Heterometallic Metal-Organic Frameworks That Catalyze Two Different Reactions Sequentially

Article abstract of DOI:10.1021/acs.inorgchem.6b00292

A series of copper- and alkaline-earth-metal-based multidimensional metal-organic frameworks, {[CuMg(pdc)₂(H₂O)₄]·2H₂O}_n (1), [CuCa(pdc)₂]_n (2), [CuSr(pdc)₂(H₂O)₃]_n (3), and {[Cuba(pdc)₂(H₂O)₅]·H₂O}_n (4), where H₂Pdc = pyridine-2,5-dicarboxylic acid, were hydrothermally synthesized and characterized. Two different metals act as the active center to catalyze two kinds of reactions, viz., olefin to its epoxide followed by epoxide ring opening to afford the corresponding vicinal diol in a sequential manner.

Full text of DOI:10.1021/acs.inorgchem.6b00292

Communication
pubs.acs.org/IC
Heterometallic Metal−Organic Frameworks That Catalyze Two
Different Reactions Sequentially
Debraj Saha,†,‡ Dipak K. Hazra,§ Tanmoy Maity,† and Subratanath Koner*,†
†Department of Chemistry, Jadavpur University, Kolkata 700032, India
§Department of Physics, Uluberia College, Howrah 711315, India
S
* Supporting Information
Herein, we report a series of metal carboxylate compounds,
ABSTRACT: A series of copper- and alkaline-earth-
metal-based multidimensional metal−organic frameworks,
{[CuMg(pdc)2(H2O)4]·2H2O}n (1), [CuCa(pdc)2]n (2),
[CuSr(pdc)2(H2O)3]n (3), and {[CuBa(pdc)2(H2O)5]·
H2O}n (4), where H2Pdc = pyridine-2,5-dicarboxylic
acid, were hydrothermally synthesized and characterized.
Two different metals act as the active center to catalyze
two kinds of reactions, viz., olefin to its epoxide followed
by epoxide ring opening to afford the corresponding
vicinal diol in a sequential manner.
{[CuMg(pdc)2(H2O)4]·2H2O}n (1), [CuCa(pdc)2]n (2),
[CuSr(pdc)2(H2O)3]n (3), and {[CuBa(pdc)2(H2O)5]·H2O}n
(4), where H2Pdc is pyridine-2,5-dicarboxylic acid. While 1 is a
2D framework compound, the rest are of the 3D variety.
Catalytic efficacy of these framework compounds in olefin
epoxidation reactions, and subsequently in situ epoxide ring-
opening reactions, has been investigated. To our knowledge,
these are the first examples of hetero-MOFs where two different
metal centers catalyze two different chemical reactions
sequentially under heterogeneous conditions.
The phase-pure crystals 1−4 were grown under hydrothermal
conditions and were characterized by single-crystal X-ray
diffraction [see the Supporting Information (SI), Tables S1
and S2]. Compound 1 features a 2D structure constructed by a
ribbonlike 1D chain of hexacoordinated CuII centers, CuO4N2,
which is connected through hexacoordinated tetraaqua MgII
centers of a MgO6 unit (Figure 1a). Each carboxylate ligand is
coordinated to two Cu centers and one Mg center through three
carboxylate O atoms and one pyridyl N atom to give rise to a 2D
framework (Figure 1a). To gain deeper insight into the structures
of 1−4, topological analysis was performed by reducing the
multidimensional structure to a simple node-and-linker net (see
the SI, Figure S2). X-ray structure analysis of 2 revealed the
etal−organic frameworks (MOFs), polymers in which
Mmetal centers are connected through organic linkers into
extended one-dimensional (1D), two-dimensional (2D), and
three-dimensional (3D) networks via coordinate bonding,
attracted immense attention in heterogeneous catalysis.1 Metal
centers of the network, especially coordinatively unsaturated
metal centers present in the catalyst or such a center generated in
situ in reaction conditions, act as active sites of catalysis.2 With
the advent of growing knowledge on the synthesis of framework
compounds, diverse predictable architectures have been
realized.3 MOFs reported so far mostly contain only one type
of metal (homometallic); hence, these are capable of catalyzing
one type of reaction when employed in catalytic reactions.
Recently, attempts were made to incorporate metal centers in
MOFs by postsynthetic functionalization of frameworks to
design heterogeneous catalysts.4 The incorporation of metal
nanoparticles in MOF materials has also been attempted to
create catalytically active centers.5 In the latter case, the metal
nanoparticles clinging to the MOF surfaces tended to migrate
and aggregate into larger particles to minimize their surface
energy. As a result, MOF materials often suffered deactivation
during continuous reactions. It is, therefore, desirable that the
prospective catalytically active metal centers remain in the
network of the MOFs. However, reports on heterometallic
MOFs (hetero-MOFs), i.e., framework compounds that contain
two different metals, are limited.6 With this in mind, we explored
the possibility of designing heterometallic frameworks connect-
ing copper-based low-dimensional (for example, 0D or 1D)
networks to alkaline-earth metals to afford 2D frameworks,
which could be further functionalized to 3D networks. While
copper(II) is an efficient catalyst for epoxidation reaction,7
alkaline-earth metals, being Lewis acidic, are capable of catalyzing
acid-catalyzed reactions subsequently.8
Figure 1. Network connectivity in 1−4 (a−d, respectively).
Received: February 3, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.6b00292
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Inorganic Chemistry
Communication
formation of a 3D network constructed by tetracoordinated CuII
centers of a CuO2N2 unit, which is connected to four
carboxylato-bridged 1D chains of hexacoordinated CaII centers
of a CaO6 unit (Figure 1b). In this case, a carboxylate ligand is
coordinated to one Cu center and three Ca centers through four
carboxylate O atoms and one pyridyl N atom to give rise to a 3D
arrangement (Figure 1b). In compound 3, a 3D framework is
constructed by pentacoordinated CuII centers of a CuO3N2 unit,
which is connected to heptacoordinated triaqua SrII centers of a
SrO7 unit (Figure 1c). The carboxylate ligand in 3 features two
types of connectivity. In one, the ligand is coordinated to two Cu
centers and two Sr centers through four carboxylate O atoms and
one N atom. In another, the ligand is coordinated to one Cu
center and two Sr centers through three carboxylate O atoms and
one N atom. Network connectivity in 3 can be easily visualized
because four corners of the basal plane of an octahedron are
occupied by Sr centers and two axial positions by Cu centers,
while the corners are connected through two pairs of carboxylate
ligands of different connectivity (Figure 1c). These octahedra are
further linked to each other in eight different directions to
construct a 3D network (see the SI, Figure S3a). Compound 4
forms a 3D framework constructed by pentacoordinated CuII
centers of a CuO3N2 unit, which is connected to non-
acoordinated pentaaqua BaII centers of a BaO9 unit (Figure
1d). The overall network connectivity of 4 is almost similar to
that of 3 (see the SI, Figure S3b). Thermogravimetric analysis of
these compounds reveals that frameworks are thermally stable up
to ∼300 °C after removal of water molecules (see the SI, Figures
S4−S7).
Table 1. Olefin-to-Diol Conversion of Olefins Catalyzed by
1−4
a
% yield of product
c
conversion
(wt %)
reaction
time (h)
TOF
b
substrate
A
epoxide diol others
(h−1
)
d
(i) 94
24
12
6
28
10
22
19
22
28
82
38
18
12
10
103
181
446
533
99
d
d
d
(ii) 100
(iii) 100
(iv) 100
(i) 90
88
6
90
B
C
D
E
24
12
6
80
(ii) 100
(iii) 100
(iv) 100
(i) 83
100
100
100
61
181
446
533
91
6
24
12
6
(ii) 100
(iii) 100
(iv) 100
(i) 70
100
100
100
51
181
446
533
77
6
24
12
6
(ii) 100
(iii) 100
(iv) 100
(i) 100
(ii) 100
(iii) 100
(iv) 100
100
100
100
46
181
446
533
110
181
446
533
6
e
24
12
6
32
14
8
e
e
e
86
92
6
94
6
a
b
Reaction condition: see the SI. A = cyclohexene, B = cyclooctene, C
c
= 1-hexene, D = 1-octene, and E = styrene. Turnover frequency
(TOF) = moles converted per moles of active site per hour. 2-
d
Cyclohexane-1-ol, 2-cyclohexane-1-one, and 2-hydroxycyclohexanone.
e
Catalytic epoxidation of alkenes and subsequent ring-opening
reactions using 1−4 were performed in an acetonitrile medium at
60 °C in the presence of hydrogen peroxide under heteroge-
neous conditions (Table 1; see also the SI, Figures S8−S11). In
every case, diol was the major product. The overall conversion
from olefin to diol increases from 1 to 4. In the case of 1 and 2,
olefin-to-epoxide conversion initially increases and reaches a
maximum, and then it decreases gradually (Figure 2; see also SI
Figure S12). Subsequently, epoxide converts to the correspond-
ing diol, and in most of the cases, conversion was 100%.
However, in some cases, the formation of side products was also
noticed. For example, in the case of cyclohexene, 2-cyclohexane-
1-ol, 2-cyclohexane-1-one, and 2-hydroxycyclohexanone were
detected, while in the case of styrene, benzaldehyde and benzoic
acid were detected in minor amounts. Therefore, it is clear that,
in the case of 1 and 2, olefins convert to diols through the
formation of the corresponding epoxides. However, in 3 and 4,
no epoxide product was detected. This difference may be
attributed to the higher catalytic activity of Sr/Ba than of Mg/Ca.
The size of the alkaline-earth-metal ions increases from Mg2+ to
Ba2+, which leads to the accommodation of a greater number of
ligands in the coordination sphere of heavier alkaline-earth-metal
ions in comparison to the lighter ones. Thereby, the possibility of
having open metal sites increased in the case of heavier alkaline-
earth metals, which, in turn, enhanced the catalytic efficiency in
the case of Sr/Ba. It is well established that open metal sites have
direct relevance in the enhancement of the catalytic activity or gas
adsorption in MOFs.9 Evidently, epoxides that were formed
during reactions catalyzed by 3 and 4 immediately converted to
the corresponding diols.
Benzaldehyde, benzoic acid. (i)−(iv) denote the catalytic activity of
compounds 1−4, respectively.
Figure 2. Conversion of olefin to diol through epoxide, catalyzed by 1.
(im)2}·2H2O]}n (im = imidazole), which catalyzes epoxidation
reactions, converting olefins to their corresponding epoxides as
major products under heterogeneous conditions (see the SI,
Table S3 and Figure S13). Recently, we explored alkaline-earth-
metal-based MOF catalysts in aldol-type reactions, exploiting the
Lewis acid character of the metal centers.10 One such compound,
[Mg(pdc)]n,11 catalyzes epoxide-to-diol conversion smoothly
under acid/base-free heterogeneous conditions at 40 °C (see the
SI, Table S4 and Figure S14). Besides, epoxide-to-diol
conversions were also investigated using compounds 1−4 as
catalysts (see the SI, Table S5 and Figures S15−S18).
To ascertain the role of the CuII and MgII centers in catalysis,
we undertook a few control experiments. It is well established
that CuII acts as an efficient catalyst in epoxidation reactions.7 To
this end, we selected a 1D copper(II) complex, {[Cu(pdc)-
B
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Interestingly, in the latter case, diols were formed comparatively
in a shorter reaction time (except for 2) and afforded lower
amounts of side products even from epoxycyclohexane. It is
worth noting that {[Cu(pdc)(im)2}·2H2O]}n does not catalyze
epoxide-to-diol conversion and [Mg(pdc)]n does not catalyze
epoxidation of olefins. From the above results, it may be
concluded that the Cu and alkaline-earth-metal centers act as
active sites for epoxidation and epoxide-to-diol conversion
reactions, respectively.
The catalytic efficacy of compounds 1−4 is not the same. A
closer look at the structural features of the active centers afforded
a plausible structure−activity correlation. In 1, the Cu centers are
hexacoordinated, while the Mg centers have the lowest Lewis
basicity in the series of alkaline-earth metals present in the
catalysts. These two factors may be cooperatively influenced by
the lowering of the efficiency of the catalyst in both epoxidation
and diol formation reactions. In 2, the Cu centers are in
coordinatively unsaturated square-planar geometry, which may
facilitate the full conversion of olefins to epoxides in only 2 h (see
the SI, Figure S9). However, the diol formation rate is not as fast
as the epoxide formation for 2, and it takes 12 h for 100% of
epoxide to convert to its corresponding diol. Barium, being the
heaviest alkaline-earth metal reported here, shows the highest
diol formation rate for catalyst 4. Nevertheless, the overall time
required for 100% conversion of olefins is the same as that for 3
and 4 (6 h). It may be realized that the formation of diol from
epoxide (intermediate) happens at a faster rate compares to the
olefin-to-epoxide conversion, which, in fact, depends on the
similar active site (Cu) environment in 3 and 4.
To confirm the heterogeneous behavior of the catalysts, a hot
filtration experiment was conducted that clearly demonstrated
that the metal was not leached out from solid catalysts during the
catalytic reactions.11 The catalysts could easily be recovered after
completion of the reaction by simple centrifugation, washed
thoroughly with acetonitrile, and dried at room temperature. The
recovered catalyst showed almost the same catalytic activity in
successive runs (see the SI, Tables S6−S8). The powder X-ray
diffraction patterns of the recovered catalysts clearly suggest that
their structures are well maintained after several cycles of
reactions (see the SI, Figures S19−S24).
In essence, we successfully prepared and structurally
characterized four new copper and alkaline-earth-metal-based
bimetallic framework compounds. Framework compounds act as
multifunctional catalysts, where copper acts as an active center
for epoxidation of olefins and, subsequently, alkaline-earth
metals act as active centers for epoxide ring-opening reactions, in
tandem, to afford vicinal diol. With an increase in the size of the
alkaline-earth metals, the diol formation rate from epoxides
rapidly enhanced because of the increase of the open metal sites.
Frameworks behaved as heterogeneous catalysts and can easily
be recovered and reused without significant catalyst deactivation
due to either leaching of the active species or degradation of the
structure.
for compounds 1−4, and additional tables and figures
(PDF)
X-ray crystallographic data in CIF format (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: snkoner@chemistry.jdvu.ac.in.
■
Present Address
‡D.S.: Department of Chemistry, Darjeeling Government
College, Darjeeling 734101, India.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial assistance received from UGC through a project (to
S.K.) is gratefully acknowledged. D.K.H. is grateful to the
Department of Inorganic Chemistry, IACS, India, for low-
temperature data collection.
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ASSOCIATED CONTENT
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S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b00292.
(11) Saha, D.; Sen, R.; Maity, T.; Koner, S. Dalton Trans. 2012, 41,
7399−7408.
Experimental details, crystallographic data and structure
refinement parameters, selected bond lengths and angles
C
DOI: 10.1021/acs.inorgchem.6b00292
Inorg. Chem. XXXX, XXX, XXX−XXX