrations as low as 10ꢀ5 M can be detected and determined using [ZnCl2(g3-N,N,O-dpkbh)] in polar sol-
vents. A provisional model for the binding of substrate to [ZnCl2(g3-N,N,O-dpkbh)] is developed and a binding constant in the
range 6000–8000 Mꢀ1 is obtained in the case of ZnCl2 in DMSO. Electrochemical measurements on [ZnCl2(g3-N,N,O-dpkbh)] in
DMF show irreversible metal and ligand-based redox processes, and electrochemical reactions of dpkbh with ZnCl2 show facile
coordination of ZnCl2 and formation of [ZnCl2(g3-N,N,O-dpkbh)]. X-ray structural analysis done on a crystal grown from a
DMF solution of [ZnCl2(g3-N,N,O-dpkbh)] confirmed the identity of [ZnCl2(g3-N,N,O-dpkbh)] and shows an extensive network
of non-covalent interactions that connects all molecules.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Hydrazone metal complexes; Di-2-pyridyl ketone benzoyl hydrazone; Sensors; Synthesis and crystal structure
1. Introduction
ketone (dpk) derivatives (see Scheme 1) and described the
synthesis and characterization of a series of compounds
of di-2-pyridyl ketone derivatives [8–27]. Spectroscopic
measurements on di-2-pyridyl ketone hydrazones revealed
their sensitivity to changes in their surroundings and X-
ray structural analysis uncovered a web of non-covalent
interactions, which may account for this sensitivity. The
ligand di-2-pyridyl ketone benzoyl hydrazone (dpkbh) is
widely used as a sensitive analytical reagent for the deter-
mination of trace amounts of metal ions in solutions
[28,29]. Bidentate N,N and tridentate N,N,O coordination
Di-2-pyridyl ketone [(C5H4N)2C@O] and its oxime, and
hydrazone derivatives (see Scheme 1) and their metal com-
pounds continue to attract much attention because of their
reactivity patterns, physical properties and relevance in
many important chemical and biological processes [1–7].
We have been interested in the chemistry of di-2-pyridyl
*
Corresponding author. Tel.: +1 876 935 8164; fax: +1 876 977 1835.
E-mail address: mohammed.bakir@uwimona.edu.jm (M. Bakir).
0022-2860/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2007.03.001

18
M. Bakir et al. / Journal of Molecular Structure 873 (2008) 17–28
Scheme 1. Di-2-pyridyl ketone and its oxime, hydrazone derivatives.
modes were reported for dpkbh [11,12,30–32]. In recent
reports, rhenium-tricarbonyl compounds of the type fac-
[Re(CO)3(g2-N,N-dpkbh)X] where X = Cl or Br were
reported with dpkbh binding to the rhenium atom through
the nitrogen atoms of the pyridine rings or through one
nitrogen atom of a pyridine ring and the nitrogen atom
of the hydrazone backbone as shown in Scheme 2
[11,12,30]. The synthesis and cytotoxicity of a series of
metal compounds of the tridentate N,N,O dpkbh anion
of the type [M(g3-N,N,O-dpkbh)2]n where M is a first-
row transition element along with their potential as lipo-
philic transport shuttles that allow entrance to the cell
and enable the delivery of both the ligand and metal to
inhibit proliferation of tumor cells were reported [31,32].
Spectroscopic measurements on dpkbh and fac-
[Re(CO)3(dpkphh)Cl] in non-aqueous solvents show two
interlocked charge transfer transitions that are sensitive
to slight changes in their surroundings and substrates in
the range 1 · 10ꢀ5–1 · 10ꢀ9 M were detected and deter-
mined using dpkbh and its rhenium derivatives [10–12].
In an effort to elucidate the molecular sensing behavior
of dpkbh and its metal compounds, herein we report on
the synthesis, spectroscopic, electrochemical, and optosen-
sing properties of a zinc compound of g3-N,N,O-dpkbh,
[ZnCl2(g3-N,N,O-dpkbh)].
2. Experimental
2.1. Reagents
All reagents were obtained from commercial sources
and used without further purification. The ligand dpkbh
was prepared following a standard literature procedure
[10].
2.1.1. Preparation of [ZnCl2(g3-N,N,O-dpkbh)]
A mixture of ZnCl2 (100 mg, 0.73 mmol), dpkbh
(150 mg, 0.81 mmol) and acetonitrile (50 mL) was allowed
to stand in an ultrasonic bath or refluxed in air for 3 h. The
resulting yellow precipitate was filtered off, washed with
hexane, diethyl ether and dried; yield 190 mg (80%) (found:
C, 49.29; H, 3.26; N, 12.79. C18H14Cl2N4OZn requires C,
49.29; H, 3.22; N, 12.77%). Infrared data (KBr disk,
cmꢀ1): m(NH) 3450 (broad), m(CH) 3057, m(C@O) 1638,
m(py) and m(ph) 1594 and 1575. UV–vis {k/nm, (e 100/
cmꢀ1 Mꢀ1)} in the concentration range 2 · 10ꢀ5
–
2 · 10ꢀ6 M: in DMF 397 (20,000) and 324 (3485); in
DMSO 397 (14,323), 324 (2880), 276 (2850); CH3CN 393
(1680), 355 (sh) 336 (15,460), 290 (sh), 283 (sh), 270
(12,960) and 235 (sh), 225 (14,460); CH2Cl2 396 (315),
358(sh), 342 (18,765), 290 (sh), 282 (sh) and 273 (12,977).
1H NMR (d, ppm): in DMF-d7 at 30 ꢁC 14.88
(s,1H,NH), 9.05 (d,1H), 8.77 (d,1H), 8.18 (t,1H), 8.14
(t,1H), 8.08 (d,21H), 7.99 (d,1H), 7.77–767 (overlapped
1
t’s and d’s, 3H) and 7.65 (t, H). H NMR (d ppm): in
DMSO-d6 at 30 ꢁC 15.05 (NH), 8.95 (d,1H), 8.65 (d,1H),
8.06 (d,t,2H), 8.02 (d,1H), 7.96–7.89 (d,31H), 7.70–7.50
(overlapped t’s and d’s, 3H) and 7.65 (t,8H).
2.2. X-ray crystallography
A single crystal isolated from the filtrate of the reaction
mixture was selected and mounted on a glass fiber with
epoxy cement. A Bruker SMART APEX CCD area detec-
tor diffractometer with a MoKa radiation and a graphite
monochromator was used for data collection and the
SHELXTL software package version 5.1 was used for
structure solution [33,34]. Cell parameters and other crys-
Scheme 2. Views of fac-[Re(CO)3(g2-N,N-dpkbh)X].

M. Bakir et al. / Journal of Molecular Structure 873 (2008) 17–28
19
tallographic information are given in Table 1, along with
additional details concerning data collection. All non-
hydrogen atoms were refined with anisotropic thermal
parameters. All other H-atom positions were refined
assuming idealized geometry, with C–H distance of
chemical measurements were performed with the use of a
Princeton Applied Research (PAR) Model 173 potentio-
stat/galvanostat and Model 276 interface in conjunction
with a 286 PC. Data were acquired with the EG&G PARC
Headstart program and manipulated using the Microsoft
Excel Program. Measurements were performed in solutions
that were 0.1 M in [N(n-Bu)4](PF6). The Ep,a, Ep,c and
Ep = (Ep,a + Ep,c)/2 values were referenced to the quasi-
reversible silver electrode at room temperature and are
uncorrected for junction potentials.
˚
˚
0.93 A and N–H distance of 0.86 A.
2.3. Optical sensing studies
A stock solution of [ZnCl2(g3-N,N,O-dpkbh)] in a polar
non-aqueous solvent, and stock solutions of a stimulus
(NaBH4, NaBF4 or ZnCl2) in a polar non-aqueous solvent
were prepared separately. Optical changes (electronic
absorption spectral changes) were measured on solutions
prepared by mixing appropriate volumes of [ZnCl2(g3-
N,N,O-dpkbh)] and stimulus solutions to prepare the
desired solutions (see figure captions for details). All mea-
surements were made at room temperature.
2.5. Molecular orbitals calculations
PM3 semi-empirical molecular orbital calculations using
default parameters on the lowest electronic state with con-
figurational interaction of 1 fi 1 in the singly excited state
were conducted on a Hyperchem molecular modeling pro-
gram. A Polak–Ribiere energy minimized model using
AMBER force field was used.
2.4. Equipment
3. Results and discussion
Electronic absorption spectra were recorded on a HP-
8452A spectrophotometer or a Perkin-Elmer UV/vis/NIR
spectrometer k-19. Baseline corrections on blank solvents
were recorded prior to measurements. A Lauda–Brink-
mann RM6 circular bath was used for temperature control.
Solution 1H NMR spectra were recorded on a Bruker ACE
500-MHz Fourier-transform spectrometer and referenced
to the residual protons in the incompletely deuteriated sol-
vent. Infrared spectra were recorded as KBr pellets on a
Perkin-Elmer Spectrum 1000 FT-IR Spectrometer. Electro-
When di-2-pyridyl ketone benzoyl hydrazone was
allowed to react with ZnCl2 in acetonitrile under reflux
or ultrasonic radiation [ZnCl2(g3-N,N,O-dpkbh)] was iso-
lated in good yield. The formulation of the compound as
[ZnCl2(g3-N,N,O-dpkbh)] was based upon the results of
its elemental analysis, a number of spectroscopic measure-
ments, and X-ray structural analysis done on a crystal
grown from a DMF solution of [ZnCl2(g3-N,N,O-dpkbh)]
when allowed to stand at room temperature for several
Table 1
Crystal data and structure refinement for [ZnCl2(g3-N,N,O-dpkbh)]
Empirical formula
Formula weight
C18H14Cl2N4OZn
438.60
Temperature
Wavelength
273(2) K
0.71073 A
˚
Crystal system, space group
Unit cell dimensions
Monoclinic, P21/n
˚
a = 8.341(2) A, a = 90ꢁ
˚
˚
b = 13.911(4) A, b = 93.864(5)ꢁ
c = 15.968(4) A, c = 90ꢁ
3
˚
Volume
1848.7(9) A
Z, calculated density
Absorption coefficient
F(000)
Crystal size
Theta range for data collection
Limiting indices
Reflections collected/unique
Completeness to h = 26.00ꢁ
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2r(I)]a,b
R indices (all data)
4, 1.576 mg/m3
1.632 mmꢀ1
888
0.48 · 0.45 · 0.30 mm3
1.94ꢁ to 26.00ꢁ
ꢀ10 ꢁ h ꢁ 10, ꢀ14 ꢁ k ꢁ 17, ꢀ18 ꢁ l ꢁ 19
9631/3600 [R(int) = 0.0183]
99.1%
0.6402 and 0.5080
Full-matrix least-squares on F2
3600/0/235
1.037
R1 = 0.0289, wR2 = 0.0749
R1 = 0.0372, wR2 = 0.0795
0.286 and ꢀ0.182 e A
ꢀ3
˚
Largest diff. peak and hole
a
R1 = RiFojꢀjFci/RjFoj.
2
2
1=2
2
wR2 ¼ fR½wðF o2 ꢀ F c2Þ ꢂ=RwðF o2Þ ꢂg ; where w ¼ ½r2ðF o2Þ þ ð0:043PÞ þ 0:410Pꢂꢀ1 and P ¼ ðF o2 þ 2F 2cÞ=3.
b

20
M. Bakir et al. / Journal of Molecular Structure 873 (2008) 17–28
days. The infrared spectrum of [ZnCl2(g3-N,N,O-dpkbh)]
shows significant changes compared to the spectrum of
the free ligand. The multiple bands observed in the metal
complex between 3600 and 3300 cmꢀ1 may be due to the
participation of the amide proton in hydrogen bonding,
and are in contrast to the non-hydrogen bonded amide
proton in the free ligand reported at 3347 cmꢀ1 [10,35]. A
notable feature that significantly changed when ZnCl2
binds to dpkbh is the amide m(C@O) stretching vibration
observed at 1680 cmꢀ1 in the free ligand shifts to
1633 cmꢀ1 in the metal complex, pointing to the coordina-
tion of the carbonyl group of dpkbh and a decrease in its
bond order and an increase in C–O bond length. In the
C=O stretching region ꢃ900 cmꢀ1, the metal complex
showed a significant decrease in the intensity of the peak
observed at 892 cmꢀ1 compared to that of the free ligand.
In the combined C@C and C@N stretching vibrations of
the pyridine rings, the broad peak at 1575 cmꢀ1 in the free
ligand splits into two sharp peaks at 1598 and 1571 cmꢀ1
may be due to the binding of one nitrogen of the pyridine
rings. In the amide II region ꢃ1530 cmꢀ1, the free ligand
shows two broad peaks between 1550–1490 cmꢀ1 and the
metal complex shows a sharp peak at 1506 cmꢀ1. The three
peaks between 1300–1215 cmꢀ1 which may be due to the
combined C–N or N–N stretching of the free ligand merg-
ing into one sharp peak at 1262 cmꢀ1 in the metal complex.
This hints at the possible coordination of the hydrazone
nitrogen atom. The N–H out-of-plane wagging observed
at 704 cmꢀ1 in the free ligand shifts to 694 cmꢀ1 in the
metal complex.
ax10
d7-dmf
a
bx10
d6-dmso
b
c
d6-dmso
16.00
12.00
8.00
Chemical shift (δ), ppm
4.00
0.00
Fig. 1. 1H NMR spectra of [ZnCl2(g3-N,N,O-dpkbh)] measured in d6-
DMSO (a), d7-DMF (b) and dpkbh in DMSO (c) at 30 ꢁC.
of the chemical shift of the amide versus (1/T)/Kꢀ1 in
DMF and DMSO gave straight lines with slopes of
The NMR spectra of [ZnCl2(g3-N,N,O-dpkbh)] in d7-
DMF and d6-DMSO and free dpkbh in d6-DMSO at
30 ꢁC are shown in Fig. 1. In the spectra of [ZnCl2(g3-
N,N,O-dpkbh)], peaks appeared in the amide, aromatic,
and solvent regions consistent with the coordination of
dpkbh. The spectra of [ZnCl2(g3-N,N,O-dpkbh)] show sen-
sitivity to solvent variations (see Section 2) and point to
strong solvent–solute interaction. The ratio of the intensity
of the amide proton to the intensity of the low field one
proton of the aromatic ring is 0.5, 0.6, and 0.7 for
[ZnCl2(g3-N,N,O-dpkbh)] in d7-DMF and d6-DMSO and
dpkbh in d6-DMSO. These results suggest a possible
exchange of the amide proton with the protons of the
incompletely deuteriated solvents with the exchange faster
in d7-DMF. The aromatic protons of [ZnCl2(g3-N,N,O-
dpkbh)] shifting to more positive values in d7-DMF com-
pared to d6-DMSO may be due to stronger hydrogen bond-
ing interaction between d7-DMF and the amide proton that
lead to a decrease in electron density about the coordinated
dpkbh [35]. A comparison of the chemical shift of the aro-
matic protons of [ZnCl2(g3-N,N,O-dpkbh)] with those of
free dpkbh in d6-DMSO shows slight changes in the split-
ting of resonances between 7.70–7.45 ppm and indicates
that the interaction between ZnCl2 and the pyridine ring
is associated with these protons. The chemical shift of the
amide proton is sensitive to temperature variations (see
Fig. 2) consistent with solvent–solute interaction. A plot
ax10
a
30 °C
bx10
b
40 °C
cx10
c
50 °C
dx10
d
60 °C
16.00
12.00
8.00
4.00
0.00
Chemical shift (δ), ppm
Fig. 2. 1H NMR spectra of [ZnCl2(g3-N,N,O-dpkbh)] measured in d6-
DMSO at 30 ꢁC (a), 40 ꢁC (b), 50 ꢁC (c) and 60 ꢁC (d).

M. Bakir et al. / Journal of Molecular Structure 873 (2008) 17–28
21
998 32 and 1209 54, respectively, and suggests the
higher sensitivity of [ZnCl2(g3-N,N,O-dpkbh)] to tempera-
ture variations in d6-DMSO compared to d7-DMF.
The electronic absorption spectra of [ZnCl2(g3-N,N,O-
dpkbh)] in different solvents are shown in Fig. 3 and a list
of the electronic transitions is included in the experimental
section. The solvent dependence of the electronic absorp-
tion confirms the strong solvent–solute interaction, as
noted from NMR results. The electronic transitions are
ILCT of the donor–acceptor type and may be due to p-
p
transitions of the pyridine rings of dpk, followed by
*
dpk to benzoyl charge transfer, or vice versa, and are sim-
ilar to those observed of di-2-pyridyl ketone hydrazones
and their metal compounds [10–12]. PM3 semi-empirical
calculations on an energy minimized model, substantiated
the aforementioned assignment (see Fig. 4), and gave a
HOMO–LUMO energy gap of 4.98 eV.
Fig. 4. Semi-empirical energy level diagram of a Hyperchem AMBER
energy minimized [ZnCl2(g3-N,N,O-dpkbh)].
The electronic transitions and relative abundance of two
complex-substrate forms of [ZnCl2(g3-N,N,O-dpkbh)] are
sensitive to changes in their surroundings. In a binary
DMSO/DMF co-solvent system, the intensity of the low-
energy electronic transition increases and the intensity of
the high-energy electronic transition decreases as the vol-
ume% of DMF/DMSO increases (Fig. 5). These results
confirm the sensitivity of the system to slight variations
in the medium and hints at the inter-conversion between
two complex-substrate forms of [ZnCl2(g3-N,N,O-
dpkbh)]. This was confirmed when [ZnCl2(g3-N,N,O-
dpkbh)] was subjected to chemical and physical stimuli that
causes the equilibrium distribution of the species corre-
sponding to the high- and low-energy electronic transitions
1.00
1
4
0.80
0.60
0.40
0.20
0.00
Fig. 5. A plot of the absorbance of [ZnCl2(g3-N,N,O-dpkbh)] versus
volume% of DMSO/DMF co-solvent at 396 nm (a) and 324 nm (b).
2
3
(henceforth denoted as b and a, respectively). Fig. 6 shows
the spectra of a mixture of [ZnCl2(g3-N,N,O-dpkbh)] and
NaBH4 in DMSO when allowed to react with increasing
amounts of KPF6. Under these conditions the electronic
spectra of the resulting solutions show the intensity of the
low-energy electronic transition to decrease and the inten-
sity of the high-energy electronic transition to increase
upon an increase in the concentration of KPF6. The same
was observed when increasing amounts of NaBF4 were
added to a DMF solution of [ZnCl2(g3-N,N,O-dpkbh)].
The reverse was obtained when a DMSO solution of
[ZnCl2(g3-N,N,O-dpkbh)] was allowed to react with
increasing amounts of NaBH4. These results support
reversible inter-conversion between the two states of
[ZnCl2(g3-N,N,O-dpkbh)] and allowed for calculation of
the extinction coefficients e of the a and b forms at
2
3
4
300.00
400.00
500.00
Wavelength, nm
Fig. 3. The electronic absorption spectra of 4.00 · 10ꢀ5 M [ZnCl2(g3-
N,N,O-dpkbh)] in DMF (1), DMSO (2), CH3CN (3) and CH2Cl2 (4).

22
M. Bakir et al. / Journal of Molecular Structure 873 (2008) 17–28
vs (103/T)/Kꢀ1 1yielded straight lines with slopes of +23.3
and ꢀ14.1 and intercepts of ꢀ51 and +31 in DMF and
DMSO, respectively.
These results substantiate that the equilibrium between
the configurational states allows for calculation of the ther-
modynamic parameters (see Table 3) pertaining to inter-
conversion between the configurations of [ZnCl2(g3-
N,N,O-dpkbh)] in DMF and DMSO. The absorbance
measurements demonstrate the facile conversion between
states and their high sensitivity to slight changes in the
composition of the surrounding medium and possible use
as molecular probes for sensing purposes.
When [ZnCl2(g3-N,N,O-dpkbh)] in DMSO (total con-
centration c = 4 · 10ꢀ5 M) was allowed to interact with
added ZnCl2, the intensity of the peak at 400 nm corre-
sponding to the electronic transition of the a form was
found to increase, while at the same time the intensity of
the peak at 320 nm (corresponding to electronic absorption
in the b form) decreased (Fig. 7). A plot of the absorbance
versus concentration of ZnCl2 is shown in Fig. 8 and shows
that ZnCl2 in concentrations as low as 1 · 10ꢀ5 M can be
detected and determined using [ZnCl2(g3-N,N,O-dpkbh)]
in DMSO. The extent of binding of ZnCl2 with
[ZnCl2(g3-N,N,O-dpkbh)] can be estimated from the fol-
lowing model
b-½ZnCl2ðg3-N; N; O-dpkbhÞꢂ $ a-½ZnCl2
Fig. 6. Electronic absorption spectra of a mixture of [ZnCl2(g3-N,N,O-
dpkbh)] 4.00 · 10ꢀ5 M and 4 · 10ꢀ5 M NaBH4 in DMSO in the presence
of (1) 5.00 · 10ꢀ4, (2) 3.00 · 10ꢀ3, (3) 8.00 · 10ꢀ3, (4) 2.00 · 10ꢀ2, (5)
4.00 · 10ꢀ2, (6) 6.00 · 10ꢀ2 and (7) 8.00 · 10ꢀ2 M KPF6.
ðg3-N; N; O-dpkbhÞꢂ
ð1Þ
ð2Þ
a-½ZnCl2ðg3-N; N; O-dpkbhÞꢂ þ ZnCl2
$ a0-½ZnCl2ðg3-N; N; O-dpkbhÞꢂ ꢄ ZnCl2
where b is the form observed at the high-energy transition,
a that observed at the low-energy transition and a0 is the
form associated with the binding of ZnCl2 to [ZnCl2(g3-
N,N,O-dpkbh)].
The total absorbance at any wavelength k, Ak ¼
eak½aꢂ þ ebk½bꢂ, can be expressed in terms of the equilibrium
constant K = [a]/[b] for the inter-conversion of a and b
and total concentration c = [a] + [b] as
arbitrary wavelengths. Values for e at the two wavelengths
of interest are given in Table 2.
Our results show that the optical behavior of [ZnCl2(g3-
N,N,O-dpkbh)] in DMF in the presence and absence of
NaBH4 and in DMSO in the presence of NaBH4 is similar
but different from that observed in pure DMSO. The exact
nature of the interaction between [ZnCl2(g3-N,N,O-
dpkbh)] and different solvents or solutions remains to be
elucidated.
1
The electronic absorption spectra of [ZnCl2(g3-N,N,O-
dpkbh)] in polar solvents showed a marked variation with
temperature. In DMF, when temperature is increased, the
intensity of the low-energy transition peak (a) decreases,
while that of the high-energy transition (b) increases. The
opposite was observed when temperature was decreased
or when DMSO was used in place of DMF. Plots of RlnK
For the following inter-conversion:
b½ZnCl2ðg3-N; N; O-dpkbhÞꢂ ¢ a½ZnCl2ðg3-N; N; O-dpkbhÞꢂ
the total absorbance at wavelength
k
equals Ak ¼ eak½aꢂ þ ebk½bꢂ ¼
ebkc þ ðeak ꢀ ebkÞ½aꢂ, where c = [a] + [b] is the total concentration. Thus,
½aꢂ ¼ ðAk ꢀ ebkcÞ=ðeka ꢀ ebkÞ, so that the equilibrium constant K = [a]/[b] at
temperature T equals
AkðTÞ ꢀ ebc
k
KðTÞ ¼
;
eak c ꢀ AkðTÞ
which should be independent of k (the occurrence of an isosbestic point in
the spectra at different temperatures indicates that the extinction coeffi-
cients are temperature independent). The data thus obtained should then
be substituted into the thermodynamic relationship
Table 2
Extinction coefficients of [ZnCl2(g3-N,N,O-dpkbh)] in DMF and DMSO
(
100, in Mꢀ1 cmꢀ1
)
DGh
DHh
¼ R ln K ¼ DSh ꢀ
;
Solvent
ea320
eb320
ea400
eb400
ꢀ
T
T
DMF
DMSO
3125
3665
15,500
15,700
20,700
20,900
1000
0
where DX = XaꢀXb, so that a plot of RlnK vs. 1/T should yield a straight
line with slope ꢀDHh and intercept DSh.

M. Bakir et al. / Journal of Molecular Structure 873 (2008) 17–28
23
Table 3
Thermodynamic parameters for the inter-conversion between the two forms of [Zn(g3-N,N,O-dpkbh)Cl2]
Solvent
DHh/kJ molꢀ1
DSh/J molꢀ1Kꢀ1
DGh/kJ molꢀ1 (25 ꢁC)
K (25 ꢁC)
DMF
DMSO
ꢀ23.3 0.5
ꢀ51
4
2
ꢀ8.16 0.02
27.0 0.2
0.16 0.04
+14.1 0.2
+31
+4.54 0.61
0.80
0.60
0.40
0.20
0.00
1.00
0.80
0.60
0.40
0.20
0.00
9
7
a
1
6
3
4
5
5
4
6
3
7
8
b
2
1
0.00
0.20
0.40
0.60
0.80
1.00
[Concentration]x102, M
300.00
400.00
500.00
Fig. 8. A plot of the absorbance of [ZnCl2(g3-N,N,O-dpkbh)] 4.00 · 10ꢀ5
M in DMSO at 396 and 324 nm, respectively, versus concentration of
ZnCl2.
Wavelength, nm
Fig. 7. Electronic absorption spectra of [ZnCl2(g3-N,N,O-dpkbh)]
4.00 · 10ꢀ5 M in DMSO in the presence of (1) 0.00, (2) 1.00 · 10ꢀ4, (3)
3.00 · 10ꢀ4
,
(4) 5.00 · 10ꢀ4
,
(5) 1.00 · 10ꢀ3
,
(6) 3.00 · 10ꢀ3
,
(7)
5.00 · 10ꢀ3, (8) 7.00 · 10ꢀ3 and (9) 9 · 10ꢀ3 M ZnCl2.
where [M] = concentration of free ZnCl2 and [M]T = total
concentration of ZnCl2, and the binding constant for (2) is
defined as
Keak þ eb
Ak ¼
k c;
ð3Þ
½a0ꢂ
1 þ K
K0 ¼
:
ð7Þ
½Mꢂ½aꢂ
so that, once the extinction coefficients e for a and b at k
(which, to simplify the notation, are taken to include the
path length l = 1 cm) are known, this can be used to find
K (which should, of course, be independent of k) from
the slope rk of a plot of Ak vs c, as obtained from a simple
dilution experiment, according to
Substitution of (6) and (7) into (5) leads to the following
quadratic equation in terms of [a],
2
K0ð1 þ Kꢀ1Þ½aꢂ þ ð1 þ Kꢀ1 þ K0ð½MꢂT ꢀ cÞÞ½aꢂ ꢀ c ¼ 0;
ð8Þ
eb ꢀ rk
k
K ¼
:
ð4Þ
rk ꢀ eak
The absorbance at some wavelength k is equal to
Ak ¼ eakð½aꢂ þ ½a0ꢂÞ þ ebk½bꢂ
Application of this model gave K values of 27.0 and 0.16 in
DMF and DMSO, respectively.
¼ eakc þ ðekb ꢀ eakÞ½aꢂ=K:
ð9Þ
When ZnCl2 is added to a solution of [ZnCl2(g3-N,N,O-
dpkbh)] the following relations are obeyed at equilibrium
After inserting the solution of (8) into (9), solving the
resulting equation for K0 and using (4) we obtain, after
some lengthy algebra,
½aꢂ þ ½a0ꢂ þ ½bꢂ ¼ ½a0ꢂ þ ð1 þ Kꢀ1Þ½aꢂ ¼ c;
ð5Þ
ðebk ꢀ ekaÞðrk ꢀ ekaÞðAk ꢀ rkcÞ
and
K0 ¼
;
ð10Þ
½Mꢂ ¼ ½MꢂT ꢀ ½a0ꢂ ¼ ½MꢂT þ ð1 þ Kꢀ1Þ½aꢂ ꢀ c;
ð6Þ
ðrk ꢀ ebkÞðAk ꢀ ekacÞðAk ꢀ rkc þ ½MꢂTðrk ꢀ eakÞÞ

24
M. Bakir et al. / Journal of Molecular Structure 873 (2008) 17–28
which should produce the same value of K0, regardless of
the chosen spectrum (i.e. ZnCl2 concentration) and value
of k (except the one corresponding to the isosbestic point
where the extinction coefficients of a and b are equal).
The values for the extinction coefficients at the wavelengths
of interest in this study, 320 and 400 nm, have been ob-
tained using the spectral shifts towards either a or b form
in different solvents via addition of NaBH4 and/or KPF6.
To check the reliability of these values, and the internal
consistency of the model, a general relationship between
extinction coefficients at different wavelengths can be de-
rived based on the following argument.
200 μA
a
b
From (9) and the mass balance equation (3) it follows
that Ak ꢀ eac ¼ ðeb ꢀ eaÞ½bꢂ and similarly Ak ꢀ ebc ¼
k
k
k
k
ðeak ꢀ ebkÞð½aꢂ þ ½a0ꢂÞ, which are valid for each spectrum in
Fig. 7. If we compare any two spectra, henceforth denoted
by superscripts (1) and (2), the first of these relations
implies that
ð1Þ
320
ð1Þ
400
ð2Þ
320
ð2Þ
400
A
A
ꢀ ea320
ꢀ ea400
c
c
A
A
ꢀ ea320
ꢀ ea400
c
c
eb320 ꢀ ea320
eb400 ꢀ ea400
¼
¼
;
ð11Þ
2.00
1.00
0.00
-1.00
-2.00
Volts vs Ag
Fig. 9. Cyclic voltammograms of [ZnCl2(g3-N,N,O-dpkbh] in DMF
solutions (0.1 M in [N(n-Bu)4]PF6 at a glassy carbon working electrode
which is clearly independent of the choice of the two spec-
tra, and so therefore is the first equality, which can be reor-
ganized to
recorded at a scan rate of 400 mV sꢀ1
.
ðAð410Þ0 ꢀ Að420Þ0Þea320 þ ðA3ð22Þ0 ꢀ Að312Þ0Þe4a00
ð2Þ ð1Þ
ꢀ Að1Þ Að420Þ0Þ=c:
ð12Þ
reductions appeared at Ep,c = ꢀ0.78 andꢀ0.93 V and clo-
sely-spaced multi-electron irreversible reductions appeared
at Ep,c = ꢀ1.47, ꢀ1.58, and ꢀ1.75 V, followed by multi-
¼ ðA
A
320 400
320
The second form of (9) leads to an identical linear equation
for eb320 and e4b00. Based on the data of Fig. 7, this relation-
a;b
a;b
ship takes the form e þ 0:58e ¼ 15; 700 for the solu-
320
400
200 μA
tions in DMSO. The analogous relationship for the
solutions in DMF is somewhat similar,
a;b
320
a;b
400
a
e
þ 0:63e ¼ 16; 130. The values for e listed in Table 2
are seen to be reasonably consistent with these equations.
This same linear relationship can be readily shown to exist
also between the slopes r320 and r400 of the dilution plots,
as a consequence of the fact that in (3), K should be inde-
pendent of k. Moreover, in view of the requirement that K0
as obtained from (10) should be invariant with respect to
changes in k, it is possible to show that this relationship
also applies to A320/c and A400/c for arbitrary [M]T. Com-
pliance with these conditions can be verified from direct
inspection of the dilution plots and spectra. Our data for
the solutions in DMSO imply K = 0.16 and support a value
of K0 somewhere in the range 6000–8000 Mꢀ1. The latter
follows after repeated application of (10) to the spectra
of Fig. 7 at the two chosen wavelengths and different con-
centrations of added ZnCl2. A similar calculation for the
case of DMF was not feasible because the a form is already
strongly favored by the solvent itself.
b
c
2.00
1.00
0.00
-1.00
-2.00
The electrochemical properties of [ZnCl2(g3-N,N,O-
dpkbh] were investigated using voltammetric techniques.
Cyclic voltammograms of [ZnCl2(g3-N,N,O-dpkbh] mea-
sured in DMF are shown in Fig. 9. In a reductively initi-
ated scan (Fig. 9a), two closely-spaced irreversible
Volts vs Ag
Fig. 10. Cyclic voltammograms of dpkbh measured in the absence (a) and
in the presence of ZnCl2 (b) along with a cyclic voltammogram of ZnCl2
(c) measured in DMF solutions 0.1 M in [N(n-Bu)4]PF6 at a glassy carbon
working electrode measured at a scan rate of 400 mV sꢀ1
.

M. Bakir et al. / Journal of Molecular Structure 873 (2008) 17–28
25
Fig. 11. The molecular structure of [ZnCl2(g3-N,N,O-dpkbh)]. The thermal ellipsoids are drawn at the 30% probability level and H atoms are shown as
small spheres of arbitrary radii.
electron irreversible oxidations at Ep,a = +1.42 and
+1.76 V. A reductive square wave voltammogram mea-
sured on the same solution confirmed the redox potentials
observed in a reductively initiated cyclic voltammogram.
The first two closely-spaced reductions can be assigned to
Zn2+/+ and Zn+/0 reductions and the other waves may be
due to ligand-based reductions. In an oxidatively-initiated
scan (see Fig. 9b), irreversible oxidations appeared at
Ep,a = +1.46 and 1.74 V, followed by an electrochemi-
cally-generated product wave at Ep,c = ꢀ0.67 (broad) and
irreversible reductions at Ep,c = ꢀ0.91, ꢀ1.26 and
ꢀ1.55 V. The appearance of an oxidatively generated prod-
uct wave upon oxidatively-initiated scan signals oxidative
decomposition of [ZnCl2(g3-N,N,O-dpkbh]. The oxidative
decomposition of [ZnCl2(g3-N,N,O-dpkbh] remains to be
explored.
The electrochemical reaction of dpkbh with ZnCl2 was
investigated. Fig. 10 shows reductively-initiated cyclic vol-
tammograms measured in DMF of dpkbh in the presence
and absence of ZnCl2, along with the voltammogram of
pure ZnCl2. These voltammograms show the addition of
ZnCl2 to a DMF solution of dpkbh leading to an electro-
chemical signature similar to that of [ZnCl2(g3-N,N,O-
dpkbh] measured in DMF, and hints at a facile reaction
between dpkbh and ZnCl2. The absence of electrochemi-
cally-generated product waves upon coordination of ZnCl2
to dpkbh, as apparent from the absence of the reductively
generated oxidative waves observed at Ep,a = +0.45 V in
free dpkbh (Fig. 10a), suggests increased stability of dpkbh
upon coordination to ZnCl2. The appearance of Zn2+/0
reduction waves at more positive values compared to the
free ZnCl2 suggests a decrease in the electron density of
ZnCl2 upon coordination to dpkbh.
The solid state structure of [ZnCl2(g3-N,N,O-dpkbh)]
was determined using X-ray structural analysis. A view of
the molecular structure of [ZnCl2(g3-N,N,O-dpkbh)] is
shown in Fig. 11 and reveals pseudo trigonal-bipyramidal
coordination about zinc with an index of trigonality
(s) = 0.33 indicating that the irregular coordination geom-
etry of [ZnCl2(g3-N,N,O-dpkbh)] in the solid state is 33%
along the pathway of distortion from square-pyramidal
to trigonal-bipyramidal [36].2 The coordinated atoms are
two chlorine atoms and one nitrogen atom of the hydra-
zone moiety of dpkbh occupy the equatorial positions
and the axial positions are occupied by a nitrogen atom
of the coordinated pyridine ring and the oxygen atom of
the benzoyl moiety. The coordinated dpkbh forms two
five-membered metallocyclic rings (Zn1–N1–C15–C–N3)
and (Zn1–N3–N4–C–O1) fused along the Zn–N junction
and leaves one pyridine ring exposed for potential binding
to other substrates. The N–N and N–O bite angles of
73.05(6)ꢁ and 71.85(6)ꢁ, respectively are significantly smal-
ler than the 90ꢁ between the axial and equatorial ligands.
The phenyl ring and hydrazone backbone are co-planar,
and non-coplanar with the pyridyine rings. Deviation from
trigonal bipyramidal geometry is due to the tridentate
binding of g3-N,N,O-dpkbh as apparent from the N–N
2
For a five-coordinate system similar to that shown in Fig. 11, the
trigonality index s = (bꢀa)/60 where b = \N1–Zn1–O1, a = \N3–Zn1–
Cl1. Thus s = 0 for a perfect tetragonal geometry and s = 1 for a perfect
trigonal-bipyramidal geometry.

26
M. Bakir et al. / Journal of Molecular Structure 873 (2008) 17–28
Table 4
3
˚
Bond lengths [A] and angles [ꢁ] for ZnCl2(g -N,N,O-dpkbh)
Bond lengths
Zn(1)–N(1)
Zn(1)–Cl(2)
Zn(1)–O(1)
N(1)–C(15)
C(12)–C(13)
C–N(3)
N(2)–C(21)
C(21)–C(22)
N(3)–N(4)
C(1)–O(1)
2.1679(16)
2.2232(8)
2.2274(14)
1.348(3)
1.363(3)
1.297(2)
1.326(3)
1.383(3)
1.354(2)
1.225(2)
1.382(3)
1.376(3)
Zn(1)–N(3)
Zn(1)–Cl(1)
N(1)–C(11)
C(11)–C(12)
C(15)–C
2.1452(16)
2.2267(8)
1.331(3)
1.379(3)
1.494(3)
1.483(3)
1.347(3)
1.366(3)
1.366(2)
1.473(3)
1.393(3)
C–C(25)
N(2)–C(25)
C(22)–C(23)
N(4)–C(1)
C(1)–C(31)
C(31)–C(36)
C(31)–C(32)
C(32)–C(33)
Bond angles
N(3)–Zn(1)–N(1)
N(3)–Zn(1)–Cl(1)
N(3)–Zn(1)–O(1)
Cl(1)–Zn(1)–O(1)
C(11)–N(1)–Zn(1)
N(1)–C(15)–C(14)
C(14)–C(15)–C
N(3)–C–C(15)
C(21)–N(2)–C(25)
N(2)–C(25)–C
C–N(3)–N(4)
N(4)–N(3)–Zn(1)
O(1)–C(1)–N(4)
N(4)–C(1)–C(31)
C(32)–C(31)–C(36)
C(36)–C(31)–C(1)
73.05(6)
124.89(5)
71.85(6)
N(1)–Zn(1)–Cl(2)
Cl(2)–Zn(1)–Cl(1)
N(1)–Zn(1)–O(1)
C(11)–N(1)–C(15)
C(15)–N(1)–Zn(1)
N(1)–C(15)–C
101.39(5)
116.07(3)
144.76(6)
118.53(17)
117.22(13)
114.02(16)
124.67(18)
122.84(16)
123.7(2)
122.23(18)
120.58(13)
115.79(15)
123.17(17)
116.36(12)
123.43(18)
120.4(2)
96.68(5)
122.85(13)
121.53(18)
124.33(17)
112.44(16)
117.97(18)
115.77(16)
121.79(16)
116.28(11)
119.66(17)
117.17(17)
118.93(19)
117.63(18)
N(3)–C–C(25)
C(25)–C–C(15)
N(2)–C(21)–C(22)
C(24)–C(25)–C
C–N(3)–Zn(1)
N(3)–N(4)–C(1)
O(1)–C(1)–C(31)
C(1)–O(1)–Zn(1)
C(32)–C(31)–C(1)
C(33)–C(32)–C(31)
and N–O bite angles of coordinated atoms (see Table 4).
The tridentate binding mode of dpkbh is in contrast to
the bidentate N,N-binding mode of dpkbh observed in
fac-[Re(CO)3(g2-N,N-dpkbh)X],
dpkbh)Cl]Æsolvate, where X = Cl or Br and sol-
fac-[Re(CO)3(g2-N,N-
vate = DMSO or DMF and dpkbh binds to the metal cen-
Fig. 12. A view of the packing of [ZnCl2(g3-N,N,O-dpkbh)].

M. Bakir et al. / Journal of Molecular Structure 873 (2008) 17–28
27
ter through the nitrogen atoms of the pyridine rings, and
fac-[Re(CO)3(g2-N,N-dpkbh)Br]Æthf where dpkbh binds
to the metal center through a nitrogen atom of a pyridine
ring and the nitrogen atom of the hydrazone backbone
and is similar to the binding mode of dpkbh in
[ZnCl2(Brdpkbh)], [Fe(dpkbh)2] and other related com-
pounds of N,N,O-tridentate di-2-pyridyl ketone hydraz-
ones [11,12,30,32,37].
ordinated pyridine ring and participates in a classical intra-
molecular hydrogen bond of the type N–Hꢄ ꢄ ꢄN (see
Fig. 13). The covalent and non-covalent bond distances
and angles are normal and similar to those observed in
dpkbh and a variety of its compounds [10–12].
4. Summary
The packing of molecules shows digitated stacks of
[ZnCl2(g3-N,N,O-dpkbh)] connected via a network of
hydrogen bonds (see Fig. 12 and Table 5). The chlorine
and oxygen atoms of [ZnCl2(g3-N,N,O-dpkbh)] participate
in non-classical intermolecular hydrogen bonds of the type
C–Hꢄ ꢄ ꢄX where X = Cl or O (see Fig. 13). The amide pro-
ton is in close proximity to the nitrogen atom of the unco-
Spectroscopic measurements on solutions of [ZnCl2(g3-
N,N,O-dpkbh)] isolated from the reaction between ZnCl2
and dpkbh in acetonitrile under reflux or ultrasonic radia-
tion show sensitivity of [ZnCl2(g3-N,N,O-dpkbh)] to slight
changes in its surroundings. Thermodynamic and optosen-
sing studies reveal that facile inter-conversion between two
ILCT transitions and substrates in concentrations as low as
1 · 10ꢀ5 M can be detected and determined using
[ZnCl2(g3-N,N,O-dpkbh)] in polar solvents. X-ray struc-
tural studies on [ZnCl2(g3-N,N,O-dpkbh)] confirmed its
identity and revealed digitated stacks of [ZnCl2(g3-
N,N,O-dpkbh)] interconnected via a network of hydrogen
bonds. In view of the convenient synthesis of di-2-pyridyl
ketone hydrazones and their metal compounds, their inter-
esting physico-chemical properties and potential use in the
development of molecular sensors, studies are underway
in our laboratory to further explore the coordination
Table 5
3
˚
Hydrogen bonds for ZnCl2(g -N,N,O-dpkbh) [A and ꢁ]
D–Hꢄ ꢄ ꢄA
d(D–H)
d(Hꢄ ꢄ ꢄA)
d(Dꢄ ꢄ ꢄA)
<(DHA)
N(4)–H(4)ꢄ ꢄ ꢄN(2)
0.86
0.93
0.93
0.93
1.98
2.57
2.78
2.82
2.605(2)
3.167(3)
3.504(3)
3.668(2)
128.9
122.4
135.1
151.5
C(13)–H(13)ꢄ ꢄ ꢄO(1)1
C(21)–H(21)ꢄ ꢄ ꢄCl(2)2
C(22)–H(22)ꢄ ꢄ ꢄCl(1)3
Symmetry transformations used to generate equivalent atoms: 1x ꢀ 1/2,
ꢀy + 3/2, z ꢀ 1/2; 2ꢀx, ꢀy + 1, ꢀz + 2 and 3ꢀx + 1/2, y ꢀ 1/2, ꢀz + 3/2.
Fig. 13. Views of the hydrogen bonds in [ZnCl2(g3-N,N,O-dpkbh)].

28
M. Bakir et al. / Journal of Molecular Structure 873 (2008) 17–28
[14] M. Bakir, C. Gyles, J. Mol. Struct. 649 (2003) 133.
[15] M. Bakir, K. Abdur-Rashid, Trans. Metal Chem. 24 (1999) 384.
[16] M. Bakir, K. Abdur-Rashid, W.H. Mulder, Talanta 51 (2000) 735.
[17] M. Bakir, C. Gyles, Talanta 56 (2002) 1117.
[18] M. Bakir, O. Brown, J. Mol. Struct. 641 (2002) 183.
[19] M. Bakir, Eur. J. Inorg. Chem. (2002) 481.
chemistry and sensing behavior of this and other di-2-pyr-
idyl ketone derivatives.
Acknowledgments
[20] M. Bakir, Inorg. Chim. Acta 332 (2002) 1.
[21] M. Bakir, O. Green, Acta Cryst. C58 (2002) 263.
[22] M. Bakir, Acta Cryst. C57 (2001) 1371.
We acknowledge Ms. Toni Johnson for her assistance
with the NMR measurements and Dr. Bohari M. Yamin
for performing the X-ray measurements.
[23] M. Bakir, Acta Cryst. C57 (2001) 1154.
[24] M. Bakir, J.A.M. McKenzie, J. Electroanal. Chem. 425 (1997) 61.
[25] M. Bakir, J.A.M. McKenzie, J. Chem. Soc. Dalton Trans. (1997)
3571.
References
[26] M. Bakir, J. Electroanal. Chem. 466 (1999) 60.
[27] M. Bakir, I. Hassan, O. Green, J. Mol. Struct. 657 (2003) 75.
[1] Y.-C. Liu, C.-I. Li, W.-Y. Yeh, G.-H. Lee, S.-M. Peng, Inorg. Chim.
Acta 359 (2006) 2361.
´
[28] J.J. Pinto, C. Moreno, M. Garcıa-Vargas, Talanta 64 (2004) 562.
[2] M.C. Tseng, J.L. Ke, C.C. Pai, S.P. Wang, W.L. Huang, Polyhedron
25 (2006) 2160.
[29] L.H.S.A. Terra, M. Guekezian, I. Gaubeur, J.R. Matos, M.E.V.
´
Suarez-Iha, Polyhedron 21 (2002) 2375.
[3] V. Philip, V. Suni, M.R.P. Kurup, M. Nethaji, Polyhedron 25 (2006)
1931.
[4] C.J. Milios, S. Piligkos, A.R. Bell, R.H. Laye, S.J. Teat, R. Vicente,
E. McInnes, A. Escuer, S.P. Perlepes, R.E.P. Winpenny, Inorg.
Chem. Commun. 9 (2006) 638.
[30] J. Grewea, A. Hagenbacha, B. Stromburga, R. Albertob, E. Vazquez-
Lopez, U. Abrama, Z. Anorg. Allg. Chem. 629 (2003) 303.
[31] P.V. Bernhardt, J. Mattsson, D.R. Richardson, Inorg. Chem. 45
(2006) 752.
[32] P.V. Bernhardt, L.M. Caldwell, T.B. Chaston, P. Chin, D.R.
Richardson, J. Biol. Inorg. Chem. 8 (2003) 866.
[33] Bruker-SHELXTL, Software Version 5.1. Brucker AXS, Inc., Mad-
ison, Wisconsin, USA, 1997.
[34] G.M. Sheldrick, SHELX97 and SHELXL97, University of Go¨ttin-
gen, Germany, 1997.
[35] R.M. Silverstein, F.X. Webster, Spectroscopic Identification of
organic Compounds, sixth ed., John Wiley and Sons Inc., New
York, 1997.
[36] A.W. Addison, T.N. Rao, J. Reedijk, J.v. Rijn, G.C. Verschoor, J.
Chem. Soc, Dalton Trans. (1984) 1349.
[37] C.E. Kyriakidis, P.C. Christidis, P.J. Rentzeperis, I.A. Tossidis, Z.
Kristallogr. 193 (1990) 261.
[5] S. Youngme, J. Phatchimkun, U. Suksangpanya, C. Pakawatchai,
G.A. van Albada, M. Quesada, J. Reedijk, Inorg. Chem. Commun. 9
(2006) 242.
[6] P.V. Bernhardt, J. Mattsson, D.R. Richardson, Inorg. Chem. 23
(2006) 752.
[7] X.-D. Chen, T.C.W. Mak, Inorg. Chim. Acta 359 (2006) 685.
[8] M. Bakir, O. Green, C. Gyles, Inorg. Chim. Acta 358 (2005) 1835.
[9] M. Bakir, C. Gyles, J. Mol. Struct. 753 (2005) 35.
[10] M. Bakir, O. Brown, J. Mol. Struct. 609 (2002) 129.
[11] M. Bakir, O. Brown, Inorg. Chim. Acta 353 (2003) 89.
[12] M. Bakir, O. Brown, T. Johnson, J. Mol. Struct. 691 (2004) 265.
[13] M. Bakir, K. Abdur-Rashid, C. Gyles, Spectrochim. Acta Part A 59
(2003) 2123.