Detection and reactivity of Pd((C8H14)PCH 2CH2P(C8H14))(CHPhCH 2Ph)(H) as determined by parahydrogen-enhanced NMR spectroscopy

Article abstract of DOI:10.1021/ja044875f

Pd(bcope)(OTf)2 (where bcope is (C₈H₁₄)PCH₂CH₂P(C₈H₁₄)) is shown to react with an alkyne in the presence of parahydrogen to form alkyl hydrides, such as Pd(bcope)(CHPhCH₂Ph)(H),

Full text of DOI:10.1021/ja044875f

Published on Web 12/02/2004
Detection and Reactivity of Pd((C8H14)PCH2CH2P(C8H14))(CHPhCH2Ph)(H) as
Determined by Parahydrogen-Enhanced NMR Spectroscopy
J. P. Dunne,† S. Aiken,† S. B. Duckett,*,† D. Konya,‡ K. Q. Almeida Len˜ero,‡ and E. Drent*,‡
Department of Chemistry, UniVersity of York, Heslington, YO10 5DD, York, United Kingdom, and Shell Research &
Technology Centre Amsterdam, Badhuisweg 3, NL-1031 CM Amsterdam, The Netherlands
Received August 25, 2004; E-mail: sbd3@york.ac.uk; edrent@wanadoo.nl
Palladium complexes are widely used in homogeneous catalysis.
They are best known for their C-C bond forming reactions but
also feature in oxidation, reduction, isomerization, and carbonylation
chemistry.1 Less well-known is the excellent performance of palla-
dium diphosphine complexes in the hydroformylation of alkenes.2
The role of palladium hydrides in many of these processes is well
established, but the direct observation of palladium hydride com-
plexes in such reactions is extremely rare. Recently, [(1,2-(CH2P-
But2)2C6H4)Pd(H)(MeOH)]+, a key intermediate in the methoxy-
carbonylation of alkenes, was observed by NMR3 with subsequent
studies detecting the corresponding [(L)2Pd(CH2CH3)]+ species,
which features a â-agostic C-H interaction.4 Deactivated species
such as Pd(PPh3)2(H)(Br) or Pd(dippp)(H)(Cl) have also been
observed.5
The parahydrogen-induced polarization (PHIP)6 effect enhances
the hydride resonances of metal-dihydride complexes directly and
those of scalar-coupled 31P and 13C heteronuclei by cross-polariza-
tion.7 The development of selective excitation methods8 and the
use of 2D methods to observe insensitive heteronuclei have also
made an impact.9 Furthermore, PHIP has been harnessed to study
the adsorption of H2 onto surfaces,10 in magnetic resonance imaging
(MRI),11 and to sensitize a hydroformylation product containing a
single atom from p-H2.12 Here we employ the PHIP effect to study
the reactions of Pd(bcope)(OTf)2 1 (where bcope is (C8H14)PCH2-
CH2P(C8H14))13 with alkynes. We show that the resonances of orga-
nic components bound within a metal’s coordination sphere can be
substantially enhanced and hence demonstrate that the PHIP effect
can be used to detect metal complexes without the need for
enhancement of a hydride resonance.
A 1 µM solution of 1 in CD2Cl2 containing a 50-fold excess of
d10-diphenylacetylene was placed under 3 atm of 100% p-H2 at
213 K and rapidly introduced into a 400 MHz NMR spectrometer.
While a 295 K based 31P NMR spectrum indicated that 1 was the
only species present, the corresponding 1H NMR spectrum showed
an emission signal at δ 6.66 for the alkene proton of the 12C
isotopomer of the kinetic hydrogenation product cis-stilbene. Since
the two-alkene protons of the cis isomer are magnetically equivalent,
the observation of this p-H2 enhanced signal indicates the involve-
ment of an undetected intermediate in which the two hydrogen
atoms of the p-H2 molecule are inequivalent14 and introduced in a
spin-correlated pathway.9c,d This intermediate most likely corre-
sponds to Pd(bcope)(CPhdCHPh)(H) as shown in Scheme 1. A
weak signal, due to the accumulated thermally polarized trans
product, slowly develops in this spectrum at δ 7.18. This product
cannot, however, be formed simply from Pd(bcope)(CPhdCHPh)-
(H) since the associated hydrogen atoms must remain cis if the
process is concerted.
3.13 and a further weakly enhanced signal at δ 2.92 (Figure 1a).
These signals contained characteristic anti-phase components due
to their origin from p-H2 protons, proved to be coupled in a COSY
spectrum, and simplified on 31P decoupling. 1H-31P HMQC spectra
revealed two 31P doublets at δ 32.3 and δ 42.9 (JPP ) 47 Hz) for
this species, while a 1H-13C HMQC spectrum produced correlations
from the proton resonance at δ 4.94 to a 13C signal at δ 63.0, and
the remaining proton resonances to a signal at δ 37.1. It can
therefore be concluded that these resonances arise from a ligand
that is attached to palladium. When this reaction was reexamined
using 13CtC enriched d10-diphenylacetylene, strong signals were
observed in one-dimensional, fully proton coupled 13C experiments
at δ 63.0 and δ 37.1; the former showed two 31P splittings of 42
and 14 Hz in addition to a single proton splitting of 147 Hz. The
δ 63.0 signal therefore corresponds to a CH group bound directly
to palladium. In this spectrum, the δ 37.1 signal appeared as a
pseudo-triplet as a consequence of the PHIP effect, with lines of
relative intensities 1, 0, and -1, and therefore corresponds to a
CH2 moiety. This suggests that cis-Ph-CHdCH-Ph has been
converted into a PdCHPhCH2Ph group, and the species giving rise
to these signals is Pd(bcope)(CHPhCH2Ph)(H), 2 (Scheme 1).15
To explore the reactivity of 2, a series of modified 1D-EXSY
experiments were recorded where a single alkyl proton resonance
was selected, and magnetization transfer from this site was
monitored as a function of mixing time.6e,16 For the δ 4.94 peak,
strong magnetization transfer into the trans-stilbene signal at δ 7.18
was observed at 295 K (Figure 1b). When the sample was warmed
to 313 K, the intensity of all of the enhanced peaks mentioned
previously increased substantially. However, under these conditions,
even greater magnetization transfer from the δ 4.94 site into trans-
stilbene was seen in conjunction with weaker transfer into cis-
stilbene and into both of the previously described CH2Ph proton
sites at δ 3.13 and δ 2.92. Simulation of these experimental traces17
suggests that the observed rate constants of formation of trans-
and cis-stilbene from 2 were 4 s-1 and 0.004 s-1, respectively. This
information confirms that the detected PdCHPhCH2Ph group of 2
transforms most readily into trans-stilbene and demonstrates that
hydride insertion is reversible and involves a discrete Pd(bcope)-
(PhCHdCHPh)(H)2 intermediate. These observations thereby ac-
count for the isomerization of the kinetic cis hydrogenation product
into the thermodynamically favored trans product, a reaction that
can be monitored for more than 50 turnovers based on 1.
When the peak at δ 3.13 was selected in the same EXSY
experiment at 313 K, exchange into free H2 and weaker exchange
into free cis- and trans-stilbene, as well as into the δ 4.94 position,
were observed. However, when the resonance at δ 2.92 was
selected, only transfer into trans-stilbene was observed. These
observations confirm that the proton yielding the resonance at δ
3.13 transfers directly into free H2, while those at δ 4.94 and δ
2.92 move into trans-stilbene. The identification of 2 as an alkyl
hydride is therefore confirmed (Scheme 1). Since the hydride
The most notable features of this NMR spectrum, however,
correspond to substantially enhanced proton signals at δ 4.94 and
† University of York.
‡ Shell Research & Technology Centre Amsterdam.
9
16708
J. AM. CHEM. SOC. 2004, 126, 16708-16709
10.1021/ja044875f CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. Hydrogenation by a Palladium Bisphosphine Complexa
a Color indicates the transformations of individual p-H2 molecules with the dominant catalytic formation of cis-stilbene indicated in bold, the secondary
isomerization to trans-stilbene competes on the NMR timescale.
the mapping of concerted catalytic hydrogenation by a palladium-
(II) bis-phosphine complex.
Acknowledgment. We are grateful to EU funding under the
HYDROCHEM network (contract HPRN-CT-2002-00176).
Supporting Information Available: Synthetic 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) Pugh, R. I.; Drent, E. In Catalytic Synthesis of Alkene-Carbon Monoxide
Copolymers and Cooligomers; Sen, A., Ed.; Kluwer Academic Publish-
ers: Dordrecht, The Netherlands, 2003; Ch. 2.
Figure 1. (a) 1H{31P} spectrum of 2 in CD2Cl2 at 295 K showing key
alkyl proton resonances. (b) Magnetization transfer from the alkyl R-H of
2 to the â-H protons and free trans-stilbene after 800 ms.
(3) Clegg, W.; Eastham, G. R.; Eiselgood, M. R. J.; Heaton, B. T.; Iggo, J.
A.; Tooze, R. P.; Whyman R.; Zacchini, S. J. Chem. Soc., Dalton Trans.
2002, 3300-3308.
(4) Clegg, W.; Eastham, G. R.; Eiselgood, M. R. J.; Heaton, B. T.; Iggo, J. A.;
Tooze, R. P.; Whyman R.; Zacchini, S. Organometallics 2002, 1832-1840.
(5) Grushin, V. V. Chem. ReV. 1996, 96, 2011-2033.
ligands transfer on the internal face of the metal-alkene, the original
orientation of the hydrogen and phenyl substituents of the alkyl
group must match those of the alkene, and hence, rotation about
the C-C bond of 2 is necessary to form trans-stilbene (Scheme
1). The strongly enhanced signals at δ 4.92 and δ 3.13 are therefore
due to the original hydrogen atoms of p-H2 activated cis-stilbene
while the weakly enhanced signal at δ 2.92 arises because of a
species where hydride exchange places two protons from the same
p-H2 molecule onto the same carbon of the alkyl; these effects can
be reproduced by simulation, and similar exchange effects have
been seen in liberated alkenes produced during related hydrogena-
tion reactions.18 The failure to detect an nOe interaction from the
δ 4.94 signal to the hydride ligand suggests the phenyl is initially
on the internal face of the alkyl, with rotation to 2′ and subsequent
â-hydrogen transfer leading to alkene isomerization (Scheme 1).
When phenylpropyne was used as a substrate, Pd(bcope)-
(CHPhCH2Me)(H) 3, was observed with enhanced proton reso-
nances at δ 1.55, 1.86, and 4.72. The observation of 2 is therefore
(6) (a) Bowers, C. R.; Jones, D. H.; Kurur, N. D.; Labinger, J. A.; Pravica,
M. G.; Weitekamp, D. P. AdV. Magn. Res. 1990, 14, 269. (b) Natterer, J.;
Bargon, J. Prog. Nucl. Magn. Reson. Spectrosc. 1997, 31, 293-315. (c)
Duckett, S. B.; Sleigh, C. J. Prog. Nucl. Magn. Reson. Spectrosc. 1999,
34, 71-93. (d) Duckett, S. B.; Blazina, D. Eur. J. Inorg. Chem. 2003,
1046-1061. (e) Blazina, D.; Duckett, S. B.; Dunne, J. P.; Godard C. J.
Chem. Soc., Dalton Trans. 2004, 2601-2609.
(7) (a) Duckett, S. B.; Newell, C. L.; Eisenberg, R. J. Am. Chem. Soc. 1993,
115, 1156-1157. (b) Haake, M.; Natterer, J.; Bargon, J. J. Am. Chem.
Soc. 1996, 118, 8688-8691.
(8) (a) Barkemeyer, J.; Bargon, J.; Sengstschmid, H.; Freeman, R. J. Magn.
Reson., Ser. A 1996, 120, 129-132. (b) Sengstschmid, H.; Freeman, R.;
Barkemeyer, J.; Bargon, J. J. Magn. Reson., Ser. A 1996, 120, 249-257.
(9) (a) Duckett, S. B.; Barlow, G. K.; Partridge, M. G.; Messerle, B. A. J.
Chem. Soc., Dalton. Trans. 1995, 3427-3430. (b) Duckett, S. B.; Mawby,
R. J.; Partridge, M. G. J. Chem. Soc., Chem. Commun. 1996, 383-384.
(c) Jang, M.; Duckett S. B.; Eisenberg. R. Organometallics 1996, 15,
2863-2865. (d) Millar, S. P.; Jang, M.; Lachicotte, R. J.; Eisenberg, R.
Inorg. Chim. Acta 1998, 270, 363-375, (e) Messerle, B. A.; Sleigh, C.
J.; Partridge, M. G.; Duckett, S. B. J. Chem. Soc., Dalton Trans. 1999,
1429-1435.
(10) Carson, P. J.; Bowers, C. R.; Weitekamp, D. P. J. Am. Chem. Soc. 2001,
123, 11821-11822.
(11) Golman, K.; Axelsson, O.; Johannesson, H.; Mansson, S.; Olofsson, C.;
Petersson, J. S. Magn. Reson. Med. 2001, 46, 1-5.
1
not unique. While enhanced H NMR signals were now observed
(12) Permin, A. B.; Eisenberg, R. J. Am. Chem. Soc. 2002, 124, 12406-12407.
(13) Eberhard, M. R. Ph.D. Thesis, University of Bristol, Bristol, U.K., 2001.
(14) Aime, S.; Gobetto R.; Canet, D. J. Am. Chem. Soc. 1998, 120, 6770-
6773.
(15) When cis-stilbene is used as the substrate, 2 becomes visible after 1000
scans in the corresponding 31P{1H} spectrum, although the corresponding
1H resonances show no polarization.
for cis- and trans-â-methylstyrene, no direct magnetization transfer
into them from the aliphatic proton signals of 3 was observed, even
at 313 K. It can therefore be concluded that 3 is less prone to
â-hydride elimination than 2.
This study has demonstrated that PHIP need not be limited to
the study of metal dihydrides since the enhancement of organic
components within a metal’s ligand sphere can be achieved. To
observe the PHIP effect in this study, the reactions must be based
on (bcope)Pd(H)2 rather than (bcope)Pd(H)(solv)+, since the
necessary pairwise H2 transfer is not possible with the latter
complex.19 It has also been demonstrated that Pd(bcope)(alkene)-
(H)2 undergoes reversible hydride transfer to the bound alkene.
These deductions are summarized in Scheme 1 and correspond to
(16) (a) Colebrooke, S. A.; Duckett S. B.; Lohman. J. A. B. Chem. Commun.
2000, 685-686. (b) Blazina, D.; Duckett, S. B.; Dyson, P. J.; Johnson,
B. F. G.; Lohman. J. A. B.; Sleigh, C. J. J. Am. Chem. Soc. 2001, 123,
9760-9768.
(17) Jones, W. D.; Rosini, G. P.; Maguire, J. A. Organometallics 1999, 18,
1754-1760.
(18) Harthun, A.; Giernoth, R.; Elsevier, C. J.; Bargon, J. Chem. Commun.
1996, 2483-2484.
(19) When this reaction was run with a mixture of D2 and p-H2, very little HD
formation was observed.
JA044875F
9
J. AM. CHEM. SOC. VOL. 126, NO. 51, 2004 16709