S. Han et al.
Journal of Solid State Chemistry 299 (2021) 122174
of readily accessible sites, we would like to examine whether different
morphologies and particle sizes of CPs with well-defined micro- or
nanostructure could use as high-efficiency heterogeneous catalysts for
performing tandem conversion reactions to generate imidazoline and
tetrahydropyrimidine rings as single products.
Table 2
a
Nanoscale 1c catalyzed aromatic nitriles and diamines by tandem reactions. .
The solvent resistance performances of 1-1c were evaluated in the
boiling toluene to implement these stoichiometric experiments. The re-
sults showed that the crystallinity of 1-1c remained their original shapes
in the boiling toluene after 36 h. Furthermore, the catalytic properties of
1-1c as catalysts for the tandem conversion process were tested with
benzonitrile (2a) and ethylenediamine (EDA, 3a) (Table S2). After
comprehensive screening of different reaction conditions (e.g., temper-
ature, catalyst loading, solvents, and reaction time, Table S2, entries
1
–14), the optimal reaction conditions were found by using 1-1c as
Entry
Catalysts
aromatic nitriles
Yield % of 4a-db
Yield % of 5a-db
ꢀ
catalysts (10 mol% loading) at 120 C after 4 h, providing the desired
imidazoline framework (4a) in 46, 61, 78, 94% yield, respectively.
It was also found that the catalytic activity of 1c was higher than that
of the microsized 1a and nanosized 1b, as well as the homogeneous
1
2
3
4
1
46
61
44
63
1a
1b
1c
78
79
94, 92c
93, 91c
catalysts (e.g., Co(NO
3
)
2
, H
4
pdpa, and their mixture in a ratio of 1 : 1)
5
6
7
8
9
10
11
Co(NO
1
1a
1b
1c
Co(NO3)2
1
1a
1b
1c
Co(NO
1
3
)
2
27
39
57
70
88
18
44
60
76
92
24
40
55
72
89
17
25
37
58
73
87
16
45
59
78
93
21
38
51
73
88
16
under identical conditions (Table S2, entries 15–17). Compared with the
bulk crystals 1, the microsized (1a) or nanosized (1b-c) possessed much
better catalytic activity under the same conditions. The yield of 4a was
obviously increased with the temperature improving (25–130 C,
Table S2, entries 5–7), while changing the amount of the catalyst 1-1c
ꢀ
1
1
1
1
2
3
4
5
(
Table S2, entries 8–10) and extending the reaction time (Table S2, en-
tries 11–13) were little influences on the yield of 4a. In addition, the as-
synthesized crystals 1-1c were activated by heating to remove the co-
ordinated water molecules, which were also used to promote the con-
version reaction from 2a and 3a to 4a. However, the results indicated
that there is no obvious difference in the final catalytic performance
before and after activation (Table S2, entry 14). In consideration of ef-
ficiency, and selectivity, nanoscale 1c/toluene/120 C was used as the
optimal reaction conditions to promote the direct tandem conversion
reactions.
Subsequently, a diverse range of substituted aromatic nitriles (2a-d)
and two diamines (3a and 3b) was used to examine the tolerance and
feasibility of the tandem conversion reactions (Table 2). In all cases,
nanosized heterogeneous catalyst 1c displayed remarkable selectivity to
generate imidazoline or tetrahydropyrimidine rings as the sole products
3
)
2
16
17
1a
1b
1c
Co(NO
1
1
2
8
9
0
3
)
2
ꢀ
a
Reaction conditions: 2a-d (1.0 mmol), 3a or 3b (4.0 mmol), catalyst 1c (0.1
ꢀ
mmol), toluene (10 mL), 120 C (4 h).
b
Isolated yield after 4 h.
Reaction conditions: 2a (10.0 mmol), 3a or 3b (40.0 mmol), catalyst 1c (0.5
c
ꢀ
mmol), toluene (50 mL), 120 C (4 h).
absorption spectroscopy (AAS) analysis confirmed that only slight
leaching (<1%, 0.56 ppm) of Co was achieved in the supernatant. Thus,
II
(
4a-d, 89–94%; 5a-d, 88–93%, respectively), while microsized 1a or
these results particularly clarified that nanoscale 1c was a real hetero-
geneous catalyst. Moreover, the reusability experiments were executed to
evaluate the long-term robustness and recyclability of nanoscale catalyst
c. The results demonstrated that it was easily recovered by simple
centrifugation from the reaction system and remained stable after at least
0 reaction cycles without distinct loss of catalytic activity (Figure S5a).
The PXRD patterns of the reused solid catalyst after 10 reaction cycles
matched well with that of as-synthesized nanoscale 1c, suggesting that
the nanoscale 1c kept their activity (Figure S5b). The mechanism for the
syntheses of imidazoline and tetrahydropyrimidine rings from aromatic
nitriles and diamines catalyzed by Co-based coordination polymers was
proposed tentatively (Scheme S1). After the activation of aromatic ni-
triles by the cobalt center on the catalyst to give I, the diamines attacks I
to produce II, followed by the cyclization of II to yield the final products.
nanosized 1b had moderate selectivity for the formation of 4a-
d (55–61% for 1a, 70–78% for 1b) or 5a-d (51–63% for 1a, 73–79% for
1b). When the bulk crystals 1 were used as heterogeneous catalysts, the
1
yields of 4a-d (39–46%) or 5a-d (37–44%) were less than these of 1a-c,
which could be ascribed to the decrease of BET surface area (1 < 1a < 1b
1
<
1c). Furthermore, the substrate with double cyano group (2d) or with
bulky naphthyl group (2b) was well tolerated to afford 4d, 4b, 5d, and
b in good or moderate yields (Table 2, Entries 19 and 9). In addition, to
5
probe the availability of this catalytic systems with 1c as heterogeneous
catalyst, a 10.0 mmol scale reaction was executed with 2a, 3a, and 3b as
substrates, and the desired products (4a, 92%; 5a, 91%) were achieved
with excellent yields (Table 2, Entry 4). Moreover, the 3-propanediamine
(
PDA, 3b) also clarified that this tandem conversion processes provided
good to prominent yields as tetrahydropyrimidine rings products (5a-d).
However, the homogeneous catalyst Co(NO , a usually utilized Lewis
3 2
)
4
. Conclusions
acid catalyst for tandem conversion of aromatic nitriles and diamines,
gave poor selectivity and low yields. Therefore, the remarkable catalytic
performance of the spheric heterogeneous catalyst 1c was attributed to
its unique structural features and nanosized morphologies, affording
much more readily accessible catalytic active sites in the nanoscale
spheric particles by reducing the diffusion distance.
To evaluate the heterogeneous features of this catalytic process, the
hot filtration test was first performed. After the tandem reaction for 1 h,
nanoscale 1c was removed by centrifugation. The tandem process was
entirely shut down, revealing no leaching of a dissolvable active catalyst
in the mother solutions to facilitate the tandem reaction. The atomic
In conclusion, an efficient catalytic system for the direct tandem
conversion of aromatic nitriles and diamines to prepare imidazoline and
tetrahydropyrimidine frameworks in the presence of stable and homo-
geneous nanoscale heterogeneous catalysts was reported. The Co-based
CP crystals with different morphologies and particle sizes, from the
large scale to the micro- or nanoscale, were precisely achieved by regu-
lating the concentration of PVP and the types of surface-supported
frameworks. The catalytic performance of nanoscale spheric 1c was
significantly better than that of bulk crystals 1, microscale 1a, nanoscale
5