10.1002/anie.201901171
Angewandte Chemie International Edition
to a 6% conversion of o-phenylenediamine to benzimidazole, (24%), 226.1 (100%); exact mass: 382.1209), which might be a
which might be due to the greater flexibility of H3tctb product of reaction of diphenylsilane with water (traces are
compared to the tctb3– ligand in the MOF structure. H3tctb can present in the reaction solvent), formed during the reaction, and
adopt a trigonal pyramidal molecular geometry, leading to a Ph2Si(OMe)2 (m/z (relative abundance): 244.0 (26%), 167.0
possibility of forming the classical tctb3–-amine adduct and (100%); exact mass: 244.0920) as a by-product.
hence quenching of the FLP activity. In SION-105, the B-C
In summary, we report a water stable and heterogeneous
bonds of tctb3– lying in one plane are restricted in movement MOF catalyst for the metal-free and FLP-mediated conversion
due to the rigidity of the framework; therefore, strong of CO2 to value-added benzimidazoles. The introduction of
interactions between the B center and the diamines are Lewis acidic B centers within the MOF structure provides
prevented. The B 1s XPS spectrum of SION-105 loaded with steric protection, suppressing irreversible binding and
o-phenylenediamine showed no interactions between the B deactivation with bases. The high stability of SION-105 in
centres and amino-groups (Figure S6). These experiments common organic solvents, CO2-responsiveness, recyclability,
demonstrate the significant impact of B, incorporated in the tolerance against a wide variety of functional groups, and
MOF structure, as the active component, thus confirming the catalytic activity achieved through the in situ formation of
role of B as a Lewis acid for FLP mediated CO2 activation.
FLPs, highlight the utilization of MOF catalysts for CO2
The next question to investigate was whether the catalytic conversion. Our findings open the way for the next generation
process occurs within the pores of SION-105 or on its surface. of heterogeneous catalysts, intricately designed via direct
From the X-ray crystal structure, the pore window of SION- assembly and functionalization for the activation of small
105 is ~4.8 Å (Figure S7), which is smaller than the sizes of molecules.
the diamine compounds used in this work (~ 5.5 Å for o-
phenylenediamine). TGA was performed on SION-105 after
Acknowledgements
immersion in acetonitrile solutions of several representative
diamines. As shown in Figure S2, the TGA profiles of the
thank Swiss National Science Foundation (SNSF) for funding
diamine-loaded samples are very similar to that of SION-105.
Hence, the catalytic reaction occurs on the surface, however
SION-105 still demonstrates high catalytic activity, making it
ideal for further investigations using isoreticular approach
allowing to increase accessibility to all the B sites.
We thank the EPFL for financial support. T.N.N and K.C.S
under the Ambizione Energy Grant n.PZENP2_166888. F.M.E,
T.N.N and K.C.S acknowledge the National Center of
Competence in Research (NCCR), Materials’ Revolution:
Computational Design and Discovery of Novel Materials
(MARVEL) of the SNSF for funding – DD4.1 and DD4.5.
Further, in order to investigate dependency of the reaction
Conflict of interest
conversion on time and quantity of diphenylsilane, a kinetic
study was performed in situ using NMR spectroscopy. The
reaction was carried out in a 10 mm sapphire NMR tube
(0.2 mmol 1c, 0.2-0.8 mmol Ph2SiH2, 3 mol% SION-105,
2 mL of CD3CN, 1.0 MPa CO2, 120°C). The methyl-
The authors declare no conflict of interest.
Keywords:! CO2! conversion,! frustrated! Lewis! pairs,! metal?
organic! frameworks,! heterogeneous! catalysis,! sustainable!
chemistry.!
1
substituted substrate 1c was used since the singlet H NMR
signal, relevant to the methyl substituent, does not overlap with
other signals in the NMR spectrum, allowing to the reaction to
be easily monitored. The use of one molar equivalent of
Ph2SiH2 led to a low reaction rate compared to when four
equivalents are used (Figures S11a-b, S12). In the first 2 hours,
no conversion was observed in both cases, possibly, due to the
generation of the active species from Ph2SiH2 (a new peak at
5.58 ppm is observed in the 1H NMR spectrum, which
disappears after complete consumption of Ph2SiH2). At the
same time, the chemical shift of amino-group of the substrate
moves to higher frequency, what may tentatively be attributed
to the formation of active intermediate from the amine,
diphenylsilane and CO2. The kinetic curves demonstrate
conversion of 24% in 24 hours for one molar equivalent of
Ph2SiH2 and 98% for four equivalents of diphenylsilane. GC-
MS for the final reaction mixture revealed formation of
(Ph2SiH)2O (m/z (relative abundance): 382.3 (2%), 303.2
!
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