Inorganic Chemistry
Communication
phenomenon could also be observed in VCD (Figure S7),
further demonstrating the enantiomeric nature of the as-
synthesized materials.26
We had also examined the recyclability of yolk−shell (R,R)-
Salen(Mn(III))@ZIF-8 for the oxidative kinetic resolution of
racemic 1-phenylethanol. The sample of yolk−shell (R,R)-
Salen(Mn(III))@ZIF-8 was readily recovered from the
catalytic reaction via centrifugation and reused. The recovered
catalyst after three cycles showed similar conversion and ee
value as those of the fresh yolk−shell (R,R)-Salen(Mn(III))@
ZIF-8 (Figure S9a). Meanwhile, the recovered catalysts
exhibited the same PXRD patterns as those of the pristine
solid catalysts of yolk−shell (R,R)-Salen(Mn(III))@ZIF-8
(Figure S9b), and the integrity of the morphology was
maintained by a relatively large extent shown from the SEM
of the supernatant showed that ee no longer changed,
indicating no leaching of chiral active species. It can be
concluded that no obvious structure degradation and loss of
chiral molecule took place during the reaction, indicating
excellent stability of the catalyst.
The chiral and amphiphilic properties of yolk−shell
Salen(Mn(III))@ZIF-8 prompted us to explore its utilization
for asymmetric catalysis. The oxidation kinetic resolution
reaction of secondary alcohols was employed to evaluate the
asymmetric catalytic performance because it was an efficient
strategy to obtain optically pure secondary alcohols27 in the
organic-aqueous two-phase reaction system.28,29 As shown in
entry 1, Table 1, the pure ZIF-8 had no chiral catalytic activity
Table 1. Oxidative Kinetic Resolution of Racemic 1-
a
Phenylethanol
Then, several representative racemic secondary alcohol
substrates were selected and evaluated for the applicability of
the yolk−shell (R,R)-Salen(Mn(III))@ZIF-8. It was found
that yolk−shell (R,R)-Salen(Mn(III))@ZIF-8 had a good
catalytic effect on α-methylbenzyl alcohol with substituents at
the 4-position (entries 1, 2 and 5, Table S1). However, the
enantioselectivity of the reaction decreased sharply when the
−CH3 group of the substrate changed from methyl to ethyl or
even bulkier substituents (entries 3, 4 and 6, Table S1). The
results showed that the electronic cooperation between the
catalyst and the substrate had a strong influence on the
enantioselectivity of the reaction. This is consistent with the
literature.30 In addition, as the size of the substrate increased
relative to 1-phenylethanol, the catalytic efficiency showed a
significant decreasing trend in Table S1, which indicated that
the MOF shell of the yolk−shell catalyst had a sieve effect on
the substrate (Table S2). The size-selectivity character of such
nanoreactor might provide a new path toward asymmetric
organic synthesis.
In conclusion, a novel yolk−shell CMOF was designed and
fabricated, in which the chiral polystyrene nanospheres served
as the hard template to form the spherical yolk−shell structure
and released the chiral Salen(Mn(III)) as the chiral yolk, and
ZIF-8 was chosen as the shell to confine the yolk and acted as
the sieve for size-selectivity. Such chiral yolk−shell CMOF was
efficient for the heterogeneous asymmetric catalysis because of
the enriched local concentration of reaction substrates within
the nanoreactor, increased catalytic stability, and confinement
effects. The yolk−shell CMOFs nanoreactor could open a new
way for synthesis of asymmetric catalysts with longer cycle-life,
excellent selectivity and catalytic performance. Furthermore, it
is conceivable that this strategy could be applied in the
heterogenization of large size homogeneous asymmetric
catalysts such as chiral porphyrin, BINOL, BINAP, Spiro,
polypeptide and even enzymes, and others.
b
c
entry
cat.
con. (%)
ee (%)
1
2
3
4
5
6
7
(R,R)-Salen(Mn(III))
(S,S)-Salen(Mn(III))
(R,R)-CPS
84
89
56
trace
21
88
96
−99
45
/
36
90
64
ZIF-8
(R,R)-CPS@ZIF-8
yolk−shell (R,R)-Salen(Mn(III))@ZIF-8
yolk−shell (S,S)-Salen(Mn(III))@ZIF-8
74
a
1.0 mol % of chiral Salen(Mn(III)), 0.4 mmol of substrate, 4 mL of
H2O, 2 mL of CH2Cl2, 0.4 mmol of NBS (1 equiv), 0.48 mmol of
KOAc (1.2 equiv), 6 h. Determined by performing GC-MS analysis.
Determined by performing HPLC analysis (Phenomenex Chiral
b
c
MJ(2)-RH).
because of the absence of chiral active sites. The chiral
Salen(Mn(III) catalysts showed excellent catalytic perform-
ance in homogeneous catalysis, with 89% conversion and
enantiomeric excesses of 96% for the (R,R)-Salen(Mn(III))
and −99% for its counterpart (S,S)-Salen(Mn(III)), respec-
tively (entries 2 and 3, Table 1). However, the (R,R)-CPS
exhibited decreased catalytic activity with 56% conversion and
45% ee value because of the high polymerization degree
between (R,R)-Salen(Mn(III)) and polystyrene (entry 4,
Table 1). The conversion and ee value of the (R,R)-CPS@
ZIF-8 catalyst were 21% and 36% respectively (entry 5, Table
1), which were lower than those of (R,R)-CPS. This is due to
the fact that the mass transfer limitation of the core−shell
structure between the ZIF-8 shell and the inner solid (R,R)-
CPS. Meanwhile, the ZIF-8 shell restricted the timely exposure
of the catalytic active center at the CPS core. Surprisingly, the
yolk−shell (R,R)-Salen(Mn(III))@ZIF-8 exhibited a greatly
enhanced heterogeneous catalytic performance compared with
that of (R,R)-CPS@ZIF-8 (entry 6, Table 1). Its 88%
conversion and 90% ee value were even comparable to those
of homogeneous (R,R)- Salen(Mn(III)) catalyst. The explan-
ation is that the hollow yolk−shell structure of such catalyst is
not only more conducive to the mass transfer, but also
provides a peculiar nanoscale reaction space as a nanoreactor.
More importantly, such amphiphilic nanoreactor could be
suspended in the interface of organic-aqueous phase and acted
as an asymmetric phase transfer catalyst that enabled the
reactant in the aqueous phase to transfer rapidly to the oil
phase through the adsorption of hydrophilic ZIF-8 shell,
resulting in the enhancement of the catalytic performance.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
Sample preparation and catalytic process, the structure
of chiral Salen(Mn(III)), macroscopic pictures, nitrogen
isotherms, BJH pore size distribution curves, UV−vis,
C
Inorg. Chem. XXXX, XXX, XXX−XXX