Investigation of prototypal MOFs consisting of polyhedral cages with accessible Lewis-acid sites for quinoline synthesis

Article abstract of DOI:10.1039/c4cc09410g

A series of prototypal metal-organic frameworks (MOFs) consisting of polyhedral cages with accessible Lewis-acid sites, have been systematically investigated for Friedl?nder annulation reaction, a straightforward approach to synthesizing quinoline and its derivatives. Amongst them MMCF-2 demonstrates significantly enhanced catalytic activity compared with the benchmark MOFs, HKUST-1 and MOF-505, as a result of a high-density of accessible Cu(II) Lewis acid sites and large window size in the cuboctahedral cage-based nanoreactor of MMCF-2.

Full text of DOI:10.1039/c4cc09410g

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Investigation of Prototypal MOFs Consisting of
Polyhedral Cages with Accessible Lewis-Acid Sites for
Quinoline Synthesis
Cite this: DOI: 10.1039/x0xx00000x
WenꢀYang Gao,a Kunyue Leng,b Lindsay Cash,a Matthew Chrzanowski,a Chavis A.
Stackhouse,a Yinyong Sunb and Shengqian Ma*,a
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
poor yields, long reaction times and tedious workup procedures.
Therefore, there is still a need to develop new types of catalysts
for efficiently catalysing the Friedlander reaction.
A series of prototypal metal-organic frameworks (MOFs)
consisting of polyhedral cages with accessible Lewis-acid
sites, have been systematically investigated for Friedlander
Metalꢀorganic frameworks (MOFs)5, emerging as a new type of
functional porous materials, have captivated tremendous attention
from both academia and industrial research over the past decades.
One of the most striking features of MOFs lies in their amenability
and modularity,6 which result from customꢀdesign of functional
organic ligands and judicious selection of secondary building units
(SBUs).7 The tubable pore sizes, controllable surface areas, and
functionalizable pore walls render MOFs the potential for a plethora
of applications, including gas adsorption,8 gas separation,9 sensor,10
catalysis11 amongst others.12 Recently, the benchmark MOF,
HKUSTꢀ113 was explored for the synthesis of quinoline and its
derivatives via Friedlander annulation reaction.4hꢀk However, its
catalytic performance is limited by the low density of accessible
Lewisꢀacid sites, and a large loading amount of catalyst is needed to
achieve high conversion. This prompts us to explore alternative
MOF catalysts for the Friedlander reaction. In continuation of our
efforts on developing polyhedralꢀcage containing MOFs as
nanoreactors for catalysis application,14 in this contribution, we
report the systematic investigation of a series of prototypal MOFs
consisting of polyhedral cages with accessible Lewisꢀacid sites for
the synthesis of quinoline derivatives.
annulation reaction,
a
straightforward approach to
synthesizing quinoline and its derivatives. Amongst them
MMCF-2 demonstrates significantly enhanced catalytic
activity compared with the benchmark MOFs, HKUST-1 and
MOF-505, as a result of a high-density of accessible Cu(II)
Lewis acid sites and large window size in the cuboctahedral
cage-based nanoreactor of MMCF-2.
Quinoline derivatives attract great interest as a major class of
nitrogen heterocyclic compounds because of various important
pharmacological and biological applications including
antimalarial, antiasthmatic, antihypertensive, antibacterial and
tyrosine kinase inhibiting agents.1 They have also been applied
for hierarchical selfꢀassembly of nanoꢀ and mesoꢀstructures
endowed with enhanced electronic and photonic properties.2
The advancement of new and efficient catalysts for quinoline
synthesis via Friedlander annulation reaction has been a longꢀ
sought goal in the last decades because this reaction is
considered to be one of the most efficient and straightforward
approaches for the synthesis of polyꢀsubstituted quinolines.3 It’s
been documented from existing studies that a strong Lewisꢀacid
catalyst plays an integral role for the Friedlander reaction
between 2ꢀaminobenzoketones and ketones. A number of
catalysts have been employed for the Friedlander condensation
reaction, including SnCl2/ZnCl2, Al2O3, H2SO4/SiO2,
It’s wellꢀknown that firstꢀrow transition metal ions exhibit
Lewis acidity, and we select three prototypal polyhedral cage
containing MOFs, HKUSTꢀ1, MOFꢀ50515 and MMCFꢀ2,16
which feature accessible Cu(II) sites, as Lewisꢀacid catalysts
for the synthesis of quinoline derivatives. MMCFꢀ216 is
assembled from the customꢀdesigned azamacrocyclic
NaHSO4/SiO2,
HClO4/SiO2,
silica
gelꢀsupported
tetracarboxylate
N,N’,N’’,N’’’ꢀtetraꢀ
ligand,
p
1,4,7,10ꢀtetraazacyclododecaneꢀ
phosphomolybdic acid, MCMꢀ41(mesoporous silica) and
HKUSTꢀ1(MOFs).4 However, earlier methods suffer from a
number of disadvantages including harsh reaction conditions,
ꢀmethylbenzoic acid (tactmb)17 and
Cu(NO3)2 under solvothermal conditions. Singleꢀcrystal Xꢀray
diffraction reveals that the nboꢀtopology network generates
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from two types of square planar nodes served by tactmb ligands (93.1% for MMCFꢀ2 v.s. 20.3% for MOFꢀ505). The dramatic
and Cu2(CO2)4 SBUs. Every six tactmb ligands link twelve enhancement of catalytic activity from MOFꢀ505 to MMCFꢀ2
copper paddlewheel SBUs to form a nanoscopic cuboctahedral for quinoline synthesis via Friedlander reaction can be
cage (Fig. 1(a)) with six Cu(II) metallated azamacrocycles tentatively ascribed to the synergetic effect of these active
residing on the six square faces. Compared to MOFꢀ505 built copper centers coupled with their high density within the
from 3,3’,5,5’ꢀbiphenyltetracarboxylate (bptc) ligand (Fig. S4, confined nanospace, as well as the larger window size of the
ESI), the addition of six centerꢀoriented copper sites per cuboctahedral cage in MMCFꢀ2 facilitating the ingress of
cuboctahedral cage can afford extra catalytically active centers reactants and the egress of products. The catalyst loading,
accessible by substrates. The nbo network of MOFꢀ505 and closely related to turnover number (TON) or turnover
MMCFꢀ2 can also be regarded as the close packing of frequency (TOF), is an important parameter to assess catalytic
nanoscopic cuboctahedral cages, illustrated in Fig. 1(b). In behaviour. HKUSTꢀ1 as investigated by Cejka et al. for
comparison, HKUST is comprised of octahedral and quinolone synthesis, showed an optimum loading amount as
cuboctahedral cages, as shown in Fig. S5, ESI. The high as ca. 4 mol%.4i The high catalytic activity of MMCFꢀ2
cuboctahedral cages in HKUSTꢀ1, MOFꢀ505 and MMCFꢀ2 are reduces the loading amount to as low as 1 mol%. In this regard,
systematically investigated as Lewisꢀacid nanoreactors for the the turnover number (TON) improves from 15 of HKUSTꢀ1 to
synthesis of polyꢀsubstituted quinolines via Friedlander 90 of MMCFꢀ2. These results thus highlight the Cu(II)ꢀ
condensation reaction.
azamacrocycle decorated cuboctahedral cage in MMCFꢀ2 as a
highly efficient nanoreactor for quinoline synthesis via
Friedlander condensation reaction.
Scheme 1. Illustrative representation of Friedlander reaction between 2ꢀ
aminoaryl ketones and ketones under solvent free conditions and at 358
K.
Fig. 1. (a) the cuboctahedral cage in MMCFꢀ2 composed of six tactmb
ligands and twelve copper paddlewheel SBUs; (b) the nboꢀtopology
MMCFꢀ2 closely packed by nanoscopic cuboctahedral cages.
The Friedlander condensation reactions were conducted
using different 2ꢀaminoaryl ketones with different carbonyl
compounds under solventꢀfree environment at 358K via loading
the same amount (0.01mmol) of Cu2(CO2)4 SBUs from
HKUSTꢀ1, MOFꢀ505 and MMCFꢀ2, as shown in Scheme 1 and
Table 1. The control experiment was conducted in absence of
catalyst. Fig. 2 depicts a time dependence of conversion for
condensation between 2ꢀaminobenzophenone and acetylacetone
catalysed by MMCFꢀ2, MOFꢀ505, and HKUSTꢀ1, and nonꢀ
catalyst test under the solventꢀfree conditions at 358K. As
shown in Fig. 2 and Table 1, MMCFꢀ2 demonstrates highly
efficient catalytic activity for quinoline synthesis via
Friedlander condensation reaction with a yield of 93.1% (Table
1, entry 4) over 24 hours. This compares favourably to the
corresponding value for the benchmarked polyhedral cageꢀ
containing copper MOF, HKUSTꢀ1 (58.2%, Table 1, entry 2).
Fig. 2. Kinetic traces of Friedlander condensation reaction between 2ꢀ
aminobenzophenone and acetylacetone under solventꢀfree conditions at
358 K catalysed by MMCFꢀ2, MOFꢀ505, HKUSTꢀ1, and in absence of
catalyst.
MMCFꢀ2 also remarkably outperforms the prototypal nbo
ꢀ
In order to generalize the results of this study, we carried out
the Friedlander condensation reaction with different 2ꢀ
aminoaryl ketones and different carbonyl compounds,
illustrated in Table 1. MMCFꢀ2 also demonstrates high
catalytic activity for the synthesis of other polyꢀsubstituted
quinolines via Friedlander condensation reaction, as indicated
by the 96.8% yield between 2ꢀaminobenzophenone and ethyl
acetoacetate (Table 1, entry 5), the 90.2% yield between 2ꢀ
aminoꢀ5ꢀchlorobenzophenone and acetylacetone (Table 1, entry
6), the 92.5% yield between 2ꢀaminoꢀ5ꢀchlorobenzophenone
and ethyl acetoacetate (Table 1, entry 7), and the 89.0% yield
between 2ꢀaminoꢀ4’ꢀchlorobenzophenone and acetylacetone
topology copper MOF, MOFꢀ505 (20.3%, Table 1, entry 3),
which possesses the similar cuboctahedral cagesꢀderived
network with MMCFꢀ2. We attribute the high catalytic activity
of MMCFꢀ2 for quinoline synthesis via Friedlander
condensation reaction to the high density of active sites with
some of them wellꢀoriented in the cuboctahedral cage,
promoting substrates and active sites interactions. Moreover,
though the number of active copper centers in the
cuboctahedral cage of MMCFꢀ2 is 1.5 times that in the
cuboctahedral cage of MOFꢀ505 (18 for MMCFꢀ2 v.s. 12 for
MOFꢀ505), the yield of quinoline synthesis of MMCFꢀ2
increases by 3.6 times when compared to that of MOFꢀ505
2 | J. Name., 2012, 00, 1-3
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N/A
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H/H
H/H
H/H
H/H
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CH3
CH3
CH3
CH3
OCH2CH3
CH3
OCH2CH3
CH3
Yield (%)b
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MMCFꢀ2
MMCFꢀ2
MMCFꢀ2
MMCFꢀ2
58.2
20.3
93.1
96.8
90.2
92.5
89.0
25.8
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polyhedral cages have been systematically investigated as
Lewis acid catalysts in the context of Friedlander annulation
reaction for quinoline synthesis. Amongst them MMCFꢀ2
demonstrates very high catalytic activity, surpassing that of
HKUSTꢀ1 and MOFꢀ505. The superior catalytic performance
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stronger Lewis acidic copper sites and large window size of its
polyhedral cages. Our studies support that creating a high
density of active sites within polyhedral cages by the use of
customꢀdesigned metallorganic ligands can be a plausible
approach to achieving high catalytic activity in MOFꢀbased
nanoreactors. Ongoing research in our laboratory focuses on
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systematic investigation of prototypal MOF platforms as
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Electronic Supplementary Information (ESI) available: [The synthesis
procedure of MOFs, powder Xꢀray diffraction patterns, catalytic details
and pictures of MOFs]. See DOI: 10.1039/c000000x/
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