DOI: 10.1002/cphc.201000407
New Perspectives for Parahydrogen-Induced Polarization in Liquid Phase
Heterogeneous Hydrogenation: An Aqueous Phase and ALTADENA Study
Igor V. Koptyug, Vladimir V. Zhivonitko, and Kirill V. Kovtunov*[a]
Since parahydrogen-induced polarization (PHIP) phenomenon
was described,[1] it has become one of the important tools for
the investigation of the mechanisms and kinetics of homoge-
neous hydrogenation and hydroformylation reactions.[2] Polari-
zation of nuclear spins provided by the utilization of parahy-
drogen in a catalytic hydrogenation reaction can be orders of
magnitude higher than the thermal one. Therefore, owing to a
significant signal enhancement in NMR spectra, it is possible to
study fast reactions and to identify intermediates of a catalytic
reaction even if their concentration in solution is low.[3] Homo-
geneous hydrogenation with parahydrogen was also success-
fully utilized for signal enhancement in magnetic resonance
imaging (MRI).[4] However, the use of homogeneous catalysts
and organic solvents in such experiments is a severe restriction
to a much broader scope of PHIP applications in MRI. Clearly,
for biomedical MRI the use of aqueous solutions is required.
Up to now, several water soluble compounds were hydrogen-
ated with parahydrogen to produce aqueous solutions of 13C
hyperpolarized contrast agents.[5] In all cases, however, homo-
geneous hydrogenation reactions were utilized. For in vivo MRI
studies, transition metal complexes used as hydrogenation cat-
alysts need to be removed from solution before the hyperpo-
larized agent is administered. Thus, a quick and facile removal
of a hydrogenation catalyst from the polarized fluid would be
essential for a successful application of PHIP in MRI experi-
ments performed in vivo.
mobilized metal complexes from the solid support into solu-
tion and aging of the immobilized catalysts represent serious
problems that complicate their use in aqueous phase hetero-
geneous hydrogenation reactions.
A recent observation of PHIP effects in heterogeneous gas-
phase hydrogenation reactions catalyzed by supported metal
catalysts[9,10] opens up new horizons for this field of research.
Later it was confirmed that supported metal catalysts are able
to produce PHIP effects in liquid phase hydrogenations as
well.[11] In order to turn these observations into useful experi-
mental protocols for MRI studies, it is essential to make further
steps in the exploration of PHIP in heterogeneous hydrogena-
tion reactions catalyzed by supported metal catalysts. More-
over, contrast agents typically used for clinical applications of
MRI are water soluble.[12] Therefore, the production of water
soluble polarized contrast agents is essential for extending
PHIP MRI applications to potential in vivo studies.
With these goals in mind, we herein compare the PHIP ef-
fects observed for two catalysts comprising rhodium particles
supported on two different porous supports and for the first
time report the use of supported metal catalysts for the obser-
vation of parahydrogen induced polarization in an aqueous
phase heterogeneous hydrogenation and of strong ALTADE-
NA[1c] effects for heterogeneous hydrogenation of liquids and
gases dissolved in several organic solvents.
For the initial study of the activity of the two supported
metal catalysts, both Rh/TiO2 and Rh/AlO(OH) catalysts were
used in a heterogeneous hydrogenation of propylene gas (see
the Supporting Information). It was established that these cata-
lysts are very active in the gas-phase hydrogenation of propyl-
ene into propane even at room temperature (RT); moreover,
the PHIP effects were clearly observed when parahydrogen
was used in the hydrogenation reaction. The results obtained
in the hydrogenation of propylene with parahydrogen employ-
ing Rh/TiO2 and Rh/AlO(OH) catalysts are shown in Figure S1 in
the Supporting Information. The observation of PHIP in the
heterogeneous hydrogenation of propylene verifies that the
heterogeneous hydrogenation reaction involves a reaction
route in which two hydrogen atoms of one hydrogen molecule
are added to the same substrate molecule. At the same time,
this pairwise hydrogenation reaction route is not a major one,
and its contribution can be estimated as several percent.[9]
Nevertheless, the observation of polarized multiplets in the
NMR spectra demonstrates that successful utilization of sup-
ported metal catalysts for PHIP production is possible.
Obviously, heterogeneous catalysts are much easier to sepa-
rate from a reaction mixture than homogeneous ones. There-
fore, the use of heterogeneous catalysts in hydrogenation reac-
tions could be an alternative route to produce catalyst-free hy-
perpolarized fluids. Besides, industrial catalytic processes are
predominantly heterogeneous, and the studies of the nature
of active sites and of the mechanisms of heterogeneous cata-
lytic reactions represent an extremely important area of re-
search that could benefit from the utilization of PHIP effects.
Recently, it was shown that it is indeed possible to observe
PHIP effects in heterogeneous hydrogenation reactions cata-
lyzed by transition metal complexes immobilized on solid sup-
ports.[6] The PHIP effects were demonstrated for heterogene-
ous hydrogenation reactions carried out in both liquid and gas
phases. The resulting hyperpolarized fluids were successfully
utilized for gas phase MRI studies.[7,8] However, leaching of im-
[a] Prof. I. V. Koptyug, Dr. V. V. Zhivonitko, Dr. K. V. Kovtunov
International Tomography Center SB RAS
3 A Institutskaya St., Novosibirsk 630090 (Russia)
Fax: (+7) 383-333-1399
For the next set of experiments, Rh/TiO2 and Rh/AlO(OH) cat-
alysts are used in a heterogeneous hydrogenation of propyl-
ene gas dissolved in a liquid. Both catalysts hydrogenate pro-
pylene into propane when the hydrogenation reaction is car-
Supporting information for this article is available on the WWW under
3086
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPhysChem 2010, 11, 3086 – 3088