Luminescent monocyclometalated cationic gold(iii) complexes: Synthesis, photophysical characterization and catalytic investigations

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Luminescent monocyclometalated cationic

gold(^III) complexes: synthesis, photophysical

characterization and catalytic investigations†

Cite this: Dalton Trans., 2014, 43,

11959

Thomas N. Zehnder, Olivier Blacque and Koushik Venkatesan*

Stable, luminescent, and cationic monocyclometalated gold(III) monoaryl complexes of the type [(ppy)Au-

(FMes)(L)]⁺[OTf]⁻[L = 4-phenylpyridine (3), quinoline (4), 4-ﬂuoroaniline (5), P(OMe)₃(6), PPh₃(7)],

bearing diﬀerent ancillary ligands, synthesized starting from the precursor complex [(ppy)Au(FMes)-

(OH₂)]⁺[OTf]⁻(2) are reported. The preliminary assignment of the structure of the complexes by various

nuclear magnetic resonance spectroscopy techniques and elemental analysis has been further corrobo-

rated by single-crystal X-ray diﬀraction studies. The complexes exhibit room temperature phosphor-

escence in solution, in neat solids and in doped PMMA ﬁlms. Detailed photophysical investigations of the

complexes in solution, in neat solids and in PMMA ﬁlms revealed the successful tuning of the emission

quantum yield (ϕ_p) based on the electronic properties of the ancillary ligands. The catalytic photo-oxi-

dation of benzylic amines to their corresponding imines using molecular oxygen as the oxidant was suc-

cessfully achieved in the presence of the luminescent Au(^III) complexes. It is also established that the

photocatalytic performance was strongly governed by the electronic properties of the ancillary ligands on

the photosensitizer as well as by the steric bulk of the substrates.

Received 23rd April 2014,

Accepted 12th June 2014

DOI: 10.1039/c4dt01187b

www.rsc.org/dalton

motes non-radiative deactivation of the excited state.¹⁴A strat-

egy to overcome this problem was reported by the group of

Introduction

In recent years, intensive investigations on phosphorescent Yam utilizing strong field ligands, especially strongly σ-donat-

transition metal complexes have grown rapidly due to their ing ligands such as N-heterocyclic carbenes (NHCs) or

interesting properties and their possible use in optoelectronic alkynes.^9,14–26These ligands decrease the probability for the

devices such as phosphorescent organic light emitting diodes thermal population of the non-emissive d–d states and thereby

(PhOLEDs) and light emitting electrochemical cells (LECs) as lead to luminescence in these complexes.

well as catalysts in photochemical reactions.^1–10A significant

Most of the reported luminescent Au(^III) complexes have a

advantage of triplet emitters over fluorescent emitters stems biscyclometalated tridentate ligand scaﬀold which helps to

from their theoretical internal quantum eﬃciency of 100% avoid molecular distortions and leads to a decrease in non-

compared to 25% for fluorescent emitters that can be achieved radiative excited state decay, therefore resulting in eﬃcient

due to the triplet harvesting enabled by the presence of a luminescence. Another reason for the use of the tridentate

heavy metal.^1,11Ir(III) and Pt(II) complexes have been the focus ligand scaﬀold is the increased stability of the complexes

of most investigations among the transition metal phosphores- towards thermal and light induced degradation of the Au(^III)

cent emitters.^12,13In contrast to the Pt(II) complexes, research complexes. Recently, our group reported the first stable and

on the isoelectronic Au(^III) complexes as emitters remained luminescent bidentate monocyclometalated Au(^III) complexes

limited. One of the reasons for the rather small number of by utilizing two pentafluorophenyl ligands.²⁷These complexes

phosphorescent Au(^III) complexes lies in the nature of the d–d were found to be more stable than the dialkyne substituted

states that are energetically low-lying and therefore very close monocyclometalated Au(^III) complexes reported independently

to the potentially emissive charge-transfer states, which pro- by Yam and co-workers and our group.^19,28In order to obtain

stable and neutral monocyclometalated Au(^III) complexes with

high luminescence eﬃciency, 1,3,5-tris(trifluoromethyl)-

benzene (FMes) was employed as a primary ancillary ligand in

combination with various anionic secondary ancillary ligands,

Department of Chemistry, University of Zurich, Winterthurerstrasse 190, CH-8057

Zürich, Switzerland. E-mail: venkatesan.koushik@chem.uzh.ch

†Electronic supplementary information (ESI) available. CCDC 985205–985210.

which resulted in complexes with good stability and photo-

physical properties as reported recently by our group.^29aTo

For ESI and crystallographic data in CIF or other electronic format see DOI:

10.1039/c4dt01187b

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gain further insight into the influence of the electronic pro-

perties of the secondary ancillary ligand and the charge of the

complex on the emission eﬃciency of the compounds, we

sought a strategy that involves the replacement of the chloride

ligand with a weakly coordinating triflate substituent, which

was anticipated to subsequently allow for further substitution

with various secondary ancillary ligands in a facile fashion.

While the introduction of ancillary ligands such as 4-phenyl-

pyridine (4-ppy) and quinoline was used to assess the eﬀect of

highly eﬃcient interligand charge transfer on the quantum

yield, ligands such as 4-fluoroaniline, PPh₃and P(OMe)₃were

used to estimate the extent of influence of both the σ-donation

and π-accepting behavior on the emission eﬃciency of the

resulting complexes. Although some examples of the related

cationic monocyclometalated Au(^III) complexes have been

reported previously,^29b–gtheir luminescent and photocatalytic

properties have not been investigated. Since the luminescence

properties of cationic monocyclometalated Au(^III) complexes

have not been previously explored, these studies were expected

to provide the first detailed insight of the choice of ancillary

ligand on the phosphorescence eﬃciency of this class of com-

plexes. Moreover, recent investigations on the isoelectronic cat-

ionic Pt(^II) cyclometalated complexes have shown very

interesting mechanochromic properties.^29hIn addition to the

exploration of the luminescence properties of monocyclometa-

lated cationic Au(^III) monoaryl complexes, we also successfully

probed the influence of the ancillary ligands with diﬀerent

electronic properties on the catalytic eﬃciency of the corres-

ponding Au(^III) complexes.

Scheme 1 Synthesis of complex 2.

parison with 9.52 ppm for the starting material 1.^29aSingle

crystals that were suitable were subjected to X-ray diﬀraction

studies, which confirmed the expected chemical identity of 2

but with a coordinated water molecule. The coordination of

H₂O is further confirmed by a singlet resonance at 5.33 ppm

in the ¹H NMR studies. Au(III) aqua complexes are very rare

and there is only one known example of the Au(^III) aqua

complex reported in the literature.²⁹ⁱ

Since the H₂O ligand bound to the Au(III) in the complex 2

is considered to be weakly coordinating, this complex was

expected to undergo facile substitution reactions and hence

was used as a precursor for the preparation of various cationic

complexes of the type [(ppy)Au(FMes)L]⁺[OTf]⁻[L = 4-phenyl-

pyridine (3), quinoline (4), 4-fluoroaniline (5), P(OMe)₃(6),

PPh₃(7)] (Scheme 2). The syntheses of the complexes 3–7 were

achieved by subjecting the complex 2 to the respective ligands

in dichloromethane (DCM) at room temperature. Analytically

pure products were isolated in moderate to good yields

(49–93%).

Reaction progress was monitored by ¹H NMR studies for

complexes 3–7 and by ³¹P NMR in the case of 6 and 7. Exten-

sive characterization of the complexes was carried out by ¹H,

¹³C, ¹⁹F, ³¹P NMR and elemental microanalyses. The formation

of the desired product was further confirmed by the obser-

vation of the relative shift of the resonance of the proton in

the C_αposition relative to N of the pyridyl ring upon coordi-

nation of the new ligand. The shift of this proton was found to

vary in the range of 9.10 ppm (6) to 8.31 ppm (7) reflecting the

diﬀerent electronic properties of the ancillary ligands.

Herein, we report on the synthesis, structural and photo-

physical investigations as well as on the catalytic behavior of a

series of stable monocyclometalated cationic Au(^III) monoaryl

complexes bearing neutral ancillary ligands that exhibit room

temperature (RT) phosphorescence in solution, neat solid and

in PMMA. These complexes were found to catalyze the photo-

oxidation of benzylic amines to their corresponding imines.

The diﬀerent electronic properties of the ancillary ligand

resulted in complexes with diﬀerent emission quantum yields

and also displayed diﬀerent behavior in the catalytic

investigations.

The ¹⁹F NMR studies revealed a resonance for the OTf⁻

counterion of the cationic complexes 3–7 at −78.9 ppm. Fur-

thermore, ¹⁹F NMR studies revealed the peak for the para tri-

fluoromethyl group of the FMes ligand as a singlet in the

range between −63.4 and −64.9 ppm for all complexes. More-

over, the two ortho bound trifluoromethyl groups also

appeared as a single resonance signal in the range between

−58.8 and −61.2 ppm for complexes 2, 3 and 5–7, while the

spectrum of 4 exhibited two separate signals for the two

groups at −57.2 and −59.8 ppm, respectively. In the phosphine

bearing complexes 6 and 7, the appearance of a new singlet

resonance at 115.5 and 37.9 ppm, respectively, in the ³¹P{¹H}

NMR studies further confirmed the product.

Results and discussions

Syntheses and characterization of the complexes

In the first step, the complex [(ppy)Au(FMes)(OH₂)]⁺[OTf]⁻

(ppy = 2-phenylpyridine, FMes = 1,3,5-tris(trifluoromethyl)benzene,

OTf = trifluoromethanesulfonate) (2) was synthesized analo-

gous to a method reported by Tilset and coworkers.³⁰Treat-

ment of [(ppy)Au(FMes)Cl] (1) with a stoichiometric amount of

AgOTf in CHCl₃at RT for 3 days gave the desired product in a

good yield of 93% (Scheme 1). ¹⁹F NMR studies revealed a new

resonance signal at −79.8 ppm confirming the presence of the

All the synthesized complexes were found to be air and

moisture stable under ambient conditions in the solid state.

Furthermore, complexes 2–4 and 6–7 appeared to be stable in

common organic solvents against decomposition for several

1

triflate group in 2. Furthermore, H NMR studies showed that

the resonances for the proton attached to the C_αto N of the

pyridyl ring significantly shifted upfield to 8.84 ppm in com-

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Scheme 2 Synthesis of complexes 3–7.

days at room temperature. In comparison with this behavior, lability of the triflate ligand confirming our initial hypothesis,

the complex 5 revealed an equilibrium between the desired since it was only possible to obtain a single crystal of the

complex and its parent compound

2 along with some complex with the H₂O molecule bound to the Au(III) center,

decomposition of the complex in solution. This was evidenced resulting in the corresponding complex [(ppy)Au(FMes)(H₂O)]⁺-

1

by H NMR studies, where besides the expected signals for the [OTf]⁻. As expected for four-coordinate d⁸complexes with four

intact complex 5, also signals of the free ancillary ligand diverse ligands, the X-ray crystal structural analysis revealed a

4-fluoroaniline as well as signals of the water coordinated pre- distorted square-planar coordination of the ligands at the gold

cursor complex 2 were found. This behavior can be attributed center. The Au–C_ppydistances were found to lie in the narrow

to the electron withdrawing fluorine group on the phenyl ring range of 2.007(3) to 2.063(5) Å reasonably reflecting the donor

that results in a weak coordination of the 4-fluoroaniline strength of the respective ligand that is bound trans to the

ligand. Although previous studies have shown to form C–C Au–C_ppy. Furthermore, the Au–C_FMesdistances lie in the

coupling products as a result of decomposition,^29j–kwe were narrow range between 2.018(3) and 2.041(5) Å. These values

not able to observe such byproducts during the photocatalytic are in good accordance with the distances reported for the

studies.

monochloro precursor complex 1, being 2.019(2) Å for Au–C_ppy

29a

and 2.013(2) Å for Au–C_FMes

.

As expected, owing to the

Structural characterization

diﬀerent electronic nature of the ligands, the bond lengths of

Single-crystal X-ray diﬀraction studies were performed for all the diﬀerent ancillary ligands trans to C_ppywere found to be

the reported complexes 2–7. Suitable single crystals were significantly diﬀerent from each other lying in between

obtained by layering hexane over a concentrated solution of 2.124(3) Å for the Au–O distance in the complex 2 and

the respective complex in DCM and subsequent slow evapor- 2.4345(7) Å for the Au–P distance in the complex 7. The bite

ation at 0–5 °C. The perspective views of the complexes 2–7 are angles of the cyclometalating ligand (N–Au–C_ppy) were found

shown in Fig. 1 and some relevant bond distances and angles to lie in the range of 80.94(10)–82.5(2)° and as a consequence

are given in Table 1. Crystallographic details are provided in the angles of the trans disposed ligands C_FMes–Au–L showed

Tables 2 and 3. The crystallographic structure of 2 revealed the large variations in the range of 88.14(19) to 92.14(12)°.

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Table 1 Selected bond distances (Å) and angles (°) of complexes 2–7

Bond distance

Bond angle

Complex 2

C(12)–Au(1)

O(1)–Au(1)

C(1)–Au(1)

N(1)–Au(1)

2.018(3)

2.124(3)

2.007(3)

2.076(3)

C(1)–Au(1)–N(1)

N(1)–Au(1)–O(1)

C(12)–Au(1)–O(1)

C(1)–Au(1)–C(12)

82.15(13)

94.37(11)

92.14(12)

91.26(14)

Complex 3

C(12)–Au(1)

N(2)–Au(1)

C(11)–Au(1)

N(1)–Au(1)

2.036(5)

2.130(5)

2.025(6)

2.075(5)

C(11)–Au(1)–N(1)

N(1)–Au(1)–N(2)

C(12)–Au(1)–N(2)

C(11)–Au(1)–C(12)

82.5(2)

94.3(2)

88.14(19)

95.3(2)

Complex 4

C(21)–Au(1)

N(2)–Au(1)

C(7)–Au(1)

N(1)–Au(1)

2.021(3)

2.171(3)

2.017(3)

2.075(3)

C(7)–Au(1)–N(1)

N(1)–Au(1)–N(2)

C(21)–Au(1)–N(2)

C(7)–Au(1)–C(21)

81.62(13)

96.47(14)

89.81(14)

92.46(13)

Complex 5

C(18)–Au(1)

N(2)–Au(1)

C(7)–Au(1)

N(1)–Au(1)

2.020(2)

2.174(2)

2.017(2)

2.0965(19)

C(7)–Au(1)–N(1)

N(1)–Au(1)–N(2)

C(18)–Au(1)–N(2)

C(7)–Au(1)–C(18)

81.59(8)

95.97(8)

90.56(8)

92.09(9)

Complex 6

C(15)–Au(1)

P(1)–Au(1)

C(11)–Au(1)

N(1)–Au(1)

2.041(5)

2.3679(12)

2.063(5)

2.085(4)

C(11)–Au(1)–N(1)

N(1)–Au(1)–P(1)

C(15)–Au(1)–P(1)

C(15)–Au(1)–C(11)

80.95(18)

96.92(12)

91.54(13)

90.59(18)

Complex 7

C(12)–Au(1)

P(1)–Au(1)

C(11)–Au(1)

N(1)–Au(1)

2.032(3)

2.4345(7)

2.051(3)

2.102(2)

C(11)–Au(1)–N(1)

N(1)–Au(1)–P(1)

C(12)–Au(1)–P(1)

C(12)–Au(1)–C(11)

80.94(10)

98.45(7)

89.22(7)

91.39(11)

Fig. 1 X-ray crystal structures of 2–7 with the selective atomic num-

bering scheme. Thermal ellipsoids are drawn at the 30% probability

level. Hydrogen atoms, solvent molecules and counterions are omitted

for clarity.

The changes in the low-energy absorption band wavelengths

are attributed to the diﬀerent electronic properties of the sec-

ondary ancillary ligands. A bathochromic shift of the low

energy bands was observed for complexes with stronger elec-

tron donating ancillary ligands, which is particularly striking

in the case of complexes 6 and 7. The molar absorption coeﬃ-

cients (ε) were found to lie in the region from 10³to 10⁴dm³

mol⁻¹cm⁻¹with the lowest being 6047 for the complex 7 and

the highest being 28 920 for compound 3. The molar absorp-

tion coeﬃcients were found to decrease with increasing wave-

length of the low-energy absorption band.

Although the cyclometalated part of the complex was found to

be essentially flat, deviations from the ideal square planar geo-

metry are found commonly as in similar Au(^III) complexes and

reflect the steric demands of the cyclometalating

ligand.^{19,27–29a,31}The intermolecular Au⋯Au distances were

found to lie in the range of 7.0628(2) Å to 8.6544(2) Å. Due to

the large distance between the gold centers, the presence of

aurophilic interactions could be ruled out.³²

UV-Vis absorption studies

The photophysical data for the complexes are given in Table 4.

The UV-Vis absorption spectra of the complexes 2 and 4–7

(Fig. 2) exhibited a low-energy absorption band ranging

Emission studies

between 293 nm and 334 nm. The shape of the bands closely All complexes, except for 4, showed emission in the solid state

resembled the bands of the free 2-phenylpyridine (ppy) ligand. at RT. Nonetheless, all the complexes showed phosphor-

The low-energy absorption of 3 was found at 293 nm and escence emission in solution as well as when doped into a

therefore at a significantly lower wavelength than the absorp- PMMA matrix. The non-emissive nature of 4 in the solid state

tion maxima of the other complexes. An additional shoulder at could be attributed to self-quenching eﬀects due to intermole-

336 nm was found in the UV-Vis spectrum of 3, which lies cular π–π stacking interactions of the quinoline moieties owing

more closely to the absorption wavelengths of the other com- to the relative close packing in the solid. These interactions

pounds. Since there are no significant changes observed in the can be minimized using low concentrated samples of the

band shape or the number of bands, the lowest significant complex, which also explains the emissive nature of 4 in solu-

singlet transition S₀→ S₁is tentatively assigned to predominantly tion and in the PMMA matrix. As elucidated in earlier studies,

involve the frontier orbitals of the cyclometalating ppy ligand. the wavelength of the emission maxima can be mainly tuned

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Table 2 Crystallographic data for compounds 2, 3 and 4^a

2

3

4

CCDC

985205

₂₀H₁₂AuF₉NO⁺·CF₃O₃S⁻

799.34

183(2)

985206

985207

Empirical formula

Formula weight (g mol⁻¹

Temperature (K)

Wavelength (Å)

Crystal system, space group

a (Å)

C

C₃₁H₁₉AuF₉N₂⁺·CF₃O₃S⁻

C₂₉H₁₇AuF₉N₂⁺·CF₃O₃S⁻

)

936.52

183(2)

0.71073

Triclinic, P1

8.6544(2)

910.50

183(2)

0.71073

Triclinic, P1

8.4101(2)

0.71073

Triclinic, P1

8.3536(2)

ˉ

b (Å)

c (Å)

α (°)

12.4389(3)

12.5317(3)

76.432(2)

11.2053(3)

16.6644(5)

96.532(2)

11.9807(2)

15.0592(5)

96.293(2)

β (°)

88.908(2)

81.078(2)

1250.30(5)

2, 2.123

6.086

93.825(2)

95.541(2)

1593.10(7)

2, 1.952

4.791

97.533(2)

γ (°)

100.257(2)

1466.40(6)

2, 2.062

5.202

876

0.25 × 0.10 × 0.02

2.36 to 32.65

36 143

9841/R_int= 0.0224

100.0

Analytical

0.881 and 0.498

9001/25/495

1.061

Volume (Å³)

Z, density (calcd) (Mg m⁻³

)

Abs coeﬃcient (mm⁻¹

F(000)

)

760

904

Crystal size (mm³)

θ range (°)

0.26 × 0.12 × 0.10

2.77 to 30.51

25 802

7588/R_int= 0.0514

99.6

Analytical

0.626 and 0.399

6904/0/367

1.057

0.07 × 0.06 × 0.04

2.76 to 28.45

18 265

6741/R_int= 0.0716

99.7

Analytical

0.859 and 0.780

5277/0/460

1.038

Reflections collected

Reflections unique

Completeness to θ (%)

Absorption correction

Max/min transmission

Data/restraints/parameters

Goodness-of-fit on F²

Final R₁and wR₂indices [I > 2σ(I)]

R₁and wR₂indices (all data)

0.0312, 0.0724

0.0361, 0.0758

1.449 and −1.178

0.0473, 0.0757

0.0725, 0.0880

0.998 and −1.243

0.0327, 0.0782

0.0375, 0.0815

3.285 and −1.546

Largest diﬀ. peak and hole (e Å⁻³

)

^aThe unweighted R-factor is R₁= ∑(F_o− F_c)/∑F_o; I > 2σ(I) and the weighted R-factor is wR₂= {∑w(F_o²− F_c²)²/∑w(F_o²)²}^1/2

.

Table 3 Crystallographic data for compounds 5, 6 and 7^a

5

6

7

CCDC

985208

985209

985210

Empirical formula

Formula weight (g mol⁻¹

Temperature (K)

Wavelength (Å)

Crystal system, space group

a (Å)

b (Å)

c (Å)

α (°)

β (°)

C

₂₆H₁₆AuF₁₀N₂⁺·CF₃O₃S⁻·(CH₂Cl₂)_0.12(C₂₃H₁₉AuF₉NO₃P⁺), 2(CF₃O₃S⁻)·H₂O C₃₈H₂₅AuF₉NP⁺·CF₃O₃S⁻·CH₂Cl₂

)

900.95

183(2)

0.71073

Triclinic, P1

10.0798(2)

12.6213(3)

12.7654(3)

94.426(2)

110.350(2)

93.672(2)

1828.81

183(2)

0.71073

1128.52

183(2)

0.71073

ˉ

Triclinic, P1

8.5077(3)

13.2966(4)

14.1736(5)

75.678(3)

76.534(3)

72.228(3)

1457.72(8)

1, 2.083

5.293

882

0.49 × 0.41 × 0.31

3.01 to 26.37

17 086

5971/R_int= 0.0349

99.9

Monoclinic, P2₁/c

15.2693(4)

17.8363(3)

16.5578(4)

90

114.620(3)

90

4099.5(2)

4, 1.828

3.904

γ (°)

Volume (Å³)

1510.92(6)

2, 1.980

5.069

Z, density (calcd) (Mg m⁻³

)

Abs coeﬃcient (mm⁻¹

F(000)

)

864

2200

Crystal size (mm³)

θ range (deg)

0.30 × 0.11 × 0.10

2.78 to 32.52

47 080

10 154/R_int= 0.0450

99.9

Analytical

0.686 and 0.333

9274/55/470

1.051

0.36 × 0.25 × 0.06

2.71 to 30.46

60 066

11 334/R_int= 0.0413

99.9

Analytical

0.811 and 0.342

9976/24/550

1.030

Reflections collected

Reflections unique

Completeness to θ (%)

Absorption correction

Max/min transmission

Data/restraints/parameters

Goodness-of-fit on F²

Final R₁and wR₂indices

[I > 2σ(I)]

Analytical

0.311 and 0.210

5550/3/433

1.081

0.0245, 0.0541

0.0356, 0.0906

0.0268, 0.0613

R₁and wR₂indices (all data) 0.0292, 0.0565

Largest diﬀ. peak and hole 1.271 and −0.690

(e Å⁻³

0.0390, 0.0933

3.914 and −0.895

0.0333, 0.0647

1.045 and −1.127

)

^aThe unweighted R-factor is R₁= ∑(F_o− F_c)/∑F_o; I > 2σ(I) and the weighted R-factor is wR₂= {∑w(F_o²− F_c²)²/∑w(F_o²)²}^1/2

.

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Table 4 Photophysical properties of complexes 2–7

Absorption

Emission

Complex

λ_max/nm (ε/(dm³mol⁻¹cm⁻¹))

Medium (T/K)

λ_max/nm ( τ₀/μs)

ϕ_P(%)

k_r× 10³s⁻¹

k_nr× 10⁵s⁻¹

1.98

2

318 (7483)

CH₂Cl₂(298)

Solid (298)

PMMA (298)

457, 488, 523 (5.0)

455, 489, 523

457, 487, 525 (98.3)

0.5

7.2

10.4

0.91

1.06

0.52

0.78

6.60

1.89

0.43

0.68

11.56

0.56

3.48

0.62

0.09

3

4

5

6

7

293 (28 920), 336 sh (8682)

314 (13 455)

CH₂Cl₂(298)

Solid (298)

PMMA (298)

460, 492, 526 (9.4)

457, 490, 526

459, 490, 526 (142.8)

0.5

0.6

11.1

1.06

0.06

CH₂Cl₂(298)

Solid (298)

PMMA (298)

455, 487, 522 (1.0)

—

457, 488, 522 (93.9)

0.6

—

17.8

10.24

0.08

330 (7991)

CH₂Cl₂(298)

Solid (298)

PMMA (298)

464, 494, 528 (5.4)

464, 498, 533

464, 493, 523 (98.1)

0.2

1.6

6.7

1.86

0.09

331 (7881)

CH₂Cl₂(298)

Solid (298)

PMMA (298)

456, 489, 522 (0.77)

460, 494, 535

457, 489, 524 (94.7)

0.9

2.1

5.3

12.87

0.10

334 (6047)

CH₂Cl₂(298)

Solid (298)

PMMA (298)

458, 487, 523 (0.92)

459, 492, 531

458, 490, 525 (86.7)

0.3

1.5

5.4

10.83

0.11

Fig. 3 Normalized emission spectra of complexes 2–7 in degassed

CH₂Cl₂at RT.

Fig. 2 Electronic absorption spectra of complexes 2–7 in CH₂Cl₂at RT.

using cyclometalating ligands with diﬀerent π–π* energies,

since the origin of the emission is π–π* ³ILCT (intraligand 5 wt% PMMA matrix were also found approximately in the

charge transfer) mainly localized on the cyclometalating same range as in solution (Fig. 4). Although the emission

ligand.^27–29aIn conjunction with this, no significant shifts in maximum of 6 in PMMA was found at the same wavelength as

the emission wavelengths of the herein presented complexes in solution, the maxima of the other compounds were found

2–7 were expected. However, the varying donor–acceptor to possess either a small hypsochromic shift in the order of

nature of the ancillary ligands was expected to aﬀect the 1–2 nm (2, 3 and 5) or a bathochromic shift in the order of

quantum yields significantly as pointed out earlier since the 1–3 nm (4 and 7). These small shifts can be attributed to the

electronic properties of the ligands would influence the rela- varying extent of rigidification of the complexes in the PMMA

tive positions of the non-emissive d–d excited states and the matrix and also to the nature of the excited state being aﬀected

emissive excited states as well as diﬀerent charge transfer by the environment, which has been previously observed.³³

states. The observed emission maxima in solution, which lay Vibrationally structured emission profiles were observed for

in the narrow range of 487–494 nm, were consistent with the the complexes, which is indicative of the emission to originate

expectations (Fig. 3). Similarly, the emission maxima in a predominantly from an intraligand charge transfer (³ILCT)

11964 | Dalton Trans., 2014, 43, 11959–11972

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for this behavior is unclear and requires further investigations.

The values for the radiative rate constants (k_r) were found to lie

in the region of 10²–10³s⁻¹, while the non-radiative rate con-

stants (k_nr) were estimated between 10⁵and 10⁶s⁻¹. The low

values of k_rwere another diagnostic hint for the emission to

originate from ³ILCT perturbed by the metal center. The

Stokes shift of the compounds is in the region between

9564 cm⁻¹(2) and 8106 cm⁻¹(7), while the solution lifetimes

of the complexes were found in the range between 0.77 and

9.40 µs, therefore indicating the origin of the emission to

come from a triplet excited state.

Cyclic voltammetry studies

The cyclic voltammograms for complexes 2–7 revealed irrevers-

ible anodic peak potentials in the range of +0.10 to +0.22 V (vs.

F_ccouple) and two reduction peaks for all the complexes

0/+

Fig. 4 Normalized emission spectra of 5 wt% complexes 2–7 in PMMA.

(Table 5). The first reduction peak was found to range from

−1.25 to −1.41 V (vs. F_c^0/+couple) in DMF at RT. This reduction

was found to be irreversible for complexes 2–5 and quasi-

reversible for complexes 6 and 7. The second reduction was

excited state based on the cyclometalating ligand, perturbed

by the metal center, as is seen in most cases of luminescent

gold(^III) complexes.^27–29aThe vibrational progressions of com-

plexes 2–7 in the emission spectra in solution as well as in

PMMA were found to lie in the range between 1300 and

1500 cm⁻¹. These values reasonably resemble the CvN and

CvC stretching frequencies in pyridyl systems and therefore

0/+

observed in the range of −1.66 to −1.92 V (vs. F_ccouple) in

DMF at RT. In contrast to the first reduction, this process was

found to be quasi-reversible for all the complexes except for 7.

The redox processes are thought to stem from the cyclometa-

lating ppy ligand, since no significant changes in the redox

potentials were observed. The quasi-reversibility of the first

reduction for 6 and 7 is thought to originate from the electron

accepting nature of the auxiliary phosphine ligands that eases

the back-oxidation of the reduced ligand. For the complex 5

the study revealed a second irreversible oxidation peak at

+0.46 V that is assigned to an oxidation process centered on

the auxiliary 4-fluoroaniline ligand. Due to the oxidizing

nature of gold(^III) complexes and the large electrochemical

band gap found for these complexes, the metal center is most

probably not involved in the redox processes.^16,17,28

3

confirm the ILCT emission based on the cyclometalating ppy

ligand. The observed quantum yields (ϕ_p) were found to lie in

the range 7.2–0.6% in the solid state and between 17.8 and

5.3% when doped into a PMMA matrix. The striking increase

of the quantum yields in the PMMA matrix could again be

attributed to the decrease in the π–π stacking interactions in

comparison with the pure crystalline form. Additionally, the

complexes bearing a conjugated aromatic secondary ancillary

ligand (3 and 4) displayed higher emission quantum yields

than the complexes with considerable π-accepting character (2,

5–7). This is indeed consistent with our hypothesis, the

quantum yields in PMMA were found to be strongly governed

by the electronic nature of the ancillary ligand in such a way

that conjugated aromatic ligands directly bound to the gold(^III)

centre as in the case of complexes 3 and 4 give rise to the

highest ϕ_Pvalues due to the presence of eﬃcient interligand

charge transfer from the ancillary ligand to the cyclometalat-

ing part of the complex, which is also supported by DFT and

TD-DFT calculations. In contrast, complexes bearing ancillary

ligands with increased π-back bonding ability exhibit low emis-

sion quantum yields. This behavior can be ascribed to the

proximity of the d–d states to the potentially emissive ³ILCT

states leading to the increased non-radiative relaxation of the

excited state due to thermal population of the non-emissive

d–d states. In contrast to the high quantum yields observed in

PMMA, the solution quantum yields were found to lie in the

region of 10⁻³. Quantum yields in solution were observed to be

in a similar order to that obtained for the complexes in the

PMMA matrix, except for the complex 6 which exhibited on the

one hand the lowest ϕ_Pin PMMA but revealed the highest

quantum yield in solution on the other hand. The exact reason

Catalytic studies

Although there have been recent reports on the employment of

luminescent Au(^III) complexes in photocatalytic reactions, they

are still scarce in comparison with the Ru(^II) and Ir(III) ana-

logues.^6,7,9In this work, we have evaluated the photocatalytic

performance of the cationic Au(^III) monocyclometalated mono-

aryl complexes. Complexes 2–7 were tested as catalysts for the

photo-oxidation of benzylic amines to the corresponding

imines (Scheme 3). In the presence of molecular oxygen and

Table 5 Cyclic voltammetry data of complexes 2–7 in 0.1 M [nBu₄N]-

[PF₆] (Au electrode; E vs. F_c^0/+; scan rate = 100 mV/s; 20 °C, DMF)

0/+

Complex

Oxidation E_pa/V vs. F_c

Reduction E_pc/V vs. F_c

2

3

4

5

6

7

0.13

0.11

0.22, 0.46

0.11

−1.41, −1.68 (ΔE_1/2

−1.40, −1.66 (ΔE_1/2

−1.25, −1.77 (ΔE_1/2

−1.39, −1.68 (ΔE_1/2

)

−1.33 (ΔE_1/2), −1.92 (ΔE_1/2

)

0.10

−1.36 (ΔE_1/2), −1.77

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Dalton Transactions

radical intermediate through hyperconjugation and/or induc-

tive eﬀects by the substituent on the benzylic nitrogen group.

In conjunction with this, it is thought that the oxidation of the

substrates proceeds over the formation of a radical intermedi-

ate at the benzylic nitrogen. In order to prove that the reactions

were actually catalyzed by the complexes 2–7 and not by nano-

particles that eventually formed through decomposition of the

catalyst during the reaction, the reaction mixture was investi-

gated for nanoparticles by diﬀerential light scattering (DLS).

The analysis indeed revealed the absence of nanoparticles in

the reaction mixture and is highly indicative of the involve-

ment of discrete molecular complexes in the aforementioned

catalytic processes.

Scheme 3 Photo-oxidation of benzylic amines.

under irradiation using a 125 W Hg-lamp as the light source, it

was indeed found that the complexes 2–7 are able to catalyze

the photo-oxidation of benzylic amines.

In these studies, the complex 6 showed the best perform-

ance of the tested catalysts, since full conversion of the sub-

strate N-benzyl-tert-butylamine (8a) to its corresponding

product N-benzylidene-tert-butylamine (9a) was achieved after

a reaction time of 4 h and with a catalyst loading of 0.50 mol%

(Table 6). The other complexes that were tested did not lead to

full conversion of the substrate 8a. The second most active

catalyst was found to be 4, where a conversion of 72% for the

substrate 8a was accomplished. The investigations pertaining

to the photo-oxidation of N,N-dibenzylamine (8b), N-benzyl-

isopropylamine (8c) and N-benzyl-methylamine (8d) to their

respective products N-benzylidene-benzylamine (9b), N-benzy-

lidene-isopropylamine (9c) and N-benzylidene-methylamine

(9d) were pursued using only complexes 4 and 6. For the sub-

strate 8c no full conversion was achieved. Even after a pro-

longed reaction time of 7 h, only a conversion of 12% was

achieved using 6 as the catalyst and 8% was achieved when 4

was employed. The conversion rates for the substrate 8d were

even poor with only 2% with 6 and no conversion at all with 4,

respectively. The oxidation of the amines to the imines is

thought to be driven by singlet oxygen, which is produced by

harvesting the triplet state of the catalyst in the presence of

light.^9,34It is quite evident from the varying conversion rates

of the diﬀerent substrates that an increase of the steric bulk of

the substituents is favorable for the product formation. This

behavior can be attributed to increased stabilization of the

Theoretical calculations

Density functional theory (DFT) and time-dependent DFT

(TD-DFT) calculations were carried out for selected molecules

with the Gaussian 03 program package³⁵to investigate the

absorption and emission properties of our series of com-

pounds [(ppy)Au(FMes)L]. The hybrid functional PBE1PBE³⁶

(also called PBE0) was applied in conjunction with the Stutt-

gart/Dresden eﬀective core potentials (SDD) basis set³⁷for the

Au center augmented with one f-polarization function (α =

1.050) and the standard 6-31+G(d) basis set³⁸for the remain-

ing atoms. Full geometry optimizations without symmetry con-

straints were carried out in the gas phase for the singlet

ground states (S₀) and the lowest triplet states (T₁). The opti-

mized geometries S₀and T₁were confirmed to be potential

energy minima by vibrational frequency calculations at the

same level of theory, as no imaginary frequency was found. On

the basis of the ground-state optimized geometries of the

selected compounds 3 (L = 4-phenylpyridine), 4 (L = quinoline)

and

7

(L

=

PPh₃), time-dependent DFT (TD-DFT)

calculations^39–41combined with the conductive polarizable

continuum model^42,43(CPCM, dichloromethane solvent) were

used to produce the ten lowest singlet–singlet and singlet–

triplet vertical excitations with the corresponding energies,

transition coeﬃcients and oscillator strengths (Table 7, Tables

S1–S3†). The energy levels and compositions of selected fron-

tier orbitals are reported in Tables S4–S6 (ESI†).

The UV-Vis absorption spectra of complexes 2 and 4–7

Table 6 Conversion rates for the photo-oxidation of benzylic amines

to imines

exhibited

a low-energy absorption band in the range

314–334 nm while the low-energy absorption of 3 was found at

a significantly lower wavelength (293 nm). An additional

shoulder at 336 nm was clearly found in the UV-Vis spectrum

of 3. The lowest-lying singlet–singlet transition S₀→S₁calcu-

lated at 307.7 nm (with an oscillator strength f = 0.224) for 3,

309.1 ( f = 0.179) for 4, and 318.4 nm ( f = 0.142) for 7 derives

mainly from the one-electron excitation HOMO → LUMO for 3

and 7 and HOMO → LUMO+1 for 4 (Table 7). The HOMO of

each compound corresponds to the π orbital of the cyclometa-

lating 2-phenylpyridine (ppy) ligand with a π-antibonding

character between the six-membered rings while the LUMO

(for 3 and 7) and LUMO+1 (for 4) are mainly the π* orbitals of

ppy showing an opposite π-bonding character between the

rings with a small contribution from the metal center (Fig. 5).

Substrate

Catalyst

Time (h)

Conversion

8a

6

4

3

2

7

5

4

100%

72%

54%

37%

25%

13%

8b

6

4

6

100%

53%

8c

6

4

7

12%

8%

8d

6

4

7

2%

—

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Table 7 Selected singlet–singlet (S₀–S_n) and singlet–triplet (S₀–T_m) excited states with vertical excitation energies (nm), transition coeﬃcients,

orbitals involved in the transitions, and oscillator strengths f for compounds 3, 4 and 7 (with f > 0.07)^a

3

4

7

S₀–S_n

n = 1

309.1 (0.179)

HOMO → L+1 (0.59)

HOMO → LUMO (0.33)

n = 1

307.7 (0.224)

HOMO → LUMO (0.67)

318.4 (0.142)

HOMO → LUMO (0.63)

n = 3

n = 2

303.9 (0.078)

HOMO → LUMO (0.43)

H−1 → LUMO (0.35)

n = 4

286.9 (0.885)

H−1 → L+1 (0.54)

H−1 → LUMO (0.36)

289.9 (0.152)

H−2 → LUMO (0.47)

n = 9

n = 3

299.1 (0.126)

H−1 → LUMO (0.52)

HOMO → L+2 (0.40)

n = 6

284.3 (0.258)

H−1 → L+1 (0.63)

255.1 (0.177)

HOMO → L+3 (0.55)

n = 8

274.5 (0.070)

H−3 → LUMO (0.57)

H−1 → L+6 (0.31)

T_n–S₀

n = 1

439.5

n = 1

462.8

n = 1

443.6

HOMO ← LUMO (0.59)

n = 2

403.6

H−1 ← LUMO (0.60)

HOMO ← LUMO (0.68)

n = 2

361.3

n = 2

438.5

H−1 ← L+1 (0.55)

HOMO ← L+1 (0.56)

H−1 ← L+6 (0.21)

^aTDDFT/CPCM (CH₂Cl₂) calculations at the optimized ground-state geometry of each compound.

Fig. 5 Spatial plots of selected frontier orbitals of the optimized ground states of 3, 4, and 7.

The S₀→S₁transition can thus be assigned as an intra-ligand the 2-phenylpyridine ligand. The S₀→S₃transition of 3 ( f =

1

charge transfer ILCT[π→π*_ppy] transition. In the case of com- 0.885) computed at 286.9 nm is more intense than the lowest-

pound 4, the singlet excited state S₁arises to a smaller extent lying transition S₀→S₁and is composed of the two excitations

from the HOMO → LUMO excitation corresponding to ligand- HOMO−1 → LUMO+1 and HOMO−1 → LUMO. Since the

1

to-ligand charge transfer LLCT[π→π*], from the quinoline to HOMO−1 and LUMO+1 frontier orbitals are mainly the π and

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Dalton Transactions

π* orbitals of the 4-phenylpyridine ligand (4-ppy), the S₀→S₃ligand ¹LLCT and intra-ligand ¹ILCT characters involving the

1

transition can be viewed as an admixture of intra-ligand ILCT ancillary ligands (4-phenylpyridine, quinoline and PPh₃) and

[π→π*_4-ppy] and ligand-to-ligand ¹LLCT[π_4-ppy→π*_ppy] charac- the chelating 2-phenylpyridine ligand.

ters. The next significant singlet–singlet transition ( f > 0.03) is

The two lowest singlet–triplet vertical excitation T₁–S₀and

the S₀→S₉transition at 255.1 nm ( f = 0.177), which corres- T₂–S₀energies obtained by TD-DFT on the ground state struc-

ponds to the HOMO → LUMO+3 excitation, and like S₀→S₁tures are reported in Table 7. The T₁–S₀transition for 3

can be assigned to a ¹ILCT[π→π*_ppy] transition. For 4, the (439.9 nm) and 7 (443.6 nm) corresponds to the HOMO–

higher-lying S₀→S₂transition at 303.9 nm ( f = 0.078) and the LUMO excitation indicating that the triplet excited state of

S₀→S₃transition at 299.1 nm ( f = 0.126) are composed of two each compound derives from the ³ILCT[π→π*_ppy] origin. The

excitations, HOMO → LUMO/HOMO−1 → LUMO and HOMO optimized triplet state of 3 and 7 (unrestricted DFT calcu-

−1 → LUMO/HOMO → LUMO+2, respectively. The former tran- lations) confirms the ³ILCT[π→π*_ppy] nature of the emissive

sition is assigned as a ¹LLCT[π_ppy→π*_quin]/¹ILCT[π→π*_quin

transition while the latter shows a manifold of charge transfer nylpyridine ligand in comparison with the corresponding

characters,

¹ILCT[π→π*_quin], ¹ILCT[π→π*_ppy], ¹LLCT ground state structure (ESI†). The spin density surfaces of the

]

state, showing the main distortions within the chelating 2-phe-

[π_ppy→π*_FMes] and ¹LMCT[π_ppy→Au] (ligand-to-metal charge lowest triplet state of 3 and 7 provide a visual display of the

transfer), since the electron density in LUMO+2 is delocalized origin of the emission (Fig. 6). The experimental emission

all over the molecule. The corresponding singlet excited states spectra are very similar in shape for all compounds which

S₂and S₃in 4 are energetically closer to the lowest-energy support the idea that the luminescence properties originate

excited state S₁(energy gap ΔE = 0.07 and 0.14 eV, respectively) from the same chromophore, i.e. the chelating 2-phenylpyri-

than in 3 (ΔE = 0.29 eV between S₃and S₁) which is in good dine ligand through the ³ILCT[π→π*_ppy] triplet excited states,

agreement with the experimental data where a shoulder at as already observed in former studies.^27,28Interestingly, the

336 nm, red-shifted in comparison with the main absorption T₁–S₀transition for 4 computed at 462.8 nm is significantly

band at 293 nm, is clearly observed in the UV-Vis absorption diﬀerent in energy from 3 and 7 and corresponds to the H−1–

spectrum of 3 and only guessed, approximately at the same LUMO excitation, two frontier orbitals for which the electron

wavelength as the shoulder of 3, within the broad absorption density is predominantly located on the quinoline ligand. Con-

band of 4 centered at 314 nm. For compound 7, the higher- sequently, the emission of 4 seems to originate from a tran-

lying singlet–singlet transitions to be considered are S₀→S₄cal- sition with a ³ILCT[π→π*_quin] character instead of the expected

culated at 289.9 nm ( f = 0.152) and S₀→S₆at 284.3 nm ( f = ³ILCT[π→π*_ppy] nature. In fact, the ³ILCT[π→π*_ppy] excited

0.258). The orbitals involved in the excitations are HOMO−2/ state, viewed as T₁for 3 and 7, becomes the T₂state for com-

LUMO for S₀→S₄and HOMO−1/LUMO+1 for S₀→S₆which are pound 4, arising from the HOMO–L+1 excitation, and is ener-

involved in ¹ILCT[π→π*_ppy]/¹LLCT[π_PPh3→π*_ppy

]

and ¹ILCT getically close to the T₁excited states of 3 and 7 (the energy of

[π→π*_PPh3]/¹ILCT[π_PPh3→π*_ppy]/¹LMCT[π_PPh3→Au] transitions. the T₂–S₀transition being computed at 438.5 nm). The DFT

It seems that the absorption band experimentally observed for optimized geometry of the triplet state of 4 is in agreement

compound 7 located at 334 nm corresponds only to the intra- with the promotion of one electron from HOMO to L+1 since

ligand charge transfer ¹ILCT[π→π*_ppy] transition S₀→S₁. Conse- the main structural variations in the triplet state in compari-

quently, we can assume that the most red-shifted absorption son with the corresponding ground state are observed within

band or shoulder for all the studied compounds 3, 4 and 7 is the quinoline ligand (ESI†). Again, the spin density surface of

1

assigned as an intra-ligand charge transfer ILCT[π→π*] tran- the lowest triplet state of 4 supports the ³ILCT[π→π*_4-ppy

]

sition involving the chelating 2-phenylpyridine ligand. The origin of the emission (Fig. 6). According to Kasha’s rule, the

corresponding band wavelength lies in the narrow range photon emission is expected only from the lowest excited state

330–336 nm showing a very small eﬀect upon the change of which forbids us to consider the second-lowest excited triplet

3

the electron donating ancillary ligand. The most intense bands state T₂(with a ILCT[π→π*_ppy] character) as the emissive state

which appear at higher energies (290–320 nm) are originating instead of the lower lying T₁state (with a ³ILCT[π→π*_4-ppy

]

mainly from excited states with metal-perturbed ligand-to- character) to match with the similarity of the emission spectra.

Fig. 6 Spin density surfaces for the optimized triplet states of 3, 4 and 7, showing the ³ILCT character of the emission processes.

11968 | Dalton Trans., 2014, 43, 11959–11972

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Nonetheless, this ambiguity between the theoretical results an Edinburgh FLS920 spectrophotometer using 450W Xenon

and the experimental luminescence properties is a point of lamp excitation by exciting at the longest-wavelength absorp-

interest for our further studies.

tion maxima. All samples for emission spectra were degassed

by at least three freeze–pump–thaw cycles in an anaerobic

cuvette and were pressurized with N₂following each cycle.

Phosphorescence lifetimes were measured by a time-correlated

single photon counting method (TCSPC) performed on an

Conclusions

In conclusion, a facile synthetic route towards the preparation Edinburgh FLS920 spectrophotometer, using an nF900 lamp

of stable, luminescent, and cationic monocyclometalated source at 30 000 Hz frequency with 15 nm excitation and

gold(^III) monoaryl complexes has been demonstrated. The 15 nm emission slit widths. Absolute quantum yields were

complex 2 can be further utilized as a precursor molecule for measured using an integrating sphere from Edinburgh Instru-

the ready preparation of other cationic gold(^III) complexes with ments. YAG:Ce (powder) was used as a calibration reference

other auxiliary ligands. It is shown that the choice of appropri- with ϕ_em= 97%. Cyclic voltammograms were obtained with a

ate ancillary ligands with diﬀerent electronic properties allows Metrohm 797 VA Computrace Voltammetric Analyzer. The cell

the emission quantum yield to be tuned, without aﬀecting the was equipped with a gold working electrode and a Pt counter

emission wavelength maxima significantly. DFT and TDDFT electrode, and a non-aqueous reference electrode. All sample

calculations carried out on selected complexes further validate solutions (DMF) were approximately 5 × 10⁻³M in the sub-

the involvement of diﬀerent charge transfer states responsible strate and 0.1 M in Bu₄NPF₆, and were prepared under nitro-

for the origin of the emission, which is significantly influ- gen. Ferrocene was subsequently added and the calibration of

enced by the electronic nature of the ancillary ligands. Further- voltammograms was done. The 797 VA Computrace program

more, the luminescent cationic complexes were successfully was employed for data analysis. Thin films were spin coated

employed as catalysts for the photo-oxidation of benzylic on a quartz slide (12 × 12 mm) from a dichloromethane solu-

amines to their corresponding imines through a C–H acti- tion consisting of a mixture of poly(methyl methacrylate)

vation process. The scope of the catalytic applications is being (PMMA) and the corresponding Au(^III) complex. Commercially

further explored in our group by tailoring the complex solubi- available reagents were purchased from Aldrich, Chemie-

lity in water by an appropriate choice of the counter anions Brunschwig and Fluorochem and were used as such without

and ancillary ligands in order to exploit the developed cationic further purification.

monocyclometalated gold(^III) monoaryl complexes as photo-

sensitizers in water oxidation catalysis.

Synthesis of [(ppy)Au(FMes)(OH₂)]⁺[OTf]⁻(2). To a solution

of 1 (0.500 g, 0.749 mmol) in dry CHCl₃(35 mL) under a

N₂atmosphere, AgOTf (0.231 mg, 0.899 mmol) was added and

the resulting suspension was shielded from light and stirred

for 72 h at RT. After evaporation of the solvent in vacuo and

subsequent dissolution in dichloromethane (DCM), the sus-

pension was filtered, washed with Et₂O and dried in vacuo to

Experimental section

General procedures and instrumentation

Unless otherwise stated, all manipulations were carried out obtain the product as a pale yellow solid. Single crystals suit-

without special precautions for excluding air and moisture. able for X-ray diﬀraction analysis were obtained from slow

¹H, ¹³C{¹H} and ³¹P{¹H} NMR spectra were recorded on either evaporation of a concentrated solution of the complex in DCM

a Bruker AV-500 or a Bruker AV2-400 spectrometer. ¹⁹F NMR with a layer of hexane at 0–5 °C. Yield = 0.547 g, 93%. ¹H NMR

spectra were recorded on either a Bruker AV2-400 spectrometer (500 MHz, CD₂Cl₂, 300 K): δ 8.84 (d, J = 5.5 Hz, 1H), 8.24–8.21

or a Varian Mercury spectrometer. Chemical shifts (δ) are (m, 1H), 8.19 (s, 2H), 8.06 (d, J = 8.5 Hz, 1H), 7.73–7.69 (m,

reported in parts per million (ppm) referenced to tetramethyl- 2H), 7.40 (td, J₁= 7.5 Hz, J₂= 1.0 Hz, 1H), 7.07–7.04 (m, 1H),

silane (δ 0.00 ppm) using the residual protio solvent peaks as 6.26 (d, J = 8.0 Hz, 1H), 5.33 (s, 2H). ¹³C{¹H} NMR (125.7 MHz,

internal standards (¹H NMR experiments) or the characteristic CD₂Cl₂, 300 K): δ 161.3, 147.7, 143.8, 143.5, 142.1, 136.4, 135.8

resonances of the solvent nuclei (¹³C NMR experiments). ¹⁹F (q, J_C–F= 31 Hz), 134.4, 132.6, 131.1 (q, J_C–F= 33 Hz), 130.0,

NMR was referenced to CFCl₃(δ 0.00 ppm) and ³¹P relative to 127.9, 126.7, 125.8, 124.1 (q, J_C–F= 273 Hz), 123.5 (q, J_C–F= 272

phosphoric acid. Coupling constants (J) are quoted in hertz Hz), 121.5, 120.4 (q, J_C–F= 317 Hz). ¹⁹F NMR (282 MHz,

(Hz), and the following abbreviations are used to describe the CD₂Cl₂, 298 K): δ −61.1 (s, 6F), −64.9 (s, 3F), −79.8 (s, 3F).

signal multiplicities:

s (singlet); br s (broad singlet); Anal. Calc. for C₂₁H₁₀AuF₁₂NO₃S·H₂O (%): C, 31.56; H, 1.51; N,

d (doublet); t (triplet); q (quartet); m (multiplet); dd (doublet 1.75. Found: C, 31.87; H, 1.39; N, 1.76.

of doublet); ddd (doublet of doublet of doublet); td (triplet of

Synthesis of [(ppy)Au(FMes)(4-ppy)]⁺[OTf]⁻(3). To a solu-

doublet); tt (triplet of triplet). Proton and carbon assignments tion of 2 (0.050 g, 0.064 mmol) in DCM (30 mL), 4-ppy

have been made using routine one- and two-dimensional NMR (0.012 g, 0.077 mmol) was added and the resulting solution

spectroscopies where appropriate. Elemental microanalysis was stirred at RT for 3 h after which the solvent was evaporated

was carried out with a Leco CHNS-932 analyzer. UV-Vis under reduced pressure. The obtained oﬀ-white solid was

measurements were carried out on a Perkin-Elmer Lambda 19 washed three times with Et₂O and then dried in vacuo to give

UV-Vis spectrophotometer. Emission spectra were acquired on the title compound as an oﬀ-white product. Single crystals

Dalton Trans., 2014, 43, 11959–11972 | 11969

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suitable for X-ray diﬀraction analysis were obtained from slow 319 Hz), 118.2 (d, J_C–F= 24 Hz). ¹⁹F NMR (376.5 MHz, CD₂Cl₂,

evaporation of a concentrated solution of the complex in DCM 300 K): δ −59.6 (s, 6F), −63.6 (s, 3F), −79.0 (s, 3F), −116.2 (s,

with a layer of hexane at 0–5 °C. Yield = 0.048 g, 81%. ¹H NMR 1F). Anal. Calc. for C₂₇H₁₆AuF₁₃N₂O₃S (%): C, 36.34; H, 1.81;

(400 MHz, CD₂Cl₂, 300 K): δ 8.64–8.62 (m, 2H), 8.30 (td, J₁= N, 3.14. Found: C, 36.50; H, 1.67; N, 2.84.

8.0 Hz, J₂= 1.6 Hz, 1H), 8.24 (s, 2H), 8.18 (d, J = 8.0 Hz, 1H),

Synthesis of [(ppy)Au(FMes)(P(OMe)₃)]⁺[OTf]⁻(6). To

a

8.09–8.06 (m, 2H), 7.94–7.92 (m, 1H), 7.85–7.80 (m, 3H), 7.72 solution of 2 (0.050 g, 0.064 mmol) in DCM (30 mL) P(OMe)₃

(ddd, J₁= 7.4 Hz, J₂= 5.8 Hz, J₃= 1.4 Hz, 1H), 7.60–7.65 (m, (0.008 mL, 0.070 mmol) was added and the resulting solution

3H), 7.45 (td, J₁= 7.6 Hz, J₂= 1.2 Hz, 1H), 7.12 (td, J₁= 7.8 Hz, was stirred at RT for 4 h. The obtained yellowish solid was

J₂= 1.2 Hz, 1H), 6.23 (dd, J = 8.0 Hz, 1.0 Hz, 1H). ¹³C{¹H} NMR washed three times with Et₂O and then dried in vacuo to give

(100.6 MHz, CD₂Cl₂, 300 K): δ 163.2, 154.9, 149.7, 148.2, 144.5, the title compound as an oﬀ-white product. Single crystals

143.6, 143.4, 141.2, 135.3, 135.1 (q, J_C–F= 31 Hz), 135.0, 132.7, suitable for X-ray diﬀraction analysis were obtained from slow

132.1, 131.7 (q, J_C–F= 35 Hz), 130.3, 130.2, 129.2, 128.2, 127.1, evaporation of a concentrated solution of the complex in DCM

127.0, 126.7, 124.2 (q, J_C–F= 274 Hz), 123.2 (q, J_C–F= 273 Hz), with a layer of hexane at 0–5 °C. Yield = 0.046 g, 80%. ¹H NMR

122.3, 121.6 (q, J_C–F= 321 Hz). ¹⁹F NMR (376.5 MHz, CD₂Cl₂, (400 MHz, CD₂Cl₂, 300 K): δ 9.11–9.08 (m, 1H), 8.35–8.30 (m,

300 K): δ −58.8 (s, 6F), −63.5 (s, 3F), −78.9 (s, 3F). Anal. Calc. 1H), 8.28 (s, 2H), 8.15 (d, J = 8.0 Hz, 1H), 7.96–7.93 (m, 1H),

for C₃₂H₁₉AuF₁₂N₂O₃S (%): C, 41.04; H, 2.04; N, 2.99. Found: 7.83–7.79 (m, 1H), 7.40 (tt, J₁= 7.6 Hz, J₂= 1.2 Hz, 1H),

C, 41.16; H, 2.06; N, 2.91.

7.17–7.12 (m, 1H), 6.03 (dd, J₁= 13.8 Hz, J₂= 6.0 Hz, 1H), 3.90

Synthesis of [(ppy)Au(FMes)(C₉H₇N)]⁺[OTf]⁻(4). To a solu- (d, J = 11.6 Hz, 9H). ¹³C{¹H} (125.7 MHz, CD₂Cl₂, 300 K): δ 165.9

tion of 2 (0.065 g, 0.083 mmol) in DCM (30 mL), quinoline (d, J_C–P= 10 Hz), 157.5, 155.9, 152.5 (d, J_C–P= 4 Hz), 145.2,

(0.012 mL, 0.100 mmol) was added and the resulting solution 144.2, 141.6 (d, J_C–P= 22 Hz), 134.2 (q, J_C–F= 31 Hz), 134.0,

was stirred at RT for 24 h. The obtained brownish solid was 132.8 (d, J_C–P= 14 Hz), 131.3 (q, J_C–F= 34 Hz), 130.0, 128.8,

washed four times with Et₂O and then dried in vacuo to give 127.8, 126.6 (d, J_C–P= 11 Hz), 123.8 (q, J_C–F= 274 Hz), 123.3 (q,

the title compound as an oﬀ-white product. Single crystals J_C–F= 273 Hz), 122.6, 121.6 (q, J_C–F= 321 Hz), 56.8 (d, J_C–P

=

suitable for X-ray diﬀraction analysis were obtained from slow 9 Hz). ¹⁹F NMR (376.5 MHz, CD₂Cl₂, 300 K): δ –61.2 (d, J = 3.5

evaporation of a concentrated solution of the complex in DCM Hz, 6F), −63.4 (s, 3F), −78.9 (s, 3F). ³¹P{¹H} NMR (202.5 MHz,

with a layer of hexane at 0–5 °C. Yield = 0.064 g, 86%. ¹H NMR CD₂Cl₂, 300 K): δ 115.5. Anal. Calc. for C₂₄H₁₉AuF₁₂NO₆PS (%):

(500 MHz, CD₂Cl₂, 300 K): δ 8.83 (d, J = 5.0 Hz, 1H), 8.80 (d, J = C, 31.84; H, 2.12; N, 1.55. Found: C, 31.51; H, 2.09; N, 1.46.

8.50 Hz, 1H), 8.50 (d, J = 8.50 Hz, 1H), 8.33 (s, 1H), 8.27–8.22

Synthesis of [(ppy)Au(FMes)(PPh₃)]⁺[OTf]⁻(7). To a solution

(m, 3H), 8.10 (s, 1H), 7.97 (t, J = 8.0 Hz, 1H), 7.90–7.87 (m, 2H), of 2 (0.050 g, 0.064 mmol) in DCM (30 mL), PPh₃(0.021 g,

7.83–7.80 (m, 1H), 7.51–7.46 (m, 2H), 7.34 (t, J = 6.5 Hz, 1H), 0.080 mmol) was added and the resulting solution was stirred

7.16 (t, J = 8.0 Hz, 1H), 6.25 (d, J = 6.25 Hz, 1H). ¹³C NMR at RT for 30 min. Thereafter, the solvent was removed in vacuo

(125.7 MHz, CD₂Cl₂, 300 K): δ 163.5, 152.9, 148.5, 144.5, 144.3, and the obtained pale yellow solid was washed with Et₂O to

143.6, 142.4, 135.1 (q, J_C–F= 31 Hz), 135.1, 134.8, 133.7, 133.0, give the title compound as an oﬀ-white solid. Single crystals

132.0, 131.8 (q, J_C–F= 35 Hz), 130.9, 130.4, 130.0, 129.7, 129.2, suitable for X-ray diﬀraction analysis were obtained from slow

127.2, 126.5, 125.5, 124.4 (q, J_C–F= 274 Hz), 123.9, 123.1 (q, evaporation of a concentrated solution of the complex in DCM

J_C–F= 273 Hz), 122.5, 121.5 (q, J_C–F= 321 Hz). ¹⁹F NMR with a layer of hexane at 0–5 °C. Yield = 0.061 g, 92%. ¹H NMR

(376.5 MHz, CD₂Cl₂, 300 K): δ −57.2 (s, 3F), −59.8 (s, 3F), −63.6 (500 MHz, CD₂Cl₂, 300 K): δ 8.34 –8.28 (m, 2H), 8.11 (d, J =

(s, 3F), −78.9 (s, 3F). Anal. Calc. for C₃₀H₁₇AuF₁₂N₂O₃S (%): 6.0 Hz, 1H), 7.99–7.96 (m, 1H), 7.84–7.78 (m, 8H), 7.65 (br s,

C, 39.57; H, 1.88; N, 3.08. Found: C, 39.38; H, 1.83; N, 3.00.

4H), 7.47 (t, J = 8.0 Hz, 1H), 7.33 (br s, 1H), 7.18–7.15 (m, 1H),

Synthesis of [(ppy)Au(FMes)(C₆H₄FN)]⁺[OTf]⁻(5). To a solu- 7.13–7.08 (m, 3H), 6.65 (br s, 2H), 6.20 (t, J = 8.0 Hz, 1H). ¹³C

tion of 2 (0.050 g, 0.064 mmol) in DCM (30 mL) 4-fluoroaniline {¹H} NMR (125.7 MHz, CD₂Cl₂, 300 K): δ 166.9 (d, J_C–P= 6 Hz),

(0.007 mL, 0.077 mmol) was added and the resulting solution 159.5, 158.5, 151.6 (d, J_C–P= 4 Hz), 146.2 (d, J_C–P= 12.7 Hz),

was stirred at RT for 16 h. The obtained solid was washed 144.7, 144.4, 136.0 (d, J_C–P= 11 Hz), 134.8, 133.3 (d, J_C–P

=

three times with Et₂O and then dried in vacuo to give the title 9 Hz), 133.1 (q, J_C–F= 33 Hz), 131.2 (q, J_C–F= 35 Hz), 131.0,

compound as an oﬀ-white solid. Single crystals suitable for 130.2, 129.3, 128.8, 127.3 (d, J_C–P= 7 Hz), 125.9, 123.6 (q, J_C–F

X-ray diﬀraction analysis were obtained from slow evaporation 274 Hz), 123.2, 123.1 (q, J_C–F= 273 Hz), 121.6 (q, J_C–F

=

of a concentrated solution of the complex in DCM with a layer 321 Hz). ¹⁹F NMR (376.5 MHz, CD₂Cl₂, 300 K): δ −59.5 (d, J =

1

of hexane at 0–5 °C. Yield = 0.028 g, 50%. H NMR (500 MHz, 3.0 Hz, 6F),−63.5 (s, 3F), −78.9 (s, 3F). ³¹P{¹H} NMR

CD₂Cl₂, 300 K): δ 9.02 (d, J = 5.5 Hz, 1H), 8.24–8.20 (m, 1H), (202.5 MHz, CD₂Cl₂, 300 K):

δ 37.9. Anal. Calc. for

8.08–8.05 (m, 1H), 7.97 (s, 2H), 7.76–7.69 (m, 2H), 7.41–7.37 C₃₉H₂₅AuF₁₂NO₃PS (%): C, 44.88; H, 2.41; N, 1.34. Found: C,

(m, 1H), 7.07–7.02 (m, 1 H), 6.88–6.82 (m, 2H), 6.79–6.76 (m, 45.19; H, 2.46; N, 1.90.

2H), 6.27–6.22 (m, 1H). ¹³C NMR (125.7 MHz, CD₂Cl₂, 300 K):

General procedure for the photo-oxidation of the benzylic

amines 8a–8d

δ 165.4 (d, J_C–F= 258 Hz), 163.4, 154.2, 148.8, 145.5, 144.3,

143.9, 140.5, 135.1 (q, J_C–F= 30 Hz), 133.7, 132.8, 131.7 (q,

J_C–F= 35 Hz), 131.5, 130.4, 128.9, 128.2, 126.7, 123.9 (q, J_C–F

273 Hz), 123.3 (q, J_C–F= 274 Hz), 121.9, 121.5 (q, J_C–F

=

To a solution of benzylic amine (0.5000 mmol) in MeCN

(9.0 mL) in a 30 cm glass cell, the respective catalyst

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Paper

(0.0025 mmol, 0.50 mol%) was added. Thereafter, O₂was

bubbled through the reaction mixture while irradiating with a

125 W Hg-lamp. After leaving the reaction run for the respect-

ive time at RT, the solvent was removed in vacuo and the

product yield was calculated by means of ¹H NMR based on

the substrate consumption.

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N. N. Ho, J. C. Thomas, J. C. Peters, R. Bau and

M. E. Thompson, Inorg. Chem., 2005, 44, 1713–1727.

14 V. W.-W. Yam, K. M.-C. Wong, L.-L. Hung and N. Zhu,

Angew. Chem., Int. Ed., 2005, 44, 3107–3110.

15 K. M.-C. Wong, X. Zhu, L.-L. Hung, N. Zhu, V. W.-W. Yam

and H.-S. Kwok, Chem. Commun., 2005, 2906–2908.

16 K. M.-C. Wong, L.-L. Hung, W. H. Lam, N. Zhu and

V. W.-W. Yam, J. Am. Chem. Soc., 2007, 129, 4350–4365.

17 V. K.-M. Au, K. M.-C. Wong, N. Zhu and V. W.-W. Yam,

J. Am. Chem. Soc., 2009, 131, 9076–9085.

18 V. K.-M. Au, K. M.-C. Wong, D. P.-K. Tsang, M.-Y. Chan,

N. Zhu and V. W.-W. Yam, J. Am. Chem. Soc., 2010, 132,

14273–14278.

19 V. K.-M. Au, K. M.-C. Wong, N. Zhu and V. W.-W. Yam,

Chem. – Eur. J., 2011, 17, 130–142.

20 V. K.-M. Au, W. H. Lam, W.-T. Wong and V. W.-W. Yam,

Inorg. Chem., 2012, 51, 7537–7545.

X-ray crystallography

The data collection and structure-refinement data for com-

pounds 2–7 are presented in Tables 2 and 3. Single-crystal

X-ray diﬀraction data were collected at 183(2) K on a Xcalibur

diﬀractometer (Ruby CCD detector) for compounds 2, 3, 5–7

and on a SuperNova area-detector diﬀractometer for com-

pound 4 using a single wavelength Enhance X-ray source with

MoK_αradiation (λ = 0.71073 Å).⁴⁴The selected suitable single

crystals were mounted using polybutene oil on the top of a

glass fiber fixed on a goniometer head and immediately trans-

ferred to the diﬀractometer. Pre-experiment, data collection,

data reduction and analytical absorption corrections⁴⁵were

performed with the program suite CrysAlis^{Pro 44}

The crystal

.

structures were solved with SHELXS97⁴⁶using direct methods.

The structure refinements were performed by full-matrix least-

squares on F²with SHELXL97.⁴⁶All programs used during the

crystal structure determination process are included in the

WINGX software.⁴⁷PLATON⁴⁸was used to check the result of

the X-ray analyses. For more details about the refinements, see

the refine_special_details and iucr_refine_instructions_details

sections in the Crystallographic Information files (ESI†).

CCDC-985205 (for 2), CCDC-985206 (for 3), CCDC-985207 (for

4), CCDC-985208 (for 5), CCDC-985209 (for 6) and

CCDC-985210 (for 7) contain the supplementary crystallo-

graphic data for this paper.

21 V. K.-M. Au, D. P.-K. Tsang, K. M.-C. Wong, M.-Y. Chan,

N. Zhu and V. W.-W. Yam, Inorg. Chem., 2013, 52, 558–567.

22 M.-C. Tang, D. P.-K. Tsang, M. M.-Y. Chan, K. M.-C. Wong

and V. W.-W. Yam, Angew. Chem., Int. Ed., 2013, 52, 446–

449.

23 C.-W. Chan, W.-T. Wong and C.-M. Che, Inorg. Chem., 1994,

33, 1266–1272.

24 K.-H. Wong, K.-K. Cheung, M. C.-W. Chan and C.-M. Che,

Organometallics, 1998, 17, 3505–3511.

Acknowledgements

This work was supported by the Swiss National Science Foun-

dation (grant no. 200021_135488) and the University of Zurich.

We are thankful to Michael Bachmann for the excited-state

lifetime measurements of the complexes in PMMA films. K.V.

is grateful to Prof. R. Alberto for his generous support.

25 W.-P. To, K. T. Chan, G. S. M. Tong, C. Ma, W.-M. Kwok,

X. Guan, K.-H. Low and C.-M. Che, Angew. Chem., Int. Ed.,

2013, 52, 6648–6652.

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Luminescent monocyclometalated cationic gold(iii) complexes: Synthesis, photophysical characterization and catalytic investigations

DOI: 10.1039/c4dt01187b

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Article abstract of DOI:10.1039/c4dt01187b

Full text of DOI:10.1039/c4dt01187b

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