Synthesis and characterization of fluorescent-active triazole-gold complexes

Article abstract of DOI:10.1007/s11426-015-5412-z

Fluorescence active fused-triazole was developed as a ligand in gold(I) cation coordination. The coordination ability was evaluated using NMR and fluorescence emission. Our previously reported N-2-aryl-triazoles, though gave good emission, could not form stable complexes with gold cations. A new N-fused-triazole (NFT) derivative was prepared and the ligand indicated good fluorescence emission, and formed strong coordination with gold cations. Both ligand and complex were characterized by X-ray crystallography. Fluorescence properties of various gold complexes (with different primary ligands) were evaluated, which suggested the feasibility of using fluorescence emission as potential tools for future mechanistic investigation.

Full text of DOI:10.1007/s11426-015-5412-z

SCIENCE CHINA
Chemistry
July 2015 Vol.58 No.7: 1–4
doi: 10.1007/s11426-015-5412-z
• COMMUNICATIONS •
Synthesis and characterization of fluorescent-active triazole-gold
complexes
Boliang Dong†, Yijin Su†, Xiaohan Ye, Jeffrey L. Petersen & Xiaodong Shi*
C. Eugene Bennett Department of Chemistry, West Virginia University, Morgantown, WV 26506, USA
Received December 4, 2014; accepted February 27, 2015
Fluorescence active fused-triazole was developed as a ligand in gold(I) cation coordination. The coordination ability was eval-
uated using NMR and fluorescence emission. Our previously reported N-2-aryl-triazoles, though gave good emission, could
not form stable complexes with gold cations. A new N-fused-triazole (NFT) derivative was prepared and the ligand indicated
good fluorescence emission, and formed strong coordination with gold cations. Both ligand and complex were characterized by
X-ray crystallography. Fluorescence properties of various gold complexes (with different primary ligands) were evaluated,
which suggested the feasibility of using fluorescence emission as potential tools for future mechanistic investigation.
fluorescence, catalysis, 1,2,3-triazole, gold, conjugation
toward alkyne, alkene and allene activation [8]. However, to
1 Introduction
the best of our knowledge, there have been no fluorescence
active gold complexes reported in literature, which make it
difficult to incorporate effective fluorophores into the com-
plexes. In this work, we report the synthesis and characteri-
zation of novel fluorescent active gold(I) complexes using a
N-fused triazole (NFT) ligand, which achieved strong coor-
dination and exhibited fluorescence emission for the first
time (Scheme 1(b)).
In recent years, our group [9] has been interested in in-
vestigating the coordination abilities of 1,2,3-triazole to-
ward transition metal cations. To explore this chemistry,
effective synthesis of various triazoles is necessary. Thus,
we have developed a series of new approaches in triazole
functionalization with good to excellent chemo-, regio-, and
stereoselectivity [10]. We have found that was the N-2-aryl-
1,2,3-triazole (NAT), which displays excellent fluorescence
emission in the high energy UV/blue region via a planar
intramolecular charge transfer (PICT) [11]. In contrast, the
N-1 derivatives, which are the typical regio isomers from
click chemistry, do not give any fluorescence emission. We
have subsequently explored their binding ability and dis-
covered several new triazole-gold complexes/ catalysts. One
Photoactive molecules play an important role in chemistry,
biology, and material science [1]. Fluorophores are widely
used as novel photoactive materials, molecular sensors and
probes, and bio-imaging [2]. The optoelectronic properties
of fluorophore can be influenced by its structure and chem-
ical environment [3]. Some particularly interesting systems
are transition metal complexes, which feature ligands that
directly influence the overall optical property [4]. Fluores-
cent metal complexes with “closed-core” coordination that
promote metal to ligand charge transfer (MLCT) [5]. A
classic example is the octahedral [Ru(bpy)3]2+ complex
shown in Scheme 1(a) [6]. Metal complexes with simple
coordination patterns tend to be less popular in fluorescence
studies due to the potential photo quenching in a rather
“open” coordination pattern.
It is known that gold(I) complexes adapt two-coordinated
linear geometries [7]. The last two decades evidenced the
great advancement of gold(I) complex promoted catalysis
†Contributed equally to this work
*Corresponding author (email: Xiaodong.Shi@mail.wvu.edu)
© Science China Press and Springer-Verlag Berlin Heidelberg 2015
chem.scichina.com link.springer.com

2
Dong et al. Sci China Chem July (2015) Vol.58 No.7
Scheme 1 Fluorescence active metal complexes.
particularly interesting system is the triazole-gold catalyst
(TA-Au), which promotes alkyne activation [12]. Thus, we
explored new fluorescence active gold complexes, which
could be developed based on fluorescence active triazoles
and might be useful for catalytic mechanism investigation
or metal sensing in the future [13].
Figure 1 Coordination of NAT with gold cation.
2 Experimental
2.1 Coordination of N-2-aryl triazole with gold cation
Scheme 2 N-fused-triazole: coordination and emission.
As shown in Figure 1, we first treated fluorescence active
NAT-1 with gold cations. The reactions were monitored by
1H NMR and 31P NMR. As discussed previously, the N-1-
aryl triazole readily coordinates with gold cations (to afford
TA-Au), which do not display fluorescence emission. When
treating the fluorescence active N-2-aryl triazole, NAT-1,
with [PPh3Au]+, a significant change of H-5 proton chemi-
cal shift was observed in 1H NMR (Figure 1(a)). This result
suggested that NAT-1 did provide some kind of interaction
with gold cation in solution. However, our efforts to isolate
complex 2 failed due to the compound decomposition. In
fact, NAT-1 failed to provide the “protecting effect” ob-
served with typical triazole ligands toward gold cations.
This brought great concern whether NAT-1 could form the
desired gold complexes with the desired stability. As indi-
cated by the 31P NMR, the mixture of NAT-1 with [PPh3Au]+
gave very similar 31P chemical shift as of “naked” [PPh3Au]+,
which suggested the weak coordination between NAT-1
and gold. Furthermore, comparing the optical property of
the complex and NAT-1 ligand, showed very little differ-
ence between their fluorescence emissions. Thus, combin-
ing all the information, it is confirmed that NAT-1, though
provided good fluorescence emission, could not form a sta-
ble complex with gold cation.
nitrogen coordination due to the steric hindrance. Although
the N-2 aryl group is “free” for rotation to reduce the steric
hindrance, breaking co-planar conformation will cause the
significant decrease of fluorescence emission, as we report-
ed previously.
To achieve our desired fluorescence active stable gold
complexes, we designed the N-fused triazole 3. Our ra-
tionale was that NFT 3 has a nitrogen coordination site with
significantly reduced steric hindrance. Thus, it will form
strong coordination as a regular triazole toward gold cation.
In addition, the extended conjugating system will likely
provide fluorescence activity, which may or may not be
influenced by the metal coordination. To explore this hy-
pothesis, both ligand 3 and complexes need to be prepared.
2.2 Synthesis and characterization of NFT 3 and Complex 4
NFT 3 was prepared as described in Figure 2. The carbox-
ylic acid 5 was reduced to alcohol 6 in 97% yield. Benzyl
alcohol 6 was then treated with PBr3 followed by SN2 az-
idation, giving 2-iodo benzyl azide 7 in 82% yield. So-
nogashira coupling followed by azide-alkyne Huisgen cy-
cloaddition gave the desired NFT ligand 3 in 53% yield.
Treating 3 with [PPh3-Au]+ under previously reported con-
dition gave the desired complex 4 in >90 % yield. The mo-
lecular structures of both ligand 3 and complex 4 are
As shown in Scheme 2, the coplanar arrangement be-
tween the N-2-aryl and the triazole ring blocks the N-1

Dong et al. Sci China Chem July (2015) Vol.58 No.7
3
data in Supporting Information online).
The emission data show that ligand 3 gave good fluores-
cence emission as expected. Complexes 4a–4f are generally
stable, and they are all fluorescence active, which made
them an interesting new class of fluorescence active gold
catalysts. When compared to ligand 3, all of these gold-
triazole complexes show higher fluorescence emission and
improved quantum efficiency. These results strongly sug-
gested that the gold cation coordination facilitated fluores-
cent decay pathway to the excited state [6,15]. The small
changes in excitation max and emission max implied that
little metal to ligand charge transfer occurred in this new
class of gold complexes. With this new class of fluores-
cence active gold complexes, ON-OFF chemodosimeter to
monitor the gold catalysis system are plausible, and are
currently under investigation in our group.
a) BH3 in THF (2.0 equiv.), THF (0.50 mol/L), 0 °C; b) PBr3 (1.8 equiv.),
DCM (0.40 mol/L), 25 °C, then NaN3, (2.0 equiv.), DMSO (0.20 mol/L),
60 °C; c) phenylacetylene (1.5 equiv.), Et3N (6.0 equiv.), CuI (7.0 mol%),
Pd(PPh3)2Cl2 (3.5 mol%), DMF (0.14 mol/L), 25 °C, then 120 °C; d)
PPh3AuCl (1.0 equiv.), AgOTf (1.0 equiv.), DCM (0.10 mol/L), 25 °C.
Figure 2 Synthesis of NFT 3 and its gold complex 4a.
confirmed by X-ray crystallography [14].
Based on the crystal structures, the dihedral angles of the
4-phenyl ring and the triazole ring are 36.4° and 37.0° in
ligand 3 and complex 4a respectively. The length of Au–N
bond is 2.065 Å, very similar to the distance of the Au–N
bond of TA–Au (2.063 Å). In addition, the 31P NMR reso-
nance of 4a was shifted downfield to  29.6 ppm. All these
results are consistent with the formation of strong coordina-
tion between ligand 3 and [L-Au]+ as proposed.
4 Conclusions
In summary, investigations of fluorescence active triazole
gold complexes have been conducted. While the fluores-
cence active N-2-aryl-triazoles (NATs) failed to produce
stable complexes with gold(I) cations, the fluorescence ac-
tive N-fused-triazole (NFT) was prepared and successfully
used as a ligand to form a new class of gold complexes.
These efforts led to the development of the first fluores-
cence active gold(I) catalysts, which enable a new approach
for mechanistic investigations and the potential sensing de-
vice development. Both areas are currently ongoing in our
group.
The characterization of compounds was shown in the
Supporting Information online.
3 Results and discussion
To evaluate the fluorescence properties of this new class of
complexes, a series of [L-Au]+ cations were prepared. The
fluorescence properties of ligand 3 and all of these com-
plexes were summarized in Table 1 (see detailed optical
Supporting information
The supporting information is available online at chem.scichina.com and
link.springer.com/journal/11426. The supporting materials are published as
submitted, without typesetting or editing. The responsibility for scientific
accuracy and content remains entirely with the authors.
Table 1 Optical properties of ligand 3 and complexes 4 a, b)
This work was supported by the Natural Science Foundation of the United
States (CHE-1362057, CHE-1336071), and the National Natural Science
Foundation of China (21228204).
Intensity
(×103)
4.45
Ex
max (nm)
285
Em
max (nm)
368
Stokes shift

b) (×103)


1
2
3
4
Haider JM, Pikramenou Z. Photoactive metallocyclodextrins: sophis-
ticated supramolecular arrays for the construction of light activated
miniature devices. Chem Soc Rev, 2005, 34: 120–132
Nolan EM, Lippard SJ. Small-molecule fluorescent sensors for inves-
tigating zinc metalloneurochemistry. Acc Chem Res, 2009, 42:
193–203
Anthony SP. Organic solid-state fluorescence: strategies for genera-
ting switchable and tunable fluorescent materials. Chempluschem,
2012, 77: 518–531
Ulbricht C, Beyer B, Friebe C, Winter A, Schubert US. Recent
developments in the application of phosphorescent iridium(III)
complex systems. Adv Mater, 2009, 21: 4418–4441
3
0.69
1.1
83
57
77
79
83
63
75
4a
4b
4c
4d
4e
4f
285
285
263
285
293
293
342
362
342
368
356
368
8.31
8.16
5.96
5.28
6.74
5.79
0.96
0.75 c)
0.75
0.87
0.95
a) Sample information: 1.0×105 mol/L in acetonitrile; b) quantum yields
(F) were determined based on 1.0×105 mol/L 9,10-diphenylanthracene in
cyclohexane (F=0.90) except 4c; c) quantum yields (F) were determined
based on 1.0×105 mol/L naphthalene in cyclohexane (F=0.23); d) calcu-
lated photo emission integration from the original spectra. All fluorescence
measured under identical conditions (see the Supporting Information online).
5
6
Zhao Q, Li F, Huang C. Phosphorescent chemosensors based on
heavy-metal complexes. Chem Soc Rev, 2010, 39: 3007–3030
2+
Teply F. Photoredox catalysis by Ru(bpy)3 to trigger transforma-

4
Dong et al. Sci China Chem July (2015) Vol.58 No.7
tions of organic molecules. Organic synthesis using visible-light
photocatalysis and its 20th century roots. Collect Czech Chem
Commun, 2011, 76: 859–917
11 a) Liu Y, Yan W, Chen Y, Petersen JL, Shi X. Efficient synthesis of
N-2-aryl-1,2,3-triazole fluorophores via post-triazole arylation. Org
Lett, 2008, 10: 5389–5392; b) Yan W, Wang Q, Lin Q, Li M,
7
Gorin DJ, Sherry BD, Toste FD. Ligand effects in homogeneous Au
catalysis. Chem Rev, 2008, 108: 3351–3378
Petersen JL, Shi X. N-2-aryl-1,2,3-triazoles: a novel class of
UV/blue-light-emitting fluorophores with tunable optical properties.
Chem Eur J, 2011, 17: 5011–5018
8
a) Xi Y, Dong B, McClain EJ, Wang Q, Gregg TL, Akhmedov NG,
Petersen JL, Shi X. Gold-catalyzed intermolecular C–S bond
formation: efficient synthesis of -substituted vinyl sulfones. Angew
Chem Int Ed, 2014, 53: 4657–4661; b) Wang D, Cai R, Sharma S,
Jirak J, Thummanapelli SK, Akhmedov NG, Zhang H, Liu X,
Petersen JL, Shi X. “Silver effect” in gold(I) catalysis: an overlooked
important factor. J Am Chem Soc, 2012, 134: 9012–9019
12 a) Duan H, Sengupta S, Petersen JL, Akhmedov NG, Shi X. Triazole-
Au(I) complexes: a new class of catalysts with improved thermal
stability and reactivity for intermolecular alkyne hydroamination. J
Am Chem Soc, 2009, 131: 12100
13 Selected reviews for the appication of fluorescent sensors on catalytic
mechanism investigation: a) Klán P, Solomek T, Bochet C, Blanc A,
Givens R, Rubina M, Popik V, Kostikov A, Wirz J. Photoremovable
protecting groups in chemistry and biology: reaction mechanisms and
efficacy. Chem Rev, 2013, 113: 119–191; b) Carter K, Young A,
Palmer A. Fluorescent sensors for measuring metal ions in living
systems. Chem Rev, 2014, 114: 4564–4601
9
a) Duan H, Sengupta S, Petersen JL, Shi X. Synthesis and
characterization of NH-triazole-bound rhodium(I) complexes:
substituted-group-controlled regioselective coordination. Organome-
tallics, 2009, 28: 2352–2355; b) Yan W, Ye X, Akhmedov NG,
Petersen JL, Shi X. 1,2,3-Triazole: unique ligand in promoting iron-
catalyzed propargyl alcohol dehydration. Org Lett, 2012, 14: 2358–
2361; c) Ye X, He Z, Ahmed T, Weise K, Akhmedov NG, Petersen
JL, Shi X. 1,2,3-Triazoles as versatile directing group for selective
sp2 and sp3 C–H activation: cyclization vs substitution. Chem Sci,
2013, 4: 3712–3716
14 Both crystals of compound 3 and complex 4a were obtained by slow
diffusion of hexane into CH2Cl2. solution of corresponding compound
15 For the analysis of “gold cation coordination facilitated fluorescent
decay pathway to the excited state” please refer to: Hattori Y,
Kusamoto T, Nishihara H. Enhanced luminescent properties of an
open-shell (3,5-dichloro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl
radical by coordination to gold. Angew Chem Int Ed, 2015, 54: 1–5;
see also references therein
10 Sengupta S, Duan H, Lu W, Petersen JL, Shi X. One step cascade
synthesis of 4,5-disubstituted-1,2,3-(NH)-triazoles. Org Lett, 2008,
10: 1493–1496