Investigations into the stability of tethered palladium(II) pincer complexes during heck catalysis

Article abstract of DOI:10.1021/om048992v

A variety of palladated PCP pincer complexes are covalently tethered onto polymeric and silica supports via either amide or ether linkages and are evaluated in the Heck reaction of iodobenzene and n-butyl acrylate. The decomposition under reaction conditions of all complexes studied is established through poisoning and kinetic studies. Furthermore, the initial steps of the decomposition pathway of PCP as well as SCS pincer Pd(II) complexes are proposed and validated using in situ NMR, mass spectroscopy, and XAS as well as computational methods. These findings together with our previous reports strongly suggest that all Pd(II) pincer complexes are simply precatalysts during the Heck reaction that decompose to form catalytically active Pd(0) species.

Full text of DOI:10.1021/om048992v

Organometallics 2005, 24, 4351-4361
4351
Investigations into the Stability of Tethered
Palladium(II) Pincer Complexes during Heck Catalysis
William J. Sommer,† Kunquan Yu,‡ John S. Sears,† Yaying Ji,§ Xiaolai Zheng,†
Robert J. Davis,§ C. David Sherrill,*,† Christopher W. Jones,*,†,‡ and
Marcus Weck*,†
School of Chemistry and Biochemistry and School of Chemical & Biomolecular Engineering,
Georgia Institute of Technology, Atlanta, Georgia 30332, and Department of Chemical
Engineering, University of Virginia, Charlottesville, Virginia 22904
Received December 19, 2004
A variety of palladated PCP pincer complexes are covalently tethered onto polymeric and
silica supports via either amide or ether linkages and are evaluated in the Heck reaction of
iodobenzene and n-butyl acrylate. The decomposition under reaction conditions of all
complexes studied is established through poisoning and kinetic studies. Furthermore, the
initial steps of the decomposition pathway of PCP as well as SCS pincer Pd(II) complexes
are proposed and validated using in situ NMR, mass spectroscopy, and XAS as well as
computational methods. These findings together with our previous reports strongly suggest
that all Pd(II) pincer complexes are simply precatalysts during the Heck reaction that
decompose to form catalytically active Pd(0) species.
Introduction
chemical industries.22 A large variety of palladium
complexes have been reported that “catalyze” this
transformation. These complexes include pallada-
cycles,23-31 Pd-carbenes,32-39 and Pd-pincer complex
soluble palladium nanoparticles.19,40-53 While use of
these catalytic or precatalytic moieties has been suc-
The Heck reaction is among the most important and
widely used reactions for the formation of carbon-
carbon bonds.1-19 In this reaction, a new bond is created
between two sp2 carbon centers that stem from an
alkene and an aryl halide. First reported in the early
1970s by Heck and Mizoroki independently,20,21 the use
of this catalytic reaction has rapidly increased, as a
result of its importance in the pharmaceutical and fine
(22) Blaser, H.-U.; Indolese, A.; Naud, F.; Nettekoven, U.; Schnyder,
A. Adv. Synth. Catal. 2004, 346, 1583.
(23) Beller, M.; Fischer, H.; Herrmann, W. A.; O¨ fele, K.; Brossmer,
C. Angew. Chem., Int. Ed. Engl. 1995, 34, 1848.
(24) Beller, M.; Riermeier, T. H.; Haber, S.; Kleiner, H.-J.; Herr-
mann, W. A. Chem. Ber. 1996.
* To whom correspondence should be addressed. E-mail:
marcus.weck@chemistry.gatech.edu (M.W.); cjones@chbe.gatech.edu
(C.W.J.); david.sherrill@chemistry.gatech.edu (C.D.S.).
† School of Chemistry and Biochemistry, Georgia Institute of
Technology.
(25) Beller, M.; Riermeier, T. H. Eur. J. Inorg. Chem. 1998, 29.
(26) Beller, M.; Riermeier, T. H.; Magerlein, W.; Muller, T. E.;
Scherer, W. Polyhedron 1998, 17, 1165.
(27) Hermann, W. A.; Bohm, V. P. W.; Reisinger, C. P. J. Organomet.
Chem. 1999, 576, 23.
‡ School of Chemical & Biomolecular Engineering, Georgia Institute
of Technology.
(28) Herrmann, W. A.; Brossmer, C.; O¨ fele, K.; Reisinger, C. P.;
Priermeier, T.; Beller, M.; Fischer, H. Angew. Chem., Int. Ed. Engl.
1995, 34, 1844.
§ Department of Chemical Engineering, University of Virginia.
(1) Beletskaya, I. P.; Cheprakov, A. V. Chem. Rev. 2000, 100, 3009.
(2) Brase, S.; deMeijere, A.; Diederich, F. In Metal-Catalyzed Cross-
Coupling Reactions; Wiley-VCH: New York, 1998.
(3) Cabri, W.; Candiani, I. Acc. Chem. Res. 1995, 28, 2.
(4) Dieck, H. A.; Heck, R. F. J. Am. Chem. Soc. 1974, 96, 1133.
(5) Dieck, H. A.; Heck, R. F. J. Org. Chem. 1975, 40, 1083.
(6) Heck, R. F. Palladium Reagents in Organic Synthesis; Academic
Press: London, 1985.
(29) Herrmann, W. A.; Brossmer, C.; Reisinger, C.-P.; O¨ fele, K.;
Beller, M.; Riermeier, T. H. Chem. Eur. J. 1997, 3, 1357.
(30) Louie, J.; Hartwig, J. F. Angew. Chem., Int. Ed. Engl. 1996,
35, 2359.
(31) Nowotny, M.; Hanefeld, U.; van Koningsveld, H.; Maschmeyer,
T. Chem. Commun. 2000, 1877-1878.
(32) Andrus, M. B.; Song, C.; Zhang, J. Org. Lett. 2002, 4, 2079.
(33) Steel, P. G.; Teasdale, C. W. T. Tetrahedron Lett. 2004, 45, 8977.
(34) Yang, C.; Lee, H. M.; Nolan, S. P. Org. Lett. 2001, 3, 1511.
(35) Andrus, M. B.; Song, C.; Zhang, J. Org. Lett. 2002, 4, 2079.
(36) Loch, J. A.; Albrecht, M.; Peris, E.; Mata, J.; Faller, J. W.;
Crabtree, R. H. Organometallics 2002, 21, 700.
(37) Lebel, H.; Janes, M. K.; Charette, A. B.; Nolan, S. P. J. Am.
Chem. Soc. 2004, 126, 5046.
(7) Heck, R. F. Org. React. 1982, 27, 345.
(8) Malleron, J. L.; Fiaud, J. C.; Legros, J. Y. Handbook of Pal-
ladium-Catalyzed Organic Reactions: Synthesis Aspects and Catalytic
Cycles; Academic Press: San Diego, CA, 1997.
(9) Link, J. T.; Overman, L. E. CHEMTECH 1998, 28, 19.
(10) Link, J. T.; Overman, L. E. In Metal-Catalyzed Cross-Coupling
Reactions; Wiley-VCH: New York, 1998.
(11) Crisp, G. T. Chem. Soc. Rev. 1998, 27, 427.
(38) Lee, H. M.; Lu, C. Y.; Chen, C. Y.; Chen, W. L.; Lin, H. C.; Chiu,
P. L.; Cheng, P. Y. Tetrahedron 2004, 60, 5807.
(39) Herrmann, W. A.; Reisinger, C. P.; Spiegler, M. J. Organomet.
Chem. 1998, 557, 93.
(12) Tietze, L. F.; Ila, H.; Bell, H. P. Chem. Rev. 2004, 104, 3453.
(13) Tsuji, J. Palladium Reagents and Catalysts: Innovations in
Organic Synthesis; Wiley: Chichester, U.K., 1995.
(14) Stille, J. K. Angew. Chem., Int. Ed. Engl. 1986, 25, 508.
(15) Ritter, K. Synthesis 1993, 735.
(40) Le Bars, J.; Specht, U.; Bradley, J. S.; Blackmond, D. G.
Langmuir 1999, 15, 7621.
(16) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457.
(17) Farina, V. Adv. Synth. Catal. 2004, 346, 1553.
(18) DeMeijere, A.; Meyer, F. E. Angew. Chem., Int. Ed. Engl. 1994,
33, 2379.
(41) Calo, V.; Nacci, A.; Monopoli, A.; Detomaso, A.; Iliade, P.
Organometallics 2003, 22, 4193.
(42) Galow, T. H.; Drechsler, U.; Hanson, J. A.; Rotello, V. M. Chem.
Commun. 2002, 1076.
(19) Amatore, C.; Jutand, A. J. Organomet. Chem. 1999, 576, 254.
(20) Mizoroki, T.; Mori, K.; Ozaki, A. Bull. Chem. Soc. Jpn. 1971,
44, 581.
(43) Gopidas, K. R.; Whitesell, J. K.; Fox, M. A. Nano Lett. 2003, 3,
1757.
(44) Mandal, S.; Roy, D.; Chaudhari, R. V.; Sastry, M. Chem. Mater.
2004, 16, 3714.
(21) Heck, R. F.; Nolley, J. P. J. Org. Chem. 1972, 37, 2320.
10.1021/om048992v CCC: $30.25 © 2005 American Chemical Society
Publication on Web 07/27/2005

4352 Organometallics, Vol. 24, No. 18, 2005
Sommer et al.
Figure 2. Pincer ligands studied by Eberhard.69
Figure 1. Commonly used types of pincer ligands.
tages of pincer ligands that have been suggested in the
literature include an unprecedented stability at high
temperatures, making them the candidate of choice for
supported catalysis.67,68 Among the most studied pincer
ligands are the ones outlined in Figure 1. When meta-
lated with a typical palladium source, all three pincer
ligands contain a Pd(II) metal center. On the basis of
this fact and their suggested stability, a controversial
PdII/PdIV cycle has been suggested as a potential cata-
lytic mechanism.28 However, a PdIV species has never
been detected to date in one of these systems. Recently,
we and Bergbreiter et al. studied the stability of
supported SCS pincer complexes independently.57,58,74
Both groups demonstrated that these complexes leach
unidentified Pd(0) species into solution that are cata-
lytically active. Furthermore, no catalytic activity could
be attributed to any intact pincer species. In view of
these results, we decided to investigate palladated PCP
complexes that are reported in the literature to be
significantly more stable.75-77 The literature concerning
these species in the Heck reaction is somewhat muddled.
Eberhard69 has demonstrated that palladated PCP
pincer complexes with oxygen atoms in place of carbons
in the arms are not stable during Heck catalysis (Figure
2), Frech78 has shown recently that the palladated
pincer ligand could be reduced by sodium to form a
bimetallic complex, whereas there are other reports
where immobilized PCP pincers are reported as “recy-
clable catalysts”.67
We have prepared the small-molecule Pd(II) PCP
pincer (1) and compared its performance in the Heck
reaction with that of Pd(II) PCP complexes tethered to
a variety of supports ranging from soluble polymers (2
and 3) to nanoporous silica, SBA-15 (4) (Figure 3). Silica-
supported catalysts allowed for easy removal of the
catalyst from the reaction mixture, while soluble poly-
mer supported catalysts allowed us to study the reactiv-
ity of the catalytic species in a homogeneous solution.
In this work, we report the synthesis of these supported
pincer complexes and a detailed study of their stability
under Heck catalysis conditions. Using a variety of
techniques such as in situ NMR and poisoning studies,
our data suggest that, in all cases studied, the pincer
species decompose, releasing Pd(0), which is the cata-
lytically active species. Finally, in situ NMR and in situ
X-ray absorption spectroscopy (XAS) are used to explore
the stability of the complexes, while mass spectroscopy
and computational experiments are used to explore the
first steps of the decomposition pathway.
cessful, several factors may limit widespread application
of the Heck reaction commercially, including the lack
of recovery of expensive palladium species, the use of
toxic phosphines during the catalysis, and the difficulty
in removing these toxic species from the final products.
Several possibilities to overcome these shortcomings
have been proposed, including the use of supported
catalysts that could be removed easily from the reaction
mixture and potentially be reused.54-61 Among the most
attractive palladium-ligand systems that have been
supported is the Pd-pincer system.54,57,58,62-66 Use of
these species for Heck reactions results in rapid conver-
sion of substrate, and metal complexes based on pincer
ligands have been suggested to be extremely stable.67,68
Pincer ligands are defined as a tridentate structure
bound to a metal. In this work our ligand can be defined
as 1,3-disubstituted benzenes with heteroatoms in the
side-chains that are able to chelate to the metal center
(Figure 1). They are abbreviated by the three atoms that
coordinate to the metal center: e.g., SCS, PCP, or NCN
pincer.
Over the past decade, extensive work has been carried
out on metalated pincer ligands.54,57,58,62,67,69-73 Advan-
(45) Rahimm, E. H.; Kamounah, F. S.; Frederiksen, J.; Christensen,
J. B. Nano Lett. 2001, 1, 499.
(46) Yeung, L. K.; Crooks, R. M. Nano Lett. 2001, 1, 14.
(47) Sablong, R.; Schlotterback, U.; Vogt, D.; Mecking, S. Adv. Synth.
Catal. 2003, 345, 333.
(48) Heidenreich, R. G.; Krauter, J. G. E.; Pietsch, J.; Kohler, K. J.
Mol. Catal. A 2002, 182-183, 499.
(49) Kohler, K.; Heidenreich, R. G.; Krauter, J. G. E.; Pietsch, J.
Chem. Eur. J. 2002, 8, 622.
(50) Kohler, K.; Wagner, M.; Djakovitch, L. Catal. Today 2001, 66,
101.
(51) Nejjar, A.; Pinel, C.; Djakovitch, L. Adv. Synth. Catal. 2003,
345, 612.
(52) Zhao, F.; Bhanage, B. M.; Shirai, M.; Arai, M. Chem. Eur. J.
2000, 6, 843.
(53) Zhao, F.; Murakami, K.; Masahiko, A. J. Catal. 2000, 194, 479.
(54) Bergbreiter, D. E.; Osburn, P. L.; Liu, Y. S. J. Am. Chem. Soc.
1999, 121, 9531.
(55) Bhanage, B. M.; Fujita, S. I.; Arai, M. J. Organomet. Chem.
2003, 687, 211.
(56) Uozumi, Y.; Kimura, T. Synlett 2002, 12, 2045.
(57) Yu, K.; Sommer, W.; Weck, M.; Jones, C. W. Adv. Synth. Catal.
2005, 347, 161.
(58) Yu, K.; Sommer, W.; Weck, M.; Jones, C. W. J. Catal. 2004,
226, 101.
(59) Deprele, S.; Montchamp, J. L. Org. Lett. 2004, 6, 3805.
(60) McNamara, C. A.; Dixon, M. J.; Bradley, M. Chem. Rev. 2002,
102, 3275.
(61) Saluzzo, C.; Lemaire, M. Adv. Synth. Catal. 2002, 344, 915.
(62) Bergbreiter, D. E.; Osburn, P. L.; Wilson, A.; Sink, E. M. J.
Am. Chem. Soc. 2000, 122, 9058.
(63) Gimenez, R.; Swager, T. M. J. Mol. Catal. A 2001, 166, 265.
(64) Koh, J. H.; Gagne´, M. R. Angew. Chem., Int. Ed. 2004, 43, 3459.
(65) Mas-Marza, E.; Segarra, A. M.; Claver, C.; Peris, E.; Fernandez,
E. Tetrahedron Lett. 2003, 44, 6595.
(66) Weissberg, A.; Portnoy, M. Chem. Commun. 2003, 1538.
(67) Chanthateyanonth, R.; Alper, H. Adv. Synth. Catal. 2004, 346,
1375.
(74) Bergbreiter, D. E.; Osburn, P. L.; Frels, J. Adv. Synth. Catal.
2005, 347, 172.
(68) Karlen, T.; Dani, P.; Grove, D. M.; Steenwinkel, P.; van Koten,
G. Organometallics 1996, 15, 5687.
(75) Miyazaki, F.; Yamaguchi, K.; Shibasaki, M. Tetrahedron Lett.
1999, 40, 7379.
(69) Eberhard, M. R. Org. Lett. 2004, 6, 2125.
(70) Morales-Morales, D.; Redon, R.; Yung, C.; Jensen, C. M. Inorg.
Chim. Acta 2004, 357, 2953.
(76) Ohff, M.; Ohff, A.; van der Boom, M. E.; Milstein, D. J. Am.
Chem. Soc. 1997, 119, 11687.
(77) Morales-Morales, D.; Grause, C.; Kasaoka, K.; Redon, R.;
Cramer, R. E.; Jensen, C. M. Inorg. Chim. Acta 2000, 958, 300.
(78) Frech, C. M.; Shimon, L. J. W.; Milstein, D. Angew. Chem., Int.
Ed. 2005, 44, 1709.
(71) Pollino, J. M.; Weck, M. Org. Lett. 2002, 4, 753.
(72) Pollino, J. M.; Weck, M. Synthesis 2002, 9, 1277.
(73) Singleton, J. T. Tetrahedron 2003, 59, 1837.

Stability of Tethered PdII Pincer Complexes
Organometallics, Vol. 24, No. 18, 2005 4353
Scheme 1. Synthesis of Palladium Chloride
N-{3,5-Bis[(diphenylphosphanyl)methyl]phenyl}-
acetamide (1)
chromatography (GC). We observed quantitative con-
version using 1 as catalyst within 20 min (Figure 5).
To examine the catalytic behavior in more detail,
poisoning studies were carried out. It has been demon-
strated that highly cross-linked poly(vinylpyridine)
(PVPy) traps soluble Pd species by coordinating to the
metal center and removing it from solution, while
leaving support-tethered Pd species untouched.56,57,79
For the poisoning test, 300 equiv of PVPy was added to
the catalytic reaction, resulting in less than 2% conver-
sion of iodobenzene after 3 h. A second poisoning test
was carried out by adding mercury. Hg(0) is known to
react with Pd(0), forming an amalgamate, thereby
quenching the activity of the leached palladium.79-81
When 100 equiv of mercury was added to the catalytic
reaction, again, less than 2% conversion of iodobenzene
was observed after 3 h. These results clearly suggest
that 1 decomposes under the reaction conditions.57,58,74
Figure 3. Immobilized palladated pincer complexes evalu-
ated in this work.
Figure 4. Heck reaction conditions.
Results and Discussion
We conducted our investigation using four different
catalysts with similar structures. Complex 1 was used
as the standard for all catalytic reactions, for compari-
son with the supported systems. The polymer-supported
catalysts 2 and 3 were synthesized to study the effect
of the linker attachment on the catalytic activity. It has
been suggested in the literature that ether derivatives
of pincer complexes are less stable than their amide
counterparts.54 Therefore, two polymeric catalysts that
are similar except for the heteroatom functionality that
attaches the pincer complex to the support were pre-
pared. For 2, the catalyst was attached to the polymer
backbone via an ether linkage, while in the case of 3,
an amide linkage was employed. Both polymers are
highly soluble under reaction conditions, allowing a
close comparison with 1. The silica-supported catalyst
4 was synthesized to study the influence of a heteroge-
neous reaction mixture on the catalysis and to compare
the catalytic activity of a solid-supported system to the
soluble polymer catalysts and the small-molecule ana-
logue.
The standard Heck reaction conditions are outlined
in Figure 4. The catalysis was carried out using 1.5
equiv of distilled triethylamine, 1 equiv of iodobenzene,
and 1.25 equiv of n-butyl acrylate in DMF at 120 °C.
The catalyst loading was 10 mol % for the polymer-
supported pincer and 0.3 mol % for the SBA-15-sup-
ported pincer.
The synthesis of 1 was carried out by reacting 5
(synthesized according to literature procedures9) with
potassium diphenylphosphide in THF under reflux for
3 h to yield 6, which was used without further purifica-
tion. Compound 6 was then dissolved in a CH2Cl2/MeCN
(1:2) mixture followed by the addition of the PdCl2-
(NCPh)2 and AgBF4 to yield 1, after purification, as a
yellow solid (Scheme 1).
Figure 5. Conversion vs time for Heck coupling using 1.
The synthesis of polymer 2 was accomplished by
adding potassium diphenylphosphide to 7, which was
synthesized using a literature procedure,9 followed by
the oxidation of the phosphorus using an aqueous
solution of 10% H2O2. The tert-butyldimethylsilyl group
was then removed using TBAF to yield 8 as a white
(79) Klingelho¨fer, S.; Heitz, W.; Oestreich, S.; Fo¨rster, S.; Antonietti,
M. J. Am. Chem. Soc. 1997, 119, 10116.
(80) Paal, C.; Hartmann, W. Chem. Ber. 1918, 51, 711.
(81) Whitesides, G. M.; Hackett, M.; Brainard, R. L.; Lavalleye, J.
P. P. M.; Sowinski, A. F.; Izumi, A. N.; Moore, S. S.; Brown, D. W.;
Staudt, E. M. Organometallics 1985, 4, 1819.
The use of 1 in Heck catalysis was investigated by
monitoring the disappearance of iodobenzene via gas

4354 Organometallics, Vol. 24, No. 18, 2005
Scheme 2. Synthesis of Polymer 2
Sommer et al.
Scheme 3. Synthesis of Polymer 3
solid, which was reacted with the cyclo[2.2.1]hept-5-ene-
2-carboxylic acid 11-bromoundecyl ester using potas-
sium carbonate in DMF to yield a brown oil. The oil was
immediately reacted with trichlorosilane and triethyl-
amine in xylenes to deprotect the phosphines. The
deprotected pincer was then reacted with PdCl2(NCPh)2
and AgBF4 to yield monomer 10 as an orange solid. The
monomer was polymerized using the third-generation
Grubbs catalyst at room temperature under air (Scheme
2).83 Quantitative conversion was obtained in 20 min.
Again, the use of polymer 2 under Heck conditions
was measured using GC by following the disappearance
of iodobenzene (Figure 6). We observed quantitative
conversion within 90 min. In analogy to the experiments
outlined above for 1, poisoning studies were carried out.
The addition of 300 equiv of PVPy resulted in the
retardation of the reaction with only 1% conversion
observed after 24 h. Furthermore, the addition of 100
equiv of mercury to the Heck reaction mixture yielded
similar results. Again, these experiments suggest that
the active species for the Heck reaction is not a well-
defined palladium species but a leached Pd moiety that
contains Pd(0) in the catalytic cycle.
Polymer 3 was synthesized by starting with the
coupling of complex 12 and 12-bicyclo[2.2.1]hept-5-en-
2-yldodecanoic acid using dicyclohexylcarbodiimide and
1-hydoxybenzotriazole to yield 13. Reduction of the
phosphorus using trichlorosilane and triethylamine in
xylenes, followed by the addition of PdCl2(NCPh)2 and
AgBF4, yielded monomer 14 (Scheme 3). The monomer
was polymerized using the third-generation Grubbs
catalyst 11 in a 50:1 monomer-to-catalyst ratio.
(82) Widegren, J. A.; Finke, R. G. J. Mol. Catal. A 2003, 198, 317.
(83) Love, J. A.; Morgan, J. P.; Trnka, T. M.; Grubbs, R. H. Angew.
Chem., Int. Ed. 2002, 41, 4035.
Figure 6. Conversion vs time for Heck coupling using 2.

Stability of Tethered PdII Pincer Complexes
Organometallics, Vol. 24, No. 18, 2005 4355
Scheme 4. Synthesis of Silica-Tethered Pincer Complex 4
Scheme 5. Proposed Initial Steps of the Decomposition of the Palladated Pincer Complex
The same reactivity experiments as outlined above
for 1 and 2 were carried out. While quantitative conver-
sion without the addition of poison was observed within
60 min (Figure 7), the two poisoning tests using 300
equiv of PVPy and 100 equiv of mercury resulted in the
near-total quenching of the catalytic reaction. Both
reactions yielded less than 1% conversion after 24 h.
These results showed that the leaching of the palladium
is independent of the tether group of the immobilized
pincer complex and correlate well with the results for
the other experiments discussed above.
To synthesize the silica-supported complex 4, com-
pound 8 was reacted with allyl bromide, followed by the
reduction of the phosphorus using trichlorosilane and
triethylamine in xylenes. Addition of PdCl2(NCPh)2 and
AgBF4 yielded 16 as a yellow powder. Complex 16 was
then reacted with (3-mercaptopropyl)trimethoxysilane
and AIBN in dry chloroform to yield 17. Complex 17
was stirred with SBA-15 silica in toluene to yield the
supported pincer complex 4 (Scheme 4).
These reactivity experiments clearly demonstrate that
palladated PCP complexes decompose under the reac-
tion conditions studied. Furthermore, the decomposition
is independent of the support or the linker attachment.
While small differences in the kinetics of the reactions
are observed for the different supports, poisoning stud-
ies for all systems studied demonstrate that no catalytic
activity stems from a support-tethered Pd(II) species;
rather, leached Pd species are the active moieties. These
results are analogous to those of our recently reported
study on supported SCS pincer complexes, clearly
demonstrating that the added stability of the PCP
ligand system is not enough to prevent decomposi-
tion.57,58
Using 4 as Heck catalyst resulted in the quantitative
conversion of iodobenzene within 2 h (Figure 8). The
PVPy and mercury leaching tests were carried out, and
no conversions were observed in the presence of either
poison.
Figure 8. Conversion vs time for Heck coupling using 4.
Investigations into the Decomposition Pathway.
A variety of decomposition pathways of these complexes
during reaction conditions are imaginable. The most
likely decomposition pathway of palladacycles starts
with the exchange of one ligand of the pincer ligand
(arm off) by the nitrogen-containing base. As outlined
in the literature, the high strain of palladated pincer
complexes as a result of their distorted-square-planar
configuration makes them likely to dechelate under
appropriate reaction conditions, thereby relieving the
strain.30 After base or solvent coordination, the resulting
Figure 7. Conversion vs time for Heck coupling using 3.

4356 Organometallics, Vol. 24, No. 18, 2005
Sommer et al.
Scheme 6. Computational Explored Ligand
Exchange Mechanism
Figure 9. Optimized structures of the calculated exchange
pathway of phosphine by a triethylamine base of palladated
pincer complexes: (A) Chemdraw representation of struc-
tures used in calculation; (B) fully intact complex; (C)
removal of one phosphine and addition of the amine base;
(D) exchange of the second phosphine by another amine
base.
energy that could be overcome at high temperatures
such as the reaction conditions (preliminary computa-
tions for the simpler SCS system indicate that the
transition state for this associative displacement is only
an additional 2-3 kcal/mol beyond the reaction energy).
Removal of the second arm and replacing it with another
ligand such as the solvent and/or the amine base is
estimated to cost an additional 13 kcal/mol relative to
the “one-arm-off configuration”. After the initial removal
of the first arm, the coordination sphere around the
palladium is a highly distorted square planar one
(Figure 9C), which is highly unfavorable. However, this
can be overcome during the second phosphine removal
that converts the metal complex back into a square-
planar configuration (Figure 9D), which explains why
the second ligand exchange is less unfavorable than the
first one. On the basis of these calculations, we hypoth-
esize that the high temperatures during the reaction
conditions allow the phosphorus ligands to come off of
the metal center and to be replaced by other ligands
such as the amine base. To support this hypothesis, we
carried out a series of 31P NMR experiments using 1.
If an arm-on/arm-off equilibrium took place in the
absence of additional ligands, one might expect to freeze
out these states, leading to two distinct 31P NMR signals
at low temperatures that should coalesce at higher
temperatures. However, only a single phosphorus signal
at 15.7 ppm at -70 °C was detected in a variety of
solvents, clearly demonstrating the stability of the Pd-P
bonds at low temperatures. To investigate the role of
the reactants toward the decomposition of the catalyst,
we carried out a series of NMR experiments with
triethylamine and/or iodobenzene present in the catalyst
solution. Initially, equimolar ratios of 1, triethylamine,
and iodobenzene were observed in situ at 120 °C in
DMF. Over a period of several hours, only one phos-
phorus signal was detected. However, addition of 3 equiv
more of triethylamine resulted in the formation of two
new signals in the phosphorus NMR at 7.7 and -4.2
ppm. Furthermore, palladium black formation was
visible, confirming the decomposition of the metal
complex. While we were not able to assign these two
new signals in the NMR unequivocally, they are clearly
indicative of a new phosphorus species.
complex resembles a half-pincer complex. Hartwig and
Louie have shown that half-pincer complexes containing
an amine base as ligand are able to undergo â-hydrogen
elimination, a common reaction pathway of palladium
amides with â-hydrogens, resulting in a palladium
hydride species.27,30 We propose that the second step of
the catalyst decomposition may follow the same path-
way of â-hydrogen elimination, as outlined in Scheme
5, creating a charged palladium hydride species with
the iminium ion as a counterion.
To investigate this proposed decomposition pathway,
we carried out a variety of in situ low- and high-
temperature NMR experiments and mass spectroscopy
experiments, supported by electronic structure compu-
tations and XAS studies. The hypothesis that the
palladated PCP pincer is in an equilibrium between an
arm-on configuration, i.e. a phosphine, bound to the
palladium via a dative bond, and an open coordination
site as a result of the breakage of this dative bond (arm-
off configuration) is essential to most potential decom-
position mechanisms.41,84,85 Therefore, initially we con-
centrated our research efforts to evaluate this hypothesis.
Electronic structure computations were carried out
to establish the energetics of the replacement of the
phosphine ligands by the amine base (Scheme 6).
Optimized geometries (computed using the BP86 den-
sity functional method with a LAV3P/6-31G* basis; see
below) are presented in Figure 9, and relative energies
for the PCP and SCS systems are depicted in Figure
10. For reasons discussed below, the reported energies
include only zero-point vibrational and solvation energy
corrections. These calculations suggested that the “one-
arm-off configuration” of palladated PCP complexes
requires about 21 kcal/mol in the presence of a coordi-
nating ligand such as the amine, an uphill reaction
(84) Poverenov, E.; Gandelman, M.; Shimon, L. J. W.; Rozenberg,
H.; Ben-David, Y.; Milstein, D. Chem. Eur. J. 2004, 10, 4673.
(85) Bassetti, M.; Capone, A.; Salamone, M. Organometallics 2004,
23, 247.
To further investigate if both iodobenzene and tri-
ethylamine are needed for decomposition or if the

Stability of Tethered PdII Pincer Complexes
Organometallics, Vol. 24, No. 18, 2005 4357
Figure 10. Free energy diagram of the relevant minima for the initial steps of the proposed decomposition pathway
computed at the BP86/LAV3P/6-31G* level of theory.
decomposition pathway might follow the one outlined
in Scheme 5, i.e. only the amine base is needed,
additional experiments were carried out to verify the
role of each reactant. Addition of iodobenzene to the
metal complex at 120 °C in DMF resulted in no changes
in the NMR over a period of 24 h and no palladium black
formation was observed, even when 20 equiv of iodo-
benzene was added. In contrast, the addition of 7 equiv
of triethylamine without the presence of an aryliodide
species resulted in the formation of a new signal, 19 ppm
upfield from the original signal. These results suggest
that triethylamine plays an essential role in the decom-
position of the palladium/pincer complex.
Theoretical investigations of the second step of the
proposed decomposition pathway, the â-hydride elimi-
nation, would be very challenging and are beyond the
scope of the present study. Nevertheless, initial com-
putations of subsequent steps for the simpler SCS
system indicate that hydride transfer to the aromatic
carbon and concomitant cleavage of the carbon-pal-
ladium bond is energetically downhill by at least 20 kcal/
mol relative to the intact palladacycle. To probe the
proposed â-hydride elimination step of the decomposi-
tion pathway, we carried out a series of mass spectros-
copy experiments. On the basis of the work of Hartwig
and Louie,30 the amine base will exist, after â-hydride
elimination, as an iminium ion that can be hydrolyzed
to yield a secondary amine. The presence of this second-
ary amine can be easily characterized using mass
spectroscopy. Therefore, characterization of a reaction
mixture using mass spectroscopy (ESI) is a facile
method to evaluate the proposed â-hydride elimination
decomposition step. For ease of characterization, dicy-
clohexylmethylamine was employed in the mass spec-
troscopy studies instead of the otherwise used triethyl-
amine. The experiments were carried out by dissolving
a mixture of catalyst 1 and dicyclohexylmethyl amine
in DMF and heating it at 120 °C for several hours. Every
1 h, an aliquot was taken from the reaction mixture and
analyzed by mass spectroscopy. All aliquots analyzed
via mass spectroscopy (ESI) showed molecular ion
signals at 182.3. This molecular ion signal is indicative
of the presence of dicyclohexylamine. This result sup-
ports the hypothesis that the decomposition pathway
goes through a â-hydrogen elimination step before the
palladium(0) leaches out.
In two recent papers, we communicated also the
decomposition of palladated SCS pincer complexes dur-
ing Heck catalysis.57,58 To investigate whether these
palladated SCS pincer catalysts also follow the decom-
position pathway identified for their PCP analogues, we
carried out XAS experiments and computational studies.

4358 Organometallics, Vol. 24, No. 18, 2005
Sommer et al.
study clearly supports these reports. It is important to
note that the majority of our computational and NMR
studies, in contrast to all XAS experiments, were carried
out in the absence of an aryl iodide. When no aryl iodide
was present during the NMR experiments, palladium
black formation was observed, suggesting the formation
of metallic palladium. However, when NMR experi-
ments were carried out in the presence of aryl iodide,
palladium black formation was significantly less pro-
nounced, suggesting the storage of the majority of
palladium in a form other than Pd(0). On the basis of
our XAS results, palladium iodide species are therefore
the likely storage species. Overall, the XAS results
clearly demonstrate that the SCS pincer complexes are
altered under reaction conditions, thereby substantiat-
ing the NMR and computational studies, while suggest-
ing that the most abundant soluble palladium species
under reaction conditions are palladium iodides.
When the experimental and computational data on
the decomposition of palladated PCP and SCS pincer
complexes are combined, it is obvious that the amine
base plays the key role in the decomposition pathway.
On the basis of the data outlined above, we suggest that
catalyst decomposition could occur through exchange of
a phosphorus or sulfur ligand (one arm of the pincer
ligand) with triethylamine, followed by a â-hydrogen
elimination of the base and a rapid second ligand
exchange, thereby changing the pincer ligands from
tridentate ligands to monodentate ones. Furthermore,
our calculations follow the reported trend of PCP-based
complexes being significantly more stable than their
SCS counterparts.86 However, at the commonly em-
ployed reaction temperatures, both complexes decom-
pose, thereby releasing soluble palladium species. Al-
though the XAS studies clearly indicate that the primary
Pd species in solution is a Pd(II) moiety, the Hg(0)
poisoning studies unequivocally show that the catalytic
cycle contains a Pd(0) intermediate, as Hg(0) addition
quenches essentially all activity. As noted in our earlier
studies, however, the nature of the true zerovalent
catalyst cannot be conclusively determined with the
data here (Pd(0) colloid40 or molecular Pd(0) species87-89),
as it is expected that Hg(0) would poison either type of
species.90 Nonetheless, a number of observations imply
Figure 11. Optimized structures of the calculated ex-
change pathway of sulfur by a trimethylamine base of
palladated pincer complexes: (A) Chemdraw representa-
tion of structures used in calculation; (B) fully intact
complex; (C) removal of one sulfur and addition of the
amine base; (D) exchange of the second sulfur by another
amine base.
Table 1. Relative BP86/LAV3P/6-31G* Energies in
kcal mol-1 for Optimized Structuresa
∆Eelec +∆Esolv
∆Eelec +∆Esolv ∆ZPVE
+
∆Eelec
compd
(kcal mol-1
)
(kcal mol-1
)
(kcal mol-1
)
PCP-Pd-Cl
0.0
17.0
25.5
0.0
0.0
19.9
31.0
0.0
0.0
21.1
34.1
0.0
PCP-Pd(Cl)(NEt3)
PCP-Pd(Cl)(NEt3)2
SCS-Pd-Cl
SCS-Pd(Cl)(NEt3)
SCS-Pd(Cl)(NEt3)2
2.6
5.6
7.0
-4.2
3.2
6.4
a Subscript letters denote noncoordinated atoms.
The electronic structure computations of the palladated
SCS pincer complex (optimized structures are outlined
in Figure 11; relative energies shown in Table 1)
suggested that the one-arm-off configuration is only 7.0
kcal/mol higher in energy than the palladated SCS
pincer complex in the presence of an amine base.
Furthermore, removal of the second arm is calculated
to be downhill relative to the one-arm-off configuration
by 0.6 kcal/mol. These calculations are supported by the
observation of palladium black formation in the reaction
vessel when adding only 1 equiv of triethylamine to a
poly(norbornene)-supported SCS Pd(II) pincer complex.
This is in stark contrast to the required 7 equiv of
triethylamine required for the visible decomposition of
the PCP analogue 1 and can be explained by the known
lesser stability of SCS complexes.54,86
The in situ XAS data on both the polymeric and silica-
immobilized palladated SCS pincer complexes, under
Heck reaction conditions, show the formation of pal-
ladium iodide species, while no metallic palladium was
found under the reaction conditions (for a detailed
description of the XAS data, see the Supporting Infor-
mation). It has been reported in the literature that
soluble palladium species are stored as palladium
halides, such as the bridged [Pd2I6]2- anion.106 Our XAS
(87) Cassol, C. C.; Umpierre, A. P.; Machado, G.; Wolke, S. I.;
Dupont, J. J. Am. Chem. Soc. 2005, 127, 3298.
(88) de Vries, A. H. M.; Mulders, J. M. C. A.; Mommers, J. H. M.;
Henderickx, H. J. W.; de Vries, J. G. Org. Lett. 2003, 5, 3285.
(89) Reetz, M. T.; De Vries, J. G. Chem. Commun. 2004, 1559.
(90) It has been argued that precursor complexes such as pallada-
cycles decompose to give active Pd(0) nanoparticles. This is because
previous Hg test results where Hg(0) poisons catalysis have been
interpreted as proof for catalysis by heterogeneous catalysts (colloids
in this case). We hypothesize here that Hg(0) will also extinguish
catalysis by molecular Pd(0) “naked” species87-89 that are not protected
by strongly bound ligands. Unfortunately, at times, the historic
literature is interpreted as saying that the Hg test distinguishes
between homogeneous and heterogeneous catalysis. This is often the
case in the context of the original studies. Indeed, the historic
literature104,105 with the Hg(0) test focuses on hydrogenation reactions
with metal complexes in elevated formal oxidation states bound by
protective ligands. Certainly, these catalysts are not affected by Hg-
(0), as they are not M(0) species and they are protected by strong
ligands. We have also shown that Pd(II) pincers are also unaffected
by Hg(0) when carrying out stoichiometric reactions where the ligand
remains intact and the complex is in a Pd(II) state.57 However, we
hypothesize that “naked” molecular Pd(0) species87-89 that have been
postulated to be the true active catalytic species in some cases are
examples of homogeneous catalysts that should be affected by Hg(0),
as a consequence of their lack of protecting strong ligands and their
M(0) state.
(86) van der Boom, M. E.; Milstein, D. Chem. Rev. 2003, 103, 1759.

Stability of Tethered PdII Pincer Complexes
Organometallics, Vol. 24, No. 18, 2005 4359
calibrated using poly(styrene) standards on a Styragel HR 4
and HR 5E column set with CH2Cl2 as an eluent. FT-Raman
spectra were obtained on a Bruker FRA-106 instrument. At
least 128 scans were collected for each spectrum, with a
resolution of 2-4 cm-1. Elemental analyses were carried out
by either Atlantic Microlabs, Norcross, GA (CHN analyses),
or Galbraith Laboratories, Inc., TN (determination of the
palladium loadings of the silica precatalysts). SBA-15 (100 Å
pore size) was synthesized following a literature method.92 The
as-prepared material was calcined using the following tem-
perature program: (1) increasing the temperature (1.2 °C/min)
to 200 °C, (2) heating at 200 °C for 1 h, (3) increasing at 1.2
°C/min to 550 °C, and (4) holding at 550 °C for 6 h. Prior to
functionalization, the SBA-15 was dried under vacuum at room
temperature overnight and then at 120 °C/min for 3 h and
stored in a drybox.
Computations. All of the calculations presented in this
work used the pseudospectral implementation93 of density
functional theory within Jaguar.94 All calculations employed
the nonlocal gradient corrected exchange functional of Becke95
and the correlation functional of Perdew,96 the combination
commonly referred to as BP86. The relativistic effective core
potentials of Hay and Wadt, LAV3P, were used for Cl, P, and
Pd atoms.97,98 A 6-31G* basis set was used for first- and second-
row atoms.99 Harmonic vibrational frequencies were computed
for each of the stationary points to verify the nature of the
optimized structures. Zero-point vibrational energy corrections
were computed from the harmonic frequencies, scaled by
1.0108 as suggested in the literature.100 Due to the difficulty
in computing accurately the changes in translational and
rotational entropy upon coordination101 and the presence of
large-amplitude vibrations in these systems, which would not
be accurately modeled in the harmonic oscillator approxima-
tions, corrections for finite temperature were not included.
Implicit solvent effects were incorporated at the gas-phase
optimized structures using the self-consistent reaction field
approach as implemented in Jaguar.102 The density of DMF
at 120 °C was taken to be 0.8521 g/cm3, and the dielectric
constant was taken to be 24.7.103 The SCRF calculations,
employing the same functional and basis set used in the rest
of the work, were used to determine energy corrections for each
structure. The energies reported throughout this work, unless
otherwise stated, include both the zero-point vibrational
energy and the solvation energy corrections.
that molecular species could be important in the Heck
reaction, including the observed reactivity and the high
concentration of [Pd2I6]2- species in ligand-free Pd
catalysts,89 our XAS observations here, kinetic and other
data presented in the literature,87,88 and the fact that
enantioselective Heck reactions are possible with chiral
ligands.91 None of these observations would be expected
if Pd(0) colloids were the primary active species.
Conclusions
We have demonstrated using a variety of leaching
experiments and kinetic studies that palladated PCP
pincer complexes (homogeneous species as well as
complexes immobilized on soluble and insoluble sup-
ports), which have been identified in the literature as
stable entities,67,68 decompose under Heck reaction
conditions. Through the employment of computational
methods, X-ray absorption spectroscopy, mass spectros-
copy, and in situ NMR spectroscopy, we have proposed
the initial steps of the decomposition pathway of these
PCP as well as SCS pincer complexes. It is important
to note that our conclusions are limited to the organic
bases used in this paper. Heck catalysis using palla-
dated pincer ligands have also been carried out using
inorganic bases such as K2CO3. We do not suggest a
decomposition pathway when inorganic bases are used.
The data outlined in this contribution combined with
recent reports from the literature57,58,69,74 call into
question whether any palladated pincer complexes that
have a palladium(II) metal center are truly stable under
the reaction conditions for carbon-carbon bond forma-
tions such as Heck and Suzuki couplings. Indeed, others
have recently observed the decomposition of Pd(II) PCP
pincers in related coupling reactions.107 Rather, these
species should be referred to as precatalysts and sup-
ported analogues as recyclable precatalyst sources.
Experimental Section
General Considerations. All reactions with air- and
moisture-sensitive compounds were carried out under a dry
nitrogen/argon atmosphere using an MBraun UniLab 2000
drybox and/or standard Schlenk line techniques. DMF, n-butyl
acrylate, and NEt3 were distilled over calcium hydride. 5-Ami-
noisophthalic acid dimethyl ester, poly(4-vinylpyridine), (3-
isocyanatopropyl)triethoxysilane, and all bases were obtained
from commercial sources and generally used without further
purification. Gas chromatographic analyses were performed
on a Shimadzu GC 14-A gas chromatograph equipped with a
flame-ionization detector with an HP-5 column (length 30 m,
inner diameter 0.25 mm, and film thickness 0.25 um). The
temperature program for GC analysis was as follows: heating
from 50 to 140 °C at 30 °C/min and heating from 140 to 300
°C at 40 °C/min under constant pressure with inlet and
detector temperatures kept constant at 330 °C. 1H (300 MHz)
and 13C NMR (75 MHz) spectra were recorded on a Varian
Mercury VX instrument. 31P NMR (162 MHz) spectra were
recorded on a Bruker AMX 400 MHz instrument using H3-
PO4 as a calibration standard. All spectra were referenced to
residual proton solvent. Mass spectral analyses were provided
by the Georgia Tech MassSpectrometry Facility using a VG-
70se spectrometer. Gel-permeation chromatography analyses
were carried out using a Waters 1525 binary pump coupled to
a Waters 2414 refractive index detector. The GPC was
Synthesis of Palladium Chloride N-{3,5-Bis[(diphen-
ylphosphanyl)methyl]phenyl}acetamide (1). Inside a ni-
trogen-filled drybox, KPPh2 (4.1 mL, 0.5 M in THF) was slowly
added to a THF solution (20 mL) of 5 (220 mg, 0.94 mmol).
The reaction mixture was removed from the drybox, refluxed
for 3 h, and cooled to room temperature. The solvent was
removed under vacuum, and dry CH2Cl2 (50 mL) was added.
The solution was washed with degassed H2O (2 × 20 mL) and
dried over anhydrous Na2SO4 and the solvent removed. The
(92) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, J. D. J.
Am. Chem. Soc. 1998, 120, 6024.
(93) Friesner, R. A. Annu. Rev. Phys. Chem. 1991, 42, 341.
(94) Jaguar 5.5; Schrodinger, LLC, Portland, OR, 1991-2003.
(95) Becke, A. D. Phys. Rev. A 1988, 38, 3098.
(96) Perdew, J. P. Phys. Rev. B 1986, 33, 8822.
(97) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270.
(98) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299.
(99) Rassolov, V. A.; Pople, J. A. J. Chem. Phys. 1998, 109, 1223.
(100) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502.
(101) Amzel, L. M. Proteins: Struct., Funct., Genet. 1997, 28, 144.
(102) Tannor, D. J.; Marten, B.; Murphy, R.; Friesner, R. A.;
Goddard, W. A., III. J. Am. Chem. Soc. 1994, 116, 11875.
(103) Technical Leaflet M5351 e; BASF, NJ, 2000.
(104) Anton, D. R.; Crabtree, R. H. Organometallics 1983, 2, 855.
(105) Foley, P.; DiCosimo, R.; Whitesides, G. M. J. Am. Chem. Soc
1980, 102, 6713.
(106) Evans, J.; O’Neill, L.; Kambhampati, V. L.; Rayner, G.; Turin,
S.; Genge, A.; Dent, A. J.; Neisius, T. Dalton 2002, 2207.
(107) Olsson, D.; Nilsson, P.; El Masnoury, M.; Wendt, O. F. Dalton
2005, 1924.
(91) Shibasaki, M.; Vogl, E. M.; Ohshima, T. Adv. Synth. Catal.
2004, 346, 1533.

4360 Organometallics, Vol. 24, No. 18, 2005
Sommer et al.
crude product was redissolved in 15 mL of CH2Cl2/CH3CN (v/v
1:2), and PdCl2(NCPh)2 (320 mg, 0.83 mmol) was added. The
reaction mixture was stirred at room temperature for 30 min,
followed by the addition of AgBF4 (485 mg, 2.49 mmol). After
it was stirred for an additional 30 min, the mixture was diluted
with CH2Cl2 (250 mL) and stirred with a saturated brine
solution (200 mL) for 5 h. The organic layer was then
separated, dried over Na2SO4, and passed through a short
silica gel column. Solvent removal and recrystallization from
CH2Cl2/Et2O yielded 1 as a yellow powder. Yield: 340 mg
cooled to room temperature, and transferred into a glovebox,
where it was poured into a degassed solution of sodium
hydroxide (2 M, 50 mL). The product was then extracted from
the aqueous reaction mixture with toluene (2 × 50 mL), dried
over MgSO4, and the solvent removed to yield an off-white oil,
which was dissolved into CH2Cl2/CH3CN (5 mL/10 mL) fol-
lowed by the addition of PdCl2(NCPh)2 (154 mg, 0.4 mmol).
The reaction mixture was stirred for 30 min before the addition
of AgBF4 (195 mg, 1 mmol). The mixture was stirred for
another 30 min, diluted with CH2Cl2 (200 mL), poured into a
saturated brine solution, and stirred for 12 h. The organic
phase was then separated and dried over MgSO4, and the
solvent was removed to yield an orange oil that was purified
via a chromatography column (1:1 ethyl acetate/hexanes).
Yield: 34%. 1H NMR (CDCl3, 300 MHz): δ 7.92-7.84 (m, 10H,
PPh2), 7.51-7.34 (m, 12H), 6.4 (s, 4H), 6.06-5.89 (m, 2H), 4.37
(br s, 4H), 2.71 (s, 2H), 2.29-2.24 (t, J ) 7.3 Hz, 2H), 1.95-
1.75 (m, 1H), 1.324 (t, J ) 6.1 Hz, 2H), 1.324-1.15 (br m, 19H),
1.08-1.00 (m, 1H), 0.92-0.83 (m, 1H), 0.51-0.45 (m, 1H).13C
NMR (CDCl3, 300 MHz): δ 171.9, 158.0, 149.8, 148.8, 133.9,
133.9, 131.9, 133.8, 132.4, 132.3, 132.2, 131.5, 131.4, 129.9,
129.7, 129.6, 128.1, 127.9, 113.8, 51.3, 49.5, 45.3, 42.4, 38.7,
37.5, 34.7, 32.4, 29.9, 29.6, 29.5, 29.3, 28.6, 25.7. HRMS
(ESI): m/z 905.3275. Anal. Calcd for C51H58ClNOP2Pd: C,
67.70; H, 6.46; N, 1.55; found C, 67.52; H, 6.42; N, 1.67.
Synthesis of Palladium Chloride 1-(Allyloxy)-3,5-bis-
[(diphenylphosphanyl)methyl]benzene (16). To a dry
DMF (20 mL) solution of complex 8 (400 mg, 0.77 mmol) were
added K2CO3 (370 mg, 2.68 mmol) and allyl bromide (0.2 mL,
2.31 mmol). The resulting mixture was heated at 90 °C for 20
h. After the mixture was cooled to room temperature, DMF
was distilled off and the product was extracted with CH2Cl2
(2 × 50 mL) and the extract washed with H2O (2 × 20 mL)
and dried over anhydrous Na2SO4. Purification through a flash
silica gel column afforded 15 as an off-white oily product.
Complex 15 (410 mg, 0.73 mmol) and NEt3 (2.5 mL, 17.9 mmol)
were then suspended in degassed m-xylene (25 mL), flushed
with argon, followed by the addition of Cl3SiH (2.2 mL, 21.8
mmol) via a syringe. The reaction vessel was then closed and
stirred at 120 °C for 14 h. After it was cooled to room
temperature, the reaction mixture was poured into a degassed
2 N NaOH (100 mL) solution, extracted with dry toluene (2 ×
100 mL), and dried over anhydrous Na2SO4. Solvent removal
afforded a light yellow oily product, which was redissolved in
a mixture of CH2Cl2 (5 mL) and CH3CN (10 mL). PdCl2(NCPh)2
(220 mg, 0.57 mmol) was added, and the resulting mixture
was stirred at room temperature for 30 min, followed by the
addition of AgBF4 (485 mg, 2.49 mmol) in one portion. After it
was stirred for an additional 30 min, the mixture was diluted
with CH2Cl2 (250 mL) and stirred with a saturated brine
solution (200 mL) for 5 h. The organic layer was separated,
dried over Na2SO4, and passed through a short silica gel
column. Solvent removal and recrystallization from CH2Cl2/
Et2O gave 16 as a yellow powder. Yield: 210 mg (55%). 1H
NMR (d6-DMSO, 300 MHz): δ 8.01-7.87 (m, 4H), 7.71-7.40
(m, 16H), 7.08 (s, 2H), 5.88 (m, 1H), 5.21 (m, 2H), 4.10 (s, 2H),
3.75 (bs, 4H). Anal. Calcd for C35H31ClOP2Pd: C, 62.61; H,
4.65; Found: C, 62.37; H, 4.52;
1
(61%). H NMR (d6-DMSO, 300 MHz): δ 9.86 (s, 1H), 8.21-
7.77 (m, 10H), 7.48-7.37 (m, 10H, Ph), 7.28 (s, 2H), 4.09 (bs,
4H), 1.96 (s, 3H). Anal. Calcd for C34H30ClNOP2Pd: C, 60.73;
H, 4.50; N, 2.08. Found: C, 60.34; H, 4.32; N, 1.94.
Synthesis of Palladium Chloride Bicyclo[2.2.1]hept-
5-ene-2-carboxylic Acid 11-{3,5-bis[(diphenylphosphanyl)-
methyl]phenoxy}undecyl Ester (10). Bicyclo[2.2.1]hept-5-
ene-2-carboxylic acid 11-bromoundecyl ester (767 mg, 2.06
mmol) was slowly added over a period of 15 min to a stirred
solution of 8 (1.08 g, 2.06 mmol) and potassium carbonate (571
mg, 4.1 mmol) in DMF. The mixture was heated to 90 °C and
stirred for 12 h. The DMF was removed in vacuo, the crude
product dissolved in methylene chloride (100 mL), and this
solution washed with 1 N HCl (50 mL), sodium bicarbonate
(50 mL), and brine (50 mL). The organic layers were dried over
magnesium sulfate and the solvent removed under vacuum
to yield 9 as a viscous brown oil, which was partially purified
via column chromatography (eluent ethyl acetate followed by
methanol) and then used without further purification. Com-
pound 9 (1.37 g, 1.7 mmol) was dissolved in degassed m-
xylenes (50 mL), and triethylamine (5.7 mL, 40.5 mmol) was
slowly added, followed by the dropwise addition of trichlorosi-
lane (4.1 mL, 40.5 mmol). The reaction mixture was heated to
120 °C for 10 h. After the mixture was cooled to room
temperature, it was poured into a degassed solution of sodium
hydroxide (500 mL). The product was then extracted from the
aqueous reaction mixture with toluene (2 × 100 mL), the
extract dried over magnesium sulfate, and the solvent removed
under vacuum to yield a yellow oil. The oil was dissolved in
CH2Cl2/CH3CN (5 mL/10 mL), and PdCl2(NCPh)2 (652 mg, 1.7
mmol) was added. The solution was stirred for 30 min before
the addition of AgBF4 (823 mg, 4.2 mmol). The mixture turned
yellow and was stirred for another 30 min, diluted with CH2-
Cl2 (200 mL), poured into a concentrated brine solution (500
mL), and stirred vigorously for 5 h. The organic layer was
separated and dried over MgSO4, and the solvent was removed
under vacuum to yield an orange oil that was purified via
column chromatography (1:1 ethyl acetate/hexanes) to give 2
1
as an orange solid. Yield: 62%. H NMR (CDCl3, 300 MHz):
δ 7.91-7.83 (m, 10H), 7.52-7.34 (m, 12H), 6.7 (s, 4H), 6.08-
5.88 (m, 2H), 4.23 (br s, 4H), 3.83 (t, 2H, J ) 6.2 Hz), 3.20-
3.13 (m, 1H), 3.00-2.99 (m, 1H), 2.94-2.82 (m, 4H), 2.22-
2.13 (m, 1H), 1.92-1.82 (m, 2H), 1.78-1.64 (m, 4H), 1.63-
1.50 (m, 4H), 1.42-1.31 (m, 5H), 1.32-1.20 (m, 28H). HRMS
(ESI): m/z 885.2534. Anal. Calcd for C51H57ClO3P2Pd: C,
66.45; H, 6.23. Found: C, 66.82; H, 6.11.
Synthesis of Palladium Chloride 12-Bicyclo[2.2.1]hept-
5-en-2-yldodecanoic Acid {3,5-Bis[(diphenylphosphanyl)-
methyl]phenyl}amide (14). To a solution of 12-bicyclo[2.2.1]-
hept-5-en-2-yldodecanoic acid (120 mg, 0.4 mmol) in CH2Cl2
were added 1-hydroxybenzotriazole (55 mg, 0.4 mmol) and
dicyclohexylcarbodiimide (85 mg, 0.4 mmol) with a few drops
of DMF. To the reaction mixture was added complex 12 (214
mg, 0.4 mmol), and this mixture was stirred for 12 h. The
reaction mixture was then filtered through a small patch of
silica and the solvent removed under vacuum. Without further
purification, compound 13 was dissolved in degassed xylenes
(50 mL) followed by the addition of triethylamine (0.56 mL, 4
mmol) and dropwise addition of trichlorosilanes (0.56 mL, 4
mmol). The reaction mixture was heated to 120 °C for 12 h,
Synthesis of Palladium Chloride 1-[3-((3-(Trimethoxy-
silyl)propyl)sulfanyl)propoxy]-3,5-bis[(diphenylphos-
phanyl)methyl]benzene (17). Complex 16 (180 mg, 0.27
mmol), (3-mercaptopropyl)trimethoxysilane (53 mg, 0.27 mmol),
and AIBN (10 mg) were mixed in dry CHCl3 (25 mL), and the
resulting mixture was refluxed under argon for 24 h. After
the mixture was cooled to room temperature, the solvent was
removed under reduced pressure and the resulting yellow solid
residue was washed with hexanes (2 × 10 mL) to afford 17 as
a yellow crystalline solid. Yield: 200 mg (85%). 1H NMR
(CDCl3, 300 MHz): δ 8.00-7.83 (m, 4H), 7.57-7.28 (m, 16H),
7.01 (s, 2H), 4.20 (m, 2H), 3.82 (bs, 4H), 3.54 (bs, 9H), 2.47
(m, 4H), 1.93 (m, 2H), 1.53 (m, 2H), 0.39 (m, 2H).

Stability of Tethered PdII Pincer Complexes
Organometallics, Vol. 24, No. 18, 2005 4361
General Procedure for the Synthesis of Polymers 2
and 3. The respective monomers were dissolved in CDCl3,
followed by the addition of the desired amount of catalyst. The
polymerization was monitored via NMR. After completion of
the polymerization a few drops of ethyl vinyl ether were added.
The polymer was then purified by reprecipitation from chlo-
roform into cold methanol. The purification procedure was
repeated until the resulting methanol solution became color-
less. The methanol was then decanted and the resulting
polymer dried in vacuo.
and DMF. A vial was loaded with the catalyst, n-butyl acrylate,
iodobenzene, and DMF. The solution was heated to 120 °C.
When the temperature reached 120 °C, triethylamine was
added in one portion to the solution, at which point time 0
was taken for the kinetic data analysis.
Mass Spectroscopy. In a reaction flask, catalyst 1 (23.5
mg, 0.035 mmol) and dicyclohexylmethylamine (13.6 mg, 0.07
mmol) were dissolved in DMF. The reaction mixture was
heated to 120 °C and stirred for 1 h before an aliquot (0.007
mL) was removed from the vessel and analyzed via ESI mass
spectroscopy (MS-ESI). Another 2 equiv (based on 1) of
dicyclohexylmethylamine was added to the reaction mixture,
which was stirred for an additional 1 h before another aliquot
was taken and analyzed by MS-ESI. The same operation was
repeated for 24 h. ESI mass spectroscopy spectra of all aliquots
showed molecular ion signals at m/z 182.3.
Polymer 2. 1H NMR (CDCl3, 300 MHz): δ 7.8 (m, 10H),
7.34 (m, 12H), 6.49 (br s, 4H), 5.50-5.17 (br m, 2H), 4.23 (br
s, 4H), 4.00-3.85 (m, 2H), 3.19-2.28 (br m, 3H), 2.01-1.55
(br m, 8H), 1.53-1.02 (m, 14H). 13C NMR (CDCl3, 300 MHz):
δ 158.0, 149.8, 148.8, 133.9, 133.9, 131.9, 133.8, 132.4, 132.3,
132.2, 131.5, 131.4, 129.9, 129.7, 129.6, 128.1, 127.9,
1
Polymer 3. H NMR (DMSO, 300 MHz): δ 7.7 (m, 10H),
7.4 (m, 12H), 6.6 (br s, 4H), 5.53-5.14 (br m, 2H), 4.5 (br m,
4H), 3.31 (s, 4H), 2.13 (br m, 2H), 1.45-0.88 (br m, 23H). 13C
NMR (DMSO, 300 MHz): δ 170.3, 158.0, 149.8, 148.8, 133.9,
133.9, 131.9, 133.8, 132.4, 132.3, 132.2, 131.5, 131.4, 129.9,
129.7, 129.6, 128.1, 127.9, 118.9, 113.7, 50.0, 36.4, 30.8, 29.7,
29.4, 25.2.
Acknowledgment. We thank the U.S. Department
of Energy, Basic Energy Sciences, for financial support
through Catalysis Science Grant/Contract No. DE-
FG02-03ER15459. The X-ray absorption spectroscopy
research was carried out in part at the National
Synchrotron Light Source, Brookhaven National Labo-
ratory, which is supported by the U.S. Department of
Energy, Division of Materials Sciences and Division of
Chemical Sciences, under Contract Nos. DE-AC02-
98CH10886 and DE-FG-03ER15460.
Synthesis of SBA-15 Palladium Chloride 1-[3-((3-(Tri-
methoxysilyl)propyl)sulfanyl)propoxy]-3,5-bis[(diphen-
ylphosphanyl)methyl]benzene (4). Complex 17 (200 mg,
0.23 mmol) was mixed with SBA-15 (1.0 g) in dry toluene (50
mL), and the mixture was stirred at room temperature for 24
h, at which point the mixture was filtered and washed
extensively with DMF and CH2Cl2 and then dried under high
vacuum to afford 4. Elemental analysis indicated a palladium
loading of 0.12 mmol of Pd/g of support. FT-Raman (spectrum
in the Supporting Information): 3061 (aromatic C-H), 2936
(aliphatic C-H), 1024 (C-O stretching, CH2OAr), 664 (C-S
Supporting Information Available: Text, figures, and
tables giving a detailed description of the XAS results and the
XYZ coordinates of all computational experiments. This mate-
rial is available free of charge via the Internet at http://
pubs.acs.org.
stretching, CH2SCH2) cm-1
.
General Procedure for the Catalysis. The reaction was
carried under an inert atmosphere using freshly distilled NEt3
OM048992V