Evidence that SCS pincer Pd(II) complexes are only precatalysts in Heck catalysis and the implications for catalyst recovery and reuse

Article abstract of DOI:10.1002/adsc.200404264

SCS-pincer Pd(II) complexes have been covalently immobilized onto porous silica, cross-linked polymer (Merrifield resin) and soluble poly(norbornene) supports via either amide or urea linkages and evaluated in the Heck coupling of iodobenzene with n-butyl acrylate. Kinetic experiments and poisoning studies indicate that in all cases the pincer complexes merely act as precatalysts. Contrary to literature reports, there is no evidence for catalysis by the intact amide-linked SCS-Pd(II) complexes under any conditions studied here.

Full text of DOI:10.1002/adsc.200404264

FULL PAPERS
Evidence that SCS Pincer Pd(II) Complexes are only
Precatalysts in Heck Catalysis and the Implications for Catalyst
Recovery and Reuse
Kunquan Yu,a William Sommer,b John M. Richardson,a Marcus Weck,b,*
Christopher W. Jonesa, b,*
a
School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA
Fax: (þ1)-404-894-2866, e-mail: cjones@chbe.gatech.edu
b
School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332, USA
E-mail: marcus.weck@chemistry.gatech.edu
Received: August 20, 2004; Accepted: September 29, 2004; Published online: January 11, 2005
Abstract: SCS-pincer Pd(II) complexes have been co- complexes merely act as precatalysts. Contrary to lit-
valently immobilized onto porous silica, cross-linked erature reports, there is no evidence for catalysis by
polymer (Merrifield resin) and soluble poly(norbor- the intact amide-linked SCS-Pd(II) complexes under
nene) supports via either amide or urea linkages any conditions studied here.
and evaluated in the Heck coupling of iodobenzene
with n-butyl acrylate. Kinetic experiments and poi- Keywords: biphasic catalysis; Heck reaction; immobi-
soning studies indicate that in all cases the pincer lization; leaching; palladium; pincer complex
Introduction
tures and they can effectively promote Heck reactions in
air. Indeed, in recent years several elegant examples of
Heck coupling is an important carbon-carbon bond Heck catalysis by “recyclable catalysts” based on poly-
forming reaction that is most often catalyzed by palladi- mer immobilized SCS pincer Pd complexes have been
um catalysts. Both homogeneous and heterogeneous[1] reported,[9] where the “catalysts” are shown to be capa-
precatalysts have been employed. Traditional heteroge- ble of promoting the Heck coupling reaction of iodoar-
neous catalysts, such as Pd(II)-exchanged oxides or res- enes and terminal olefins even in the presence of air.
ins[2] or Pd(0) on standard supports such as activated car- Furthermore, the supported species was used repeatedly
bon[3] or oxides[4] have been used. In addition, metallic with little or no decrease in reaction yields.[9] However,
Pd nanoparticles[5] have also been employed as homoge- some SCS pincer Pd(II) complexes, such as those with
neous or quasi-homogeneous systems. Despite the pre- an ether linkage between the complex and the support
ponderance of solid catalysts for Heck reactions, the de- (Scheme 2, a), have been shown to decompose under re-
velopment of stable, recyclable supported catalysts re- action conditions.[9a,10]
mains an important goal. This is due to the fact that,
Despite the observations of complex decomposition
for many heterogeneous “catalysts”, there is substantial with SCS pincer Pd(II) complexes with ether linkages
evidence that leached/dissolved Pd species are the true (SCS-O), similar complexes with subtly different ligand
active species.[3,6] However, in other studies it is main- structures including an amide-containing linkage
tained that all the evidence supports heterogeneous ac- (Scheme 2, b) to the support are reported to promote
tive species.[4a,7] If a truly heterogeneous catalyst that can Heck reactions in multiple uses, implying that they are
be conclusively shown to impart no leached metal spe- recyclable catalysts.[9] Indeed, results similar to those re-
cies into solution were developed, this would be a major
advance, as residual metal contamination is the primary
drawback of current Heck catalysis techniques.
One of the most promising, well-defined transition
metal catalysts for Heck and Suzuki coupling reactions
are Pd(II) pincer complexes (SCS, NCN, PCP)
(Scheme 1).[8] It has been suggested that these catalysts
are quite stable compared to other precatalysts because
they are stable in organic solvents at elevated tempera- Scheme 1. Pincer palladium(II) complexes.
Adv. Synth. Catal. 2005, 347, 161–171
DOI: 10.1002/adsc.200404264
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Scheme 2. SCS-N pincer Pd(II) complexes.
ported in the literature have been replicated on polymer
and silica supported amide-linked SCS pincer Pd(II)
complexes in our laboratories. As in previous reports,
solids containing tethered Pd(II) SCS-N pincer com-
plexes can be added to reaction media to promote the
Heck reaction of iodoarenes and acrylates and following
the reaction, the solids can be recovered and subse-
quently used to promote additional Heck reactions giv-
ing similar yields. However, further experimentation
presented here indicates that the notion that these im-
mobilized complexes are acting as recyclable catalysts
is false.
Scheme 3. Synthesis of precatalyst 5.
By combining the study of homogeneous, soluble poly-
(norbornene), insoluble Merrifield resin and insoluble
porous silica materials as supports for SCS pincer
Pd(II) complexes, we have obtained compelling evi-
dence that immobilized Pd-SCS pincer complexes mere-
ly serve as recoverable and reusable Pd(II) precatalyst
sources that effectively leach Pd species into solution.
The resulting soluble Pd(0) species, as in the related
half-pincer complexes previously described,[11] are the
truecatalytic speciesandwefindnoevidenceforcatalytic
activity by intact SCS pincer Pd complexes. These results
also rule out a possible Pd(II)-Pd(IV) catalytic cycle[8f, 12]
that has been proposed as the catalytic cycle for SCS
Scheme 4. Supported SCS-N pincer complexes.
Porous silica, soluble poly(norbornene) and insoluble
Pd(II) Heck reactions with these “catalysts”. This be- Merrifield resin-supported SCS-N pincer Pd(II) com-
havior was not likely apparent in previous studies due plexes with either an amide or urea linkage were pre-
to the more limited scope of the catalysts employed. Es- pared, as shown in Schemes 3 and 4. Reaction between
pecially with soluble polymeric precatalysts,[9] this obser- the SCS-NH2 complex 2 with 3-isocyanopropyltriethoxy-
vation would be difficult to discern because critical tests silane 3 yields an SCS pincer complex with an Si(OEt)3
such as filtration tests could not be performed reliably.
end group in quantitative yield; palladation was carried
out in refluxing acetonitrile to afford complex 4 with a
urea linkage at the 4 position in its aromaticring. The ho-
mogeneous SCS pincer Pd(II) complex 4 was covalently
immobilized onto mesoporous silica support, e.g., SBA-
15, utilizing a reaction between the -Si(OEt)3 group with
Results and Discussion
Synthesis
ꢀ ꢀ
surface silanols to form Si O Si bonds with the con-
comitant release of ethanol (Scheme 3). A typical load-
As noted above, simply changing the support linkage ing for the SBA-15 supported complex 5 is approximate-
from an ether group to an amide one (Scheme 2, b and ly 0.10 mmol Pd per gram support. Extensive washing
1) has been suggested to drastically alter the behavior with DMF and CH2Cl2 and filtration tests[13] indicated
of SCS-Pd complexes and yield stable and recyclable that essentially all the SCS pincer Pd(II) in 5 is covalent-
catalysts.[9a] To this end, complex 1[9a] and its supported ly bonded to the silica surface.
derivatives were synthesized and evaluated in the
Heck reaction as described below.
Similarly, contacting 2 with isocyanate modified Mer-
rifield resin, followed by metallation with Pd(PhCN)2Cl2
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SCS Pincer Pd(II) Complexes are Precatalysts in Heck Catalysis
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in refluxing MeCN afforded an insoluble Merrifield res- benzene in merely 10 min. However, in the presence
in supported pincer Pd complex 6 (Scheme 4). The poly- of excess Hg(0) (molar ratio to [Pd] ~ 300), 1 was com-
(norbornene)-immobilized 7 (Scheme 4) was synthe- pletely inactive, as shown in Figure 1D. If the Heck reac-
sized in 5 steps starting from 2 and cyclobutadiene. Com- tion were promoted by the intact Pd(II) SCS-N pincer
plex 7 is a close replicate to the PEG supported materi- complex, a hypothetical Pd(II)-Pd(IV) cycle would be
als previously reported,[9a] as both polymer supports expected and Hg(0) should not affect the reaction, as
have an amide linkage (in contrast to the urea linkage it is used as a poison for Pd(0) species. The ability of
in 5 and 6) and are soluble under reaction conditions.
Hg(0) to poison metal particle heterogeneous catalysts,
by amalgamating the metal or adsorbing on the metal
surface, has been known for many years.[14,15] However,
Hg(0) is not expected to have a poisoning effect on mo-
lecular homogeneous organometallic complexes con-
Heck Catalysis
All Heck coupling reactions were carried out using n- taining metals in high oxidation states that are tightly
butyl acrylate (2.74 mmol) and iodobenzene bound by protective ligands, especially pincer com-
(1.37 mmol) in DMF (5 mL) at 1208C using NEt3 plexes that utilize a hypothetical Pd(II)-Pd(IV) cycle.[12]
(2.06 mmol) as base.
Although this assumption about Hg(0) is widely held, it
is rarely probed. To probe this, a chemical transforma-
tion where the complex is known to remain bound to or-
ganic ligands in an elevated oxidation state must be car-
ried out in the presence of Hg(0). In the case of Pd SCS
pincers, Weck and Pollino[16] have shown that poly(nor-
Distinguishing Between Homogeneous and Heteroge-
neous Catalysis
There is a substantial body of work dedicated toward bornene) supported cationic pincer SCS Pd(II) species
distinguishing between homogeneous and heterogene- can serve as a recognition unit for self-assembly with
ous catalysis. For example, Finke has recently reviewed pyridines. We find that this coordination is not disrupted
the methodologies for distinguishing between catalysis in the presence of excess Hg(0). For example, after Cl
by metal nanoparticles and metal complexes when start- abstraction of 8 with AgBF4, coordination still occurs
ing with metal complexes of easily reducible metals (Pd, between the resulting cationic Pd(II) pincer complex
Pt, Ru, etc.) in hydrogenation reactions.[14] By utilizing a and pyridine molecule (Scheme 5), as determined by
combination of experimental tests including kinetic in- 1H NMR. This gives clear evidence that Hg(0) is not in-
vestigations, filtration tests, and poisoning studies, in teracting with the Pd(II) metal center in the pincer com-
many cases it is possible to conclusively determine the plex and this is unlikely the mechanism for catalyst poi-
nature of the true active species. A number of the tech- soning in the presence of Hg(0).
niques outlined in the excellent review by Finke are used
here to show that the intact Pd(II) SCS-N pincer com-
plex is not the active species in Heck catalysis.
Complex 1 was found to be highly active in the Heck
coupling of n-butyl acrylate and iodobenzene, giving
quantitative conversion of the limiting reactant iodo-
Scheme 5. Coordination between poly(norbornene)-SCS-
PdCl (8) and pyridine. Self-assembly experiment was carried
out by adding 1 equiv. of AgBF4 to a CD2Cl2 solution of 8 and
Figure 1. Conversion of iodobenzene as a function of time for
the Heck catalysis with 1 under normal conditions (A), in the
presence of 150 equivs. pyridine (B), in the presence of 300
equivs. PVPy (C), and in the presence of excess Hg(0)
(~300 equivs.) (D). All reagents ratios are relative to complex
1.
the solution was filtered 3 times through celite to remove the
AgCl precipitate. The cationic Pd(II) pincer complex was
then added into a stirred mixture of Hg(0) (20 equivs.) and
pyridine (1 equiv.) in CD2Cl2. The solution turned light green.
Solvent removal and drying under vacuum yielded a light
1
green complex. Coordination was confirmed by H NMR.[16]
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filtrate and it was observed that the Heck reaction con-
tinued at about half the reaction rate prior to the solid
removal. In addition to this filtrate activity, the recov-
ered solid is also shown to retain activity for the Heck re-
action, as has been reported in previous works.[9a,f]
The above data clearly show that some complex de-
composition takes place and that an active, soluble
Pd(0) species is likely formed. However, the data de-
scribed above do not rule out the possibility of a fraction
of the catalysis occurring at intact Pd(II) SCS-N pincer
sites that are still attached to the solid support. To probe
this, a poison that effectively quenches only homogene-
ous catalysis is needed. Insoluble cross-linked poly(vi-
nylpyridine) (PVPy) has been shown to be an excellent
scavenger for soluble molecular Pd[10,18] [and potentially
also for soluble Pd(0) nanoparticles[10]], most likely due
Figure 2. Conversion of iodobenzene as a function of time for
the first (A) and second (B) Heck reaction runs with 5.
The lack of activity of complex 1 in the presence of to its excellent ligand binding ability to Pd and insolubil-
Hg(0) provides the first evidence that 1 likely decom- ity under reaction conditions. The addition of insoluble
poses under reaction conditions to form soluble Pd(0) PVPy to the reaction mixture containing 5 should pref-
species and that Hg(0) interactions with these Pd moiet- erentially shut down catalysis due to homogeneous Pd,
ies completely terminate the reaction. In addition, a col- without affecting sites that are covalently bound to the
or change from light yellow to reddish-orange was ob- SBA-15 support if the assumption that PVPy cannot ac-
served during the Heck reaction with 1 and the intensity cess the pores of SBA-15 is correct. To probe this as-
of the reddish-orange color correlates with the increased sumption, PVPy was used as a basic poison in the acid-
conversion of the reactants. As soluble colloidal palladi- catalyzed dimerization of a-methylstyrene.[19] As shown
um nanoparticles often impart a reddish-orange tint in in Table 1, when a silica-tethered sulfonic acid catalyst is
DMF,[11f] this further suggests a breakdown of the pin- used (with an average pore size larger than the SBA-15
cer-Pd bonds to liberate metallic Pd species. This hy- used here), the addition of PVPy (150 equivs., molar ra-
pothesized decomposition motivated further study of tio to acid sites) results in only a slight decrease of its ac-
supported analogues.
tivity, likely due to some poisoning of acid sites on the
The immobilized complex 5 exhibited high activity for external surface. In contrast, molecular pyridine (150
the Heck reaction, and a 93% GC conversion of iodo- equivs.) almost completely quenches its activity. This
benzene was achieved in 2 h. After recovery, a similar gives evidence that PVPy cannot access the acid sites
yield of 92% conversion was also obtained in 2 h when in the mesopores of the support and provides some foun-
the recovered 5 was reused in a subsequent Heck reac- dation to the notion that it may be used as an effective
tion. However, a closer look at the kinetic plots (Fig- poison for exclusively soluble, accessible species.[20]
ure 2) revealed that the recovered complex 5 has a
It was found that in the presence of excess PVPy (150
much longer induction period and much lower early equivs.), less than 1% iodobenzene conversion was ob-
stage activity in comparison to the first run. Indeed, served for 5 even after 2.5 h. This suggests that the cata-
the sigmoidal kinetic plot shown in Figure 2 is consistent lytic activity from 5 stems solely from leached/unsup-
with catalysis by Pd nanoparticles whereby the active ported Pd species that are soluble in DMF. Furthermore,
nanoparticles are generated by an autocatalytic growth in the presence of excess of Hg(0), no Heck activity was
process.[14,17] Elemental analysis of the solid before and observed with 5. This provides additional evidence that
after the 1st use indicated that 6.7% of the immobilized 5 likely decomposes under reaction conditions to form
palladium was lost. Since all the palladium is covalently soluble Pd(0) species.
bonded to the support before reaction,[13] one must con-
clude that some of the supported Pd metal is leached out cer-Pd 5 under any conditions used here. As the pincer
into the solution during the reaction. Pd complex in 5 is immobilized on a silanol-containing
Thus, no Heck activity is observed from the intact pin-
Thus, if proof of recyclability is based solely on reac- oxide surface and these groups could interact with the
tion yields, one might suggest that 5 is a recyclable cata- Pd complex, potentially aiding complex decomposition,
lyst. However, the kinetic data clearly indicate that de- the examination of other, more inert supports is also im-
composition took place. A filtration test confirms the portant. Therefore, the Heck activity of the insoluble
presence of soluble leached active palladium species. Merrifield resin supported 6, where pincer Pd com-
In this control experiment, a Heck reaction performed plexes are attached to a cross-linked polymeric network,
with 5 was interrupted, and the solid was filtered while was also evaluated.[21] Material 6 is also highly active for
hot under argon after achieving 38% conversion of iodo- the Heck reaction of n-butyl acrylate and iodobenzene,
benzene. Subsequently, fresh reagents were added to the a 92% GC conversion of iodobenzene was achieved in
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Table 1. Summary of tests and observations.
Case
i
Catalyst
Test
Observations and Implications
1
Hg(0) – Heck
No reactivity. Catalysis occurs via a classic Pd(0)-Pd(II) cycle or
Hg(0) reacts with the Pd(II) precatalyst.
ii
iii
iv
1
1
8
PVPy – Heck
No reactivity. PVPy blocks access to Pd sites or removes Pd
species from solution.
High activity. PVPy must poison by removing Pd sites from so-
lution.
Hg(0) does not inhibit the coupling, implying Hg(0) has no affect
on Pd(II) intermediates.
No reactivity. Catalysis occurs via a classic Pd(0)-Pd(II) cycle.
No reactivity. PVPy blocks access to Pd sites. All sites are
homogeneous, no evidence for catalysis by intact Pd(II) SCS-N
pincers.
Pyridine – Heck
Hg(0) – pyridine
self-assembly
Hg(0) – Heck
PVPy – Heck
v
vi
5
5
vii
5
5
Filtration test – Heck
Filtrate shows high activity, giving evidence for catalysis by leach-
ed, soluble species.
Slightly reduced activity. Solid is a reusable precatalyst or cata-
lyst. Kinetics are indicative of potential formation of a Pd(0)
nanoparticle catalyst.
viii
Recycle of 5 – Heck
ix
x
xi
xii
xiii
xiv
6
6
7
7
Hg(0) – Heck
PVPy – Heck
Hg(0) – Heck
PVPy – Heck
AMS coupling
PVPy – AMS coupling
No reactivity. Catalysis occurs via a classic Pd(0)-Pd(II) cycle.
No reactivity. PVPy blocks access to Pd sites.
No reactivity. Catalysis occurs via a classic Pd(0)-Pd(II) cycle.
Very little reactivity. PVPy blocks access to Pd sites.
High activity. 97% conversion of AMS in 2 h.
High activity, 83% conversion of AMS in 2 h. PVPy cannot enter
the pores of the support.
Silica-propyl-SO3H
Silica-propyl-SO3H
xv
Silica-propyl-SO3H
Pyridine – AMS coupling
Low activity, 4% conversion of AMS in 2 h. Pyridine can enter
the pores of the support and neutralize the acid sites.
only 25 min. Similar poisoning studies on 6 showed that mechanism was also adopted on the homogeneous com-
in the presence of PVPy (150 molar equivs.), less than plex 1 (Figure 1C), even though it is less straightforward
1% iodobenzene conversion was achieved after 3 h, in light of the soluble nature of 1. In the presence of 300
and with excess Hg(0), no Heck product was detected equivs. PVPy (molar ratio of pyridine unit to [Pd]), less
even after prolonged reaction times. This gives evidence than 1% iodobenzene conversion was achieved even af-
that 6 behaves like its silica supported analogue 5, and ter 100 min (Figure 1C). As complex 1 was soluble un-
that all the Heck activity stems from the leached/unsup- der reaction conditions, the pyridine unit in PVPy could
ported Pd(0) species.
potentially block the coordination site of the Pd active
Soluble poly(norbornene)-immobilized 7 also exhib- species, inhibiting any catalytic reaction. If this assump-
ited high activity for the Heck coupling of iodobenzene tion is true, addition of molecular pyridine during the re-
and n-butyl acrylate, as 99% GC conversion of iodo- action should suppress the catalytic activity of 1 even
benzene was achieved within 60 min. However, in the more, since the interaction between 1 with soluble pyri-
presence of excess Hg(0) (~500 equivs.) no Heck activ- dine would be much easier than that with the insoluble
ity was observed and with PVPy (150 equivs.), only 6% PVPy. To test these possibilities, a Heck reaction was
GC conversion of iodobenzene was achieved with a re- performed with 1 in the presence of 150 equivs. pyridine
action time of 3 h in both cases. This suggests that 7 acts and 100% conversion of iodobenzene was observed with
like its silica and Merrifield resin supported insoluble only a slightly slower rate than a Heck reaction under
analogues 5 and 6. Contrary to literature reports that normal conditions (Figure 1A and B). This rules out
SCS pincer Pd(II) complexes with an amide linkage the possibility that the mechanism by which PVPy is poi-
are stable and recyclable, the soluble polymer-support- soning the reactivity is by interacting with intact SCS
ed catalyst used here (7) merely acts as a precatalyst. pincer Pd(II) sites. More likely, soluble Pd species are
Indeed, more recent reports of Heck catalysis by poly- leached from the SCS complex and these species are re-
mer-bound SCS Pd pincer complexes[22] indicate that moved from solution by the insoluble PVPy and the ac-
catalysis by homogeneous Pd colloids cannot be ex- tivity difference (Figure 1B and C) observed is due to
cluded.
the solubility difference of the as formed Pd-pyridine
The fact that PVPy shuts down the Heck activity of and Pd-PVPy complexes under the reaction conditions
both insoluble 5 and 6 suggests that likely a similar used here.
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kinetic models can also lead to sigmoidal kinetics.[14] In-
deed, a simple kinetic model for catalysis by molecular
Pd(0) species: A ! B, B þolefinþiodoarene ! Bþ
HIþcoupling product [where A is the pincer Pd com-
plex and B is soluble molecular Pd(0)] can also give a
reasonable fit, with k1 six orders of magnitude smaller
than k2. One might argue that there should be very little
molecular Pd(0) species present in the reaction media,
as the formation of nanoparticles from molecular
Pd(0) species is thermodynamically downhill,[26] and
thus most Pd in solution is likely in the form of nanopar-
ticles.[27] However, formation of palladium black from
soluble nanoparticles is also downhill yet in many cases
nanoparticles are kinetically stable species that are
metastable for long periods of time. Thus, given the tre-
mendous turnover frequencies that are reported for
some Heck reactions, it is possible that short-lived,
metastable molecular Pd species in low concentration
are the true active catalysts. Indeed, the observation
that PVPy poisons the reaction may be more compatible
Figure 3. Iodobenzene loss vs. time curve and curve fit dem-
onstrating the reasonable fit of the data to the nanoparticle-
catalyzed reaction. The experimental data were collected
with complex 1 (4.03ꢁ10ꢀ6 mol) at 858C using 1.37 mmol io-
dobenzene, 2.74 mmol n-butyl acrylate, NEt3 (2.06 mmol)
and n-dodecane (0.1 mL) in DMF (5 mL).
As noted above, the kinetics of the reaction are sig- with molecular active species that are easily ligated by
moidal and this is consistent with catalysis by Pd nano- the polymer than with nanoparticles. Additionally, the
particles.[14,17] Although it is not our intent to conclusive- observation that turnover frequencies often increase
ly prove that molecular Pd(0) “naked” species[3b–d,23] or as the amount of added palladium decreases also sup-
Pd(0) nanoparticles[5a,24] are the true active species ports catalysis by molecular Pd.[23] Overall, there is com-
(rather, we simply aim to show that intact Pd(II) SCS- pelling evidence both for catalysis with molecular Pd
N pincer species are not active), a brief discussion of species as well as by soluble Pd nanoparticles. Nonethe-
the kinetics of the reaction is warranted.
less, based on all the data provided, it is clear that the
Following the work of Finke[14,17] and others,[25] for cat- Pd(II) SCS-N pincer complex is not the true catalytic
alysis by in situ formed metal nanoparticles, the rate is species; rather it is a convenient precatalyst for catalytic
expected to be limited by the formation of the catalytic Pd(0) species.[28]
nanoparticles. In this case, the rate of product formation
will follow the kinetic model for metal-particle forma-
tion. Typically, nanoparticle formation can be modeled Conclusions
using the combination of a nucleation pseudo-elementa-
ry step, A ! B (rate constant k1) followed by an autoca- In summary, we have provided compelling evidence us-
talytic surface-growth pseudo-elementary step, AþB ing a number of soluble and insoluble supported SCS-N
! 2B (rate constant k2), whereby metal species are add- pincer palladium complexes that SCS-pincer complexes
ed to a soluble nanoparticle [where A is the pincer com- with amide linkages are not stable during Heck reac-
plex and B is soluble Pd(0)]. When combined with the tions and that tethered derivatives are not recyclable
pseudo-elementary step for the Heck reaction, Bþ catalysts. In all cases studied, these SCS-N pincer Pd
olefinþiodoarene ! BþHIþcoupling product, these complexes simply act as precatalysts, decomposing
steps can be fit to the kinetic data. Figure 3 depicts the into soluble palladium species that perform the catalytic
concentration of iodobenzene as a function of time using reaction. Furthermore, these results suggest that the
complex 1. In addition, the least-squares fit of the data leached and soluble palladium species do not follow
using the above pseudoelementary kinetic model[17] is the suggested Pd(II)/Pd(IV) catalytic cycle.[12] These re-
also plotted in Figure 3. It is evident that a close match sults in conjunction with our recently published results
is possible, giving pseudo-elementary rate constants of on SCS-O pincer complexes[10] indicate that all SCS pin-
k1 ¼0.014 minꢀ1 and k2 ¼216 Mꢀ1 minꢀ1. It is notewor- cer complexes are likely not stable during the Heck re-
thy that the induction period that is commonly observed action and are therefore neither recyclable nor reusable
in these Heck reactions could be due to the slow forma- Heck catalysts. Recyclable catalysts may nonetheless be
tion of catalytic Pd(0) nanoparticles. This kinetic fit, derived from immobilized Pd(II) SCS-N pincer species
along with the observation that Pd solids form under re- if, and only if, the in situ produced Pd(0) species remain
action conditions with 1, is strong evidence that Pd nano- wholly with the support and do not freely leach into sol-
clusters may be the true catalytic species.
ution.
However, these observations cannot be taken as con-
clusive proof for catalysis by Pd nanoparticles, as other
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(100 mL) slurry of LiAlH4 (2.2 g, 58.0 mmol) at 08C under vig-
orous stirring. After stirring at 08C for 30 min, the mixture was
allowed to reflux for 14 h, at which point it was cooled to 08C.
Ethyl acetate (total ~20 mL) was then slowly added into the
grayish mixture under vigorous stirring to quench excess
LiAlH4 and ca. 15 mL H2O were then added to hydrolyze the
alumina salt. A color change from grayish to green then yellow-
ish was observed. After stirring for another 1 h, the resulting
slurry was filtered through a pad of silica gel using a coarse
frit and washed with several portions of THF (3ꢁ100 mL). Sol-
vent removal of the combined filtrate and washings afforded a
yellow-brownish crude compound a. Further purification was
achieved by recrystallization from THF/hexane; yield: 1.6 g
(96%). 1H NMR (THF-d8): d¼6.58 (1H, s, ArH), 6.53 (2H, s,
ArH), 4.47 (4H, d, J¼5.7 Hz, OCH2), 4.40 (2H, br, OH), 3.93
(2H, t, J¼5.85 Hz, NH2).
Experimental Section
General Procedures
All reactions with air- and moisture-sensitive compounds were
carried out under dry nitrogen/argon atmosphere using an
MBraun UniLab 2000 dry box and/or standard Schlenk line
techniques. DMF, n-butyl acrylate and NEt3 were distilled
over calcium hydride. 5-Amino-isophthalic acid dimethyl ester
(Aldrich Co.), poly(4-vinylpyridine) (2% cross-linked, Aldrich
Co.), isocynate modified Merrifield resin (2% DVB, 200–400
mesh, ~2 mmol N/g) (Aldrich Co.) and 3-isocyanatopropyltri-
ethoxysilane (Gelest, Inc.) were used as received. All other re-
agents were obtained from commercial sources and generally
used without further purification. Gas chromatographic analy-
ses were performed on a Shimadzu GC 14-A gas chromato-
graph equipped with a flame-ionization detector with an HP-
5 column (length¼30 m, inner diameter¼0.25 mm, and film
2. Synthesis of N-acetyl-3,5-bis(hydroxymethyl)aniline (b)
and N-acetyl-3,5-bis(chloromethyl)aniline (c): Species b and
c were synthesized according to a previously reported proce-
dure.[9a]
thickness¼0.25 mm). The temperature program for GC analy-
sis was the following: heating from 508C to 1408C at 30 K/min
and heating from 1408C to 3008C at 40 K/min under constant
pressure with inlet and detector temperatures kept constant
at 3308C. 1H and 13C NMR (300 MHz) spectra were recorded
on a Mercury VX instrument. FT-Raman spectra were ob-
tained on a Bruker FRA-106. At least 128 scans were collected
for each spectrum, with a resolution of 2–4 cmꢀ1. Elemental
analyses (Galbraith Laboratories, Inc., TN) were used to deter-
mine the palladium loadings of silica and Merrifield resin sup-
ported precatalysts. SBA-15 (100 ꢁ pore size) was synthesized
following a literature method.[29] The as-prepared material was
calcined using the following temperature program: 1) increas-
ing the temperature (1.2 8C/min) to 200 8C, 2) heating at 200 8C
for 1 h, 3) increasing at 1.2 8C/min to 550 8C, and 4) holding at
550 8C for 6 h. Prior to functionalization, the SBA-15 was dried
under vacuum at room temperature overnight and then at
120 8C/min for 3 h and stored in a dry box.
3. Synthesis of N-acetyl-3,5-bis(phenylthiomethyl)aniline
(d): A modified literature procedure was used for the synthesis
of compound d.[9a] Species c (500 mg, 2.14 mmol) was mixed
with NaSPh (800 mg, 6.06 mmol) in 30 mL dry THF and re-
fluxed for 16 h. At this point, the reaction mixture was cooled
to room temperature and solvent was removed under reduced
pressure. The residual material was extracted with CH2Cl2
(40 mL) and washed with saturated aqueous NaCl (2ꢁ
15 mL) and NaOH (2 N, 1ꢁ15 mL) and dried over anhydrous
MgSO4. Solvent was removed under vacuum and it was then
purified through a flash silica gel column to afford an orange-
brown oily product d; yield: 760 mg (94%). 1H NMR
(CDCl3): d¼7.50 (2H, m, Ph), 7.36 (2H, s, ArH), 7.31–7.10
(8H, m, Ph), 6.99 (1H, s, ArH), 4.04 (4H, s, SCH2), 2.14 (3H,
s, CH3).
4. Synthesis of 3,5-Bis(phenylthiomethyl)aniline (2): To a
solution of compound d (760 mg, 2.01 mmol) in 15 mL metha-
nol was added 1.2 mL 40% aqueous NaOH, and the resulting
mixture was allowed to reflux for 20 h. After cooling to room
temperature, solvent was removed under reduced pressure
and 50 mL CH2Cl2 were added. The organic phase was washed
with 3ꢁ10 mL H2O and dried over anhydrous MgSO4. Passing
through a short silica gel column and recrystallization from
THF/ hexanes afforded pure 2 as an orange yellow solid; yield:
560 mg (83%). 1H NMR (CDCl3): d¼7.34–7.16 (10H, m, Ph),
6.64 (1H, s, ArH), 6.52 (2H, s, ArH), 3.99 (4H, s, SCH2), 3.61
(2H, br, NH2); 13C NMR (CDCl3): d¼146.90, 138.91, 136.77,
SCS-NH2 Pincer Ligand (2)
The ligand 2 was prepared in five steps as described in
Scheme 6.
1. Synthesis of 3,5-bis(hydroxymethyl)aniline (a): A modi-
fied literature procedure was used for the synthesis of species
a.[9a] 5-Amino-isophthalic acid dimethyl ester (2.0 g,
9.57 mmol) in 20 mL dry THF was slowly added into a THF
Scheme 6. Synthesis of SCS-NH2 (2).
Adv. Synth. Catal. 2005, 347, 161–171
asc.wiley-vch.de
ꢀ 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
167

Kunquan Yu et al.
FULL PAPERS
129.68, 129.02, 126.39, 119.88, 114.55, 39.13 ppm; anal. calcd.
for C20H19NS2: C 71.17, H 5.67, N 4.15; found: C 71.09, H
5.63, N 4.12.
Silica-Immobilized Precatalyst (5)
Compound 2 (200 mg, 0.593 mmol) wasmixed with 3-isocyana-
topropyltriethoxysilane (95%, 155 mg, 0.595 mmol) in 20 mL
dry CH2Cl2. The mixture was allowed to stir at room tempera-
ture overnight and after solvent removal under reduced pres-
sure, silane functionalized SCS ligand was obtained in a quan-
1
titative yield. H NMR (CDCl3): d¼7.30–7.16 (10H, m, Ph),
7.13 (2H, s, ArH), 6.95 (1H, s, ArH), 6.27 (1H, s, NH), 4.86
(1H, t, J¼11.4 Hz, NH), 4.01 (4H, s, SCH2), 3.81 [6H, q, J¼
7.0, Si(OCH2CH3)], 3.21 (2H, q, J¼6.6 Hz, CH2), 1.62 (2H,
m, J¼7.6 Hz, CH2), 1.21 [9H, t, J¼6.9 Hz, Si(OCH2CH3)],
0.64 (2H, t, J¼8.1 Hz, SiCH2); 13C NMR (CDCl3): d¼155.41,
139.10, 136.27, 129.93, 129.03, 126.56, 124.61, 119.91, 58.78,
43.06, 39.05, 23.83, 18.68, 8.01; anal. calcd. for C30H40O4N2S2Si:
C 61.61, H 6.89, N 4.79; found: C 61.34, H 7.12, N 4.72.
Scheme 7. Synthesis of poly(norbornene)-immobilized com-
plex 7.
ꢀ
ꢀ
matic C H), 3003–2851 (aliphatic C H), and 1583 (urea link-
Silane-SCS (347 mg, 0.593 mmol) and Pd(PhCN)2Cl2
(227 mg, 0.593 mmol) were mixed in 20 mL dry acetonitrile
and the resulting mixture was stirred at room temperature
for 30 min and then refluxed for 16 h. The reaction mixture
was filtered while hot to remove any insoluble materials. The
solvent of the filtrate was removed under vacuum and the re-
siduals were then washed with 2ꢁ10 mL hexanes to afford 4
age) cmꢀ1
.
Poly(norbornene)-immobilized precatalyst (7)
The polymer 7 was prepared in five steps as described in
Scheme 7.
1
as an orange yellow powder; yield: 330 mg (77%). H NMR
5. Synthesis of 5-(11-bromoundecyl)bicyclo[2.2.1]hept-2-
ene (g): To a stirred solution of THF (50 mL) and Mg
(1.652 g, 67 mmol) was added norbornene methyl bromide
(9.35 g, 50 mmol). The solution was refluxed for 10 h before be-
ing slowly added to a mixture of 1,10-dibromodecane (10 g,
100 mmol) and Li2CuCl4 (9 mL, 0.1 M in THF) in THF
100 mL) at ꢀ108C. The solution was allowed to stir for 24 h
at room temperature, at which point it was then washed with
ammonium chloride, extracted with diethyl ether (3ꢁ
100 mL), dried over MgSO4 and the solvent was removed un-
der vacuum. The product was further purified by column chro-
matography (hexanes); yield: 7.48 g (50%). 1H NMR (CDCl3,
(DMSO-d6): d¼8.40(1H, s, NH), 7.81 (2H, m, Ph), 7.46–7.15
(8H, m, Ph), 6.90 (2H, s, ArH), 6.10 (1H, s, NH), 4.79 (4H, s,
SCH2), 3.72 [6H, m, Si(OCH2CH3)], 3.43 (2H, q, J¼7.0 Hz,
CH2), 1.48 (2H, m, CH2), 1.16 [9H, t, J¼6.9 Hz, Si(OCH2
CH3)], 0.51 (2H, t, J¼8.1 Hz, -SiCH2); anal. calcd. for
C30H39O4N2S2SiPdCl: C 49.65, H 5.42, N 3.86; found: C 49.87,
H 5.38, N 3.76.
Complex 4 (200 mg, 0.276 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 fil-
tered and washed extensively with DMF and CH2Cl2 and then
dried under high vacuum to afford silica immobilized precata-
lyst 5. Elemental analysis indicated a palladium loading of
0.10 mmol Pd per gram support. FT-Raman: d¼3057 (aro-
¼
300 MHz): d¼6.08–6.05 (m, 2H, CH CH), 3.37 (t, J¼
6.9 Hz, 2H, CH2-Br), 2.72 (t, J¼3.2 Hz, 2H), 1.93 (m, 1H),
1.83 (m, 2H), 1.92 (m, 19H), 1.03 (m, 1H), 0.46 (m, 1H);
13C NMR (CDCl3, 300 MHz): d¼136.6, 135.9, 132.3, 49.5,
46.3, 45.4, 42.5, 41.8, 38.7, 36.6, 34.8, 33.9, 32.8, 32.4, 29.9,
29.6, 29.5, 28.8, 28.7, 28.2. HRMS (EI): m/z¼327.1; anal. calcd.
for C18H31Br: C 66.04, H 9.54; found: C 66.33, H 9.59.
ꢀ
ꢀ
matic C H), 2928 (aliphatic C H) and 1582 (urea linkage)
cmꢀ1
.
6. Synthesis of 12-bicyclo[2.2.1]hept-5-en-2-yl-dodecano-
ic acid (h): Magnesium (474 mg, 19.5 mmol) and THF (8 mL)
were added to a flame-dried flask. Species g (5.8 g, 17.7 mmol)
was then added in portions of 1 g each. A catalytic amount of
1,2-dibromoethane (1 mL) was added to activate the reaction.
The reaction mixture was then refluxed for 12 h under argon.
Then CO2 gas was bubbled into the solution for 15 minutes
and the color of the solution turned off-white. At this point,
the solution was cooled to room temperature, the unreacted
magnesium was removed and then solvent was removed under
vacuum. The product was further purified using column chro-
matography (CH2Cl2) to yield h as an orange oil; yield: 3.1 g
Merrifield Resin-Immobilized Precatalyst (6)
Merrifield resin (2% DVB cross-linked, typical loading
2.0 mmol N/g, Aldrich Co.) (700 mg, ~1.4 mmol N) was mixed
with 2 (100 mg, 0.296 mmol) in 20 mL CH2Cl2 and the mixture
was stirred at room temperature for 24 h, at which point, it was
filtered and washed with CH2Cl2 and then dried under high
vacuum to afford the polymer-bound SCS ligand, which was
then mixed with Pd(PhCN)2Cl2 (110 mg, 0.287 mmol) in
20 mL MeCN and refluxed for 16 h. After cooling to room tem-
perature, the reaction mixture was filtered and washed exten-
sively with DMFand CH2Cl2 to afford the polymer-bound pre-
catalyst 6. Elemental analysis indicated a palladium loading of
0.135 mmol Pd per gram support. FT-Raman: d¼3054 (aro-
1
(60%). H NMR (CDCl3, 300 MHz): d¼9.05 (br, 1H, OH),
¼
6.06–5.88 (m, 2H, CH CH), 2.71 (m, 2H), 2.30 (m, 2H), 1.93
168
ꢀ 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
Adv. Synth. Catal. 2005, 347, 161–171

SCS Pincer Pd(II) Complexes are Precatalysts in Heck Catalysis
FULL PAPERS
(m, 1H), 1.59 (m, 1H), 1.79 (m, 1H), 1.59 (m, 2H), 1.22 (br,
19H), 0.47 (m, 1H); 13C NMR (CDCl3, 300 MHz): d¼179.7,
136.6, 135.9, 132.3, 49.5, 46.3, 45.4, 45.2, 42.5, 41.8, 38.7, 36.6,
34.8, 34.4, 33.1, 32.4, 29.9, 29.7, 29.5, 29.3, 29.1, 28.9, 28.7,
24.8; HRMS (ESI): m/z¼291.0.
Heck Reactions of n-Butyl Acrylate and Iodobenzene
All Heck coupling reactions were carried out using n-butyl
acrylate (2.74 mmol) and iodobenzene (1.37 mmol) in DMF
(5 mL) at 1208C using NEt3 (2.06 mmol) as base and dodecane
(~0.15 g) as GC internal standard. Typically, pincer complexes
1, 5–7 (molar ratio of [Pd] toiodobenzene ~1:350)were mixed
with all the reagents except NEt3 and preheated at 1208C for
ca. 15 min, at which point NEt3 in 0.5 mL DMF was syringed
into the reaction mixture. Conversion was followed through
GC analysis by taking aliquots from the reaction mixture peri-
odically (diluted with THF or acetone). Preheating under vig-
orous stirring was found to be critical to minimize induction pe-
riods as well as insure good saturation of the reaction media
with PVPy and Hg(0) when it were used (vide supra). All reac-
tions were carried out at 1208C unless otherwise noted.
7. Synthesis of 12-bicyclo[2.2.1]hept-5-en-2-yl-dodecano-
ic acid (3,5-bis-pheny lsulfanylmethylphenyl)-amide (i): Spe-
cies h (242 mg, 0.83 mmol) and 1-hydroxybenzotriazole
(112 mg, 0.83 mmol) were stirred in a mixture of methylene
chloride (10 mL) and dimethyl formamide (0.5 mL) until com-
plete dissolution of the benzotriazole was observed. N,N’-Di-
cyclohexylcarbodiimide (171 mg, 0.83 mmol) was then added
to the solution followed by the addition of 2. The reaction mix-
ture was stirred for 9 h at room temperature. A precipitate was
observed and filtered through a pad of celite. The solvent of the
filtrate was removed and the product was further purified by
column chromatography (v/v 1:10 ethyl acetate:hexane) to af-
1
ford i as a yellow oil; yield: 330 mg (65%). H NMR (CDCl3,
Acid-Catalyzed Dimerization of a-Methylstyrene
(AMS)
300 MHz): d¼7.39 (s, 2H, ArH), 7.31–7.13 (m, 10 H, SPh),
¼
6.97 (s, 1H, ArH), 6.11–5.89 (m, 2H, CH CH), 4.01 (s, 4H),
2.7 (m, 2H), 2.32–2.27 (t, J¼7.4 Hz, 2H), 1.97–1.93 (m, 1H),
Porous silica supported sulfonic acid was prepared according to
1.86–1.78 (m, 1H), 1.73–1.66 (m, 2H), 1.39–1.15 (br m,
a
literature method.[30] 3-Mecaptopropyltrimethoxysilane
(1.0 g) was added into a toluene suspension of silica (MS-
3050, PQ Corporation, pore volume: 3.0 mL/g; BET surface
area: 500 m2/g; pore diameter 240 ꢁ; and average particle
size ~ 90 m). The reaction mixture was stirred at room temper-
ature for 24 h and was then filtered and washed with toluene
and dichloromethane. TGA analysis on the dried solid gave a
thiol loading at about 0.55 mmol/g and FT-Raman showed
19H), 1.08–1.00 (m, 1H), 0.92–0.83 (m, 1H), 0.51–0.45 (m,
1H); 13C NMR (CDCl3, 300 MHz): d¼171.2, 138.4, 138.1,
136.7, 136.6, 136.0, 132.2, 129.4, 128.7, 126.1, 124.7, 118.7,
49.5, 46.3, 45.4, 45.2, 42.5, 41.8, 38.7, 37.7, 36.6, 34.8, 33.1,
32.8, 32.4, 32.0, 29.9, 29.7, 29.5, 29.4, 29.3, 28.7, 25.6; HRMS
(ESI): m/z¼612.3.
8. Synthesis of Pd-Cl, 12-bicyclo[2.2.1]hept-5-en-2-yl-do-
decanoic acid (3,5-bis-phenylsulfanylmethylphenyl)-amide
(j): Pd(PhCN)2Cl2 (92 mg, 0.24 mmol) was added to a stirred
solution of i (150 mg, 0.24 mmol) in 15 mL CH2Cl2/CH3CN
(v/v: 1/2). The solution was stirred for 30 min before the addi-
tion of AgBF4 (119 mg, 0.61 mmol) in one portion. After being
stirred for another 30 min, the mixture was added to a brine sol-
ution (200 mL) and stirred for another 6 h. The organic layer
was separated, dried over MgSO4 and the solvent removed un-
der vacuum to yield a yellow powder that was further purified
by column chromatography (v/v¼9:1, CH2Cl2:methanol);
yield: 138 mg (59%). 1H NMR (CDCl3, 300 MHz): d¼8.17 (s,
1H, ArH), 7.72 (d, J¼9.2 Hz, 4H, SPh, ArH), 7.29 (m, 8H),
the presence of the SH stretching at 2058 cmꢀ1. The immobi-
lized mercaptopropyl groups were then oxidized by 30% H2O2
(in methanol/H2O mixture) and followed by acidification by
0.1 H2SO4 to yield the immobilized sulfonic acid. FT-Raman
ꢀ
ꢀ
showed the complete disappearance of the SH stretching at
2058 cmꢀ1, indicating conversion of SH to SO3H.
In a typical AMS dimerization experiment, 60 mg (~3.3ꢁ
10ꢀ5 mol) of the silica supported sulfonic acid was added into
a 0.50 g AMS and 4.5 g cumene mixture along with ~0.2 g do-
decane (GC internal standard) and the reaction was carried
out at 508C. Conversion of AMS was analyzed through GC
chromatography.
ꢀ
ꢀ
¼
6.08–5.88 (m, CH CH, 2H), 4.37 (br s, 4H), 2.71 (s, 2H),
2.29–2.24 (t, J¼7.3 Hz, 2H), 1.95–1.75 (m, 1H), 1.324 (t, J¼
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): d¼171.9, 149.3, 136.6, 136.0, 132.2, 131.8, 130.9,
129.4, 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/z¼782.9; anal.
calcd. for C40H50ClNO2PdS2: C 61.37, H 6.44, N 1.79; found:
C 61.06, H 6.48, N 1.84.
Acknowledgements
The US DOE Office of Basic Energy Sciences is acknowledged
for financial support through Catalysis Science Contract No.
DE-FG02–03ER15459. This work was also partially supported
under a National Science Foundation Graduate Research Fel-
lowship, grant No. 0228234, to JMR. The authors thank Prof.
Robert J. Davis at the University of Virginia for helpful discus-
sions.
9. Polymerization procedure for the synthesis of 7: Mono-
mer j (85 mg, 0.11 mmol) was dissolved in CDCl3 and stirred
before the addition of the ruthenium catalyst (2 mg,
0.002 mmol). The polymer precipitated out almost instantly.
The reaction was stirred for another 20 min before the addition
of ethyl vinyl ether (2 drops). The polymer was then washed
with methanol to afford 7 as an orange solid; yield: 70 mg
References and Notes
1
[1] Here we use the traditional definitions of homogeneous
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(br m, 4H), 3.31 (s, 4H), 2.13 (br m, 2H), 1.45–0.88 (br m,
23H); 13C NMR (DMSO, 300 MHz): d¼170.3, 149.4, 136.2,
131.9, 130.4, 129.3, 113.0, 50.0, 36.4, 30.8, 29.7, 29.4, 25.2.
Adv. Synth. Catal. 2005, 347, 161–171
asc.wiley-vch.de
ꢀ 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
169

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ꢀ
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