Luminescent organogold(III) complexes with long-lived triplet excited states for light-induced oxidative C-H bond functionalization and hydrogen production

Article abstract of DOI:10.1002/anie.201108080

All that glitters is gold: Highly phosphorescent gold(III) complexes (see picture) with extended π-conjugated cyclometalating ligands exhibit rich photophysical and photochemical properties. They act as efficient photocatalysts/photosensitizers for oxidative functionalizations of secondary and tertiary benzylic amines and homogeneous hydrogen production from a water/acetonitrile mixture. Copyright

Full text of DOI:10.1002/anie.201108080

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DOI: 10.1002/anie.201108080
Organogold(III) Photochemistry
Luminescent Organogold(III) Complexes with Long-Lived Triplet
ꢀ
Excited States for Light-Induced Oxidative C H Bond
Functionalization and Hydrogen Production**
Wai-Pong To, Glenna So-Ming Tong, Wei Lu, Chensheng Ma, Jia Liu, Andy Lok-Fung Chow,
and Chi-Ming Che*
Transition-metal complexes with high-energy, long-lived, and
highly emissive triplet excited states have profound applica-
tions in photocatalysis[1] and biosensing.[2] In this regard,
luminescent gold(III) complexes have recently received much
interest as complementary species to their isoelectronic
platinum(II) congeners.[3] Although there have been consid-
erable efforts to develop organogold(III) photochemistry
since the 1990s,[4] cyclometalated gold(III) complexes that are
highly emissive in fluid solutions are still sparse.[5,6] The high
electrophilicity of gold(III) ion renders the unoccupied ds*
(dx2ꢀy2) orbital low-lying and thus the low-energy ligand-to-
metal charge transfer (LMCT) excited state(s) close to the
emissive intraligand excited states, leading to effective
luminescence quenching.[5a] To increase the luminescence
efficiency of gold(III) complexes, strong-field ligands are
desired to push the ds* orbital to higher-lying energy. The
incorporation of a strong s-donating auxiliary ligand L, such
as an acetylide or an N-heterocyclic carbene (NHC), to the
bis-cyclometalated [AuIII(C^N^C)L]+ (HC^N^CH = 2,6-
diphenylpyridine) system[6a] was found to be successful,
resulting in phosphorescence in solutions with emission
Figure 1. a) Chemical structure of 1 and 2; b) ORTEP of 1 with
ellipsoids set at 50% probability and hydrogen atoms omitted for
quantum yields of up to 1.0% and lifetimes up to 0.6 ms,[6b–g]
indicating fast nonradiative decay (knr ꢁ 106–107 sꢀ1) and
clarity; c) Calculated HOMO and LUMO of 1.
moderate radiative decay (kr ꢁ 103–104 sꢀ1).
Herein we present the photophysical and photocatalytic
properties of two novel gold(III) complexes bearing an
extended p-conjugated bis-cyclometalated ligand (Figure 1a).
A p-extention in bis-cyclometalated ligand is anticipated to
enhance radiative decay by increasing the transition dipole
(m) and oscillator strength (f) of the S0!S1 transition whereas
suppressing nonradiative decay by decreasing the structural
excited-state distortion. Indeed, emissive excited states with
lifetimes of 282 and 506 ms in solutions at room temperature
were recorded with these two complexes and, to our best
knowledge, unprecedented for luminescent organogold(III)
complexes. We also demonstrated that these long-lived triplet
intraligand excited states can be harnessed to catalyze
oxidative functionalization of secondary and tertiary benzylic
amines and homogeneous hydrogen production from a water/
acetonitrile mixture.
[*] W.-P. To, Dr. G. S.-M. Tong, Dr. W. Lu, C. Ma, Dr. J. Liu,
Dr. A. L.-F. Chow, Prof. Dr. C.-M. Che
State Key Laboratory of Synthetic Chemistry, Institute of Molecular
Functional Materials, HKU-CAS Joint Laboratory on New Materials,
and
Department of Chemistry, The University of Hong Kong
Pokfulam Road, Hong Kong (China)
E-mail: cmche@hku.hk
Dr. W. Lu
Department of Chemistry
South University of Science and Technology of China
1088 Xueyuan Boulevard, Shenzhen, Guangdong 518055 (China)
Details for the synthesis and characterization of [AuIII(R-
Cnp^N^Cnp)(NHC)](OTf) (R-HCnp^N^CnpH = 4-R-2,6-di-
naphthalen-2-yl-pyridine; R = C6H4-4-OCH3 (1), and R = H
(2); NHC = 1,3-dimethylimidazol-2-ylidene) are given in the
Supporting Information. The structure of 1 has been con-
firmed by single-crystal X-ray crystallography.[7] As shown in
Figure 1b, the (Cnp^N^Cnp) ligand and gold atom are in
a coplanar geometry, revealing the structural rigidity of this
complex. The dihedral angle between the Au(Cnp^N^Cnp)
[**] This work was supported by the Hong Kong Research Grants
Council (HKU 7008/09P and AoE/P-03/08), NSFC/RGC Joint
Research Scheme (N_HKU 752/08), and the CAS-Croucher Funding
Scheme for Joint Laboratories. We thank Dr. Yong Chen for solving
the crystal structure.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201108080.
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Chemie
coordination plane and the NHC ligand is about 898, which is
presumably due to the steric hindrance between the two
methyl groups and the two naphthalenyl hydrogen atoms
close to the gold atom.
Density functional theory (DFT) and time-dependent
DFT (TDDFT) calculations have been performed on both the
prototype complexes [AuIII(C^N^C)(NHC)]+ and [AuIII(R-
Cnp^N^Cnp)(NHC)]+ (1+) at their respective optimized S0
ground state and T1 excited state geometries. The oscillator
strengths (f) of the low-lying singlet excited states from which
the triplet excited state can borrow intensity is larger in 1+ and
the change in bond lengths between the singlet ground state
(S0) and the first triplet excited state (T1) is smaller for 1+ (see
Supporting Information). The singlet–triplet energy gap at
the Franck–Condon state is smaller in 1+ (1000–2500 cmꢀ1,
depending on excitation wavelength) than in [AuIII(C^N^C)-
(NHC)]+ (ca. 3600 cmꢀ1). Accordingly, 1+ has a faster inter-
system crossing (ISC) rate and higher triplet quantum yield.
At the Franck–Condon state, there is a minor contribution of
ligand-to-metal charge-transfer (LMCT) character in the low-
lying singlet excited states of [AuIII(C^N^C)(NHC)]+ but
none in those of 1+. Thus, it is expected that 1+ has a slower
nonradiative decay rate than [AuIII(C^N^C)(NHC)]+.
Indeed, the nonradiatve decay of 1+ is more than 300-fold
slower than that of [AuIII(C^N^C)(NHC)]+ (see below).
The low-energy absorption bands of 1 and 2 in dichloro-
methane with lmax at 403 nm and 402 nm (emax ꢁ 1.7–2.6 ꢀ
104 dm3 molꢀ1 cmꢀ1) are assigned to intra-ligand transitions
localized at the (R-Cnp^N^Cnp) ligand. Dual emission bands
with lmax at 420 and 520 ꢂ 1 nm are found in the emission
spectra of both 1 and 2 in degassed dichloromethane at
a complex concentration of 1.0 ꢀ 10ꢀ5 moldmꢀ3 at 298 K
(Figure 2a). The vibronically structured low-energy emission
band reveals a spacing of about 1400 cmꢀ1 corresponding to
stretching frequencies of the dinaphthalen-2-yl-pyridine
moiety. The quantum yields of the lower-energy emission
are 11.4% and 5.5% for 1 and 2, respectively; the lifetimes of
the lower-energy (519–521 nm) emission bands are 506 ms for
1 and 282 ms for 2. Both the excitation spectra monitored at
lem = 420 nm and 519–521 nm show well-resolved vibronic
structures with lmax at 402–403 nm, which is identical to that of
the absorption spectrum (Figure 2a). There is negligible
solvent effect on the emission spectra of 1 and 2. Upon
excitation at 380 nm, femtosecond time-resolved emission
measurements with dichloromethane solutions of 1 and 2
established that the lifetime of the higher-energy emission
(420 nm) is in the picosecond region (3.3 ps for 1 and 3.7 ps
for 2; see Supporting Information) and the excited states of
which rapidly decay by intersystem crossing with a rate
constant estimated to be about 3 ꢀ 1011 sꢀ1 to the triplet
emissive excited state having the emission lmax at about
520 nm. We thus assign the high- and low-energy emission
bands to the respective prompt fluorescence and phosphor-
escence from metal-perturbed intraligand excited state.
There is a more than 100-fold decrease in emission
intensity at 519–521 nm upon exposure of solutions of 1 and 2
to air. The 420 nm emission band is not changed during the O2
diffusion process (Figure 2b). Given the long emission life-
times of 506 and 282 ms, it is not surprising to find the
Figure 2. a) Electronic absorption, emission, and excitation spectra of
1 (concentration ca. 1.0ꢀ10ꢀ5 moldmꢀ3) in degassed CH2Cl2 at room
temperature. The asterisk indicates a second-order transmission of the
380 nm excitation. b) Emission traces showing the decrease of emis-
sion intensity upon slow diffusion of air into a thoroughly degassed
dichloromethane solution of 1. Only 1% of the emission intensity at
519 nm was retained after exposure to air. Excitation wavelength:
380 nm; asterisk indicates an instrumental artifact. Inset: the air
diffusion process under a 365 nm lamp.
significant O2 quenching effect on the emission intensity at
519–521 nm. The sensitivity of the emission of 2 towards
oxygen can be harnessed to visualize the living cells (see video
in Supporting Information). When nitrogen was blown over
the living cells incubated with 2 in buffer solution, these cells
became yellowish green upon illumination under a fluorescent
microscope. This reveals that 2 can be taken into the living
cells and serves as an O2 sensor inside.
The high emission quantum yields and long-lived triplet
excited states render 1 and 2 good candidates for photo-
chemical reactions. By employing the electrochemical data
plus the spectroscopic measurements, we estimated the
excited-state potential of 1 by using the following equation:
E(*1+/0) = E(1+/0) + E0-0(1+). The cyclic voltammogram of
1 reveals the first quasi-reversible one-electron reduction
wave [E(1+/10), (ipc/ipa) ꢁ 1] at ꢀ1.73 V vs. Cp2Fe+/0 (ꢀ1.04 V
vs. NHE). The 0–0 transition energy of 1, E0–0(1+), can be
estimated from the onset of the triplet emission band, which
occurs at about 500 nm (2.48 eV). Thus E(*1+/0) is estimated
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to be about 0.75 V vs. Cp2Fe+/0 (1.43 V vs. NHE), signifying
that 1 is a powerful photooxidant. Complex 1 was chosen in
the present study to test for its photocatalytic activities.
Oxidation of secondary amines to imines and a-substitu-
tion of tertiary amines have important applications in organic
synthesis.[8] In the presence of oxygen, 1 was found to catalyze
oxidation of secondary amines under light irradiation at room
temperature. As summarized in Table 1, secondary benzyl
Table 2: Conversion and yield of oxidative cyanation of tertiary amines by
complex 1.
Entry
Substrate
R
Product
Conversion,
yield [%]
1
2
3
4
5
6
7
8
5a
5a
5a
5a
5a
5b
5b
5c
5c
5d
5d
5e
5e
H
H
H
H
6a
6a
6a
6a
6a
6b
6b
6c
6c
6d
6d
6e
6e
100, 89[a]
100, 50[b]
100, 87[c]
6, n.d.[d]
2, n.d.[e]
Table 1: Conversion and yield of oxidation of secondary amines to imines
by complex 1.[a]
H
Me
Me
OMe
OMe
Cl
Cl
Br
Br
100, 86[a]
100, 50[b]
100, 84[a]
100, 43[b]
99, 92[a]
Entry
Substrate
R1
R2
Product
Conversion,
yield [%]
9
10
11
12
13
100, 27[b]
100, 82[a]
100, 33[b]
1
2
3
4
5
6
7
8
3a
3b
3c
3d
3e
3 f
H
tBu
tBu
tBu
tBu
tBu
tBu
Bn
4a
4b
4c
4d
4e
4 f
100, 98
100, 98
100, 97
100, 94
100, 96
100, 97
100, 98
100, 89
Me-4
OMe-4
(OMe)2-3,4
F-4
Cl-4
H
[a] The substrate (0.107 mmol), NaCN (0.214 mmol), HOAc
(0.161 mmol), and catalyst 1 were dissolved in 1.6 mL CH3CN/CH3OH
(1:1, v/v). Solvent-saturated O2 was bubbled into the solution and light
(l>385 nm) was focused in front of the quartz cell. After 1.5 h, the
3g
3h
4g
4h
H
iPr
1
solvent was evaporated and the residue was subjected to H NMR
[a] The substrate (0.107 mmol) and catalyst 1 were dissolved in 1.6 mL
CH3CN. Solvent-saturated O2 was bubbled into the solution and light
(l>385 nm) was focused in front of the quartz cell. After 2.5 h, the
solvent was evaporated and the residue was subjected to H NMR
analysis. Conversions and yields were calculated on the basis of the
consumption of the substrates.
analysis. [b] The same reaction conditions as described in [a] except that
no acetic acid was added and irradiation time was 3 h. [c] Same reaction
conditions as that in [a] except that gaseous HCN was used instead of
NaCN/acetic acid. [d] Similar conditions with that in [a] but in the
absence of 1. [e] Similar conditions as in [a] except that the solution was
kept in darkness for 24 h. n.d.=not determined.
1
amines with a variety of functional groups (R1 and R2) can be
converted to imines efficiently in the presence of 1; light
irradiation and oxygen were used together as the oxidant.
Notably, all entries reached 100% conversion with excellent
product yields (89–98%), but only 0.15 mol% of 1 as catalyst
and a short irradiation time of 2.5 h were required. Singlet
oxygen sensitized from the triplet state of 1 is suggested to be
an active oxidant in this reaction.
Complex 1 can also catalyze light-induced generation of
hydrogen upon irradiation of a three-component system in
a degassed acetonitrile/water mixture comprising 1, trietha-
nolamine, and [Co(dmgH)2(py)Cl].[9] Upon irradiation,
a bright yellow color, which signified the reduction of CoIII
to CoII, developed within 30 s. Quenching experiments
revealed that oxidative quenching of 1 by [Co(dmgH)2(py)Cl]
has a close to diffusion-controlled rate constant of 4.9 ꢀ
109 mꢀ1 sꢀ1. However, reductive quenching with TEOA has
not been observed, implying that TEOA acted as a sacrificial
donor to reduce the 1+ generated by oxidative quenching of
1 with [Co(dmgH)2(py)Cl]. As depicted in Figure 3, the
maximum turnover of hydrogen production was more than
350 after 4 h of irradiation. We also allowed the three-
component system in an acetonitrile/water mixture to be
irradiated by sunlight. After 4 h, 250 turnovers of hydrogen
were obtained. For the photolysis using xenon lamp, we used
a glass filter to block the high-energy irradiation (l < 385 nm
from the xenon lamp), but no such protection was adopted in
the experiment when sunlight was used. Light photons with
wavelengths ranging from 315 to 385 nm (UV-A) from
sunlight are high in energy facilitating the photo-decompo-
sition of 1. This may account for the reduced amount of
hydrogen production. Another well-known method to gen-
erate hydrogen from water by light irradiation is to use an
electron relay and a hydrogen-generating catalyst.[8b] Such
system involves electron transfer from the photosensitizer to
We also turned our attention to the oxidative cyanation of
tertiary amines, such as N-aryltetrahydroisoquinoline
(Table 2). When N-phenyl-1,2,3,4-tetrahydroisoquinoline is
ꢀ
used, the a-C H bond reacts with singlet oxygen to give
iminium ions as the intermediate. The iminium ion is unstable
and rapidly reacts with nucleophiles to give the corresponding
a-substituted products. In this work, all of the N-aryl
tetrahydroisoquinoline substrates reacted with sodium cya-
nide in the presence of acetic acid to give the products in 82–
92% yields after 1.5 h of irradiation (l > 385 nm). This
reaction is tolerable to a variety of substituents R on the N-
aryl group. The addition of acetic acid is crucial, as the
product yield obtained without acetic acid is significantly
lower. Murahashi and co-workers have recently reported that
the oxidative cyanation of tertiary amines does not proceed in
the absence of acetic acid (we noted that their reactions are
not photocatalyzed).[8l] The possible role of acetic acid in our
case is to provide H+ so that CNꢀ could be more soluble in the
solvent, which facilitates nucleophilic attack of CNꢀ on
iminium ion.
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[2] a) I. Klimant, M. Kꢁhl, R. N. Glud, G. Holst, Sens. Actuators B
1997, 38, 29 – 37; b) F. N. Castellano, J. R. Lakowicz, Photochem.
Photobiol. 1998, 67, 179 – 183; c) Q. Zhao, C. H. Huang, F. Y. Li,
Chem. Soc. Rev. 2011, 40, 2508 – 2524.
[3] a) R. A. Taylor, D. J. Law, G. J. Sunley, A. J. P. White, G. J. P.
Britovsek, Angew. Chem. 2009, 121, 6014 – 6017; Angew. Chem.
Int. Ed. 2009, 48, 5900 – 5903; b) P. W. Du, J. Schneider, P. Jarosz,
J. Zhang, W. W. Brennessel, R. Eisenberg, J. Phys. Chem. B 2007,
111, 6887 – 6894; c) S. C. F. Kui, S. S.-Y. Chui, C.-M. Che, N. Zhu,
J. Am. Chem. Soc. 2006, 128, 8297 – 8309; d) J. A. G. Williams,
Top. Curr. Chem. 2007, 281, 205 – 268.
[4] C.-W. Chan, W.-T. Wong, C.-M. Che, Inorg. Chem. 1994, 33, 1266 –
1272.
[5] a) C. Bronner, O. S. Wenger, Dalton Trans. 2011, 40, 12409 –
12420; b) W. Lu, K. T. Chan, S.-X. Wu, Y. Chen, C.-M. Che,
Chem. Sci. 2011, DOI: 10.1039/c2sc00947a.
[6] a) K.-H. Wong, K.-K. Cheung, M. C.-W. Chan, C.-M. Che,
Organometallics 1998, 17, 3505 – 3511; b) K. M.-C. Wong, L.-L.
Hung, W. H. Lam, N. Zhu, V. W.-W. Yam, J. Am. Chem. Soc. 2007,
129, 4350 – 4365; c) V. K.-M. Au, K. M.-C. Wong, N. Zhu, V. W.-
W. Yam, J. Am. Chem. Soc. 2009, 131, 9076 – 9085; d) J. J. Yan,
A. L.-F. Chow, C. H. Leung, R. W.-Y. Sun, D.-L. Ma, C.-M. Che,
Chem. Commun. 2010, 46, 3893 – 3895; e) J. A. Garg, O. Blacque,
T. Fox, K. Venkatesan, Inorg. Chem. 2010, 49, 11463 – 11472;
f) J. A. Garg, O. Blacque, K. Venkatesan, Inorg. Chem. 2011, 50,
5430 – 5441; g) V. K.-M. Au, K. M.-C. Wong, N. Zhu, V. W.-W.
Yam, Chem. Eur. J. 2011, 17, 130 – 142.
Figure 3. The hydrogen production turnovers of the three-component
system (1, [Co(dmgH)2(py)Cl] and triethanolamine) in degassed aceto-
nitrile/water (4:1, v/v) versus time of irradiation by a 300 W Xenon
~
&
lamp ( ), under sunlight ( ), and in the presence of an electron relay
(1,1’-dimethyl-4,4’-bipyridinium hexafluorophosphate, [MV](PF6)2), col-
*
loidal Pt and triethanolamine ( ).
the mediator and then to the hydrogen-generating catalyst.
When 1 was used as the photosensitizer, the blue color
characteristic of the MV+ radical (MV2+ = 1,1’-dimethyl-4,4’-
bipyridinium) developed within 30 s of irradiation. The
evolution of hydrogen maintained for one hour and about
75 turnovers were recorded.
In summary, two organogold(III) complexes have been
designed to display a long-lived and highly emissive triplet
excited state in solutions under ambient conditions. The
results of using sunlight and low catalyst loadings for aerobic
[7] CCDC 841693 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif.
[8] a) S. Minakata, Y. Ohshima, A. Takemiya, I. Ryu, M. Komatsu, Y.
Ohshiro, Chem. Lett. 1997, 311 – 312; b) K. Yamaguchi, N.
Mizuno, Angew. Chem. 2003, 115, 1518 – 1521; Angew. Chem.
Int. Ed. 2003, 42, 1480 – 1483; c) Y. Maeda, T. Nishimura, S.
Uemura, Bull. Chem. Soc. Jpn. 2003, 76, 2399 – 2403; d) M.-H. So,
Y. Liu, C.-M. Ho, C.-M. Che, Chem. Asian J. 2009, 4, 1551 – 1561;
e) T. Naota, H. Takaya, S.-I. Murahashi, Chem. Rev. 1998, 98,
2599 – 2660; f) S. Singhal, S. L. Jain, B. Sain, Chem. Commun.
2009, 2371 – 2372; g) X.-Z. Shu, X.-F. Xia, Y.-F. Yang, K.-G. Ji, X.-
Y. Liu, Y.-M. Liang, J. Org. Chem. 2009, 74, 7464 – 7469; h) W.
Han, A. R. Ofial, Chem. Commun. 2009, 5024 – 5026; i) A. G.
Condie, J. C. Gonzꢂlez-Gꢃmez, C. R. J. Stephenson, J. Am. Chem.
Soc. 2010, 132, 1464 – 1465; j) S.-I. Murahashi, Angew. Chem.
1995, 107, 2670 – 2693; Angew. Chem. Int. Ed. Engl. 1995, 34,
2443 – 2465; k) G. Jiang, J. Chen, J.-S. Huang, C.-M. Che, Org.
Lett. 2009, 11, 4568 – 4571; l) S.-I. Murahashi, T. Nakae, H. Terai,
N. Komiya, J. Am. Chem. Soc. 2008, 130, 11005 – 11012; m) Z. Li,
C.-J. Li, Eur. J. Org. Chem. 2005, 3173 – 3176; n) Z. Li, C.-J. Li, J.
Am. Chem. Soc. 2005, 127, 3672 – 3673.
ꢀ
oxidative C H bond functionalization and hydrogen produc-
tion photocatalyzed by complex 1 highlight the richness of
organogold(III) photochemistry that has yet to be developed.
Received: November 17, 2011
Revised: January 3, 2012
Published online: February 3, 2012
Keywords: gold · hydrogen production · luminescence ·
.
N-heterocyclic carbenes · photooxidation
[1] a) D. Li, C.-M. Che, H.-L. Kwong, V. W.-W. Yam, J. Chem. Soc.
Dalton Trans. 1992, 3325 – 3329; b) X.-H. Li, L.-Z. Wu, L.-P.
Zhang, C.-H. Tung, C.-M. Che, Chem. Commun. 2001, 2280 –
2281; c) D. M. Roundhill, H. B. Gray, C.-M. Che, Acc. Chem.
Res. 1989, 22, 55 – 61; d) A. J. Esswein, D. G. Nocera, Chem. Rev.
2007, 107, 4022 – 4047; e) D. A. Nicewicz, D. W. C. MacMillan,
Science 2008, 322, 77 – 80; f) J. M. R. Narayanam, J. W. Tucker,
C. R. J. Stephenson, J. Am. Chem. Soc. 2009, 131, 8756 – 8757.
[9] a) X. L. Hu, B. M. Cossairt, B. S. Brunschwig, N. S. Lewis, J. C.
Peters, Chem. Commun. 2005, 4723 – 4725; b) M. Razavet, V.
Artero, M. Fontecave, Inorg. Chem. 2005, 44, 4786 – 4795; c) P. W.
Du, K. Knowles, R. Eisenberg, J. Am. Chem. Soc. 2008, 130,
12576 – 12577.
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