Oxidative addition of the imidazolium cation to zerovalent Ni, Pd, and Pt: A combined density functional and experimental study

Article abstract of DOI:10.1021/ja010628p

Oxidative addition of different imidazolium cations to zerovalent group 10 metals, to afford heterocyclic carbene complexes, has been investigated by both density functional theory (DFT) and experimental studies. The theoretical analysis shows that addition of imidazoliums to Pt⁰ and Ni⁰ is more exothermic than to Pd⁰, and Ni⁰ is predicted to react with a much lower barrier than either Pt⁰ or Pd⁰ Strongly basic supporting ligands on the metal, as well as cis-chelating ligands, increase the exothermicity of the reaction and also lower the activation barrier. The addition of 2-H imidazoliums is easier and more exothermic than addition of 2-alkylimidazoliums, and a halo-imidazolium is expected to further lower the barrier to oxidative addition and increase the exothermicity. The DFT results show that all three of the metals should be able to oxidatively add imidazolium cations under appropriate conditions. Experimental studies confirmed that oxidative addition is possible, and a number of Pt- and Pd-carbene complexes were prepared via oxidative addition of imidazolium salts to M⁰ precursors. Most significantly, oxidative addition of 2-H azolium salts was found to readily occur, and the reaction of 1,3-dimethylimidazolium tetrafluoroborate with Pt(PPh₃)₂ and Pt(PCy₃)₂ affords [PtH(dmiy)(PPh₃)₂]BF4 (10) and [PtH(dmiy)(PCY₃)₂]BF4 (11), while reaction between 3,4-dimethylthiazolium tetrafluoroborate and Pt(PCy₃)₂ yields [PtH(dmty)(PCy₃)₂]BF4 (12) (dmiy = 1,3-dimethylimidazolin-2-ylidene, dmty = 3,4-dimethylthiazolin-2-ylidene). Addition of 2-iodo-1,3,4,5-tetramethylimidazolium tetrafluoroborate to Pt(PPh₃)₄ or Pd(dcype)(dba) yields [PtI(tmiy)(PPh₃)₂]BF4 (9) and [PdI(tmiy)(dcype)]BF4 (14), respectively (tmiy = 1,3,4,5-tetramethylimidazolin-2-ylidene, dcype = 1,3-bis(dicyclohexylphosphino)ethane)). X-ray crystal structures are reported for complexes 9 and 11 (cis and trans). These studies clearly show for the first time that oxidative addition of imidazolium and thiazolium cations is possible, and the results are discussed in terms of the ramifications for catalysis in imidazolium-based ionic liquids with both carbene-based and non-carbene-based complexes.

Full text of DOI:10.1021/ja010628p

J. Am. Chem. Soc. 2001, 123, 8317-8328
8317
Oxidative Addition of the Imidazolium Cation to Zerovalent Ni, Pd,
and Pt: A Combined Density Functional and Experimental Study
David S. McGuinness,†, Kingsley J. Cavell,*,‡ Brian F. Yates,*,† Brian W. Skelton,§ and
Allan H. White§
Contribution from the School of Chemistry, UniVersity of Tasmania, GPO Box 252-75 Hobart, Tasmania
7001, Australia, Department of Chemistry, Cardiff UniVersity, PO Box 912, Cardiff, CF1 3TB, U.K., and
Department of Chemistry, UniVersity of Western Australia, Nedlands, Western Australia 6907, Australia
ReceiVed March 8, 2001
Abstract: Oxidative addition of different imidazolium cations to zerovalent group 10 metals, to afford
heterocyclic carbene complexes, has been investigated by both density functional theory (DFT) and experimental
studies. The theoretical analysis shows that addition of imidazoliums to Pt0 and Ni0 is more exothermic than
to Pd0, and Ni0 is predicted to react with a much lower barrier than either Pt0 or Pd0. Strongly basic supporting
ligands on the metal, as well as cis-chelating ligands, increase the exothermicity of the reaction and also lower
the activation barrier. The addition of 2-H imidazoliums is easier and more exothermic than addition of
2-alkylimidazoliums, and a halo-imidazolium is expected to further lower the barrier to oxidative addition and
increase the exothermicity. The DFT results show that all three of the metals should be able to oxidatively add
imidazolium cations under appropriate conditions. Experimental studies confirmed that oxidative addition is
possible, and a number of Pt- and Pd-carbene complexes were prepared via oxidative addition of imidazolium
salts to M0 precursors. Most significantly, oxidative addition of 2-H azolium salts was found to readily occur,
and the reaction of 1,3-dimethylimidazolium tetrafluoroborate with Pt(PPh3)2 and Pt(PCy3)2 affords
[PtH(dmiy)(PPh3)2]BF4 (10) and [PtH(dmiy)(PCy3)2]BF4 (11), while reaction between 3,4-dimethylthiazolium
tetrafluoroborate and Pt(PCy3)2 yields [PtH(dmty)(PCy3)2]BF4 (12) (dmiy ) 1,3-dimethylimidazolin-2-ylidene,
dmty ) 3,4-dimethylthiazolin-2-ylidene). Addition of 2-iodo-1,3,4,5-tetramethylimidazolium tetrafluoroborate
to Pt(PPh3)4 or Pd(dcype)(dba) yields [PtI(tmiy)(PPh3)2]BF4 (9) and [PdI(tmiy)(dcype)]BF4 (14), respectively
(tmiy ) 1,3,4,5-tetramethylimidazolin-2-ylidene, dcype ) 1,3-bis(dicyclohexylphosphino)ethane)). X-ray crystal
structures are reported for complexes 9 and 11 (cis and trans). These studies clearly show for the first time
that oxidative addition of imidazolium and thiazolium cations is possible, and the results are discussed in
terms of the ramifications for catalysis in imidazolium-based ionic liquids with both carbene-based and non-
carbene-based complexes.
1. Introduction
and hydroformylation (Rh),10-15 copolymerization (Pd),16 and
C-C coupling reactions (Pd, Ni).17-28 Consequently, the
fundamental reaction chemistry of these species is of consider-
In the 10 years since the discovery of free N-heterocyclic
carbenes by Arduengo and co-workers,1 there has been an
increasing number of reports of transition metal complexes of
these ligands being used in homogeneous catalysis.2 Reactions
in which carbene complexes have found application include
furan synthesis (Ru),3 olefin metathesis (Ru),4-9 hydrosilylation
(8) Huang, J.; Schanz, H.-J.; Stevens, E. D.; Nolan, S. P. Organometallics
1999, 18, 5375.
(9) Jafarpour, L.; Schanz, H.-J.; Stevens, E. D.; Nolan, S. P. Organo-
metallics 1999, 18, 5416.
(10) Hill, J. E.; Nile, T. A. J. Organomet. Chem. 1977, 137, 293.
(11) Lappert, M. F.; Maskell, R. K. J. Organomet. Chem. 1984, 264,
217.
* To whom correspondence should be addressed. E-mail: cavellkj@
cf.ac.uk, brian.yates@utas.edu.au.
(12) Herrmann, W. A.; Goossen, L. J.; Ko¨cher, C.; Artus, G. R. J. Angew.
Chem., Int. Ed. Engl. 1996, 35, 2805.
(13) Enders, D.; Gielen, H.; Breuer, K. Tetrahedron: Asymmetry 1997,
8, 3571.
(14) Enders, D.; Gielen, H.; Runsink, J.; Breuer, K.; Brode, S.; Boehn,
K. Eur. J. Inorg. Chem. 1998, 913.
(15) Chen, A. C.; Decken, A.; Crudden, C. M. Organometallics 2000,
19, 3459.
(16) Gardiner, M. G.; Herrmann, W. A.; Reisinger, C.-P.; Schwarz, J.;
Spiegler, M. J. Organomet. Chem. 1999, 572, 239.
(17) Herrmann, W. A.; Elison, M.; Fischer, J.; Ko¨cher, C.; Artus, G. R.
J. Angew. Chem., Int. Ed. Engl. 1995, 34, 2371.
(18) Herrmann, W. A.; Reisinger, C.-P.; Spiegler, M. J. Organomet.
Chem. 1998, 557, 93.
(19) Schwarz, J.; Bo¨hm, V. P. W.; Gardiner, M. G.; Grosche, M.;
Hermann, W. A.; Hieringer, W.; Raudaschl-Sieber, G. Chem. Eur. J. 2000,
6, 1773.
† University of Tasmania.
‡ Cardiff University.
§ University of Western Australia.
Present address: Institute for Technical and Macromolecular Chemistry,
Worringer Weg 1, D-52074 Aachen, Germany.
(1) Arduengo, A. J., III; Harlow, R. L.; Kline, M. J. Am. Chem. Soc.
1991, 113, 361.
(2) Herrmann, W. A.; Ko¨cher, C. Angew. Chem., Int. Ed. Engl. 1997,
36, 2162.
(3) Ku¨cu¨kbay, H.; Cetinkaya, B.; Salaheddine, G.; Dixneuf, P. H.
Organometallics 1996, 15, 2434.
(4) Weskamp, T.; Schattenmann, W. C.; Spiegler, M.; Herrmann, W.
A. Angew. Chem., Int. Ed. 1998, 37, 2490.
(5) Weskamp, T.; Kohl, F. J.; Hieringer, W.; Gleich, D.; Herrmann, W.
A. Angew. Chem., Int. Ed. 1999, 38, 2146.
(6) Ackermann, L.; Fu¨rstner, A.; Weskamp, T.; Kohl, F. J.; Herrmann,
W. A. Tetrahedron Lett. 1999, 40, 4787.
(7) Huang, J.; Stevens, E. D.; Nolan, S. P.; Petersen, J. L. J. Am. Chem.
Soc. 1999, 121, 2674.
(20) Enders, D.; Gielen, H.; Raabe, G.; Runsink, J.; Teles, J. H. Chem.
Ber. 1996, 129, 1483.
(21) McGuinness, D. S.; Green, M. J.; Cavell, K. J.; Skelton, B. W.;
White, A. H. J. Organomet. Chem. 1998, 565, 165.
10.1021/ja010628p CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/01/2001

8318 J. Am. Chem. Soc., Vol. 123, No. 34, 2001
McGuinness et al.
able interest. While the high activity of heterocyclic carbene
complexes in various reactions is often attributed to strong
M-carbene bonding, we have shown in several recent reports
that heterocyclic carbene complexes of PdII and NiII which
contain alkyl, aryl, and acyl groups may decompose via
elimination of 2-organyl imidazolium salts.21,22,24,29 A combined
kinetic and density functional study of the reaction (M ) Pd)
has confirmed that this process can occur with a low activation
barrier, proceeding via a concerted reductive elimination of the
hydrocarbyl and carbene moieties (reaction 1).30 Inasmuch as
immobilize metal-based catalysts while being nonvolatile. These
properties make them suited to the development of cleaner
technologies, and the term “green solvents” has been used to
describe them.36-38 As such, the reaction chemistry of these
salts is presently of much interest. Among the reactions that
have been performed in ionic liquids are olefin dimerization
(Ni),39,40 Heck-type reactions (Pd),41,42 and hydroformylation
(Pt, Rh).43,44 It is generally believed that the anion in such
systems usually determines their chemical properties. However,
while the imidazolium cation is usually considered inert when
used as an ionic liquid, there has been some recent speculation
as to the possible involvement of carbene complexes in
catalysis.45 Consequently, the nature of the catalytically active
species in these solvents is of critical importance.42,46 Very
recently, Xiao et al. isolated Pd-carbene complexes from an
ionic liquid solvent in which Pd(OAc)2 was used as a precatalyst
for the Heck reaction.46 The formation of carbene complexes
in this case most likely resulted from deprotonation of the
imidazolium C2 proton by Pd(OAc)2, which is known to react
with various imidazoliums to give carbene complexes.17,20
However, the possibility that the imidazolium cation may
oxidatively add to low-valent metals raises the question as to
whether carbene metal complexes may form without the aid of
a basic ligand to deprotonate the imidazolium. In many cases,
the catalytic system used in ionic liquids consists of a group 10
metal in combination with phosphine ligands,39,42,43,45 giving
an opportunity for M0Ln species to form that would be amenable
to oxidative addition.
hydrocarbyl-metal species are considered to be intermediates
in many catalytic reactions, this reaction represents a significant
pathway for catalyst deactivation. Once a problem has been
identified, the development of strategies by which it can be
impeded becomes of major importance. Of the methods that
can be envisaged as hindering reductive elimination of the
imidazolium cation,30 shifting the energetics of the reaction such
that the oxidative addition of imidazolium to M0 is favored over
reductive elimination provides a possible approach. Thus, an
understanding of which factors favor oxidative addition of the
imidazolium cation will lead to the development of more stable
catalysts. Furthermore, if it were found that oxidative addition
of imidazolium cations was possible, this would suggest that
operating in an imidazolium-based ionic liquid31,32 should
prevent catalyst decomposition by reductive elimination. The
large excess of imidazolium salt, as present when it is used as
the solvent, would be expected to favor reoxidation of the metal
to give a carbene complex.
A number of studies have been performed on conventional
oxidative addition-reductive elimination of alkanes to low-
valent metals, leading to an understanding of factors which affect
this reaction. While the imidazolium system is considerably
different from those previously studied, it seems possible that
similar factors will influence the reaction. It is known that
chelating ligands on zerovalent Pd and Pt favor oxidative
addition, in terms of both activation barrier and exothermicity
of the reaction.47-52 This is a consequence of a bent geometry
for the M0(L∧L) fragment, which has a high d-orbital energy
relative to linear ML2. Thus, the reverse reaction (in this case,
reductive elimination from a carbene complex containing a
chelating ligand) is expected to be disfavored.
Concomitantly, the possible interaction of the imidazolium
cation with low-valent transition metals during catalysis in ionic
liquids using non-carbene-based complexes is of interest.
Imidazolium-based ionic liquids33-35 have found increasing
application in catalysis as nonaqueous alternatives for biphasic
catalysis.31,32 These salts are highly polar and readily dissolve/
It has been established that oxidative addition to Pt0 is favored
both kinetically and thermodynamically over oxidative addition
(36) Freemantle, M. Chem. Eng. News 1998, March 30, 32.
(37) Freemantle, M. Chem. Eng. News 2000, May 15, 37.
(38) Freemantle, M. Chem. Eng. News 2001, January 1, 21.
(39) Chauvin, Y.; Einloft, S.; Olivier, H. Ind. Eng. Chem. Res. 1995,
34, 1149.
(40) Ellis, B.; Keim, W.; Wasserscheid, P. J. Chem. Soc., Chem.
Commun. 1999, 337.
(22) McGuinness, D. S.; Cavell, K. J.; Skelton, B. W.; White, A. H.
Organometallics 1999, 18, 1596.
(23) McGuinness, D. S.; Cavell, K. J. Organometallics 2000, 19, 741.
(24) Magill, A. M.; McGuinness, D. S.; Cavell, K. J.; Britovsek, G. J.
P.; Gibson, V. C.; White, A. J. P.; Williams, D. J.; White, A. H.; Skelton,
B. W. J. Organomet. Chem 2001, 617-618, 546.
(25) Zhang, C.; Huang, J.; Trudell, M. L.; Nolan, S. P. J. Org. Chem.
1999, 64, 3804.
(41) Herrmann, W. A.; Bo¨hm, V. P. W. J. Organomet. Chem. 1999,
572, 141.
(26) Calo´, V.; Del Sole, R.; Nacci, A.; Schingaro, E.; Scordari, F. Eur.
J. Org. Chem. 2000, 869.
(27) Tulloch, A. A. D.; Danopoulos, A. A.; Tooze, R. P.; Cafferkey, S.
M.; Kleinhenz, S.; Hursthouse, M. B. J. Chem. Soc., Chem. Commun. 2000,
1247.
(28) Peris, E.; Loch, J. A.; Mata, J.; Crabtree, R. H. J. Chem. Soc., Chem.
Commun. 2001, 201.
(29) McGuinness, D. S.; Cavell, K. J. Organometallics 2000, 19, 4918.
(30) McGuinness, D. S.; Saendig, N.; Yates, B. F.; Cavell, K. J. J. Am.
Chem. Soc. 2001, 123, 4029.
(42) Mathews, C. J.; Smith, P. J.; Welton, T. J. Chem. Soc., Chem.
Commun. 2000, 1249.
(43) Wasserscheid, P.; Waffenschmidt, H. J. Mol. Catal. A: Chem. 2000,
164, 61.
(44) Keim, W.; Vogt, D.; Waffenschmidt, H.; Wasserscheid, P. J. Catal.
1999, 186, 481.
(45) Carmichael, A. J.; Earle, M. J.; Holbrey, J. D.; McCormac, P. B.;
Seddon, K. R. Org. Lett. 1999, 1, 997.
(46) Xu, L.; Chen, W.; Xiao, J. Organometallics 2000, 19, 1123.
(47) Sakaki, S.; Biswas, B.; Sugimoto, M. Organometallics 1998, 17,
1278.
(48) Su, M.-D.; Chu, S.-Y. Inorg. Chem. 1998, 37, 3400.
(49) Hackett, M.; Ibers, J. A.; Jernakoff, P.; Whitesides, G. M. J. Am.
Chem. Soc. 1986, 108, 8094.
(31) Welton, T. Chem. ReV. 1999, 99, 2071.
(32) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772.
(33) Wilkes, J. S.; Levisky, J. A.; Wilson, R. A.; Hussey, C. L. Inorg.
Chem. 1982, 21, 1263.
(34) Bonhoˆte, P.; Dias, A.-P.; Papageorgiou, N.; Kalyanasundaram, K.;
Gra¨tzel, M. Inorg. Chem. 1996, 35, 1168.
(35) Larsen, A. S.; Holbrey, J. D.; Tham, F. S.; Reed, C. A. J. Am. Chem.
Soc. 2000, 122, 7264.
(50) Hackett, M.; Ibers, J. A.; Whitesides, G. M. J. Am. Chem. Soc. 1988,
110, 1436.
(51) Hackett, M.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 1449.
(52) Sakaki, S.; Kai, S.; Sugimoto, M. Organometallics 1999, 18, 4825.

OxidatiVe Addition of Imidazolium to Group 10 Metals
J. Am. Chem. Soc., Vol. 123, No. 34, 2001 8319
Figure 1. Optimized geometries corresponding to oxidative addition to give complex 2 (1) (left) and complex 3 (right).
to an equivalent Pd0 system.48,53,54 This has been attributed to
and M-C bond formation. Experimental observations support
Pt having a more accessible d9s1 state, as required for M-H
this idea.49-51 The behavior of Ni0 is less predictable.
In a previous investigation, the reaction of a 2-methylimi-
dazolium cation with Pd was studied.30 However, ionic liquids
(53) Low, J. J.; Goddard, W. A., III. J. Am. Chem. Soc. 1986, 108, 6115.
(54) Dedieu, A. Chem. ReV. 2000, 100, 543 and references therein.

8320 J. Am. Chem. Soc., Vol. 123, No. 34, 2001
McGuinness et al.
6-311+G(2d,p) basis set for Pd, which incorporates the LANL2
effective core potential and the large f-polarized valence basis
set of Bauschlicher and co-workers63 on Pd together with the
6-311+G(2d,p) basis set on all other atoms, and
a
LANL2DZaugmented: 6-311+G(2d,p) basis set for Pt, which
incorporates the LANL2TZ+(3f) basis set on Pt64 and 6-311+G-
(2d,p) on all other atoms. The energy values in the text refer to
these final levels of theory together with thermodynamic
corrections from B3LYP/LANL2DZ frequency calculations.
2.2. Effect of Metal. First calculations were aimed at
evaluating the effect of different group 10 metals on the reaction.
As this work was undertaken to see whether oxidative addition
of imidazolium cations to M0 is possible, the reactions modeled
are depicted throughout as proceeding as an oxidative addition
as opposed to reductive elimination.30 Previous theoretical results
predict that oxidative addition of 1,2,3-trimethylimidazolium to
Pd(PH3)2 to yield complex 1 is endothermic, with the product
lying 3.7 kcal mol-1 above the reactants (reaction 2, M ) Pd).30
Figure 2. Potential energy profile for the oxidative addition of 1,2,3-
trimethylimidazolium to Pd, Pt, and Ni, giving complexes 1, 2, and 3,
respectively.
are usually composed of an imidazolium cation with hydrogen
in the 2-position, and it is known that C-H oxidative addition
is more facile than C-C activation. Hence, we present here
theoretical and experimental studies aimed at elucidating the
factors that favor oxidative addition of imidazolium and
thiazolium cations to M0. The effects of different metals (Ni,
Pd, Pt), ligands, and imidazolium salts have been evaluated
computationally, and the first isolated examples of C-H
oxidative addition of an imidazolium cation are reported. Crystal
structures of several products are provided. A preliminary
communication concerning oxidative addition of the imidazo-
lium cation to Pt0 has recently been published.55
Calculations on the analogous Pt and Ni systems, to yield
complexes 2 and 3, respectively (reaction 2), were performed.
The optimized geometries of the stationary points along the
potential energy surface are shown in Figure 1. The geometry
of the Pd system is also given in parentheses on the Pt stationary
point structures for comparison. In each case, a precursor
complex was found to precede the transition structure. In the
case of Pd and Pt, the geometry of M(PH3)2 is distorted only to
a small extent in the precursor complexes (PC1 and PC2), and
the imidazolium cation remains unperturbed and distant to the
metal. This indicates a weak interaction between Pt and the
imidazolium, as indicated for Pd with previous NBO results.
In contrast, both reactant fragments are distorted to a large extent
in the Ni case, PC3. The angle between the phosphines is
reduced to 102.9°, and bonds are formed with Ni between
Ccarbenoid and a nitrogen of the imidazolium ring, both bond
lengths being 1.97 Å (Figure 1). This interaction causes the
methyl groups attached to Ccarbenoid and the bonding nitrogen
to be bent out of the imidazolium plane.
2. Theoretical Studies
2.1. Theoretical Methods. Full geometry optimizations were
carried out with the B3LYP56,57 density functional level of theory
combined with the LANL2DZ basis set.58,59 Sets of five
d-functions were used in the basis sets throughout these
calculations. For the optimized geometries, harmonic vibrational
frequencies were calculated at the B3LYP level, and zero-point
vibrational energy corrections were obtained using unscaled
frequencies. The vibrational frequencies were also used to obtain
thermodynamic corrections and entropies. All transition struc-
tures possessed one and only one imaginary frequency, and they
were further characterized by following the corresponding
normal mode toward each product and reactant. All structures
were treated as singlets, and electronic wave function stability
optimizations reveal no singlet-triplet instability. Single-point
energies on B3LYP/LANL2DZ optimized geometries were
calculated at the B3LYP level with the 6-311+G(2d,p) basis
set60-62 on all atoms for the Ni system, a LANL2DZaugmented:
The stronger interaction of imidazolium with Ni in PC3 is
reflected in the potential energy profile of the reaction, which
is shown in Figure 2. The Pt and Pd precursor complexes have
stabilization energies of -5.3 and -5.7 kcal mol-1, respectively,
compared to a stabilization of -9.7 kcal mol-1 in the Ni case.
The geometry about the transition structure is very similar in
TS1, TS2, and TS3, the main differences being shorter bond
lengths to the metal in the Ni structure. The geometries are
likewise related in the products 1, 2, and 3. The major
differences between the metals comes in the relative energies
of the stationary points on the potential energy surface. For the
Pt system, the activation barrier (∆Hq) is predicted to be +27.8
kcal mol-1, which is similar to that predicted for the Pd system,
+26.1 kcal mol-1. However, the product of oxidative addition
to Pt lies 13.5 kcal mol-1 below the reactants. Thus, in contrast
to the reaction with Pd, addition of the imidazolium cation to
Pt(PH)3 is predicted to be exothermic. Oxidative addition to Ni
(55) McGuinness, D. S.; Cavell, K. J.; Yates, B. F. J. Chem. Soc., Chem.
Commun. 2001, 355.
(56) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(57) Stephens, P. J.; Devlin, J. F.; Chabalowski, C. F.; Frisch, M. J. J.
Phys. Chem. 1994, 98, 11623.
(58) Hay, P. J.; Wadt, W. R. J. Chem. Phys 1985, 82, 299.
(59) Dunning, T. H.; Hay, P. J. Modern Theoretical Chemistry; Schaefer,
H. F., Ed.; Plenum: New York, 1976; Vol. 3, p 1.
(60) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys.
1980, 72, 650.
is likewise predicted to be exothermic by 11.2 kcal mol-1
.
(61) McLean, A. D.; Chandler, G. S. J. Chem. Phys. 1980, 72, 5639.
(62) Frisch, M. J.; Pople, J. A.; Binkley, J. S. J. Chem. Phys. 1984, 80,
3265.
(63) Langhoff, S. R.; Petterson, L. G.; Bauschlicher, C. W.; Partridge,
H. J. J. Chem. Phys. 1987, 86, 268.
(64) Yates, B. F. J. Mol. Struct. (THEOCHEM) 2000, 506, 223.

OxidatiVe Addition of Imidazolium to Group 10 Metals
J. Am. Chem. Soc., Vol. 123, No. 34, 2001 8321
Figure 3. Optimized geometries corresponding to oxidative addition to give complexes 4 (left) and 5 (right).
However, in this case the activation barrier (∆Hq ) +7.1 kcal
mol-1) is predicted to be much lower than that for Pd and Pt.
These results show that oxidative addition of imidazoliums to
Pt and Ni should be much more favorable than to Pd, and in
the case of Ni the activation barrier will be much smaller.
effect of a bidentate, chelating phosphine. The optimized
geometries corresponding to oxidative addition of 1,2,3-tri-
methylimidazolium are shown in Figure 3 (left column). In the
precursor complex, PC4, the imidazolium cation is somewhat
nearer the Pd center than is the case when unidentate phosphines
are coordinated (PC1). This is understandable, as the P-Pd-P
angle in Pd(dpe) is constrained at 94.3° by chelation of the
2.3. Effect of Chelation. The model phosphine 1,2-diphos-
phinoethane (dpe) was used in initial calculations to mimic the

8322 J. Am. Chem. Soc., Vol. 123, No. 34, 2001
McGuinness et al.
lium cation closely interacts with the Pd center, resulting in
distortion of the proton on Ccarbenoid out of the imidazolium plane
and elongation of the Ccarbenoid-H bond. This is reflected in
the potential energy profile (Figure 4), which shows a large
stabilization energy of 17.2 kcal mol-1 for PC5. The transition
structure is similar to the precursor complex, the main difference
being a further elongation of the Ccarbenoid-H bond and
shortening of the Pd-Ccarbenoid and Pd-H distances. This
geometrical similarity is consistent with the similar relative
energies of TS5 (-18.6 kcal mol-1) and PC5.66 Importantly,
the results therefore predict that oxidative addition of 2-H
imidazolium to Pd(dpe) occurs without an energy barrier. The
energy of the product of oxidative addition 5 lies 27.5 kcal mol-1
below that of the reactants, showing that addition of 2-H
imidazolium is thermodynamically favored over addition of
2-alkylimidazolium.
The apparent oxidative addition of 2-halogenated thiazoliums
to Pd0 was reported some time ago by Stone and co-workers.67
As oxidative addition of alkyl and aryl halides is facile compared
to C-C or C-H activation, it was of interest to study this
reaction using halo-imidazolium species for comparison with
the oxidative addition using 2-H imidazolium. Thus, oxidative
addition of the 2-bromo-1,3-dimethylimidazolium cation to Pd-
(dpe) was studied. The optimized geometries of the imidazolium
cation and the product 6 are shown in Figure 5. No precursor
complex or transition structure was found for the reaction.
Optimization of the reactants when the imidazolium cation was
positioned in the vicinity of Pd(dpe) (within 5 Å) always led to
complex 6. It is therefore concluded that the reaction occurs
without barrier, as shown on the potential energy surface (Figure
4). This also shows that oxidative addition is strongly exother-
mic, with the product lying 65.1 kcal mol-1 below the reactants.
This result demonstrates that oxidative addition of 2-haloimi-
dazoliums (halo ) bromo, iodo, chloro) should occur and, in
the case of bromo and iodo at least, should be very facile and
easier than C-H or C-C oxidative addition.
2.5. Effect of Ligand Substituent. It was shown in a previous
study30 that placing hydrogens on the phosphine as in PH3 is
adequate for modeling the oxidative addition reaction, and that
replacing PH3 with a more realistic phosphine [P(OPh)3] leads
to the same mechanism, but with different energetics. It is known
that a less basic phosphine favors reductive elimination, as does
increased steric bulk. It was therefore of interest to study the
effect of a compact and highly basic phosphine on oxidative
addition. The addition of the 1,3-dimethylimidazolium species
to Pd[1,3-bis(dimethylphosphino)ethane] [Pd(dmpe)] to yield
complex 7 was thus studied, as shown in Figure 6 (left column).
Comparison of the geometry of PC7 with PC5 (which contains
hydrogens in place of methyl groups on the phosphine) shows
that the two are very similar, the imidazolium being slightly
closer to Pd in PC7. The transition structures TS7 and TS5 are
likewise very similar in geometry, as are the products. However,
the potential energy profile for the reaction (Figure 7) reveals
a stabilization energy of 25.0 kcal mol-1 for PC7 (cf. 17.2 kcal
mol-1 for PC5) and a transition structure that lies 27.3 kcal
mol-1 below the products.66 The product of oxidative addition
lies 39.9 kcal mol-1 below the reactants. Thus, oxidative addition
to give 7 is also predicted to occur with no barrier, but with a
much greater exothermicity than that to give 5. This is an
important result, as it shows that oxidative addition of imida-
Figure 4. Potential energy profile for the oxidative addition of different
imidazolium cations to Pd(dpe).
phosphine, such that in PC4 the phosphine has to undergo little
distortion for the imidazolium to interact. Similar trends were
observed in theoretical studies on CH4 oxidative addition.48,65
In the transition structure TS4, the Pd-Me, Pd-Ccarbenoid, and
Me-Ccarbenoid bonds are virtually the same as in the nonchelated
model TS1. The only difference observed is in the P-Pd-P
angle, which is 83.1° in TS4 compared to 98.3° in TS1. In the
product 4, the bond distances to Pd are also very similar to
those in 1, and again the only real difference between geometries
is in the P-Pd-P angle. The potential energy changes for the
reaction (Figure 4) reveal a stabilization energy of -8.2 kcal
mol-1 for PC4 (cf. -5.3 kcal mol-1 for PC1), which is
consistent with the closer approach of the imidazolium cation
relative to PC1. The major difference, however, is in ∆Hq,
which is reduced to +7.3 kcal mol-1 for TS4, compared to
+26.1 kcal mol-1 for TS1. Furthermore, the reaction is predicted
to be exothermic by 15.8 kcal mol-1, compared to an endot-
hermic reaction when monodentate phosphines are coordinated.
The results thus show that oxidative addition of imidazolium
to chelated Pd0 is strongly favored over that to linear Pd(PR3)2
species, and that the barrier to this reaction is also much lower
when a chelating phosphine is present.
2.4. Effect of Imidazolium Substituent. The effect of
different substituents at C2 on the oxidative addition of
imidazoliums was studied initially by calculation of the reaction
between Pd(dpe) and 1,3-dimethylimidazolium. Oxidative ad-
dition of the C-H bond to yield complex 5 is shown in Figure
3 (right column). In the precursor complex, PC5, the imidazo-
(66) At the level of geometry optimisation (B3LYP/LANL2DZ), the
precursor complex is slightly lower in energy than the transition structure.
(67) Fraser, P. J.; Roper, W. R.; Stone, F. G. A. J. Chem. Soc., Chem.
Commun. 1974, 102.
(65) Sakaki, S.; Biswas, B.; Sugimoto, M. J. Chem. Soc., Dalton Trans.
1997, 803.

OxidatiVe Addition of Imidazolium to Group 10 Metals
J. Am. Chem. Soc., Vol. 123, No. 34, 2001 8323
Figure 5. Optimized geometries of 2-bromo-1,3-dimethylimidazolium and the product of oxidative addition to Pd(dpe), 6.
zolium to an actual Pd0 complex may be strongly favored over
reductive elimination.
tetrafluoroborate was prepared following the method of Ar-
duengo.68 When the iodoimidazolium salt was suspended in a
THF solution of Pt(PPh3)4 and heated to 60 °C, the iodo Pt-
carbene complex [PtI(tmiy)(PPh3)2]BF4 (9) (tmiy ) 1,3,4,5-
tetramethylimidazolin-2-ylidene) was formed in 89% yield
(reaction 3). The 31P NMR spectrum of 9, which contains a
Oxidative addition to Pd(PMe3)2 was also studied to see if
the reaction would be possible without the benefit of chelation.
The optimized geometries of the stationary points are shown in
the right column in Figure 6. As expected, the P-Pd-P angles
in the precursor complex, transition structure, and product are
greater than those in the chelated analogues shown on the left
of Figure 6. In the precursor complex PC8, the imidazolium is
slightly farther away from the Pd center than in PC7, but the
Pd-ligand bond lengths in TS8 and 8 are almost identical to
those in TS7 and 7. The potential energy profile (Figure 7)
shows a stabilization energy of 6.8 kcal mol-1 for PC8 and a
transition structure that lies 9.0 kcal mol-1 below the reactants.
The reaction is therefore predicted to occur with no energy
barrier, with the product lying 18.7 kcal mol-1 below the
reactants. Accordingly, it should be possible to carry out
oxidative addition of the imidazolium cation to an appropriate
Pd0L2 complex, even without the aid of a chelating phosphine.
2.6. Summary of Theoretical Studies. The results of the
density functional studies show that those factors affecting
conventional oxidative addition likewise affect oxidative addi-
tion of imidazolium cations. The factors that are predicted to
favor oxidative addition for the group 10 metals can be
summarized as follows:
(i) Oxidative addition of imidazolium cations to Pt0 and Ni0
is more exothermic than that to Pd0, and Ni0 will react with a
lower energy barrier.
(ii) A cis-chelating ligand on the M0 complex both increases
the exothermicity of the reaction and lowers the activation
barrier relative to monodentate ligands.
(iii) Addition of 2-H imidazolium groups is easier and more
exothermic than addition of the 2-alkylimidazolium moiety. A
halo-imidazolium group should further lower the barrier and
increase the exothermicity.
singlet at 13.8 ppm with Pt satellites (JP-Pt ) 1234 Hz), indicates
a trans arrangement of the phosphine ligands. Evidently,
isomerization of the cis complex that would initially result from
oxidative addition has occurred. Isomerization of related Pt-
carbene complexes has been previously reported.55,69
Single crystals suitable for an X-ray structure determination
were grown by vapor diffusion of ether into a dichloromethane
(DCM) solution of 9. The X-ray study confirms a trans
arrangement of the phosphine ligands and a nearly perfect square
planar coordination geometry (Figure 8), the cations lying with
their [IPt(N2C3(methyl-CH)4)] components disposed in the
crystallographic mirror planes of space group Pnma (anionic
F2BF2 disposed similarly). The Pt-C distance (1.995(5) Å) is
similar to that found in other Pt-carbene complexes;70,71 the
carbene ligand skeleton is obligate planar and coplanar with
Pd, I and normal to the IPtP2C plane.
The reactivity of Pt(PPh3)4 toward 1,3-dimethylimidazolium
tetrafluoroborate was also investigated.55 Heating equimolar
quantities of the imidazolium salt and Pt(PPh3)4 in THF/acetone
(60 °C) resulted in the formation of ca. 15% (NMR) of the
oxidative addition product cis-[PtH(dmiy)(PPh3)2]BF4 (10)
(dmiy ) 1,3-dimethylimidazolin-2-ylidene) relative to unreacted
imidazolium and Pt(PPh3)4 (reaction 4). It was concluded that
(iv) A compact, strongly basic phosphine lowers the activation
barrier and increases the exothermicity of the reaction.
From this it can be seen that the ideal candidate for oxidative
addition of imidazolium would consist of a highly basic,
chelating phosphine ligated to Pt0 or Ni0. However, the
theoretical results show that not all of these conditions should
be required, and that Pd0 in combination with basic phosphines,
even unidentate, may be able to oxidatively add the 2-H
imidazolium cation.
the low amount of 10 resulting was not a consequence of a
high energy barrier to oxidative addition but, rather, a repre-
sentation of the equilibrium concentration of the reactants and
(68) Arduengo, A. J., III; Tamm, M.; Calabrese, J. C. J. Am. Chem. Soc.
1994, 116, 3625.
(69) C¸ etinkaya, B.; C¸ etinkaya, E.; Lappert, M. F. J. Chem. Soc., Dalton
Trans. 1973, 906.
(70) Cardin, D. J.; C¸ etinkaya, B.; C¸ etinkaya, E.; Lappert, M. F.;
Manojlovic-Muir, L. J.; Muir, K. W. J. Organomet. Chem. 1972, 44, C59.
(71) Michelin, R. A.; Zanotto, L.; Braga, D.; Sabatino, P.; Angelici, R.
J. Inorg. Chem. 1988, 27, 93.
3. Oxidative Addition: Experimental Studies
As, based on the theoretical studies, oxidative addition of a
2-haloimidazolium moiety was expected to be facile, and as
Stone et al.67 had previously reported the oxidation addition of
2-halothiazolium salts, 2-iodo-1,3,4,5-tetramethylimidazolium

8324 J. Am. Chem. Soc., Vol. 123, No. 34, 2001
McGuinness et al.
Figure 6. Optimized geometries corresponding to oxidative addition of 1,3-dimethylimidazolium to Pd(dmpe) and Pd(PMe3)2.
product, so that the ∆Greact value is therefore close to zero.
Oxidative addition to Pt(PPh3)4 requires concomitant dissociation
of two phosphine ligands, which is expected to result in the
reaction being less exothermic (or more endothermic). It was
therefore anticipated that starting with the unsaturated 14-
electron complex Pt(PPh3)2 may be enough to drive the reaction
to completion. Indeed, when Pt(PPh3)2 was reacted with 1,3-
dimethylimidazolium tetrafluoroborate in THF/acetone, complex
cis-10 was isolated pure in 63% yield as an air-stable white
powder. Thus, by eliminating two phosphines from the coor-
dination sphere of Pt, the equilibrium is driven substantially in
the forward direction, allowing isolation of the product in good

OxidatiVe Addition of Imidazolium to Group 10 Metals
J. Am. Chem. Soc., Vol. 123, No. 34, 2001 8325
Figure 7. Potential energy profile for the oxidative addition of 1,3-
dimethylimidazolium to Pd(dmpe) and Pd(PMe3)2.
Figure 9. Projection of cis-11. Selected bond lengths (Å) and angles
(°): Pt-C(1) 2.028(3), Pt-P(1) 2.3076(9), Pt-P(2) 2.3693(9), C(1)-
Pt-P(1) 154.44(9), C(1)-Pt-P(2) 97.91(9), P(1)-Pt-P(2) 107.61-
(3).
Figure 8. Projection of 9, normal to the coordination plane. Thermal
ellipsoids with 50% probability are shown for the non-hydrogen atoms,
hydrogen atoms having arbitrary radii of 0.1 Å. Selected bond lengths
(Å) and angles (°): Pt-I 2.6449(5), Pt-C(1) 1.995(5), Pt-P(1) 2.315-
(1), C(1)-N(2) 1.343(6), C(1)-Pt-I 177.8(1), C(1)-Pt-P(1) 90.98-
(2), P(1)-Pt-I 89.08(2), P(1)-Pt-P(1′) 176.20(3).
yield. Complex 10 has been characterized by 1H, 13C, 31P NMR,
high-resolution MS, and elemental analysis.
The unsaturated Pt0 complex Pt(PCy3)2 is also available by
way of a straightforward synthesis.72 It was therefore of interest
to determine if this complex underwent oxidative addition of
the imidazolium cation, as the bulky PCy3 ligand has been
shown to promote reductive elimination of methyl-imidazolium
from PdII.30 When Pt(PCy3)2 and 1,3-dimethylimidazolium
tetrafluoroborate were heated together in THF/acetone at 60 °C,
[PtH(dmiy)(PCy3)2]BF4 (11) was formed in 60% isolated yield.
A cis geometry for 11 was inferred from the 31P and 1H NMR
data. The Pt-H resonance appears as a doublet of doublets at
-7.95 ppm with Pt satellites (JP-H ) 160, 21 Hz, JPt-H ) 880
Hz). When crystals of 11 were grown from DCM/pentane over
a number of days, two different forms of crystals were obtained,
trans and cis. An X-ray study of a prismatic specimen showed
that phase to be cis-11, consistent with the spectroscopic data
(Figure 9). A significantly distorted square planar core geometry
is displayed, with a P-Pt-P angle of 107.61(3)°. This distortion
Figure 10. Projection of trans-11. Selected bond lengths (Å) and angles
(°): Pt-C(1) 2.058(3), Pt-P(1) 2.3020(8), Pt-P(2) 2.2976(8), C(1)-
Pt-P(1) 98.03(8), C(1)-Pt-P(2) 97.35(8), P(1)-Pt-P(2) 163.28(2).
from square planar geometry is attributed to the bulk of the
PCy3 ligands, which push each other apart, crowding the carbene
ligand to such an extent that the P(2)-Pt-C angle is opened to
97.91(9)°. The crystal structure of 11 is thus consistent with
the notion that bulky cis ligands will push cis carbene and
hydrocarbyl groups together and facilitate reductive elimina-
tion.30 The Pt-C distance (2.028(3) Å) is longer than that in 9,
reflecting steric crowding and a higher trans influence of PCy3
relative to that of the iodide ligand. Also noticeable is the
difference between the two Pt-P distances, the bond length trans
to the hydride (2.3693(9) Å) being longer than Pt-P trans to
the carbene (2.3076(9) Å), a reflection of the higher trans
influence of the hydride ligand. A similar trend is observed in
the theoretical geometries of the hydride complexes 5, 7, and
8. An X-ray study of a specimen of the needle phase showed it
(72) Yoshida, T.; Otsuka, S. Inorg. Synth. 1990, 28, 113.

8326 J. Am. Chem. Soc., Vol. 123, No. 34, 2001
McGuinness et al.
to be trans-11, as shown in Figure 10. A distorted square planar
core geometry is displayed, the distortion less than that found
in cis-11 due to the trans arrangement of the phosphines. The
phosphine ligands are both bent away from the carbene ligand,
reflecting the low steric demand of the hydride. The Pt-C
distance (2.058(3) Å) is slightly elongated relative to that in
cis-11 due to the high trans influence of the hydride ligand.
The formation of the trans isomer must result from a slow cis-
trans isomerization in solution over the period of crystallization.
Crystal structures of hydrido carbene complexes are rare,73,74
and the two structures above represent the first examples for
the group 10 metals. The formation of 11, which contains a
very bulky phosphine, can be attributed to the greater tendency
of Pt toward oxidative addition relative to Pd, the strong donor
nature of PCy3, and the low steric demand of the hydride ligand.
Notably, the above results demonstrate for the first time that
oxidative addition of the imidazolium cation to yield a M-hy-
dride complex is possible. Oxidative addition of the imidazolium
ion to give complexes 10 and 11 is the first example of oxidative
addition of a C-H bond to a Pt0 complex which does not contain
a chelating ligand,49-51 hitherto thought to be requisite for this
reaction.54
sociation of the weakly coordinating dba ligand from 13 would
generate the reactive Pd0(P∧P) fragment. The Pt analogue to
this species has been used by Whitesides et al.49-51 to carry
out C-H bond oxidative addition of hydrocarbons. Complex
1
13 gives broad signals in the H NMR spectrum at ambient
temperature, ascribed to fluxional behavior of the dba ligand.
At -40 °C, the signals sharpen and the coordinated olefinic
protons appear as two multiplets at 4.83 and 5.01 ppm. The
same behavior has been described for similar Pd0-dba com-
plexes.75,76 Recrystallization of 13 from DCM/pentane afforded
yellow crystals which were identified as Pd(dcype)Cl2:dba 1:1
“solvate” by an X-ray study.77 This complex is most likely
formed after an initial oxidative addition of DCM,78 indicating
that 13 has a high reactivity toward oxidative addition.
Complex 13 was found to react with 2-iodo-1,3,4,5-tetram-
ethylimidazolium tetrafluoroborate under conditions analogous
to those used with Pt(PPh3)4 to yield [PdI(tmiy)(dcype)]BF4 (14)
in 72% yield (reaction 7). It was therefore decided to study its
Stone et al.67 had previously reported oxidative addition of
2-halothiazolium cations to M0, and the above theoretical studies
have shown that this reaction probably occurs via the same
mechanism of concerted oxidative addition as that of 2-H
imidazolium addition. Consequently, it was decided to inves-
tigate the reaction of 3,4-dimethylthiazolium tetrafluoroborate
with Pt(PCy3)2. The heating of a THF solution of these reactants
to 60 °C afforded [PtH(dmty)(PCy3)2]BF4 (12) (dmty ) 3,4-
dimethylthiazolin-2-ylidene), isolated in 23% yield as a yellow
powder (reaction 5). A cis geometry is indicated by the
reactivity toward 1,3-dimethylimidazolium tetrafluoroborate.
When 13 was treated at room temperature with an equimolar
quantity of the imidazolium salt, no change was evident by
NMR. When excess imidazolium was added (4 equiv added to
push the equilibrium in favor of oxidative addition), a reaction
took place that resulted in decomposition and formation of Pd
metal and free dba. The fate of the other organic groups could
not be reliably determined by NMR. Therefore, it cannot be
confidently stated at this stage whether 13 oxidatively adds the
imidazolium ion and then subsequently decomposes, or whether
it decomposes from Pd0 by some interaction of the imidazolium
ion. It is known, however, that hydrido Pd complexes are less
stable than their Pt equivalents.79 Further work on the oxidative
addition of imidazoliums to Pd and Ni is underway.
spectroscopic data. The Pt-H resonance appears as a doublet
of doublets (JP-H ) 24, 159 Hz) centered at -7.49 ppm with
Pt satellites (JPt-H ) 444 Hz). The formation of complex 12
confirms that the thiazolium cation shares similar reactivity with
the imidazolium cation, in that it is also capable of oxidative
additionsand probably reductive eliminationsreactivity with
metals. This is an important result, as it shows that hydrocarbyl
complexes of thiazolin-2-ylidenes may also be prone to decom-
position via carbene-hydrocarbyl coupling. Complexes of
thiazole-based carbenes have recently been investigated as
catalysts for C-C coupling.26
(75) Herrmann, W. A.; Thiel, W. R.; Brossmer, C.; O¨ fele, K.; Priermeier,
T.; Scherer, W. J. Organomet. Chem. 1993, 461, 51.
(76) Herrmann, W. A.; Thiel, W. R.; Priermeier, T.; Herdtweck, E. J.
Organomet. Chem. 1994, 481, 253.
To study the oxidative addition behavior of Pd0, complex 13
was prepared (reaction 6). Complex 13 contains the chelating
(77) Ganguly, S.; Mague, J. T.; Roundhill, D. M. Acta Crystallogr., Sect.
C 1994, 50, 217 reports the unsolvated form. In this work, [(dcype)PdCl2]‚
dba, C43H62Cl2OP2Pd, Mr ) 834.2, is orthorhombic, space group Pna21, a
) 13.4790(7) Å, b ) 19.785(4) Å, c ) 15.697(3) Å, V ) 4186 Å3, Dc )
1.324 g cm-3 (Z ) 4), T ≈ 153 K; 78 429 total reflections (full sphere)
were merged after “empirical”/multiscan absorption correction (µMo ) 6.8;
specimen 0.60 × 0.30 × 0.22 mm, “T ”min, max ) 0.66, 0.80) to 10 658 (Rint
) 0.030) unique (xabs being found indeterminate of chirality), 8402 (F >4σ-
(F)) used in full-matrix least-squares refinement (R ) 0.044, (Rw ) 0.050;
|∆Fmax| ) 2.3(1) e Å-3.) One Cy ring was disordered over two sets of sites
(occupancy, 0.758(7) and complement); the olefinic carbons of dba were
also modeled as disordered over two sets of (trans) sites of equal occupancy.
Results of the two determinations are generally harmonious.
(78) Huser, M.; Youinou, M.-T.; Osborn, J. A. Angew. Chem., Int. Ed.
Engl. 1989, 28, 1386.
phosphine 1,3-bis(dicyclohexylphosphino)ethane (dcype). Dis-
(73) Francis, M. D.; Hibbs, D. E.; Hursthouse, M. B.; Jones, C.; Smithies,
N. A. J. Chem. Soc., Dalton Trans. 1998, 3249.
(74) Huang, J.; Stevens, E. D.; Nolan, S. P. Organometallics 2000, 19,
1194.
(79) Hartley, F. R. The Chemistry of Platinum and Palladium; Applied
Science Publishers: Essex, 1973.

OxidatiVe Addition of Imidazolium to Group 10 Metals
J. Am. Chem. Soc., Vol. 123, No. 34, 2001 8327
Table 1. Crystal Data, Collection, and Refinement Parameters for Complexes 9, cis-11, and trans-11
crystal data
9
cis-11
2CH2Cl2
C43H79BCl4F4N2P2Pt
1109.8
trans-11
solvation component
formula
molecular weight
crystal size (mm)
crystal system
space group
a (Å)
b (Å)
c (Å)
R (°)
2CH2Cl2
C45H46BCl4F4IN2P2Pt
1227.4
0.2 × 0.17 × 0.1
orthorhombic
Pnma
C41H75BF4N2P2Pt
939.9
0.45 × 0.12 × 0.10
monoclinic
P21/n
12.245(3)
16.533(4)
0.12 × 0.10 × 0.07
triclinic
P1h
18.989(1)
15.970(1)
15.586(1)
11.5338(7)
15.0977(9)
15.1892(9)
85.795(2)
75.590(2)
75.522(2)
2480
21.360(4)
â (°)
96.348(5)
γ (°)
V (Å3)
4726
4
1.725
4298
4
Z
2
Fc (g cm-3
)
1.486
1.453
Data Collection and Refinement Parametersa
µ (cm-1
)
39.7
75
31.6
33.9
76
87 494
2θmax (°)
76
51 715
total reflctns
97 594
indepdnt reflctns
“observed” reflctns
“T ” (min, max)
R, Rw
12 903
25 611
22 535
8555 (F > 4σ(F))
0.51, 0.77
0.045, 0.053
16 855 (F > 4σ(F))
0.63, 0.78
0.045, 0.039
15 025 (F > 4σ(F))
0.48, 0.76
0.038, 0.040
a Full spheres of CCD area detector data (T ≈ 153 K), merged after empirical/multiscan absorption correction. Anisotropic thermal parameters
form refinement (non-H atom), (x, y, z, Uiso)H constrained at estimated values (hydrido-H observed in difference maps for 11). Reflection weights:
(σ2(F) + 0.0004F2)-1; neutral atom complex scattering factors. Monochromatic Mo KR radiation, λ ) 0.71073 Å. In 9, residues modeled as CH2Cl2
were modeled as disordered over two sets of sites, occupancy 0.750(7) and complement. In cis-11, one Cy ring was disordered over two sets of
sites, occupancies 0.5, with three of the BF4 fluorines rotationally disordered over two sets of sites, occupancy 0.729(3) and complement.
4. Conclusions
5. Experimental Section
5.1. General Comments. All manipulations were carried out using
standard Schlenk techniques or in a nitrogen glovebox. All solvents
were purified and dried by standard procedures and distilled under
nitrogen immediately before use. Nuclear magnetic resonance spectra
were recorded at ambient temperature unless otherwise stated, and peaks
are labeled as singlet (s), doublet (d), triplet (t), multiplet (m), and
broad (br). Elemental analysis and MS were carried out by the Central
Science Laboratory, University of Tasmania. X-ray structure determina-
tions are summarized in Table 1. Pt(PPh3)4,80 Pt(PPh3)2,81 and
Theoretical density functional studies have shown which
factors can be expected to favor oxidative addition of the
imidazolium cation to zerovalent group 10 metals. Pt and Ni
should react most readily; however, the results suggest that with
appropriate supporting ligands and under appropriate reaction
conditions, all three of the metals should undergo oxidative
addition. The prediction that oxidative addition can occur has
been confirmed by experiment for the first time, and three new
hydrido Pt-carbene complexes have been prepared by this
reaction.
72
Pt(PCy3)2 were prepared by literature methods.
5.2. Synthesis. 2-Iodo-1,3,4,5-tetramethylimidazolium Tetrafluo-
roborate. This compound was prepared via a method similar to that
used by Arduengo et al.68 to prepare related imidazolium salts. The
free carbene tmiy (2.03 g, 16.3 mmol) was dissolved in THF (20 mL),
and a solution of iodine (4.15 g, 16.3 mmol, 40 mL of THF) was added
dropwise over 1 h. After addition, the solution was stirred for a further
30 min before the precipitate was collected by filtration. The product
was washed with THF followed by ether and then dried in vacuo to
yield 2-iodo-1,3,4,5-tetramethylimidazolium iodide. The iodide (1.71
g, 4.52 mmol) was suspended in water/MeOH (1:3), and an aqueous
solution of AgBF4 was added (prepared by stirring 0.52 g (2.24 mmol)
of Ag2O in water with 0.79 g of 50% HBF4). The solids were collected
on a filter paper and extracted with THF followed by DCM. The
organics were then removed in vacuo, and the product was washed
with THF/ether. 1H NMR (200 MHz, DMSO-d6): δ 3.75 (s, 6H, NCH3),
2.32 (s, 6H, CCH3). 13C NMR (50 MHz, DMSO-d6): δ 129.6
(H3CCCCH3), 100.1 (CI), 36.4 (NCH3), 9.1 (H3CCCCH3).
3,4-Dimethylthiazolium Tetrafluoroborate. Thiazole (4.81 g,
0.0485 mol) and MeI (4 mL, 0.06 mol) were added to a reaction bomb
along with 10 mL of THF and heated to 90 °C for 2 days. A solid
yellow cake was obtained that was collected on a Schlenk filter and
washed with THF (50 mL). The compound was dried in vacuo to yield
a yellow powder, 3,4-dimethylthiazolium iodide. Yield: 11.9 g (100%).
The iodide (6.30 g, 0.0261 mol) was taken up in EtOH/MeOH (50/50,
These results confirm that a potentially important method of
limiting decomposition of M-carbene catalysts will be to
operate in imidazolium-based ionic liquids. For this reason,
catalysts based on heterocyclic carbene complexes should be
particularly suited to reactions in ionic liquids. Studies on the
application of Ni-carbene complexes for olefin dimerization
in an imidazolium-based ionic liquid will be reported shortly.
Furthermore, these results show unambiguously for the first time
the “noninnocent” role of ionic liquid solventssthe imidazolium
cation can react with low-valent metal complexes to generate a
carbene complex in situ. While interaction of the imidazolium
cation with Pd(OAc)2 has been shown to yield carbene
complexes,46 this work shows that a basic metal salt is not
necessarily required to deprotonate the imidazolium. Thus, when
group 10 metal complexes, particularly M0 complexes, are used
in imidazolium-based ionic liquids, the possibility that carbene
complexes are generated and act as the active species must be
considered. The large excess of imidazolium present under these
conditions can be expected to drive the oxidative addition
reaction. Furthermore, the supporting ligands studied herein and
found using both theoretical and experimental methods to
promote oxidative addition are commonly used ligands in
catalysis. It is noted that oxidative addition of an imidazolium
ion would generate a reactive metal hydride that could initiate
many catalytic cycles involving unsaturated substrates.
(80) Herrmann, W. A.; Salzer, A. Synthetic Methods of Organometallic
and Inorganic Chemistry; Thieme: Stuttgart, 1996; Vol 1.
(81) Ugo, R.; La Monica, G.; Cariati, F.; Cenini, S.; Conti, F. Inorg.
Chim. Acta 1970, 4, 390.

8328 J. Am. Chem. Soc., Vol. 123, No. 34, 2001
McGuinness et al.
100 mL), and a MeOH solution of AgBF4 (prepared from 3.20 g, 0.0138
mol of Ag2O and 3.4 mL of 50% HBF4 solution) was added. The
solution was dried over molecular sieves and filtered to remove AgI,
and the solvent was removed in vacuo until ca. 20 mL remained. Ether
(20 mL) was added, and the solvent was decanted. The remaining oil
was washed with ether, dried in vacuo, and washed again with ether.
Drying in vacuo over a warm water bath gave a white powder. Yield:
CyC). 31P NMR (162 MHz, CD2Cl2): δ 29.1 (d with Pt satellites, JP-P
) 18 Hz, JP-Pt ) 1265 Hz), 29.7, 29.8 (two doublets attributed to two
conformations of the bulky PCy3 ligand, with Pt satellites, JP-P ) 18
Hz, JP-Pt ) 990 Hz).
[PtH(dmty)(PCy3)2]BF4 (12). This was prepared in the same manner
as 10, from Pt(PCy3)2 (0.080 g, 0.10 mmol) and 3,4-dimethylthiazolium
tetrafluoroborate (0.087 g, 0.43 mmol) to yield a yellow powder.
Yield: 0.022 g (23%). Anal. Calcd for C41H74NSP2PtBF4: C, 51.46;
H, 7.79; N, 1.46. Found: C, 51.33; H, 8.38; N, 1.38. MS (LSIMS):
m/z (relative intensity) 869 [M]+ (80), 755 [Pt(PCy3)2]+ (55), 588 [Pt-
(PCy3)2 - Cy]+ (100), 504 [Pt(PCy3)2 - 2Cy]+ (25), 281 [HPCy3]+
1
2.79 g (53%). H NMR (200 MHz, D2O): δ 9.75 (s, 1H, SC(H)N),
7.76 (s, 1H, SCH), 4.10 (s, 3H, NCH3), 2.54 (s, 3H, CCH3). 13C NMR
(50 MHz, D2O): δ 160.5 (SCN), 123.0 (NCCH3), 116.0 (SC), 42.5
(NCH3), 15.3 (CCH3).
1
[PtI(tmiy)(PPh3)2]BF4 (9). 2-Iodo-1,3,4,5-tetramethylimidazolium
tetrafluoroborate (0.0403 g, 0.119 mmol) and Pt(PPh3)4 (0.144 g, 0.116
mmol) were taken up in THF (30 mL) and heated to 65 °C for 24 h.
The solvent was removed until ca. 2 mL remained, ether (10 mL) was
added, and the solvent was decanted off the precipitate. After washing
with THF/ether (2 mL/10 mL) and ether (2 × 5 mL), the product was
dried in vacuo to give a white powder. Yield: 0.109 g (89%). Anal.
Calcd for C43H42N2IP2PtBF4: C, 48.83; H, 4.00; N, 2.65. Found: C,
48.09; H, 4.24; N, 2.42. MS (LSIMS): m/z (relative intensity) 970
(70). H NMR (200 MHz, CD2Cl2): δ 7.27 (s with Pt satellites, JPt-H
) 7 Hz, 1H, SCH), 3.96 (s, 3H, NCH3), 2.44 (s, 3H, CCH3), 0.8-2.2
(m, 66H, CyH), -7.49 (dd with Pt satellites, JP-H ) 24, 159 Hz, JPt-H
) 444 Hz, 1H, PtH). 13C NMR (100 MHz, CD2Cl2): δ 38.2 (NCH3),
35.9 (d, CyC), 30.6 (CyC), 30.4 (m, CyC), 27.6 (m, CyC), 27.2 (CyC),
26.5 (d, CyC), 14.6 (CCH3). 31P NMR (161 Hz, CD2Cl2): δ 28.9 (d,
with Pt satellites, JP-P ) 15 Hz, JPt-P ) 1278 Hz), 27.0 (“t” attributed
to conformations of the PCy3 ligand, with Pt satellites, JP-P ) 15 Hz,
JPt-P ) 985 Hz).
1
[M]+ (11); 708 [M - PPh3]+ (10); 580 [M - PPh3I]+ (25). H NMR
Pd(dcype)(dba) (13). Pd(dba)2 (0.59 g, 1.03 mmol) and dcype (0.44
g, 1.04 mmol) were dissolved in toluene (30 mL) and stirred for 3
days. The solvent was removed in vacuo, and the residue was taken
up in THF (25 mL) and filtered through Celite. The THF was removed
in vacuo until ca. 5 mL remained, and pentane (20 mL) was added to
precipitate the product. The solvent was decanted off, the solid washed
with pentane/ether (10 mL/10 mL, 3×), and the product dried in vacuo
to give an orange powder. Yield: 0.426 g (54%). MS (LSIMS): m/z
(relative intensity) 763 [M]+ (20), 528 [M - dba]+ (100), 445 [M -
dba - Cy]+ (33), 363 [M - dba - 2Cy]+ (35), 281 [M - dba -
(200 MHz, CD2Cl2): δ 7.6-7.4 (m, 30H, phenylH), 3.11 (singlet with
Pt satellites, JPt-H ) 3 Hz, 6H, NCH3), 1.62 (s, 6H, CCH3). 13C NMR
(100 MHz, CD2Cl2) δ: 134.3 (singlet with Pt satellites, JPt-C ) 6 Hz,
phenylC), 131.6 (phenylC), 128.9 (m, phenylC), 128.7 (singlet with Pt
satellites, JPt-C ) 5 Hz, CdC), 127.0 (phenylC), 34.3 (NCH3), 8.6
(CCH3). 31P NMR (161 Hz, CD2Cl2): δ 13.8 (singlet with Pt satellites,
JP-Pt ) 1234 Hz).
[PtH(dmiy)(PPh3)2]BF4 (10). Pt(PPh3)2 (0.247 g, 0.343 mmol) was
dissolved in 10 mL of THF, and a solution of 1,3-dimethylimidazolium
tetrafluoroborate (0.126 g, 0.68 mmol) in 10 mL of acetone was added.
The heterogeneous solution was stirred at 60 °C for 18 h and filtered
through Celite. The solvent was removed in vacuo, and the residue
was taken up in 2.5 mL of DCM and extracted with water (3 × 4 mL).
The solvent was then removed in vacuo, and the product was washed
further with water (2 × 4 mL) and ether (2 × 5 mL). Drying in vacuo
gave a white powder. Yield: 0.196 g (63%). Anal. Calcd for
C41H39N2P2PtBF4: C, 54.50; H, 4.35; N, 3.10. Found: C, 54.32; H,
4.32; N, 3.02. HRMS (LSIMS): m/z calcd for C41H39N2P2195Pt,
816.22361; found, 816.22419. MS (LSIMS): m/z (relative intensity)
816 [M]+ (100); 719 [Pt(PPh3)2]+ (50); 553 [M - PPh3H]+ (80); 455
1
3Cy]+ (28). H NMR (200 MHz, CD2Cl2, -40 °C): δ 6.8-7.8 (m,
12H, PhH, HCdCH), 5.01 (m, 1H, HCdCH), 4.83 (m, 1H, HCdCH),
0.5-2.2 (br, m, 48H, CyH, PCH2). 13C NMR (50 MHz, CD2Cl2, -60
°C): δ 129.7, 129.6, 129.1 (PhC), 25.9-27.9 (m, CyC). 31P NMR (161
Hz, CD2Cl2, 15 °C): δ -1.29, -3.56 (s, br).
[PdI(dcype)(tmiy)]BF4 (14). This complex was prepared by a
method similar to that employed for the synthesis of 9, from Pd(dcype)-
(dba) (13) (0.095 g, 0.12 mmol) and 2-iodo-1,3,4,5-tetramethylimida-
zolium tetrafluorborate (0.0426, 0.126) to afford an orange powder.
Yield: 0.075 g (72%). Anal. Calcd for C33H60N2P2IPdBF4: C, 45.72;
H, 6.98; N, 3.23. Found: C, 45.47; H, 6.45; N, 3.44. MS (LSIMS):
m/z (relative intensity) 779 [M]+ (10), 655 [M - tmiy]+ (65). 1H NMR
(400 MHz, CD2Cl2): δ 3.76 (s, 6H, NCH3), 2.23 (s, 6H, CCH3), 1.2-
2.2 (broad humps, 48H, CyH, PCH2). 13C NMR (100 MHz, CD2Cl2):
δ 127.3 (CdC), 33.6 (NCH3), 30.6, 29.4, 26.9 (br), 25.8 (CyC, PCH2),
8.2 (H3CCCCH3).
1
[PPh3]+ (25). H NMR (200 MHz, CD2Cl2): δ 6.79 (singlet with Pt
satellites, JPt-H ) 6 Hz, 2H, HCdCH), 3.50 (singlet with Pt satellites,
JPt-H ) 2 Hz, 6H, NCH3), -5.23 (dd with Pt satellites, JP-H ) 19, 176
Hz, JPt-H ) 511 Hz, 1H, PtH). 13C NMR (100 MHz, CD2Cl2): δ 134.3
(d, J ) 12 Hz, PhC), 133.6 (d, J ) 12 Hz, PhC), 131.2 (s, br, PhC),
128.8 (“t”, J ) 11 Hz, PhC), 122.9 (s with Pt satellites, JP-C ) 4 Hz,
JPt-C ) 16 Hz, HCdCH), 37.6 (s with Pt satellites, J ) 20 Hz, NCH3).
Acknowledgment. We are indebted to the Australian
Research Council for financial support and for providing an
Australian Postgraduate Award for D.S.M. We also thank
Johnson-Matthey for the generous loan of Pd and Pt salts. The
staff of the Central Science Laboratory (University of Tasmania)
are gratefully acknowledged for their assistance in a number of
instrumental techniques.
31P NMR (161.8 MHz, CD2Cl2): δ 24.0 (d with Pt satellites, JP-P
)
18 Hz, JP-Pt ) 654 Hz), 20.1 (d with Pt satellites, JP-P ) 18 Hz, JP-Pt
) 663 Hz).
[PtH(dmiy)(PCy3)2]BF4 (11). This was prepared in the same manner
as 10 from Pt(PCy3)2 (0.099 g, 0.13 mmol) and 1,3-dimethylimidazo-
lium tetrafluoroborate (0.0248 g, 0.134 mmol) to give a white solid.
Yield: 0.074 g (60%). Anal. Calcd for C41H75N2P2PtBF4: C, 52.39;
H, 8.04; N, 2.98. Found: C, 52.29; H, 8.03; N, 2.89. MS (LSIMS):
m/z (relative intensity) 852 [M]+ (65), 756 [M - dmiyH]+ (10), 571
[M - PCy3H]+ (100), 281 [HPCy3]+ (15). 1H NMR (CD2Cl2, 400
MHz): δ 7.16 (s with Pt satellites, JPt-H ) 4 Hz, 2H, HCdCH), 3.67
(s, 6H, NCH3), 1.0-2.2 (m, 66H, CyH), -7.95 (dd with Pt satellites,
JP-H ) 160, 21 Hz, JPt-H ) 880 Hz, 1H, PtH). 13C NMR (CD2Cl2, 100
MHz): δ 123.1 (d with Pt satellites, JP-C ) 3 Hz, JPt-C ) 16 Hz,
HCdCH), 38.0-38.5 (m, CyC), 37.7 (s, NCH3), 30.8 (s with Pt
satellites, CyC), 30.7 (s, CyC), 27.9-27.7 (m, CyC), 26.6, 26.3 (s,
Supporting Information Available: Complete listing of the
optimized geometries with absolute energies of all stationary
points, and a thermal ellipsoid plot of PdCl2(dcype)‚dba (PDF).
X-ray parameters, fractional coordinates and displacement
parameters, bond lengths and bond angles for complexes 9, cis-
11, trans-11, and PdCl2(dcype)‚dba (CIF). This material is
available free of charge via the Internet at http://pubs/acs.org.
JA010628P