[Pd{2-CH2-5-MeC6H3C(H){double bond, long}NN{double bond, long}C(S)NHEt}]3: An unprecedented trinuclear cyclometallated palladium(II) cluster through induced flexibility in the metallated ring

Article abstract of DOI:10.1016/j.jorganchem.2008.11.062

The reaction of 1-(2,5-dimethylbenzylidene)-3-ethylthiosemicarbazone and palladium acetate in acetic acid yields a trinuclear cyclometallated palladium(II) compound. Each thiosemicarbazone ligand is tridentate with the metal bonded to the carbon atom from

Full text of DOI:10.1016/j.jorganchem.2008.11.062

Journal of Organometallic Chemistry 694 (2009) 747–751
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
[Pd{2-CH2-5-MeC6H3C(H)@NN@C(S)NHEt}]3: An unprecedented trinuclear
cyclometallated palladium(II) cluster through induced flexibility
in the metallated ring
Luis Adrio a, José M. Antelo a, Jesús J. Fernández b, King Kuok (Mimi) Hii c, Ma Teresa Pereira a, José M. Vila a,
*
a Departamento de Química Inorgánica, Universidad de Santiago de Compostela, E-15782 Santiago de Compostela, Spain
b Departamento de Química Fundamental, Facultad de Ciencias, Universidad de La Coruña, 15071 La Coruña, Spain
c Department of Chemistry, Imperial College, South Kensington, London SW7 2AZ, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 October 2008
Received in revised form 26 November 2008
Accepted 27 November 2008
Available online 7 December 2008
The reaction of 1-(2,5-dimethylbenzylidene)-3-ethylthiosemicarbazone and palladium acetate in acetic
acid yields a trinuclear cyclometallated palladium(II) compound. Each thiosemicarbazone ligand is tri-
dentate with the metal bonded to the carbon atom from the 2-methyl group, to the azomethine nitrogen
and to the sulfur atom, which bridges to an adjacent palladium center. The crystal structure confirms the
presence of a non-planar hexagonal metallated ring plus a central six-membered palladium–sulfur core
within the trimer, which also displays a rather deep intramolecular cavity.
Keywords:
Palladium
Ó 2009 Elsevier B.V. All rights reserved.
Cyclometallation
Trinuclear cluster
Thiosemicarbazone
1. Introduction
We have also found [10,14] that the strength of the palladium–sul-
fur bond, following Pearson’s concept [15], allows the ligand to be
The cyclometallation reaction has been profusely investigated
in view of the rich chemistry it renders and is well documented
for a great variety of metal centers and ligands [1]. The ensuing
compounds are successfully used in organic synthesis [2], catalysis
[3], photochemistry [4] and optical resolution process [5], and are
rather promising as potential biologically active materials [6] and
liquid crystals [7]. Among the numerous ligands prone to undergo
metallation, thiosemicarbazones in particular, are most interesting
because they are readily metallated by Ru(II) [8], Rh(III) [9], Pd(II)
[10] and Pt(II) [11] with activation of a C–H bond. Palladium(II)
salts, such as palladium(II) acetate or potassium tetrachloropalla-
date(II) give tetranuclear species with a Pd4S4 core [10,12]. Fur-
thermore, the previous studies by us and others related to
thiosemicarbazone palladacycles and platinacycles have shown
that tetrameric species are obtained in all cases with two pairs of
parallel metallated units set mutually at ca. 90° [10,12] (Fig. 1).
Even in the absence of metalÀcarbon bond, with the metal only
bonded to the nitrogen and sulfur atoms, the tetrameric nature
of the structure persists [13]. The ability shown by these ligands
to produce such compounds is due, in part, to the bonding proper-
ties of the sulfur atom in its thiolate form which enables it to bind
two metal atoms in a chelating and bridging mode simultaneously.
tightly bonded to the metal as terdentate [C,N,S] clasping firmly
three of the metal coordination positions, preventing reaction of
the Pd–N bond situated in the central part of the two fused five-
membered rings at the metal center; upon reaction with diphos-
phines with small bite angles the new cyclometallated bidentate
P,S metaloligands have been obtained [16]. Therefore, having pre-
viously studied the tetranuclear thiosemicarbazone species our
perspectives for the advancement into further aspects related to
the chemistry of these compounds was to seek new structural fea-
tures induced by alterations in the nature of the ligand and of its
reactivity towards the palladium substrate; the alternative thus
being modifications in the central palladium/sulfur ring. We rea-
soned that altering the size and the flexibility of the five-mem-
bered cyclometallated ring, [PdÀC@CÀC@N], should change the
angle between the palladium coordination plane and the metallat-
ed phenyl ring, as well as modifying the angles between the palla-
dium coordination planes themselves, thus hindering formation of
the tetrameric structure and bringing about alterations in the num-
ber of atoms and in the geometry of the PdnSn core. In order to
achieve this, Pd–C bond formation should arise at a carbon atom
other than a phenyl ring, as for example, a methyl aliphatic carbon
atom. For the study we sought out the ligand 2,5-
Me2C6H3C(H)@NN(H)C(@S)NHEt and attempted to metallate the
methyl group to give the more flexible six-membered cyclometal-
lated ring, in the hope that the steric requirements of the 5-Me
* Corresponding author.
E-mail address: josemanuel.vila@usc.es (J.M. Vila).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.11.062

748
L. Adrio et al. / Journal of Organometallic Chemistry 694 (2009) 747–751
Fig. 1. (a) Tetranuclear structure of a cyclometallated palladium(II) compound with a terdentadate [C,N,S] thiosemicarbazone. (b) View along the c axis showing the
S4-symmetry.
group would sufficiently hinder metallation of the C6 atom. Palla-
dacycles bearing metallated sp3 carbon atoms are known with, e.g.,
phosphorus donor ligands [17], methylquinoline [18]; and metalla-
tion of methyl groups attached to phenyl rings rendering six-mem-
bered metallacycles is scarce and limited to Schiff base ligands
where methyl substituents occupy the two possible metallation
sites [19]. Furthermore, although several examples of trinuclear
cyclometallated palladium(II) compounds are known, such as
ture for 4 h. The solid formed was filtered off, washed with cold
water and dried in vacuo.
Yield: 957 mg, 97%. Anal. Found: C, 61.3; H, 7.3; N, 17.9; S,
13.5%; C12H17N3S (235.35) requires C, 61.2; H, 7.3; N, 17.8; S,
13.6. IR:
m
m ; m ;
(N–H) 3412s, 3153s cmÀ1 (C@N) 1586s, br cmÀ1
(C@S) 806m cmÀ1 1H NMR (CDCl3, d ppm, J Hz): 1.31 (t, 3H,
.
NHCH2CH3, 3J(HH) = 7.3); 2.34, 2.40 (2s, 6H, Me); 3.76 (qd, 2H,
NHCH2CH3, 3J(HH) = 7.3, 3J(NHCH2) = 4.9); 7.08 (b, 2H, H3 y H4);
7.42 (s, 1H, NHCH2CH3); 7.54 (s, 1H, H6); 8.23 (s, 1H, HC@N);
10.31 (s, 1H, NH).
´
Steels compound [20], and also trimers that display structures
which may described crudely as an equilateral triangle with the
palladium atom at each vertex [21], the present investigation
brings about an altogether different structure with the metal and
the sulfur donor atoms linked together in a crown-type nucleus.
What follows are our findings in the synthesis, structure and char-
acterization of a new type of trinuclear palladacycle.
2.3. [Pd{2-CH2-5-MeC6H3C(H)@NN@C(S)NHEt}]3 (2)
The ligand 1 (235 mg, 1.0 mmol) and palladium(II) acetate
(213 mg, 0.95 mmol) were added to 40 cm3 of acetic acid to give
a clear solution, which was stirred at 60 °C for 8 h. After cooling
to room temperature the solution obtained was filtered through
celite to remove the black palladium formed, the solvent removed
and the residue extracted with dichloromethane. The combined
extracts were dried overnight with anhydrous magnesium sul-
phate, and chromatographed on a column packed with silica gel.
Elution with dichloromethane afforded the final product as an or-
ange solid after concentration. Yield 290 mg 90%. Anal. Found: C,
42.4, H, 4.4, N, 12.4, S, 9.3%. C36H45N9S3Pd3 (1019.21) requires C,
2. Experimental
2.1. General comments
Solvents were purified by standard methods [22]. Chemicals
were used as supplied from commercial sources. Elemental analy-
ses were carried out by the Unidade de Analise Elemental de la
Universidad de Santiago de Compostela using a Carlo-Erba elemen-
tal analyzer, Model 1108. IR spectra were recorded as Nujol mulls
or polythene discs on Perkin–Elmer 1330, Mattson Model Cygnus-
100 and Bruker Model IFS-66V spectrophotometers. NMR spectra
were obtained as CDCl3 solutions and referenced to SiMe4, and
were recorded on a Bruker AMX 300 spectrometer. All chemical
shifts are reported downfield from standards. The FAB mass spec-
tra were recorded with a Fisons Quatro mass spectrometer with a
Cs ion gun; 3-nitrobenzyl alcohol was used as the matrix.
42.4, H, 4.5, N, 12.4, S, 9.4. IR:
m .
(C@N) 1565m, cmÀ1 1H NMR
(CDCl3, d ppm, J Hz): 1.29 (t, 2H, CH2CH3); 2.23 (s, 3H, Me); 3.10
(s, 2H, CH2); 3.47 (m, 2H, CH2CH3); 4.99 (m, 1H, NHEt); 6.89 (s,
1H, H6); 6.93 (d, 1H, H4); 7.10 (d, 1H, H3, 3J(H3H4) = 7.9); 7.91
(s, 1H, HC@N). FAB-MS: m/z 1019 [M]+.
2.4. Crystal structure of 2
For 2 room temperature X-ray data were collected on a BRUKER
SMART-CCD-1000 diffractometer using monochromated Mo K
a
2.2. 2,5-Me2C6H3C(H)@NN(H)C(@S)NHEt (1)
radiation by the omega method. All the measured reflections were
corrected for Lorentz and polarization effects and for absorption by
semiempirical methods (SADABS) based on symmetry-equivalent
and repeated reflections. The structures were solved by direct
methods and refined by full matrix least-squares on F2. Hydrogen
To a suspension of the thiosemicarbazide (500 mg, 4.2 mmol) in
water (40 cm3) 35% hydrochloric acid (0.6 cm3) was added to give a
clear solution. Then 2,5-dimethylbenzaldehyde (563 mg, 4.2 mmol)
was added and the resulting mixture was stirred at room tempera-

L. Adrio et al. / Journal of Organometallic Chemistry 694 (2009) 747–751
749
Table 1
atoms were included in calculated positions and refined in riding
Crystal data and structure refinement data for 2.
mode. Refinement converged at
a final R = 0.0454 and wR2
= 0.0829 (all unique data, F2), with allowance for thermal anisot-
ropy of all non-hydrogen atoms, except C(23), C(24), C(35) and
C(36). The structure solutions and refinements were carried out
with the SHELX-97 program package [23].
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
C36H45N9Pd3S3
1019.19
293(2)
0.71069
Monoclinic
P2(1)/c
3. Results and discussion
10.562(5)
20.083(5)
19.459(5)
100.721(5)
4056(2)
4
1.649
1.508
2040
b (Å)
c (Å)
The neutral complex, [Pd{2-CH2-5-MeC6H3C(H)@NN@C(S)N-
HEt}]3 (2), is homoleptic presenting one thiosemicarbazone unit
for each palladium atom. This compound was made from the reac-
tion of 2,5-Me2C6H3C(H)@NN(H)C(@S)NHEt and Pd(O2CMe)2 in
90% yield after recrystallization (Scheme 1). Compound 2 was
unambiguously analyzed by CHN and mass spectrometry and fur-
ther characterized by 1H NMR spectroscopy. In the trimer each thi-
osemicarbazone ligand is terdentate [C,N,S] with the metal bonded
to the 2-methyl group, to the azomethine nitrogen and to the sul-
fur atom, which bridges to an adjacent palladium center; the
fourth coordination site of the metal is occupied by a sulfur from
a neighboring metallated unit. The mass spectrum (FAB) showed
a peak at m/z 1019 for the molecular ion whose isotopic composi-
b (°)
Volume (Å3)
Z
Dcalc (Mg/m3)
Absorption coefficient (mmÀ1
F(000)
)
Crystal size (mm3)
0.30 Â 0.9 Â 0.05
h Range for data collection (°)
Index ranges
Reflections collected
1.47–21.97
À11 6 h 6 10, 0 6 k 6 21, 0 6 l 6 20
23710
Independent reflections
Completeness to h = 21.97°
Absorption correction
Refinement method
4953 [Rint = 0.0915]
100.0%
Multi-scan
Full-matrix least-squares on F2
4953/0/450
Data/restraints/parameters
tion is in agreement with
a trinuclear complex of formula
Goodness-of-fit on F2
1.096
C36H45N9Pd3S3, and there is no evidence of tetramers or higher or-
der aggregates. The main features in the IR spectrum are: absence
of the m(N–H) and m(C@S) stretches, the latter in accordance with
Final R indices [I > 2
R indices (all data)
r
(I)]
R1 = 0.0454, wR2 = 0.0829
R1 = 0.1186, wR2 = 0.1171
0.614 and À0.732
Largest difference in peak and hole (e Å–3
)
loss of the double-bond character upon deprotonation of the NH
group, which is reflected in the lengthening of the C–S bond in
the structure (vide infra); shift of the m(C@N) band to lower wave-
Fig. 2 reveals that the highly symmetric Pd3S3 core consists of a
six-membered ring of alternating palladium and sulfur atoms,
which may be viewed as two staggered Pd3 and S3 quasi parallel
triangles at 175.45(0.06)°.
numbers upon complex formation [25]. The 1H NMR spectrum is
indicative of a C3-symmetry species in solution with only one set
of signals being observed. The data are in agreement with the pres-
ence of only one methyl group and of the expected methylene
group, consequent upon metallation, per monomeric unit, with a
singlet at d 2.23 (3H, Me) and a doublet of doublets at d 3.10
(2H, CH2) for the non-equivalent methylene protons. Three aro-
matic proton signals at d 6.89s (H6), d 6.93d (H4) and d 7.10d
(H3), evidences non-metallation of the phenyl ring carbon atoms.
The absence of the signal for the NH group in the 1H NMR spectrum
reveals deprotonation as observed in coordination compounds of
these ligands [24], and the HC@N signal is upfield shifted conse-
quent upon Pd–N bond formation [12b].
The remaining two sites of each square-planar palladium coor-
dination sphere are occupied by the alkyl carbon atom from the
methylene group and the nitrogen atom of the C@N unit. As with
the Pd4S4 complexes, this structure depicts the bonding inherent
to thiosemicarbazone palladacycles where each sulfur atom within
the metallacycle bridges to another palladium atom. Each of the
three palladium atoms belongs to two fused six and five-mem-
bered chelate rings: the C,N metallacycle and the N,S-chelate moi-
ety, respectively, as a result of bonding to a tridentate C,N,S ligand
(Fig. 2). The angles between adjoining atoms in the coordination
sphere of each metal atom are close to the theoretical value of
90°; with the most noteworthy strains in the N–Pd–S values, ca.
82°, due to formation of the N,S chelate ring. The Pd–C bond dis-
tances [2.038(11)–2.058(11) Å] are shorter than the expected value
of 2.081 Å [26], but nevertheless somewhat longer than those
found earlier in related tetranuclear palladium structures [cf.
2.021(5) Å [12b], 1.999(9)–2.016(9) Å [10], 1.990(7) Å [12a]. The
C–S [1.775(13)–1.804(12) Å] and the C–Nhydrazynic [1.276(14)–
1.300(15) Å] bond distances are consistent with increased single-
bond and double-bond character, respectively, in the deprotonated
form. The Pd–Sbridging bond lengths [2.316(13)–2.322(4) Å], trans to
nitrogen, are shorter than the Pd–Schelating bonds [2.383(3)–
2.421(3) Å], trans to carbon, reflecting the differing trans influence
of the methylene carbon and nitrogen atoms of the coordinated li-
gand. Contrary to a recent example where the metallated ring is
essentially planar [12b], herein the metallated rings are puckered
with the palladium atoms deviating from the mean planes defined
by the carbon and nitrogen atoms by 0.882–1.064 Å. The Cremer
and Pople puckering parameters [27] are in accordance with a con-
3.1. Crystal structure of complex 2
Crystals of 2 were obtained by slow evaporation from a chloro-
form solution, space group P2(1)/c, with four independent mole-
cules in the asymmetric unit. Crystal data are shown in Table 1
and ORTEP views of the structure are depicted in Figs. 2 and 3.
The result from the X-ray crystallographic study on 2 confirms
the NMR data.
4
5
Me
Me
3
6
2
Me
eq. Pd(AcO)2
MeCOOH
Pd
N
N
S
N
HN
S
NHEt
formation
midway between half–boat and twist-boat
3
[Q = 0.598(13)–0.730(12), h = 115.6(13)–118.2(13), u = 146.9(16)–
149.0(12)]; cf. with the values for the Pd3S3 ring with a chair con-
formation [Q = 1.546(3), h = 5.07(11), u = 280.1(12)]. The palladium
coordination planes at Pd(1), Pd(2) and Pd(3) are essentially planar,
NHEt
1
2
Scheme 1. Formation of the trinuclear cyclometallated Pd(II) cluster 2.

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L. Adrio et al. / Journal of Organometallic Chemistry 694 (2009) 747–751
Fig. 2. The monomeric cyclometallated unit showing bonding of the tridentate [C,N,S] thiosemicarbazone.
Fig. 3. Molecular structure of (2) [Pd{2-CH2-5-MeC6H3C (H)@NN@C(S)NHEt}]3. Thermal ellipsoids drawings are shown at the 30% probability level. Selected bond lengths and
angles: Pd(1)ÀC(1) 2.038(11); Pd(1)ÀN(1) 2.001(11); Pd(1)ÀS(1) 2.403(3); Pd(1)ÀS(2) 2.322(4); S(1)ÀC(10) 1.782(13); N(1)ÀC(9) 1.216(13); N(2)ÀC(10) 1.299(14);
Pd(2)ÀC(13) 2.053(11); Pd(2)ÀN(4) 2.025(10); Pd(2)ÀS(2) 2.421(3); Pd(2)ÀS(3) 2.316(3); S(2)ÀC(22) 1.775(13); N(4)ÀC(21) 1.270(14); N(5)ÀC(22) 1.300(15) Å. Pd(3)ÀC(25)
2.038(12); Pd(3)ÀN(7) 2.051(9); Pd(3)ÀS(1) 2.316(3); Pd(3)ÀS(3) 2.383(3); S(3)ÀC(34) 1.804(12); N(7)ÀC(33) 1.286(14); N(8)ÀC(34) 1.276(14) Å. N(1)–Pd(1)–C(1) 86.4(5);
N(1)–Pd(1)–S(2) 176.5(3); C(1)–Pd(1)–S(2) 96.2(4); N(1)–Pd(1)–S(1) 82.7(3); C(1)–Pd(1)–S(1) 165.9(4); S(2)–Pd(1)–S(1) 94.97(12); N(4)–Pd(2)–C(13) 86.3(4); N(4)–Pd(2)–
S(3) 175.8(3); C(13)–Pd(2)–S(3) 97.3(4); N(4)–Pd(2)–S(2) 81.7(3); C(13)–Pd(2)–S(2) 167.6(3); S(3)–Pd(2)–S(2) 94.64(11); C(25)–Pd(3)–N(7) 85.3(4); C(25)–Pd(3)–S(1)
98.2(4); N(7)–Pd(3)–S(1) 173.8(3); C(25)–Pd(3)–S(3) 166.3(4); N(7)–Pd(3)–S(3) 82.3(3); S(1)–Pd(3)–S(3) 94.70(11).
with rms values of 0.0981, 0.0238 and À0.0881, respectively, with
angles between planes from 81.41(10)° to 87.56(10)°, as opposed
to the parallel/perpendicular arrangement in the tetranuclear moi-
eties. The intramolecular PdÁ Á ÁPd distances range form 3.281(3) to
3.565(3) Å. There are no noteworthy intermolecular hydrogen-
bonding contacts.
the six-membered Pd3S3 ring and the approximate C3-symmetry in
the solid state [cf. S4-symmetry in Fig. 1b]. This perspective shows
a rather deep intramolecular cavity enclosed within the three met-
allated rings and the three phenyls rings. The distance from the
S(1)–S(2)–S(3) plane to the C(5)–C(18)–C(19) plane is 6.626 Å
Fig. 4. The cavity is somewhat narrower at the phenyl end, with
distances between the ring centers estimated from 4.849 to
5.379 Å, and angles between planes from 55.78° to 66.16°.
Fig. 3 shows a second view of 2, namely along the axis perpen-
dicular to the plane formed by the three palladium atoms; showing

L. Adrio et al. / Journal of Organometallic Chemistry 694 (2009) 747–751
751
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We have produced the first example of a trinuclear homoleptic
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solution and in the solid state. Thus, palladacycles derived from
thiosemicarbazones that behave as [CNS] pincer-type ligands, are
not only capable of yielding tetranuclear compounds bearing a
Pd4S4 core, but properly designed as in the present case, may gen-
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5. Supplementary material
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5975;
CCDC 278846 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
(j) J. Taquet, O. Siri, P. Braunstein, R. Welter, Inorg. Chem. 43 (2004) 6944.
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Acknowledgments
(b) R. Bosque, J. Granell, J. Sales, M. Font-Bardia, X. Solans, J. Organomet. Chem.
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(c) C. López, S. Pérez, X. Solans, M. Font-Bardia, J. Organomet. Chem. 650
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We thank the Ministerio de Educación y Ciencia (Project
CTQ2006-15621-C02-01/BQU) for financial support. L.A. and
J.M.A. acknowledge fellowships from the Xunta de Galicia (Spain).
(d) J. Albert, J.M. Cadena, A. González, J. Granell, X. Solans, M. Font-Bardia,
Chem. Commun. (2003) 528;
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