A new organometallic palladium(II) compound derived of 2,6-diacetylpyridine mono(thiosemicarbazone): Synthesis, spectroscopic properties and crystal structure of two solvatomorphic forms

Article abstract of DOI:10.1016/j.jorganchem.2013.09.022

Preparation and characterization of a novel 2,6-diacetylpyridine mono(4-methoxyphenyl-thiosemicarbazone), H₂L, is described. Treatment of H₂L with PdCl₂(PPh₃)₂ gave the neutral mononuclear complex [Pd

Full text of DOI:10.1016/j.jorganchem.2013.09.022

Accepted Manuscript
A new organometallic palladium(II) compound derived of 2,6-diacetylpyridine
mono(thiosemicarbazone): synthesis, spectroscopic properties and crystal structure
of two solvatomorphic forms
Ana I. Matesanz, Carolina Henández, Pilar Souza
PII:
S0022-328X(13)00690-6
10.1016/j.jorganchem.2013.09.022
JOM 18268
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 28 May 2013
Revised Date: 13 September 2013
Accepted Date: 16 September 2013
Please cite this article as: A.I. Matesanz, C. Henández, P. Souza, A new organometallic palladium(II)
compound derived of 2,6-diacetylpyridine mono(thiosemicarbazone): synthesis, spectroscopic properties
and crystal structure of two solvatomorphic forms, Journal of Organometallic Chemistry (2013), doi:
10.1016/j.jorganchem.2013.09.022.
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ACCEPTED MANUSCRIPT

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A new organometallic palladium(II) compound derived of 2,6-diacetylpyridine
mono(thiosemicarbazone) has been synthesized and structurally characterized by single
crystal X-ray diffraction. The molecular structure shows that The the diacetylpyridine
ring undergoes an interesting and unexpected C-H bond activation.

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Highlights
-
-
-
A new 2,6-diacetylpyridine mono(4-methoxyphenyl-thiosemicarbazone)
cyclopalladated diacetylpyridine/thiosemicarbazone compound with has
been prepared
Cyclopalladation occurs at theThe diacetylpyridine ring which undergoes an
interesting and unexpected C-H bond activation
Two solvatomorphic forms, of [PdL(PPh3)] and [PdL(PPh3)]·DMSO, have
been studied by X-ray crystallography

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A new organometallic palladium(II) compound derived of 2,6-diacetylpyridine
mono(thiosemicarbazone): synthesis, spectroscopic properties and crystal
structure of two solvatomorphic forms
Ana I. Matesanza, Carolina Henándeza,b and Pilar Souzaa*
aDepartamento de Química Inorgánica (Módulo 07), Facultad de Ciencias, c/ Francisco
Tomás y Valiente nº 7, Universidad Autónoma de Madrid, 28049-Madrid, Spain.
bDepartamento de Química Inorgánica, Orgánica y Bioquímica, Facultad de Ciencias
del Medio Ambiente, Avd. Carlos III s/n, Universidad de Castilla-La Mancha, 45071-
Toledo, Spain.
* Corresponding author. Tel.: +34 914975146; fax: +34 914974833. E-mail address: pilar.souza@uam.es
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Keywords: Cyclopalladated, 2,6-diacetylpyridine, solvatomorph, thiosemicarbazone,
X-ray.
Summary
Preparation and characterization of
a
novel 2,6-diacetylpyridine mono(4-
methoxyphenyl-thiosemicarbazone), H2L, is described. Treatment of H2L with
PdCl2(PPh3)2 gave the neutral mononuclear complex [PdL(PPh3)]. Both compounds
have been characterized by elemental analysis and FAB+ spectrometry and by IR and 1H
NMR spectroscopy. Recrystallization of [PdL(PPh3)] in DMSO led to the isolation of
two solvatomorphic forms of this compound which were studied by X-ray
crystallography. In each solvatomorph, the thiosemicarbazone ligand acts as dianionic
[CNS] tridentate donor coordinating to the palladium(II) ion in a square planar
geometry through the iminic nitrogen atom, the sulfur atom and one ortho carbon atom
of the pyridine ring and the fourth site is occupied by P atom from the PPh3 coligand.
The overall molecular conformation of the two solvatomorphs is very similar and the
largest difference lies in the orientation of the methyl group of the 4-methoxyphenyl
substituent with respect to the sulfur atom.
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1. Introduction
Metal mediated activation of aromatic C-H bonds represents an active research
area in organometallic chemistry due to the numerous applications that the resulting
cyclometallated compounds have found in organic synthesis, catalysis, material sciences
and as biologically active compounds [1-4]. The cyclometallation process occurs quite
readily with platinum group metals and consists of two consecutive steps: initial
coordination of the metal center via a donor group such as N, O, P, S, etc; and
subsequent intramolecular activation of the C-H bond, which closes the metallacycle [5-
8].
Thiosemicarbazones (Scheme 1, left) are important and versatile N/S donor
ligands. They may exist as thione/thiol tautomeric forms owing to the intramolecular
proton transfer and thus can bind to a metal center via a variety of coordination modes
in both neutral and anionic forms. Usually the thiosemicarbazone moiety coordinates to
the metal ion through the sulfur atom and the azomethine nitrogen atom forming five
membered chelate rings [9,10]. The coordination capacity of thiosemicarbazones can be
further increased, if the parent aldehyde or ketone contains, at R1, an additional donor
atom in position suitable for chelation (Scheme 1, right).
Scheme 1.
In this regard, heterocyclic thiosemicarbazone coordination compounds have
been studied extensively and it is well established that both, the nature of the ligand
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substituents, the metal ion, the type of anion, co-ligands and solvents have high
influence in the bonding and nuclearity of the resulting complexes [11-15].
Cyclopalladated compounds with tridentate thiosemicarbazones prepared via C-
H activation of an aryl group have also been previously described [16-20] and the
cyclometallation has been attributed to the steric effect of R2 substituents. However,
there are limited reports concerning the C(sp2)-H bond activation in electron-deficient
aromatic systems such as in pyridines since α-(N)-heterocyclic thiosemicarbazones
usually give rise to chelate complexes [1, 21-22].
As part of our systematic investigation on the chemistry of thiosemicarbazone
derivatives, we have reported both cyclopalladated and coordination palladium(II)
complexes derived of α-diphenyletanodione bis(thiosemicarbazones) [23-25] as well as
numerous coordination palladium(II) complexes derived from different α-(N)-
heterocyclic bis(thiosemicarbazones) [26-28].
Recently, we have prepared a series of palladium(II) and platinum(II) complexes
derived from 2,6-diacetylpyridine bis(4N-tolyl-thiosemicarbazones) and structural study
revealed the mononuclear neutral nature of the complexes and the dianionic [SNNN]
tetradentate behavior of the α-(N)-heterocyclic bis(thiosemicarbazone) ligands [29].
To extend the knowledge in this research field, in this study we report on the
synthesis and chemical characterization of the new 2,6-diacetylpyridine mono(4-
methoxyphenyl-thiosemicarbazone) ligand, H2L (Scheme 2), and its cyclopalladated
complex [PdL(PPh3)].
In principle, the new ligand could give rise to [SNNO] or [SNN] donor sites
forming, by reaction with palladium(II) salts, square planar coordination complexes in
a similar manner to the related 2,6-diacetylpyridine bis(4N-tolyl-thiosemicarbazones).
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Scheme 2. Structural drawings of 2,6-diacetylpyridine mono(4-methoxyphenyl-
thiosemicarbazone), H2L (left) and 2,6-diacetylpyridine bis(4N-tolyl-thiosemicarbazone)
(right) ligands
However we found that 2,6-diacetylpyridine mono(4-methoxyphenyl-
thiosemicarbazone) ligand undergoes an unexpected and interesting C-H bond
activation of the pyridine ring leading to the formation of an orthopalladated compound.
2. Experimental
2.1.
Measurements
Elemental analyses were performed on a LECO CHN-932 microanalyzer. Fast
atom bombardment (FAB) mass spectra were performed on a VG AutoSpec
spectrometer (nitrobenzyl alcohol matrix). Nuclear Magnetic Resonance (NMR) spectra
were recorded on a BRUKER AMX-300 spectrometer. All cited physical measurements
were obtained out by the Servicio Interdepartamental de Investigación (SIdI) of the
Universidad Autónoma de Madrid.
Melting points were determined with a Stuart Scientific SMP3 apparatus.
Infrared spectra (KBr pellets) were recorded on a Bomen–Michelson spectrophotometer
(4000–400 cm-1).
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2.2.
Materials
Solvents were purified and dried according to standard procedures. Hydrazine
hydrate, 2,6-diacetylpyridine, 4-methoxyphenyl isothiocyanate, PdCl2(PPh3)2 were
commercially available.
2.3.
Synthesis of compounds
2,6-Diacetylpyridine mono(4-methoxyphenyl-thiosemicarbazone), H2L. An
ethanolic solution of hydrazine hydrate (0.250 g, 5 mmol) was added dropwise with
constant stirring to an ethanolic solution of 4-methoxyphenyl isothiocyanate (0.826 g, 5
mmol). The reaction mixture was stirred for one more hour and then the white product
4-methoxyphenyl-thiosemicarbazide formed was filtered, washed with cold ethanol and
diethyl ether, dried in vacuo and recrystallized from ethanol. An ethanolic solution of
the 4-methoxyphenyl-thiosemicarbazide (0.346 g, 1 mmol) was then stirred with 2,6-
diacetylpyridine (0.163 g, 1 mmol) for 5 h. The resulting solution was reduced to half
volume and the pale yellow solid formed was filtered, washed several times with hot
water, diethyl ether and finally dried in vacuo.
Yield (55%), mp 196 ºC (decomposes). Elemental analysis found, C, 56.6; H,
5.5, N, 16.0; C17H18N4O2S·H2O requires C, 56.5; H, 5.5, N, 15.5 %. MS (FAB+ with
mNBA matrix) m/z 343.1 for [H2L+H]+. IR (KBr pellet): n/cm-1 3192, 3114 (s, NH);
1
1690 (s, CO); 1590 (s, CN); 1518 (s, CN-thioamide I); 805 (w, CS-thioamide IV). H
NMR (300.14 MHz, DMSO-d6): δ (ppm) 10.60 (s, 2NH); 10.15 (s, 4NH); 8.70 (m, H12,
H16); 8.25 (m, H13, H15); 7.90 (m, H2); 7.40 (d, H1); 6.90 (d, H3); 3.80 (s, H17); 2.50
and 2.30 (s, H6 and H9).
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2,6-Diacetylpyridine mono(4-methoxyphenyl-thiosemicarbazonato-κ3CNS)triphe-
nylphosphinepalladium(II), [Pd(L)(PPh3)]. The reaction of H2L ligand with
PdCl2(PPh3)2, in toluene, in presence of Et3N, in 1:1 molar ratios over 2 h at room
temperature led to the formation of an orange solution which was filtered and left to
stand at ambient temperature for two days. The solid formed was filtered, washed
several times with hot water, recrystallizated from DMSO and finally dried in vacuo.
Yield (35%), mp >250 ºC. Elemental analysis found, C, 56.7; H, 4.6, N, 7.1;
C35H31N4O2SPPd·DMSO requires C, 56.5; H, 4.7, N, 7.1 %. IR (KBr pellet): n/cm-1
3418 (s, NH); 3048 (w, PPh3); 1685 (s, CO); 1591 (s, CN); 1537 (s, CN-thioamide I);
1
1076 (w, PPh3); 798 (vw, CS-thioamide IV). H NMR (300.14 MHz, DMSO-d6): δ
4
(ppm) 9.40 (s, NH), 7.65-7.45 (m, H12, H13, H15, H16 and -PPh3 aromatic protons);
7.00 (d, H2); 6.85 (d, H3); 3.70 (s, H17); 2.55 and 2.50 (s, H6 and H9).
Rerystallization from DMSO led to the isolation of two types of crystals: (I)
clear yellow plates and (II) clear yellow prismatic. The molecular structures of both
crystals were determined by single crystal X-ray diffraction.
Crystallography
Data were collected on a Bruker X8 APEX II CCD. Crystallographic data and
selected interatomic distances and angles are listed in Tables 1 and 2. The software
package SHELXTL was used for space group determination, structure solution, and
refinement [30]. The structures were solved by direct methods, completed with
difference Fourier syntheses, and refined with anisotropic displacement parameters.
CCDC 940829 and 940830 contain the supplementary crystallographic data.
These data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html
[or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2
1EZ, UK; fax: +44-1223/336-033; e-mail: deposit@ccdc.cam.ac.uk].
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3. Results and discussion
3.1.
Synthesis and spectroscopic characterization
A new multidentate ligand, H2L, was synthesized as a platform for metal
arrangement. Complexation with PdCl2(PPh3)2 led to the isolation of an orange solid.
The new palladium(II) complex obtained which is stable to air and moisture was
characterized by elemental analysis, IR and 1H NMR spectroscopy and single crystal X-
ray diffraction. It is noteworthy that only two reports on structural properties of
mono(thiosemicarbazone) complexes derived of a (N)-hetrocyclic diketones have been
found in literature [31-32].
Analytical data suggests [PdL(PPh3)] stoichiometry and this requires that a
1
dideprotonate tridentate ligand behavior. The HNMR spectroscopy was used to
confirm the dideprotonation. Thus while the spectrum of the ligand shows two peaks at
low field assigned to the hydrazinic and thioamidic protons (10.60 and 10.15 ppm
respectively) and three signals assigned to the three pyridinic protons of the ligand, in
the spectrum of the complex the absence of N2H proton signal and the presence of only
two signals assigned to pyridinic protons supported the dideprotonation.
The infrared spectral bands most useful for determining the mode of
coordination of the ligand are the ν(C=O), ν(C=N) iminic and ν(C=S) thioamide IV
vibrations. In the IR spectrum of palladium(II) complex, the ν(C=O) band appears at
1685 cm-1 indicating the no participation of the carbonyl oxygen in coordination. On the
other hand, the shift of ν(C=S) thioamide IV to lower wavenumbers suggests
coordination of the sulfur atom. Moreover, the presence of the triphenylphosphine
ligand is confirmed by the existence of the characteristic bands around 3050 and 1070
cm−1 for ν(C-H) and ν(P-C), with no significant change when compared to the precursor
PdCl2(PPh3)2.
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Thus analytical and spectroscopic findings point to an unexpected dianionic
CNS tridentate behavior of the ligand in complex which could be confirmed from the
structure of the complex determined by single-crystal X-ray diffraction techniques.
Lobana et al. [21] prepared a series of palladium thiosemicarbazone compounds
containing both metallated thiophene or phenyl rings and the corresponding chelated
derivatives. Thus, they demonstrated that the nature of R2 thiosemicarbazone
substituents (see Scheme 1) might induce cyclopalladation by changing electronic and
steric properties. Similarly we achieved the activation of C-H bond of the aromatic
pyridine ring and this unusual cyclopalladation can be tentatively attributed to the steric
effect of acetyl group at R2 which drives to R1 substituent closer to palladium atom.
3.2.
[Pd(L)(PPh3)]
Recrystallization of [Pd(L)(PPh3)] compound in DMSO led to the isolation,
Crystallographic characterization of two polymorphs of
from the same crystallization vial, of two types of clear yellow crystals (designated here
as forms I and II) which differing in shape: I is plate and II is prismatic. The molecular
structures of both crystals were determined by single crystal X-ray diffraction and the
ORTEP view together with the atomic numbering scheme is shown in Fig. 1 and Fig. S1
(Supporting Information).
Interestingly, both forms, I and II, contain [Pd(L)(PPh3)] molecules but while the
first is DMSO solvated, the second do not contain any crystallization solvent molecules.
Crystallographic data is shown in Table 1 and selected bond lengths are listed in Table
2.
Insert Figure 1
Insert Table 1
Insert Table 2
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Both solvatomorphs crystallize in the same space group but show different
lattice parameters. In each molecule, the palladium(II) ion is tetracoordinated, bound to
the Niminic and the S atoms from the thiosemicarbazone moiety, one ortho C atom of the
pyridine ring and the P atom from the PPh3 coligand. Therefore the thiosemicarbazone
ligand behaves as dianionic [CNS] tridentate after metallation through C(1) carbon of
pyridine ring and deprotonation of N(2).
It is important to note that upon coordination, the deprotonated
thiosemicarbazone ligand undergoes significant evolution from the thione to the thiol
form which is reflected in a C–S distance of 1.765(3) Å (form I) and 1.764(4) Å (form
II). The C–N and N–N bond distances (Table 2) are intermediate between formal single
and double bonds, pointing to extensive delocalization over the entire
thiosemicarbazone skeleton.
The Pd–C 2.038(3) Å, Pd-N 2.025(2) Å, Pd–S 2.3249(7) Å and Pd–P 2.2469(7)
Å (form I) and Pd–C 2.042(4) Å, Pd-N 2.036(3) Å, Pd–S 2.3222(11) Å and Pd–P
2.2484(10) Å (form II) bond distances are comparable with those reported for Pd(II)
thiosemicarbazones [18,33]. The overall molecular conformation of the two polymorphs
is very similar and the largest difference lies in the orientation of the methyl group of
the 4-methoxyphenyl substituent with respect to the sulfur atom, namely syn (form II)
or anti (form I) [34].
The bond angle data indicate that the stereochemistry around the palladium (II)
ion is almost planar. The angles deviate slightly from that expected for a regular square–
planar geometry, this distortion may be attributed to the restricted bite angle of the
tridentate moieties. Coordination results in the formation of two five membered chelate
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rings with bite angles C(1)-Pd(1)-N(3) = 81.66(10)º and N(3)-Pd(1)-S(1) = 82.87(7)º
(form I) and C(1)-Pd(1)-N(3) = 81.47(15)º and N(3)-Pd(1)-S(1) = 83.12(10)º (form II).
In both polymorph supramolecular association patterns are basically
characterized by π-π stacking interactions (Fig. 2 and S2) however in the I-polymorph
4
DMSO solvent molecules are involved in intermolecular hydrogen bond with the N-H
atoms as is common in the crystal structures of thiosemicarbazones [21,24].
Figure 2
Acknowledgments
We are grateful to Ministerio de Economía y Competitividad, Instituto de Salud
Carlos III of Spain (PI1100659) for financial support.
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Table 1 Crystal data and structure refinement for I and II.
I
II
Molecular formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
C37H37N4O3PPdS2
787.20
100(2)
0.71073
Monoclinic
P21/n
C35H31N4O2PPdS
709.07
100(2)
0.71073
Monoclinic
P21/c
a(Å)
b(Å)
c(Å)
α/º
16.9191(11)
9.4437(6)
22.6259(12)
90
9.1218(10)
14.8577(17)
23.660(3)
90
β/º
γ/º
102.292(3)
90
3532.3(4)
4
99.302(5)
90
3164(6)
4
Volume(Å3)
Z
Density (calculated) (g/cm3)
1.480
1.488
1
-
0.731
0.742
Absorption coefficient (mm )
F(000)
1616
1448
3
0.05 x 0.40 x 0.40
0.08 x 0.10 x 0.25
Crystal size (mm )
Index ranges
-21≤h≤21, -11≤k≤11, -28≤l≤28
68535
7487 [R(int) = 0.0442]
7487 /0 /438
-10≤h≤10, -17≤k≤17, -28≤l≤28
55369
5776 [R(int) = 0.0890]
5776 /1 /400
Reflections collected
Independent reflections
Data / restraints / parameters
2
1.006
1.002
Goodness-of-fit on F
Final R indices [I>2σ(I)]
R1 = 0.0336, wR2 = 0.0952
R1 = 0.0455, wR2 = 0.1127
2.182 and -0.716
R1 = 0.0373, wR2 = 0.1027
R1 = 0.0610, wR2 = 0.1297
0.591 and -0.876
R indices (all data)
Largest diff. peak and hole, e.Å-3
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Table 2. Selected bond distances (Å) and angles (º) for I and II.
I
II
S(1)-C(10)
C(8)-N(2)
C(10)-N(3)
C(10)-N(4)
Pd(1)-C(1)
Pd(1)-N(2)
Pd(1)-S(1)
Pd(1)-P(1)
1.765(3)
1.295(4)
1.312(4)
1.361(4)
2.038(3)
2.025(2)
2.3249(7)
2.2469(7)
1.764(4)
1.301(5)
1.305(5)
1.367(5)
2.042(4)
2.036(3)
2.3222(11)
2.2484(10)
N(2)-Pd(1)-C(1)
N(2)-Pd(1)-P(1)
N(2)-Pd(1)-S(1)
C(1)-Pd(1)-P(1)
C(1)-Pd(1)-S(1)
P(1)-Pd(1)-S(1)
81.66(10)
174.08(7)
82.87(7)
97.69(8)
164.39(8)
97.49(3)
81.47(15)
176.78(9)
83.12(10)
104.4(5)
164.46(12)
97.61(4)
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Captions
Scheme 1. General structure of thiosemicarbazones.
Scheme 2. General structure of diacetylpyridine mono and bis(thiosemicarbazone)
ligands.
Figure 1. Molecular structure of I.
Figure 2. Crystal packing view of I along a axis.
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Figure 1
19

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Figure 2
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Figure S1
Figure S1. Molecular structure of II.

ACCEPTED MANUSCRIPT
Figure S2
Figure S2. Crystal packing view of II along c axis.