Synthesis and structural characterization of novel bridged platinum(II) biscarbene complexes

Article abstract of DOI:10.1016/S0022-328X(02)01670-4

Novel bridged platinum(II) biscarbene complexes are reported: platinum(II) (3) and 1,1′-dimethyl-3,3′-ethylene-4-diimidazolin-2,2′-diylidene platinum(II) complexes 4 are directly accessible in high yields starting from platinum halides. The one-pot synthe

Full text of DOI:10.1016/S0022-328X(02)01670-4

Journal of Organometallic Chemistry 660 (2002) 121ꢀ126
/
www.elsevier.com/locate/jorganchem
Synthesis and structural characterization of novel bridged
platinum(II) biscarbene complexes
Michael Muehlhofer, Thomas Strassner *, Eberhardt Herdtweck, Wolfgang
A. Herrmann
Anorganisch-chemisches Institut der Technischen Universitat Munchen, Lichtenbergstrasse 4, 85747 Garching, Germany
¨ ¨
Received 13 May 2002; accepted 7 June 2002
Dedicated to Professor Dr J. Strahle on the occasion of his 65th birthday
¨
Abstract
Novel bridged platinum(II) biscarbene complexes are reported: 1,1?-dimethyl-3,3?-methylene-4-diimidazolin-2,2?-diylidene
platinum(II) (3) and 1,1?-dimethyl-3,3?-ethylene-4-diimidazolin-2,2?-diylidene platinum(II) complexes 4 are directly accessible in
high yields starting from platinum halides. The one-pot synthesis obviates the need for multi-step reactions via metal precursors or
free carbenes. An X-ray crystal structure of 1,1?-dimethyl-3,3?-methylene-4-diimidazolin-2,2?-diylidene platinum(II) dibromide (3b)
confirmed the structural similarity to the known corresponding palladium complexes. Since free 1,1?-di-R-3,3?-methylene-4-
diimidazolin-2,2?-diylidenes are only available in low yields this synthetic route provides an easy access to the corresponding carbene
complexes.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Platinum(II) biscarbene complexes; NHC; X-ray structures
1. Introduction
imidazolin-2-ylidene by Arduengo et al. in 1991 [15].
With free carbenes a huge number of NHC complexes
was now accessible which could not be synthesized
before due to a lack of basic metal precursors.
In the last decade N-heterocyclic carbenes (NHC)
have been the subject of intense research in the field of
organometallic chemistry. Because of their extraordin-
ary properties they have found access to a great variety
But since free NHCs are sensitive to air and moisture
and therefore hard to handle, alternative routes for the in-
situ formation of NHC complexes have been developed,
like the ammonia- and the alcoholate-method [16,6].
Metal acetates also allow the synthesis of NHC
complexes without isolation of the free carbene. Palla-
dium(II) acetate in wet dimethyl sulphoxide at elevated
temperatures reacts via the deprotonation of the imida-
zolium salt. This route is very useful in cases where the
metal acetates are easily available and was used for the
synthesis of palladium(II) NHC complexes, which have
shown to be active in the CH-activation of methane [12].
Even methylene bridged bisimidazolium salts (Scheme
1) can be converted to the corresponding biscarbene
of catalytic processes which include Cꢀ
/C-coupling
reactions [1ꢀ3], olefin metathesis [4ꢀ7], hydroformyla-
/
/
tion [8,9], polymerization reactions [10,11] and CH-
activation [12]. The key for this development have been
improved synthetic methods for the preparation of
NHC complexes, which have been reported in recent
reviews [13,14].
While the NHC complex formation via multi compo-
nent reaction only plays an inferior role, many attempts
were made to improve the deprotonation reaction and
isolate free carbenes for consecutive reactions. A break-
through was the isolation of the first free 1,3-di-R-4-
complexes in high yields (85ꢀ90%) [10,11,17].
/
Since the acidic methylene protons in bisimidazolium
salts are also attacked under common deprotonation
conditions, a pathway via free biscarbenes is only
* Corresponding authors. Tel.: ꢁ
28913473
E-mail address: thomas.strassner@ch.tum.de (T. Strassner).
/
49-89-28913174; fax: ꢁ49-89-
/
0022-328X/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 3 2 8 X ( 0 2 ) 0 1 6 7 0 - 4

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M. Muehlhofer et al. / Journal of Organometallic Chemistry 660 (2002) 121ꢀ
/126
Scheme 1. The ‘acetate route’.
possible by using sterically extremely demanding bases,
but the yields are moderate [18,19].
For the syntheses of platinum NHC complexes so far
either platinum precursor molecules or free carbenes
had to be prepared, which usually have to be handled
1H-NMR spectra of the product solutions reveal that
the characteristic signals for the acidic proton in the
imidazolium salt at 9.65 ppm for 1a and 9.83 ppm for 1b
have disappeared. Furthermore the 13C spectra show a
significant shift for the carbon atom in position 2,
indicating the formation of the NHC complexes 3a and
3b.
under inert gas atmosphere [20ꢀ25]. To circumvent
/
multi step pathways we developed a new synthetic route.
While platinum acetate is not commercially available
and difficult to synthesize [26,27], the corresponding
halide salts are readily available.
We would like to report a synthetic route to novel
platinum NHC complexes via an in-situ method, which
eliminates the need for the metal precursors or metal
acetates and will be useful for all cases where the metal
acetates are not readily available.
1
In addition the H signals of the methylene groups of
3a and 3b split into two signals, which has already been
reported for several palladium(II) NHC complexes
bearing bridged NHC ligands. The new synthetic
method was also successful for the synthesis of the
1
ethylene bridged complex 4. Again the H signals of the
bridging ethylene group split up, indicating the forma-
tion of the desired product.
2. Results and discussion
3. X-ray structure determination of complex 3b
Using the halide salts we developed a new direct, easy
synthetic route for platinum(II) NHC complexes, com-
bining the advantages of readily available starting
materials with the in-situ deprotonation of the imida-
zolium salts and avoiding free carbenes or expensive
syntheses of air-sensitive organometallic precursors.
We found that sodium acetate does not compete in
carbene complex formation. In order to avoid halogen
scrambling during the reaction, the platinum(II) halides
were reacted with imidazolium salts bearing the same
halide counter anions. The methylene bridged species
Details of the X-ray experiment, data reduction, and
final structure refinement calculation are summarized in
Table 2. Crystals of complex 3b suitable for an X-ray
structure determination were grown from a saturated
solution of 3b in acetonitrile. Preliminary examination
and data collection were carried out on a CAD4
diffractometer (Nonius) at the window of a sealed tube
(NONIUS FR590; 51 kV; 35 mA; 1.8 kW) and graphite
˚
monochromated Moꢀ
/
Ka radiation (lꢂ 0.71073 A).
/
The unit cell parameters were obtained by full-matrix
least-squares refinement of 25 high angle reflections.
Data collection was performed at 293 K with an
exposure time of maximum 60 s per reflection (omega
3,3?-dimethyl-1,1?-methylene-diimidazolium
diiodide
(1a) and 3,3?-dimethyl-1,1?-methylene-diimidazolium di-
bromide (1b), as well as 3,3?-dimethyl-1,1?-ethylene-
diimidazolium diiodide (2) have been synthesized by a
known procedure [28], additional data of the character-
ization is given in Section 6.
In the complex synthesis with platinum(II) iodide one
equivalent of 1a and sodium acetate, respectively, were
solved in dimethyl sulphoxide and heated to 75 8C
(Scheme 2). After one hour, the dark color originating
from platinum(II) iodide disappears and a yellow
solution is obtained. After subjection to the work-up
procedure described below, 3a was isolated in 70.1%
yield. The same procedure was used for the reactions of
1b with platinum(II) bromide, leading to 3b in 64.2%
yield (Fig. 1, Table 1).
scans; scan width: (1.10ꢁ
/
0.15 tan U)8925% to each
/
side of the reflection to determine the background). A
total number of 10 100 reflections were collected. Raw
data were corrected for Lorentz and polarization effects
[29]. Corrections for absorption (based on psi-scans)
and decay effects were applied [30]. After merging a sum
of 2850 independent reflections remained and were used
for all calculations. The structure was solved by a
combination of direct methods and difference Fourier
syntheses [31]. All non-hydrogen atoms of the asym-
metric unit were refined with anisotropic thermal
displacement parameters. All hydrogen atoms were
placed in calculated positions (dCÃH
ꢂ
/
0.93, 0.96, 0.97
˚
A). Isotropic displacement parameters were calculated

M. Muehlhofer et al. / Journal of Organometallic Chemistry 660 (2002) 121ꢀ
/126
123
Scheme 2. Synthesis of bridged PtꢀNHC complexes.
/
Table 2
Crystallographic data for complex 3b
3b
Chemical formula
Formula weight
Temperature (K)
Color/shape
C9H12Br2N4Pt
531.11
293
Pale yellow/plate
Monoclinic
P21/n (No. 14)
8.5813(8)
15.9405(7)
10.4418(12)
112.385(7)
1320.7(2)
4
Crystal system
Space group
˚
a (A)
˚
b (A)
˚
c (A)
b (8)
3
V (A )
˚
Z
rcalc (g cmꢃ3
)
2.671
16.655
m (mmꢃ1
F(000)
Crystal size (mm)
)
968
0.46ꢄ0.28ꢄ0.15
Fig. 1. ORTEP [34] representation of complex 3b in the solid state.
Thermal ellipsoids are drawn at the 50% probability level.
u-Range (8)
Data collected
2.47ꢀ25.93
/
ꢃ105h 510, ꢃ195k 519,
ꢃ125l 512
10 100
Table 1
˚
Selected interatomic distances (A) and angles (8) for complex 3b
Reflections collected
Reflections observed
[I ꢀ2s(I)]
Independent reflections
Parameters refined
R1 (observed/all)
wR2 (observed/all)
Goodness-of-fit (observed/
all)
2100 (observed)
Bond distances
PtÃBr1
PtÃBr2
PtÃC1
PtÃC6
2.4976(11)
2.4883(13)
1.963(10)
1.950(10)
2580 (all) [Rintꢂ0.0907]
148
0.0439/0.0604
0.0897/0.0963
1.030/1.030
Bond angles
Br1ÃPtÃBr2
Br1ÃPtÃC1
Br1ÃPtÃC6
Br2ÃPtÃC1
Br2ÃPtÃC6
C1ÃPtÃC6
91.14(4)
175.0(3)
92.5(3)
ꢃ3
˚
Max/min Dr (e A
)
ꢁ3.89/ꢃ3.73
92.2(3)
173.3(3)
83.8(4)
4. Conclusion
from the parent carbon atom (UHꢂ1.2UC). Full-matrix
/
least-squares refinements were carried out by minimiz-
ing a w(Fo2 ꢃFc2)2 with a SHELXL-97 weighting scheme
We present a new synthetic pathway which directly
leads to novel platinum(II) biscarbene complexes start-
ing from inexpensive and easily available platinum(II)
halides. The reactions with sodium acetate and imida-
zolium salts can be carried out without special precau-
tions concerning atmosphere and solvents. This was
demonstrated for several bridged biscarbene complexes,
which can be synthesized in high yields under these
reaction conditions.
and stopped at maximum shift/errB0.001 [32]. Neutral
/
atom scattering factors for all atoms and anomalous
dispersion corrections for the non-hydrogen atoms were
taken from International Tables for Crystallography [33].
All other calculations (including ORTEP graphics) were
done with the program PLATON [34]. Calculations were
performed on a PC workstation (Intel Pentium II)
running LINUX
.

124
M. Muehlhofer et al. / Journal of Organometallic Chemistry 660 (2002) 121ꢀ126
/
5. Experimental
5.2.3. 1,1?-Dimethyl-3,3?-methylene-4-diimidazolin-2,2?-
diylidene platinum(II) dibromide (3b)
Platinum(II) bromide (200 mg, 0.56 mmol), sodium
acetate trihydrate (153 mg, 1.12 mmol) and 1,1?-
dimethyl-3,3?-methylene-4-imidazolium dibromide (190
mg, 1.12 mmol) are solved in 4 ml Me2SO and are
heated to 80 8C. After 2 h the solution is heated to
100 8C for 1 h. The mixture is cooled to r.t. and the
solution filtered off from a small amount of platinum
black that has formed. Afterwards the solvent is
evaporated and the residue washed twice with 2 ml
portions of CH2Cl2. After extraction of the sodium salt
with a small amount of water the residue is washed with
THF and dried under high vacuum. One hundred and
ninety-one milligrams (64.2%) of a very light yellow
powder are obtained.
5.1. Reagents
Platinum(II) bromide (98%) and platinum(II) iodide
(98%) were purchased from Aldrich, platinum(II) chlo-
ride was a generous donation from Degussa. Dibromo-
methane (ꢀ
diiodoethane (ꢀ
methyl imidazole was available from Fluka. Solvents
were purchased from common suppliers and used with-
out further purification.
/
99%), diiodomethane (ꢀ
/
99%) and
/
99%) were supplied by Merck, while
5.2. Reactions
1H-NMR (400 MHz, 25 8C, d6-Me2SO): dꢂ
7.56 (s,
/
5.2.1. 1,1?-Dimethyl-3,3?-ethylene-diimidazolium diiodide
(2)
2H, NCH), 7.33 (s, 2H, NCH), 6.23 (d, 1H, NCH2), 5.94
(d, 1H, NCH2), 3.83 ppm (s, 6H, CH3). 13C-NMR (100
In an ACE pressure tube 1.00 g (12.2 mmol) methyl
imidazole and 1.89 g (6.7 mmol) diiodoethane are solved
in 5 ml THF and heated to 130 8C for 16 h. After
cooling to room temperature (r.t.) the resulting pre-
cipitate is filtered off and washed twice with 5 ml THF.
The product is dried under vacuum, yielding 2.13 g
(78.3%) of a yellow powder.
MHz, 25 8C, d6-dimethyl sulphoxide): dꢂ144.8
/
(NCN), 122.6 (NCH), 120.4 (NCH), 61.8 (CH2), 36.3
ppm (CH3). EA: Anal. Calc.: C, 20.3; H, 2.3; N, 10.5.
Found: C, 20.4; H, 2.4; N, 10.3%. MS: m/u 531 [Mꢁ],
451 [Mꢁꢃ
/
Br], 369 [Mꢁꢃ
2Br].
/
1H-NMR (400 MHz, 25 8C, d6-Me2SO): dꢂ
9.24 (s,
/
5.2.4. 1,1?-Dimethyl-3,3?-ethylene-4-diimidazolin-2,2?-
diylidene platinum(II) diiodide (4)
Platinum(II) iodide (224 mg, 0.5 mmol), sodium
acetate trihydrate (136 mg, 1.0 mmol) and 1,1?-di-
methyl-3,3?-ethylene-4-imidazolium diiodide (223 mg,
0.5 mmol) are solved in 6 ml Me2SO and are heated to
2H, NCHN), 7.78 (s, 2H, NCH), 7.53 (s, 2H, NCH),
4.65 (s, 4H, CH2CH2), 3.74 ppm (s, 6H, CH3). 13C-
NMR (100 MHz, 25 8C, d6-Me2SO): dꢂ137.4
/
(NCHN), 124.3 (NCH), 122.7 (NCHN), 48.7
(CH2CH2), 36.6 ppm (CH3). EA: Anal. Calc.: C, 26.9;
H, 3.6; N, 12.6. Found: C, 27.0; H, 3.6; N, 12.6%.
75 8C. The dark suspension clears to a redꢀbrown
/
solution within 1 h. After further 2 h the solvent is
evaporated and the residue washed twice with 2 ml
portions of CH2Cl2. After extraction of the sodium salt
with a small amount of water the residue is washed with
THF and dried under high vacuum. Two hundred and
thirty-one milligrams (72.4%) of a slightly yellow
powder are obtained.
5.2.2. 1,1?-Dimethyl-3,3?-methylene-4-diimidazolin-2,2?-
diylidene platinum(II) diiodide (3a)
Platinum(II) iodide (224 mg, 0.5 mmol), sodium
acetate trihydrate (136 mg, 1.0 mmol) and 1,1?-di-
methyl-3,3?-methylene-4-imidazolium diiodide (216 mg,
0.5 mmol) are solved in 6 ml Me2SO and are heated to
1H-NMR (400 MHz, 25 8C, d6-Me2SO): dꢂ
7.33 (s,
/
75 8C. The dark suspension clears to a redꢀbrown
/
2H, NCH), 7.29 (s, 2H, NCH), 5.16 (m, 2H, NCH2),
4.44 (m, 2H, NCH2), 3.79 ppm (s, 6H, CH3). 13C-NMR
solution within 1 h. After further 2 h the solvent is
evaporated and the residue washed twice with 2 ml
portions of CH2Cl2. After extraction of the sodium salt
with a small amount of water the residue is washed with
THF and dried under high vacuum. Two hundred and
nineteen milligram (70.1%) of a slightly yellow powder
are obtained.
(100 MHz, 25 8C, d6-dimethyl sulphoxide): dꢂ143.0
/
(NCN), 122.7 (NCH), 121.7 (NCH), 46.8 (CH2), 37.4
ppm (CH3). EA: Anal. Calc.: C, 18.8; H, 2.2; N, 8.8.
Found: C, 18.8; H, 2.2; N, 9.5%.
1H-NMR (400 MHz, 25 8C, d6-Me2SO): dꢂ
7.52 (s,
/
6. Additional experimental data
2H, NCH), 7.29 (s, 2H, NCH), 6.14 (d, 1H, NCH2), 5.91
(d, 1H, NCH2), 3.83 ppm (s, 6H, CH3). 13C-NMR (100
MHz, 25 8C, d6-Me2SO): dꢂ
/
143.4 (NCN), 122.7
(NCH), 120.4 (NCH), 61.8 (CH2), 37.3 ppm (CH3).
6.1. 1,1?-Dimethyl-3,3?-methylene-diimidazolium diiodide
(1a)
EA: Anal. Calc.: C, 17.3; H, 1.9; N, 9.0. Found: C, 17.8;
H, 2.2; N, 9.5%. MS: m/u 498 [Mꢁꢃ
/
I], 370 [Mꢁꢃ
/2I],
In an ACE pressure tube 1.00 g (12.2 mmol) methyl
imidazole and 0.54 mL (6,7 mmol) diiodomethane are
289 [Mꢁꢃ
/
2Iꢃ
/
C4H5N2], 176 [Mꢁꢃ
/
2IꢃPt].
/

M. Muehlhofer et al. / Journal of Organometallic Chemistry 660 (2002) 121ꢀ
/126
125
solved in 5 mL tetrahydrofurane and heated to 130 8C
for 16 h. After cooling to room temperature the
resulting precipitate is filtered off and washed twice
with 5 mL tetrahydrofurane. The product is dried under
vacuum, yielding 1.91 g (72.3%) of a light yellow
powder.
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/
dꢂ9.65 ppm (s, 2H, NCHN), 7.97 ppm (s, 2H, NCH),
/
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7.79 ppm (s, 2H, NCH), 6.65 ppm (s, 2H, CH2), 3.89
ppm (s, 6H, CH3).
13C NMR (100 MHz, 25 8C, d6-dimethyl sulphoxide):
dꢂ138.4 ppm (NCHN), 124.7 ppm (NCH), 122.3 ppm
/
(NCHN), 58.5 ppm (CH2), 36.7 ppm (CH3).
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In an ACE pressure tube 1.00 g (12.2 mmol) methyl
imidazole and 0.95 mL (6.7 mmol) dibromomethane are
solved in 5 mL tetrahydrofurane and heated to 130 8C
for 16 h. After cooling to room temperature the
resulting precipitate is filtered off, washed twice with 5
mL tetrahydrofurane. The product is dried under
vacuum, yielding 1.56 g (75.8%) of a white powder.
1H NMR (400 MHz, 25 8C, d6-dimethyl sulphoxide):
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dꢂ9.83 ppm (s, 2H, NCHN), 8.34 ppm (s, 2H, NCH),
/
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/
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16.6%.
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3b. Copies of the data may be obtained free of charge
from The Director, CCDC, 12 Union Road, Cambridge
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We are grateful for the financial support by Sud-
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