Transition-Metal-Free Oxidative C(sp2)?H Hydroxylation of Terpyridines: A HOMO-Raising Strategy for the Construction of a New Super-Stable Terpyridine Chromophores

Article abstract of DOI:10.1002/chem.201605793

Direct functionalization of terpyridines is an increasingly important topic in the field of dyes and catalysis as well as supramolecular chemistry, but its synthesis and transformation is usually challenging. Herein, a HOMO-raising strategy is reported for the construction of a super-stable novel terpyridine chromophores, in which the selective oxidation of terpyridines at its 3-position was determined successfully to the synthesis of phenol-functionalization of terpyridines (TPyOHs) bearing a hydrogen bonding group. The corresponding TPyOHs displayed strong aggregation-induced emission and exhibited highly selective and visual detection of Zn^II cation with a record green terpyridine-based luminophore with nanomolar sensitivity (125 nm).

Full text of DOI:10.1002/chem.201605793

A Journal of
Accepted Article
Title: Transition-Metal-free Oxidative C(sp2)-H Hydroxylation of
Terpyridines: A HOMO-Raising Strategy for the Construction of A
Super-Stable Novel Terpyridine Chromphores
Authors: Li-Wen Xu, Jiang-Bo Huang, Xing-Feng Bai, Li Li, Zhan-Jiang
Zheng, Zheng Xu, Yu-Ming Cui, and Jiao Cao
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To be cited as: Chem. Eur. J. 10.1002/chem.201605793
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COMMUNICATION
2
Transition-Metal-free Oxidative C(sp )-H Hydroxylation of
Terpyridines: A HOMO-Raising Strategy for the Construction of A
Novel Super-Stable Terpyridine Chromophores
Jiang-Bo Huang, Xing-Feng Bai, Li Li, Zhan-Jiang Zheng, Zheng Xu*, Yu-Ming Cui, Jian Cao, and Li-
Wen Xu*
(
(optional))
Abstract: Direct functionalization of terpyridines is an increasingly
important topic in the field of dyes and catalysis as well as
supramolecular chemistry, but its synthesis and transformation is
usually challenging. In this manuscript, we reported a HOMO-raising
strategy for the construction of a super-stable novel terpyridine
chromophores, in which the selective oxidation of terpyridines at its
electrochemical and optical properties as well as transition-
[
3,4]
metal-complexed molecular machine prototyes.
However, by
and large, functionalized terpyridines or its complexes have
been underutilized due to their limited accessibility, difficult
introduction of functional substituents because of the super-
stable terpyridine rings, and low chemoselectivity and the
[
5]
3-position was determined successfully to the synthesis of phenol-
generally poor yields of multi-step group-transformations. On
the other hand, although many compounds have been reported
as good sensors for metal cations, only a limited number of
terpyridines could be applied as fluorescent probes. Notably,
there are no reports on AIE-active terpyridine compounds
without any substituted groups because their special structures
without the commonly known AIE moieties, such as silole,
functionalization of terpyridines (TPyOHs) bearing hydrogen
a
bonding group. The corresponding TPyOHs displayed strong
aggregation-induced emission and exhibited highly selective and
visual detection of Zn(II) cation with a record green terpyridine-based
luminophore with nanomolar sensitivity (125 nM).
[
6]
anthracene, tetraphenyl ethylene, and carbazole. Therefore,
the design and functionalization of terpyridines which can be
used in selective enhancement of optical and physical properties,
especially work as a fluorescent probe, is highly desirable.
Accordingly, our goal is to develop a one-step synthesis of a
functionalized terpyridine ligand that had the potential to be a
new fluorescent probe for metal ions.
It is well known that reductive PET (photoinduced electron
transfer) is used frequently in the design and application of
BODIPY-based sensors, in which the BODIPY fluorophore acts
2
,2’:6’,2’’-Terpyridines consist of three linked pyridine, could be
synthesized easily from commercially available starting materials,
which provide three strong N-binding centers for coordination of
metal ion to form stable metal complex. Now, the construction of
terpyridine-containing architectures has great importance in
chemistry, materials science, and biology owing to the ability of
coordination-driven
self-assembly
and
ranging
from
mononuclear metal-coordinated compounds to higher-order
domain structures, for the potentially significant applications in
photoelectronics, catalysis, sensors, functional materials, and so
as the electron acceptor and the chelator (for example, the
[
1]
on.
The planar structure of terpyridine is
a
unique
[7]
amine group) is the electron donor.
In addition, the most
hemispherical molecule containing three heterocyclic nitrogen
centers on the backbone, and this rigid and stable structural
motif and its coordinated chemistry have attracted extensive
attention in the past decades. Especially in the past years,
there has been considerable research work conducted on
octahedral terpyridine-based transition metal complexes
featured with capability of fine-tuning of corresponding
important feature of -conjugated fluorophore compounds with
electron-accepting (A) and electron-donating (D) groups is the
intramolecular charge-transfer (ICT), which resulted into strong
emission, photoabsorption, electroluminescence, and other
optical properties, making them promising candidate for two-
[
2]
[8]
photon fluorescent probes and OLED. In general, for both
reductive PET and ICT systems, the HOMO (highest occupied
molecular orbital) of the fluorophore provides a key information
for the design a novel fluorescent sensor upon photo-excitation.
Thus we hypothesized that the introduction of a hydroxyl group
to the terpyridine rings would be a simple and direct strategy to
promote HOMO level of terpyridines. To test the hypothesis of
hydroxyl group-raising HOMO of general terpyridines, theoretical
calculations of molecular orbitals for all the possibly
hydroxylated terpyridines were also performed using Gaussian
[*]
Mr. J.B. Huang, Dr. X.F. Bai, Dr. L Li, Dr. Z.J. Zheng, Dr. Z. Xu, Dr.
Y.M. Cui, Dr. J. Cao, Prof. Dr. L.W. Xu
Key Laboratory of Organosilicon Chemistry and Material Technology
of Ministry of Education, Hangzhou Normal University, P. R. China
E-mail: liwenxu@hznu.edu.cn
Prof. Dr. L.W. Xu
State Key Laboratory for Oxo Synthesis and Selective Oxidation
Lanzhou Institute of Chemical Physics, Chinese Academy of
Sciences, P. R. China
[
9]
0
9 software package (Figure 1) . It is noteworthy that the
[**]
This Project was supported by the National Natural Science
Founder of China (No. 21472031 and 21503060), and Zhejiang
Provincial Natural Science Foundation of China (LY17E030003 and
LY17B030005). This work is also supported partially by Science and
Technology Department of Zhejiang Province (2015C31138), and
Hangzhou Science and Technology Bureau of China (20170533B08
and 20160432B08).
electron density of the HOMO of only the hydroxyl-substituted
terpyridine T5 and T6 is spread over the donor aryl rings, while
the HOMO of its terpyridine precursor and other hydroxylated
terpyridines is mainly localized on the three pyridine units or
phenyl ring. Encouraged by our hypothesis and corresponding
calculation, we paid much attention to the hydroxylated
functionalization of general terpyridines to the synthesis of
terpyridine T5 with the highest level of HOMO showed in Figure
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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COMMUNICATION
1
. It should be noted that, no reported effort has been devoted to
R
) t-BuLi ( 2 equiv), -78 oC, 1 h
1
2
R
replacing C-H by C-OH bond via oxidation of terpyridines, and
most methods for the synthesis of functional terpyridines are
fulfilled on the modification of backbone of N-para-position that
inherited from the building block of starting aldehyde. Herein, we
report our finding on the unprecedented functionalization of
terpyridines via strong base-mediated air oxidation reaction. And
our results show that the corresponding terpyridine derivatives,
simplified as TPyOHs, exhibit excellent Zn(II) cation binding
selectivity over other structurally analogues terpyridines without
ortho-OH-functional groups.
o
) Air, -78 C, 1 d and then
o
-
20 C, 10 h
OH
N
THF
Air Oxidation
N
3
N
N
N
N
4
Ph
OMe
OH
OH
OH
N
N
N
N
N
N
N
N
N
4a
4b
32% yield
4c
5
4% yield
54% yield
OH
N
OH
N
N
N
N
N
4d
4e
8
4% yield
78% yield
Scheme 2. Substrate scope of strong base (t-BuLi) -promoted air oxidation of
,2’:6’,2’’-terpyridines.
2
Figure 2. The crystal structure of 4-phenyl-2,6-di(pyridin-2-yl)pyridin-3-ol 4a
CCDC 1520339). H atoms have been omitted for clarity.
(
Figure 1. Hydroxyl group-raising HOMO level of general 2,2’:6’,2’’-terpyridine
molecules: A motivation for the design of a novel HOMO-raising terpyridine-
conjugated–donor-acceptor (TCDA) chromophore.
Initially, we found that the metal catalysts, especially palladium
and copper complex have no enough ability to activate C-H
bond on the aromatic ring of 2,2’:6’,2’’-terpyridines to occur C-H
[
10]
hydroxylation under the reported reaction conditions (Scheme
). Due to the high stability of terpyridines, we felt the need to
R
O
1
R
KOH
establish a facile strategy aiming at the straightforward C-H
+
+
NH3
(1)
N
1
Reflux, ethanol
functionalization on the pyridine rings of terpyridines. Then on
CHO
N
3
[11]
the basis of our previous work on lithium chemistry,
we
2
N
N
hypothesized that ortho- lithiation and subsequent hydroxylation
could be a possible strategy for the facile construction of
structurally defined terpyridine derivatives bearing new OH-
functional groups. In order to improve the ability of lithiation of
terpyridine, we used t-BuLi as a strong base to initiate the
lithiation/air oxidation. Fortunately, the dehydrogenative
oxidation occurred under the air atmosphere after lithiation of
terpyridine with t-BuLi. The air oxidation of terpyridine 3a
resulted in the formation of desired OH-functionalized terpyridine
PdCl2 (10 mol%)
aq. H2O2 (5 eq.)
4
-methyl-2-pentanone
OH
(2)
(
solvent)
N
4 h, 100 oC
2
N
N
N
no reaction
N
N
3a
4a
Scheme 1. Classic method for the synthesis of general terpyridines
equation 1) and the initial and unsuccessful exploring of palladium-catalyzed
hydroxylation of terpyridine 3a (equation 2).
3
4a in promising yield (see Table S1 of Supporting Information).
(
Encouraged by this preliminary and unprecedented finding, we
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COMMUNICATION
continued to optimize the reaction conditions for the high-yield
never reported. Considering that the terpyridine core (2,2’:6’,2’’-
synthesis of OH-functionalized terpyridines in the t-BuLi
-
terpyridine) has been extensively used as a metal ion receptors
[2,3,12]
promoted air oxidation reaction of terpyridine 3a. Fortunately,
under the optimized reaction conditions, the lithiation-oxidation
of terpyridine 3a resulted into the desired product (TPyOH 4a) in
in coordination chemistry and fluorescent sensor,
we
initially tested the chemoselective and fluorescent sensing ability
of 4-phenyl-2,6-di(pyridin-2-yl)pyridin-3-ol (TPyOH 4a) to detect
2
+
2+
+
+
2+
2+
+
54% isolated yield. The obtained novel OH-functionalized
various metal ions, such as Cu , Mg , K , Na , Fe , Co , Ag ,
2
+
2+
terpyridine 4a was characterized by NMR, mass spectroscopy,
and confirmed by X-ray crystallography (Figure 2).
Ni , Cd . As seen from Figure 3a and 3b, the TPyOH 4a
2
+
displayed obvious fluorescent enhancement toward Zn ,
whereas most of ions evaluated in this work caused fluorescent
quenching of TPyOH 4a. To further illustrate the quantitative
sensing of TPyOH (5 M) toward Zn , fluorescent titrations
were carried out carefully. As shown in Figure 3c, the intensity of
the fluorescent peak of TPyOH 4a at 510 nm was remarkably
We further briefly examined the substrate scope of this novel air
oxidation reaction of terpyridines under the optimized reaction
conditions. As shown in Scheme 2, versatile products 4a-4e
could be achieved in moderate to good yields (up to 84%
isolated yield). The facile one-step oxidation of terpyridines at 3-
position to TPyOHs 4a-4e provided an unprecedented method
for the functionalization of general terpyridines as well as newly
designed fluorescence probe.
2
+
2
+
increased upon the addition of Zn (0-20 eq.). Similarly, the
addition of Zn to other TPyOHs (4b-4e) was also observed as
2
+
fluorescent enhancement at nanomolar level of concentration
(
125 nM, Figure 3d). The above results suggested that when
2
+
TPyOH 4a was bonded with Zn , a novel green and stable
luminophore was formed, and the electron transfer from the
phenol electron donor to the terpyridine core became more
favorable and finally caused fluorescence enhancement in the
presence of Zn ion, which is largely different from that of
unmodified terpyridines 3 (see ESI, Figure S8).
2
+
As shown in Figure 1, the incorporation of a OH-functional group
at the 3’-position of
a terpyridine unit possibly has the
(
a)/(b)
pronounced effect on electronic and physical properties of
terpyridine-based metal complex or intermolecular interaction.
The electron density diagrams for LUMO and HOMO orbitals of
TPyOH 4a as well as the Zn-TPyOH (4a) complex were
provided in Table S2. It is noteworthy that the electron density of
the HOMO of the TPyOH 4a was spread over the donor aryl
rings and the LUMO predominantly distributed dispersively along
the two pyridine rings. Although the electron density of the
LUMO of terpyridine 3a without phenol group at 3-position was
also mainly localized on the acceptor pyridine rings, different
from that of 4a, its HOMO was mainly localized on the three
pyridine units. The calculations showed that the lowest energy
absorption band of 4a represented the HOMOLUMO transition,
which resulted into the strong fluorescent sensing ability of 4a to
zinc ion and was probably assigned to a combination of
intramolecular charge-transfer and -* excitation. Similarly, the
HOMO/LUMO orbitals of Zn-3a and Zn-4a were also reported in
Table 1, and it is worth noting that the LUMO value of the Zn-4a
complex is the lowest in comparison to that of Zn-3a. This
feature indicates that the energies of orbitals could be affected
by phenol unit largely.
(c)/(d)
We further investigated the applicability of aggregation-induced
Figure 3. (a) Fluorescence emission spectrum of TPyOH/cation mixtures (5 M,
[
13,14]
emission (AIE)
in the two system of TPyOH 4a and Zn-
1
F
:1) in EtOH/water (99:1) solution (λex = 430 nm). (b) Fluorescence enhancement
0
0
510 nm/F 510 nm (F 510 nm:TpyOH. F510 nm: TpyOH with metal cations.). (c) Fluorescence
TPyOH (4a) complex respectively because of the strong -
interaction aroused from planar terpyridine molecular unit and
possible hydrogen bonding of phenol group on the TPyOH
(Figure 4). The corresponding emission spectra of TPyOH 4a in
aqueous THF with different THF/water ratios are shown in
Figure 5. As expected, fluorescence emission enhancement
from pure THF solution to THF/water solutions of corresponding
TPyOH 4a and Zn-TPyOH (4a) respectively were observed
simultaneously. Notably, luminescence was significantly
enhanced for approximately 20:80 (v/v) and 10:90 (v/v)
THF/water mixtures for TPyOH 4a and Zn-TPyOH (4a).
2+
emission spectra of TpyOH (5 M, EtOH/water = 99 /1) in the presence of Zn at
various concentrations including 0, 0.0025, 0.025, 0.25, 0.5, 0.75, 1.0, 2.0, 3.0, 4.0, 5.0,
1
0.0, and 20.0 equiv (λex = 430 nm). (d) Fluorescence emission spectra of other
2+
TpyOHs (4b, 4c, 4d, and 4e) (5 M, EtOH/water = 99/1) in the presence of Zn (125
nM), λex = 430 nm.
While a number of recognition-based multitopic receptors have
been studied in the field of supramolecular chemistry, OH-
functionalized terpyridines at 3’-postion of center pyridine unit for
binding of metal cations and hydrogen bonding activation have
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Especially, almost no PL signs were recorded for TPyOH 4a in
the pure THF, but the PL value increased to about 1100 in 10:90
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terpyridines dramatically increased the fluorescent ability
because of their molecular - packing structures assisted by
intermolecular hydrogen bonding activation (Figure 4).
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Figure 5. (a) Fluorescence emission spectra of TpyOH in THF–water mixtures (fw =
2+
0
and 90%). (b) Fluorescence emission spectra of TpyOH /Zn (5 M, 1:1) mixture in
THF–water mixtures (fw = 0 and 90%). λex = 280 nm, λes = 545 nm.
[
In conclusion, on the basis of the hypothesis that the
introduction of a hydroxyl group to the terpyridine rings would be
a simple and direct strategy to promote HOMO level of
terpyridines, we have successfully developed a strong base (t-
BuLi) -assisted air oxidation of simple terpyridine, in which a
series of novel terpyridine molecules (simplified as TPyOHs)
bearing a phenol group at 3-position were prepared in good
yields. In addition, it was found that these TPyOHs displayed
strong fluorescence in the presence of nanomolar zinc ions. And
the TPyOHs 4 displayed strong aggregation-induced emission in
THF/water solution. These novel terpyridine derivatives, which
2
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Keywords: Selective oxidation • terpyridine • molecular
recognition • aggregation-induced emission • lithium
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Chemistry - A European Journal
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COMMUNICATION
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Chemistry - A European Journal
10.1002/chem.201605793
COMMUNICATION
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COMMUNICATION
A HOMO-Raising Strategy:
It is the first example to
reveal clearly that a new
family of terpyridine
Jiang-Bo Huang, Xing-Feng
Bai, Li Li, Zhan-Jiang Zheng,
Zheng Xu*, Yu-Ming Cui, Jian
Cao, Li-Wen Xu*
molecules bearing a phenol
group at 3-position (simplified
as TPyOHs) could be
prepared via t-BuLi promoted
air oxidation of simple
terpyridines, which provide a
high useful fluorescent probe
to zinc cation and AIE active
molecules and Zn complex
as a green luminophore.
Page No. – Page No.
Transition-Metal-free
2
Oxidative C(sp )-H
Hydroxylation of
Terpyridines: A HOMO-
Raising Strategy for the
Construction of A Novel
Super-Stable Terpyridine
Chromophores
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