M. Sohail Ahmad et al. / Journal of Catalysis 365 (2018) 344–350
347
by comparing the FT-IR spectra with a standard spectrum of pyrrole
and guanidine (Fig. S3). Other characteristic peaks at 1660, 1575,
1180, and 1113 cmÀ1 were assigned for AC@O, C@C, epoxy, and
CAN groups, respectively (Fig. 3c) [50].
[59]. However, determination of the active sites in liquid phase
organic reactions are quite difficult; previous reports suggest that
quaternary nitrogen sites are catalytically active [39,60], while in
other studies, pyridinic sites are effective [61,62]. To understand
which type of nitrogen atoms contribute to the catalytic site, we
plotted the ratio of nitrogen structure (pyridinic, NH group, quater-
nary, or N-oxide) versus product yield, and found that there was a
clear relationship between the amount of NH groups and product
yield (Figs. 4 and S4). Therefore, we hypothesized that NH groups
contributes to the catalytic cycle. To confirm that NH groups inter-
act with substrates during the reaction, we conducted in situ FT-IR
analysis. A peak of NH group of NrGO(G) at 3410 cmÀ1 (Fig. 5a)
shifted to a lower wavenumber by the reaction with tBuOK
(Fig. 5b), of which phenomena was also observed in the case of
standard pyrrole molecule (Fig. 5c and d). These results that NH
groups contribute to the reaction were also supported by density
functional theory (DFT) calculation. We compared the stabilization
energy of pyridine and pyrrole for tBuOK, and found that pyrrole
showed better stabilization effect (Fig. S5).
3.2. Catalytic activity of NrGOs for coupling reactions
The NrGOs were tested as a catalyst for the coupling reaction
with iodobenzene (1a) and benzene (2a) in the presence of tBuOK.
Among them, NrGO(G) showed the highest yield of 85% (Table 1,
Entry 1). Other nitrogen-doped catalysts, NrGO(A) and NrGO(U),
showed lower yield of 66% and 60%, respectively (Table 1, Entries
2 and 3). Without catalyst under the same reaction condition gave
9% yield of 3a (Table 1, Entry 4), which was also observed in the
previous research [14]. In addition, GO and rGO prepared by the
same procedure without nitrogen source was tested, and 35%
44% of 3a was obtained, respectively (Table 1, Entries 5 and 6).
These results confirm that the nitrogen doping plays an impor-
tant role in the catalysis. Compared with the previous reports using
metal-based catalysts, such as Co [51,52], Ir [53,54], Rh [55,56], Fe
[11,57], and metal-free catalysts, such as GO [16], phenanthroline
[18], and DMEDA [58], our NrGO catalysts showed higher or
comparable performance compared with the above conditions
(Table 2).
As a result of ESR analysis, NrGO(G) contained 1021/g of radicals.
Suppose that all the radicals are active, TON was calculated to be
0.2, and TOF was 0.008. These values are extremely low, therefore,
we believe only a few of specific radicals can contribute to the
reaction, since ESR spectra of NrGO is so broad, and after addition
of tBuOK, the ESR spectra has a sharp peak (Fig. S6).
3.3. Determination of active sites of catalyst
3.4. Substrate scope
It has been unclear about which site can work as catalyst in
nitrogen-doped carbons. As for electrocatalysts in oxygen
reduction reaction, pyridinic sites are determined to be active sites
To explore the scope of the NrGO(G) catalyzed reaction, a series
of aryl halide and aromatic compounds were examined (Table 3).
Iodobenzene (1a) and aryl iodides with an electron-donating sub-
stituent at the para position (1b and 1c) successfully gave desired
biaryl products (3a, 3b, and 3c) (Table 3, Entries 1–3), while an aryl
iodide with an electron-withdrawing substituent (1d and 1e) did
Table 1
Catalyst screening.a
Entry
Catalyst
Yield/%b
1
2
3
4
5
6
7
NrGO(G)
NrGO(A)
NrGO(U)
–
85
66
60
9
35
44
N.D
GO
rGO
NrGO(G) + TEMPOc
a
Reaction conditions: 1a (0.4 mmol), 2a (4 mL), catalyst (20 mg), tBuOK
(1.2 mmol) 120 °C, 24 h.
Yields were determined by GC using dodecane as an internal standard.
TEMPO (0.4 mmol) was added.
b
c
Fig. 4. Relationship between the amount of NH groups and product yield.
Table 2
Catalytic performance of NrGOs and other reported catalysts for the cross-coupling reactions of iodobenzene and benzene.
Entry Catalyst Reaction conditions
Cobalt acetylacetonato complex (15 mol %) Iodobenzene (0.5 mmol), LiHMDS (3 equiv), benzene (6 mL), 80 °C, 48 h
Yield/% Ref
1
72
Iodobenzene (0.224 mmol), benzene (2.0 mL) KOH (2.24 mmol), tBuOH (2.24 mmol), 200 °C 70
[51]
[52]
2
Cobalt porphyrin complex (5 mol%)
Iridium Cp* complex (5 mol%)
Fluorous ethylenediamine
1,10-Phenanthroline derivative (10 mol%)
NrGO(G) (20 mg)
3
4
5
6
Iodobenzene (0.50 mmol), benzene (20 mmol), tBuOK (1.65 mmol), 80 °C, 30 h
Iodobenzene (0.5 mmol), benzene (6 mL), tBuOK (5 equiv), ligand (3 equiv), 120 °C, 24 h
Iodobenzene (0.225 mmol), benzene (27 mmol), tBuONa (0.450 mmol), 185 °C, 6 h
Iodobenzene (0.4 mmol), benzene (4 mL), tBuOK (1.2 mmol), 120 °C, 24 h
Iodobenzene (0.4 mmol), benzene (4 mL), tBuOK (1.2 mmol), 120 °C, 24 h
Iodobenzene (0.4 mmol), benzene (4 mL), tBuOK (1.2 mmol), 120 °C, 24 h
Iodobenzene (0.4 mmol), benzene (4 mL), tBuOK (1.2 mmol), 120 °C, 24 h
Iodobenzene (0.4 mmol), benzene (4 mL), tBuOK (1.2 mmol), 120 °C, 24 h
Iodobenzene (0.4 mmol), benzene (4 mL), tBuOK (1.2 mmol), 120 °C, 24 h
72
70
65
85
66
60
44
35
9
[53]
[54]
[18]
This work
7
NrGO(A) (20 mg)
8
NrGO(U) (20 mg)
9
rGO (20 mg)
10
11
GO (20 mg)
No catalyst