Photochemical pump and NMR probe to monitor the formation and kinetics of hyperpolarized metal dihydrides

Article abstract of DOI:10.1039/C6SC01956K

On reaction of IrI(CO)(PPh₃)₂1with para-hydrogen(p-H₂),Ir(H)₂I(CO)(PPh₃)₂2 is formed which exhibits strongly enhanced ¹H NMR signals for its hydride resonances. Complex 2 also shows similar enhancement of its NMR spectra when it is irradiated under p-H₂. We report the use of this photochemical reactivity to measure the kinetics of H₂ addition by laser-synchronized reactions in conjunction with NMR. The single laser pulse promotes the reductive elimination of H₂ from Ir(H)₂I(CO)(PPh₃)₂2 in C₆D₆ solution to form the 16-electron precursor 1, back reaction with p-H₂ then reforms 2 in a well-defined nuclear spin-state. The build up of this product can be followed by incrementing a precisely controlled delay (τ), in millisecond steps, between the laser and the NMR pulse. The resulting signal vs. time profile shows a dependence on p-H₂ pressure. The plot of k_obs against p-H₂ pressure is linear and yields the second order rate constant, k₂, for H₂ addition to 1 of (3.26 ± 0.42) × 10² M^?1 s^?1. Validation was achieved by transient-UV-vis absorption spectroscopy which gives k₂ of (3.06 ± 0.40) × 10² M^?1 s^?1. Furthermore, irradiation of a C₆D₆ solution of 2 with multiple laser shots, in conjunction with p-H₂ derived hyperpolarization, allows the detection and characterisation of two minor reaction products, 2a and 3, which are produced in such low yields that they are not detected without hyperpolarization. Complex 2a is a configurational isomer of 2, while 3 is formed by substitution of CO by PPh₃

Full text of DOI:10.1039/C6SC01956K

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Photochemical pump and NMR probe to monitor
the formation and kinetics of hyperpolarized metal
Cite this: DOI: 10.1039/c6sc01956k
dihydrides†
b
Barbara Procacci,ab Pedro M. Aguiar,b Meghan E. Halse,ab Robin N. Perutz
*
ab
*
and Simon B. Duckett
On reaction of IrI(CO)(PPh3)2 1 with para-hydrogen (p-H2), Ir(H)2I(CO)(PPh3)2 2 is formed which exhibits
strongly enhanced 1H NMR signals for its hydride resonances. Complex
also shows similar
2
enhancement of its NMR spectra when it is irradiated under p-H2. We report the use of this
photochemical reactivity to measure the kinetics of H2 addition by laser-synchronized reactions in
conjunction with NMR. The single laser pulse promotes the reductive elimination of H2 from
Ir(H)2I(CO)(PPh3)2 2 in C6D6 solution to form the 16-electron precursor 1, back reaction with p-H2 then
reforms 2 in a well-defined nuclear spin-state. The build up of this product can be followed by
incrementing a precisely controlled delay (s), in millisecond steps, between the laser and the NMR pulse.
The resulting signal vs. time profile shows a dependence on p-H2 pressure. The plot of kobs against p-H2
pressure is linear and yields the second order rate constant, k2, for H2 addition to 1 of (3.26 ꢀ 0.42) ꢁ
102 Mꢂ1 ꢂ1. Validation was achieved by transient-UV-vis absorption spectroscopy which gives k2 of
s
(3.06 ꢀ 0.40) ꢁ 102 Mꢂ1
s
ꢂ1. Furthermore, irradiation of a C6D6 solution of 2 with multiple laser shots, in
Received 4th May 2016
Accepted 3rd August 2016
conjunction with p-H2 derived hyperpolarization, allows the detection and characterisation of two minor
reaction products, 2a and 3, which are produced in such low yields that they are not detected without
hyperpolarization. Complex 2a is a configurational isomer of 2, while 3 is formed by substitution of CO
by PPh3.
DOI: 10.1039/c6sc01956k
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necessary increase in sensitivity and the benet of enhance-
ment of resonances that oen allow structure determination.5,6
Introduction
We recently described a laser pump-NMR probe technique in
which the laser initiation of a reaction is synchronized to NMR
detection, such that NMR spectra can be recorded at well-
dened intervals aer a laser pulse (Fig. 1). In this method, p-H2
hyperpolarization5,6 enhances the sensitivity of the NMR
detection method so much that the dihydride complexes can be
observed in optically dilute solutions with a single NMR scan at
intervals as short as 10 ms aer the laser pulse. When dihy-
drogen addition is very fast, the magnetisation that is created
through the use of p-H2 evolves coherently during the well-
dened interval s such that sinusoidal oscillations can subse-
quently be detected through radio frequency excitation. These
oscillations have periods corresponding to the frequency
difference between the inequivalent hydrides and/or their
difference in spin–spin coupling to a heteronucleus such as
phosphorus.7 Notably, the laser method generates the hyper-
polarized molecules synchronously unlike the usual PHIP
methods which are inherently asynchronous.8,9 We now wish to
validate the method for the measurement of chemical reaction
kinetics on timescales where these initial oscillations are de-
phased, but short enough that substantial para-hydrogen-
induced-polarization (PHIP) is retained despite the effects of
A number of pump-probe time-resolved techniques have been
developed that allow chemical reactions that happen on
extremely fast timescales (milliseconds down to femtoseconds)
to be followed and understood. Whilst various probe methods
have been successfully exploited,1–4 the level of structurally
diagnostic information available is oen limited compared to
that provided by nuclear magnetic resonance (NMR). Therefore,
the corresponding time-resolved NMR spectroscopy method
would be a very powerful analytical tool capable of providing
complementary structural and kinetic information. The existing
sensitivity limitation of NMR spectroscopy would, however,
need to be overcome in order to achieve this aim. para-
Hydrogen (p-H2) induced polarization (PHIP) provides the
aCentre for Hyperpolarisation in Magnetic Resonance, Department of Chemistry, York
Science Park, University of York, Heslington, York, YO10 5NY, UK. E-mail: simon.
duckett@york.ac.uk
bDepartment of Chemistry, University of York, Heslington, York YO10 5DD, UK. E-mail:
robin.perutz@york.ac.uk
† Electronic supplementary information (ESI) available: Experimental details;
synthesis and characterisation of compounds, photochemical experiments,
kinetic data, UV-vis data. See DOI: 10.1039/c6sc01956k
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spectroscopy on solutions of IrCl(CO)(PPh3)2 and IrCl(H)2-
(CO)(PPh3)2 (lexc > 254 nm, ash lamp) suggest CO photo-
dissociation as a primary step17 while time resolved IR spec-
troscopy (lexc ¼ 308 nm, laser) excludes any loss of CO on the
basis of a lack of evidence for the bleaching of the parent CO
stretch.18 Both studies detected a short-lived transient (ꢃ10 ms)
which was speculated to be a dimeric species. Studies on the
thermal addition of p-H2 to a series of related complexes,
(IrCl(CO)(PPh3)2 and RhCl(CO)(PPh3)2) have been previously
reported.19,20 These experiments yielded large PHIP enhance-
ments for the iridium and rhodium dihydride products visible
for tens of minutes, hence demonstrating the reversibility of H2
addition and permitting signal averaging. Using both theoret-
ical and experimental approaches, Bargon assessed the purity of
the singlet state that is created between the hydride ligands of
IrCl(H)2(CO)(PPh3)2 upon the addition of p-H2 as greater than
50%.21 In the case of IrCl(CO)(PPh3)2, thermal reaction studies
demonstrated that a minor cis–cis isomer of IrCl(H)2(CO)(PPh3)2
can be detected in addition to the more usual trans–cis
product.22,23 In contrast, for derivatives of RhCl(CO)(PPh3)2,
a series of binuclear products were detected which contain
either bridging or terminal hydride or halide ligands. These
products were proposed to form via CO loss from M(H)2-
X(CO)(PPh3)2 [X ¼ halide] with the result that a 16-electron
M(H)2X(PPh3)2 species is created. The presence of this inter-
mediate was conrmed by trapping with added phosphine in
a series of experiments which revealed that IrCl(H)2(PPh3)2
exhibits a square pyramidal geometry.20 Furthermore, extensive
studies on IrX(CO)(dppe) have been reported establishing that
dihydrogen adds to these complexes in a stereoselective way
controlled by the X and the CO ligands.24
Fig. 1 Pulse sequences used in the laser pump-NMR probe experi-
ments. (a) In the single-shot experiments, a single laser pulse (5 ns) is
followed by a delay (s) for evolution of the chemical system before
observation is achieved through the application of a 45ꢄ RF pulse. (b)
The multiple laser pulse experiments apply a train of laser pulses,
separated by an additional delay of 100 ms between each of the n
pulses.
relaxation. The method we illustrate involves a degenerate
reaction, where H2 is photodissociated from the parent and p-
H2 adds to reform the same molecule but now in a dened
nuclear spin state (indicated by asterisk, eqn (1)). In our
experiments, p-H2 addition occurs in the high magnetic eld of
the spectrometer, a situation sometimes referred to as PASA-
DENA conditions.5,8
In this work, we report the investigation of the thermal and
(1)
photochemical reactions of IrI(CO)(PPh3)2
1 and Ir(H)2-
I(CO)(PPh3)2 2 in the presence of p-H2. We use our laser pump-
NMR probe method to determine the kinetics of reaction of 2
with H2 and compare with the results of laser ash photolysis
with UV-vis detection.
In order to validate the proposed kinetic measurements, the
rate of reaction of the intermediate [M] with H2 must be known
already (or be capable of independent measurement) and
ideally have a pseudo rst-order rate constant of ca. 0.1 to 10 sꢂ1
at 3 bar H2 to satisfy the NMR criteria above. The thermal
reactions of these complexes with p-H2 should also be under-
stood. For this purpose, we selected Ir(H)2I(CO)(PPh3)2 2,
derived from the iodide analogue of Vaska's complex,
IrI(CO)(PPh3)2 1, for which kinetic data for reaction with dihy-
drogen10–12 and underlying reactivity are known.13–15 Complex 2
can be readily prepared by thermal reaction of the d8 square-
planar precursor IrI(CO)(PPh3)2 1 with H2 in C6D6 solution.16
The steady state photochemistry of Ir(H)2X(CO)(PPh3)2 (X ¼
Cl, I) complexes has been reported (366 nm, C6H6) previously
and shown to access their Ir(I) counterparts by reductive elim-
ination of H2, which then undergo oxidative addition of H2 to
reform the parent dihydride.15 In this context, the photochem-
ical reductive elimination step allows access to its thermal
microscopic reverse, oxidative addition, in a controlled way.
While time-resolved spectroscopy has been used to examine
these chloride complexes, the literature is not denitive on the
primary photo-process. Measurements by time resolved UV-vis
Experimental‡
Complex 1 was prepared using the procedure reported in the
literature.10 Complex 2 was obtained by hydrogenation of 1 at 50
ꢄC and complete conversion to the 18-electron dihydride was
established from NMR spectra recorded at 298 K. p-H2 was
generated by cooling hydrogen gas over charcoal in a copper
block at 26 K. The proportion of p-H2 at 26 K was calculated as
>99%. Pressures of p-H2 were measured with an MKS Baratron
capacitance manometer. All NMR spectra were recorded on
a Bruker Avance II 600 MHz spectrometer with a 14 T widebore
magnet tted with a 5 mm BBO probe. In situ laser photolysis
was carried out with a pulsed Nd:YAG laser (Continuum Surelite
II) tted with a frequency tripling crystal (output 355 nm).
Operating conditions were typically: 10 Hz repetition rate, ash
lamp voltage 1.49 kV, and Q-switch delay increased from the
standard to 320 ms yielding a laser power of 75 mW in internal
mode. The energy of a single laser pulse was measured using an
energy meter calibrated for 355 nm to be ꢃ29.8 mJ at our
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operating conditions (external triggering with Q-switch delay
set to 150 ms). The unfocused laser beam is directed at the base
of the spectrometer and reected up into the probe via a mirror
as previously reported.7 Adjustment screws control the vertical
and horizontal position of the mirror which is on a kinematic
mount. The system is fully shielded from the operator and the
screws of the kinematic mount can be adjusted remotely. The
laser radiation is incident on a xed mirror that is level with the
sample and passes through a hole in the probe onto the NMR
tube. Standard NMR tubes tted with Young's taps were used.
The samples contained 1–2 mg of compound (Abs355 ꢃ 0.7) and
approximately 0.4 mL of solvent. A sample of Ru(dppe)2H2 in
C6D6 was used for laser alignment with p-H2 amplication in
real time. Standard NMR pulse sequences were modied for
use with p-H2 by including a synchronized laser initiation
sequence prior to NMR excitation. A purpose-written program
was used to control the laser ring from the NMR console with
the laser set on external triggering. The program sets the laser
to re one warm-up shot before the re signal. The NMR pulse
is initiated at a set delay time (s) following the re signal. The
intrinsic time delay between sending the re signal from the
spectrometer and the actual ring of the laser pulse was
measured with a photodiode and an oscilloscope to be 150 ms
(equal to the Q-switch delay for the generation of the pulse).
This signal delay was incorporated into the pulse sequence
such that synchronized measurements with a time delay, s,
were achieved by setting the spectrometer delay to: s + 150 ms.
The precision of this delay between the laser and radio
frequency (RF) pulses is controlled by the 200 ns clock of the
spectrometer.
The samples for laser ash photolysis (LFP) were prepared
exclusively in a nitrogen glove box. It was loaded into a quartz
cuvette (10 mm path-length) tted with a J-Young's PTFE stop-
cock, a degassing bulb, and a greaseless Young's connection.
The complex was dissolved in benzene (5 mL) with a concen-
tration selected to have an absorbance at the laser wavelength
(355 nm) between 0.6 and 0.85. The solution was then degassed
by repeated freeze–pump–thaw cycles (3 times) on a high-
vacuum Schlenk line before being backlled with hydrogen. For
high pressure work the window edges of the cuvette were amed
to secure the seal and the Young's connection was replaced by
a glass-to-metal seal and a Swagelok™ tting. The gas was
admitted on a high pressure line and the pressure measured
with an MKS Baratron capacitance manometer. The cell was
held in a metal container for safety. A single sample was used
for each run with increasing gas pressure. Hydrogen was of
Research Grade N5.5 (BOC).
Results and discussion
When IrI(CO)(PPh3)2 1 reacts with normal H2 at 298 K only two
signals (1 : 1) are observed in the hydride region of the corre-
sponding 1H NMR spectrum due to a pair of chemically in-
equivalent hydride ligands. They resonate at d ꢂ8.5 (dt, with
2
2JHH ¼ 4.0 Hz and JPH ¼ 17.2 Hz) for the site trans to CO and
d ꢂ14.9 (dt, with 2JHH ¼ 4.0 Hz and 2JPH ¼ 13.8 Hz) for the site
trans to I (Fig. 2a and b). This product is therefore readily
assigned as dihydride 2. When the same reaction is repeated
with p-H2 at 298 K, these two hydride resonances exhibit strong
PHIP signals (SNR ꢃ 700 on a 5 mg solution, SNR ꢃ 280 on an
optically dilute solution) and 2JHH is conrmed to be negative26
(see ESI†). At 263 K, very little PHIP (SNR ꢃ 2.2 on an optically
dilute solution at 273 K with a single scan) is observed for the
hydride resonances of 2 due to slow addition and no evidence
for its minor cis–cis isomer was evident in 1H NMR spectra even
at low temperature.
When a sample of the pure dihydride Ir(H)2I(CO)(PPh3)2 (2)
1
was examined by H NMR spectroscopy at 298 K under p-H2 (3
bar), no p-H2-enhanced hydride resonances were observed
either with the standard PHIP protocol (45ꢄ pulse) or with the
OPSY approach,27 thereby conrming its stability with respect to
H2 loss on the time scale of these measurements. However, at
temperatures of 335 K and above, PHIP-enhanced hydride
resonances of 2 were observed, indicating that the thermal
exchange of H2 now becomes accessible.
In contrast to this situation, upon photo-initiation by
a single laser shot at 355 nm (298 K, 3 bar p-H2), the same two
hydride resonances are seen with large PHIP (SNR ꢃ 280) in
a single scan 1H{31P} NMR measurement that is recorded aer
a 45ꢄ RF pulse (Fig. 2c and Scheme 1).
The nuclear spin state description of the two hydride ligands
in 2*, formed aer p-H2 addition, reects a longitudinal two
spin order term (IzSz) which depends on sin 2q (where q is the
pulse angle) and is therefore optimally encoded for detection by
a 45ꢄ pulse.9 This contrasts with the situation that was reported
when the very rapid H2 addition to [Ru(CO)(PPh3)3] was
The LFP apparatus was previously described.25 Briey, it
consists of an Nd:YAG laser (Quanta Ray, GCR3-30) operating at
355 nm as the exciting source, coupled to an Applied Photo-
physics laser kinetic spectrometer with a Xe arc lamp as a white
light source. The unfocused laser beam is directed at 90ꢄ to the
sample. The laser runs at 30 Hz with individual pulses (ca. 5 ns)
selected with a synchronised shutter. Light falling on the photo-
multiplier detector is sampled by a Tektronix TDS 540B oscil-
loscope. Transient decays are usually analysed as 35 shot aver-
ages. The samples were maintained at 295 K.
Fig. 2 Hydride region of a series of 1H NMR spectra of 2 in C6D6
where: (a) the signals are fully relaxed prior to collection (32 scans, SNR
¼ 14); (b) the relaxed signals are 31P decoupled (4 scans, SNR ¼ 6) and
(c) the signals are hyperpolarized and 31P decoupled (1 laser pulse, 1
scan, SNR ¼ 280). The values of SNR in (b) and (c) yield an enhance-
ment factor of 93.
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Scheme 2 Schematic diagram showing photodissociation of H2 from
2, oxidative addition of p-H2 to 1, and nuclear spin relaxation reactions
of 2*.
Scheme 1 Photochemical reaction to prepare 2 in a hyperpolarized
state, 2*.
versus time prole (Fig. 3) shows that an exponential growth is
followed by an exponential decay which ultimately restores the
initial intensity. Such a build-up and decay prole is typical of
that expected for two consecutive reactions where the rst
product is depleted as it takes part in the subsequent one. Here,
the chemical process is the formation of 2* by p-H2 addition to
its photochemically formed precursor IrI(CO)(PPh3)2 1 and
proceeds with a pseudo rst order-rate constant kobs. The
subsequent decay is due to nuclear spin-lattice relaxation of the
hyperpolarized NMR signal that proceeds with the rst order-
rate constant R1 (Scheme 2). These experimental data can be
tted to the function in eqn (2)28 to yield kobs of 2.1 ꢀ 0.2 sꢂ1 for
H2 oxidative addition under 3.31 bar of p-H2.
followed. In this latter case, the original singlet state was
retained (IzSz + ZQx) and a 90ꢄ probe pulse allowed the moni-
toring of the evolution of the resulting zero-quantum coher-
ences (ZQy).7 This different behaviour arises because the
relatively slow H2 addition to the [IrI(CO)(PPh3)2], 1, causes the
zero quantum terms (ZQ) to lose coherence while the longitu-
dinal two-spin order term (IzSz) is preserved.8,9 The resulting IzSz
amplitude accumulates as the reaction proceeds until all of the
intermediate 1 is converted into Ir(H)2I(CO)(PPh3)2 (2).9
The intensity of the hyperpolarized single scan 1H{31P} NMR
signal in this photochemical experiment allows us to work with
an optically dilute solution (0.7 mM in C6D6, Abs(355) z 0.7
across the 5 mm NMR tube) whilst still obtaining a good
hydride resonance signal-to-noise ratio (SNR ¼ 280). It might be
expected that this dilution ensures that a constant amount of
p-H2 is available in solution even if multiple observations are
required. However, a control measurement showed that the
detected signal intensities decreased slightly aer four single
laser shot experiments. We therefore refreshed the dissolved
p-H2 by removing the sample from the magnet/probe and re-
equilibrating it with the p-H2 in the headspace every four laser
shots in order to ensure a reproducible response (see ESI†).
The evolution of the hydride signal intensity was followed as
a function of the time, s, between the laser pump and NMR
probe steps. This corresponds to following the change in
amplitude of the longitudinal two-spin order term (IzSz) during
the pump probe delay which, in the absence of NMR relaxation,
is proportional to the concentration of 2 (Scheme 1). The signal
ꢀ
ꢁ
d½2*ꢅ
dt
¼ kobs ½2ꢅ ꢂ R1½2*ꢅ
(2)
À
Á
kobs
1
½2*ꢅ ¼ ½2ꢅ
ꢁ eꢂk ꢂ eꢂR t
obst
0
R1 ꢂ kobs
The reformation of 2 follows pseudo-rst-order kinetics
under these conditions where p-H2 is in excess. We therefore
varied the hydrogen pressure from 3 to 6 bar to extract k2 from
the resulting series of kobs values via the associated linear
dependence analysis (Table 1 and Fig. 4).
The second order rate constant k2, for the regeneration of 2
in the presence of hydrogen was determined from this analysis
as (3.26 ꢀ 0.42) ꢁ 102 Mꢂ1 sꢂ1 (with solubility of H2 taken as
2.9 ꢁ 10ꢂ3 M atmꢂ1) at 298 K.29 In parallel to this NMR study, we
also determined the kinetics of H2 addition to 1 by transient
absorption UV-vis spectroscopy§ starting from photo-excitation
(355 nm) of a C6H6 solution of 2. Once more, the plot of kobs
against H2 pressure was linear with a corresponding second
order rate constant of (3.06 ꢀ 0.40) ꢁ 102 Mꢂ1 sꢂ1. Literature
Table 1 kobs determined at different hydrogen pressures by photo-
chemical pump-NMR probe
H2 pressurea bar
Hydride at d ꢂ8.5 kobsb, sꢂ1
3.31
4.42
5.10
6.02
2.1 ꢀ 0.2
3.5 ꢀ 0.4
3.7 ꢀ 0.5
4.8 ꢀ 0.3
a
b
The amount of p-H2 at 26 K was calculated as >99%.5 Polynomial
tting involved using R1 (Scheme
2 and eqn (2)) as a shared
Fig. 3 Hydride signal integral values of 2* versus s (time) taken from
a series of 1H{31P} NMR spectra that were recorded under 3.31 bar of p-
H2 pressure. Coloured squares/circles are the experimental points
while the lines show the fit according to eqn (2) where R1 has been
optimised as a linked parameter in a multiple dataset fit. Points were
collected in random order.
parameter in a series of traces mapping d(2*)/dt as a function of [p-
H2], the rate of relaxation of the double quantum term (IzSz) is not
affected by the change in [H2]. 1/R1 was determined to be 0.67 ꢀ 0.11
s. This value was validated by measuring the rate of relaxation of the
IzSz term independently employing a different method (See ESI)†.
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Fig. 5 Hydride region of the 1H NMR spectrum of a C6D6 solution of 2
exposed to 32 laser shots. The major peaks belong to 2* (diamonds);
insets: same part of the spectrum magnified to observe the newly
formed species 2a* and 3*.
Fig. 4 Plots of pseudo-first-order rate constants kobs determined
from the decay of the transients obtained by TR-NMR (black squares)
and TR-UV-vis (red circles) after 355 nm excitation of complex 2 in
benzene vs. the pressure of quenching gas (H2). The corresponding
lines of best fit are shown in black and red.
phosphorus splitting conrms its identity as the ligand
exchange product Ir(H)2(I)(PPh3)3. Their relative signal
strengths are 2 and 1% of those of 2* respectively. Photo-
isomerization of square planar complexes is a well-known
process.32 We therefore propose that following the formation of
1 by reductive elimination of H2 from 2*, secondary photolysis
takes place to induce geometrical rearrangement and ligand
dissociation in 1. The resulting photo-products react promptly
with p-H2 giving rise to the observed enhanced signals of 2a*
and 3*. Their detection is completely suppressed by decreasing
the temperature by 10ꢄ (285 K).
The identity of 3* was also conrmed by trapping the inter-
mediate with excess PPh3 (see ESI†). The addition of 20 fold
excess PPh3 to an optically dilute C6D6 solution of 2 resulted in
the observation of 3* at room temperature aer addition of p-H2.
The hyperpolarized hydride signals of 3* could be made stronger
by increasing the temperature to 50 ꢄC. When the same solution
was exposed to 32 laser shots, once again 2* was the major
species with 3* being the only by-product present in solution.
Under these conditions, the formation of 2a* was prevented by
the excess PPh3 which acted as a trap for any unsaturated species
formed in solution. Interestingly, the minor isomer observed by
thermal reaction of the chloride analogues exhibited a cis–cis–
trans structure that is not seen here.23,33
When the temperature of an optically dilute solution of 2
under p-H2 was raised to 335 K, thermal exchange was observed
to result in small PHIP enhancements for the hydride reso-
nances of 2 (SNR ꢃ 40) in the absence of laser irradiation. In
addition to these 1H NMR signals, new enhanced hydride
signals in the region d ꢂ18 to ꢂ22 were detected which
remained visible by PHIP for several minutes (see ESI†). These
signals could also be accessed aer multiple laser shots at lower
temperatures (>16 laser shots at 315 K). The greater shielding of
these hydride resonances is characteristic of those observed for
dimers, such as Rh(H)2(PPh3)2(m-I)2Rh(CO)(PPh3);20 we there-
fore assign these products to dimeric complexes with bridging
iodine atoms (see ESI†).20
values for hydrogen uptake by 1 are reported as 430 Mꢂ1 sꢂ1 in
chlorobenzene and >102 Mꢂ1 sꢂ1 in C6D6 solution at 30 C, as
ꢄ
determined by spectrophotometry12 and manometry10 tech-
niques, respectively. These values are consistent with our TR-
NMR and TR-UV-vis measurements. In Fig. 4, we use the pres-
sure of H2 as the x-axis quantity for UV/vis and NMR measure-
ments. The underlying assumption is that 2 is formed with
100% spin-state purity. The non-zero intercept for the line
through the NMR data may result from lower purity, but any
such reduction has an insignicant effect on the rate k2,
because the rate is derived from the gradient of the lines.21,30
The effect of temperature on the reactivity of 2 towards H2 was
also tested. In order to do this, a set of similar photochemical
pump-NMR probe experiments was undertaken at 315 K which
gave a k2 value of (6.0 ꢀ 0.3) ꢁ 102 Mꢂ1 sꢂ1 from the corre-
sponding plot of kobs vs. H2 pressure (See ESI†); the gain in rate for
this 17ꢄ increase in temperature was ꢃ2-fold. Additionally, the
rate of the same reaction was measured as a function of temper-
ature for a C6D6 solution of 2 under 4.42 bar of H2. The resulting
activation parameters31 (DH‡ ¼ 42 ꢀ 1 kJ molꢂ1, DS‡ ¼ ꢂ53 ꢀ 1)
(see ESI†) were consistent with the values previously reported.10
We also obtained further insight into the mechanism of H2
addition to the metal centre using our pump-probe system. To
clarify the mechanism, we exposed an optically dilute C6D6
solution of 2 under p-H2 atmosphere to 48 laser shots (355 nm,
repetition frequency 10 Hz, at 295 K, 3 bar p-H2). The major
detectable species was 2* (Fig. 5) but by-products started to
form aer more than 32 laser shots. These arise from two
additional photo-generated dihydride species 2a* and 3*. The
splitting of the hydride peaks and their chemical shis indicate
that both of these species contain pairs of cis hydrides and cis
phosphines (Fig. 5, see ESI† for d and J values). In 2a*, a bis
phosphine complex, one hydride lies trans to carbonyl (d ꢂ9.2)
and the second (d ꢂ11.5) lies trans to phosphorus. In 3*, one
hydride ligand lies trans to iodine (d ꢂ17.6) while the second
(d ꢂ11.2) is trans to phosphorus, but now an additional
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Conclusions
Notes and references
‡ Data created during this research are available by request from the University of
York Data Catalogue (http://dx.doi.org/10.15124 /282717d1-d7d9-428f-8b77-
b98b6a8b6f80).
The observations that have been described here establish that
the combination of in situ photochemistry and p-H2 derived
signal amplication allows the quantication of H2 addition
rates on a ms timescale by NMR spectroscopy. In this case,
IrI(CO)(PPh3)2 (1) reacts with p-H2 at room temperature to
generate Ir(H)2I(CO)(PPh3)2 (2) which is characterised by the
PHIP-enhancement of its hydride resonances. While 2 is ther-
mally stable to H2 loss at 298 K, it is photoactive under 355 nm
irradiation and in the presence of p-H2 PHIP-enhancement is
observed. Because of the signal gain associated with PHIP, the
accumulation of 2 could be easily followed between 50 ms and
1 s despite the inherently low sensitivity of NMR spectroscopy.
Data analysis of the kinetics of formation of 2 from 1 as
a function of H2 pressure and temperature yielded second order
rate constants and activation parameters (e.g. k2 ¼ (3.26 ꢀ 0.42)
ꢁ 102 Mꢂ1 sꢂ1). This approach was validated by laser ash
photolysis which yielded similar second order rate constants
(e.g. (3.06 ꢀ 0.40) ꢁ 102 Mꢂ1 sꢂ1 at 298 K).
§ TR-UV-vis experiments were run in the presence of H2. The use of p-H2 does not
change the results in this type of measurement.
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Acknowledgements
We are grateful to EPSRC (grant EP/K022792/1) for funding,
Simon Colebrooke for helpful discussions and Dr Soumya S.
Roy for the thermal measurement of the IzSz states lifetime in 2.
Chem. Sci.
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