Catalytic C–C cross-coupling and hydrogen evolution by two Pd(II)-complexes of di-2-pyridyl ketone benzoyl hydrazones

Article abstract of DOI:10.1080/00958972.2019.1645329

The reactions between [PdCl₂(CH₃CN)₂] and dpk-h [dpk-h = di-2-pyridyl ketone benzoyl hydrazone (dpkbh) or di-2-pyridyl ketone 4-aminobenzoyl hydrazone (dpk-4-abh)] at room temperature in air produced [PdCl₂(κ³-N_py,N_im,O-dpkbh)] (1) and PdCl₂(κ³-N_py,N_im,O-dpk-4-abh)] (2). X-ray structural analysis on a single crystal of PdCl(κ³-N,N,O-dpk-4-abh-H)]·H₂O·dmf (3) established its authenticity, points to keto-enol tautomerization due to solvent-compound interactions and revealed pseudo-coordination of the carbonyl group to the Pd ion. Solid-state infrared measurements confirmed the pseudo coordination of the carbonyl group of dpk-h as evident from the appearance of the ν(C=O) of coordinated dpk-h in close proximity to the ν(C=O) of uncoordinated dpk-h. ¹H NMR measurements on protophilic solutions of 1 and 2 disclosed solvent dependence and keto-enol tautomerization while variable temperature studies established the predominance of the keto form. The electronic absorption spectra of 1 and 2 measured in protophilic solvents confirmed the coordination of the Pd ion to dpk-h as evident from the appearance of d-d and ligand based electronic transitions. The catalytic C–C cross-coupling reactions and electro-catalytic behavior of 1 and 2 toward a proton reduction were investigated and revealed good catalytic properties. Overall rate constants (k_app) for the electrocatalytic H₂ evolution of (3.39 ± 0.3) × 10³ and (4.04 ± 0.2) × 10³ M^?1 s^?1 were estimated and overpotentials of 157 and 67 mV, and turnover numbers (TON) of 2.0 and 3.2 for 1 and 2, respectively, were determined.

Full text of DOI:10.1080/00958972.2019.1645329

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
Catalytic C–C cross-coupling and hydrogen
evolution by two Pd(II)-complexes of di-2-pyridyl
ketone benzoyl hydrazones
Mohammed Bakir, Mark W. Lawrence, Peter Nelson & M. Bohari Yamin
To cite this article: Mohammed Bakir, Mark W. Lawrence, Peter Nelson & M. Bohari Yamin
(2019): Catalytic C–C cross-coupling and hydrogen evolution by two Pd(II)-complexes
of di-2-pyridyl ketone benzoyl hydrazones, Journal of Coordination Chemistry, DOI:
10.1080/00958972.2019.1645329
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JOURNAL OF COORDINATION CHEMISTRY
https://doi.org/10.1080/00958972.2019.1645329
Catalytic C–C cross-coupling and hydrogen evolution
by two Pd(II)-complexes of di-2-pyridyl ketone
benzoyl hydrazones
Mohammed Bakira, Mark W. Lawrencea,b, Peter Nelsona and M. Bohari Yaminc
aDepartment of Chemistry, The University of the West Indies-Mona Campus, Jamaica, West Indies;
bSchool of Natural and Applied Sciences, University of Technology, Jamaica, West Indies;
cPublication Enhancement Unit, Publishing Centre, Universiti Kebangsaan Malaysia, Bangi,
Selangor, Malaysia
ABSTRACT
ARTICLE HISTORY
Received 1 May 2019
Accepted 11 June 2019
The reactions between [PdCl2(CH3CN)2] and dpk-h [dpk-h ¼ di-2-
pyridyl ketone benzoyl hydrazone (dpkbh) or di-2-pyridyl ketone
4-aminobenzoyl hydrazone (dpk-4-abh)] at room temperature in
air produced [PdCl2(j3-Npy,Nim,O-dpkbh)] (1) and PdCl2(j3-
Npy,Nim,O-dpk-4-abh)] (2). X-ray structural analysis on a single
crystal of PdCl(j3-N,N,O-dpk-4-abh-H)]ꢀH2Oꢀdmf (3) established its
authenticity, points to keto-enol tautomerization due to solvent-
compound interactions and revealed pseudo-coordination of the
carbonyl group to the Pd ion. Solid-state infrared measurements
confirmed the pseudo coordination of the carbonyl group of dpk-
h as evident from the appearance of the m(C¼O) of coordinated
dpk-h in close proximity to the m(C¼O) of uncoordinated dpk-h.
1H NMR measurements on protophilic solutions of 1 and 2 dis-
closed solvent dependence and keto-enol tautomerization while
variable temperature studies established the predominance of the
keto form. The electronic absorption spectra of 1 and 2 measured
in protophilic solvents confirmed the coordination of the Pd ion
to dpk-h as evident from the appearance of d-d and ligand based
electronic transitions. The catalytic C–C cross-coupling reactions
and electro-catalytic behavior of 1 and 2 toward a proton reduc-
tion were investigated and revealed good catalytic properties.
Overall rate constants (kapp) for the electrocatalytic H2 evolution
of (3.39 0.3) ꢁ 103 and (4.04 0.2) ꢁ 103 Mꢂ1 sꢂ1 were esti-
mated and overpotentials of 157 and 67 mV, and turnover num-
bers (TON) of 2.0 and 3.2 for 1 and 2, respectively, were
determined.
KEYWORDS
Palladium; catalysis; C-C
cross-coupling; di-2-pyridyl
ketone benzoyl hydrazones;
spectroscopy;
electrochemistry
CONTACT Mohammed Bakir
mohammed.bakir@uwimona.edu.jm
Department of Chemistry, The University of
the West Indies, Mona campus, Kingston 7, Jamaica, West Indies
Supplemental data for this article can be accessed here.
ß 2019 Informa UK Limited, trading as Taylor & Francis Group

2
M. BAKIR ET AL.
1. Introduction
Palladium and its coordination compounds continue to attract research activities for
their industrial [1–7], environmental [8–12], chemical [13–30], and biological relevance
[26–30]. Pd-catalysts are widely used in organic synthesis to affect a variety of transfor-
mations that include C-H bond activation, cross-coupling reactions, hydrogenation/
dehydrogenation, decarboxylation, carbonylation, aminocarbonylation, amidation, etc.
[14–23]. Among the various types of Pd cross-coupling reactions, the Suzuki-Miyaura,
Negishi, Mizoroki-Heck, Sonogashira, and Hiyama are the most powerful [16–21]. Pd
coordination compounds of Schiff bases that include di-2-pyridyl imines such as 2,4,6-
trimethyl-(di-2-pyridylmethylene)aniline and 2,6-di-isopropyl-(di-2-pyridylmethylene)ani-
line and ethanedial bis[methyl(pyridin-2-yl)hydrazone are of current interest due to
their rich properties, resistance to oxidation reactions, and catalytic and medicinal
applications [22–30]. Pd(II) complexes of chelating j2-N,N-a-diimine and j3-NNX-Schiff
bases (X ¼ N, O or S) exhibit cytotoxicity against various cancers and emerged as alter-
natives to Pt therapeutic agents due to their higher solubility, lower toxicity, and facile
synthesis compared to Pt complexes [28–30]. In addition to the catalytic and thera-
peutic uses of Pd coordination compounds, Pd plays an important role in hydrogen
economy for purification and storage of hydrogen and use in fuel cells [4–7]. The
widespread use of Pd and its coordination compounds resulted in environmental and
medicinal pollution that promoted research to develop highly sensitive reagents and
analytical methods for sequestering, detection and determination of trace amounts of
Pd due to its high toxicity [9, 10].
We have been interested in the chemistry of di-2-pyridyl ketone (dpk) and di-2-thienyl
ketone (dtk) and their hydrazonic derivatives (dxk-h where x¼ pyridyl or thienyl) contain-
ing the C¼N-NH-(C¼X)n-Y (X¼O or S, n ¼ 0 or 1 and Y ¼ R, Ar or NH2) moiety and reported
on their synthesis, spectroscopic, electrochemical and solid state structures [31–39]. Our
interest in these systems arose from their physical properties, diverse coordination pat-
terns and applications in several areas that include catalysis, medicine, molecular sensors,
etc. In the course of our studies, we reported on the synthesis, solution chemistry

JOURNAL OF COORDINATION CHEMISTRY
3
Scheme 1. Representations of Pd2Cl3(j5-Npy,Nim,S,Nam,Npy-dpktsc-H)] (1) (dpktsc-H ¼ thioamide
deprotonated di-2-pyridyl ketone thiosemicarbazone anion and [PdCl2(j3-Npy,Nim,O-dpk-h)] (2)
(dpk-h with n ¼ 0 ¼ di-2-pyridyl ketone benzoyl hydrazone and n ¼ 1 dpk-h ¼ di-2-pyridyl ketone
4-aminobenzoyl hydrazone.
(spectroscopic and electrochemical measurements) and solid state structures of a variety
of Groups 7 and 12 metal complexes with dpk-h in bidentate j2-N,N-, tridentate j3-N,N,X-
and tetradentate j4-N,N,X,N coordination modes. Spectroscopic and electrochemical
measurements on protophilic solutions (hydrogen bond acceptors) revealed the sensitivity
of dxk-h and their metal complexes to their surrounding and use for the detection and
determination of trace amounts of substrates. The solid-state structures of several of these
compounds revealed extensive networks of noncovalent interactions that account in part
for their stability and molecular sensing behavior. In a recent report, we described the syn-
thesis and solution chemistry of a novel dinuclear Pd-compound of j5-N,N,S,N,N-dpktsc-H,
Pd2Cl3(j5-Npy,Nim,S,Npy,Nam-dpktsc-H)]ꢀsolvate [solvate ¼ CH3CN or H2O and dpktsc-H– ¼
amide deprotonate di-2-pyridyl ketone thiosemicarbazone anion] (Scheme 1) [37]. In this
report, we describe the synthesis and solution chemistry along with the catalytic behavior
of two Pd(II)-compounds of di-2-pyridyl ketone hydrazones [di-2-pyridyl ketone benzoyl
hydrazone (dpkbh) and di-2-pyridyl ketone 4-amino benzoyl hydrazone (dpk-4-abh)]
(Scheme 1). This is, in part, to explore how slight variation in the backbone of hydrazonic
compounds affect their coordination behavior and physicochemical properties.
2. Experimental
2.1. Reagents
PdCl2 was purchased from BDH, Ltd., di-2-pyridyl ketone (dpk), benzoyl hydrazide (bh),
and 4-aminobenzoyl hydrazide (4-abh) were obtained from Sigma-Aldrich and all other
reagents were obtained from commercial sources and used as received. The ligands
di-2-pyridyl ketone benzoylhydrazone, N’-(dipyridin-2-ylmethylene)benzohydrazide,
(dpkbh), and 4-amino-N’-(dipyridin-2-ylmethylene)benzohydrazide, di-2-pyridyl ketone
4-aminobenzoylhydrazone (dpk-4-abh) were prepared from the acid catalyzed conden-
sation reactions between dpk and the desired hydrazide in refluxing acidified ethanol
as described in the literature [32, 34]. Solvents used in spectroscopic and electrochem-
ical measurements were analytical grade. All manipulations were performed in open

4
M. BAKIR ET AL.
air except for the electrochemical measurements. Pd2Cl3(j5-N,N,S,N,N-dpktsc-H) [37],
[PdCl(pdcta)] (pdcta ¼ bis-N-(2,5-dimethoxyphenyl)pyridine-2,6-dicarbothioamide) [40],
and [PdCl(pbcta)].HCl (pbcta ¼ 6-(4,7-dimethoxy-2-benzothiazolyl)-N-(2,5-dimethoxy-
phenyl)-2-pyridinecarbothioamide) [41] were prepared as reported previously.
2.2. Preparation of [N’-(dipyridin-2-ylmethylene)benzohydrazide-j3Npy,Nim,O-
palladium(II)chlorido], [PdCl2(j3-Npy,Nim,O-dpkbh)] (1)
A mixture of PdCl2 (140 mg, 0.80 mmol) and CH3CN (60 mL) was stirred at room tem-
perature for 24 h and dpkbh (300 mg, 0.95 mmol) dissolved in CH3CN (25 mL) was
added. The mixture was allowed to react at room temperature for 2 h. The volume of
the reaction mixture was reduced to ꢃ20 mL under reduced pressure. The addition of
diethyl ether to the reaction mixture gave a yellow solid that was filtered off, washed
with hexanes and diethyl ether, and dried in air; yield 320 mg (84%). Anal. Calcd for
C18H14Cl2N4OPd (%): C, 45.07; H, 2.94; N, 11.68. Found: C, 45.45; H, 2.73; N, 12.36. IR
(ATR, neat/cmꢂ1): m(N-H) 3318, m(C-H) 3200–2800 (w), m(C¼O) 1676 (s), m(N-H)bending
1600, m(C¼N) 1590, m(C-N) 1150, m(N-N) 1105, m(N-H)wagging 760. 1H NMR (at 303 K,
d/ppm) in d7-dmf: 12.49 (s, 1H, NH), 9.14 (d, 1H), 9.02 (d, 1H), 8.41 (d, 1H), 8.35 (t, 1H),
8.32 (t, 1H), 8.11 (d, 1H), 7.79 (t, 1H), 7.74 (t, 1H), 7.64 (t, 1H), and 7.55 (overlapped d,
1
2H), [minor peaks due to enol form: 8.90, 8.66, 8.26, 8.20, 7.97, 7.70, 7.59 and 7.46]. H
NMR (at 303 K, d/ppm) in d6-dmso: 12.41 (s, NH), 9.01 (d, 1H), 8.80 (d, 1H), 8.23 (m,
3H), 7.88 (m, 4H), 7.69 (m, 3H), 7.54 (m, 3H), [minor peaks due to enol form: 8.97, 8.85,
8.66, 8.54, 8.16, 8.00, 7.76, and 7.45]. UV-vis (k/nm, e 500/cmꢂ1Mꢂ1) in CH2Cl2: 309
(13,440), 273 (13,800), dmso: 465 (8160), 440 (9670), 420 (7920), 328 (18,800) and 290
(22,190), and in dmf: 467 (5085), 444 (6000), 420 (4900), 325 (17,230), and
290 (19,100).
2.3. Preparation of [4-amino-N’-(dipyridin-2-ylmethylene)benzohydrazide-
j3Npy,Nim,O-palladium(II)chlorido], [PdCl2(j3-Npy,Nim,O-dpk-4-abh)] (2)
A mixture of PdCl2 (195 mg, 1.10 mmol) and CH3CN (60 mL) was stirred at room tem-
perature for 24 h and to the resulting yellow solution dpk-4-abh (473 mg, 1.41 mmol)
dissolved in CH3CN (25 mL) was added and the mixture was allowed to react at room
temperature for 2 h. When the volume of the reaction mixture was reduced to ꢃ40 mL
a brown solid formed that was filtered off, washed with hexanes and diethyl ether,
and dried in air; yield 320 mg (87%). Anal. Calcd for C18H15Cl2N5OPd (%): C, 43.70; H,
3.06; N, 14.16. Found: C, 43.16; H, 3.12; N, 14.16. IR (ATR, neat/cmꢂ1): m(N-H of NH2)
3462 and 3360, m(N-H of NH) 3210, m(C-H) 3100-2800 (w), m(C¼O) 1650 (s), m(N-
1
H)bending 1620, m(C¼N) 1600, m(C-N) 1145, m(N-N) 1105, m(N-H)wagging 760. H NMR (at
303 K, d/ppm) in d7-dmf: 12.17 (s, NH), 9.14 (d, 1H), 9.00 (d, 1H), 8.36 (m, 3H), 8.04 (m
1
plus solvent proton), 7.77 (m, 4H), 6.72 (s, 2H). H NMR (at 303 K, d/ppm) in d7-dmso:
12.00 (s, NH), 8.97 (d, 1H), 8.86 (d, 1H), 7.66 (m, 5H), 6.64 (overlapped s, 2H, NH2)
[minor peaks due to the enol form: 14.78, 8.82, 8.64, 8.50, 8.05, 7.54, 6.73 and 6.54].
UV-vis (k/nm, e 500/Mꢂ1cmꢂ1): in dmso: 490 (20,870), 366 (6900), 287 (23,700), and
in dmf: 490 (11,300), 465 (10,200), 360 (11,700) and 290 (22,470).

JOURNAL OF COORDINATION CHEMISTRY
5
Table 1. Crystal data and structure refinement for 3.
Empirical formula
Formula weight
C21H23ClN6O3Pd
549.30
Temperature
302(2) K
Wavelength
Crystal system, space group
Unit cell dimensions
0.71073 Å
Triclinic, P-1
a ¼ 7.4032(6) Å, a ¼ 81.689(2) (ꢄ)
b ¼ 12.3047(10) Å, b ¼ 85.122(2) (ꢄ)
c ¼ 12.3760(10) Å, c ¼ 83.113(2) (ꢄ)
1104.86(16) Å3
Volume
Z, Calculated density
Absorption coefficient
F(000)
2, 1.651 mg/m3
0.997 mmꢂ1
556
Crystal size
h range for data collection
Limiting indices
0.49 ꢁ 0.13 ꢁ 0.07 mm
2.78 to 28.00 (ꢄ)
–9< ¼h< ¼9, –16< ¼k< ¼16, –16< ¼1< ¼16
88,546/5336 [R(int) ¼ 0.1268]
99.9%
Reflections collected/unique
Completeness to h ¼ 28.00
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2r(I)]
R indices (all data)
Largest diff. peak and hole
CCDC#
0.9335 and 0.6407
Full-matrix least-squares on F2
5336/4/305
1.110
R1 ¼ 0.0537, wR2 ¼ 0.1008
R1 ¼ 0.0972, wR2 ¼ 0.1202
2.132 and –0.782 e.Å–3
1910124
2.4. X-ray crystallography
Crystals of [PdIICl(j3-Npy,Nim,O-dpk-4-abh-H)]ꢀH2Oꢀdmf (3) were grown from a dmf solu-
tion of 2 when left to stand at room temperature in air for several days. A single crys-
tal was selected, mounted on a glass fiber and placed in the goniometer. A Bruker
(2009) SMART APEX CCD area-detector diffractometer with Mo-Ka radiation and a
graphite monochromator was used for data collection and the Bruker (2009) SAINT
program was used for cell refinement and data reduction (see CIF file) [42].
Absorption correction was performed by the multi-scan method implemented in
SADABS [43]. The SHELXS-97 [44], SHELXTL 2014/7 [45, 46] and PLATON 240413 soft-
ware versions were used for structure analysis [47]. Table 1 shows cell parameters and
other crystallographic information. All hydrogen atoms were assigned by assuming
idealized geometries with d(C-H) ¼ 0.93, 0.96 for aromatic and aliphatic hydrogen
atoms, respectively, and d(N-H) ¼ 0.86 Å with Uiso(H) ¼ 1.2 Ueq(N) and Uiso(H) ¼ 1.5
Ueq(C). All nonhydrogen atoms were refined anisotropically. In the final refinement, a
region of electron density (2.14 e–Å3) at 0.63 Å from C35 points to the potential partial
charge on the complex due to keto-enol tautomerization during crystallization.
2.5. Density functional theory calculations
Density functional theory calculations were carried out using the Gaussian 16 software
package at the 6-311G(d, p) level [48]. The calculations employed the Becke-3
exchange hybrid function and Lee-Yang-Parr (LYP) correlation function augmented
with Coulomb Attenuating Method (CAM-B3LYP) for long-range interactions and
charge transfer. The conductor-like polarizable continuum model (C-PCM) was used for
phase calculations.

6
M. BAKIR ET AL.
2.6. Physical measurements
Electronic absorption spectra were recorded on an HP-8452A spectrophotometer and
a Perkin-Elmer k-19 UV/VIS/NIR spectrometer. Baseline corrections on blank solvents
were recorded prior to measurements. A Lauda-Brinkmann RM6 circular bath was used
1
for temperature control. Solution H and 13C NMR spectra were recorded on a Bruker
500 DRX spectrometer and were referenced to the residual proton and carbon in the
incompletely deuterated solvent. Infrared spectra of neat 1 and 2 were recorded on a
Bruker Vector 22 FT-IR spectrometer with ATR accessory. Electrochemical measure-
ments were performed with the use of a Princeton Applied Research (PAR) Model 173
potentiostat/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 on solutions that were 0.1 M
in [nBu4N]PF6. Solvents used in electrochemical measurements were deoxygenated
prior to use. The Ep,a, Ep,c, and Ep ¼ (Ep,a þ Ep,c)/2 values were referenced to a Ag wire
as a quasi-reference electrode at room temperature and are uncorrected for junction
potentials. The number of electrons on the redox waves was determined using the
þ
oxidative peak current of the reversible one-electron couple of FeCp2/FeCp2 as an
internal standard. Electrochemical cells were of conventional design based on scintilla-
tion vials or H-cells. Glassy-carbon and a Pt-wire were used as the working and coun-
ter electrodes, respectively.
2.7. Suzuki-Miyaura C-C coupling reactions
In a typical reaction, a mixture of 1 or 2 (1.9 mmol), phenyl boronic acid (74 mg,
0.61 mmol), anhydrous K2CO3 (225 mg, 1.63 mmol), aryl halide (0.38 mmol), and ethanol
was added to a pressure tube, the mixture was purged with N2 and refluxed for 6 h.
The reaction mixture was allowed to cool to room temperature and filtered to remove
the catalyst. The filtrate was passed through a silica column that was eluted with hex-
anes/ethyl acetate in a ratio of 3:1. Evaporation of the solvent from the eluted
fractions gave the C-C coupled products. The resulting C-C cross-coupling products
were identified from the results of their 1H and 13C NMR spectra (see
Supplemental material).
2.8. Electrocatalytic hydrogen evolution
Controlled-potential electrolysis (CPE) measurements for the production of H2 were
conducted under an Ar atmosphere at –0.60 V on stirred solutions (200 rpm) of 1 or 2
for 20 min in a sealed H-cell separated by a fine frit. In one chamber, a glassy carbon
plate (contact area ꢃ2 cm ꢁ 1 cm ꢁ 3 mm) working electrode and Ag wire reference
electrode were suspended in 10 mL of electrolyte containing 75 mM of 1 or 2 and
20 mM p-toluenesulfonic acid monohydrate (p-TSOH) while the auxiliary electrode (Pt-
wire) was suspended in 5 mL of the electrolyte in the other chamber. The electro-
chemically generated gas in the headspace (50 mL) was analyzed using an HP (Agilent)
5890 gas chromatograph series II equipped with an ECD detector. The column was

JOURNAL OF COORDINATION CHEMISTRY
7
Figure 1. View of the molecular structure of 3. The thermal ellipsoids are drawn at the 30% prob-
ability level.
kept at 40 ꢄC and eluted with He at 1.0 mL minꢂ1 flow rate [41]. The remainder of the
headspace was analyzed with a PGas-22 H2 gas detector.
2.9. Analytical procedures
Elemental microanalyses were performed by MEDAC Ltd., Department of Chemistry,
Brunel University, Uxbridge, United Kingdom.
3. Results and discussion
3.1. Synthesis and characterization
The reactions between [PdCl2(CH3CN)2], generated in situ from the reaction between
PdCl2 and CH3CN at room temperature in air, and CH3CN solutions of dpk-h (dpk-
h ¼ dpkbh or dpk-4-abh) produced [PdCl2(j3-Npy,Nim,O-dpkbh] (1) and [PdCl2(j3-
Npy,Nim,O-dpk-4-abh] (2) in good yield. The isolation of 1 and 2 is in contrast to the
isolation of the dinuclear [Pd2Cl3(j5-Npy,S,Npy,Npy,Nam-dpktsc-H)]ꢀsolvate complex
(solvate ¼ CH3CN or H2O) reported when dpktsc was used in place of dpk-h [37].
When 2 was allowed to stand in dmf at room temperature for several days, crystals of
[PdIICl(j3-Npy,Nim,O-dpk-4-abh-H)]ꢀH2Oꢀdmf (3) were isolated. The isolation of 3 hints
to keto enol tautomerization due to solvent-compound interaction. The authenticities
of 1 and 2 were established from the results of their elemental analysis and a number
of spectroscopic measurements (see Experimental section). X-ray crystallographic ana-
lysis confirmed the identity of 3.
A view of the molecular structure of 3 is shown in Figure 1 and selected bond dis-
tances and angles are given in Supplementary material Table S2. The coordination
about the Pd ion is pseudo square planar. Deviation from square planar geometry is
due to the constraints associated with the j3-Npy,Nim,O- coordination of dpk-4-abh.
This is evident from the Npy-Pd-Nim and Nim-Pd-O1 bite angles of 80.93(15) and 80.
58(13)ꢄ, respectively, and Cl1-Pd-Nim angle of 178.38(10)ꢄ. The C-O bond distance of 1.
297(5) Å is midway between C¼O and C-O bond distances of uncoordinated dpk-4-
abh and other related compounds [32–35, 40, 45]. Bond distances of 1.226(18) Å and

8
M. BAKIR ET AL.
Figure 2. Selected views of the hydrogen bonds in the extended structure of 3.
1.225(2) Å were reported for C¼O bond distances in uncoordinated dpk-4-abh and j3-
Npy,Nim,O-coordinated dpkaah of MCl2(j3-Npy,Nim,O-dpkaah) (dpkaah ¼ di-2-pyridyl
ketone acetic acid hydrazone, M ¼ Zn, Cd or Hg), respectively [35, 38]. A bond distance
of 1.413(6) Å was reported for the C-O single bond in fac-Re(CO)3(j3-N,O,N-dpkO,OH)
[49]. The extended structure of 3 (Supplementary material Figure S1) shows stacks of
antiparallel chains of 3 connected via a web of noncovalent interactions (Figure 2 and
Supplementary material Table S2). For example, hydrogen bond parameters of 0.
857(10), 2.194(15), 2.194(15), and 2.194(15) were observed for the d(D-H), d(H … A),
d(D … A), and <(DHA) of N(5)-H(5B) … O(2). The bond distances and angles of the
noncovalent bonds are normal and similar to those reported for a variety of com-
pounds containing such bonds [32–35, 40, 49].
The infrared spectra of neat 1 and 2 show peaks in the m(NH), m(C¼O) (see
Experimental section and Supplementary material Figure S2) consistent with the weak
coordination of a carbonyl group of dpk-h to the Pd-ion. This is evident from the
appearance of m(NH) and m(C¼O) vibrations of the metal complexes at close proximity
( 5 cmꢂ1) to those of m(NH) and m(C¼O) of uncoordinated dpk-h [34–36, 40]. Changes
appeared in the combined m(C¼C) and m(C¼N) vibrations pointing to the imine and
1
pyridyl nitrogen coordination of dpk-h. The H NMR spectra of 1 and 2 measured in
protophilic solvents (d6-dmso or d7-dmf) are solvent and temperature dependent (see

JOURNAL OF COORDINATION CHEMISTRY
9
Figure 3. 1H NMR spectra of 1 measured in d7-dmf and d6-dmso at 303 K.
Experimental section). Figure 3 shows the 1H NMR spectra of 1 in d7-dmf and
d6-dmso; the spectra of 2 in the same solvents are shown in Supporting material
Figure S3. These spectra disclose the sensitivity of 1 and 2 to solvent variations and
hints to keto-enol tautomerization. This is evident from the appearance of minor peaks
in close proximity to the major peaks and is similar to those reported previously for
the keto-enol tautomerization of di-2-pyridyl ketone acetic acid hydrazone (dpkaah)
1
and its group 12 metal complexes [40]. Variable temperature H NMR measurements
disclosed the temperature dependence (Figure 4) and allowed for calculation of the
thermodynamic parameters for the amidic proton exchange and the amide-imidol tau-
tomerization of 1 and 2 in d7-dmf and d6-dmso (Table 2). The results show that the
keto form of coordinated dpk-h is more favorable than the enol form. This is consist-
ent with solid state infrared data that show weak coordination of the carbonyl group
of coordinated dpk-4-abh to the Pd ion.
The electronic absorption spectra of 1 and 2 are solvent dependent pointing to
solvent-compound interaction. This is similar to those reported previously for a variety
of di-2-pyridyl ketone hydrazones and their metal compounds and further confirms
the solvent dependence observed in the 1H NMR measurements [32–36]. Figure 5

10
M. BAKIR ET AL.
1
Figure 4. Variable temperature H NMR spectra of 1 measured in d6-dmso.
shows the electronic absorption spectra of 1 in dmso and dmf along with the spec-
trum of 1 in dmso in the presence of NaBH4 and the spectrum of uncoordinated
dpkbh in dmso. The ratios of the low energy electronic transitions centered at 445 nm
to the high-energy electronic transition at 325 nm of 0.96:1.87 and 0.60:1.70 in dmso
and dmf, respectively, confirmed the solvent-compound interaction. The low-energy
electronic transition of 1 is red shifted compared to the low-energy electronic transi-
tion of the amide deprotonated conjugate base of dpkbh pointing to the increase in
electron density of 1 compared to amide deprotonated dpkbh upon coordination to
the Pd(II)Cl moiety [33]. The broad character of the low-energy electronic transitions of
1 observed between 550 and 390 nm hints to charge transfer and is assigned to ligand
to metal charge transfer (LMCT) mixed with metal-ligand charge transfer (MLCT) and
intra-ligand charge transfer (ILCT) along with d-d electronic transitions. The d-d contri-
bution to the low-energy electronic transition is evident from the splitting of the low-
energy peak centered at 445 nm with shoulders at 470 and 415 nm and is in accord
1
1
1
1
1
1
with the spin allowed A1g! Eg, A1g! B1g and A1g! A2g electronic transitions of a
square planar d8-electronic configuration. This is similar to those previously reported
for other square planar Pd(II)-hydrazone complexes [50, 51]. The intra-ligand charge

JOURNAL OF COORDINATION CHEMISTRY
11
Table 2. Thermodynamic data for the amidic proton exchange and the amide-imidol tautomeriza-
tion of 1 and 2 in dmf-d7 and dmso-d6.
Species
Solvent
DH/kJ molꢂ1
DS/J K–1mol–1
DG298K/kJ molꢂ1
NH () ND
PdCl2(dpk4abh)
dmf
dmso
37
23
2
2
97
62
5
6
8.6 0.6
4.6 0.6
Amide () imidol
PdCl2(dpkbh)
dmf
dmso
dmf
dmso
26
21
18
2
1
2
76
54
50
12
6
2
6
2
3.6 0.4
4.4 0.2
3.4 0.5
3.5 0.7
PdCl2(dpk4abh)
7.2 0.7
Figure 5. Electronic absorption spectra for 1 measured in dmf (4.42 x 10-5 M) and dmso (3.56 x
10-5 M) along with the spectrum of 1 in dmso (3.56 x 10-5 M) in the presence of NaBH4 (1.00 x 10-
4
M) and the spectrum of uncoordinated dpkbh in dmso (3.00 x 10-5 M).
transfer contribution to the low-energy electronic transition of 1 is evident from the
overlap of the low-energy electronic transition of the amide deprotonated form of the
free ligand with the low-energy electronic transition of 1 in the presence and absence
of NaBH4. The low-energy electronic transition of the free ligand is due to intra-ligand
charge transfer of the amide deprotonated conjugate base of dpkbh [33]. The appear-
ance of the highly intense charge transfer electronic transition when a dmso solution

12
M. BAKIR ET AL.
Scheme 2. Suzuki-Miyaura C–C cross-coupling of aryl bromides and phenyl boronic acid.
of NaBH4 was added to a dmso solution of 1 confirmed the ILCT contribution and
hints to the hydrogenation of the Pd(II)Clmoiety and release of the dpkbh-H anion.
The high-energy electronic transitions observed at 325 and 290 nm are intra-ligand
ꢅ
charge transfer (ILCT) due to p!p of the pyridyl ring of dpk followed by dpk to ben-
ꢅ
ꢅ
zoyl charge transfer mixed with p!p and n!p of the hydrazone moiety. The ILCT
of uncoordinated dpkbh observed at 325 nm is unaffected by coordination to the
Pd(II)Cl moiety while changes appeared on the electronic transitions observed at
290 nm. This hints that the electronic transitions at 290 nm is due to the hydrazone
ꢅ
ꢅ
(C¼N) p!p and n!p consistent with the coordination the imine (C¼N) nitrogen to
the metal moiety. The LMCT, MLCT and ILCT contribution to the low-energy electronic
transition of 1 were confirmed from density functional theory calculations (DFT).
Supplementary material Figure S4 shows the HOMO-LUMO orbitals of 1 showing the
contribution of the LMCT, MLCT and ILCT to electronic transition of 1. In the case of 2,
the electronic transitions are more sensitive to solvent variations than 1.
Supplementary material Figure S5 shows the electronic transition of 2 in dmso and
dmf along with the spectrum of uncoordinated dpk-4-abh in dmso. This is consistent
with DFT calculations (Supplementary material Figure S4) that show the electron dens-
ity on the highest occupied molecular orbital HOMO is centered on the 4-aminophenyl
ring with significant portion of electron density is localized of the amine nonbonding
electrons and the electronic transition HOMO ! LUMO is composed of LMCT and ILCT.
In contrast to 1, where the electron density on the chlorine atom makes significant
contribution to its electronic transitions, the contribution of the electron density on
the chlorine atom to the electronic transitions of 2 is insignificant. The low energy
electronic transition of 2 is red shifted compared to 1 and the uncoordinated dpk-4-
abh in accord with the increase in electron density of 2 compared to 1.
3.2. C–C cross-coupling catalysis
The catalytic behavior of 1 and 2 along with [Pd2Cl2(j5-Npy,S,Nim,Npy,Nam-dpktsc-H)]
toward the Suzuki-Miyaura cross-coupling reaction of 4-bromoacetophenone with
phenyl boronic acid (Scheme 2) was investigated under similar conditions as
reported for [PdCl(pdcta]ꢀnHCl (n ¼ 0 or 1) [52]. The results of these studies
(Table 3) revealed that 1 and 2 outperformed [Pd2Cl3((j5-Npy,S,Nim,Npy,Nam-dpktsc-
H)] and [PdCl(pdca)]ꢀnHCl.
The application of the mononuclear pre-catalysts in the coupling of the less acti-
vated 1-bromonaphthalene and phenyl boronic acid gave similar isolated yields as
those of 4-bromoacetophenone. These results confirmed that 1 and 2 gave better

JOURNAL OF COORDINATION CHEMISTRY
13
Table 3. Evaluation of the palladium(II) complexes as precatalysts for the Suzuki–Miyaura cross-
coupling reaction.
Entry
Precatalyst
% Yield (1)a
% Yield (2)a
1
2
3
4
5
PdCl2(dpk4abh)
PdCl2(dpkbh)
[PdCl(pdcta)]
87
88
78
61
15
88
77
65
66
–
[PdCl(pbcta)]ꢀHCl
Pd2Cl3(dpktsc)
aIsolated yields, an average of two runs.
pdcta ¼ Bis-N-(2,5-dimethoxyphenyl)pyridine-2,6-dicarbothioamide and pbcta ¼ 6-(4,7-dimethoxy-2-benzothiazolyl)-
N-(2,5-dimethoxyphenyl)-2-pyridinecarbothioamide.
Figure 6. Cyclic voltammograms of 1 (a and b) and 2 (c and d) measured in dmf at a glassy car-
bon working electrode at scan rate of 200 mV s–1.
yields than the Pd(II)-pincer type systems and the isolated yield utilizing 2 was bet-
ter than 1. The Suzuki-Miyaura cross-coupled reaction did not occur when 4-chlor-
oacetophenone was used in place of the bromo-aryl substrates. This is consistent
with those reported previously for the cross-coupling reactions of aryl chlorides
with a variety of aryl and alkyl boronic acids. This may be due to the stronger C-Cl

14
M. BAKIR ET AL.
bond compared to C-Br bond and ease of alkyl chlorides reduction compared to
alkyl bromides [53, 54].
3.3. Electrochemical properties
The electrochemical properties of 1 and 2 measured in dmf in the absence and pres-
ence of p-TSOH were investigated using voltammetric techniques. Cyclic voltammo-
grams of 1 and 2 (Figure 6) show close similarities and subtle differences. On a
reductively initiated scan on 1 [Figure 6(a)] a pre-wave appeared at –0.25 V, followed
by quasi-reversible reductions at E1/2 ¼ –0.55 V (Ep,c ¼ –0.62 and Ep,a ¼ –0.48 V), –1.18
V (Ep,c ¼ –1.26 V and Ep,a ¼ –1.10 V). Further, a broad reductively generated product
wave appeared at Ep,a ¼ þ0.29 V and an irreversible oxidation appeared at Ep,a
¼
þ1.15 V. The reductively generated product wave observed at Ep,a ¼ þ0.29 V disap-
peared and the oxidative current (Ip,a) on the wave observed at Ep,a ¼ þ1.15 V
decreased on oxidatively initiated scan [Figure 6(b)]. The first reduction wave at E1/2
¼
–0.55 V is a two electron PdII!Pd0 reduction and the reduction wave at Ep,c ¼ –1.18
V is a one electron ligand based reduction. The oxidative wave at Ep,a ¼ þ1.15 V is
due to 2Cl-/Cl2 oxidation [31]. Cyclic voltammograms of dpkbh measured in dmf under
the same conditions in the absence and presence of PdCl2(CH3CN)2 (Supporting mater-
ial Figure S5) confirmed the assigned potentials and show that the ligand based
reduction of coordinated dpkbh shifts 0.20 V more positive compared to the uncoor-
dinated dpkbh. The PdII!Pd0 and ligand-based reductions of 2 [Figure 6(c,d)]
appeared at similar potentials as those of 1. The oxidation waves observed more posi-
tive than þ1.25 V in the voltammograms of 2 and the product wave at Ep,c ¼ -0.28 V
are due to the oxidation of the amine group.
The electrochemical behavior of 1 and 2 in the presence of stoichiometric amounts
of p-TSOH were investigated. Figure 7 shows cyclic voltammograms of 2 in the
absence and presence of stoichiometric amounts of 4mM p-TSOH on reductively initi-
ated scans and the voltammograms of 1 measured under similar conditions are shown
in Supporting material Figure S6. In these voltammograms, changes appeared pointing
to structural as well as Hþ reduction. The structural changes are due to the proton-
ation of the uncoordinated pyridyl ring and the amine group of 2 as well as the
coordination of p-TSO- to the four-coordinate complex. This is evident from the
appearance of new reduction and oxidation waves at Ep,c ¼ –0.19 V, Ep,a ¼ þ0.04 V
and Ep,c ¼ þ0.55 and þ0.40 V in the presence of p-TSOH compared to the absence of
p-TSOH. The current enhancement at the metal based reduction and the diminishment
of the ligand based reduction wave of 2 hint to metal based electrocatalytic reduction
of Hþ. The reduction wave at Ep,c ¼ –0.19 V is assigned to the protonated complex
and the oxidation wave at Ep,a ¼ þ0.04 V is due to the oxidation of the electrochem-
ically generated H2. As the concentration of p-TSOH increased, the current on the
reduction wave at –0.19 V increased and the wave shifts more negative. The oxidative
currents (Ip,a) of the oxidations observed between Ep,a ¼ –0.19 to þ0.5 V, and þ0.65
to 1.40 V, and the reductive current (Ip,c) on the wave observed between Ep,c ¼ þ0.80
and þ0.1 V increased as the concentration of p-TSOH increased (Figure 7). The overall
1
rate constants (kapp
)
for H2 evolution were estimated from the height of the catalytic

JOURNAL OF COORDINATION CHEMISTRY
15
Figure 7. Cyclic voltammograms of 2 (2.2 mM) measured in dmf solution containing 0.1 M
[nBu4N]PF6 measured in the absence and presence of p-TSOH: (a) 0 mM, (b) 2.7, (c) 8.3, (d) 16 (e)
22, (f) 30, and (g) 40 mM at a glassy carbon working electrode at scan rate of 200 mV s–1.
plateau current and gave values of (3.4 0.3) ꢁ 103 and (4.0 0.2) ꢁ 103 Mꢂ1 sꢂ1 for
1 and 2, respectively [55, 56]. Controlled potential electrolysis (CPE) in the presence
and absence of p-TSOH confirmed the electro-catalytic properties of 1 and 2
(Supporting material Figure S11). In a 20 min CPE at –0.60 V on the catalyst free solu-
tion and 1 and 2 charges of 7.6, 13.3, and 12.6 C were consumed. Gas chromato-
graphic analysis and PGas-22 H2 gas detector confirmed the identity the H2 gas
produced. Gas volumes of 405, 697, and 792 mL for the catalyst free solution, 1 and 2
were obtained. These results correspond to 41 and 61% Faradaic efficiency with asso-
ciated overpotentials of 157 and 67 mV and turnover numbers (TON) of 2.0 and 3.2,
respectively [57, 58]. On completion of the CPE experiment, Pd-black was observed on
the electrode surface, signifying the eventual decomposition of the complexes in the
presence of the proton source. This is similar to those reported for the electrolysis of
other Pd(II)-complexes [41]. The area under the I-t curve of a CPE experiment con-
ducted in the absence of 1 or 2 was significantly less than that in the presence of the
catalyst. The eventual decomposition of the complexes hints to the long-term stability
of the catalytic system despite the relative stability of the Pd-H transient observed in
the voltammograms of the electrocatalytic experiments.

16
M. BAKIR ET AL.
4. Conclusion
In contrast to the dinuclear complex [Pd2Cl3(j5-Npy,Nim,S,Npy,Nam-dpktsc-H] reported
from the facile reaction between dpktsc and [PdCl2(CH3CN)2] at room temperature
in air, mononuclear complexes 1 and 2 were isolated when dpktsc was replaced
with dpk-h. Solid-state infrared measurements disclosed pseudo coordination of the
carbonyl group of dpk-h to the Pd ion and solution (1H NMR and electronic absorp-
tion) measurements in protophilic solutions disclosed keto-enol tautomerization.
Variable temperature 1H NMR measurements disclosed the predominance of the
keto form in protophilic solutions. Crystals of 3 were isolated when 2 was allowed
to stand in dmf for several days. The isolation of 3 is consistent with the keto-enol
tautomerization of 2 in protophilic solutions. The catalytic behavior of 1 and 2
toward C-C cross-coupling and proton reduction were investigated and revealed
good catalytic activity.
Note
1. The reaction rate constant, kapp (M–1 s–1), can be estimated from
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ic ¼ nFA½catꢆ Dkapp½Hþꢆ (1)
where n is the number of electrons, ic is the plateau current (mA) at very fast scan rates, F
is the Faraday’s constant (96,485 C mol–1), A is the area of the electrode surface (cm2),
[cat] is the bulk concentration of the catalyst (M), D is the diffusion coefficient (cm2 s–1)
and [Hþ] is the bulk concentration of acid (M).
Acknowledgement
The authors acknowledge Ms. Toni Johnson for NMR measurements.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This work was supported by the University of the West Indies for financial support.
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