Rhenium tricarbonyl chloro of di-2-pyridylketone 4-aminobenzoyl hydrazone (dpk4abh), fac-[Re(CO)3(κ2-N,N-dpk4abh)Cl]: Synthesis, spectroscopic and electrochemical properties

Article abstract of DOI:10.1016/j.molstruc.2011.03.009

The reaction between di-2-pyridyl ketone 4-aminobenzoylhydrazone hydrate (dpk4abh.H₂O) and [Re(CO)₅Cl] in refluxing toluene gave fac-[Re(CO)₃(κ²-N,N-dpk4abh)Cl]. Hydrolysis of fac-[Re(CO)₃(κ²-N,N-dpk4abh)Cl] to form fac-[Re(CO)₃(κ³-N,O,N-dpkO,OH)] occurs in wet solvents. The identity of fac-[Re(CO)₃(κ²-N,N- dpk4abh)Cl] was established from the results of its elemental analysis, spectroscopic measurements, and electrochemical properties. The infrared spectra of fac-[Re(CO)₃(κ²-N,N-dpk4abh)Cl] show the facial coordination of the carbonyl (CO) groups, the coordination of dpkabh and none coordination of the carbonyl (CO) group. ¹H-NMR measurements on fac-[Re(CO)₃(κ²-N,N-dpk4abh)Cl] in d ₆-dmso and d₇-dmf confirmed the coordination of dpkabh, and demonstrate sensitivity of fac-[Re(CO)₃(κ²-N,N- dpk4abh)Cl] to its surroundings as manifested by the chemical shift variations observed in different solvents. Variable temperature ¹H-NMR studies showed stronger interaction between d₇-dmf and fac-[Re(CO) ₃(κ²-N,N-dpk4abh)Cl] compared to d₆-dmso and the amine protons of coordinated dpk4abh undergo faster exchanges with solvated water protons compared to the amide proton. The electronic absorption spectra of fac-[Re(CO)₃(κ²-N,N-dpk4abh)Cl] showed significant solvent dependence. In CH₃CN, a single intra-ligand charge transfer (ILCT) transition appeared at 332 nm and assigned to π → ^π of dpk followed by dpk → hydrazone charge transfer. In a protophilic solvent, such as dmf, two intra-ligand charge transfer (ILCT) transitions assigned to fac-[Re(CO)₃(κ²-N,N-dpk4abh- H)Cl] and fac-[Re(CO)₃(κ²-N,N-dpk4abh)Cl] appeared at 464 and 350 nm, respectively. Reversible interconversion between fac-[Re(CO)₃(κ²-N,N-dpk4abh)Cl] and its conjugate base fac-[Re(CO)₃(κ²-N,N-dpk4abh-H)Cl] was established in protophilic solvents by using a base to shift the equilibrium to the conjugate base and an acid to shift the equilibrium or conjugate base to the neutral form. Acid-base titrations of fac-[Re(CO)₃(κ ²-N,N-dpk4abh-H)Cl] or fac-[Re(CO)₃(κ²-N, N-dpk4abh)Cl] show acids or bases in concentrations as low as 1x10^-6 M can be detected and determined using protophilic solutions of fac-[Re(CO) ₃(κ²-N,N-dpk4abh)Cl]. Electrochemical measurements on fac-[Re(CO)₃(κ²-N,N-dpk4abh)Cl] in CH ₃CN and CH₂Cl₂ showed sequential irreversible redox processes consistent with the electrochemical reduction or oxidation of the hydrazone backbone.

Full text of DOI:10.1016/j.molstruc.2011.03.009

Journal of Molecular Structure 996 (2011) 24–30
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Rhenium tricarbonyl chloro of di-2-pyridylketone 4-aminobenzoyl
hydrazone (dpk4abh), fac-[Re(CO)3(j
2-N,N-dpk4abh)Cl]: Synthesis,
spectroscopic and electrochemical properties
⇑
Mohammed Bakir , Orville Green
Department of Chemistry, The University of the West Indies – Mona Campus, Kingston 7, Jamaica, WI
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction between di-2-pyridyl ketone 4-aminobenzoylhydrazone hydrate (dpk4abh.H2O) and
[Re(CO)5Cl] in refluxing toluene gave fac-[Re(CO)3(
N,N-dpk4abh)Cl] to form fac-[Re(CO)3(
3-N,O,N-dpkO,OH)] occurs in wet solvents. The identity of fac-
[Re(CO)3(
2-N,N-dpk4abh)Cl] was established from the results of its elemental analysis, spectroscopic
measurements, and electrochemical properties. The infrared spectra of fac-[Re(CO)3(
2-N,N-dpk4abh)Cl]
show the facial coordination of the carbonyl (C„O) groups, the coordination of dpkabh and none coor-
dination of the carbonyl (C@O) group. 1H-NMR measurements on fac-[Re(CO)3( 2-N,N-dpk4abh)Cl] in
d6-dmso and d7-dmf confirmed the coordination of dpkabh, and demonstrate sensitivity of fac-
[Re(CO)3(
2-N,N-dpk4abh)Cl] to its surroundings as manifested by the chemical shift variations observed
in different solvents. Variable temperature 1H-NMR studies showed stronger interaction between d7-dmf
and fac-[Re(CO)3(
2-N,N-dpk4abh)Cl] compared to d6-dmso and the amine protons of coordinated
dpk4abh undergo faster exchanges with solvated water protons compared to the amide proton. The elec-
tronic absorption spectra of fac-[Re(CO)3(
2-N,N-dpk4abh)Cl] showed significant solvent dependence. In
CH3CN, a single intra-ligand charge transfer (ILCT) transition appeared at 332 nm and assigned to
of dpk followed by dpk ? hydrazone charge transfer. In a protophilic solvent, such as dmf, two intra-
ligand charge transfer (ILCT) transitions assigned to fac-[Re(CO)3(
2-N,N-dpk4abh-H)Cl] and fac-
[Re(CO)3(
2-N,N-dpk4abh)Cl] appeared at 464 and 350 nm, respectively. Reversible interconversion
between fac-[Re(CO)3(
2-N,N-dpk4abh)Cl] and its conjugate base fac-[Re(CO)3( 2-N,N-dpk4abh-H)Cl]
j -
2-N,N-dpk4abh)Cl]. Hydrolysis of fac-[Re(CO)3(j2
Received 24 January 2011
Received in revised form 4 March 2011
Accepted 5 March 2011
j
j
Available online 23 March 2011
j
Keywords:
Rhenium
Di-2-pyridyl ketone 4-amino benzoyl
hydrazone
Synthesis
j
j
j
Spectroscopy
Electrochemistry
j
p
?
pꢀ
j
j
j
j
was established in protophilic solvents by using a base to shift the equilibrium to the conjugate base
and an acid to shift the equilibrium or conjugate base to the neutral form. Acid–base titrations of
fac-[Re(CO)3(
concentrations as low as 1x10ꢁ6 M can be detected and determined using protophilic solutions of
fac-[Re(CO)3(
2-N,N-dpk4abh)Cl]. Electrochemical measurements on fac-[Re(CO)3( 2-N,N-dpk4abh)Cl]
j j
2-N,N-dpk4abh-H)Cl] or fac-[Re(CO)3( 2-N,N-dpk4abh)Cl] show acids or bases in
j
j
in CH3CN and CH2Cl2 showed sequential irreversible redox processes consistent with the electrochemical
reduction or oxidation of the hydrazone backbone.
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
tate, or multidentate fashion to form novel mononuclear and
polynuclear compounds [1–12,29]. The biological activity of sev-
The diverse reactivity patterns and rich physical properties of
di-2-pyridyl ketone derivatives (see Scheme 1) and their metal
compounds continue to attract research interest because of their
potential applications in several areas that include medicine, catal-
ysis, environment, pharmaceuticals, materials science, etc. [1–37].
Di-2-pyridyl ketone derivatives possess a variety of potential bind-
ing sites, hence may coordinate to metal moieties in a monoden-
eral di-2-pyridyl ketone derivatives and their metal compounds
were investigated and revealed anti-mycobacterium tuberculosis
activity, cytotoxicity, and potent and selective antitumor activity
[14–19]. Although extensive efforts were devoted to exploring
the use of di-2-pyridyl ketone hydrazones as sensitive analytical
reagents for the detection and determination of trace amounts of
a variety of substrates in biological and industrial samples, the
coordination chemistry of di-2-pyridyl ketone hydrazones
(dpk4abh) is scarce [20–27]. To our knowledge, only one Fe(II)
compound of the conjugate base of dpk4abh (dpk4abh-H), [Fe(dp-
k4abh-H)2], isolated from the interaction between FeSO4ꢂ7H2O,
⇑
Corresponding author. Tel.: +1 876 935 8164; fax: +1 876 977 1835.
E-mail address: mbakir@uwimona.edu.jm (M. Bakir).
0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.03.009

M. Bakir, O. Green / Journal of Molecular Structure 996 (2011) 24–30
25
Scheme 1. Selected di-2-pyridyl ketone derivatives.
dpk4abhꢂH2O and triethylamine in aqueous media, was reported
2.2. Preparation of fac-[Re(CO)3(j
2-N,N-dpk4abh)Cl]
[17]. In [Fe(dpk4abh-H)2], dpkabh-H coordinates to Fe through a
nitrogen atom of a pyridine ring, a nitrogen atom of the hydrazone
backbone, and the oxygen atom of the carbonyl group. Tin
compounds of the closely related ligand, di-2-pyridyl ketone 2-amino-
benzoyl hydrazone (dpk2abh), of formula [Sn(C6H5)(dpk2abh-
H)Cl2], [Sn(C6H5)2(dpk2abh)Cl2], [Sn(C6H5CH2)(dpk2abh-H)Cl2]
and [Sn(C6H5)3Cl(H2O)ꢂdpk2abh)] were reported [19,28]. In the
case of [Sn(C6H5)(dpk2abh-H)Cl2] and [Sn(C6H5)2(dpk2abh)Cl2],
and [Sn(C6H5CH2)(dpk2abh-H)Cl2] the ligand (dpk2abh) or its con-
jugate base (dpk2abh-H) bind to tin through one nitrogen atom of
a pyridine ring, a nitrogen atom of the hydrazone backbone, and
the oxygen atom of the carbonyl group [28]. In the case of
[Sn(C6H5)3Cl(H2O)ꢂdpk2abh)], the ligand does not coordinate to
tin [19]. The in vitro studies of the reported tin compounds re-
vealed strong antimicrobial activity of [Sn(C6H5)2(dpk2abh)Cl2]
and [Sn(C6H5)3Cl(H2O)ꢂdpk2abh]. None of the tin compounds of
dpk2abh or alk2abh-H showed genotoxicity in Bacillus subtilis
rec-assay and in the Salmonella-microsone test [19]. We have been
interested in the chemistry of di-2-pyridyl ketone derivatives and
reported on the synthesis, characterization and structures of sev-
eral transition metal compounds containing N,N-bidentate and
N,O,N- or N,N,O-tridentate di-2-pyridyl ketone derivatives [29–
36]. In our efforts to explore how slight variations in the backbone
of di-pyridyl ketone derivatives affect their coordination chemistry
and physical properties, in this report we describe the synthesis
and physical properties of the first rhenium compound of dpk4abh
and compare the results with those reported for related com-
pounds. In a recent report, we described the molecular sensing
behavior of dpk4abhꢂH2O in non-aqueous media and revealed the
presence of two intra-ligand charge transfer transitions (ILCT)
associated with dpk4abhꢂH2O and its conjugate base. Spectroscopic
and thermodynamic measurements done on non-aqueous solu-
tions of dpk4abhꢂH2O revealed reversible interconversion between
dpk4abh and its conjugate base dpk4abh-H and showed group 12
metal ions in concentrations as low as 1.0 ꢃ 10ꢁ6 M can be de-
tected using protophilic solutions of dpk4abhꢂH2O [34].
A mixture of Re(CO)5Cl (200 mg, 0.55 mmol), dpk4abhꢂH2O
(250 mg, 0.74 mmol) and toluene (50 mLs) was refluxed for 10 h.
The resulting reaction mixture was allowed to cool to room tem-
perature and reduced in volume to ꢄ25 mL. An off-yellow solid
was filtered off, washed with hexane, diethyl ether, and dried; yield
250 mg (73%) (Found: C, 41.18; H, 2.37; Cl, 8.99. C21H14Cl N4O4Re
requires C, 41.48; H, 2.32; Cl, 9.22 %). Infrared data (KBr disk,
cmꢁ1):
3484, 3466, and 3333 and
m(C„O) 2026, 1924, 1918;
m(C@O) 1684, and m(NAH)
m(CAH) 3278, 3242, 3183, 3088 and
2925. 1H-NMR (d ppm): in dmso-d6 at 303 K: 11.67 (s, 0.7 H,
NH), 9.03 (d, 1H, H14-dpk), 8.90 (d, 1H, H140-dpk), 8.25 (t, 1H,
H13-dpk), 8.22 (t, 1H, H130-dpk), 8.03 (d, 1H, H11-dpk), 7.99 (d,
1H, H110-dpk), 7.72 (t, 2H, H12 and H120-dpk), 7.58 (d, 2H,H32
and H36-phen), 6.60 (d, 2H, H33 and H35-phen), 5.89 (broad, 1.0
H, NH2) and 3.30 (water); in dmf-d7 at 303 K 11.90 (s, 0.9 H,
NH), 9.17 (d, 1H, H14-dpk), 9.04 (d, 1H, H140-dpk), 8.36 (t, 1H,
H13-dpk), 8.31 (t, 1H, H130-dpk), 8.25 (d, 1H H11-dpk), 8.12 (d,
1H, H110-dpk), 7.82 (t, 1H, H12-dpk), 7.80 (t, 1H, H120-dpk), 7.76
(d, 2H, H32andH36-phen), 6.70 (d, 2H, H33 and H35-phen), 5.98
(broad, 1.5 H, NH2) and 3.45 (water). UV–vis {k 2/nm, (
e 200/
cmꢁ1 Mꢁ1)} CH2Cl2: 342 (13,794); CH3CN: 346 (13,414); dmf:
476 (27,260), 362 (15,000).
2.3. Hydrolysis of fac-[Re(CO)3(j
2-N,N-dpkabh)Cl]
When a sample of fac-[Re(CO)3(j
2-N,N-dpkabh)Cl] was refluxed
in CH3CN containing a few drops of H2O, yellow fac-[Re(CO)3(j3
N,O,N-dpkO,OH)] was isolated. Yellow crystals of fac-[Re(CO)3(j3
-
-
N,O,N-dpkO,OH)] were obtained when fac-[Re(CO)3(
j
2-N,
N-dpkabh)Cl] was allowed to stand in CH3CN or dmf for a few days
in air. The spectroscopic and electrochemical properties of the iso-
lated fac-[Re(CO)3(
the previously reported fac-[Re(CO)3(
crystal X-ray studies on several crystals isolated confirmed the
j
3-N,O,N-dpkO,OH)] were similar to those of
j
3-N,O,N-dpkO,OH)]. Single-
identity of the hydrolysis product as fac-[Re(CO)3(j
3-N,O,N-
dpkO,OH)].
2. Experimental
2.4. Physical measurements
2.1. Reagents and reaction procedures
Electronic absorption spectra were recorded on a Perkin-Elmer
Lambda 19 UV/VIS/NIR spectrometer. Solution 1H-NMR spectra
were recorded on a Bruker ACE 500-MHz Fourier-transform spec-
trometer and referenced to the residual protons in the incom-
pletely deuteriated solvent. Infrared spectra were recorded as
KBr pellets on a Perkin–Elmer Spectrum 1000 FT-IR Spectrometer.
Electrochemical measurements were performed with the use of a
Princeton Applied Research (PAR) Model 173 potentiostat/galvano-
stat and Model 276 interface in conjunction with a 286 PC. Data
The ligand di-2-pyridyl ketone p-aminobezoyl hydrazone hy-
drate (dpk4abhꢂH2O) was prepared from the reaction between
dpk, p-aminobenzoylhydrazine, ethanol and three drops of concen-
trated HCl under reflux. Found C, 64.37; H, 5.08; N, 20.71.
C
18H17N5O2 requires C, 64.47; H, 5.11; N, 20.77. IR (KBr disk):
m
(NH) 3220, (OH) 3394 and
m
m
(C@O) 1642 cmꢁ1 [34]. All other re-
agents were obtained from commercial sources and used without
further purification.

26
M. Bakir, O. Green / Journal of Molecular Structure 996 (2011) 24–30
were acquired with the EG&G PARC Headstart program and manip-
ulated using Microsoft Excel. 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 potassium chloride
saturated calomel electrode, SCE, at room temperature and were
uncorrected for junction potentials. The number of electrons on
the redox waves was determined using the oxidative peak current
served in a variety of rhenium compounds of the type fac-[Re
(CO)3(LAL)Cl], where LAL = bidentate nitrogen ligand, thus confirm
the fac-geometry of coordinated carbonyl groups [29–31]. The
m(C@O) stretching vibration of coordinated dpk4abh shifts to
higher wave number compared to free dpk4abh pointing to the
non-coordination of the acyl group and delocalization of the
electron dentistry about the hydrazone moiety upon coordination
of the reversible one-electron couple of FeCp2/FeCpþ couple as an
of dpk4abh to rhenium moiety [34]. The combined
(C@N) of the pyridine vibrations of fac-[Re(CO)3(
dpk4abh)Cl] appeared at 1624 and 1604 cmꢁ1. In the spectrum of
fac-[Re(CO)3(
2-N,N-dpkphh)Cl] where dpkphh is di-2-pyridyl ke-
m
(C@C) and
2
internal standard. Electrochemical cells were of conventional de-
sign based on scintillation vials or H-cells. A glassy-carbon disc
was used as the working electrode and a Pt-wire was used as the
counter electrode.
m
j
2-N,N-
j
tone phenyl hydrazone, the combined C@N and C@C stretching ap-
peared at 1606 and 1561 cmꢁ1 [34]. In the free ligand, these bands
appeared at 1606 cmꢁ1. The shift of the combined
m
(C@C) and
2.5. Analytical procedures
m(C@N) in fac-[Re(CO)3(j
2-N,N-dpk4abh)Cl] to a higher wave-
length, compared to the free ligand, hints to a decrease in electron
density of the pyridine rings upon coordination, and binding
Elemental microanalyses were performed by MEDAC Ltd.,
Department of Chemistry, Brunel University, Uxbridge, United
Kingdom.
through the nitrogen atoms of the pyridine rings. In the
stretching region of the amine group between 3500–3300 cmꢁ1
two sharp bands appeared at 3469 and 3350 cmꢁ1, confirming
the presence of free amine group. The amide (NAH) and the aro-
m(NH)
,
3. Results and discussion
m
matic m(CAH) of the dpk moiety show weak absorption bands be-
Following a procedure similar to those reported for the synthe-
tween 3200–2800 cmꢁ1. In the spectrum of free dpk4abhꢂH2O, the
sis of rhenium tricarbonyl compounds of
type fac-[Re(CO)3(
2-LAL)Cl] where LAL is N,N -bidentate
mine ligand, yellow fac-[Re(CO)3(
2-N,N-dpk4abh)Cl] (see
a-diimine ligands of the
amine m
(NH) vibrations appeared at 3460 and 3406 cmꢁ1, amide
j
a-di-
m
(NAH) vibration appeared at 3244 and
between 3100–2800 cmꢁ1. In the
(C@N),
d(NH) region in the spectrum of fac-[Re(CO)3(
bands appeared similar to those observed in the free ligand, fac-
[Re(CO)3(
2-N,N-dpkphh)Cl] and other rhenium compounds con-
taining di-2-pyridyl ketone derivatives [36].
The 1H-nmr spectra of fac-[Re(CO)3( 2-N,N-dpk4abh)Cl] in d6-
m
(CAH) stretches appeared
(CAC), (CAN), (NAN),
2-N,N-dpk4abh)Cl],
j
m
m
m
m
Scheme 2) was isolated in good yield when dpk4abhꢂH2O was al-
lowed to react with [Re(CO)5Cl] in refluxing toluene [29,31,36].
When the reaction was carried out in refluxing CH3CN containing
j
a few drops of water, or when fac-[Re(CO)3(
was allowed to stand in CH3CN in air for an extended period,
fac-[Re(CO)3(
3-N,O,N-dpkO,OH)] was isolated [29]. The hydro-
lysis product, fac-[Re(CO)3(
3-N,O,N-dpkO,OH)], exhibits physical
properties similar to those of the previously reported fac-
[Re(CO)3(
3-N,O,N-dpkO,OH)] isolated from the reaction between
[Re(CO)5Cl] and dpk in refluxing CH3CN containing a few drops
of water [29]. Attempts to grow crystals of fac-[Re(CO)3(
2-N,
j
2-N,N-dpk4abh)Cl]
j
j
j
dmso and d7-dmf at 303 K are shown in Fig. 1, and the chemical
shifts are listed in the experimental section. These spectra,
along with the chemical shifts, show significant variation in the
chemical shifts of the protons of coordinated dpk4abh, pointing
to strong solvent–complex interaction and the sensitivity of
j
j
j
fac-[Re(CO)3(
all protons of fac-[Re(CO)3(
downfield compared to d6-dmso, and suggest
j
2-N,N-dpk4abh)Cl] to its surroundings. In d7-dmf,
2-N,N-dpk4abh)Cl] were observed
stronger
N-dpk4abh)Cl] from CH3CN or dmf solutions of fac-[Re(CO)3-
j
(
(
j
j
2-N,N-dpk4abh)Cl] were futile and crystals of fac-[Re(CO)3-
3-N,O,N-dpkO,OH)] were obtained and characterized using
a
single-crystal X-ray crystallography.
The identity of fac-[Re(CO)3(
j
2-N,N-dpk4abh)Cl] was estab-
lished from the results of its elemental analysis, and a number of
spectroscopic measurements, and a comparison of the spectro-
scopic results with those reported for other rhenium compounds
of the type fac-[Re(CO)3(
fac-[Re(CO)3(
2-N,N-dpk4abh)Cl] display peaks consistent with
the coordination of dpk4abh (see experimental section) and facial
coordination of the carbonyl groups. In the carbonyl (C„O)
j
2-LAL)Cl] [29–31]. The IR spectra of
j
m
stretching region, three strong bands appeared similar to those ob-
Fig. 1. 1H-NMR spectrum of fac-[Re(CO)3( 2-N,N-dpk4abh)Cl] in d6-dmso and d7-
j
Scheme 2. fac-[Re(CO)3(dpk4abh)Cl].
dmf.

M. Bakir, O. Green / Journal of Molecular Structure 996 (2011) 24–30
27
Table 1
solvent–complex interaction in d7-dmf compared to d6-dmso. Four
Slopes of d vs. 1/T ꢃ 103, Kꢁ1 of fac-[Re(CO)3(dpk4abh)Cl] in d6-dmso and d7-dmf.
doublets, one proton each, and four triplets, one proton each, were
observed and assigned to the coordinated pyridine rings and sug-
gest that the pyridine rings are not equivalent. The phenyl protons
of the benzoyl hydrazone moiety were observed as two doublets,
two protons each, up-field compared to dpk protons. These spectra
show that in d6-dmso at 303 K, the amide proton (NAH) and the
amine protons (NH2) undergo faster exchange with the solvated
water protons compared d7-dmf (see experimental section). Chem-
ical shift separations of 0.06 ppm and 0.13 ppm were observed be-
tween H11 and H110 in d6-dmso and d7-dmf, respectively and
Solvent
(NH)
(NH2)
Hydrate
d6-dmso
d7-dmf
0.44
0.60
0.56
0.62
0.51
0.69
points to significant structural changes of fac-[Re(CO)3(j
2-N,N-
dpk4abh)Cl] when dissolved in protophilic solvents. In d6-dmso
and d7-dmf a slight change in the chemical shifts of H12 and
H120 was noticed.
Variable temperature 1H-NMR studies done on fac-[Re(CO)3(j2
-
N,N-dpk4abh)Cl] dissolved in d7-dmf and d6-dmso show tempera-
ture dependent and temperature independent protons of
coordinated dpkabh. Fig.
fac-[Re(CO)3(
2-N,N-dpk4abh)Cl] in d6-dmso at different tempera-
2
shows the 1H-NMR spectra of
j
ture. These spectra show the amide (NH), amine (NH2) and hydrate
(H2O) protons to exhibit higher sensitivity to temperature varia-
tions, compared to the aromatic protons. Upon a gradual increase
in temperature, the amide, amine and solvated water protons shift
up-field, and the intensity of the amine protons diminished. The
decrease in intensity of the amine proton upon increase in temper-
ature points to exchange of the amine protons with water protons.
In d7-dmf, similar behavior as in d6-dmso was observed, and a slow
exchange of the amine protons with the solvated water protons
was noticed. Solvent-exposed protons are temperature indepen-
dent and solvent shielded protons show temperature dependence.
A plot of the chemical shift of the amide, amine, and water protons
vs. 1/T ꢃ 103, Kꢁ1 gave straight lines with slopes as shown in
Table 1. The higher the slope of the line, the more sensitive the
proton is to temperature variations, hinting at the shielding of
the proton from its surroundings, and participation in hydrogen
bonding. The results obtained in this study point to stronger
Fig. 3. Electronic absorption of 2.00 ꢃ 10ꢁ5 M fac-[Re(CO)3(
j
2-N,N-dpk4abh)Cl] in
dmf, dmso, and CH3CN.
served electronic transitions are similar to those reported for free
dpk4abh and are due to
pꢀ of dpk followed by dpk ? amino-
p
?
benzoyl charge transfer. In the free ligand, dpk4abh, two electronic
transitions appeared at 426 and 346 nm in protophilic solvents
(dmf and dmso) and in CH2Cl2 a single electronic transition
appeared at 338 nm [34]. In protophilic solvents, the low energy
electronic transitions are assigned to fac-[Re(CO)3(
dpk4abh-H)Cl], the conjugate base of fac-[Re(CO)3(
interaction between fac-[Re(CO)3(
j
2-N,N-dpk4abh)Cl] and d7-dmf
j
j
2-N,N-
compared to d6-dmso.
2-N,N-
The electronic absorption spectra of fac-[Re(CO)3(
j
2-N,N-
dpk4abh)Cl], resulted from deprotonation of amide proton,
dpk4abh)Cl] in dmf, dmso, and CH3CN are shown in Fig. 3, and
the extinction coefficients are given in the experimental section.
In CH3CN, a single electronic transition appeared at 332 nm and
in protophilic solvent (dmf or dmso), two electronic transitions ap-
peared at 464 and 350 nm (see experimental section). The ob-
and the high energy electronic transition is assigned to fac-
[Re(CO)3(j
2-N,N-dpk4abh)Cl]. The ratio of the intensity of the
low energy electronic transition observed at 464 nm to the inten-
sity of the high energy electronic transition observed at 350 nm in-
creases as the concentration of fac-[Re(CO)3(
dissolved in dmso decreases. These results show sensitivity of
j
2-N,N-dpk4abh)Cl]
fac-[Re(CO)3(j
2-N,N-dpk4abh)Cl] to its surroundings and confirm
the solvent–complex interaction observed in the 1H-NMR spectra.
When a dmf solution of NaBH4 was allowed to interact with fac-
[Re(CO)3(j
2-N,N-dpk4abh)Cl] dissolved in dmf, the intensity of
the low energy electronic transition increased and the intensity
of the high energy electronic transition decreased (see Fig. 4).
The reverse was observed when excess NaBF4 was added to a
dmf solution of fac-[Re(CO)3(
hint to acid–base interconversion between fac-[Re(CO)3(
N-dpk4abh)Cl] and its conjugate base fac-[Re(CO)3(
2-N,N-
dpk4abh-H)Cl] in dmf (see Scheme 3). The acid–base interconver-
sion between fac-[Re(CO)3(
2-N,N-dpk4abh)Cl] and its conjugate
base fac-[Re(CO)3(
2-N,N-dpk4abh-H)Cl] was further confirmed
when stoichiometric amounts of benzoic acid dissolved in dmso
were added to a mixture of a dmso solution of fac-[Re(CO)3(j2
j
2-N,N-dpk4abh)Cl]. These results
2-N,
j
j
j
j
-
N,N-dpk4abh)Cl] that contain excess NaOH (see Fig. 5). These re-
sults showed benzoic acid in concentrations as low as
1.00 ꢃ 10ꢁ6 M can be detected and determined using a protophilic
Fig. 2. Variable temperature 1H-NMR spectra of fac-[Re(CO)3( 2-N,N-dpk4abh)Cl]
j
in d6-dmso.
solution of fac-[Re(CO)3(j
2-N,N-dpk4abh)Cl] and allowed for

28
M. Bakir, O. Green / Journal of Molecular Structure 996 (2011) 24–30
Fig. 4. Electronic absorption spectra of 2.00 ꢃ 10ꢁ5
M fac-[Re(CO)3(j
2-N,N-
dpk4abh)Cl] (1) in dmf in the presence of excess (1.00 ꢃ 10ꢁ3 M) NaBH4 (2) and
excess (1.00 ꢃ 10ꢁ3 M) NaBF4 (3).
Fig. 5. Electronic absorption spectra of a mixture of 2.00 ꢃ 10ꢁ5 M fac-[Re(CO)3(j2
-
N,N-dpk4abh)Cl] and 1.75 ꢃ 10ꢁ4 M NaOH in dmso (1) in the presence 1.00 ꢃ 10ꢁ6
(2), 1.00 ꢃ 10ꢁ5 (3), 2.00 ꢃ 10ꢁ5 (4), 3.00 ꢃ 10ꢁ5 (5), 4.00 ꢃ 10ꢁ5 (6) , 5.00 ꢃ 10ꢁ5
(7), 6.00 ꢃ 10ꢁ5 (8), 7.00 ꢃ 10ꢁ5 (9) M benzoic acid.
calculation of the extinction coefficients of fac-[Re(CO)3(
N-dpk4abh)Cl] observed at 350 nm and its conjugate base,
fac-[Re(CO)3(
2-N,N-dpk4abh-H)Cl], observed at 464 nm (see
experimental section). The electronic transitions of fac-[Re(CO)3-
2-N,N-dpk4abh)Cl] and its conjugate base, fac-[Re(CO)3( 2-N,
N-dpk4abh-H)Cl] in protophilic solvents shift to lower energy,
compared to the electronic transitions of fac-[Re(CO)3(
2-N,N-
dpkbh)Cl] and its conjugate base fac-[Re(CO)3(
2-N,N-dpkbh-
j
2-N,
j
CH2Cl2 (see Fig. 6d), an irreversible reduction appeared at
Ep,c = ꢁ1.42 V followed by an electrochemically generated product
wave at Ep,a = + 0.58 V, a reversible oxidation at E1/2 = +1.35 V and
an irreversible oxidation at Ep,a = +1.76 V. Scan rate studies on
the first reduction waves show no sign of reversibility in CH3CN
or CH2Cl2. On oxidatively initiated scan in CH3CN (Fig. 6b), voltam-
metric behavior similar to the reductively initiated scan was ob-
served except new oxidatively generated product waves
appeared at Ep,c = ꢁ0.92 and ꢁ1.17 V and the reductively generated
product wave observed at Ep,a = +0.58 V on a reductively initiated
scan disappeared. On oxidatively initiated scan in CH2Cl2
(Fig. 6e), a reversible oxidation wave appeared at E1/2 = +1.35 V fol-
lowed by an oxidatively generated product wave at Ep,c = ꢁ1.04 V
and an irreversible reduction at Ep,c = ꢁ1.70 V. The first reduction
wave observed in reductively generated scans in CH3CN and CH2Cl2
is assigned to the to ReI/0 couple, the quasi-reversible/reversible
(j
j
j
j
H)Cl] where dpkbh = di-2-pyridyl ketone benzoyl hydrazone [31].
This is consistent with the electron donating properties of the ami-
no group. In dmf, two electronic transitions appeared at 318 and
440 nm due to to fac-[Re(CO)3(
base fac-[Re(CO)3(
2-N,N-dpkbh-H)Cl], respectively.
The electrochemical properties of fac-[Re(CO)3(
j
2-N,N-dpkbh)Cl] and its conjugate
j
j
2-N,N-
dpk4abh)Cl] measured in non-aquesous solvents were investigated
using voltammetric techniques. Cyclic voltammograms of fac-
[Re(CO)3(j
2-N,N-dpk4abh)Cl] in CH3CN and CH2Cl2 are shown in
Fig. 6. These voltammograms show close similarities, although
subtle variations were noticed. On a reductively initiated scan in
CH3CN (Fig. 6a), an irreversible one electron reduction wave ap-
peared at Ep,c = ꢁ1.40 V, followed by electrochemically generated
product waves at Ep,a = ꢁ0.64, +0.54, a quasi-reversible one elec-
tron oxidation at E1/2 = +1.42 V (Ep,a = +1.38 and Ep,c = + 1.46 V),
and an irreversible multi-electron oxidation at Ep,a = +1.80 V. In
oxidation waves observed at E1/2 = + 1.42 V in CH3CN and E1/2
=
+ 1.35 V are assigned to the ReI/II oxidation, and the multi-electron
irreversible oxidation wave that appeared at +1.80 V in CH3CN
could be assigned to the oxidation of hydrazone backbone. The
assigned ReI/0 and ReI/II couples fall in the potential region as
Scheme 3. Acid–base interconversion between fac-[Re(CO)3(dpkabh)Cl] and its conjugate base fac-[Re(CO)3(dpkabh-H)Cl] in the presence of NaBH4 and NaBF4.

M. Bakir, O. Green / Journal of Molecular Structure 996 (2011) 24–30
29
shows sequential irreversible reductions at Ep,c = ꢁ1.35, ꢁ1.98
and ꢁ2.74 V and an irreversible oxidation appeared at
Ep,a = ꢁ1.80 V. In
a reversible scan between +1.00 ? +3.00 V
sequential irreversible oxidations appeared at Ep,a = +1.33, +1.83,
+2.32 and +2.58 V. A comparison of the peak currents of the reduc-
tions and oxidation waves hint that the reduction wave at ꢁ2.74 V
and second oxidative waves at +1.83 V are multi-electron in nature
at least two electrons compared to the first electronic reductive or
oxidative waves. The irreversibility of the reduction and oxidation
waves in CH3CN and CH2Cl2 and appearance of electrochemically
generated product waves hint to electrochemical decomposition
of fac-[Re(CO)3(
dation. Although detailed analysis of the electrochemical behavior
of fac-[Re(CO)3(
2-N,N-dpk4abh)Cl] remains to be explored, plau-
sible mechanisms for the electrochemical reduction and oxidation
of fac-[Re(CO)3(
2-N,N-dpk4abh)Cl] is shown in Scheme 4. The
j
2-N,N-dpk4abh)Cl] following its reduction or oxi-
j
j
proposed mechanisms are analogous to those reported for the
oxidation and reduction of coordinated dpkoxime in fac-
[Re(CO)3(j
2-N,N-dpkoxime)Cl] in non-aqueous media and are con-
sistent with the sequential electronic transfer observed.37 In
CH3CN, cyclic voltammogram of free dpkabh is show in Fig. 6c,
and shows an irreversible reduction at Ep,a = ꢁ1.85 V followed by
Fig. 6. Cyclic voltammograms of fac-[Re(CO)3(
j
2-N,N-dpk4abh)Cl] in CH3CN (a and
2-N,N-dpk4abh)Cl] in CH2Cl2 (d
a
reductively generated product waves at Ep,a = ꢁ0.60 and
b) and dpkabhꢂH2O in CH3CN (c) and fac-[Re(CO)3(
j
+0.28 V and oxidative waves at Ep,a = +1.34 and +1.60 V.
Due to their convenient synthesis, rich physico-chemical prop-
erties, and molecular sensing capabilities, work is in progress in
our laboratory to isolate a variety of metal compounds of polypyr-
idyl-like ligands, to explore their electro-optical properties, solid
state structures, and conformations.
and e) in the presence of 0.1 M in [N(n-Bu)4](PF6) at a glassy carbon working
electrode at a scan rate of 400 mV sꢁ1 vs. SCE.
observed for these couples in a variety of rhenium carbonyl com-
pounds of polypyridyl-like ligands of the type fac-[Re(CO)3(LAL)Cl]
where LAL = dpk, dpk.oxime, dpknph and others [29,31,36]. The
oxidatively generated product waves in CH3CN and CH2Cl2 fall in
the same vicinity as those observed in cyclic voltammorams of
coordinated dpk in rhenium tricarbonyl compounds measured in
4. Conclusion
The first rhenium complex of dpk4abh has been isolated. Spec-
non-aqueous solvents. Cyclic voltammogram of fac-[Re(CO)3(j2
N,N-dpk4abh)Cl] measured in CH3CN between ꢁ1.00 ? ꢁ3.00 V
-
troscopic measurements on fac-[Re(CO)3(j
2-N,N-dpk4abh)Cl] re-
vealed sensitivity to its surroundings and electrochemical studies
-
O
O
H
H
N
N
+ 2e-
N
N
+ e-
C6H5NH2
N
C6H5NH2
OC
OC
Cl
N
N
N
Re0
CO
N
OC
OC
Cl
N
OC
OC
Cl
N
ReI
CO
-[NHCOC6H5NH2]-
Re0
CO
+ 2e-, + 3H+
NH
2
-e-
NH
2
N
N
H
OC
OC
Cl
H
OC
OC
Cl
Re0
CO
ReI
CO
N
N
Scheme 4. Plausible mechanisms for the reduction and oxidation of fac-[Re(CO)3(dpkabh)Cl].

30
M. Bakir, O. Green / Journal of Molecular Structure 996 (2011) 24–30
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