2
S. Yin et al. / Journal of Molecular Structure 1198 (2019) 126925
complexes has seldom to been reported yet.
Since triphenylenes can be synthesised from readily available 2-
iodobiphenyls and iodobenzenes under the conditions of Pd
catalysis [41], we attempted to employ A1 and A2 as catalysts of the
cross-coupling reaction between 2-iodobiphenyl and iodobenzenes
under mild conditions without the use of expensive and toxic dppf
ligand and a protective N2 atmosphere. To our delight, the reaction
of 2-iodobiphenyl with iodobenzene catalysed by A1([PdClL1]∙sol-
vent) and A2 ([PdClL2]∙2H2O) in dimethylformamide (DMF) under
mild conditions afforded triphenylene in isolated yields of 34 and
22%, respectively (Table 1, entry 1). The obtained product was
characterised by NMR, and its physical and spectral data were
compared to those reported in literature (Fig. S7) [41]. The above
coupling reaction was used to determine the optimal reaction
conditions. First, we investigated the effect of catalyst loading
(Table 1), showing that the highest yield was obtained at 10 at% A1
or A2, with A1 featuring a higher catalytic activity than A2. Hence,
subsequent experiments were performed with 10 at% A1.
Scheme 1. Structures of complexes A1, A2 and precursor ligands L1a and L2a
.
CeO (1399e1608 cmꢀ1) and C]O (1696.6 cmꢀ1) bands in the FT-IR
spectrum of A1 (Fig. S3) demonstrated that A1 comprised Pd ions
and 4’-(4-carboxyl-phenyl)-2,2’:60,200-terpyridine (L1), i.e., the eCN
group was converted into a eCOOꢀ group [34]. In support of this
assumption, the C]O band was blue-shifted because of the
conjugation between the carboxyl group and the benzene ring. The
absence of characteristic eCOOH bands at 1710 cmꢀ1 indicated that
carboxylate groups in A1 were completely deprotonated, as was
further confirmed by single-crystal XRD.
Single-crystal XRD analysis revealed that A1 crystallised in the
monoclinic C2/c space group and featured a structural unit con-
taining one crystallographic Pd2þ ion, one L1 ligand, and one Clꢀ
anion (Fig. 1). Each Pd2þ ion exhibited a distorted square-planar
geometric configuration and was coordinated by Clꢀ (Cl1) and
three nitrogen atoms (N1, N2, and N3) of the terpyridine moiety of
L1. Thus, the Pd2þ ion was coordinatively unsaturated, which was of
key importance for the catalytic activity of A1. L1 acted as a tri-
dentate ligand, and its carboxylate group was non-coordinated but
deprotonated to achieve charge balance. PdeN bond lengths lied in
the range of 1.929(6)e2.016(7) Å. The torsion angle between the
phenyl and the terpyridine planes was 37.53(8) degree, and the
torsion angle between the carboxylate and the phenyl was
174.61(7) degree. The structure of A2 was found to be similar to that
of A1 (Figs. S4eS6). The PdeCl bond lengths in A1 and A2 were
2.284(2) Å and 2.025 (3) Å, respectively.
Second, we probed the effect of base type/loading, revealing that
although strong bases were more effective than weak bases,
decreased yields were observed for very strong bases (Table 2,
entry 2). As has been described in literature [41], the highest yield
was obtained in the presence of Na2CO3 and CsOAc (entry 1).
Finally, we optimised reaction time, temperature, and solvent, and
the optimum reaction conditions were identified as iodobenzene (2
equiv), 2-iodobiphenyl (1 equiv), Na2CO3 (2 equiv), CsOAc (3 equiv),
A1 or A2 (10 at%) in DMF at 120 ꢁC.
Having developed an efficient procedure for the synthesis of
triphenylene, we investigated the corresponding substrate scope.
As A1 featured a higher catalytic activity than A2, only the former
catalyst was employed in the following reactions.
2.2. Dual CeH bond activation and double CeC bond formation
Table 1
There has been ongoing interest in the development of triphe-
nylenes [35], as triphenylenes were widely employed to prepare
functional organic materials such as discotic liquid crystals and
organic light-emitting diodes [36e39]. Among the variety of tri-
phenylene synthesis strategies, those based on Pd catalysis have
gained great attention, offering ease of implementation and high
efficiency [40]. However, CeC coupling catalysed by Pd-terpyridine
Effect of catalyst loading on triphenylene yield.
Entry
Catalyst (mmol%)
Product (A3)
yield A1 (%)
yield A2 (%)
1
2
3
5
10
15
3aa
3aa
3aa
34.2
50.8
45.4
22(0.5a)
38(0.8)
26(0.6)
a
Margins of error for multiple experiments.
Fig. 1. Coordination environment of Pd(II) in A1 (left) and A2 (right), with displacement ellipsoids drawn at the 30% probability level.