Expedient, direct synthesis of (L)Pt(0)(1,6-diene) complexes from H 2PtCl6

Article abstract of DOI:10.1021/om7007088

The one-pot synthesis of useful [Pt₂(0)(η⁴-1,6- diene)₃] complexes, directly from H₂PtCl ₆·xH₂O, has remained an unaddressed problem. We have found that the treatment of an i-PrOH solution of H₂PtCl ₆-XH₂O by (Me₃SiO)₂MeSi(CH=CH ₂), in the presence of allyl ether (AE), followed by reaction of the in situ generated Pt(O) species with IPr carbene (IPr =1,3-bis(2,6- diisopropylphenyl)imidazol-2-ylidene) enables the isolation of (IPr)Pt(AE) (I) in 50-70% yield. The scope of this method has been extended to other (L)Pt(1,6-diene) complexes (L = 1,3-dicyclohexylimidazol-2-ylidene, triphenylphoshine; 1,6-diene = diethyl 2,2-diallylmalonate (DAM)), and the molecular structure of the (IPr)Pt(DAM) (4) complex has been unequivocally determined by a single-crystal X-ray diffraction analysis. These results are significant for the formation of active L-Pt(O) fragments in catalysis.

Full text of DOI:10.1021/om7007088

Organometallics 2007, 26, 5731-5734
5731
Expedient, Direct Synthesis of (L)Pt(0)(1,6-diene) Complexes from
H2PtCl6
Guillaume Berthon-Gelloz,† Jean-Marc Schumers,† Fabio Lucaccioni,† Bernard Tinant,†
Johan Wouters,‡ and Istva´n E. Marko´*,†
De´partement de Chimie, UniVersite´ catholique de LouVain, Place Louis Pasteur 1,
1348 LouVain-la-NeuVe, Belgium, and De´partement de Chimie, Faculte´s UniVersitaires Notre-Dame de la
Paix, Rue de Bruxelles 61, 5000 Namur, Belgium
ReceiVed July 16, 2007
Summary: The one-pot synthesis of useful [Pt2(0)(η4-1,6-diene)3]
complexes, directly from H2PtCl6‚xH2O, has remained an
unaddressed problem. We haVe found that the treatment of an
i-PrOH solution of H2PtCl6‚xH2O by (Me3SiO)2MeSi(CHdCH2),
in the presence of allyl ether (AE), followed by reaction of the
in situ generated Pt(0) species with IPr carbene (IPr ) 1,3-
bis(2,6-diisopropylphenyl)imidazol-2-ylidene) enables the isola-
tion of (IPr)Pt(AE) (1) in 50-70% yield. The scope of this
method has been extended to other (L)Pt(1,6-diene) complexes
(L ) 1,3-dicyclohexylimidazol-2-ylidene, triphenylphoshine; 1,6-
diene ) diethyl 2,2-diallylmalonate (DAM)), and the molecular
structure of the (IPr)Pt(DAM) (4) complex has been unequiVo-
cally determined by a single-crystal X-ray diffraction analysis.
These results are significant for the formation of actiVe L-Pt-
(0) fragments in catalysis.
Scheme 1. Formation of Karstedt’s Catalyst (dvtms )
divinyltetramethyldisiloxane)
will and the dvtms ligand can be displaced only by electron-
poor alkynes, such as dimethylacetylenedicarboxylate.14 This
unexpected inertia is due to the strongly coordinating nature of
the dvtms ligand to the Pt(0) center. The thermodynamic stability
of (L)M(0)(η4-1,6-diene) complexes has been established in a
study by Po¨rschke et al. and is depicted in Figure 1.15
Consequently, (L)Pt(0)(η4-1,6-diene) complexes cannot be ac-
cessed by the simple displacement of the dvtms moiety in (L)-
Pt(0)(η4-dvtms) derivatives by another diene.
Introduction
Platinum(0) complexes play a prominent role in homogeneous
catalysis. They are also useful starting materials for the synthesis
of various organometallic species.1 Organoplatinum derivatives
bearing alkene ligands have been found to be uniquely active
catalysts for the hydrosilylation of olefins and alkynes.2 The
most notable homoleptic Pt(0)(alkene)3 complex is Karstedt’s
catalyst (Scheme 1, 1)3 which is one of the most widely used
organometallic reagents for the hydrosilylation reaction. It is
routinely prepared on the kilogram scale, in high yield and in
a single step, from H2PtCl6 (Scheme 1).2,4,5
The straightforward access to Karstedt’s catalyst has made it
the source of choice for the synthesis of a plethora of (L)Pt-
(0)(dvtms) complexes (L ) ligand, dvtms ) divinyltetramethyl-
disiloxane).3,6-13 However, one particularly limiting factor in
this methodology is that the diene fragment cannot be varied at
Results from Osborn, Elsevier, and our group have shown
that the nature of the alkene fragment is a key parameter in
determining the stability and the hydrosilylating activity of (L)-
Pt(0)(η2-alkene)2 and (L)2Pt(0)(η2-alkene) complexes, the rate-
limiting step of the catalyst activation being the decoordination
of one of the alkene moieties.11,13,16-19 The influence of so-
called “spectator” dienes (e.g., cis,cis-1,5-cyclooctadiene (cod),
trans,trans-dibenzylidene acetone (dba), 2,5-bicyclo[2.2.1]-
heptadiene) on the overall catalytic activity of precatalysts
bearing these ligands has recently become widely recognized.20
For example, the influence of spectator dba diene has been
elegantly demonstrated by Fairlamb et al. for cross-coupling
reactions catalyzed by Pd-dba-type complexes.21,22 In these
studies, the electronic nature of the substituents on the aromatic
rings of dba was shown to have significantly altered the overall
* To whom correspondence should be addressed. E-mail: marko@
chim.ucl.ac.be. Phone: +32/10478773. Fax: +32 10472788.
† Universite´ catholique de Louvain.
(11) Steffanut, P.; Osborn, J. A.; DeCian, A.; Fisher, J. Chem.-Eur. J.
1998, 4, 2008.
(12) Aneetha, H.; Wu, W.; Verkade, J. G. Organometallics 2005, 24,
2590.
‡ Faculte´s Universitaires Notre-Dame de la Paix.
(1) Clarke, M. L. Polyhedron 2001, 20, 151.
(13) Berthon-Gelloz, G.; Brie`re, J. F.; Buisine, O.; Ste´rin, S.; Michaud,
G.; Mignani, G.; Tinant, B.; Declercq, J.-P.; Chapon, D.; Marko´, I. E. J.
Organomet. Chem. 2005, 690, 6156.
(14) De Bo, G.; Berthon-Gelloz, G.; Tinant, B.; Marko´, I. E. Organo-
metallics 2006, 25, 1881.
(2) Marciniec, B.; Gulinski, J.; Urbaniac, W.; Kornetka, Z. W. Com-
prehensiVe Handbook on Hydrosilylation; Pergamon: Oxford, UK, 1992.
(3) Hitchcock, P. B.; Lappert, M. F.; Warhurst, N. J. W. Angew. Chem.,
Int. Ed. Engl. 1991, 30, 438.
(4) Karstedt, B. D. (General Electric) U.S. Patent 3,715,334, 1970.
(5) Although 1 is the major component of Karstedt’s catalyst, the reaction
product from the synthesis is a complex and dynamic mixture of different
Pt(0) species, which must be stored with several equivalents of the
divinyltetramethyldisiloxane (dvtms) ligand to ensure long-term stability.
(6) Chandra, G.; Lo, P. Y.; Hitchcock, P. B.; Lappert, M. F. Organo-
metallics 1987, 6, 191.
(7) Beuter, G.; Heyke, O.; Lorenz, I. P. Z. Naturforsch., B: Chem. Sci.
1991, 46, 1694.
(8) Lewis, L. N.; Stein, J.; Colborn, R. E.; Gao, Y.; Dong, J. J.
Organomet. Chem. 1996, 521, 221.
(15) Krause, J.; Cestaric, G.; Haack, K.-J.; Seevogel, K.; Storm, W.;
Po¨rschke, K.-R. J. Am. Chem. Soc. 1999, 121, 9807.
(16) Sprengers, J. W.; Mars, M. J.; Duin, M. A.; Cavell, K. J.; Elsevier,
C. J. J. Organomet. Chem. 2003, 679, 149.
(17) Sprengers, J. W.; Greef, M. d.; Duin, M. A.; Elsevier, C. J. Eur. J.
Inorg. Chem 2003, 3811.
(18) Sprengers, J. W.; Agerbeek, M. J.; Elsevier, C. J.; Kooijman, H.;
Spek, A. L. Organometallics 2004, 23, 3117.
(19) Duin, M. A.; Lutz, M.; Spek, A. L.; Elsevier, C. J. J. Organomet.
Chem. 2005, 690, 5804.
(20) Glorius, F. Angew. Chem., Int. Ed. 2004, 43, 3364.
(21) Fairlamb, I. J. S.; Kapdi, A. R.; Lee, A. F. Org. Lett. 2004, 6, 4435.
(22) Fairlamb, I. J. S.; Kapdi, A. R.; Lee, A. F.; McGlacken, G. P.;
Weissburger, F.; H. M. de Vries, A.; Schmieder-van de Vondervoort, L.
Chem.-Eur. J. 2006, 12, 8750.
(9) Hitchcock, P. B.; Lappert, M. F.; MacBeath, C.; Scott, F. P. A. J.
Organomet. Chem. 1997, 534, 139.
(10) Hitchcock, P. B.; Lappert, M. F.; MacBeath, C.; Scott, F. P. A.;
Warhurst, N. J. W. J. Organomet. Chem. 1997, 528, 185.
10.1021/om7007088 CCC: $37.00 © 2007 American Chemical Society
Publication on Web 10/09/2007

5732 Organometallics, Vol. 26, No. 23, 2007
Notes
Table 1. Optimization of the Two-Step Reaction Sequence
for the Synthesis of (IPr)Pt(η4-AE) (2)a
Figure 1. Sequence of increasing acceptor strength of alkenes,
1,6-dienes, and alkynes and of increasing stability of the corre-
sponding (L)M(η4-1,6-diene), (L)M(η4-alkene)2, and (L)M(η4-
alkyne)2 complexes.
AE
t
IPr‚HCl/t-BuOK yield
entry
reducing agent (equiv)
(equiv) (h)
(1:1, equiv)
(%)b
1c
2
Et3SiVi (5)
5
5
5
10
10
12
10g
12
10
10
10
2
2
6
6
10
10
48
7
c
60d
f
18
40
47
43
70
i
catalytic activity and this, even in the presence of other ancillary
ligands on palladium.
(EtO)Me2SiVi (5)
(Me3SiO)Me2SiVi (5)
(Me3SiO)Me2SiVi (2.5)
(Me3SiO)3SiVi (5)
(Me3SiO)2MeSiVi (5)
(Me3SiO)2MeSiVi (5)
(Me3SiO)2MeSiVi (5)
(Me3SiO)2MeSiVi (5)
(Me3SiO)2MeSiVi (5)
+ KFj
3e
4
1.1
1.1
1.2
1.1
1.6
1.1
1.1
Existing syntheses of (L)Pt(0)(alkene)2 derivatives employ
either Pt(0)(cod)2 or Pt(0)(nbn)3 (nbn ) bicyclo[2.2.1]hept-2-
ene) as starting materials. The latter complexes are prepared
by the reduction of PtCl2(cod) by Li2(cot) (cot ) cyclooctatet-
raene), in the presence of a large excess of alkene (e.g., nbn),
to yield the homoleptic Pt(η2-alkene)3 complexes.23 Such an
approach not only is fastidious but also requires the rigorous
exclusion of oxygen and water. A report by Ogoshi and
Kurosawa describes the use of SmI2 instead of Li2(cot) as the
reducing agent. Again, this method employs an expensive and
sensitive reducing agent.24 Pregosin and Caseri described an
interesting process based upon the reduction of PtCl2(CH2d
CHPh)2 by Ph3SiH, in the presence of excess styrene, to afford
the extremely sensitive Pt(0)(CH2dCHPh)3 complex. However,
the scope of this method remains to be delineated.25,26 Another
reduction protocol relies upon the electron-transfer reaction
between a Ni(0) derivative ([Ni(bpy)(cod)] (bpy ) 2,20-
bipyridine)) and a Pt(II) complex, to yield the corresponding
Pt(0) species.27 Unfortunately, this approach requires the use
of Ni(cod)2, which is difficult to handle.
At this stage, and to the best of our knowledge, no efforts
have been made to expand the scope of the reduction process
involved in the formation of Karstedt’s catalyst to dienes other
than divinylsilanes. Detailed studies by Lewis and Lappert on
the mode of formation of Karstedt’s catalyst 1 have shown that
the dvtms diene acts not only as a stabilizing ligand for Pt(0)
but also as a reducing agent for Pt(IV) or Pt(II) species to Pt-
(0).3,28,29 It should therefore be possible to use a poorly
coordinating vinylsilane, in the presence of a diene, to effect
the reduction of H2PtCl6 to Karstedt-type complexes.
5
6
7
8
9h
10
15
a Reaction conditions: (1) H2PtCl6‚xH2O (40% Pt; 150 mg, 0.30 mmol),
i-PrOH (0.3 M), 0.5 h, 60 °C; NaHCO3 (200 mg, 8 equiv), 10 min; reducing
b
agent, AE, 60 °C; (2) THF (0.15 M), IPr‚HCl; t-BuOK, Ar, 16 h. Isolated
c
d
yield after column chromatography. No reaction. Only (IPr)Pt(dvtms) was
isolated. Reaction temperature ) 75 °C. Formation of platinum black.gAE
e
f
h
added prior to reducing agent addition. H2PtCl6‚xH2O, AE, vinylsilane,
i
j
60 °C, 4 h; NaHCO3. Complex reaction mixture. KF (6.5 equiv). AE )
diallyl ether, Vi ) vinyl.
starting material, and can be carried out on a relatively large
scale (20 mmol).
Results and Discusion
The direct characterization of Karstedt-type complexes (Pt2-
(1,6-diene)3) is difficult due to their dynamic properties in
solution and to the unstable nature of the isolated Pt2(1,6-diene)3
species (without excess diene). It is operationally simpler to
react the Pt(0)2(1,6-diene)3 solution with a coordinating ligand
and to isolate the more stable, mononuclear (L)Pt(1,6-diene)
complex, to evaluate the efficiency of the reduction process.
Therefore, we focused our efforts on the synthesis of the (IPr)-
Pt(η4-AE) (IPr ) 1,3-bis(2,6-diisopropylphenyl)imidazol-2-
ylidene), AE ) allyl ether) derivative (2), which displayed
interesting catalytic activities in the hydrosilylation of alkynes
and which was required in substantial amounts. The intermediate
complex Pt2(AE)3 (3) has been previously fully characterized
by Po¨rschke and co-workers.15,30
At the onset of our study, we adapted conditions from the
synthesis of Karstedt’s catalyst by replacing the dvtms ligand
by allyl ether and by employing triethylvinylsilane as the
reducing agent (Table 1, entry 1). After prolonged heating at
60 °C, no formation of Pt(0) was observed, as judged by the
absence of nonpolar UV-active Pt(0) species on the TLC plate.31
Hiyama and Denmark have demonstrated the necessity for the
silicon center to bear at least one heteroatom for successful
transmetalation to occur in palladium-catalyzed cross-coupling
reactions of organosilicon compounds.32 In analogy, we used
(EtO)Me2SiVi (Vi ) vinyl) as a reducing agent (Table 1, entry
2) and were gratified to observe the smooth formation of Pt(0)
species. However, even under these mild conditions, (EtO)Me2-
SiVi redistributed to form the (Me2ViSi)2O (dvtms) diene.13 This
Herein, we wish to report a practical and widely applicable
two-step sequence for the synthesis of a family of (L)Pt(0)(η4-
1,6-diene) complexes starting from H2PtCl6. In these organo-
metallic derivatives, L can be a donor ligand such as a phosphine
or an N-heterocyclic carbene (NHC). The key to our successful
approach is the use of a monomeric and hydrolytically stable
vinylsiloxane, instead of dvtms, as a mild reducing agent. This
synthesis has several significant advantages: it does not require
glovebox or Schlenck techniques, employs readily available
(23) Green, M.; Howard, J. A. K.; Spencer, J. L.; Stone, F. G. A. J.
Chem. Soc., Dalton Trans. 1977, 271.
(24) Ogoshi, S.; Morita, M.; Inoue, K.; Kurosawa, H. J. Organomet.
Chem. 2004, 689, 662.
(25) Caseri, W.; Pregosin, P. S. Organometallics 1988, 7, 1373.
(26) Huber, C.; Kokil, A.; Caseri, W. R.; Weder, C. Organometallics
2002, 21, 3817.
(27) Walther, D.; Heubach, L.; Bo¨ttcher, H.; Schreer, H.; Go¨rls, H. Z.
Anorg. Allg. Chem. 2002, 628, 20.
(28) Lewis, L. N.; Colborn, R. E.; Grade, H.; Garold L. Bryant, J.;
Sumpter, C. A.; Scott, R. A. Organometallics 1995, 14, 2202.
(29) Lappert, M. F.; Scott, F. P. A. J. Organomet. Chem. 1995, 492,
C11.
(30) Krause, J.; Haack, K.-J.; Cestaric, G.; Goddard, R.; Po¨rschke, K.-
R. Chem. Commun. 1998, 1291.
(31) Pt(0)-alkene complexes display very strong absorption in their UV
spectra. Karstedt’s catalyst is an apolar complex, Rf(Hex) ) 0.6.
(32) Hiyama, T.; Shirakawa, E. Top. Curr. Chem. 2002, 219, 61.

Notes
Scheme 2. Proposed Mechanism for the Main Reduction
Organometallics, Vol. 26, No. 23, 2007 5733
Table 2. Synthesis of (L)Pt(1,6-diene) Complexesa
Pathway of H2PtCl6 into the Pt(0) Complex 3
reorganization of the vinylsilane led to the isolation of the (IPr)-
Pt(η4-dvtms) complex, which is more stable than the corre-
sponding (IPr)Pt(η4-AE) derivative. The driving force behind
the formation of dvtms diene from (EtO)Me2SiVi is the highly
stable Si-O-Si linkage. To overcome this issue, we opted for
the use of (Me3SiO)Me2SiVi, which already contains the Si-
O-Si motif. Although, initial trials at 75 °C were unsuccessful
(Table 1, entry 3), due to decomposition of the Pt(0) species
formed, lowering the reaction temperature to 60 °C and using
10 equiv of AE led to the isolation of the desired complex in a
modest but encouraging 18% yield (Table 1, entry 4). Next,
we examined the influence of increasing amounts of vinylsilane
(5 equiv) and varied the number of heteroatoms present on the
silicon atom (Table 1, entries 5 and 6). Both changes had a
positive effect, resulting in a 47% isolated yield of 2 (Table 1,
entry 6), although the electronic influence on the silicon center
proved to be moderate. The (Me3SiO)2MeSiVi vinylsilane was
preferred over (Me3SiO)Me2SiVi and (Me3SiO)3SiVi because
it is more readily available. Adding the allyl ether to form the
PtCl2(η4-AE) complex prior to the reducing agent had no
significant effect on the yield (Table 1, entry 7). Gratifyingly,
by performing the reduction during 48 h and by generating 1.6
equiv of the IPr carbene in the second step, the desired complex
could be isolated in 70% yield over the two steps (Table 1,
entry 8). A control experiment (Table 1, entry 9) was conducted
in which (Me3SiO)2MeSiVi and AE were reacted for 4 h at
60 °C before addition of the sodium bicarbonate. This led to a
complex reaction mixture that contained none of the desired
complex 2. The highly acidic nature of H2PtCl6 is presumably
responsible for the redistribution of the vinyl silane and the
isomerization and subsequent hydrolysis of the allyl ether.28 We
sought to further improve the transmetalation process by adding
a fluoride source (Table 1, entry 10). This led to the observation
of scores of Pt(0) species (TLC analysis). After reaction with
the IPr carbene, an intractable mixture of complexes was
obtained probably due to a fluoride-catalyzed redistribution of
(Me3SiO)2MeSiVi into silicone oligomers.
a Reaction conditions: (1) H2PtCl6‚xH2O (40% Pt), i-PrOH (0.2 M), 0.5
h, 60 °C; NaHCO3 (8 equiv), 10 min; (Me3SiO)2MeSiVi (5 equiv), 1,6-
diene (10 equiv), H2O (4 equiv), 16 h; filtration (SiO2), evaporation; (2)
b
THF (0.15 M), ligand (1.5 equiv), 16 h, Ar. Isolated yield after column
c
chromatography. 1.0 equiv of ligand added. ICy ) 1,3-dicyclohexylimi-
dazol-2-ylidene.
During the scale-up of this reaction on the gram scale,
reproducibility issues were unveiled that were eventually traced
to the amount of water present in the reaction mixture. Thus,
the addition of 4 equiv of water, relative to platinum, enhanced
the rate and the amount of Pt(0) complexes formed during the
reaction. We believe that the presence of water significantly
increases the rate of hydrolysis of the chlorosilane byproduct,
formed from the σ-bond metathesis of the Pt-Cl and the Si-
Vi linkages (cf. Scheme 2), to yield a stable polysiloxane.
Therefore, the formation of the polysiloxane also acts as a
thermodynamic driving force for the reaction (DSi-O ) 136 kcal
mol-1 in siloxanes). The use of a mechanical stirrer for larger
scale reaction (>5 mmol of Pt), which ensured proper mixing
of the heterogeneous reaction media, became mandatory. The
sturdiness of this method was probed by carrying out the
synthesis on 10 g of H2PtCl6; the desired (IPr)Pt(AE) (2)
complex was obtained in more than 60% overall yield.
A proposed mechanism for this reduction process is outlined
in Scheme 2. The first step involves the reduction of H2PtCl6
by i-PrOH to form the Pt(II) complex A (an analogue of Zeise’s
salt) with concomitant generation of acetone and HCl.33,34
Organometallic A can then undergo a ligand exchange with
diallyl ether to produce PtCl2(AE) (B). Then, a double trans-
metalation of the Pt-Cl bonds with the vinyl silane occurs,
leading to C. The driving force of this process lies in the
formation of the Si-Cl bond (DSiCl ) 117 kcal mol-1 vs DSi-C
) 90 kcal mol-1).35 The resulting chlorosilanes can react further
with the isopropyl alcohol or water, in the presence of NaHCO3.
Interestingly, this transmetalation appears to occur under neutral
conditions (NaHCO3 is added to neutralize the HCl generated
so far) and without any specific additives, the only Lewis base
present to activate the vinylsilane being the chloride anion. The
intermediate C can then undergo a reductive elimination to form
1,3-butadiene and the desired Pt(0) species. Evidence for this
mechanism is provided by the observation of 1,3-butadiene and
Although the exact nature of the Pt(0)2(AE)3 (3) formed was
difficult to elucidate by 1H and 13C NMR, due to the fluxional
nature of such complexes, the 195Pt NMR displayed only one
Pt signal with a chemical shift of -6365 ppm. This shift is
perfectly in line with the 195Pt chemical shift of the analogous
Karstedt’s catalyst (-6156 ppm). The lower chemical shift of
the homoleptic complex Pt2(AE)3 (3) relative to Pt2(dvtms)3 is
indicative of a more electron-rich platinum center due to lesser
back-bonding onto the olefins.3
(33) Zeise, W. C. Mag. Pharm. 1930, 35, 105.
(34) Benkeser, R. A.; Kang, J. J. Organomet. Chem. 1980, 185, C9.
(35) Walsh, R. Acc. Chem. Res. 1981, 14, 246.

5734 Organometallics, Vol. 26, No. 23, 2007
Scheme 3. Product Distribution Observed by 31P NMR with L ) Ph3P
Notes
higher order polysiloxanes in earlier studies with dvtms as the
reducing agent.28 The effectiveness of the present method might
be partially attributed to the formation of 1,3-butadiene during
the reductive elimination of the Pt(II) species to yield Pt(0)
derivative 3. Hence, 1,3-butadiene can act as a transient alkene
ligand for Pt(0), preventing the decomposition of this highly
reactive species, before being displaced by the more coordinating
and stabilizing 1,6-diene.
With a robust method in hand, we set out to synthesize various
members of the (L)Pt(1,6-diene) complexes family. The results
are depicted in Table 2. The synthesis of Pt(0) derivatives
bearing the bulky IPr ligand proceeded smoothly and with
reasonable yields (Table 2, entries 1 and 2). However, for
complexes 6 and 7, in which the ICy and Ph3P ancillary ligands
are not sterically hindered, the product distribution becomes
more complicated because two or three of these ligands can be
easily accommodated around the platinum center and the AE
can be readily displaced. This problem is illustrated in Scheme
3. The addition of 1.5 equiv of Ph3P to the crude Pt2(AE)3
complex leads to the observation of three types of complexes
(determined by 31P NMR), the desired product 7 (69%), along
with complexes 8 (21%) and 9 (10%), bearing two and three
Ph3P ligands, respectively. This ligand displacement does not
arise with the hindered IPr carbene and is absent if the 1,6-
diene ligand is more strongly coordinated to the platinum center
(e.g., dvtms). To minimize this problem, exactly 1 equiv (or
slightly less) of ligand must be added.
complex. Thus, the 195Pt NMR chemical shifts of 2 and 4, which
are -5574 and -5486 ppm, respectively, point to a higher
stability of 4 over 2. A slightly lower-field chemical shift, -5453
ppm, is observed for 5, thereby suggesting that the presence of
the cyclic acetonide further reinforces the “Thorpe-Ingold”
effect, whereby the gem-disubstitution of the two ethyl ester
moieties on the diallyl fragment limits their degrees of rotational
freedom. We have also correlated the 195Pt shift of a number of
complexes with their hydrosilylation activity. These and other
observations will be fully detailed in a forthcoming report.
A crystal of 4, suitable for X-ray diffraction studies, was
grown from a saturated i-PrOH solution (Figure 2). The 1,6-
heptadiene portion of the diethyl diallylmalonate (DAM) ligand
wraps around the Pt(0) center in a pseudo-chair conformation,
enabling an almost perfect trigonal-planar arrangement. The Pt-
carbene distance is 2.067(4) Å. The plane formed by the alkene
coordinated to platinum and the plane of the imidazole ring are
tilted by -51.2(4)°. The olefinic bond lengths are 1.427(8) and
1.436(7) Å, indicating extensive back-bonding of the electron-
rich Pt(0) center onto the π* of the alkenes. These features are
in line with the closely related (IPr)Pt(dvtms) derivative.13
Initial attempts to generate Pt(alkene)3 complexes with
monodentate alkenes, using our method, proved unsuccessful,
and precipitation of Pt black readily occurred during the
reduction. It is likely that these complexes are too unstable in
solution at 60 °C. Thus, our method is complementary to Stone’s
reduction with Li2(cot). Preliminary experiments suggest that
the reduction process presented here is also applicable to
palladium and nickel, but the low stability of the resulting
complexes renders their isolation difficult and manipulations
must be carried out with the rigorous exclusion of oxygen.
The electronic effect of the modification of the 1,6-diene
fragment in the (L)Pt(0) unit can be monitored by 195Pt NMR.
We have previously observed that, within a series in which the
nature of one of the ligands remains constant, there was a
correlation between the thermodynamic stability of a L-Pt(0)-
(η4-1,6-diene) complex and its 195Pt chemical shift.13 In other
words, the higher the chemical shift, the more stable the
Conclusions
We have devised an efficient and operationally simple two-
step process for the formation of (L)Pt(η4-1,6-diene) complexes
from commercially available H2PtCl6‚xH2O. Our approach led
us to expand the scope of the reduction process involved in the
formation of Karstedt’s catalyst to dienes other than dvtms. The
variation of the diene fragment enables the straightforward
modulation of the stability of the resulting complex and the ease
with which the L-Pt(0) fragment is generated. This method
should find applications in the generation of new Pt(0)
complexes and should expand the scope of catalytic processes
employing the reactive L-Pt(0) fragment.
Acknowledgment. The authors wish to acknowledge Dr D.
Chapon for performing 195Pt NMR measurements and Umicore
AG & Co. KG. (Dr. R. Karch) for a generous gift of H2PtCl6.
Financial support by Rhodia Corporate (Ph.D. fellowship to
G.B.-G.), the Universite´ catholique de Louvain, and Merck
Sharp & Dohme (unrestricted grant to I.E.M.) is gratefully
acknowledged.
Supporting Information Available: Detailed experimental
procedures, complete characterization of the complexes including
1H and 13C NMR spectra, and crystallographic data for 4, including
atomic coordinates, displacement parameters, bond lengths, and
bond angles, as a CIF file. This material is available free of charge
via the Internet at http://pubs.acs.org.
Figure 2. ORTEP plot of (IPr)Pt(DAM) (4) with thermal ellipsoids
at the 50% probability level. Selected bond distances (Å) and angles
(deg): Pt51-C4, 2.067(4); Pt51-C31, 2.122 (4); Pt51-C32, 2.123
(4); Pt51-C34, 2.143 (5); Pt51-C35, 2.144(5); C31-C32, 1.427-
(8); C34-C35, 1.436(7); C31-C32-C33-C35, -2.2(4); N1-C4-
Pt51-C32, -51.2(4).
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