Palladium-catalyzed hydrogenation: Detection of palladium hydrides. A joint study using para-hydrogen-enhanced NMR spectroscopy and density functional theory

Article abstract of DOI:10.1021/ja0630137

Pd(PEt₃)₂(OTf)₂, acting as an in situ source of Pd(PEt₃)₂, reacts with an alkyne and hydrogen via phosphine loss to form the detectable hydride-containing species Pd(PEt₃)₂(H)(CHPhCH₂Ph), cis- and trans-Pd(PEt₃)₂(H)(CPh=CHPh), and Pd₂(PEt₃)₃(H)(CHPhCH₂Ph)²⁺, which map onto the reaction scheme predicted by density functional theory. Copyright

Full text of DOI:10.1021/ja0630137

Published on Web 07/06/2006
Palladium-Catalyzed Hydrogenation: Detection of Palladium Hydrides.
A Joint Study Using Para-Hydrogen-Enhanced NMR Spectroscopy and
Density Functional Theory
Joaqu´ın Lo´pez-Serrano,† Simon B. Duckett,*,† and Agust´ı Lledo´s‡
Department of Chemistry, UniVersity of York, Heslington, YO10 5DD York, United Kingdom, and
Department de Qu´ımica, UniVersitat Auto`noma de Barcelona, 08193 Barcelona, Spain
Received May 1, 2006; E-mail: sbd3@york.ac.uk
Palladium complexes containing two monodentate phosphine
ligands are widely used in homogeneous catalysis.1 Their conversion
during catalysis to palladium monophosphine complexes2 or pal-
ladium colloids3 means that identifying the active catalyst is not
straightforward. In the case of hydrogenation, a role for a palladium
hydride is often proposed.4 An example of such a species is given
by Pd(bcope)(CHPhCH2Ph)(H),5 which was seen through the para-
hydrogen-induced polarization (PHIP)6 effect. In this Communica-
tion, we report on the catalytic activity of cis-[Pd(PEt3)2(OTf)2].7
Although many palladium-based reactions have been studied by
PHIP,6,8 we show here for the first time that palladium hydride
resonances can themselves be enhanced and, in conjunction with
density functional theory (DFT), rationalize the hydrogenation
activity of a palladium-bisphosphine based hydrogenation catalyst.
Earlier ab initio studies on Pd(PH3)(H)(C2H5) have considered the
role of monophoshine species in alkene hydrogenation.9
A number of 9 µM solutions of cis-[Pd(PEt3)2(OTf)2] (1) in Me-
OD-d4 containing a 25-fold excess of diphenylacetylene-d10 under
2-4 atm of 100% p-H2 have been examined by 700 MHz NMR
spectroscopy. This reaction would be expected to provide a route to
the analogous Pd(0) species Pd(PEt3)2.10 The resulting 1H NMR spec-
tra at 295 K revealed three new signals at δ 7.25, 6.66, and 2.91, due
to the three expected organic hydrogenation products, trans-stilbene,
cis-stilbene, and 1,2 diphenylethane, respectively. The δ 6.66 signal is
due to the kinetic cis-hydrogenation product and appears in emission
due to its formation via an intermediate with inequivalent hydride
ligands.11 While the two other signals appear in absorption, they also
show PHIP when 13CtC-enriched diphenylacetylene-d10 is employ-
ed and therefore correspond to products formed via a hydrogenation
pathway that places two protons from the same p-H2 molecule on ad-
jacent sites. When this reaction is examined by GCMS, the ratio of
cis-stilbene:trans-stilbene:1,2 diphenylethane was typically 78:11:11.
In these spectra, a number of metal-based species are also detec-
ted through the PHIP effect. Notably, three enhanced 1H NMR sig-
nals appear immediately at δ 4.47, 3.22, and 2.99 (Figure 1a). These
three coupled signals, arising from species 2 (Chart 1), also couple
to two 31P nuclei which resonate at δ 8.40 (JPP ) 54 Hz) and 25.90
(JPP ) 54 Hz). Use of 13CtC-enriched diphenylacetylene-d10 (310 K)
revealed that the 1H resonance of 2 at δ 4.47 connected to a 13C sig-
nal at δ 67.14 (JHC ) 157 Hz, JCP ) 44 Hz), while both the δ 3.22
and 2.99 signals connected to a single δ 35.30 resonance (JHC ) 130
Hz, JCP ) 15 Hz). These data confirm that 2 is Pd(PEt3)2(CHPhCH2-
Ph)(H). When an EXSY spectrum was recorded at 308 K, magneti-
zation transfer from the δ 4.47 signal of 2 to the δ 7.25 signal of
trans-stilbene confirmed a role for 2 in the hydrogenation of
diphenylacetylene.
Figure 1. 1H NMR spectra for the reaction of 1 with diphenylacetylene-
10 and p-H2 in MeOD-d4: (a) 32 scans (295 K) showing key alkyl proton
resonances; (b) 1224 scans (312 K) showing hydride resonances of 3, 4a,
and 4b, with the DFT model for 3 (3′) inset.
d
Chart 1. Structures of 2, 3, 4a, and 4b
-9.18 due to 3 (Chart 1). This hydride resonance appears as a
doublet of doublets of doublets of antiphase doublets due to
couplings to three inequivalent phosphine ligands (JPH couplings
19, 71, and 89 Hz) and a single proton (JHH ) -3.50 Hz) (Figure
1b); the proton providing this coupling was located by COSY
spectroscopy at δ 3.34 and coupled, in turn, to two signals at δ
3.52 and 3.38. The corresponding 1H-31P HMQC spectrum
revealed three 31P signals for 3 at δ 3.41 (d, JPP ) 75 Hz), 4.63 (d,
1
JPP ) 75 Hz), and 7.64. A H-13C HMQC measurement yielded
two 13C signals for 3, one due to CH2 a moiety at δ 49.70 (JHC
124 Hz) and the other due to a CH moiety at δ 49.58 (dd, JHC
)
)
124 and 134 Hz, JPC ) 24 and 55 Hz). When the hydride resonance
of 3 was monitored by EXSY spectroscopy, magnetization transfer
into both cis-stilbene and 1,2-diphenylethane was seen at 313 K.
Species 3 therefore contains three phosphines, one alkyl ligand,
and one hydride ligand. DFT calculations, however, reveal that the
monomeric trisphosphine complex Pd(PH3)3(H)(CH2CH3) is unsta-
1
The most remarkable feature of these H NMR spectra was,
however, the observation of a PHIP-enhanced hydride signal at δ
† University of York.
‡ Universitat Auto`noma de Barcelona.
9
9596
J. AM. CHEM. SOC. 2006, 128, 9596-9597
10.1021/ja0630137 CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. DFT-Based Reaction Scheme (with Energies) for Hydrogenation by a Palladium-Bisphosphine Complexa
a All energies are in kJ/mol and relative to Pd(PH3)2 + HsCtCsH + 2H2; labels 2′ and 4a′ indicate where experimentally observed complexes map
onto the DFT model system.
ble with respect to phosphine dissociation and the formation of the
corresponding PH3 analogue of 2, 2′. The NMR characteristics of the
hydride signal of 3, and in particular the observation of three distinct
phosphine resonances without a characteristically large trans PP coupl-
ing, support this suggestion and indicate that 3 is most likely a dipal-
ladium species. This is consistent with the fact that the size of the
PHIP-enhanced signal for 3 depends critically on the concentration of
1. DFT calculations on potential dipalladium species reveal that Pd2-
(PH3)3(H)(CH2CH3)2+ (3′) is stabilized by 489 kJ mol-1 relative to
Pd(PH3)22+ and Pd(PH3)(H)(CH2CH3). Hence, under the reaction con-
ditions where Pd(PEt3)22+ is present, the formation of 3 is expected.
Remarkably, in the high metal concentration experiments at 312
K, two further PHIP-enhanced hydride signals (ratio 1:0.7) are
detected in the early stages of the reaction, where there are high
CH3) upon PH3 coordination, a species that is directly analogous to
2. The detection of 3, 4a, and 4b is therefore fully consistent with
the DFT studies.
An additional hydride resonance appears in these NMR spectra
as an emission signal at δ -18.69 which does not contain any 31
P
splittings. The formation of a cluster containing two equivalent
hydrides that are not phosphorus-coupled is therefore indicated. This
confirms that phosphine loss occurs. We further note that the
addition of free PEt3 slows down both hydrogenation and cluster
formation; activity is totally suppressed by 5 equiv.
The key deductions outlined in this paper are summarized in
Scheme 1 and correspond to the mapping of the hydrogenation of
an alkyne by a palladium-bisphosphine complex.
Acknowledgment. We are grateful to EU for funding (HYDRO-
CHEM HPRN-CT-2002-00176) and to K. Q. Almeida Len˜ero and J.
Dunne for discussions. This paper is dedicated to Dr. Jose´ Antonio
Abad Ban˜os on his retirement.
levels of substrate and H2, at δ -7.77 (antiphase ddd with JPH
82 and 18 Hz) (4a) and -10.65 (antiphase dt triplets with JPH
)
)
15.8 Hz) (4b).12 Species 4a yielded two distinct 31P resonances at
δ 11.92 and 10.72, while 4b produced a single signal at δ 19.29.
Additional COSY spectra located a proton at δ 6.35, which
accounted for the antiphase JHH splitting of -4 Hz in 4a, and a
further signal at δ 5.35 (JHH ) -4 Hz) in 4b. While the weak and
transient nature of the NMR signals seen for these species prevented
the collection of 13C data, they can be unambiguously be assigned
to cis and trans isomers of Pd(PEt3)2(H)(CPhdCPhH), respectively.
The role of these species in the hydrogenation chemistry of 1 is
illustrated in Scheme 1 and has been rationalized through this and
other theoretical studies, which reveal that Pd(PH3)2 adds H2 in a
high-energy process while coordination of the alkyne is exother-
mic.13 The catalytic cycle features palladium-monophosphine
species, with Pd(PH3)(H)2(HsCtCsH) reacting via hydride trans-
fer to form the three-coordinate vinyl hydride Pd(PH3)(H)(CHd
CH2), which is analogous to Pd(PH3)(H)(CH2CH3).9 This T-shaped
species coordinates phosphine to form cis and trans isomers of Pd-
(PH3)2(H)(CHdCH2) (4a′, 4b′), which differ in energy by 20.6 kJ
mol-1. Experimentally, the observation of NMR signals for 4a and
4b in the ratio 1:0.7 suggests that their formation proceeds under
kinetic control. In view of the fact that the [4b] remains low, C-H
bond formation via phosphine dissociation must be rapid.
Supporting Information Available: Synthetic and computational
details and key NMR observations. This material is available free of
charge via the Internet at http://pubs.acs.org.
References
(1) Negishi, E.-I. Handbook of Organopalladium Chemistry for Organic Synthesis;
Wiley: New York, 2002.
(2) (a) Stambuli, J. P.; Incarvito, C. D.; Buhl, M.; Hartwig, J. F. J. Am. Chem.
Soc. 2004, 126, 1184-1194. (b) Ahlquist, M.; Fristrup, P.; Tanner, D.; Norrby,
P.-O. Organometallics 2006, 25, 2066-2073.
(3) Moiseev, I. I.; Vargaftik, M. N. New J. Chem. 1998, 22, 1217-1227.
(4) Grushin, V. V. Chem. ReV. 1996, 96, 2011-2034.
(5) Dunne, J. P.; Aiken, S.; Duckett, S. B.; Konya, D.; Almedia Lenero, K. Q.;
Drent, E. J. Am. Chem. Soc. 2004, 126, 16708-16709.
(6) (a) Bowers, C. R.; Jones, D. H.; Kurur, N. D.; Labinger, J. A.; Pravica, M. G.;
Weitekamp, D. P. AdV. Magn. Reson. 1990, 14, 269. (b) Natterer, J.; Bargon,
J. Prog. Nucl. Magn. Reson. Spectrosc. 1997, 31, 293-315. (c) Blazina, D.;
Duckett, S. B.; Dunne, J. P.; Godard, C. Dalton 2004, 2601-2609.
(7) cis-[Pd(PEt3)2(OTf)2] was synthesized from cis-[Pd(PEt3)2(Cl)2] according to
literature procedures. Stang, P. J.; Cao, D. H.; Saito, S.; Arif, A. M. J. Am.
Chem. Soc. 1995, 117, 6273-6283.
(8) (a) van Laren, M. W.; Duin, M. A.; Klerk, C.; Naglia, M.; Rogolino, D.;
Pelagatti, P.; Bacchi, A.; Pelizzi, C.; Elsevier, C. J. Organometallics 2002, 21,
1546-1553. (b) Dedieu, A.; Humbel, S.; Elsevier, C. J.; Grauffel, C. Theor.
Chem. Acc. 2004, 112, 305-312. (c) Kluwer, A. M.; Koblenz, T. S.; Jonischkeit,
T.; Woelk, K.; Elsevier, C. J. J. Am. Chem. Soc. 2005, 127, 15470-15480.
(9) Koga, N.; Obara, S.; Kitaura, K.; Morokuma, K. J. Am. Chem. Soc. 1985, 107,
7109-7116.
(10) Budzelaar, P. H. M.; Leeuwen, P. W. N. M.; Roobeek, C. F. Organometallics
1992, 11, 23-25.
(11) Aime, S.; Gobetto R.; Canet, D. J. Am. Chem. Soc. 1998, 120, 6770-6773.
(12) (a) Abis, L.; Santi, R. J. Orgomet. Chem 1981, 215, 263-267. (b) Portnoy,
M.; Milstein, D. Organometallics 1994, 13, 600-609.
The DFT studies predict that Pd(PH3)(H)(CHdCH2) isomerizes
to Pd(PH3)(CH2dCH2) prior to addition of H2. The corresponding H2
addition product then forms the alkyl hydride Pd(PH3)(H)(CH2CH3)
after hydride migration. This alkyl species forms Pd(PH3)2(H)(CH2-
(13) Low, J. J.; Goddard, W. A. J. Am. Chem. Soc. 1986, 108, 6115-6128.
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