Polyoxometalate-based N-Heterocyclic Carbene (NHC) complexes for palladium-mediated C-C coupling and chloroaryl dehalogenation catalysis

Article abstract of DOI:10.1002/chem.201001009

Better together: A novel hybrid N-heterocyclic carbene (NHC) palladium complex, integrating a totally inorganic and polyanionic decatungstate unit, has been synthesized following a convergent strategy. The interplay of the Pd binding domains with the inorganic scaffold is instrumental in accessing multi-turnover catalysis in C-C crosscoupling and aromatic dehalogenation reactions under MW-assisted protocols (see scheme). (Figure Presented).

Full text of DOI:10.1002/chem.201001009

DOI: 10.1002/chem.201001009
Polyoxometalate-Based N-Heterocyclic Carbene (NHC) Complexes for
À
Palladium-Mediated C C Coupling and Chloroaryl Dehalogenation Catalysis
Serena Berardi,[a] Mauro Carraro,[a] Manuel Iglesias,[b] Andrea Sartorel,[a]
Gianfranco Scorrano,[a] Martin Albrecht,*[b] and Marcella Bonchio*[a]
Molecular hybrids are of particular interest in catalysis,
due to the interplay of joint organic–inorganic domains with
very diverse functional environments. The hybrid strategy is
expected to allow for fine-tuning of the stereo-electronic
properties of the catalyst through a tailored choice of the in-
teracting organic–inorganic moieties. Such a hybrid upgrade
of the catalytic system can be readily accessed, through the
covalent functionalization of molecular polyoxometalates
(POMs).[1] Indeed, covalent POM hybrids are characterized
by discrete, nanosized, multi-metal oxides as polyanionic
scaffolds, which allow for anchoring on-surface organic
pendants, including chiral residues.[1,2] The hybrid structures
display a remarkable stability, and resist harsh catalytic con-
ditions, enabling a multi-turnover performance and sequen-
tial recycling in ionic-liquid media exposed to MW irradia-
tion.[3] Interestingly, the hybrid modification of the POM
framework has been successfully used to expand the coordi-
nation potential by introduction of tailored organic domains.
In this respect, polydentate salen-type or thiol/phosphine-
terminated ligands have been immobilized on the POM sur-
face to bind Mn, Pd or Rh ions, or to implement the stabili-
zation of Pd0 colloids.[4–6] We present herein the synthesis,
characterization, and catalytic applications of new POM-ap-
pended N-heterocyclic carbene (NHC) palladium complexes
coupling and aromatic dehalogenation reactions.[7] To this
end, imidazolium moieties have been successfully grafted on
the defect site of the divacant Keggin polyanion [g-
SiW10O36]8À.[8] These organic domains are useful precursors
of N-heterocyclic (NHC) carbenes, which are known to pro-
À
vide strong M NHC bonds, thus imparting high thermal ro-
bustness and stability to organometallic intermediates in
multi-turnover palladium catalysis.[9]
In addition to the specifically designed Pd binding site,
such POM-based hybrids display an extended polyanionic
surface. This setup is expected to contribute sterically and
by virtue of electrostatic repulsions to prevent agglomera-
tion of incipient Pd0, which generally deteriorates the turn-
over efficiency of the system.[4c] The molecular nature of
such composite structures is a further point of interest, offer-
ing a single-site model for extended materials. Specifically,
the molecularly defined structure of the hybrid allows for
selective tailoring of the active site as well as for detecting
subtle changes by solution characterization techniques, in-
cluding heteronuclear NMR spectroscopy and ESI-MS.
The synthetic route to Keggin-type hybrids involves the
reaction of organosilane reagents with the nucleophilic
oxygen atoms that border a defect site on the POM surface.
Grafting strategies have been optimized in acetonitrile in
which the presence of excess nBu4NBr promotes the solubi-
lization of the POM by counterion metathesis (Scheme 1).
Under these conditions, decoration of the lacunary POM is
known to yield hybrids with two surface-anchored organosil-
yl (RSi-) groups, each one linked to two oxygen atoms of
two edge-shared WO6 octahedra.[4,10]
À
and their remarkable performance in catalyzing C C cross-
[a] Dr. S. Berardi, Dr. M. Carraro, Dr. A. Sartorel, Prof. G. Scorrano,
Dr. M. Bonchio
ITM-CNR and Department of Chemical Sciences
University of Padova, via Marzolo 1, 35131 Padova (Italy)
Fax : (+39)0498275239
A first attempt focused on a divergent approach, includ-
ing the covalent attachment of the triethoxysilyl-functional-
ized 4,5-dihydro-imidazolium bromide (1) to the divacant
decatungstosilicate. The hybrid POM (2) was isolated and
displayed spectroscopic data (1H, 13C, 29Si and 183W NMR,
FT-IR) that are in agreement with the expected bis-func-
tionalized structure. However, palladation of 2 met with
little success (Scheme 1). Neither direct metalation using
E-mail: marcella.bonchio@unipd.it
[b] Dr. M. Iglesias, Prof. M. Albrecht
Department of Chemistry, University of Fribourg
Current address: School of Chemistry and Chemical Biology
University College Dublin, Belfield, Dublin 4 (Ireland)
Fax : (+353)17162501
E-mail: martin.albrecht@ucd.ie
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201001009.
PdACTHNUTRGNE(UGN OAc)2 nor transmetalation using Ag2O resulted in the
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COMMUNICATION
[g-SiW10O36]8À in acetonitrile
under phase transfer conditions
(Scheme 1), leading to the func-
tionalized hybrid
5 in 75%
yield.[10]
Complex 5 was characterized
both in the solid state (by FT-
IR spectroscopy and elemental
analysis) and in solution (1H,
13C, 29Si and 183W NMR spec-
troscopy and ESI-MS). All
spectroscopic results confirmed
the proposed structure (Sup-
porting
A
Information,
The
Fig-
ures S18–S23).[10]
183W
NMR spectrum shows three
resonances at À107.4, À135.8,
and À142.1 ppm in 2:1:2 ratio,
in agreement with the expected
C2v symmetry. The 29Si NMR
spectrum reveals two signals at
À62.8 and À88.4 ppm with inte-
gration ratio 2:1. These results
are consistent with a double
substitution of the POM sur-
Scheme 1. Synthetic strategies to the hybrid POMs 2 and 5.
face. Coordination of PdII has
been confirmed by X-ray pho-
toelectron spectroscopy (XPS)
formation of the desired palladium NHC complex.[11–13] Sim-
ilarly, base-mediated palladation with [PdCl2ACTHNUTRGNEU(GN CH3CN)2] in
of a solid sample of 5, for which the expected Pd 3d 5/2
peaks centered at 338 and 343 eV in binding energy were
observed (Supporting Information, Figures S24–S26). The
DFT optimized structure of 5 suggests a planar distorted co-
ordination for Pd, with trans halide ligands and bond lengths
the presence of potassium bis(trimethylsilyl)amide
(KHMDS) or NaH failed, as evidenced by the persistence
1
of the H NMR resonance of the C2-bound protons on the
À
À
dihydroimidazolium ring (dH =8.1 ppm). Analysis of the re-
action by UV/Vis spectrophotomeric titration suggests a
possible electrostatic association of Pd2+ ions to the anionic
POM surface.[14] To avoid such complications, we concen-
trated on a convergent approach involving the immobiliza-
tion of preformed palladium complexes on the POM.
Hence, the trialkoxysilyl-tagged imidazolium precursor 3,
containing an unsaturated heterocycle and a less hindered
n-butyl substituent at the nitrogen atom,[9,15] was synthesized
under strictly anhydrous conditions, from (3-chloropropyl)-
triethoxysilane and 1-butylimidazole. Reaction of 3 with
around 2.07–2.11 (Pd C) and 2.59 ꢁ (Pd Br) (Scheme 2).
Complex 5 has been tested in the palladium-catalyzed
cross-coupling of aryl halides with phenylboronic acids, ac-
cording to established Suzuki–Miyaura protocols (Scheme 2,
path a).[16] This reaction represents one of the key tools for
À
selective C C bond formation. The performance of catalyst
5 in this chemistry has been screened in 1:1 DMF/H2O mix-
tures and under microwave (MW)-induced dielectric heating
(Table 1).[17,18] This method is particularly efficient for poly-
charged catalysts, which behave as MW-activated molecular
heat-carriers.[19] Indeed, hybrid POMs, are known to display
a remarkable thermal stability and high catalytic efficiency
in MW-assisted processes.[3]
The cross-coupling catalytic activity of 5 was initially eval-
uated using a representative range of aryl halides as sub-
strates and MW irradiation at 10 W for 10–30 min (Table 1).
Under these conditions, both activated and deactivated
aryl iodides react smoothly with 0.05 mol% catalyst loading,
leading to the corresponding substituted biphenyl products
with up to 99% yield, 1980 turnover numbers (TON), and
frequencies (TOF) up to 11880 hÀ1 (entries 1–4, Table 1,
comparison with literature benchmarks in Tables S1 and S2
in the Supporting Information). In all reactions, the bulk
temperature is kept at about 808C by simultaneous cooling
Ag2O followed by transmetallation with [PdCl2ACHTNUTRGNEUNG
(CH3CN)2][13]
afforded the Pd–biscarbene complex 4, as confirmed by
1
ESI-MS, H and 13C NMR analyses. The absence of a reso-
nance due to the C2-bound imACTHUNGTRENNiGU dACHTUGTNRENNaGU zolium proton (dH =
10.3 ppm) is corroborated by the appearance of a new signal
at dC =170.6 ppm, which is characteristic of a palladium-
bound carbene carbon (see the Supporting Information, Fig-
ures S16 and S15). ESI-MS (positive mode) shows signals
centered at m/z=799.3 and 843.3, corresponding to the
monochloride and monobromide cationic fragments
[{C10H17N2SiACHTUNGTRENNUNG ACHUTNGTRENNGUN
(OEt)3}2PdCl]+ and [{C10H17N2Si(OEt)3}2PdBr]+,
respectively (Supporting Information, Figure S17). Complex
4 was finally reacted with the divacant Keggin POM
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M. Bonchio, M. Albrecht et al.
with similar yields and reaction
time (yield up to 99%, TON=
990, TOF=5940 hÀ1, entry 5,
TaACTHNUTRGNEUGNble 1). A remarkable 87%
yield of 4-methylbiphenyl was
obtained after 30 min irradia-
tion when using the unactivated
4-bromotoluene as substrate
(TOF=1740 hÀ1, entry 8, Ta-
AHCTUNGTRENNUNG
also effective in coupling aryl
chlorides, which are known to
be poorly reactive but syntheti-
cally highly appealing.[22,23] An
increased catalyst loading (1.0
mol%), and a larger excess of
phenylboronic acid and Na2CO3
(1.3 and 3 equivalents with re-
spect to aryl chloride), were
Scheme 2. Cross-coupling (path a) and dehalogenation reactions (path b) catalyzed by 5 under MW irradiation.
DFT optimized geometry is presented for the proposed structure of 5.
Table 1. Suzuki coupling catalyzed by
(Scheme 2, path a). In all reactions: aryl halide, ArX (0.25 mmol), phen-
ACHTUNGTRENNUNGylboronic acid, PhB(OH)2 (1.1 equiv, 0.275 mmol), Na2CO3 (2 equiv,
0.5 mmol), DMF:H2O=1:1 (0.45:0.45 mL); MW irradiation: 10 W, com-
pressed air at 30–40 psi; Tbulk =75-858C.
5
under MW-irradiation
pivotal for ensuring reactivity. Under such conditions, 4-
chloroacetophenone gave the corresponding coupling prod-
uct in 98% yield when subjected to MW irradiation for
30 min (TON=98, TOF=196 hÀ1, entry 9, Table 1).[24] ortho-
Substituted 2-chloroacetophenone was converted only slug-
gishly, presumably due to steric factors (entry 10, Table 1).
The POM residue may be sufficiently bulky to hamper the
oxidative addition and/or transmetallation step of the cata-
lytic cycle.[25] Other activated aryl chlorides reacted with less
success, as for 4-chloronitrobenzene, affording a significant
amount (18%) of biphenyl originating from homo-coupling
together with 38% of cross-coupled 4-nitrobiphenyl
(entry 12, Table 1). In this case, the selectivity of the process
did not improve upon increasing the amount of base.[26]
Interestingly, the reaction of the unactivated 4-chloro-
Entry
X
R1
5 loading [%]
product [%]
t [min]
1[a]
2
3
4
5
I
I
I
I
p-COCH3
H
0.05
0.05
0.05
0.05
0.1
0.1
0.1
0.1
1
1
1
1
1
99
85
10
30
20
20
10
10
35
30
30
150
30
30
60
60
p-CH3
p-OCH3
p-COCH3
p-NO2
H
p-CH3
p-COCH3
o-COCH3
p-CHO
p-NO2
H
80[c]
95[c]
99
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Cl
Cl
6
93[c]
94
7[b]
8[b]
9[b]
10[b]
11[b]
12
87[c]
98[c]
10[d]
20[c]
38[e]
57
AHCTUNGTREGtNNUN oluene turned out to yield 33% of the desired product,
13[b]
14[b]
33[f]
with concurrent homocoupling (<9%) and dehalogenation
to toluene (<10%).[27] Catalytic dehalogenation was previ-
ously reported as a parallel reaction in Suzuki coupling;
however, few papers provide details on this topic.[28] Such re-
action is of major importance from an environmental point
of view, in particular with regards to the degradation of
highly toxic halogenated organic compounds.[29]
p-CH3
1
[a] Under analogous conditions, the coupling reaction catalyzed by 4 pro-
ceeds with 32% yield. [b] ArX (0.25 mmol), PhB(OH)2 (1.3 equiv,
0.32 mmol), Na2CO3 (3 equiv, 0.75 mmol). [c] Biphenyl (1–7%) and de-
ACHTUNGTRENNUNGhalACHTUNGTRENNUNGogenation products observed in traces. [d] Biphenyl (4%) and aceto-
phenone (14%) were detected. [e] Biphenyl (18%) was detected. [f] Bi-
phenyl (9%) and toluene (6%) were detected.
The proposed mechanism of palladium-catalyzed dehalo-
genation involves the oxidative addition of the aryl halide to
the Pd0–NHC intermediate, and the formation of a PdII–hy-
dride species, generated by a strong base such as an alkox-
ide.[29,30] In line with this proposal, the dehalogenation reac-
tion is strongly promoted upon changing the base from
Na2CO3 to nBu4NOH.[31] Under conditions that are other-
wise identical to the Suzuki protocol, catalytic dehalogena-
tion of aryl chlorides was readily accomplished (Scheme 2,
with compressed air. Control experiments performed with
iodobenzene and phenylboronic acid indicate no coupling
products in the Pd-free reaction, nor in the presence of the
imidazolium-functionalized POM 2. It is worth noting that
the POM-free NHC complex 4 deactivates rapidly under
analogous catalytic regime, thus yielding poor conversion
and turnover efficiency (see note [a] in Table 1). Addition of
Na2CO3 (2 or 3 equivalents) is an essential prerequisite of
the catalytic protocol, in agreement with the generally ac-
cepted mechanism involving a reactive borate anion for the
transmetallation step.[20] Coupling of aryl bromides is readily
achieved by increasing the catalyst loading to 0.1 mol%,
path b). In the presence of nBu4NOH as the base, 4-chlo
ACHTUNGTRENNUNGro-
AHCTUNGTREGaNNNU cetophenone and phenyl boronic acid, dehalogenated ace-
tophenone was formed as the major product (63% yield),
together with 4-acetylbiphenyl in 35% yield. The competi-
tive dehalogenation reaction has thus been investigated in
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Polyoxometalates
COMMUNICATION
more detail. Highest selectivities were achieved when using
two equivalents of nBu4NOH in DMF, under MW irradia-
tion (Table 2). Under these conditions, a formate ion is sur-
Keywords: carbenes · homogeneous catalysis · palladium ·
polyoxometalates
Table 2. Dehalogenation of aryl chlorides (ArCl), catalyzed by 5 under
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MW-irradiation (Scheme 2, path b).[a]
Entry
ArCl
ArH Yield [%]
t [min]
1
2
3
4
5
6
7
4-chloroacetophenone
2-chloroacetophenone
3-chloroacetophenone
4-chloronitrobenzene
4-chlorophenol
>99
57
86
>99
12
43
30
120[b]
60
40
60
4-chlorotoluene
1-chloronaphtalene
60
60
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87
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occurs smoothly and quantitative yields were reached within
30–40 min (entries 1 and 4, Table 2).[33] Sterically hindered 2-
chloroacetophenone was converted at a slower rate and af-
forded 57% acetophenone after 2 h (entry 2, Table 2). Aryl
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genated less efficiently (entries 5 and 6, Table 2). Neverthe-
less, 4-chlorotoluene yielded appreciable 43% of toluene
after irradiation for 1 h (entry 6, Table 2).
In conclusion, a novel synthetic route to hybrid Pd cata-
lysts has been developed through decoration of a POM sur-
face with imidazolium-based NHC palladium complexes.
The interplay of the Pd binding domains with the inorganic
scaffold provides new opportunities to access multi-turnover
catalysis with good to excellent performance under MW-as-
sisted protocols. Depending on the reaction conditions, both
cross-coupling and dechlorination of aromatic compounds
are efficiently accomplished. Future studies will concentrate
on optimizing the carbene domain, in particular with respect
to steric effects and the Pd/carbene ratio. The introduction
of the POM platform is potentially amenable to nanofiltra-
tion techniques for catalyst recovery and product purifica-
tion, while being instrumental for heterogeneization strat-
egies on charged surfaces/supports.[34] In this respect, a
straightforward upgrade of the system in ionic-liquid phases
can be envisaged due to the implemented affinity of such
POM-based hybrids for these media.[35]
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tion of [PdACTHNUGRTENUNG(OAc)2] in DMSO and incubation at 508C for 4 h and at
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Acknowledgements
Financial support from CNR, MIUR, University of Padova, (Progetto
Strategico 2008, HELIOS, prot. STPD08RCX and PRAT CPDA084893),
the Alfred Werner Foundation (to M.A.) and ESF COST D40 action are
gratefully acknowledged. We thank Dr. Marta M. Natile for the XPS
analysis.
[20] A. de Meijere, F. Diederich in Metal Catalyzed Cross-Coupling Re-
actions, Wiley-VCH Weinheim, 2004.
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M. Bonchio, M. Albrecht et al.
[21] Average TON and TOF values in the range 1100–1900 and 314–
543 hÀ1, respectively, have been reported for classic NHC ligands
employed in the homogeneous Suzuki coupling of deactivated aryl
bromides: N. Marion, O. Navarro, J. Mei, E. D. Stevens, N. M. Scott,
S. P. Nolan, J. Am. Chem. Soc. 2006, 128, 4101–4111.
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[23] Ligand 2 in combination with [Pd(Ac)2] (1 equiv) was found to be
inactive with chloroaryl substrates, thus confirming the superior per-
formance of the POM-based NHC ligand in this chemistry.
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69, 3173–3180.
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[31] FT-IR analysis of pre-catalyst 5 (5 mm in DMF) after incubation
with nBu4NOH·30H2O (5 equiv) under microwave irradiation for
1 h and subsequent precipitation with water shows retention of the
main spectral features associated to the POM scaffold, with bands at
n˜ =946 (w), 883 (s), 825 (w), 741 cmÀ1 (s), indicating that the POM
support is stable under catalytically relevant reaction conditions.
[32] a) E. Buncel, E. A. Symons, J. Chem. Soc. Chem. Commun. 1970,
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[24] Literature TON and TOF values, for Suzuki coupling of aryl chlo-
ACHTUNGTRENNUNG
rides by Pd–NHC catalysts are in the range 40–100 hÀ1; see, for ex-
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Received: April 19, 2010
Published online: July 29, 2010
10666
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