Solvent responsive catalyst improves NMR sensitivity: Via efficient magnetisation transfer

Article abstract of DOI:10.1039/c6cc03185d

A bidentate iridium carbene complex, Ir(κC,O-L₁)(COD), has been synthesised which contains a strongly electron donating carbene ligand that is functionalised by a cis-spanning phenolate group. This complex acts as a precursor to effective magne

Full text of DOI:10.1039/c6cc03185d

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Solvent responsive catalyst improves NMR
sensitivity via efficient magnetisation transfer†
Cite this:DOI: 10.1039/c6cc03185d
Amy J. Ruddlesden and Simon B. Duckett*
Received 15th April 2016,
Accepted 2nd June 2016
DOI: 10.1039/c6cc03185d
www.rsc.org/chemcomm
A bidentate iridium carbene complex, Ir(jC,O-L1)(COD), has been ligands and the substrate ligands.3 Exchange with free substrate
synthesised which contains a strongly electron donating carbene and fresh p-H2 enables the build-up of polarisation in the substrate
ligand that is functionalised by a cis-spanning phenolate group. pool through multiple visits to the catalyst. The most commonly
This complex acts as a precursor to effective magnetisation transfer used catalysts are cationic species which contain either phos-
catalysts which form after reaction with H2 and a suitable two electron phine4,5 or N-heterocyclic carbene (NHC) ligands.6,7
donor. In solvents such as benzene, containing pyridine, they are
In fact, one of the most effective magnetisation transfer
exemplified by neutral, chiral Ir(H)2(jC,O-L1)(py)2 with inequivalent catalysts is Ir(COD)(IMes)Cl7 [where IMes = 1,3-bis(2,4,6-
hydride ligands and Ir–O bond retention, whilst in methanol, Ir–O trimethylphenyl)imidazole-2-ylidene, COD = cyclooctadiene]
bond cleavage leads to zwitterionic [Ir(H)2(jC,OÀ-L1)(py)3]+, with which forms the charged complex, [Ir(H)2(IMes)(substrate)3]Cl
chemically equivalent hydride ligands. The active catalyst’s form is once activated with H2 and a substrate. This SABRE catalyst contains
therefore solvent dependent. Both these complexes break the chemically equivalent but magnetically inequivalent hydride ligands
magnetic symmetry of the hydride ligands and are active in the and polarisation transfer has proven particularly efficient in polar
catalytic transfer of polarisation from parahydrogen to a loosely protic solvents such as methanol. Furthermore, using this
bound ligand. Test results on pyridine, nicotinaldehyde and nicotine catalyst a wide range of substrates including nicotinamide,8–10
reveal up to E1.2% single spin proton polarisation levels in their isoniazid11 and pyrazole12 have been shown to become hyper-
1H NMR signals which compare to the normal 0.003% level at polarised. This type of approach has been exemplified for
9.4 Tesla. These results exemplify how rational catalyst design 1H, 13C, 31P, 19F and 15N nuclei.5,13–15 However, due to the charged
yields a solvent dependent catalyst with good SABRE activity.
nature of such a species, it has proven less efficient in the range of
low polarity solvents commonly used in NMR analysis.
The use of hyperpolarisation methods to overcome the inherent
It has also been shown that species with chemically inequi-
insensitivity of NMR spectroscopy reflects an area of research valent hydride ligands can act as SABRE catalysts. One reported
where Parahydrogen Induced Polarisation (PHIP) features heavily.1 example of this class of catalyst is given by [Ir(H)2(CH3CN)(IMes)-
The incorporation of parahydrogen (p-H2), a nuclear singlet, into a (PCy3)(pyridine)]Cl.16 Recently, a system that exhibits a wider
molecule was first shown to produce an enhanced NMR signal in solvent tolerance has been developed. It contains a bidentate
1987.2 The increase in signal intensity for resonances arising from, ligand which binds through carbene and phenolate centres,
or coupled to, p-H2 derived protons, has since been the subject of resulting in a neutral catalyst in all solvents tested. While it
intense investigation. Recently a p-H2 technique that does not was found to exhibit greater activity in non-polar solvents such
chemically change a molecule, called Signal Amplification By as benzene,17 it was not efficient in methanol due to much
Reversible Exchange (SABRE), has been developed.3 Polarisation slower ligand exchange.
is transferred through J-coupling between the p-H2 derived hydride
In this study, the related neutral iridium complex, Ir(kC,O-L1)-
(COD), 1, [where L1 = 3-(2-methylene-phenolate)-1-(2,4,6-trimethyl-
phenyl)imidazolylidene], has been synthesised starting from
salicylaldehyde. It contains a cis-spanning phenolate-substituted
bidentate NHC (see Scheme 1). Benzyl protection of the phenol18
allowed conversion of the aldehyde to the bromide19 via the
alcohol.20 Addition of 1-(2,4,6-trimethylphenyl)-1H-imidazole then
formed the imidazolium bromide salt.21 Deprotection followed
by silver carbene formation and subsequent transmetallation22
Centre for Hyperpolarisation in Magnetic Resonance (CHyM), York Science Park,
University of York, Heslington, York, YO10 5NY, UK.
E-mail: simon.duckett@york.ac.uk; Tel: +44 (0)1904 322564
† Electronic supplementary information (ESI) available: Experimental details;
synthesis and characterisation of compounds, SABRE experiments, kinetic data,
activation parameters, UV-vis data. The underlying research data for this paper is
available in accordance with EPSRC open data policy from DOI: 10.15124/
cdaf969f-ba47-401e-876b-512c0cece55c. See DOI: 10.1039/c6cc03185d
This journal is ©The Royal Society of Chemistry 2016
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under N2. It also forms stable complexes once fully reacted with
substrate and hydrogen as detailed in the following reactions.
Upon cooling a CD2Cl2 solution of complex 1 to 243 K all of
its NMR signals become broad and undefined due to fluxional
behaviour and when pyridine is added to this solution no
reaction is evident. If, however, H2 gas is added at 255 K, a
limited reaction occurs as two pairs of hydride signals are now
seen at d À12.35 and À18.25 (1.7% conversion) and at d À12.39
and À17.64 (0.6%). These minor hydride containing products
are conformational isomers of compound 2 (see Scheme 3)
which arise due to differing metallocycle orientations;30 analo-
gous behaviour has been reported.17
When p-H2 is used in this reaction, these hydride ligand signals
1
do not show any significant H NMR signal enhancement. How-
ever, the free H2 signal in these spectra is substantial which
suggests that 1 undergoes rapid and reversible oxidative addition
of H2 to form 2 which consumes the p-H2. There is no indication of
the hydrogenation of COD at 255 K and upon warming to 298 K,
these hydride signals broaden into the baseline of the corres-
ponding NMR spectra and strong signals for 1 are always visible.
When a CD2Cl2 solution of 1 reacts with both pyridine and
H2 at 298 K a new neutral iridium species, Ir(H)2(kC,O-L1)(py)2,
3 is observed to slowly form containing two bound pyridine
environments in a ratio of 1 : 1 (see ESI† for details). This
species is also formed in C6D6. The mutually coupled inequi-
valent hydride signals appear in CD2Cl2 at d À22.55 and À25.49
(2J(HH) = 8.1 Hz) and in C6D6 at d À21.94 and À24.52 (2J(HH) =
7.7 Hz). Their inequivalence suggests Ir–O bond retention,
a fact which is further supported by the diastereotopic nature
of the CH2 linker protons which appear as doublets in both
CD2Cl2 and C6D6. Both complexes undergo pyridine and H2
exchange as highlighted in Scheme 2, although only the pyridine
ligand trans to hydride dissociates. The use of EXSY NMR31
experiments enabled determination of experimental rate con-
stants for pyridine loss in 3 at 294 K of 3.74 Æ 0.06 sÀ1 in CD2Cl2
and 13.5 Æ 0.6 sÀ1 in C6D6. The corresponding H2 loss rates were
0.80 Æ 0.01 sÀ1 and 3.02 Æ 0.07 sÀ1 respectively and are therefore
much slower than those of pyridine loss.
Scheme 1 Synthetic route to 1.
afforded the product, the phenoxide iridium carbene complex, 1.
Key compounds have been characterised by NMR and MS as
illustrated in the ESI.†
In solution, complex 1 appears yellow/orange in colour.
UV-Vis analysis in DCM showed it exhibits three absorption
bands in the visible region of the spectrum (373, 425 and
490 nm) with absorption coefficients in the range of charge transfer
transitions (1017, 1326 and 249 dm3 molÀ1 cmÀ1). Work by Perutz
et al.23–26 has assigned similar bands in related square planar metal
complexes to metal d–p transitions although earlier assignments
suggested they were metal-to-ligand charge transfer bands.27–29
At room temperature, the 1H NMR spectrum of complex 1 in
CD2Cl2 yields well resolved resonances (see Fig. 1). Two doublets
at d 6.53 and 5.17 are observed for the CH2 linker protons which
2
have a common J(HH) coupling of 14.9 Hz. These two protons
are diastereotopic due to the seven-membered metallocycle
which is indicative of the retention of the Ir–O bond.30 Complex
1 is air/moisture sensitive but stable as a solid and in solution
This behaviour changes significantly upon moving to protic
methanol. Now, the addition of H2 to a CD3OD solution of
complex 1 at 250 K, results in a very limited reaction to form a
dihydride (o1%, with resonances at d À12.65 and À18.27 and
the low concentration presumably prevents observation of its
conformational isomer) but upon warming further, rapid and
total decomposition of 1 follows. In contrast, the addition of
pyridine to a CD3OD solution of complex 1 at 243 K forms the
Scheme 2 Transfer of hydride polarisation into the indicated pyridine
ligand is followed by ligand exchange to build-up hyperpolarised pyridine
in solution.
Fig. 1 1H NMR spectrum of 1 showing evidence for the diastereotopic
CH2 linker protons with key CH resonances labelled.
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Scheme 3 Species formed during the reaction of 1 with H2 and pyridine
in CD3OD (4, 5 and 6) and CD2Cl2 (2 and 3) or C6D6 (3) solution.
Fig. 2 Thermally polarised and hyperpolarised 1H NMR spectra of a
CD2Cl2 solution containing 0.06 M pyridine, 11 mol% 1 and 3 bars p-H2.
The enhanced signals for free and bound pyridine (in 3) and the corres-
ponding hydride ligands are indicated.
phenolate dissociation product, square planar 4 quantitatively
(see Scheme 3 and ESI†) where the COD ligand yields four
inequivalent alkene proton resonances.
Upon the addition of p-H2 and pyridine to 4 at 243 K, two
PHIP enhanced hydride signals become immediately visible at
d À12.34 and À17.50 that are shown to couple via COSY. They
arise from H2 addition to 4 which initially forms the alkene
dihydride complex, 5. The CH2 linker protons of 5 remain
diastereotopic on phenolate rotation due to the absence of a
mirror plane. Upon warming to 265 K, this system evolves further
and a single hydride signal becomes visible at d À22.18 due to the
formation of [Ir(H)2(kC,OÀ-L1)(py)3]+, 6; the COD is converted to
cyclooctene. Further NMR analysis confirms that 6 contains two
bound pyridine ligand environments, in a 2 : 1 ratio, two hydride
ligands and an iridium–carbene bond. Its CH2 linker protons are
now equivalent, appearing as a singlet at d 4.83 due to the
existence of a mirror plane as detailed in Scheme 3.
Complex 6 is zwitterionic, and its cationic Ir(III) centre is
balanced by phenoxide ion formation. Hence the final reaction
product formed from 1 with pyridine and H2 is solvent dependent.
The two pyridine ligands of 6 that lie trans to hydride are shown to
dissociate with an experimental rate constant of 1.35 Æ 0.03 sÀ1 at
294 K, although no exchange of the pyridine trans to the carbene
is observed. In CD3OD, rapid H/D exchange, accompanied by HD
formation, is observed which prevents the quantification of the H2
loss rate in this solvent. At 294 K, the ligand exchange rates of
species 3 in C6D6 are therefore much faster than those in CD2Cl2,
but both are faster than those of 6 in CD3OD as shown in the ESI.†
The ligand exchange rates of 6 in CD3OH were examined as a
function of temperature and activation parameters for these
processes were calculated (see ESI†). The activation enthalpy
values for both pyridine and hydride loss are very similar to
each other (90.7 Æ 1.6 kJ molÀ1 and 88.3 Æ 9.1 kJ molÀ1
respectively). The entropy of activation values of 71.3 Æ
5.3 J KÀ1 and 56.1 Æ 30.6 J KÀ1 for pyridine and hydride loss
respectively are also similar and positive thereby confirming the
dissociative character of these steps.7 Similar ligand exchange
processes, in a series of related complexes, yield values of similar
size.7,32 A ligand exchange mechanism featuring reversible
pyridine dissociation with H2 loss via [Ir(kC,OÀ-L1)(H)2(H2)(py)2]+
is therefore indicated.33
catalysts. To test their substrate signal enhancing performance,
samples were prepared containing catalyst 1 and a chosen
substrate in the desired NMR solvent. These were rigorously
degassed before the addition of 3 bars of H2. Samples were
tested by reintroducing p-H2 into the headspace of the NMR
tube, shaking the sample in the low field outside of the
spectrometer and then rapidly transferring the sample into
the spectrometer for examination. This resulted in the observa-
tion of enhanced signals in the corresponding single scan
1H NMR spectra for the hydride ligands, bound substrate and
free substrate in solution as exemplified by Fig. 2. The total
enhancement value seen for the five protons of pyridine in C6D6
proved to be 1850-fold under the conditions detailed. For
CD2Cl2 this enhancement level was reduced to ca. 1660-fold
whilst for CD3OD solution it became 710-fold. Given the corres-
ponding gain in signal-to-noise levels that these enhancements
Table 1 Maximum 1H NMR SABRE enhancements (fold) observed for
solutions containing pyridine (0.06 M and 11 mol% 1), nicotinaldehyde
(0.05 M and 14 mol% 1) and nicotine (0.04 M and 16 mol% 1) in CD2Cl2,
C6D6 and CD3OD under 3 bars p-H2
Pyridine 1H NMR SABRE enhancement (fold)
Solvent
ortho
meta
para
C6D6
CD2Cl2
CD3OD
843 Æ 27
877 Æ 32
426 Æ 5
600 Æ 30
337 Æ 57
47 Æ 28
404 Æ 13
450 Æ 22
243 Æ 3
Nicotinaldehyde 1H NMR SABRE enhancement (fold)
Solvent HA
HB
HC
HD
C6D6
CD2Cl2
CD3OD
259 Æ 17
209 Æ 22
400 Æ 32
59 Æ 10
118 Æ 22
148 Æ 34
26 Æ 7
244 Æ 18
396 Æ 65
65 Æ 6
486 Æ 36
62 Æ 7
Nicotine 1H NMR SABRE enhancement (fold)
Solvent
HA
HB
HC
HD
C6D6
CD2Cl2
400 Æ 54
419 Æ 51
29 Æ 4
315 Æ 48
381 Æ 48
28 Æ 3
88 Æ 30
338 Æ 80
386 Æ 55
31 Æ 3
Both catalysts 3 and 6 therefore exhibit the substrate and H2
146 Æ 44
3 Æ 3
exchange characteristics required for them to act as SABRE CD3OD
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provide, the resulting time savings are substantial in all solvents.
For comparison purposes, the site specific enhancement values
for the substrates pyridine, nicotinaldehyde and nicotine are
summarised in Table 1. The performance of 1 as a SABRE catalyst
for pyridine like substrates has therefore been established.
It is known that the amount of signal enhancement observed
1
in the H NMR spectrum of a substrate under SABRE is signifi-
cantly affected by the ligand exchange rates, as previously
explained by Green et al.34 For comparison, [Ir(H)2(IMes)(py)3]Cl,
is commonly used as the catalyst benchmark for SABRE and its
pyridine dissociation rate32 was found to be 23 sÀ1. While the
ligand dissociation rates for 3 and 6 are much lower, they still
achieve good hyperpolarisation levels. In fact, these data show
that one pyridine substrate trans to hydride in CD2Cl2 or C6D6 is
more efficient at receiving SABRE than the two equivalent pyridine
ligands of 6 in CD3OD. Furthermore, the concept of an active
solvent responsive catalyst is illustrated.
In summary, the iridium precatalyst Ir(kC,O-L1)(COD), 1 con-
taining a phenolate substituted NHC has been synthesised and
shown to act as an efficient SABRE catalyst precursor. The active
catalytic species is solvent dependent. Complex 1 contains an Ir–O
bond which is affected by solvent polarity and proton availability;
in non-polar C6D6 and polar aprotic CD2Cl2, this bond is strong
and substitution resistant with 1 forming Ir(kC,O-L1)(H)2(py)2 (3)
on reaction with H2 and pyridine. In contrast, on changing to polar
protic methanol, the Ir–O bond becomes labile and the phenolate
easily dissociates from the iridium centre, such that zwitterionic
[Ir(kC,OÀ-L1)(H)2(py)3]+ (6) forms. 6 is directly analogous to the
efficient SABRE catalyst [Ir(H)2(IMes)(py)3]Cl which performs well
in CD3OD but has lower activity in non-polar CD2Cl2 and C6D6.
Both 3 and 6 undergo pyridine and H2 exchange thereby enabling
them to act as SABRE catalysts. Whilst 6 works well in CD3OD,
catalyst neutrality in the non-polar solvents CD2Cl2 and C6D6
results in the formation of 3 which is highly active for SABRE
catalysis. This study therefore shows that catalyst design and
control can lead to improved magnetisation transfer in a range
of solvents, a requirement for future studies that seek to identify
low concentration analytes35–37 and to produce hyperpolarised
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