Molecular structure and optosensing behavior of di-2-pyridyl ketone benzoylhydrazone in non-aqueous solvents

Article abstract of DOI:10.1016/S0022-2860(01)00973-5

Crystals of di-2-pyridyl ketone benzoylhydrazone (dpkbh) obtained from a dimethylsulfoxide (DMSO) solution of dpkbh are in the monoclinic space group, P_l/c. Structural analysis reveals anti-parallel, non-planar, one-dimensional tapes of dpkbh w

Full text of DOI:10.1016/S0022-2860(01)00973-5

Journal of Molecular Structure 609 )2002) 129±136
www.elsevier.com/locate/molstruc
Molecular structure and optosensing behavior of di-2-pyridyl
ketone benzoylhydrazone in non-aqueous solvents
*
Mohammed Bakir , Ordel Brown
Department of Chemistry, The University of the West Indies, Mona Campus, Kingston 7, Jamaica
Received 25 July 2001; revised 27 November 2001; accepted 27 November 2001
Abstract
Crystals of di-2-pyridyl ketone benzoylhydrazone )dpkbh) obtained from a dimethylsulfoxide )DMSO) solution of dpkbh are
in the monoclinic space group, P21/c. Structural analysis reveals anti-parallel, non-planar, one-dimensional tapes of dpkbh with
a network of intra- and intermolecular hydrogen bonds. Optical measurements on dpkbh in polar non-aqueous solvents revealed
the presence of interlocked low-)a-) and high-)b-) energy intraligand charge-transfer electronic states may be due to dpk p±pp
followed by dpk to benzoyl charge-transfer. The equilibrium distribution of the low-)a-) and high-)b-) energy electronic states
is solvent dependent, controlled by the hydrogen bonding capacity of surrounding solvent molecules. The interplay between the
a- and b-conformations of dpkbh allowed calculations of their extinction coef®cients by forcing the equilibrium to shift to one
conformation using chemical stimuli. In N,N-dimethylformamide )DMF), extinction coef®cients of 45,000 ^ 2000 and
15,000 ^ 2000 M21 cm21 were calculated for the a- and b-conformations of dpkbh at lmax, respectively, when excess acid
was added to a DMF solution of dpkbh. The weak bond energies of the hydrogen bonds, the reversibility and the sensitivity of
the high- and low-energy electronic states to their surroundings in polar non-aqueous solvents allow for the use of this system as
molecular sensor for a variety of stimuli that include metal ions, acids and bases. Metal ions in concentrations ,1027 M can be
detected and determined using this system )dpkbh and surrounding solvent molecules). q 2002 Elsevier Science B.V. All rights
reserved.
Keywords: Di-2-pyridyl ketone benzoylhydrazone; Optosensing; Electronic absorption spectrum
1. Introduction
Fe)III), Co)II), Pd)II), Ni)II) and Cu)II) in aqueous
media, its optosensing properties in non-aqueous
media and solid state structure were not reported
[7±18]. We have been interested in the chemistry of
polypyridyl-like ligands and described the synthesis
and characterization of di-2-pyridyl ketone p-nitro-
phenyl hydrazone )dpknph) and its rhenium carbonyl
compound fac-Re)CO)3)dpknph)Cl [19±23]. Optical
and thermodynamic measurements on dpknph and
fac-Re)CO)3)dpknph)Cl in polar non-aqueous
solvents revealed reversible interconversion between
two interlocked electronic states due to intermolecular
interactionbetweenthesespeciesandtheir surroundings
Hydrazones and their metal compounds have
attracted a lot of research activities in recent years
for their physicochemical properties, reactivity
patterns and applications in many important chemical
processes [1±6]. Although di-2-pyridyl ketone
benzoylhydrazone )dpkbh) has been used as a spectro-
photometric reagent for the determination of Fe)II),
* Corresponding author. Tel.: 11-876-977-1910; fax: 11-876-
977-1835.
E-mail address: mbakir@uwimona.edu.jm )M. Bakir).
0022-2860/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved.
PII: S0022-2860)01)00973-5

130
M. Bakir, O. Brown / Journal of Molecular Structure 609 32002) 129±136
Table 1
Crystal data and structure re®nement for dpkbh
[22,23]. The high values for the extinction coef®cients
and low values for the activation parameters for the
interconversion between the low- and high-energy
conformations of fac-Re)CO)3)dpknph)Cl in polar
non-aqueous solvents allowed the use of these
systems as molecular sensors for a variety of chemical
and physical stimuli that include metal ions [22]. In
this report, we describe the X-ray structure and opto-
sensing properties of dpkbh in non-aqueous solvents
and compare the results with those reported for the
spectrophotometric determination of metal ions using
dpkbh in aqueous media.
Empirical formula
Formula weight
C18H14N4O
302.33
Temperature
293)2) K
Ê
0.71073 A
Wavelength
Crystal system, space group
Unit cell dimensions
Monoclinic, P21/c
Ê
a ˆ 8.2640)10) A, a ˆ 908
Ê
b ˆ 22.107)10) A,
b ˆ 108.940)10)8
Ê
c ˆ 8.7820)10) A, g ˆ 908
Ê 3
Volume
1517.5)7) A
Z, calculated density
Absorption coef®cient
F)000)
4, 1.323 mg/m3
0.086 mm21
632
Crystal size
0.4 £ 0.25 £ 0.1 mm3
u range for data collection
Limiting indices
1.84±27.598
2 1 # h # 10, 228 # k # 1,
211 # l # 11
Re¯ections collected/unique
Completeness to u ˆ 27.59
Absorption correction
Re®nement method
4412/3488 [R)int) ˆ 0.0313]
99.4%
None
Full-matrix least-squares on F2
Data/restraints/parameters
Goodness-of-®t on F2
Final R indices [I . 2s )I)]
R indices )all data)
3488/0/265
1.127
R1 ˆ 0:0587; wR2 ˆ 0:0894
R1 ˆ 0:1577; wR2 ˆ 0:1031
0.0076)9)
Extinction coef®cient
Largest diff. peak and hole
Ê 23
0.203 and 20.199 eA
2. Experimental
cement. A Brucker AXS 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 [24,25]. Cell parameters
and other crystallographic information are given in
Table 1 along with additional details concerning
data collection, and Table 2 gives a list of atomic
coordinates. All non-hydrogen atoms were re®ned
with anisotropic thermal parameters.
2.1. Reagents
Solvents were reagent grade and thoroughly deox-
ygenated prior to use. All other reagents were
obtained from commercial sources and used without
further puri®cation. The compound dpkbh melting
point 141±142 8C was prepared following a procedure
similar to that described for the synthesis of dpknph
by re¯uxing a mixture of di-2-pyridyl ketone )502 mg,
2.72 mmol), benzhydrazide )695 mg, 5.10 mmol),
EtOH )100 ml) and two drops of concentrated HCl
for 72 h. Slow evaporation of the solvent gave off-
yellow crystals that were ®ltered off, washed with
hexane, Et2O and dried [23].
2.3. Optical sensing studies
A stock solution of dpkbh in DMF and stock solu-
tions of stimuli )acids, bases or salts) in DMF were
prepared separately. Optical changes )electronic
absorption spectral changes) were measured on solu-
tion prepared by mixing appropriated volumes of
dpkbz and stimili solutions to prepare the desired
solution )see ®gure captions for details). All measure-
ments were made at room temperature.
2.2. X-ray crystallography
Single crystals of dpkbh were obtained from a
DMSO solution of dpkbh when allowed to stand at
room temperature for several days. A single crystal
was selected and mounted on a glass ®ber with epoxy

M. Bakir, O. Brown / Journal of Molecular Structure 609 32002) 129±136
131
Table 2
2.4. Physical measurements
Atomic coordinates ) £ 104) and equivalent isotropic displacement
parameters )A2 £ 103) for dpkbh. U)eq) is de®ned as one-third of the
trace of the orthogonalized Uij tensor
Electronic absorption spectra were recorded on a
Perkin±Elmer UV/VIS/NIRSpectrometer l 19 or
HP-8452A spectrophotometer. Baseline corrections
on blank solvents were recorded prior to measure-
ments.
x
y
z
U)eq)
N)1)
2235)2)
2137)3)
3076)3)
4169)3)
4310)3)
3335)3)
3524)3)
4145)2)
5030)4)
6261)3)
6583)4)
5691)3)
4487)3)
2995)2)
2128)2)
1590)3)
1764)2)
788)3)
5286)1)
5215)1)
5543)1)
5965)1)
6055)1)
5700)1)
5746)1)
6816)1)
7278)1)
7222)1)
6655)1)
6170)1)
6266)1)
5362)1)
4863)1)
4439)1)
4490)1)
3898)1)
3721)1)
3209)1)
2865)1)
3039)1)
3545)1)
5170)2)
6653)3)
7965)3)
7738)3)
6228)3)
4957)2)
3328)2)
3414)2)
3087)3)
2370)3)
1899)3)
2187)3)
2957)2)
2146)2)
2356)2)
1172)3)
49)1)
52)1)
50)1)
54)1)
50)1)
40)1)
41)1)
53)1)
62)1)
62)1)
60)1)
47)1)
41)1)
45)1)
47)1)
46)1)
66)1)
42)1)
50)1)
59)1)
59)1)
56)1)
50)1)
C)11)
C)12)
C)13)
C)14)
C)15)
C)1)
3. Results and discussion
3.1. Structural studies
N)2)
C)21)
C)22)
C)23)
C)24)
C)25)
N)3)
An ORTEP view of dpkbh is shown in Fig. 1 and
Table 3 shows selected bond distances and angles.
The bond distances and angles are normal and similar
to those reported for di-2-pyridylketone-2-aminoben-
zoylhydrazone )Hdpa), di-2-pyridyl ketone 2-pyridyl-
hydrazone )dpkph) and other related species [5,26].
For example, d)CyN) and d)CyO) bond distances of
N)4)
C)2)
O
2 152)2)
1633)2)
3219)3)
3570)3)
2342)3)
773)3)
Ê
1.29 and 1.23 A were reported for the azomethine
C)31)
C)32)
C)33)
C)34)
C)35)
C)36)
1073)3)
303)3)
and carbonyl double bonds of Hdpa, respectively
[5]. Although the hydrazone backbone )C1yN3±
N4±C2O) is planar with the amide hydrogen and
oxygen atoms are in anti-positions, the molecule is
non-planar due to the non-coplanarity of the aromatic
rings.
2 761)3)
2 1077)3)
2 299)3)
419)3)
The packing of molecules revealed a zig-zag pattern
of anti-parallel one-dimensional tapes of dpkbh with a
network of intermolecular and intramolecular
Fig. 1. ORTEP drawing of the structure of dpkbh. The thermal ellipsoids are drawn at the 30% probability level.

132
M. Bakir, O. Brown / Journal of Molecular Structure 609 32002) 129±136
Table 3
Selected bond distances )A), angles )8) and hydrogen bonds for dpkbh
Ê
N)1)±C)11)
N)1)±C)15)
C)11)±C)12)
C)15)±C)1)
C)1)±C)25)
C)1)±N)3)
N)3)±N)4)
N)4)±C)2)
C)2)±O
1.34)3)
1.34)2)
1.37)3)
1.49)3)
1.49)3)
1.30)2)
1.36)2)
1.36)3)
1.22)2)
1.49)3)
1.39)3)
C)11)±N)1)±C)15)
N)1)±C)11)±C)12)
N)1)±C)15)±C)1)
N)3)±C)1)±C)25)
N)3)±C)1)±C)15)
C)25)±C)1)±C)15)
N)4)±C)2)±C)31)
C)1)±N)3)±N)4)
N)3)±N)4)±C)2)
O±C)2)±N)4)
118.2)2)
124.0)2)
117.8)2)
113.1)2)
127.4)2)
119.5)2)
114.2)2)
118.5)2)
120.6)2)
123.6)2)
122.2)2)
C)2)±C)31)
C)31)±C)32)
O±C)2)±C)31)
Donor-H´´´acceptor
d)D±H)
d)H´´ ´A)
d)D´ ´ ´A)
, )D±H´´´A)
N)4)±H)4)´´´N)1)
C)12)±H)12)´´´Oa
C)14)±H)14)´´´N)2)
0.91)3)
1.00)3)
0.99)2)
1.85)2)
2.50)2)
2.36)2)
2.62)3)
3.24)3)
2.96)3)
141.0)2)
130.6)2)
118.0)2)
a
Symmetry transformations used to generate equivalent atoms: x, y, 1 1 z.
hydrogen bonds. Table 3 gives a list of hydrogen bonds
and Fig. 2 shows a view of the inter- )O´´´H±C) and
intra- )N2´´´H14±C14) and )N1´´´H4±N4) hydrogen
bonds. The bond distances and angles of these bonds
are of the same order as those reported in variety of
compounds containing such bonds [27,28]. For
hydrogen-bond in Na1´C5H3N2O32´3H2O [29] and
Ê
hydrogen bond parameters of 2.61 A and 1298 were
reported for the )N±H´´´N) in dpkph [26].
3.2. Molecular sensing behavior
Ê
example, d)H´´´A) and , )D±H´´´A) of 2.46 A and 1368
were observed for the soft non-classic C±H´´´O
The electronic absorption spectra of dpkbh in non-
aqueous solvents are shown in Fig. 3. In these spectra,
Fig. 2. A view of the hydrogen bonds in dpkbh. Symmetry transformations used to generate equivalent atoms: x, y, 1 1 z.

M. Bakir, O. Brown / Journal of Molecular Structure 609 32002) 129±136
133
Fig. 3. The electronic absorption spectra of dpkbh 3.0 £ 1025 M in DMF )1), DMSO )2) and CH3CN )3).
a low-)a-) and a high-)b-) energy intraligand charge-
transfer )ITLC) absorption bands may be due to dpk
p±pp transition followed by dpk to benzoyl charge-
transfer appeared that are solvent dependent. These
bands shift to higher energy compared to those
reported for high- and low-energy conformations of
dpknph due to the decrease in electron withdrawing
and delocalization of the banzoyl moiety compared to
the p-nitrophenyl moiety [22,23]. The ratio of Aa/Ab
)seeFig. 3)issolventdependentpointstointermolecular
Fig. 4. Electronic absorption spectra of a DMF solution of dpkbh 3.0 £ 1025 M )1) in the presence of NaOH 5.6 £ 1022 M )2) and citric acid
6.0 £ 1022 M )3).

134
M. Bakir, O. Brown / Journal of Molecular Structure 609 32002) 129±136
Fig. 5. Electronic absorption spectra of a DMF solution of dpkbh 1.25 £ 1025 M )1) in the presence 1.00 £ 1026 M )2), 5.00 £ 1026 M )3),
1.00 £ 1025 M )4), 2.00 £ 1025 M )5), 3.00 £ 1025 M )6) and 4.00 £ 1025 M )7) CdCl2.
interaction between dpkbh and surrounding solvent
molecules. A plot of the absorbance of the high-)b-)
and low-)a-) energy absorption bands of dpkbh
increased. In the absence of metal ions the electronic
absorption spectra of dpkbh in aqueous media )ethanol
50% v/v and pH 6) are similar to those observed in
acidic non-aqueous media and in the presence of
metal ions the aqueous spectra are similar to those
observed in basic non-aqueous media [8].
in DMF versus its total concentration )5.0 £ 1025
±
1.0 £ 1026 M) is non-linear, con®rming the solvent 2
solute interaction observed from solvent variation and
hints to deviation from Beer's Law. As the concentra-
tion of dpkbh decreases, the ratio Aa/Ab increases. In
previous studies on the electronic absorption spectra
of dpkbh in non-aqueous media that include DMF and
CHCl3, there have been no reports on the appearance
of the low-energy absorption band [7,16].
The reversibility and high sensitivity of the high-
and low-energy absorption bands of dpkbh to their
surroundings allowed for the use of this system
)dpkbh and surrounding solvent molecules) as opto-
sensors for a variety of species that include main
group and transition metal ions. When metal ions of
CaCl2, ZnCl2 or CdCl2 were allowed to interact with
dpkbh in DMF the intensity of the high-energy
absorption band decreased and the intensity of the
low-energy absorption band increased. The electronic
absorption spectra of dpkbh in DMF in the presence
and absence of CdCl2 are shown in Figs. 5 and 6
shows a plot of the absorbance of the high- and
low-energy electronic states of dpkbh, versus concen-
tration of CdCl2. These results con®rm the high
sensitivity of the a- and b-electronic states to slight
variations in their surroundings and metal ions in
concentrations ,1.0 £ 1027 can be detected and
determined using this system. In previous reports on
the spectrophotometric determination of transition
metal ions in aqueous media using dpkbh, competing
Manipulation of the solution composition allows us
to push the equilibrium distribution of the low-)a-) and
high-)b-) conformations into one favorable conforma-
tion and thus calculate the extinction coef®cients of the
a- and b-electronic states of dpkbh. The electronic
absorption spectra of dpkbh 3.0 £ 1025 M in the
presence and absence of NaOH 5.6 £ 1022 M and citric
acid 5.9 £ 1022 M in DMF are shown in Fig. 4 and gave
extinction coef®cients of 45,000 ^ 2000 and
15,000 ^ 2000 M21 cm21 for the a- and b-electronic
states of dpkbh, respectively. When NaBH4 was used in
place of NaOH, the low-energy electronic state
becomes favorable and when KPF6 was used in place
of NaBH4, the intensity of the low-energy band
decreased and the intensity of the high-energy band

M. Bakir, O. Brown / Journal of Molecular Structure 609 32002) 129±136
135
Fig. 6. A plot of the absorbance of a-)series 1) and b-)series 2) conformations of dpkbh 1.25 £ 1025 M in DMF at 392 and 324 nm, respectively,
versus concentration of CdCl2.
species such as HClO4, NaOH, CH3COOH and others,
all of which interact with dpkbh, were used to adjust
the pH of the solution. The interaction between the
competing species and dpkbh affect the equilibrium
distribution of the high- and low-energy electronic
states of dpkbh and its optosensing properties.
Due to its convenient synthesis, rich physicochem-
ical properties and current interest in molecular
sensors, the coordination chemistry and optosensing
behavior of dpkbh are under investigation.
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