Ruthenium complexation in an aluminium metal-organic framework and its application in alcohol oxidation catalysis

Article abstract of DOI:10.1002/chem.201200885

A ruthenium trichloride complex has been loaded into an aluminium metal-organic framework (MOF), MOF-253, by post-synthetic modification to give MOF-253-Ru. MOF-253 contains open bipyridine sites that are available to bind with the ruthenium complex. MOF-253-Ru was characterised by elemental analysis, N₂ sorption and X-ray powder diffraction. This is the first time that a Ru complex has been coordinated to a MOF through post-synthetic modification and used as a heterogeneous catalyst. MOF-253-Ru catalysed the oxidation of primary and secondary alcohols, including allylic alcohols, with PhI(OAc) ₂ as the oxidant under very mild reaction conditions (ambient temperature to 40 C). High conversions (up to >99 %) were achieved in short reaction times (1-3 h) by using low catalyst loadings (0.5 mol % Ru). In addition, high selectivities (>90 %) for aldehydes were obtained at room temperature. MOF-253-Ru can be recycled up to six times with only a moderate decrease in substrate conversion. In the frame: A ruthenium trichloride complex is loaded into an aluminium metal-organic framework by post-synthetic modification (see figure). This material is used in the heterogeneous catalysis of the oxidation of primary and secondary alcohols, including allylic alcohols, with PhI(OAc)₂ as the oxidant under mild reaction conditions. High selectivities for aldehydes are obtained at room temperature. Copyright

Full text of DOI:10.1002/chem.201200885

FULL PAPER
DOI: 10.1002/chem.201200885
Ruthenium Complexation in an Aluminium Metal–Organic Framework and
Its Application in Alcohol Oxidation Catalysis
Fabian Carson,[a, b] Santosh Agrawal,[a, c] Mikaela Gustafsson,*[a, b]
Agnieszka Bartoszewicz,[a, c] Francisca Moraga,[a, b] Xiaodong Zou,*[a, b] and
Belꢀn Martꢁn-Matute*[a, c]
Abstract:
complex has been loaded into an alu-
minium metal–organic framework
A
ruthenium trichloride
through post-synthetic modification
and used as a heterogeneous catalyst.
MOF-253-Ru catalysed the oxidation
of primary and secondary alcohols, in-
cluding allylic alcohols, with PhIACTHNUGTRENUNG(OAc)2
as the oxidant under very mild reaction
conditions (ambient temperature to
408C). High conversions (up to
>99%) were achieved in short reac-
tion times (1–3 h) by using low catalyst
loadings (0.5 mol% Ru). In addition,
high selectivities (>90%) for alde-
hydes were obtained at room tempera-
ture. MOF-253-Ru can be recycled up
to six times with only a moderate de-
crease in substrate conversion.
(MOF), MOF-253, by post-synthetic
modification to give MOF-253-Ru.
MOF-253 contains open bipyridine
sites that are available to bind with the
ruthenium complex. MOF-253-Ru was
characterised by elemental analysis, N2
sorption and X-ray powder diffraction.
This is the first time that a Ru complex
Keywords: alcohol oxidation · het-
erogeneous catalysis · immobiliza-
tion · metal–organic frameworks ·
ruthenium
has been coordinated to
a MOF
Introduction
MOFs will find use in areas such as gas separation and stor-
age, fine chemicals catalysis and drug delivery.[6]
The last decade has witnessed an explosion in the field of
metal–organic frameworks (MOFs) and porous coordination
polymers (PCPs).[1] These materials are constructed from
metal ions or clusters connected by polytopic organic li-
gands that assemble into three-dimensional, crystalline,
porous frameworks. Much enthusiasm towards these materi-
als has stemmed from their huge surface areas, which are
typically higher than those of traditional porous solids, such
as zeolites[2,3] and mesoporous materials.[4,5] An almost end-
less combination of metal ions and organic linkers has led to
a vast library of MOF structures in recent years. As the
focus shifts from synthesis to applications, it is expected that
MOFs were recognised as potential catalysts as early as
1990,[7] and several years later, Fujita and co-workers dem-
onstrated for the first time the use of MOFs as heterogene-
ous catalysts.[8] Recently, there has been extensive activity in
this field, and numerous reviews have appeared in the litera-
ture.[9]
As MOFs are hybrid materials, different parts of the
framework can function as the active site. First, the metal
node can act as a Lewis acid catalyst if coordinatively unsa-
turated sites are available.[8,10] Furthermore, organic mole-
cules with coordinating groups can be grafted to these metal
sites to introduce additional catalytic functionality to the
MOF.[11] Second, the linker may contain functional groups,
or metal complexes, that can act as a catalyst;[12] examples
include pyridyl groups,[13] metalloporphyrins,[14] binaphthyl[15]
and metal–salen complexes.[16] Third, guest-encapsulated
species, such as metal[17] or metal oxide nanoparticles,[18] pol-
yoxometalate clusters,[19] or metal complexes,[20] can be sup-
ported in the void spaces of the MOF.
[a] F. Carson, Dr. S. Agrawal, Dr. M. Gustafsson, A. Bartoszewicz,
F. Moraga, Prof. X. Zou, Assoc. Prof. B. Martꢀn-Matute
Berzelii Center EXSELENT on Porous Materials
Stockholm University, SE-10691 Stockholm (Sweden)
Fax : (+46)8-15-4908
E-mail: mikaela.gustafsson@gmail.com
xzou@mmk.su.se
The oxidation of alcohols to carbonyl compounds is one
of the most fundamental processes in organic synthesis.[21]
During the last decade, a number of green heterogeneous
catalytic systems have been developed that use molecular
oxygen or air as the oxidant.[22] These typically employ
metal-based heterogeneous catalysts containing rutheni-
um,[23] gold[24] or platinum.[25] However, many of these sys-
tems require harsh reaction conditions and substrate selec-
tivity can be difficult to control. The selective oxidation of
primary alcohols to aldehydes at room temperature with
belen@organ.su.se
[b] F. Carson, Dr. M. Gustafsson, F. Moraga, Prof. X. Zou
Inorganic and Structural Chemistry
Department of Materials and Environmental Chemistry, Arrhenius
Laboratory
Stockholm University, SE-10691 Stockholm (Sweden)
[c] Dr. S. Agrawal, A. Bartoszewicz, Assoc. Prof. B. Martꢀn-Matute
Department of Organic Chemistry, Arrhenius Laboratory
Stockholm University, SE-10691 Stockholm (Sweden)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200885.
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heterogeneous catalysts is still rather limited and represents
a challenging area in green chemistry.[26] Oxidation catalysis
with MOFs has been particularly prolific over the past sev-
eral years.[27] The majority of these transformations have
employed either metal nodes or encapsulated nanoparticles
in the MOF as catalysts. For example, Van der Voort and co-
workers used V-MIL-47 as a catalyst for the oxidation of cy-
clohexene with tert-butylhydroperoxide as the oxidant; how-
ever, the catalyst showed low selectivity.[28] Li, Tang and co-
workers employed gold nanoparticles in Cr-MIL-101 for the
selective aerobic oxidation of alcohols to the corresponding
carbonyl compounds, including benzylic alcohols.[29] In this
case, the oxidation of 1-phenylethanol with molecular
oxygen, albeit at relatively high temperatures (1608C),
showed spectacular results with a turnover frequency (TOF)
of 29300 hÀ1. There are also examples of enantioselective
oxidation by using MOFs containing chiral linkers that can
coordinate to transition-metal complexes.[9c,30] Hupp and co-
workers synthesised a Zn-MOF containing a chiral Mn–
salen linker, which catalysed the enantioselective epoxida-
tion of 2,2’-dimethyl-2H-chromene, achieving a turnover
number (TON) of 1430 with 82% ee.[16a] Lin and co-workers
used a similar Mn–salen linker as a catalyst for the enantio-
selective epoxidation of a variety of olefins, with up to
92% ee.[16b]
Results and Discussion
Synthesis and characterisation of ruthenium complexes: Sri-
vastava and co-workers previously reported that reaction of
[RuCl
formation of [Ru
procedure to synthesise a similar complex, [Ru
(dmso)] (2; bpydc=2,2’-bipyridine-5,5’-dicarboxylic acid), in
4ACHTUGTNRNENUG(dmso)2]ACTHNUGTRENNNUG
A
CHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
which bpy was replaced with bpydc (Figure 1a). Unfortu-
Figure 1. a) Ru-bipyridyl complexes. b)Schematic representation of Ru
complex coordinated to the bpy site of MOF-253 in a post-synthetic step.
nately, attempts to synthesise MOFs with this ruthenium-
functionalised linker were unsuccessful. It was then realised
that ruthenium could be introduced into MOF-253, which
contains the appropriate bpy ligand to coordinate rutheni-
um, by PSM.[15,40,41] Thus, we first synthesised MOF-253 as
previously reported.[35] The structure of MOF-253 is built of
infinite, one-dimensional chains of AlO6 corner-sharing oc-
tahedra that connect through bpydc linkers to give rhombic-
shaped pores with a free diameter of 13ꢂ11 ꢃ (Figure 2).
The structure crystallises in an orthorhombic unit cell (a=
22.63, b=5.58, c=18.60 ꢃ; V=2348.84 ꢃ3).[42] The X-ray
powder diffraction (XRPD) patterns of as-synthesised
MOF-253 are shown in Figure S1 in the Supporting Informa-
Ruthenium compounds represent a versatile group of cat-
alysts that are important in various organic transforma-
tions.[31,32] Ru complexes containing N-donor ligands, such as
bipyridine (bpy) and phenanthroline, are particularly active
as homogeneous oxidation and isomerisation catalysts.[33] It
has been shown that certain ruthenium–bpy complexes can
undergo deactivation through a ligand redistribution mecha-
nism, in which multiple bpy ligands bind to the metal.[34]
Supporting Ru in a MOF, on the other hand, would over-
come this issue. As the bpy ligands are remotely distributed
throughout the framework, inactive [Ru
G
tion. Reaction of MOF-253 with [RuCl4ACHTUNGRTENNU(G dmso)2]ACHTNUGTREN[NNUG (dmso)2H]
ligand) complexes should not form. MOF-253, recently re-
ported by Yaghi and co-workers,[35] was selected as a suita-
ble host, as it has a reasonably large Langmuir surface area
(2490 m2 gÀ1) and contains accessible bpy units in the frame-
work. Furthermore, high chemical and thermal stability is
expected of MOF-253 due to the Al3+ nodes.[36,37] In fact,
the recyclability of this MOF as a catalyst was recently dem-
onstrated for C–C cross-coupling reactions.[38]
(Figures 1b and 2) resulted in the formation of a ruthenium-
functionalised MOF (MOF-253-Ru), as confirmed by
XRPD, elemental analysis, thermogravimetric analysis
(TGA) and N2 sorption measurements. The XRPD patterns
of MOF-253 before and after ruthenium complexation are
shown in Figure 3. Despite the low quality of these pat-
terns—typical of Al-MOFs and most likely due to disorder
within the crystal structure[43]—the peaks correspond closely
with the strong peaks in the simulated pattern,[36] although
there is a small shift towards higher 2q values. As-synthes-
ised MOF-253 contained amorphous species that were re-
moved by washing with DMF and MeOH (Figure S1 in the
Supporting Information).
To gain some insight into the number and type of ligands
coordinated to ruthenium in MOF-253-Ru, we compared its
characterisation data (CHSN and Cl analyses and inductive-
ly coupled plasma–optical emission spectrometry (ICP-
OES)) with those of homogeneous ruthenium complexes 1–
3 (Figure 1a and the Supporting Information). Because
complex 1 is formed immediately at room temperature from
Herein, we report the immobilisation of a ruthenium com-
plex in a MOF (MOF-253-Ru) by post-synthetic modifica-
tion (PSM). This material is an efficient catalyst for the oxi-
dation of alcohols with PhIACHTNUTRGNENG(U OAc)2 as the oxidant. A wide
range of secondary alcohols were oxidised to ketones in
good to excellent yields under mild conditions (RT to
408C). In addition, primary alcohols were selectively oxi-
dised to aldehydes at room temperature within 1–2 h. The
MOF catalyst can be easily recycled and displays only a
minor loss of catalytic activity over six cycles.
[RuCl4ACHTNUTRGENNUG(dmso)2]ACHTUNGTERNN[UGN (dmso)2H] and bpy in ethanol, we anticipat-
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Ruthenium Complexation in an Aluminium Metal–Organic Framework
FULL PAPER
Figure 2. Representative structure and post-synthetic modification of MOF-253. Blue octahedra represent Al atoms; grey, red and green spheres repre-
sent C, O and N atoms, respectively; orange spheres represent the Ru complex, [RuCl
figure was drawn by using structural data taken from the literature, [36].
3ACHTUNGRTEN(NUNG dmso)]. H atoms and guest molecules are omitted for clarity. The
Figure 3. From bottom to top: XRPD patterns of simulated MOF-253,
washed MOF-253, MOF-253-Ru7, MOF-253-Ru13 and MOF-253-Ru7
after six catalytic cycles.
ed that reaction of [RuCl4ACHTNUGTRENN(UG dmso)2]ACHTUNTGERN[NUGN (dmso)2H] with MOF-
253 would result in formation of a complex with similar stoi-
chiometry inside MOF-253. Indeed, a stoichiometry match-
ing that of [RuCl
ysis, which indicated formation of the complex
[Ru(bpydc)Cl3A(dmso)] (Figure 1b) within the MOF. Differ-
3ACHTUNGTRENNUNG(dmso)] was confirmed by elemental anal-
A
CHTUNGTRENNUNG
ent amounts of Ru were loaded into MOF-253 to determine
the maximum possible catalyst loading. Two samples were
prepared containing 7 and 13 mol% Ru (i.e., Ru complexes
occupied 7 and 13% of the bpydc linkers in the MOF, re-
spectively),[44] and named as MOF-253-Ru7 and MOF-253-
Ru13. MOF-253-Ru13 was prepared using a large excess of
Ru (Ru/linker=1.5:1). Remarkably, the sample contained
only 13 mol% Ru, which indicated that only a fraction of
the bpydc sites were functionalised. The material remained
crystalline upon incorporation of Ru (Figure 3).
The thermal stability of MOF-253 and MOF-253-Ru in air
was investigated by TGA. The thermogravimetry (TG)
curves of the MOF-253-Ru samples, displayed in Figure 4
(top), showed 7–9% weight loss between 100 and 3008C,
which corresponds to decomposition of the Ru complex.
Collapse of the framework occurred at significantly lower
*
*
)
Figure 4. Top: TG curves; bottom: N2 adsorption ( ) and desorption (
isotherms for MOF-253 (a), MOF-253-Ru7 (b) and MOF-253-Ru13 (c).
temperatures (3308C) for MOF-253-Ru relative to MOF-
253 (4008C).
The N2 sorption isotherms of MOF-253 and MOF-253-Ru
samples are shown in Figure 4 (bottom). The Langmuir sur-
face areas of MOF-253, MOF-253-Ru7 and MOF-253-Ru13
were calculated to be 1202, 1145 and 80 m2 gÀ1, and the
pore volumes were estimated to be 0.51, 0.47 and
0.05 cm3 gÀ1, respectively. The surface area of MOF-253 was
considerably lower than the reported value of 2490 m2 gÀ1.[35]
The CHN analysis (see the Supporting Information) and
observed weight losses in the TGA (Figure 4) matched
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M. Gustafsson, X. Zou, B. Martꢀn-Matute et al.
closely with the chemical formula of MOF-253,
[Al(bpydc)(OH)·0.5H2O], thereby indicating that no excess
Ru7. The results are displayed in Table 1. The reaction in
the absence of Ru with or without MOF-253 gave only
traces of acetophenone (Table 1, entries 1 and 2). Addition
of 1 mol% MOF-253-Ru7 resulted in complete oxidation to
acetophenone within 3 h (Table 1, entry 3). The catalyst
loading could be decreased to 0.25 mol% Ru, which afford-
ed conversions of >95% within 3 h (Table 1, entries 4–6).
Almost quantitative oxidation was achieved after 2 h with
0.5 mol% Ru (Table 1, entry 5 in parentheses). A catalyst
loading of 0.05 mol% Ru (Table 1, entry 7) gave a TON of
1980 (10 h). Moreover, the reaction could be performed at
room temperature with 0.5 mol% Ru (Table 1, entry 8) or
by using 1 equivalent of oxidant (Table 1, entry 9), although
longer times were required (4 and 4.5 h, respectively).
ACHTUNGTRENNUNG
linker was present in the pores of MOF-253. Therefore, this
lower surface area is not related to the activation procedure,
but rather to the crystal defects of the MOF. The surface
area decreased slightly upon low Ru loading, whereas
higher loading led to a rapid drop in surface area. The pore
openings in MOF-253-Ru13 are most likely blocked by the
[RuACHTUNGTRENNUNG(bpydc)Cl3ACHTUNGTRENNUNG(dmso)] complexes so that neither more Ru
complexes nor N2 molecules could enter the pores, thus re-
sulting in the low surface area. SEM images of MOF-253 re-
vealed uniform spherical particles, around 0.2–0.5 mm in di-
ameter (Figure S2a in the Supporting Information). There
was little change in particle size or morphology for MOF-
253-Ru7 (Figure S2b in the Supporting Information). How-
ever, at higher Ru loading (13 mol%), aggregation of the
MOF particles occurred, thereby leading to larger particles
1 mm in diameter (Figure S2c in the Supporting Informa-
tion). In addition, the MOF-253-Ru13 particles were inho-
mogeneous and the surfaces were covered with smaller rod-
shaped crystallites. This is evidently related to the increase
in the amount of the ruthenium complex present in the
sample, and may explain the low surface area of MOF-253-
Ru13.
The method was expanded to other secondary alcohols by
using the reaction conditions shown in Table 1, entry 5 and
Table 2. Oxidation of various alcohols by using MOF-253-Ru as the cata-
lyst.[a]
Entry Product
t
T
Conversion/yield[b] TOF
[h] [8C] [%]
[hÀ1
N
]
Oxidation catalysis with MOF-253-Ru: A preliminary study
was carried out using the homogeneous Ru complexes
[RuACHTUNGTRENNUNG(bpy)Cl3ACHTUNGTRENNUNG(dmso)] (1) and [RuCl3ACHTUNEGRTG(NNUN dbpydc)ACHTNUGTRNE(NUGN dmso)] (3;
1
2
2
2
40
40
97/97
99/99
97
99
dbpydc=diethyl 2,2’-bipyridine-5,5’-dicarboxylate) to find
suitable conditions for the oxidation of alcohols (Scheme S1
in the Supporting Information). The oxidation of 1-phenyle-
thanol was tested as a model reaction. Over 99% conversion
was achieved by using 1 mol% Ru (1 and 3) and (diacetox-
y)iodobenzene as the oxidant in dichloromethane at 408C in
1.5 h (Table S1 in the Supporting Information). This reaction
was then extended to the heterogeneous catalyst MOF-253-
3
4
3
40
93/93 (89)
95/95 (90)
62
63
3
2
40
40
Table 1. Optimisation of reaction conditions for the oxidation of 1-phe-
nylethanol with MOF-253-Ru7 as the catalyst.[a]
5
6
7
92/92 (88)
98/98
92
56
43
3.5 40
Entry
Ru
[mol%]
t
Yield
[%][b]
N
[h]
4.5 40
4.5 40
97/97
1
2
3
4
5
6
–
3
3
3
3
4
3
99
99
MOF-253 (no Ru)
8
9
99/99
44
99
1.00
0.75
0.50
0.25
0.05
0.50
0.50
2
2
40
40
99/95[d]
3 (2)
99[c] (97)
3
10
4
95
99
97
98
10
95/88[d]
96
41
7[d]
8[e]
9[f]
4.5
11[c,d]
4
5
40
40
82/78 (75)[e]
[a] All reactions were conducted by using MOF-253-Ru7 (7 mol% or
2.2 wt% Ru), 1-phenylethanol (0.5 mmol, 60 mL), PhI(OAc)2 (0.75 mmol,
242 mg) and dichloromethane (1 mL) at 408C. [b] Based on analysis by
gas chromatography (GC). [c] Initial turnover frequency (TOF)=480 hÀ1
[d] 2.5 mmol of 1-phenylethanol (1m) was used. [e] Reaction at 228C.
[f] Reaction was conducted with 1 equivalent of PhI(OAc)2.
AHCTUNGTRENNUNG
12[c,d]
64/60 (54)[e]
16
.
AHCTUNGTRENNUNG
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Ruthenium Complexation in an Aluminium Metal–Organic Framework
Table 2. (Continued)
FULL PAPER
Ru (Table 2, entries 14–17). Therefore, this work represents
a considerable step forward in the application of MOFs as
heterogeneous catalysts for the selective oxidation of alco-
hols under mild conditions. On the other hand, these previ-
Entry Product
t
T
Conversion/yield[b] TOF
[h] [8C] [%]
[hÀ1
N
]
[29,46]
[47]
ous MOF catalysts employed molecular O2
as the oxidant, which results in fewer byproducts and less
waste in comparison to PhI(OAc)2. Unfortunately, oxidation
of primary aliphatic alcohols, such as 1-nonanol or 2,2-dime-
thylpropan-1-ol, afforded only traces of the corresponding
aldehydes.
A more complex alcohol, cholestanol, was oxidised to the
corresponding ketone in excellent yield after 4 h in the pres-
ence of 0.5 mol% Ru by using MOF-253-Ru7 (Scheme 1).
or H2O2
13[c,d]
5
40
55/51 (43)[e]
11
AHCTUNGTRENNUNG
14[c,f]
15[c,f]
1.5 22
99/90[g]
98/91[g]
66
49
2
2
22
22
16[c,f]
17[c,f]
95/88[g]
90/84[g]
48
60
1.5 22
[a] All reactions were conducted by using MOF-253-Ru7 (0.5 mol% Ru,
0.0025 mmol, 11.5 mg), the substrate alcohol (0.5 mmol, 60 mL), PhI-
ACHTUNGTRENNUNG(OAc)2 (0.75 mmol, 242 mg) and CH2Cl2 (1 mL) at the temperature indi-
cated. [b] Based on GC analysis with dodecane as the internal standard;
yield of the isolated product is given in parentheses. [c] 1 mol% Ru used
1
as the catalyst. [d] Yield based on H NMR spectroscopy of the crude re-
action mass in the presence of 1,4-dimethoxybenzene as the internal
standard; yield of the isolated product is given in parentheses. [e] a,b-Un-
saturated aldehydes were formed in small quantities (up to 4%).
[f] 1.5 mL of CH2Cl2 was used. [g] Carboxylic acids formed in 6—9%
yield.
Scheme 1. Oxidation of cholestanol to cholestanone catalysed by MOF-
253-Ru7.
Table 2, entry 1. Benzylic sec-alcohols[45] containing aromatic
rings bearing electron-donating and electron-accepting
groups were also oxidised to the corresponding ketones with
nearly full conversion after 2–3 h (Table 2, entries 2–5). A
variety of aliphatic cyclic and acyclic sec-alcohols were oxi-
dised in excellent yield within 2–5 h (Table 2, entries 6–10).
Chemoselective oxidation of the alcohol moiety for several
allylic alcohols was achieved with moderate to good yields
in 4–5 h by using 1 mol% Ru (Table 2, entries 11–13).
Since MOF-253 possesses one-dimensional channels, only
the two shortest dimensions of the molecule need to be con-
sidered. For cholestanol these are 8.1ꢂ6.5 ꢃ, significantly
smaller than the channels of the MOF (11ꢂ13 ꢃ). There-
fore, the substrates should be able to enter the pores of
MOF-253. According to the proposed mechanisms, the reac-
tion proceeds through initial coordination of the oxidant to
the metal complex, followed by reaction of this complex
with the substrate.[48] The pore size is large enough to ac-
commodate the oxidant–Ru substrate complex.
Oxidation of primary alcohols to the corresponding alde-
hydes in high yields is a challenging task. A major problem
usually encountered is the formation of variable amounts of
carboxylic acids, particularly when using heterogeneous cat-
alysts that typically require high reaction temperatures.
High yields and selectivities for aldehydes at room tempera-
ture with MOFs as catalysts have only been achieved by
Pt@MOF-177,[46] which required reaction times of 24 h. Un-
fortunately, Pt@MOF-177 collapsed after the first reaction
cycle and could not be reused. A MOF containing Ru pad-
dlewheels also demonstrated high selectivities for the oxida-
tion of alcohols to aldehydes; however, relatively low TONs
(ꢀ10) were attained and long reaction times (24 h) were re-
quired.[47] Alcohols were also selectively oxidised to alde-
hydes with high conversions by Au/MIL-101 in short reac-
tion times (1–3 h) at relatively high temperatures (808C).[29]
By using similar conditions to those used with secondary al-
cohols (408C), oxidation of primary benzylic alcohols cata-
lysed by MOF-253-Ru7 gave low selectivity for the corre-
sponding aldehyde as a result of over-oxidation to the car-
boxylic acid. However, excellent conversions and selectivi-
ties were achieved at room temperature by using 1 mol%
The heterogeneous nature of the catalyst was verified by
hot filtration tests. At 72% conversion of 1-phenylethanol
into acetophenone (30 min), the catalyst was removed by fil-
tration and the reaction was continued. After 3 h, no signifi-
cant increase in acetophenone formation was observed (a
reaction profile is shown in Figure S4 in the Supporting In-
formation). ICP-OES, however, revealed 0.9 ppm Al and
1.6 ppm Ru present in the supernatant, corresponding to 0.1
and 0.7% of the initial amounts of Al and Ru, respectively,
in MOF-253-Ru7. Consequently, although metal leaching
does occur, it does not contribute significantly to the oxida-
tion of the alcohol. The evolution of ruthenium leaching
over time upon heating at 358C in CH2Cl2 or in a mixture of
CH2Cl2 and AcOH (1m) is presented in the Supporting In-
formation (Figure S5). It was observed that in the former
case, a ruthenium leaching of 0.6% occurs during the first
2 h. Interestingly, after this time the leaching slows down
considerably, and the amount of ruthenium remains constant
at least up to 10 h under the same conditions. Similar results
were obtained when MOF-253-Ru7 was heated at 358C in
the mixture CH2Cl2/AcOH (1m), the difference being a
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M. Gustafsson, X. Zou, B. Martꢀn-Matute et al.
slightly increased ruthenium leaching (1.4%) during the first
couple of hours. These results may indicate that the material
contains some ruthenium species weakly bound to the MOF,
which are easily displaced. However, most of the ruthenium
species remain within the MOF. These species are catalyti-
cally active, since the material could be recycled for at least
six runs (see below).
and morphology between the samples (Figure S4 in the Sup-
porting Information).
Conclusion
In this work, we have established a method for introducing
ruthenium complexes into MOF-253. The MOF remains
crystalline and retains a high surface area upon coordination
Recycling experiments were performed by using
a
6.5 mmol scale of 1-phenylethanol. After the required reac-
tion time, the catalyst was recovered, washed with solvent
and dried under vacuum at room temperature for 16 h be-
tween each cycle. The catalyst was reused six times with
only a moderate decrease in conversion (Table S2 in the
Supporting Information, first cycle: 97%, sixth cycle: 85%).
Analysis of the used catalyst revealed no change in either
crystallinity or surface area (Figures 3 and 5), which indicat-
of small amounts of [RuCl3ACTHNUTRGNE(UNG dmso)]. In addition, we have
shown that our synthesised MOF-253-Ru can catalyse the
oxidation of a wide range of alcohols with high selectivities
for ketones and aldehydes. Oxidation of primary alcohols to
aldehydes at room temperature and in short reaction times
is unprecedented in MOF catalysis. The MOF-253-Ru cata-
lyst is truly heterogeneous and can be successively recycled
up to six times without significant loss of activity, crystallini-
ty or surface area. In addition, samples containing low Ru
loadings and higher surface areas were significantly more
active as oxidation catalysts than samples with high Ru load-
ings and low surface areas. Because the bpydc ligands are
fixed to the MOF, deactivation of the catalyst through for-
mation of inactive [RuACTHNUTRGNE(NUG bpy)3ÀxL2x] species is avoided. Al-
though the coordination of metal complexes into MOFs by
PSM is a practical method that leads to materials with en-
hanced functionality, this work demonstrates that even
stronger metal binding sites in the framework are required
for practical applications associated with heterogeneous cat-
alysis.
~
~
*
,
*
Figure 5. N2 adsorption (
,
) and desorption (
) isotherms for MOF-
~ ~
* *
253-Ru7 before catalysis (
,
) and MOF-253-Ru7 after six cycles ( , ).
Experimental Section
General: All chemicals and solvents were used as received without fur-
ther purification. XRPD was carried out on a PANalytical XꢄPert PRO
diffractometer in reflectance Bragg–Brentano geometry equipped with a
pixel detector and using CuKa1 radiation. The samples were dispersed on
zero-background Si plates. Nitrogen sorption was carried out at 77 K on
a Micromeritics ASAP2020 device. Samples of MOF-253 and MOF-253-
Ru were degassed at 250 and 508C for 16 h, respectively, prior to sorp-
tion measurements. TGA was performed on a PerkinElmer TGA 7 appa-
ratus equipped with a platinum pan, and all samples were heated at a
rate of 28CminÀ1 in air. CHSN analysis was performed on a Carlo Erba
Flash 1112 elemental analyser, Cl analysis was carried out by Schçniger
flask combustion followed by titration, and ICP-OES was used for metal
determination with a Varian Vista MPX ICP-OES instrument. Medac
Ltd. (UK) carried out the elemental analysis. Field-emission SEM was
performed on a JEOL JSM-7000F microscope operated at 5 kV. The
samples were pre-coated with a thin carbon film. High-resolution mass
spectra were recorded on a Bruker microTOF ESI–time-of-flight mass
spectrometer. GC was performed on a Varian 3800 chromatograph with
a CP-CHIRASIL-DEX CB chiral column (30 m) and a flame ionisation
detector. Dodecane was used as an internal standard. 1H NMR spectra
were recorded at 400 MHz and 13C NMR spectra at 100 MHz, on a
Bruker Avance 400 spectrometer. Chemical shifts (d) are reported in
ppm, with the residual solvent peaks in CDCl3 (dH =7.28; dC =
77.00 ppm) as an internal reference; coupling constants (J) are given in
hertz.
ed that the Al leaching did not affect the integrity of the
framework. On the other hand, the Ru content decreased
from 7 to 5 mol% after six cycles, according to elemental
analysis. SEM images of MOF-253-Ru7 after six cycles
showed little change in the particle size (Figure S2d in the
Supporting Information). Elemental analysis also revealed
that the dmso and Cl ligands on the Ru complex in the
MOF had been exchanged during catalysis (Supporting In-
formation).
Finally, the effect of porosity on the catalytic properties of
MOF-253-Ru was examined. Since MOF-253-Ru13 has a
significantly lower surface area (80 m2 gÀ1) than MOF-253-
Ru7 (1145 m2 gÀ1), we expected it would be an inferior cata-
lyst. MOF-253-Ru13 was employed as a catalyst for the oxi-
dation of 1-phenylethanol by using the reaction conditions
from Table 1, entry 5 (0.5 mol% catalyst loading). As we
suspected, after 1 h only 36% of the substrate had been con-
verted to acetophenone, compared with 62% conversion
achieved with MOF-253-Ru7 after 1 h. This suggests that
ruthenium within the pores of MOF-253-Ru13 is less acces-
sible to the substrate, as a result of pore blockage. Catalysis
will therefore mainly occur on the surface of the MOF-253-
Ru13 particles. However, this variation in catalytic activity
may also be attributable to the difference in particle size
Synthesis of MOF-253: The synthesis conditions used were similar to
those described in the literature, [35]. AlCl3·6H2O (151 mg, 0.625 mmol)
and bpydc (153 mg, 0.625 mmol) were added to a Teflon-capped 20 mL
glass vial containing N,N’-dimethylformamide (DMF, 10 mL) and stirred
15342
www.chemeurj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 15337 – 15344

Ruthenium Complexation in an Aluminium Metal–Organic Framework
FULL PAPER
for 30 min at room temperature, followed by sonication for 10 min. The
mixture was heated at 1208C for 24 h in an oil bath on a hotplate. The re-
sulting white powder was rinsed in fresh DMF and the supernatant deca-
nted. The Al-MOF was washed in DMF at 808C for 3 h, followed by
washing in methanol by Soxhlet extraction for 24 h, after which the solid
was rinsed in fresh methanol and heated overnight at 2008C under
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vacuum to give [Al
Synthesis of MOF-253-Ru7 and MOF-253-Ru13: The preparation of
MOF-253-Ru7 was carried out as follows: [Al(bpydc)(OH)] (1.078 g,
3.652 mmol) and [RuCl4A(dmso)2][(dmso)2H] (0.163 g, 0.292 mmol) were
ACHTUNGTRENNUNG(bpydc)(OH)], MOF-253 (165 mg, 90%).
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AHCTUNGTRENNUNG
C
ACHTUNGTRENNUNG
heated at reflux in absolute ethanol (100 mL) for 24 h while stirring. A
dark-brown solid was collected by centrifugation. The solid was washed
by stirring in fresh ethanol for 2 h, followed by centrifugation. This proc-
ess was repeated three times. The powder was then dried at 508C under
vacuum for 16 h to yield the product, [AlACTHNUTRGENN(GU bpydc)(OH)]·0.07RuCl3ACHTUNGTRNE(NUGN dmso)
(1.085 g, 97%). MOF-253-Ru13 was prepared in a similar manner; exact
amounts are supplied in the Supporting Information.
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for the appropriate reaction time. After cooling to room temperature the
mixture was centrifuged and the conversion was determined by GC.
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Acknowledgements
This project is supported by the Swedish Research Council (VR) and the
Swedish Governmental Agency for Innovation Systems (VINNOVA)
through the Berzelii Center EXSELENT. S.A. is grateful for a post-doc-
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from the Gçran Gustafsson Foundation for Nature Sciences and Medical
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