Synthesis, characterisation and coordination chemistry of novel chiral N,N-dialkyl-N′-menthyloxycarbonylthioureas. Crystal and molecular structures of N,Ndiethyl-Ar'-(-)-(3/?)-menthyloxycarbonylthiourea and m-(S,S)-[Pt(L)CI(DMSO)] [where HL = N-(+)(3JR)-menthyloxycarbonyl-A'-morpholinothiourea or W-benzoylAr',W-diethylthiourea]

Article abstract of DOI:10.1039/b007589m

The novel ligands N,N-diethyl-N-(-)-(3fl)-menthyloxycarbonylthiourea (HL3), N,N-dklhyl-N'-(+)-(3R)-menthyoxycarbonylthiourea (HL4), AH-HS/-menthyloxycarbonyl-N'-morpholinothiourea (HL5) and N-(+)-(3R)menthyloxycarbonyl-W-morpholinothiourea (HL6) have been prepared and characterised. The molecular structure of HL3 has been confirmed by X-ray crystallography. The reaction of these Ar,Ar-disubstituted-Ar'-menthyloxycarbonylthiourea ligands with m-[PtQ2(DMSO)J in the presence of sodium acetate yields geometric isomers of the resultant (Pt(L)Cl(DMSO)] complexes, that is the DMSO is sulfur bonded to the platinum in either a cw-(S,S) or trans-(S,S) arrangement with respect to the sulfur donor atom of the chelated ligand. This is in contrast to the complexation reaction of c-[PtCI2(DMSO)2] with A'-benzoyl-AN'-diethylthiourea (HL1) or W-benzoyl-A morpholinothiourea (HL2) which yields only one [Pt(L)Cl(DMSO)] complex in which the DMSO is in a ra-(S,S) arrangement with respect to the sulfur donor atom of the chelated ligand. The molecular structures of c/s-(S,S)(Pt(L)Cl(DMSO)], where L = (L')- or (L6)-, have been determined by X-ray crystallography. The difference in the coordination chemistry of the acylthiourea and alkoxycarbonylthiourea ligands has been examined further by treating the [Pt(L)Cl(DMSO)] complexes with PPh3 to give the corresponding mono- and bis-(phosphine) complexes, [Pt(L)Cl(PPhj)] and [Pt(L)(PPh3)2] The 3IP NMR studies of these complexes reveal that the alkoxycarbonylthiourea ligands bind less strongly than the acylthiourea ligands, which is consistent with the crystallographic studies. The weaker binding properties of the alkoxycarbonylthiourea ligands might be a possible explanation for the observed geometric isomerisation of the complexes and that the mechanism could involve a chelate ring opening step. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/b007589m

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Synthesis, characterisation and coordination chemistry of novel
chiral N,N-dialkyl-NЈ-menthyloxycarbonylthioureas. Crystal and
molecularstructuresofN,N-diethyl-NЈ-(؊)-(3R)-menthyloxycarbonyl-
thiourea and cis-(S,S)-[Pt(L)Cl(DMSO)] [where HL ؍ N-(؉)-
(3R)-menthyloxycarbonyl-NЈ-morpholinothiourea or N-benzoyl-
NЈ,NЈ-diethylthiourea]
Cheryl Sacht,*a Michael S. Datt,a Stefanus Ottob and Andreas Roodt*b
a Department of Chemistry, Rhodes University, PO Box 94, Grahamstown, 6140, South Africa.
E-mail: C.Sacht@ru.ac.za
b Department of Chemistry, University of the Free State, Bloemfontein, 9300, South Africa
Received 19th September 2000, Accepted 27th October 2000
First published as an Advance Article on the web 4th December 2000
The novel ligands N,N-diethyl-NЈ-(Ϫ)-(3R)-menthyloxycarbonylthiourea (HL3), N,N-diethyl-NЈ-(ϩ)-(3R)-menthyl-
oxycarbonylthiourea (HL4), N-(Ϫ)-(3R)-menthyloxycarbonyl-NЈ-morpholinothiourea (HL5) and N-(ϩ)-(3R)-
menthyloxycarbonyl-NЈ-morpholinothiourea (HL6) have been prepared and characterised. The molecular structure
of HL3 has been confirmed by X-ray crystallography. The reaction of these N,N-disubstituted-NЈ-menthyloxy-
carbonylthiourea ligands with cis-[PtCl2(DMSO)2] in the presence of sodium acetate yields geometric isomers of
the resultant [Pt(L)Cl(DMSO)] complexes, that is the DMSO is sulfur bonded to the platinum in either a cis-(S,S)
or trans-(S,S) arrangement with respect to the sulfur donor atom of the chelated ligand. This is in contrast to the
complexation reaction of cis-[PtCl2(DMSO)2] with N-benzoyl-NЈ,NЈ-diethylthiourea (HL1) or N-benzoyl-NЈ-
morpholinothiourea (HL2) which yields only one [Pt(L)Cl(DMSO)] complex in which the DMSO is in a cis-(S,S)
arrangement with respect to the sulfur donor atom of the chelated ligand. The molecular structures of cis-(S,S)-
[Pt(L)Cl(DMSO)], where L = (L1)Ϫ or (L6)Ϫ, have been determined by X-ray crystallography. The difference in the
coordination chemistry of the acylthiourea and alkoxycarbonylthiourea ligands has been examined further by
treating the [Pt(L)Cl(DMSO)] complexes with PPh3 to give the corresponding mono- and bis-(phosphine) complexes,
[Pt(L)Cl(PPh3)] and [Pt(L)(PPh3)2]ϩ. The 31P NMR studies of these complexes reveal that the alkoxycarbonylthiourea
ligands bind less strongly than the acylthiourea ligands, which is consistent with the crystallographic studies. The
weaker binding properties of the alkoxycarbonylthiourea ligands might be a possible explanation for the observed
geometric isomerisation of the complexes and that the mechanism could involve a chelate ring opening step.
alkoxycarbonylthiourea ligand systems stems from the fact
that we could prepare chiral ligands from optically pure alkoxy-
Introduction
We are currently investigating the design and synthesis of a
novel series of platinum() complexes, of the type [Pt(L)-
Cl(RRЈSO)] (where L = O,S chelate), for potential use as
chemotherapeutic agents.1,2 Our approach has been to prepare
complexes that are not structurally analogous to cisplatin [Pt-
(NH3)2Cl2] in an attempt to develop new platinum()-based
antitumour drugs that will hopefully overcome its major draw-
backs. We have, to date, focused on the use of the oxygen–sulfur
acylthiourea, RCONHCSN(RЈ)2, chelate ligand systems.1,2
Interest in these ligands was prompted by their versatility in
that one can readily vary the functional groups R and RЈ to give
a wide range of different ligand systems with different physical
and chemical properties. We have since extended these studies
to include the alkoxycarbonylthioureas, ROCONHCSN(RЈ)2,
the only difference between the two ligand systems being the
O atom between the R and carbonyl groups. While the co-
ordination chemistry of acylthiourea ligands with platinum()
is fairly well established,3 the corresponding complexation
behaviour of alkoxycarbonylthiourea ligands has, to the best of
our knowledge, hardly been explored.
carbonyl chloride precursors, such as menthyloxycarbonyl
chlorides. Interest in preparing chiral platinum() metal-based
drugs is motivated by the fact that enzymes and other natural
binding sites recognise substrates with a particular chirality to
generate a variety of biological functions. Although chirality
is not a prerequisite for biological activity, when bioactive
molecules contain a stereogenic centre large differences are
usually observed in the activities of the individual enantiomers.
This is a general phenomenon and applies to all bioactive
substances, including metal-based drugs. For example, dif-
ferent activities have been observed for isomeric platinum()
complexes, and this difference could be ascribed to the chiral
recognition between the active platinum() species and their
final target, DNA.4,5
The anionic ligands or leaving groups in the platinum
complexes also have considerable influence on antitumour
activities, and are believed to play important roles in drug
metabolism. Unsymmetrical sulfoxides are chiral, and therefore
special regard has to be paid not only to the chiralities of
the carrier ligands but also to those of the sulfoxide leaving
groups. The effect of the chirality of the leaving group has
recently been reported by Farrell et al.; they noted that for
a series of complexes of the type [Pt(diamine)Cl(RRЈSO)]ϩ
Since platinum() forms 4-coordinate square planar com-
plexes it is only possible to prepare chiral platinum()
complexes using chiral ligands. Hence further interest in the
DOI: 10.1039/b007589m
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prepared using a modified method of Douglass and Dains.7
To dry potassium thiocyanate (9.53 mmol) in dry acetone
(50 ml) was added dropwise an equimolar amount of menthyl
chloroformate (9.57 mmol). The solution was refluxed under
nitrogen for 2 hours. The amine (9.58 mmol) was then added
dropwise to the cooled reaction mixture and the solution
refluxed for 2 hours then cooled to room temperature. KCl was
removed by filtration and the filtrate concentrated to half
the volume under reduced pressure. Water (100 ml) was added
whereupon a yellow oil formed. The aqueous layer was
decanted and the oil extracted into diethyl ether (40 ml). The
aqueous layer was extracted twice with ether (40 ml) and the
ether fractions were combined, dried over anhydrous MgSO4
and evaporated to dryness under reduced pressure to yield a
yellow oil. The oil was dissolved in a small volume of ethanol
and the solution placed in a freezer. Colourless crystals were
collected by filtration, washed with water, ethanol and dried
in vacuo over silica gel. The ligands were fully characterised
and the experimental data are summarised in Tables 1 and 2.
Recrystallisation of HL3 1 from EtOH yielded crystals suitable
for X-ray analysis.
Fig. 1 Structural formulae of ligands. The atom numbering scheme
adopted for NMR assignments is also given.
enhanced biological activity was observed when RRЈSO = (S)-
methyl (p-tolyl) sulfoxide (S-MTSO).5
To this end we have synthesized the chiral carrier ligands,
N,N-diethyl-NЈ-(Ϫ)-(3R)-menthyloxycarbonylthiourea, N,N-
diethyl-NЈ-(ϩ)-(3R)-menthyloxycarbonylthiourea, N-(Ϫ)-(3R)-
menthyloxycarbonyl-NЈ-morpholinothiourea and N-(ϩ)-(3R)-
menthyloxycarbonyl-NЈ-morpholinothiourea (Fig. 1) and their
corresponding platinum() complexes, [Pt(L)Cl(RRЈSO)],
where RRЈSO = DMSO or S-MTSO. To the best of our
knowledge the synthesis and characterisation of these chiral
ligands and their corresponding platinum() complexes are
being reported for the first time. A comparison of the co-
ordination chemistry of these novel ligands to that of the
N,N-disubstituted-NЈ-benzoylthioureas is also presented.
Platinum sulfoxide complexes
[Pt(L3)Cl(DMSO)] and [Pt(L4)Cl(DMSO)]. A solution of
the respective ligand (0.48 mmol) in acetonitrile (10 ml) was
added dropwise to a solution of cis-[PtCl2(DMSO)2] (0.48
mmol) in acetonitrile–DMSO (6 ml, 1:1 v/v), followed by
sodium acetate (0.74 mmol) in water (1.5 ml), and stirred for
3 days at room temperature. Water (100 ml) was then added
to the bright yellow solution whereupon a yellow oil formed.
The mixture was placed in a refrigerator (4 ЊC) overnight. The
yellow oil was extracted into ether (40 ml), dried over MgSO4
and evaporated to dryness under reduced pressure to yield a
yellow precipitate. The complexes were fully characterised and
the experimental data are summarised in Tables 5 and 6.
Experimental
[Pt(L5)Cl(DMSO)] and [Pt(L6)Cl(DMSO)] 2. A solution of
the respective ligand (0.47 mmol) in acetonitrile (10 ml) was
added dropwise to a solution of cis-[PtCl2(DMSO)2] (0.47
mmol) in acetonitrile–DMSO (6 ml, 1:1 v/v), followed by
sodium acetate (0.72 mmol) in water (1.5 ml), and stirred for
3 days at room temperature. After a few hours a yellow precipi-
tate was observed. Water (100 ml) was then added and the
mixture placed in a refrigerator (4 ЊC) overnight. The yellow
precipitate was collected by filtration, washed with water,
ethanol, and dried in vacuo over silica gel. The complexes were
fully characterised and the experimental data are given in
Tables 5 and 6. Slow evaporation of a solution of [Pt(L6)-
Cl(DMSO)] 2 in benzene–hexane yielded crystals suitable for
X-ray crystallography.
Materials and physical methods
Diethylamine, morpholine and acetone were dried over 4 Å
molecular sieve and freshly distilled before use. All other
reagents were commercial grade and used as received. The com-
plexes cis-[PtCl2(DMSO)2] and cis-[PtCl2(S-MTSO)2] were pre-
pared according to a literature procedure.6 IR spectra were
obtained as KBr disks on a Perkin-Elmer FT-IR Spectrum
2000, between 4000 and 250 cmϪ1, 1H, 13C, 31P and 195Pt NMR
spectra on a Bruker 400AMX Spectrometer at 30 1 ЊC. All
samples were prepared using deuteriated solvents purchased
from Aldrich Chemical Company, and 5 mm NMR tubes were
used throughout. Chemical shifts for 1H NMR are reported in
parts per million (ppm) relative to the centre line of the solvent
proton resonance of known shifts relative to TMS, those for
31P NMR relative to an external standard (85% H3PO4) and
195Pt shifts relative to the external standard H2PtCl6 [500 mg
in 1 ml 30% (v/v) D2O–HCl (1 M)]. Melting points were
determined using a Reichert hot-stage microscope and are
uncorrected. Elemental analyses were carried out at the micro-
analytical unit at the University of Cape Town, South Africa.
Thin-layer chromatography was performed on silica sheets
60F254 (Merck, Darmstadt). The optical rotations of chiral
ligands were determined using a Perkin-Elmer 141 Polarimeter,
with dry acetone as solvent. High performance liquid chroma-
tography was performed with a Spectro-Physics P100 iso-
cratic pump, Whatman Partisil 10 column (NP), Waters 401
Differential Refractometer and Rikadenki chart recorder
(5 mm minϪ1; 10 mV).
[Pt(L3)Cl(S-MTSO)]. A solution of HL3 (0.115 g, 0.364
mmol) in acetonitrile (10 ml) was added dropwise to a solution
of cis-[PtCl2(S-MTSO)2] (0.209 g, 0.364 mmol) in acetonitrile
(6 ml), followed by sodium acetate (0.045 g, 0.546 mmol) in
water (1.5 ml), and stirred for 3 days at room temperature. Water
(100 ml) was then added to the orange solution whereupon a
fine yellow precipitate formed. This mixture was placed in a
refrigerator (4 ЊC) for a week. The precipitate was extracted
into CHCl3 (40 ml), dried over MgSO4 and evaporated to dry-
ness under reduced pressure to yield a brown oil. The oil was
dissolved in ethanol and placed in a freezer for several days. The
resultant yellow precipitate was collected by filtration, washed
with water, ethanol and dried in vacuo over silica gel. The com-
plex was fully characterised and the experimental data are
summarised in Tables 5 and 6.
Preparation of ligands
The synthesis and full characterisation of the ligands HL1
cis-(S,S)-[Pt(L1)Cl(DMSO)]
(DMSO)]. The preparation and full characterisation of these
3
and cis-(S,S)-[Pt(L2)Cl-
and HL2 have recently been reported.1,2 The ligands HL3–6 were
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Table 1 Analytical data for N,N-diethyl-NЈ-menthyloxycarbonylthiourea and N-menthyloxycarbonyl-NЈ-morpholinothiourea ligands
IR
δ(NH) ϩ
ν(C᎐O) ϩ
δ(NH)
᎐
Ligand
mp/ЊC
Yield (%)
αD25/Њ
Analytical data (% C/H/N/S)a
ν(NH)
ν(C᎐O)
᎐
HL3
HL4
HL5
HL6
88–90
89–90
128–130
127–129
26
32
42
42
Ϫ42.4
ϩ36.5
Ϫ45.8
ϩ45.4
60.6; 10.3; 9.0; 9.8 (61.1; 9.6; 8.9; 10.2)
61.0; 10.3; 9.1; 10.2 (61.1; 9.6; 8.9; 10.2)
58.9; 8.8; 8.55; 9.3 (58.5; 8.6; 8.5; 9.8)
59.0; 8.9; 8.4; 9.1 (58.5; 8.6; 8.5; 9.8)
3179
3179
3209
3209
1536
1536
1540
1540
1737
1737
1737
1737
a Required values are given in parentheses.
Table 2 Selected 1H NMR data for the N,N-diethyl-NЈ-menthyloxycarbonylthiourea and N-menthyloxycarbonyl-NЈ-morpholinothiourea ligands
Com-
pound
H1
H2
H3
H4
H5
H6
H7
H8
H9
H10
Hα
Hβ
HNH
HL3
HL4
HL5
HL6
4.575
4.575
4.557
4.557
1.341
1.341
1.349
1.349
1.676
1.030
1.676
1.030
1.671
1.024
1.671
1.024
1.676
0.854
1.676
0.854
1.671
0.837
1.671
0.837
1.466
1.466
1.460
1.460
2.038
1.030
2.038
1.030
2.002
1.024
2.002
1.024
0.906
0.906
0.894
0.894
1.898
1.898
1.868
1.868
0.783
0.783
0.760
0.760
0.894
0.894
0.894
0.894
3.639
3.639
3.785
3.785
1.274
1.274
3.785
3.785
6.998
6.998
7.346
7.346
complexes have recently been reported.1,2 Recrystallisation of
3 from chloroform–hexane yielded crystals suitable for X-ray
crystallography.
Phosphine studies
The phosphine studies were carried out by adding stoichio-
metric amounts of triphenylphosphine to a solution of the
[Pt(L)Cl(DMSO)] (where L = L1, L2, L4, or L6) complex (5–10
mg) in CDCl3 in an NMR tube. The mixture was shaken
vigorously and then the 1H and 31P NMR spectra were
recorded. The 31P NMR data for the [Pt(L)Cl(PPh3)] and
[Pt(L)(PPh3)2]ϩ complexes are given in Tables 9 and 10,
respectively.
Crystal structure determinations
The three dimensional intensity data were collected on a Syntex
P-1 for HL3 1 and those for cis-(S,S)-[Pt(L6)Cl(DMSO)] 2
and cis-(S,S)-[Pt(L1)Cl(DMSO)] 3 on a Bruker SMART CCD
diffractometer. All reflections were corrected for Lorentz
and polarisation effects. Data reduction was performed using
PROFIT8 for 1 and SHELXTL9 for 2, while absorption
correctionsfor2and3weredoneusingSADABS.10 Thestructure
of HL3 was solved by the direct method and those of 2 and 3
by Patterson and successive Fourier synthesis (SHELXS 9711
and SHELXL 9712). The crystal of HL3 was covered with
Canada Balsam after signs of decay were detected, hence a data
reduction was done to eliminate weak reflections and identical
reflections were merged. All hydrogen atoms were included in
calculated positions with fixed isotropic thermal parameters
except for the amine hydrogens of HL3 that were detected from
the Fourier map. All the relevant structural details and refine-
ment parameters are given in Table 3. The molecular graphics
were produced using ORTEP.13
Fig. 2 Molecular structure showing the atom numbering scheme and
displacement ellipsoids (30% probability) for molecule 1 of N,N-
diethyl-NЈ-(Ϫ)-(3R)-menthyloxycarbonylthiourea 1. Hydrogen atoms
have been omitted for clarity.
method of Douglass and Dains.7 The synthesis involves reac-
tion of menthyl chloroformate with potassium thiocyanate
followed by condensation with the appropriate secondary
amine. The ligands were characterised using infrared (IR) and
1H NMR spectroscopy, polarimetry and elemental analysis.
1
The analytical data, principal infrared bands and H NMR
data are summarised in Tables 1 and 2. The assignments of
1
1
the H NMR spectra were accomplished using 2-D H and
13C-{1H} NMR and HMQC (heteronuclear multiple quantum
correlation) experiments. The elemental analyses, IR and H
1
NMR data are consistent with the proposed structures and the
structure of HL3 has been confirmed by X-ray crystallography.
CCDC reference number 186/2253.
See http://www.rsc.org/suppdata/dt/b0/b007589m/ for crys-
tallographic files in .cif format.
Crystal structure of N,N-diethyl-NЈ-(؊)-(3R)-menthyloxy-
carbonylthiourea (HL3)
The structure of HL3 consists of two discrete molecules per
asymmetric unit, thus four chemically equivalent molecules in
the unit cell. The structure of molecule 1, together with the
atom numbering scheme, is given in Fig. 2. The conformations
of the two molecules differ as illustrated in Fig. 3, wherein the
two molecules are superimposed on one another. Relevant
bond lengths and angles are given in Table 4, and it is clear
Results and discussion
Ligand synthesis and characterisation
The N,N-disubstituted NЈ-menthyloxycarbonylthiourea ligands
(HL3-6,), as shown in Fig. 1, were prepared using a modified
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Table 3 Crystal data and structure refinement for compounds 1, 2 and 3
1
2
3
Empirical formula
Formula weight
T/K
C32H60N4O4S2
628.96
293(2)
C18H35ClN2O4PtS2
638.14
293(2)
C14H21ClN2O2PtS2
543.99
293(2)
Wavelength/Å
Crystal system
Space group
a/Å
b/Å
c/Å
α/Њ
β/Њ
0.71073
Monoclinic
P21
8.889(2)
13.207(3)
16.911(3)
90
100.51(3)
90
1952.0(7)
2
0.71073
Orthorhombic
P212121
9.8964(6)
13.2721(8)
18.0016(11)
90
90
90
2364.4(2)
4
6.249
0.71073
Triclinic
¯
P1
8.7436(4)
10.0717(5)
11.5723(6)
76.263(10)
89.207(10)
68.205(10)
916.05(8)
2
γ/Њ
V/Å3
Z
µ/mmϪ1
0.172
8.039
Reflections collected
Independent reflections
Rint
2910
2770
0.031
0.0385, 0.0925
0.18(11)
15355
5462
0.0369
0.0296, 0.0535
0.013(6)
9904
4433
0.0365
0.0299, 0.0725
Final R, Rw indices [I > 2σ(I)]
Absolute structure parameter14
Table 4 Selected bond lengths (Å), angles (Њ) and torsion angles (Њ) for the ligands in compounds 1, 2 and 3
1
Molecule 1
Molecule 2
2
3
S(1)–C(11)
1.677(4)
1.200(5)
1.333(5)
1.463(5)
1.399(5)
1.370(5)
1.329(6)
1.673(4)
1.186(5)
1.343(5)
1.458(4)
1.396(5)
1.382(5)
1.326(6)
1.734(5)
1.254(5)
1.329(5)
1.464(5)
1.345(6)
1.309(5)
1.343(6)
1.735(4)
1.271(5)
O(11)–C(12)
O(12)–C(12)
O(12)–C(1A)
N(11)–C(11)
N(11)–C(12)
N(12)–C(11)
1.348(5)
1.312(6)
1.335(5)
C(12)–O(12)–C(1A)
C(12)–N(11)–C(11)
N(12)–C(11)–N(11)
N(12)–C(11)–S(1)
N(11)–C(11)–S(1)
O(11)–C(12)–N(11)
O(11)–C(12)–O(12)
N(11)–C(12)–O(12)
117.2(3)
125.6(4)
116.8(4)
124.6(3)
118.5(3)
125.3(4)
126.0(4)
108.7(4)
117.1(3)
122.3(3)
117.5(4)
124.0(3)
118.5(3)
126.4(4)
126.0(4)
107.7(3)
120.1(3)
125.0(4)
114.8(4)
115.3(4)
129.8(4)
132.6(4)
115.7(4)
111.7(4)
127.3(4)
113.8(4)
117.4(3)
128.8(3)
130.3(4)
O(11)–C(12)–O(12)–C(1A)
C(1G)–C(1F)–C(1A)–O(12)
O(11)–C(12)–C(11)–S(1)
C(13)–N(12)–C(11)–N(11)
C(15)–N(12)–C(11)–N(11)
4.7(7)
59.2(5)
115.0(4)
173.9(3)
Ϫ19.7(6)
3.3(6)
58.8(5)
Ϫ109.3(4)
19.5(6)
Ϫ171.5(3)
that most of the bond distances, angles and torsion angles for
molecules 1 and 2 of compound 1 are essentially the same.
The largest difference concerns the orientation of the sulfur
[S(1) and S(2)] and carbonyl groups [C(12)–O(11) and C(22)–
O(21)] with respect to one another, as illustrated by the torsion
angles O(11)–C(12)–C(11)–S(1) and O(21)–C(22)–C(21)–S(2)
of 115.0(4) and Ϫ109.3(4) Њ, respectively.
The C–N bond lengths range from 1.326(6) to 1.399(5) Å and
are shorter than the average single C–N bond length of 1.479 Å,
thus showing varying degrees of double bond character.
The partial double bond character of the C(11)–N(12) bond
is further manifested by the magnetic inequivalence of the
methylene protons adjacent to the nitrogen atom of the
diethylamino moiety.
Synthesis of [Pt(L)Cl(RRЈSO)] complexes
Fig. 3 Molecular structures of molecules 1 and 2 of N,N-diethyl-NЈ-
(Ϫ)-(3R)-menthyloxycarbonylthiourea superimposed. Molecule 2 is
numbered with the first digit referring to the number of the molecule
and the second to the number of the atom. Hydrogen atoms have been
omitted for clarity.
Dimethyl sulfoxide complexes were prepared using all four
alkoxycarbonylthiourea ligands, while the complex with an
unsymmetrical sulfoxide (S-MTSO) was prepared using N,N-
diethyl-NЈ-(Ϫ)-(3R)-menthyloxycarbonylthiourea only. The
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Table 5 Analytical data for platinum() sulfoxide complexes
Complex
mp/ЊC
Yield (%)
Analytical data (% C/H/N/S)a
ν(SO)/cmϪ1
[Pt(L3)Cl(DMSO)]
[Pt(L4)Cl(DMSO)]
[Pt(L5)Cl(DMSO)]
[Pt(L6)Cl(DMSO)]
[Pt(L3)Cl(S-MTSO)]
72–75
74–80
200–(decomp.)
200–(decomp.)
170–171
29
64
75
80
48
35.1; 5.8; 4.5; 10.25 (34.75; 5.7; 4.5; 10.3)
35.1; 5.8; 4.5; 10.5 (34.75; 5.7; 4.5; 10.3)
34.2; 5.3; 4.6; 10.0 (34.0; 5.2; 4.4; 10.1)
34.3; 5.2; 4.5; 9.95 (34.0; 5.2; 4.4; 10.1)
41.1; 6.1; 4.0; 9.1 (41.3; 5.6; 4.0; 9.2)
1142
1142
1154
1154
1138
a Required values are given in parentheses.
complexes were prepared in a two step synthesis according
to eqns. (1) and (2), where HL represents the alkoxycarbonyl-
thiourea ligands.
K2PtCl4 ϩ 3 RRЈSO → cis-[PtCl2(RRЈSO)2] (1)
NaOAc
(2)
[Pt(L)Cl(RRЈSO)]
cis-[PtCl2(RRЈSO)2] ϩ HL
Characterisation of complexes
All the complexes were fully characterised by elemental
1
analysis, IR and H, 195Pt NMR spectroscopy. The elemental
analyses, principal infrared bands, 1H and 195Pt NMR data are
given in Tables 5 and 6.
The strong IR band in the region 1130–1154 cmϪ1 is assigned
to the ν(SO) stretching frequency. The ν(SO) peak is shifted
to a higher wavenumber upon complexation compared to the
corresponding vibration of the unbound sulfoxide ν(SO) 1055
cmϪ1, and is indicative of sulfur bonded sulfoxide.6 Upon
complexation of the alkoxycarbonylthiourea ligands the N–H
stretching vibrations disappear and the ligand ν(CO) peak
shifts to higher wavenumbers. Such a shift is also expected for
Fig.
4
Molecular structure of cis-(S,S)-[Pt(L6)Cl(DMSO)] 2,
showing the atom numbering scheme and displacement ellipsoids
(30% probability). Hydrogen atoms have been omitted for clarity.
the C᎐S stretch vibration, but this could not be assigned
᎐
unambiguously.
In the 1H NMR spectra for the complexes two proton reson-
ances for the methyl protons of the sulfoxide ligand are
observed in the region δ 3.6–3.4. These are shifted 1 ppm down-
field relative to the “free” sulfoxide ligand and are thus typical
for sulfur bonded sulfoxides.6 Moreover, the signals due to the
methyl protons of the sulfoxide, in cases where there were no
overlapping peaks, show a 195Pt satellite doublet with 3J(PtH) of
about 23 Hz, due to vicinal coupling with the platinum centre,
195Pt (33.7%, I = ¹), the same order of magnitude as found for
¯
²
sulfur bonded sulfoxide complexes.6 The presence of two reson-
ances for the sulfoxide ligand was unexpected as it suggested
that there are two sulfur-bonded sulfoxide species present in
solution, which is a phenomenon not observed for the corre-
sponding complexes with acylthiourea ligands.1,2 Attempts
were made to separate the two species by normal phase (NP)-
HPLC [1:1 v/v EtOAc–hexane] and although two HPLC peaks
were obtained with relative peak heights of 1.66:1 respectively
the 1H NMR spectra of the two fractions were identical to the
original spectrum with two sulfoxide resonances, suggesting
that the separated species re-equilibrated to give both species in
solution. Using 195Pt NMR spectroscopy it was possible to
establish that the two species in solution were geometric
isomers, that is with the sulfoxide ligand in a trans-(S,S) and
cis-(S,S) arrangement with respect to the sulfur donor atom of
the chelated ligand. This is notably different to the complex-
ation reaction of cis-[PtCl2(RRЈSO)2] with acylthiourea ligands
which yields a single complex with the sulfoxide ligand in a
cis-(S,S) arrangement with respect to the sulfur donor atom of
the chelated ligand.
Fig. 5 Molecular structure of cis-(S,S)-[Pt(L1)Cl(DMSO)] 3. Details
as in Fig. 4.
equilibrium with an approximate cis-(S,S):trans-(S,S) ratio of
1:2 (Table 6), the cis-(S,S) and trans-(S,S) isomers being the
complexes with the sulfoxide ligand in a cis and trans arrange-
ment with respect to the sulfur donor atom of the chelated
ligand, respectively. The trans isomer is the thermodynamically
favoured isomer in CDCl3, for all the complexes prepared,
which is in contrast to what is found for the acylthiourea com-
plexes. The polarity of the solvent appears to have a marginal
effect on the cis/trans equilibrium.
Crystal structures of cis-(S,S)-[Pt(L6)Cl(DMSO)] and cis-(S,S)-
[Pt(L1)Cl(DMSO)]
The crystal and molecular structures of cis-(S,S)-[Pt(L6)-
Cl(DMSO)] 2 and cis-(S,S)-[Pt(L1)Cl(DMSO)] 3 are shown in
Figs. 4 and 5 respectively, together with the atom numbering
scheme, while selected bond lengths and angles are given in
Tables 4, 7 and 8.
It is well known that the 195Pt chemical shift is sensitive to
the coordination sphere of the metal centre and it is generally
accepted that the chemical shifts of cis isomers are upfield
relative to those of trans.15 For all the alkoxycarbonylthio-
urea complexes prepared two 195Pt signals were observed at
J. Chem. Soc., Dalton Trans., 2000, 4579–4586
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Table 7 Coordination polyhedron (distances in Å, angles in Њ) for
complexes 2 and 3
2
3
Pt(1)–O(11)
Pt(1)–S(2)
Pt(1)–S(1)
Pt(1)–Cl(1)
S(1)–C(11)
S(2)–O(2)
S(2)–C(21)
S(2)–C(22)
2.018(3)
2.010(3)
2.1839(12)
2.2595(13)
2.3276(14)
1.734(5)
1.457(4)
1.752(5)
2.1885(10)
2.2586(11)
2.3337(11)
1.735(4)
1.462(3)
1.764(5)
1.775(5)
1.759(5)
O(11)–Pt(1)–S(2)
176.40(11)
92.66(10)
90.71(4)
86.00(10)
90.66(5)
178.15(6)
117.16(14)
111.26(19)
109.1(2)
175.38(10)
93.75(10)
90.87(4)
84.94(10)
90.44(4)
178.64(4)
119.13(15)
110.36(18)
109.2(2)
O(11)–Pt(1)–S(1)
S(2)–Pt(1)–S(1)
O(11)–Pt(1)–Cl(1)
S(2)–Pt(1)–Cl(1)
S(1)–Pt(1)–Cl(1)
O(2)–S(2)–Pt(1)
C(21)–S(2)–Pt(1)
C(22)–S(2)–Pt(1)
O(2)–S(2)–C(21)
O(2)–S(2)–C(22)
C(21)–S(2)–C(22)
108.4(3)
108.5(3)
101.1(3)
107.4(2)
108.2(3)
101.0(3)
The bond lengths and angles all fall within the expected
limits.1 Both the Pt–Cl and the Pt–S᎐O bond lengths in complex
᎐
2, 2.3276(14) and 2.1839(12) Å, are however statistically signif-
icantly shorter than in 3, that is 2.3337(11) and 2.1885(10) Å,
respectively. The opposite however holds (although not stat-
istically significant, the tendency is observed) for the corre-
sponding sulfur and oxygen donor atoms of the bidentate
ligands as manifested in the Pt–S(1) and Pt–O(11) distances of
2.2595(13) and 2.018(3) Å in 2 versus 2.2586(11) and 2.010(3) Å
for complex 3, respectively. This implies that both the sulfur and
oxygen donor atoms of ligand HL1 have larger trans influences
than the corresponding donor atoms in ligand HL6. In general
this implies further that the (L1)Ϫ ligand as such is more
strongly bonded than the (L6)Ϫ moiety, which is in excellent
agreement with what is observed from the NMR experiments
described below. Small differences exist in the bond angles
of the chelate in complexes 2 and 3, but these may be due to
packing effects and are thus not discussed further.
The Pt–MPSO bond length [2.192(3) Å] in complex 4 shows
a tendency to slight elongation (although not statistically sig-
nificant) compared to the Pt–DMSO bond length [2.1885(10)
Å] in 3 (see Table 8). This is in agreement with the expected
labilities of the coordinated sulfoxides, in which MPSO is more
labile than DMSO.5 The other bond distances of the chelate
rings in 3 and 4 do not differ significantly.
However, the bond distances in ligand HL3 and both the
(L1)Ϫ and (L6)Ϫ moieties show significant differences. Sub-
stantial elongation of the S–C and C–O ligand fragments upon
coordination to the platinum() centre (by as much as 0.06 Å in
both cases) compared to the “free” ligand, is observed (Table 4).
On the other hand, the C(11)–N(11) and C(2)–N(11) bonds are
very similar in complexes 2 and 3 [1.345(6) vs. 1.348(5) and
1.309(5) vs. 1.312(6) Å respectively], and thus seem to be
independent of the R and RЈ groups of the ligand systems.
However, these two bonds are significantly shortened (by 0.04–
0.06 Å) compared to the corresponding ones [average 1.397(5)
and 1.376(5) Å, respectively] in the “free” HL3 ligand 1, thus
indicating substantial delocalisation of electrons within the
chelate ring upon coordination.
Reactions with PPh3
Further evidence for the difference in the coordination chem-
istry of the acylthiourea and alkoxycarbonylthiourea ligands
was obtained by substitution of the bound sulfoxide by tri-
phenylphosphine (PPh3). 31P NMR data for the monophos-
4584
J. Chem. Soc., Dalton Trans., 2000, 4579–4586

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Table 8 Comparison of selected bond lengths (Å) of acylthiourea and alkoxycarbonylthiourea platinum() sulfoxide complexes
Complex
Pt–O
Pt–S
Pt–Cl
Pt–S᎐O
Reference
᎐
2 cis-(S,S)-[Pt(L6)Cl(DMSO)]
3 cis-(S,S)-[Pt(L1)Cl(DMSO)]
4 cis-(S,S)-[Pt(L1)Cl(MPSO)]
2.018(3)
2.010(3)
2.016(9)
2.2595(13)
2.2586(11)
2.257(4)
2.3276(14)
2.3337(11)
2.334(3)
2.1839(12)
2.1885(10)
2.192(3)
This work
This work
1
Table 9 31P NMR data for [Pt(L)Cl(PPh3)] complexes
Integration of the cis-(P,S)- and trans-(P,S)-[Pt(L4)Cl(PPh3)]
signals gives rise to a cis:trans ratio of approximately 2:1. The
opposite is observed for the sulfoxide complexes, where the
trans-(S,S) isomers are favoured, and this could be due to the
higher basicity and π-acceptor properties of PPh3 compared to
those of DMSO.
Complex
δ31P (1J(PtP)/Hz)
cis-(P,S)-[Pt(L1)Cl(PPh3)]
cis-(P,S)-[Pt(L2)Cl(PPh3)]
cis-(P,S)-[Pt(L4)Cl(PPh3)]
trans-(P,S)-[Pt(L4)Cl(PPh3)]
cis-(P,S)-[Pt(L6)Cl(PPh3)]
trans-(P,S)-[Pt(L6)Cl(PPh3)]
8.137 (4040)
7.729 (4033)
8.001 (4181)
5.704 (3974)
7.769 (4189)
5.381 (3956)
The 1J(PtP) coupling constants for cis-(P,S)-[Pt(L1)Cl(PPh3)]
and cis-(P,S)-[Pt(L4)Cl(PPh3)] are 4040 and 4181 Hz respect-
ively. The increase of 141 Hz on going from an acylthiourea to
alkoxycarbonylthiourea ligand suggests an increase in the Pt–P
bond strength trans to the carbonyl oxygen donor atom of
the alkoxycarbonylthiourea ligand. This implies that for the
monophosphine complexes the acylthiourea carbonyl oxygen
donor atom is relatively softer and therefore has a greater trans
influence than the carbonyl oxygen donor atom of the alkoxy-
carbonylthiourea ligand, which is in agreement with the crystal-
lographic data discussed above. Similar results were obtained
for the morpholine containing complexes, except that the
cis:trans ratio for the [Pt(L6)Cl(PPh3)] complex is 1:2. The
inversion of the cis:trans ratio for the morpholine containing
complexes is as yet not clearly understood.
Table 10 31P NMR data for [Pt(L)(PPh3)2]ϩ complexes
δ31P (1J(PtP)/Hz)
Complex
trans sulfur
trans oxygen
2J(PP)/Hz
[Pt(L1)(PPh3)2]ϩ
[Pt(L2)(PPh3)2]ϩ
[Pt(L4)(PPh3)2]ϩ
[Pt(L6)(PPh3)2]ϩ
21.912 (3088)
22.249 (3129)
22.010 (3064)
Not present
11.371 (3836)
10.493 (3835)
10.512 (3945)
26
25
24
—
phine and bis(phosphine) complexes are given in Tables 9 and
10, respectively. The phosphine complexes were prepared
according to eqns. (3) and (4).
Bis(phosphine) complexes. In the 31P NMR spectrum of
[Pt(L1)(PPh3)2]ϩ two doublets are observed at δ 21.912 and
11.371, with J(PtP) coupling constants of 3088 and 3836 Hz,
1
respectively. Similarly, two doublets are observed in the spec-
[PptCl(DMSO)(L)] ϩ PPh3 →
[PtCl(L)(PPh3)] ϩ DMSO (3)
1
trum of [Pt(L2)(PPh3)2]ϩ at δ 22.249 and 10.493, with J(PtP)
coupling constants of 3129 and 3835 Hz, respectively. A signifi-
cant difference between [Pt(L1)(PPh3)2]ϩ and [Pt(L2)(PPh3)2]ϩ is
that while the former is stable in solution the latter is not,
as manifested by the changes in the 31P NMR spectrum of
[Pt(L2)(PPh3)2]ϩ with time. After 12 hours the intensity of the
doublets at δ 22.249 and 10.493 decreased dramatically and the
solution only contained ca. 10% of the original complex. This
decrease in intensity was accompanied by the appearance of
singlets at δ 15.550 [1J(PtP) 3674 Hz and ca. 21% abundance]
and 7.720 [1J(PtP) 4030 Hz and ca. 21% abundance], two
singlets with no platinum satellites at δ 30.169 (ca. 12% abun-
dance) and 44.582 (ca. 22% abundance), as well as two doublets
at δ 12.553 and 20.096 [2J(PP) 20 Hz; ca. 8% abundance] with
unresolved platinum satellites. The signals at δ 7.720, 15.550
and 30.169 have been assigned as [Pt(L2)Cl(PPh3)], cis-
[PtCl2(PPh3)2] and triphenylphosphine oxide, respectively. The
signals of cis-[PtCl2(PPh3)2] and triphenylphosphine oxide were
confirmed by recording the 31P NMR spectra of the pure com-
pounds. The signal at δ 44.582 appears to be due to the reaction
between the deprotonated ligand and triphenylphosphine. The
doublets at δ 20.096 and 12.553 have been tentatively assigned
to the ring opened product [Pt(L2-S)Cl(PPh3)2]ϩ, as the chem-
[PtCl(DMSO)(L)] ϩ 2 PPh3 →
[Pt(L)(PPh3)2]ϩ ϩ DMSO ϩ Cl (4)
The substitution patterns of the chloride and dimethyl
sulfoxide leaving groups were confirmed by 1H NMR spec-
troscopy. Upon addition of 1 equivalent of PPh3 to the
platinum() sulfoxide complexes in CDCl3 the proton reson-
ance of the bound sulfoxide disappeared and a resonance peak
due to free dimethyl sulfoxide was observed. This suggests that
upon addition of one equivalent of PPh3 the dimethyl sulfoxide
ligand is displaced and further addition of PPh3 results in
exchange of the chloride ligand.
Monophosphine complexes. The 31P NMR spectra for the
complexes [Pt(L1)Cl(PPh3)] and [Pt(L2)Cl(PPh3)] each showed
only one 31P signal with 1J(PtP) satellites of 4040 and 4033 Hz,
1
respectively. On the other hand, two 31P signals with J(PtP)
satellites were observed for the platinum complexes containing
the alkoxycarbonylthiourea ligands (L4)Ϫ and (L6)Ϫ, indicating
that there are two monophosphine species present in solution.
Likewise, these two species have, for convenience, been desig-
nated as cis-(P,S) and trans-(P,S) isomers, where cis refers to the
complex with the phosphine ligand cis with respect to the sulfur
donor atom of the chelate ligand. The signals could unequiv-
ocally be assigned to either the cis-(P,S) or trans-(P,S) isomers as
it is well accepted for complexes of the type [PtX2(PR3)2] that
2
ical shift positions and J(PP) of 20 Hz correspond to the ring
opened species of [Pt(L6)(PPh3)2]ϩ discussed below.
The reaction of [Pt(L4)Cl(DMSO)] with two equivalents of
PPh3 yielded a 31P NMR spectrum that consisted of three
singlets (abundance) at δ 8.001 [1J(PtP) 4181 Hz; ca. 11%],
1
1
the cis isomers have significantly larger J(PtP) coupling con-
5.704 (ca. 7%) and 15.562 (ca. 4%); J(PtP) satellites for the
stants than those of the corresponding trans isomers.16–18 It is
clear from Table 9 that the platinum–phosphorus coupling con-
stants for the cis-(P,S) isomers are larger than for the trans-(P,S)
isomers, which is consistent with the greater trans influence of
sulfur compared to oxygen. These results are also consistent
with the work reported by Cavell et al. on platinum() com-
plexes with monothio-β-diketone and phosphine ligands.19
signals at δ 5.704 and 15.562 could not be resolved due to over-
lapping peaks. Two sets of broad doublets were also observed at
δ 22.010 and 10.512 [1J(PtP) 3064 and 3945 Hz; 39%] and
15.562 and 8.855 [1J(PtP) 3346 and 3512 Hz; 13%]. Based on
the chemical shift positions and the 1J(PtP) coupling constants
observed for [Pt(L1)(PPh3)2]ϩ, the doublets at δ 22.010 and
10.512 have been assigned to [Pt(L4)(PPh3)2]ϩ. It is important to
J. Chem. Soc., Dalton Trans., 2000, 4579–4586
4585

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note that the difference in the coupling constants for the one set
of doublets (881 Hz) is far greater compared to that observed
for the other set of signals (166 Hz). Since the 1J(PtP) coupling
is diagnostic of the trans ligand, this suggests that the difference
in the trans influences of the two donor atoms trans to the
phosphine ligands in the latter complex is much smaller than
the difference in the trans influences of the oxygen and sulfur
donor atoms in the chelated ligand. Moreover, the increase
in the coupling constant from 3064 to 3346 Hz for the peak
at δ 15.562 implies that a donor atom with a greater trans
influence than oxygen has replaced the oxygen donor atom of
the chelated ligand. These results suggest that ring opening
has occurred and that the second set of doublets corresponds to
a cis-[Pt(L4-S)Cl(PPh3)2]ϩ complex, where L4-S represents the
ring-opened ligand. The signals at δ 8.001 and 5.704 are the
same as those products formed when one equivalent of PPh3 is
used and have been assigned to cis-(P,S)- and trans-(P,S)-[Pt(L4)-
Cl(PPh3)], respectively. The singlet at δ 15.562 corresponds to
cis-[PtCl2(PPh3)2].
Further studies need to be carried out to establish the mech-
anism involved in the isomerisation of the alkoxycarbonyl-
thiourea containing complexes. However, the weaker binding
properties of these ligands might be a possible explanation
for the observed geometric isomerisation of the complexes and
that the mechanism could involve a chelate ring opening step.
Acknowledgements
We wish to thank Rhodes University, the University of the Free
State and the National Research Foundation for financial
support and Johnson Matthey for a generous loan of K2PtCl4.
References
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By contrast, the reaction of [Pt(L6)Cl(DMSO)] with two
equivalents of PPh3 gave rise to a 31P NMR spectrum that con-
sisted of three singlets (abundance) at δ 7.766 [1J(PtP) 4189 Hz;
13%], 5.381 [1J(PtP) 3956 Hz; 16%] and 15.572 [1J(PtP) 3673
Hz; 37%] and only one set of doublets at δ 20.083 and 12.971
[1J(PtP) 3135 and 3477; 2J(PP) 20 Hz; 15%]. Since the difference
in the 1J(PtP) coupling constants is 242 Hz, the set of doublets
at δ 20.083 and 12.971 has been assigned to cis-[Pt(L6-S)Cl-
(PPh3)2]ϩ, where L6-S represents the ring-opened ligand. The
three singlets at δ 7.766, 5.381 and 15.572 have been assigned
to cis-(P,S)- and trans-(P,S)-[Pt(L6)Cl(PPh3)] and cis-[PtCl2-
(PPh3)2], respectively. The signals due to [Pt(L6)(PPh3)2]ϩ were
not detected in the spectrum, implying that this ligand, (L6)Ϫ, is
weakly bound to the platinum() ion.
12 G. M. Sheldrick, SHELXL 97, Program for Structure Refinement,
University of Göttingen, 1997.
Conclusion
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The phosphine studies highlight the difference in the coordin-
ation chemistry of the acylthiourea and alkoxycarbonylthio-
urea ligands illustrating that geometric isomers are formed
when using the latter ligand systems and that these ligands bind
less strongly to the platinum() ion. Moreover, the morpholine
derivatives for both ligand systems appear to bind less tightly to
the metal ion. These results are in agreement with the stability
constant measurements reported by Beyer and co-workers
which show that the Ni2ϩ, Zn2ϩ Cd2ϩ and Co2ϩ complexes of
HL1 are more stable than the corresponding complexes of
HL2.20
4586
J. Chem. Soc., Dalton Trans., 2000, 4579–4586