Catalytic hydrogenation of liquid alkenes with a silica-grafted hydride pincer iridium(III) complex: Support for a heterogeneous mechanism

Article abstract of DOI:10.1039/c5cy00837a

The previously reported silica-grafted iridium(III) hydride complex [IrH(O-SBA-15)(POCOP)] (2), prepared by treating [IrH₂(POCOP)] (1) (POCOP is 1,3-bis((di-tert-butylphosphino)oxy)benzene) with SBA-15 (mesoporous silica), hydrogenates liquid alkenes (1-decene, trans-5-decene, cyclohexene, styrene, and 4-phenyl-1-butene) at room temperature and under 1 atm H₂. Internal alkenes react at a lower rate than the terminal ones. For the sake of comparison, the hydrogenation of the same substrates was studied with the homogeneous catalyst [IrH₂(POCOP)] (1). The heterogeneous catalyst 2 hydrogenates 1-decene, cyclohexene, and 4-phenyl-1-butene faster than 1, whereas the opposite is true for styrene and trans-5-decene, which suggests that different active species are involved in the heterogeneous and homogeneous reactions. Catalysis by a truly heterogeneous species is supported by a series of "hot filtration tests". NMR spectroscopic studies showed that 2 does not undergo degrafting upon longer exposure to alkenes compared to ethene under hydrogenation conditions.

Full text of DOI:10.1039/c5cy00837a

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Catalytic hydrogenation of liquid alkenes with a
silica-grafted hydride pincer iridiumIJIII) complex:
support for a heterogeneous mechanism†
Cite this: DOI: 10.1039/c5cy00837a
M. Rimoldi,‡ D. Fodor, J. A. van Bokhoven and A. Mezzetti*a
a
a
ab
The previously reported silica-grafted iridiumIJIII) hydride complex ijIrHIJO–SBA-15)IJPOCOP)] (2), prepared by
treating ijIrH
porous silica), hydrogenates liquid alkenes (1-decene, trans-5-decene, cyclohexene, styrene, and 4-phe-
nyl-1-butene) at room temperature and under 1 atm H . Internal alkenes react at a lower rate than the ter-
minal ones. For the sake of comparison, the hydrogenation of the same substrates was studied with the
homogeneous catalyst ijIrH IJPOCOP)] (1). The heterogeneous catalyst 2 hydrogenates 1-decene, cyclo-
2
IJPOCOP)] (1) (POCOP is 1,3-bisIJ(di-tert-butylphosphino)oxy)benzene) with SBA-15 (meso-
2
2
Received 8th June 2015,
Accepted 7th July 2015
hexene, and 4-phenyl-1-butene faster than 1, whereas the opposite is true for styrene and trans-5-decene,
which suggests that different active species are involved in the heterogeneous and homogeneous reac-
tions. Catalysis by a truly heterogeneous species is supported by a series of “hot filtration tests”. NMR
spectroscopic studies showed that 2 does not undergo degrafting upon longer exposure to alkenes com-
pared to ethene under hydrogenation conditions.
DOI: 10.1039/c5cy00837a
www.rsc.org/catalysis
1
6–20
and alkene hydrogenation under mild conditions.
Con-
Introduction
comitantly, Marks and his co-workers explored the grafting of
Over the past years, many attempts have been made to com-
bine the selectivity of molecular homogeneous catalysts with
the robustness and reusability of heterogeneous systems.
Among all these attempts, one of the most widespread
methods to heterogenise a homogeneous catalyst is to
bisIJcyclopentadienyl)
complexes
of
actinides
which gave
2
1–24
ijMIJCH ) IJC Me ) ] (M = Th, U) onto alumina,
3
2
5
5 2
a material that is highly active in the hydrogenation of
2
5,26
alkenes.
Different from the early TMs, the late ones have been
investigated to a lesser extent in the preparation and charac-
terisation of well-defined species featuring a metal–OSi
1
,2
immobilise it onto a surface, generally an inorganic oxide.
The covalent attachment to the surface is achieved either via
anchoring, in which the ligand backbone of the complex is
linked to the surface, or via grafting, which involves the for-
2
7–33
bond.
A specific difference from the early TMs is that
oxide-grafted hydrides of the late TMs are unstable and tend
3
,4
3,34,35
mation of a bond between the metal and a surface oxygen.
to aggregate into metal(0) nanoparticles.
recently reported that the dihydride pincer complex
ijIrH IJPOCOP)] (1) reacts with SBA-15 to give the silica-grafted
We have
The advantage of grafting is that no ligand functionalisation
is needed to heterogenise the soluble catalysts.
2
Oxide-supported grafted complexes of the early transition
iridiumIJIII) hydride ijIrHIJO–SBA-15)IJPOCOP)] (2) (POCOP is
the pincer ligand 1,3-bisIJdi-tert-butylphosphino)oxy)benzene
(Scheme 1).
5
–7
metals (TMs)
have been successfully developed and have
8
–15
found application in various catalytic transformations.
Earlier examples are zirconium and titanium alkyl complexes
grafted onto silica, whose reaction with dihydrogen gives
polyhydride species that are active in alkene polymerisation
This compound, which is a rare example of a well-defined
and stable hydride complex of a late TM grafted onto an
a
Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir-Prelog-
Weg 2, CH-8093, Switzerland. E-mail: mezzetti@inorg.chem.ethz.ch
b
Laboratory for Catalysis and Sustainable Chemistry, Paul Scherrer Institute,
5335 PSI Villigen, Switzerland
†
Electronic supplementary information (ESI) available: Additional experimental
details, NMR spectra and crystallographic data for 6a and 6b. See DOI: 10.1039/
c5cy00837a
‡
Present address: Department of Chemistry, Northwestern University, 2145
Sheridan Road, Evanston, Illinois 60208, USA.
2
Scheme 1 Synthesis of 2 from ijIrH IJPOCCOP)] (1).
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oxide surface, catalyses the hydrogenation of gaseous alkenes
3
6
(
ethene and propene) at a total pressure of 2 bar (absolute).
−
1
At room temperature, TOFs of 7.3 and 5.3 min were
achieved for ethene and propene, respectively, whereas the
reaction at 70 °C gave TOFs of 28.7 and 24.7 min . Interest-
ingly, the catalyst was active without any chemical or thermal
activation, was recovered without any sign of decomposition
after catalysis, and was reused without loss of activity.
−
1
Some of us have recently reported that 2 and its soluble
analogues ijIrHIJiBu-POSS)IJPOCOP)] (3) (iBu-POSS is isobutyl-
substituted polysilsesquioxane, –OSi O iBu ) and ijIrH-
8
12
7
3
IJOSiMe )IJPOCOP)] (4) (Fig. 1) react with carbon monoxide to
give the corresponding six-coordinated adducts, which then
undergo reductive elimination of silanol and release the
3
7
degrafted iridiumIJI) complex ijIrIJCO)IJPOCOP)].
The reductive elimination of silanol is a major potential
problem which the application of such grafted complexes in
the hydrogenation of liquid olefins faces. The present work
addresses the issue of degrafting during the catalytic hydro-
genation of liquid alkenes, such as 1- and trans-5-decene,
cyclohexene, styrene, and 4-phenyl-1-butene, which all
underwent leaching tests. Furthermore, we report the stoi-
chiometric reactions of 2 and its homogeneous analogues 3
and 4 with representative liquid alkenes, dihydrogen, as well
as in the presence of an alkene–dihydrogen mixture.
Fig. 2 Hydrogenation of 1-decene with 2 and its homogeneous ana-
logues 1 and 3.
−
1
slower (TOF = 0.72 h ).§ As the TOF observed for the silica-
grafted catalyst 2 exceeds by 11.5 times that obtained with
the maximum concentration of 1 which can be acquired
upon complete leaching of the iridium complex, we rule out
that the active species is a portion of the grafted catalyst 2
that has been released as 1 in the solution during catalysis.
To double-check the absence of leaching, a “hot filtration
test” was performed. The hydrogenation reaction was run for
90 min up to a conversion of 56.7%, and then stirring was
interrupted. After the heterogeneous catalyst had been depos-
ited, which occurred within a few seconds, most of the
colourless supernatant solution was transferred by using a
syringe into a new reaction vessel under dihydrogen (1 bar
absolute) to maintain the catalysis conditions. Monitoring of
the reaction in this portion of the solution for 1.5 h showed
that 1-decene was not being hydrogenated further (Fig. 2,
dashed line), which confirms that the active catalyst is a
silica-bound complex and not a species released into the
solution.
Results and discussion
Hydrogenation of 1-decene
For the sake of comparison with the hydrogenation of ethene
3
6
and propene, a linear terminal alkene such as 1-decene was
studied first with ijIrHIJO–SBA-15)IJPOCOP)] (2), the dihydride
2
complex ijIrH IJPOCOP)] (1), and the soluble analogue ijIrH-
3
7
IJiBu-POSS)IJPOCOP)] (3) as catalysts in pentane solution. To
enable comparison between data obtained from catalytic runs
under homogeneous and heterogeneous conditions, the same
amounts of the pincer complex and substrate were used in
all reactions. The dihydrogen pressure was kept constant at 1
bar (absolute) in all reactions and over the entire reaction
time, which allowed periodic sampling of the reaction mix-
ture. The reaction profiles are shown in Fig. 2. The rate is
approximately constant over the conversion range studied
and gives the TOF values (Fig. S1†) reported in Table 1.
After the catalytic run, the solid catalyst was separated
3
1
The silica-grafted complex ijIrHIJO–SBA-15)IJPOCOP)] (2)
catalysed the hydrogenation of 1-decene with a TOF of 8.4
from the liquid phase by filtration, and its solid-state
P
NMR spectrum was measured and found to be identical to
−
1
h , whereas the reaction with dihydride 1 was significantly
§
Worthy of note is the value of the intercept at 8.33, which indicates a virtual 1-
decene conversion of 8.3% at time zero. This fact, in addition to the appearance
of an orange-red colour of the solution upon addition of the substrate (1-decene)
and its persistence during catalysis, speaks in favour of the predominant forma-
tion of the alkene complex ijIrIJalkene)IJPOCOP)], instead of the di/tetrahydride
complex. Therefore, considering the catalyst loading and the formation of the
4
tetrahydride complex ijIrH IJPOCOP)] (5) under hydrogen atmosphere, a conver-
sion of 8.25% can be calculated as a result of the effect of dihydrogen already
coordinated to the metal center as a tetrahydride complex. The conversion value
of 8.3% is well in agreement with the calculated one and is consistent with the
rapid formation of the alkene complex, which hydrogenates the alkene at a
lower rate than 5.
Fig. 1 Soluble siloxo derivatives 3 and 4.
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−1 ,
ab
Table 1 TOF values (h )
Substrate
2
ijIrHIJO–SBA-15)IJPOCOP)] (2) ijIrH IJPOCOP)] (1)
c
d
Ethene
438
306
8.4
—
—
0.72
0.70
1.1 f
18.3
c
e
Propene
-Decene
Trans-5-decene
Cyclohexene
Styrene
f
1
0.42
1.4
2.6
f
4
-Phenyl-1-butene 3.8
1.1
a
b
Calculated from Fig. S1, S3, S7–S10, and S12–S15. Reaction
conditions: pincer complex:
5 mg, 4.2 mol% catalyst loading,
substrate: approximately 20c0 μmol, solvent: pentane (5 mL),
d
dihydrogen: 1 bar absolute. From ref. 36. Determined after 30
e
min and at 20% ethene conversion. Determined after 30 min and
f
at 14% propene conversion. The virtually instantaneous alkene
conversion observed at time zero (Y intercept) was not considered in
the calculation of the TOF (h ) values.
−1
Fig. 4 Recycling of catalyst 2 in the hydrogenation of 1-decene.
that of an unused sample of 2 (Fig. 3). Consistent with this
finding, a recycling test of the solid catalyst showed only min-
imal differences between the first and the second run
dihydride 1 was used with the other substrates. Therefore,
the POSS derivative 3 was not investigated further.
Finally, to simulate the excess of free silanols present in
the reaction catalysed by the silica-grafted complex 2, a 10-
fold excess of POSS was added to the POSS complex 3, which
gave similar results in the hydrogenation of 1-decene
(Fig. 2, d) and the same visual changes in the colour of the
reaction solution. The comparison in Fig. 2b–d shows no sig-
nificant differences between the homogeneous reactions with
soluble complexes 1 and 3 under different conditions, which
suggests that they are all catalysed by the same species.
Besides the activity, a further difference between the silica-
grafted catalyst 2 and its homogeneous analogues 1 and 3
was observed which concerns the isomerisation of 1-decene
to internal alkenes (see below).
(Fig. 4).
Since POSS complexes are generally used as homogeneous
analogues of silica-grafted complexes, we tested the POSS
derivative ijIrHIJiBu-POSS)IJPOCOP)] (3) in the hydrogenation
of 1-decene for the sake of comparison. The reaction profile
of 3 is essentially indistinguishable from that of the catalytic
reaction with dihydride 1 (Fig. 2b–c and S4–S5†).
In fact, the pentane solution of 3 turned from red to light
yellow when placed under dihydrogen (1 atm), which indi-
4
cates the formation of the tetrahydride ijIrH IJPOCOP)] (5) (see
Scheme 8 below). This suggests that the exposure of 3 to
dihydrogen produces free silanol and dihydride 1 to eventu-
ally form the tetrahydride complex 5, as independently dem-
onstrated by the stoichiometric reaction of 3 with dihydrogen
Isomerisation of 1-decene
(
see below).
GC-MS analysis of the products of 1-decene hydrogenation
catalysed by the grafted complex 2 showed that three isomers
of 1-decene are formed in small, but detectable amounts dur-
ing the reaction. The total yield of these isomers reaches
approximately 5% at about 57% yield of decane, and the iso-
merisation stops upon removal of the solid catalyst (Fig. S4†),
which is consistent with the lack of leaching described above.
When monitored to completeness, the concentration of
the isomerised products declined after the yield of decane
had reached ca. 85% (Fig. S5 and S6†). Different from the
silica-grafted catalyst 2, no internal alkenes were detected
Upon addition of 1-decene, the solution turned red as the
corresponding alkene complex ijIrIJ1-decene)IJPOCOP)] (6c) was
formed,‡ as confirmed by P NMR spectroscopy. As the cata-
lytic behaviour of 1 and 3 is essentially identical (Fig. 2), only
3
1
with the soluble complexes ijIrH IJPOCOP)] (1) and ijIrHIJiBu-
2
POSS)IJPOCOP)] (3), although dihydride 1 is known to iso-
38
merise terminal alkenes at room temperature. The different
isomerisation behaviour of the silica catalyst 2 compared to 1
and 3 further supports a different hydrogenation mechanism
for the heterogeneous and soluble catalysts.¶
Hydrogenation of internal alkenes
1
31
To understand the different isomerisation behaviours of the
grafted and soluble catalysts, trans-5-decene was hydrogenated
Fig. 3 { H} P MAS NMR spectra of catalyst 2 before (A) and after (B)
-decene hydrogenation (“*” denotes spinning side bands).
1
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Scheme 6 Reactivity of 6a and 6b with iBu-POSS.
Scheme 2 Reactivity of 2 in solution with liquid and gaseous alkenes.
with 1 and 2 under the same conditions used for 1-decene
Fig. 5a). The TOF values in Table 1 show that the grafted
(
complex 2 hydrogenates trans-5-decene at a much slower rate
than that for 1-decene, whereas the homogeneous catalyst 1
gives the same rate for both isomers. Therefore, the accumu-
lation of internal isomers is possible during the hydrogena-
tion of 1-decene with 2, but not with 1. Also with 5-decene,
the hot filtration test indicated that the solution is not cata-
lytically active.
Cyclohexene, which was tested as a model for cis internal
alkenes, was hydrogenated by the grafted catalyst 2 about
three times faster than trans-5-decene (Table 1). With TOFs
−
1
−1
of 1.1 h and 1.4 h , respectively, the activity of the soluble
dihydride 1 is comparable to that of 2 (Fig. 5b). Also, the
phase separation test showed that there is no leaching into
the solution, in agreement with the findings described above
for 1-decene and trans-5-decene.
Scheme 3 Reactivity of 2 in the gas phase with ethene and propene.
Scheme 4 Reactivity of 3 with liquid and gaseous alkenes.
Scheme 7 Reactivity of siloxo species with dihydrogen.
Scheme 5 Reactivity of 6a a) and 6b b) with SBA-15.
¶ In a control experiment, SBA-15 was inactive in 1-decene isomerisation.
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studied the reaction of 2 and its soluble analogues with eth-
ene, propene, 1-dodecene, and neohexene.
8
The silica-grafted complex 2 was suspended in [D ]toluene
in a gas-tight NMR tube and the liquid alkenes (1-dodecene
or neohexene, approx. 20 μL) were added. For ethene and
propene, the solvent was saturated with the gaseous alkene.
1
31
The H and P NMR spectra of the liquid phase showed that,
with propene, 1-dodecene, and neohexene, no iridiumIJI) com-
plex was released into the solution, whereas the signals of
the iridiumIJI) ethene complex ijIrIJC H )IJPOCOP)] (6a) were
2
4
detected a few minutes after the addition of ethene (1 bar) to
the suspension of the grafted complex 2 (Scheme 2).
To double-check that only ethene triggers the reductive
elimination of silanol from the silica-grafted complex 2, the
reactions were repeated with gaseous ethene and propene.
When a sample of the silica-grafted hydride 2 was exposed to
an ethene atmosphere (1.5 bar) for 6 h at room temperature,
Scheme 8 Reactivity overview.
31
a new signal at δ 182 appeared in the solid-state P NMR
spectrum of the recovered sample, which indicates the forma-
tion of the iridiumIJI) ethene complex ijIrIJC
2 4
H )IJPOCOP)] (6a)
Styrene and 4-phenyl-1-butene
by reductive elimination of silanol (Scheme 3 and Fig.
S17†).
4
2
Finally, the effect of aromatic substituents was studied. Dif-
ferent from the other substrates tested, styrene was hydroge-
nated by the homogeneous catalyst 1 at a rate significantly
higher than that when the silica-grafted 2 was employed
The residual signal of the starting material 2 indicates
that the conversion into 6a is not quantitative. However,
when propene was used instead of ethene under otherwise
similar conditions, no reaction was observed, and the solid-
−
1
(
TOF was 18.3 vs. 2.6 h ) (Fig. 5c). Again, 2 did not leach into
the reaction solution, as indicated by the filtration test
dashed line in Fig. 5c). GC-MS analysis of the reaction prod-
3
1
state P NMR spectrum showed only the signal of the
starting material 2.
(
The reactions with alkenes were studied also with
the siloxo complexes ijIrHIJOSi O12iBu )IJPOCOP)] (3) and
8 7
ucts showed that both the heterogeneous (2) and the homo-
geneous (1) catalysts gave ethyl benzene as the only detect-
able product. No species derived from the hydrogenation of
the aromatic ring were observed, which may have suggested
the formation of metal nanoparticles that are known to catal-
yse the hydrogenation of aromatics under mild conditions in
the case of rhodium, iridium, and Pd/Pt.
Different from styrene, 4-phenyl-1-butene was hydroge-
nated by the silica-grafted catalyst 2 at a rate higher than that
ijIrHIJOSiMe )IJPOCOP)] (4), which are soluble analogues of 2.
3
When a [D10]p-xylene solution of the POSS complex 3 was sat-
urated with ethene (1 bar absolute), the conversion into the
iridiumIJI) complex ijIrIJC H )IJPOCOP)] (6a) was almost quanti-
2
4
3
4,39,40
tative within 30 h, as indicated by the signal at δ 180.6 in the
3
1
P NMR spectra of the reaction solution (Scheme 4, see Fig.
S18†).
Analogous to the silica-grafted complex 2, the POSS deriva-
−
1
when the soluble dihydride 1 was used (TOF was 3.8 h vs.
−
1
tive 3 failed to react with propene or the longer terminal
alkene 1-dodecene (Scheme 4). The trimethylsiloxo complex 4
reacted with ethene in the p-xylene solution much faster than
the POSS derivative 3 to give the reductive elimination prod-
uct 6a quantitatively. Upon leaving to stand, the ethene com-
plex 6a crystallised out, and single crystal X-ray analysis con-
firmed its identity (Fig. 6). For the sake of comparison, the
propene analogue ijIrIJC H )IJPOCOP)] (6b) was prepared (see
1
.1 h ) (Fig. 5d), as observed for the other aliphatic alkenes
discussed above. As for 1-decene, the hydrogenation of 4-phe-
nyl-1-butene was also accompanied by the formation of prod-
ucts of double bond isomerisation. With the silica-grafted
complex 2, the total amount of internal isomers was approxi-
mately 1.5% (at 31.9% of the 4-phenylbutane yield (Fig.
S11†)), whereas the homogeneous catalyst 1 gave a roughly
stationary isomer concentration (0.2%).
3
6
3
8
the ESI†), and its X-ray structure was determined.‖
The reactivity described above clearly shows that ethene
behaves differently from a higher alkene, including propene,
whose reactivity is analogous to that of the liquid alkenes.
The X-ray structures of 6a and 6b (Fig. 6) provide some
insight into the different tendencies of ethene and propene
to induce reductive elimination with the siloxo complexes 2
and 3.
Stoichiometric reaction of 2–4 with alkenes
Complexes of the type ijIrH IJpincer)] and ijIrIJC H )IJpincer)]
2
2
4
with aryl-substituted POCOP ligands have been recently
reported to catalyse the hydrogenation of gaseous alkenes.
Thus, in the case of 2, one might speculate that the compe-
tent catalyst is a physisorbed, non-chemically bound species
formed upon degrafting of 2 under hydrogenation condi-
tions, rather than a silica-grafted species. Therefore, we
4
1
In both alkene complexes 6a and 6b, the CC double
bond is rotated out of the plane of the complex as usually
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Fig. 5 Hydrogenation of alkenes with 1 and 2 (the dashed line refers to “hot filtration tests”).
observed in square-planar complexes. The alkene ligand and
two tert-butyl groups of the propene derivative 6b are affected
by disorder (not shown in Fig. 6, see Fig. S19†), which hin-
ders the structural comparison with 6a. However, the irid-
ium–alkene distances are appreciably longer in the propene
complex 6b (2.20 and 2.21 Å for Ir–CIJ23) and Ir–CIJ24), respec-
tively) than in the ethene analogue 6a (2.178(3) and 2.185(3)
Å). We conclude that the propene complex 6b is more
crowded than the ethene analogue 6a. Therefore, the failure
of the bulky SBA-15 (2) and POSS (3) derivatives to react with
higher alkenes can be attributed to steric effects. This is fur-
ther supported by the sluggish reaction of the POSS derivative
3 with ethene as compared to the trimethylsiloxo derivative 4,
which reacts much faster (see above).
Overall, these results suggest that, compared to its ethene
analogue 6a, the propene complex 6b is not stable enough to
trigger the reductive elimination of silanol. Interestingly, it
has been reported previously that the anilido complex ijIrH-
IJNHAr)IJPOCOP)] reacts with ethene to give aniline and the
4
3
reductive elimination product 6a, but we are not aware of
analogous reactions with higher alkenes. The implications of
these observations for catalysis will be discussed below.
Reaction of 6a and 6b with silanols
To assess the relative stability of the siloxo hydride complexes
of iridiumIJIII) as compared to the corresponding IrIJI) alkene
derivatives, the inverse reactions were studied by treating the
‖
Details of the crystallographic analyses are given in the ESI .† CCDC 988733
(6a) and 988734 (6b) contain the supplementary crystallographic data for this
paper.
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five-coordinated POSS–hydride iridium complex 3 quantita-
tively. These observations conclusively support the lower sta-
bility of the propene complex 6b in comparison to that of the
ethene analogue 6a.
Reaction of 2 and 3 with dihydrogen
Besides alkenes, dihydrogen might trigger the degrafting of
the silica-grafted pincer complex ijIrHIJO–SBA-15)IJPOCOP)] (2)
(
Scheme 7a). In a preliminary study, a [D
the POSS analogue 3 was exposed to dihydrogen (1 bar),
which resulted in the formation of a mixture of ijIrH
IJPOCOP)] (1) and the corresponding tetrahydride complex
8
]toluene solution of
2
-
3
1
1
ijIrH IJPOCOP)] (5), as determined by P and H NMR spectro-
4
scopy (Scheme 7b). In contrast, when the silica-grafted com-
8
plex ijIrHIJO–SBA-15)IJPOCOP)] (2) was suspended in [D ]tolu-
1
ene in an NMR tube under dihydrogen (1 bar), the H NMR
spectrum of the reaction solution showed a small signal at δ
−
8.4 in the hydride region, which indicates that only trace
1
Fig. 6 ORTEP diagrams of alkene complexes 6a (ethene, top) and 6b
amounts (approximately 4% found by H NMR spectroscopy)
of the grafted complex 2 are released into the solution as the
tetrahydride complex 5 under such conditions. Reaction
(
propene, bottom) (30% probability ellipsoids). Only one set of the
disordered propene and tert-butyls is shown. Hydrogen atoms, except
those of the alkene ligands, are omitted for clarity.
1
monitoring by H NMR spectroscopy indicates that the traces
of tetrahydride complex 5 are formed instantaneously upon
addition of hydrogen and no progress of the reaction is
observed over 30 min.
Interestingly, however, when a suspension of the silica-
grafted hydride 2 was exposed to a gaseous mixture of hydro-
gen and propene (1 : 1), neither dihydride 1 nor tetrahydride
2 4 3 6
alkene complexes ijIrIJC H )IJPOCOP)] (6a) and ijIrIJC H )-
IJPOCOP)] (6b) with SBA-15 and iBu-POSS. When SBA-15 was
added to a pentane solution of 6a or 6b (Scheme 5) under
identical experimental conditions, the colour of the solution
decreased in intensity over time, and SBA-15 turned from
white to orange. In both reactions, the solid recovered by
removing the solvent in vacuum was found to be the silica-
grafted species ijIrHIJO–SBA-15)IJPOCOP] (2), as indicated by
5
was detected in the solution by NMR spectroscopy after 45
min (Scheme 7c). The lack of detectable degrafting under
dihydrogen/propene atmosphere is in agreement with the
results of the hot filtration tests for the liquid alkenes
described above.
3
1
36
P MAS NMR spectroscopy.
However, the solution turned completely colourless with
the propene complex 6b, whereas the ethene analogue 6a did
3
1
Stoichiometric reaction: relevance to catalysis
not react quantitatively. Accordingly, the P solid-state NMR
spectrum (Fig. S20†) of reaction a in Scheme 5 showed the
presence of unreacted ethene complex 6a, in agreement with
the colour changes of the reaction solution described above.
This behaviour confirms that the ethene complex 6a is more
stable than its propene analogue 6b. It should be noted that
the reaction of an iridiumIJI) alkene complex with surface
silanols represents an alternative synthetic pathway to 2,
which has been originally prepared by treating dihydride
The stoichiometric reactions summed up in Scheme 8 reveal
a number of intriguing features that can explain the lack of
degrafting observed in catalysis. The reactions with alkenes
show that, although both the ethene complex ijIrIJC H )-
2
4
IJPOCOP)] (6a) and its propene analogue 6b oxidatively add
silanol to give the grafted complex 2, only ethene triggers the
inverse reaction, that is, the reductive elimination of silanol
from 2 releasing 6a into the solution. The X-ray structures of
the ethene (6a) and propene (6b) derivatives suggest that the
iridiumIJI) complexes ijIrIJalkene)IJPOCOP)] with higher alkenes
are too crowded to be stable with respect to the grafted
hydride 2. We conclude that higher alkenes cannot induce
reductive elimination of silanol and thus degrafting.
Under hydrogenation conditions, however, the reaction
with H2 offers an alternative possibility for degrafting.
Concerning this issue, it is of particular importance that the
silica-grafted siloxo complex 2 barely reacts with dihydrogen
to eliminate silanol, as mentioned above. In fact, NMR
spectroscopic monitoring of the reaction of 2 with dihydro-
gen (1 atm) (see above) shows that only a small amount (4%)
3
6
2
ijIrH IJPOCOP)] with SBA-15.
The reactivity of alkene complexes 6a and 6b with POSS in
[
D ]toluene (Scheme 6) parallels that with SBA-15. The ethene
8
complex 6a did not react with the homogeneous POSS silanol
at room temperature, whereas the propene derivative 6b gave
a conversion to ijIrHIJiBu-POSS)IJPOCOP] (3) of 22% after 10
1
31
min of reaction, as determined by H and P NMR spectro-
scopy. However, the equilibrium can be shifted toward the
siloxo complexes by heating the reaction solution at 80 °C
under static vacuum. Under these conditions, the ethene
complex 6a reacted with POSS to give 3 in 29% yield after
1
2 h, whereas the propene complex 6b formed the
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of 2 undergoes rapid degrafting and that the concentration of
the degrafted species does not increase with time. These
observations indicate that the equilibrium (a) in Scheme 8 is
rapidly established and that the grafted complex 2 is thermo-
dynamically favored as compared to 1 (at least under 1 atm H2).
In contrast, the iridiumIJIII) POSS complex 3 gives mainly
dihydride 1, tetrahydride 5, and silanol upon exposure to
dihydrogen (1 atm), which can be explained with subtle dif-
ferences in the overall donor ability of iBu-POSS and SBA-
3
7
1
5. We conclude that 2 is more stable than its soluble
siloxo analogues 3 and 4 toward silanol elimination in H2
under the experimental conditions applied.
Even more importantly, the formation of ijIrH IJPOCOP)]
4
(
5) or ijIrH IJPOCOP)] (1) is completely suppressed in the pres-
2
2
ence of H and propene, that is, under hydrogenation condi-
tions (Scheme 8b). This is a very important observation,
because, in the hydrogenation catalysed by the soluble
dihydride ijIrH IJPOCOP)] (1), the iridiumIJI) alkene complex
2
ijIrIJalkene)IJPOCOP)] is the resting species in the hydrogena-
tion of alkenes.** Hence, its signals should be observable in
1
31
the H and P NMR spectra of the reaction solution of the
hydrogenation catalysed by the grafted complex 2 if leaching
occurred during catalysis, which is not the case here.
Scheme 9 Mechanistic working hypothesis.
Mechanism of catalysis: a working hypothesis
Although a mechanistic study is beyond the scope of this
paper, the experimental observations in the catalytic and stoi-
chiometric reactions reported above are completely compati-
ble with the working hypothesis for the reaction pathway that
we reported previously (Scheme 9). In particular, it offers
an explanation for the lack of degrafting of 2 during the
hydrogenation of liquid olefins, which is the most striking
feature of the present study. The direct involvement of the
the corresponding alkene complex ijIrIJalkene)IJPOCOP)] (in
red in Scheme 9).
There is barely a literature precedent of iridium-catalysed
hydrogenation reactions based on monohydride complexes,
and the alternative dihydride pathway which involves oxida-
3
6
2
tive addition of H , alkene insertion into the M–H bond, and
reductive elimination of the alkane is nearly ubiquitous. A
possible monohydride pathway might encompass alkene
insertion into the Ir–H bond of a monohydride complex (b
and c), followed by H2 coordination to the resulting
1
6-electron, silica-grafted monohydride complex 2 in the cata-
lytic cycle is corroborated by the fact that it is active without
prior activation, is isolated unchanged after catalysis, and is
equally active when reused. This conclusion is of paramount
importance, as it implies a monohydride mechanism in
which the silica-grafted complex 2 is the active species.
Then, the observation that propene suppresses the elimi-
iridiumIJIII) alkyl complex B (d). Finally, the intramolecular
2
deprotonation of the η -H
2
ligand by the alkyl ligand in C
might form the alkane and regenerate the monohydride
iridiumIJIII) complex 2 according to (e). Interestingly, the latter
step has been determined to be the lowest energy pathway
nation of silanol from 2 under H suggests that the insertion
2
of the alkene into the Ir–H bond of 2 (Scheme 9B and C) is
for the asymmetric hydrogenation reactions of alkenes
faster than the
Scheme 9A). Furthermore, the latter reaction is also thermo-
dynamically disfavored at PIJH ) = 1 atm, as indicated by the
H
2
-induced elimination of silanol
+
catalysed by ijIrH
zoline ligand).
2
IJH
2
)IJC
2
H
4
)IJC–N)] (C–N is a carbine oxa-
(
44
2
The fact that the monohydride complex ijIrHIJO–SBA-15)-
IJPOCOP)] (2) is the active species in catalysis implies that the
mechanism of the heterogeneous reactions is fundamentally
different from that of the homogeneous reaction catalysed by
corresponding stoichiometric reaction, in which only a small
amount of dihydride 1 is released into the solution (see
above). Also, both the failure of 2 to react with higher alkenes
and the absence of ijIrIJalkene)IJPOCOP)] in the solution dur-
ing catalysis suggest that intermediate A undergoes alkene
insertion faster than reductive elimination of silanol to give
2
ijIrH IJPOCOP)] (1), which is a dihydride species. The data in
Table 1 support such a mechanistic difference, as they show
different trends for the silica-grafted complex 2 and its homo-
geneous counterpart 1 with respect to the different alkene
classes. In fact, the heterogeneous catalyst 2 gives the highest
hydrogenation rate for 1-decene (the least bulky alkene,
*
* In the hydrogenation of the liquid alkenes with the soluble dihydride
complex 1, the iridiumIJI) alkene complex is the only species detected in the
3
1
solution by
P NMR spectroscopy after the initial hydrogenation activity
−
1
provided by the tetrahydride complex 5 (see also footnote ‡).
TOF = 8.4 h ), followed by a bulky terminal olefin such as
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−
1
−1
4
-phenyl-1-butene (3.8 h ), and finally styrene (2.6 h ),
structure, wealth of spectroscopic information, and (in princi-
ple) finely tunable catalytic activity, 2 opens new perspectives
in the endeavour of closing the gap between homogeneous
and heterogeneous catalysis.
which suggests that steric effects play a major role. The TOF
values of cyclohexene, a cis cyclic alkene (1.4 h ), and
−
1
−
1
trans-5-decene (0.42 h ) support a trend influenced by steric
effects for 2. In contrast, the homogeneous catalyst 1 gives
the fastest rate with styrene (18.3 vs. 1.1–0.70 h for aliphatic
alkenes), which suggests that electronic effects are predomi-
nant in this case.
In the field of heterogenised catalysts, monohydride
mechanisms have been proposed for a number of catalytic
systems. Thus, the catalytic alkene hydrogenation with
alumina-supported complexes of the f-elements has been
suggested to involve an intermediate alkyl complex that takes
part in the heterolytic splitting of coordinated dihydrogen by
σ-bond metathesis. The protonation of the alkyl ligand
releases the alkane and regenerates the metal hydride spe-
cies, as suggested in Scheme 9C.
−
1
Experimental section
General
4
7
Complexes ijIrH
IJPOCOP)] (1),
ijIrHIJO–SBA-15)IJPOCOP)]
2
3
6
37
(2), ijIrHIJOSi
8
O
7 3
12iBu )IJPOCOP)] (3), ijIrHIJOSiMe )IJPOCOP)]
3
6
42
2 4
(4), and ijIrIJC H )IJPOCOP)] (6a) were prepared according
to literature procedures.
Catalytic hydrogenation with ijIrH IJPOCOP)] (1)
2
2
2
2
The homogeneous catalyst ijIrH IJPOCOP)] (1) (5 mg, 8.4
μmol) was dissolved in pentane (5 mL) in a 20 mL Young–
Schlenk tube equipped with a Teflon-coated stirring bar
under argon. The Schlenk tube was purged with dihydrogen
More recently, a silica-grafted zinc complex has been pro-
posed to hydrogenate propene with the intermediacy of a
hydride complex of zinc generated by heterolytic splitting of
with concomitant SiOH formation. The key steps of this
mechanism are alkene insertion into the Zn–H bond and pro-
(
1 bar absolute) for 1 min, and the reaction solution was
stirred at room temperature for 10 min. The solution colour
changed from brown to light yellow, which indicates the for-
mation of the tetrahydride complex ijIrH IJPOCOP)] (5). An
4
appropriate amount of alkene (1-decene: 38 μL, 201 μmol;
trans-5-decene: 38 μL, 201 μmol; cyclohexene 20 μL, 197
μmol; styrene: 23 μL, 201 μmol; 4-phenyl-1-butene: 30 μL,
H
2
4
5
ton transfer from silanol to release the alkane.
The common features of these three systems are the
mechanism based on a constant oxidation state and the
occurrence of σ-bond metathesis involving alkyl and metal-
2
. Four-center transition states have been widely pro-
posed and represent a well-established reaction pattern for
the late TM centers. Still, it should be kept in mind that the
reaction pathway sketched in Scheme 9 is only a working
hypothesis which requires deeper investigation in the future.
200 μmol) was added to the stirred reaction mixture by
bound H
means of a microsyringe through a rubber septum. Samples
of the reaction mixture (approximately 0.15 mL) were periodi-
cally taken, filtered over silica, and analysed by GC to evalu-
ate the conversion of the alkene into the corresponding
alkane (Fig. S3 and S12–S15†). Fitting of these approximately
linear plots gave the TOF values reported in Table 1. The
POSS complex 3 gave similar results with 1-decene (Fig. 2
and S2†).
4
6
Conclusions
Besides gas-phase reactions, the silica-grafted hydride ijIrH-
IJO–SBA-15)IJPOCOP)] (2) catalyses the hydrogenation of liquid
alkenes when suspended in a suitable solvent under dihydro-
gen (1 bar). Although it is stable, the silica-grafted complex 2
is intrinsically reactive and does not require activation before
catalyzing alkene hydrogenation, which is carried out at room
temperature and at atmospheric pressure of dihydrogen. The
catalyst was recovered unchanged and reused in catalysis
with similar results. With all the alkenes tested (1-decene,
trans-5-decene, cyclohexene, styrene, and 4-phenyl-1-butene),
the hot filtration tests showed that the complex does not
leach into the solution. Also, the silica-grafted hydride of
iridiumIJIII) has a lower tendency to undergo hydrogenolysis
of the Ir–siloxo bond, and the reaction is completely
suppressed in the presence of propene. Finally, the reactivity
pattern of the grafted species 2 differs significantly from that
Catalytic hydrogenation with ijIrHIJO–SBA-15)IJPOCOP)] (2)
The silica-grafted catalyst 2 (loading of 1 : 5.6% w/w, 8.4
μmol) was suspended in pentane (5 mL) in a 20 mL Schlenk
tube equipped with a Teflon-coated stirring bar under argon.
The Schlenk tube was purged with dihydrogen (1 bar abso-
lute) for 1 minute, and the suspension was stirred at room
temperature for 10 min. An appropriate amount of alkene
(1-decene: 38 μL, 201 μmol; trans-5-decene: 38 μL, 201 μmol;
cyclohexene 20 μL, 197 μmol; styrene: 23 μL, 201 μmol;
4-phenyl-1-butene: 30 μL, 200 μmol) was added to the stirred
reaction mixture by means of a microsyringe through a rub-
ber septum. Samples of the reaction mixture (approximately
0.15 mL) were periodically taken, filtered over silica, and
analysed by GC to determine the conversion of the alkene
into the corresponding alkane. During the reaction, stirring
was interrupted and the solid catalyst deposited on the bot-
tom of the Schlenk tube within a few seconds. Most of the
supernatant reaction solution was separated by means of a
syringe and transferred into a Schlenk tube set under hydro-
gen (1 bar absolute). Monitoring of the reaction progress by
2
of its soluble dihydride analogue ijIrH IJPOCOP)] (1). Overall,
although the present work is only the first application of the
silica-grafted complex 2 to catalytic reactions in solution, the
present results support the notion that the silica-grafted com-
plex is the active species in the catalytic hydrogenation and
that it resists leaching. With its uniform mononuclear
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GC in the new reaction vessel showed that the concentrations
of the alkane and alkene remained constant after separation
of the solution from the solid catalyst (Fig. S1 and S7–S10†).
Fitting of these approximately linear plots gave the TOF
values reported in Table 1.
solution showed exclusively the signal of unreacted 3 even
after 24 h.
Reaction of ijIrHIJOSiMe )IJPOCOP)] (4) with ethene
3
Complex 4 (34 mg, 0.06 mmol) was dissolved in [D10]p-xylene
(
1 mL) in a 20 mL Young–Schlenk tube under argon. The
Catalytic hydrogenation with 3
argon atmosphere was replaced with ethene (1.5 bar abso-
1
31
See the ESI.†
lute), and the solution was stirred for 1 h. The H and
NMR spectra indicated the formation of the ethene complex
a. Crystals suitable for X-ray analysis were obtained from
P
6
Reaction of ijIrHIJO–SBA-15)IJPOCOP)] (2) with gaseous alkenes
the solution and the structure of 6a was thus solved.
Compound 2 (complex loading of 21% w/w, 38 mg) was
placed in a 20 mL Young–Schlenk tube, the argon atmo-
sphere was replaced with ethene (1.5 bar absolute), and the
solid was stirred at room temperature for 6 h. The solid-state
Reaction of ijIrIJC H )IJPOCOP)] (6a) with SBA-15
2
4
ijIrC H IJPOCOP)] (6a) (6 mg, 9.7 μmol) and SBA-15 (97 mg)
2
4
3
1
were introduced into a 20 mL Young–Schlenk tube and then
pentane (4 mL) was added under argon. The resulting mix-
ture was stirred at room temperature for 50 min. The solvent
is removed in vacuum and the resulting solid (87 mg) was
dried in vacuum for 2 h. The P solid-state MAS NMR spec-
trum of the reaction product shows the formation of the spe-
cies ijIrHIJO–SBA-15)IJPOCOP)] (2) (δ 171) along with unreacted
P NMR spectrum of the resulting material shows a signal at
δ ~182, which is evidence of the formation of the ethene com-
plex ijIrIJC )IJPOCOP)] (6a) (an impurity gives a signal at
2
H
4
approximately δ 71). The same procedure was repeated using
propene as the alkene. Different from the reaction performed
with ethene, the formation of the corresponding iridiumIJI)
propene complex was not observed and unreacted 2 was
recovered.
3
1
6
a (δ 182).
Reaction of ijIrIJC H )IJPOCOP)] (6b) with SBA-15
3
6
Reaction of ijIrHIJO–SBA-15)IJPOCOP)] (2) with alkenes in
solution
ijIrC H IJPOCOP)] (6b) (5.5 mg, 8.7 μmol) and SBA-15 (95 mg)
3 6
were introduced into a 20 mL Young–Schlenk tube and then
pentane (4 mL) was added under argon. The resulting mix-
ture was stirred at room temperature for 50 min. The solvent
is removed in vacuum and the resulting solid was dried in
vacuum for 2 h. The P solid-state MAS NMR spectrum of
the reaction product shows the exclusive formation of the
species ijIrHIJO–SBA-15)IJPOCOP)] (2) (δ 171).
Compound 2 (approximately 10–15 mg with a loading of
1
3%) was introduced into a Young NMR tube, and [D ]tolu-
8
ene (0.6 mL) was added under argon. The alkene was then
introduced into the NMR tube. The addition of gaseous
alkenes (propene or ethene) was performed by evacuating the
NMR tube and then introducing 1.4 bar (absolute) of the gas-
eous alkene. Alternatively, the liquid alkene (1-dodecene,
approximately 20 μL) was introduced into the NMR tube
under argon with a microsyringe. After the addition of the
alkene, the tube was shaken to mix the reagents, and the
solid was deposited on the bottom by centrifugation prior to
3
1
2 4
Reaction of ijIrIJC H )IJPOCOP)] (6a) with iBu-POSS-OH
The complex ijIrIJC H )IJPOCOP)] (6a) (18.9 mg, 31 μmol) and
2
4
iBu-POSS-OH (51 mg, 61 μmol) were dissolved in [D ]toluene
8
3
1
1
31
1
measurement. The P and H NMR spectra were recorded to
analyse the soluble species. The reaction with ethene gave a
new species in the solution, the complex ijIrIJC H )IJPOCOP)]
(0.6 mL) in a Young NMR tube under argon. The P and H
NMR spectra of the reaction solution showed no formation of
new products after some hours. Alternatively, the reaction
was performed in a 20 mL Young–Schlenk tube. In a typical
procedure, ijIrIJC H )IJPOCOP)] (6a) (18.2 mg, 29 μmol) and
2
4
3
1
(
6a), identified by the P NMR chemical shift at δ 180.6 and
the H NMR shift of the coordinated ethene ligand at δ 3.05.
1
2
4
No dissolved species were observed for all the other alkenes.
iBu-POSS-OH (54 mg, 65 μmol) were dissolved in [D ]toluene
8
(
1 mL) under argon, and the resulting solution was heated at
3
1
1
Reaction of ijIrHIJOSi
8
O
12iBu
7
)IJPOCOP)] (3) with alkenes
80 °C under static vacuum. After 2 h, the P and H NMR
spectra of the reaction mixture showed the formation of ijIrH-
IJOSi O iBu )IJPOCOP)] (3) (14%). Upon an additional 10 h of
Complex 3 (10 mg) was introduced into a Young NMR tube
and then dissolved in [D10]p-xylene under argon. The argon
atmosphere was replaced with ethene (1 bar absolute), and
the progress of the reaction was monitored by NMR spectro-
8
12
7
heating at 80 °C under static vacuum, the yield of ijIrH-
IJOSi O iBu )IJPOCOP)] (3) reached 29% (based on the inte-
gration of the P NMR spectrum).
8
12
7
3
1
3
1
scopy. Just after the addition of ethene, the P NMR spec-
trum showed only the presence of the starting material 3
Reaction of ijIrIJC )IJPOCOP)] (6b) with iBu-POSS-OH
3
H
6
(
singlet at δ 170.2). After 2.3 h, a signal corresponding to ijIr-
IJC )IJPOCOP)] (6a) appeared (singlet at δ 180.6, 6%). A con-
version of 84% was observed after 29.8 h. With propene or
2
H
4
The complex ijIrIJC H )IJPOCOP)] (6b) (7.0 mg, 11 μmol) and
3 6
iBu-POSS-OH (21 mg, 25 μmol) were dissolved in [D ]toluene
8
3
1
1
-dodecene as alkenes, the P NMR spectra of the reaction
(0.8 mL) in a Young NMR tube under argon. After 10 min,
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3
1
1
31
the P and H NMR spectra of the reaction solution showed
2% conversion to the product ijIrHIJOSi O iBu )IJPOCOP)] (3)
(
18H; CIJCH ) ), 1.36 (s, 18H; CIJCH ) ); P NMR (202 MHz,
3 3 3 3
1
3
2
[D ]p-xylene, RT): δ = 180.4 (s); C NMR (125 MHz, [D ]p-
10 10
8
12
7
3
1
P NMR shift at δ 170.5). Alternatively, the reaction was
xylene, RT): δ = 168.5 (t, J = 8.2 Hz, 2C; C
q
C–O), 143.1 (t, J =
performed in a 20 mL Young–Schlenk tube. In a typical pro-
cedure, ijIrIJC H )IJPOCOP)] (6b) (6.5 mg, 10 μmol) and iBu-
8.6 Hz, 1C; arom), 127.5 (m, 1C; CH arom), 104.5 (m, 2C; CH
arom), 46.0 (s, 1C; CH CHCH ), 43.4 (t, J = 11.2 Hz, 2C; C
3
6
2
3
q
POSS-OH (21 mg, 25 μmol) were dissolved in [D
8
]toluene (0.8
CIJCH
3
)
3
), 42.8 (t, J = 11.0 Hz, 2C; C
q
CIJCH
3
)
3
), 41.2 (s,
mL) under argon, and the resulting solution was heated at 80
CH CHCH ), 30.3 (t, J = 2.8 Hz, 6C; CIJCH ) ), 29.3 (t, J = 2.9
Hz, 6C; CIJCH ) ), 22.5 (s, 1C; CH CHCH ); elemental analysis
3 3 2 3
2
3
3 3
3
1
1
°C under static vacuum for two hours. The P and H NMR
spectra showed complete conversion into ijIrHIJOSi
IJPOCOP)] (3).
8
O
12iBu
7
)-
calcd (%) for C25
H 7.24.
2 2
H45IrO P : C 47.53, H 7.18; found: C 47.72,
Reaction of ijIrHIJO–SBA-15)IJPOCOP)] (2) with dihydrogen
Synthesis of ijIrIJC10H20)IJPOCOP)] (6c)
Complex 2 (10 mg, 5.6% w/w loading) was suspended in deu-
terated toluene (0.6 mL) in a Young NMR tube under argon.
The argon atmosphere was replaced with dihydrogen (1 bar
absolute), the tube was shaken several times, and the solid
was deposited on the bottom of the tube by centrifugation.
Complex 1 (26.4 mg, 44.6 μmol) was dissolved in pentane (2
mL) in a 20 mL Young–Schlenk tube under argon. Upon
adding 1-decene (18 μL), the colour of the solution changed
from brown to red. After stirring at room temperature for 1
1
h, removal of the volatiles under vacuum gave a red solid. H
1
The H NMR spectrum of the solution recorded immediately
NMR (500 MHz, [D ]p-xylene, RT): δ = 7.1 (t, J = 7.8 Hz, 1H;
1
0
thereafter showed a very weak signal at δ ~8.4, corresponding
arom), 6.94 (d, J = 7.9 Hz, 2H; arom), 4.73 (br m, 1H;
CH CHCH –), 4.00 (m, 1H; CH CHCH –), 2.66 (d, J = 8.0 Hz,
1H; CH CHCH –), 2.43 (br m, 1H), 1.77 (br m, 2H), 1.70–1.43
to the tetrahydride complex ijIrH
4
IJPOCOP)] (5) whose inten-
2
2
2
2
sity remained constant over 30 min. Although hardly detect-
able, the tetrahydride complex 5 was quantified to be approx-
imately 4% of the initial complex loading by using
trimethylbenzene as the internal standard. When the same
2
2
(br m, 11H), 1.56 (t, J = 6.5 Hz, 18H; CIJCH
Hz, 18H; CIJCH ), 1.11 ppm (t, J = 6.7 Hz, 3H; –CH
NMR (202 MHz, [D ]p-xylene, RT): δ = 179.6 ppm (s);
3 3
) ), 1.40 (t, J = 6.6
3
1
3
)
3
2
CH
3
);
P
1
3
C
1
0
test was performed under dihydrogen/propene atmosphere
NMR (125 MHz, [D10]p-xylene, RT): δ = 168.6 (t, J = 8.5 Hz,
2C; C C–O), 143.6 (t, J = 8.7 Hz, 1C; arom), 127.7 (s, 1C; CH
arom), 104.5 (t, J = 5.9 Hz, 2C; CH arom), 52.7 (s, 1C;
CH CHCH –), 43.5 (t, J = 11.2 Hz, 2C; C CIJCH ), 42.8 (t, J =
10.8 Hz, 2C; C , CIJCH ), 40.3 (s, 1C; CH CHCH –), 38.2 (s,
1
(1 : 1, 2 bar absolute), the signal of 5 was not detected by H
q
NMR spectroscopy over 45 min.
2
2
q
3 3
)
Reaction of ijIrHIJOSi 12iBu )IJPOCOP)] (3) with dihydrogen
8
O
7
q
3
)
3
2
2
1
CH
C; CH ), 35.3 (s, 1C; CH ), 33.5 (s, 1C; CH ), 31.1 (s, 1C;
2
2
2
Complex 3 (12 mg, 8.4 μmol) was dissolved in pentane (5
mL) in a Schlenk tube (20 mL) under argon. After purging
with dihydrogen (1 bar absolute) for 1 minute and stirring at
room temperature for 10 min, the colour of the catalyst solu-
tion changed from red to light yellow. The P NMR spectrum
of the sample thereof indicated the presence of a mixture of
2
), 30.9 (s, 1C; CH
2
), 30.9 (s, 1C; CH
2
), 30.3 (t, J = 2.3 Hz,
3 3 3 3
6C; CIJCH ) ), 29.3 (t, J = 2.6 Hz, 6C; CIJCH ) ), 24.2 (s, 1C;
CH ), 15.3 (s, 1C; –CH CH ); elemental analysis calcd (%) for
C
2
2
3
3
1
32
H
59IrO
2
P
2
: C 52.65, H 8.15; found: C 52.59, H 8.07.
Notes and references
ijIrH
(
(
2
IJPOCOP)] (1) (singlet at δ 203.0, 42%), ijIrH
singlet at δ 181.7, 38%), and ijIrHIJOSi 12iBu )IJPOCOP)] (3)
singlet at δ 170.0, 19%) in the solution. Then, 1-decene (38
4
IJPOCOP)] (5)
8
O
7
1
J. M. Basset, R. Psaro, D. Roberto and R. Ugo, Modern
Surface Organometallic Chemistry, Wiley-VCH, Weinheim,
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2
009.
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within a few seconds, and its P NMR spectrum showed the
presence of the alkene complex ijIrIJC10H20)IJPOCOP)] (δ 179.0)
as the only product.
3
4
3 6
Synthesis of ijIrIJC H )IJPOCOP)] (6b)
5
R. Anwander, Chem. Mater., 2001, 13, 4419–4438.
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evaporating the solvent in vacuum. Red crystals suitable for
X-ray analysis were obtained by slow evaporation of a toluene
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solution. H NMR (500 MHz, [D ]p-xylene, RT): δ = 7.10 (m,
1
0
2
H; arom), 6.93 (m, 1H; arom), 4.68 (m, 1H; CH
2 3
CHCH ),
3
.95 (d, J = 9.9 Hz, 1H; CH CHCH ), 2.62 (d, J = 8.1 Hz, 1H;
9 V. Vidal, A. Théolier, J. Thivolle-Cazat and J.-M. Basset,
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CH CHCH ), 1.86 (d, J = 5.9 Hz, 3H; CH CHCH ), 1.53 (s,
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