On the borderline between cis and trans in organometallic (phosphane)platinum(II) complexes

Article abstract of DOI:10.1002/ejic.200500251

The borderline between the cis and trans configurations in square-planar diarylplatinum(II) complexes with triethylphosphane ligands [Pt(Ar) ₂(PEt₃)₂] [Ar = 2,4,6-trimethylphenyl (mesityl), 2,6-dimethylphenyl, 2-methylphenyl, 3-methylphenyl, 4-methylphenyl and phenyl] has been investigated by a combination of multinuclear ( ¹H, ¹³C, ³¹P and ¹⁹⁵Pt) NMR spectroscopy, X-ray crystallography and quantum chemical (DFT) calculations. When formed under thermodynamic conditions, the complexes show a clear cut-off between cis (tolyl and phenyl complexes) and trans (2,6-xylyl and mesityl complexes). The syn and anti stereoisomers were identified for the 2-methylphenyl and 3-methylphenyl derivatives. Calculated data (structural isomers or NMR) are in excellent agreement with experimental findings. Wiley-VCH Verlag GmbH & Co. KGaA, 2005.

Full text of DOI:10.1002/ejic.200500251

FULL PAPER
On the Borderline between cis and trans in Organometallic
(Phosphane)platinum(II) Complexes
[a]
[b]
[c]
[c]
ˇ[d]
Dietrich Gudat, Vimal K. Jain, Axel Klein,* Thilo Schurr, and Stanislav Zális
Keywords: Density functional calculations / Isomers / NMR spectroscopy / Phosphane ligands / Platinum
The borderline between the cis and trans configurations in
square-planar diarylplatinum(II complexes with triethyl-
phosphane ligands [Pt(Ar)2(PEt3)2] [Ar 2,4,6-trimeth-
ylphenyl (mesityl), 2,6-dimethylphenyl, 2-methylphenyl, 3-
methylphenyl, 4-methylphenyl and phenyl] has been investi-
gated by a combination of multinuclear (1H, 13C, 31P and
195Pt) NMR spectroscopy, X-ray crystallography and quan-
tum chemical (DFT) calculations. When formed under
thermodynamic conditions, the complexes show a clear cut-
off between cis (tolyl and phenyl complexes) and trans (2,6-
xylyl and mesityl complexes). The syn and anti stereoisomers
were identified for the 2-methylphenyl and 3-methylphenyl
derivatives. Calculated data (structural isomers or NMR) are
in excellent agreement with experimental findings.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
)
=
platinum [PtCl2(NH3)2]: the cis derivative is an effective
anti-cancer drug (“cisplatin”) with sales of around 2 million
j a year, whereas the trans derivative is inactive in that re-
spect.[4–6]
Introduction
The stability of the cis or trans conformation of square-
planar complexes with a d8 configuration is usually gov-
erned by an interplay of steric and electronic factors:[1,2]
bulky ligands tend to avoid each other, and the presence of
two of them usually leads to a trans configuration, whereas
strong donor ligands prefer a cis orientation towards other
strong ligands due to the trans influence. The latter should
not be confused with the trans effect. The former is respon-
sible for ground state-properties (structure, thermodynamic
stability etc.), whereas the latter governs the behaviour in
chemical reactions.[1] Both the trans influence and the trans
effect are of high importance since both stability and for-
mation reactions predetermine the occurrence of cis and
trans configurations in such complexes, which is very often
crucial for their applications. This is of enhanced impor-
tance for platinum() complexes, since in the series of d8-
configured transition metals like RhI, IrI, NiII, PdII, PtII
and AuIII platinum() derivatives are markedly more resist-
ant to isomerisation reactions than others.[1–3] The most
prominent example of the different behaviour of two iso-
meric platinum() complexes is probably diamminedichloro-
Phosphane ligands have been explored in organoplati-
num complexes for more than half a century.[2,7–9] Besides
their numerous applications in organometallic catalysis,
they have been found to be ideally suited for the determi-
nation of their configurations or configurational equilibria
since the 31P–195Pt coupling constants are very sensitive to
the trans and cis influence of further co-ligands.[10,11] Aryl
ligands can be used to probe the subtle interplay of steric
and electronic factors of the configuration since they are (i)
strongly donating ligands; (ii) their electronic properties can
be tuned by various substituents and the observed changes
can be easily rationalised in terms of inductive or meso-
meric influences; (iii) they can be easily introduced by estab-
lished metathesis reactions;[9] (iv) their steric demand can
be varied almost deliberately;[12] and (v) since their ring pla-
nes are usually oriented almost perpendicular with respect
to the binding plane they allow discrimination of the two
hemispheres above and below the binding plane, for exam-
ple by unsymmetrical aryl ligands like 2-R,6-RЈ-C6H4 that
give rise to further isomerism.[13]
We therefore decided to prepare a series of (triethylphos-
phane)platinum complexes [Pt(Ar)2(PEt3)2] [Ar = 2,4,6-tri-
methylphenyl (Mes), 2,6-dimethylphenyl (Xyl), 2-meth-
ylphenyl (2-Tol), 3-methylphenyl (3-Tol), 4-methylphenyl (4-
Tol) and phenyl (Ph)] with various alkyl substitution pat-
terns on the aryl ligands.
A similar study was undertaken by Rieger and co-
workers some years ago[14] and some of the compounds (Ar
= Ph, 2-Tol, 4-Tol) were reported in 1959 by Chatt and
Shaw.[9] However, their preparation routes, involving me-
[a] Institut für Anorganische Chemie, Universität Stuttgart,
Pfaffenwaldring 55, 70569 Stuttgart, Germany
E-mail: gudat@iac.uni-stuttgart.de
[b] Novel Materials and Structural Chemistry Division, Bhabha
Atomic Research Centre,
Mumbai 400 085, India
E-mail: jainvk@apsara.barc.ernet.in
[c] Institut für Anorganische Chemie, Universität zu Köln,
Greinstraße 6, 50939 Köln, Germany
E-mail: axel.klein@uni-koeln.de
[d] J. Heyrovsky Institute of Physical Chemistry, Academy of Sci-
ences of the Czech Republic,
Dolejsˇkova 3, 18 000 Prague 8, Czech Republic
E-mail: stanislav.zalis@jh-inst.cas.cz
4056
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/ejic.200500251
Eur. J. Inorg. Chem. 2005, 4056–4063

The Borderline between cis and trans in (Phosphane)platinum() Complexes
FULL PAPER
tathesis reactions between cis- or trans-[PtCl2(PEt3)] and an NMR Spectroscopy
appropriate Grignard reagent, did not allow synthesis of
the sterically highly crowded dimesityl or bis(2,6-xylyl) de-
rivatives and often led to mixtures of isomers.[9,14] To pre-
vent this we have chosen a different preparation route to
ensure the thermodynamically controlled selective forma-
tion of only one isomer. To this end we applied ligand-ex-
change reactions at elevated temperatures using the DMSO
precursor complexes [Pt(Ar)2(DMSO)2] to obtain isomer-
ically pure compounds.
The thoroughly recrystallised complexes were submitted
to multinuclear (1H, 13C, 31P and 195Pt) NMR studies. Se-
lected data for the determination of the structure by NMR
spectroscopy are summarised in Table 1 together with
ADF/BP calculated data. Full NMR spectroscopic data can
be found in the Experimental Section.
From the NMR spectroscopic data, especially from the
1
1JPt,P and JPt,C(1) coupling constants (Table 1), a clear line
can be drawn between the Ph and Tol derivatives on one
side and the Xyl and Mes derivatives on the other side. The
1JPt,P coupling constants of the former are markedly smaller
Results and Discussion
1
and the JPt,C(1) couplings markedly larger than the values
Preparation
found for the more substituted derivatives. Both findings
are indicative of shorter Pt–P and longer Pt–C(1) bonds,
and thus the less substituted derivatives are assigned to have
cis configurations whereas the more substituted ones are
The preparation of the complexes was performed as
shown in Scheme 1 from the corresponding DMSO deriva-
tives [Pt(Ar)2(DMSO)2][15] by ligand-exchange reactions at
elevated temperatures (boiling toluene, see Exp. Sect.). With
the exception of [Pt(2-Tol)2(PEt3)2], where 1H and 31P
NMR spectra showed the presence of two isomers, we ob-
served in all cases the formation of only one isomer in the
crude reaction product as well as in the recrystallised pro-
ducts.
1
trans-configured. The calculated JPt,C coupling constants
are in excellent agreement with the experimental ones and
clearly support the assignment of cis and trans configura-
tions. Due to the simplification of the phosphane ligand
1
(PMe3 instead of PEt3), the calculated JPt,P coupling con-
stants reflect the cis/trans variation only qualitatively.
Two sets of signals (ca. 2:1 ratio) were found for the 2-
Tol derivative (Figure 1), both clearly showing a cis configu-
ration. The two isomers correspond to species with a syn
(CS symmetry) and anti (C2 symmetry) orientation of the
two ortho-methyl groups (above and below the coordination
plane), as has been reported previously for these systems[14]
and related ditolyl complexes.[16,17] Detailed NMR experi-
ments, including 1H gsNOESY, 1H,13C HMQC and
1H,195Pt HMQC experiments, allowed the assignment of all
signals to the two stereoisomers. Furthermore, the absence
of exchange correlations in the 1H NOESY spectra dis-
closed that the stereoisomers do not undergo any mutual
exchange on a second time-scale, even in the presence of
Scheme 1. Preparation of the platinum complexes [Pt(Ar)2(PEt3)2]. additional PEt3. This stands in contrast to similar results
Table 1. Experimental[a] and calculated (DFT)[b] NMR spectroscopic data for complexes [Pt(Ar)2(PEt3)2].
[c]
195Pt
δ
Ar
1JPt,C(1) exp.
1JPt,C calcd.
1JPt,P exp.
1JPt,P calcd.
Conformation from
[Hz]
[Hz]
[Hz]
[Hz]
[ppm]
NMR spectroscopic data
Ph
2-Tol (syn)
2-Tol (anti)
3-Tol
879
844
832
812
1090
591
536
cis: 840.1
trans: 512.1
cis: 888.8
trans: 541.6
cis: 874.7
trans: 514.8
cis: 856.0
trans: 537.6
cis: 846.7
trans: 517.8
cis: 937.4
trans: 526.8
cis: 922.7
1775
1751
1739
1754
1762
2835
2841
cis: 1093.8
trans: 2575.3
cis: 942.2
trans: 2566.8
cis: 933.9
trans: 2566.9
cis:1005.8
trans: 2455.4
cis: 959.8
trans: 2346.9
cis: 985.1
trans: 2595.5
cis: 875.1
–4568
cis
cis-syn
cis-anti
cis
–4471
–4466
–4570
–4566
–4311
–4306
4-Tol
cis
Xyl
trans
trans
Mes
trans: 530.2
trans: 2737.8
[a] As measured in CDCl3 at 303 K. [b] Calculated for [Pt(Ar)2(PMe3)2]. [c] From 31P NMR measurements.
www.eurjic.org
Eur. J. Inorg. Chem. 2005, 4056–4063
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4057

D. Gudat, V. K. Jain, A. Klein, T. Schurr, S. Zálisˇ
FULL PAPER
with the related DMSO complexes cis-[Pt(2-Tol)2- have reported line-broadening on related bis(3-Tol) and
(DMSO)2], where syn/anti isomerisation takes place pre- bis(4-Tol) complexes, thus indicating fluxional behaviour.[16]
sumably by a dissociative pathway.[15] Analogous stereoiso- We therefore performed quantum-chemical calculations on
mers were expected for the 3-Tol derivative. However, we the energy barrier of syn/anti isomerisation of the 2-Tol and
found only one set of signals in the NMR spectra at ambi- 3-Tol derivatives. The barrier calculated for [Pt(2-Tol)2-
ent temperature (298 K). Recently, Hashemi and Rashidi (PMe3)2] (111.7 kcalmol–1) turns out to be much larger
than for [Pt(3-Tol)2(PMe3)2] (34.1 kcalmol–1), which seems
to be reasonable. However, both numbers indicate the oc-
currence of syn and anti isomers for 2-Tol as well as for
3-Tol. We therefore examined the 3-Tol derivative at low
temperatures (223 and 193 K) and found two sets of signals
in the 1H, 13C, 31P and 195Pt NMR spectra. Due to the
rather small splitting of the corresponding signals of the
two isomers (some signals are just broadened; see, for exam-
ple, Tables 6 and 7) and signal ratios of approximately 1:1,
we could not assign either the syn or anti isomers or calcu-
late the energy difference. We also refrained from measuring
exact coalescence temperatures since the two measurements
(at 223 and 193 K) indicated already that different coalesc-
ence temperatures will be found for the various signals.
Comparing the syn/anti ratios for the 2-Tol and 3-Tol
derivatives we found a ratio of approx. 1:1 for 3-Tol,
whereas for the 2-Tol derivative the ratio is about 2:1. This
ratio is reversed (syn/anti = 1:2) in the precursor complex
cis-[Pt(2-Tol)2(DMSO)2].[15] Since for the DMSO derivative
we found isomerisation already at ambient temperature
(NMR), we assume that the syn derivative of the DMSO
precursor complex is more reactive than the anti, which is
(thermodynamically) slightly more stable. It is also worth-
Figure 1. Magnitude-mode two-dimensional 1H,195Pt gs-HMQC
spectrum (defocusing time 8.3 ms), showing various long-range
correlations between the protons in the 2-Tol and PEt3 groups and
the metal atom, and the 1H NMR spectrum (top projection; the
solvent signal is denoted by * and the resonances of an impurity
of OPEt3 by o) of a mixture of the syn and anti stereoisomers of
cis-[Pt(2-Tol)2(PEt3)2]. The F1 projection (left) shows the 195Pt sig- while mentioning that the PEt3 complexes retain the cis or
1
nals of both isomers as interleaved triplets split by JPt,P
.
Table 2. Crystallographic and structure refinement data of complexes [(PEt3)2Pt(Ar)2].[a]
Ar
Ph (cis)
3-Tol (cis)
4-Tol (cis)
Xyl (trans)
Mes (trans)
Empirical formula
Formula mass
Crystal system
Space group
a [Å]
b [Å]
c [Å]
C24H40P2Pt
585.59
monoclinic
Cc
20.411(5)
9.8288(14)
15.3942(17)
90
124.411(13)
90
2547.9(7)
4
1.527
5.639/1168
0 Յ h Յ 27
0 Յ k Յ 13
–21 Յ l Յ 17
3463/3463
0.0639
C26H44P2Pt
613.64
monoclinic
P21/c
15.951(3)
10.111(2)
17.737(4)
90
104.37(3)
90
2771.1(10)
4
1.471
5.189/1232
–17 Յ h Յ 20
–8 Յ k Յ 13
–23 Յ l Յ 22
6596/6371
0.0614
C26H44P2Pt
613.64
monoclinic
P21/n
13.3174(15)
14.8753(19)
14.5993(18)
90
99.662(9)
90
2851.1(6)
2
1.430
5.043/1232
0 Յ h Յ 17
0 Յ k Յ 19
–18 Յ l Յ 18
6423/6162
0.0460
C28H48P2Pt
641.69
C30H52P2Pt
669.75
orthorhombic
Pca21
22.885(5)
9.2381(18)
14.246(3)
90
triclinic
¯
P1
8.8835(18)
10.314(2)
15.804(3)
90.54(3)
91.09(3)
104.44(3)
1401.9(5)
2
α [°]
β [°]
90
90
γ [°]
V [Å3]
3011.8(10)
4
1.477
4.781/1360
–1 Յ h Յ 30
–1Յ k Յ 12
–1 Յ l Յ 18
4718/4054
0.0369
Z
ρcalcd. [gcm–3]
µ [mm–1]/F(000)
Limiting indices
1.520
5.132/648
–10 Յ h Յ 11
–13Յ k Յ 13
–20 Յ l Յ 20
7200/6769
0.0388
Reflections collected/unique
Rint
Data/restraints/parameters
3463/2/250
1.031
6371/11/306
1.050
6160/0/270
1.007
6769/0/293
1.179
4054/1/322
1.212
Goof. on F2
Final indices: R1
[I Ͼ 2σ(I)] wR2
0.0323
0.0730
0.0417
0.0769
0.0691
0.1425
0.1270
0.1718
0.0650
0.1326
0.1416
0.1617
0.0295
0.0824
0.0398
0.0919
0.0364
0.0758
0.0528
0.0822
R value all data: R1
wR2
Largest diff. peak/hole [eÅ–3]
1.505/–1.427
1.976/–2.538
3.419/–2.313
1.122/–1.445
1.378/–1.069
[a] Measurement temperature: 173(2) K; radiation wavelength: 0.71073 Å; refinement method: full-matrix least squares on F2; absorption
correction: empirical, ψ-scans.
4058
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
Eur. J. Inorg. Chem. 2005, 4056–4063

The Borderline between cis and trans in (Phosphane)platinum() Complexes
FULL PAPER
trans configuration of their DMSO precursors. This is not No significant intermolecular contacts were found in any
trivial, since the DMSO precursor complexes may undergo of the crystal structures.
facile isomerisation, which comprises not only syn/anti (vide
The molecular structures confirm the conclusions drawn
supra) but also cis/trans isomerisation, for example during from spectroscopic studies in solution: The Ph, 3-Tol and
the formation of the cis-configured (diimine)platinum com- 4-Tol derivatives exhibit a cis conformation (Figure 2) as
plexes cis-[Pt(Ar)2(_NN)] (_NN = α-diimine; Ar = Mes or has been found before for the 2-Tol derivative.[14] The Xyl
Xyl) from trans-configured DMSO precursor com- and Mes derivatives show a trans orientation of the two aryl
plexes.[18–20]
groups (Figure 3), and the platinum atom lies on a centre
of symmetry for Ar = Xyl. The 3-Tol derivative shows a syn
orientation of the two methyl substituents. All structures
show nearly perfect planar arrangements of ligands around
the platinum atom, as can be seen from the sum of angles
and the small CCPt/PPPt tilt angles (Table 3).
The bond lengths and angles around the platinum atom
in the series of complexes nicely reflect the trans influence.[1]
For instance, the Pt–P distances are slightly longer for the
cis-configured compounds while the Pt–C distances are
markedly shorter. The bond lengths and angles of the pre-
viously reported syn and anti isomers of cis-[Pt(2-Tol)2-
(PEt3)2][14] fit nicely into our series. Interestingly, Osakada
et al.[21] have recently reported the structure of the trans-
Crystal Structures
The crystal and molecular structures of [Pt(Ph)2(PEt3)2],
[Pt(3-Tol)2(PEt3)2], [Pt(4-Tol)2(PEt3)2], [Pt(Xyl)2(PEt3)2] and
[Pt(Mes)2(PEt3)2] were determined from single-crystal XRD
experiments. The structures of the syn and anti isomers of
cis-[Pt(2-Tol)2(PEt3)2] have been reported previously by
Rieger et al.[14] The results of the structure determinations
are summarised in Table 2, with bond lengths and angles
given in Table 3. The first three compounds crystallise in
monoclinic space groups, the Xyl derivative is found in tri-
¯
clinic P1 and the Mes compound in orthorhombic Pca21.
Figure 2. Molecular structures (30% thermal ellipsoids) of cis-[(Pt(Ar)2(PEt3)2] with Ar = Ph (left, with full numbering) and 3-Tol (right).
H atoms omitted for clarity.
Figure 3. Molecular structures (30% thermal ellipsoids) of trans-[(Pt(Ar)2(PEt3)2] with Ar = Xyl (left) or Mes (right). H atoms omitted
for clarity.
Eur. J. Inorg. Chem. 2005, 4056–4063
www.eurjic.org
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4059

D. Gudat, V. K. Jain, A. Klein, T. Schurr, S. Zálisˇ
FULL PAPER
Table 3. Selected bond lengths [Å] and bond angles [°] of complexes [(PEt3)2Pt(Ar)2].[a]
Ph (cis)
4-Tol (cis)
3-Tol (cis)
Xyl (trans) (1)[b]
Xyl (trans) (2)[b]
Mes (trans)
Pt–P(1)
Pt–P(2)
Pt–C(1)
Pt–C(11)
Pt–C(7/17)
Pt–C(8/18)
P–Pt–P
2.318(2)
2.328(2)
2.061(9)
2.073(8)
–
2.309(3)
2.324(4)
2.057(12)
2.057(9)
–
2.301(3)
2.316(3)
2.067(12)
2.083(11)
–
2.2991(12)
2.2991(12)
2.113(4)
2.113(4)
3.346(5)
3.377(5)
180.00(7)
180.0(3)
90.45(11)
89.55(11)
90.45(11)
89.55(11)
360.00
2.2954(13)
2.2954(13)
2.104(3)
2.104(3)
3.360(4)
3.367(3)
180.00(6)
180.0(2)
91.58(10)
88.42(10)
91.58(10)
88.42(10)
360.00
2.296(2)
2.290(2)
2.101(8)
2.104(7)
3.319(7), 3.375(7)
3.336(8), 3.377(8)
179.66(10)
178.1(6)
–
–
–
103.72(8)
84.3(3)
86.2(2)
169.1(2)
86.3(2)
169.1(2)
360.52
6.7
99.32(11)
83.4(4)
90.2(3)
170.2(3)
87.3(3)
172.4(4)
360.22
4.8
102.12(11)
84.2(5)
88.3(3)
169.1(3)
85.6(3)
171.8(3)
360.22
4.5
C–Pt–C
C(1)–Pt–P(1)
C(1)–Pt–P(2)
C(11)–Pt–P(2)
C(11)–Pt–P(1)
Sum of angles
Tilt CCPt/PPPt
CCPtPP/aryl
90.5(2)
89.7(2)
90.7(3)
89.1(3)
360.00
[c]
[c]
[c]
0.00
89.3
0.00
88.7
1.9
86.0, 76.3
89.9, 83.4
80.3, 86.6
85.5, 89.9
[a] Numbering scheme employed in figures and text is as shown in Scheme 2. [b] Two molecules (1) and (2) were found for Ar = Xyl,
each with a centre of inversion on the platinum atom. [c] Tilt angles PCPt/PCPt.
Essential structural data marking the differences between
cis and trans conformers are summarised in Table 4 to-
gether with DFT-calculated energies for possible isomers.
Table 4. Experimental and ADF/BP calculated structural data for
complexes [Pt(Ar)2(PEt3)2].
Exp. structure
from
single-crystal
XRD
Lowest-energy
structure[a]
/
stabilisation energy
[kcalmol–1]
Scheme 2. Numbering scheme.
[Pt(Ph)2(PEt3)2]
[Pt(2-Tol)2(PEt3)2]
(syn)
[Pt(2-Tol)2(PEt3)2]
(anti)
[Pt(3-Tol)2(PEt3)2]
(syn)
[Pt(3-Tol)2(PEt3)2]
(anti)
cis (173 K)
cis/10.7
cis-syn/6.2
cis-syn (293 K)[b]
cis-anti (293 K)[b]
cis-syn (173 K)
–
cis-anti/5.8
cis-syn/3.5
cis-anti/3.5
configured complex [Pt(4-CF3Ph)2(PEt3)2], which they ob-
tained by an oxidative addition/reductive elimination se-
quence using [PtBr(PEt3)3]BF4 and the silanol 4-
CF3PhSi(Me)2OH. The Pt–C bonds are markedly longer
[2.089(8) Å] and the Pt–P bonds slightly shorter
[2.287(3) Å] than what we found for the cis-4-Tol derivative.
The distances between the ortho-methyl groups and the
platinum atoms in compounds Ar = Xyl or Mes range from
3.32 to 3.38 Å, with the upper value representing approxi-
mately the sum of the van der Waals radii of Pt and CH3,[17]
values which have also been found for the series of phos-
phane(2-Tol)platinum complexes reported by Rieger et
al.[14] They are slightly lower for the (diimine)platinum com-
plexes cis-[Pt(Ar)2(_NN)] (_NN = α-diimine; 3.2–3.3 Å),[19,20]
[Pt(4-Tol)2(PEt3)2]
[Pt(Xyl)2(PEt3)2]
[Pt(Mes)2(PEt3)2]
cis (173 K)
trans (173 K)
trans (173 K)
cis/3.0
trans/4.7
trans/7.3
[a] Calculated data for [Pt(Ar)2(PMe3)2] model systems, stabilisa-
tion energy calculated as the energy difference between the highest
and lowest bonding energies of the corresponding cis and trans
isomers. [b] Data for cis-[Pt(2-Tol)2(PEt3)2] (cis-syn: monoclinic
P21/n, cis-anti: monoclinic P21/c) from ref.[14]
The ADF/BP calculated ground-state molecular struc-
probably caused by shorter Pt–C(1) bond lengths due to the tures agree fully with the experimental structures The syn
weaker trans influence of the diimine ligands. The tilt angles isomer was calculated to be slightly more stable
between the aryl rings and the CCPtPP coordination plane (1 kcalmol–1) for the cis-2-Tol complex, which is in agree-
range from 76 to 90° for the cis derivatives but are almost ment with the observed ratio of syn/anti stereoisomers of
90° for the two trans-configured analogues. Since 90° is the approximately 2:1 in solution (see NMR). The calculated
optimum value for steric reasons, this is remarkable. We difference in bond energy for the syn and anti isomers of
have previously made a similar observation for related cis- the 3-Tol derivative is negligible, which fully agrees with the
configured (diimine)platinum complexes (angles around approximate 1:1 ratio found in the low-temperature NMR
70°) and have ascribed this effect to an optimisation of li- spectra. The fact that we found the syn form for the 3-Tol
gand-to-metal overlap in conjunction with a metal-medi- derivative in the crystal structure determination can thus
ated ligand-to-ligand interaction.[19] Such interactions seem not be ascribed to its higher stability; it might instead be
to be absent for the trans-configured compounds.
due to a higher tendency to crystallise.
4060
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org Eur. J. Inorg. Chem. 2005, 4056–4063

The Borderline between cis and trans in (Phosphane)platinum() Complexes
FULL PAPER
or 193(5) K; chemical shifts were referenced to external TMS (1H,
13C) or K2PtCl6 (Ξ = 21.496784 MHz, 195Pt). The assignment of
13C and 195Pt resonances was obtained from two-dimensional gra-
Conclusions
The combination of structural and spectroscopic data
with quantum chemical calculations on diarylplatinum dient selected (gs) 1H,13C HMQC and HMBC and 1H,195Pt
HMQC spectra. Conformational assignments and the analysis of
dynamic properties were derived from two-dimensional 1H gs-
NOESY NMR spectra using mixing times of between 500 and
750 ms. All 2D NMR spectra were obtained by using standard
pulse sequences from the Bruker pulse program library.
complexes with triethylphosphane ligands has revealed that
the borderline between cis- and trans-configured ground-
state geometries is crossed when two ortho-methyl groups
are introduced on the aryl ligands. Under the applied prep-
aration conditions (high-temperature ligand exchange using
the [Pt(Ar)2(DMSO)2] precursor complexes) we found the
selective formation of either the cis (Ph or Tol) or trans
(Xyl or Mes) isomers. In case of the 2-Tol and the 3-Tol
derivatives, the formation of a mixture of the syn and anti
stereoisomers was observed. Detailed NMR experiments al-
lowed the structural assignment of the individual isomers
and within these experiments we did not observe any iso-
merisation reaction. Therefore, we postulate that our cho-
Synthesis of the Complexes [Pt(Ar)2(PEt3)2]: An excess of triethyl-
phosphane (1 mmol) was added to a stirred toluene (20 mL) solu-
tion of [Pt(Ar)2(DMSO)2] (Ar = Ph, 2-Tol, 3-Tol, 4-Tol, Xyl, Mes;
typically 0.28 mmol). The mixture was stirred with refluxing for
24 h. The solvent, DMSO and the excess of PEt3 were distilled off
and the residue washed with cold pentane. The colourless products
were dried in vacuo. Recrystallisation of the products from warm
pentane gave colourless crystals. Yields and elemental analyses are
given in Table 5, and full NMR spectroscopic data in Tables 6, 7
sen reaction conditions are appropriate to obtain the ther- and 8.
modynamically most stable forms. The results of quantum
Table 5. Yields and analytical data of complexes [Pt(Ar)2(PEt3)2].
chemical calculations are in excellent agreement with the
findings, even in predicting the relative energies of syn or
anti stereoisomers with relatively small energy differences.
Ar
Yield
[mg]
Yield
[%]
Calculated
[%]
Found
[%]
Ph
105
167
168
165
154
161
64
97
98
96
86
86
C 49.22, H 6.89
C 50.89, H 7.23
C 50.89, H 7.23
C 50.89, H 7.23
C 52.41, H 7.54
C 53.80, H 7.83
C 50.01, H 6.90
C 50.93, H 7.28
C 50.90, H 7.24
C 50.94, H 7.27
C 52.53, H 7.58
C 53.83, H 7.83
2-Tol
3-Tol
4-Tol
Xyl
Experimental Section
General: The precursor complexes [Pt(Ar)2(DMSO)2] (Ar = Ph, 2-
Tol, 3-Tol, 4-Tol, 2,6-Xyl or Mes) were prepared according to lit-
erature methods.[15,18] Triethylphosphane was obtained from Ald-
rich. All reactions were carried out under argon in dry and distilled
Mes
1
analytical-grade solvents. One-dimensional H, 13C{1H}, 31P{1H},
Crystallography: Single crystals of [Pt(Ar)2(PEt3)2] (Ar = Ph, 3-Tol,
4-Tol, 2,6-Xyl or Mes) were obtained by slow evaporation of the
solvent from saturated solutions in CH2Cl2 or pentane. The X-ray
data of these complexes were collected at 173(2) K with a Siemens
P4 or P3 diffractometer, using graphite-monochromated Mo-Kα ra-
diation (λ = 0.71073 Å) and employing Wyckoff scans. The struc-
tures for Ar = Xyl or Mes were solved by direct methods; for all
others the Patterson method was employed, in each case using the
SHELXTL package,[22] while refinement was carried out with
SHELXL97 employing full-matrix least-squares methods on F2
with Fo2 Ն 2σ(Fo2).[23] All non-hydrogen atoms were refined aniso-
and 195Pt{1H} NMR spectra were recorded with a Bruker DPX-
300 NMR spectrometer (1H: 300.13; 13C: 75.47; 31P: 121.49; 195Pt:
64.52 MHz). Chemical shifts are relative to the internal chloroform
peak at δ = 7.26 ppm for 1H and δ = 77.0 ppm for 13C, external
85% H3PO4 for 31P and Na2PtCl6 in D2O for 195Pt. A 90° pulse
was used in every case. NMR measurements for the assignment of
the syn and anti conformations of [Pt(2-Tol)2(PEt3)2] were carried
out with a Bruker DPX 400 spectrometer (1H: 400.13 MHz; 195Pt:
86.01 MHz; 13C: 100.3 MHz) in CDCl3 at 303(1) K; for [Pt(3-Tol)2-
(PEt3)2] these measurements were performed in CD2Cl2 at 223(5)
1
Table 6. H NMR spectroscopic data of complexes [Pt(Ar)2(PEt3)2].[a]
δ (JH,Pt
CH3(o)
)
Ar
Ph
H(2)
H(6)
H(3)
H(5)
H(4)
6.63
CH3(m)
CH3(p)
CH2(Et)
1.50
CH3(Et)
1.08
7.31
(55.5)
–
7.31
(55.5)
7.4
(56.0)
7.37
(54.5)
7.10
(56.8)
6.95
(br)
6.84
(21.6)
6.77
6.84
(21.6)
6.67
–
–
–
2-Tol (syn)[b]
2-Tol (anti)[b]
3-Tol (303 K)
3-Tol (193 K)
4-Tol
6.77
6.77
6.46
2.45
(5.7)
2.60
(5.9)
–
–
–
–
–
1.47
1.47
1.48
1.07
1.07
1.07
–
6.77
–
6.67
7.16
(56.7)
7.00
(br)
7.16
(56.9)
–
6.74
(25.2)
6.63/
6.60
2.15
–
–
6,33
(br)
–
–
–
1.98/
2.01
–
–
1.27
(br)
1.51
0.88
(br)
1.07
7.16
(56.9)
–
6.68
6.80
6.64
6.68
2.10
–
Xyl
6.80
6.64
6.80
2.60
(5.1)
2.54
(4.7)
–
–
1.32
1.29
0.89
0.89
Mes
–
–
–
2.17
[a] Chemical shifts in ppm (JH,Pt in Hz), as measured in CD2Cl2. [b] Measured in CDCl3.
Eur. J. Inorg. Chem. 2005, 4056–4063
www.eurjic.org
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4061

D. Gudat, V. K. Jain, A. Klein, T. Schurr, S. Zálisˇ
FULL PAPER
Table 7. 13C NMR spectroscopic data of complexes [Pt(Ar)2(PEt3)2].[a]
δ (JC,Pt
C(4)
)
[b]
Ar
Ph
C(1)
C(2)
C(6)
C(3)
C(5)
CH3(o)
CH3(p)
CH2(Et)
CH3(Et)
155.1
(878.9)
162.5
(844.0)
161.4
(832.2)
164.4
(812)
166.6/
166.5
160.8
(1090)
160.2
(591.1)
155.5
127.1
(63.5)
127.6
(43.0)
128.2
(40.0)
137.1
(34.6)
136.9/
137.2
127.7
(66.2)
125.2
(24.9)
126.3
127.1
(63.5)
123.8
(62.4)
123.7
(62.7)
133.4
(32.9)
133.0/
133.1
127.7
(66.2)
125.2
(24.9)
126.3
136.8
(34.1)
142.2
(25.4)
142.6
(20.4)
135.2
(63.4)
135.6
(br)
136.8
(34.1)
135.8
(24.3)
136.8
(37.9)
126.3
(66.1)
126.3
(br)
121.0
(12.3)
121.0
(9.6)
120.9
(9.2)
121.2
(11.5)
121.0/
121.0
129.1
(5.8)
–
–
15.9
(26.4)
15.8
8.2
(17.6)
8.0
(29.1)
2-Tol (syn)
2-Tol (anti)
3-Tol (303 K)[c]
3-Tol (193 K)[c]
4-Tol
27.5
(74.6)
25.9
–
–
15.8
8.0
(67.3)
21.7
15.4
(21.8)
14.6
(br)
15.6
(25.6)
15.0
(32.9)
15.0
7.7
(17.4)
8.1
[d]
21.6/
21.6
–
135.8
(35.4)
145.0
135.8
(35.4)
145.0
20.9
–
8.2
(17.7)
7.83
(16.24)
7.87
(28.6)
(41.0)
(39.6)
Xyl
121.9
26.6
(55.5)
26.4
Mes
144.7
144.7
131.1
20.8
(536.0)
(59.0)
(33.5)
(17.7)
2
[a] Chemical shifts in ppm (JC,Pt in Hz), as measured in CDCl3. [b] Further coupling J
(Hz). [c] In CD2Cl2. [d] mCH3.
CH2,P
Table 8. 31P NMR and 195Pt NMR spectroscopic data of complexes
[Pt(Ar)2(PEt3)2].[a]
ployed for geometry optimisation. The inner shells were repre-
sented by the frozen-core approximation (1s for C, 2p for P and
1s–4d for Pt were kept frozen). Core electrons were included for
the calculation of NMR parameters, and here the quadruple-ζ with
four polarisation functions (QZ4P)[24b] basis set was used for Pt.
The calculations were carried out with the functional including
Becke’s gradient correction[26] to the local exchange expression in
conjunction with Perdew’s gradient correction[27] to the local corre-
lation (BP86). The scalar relativistic (SR) zero-order regular
approximation (ZORA) was used within this study. The energy bar-
riers of syn/anti isomerisation within [Pt(2-Tol)2(PMe3)2] and [Pt(3-
Tol)2(PMe3)2] model complexes were calculated as the lowest en-
ergy rotation barrier associated with the rotation of one 2-Tol or
3-Tol group, respectively.
31P
195Pt
δ
Ar
Ph
δ
1
1
5.4 (t, JP,Pt
1775 Hz)
0.8 (t, JP,Pt
1751 Hz)
1.0 (t, JP,Pt
1739 Hz)
=
=
=
=
–4568 (t, JP,Pt
=
=
=
=
Ϸ
Ϸ
=
=
=
1790 Hz)[b]
1
1
2-Tol (syn)
2-Tol (anti)
–4471 (t, JP,Pt
1751 Hz)
1
1
–4466 (t, JP,Pt
1739 Hz)
1
1
3-Tol (303 K) 3.9 (t, JP,Pt
–4570 (t, JP,Pt
1754 Hz)
1764 Hz)
1
3-Tol (193 K) 2.42 (1JP,Pt
=
–4587 (t, JP,Pt
[c]
1754 Hz)
1.77 kHz)
1
2.45 (1JP,Pt
=
–4589 (t, JP,Pt
1754 Hz)
4.0 (t, JP,Pt
1762 Hz)
1.1 (t, JP,Pt
2845 Hz)
1.8 (t, JP,Pt
1.77 kHz)
1
1
4-Tol
Xyl
=
=
=
–4566 (t, JP,Pt
1780 Hz)
Acknowledgments
1
1
–4311 (t, JP,Pt
2848 Hz)
1
1
Dr. M. Bader and Dr. M. Niemeyer (University of Stuttgart) are
thanked for collecting single-crystal XRD data. Support for this
work under the Indo-German bilateral program from the BMBF
(Project No. IND 99/060) and the COST D14 action is also ac-
knowledged. We are also grateful for a loan of K2PtCl4 by Johnson
Matthey. S. Z. also acknowledges financial support by the Grant
Agency of the Academy of Sciences of the Czech Republic (grant
1ET400400413).
Mes
–4306 (t, JP,Pt
2841 Hz)
2841 Hz)
[a] Chemical shifts in ppm (JP,Pt in Hz), as measured in CDCl3. [b]
Measured in (CD3)2CO. [c] In CD2Cl2.
tropically, including the disordered C atoms of the ethyl groups in
the compounds Ar = 3-Tol or Mes. Hydrogen atoms were intro-
duced using appropriate riding models. An empirical absorption
correction was performed using ψ-scans. CCDC-266186 for
[Pt(Ph)2(PEt3)2], -266187 for [Pt(3-Tol)2(PEt3)2], -266188 for [1] F. R. Hartley, The Chemistry of Platinum and Palladium, John
Wiley & Sons, New York, 1973.
[Pt(4-Tol)2(PEt3)2], -266189 for [Pt(2,6-Xyl)2(PEt3)2] and -266190
for [Pt(Mes)2(PEt3)2] contain 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.a-
c.uk/data_request/cif.
[2] J. N. Harvey, K. M. Heslop, A. G. Orpen, P. G. Pringle, Chem.
Commun. 2003, 278–279 and references cited therein.
[3] G. K. Anderson, R. J. Cross, Chem. Soc. Rev. 1980, 9, 185–215.
[4] G. A. Petsko, Nature 1995, 377, 580–581.
[5] P. J. Sadler, Adv. Inorg. Chem. 2000, 49, 183–306.
[6] Cisplatin – Chemistry and Biochemistry of a Leading Anticancer
Drug (Ed.: B. Lippert), 1st ed., Helvetica Chimica Acta
(Zürich), Wiley-VCH (Weinheim), 1999.
Quantum Chemical Calculations: Ground-state electronic structure
calculations were performed by density-functional theory (DFT)
methods using the ADF2004.01 program package.[24] Nuclear spin-
spin coupling constants were calculated with the CPL module[25]
within ADF2004.01. The calculations were performed on the
model systems [Pt(Ar)2(PMe3)2] due to the size of the [Pt(Ar)2-
(PEt3)2] complexes. Slater-type orbital (STO) basis sets of triple-ζ
quality with one polarisation function (TZ2P)[24b] for Pt and two
polarisation functions (TZP)[24b] for the remaining atoms were em-
[7] J. Chatt, R. G. Wilkins, J. Chem. Soc. 1952, 273–278.
[8] J. Chatt, R. G. Wilkins, J. Chem. Soc. 1952, 4300–4306.
[9] J. Chatt, B. L. Shaw, J. Chem. Soc. 1959, 4020–4033.
[10] T. G. Appleton, M. A. Bennett, Inorg. Chem. 1978, 3, 738–747.
[11] R. Romeo, M. R. Plutino, L. M. Scolaro, S. Stoccoro, Inorg.
Chim. Acta 1997, 265, 225–233.
[12] P. Power, J. Organomet. Chem. 2004, 689, 3904–3919.
4062
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
Eur. J. Inorg. Chem. 2005, 4056–4063

The Borderline between cis and trans in (Phosphane)platinum() Complexes
[13] M. C. Biagini, M. Ferrari, M. Lanfranchi, L. Marchio, M. A. [22] G. M. Sheldrick, SHELXTL, Bruker Analytical X-ray Systems,
FULL PAPER
Pellinghelli, J. Chem. Soc., Dalton Trans. 1999, 1575–1580.
[14] A. L. Rieger, G. B. Carpenter, P. H. Rieger, Organometallics
1993, 12, 842–847.
[15] A. Knödler, T. Schurr, A. Klein, W. Kaim, V. K. Jain, K.-W.
Klinkhammer, D. Gudat, S. Zálisˇ, Organometallics 2005, 24,
4125–4131.
Madison, Wisconsin, USA, 1998.
[23] G. M. Sheldrick, SHELXL-97: A program for Crystal Structure
Determination, Universität Göttingen, Göttingen, Germany,
1997.
[24] a) G. te Velde, F. M. Bickelhaupt, S. J. A. van Gisbergen, C.
Fonseca Guerra, E. J. Baerends, J. G. Snijders, T. Ziegler, J.
Comput. Chem. 2001, 22, 931–967; b) ADF2004.01, SCM,
Theoretical Chemistry, Vrije Universiteit, Amsterdam, The Ne-
therlands, http://www.scm.com.
[25] a) J. Autschbach, T. Ziegler, J. Chem. Phys. 2000, 113, 936–947;
b) J. Autschbach, T. Ziegler, J. Chem. Phys. 2000, 113, 9410–
9418.
[16] M. Hashemi, M. Rashidi, J. Organomet. Chem. 2005, 690, 982–
989.
[17] A. T. Hutton, P. G. Pringle, B. L. Shaw, J. Chem. Soc., Dalton
Trans. 1985, 1677–1682.
[18] C. Eaborn, K. Kundu, A. Pidcock, J. Chem. Soc., Dalton
Trans. 1981, 933–938.
[19] A. Klein, J. van Slageren, S. Zálisˇ, Eur. J. Inorg. Chem. 2003,
1917–1938.
[20] W. Kaim, A. Klein, Organometallics 1995, 14, 1176–1186.
[21] N. Mintcheva, Y. Nishihara, M. Tanabe, K. Hirabayashi, A.
Mori, K. Osakada, Organometallics 2001, 20, 1243–1246.
[26] A. D. Becke, Phys. Rev. A 1988, 38, 3098–3100.
[27] J. P. Perdew, Phys. Rev. B 1986, 33, 8822–8824.
Received: March 23, 2005
Published Online: September 5, 2005
Eur. J. Inorg. Chem. 2005, 4056–4063
www.eurjic.org
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4063