Kinetic and mechanistic examination of NBu4[IrH2(CO)2I2] and NBu4[RhH2(CO)2I2] via para-hydrogen enhanced NMR spectroscopy

Article abstract of DOI:10.1039/a900961b

para-Hydrogen enhanced NMR signals are used to show that NBu₄[M(CO)₂I₂](M = Rh, Ir) add hydrogen to form NBu₄-{all-cis-[M(H)₂(CO)₂I₂]} which for M = Ir undergoes H₂ elimination in a step where ΔH≠ 106 ± 10 kJ mol^-1 and ΔS≠ 60 ± 6 J K^-1 mol^-1 while showing a rich substitution chemistry with PPh₃ that leads to both charged and neutral products via square pyramidal Ir(H)₂(CO)₂I.

Full text of DOI:10.1039/a900961b

Kinetic and mechanistic examination of NBu4[IrH2(CO)2I2] and
NBu4[RhH2(CO)2I2] via para-hydrogen enhanced NMR spectroscopy
Sarah K. Hasnip,a Simon B. Duckett,*a Diana R. Taylor,b Graham K. Barlowb and Mike J. Taylorb
a Department of Chemistry,University of York, Heslington, York, UK YO10 5DD. E-mail: sbd3@york.ac.uk
b BP Chemicals, BP Chemicals Limited, Salt End, Hull, UK HU12 8DS
Received (in Cambridge, UK) 4th February 1999, Accepted 12th April 1999
para-Hydrogen enhanced NMR signals are used to show that
NBu4[M(CO)2I2] (M = Rh, Ir) add hydrogen to form NBu4-
{all-cis-[M(H)2(CO)2I2]} which for M = Ir undergoes H2
elimination in a step where DH‡ 106 10 kJ mol21 and DS‡
[Ir(CO)2I2]} should only yield 3a. We therefore investigated the
dynamic behaviour of 3a using gradient-assisted EXSY spec-
troscopy. This revealed that at 323 K 3a undergoes simple
reductive elimination to form free H2, which at temperatures
beyond 350 K is complicated by both intermolecular hydride
interchange and intramolecular exchange into 3b. Under these
conditions, addition of an excess of free NBu4I, such that the
solution is saturated, results in a 5% increase in the rate of H2
loss and suppression of the intermolecular and intramolecular
exchange peaks. In view of this we can state that H2 loss occurs
from 3a rather than the 16-electron complex [Ir(H)2(CO)2I].
Analysis of the exchange peak intensity as a function of mixing
time allowed the rate of H2 elimination from 3a to be
determined; at 325 K this corresponds to 0.17 s21. Monitoring
this process as a function of reaction temperature enabled the
activation parameters DH‡ 106 ± 10 kJ mol21 and DS‡ 60 ± 6
J K21mol21 to be calculated.
60
6 J K21 mol21 while showing a rich substitution
chemistry with PPh3 that leads to both charged and neutral
products via square pyramidal Ir(H)2(CO)2I.
When transition metal complexes are reacted with para-
hydrogen (p-H2) the size of the detectable NMR signatures of
many hydride reaction products are dramatically enhanced
owing to the generation of non-Boltzmann spin populations.1–4
This phenomenon provides sufficient sensitivity to facilitate the
observation of intermediates in catalytic reactions, species in
minor reaction pathways such as RhH2(PMe3)2I2Rh(P-
Me3)(CO) and minor constituents in equilibria, for example all-
cis-[Ru(PMe3)2(CO)2(H)2].5 Here we examine the reactivity of
NBu4[M(CO)2I2] (M = Ir, Rh) towards hydrogen. While these
complexes are known to react with MeI to form NBu4[MI3-
Me(CO)2], and play a substantial role in acetic acid generation,
their reactivity towards hydrogen, a substrate also present in this
process, is far less well understood.6 We show that two isomers
of NBu4[IrH2(CO)2I2] are formed upon H2 addition to NBu4[Ir-
(CO)2I2], and report for the first time kinetic and thermody-
namic parameters for H2 elimination from a p-H2 enhanced
dihydride. The lability of the ligand sphere of the all-cis isomer
is further explored by employing PPh3 as a vacant coordination
site scavenger.
Importantly, when a 0.1 mM solution of the rhodium
analogue 2 is warmed with p-H2 to 350 K and monitored by 1H
NMR spectroscopy [Fig 1(b)] two rhodium coupled hydride
resonances are observed at d 29.75 (JRhH 17.3, JHH 24 Hz) and
214.15 (JRhH 16.5 Hz) which are consistent with the formation
of
the
previously
unseen
species
NBu4{all-cis-
[RhH2(CO)2(I)2]}. Clearly the extent to which NBu4{all-
When a 0.1 mM solution of NBu4[Ir(CO)2I2] 1 in benzene-d6
is monitored by 1H NMR spectroscopy at 295 K while under 3
atm of p-H2 substantial signals arise from a previously
undetected complex at d 29.13 and 213.30 [Fig. 1(a)].† These
resonances are assigned to the hydride ligands Ha and Hb of 3a,
and their chemical shifts indicate they are trans to carbonyl and
iodide respectively.† The two carbonyl resonances of 3a were
detected at d 167.2 and 160.9 by a 1H–13C HMQC experiment,
and when a sample containing 1 and p-H2 was monitored with
an excess of NBu4I present the product signal intensities were
unchanged. This information suggests that 3a corresponds to
the all-cis isomer of NBu4[Ir(H)2(CO)2I2] 3a as shown in
eqn. (1).
–
–
–
–
CO
Ir
I
–
CO
CO
Ir
H
I
H
H
Ha
Ir Hb
H
+
I
(1)
+
+
I
I
OC Ir
I
I
H
I
Ir CO
OC
OC
I
OC
CO
3c
I
I
CO
3b
3a
3d
Weak hydride resonances attributable to species 3b and 3c of
eqn. (1) were also observed in the 1H NMR spectra.7
Resonances attributable to 3a are also observable in methanol-
d4 and acetic acid-d4. We further note, that 3a is only visible
with p-H2, and that isomer 3d is not seen even when 13CO
labelled 1 is employed. Additionally, when the sample is left
under H2 at room temperature, the resonance for 3c disappears
leaving 3b as the only detectable hydride containing species.
The mechanism of generation of these species requires
further comment since concerted H2 addition to NBu4{cis-
Fig. 1 (a) 1H NMR spectrum (400 MHz, 295 K) of a 0.1 mM solution of 1
in benzene-d6 under 3 atm of p-H2. The antiphase resonances arise from the
parahydrogen enhanced hydride resonances in the all-cis isomer of
NBu4[Ir(H)2(CO)2I2], 3a. (b) 1H NMR spectrum (400 MHz, 350 K) of a 0.1
mM solution of 2 in benzene-d6 under 3 atm of p-H2 showing resonances
due to the all-cis isomer of [Rh(H)2I2(CO)2]. (c) H NMR spectrum (400
MHz, 295 K) of a 0.1 mM solution of 1 in benzene-d6 under 3 atm of p-H2,
in the presence of 0.1 mM of PPh3 with resonance assignments indicated.
1
Chem. Commun., 1999, 889–890
889

cis[RhH2(CO)2(I)2]} forms is much lower than that of 3a, and
the temperatures required to observe it are at least 60 K
higher.
trans to phosphine trans to iodide in the product, 4b. While both
phosphine and CO loss from 4c are possible in the first step, the
data suggests CO loss is more likely since different inter-
mediates to those shown in Scheme 1 are required to account for
the dramatically different hydride exchange preferences seen in
the two interconversion processes. This second reaction there-
fore most likely involves square pyramidal Ir(H)2(I)-
(CO)(PPh3).
Here, we have shown that p-H2 derived spectral amplification
can be used to examine the hydrogen addition chemistry of both
NBu4[IrI2(CO)2] and NBu4[RhI2(CO)2]. Products not pre-
viously observed have been characterised, and for the first time
activation parameters have been determined for H2 elimination
from a p-H2 enhanced product. Additionally, a new series of
iridium dihydrides containing phosphine and carbonyl ligands
have been characterised, and the intermediates, IrH2(I)(CO)2
and Ir(H)2(I)(CO)(PPh3), involved in their formation shown to
be square pyramidal.
The observation of intramolecular and intermolecular ex-
change pathways indicated that the substitution reactivity of 3a
was worthy of examination. We therefore monitored benzene-
d6 solutions containing 1, p-H2 and PPh3 ( < 2 equivalents). A
typical 1H spectrum is shown in [Fig. 1(c)]. The new
mononuclear products, identified by 1H–1H COSY, 1H–31P and
1H–13C HMQC techniques, correspond to four isomeric forms
of IrH2(CO)2(PPh3)I, three isomers of NBu4[IrH2(CO)(P-
Ph3)I2], and IrH2(CO)(PPh3)2I with trans phosphines are shown
below.† Surprisingly, when NBu4[Rh(CO)2I2] 2, p-H2 and PPh3
were examined at 350 K no new dihydride products were
detected.
PPh3
PPh3
CO
I
H
H
H
H
H
H
H
H
O
13C Ir
OC Ir
Ph3P Ir
Ph3P Ir
OC
O13
C
I
I
Financial support from the EPSRC (Spectrometer and
S.K.H.), BP Chemicals (CASE award S.K.H.), the Royal
Society, NATO and Bruker UK, and discussions with Dr C.
Sleigh, Professor R. Eisenberg, Profesor R. N. Perutz, Dr P.
Dyson and Dr R. J. Mawby are gratefully acknowledged.
CO
CO
4b
CO
I
4c
4a
4d
–
–
PPh3
–
PPh3
Ir
I
CO
H
H
H
H
H
H
H
OC Ir
I
H
Ph3P Ir
I
Ph3P Ir
OC
OC
I
PPh3
I
I
I
Notes and references
5c
5a
6
5b
† Selected spectroscopic data in C6D6 at 295 K unless otherwise indicated
with 400.13 MHz (1H), 161.45 MHz (31P) and 100.2 MHz (13C). 3a: 1H d
29.13 [Ha, J(HH) 24.4, J(H13CO) 58.4, 6.5 Hz], d 213.30 [Hb, J(HH)
24.5, J(H13CO) 4.5 Hz], 13C d 167.2 (COa), 160.9 [COb, J(CC) 11.8 Hz].
3b: 1H d 215.40 [J(H13CO) 5.6 Hz}], 13C d 171.6 (CO). 3c: 1H d 210.94
When samples containing 1, PPh3 and H2 were monitored by
EXSY spectroscopy at 295 K the signals observed for each
hydride of 3a connected to both hydride resonances of 4b via
cross peaks arising from chemical exchange. Examination of
the intensity of these peaks indicated that interconversion of 3a
to 4b places 3a-Ha trans to phosphine in 4b more often than
trans to iodide; this process is suppressed upon addition of
NBu4I. As expected, the reverse situation is true when cross
peaks from Hb-3a are considered and the difference in intensity
between the corresponding hydride cross peaks falls as the
temperature rises. This information suggests that 4b forms from
3a via a process involving iodide loss to yield an intermediate
with inequivalent hydrides which undergoes rearrangement to
form the square pyramidal intermediate IrH2(I)(CO)2 with trans
carbonyls. Coordination of phosphine then generates 4b, or
iodide 3b, as shown in Scheme 1. Furthermore, while the two
hydride ligands of IrH2(I)(CO)2 are inequivalent they must be
able to interchange their positions on the same time scale as
phosphine coordination. Significantly, weaker cross peaks,
connect the hydride resonances of 4b to 3a which indicates that
they are in equilibrium.
1
[J(H13CO) 2.9 Hz], 13C d 155.1 (CO). 4a: H d 7.73 (o-phenyl H of P),
29.63 [H, J(PH) 17.0, J(13COH) 44.9, 5.6, J(HH) 24.3 Hz], 214.12 [H,
J(PH) 16.4, J(13COH) 4.5, J(HH) 24.3 Hz], 31P d 0.2 (P, s), 13C d 164.3
[COa, J(PC) 118 Hz], 167.5 (COb). 4b: 1H d 210.65 [H, J(PH) 115.6,
J(13COH) 4.0, J(HH) 24.7 Hz], 214.37 [H, J(PH) 8.8, J(13COH) 4.0,
J(HH) 24.7 Hz], 31P d 210.9 (P, s), 13C d 168.4 [CO, J(PC) 9 Hz]. 4c: (T
= 350 K) 1H d 29.62 [H, J(PH) 14.7, J(13COH) 50.7, 3, J(HH) 23.5 Hz],
210.65 [H, J(PH) 151.1, J(HH) 23.5 Hz}, 31P d 216.3 (P, s), 13C d 164.3
[CO, J(PC) 118 Hz]. 4d: (T = 350 K) 1H d 210.00 [H, J(PH) 17.1,
J(13COH) 45, 12 J(HH) 25.3 Hz], 31P d 3.7 (P, s), 13C d 166.1 [CO, J(PC)
18.8 Hz]. 5a 1H d 28.13 [H, J(PH) 20.9, J(HH) 24.6 Hz], 216.45 [H,
J(PH) 12.6, J(HH) 24.6 Hz], 31P d 218.1 (P, s). 5b 1H d 210.20 [H, J(PH)
170.5, J(HH) 25.6 Hz], 214.80 [H, J(PH) 8.5, J(HH) 25.6 Hz], 31P 214.5
(P, s). 5c 1H d 29.89 [H, J(PH) 17.3, J(HH) 21.8 Hz], 210.60 [H, J(PH)
122.1, J(HH) 22.6 Hz], 31P d 3.00 (P, s). 6: 1H d 8.02 (o-phenyl H of P),
28.49 {H, J(PH) 17.0, J(
13COH) 42.6, J(HH) 24.4 Hz], d 214.95 [H,
J(PH) 13.8, J(13COH) 4, J(HH) 24.4 Hz], 31P d 6.1 (P, s), 13C d 175.4 [CO,
J(PC) 8 Hz].
1 C. R. Bowers and D. P. Weitekamp. J. Am. Chem. Soc., 1987, 109,
5541.
2 R. Eisenberg. Acc. Chem. Res., 1991, 24, 110.
3 J. N. Atterer and J. Bargon. Prog. Nucl. Magn. Reson. Spectrosc., 1997,
31, 293.
The only other exchange peaks visible in the hydride region
of the spectrum at 350 K connect the hydride resonances of 4c
to those of 4b. This interconversion process occurs with a
twofold preference for placing the hydride ligand of 4c that was
4 C. J. Sleigh and S. B. Duckett. Prog. Nucl. Magn. Reson. Spectrosc.,
1999, 34, 71.
–
CO
Ir
CO
Ir
isomerisation
+I
H
H
H
H
H
H
5 S. B. Duckett, C. L. Newell and R. Eisenberg. J. Am. Chem. Soc., 1994,
116, 10548; S. B. Duckett and R. Eisenberg. J. Am. Chem. Soc., 1993,
115, 5292; S. B. Duckett, R. J. Mawby and M. G. Partridge. Chem.
Commun., 1996, 383; P. D. Morran, S. A. Colebrooke, S. B. Duckett,
J. A. B. Lohmann and R. Eisenberg. J. Chem. Soc., Dalton. Trans., 1998,
3363.
6 P. M. Maitlis, A. Haynes, G. J. Sunley and M. J. Howard, J. Chem. Soc.,
Dalton Trans., 1996, 2187.
7 When 13CO labelled 1 is used both these resonances are split into a triplet.
The hydride resonance attributable to 3c is observed when HI is added to
1.
I
OC Ir
I
I
I
CO
CO
CO
3b
–PPh3 +PPh3
–I
+I
–
CO
H
I
H
H
Ph3P Ir
I
H
OC Ir
I
CO
4b
CO
3a
Scheme 1
Communication 9/00961B
890
Chem. Commun., 1999, 889–890