Synthesis of electron-rich platinum centers: Platinum0(carbene) (alkene)2 complexes

Article abstract of DOI:10.1016/j.jorganchem.2005.07.059

The synthesis and molecular structure of the zero-valent platinum-mono-carbene-bis-alkene complexes [Pt⁰(NHC)(dimethyl fumarate)₂] (NHC = 1,3-dimesityl-imidazol-2-ylidene (1a); 1,3-dimesityl-dihydroimidazol-2-ylidene (2a); diphenyl-d

Full text of DOI:10.1016/j.jorganchem.2005.07.059

Journal of Organometallic Chemistry 690 (2005) 5804–5815
www.elsevier.com/locate/jorganchem
Synthesis of electron-rich platinum centers:
Platinum0(carbene)(alkene)2 complexes
a
b
b
a,
*
Marcel A. Duin , Martin Lutz , Anthony L. Spek , Cornelis J. Elsevier
a
Van Ôt Hoff Institute of Molecular Sciences, Molecular Inorganic Chemistry, Universiteit van Amsterdam,
Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands
b
Bijvoet Center for Biomolecular Research, Vakgroep Kristal- en Struktuurchemie, Utrecht University, Padualaan 8, NL-3584 CH Utrecht,
The Netherlands
Received 2 June 2005; received in revised form 15 July 2005; accepted 15 July 2005
Available online 16 September 2005
Abstract
The synthesis and molecular structure of the zero-valent platinum-mono-carbene-bis-alkene complexes [Pt0(NHC)(dimethyl
fumarate)2] (NHC = 1,3-dimesityl-imidazol-2-ylidene (1a); 1,3-dimesityl-dihydroimidazol-2-ylidene (2a); diphenyl-dihydroimida-
zol-2-ylidene (2b) are described. Two routes have been evaluated for the synthesis of 1a and 2a, involving reaction of a zero-valent
platinum compound either with an isolated carbene ligand, or with an in situ generated carbene ligand. The in situ method proved to
be easier and gave similar yields of about 50% after crystallization. Attempts have been made to synthesize similar compounds with
N-phenyl and N-alkyl groups, of which the latter met with little success. However, (1,3-diphenyl-dihydroimidazol-2-ylidene)-bis(g2-
dimethyl fumarate) platinum(0) (2b) could be obtained in 49% yield, after crystallization, from the appropriate Wanzlick dimer.
Compound 1a reacts with H2 and D2 in sequences of oxidative addition, migration–insertion involving dimethyl fumarate, and
reductive elimination to form neutral hydrido platinum (II) carbene complexes, probably containing a metallacyclic (R)–C@O . . . Pt
unit.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: N-Heterocyclic carbene; Alkene; Low-valent; Insertion; Addition; C–H activation
1. Introduction
advantages and potential for catalysis [4]. Promising
features are, apart from generally good to high turnover
numbers and high rates, the excellent catalyst stability,
due to high thermal and hydrolytic durability resulting
from exceptionally stable M–C bonds, i.e., long shelf-life
and stability to oxidation. Fortunately, many NHC and
their metal complexes are readily accessible, requiring
only stoichiometric amounts of the free ligand; there is
generally no need to employ an excess of the NHC.
Despite the increasing interest in this class of NHC
d10-metal complexes, relatively few examples of isolated
and well-characterized nickel(0)-, palladium(0)- or plat-
inum(0)-carbene complexes are known [10–19]. One of
these compounds (Fig. 1, left) is a highly efficient cata-
lyst for for telomerization of 1,3-dienes with alcohols
N-Heterocyclic carbenes (NHCs) are being applied
more and more frequently as ligands in metal-mediated
synthesis and homogeneous catalysis [1,2]. For almost
all d10-metals, complexes with NHC ligands have been
reported, which are able to catalyze reactions, such as
Heck and Suzuki coupling (Pd,Ni) [3–7], aryl amination
(Pd,Ni) [8,9], hydrosilylation (Pt) [10,11], Grignard
cross-coupling (Ni) [12] and Stille coupling (Pd) [13].
These transition-metal-NHC-catalysts have several
*
Corresponding author. Tel.: +31
0 205255653; fax: +31 0
205256456.
E-mail address: elsevier@science.uva.nl (C.J. Elsevier).
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.07.059

M.A. Duin et al. / Journal of Organometallic Chemistry 690 (2005) 5804–5815
5805
Mes
N
thermally stable, they could yet be prone to fumarate
dissociation, opening pathways to activity in catalysis.
The pre-eminent method for the synthesis of these
platinum complexes in our hands started from Pt(nbe)3
(nbe = norbornene) or Pt(cod)2 (cod = 1,5-cyclooctadi-
ene). In this paper, we describe the synthesis of novel
compounds [Pt0(NHC)(dmfu)2], which were prepared
by this approach [21]. Furthermore, the reactivity of
these novel compounds towards dihydrogen will be
reported.
Si
O
Si
N
N
N
N
Ni
Pd
N
Mes
Fig. 1. Pd(0)- and Ni(0)-NHC complexes reported by Beller [14] and
Cavell [15].
(TON = 2.67 · 105) [14]. Concerning platinum (0) com-
plexes with NHCs, apart from the seminal report by
Arduengo et al. describing the synthesis of a platinum
(0) biscarbene compound [19], only recently several
isolated complexes (Fig. 2) have been reported by Marko
and by our group, that are active in hydrosilylation
[10,11]. Although the Arduengo complex is formally a
14-electron species, it is thermally rather stable, which
is probably due to the large mesityl-substituents on the
N-atoms of the imidazolium-based carbene. In this
way the platinum center is shielded by these large
ligands and thereby very difficult to approach by other
ligands or reagents. Since the Pt0(IMes)2 complex lacks
reactivity, we reasoned that one of its carbene ligands
should be replaced by one or two more labile ligands,
thereby facilitating dissociative routes into active plati-
num complexes for, for instance, oxidative addition
and catalysis.
As an approach to compounds having an electron-
rich platinum center, we endeavored to synthesize a
new class of platinum (0) complexes containing one
carbene ligand and two alkenes. The carbene ligand is
deemed to endow a rather high electron density on the
metal center, which may have an accelerating effect on
oxidative addition of C–X, C–H and H–H bonds to such
compounds. The alkene should dissipate some of the
electron density and provide reasonable thermal stabil-
ity, yet be an easily (relative to the NHC) dissociating
ligand.
2. Results and discussion
2.1. Synthesis of [Pt(IMes)(dmfu)2] and
[Pt(SIMes)(dmfu)2]
These zero-valent platinum(NHC)bis(dimethyl fuma-
rate) compounds were prepared via two methods. The
first method (A, see Scheme 1) consists of two steps:
First, the free carbene, 1,3-dimesityl-imidazol-2-ylid-
ene (IMes) or 1,3-dimesityl-dihydroimidazol-2-ylidene
(SIMes) has been prepared [22]. Second, one equivalent
of [Pt(cod)2] (cod = 1,5-dicyclo-octadiene) and two
equivalents of the alkene, dimethyl fumarate (dmfu),
were added to the free carbene in THF at room
temperature. Immediate reaction resulted in the forma-
tion of thermally stable, white products in good yields
(55–73%).
The second method (B, see Scheme 1) concerns the
reaction of Pt(cod)2 with two equivalents of dmfu, one
equivalent of a imidazolium salt (IMesHCl) or imidazo-
linium salt (SIMesHCl) and sodium hydride. The latter
is used as a base for deprotonation of (S)IMesHCl, pro-
viding the free carbene in situ. This method is somewhat
more time consuming than the first one, but obviates the
additional step of isolating the free carbene; overall this
method benefits from easier workup. Pt(nbe)3 can be
used as the starting material instead of [Pt(cod)2]. How-
ever, according to 1H NMR spectroscopy, overnight
stirring yields a mixture of [Pt(carbene)(dmfu)(nbe)]
and Pt(carbene)(dmfu)2. So, the substitution of the last
nbe from Pt(carbene)(dmfu)(nbe) to Pt(carbene)(dmfu)2
is not complete at room temperature. It appeared to be
necessary to add more dmfu, as well as to stir the reac-
tion mixture at elevated temperature (40–50 ꢁC). In situ
preparation of Pt(cod)2 by addition of cod to Pt(nbe)3
[23] before adding the other reagents, can overcome this
problem. However, employing Pt(cod)2 is to be pre-
ferred, and leads to overall exhaustive displacement of
cod by using two equivalents of dimethyl fumarate.
The [Pt(carbene)(g2-alkene)2] complexes [Pt(IMes)-
(dmfu)2], (1a, IMes = 1,3-dimesityl-imidazol-2-ylidene)
and [Pt(SIMes)(dmfu)2], (2a, SIMes = 1,3-dimesityl-
dihydroimidazol-2-ylidene) can be handled in air
without significant decomposition and are stable for
Previously, the efficient synthesis of a number of zero-
valent [Pt0(L)2(fumarate)] and [Pt0(L)(fumarate)2]
complexes has been detailed [20]. We reasoned that,
although the fumarate will – due to p-backdonation
from the metal center – be relatively strongly bonded
in many [Pt0(L)2(fumarate)] and [Pt0(L)(fumarate)2]
compounds, it will probably be less strongly bonded
than an NHC type carbene ligand in [Pt0(NHC)(fuma-
rate)2]. So, although such compounds would be
Mes
N
Mes
N
R
N
Si
O
Pt
Pt
N
N
N
R
Si
Mes
Mes
R = Me, Cy, tBu
´
Fig. 2. Pt(0) NHC-complexes reported by Marko [10] and Arduengo
[19].

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M.A. Duin et al. / Journal of Organometallic Chemistry 690 (2005) 5804–5815
R
R'
N
-2 cod
20 oC, 1 hr
(A)
(B)
(A)
+
Pt(cod)2
+
2
R
N
R
R'
R'
Pt
N
R'
R'
R
N
R
N
+
N
R'
-2 cod, -NaX
R'
2
H
+
Pt(cod)2
+
NaH
+
20 oC, 16 hrs.
R'
X-
1a
R
R
N
R'
-2 cod
20 oC, 1 hr
+
Pt(cod)2
+
2
R
N
R'
R'
Pt
N
R'
R'
R
R
N
N
R
R'
R'
-2 cod, -NaX
+
H
X-
(B) Pt(cod)2 +
2
NaH
+
+
20 oC, 16 hrs.
N
2a, 2b
R'
R
R = Mes (a), Ph (b), R' = COOMe
Scheme 1. Synthesis of platinum(carbene)(g2-alkene)2 complexes.
extended periods of time in solution, even in reflux-
ing acetone. Their stability is surprising, taking into
account that similar complexes are known to easily
undergo alkene dissociation [11,20,21].
imines (Ar–N@CH–)2. Under normal conditions, the
condensation of glyoxal with two equivalents of aniline
in methanol, did not result in phenyl-DAB. The 1H
NMR of the resulting tar shows that this method is
not suitable for the synthesis of the phenyl-substituted
imidazolium salt, hence the corresponding platinum
complex Pt(IPh)(dmfu)2 (1b, IPh = 1,3-diphenyl-imi-
dazol-2-ylidene), cannot be obtained by this route.
An early method for synthesizing saturated N-hetero-
cyclic carbenes had been discovered by Wanzlick and
co-workers [24]. Recently, the equilibrium between
monomer and dimer proposed by Wanzlick [25] has in-
deed been observed by Hahn et al. and Lemal et al. [26–
28]. These authors were actually able to observe a
2.2. Decreasing the size of the carbene ligands and
synthesis of Pt(SIPh)(dmfu)2
In order to evaluate the influence on C–H activation
reactions, we would also like to decrease the steric bulk
on the N-atom of the carbenes. Therefore, we used sev-
eral imidazolium salts like 1,3-dimethyl-imidazolium io-
dide (IMeHI), 1,3-diisopropyl-imidazolium chloride
(IiPrHCl) and 1,3-(di(t-butyl)-imidazolium chloride (It-
BuHCl) as starting salt. We used method B (in situ
method) for making the (IR)Pt(dmfu)2-complexes
(Scheme 2). All reactions resulted in off-white solids,
but products containing a carbene ligand were not
found for R = iPr, tBu. In the case of R = Me, a car-
1
mixture of the dimer and monomer by H NMR spec-
troscopy, by using sterically larger aromatic N-substitu-
ents. We used this approach for the synthesis of the
saturated phenyl-NHC and its corresponding platinum
complex [Pt(SIPh)(dmfu)2] (2b, SIPh = 1,3-diphenyl-di-
hydroimidazol-2-ylidene). Various metal-carbene com-
plexes have already been prepared by this method [29].
1,3-Diphenyl-2-trichloromethyl-imidazoline (3; see Sec-
tion 4) has been used for the straightforward synthesis
of the ‘‘Wanzlick-dimer’’ 4 by a slightly modified litera-
ture method [24].
1
bene-containing product was formed according to H
NMR spectroscopy. However, washing and crystalliza-
tion did not yield pure products.
Apparently, contrary to the N-mesityl analogues, N-
alkyl NHCs did not result in Pt(carbene) complexes
using method B. We then decided to synthesize phe-
nyl-substituted imidazolium and imidazolinium salts,
which were synthesized from the corresponding a-di-
When the dimer 4 is treated with Pt(cod)2 and two
equivalents of dmfu, no platinum(carbene) complex
R
N
R
N
-2 cod, -NaX
R'
Pt
R'
R'
R'
R'
+
N
2
H
+
Pt(cod)2
+
NaH
+
X-
N
R
R'
R
R = Me, iPr, tBu
R' = COOMe
Scheme 2. Attempted synthesis of (N-alkyl-NHC)Pt(dmfu)2 complexes.

M.A. Duin et al. / Journal of Organometallic Chemistry 690 (2005) 5804–5815
5807
was obtained, even gentle heating did not result in the
formation of 2b (Scheme 3). Using Pt(nbe)3 instead of
Pt(cod)2/dmfu, i.e., a more labile alkene, did not result
in a platinum(carbene) complex either. Apparently, the
Wanzlick-dimer is either too stable in this case (no for-
mation of 40), or the in situ formed Pt(cod)(dmfu) (or
Pt(nbe)2(dmfu)) is not reactive enough towards the free
carbene 40. From the observed failure to form 2b accord-
ing to Scheme 3, it can be concluded that for saturated
carbenes, the Wanzlick-dimer 4 either does not dissoci-
ate at all to give 40, or that the Wanzlick equilibrium
as shown in Scheme 3 lies extremely far to the left.
The alternative, that the carbene 40 does form but can-
not substitute an alkene from the Pt0(alkene)n precursor
can be excluded. It is known and has amply been dem-
onstrated that NHCs can substitute all kinds of ligands
and certainly one alkene [2].
dry conditions. Adding this strong acid in THF resulted
in the fast precipitation of imidazolinium salt 5. In a
slow reaction, the addition of NaH, Pt(cod)2 and two
equivalents of dmfu to the imidazolinium salt 5 indeed
gave 2b, c.f. Scheme 4 [30].
This successful formation of 2b according to Scheme
4 may have two reasons. The first possibility is, that
NaH slowly deprotonates 5 to give the free carbene 40,
which then immediately reacts with Pt0(alkene)n. How-
ever, we doubt that this explanation is correct. Wanzlick
[31] noted already 40 years ago, and Denk et al. [32] re-
cently confirmed, that the free carbene 40 dimerizes in a
very fast reaction to the unreactive dimer 4. So, the fol-
lowing alternative seems more likely. According to
Scheme 5, the Pt0(alkene)n precursor may react with
the imidazolinium salt 5 by C–H activation at the 2-po-
sition, giving the probably unstable hydridoPtII(car-
bene) complex A, which is subsequently deprotonated
by NaH to form 2b and dihydrogen.
As the direct synthesis of 2b was not possible starting
from the dimer 4, we treated 4 with HBF4 in ether under
Pt(cod)2 / dmfu
or Pt(nbe)3
R'
Pt
R'
N
N
N
N
N
N
N
N
R'
R'
?
1/2
R' = COOMe
2b
4
4'
Scheme 3. First approach to the synthesis of 2b via the ‘‘Wanzlick dimer’’ 4.
Pt(cod)2
NaH, 2 dmfu
R'
Pt
R'
N
N
N
N
N
N
N
+
N
R'
R'
HBF4
1/2
H
-2 cod -NaBF4
-
BF4
R' = COOMe
4
5
2b
Scheme 4. Synthesis of 2b via an imidazolinium salt.
+
-
+Pt(cod)2
+dmfu
- 2 cod
BF4
+dmfu
+NaH
R
H
R
R
N
N
N
+
N
N
N
Pt
Pt
H
- NaBF4
R
O
-
R
BF4
OMe
R = COOMe
2b
5
A
Scheme 5. Proposed intermediate A.

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M.A. Duin et al. / Journal of Organometallic Chemistry 690 (2005) 5804–5815
1
2.3. H NMR spectroscopy
the coordination angles as well as the donor/acceptor
properties of the ligands have a large influence on the
chemical shift in 195Pt NMR, we refrain from discussing
the differences between these shifts. However, concern-
ing the range of the 195Pt chemical shifts, comparison
to another, recently published, Pt(0)-monocarbene com-
plex with alkene with a chemical shift of ꢀ5343 ppm and
with Pt0(IMes)2 (Fig. 2) [19], shows that such low shifts
are not uncommon for this type of compounds [10] and
that the electron density on the platinum centers of these
molecules is quite similar. Other tris-coordinated plati-
num(0) complexes, like the [Pt0(NN)(alkene)] com-
plexes, have 195Pt chemical shifts around ꢀ4000 ppm
[20]. Compared to the latter, the present [platinum(car-
bene)bis(alkene)] compounds 1a, 2a, 2b have their
195Pt resonances at much lower frequencies, which must
be attributed to the fact that carbenes are carbon donor
ligands, which cause larger ligand fields than chelating
didentate nitrogen ligands, hence lower 195Pt chemical
shifts [35,36].
In agreement with the C2-symmetry of complexes 1a,
2a and 2b, the H NMR spectra of the zero-valent plat-
1
inum compounds show two different signals for the
anisochronous alkene-protons (Table 1). Two of the
protons of the alkenes are pointing toward the aromatic
part of the carbene-ligand, these are found at lower
frequency due to the anisotropic shielding of the aryl
ring. The other two protons of the alkenes are pointed
away from the aromatic part of the carbene-ligand
and are found at normal frequencies for coordinated
alkenes [20] (see Fig. 3).
The saturated carbene is a stronger r-donor than the
unsaturated carbene [33,34], which leads to more p-
backbonding to the dmfu in 2a as compared to 1a,
2
and hence lower bond order and smaller JPtH for 2a.
Furthermore the Ph-carbene b is a better acceptor than
carbene a because of the fact that the unsubstituted
phenyl-group has significantly more p-overlap with the
NHC frame, thereby increasing the alkene C–C double
bond character leading to higher values for 2JPtH in case
of 2b.
2.5. Variable temperature NMR spectroscopy of 2a and
2b
2.4. 195Pt NMR spectroscopy
When investigating the chemical and physical proper-
ties of 2b, line-broadening of the alkene hydrogen atoms
was observed in the 1H NMR spectrum of 2b in
benzene-d6 slightly above room temperature (303 K).
In order to probe the fluxional process, we measured
More information on the chemical properties of the
metal center was sought by means of 195Pt NMR. The
195Pt chemical shift is very sensitive to the ligands pres-
ent in the coordination sphere in the Pt-compound and
is therefore a useful probe of the electronic environment
of the metal [35,36]. In the 195Pt NMR spectra the chem-
ical shifts are found at ꢀ5184 and ꢀ5200 ppm for 1a
and 2a, respectively; for 2b the value is ꢀ5129 ppm.
Since the chemical shift differences are only small and
1
the H NMR spectra of 2b and 2a in toluene-d8 in the
temperature range from 298 to 378 K. At 298 K two
doublets are seen for the alkene protons of the dmfu
ligands in 2b around 4.1 and 3.6 ppm with
2J{1H,195Pt}-couplings superimposed (see Fig. 4). A
gradual increase of the temperature leads to broadening
of the signals (at 308 K), coalescence and disappearing
of the signals in the base line (318 K). No average
signals for the alkene-protons between the former al-
kene-resonances (at 298 K) were observed when the tem-
perature was further increased up to 378 K. After
cooling down to room temperature the starting spec-
trum was found, but also some depletion of platinum
metal was found. The same experiments were also
carried out with a solution of 2a in toluene-d8. For 2a
Table 1
1H NMR spectroscopic data of the alkene-protons
Compound
d(HA) (ppm)
[2JHPt (Hz)]
d(HB) (ppm)
[2JHPt (Hz)]
Pt(IMes)(dmfu)2 (1a)
Pt(SIMes)(dmfu)2 (2a)
Pt(SIPh)(dmfu)2 (2b)
3.92 [46.0]
3.89 [43.2]
3.49 [57.0]
3.13 [63.0]
3.17 [52.8]
3.10 [60.6]
N
N
N
N
N
N
R
R
R
Pt
Pt
Pt
R
R
R
HB
HB
HB
A
A
A
HA
HA
HA
HB
HB
HB
H
H
H
R
R
R
R
R
R
1a
2a
R = COOMe
2b
Fig. 3. Prepared Pt(carbene)(dmfu)2 complexes.

M.A. Duin et al. / Journal of Organometallic Chemistry 690 (2005) 5804–5815
5809
*
*
298 K
318 K
338 K
358 K
4.40
4.00
Fig. 4. VT H NMR spectra of 2b (* alkene protons region).
3.60
4.20
3.80
1
the signals of the alkene-protons remain sharp over the
whole temperature range (298–378 K).
2.6. X-ray crystal structure determination of 1a
The most plausible explanation for the absence of an
average signal for the alkene-protons at higher tempera-
tures is an exchange of free dmfu and coordinated dmfu
at higher temperatures for 2b. The reason why this is not
observed for a solution of 2a may be that the SIMes-car-
bene is the better r-donor relative to IMes, resulting in
more p-backbonding to the dmfuÕs in case of the former
and, as a result, stronger coordination of the dmfuÕs in
2a. Moreover, in contrast to the situation for the
SIMes-carbene, the phenyl-group of the SIPh-carbene
is in plane of the imidazol-2-ylidene, resulting in better
p-accepting properties and less p-donation for this car-
bene-ligand. This feature causes weaker coordination
of dmfu.
The molecular structure of [Pt(IMes)bis(dimethyl
fumarate)], 1a, was unambiguously proven by a single
crystal X-ray structure analysis, which is depicted in
Fig. 5. Selected bond lengths and angles are given in Ta-
ble 2.
Compound 1a is monoclinic, space group P21/c. The
platinum has a distorted square-planar environment
with a sum of cis angles of 360.4ꢁ. The compound is
approximately C2v symmetric. The C@C bonds of the
˚
coordinated dmfuÕs (1.422(4) and 1.427(4) A) are elon-
˚
gated compared to free dmfu (1.318(2) A) [37] caused
by the donation of electron density from platinum into
the p*-orbitals of the dimethyl fumarate. The Pt–g2-
C@C planes are oriented in an angle of 56.05(19)ꢁ and
Fig. 5. Displacement ellipsoid plot of 1a with ellipsoids drawn at the 50% probability level. Hydrogens are omitted for clarity.

5810
M.A. Duin et al. / Journal of Organometallic Chemistry 690 (2005) 5804–5815
Table 2
and the Pt–C(alkene) bonds are slightly longer (2.103
and 2.178 A) compared to 1a.
˚
Selected bond lengths (A) and angles (deg) for 1a (e.s.d. in parentheses)
˚
Pt(1)–C(1)
2.072(2)
2.098(3)
2.101(3)
1.422(4)
2.130(3)
2.120(3)
1.427(4)
Pt(1)–C(23)
Pt(1)–C(29)
C(28)–C(29)
Pt(1)–C(22)
Pt(1)–C(28)
C(22)–C23)
2.7. Reactivity of platinum(carbene)(alkene)2 towards
H2
The new platinum(carbene)(alkene)2-complexes are
reactive towards C–H bonds of imidazolium salts
(Scheme 6) [21]. We were also interested to see whether
these compounds are reactive towards C–C bonds, for
instance of 2-methyl-imidazolium salts. MO-calcula-
tions have shown that C–C bond fission is in principle
feasible, insertion of Pt into this C–C bond is an
exothermic reaction [38].
Therefore, we investigated the reactivity of Pt
(IMes)(dmfu)2 towards (x) (Scheme 7). This should in
principle result in a thermodynamically stable product
(biscarbene)platinum(methyl)(iodide) (6ax). However,
when carrying out this reaction under various condi-
tions (in benzene or acetone at reflux temperatures),
we did not observe any other compounds than the start-
ing materials. We added dihydrogen gas to the reaction
mixture to see if the putative compound 6ax could be
trapped by reaction with H2. This reaction was carried
out in acetone-d6 in an NMR tube and was followed
C(1)–Pt(1)–C(22)
C(1)–Pt(1)–C(28)
C(1)–Pt(1)–C(23)
C(1)–Pt(1)–C(29)
96.96(10)
94.21(10)
136.16(10)
133.38(10)
56.7(2)ꢁ with respect to the NHC-plane. These Pt–g2-
C@C planes are mutually oriented almost in one plane,
making an angle of 10.8(2)ꢁ. The mesityl substituents on
the nitrogens of the NHC-ligand are tilted and make an
angle of 68.77(14)ꢁ and 66.28(14)ꢁ, respectively, relative
to the plane of the imidazole ring.
Recently, the X-ray crystal structures of zero-valent
platinum(carbene) compounds with a diene ligand
[Pt(1,3-dimethyl-imidazol-2-ylidene)(g4-divinyltetramet-
hylsiloxane)] [10] has been reported. The Pt–C(carbene)
˚
bond of this compound isis slightly shorter (2.051 A)
R
R
N
R'
Pt
N
R'
+
I-
I
N
+
N
- 2 dmfu
H
Pt
R
R'
R'
N
N
R
N
H
N
R = Mes, R' = COOMe
Scheme 6. Example of C–H activation of imidazolium salts [21].
Mes
Me
N
Mes
N
R
N
R
+
I-
Me
I
-2 dmfu
N
+
Pt
Pt
Mes
Me
R
∆
N
N
N
CH3
Me
Mes
N
Me
R
1a
x
6ax
R = COOMe
H2
-CH4
∆
H2
Mes
Pt
N
a range of platinum hydrides
I
N
Mes
Me
N
H
N
Me
6ay
Scheme 7. Reactivity of 1a towards C–C bonds and dihydrogen.

M.A. Duin et al. / Journal of Organometallic Chemistry 690 (2005) 5804–5815
5811
H
R
Pt
COOMe
N
N
N
N
R
R
COOMe
H2
Pt
O
+
55 oC
MeOOC
R
O
1a
R = COOMe
Scheme 8. Formation of a platinum hydride by addition of H2 to 1a and multiple migration–insertions of coordinated dmfu.
7a
1
by H NMR spectroscopy. Indeed platinum hydrides
of 1,2,3-trimethyl-imidazolium iodide (x) showed more
than one platinum hydride signal in the H NMR spec-
1
were observed in the expected chemical shift-range sim-
ilarly to known analogues [21] at about ꢀ14.5 ppm,
indicating a hydride trans to an iodide, as in trans-hydr-
ido 3,5-dimethyl-imidazol-2-yliden iodo 3,5-dimesityl-
imidazol-2-yliden platinum(II) (6ay) after reductive
elimination of methane.
trum. This reactivity can be rationalized if one takes the
coordination site that is occupied by the weakly coordi-
nating carbonyl into account, which can be substituted
for example by a C–H bond. Then, C–H bond activation
of one of the methyl groups is feasible, which would be
the first one taking place for a non-cationic Pt(II) com-
pound [40].
In order to decrease the reactivity of 7a towards
hydrocarbons, we added 1 equivalent of pyridine in ace-
tone-d6 to stabilize the PtII center (Scheme 9). Most of
7a (dPt = ꢀ3920 ppm in acetone-d6) was converted in
8a (dPt = ꢀ4081 ppm). With an excess of pyridine, 8a
is the only platinum complex observed by NMR. Evap-
oration of the solvent and the excess of pyridine gave a
mixture of 7a and 8a instead of the expected pure 8a,
indicating that pyridine does not coordinate very
strongly to platinum. The hemilabile coordination of
the carbonyl is apparently quite strong. This is reminis-
cent of known cases for Pt [41] and Pd [42]-metallacy-
cles, however these are cationic complexes.
As a reason for the observation of several platinum
hydrides, we considered direct reaction of dihydrogen
with 1a to give a product that is reactive towards x.
So, H2 was bubbled through a solution of 1a in ben-
zene-d6 in an NMR-tube at 55 ꢁC for 30 min. At this
temperature a reaction occurred and, surprisingly, no
platinum metal was observed.[39]. After the reaction, a
1H NMR spectrum was taken and dimethyl succinate
and platinum complex 7a were observed as the main
products. This reaction takes place surprisingly cleanly,
and yields mononuclear (hydrido)platinum(carbene) 7a
(Scheme 8). Apparently, successive oxidative addition
of dihydrogen, migration–insertion involving the coor-
dinated dmfu and reductive elimination of the second
hydride with the incipient alkylplatinum species (rather
than b-elimination) take place. After formation of di-
methyl succinate, another sequence of addition and
migration–insertion takes place, but this fails to elimi-
nate and terminates as compound 7a.
Compound 7a has only been characterized in solution
by 1H NMR, 195Pt NMR and IR spectroscopy. The hy-
dride is found at ꢀ27.07 ppm with a large 1JPtH coupling
of 1900 Hz, which is in agreement with a very weak
trans-ligand such as the carbonyl in this case. The
195Pt-resonance of 7a is found at ꢀ3940 ppm, indicative
of the +2 oxidation state for the platinum center of 7a
[35], which implies an oxidative shift of +1300 ppm
upon conversion of 1a into 7a. 13C NMR spectroscopy
failed due to the limited stability of 7a in organic sol-
vents like benzene or acetone. The IR of the product re-
vealed, apart from carbonyl signals from coordinated
and free dimethyl fumarate and succinate, a band at
1646 cmꢀ1, which is ascribed to a carbonyl: Pt . . . O@C.
Attempts to isolate complex 7a failed; it probably reacts
with C–H bonds of hydrocarbons, because the addition
In order to assure that the hydride in 7a originates
from the dihydrogen that was added to 1a, we added
D2 to 1a. Indeed, in this case the deuteride Pt-2H was
2
observed at ꢀ26.6 ppm in the H NMR spectrum. Fur-
thermore, deuterated dimethyl succinate was formed.
Hence, reaction of 1a with dihydrogen constitutes a
rather gentle method to arrive at a neutral hydrido plat-
inum(II) carbene complex with a hemilabile coordinat-
ing carbonyl group, from which a free coordination
site for bond activation reactions can be readily created.
R
N
R
N
H
Pt
O
H
Pt
N
COOMe
COOMe
COOMe
+ py
- py
N
R
N
R
O
8a
7a
R= Mes
Scheme 9. Addition of pyridine to 7a.

5812
M.A. Duin et al. / Journal of Organometallic Chemistry 690 (2005) 5804–5815
3. Conclusion
on a JEOL JMS SX/SX102A four sector mass spec-
trometer, coupled to a JEOL MS-MP9021D/UPD sys-
tem program. For Fast Atom Bombardment (FAB
mass spectrometry, the samples were loaded in a matrix
solution (3-nitrobenzyl alcohol) onto a stainless steel
probe and bombarded with xenon atoms with an energy
of 3 KeV. During the high resolution FAB-MS a resolv-
ing power of 10,000 (10% valley definition) was used.
Several zero-valent platinum-mono-carbene-bis-al-
kene complexes [Pt0(NHC)(dimethyl fumarate)] have
been isolated and characterized. Importantly, impurities
can be easily removed with apolar solvents, facilitating
the purification of these compounds. The complexes
are air-, moisture- and thermally stable for months in
solution and in the solid state. Two routes for the syn-
thesis of these complexes have been implemented, the
route involving in situ preparation of the carbene is
the most facile one for obtaining the [plati-
num(NHC)bis(alkene)] complexes in reasonable to good
yield. The new platinum complexes could be valuable
compounds for some Pt0-catalyzed reactions [11,20]
and may provide information about intermediates of
known Pt0(NHC)-catalyzed reactions.
4.2. Synthesis
4.2.1. Method A: Synthesis of 1a via the preparation of
the free carbene
4.2.1.1. 1,3-Dimesityl-imidazol-2-ylidene. The free
carbene was synthesized by a slightly changed procedure
described by Arduengo et al. [22]; 0.80 g (2.3 mmol) 1,3-
bis(2,4,6-trimethylphenyl)-imidazolium chloride was
dissolved in 50 ml THF at room temperature and one
equivalent of KOtBu (1 M in THF, 2.35 ml) was added
to the mixture. The solution was stirred for 4 h during
which time a white solid precipitated from the solution.
The mixture was then filtered over Celite filter aid and
the filter was washed with 10 ml hexane. From the
resulting clear yellowish filtrate the solvent was removed
in vacuo. The residue was extracted with four portions
of hot hexane (around 60 ꢁC). The extracts were
collected concentrated until precipitation of the carbene
occurred. A small volume of dry hexane was added in
order to dissolve the precipitated solids and the flask
was left to stand overnight at ꢀ80 ꢁC for crystalliza-
tion. The supernatant was removed with a cannulae
and the solid was dried in vacuo to give 0.52 g (74%)
of the free carbene as an off white slightly sticky
material.
The [Pt0(NHC)(dimethyl fumarate)] complex 1a re-
acts under mild conditions with dihydrogen in a se-
quence of oxidative addition, migration–insertion
involving the coordinated dmfu and reductive elimina-
tion, to form dimethyl succinate and, after a second se-
quence, the corresponding neutral hydrido platinum(II)
carbene complex in which the carbonyl group of the es-
ter moiety of the inserted second dmfu probably consti-
tutes a cyclic (R)–C@O . . . Pt unit. Pyridine is able to
break the carbonyl coordination, however, the coordi-
nation of pyridine is also quite weak. The presumed
hemilability of the carbonyl group may facilitate the
reaction of the neutral hydrido platinum(II) carbene
complex towards certain C–H bonds, to give C–H bond
activation.
4. Experimental
4.2.1.2.
(1,3-Dimesityl-imidazol-2-ylidene)-bis-(g2-di-
4.1. General
methyl fumarate) platinum(0) (1a). An amount of
149.8 mg (0.4911 mmol) 1,3-dimesityl-imidazol-2-ylidene
is carefully transferred in a Schlenk tube under dinitrogen
atmosphere to avoid any moisture. To this Schlenk
tube 201.9 mg (0.4912 mmol) Pt(cod)2 and 141.5 mg
(0.9826 mmol) dmfu were added. The solids were dis-
solved in 20 ml THF and the resulting solution was stirred
for 1 h at room temperature. Then 20 ml hexane is added
and the amount of solvent is reduced to 10 ml in vacuo.
Another 20 ml hexane is added and the volume of solvent
is reduced to 5 ml during which an off white solid came out
of the solution. The solvent is removed using a cannulae
and the solids were washed twice with 5 ml of ether/hex-
anes 1:1 v/v to give 198.8 mg (51%) of an off white solid.
1H NMR (500 MHz, acetone-d6, d (ppm)): 7.61 (2H, s,
4JHPt = 10.5 Hz), 7.08 (2H, s), 6.96 (2H, s), 3.92 (2H, d,
All reactions involving air-sensitive compounds were
carried out under a dinitrogen atmosphere using stan-
dard Schlenk techniques. Solvents were dried and dis-
tilled prior to use, according to standard methods [43].
Pt(cod)2 [22], Pt(nbe)3 [22], 1,3-dimesityl-imidazolium
chloride [30], 1,3-dimesityl-dihydroimidazolium chloride
[30] were prepared according to literature procedures.
NMR measurements were performed on a Varian
Mercury300 spectrometer (1H: 300.13 MHz, 13C
75.47 MHz), a Varian Inova500 spectrometer (1H:
499.88 MHz, 13C: 125.70 MHz) and Bruker DRX300
spectrometer (1H: 300.13 MHz, 13C: 75.47 MHz, 195Pt:
64.13 MHz). 195Pt NMR spectra were measured via a
normal HMQC sequence at 298 K. 13C NMR spectra
1
2
were measured with H decoupling. Positive chemical
3JHH = 9.0 Hz, JHPt = 46.0 Hz), 3.40 (6H, s), 3.30
3
2
shifts (d) are denoted for high-frequency shifts relative
to the external TMS reference (1H, 13C) or a Na2PtCl6
reference (195Pt). HRMS measurements were performed
(6H, s), 3.13 (2H, d, JHH = 9.0 Hz, JHPt = 63.0 Hz),
2.39 (6H, s), 2.35 (6H, s), 1.80 (6H, s). 13C NMR
(125.7 MHz, acetone-d6, d (ppm)): 172.27, 170.69

M.A. Duin et al. / Journal of Organometallic Chemistry 690 (2005) 5804–5815
5813
(2J{195Pt,13 C} = 38.0 Hz), 169.14 (2JCPt = 36.0 Hz),
138.38, 136.21 (3JCPt = 9.0 Hz), 135.40, 135.34, 129.42,
129.13, 124.90 (3JCPt = 42.2 Hz), 50.22 (1JCPt = 146.6
Hz), 50.16, 50.13, 48.98 (1JCPt = 191.1 Hz), 20.41, 18.75,
17.51. 195Pt (64.3 MHz, acetone-d6, d (ppm)): ꢀ5184.
HRMS(FAB) (m/z): Obs: 788.2512, Calc: 788.2514.
7.03 (2H, s), 6.87 (2H, s), 4.20 (2H, m), 4.05 (2H,
3
2
m), 3.89 (2H, d, JHH = 9.0 Hz, JHPt = 43.2 Hz), 3.49
3
(6H, s), 3.21 (6H, s), 3.17 (2H, d, JHH = 9.0 Hz,
2JHPt = 52.8 Hz), 2.65 (6H, s), 2.29 (6H, s), 1.91 (6H,
s). 13C NMR (125.7 MHz, acetone-d6, d (ppm)):
171.51 (2JCPt = 40.0 Hz), 169.68 (2JCPt = 36.0 Hz),
138.14, 137.33, 136.90, 136.86, 130.26, 129.98, 52.93
(1JCPt = 136.5 Hz), 51.02, 50.89, 49.89 (1JCPt = 136.5
Hz), 21.13, 19.13, 18.17. 195Pt NMR (64.3 Hz, ace-
tone-d6, d (ppm)): ꢀ5200. EA: Found: 50.26 (C), 5.31
(H), 3.48 (N) Calc: 50.19, 5.36, 3.55.
4.2.2. Method B: synthesis of 1a via one-pot synthesis by
in situ generation of the free carbene
4.2.2.1.
(1,3-Dimesityl-imidazol-2-ylidene)-bis-(g2-di-
methyl fumarate) platinum(0) (1a). A Schlenk tube
was charged with 132.8 mg (0.450 mmol) 1,3-bis(2,4,6-
trimethylphenyl)-imidazolium chloride, 166.0 mg (0.404
mmol) Pt(cod)2, 128.3 mg (0.891 mmol) dimethyl fuma-
rate (dmfu) and 63.5 mg (1.59 mmol) of 60% NaH in
mineral oil. Then 20 ml THF was added to the solids.
The mixture was stirred overnight at room temperature.
The mixture was then filtered over Celite filter aid. The
remaining solids were washed with another 10 ml THF
and also filtered over the filter. 20 ml hexane was added
to the combined filtrates, and the solvent was reduced to
5 ml under reduced pressure. Then 20 ml hexane was
added to the residue and the mixture was centrifuged
for 10 min at 3500 RPM. The solvent was decanted
and the resulting off-white solid was washed with
hexanes/ether 3:1 (2 · 20 ml) removing the solvents
again via centrifugation. The solids were dried further
under vacuum yielding 152.6 mg (48%) of a pale yellow
powder. Eventually this powder can be recrystallized
from acetone. A second crop of product can be obtained
from the combined washings. The products of methods
A and B have identical 1H NMR spectra. Single crystals
suitable for X-ray structure analysis could be obtained
by slowly cooling down a saturated acetone solution
from 55 ꢁC to room temperature.
4.2.2.3. 1,3-Diphenyl-2-trichloromethyl-imidazolidine (3)
[24]. Instead of chloral as used by Wanzlick et al.,
chloral hydrate was used. 2.31 g (10.9 mmol) 1,2-dianil-
inoethane was dissolved in 5 ml glacial acid. 1.86 g
(11.2 mmol) chloral hydrate was added and the resulting
mixture was stirred overnight at room temperature. A
red solid was filtered off and the solid was washed with
methanol (2 · 5 ml) yielding 0.98 g (27%) of a white
1
solid, which was identified by H NMR spectroscopy
1
as pure 3. H NMR (300 MHz, acetone-d6, d (ppm)):
3
4
7.16–7.28 (8H, m), 6.80 (2H, tt, JHH = 6.9 Hz, JHH
=
3
1.5 Hz), 6.86 (1H, s), 4.21 (2H, q, JHH = 4.2 Hz), 4.87
3
(2H, q, JHH = 4.2 Hz).
4.2.2.4. Bis-(1,3-diphenyl-2-imidazolidine) (4) [24]. An
amount of 495.3 mg (1.45 mmol) 1,3-diphenyl-2-trichlo-
romethyl-imidazolidine (3) was dissolved in a mixture of
4 ml xylenes and 1 ml collidine. This solution is heated
on a oil bath to 185 ꢁC for 1 h. After cooling down to
room temperature, the solvents were removed by decan-
tation and the yellow solid was washed with diethyl
ether (2 · 4 ml). A pale yellow solid was obtained in a
yield of 225.4 mg (70%). As reason of the reactivity of
this compound towards air and moisture combined with
the bad solubility in common organic solvents, the
dimer of 1,3-diphenyl-2-imidazolidine was used without
identifications.
4.2.2.2. (1,3-Dimesityl-dihydroimidazol-2-ylidene)-bis-
(g2-dimethyl fumarate) platina(0) (2a). A Schlenk
tube was charged with 1.07 g (3.14 mmol) SIMesHCl,
1.09 g (2.65 mmol) Pt(cod)2, 0.79 g (5.49 mmol) di-
methyl fumarate (dmfu) and 0.40 g (9.91 mmol) of
60% NaH in mineral oil. Then 30 ml THF was added
to the solids. The mixture is stirred overnight at room
temperature. The mixture was filtered over Celite. The
solids were washed with another 10 ml THF and also
filtered. Hexane (20 ml) was added to the combined
filtrates, and the solvent was reduced to 5 ml under
reduced pressure. Then 20 ml hexanes were added to
the residue and the mixture was centrifuged for
10 min at 3500 RPM. The solvent was decanted and
the resulting off-white solid was washed with hexanes/
ether 3:1 (2 · 20 ml) removing the solvents again via
centrifugation. The solids were dried further under
vacuum yielding 0.87 g (55%) of an off-white powder.
Eventually this powder can be recrystallized from
acetone. 1H NMR (500 MHz, acetone-d6, d (ppm)):
4.2.2.5. 1,3-Diphenyl-2-imidazolidinium tetrafluoroborate
(5). An amount of 0.18 g (0.40 mmol) bis-(1,3-
diphenyl-2-imidazolidine) (4) was suspended in 40 ml
THF. To this solution 0.10 ml 54% HBF4 (0.73 mmol)
in diethyl ether was added. An orange glow could be
seen and immediately a yellow solid precipitated from
the solution. After 30 min stirring at room tempera-
ture, the solid was filtered off and was washed with
diethyl ether (2 · 10 ml). This yielded 0.20 g (80%)
of a pale yellow solid. 1H NMR (300 MHz, ace-
tone-d6, d (ppm)): 9.86 (1H, s), 7.72 (4H, m), 7.60
(4H, m), 7.46 (2H, m), 4.92 (4H, s). 13C NMR
(75.47 MHz, dmso-d6,
d
(ppm)): 152.65 (NCH),
136.97 (C), 130.59 (m-CH), 127.93 (p-CH), 119.31
(o-CH), 49.15 (CH2). 19F NMR (282.4 MHz, ace-
tone-d6, d (ppm): ꢀ151.7.

5814
M.A. Duin et al. / Journal of Organometallic Chemistry 690 (2005) 5804–5815
Table 3
4.2.2.6.
(1,3-Diphenyl-dihydroimidazol-2-ylidene)-bis-
(g2-dimethyl fumarate) platinum(0) (2b). To 59.2 mg
(0.191) 1,3-diphenyl-2-imidazolidinium tetrafluorobo-
rate (5), 73.6 mg (0.179 mmol) Pt(cod)2, 55.7 mg
(0.387 mmol) dimethyl fumarate and 32 mg (0.80 mmol)
60% NaH in mineral oil in a Schlenk tube, 20 ml THF
was added. This mixture was stirred overnight at room
temperature. The mixture was filtered over Celite and
the solids were washed with THF (2 · 5 ml). To the
combined filtrates 15 ml of hexanes was added, and
the volatiles were removed in vacuo. The solids were
scraped from the wall and were washed with ether
(2 · 5 ml) giving 110.7 mg (87%) of an off white solid.
The impure compound was recrystallized from acetone
yielding pure 2b (yield: 65.0 mg, 49%). 1H NMR
Crystal data and details of the structure determination of 1a
Empirical formula
Formula weight
Crystal color and shape
Crystal size (mm3)
Crystal system
C33H40N2O8Pt
787.76
Yellow block
0.24 · 0.18 · 0.12
Mmonoclinic
P21/c (no. 14)
18.362(3)
10.5379(14)
16.5493(17)
92.063(15)
3200.2(8)
150
Space group
˚
a (A)
˚
b (A)
˚
c (A)
b (ꢁ)
3
˚
V (A )
T (K)
Z
4
Dcalc (g/cm3)
1.635
4.438
(l(Mo Ka) (mmꢀ1
)
Absorption correction range
Reflections collected/unique
Parameters
0.63–0.77
46,720/11,514
424
(300 MHz, acetone-d6,
d (ppm)): 7.47 (4H, dd,
4
3JHH = 8.4 Hz, JHH = 1.5 Hz), 7.2–7.3 (6H, m), 4.70
(2H, m), 4.34 (2H, m), 3.54 (12H, s), 3.49 (2H, d,
3JHH = 9.9 Hz, 3JHPt = 57.0 Hz), 3.10 (2H, d,
R1 (obs./all refl.)
wR2 (obs./all refl.)
GoF
0.0255/0.0371
0.0467/0.0498
1.031
ꢀ0.81/0.73
3
3JHH = 9.9 Hz, JHPt = 60.6 Hz). 13C NMR (125.7
3
˚
qmin/max (e/A )
1
MHz, CD2Cl2, d (ppm)): 196.89 JCPt = 1332 Hz,
172.20 (2JCPt = 41 Hz), 171.84 (2JCPt = 44 Hz), 171.39,
129.04 (4JCPt = 39.7 Hz), 126.47, 122.35, 52.10
(1JCPt = 164.4 Hz), 51.75 (3JCPt = 42.0 Hz), 51.41,
51.11, 46.06 (1JCPt = 172.0 Hz). 195Pt NMR (64.3 Hz,
acetone-d6, d (ppm)): ꢀ5129.
[48]. An analytical absorption correction was applied
with the program PLATON [46]. Reflections, equivalent
with respect to the twin situation, were merged. The
refinement used the HKLF5 option [49] of SHELXL97
[45] resulting in a twin ratio of 0.40:0.60. Details of
the structure determinations are given in Table 3.
CCDC 273661 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif
(or from the CCDC, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033; e-mail: deposit@
ccdc.cam.ac.uk).
4.2.2.7. cis-[3-Methoxy-1-(methoxycarbonyl)-3-oxopro-
pyl][hydrido][1,3-dimesityl-imidazol-2-ylidene] platina(II)
(7a). H2 was bubbled through a solution of 1a in ben-
zene-d6 in an NMR-tube at 55 ꢁC for 30 min. After the
1
reaction a H NMR spectrum was taken, indicating that
1a had completely been converted to dimethyl succinate
1
and platinum complex 7a. H NMR (300 MHz, benzene-
d6, d (ppm)): 6.64 (4H, s, ArH), 6.08 (2H, ImH), 3.47
(3H, br, CHCH2), 3.41 (3H, COOCH3), 2.95 (3H, s,
COOCH3), 2.14 (6H, s, CCH3), 2.08 (6H, s, CCH3), 1.99
(6H, s, CCH3),ꢀ27.07 (1H, s,1JHPt = 1900 Hz). 195Pt
(64.3 MHz, benzene-d6, d (ppm)):ꢀ3940. IR(benzene):
Acknowledgement
m(C@O . .. Pt) 1646 cmꢀ1
.
M.A.D. and C.J.E. would like to thank Prof. K.J.
Cavell and Dr. N.D. Clement for their helpful sugges-
tions and comments, especially in the early stage of this
research.
4.2.2.8. X-ray crystal structure determination of 1a. X-
ray intensities were measured on a Nonius KappaCCD
diffractometer with rotating anode (Mo Ka,
˚
k = 0.71073 A). The structures were solved with auto-
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