Journal of the American Chemical Society
Communication
into the ultrastable imidazole rings. Meanwhile, the diversified
functionalities have been integrated from the readily available 2-
aminopyridine monomers. In addition, the alternation of
isocyanide monomers has brought the extra structural diversity.
We initially selected the monomers of 2-aminopyridine (1a),
1,3,5-tris(3-fluoro-4-formyl phenyl)benzene (2),19 and 1,3,5-
tris(4-isocyanophenyl)benzene (3) to test the feasibility of
constructing the pyrimidazole-based COF, LZU-561, under
solvothermal conditions. After extensive screening, the opti-
mized conditions for constructing highly crystalline LZU-561
were established as crystallization at 120 °C for 5 days with
S3). The crystalline structure of LZU-561 was first determined
by powder X-ray diffraction (PXRD) measurement (Figure 1a).
Simulation of the PXRD pattern using Materials Studio20
suggested that LZU-561 possesses preferably the eclipsed
stacking structure (Figure 1a). The image recorded by scanning
electron microscopy (SEM) (Figure S11) showed uniform
morphology of LZU-561 and thus confirmed its phase purity.
The nitrogen adsorption−desorption isotherms of LZU-561
derived the Brunauer−Emmett−Teller (BET) surface area as
321 m2 g−1 (Figure S6) and the total pore volume (P/P0 = 0.99)
as 0.58 cm3 g−1. Formation of the pyrimidazole linkages was
confirmed by FT-IR and, conclusively, by solid-state 13C NMR
spectroscopy. The FT-IR spectrum of LZU-561 (Figure S7)
showed the typical21 pyrimidazole and N−H bands at 1617 and
3433 cm−1, respectively, the assignments of which have further
been supported by the FT-IR data of the model compound
(Figure S1). The solid-state 13C cross-polarization magic-angle
spinning (CP/MAS) NMR spectrum of LZU-561 (Figure 2a)
functionalities could be integrated into the pyrimidazole-based
COFs by simple replacement of the 2-aminopyridine mono-
mers. Note that 2-aminopyridines are either commercially
available or readily synthesized via well-established trans-
formation.23 In addition, the structural diversity can be
simultaneously achieved, for example, by switching the C3
symmetrical (3) isocyanide monomer to that with C2 symmetry
(4) (Figure 1b). Owing to these synthetic advantages, a series of
pyrimidazole-based COFs have been facilely constructed with
diversified functionalities (Scheme 2). The synthetic procedure
and characterization data have been presented in detail in the
tional groups could be selectively installed in the meta- (for
example, R1 = OCH3/R2 = H) or para-position (for example, R1
= H/R2 = Cl); (ii) the bifunctional groups (for example, R1 =
Br/R2 = CH3) could coexist in the same pore for synergetic
applications; (iii) the amount of pyrimidazole moieties in each
unit could be tuned as three or six by alternating the isocyanide
monomer; (iv) pyrimidazoles possess rigid and planar structures
with electron-withdrawing properties, rendering these COFs
attractive for optoelectronic24 applications; (v) the different
functionality introduced by diversified 2-aminopyridines could
be distinguished by 13C CP/MAS NMR measurements (see, for
example, Figure 3a); (vi) the properties of pyrimidazole-based
COFs can be tuned by alternating the substituent groups (R1
and R2). For example, LZU-562 displayed a narrower optical
band gap (1.88 eV) than those of LZU-561 (2.06 eV), LZU-563
(2.64 eV), and LZU-564 (2.68 eV) (Figure 3b), indicating that
the charge transfer has been facilitated by the introduction of a
methoxy group.25
Moreover, with the ubiquitous existence of fused imidazole
rings within the framework, these COFs exhibit excellent
thermal and chemical stability. Thermogravimetric analysis
(TGA) indicated that they are thermally stable up to 250 °C
Meanwhile, their ultrastability has been identified in water,
N,N-dimethyl-formamide (DMF), aqueous HCl (9 M), and
aqueous NaOH (9 M). For example, after three-day treatment
of LZU-563 in the above-mentioned solvents, the PXRD
patterns (Figure 4a) and 13C CP/MAS NMR spectra (Figure
S108) remained unchanged, indicating the retention of the
original crystallinity and composition. The SEM images showed
well-maintained morphologies (Figure S107), and N2 adsorp-
tion−desorption analysis (Figure 4b) revealed slight changes in
BET surface areas. The residual weight percentages of LZU-563
were 89% and 93% after harsh treatments in NaOH (9 M) and
In summary, we explore herein the isocyanide chemistry to
systematically realize the integration of functionality and
ultrastability in the COF synthesis. As the proof of concept,
one of the representative reactions based on isocyanide
chemistry, the GBB reaction, has been successfully applied to
construct a series of pyrimidazole-based COFs. As new members
in the COF family, these pyrimidazole-based COFs possess not
only the ultrastability toward harsh conditions due to the
ubiquitous existence of imidazole cyclic linkages but also tunable
functionality due to the participation of diversified and easily
available 2-aminopyridine monomers. In addition, the structural
diversity could be further realized by predesigned isocyanide or
aldehyde monomers. Given the ready availability and rich
reactivity15 (for example, with carbon-based electrophiles shown
in Scheme 1a) of isocyanides, we expect that isocyanide
chemistry introduced herein will inspire new approaches (based
Figure 2. Solid-state 13C CP/MAS NMR spectra of LZU-561 (a) and
selectively 13C-labeled LZU-561 (b). The assignments of the 13C
chemical shifts to the pyrimidazole rings are shown in the chemical
structures. The presence of the signal at 121 ppm verifies the formation
of pyrimidazole linkages in which the selectively 13C-labeled atom was
marked with an asterisk in (b).
contained the signals at 142 and 121 ppm which indicated the
formation of pyrimidazole rings. To further identify this key
issue, we synthesized the selectively 13C-labeled LZU-561 from
13
monomer 3 with the 13C-labeling at the −N C position. The
dominating signal at 121 ppm in the 13C CP/MAS NMR
spectrum (Figure 2b) thus verified the formation of selectively
13C-labeled pyrimidazole linkages via the complete trans-
formation22 of 13C-labeled isocyanides.
Facilitated by the present strategy based on isocyanide
chemistry, we were able to establish a combinatorial library of
functional COFs in one step. As shown in Scheme 2, tunable
C
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Liu, Jiao
