Para-hydrogen induced polarization without incorporation of para-hydrogen into the analyte

Article abstract of DOI:10.1021/ic8020029

The cationic iridium complexes [lr(COD)(PR₃)₂]BF ₄ (1a-c) (a, R = Ph; b, R = p-tolyl; c, R = p-C₆H ₄-OMe) react with parahydrogen in the presence of pyridine to give trans, cis, cis-[lr(PR₃

Full text of DOI:10.1021/ic8020029

Inorg. Chem. 2009, 48, 663-670
Para-Hydrogen Induced Polarization without Incorporation of
Para-Hydrogen into the Analyte
Kevin D. Atkinson,† Michael J. Cowley,† Simon B. Duckett,*,† Paul I. P. Elliott,‡ Gary G. R. Green,§
Joaqu´ın Lo´pez-Serrano,† Iman G. Khazal,† and Adrian C. Whitwood†
Department of Chemistry, UniVersity of York, Heslington, York YO10 5DD, U.K., and
York Neuroimaging Centre, The Biocentre, York Science Park, InnoVation Way, Heslington,
York YO10 5DD, U.K.
Received October 20, 2008
The cationic iridium complexes [Ir(COD)(PR3)2]BF4 (1a-c) (a, R ) Ph; b, R ) p-tolyl; c, R ) p-C6H4-OMe) react
with parahydrogen in the presence of pyridine to give trans, cis, cis-[Ir(PR3)2(py)2(H)2]+ (2a-c) and small amounts
of fac, cis-[Ir(PR3)(py)3(H)2]+ (3a-c), each of which exhibit polarized hydride resonances due to the magnetic
inequivalence associated with the resultant AA“XX” spin system when 15N-labeled pyridine is employed. The pyridine
ligands in 2 are labile, exchanging slowly into free pyridine with a rate constant of 0.4 s-1 for 2a at 335 K in a
dissociative process where ∆Hq ) 134 ( 1 kJ mol-1 and ∆Sq ) 151 ( 5 J mol-1 K-1. Pyridine ligand exchange
in 2 proves to be slower than that determined for 3. Parahydrogen induced polarization (PHIP) based on the
hydride ligands of 2 and 3 is transferred efficiently to the 15N nuclei of the bound pyridine ligand by suitable
insensitive-nuclei-enhanced-by-polarization-transfer (INEPT) based procedures. Related methods are then used to
facilitate the sensitization of the free pyridine 15N signal by a factor of 120-fold through ligand exchange even
though this substrate does not contain parahydrogen. This therefore corresponds to the successful polarization of
an analyte by parahydrogen induced polarization methods without the need for the actual chemical incorporation
of any parahydrogen derived nuclei into it.
Introduction
present in H2 addition equilibria have also been detected
using this approach.11,12 This method, relies on the fact that
parahydrogen (p-H2), the nuclear spin isomer of dihydrogen
with the spin configuration Rꢀ-ꢀR, can be easily isolated.
Reactions with p-H2 then lead to products that are initially
formed with a nuclear spin distribution that at best reflects
that of the original dihydrogen molecule. In this way non-
Boltzmann spin distributions can be produced in such
reaction products which lead to significant signal enhance-
Over the past two decades parahydrogen enhanced NMR
spectroscopy1 has proved to be a powerful tool for the study
of reaction mechanisms involving H2.2-4 During these
studies several reaction intermediates have been detected and
their kinetic fate monitored.5-10 Low concentration products
*
To whom correspondence should be addressed. E-mail:
sbd3@york.ac.uk.
†
1
University of York.
Present Address: Department of Chemical & Biological Sciences,
ments in their H NMR spectra. It is for this reason that
‡
University of Huddersfield, Queensgate, Huddersfield HD1 3DH, U.K.
§
York Neuroimaging Centre.
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10.1021/ic8020029 CCC: $40.75  2009 American Chemical Society
Inorganic Chemistry, Vol. 48, No. 2, 2009 663
Published on Web 12/15/2008

Atkinson et al.
parahydrogen induced polarization (PHIP) has facilitated the
characterization of many species that would otherwise be
unobservable by NMR spectroscopy.
complexes by polarization transfer. An early example of such
a study dealt with the addition of p-H2 to Rh(PMe3)4Cl and
Rh(PMe3)3Cl, where the dihydride products were used to
demonstrate that two-dimensional NMR techniques such as
COSY, HSQC, HMQC, and NOESY can be used with p-H2.24
Other one-dimensional methods of characterizing hydrogenation
products have been developed by Bargon, for example, using
a number of modified insensitive-nuclei-enhanced-by-polariza-
tion-transfer (INEPT) experiments to obtain 13C spectra of 1,4-
diphenylbut-1-en-3-yne, formed via the hydrogenation of
1,4-diphenylbutadiyne.25
Here we report on p-H2 studies dealing with the interaction
of cationic dihydride iridium phosphine complexes and pyridine.
The complexes employed here are based on the well-known
[Ir(COD)L2]BF4 (COD ) 1,3- cyclooctadiene and L ) phos-
phine) system of Crabtree.26 Groups have prepared and
conducted very extensive studies on the reactivity of these and
the related complexes [IrH2S2L2]BF4 and [IrH2(ol)2L2]BF4 (S
) solvent; ol ) olefin) in both hydrogenation and dehydroge-
nation reactions. Related [IrH2S3L]BF4 complexes have also
proved to be active in a variety of situations including alkyne
dimerization and hydrogenation.27,28 Our experiments demon-
strate that polarization transfer from the hydride ligands to the
15N center of pyridine in [IrH2S2L2]BF4 (S ) pyridine) is
possible. We also demonstrate that the PHIP effect can be used
to enhance the signal strength of the 15N resonance of free
pyridine via ligand exchange. This contrasts with the situation
found in other, well-reported approaches, to polarization transfer
with p-H2, where the NMR signature of organic materials is
enhanced through their formation in a hydrogenation reaction
involving an unsaturated version of the substrate.2-4 This report
therefore presents a significant new development in the in situ
preparation of hyperpolarized materials.
The best example of this enhancement is provided by the
hydride resonances of the complex [Ru(dpae)(CO)2(H)2]
(dpae ) bis(diphenylarsino)ethane) when it is generated by
photolysis of [Ru(dpae)(CO)3] under a p-H2 atmosphere.13,14
The hydride resonances were shown to yield the maximum
theoretical signal enhancement of 31,200 at 9.4 T. This
situation actually corresponds to the generation of a pure
singlet spin state and therefore means that every complex
molecule contributes to the detected NMR signal. New
insights into reactivity that have resulted from this effect
include a more detailed delineation of the role of ruthenium
clusters and their fragmentation products in hydrogenation
catalysis.15 Other key species have been detected in cobalt
and iridium catalyzed hydroformylation processes through
the incorporation of the p-H2 label into alkyl and acyl ligands
within the ligand sphere.16-18 Similarly, several palladium
alkyl and vinyl complexes that play a direct role in alkyne
hydrogenation have been detected, characterized, and stud-
ied.10,19 Many of these results have been reviewed.20,21
The detection of transition metal dihydride complexes
through their hydride resonances is often facilitated because
1
they appear in a normally vacant part of the H NMR
spectrum, typically between -5 and -25 ppm. The polariza-
tion afforded by the PHIP effect therefore allows suitable
metal dihydride complexes to act as sensors for substrates
in low concentration through the detection of such signals.
Here, the chemical shift of the hydride signal is diagnostic
of the substrate. We recently reported studies dealing with
the detection and dynamics of pyridine coordination deriva-
tives of Wilkinson’s complex and its tribenzylphosphine and
tricyclohexylphosphine analogues as part of such a study.22
More significantly, when it was extended to the receptor
complex [Ir(H)2Cl(PPh3)2] the binding of purine and adenine
substrates demonstrated that their detection at pico-mole
levels was possible through such a hydride reporter.23
In these and other studies, the hydride-based polarizations
have been used to illuminate the coordination sphere of metal
Results and Discussion
Reactivity of [Ir(COD)(PPh3)2]+ Toward H2 and Py-
ridine. When H2 was bubbled through a solution of
[Ir(COD)(PPh3)2]BF4 (1a) in dichloromethane, in the presence
of a 2-fold excess pyridine, complex [Ir(PPh3)2(py)2(H)2]+ (2a)
was generated. Complex 2a was isolated, and appropriate NMR
based spectroscopic data obtained to confirm its identity on the
basis of agreement with previously described data of Rosales
et al.29 The NMR parameters for 2a can be found in Table 1.
2a was then generated from 1a in CD3OD rather than
CD2Cl2 by adding an 8-fold excess of pyridine and H2 to
this solution at 330 K. When the time course of this reaction
was monitored by in situ 1H NMR spectroscopy, the
(13) Anwar, M. S.; Blazina, D.; Carterer, H.; Duckett, S. B.; Halstead,
T. K.; Jones, J. A.; Kozak, C. M.; Taylor, R. J. K. Phys. ReV. Lett.
2004, 93, 040501–040504.
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R. J. K.; Anwar, M. S.; Jones, J. A.; Carteret, H. A. Magn. Reson.
Chem. 2005, 43, 200–208.
(15) Blazina, D.; Duckett, S. B.; Dyson, P. J.; Lohman, J. A. B. Dalton
Trans. 2004, 2108–2114.
(16) Godard, C.; Duckett, S. B.; Henry, C.; Polas, S.; Toose, R.; Whitwood,
A. C. Chem. Commun. 2004, 1826–1827.
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J. Am. Chem. Soc. 2005, 127, 4994–4995.
(18) Permin, A. B.; Eisenberg, R. J. Am. Chem. Soc. 2002, 124, 12406–
12407.
(19) Lopez-Serrano, J.; Lledos, A.; Duckett, S. B. Organometallics. 2008,
27, 43–52.
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H. Organometallics. 1999, 18, 3534–3546.
(22) Zhou, R. R.; Aguilar, J. A.; Charlton, A.; Duckett, S. B.; Elliott, P. I. P.;
Kandiah, R. Dalton Trans. 2005, 3773–3779.
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J. J. Am. Chem. Soc. 2007, 129, 11012–11013.
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Trans. 2004, 2952–2956.
664 Inorganic Chemistry, Vol. 48, No. 2, 2009

Para-Hydrogen Induced Polarization
Table 1. NMR Data for Complexes 2a-c and 3a-ba
complex
1H/δ
31P/δ
13C/δ
15N/δb
2
2a
H
-21.64 (2JPH )17.4 Hz, JNH ) 20,
JHH ) -6.4)
PPh3
7.37 (m, JHH ) 8.3 Hz, ortho), 7.26 δ 23.0 (s)
(t, J ) 8.3, meta), 7.36 (m, para)
8.05 (d, J ) 7.0 Hz, o), 6.78 (t, J )
7.0, m), 7.49 (t, J ) 7.7, p)
131.7 (1JPC 16 Hz, ipso), 133.7 (o),
128.0 (m), 130.0 (p)
155.8 (o), 125.3 (m), 136.1 (p)
Py
δ -62.2
δ -62.8
2
2
2b
2c
3a
H
-21.47 (t, JPH ) 17.6 Hz, JNH
)
2
24, JHH ) -6.5)
P(p-Tol)3
7.18 (dd, 7.7 and 6.8 Hz, o), 7.06
(d, 7.7, m), 2.33 (s, CH3)
δ 22.8 (s)
133.5 (o), 128.6 (m), 140.5 (p),
19.9 (s, CH3)
155.9 (o), 125.3 (m), 131.7 (p)
Py
H
8.02 (d, J ) 5.6 Hz, o), 6.77 (t, J )
7.0, m), 7.50 (t, J ) 6.5, p)
-21.73 (2JPH ) 17.1 Hz, JNH
)
2
2
23.3, JHH ) -6.5)
P(p-C6H4OMe)3 7.22 (dt, J ) 8.0, 5.4 Hz, o), 6.79
(d, J ) 8.0 Hz, m), 3.77 (s, OCH3).
18.2 (s)
127.0 (o), 105.7 (m), 153.4 (p),
115.55 (i), 46.58 (OCH3)
148.07 (s, o), 117.6 (s, m), 128.7 (s, δ -62.0
p)
Py
8.05 (d, J ) 6 Hz, o), 6.82 (t, J )
7.0, m), 7.54 (t, J ) 7.0, p)
2
H
-21.8 (2JPH ) 22.7 Hz, JNH ) 22,
JHH ) -6.6)
2
PPh3
trans-Py
cis-Py
7.40 (m, JHH ) 8.3 Hz, ortho), 7.23 12.6 (d, JPN ) 42 Hz) 133.1 (o), 127.8 (m), 129.8 (p)
(t, J ) 8.3, meta), 7.33 (m, para)
8.48 (d, J ) 7.0 Hz, o), 7.15 (t, 7.0,
154.5 (s, o), 125.8 (s, m), 136.7 (s, -48.2
p)
155.6 (s, o), 124.1 (s, m), 137.0 (s, -64.0
p)
m), 7.73 (t, 7.7, p)
8.90 (d, J ) 7.0 Hz, o), 7.40 (t, J )
7.0, m), 7.96 (t, J ) 7.7, p)
2
3b
3c
H
-21.75 (2JPH ) 24 Hz, JNH ) 22)
2
P(p-Tol)3
7.21 (t, J ) 8.3 Hz, o), 7.02 (t, J ) 9.74 (d, JPN ) 43 Hz) 133.15 (o), 128.14 (m), 19.78 (CH3)
8.3,m), 2.32 (s, CH3)
trans-Py
cis-Py
H
8.43 (d, J ) 7.0 Hz, o), 7.13 (t, 7.0,
154.65 (o), 125.58 (m), 136.8 (p)
154.9 (o), 125.83 (m), 137.9 (p)
-48.7
m), 7.78 (t, 7.7, p)
2
8.97 (d, J ) 7.0 Hz, o), 7.39 (t, J )
-64.6 (d, JNP ) 43 Hz)
7.0, m), 7.97 (t, J ) 7.7, p)
-21.71 (2JPH ) 23.2 Hz, JNH
)
2
22)
2
P(p-C6H4OMe)3 7.28 (dd, J ) 8.9. 10.9 Hz, o), 6.78 6.8 (d, JPN ) 49 Hz) 134.6 (o), 113.1 (m), 160.9 (CO),
(dd, J ) 1.7, 9.0 Hz, m), 3.79 (s,
OCH3)
8.47 (d, J ) 4.9 Hz, o), 7.18 (m,
6.6, m), 7.80 (tt, 7.7, 1.6, p)
8.88 (m, J ) 6.0 Hz, o), 7.31 (t, J
) 7.1, m), 7.94 (tt, J ) 7.8,1.6, p)
123.4 (d, JCP ) 66 Hz) 54.4 (CH3)
154.7 (o), 125.65 (m), 136.84 (p)
155.4 (o), 125.9 (m), 137.8 (p)
trans-Py
cis-Py
-48.3
2
-64.1 (d, JNP ) 49 Hz)
a All chemical shifts were measured at 295 K in CD3OD solution. Relative to 15N-pyridine ) 0 ppm. trans-Py ) trans to H, cis-Py ) cis to H.
b
singlet at δ 12.6. When 15N-labeled pyridine was employed
Scheme 1. Reaction of 1 with H2 in the Presence of Excess Pyridine for
Form Complexes 2 and 3 (R) Ph (a), p-tolyl (b) and p-C6H4-OMe (c))
in this reaction the corresponding 31P resonance now appears
as a nitrogen coupled doublet of 42 Hz. This is indicative of
a trans phosphine-pyridine ligand arrangement in this mono-
phosphine complex.
More importantly, when this reaction was repeated with
15N-labeled pyridine and p-H2 rather than H2 the hydride
formation of 2a was indicated by the observation of a
dominant hydride signal at δ -21.64 (t, JHP ) 17.4 Hz) after
a few minutes. The structure of 2a is indicated in Scheme
1, and it should be noted that the two hydride ligands are
chemically equivalent and lie trans to pyridine.
resonances of both 2a and 3a appear PHIP enhanced. These
signals are illustrated in Figure 1 where the upper trace
exhibits both 31P and 15N splittings while the lower trace is
31P decoupled thereby revealing apparent 15N splittings of
20 and 22 Hz, respectively, in addition to the antiphase
components, which are separated by -6.4, and -6.6 Hz,
respectively. This observation is expected for the hydride
resonance of the 15N-labeled isomer of the dihydride complex
2a since it now belongs to a second order spin system of
the type AA“XX”. It is the 15N label that makes the hydride
ligands magnetically inequivalent and therefore PHIP active.
The formation of a small amount of a second complex
was also indicated in this NMR spectrum, as evidenced by
a second hydride signal at δ -21.8. This second hydride
signal, due to species 3a, appears as a simple phosphorus-
coupled doublet of 22.7 Hz. The presence of a single
phosphorus-hydride splitting suggests that 3a is a mono-
phosphine complex, and the small value of the JPH coupling
indicates that the phosphine ligand is arranged cis to the
hydride groups. The 31P resonance of the phosphorus center
1
Indeed, when a H NMR spectrum is recorded that is 15N
decoupled the second order effect is quenched and the PHIP
enhancement disappears.
1
in 3a was located in an H-31P HMQC experiment as a
Inorganic Chemistry, Vol. 48, No. 2, 2009 665

Atkinson et al.
Figure 2. Molecular structure of the cation 2a (ORTEP). Selected bond
distances [Å] and angles (deg): Ir(1)-N(2) 2.177(2), Ir(1)-N(1) 2.196(2),
Ir(1)-P(1) 2.2987(6), Ir(1)-P(2) 2.3038(6), N(2)-Ir(1)-N(1) 92.94(8),
N(2)-Ir(1)-P(1) 92.89(6), N(1)-Ir(1)-P(1) 100.45(5), N(2)-Ir(1)-P(2)
97.82(6), N(1)-Ir(1)-P(2) 91.61(5), P(1)-Ir(1)-P(2) 163.44(2).
Figure 1. (top) 1H NMR spectrum of the hydride region of a sample
containing [Ir(COD)(PPh3)2]+ in CD3OD with 15N-pyridine under p-H2,
proved to connect to the corresponding para protons which
resonate at δ 7.49 in the same experiment. In contrast, the δ
7.37 signal for the phenyl ring connected to a signal at δ 7.26
due to a meta proton which connected in turn to a para proton
signal which appears at δ 7.36. The remaining heteronuclear
signals of 2a were located by HMQC methods, and the
corresponding signals for 3a were attributed by a similar
procedure.
1
(bottom) H{31P} NMR spectrum.
The hydride signals for 3a show a similar enhancement
profile to those of 2a when the 1H NMR spectrum is recorded
with 31P decoupling (Figure 1 (bottom)) and also disappear on
15N decoupling. These observations are therefore indicative of
similar ligand arrangements in both 2a and 3a. 3a is therefore
confirmed as the dihydride monophosphine tris-pyridine com-
plex fac, cis-[Ir(PPh3)(py)3(H)2]+. It is formed by exchange of
one of the phosphine ligands in 2a with pyridine. The related
complex fac, cis-[Ir(NCMe)3(PiPr3)(H)2]+ has been described
in the literature.27 One interesting feature that manifests itself
in Figure 1 is the relative size of the PHIP enhanced hydride
resonances of 3a and 2a. In the PHIP enhanced spectra, the
hydride signal for 3a is much larger than that seen for 2a even
though 2a has the higher real concentration. This effect is
reflected in the corresponding 31P{1H} NMR spectrum where
no signal for 3a is visible after 128 scans, although that for 2a
is. This situation also agrees with observations when a 9-fold
excess of unlabeled pyridine is present since the relative hydride
resonance intensities now prove to be 1 to 0.04, respectively.
This information suggests therefore that the hydride ligands of
complex 3a exchange with p-H2 at a much greater rate than
those of 2a.
When 2 µL of 15N-labeled pyridine was added to a sample
in which 2a and 3a had been generated previously using
pyridine where the 15N label was present at isotopic abundance,
only small levels of PHIP enhancement are observed for the
corresponding hydride resonance of 3a and essentially no
enhancement is observed for the corresponding hydride reso-
nance of 2a. This suggests that the pyridine ligands of both
these complexes do not exchange rapidly with free pyridine at
295 K. The enhancement observed for 3a is, however, compat-
ible with it having a higher rate of exchange than that of 2a.
X-ray Structure of [Ir(PPh3)2(py)2(H)2]+ (2a). The crystal
structure of 2a with a BF4- counterion is illustrated in Figure
2. Previously Rosales et al. reported the same structure with
the PF6- counterion.29 We note that with BF4- the P-Ir-P
bond angle is 163.44(7)° but with PF6- it is 161.00(4)°.
Furthermore, the reported Ir-N distances are 2.212(6) and
2.193(7) while here they are 2.177(2) and 2.196(2).
While there are, however, no significant deviations be-
tween the Ir-P bond lengths, the N-Ir-N bond angles also
vary from 92.94(8)° to 96.3(3)°, respectively. These changes
may simply reflect crystal packing effects, but it should be
noted that the ability of the precursor complexes as hydro-
genation catalysts depends critically on the identity of the
anion.30-32
The presence of pyridine within the coordination spheres
of complexes 2a and 3a was also confirmed by 15N-based
1
NMR spectroscopy. H-15N HMQC spectra of the 15N-
labeled samples contain correlations between the hydride
signals of 2a and 3a and resonances in the 15N domain at δ
-62.2 and -64.0, respectively. These 15N signals arise from
the nitrogen center of the bound pyridine ligands that lie
trans to hydride.
Reactions of [Ir(COD)(P{p-tolyl}3)2]+ (1b) and [Ir(COD)-
(P{C6H4-4-OMe}3)2]+ (1c) with Pyridine and H2. To probe
the effect of the identity of the phosphine on this reaction,
analogues of 2a and 3a containing P{p-tolyl}3 and P{C6H4-p-
OMe}3 were generated, in situ, using the directly related
precursors [Ir(COD)(P{p-tolyl}3)2]+, 1b, and [Ir(COD)(P{C6H4-
The NMR parameters for these two complexes are listed in
Table 1. The following text describes more fully how complexes
2a and 3a were characterized by NMR spectroscopy. First,
when a 1D NOE experiment was recorded, the hydride signal
at δ -21.64 for 2a proved to connect with proton signals at δ
8.05 and 7.37, where the latter signal couples strongly to 31P
and therefore arises from an ortho phenyl proton and the former
signal belongs to the ortho protons of bound pyridine. When
the ortho pyridine protons at δ 8.05 are interrogated in a similar
way in a 2D experiment, NOE connections to the corresponding
meta protons at δ 6.78 and the ortho phenyl protons of the
phosphine at δ 7.37 are visible. The meta pyridine protons
(30) Crabtree, R. H.; Anton, D. R.; Davis, M. W. Ann. N.Y. Acad. Sci.
1983, 415, 268–270.
(31) Crabtree, R. H.; Demou, P. C.; Eden, D.; Mihelcic, J. M.; Parnell,
C. A.; Quirk, J. M.; Morris, G. E. J. Am. Chem. Soc. 1982, 104, 6994–
7001.
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Organomet. Chem. 1978, 161, C67–C70.
666 Inorganic Chemistry, Vol. 48, No. 2, 2009

Para-Hydrogen Induced Polarization
p-OMe}3)2]+, 1c, respectively. The NMR data for these species
was then collected and assigned in a similar way to that
described for 2a. This information is presented in Table 1.
Table 2. Example Rate Constants for Equatorial Pyridine Ligand
Exchange and Associated Activation Parameters for Exchange in Both 2
and 3
Rate
complex (335 K)/s-1 mol-1 mol-1 K-1
∆Hq/kJ
∆Sq/J
∆G295q/kJ
∆G335q/kJ
mol-1
mol-1
Reactivity of [Ir(PPh3)2(py)2(H)2]+ (2a), [Ir(P{p-tolyl}3)2(py)2
(H)2]+ (2b) and [Ir(P{C6H4-4-OMe}3)2(py)2(H)2]+ (2c). Hy-
dride Ligand Exchange with Dihydrogen. To compare the
relative ability of 2 to undergo H2 exchange, two samples, one
containing 1a and 1b and the second 1a and 1c, were prepared
in CD3OD. They were then exposed to a 15-fold excess of 15N-
pyridine and p-H2 and then examined by 1H NMR spectroscopy
at 295 K. Neither sample showed any PHIP enhancement in
the corresponding hydride resonances of 2 or 3 although they
were detectable. The samples were then warmed to 330 K and
re-examined. At this point, the hydride signals of 2 and 3 were
2a
2b
2c
3a
3b
0.40
0.82
0.90
1.16
2.00
134 ( 1 151 ( 5
125 ( 3 132 ( 9
89.1 ( 0.2 83.1 ( 0.02
86.2 ( 0.3 81.0 ( 0.03
155 ( 9 222 ( 28 89.7 ( 1
80.8 ( 0.14
91 ( 8 34 ( 25
107 ( 3 85 ( 8
81.1 ( 1.0 79.7 ( 0.05
81.9 ( 0.2 78.5 ( 0.08
complexes 3 undergo significantly more rapid H2 exchange
than that of 2. We have also examined analogous samples
of 3a and 3b, and 3a and 3c at 330 K. The associated NMR
spectra revealed that the relative signal enhancements for
the hydride ligands of 3a/3b/3c were 2.25:1:0.79. It therefore
appears that 3a undergoes more rapid exchange with H2 than
either 3b or 3c. In this case therefore the extent of
enhancement follows the reduction in electron donating
power of the phosphine.33
We note that when EXSY spectroscopy was used to probe
the hydride ligands in these complexes no exchange into free
hydrogen was observed to occur in 2a on the NMR time
scale at 335 K. In contrast, around 4% of the hydride signal
intensity transferred into that of H2 when 3c was examined
at 335 K. It is therefore clear that the reductive elimination
of H2 from 2 or 3 under these conditions is not facile.
Previous studies have suggested that H2 elimination proceeds
dominantly from 16-electron complexes such as
Ir(H)2Cl(PPh3)2 rather than 18-electron Ir(H)2Cl(PPh3)3.20
This process would therefore be expected to exhibit complex
kinetic behavior, and given that its onset already pushes the
temperature envelope of the solvent we have not explored it
in more detail.20
1
1
polarized and a series of H, H{31P}, integrateable 31P{1H}
and 2D HMQC data sets were recorded. These spectra
demonstrated that both samples contained signals for the
corresponding mixed phosphine dihydride complex. These
were [Ir(PPh3)(P{p-tolyl}3)(py)2(H)2]+ and [Ir(PPh3)(P{p-C6H4-
OMe}3)(py)2(H)2]+ respectively. NMR data for these com-
plexes can be found in the Experimental Section. Key points
of interest in these data include the observation of two distinct
phosphorus signals for these species which exhibit a strong
and mutual PP coupling of around 350 Hz. Furthermore, the
dominant hydride signals detected in both samples under
PHIP conditions actually corresponded to those of the mono
phosphine complexes 3a, 3b, and 3c.
The relative PHIP enhanced hydride signal intensities for
2 obtained from these samples were then determined, and
then normalized by dividing them by the relative 31P integral
values for these species. The normalized hydride signals
intensity ratios determined in this way proved to be 1:1.42:
1.9:2.32:3.60 for the species 2a/2b/2c/[Ir (PPh3)(P{p-
tolyl}3)(py)2(H)2]+/[Ir(PPh3)(P{C6H4-4-OMe}3)(py)2(H)2]+,
respectively. It should be noted that care needs to be
employed when interpreting PHIP enhancements since the
hydride signal strengths for two dihydride complexes with
the same concentration need not be the same because they
will reflect both their concentration and their ability to
incorporate fresh parahydrogen. This means that the normal-
ized signal intensities for 2 will only reflect the associated
rate of introduction of fresh p-H2 into them. We therefore
conclude that the extent of H2 exchange increases across the
series PPh3 < P{p-tolyl}3 < P{C6H4-4-OMe}3 for 2a, 2b,
and 2c. This trend parallels that associated with increase in
electron donating ability of the phosphine.33 The anomalous
behavior of the mixed phosphine systems reveals, however,
that care needs to be taken before drawing any more precise
conclusions about how the electronic properties of the
phosphine relate to these observations.
Exchange of Bound and Free Pyridine Ligands by 2.
EXSY spectroscopy was then employed to rigorously moni-
tor the exchange rates of the bound pyridine ligand with free
pyridine in 2 between 328 and 341 K. In all cases, the
pyridine ligand that is trans to hydride was observed to
undergo exchange with free pyridine.
For 2a, the corresponding first order rate constant for
pyridine dissociation proved to be 0.4 s-1 at 335 K, and
values of ∆Hq ) 134 ( 1 kJ mol-1 and ∆Sq ) 151 ( 5 J
q
mol-1 K-1 with ∆G295 ) 89.1 ( 0.2 kJ mol-1 were
determined for this process. For 2b the corresponding rate
constant proved to be 0.82 ( 0.008 s-1 at 335 K, and it can
therefore be deduced that this process is faster in 2b than
2a. When this analysis method was repeated for 2c, the
corresponding rate constant at 335 K proved to be 0.90 s-1.
The associated activation parameters for this process are
listed in Table 2.
The contributions of both the ∆Sq and the ∆Hq terms are
largest for the most electron rich phosphine, P{C6H4-4-
OMe}3 of 2c. However, their balance results in 2c having
the greatest pyridine exchange rate at 335 K. In the case of
the related complex, [Ir(NCMe)2(PPh3)2(H)2]+, the corre-
sponding values are reported as 95.6 ( 2.3 kJ mol-1 and
The relative enhancements of the hydride resonance of
2a relative to those of 3a and 3b, and 2a relative to those of
3a and 3c were also determined from these data. They proved
to be 1:13.91:10.71 and 1:8:7.7, respectively. These data are
not normalized since the 31P signals for 3 could not be seen,
but they clearly confirm that the family of tris-pyridine
(33) Tolman, C. A. Chem. ReV. 1977, 77, 313–348.
Inorganic Chemistry, Vol. 48, No. 2, 2009 667

Atkinson et al.
28.2 ( 7.1 J mol-1 K-1, respectively.34 The higher values
of ∆Hq for pyridine loss in 2a, than acetonitrile in
[Ir(NCMe)2(PPh3)2(H)2]+, agrees with the fact that the
stronger pyridine donor should lead to the more costly rupture
of a stronger Ir-N bond.
Exchange of Bound Pyridine and Free Pyridine in 3.
A similar EXSY based procedure was used to study the
exchange of pyridine in 3. This process proved to be faster
than that found for 2. The associated rate constants and
activation parameters for 3 are listed in Table 2. When 3a
was examined using this approach the dominant pathway
again corresponded to exchange of the equatorial pyridine
ligands with free pyridine. At the higher temperatures,
however, there was evidence for the intercoversion of the
axial and equatorial pyridine ligands within 3a. At 340 K,
and with a mixing time of 0.8 s, the proportion of pyridine
loss was 60% while the level of axial-equatorial site
exchange was only 4%. There were, however, no exchange
peaks that connected the axial pyridine and free pyridine
signals. These data therefore suggest that this process must
occur via ligand interchange within the 16 electron inter-
mediate [Ir(PPh3)(py)2(H)2]+ that is generated by pyridine
loss trans to hydride in 3a rather than by direct dissociation
of the pyridine ligand which is trans to phosphine. These
observations also require that in the 16-electron intermediate
[Ir(PPh3)(py)2(H)2]+, the two pyridine ligands remain in-
equivalent, a situation that would certainly hold true in the
solvent complex [Ir(methanol)(PPh3)(py)2(H)2]+ which might
be expected in methanol. No such complications were evident
in the case of 3b, and the corresponding rates and activation
parameters are listed in Table 2. In the case of 3b, the
corresponding rate constant for pyridine loss is 2.00 s-1 at
335 K and therefore a factor of 2.4 faster than that of 2b.
As a consequence the values of both ∆Hq and ∆Sq are
reduced relative to those of 2.
Much lower thermal stability was observed in the case of
3c with decomposition occurring over time at temperatures
above 308 K. We measured a rate for pyridine dissociation
at this temperature of 0.066 s-1 but were unable to obtain
activation parameters because of the decomposition. This
situation was further complicated because weak exchange
into a new species which was tentatively assigned to
[Ir(methanol)(P{C6H4-4-OMe}3) (py)2(H)2]+ was observed.
When the Eyring data are used to estimate the corresponding
rate constants at 308 K for 3a and 3b, the values obtained
are 0.068 ( 0.016 s-1 and 0.064 ( 0.004 s-1, respectively.
This confirms that at the lower temperatures, 3c undergoes
comparable exchange to 3a, but clearly at the higher
temperatures, where a large positive ∆Sq would work to
enhance the difference, we cannot predict an order.
Figure 3. (a) 1024 scan 15N control spectrum: (b) 16 scan 15N detected
PHIP-INEPT spectrum during the reaction of 1b with parahydrogen in the
presence of 15N-labeled pyridine in methanol-d4 (c) 64 scan 15N detected
PH-INEPT+EXSY spectrum of the same sample
pyridine exchange step would lower ∆Sq, as would increased
ordering of the phosphine in the TS. Oro et al. have
determined that the activation parameters for acetonitrile
dissociation from the site trans to hydride in fac-cis-
[Ir(NCMe)3(PiPr3)(H)2]+ are ∆Hq ) 112 ( 4 kJ mol-1 and
∆Sq ) 135 ) ( 8 J mol-1 K-1.27 The ∆Sq terms determined
here for 3 are therefore not unusual.
In the case of the monophosphine systems 3a and 3b, both
the values of ∆Hq are consistently smaller than those found
for their bis-phosphine analogues. This suggests the Ir-py
bond energy in 2 is higher than that in 3 thereby indicating
that the second phosphine acts to strengthen the Ir-N bond
when compared to pyridine. We note that while both pyridine
and phosphine are σ-donors and π-acceptors, phosphine is
the higher field ligand.
Magnetization Transfer from the PHIP Enhanced
Hydride Signals to that of the 15N Centre in Bound
Pyridine. A series of heteronuclear polarization transfer
experiments were then tested for their ability to transfer the
hydride polarizations exhibited by 2 and 3 into the 15N nuclei
of the bound pyridine ligand. The NMR pulse sequences used
were PH-INEPT, refocused PH-INEPT+, and PH-INEPT(+π/
4) and have been reported previously.25
The associated NMR spectra were recorded for the same
experimental setup and then the signal-to-noise ratios of the
observed 15N resonances measured and normalized according
to the relative signal strengths of the hydride resonances seen
1
in the corresponding H NMR spectra that were recorded
immediately before each 15N magnetization transfer experi-
ment. Without the p-H2 based polarization transfer essentially
no 15N signals were visible after 1024 scans as shown in
Figure 3a. The most efficient transfer of magnetization
proved to be achieved using the PH-INEPT pulse sequence,
and the corresponding 16 scan experiment is illustrated in
Figure 3b. This situation arises because in the refocused
version of this experiment, PH-INEPT+, relaxation during
the refocusing period results in only a third of the potential
signal intensity surviving to the point of observation. The
There is a significant difference in the relative magnitudes
of the activation entropies for 3 when compared to 2, and
3a when compared to 3b, even though they are essentially
monitoring the same dissociation of pyridine. We note,
however, that a higher level of associative character for the
(34) Howarth, O. W.; McAteer, C. H.; Moore, P.; Morris, G. E. Dalton
Trans. 1981, 1481–1485.
668 Inorganic Chemistry, Vol. 48, No. 2, 2009

Para-Hydrogen Induced Polarization
Table 3. Comparison of 15N Signal to Noise Ratios for the Indicated
Heteronuclear Polarization Transfer Experiments Employing Complex
1a
experiment
PH-INEPT
PH-INEPT+
PH-INEPT(+π/4)
signal/noisea
27.9
10.5
7.2
a Normalized using relative intensities of 1H hydride signals recorded
immediately before the polarization transfer experiments were conducted.
Figure 5. Plot of magnetization, as a mole fraction, for the equatorial
pyridine signal sites, starting from 3b versus reaction time obtained from
PH-INEPT+EXSY data for a sample prepared from 1b, 15N-pyridine and
p-H2 at 330 K.
In the corresponding control experiments, using INEPT,
and PH-INEPT sequences, no signal for free pyridine can
be observed on this time scale. The control experiments
therefore confirm that it is magnetization transfer from p-H2
derived hyperpolarization sitting on the hydride ligands of
3b that is responsible for these observations rather than
magnetization transfer from the ring protons of free pyridine.
These results represent the successful demonstration of the
PHIP sensitization of an organic molecule that contains no
nuclei that were previously located in parahydrogen.
Figure 4. Schematic of the pulse sequence PH-INEPT+EXSY used for
magnetization transfer where (τ ) 1/4JHN).
PH-INEPT(+π/4) experiment was found to be least effective.
These data are summarized in Table 3.
Magnetization Transfer from the PHIP Enhanced
Hydride Signals to that of the 15N Centre in Free
Pyridine. Our main goal in these experiments was to transfer
p-H2 derived polarization from bound to free pyridine in a
second step. This process would therefore lead to the
polarization of a substrate that did not contain any protons
that originated in the source of polarization, p-H2. We
therefore modified these polarization transfer experiments
to store the 15N magnetization in an Iz term prior to the
introduction of a variable delay to facilitate magnetization
transfer into the signal for free pyridine. This situation can
be simply thought of as a heteronuclear EXSY experiment.
The new pulse sequence, PH-INEPT+EXSY, is illustrated
in Figure 4 where the polarization transfer sequence is
followed by a 90°-y storage pulse, and the magnetization
transfer delay (d) is followed by a 90° observation pulse.
When this process was carried out on a sample containing
enhanced proton signals for 2a and 3a it became clear that
while polarization transfer to bound 15N can be achieved,
little or no exchange of magnetization corresponding to the
ligated ligand exchanging into free pyridine occurs at 330
K. This is in direct agreement with the quantitative results
described earlier.
The PH-INEPT+EXSY based data collection was then
repeated on samples containing both 1b and 1c. The 15N
spectra now show significant signals for both bound pyridine
and free pyridine. As the delay time used to encode ligand
dissociation is increased from 0.1 to 1 s the proportion of
signal that resides at the position of free pyridine clearly
increases at the expense of the signals for the bound pyridine
ligands of 3b. These data are illustrated as a time course
plot in Figure 5. A 64 scan spectrum with a reaction delay
of 1 s is illustrated in Figure 3c for a system based on the
precursor 1b. The signal-to-noise ratio for the pyridine
resonance in this 64 scan experiment is 198.4 and compares
with the 1024 scan control experiments value of 6.64. This
equates to a 120-fold increase in signal strength via polariza-
tion transfer.
Summary
In this paper we have demonstrated that [Ir(PR3)2(py)2(H)2]BF4
and fac, cis-[Ir(PR3)(py)3(H)2]BF4 where R ) Ph, p-tolyl and
p-methoxyphenyl, undergo pyridine loss in a process that is
influenced by the identity of the phosphine. The rate of pyridine
ligand dissociation at 335 K proved to be highest for the most
electron rich p-methoxyphenyl phosphine in the bis-pyridine
complexes 2 which corresponds to the situation where both ∆Hq
and ∆Sq are maximized. In contrast for 3, the rate constants for
pyridine loss in the tris-pyridine systems are all similar at 308 K,
although they are larger than those determined for 2 (Table 2).
When these complexes are prepared using p-H2 and 15N-
labeled pyridine was employed as the substrate, the parent
complexes 2 and 3 show substantial PHIP enhancements in
their hydride resonances. Magnetization transfer from the
enhanced hydride signals of these species has been demon-
strated to provide access to substantial 15N signal enhance-
ments in the bound pyridine ligand via the application of an
INEPT based protocol. When such an approach is used in
conjunction with a magnetization transfer step, the hyperpo-
larization that was originally based in p-H2 is transferred first to a
bound pyridine ligand in fac, cis-[Ir(PR3)(py)3(H)2]BF4 and then
to free pyridine itself. This relayed magnetization transfer
demonstrates that it is possible to employ the p-H2 to sensitize
the NMR signals of materials that have not been function-
alized by an H2 addition reaction. We have estimated that a
120-fold gain in the signal strength of the free pyridine
resonances can be achieved via this route. It should be noted
that since this means 1 scan equates to 14,400 scans without
polarization there is therefore a significant reduction in the
time necessary to record such information. Furthermore, since
sensitivity relates to the square of the size of the magnetic
Inorganic Chemistry, Vol. 48, No. 2, 2009 669

Atkinson et al.
as supplied and stored under N2. [Ir(COD)(PPh3)2]BF4 (1a) and
[Ir(COD)(NCMe)2]BF4 were prepared by literature meth-
ods.26,29,36,37
field, similar results could be achieved on a 103 T magnet
which is around five times larger than that found in a 900
MHz system.
[Ir(COD)(P{p-tolyl}3)2]BF4 (1b). The known complex 1b was
prepared as follows: [Ir(COD)(NCMe)2]BF4 (150 mg, 0.32 mmol)
was dissolved in dichloromethane (5 mL) to which was added by
canula P(p-tolyl)3 (195 mg, 0.64 mmol) in dichloromethane (5 mL).
After stirring for 15 min, the volume was reduced in vacuo and
diethyl ether added to precipitate the product. The dark pink/red
solid was then washed with diethyl ether (5 mL) and dried under
vacuum. Yield: 197 mg (62%): 31P NMR: δ 16.1.
[Ir(COD)(P{C6H4-4-OMe}3)2]BF4 (1c). The known complex 1c
was prepared as follows: A Schlenk tube was charged with
[Ir(COD)Cl]2 (250 mg, 0.37 mol) and AgBF4 (160 mg, 0.82 mmol).
To this was added a degassed methanol (10 mL) solution of P(C6H4-
4-OMe)3 (521 mg, 1.48 mmol), and the resulting suspension stirred
overnight in the absence of light. The solvent was then completely
removed under vacuum, and the product extracted into CH2Cl2 (10
mL). The suspension was filtered through celite, and the red filtrate
concentrated to about 1 mL before Et2O (10 mL) was added to
precipitate the product. After washing with Et2O (3 × 2 mL) and
drying in vacuo for 1 h a dark red powder was obtained. Yield:
640 mg, 79%. 31P NMR: δ 14.9
Previously, reactions with p-H2 have been shown to enable
the detection of metal dihydride complexes. It is not
surprising, however, that when the protons that were previ-
ously located in p-H2 are transferred into a hydrogen-acceptor
such as an alkene, strong observable magnetization is also
produced in the corresponding hydrogenation products.
Recently this approach has been used to prepare hyperpo-
larized organic molecules for use as contrast agents in
magnetic resonance imaging (MRI).35 However, the need
for the existence of a suitable hydrogen-acceptor reflects a
significant limitation in the existing p-H2 approach. The
novelty of the procedure illustrated here is that it does not
require the functionalization of a substrate through a formal
hydrogenation step. Consequently it offers the opportunity
to sensitize substrates where there is no dehydro-analogue
available. We are continuing to explore the potential of this
and other routes to sensitize the NMR experiment more
generally.
Exchange of IrH for IrD. It should be noted that during the
course of all of these reactions in CD3OD, the slow exchange of a
deuterium label into the hydride sites of 2 and 3 was observed.
After 24 h, the corresponding hydride resonances were very weak,
or no longer visible, because of total exchange to the corresponding
deuterides. Both CD3OH and HD signals were also visible in these
spectra.
Selected NMR data for [Ir(py)2(PPh3)(P{p-tolyl}3)(H)2]BF4.
1H: Hydride: δ -21.72, (JPH ) 19 Hz, JNH ) 23 Hz); 31P: PPh3 δ
23.17 (d JPP ) 344 Hz), P{p-tolyl}3, δ 20.47 (d, JPP ) 344 Hz),
15N: δ -63.3.
Experimental Details
NMR measurements were made on Bruker DRX400 (1H at
400.13 MHz, 31P at 161 MHz, 15N at 40 MHz), Bruker AV500 (1H
at 500.22 MHz, 31P at 202.50 MHz, 15N at 50.69 MHz), Bruker
wide-bore AV600 (1H at 600.12 MHz, 31P at 242.93 MHz, 15N at
60.81 MHz) and Bruker AV 700 (1H at 700.12 MHz) NMR
spectrometers. NMR samples were made up in 5 mm Young’s
tapped NMR tubes. Typical samples contained 1 mg of the iridium
complex dissolved in CD3OD (600 µL) with the other reagents
being added by microsyringe. Samples were degassed at -78 °C
and then pressurized with 3.5 bar p-H2 before appropriate NMR
measurements were recorded. p-H2 was prepared by cooling the
gas to 20 K over activated charcoal as described previously.20
X-ray data were obtained for a crystal of 2a at 110 K using a
Bruker Smart Apex diffractometer with Mo KR radiation (ν )
0.71073 Å) in conjunction with a SMART CCD camera. Diffrac-
tometer control, data collection, and initial unit cell determination
were performed using “SMART” (v5.625 Bruker-AXS). Frame
integration and unit-cell refinement were carried out with the
“SAINT+” (v6.22, Bruker AXS) package. Absorption corrections
were applied by SADABS (v2.03, Sheldrick). The structure was
solved by Patterson map using ShelXS (Sheldrick, 1997) and refined
using ShelXL (Sheldrick, 1997). Crystals were grown from a
benzene solution of 1a which contained H2 and pyridine.
Selected NMR data for [Ir(py)2(PPh3)(P{C6H4-4-OMe}3)(H)2]
1
BF4. H: Hydride: δ -21.72, (JPH ) 21 Hz, JNH ) 21 Hz); 31P:
PPh3 δ 22.86 (d JPP ) 348 Hz), P{C6H4-4-OMe}3, δ 18.35 (d, JPP
) 348 Hz), 15N: δ -61.1.
Acknowledgment. We wish to thank the University of
York and a White Rose initiative for part funding this work
through its technology transfer scheme. Other financial
support from the Spanish MEC (Project Consolider ORFEO
(CSD 2007-00006)) and the MRC (K.D.A.) are also ac-
knowledged. We are also grateful to Bruker BioSpin and
Mark Mortimer for particularly helpful discussions.
Supporting Information Available: Tables containing rates and
activation parameters for pyridine exchange and X-ray information
for 2a (CCDC 699760). This material is available free of charge
via the Internet at http://pubs.acs.org.
Syntheses. All reactions were carried out under N2 using
glovebox or Schlenk line techniques. Dichloromethane and diethyl
ether were distilled over calcium hydride and sodium benzophenyl
ketyl, respectively, prior to use. Tri(phenyl)phosphine (Aldrich),
tri(p-tolyl)phosphine (Strem), tri(anisolyl)phosphine (Strem), py-
ridine (Aldrich), and 15N-pyridine (Cambridge Isotopes) were used
IC8020029
(36) Crabtree, R. H.; Mellea, M. F.; Mihelcic, J. M.; Sen, A.; Chebolu, V.
Inorg. Synth. 1989, 26, 122–126.
(35) Goldman, M.; Johannesson, H.; Axelsson, O.; Karlsson, M. C. R. Chim.
2006, 9, 357–363.
(37) Crabtree, R. H.; Hlatky, G. G.; Parnell, C. P.; Segmuller, B. E.; Uriarte,
R. J. Inorg. Chem. 1984, 23, 354–358.
670 Inorganic Chemistry, Vol. 48, No. 2, 2009