Inorganic Chemistry
Article
systems, a rational effort in this direction is key for the
advancement of this field. The metal-catalyzed nitrene transfer
reactions to alkenes affording aziridines, nitrogen analogues of
epoxide, are highly valued due to the potential of aziridine to
undergo stereo- and regioselective ring-opening transformation
forming a wide range of biologically important molecules.51
Although many different catalytic systems involving costly
metals like Pd, Ru, and Rh have been reported to catalyze
aziridination, it is the first-row transition metal complexes that
have been largely used as a homogeneous catalyst in
aziridination reactions.52−54 However, the development of an
environmentally friendly, commercially available, cost-effective,
and reusable heterogeneous catalytic system for aziridination of
olefins has not yet been achieved. It may be noted that
although Cu(salen)-based ligand-centric Cd-MOF-catalyzed
aziridination of styrene and IRMOF-3 (Zn)-catalyzed ring
opening of aziridine with aniline by Cui et al.55 and Yaghi et
al.,56 respectively, are known, MOF-catalyzed one-pot
aziridination and its ring opening using a single catalyst have
not been reported till date (Scheme 1).
structure with four BTC3− and two H2O molecules in which
each Cu(II) ion is present in a pseudo-octahedral geometry
with an H2O molecule at the axial position. The coordinated
H2O molecule was removed successfully (activation) by
heating 1 at 120 °C under a dynamic vacuum for 12 h. The
color changes from turquoise in 1 to navy blue in activated
Cu3(BTC)2, 1act, which reverts to its initial color upon
exposure to air and moisture (Figure 1a).
The generation of CUS (1 to 1act) is evident in (i) the TGA
plot showing the loss of one H2O molecule per Cu(II) unit
(Figure 1b), (ii) the loss of a characteristic O−H band (3690−
3314 cm−1) for the H2O molecule in Fourier transform
infrared (FT-IR) (Figure 1c), and (iii) an increase in the
experimental Brunauer−Emmett−Teller (BET) surface area
from 836 m2 g−1 in 1 to 1667 m2 g−1 in 1act (Figure 1d).58 The
powder X-ray diffraction (PXRD) patterns of 1 and 1act are
identical with no new peaks, showing the strong structural
integrity of 1act, despite the loss of H2O molecules and the
creation of CUS (Figure 1e). To understand the importance of
the generation of open metal sites in a MOF, we performed
aziridination reaction with both 1 (no open metal sites) and
1act (thermally generated CUS) as catalysts. The reaction was
initiated by reacting phenyl-substituted alkenes with [N-(p-
tolylsulfonyl)imino]phenyliodinane (PhINTs) taken in 150 μL
of dry CH3CN and MOFs as catalysts under inert conditions
inside a glovebox. High reaction yields (70−98%) of aziridines
(3a−3m) were observed using 1act (Scheme 2), whereas no
product was formed when 1 was used as a catalyst under
identical reaction conditions. Furthermore, when the reaction
was performed with different catalysts, including MOF, 1act was
found to be best heterogeneous catalyst among many other
MOFs (Table S1). To rule out the effect of apical H2O
molecules on the reactivity of the catalyst without the open
sites, following a standard literature procedure, we synthesized
Scheme 1. Separate MOF-Catalyzed Aziridine Ring
Opening55 and Aziridination56 Reported Previously Shown
in Comparison to HKUST-1 (1act)-Catalyzed One-Pot
Aziridination and Its Ring Opening Using a Single Catalyst
Achieved in This Work
2
2
H2O is replaced by a methylene chloride group.59 Reaction
under identical conditions using 1CH Cl also resulted in a trace
2
2
(<15%) amount of the desired product, highlighting the
significance for the generation of OMS in aziridination of
alkenes. Furthermore, aziridination of styrene under identical
conditions using homogeneous Cu(II) salts such as Cu(NO3)2
and Cu(OAc)2 gave 1% and 15% aziridines, respectively,
showing catalytic attributes of 1act (Table S1). The yields are
higher than those of all previously reported aziridines with
MOFs, CPF-5 (15%)60 and [Cd4(Cu(L4)4(DMF)4)DMF·
4H2O] (81%).55 The optimization of the catalytic reaction
reveals that the reaction is completed within 2 h and best
suited for an alkene:PhINT ratio of 5:1 (Table S2).
Therefore, designing an efficient catalytic system for
synthesis and regioselective ring opening of aziridines would
be more important both synthetically and pot economically.
Taking aziridination and its ring-opening reaction as a model
system, we demonstrate the LA sites in MOF can work in the
presence and absence of open metal sites depending upon the
demand of the reaction. Furthermore, after a detailed
understanding of the importance of the generation of
unsaturation in LA sites, we have designed a catalytic system
based on a MOF (HKUST-1) for direct synthesis of C−C, C−
N, C−O, and C−S bonds from alkene in a single pot via
intermediate formation of activated aziridines. We report our
findings in detail herein.
It is noteworthy that Lewis acids like BF3,61 AlCl3,61 TiCl4,61
CuOTf,62a Cu(acac)2,62b AgNO3,63 etc., are also known to
catalyze aziridination of styrene. In these salts, the distinction
in the working mode of LA sites is difficult to make due to the
nonrigid structure of the catalyst complex. However, in
principle, with MOFs such a distinction can be made due to
their structural integrity even after the removal of liable ligands.
Our result suggests that the presence of Lewis acidity is not
sufficient to catalyze aziridination reaction and the generation
of free binding sites (or OMS) as in 1act is equally essential.
The aziridination reaction performed in the presence of 1act
shows an excellent substrate scope (Scheme 2) and efficient
recyclability for up to four catalytic cycles with a minimal
change in the final yield (Figure S3). The decrease in the yield
RESULTS AND DISCUSSION
■
The MOF HKUST-1 (1) [Cu3(BTC)2·(H2O)3]n (BTC =
benzene 1,3,5-tricarboxylate) was synthesized using the
The secondary building unit (SBU) consists of the Cu2 dimer
at six vertices bridged with a triconnected four-BTC unit,
which results in fcc crystals with large square-shaped pores of 9
Å × 9 Å.57a Here, each copper(II) dimer forms a paddle wheel
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Inorg. Chem. 2021, 60, 7794−7802