Cis-trans Isomerism at Square-Planar MN2S2 Centers (M = Pd, Pt): Crystal Structures of N-Phenyl-N-(2-thiazoyl)thiourea Complexes trans-Pd(S2N3C10H8)2 and cis-Pt(S2N3C10H8)2 and Density Functional Calculations

Article abstract of DOI:10.1007/s10870-013-0429-7

Simple palladium(II) and platinum(II) complexes, ML₂ (1-2), of N-phenyl-(2-thiazoyl)thiourea have been prepared and fully characterized. The structure of Pd(S₂N₃C₁₀H₈) ₂ (1) is monoclinic P2₁ /c, a = 12.510(2), b = 5.6963(6), c = 15.322(2) A, b = 90.07(2)^o and Pt(S₂N ₃C₁₀H₈)₂ (2) is orthorhombic P2 ₁ 2 ₁ 2 ₁, a = 7.3021(5), b = 11.8025(9), c = 25.628(2) A. In both complexes the ligands bind in a chelate fashion through the nitrogen atom of the thiazole ring and the sulfur atom of the thiourea to give six-membered chelate rings. In 1 the metal ion sits on an inversion center and the two ligands adopt a relative trans-disposition of like atoms, while in 2 all atoms are unique and the ligands are in a relative cis-disposition. DFT calculations on the cis and trans isomers based on 1 and 2 reveal that each pair of isomers is isoenergetic in the gas phase. Graphical Abstract: Simple palladium(II) and platinum(II) complexes, M(S₂N ₃C₁₀H₈)₂, of N-Phenyl-(2-thiazoyl) thiourea have been prepared and fully characterized. In both complexes the ligands bind in a chelate fashion through the nitrogen atom of the thiazole ring and the sulfur atom of the thiourea to give six-membered chelate rings. In the palladium complex the metal ion sits on an inversion center and the two ligands adopt a relative trans-disposition of like atoms, while for platinum all atoms are unique and the ligands are in a relative cis-disposition. DFT calculations on the cis and trans isomers based on 1 and 2 reveal that each pair of isomers is isoenergetic in the gas phase. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10870-013-0429-7

J Chem Crystallogr (2013) 43:365–372
DOI 10.1007/s10870-013-0429-7
ORIGINAL PAPER
cis–trans Isomerism at Square-Planar MN2S2 Centers (M 5 Pd,
Pt): Crystal Structures of N-Phenyl-N-(2-thiazoyl)thiourea
Complexes trans-Pd(S2N3C10H8)2 and cis-Pt(S2N3C10H8)2
and Density Functional Calculations
•
Subhi A. Al-Jibori Noor A. Dayaaf Modher Y. Mohammed Kurt Merzweiler
•
•
•
•
•
Christoph Wagner Graeme Hogarth Michael G. Richmond
Received: 23 January 2013 / Accepted: 28 May 2013 / Published online: 13 June 2013
Ó Springer Science+Business Media New York 2013
Abstract Simple palladium(II) and platinum(II) com-
plexes, ML2 (1–2), of N-phenyl-(2-thiazoyl)thiourea have
been prepared and fully characterized. The structure of
Pd(S2N3C10H8)2 (1) is monoclinic P21/c, a = 12.510(2),
Keywords Palladium ꢀ Platinum ꢀ N-Phenyl-N-
(2-thiazoyl)thiourea ꢀ DFT ꢀ Chelate ꢀ Geometric isomerism
b = 5.6963(6), c = 15.322(2) A, b = 90.07(2)o and
Introduction
˚
Pt(S2N3C10H8)2 (2) is orthorhombic P212121, a = 7.3021(5),
˚
b = 11.8025(9), c = 25.628(2) A. In both complexes the
The thiazole ring is found in a wide range of living
organisms [1, 2] and common foodstuffs [3, 4], while
several antibiotics [5, 6] also contain this simple organic
group. Recently thiazole-containing compounds called
epothilones have been identified as a new class of anti-
cancer drugs, working by preventing cancer cells from
dividing by interfering with tubulin [7]. A range of plati-
num-containing complexes as exemplified by cisplatin [8, 9],
carboplatin [9] and oxaliplatin [10] also find widespread
use as anti-cancer drugs, reacting in vivo with DNA
resulting in cross-linking which ultimately triggers pro-
grammed cell death. Cytotoxic palladium(II) complexes
are less well-developed [11–17] but Das and Livingstone
[17] have reported that the palladium(II) complex of S-
methyldithiobarbazate and related derivatives which pos-
sess a Pd(NS)2 core show good cytotoxic activity .
ligands bind in a chelate fashion through the nitrogen atom of
the thiazole ring and the sulfur atom of the thiourea to give
six-membered chelate rings. In 1 the metal ion sits on an
inversion center and the two ligands adopt a relative trans-
disposition of like atoms, while in 2 all atoms are unique and
the ligands are in a relative cis-disposition. DFT calculations
on the cis and trans isomers based on 1 and 2 reveal that each
pair of isomers is isoenergetic in the gas phase.
S. A. Al-Jibori ꢀ N. A. Dayaaf
Department of Chemistry, College of Science, University
of Tikrit, Tikrit, Iraq
We have recently prepared a range of platinum and
palladium complexes with ligands containing simple
heterocyclic substituents [18–21] as such complexes are
anticipated to show interesting biological activity. In
seeking to further develop this approach we were drawn to
an earlier report concerning the coordination chemistry of
N-phenyl-N-(2-thiazole)thiourea (LH) [22], which is easily
prepared from commercially available 2-aminothiazole and
phenylisothiocyanate in a single step, as it is a monoan-
ionic NS-chelating ligand. While a number of complexes of
N-phenyl-N-(2-thiazoyl)thiourea (L) (Chart 1) have been
reported [22–27], a search of the literature revealed that
none had been crystallographically characterized. Thus in
M. Y. Mohammed
Department of Chemistry, College of Education for Women,
University of Tikrit, Tikrit, Iraq
K. Merzweiler ꢀ C. Wagner
Institut fu
¨r Chemie, Martin-Luther-Universita¨t, Halle-
Wittenberg, Kurt-Mothes-Str. 2, 06120 Halle, Germany
G. Hogarth (&)
Department of Chemistry, University College London,
20 Gordon Street, London WC1H 0AJ, UK
e-mail: g.hogarth@ucl.ac.uk
M. G. Richmond
Department of Chemistry, 1155 Union Circle, Box 305070,
Denton, TX 76203, USA
123

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J Chem Crystallogr (2013) 43:365–372
Chart 1 Possible trans and cis
[M(NS)2] isomers of N-phenyl-
(2-thiazoyl) thiourea (NS)
S
N
S
N
N
S
H
S
N
N
M
M
Ph
C
S
N
S
N
trans
cis
N-phenyl-N-(2-thiazoyl)thiourea
order to confirm the assumed coordination mode of this
ligand, namely in a chelate fashion through the nitrogen of
the thiazole ring and the sulfur of the thiourea, we have
prepared and carried out single crystal X-ray diffraction
studies on M(S2N3C10H8)2 (M = Pd, Pt) (1–2). These
show that the relative coordination geometry at the metal
center is a function of the metal type as found for trans-
Pd(S2N3C10H8)2 (1) but cis-Pt(S2N3C10H8)2 (2). As far as
we are aware this is the first crystallographically estab-
lished example of cis–trans isomers (Chart 1) based on a
M(NS)2 core (M = Pd, Pt), and in order to probe this
further we have carried out a series of density functional
calculations on all four possible isomers.
integrating for 1H, Ph), 7.30 (d, J 3.6 Hz, 1H, thiazole),
6.81 (d, 3.6 Hz, 1H, thiazole). Melting point:
168–170 °C.
J
Preparation of M(S2N3C10H8)2 (1, M = Pd, 2, M = Pt)
A solution of N-phenyl-N-(2-thiazole)thiourea (0.15 g,
0.64 mol) in acetone (5 cm3) was added to a warm solution
of Na2PdCl4 (0.10 g, 0.32 mol) in acetone (10 cm3). An
orange yellow solid was formed directly. The mixture was
stirred for 3 h and then chloroform (10 cm3) was added and
the mixture was refluxed for a further 3 h. The yellow
precipitate was collected by filtration, washed with cold
CHCl3, and dried under vacuum to give yellow crystals of
Pd(S2N3C10H8)2 (1) (0.18 g, 97 %). Single crystals suitable
for X-ray diffraction were grown upon slow evaporation of
a DMF and ethanol mixture containing 1. Anal. Calc. for
C20H16N6PdS4: C, 41.5; H, 3.5; N, 14.5. Found: C, 41.6; H,
3.2; N, 14.3 %. IR (KBr): 3300m, 3120w, 3099w, 2923w,
1594m, 1535s, 1490s, 1443s, 1311m, 1173m, 882m, 754m,
Experimental
Materials and Methods
1H NMR spectra were recorded at the Institut fu¨r Chemie,
¨
Martin-Luther-Universitat, Halle-Wittenberg, Germany on
1
a Varian Unity 500 spectrometer with CDCl3, DMF-d7 or
DMSO-d6 as solvent and internal reference. IR spectra were
recorded on a Shimadzu FT-IR 8400 spectrophotometer in
the 400–4,000 cm-1 range using KBr discs. Elemental
539w, 495w cm-1. H NMR (DMF-d7): d 10.47 (s, 1H,
NH), 7.98 (d, J 7.6 Hz, 2H, Ph), 7.52 (m, 3H, 2Ph?thia-
zole), 7.26 (t, J 7.2 Hz, 1H, Ph), 7.23 (d, J 4.0 Hz, 1H,
thiazole). Melting point: 200–202 °C.
¨
analysis was carried out at Martin-Luther-Universitat.
A solution of N-phenyl-N-(2-thiazole)thiourea (0.10 g,
0.43 mol) in warm MeOH (10 cm3) was added to a solu-
tion of K2PtCl4 (0.10 g, 0.206 mol) in water (2 cm3). A
yellow solid was formed directly. The mixture was stirred
for 3 h and then chloroform (10 cm3) was added and the
mixture was refluxed for 8 h. The resulting yellow pre-
cipitate was collected by filtration, washed with water, and
dried under vacuum to give Pt(S2N3C10H8)2 (2) (0.11 g,
70 %). Crystals suitable for X-ray diffraction were grown
upon slow evaporation of an acetone/ethanol mixture
containing 2. Anal. Calc. for C20H16N6PtS4: C, 36.0; H,
3.0; N, 12.6. Found: C, 36.3; H, 3.1; N, 12.9 %. IR (KBr):
3367s, 332m, 3106w, 2924w, 1595m, 1535s, 1480s, 1445s,
1313m, 1168m, 881m, 756m, 546w, 536w, 505w, 476w
Melting points were measured on a Gallenkamp melting
point apparatus and were uncorrected. Na2PdCl4, K2PtCl4,
and other commercial products were used as supplied.
Preparation of N-Phenyl-N-(2-thiazole)thiourea
Phenylisothiocyanate (6 cm3, 0.50 mol) was added to a
solution of 2-aminothiazole (5.00 g, 0.50 mmol) in ben-
zene (20 cm3). The mixture was refluxed for 6 h. The
resulting precipitate was filtered off, dried and recrystal-
lized from benzene–hexane to afford a white crystalline
solid (8.50 g, 70 %). Anal. Calc. for C10H9N3S2: C, 51.0;
H, 3.9; N, 17.7. Found: C, 51.2; H, 3.8; N, 18.0 %. IR
(KBr): 3170m, 3020m, 2945m, 1625s, 1575s, 1504s,
1
cm-1. H NMR (DMSO-d6): d 10.22 (s, 1H, NH), 7.65
1
1375s, 1180s, 754m, 721m, 605w, 435w cm-1. H NMR
(d, J 7.6 Hz, 2H, Ph), 7.30 (dd, J 7.5 Hz, 2H, Ph), 7.06
(t, J 7.6 Hz, 1H, Ph), 7.25 (d, 1H, J 4.0 Hz, thiazole), 7.11
(d, J 4.0 Hz, 1H, thiazole). Melting point: 256–258 °C.
(CDCl3): d 7.56 (bs, 1H, NH), 7.54 (bs, 1H, NH), 7.73,
7.35, 7.33, 7.22, 7.21 (individual multiplets each
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J Chem Crystallogr (2013) 43:365–372
367
using full-matrix least-square routines against F2 with
SHELXL-97 [30]. Non-hydrogen atoms were refined with
anisotropic displacement parameters. The absolute structure
parameter for 2 was -0.013(8).
Table 1 Crystallographic data and structure refinement details for
1–2
Compound
1
2
Empirical formula
C20H16S4H16Pd
575.03
C20H16S4H16Pt
663.73
˚
Formula weight (A)
Temperature (K)
Computational Details
220(2)
220(2)
˚
Wavelength (A)
0.71073
0.71073
The DFT calculations were performed with the hybrid DFT
functional B3LYP, as implemented by the Gaussian 09
program package [31]. This functional utilizes the Becke
three-parameter exchange functional (B3) [32], combined
with the correlation functional of Lee, Yang and Parr
(LYP) [33]. The Pd and Pt atoms were described by
Stuttgart–Dresden effective core potentials (ecp) and a
SDD basis set, while the 6-31G(d0) basis set was employed
for the remaining atoms. The geometries found for each
isomeric pair of compounds based on 1 and 2 were fully
optimized, and the analytical Hessian afforded only posi-
tive eigenvalues for the ground-state minima. The com-
puted frequencies were used to make zero-point and
thermal corrections to the electronic energies. The natural
charges and Wiberg bond indices reported here were
computed using Weinhold’s natural bond orbital (NBO)
program, as executed by Gaussian 09 [34, 35]. The
geometry-optimized structures have been drawn with the
JIMP2 molecular visualization and manipulation program
[36, 37].
Crystal system
Space group
Monoclinic
P21/c
Orthorhombic
P212121
Unit cell dimensions
˚
a (A)
12.510(2)
5.6963(6)
15.322(2)
90
7.3021(5)
11.8025(9)
25.628(2)
90
˚
B (A)
˚
c (A)
a (°)
b (°)
c (°)
90.07(2)
90
90
90
3
˚
Volume (A )
1091.9(3)
2
2208.7(3)
4
Z
Density (calculated)
(Mg/m3)
1.749
1.996
Absorption coefficient
(mm-1
1.254
576
6.752
1280
)
F(000)
Crystal size (mm)
0.38 9 0.07 9 0.02 0.22 9 0.15 9 0.11
h Range for data
collection (°)
Index ranges
2.66–26.13
2.35–26.10
-14 B h C 15
-7 B k C 7
-19 B l C 18
7970
-8 B h C 8
-14 B k C 14
-31 B l C 31
17267
Results and Discussion
Reflections collected
Independent reflections 2127
[Rint
4191
[Rint = 0.0705]
N-phenyl-N-(2-thiazole)thiourea (LH) was prepared by a
modified literature method upon refluxing equimolar
amounts of phenylisothiocyanate and 2-aminothiazole in
benzene; the desired product was isolated as a white
crystalline solid in 70 % yield. Addition of two equivalents
to Na2PdCl4 in acetone, followed by heating to drive the
reaction to completion, gave Pd(S2N3C10H8)2 (1) as a
yellow solid in 97 % yield after recrystallization from a
mixture of DMF and ethanol. Likewise addition of a slight
excess of N-phenyl-N-(2-thiazole)thiourea in methanol to
Na2PtCl4 in water, followed by heating for 8 h, gave
Pt(S2N3C10H8)2 (2) also as a yellow solid in 70 % yield
(Scheme 1). In this case crystallization was carried out
using a binary solvent system composed of acetone and
ethanol. While N-phenyl-N-(2-thiazole)thiourea and
Pd(S2N3C10H8)2 (1) [22] have previously been reported,
preparation of Pt(S2N3C10H8)2 (2) appears novel.
]
[Rint = 0.0675]
Max. and min.
transmission
0.9754 and 0.6472
0.5238 and 0.3181
4191/0/280
Data/restraints/
parameters
2127/1/146
Goodness-of-fit on F2
0.967
0.894
Final R indices
[I [ 2r(I)]
R1 = 0.0403,
wR2 = 0.0817
R1 = 0.0699,
wR2 = 0.0900
1.034 and -0.396
R1 = 0.0275,
wR2 = 0.0452
R1 = 0.0392,
wR2 = 0.0475
1.059 and -0.618
R indices (all data)
Largest diff. peak and
-3
˚
hole (e. A
)
Crystal Structure Determination
Table 1 gives the crystallographic data and collection param-
eters for 1 and 2. Intensity data were collected on a STOE-
˚
Characterization of both products was straightforward
based on 1H NMR and IR spectroscopy and elemental
IPDS diffractometer with Mo-Ka radiation (k = 0.7103 A,
1
analysis. The H NMR spectra are most informative, all
graphite monochromater). Absorption corrections were made
using the IPDS software package [28]. All structures were
solved by direct methods with SHELX-97 [29] and refined
showing the expected resonances for the phenyl group. For
N-phenyl-N-(2-thiazoyl)thiourea (LH) there are also two
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J Chem Crystallogr (2013) 43:365–372
Ph
C
NH
S
S
S
N
N
S
N
S
C
N
Na2PdCl4
K2PtCl4
2
Ph
Pt
N
S
N
N
Pd
N
N
N
S
H
H
C
S
S
C
N
C
S
NH
HN
HN
Ph
Ph
Ph
2
1
Scheme 1 Reactions of N-phenyl-(2-thiazoyl) thiourea with [MCl4]2- (M = Pd, Pt)
and 7.23 (3JHH 4.0 Hz), while for 2 they are shifted up-
field, slightly appearing at d 7.25 and 7.11 (3JHH 4.0 Hz).
The molecular structures of 1 and 2 are shown in Figs. 1
and 2, respectively, with selected bond lengths and angles
given in the captions. The palladium complex 1 crystallizes
in the monoclinic space group P21/c. The unique angle of
90.06(1)° first suggested orthorhombic symmetry, how-
ever, the merging R-value for the orthorhombic Laue group
mmm was ca. 50 % indicating a monoclinic crystal system
The molecule is centrosymmetric, the ligands thus adopting
a relative trans-disposition (of like atoms) [N1–Pd–N1A
180.0(3), S1–Pd–S1A 180.0(7)°]. The platinum complex 2
crystallizes in the orthorhombic space group P212121 and
all atoms are unique. Here the ligands adopt a relative cis-
disposition [S1–Pt–S3 87.77(8), N1–Pt–N4 92.8(2)°].
The structures confirm the supposition of Shoukry et al.
[22] based on spectroscopic data that the N-phenyl-N-(2-
thiazoyl)thiourea ligands bind to metal centers in a chelate
fashion through ring nitrogen and thiocarbonyl sulfur
atoms. This results from the selective deprotonation of the
thiourea at the thiazole ring bound nitrogen atom. As seen
from the metal-centered bond angles in both complexes,
the metal is in a square-planar coordination environment.
The bite angles of the ligands in both complexes are quite
similar and close to 90°, that in 1 of 91.7(1)° being
somewhat larger than those of 89.6(1) and 90.0(1)° in 2.
Metal-nitrogen bond lengths do not vary significantly
between the two complexes [Pd–N1 2.038(4), Pt–N1
Fig. 1 Two ORTEP views of the molecular structure of 1 (hydrogen
atoms omitted for clarity). Ellipsoids are drawn at 50 % probability
o
level. Selected bond lengths (A) and angles ( ); Pd–N1 2.038(4), Pd–
˚
S1 2.312(1), N1–Pd–S1 88.2(1), N1–Pd–N1A 180.0(3), S1–Pd–S1A
180.0(7), N1–Pd–S1 91.8(1), N1–C1 1.323(6), N1–C2 1.364(6), N2–
C1 1.364(6), N2–C4 1.304(6), N3–C4 1.367(6), S1–C4 1.744(5), S2–
C1 1.722(5), S2–C3 1.708(5), C2–C3 1.333(7), Pd–S1–C4 104.1(2),
Pd–N1–C1 125.9(3), C1–N2–C4 122.3(4), N2–C4–N3 118.7(5)
˚
2.042(6), Pt–N4 2.033(6) A] but there is a slight variation
in the metal-sulfur distances [Pd–S1 2.3123(1), Pt–S1
˚
2.272(2), Pt–S3 2.285(2) A]. Bond lengths and angles
within the chelate rings are fully consistent with the reso-
nance structures shown in Scheme 1 (vide infra). Most
important in this respect are the bond lengths to the central
ring nitrogen atoms [N2 and N5], for example as shown in
broad singlets at d 7.56 and 7.54 assigned to the inequiv-
alent amine protons and a pair of doublets at d 7.30 and
6.81 (3JHH 3.6 Hz) associated with the vicinal protons on
the thiazole ring. For both complexes, a single broad band
associated with the remaining amine proton is seen at d
10.47 and 10.23 for 1 and 2, respectively. The patterns of
the phenyl and thiazole protons in 1 and 2 are similar. For 1
the thiazole protons appeared as a pair of doublets at d 7.52
˚
1 [N2–C1 1.364(6), N2–C4 1.304(6) A], which are clearly
indicative of single and double bonds, respectively. These
bond-length trends rule out any significant contributions
from zwitterionic resonance structures associated with
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J Chem Crystallogr (2013) 43:365–372
369
Fig. 3 Optimized structures for the ground-state minima of 1 (top)
and 2 (bottom)
Since the pioneering work of Chatt and Wilkins on cis–
trans isomerism at a square-planar platinum(II) center in
the 1950s [38], a great deal of subsequent work has been
carried out in this area due to the importance of this
isomerism in catalysis and anti-cancer activity. Energy
differences between isomers are generally found to be
small with steric effects, electrostatic interactions, differ-
ences in bond energies, and solvation being key determi-
nants [39]. Harvey and co-workers have probed the relative
stabilities of isomers of MX2L2 complexes using density
functional theory (DFT) [40]. Treating the MX2L2 core as
five point charges interacting in a vacuum allows an elec-
trostatic energy difference (EED) between cis and trans
isomers to be determined and solvation effects can be
added to this. Interesting in CH2Cl2 the free energy of the
cis isomer of PtCl2(PMe3)2 is predicted to be 4.5 kcal
mol-1 more stable than the trans-derivative, while for
palladium the trans isomer that is calculated to lie
0.6 kcal mol-1 lower in energy than cis-PdCl2(PMe3)2.
The preference for the trans isomer in the palladium
compound is attributed to the greater importance of ionic
bonding at the slightly harder Pd(II) metal. However, the
present situation is more complex and a key finding of this
work is that the bonding in complexes containing two
N-phenyl-N-(2-thiazoyl)thiourea ligands has both ionic and
covalent character, the latter resulting from r-donation
from the ligands into a metal-based sd-hydrid orbital
through a three-center, three-orbital, four-electron interac-
tion [40]. Consequently where two mutually trans ligands
are not identical they compete as donors, and softer ligands
higher in the trans-influence scale donate electrons more
effectively, leading to an antisymbiotic effect whereby
stronger ligands prefer to bind trans to weaker ligands [41].
Fig. 2 Two ORTEP views of the molecular structure of 2 (hydrogen
atoms omitted for clarity). Ellipsoids are drawn at 50 % probability
o
level. Selected bond lengths (A) and angles ( ); Pt–S1 2.272(2), Pt–S3
˚
2.285(2), Pt–N1 2.042(6), Pt–N4 2.033(6), S1–Pt–S3 87.77(8),
N1–Pt–N4 92.8(2), N1–C1 1.345(8), N1–C2 1.39(1), N2–C1
1.365(9), N2–C4 1.30(1), N3–C4 1.356(9), S1–C4 1.761(6), S2–C1
1.722(8), S2–C3 1.713(7), C2–C3 1.34(1), N4–C11 1.347(8), N4–
C12 1.390(9), N5–C11 1.373(8), N5–C14 1.294(8), N6–C14
1.357(8), S3–C14 1.769(7), S4–C11 1.710(7), S4–C13 1.707(9),
C12–C13 1.32(1), Pt–S1–C4 104.4(2), Pt–N1–C1 126.1(5), C1–N2–
C4 122.1(6), N2–C4–N3 119.7(6), Pt–S3–C14 103.0(2), Pt–N4–C11
126.3(4), C11–N5–C14 123.7(6), N5–C14–N6 118.6(6)
delocalisation of the lone-pair of electrons from the exo-
cyclic nitrogen atom into the chelate ring.
While the MN2S2 subunit lies approximately in a plane,
the chelate rings are far from planar. In both complexes the
SN2C2 subunit is approximately planar, the largest devia-
˚
tions from planarity being of the order of 0.164 A (C1) in 1
˚
and 0.176 (C1)/0.164 A(C11) in 2, with the metal ions
˚
lying around 0.90–0.94 A out of this plane. This then leads
to an ‘‘up-down’’ orientation of the two ligands (as seen
best by the phenyl rings) out of the MN2S2 plane. In each
structure there are intermolecular interactions primarily
between sulfur and hydrogen atoms. In 1 the shortest of
˚
these is between S1 and H3 of 2.681 A, while the same
sulfur atom also displays a longer interaction with H4
˚
[S1_H4 2.951 A]. In 2 the shortest intermolecular inter-
˚
action is between Pt and H4 of 2.825 A, while there are
also a number of sulfur-hydrogen interactions [S1_H3
˚
2.976, S2_H6 2.903, S3_H11 2.976 A]. None of these
appear to influence significantly the overall molecular
structure or isomer type.
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J Chem Crystallogr (2013) 43:365–372
Fig. 4 Wiberg bond indices for
compounds 1 and 2. Due to the
two-fold symmetry inherent in
these compounds, only one half
of the bond indices are
1.69
1.15
S
1.69
1.15
S
S
displayed
NH
N
1.15
1.17
1.15
1.17
N
S
N
S
N
N
0.45
0.43
1.26
1.36
1.15
1.25
1.37
1.16
Pt
N
N
Pd
N
1.41
1.20
0.77
1.42
1.19
0.66
S
S
HN
NH
HN
S
1.04
1.05
Thus based on a predominance of covalent bonding, sul-
phur-nitrogen chelate ligands would be expected to adopt a
relative trans arrangement as sulphur is much softer than
nitrogen. Thus it appears that for palladium, the ionic
component of the bonding outweighs the covalent contri-
bution and a trans geometry is favoured, while for platinum
the converse is true and covalency dominates the metal–
ligand bonding, affording the observed cis structure.
that the crystallographic characterization of trans-
Pd(S2N3C10H8)2 (1) and cis-Pt(S2N3C10H8)2 (2) may simply
be a solid-state effect, with both isomers co-existing in
solution as has been found to be the case for related amino-
phenolate complexes [42]. Careful inspection of the 1H NMR
spectra of both 1 and 2 did not, however, reveal the presence
of a second isomer and thus we conclude that if this is the
case the second isomer as only an extremely minor
component.
In order to probe this further we have carried out DFT
calculations on all four possible isomeric arrangements of the
N-phenyl-N-(2-thiazoyl)thiourea complexes M(S2N3C10H8)2.
The DFT computed geometries for both observed compounds
closely match the X-ray diffraction data. Figure 3 shows the
DFT-optimized structures for trans-1 and cis-2 and Fig. 4
shows the Wiberg bond indices for 1 and 2, where the bond-
length trends are in excellent agreement with the principal
resonance contributor of each compound. Here theory is again
fully consistent with the resonance structures shown in
Scheme 1 and a natural bond order (NBO) analysis provides
additional support for the bonding within each diffraction
structure. The corresponding isomer of each of these com-
plexes (cis-1 and trans-2) was also investigated computa-
tionally, and the energy difference between the respective
pairs of isomers was found to be isoenergtic
(DG\1.0 kcal mol-1). Given the small calculated energy
differences between the cis and trans isomers we considered
There are crystallographically characterized examples of
Pd(NS)2 complexes in both trans [43–45] and cis [45–52]
arrangements, although the latter is more prevalent. Thus
the preferential formation of trans-Pd(S2N3C10H8)2 (1) is
somewhat unexpected. Of the three examples we found in
the literature containing a trans-Pd(NS)2 core, two [43, 44]
might be rationalised on the basis of the minimisation of
adverse steric effects between ligand substituents, but in
the third [45] no such effects can exist as both nitrogen and
sulphur atoms bear no other substituents. Interestingly,
Yoda et al. [52] have reported crystal structures of three
palladium 1,1-dimethyl-3-(2-thiazolyl)thiourea complexes
all found in the cis form. There are fewer examples of
crystallographically characterized complexes containing a
Pt(NS)2 core and the majority contain a trans arrangement
[42, 53–58]. The cis-Pt(NS)2 core geometry has been
shown [42] or postulated [58] to exist in solution but the
Chart 2 A comparison of the
metal binding of N-phenyl-(2-
thiazoyl) thiourea and methyl-2-
amino-cyclopentenedithi
ocarboxylate
H
S
S
N
N
Me
C
S
Ph
C
S
N
N
H
M
M
N-phenyl-N-(2-thiazoyl)thiourea
methyl-2-amino-cyclopentenedithiocarboxylate
123

J Chem Crystallogr (2013) 43:365–372
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