Synthesis and solid state characterisation of mononuclear 2-benzoylpyridine N-methyl-N-phenylhydrazone palladium(II) complexes

Article abstract of DOI:10.1039/b316843c

2-Benzoylpyridine N-methyl-N-phenylhydrazone, HL, is a versatile ligand which reacts with [Pd(PhCN)₂Cl₂] forming the coordination compound [HLPdCl₂, 1, characterized by the presence of the N(py)∧N (im) chelate ring. When H

Full text of DOI:10.1039/b316843c

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Synthesis and solid state characterisation of mononuclear
2-benzoylpyridine N-methyl-N-phenylhydrazone palladium(II)
complexes
Mauro Ghedini,* Iolinda Aiello, Alessandra Crispini and Massimo La Deda
LASCAMM, Unità INSTM della Calabria, Dipartimento di Chimica,
Università degli Studi della Calabria, I-87036 Arcavacata di Rende (CS), Italy.
E-mail: m.ghedini@unical.it; Fax: ϩ39(0)984 492044; Tel: ϩ39(0)984 492062
Received 23rd December 2003, Accepted 11th March 2004
First published as an Advance Article on the web 26th March 2004
2-Benzoylpyridine N-methyl-N-phenylhydrazone, HL, is a versatile ligand which reacts with [Pd(PhCN)2Cl2]
forming the coordination compound [HLPdCl2], 1, characterized by the presence of the N(py)^N(im) chelate ring.
When HL reacts with [Pd3(OAc)6] this gives rise to the orthometallated complex [LPd(OAc)], 2. In this case the
Pd() environment consists of a N(py)^N(im) ring fused to the N(im)^C palladacycle and a monodentate acetate
anion. Complex 2 undergoes methatetical reactions with alkaline halides and complexes of general formula [LPdX]
(3: X = Cl; 4: X = Br; 5: X = I) are obtained. The molecular structures of 3–5 determined by single-crystal X-ray
analysis proved the formation in all cases of mononuclear Pd() complexes containing a N(py)^N(im)^C terdentate
ligand. As solid samples only compounds 3–5 exhibited luminescence at room temperature (λmax ≈ 610 nm). This
property, quite unusual in Pd() complexes, is discussed in terms of π–π interactions, which are mainly responsible
for the existence in the crystalline solid state of dimeric units.
metal complexation, they can act as both bi- and terdentate
Introduction
ligands.13,14 In one of our previous works we investigated
the reaction between a Pd() source and the ligand phenyl-2-
pyridylketone-2,4-dinitrophenylhydrazone (I in Chart 1) and
we proved that the presence of a N–H bond prevented the
formation of a five-membered N^C metallacycle. In particular,
N–H metallation rather than C–H metallation occurred and a
six-membered N(py)^N(am) ring formed.15
The great interest in materials potentially useful for new
technologies is presently orientated towards investigations in
organometallic and coordination compounds. In this regard,
cyclometallation has been widely studied1,2 and is still
generating much interest. Cyclopalladated species have been
designed for various applications in materials science ranging
from liquid crystalline materials3–5 to potential candidates for
optical and electro-optical devices.5–9 N-Donor substrates such
as azobenzenes, imines, phenylpyridine and phenylpyrimidine
have been extensively used in cyclopalladation reactions and
have shown a strong tendency to form cyclopalladated five-
membered rings containing square planar Pd(). The partial
electron delocalisation within the ring, which implies a certain
degree of “aromaticity”,10 could influence intermolecular inter-
actions important for the physical properties that characterize
this class of molecular materials. Another area of research is
the synthesis of N^N^C terdentate cyclometallated species
where two coordinating groups hold a C–H bond next to the
metal ion. In this context some interesting studies of the
luminescent properties of complexes obtained using 6-phenyl-
2,2Ј-bipyridine ligands, have been recently reported.11,12
In the present study we extended our research to the Pd()
complexes obtained from 2-benzoylpyridine N-methyl-N-
phenylhydrazone (HL in Chart 1). In this ligand, the N–H
group is replaced with a N–CH3 group so that the nitrogen
atom deprotonation is prevented and upon reaction with Pd(),
HL was found to act as both a N(py)^N(im) bidentate and
N(py)^N(im)^C terdentate ligand. The structure of the result-
ing products was determined by the nature of the Pd()
precursor. In fact, the reaction of HL with [Pd(PhCN)2Cl2] gave
the N(py)^N(im) [HLPdCl2] complex, 1, while a N(py)^N(im)^C
derivative, [LPd(OAc)], 2, was obtained on reaction with
[Pd3(OAc)6]. The synthesis, characterization and photophysical
properties of 1 and of a series of mononuclear complexes
of general formula [LPdX] (2: X = OAc; 3: X = Cl; 4: X = Br; 5:
X = I) are reported here. The formation of the N(py)^N(im)^C
terdentate species was confirmed by the single-crystal X-ray
structural analysis performed on 3, 4 and 5. The new
synthesized complexes were characterized for photophysical
properties. Interestingly, microcrystalline samples of 3–5
showed room-temperature luminescence. Since radiationless
decay of Pd() complexes excited states are usually expected
because of the presence of thermally accessible low-lying metal
centered states (MC),16 the emission observed for 3–5 is
discussed on the basis of intermolecular interactions observed
in the crystal packing, which could modify the energy of the
MC states.
As part of our interest in cyclopallated complexes and their
application in material sciences, we decided to investigate the
coordination capabilities of phenylhydrazones derived from
2-benzoylpyridine (Chart 1). Phenylhydrazone ligands are very
versatile in the number of possible metal binding sites they have
and, depending on the portion of the ligand involved in the
Results and discussion
Compounds and reactions are shown in Scheme 1. 2-Benzoyl-
pyridine N-methyl-N- phenylhydrazone HL, a red–orange
solid, was synthesized by direct coupling of N-methyl-N-phe-
nylhydrazine and 2-benzoylpyridine. The structure of HL was
inferred from satisfactory elemental analyses, IR and 1H NMR
Chart 1
D a l t o n T r a n s . , 2 0 0 4 , 1 3 8 6 – 1 3 9 2
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
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Table 1 Dihedral angles (Њ) between the best mean planes of the
phenyl and pyridine rings in HL
C(1)/C(6) and N(3)/C(12) rings
C(1)/C(6) and C(13)/C(18) rings
N(3)/C(12) and C(13)/C(18) rings
72.69(6)
82.49(7)
63.93(6)
trum displayed signals which accounted for all the expected
protons with a down-field shift of the N–CH3 resonance (in
DMSO solutions, 3.27 ppm for 1 and 2.89 ppm for HL)
indicating N–Pd coordination. These data proved that no
cyclopalladation occurred and therefore in complex 1 HL is
N(py)^N(im) chelated to the PdCl2 fragment. The synthesis of a
different species 2, was accomplished (80% yield) by reaction of
HL with [Pd3(OAc)6] (Scheme 1). The results of the elemental
analysis of 2 were correct for the [LPd(OAc)] stoichiometry and
indicated the formation of an orthopalladated derivative. The
IR spectrum of 2, compared to HL, exhibited C᎐N stretching
᎐
decreased at 1591 cmϪ1 which indicated that metal complex-
ation induced a reduction of the C᎐N double bond character
᎐
more significant than the reduction observed for 1. In addition,
the two strong bands recorded at 1627 and 1368 cmϪ1, assigned
to νasym(CO2) and νsym(CO2) stretching vibrations, are represent-
ative of an acetato ligand coordinated in a monodentate
fashion.13 The success of the cyclopalladation reaction was
1
confirmed by H NMR spectroscopy (Experimental section)
which showed both the disappearance of signals attributable to
the H14 proton (Scheme 1) and the down-field shift of the
doublet concerning the H13 proton (from 6.97 ppm in 1 to 7.46
ppm in 2) consistent with the metallation of the adjacent carb-
on atom. Moreover, the methyl group of the acetato ligand gave
rise to a singlet at 2.23 ppm, while the singlet observed at 2.80
ppm was assigned to the N–CH3 group of the phenylhydrazone
ligand. The latter signal was found to be high-field shifted with
respect to the corresponding resonance in HL (2.95 ppm in
CDCl3). This shift can probably be ascribed to the deshielding
effect that the C(7)-bonded phenyl ring exerts on the N–CH3
Scheme 1
spectra (Experimental section). Moreover, the crystal structure
analysis performed by single-crystal X-ray diffraction study,
has confirmed the success of the synthesis and revealed as a
primary structural characteristic a severe deviation from
planarity of the overall geometry of the molecule (Fig. 1).
Despite the co-planarity of the methyl group and the phenyl
ring bonded to N(1) (dihedral angle C(2)–C(1)–N(1)–C(19) of
Ϫ5.2(3)Њ), the pyridine and the C(7)-bonded phenyl rings are
nearly orthogonal with respect to both the C(1)/C(6) ring and
to each other (Table 1). The bond distances and angles are
found to be in good agreement with those reported for similar
hydrazone ligands.17 An interesting feature of HL is its
versatility in the Pd() coordination mode. Through reaction
with different Pd() salts, different metal complexes are
obtained which differ from each other in stoichiometry and
coordination environment. In fact, when HL reacted with
[Pd(PhCN)2Cl2] (Scheme 1), a red–brown solid 1, the elemental
analysis of which was in agreement with the [(HL)PdCl2]
protons, closely blocked by the [PdCCNN] ring formation
(Fig. 2). The acetato ligand in complex 2 was replaced easily
with different halogenide ligands and new cyclopalladated
species were prepared. The treatment of 2 with the appropriate
lithium or sodium halide, in a typical methatetical reaction,
gave the halide complexes [LPdX] 3–5, (3, X = Cl; 4, X = Br; 5,
X = I). These new phenylhydrazone complexes were isolated as
air- and thermally-stable red microcrystalline solids (yields
ranging from 83%, X = I, to 89%, X = Br). In all cases, the IR
spectra exhibited a sharp intense band at about 1590 cmϪ1
(C᎐N stretching). Regarding the 1H NMR signals, a particular
᎐
attention needs to be focused on the H13 proton (Scheme 1),
since the resonance frequency is very sensitive to the nature of
the halide ligand. In fact, the H13 signal, found at 7.46 ppm for
1
stoichiometry, was isolated in 78% yield. The H NMR spec-
Fig. 1 Molecular structure of HL showing the atom-labelling scheme
and displacement ellipsoids at the 50% probability level.
Fig. 2 Molecular structure of 3 showing the atom-labelling scheme
and displacement ellipsoids at the 50% probability level.
D a l t o n T r a n s . , 2 0 0 4 , 1 3 8 6 – 1 3 9 2
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Table 2 Absorption and emission data for Pd() complexes 1–5
CH3CN solution
Solida
Compound
Abs: λmax/nm (ε/MϪ1cmϪ1
)
Abs: λmax/nm
Em: λmax/nm
b
b
1
2
3
4
5
285 (10400), 330 (6300), 420 (3000)
290 (21000), 346 (sh),c 470 (4000)
290 (23700), 350 (sh), 470 (4300)
290 (21500), 350 (sh), 470 (4000)
290 (22300), 350 (sh), 470 (4300)
290, 337, 440
290, 375 (sh), 490
293, 360 (sh), 530
293, 360 (sh), 530
295, 372 (sh), 530
610
613
618
a KBr pellet. b Emission not detectable. c sh = Shoulder.
3, is deshielded by 0.24 and 0.64 ppm for 4 and 5, respectively,
and moves down-field along the series 3–5 as the electron-with-
drawing power of the X ligand decreases.
Spectroscopic characterization of the compounds was per-
formed in solution, and no noticeable variations were observed
in their spectra after 3 days.
The absorption spectrum of HL, dissolved in acetonitrile,
showed two intense bands at 250 nm (ε = 19600 MϪ1 cmϪ1) and
360 nm (ε = 11000 MϪ1 cmϪ1) which, in dichloromethane
solution, are shifted at 255 and 368 nm, respectively. In agree-
ment with previous assignments,18–20 the high-energy band is
ascribed to a π–π* transition mainly located on the phenyl
rings, while the low-energy one, with a shoulder at 296 nm, is
attributed to an intraligand charge-transfer (ILCT) transition,
with a π–π* character, from the imino (donor) moiety to the
spectra of 2–5 can reasonably be associated only to ILCT tran-
sitions. The red-shift of ca. 5500 cmϪ1 that these ILCT bands
show with respect to the ILCT band of HL can be explained by
the strong perturbation induced by cyclometallation which
force the pyridine ring, the imino moiety and the phenyl ring to
lie on the same plane. This geometry allows extension of the
conjugation to the whole ligand moiety, lowering the energy of
these π–π* type transitions. The red-shift of the ILCT band in 1
(ca. 3500 cmϪ1) is lower than in 2–5 and is in accord with the
absence of the cyclometallated ring which results in a reduced
conjugation of the ligand moiety.
All complexes showed a very weak emission around 610 nm
in acetonitrile solution; unfortunately, the emission output was
close to the detection limit of the available equipment and
for 3–5 only an estimation of the emission quantum yield
(about 10Ϫ5) was possible. Luminescence intensity of palladium
complexes in fluid solution at room temperature is very low,
usually because of the presence of low-lying MC excited
states24,25 which deactivate the potentially emitting metal-
to-ligand charge transfer and ligand-centred levels through
thermally activated processes.15 Comparison with reported
data26 proved that the luminescent band exhibited by 1–5 may
be due to the emission from ILCT excited states the CT
character of which is confirmed by the relatively high Stokes
shift (4900 cmϪ1 in acetonitrile solution).
pyridyl and phenyl (acceptor) rings conjugated with the C᎐N
᎐
group. This assignment is supported by the relatively high
extinction coefficient and the red-shift observed with the
decreasing of solvent polarity is indicative of a dipole moment
larger in the ground state than in the excited one.21
The absorption spectra of the Pd() complexes were recorded
in acetonitrile solution (Table 2), and some examples are
reported in Fig. 3.
Crystals of 3 suitable for X-ray analysis were obtained from
slow evaporation of an ethanol solution. A view of the
structure of 3 is shown in Fig. 2. Table 3 lists relevant bond
lengths and angles. X-Ray structural analysis showed the
monuclear nature of the investigated complexes, with the
phenylhydrazone ligand bonded to the metal center through the
N(py)^N(im)^C atoms. The Pd center revealed a distorted
square-planar geometry with the bite angle N(3)–Pd–C(2) of
161.9(2)Њ deviating from linearity. This type of distortion is
typical for Pd() and Pt() complexes of cyclometallating
6-phenyl-2,2Ј-bipyridine ligands and related terdentate ligands
that form two fused five-membered chelate rings.11,27,28 The
molecule is nearly flat, except for the rotationally free phenyl
ring C(13)/C(18) (tilt angle of 62.0(6)Њ) and the methyl group, as
shown by the dihedral angle C(19)–N(1)–N(2)–C(7) of 23.6(7)Њ.
The bond distances and angles within the two chelate rings and
Fig. 3 Absorption spectra of 1 (solid line) and 3 (dashed line) in
acetonitrile solution.
[PdNCCN] and [PdCCNN] are in good agreement with those
reported for Pd() complexes containing bi- and terdentate
phenylhydrazone ligands.12,29,30 Moreover, the presence of a
short Pd–N bond length to the central nitrogen atom [1.981(4)
Å] and a larger one to the lateral pyridine ring [2.120(4) Å]
resulting from the strong trans influence of the phenyl carbon
atom C(2), has already been observed in the case of a similar
terdentate systems.31 The crystal structure of 3 showed inver-
sion-related dimers with a Pd ؒ ؒ ؒ Pd intradimer distance of
3.623(2) Å (Fig. 4(a)). This value, significantly larger than the
sum of their van der Waals radii,32 together with a strong
deviation from orthogonality of the Pd ؒ ؒ ؒ Pd vector and the
coordination mean plane (19Њ angle with the plane normal),
suggested that the metal–metal interaction does not signifi-
cantly contribute to the intermolecular interactions in the
crystal packing arrangement of 3. The presence of an extended
Electronic spectra of 1–5 show three maxima of absorption,
at 285, 330 and 420 nm for 1 and red-shifted to 290, 350 and 470
nm for 2–5, respectively.
In order to discuss the nature of the lowest energy bands, it
is useful to comment first on the transitions concerning the
cyclometallated complexes 2–5. The profile and the energy of
these bands are both independent of the σ-donor ability of the
ancillary ligand;22 thus charge transfer (CT) transitions involv-
ing the metal, the energies of which are expected to depend on
the actual Pd() electron density, could not be responsible for
these transitions. A metal centred (MC) character can also be
excluded, since MC transitions, which are expected to be
sensitive to the nature of the ancillary ligand, usually, as pre-
viously reported for other Pd() complexes,23 fall at higher
energy. Therefore the lowest energy bands of the absorption
D a l t o n T r a n s . , 2 0 0 4 , 1 3 8 6 – 1 3 9 2
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Fig. 4 (a) Diagram showing the slipped stacking, Pd ؒ ؒ ؒ Pd shortest distance and C–H ؒ ؒ ؒ Cl–Pd hydrogen bond [Cl ؒ ؒ ؒ H18A(Ϫx, Ϫy, Ϫz) 2.84
Å, Cl ؒ ؒ ؒ C18A 3.760(3) Å, C18A–H18A ؒ ؒ ؒ Cl 167Њ] within the dimer of 3. (b) Crystal packing of 3 viewed along the a axis showing the Pd–
Cl ؒ ؒ ؒ H contacts between dimers [Cl ؒ ؒ ؒ H14A(Ϫx, y Ϫ 0.5, 0.5 Ϫ z) 2.87 Å, Cl ؒ ؒ ؒ C14A 3.558(4) Å, C14A–H14A ؒ ؒ ؒ Cl 132Њ].
Table 3 Selected bond lengths (Å) and angles (Њ) for 3, 4ؒCHCl3 and
5ؒCHCl3
the association of two inversion related molecules with the
formation of dimers characterized the crystal packing of both
complexes 4 and 5. The Pd ؒ ؒ ؒ Pd distances within the dimers
of 4 and 5 became slightly longer than the value found in 3
[3.951(1) and 3.825(2) Å, respectively] and the π–π stacking
between the cyclometallated phenyl ring and the centro-
symmetric coordinated pyridine ring, characterized by
interplanar distances of 3.51 Å for 4 and 3.56 Å for 5, was
found to be significantly less displaced than in 3, with displace-
ment angles and offsets 16Њ and 0.99 Å in 4, and 17Њ and 1.05 Å
in 5. Furthermore, the C–H ؒ ؒ ؒ X–Pd non-classical hydrogen
bonds between centrosymmetric molecules, are still present
in the case of 4 and 5 with X ؒ ؒ ؒ H distances indicative for
these type of intermolecular interactions (Fig. 5).36–39 In the
3-D space, complex 4 gave rise to a quasi-bilayered packing
arrangement in which two symmetrical rows of planar cores
of molecules are spaced by about 8 Å through a row of rota-
tionally free phenyl rings π–π stacked between each other
(interplanar distance of 3.07 Å). Chloroform guest molecules
occupy interstitial channels between stacked aromatic rings and
contributed to crystal packing through additional C–H ؒ ؒ ؒ Cl
short contacts. The crystal packing of 5 is influenced by the
presence of the iodide ligand on the Pd() metal ion. In fact,
the herringbone arrangement is reached meanly through
C–H ؒ ؒ ؒ Cl contacts between dimers and chloroform mole-
cules rather than π–π contacts between the rotationally free
aromatic rings, as observed in 4.
The absorption spectra of 1–5 recorded on a microcrystalline
sample showed a substantial uniformity and a strong corre-
lation with the spectra recorded from solutions. The main
difference is the position of the lowest energy band, ascribed
to an ILCT transition, which, for 1 and 2, in the solid state, is
red-shifted by 20 nm (Table 2). This bathochromic shift
increased to 60 nm for 3–5 and can be symptomatic of signifi-
cant differences in the crystal packing of 3–5 compared to 1 and
2. The observed “slipped stacking” between cyclometallated
phenyl rings in the dimeric solid arrangement, could lead to an
extended electronic delocalisation, which decreases the ILCT
transition energy, and, consequently, induces the red shift of the
related band.
3
4ؒCHCl3
5ؒCHCl3
Pd–X
Pd–C(2)
2.310(2)
1.966(5)
1.981(4)
2.120(4)
1.352(5)
1.412(6)
1.399(7)
1.314(6)
1.482(7)
1.357(6)
2.412(1)
1.940(7)
1.964(5)
2.115(5)
1.371(7)
1.419(8)
1.394(9)
1.300(8)
1.474(9)
1.356(8)
2.574(1)
1.970(6)
1.970(5)
2.114(5)
1.366(6)
1.405(7)
1.413(8)
1.300(7)
1.469(8)
1.355(7)
Pd–N(2)
Pd–N(3)
N(1)–N(2)
N(1)–C(1)
C(1)–C(2)
N(2)–C(7)
C(7)–C(8)
N(3)–C(8)
X–Pd–C(2)
X–Pd–N(2)
X–Pd–N(3)
N(2)–Pd–N(3)
N(2)–Pd–C(2)
N(3)–Pd–C(2)
98.6(2)
175.7(1)
99.5(1)
79.7(2)
82.1(2)
161.9(2)
98.7(2)
176.1(2)
99.4(1)
79.5(2)
82.4(3)
161.9(3)
98.1(2)
174.2(1)
100.3(1)
78.9(2)
82.7(2)
161.6(2)
aromatic system constituted by the fusion of pyridine and
phenyl rings, plus two five-membered metallacycles, indicated
that, in this type of structures, π–π interactions can play an
important role in controlling the crystal packing. In fact, within
the dimer, the cyclometallated phenyl ring showed a so-called
“slipped stacking”33 with the coordinated pyridine ring N(3)/
C(12) of the centrosymmetrical related molecule. The two rings
are parallel though displaced with characterizing parameters of
interplanar distance, displacement angle and transverse slip
of 3.23 Å, 32Њ and 2.01 Å, respectively. Moreover, the two
molecules are held together through intermolecular contacts
of metal-assisted hydrogen bond type34 between the acceptor
Pd–Cl fragment and the hydrogen atom of the rotationally free
phenyl ring (Fig. 4(a)).35 As shown in Fig. 4(b), in the crystal
packing the dimers, found in a herringbone arrangement in the
bc plane, interact between each other mainly through additional
C–H ؒ ؒ ؒ Cl–Pd metal-assisted hydrogen bond contacts.
Crystals of complexes 4 and 5 suitable for single crystal X-ray
analysis were obtained from a chloroform–n-hexane mixture. In
both cases one molecule of CHCl3 was found in the asymmetric
unit. Only small variations in the geometrical parameters of the
molecule were observed, caused by the change of the ancillary
ligand X on the palladium atom more than the presence of
molecules of co-crystallized chloroform (see Table 3). Again,
Luminescence investigations carried on microcrystalline
samples of 3–5 showed an emission more intense than in solu-
tions, and a sharp band located in the 610–618 nm range (Table
2 and Fig. 6) was recorded. As for the absorption spectra, the
luminescence data confirmed that in the solid state, because of
D a l t o n T r a n s . , 2 0 0 4 , 1 3 8 6 – 1 3 9 2
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Fig. 5 Crystal packings viewed along the b and a axis showing (a) Pd–Br ؒ ؒ ؒ H [Br ؒ ؒ ؒ H11A(1 Ϫ x, Ϫy, Ϫz) 2.98 Å, Br ؒ ؒ ؒ C11A 3.750(8) Å,
C11A–H11A ؒ ؒ ؒ Br 141Њ] and C–H ؒ ؒ ؒ Cl [Cl2 ؒ ؒ ؒ H6(x, y, z) 2.99 Å, Cl2 ؒ ؒ ؒ C6 3.819(8) Å, C6–H6 ؒ ؒ ؒ Cl 149Њ] hydrogen bonds in complex 4 and
(b) Pd–I ؒ ؒ ؒ H [I ؒ ؒ ؒ H14A(Ϫx, Ϫy, Ϫz) 3.23 Å, I ؒ ؒ ؒ C14A 3.992(7) Å, C14A–H14A ؒ ؒ ؒ I 140Њ] and C–H ؒ ؒ ؒ Cl [Cl3 ؒ ؒ ؒ H19A(x Ϫ 1, y, z) 2.87
Å, Cl3 ؒ ؒ ؒ C19A 3.825(7) Å, C6–H6 ؒ ؒ ؒ Cl3 166Њ; Cl3 ؒ ؒ ؒ H11B(1 Ϫ x, y Ϫ 0.5, 0.5 Ϫ z) 2.94 Å, Cl3 ؒ ؒ ؒ C11B 3.825(7) Å, C11B–H11B ؒ ؒ ؒ Cl3
132Њ] hydrogen bonds in complex 5.
arranged in dimers, compared to metal–metal interactions, less
common in Pd() derivatives (non-bonding Pd ؒ ؒ ؒ Pd dis-
tances of 3.623(2), 3.951(1) and 3.825(2) Å for X = Cl, Br and I,
respectively).
The photophysical characterisation of complexes 1–5 showed
that, when HL acted as a terdentate ligand, the Pd() halide
derivatives (3–5) gained, in the solid state, good luminescence
properties at room temperature (λmax at about 610–618 nm).
The crystallographic results concerning 3–5 showed the
presence of intermolecular interactions mainly due to “slipped
stacking” involving aromatic rings. This type of non-covalent
interactions could be considered responsible for the delocalis-
ation which is necessary to decrease the energy of the related
ILCT electronic state. Therefore, in the solid samples, a radia-
tive deactivation could be allowed since the energy gap between
ILCT and MC states is raised enough to prevent a thermal
Fig. 6 Emission spectrum of 3 recorded on microcrystalline sample.
access to MC states. Even though in complex 2 HL is found to
bind the metal ion in a terdentate fashion, and its absorption
the presence of π–π interactions, the ILCT excited states are
spectrum recorded in solution is comparable with those of 3–5,
positioned at lower energy compared to the solution spectrum.
no detectable emission is observed in the solid state. It should
Therefore the energy gap between ILCT and MC states is large
be considered that in the case of complex 2, the intermolecular
interactions in the crystalline state are expected to be weaker
enough to prevent non-radiative deactivating processes.40
considering the greater steric demand of the ancillary ligand
(i.e. acetato vs. halide)41 and therefore not able to promote
Conclusion
radiative deactivation of the excited states.
The 2-benzoylpyridine-phenylhydrazones form an interesting
Concluding, the present work illustrates some of the
class of ligands able to display different coordination modes on
ligand capabilities of the 2-benzoylpyridine N-methyl-N-
reaction with Pd() salts. In particular, the reaction between
phenylhydrazone and proves that Pd() complexes, photo-
phenyl-2-pyridylketone-2,4-dinitrophenylhydrazone, I, and
luminescent in the solid state, can be obtained on promoting
[Pd3(OAc)6] gave rise to a µ-OAc dinuclear complex character-
stacking interactions in their crystal structures. In this light,
ized by the presence of a six-membered N(py)^N(am) metalla-
phenylhydrazone ligands can be useful molecules to design new
cycle, in which the pyridinic and the deprotonated hydrazine
molecular materials.
nitrogen atoms are involved.14 When I is substituted by HL,
for which the N-deprotonation is prevented, different metal
Experimental
complexes are obtained, wherein the Pd() environment
depends on the nature of the Pd() source. In fact, through
reaction with [Pd(PhCN)2Cl2] the coordination compound 1
is obtained in which a “PdCl2” fragment, chelated to N(py)
and N(im) in a five-membered N(py)^N(im) ring, is present.
However, when HL reacts with [Pd3(OAc)6], which is considered
comparatively more reactive than [Pd(PhCN)2Cl2] in the cyclo-
palladation process,1 [LPd(OAc)], 2, is isolated. In this case an
acetato anion is coordinated in a monodentate fashion, and
HL behaves as a terdentate ligand, forming a N(py)^N(im)^C
chelated ring. Complex 2 undergoes methatetical reactions with
halides and the molecular structures of the resulting [LPdX]
complexes (3–5: X = Cl, Br, I, respectively) are determined by
single crystal X-ray analysis.
General data
All the starting materials were purchased from Aldrich and used
without further purification. Solvents for syntheses, chromato-
graphy and spectroscopic characterization of compounds were
pure chemicals (Aldrich and Across Organics). [Pd(PhCN)2Cl2]
was prepared according to a literature procedure.42
All the 1H NMR spectra were recorded on a Bruker AC 300
spectrometer, at 25 ЊC, and all chemical shifts are reported
in parts per million (ppm) relative to internal TMS with the
residual solvent proton resonance as internal standards.
IR spectra were recorded on a Perkin-Elmer Spectrum One
FT-IR spectrometer equipped for reflectance measurements.
Elemental analyses were performed using a Perkin-Elmer 2400
analyzer. The thermal behaviour was monitored with a Zeiss
The main feature of these compounds is the predominance
of π–π contacts between adjacent antiparallel molecules
D a l t o n T r a n s . , 2 0 0 4 , 1 3 8 6 – 1 3 9 2
1390

View Article Online
Table 4 Crystallographic data for HL and 3–5
HL
3
4ؒCHCl3
5ؒCHCl3
Empirical formula
Mr
Crystal system
Space group
C19H17N3
287.36
C19H16N3ClPd
428.20
Monoclinic
P21/c
C20H17N3BrCl3Pd
592.03
Monoclinic
P21/c
C20H17N3ICl3Pd
639.02
Monoclinic
P21/c
Triclinic
¯
P1
a/Å
b/Å
c/Å
α/Њ
9.189(3)
9.501(2)
10.119(3)
99.16(2)
109.48(2)
105.28(2)
773(4)
2
12.116(2)
11.728(2)
12.754(3)
90
111.44(3)
90
1687(1)
4
1.686
11.057(2)
9.028(2)
21.759(6)
90
103.70(2)
90
2110(1)
4
1.864
9.081(2)
16.537(4)
14.720(5)
90
95.20(2)
90
2202(1)
4
1.928
β/Њ
γ/Њ
V/Å3
Z
Dc/g cmϪ3
1.234
0.074
µ/mmϪ1
1.263
3.164
2.621
T /K
λ/Å
F(000)
298
0.71073
304
298
0.71073
856
298
0.71073
1160
298
0.71073
1232
Scan type
θ–2θ
θ–2θ
0.53–0.61
3141
2993
2172
ω
θ-2θ
0.10–0.13
3723
3480
2377
Transm. factors
No. measd reflns.
No. unique reflns.
No. of reflns. (I > 2σ(I ))
No. refined params.
R Indicesa, b
R Indices (all data)
Goodness-of-fit
0.06–0.13
4394
4169
2892
253
0.056, 0.134
0.081, 0.146
0.936
2921
2734
1898
199
0.041, 0.110
0.063, 0.118
1.08
217
253
0.041, 0.100
0.059, 0.104
0.904
0.035, 0.076
0.059, 0.081
0.871
2
a R1 = Σ(|Fo| Ϫ |Fc|)/Σ|Fo|. b wR2 = [Σw(Fo2 Ϫ Fc2)2/Σw(Fo )2]1/2
.
Axioscope polarizing microscope equipped with a Linkam C0
600 heating stage.
Synthesis of [(HL)PdCl2] 1
A solution of HL (0.20 g, 0.70 mmol) in diethyl ether (20 cm3)
was added to a suspension of [Pd(PhCN)2Cl2] (0.27 g, 0.70
mmol) in ethanol (10 cm3). Stirring of the mixture at room
temperature resulted in the formation of a brownish precipitate
within 2 h. The formed solid was removed by filtration and
washed with ethanol. A brown soluble impurity was removed
by further washing with chloroform, leaving the product as a
microcrystalline bright red solid on the frit. Yield 78%. Mp
>285 ЊC. Calc. for C19H17N3PdCl2: C, 49.11; H, 3.69; N, 9.04.
Found: C, 48.95; H, 3.73; N, 8.91%; νmax/cmϪ1 (C᎐N) 1596; δ
Spectrofluorimetric grade solvent (Acros Organics) was used
for the photophysical investigations in solution at room
temperature. Solid state spectra were obtained from micro-
crystalline samples in KBr pellets. Absorption spectra were
recorded with a Perkin Elmer Lambda 900 spectrophotometer
and corrected emission and excitation spectra were obtained
with a Perkin Elmer LS 50B spectrofluorimeter, equipped with
a Hamamatsu R-928 photomultiplier tube. Emission quantum
yields were determined using the optically dilute method43
on aerated acetonitrile solutions (Ru(bipy)3Cl2 in aqueous
aerated solution was used as standard (Φ = 0.02844)). The
experimental uncertainty on the band maximum for absorption
and luminescence spectra was 2 nm while that of the extinction
coefficient was 10% and of the luminescent intensity was 20%.
᎐
H
(DMSO) 9.27 (1H, dd, H1); 8.24 (1H, td, H3), 7.95 (1H, td, H2),
7.58–7.45 (5H, m, H5,6,7,8,9), 7.34 (1H, dd, H4), 7.22 (2H, t,
H11,13), 6.97 (2H, d, H10,14), 6.86 (1H, t, H12) and 3.27 (3H, s, N–
CH3).
Synthesis of [LPd(OAc)] 2
Synthesis of 2-benzoylpyridine N-methyl-N-phenyldhydrazone
Palladium() acetate 0.195 g, 0.87 mmol) was added to a hot
red solution of HL (0.25 g, 0.87 mmol) in acetic acid (25 cm3).
The mixture was allowed to stir at 60 ЊC for 5 h, while the
colour of the solution changed from dark red to brick red. The
solvent was removed under reduced pressure, and the residue
azeotrope cyclohexane–methanol. The resulting red–orange
solid was suspended in ethanol–diethyl ether and the obtained
complex was collected by filtration and dried in vacuo. Yield
80%. Mp. 271–275 ЊC. Calc. for C21H19N3O2Pd: C, 55.83; H,
4.24; N, 9.30. Found: C, 55.62; H, 4.36; N, 9.18%; νmax/cmϪ1
HL
A solution of N-methyl-N-phenylhydrazine (1.54 cm3, 1.60 g,
13.1 mmol) and 0.3 cm3 of concentrated hydrochloric acid in
ethanol (25 cm3) was heated at reflux and stirred for 15 min;
2-benzoylpyridine (2.0 g, 10,92 mmol) in 20 cm3 of ethanol
was added dropwise to this solution. The obtained red crimson
mixture was heated at reflux for 2 h and then it was reduced
to dryness in vacuo. The residue oil was poured into water and
repeatedly extracted with chloroform. The combined organic
extracts were dried over anhydrous sodium sulfate, filtered and
then concentrated under vacuum to give a crude product, which
was chromatographed on silica gel. Elution with diethyl ether–
petroleum ether (7 : 3, v/v) allowed the product to be recovered.
The analytically pure product was obtained as an orange solid
upon removal of the solvent in vacuo and suspension in diethyl
ether. The solid was filtered off and vacuum dried. Yield 70%.
Mp. 100–103 ЊC; Calc. for C19H17N3: C, 79.41; H, 5.96;
(asym. CO ) 1627, (C᎐N) 1591 and (sym. CO ) 1369; δ
᎐
2
2
H
(CDCl3) 8.34 (1H, dd, H1), 7.65–7.56 (6H, m, H3,5,6,7,8,9), 7.46
(1H, dd, H13), 7.27 (1H, td, H2), 7.01 (1H, dd, H4), 6.91 (1H, td,
H11), 6.72 (1H, td, H12), 6.18 (1H, dd, H10), 2.80 (3H, s, N–CH3)
and 2.23 (3H, s, –COOCH3).
Synthesis of [LPdX] 3–5
N, 14.62. Found: C, 79.41; H, 5.91; N, 14.95%; νmax/cmϪ1 (C᎐N)
All these analogous complexes were prepared in the same way;
the procedure for 3 is described below. Lithium chloride (0.13 g,
3.1 mmol) was added to a suspension of complex 2 (0.07 g, 0.15
mmol) in ethanol (15 cm3). The mixture was stirred at room
᎐
1598; δH (CDCl3) 8.54 (1H, d, H1), 8.03 (2H, d, H10,14), 7.70 (2H,
td, H11,13), 7.55–7.35 (5H, m, H5,6,7,8,9), 7.34–7.19 (3H, m,
H2,4,12,), 6.91 (1H, td, H3) and 2.95 (3H, s, N–CH3).
D a l t o n T r a n s . , 2 0 0 4 , 1 3 8 6 – 1 3 9 2
1391

View Article Online
9 I. Aiello, U. Caruso, M. Ghedini, B. Panunzi, A. Quatela, A. Rovello
and F. Sarcinelli, Polymer, 2003, 44, 7635.
10 A. Crispini and M. Ghedini, J. Chem. Soc., Dalton Trans., 1997, 75.
11 S.-W. Lai, T.-C. Cheung, M. C. W. Chan, K.-K. Cheung, S.-M. Peng
and C.-M. Che, Inorg. Chem., 2000, 39, 255.
12 F. Neve, A. Crispini, C. Di Pietro and S. Campagna,
Organometallics, 2002, 21, 3511.
13 G. García-Herbosa, A. Muñoz, D. Miguel and S. Garcia-Granda,
Organometallics, 1994, 13, 1775.
14 A. Bacchi, M. Carcelli, M. Costa, P. Pelagatti, C. Pelizzi and
G. Pelizzi, J. Chem. Soc., Dalton Trans., 1996, 4239.
15 I. Aiello, A. Crispini, M. Ghedini, M. La Deda and F. Barigelletti,
Inorg. Chim. Acta, 2000, 308, 121.
16 G. R. Crosby, Acc. Chem. Res., 1975, 8, 231.
17 R. Butler and S. M. Johnston, J. Chem. Soc., Perkin Trans. 1, 1984,
2109.
temperature for 12 h, after which the red product was filtered
off and washed several times with water, ethanol and diethyl
ether and dried in vacuo. Yield 87%. Mp >300 ЊC. Calc. for
C19H16N3PdCl: C, 53.29; H, 3.77; N, 9.81. Found: C, 52.94; H,
3.61; N, 9.69%; νmax/cmϪ1 (C᎐N) 1588; δH (CDCl3) 8.53 (1H, d,
᎐
H1), 7.63–7.54 (6H, m, H3,5,6,7,8,9), 7.45 (1H, d, H13), 7.17 (1H, t,
H2), 6.92 (1H, d, H4), 6.86 (1H, td, H11), 6.64 (1H, t, H12), 6.15
(1H, d, H10) and 2.83 (3H, s, N–CH3).
[LPdBr] 4. Bright red solid. Yield 89%. Mp >300 ЊC. Anal.
Calc. for C19H16N3PdBr: C, 48.28; H, 3.41; N, 8.89. Found: C,
48.61; H, 3.35; N, 8.81; νmax/cmϪ1 (C᎐N) 1591; δH (CDCl3) 8.69
᎐
(1H, d, H1), 7.69 (1H, dd, H13), 7.62–7.57 (6H, m, H3,5,6,7,8,9),
7.18 (1H, td, H2), 6.93 (1H, d, H4), 6.87 (1H, td, H11), 6.62 (1H,
td, H12), 6.16 (1H, d, H10) and 2.83 (3H, s, N–CH3).
18 S. Campagna, M. Cusumano, A. Giuffrida, G. Guglielmo and
V. Ricevuto, Polyhedron, 1988, 7, 207.
19 A. Giannetto, G. Guglielmo, V. Ricevuto, A. Giuffrida and
S. Campagna, J. Photochem. Photobiol. A: Chem., 1990, 53, 23.
20 I. Aiello, M. Ghedini and M. La Deda, J. Lumin., 2002, 96, 249.
21 C. Reichartd, Solvents and Solvent Effects in Organic Chemistry,
VCH, Weinheim, 2nd edn., 1988.
22 M. N. Golovin, M. M. Rahman, J. E. Belmonte and W. P. Giering,
Organometallics, 1985, 4, 1981.
[LPdI] 5. Bright red solid. Yield 83%. Mp >285 ЊC. Calc. for
C19H16N3PdI: C, 43.91; H, 3.10; N, 8.09. Found: C, 44.27; H,
3.06; N, 8.05; νmax/cmϪ1 (C᎐N) 1591; δ (CDCl3) 8.97 (1H, dd,
᎐
H
H2), 6.94 (1H, d, H4), 6.86 (1H, td, H11), 6.54 (1H, td, H12), 6.19
(1H, dd, H10) and 2.83 (3H, s, N–CH3).
23 Y. Wakatsuki, H. Yamazaki, P. A. Grutsch, M. Santhanam and
C. Kutal, J. Am. Chem. Soc., 1985, 107, 8153.
Crystallography
24 D. Sandrini, M. Maestri, V. Balzani, A. Chassot and A. von
Zelewsky, J. Am. Chem. Soc., 1987, 109, 7720.
25 P. J. Spellane, R. J. Watts and A. Vogler, Inorg. Chem., 1993, 32,
5633.
26 G. García-Herbosa, A. Muñoz and M. Maestri, J. Photochem.
Photobiol. A: Chem., 1994, 83, 165.
27 L. Barloy, R. M. Gauvin, J. A. Osborn, C. Sizun, R. Graff and
N. Kyritsakas, Eur. J. Inorg. Chem., 2001, 1699.
28 C. J. Isaac, C. Price, B. R. Horrocks, A. Houlton, M. R. J. Elsegood
and W. Clegg, J. Organomet. Chem., 2000, 598, 248.
29 J. Albert, A. Gonzàlez, J. Granell, R. Moragas, C. Puerta and
P. Valerla, Organometallics, 1997, 16, 3775.
Diffraction measurements were carried out on a Siemens R3m/
V automated four-circle diffractometer equipped with graphite-
monochromated Mo-Kα radiation (λ = 0.71073 Å). The data
were corrected for Lorentz and polarization effects. An
empirical absorption correction was applied using a method
based upon azimuthal (Ψ) scan data in the case of complexes
3, 4ؒCHCl3 and 5ؒCHCl3.45 Details of crystal data collection
are listed in Table 4. Crystal structures were solved by direct
(HL, SIR92 program in WINGX software package)46,47 and
Patterson (3, 4ؒCHCl3 and 5ؒCHCl3, SHELXS/L program
in SHELXTL-NT software package)48 methods and refined
by full-matrix least-squares based on F 2. In all cases, all
the non-hydrogen atoms were refined anisotropically, and
the hydrogen atoms were included as idealized atoms riding
on the respective carbon atoms with C–H bond lengths
appropriate to the carbon atom hybridization.
30 J. Albert, A. Gonzàlez, J. Granell, R. Moragas, X. Solans and
M. Font-Bardìa, J. Chem. Soc., Dalton Trans., 1998, 1781.
31 F. Neve, M. Ghedini, O. Francescangeli and S. Campagna, Liq.
Cryst., 1998, 24, 673.
32 A. Bondi, J. Chem. Phys., 1964, 68, 441.
33 C. Janiak, J. Chem. Soc., Dalton Trans., 2000, 3885.
34 (a) G. Aullòn, D. Bellamy, L. Brammer, E. A. Bruton and
A. G. Orpen, Chem. Commun., 1998, 653; (b) A. L. Gillon, G. R.
Lewis, A. G. Orpen, S. Rotter, J. Starbuck, X.-M. Wang,
Y. Rodrìguez-Martìn and C. Ruiz-Pèrez, J. Chem. Soc., Dalton
Trans., 2000, 3897.
CCDC reference numbers 227423–227426.
See http://www.rsc.org/suppdata/dt/b3/b316843c/ for crystal-
lographic data in CIF or other electronic format.
35 P. K. Thallapally and A. Nangia, CrystEngComm, 2001, 27, 1.
36 G. R. Desiraju, Acc. Chem. Res., 1996, 29, 441.
37 T. Steiner, Chem. Commun., 1997, 727.
Acknowledgements
38 L. Brammer, E. A. Bruton and P. Sherwood, New J. Chem., 1998, 23,
This work was partly supported by CIPE grants (Clusters 14
and 26) from the Ministero dell’Istruzione, dell’Università e
della Ricerca.
965.
39 F. Neve and A. Crispini, Cryst. Growth Des., 2001, 1, 387.
40 M. Ghedini, I. Aiello, M. La Deda and A. Grisolia, Chem.
Commun., 2003, 2198.
41 A search in the CSD (Cambridge Structural Database, Version 5.24)
for mononuclear Pd() complexes containing monodentate acetate
ligand has demonstrated that, on average, the mean plane containing
the ligand and the mean coordination plane are nearly orthogonal to
each other with an evident lack of planarity of the molecule.
42 M. S. Karash, R. C. Seyler and F. R. Mayo, J. Am. Chem. Soc., 1938,
60, 882.
43 J. N. Demas and G. A. Crosby, J. Phys. Chem., 1971, 75, 991.
44 K. Nakamaru, Bull. Soc. Chem. Jpn., 1982, 5, 2697.
45 A. C. T. North, D. C. Phillips and F. S. Mathews, Acta Crystallogr.,
Sect. A, 1968, 24, 351.
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D a l t o n T r a n s . , 2 0 0 4 , 1 3 8 6 – 1 3 9 2
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