Synthesis and study of Au(iii)-indolizine derivatives: Turn-on luminescence by photo-induced controlled release

Article abstract of DOI:10.1039/c8cc10177a

The photo- and structural properties of a series of Au(iii) indolizine complexes were determined. Controlled release of halogenated indolizine derivatives from the corresponding Au(iii) complexes was achieved by photoinduced C-X bond formation, which provided turn-on luminescence with an increase in emission intensity of up to 67 times.

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Synthesis and study of Au(III)–indolizine
derivatives: turn-on luminescence
Cite this:DOI: 10.1039/c8cc10177a
by photo-induced controlled release†
Received 24th December 2018,
Accepted 25th February 2019
a
a
b
a
a
Jie Yang, Yifan Zhu, Anfernee Kai-Wing Tse, Xinghua Zhou, Yang Chen,
c
a
a
Yu Chung Tse, * Keith Man-Chung Wong * and Chun-Yu Ho
*
DOI: 10.1039/c8cc10177a
rsc.li/chemcomm
The photo- and structural properties of a series of Au(III) indolizine
complexes were determined. Controlled release of halogenated
indolizine derivatives from the corresponding Au(III) complexes
was achieved by photoinduced C–X bond formation, which provided
turn-on luminescence with an increase in emission intensity of up to
67 times.
Indolizine derivatives and Au(III) complexes are both interesting
1
classes of chemicals because of their fascinating biological
2
,3
Scheme 1 Reactivity of Au(III) indolizine derivatives.
and photophysical properties, which lead to potential appli-
4
,5
cations as drugs and organic sensitizers. Cycloisomerization
6
–9
of 2-propargylic pyridines, additions, and cross-coupling are
indolizine derivatives and reveal their reactivity and properties.
These Au(III) complexes display highly efficient and unusual
photoinduced elimination. Such controlled release of indolizine
derivatives from Au(III) has potential photochemical and biological
1
0
the most common pathways to synthesize organic indolizines.
However, no Au(III) indolizinyl complexes have been reported to
1
1
date. The protons of simple organic indolizinium salts are
generally less acidic than those of most imidazolium salts and
thus the corresponding complexes cannot be accessed easily by
deprotonation in an analogous process to that used to synthesize
Au(III) imidazolium complexes. This situation has hampered the
study and use of indolizine derivatives as ligands or photosensitizers
for Au(III) as well as other metals for a broad range of applications
1,14
applications (Scheme 1).
Our study commenced with the synthesis of Au(III) indolizine
derivatives using propargyl pyridine 1 (Scheme 2). Intercepting
the H shifts or deprotonation step in typical cycloisomerization
at the propargylic position could block the transition-metal
elimination from the indolizine derivatives. Therefore, a quaternary
center was introduced onto 1 in the hope of providing the desired
complexes 3 with sufficient stability. To our delight, Au(III) com-
plexes with a general structure of 3 were obtained in one step from 1
and 2 under operationally simple, mild, and general conditions (see
the ESI,† S9). Highly substituted 3 with different functional groups
12a–h
like catalysis,
transmetallation, sensing, cell coloring, photo-
12j–m
driven disease therapy, and reducing environment therapy.
3b–d
Our team has a common interest in Au(III)
and other d8
1
3
catalysts with NHCs or vinyl ligands. In connection with this,
we develop an efficient and general method to synthesize Au(III)
1
2
3
(
R , R , and R ), such as silyl, aryl, heteroaryl, methoxy, and alkyl,
a
Shenzhen Grubbs Institute, Department of Chemistry, Southern University of
Science and Technology (SUSTech), Shenzhen 518055, China.
E-mail: keithwongmc@sustech.edu.cn, jasonhcy@sustech.edu.cn
b
Academy for Advanced Interdisciplinary Studies, Southern University of Science
and Technology (SUSTech), Shenzhen 518055, China
c
Guangdong Provincial Key Laboratory of Cell Microenvironment and Disease
Research, Department of Biology, Southern University of Science and Technology
(
SUSTech), Shenzhen, 518055, China. E-mail: tseyc@sustech.edu.cn
†
Electronic supplementary information (ESI) available: Characterization data and
experimental details. CCDC 1861778, 1861779 and 1861825–1861827. The structure of
1
13
5
was characterized by H and C NMR spectroscopy and HRMS. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/c8cc10177a
Scheme 2 General synthetic route to Au(III) indolizine derivatives.
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were obtained in good yield. Moreover, salts like 2a and 2c–e were catalysts and suggests the potential of these Au(III) complexes in
1
2
3
found to be compatible with the conditions (AuCl, AuBr as well as catalyst development, especially in view of their chiral alcohol
CuCl, and CuBr), giving the corresponding Au(III) complexes 3, and center on the ring. At the same time, it reduced the degree of
Au(I) and Cu(I) complexes 4. This allowed us to compare their conjugation between the two p-systems. This structure might
properties as a function of M center, oxidation state, and counter be helpful for turn-on luminescence development because such
ion later on.
conjugation was found to be important in the corresponding
2
Au(III) indolizine derivatives were isolated and structurally organic dye.
characterized by X-ray crystallography (3ab, 3bb, 3cb, 3fb, and
Controlled release of organic species from transition metal
1
5
3bc). Perspective views of the complexes are depicted in Fig. 1 complexes is an important topic in a number of areas. Controlled
and important structural information is tabulated in the ESI,† release is particularly interesting when the species is a fluorophore
S19. In general, (a) Au(III) adopts a slightly distorted square with potential applications in biological systems and is photo-
16a–c
planar geometry to avoid unfavorable steric interactions with inactive unless a desired stimulus is applied (Scheme 1).
There
R ; (b) the Me and OH groups tend to sit at equatorial positions; has been a surge of interest in the development of luminescence
c) the Au–C bond distances are indicative of single bond detection techniques with high sensitivity and low instrumental
1
(
character; (d) the strong trans influence of the indolizinyl needs. Materials with turn-on luminescence ability are attractive
ligand is revealed by the long Au–Cltrans bond length; (e) the for practical applications such as bioimaging/assays, chemo-
1
6d–k
use of OMe or OH does not strongly affect the core geometry; sensing, and super-resolution imaging.
f) the indolizine plane is almost orthogonal to the AuX plane. strong emission of the indolizine derivative was suppressed
Several notable aspects of those non-cyclometalated Au(III) and there was a change in the absorbance intensity of 3 with
In complexes 3, the
(
3
complexes were revealed by further analysis. First, the bond AuX upon excitation (ESI,† S20–24 for spectra). We suspected
3
angles of the two C–Au(III)–Clcis are different (ESI,† S19, entries that indolizine luminescent quenching was brought by Au(III).
1–3). One Clcis was bent towards the indolizinyl core, which That exact combination was not studied before and we thought
gave a relatively small Clcis–Au(III)–C angle. As a result, the that might have potential to display turn-on luminescence. In
structure of 3 is less symmetrical than expected because it does this regard, the reactivity and photophysical properties of 3
not minimize undesired steric interactions. This suggests that were studied (ESI,† S25). It is also because (1) selective reductive
there might be a favorable electronic and/or orbital interaction elimination of Xcis and the indolizine-like core over a normal
between one Cl and the indolizine on Au(III) (see below). elimination of X might be possible under appropriate conditions
cis
3
2
17
Second, for OR = OMe or OH (3bb and 3fb), the Clcis bent in view of the structural information obtained from the crystals;
towards the indolizinyl core is always located on the same face and (2) the success of such a transformation might allow con-
3
as OR (i.e., the opposite face to Me, see Scheme 2 and Fig. 1). trolled release of highly emissive indolizine derivatives (Scheme 3).
This indicates that the bending of Clcis towards the indolizinyl
3
The indolizinyl–AuBr complex 3bc was subjected to photo-
core was not a result of simple hydrogen bonding between OH induced reductive elimination conditions similar to those employed
18
3
and the halogen and was not controlled by the uneven steric for related NHC–AuBr species. First, the chemical and photo-
hindrance provided by the tertiary alcohol center. Also, both Cl reactivity of 3bc were examined. The desired brominated indolizine
and the bulkier Br complexes of 3 showed the same bending derivative 5bc was released for the first time. The reactivity of
trend (3bb and 3bc, entries 2 and 5), although Brcis was bent 3bc in this elimination process is notable because C–X bond
more than Cl . These observations collectively indicate that formation in NHC–Au(III)X3 is reported to be challenging.
cis
the origin of the unusual bending of halogens might be related In addition, neither Br
to an electronic effect rather than sterics. Third, the dihedral (Schemes 1 and 3).
angles of the indolizinyl plane and aromatic R plane are in the
2
nor indolizinyl–Au(I)Br 6bc formed
1
range of B401 to 551 for these Au(III) complexes. This structural
feature resembles those of most N-aryl NHC transition metal
Scheme 3 Controlled release of indolizine derivatives through photo-
induced reductive elimination to achieve turn-on luminescence. Graph on
the right: emission intensity changes in complexes 3 and 4 upon 330 nm
À5
excitation (concentration: 1 Â 10 M). No observable changes were noted
for Au(I) complexes: 4aa, 4ba, and 4ca. Emission intensity increased for all
Au(III) complexes examined. 5 was obtained and identified by NMR
spectroscopy and HRMS and by comparison with relevant literature and
1
the data for Au(I) halides (see the ESI†). However, complexes with R = TMS
3
Fig. 1 Perspective views of 3ab, 3bb, 3cb, 3fb, and 3bc represented by
0% probability ellipsoids; hydrogen atoms are omitted for clarity.
or R = OMe decomposed upon prolonged excitation, which prohibited
3
direct comparison of the changes in rate and intensity.
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The elimination process to give 5 was highly efficient, elimination of halogenated indolizine derivatives from Au(III)
1
7
general, and presented good substrate scope. Indeed, our other was observed, which enabled tunable turn-on luminescence
AuCl complexes also showed similar behavior to that of 3bc (e.g., with an associated increase in emission intensity of up to 67 times.
3
19,20
3
bb and 3cb),
giving the corresponding chlorinated indolizines The results were correlated with an observed singlet oxygen
5
in 492% yield. In contrast, the control experiments that used emission signal and the unusual molecular structures were
Au(I) or Cu(I) under otherwise the same conditions did not provide compared with those of NHC analogues. We believe that the
the halogenated indolizines. Instead, the starting complexes were Au(III) indolizine complexes may have potential in materials
2
2
recovered in good yield as expected, which is attributed to their and biological applications. In terms of synthesis and ligand
linear geometry and redox potential. From a spectroscopic point of development, a general method to synthesize Au(III) indolizine
view, the inertness of 4aa, 4ba, and 4ca towards such a transforma- derivatives was established. The high yield, broad substrate
tion is probably caused by their low-lying singlet excited state, as scope, complex stability, and good functional group compatibility
indicated by their low-energy absorption bands at 334–350 nm observed here were attributed to the mild reaction conditions
(ESI,† S22). All complexes 3 were efficiently converted to 5 according (neutral pH, which is different from the pH needed for depro-
to NMR spectroscopy results, so their differences in emission tonation of imidazolium salts in NHCs) and the quaternary
enhancement were attributed to substituent effects, in accordance stereocenter employed. The obtained structural information
2
with the related literature.
showed that the Au(III) indolizine complexes share a number
1
2
Second, the halogenated indolizine derivatives 5 were found of similarities with NHC complexes, which suggests that they
to display tunable photoactivity, showing good potential for use may be utilized as catalysts,
in turn-on luminescence applications (Scheme 3 and ESI,† S25). catalysis by using the possibly chiral alcohol center.
1
3,15c,21
especially in asymmetric
12h,i
Applications
The elimination process induced an increase in emission of the Au(III) indolizine complexes along these lines are now being
intensity by up to 67 times. Both the choice of halogen X and explored.
substituents R on the indolizine-like skeleton could affect the
We thank Shenzhen Nobel Prize Scientists Laboratory
magnitude of the increase in emission intensity induced by Project (C17213101), NSFC (21602099), Shenzhen Technology
elimination. In particular, the increase was larger for chlori- and Innovation Committee (JCYJ 20170817105041557, JCYJ2017
nated 5bb than the corresponding brominated analog 5bc, and 0817110721105), Guangdong Provincial Key Laboratory of Cell
1
R = Ph gave larger emission enhancement than 3-thiophene. Microenvironment and Disease Research (2017B030301018).
Similar to related organofluorophores, lEm of 5 could be easily We thank C. Y. Wong for helpful discussion and early attempts.
tuned from 383 to 538 nm according to the substituents (ESI,†
S25, entries 2, 5, 6, 8, 10, and 11). Notably, lEm before and after
elimination were nearly the same for these entries. This result
strongly suggests that the main function of Au(III) here is the
emission suppression of the organofluorophore. Such emission
Conflicts of interest
There are no conflicts to declare.
suppression was likely a result of an enhanced spin–orbit coupling
in complex 3 by the Au(III) heavy atom through the generation of an
Notes and references
indolizine derivative non-emissive triplet state. The presence of
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oxygen emission (see the ESI†). Thus, we tentatively believe that
the intersystem crossing (ISC) of Au(III) and indolizine derivatives
at their excited states, converted the emissive singlet state to a
non-emissive dark triplet excited state in the complexes.
The reaction outcomes and photophysical changes depicted
in Scheme 3 may be explained as follows. First, one Xcis on 3 was
bent towards the indolizine derivative by a favorable electronic
and/or orbital interaction, as suggested by the crystallographic
information. Second, the selective photoinduced elimination of
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5
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1
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