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four-membered ring cavity has 5.89 ꢁ edges. The ZnÀN distan-
ces in the calculated structures vary from 2.019 to 2.022 ꢁ in
the ring and 1.974 ꢁ for the pendent ICAs; these values are
similar to those obtained by means of X-ray diffraction. Optimi-
zation of the possible reactant complex in which CO2 and PO
are encapsulated in the four-membered ring cavity was not
successful owing to the small size of the cavity.
transition state (TS-1) resulted in the interaction of Zn atom of
ZIF-90 with the O atom of PO (ZnÀO=1.94 ꢁ), thereby leading
to ring opening at the b carbon (bC) of the PO. The distance
between the ring-opened bC and O was found to be 1.76 ꢁ. In
the next step, TS-1 dropped to a more stable Int-2 (À39.9 kcal
molÀ1), as a result of which the ring-opened bC(PO) and O(PO) dis-
tances became weaker (2.25 ꢁ) and the ZnÀO interaction be-
tween ZIF-90 and PO became stronger (1.86 ꢁ). The transition
from Int-2 to TS-2 produced the following events: (a) The ring-
opened bC(PO) and O(PO) distance became even longer (2.43 ꢁ);
(b) the ZnÀO interaction became stronger (1.78 ꢁ); and (c) the
C atom of CO2 moved closer to the O atom of the aldehyde
group of ZIF-90 (i.e., from 2.78 to 1.72 ꢁ), whereby the geome-
try of the CO2 molecule transformed from a linear to a bent
structure. Then TS-2 dropped to intermediate Int-3 (À23.6 kcal
molÀ1), whereby the CO2 molecule became tethered between
PO and ZIF-90 to create an eight-membered stable cyclic inter-
mediate ring. In TS-3, the eight-membered intermediate was
The lowest-energy structure of the six-membered ring (hexa-
mer) in which the ICAs are alternately oriented up and down
was optimized by using a semiempirical (PM3) method. The
structure was reoptimized using DFT at the B3LYP/6-31G(d,p)
level, as shown in Figure 8. The ZnÀN distances in the calculat-
ruptured by the interaction of the OCO atom in the eight-mem-
2
bered ring with the Zn of ZIF-90. Cycloaddition occurred in the
next step to form another stable intermediate (Int-4;
À26.1 kcalmolÀ1), which ultimately rendered the desired PC.
On the basis of the above DFT results and various previous
studies, we put forth a plausible mechanism for the ZIF-90-cat-
alyzed PO–CO2 cycloaddition.[5,6,8] The TPD results clearly quan-
tified the amount of acidic and basic sites in ZIF-90 (Table 1). In
general, the zinc (Lewis acidic center) of ZIF-90 interacts with
the oxygen atom of epoxide, which activates the epoxide ring.
Meanwhile, the aldehyde group of ZIF-90 activates the CO2,
whereby the ring-opened intermediate is formed,[9] which sub-
sequently leads to the ring-closure step to produce PC
(Scheme 2).
Figure 8. Lowest-energy conformation of ZIF-90 as a hexamer.
ed structure of the hexamer are 2.016 ꢁ in the ring and
1.984 ꢁ in the pendant ICAs; these values are also similar to
those obtained using X-ray diffraction. The distance between
two Zn atoms at alternate corners of the hexagon is 10.30 ꢁ,
thus indicating that the cavity formed by the hexamer is large
enough to accommodate PO. As shown in the DFT structures
and other reports,[1,2a,7h] ZIF-90, which is highly porous and has
a large surface area, easily encapsulates small molecules such
as PO and CO2 in its large cage, which results in increased cata-
lyst–substrate interactions and thus, higher activity.
The reusability performance of ZIF-90 is illustrated in
Figure 9. The spent catalyst was regenerated by centrifugation
and washing with methanol. It was then filtered and dried at
608C for 24 hours. The regenerated catalyst was again used for
the cycloaddition of PO and CO2 under similar conditions. The
results in Figure 9 show that the catalyst could be reused for
at least five cycles without any considerable loss in its activity.
The post-XRD analysis done after the first recycle of the reac-
tion at 1008C is shown in the Supporting Information (Fig-
ure S4).
Reaction mechanism and reusability
For simplicity, a single repeating unit of ZIF-90 was chosen to
explain the mechanism of the PO–CO2 cycloaddition reaction
through DFT. On the basis of DFT calculations, the activation
energy required for the noncatalyzed cycloaddition of PO with
CO2 to produce PC was found to be 55–59 kcalmolÀ1, which is
high for the reaction to proceed spontaneously and demands
the use of a catalyst. Initially, the total energy of the reactant
complex (PO, CO2, and ZIF-90) was preset to zero; the opti-
mized geometrical arrangement of the reactant complex is
Conclusion
In conclusion, the catalytic ability of ZIF-90 toward PO–CO2 cy-
cloaddition was explored. ZIF-90, which is highly porous and
contains imidazole-based ligands, produced a conversion of
88% under moderate reaction conditions of 1208C and
1.2 MPa CO2 for a duration of eight hours without any co-cata-
lysts or solvents. The catalytic activity of ZIF-90 was compared
to that of MOF-based catalysts reported to date for the cyclo-
addition of CO2 and PO; ZIF-90 was an efficient catalyst in the
absence of co-catalysts and solvents. On the basis of the DFT
calculations, the role played by ZIF-90 in providing a favorable
environment has been theoretically explained. A plausible
mechanism in which ZIF-90 catalyzes the PO–CO2 cycloaddition
shown in Scheme 2. An intermediate (Int-1) (À16.7 kcalmolÀ1
)
originated from the reactant complex. The O atom of PO was
at a distance of 2.12 ꢁ from the Zn of ZIF-90, whereas the O
atom of the aldehyde group of ZIF-90 was at a distance of
2.81 ꢁ from the C atom of CO2. The transition from Int-1 to
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