Synthesis, structure and stability of new PtII-bis(N- heterocyclic carbene) complexes

Article abstract of DOI:10.1002/ejic.200500888

The use of metal acetates has allowed the synthesis of NHC complexes without isolation of the free carbene. This route is very useful in cases where the metal acetates are readily available and has been used, for example, for the synthesis of bis(N-hetero

Full text of DOI:10.1002/ejic.200500888

FULL PAPER
DOI: 10.1002/ejic.200500888
Synthesis, Structure and Stability of New PtII–Bis(N-Heterocyclic Carbene)
Complexes
Sebastian Ahrens,[a] Eberhardt Herdtweck,[b][‡] Sigrid Goutal,[a][‡‡] and
Thomas Strassner*[a]
Keywords: Carbene ligands / NMR spectroscopy / Platinum / Synthesis / Ligand design / Solid state structure
The use of metal acetates has allowed the synthesis of NHC
complexes without isolation of the free carbene. This route
is very useful in cases where the metal acetates are readily
available and has been used, for example, for the synthesis
of bis(N-heterocyclic carbene) complexes of palladium(II). In
the case of platinum, however, the corresponding acetate is
not commercially available and is difficult to synthesize. Pre-
viously published syntheses require either an external base
(e.g. NaOAc or KOtBu) or a transmetalation step. Here we
present a new general synthetic pathway for platinum–
bis(carbene) complexes using platinum(II) acetylacetonate as
a commercially available metal precursor. Aromatic substitu-
ents greatly enhance the stability of the metal complexes
against acidic media.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
aromatic substituents, which greatly enhance the stability of
the corresponding platinum carbene complexes.
Introduction
In the last decade N-heterocyclic carbene (NHC) com-
plexes of transition metals have found applications in many
fields of chemistry. Metal complexes of imidazoline-2-ylid-
Results and Discussion
enes have recently been shown to be extremely versatile and Preparation of the Platinum(II)–Bis(carbene) Complexes
stable catalysts for a wide range of reactions, including
The previously reported syntheses of platinum–NHC
C–C coupling reactions,[1,2] olefin metathesis,[3–6] hydrofor-
complexes involve the preparation of either platinum pre-
mylation,[7,8] and, most recently, CH activation.[9,10] The
cursors or free carbenes, which usually have to be handled
key for this development has been the number of improved
under an inert gas atmosphere.[24–27] Since the acidic meth-
synthetic methods for the preparation of NHC complexes,
ylene protons in bis(imidazolium) salts might be attacked
which have been summarized in recent reviews.[11–13]
under common deprotonation conditions, a pathway via the
Metal acetates allow the synthesis of NHC complexes
free bis(carbenes) is only possible by using sterically de-
without isolation of the free carbene. Palladium() acetate
manding bases, but the yields are only moderate.[28,29] To
in wet dimethyl sulfoxide (DMSO) at elevated temperatures
circumvent the need for multi-step pathways we recently in-
directly forms carbene complexes with imidazolium salts.
troduced a new synthetic route using the corresponding
Even methylene-bridged bis(imidazolium) salts can be con-
platinum() halide salts and sodium acetate as an external
verted into the corresponding bis(carbene) complexes in
base for the synthesis of the metal complexes. However, the
high yields (85–90%).[14–16] We and others have been par-
use of external bases like sodium acetate often leads to the
ticularly interested in these chelate ligands as a way of ex-
formation of small amounts of platinum black, which pro-
tending the area to more complex ligands and more stable
duces lower yields in the reaction and is difficult to remove
complexes.[17–23] Here we present a new general synthetic
from the product.[22]
pathway towards platinum–bis(carbene) complexes with
If platinum() acetylacetonate is used as metal precursor,
no formation of platinum black is observed during the reac-
[a] Department Chemie – Physikalische Organische Chemie, Tech-
nische Universität Dresden,
tion. Like the platinum() halides used in our first proto-
col,[22] it is commercially available and can be handled in
air. Furthermore, there are two major advantages for the
use of platinum() acetylacetonate compared to plati-
num() halides. First, only platinum() halides with the
same halides as the imidazolium salts can be used in the
reaction to avoid anion scrambling in the complex, and
platinum() halides are also much more expensive than
platinum() acetylacetonate. Secondly, different from our
Mommsenstr. 13, 01062 Dresden, Germany
Fax: +49-351-463-39679
E-mail: Thomas.Strassner@chemie.tu-dresden.de
[b] Department Chemie – Lehrstuhl für Anorganische Chemie,
Technische Universität München,
Lichtenbergstr. 4, 85747 Garching b. München, Germany
Fax: +49-89-289-13473
[‡] Solid-state structure of compound 3b.
[‡‡] Solid-state structure of compound 3d.
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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New PtII–Bis(N-Heterocyclic Carbene) Complexes
FULL PAPER
older procedure, no other external base has to be added as reported previously.[22] The preparation of the imidazoles
since the basicity of the acetylacetonate anion is sufficient and the following bridging step of two of these imidazoles
to deprotonate the imidazolium salt. When platinum() ha- with dibromomethane are compatible with a wide range of
lides are used as metal precursors, sodium acetate has to be functional groups at the aromatic ring. Yields are given in
added to the reaction mixture, which leads to the formation Table 2.
of sodium halide salts during the reaction which have to be
Table 2. Isolated yields of 2a–2e.
removed by washing the product with water several
times.[22] In this case only pentan-2,4-dione emerges during
the reaction, and this can be removed in vacuo along with
the solvent. We were able to synthesize several new plati-
num–N-heterocyclic bis(carbene) complexes with different
substituents at the aromatic ring following this method.
The synthesis of these new ligands can be accomplished
in only two steps. First, the aromatic-substituted imidazoles
were synthesized according to Scheme 1 from substituted
anilines, which are commercially available in great diversity:
4-nitroaniline for 1a, 4-chloroaniline for 1b, 4-bromoaniline
for 1c, 4-methoxyaniline for 1d, and ethyl 4-aminobenzoate
for 1e. The aromatic amine, glyoxal and formaldehyde are
converted into an aromatic-substituted imidazole in a one-
pot reaction. Additionally to the imidazoles prepared by
Zhang et al.,[30] we extended this method to anilines substi-
tuted by different halogens. Both electron-donating and
-withdrawing substituents are tolerated, although the yields
of product are quite different (Table 1).
2a
2b
2c
2d
2e
Yield
94.8%
64.0%
84.0%
88.7%
86.4%
Finally, we succeeded in synthesizing the platinum–
bis(carbene) complexes under very mild conditions, thus
proving the application of our new compounds as suitable
NHC ligands for late transition metals. In this case, the
traditional metalation procedure involving deprotonation
of the imidazolium precursor with strong bases produces
unwanted side reactions because of the acidic protons in
the bridging CH2 group and in the functional groups at the
aromatic ring.[31] Milder procedures were therefore needed
that are compatible with more sensitive functionalities.
Base-sensitive methylene-bridged bis(imidazolium) salts can
be converted into the corresponding palladium–bis-
(carbene) complexes by treatment with Pd(OAc)2 in wet di-
methyl sulfoxide (DMSO) at elevated temperatures in high
yields (Scheme 3).[14–16] This route is very useful in cases
where the metal acetates are easily available. In the case of
platinum, however, the corresponding acetate is not avail-
able and is difficult to synthesize.[32,33] So until now plati-
num() halides had to be used and sodium acetate had to
be added to the reaction mixture as an external base, lead-
ing to the formation of sodium halides as by-products dur-
ing the reaction.[22] Here we present a new synthetic ap-
proach that uses platinum() acetylacetonate as a commer-
cially available metal precursor and base for the deproton-
ation of the imidazolium salt. The synthesis of the platinum
complexes is illustrated in Scheme 4 and yields of the syn-
theses are given in Table 3.
Scheme 1. Synthesis of imidazoles with different aromatic substitu-
ents [R = NO2 (1a), R = Cl (1b), R = Br (1c), R = OCH3 (1d), R
= COOCH2CH3 (1e)].
Table 1. Isolated yields of 1a–1e.
1a
1b
1c
1d
1e
Yield
28.0%
77.0%
60.7%
75.0%
73.7%
The low yield for 1a can be explained by its lower solubil-
ity in organic solvents, so the extraction of the product is
the limiting factor here. This reaction gives access to a wide
range of N-substituted imidazoles with different functional
groups at the aromatic ring, thereby allowing us to fine-
tune the electronic situation in the N-heterocyclic carbene
ligand.
These N-substituted imidazoles can be converted in good
yields into the corresponding bis(imidazolium) salts by a
nucleophilic substitution reaction with dibromomethane
(Scheme 2). The reaction also works with diiodomethane,
Scheme 3. Synthesis of bridged palladium complexes from imid-
azolium salts and palladium() acetate.
To obtain these complexes in good yields it is necessary
to follow the synthetic protocol described below. It is cru-
cial that the halide salt of the bis(imidazolium) compound
is completely converted into the platinum–monocarbene
species by slowly heating the reaction mixture over several
hours first to 60 °C, thereby deprotonating only one of the
imidazolium rings, otherwise a substantial amount of plati-
num black formation is observed.[22] The reaction mixture
should then be heated up to 130 °C to deprotonate the sec-
ond imidazolium ring. This is consistent with the earlier
findings of Herrmann et al. regarding palladium–bis(car-
bene) complexes,[34] but had not been observed for platinum
carbenes before.
Scheme 2. Synthesis of the imidazolium salts [R = NO2 (2a), R =
Cl (2b), R = Br (2c), R = OCH3 (2d), R = COOCH2CH3 (2e)].
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S. Ahrens, E. Herdtweck, S. Goutal, T. Strassner
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Scheme 4. Synthesis of the platinum complexes [R = NO2 (3a), R = Cl (3b), R = Br (3c), R = OCH3 (3d), R = COOCH2CH3 (3e)].
Table 3. Isolated yields of 3a–3e.
3a
3b
3c
3d
3e
Yield
62.1%
86.8%
67.0%
69.9%
71.8%
Solid-State Structures and NMR Studies of the
Platinum(II)–Bis(carbene) Complexes
Important geometrical parameters of the solid-state
structures are shown in Table 4 and the Supporting Infor-
mation. Details of the structure determinations are given in
the Experimental Section.
Table 4. Selected bond lengths and bond and torsional angles of
3b–3e.
3b
3c
3d
3e
Pt–C1
Pt–C11
1.968(12)×2
1.968(5)×2
1.961(6)
1.971(6)
1.967(12)
1.976(13)
Figure 1. ORTEP-style plot of the solid-state structure of com-
pound 3d. Thermal ellipsoids are drawn at the 50% probability
level. The disordered DMSO molecule in the crystal has been omit-
ted for clarity.
Pt–Br1
Pt–Br2
C1–N1
C11–N3
C1–Pt–C1A/C11
C1–Pt–Br1
C1–N2–C5–C10
2.4685(16)×2 2.4833(6)×2 2.4926(7) 2.4746(13)
2.4831(7) 2.4812(14)
1.366(16)×2
1.351(7)×2
1.349(8)
1.333(8)
84.4(3)
92.34(18)
128.0(7) –143.8(12)
1.377(15)
1.359(15)
85.6(5)
86.4(5)
92.2(4)
137.0(12)
84.9(2)
92.75(15)
–147.0(5)
93.1(4)
The molecular structures of complexes 3b–3e are nearly
identical with each other and with 4; they contain a bowl-
shaped NHC ligand. A DMSO solvent molecule, which co-
vers the cavity, is found inside this bowl in all structures.
The solid-state structure of complex 3d is given in Figure 1
as an example, although the DMSO solvent molecule has
been omitted for clarity. Pictures of the solid-state struc-
tures of the other complexes are given in the Supporting
Information.
The Pt–carbene-C distances of the aromatic-substituted
complexes 3b–3e are equivalent within the margin of error.
The biggest differences between these structures are found
in the torsional angles of the plane of the aromatic rings
towards the plane of the imidazole rings. Figure 2 shows a
superposition of all four crystal structures; the DMSO and
methanol solvent molecules found in the crystal structures
have been omitted for clarity. The figure shows that the co-
ordination environment of the metal center is highly pre-
Figure 2. Superposition of the solid-state structures of 3b–3e. Sol-
vents omitted for clarity.
served and so there are nearly no differences in the steric aromatic substituents R1 on the 13C chemical shifts for the
situation of the metal, even though the complexes crys- carbene carbon atoms are shown in Figure 3. Therefore it
tallize in different space groups.
is possible to fine-tune the electronic situation at the metal
Although structurally quite similar these complexes have center by varying the functional groups at the aromatic
different electronic properties at the metal center, as can be ring. Electron-withdrawing groups like NO2 or halogens at
seen from the 13C NMR spectra. The effects of the different the aromatic ring lead to a shift of the 13C signal of the
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New PtII–Bis(N-Heterocyclic Carbene) Complexes
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Figure 3. Dependence of the 13C chemical shift of the carbene carbon atom on the aromatic substituents R1.
carbene C to lower field, which is indicative of a higher
partial negative charge at the carbene carbon atoms of the
Conclusion
We have shown that platinum()–bis(carbene) complexes
with various aromatic substituents can be synthesized fol-
lowing our new protocol with platinum() acetylacetonate
as metal precursor. Together with our new aromatic ligands
with various substituents, acid-stable platinum()–bis(car-
bene) complexes are now synthetically available in great di-
versity. The electronic situation at the metal center can be
fine-tuned by varying the functional groups at the aromatic
ring without changing the steric situation of the metal cen-
ter. This is important for future applications of these com-
plexes in various catalytic reactions, since many known
homogeneous catalytic processes depend on a particular
electronic situation at the metal center.
complex. As expected, the electron-withdrawing effects of a
4-chlorophenyl and a 4-bromophenyl group at the imida-
zole are nearly equivalent. On the other hand, electron-do-
nating groups like the methoxy group at the aromatic ring
lead to a high-field shift of the 13C signal of the carbene
carbon atom.
This type of complex also shows an unusually high ther-
mal stability combined with a surprising resistance to acidic
media. We have observed this stability previously for palla-
dium()–NHC complexes like 1,1Ј-dimethyl-3,3Ј-methyl-
ene-4-diimidazoline-2,2Ј-diylidenepalladium() dibromide
(4), which are catalysts for the activation of methane, while
the analogous platinum()–NHC complex 5 decomposes
quickly in trifluoroacetic acid (Figure 4).[9,10,22]
Experimental Section
General Procedures: Solvents of 99.5% purity were used through-
out this study. Platinum() acetylacetonate (98%) was purchased
from Acros. All other chemicals were obtained from common sup-
pliers and used without further purification. Ligands 2a–2e were
prepared according to the procedures given in the Supporting In-
Figure 4. Methyl-substituted bis-NHC complexes of palladium (4)
and platinum (5).
1
formation. H and 13C NMR spectra were recorded with a Bruker
AC 300 P, Jeol JNM GX-270, or Jeol JNM GX-400 spectrometer.
As the 1H and 13C NMR spectra of one compound were not always
measured on the same spectrometer, spectrometer frequencies of
the measurements are given. The spectra were referenced internally
to the resonances of the solvent (1H, 13C). Elemental analyses were
performed by the microanalytical laboratory at our institute using
an EuroVektor Euro EA-300 Elemental Analyzer. Mass spectra
were recorded with a Finnigan MAT 90 spectrometer.
With the introduction of aromatic ligands and the novel
platinum() acetylacetonate synthesis presented here, we
have been able to make several new platinum–bis(carbene)
complexes with increased stability against strongly acidic
conditions. While the analogous N-methyl-substituted com-
plex 5 is stable towards air and moisture even at elevated
temperatures, it decomposes within minutes to platinum
black in trifluoroacetic acid.[9] However, the new complex
3b does not decompose even after weeks in TFA or at ele-
vated temperatures of up to 90 °C in the presence of strong
oxidants like K2S2O8. Complex 3e can also be heated to
110 °C in 2  HCl solution for 4 h with no change in the
NMR spectrum. Neither reprotonation to the imidazolium
salt nor hydrolysis of the ester functionality are observed
under these conditions.
Synthesis of Platinum(II) Complex 3a: 1,1Ј-Bis[1-(4-nitrophenyl)]-
3,3Ј-methylenimidazolium dibromide (2a; 280.5 mg, 0.51 mmol)
and platinum() acetylacetonate (200 mg, 0.51 mmol) were dis-
solved in 3 mL of dimethyl sulfoxide in a nitrogen-flushed Schlenk
tube and heated for 2 h to 60 °C, 2 h to 80 °C, 2 h to 100 °C, and
1 h to 130 °C. At approximately 80 °C the reaction mixture became
a clear, yellow solution. After removal of the solvent in vacuo the
resulting residue was washed twice with 5 mL of diethyl ether and
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5 mL of dichloromethane. The product was obtained as a yellow
powder. Yield: 0.236 g (62.1%). 1H NMR (300 MHz, [D6]DMSO):
δ = 6.31 (d, J = 14.4 Hz, 1 H, NCH2N), 6.38 (d, J = 14.4 Hz, 1 H,
H, arom. H) ppm. 13C NMR (75 MHz, [D6]DMSO): δ = 55.3
(OCH3), 62.4 (NCH2N), 113.7 (arom. CH), 122.5 (NCHCHN),
126.1 (arom. CH), 126.3 (NCHCHN), 126.7 (C1 of C6H4OMe),
NCH2N), 7.79 (d, J = 9.0 Hz, 2 H, NCH2CH2N), 7.97–8.03 (m, 4 132.5 (Pt–C), 158.8 (C–OMe) ppm. C21H20Br2N4O2Pt (715.30):
H, arom. H), 8.36 (d, J = 9.0 Hz, 2 H, NCH2CH2N), 8.51–8.55 calcd. C 35.26, H 2.81, N 7.86; found C 35.34, H 2.87, N 7.52. MS
(m, 4 H, arom. H) ppm. 13C NMR (75 MHz, [D6]DMSO): δ = 62.4
(FAB): m/z = 635.1 [M – Br]+, 554.3 [M – 2Br]+, 448.2 [M – 2Br –
(NCH2N), 121.0 (NCHCHN), 122.6 (NCHCHN), 124.5 (C2, C6 C6H4OCH3]+, 341.3 [M – 2Br – 2C6H4OCH3]+.
of C6H4NO2), 125.5 (C3, C5 of C6H4NO2), 126.4 (C1 of
Synthesis of Platinum(II
boxyphenyl)]-3,3Ј-methylenimidazolium dibromide (3e; 100.0 mg,
0.165 mmol) and platinum() acetylacetonate (64.9 mg,
) Complex 3e: 1,1-Bis[1-(4-ethylcar-
C6H4NO2), 146.4 (C–Pt), 146.8 (C–NO2). C19H14Br2N6O4Pt·
0.5DMSO (745.24 + 39.07): calcd. C 30.63, H 2.18, N 10.72; found
C 30.94, H 2.46, N 10.79. MS (FAB): m/z = 584.2 [M – 2Br]+,
462.2 [M – 2Br – C6H4NO2]+, 341.2 [M – 2Br – 2C6H4NO2]+, 147.3
[M – 2Br – Pt – 2C6H4NO2]+.
0.165 mmol) were dissolved in 6 mL of dimethyl sulfoxide in a ni-
trogen-flushed Schlenk tube and stirred for 3 h at 40 °C, 2 h at
60 °C, 2 h at 70 °C, 2 h at 100 °C, and 1 h at 130 °C. The solvent
was removed in vacuo and the product washed once with 5 mL of
ethanol. The product was obtained as a white powder. Yield:
Synthesis of Platinum(II) Complex 3b: 1,1-Bis[1-(4-chlorophenyl)]-
3,3Ј-methylenimidazolium dibromide (2b; 500 mg, 0.94 mmol) and
platinum() acetylacetonate (370 mg, 0.94 mmol) were dissolved in
10 mL of dimethyl sulfoxide in a nitrogen-flushed Schlenk tube and
heated for 2 h to 40 °C, 2 h to 60 °C, 2 h to 80 °C, 2 h to 100 °C
and 1 h to 130 °C. At 60 °C the reaction mixture became a clear,
yellow solution. After removal of the solvent in vacuo the resulting
residue was washed twice with 5 mL of ethanol. The product was
obtained as a white powder. Yield: 0.591 g (86.8%). 1H NMR
(300 MHz, [D6]DMSO): δ = 6.10 (d, J = 14.6 Hz, 1 H, NCH2N),
6.31 (d, J = 14.6 Hz, 1 H, NCH2N), 7.64 (d, J = 9.0 Hz, 4 H, arom.
H), 7.73 (d, J = 2.2 Hz, 2 H, NCHCHN), 7.77 (d, J = 2.1 Hz, 2
H, NCHCHN), 7.81 (d, J = 9.0 Hz, 4 H, arom. H) ppm. 13C NMR
(75 MHz, [D6]DMSO): δ = 67.0 (NCH2N), 121.4 (NCHCHN),
122.2 (NCHCHN), 127.0 (C2, C6 of C6H4Cl), 128.6 (C3, C5 of
C6H4Cl), 132.3 (C–Cl), 138.2 (C1 of C6H4Cl), 145.5 (C–Pt) ppm.
C19H14Br2Cl2N4Pt·DMSO (724.13 + 78.13): calcd. C 31.44, H 2.51,
N 6.98, S 4.00; found C 31.16, H 2.51, N 6.65, S 4.00. MS (FAB):
m/z = 643.3 [M – Br]+, 563.3 [M – 2Br]+, 176.3 [C9H6N2Cl+].
1
0.095 g (72%). H NMR (300 MHz, [D6]DMSO): δ = 1.43 (t, J =
7.0 Hz, 6 H, CH2CH3), 4.40 (q, J = 7.0 Hz, 4 H, CH2CH3), 6.14
(d, J = 13.2 Hz, 1 H, NCH2N), 6.35 (d, J = 13.2 Hz, 1 H, NCH2N),
7.84 (d, J = 2.7 Hz, 2 H, NCHCHN);7.91 (d, J = 2.7 Hz, 2 H,
NCHCHN), 7.98 (d, J = 8.4 Hz, 4 H, arom. H), 8.19 (d, J = 8.4 Hz,
4 H, arom. H) ppm. 13C NMR (75 MHz, [D6]DMSO): δ = 14.1
(CH2CH3), 61.1 (CH2CH3), 62.7 (NCH2N), 120.3 (arom. CH),
125.2 (NCHCHN), 129.8 (arom. CH), 130.7 (NCHCHN), 138.4
(C1 of C6H4COOEt), 139.1 (C4 of C6H4COOEt), 142.9 (Pt–C),
165.3 (COOEt) ppm. C25H26Br2N4O2Pt·0.5DMSO (799.37
+
39.07): calcd. C 37.25, H 3.25, N 6.68, S 1.91; found C 37.15, H
3.38, N 6.29, S 2.11.
X-ray Crystal-Structure Determination for 3b–3e: Crystals suitable
for an X-ray investigation were obtained either by allowing meth-
anol to condense into a saturated solution of the metal complex in
DMSO (3b, 3c) or by slowly cooling a hot, saturated solution of
the complex in DMSO (3d, 3e). The crystals were stored under
perfluorinated ether, transferred to a Lindemann capillary, fixed,
and sealed. Preliminary examination and data collection were car-
ried out on an area-detection system (Kappa-CCD; Nonius) with
graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). Data
collections were performed at 173 K (3b) or 198 K (3c–3e) in the
Θ range 2.27° Ͻ Θ Ͻ 21.02° (3b) or 3.00° Ͻ Θ Ͻ 27.40° (3c–3e).
The reflections were integrated, raw data were corrected for Lo-
rentz, polarization, decay, and absorption effects. Due to the small
size of the crystal the data set for 3d was reduced to 25.00 and that
of 3e to 26.40. The absorption correction was applied using the
PLATON DELABS algorithm for 3b and SADABS for 3c–3e. The
independent reflections were used for all calculations after merging.
The structures were solved by a combination of direct methods and
difference Fourier syntheses. All non-hydrogen atoms were refined
with anisotropic displacement parameters. All hydrogen atoms
were calculated in ideal positions as riding on the parent carbon
atoms. During the final stage of the refinements it becomes evident
that the crystal of 3b contains unresolvable solvent molecules. This
problem was cured by using the PLATON CALC SQUEEZE pro-
cedure. All other solvent molecules in 3c–3e could be fully resolved.
Synthesis of Platinum(II) Complex 3c: 1,1-Bis[1-(4-bromophenyl)]-
3,3Ј-methylenimidazolium dibromide (2c; 315.0 mg, 0.51 mmol)
and platinum() acetylacetonate (200.0 mg, 0.51 mmol) were dis-
solved in 3 mL of dimethyl sulfoxide in a nitrogen-flushed Schlenk
tube and stirred for 2 h at room temperature. The reaction mixture
was then heated for 2 h to 60 °C, 1 h to 85 °C, and finally 1 h to
110 °C. The solvent was then removed in vacuo until only 0.5 mL
of the solvent remained. After the addition of 3 mL of water a
white solid precipitated. The aqueous solution was filtered off and
the resulting residue was washed twice with 5 mL of dichlorometh-
ane. The product was obtained as a white powder. Yield: 0.278 g
(67.0%). 1H NMR (270 MHz, [D6]DMSO): δ = 6.10 (d, J =
12.7 Hz, 1 H, NCH2N), 6.31 (d, J = 12.4 Hz, 1 H, NCH2N), 7.53–
7.95 (m, 12 H, remaining H) ppm. 13C NMR (67.9 MHz, [D6]
DMSO): δ = 62.5 (NCH2N), 120.7 (NCHCHN), 121.5 (C–Br),
122.2 (NCHCHN), 127.3 (C2, C6 of C6H4Br), 131.6 (C3, C5 of
C6H4Br), 138.8 (C1 of C6H4Br), 145.4 (C–Pt) ppm. MS (FAB):
m/z = 733.0 [M – Br]+, 653.1 [M – 2Br]+, 572.2 [M – 3Br]+, 419.5
[M – 3Br – C6H4Br]+.
Synthesis of Platinum(II
)
Complex 3d: 1,1-Bis[1-(4-meth- Full-matrix least-squares refinements were carried out by minimiz-
2 2
oxyphenyl)]-3,3Ј-methylenimidazolium dibromide (2d; 250.0 mg,
0.48 mmol) and platinum() acetylacetonate (188.3 mg, 0.48 mmol)
were dissolved in 7 mL of dimethyl sulfoxide in a nitrogen-flushed
Schlenk tube and stirred for 4 d at 60 °C. The solvent was then
removed in vacuo and the product washed twice with 5 mL of water
and twice with 5 mL of diethyl ether. The product was obtained as
ing Σw(Fo2 – Fc ) with SHELXL-97 weighting scheme and stopped
at shift/err Ͻ 0.001. The final difference Fourier map showed high
residual electron density peaks for 3b (+1.610 and –2.096 eÅ–3)
caused by the incomplete absorption correction and the incomplete
removal of solvent molecules and 3c (1.546 and –0.959 eÅ–3; dis-
tance to platinum 0.783 and 0.976 Å) caused by an incomplete ab-
sorption correction. Details of the structure determinations are
given in Table 5. Neutral-atom scattering factors for all atoms and
anomalous dispersion corrections for the non-hydrogen atoms were
taken from the International Tables for Crystallography. All calcu-
1
a white powder Yield: 0.240 g (69.9%). H NMR (300 MHz, [D6]
DMSO): δ = 3.86 (s, 6 H, OCH3), 6.07 (d, J = 14.6 Hz, 1 H,
NCH2N), 6.26 (d, J = 14.6 Hz, 1 H, NCH2N), 7.08 (m, 4 H, arom.
H), 7.67 (s, 2 H, NCHCHN), 7.72 (s, 2 H, NCHCHN), 7.78 (m, 4
1272
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© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2006, 1268–1274

New PtII–Bis(N-Heterocyclic Carbene) Complexes
FULL PAPER
Table 5. Selected parameters for the crystal-structure determination of 3b–3e.
3b
3c·DMSO,·MeOH
3d·DMSO
3e·DMSO
Measured by
Crystal color and
form
E.H.
colorless needle
S.A.
colorless needle
S.G.
colorless needle
S.A.
colorless fragment
Crystal size [mm3]
Unit cell
Space group
Θ range [°]
a [Å]
0.05×0.08×0.43
orthorhombic
Pnma (no. 62)
2.27–21.02
13.2647(2)
16.8268(3)
0.38×0.08×0.03
monoclinic
P21/m (no. 11)
3.91–27.40
8.108(1)
17.395(1)
10.258(1)
0.18×0.08×0.07
0.25×0.16×0.10
monoclinic
P21 (no. 4)
3.96–26.40
8.2480(3)
17.8570(15)
10.5310(7)
triclinic
¯
P1 (no. 2)
3.61–25.00
9.840(1)
10.050(1)
14.054(1)
101.46(1)
106.39(1)
95.43(1)
1289.9(2), 2
760
b [Å]
c [Å]
α [°]
β [°]
10.6220(2)
109.56(1)
103.150(5)
γ [°]
V [Å3], Z
F000
2370.86(7), 4
1352
1363.3(2), 2
868
1510.38(17), 2
848
Rflns. measured
Rflns. independent
Refined
47273
18489
3184
23987
4533
20357
6110
1336
130-parameter full-matrix 168-parameter full-matrix 312-parameter full-matrix 373-parameter full-matrix
refinement
refinement
refinement
refinement, 8 restraints
(disordered solvent)
Refinement
least-squares refinement of least-squares refinement of least-squares refinement of least-squares refinement of
1336 reflections with
Denzo/HKL
+1.610, –2.096 (insuff.
CALC SQEEZE)
0.0498
3184 reflections with
Collect/SADABS
+1.546, –0.959 (near plati- +0.668, –0.621
num)
0.0283
4533 reflections with
Collect/SADABS
6110 reflections with
Collect/SADABS
+0.899, –0.596
Remaining electron
density [eÅ–3
]
[a]
R1 [Fo Ͼ 4σ(Fo)]
0.0330
0.0379
[all reflections]
wR2 [Fo Ͼ 4σ(Fo)]
0.0530
0.1229
0.0426
0.0560
0.0568
0.0502
0.0516
0.0741
[b]
[all reflections]
GOF (on F2)[a]
Diffractometer
0.1252
1.172
Kappa CCD (Area Dif-
0.0611
1.137
Kappa CCD (Area Dif-
0.0537
0.995
Kappa CCD (Area Dif-
0.0789
1.065
Kappa CCD (Area Dif-
fraction System; Nonius); fraction System; Nonius); fraction System; Nonius); fraction System; Nonius);
Mo-Kα; rotating anode
Mo-Kα; sealed tube
Mo-Kα; sealed tube
Mo-Kα; sealed tube
[a] R1 = Σ(||Fo| – |Fc||)/Σ|Fo|. [b] R2 = [Σw(Fo – Fc ) /Σw(Fo2)2]1/2; GOF = {Σ[w(Fo – Fc ) ]/(n – p)}1/2
.
2
2 2
2
2 2
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