Journal of the American Chemical Society
Article
exhibited a large red shift to 500 nm from 348 nm in free DB.
The magnitude of the bathochromic shift is approximately 150
nm, which could be ascribed to the formation of a greater
results suggested that COF−DB might serve as a promising
and versatile platform to obtain different metalated COFs
through a postsynthetic metal exchange.
degree of π-conjugation via the coordination with Pd(OAc) . A
2
also observed (Figure S38). Such a phenomenon could be
attributed to different coordination modes between the organic
ligands and metal ions. The main feature of DB lies in its
distinct coordination mode through the pyridine and
hydrazone nitrogen atoms to serve as a bidentate ligand to
form a five-membered ring conformation upon complexation
with a metal species. This complexation mode can effectively
increase the degree of π-conjugation of the system. Compound
DT, on the other hand, only depends on the nitrogen atoms of
the hydrazone linkages to form a sandwich-like structure with
metal ions, which had hardly any influence on the degree of π-
electron delocalization.
Heterogeneous Catalysis for the Suzuki−Miyaura
Cross-Coupling Reaction. The development of recyclable
heterogeneous catalytic systems is the impetus for many
industrial applications. In terms of their practical use,
heterogeneous catalysts outperform their homogeneous
counterparts due to their easy separation from reaction
mixtures and efficient performance in continuous flow
processes. The metalated COFs may serve as promising
systems for heterogeneous catalytic reactions. Given its good
crystallinity, stability, and functionality, Pd/COF−DB was
selected as a heterogeneous catalyst to investigate the Suzuki−
Miyaura cross-coupling reaction, which was well-known as a
useful method to form C−C bonds. As presented in Table 1,
A similar phenomenon was also observed for π-conjugated
COFs, and these interactions between metal-binding units and
metal ions were further amplified to exert an important
influence on the properties of the resulting materials. The
lowest-energy absorption band of Pd/COF−DB red-shifted to
a larger extent as compared to that of Pd/COF−DT, resulting
in a distinct color change from yellow to red (Figures 3a and
S39). The difference could be ascribed to the different
coordination modes between Pd/COF−DB and Pd/COF−
DT. For M/COF−DB, such as Pd/COF−DB, the metal
species coordinated with the metal-binding units (nitrogen
atoms) distributed at linkers and linkages could suppress the
bond rotation of the linkages and lock the hexagonal 2D sheets
in a more planar conformation. This arrangement could be
verified by comparing the PXRD patterns of COF−DB and
M/COF−DB. The presence of the diffraction peak assigned to
the (001) facet revealed that the M/COF−DB samples had an
ordered arrangement of multilayers, which corresponded to the
π−π stacking distance between adjacent layers. The π−π
stacking distance in all M/COF−DBs, ranging from 3.43 to
Table 1. Catalytic Activity Tests of Pd/COF−DB in the
a
Suzuki−Miyaura Cross-Coupling Reaction
b
entry
R
X
time (h)
yield (%)
1
2
3
4
5
6
7
8
H
CH3
CH O
NO2
H
CH3
CH O
NO2
CHO
H
I
I
I
I
Br
Br
Br
Br
Br
Br
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
99
99
95
99
97
95
97
96
99
0
3
3
9
c
1
0
a
Reaction conditions (unless otherwise noted) are as follows: a
solution of aryl halide (1.0 mmol), phenylboronic acid (183 mg, 1.5
mmol), K CO (276 mg, 2 mmol), and Pd/COF−DB (5 mg) in 5
2
3
3
.35 Å, was smaller than that of the parent COF−DB (3.45 Å)
b
mL of EtOH/H O (1:1 v/v) was stirred at 80 °C for 30 min. Yields
were determined by gas chromatography. No catalyst was added.
2
(Figure 3b). Therefore, M/COF−DBs should have more
c
compact structures as a result of the synergistic supramolecular
interactions that increased the stacking interactions between
adjacent 2D layers in M/COF−DB, leading to extended π-
conjugation, improved crystallinity, and enhanced stability. By
contrast, the π−π stacking distance of M/COF−DTs, ranging
from 3.37 to 3.38 Å, was slightly larger than that of the parent
introduction of the routine coordination mode, where metal
species coordinate with metal-binding units (nitrogen atoms)
distributed at the linkages, could only weaken the interlayer
interactions to some extent. The loosely stacked layers may
account for the obvious decrease of the crystallinity and
stability of Pd/COF−DT.
Postsynthetic Metal Exchange. The dynamic nature of
coordination bonding inspired us to explore the possibility of
metal exchange in M/COF−DB. M/COF−DB (M = Mn and
Co) samples were selected and immersed in saturated
solutions containing Pd(II) at ambient conditions for 12 h.
The successful transformation of M/COF−DB (M = Mn and
Co) into M′/COF−DB (M′ = Pd) was demonstrated by the
dramatic change in the color of the samples and a negligible
analysis also revealed the homogeneous distribution of Pd(II)
ions in the exchanged Pd/COF−DB sample, indicating that
both electron-donating (entry 3) and electron-withdrawing
(entry 4) aryl iodide substrates gave rise to the expected biaryl
products in excellent yields (95−99%) within 30 min (entries
1−4). Similarly, high yields (95−99%) could also be obtained
for the less active aryl bromides (entries 5−9), suggesting the
superb efficiency of Pd/COF−DB for the Suzuki−Miyaura
reaction.
The recyclability and durability of Pd/COF−DB was further
assessed by subjecting it to four consecutive catalytic cycles
using bromobenzene and phenylboronic acid as reactants. It
was found that the catalytic activity of Pd/COF−DB could be
well maintained at a high level. However, Pd/COF−DT and
amorphous Pd/COF−DB showed a significant loss of catalytic
exhibited good catalytic activities (Figure S43). The well-
preserved ordered structure of the recycled Pd/COF−DB was
also verified by PXRD and SEM analysis, showing an
insignificant decrease in the diffraction intensity and an almost
unchanged morphology. In contrast, a significant decrease in
the intensity of the PXRD pattern was observed for both Pd/
lower stability compared to Pd/COF−DB (Figure S44). The
H
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX