Abstract:
A series of three new isostructural metal
−
organic frameworks (MOFs) of nickel(II), [{Ni(muco)(bpa)-
(2H
2
O)}
·
2H
2
O] (
1
), [{Ni(muco)(bpe)(2H
2
O)}
·
2.5H
2
O]
(
2
), and [{Ni(muco)(azopy)(2H
2
O)}
·
2H
2
O] (
3
)[where
muco =
trans
,
trans
-muconate dianion, bpa = 1,2-bis(4-
pyridyl)ethane, bpe = 1,2-bis(4-pyridyl)ethylene, and azopy
=4,4
′
-bis(azobipyridine)], have been synthesized and
characterized by single-crystal X-ray di
ff
raction analysis and
other physicochemical methods. Compounds
1
−
3
exhibit an
interesting 3-fold-interpenetrated three-dimensional pillar-
layered framework structure constituted of 4-coordinating
(4-c) Ni
II
nodes with {6
6
}-neb net topology. Remarkably, in
spite of 3-fold interpenetration, the structures possess one-dimensional channels with dimensions of
∼
8.05
×
5.25 Å
2
. Gas (N
2
,
Ar, H
2
, and CO
2
) adsorption studies of compounds
2
and
3
revealed selective adsorption properties for CO
2
over other gases. In
all three structures, the 4-c Ni
II
node has two coordinated H
2
O molecules that can be reversibly removed by high-temperature
treatment to generate a dehydrated framework composed of highly unsaturated, Lewis acidic Ni
II
ions. Further, the activated
compounds of
1
−
3
act as e
ffi
cient recyclable catalysts for heterogeneous cycloaddition of CO
2
with styrene oxide, resulting in
cyclic carbonate with high conversion and selectivity. Interestingly, the cycloaddition reactions of CO
2
with bulky epoxides show
a decrease in the activity with an increase in the alkyl chain length of the substrate due to con
fi
nement of the pore size of the
MOF. The high catalytic e
ffi
ciency and size-dependent selectivity for smaller epoxides show the potential utility of
1
as a
promising heterogeneous catalyst for the cycloaddition of CO
2
. Furthermore, the catalyst can be easily separated and reused for
several cycles without signi
fi
cant reduction in the catalytic activity as well as structural rigidity. Compounds
1
−
3
represent rare
examples of interpenetrated MOFs exhibiting selective storage and conversion of CO
2
at mild conditions.
