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DOI: 10.1002/cctc.201402164
Tuning Regioisomer Reactivity in Catalysis using
Bifunctional Metal–Organic Frameworks with Mixed
Linkers
Xiaoying Xu,[a, b] Jeroen A. van Bokhoven,*[a, b] and Marco Ranocchiari*[b]
The activity of two bifunctional metal–organic frameworks
(MOFs) with IRMOF-9 topology that contain amino, phosphine
oxide and methyl groups is described. The amino group acts
as an active site for the Knoevenagel condensation of ortho,
meta or para nitrobenzaldehyde and malononitrile, while the
non-active site diphenylphosphoryl or methyl group moderate
the spatial characteristics inside the MOF pores. The functional
groups induce a unique catalytic response. The reaction activi-
ty was enhanced when catalysed by MOFs comparing with ani-
line, whereas the phosphine oxide retards formation of the
ortho isomer product.
densation of benzaldehyde with ethyl cyanoacetate with
IRMOF-3 and MIL-53-NH2. Hartmann and Fischer reacted ben-
zaldehyde with malononitrile on stable Al-MIL-101-NH2.[18] Kita-
gawa et al.[19] found the Knoevenagel condensation reactions
catalysed by [Cd (4-btapa)2(NO3)2]·6H2O·2DMF (4-btapa=1,3,5-
benzene tricarboxylic acid tris[N-4-(pyridyl)amide]) with selec-
tivity by changing the size of active methylene compounds.
Lin and co-worker reported that catalytic activity of isoreticular
chiral MOFs (CMOFs) was tuned by varying the channel size,
by changing the length of the organic linker and by using dif-
ferent solvent (dimethylformamide, DMF or diethylformamide,
DEF) in the synthesis.[20]
Here, we report the synthesis of MOFs with two functionali-
ties, amino (NH2) active group, inert methyl (Me) and diphenyl-
phosphoryl (DPP) groups. The diphenylphosphoryl group has
been chosen because of its big steric bulk combined with the
easy availability of the linker in our group. Their topology is
that of IRMOF-9, which consists of R-BPDC (R=NH2, Me and
Metal–organic frameworks (MOFs) are crystalline, hybrid mate-
rials that are composed of inorganic nodes and organic build-
ing blocks.[1,2] The high surface area and thermal stability of
MOFs makes them appropriate materials for many applications,
including gas storage,[3] separation,[4] heterogeneous catalysis,[5]
and drug delivery.[6] MOFs work as heterogeneous catalysts,
and their active sites may reside on the functional groups of
their organic linkers or at unsaturated inorganic node vacan-
cies.[5,7] The chemical attributes of organic linkers can be varied
by pre-synthetic and post-synthetic modification, to introduce
nucleophilic and electrophilic active sites on MOFs.[8]
DPP; BPDC=(1,1’-biphenyl)-4,4’-dicarboxylic acid) and Zn4O6+
.
(Scheme 1) The strategy was to make the amino functionalised
group work as the catalytic active centre for the Knoevenagel
condensation and the second moiety (Me or DPP) to tune the
pore opening by changing the size within the pores. We ex-
ploited this to tune regioisomer reactivity in the Knoevenagel
condensation of ortho, meta and para nitrobenzaldehye with
malononitrile.
Several research groups, have successfully prepared individ-
ual MOFs containing different functional groups mixed-linker
MOFs (MIXMOFs) or multivariate MOFs (MTV-MOFs).[9–11] MIX-
MOFs are prepared by using the same inorganic node varying
the functionality of the organic building blocks with the same
size. Thus, the framework is identical, while the functionality of
the framework can be modified.
DPP-BPDC was synthesised based on a previously reported
method.[21] Inspired by the synthesis of DPP-BPDC, NH2-BPDC
and Me-BPDC were synthesised in H2O on Pd/C catalyst at
1008C with the aid of microwave radiation. After 2 h reaction
under such condition and easy workup, NH2-BPDC and Me-
BPDC were obtained in 78 and 71% yield, respectively (see
Supporting Information). Bifunctional MOFs, LSK-6 [Zn4O(NH2-
BPDC)1.5(DPP-BPDC)1.5] and LSK-9 [Zn4O(NH2-BPDC)1.2(Me-
BPDC)1.8] were synthesised in DMF with an equal amount of
two linkers and Zn(NO3)2·4H2O under solvothermal condition
at 858C for 72 h (Scheme 1). LSK represents the German acro-
nym of the Laboratory for Catalysis and Sustainable Chemistry
at the Paul Scherrer Institute. To make each active centre in
the structure with a maximum equivalent environment, we
chose the ratio of two linkers as 1 to 1 in the synthesis of
MOFs. After synthesis, the crystals were washed three times
with fresh DMF and soaked in CHCl3 for three days, with a re-
placement of fresh CHCl3 every 24 h.
Among the catalytic active MOFs, amino functionalised
MOFs, such as IRMOF-3,[12,13] UMCM-1-NH2,[14] MIL-101(Al)-
NH2,[15,16] and PCN-124,[17] catalyse Knoevenagel condensation
reactions. Gascon et al.[12] have studied the Knoevenagel con-
[a] X. Xu, Prof. J. A. van Bokhoven
Department of Chemistry and Applied Biosciences
ETH Zꢀrich
8093 Zꢀrich (Switzerland)
[b] X. Xu, Prof. J. A. van Bokhoven, Dr. M. Ranocchiari
Department of Synchrotron Radiation and Nanotechnology
Paul Scherrer Institute
CH-5232, Villigen PSI (Switzerland)
Powder X-ray diffraction (PXRD) analysis indicated that
LSK-6 and LSK-9 were crystalline materials featuring similar
structure as LSK-3 and IRMOF-9, respectively (Figure S1).[21,22]
Supporting information for this article is available on the WWW under
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 1891
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