X-ray crystallographic and molecular sensing properties of [ZnCl2(κ2-N,N′-dpknph)] (dpknph = di-2-pyridyl ketone-p-nitrophenyl hydrazone)

Article abstract of DOI:10.1016/j.molstruc.2008.01.015

When ZnCl₂ was allowed to react with di-2-pyridyl ketone p-nitrophenylhydrazone (dpknph) in acetonitrile under reflux, [ZnCl₂ (κ² - N, N^′ -dpknph)] was isolated in good yield. The formulation of the isolated product was established from the results of its elemental analysis and a number of spectroscopic measurements. Single crystal X-ray structural analysis done on [ZnCl₂ (κ² - N, N^′ -dpknph)].dmf isolated from a dmf solution of [ZnCl₂ (κ² - N, N^′ -dpknph)] shows the pyridine N,N′-bidentate binding of dpknph and revealed a serpentine polymeric network of [ZnCl₂ (κ² - N, N^′ -dpknph)].dmf units locked via a web of hydrogen bonds. ¹H NMR spectra of [ZnCl₂ (κ² - N, N^′ -dpknph)] measured in d₆-dmso, d₇-dmf and d₆-acetone confirmed the coordination of dpknph and showed partial exchange of the amide proton with solvent protons in d₆-acetone and that [ZnCl₂ (κ² - N, N^′ -dpknph)] is stable in dmso and dmf. ¹H NMR variable temperature studies confirmed the stability of [ZnCl₂ (κ² - N, N^′ -dpknph)] in d₆-dmso and d₇-dmf and showed the presence of temperature-dependent shielded protons due to hydrogen bonding. The electronic absorption spectra of [ZnCl₂ (κ² - N, N^′ -dpknph)] in non-aqueous media show a single solvent-dependent intra-ligand-charge transfer (ILCT) transition of the donor-acceptor type between 300 and 700 nm. Thermo-optical measurements in protophilic solvent (dmso or dmf) between 303.15 and 353.15 K show the stability of [ZnCl₂ (κ² - N, N^′ -dpknph)] at those temperatures. In the presence of a base, a new ILCT peak appeared at 580 ± 2 nm in dmso, and the addition of an acid to a mixture of [ZnCl₂ (κ² - N, N^′ -dpknph)] and a base showed the disappearance of the peak at 580 ± 2 nm and reappearance of the peak due to [ZnCl₂ (κ² - N, N^′ -dpknph)]. These results established reversible inter-conversion between [ZnCl₂ (κ² - N, N^′ -dpknph)] and its conjugate base, and allow for the use of [ZnCl₂ (κ² - N, N^′ -dpknph)] in dmso to measure and detect substrates in concentrations as low as 1.00 × 10^-5 M. Thermo-optical measurements on a mixture of [ZnCl₂ (κ² - N, N^′ -dpknph)] and NaBH₄ in dmso confirmed the reversible inter-conversion between [ZnCl₂ (κ² - N, N^′ -dpknph)] and its conjugate base, and gave changes in enthalpy of 43.32 kJ/mol and entropy of 138.87 JK^-1 mol^-1 at 298.15 K.

Full text of DOI:10.1016/j.molstruc.2008.01.015

Available online at www.sciencedirect.com
Journal of Molecular Structure 889 (2008) 35–44
www.elsevier.com/locate/molstruc
X-ray crystallographic and molecular sensing properties
of [ZnCl2(j2-N,N0-dpknph)] (dpknph = di-2-pyridyl
ketone-p-nitrophenyl hydrazone)
a,
b
a
*
Mohammed Bakir , Rebecca R. Conry , Orville Green
a Department of Chemistry, The University of the West Indies-Mona Campus, Kingston 7, Jamaica, West Indies
b Department of Chemistry, Colby College, 5764 Mayflower Hill, Waterville, ME 04901-8857, USA
Received 10 December 2007; received in revised form 8 January 2008; accepted 9 January 2008
Available online 17 January 2008
Abstract
When ZnCl2 was allowed to react with di-2-pyridyl ketone p-nitrophenylhydrazone (dpknph) in acetonitrile under reflux,
½ZnCl2ðj2-N; N0-dpknphÞꢀ was isolated in good yield. The formulation of the isolated product was established from the results of its ele-
mental analysis and
a
number of spectroscopic measurements. Single crystal X-ray structural analysis done on
½ZnCl2ðj2-N; N0-dpknphÞꢀ.dmf isolated from a dmf solution of ½ZnCl2ðj2-N; N0-dpknphÞꢀ shows the pyridine N,N0-bidentate binding
of dpknph and revealed a serpentine polymeric network of ½ZnCl2ðj2-N; N0-dpknphÞꢀ.dmf units locked via a web of hydrogen bonds.
1H NMR spectra of ½ZnCl2ðj2-N; N0-dpknphÞꢀ measured in d6-dmso, d7-dmf and d6-acetone confirmed the coordination of dpknph
and showed partial exchange of the amide proton with solvent protons in d6-acetone and that ½ZnCl2ðj2-N; N0-dpknphÞꢀ is stable in dmso
and dmf. 1H NMR variable temperature studies confirmed the stability of ½ZnCl2ðj2-N; N0-dpknphÞꢀ in d6-dmso and d7-dmf and showed
the presence of temperature-dependent shielded protons due to hydrogen bonding. The electronic absorption spectra of
½ZnCl2ðj2-N; N0-dpknphÞꢀ in non-aqueous media show a single solvent-dependent intra-ligand-charge transfer (ILCT) transition of
the donor–acceptor type between 300 and 700 nm. Thermo-optical measurements in protophilic solvent (dmso or dmf) between
303.15 and 353.15 K show the stability of ½ZnCl2ðj2-N; N0-dpknphÞꢀ at those temperatures. In the presence of a base, a new ILCT peak
appeared at 580 2 nm in dmso, and the addition of an acid to a mixture of ½ZnCl2ðj2-N; N0-dpknphÞꢀ and a base showed the disap-
pearance of the peak at 580 2 nm and reappearance of the peak due to ½ZnCl2ðj2-N; N0-dpknphÞꢀ. These results established reversible
inter-conversion between ½ZnCl2ðj2-N; N0-dpknphÞꢀ and its conjugate base, and allow for the use of ½ZnCl2ðj2-N; N0-dpknphÞꢀ in dmso to
measure and detect substrates in concentrations as low as 1.00 ꢁ 10ꢂ5 M. Thermo-optical measurements on a mixture of
½ZnCl2ðj2-N; N0-dpknphÞꢀ and NaBH4 in dmso confirmed the reversible inter-conversion between ½ZnCl2ðj2-N; N0-dpknphÞꢀ and its
conjugate base, and gave changes in enthalpy of 43.32 kJ/mol and entropy of 138.87 JKꢂ1 molꢂ1 at 298.15 K.
Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Synthesis; Di-2-pyridyl ketone p-nitrophenylhydrazone metal complexes; Sensors; Zinc; X-ray diffraction
1. Introduction
compounds continue to attract research activities because
of their physical properties, reactivity patterns and
potential applications in medicine, catalysis and molecu-
lar sensing, etc. [1–26]. The pharmacological activity of
several hydrazone, semicarbazone and thiosemicarbazone
derivatives of di-2-pyridyl ketone has been demonstrated
and several reports appeared on the metal chelating
behavior of di-2-pyridyl ketone hydrazones and
Di-2-pyridyl ketone (dpk) and a range of its deriva-
tives that include oxime, hydrazone, semicarbazone, thio-
semicarbazone (see Scheme 1) and their metal
*
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 Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2008.01.015

36
M. Bakir et al. / Journal of Molecular Structure 889 (2008) 35–44
2. Experimental
2.1. Reagents
All reagents were obtained from commercial sources and
used without further purification. The ligand dpknph was
prepared following a standard literature procedure [40].
2.1.1. Preparation of ½ZnCl2ðj2-N; N0-dpknphÞꢀ
A mixture of ZnCl2 (300 mg, 2.20 mmol), dpknph
(772 mg, 1.32 mmol) and acetonitrile (100 mL) was allowed
to reflux in air for 3 h. The resulting yellow precipitate was
filtered off, washed with hexane, diethyl ether and dried;
yield 620 mg (62%) (found: C, 44.36; H, 2.86; N, 15.20.
C17H13Cl2N5O2Zn requires C, 44.81; H, 2.88; N, 15.37%).
Infrared data (KBr disk, cmꢂ1): m(NH) 3220, m(CH)
3097–2900, m(py) 1597, m(NO2) asym 1557, m(NO2) sym
Scheme 1. dpk and its oxime and hydrazone derivatives.
di-2-pyridyl ketone thiosemicarbazone [18–20]. Di-2-pyri-
dyl ketone derivatives are employed in analytical chemis-
try for the extraction and spectrophotometric
determination of metal ions in solution and a recent
report showed copper in concentrations as low as
7.24 ꢁ 10ꢂ8 M in natural water can be detected and
determined using di-2-pyridyl ketone benzoylhydrazone
[21–26]. Although numerous metal compounds of triden-
tate N,N0,O-di-2-pyridyl ketone benzoyl hydrazone deriv-
atives have been reported only three reports appeared on
the N,N0-bidentate coordination of di-2-pyridyl ketone
benzoyl hydrazone (see Scheme 2) [27–37]. We have been
interested in the chemistry of di-2-pyridyl ketone and its
derivatives and reported on the synthesis, spectroscopic
and electrochemical properties, and structure of a variety
of their derivatives and metal compounds [11,22,34,36–
48]. Optical measurements on dpknph and its rhenium
and ruthenium complexes in protophilic solvents (dmso
or dmf) show high sensitivity of dpknph and its metal
complexes to changes in their surroundings [22,39–41].
In order to elucidate the molecular sensing behavior of
di-2-pyridyl ketone hydrazones, we embarked on the syn-
thesis of their MCl2 compounds and explored their opto-
sensing behavior. In recent reports, we described the syn-
thesis and structure of ZnCl2 compounds of di-2-pyridyl
ketone benzoyl hydrazone and di-2-pyridyl ketone thio-
nyl hydrazone as forming a five-coordinate zinc com-
pounds of the type [ZnCl2(j3-N,N0,O-dpkhydrazone)]
and here, we report on the synthesis of the four coordi-
nate ZnCl2 compound of di-2-pyridyl ketone p-nitro-
phenyl hydrazone [47,48].
1345 and m(NO2)c 749 cmꢂ1
cmꢂ1 Mꢂ1)}
in the concentration
.
UV–vis {k/nm, (e/
range
2 ꢁ
10ꢂ5ꢂ2 ꢁ 10ꢂ6 M: in dmf 397 (20,000 100) and 324
(3485 100); in dmso 397 (14,323 100), 324 (2880
100), 276 (2,850 100); CH3CN 393 (1680 100), 355
(sh) 336 (15,460 100), 290 (sh), 283 (sh), 270
(12,960 100) and 235 (sh), 225 (14,460 100); CH2Cl2
396 (315 100), 358 (sh), 342 (18,765 100), 290 (sh),
282 (sh) and 273 (12,977 100). 1H NMR (d ppm): in
dmf-d7 at 30 °C 13.52 (s, 1H, NH), 9.04 (d, 1H), 8.76 (d,
1H), 8.41 (d, 2H), 8.34 (d, 1H), 8.20 (t, 1H), 8.18 (t, 1H),
7.89 (d, 1H) and 7.76 (d, 2H), 7.74 (t, 1H) and 7.64 (t,
1
1H). H NMR (d ppm): in dmso-d6 at 30 °C 12.96 (NH,
1H), 8.84 (d, 1H), 8.56 (d, 1H), 8.20 (d, 2H), 8.08 (d,
1H), 7.98 (t, 1H), 7.96 (t, 1H), 7.58 (d, 1H), 7.56 (t, 1H),
7.50 (d, 2H) and 7.43 (t, 1H).
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 Mo Ka radiation and a graphite
monochromator was used for data collection and the
SHELXTL software package version 5.1 was used for
structure solution [49–51]. Cell parameters and other crys-
tallographic information are given in Table 1, along with
additional details concerning data collection. All non-
hydrogen atoms were refined with anisotropic thermal
parameters. The water and the amide N4 hydrogen atoms
were located from Fourier difference maps and refined
without constraints. All other H-atom positions were
refined assuming idealized geometry, with C–H distance
˚
˚
of 0.93 A and N1–H1A distance of 0.86 A.
2.3. Optical sensing studies
A stock solution of ½ZnCl2ðj2-N; N0-dpknphÞꢀ in a prot-
ophilic solvent, and stock solutions of a stimulus (NaBH4,
NaBF4 or ZnCl2) in a protophilic solvent were prepared
2
fac-[Re(CO)3(κ2-N,N’-dpkbh)Br]
fac
-[Re(CO)3(κ -N,N’-dpkbh)Cl]
Scheme 2. Views of fac-½ReðCOÞ3ðj2-N; N0-dpkbhÞXꢀ.

M. Bakir et al. / Journal of Molecular Structure 889 (2008) 35–44
37
Table 1
3. Results and discussion
Crystal data and structure refinement for ½ZnCl2ðj2-N; N0-dpknphÞꢀ.dmf
Empirical formula
C20H20Cl2N6O3Zn
The reaction between ZnCl2 and dpknph in acetonitrile
under reflux gave ½ZnCl2ðj2-N; N0-dpknphÞꢀ. The identity
of the isolated compound was elucidated from the results
of its elemental analysis, a number of spectroscopic mea-
surements and an X-ray structural analysis done on a crys-
tal of its dimethyl foramide (dmf) solvate grown from a
dmf solution of ½ZnCl2ðj2-N; N0-dpknphÞꢀ. The infrared
spectrum of ½ZnCl2ðj2-N; N0-dpknphÞꢀ show the coordina-
tion of dpknph as evident from the appearance of peaks
associated with NH, CH, NO2 and C@C and C@N stretch-
ing vibrations [53]. The 1H NMR spectra of
½ZnCl2ðj2-N; N0-dpknphÞꢀ measured in d6-dmso, d7-dmf
and d6-acetone (see Fig. 1) confirmed the coordination of
dpknph to ZnCl2 and revealed the sensitivity of
½ZnCl2ðj2-N; N0-dpknphÞꢀ to solvents as apparent from
the significant change in chemical shift of the NH proton
and slight changes in the chemical shifts of its aromatic
protons in different solvents. While partial exchange of
the NH proton occurs in d6-acetone, as noted by the broad
character of the NH peak and the relative integration of
the NH peak at 10.97 ppm to one CH peak (1H of the
pyridine ring at 8.94 ppm) that gave a ratio of 0.5:1.0,
the spectra show no exchange of the hydrazone NH proton
in d6-dmso or d7-dmf as integration of the NH proton to
the CH proton of coordinated pyridine ring for the
complex in these solvents gave a ratio of 1:1. Variable tem-
perature measurements in d6-dmso solution of
½ZnCl2ðj2-N; N0-dpknphÞꢀ (see Fig. 2) show the NH proton
and water protons exhibit higher temperature sensitivity
compared to the other protons in the molecule. Minor
temperature dependence was noted for the C11 and C21
protons while the other protons do not show any tempera-
ture dependence. In d7-dmf, the NH proton, solvent
protons, phenyl protons and C11 and C21 show greater
temperature dependence than the other protons. In d6-acte-
one, the C12, C13, C22 and C23 protons show higher
temperature sensitivity compared to the other protons.
Temperature-dependent protons reflect shielding from sol-
vent due to hydrogen bonding, while solvent-exposed pro-
tons show no temperature dependence. The results confirm
the sensitivity of ½ZnCl2ðj2-N; N0-dpknphÞꢀ to its surround-
ings. Plots of chemical shifts (d) of NH versus 1/T, K in d6-
dmso and d7-dmf gave straight lines with slopes of ꢂ405
and ꢂ282 per unit decade showing the NH proton of
½ZnCl2ðj2-N; N0-dpknphÞꢀ to be more sensitive to tempera-
ture variations in d6-dmso than in d7-dmf.
Formula weight
Temperature (K)
528.69
296(2)
0.71073
Monoclinic, P21
˚
Wavelength (A)
Crystal system, space group
Unit cell dimensions
˚
˚
a = 7.0183(4) A
a = 90°
b = 96.8870(10)°
c = 90°
b = 8.4161(5) A
c = 19.6681(12) A
˚
3
˚
Volume (A )
1153.35(12)
Z, Calculated density (mg/m3) 2, 1.522
Absorption coefficient (mmꢂ1
)
1.331
540
F (000)
Theta range for data collection 1.04–28.24°
Limiting indices
ꢂ8 6 h 6 9, ꢂ11 6 k 6 10,
ꢂ25 6 l 6 25
Reflections collected/unique
Completeness to h = 28.21
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2r(I)]a,b
R indices (all data)
10376/5184 [R(int) = 0.0169]
96.1%
Full-matrix least-squares on F2
5184/1/295
1.084
R1 ¼ 0:0279, wR2 ¼ 0:0731
R1 ¼ 0:0301, wR2 ¼ 0:0799
0.002(8)
0.362 and ꢂ0.217 eAꢂ3
Absolute structure parameter
Largest diff. peak and hole
P
P
a
R1 ¼ R1 ¼ kF oj-jF 9M k= jF oj.
P
P
2
2
1=2
b
wR2 ¼ f ½wðF 2o-F c2Þ ꢀ= wðF 2oÞ ꢀg
where
w ¼ 1=½r2ðF 2oÞ þ
2
ð0:0546PÞ þ 0:0000Pꢀ and P ¼ ðF 2o þ 2F 92M Þ=3.
separately. Optical changes (electronic absorption spectral
changes) were measured on solutions prepared by mixing
appropriate volumes of ½ZnCl2ðj2-N; N0-dpknphÞꢀ and
stimulus solutions to prepare the desired solutions (see fig-
ure captions for details). All measurements were made at
room temperature.
2.4. Equipment
Electronic absorption spectra were recorded on a HP-
8452A spectrophotometer or a Perkin-Elmer UV/VIS/
NIR spectrometer kꢂ19. Baseline corrections on blank sol-
vents were recorded prior to measurements. A Lauda-
Brinkmann 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 solvent. Infrared spectra were recorded as
KBr pellets on a Perkin-Elmer Spectrum 1000 FT-IR Spec-
trometer. A Lauda-Brinkmann RM6 Circular Bath was
used for temperature control.
The
electronic
absorption
spectra
of
½ZnCl2ðj2-N; N0-dpknphÞꢀ measured in dmso, dmf, CH3CN
and CH2Cl2 are shown in Fig. 3. These spectra show
½ZnCl2ðj2-N; N0-dpknphÞꢀ exhibits solvatochromic behav-
ior. Further, single absorption peaks appeared at 420 nm
in dmso and dmf, 410 nm in CH3CH and 396 nm in
CH2Cl2. The0 electronic absorption transitions of
½ZnCl2ðj2-N; N -dpknphÞꢀ are intra-ligand-charge transfer
(ILCT) due mainly to p ? p* of dpk followed by
2.5. Molecular orbitals calculations
Restricted Hartree-Fock (RHF) semi-empirical molecu-
lar orbital calculations (PM3) on the lowest electronic state
were conducted on a Hyperchem molecular modeling pro-
gram using default parameters on singlet and triplet states.
A Polak-Ribiere energy minimized model using MM+
force field was used [52].

38
M. Bakir et al. / Journal of Molecular Structure 889 (2008) 35–44
1
Fig. 1. H NMR spectra of ½ZnCl2ðj2-N; N0-dpknphÞꢀ measured in d6-dmso (a), d7-dmf (b) and d6-acetone (c) at 30 °C.
1
Fig. 2. H NMR spectra of ½ZnCl2ðj2-N; N0-dpknphÞꢀ measured in d6-dmso at 30 (a), 40 (b), 50 (c) and 60 (d) °C.

M. Bakir et al. / Journal of Molecular Structure 889 (2008) 35–44
39
Fig. 3. The electronic absorption spectra of 1.00 ꢁ 10ꢂ5 M ½ZnCl2ðj2-N; N0-dpknphÞꢀ in dmso, dmf, CH3CN and CH2Cl2.
dpk ? NO2 charge transfer mixed with Clp ? dpknph
charge transfer. Semi-empirical PM3 molecular calcula-
tions (see Fig. 4) confirmed the assigned electronic transi-
tions and gave a HOMO ? LUMO energy gap of
dmf, and the absence of any other absorbing species under
these conditions. These results are in contrast to those
reported for the optical behavior of the free ligand
(dpknph) and its rhenium and ruthenium complexes
fac-ReðCOÞ3(dpknph)Cl and ½RuðbipyÞ2ðdpknphÞꢀCl2 in
dmso and dmf, where two ILCT transitions appeared
between 300 and 800 nm and plots of absorbance versus
concentration showed non-linearity pointing to strong
complex–solvent interactions [22,42]. A comparison of
3.53 eV (351 nm).
A
plot of the absorbance of
½ZnCl2ðj2-N; N0-dpknphÞꢀ versus concentration in dmso
and dmf gave straight lines with a slope of 33,240 200
for dmso and 33,560 200 in dmf. These results hint to
the stability of ½ZnCl2ðj2-N; N0-dpknphÞꢀ in dmso and
Fig. 4. Semi-empirical energy level diagram of ½ZnCl2ðj2-N; N0-dpknphÞꢀ.

40
M. Bakir et al. / Journal of Molecular Structure 889 (2008) 35–44
the electronic absorption properties of ½ZnCl2ðj2-N;
N0-dpknphÞCl2ꢀ with those reported for [ZnCl2(j3-
N,N0,O-dpktah)] show the latter to exhibit two absorption
peaks at 406 and 330 nm in dmso and a single absorption
peak at 405 nm in dmf [51]. The absorption peaks in spec-
tra of [ZnCl2(j3-N,N0,O-dpktah)] were assigned to
[ZnCl2(j3-N,N0,O-dpktah)] and its conjugate base. In
dmf, complete proton transfer from [ZnCl2(j3-N,N0,O-
dpktah)] to dmf takes place, and in dmso, partial proton
transfer occurs as noted by the appearance of two absorp-
tion peaks in dmso and one absorption peak in dmf. In
CHCl3 and CH2Cl2, a single absorption peak was observed
in the high energy region in the spectra of [ZnCl2(j3-
N,N0,O-dpktah)] and [ZnCl2(j2-N,N0-dpknph)Cl2], point-
ing to the absence of any solvent complex interaction.
When ½ZnCl2ðj2-N; N0-dpknphÞꢀ was allowed to interact
with stoichiometric amounts of NaBH4 in dmso, a new
absorption peak appeared at 580 2 nm, and as the con-
centration of NaBH4 increased, the intensity of the absorp-
tion peak at 420 2 nm diminished, and the intensity of
the absorption peak at 580 2 nm increased (see Fig. 5).
The reverse was observed when stoichiometric amounts
of NaBF4 were added to a mixture of ½ZnCl2ðj2-N;
N0-dpknphÞꢀ and NaBH4 dissolved in dmso. Under our
Fig. 5. Electronic absorption spectra of ½ZnCl2ðj2-N; N0-dpknphÞꢀ 1.00 ꢁ 10ꢂ5 M in dmso in the presence of (1) 0.00, (2) 1.00 ꢁ 10ꢂ4, (3) 5.00 ꢁ 10ꢂ4, (4)
1.00 ꢁ 10ꢂ3, (5) 2.00 ꢁ 10ꢂ3, (6) 3.00 ꢁ 10ꢂ3, (7) 5.00 ꢁ 10ꢂ3, (8) 1.00 ꢁ 10ꢂ2, (9) 2.00 ꢁ 10ꢂ2 and (10) 3.00 ꢁ 10ꢂ2 M NaBH4.
[ZnCl2(κ2-N,N’-dpknph)]
[ZnCl 2( 2-N,N’-dpknph-H)]
κ
Scheme 3. The inter-conversion between [ZnCl2(j2-N,N0-dpknph)] and its conjugate base.

M. Bakir et al. / Journal of Molecular Structure 889 (2008) 35–44
41
Fig. 6. A plot of absorbance of a mixture of ½ZnCl2ðj2-N; N0-dpknphÞꢀ (1.00 ꢁ 10ꢂ5 M) and NaBH4 (5.00 ꢁ 10ꢂ3 M) in dmso versus concentration of
glucose.
Table 2
2
0
˚
Bond lengths [A] and angles [°] for ZnCl2(j -N,N -dpknph)].dmf
Bond lengths
Zn–N(2)
Zn–Cl(1)
2.0348(18)
2.1752(7)
1.338(3)
1.338(3)
1.372(4)
1.487(3)
1.287(3)
1.377(3)
1.396(3)
1.210(4)
1.229(4)
1.455(4)
Zn–N(1)
Zn–Cl(2)
2.0621(19)
2.2097(7)
1.349(3)
1.345(3)
1.387(5)
1.376(4)
1.350(3)
0.77(3)
1.458(3)
1.225(5)
1.316(3)
1.442(4)
N(1)–C(11)
N(2)–C(21)
C(11)–C(12)
C(15)–C(01)
C(01)–N(3)
N(4)–C(31)
C(31)–C(32)
N(5)–O(2)
C(1)–O(3)
C(3)–N(6)
N(1)–C(15)
N(2)–C(25)
C(12)–C(13)
C(22)–C(21)
N(3)–N(4)
N(4)–H(4)
C(34)–N(5)
N(5)–O(1)
C(1)–N(6)
C(2)–N(6)
Bond angles
N(2)–Zn–N(1)
89.97(7)
112.97(6)
105.32(5)
119.4(2)
116.91(14)
122.39(15)
122.8(3)
118.01(17)
119.05(18)
114.48(18)
120.09(19)
122.0(2)
N(2)–Zn–Cl(1)
N(2)–Zn–Cl(2)
Cl(1)–Zn–Cl(2)
C(11)–N(1)–Zn
C(21)–N(2)–C(25)
C(25)–N(2)–Zn
C(11)–C(12)–C(13)
N(2)–C(25)–C(24)
N(2)–C(21)–C(22)
C(15)–C(01)–C(25)
N(3)–N(4)–C(31)
C(32)–C(31)–C(36)
O(1)–N(5)–C(34)
C(1)–N(6)–C(2)
118.67(6)
N(1)–Zn–Cl(1)
N(1)–Zn–Cl(2)
C(11)–N(1)–C(15)
C(15)–N(1)–Zn
C(21)–N(2)–Zn
N(1)–C(11)–C(12)
N(1)–C(15)–C(01)
N(2)–C(25)–C(01)
N(3)–C(01)–C(15)
C(01)–N(3)–N(4)
N(4)–C(31)–C(36)
O(2)–N(5)–O(1)
O(3)–C(1)–N(6)
105.68(5)
119.67(3)
120.95(17)
118.98(19)
118.57(14)
117.9(2)
121.1(2)
122.6(2)
121.04(18)
119.2(2)
120.0(2)
Fig. 7. The molecular structure of ½ZnCl2ðj2-N; N0-dpknphÞꢀ. The thermal
ellipsoids are drawn at the 30% probability level and H atoms are shown
as small spheres of arbitrary radii.
123.4(3)
124.5(3)
118.1(3)
121.4(3)

42
M. Bakir et al. / Journal of Molecular Structure 889 (2008) 35–44
conditions, NaBH4 did not reduce the hydrazone backbone
but instead acted as a base while NaBF4 dissolved in dmso
behaved as strong acid. This was further confirmed when
excess benzoic acid wa0s added to a dmso solution of a mix-
ture of ½ZnCl2ðj2-N; N -dpknphÞꢀ and NaBH4 and the peak
at 580 2 nm disappeared. These reactions hint to revers-
ible inter-conversion between the ½ZnCl2ðj2-N;N0-dpknphÞꢀ
and its conjugate base (see Scheme 3) when allowed to
interact with NaBH4 and NaBF4 and are similar to those
reported for the acid–base inter-conversion between the
free ligand and its conjugate base in dmso and dmf. In
the presence of excess NaBH4, the absorption peak at
420 2 nm disappeared and the absorption peak at
580 2 nm reached its maximum intensity, giving the
extinction coefficient of 53000 200 Mꢂ1 cmꢂ1 for conju-
gate base in dmf and dmso. A residual absorption peak
appeared at 400 2 nm when excess NaBH4 was added
to a dmso solution of ½ZnCl2ðj2-N; N0-dpknphÞꢀ and that
may be assigned to Clp ! dpknph CT.
increased and the intensity of the absorption peak at
420 2 to decreased as the temperature was increased.
The reverse was observed when the temperature was
allowed to decrease. These results confirm reversible
inter-conversion between ½ZnCl2ðj2-N; N0-dpknphÞꢀ and
its conjugate base. In the absence of NaBH4, a slight
decrease and shift to high energy of the absorption peak
at 420 nm was observed and no evidence was noted for
the appearance of an absorption peak due to the conjugate
base. A plot of ln(A580/A420) versus 1/T ꢁ 103 of a mixture
of ½ZnCl2ðj2-N; N0-dpknphÞꢀ and NaBH4 in dmso gave a
Table 3
0
2
˚
Hydrogen bonds for ½ZnCl2ðj -N; N -dpknphÞꢀ.dmf [A and °]
D–H. . .A
d(D–H)
d(H. . .A)
d(D. . .A)
<(DHA)
N(4)–H(4). . .O(3)
0.77(3)
0.93
0.96
0.93
0.93
2.06(3)
2.81
2.40
2.56
2.80
2.815(3)
3.490(3)
2.773(4)
3.423(3)
3.612(2)
165(3)
130.4
102.6
155.1
146.9
C(11)–H(11). . .Cl(2)1
C(2)–H(2A). . .O(3)
C(24)–H(24). . .O(1)2
C(21)–H(21). . .Cl(1)3
Thermo-optical measurements on
a
mixture of
½ZnCl2ðj2-N; N0-dpknphÞꢀ and NaBH4 dissolved in dmso
1
Symmetry transformations used to generate equivalent atoms: x þ 1; y; z;
2
3
show the intensity of the absorption peak at 580 2 nm
ꢂx þ 1; y þ 1=2; ꢂz þ 1 and ꢂx þ 2; y þ 1=2; ꢂz.
Fig. 8. Packing of molecules of ½ZnCl2ðj2-N; N0-dpknphÞꢀ. Non-covalent hydrogen bonds are represented by dashed lines.

M. Bakir et al. / Journal of Molecular Structure 889 (2008) 35–44
43
was allowed to interact with stoichiometric amounts of glu-
cose, and showed a decrease in the intensity of the absorp-
tion peak at 580 2 nm and an increase in the intensity of
the absorption band at 420 2 nm as the concentration of
glucose increased. A plot of absorbance versus concentra-
tion of added glucose (see Fig. 6) shows glucose concentra-
tions as low as 1 ꢁ 10ꢂ6 M can be detected and determined
using a mixture of ½ZnCl2ðj2-N; N0-dpknphÞꢀ and NaBH4
in dmso.
The solid state structure of ½ZnCl2ðj2-N; N0-dpknphÞꢀ.
dmf was determined using a single crystal X-ray crystal-
lography.
A
0 view of the molecular structure of
½ZnCl2ðj2-N; N -dpknphÞꢀ.dmf is shown in Fig. 7 and
selected bond angles and distances are given in Table 2.
The coordination about zinc is pseudo-tetrahedral with
two nitrogen atoms from the pyridine rings of dpknph
molecule and two chlorine atoms occupying the coordi-
nation sites. The pyridine N,N0-chelating dpknph mole-
cule forms a six-membered Zn–N1–C15–C01–C25–N2
metallocyclic ring in a boat conformation. Some or all
of this distortion from tetrahedral geometry about zinc
is due to the constraints associated with the pyridine
N,N0-bite angle of 90° in this structure, which is signifi-
cantly larger than those reported for other transition
metal compounds that contain pyridine N,N0-bidentate
dpk-derivatives [45–47]. The bond distances of the
coordinated atoms and those of the coordinated dpknph
molecule appear normal as they are similar to those
reported for a variety of zinc compounds that contain
such bonds.
Fig. 9. Views of the hydrogen bonds in ½ZnCl2ðj2-N; N0-dpknphÞꢀ.
The packing of molecules shows a serpentine polymeric
network of ½ZnCl2ðj2-N; N0-dpknphÞꢀ.dmf units linked via
a network of hydrogen bonds (Fig. 8 and Table 3). Each
molecule within the bulk of the network binds to four adja-
cent molecules using its chlorine atoms, oxygen atom (O1)
of the nitro-group, and hydrogen atoms of C11 and C21.
The dmf molecule binds to the NH proton of the hydra-
zone via a classic O. . .H–N hydrogen bond (see Fig. 9a),
and the chlorine atoms and oxygen atom (O1) of the
nitro-group for non-classic hydrogen bonds of the type
C–H. . .X (X = Cl or X) with neighboring molecules (see
Fig. 9b and c), thus linking all ½ZnCl2ðj2-N;
N0-dpknphÞꢀ.dmf molecules together. The bond distances
and angles of the hydrogen bonds are normal and similar
to those reported for other molecules containing such
bonds.
straight line with a slope of ꢂ5.21 per unit decade and an
intercept of 17.17 that gave changes in enthalpy of
43.32 kJ/mol and entropy of 138.87 JKꢂ1 molꢂ1 1
The
.
low values for the thermodynamic parameters hint to facile
inter-conversion between ½ZnCl2ðj2-N; N0-dpknphÞꢀ and its
conjugate base in the presence of NaBH4, and possible use
as molecular sensor. This 0was further illustrated when a
mixture of ½ZnCl2ðj2-N; N -dpknphÞꢀand NaBH4 in dmso
1
For the acid–base inter-conversion between [ZnCl2ðj2-N-N0-dpknph)]
and its conjugate base [ZnCl2ðj2-N-N0-dpknph-1)]:
½ZnCl2ðj2-N ꢂ N0-dpknphÞꢀ $ ½ZnCl2ðj2-N ꢂ N0-dpknph ꢂ 1Þꢀ
ð1Þ
Application of Beer’s law gives:
Aa=Ab ¼ eaca=ebcb
ð2Þ
where a = [ZnCl2ðj2-N-N0-dpknph-1)] and b = [ZnCl2ðj2-N-N0-dpknph)]
and lnðAa=AbÞ ¼ lnðea=ebÞ þ ln K
ð3Þ
4. Conclusion
The equilibrium constant is related to the thermodynamic parameters as
The first zinc complex of dpknph has been isolated and
structurally characterized. Spectroscopic measurements of
½ZnCl2ðj2-N; N0-dpknphÞꢀ in protophilic solvents disclose
sensitivity to its surroundings and substrates in concentra-
tions as low as 1 ꢁ 10ꢂ6 M can be detected and determined
using a mixture of ½ZnCl2ðj2-N; N0-dpknphÞꢀ and NaBH4
in dmso.
shown in Eq. (4):
ln K ¼ DSø=R ꢂ DHø=RT
and substitution of (4) into (3) gives:
ð4Þ
ð5Þ
lnðAa=AbÞ ¼ lnðea=ebÞ þ DSø=R ꢂ DHø=RT
A plot of lnðAa=AbÞ versus 1/T gives a straight line with a slope of ꢂDHø/R
and an intercept of {ln ðea=ebÞ þ DSø/R}.

44
M. Bakir et al. / Journal of Molecular Structure 889 (2008) 35–44
[24] I. Gaubeur, M.C.D. Areias, U.H.S.V. Terra, M.E.V. Suarez-Iha,
Acknowledgments
Spec. Lett. 35 (2002) 455.
[25] J.P. Juan, C. Moreno, M. Garcia-Vergas, Anal. Bioanal. Chem. 373
(2002) 844.
[26] S. Pehkonen, Analyst 120 (1995) 2655.
[27] S. Ianelli, M. Orcesi, C. Pelizzi, G. Pelizzi, G. Predieri, J. Organomet.
Chem. 451 (1993) 59.
We wish to thank Ms. Toni Johnson for NMR measure-
ments and The University of the West Indies for financial
support.
[28] M. Carcelli, C. Pelizzi, G. Pelizzi, P. Mazza, F. Zani, J. Organomet.
Chem. 488 (1995) 55.
References
[29] C.C. Gatto, E.S. Lang, A. Kupfer, A. Hagenbach, D. Wille, U.
Abram, Z. Anorg. Allg. Chem. 630 (2004) 735.
[30] A. Bacchi, L.P. Battaglia, M. Carcelli, C. Pelizzi, G. Pelizzi, C.
Solianas, M.A. Zoroddu, J. Chem. Soc. Dalton Trans. (1993)
775.
[1] M. Toyama, M. Nakahara, N. Nagao, Bull. Chem. Soc. Jpn. 80
(2007) 937.
[2] C. Papatriantafyllopoulou, C.P. Raptopoulou, A. Escuer, C.J. Milios,
Inorg. Chim. Acta 360 (2007) 61.
[31] L.P. Battaglia, M.B. Ferrari, R. Boggia, Inorg. Chim. Acta 215 (1994)
85.
[32] L. Zhao, L.K. Thompson, Z. Xu, D.O. Miller, D.R. Stirling, J. Chem.
Soc. Dalton Trans. (2001) 1706.
[33] L.P. Battaglia, P.G. Berzollo, A.B. Corradi, C. Pelizzi, J. Crystallogr.
Spectrosc. Res. 23 (1993) 973.
[34] M. Bakir, M.A.W. Lawrence, M. Singh-Wilmot, J. Coord. Chem. 60
(2007) 2385.
[3] Z. Chen, F. Liang, S. Zhou, C. Xia, R. Hu, J. Mol. Struct. 827 (2007) 20.
[4] D.-H. Lee, M.N. Murthy, K.D. Karlin, Bull. Chem. Soc. Jpn. 80
(2007) 732.
[5] A. Grirrane, A. Pastor, E. Alvarez, C. Mealli, A. Ienco, P. Rosa, A.
Galindo, Eur. J. Inorg. Chem. 22 (2007) 3543.
[6] V. Suni, M.R.P. Kurup, M. Nethaji, Polyhedron 26 (2007) 3097.
[7] B.F. Abrahams, T.A. Hudson, R. Robson, Chem. A Eur. J. 12 (2006)
7095.
[35] J. Grewea, A. Hagenbacha, B. Stromburga, R. Albertob, E. Vazquez-
Lopez, U. Abrama, Z. Anorg. Allg. Chem. 629 (2003) 303.
[36] M. Bakir, O. Brown, Inorganica Chim. Acta 353 (2003) 89.
[37] M. Bakir, O. Brown, T. Johnson, J. Mol. Struct. 691 (2004) 265.
[38] M. Bakir, O. Green, C. Gyles, Inorganica Chim. Acta 358 (2005)
1835.
[8] C.J. Milios, A. Prescimone, J. Sanchez-Benitez, S. Parsons, M.
Murrie, E.K. Brechin, Inorg. Chem. 45 (2006) 7053.
[9] F.B. Zhang, E.M. Prokopchuk, M.E. Broczkowski, M.C. Jennings,
R.J. Puddephatt, Organometallics 25 (2006) 1583.
[10] Y.-C. Liu, C.-I. Li, W.-Y. Yeh, G.-H. Lee, S.-M. Peng, Inorg. Chim.
Acta 359 (2006) 2361.
[39] M. Bakir, K. Abdur-Rashid, C. Gyles, Spectrochim. Acta Part A 59
(2003) 2123.
[11] M. Bakir, I. Hassan, O. Green, J. Coord. Chem. 59 (2006) 1953.
[12] Y.-M. Li, S.-C. Xiang, T.-L. Sheng, J.-J. Zhang, S.-M. Hu, R.-B. Fu,
X.-H. Huang, X.-T. Wu, Inorg. Chem. 45 (2006) 6577.
[13] M.C. Suen J.C. Wang, Inorg. Chem. Commun. 9 (2006) 478.
[14] T.C. Stamatatos, J.C. Vlahopoulou, Y. Sanakis, C.P. Raptopoulou,
V. Psycharis, A.K. Boudalis, S.P. Perlepes, Inorg. Chem. Commun. 9
(2006) 814.
[40] M. Bakir, K. Abdur-Rashid, Trans. Metal Chem. 24 (1999) 384.
[41] M. Bakir, O. Brown, J. Mol. Struct. 641 (2002) 183.
[42] M. Bakir, Eur. J. Inorg. Chem. (2002) 481.
[43] M. Bakir, Inorg. Chim. Acta 332 (2002) 1.
[44] M. Bakir, Acta Cryst. C57 (2001) 1371.
[45] M. Bakir, J.A.M. McKenzie, Electroanal. Chem. 425 (1997) 61.
[46] M. Bakir, J. Electroanal. Chem. 466 (1999) 60.
[47] M. Bakir, O. Green, W.H. Mulder, J. Mol. Struct. 873 (2008) 17.
[48] M. Bakir, R.R. Conry, O. Green, W.H. Mulder, J. Coord. Chem.
(2008), in press.
[15] B. Asc, G. Alpdogan, S. Sungur, Anal. Lett. 39 (2006) 997.
[16] Y.-C. Liu, C.-I. Li, W.-Y. Yeh, G.-H. Lee, S.-M. Peng, Inorg. Chim.
Acta 359 (2006) 2361.
[17] X.-D. Chen, T.C.W. Mak, Inorg. Chim. Acta 359 (2006) 685.
[18] D.R. Richardson, P.C. Sharpe, D.B. Lovejoy, D. Senaratne, D.S.
Kalinowski, M. Islam, P.V. Bernhardt, J. Med. Chem. 49 (2006)
6510.
[49] Bruker-SMART, Software Version 5.625, Brucker AXS, Inc., Mad-
ison, Wisconsin, USA, 2001.
[50] Bruker-SHELXTL, Software Version 5.1, Brucker AXS, Inc., Mad-
ison, Wisconsin, USA, 1997.
[19] P.V. Bernhardt, P. Chin, P.C. Sharpe, D.R. Richardson, J. Chem.
Soc. Dalton Trans. 30 (2007) 3232.
[51] G.M. Sheldrick, SHELX97 and SHELXL97, University of Go¨ttin-
gen, Germany, 1997.
[20] P.V. Bernhardt, L.M. Caldwell, T.B. Chaston, P. Chin, D.R.
Richardson, J. Biol. Inorg. Chem. 8 (2003) 866.
[52] Hyper Chem 7.5 Professional Edition, Hyper Cube, Inc., 1115 NW
Fourth Street, Gainesville, Florida 32601, USA.
[53] R.M. Silverstein, F.X. Webster, Spectroscopic Identification of
Organic Compounds, Sixth ed., John Wiley and Sons, Inc, 1997.
[21] I. Gaubeur, L.S.S. Avila-Terra, J.C. Masini, M.E.V. Suarez-Iha,
Anal. Sci. 23 (2007) 1227.
[22] M. Bakir, K. Abdur-Rashid, W.H. Mulder, Talanta 51 (2000) 735.
[23] J.J. Pinto, C. Moreno, M. Garcia-Vargas, Talanta 64 (2004) 562.