A Bifunctional MOF Catalyst Containing Metal–Phosphine and Lewis Acidic Active Sites

Article abstract of DOI:10.1002/chem.201803094

Post-synthetic modification of the hafnium metal–organic framework MOF-808(Hf) to include triarylphosphine ligands is reported. Sulfonated phenylphosphines are incorporated without oxidation to give a “MOF ligand” that can complex late transition metals such as Ir and Rh to give a bifunctional catalyst containing both metal–phosphine complexes and the Lewis acidic framework hafnium metal sites. The metallated phosphine-bearing MOFs act as fully heterogeneous bifunctional catalysts for tandem reductive amination and hydroaminomethylation reactions.

Full text of DOI:10.1002/chem.201803094

A Journal of
Accepted Article
Title: A Bifunctional MOF catalyst containing Metal-Phosphine and
Lewis Acidic Active Sites
Authors: Ram R. R. Prasad, Daniel M. Dawson, Paul A. Cox, sharon E.
Ashbrook, Paul A. Wright, and Matthew Lee Clarke
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A Bifunctional MOF Catalyst containing Metal-Phosphine and
Lewis Acidic Active Sites
Ram R. R. Prasad,[a] Daniel M. Dawson,[a] Paul A. Cox,[b] Sharon E. Ashbrook,[a] Paul A. Wright*[a]
and Matthew L. Clarke*[a]
Abstract: Post-synthetic modification of the hafnium metal-organic
framework MOF-808(Hf) to include triarylphosphine ligands is
reported. Sulfonated phenylphosphines are incorporated without
oxidation to give a ‘MOF-ligand’ that can complex late transition
metals such as Ir and Rh to give a bifunctional catalyst containing
both metal-phosphine complexes and the Lewis acidic framework
hafnium metal sites. The metallated phosphine-bearing MOFs act as
fully heterogeneous bifunctional catalysts for tandem reductive
amination and hydroaminomethylation reactions.
The ubiquity of metal-phosphine catalysts stems from the
relative ease with which a collection of catalysts with differing
steric and electronic properties can be tested in a desired
transformation. Typically, many different phosphorous ligands
are tested, leading to the identification of a promising catalyst
that is then further optimized for application. Relatively few
synthetic routes to phosphine-functionalized MOFs have been
developed so far. Pioneering works on MOFs containing
phosphorous compounds report preparation of novel phosphine-
functionalised linkers and their incorporation into MOFs via
[5,8]
coordination with a suitable metal.
However, this route is
inconsistent with the practically-important requirement to
examine various ligand parameters by testing libraries of new
ligands. Neither does it lend itself to the reliable preparation of a
series of MOFs with coordinatively unsaturated metal cation
sites and similar pore sizes, where the effect of varying a single
parameter - the metal-phosphine active site - can be monitored.
To establish bifunctional MOFs as versatile and tunable
catalysts, we therefore had to address this challenge of
introducing known phosphines into specific Lewis acidic MOFs.
Here we show how post-synthetic modification can be used to
make bifunctional MOFs based on MOF-808 (Figure 1)[9] that
contain both Lewis acidic and metal-phosphine catalytic sites.
Introduction
Metal-organic frameworks (MOFs)[1] are now investigated
intensively as catalysts.[2] Some possess inherent Lewis acidity
due to the presence of coordinatively unsaturated metal sites in
their frameworks,[3] while in others catalytically-active species
can be introduced post synthesis. These may be transition metal
atoms bound at sites pre-designed by linker choice, intact
transition metal pre-catalysts, precious metal nanoparticles or
even enzymes. [4-6] In these cases, a catalyst known to be active
under homogeneous conditions is immobilized in a well-defined,
highly porous and tunable (but otherwise inert) crystalline
environment. Attractive opportunities arise for the wider
application of MOF catalysis where synergistic use can be made
of active sites at framework metal cations and additional
catalytic species incorporated by design into the MOF. A few
initial papers have appeared that point towards a bright future for
this approach.[7] However, none of these bifunctional catalysts
feature arguably the mainstay of homogeneous catalysis, metal-
phosphine complex catalysts. It can be envisaged that new
reactions with unusual selectivity, new tandem processes and
reaction cascades will become possible using porous Lewis
acidic MOFs that contain catalytically active metal-phosphine
complexes. The diverse reactivity patterns and the industrial
importance of catalysts of this type make the introduction of
bifunctional MOFs containing metal-phosphine complexes of
special significance.
[a]
R. R. R. Prasad, Dr. D. M. Dawson, Prof. S. E. Ashbrook, Prof. P. A.
Wright, Prof. M. L. Clarke
EaStCHEM School of Chemistry, University of St Andrews,
Purdie Building, North Haugh, St Andrews, KY16 9ST
United Kingdom
Figure 1. (Above) Polyhedral and ball-stick representation of the structure of
MOF-808[9]
. Each M6O8 (M = Zr, Hf) cluster is coordinated to six
E-mail: paw2@st-andrews.ac.uk, mc28@st-andrews.ac.uk
benzenetricarboxylate (BTC) ligands and six formates (HCO2). (Below) The 6-
connected M6 cluster of MOF-808. The carboxylate groups in axial and
equatorial positions are from BTC and formate groups respectively. Hydrogen
atoms of formate groups are omitted from the structure for clarity but shown in
the cluster.
[b]
Dr. Paul A. Cox
School of Pharmacy and Biomedical Sciences,
University of Portsmouth, St Michael’s Building
White Swan Road, Portsmouth, PO1 2DT
United Kingdom
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Results and Discussion
Figure 2. (Left) Powder X-ray diffraction patterns (PXRDs) of (b) activated MOF-808(Zr) and (c) activated MOF-808(Hf) compared with (a) the simulated pattern of
MOF-808(Hf).[12c] (Middle) N2 adsorption isotherm of MOF-808(Zr) (black) and MOF-808(Hf) (red) at -196 °C after activation (heated under vacuum) at 150 °C for
16 h. (Right) TGA of MOF-808(Zr) (black) and MOF-808(Hf) (red).
Table 1. Evaluation of MOFs as Lewis acid catalysts for imine synthesis
Our requirements for the platform MOF were that it should
possess strong Lewis acidity and large pore size, and that it
O
NH2
should be readily functionalised with the phosphine. After an
initial screen through a series of reported early transition metal-
based Lewis acidic MOFs for their activity in imine formation
(Table 1), the large pore (1.8 nm) structure of MOF-808(Zr)[9]
(Figure 1), originally prepared as a zirconium 1,3,5-benzene
Catalyst
N
Toluene
90 oC, 24 h
F
F
Entry[a]
Catalyst
None
Cluster[b]
Imine(%)[c]
~8
tricarboxylate,
Zr6(µ3-O)4(µ3-OH)8(HCO2)6((O2C)3C6H3)6/3
seemed ideal, in light of its high activity and the recent
demonstration that formate groups on the secondary building
units (SBUs) can be interchanged with other anions, such as
carboxylates or sulfonates, via post-synthetic modification.[10,11]
We speculated that it might be possible to conduct this
exchange using known sulfonated phosphine ligands, the strong
Zr-O bonds being expected to ensure that the cluster remains
intact throughout the post-synthetic modification. Inspired by the
higher Lewis acidity of PCN-777(Hf) compared with its Zr
analogue (Table 1, Entries 7 and 8), the Hf version of MOF-
808(Zr), MOF-808(Hf), was synthesised as micron-sized crystals
by a method similar to that subsequently reported by Liu et al [12]
(Figure 2). Nitrogen porosimetry at -196 °C showed it had lower
specific uptake (BET surface area of 1201 m2 g-1) than the Zr
form (1546 m2 g-1) in line with its higher formula weight (Figure 2).
The presence of formate groups in MOF-808(Hf) reported as
being attached to the Hf6O8 clusters was indicated by
thermogravimetric analysis/mass spectrometry, TGA-MS, where
a mass loss at ca. 250 °C is accompanied by the evolution of
CO2 (m/z = 44) (Figure S29). MOF-808(Hf) demonstrated the
Lewis acidity required to promote imine synthesis, where it was
among the most active large pore MOFs we have tested for this
reaction, including PCN-777(Hf), which possesses the same
Hf6O8 clusters, and it was certainly more active than its Zr
analogue (Table 1, Entries 9 and 10).
1
-
2
3
4
5
6
7
8
9
UiO-66(Zr)
Zr-ABDC
Zr6(µ3-O)4(µ3-OH)4(O2C-)12
Zr6(µ3-O)4(µ3-OH)4(O2C-)12
Zr6(µ3-OH)8(OH)8(O2C-)8
Zr6(µ3-O)4(µ3-OH)4(OH6)(H2O)6(O2C-)6
Zr6(µ3-O)4(µ3-OH)8(H2O)8(O2C-)8
Zr6(µ3-O)4(OH)10(H2O)6(O2C-)6
Hf6(µ3-O)4(OH)10(H2O)6(O2C-)6
Zr6(µ3-O)4(µ3-OH)8(HCO2)6(O2C-)6
Hf6(µ3-O)4(µ3-OH)8(HCO2)6(O2C-)6
Sc3O(HCO2)(O2C-)6
~7
10
18
28
51
~5
32
65
85
70
80
PCN-222(Zr)
PCN-224(Zr)
NU-1200(Zr)
PCN-777(Zr)
PCN-777(Hf)
MOF-808(Zr)
MOF-808(Hf)
ScBTB
10
11
12
MIL-100(Sc)
Sc3O(OH,OH2)3(O2C-)6
[a]
Reaction conditions: 1 mmol ketone, 1.3 mmol benzyl amine and
1.5 mol % catalyst in 5 mL toluene at 90 ºC for 24 h. The mol% is
calculated according to the molecular weight based on a formula
unit containing one metal-oxygen cluster(M6O8 or M3O).[b](O2C-)
refers to carboxylate groups from the linkers or modulators used in
the synthesis of MOFs. For linker structure, see ESI section 1.
[c] Conversions were determined by 19F NMR.
half of the formates calculated to be present in the MOF
according to the reported chemical formula.[9] The water was
replaced with acetone and the solid was kept in this solvent for
24 h before activation under vacuum. The crystallinity of the
product MOF-808(Hf)-PTSA, obtained in this way was confirmed
by PXRD and N2 porosimetry showed the expected reduction in
porosity due to incorporation of PTSA (Figures 3A & 3B).
Furthermore, TGA of MOF-808(Hf)-PTSA showed the mass loss
corresponding to the formate groups has been considerably
reduced with respect to the parent MOF-808(Hf) by the tethering
of PTSA, indicating successful post-synthetic modification
(Figure 3C). 13C CP MAS NMR of MOF-808(Hf)-PTSA after the
activation showed peaks characteristic of PTSA along with those
for linker BTC (Figure 3D). MOF-808(Hf)-PTSA retained its
crystallinity even after two weeks when kept under ambient
conditions as shown by PXRD (Figure S41).
Using the method of exchanging formate groups on the M6O8
clusters by sulfate groups using aqueous sulfuric acid by Jiang
[11]
et al.
for MOF-808(Zr), it was found that for MOF-808(Hf) the
resulting solid showed no mass loss at 250 °C in the TGA,
supporting the assignment of this loss to thermal removal of
carbon dioxide by formate decomposition (Figure S38). This
formate-sulfate exchange is possible without the loss of
crystallinity or porosity (Figures S39 and S40). Subsequently,
similar reaction using p-toluenesulfonic acid (PTSA) was
attempted to prove that aryl sulfonates could be included in the
same way. MOF-808(Hf) was soaked for 24 h in an aqueous
solution of PTSA containing enough sulfonates to exchange one
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(A)
(B)
(C)
(D)
(F)
(E)
*
*
Figure 3. (A) PXRD pattern of (b) as-synthesised MOF-808(Hf)-PTSA and (c) activated MOF-808(Hf)-PTSA compared with (a) the simulated pattern of MOF-
808(Hf) [12c] , (B) N2 adsorption isotherm of MOF-808(Hf) (black) and MOF-808(Hf)-PTSA (red) at -196 °C after activation at 150 °C for 16 h, (C) TGA of MOF-
808(Hf) (black) with TGA of MOF-808(Hf)-PTSA (red), (D) 13C CP MAS NMR spectra of (a) MOF-808(Hf) with (b) MOF-808(Hf)-PTSA (PTSA peaks marked
with asterisk), (E) PXRD pattern of (b) as-synthesised SulP1-MOF-808(Hf) and (c) SulP2-MOF-808(Hf) with the (a) simulated pattern of MOF-808(Hf) [12c] and
(F) 31P MAS NMR of SulP1-MOF-808(Hf) (red) and SulP2-MOF-808(Hf) (blue).
MOFs
containing
sulfonated
phenylphosphines
were
correlation functional indicates that ligand can be included in the
cages and space is still available for the substrates to access
and products to leave the pores.
subsequently prepared by this ligand exchange protocol. To
provide sufficient accessible pore volume for the free movement
of catalytic substrates and to access the active sites distributed
throughout the framework, we thought it ideal to incorporate
approximately one phosphine moiety per cage of MOF-808(Hf).
SO3M
The
MOF
and
either
the
sodium
salt
of
3-
Hf
P
R
Hf
Hf
H
O
O
O
(diphenylphosphino)benzene sulfonic acid (SulP1 for anion) or
the dipotassium salt of bis(p-sulfonatophenyl) phenylphosphine
dihydrate (SulP2) were kept in degassed water for 24 h with
occasional stirring and the water was replaced for degassed
acetone and kept for 24 h prior to activation. Solvent exchange
with acetone removes water from the MOF pores, thereby
eliminating the capillary forces of water than can result in pore
wall collapse and degradation of MOF structure upon activation.
This gave SulP1-MOF-808(Hf) and SulP2-MOF-808(Hf)
respectively (Scheme 1).
Hf
O
Hf
Hf
Hf
Hf
Hf
O
P
H
S
O
Hf
O
-HCOONa
Hf
R
Hf
R = H;SulP1, M= Na
R = SO3M; SulP2, M = K
Scheme 1. Post-synthetic exchange of sulfonated phosphines for formate
NMR of concentrated reaction solution indicated negligible
amounts of phosphine were left in solution after the modification,
so
that
the
solids
had
empirical
formulae
of
Hf6O4(OH)4(BTC)2(HCO2)5.75(SO3-PPh3R)0.25 (R = H, SO3). EDX
(Figure S44) and ICP analysis gave a P/Hf value of 0.04
consistent with one phosphine per cage on average. PXRD
(Figure 3E) of SulP1/SulP2-MOF-808(Hf) showed high
crystallinity and 31P MAS NMR spectroscopic analysis of both
(Figure 3F) showed a peak at ca. -4 ppm characteristic of free
phosphines. This simple procedure directly delivers phosphine
introduction to MOF via post-synthetic modification and without
the formation of any phosphine oxide species. A model structure
(Figure 4) optimised using periodic density functional theory
(DFT) code CASTEP[13] using the generalised gradient
approximation with Perdew-Burke-Ernzerhof (PBE) exchange
Figure 4. The DFT optimized model structure of SulP1-MOF-808(Hf),
in which a formate group has been replaced by SulP1.
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Table 2. Optimisation of Reductive Amination[a]
To establish that this phosphine-modified MOF can act as a
platform for a range of bifunctional catalysts, we selected the
direct reductive amination of ketones as a test reaction. This
important reaction requires both Lewis acidity and hydrogenation
catalysis.[14] Most studies in this area have focused on
homogeneous catalysts because of the requisite functional
group tolerance in pharmaceutical synthesis, along with ongoing
development of enantioselective reactions. In addition,
hydrogenation is one of the most important applications of
metal-phosphine catalysts.
Con[b]
(%)
Imine
(%)
2oamine
(%)
Alcohol
(%)
Entry
Catalyst
To mimic the protocol by which many homogeneous catalytic
processes are conducted, the MOF-phosphine was mixed with
an appropriate metal precursor, [IrCl(COD)]2 or [Rh(acac)(CO)2],
with a metal:P ratio of 1:1 and then tested directly afterwards in
the catalytic tandem reaction. The inclusion of the Ir precursor in
the MOF-phosphine is visually apparent with the solution color
associated with the homogeneous metal precursors
disappearing after the reaction with the MOF and the MOF
1
2
3
4
5
6
7
8
9
SulP1-MOF-808(Hf)
SulP2-MOF-808(Hf)
Rh(acac)(CO)2
SulP1-MOF-808(Hf)-Rh
[IrCl(COD)]2
83
86
6
80
75
79
62
80
98
96
83
86
6
80
~2
0
0
0
~3
0
~4
4
0
0
0
0
~3
79
62
62
95
96
76
76
0
0
0
0
70
0
0
18
0
0
0
MOF-808(Hf)+[IrCl(COD)2] [c]
MOF-808(Hf)+[IrCl(COD)2] [d]
MOF-808(Hf)+[IrCl(COD)2] [e]
SulP1-MOF-808(Hf)-Ir
SulP2-MOF-808(Hf)-Ir
SulP1-MOF-808(Hf)-Ir [f]
SulP1-MOF-808(Hf)-Ir [g]
becoming coloured (Figure S47).
A similar change was
10
11
12
observed with the Rh complex. EDX reveals that the Ir is
incorporated in the MOF (Figure S48). The pre-catalysts can be
characterised by 31P MAS NMR and show the expected
downfield coordination shift to 25 ppm relative to -4 ppm for the
free phosphine (relative to +40 ppm for oxidised phosphine) in
the 31P MAS NMR spectrum for Ir (Figure 5) and similar shifts
was observed for Rh coordination (Figure S49). If the pre-
catalyst is removed at this point, PXRD reveals it is crystalline
while its specific porosity is lowered by around 25% compared to
the parent due to increase in the mass and occupation of pore
space from ligand addition (Figures S50 and S51).
~80
80
0
[a]
Standard Reaction Conditions: 1 mmol of ketone, 1.3 mmol of benzyl
amine and 1.5 mol % catalyst in 5 mL toluene under 50 bar of H2 at 90
[b]
ºC for 24 h. Conversions are determined by 19 F NMR.[c] MOF-808(Hf)
[d]
with 0.37 mol %Ir as [IrCl(COD)]2
MOF-808(Hf)+[IrCl(COD)]2 when
Subsequent heating of filtrate from
[e]
stopped after 6 h for hot filtration
[d] under 50 bar of H2 at 90 ºC for 24h. [f]SulP1-HMOF-808(Hf) stopped
[g]
[f]
at 6 h for hot filtration Filtrate from after 24 h for 90 ºC under 50 bar
H2.
A series of reductive aminations of 4’-fluoroacetophenone were
performed using different catalysts, the results of which are
given in Table 2. As expected, if MOF-808(Hf) (Table 1, Entry
10) or SulP1/SulP2-MOF-808(Hf) is used alone, the only product
detected is imine (Table 2, Entry 1,2). If the MOF ligand, SulP1-
MOF-808(Hf) is combined with [Rh(acac)(CO)2], only imine
formation takes place with no hydrogenation and use of
[Rh(acac)(CO)2] itself has no effect above background levels of
imine formation (Table 2, Entry 3,4).
(a)
(b)
(c)
(d)
If [IrCl(COD)]2 is added to a solution of ketone, amine and the
reaction performed at 50 bar, a black suspension of Iridium
nanoparticles forms, and negligible imine formation takes place
(Table 2, Entry 5). Instead, the major product is from the partial
reduction of the ketone starting material to 2-(4-
fluorophenyl)ethanol. MOF-808(Hf) combined with [IrCl(COD)]2
does gives secondary amine as the major product (Table 2,
Entry 6), but hot filtration and subsequent re-exposure of the
filtrate to the reaction conditions gives 2-(4-fluorophenyl)ethanol,
indicating the presence of leached iridium (Table 2, Entry 8). By
contrast, both SulP1- and SulP2-MOF-808(Hf) with [IrCl(COD)]2
showed high conversions and high selectivity towards secondary
amine (Table 2, Entries 9,10), and demonstrate no observable
leaching upon hot filtration, as measured by lack of filtrate
activity for hydrogenation (Table 2, Entry 12, Figure S52). Thus,
it is possible to form a permanently immobilised Ir metal catalyst
if a phosphine is first tethered to the MOF. Furthermore, the Ir-
phosphine catalysts are more active for reductive amination than
those without phosphine. While it is well known that various
(e)
Figure 5. (a) 31P solution state NMR of SulP1, 31P MAS NMRs of (b)
SulP1-MOF-808(Hf), (c) SulP2-MOF-808(Hf), (d) SulP1-MOF-808(Hf)-Ir
and (e) SulP1-MOF-808(Hf) prepared in non de-oxygenated solvents that
gave both free and oxidised phosphines.
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alkenes and is followed by reductive amination, is an important
and efficient synthesis of amines.[17] In our chosen exemplar
reaction between cyclopentene and aniline (Scheme 4) with
SulP1-MOF-808(Hf)-Rh (using Rh(acac)(CO)2 as the metal
precursor), N-(cyclopentylmethyl)aniline was obtained in good
yield (75%).
combination of homogeneous catalysts can promote this
reaction,[15] this system has the advantage of an easy to remove
catalyst. In fact, attempting to do this reaction homogeneously
by replacing the MOF phosphine with [IrCl(COD)]2 and PPh3, or
even mixture of [IrCl(COD)]2, PPh3 and HfCl4 delivers mainly the
alcohol side product.(See Table S1 ESI). SulP1-MOF-808(Hf)
showed excellent conversion and selectivity towards secondary
amine even after 5 catalytic cycles (Figure 6). No nanoparticle
deposition was observed by transmission electron microscopic
analysis on the MOF after catalysis (Figure S54). PXRD of
SulP1-MOF-808(Hf)-Ir after catalysis shows the MOF retains its
crystallinity under catalytic conditions (Figures S55 and 56). The
N2 adsorption of a sample retrieved after catalytic testing shows
the same type of isotherm as SulP1-MOF-808(Hf)-Ir before
catalysis, although the uptake has been reduced by ca. one third
due to a combination of structural loss during the catalysis,
handling in air and subsequent activation and possible
occupation of pore volume by adsorbed species.
SulP1-MOF-808(Hf)-Rh gave good yields of ortho-substituted
alkyl derivatives of anilines with cyclopentene (72-75%) and
excellent yields (81-87%) were observed for ortho and para
substituted halide derivatives of aniline with cyclohexene. The
resonance at +30 ppm in the 31P MAS NMR spectrum of the
MOF post-catalysis, shows the metal complex remains intact
(Figure S58).[18] For comparison, the 31P chemical shift for the
original, unbound phosphine is -3 ppm and that of the
corresponding phosphine oxide is +40 ppm. The tunable nature
of SulP1-MOF-808(Hf) to act as a bifunctional ligand with
judicious choice of metal sources was therefore, demonstrated
in
tandem
reductive
amination
and
domino
hydroaminomethylation reactions.
Figure 6. Plots of yields(%) of N-benzyl-1-(4-fluorophenyl)ethan-1-amine
at different runs in the recycle experiments of SulP1-MOF-808(Hf)-Ir for
reductive amination of 4’-fluoroacetophenone with benzylamine.
SulP2-MOF-808(Hf)-Ir showed productivity towards 2o amine
similar to that of SulP1-MOF-808(Hf)-Ir, but the lower cost of the
phosphine precursor made SulP1-MOF-808(Hf)-Ir our preferred
catalyst for further expansion of the scope to a range of other
acetophenone derivatives and primary amines (Table S1 and
Scheme 2). High conversions and selectivities were observed.
There is no hydrogenolysis of the benzyl group or, in 1d, of the
C-Br, which would be expected using typical heterogeneous
hydrogenation catalysts.[16]
F
NH
NH
NH
F
F
1h:96% (80%)
1i:72% (65%), dr: 80:20
Scheme 2. Reductive amination of a range of functionalised aromatic
ketones by SulP1-MOF-808(Hf)-Ir. Reaction conditions: 1 mmol of
ketone, 1.3 mmol of benzyl amine, 1.5 mol % SulP1-MOF-808(Hf) and
While the SulP1-MOF-808(Hf)-Ir gave good results in reductive
amination of ketones, it is well known that reduction of aldimines
is significantly easier than that of ketimines and we found that
SulP1-MOF-808(Hf)-Rh was able to reductively aminate
benzaldehyde (Scheme 3). With this precedent, we aimed to
establish the versatile nature of SulP1-MOF-808(Hf) as an
insoluble “porous phosphine ligand” with Lewis acidity, whose
catalytic properties can be modified by the choice of metal
0.37 mol% of Ir as [Ir(COD)Cl]2 in 5 mL toluene under 50 bar of H2 at
[b]
90 ºC for 24 h.
Conversions are determined by 19 F NMR and 1H
NMR in the presence of 1-methylnaphthalene as internal standard.
Isolated yields in brackets.
precursor
added.
Therefore,
we
studied
the
hydroaminomethylation of alkenes to see if a bifunctional Rh
MOF could promote such a sequential reaction. This atom-
economic domino reaction, which starts with hydroformylation of
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Experimental Section
All chemicals were purchased from chemical suppliers and used without any
further purifications. Zirconyl chloride octahydrate (ZrOCl2.8H2O) 98 %,
hafnium dichloride oxide octahydrate (HfOCl2.8H2O), 98+%, 1,3,5-benzene
tricarboxylic acid, 98%, (BTC), 1,3,5-tri(4-carboxyphenyl)benzene, 97%,
formic acid 97%, and N,N-diethylformamide 99%, were purchased from Alfa
Aesar. Benzene-1,4-dicarboxylic acid (BDC) was purchased from Sigma-
Aldrich. N,N- Dimethylformamide (DMF) was purchased from Acros Organics.
Tetracarboxyphenyl porphyrin from Frontier Scientific and acetone from Fisher
Scientific. Azobenzenedicarboxylic acid (ABDC),[19] 4,4’,4’’-(1,3,5-triazine-
2,4,6-triyl)tribenzoic acid (TATB),[20,21] and 4,4’,4’’-(2,4,6-trimethylbenzene-
1,3,5-triyl)tribenzoic acid (TMTB),[22,23] were synthesised in the lab. Room
temperature studies refers to temperature range from 15-25 °C. All the MOFs
were synthesised using a Carbolite programmable oven equipped with 3216
programmable controllers. Heating the reaction mixture for linker synthesis
and/or catalysis is performed by using an oil bath (paraffin oil) on Heidolph MR
3003 magnetic stirring hotplate or IKA® C-MAG HS7 hotplate. Powder X-ray
Diffraction (PXRD) patterns on finely ground powder of the MOFs were
collected in Debye-Scherrer geometry from Stoe STAD i/p diffractometers with
primary monochromation (Cu Kα1, λ = 1.54056 Å), using 0.5 or 0.7 mm quartz
glass capillaries. Thermogravimetric analysis (TGA) of all MOF samples were
carried out on a Netzsch TGA 760 for a temperature range from 20 - 900 °C at
a heating rate of 5 °C min-1 in a continuous air flow. ICP analysis was
performed on Thermo Fisher Scientific ICP-OES iCAP 6000 series,using
hafnium and phosphorus standard for ICP from TraceCERT®, Sigma-Aldrich.
N2 adsorption isotherms for all samples were measured volumetrically on a
Micrometrics Tristar after heating the sample under vacuum at optimised
temperatures (see synthesis of MOFs for exact activation temperatures for
each MOF). Solid-state nuclear magnetic resonance (SS-NMR) spectra were
recorded using a Bruker Avance III spectrometers equipped with either 9.4 or
14.1 T superconducting magnets (Larmor frequencies of 161.98 and 242.99
MHz, respectively for 31P). Samples were packed into standard ZrO2 rotors
with outer diameters of 4 mm and rotated at the magic angle at a rate of 10-14
kHz. Spectra were recorded with signal averaging for between 8 and 128
transients with a recycle interval of between 30 and 120 s. Chemical shifts are
reported in ppm relative to 85% aqueous H3PO4, using BPO4 as a secondary
solid reference (δ = –29.6 ppm). A first principles periodic Density Functional
Theory (DFT) calculation was performed based on the plane wave
pseudopotential method as implemented in the program CASTEP. The
interaction between core and valence electrons was described by ultrasoft
pseudopotentials and the Perdew-Burke-Ernzerhof (PBE) generalised gradient
approximation (GGA) was used to describe the exchange-correlation energy
for the valence electrons. A cut-off energy of 500 eV was used for the plane
wave basis sets. The convergence criteria used for geometry optimisation
were 2 x 10-5 eV/atom for the maximum energy change, 0.05 eV/Å for the
maximum change in force and 0.002 Å for the maximum atom displacement.
Solution-state NMR analyses were carried out at room temperature on
deuterated solvents. Chemical shifts are quoted as parts per million (ppm).
Coupling constants, J, are quoted in Hz. Multiplicities are indicated by: s
(singlet), d (doublet), t (triplet) and m (multiplet). 1H, 13C, 31P, 19F NMR were
carried out using either a Bruker Avance 300 (300 MHz for 1H, 75MHz for 13C,
121 MHz for 31P and 282 MHz for 19F) or Bruker Ultrashield 500 (500 MHz for
1H, 125MHz for 13C, 201 MHz for 31P and 470 MHz for 19F). TLC visualization
was carried out using a UV lamp (254 nm) or using a 1 % potassium
permanganate aqueous solution. Flash silica chromatography was performed
using Kieselgel 60 silica.
Scheme 3. Reductive amination of aromatic aldehydes and ketones with
benzylamine
using SulP1-MOF-808(Hf)-Rh. Standard reaction
conditions: 1mmol of benzaldehyde, 1.3 mmol of benzyl amine, 1.5 mol%
SulP1-MOF-808(Hf) and 0.3 mol% Rh(acac)(CO)2. Conversion
determined 1H NMR using 1-methylnaphthalene as internal standard.
Isolated yields in brackets.
n
n
NH2
Catalyst
n
R1
30bar CO/H
2
HN
N
Toluene
70-90oC,16 h
R1
R2
R2
n =0, 1
HN
HN
HN
2c (75%)
2b(72%)
2a(75%)
HN
F
HN
HN
Br
2d(83%)
2e (81%)
2f(87%)
Scheme 4. Hydroaminomethylation of alkenes. Reaction Conditions: 1
mmol of cyclopentene/cyclohexene, 1.1 mmol of aniline, 1.5 mol %
SulP1-MOF-808(Hf) and 0.3 mol% Rh(acac)(CO)2 in 5 mL toluene under
30 bar of CO/H2 at 70 ºC(cyclopentene) /90 °C(cyclohexene) for 16 h.
Isolated yields in brackets.
Conclusion
We have developed
a one-step post-synthetic method to
incorporate triarylphosphine moieties into the highly Lewis
acidic MOF-808(Hf) without the phosphine undergoing oxidation,
using aryl sulfonated phosphines as precursors. The SulP1-
MOF-808(Hf) thus obtained provides a versatile platform for
subsequent functionalisation with desired metals by coordination
to generate bifunctional catalysts with inherent Lewis acidic and
metal-phosphine complex properties. Upon reaction with
[IrCl(COD)]2 the SulP1-MOF-808(Hf) forms SulP1MOF-808(Hf)-
Ir, which was found to be a highly active pre-catalyst for
reductive aminations of functionalised acetophenones with
benzyl amine derivatives. Furthermore, SulP1MOF-808(Hf)-Rh
obtained via coordination of Rh(acac)(CO)2 with SulP1MOF-
808(Hf) acts as an excellent catalyst for hydroaminomethylation
of alkenes.
Synthesis of MOFs
1. UiO-66(Zr)
UiO-66(Zr) was synthesised by following the method reported by Vermoortele
et al.[24] without any further modifications. To a 1 L Schott DURAN pressure
plus bottle equipped with a magnetic stirrer bar, 3.5 g of ZrCl4 (15 mmol), 2.5 g
BDC (15 mmol) and 155 mL of DMF was added and stirred until the solution
became homogeneous. To which, 1.5 mL of HCl (17 mmol) and 11.5 mL of
CF3COOH was added and stirred for 5 min. After the removal of magnetic
stirrer bar, the bottle was capped and placed in a preheated oven at 120 °C for
21 h. After cooling to room temperature, the white powder formed was
collected by centrifugation and washed 3 times each with DMF and methanol.
The precipitate was dried at 80 °C overnight and activated at 150 °C for N2
adsorption.
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10.1002/chem.201803094
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2. Zr-ABDC (Zirconium-Azobenzenedicarboxylic acid)
with a syringe and the crystalline solid was kept in fresh DMF for 48 h with
DMF exchanged in every 12 h. The DMF exchanged solid was then immersed
in acetone for 48 h with acetone exchanged in every 12 h before being filtered
and washed with acetone for multiple times and dried overnight at 80 °C.
Sample was activated at 150 °C for 16 h under vacuum prior to N2 adsorption .
For removal of cluster coordinated benzoic acid groups, following the
procedure by Farha et al. 80 mg of NU-1200(Zr) was soaked in 12 mL of DMF
and 0.5 mL of 4 M aq.HCl was added. The mixture was subsequently heated
in a preheated oven at 100 °C for 24 h. After cooling to room temperature, the
white solid was collected by centrifugation (10 min, 15000 rpm), washed
multiple times with DMF and acetone and immersed in acetone for 48 h with
acetone replaced every 24 h. After removal of acetone, the solid was dried at
80 °C overnight. Sample was activated at 150 °C for 16 h under vacuum prior
to N2 adsorption studies.
Zr-ABDC was synthesised according to the procedure reported by Schaate et
al. [19] 120 mg of ZrCl4 (0.51 mmol) with 1.884 g of benzoic acid (15.43 mmol)
was added to 20 mL DMF in a screw capped scintillation vial. After sonication
for 40 min, 139 mg of 4,4-azobenzenedicarboxylic acid was added to the clear
solution obtained and further sonicated for 10 min. The resulting solution after
addition of 0.05 mL of water was heated to 120 °C in a programmable oven at
a ramp rate of 5 °C/min and kept at the same for 48 h before being cooled to
room temperature. The crystalline product obtained was filtered, washed
multiple times with DMF and acetone and kept in acetone for 36 h with
exchanged once in every 12 h. After removal of acetone, the solid was dried
overnight at 80 °C. Sample was activated at 150 °C for 16 h under vacuum
prior to N2 adsorption studies.
3. PCN-222(Zr)
The published work by Liu et al. [25] was followed for the synthesis of PCN-
222(Zr). Two different stock solutions were made in 30 mL scintillation vials:
6. PCN-777(Zr)
PCN-777(Zr) was synthesised by modifying the procedure reported by Feng et
al. [21] 60 mg (0.135 mmol) of TATB and 200 mg (0.620 mmol) of ZrOCl2.8H2O
is weighed out to a glass vial. 12 mL of DEF was measured out separately, to
which 0.6 mL of trifluoroacetic acid was added. The resulting solution was
immediately added to the precursors and sonicated at room temperature for
50 min. The clear solution thus obtained is then heated to 120 °C at a ramp
rate of 5 °C/min in a programmable oven and kept at the same for 48 h under
static conditions before being cooled down to room temperature. The
supernatant solution was removed, and the white precipitate obtained was
washed multiple times with DMF before being kept in fresh DMF for 48 h with
DMF replaced in every 12 h. The DMF immersed sample is then kept in
acetone for 48 h with acetone exchanged once in every 12 h, before being
filtered and washed multiple times with acetone and dried at 80 °C overnight.
Sample was activated at 160 °C for 16 h under vacuum prior to N2 adsorption
studies.
Stock 1: 200 mg ZrOCl2.8H2O (0.620 mmol), 3.0 g of benzoic acid (25 mmol)
in 20 mL of DMF.
Stock 2: 100 mg of 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin (0.126
mmol) in 20 mL DMF.
After sonicating for 30 min, both solutions were heated at 100 °C in a
preheated non-programmable oven for an hour. After that in a 20 mL glass vial,
4 mL of solution from each stock together with 0.2 mL of trifluoroacetic acid
were mixed and heated in a preheated oil bath at 120 °C for an hour. The
purple coloured precipitate thus obtained was collected by centrifugation (10
min, 15000 rpm). The solid was washed multiple times with DMF and
immersed in DMF for 48 h with DMF exchanged once in every 16 h followed
by solvent exchange with acetone and kept in acetone for 48 h with acetone
replaced in every 24 h. After removal of the solvent, the solid was dried at
80 °C overnight. Sample was activated at 150 °C for 16 h under vacuum prior
to N2 adsorption studies.
7. PCN-777(Hf)
For removal of cluster coordinated benzoic acid groups, following the
procedure by Farha et al. [25] 100 mg of PCN-222(Zr) was soaked in 15 mL of
DMF and 0.75 mL of 8M aq.HCl was added. The mixture was subsequently
heated in a preheated oven at 120 °C for 12 h. After cooling to room
temperature, the white solid was collected by centrifugation (10 min, 15000
rpm), washed multiple times with DMF and acetone and immersed in acetone
for 48 h with acetone replaced in every 24 h. After removal of acetone, the
solid was dried at 80 °C overnight. Sample was activated at 150 °C for 16 h
under vacuum prior to N2 adsorption studies.
We here report the synthesis of PCN-777(Hf) for the first time as per our
knowledge. The procedure used for the synthesis of PCN-777(Zr) was
modified by replacing ZrOCl2.8H2O with HfOCl2.8H2O for PCN-777(Hf). 60 mg
(0.135 mmol) of TATB and 254 mg (0.620 mmol) of HfOCl2.8H2O is weighed
out to a glass vial. To 12 mL of DEF measured out separately, 0.6 mL of
trifluoroacetic acid was added. The resulting solution was immediately added
to the precursors and sonicated at room temperature for 1 h. The clear
solution obtained is then heated to 120 °C at a ramp rate of 5 °C/min in a
programmable oven and kept at the same for 48 h under static conditions
before being cooled down to room temperature. The supernatant solution was
removed, and the white precipitate obtained was washed multiple times with
DMF before being kept in fresh DMF for 48 h with DMF replaced in every 12 h.
The DMF immersed sample is then kept in acetone for 48 h with acetone
exchanged once in every 12 h, before being filtered and washed multiple times
with acetone and dried at 80 °C overnight. Sample was activated at 160 °C for
16 h under vacuum prior to N2 adsorption studies.
4. PCN-224(Zr)
PCN-224(Zr) was synthesised by following the procedure developed by Feng
[26]
et al.
60 mg of ZrCl4 (0.25 mmol), 20 mg of TCPP (0.025 mmol) and
benzoic acid were ultrasonically dissolved in a 4 mL DMF on a 35 mL glass
vial. The resulting solution was gradually heated to 120 °C with a ramp rate of
5 °C/min and kept there for 24 h before being cooled to room temperature.
After cooling down to room temperature at 5 °C/min, the supernatant solution
was removed via pipette and the reddish-purple solid was kept in fresh DMF
for 48 h with DMF exchanged in every 24 h. The DMF exchanged solid was
then immersed in acetone for 48 h with acetone exchanged in every 24 h
before being filtered and washed with acetone for multiple times and dried
overnight at 80 °C.
8. MOF-808(Zr)
Procedure was slightly modified in comparison with the reported. [11] 45 mg
(0.214 mmol) of BTC and 209 mg (0.649 mmol) of ZrOCl2.8H2O is weighed out
to a glass vial equipped with a stirrer bar. 10 mL each of DMF and formic acid
is measured out separately and mixed thoroughly before addition to the
precursors. The resulting solution after being stirred at room temperature for at
least 1 h, till the solution become homogeneous is heated to 120 °C at a ramp
rate of 3 °C/minute in a programmable oven and kept at the same for 48 h
under static conditions before being cooled down to 35 °C. The supernatant
solution was removed, and the white precipitate obtained was washed with
DMF for 20 h and followed by filtration and an acetone wash for 20 h before
drying at 80 °C for overnight. Sample was activated at 150 °C for 16 h under
vacuum prior to N2 adsorption studies.
For removal of cluster coordinated benzoic acid groups, following the
procedure by Farha et al. [25] 100 mg of PCN-224(Zr) was soaked in 15 mL of
DMF and 0.75 mL of 8M aq.HCl was added. The mixture was subsequently
heated in a preheated oven at 120 °C for 12 h. After cooling to room
temperature, the white solid was collected by centrifugation (10 min, 15000
rpm), washed multiple times with DMF and acetone and immersed in acetone
for 48 h with acetone replaced in every 24 h. After removal of acetone, the
solid was dried at 80 °C overnight. Sample was activated at 150 °C for 16 h
under vacuum prior to N2 adsorption studies.
9. MOF-808(Hf)
MOF-808(Hf) can be synthesised under conditions similar to MOF-808(Zr) with
reasonable crystallinity. Moreover, as mentioned in the main text, different
groups have also reported procedures for the synthesis of the same. We have
developed our own method for MOF-808(Hf). 45 mg (0.214 mmol) of BTC and
266 mg (0.649 mmol) of HfOCl2.8H2O is weighed out to a glass vial equipped
with a stirrer bar. 10 mL each of DMF and formic acid is measured out
separately and mixed thoroughly before addition to the precursors. The
resulting solution after being stirred at room temperature for at least 1 h, till the
solution become homogeneous is heated to 100 °C at a ramp rate of
5. NU-1200(Zr)
The published work by Liu et al. [27] was slightly modified to obtain NU-1200(Zr).
48 mg of ZrCl4 (0.20 mmol), 40 mg of TMTB (0.08 mmol) and 1.40 g of
benzoic acid or 1.61 g 2-fluorobenzoic acid (11.5 mmol) were ultrasonically
dissolved in 8 mL of DMF in a glass vial (total volume of the vial ≈ 35 mL). The
resulting solution was then heated to 120 °C at a ramp rate of 5 °C/min in a
programmable oven and kept for 48 h under static conditions before being
cooled down to room temperature. The supernatant solution was extracted
This article is protected by copyright. All rights reserved.

10.1002/chem.201803094
Chemistry - A European Journal
FULL PAPER
3 °C/minute in a programmable oven and kept at the same for 24 h under
static conditions before being cooled down to 35 °C. The supernatant solution
was removed, and the white precipitate obtained was washed with DMF for 20
h and followed by filtration and an acetone wash for 20 h before drying at
80 °C for overnight. Sample was activated at 150 °C for 16 h under vacuum
prior to N2 adsorption studies.
Acknowledgements
We would like to thank the Engineering and Physical Sciences
Research Council and CRITICAT Centre for Doctoral Training
for financial support [Ph.D studentship to R. R. R. P; Grant code:
EP/L016419/1].
10. ScBTB
We here report the scandium version of InPF-110 reported by Daniel et al.[28]
.
Conflict of Interest
Prior to synthesis Sc(NO3)3.xH2O was dried in at 80 °C overnight. 76 mg (0.33
mmol) of Sc(NO3)3.xH2O and 66 mg (0.15 mmol) BTB was added to a Teflon-
liner. After the addition of 2.5 mL of DMF, 0.8 mL nitric acid was added and
stirred at room temperature for an hour. The Teflon-liner was then sealed
inside a stainless-steel autoclave and placed in a preheated oven at 150 °C
and kept at the same for 4 h. After cooling to room temperature, the needle
shaped crystals obtained was collected by filtration, washed multiple times
with DMF, followed by ethanol wash and kept in methanol overnight. The
solvent was removed on the next day and dried at 80 °C for 24 h. Sample was
activated at 160 °C under vacuum for 16 h, before nitrogen adsorption studies.
The authors declare no conflict of interest.
Keywords: Metal-organic frameworks • MOF-808(Hf) • PSM •
phosphines • reductive amination.
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For example: Between 45.1 - 45.4 mg (max) of BTC and 266.1-266.5 mg of
HfOCl2.8H2O were accurately weighed out for the synthesis of MOF-808(Hf) in
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tested in catalysis.
Post-synthetic modifications (PSM) on MOF-808/Hf-MOF-808
12. Synthesis of MOF-808(Hf)-PTSA
To 300 mg of MOF-808(Hf) activated at 150 °C for 16 h, 0.5 mmol p-
toluenesulfonic acid in 50 mL distilled water was added and kept at room
temperature for 24 h with occasional stirring every 2-3 h. After 24 h, the water
was exchanged with acetone and kept for another 24 h with occasional stirring
every 4 h. After removal of acetone, the MOF was activated at 100 °C and
stored at ambient conditions prior to characterization studies.
13. Synthesis of SulP1-MOF-808(Hf)
To 300 mg of MOF-808(Hf) activated at 150 °C for 16 h, 0.04 mmol (14.5 mg)
sodium salt of 3-(Diphenylphosphine)benzenesulfonic acid in 50 mL degassed,
distilled water was added and kept at room temperature for 24 h with
occasional stirring every 2-3 h under inert atmosphere. After 24 h, the water
was exchanged (31P-NMR of the solution after PSE showed no characteristic
peaks for free phosphine, confirming successful exchange process) with
degassed acetone and kept for another 24 h with occasional stirring every 4 h.
After removal of acetone, the MOF was activated at 100 °C and stored at
ambient conditions before further characterization studies.
[5]
14) Synthesis of SulP2-MOF-808(Hf)
To 300 mg of MOF-808(Hf) activated at 150 °C for 16 h, 0.04 mmol (21.16 mg)
dipotassium salt pf bis(p-sulfonatophenyl)phenylphosphine dihydrate in 50 mL
degassed, distilled water was added and kept at room temperature for 24 h
under inert atmosphere with occasional stirring every 2-3 h. After 24 h, the
water was exchanged with degassed acetone and kept for another 24 h with
occasional stirring every 4 h. After removal of acetone, the MOF was activated
at 100 °C and stored at ambient conditions before further characterization
studies.
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FULL PAPER
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10.1002/chem.201803094
Chemistry - A European Journal
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Post-synthetic modification of hafnium
Ram R. R. Prasad, Daniel M. Dawson,
Paul A. Cox, Sharon E. Ashbrook, Paul
A. Wright* and Matthew L. Clarke*
MOF-808
with
sulfonated
ar-
ylphosphines and precious metal cata-
lyst pre-cursors generates highly ac-
tive and tunable bifunctional catalysts
for reductive amination reactions.
A Bifunctional MOF Catalyst contain-
ing Metal-phosphine and Lewis Acid-
ic Active Sites
This article is protected by copyright. All rights reserved.