Parahydrogen derived illumination of pyridine based coordination products obtained from reactions involving rhodium phosphine complexes

Article abstract of DOI:10.1039/b508149a

The reactions of RhCl(PBz₃)₃ with H₂ and pyridine or 4-methylpyridine yield RhCl(H)₂(PBz₃) ₂(py) and RhCl(H)₂(PBz₃)₂(4-Me-py), respectively. These species undergo hydride site exchange via the loss of the pyridyl donor and formation of RhCl(H)₂(PBz₃)₂ which contains equivalent hydride ligands; for the py system the activation free energy, ΔG₃₀₀,^? is 57.4 ± 0.1 kJ mol^-1 while for 4-Me-py the value is 59.6 ± 0.3 kJ mol ^-1. These products only showed parahydrogen enhancement in the corresponding hydride resonances when a sacrificial substrate was added to promote hydrogen cycling. When RhCl(PPh₃)₃ was used as the precursor similar observations were made, while when RhCl(PCy ₃)₂(C₂H₄) was examined, H ₂ addition led to the formation of the binuclear complex (H) ₂Rh(PCy₃)₂(μ-Cl)₂Rh(H) ₂(PCy₃)₂ which was differentiated from RhCl(H)₂(PCy₃)₂ on the basis of the similarity in diffusion coefficient (5.5 × 10^-9 m² s ^-1) to that of (H)₂Rh(PPh₃) ₂(μ-Cl)₂Rh(PPh₃)₂ (5.3 × 10^-9 m² s^-1). The detection of RhCl(H) ₂(PCy₃)₂(py) was facilitated when pyridine was added to a solution of RhCl(PCy₃)₂(C₂H ₄) before the introduction of H₂. During these reactions trace amounts of the double substitution products, RhCl(H) ₂(phosphine)(py)₂, were also detected. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b508149a

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F U L L P A P E R
Parahydrogen derived illumination of pyridine based coordination
products obtained from reactions involving rhodium phosphine
complexes
Rongrong Zhou,a Juan A. Aguilar,a Adrian Charlton,b Simon B. Duckett,*a Paul I. P. Elliotta
and Rathika Kandiaha
a Department of Chemistry, University of York, Heslington, York, UK YO10 5DD.
E-mail: sbd3@york.ac.uk; Fax: (+44)1904-432564
b Central Science Laboratories, Dept Environment Food & Rural Affairs, York, N. Yorkshire,
UK YO41 1LZ. E-mail: sbd3@york.ac.uk; Fax: (+44)1904-432564
Received 9th June 2005, Accepted 27th July 2005
First published as an Advance Article on the web 8th September 2005
The reactions of RhCl(PBz3)3 with H2 and pyridine or 4-methylpyridine yield RhCl(H)2(PBz3)2(py) and
RhCl(H)2(PBz3)2(4-Me-py), respectively. These species undergo hydride site exchange via the loss of the pyridyl
donor and formation of RhCl(H)2(PBz3)2 which contains equivalent hydride ligands; for the py system the activation
free energy, DG‡300, is 57.4 0.1 kJ mol−1 while for 4-Me-py the value is 59.6 0.3 kJ mol−1. These products only
showed parahydrogen enhancement in the corresponding hydride resonances when a sacrificial substrate was added
to promote hydrogen cycling. When RhCl(PPh3)3 was used as the precursor similar observations were made, while
when RhCl(PCy3)2(C2H4) was examined, H2 addition led to the formation of the binuclear complex
(H)2Rh(PCy3)2(l-Cl)2Rh(H)2(PCy3)2 which was differentiated from RhCl(H)2(PCy3)2 on the basis of the similarity in
diffusion coefficient (5.5 × 10−9 m2 s−1) to that of (H)2Rh(PPh3)2(l-Cl)2Rh(PPh3)2 (5.3 × 10−9 m2 s−1). The detection
of RhCl(H)2(PCy3)2(py) was facilitated when pyridine was added to a solution of RhCl(PCy3)2(C2H4) before the
introduction of H2. During these reactions trace amounts of the double substitution products,
RhCl(H)2(phosphine)(py)2, were also detected.
when the ligand is present in very low concentration. Wilkinson’s
catalyst, RhCl(PPh3)3, provides an ideal starting point for
Introduction
Hydrogen exists in two forms, parahydrogen (p-H2) and ortho-
such a study since it is known to access RhCl(H2)(PPh3)3,
hydrogen (o-H2).1 The para form of H2 exists as a nuclear singlet
a species which contains a labile phosphine.10 This system
state where both hydrogen atoms have opposite spin. It can be
has been studied with parahydrogen and yields very broad
prepared by cooling H2 to 20 K in the presence of a suitable
hydride signals even at 295 K in the absence of any substrate.11
paramagnetic catalyst. When this form of hydrogen is involved
in a chemical reaction where the two nuclei become distinct
and maintain their original nuclear spin alignment, enhanced
NMR signals can be observed. It has been recently shown for the
Heaton et al.12 have described how RhCl(PPh3)3 reacts with
pyridine (py) and H2, characterising RhCl(H)2(PPh3)2(py) and
the double substitution products Rh(H)2(PPh3)2(py)2 Cl− and
+
RhCl(H)2(PPh3)(py)2. Dragon et al.13 have also reported on
situation where pure parahydrogen is employed as the reactant
a similar series of reactions with the corresponding para-
tolylphosphine (P(p-tol)3) containing system.
that Ru(dpae)(CO)2(H)2 (dpae = bisdiphenylarsinoethane) can
be formed in a spin-selective reaction with Ru(dpae)(CO)2 and
Here we report
a study where the interactions of
that the two hydride ligands yield NMR signals that show a
31 000 fold increase in size over normal level.2 The formation of
a product in a pure spin state has therefore been achieved.
The parahydrogen (p-H2) effect has also been called
PHIP (parahydrogen induced polarisation)3 and PASADENA
(parahydrogen and synthesis allow dramatically enhanced nu-
clear alignment).1 It has been extensively reviewed.4 The PHIP
approach has enabled the observation of several reaction
intermediates which range from classical dihydrides5 to species
that contain agostic hydrides.6 More recently, PHIP has been
employed in the sensitisation of a hydroformylation product
containing a single p-H2 atom7 and the transfer of polarisation
via a 13C nucleus to deuterium after the hydrogenation of a
perdeuterated substrate.8 This technique has also been used to
enhance organic components that lie within a metal’s ligand
sphere during catalysis.9
Rh(H)2(PBz3)2Cl, Rh(H)2(PPh3)2Cl and Rh(H)2(PCy3)2Cl with
pyridine (and 4-methyl pyridine) are followed. We employ
Rh(PBz3)3Cl (where PBz3 = tribenzylphosphine), Rh(PPh3)3Cl
and Rh(C2H4)(PCy3)2Cl] (where PCy3 = tricyclohexylphos-
phine) as the precursors. NMR measurements that include
1H–15N HMQC, 1H–103Rh HMQC, EXSY, DOSY and nOe
techniques have been used to characterise the products and study
their reactivity. In addition it is shown that the addition of the
sacrificial hydrogen acceptor, 1-phenyl-1-propyne, dramatically
increases the size of hydride signals that are detected for the key
pyridine containing complexes.
Experimental
General methods and chemicals
One reason that metal hydride complexes are normally visible
through this approach stems from the fact that hydride chemical
shifts often fall in the normally clear region of the H NMR
spectrum between d −5 and −25. Opportunities therefore exist
to utilise the ability of metal complexes to bind stable ligands
and then employ the associated hydride chemical shifts as
information that is diagnostic of the ligand. If parahydrogen is
employed in this approach, these signals should be visible even
All reactions and purifications were carried out under N2
using glove box or Schlenk line techniques. Tetrahydrofuran
(THF), toluene and hexane were distilled over sodium ben-
zophenone ketyl under nitrogen prior to use. Ethanol was
distilled over molecular sieves. Benzophenone (Aldrich), sodium
(Lancaster), hydrogen (99.99%, BOC), tri(methyl)phosphine
(PMe3) (Aldrich) and RhCl(PPh3)3 (Aldrich) were used
as received. Tri(benzyl)phosphine (PBz3) (Aldrich) and
1
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
D a l t o n T r a n s . , 2 0 0 5 , 3 7 7 3 – 3 7 7 9
3 7 7 3
©

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tri(cyclohexyl)phosphine (PCy3) (Aldrich) were purified by
crystallisation from hot ethanol. 1-Phenyl-1-propyne (Aldrich)
was degassed and stored under nitrogen and used without
further purification. Rh2Cl2(C2H4)4 was synthesised according
to established methods14 and then used for the synthesis
of tris(tribenzylphosphine)rodium(I) chloride RhCl(PBz3)3 and
RhCl(C2H4)(PCy3)2 which were stored in a glove box.15
Parahydrogen were prepared by cooling H2 to 18 K using a
helium refrigeration system in the presence of a paramagnetic
material using the system described previously.4e
16.2 (JPP = 38 Hz, JPRh = 139 Hz). These features match those
5f
found for the analogous complex RhCl(PPh3)3 and confirm
that RhCl(PBz3)3 was prepared.
When a 1 mM solution of RhCl(PBz3)3 in toluene-d8 was
mixed with ca. 3 atm p-H2 and the resultant reaction monitored
1
by H NMR spectroscopy at 295 K, two sets of hydride reso-
nances were detected at d −10.6 and −19.2. These resonances,
illustrated in Fig. 1, are assigned to the hydride ligands, Ha and
Hb, respectively of RhCl(H)2(PBz3)3. The hydride resonance of
RhCl(H)2(PBz3)3, for the ligand trans to PBz3, Ha, resonates at
d −10.6 and possesses a large coupling to a single 31P centre of
163 Hz, while the hydride ligand trans to chloride, Hb, appears
at d −19.2 and couples to three cis phosphine ligands, with
essentially identical JPH values of 14 Hz (Table 1). This reaction
is similar to the known reaction of H2 with RhCl(PPh3)3, and
related systems, that forms analogous Rh(III) product such as
Synthesis of RhCl(PBz3)3
0.195 g (0.5 mmol) of [RhCl(C2H4)2]2 were reacted with 0.918 g
(3 mmol) of PBz3 in 10 ml toluene. After stirring for 30 min
the solution was filtered and 20 ml of dry hexane added to
precipitate the product. The orange–yellow product was col-
lected, washed with dry hexane, and dried under vacuum. Yield
0.890 g (80%). FAB mass (m/z): 746 [RhCl(P(CH2Ph)3)2]+; 711
31
RhCl(H)2(PPh3)3.9 The corresponding 1H{ P} NMR spectrum
(Fig. 1(b)) simplifies the original hydride resonances into dou-
blets of antiphase doublets where JRhH = 12 Hz, JHH = 8 Hz, and
a
[Rh(P(CH2Ph)3)2]+; 406 [RhP(CH2Ph)3]+; 315 [RhP(CH2Ph)2]+;
1
JRhH = 20 Hz. A gradient assisted H–31P HMQC experiment
was bused to probe the 31P nuclei in this product. Two phosphorus
signals were observed due to Pa and Pb at d 32.3 (a doublet of
doublets) and at d −1.95 (a doublet of triplets) with JP = 22,
+
=
223 [RhP CH–Ph] .
NMR methods
P
a
1
b
JP = 113, and JP
and confirmed that the molecule contained three 31P centres.
= 93 Hz, respectively. A 2D H–103Rh
Rh
Rh
a
b
The deuterated toluene and benzene (Aldrich) were dried over
potassium and degassed prior to use. Pyridine-d5 was dried
over molecular sieve, while 15N-labelled pyridine (Aldrich) was
used as received. In a typical experiment 1 mg of the specified
rhodium complex was transferred into an NMR tube fitted
with a Young’s valve in a nitrogen-filled glove-box. Pyridine and
phenyl propyne were then added by micro-syringe in the glove
box when necessary. Samples were then transferred to a high
vacuum line where the NMR solvent was introduced by vacuum
transfer. The resealable NMR tubes were then filled with 3 atm
p-H2, shaken and introduced into the NMR spectrometer which
was pre-set to a specific temperature. In some cases, the solvent
and p-H2 were added before the ligand or substrate.
HMQC spectrum located the 103Rh centres resonance at d −393,
NMR spectra were recorded on Bruker DRX-400 spectrome-
ters with 1H at 400.13 MHz, 31P at 161.9 MHz, 15N at 40.6 MHz
and 103Rh at 15.737 MHz, respectively. 1H NMR chemical
shifts are reported in ppm relative to residual 1H signals in
the deuterated solvents (benzene-d5, 7.13 and toluene-d7, 2.13),
1
Fig. 1 Selected hydride regions of H NMR spectra showing hydride
resonances for RhCl(H)2(PBz3)3 that were obtained with p-H2 at 295 K
in toluene-d8. The antiphase components arise in transitions involving
protons that originate from p-H2. (a) 1H NMR spectrum and (b) 1H{ P}
31
NMR spectrum.
1
31P{ H} NMR in ppm downfield of an external 85% solution
of phosphoric acid, and 103Rh chemical shift are reported in
ppm from the absolute frequency standard of 3.16 MHz. 15N
spectra are relative to pyridine at d −60.6.16 Modified 1H–
31P, 1H–103Rh, 1H–15N HMQC (heteronuclear multiple quantum
The dynamic behaviour of this complex was then monitored
by EXSY spectroscopy. The data from these experiments re-
vealed that no exchange processes involving the hydride ligands
of RhCl(H)2(PBz3)3 occurred at 273 K. However, when the
measurements were repeated at 295 K (Fig. 2), mutual hydride
site interchange and hydride site exchange into the signal at d
4.57 due to free hydrogen were observed. In addition, a strong
nOe connection from the hydride signal at d −10.6 to an ortho-
phenyl proton at d 7.2 was detected; this corresponds to a
resonance in the two equivalent axial phosphines. The mixing
time used in this experiment was then varied and the intensity of
the hydride signals modelled as a function of mixing time.18 The
associated rate constant for hydride-hydride exchange at 295 K
was determined to be 1.9 s−1, while that for the formation of free
H2 was found to be 1.38 s−1.
A study of phosphine dissociation from RhCl(H)2(PPh3)3,
revealed that the PPh3 ligand that is trans to hydride dissociates at
a rate of 400 s−1 at 298 K.20 This process was shown to proceeded
via an intermediate with C2v symmetry and results in hydride
site interchange. The crystal structure of RhCl(H)2(PtBu3)2
confirmed this view.21 It is therefore likely that hydride site
interchange in RhCl(H)2(PBz3)3 proceeds via phosphine loss.
When 5 lL of pyridine was introduced into a toluene-
d8 solution containing preformed RhCl(H)2(PBz3)3, and the
resulting sample degassed and refilled with 3 atm. p-H2 the
hydride signals from RhCl(H)2(PBz3)3 disappeared. Two new
hydride signals were, however, seen at d −18.1 (Ha) and d −18.6
31
correlation) and 1D 1H{ P}, and nOe pulse sequences were used
as previously described.17 NOe spectra were analysed according
to standard methods.18
A series of diffusion ordered spectra (DOSY) were collected
on samples using the LEDbp pulse sequence.19 Pulse-field
gradients were incremented in 20 steps from 2% up to 95% of
the maximum gradient strength in a linear ramp. Between 2400
and 3200 transients were recorded per experiment. Gradient
lengths were selected between 2.8 and 3.0 ms, with a diffusion
time of 75 ms, and an eddy current delay of 5 ms. After Fourier
transformation and baseline correction, the diffusion dimension
was processed using the Bruker Xwinnmr package (version 3.5)
and the diffusion values were read directly from the spectra
(referenced relative to 1.9 × 10−9 m2 s−1 for D2O at 298 K).
Results and discussion
Studies involving RhCl(PBz3)3
When [RhCl(C2H4)2]2 was reacted with 6 equivalents of triben-
zylphosphine, PBz3, only one species was produced according to
31P NMR spectroscopy. This product yielded two 31P resonances,
one appearing as a doublet of triplets at d 22.2 (JPP = 38 Hz,
JPRh = 196 Hz) and the second as a doublet of doublets at d
3 7 7 4
D a l t o n T r a n s . , 2 0 0 5 , 3 7 7 3 – 3 7 7 9

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31
Fig. 2 (a) 1H{ P} nOe spectrum obtained when the hydride resonance
for the site trans to chloride in RhCl(H)2(PBz3)3 is excited (mixing time
0.25 s, at 295 K). Magnetisation transfer to H2 and the hydride site trans
to phosphine is indicated. The * indicates an nOe connection to the
ortho phenyl protons of the benzyl group. (b) Kinetic profile showing
the change in peak area as a function of mixing time for indicated species,
with the continuous line indicating data obtained by simulation.
(Hb) which showed no enhancement. At 255 K, these hydride
resonances appeared as pairs of doublets of doublets with JHH
=
11 Hz, JH = 14 Hz, and JH = 25 Hz in the corresponding
Rh
Rh
a
b
31
1H{ P} NMR spectrum. In the fully coupled 1H spectrum each
hydride showed an additional triplet splitting due to two 31P
nuclei where JH = 15 Hz and JH = 13 Hz respectively. The
P
P
a
b
31P signal associated with the single phosphine environment in
this product was located at d 36.6 and appeared as a rhodium
coupled doublet where JRhP = 120 Hz. In addition, a signal
for free phosphine was detected at d −13.2. The corresponding
1
2D H–103Rh HMQC spectrum, shown in Fig. 3(a) (275 K),
located a single 103Rh resonance for this product at d 119 which
showed couplings to two 31P centres. In order to confirm the
presence of pyridine in this product, and define the chemical
shift of the hydride ligand that is trans to pyridine, a further
sample was prepared containing 5 lL of 15N labelled pyridine.
1
31
The corresponding H{ P} NMR spectrum revealed that the
Fig. 3 Hydride region of 2D NMR spectra of RhCl(H)2(PBz3)2(py)
obtained via the reaction of RhCl(H)2(PBz3)3 with pyridine in toluene-d8
at 275 K. (a) 1H–103Rh HMQC spectrum obtained via hydride couplings,
(b) 1H–15N HMQC spectrum of the 15N labelled complex at 255 K.
D a l t o n T r a n s . , 2 0 0 5 , 3 7 7 3 – 3 7 7 9
3 7 7 5

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resonance for Ha now possessed an additional 18 Hz coupling
and thereby confirmed that this ligand is trans to pyridine.22
A
1H–15N HMQC spectrum located the corresponding pyridine
15N resonance at d −117 (Fig. 3(b)). A signal for free pyridine
was also located in this spectrum at d −60.6. The product
detected in these experiments has therefore been confirmed as
RhCl(H)2(PBz3)2(py), with the ligand arrangement shown in
Scheme 1.
Scheme 1 Reaction of RhCl(PR3)3 (R = benzyl, phenyl) with pyridine
and H2.
The hydride signals for RhCl(H)2(PBz3)2(py) failed to show
any parahydrogen enhancement between 255 and 325 K,
although the resonances themselves were strongly affected by
temperature, coalescing at ca. 300 K. The hydride regions of
31
a series of 1H{ P} NMR spectra recorded for this species
between 265 and 325 K are shown in Fig. 4. The hydride ligands
in this complex can therefore be concluded to interchange
their positions on the NMR timescale. EXSY examination of
RhCl(H)2(PBz3)2(py) system at 273 K, revealed the presence
of only simple exchange from one hydride site to the other
(Fig. 5). In addition, an nOe connection to a peak at d 7.2
corresponding to a resonance for an ortho-phenyl proton of the
phosphine was detected. Analysis of the magnetisation transfer
data yielded a rate constant for hydride site interchange of
10.68 s−1 at 275 K when 5 lL of pyridine was employed. When
this observation was repeated on a sample containing the same
metal concentration and 40 lL of pyridine, the corresponding
rate constant for hydride site exchange proved to be 10.38 s−1
and was hence identical to the earlier value within experimental
error. This process is therefore independent of pyridine. This
suggests the involvement of RhCl(H)2(PBz3)2 and supports the
view that it too has C2v symmetry (Scheme 2). In order to
31
Fig. 5 (a) 1H{ P} nOe spectrum obtained when the resonance for
the hydride ligand trans to chloride in RhCl(H)2(PBz3)2(py) is excited
(mixing time 0.25 s, 275 K). The * indicates an nOe connection to the
ortho phenyl protons of the benzyl group. (b) Kinetic trace showing the
change in peak area as a function of mixing time for indicated species,
with the continuous line indicating data obtained by simulation.
explore the dynamic behaviour of this system more fully, a
combination of EXSY spectroscopy and line-shape analysis
methods were employed. These data yielded the activation
parameters DH‡ = 98.6 3.3 kJ mol−1 and DS‡ = 137 12 J
K−1 mol−1 for the hydride–hydride exchange process. It should
be noted, that at 273 K no hydride ligand exchange was observed
for RhCl(H)2(PBz3)3.
Pyridine was then replaced with 4-methylpyridine, and the
analogous complex, RhCl(H)2(PBz3)2(4-methylpyridine) char-
acterised. The NMR data for this species can be found in Table 1.
The variable temperature NMR spectra of RhCl(H)2(PBz3)2(4-
methylpyridine) revealed evidence for a similar dynamic process
whereby the hydride ligands interchange identities. Line-shape
analysis of appropriate spectra between 265 and 325 K yielded
the activation parameters for the hydride-hydride exchange
process, DH‡ = 95.8
7.5 kJ mol−1 and DS‡ = 121
27 J
K−1 mol−1. Unfortunately, the errors in these data suggest that
no substantial differences exist between the DH‡ and DS‡ values
for the two substrates. The rate of hydride exchange for the
4-methyl pyridine complex proved to be 1.6 s−1 while that of
the pyridine system was 3.22 s−1 at 265 K. It is therefore clear
that the rhodium 4-methylpyridine bond is less labile than the
rhodium pyridine bond. This reflects the fact 4-methylpyridine
is the stronger base and hence a better electron donor.
When 100% pyridine-d5 was employed as the solvent, the
NMR spectra obtained for the reaction of RhCl(PBz3)3 with H2
contained evidence for a further product. In the corresponding
1H NMR spectra (255 K) two pairs of hydride resonances were
seen at d −18.1, −18.6, due to RhCl(H)2(PBz3)2(py) and at
d −17.3 (Hc) and d −18.3 (Hd) due to the new species. The
resonances due to the new species appear as doublets of doublets
1
31
Fig. 4 The hydride region of H{ P} NMR spectra obtained from a
pyridine doped sample of RhCl(H)2(PBz3)3 and 3 atm pH2 in toluene-d8
between 265 and to 325 K Resonances for RhCl(H)2(PBz3)2(py) are
visible.
upon 31P decoupling with JHH = 11 Hz, and JHRh = 25 and JHRh
=
14 Hz, respectively. Comparison of the 31P decoupled and fully
coupled 1H NMR spectra confirm that the new species contains
one phosphine ligand, that is cis to both hydride ligands, and
it is therefore identified as the double substitution product
RhCl(H)2(PBz3)(py)2 (Scheme 1).
Scheme
2 Mechanism for hydride site exchange observed in
RhCl(H)2(PR3)2(pyridine) (R = benzyl, phenyl).
3 7 7 6
D a l t o n T r a n s . , 2 0 0 5 , 3 7 7 3 – 3 7 7 9

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We have previously reported that the size of the parahydrogen
amplification observed for a series of binuclear dihydride com-
plexes that were detected when RhCl(CO)(PPh3)2 was reacted
with p-H2 increased when a suitable hydrogen receptor such
as styrene was added to the solution.5g We therefore examined
the effect of adding styrene and 1-phenyl-1-propyne to the
RhCl(PBz3)3–pyridine system. The addition of styrene proved
to have no effect on the associated NMR spectra at 295 K,
although, the addition of Ph–C≡C–Me led to the observation
of broad parahydrogen enhanced hydride resonances at d −18.3
and −18.6 at 295 K. When the reaction temperature was lowered
to 273 K, the hydride signals resolved such that for the d −18.1
signal (moved from d −18.3 at 295 K) the coupling JHH = 11 Hz,
JHRh = 15 Hz and JHP = 14 Hz could be measured while for
the signal at d −18.6 the couplings JHH = 11 Hz, JHRh = 25 Hz
and JHP = 13 Hz could be determined (Fig. 6(a)). These spectral
features, and additional 15N, 31P and 103Rh data obtained for this
species, match those obtained above for RhCl(H)2(PBz3)2(py)
exactly. The observation of RhCl(H)2(PBz3)2(py) with parahy-
drogen enhanced hydride resonances therefore arises because the
substrate, in this case 1-phenyl-1-propyne, promotes hydrogen
cycling, and allows the RhCl(H)2(PBz3)2(py) to be detected
before relaxation quenches the p-H2 enhancement.
hydride resonances at d −16.5 and −17.0 which showed no
parahydrogen enhancement. At 263 K, these signals resolved
upon 31P decoupling into doublets of doublets for which JHH
=
12 Hz, JHRh = 17 Hz and JHRh = 21 Hz respectively. Additional
JHP triplet couplings of 12 and 14 Hz were then determined
1
for the fully coupled spectra. A H–103Rh HMQC spectrum,
recorded at 275 K, connected these two hydride signals to a
103Rh signal at d 304 which coupled to two 31P centres. When
15N labelled pyridine was employed, the d −16.5 hydride showed
an additional 15 Hz coupling at 263 K due to the trans 15N
label; the corresponding 1H–15N HMQC located a 15N signal for
this species at d −111. The formation of RhCl(H)2(PPh3)2(py) is
therefore indicated.10,12
This NMR tube was then opened and 10 lL of 1-phenyl-
1-propyne, added, before being refilled with p-H2. Monitoring
of this sample by NMR spectroscopy then revealed that the
hydride resonances of RhCl(H)2(PPh3)2(py) were polarised for
ten minutes at 263 K (Fig. 6(b)). The NMR characteristics of
this species were, however, again unaffected by the addition
of the substrate. Small antiphase signals were now visible in
these spectra at d 6.4 and 5.6 (dq), due to the formation of the
=
hydrogenation product Ph–CH CH–Me (Fig. 6(b)).
When the hydride exchange process was examined in the
presence of Ph–C≡C–Me, the corresponding hydride exchange
rate (275 K) proved to be 11.64 s−1 regardless of the excess of Ph–
C≡C–Me (5–40 lL). The size of the observed parahydrogen en-
hancement seen for the hydride signals of RhCl(H)2(PBz3)2(py),
however, increased as the proportion of Ph–C≡C–Me increased
from 2 to 222 relative to RhCl(PBz3)3. There was, however,
no evidence in these spectra for magnetisation transfer from
Reactions of Rh(Cl)(C2H4)(PCy3)2
When [RhCl(C2H4)2]2 was reacted with four equivalents
of the phosphine PCy3 in toluene, the known complex
RhCl(PCy3)2(C2H4) was formed. NMR spectroscopy revealed
that RhCl(PCy3)2(C2H4) gave rise to a 31P signal at d 24.1
which possessed a 119 Hz Rh–P coupling to a 103Rh signal that
resonated at d 357.
=
the rhodium species into PhCH CHMe, although, clearly
A toluene-d8 solution containing RhCl(PCy3)2(C2H4) was
placed in an NMR tube fitted with a Young’s tap, and 3 atm.
of p-H2 introduced. Spectra were then collected on this sample
at 295 K. Normal hydride signals were immediately observed
more rapid hydrogen cycling is necessary to account for the
parahydrogen amplification seen for the hydride resonances of
RhCl(H)2(PBz3)2(py).
1
in the high field region of the H NMR spectrum at d −22.6
due to the formation of a stable product. This signal appears
as a distorted doublet of triplets due to couplings to one
rhodium (JHRh = 26.8 Hz) and two phosphorus nuclei (JHP
=
13.7 Hz). In the corresponding 31P NMR spectrum, a single
peak is observed at d 51.8 (JRhP = 114 Hz) for this species
while in the corresponding 103Rh spectrum a triplet is observed
at d 194. Literature suggests that this product corresponds to
Rh(Cl)(H)2(PCy3)2,23 but a tetrahydride dimer, analogous to
5g
(H)2Rh(PPh3)2(l-Cl)2Rh(H)2(PPh3)2 would also fit these data.
NMR methods involving diffusion measurements have been
used to examine problems relating to molecular volume, hy-
drogen bonding and ion pairing.24 This DOSY approach has
recently been used to distinguish between mononuclear and
binuclear products.25 A series of DOSY spectra were therefore
recorded on a sample containing hydrogen and a mixture
of RhCl(PPh3)3 and RhCl(PCy3)2(C2H4) at 295 K. Reaction
with H2 led to the rapid formation of (H)2Rh(PPh3)2(l-
Cl)2Rh(PPh3)2, and the product yielding the signal at d −22.6.
The diffusion coefficient of the known species (H)2Rh(PPh3)2(l-
Cl)2Rh(PPh3)2 proved to be 5.3 × 10−9 m2 s−1 while that for the
unknown species yielded a very similar diffusion coefficient of
4.8 × 10−9 m2 s−1. The similarities in these values suggest that
the molecules concerned have similar volumes. We therefore
conclude that Rh(Cl)(H)2(PCy3)2 is actually the tetrahydride
dimer (H)2Rh(PCy3)2(l-Cl)2Rh(H)2(PCy3)2. As a further control
experiment, a sample of RhCl(PCy3)2(C2H4) and H2 alone was
also examined under the same conditions, the d −22.6 signal
now corresponded to a species with diffusion coefficient 5.5 ×
10−9 m2 s−1. The reproducibility of these data supports this
conclusion.
Fig. 6 (a). Hydride region of a 1H NMR spectrum obtained when
a sample of RhCl(H)2(PBz3)3 was reacted with pyridine-15N, phenyl-
propyne and 3 atm. of H2 at 275 K (b) Hydride and alkyl regions of a
31
1H{ P} NMR spectrum obtained when a sample of RhCl(H)2(PPh3)3
was reacted with pyridine-15N, phenylpropyne and 3 atm of H2 at 263 K.
Studies involving RhCl(PPh3)3
When a toluene-d8 solution of RhCl(PPh3)3 was mixed with
3 atm H2, and observed by NMR spectroscopy, the known
dihydride RhCl(H)2(PPh3)3 was detected via the associated
1
hydride resonances at d −9.3 and −16.7. A H–103Rh HMQC
spectrum recorded on this sample at 275 K located a 103Rh
signal at d 98.5 for this species. This samples was then taken
into the glovebox and 5 lL of pyridine added before the
When pyridine was added to this sample, the hydride
resonance at d −22.6 remained even when the sample was
warmed to 333 K. This suggests that once (H)2Rh(PCy3)2(l-
Cl)2Rh(H)2(PCy3)2 is formed it is relatively stable. However,
1
1
31
NMR tube was refilled with 3 atm. of p-H2. H and H{ P}
NMR spectra recorded at 295 K now contained two broad
D a l t o n T r a n s . , 2 0 0 5 , 3 7 7 3 – 3 7 7 9
3 7 7 7

View Article Online
when 5 lL of pyridine was added to a toluene-d8 solution of
the precursor, RhCl(PCy3)2(C2H4), and the subsequent reaction
with p–H2 monitored at 235 K, three hydride resonances were
observed; the signal at d −22.6 described above, and a pair of
doublets of triplets of doublets at d −18.9 and −19.7. The logical
identity of this species is Rh(Cl)(H)2(PCy3)2(py).
In this study we aimed to demonstrate that parahydrogen can
be used to facilitate the detection of weakly bound ligands via the
observation of substantially enhanced and diagnostic hydride
signals in a suitable receptor complex. The intrinsic chemistry
of these systems however means that they proved unsuitable for
this role since they fail to exchange free and bound hydrogen at a
sufficient rate. Nonetheless, the addition of a sacrificial substrate
that acts to pull hydrogen through the metal-based manifold
does enable enhanced hydride resonance to be observed for the
pyridine containing dihydride adducts.
In an effort to activate the system to H2 addition, we also
studied the effect of replacing PPh3 with both PCy3 and PMe3.
Because of the large cone angle of PCy3 and the desire to
start with a pure complex we selected RhCl(PCy3)2(C2H4)
as a suitable precursor. We were therefore surprised when
the preformed Rh(Cl)(H)2(PCy3)2 we expected to obtain via
H2 addition to RhCl(PCy3)2(C2H4) failed to coordinate the
added pyridine ligand. This proved to be consistent with a
revised formulation of the stable H2 addition product as the
tetrahydride dimer (H)2Rh(PCy3)2(l-Cl)2Rh(H)2(PCy3)2.23 The
order of ligand addition, proved to be critical since when
RhCl(PCy3)2(C2H4) was reacted with pyridine first and H2 added
later, the formation of Rh(Cl)(H)2(PCy3)2(py) was indicated.
This suggests that RhCl(PCy3)2(py) was formed,12,13 and H2
addition to form Rh(Cl)(H)2(PCy3)2(py) becomes possible.
Neither Rh(Cl)(H)2(PMe3)3 nor [Rh(H)2(PMe3)4]Cl, however,
showed any sign of reacting with pyridine.
When the pyridine concentration was increased to
30 lL, the d −22.6 signal disappeared, while those for
Rh(Cl)(H)2(PCy3)2(py) remained, and a second pair of hydride
resonances were observed at d −18.2 and −18.7 which ap-
peared as simple doublets of doublets of doublets. Signals for
this species were clearly visible in neat d5-pyridine solution.
The corresponding hydride resonances appeared at d −18.3
and −19.0 in pyridine-d5, and showed strong parahydrogen
enhancement at 235 K. Under these conditions, the signals
appear as distorted antiphase doublets of doublets of doublets
1
and proved to be coupled in the corresponding H–1H COSY
NMR dataset. In the corresponding 1H–31P HMQC correlation,
a single 31P signal was located at d 71.6 (JRhP = 142 Hz). When a
sample was prepared using 15N labelled pyridine, the resonance
at d −18.3 showed an additional 20 Hz coupling due to the
trans 15N centre which resonates at d −101. The structure of
Rh(Cl)(H)2(PCy3)(py)2 is indicated in Scheme 1.
Upon warming this sample to 335 K, a new species yielding
a single hydride resonance at d −19.6 was observed. This signal
appeared as doublet of triplets due to a single HRh coupling and
1
two equivalent cis HP splittings. A H–31P HMQC correlation
spectrum revealed that this hydride resonance coupled to a 31
P
signal at d 46.8. On the basis of this data and studies of Heaton
et al.,12 the product yielding these signals in assigned to the ionic
complex [Rh(H)2(PCy3)2(py)2]+Cl−.
Acknowledgements
S. B. D., R. Z., P. I. P. E. and J. A. A. would like to thank the Basic
Technology Programme for financial support. R. K. would like
to thank CSL for the sponsorship. S. B. D. is also grateful to the
University of York, SCL, BBSRC and Bruker UK (sponsorship
and spectrometer) for financial support.
In-situ reaction of RhCl(PCy3)2(C2H4) with p-H2 in the presence
of PMe3 and pyridine
1H NMR observation of a toluene-d8 solution containing 1
equivalent of PMe3, 3 atm. p-H2, and a mixture of [Rh(Cl)-
(PCy3)2]2 and RhCl(PCy3)2(C2H4), led to the observation of
strong p-H2 based signals at d −9.4 and −18.6. The NMR data
for the product giving rise to these signals matched that previ-
ously reported for Rh(Cl)(H)2(PMe3)3 with [Rh(H)2(PMe3)4]Cl
being detected at greater PMe3 loadings.14 The addition of
pyridine to these systems failed to enable the detection of
any further species. This suggests that the Rh–PMe3 bond is
too strong to be readily broken and that systems based on
Rh(Cl)(PMe3)3 or [Rh(PMe3)4]Cl are unsuitable as precursors
for a pyridine sensor.
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Conclusions
In this paper, we have used the fact that both RhCl(PBz3)3
and RhCl(PPh3)3 add H2 to form Rh(III) dihydrides
with labile phosphine ligands to enable the detection of
RhCl(H)2(PR3)2(pyridine). The corresponding pyridine adducts
have been characterised by multinuclear NMR spectroscopy.
In the case of the PBz3 ligand system we also employed 4-
methylpyridine as a substrate. EXSY spectroscopy revealed
that these RhCl(H)2(PBz3)2(L) systems undergo hydride site
interchange via nitrogen donor dissociation and the formation
of the 16-electron intermediate RhCl(H)2(PR3)2 with equivalent
hydride ligands. The activation parameters for this process
were very similar for both pyridine and 4-methylpyridine.
The large positive DS‡ values of 137
12 and 121
27 J
K−1 mol−1 respectively are consistent with a dissociative process.
For these systems, the activation free energy, DG‡300, proved
better defined, and corresponds to 57.4
for 4-methylpyridine the value is 59.6
0.1 kJ mol−1 while
0.3 kJ mol−1. This
confirms that the barrier to the loss of 4-methylpyridine is 2.2
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higher kinetic stability of such complexes.26
3 7 7 8
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3 7 7 9