A metal-organic framework immobilised iridium pincer complex

Article abstract of DOI:10.1039/c6sc01376g

An iridium pincer complex has been immobilised in the metal-organic framework NU-1000 using a technique called solvent assisted ligand-incorporation (SALI). The framework proved to be stable under the conditions required to activate the iridium complex and spectroscopic investigations showed formation of the catalytically active iridium dihydride. The Ir-pincer modified NU-1000 is an active catalyst for the condensed phase hydrogenation of a liquid alkene (1-decene and styrene) and shows enhanced activity with respect to a homogeneous analogue. Additionally, the Ir-pincer immobilised inside NU-1000 operated as an efficient heterogenous catalyst under flow conditions.

Full text of DOI:10.1039/c6sc01376g

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Nakamura, N. A. Vermeulen, J. J. Henkelis, A. Blackburn, J. T. Hupp, J. F. Stoddart and O. K. Farha, Chem.
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EDGE ARTICLE
A Metal-Organic Framework Immobilised Iridium Pincer Complex
Martino Rimoldi,a Akitake Nakamura,a Nicolaas A. Vermeulen,a James J. Henkelis,a Anthea K.
Blackburn,a Joseph T. Hupp,a J. Fraser Stoddart,*a and Omar K. Farha*ab
Received 00th January 20xx,
Accepted 00th January 20xx
An iridium pincer complex has been immobilised in the metal-organic framework NU-1000 using a technique called solvent
assisted ligand-incorporation (SALI). The framework proved to be stable under the conditions required to activate the
iridium complex and spectroscopic investigations showed formation of the catalytically active iridium dihydride. The Ir-
pincer modified NU-1000 is an active catalyst for the condensed phase hydrogenation of a liquid alkene (1-decene and
styrene) and shows enhanced activity with respect to a homogeneous analogue. Additionally, the Ir-pincer immobilised
inside NU-1000 operated as an efficient heterogenous catalyts under flow conditions.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Herein, we describe the incorporation of a catalytically active
iridium pincer complex into a metal-organic framework (MOF).
We selected NU-10009 — a mesoporous MOF (Figure 1)
Introduction
Pincer complexes have been widely used and investigated for
application in homogeneous catalytic transformations.1 Iridium
pincer complexes are of special interest and were first
prepared two decades ago.2 Their chemistry is still the object
of numerous investigations3 and this class of complexes are
extremely effective in applications involving hydrocarbon
conversions.4 Heterogenisation of organometallic complexes
has been under extensive study, especially employing
amorphous metal oxides as supports.5
featuring 30
Å channels with Zr6 oxide metal nodes and
1,3,6,8,-tetrakis(p-benzoic acid)pyrene as the tetratopic linkers
— as the heterogenization platform. Detailed physical and
structural characterisations, as well as the elucidation of the
proton topology around the node, have been described9-10
recently and a post-synthetic modification, called solvent
assisted ligand-incorporation11 (SALI) has been used to
decorate the metal nodes of NU-1000 with carboxylic acid
containing molecules. SALI makes use of carboxyl groups to
displace labile -OH and H2O ligands on the Zr6 node and form
strong chelating metal-ligand bond. In this report, an Ir(III)
complex featuring a bis-phosphinite pincer ligand, similar to
those introduced and developed by Brookhart and co-
workers,12 has been incorporated into NU-1000 using SALI and
its catalytic activity tested in a hydrogenation reaction.
Generally, supporting an organometallic species requires the
use of organic ligands designed to enable anchoring on a
heterogeneous substrate.6 The ligands are often complex and
require a significant investment in synthesis. This approach
aims to yield single sites or usually well-defined systems,
whose characterisation is not always straightforward.
Recently this approach has also been applied to the post-
synthetic metalation of MOFs to afford, amongst others, Co(II)-
and Fe(II)-based single-site catalysts that were found active in
various transformations. In particular, high turnover numbers
were obtained in alkene hydrogenations. Catalytic tests where
typically conducted at 40 bar, room temperature and using
THF as solvent.7
To date, only a few heterogeneous materials containing pincer
complexes have been prepared and used in catalytic
processes.8
Figure 1. Unfunctionalised NU-1000 showing the linker (left) and node (right) with the
displaceable -OH and H2O ligands highlighted in yellow.
Results and discussion
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complexes are known14 to activate a variety of unreactive
bonds and to prevent the pincer complex from reacting in an
uncontrolled fashion with the zirconium nodes or with the
organic linkers in the MOF, we decided to perform SALI with
R
H
tBu2P
O
PtBu2
O
tBu2P
O
PtBu2
O
Ir
[IrCl(COD)]2
R
Cl
H
PhMe
1
1a
the iridium-chloride complex
in a subsequent step.15
3
and to proceed to its activation
TBDMSO
1) CuCl2
OTBDMS
86%
68%
Me2CO / H2O
2) Air
NU-1000 was suspended in a PhMe solution of
3
and allowed
to react at room temperature (see ESI). After 24 h, the MOF
changed colour from yellow to orange, indicating (Scheme 2)
the incorporation of the iridium complex and the formation of
Cl
Ir
O
tBu2P
O
PtBu
O
Cl
Ir
tBu2P
O
Cl
PtBu
O
OMe
4
. The solid was thoroughly washed16 with PhMe. Inductively
1) K2CO3 / Me2CO
2) HCl / THF
51%
coupled plasma – optical emission spectrometry (ICP-OES) of
4
O
showed an Ir : P : Zr ratio of 0.8 : 1.5 : 6 (in theory a 1:2:6 ratio
is expected for incorporation of one iridium pincer complex
per node) and confirmed the incorporation of the iridium
complex into the framework with a loading of 0.8 iridium
complexes per Zr6 node. Scanning electron microscopy –
energy dispersive X-ray spectroscopy (SEM-EDS) confirms that
following SALI, crystallites of NU-1000 retain their morphology
while the iridium complex is evenly distributed (see ESI) within
the crystal. The identity of the heterogenised complex was
probed by solid-state 31P MAS NMR spectroscopy. A set of two
OH
O
OH
3
2
Scheme 1. Synthesis of the complex 3
The synthetic route outlined in Scheme 1 was used to prepare
the carboxylic acid functionalised pincer complex . Firstly, the
TBDMS-protected bis-phosphinite ligand (see ESI) was reacted
with [IrCl(COD)]2 to afford the complex . Deprotection of the
phenol silyl ether using Cu(II) chloride led to the formation of
the phenolic derivative as the major product. At this stage
3
1
2
signals at
with those found (
homogenous starting material
δ
158 and 116 was observed, in good agreement
160.9 and 119.5 see Figure 3) for the
. This spectroscopic evidence
we observed that one of the tert-butyl phosphinite
δ
substituents had reacted with the metal center to form a
3
cyclometallated four-membered ring compound
complexes have been reported13 and their distinctive
characterisation data described. The 31P NMR spectrum of
(and of the complexes obtained in the subsequent steps)
shows a set of resonances (two doublets) at 160.4 and 119.3,
2. Similar
proves that the pincer complexes remain intact following SALI.
Employing procedures previously reported12 for the activation
2
of homogenous iridium pincer complexes, 4 was exposed to
tBuOK in THF under an atmosphere of H2 at room temperature
δ
confirming the asymmetry of the complex and the existence of
two heterotopic phosphorous atoms. Notably, the hydride
signal (
observed. Single crystal X-ray diffraction analysis confirms
(Figure 2) the structural assignment of
δ -41.9) that characterises the complex 1 is no longer
2.
Figure 2. ORTEP drawing of 2. Hydrogen atoms are omitted for clarity, and thermal
ellipsoids are set to 20% probability. Only one set of disordered tert-butyl groups and
chlorides are shown for clarity.
Etherification, using methyl chloroacetate and subsequent
hydrolysis completes the synthesis of the desired carboxylic
Scheme 2. Reactivity of 4 with tBuOK and H2 to give the NU-1000 supported dihydride
iridium pincer complex, 5.
acid functionalised pincer complex 3. Hydride iridium pincer
2 | J. Name., 2012, 00, 1-3
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5, confirming the presence of analogous Ir(III) complexes in
both a homogeneous solution and a heterogeneous material.
Condensed-phase hydrogenation of 1-decene and styrene,
catalysed by 1a and
5 at 1 bar (abs.) and 23 °C, were used to
examine the relative catalytic activities of the homogenous
and heterogeneous Ir(III) catalysts (see ESI). With 1-decene as
the substrate, turnover frequencies (TOFs) of 0.3 and 2.3 h-1
for 1a and
styrene reveals TOFs of 0.1 and 7.2 h-1 in the case of 1a and
respectively. In both examples, proved (Figure 4) to be more
active than the homogeneous catalyst 1a
5, respectively, were calculated. Catalysis with
5,
5
Figure 3. Solution phase 31P NMR of 3 and 1a in C6D6 at 243 MHz. Solid-state 31P MAS
NMR of 4 and 5 at 162 MHz. Asterisks denote spinning side bands.
.
Leaching tests on the heterogeneously catalysed reactions
showed that the liquid phase is not catalytically active,
confirming that the Ir(III) pincer complex is not released into
the solution.
(see ESI). Adoption of this procedure (Scheme 2) allowed us to
(i) activate the supported iridium chloride complex17, 18 and (ii)
form the corresponding metal hydride 5.
Solid-state 31P MAS NMR spectroscopy (Figure 3) revealed that
Although a thorough explanation of such enhanced activity is
not trivial, we speculate that site isolation caused by the
heterogenization of the molecular complex reduces
intermolecular interactions that could deactivate the catalyst
or interfere in the catalytic cycle. Similar behaviour has
recently been reported.8f, 19
t
treatment of
resonances at
4
with BuOK and H2 produces a material with
206 and 185. These signals correspond18 to the
δ
dihydride complex
(
δ
185, minor). Additionally, infrared spectroscopy
206, major) and the tetrahydride
complex (
δ
revealed (see ESI) the appearance of a band at 2006 cm-1 in
good agreement with the formation of a metal hydride. In
In order to probe the activity of catalyst 5, we tested gas-phase
ethene hydrogenation in a tubular flow reactor. The TOF was
determined under differential conditions at conversions below
addition, powder X-ray diffraction of 4 confirmed the retention
of crystallinity following SALI and, more importantly,
5% (see ESI). Catalyst 5
showed a TOF value of 0.32 s-1 at 50 °C,
demonstrated that the framework’s extended structure is
t
maintained even after the activation step with BuOK to give
1 bar (rel.), and with an ethene : hydrogen ratio of 1 : 1.20 Little
deactivation was observed over time and stability tests (Figure
5) showed only minor losses (4 %) of activity over the course of
24 h.
5
(see ESI). Moreover, the porosity of
Figure S2-3.
5 was retained as shown in
Figure 5. Stability test of ethene hydrogenation catalysed by 5 in flow reactor at 0.5 bar
(rel.) and at 23 °C, with an ethene : hydrogen ratio of 1 : 1 and a total flow of 80
mL/min.
Conclusions
We report attaching an iridium pincer complex inside a MOF
without having to synthesise the extended structure de novo.
The complex can be activated after its incorporation to yield
the active dihydrido-iridium catalyst. Despite the basic
conditions required to carry out the chloride-abstraction, the
crystallinity and porosity of the framework are retained and
spectroscopic investigations corroborate the similarity
between the solid and the related homogeneous catalyst. This
Figure 4. Reaction profile of styrene (A) and 1-decene (B) hydrogenation catalysed by
1a, and 5. Leaching tests performed using 5 indicate that the solution did not contain
active catalyst
For comparison, we prepared (Scheme 1) the homogeneous
dihydride 1a (see ESI) from
. Solution and solid-state 31P MAS
1
NMR spectra showed (Figure 3) similar resonances for 1a and
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observation establishes that the catalytically active site 3.
characteristic of the molecular compound has been preserved
upon its heterogenization. In order to demonstrate the 4.
accessibility to the active sites, we tested this material in the
hydrogenation of liquid substrates and with ethene (gas)
under flow conditions – the latter being possible only with the
catalyst in heterogeneous form. It is noteworthy that the
heterogeneous catalyst shows enhanced activity with respect 5.
to its homogeneous counterpart and reveals stability upon
prolonged use. This work serves as a proof-of-concept that
well studied organometallic complexes can be installed and 6.
activated within a crystalline metal-organic framework.
M. C. Haibach, S. Kundu, M. Brookhart and A. S. Goldman,
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Acknowledgments
This work was supported as part of the Inorganometallic
Catalyst Design Center, an Energy Frontier Research Center
funded by the U.S. Department of Energy (DOE), Office of
Science, Basic Energy Sciences (BES), under award DE-
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SC0012702. This research is part of the Joint Center of
Excellence in Integrated Nano-Systems (JCIN) at King Abdulaziz
City for Science and Technology (KACST) at Northwestern
University (NU). The authors would like to thank both KACST
and NU for their continued support of this research which
made use of the IMSERC facility supported by the National
Science Foundation (NSF, DMR-0521267); the J.B.Cohen X-Ray
Diffraction Facility at the Materials Research Center of NU
supported by the MRSEC (NSF, DMR-1121262); the EPIC facility
of the NUANCE Center at NU, which has received support from
the Soft and Hybrid Nanotechnology Experimental (SHyNE)
Resource (NSF, NNCI-1542205); the International Institute for
Nanotechnology (IIN); the Keck Foundation; and the State of
Illinois, through the IIN. Metal analysis was performed at the
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NU Quantitative Bio-element Imaging Center. M.R. was
supported by the Swiss National Science Foundation with an
“Early Postdoc.Mobility Fellowship”. A.K.B. thanks Fulbright
New Zealand for a Fulbright Graduate Award and the New
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Zealand Federation of Graduate Women for a Postgraduate
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15.
To obtain an active species, iridium-chloride pincer
complexes are commonly activated in solution by halogen
abstraction under hydrogen atmosphere. This procedure
usually affords the corresponding highly reactive
dihydride species.
16.
17.
The remaining reaction solution was shown by 31P NMR to
contain exclusively unreacted starting material.
Phosphinite iridium hydride pincer complexes are known
to be stable in the form of dihydride and to loosely
coordinate an additional dihydrogen molecule when
exposed to hydrogen.
18.
19.
20.
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A TOF of 0.14 s-1 was obtained at 23 °C and 0.5 bar (rel.).
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