A well-defined Pd hybrid material for the Z-selective semihydrogenation of alkynes characterized at the molecular level by DNP SENS

Article abstract of DOI:10.1002/chem.201302484

Direct evidence of the conformation of a Pd-N heterocyclic carbene (NHC) moiety imbedded in a hybrid material and of the Pd-NHC bond were obtained by dynamic nuclear polarization surface-enhanced NMR spectroscopy (DNP SENS) at natural abundance in short experimental times (hours). Overall, this silica-based hybrid material containing well-defined Pd-NHC sites in a uniform environment displays high activity and selectivity in the semihydrogenation of alkynes into Z-alkenes (see figure). Copyright

Full text of DOI:10.1002/chem.201302484

DOI: 10.1002/chem.201302484
A Well-Defined Pd Hybrid Material for the Z-Selective Semihydrogenation
of Alkynes Characterized at the Molecular Level by DNP SENS**
Matthew P. Conley,[a] Ruben M. Drost,[b] Mathieu Baffert,[c] David Gajan,[a]
Cornelis Elsevier,[b] W. Trent Franks,[d] Hartmut Oschkinat,[d] Laurent Veyre,[c]
Alexandre Zagdoun,[e] Aaron Rossini,[e] Moreno Lelli,[e] Anne Lesage,[e] Gilles Casano,[f]
Olivier Ouari,[f] Paul Tordo,[f] Lyndon Emsley,*[e] Christophe Copꢀret,*[a] and
Chloꢀ Thieuleux*[c]
The detailed characterization of catalytic materials lays
the foundation for quantitative structure–activity relation-
ships that lead to rational design of catalysts. However, most
classical solid catalysts are difficult to characterize at the
molecular level, even when using the most advanced analyti-
cal techniques available to surface science. Thus, the vast
majority of chemical reactions that are performed with het-
erogeneous catalysts lack characterized molecular structures.
In contrast, homogeneous catalysts contain single metal sites
in well-defined ligand environments, structures of which can
be readily solved by spectroscopic or diffraction methods.
The knowledge of structure gives fundamental insight into
the behavior of a homogeneous complex under catalytic
conditions, which then directly leads to improved catalyst
design. Construction of materials that contain metal com-
plexes supported on a material surface and having control-
led first-coordination-sphere environments bridges this gap
between homogeneous and heterogeneous catalysis,[1] giving
catalysts that are readily separable from the reaction
medium and that avoid metal contamination of the end
product.
However, one of the critical barriers to overcome in the
production of these molecularly defined materials is the de-
termination of the structure of the surface sites. Though
NMR spectroscopy is a powerful method to determine the
structure of such solids,[2] the low inherent sensitivity of
NMR spectroscopy coupled with the small quantity of cata-
lytic sites in these materials renders conventional NMR
characterization extremely difficult. Recently, we have
shown in model systems how dynamic nuclear polarization
(DNP) could be used to selectively amplify the NMR signal
from surface species, leading to remarkable signal enhance-
ments (up to 100) and to large reduction in experimental
times.[3] Herein, we describe the first application of this ap-
proach to the characterization of a well-defined catalytic
system, namely, a hybrid material containing uniformly dis-
tributed Pd–N heterocyclic carbenes (NHC) complexes
within the mesoporous network of a silica matrix. We show
that by using an appropriate solvent/radical system for im-
pregnation and by taking care that the surface is correctly
passivated, the DNP–surface-enhanced NMR spectroscopy
(SENS) method leads to the full structural determination of
the Pd surface complexes, in a manner analogous to what
would be done for homogeneous catalysts with solution
NMR. We then show that, as was expected from its well-de-
fined structure and environment, this material is an efficient
precatalyst for the Z-selective semihydrogenation of al-
kynes.
[a] Dr. M. P. Conley, Dr. D. Gajan, Prof. Dr. C. Copꢀret
Laboratory of Inorganic Chemistry, ETH Zꢁrich
Wolfgang Pauli Strasse 10, 8093 Zꢁrich (Switzerland)
E-mail: ccoperet@inorg.chem.ethz.ch
[b] R. M. Drost, Prof. Dr. C. Elsevier
Vanꢂt Hoff Institute for Molecular Sciences
University of Amsterdam
Science Park 904, 1098XH Amsterdam (The Netherlands)
[c] Dr. M. Baffert, L. Veyre, Dr. C. Thieuleux
Universitꢀ Lyon, Institut de Chimie de Lyon
UMR C2P2 CNRS-UCBL-ESCPELyon
Equipe Chimie Organomꢀtallique de Surface
43 Bd. du 11 Novembre 1918, 69616 Villeurbanne (France)
E-mail: thieuleux@cpe.fr
[d] Dr. W. T. Franks, Prof. Dr. H. Oschkinat
NMR Supported Structural Biology
Leibniz-Institute fꢁr Molekulare Pharmakologie (FMP)
Robert-Roessle-Strasse 10, 13125 Berlin (Germany)
[e] A. Zagdoun, Dr. A. Rossini, Dr. M. Lelli, Dr. A. Lesage,
Prof. Dr. L. Emsley
Centre de RMN ꢃ Trꢄs Hauts Champs
Institut des Sciences Analytiques
Universitꢀ de Lyon (CNRS/ENS Lyon/UCB Lyon 1)
5, rue de la Doua, 69100 Villeurbanne (France)
E-mail: lyndon.emsley@ens-lyon.fr
[f] Dr. G. Casano, Dr. O. Ouari, Prof. Dr. P. Tordo
The catalytic reduction of alkynes to Z-olefins (Eq. [1]) is
classically performed with the Lindlar catalyst,[4] a heteroge-
neous catalyst that consists of palladium particles supported
on CaCO3 doped with lead acetate. Quinoline is also added
to this mixture to poison most of the surface palladium sites,
preventing over-reduction of Z-ene products. Though recent
Aix-Marseille Universitꢀ, CNRS, ICR UMR 7273
Facultꢀ de Saint Jꢀrꢅme case 521, 13397 Marseille cedex 20 (France)
[**] DNP SENS=Dynamic nuclear polarization surface-enhanced spec-
troscopy.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201302484.
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Chem. Eur. J. 2013, 19, 12234 – 12238

COMMUNICATION
progress has shown that Pd particles[5] and even gold surfa-
ces[6] can catalyze the selective semihydrogenation of al-
kynes,[7] the catalytically active site in Lindlarꢂs catalyst re-
mains unknown,[8] and optimizing this system often relies on
empirical observations as opposed to quantitative structure
activity relationships.
The ligand precursor (Mat-Im) and the Pd–NHC catalyst
(Mat-Pd) were fully characterized by DNP SENS analy-
sis.[3,13] Spectra were recorded on a commercial Bruker
400 MHz/263 GHz gyrotron DNP system (9.4 T) by using
samples impregnated with
a 16 mm solution of bis-
cyclohexyl-TEMPO-bisketal (bCTbK; TEMPO=2,2,6,6-
tetramethylpiperidin-1-yl)oxyl) in 1,1,2,2-tetrachloroethane
(C2H2Cl4).[14] Microwave irradiation of the NMR sample at
100 K saturates the EPR transitions of bCTbK, and subse-
quent transfer of polarization from the electrons to the
nuclei of interest on the surface resulted in large NMR
signal enhancements. Figure 1b shows the 13C CPMAS spec-
tra of [D9]Mat-Im (top spectrum) and [D9]Mat-Pd (bottom
spectrum) at natural isotopic abundance. Surface signal en-
In selective alkyne semihydrogenation, homogeneous Pd
complexes that contain bis(arylimino)acenaphthene (Ar-
bian) or NHC ligands have also been developed to selective-
ly give Z-alkenes.[9] Herein, we have prepared the corre-
sponding immobilized system that contains palladium–NHC
complexes incorporated into a mesostructured silica materi-
al (Mat-Pd) by reaction of [p-C3H5PdCl]2 with the imidazoli-
um functionalities distributed in the pore channels of a mes-
oporous hybrid material (Scheme 1).[10] The material con-
taining imidazolium units (Mat-Im) was prepared by 1) the
hancement factors e
(defined as the ratio of integrated
13C,CP
intensities in spectra acquired with and without microwave
irradiation) of 25 and 15 were obtained for the aromatic car-
bons of [D9]Mat-Im and [D9]Mat-Pd, respectively. These
signal amplification factors (illustrated in Figure 1b for the
ligand precursor) translate into a spectacular reduction in
experimental time (up to a factor of 625 with respect to
standard NMR instrumentation) and allowed the complete
structural characterization of
both materials by using multidi-
mensional NMR methods. For a
comparison, the 13C CPMAS
spectrum of Mat-Pd is included
in the Supporting Information,
the signal-to-noise ratio of this
spectrum is lower for the sur-
face nuclei after 24 h of acquisi-
tion than the SENS spectrum of
[D9]Mat-Pd after 35 min (Fig-
ure S11 in the Supporting Infor-
mation). Clearly, the multi-
AHCTUNGTERGdNNUN imensional data presented
Scheme 1. Synthesis of Mat-Pd: i) TEOS (28 equiv), HCl/H2O (pH 1.5), P123, NaF, 458C, 4 days; ii) mesitylimi-
dazole (10 equiv), toluene, reflux, 16 h; iii) TMSCl, Et3N, toluene, 258C, 16 h; iv) KHMDS (1 equiv), THF fol-
lowed by [p-C3H5PdCl]2 (2 equiv Pd).
below would not be possible
without SENS technology.
The 13C CPMAS spectrum of
co-hydrolysis and co-condensation of Si
(OEt)4 and (para-
[D9]Mat-Im was fully assigned from a 2D 1H–13C heteronu-
clear correlation (HETCOR) spectrum (Figure S9 in the
Supporting Information). The complete 1H, 13C, and 29Si
chemical-shift values are reported in Table S2 in the Sup-
porting Information.
chlorophenylmethyl)trimethoxysilane in the presence of Plu-
rionic 123, a structure-directing agent;[11] 2) the subsequent
treatment of this material with mesitylimidazole; and 3) the
passivation of the surface silanols with (CH3)3SiCl/Et3N to
give Mat-Im. We also prepared the corresponding materials,
in which the surface was passivated with (CD3)3SiCl/Et3N in
place of (CH3)3SiCl/Et3N to give [D9]Mat-Im, because we
have observed that trimethylsilyl (TMS)-passivated materi-
als, which are essential for metal-complex incorporation, ob-
literate the DNP NMR signal enhancements necessary for
the characterization discussed below, due to the high density
of fast-relaxing methyl protons.[12] Deuteration of the surface
methyl groups restores the DNP performance. Treatment of
Mat-Im and [D9]Mat-Im with potassium hexamethyldisila-
zide (KHMDS) in THF suspension and subsequent addition
of [p-C3H5PdCl]2 gave Mat-Pd and [D9]Mat-Pd, respectively
(Scheme 1, Figure 1a).
Clear changes were observed in the 13C CPMAS spectrum
upon the conversion of [D9]Mat-Im to [D9]Mat-Pd (Fig-
ure 1c). The 13C NMR spectrum contains a new signal at d=
109 ppm together with a significant increase in the reso-
nance intensity of the peaks at 0 and 54 ppm relative to
[D9]Mat-Im. The new resonances at d=54 and 110 ppm cor-
respond to two of the expected signals from the Pd–allyl
fragment (carbons 9’ and 10’). The peak at d=0 ppm is as-
signed to the carbons of the surface OSiACTHNUTGRNEG(UN CD3)3 groups, and
to the methyl carbons of a residual amount of unreacted
KHMDS used in the synthesis of [D9]Mat-Pd. By using a
longer cross-polarization contact time of 6 ms, the NHC–Pd
carbon resonance is observed at d=181 ppm, which consti-
Chem. Eur. J. 2013, 19, 12234 – 12238
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12235

C. Copꢀret, C. Thieuleux et al.
Figure 2. a)Contour plot of a 2D 1H–13C HETCOR spectra recorded on
[D9]Mat-Pd. e) Contour plot of a 2D 1H–29Si spectrum recorded on
[D9]Mat-Pd.
Figure 1. a) Structures of [D9]Mat-Im and [D9]Mat-Pd. b) 1D 13C CPMAS
spectra of [D9]Mat-Im with microwaves on (mW; top, black) and mW off
(bottom, grey). c) 1D 13C CPMAS spectra of [D9]Mat-Pd with mW.
of the surface tether (C7’) and the mesityl ligand (in particu-
lar C2 and C2’), as well as the carbons of the NHC ligand
(C3’ and C4’). The cross-peak at [d1H, d13C]=[2.0, 17 ppm] is
assigned to the methyl groups (C1’,C1’’ and C1’’’) of the me-
sityl function. Finally, the correlation at [d1H, d13C]=[0,
0 ppm] corresponds to surface TMS groups.
tutes clear evidence for the attachment of the Pd–allyl frag-
ment to the surface-bound NHC. Previous work on similar
metal-containing hybrid materials required tedious and ex-
pensive 13C isotopic-labeling strategies to observe this key
resonance.
1
At longer contact times, H–13C HETCOR spectra (Fig-
ure S10 in the Supporting Information) display correlations
between the terminal Pd–allyl protons (at around 3 ppm)
and the central Pd–allyl carbon (C10’) at 110 ppm, as was
The assignment of the 13C CPMAS spectrum of [D9]Mat-
1
Pd was confirmed by a series of H–13C 2D HETCOR NMR
expected. Long-range correlations are also observed at [d1H
,
experiments recorded with various CP contact times (250 ms,
2, and 6 ms). The short contact time primarily allows the de-
tection of correlations between bonded pairs of nuclei (Fig-
ure 2a). The correlation at [d1H, d13C]=[5.0, 110 ppm] corre-
sponds to the central Pd–allyl carbon (C10’). The broad
shoulder centered at around [d1H, d13C]=[5.5, 54 ppm] corre-
sponds to both the correlations involving the CH2N (C6’)
and the terminal Pd–allyl (C9’) carbons. The third Pd–allyl
resonance (expected at ca. [d1H, d13C]=[4, 72 ppm] by com-
parison with molecular complexes) likely resides under the
NMR solvent signal (C2H2Cl4).
d
13C]=[3.0, 0 ppm] between these terminal Pd–allyl protons
and the carbons of surface trimethylsilyl groups. Similar
cross-peaks are visible between the mesityl–methyl protons
and carbons of trimethylsilyl moieties at [d1H, d13C]=[2.2,
0 ppm]. These correlations reflect spatial proximities be-
tween the allyl and mesityl groups on the one hand and the
TMS surface groups on the other hand, and suggest that the
ligand is bent, interacting with the surface, and that the Pd
metal center, therefore, lies close to the silica surface.[13d,15]
Finally, we note that the Pd–NHC carbene resonance has
weak correlations with aromatic and the C10’-Pd–allyl pro-
tons, which is also consistent with the structure of the cata-
lyst.
The cross-peak at [d1H, d13C]=[7, 133 ppm] in the 2D HET-
COR NMR spectrum corresponds to the aromatic carbons
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Chem. Eur. J. 2013, 19, 12234 – 12238

Pd Hybrid Material for the Selective Semihydrogenation
COMMUNICATION
1
Figure 2b shows a H–29Si 2D HETCOR NMR spectrum
Table 1. Mat-Pd-catalyzed alkyne semihydrogenation.
recorded on the [D9]Mat-Pd material. The 1D 29Si NMR
spectrum (on top of the 2D plot) contains four resonances
corresponding to bulk silicon Q sites at d=À98 ppm, the sil-
icon T sites at À77 ppm, the residual unreacted KHMDS
signal at À15 ppm, and surface passivating [D9]TMS groups
Entry
Alkyne
TOF [hÀ1
]
TON[a]
Selectivity[b]
1
2
3
4
1800
1100
3100
2200
3300
93 (89)
5100
81 (56)
1200[c]
13200
94[c] (n.d.)[d]
94 (92)
at d=16 ppm. The H–29Si 2D HETCOR spectrum contains
1
the expected correlation between the 29Si T site and aromat-
ic proton resonances at around 8 ppm. The [D9]TMS groups
show long-range cross-peaks with aromatic protons at d=
7.3 ppm, as well as correlations with the mesityl methyl
groups at 2.0 ppm. The 29Si nuclei of KHMDS at d=
À15 ppm correlate only with their covalently bound methyl
protons at 0 ppm. No long-range correlations with other
protons were observed, establishing that the KHMDS
groups adsorbed onto the surface are well isolated from the
rest of the organic residues.
5
4500
2100
93 (87)
[a] mol product/mol catalyst. [b] Measured at 50% alkyne consumption,
number in parentheses measured at 99% alkyne consumption. [c] Termi-
nal alkene selectivity. [d] C16 byproducts observed at high alkyne (>
70%) conversion. n.d.=not determined.
uct is often a critical problem. Finally, the formation of the
overhydrogenated byproduct (Ane-1) occurs only at high
alkyne conversion (>90%) and at >99% consumption of
Yne-1 the Z-selectivity decays only slightly to 89%.
We should note that Mat-Pd is much more active and se-
lective than similar homogeneous complexes. For example,
related discrete NHC–Pd0 complexes have a TOF of about
45 hÀ1 with a selectivity of 75% for the Z-alkene at 99%
conversion.[9a] Under similar conditions, the homogeneous
Only two days of measurement time were required to ac-
quire this complete set of experiments (including multiple
1
13C CP magic-angle spinning (MAS), H–13C HETCOR, 29Si
1
CP MAS, H–29Si HETCOR NMR experiments) for the un-
ambiguous assignment of each 1 H, 13C, and 29Si NMR spec-
trum, and for the full structural characterization of the orga-
nometallic fragment. Note that high-resolution MAS
(HRMAS) NMR experiments fail on these types of materi-
als. For example, HRMAS on Mat-Im only gave broad spec-
tral lines with poor signal-to-noise ratio in the 1H and
13C NMR spectra, probably because of the low mobility of
surface species, which resulted in strong residual dipolar
coupling (see the Supporting Information for details). The
NMR data clearly show that the NHC–Pd fragment is in a
well-defined molecular environment on the silica surface,
that there are no significant side products to the reaction or
unreacted ligands, and that the 3D conformation of the sur-
face-bound complex is bent over close to the surface. Such
atomic level of characterization was not accessible previous-
ly by using any other technique, and would be inconceivable
at natural-isotopic abundance without DNP SENS NMR
technology.
analogue [BnMesNHC]PdACTHNUTRGNE(NUG C3H5)Cl transforms Yne-1 to Z-
Ene-1 with 90% selectivity at 50% conversion, although the
selectivity drops to 84% at 95% conversion. The dominant
byproduct of the [BnMesNHC]PdACTHUNTRGNE(UNG C3H5)Cl-catalyzed semi-
hydrogenation is Ane-1, which appears only at high alkyne
consumption (see the Supporting Information for details).
We also decomposed Mat-Pd in the absence of alkyne to
give Pd0 particles (ca. 20 nm) that also catalyze the semihy-
drogenation of Yne-1, although with different rate (TOF=
700 hÀ1) and selectivity (74% Z-Ene) profiles, suggesting
that these particles are not the active catalyst in Mat-Pd
(see the Supporting Information).
The scope of the Mat-Pd-catalyzed semihydrogenation of
alkynes is shown in Table 1. Mat-Pd efficiently reduces di-
phenylacetylene (Yne-2), which is a challenging substrate
for this reaction, to give Z-stilbene with 81% selectivity at
50% alkyne conversion, and 56% at >99% conversion
(entry 2). This loss of selectivity at high alkyne conversion is
common for Yne-2, because the product Z-ene-2 is known
to be readily hydrogenated. 1-Octyne (Yne-3) was hydro-
genated in the presence of 0.05 mol% Mat-Pd to give 1-
octene in 93% selectivity at 50% conversion (entry 3). Al-
though the semihydrogenation of 1-octyne is 86% selective
for 1-octene in the C8 fraction at 97% alkyne conversion,
the formation of C16 byproducts at high alkyne conversion
(>70%) reduces the yield of 1-octene. 4-Octyne (Yne-4)
and hex-3-yne-1-ol (Yne-5) were clearly semihydrogenated
to give their respective Z-ene products with high selectivity
and activity (entries 4 and 5).
The catalytic performance of Mat-Pd was evaluated in the
stereoselective semihydrogenation of alkynes to Z-alkenes
in the presence of dihydrogen (Table 1).[9a,c] The hydrogena-
tion of alkynes (Yne) can give three products: the desired
Z-alkene (Z-Ene), the unwanted E-alkene isomer (E-Ene),
or the over-reduced alkane product (Ane). Mat-Pd is active
in semihydrogenation of 1-phenylpropyne (Yne-1) with a
turnover frequency (TOF) of 1800 hÀ1, and kinetically selec-
tive towards Z-Ene-1 (93%, entry 1). The catalyst is fairly
stable under these conditions, allowing a turnover number
(TON) of 1100 to be reached for this substrate. However,
increasing the substrate to Pd ratio beyond this limit did not
lead to full alkyne conversion indicating catalyst deactiva-
tion. ICP analysis of the reaction mixtures contains less than
1 ppm palladium, indicating that leaching from the silica
support does not occur during catalysis, preventing metal
contamination of the final product. This is particularly im-
portant, because removing Pd traces from the organic prod-
In summary, we have described the rational design and
detailed DNP SENS characterization of a mesostructured
silica hybrid material that contains Pd–NHC functionalities
regularly distributed within the pore channels. The SENS
Chem. Eur. J. 2013, 19, 12234 – 12238
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Keywords: carbenes · hybrid materials · palladium · NMR
spectroscopy · semihydrogenation
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Published online: August 19, 2013
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