Activation of H2 by halocarbonyl bis-phosphine and bis-arsine iridium(I) complexes. The use of parahydrogen induced polarisation to detect species present at low concentration and investigate their reactivity

Article abstract of DOI:10.1039/b107444j

The iridium phosphine complexes Ir(CO)Cl(L)₂ [L = PPh₃, PMe₃, AsPh₃ and PPh₂Cl, and L₂ = (PPh₂Cl)(PPh₃)] add H₂ to form the corresponding dihydrides IrH₂(CO)Cl(L)₂. These products are detected at enhanced levels of sensitivity through the ¹H NMR signatures of their hydride resonances via para-hydrogen (p-H₂) based spin state synthesis. Products corresponding to addition across both the Cl-Ir-CO and L-Ir-L axes are detected. For L = PPh₃, there is a 100 fold preference for the former pathway at 295 K, while for L = AsPh₃ the second product is favoured by a factor of 2.85. At elevated temperatures a third product corresponding to addition over the Cl-Ir-L axis is detected for L = AsPh₃ and PPh₂Cl. Under these conditions, the CO and HCl transfer products Ir(H)₃(CO)₂(AsPh₃), and IrH(CO)Cl₂(AsPh₃)₂ are also formed in a thermal reaction. When IrH₂(CO)Cl(L)₂ is warmed or photolysed with H₂ and CO, the corresponding products are produced for L = PPh₃ and PMe₃. However after photolysis with H₂ alone Ir(H)₃(CO)(L)₂ is the favoured product. Additional products detected during the photochemical studies include Ir(H)₂(PPh)₃(PPh₂C₅H₄ CO), an unusual orthometallation product containing an η²-acyl ligand, and the binuclear products H(Cl)Ir(PMe₃)₂(μ-H)(μ-Cl)Ir(PMe₃)(CO) and (H)₂Ir(PMe₃)₂(μ-Cl)₂Ir(PMe) ₃(CO).

Full text of DOI:10.1039/b107444j

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Activation of H2 by halocarbonyl bis-phosphine and bis-arsine
iridium(I) complexes. The use of parahydrogen induced polarisation
to detect species present at low concentration and investigate their
reactivity†
Sarah K. Hasnip,a Simon A. Colebrooke,a Christopher J. Sleigh,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
b BP Chemicals, BP Chemicals Limited, Salt End, Hull, UK HU12 8DS
Received 16th August 2001, Accepted 15th October 2001
First published as an Advance Article on the web 29th January 2002
The iridium phosphine complexes Ir(CO)Cl(L)2 [L = PPh3, PMe3, AsPh3 and PPh2Cl, and L2 = (PPh2Cl)(PPh3)]
add H2 to form the corresponding dihydrides IrH2(CO)Cl(L)2. These products are detected at enhanced levels of
sensitivity through the 1H NMR signatures of their hydride resonances via para-hydrogen (p-H2) based spin state
synthesis. Products corresponding to addition across both the Cl–Ir–CO and L–Ir–L axes are detected. For L = PPh3,
there is a 100 fold preference for the former pathway at 295 K, while for L = AsPh3 the second product is favoured by
a factor of 2.85. At elevated temperatures a third product corresponding to addition over the Cl–Ir–L axis is detected
for L = AsPh3 and PPh2Cl. Under these conditions, the CO and HCl transfer products Ir(H)3(CO)2(AsPh3), and
IrH(CO)Cl2(AsPh3)2 are also formed in a thermal reaction. When IrH2(CO)Cl(L)2 is warmed or photolysed with H2
and CO, the corresponding products are produced for L = PPh3 and PMe3. However after photolysis with H2 alone
Ir(H)3(CO)(L)2 is the favoured product. Additional products detected during the photochemical studies include
Ir(H)2(PPh3)(PPh2C5H4CO), an unusual orthometallation product containing an η2-acyl ligand, and the binuclear
products H(Cl)Ir(PMe3)2(µ-H)(µ-Cl)Ir(PMe3)(CO) and (H)2Ir(PMe3)2(µ-Cl)2Ir(PMe3)(CO).
possible isomers, cis,trans-IrH2Cl(CO)(PPh3)2 and cis,cis-IrH2-
Cl(CO)(PPh3)2, might result from this process, observations
suggest that stereospecific addition across the Cl–Ir–CO axis
Introduction
The examination of the oxidative addition of H2 to d8 square-
planar rhodium and iridium complexes has provided a number
leads to the former species. Many analogues of Vaska’s com-
of key observations that have been essential to the development
plex have been synthesised in order to examine how the ligands
of organo-transition metal catalysis. trans-Ir(CO)Cl(PPh3)2, or
affect this selectivity. Studies by Crabtree and co-workers
Vaska’s complex1 has played a particularly important role in
this process via studies of stoichiometric reactions with species
demonstrated that when the Cl ligand of Vaska’s complex is
replaced by an alkyl group10 addition at low temperature occurs
such as H2, O2, SO2, CO, HCl, CS2 and CH3I. These additions
across the P–Ir–P axis. In the case of IrPh(CO)(PMe3)2 the
can generally be categorised as either coordinative addition or
corresponding low-temperature product isomerises at 25 ЊC
to the trans isomer. Consequently, it can be deduced that when
this reaction proceeds under kinetic control addition across the
P–Ir–P axis results, with the thermodynamic product, corre-
sponding to addition over the Ph–Ir–CO axis, being formed by
subsequent isomerisation.
This trend has also been observed experimentally in a
number of related systems. For example, Anton and Crabtree’s
oxidative addition.2–4 For instance, the observation that trans-
Ir(CO)Cl(PPh3)2 acts as a reversible oxygen carrier,5 led to a
crystal structure of the oxygenated complex Ir(CO)Cl(O2)-
6
(PPh3)2 that revealed an O–O distance of 130 pm. Con-
sequently this complex can be regarded as an example of either
an Ir() peroxide or an Ir() O2 adduct. The binding of H2 to a
transition metal centre provides another example of where the
distinction of product type is blurred. In this case, dihydride
synthesis of [(dct)Ir(PPh3)2]BF4, where dct = dibenzo[a,e]-
and dihydrogen complexes with intact H–H bonds, mark the
cyclooctatetraene, enabled them to show that H2 addition
boundaries.7 Many workers have studied this phenomenon,
led to an all-cis kinetic product11 which rearranged via a base
with Jessop and Morris8 defining dihydrogen complexes as
catalysed deprotonation to the thermodynamically more stable
having the following characteristics: atomic distance, d(H–H) =
cis, trans-phosphine isomer. Additionally, Eisenberg and co-
0.8–1.0 Å, IR stretches ν(H–H) = 3100–2400 cmϪ1, ν(M–H2) =
workers investigated a series of complexes12 containing the
850–950 cmϪ1 and relaxation delay, T 1 (200 MHz) 5–22 ms.
chelating ligand dppe (dppe = 1,2-bis(diphenylphosphino)-
ethane) that are constrained to have cis phosphine ligands. Elec-
tronic factors were demonstrated to control the H2 addition
pathway, with the cis,cis-isomer containing a hydride trans to
CO forming initially with >99% selectivity. However, over a
period of hours this isomer rearranged, via reversible H2 add-
ition, to place the hydride trans to chloride. In other words,
whilst addition across the P–Ir–CO axis is kinetically preferred,
There are, however, still some complexes, such as [RhH2(pp3)]ϩ
(where pp3 = tetraphos-2, P(CH2CH2PPh2)3),9 whose character-
istics lie on the edge of these boundaries.
As a result of early work by Vaska and others, the generally
accepted mechanism of H2 addition to square-planar Ir(CO)-
Cl(PPh3)2 proceeds via a concerted route and leads to the form-
ation of an octahedral product with cis hydrides. Whilst two
addition the P–Ir–Cl axis leads to the more thermodynamically
stable product.
† Based on the presentation given at Dalton Discussion No. 4, 10–13th
January 2002, Kloster Banz, Germany.
DOI: 10.1039/b107444j
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This journal is © The Royal Society of Chemistry 2002
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Sargent, Hall and Guest investigated the oxidative addition
of H2 to IrCl(CO)(dppe) by ab initio MO techniques13 using
two cis PH3 groups to mimic the dppe ligand. They found that
while addition over the P–Ir–Cl axis led to the formation of the
more stable isomer, the more stable transition state involved
reaction across the P–Ir–CO axis. They too concluded that
strong σ- and π-donor ligands destabilise the associated five-
coordinate transition state by increasing the electronic repul-
sion between the ligands and the metal.12 In contrast, electron-
withdrawing ligands stabilise the transition state by reducing
this repulsion. Similar model calculations for Vaska’s com-
plex14 predicted that addition across the P–Ir–P axis proceeded
via the more stable transition state for PH3. However, upon
improving the accuracy of the simulation by utilisation of
PMe3, the relative stabilities of the transition states were
reversed. Sargent and Hall therefore predicted that complexes
of the type IrCl(CO){P(OR)3}2 might give H2 addition across
the P–Ir–P axis due to the π-accepting ability of the phos-
phite ligand. However, when Crabtree, Hall and co-workers
tested this prediction15 addition across the Cl–Ir–CO axis was
still observed.
It has been shown that the addition of H2 enriched in the
para spin state to a transition metal centre leads to greatly
enhanced hydride signals in associated 1H NMR spectra.16 This
phenomenon has resulted in the development of a powerful
tool for the characterisation of minor reaction products such as
17
all-cis Ru(H)2(CO)2(PMe3)2 and the investigation of hydro-
genation products and kinetics.18 Here we describe how para-
hydrogen has been used to probe the reactions of Vaska’s
complex and a number of analogues by NMR spectroscopy.
In particular we report on the observation of cis,cis-Ir(H)2-
(CO)Cl(L)2 (L = PPh3, PMe3, AsPh3, PPh2Cl) and demon-
strate that at elevated temperatures Ir(H)2(CO)Cl(L)2 reacts to
form a series of iridium trihydride complexes such as IrH3-
(CO)2L via an HCl transfer process that is promoted by CO or
UV irradiation. Complementary formation of the correspond-
ing monohydride-dichloride complex Ir(H)(CO)(Cl)2(L)2 is also
observed. Some of this work has been communicated.19
Fig. 1 (a) 1H NMR spectrum (400 MHz, 295 K) of a 0.1 mM solution
of 1-[Cl-Ph] in benzene-d6 under 3 atm of p-H2. The weak second
order resonance arises from cis,cis-IrH2(CO)Cl(PPh3)2 3-[Cl-Ph]. (b)
1H NMR spectrum (400 MHz, 333 K) of a 0.1 mM solution of
1-[Cl-Me] in benzene-d6 under 3 atm of p-H2 with resonances due to
2-[Cl-Me] and 3-[Cl-Me] indicated. (c) 1H NMR spectrum (400 MHz,
338 K) of a 0.1 mM solution of 1-[Cl-As] in benzene-d6 under 3 atm
of p-H2 showing resonances due to 2-[Cl-As], 3-[Cl-As] and 8-[Cl-As].
Results and discussion
Oxidative addition of H2 to Ir(CO)Cl(PPh3)2, 1-[Cl-Ph]
1
It has been reported previously that when a 0.1 mM solution
containing Ir(CO)Cl(PPh3)2, 1-[Cl-Ph], in toluene-d8 under
3 atm p-H2 is examined by 1H NMR spectroscopy at 343 K the
hydrogen addition product IrH2(CO)Cl(PPh3)2, 2-[Cl-Ph], is
produced and the associated hydride resonances at δ Ϫ7.02 and
Ϫ17.6 are parahydrogen enhanced.20 When this system is sub-
sequently cooled to 295 K the hydride resonances no longer
show parahydrogen-based enhancement and it can be con-
cluded that hydrogen exchange at 295 K is very slow. In con-
trast, when H2 addition to a 0.1 mM solution of 1-[Cl-Ph] in
benzene-d6 under 3 atm p-H2 is observed directly by NMR spec-
troscopy at 295 K the associated reaction products are observed
as they form and prior to nuclear spin relaxation. Conse-
quently, the resulting 1H NMR spectrum again contains
enhanced hydride resonances that are due to 2-[Cl-Ph]. How-
ever, under these conditions a somewhat unexpected enhanced
hydride resonance at δ Ϫ8.10 is also observed with substantially
lower signal intensity. The appearance of the hydride resonance
of this second product, 3-[Cl-Ph], (Fig. 1(a)) indicates that it
arises from a pair of chemically equivalent protons that are
magnetically distinct and belong to [AX]2 spin system. This
situation matches that which would exist in the isomer of H2
(d) H NMR spectrum (400 MHz, 333 K) of a 0.1 mM solution of
1-[Cl-Ph] in benzene-d6 in the presence of a four-fold excess of PPh2Cl
under 3 atm of p-H2 showing resonances due to 2-[Cl-Ph], 2-[Cl-Ph-Cl],
2-[Cl-Cl] and 3-[Cl-Ph-Cl].
be noted at this point that the complexes described in this paper
are referred to according to the designation X-[Y-Z] where X is
the compound number, Y indicates whether the H2 addition
product contains additional chloride or hydride, and Z refers
to the substituent on L, except for L = AsPh3 where Z = As.
These complexes are illustrated in Scheme 1 and the reaction is
illustrated in Scheme 2.
In an attempt to quantify the ratios of these two products an
NMR experiment was constructed that allowed the products of
p-H2 addition to be observed only once. This constraint was
achieved by applying a 90Њ 1H pulse to the sample followed by a
pulsed field gradient to de-phase the initially coherent spin
1
state. After a 100 ms delay a 45Њ H read pulse was applied to
enable the detection of any p-H2 enhanced product formed dur-
ing the 100 ms reaction time. The ratio of the hydride signal
intensities for the two products under these conditions was
50 : 1. If similar enhancement levels are assumed for these two
products, the ratio of 2-[Cl-Ph] : 3-[Cl-Ph] is 100 : 1 at 295 K.
This equates to a difference in free energies of activation
between the two H2 addition pathways of 11 kJ molϪ1 at 295 K.
We have already described how Sargent and Hall have investi-
gated the oxidative addition of H2 to Vaska’s complex with ab
addition that is generated by reaction across the P–Ir–P axis. 31
P
and 13C chemical shift information was determined for the CO
and phosphine ligands of this product via suitable p-H2
enhanced 2D NMR experiments.20b These data confirm that
3-[Cl-Ph] corresponds to cis,cis-IrH2(CO)Cl(PPh3)2. It should
744
J. Chem. Soc., Dalton Trans., 2002, 743–751

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Scheme 1 Product identities.
Scheme 2 Reaction products detected by NMR spectroscopy when
1-[Cl-Ph], 1-[Cl-Me], 1-[Cl-As], 1-[Cl-Ph-Cl] and 1-[Cl-Cl] are warmed
with p-H2.
initio MO techniques.21 They found that for PH3 addition across
the P–Ir–P axis proceeded via the more stable transition state
whilst for PMe3 this situation was reversed. In absolute terms,
PH3 addition across the P–Ir–P axis was favoured by 37.21 kJ
molϪ1, while for PMe3, addition across the CO–Ir–Cl axis was
favoured by 9.51 kJ molϪ1. Consequently the value of 11 kJ
molϪ1 estimated here for favouring addition over the CO–Ir–Cl
axis of 1-[Cl-Ph] at 295 K would seem to be reasonable. In
addition, we used a series of 1D EXSY NMR spectra to dem-
onstrate that neither of these species interconvert over a 1-s
reaction window. It can therefore be stated that 3-[Cl-Ph]
corresponds to a minor addition product which is not observed
at 333 K. NMR data for 2-[Cl-Ph] and 3-[Cl-Ph] is presented
in Table 1.
Exchange of H2 with IrH2(CO)Cl(PPh3)2, 2-[Cl-Ph], in the
presence of CO
In order to monitor the reaction of 2-[Cl-Ph] with p-H2 in the
presence of CO an NMR sample of 13C labelled 1-[Cl-Ph-13CO]
was first converted to 2-[Cl-Ph-13CO] by reaction with H2 in
benzene-d6. The sample of 2-[Cl-Ph-13CO] prepared in this way
was then placed under 300 mm Hg of 13CO, the solution frozen,
and the headspace of the NMR tube filled with 3 atm of p-H2.
The sample was then thawed and the subsequent reaction
observed by multinuclear NMR spectroscopy at 313 K. The
Fig. 2 (a) 1H NMR spectrum (400 MHz, 333 K) of a 0.1 mM solution
of 1-[Cl-Ph] in benzene-d6 under CO and 3 atm of p-H2 recorded after
5 minutes UV irradiation. Resonances for the fac-cis isomer of 4-[H-Ph]
are clearly seen. (b) 1H NMR spectrum (400 MHz, 338 K) of a 0.1 mM
solution of 1-[Cl-Ph] in benzene-d6 under 3 atm of p-H2 recorded after
5 minutes of UV irradiation. Resonances for the mer-trans and fac-cis
isomers of 7-[H-Ph] and the fac-cis isomer is 4-[H-Ph] is revealed.
1
(c) H NMR spectrum (400 MHz, 295 K) of the same sample used
to record the spectrum shown in part (a). (d) 1H NMR spectrum
(400 MHz, 300 K) of a 0.1 mM solution of 1-[Cl-Me] in benzene-d6
under 3 atm of p-H2 after 5 minutes UV irradiation showing resonances
due to H(Cl)Ir(PMe3)2(µ-H)(µ-Cl)Ir(PMe3)(CO), and (H)2Ir(PMe3)2-
(µ-Cl)2Ir(PMe3)(CO) (structures inset).
1
corresponding H NMR spectrum contained evidence for 2-
[Cl-Ph-13CO], and a number of new hydride containing species
(Fig. 2(a)) The most significant of these extra species yielded
a pair of hydride signals at δ Ϫ9.88 and Ϫ10.52 with overall
intensity 2 : 1, respectively. The observation of hydride–hydride
splittings for these two peaks of Ϫ2.4 and Ϫ4.8 Hz, respec-
tively, suggested that these signals arise from a trihydride
complex. This conclusion can be drawn on the basis that when
trihydride complexes are examined with the p-H2 the central
feature of the triplet disappears due to cancellation of emission
and absorption contributions and results in the apparent obser-
vation of two different couplings.22 The identification of this
product is made relatively easy by the observation that these
hydride signals are split by a single 31P nucleus, consequently
the product must correspond to IrH3(CO)2(PPh3) 4-[H-Ph-
13CO]. Furthermore, since these protons couple to a single 13CO
environment that resonates at δ 172.2, and the hydride ligand
providing the δ Ϫ10.48 signal is trans to phosphine, a facial
ligand arrangement can be deduced. This information matches
that previously reported for these complexes.23
The formation of fac-4-[H-Ph] requires both the replacement
of a phosphine ligand in 2-[Cl-Ph] by CO, and a chloride ligand
by hydride. Notable features in these spectra also include the
J. Chem. Soc., Dalton Trans., 2002, 743–751
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Table 1 Selected NMR data for complexes 2–9 in benzene-d6 unless specified
Complex, temp.
Nucleus δ (multiplicity)
Assignment
Coupling constant/Hz, assignment
2-[Cl-Ph], 295 K
1H
7.92 (m)
Ϫ6.69 (td)
Ϫ17.48 (td)
9.9
177.8 (t)
1.40 (m)
Ϫ7.86 (td)
Ϫ18.98 (td)
Ϫ41.1
177.6 (t)
7.93 (m)
7.01 (m)
Ϫ7.11 (d)
Ϫ18.91 (d)
177.2 (s)
7.85 (m)
Ϫ6.81 (ddd)
Ϫ16.99 (td)
9.0 (d)
PPh3 (o-H)
Ir–H
Ir–H
PPh3
CO
CH3
Ir–H
Ir–H
PMe3
CO
AsPh3 (o-H)
AsPh3 (m,p-H)
Ir–H
Ir–H
CO
PPh3 (o-H)
Ir–H
Ir–H
PPh3
PPh2Cl
CO
Ϫ4.6 2JHH, 17.4 |2JPH|, 43.7 |2JCH
Ϫ4.6 2JHH, 17.4 |2JPH|, 4.1 |2JCH
15 |2JCH
5 |2JCP
|
|
31P]
13C
1H
|
|
2-[Cl-Me], 295 Ka
2-[Cl-As], 295 K
7.3 |2JPH ϩ 4JPH
|
Ϫ5.2 2JHH, 20.8 |2JPH
Ϫ5.2 2JHH, 14.7 |2JPH
|
|
31P
13C
1H
14.6 |2JPC
14.6 |2JCP
|
|
Ϫ5.0 2JHH, 44.5 |2JCH
Ϫ5.0 2JHH, 3.7 |2JCH
|
|
13C
1H
2-[Cl-Ph-Cl], 295 K
Ϫ4.7 2JHH, 17.3 |2JPH|, 19.7 |2JPH|, 44.0 |2JCH
|
Ϫ4.7 2JHH, 17.3 |2JPH|, 17.3 |2JPH|, 4.4 |2JCH
|
31P
398.6 |2JPP
398.6 |2JPP
|
|
66.2 (d)
13C
1H
176.3 (t)
Ϫ7.24 (td)
Ϫ16.73 (td)
65.1
174.8 (t)
Ϫ8.10 (2nd order) Ir–H
Ϫ5.9
167.1
1.23
Ϫ8.00 (2nd order) Ir–H
Ϫ51.0
173.7
7.52 (m)
6.93 (m)
Ϫ9.62 (s)
165.7 (s)
Ϫ8.13 (ddd)
Ϫ8.34 (ddd)
Ϫ4.0 (d)
65.2 (d)
165.3
Ϫ8.37
62.9 (s)
Ϫ8.8 (dd)
Ϫ9.7 (dd)
25.4 (s)
Ϫ9.88 (dd)
Ϫ10.52 (dd)
3.29 (s)
6 |2JPC
|
2-[Cl-Cl], 333 K
Ir–H
Ir–H
PPh2Cl
CO
Ϫ5.0 2JHH, 20.3 |2JPH|, 44 |2JCH
Ϫ5.0 2JHH, 15.0 |2JPH|, 3.5 |2JCH
|
|
31P
13C
1H
31P
13C
1H
7 |2JCP
|
3-[Cl-Ph], 295 K
3-[Cl-Me], 295 K
179 |2JPH trans ϩ 2JPH cis
|
PPh3
CO
PMe3
Obscured
31P
13C
1H
PMe3
CO
AsPh3 (o-H)
AsPh3 (m,p-H)
Ir–H
CO
Ir–H
Ir–H
PPh3
PPh2Cl
CO
Ir–H
PPh2Cl
Ir–H
Ir–H
PPh3
Ir–H
Ir–H
PPh3
CO
Ir–H
Ir–H
PMe3
Ir–H
Ir–H
PMe3
CO
AsPh3 (o-H)
Ir–H
Ir–H
CO
Ir–H
PPh3
CO
Ir–H
PPh3
CO
14.6 |2JPC
|
3-[Cl-As], 295 K
6.4 |2JCH
Ϫ1.8 2JHH, 15.0 |2JPH|, 224.7 |2JPH|, 6.5 |2JCH
|
|
13C
1H
3-[Cl-Ph-Cl], 333 K
Ϫ1.8 2JHH, 160.8 |2JPH|, 15.9 |2JPH|, 6.7 |2JCH
|
31P
28.5 |2JPP
28.5 |2JPP
|
|
13C
1H
31P
1H
6 |2JPC
|
3-[Cl-Cl], 333 K
mer-trans-4-[H-Ph], 333 K
5.2 2JHH, 136.9 |2JPH
5.2 2JHH, 14.8 |2JPH
|
|
31P
1H
fac-cis-4-[H-Ph], 333 K
Ϫ2.4 2JHH, 17.2 |2JPH|, 39.5, 5.8 |2JCH
Ϫ2.4 2JHH, 121.6 |2JPH|, 4 |2JCH
|
|
31P
13C
1H
172.2 (s)
mer-cis-4-[H-Me], 333 K
fac-cis-4-[H-Me], 333 K
Ϫ10.06 (dd)
Ϫ10.97 (dd)
Ϫ50.7 (s)
Ϫ10.60 (dd)
Ϫ10.64 (dd)
Ϫ60.7 (s)
171.9 (s)
Ϫ4.7 2JHH, 21.4 |2JPH|, 8.7 |2JCH
|
Ϫ4.7 2JHH, 22.0 |2JPH|, 31.3 |2JCH
|
|
31P
1H
Ϫ2.9 2JHH, 19.1 |2JPH|, 40.6 |2JCH
Ϫ2.9 2JHH, 116.5 |2JPH
|
31P
13C
1H
fac-cis-4-[H-As], 338 K
7.54
Ϫ9.99 (d)
Ϫ11.44 (d)
171.9 (s)
Ϫ14.51 (t)
Ϫ2.9 (s)
163.2 (br)
Ϫ7.62 (td)
4.0 (d)
Ϫ1.5 2JHH, 45.2 |2JCH
Ϫ1.5 2JHH, 4.9 |2JCH
|
31P
13C
1H
31P
13C
1H
31P
13C
1H
31P
13C
1H
|
cis-trans-5-[Cl-Ph], 295 K
11.6 |2JPH|, 5.2 |2JCH
7.9 |2JPC
|
|
trans-trans-5-[Cl-Ph], 295 K
trans-cis-5-[Cl-Me], 295 K
13.7 |2JPH|, 57.4 |2JCH
5.3 |2JPC
|
|
173.4 (s)
Ϫ16.31 (dt)
Ϫ33.1 (s)
164.7 (t)
8.12
Ϫ15.06 (d)
162.4 (s)
Ϫ7.97 (br dd)
Ϫ16.51 (brdd)
5.2 (d)
170.8 (d)
166.1 (d)
Ir–H
PMe3
CO
AsPh3 (o-H)
Ir–H
CO
Ir–H
Ir–H
PPh3
CO
13.1 |2JPH|, 5.8 |2JCH
8.1 |2JPC
5.4 |2JCH
Ϫ5.0 2JHH, 17.4 |2JPH|, 45.5 |2JCH
|
|
trans-cis-5-[Cl-As], 295 K
|
13C
1H
cis-cis-6-[Cl-Ph], 313 K (H trans to CO)
|
Ϫ5.0 2JHH, 16.0 |2JPH|, 2.9 |2JCH
123.4 |2JPC
8 |2JPC
123 |2JPC
|
31P
13C
|
|
CO
|
746
J. Chem. Soc., Dalton Trans., 2002, 743–751

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Table 1 (Contd.)
Complex, temp.
Nucleus δ (multiplicity)
Assignment
Coupling constant/Hz, assignment
cis-cis-6-[Cl-Ph], 313 K (H trans to PPh3)
cis-trans-6-[Cl-Ph], 313 K
1H
Ϫ7.42 (dddd)
Ϫ8.37 (dtd)
Ϫ5.7 (s)
169.8 (d)
161.7 (d)
Ϫ9.43 (dd)
Ϫ17.03 (dd)
Ϫ2.6
Ir–H
Ir–H
PPh3
CO
Ϫ7.0 2JHH, 14.5 |2JPH|, 56.6, 7 |2JCH
Ϫ7.0 2JHH, 162.1 |2JPH|, 5 |2JCH
|
|
31P
13C
8 |2JPC
8 |2JPC
|
|
CO
1H
Ir–H
Ir–H
PPh3
Ir–H
Ir–H
Ir–H
Ir–H
Ir–H
CO
6.4 2JHH, 120.9 |2JPH
|
6.4 2JHH, 9.2 |2JPH
|
31P
1H
1H
cis-cis-6-[Cl-Me], 313 K
cis-cis-6-[Cl-Me], 333 K (H trans to PMe3)
Ϫ8.09
18.8 |2JPH|, 52.7 |2JCH|, 6 |2JCH
|
Ϫ9.12 (dd)
Ϫ9.15 (dd)
Ϫ7.38 (d)
Ϫ9.49 (d)
169.7
161.2
7.91
Ϫ9.26 (td)
Ϫ9.97 (dtd)
18.0 (s)
179.6 (t)
7.90 (m)
Ϫ4.1 2JHH 16.7 |2JPH
Ϫ4.1 2JHH 113.6 |2JPH
Ϫ2 2JHH, 56.5, 7.1 |2JCH
Ϫ2 2JHH, 6.1, 4.2 |2JCH
|
|
cis-cis-6-[Cl-As], 338 K
1H
13C
1H
|
|
CO
mer-trans-7-[H-Ph], 323 K
PPh3 (o-H)
Ir–H
Ir–H
PPh3
CO
PPh3 (o-H)
Ir–H
Ir–H
PPh3
CO
Ir–H
Ir–H
PPh3
PPh3
Ir–H
Ϫ4.0 2JHH, 16.9 |2JPH|, 5 |2JCH
|
Ϫ4.0 2JHH, 19.5 |2JPH|, 36 |2JCH
10.3 |2JPC
ϩ2.4 2JHH, 18.3 |2JPH|, 37.7 |2JCH
|
31P
13C
1H
|
fac-cis-7-[H-Ph], 323 K
mer-cis-7-[H-Ph], 333 K
Ϫ9.51 (td)
Ϫ10.43 (2nd order)
8.8 (s)
|
ϩ2.4 2JHH, 106 |2JPH trans ϩ 2JPH cis
|
31P
13C
1H
179.3 (t)
10.8|2JPC
|
Ϫ9.08 (ddd)
Ϫ10.10 (obs)
20.5 (b)
5.1 (b)
Ϫ10.82 (dt)
4.6 2JHH, 17.6 |2JPH|, 136.4 |2JCH
5.8 |2JHH
|
|
31P
1H
mer-cis-7-[H-Me], 333 K
mer-trans-7-[H-Ph], 333 K
cis-cis-8-[Cl-As], 338 K
3 2JHH, 21.0 |2JPH
104.8 |2JPH trans ϩ 2JPH cis
|
Ϫ11.32 (2nd order) Ir–H
|
31P
1H
Ϫ50.41 (s)
Ϫ10.95 (dt)
Ϫ10.74 (dt)
Ϫ60.06 (s)
7.65
PMe3
Ir–H
Ir–H
PMe3
AsPh3 (o-H)
AsPh3 (o-H)
Ir–H
Ir–H
CO
Ϫ5 2JHH, 22.0 |2JPH
Ϫ5 2JHH, 15.5 |2JPH
|
|
31P
1H
7.60
Ϫ11.69 (d)
Ϫ18.50 (d)
170.3 (s)
Ϫ7.52 (ddd)
Ϫ9.50 (ddd)
5.87 (s)
Ϫ5.9 2JHH, 3.4 |2JCH
Ϫ5.9 2JHH, 4.7 |2JCH
|
|
13C
1H
cis-cis-8-[Cl-Ph-Cl], 333 K
Ir–H
Ir–H
PPh3
Ϫ2.9 2JHH, 17.0, 23.0 |2JPH|, 48.0 |2JCH
|
Ϫ2.9 2JHH, 202, 23.0 |2JPH|, 4.4 |2JCH
(trans to chloride)
|
31P
65.3 (s)
174.0
PPh2Cl
CO
(trans to hydride)
cis-cis-8-[Cl-Cl], 333 K
1H
Ϫ7.25 (obs)
Ϫ9.72 (ddd)
72.3 (br)
7.65
7.39
Ϫ14.01 (d)
Ϫ22.72 (d)
Ϫ15.53 (m)
Ϫ22.13 (td)
Ϫ37.3 (br)
Ϫ23.32 (dt)
Ϫ23.54 (dt)
Ϫ32.71 (s)
Ϫ7.45 (m)
Ϫ18.49 (m)
14.7 (dd)
80.8 (d)
Ir–H
Ir–H
PPh2Cl
AsPh3 (o-H)
AsPh3 (o-H)
Ir–H
Ir–H
Ir–H–Ir
Ir–H
PMe3
Ir–H
Ir–H
PMe3
Ir–H
Ir–H
PPh3
PPh2PhCO
Ϫ4.0 2JHH, 21.8 |2JPH|, 201.6 |2JPH
|
31P
1H
trans-9-[Cl-As], IrClH2(AsPh3)3, 338 K
Ϫ5.7 2JHH
Ϫ5.7 2JHH
H(Cl)Ir(PMe3)2(µ-H)(µ-Cl)Ir(PMe3)(CO), 295 Ka 1H
31P
Ϫ6.4 2JHH, 15.5 |2JPH
Ϫ6.4 2JHH, 15.5 |2JPH
|
|
(H)2Ir(PMe3)2(µ-Cl)2Ir(PMe3)(CO), 295 Ka
1H
Ϫ4.5 2JHH, 8.5, 22 |2JPH
Ϫ4.5 2JHH, 17.6 |2JPH
|
|
31P
1H
Acylated metallation product, Scheme 3, 333 K
31P
380 |2JPP|, 11.9 |2JPC
380 |2JPP
|
|
a In toluene-d8.
observation of weak signals for the hydride ligands of the
dichloromonohydride complex, trans,cis-IrH(CO)(Cl)2(PPh3)2
5-[Cl-Ph] at δ Ϫ14.51 and all four of the possible isomers of
IrH2(CO)2Cl(PPh3) 6-[Cl-Ph] with cis-hydrides. Consequently,
the formation of 5-[Cl-Ph] when 2-[Cl-Ph] is reacted with H2 in
the presence of CO suggests that HCl can be eliminated from
6-[Cl-Ph]. (We note that the addition of HCl to a sample of
1-[Cl-Ph] does indeed form 5-[Cl-Ph].) Trapping of the result-
ant 16-electron intermediate IrH(CO)2(PPh3) with H2 accounts
for the formation of 4-[H-Ph]. The spectral characteristics of
these new complexes are reported in Table 1. Since PPh3 is
liberated during the formation of 4-[H-Ph] it would therefore
seem sensible to suggest that IrH2(PPh3)3Cl, a known p-H2
active product, should also be detected.20 We note that prior
work has demonstrated that at this temperature the spectral
features of IrH2(PPh3)3Cl are not substantially enhanced, it is
however observable in spectra recorded at higher temperatures.
These reactions are summarised in Scheme 3.
J. Chem. Soc., Dalton Trans., 2002, 743–751
747

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arrangements. An example of such a complex is given by
IrH(PPh3)2(P(C6H5)2(C6H4))Cl, A, where JPP has been reported
to be 376 Hz.25 However, in such cases, the resonance due to the
phosphorus centre of the four-membered ring is normally
shifted considerably upfield (δ Ϫ69.6 (A)) relative to that of the
simple phosphine (δ 3.1 and Ϫ2.5 in A). In contrast, the new
product is characterised by a marked downfield shift of one of
the phosphorus resonances. This situation is typical of that
found in phosphorus centres that are located in five-membered
rings.26,27 When these experiments were repeated with 2-[Cl-Ph-
13CO] the corresponding hydride resonances of this product
appeared to be unaffected by the presence of the 13C label.
However, the corresponding phosphorus resonance at δ 14.7
contained an additional splitting of 11.9 Hz due to 13C.
Although this requires the product to contain a 13CO unit we
1
were unable to locate its chemical shift by 2D H–13C HMQC
experiments. This reaction product can therefore be deduced to
contain the five-membered ring Ir(CO–C5H4–P), an acylated
metallation product. A further key feature of this complex can
be deduced on the basis of the chemical shifts of the two
hydride ligands which resonate at δ Ϫ7.45 and Ϫ18.49. While
the former is typical of a hydride ligand trans to a soft donor,
the latter is typical of a hydride trans to a hard group. The only
hard ligand in this system is Cl, but filling the equatorial
vacancy with a Cl ligand would lead to a paramagnetic product.
In contrast Caulton et al.28 have shown that when an iridium
hydride is located trans to a vacant site, the corresponding
resonance is dramatically shifted to high field which in the case
of Ir(H)2(PtBu2Ph)2I corresponds to δ Ϫ44.4. The hydride of
our product resonates at δ Ϫ18.49 and is therefore unlikely to
be trans to a vacant site, this suggests that η2 binding of the acyl
is required (a loosely bound solvent molecule is unlikely). The
spectral evidence described here suggests that this product
adopts the structure indicated in Scheme 3. NMR data for this
product is presented in Table 1.
Scheme 3 Reaction pathways evident when 2-[Cl-Ph], 2-[Cl-Me] and
2-[Cl-As] are warmed with p-H2 in the presence of CO, or photolysed.
Products specific to the PPh3 and PMe3 systems are expressly indicated.
Oxidative addition of H2 to Ir(CO)2Cl(PPh3)2
In order to explore the role of CO more fully, a sample of
1-[Cl-Ph], in benzene-d6, was first placed under an atmosphere
of CO in order to form the known complex Ir(CO)2Cl(PPh3)2.2
The solution was then placed under CO and 3 atm of p-H2
according to the previous procedure. When the associated reac-
tion at 295 K was monitored by multinuclear NMR spectro-
scopy, p-H2 enhanced hydride resonances were readily detected
for 2-[Cl-Ph], 3-[Cl-Ph] and the facial isomer of 4-[H-Ph].
Much weaker hydride signals were also present for the meri-
donal isomer of IrH3(CO)(PPh3)2 7-[H-Ph] and the trans,cis
(major) and trans,trans (minor) isomers of the HCl addition
product 5-[Cl-Ph]. Upon warming this sample to 313 K the
hydride signals for 3-[Cl-Ph] were observed to disappear while
those for 4-[H-Ph] showed substantial enhancement. It can
therefore be concluded that H2 addition to Ir(CO)2Cl(PPh3)2
leads to the generation of both IrH2(CO)2Cl(PPh3) (minor)
and IrH2(CO)Cl(PPh3)2 (major). It can also be deduced that
the former complex, IrH2(CO)2Cl(PPh3), then reacts rapidly to
form 4-[H-Ph] via HCl elimination. These findings are summar-
ised in Scheme 3.
Using normal NMR methods to monitor the effect of UV
irradiation on a sample of 2-[Cl-Ph]
In order to explore whether 2-[Cl-Ph] eliminates HCl that is
subsequently trapped by 1-[Cl-Ph] under photolysis a sample
of 2-[Cl-Ph] was first prepared in benzene-d6. The large excess
of H2 present in the original solution was then reduced to 0.5
equivalents by degassing. At this point the solution of 2-[Cl-
Ph] was photolysed for 5 minutes before being re-examined by
1
Monitoring H2 exchange within IrH2(CO)Cl(PPh3)2, 2-[Cl-Ph],
NMR spectroscopy. The H spectrum shown in Fig. 2(c) was
after UV irradiation of the precursor
obtained. Evidence for conversion of 6% of 2-[Cl-Ph] was
obtained with detected products including mer-trans- and fac-
cis-7-[H-Ph], and fac-cis-4-[H-Ph] (ratio 5 : 5 : 1). In addition,
equal amounts of both the cis,cis and cis,trans isomers of the
HCl addition product 5-[Cl-Ph] were observed.
When this sample was subsequently warmed to 338 K with
p-H2, the ratio of the enhanced hydride peak intensities for mer-
trans- and fac-cis-7-[H-Ph], and fac-cis-4-[H-Ph] corresponded
to 1 : 0.2 : 0.35. However, on cooling to 300 K the ratio of the
hydride preak intensities matched those obtained immediately
after photolysis. It can therefore be concluded that at 338 K
exchange of free H2 into fac-cis-4-[H-Ph] proceeds at a faster
rate than mer-trans isomer of 7-[H-Ph]. Evidence was also
observed for the conversion of the cis,trans isomer of the HCl
addition product 5-[Cl-Ph] into the cis,cis form.
In view of the ability of 1-[Cl-Ph] to act as a photocatalyst
for the carbonlyation of benzene we decided to investigate how
UV irradiation affected the identity of the species present in
solution.24 A solution of 2-[Cl-Ph] under p-H2 was therefore
first exposed to UV light for 5 minutes and then observed by
1
NMR spectroscopy. The corresponding H NMR spectrum,
recorded at 300 K, contained hydride resonances for 2-[Cl-Ph]
that were enhanced due to photochemically driven dihydrogen
exchange. However, when this sample was heated to 333 K and
re-examined enhanced hydride resonances were seen for both
the facial and meridonal isomers of 7-[H-Ph], the facial isomer
of 4-[H-Ph] and IrH2(PPh3)3Cl as illustrated in Fig. 2(b). The
observation of both isomers of the dichloromonohydride
complex 5-[Cl-Ph] accompanied these changes. UV irradiation
therefore facilities reaction pathways that lead to CO and HCl
transfer products as indicated in Scheme 3.
More significantly, two highly complicated, and mutually
coupled, hydride resonances were observed at δ Ϫ7.45 and
Ϫ18.49 in these spectra. 2D 1H–31P HMQC NMR spectroscopy
showed that both these hydride resonances coupled to phos-
phorus signals resonating at δ 14.7 and 80.8 which were split
into doublets of 380 Hz. This large coupling is typical of that
seen in a complex that contains inequivalent trans phosphine
Effect of UV irradiation of 2-[Cl-Ph] in the presence of both CO
and H2
When a sample of 2-[Cl-Ph] in the presence of H2 and CO is
irradiated, and subsequently monitored by NMR spectroscopy
at 295 K enhanced resonances are observed initially due to 2-
[Cl-Ph]. Upon warming the formation of 4-[H-Ph] became
readily apparent. Surprisingly, however, at 333 K, the signals
for fac-cis-4-[H-Ph] were around 1000-fold larger than those
748
J. Chem. Soc., Dalton Trans., 2002, 743–751

View Article Online
observable in the absence of CO. In addition, the ratio of
hydride signals for fac-cis-4-[H-Ph] and mer-trans-7-[H-Ph]
under these conditions was 13 : 1 rather than the 1 : 5 observed
without CO. It can therefore be concluded that in the presence
of CO, IrH3(CO)(PPh3)2 7-[H-Ph] readily converts into IrH3-
(CO)2(PPh3) 4-[H-Ph].
at δ Ϫ16.31, due to the HCl transfer product IrH(CO)Cl2-
(PMe3)2 5-[Cl-Me] and complex features in the region δ Ϫ9.5
and Ϫ11, accompanied these changes.10 On warming to 333 K,
the hydride resonances due to (H)2Ir(PMe3)2(µ-Cl)2Ir(PMe3)-
(CO) disappeared while those due to H(Cl)Ir(PMe3)2(µ-H)-
(µ-Cl)Ir(PMe3)(CO) and the mer-trans-P isomer of 7-[H-Ph]
grew in intensity. Signals due to 3-[Cl-Me] could also be picked
out above the noise in the spectral baseline at this point. Inter-
estingly, no evidence was obtained in these spectra to suggest
that 4-[H-Ph] was formed by photolysis.
Oxidative addition of H2 to Ir(CO)Cl(PMe3)2, 1-[Cl-Me]
When the addition of hydrogen to the iridium() centre in 1-[Cl-
Me] was monitored at 295 K new polarised hydride resonances
were initially seen at δ Ϫ7.86, Ϫ18.98 and Ϫ8.00 in the corre-
When this reaction is repeated with 13CO labelled 1-[Cl-Me] a
new product resonance was observed at δ Ϫ9.56 that failed to
couple to 31P. This signal has been tentatively assigned to the
facial isomer of H3Ir(CO)3 and only becomes visible through
the complex effects associated with the 13C labelling that enable
the formation of a second-order spin system containing the H3
group. NMR data for these species are listed in Table 1. Scheme
3 summarises the reaction products observed with the PMe3
based system.
1
sponding H NMR spectrum that are indicative of the form-
ation of the analogous products 2-[Cl-Me] and 3-[Cl-Me],
respectively. In these spectra cross-relaxation between the
hydrides, and the methyl protons of the phosphines, leads to the
observation of enhanced signals for the methyl protons of 3-
[Cl-Me] at δ 1.23. When this sample was subsequently warmed
to 333 K polarised hydride resonances could still be observed
for both 2-[Cl-Me] and 3-[Cl-Me] in contrast to the situation
described earlier for 1-[Cl-Ph]. This suggests that the addition
of H2 to trans-1-[Cl-Me] is reversible at 333 K, with both iso-
mers of H2 addition being readily formed according to Scheme
2. The hydride resonances of 2-[Cl-Me] and 3-[Cl-Me] were
present in the corresponding spectra in the ratio 93 : 1 at 333 K
as revealed in Fig. 1(b).
Oxidative addition of H2 to Ir(CO)Cl(AsPh3)2, 1-[Cl-As]
Addition of H2 to the triphenylarsine analogue Ir(CO)-
Cl(AsPh3)2 1-[Cl-As] at 295 K produces three hydride reson-
1
ances in the corresponding H NMR spectrum at δ Ϫ7.11,
Ϫ18.91 and Ϫ9.62 with relative intensities of 1 : 1 : 5.7. These
correspond to 2-[Cl-As] and 3-[Cl-As] and are formed via add-
ition over the Cl–Ir–CO and As–Ir–As axes, respectively. The
initial ratio of 2-[Cl-As] : 3-[Cl-As] of 1 : 2.85 suggests that
at 295 K the associated difference in free energy of activation
corresponds to 2.5 kJ molϪ1 in favour of addition over the As–
Ir–As axis. However, after five days at 295 K, these product
equilibrate and the ratio of 2-[Cl-As] : 3-[Cl-As] changes to
1 : 0.1. This suggests that 2-[Cl-As], the thermodynamic pro-
duct, is 5.6 kJ molϪ1 lower in free energy than 3-[Cl-As] at
295 K. Key resonances for 2-[Cl-As] and 3-[Cl-As] are listed in
Table 1.
When this reaction was repeated with p-H2 at 295 K, the
hydride resonances for 2-[Cl-As] were initially enhanced. How-
ever, once all the 1-[Cl-As] was consumed normal signals were
observed. H2 addition is therefore irreversible on the NMR
timescale. Furthermore, in this reaction, the isomer formed by
H2 addition over the As–Ir–As axis is not p-H2 enhanced since
the two hydrides are both chemically and magnetically equiv-
alent. A significant consequence of this simplification is the
lack of structural information associated with the correspond-
ing hydride resonance. However, the identity of 3-[Cl-As] was
secured by a series of NMR measurements. For example, the
corresponding hydride resonance for 3-[Cl-As] connects to
ortho-phenyl protons at δ 7.52 (strong connection), and coin-
cident meta and para protons δ 6.93 (weaker connection), in
a 2D nOe spectrum.
Interestingly, at 338 K observation with p-H2 revealed the
formation of a third H2 addition product, 8-[Cl-As] with
hydride resonances at δ Ϫ11.69 and Ϫ18.50 (Fig. 1(c)). The
corresponding 2D nOe NMR spectrum revealed that the
hydride yielding the resonance at δ Ϫ11.69 connected to a single
ortho-phenyl resonance at δ 7.65, while that at δ Ϫ18.50 was
close to this group and a second whose ortho phenyl proton
resonates at δ 7.60. This product must therefore contain two
inequivalent arsine ligands that are cis to the hydride which
resonates at δ Ϫ18.50. When 13CO 1-[Cl-As] was employed,
both the hydride resonances of 8-[Cl-As] are split by an add-
itional 13C coupling that is typical of a cis carbonyl–hydride
arrangement. Consequently, the hydrides lie trans to arsine
and halide, respectively, and the structure shown in Scheme 1 is
supported.
Monitoring the exchange of H2 with IrH2(CO)Cl(PMe3)2,
2-[Cl-Me], in the presence of CO
In order to monitor the effect of CO on the reaction of
2-[Cl-Me] with p-H2 a solution was made up that contained 2-
[Cl-Me], 3 atm p-H2, and CO in a similar way to that described
previously for 1-[Cl-Ph]. At 333 K several new hydrides were
observed by NMR spectroscopy of which the most intense
appeared at δ Ϫ10.64 and Ϫ10.60. This product proved to be
the facial isomer of the dicarbonyltrihydride complex IrH3-
(CO)2(PMe3), 4-[H-Ph]. The NMR characteristics of this
product are described in Table 1. Additional signals at δ Ϫ10.06
and Ϫ10.97 corresponding to the meridonal isomer of 4-[H-Ph]
with 10% of the intensity of the facial peaks, and the dichloro-
monohydride complex 5-[Cl-Me] were also observed in these
spectra.10 The addition of CO therefore again enables the
observation of a CO substituted HCl transfer product.
Monitoring the exchange of H2 with IrH2(CO)Cl(PMe3)2,
2-[Cl-Me], after UV irradiation of the precursor
The role of photochemistry in enhancing ligand exchange was
also investigated for 2-[Cl-Me]. A solution of Ir(CO)Cl(PMe3)2,
in benzene-d6 was placed under 3 atm of p-H2 in an NMR tube,
left to react to form 2-[Cl-Me], and then exposed to UV light
for 5 minutes. The resulting solution was then observed by
NMR spectroscopy. The hydride resonances of 2-[Cl-Me] at
δ Ϫ7.86 and Ϫ18.98 now showed enhancement in the corre-
1
sponding H NMR spectrum at 295 K in agreement with the
observations made on 2-[Cl-Ph]. Such an enhancement has not
been seen previously at temperatures less than 333 K and dem-
onstrates that photolysis causes the loss of H2 from 2-[Cl-Me].
In addition, pairs of enhanced resonances are also observed at
δ Ϫ15.53, Ϫ22.13 and δ Ϫ23.32, Ϫ23.54 as shown in Fig. 2(d).
The former pair appear as anti-phase doublet of doublet of
triplets (JHH = Ϫ6.4, JPH = 8.5 and 22 Hz) and anti-phase doub-
let of triplets (JHH = Ϫ6.4, and JPH = 14 Hz), respectively, while
the latter pair both appear with anti-phase doublet of triplet
multiplicities (JHH = Ϫ4.5, and JPH = 18.5 and 17 Hz, respect-
ively). The 1H NMR chemical shifts of these products are con-
sistent with those expected for a binuclear species contain-
ing bridging (δ Ϫ15) and terminal (δ Ϫ21) hydrides.29 These
resonances are therefore assigned to the binuclear species
H(Cl)Ir(PMe3)2(µ-H)(µ-Cl)Ir(PMe3)(CO) and (H)2Ir(PMe3)2-
(µ-Cl)2Ir(PMe3)(CO), respectively. The observation of a triplet
Under these conditions, the nOe spectra also revealed that
while 8-[Cl-As] undergoes exchange with free H2 no exchange
with 2-[Cl-As] or 3-[Cl-As] occurs. This observation requires
J. Chem. Soc., Dalton Trans., 2002, 743–751
749

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the formation of 8-[Cl-As] by direct H2 addition to 1-[Cl-As]
and suggests that 1-[Cl-As] can exist in both cis and trans con-
figurations at this temperature (Scheme 1). Direct H2 addition
across the Cl–Ir–As axis of the cis isomer would be expected to
yield 8-[Cl-As]. At 338 K, four further hydride resonances are
observed, corresponding to coupled pairs at δ Ϫ14.01, Ϫ22.72
and δ Ϫ9.99 and Ϫ11.44. These resonances correspond to the
complexes IrH2(AsPh3)3Cl, 9-[Cl-As], and IrH3(CO)2(AsPh3),
4-[H-As], with meridonal and facial geometries respectively.
Examination of the hydride and exchange peak integrals
enabled the rate of reductive elimination of H2 in 9-[Cl-As] to
be estimated at 343 K as 5.16 sϪ1 which compares with that for
IrH2(PPh3)3Cl of 4.7 sϪ1 under the same conditions.29c
Experimental
All sample preparations were completed using either a nitrogen
filled glove box or a Schlenk line. Solvents were dried over
potassium and degassed prior to use. NMR measurements were
made using NMR tubes that were fitted with J. Young Teflon
valves and solvents were added by vacuum transfer on a high
vacuum line. Triphenylphosphine (Aldrich), trimethylphos-
phine (Aldrich) and hydrogen (99.99%, BOC) were used as
received. Ir(CO)Cl(PPh3)2, 1-[Cl-Ph] and Ir(CO)Cl(PMe3)2, 1-
[Cl-Me] were prepared according to established methods.30 An
attempt to prepare Ir(CO)Cl(AsPh3)2 1-[Cl-As] by the same
synthetic route used to prepare Ir(CO)Cl(PPh3)2 gave only a low
yield of Ir(AsPh3)3Cl. 1-[Cl-As] was successfully synthesised
by bubbling CO through a 2-methoxyethanol solution of
Na2IrCl6ؒ6H2O for 3.5 hours.31 AsPh3 was then added to the
reaction mixture at room temperature and the solution was
then refluxed for 15 minutes. The solution was filtered hot and
crystals of 1-[Cl-As] precipitated on cooling: 13C (CO) δ 207.0
in CD2Cl2, ν(CO) = 1950 cmϪ1 (CH2Cl2).
Monitoring the exchange of H2 with IrH2(CO)Cl(AsPh3)2,
2-[Cl-As], in the presence of CO
When the reaction of 2-[Cl-As] with H2 was monitored in the
presence of CO, peaks due to Ir(H)2Cl(CO)2(AsPh3), 6-[Cl-As],
4-[H-As], cis,cis 6-[Cl-H], and 5-[Cl-As] were observed. These
complexes have been characterised by nOe and 13C NMR
measurements, key assignments are listed in Table 1.
For the parahydrogen experiments, hydrogen enriched in the
para spin state was prepared by cooling H2 to 77 K over a
paramagnetic catalyst as described previously. An atmosphere
of H2 equivalent to ca. 3 atm. pressure at 298 K was introduced
into the resealable NMR tube on a high vacuum line. The
samples were thawed immediately prior to use and introduced
into the NMR spectrometer at the pre-set temperature.
Parahydrogen-enhanced NMR spectra were recorded on
Bruker DRX-400 spectrometers with 1H at 400.13, 31P at
Observation of H2 exchange in IrH2(CO)Cl(APh3)2,
2-[Cl-AsPh], after UV irradition
A solution of IrH2(CO)Cl(AsPh3)2 in benzene-d6 was placed
under 3 atm of p-H2 and exposed to UV light for 5 minutes.
At 333 K, the following products were observed: 2-[Cl-AsPh],
3-[Cl-As], 9-[Cl-As], 4-[H-As], 6-[Cl-As] and 5-[Cl-As].
161.92, 13C at 100, respectively. H NMR chemical shifts are
1
Utilisation of PPh2Cl as a probe to explore the effect of
phosphine on the addition of H2 to Ir(CO)Cl(L)2
reported in ppm relative to residual 1H signals in the deuterated
solvents (benzene-d6, δ = 7.13, and toluene-d8, δ = 2.13). 31P{1H}
NMR data are reported in ppm downfield of an external 85%
solution of phosphoric acid, 13C NMR data are reported
relative to benzene-d6, δ = 128.0, and toluene-d8, δ = 21.3.
In order to test the assertion of Sargent and Hall that increas-
ing the π-acceptor ability of the phosphine in analogues of
Vaska’s complex would increase the proportion of addition of
hydrogen across the P–Ir–P axis we also examined the reactivity
of the complexes Ir(CO)Cl(PPh3)(PPh2Cl) 1-[Cl-Ph-Cl] and
Ir(CO)Cl(PPh2Cl)2 1-[Cl-Cl]. Here, -[Cl-Ph-Cl] is used to
indicate the presence of both PPh3 and PPh2Cl, while -[Cl-Cl]
corresponds to the fully substituted PPh2Cl product. These com-
plexes were formed in situ by taking a solution of 1-[Cl-Ph] in
benzene-d6 under 3 atm p-H2 and heating to 333 K with a four-
fold excess of PPh2Cl. This process inevitably led to the form-
ation of a number of products. However, NMR spectroscopy
enabled the reactivity profiles of 1-[Cl-Ph-Cl] and 1-[Cl-Cl] to
be deduced.
Firstly, hydride resonances at δ Ϫ6.81 and Ϫ16.99 could be
readily assigned to the mono-substituted product 2-[Cl-Ph-Cl],
while those at δ Ϫ7.24 and Ϫ16.73 correspond to signals from
of 2-[Cl-Cl], the bis-substituted counterpart. However, of
greater significance was the observation of strong signals at
δ Ϫ8.13 and Ϫ8.34 due to 3-[Cl-Ph-Cl], the cis,cis isomer of the
monosubstiuted complex, and at δ Ϫ8.37 due to 3-[Cl-Cl]
respectively. These signals can be viewed in Fig. 1d. The former
species is formed by H2 addition across the P–Ir–P axis of
Ir(CO)(PPh3)(PPh2Cl)Cl. Since 3-[Cl-Ph-Cl] now contains
inequivalent phosphines two hydride resonances result. Con-
sequently, each resonance appears as doublet of doublet of
doublets with couplings typical for cis and trans phosphine
orientations. The ratio of hydride signals for 2-[Cl-Ph-Cl] and
3-[Cl-Ph-Cl] at 333 K suggests that these species are formed in
the ratio 4 : 1; this corresponds to an activation barrier differ-
ence of 3.8 kJ molϪ1. Pairs of much weaker hydride resonances
were observed, at δ Ϫ7.52, Ϫ9.50 and Ϫ7.25, Ϫ9.72 due to the
analogues of 8-[Cl-As], 8-[Cl-Ph-Cl] and 8-[Cl-Cl], respect-
ively. In 8-[Cl-Ph-Cl], the PPh3 ligand lies trans to chloride. It
should be noted that this is by no means a synthetic route to the
PPh2Cl substituted analogues of Vaska’s complex since they
react further to produce a multitude of hydride dichloride
complexes.
1
Modified H–1H-COSY, HMQC and NOESY pulse sequences
were used as previously described.20b,32,33
Conclusions
Our studies of the complexes Ir(CO)Cl(L)2 [L = PPh3, PMe3,
AsPh3 and PPh2Cl, and L2 = (PPh2Cl)(PPh3)] have revealed a
strong dependence of the relative stabilities of the cis,cis and
cis,trans isomers of the H2 addition product, Ir(H)2(CO)Cl(L)2,
on the nature of the phosphine ligand. For L = PPh3, the
cis,trans isomer produced by H2 addition over the Cl–Ir–CO
axis is the more stable by such a wide margin that the cis,cis was
previously undetected. At 295 K, the difference in activation
barriers between addition over the Cl–Ir–CO and L–Ir–L axes
has been estimated to be at least 11 kJ molϪ1. For L = PMe3,
at 333 K the ratio of cis,trans to cis,cis addition products was
93 : 1. This corresponds to a difference in activation barriers of
12.5 kJ molϪ1. Both these values are consistent with that of
9.5 kJ molϪ1 calculated by Sargent and Hall for L = PMe3.14 The
gap narrows for L = AsPh3, where both isomers can be detected
without the need for p-H2. At 295 K, the initially observed ratio
of cis,trans and cis,cis addition products corresponds to a free
energy of activation difference of 2.5 kJ molϪ1 in favour of
the cis,cis form. However, on standing these two isomers equi-
librate, with the equilibrium position interchanging the dom-
inant form via a 5.6 kJ molϪ1 free energy difference in favour
of the cis,trans product. Here matters are further complicated
by the observation of a third isomer corresponding to addition
over the Cl–Ir–As axis at 338 K. This suggests that the pre-
cursor, Ir(CO)Cl(AsPh3)2, can exist in both cis and trans
isomers at high temperatures. Further reaction products corre-
sponding to IrH3(CO)2(AsPh3), IrH(CO)(Cl)2(AsPh3)2 and
IrH2(AsPh3)3 are also observed to grow in under these
conditions. These can be categorised as the products of CO
and AsPh3 exchange, and the product of HCl transfer to
750
J. Chem. Soc., Dalton Trans., 2002, 743–751

View Article Online
8 P. G. Jessop and R. H. Morris, Coord. Chem. Rev., 1992, 121,
Ir(CO)Cl(AsPh3)2. When Ir(CO)Cl(PPh3)2 was warmed in the
presence of PPh2Cl, products corresponding to H2 addition to
Ir(CO)Cl(PPh3)(PPh2Cl) were observed. In this case, the ratio
of hydride signals for the cis,trans and cis,cis isomers of
Ir(H)2(CO)(Cl)(PPh3)(PPh2Cl) was 4 : 1 at 333 K. This corres-
ponds to an activation barrier difference of 3.8 kJ molϪ1 and
suggests that substituents on the phosphine that enhance its π
accepting capabilities decrease the barrier to addition over the
L–Ir–L axis in accordance with the suggestions of Sargent and
Hall.14
155.
9 C. Bianchini, C. Mealli, M. Peruzzini and F. Zanobini, J. Am. Chem.
Soc., 1987, 109, 5548.
10 M. J. Burk, M. P. McGrath, R. Wheeler and R. H. Crabtree, J. Am.
Chem. Soc., 1988, 110, 5034.
11 D. R. Anton and R. H. Crabtree, Organometallics, 1983, 2, 621.
12 (a) C. E. Johnson, B. J. Fisher and R. Eisenberg, J. Am. Chem. Soc.,
1983, 105, 7772; (b) C. E. Johnson and R. Eisenberg, J. Am. Chem.
Soc., 1985, 107, 3148; (c) C. E. Johnson and R. Eisenberg, J. Am.
Chem. Soc., 1985, 107, 6531; (d ) P. P. Deutsch and R. Eisenberg,
Chem. Rev., 1988, 88, 1147.
We have obtained evidence to suggest that in the case of L =
PPh3 and PMe3 the formation of IrH3(CO)2(L) occurs via the
initial formation of IrH2(CO)2Cl(L). This reaction product is
clearly formed when Ir(H)2(CO)Cl(L)2 is warmed under an
atmosphere containing both H2 and CO. Subsequently, IrH2-
(CO)2Cl(PPh3) undergoes HCl elimination which accounts for
the observation IrH(Cl)2(CO)(PPh3)2. The 16-electron complex
IrH(CO)2(L) formed by this process rapidly reacts with H2 to
yield IrH3(CO)2(L). Additional products corresponding the
meridonal and facial isomers of IrH3(CO)(L)2 are also observed
as minor reaction products in these reactions. However, when
samples of Ir(H)2(CO)Cl(L)2 are subject to UV irradiation
prior to warming with p-H2 these species correspond to the
dominant photo-products when L = PPh3.
The photochemical studies are complicated by the observ-
ation of signals that we have attributed to Ir(H)2(PPh3)(PPh2-
C5H4CO) containing an unusual η2-acyl ligand when L = PPh3.
Furthermore, with L = PMe3, signals for the binuclear products
H(Cl)Ir(PMe3)2(µ-H)(µ-Cl)Ir(PMe3)(CO) and (H)2Ir(PMe3)2-
(µ-Cl)2Ir(PMe3)(CO) are detected after irradiation at 295 K.
It has therefore been demonstrated that p-H2 enhanced
NMR studies can be used to provide experimental proof that
complements theoretical investigations.
13 A. L. Sargent, M. B. Hall and M. F. Guest, J. Am. Chem. Soc., 1992,
114, 517.
14 A. L. Sargent and M. B. Hall, Inorg. Chem., 1992, 31, 317.
15 X. Luo, D. Michos, R. H. Crabtree and M. B. Hall, Inorg. Chim.
Acta, 1992, 200, 429.
16 C. R. Bowers and D. P. Weitekamp, J. Am. Chem. Soc., 1987, 109,
5541; R. Eisenberg, Acc. Chem. Res., 1991, 24, 110; J. Natterer and
J. Bargon, Prog. Nucl. Magn. Reson. Spectrosc., 1997, 31, 293; C. J.
Sleigh and S. B. Duckett, Prog. Nucl. Magn. Reson. Spectrosc., 1999,
34, 71.
17 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.
18 P. Hubler, R. Giernoth, G. Kummerle and J. Bargon, J. Am. Chem.
Soc., 1999, 121, 5311.
19 S. K. Hasnip, S. B. Duckett, C. S. Sleigh, D. R. Taylor, G. K. Barlow
and M. J. Taylor, Chem. Commun., 1999, 1717.
20 (a) C. J. Sleigh, S. B. Duckett and B. A. Messerle, Chem. Commun.,
1996, 2395; (b) B. A. Messerle, C. J. Sleigh, M. G. Partridge and
S. B. Duckett, J. Chem. Soc., Dalton Trans., 1999, 1429.
21 A. L. Sargent and M. B. Hall, Inorg. Chem., 1992, 31, 317.
22 (a) S. Hasnip, S. B. Duckett, D. R. Taylor and M. J. Taylor, Chem.
Commun., 1998, 923; (b) S. P. Millar, D. L. Zubris, J. E. Bercaw and
R. Eisenberg, J. Am. Chem. Soc., 1998, 120, 5329.
23 L. Malatesta, M. Angoletta and F. Conti, J. Organomet. Chem.,
1971, C43.
24 A. J. Kunin and R. Eisenberg, Organometallics, 1998, 7, 2124.
25 M. A. Bennett and J. L. Latten, Inorg. Synth., 1989, 26, 202.
26 P. E. Garrou, Chem. Rev., 1981, 81, 229.
27 C. A. Gilhardi, S. Midollini, S. Moneti and A. Oslandini, J. Chem.
Soc., Dalton Trans., 1988, 1833.
28 K. Caulton, B. E. Hauger and D. Gusev, J. Am. Chem. Soc., 1994,
116, 208.
29 (a) S. B. Duckett, R. Eisenberg and A. S. Goldman, J. Chem. Soc.,
Chem Commun., 1993, 1185; (b) S. B. Duckett and R. Eisenberg,
J. Am. Chem. Soc., 1993, 115, 5292; (c) P. D. Morran, S. A.
Colebrooke, S. B. Duckett, J. A. B. Lohmann and R. Eisenberg,
J. Chem. Soc., Dalton. Trans., 1998, 3363.
Acknowledgements
S. B. D. is grateful to the University of York, the EPSRC
(C. J. S., S. A. C. and S. K. H.), the JREI scheme, Bruker UK
(CASE award SAC and spectrometer) and BP Chemicals
(CASE award SKH) for financial support. Discussions with Dr
R. Watt, Dr M. Payne, Prof. R. N. Perutz and Dr R. J. Mawby
are gratefully acknowledged.
References
1 L. Vaska and J. W. DiLuzio, J. Am. Chem. Soc., 1961, 83, 2784.
2 L. Vaska, Science, 1966, 152, 769.
3 L. Vaska and S. S. Bath, J. Am. Chem. Soc., 1966, 88, 1333.
4 S. J. La Placa and J. A. Ibers, Inorg. Chem., 1966, 5, 405.
5 L. Vaska, Science, 1963, 140, 809.
6 S. J. La Placa and J. A. Ibers, J. Am. Chem. Soc., 1965, 87, 2581.
7 G. J. Kubas, R. R. Ryan, B. I. Swanson, P. J. Vergamini and
H. J. Wasserman, J. Am. Chem. Soc., 1984, 106, 451.
30 J. P. Collman, C. T. Sears Jnr. and M. Kubota, Inorg. Synth., 1990,
28, 92.
31 G. Yagupsky and G. Wilkinson, J. Chem. Soc. A, 1969, 725.
32 (a) W. P. Aue, E. Bartholdi and R. R. Ernst, J. Chem. Phys., 1976, 64,
2229; (b) K. Nagayama, A. Kumar, K. Wüthrich and R. R. Ernst,
J. Magn. Reson., 1980, 40, 321.
33 A. Bax, R. H. Griffey and B. L. Hawkins, J. Magn. Reson., 1983, 55,
301.
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