(Diphenylphosphino)alkyl-functionalized nucleophilic carbene complexes of palladium

Article abstract of DOI:10.1021/om034061s

The (diphenylphosphino)alkyl-functionalized nucleophilic heterocyclic carbene (NHC) complexes of palladium LPdX₂ (L = (3-R¹)[1-Ph₂P(CH₂)₂]-imidazol-2-ylid ene; R¹ = 2,6-Pr₂ⁱC₆H₃, 2,4,6-Me₃C₆H₂; X = CH₃ (3a,b), X = Br (4a,b)) have been synthesized by the reaction of the in situ generated functionalized NHC ligand L^a or L^b with Pd(tmed)(CH₃)₂ and Pd(COD)Br₂, respectively, and structurally characterized. Interaction of 3a with H(Et₂O)- {B[3,5-(CF₃)₂C₆H₂]₄} and pyridine or with (CF₃)₂CHOH and pyridine in CH₂Cl₂ gave the monocationic complexes [(L^a)Pd(CH₃)(pyridine)]⁺(A)^- (A^- = {B[(3,5-CF₃)₂C₆H₂]₄} ^-, (CF₃)₂CHO^-); acetonitrile and benzonitrile analogues can be prepared in an analogous way. Reaction of 4a with AgBF₄ in MeCN gave the dicationic complexes [(L^a)Pd(MeCN)₂](BF₄)₂. Complexes 3 show moderate catalytic activity for the coupling of acrylates with aryl bromides but not chlorides. The cationic species generated in situ from 3a and H(Et₂O){B[(3,5-CF₃) ₂C₆H₂]₄} in CH₂Cl₂ under CO/ethylene acts as a copolymerization catalyst under mild conditions.

Full text of DOI:10.1021/om034061s

4750
Organometallics 2003, 22, 4750-4758
(Dip h en ylp h osp h in o)a lk yl-F u n ction a lized Nu cleop h ilic
Ca r ben e Com p lexes of P a lla d iu m
Nikolaos Tsoureas,† Andreas A. Danopoulos,*,† Arran A. D. Tulloch,‡ and
Mark E. Light†
Department of Chemistry, University of Southampton, Highfield,
Southampton, U.K. SO17 1BJ , and J ohnson Matthey Catalysts,
Belasis Avenue, Billingham, Cleveland, U.K. TS23 1LB
Received J uly 22, 2003
The (diphenylphosphino)alkyl-functionalized nucleophilic heterocyclic carbene (NHC)
complexes of palladium LPdX2 (L ) (3-R1)[1-Ph2P(CH2)2]-imidazol-2-ylidene; R1 ) 2,6-
Pri2C6H3, 2,4,6-Me3C6H2; X ) CH3 (3a ,b), X ) Br (4a ,b)) have been synthesized by the reaction
of the in situ generated functionalized NHC ligand La or Lb with Pd(tmed)(CH3)2 and
Pd(COD)Br2, respectively, and structurally characterized. Interaction of 3a with H(Et2O)-
{B[3,5-(CF3)2C6H2]4} and pyridine or with (CF3)2CHOH and pyridine in CH2Cl2 gave the
monocationic complexes [(La)Pd(CH3)(pyridine)]+(A)- (A- ) {B[(3,5-CF3)2C6H2)]4}-, (CF3)2CHO-);
acetonitrile and benzonitrile analogues can be prepared in an analogous way. Reaction of
4a with AgBF4 in MeCN gave the dicationic complexes [(La)Pd(MeCN)2](BF4)2. Complexes 3
show moderate catalytic activity for the coupling of acrylates with aryl bromides but not
chlorides. The cationic species generated in situ from 3a and H(Et2O){B[(3,5-CF3)2C6H2)]4}
in CH2Cl2 under CO/ethylene acts as a copolymerization catalyst under mild conditions.
In tr od u ction
We have recently reported the synthesis of several
thermally stable N-functionalized NHCs with various
substituted pyridines, ethers, and arylphosphines.5 Fur-
thermore, a range of complexes with platinum-group
metals, mainly Pd, Ru, Ir, and Rh, have been also pre-
pared by the interaction of the isolated or in situ gen-
erated functionalized NHCs or their silver adducts with
transition-metal precursors.6 Herein, we wish to report
the synthesis, structural characterization, and selected
reactions of 3-aryl-1-(â-(diphenylphosphino)ethyl)imi-
There is a continuing effort to improve our mecha-
nistic understanding of the organometallic transforma-
tions and catalytic reactions involving NHC complexes.1
The reductive elimination of methylimidazolium salts
from [Pd(tmim)CH3(L)2]+ (tmim ) tetramethylmeth-
ylimidazol-2-ylidene, L ) phosphine, phosphite) and the
microscopic reverse, oxidative addition, have been ob-
served in simple systems and studied by theoretical
methods;2 methyl migration to the NHC carbon has also
been observed in a Pd complex with a preorganized
“pincer” NHC ligand and studied theoretically.3 The
extent of these reactions is critically dependent on the
nature of the coligating functionalities, the presence and
size of chelating rings, and other structural variables.
Consequently, the design of successful carbene catalysts
should be based on ligands promoting productive cata-
lytic steps while inhibiting decomposition or deactiva-
tion. The use of bulky groups on imidazole and imida-
zolin-2-ylidene is becoming a common feature of most
catalytically active complexes. In addition, incorporation
of the carbene functionality into ligand systems contain-
ing other “classical” donor groups facilitates electronic
tuning of the metal coordination sphere.4
dazol-2-ylidene (aryl ) 2,6-Pri -C6H3, La ; aryl ) 2,4,6-
2
trimethylphenyl, Lb) complexes of palladium. The com-
pounds described in the paper are summarized in
Scheme 1.
Resu lts a n d Discu ssion
Liga n d Syn th esis. The ligand synthetic strategy is
shown in Scheme 2.
It is based on the quaternization of 1-arylimidazoles
with (ω-bromoalkyl)diphenylphosphine oxide followed
by reduction with SiHCl3. A similar procedure was used
for the synthesis of the 1-(â-(diphenylphosphino)ethyl)-
3-methylimidazolium salt.4 However, under the previ-
ously reported conditions both reaction steps failed with
the bulky, less nucleophilic 1-arylimidazoles. When the
* To whom correspondence should be addressed. E-mail: ad1@
soton.ac.uk.
† University of Southampton.
(4) Herrmann, W. A.; Ko¨cher, C.; Goossen, L. J .; Artus, G. R. J .
Chem. Eur. J . 1996, 2, 1627.
‡ J ohnson Matthey Catalysts.
(1) For recent reviews of the area see: (a) Herrmann, W. A. Angew.
Chem., Int. Ed. 2002, 41, 1290. (b) Bourisou, D.; Guerret, O.; Gabbai,
F. P.; Bertrand, G. Chem. Rev. 2000, 100, 39.
(2) (a) McGuinness, S.; Saendig, N.; Yates, B. F.; Cavell, K. J . J .
Am. Chem. Soc. 2001, 123, 4029. (b) McGuiness, S.; Cavell, K. J .; Yates,
B. F.; Skelton, B. W.; White, A. H. J . Am. Chem. Soc. 2001, 123, 8317.
(c) Gru¨ndemann, S.; Albrecht, M.; Kovacevic, A.; Faller, J . W.; Crabtree,
R. H. J . Chem. Soc., Dalton Trans. 2002, 2163.
(5) Danopoulos, A. A.; Gelbrich, T.; Hursthouse, M. B.; Winston, S.
Chem. Commun. 2002, 482.
(6) (a) Danopoulos, A. A.; Tulloch, A. A. D.; Winston, S.; Hursthouse,
M. B.; Eastham, G. Dalton Trans. 2003, 699 and references therein.
(b) Danopoulos, A. A.; Tulloch, A. A. D.; Winston, S.; Hursthouse, M.
B.; Eastham, G. Dalton Trans. 2003, 1009. (c) Lappert, M. F. J .
Organomet. Chem. 1993, 451, 389. (d) Arnold, P. L.; Scarisbrick, A.
C.; Blake, A. J .; Wilson, C. Chem. Commun. 2001, 2340. (e) Fu¨rstner,
A.; Krause, H.; Ackermann, L.; Lehmann, C. W. Chem. Commun. 2001,
2240.
(3) Danopoulos, A. A.; Tsoureas, N.; Green, J . C.; Hursthouse, M.
B. Chem. Commun. 2003, 756.
10.1021/om034061s CCC: $25.00 © 2003 American Chemical Society
Publication on Web 10/14/2003

Carbene Complexes of Palladium
Organometallics, Vol. 22, No. 23, 2003 4751
Sch em e 1. P a lla d iu m Com p lexes a n d Ch em ica l Tr a n sfor m a tion s Descr ibed in Th is P a p er a
a
Compound given in boldface type have been structurally characterized. Reagents and conditions: (i) 1.1 equiv of KN(SiMe3)2,
THF, -78 °C and then 1 equiv of tmedPd(CH3)2; (ii) 1.1 equiv of KN(SiMe3)2, THF, -78 °C and then 1 equiv of (1,5-COD)PdCl2;
(iii) CH3I excess in THF; (iv) 1 equiv of H(Et2O){B[(3,5-CF3)2C6H2)]4} at -78 to -30 °C and then 1 equiv of Py; (v) 1 equiv of
H(Et2O){B[(3,5-CF3)2C6H2)]4} at -78 to -30 °C and then 1 equiv of CH3CN or PhCN; (vi) 1 equiv of H(Et2O){B[(3,5-CF3)2C6H2)]4}
at -78 to -30 °C and then 1 equiv of PMe3; (vii) 2 equiv of AgBF4 in CH3CN.
Sch em e 2. Syn th esis of th e Liga n d P r ecu r sor
Im id a zoliu m Sa ltsa
Syn th esis of th e P a lla d iu m Com p lexes. Neu tr a l
Sp ecies. The best method for the introduction of the
new functionalized NHC ligands onto the palladium
center is by trapping of the in situ generated L with a
suitable metal precursor. The generation of carbenes by
deprotonation of the imidazolium precursors with
KN(SiMe3)2 in THF is complete within a few minutes
at -78 °C. Reaction of L with Pd(tmed)(CH3)2 or
Pd(1,5-COD)Br2 gave good yields of complexes 3 and 4.
The isolation and characterization of the free La has
already been communicated,5 and details on its reactiv-
ity together with other N-functionalized NHCs will be
reported in a forthcoming paper. The air-stable, colorless
3a ,b are soluble in THF and CH2Cl2, although the two
solutions in the latter develop a dark coloration on
standing at room temperature for 2-3 h. Complexes
4a ,b are yellow and soluble in THF and CH2Cl2 but
insoluble in CHCl3. They were characterized by spec-
troscopic and analytical methods.
a
Reagents and conditions: (i) Ph2P(dO)(CH2)nBr, 150-160
°C, 4-5 days; (ii) SiHCl3 in chlorobenzene, 120 °C (n ) 2, 3).
quaternization reactions were carried out at 150-160
°C (most conveniently in the melt in the absence of
solvent), they gave good yields. Furthermore, the yield
of the subsequent reduction was also dramatically
improved when using SiHCl3 in refluxing chlorobenzene
as a reducing agent. The synthesis of the 1-(â-(diphen-
ylphosphino)ethyl)-3-mesitylimidazolium salt (LbH)Br
from 1-(â-bromoethyl)-3-mesitylimidazolium and PPh2H
in DMSO in the presence of KOBut has recently been
communicated, although the reported yields were poor.7
All imidazolium phosphines were isolated as white, air-
sensitive powders and characterized by spectroscopic
and analytical methods. The microanalysis data support
the presence of mixed chloride/bromide salts; the chlo-
ride ion originates from the hydrolytic workup of the
excess SiHCl3 during the reduction step. Mixed halide
imidazolium salts and palladium NHC complexes have
been observed before.6a
1
The observed H NMR spectra of the new complexes
are consistent with nonsymmetric structures. The doub-
lets at δ -0.4 to -0.6 in the spectra of 3 are attributable
to inequivalent Pd-CH3 groups. They are almost iso-
chronous with the methyls of Pd(tmed)(CH3)28 but more
shielded relative to the methyls of cis-Pd(P)2(CH3)2 (P
) tertiary phosphine)9 and the recently reported cis-
Pd(CH3)2[(carbene-CH2)2].10 The ethylene bridge protons
appear as two sets of multiplets. In 3a the o-isopropyl
methyls are diastereotopic and appear as two doublets.
Characteristic signals for the carbene carbons are
(8) de Graaf, W.; Boersma, J .; Smeets, W. J . J .; Spek, A. L.; van
Koten, G. Organometallics 1989, 8, 2907.
(9) Tooze, R.; Chiu, K. W.; Wilkinson, G. Polyhedron 1984, 3, 1025.
(10) Douthwaite, R. E.; Green, M. L. H.; Silcock, P. J .; Gomes, P. T.
J . Chem. Soc., Dalton Trans. 2002, 1386.
(7) Yang, C.; Lee, H. M.; Nolan, S. P. Org. Lett. 2001, 3, 1511.

4752 Organometallics, Vol. 22, No. 23, 2003
Tsoureas et al.
F igu r e 2. ORTEP representation of the structure of 4b
showing 50% probability ellipsoids. H atoms are omitted
for clarity. Selected bond lengths (Å) and angles (deg) with
estimated standard deviations: C(1)-Pd(1) ) 1.996(3),
P(1)-Pd(1) ) 2.2461(7), Pd(1)-Br(1) ) 2.4989(3), Pd(1)-
Br(2) ) 2.4708(3), C(1)-N(1) ) 1.349(3), C(1)-N(2) )
1.349(3); N(1)-C(1)-Pd(1) ) 129.9(9), N(2)-C(1)-Pd(1) )
124.8(2), C(1)-Pd(1)-Br(2) ) 176.63(7), C(1)-Pd(1)-P(1)
) 89.24(8), C(1)-Pd(1)-Br(1) ) 90.37(7), P(1)-Pd(1)-
Br(1) ) 168.96(2), P(1)-Pd(1)-Br(2) ) 89.32(2), Br(2)-
Pd(1)-Br(1) ) 91.622(11).
F igu r e 1. ORTEP representation of the structure of 3a
showing 50% probability ellipsoids. H atoms are omitted
for clarity. Selected bond lengths (Å) and angles (deg) with
estimated standard deviations: C(1)-N(1) ) 1.357(6),
C(1)-N(2) ) 1.389(6), C(1)-Pd(1) ) 2.088(5), P(1)-Pd(1)
) 2.2994(13), C(30)-Pd(1) ) 2.098(5), C(31)-Pd(1) )
2.111(5); N(1)-C(1)-Pd(1) ) 124.4(4), N(2)-C(1)-Pd(1) )
131.6(4), C(1)-Pd(1)-P(1) ) 91.06(14), C(30)-Pd(1)-P(1)
) 89.23(15), C(31)-Pd(1)-P(1) ) 172.31(14), C(1)-Pd(1)-
C(31) ) 95.84(18), C(1)-Pd(1)-C(30) ) 177.0(2), C(30)-
Pd(1)-C(31) ) 84.05(19).
palladium colloids and a colorless solid. The NMR
spectrum shows only the existence of 3a in solution,
while formation of ethane or methane was not observed.
Removal of the volatiles, extraction in dichloromethane,
and filtration through Celite resulted in a beige-yellow
solution. The ES+ mass spectrum of this solution in
acetonitrile showed the presence of 2-methyl-[3-(2,6-
diisopropylphenyl)-1-(â-((diphenylphosphino)oxy)ethyl)]-
imidazolium as well as the cationic [(La)PdMe]+ and
[(La)PdMe+MeCN]+ in a 1:1 ratio.
observed as singlets in the range δ 185-187 for 3 and
δ 160-165 for 4.
X-ray-quality crystals of 3a were obtained by layering
a THF solution with ether. The structure of 3a as
determined by single-crystal X-ray diffraction is shown
in Figure 1. The geometry around the palladium is
square planar, and the ligand acts as a chelate occupy-
ing two cis positions. The two Pd-CH3 bond lengths,
2.098(5) and 2.111(5) Å, are equal within the esds. The
Pd-C(carbene) bond length at 2.088(5) Å is comparable
to that recently reported, where the NHC is trans to
an alkyl group.10 The six-membered chelate ring is
puckered, and the ligand bite angle is 91.06(14)°.
It is interesting to note that, due to the fact that the
NHC ligands are better σ-donors than trialkylphos-
phines, they should be exerting a higher trans influence,
which is not evidenced by the structural data of 3a .
However, the reactivity reported below shows that the
two cis methyls are kinetically different.
X-ray-quality crystals of 4b have been obtained by
slow diffusion of ether in CH2Cl2 solution of the complex.
The structure of the molecule, as determined by single-
crystal X-ray diffraction, is shown in Figure 2. The
geometry around the metal is square planar, with the
ligand occupying two cis sites. The Pd-Br bond trans
to the P atom is longer than that trans to the NHC by
ca. 0.03 Å. This observation is in contrast to that
expected on the basis of the relative trans influence of
the phosphine and NHC ligands and is not understood
at present. The Pd-carbene bond length is the same as
that observed in 3a within the observed esds. Finally,
the Pd-P bond length in 3a is longer in 4b, as expected.
The existence of two types of Pd-C bonds (i.e. Pd-
CH3 and Pd-carbene) in 3a , arranged in a way that
one Pd-CH3 bond is flanked by one Pd-CH3 bond and
one Pd-carbene bond, provides an opportunity to test
the preferred decomposition pathway of the complexes
by reductive elimination of either ethane or methylimi-
dazolium. Complexes 3 are stable at room temperature
in the solid state or in solution. Heating a d8-THF
solution of 3a to 60 °C results in the formation of
Attempts were also made to explore the reactivity of
3a with CH3I. It is established that Pd(L-L)(CH3)2
(L-L ) tmed, bipy, phen) react with CH3I to form
Pd(IV) complexes. Some of them have been character-
ized structurally.8,11 The Pd(IV) methyls can reductively
eliminate ethane and form new Pd(II) complexes. Reac-
tion of 3a with excess CH3I in CH2Cl2 at room temper-
ature gave after crystallization from dichloromethane/
ether the product 5a . Its identity was unequivocally
established by single-crystal X-ray diffraction and is
shown in Figure 3. The molecule comprises a square-
planar Pd center coordinated by one imidazolium-func-
tionalized phosphine, two iodides, and one chloride. The
bond lengths and angles are not unusual. We believe
that the source of chloride is chlorinated solvent. A
plausible mechanism for the formation of 5a is given in
Scheme 3. It involves successive Pd(II)-Pd(IV) oxidative
addition of CH3I and reductive elimination of CH3CH3
leading to LaPdI2. The last oxidative addition is followed
by reductive elimination of methylimidazolium cation,
leading to the observed species. Reductive elimination
of methylimidazolium after the first oxidative addition
step is also plausible, leading to iodo dimethyl species,
which can undergo further oxidative addition steps to
the observed product. An alternative pathway involving
Pd(II)-Pd(0) cycles is less likely, on the basis of the high
thermal stability of 3a at room temperature. Attempts
to monitor the reaction by VT NMR spectroscopy (1H
and 31P{1H}) showed that the formation of methylimi-
dazolium (singlet at 2.4 ppm) occurs at room tempera-
(11) Canty, A. J . Acc. Chem. Res. 1992, 25, 83.

Carbene Complexes of Palladium
Organometallics, Vol. 22, No. 23, 2003 4753
and Lb synthesized in this paper could give rise to the
first cations incorporating chelates with nonsymmetric
NHC and “classical” donors and provide useful insight
into the mechanism of their formation and possible
catalytic activity.
Recent reports2a provide experimental and theoretical
evidence that cationic cis-[L2Pd(NHC)(CH3)]+ complexes
undergo facile concerted reductive elimination, resulting
in the formation of alkylimidazolium salts. However,
chelating ligands (i.e. L2 ) dppp) suppress this transfor-
mation.2a The results presented below amplify the fact
that Pd(II) cationic complexes with NHC donors as part
of a chelate ring are resistant to reductive elimination
of alkylimidazolium salts and show that cation forma-
tion occurs with unexpected regiospecificity.
F igu r e 3. ORTEP representation of the structure of the
cation in 5a showing 50% probability ellipsoids. H atoms
are omitted for clarity. Selected bond lengths (Å) and angles
(deg) with estimated standard deviations: Pd(1)-I(1) )
2.6613(7), Pd(1)-P(4) ) 2.2483(18), Pd(1)-I(2) ) 2.6674-
(7), C(1)-C(4) ) 1.470(10), C(1)-C(4) ) 1.470(10), Pd(1)-
I(3) ) 2.360(5), C(1)-N(1) ) 1.353(9), C(2)-C(3) ) 1.335-
(10), C(1)-N(2) ) 1.335(9); I(3)-Pd(1)-I(1) ) 168.10(14),
I(1)-Pd(1)-I(2) ) 92.25(2), I(3)-Pd(1)-I(2) ) 86.37(11),
P(4)-Pd(1)-I(1) ) 90.98(5), P(4)-Pd(1)-I(3) ) 90.07(12).
The interaction of 3a with 1 equiv of (CF3)2CHOH in
CH2Cl2 or CH2Cl2/ether at -78 °C and warming to -20
°C results in the formation of Pd black and unidentified
decomposition products. However, addition to the above
reaction mixture of 1 equiv of pyridine at -30 °C,
warming to room temperature, and workup gave a
1
white, air-stable solid. The H NMR spectrum of this
solid shows the presence of one methyl attached to the
Pd (doublet at δ -0.3), in addition to pyridine and reso-
nances assignable to the La and the anion [(CF3)2CHO-].
The appearance of one single Pd-CH3 peak and one
signal in the 31P NMR spectrum implies the presence
of one isomer in solution. An identical spectrum was
obtained by replacing (CF3)2CHOH with H(Et2O){B[(3,5-
CF3)2C6H3)]4}, in which case 6a was isolated and
crystallized by slow evaporation of the ether solution.
Its structure in the solid state was determined by a
single-crystal X-ray diffraction study. A diagram of the
molecule is shown in Figure 4.
Sch em e 3. P r op osed Mech a n ism for th e
F or m a tion of 5a
The geometry around the Pd center in the cation is
square planar; its coordination sphere comprises the
ligand La , one methyl group, and one pyridine ligand.
The methyl group is trans to the NHC functionality. The
Pd-carbene and Pd-CH3 bond lengths and ligand bite
angle compare with the corresponding values observed
in 3a .
Monitoring the reaction of 3a with (CF3)2CHOH by
1H NMR spectroscopy provides evidence that the only
one isomer is formed under kinetic control and, and this
is the isomer characterized crystallographically. The
reason for the selective abstraction of the palladium
methyl group trans to the phosphorus, in the absence
of any apparent trans influence as established by the
crystal structure of 3a , could lie in the different steric
demands of the two electronically dissimilar parts of La .
The steric bulk imposed by the phosphine end could
hinder the coordination of a donor ligand, thus making
the attack to the methyl trans to it more kinetically
favored.
ture overnight and is accompanied by disappearance of
the two Pd - Me doublets and the formation of ethane.
However, a complex reaction mixture prevented the full
identification of all products from the reaction.
Ca tion ic Sp ecies. Square-planar cationic complexes
of palladium and nickel stabilized by bidentate chelating
ligands have received much attention, due to their
involvement in catalytic C-C bond-forming reac-
tions.10,12 The most studied ligands include diphosphines
and diimines, although in recently published reports
cationic complexes with chelating dicarbenes10 have
been observed spectroscopically and in some cases
structurally characterized.13 Furthermore, mixed donor
nonsymmetric chelating neutral or anionic ligands with
N-O, P-P′, P-N, and P-O donor atoms have also been
studied.14 We thought that the mixed donor chelates La
Complex 6a is stable in the solid state and in solution.
On prolonged standing at room temperature, it develops
a yellow coloration, even though the compound is
unchanged spectroscopically. Its stability could possibly
(12) For a recent review on late-transition-metal cationic complexes
as alkene polymerization catalysts see: Ittel, S. D.; J ohnson, L. K.;
Brookhart, M. Chem. Rev. 2000, 100, 1169.
(13) Gardiner, M. A.; Herrmann, W. A.; Reisinger, C.-P.; Schwarz,
J .; Spiegler, M. J . Organomet. Chem. 1999, 572, 239.
(14) For recent work in this area see: Liu, W.; Malinowski, J . M.;
Brookhart, M. Organometallics 2002, 21, 2836 and references therein.

4754 Organometallics, Vol. 22, No. 23, 2003
Tsoureas et al.
F igu r e 5. ORTEP representation of the structure of the
cation in 7a showing 50% probability ellipsoids. The BArf
F igu r e 4. ORTEP representation of the structure of the
cation in 6a showing 50% probability ellipsoids. The
asymmetric unit consists of two cations and two anions and
4
anion and H atoms are omitted for clarity. Selected bond
lengths (Å) and angles (deg) with estimated standard
deviations: C(1)-Pd(01) ) 2.090(4), C(6)-Pd(01) ) 2.070-
(4), N(3)-Pd(01) ) 2.091(3), P(1)-Pd(01) ) 2.2043(9),
C(1)-N(1) ) 1.369(4), C(1)-N(2) ) 1.358(4), C(7)-N(3) )
1.139(5), C(7)-C(8) ) 1.464(5); C(1)-Pd(01)-C(6) ) 85.68-
(14), C(1)-Pd(01)-P(1) ) 93.56(9), P(1)-Pd(01)-N(3) )
169.48(9), N(3)-C(7)-C(8) ) 179.4(4), P(1)-Pd(01)-C(6)
) 84.32(11), N(2)-C(1)-N(1) ) 103.1(3).
two molecules of ether. The BArf anion, the ether mol-
4
ecules, and the H atoms are omitted for clarity. Selected
bond lengths (Å) and angles (deg) with estimated standard
deviations: C(36)-Pd(1) ) 2.092(6), C(36)-N(4) ) 1.356-
(6), C(65)-Pd(1) ) 2.099(6), N(6)-Pd(1) ) 2.128(5), C(36)-
N(5) ) 1.375(6), P(1)-Pd(1) ) 2.2221(18); N(5)-C(36)-
Pd(1) ) 129.0(4), N(6)-Pd(1)-P(1) ) 171.75(13), C(36)-
Pd(1)-P(1) ) 89.86(17), C(36)-Pd(1)-C(65) ) 173.9(2),
C(65)-Pd(1)-P(1) ) 89.12(17), N(4)-C(36)-Pd(1) ) 125.7-
(4), C(65)-Pd(1)-N(6) ) 84.13(17).
and reduced thermal stability complex 9a was charac-
terized by multinuclear NMR spectroscopy, which sup-
ports the structure proposed in Scheme 1. Attempts to
crystallize this complex were unsuccessful.
arise from the trans arrangement of the methyl and the
NHC groups, rendering methyl migration to the NHC
carbon or reductive elimination of methylimidazolium
salts unfavorable. Thermal decomposition of 6a occurs
at 90 °C to colloidal Pd and intractable mixtures of
Ca ta lytic Stu d ies. Heck Cou p lin g Rea ction s. It
has been claimed that the imidazolium salts LbH in the
presence of Pd(dba)2 and KOBut or Cs2CO3 are highly
active catalysts for the Heck coupling of arylbromides
with acrylates.7 Our studies using 3a ,b and 4b as
catalysts are given in Table 1. All data given in Table 1
are the average of at least two runs. From the informa-
tion given, it is clear that 3a ,b show good activity, albeit
not as high as previously claimed for catalysts formed
in situ. In addition, after the completion of the reaction
the formation of Pd colloids was evident in the reaction
mixture. Attempts to reproduce the activity reported in
the literature under identical conditions met also with
limited success in our hands.
1
organic products (by H NMR).
Complexes 7a , 8a , and 9a were prepared in a way
analogous to 6a using acetonitrile, benzonitrile, and
trimethylphosphine instead of pyridine, respectively. All
1
three complexes are colorless, air-stable solids. The H
NMR spectra show that in these cations as well, the
methyl groups are located trans to the NHC. There is
no evidence of the presence of any other isomers in
solution. This was further established by a single-crystal
X-ray diffraction study of 7a . A diagram of the molecule
is shown in Figure 5. Here, too, the Pd center in the
cation is square planar with chelating La , one methyl
group trans to the NHC end, and one acetonitrile ligand
trans to the phosphine. The Pd-carbene and Pd-CH3
bond lengths and ligand bite angle compare with those
observed in 6a . Complex 7a is stable in d3-acetonitrile
up to 65 °C; the only process that can be detected
involves exchange of coordinated and free acetonitrile.
Dications are also accessible by the reaction of 4 with
AgBF4 in MeCN. Due to their air and light sensitivity
Cop olym er iza tion of Eth ylen e a n d CO. A dichlo-
romethane solution of complex 3a when pressurized at
5 bar with a mixture of CO and ethylene (1:1) at room
temperature, in the presence of Brookhart’s acid, yielded
the desired copolymer over a period of 2 h.
Con clu sion s
The high-yield synthesis of chelate (diphenylphosphi-
no)ethyl carbene complexes of palladium(II) established
Ta ble 1. Rep r esen ta tive Ca ta lytic Da ta for th e Heck Cou p lin g of Ar yl Ha lid es w ith Meth yl Acr yla te in th e
P r esen ce of th e P a lla d iu m Com p lexes Rep or ted in Th is P a p er
catalyst
aryl halide
base
solvent
time (h)
temp (°C)
loading (M)
yield (%)
TON
p-C(O)MePhBr
PhBr
p-C(O)MePhBr
p-C(O)MePhBr
PhBra
NEt3
Cs2CO3
NEt3
Cs2CO3
NEt3
NEt3
NMP
DMAC
NMP
NMP
NMP
NMP
NMP
NMP
6
6
6
6
12
6
120
120
120
120
140
120
140
120
2 × 10-4 (3a )
2.5 × 10-4 (3a )
2.5 × 10-4 (3b)
2.5 × 10-4 (3b)
5 × 10-5 (3b)
2.5 × 10-4 (3b)
5 × 10-5 (3b)
2.5 × 10-4 (3b)
92
60
66
54
19
22
22
14
184
122
134
110
1920
44
PhBr
PhBrc
NEt3
NEt3
12
6
2242
28
PhBrb
a
b
Alkene is n-butyl acrylate; 2 mL of [2.2.1]-1,5-norbonadiene was added. In the presence of 1.1 equiv of N2H4‚H2O. c Alkene is n-butyl
acrylate.

Carbene Complexes of Palladium
Organometallics, Vol. 22, No. 23, 2003 4755
aromatic), 10.3 (s, 1H, NC(H)N proton); 13C{1H}, δ 24.41 (s,
CH(CH3)2), 24.47 (s, CH(CH3)2), 28.7 (s, Ph2P(O)CH2CH2-
imidazole), 44.6 (s, Ph2P(O)CH2CH2-imidazole), 123.9 (s,
imidazole backbone), 124.8 (s, aromatic), 125 (s, imidazole
backbone), 129.2 (d, J PC ) 12.08 Hz, aromatic), 130.2 (s,
aromatic), 130.9 (d, J PC ) 9.81 Hz), 132 (s, aromatic), 132.1
(s, aromatic), 132.5 (s, aromatic), 138.4 (s, ipso at imidazole
moiety), 145.4 (s, NCN); 31P{1H}, δ 33.4 ppm (s, Ph2P(O)-
(CH2)2-imidazole). Anal. Found: C, 61.93; H, 6.14; N, 5.12.
Calcd for C29H34BrN2OP‚H2O: C, 62.70; H, 6.63; N, 5.04.
3-Me sit yl-1-(â-((d ip h e n ylp h osp h in o)oxy)e t h yl)im i-
d a zoliu m Br om id e (1b). This was prepared by a method
analogous to 1a from 10 g (32.5 mmol) of Ph2P(O)(CH2)2Br
and 6.1 g (32.6 mmol) of mesitylimidazole. Yield: 13.4 g, 83%.
NMR (CDCl3): 1H, δ 1.8 (s, 6H, o-methyls of mesityl), 2.7 (s,
3H, p-methyl of mesityl), 3.4 (m, 2H, Ph2P(O)CH2CH2-
imidazole), 4.8 (m, 2H, Ph2P(O)CH2CH2-imidazole), 6.6 (s, 1H,
imidazole backbone), 6.9 (s, 1H, imidazole backbone), 7.2-7.5
(m, 5H, aromatics), 7.7-7.9 (m, 4H, aromatics), 8.4 (s, 1H,
aromatic), 10.1 (s, 1H, NCN proton); 13C{1H}, δ 17.8 (s,
o-methyls of mesityl), 21.2 (s, p-methyl of mesityl), 44.5 (s,
Ph2P(O)CH2CH2-imidazole), 122.8 (s, imidazole backbone),
124.8 (s, imidazole backbone), 129.1 (d, J PC ) 12.83 Hz,
aromatic), 129.8 (s, aromatic), 131 (d, J PC ) 18.12 Hz,
aromatic), 131.4 (s, aromatic), 132.2 (s, aromatic), 134.7 (s,
aromatic), 138.3 (s, ipso to the imidazole moiety), 141.3 (s,
NCN); 31P{1H}, δ 31.0 (s, Ph2P(O)(CH2)2-imidazole). Anal.
Found: C, 62.90; H, 5.70; N, 5.64. Calcd for C26H28BrN2OP:
C, 63.04; H, 5.70; N, 5.64.
a new family of well-defined, electronically nonsymmet-
ric, square-planar complexes. The thermally stable
dimethyl analogues served as useful starting materials
for the synthesis of cationic species by their reaction
with Lewis acids containing noncoordinating anions. In
all studied cases only one cationic isomer in which the
methyl is trans to the carbene is spectroscopically
observed and isolated. The reasons for this selectivity
are not well understood.
Furthermore, the neutral dimethyl complexes showed
moderate catalytic activity in the Heck coupling of aryl
halides with acrylates. This activity is lower than that
previously reported for systems generated in situ from
Pd(dba)2 and the (diphenylphosphino)ethyl imidazol-
ium salts in the presence of Cs2CO3. The synthesis of
well-defined chelate (diphenylphosphino)ethyl carbene
Pd(0) complexes, which is under way, may provide an
explanation for the reduced catalytic activity observed
with the Pd(II) complexes.
Exp er im en ta l Section
Gen er a l Meth od s. Elemental analyses were carried out
by the University College, London, microanalytical laboratory.
All manipulations were performed under nitrogen in a Braun
glovebox or using standard Schlenk techniques, unless stated
otherwise. Solvents were dried using standard methods and
distilled under nitrogen prior to use. The light petroleum used
throughout had a boiling point of 40-60 °C.
3-Mesityl-1-(γ-((d ip h en ylp h osp h in o)oxy)-n -p r op yl)im i-
d a zoliu m Br om id e (1c). This was prepared by following a
method analogous to that for 1a from 4.34 g (13.5 mmol) of
Ph2P(O)(CH2)3Br and 2.80 g (15 mmol) of mesitylimidazole.
Yield: 6.09 g (89%). NMR (CDCl3), 1H, δ 1.9 (s, 6H, o-methyls
of mesityl), 2,4 (s, 3H, p-methyl of mesityl), 2.3-2.5 (m, 2H,
Ph2P(O)CH2(CH2)2-imidazole), 2.6-2.8 (m, 2H, Ph2P(O)-
CH2CH2CH2-imidazole), 5.0 (t, 2H, Ph2P(O)(CH2)2CH2-imi-
dazole), 6.9 (s, 2H, aromatics of mesityl), 7.1 (s, 1H, imidazole
backbone), 7.3 (s, 1H, imidazole backbone), 7.5 (br s, 5H,
aromatics of phosphine oxide), 7.7-7.9 (m, 4H, aromatic), 8.2
(s, 1H, aromatic), 10.4 (s, 1H, NC(H)N proton); 13C, δ 17.9 (s,
o-methyls of mesityl), 21 (s, p-methyl of mesityl), 24.6 (d,
The starting materials 1-(2,6-diisopropylphenyl)imidazole,
1-(2,4,6-trimethylphenyl)imidazole,15 Ph2POEt,16 Ph2P(O)-
(CH2)2Br,16 Pd(tmed)Me2,8 and Pd(COD)Br2 were prepared
17
according to literature procedures. Ph2P(O)(CH2)3Br was pre-
pared from Ph2POEt and 1,3-C3H6Br2 by a method analogous
to that for Ph2P(O)(CH2)2Br and was recrystallized from ben-
zene/petroleum. NMR data were recorded on Bruker AMX-
300 and DPX-400 spectrometers, operating at 300 and 400
MHz (1H), respectively. The spectra were referenced internally
using the signal from the residual protio solvent (1H) or the
signals of the solvent (13C). 31P{1H} spectra were recorded on
a Bruker AC-300 spectrometer at 121.44 MHz and referenced
externally relative to 85% H3PO4 in D2O. 19F spectra were
recorded on a Bruker AC-300 instrument at 282.36 MHz and
were referenced externally relative to C6F6. The phosphine ox-
ide-imidazolium salts are hygroscopic, while the ligand pre-
cursors 2a -c are mixtures of the bromide and chloride salts.
3-(2,6-Diisop r op ylp h en yl)-1-(â-((d ip h en ylp h osp h in o)-
oxy)eth yl)im id a zoliu m Br om id e (1a ). A 9.7 g (31.5 mmol)
portion of Ph2P(O)(CH2)2Br and 7.2 g (31.6 mmol) of (2,6-
diisopropylphenyl)imidazole were placed in a glass ampule,
which was then sealed under vacuum and immersed in an oil
bath heated at 150-160 °C for 1 week. After the ampule was
opened, the brown solid was dissolved in dichloromethane and
the volatiles were removed under reduced pressure to yield a
beige solid, which was washed with ether (2 × 50 mL). It was
then redissolved in the minimum amount of dichloromethane
and reprecipitated by addition of ether, giving the product as
a white powder. Yield: 15.2 g (90%). NMR (CDCl3): 1H, δ 0.9
1
J PC ) 11.57 Hz, Ph2P(O)CH2CH2CH2-imidazole), 27.0 (s,
Ph2PCH2CH2CH2-imidazole), 50.8 (s, Ph2PCH2CH2CH2-imi-
dazole), 122.5 (s, imidazole backbone), 123.9 (s, imidazole
backbone), 127.5 (d, J PC ) 6.63 Hz, aromatic), 128.3 (s,
aromatic), 129.1 (s, aromatic), 131.3 (d, J PC ) 17.34 Hz,
aromatic), 131.6 (s, aromatic), 132.9 (s, aromatic), 133.6 (s,
aromatic), 138.8 (s, ipso to the imidazole moiety), 142.3 (s,
NCN); 31P{1H}, δ 33.8 (s, Ph2P(O)(CH2)3-imidazole).
3-(2,6-Diisop r op ylp h en yl)-1-(â-(d ip h en ylp h osp h in o)et-
h yl)im id a zoliu m Br om id e (2a ; La H). The phosphine oxide
1a (4.5 g, 8.4 mmol) was placed in a three-neck 1 L flask
(Schlenk adapter, double-jacket condenser, and a pressure-
equalizing funnel) and was dissolved in 110 mL of chloroben-
zene by heating to the reflux temperature under a nitrogen
atmosphere. The temperature was then maintained at 120 °C
(oil bath), and excess trichlorosilane was added in small
portions (14 mL, 100.7 mmol, 12-fold excess). After completion
of the addition, the reaction mixture was heated at 120 °C for
3 h and then cooled to room temperature. After addition of
100 mL of CH2Cl2 the excess of trichlorosilane was quenched
by careful dropwise addition of 300 mL of degassed 10% NaOH
(aqueous) at 0 °C. The organic phase was separated, and the
aqueous phase was washed with CH2Cl2 (3 × 50 mL). After
the combined organic extracts were dried over MgSO4, the
volatiles were removed under reduced pressure and the
resulting white solid was washed with ether (2 × 150 mL) and
dried under vacuum overnight. The imidazolium salt was
further dried azeotropically with toluene and was stored in
the glovebox. Yield: 3.3 g (75%). NMR (CDCl3): 1H, δ 1.1 (d,
3
3
(d, J HH ) 6.9 Hz, 6H, CH(CH3)2), 1.1 (d, J HH ) 6.9 Hz, 6H,
CH(CH3)2), 2.1 (septet, 2H, 3J HH ) 6.9 Hz, CH(CH3)2), 3.4 (2H,
m, Ph2P(O)CH2CH2-imidazole), 5.1 (m, 2H, Ph2P(O)CH2CH2-
imidazole), 6.9 (s, 1H, imidazole backbone), 7.1-7.2 (m, 3H,
aromatics and the other imidazole backbone), 7.4-7.7 (m,
8.5H, aromatics), 7.8-8.0 (m, 4.5H, aromatics), 8.7 (s, 1H,
(15) (a) J ohnson, A. L.; Del, W. U.S. Patent 3,637,731, 1972; Chem.
Abstr. 1972, 76, 113215. (b) Arduengo, A. G., III. U.S. Patent 6,177,-
575, 2001; Chem. Abstr. 2001, 134, 115958.
(16) Struck, R. F.; Sheally, Y. F. J . Org. Chem. 1965, 30, 414.
(17) Drew, D.; Doyle, J . R. Inorg Synth. 1972, 13, 53.

4756 Organometallics, Vol. 22, No. 23, 2003
Tsoureas et al.
6H, CH(CH3)2), 1.2 (d, 6H, CH(CH3)2), 2.4 (sept, 2H, CH(CH3)2),
2.9 (m, 2H, PPh2CH2CH2-imidazole), 4.9 (m, 2H, PPh2-
CH2CH2-imidazole), 6.9 (s, 1H, imidazole backbone), 7.1-7.8
(m, 13H, aromatics), 8.0 (s, 1H, aromatic), 10.4 (s, 1H, NCN
proton); 13C{1H}, δ 24.5 (s, CH(CH3)2), 28.8 (s, CH(CH3)2), 30.0
(d, J P-C ) 12.98 Hz, PPh2CH2CH2-imidazole), 48.2 (d, J P-C
) 18.12 Hz, PPh2CH2CH2-imidazole), 123.8 (s, imidazole
backbone), 124.0 (s, imidazole backbone), 124.8 (s, aromatic),
129.05 (d, J P-C ) 6.78 Hz, aromatic), 130.3 (s, aromatic), 130.8
(s, aromatic), 132 (s, aromatic), 132.9 (d, J P-C ) 19.55 Hz,
aromatic), 136.2 (d, J P-C ) 11.32 Hz, aromatic), 138.6 (s, ipso
to imidazole moiety, aromatic), 145.5 (s, NCN); 31P{1H}, δ
-22.4 (s, Ph2P(CH2)3-imidazole). Anal. Found: C, 69.07; H,
6.63; N, 5.08. Calcd for C58H68BrClN4P2 (1:1 bromide/chloride
salt): C, 69.77; H, 6.86; N, 5.61.
3-Me sit yl-1-(â-(d ip h e n ylp h osp h in o)e t h yl)im id a zo-
liu m Br om id e, LbH (2b). This was prepared by a method
analogous to 2a from 5 g (10.1 mmol) of the phosphine oxide
and 18 mL of trichlorosilane. Yield: 3.6 g, 75%. NMR
(CDCl3): 1H, δ 2.1 ppm (s, 6H, o-methyls of mesityl), 2.4 ppm
(s, 3H, p-methyl of mesityl), 2.9 (m, 2H, Ph2PCH2CH2-
imidazole), 4.9 (m, 2H, Ph2PCH2CH2-imidazole), 6.7 (s, 1H,
imidazole backbone), 6.9 (s, 1H, imidazole backbone), 7.0
(aromatics of mesityl), 7.5 (5H, br, aromatics), 7.6 (br, 4H,
aromatic), 7.9 (s, 1H, aromatic) 10.6 (s, 1H, NCN proton);
13C{1H}, δ 17.8 (s, o-methyls of mesityl), 21.2 (s p-methyl of
mesityl), 29.8 (d, 1J PC ) 15.99 Hz, Ph2PCH2CH2-N), 48.15 (d,
2J PC ) 21.89 Hz, Ph2PCH2CH2-imidazole), 123 (s, imidazole
backbone), 123.5 (s, imidazole backbone), 129 (d, J PC ) 6.80
Hz, aromatic), 129.15 (d, J PC ) 8.31 Hz, aromatic), 129.9 (s,
aromatic), 130.8 (s, aromatic), 132.9 (d, J PC ) 18.87 Hz,
aromatic), 134.4 (s, aromatic), 136.3 (d, J PC ) 11.32 Hz,
aromatic), 138.5 (s, ipso to the imidazole), 141.3 (s, NCN);
31P{1H}, δ -22.4 (s, Ph2P(CH2)2-imidazole). Anal. Found: C,
63.57; H, 5.69; N, 5.31. Calcd for C52H56BrClN4P2‚CH2Cl2 (1:1
bromide/chloride salt): C, 63.70; H, 5.85; N, 5.61.
[La ]P d Me2 (3a ). This was prepared according to the general
method using 0.11 g (0.44 mmol) of Pd(tmed)Me2, 0.23 g (0.46
mmol) of La H, and 0.10 g of KN(SiMe3)2 (0.50 mmol). Yield:
0.18 g (70%). X-ray-quality crystals were grown by layering a
3
THF solution with Et2O. NMR (CD2Cl2): 1H, δ -0.6 (d, J PH
1
2
3
) 7.15 Hz, 3H, PdMe), -0.4 (d, J PH ) 8.24 Hz, 3H, PdMe),
3
3
1.0 (d, J HH ) 6.7 Hz, 6H, CH(CH3)2), 1.1 (d, J HH ) 6.7 Hz,
6H, CH(CH3)2), 2.3 (m, 2H, (PPh2CH2CH2-ylidene)PdMe2), 2.7
3
(sept. J HH ) 6.7 Hz, 2H, CH(CH3)2), 4.3 and 4.4 (each br dt,
1H, (PPh2CH2CH2-ylidene)PdMe2), 6.8 (d, 1H, ylidene back-
bone), 7.0 (d, 1H, ylidene backbone), 7.1 and 7.2 (each s, 1H,
aromatics), 7.3-7.4 (m, 7H, aromatics), 7.6-7.7 (m, 4H,
aromatics); 13C{1H} (C5D5N), δ 0.05 (d, 2J PC ) 9.96 Hz, PdCH3),
2
2.4 (d, J PC ) 9.34 Hz, PdCH3), 24.4 (s, CH(CH3)2), 26 (s,
CH(CH3)2), 29.7 (s, CH(CH3)2), 30.3 (d, (PPh2CH2CH2-ylidene)-
1
PdMe2, J C-P ) 14.61 Hz), 49.35 (d, (PPh2CH2CH2-ylidene)-
2
PdMe2, J C-P ) 10.62 Hz), 121 (s, imidazole backbone), 125.2
(s, aromatic), 125.4 (s, aromatic), 129.69 (d, J PC ) 9.06 Hz,
aromatic), 130.34 (d, J PC ) 7.04 Hz, aromatic), 130.8 (s,
aromatic), 134.74 (s, aromatic), 134.83 (d, J PC ) 5.03 Hz,
aromatic), 147.15 (ipso carbon of aryl), 187.77 (s, NCN).
31P{1H} (CD2Cl2), δ 12.45 (s, (PPh2CH2CH2-ylidene)PdMe2). MS
(ES+): m/z 561 [(P-C)PdMe]+, 602 [M + MeCN]+. Mp: 154-
156 °C dec. Anal. Found: C, 64.50; H, 6.77; N, 4.81. Calcd for
C
31H39N2PPd: C, 64.52; H, 6.81; N, 4.85.
[Lb]P d Me2 (3b). This was prepared according to the general
synthetic method from 0.10 g (0.44 mmol) of Pd(tmed)Me2, 0.24
g (0.45 mmol) of LbH, and 0.09 g (0.46 mmol) of KN(SiMe3)2.
3
Yield: 0.16 g (75%). NMR (CD2Cl2): 1H, δ -0.6 (d, J P-H
)
3
7.68 Hz, 3H, PdCH3), -0.5 (d, J P-H ) 6.50 Hz, 3H, PdCH3),
2.0 (s, 6H, 2,6-methyls of mesityl), 2.1 (s br, 2H, (PPh2CH2-
CH2-ylidene)PdMe2), 2.3 (s, 3H, 4-methyl of mesityl), 4.4 and
4.5 (two sets of dt 1H each, (PPh2CH2CH2-ylidene)PdMe2), 6.7
3
(d, J PH ) 7.68 Hz, 1H, ylidene backbone), 6.9 (s. br., 2H,
3
aromatics of mesityl), 7.0 (d, J PH ) 7.68 Hz, 1H, ylidene
backbone), 7.4 (m, 5H, aromatics), 7.5-7.7 (m, 5H, aromatics);
13C{1H} (C5D5N), δ 0.04 (d, J PC ) 9.87 Hz, PdCH3), 1.9 (d,
2
3-Mesityl-1-(γ-(d ip h en ylp h osp h in o)-n -p r op yl)im id a zo-
liu m Br om id e (2c). This compound was prepared by following
a method analogous to that for 2a from 4.5 g (8.8 mmol) of
the oxide. Yield: 3.30 g (75%). NMR (CDCl3): 1H, δ 2.0 (s, 6H,
o-methyls of mesityl), 2.1-2.3 (m, 4H, Ph2P(O)CH2(CH2)2-
imidazole and Ph2P(O)CH2CH2CH2-imidazole), 2.4 (s, 3H,
p-methyl of mesityl), 5 (t, 2H, Ph2P(O)(CH2)2CH2-imidazole),
6.9 (s br, 2H, aromatics of mesityl), 7.1 (s br, 2H, imidazole
backbone), 7.4-7.7 (m, 10H, aromatics of phosphine), 10.9 (s,
1H, NCN proton); 13C{1H}, δ 17.8 (s, o-methyls of mesityl), 21.2
2J PC ) 9.26 Hz, PdCH3), 18.1 (s, o-methyls of mesityl), 22.2 (s,
1
p-methyl of mesityl), 31.1 (d, J PC ) 14.22 Hz, (PPh2CH2CH2-
ylidene)PdMe2), 49.2 (d, J PC ) 10.31 Hz, (PPh2CH2CH2-
2
ylidene)PdMe2), 120 (s, imidazole backbone), 120.5 (s, imida-
zole backbone), 122.5 (s, aromatic), 123.8 (d, J PC ) 8.05 Hz,
aromatic), 124.2 (s, aromatic), 124.7 (s, aromatic), 126.20 (d,
J PC ) 9.06 Hz, aromatic), 126.38 (d, J PC ) 5.03 Hz, aromatic),
128.6 (s, aromatic), 148.5 (s, ipso carbon of the aryl), 185.8 (s,
NCN); 31P{1H}, δ 12.24 (s, (PPh2CH2CH2-ylidene)PdMe2).
MS (ES+): m/z 519 [LbPdMe]+ (M), 560 [M + MeCN]+. Mp:
133-134 dec. Anal. Found: C, 57.33; H, 5.73; N, 4.49. Calcd
for C28H34N2PPd‚CH2Cl2: C, 56.10; H, 5.84; N, 4.51.
1
(s, p-methyls of mesityl), 24.4 (d, J PC ) 10.56 Hz, Ph2PCH2-
2
CH2CH2-imidazole), 27.1 (d, J PC ) 18.09 Hz, Ph2PCH2CH2-
CH2-imidazole), 50.6 (d, 3J PC ) 19.06 Hz, Ph2PCH2CH2CH2-
imidazole), 122.3 (s, imidazole backbone), 123.3 (s, imidazole
backbone), 128.8 (d, J PC ) 6.78 Hz, aromatic), 129.1 (s,
aromatic), 130 (s, aromatic), 130.8 (s, aromatic), 132.9 (d, J PC
) 19.22 Hz, aromatic), 134.3 (s, aromatic), 137.6 (d, J PC ) 12.44
Hz, aromatic), 139.1 (s, ipso to the imidazole moiety), 141.5
(s, NCN); 31P{1H}, δ -16.6 (s, Ph2P(CH2)3-imidazole) Anal.
Found: C, 68.66; H, 6.28; N, 6.39. Calcd for C54H60BrClN4P2
(1:1 bromide/chloride salt): C, 68.82; H, 6.42; N, 5.95.
P r ep a r a t ion of t h e P a lla d iu m Com p lexes. Gen er a l
Meth od . To a mixture of solid imidazolium salt (1 equiv) and
KN(Si(CH3)3)2 (1.1 equiv) was added precooled (-78 °C) THF
with vigorous stirring. The solution of the free carbene
generated in this way was added after 10 min via cannula to
a solution of palladium precursor in THF at -78 °C. After the
mixture was stirred at -78 °C for 10 min, it was allowed to
reach room temperature and stirred for 2 h. Evaporation of
the volatiles under reduced pressure, extraction of the solid
residue into cold (0 °C) dichloromethane, filtration of the cold
solution through Celite, concentration of the filtrates to 1-3
mL, and addition of ether precipitated the product as a white
to beige solid, which was collected by filtration and dried under
vacuum.
[La ]P d Br 2 (4a ). This compound was prepared by following
the general method from 0.10 g of Pd(COD)Br2 (0.27 mmol),
0.14 g (0.28 mmol) of La H, and 0.06 g (0.30 mmol) of
KN(SiMe3)2. Yield: 0.14 g (70%). NMR (CD2Cl2): 1H, δ -0.4
3
3
(d, J HH ) 6.8 Hz, 6H, CH(CH3)2), 1.3 (d, J HH ) 6.8 Hz, 6H,
CH(CH3)2), 1.9 (m, 2H, (PPh2CH2CH2-ylidene)PdBr2), 2.1
(sept, 3J HH ) 6.8 Hz, 2H, CH(CH3)2), 4.3 and 4.4 (each dt, 1H,
(PPh2CH2CH2-ylidene)PdBr2), 6.9 (s, 1H, ylidene backbone),
7.4-7.7 (m, 11H, aromatics), 7.7-7.8 (m, 3H, aromatics); 13C-
{1H}, δ 20.0 (s, CH(CH3)2), 23.5 CH(CH3)2), 26.2 (s, CH(CH3)2),
29.8 (s, (PPh2CH2CH2-ylidene)PdBr2), 48.0 (s, (PPh2CH2CH2-
ylidene)PdBr2), 122.2 (s, ylidene backbone), 124.6 (s, ylidene
backbone), 127.1 (s, aromatic), 129.35 (d, J PC ) 10.69 Hz,
aromatic), 130.3 (s, aromatic), 131.5 (s, aromatic), 132.13 (d,
J PC ) 2.92 Hz, aromatic), 134.38 (d, J PC ) 9.72 Hz, aromatic),
136.8 (s, aromatic), 146.1 (s, ipso carbon of the aryl group),
160.8 (NCN); 31P{1H}, δ 21 (s, (PPh2CH2CH2-ylidene)PdBr2)
Anal. Found: C, 50.88; H, 4.94; N, 3.55. Calcd for C29H34Br2N2-
PPd‚Et2O: C, 50.69; H, 5.67; N, 3.58.
[Lb]P d Br 2 (4b). This was prepared by following the general
method from 0.10 g (0.27 mmol) of Pd(COD)Br2, 0.13 g (0.28
mmol) of LbH and 0.06 g (0.30 mmol) of KN(SiMe3)2. Yield:

Carbene Complexes of Palladium
Organometallics, Vol. 22, No. 23, 2003 4757
0.12 g (69%). X-ray-quality crystals were grown in an NMR
tube by slow diffusion of Et2O into a CH2Cl2 solution. NMR
(CD2Cl2): 1H, δ 0.9 (s, 6H, o-methyls), 1.3 (s, 3H, p-methyl),
1.8 (m, 2H, (PPh2CH2CH2-ylidene)PdBr2), 4.3 and 4.4 (each
dt, 1H, (PPh2CH2CH2-ylidene)PdBr2), 6.8 (d, 1H, imida-
zolylidene backbone), 7.1 (s br, 1H, imidazolylidine backbone),
7.2 (s, 2H, mesityl aromatic protons), 7.2-7.7 (m, 10H,
aromatics); 13C{1H}, δ 17.9 (s, o-methyls of mesityl), 21.8 (s,
p-methyl of mesityl), 30.1 (s, (PPh2CH2CH2-ylidene)PdBr2),
48.0 (s, (PPh2CH2CH2-ylidene)PdBr2), 121.0 (s, ylidene back-
bone), 124.4 (s, ylidene backbone), 127 (s, aromatic), 128.85
(d, J PC ) 9.06 Hz, aromatic), 130.3 (s, aromatic), 132.3 (s,
aromatic), 133.5 (s, aromatic), 134.15 (d, J PC ) 3.02 Hz,
aromatic), 135.03 (d, J PC ) 12.08 Hz, aromatic), 138.2 (s,
aromatic), 147.1 (s, ipso carbon of the aryl group attached to
the ylidene backbone), 162.4 (NCN); 31P{1H}, δ 19 (s, (PPh2-
CH2CH2-ylidene)PdBr2). Anal. Found: C, 44.24; H, 3.93; N,
3.85. Calcd for C26H28Br2N2PPd‚CH2Cl2: C, 43.20; H, 4.03; N,
3.73.
P r ep a r a t ion of [La P d (CH3)(C6H 5CN)]{B[(3,5-CF 3)2-
C6H2)]4} (8a ). This was prepared by following the above
general method from 3a (0.06 g, 0.1 mmol), H(Et2O){B[(3,5-
CF3)2C6H2)]4} (0.09 g, 0.1 mmol), and C6H5CN (10 µL, 0.1
3
mmol). Yield: 0.14 g, 88%. NMR (CD2Cl2): 1H, δ 0.0 (d, J PH
3
) 2.73 Hz, 3H, PdMe), 1.1 (d, J HH ) 7.29 Hz, 6H, CH(CH3)2),
3
1.3 (d, J HH ) 6.38 Hz, 6H, CH(CH3)2), 2.7 (m, 4H, CH(CH3)2
and [Ph2PCH2CH2-ylidene]PdMe(NCPh)+), 4.1 and 4.2 (m, 1H
each, [Ph2PCH2CH2-ylidene]PdMe(NCPh)+), 6.9 (s, 1H, ylidene
backbone), 7.1 (s, 1H, ylidene backbone), 7.2 (s, 4H, aromatic),
7.3 (broad s, 3H, aromatic), 7.4 (d, 2H, aromatic), 7.5-7.6 (m,
9H, aromatic), 7.7-7.9 (m, 12H, aromatic); 31P{1H}, δ 36.5
[Ph2PCH2CH2-ylidene]PdMe(NCPh)+); 19F, δ 100.2 (s, B[(3,5-
CF3)2C6H3]4-). 13C{1H}, δ 4.9 (s, PdCH3), 24.4 (s, CH(CH3)2),
1
25.2 (s, CH(CH3)2), 29.3 (s, CH(CH3)2), 31.6 (d, J PC ) 34.41
Hz, [Ph2PCH2CH2-ylidene]PdMe(NCPh)+)), 47.3 (s, [Ph2PCH2-
CH2-ylidene]PdMe(NCPh)+), 117.5 (broad CF3’s of BAr4F-),
118.2 (s, PhCN), 121.8 (s, ylidene backbone), 122.8 (s, ylidene
backbone), 124.0 (s, aromatic), 124.2 (s, aromatic), 124.8 (s,
aromatic), 126.7 (s, aromatic), 129.4 (s, aromatic), 129.9 (broad
doublet J ) 11 Hz, aromatic of BAr4F), 131.5 (s, aromatic),
132.6 (s, aromatic), 133.2 (s, aromatic), 134.16 (d, J PC ) 11
Hz, aromatic), 135.5 (s, aromatic), 136.3 (s, aromatic), 146.8
(s, ipso aromatic to the ylidene moiety), 161.7 (s, aromatic),
162.2 (s, aromatic), 162.7 (s, aromatic), 163.2 (s, NCN carbon);
31P{1H}, δ 36.5 ([Ph2PCH2CH2-ylidene]PdMe(NCPh)+), 36.3 (s,
[Ph2PCH2CH2-ylidene]PdMe(NCMe)+); 19F, δ 100.2 (s, B[(3,5-
CF3)2C6H3]4-).
Rea ction of 3a w ith CH3I. F or m a tion of 5a . To a
precooled (0 °C) solution of 0.08 g (0.14 mmol) of compound
3a dissolved in 15 mL of dichloromethane was added via a
microliter syringe a 3-fold excess of MeI (26 µL, 0.42 mmol).
The solution was brought to room temperature and was stirred
overnight. A color change occurred within the first 30 min of
stirring at room temperature, from pale yellow to deep orange.
The solution was filtered through Celite, and the volatiles were
removed under vacuum. Repeated recrystallizations from
dichloromethane/ether at -35 °C gave orange crystals.
P r ep a r a tion of th e Sa lts 6a , 7a , 8a , a n d 9a . Gen er a l
Meth od . To a solution of 3a in dichloromethane (20 mL) at
-78 °C was added a solution of H(Et2O){B[(3,5-CF3)2C6H2)]4}
(1 equiv in 20 mL of dichloromethane). The mixture was
warmed to -40 to -50 °C when 1 equiv of pyridine, nitrile,
or trimethylphosphine was added. After the mixture was
warmed to room temperature, the volatiles were removed
under reduced pressure and the residue was washed three
times with ca. 10 mL of petroleum ether and dried under
vacuum.
P r ep a r a tion
of
[La P d (CH3)(P Me3)]{B[(3,5-CF 3)2-
C6H2)]4} (9a ). This was prepared by following the above
general method from 3a (0.058 mg, 0.1 mmol), H(Et2O){B[(3,5-
CF3)2C6H2)]4} (0.094 mg, 0.1 mmol), and PMe3 (8 µL, 0.11
mmol). Yield: 140 mg, 93.3%. NMR (CD2Cl2): 1H, δ 0.0 (t, 3J PH
) 7.29 Hz, 3H, PdCH3), 0.9 (m, 15H, PMe3 and CH(CH3)2),
3
1.2 (d, J HH ) 7.29 Hz, 6H, CH(CH3)2), 2.4 (m, 2H, [Ph2PCH2-
CH2-ylidene]PdMe(PMe3)+), 2.6 (sept, 3J HH ) 6.38 Hz), 4.3 and
4.4 (m, 1H each [Ph2PCH2CH2-ylidene]PdMe(PMe3)+), 7.0 (s,
1H, ylidene backbone), 7.15 (s, 1H, ylidene backbone), 7.2 (d,
2H, aromatic), 7.3-7.6 (m, 16H, aromatic), 7.7 (broad s., 7H,
2
aromatic); 31P{1H}, δ 4.6 (d, J PP ) 32.81 Hz, [Ph2PCH2CH2-
P r ep a r a tion
of
[La P d (CH3)(C5H5N)]{B[(3,5-CF 3)2-
2
ylidene]PdMe(PMe3)+), -15.8 (d, J PP ) 32.81 Hz, [Ph2PCH2-
C6H2)]4} (6a ). This was prepared by following the above
general method from 3a (0.06 g, 0.1 mmol), H(Et2O){B[(3,5-
CF3)2C6H3)]4} (0.09 g, 0.1 mmol), and pyridine (8 µL, 0.1 mmol).
CH2-ylidene]PdMe(PMe3)+); 19F, δ 100.2 (s, B[(3,5-CF3)2C6H3]4-).
P r ep a r a tion of 10a . To a cold solution of 4b (60 mg, 0.085
mmol) in 20 mL of acetonitrile was added by cannela a solution
of AgBF4 (33 mg, 0.17 mmol in 5 mL of acetonitrile). Upon
addition a gray precipitate was formed, which was filtered off
through Celite while the mixture was kept at 0 °C. The
resultant bright yellow solution was evaporated to dryness,
and the yellow solid was characterized by NMR spectroscopy
after dissolution in d3-acetonitrile. NMR (CD3CN): 1H, δ 0.8
(d, 6H, CH(CH3)2), 0.9 (d, 6H, CH(CH3)2), 2.5 (2H, sept,
CH(CH3)2), 2.8 (m, 2H, (PPh2CH2CH2-ylidene)Pd(MeCN)22+),
4.4 and 4.6 (each dt, 1H, (PPh2CH2CH2-ylidene)Pd(MeCN)22+),
7.1-8.0 (15H, aromatics and imidazole backbone); 31P{1H}, δ
29.9 (s). The compound was unstable for further characteriza-
tion.
3
Yield: 0.15 g, 96%. NMR (CD2Cl2): 1H, δ -0.3 (d, J PH ) 2.73
Hz, 3H, PdCH3), 0.8 (d, 6H, CH(CH3)2), 1.0 (d, 6H, CH(CH3)2),
2.5 (m, 4H, CH(CH3)2 and [Ph2PCH2CH2-ylidene]PdMe(py)+),
4.3 and 4.4 (m, 1H each [Ph2PCH2CH2-ylidene]PdMe(py)+), 6.9
(s, 1H, imidazole backbone), 7.0-7.2 (m, 4H, aromatic), 7.3-
7.4 (d, 2H, aromatic), 7.5-7.9 (m, 25H, aromatic); 13C{1H}, δ
7.6 (s, PdCH3), 23.3 (s, CH(CH3)2), 26.1 (s, CH(CH3)2), 29.1 (s,
CH(CH3)2), 32.0 (s, [Ph2PCH2CH2-ylidene]PdMe(py)+), 47.9 (s,
[Ph2PCH2CH2-ylidene]PdMe(py)+); 31P{1H}, δ 34.8 (s, [Ph2PCH2-
CH2-ylidene]PdMe(py)+); 19F, δ 100.2 (s, B[(3,5-CF3)2C6H3]4-).
Anal. Found: C, 51.77; H, 3.77; N, 2.49. Calcd for C67H54
-
BF24N3PPd‚CH2Cl2: C, 51.36; H, 3.55; N, 2.64.
P r ep a r a t ion of [La P d (CH3)(CH3CN)]{B[(3,5-CF 3)2-
C6H2)]4} (7a ). This was prepared by following the above
general method from 3a (0.06 g, 0.1 mmol), H(Et2O){B[(3,5-
CF3)2C6H2)]4} (0.09 g, 0.1 mmol), and CH3CN (6 µL, 0.11
mmol). Yield: 0.110 g, 75%. NMR (CD2Cl2, 300 MHz): 1H, δ
Mon itor in g of th e Rea ction of 3a w ith (CF 3)2CHOH by
NMR Sp ectr oscop y. In the glovebox 20 mg of complex 3a
was dissolved in C5D5N. The solution was then cooled to -78
°C, and 3-4 drops of hexafluoro-2-propanol using a microsy-
ringe were added. The solution was brought to room temper-
3
3
-0.1 (d, J PH ) 1.83 Hz, 3H, PdMe), 1.1 (d, J HH ) 6.37 Hz,
3
1
6H, CH(CH3)2), 1.2 (dd, J HH ) 6.37 Hz, 6H, CH(CH3)2), 1.9
ature, and the NMR spectra were acquired. H NMR (C6D5N,
3
(s, 3H, MeCN), 2.6 (m, 4H, CH(CH3)2 and [Ph2PCH2CH2-
ylidene]PdMe(NCMe)+), 4.1 and 4.2 (m, 1H each, [Ph2PCH2CH2-
ylidene]PdMe(NCMe)+), 6.9 (s, 1H, ylidene backbone), 7.0
(s, 1H, ylidene backbone), 7.3 (d, 2H, aromatics), 7.4-7.6
(m, 8H, aromatic), 7.7-7.9 (m, 15H, aromatic). No 13C{1H}
NMR data are available, due to sample decomposition during
acquisition. Anal. Found: C, 49.76; H, 3.29; N, 4.26. Calcd
for C64H52BF24N3PPd‚CH2Cl2‚2CH3CN: C, 50.71; H, 3.70; N,
4.29.
400 MHz): δ 0.0 (d, J P-H ) 2.68 Hz, 3H, PdCH3), 0.2 (CH4),
0.9 (d, 6H, CH(CH3)2), 1.1 (d, 6H, CH(CH3)2), 2.6 (2H, sept,
CH(CH3)2), 2.8 (m, 2H, (Ph2PCH2CH2-ylidene)PdMe+), 4.4 and
4.6 (each dt, 1H, (Ph2PCH2CH2-ylidene)PdMe+), 5.4 (sept,
(CF3)2CHO-), 7.0-7.8 (m, 10H, aromatic and imidazolylidene
backbone), 7.9-8.1 (m, 5H, aromatic). 13C NMR (C6D5N, 100.65
2
MHz): δ 7.98 and 8.03 (d, J P-C ) 4.6 Hz, PdCH3), 23.7 (s,
CH(CH3)2), 26.4 (s, CH(CH3)2), 29.4 (s, CH(CH3)2), 31.53 and
1
31.87 (d, J P-C ) 34.2 Hz (PPh2CH2CH2-ylidene)PdMe+), 47.7

4758 Organometallics, Vol. 22, No. 23, 2003
Ta ble 2. Cr ysta llogr a p h ic Da ta for Com p lexes 3a , 4b, 5a , 6a , a n d 7a
Tsoureas et al.
3a
4b
5a
6a
7a
chem formula
formula wt
cryst syst
space group
a/Å
b/Å
c/Å
C
31H39N2PPd
C26H27Br2N2PPd
664.69
triclinic
P1h
9.3880(2)
10.4022(2)
13.8831(3)
69.2150(10)
88.9380(10)
77.9220(10)
1237.17(4)
2
C31H38Cl2I3N2PPd
999.64
C136H105B2F48N6O0.50P2Pd2
3039.62
triclinic
P1h
14.258(5)
17.042(5)
27.324(5)
92.213(5)
94.633(5)
91.630(5)
6610(3)
2
C
64H51BF24N3PPd
577.01
monoclinic
P21/n
12.7219(5)
15.2549(8)
15.3658(6)
90
110.421(3)
90
2794.7(2)
4
1466.26
triclinic
P1h
monoclinic
I2/a of C2/cc
18.5870(4c
14.5626(3))
27.2787(6
90
108.7830(10)
90
6990.4(3)
4
10.2531(2)
15.1598(5)
22.1848(7)
74.158(2)
80.735(2)
73.122(2)
3161.89(16)
2
R/deg
â/deg
γ/deg
V/Å3
Z
T/K
150(2)
0.743
28 963
6290
0.1825
0.0674
150(2)
4.062
10 490
5607
0.0188
0.0283
120(2)
2.596
34 885
7990
0.0484
0.0576
153(2)
0.417
60 460
18 875
0.1037
0.0669
120(2)
0.433
56 165
14 437
0.1028
0.0561
µ/mm-1
no. of data collected
no. of unique data
Rint
final R(|F|) for
Fo > 2σ(Fo)
final R(F2) for all data
0.1051
0.0347
0.0877
0.1194
0.1117
(s, (PPh2CH2CH2-ylidene)PdMe+), 71.7 (quintet, 1J CF ) 31 Hz,
(CF3)2CHO-), 123.5 (s, ylidene backbone), 123.8 (s, ylidene
backbone), 124.8 (s, aromatic), 125.5 (s, aromatic), 130.22 and
130.32, 131.4 (s, aromatic), 131.7 (s, aromatic), 132.6 (s,
aromatic), 134.53 and 134.65, 146.6 (s, ipso aromatic carbon
attached to the ylidene backbone), 180.3 (NCN carbon);
31P{1H} NMR (C6D5N, 121.44): δ 35.26 (s, (PPh2CH2CH2-
ylidene)PdMe+).
Heck Ca ta lytic Stu d ies. A 1 mmol portion of aryl halide,
1.4 equiv of alkene (methyl acrylate unless otherwise stated),
2 equiv of base (NEt3 or dried Cs2CO3), and 0.3 equiv of
diethylene glycol-dibutyl ether (internal standard) were
placed in a 50 mL Rotaflo ampule using a microliter syringe.
A 1 mL portion of N-methyl-2-pyrrolidinone or N,N′-dimethyl-
acetamide was added to dissolve organics and 1 mL of solution
of catalyst in the same solvent so that 0.5 or 0.01 mol % of
catalyst/substrate was obtained. The ampule was then evacu-
ated and placed in a preheated oil bath fitted with a thermo-
stat. After the end of the catalytic run the ampule was cooled
to room temperature and opened and the reaction mixture
worked up by addition of water and dichloromethane, extrac-
tion, phase separation, drying and GC of the organic phases.
GC yields (average of two runs) were calculated using dieth-
ylene glycol-dibutyl ether as an internal standard.
Cop olym er iza tion of CO a n d Eth ylen e. In a typical run
11 mg of complex 3a was dissolved in 10 mL of dichlo-
romethane, and the solution was transferred via cannula to a
glass pressure reactor and cooled to -78 °C. To this solution
was added 18 mg of Brookhart’s acid dissolved in 10 mL of
dichloromethane and cooled to -78 °C. After the addition was
complete, the temperature was raised slowly to -30 °C and
the apparatus was slowly pressurized with a 1:1 mixture of
CO and ethylene up to about 20 psi. The slush bath was then
removed, and the reaction mixture was pressurized to 5 bar.
After 30 min the solution became cloudy. The reaction was
quenched after 2 h with 30 mL of methanol. The precipitated
polymer was filtered and dried under vacuum to yield 69 mg
of poly(CO-block-ethylene). The polymer was dissolved in a 1:1
mixture of C6D6 and (CF3)2CHOH, and the NMR spectra were
recorded. NMR (C6D6): δ 2.4 (s, poly(CO-block-(CH2)2), 3.2
(broad s, (CF3)2CHOH), 3.7 (sept, (CF3)2CHOH); 13C{1H}, δ 35.9
2
(s, poly(CO-block-(CH2)2), 69.8 (sept. J FH ) 33.70 Hz, (CF3)2-
1
CHOH), 122.5 (q, J FH ) 281.80 Hz, (CF3)2CHOH), 212.3 (s,
poly(CO-block-(CH2)2). IR: 2906 cm-1 (CO stretching).
X-r a y Cr ysta llogr a p h y. A summary of the crystal data
and data collection and refinement details for compounds 3a ,
4b, 5a , 6a , and 7a are given in Table 2. All data sets were
collected on a Enraf-Nonius Kappa CCD area detector diffrac-
tometer with an FR591 rotating anode and an Oxford Cryo-
systems low-temperature device operating in ω scanning mode
with ψ and ω scans to fill the Ewald sphere. The programs
used for control and integration were Collect, Scalepack, and
Denzo.18 The crystals were mounted on a glass fiber with
silicon grease, from Fomblin vacuum oil. All solutions and
refinements were performed using the WinGX package19 and
all software packages within. All non-hydrogen atoms were
refined using anisotropic thermal parameters, and hydrogens
were added using a riding model.
Ack n ow led gm en t. We thank J ohnson Matthey
Catalysts and the University of Southampton for finan-
cial support (to N.T.) and Prof. R. P. Tooze for initiating
this collaborative research project and for helpful dis-
cussions.
Su p p or tin g In for m a tion Ava ila ble: Full details of the
X-ray crystal structures, including complete tables of crystal
data, atomic coordinates, bond lengths and angles, and posi-
tional and anisotropic thermal parameters; these data are also
available as CIF files. This material is available free of charge
via the Internet at http://pubs.acs.org.
OM034061S
(18) Hooft, R. COLLECT; Nonius BV, 1997-2000. Otwinowski, Z.;
Minor, W. SCALEPACK, DENZO. Methods Enzymol. 1997, 276, 307.
(19) Farrugia, L. J . J . Appl. Crystallogr. 1999, 32, 83.