Synthesis, spectroscopic characterization and catalytic activity of platinum(II) carbene complexes

Article abstract of DOI:10.1016/j.ica.2016.02.036

A novel platinum complex with 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidenyl ligand has been synthesized and characterized on the basis of elemental analysis, MS, ¹H and ¹³C NMR spectroscopy, X-ray Absorption Spect

Full text of DOI:10.1016/j.ica.2016.02.036

Accepted Manuscript
Synthesis, spectroscopic characterisation and catalytic activity of platinum(II)
carbene complexes
Ekaterina Bolbat, Karina Suarez-Alcantara, Sophie E. Canton, Ola F. Wendt
PII:
S0020-1693(16)30052-4
http://dx.doi.org/10.1016/j.ica.2016.02.036
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Please cite this article as: E. Bolbat, K. Suarez-Alcantara, S.E. Canton, O.F. Wendt, Synthesis, spectroscopic
characterisation and catalytic activity of platinum(II) carbene complexes, Inorganica Chimica Acta (2016), doi:
http://dx.doi.org/10.1016/j.ica.2016.02.036
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Synthesis, spectroscopic characterisation and catalytic activity
of platinum(II) carbene complexes
Ekaterina Bolbat,a Karina Suarez-Alcantara,b Sophie E. Cantonc and Ola F. Wendta*
a. Centre for Analysis and Synthesis, Department of Chemistry, Lund University,
P.O. Box 124, S-221 00 Lund, Sweden
b. Unidad Morelia del Instituto de Investigaciones en Materiales, Universidad
Nacional Autónoma de México, Antigua Carretera a Pátzcuaro No. 8701, Col. Ex
Hacienda de San José de la Huerta, CP 58190 Morelia, Michoacán, México
c. FS-SCS, Deutsches Elecktronen-Synchrotron (DESY), Notkestr.85, 22607
Hamburg, Germany and Max Planck Institute for Biophysical Chemistry, Am
Fassberg 11, 37077, Göttingen, Germany
Keywords: NHC carbene complexes, platinum, EXAFS, XRD, hydrosilylation.
Abstract
A novel platinum complex with 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-
imidazol-2-ylidenyl ligand has been synthesized and characterized on the basis of
1
elemental analysis, MS, H and 13C NMR spectroscopy, X-ray absorption
spectroscopy and single crystal X-ray diffraction studies. The XRD determination of
the complex (Monoclinic, C 2/c) revealed a structure in which platinum (II) centre
coordinates two chlorides, a carbon atom of the N-heterocyclic carbene (NHC) and
is stabilized by coordinating the nitrogen atom of a 3-chloropyridine molecule,
forming an over-all square planar geometry. By prolonging the time of the reaction,
it was possible to obtain trans-dichloridobis(3-chloropyridine) platinum(II) complex
under the same reaction conditions. The electronic and molecular properties of both
complexes were investigated and compared by means of Near Edge X-ray
Absorption Fine Structure spectroscopy (NEXAFS), supported by numerical
simulations. The platinum carbene complex obtained was tested in a series of C-H
activation and hydrosilylation reactions.

Graphical abstract
Highlights
• Synthesis
of
[1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene](3-
chloropyridyl)-platinum(II) dichloride and trans-dichlorobis(3-chloropyridine)
platinum(II)
• Characterization of the complexes using XRD and XAS techniques
• Determination of the local binding environment around the platinum site
using XANES data
• Catalytically active in hydrosilylation
1. Introduction
Over the last decade N-heterocyclic carbene complexes of late transition metals
have gained significant attention in modern chemistry due to their truly unique
features [1-5]. These compounds are widely utilized across the chemical field
including the use in materials [6, 7], as metallopharmaceuticals [8-10] and
extensively as both homogeneous and heterogeneous catalysts [11-15]. Among
them is a family of catalytically active so-called PEPPSI-complexes wherein a
palladium(II) centre is strongly coordinating an NHC ligand and is stabilized by 3-
chloropyridine as an ancillary ligand [16-18]. First synthesized by Organ and co-
workers, these complexes have been shown to possess spectacular reactivity in a

range of cross coupling reactions such as Suzuki-Miyaura [19-21], Negishi[22-24],
Buchwald-Hartwig [25-28] and Kumada reactions [29].
Compared to the extensive coordination chemistry of palladium-NHC compounds,
similar platinum complexes have been much less studied until the recent discovery
of their high anti-cancer activity [30-33]. However, interactions of Pt-NHC with
biological systems and their therapeutic properties remain a substantially
unexplored field of great interest [34]. Similarly, Pt-NHC complexes have
considerable potential for use in catalysis, e.g. hydrosilylation, cycloisomerization,
C-H activation processes, etc [35-38]. Thus, the development of novel Pt complexes
with diverse range of NHC ligands is in high demand. In the present work, we report
the first PEPPSI type Pt complex together with its catalytic activity in C-H
functionalization and hydrosilylation reactions.
A general trend to extend and significantly improve the application of transition
metal complexes, including Pt carbene compounds, is the creation of hybrid
materials where metal complexes are covalently grafted to suitable kind of support
as organic polymers, metal oxides, nanotubes or plastics [39-42]. Despite the many
benefits of such method, the main drawback arises from the lack of analytical
techniques for characterization of the obtained materials. For such hybrids, it is
important to monitor closely the initial stability, the changes in oxidation state, and
the nature of the chemical bonding during the activation step in order to optimize
these systems. In this regard, the near edge region of the X-ray absorption (XA)
spectrum is an extremely sensitive probe of the electronic and structural properties
around a specific metal centre for samples in any physicochemical phase of matter,
unlike X-ray diffraction that relies on long-range crystalline ordering. In this study,
we introduce the comparative analysis of electronic and structural properties of [1,3-
bis(2,6-diisopropylphenyl)imidazol-2-ylidene](3-chloropyridyl) platinum(II) dichloride
and trans-dichlorobis(3-chloropyridine) platinum(II) complexes using a combination
of XRD and XAS techniques. This gives a convenient reference material when
using such compounds in study of the catalysis mechanism.
2. Experimental
2.1. General considerations

Commercially available reagents and deuterated solvents were purchased from
Sigma Aldrich and Acros Organics and used as received. 1,3-Bis(2,6-
diisopropylphenyl)imidazolium chloride was prepared according to the previously
1
13
published procedure [43]. H and C NMR spectra were recorded on a Bruker
Avance 400 spectrometer working at 400 MHz. Chemical shifts are given in ppm
downfield from TMS using residual peak of the solvents as a reference. Multiplicities
are abbreviated as follows: (s) singlet, (d) doublet, (t) triplet, (m) multiplet. Elemental
analysis were performed by H. Kolbe Mikroanalytisches Laboratorium, Mülheim an
der Ruhr, Germany. IR spectra were recorded on a Bruker Alpha spectrometer, with
diamond ATR–FT IR detection.
2.1 Synthesis of [1,3-Bis(2,6-diisopropylphenyl)imidazol-2-ylidene](3-chloropyridyl)-
platinum(II) dichloride (1)
Platinum(II)
chloride
(0.150
g,
0.564
mmol),
1,3-Bis(2,6-
diisopropylphenyl)imidazolium chloride (0.264 g, 0.621 mmol) and K2CO3 (0.389 g,
2.81 mmol) were added to 2.25 mL of 3-chloropyridine in 4 ml vial capped with a
Teflon-lined screw cap. The resulting mixture was vigorously stirred for 48 h at
80°C. After cooling to RT, the reaction mixture was diluted with dry dichloromethane
and filtered through a short pad of celite. The solvent was removed on the high
vacuum line and the crude was stirred in dry n-pentane during the night. The beige
residue was separated by decanting the liquid and purified with column
chromatography (eluent: n-hexane:ethyl acetate = 8:9). The solvent was evaporated
and the yellow-orange solid was dried in high vacuum (0.317 g, 73 %).
1H NMR (400 MHz, CD2Cl2): [ppm] = 8.63 (d, 1H), 8.56 (dd, 1H), 7.67 (dt, 1H)), 7.49
(dd, 1 H), 7.34 (d, 4H), 7.15 (dd, 1H), 7.12 (s, 2H), 3.14 (m, 4H), 1.42 (d, 12H), 1.11
(d, 12H).
13C-NMR (101 MHz, CDCl3, 25 °C): [ppm] =150.5, 149.5, 146.6, 137.5, 135.4,
132.5, 130.2, 128.0, 126.0, 124.9, 124.3, 124.0, 29.9, 28.8, 26.4, 23.1.
Anal. Calcd. for C32H40Cl3N3Pt: C, 50.04; H, 5.25; N, 5.47. Found: C, 50,24; H, 5,91;
N, 5,12.
2.2 Synthesis of trans-dichlorobis(3-chloropyridine) platinum(II) (2)
Platinum(II) chloride (0.100 g, 0.376 mmol), 1,3-Bis(2,6-diisopropylphenyl)
imidazolium chloride (0.176 g, 0.414 mmol) and K2CO3 (0.214 g, 1.54 mmol) were

added to 1.5 mL of 3-chloropyridine in 4 ml vial capped with a Teflon-lined screw
cap. The resulting mixture was vigorously stirred for 64 h at 80°C. After cooling to
RT, the reaction mixture was diluted with dry dichloromethane and filtered through a
short pad of silica gel covered with a pad of celite eluting with CH2Cl2 until the
product was completely recovered. The solvent was removed on the high vacuum
line and the crude was stirred in dry n-pentane during the night. The residue was
separated by decanting the liquid and purified with column chromatography (eluent:
n-hexane:ethyl acetate = 1:10). The solvent was evaporated and a yellow solid was
dried in high vacuum (0.060 g, 32 %).
1H NMR (400 MHz, CD2Cl2): [ppm] = 8.98 (s, 2H), 8.86 (d, 2H), 7.89 (d, 2H), 7.36
(t, 2H). A preliminary XRD data set was recorded and shown to be in accordance
with the literature [44].
2.3. General procedure for the C-H acetoxylation of 4-(2-pyridyl)benzaldehyde
4-(2-Pyridyl)benzaldehyde (25 mg, 0.137 mmol, 1 equiv), PhI(OAc)2 (54.3 mg, 0.164
mmol, 1.2 equiv), and Pt-IPr catalyst (5.2 mg, 0.007 mmol, 0.05 equiv) were
combined in CH3CN (2 mL) in a 8 mL vial. The vial was sealed with a Teflon lined
cap, the reaction mixture was heated at 100 ºC for 24 hr. The reaction mixture was
analyzed by GC.
2.4. General procedure for the hydrosilylation of styrene
A Straus flask was charged with styrene (0.416 g, 4mmol, 1 equiv),
bis(trimethylsiloxy)methylsilane (0.979 g, 4.4 mmol, 1.1 equiv) and Pt-IPr catalyst (
15.4 mg, 0.02 mmol, 0.005 equiv) under an atmosphere of N2 and then was put on a
preheated oil bath for 6hr. For analysis, an aliquote (0.1 mL) was removed from the
reaction mixture, diluted with deuterated chloroform in an NMR tube and analyzed
by 1H NMR spectroscopy.
2.5 Single-crystal X-ray diffraction
Single crystals suitable for X-ray analysis of complex 1 were grown from a
concentrated solution by slow evaporation in chloroform. Intensity data were
collected with an Oxford Diffraction Excalibur 3 system, using -scans and MoK ( =
0.71073 Å) radiation [45]. The data were extracted and integrated using Crysalis

RED [46]. The structure was solved by direct methods and refined by full-matrix
least-squares calculations on F2 using SHELXTL5.1 [47]. Molecular graphics were
generated using CrystalMaker® 8.3.5. Crystallographic data and details of the data
collection and structure refinements are listed in Table 1.
2.6 X-ray Absorption Spectroscopy
The X-ray Absorption Spectroscopy (XAS) experiments were carried out at the
beamline I811 of the MAX-II storage ring in Lund, Sweden, operating at 1.5 GeV
with a current maximum at 250 mA (2 injections per day). This is a superconducting
multipole wiggler beamline equipped with a water-cooled channel cut Si(1 1 1)
double crystal monochromator delivering at 10 keV, approximately 2 x 1015
photons/s/0.1% bandwidth with horizontal and vertical FWHM of 7 and 0.3mrad,
respectively [48]. A beamsize of 0.5 mm x 0.5 mm (width x height) was used.
Samples were compacted in small pellets and fixed in the sample chamber at 45°
respective to the beam detector. The ionisation and sample chambers were filled
with a mixture of He/N2. Higher order harmonics were reduced by detuning the
second monochromator to 40% of the maximum intensity. The XAS spectra were
collected in fluorescence mode at the Pt L3-edge (11.56 keV); a 3mm thick Zn filter
was used.
Five scans were collected for each compound in order to check for possible
radiation damage, none was observed. The data were then averaged. The energy
calibration was performed by setting the first derivative of the L3 edge of a Pt foil
recorded in transmission to 11.564 keV. The spectra were normalized to the edge
jump after removal of a linear background. Data reduction and analysis were
performed with the Athena software.
3. Results and discussions
The 1,3-bis(2,6-diisopropylphenyl)imidazolium (IPr) chloride ligand was synthesized
according to the previously reported procedure [43]. In order to obtain a Pt-IPr
complex (1), the same method as in the synthesis of the analogous palladium
complex was applied [18]. For this purpose, PtCl2 was reacted with the IPr
imidazolium precursor in the presence of potassium carbonate in an excess of 3-
chloropyridine.

Scheme 1. Synthesis of Pt(IPr)(3-ClPy)Cl2 (1) and trans-[Pt(3-ClPy)2Cl2] (2)
complexes.
While both Pt(II) and Pd(II) metal ions readily form square planar complexes,
reactions on Pt(II) centres typically proceed many orders of magnitude slower
compared to Pd(II) [49]. Therefore, TLC was used to monitor the progress of the
reaction. After 48h, the desired Pt complex of PEPPSI type was formed. It is
interesting to note that prolongation of the reaction time up to 64 hours led to
displacement of the carbene and formation of trans-Pt(3-ClPy)2Cl2 (2) complex as
the thermodynamic product despite the strong Pt-carbene bond. Compound 2 is a
known compound displaying crystal data in good agreement with the literature. The
reactions are described in Scheme 1.
A single crystal X-ray diffraction experiment was performed and the molecular
structure of complex 1 is given in Figure 1. The geometry around the Pt(II) centre is
as expected a slightly distorted square-plane with a trans arrangement of the two
chlorine ligands and angles between cis-ligands ranging from 87.4(3)° to 94.2(3)°.
The NHC ring is situated at a 66°angle to the coordination plane of Pt, whereas the
angle for the pyridine ring is 59°. The carbene C−Pt distance is 1.986(8) Å and the
Pt−N 3-chloropyridine bond distance is 2.089(7) Å, which is in agreement with
previously published data for similar complexes [36,38,50].

Figure 1. Molecular structure of the compound 1. Hydrogen atoms omitted for
clarity.
Further information about the influence of the specific chemical environment around
the metal ion on the electronic and geometric structures of 1 and 2 were obtained
with XAS. Figure 2 displays the X-ray absorption spectra of 1 and 2 at the Pt L3-
edge. The inset zooms on the near-edge region and shows the FEFF 9.0
simulations for complex 1 (Sim 1) and 2 (Sim 2) based on the crystal structures,
with an arbitrary offset in the y axis for clarity.

Figure 2. Normalized X-ray absorption coefficient µ(E) for complex 1 (red) and 2
(black) at the Pt L3 edge. The white line and the hybridization peak are indicated by
WL and HP respectively. The inset zooms on the near-edge region and shows the
FEFF 9.0 simulations for complex 1 (orange) and 2 (grey) based on the crystal
structures, with an arbitrary offset in the y axis for clarity.
We focused on the analysis of XANES region of the spectra due to its high
sensitivity to the local binding environment around the particular absorbing atom.
The first observable sharp spectral feature at the Pt L3 edge (the so-called white line
(WL)) is attributed to the dipole-allowed transition from Pt 2p3/2 to the unoccupied 5d
levels that lie above the Fermi level. It represents a rather precise fingerprint of the
oxidation state of the Pt centre – the higher intensity of the spectral line is attributed
to the higher oxidation state of the metal. Although precautions must be taken when
comparing XAS measurements acquired at different beamlines, the XANES
features for [PtCl4]2- [51] and 2 present some similarities. In both complexes, the Pt
centre is in oxidation state +2 and they display a relatively sharp white line of
comparable intensities with a maximum at 11567 eV and 11562 eV respectively.
Replacing two chlorine atoms by two 3-chloropyridine ligands does not seem to

alter the electronic distribution significantly despite the π-acceptor properties of the
pyridine; such an effect may be weakened by the fact that the pyridine is not co-
planar with the coordination plane and that electronic effects are known to be
transmitted primarily in a trans fashion. However, a very small effect can possibly be
seen in the crystallographic structure: the Pt−Cl bond distances are known to be
2.310(2) Å for [PtCl4]2-[52] and 2.3001(9) Å for 2 [44]. In other words, the
coordination of 3-Cl-pyridine causes a slight Pt−Cl bond contraction.
On the other hand, comparing the shapes of the white line for 1 and 2 reveals the
specific changes induced upon substitution of a 3-chloropyridine for the IPr carbene
ligand. As seen in Figure 2, the white line gets broader and less intense. The
intensity of the WL reflects the unoccupied density of state at the PtII centre [53,54].
Therefore, it can be deduced that the additional electron density associated with the
strong -donor ability of the NHC ligand does not completely redistribute onto the
trans 3-ClPy ligand, which results in a higher d-electron density at the Pt centre.
This observation can be correlated with the crystal data: the Pt−N of 3-ClPy is
2.089(7) Å in 1 instead of 2.015 Å in 2, which is expected from the higher trans
influence of the NHC ligand giving an additional effective charge on the PtII centre.
The second noticeable feature in the XANES spectra is the shoulder found after the
white line, the so-called hybridization peak (HP) [55]. It arises from a mixture
between the Pt ε photoelectron states of the outgoing photoelectron and the Cl 3d
unoccupied states. This hybridization is mediated by the multiple scattering paths
that connect these two atomic sites (e.g. Pt-Cl-Pt and Pt-Cl-Cl-Pt). As seen in the
inset of Figure 2, HP is more diffuse for 1 than for 2. This mirrors the increased
average Pt−Cl distance of 2.311(3) Å in 1, while it is only 2.3001 Å in 2.
This comparative study shows that the XANES region at the Pt L3 edge is a
sensitive spectral fingerprint of the Pt-NHC bond. As pointed out in the introduction,
these markers can be followed in any physicochemical phase, hence operando. As
such, it can clearly be applied to the diagnostic of the long-term hydrolytic durability
of this type of complexes under catalytic conditions.
In order to evaluate the catalytic activity of this type of compounds, complex 1,
obtained in this work, was tested as a catalyst in a range of ligand directed C-H
functionalization reactions (Table 3) [56]. So far, palladium complexes have been
overwhelmingly used as catalysts for such transformations, leaving ample space to

investigate other metal for such applications. In a model reaction, 4-(2-
pyridyl)benzaldehyde was reacted with PhI(OAc)2, NCS or Ph2IPF6 and 5 mol %
catalyst and heated at 100oC for 24 hours. Unfortunately, no conversion of the
starting material was observed.
Catalytic studies of the Pt-IPr complex performance were continued with a
benchmark
reaction
of
styrene
hydrosilylation
using
bis(trimethylsilyloxy)methylsilane (Table 4) [36]. The hydrosilylation of carbon-
carbon double or triple bonds employing platinum complexes is a well-known
process for producing silicon derivatives on an industrial scale. Pioneering work in
this field was done with highly active Speier and Karstedt catalysts [57, 58].
Recently several platinum N-heterocyclic carbene complexes were found to be
active in this transformation showing excellent efficiency and selectivity [36, 38, 59,
60]. We were pleased to find that the 0,5 mol % of Pt-IPr complex catalyses up to
°
96 % conversion of styrene to the products A and B at 100 C after 6 hours
according to the NMR data. The ratio between the hydrosilane addition products A
vs. B was 85 to 15 per cent respectively, which is within the same range as reported
°
by Strassner et al. Increase of the reaction temperature to 140 C led to the
decrease of the styrene conversion and the formation of platinum black was
observed.
Table 1.Crystal data and refinement details for compound 1.
Formula
Fw
C32H40Cl3N3Pt
768.11
Space group
Crystal system
T/K
a•/Å
b/Å
c/Å
β/deg
V/ų
C2/c
monoclinic
2/9Å3
34.0085(16)
15.9935(8)
12.7181(7)
105.777(4)
6657.0(6)
8
Z
Dcalcd /g cm-3
1.533
4.481
µ /mm-1
2.487-28.881
13749
7501
θ•-range /deg
No. reflns collected
No. of unique reflns
R(F) (I >2•(I))a
0.0407

wR2(F2) (all data)b
0.1133
1.113
0.0394
1441883
Sc
Rint
CCDC
___________________
a R = Σ(Fo - Fc)/ ΣFo. b wR2 = [Σw(Fo2 - Fc2)2/ Σ(Fo2)2]1/2.
c S = [Σw Fo2 - Fc2)2/(n-p)]1/2.
Table 2. Selected bond lengths and angles for complex 1.
Bond lengths/Å
Pt1-Cl1
Pt1-Cl2
Pt1-N1
Bond angles/deg
Cl1-Pt1-Cl2
N1-Pt1-C1
2.3153(15)
2.3062(16)
2.089(5)
1.984(5)
1.370(6)
1.366(6)
176.67(6)
177.8(2)
C1-Pt1-Cl1
C1-Pt1-Cl2
N1-Pt1-Cl1
N1-Pt1-Cl2
93.96(14)
89.36(15)
87.51(14)
89.18(14)
Pt1-C1
N2-C1
N3-C1
Table 3. C-H functionalization of 4-(2-pyridyl)benzaldehyde.
X-reagent
PhI(OAc)2
PhI(OAc)2
NCS
Solvent
MeCN
AcOH
MeCN
AcOH
Catalyst, %
Time
24
Conversion
5
5
5
5
0
0
0
0
24
24
Ph2IPF6
24
Table 4. Hydrosilylation of styrene.
Conversion
Entrya
T oC
Ratio A:Bb
b
(styrene) [%]

1
2
100
140
96
87
85:15
87:13
aReaction conditions: 0,5 mol % of catalyst, styrene (4 mmol), bis(trimethylsilyloxy)methylsilane (4.4
mmol), 6h.bDeterminded by 1H NMR.
Acknowledgements
We are grateful for support from the FP7 Marie Curie Actions of the European
Commission, via the Initial Training Network SMALL (MCITN-238804), the Swedish
Research Council, the Knut and Alice Wallenberg Foundation and the Royal
Physiographic Society in Lund. S.E.C acknowledges financial support from SFB
1073. We thank Dr. Alexey Polukeev for collecting the XRD data.
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Graphical abstract
An NHC platinum complex is characterised by inter alia XANES and is catalytically
active in hydrosilylation

Highlights
• Synthesis
of
[1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene](3-
chloropyridyl)-platinum(II) dichloride and trans-dichlorobis(3-chloropyridine)
platinum(II)
• Characterization of the complexes using XRD and XAS techniques
• Determination of the local binding environment around the platinum site
using XANES data
• Catalytically active in hydrosilylation