NMR detection of thermal and photochemical dihydrogen addition products of mono- and tri-nuclear ruthenium complexes containing carbonyl and triphenylphosphine ligands through para-hydrogen induced polarisation

Article abstract of DOI:10.1039/a903321a

Enhancement of NMR signals by para-hydrogen induced polarisation is shown to facilitate the detection of isomers of Ru(CO)₂(H)₂(PPh₃)₂ and Ru(CO)₃(H)₂(PPh₃) which contain inequiv

Full text of DOI:10.1039/a903321a

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NMR detection of thermal and photochemical dihydrogen addition products of
mono- and tri-nuclear ruthenium complexes containing carbonyl and
triphenylphosphine ligands through para-hydrogen induced polarisation
Christopher J. Sleigh, Simon B. Duckett,* Roger J. Mawby and John P. Lowe
Department of Chemistry, University of York, York, UK YO10 5DD. E-mail: sbd3@york.ac.uk
Received (in Cambridge, UK) 26th April 1999, Accepted 25th May 1999
Enhancement of NMR signals by para-hydrogen induced
polarisation is shown to facilitate the detection of isomers of
Ru(CO)2(H)2(PPh3)2 and Ru(CO)3(H)2(PPh3) which contain
inequivalent hydride ligands, and to demonstrate that
Ru3(CO)9(PPh3)3 adds H2 to form Ru3(CO)8(H)(m-
H)(PPh3)3, as well as undergoing fragmentation to form
Ru(CO)3(H)2(PPh3).
authentic sample (see above). Under normal H2, and after
prolonged photolysis, 2a was the only detectable product.
When the solution containing Ru(CO)3(PPh3)2 and 1a
enhanced with p-H2 was warmed to 328 K, the hydride
resonances for 1a broadened substantially, and a second pair of
polarised hydride resonances appeared at d 26.27 (ddd, JPH
26.4, 19.1, JHH = 27.1 Hz) and 27.52 (ddd, JPH 75.1, 33.0,
JHH 27.1 Hz). Resonances for two inequivalent 31P nuclei, both
doublets with JPP 32.0 Hz, were located in the 1H–31P spectrum
at d 50.2 and 41.6. It was concluded that the species responsible
for the new resonances was the previously undetected all-cis
isomer of Ru(CO)2(H)2(PPh3)2 2b.
We have shown that the enhanced absorption and emission
signals observed in NMR spectra of complexes incorporating
hydrogen nuclei derived from para-enriched hydrogen1 (p-H2)
can be used to detect and characterise materials present at
concentrations too low for detection by normal NMR methods.2
In particular, using this approach we have been able to identify
minor all-cis isomers of Ru(CO)2(H)2L2 (L = PMe2Ph or
PMe3) that were previously unknown.3 Here we extend this
approach by demonstrating how both Ru(CO)3(H)2(PPh3) 1 and
Ru(CO)2(H)2(PPh3)2 2 behave with p-H2. We show that two
isomers of both 1 and 2 are detectable, and demonstrate that
Ru3(CO)9(PPh3)3 reacts with hydrogen to yield new clusters
containing bridging and terminal hydride ligands and also
fragments to Ru(CO)3(H)2(PPh3). The precursors required for
these studies, Ru(CO)3(PPh3)2, Ru3(CO)9(PPh3)3 and trans-
cis–cis-Ru(CO)2(H)2(PPh3)2 2a, were prepared by standard
methods.4 These complexes were selected for study because,
despite the fact that ruthenium/carbonyl/triphenylphosphine
species have been implicated in catalytic cycles involving H2,
their reactivity towards H2 is only poorly understood.5–7
A
1H–1H PHIP–EXSY experiment, recorded at 308 K,
contained exchange peaks that indicated that the hydride ligands
of mer-Ru(CO)3(H)2(PPh3) 1a were interchanging positions
with a rate constant of 40 s21. Even at higher temperatures,
however, no peaks connected the resonances of 1a to those of
2b, suggesting that the interchange may be intramolecular,
When a benzene-d6 solution of Ru(CO)3(PPh3)2 under 3 atm
1
of p-H2 was warmed to 353 K, no change in the H NMR
spectrum of the solution was detected. However, when a similar
solution, also under p-H2 was photolysed for 2 min, an NMR
spectrum recorded at 308 K contained enhanced resonances at d
26.67 (Ha, dd, JPH 15.9, JHH 25.4 Hz) and d 27.34 (Hb, dd, JPH
61.6, JHH 25.4 Hz) [Fig 1(a)]. The values for JPH suggested that
the species responsible for these resonances, 1a, contained a
single PPh3 ligand lying cis to Ha and trans to Hb. The p-H2
enhanced complex 1a was further characterised by 2D NMR,8
allowing the 31P resonance to be located at d 20.3. In order to
obtain 13C data, 13CO-labelled Ru(CO)3(PPh3)2 was photolysed
with p-H2. In the resulting 1H NMR spectrum of 13CO-labelled
1a, the Ha resonance showed an additional doublet splitting
(JCH 27.8 Hz) indicative of a trans H–Ru–CO arrangement.
Two 13CO resonances were detected at d 197.6 and 199.5 by a
1H–13C HMQC experiment. The data indicated that 1a was the
mer isomer of Ru(CO)3(H)2(PPh3). This complex has not
previously been observed by NMR, although high pressure IR
studies have indicated its existence.6 Surprisingly, in the
corresponding 1H–31P correlation, additional resonances corre-
sponding to the fac isomer of Ru(CO)3(H)2(PPh3) 1b were
detected which were missed in the original 1H NMR spectrum
owing to coincidental overlap with the resonance due to Ha of
1a.
Fig 1 NMR spectra of reaction products obtained with p-H2 in C6D6
showing the hydride region only. The antiphase components arise in
transitions involving protons that originate from p-H2. (a) 1H spectrum of 1a
generated after UV irradiation of a sample of Ru(CO)3(PPh3)2 at 308 K; (b)
1H spectrum of 2b at 328 K; (c) 1H spectrum of a sample of 2a warmed with
p-H2 in the presence of CO at 318 K; (d) 1H spectrum of 3 generated from
a sample of Ru3(CO)9(PPh3)3 at 308 K.
A very weak hydride resonance could also be seen in the 1H
NMR spectra of the photolysed solutions at d 26.35 (t, JPH 23.2
Hz). This was shown to be due to the trans–cis–cis isomer of
Ru(CO)2(H)2(PPh3)2 2a by comparison with the spectrum of an
Chem. Commun., 1999, 1223–1224
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PPh3
CO
CO
Ru
CO
CO
Ru
Ru3(CO)8H(m-H)(PPh3)3 fragment under hydrogen to yield
mononuclear ruthenium complexes under relatively mild condi-
tions. This observation is interesting, given that mixtures of
Ru3(CO)12 and PPh3 have been used in hydroformylation
catalysis and Ru(CO)3(H)2(PPh3) has been implicated as an
intermediate in hydroformylation.5,7 No para-hydrogen en-
hanced products, either tri- or mono-nuclear, were detected on
treating Ru3(CO)12 with p-H2 either in the presence or the
absence of UV irradiation.
Here, we have established some features of the ruthenium
carbonyl/triplenylphosphine/hydrogen system, which shows
some important differences from those with PMe2Ph and PMe3.
Furthermore, we have established the feasibility of using p-H2
to probe photochemical reactions, H2 addition to clusters, and
cluster fragmentations.
Ph3P
OC
CO
Ru PPh3
PPh3
CO
+CO/-PPh3
CO
Ru
Ru
H
CO
Ha
Hb
H
PPh3
Fragmentation
+H2, 20 min
CO
CO
CO
hν,
+H2
CO
Ph3P
1a
2a
CO
Ru(CO)3(PPh3)2
+H2 308 K,
immediate
Isomerisation
Isomerisation
CO
OC
CO
PPh3
CO
PPh3
CO
Ru
H
PPh3
CO
Ru
Ru
PPh3
Ph3P
Ru
H
H
CO
H
Ru
CO
OC
CO
PPh3
H
H
CO
CO
CO
3
1b
2b
Scheme 1
We are grateful to the EPSRC (C. J. S. and spectrometer), the
University of York (J. P. L.), Bruker UK (spectrometer) and the
Royal Society for financial support. We appreciated helpful
discussions with Professor R. N. Perutz, Dr M. K. Whittlesey
and Dr J. M. Lynam. A generous loan of ruthenium trichloride
from Johnson Matthey is also gratefully acknowledged.
perhaps a trigonal twist process similar to that reported for
Ru(CO)(H)2(PPh3)3.9
The interconversions between 2a, 2b and 1a were further
investigated using an authentic sample of 2a. When a benzene-
d6 solution of 2a was heated under 3 atm of p-H2 to 328 K, no
1
signals for 1a were visible in the H NMR spectrum, but the
hydride resonances for 2b were again observed [Fig. (1b)]. An
NMR tube containing a fresh solution of 2a was then placed
under 1 atm of CO, frozen and then filled with 3 atm of p-H2.
When the solution was warmed to 318 K, the enhanced hydride
resonances of 1a and 2b were observed [Fig. 1(c)]. Evidently 2a
readily undergoes replacement of PPh3 by CO. We have
recently adapted an NMR probe to allow a sample to be
irradiated by UV light from a HgXe arc while NMR spectra are
recorded.10 1H NMR spectra recorded during photolysis of a
benzene-d6 solution Ru(CO)3(PPh3)2 under 3 atm of p-H2 at 292
K showed large enhanced hydride signals due to 1a and a small
non-enhanced signal due to 2a. The signals for 1a were not
observed in the absence of UV radiation, confirming that, in this
case, hydrogen exchange between 1a and free H2 is photo-
chemical rather than thermal.
Many phosphine-substituted derivatives of Ru3(CO)12 have
been characterised and studied, but the reactions of these
complexes with hydrogen are less well understood.11 We have
therefore used p-H2 to monitor the reaction of Ru3(CO)9(PPh3)3
with hydrogen. Nevinger et al. used a complex synthetic
procedure to obtain a species believed to be Ru3(CO)11H(m-H),
whose 1H NMR spectrum contained hydride resonances at
d 211.79 and 18.55.12 We recorded the 1H NMR spectrum of
a benzene-d6 solution of Ru3(CO)9(PPh3)3 under p-H2 at 308 K,
and detected enhanced resonances at d 210.20 (dd, JPH 11.2,
JHH 26.0 Hz) and 217.64 (ddd, JPH 21.2,10.4, JHH 26.0 Hz)
for two mutually coupled hydrides in a species 3 [Fig. 1(d)]. The
chemical shifts and couplings to phosphorus suggested that the
former resonance represented a terminal hydride cis to a
phosphine and the latter a hydride bridging two ruthenium
atoms and cis to a phosphine ligand on each ruthenium. A 2D
experiment located the resonances for the corresponding 31P
nuclei at d 44.4 (coupled to both hydrides) and 217.6 (coupled
only to the terminal hydride). From this evidence it appeared
that 3 was the 48-electron cluster Ru3(CO)8H(m-H)(PPh3)3,
with the structure shown in Scheme 1. The third 31P nucleus was
not detected by the 2D experiment because it is not coupled to
either hydride. Complex 3, which is comparable to both
Notes and references
† Selected spectroscopic data for 1a, 1b, 2a, 2b and 3: NMR spectra in C6D6
at 400.13 MHz (1H) and 202.45 MHz (31P) recorded on 5 mm samples in a
5 mm inverse geometry probe. 1a: 1H dH(328 K) 26.67 [Ha, J(PH) 15.9,
J(COH) 27.8, J(HH) 25.4 Hz], 27.34 [Hb, J(PH) 61.6, J(HH) 25.4 Hz].
dP(328 K) 20.3 (s). dC(328 K); 199.5 d [J(PC) 8.4 Hz], 197.6 [d J(PC) 4.6
Hz]. 1b: dH(308 K) 26.68 [J(PH) 23 Hz], dP(328 K) 55.1 (s). 2a: dH(295 K)
26.35 [H, J(PH) 23.2 Hz]. dP(295 K) 57.7 (s). dC(295 K) 202.0 [t, J(PC)
8.3 Hz]. 2b: dH(328 K) 26.27 [Hc, J(PH) 26.4, 19.1, J(HH) 27.1 Hz],
27.52 [H, J(PH) 75.1, 33.0, J(HH) 27.1 Hz]. dP(328 K) 50.2 d [J(PP) 32
Hz], 41.6 [d, J(PP) 32 Hz]. 3: dH(300 K) 210.20 [H, J(PH) 11.2, J(HH 26.0
Hz], 217.64 [H, J(PH) 21.2, 10.4, J(HH) 26.0 Hz]. dP (308 K) 44.4 (s) and
31.2 (s).
1 C. R. Bowers and D. P. Weitekamp, J. Am. Chem. Soc., 1987, 109, 5541;
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We note that although we have no direct evidence that 1a and 1b
interconvert, this is the case for the PMe3, AsMe2Ph and PMe2Ph
analogues of 2.
10 This will be reported fully elsewhere, however, we note that two UV
transmitting liquid light guides were employed in conjunction with a
modified narrow-bore probe that was used in a wide-bore magnet.
11 M. I. Bruce, M. J. Liddell, O. bin Shawkataly, C. A. Hughes, B. W.
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Ru3(CO)11H(m-H) and Os3(CO)10H(m-H)L (L
= PPh3 or
CD3CN),13 is presumably formed by a simple substitution of
CO by H2.
13 S. Aime, R. Gobetto and D. Canet, J. Am. Chem. Soc., 1998, 120, 6770;
A. J. Deeming, Adv. Orgnaomet. Chem., 1986, 26, 1.
After 20 min at 308 K, resonances for isomer 1a of
Ru(CO)3H2(PPh3) had appeared in the 1H NMR spectrum of the
solution. Consequently, either unreacted Ru3(CO)9(PPh3)3 or
Communication 9/03321A
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Chem. Commun., 1999, 1223–1224