Inorganic Chemistry
Article
Crystal Structure of the Iridium Complex 3 (CCDC
2085201). X-ray analysis of the iridium complex 3 was carried
out to further confirm the structure of the iridium complex.
Complex 3 crystallized in the monoclinic space group P21c
(Figure 3). The geometric data of the iridium complex are
legged piano-stool environment.34 Generation of the six-
membered ring Ir1−N1−C4−C3−C2−O1 possibly caused
good air and moisture stability. The Ir−N bond length of
2.109(2) Å in complex 3 is a normal value compared with the
bond distance in similar N,O-chelate iridium complexes.35 The
Ir−O distance of 2.083(2) Å in the iridium complex is also in
the known range.36 The six-membered ring N1−C4−C3−
C2−O1−Ir1 of complex 3 is folded with a dihedral angle of
14.8° between the planes of O1−Ir1−N1 and C2−C3−C4.
Synthesis of Primary Amides from Aldehydes under
Catalysis of Iridium Complexes. We initially studied the
reaction by using benzaldehyde with the substrate NH2OH·
HCl at 60 °C in the presence of iridium complex 1 (0.5 mol
%). The reaction has used different solvents, and the results
indicated that benzamide was given in good yields in MeOH or
ethanol (EtOH) by using NaHCO3 (1.2 equiv) as the base
(Table 2, entries 1−7). The process with an inorganic base is
more available, especially in the reaction in the presence of
NaHCO3, and is the best choice compared with NaOAc,
Na2CO3, Cs2CO3, and KHCO3 (Table 2, entries 8−11). This
may be due to the decreased catalytic activity of the iridium
complex, which resulted from the coordination interaction
between the nitrogen atom of these organic bases and the
metal ion center (Table 2, entries 12 and 13). The reaction
offered good yields when the loading of complex 1 was
decreased from 0.5 to 0.1 mol % (Table 2, entry 14). However,
benzamide was furnished in lower yield when the catalyst
loading of 1 was decreased to 0.02 mol % (Table 2, entry 15).
Moreover, little difference was exhibited when the reaction
temperature was adjusted from 60 to 50 °C (Table 2, entry
16), but poor results were given when the reaction was
performed at room temperature (Table 2, entry 17). Similar
catalytic activity for complexes 2−4 was exhibited compared
with that of complex 1, and good yields (90−91%) of 5a were
observed (Table 2, entries 18−20). No product was generated
in the absence of catalyst or base (Table 2, entries 21 and 22).
A low yield of the product was observed when a [Cp*IrCl2]2
precursor was used as the catalyst (Table 2, entry 23).
Figure 3. Crystal structures of complex 3 with thermal ellipsoids
drawn at the 30% level. Selected bond lengths (Å) and angles (deg):
Ir1−N1, 2.109(2); Ir1−O1, 2.083(2); Ir1−Cl1, 2.4242(7); C6−N1,
1.442(4); C2−O1, 1.279(4); C2−C3, 1.375(5); C3−C4, 1.418(4);
N1−Ir1−O1, 89.16(9); O1−Ir1−Cl1, 83.45(6); Cl1−Ir1−N1,
88.50(6); Ir1−N1−C4, 124.3(2); C2−O1−Ir1, 124.5(2); C2−C3−
C4, 128.5(3).
shown in Table 1, and the characteristic bond lengths and
angles are shown in Figure 3. The metal center of the iridium
complex displayed a normal octahedral geometry with a three-
Table 1. Crystallographic Data and Structure Refinement
a
Parameters for 3
chemical formula
fw
C22H29ClIrNO2
567.11
A series of reactions under optimal conditions have been
surveyed to explore the substrate scope of the amidation
reaction. Various types of aromatic aldehydes have been
utilized under the catalysis of iridium complex 1. Good yields
(85−96%) were obtained with all of the amides at 50 °C under
open-flask conditions (Table 3). This reaction allowed
different benzaldehyde derivatives with either electron-
donating (Table 3, 5b−5e, 5j, and 5k) or electron-withdrawing
(Table 3, 5f−5i, 5l, and 5m) groups. However, poor
performances were offered by ketones under the catalysis
system. To investigate the impact of different substituent
positions on the catalytic system, reactions using o-, m-, and p-
methylaldehydes were carried out. Good yields were obtained
with all of the corresponding products (Table 3, 5b−5d).
Hydroxyl groups and halogen are insensitive under the
catalysis of iridium complexes (Table 3, 5g−5j). Heterocyclic
substrates such as aldehydes with pyridine, furan, and
thiophene structure also gave amides in acceptable yields
(Table 3, 5n−5p). Aliphatic aldehydes also have been used as
substrates, and the products were furnished smoothly (Table 3,
5q−5t). The possible mechanism of the catalytic cycle is
shown in Figure 4 on the basis of previous results.18,19 The
oxidative addition of N−OH in the aldoxime generated from
aldehyde with NH2OH to the iridium center gave iridium(IV)
species I, followed by the formation of iridacycle intermediate
T (K)
λ (Å)
cryst syst
space group
a (Å)
173(2)
1.34138
monoclinic
P21/c
7.8567(5)
17.6078(10)
15.2387(9)
90
96.651(2)
90
2093.9(2)
4
1.799
9.052
1112
4.369−58.220
32004
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
ρ (Mg m−3
μ (mm−1
F(000)
θ range/deg
reflns collected
data/restraints/param
GOF on F2
)
)
4458/0/252
1.147
R1 = 0.0225, wR2 = 0.0543
0.494/−1.298
a
final R indices [I > 2σ(I)]
largest diff peak/hole (e Å−3
)
a
2
R1 = ∑||Fo| − |Fc||/∑|Fo| (based on reflections with Fo > 2σF2).
wR2 = [∑[w(Fo − Fc2)2]/∑[w(Fo )2]]1/2; w = 1/[σ2(Fo ) +
2
2
2
(0.095P)2]; P = [max(Fo ,0) + 2Fc2]/3 (also with Fo > 2σF2).
2
2
11516
Inorg. Chem. 2021, 60, 11514−11520