Luminescent gold(III) alkynyl complexes: Synthesis, structural characterization, and luminescence properties

Article abstract of DOI:10.1002/anie.200500253

(Chemical Equation Presented) Luminous: The Au^III alkynyl complexes [Au(C∧N∧C)(C≡CR)] [HC^N^CH=2,6-diphenylpyridine, R = C₆H₅ (1), C₆H₄-Cl-p (2), C ₆H₄-OCH₃-P (3), C

Full text of DOI:10.1002/anie.200500253

Angewandte
Chemie
Luminescent Gold Complexes
Luminescent Gold(iii) Alkynyl Complexes:
Synthesis, Structural Characterization, and
Luminescence Properties**
Vivian Wing-Wah Yam,* Keith Man-Chung Wong,
Ling-Ling Hung, and Nianyong Zhu
Scheme 1.
In contrast to the related gold(i)[1–3] and the isoelectronic
platinum(ii) compounds,[4,5] which are known to show rich
luminescence properties, luminescent gold(iii) compounds are
rare, with very few exceptions that emit at room temperature
in solution.[6] The reasons for the lack of luminescence in
gold(iii) species are probably the presence of low-energy d–d
ligand field (LF) states and the electrophilicity of the gold(iii)
center. The presence of a nonemissive low-lying d–d state
would quench the luminescence excited state by thermal
equilibration or energy transfer.[7] Coupling of strong s-
donating alkynyl ligands to gold(iii) should render the metal
center more electron rich, with the additional advantage of
raising the energy of the d–d states, which would result in
enhanced luminescence by increasing the chances of populat-
ing the emissive state for the construction of luminescent
organometallic materials. Despite the fact that alkynyl com-
plexes of gold(i)[3] and the isoelectronic platinum(ii)[5] are
known and have been shown to display rich photolumines-
cence properties, to our surprise, alkynyl complexes of
gold(iii) are extremely rare.[8] Although there is increasing
interest in the use of gold(iii) compounds for catalysis of
organic synthetic reactions of alkynes,[9] the chemistry of
gold(iii) alkynyls is essentially unexplored and underdevel-
oped, and their luminescence properties are virtually
unknown.
Unlike most other gold(iii) compounds, which exhibit lumi-
nescence only at low temperature or are nonemissive,
complexes 1–5 display luminescence in various media at
both low and ambient temperature.
Figure 1 shows the crystal structure of 1, in which the
gold(iii) center adopts a distorted square-planar coordination
geometry with C(9)-Au(1)-C(25) and N(1)-Au(1)-C(1) angles
of 162.0(3) and 178.5(3)8, respectively, as is characteristic of d8
metal complexes. The C^N^C ligand and the gold(iii) center
are essentially coplanar, with the plane lying at a dihedral
angle of 64.978 with respect to the ethynylbenzene moiety.
The bond lengths about the gold(iii) center and the C^N^C
Herein we describe the synthesis of a novel series of bis-
cyclometalated gold(iii) alkynyl complexes [Au(C^N^C)-
ꢀ
(C CR)] [HC^N^CH = 2,6-diphenylpyridine, R = C6H5 (1),
C6H4-Cl-p (2), C6H4-OCH3-p (3), C6H4-NH2-p (4);
HC^N^CH = 2,6-bis(4-tert-butylphenyl)pyridine, R = C6H5
(5)], which are the first of their kind (Scheme 1). The
molecular structure of 1 was determined by X-ray crystallog-
raphy. The photophysical properties of 1–5 were also studied.
[*] Prof. Dr. V. W.-W. Yam, Dr. K. M.-C. Wong, L.-L. Hung, Dr. N. Zhu
Centre for Carbon-Rich Molecular and
Nano-Scale Metal-Based Materials Research
Department of Chemistry
The University of Hong Kong
Pokfulam Road, Hong Kong (P. R. China)
Fax: (+852)2857-1586
E-mail: wwyam@hku.hk
[**] V.W.-W.Y. acknowledges support from the University Development
Fund of The University of Hong Kong and The University of Hong
Kong Foundation for Educational Development and Research
Limited. The work described in this paper has been supported by a
CERG Grant from the Research Grants Council of Hong Kong
Special Administrative Region, China (Project No. HKU 7022/03P).
L.-L.H. acknowledges the receipt of a postgraduate studentship
from The University of Hong Kong.
Figure 1. Top: Crystal structure of 1 with atomic numbering scheme.
Hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn
at the 40% probability level. Selected bond lengths [ꢀ] and angles [8]:
ꢁ
ꢁ
ꢁ
Au(1) C(1) 1.979(7), Au(1) C(9) 2.073(7), Au(1) C(25) 2.071(7),
ꢁ
ꢁ
Au(1) N(1) 1.999(5), C(1) C(2) 1.185(9); C(9)-Au(1)-C(25) 162.0(3),
N(1)-Au(1)-C(1) 178.5(3), Au(1)-C(1)-C(2) 176.6(7), C(1)-C(2)-C(3)
177.6(9). Bottom: Crystal packing showing the p···p stacking arrange-
ment in different views.
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DOI: 10.1002/anie.200500253
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ꢁ
ꢁ
ꢁ
ligand [Au(1) C(9) 2.073(7), Au(1) C(25) 2.071(7), Au(1)
ꢁ
ꢁ
N(1) 1.999(5) ꢀ] are similar to the Au C and Au N distances
in related gold(iii) bis-cyclometalated complexes[10] and are
ꢁ
ꢁ
also comparable to the Pt C and Pt N distances in analogous
platinum(ii) complexes.[5c–f] The Au C(1) and C(1) C(2)
bond lengths of 1.979(7) and 1.185(9) ꢀ, respectively, as
well as the Au(1)-C(1)-C(2) bond angle of 176.6(7)8 are
comparable to those found in gold(i) alkynyl[3] and analogous
platinum(ii) alkynyl complexes.[5c–f] The shortest Au···Au
distance of 5.003(1) ꢀ indicates the absence of significant
Au···Au interaction. However, the crystal packing of 1
(Figure 1) reveals the presence of partially staggered [AuIII-
C^N^C] units with interplanar distances of 3.384 ꢀ between
the adjacent C^N^C ligands, suggestive of the presence of
aromatic p···p stacking interactions.
ꢁ
ꢁ
Figure 2. Electronic absorption spectra of 1 (c), 4 (a), and
5 (g) in dichloromethane solution at room temperature.
All complexes exhibit an intense absorption band at 312–
323 nm and a moderately intense vibronic-structured absorp-
tion band at 362–412 nm in dichloromethane. The photo-
physical data of 1–5 are collected in Table 1, and the
electronic absorption spectra of 1, 4, and 5 are shown in
Figure 2. The vibrational progressional spacings of 1310–
1380 cmꢁ1 in the lower energy absorption band correspond to
the skeletal vibrational frequency of the C^N^C ligand. The
absorption energy was found to be rather insensitive to the
nature of the alkynyl ligand. In addition, the observation of
similar absorption bands for the chloro counterpart [Au-
(C^N^C)Cl] is suggestive of the lack of involvement of the
alkynyl ligands in this transition. In view of this, and the
nonreducing nature of gold(iii), such low-energy absorptions
are assigned as metal-perturbed intraligand (IL) p–p*
transition of the C^N^C ligand, involving some charge
transfer from the phenyl moiety to the pyridyl unit. This is
further supported by the lower absorption energy of 5
compared to 1 with the same alkynyl ligand. The observation
of a lower energy metal-perturbed IL p–p* transition in 5 is
ascribed to the better electron-donating properties of the tert-
butyl group on the phenyl rings, which raises the energy of the
HOMO localized on the phenyl ring and thus narrows the gap
between the HOMO and the pyridine-localized LUMO.
Similar assignments were also suggested for other related
gold(iii) complexes.[10] Interestingly, an additional shoulder
was observed in the electronic absorption spectrum of 4 at
about 415 nm (Figure 2). Since the alkynyl ligand with an
electron-rich amino substituent has better electron-donating
ability, the presence of a low-lying alkynyl-to-diarylpyridine
Table 1: Photophysical data for 1–5.
Complex
Absorption
Emission
lmax [nm] (t0 [ms])
lmax [nm] (emax [dm3 molꢁ1 cmꢁ1])[a]
Medium (T [K])
Fem
[b]
1
312 (19890), 322 (19980), 364 (5050), 381 (5870), 402 (4870)
CH2Cl2 (298)[c]
solid (298)
476, 506, 541, 582 (<0.05)
588
1.0ꢁ10ꢁ3
thin film (298)[d]
solid (77)
568
496 (28.3, 310.4)[f]
470, 503, 537, 585 (194)
glass (77)[c,e]
2
3
4
5
312 (19400), 322 (19640), 365(4640), 382 (5170), 402 (4305)
312 (13820), 322 (13455), 362 (6400), 380 (6245), 400 (4190)
CH2Cl2 (298)[c]
solid (298)
solid (77)
476, 506, 539, 584 (<0.05)
550 (<0.05)
0.2ꢁ10ꢁ3
0.5ꢁ10ꢁ3
2.0ꢁ10ꢁ3
6.0ꢁ10ꢁ3
500 (25.3, 256.6)[f]
glass (77)[c,e]
470, 504, 534, 590 (151)
CH2Cl2 (298)[c]
solid (298)
solid (77)
474, 505, 539, 584 (<0.05)
555 (<0.05)
485 (28.4, 268.2)[f]
glass (77)[c,e]
470, 503, 536, 585 (137)
310 (19195), 322 sh (15680), 365 (8855), 381 (10100), 399 (8300),
415 sh (3410)
CH2Cl2 (298)
solid (298)
solid (77)
611 (0.3)
585 (0.2)
548 (0.3, 1.7)[f]
470, 503, 535, 587 (107)
glass (77)[c,e]
313 (26190), 323 (24460), 374 (6130), 392 (8035), 412 (7465)
CH2Cl2 (298)[c]
solid (298)
solid (77)
484, 514, 548, 593 (0.1)
550 (<0.05)
503 (18.2, 171.9)[f]
478, 512, 548, 600 (200)
glass (77)[c,e]
[a] In dichloromethane at 298 K. [b] Luminescence quantum yield, measured at room temperature with [Ru(bpy)3]2+ as standard. [c] Vibronic-
structured emission band. [d] Prepared by vacuum deposition. [e] In CH2Cl2/EtOH/MeOH (1:40:10). [f] Double-exponential decay.
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ligand-to-ligand charge-transfer (LLCT) transition is possi-
p···p stacking of the C^N^C ligand, probably due to the
ble. Thus, the low-energy absorptions in 4 are assigned as an
ordered packing of the molecules in the solid state, as
supported by the observation of such p···p stacking in the
crystal packing of 1 (Figure 1).
ꢀ
admixture of intraligand (IL) p–p*(C^N^C)/LLCT p(C C-
C6H4-NH2-p)!p*(C^N^C) transitions.
Unlike most other gold(iii) compounds, which are non-
emissive or only show luminescence at low temperature, 1–5
display intense luminescence at 468–611 nm in solution on
excitation at l ꢂ 360 nm at room temperature (Table 1). This
supports the idea that introduction of strong s-donating
alkynyl ligands into gold(iii) compounds would enhance the
luminescence properties by increasing the d–d splitting, as
opposed to the observation of emission only in low-temper-
ature glasses of the chloro analogue [Au(C^N^C)Cl].[10] The
long-lived emission with lifetimes in the microsecond range is
suggestive of a triplet parentage. In general, the emission
energies of the compounds were found to be rather insensitive
to the nature of the alkynyl ligands. A vibronic-structured
emission band with band maximum at around 480 nm was
observed for 1–3 and 5 in dichloromethane at room temper-
ature (Figure 3). The vibrational progressional spacings of
Experimental Section
Complexes 1–5 were synthesized by reaction of [Au(C^N^C)Cl] with
various terminal alkynes in the presence of a catalytic amount of
copper(i) iodide in triethylamine and dichloromethane. Pale yellow
crystals were obtained by slow diffusion of diethyl ether vapor into a
dichloromethane solution of the complexes after column chromatog-
raphy on silica gel with dichloromethane as eluent. 1: Yield: 88%.
1H NMR (300 MHz, CH2Cl2, 298 K, relative to Me4Si): d = 8.04 (dd,
J = 7.4, 1.0 Hz, 2H, C^N^C), 7.92 (t, J = 8.0 Hz, 1H, C^N^C), 7.62
(m, 4H, C^N^C and C6H5), 7.54 (d, J = 8.0 Hz, 2H, C^N^C), 7.26–
7.44 ppm (m, 7H, C^N^C and C6H5); positive EI-MS: m/z: 527 [M]+;
IR (KBr): n˜ = 2147 cmꢁ1 n(C C); elemental analysis (%) calcd for
ꢀ
C25H16NAu (found): C 56.93 (56.57), H 3.04 (3.05), N 2.66 (2.66). 2:
Yield: 85%. 1H NMR (300 MHz, CH2Cl2, 298 K, Me4Si): d = 8.00 (dd,
J = 7.2, 1.0 Hz, 2H, C^N^C), 7.90 (t, J = 8.0 Hz, 1H, C^N^C), 7.50–
7.60 (m, 6H, C^N^C and C6H4), 7.25–7.42 ppm (m, 6H, C^N^C and
C6H4); positive EI-MS: m/z: 562 [M]+; IR (KBr): n˜ = 2157 cmꢁ1 n(C
ꢀ
C); elemental analysis (%) calcd for C25H15NClAu·0.5H2O (found): C
52.59 (52.85), H 2.80 (2.66), N 2.45 (2.40). 3: Yield: 86%. H NMR
1
(400 MHz, CH2Cl2, 298 K, Me4Si): d = 8.02 (dd, J = 7.6, 1.0 Hz, 2H,
C^N^C), 7.90 (t, J = 8.0 Hz, 1H, C^N^C), 7.60 (dd, J = 7.6, 1.0 Hz,
2H, C^N^C), 7.50–7.56 (m, 4H, C^N^C and C6H4), 7.40 (dt, J = 7.3,
1.3 Hz, 2H, C^N^C), 7.27 (dt, J = 7.3, 1.3 Hz, 2H, C^N^C), 6.91 (d,
J = 8.9 Hz, 2H, C6H4), 3.88 ppm (s, 3H, OCH3); positive EI-MS: m/z:
557 [M]+; IR (KBr): n˜ = 2157 cmꢁ1 n(C C); elemental analysis (%)
ꢀ
calcd for C26H18NOAu·0.5H2O (found): C 55.12 (55.15), H 3.36 (3.28),
N 2.47 (2.48). 4: Yield: 80%. 1H NMR (300 MHz, CH2Cl2, 298 K,
Me4Si): d = 8.07 (dd, J = 7.4, 1.0 Hz, 2H, C^N^C), 7.92 (t, J = 8.0 Hz,
1H, C^N^C), 7.65 (dd, J = 7.4, 1.0 Hz, 2H, C^N^C), 7.56 (d, J =
8.0 Hz, 2H, C^N^C), 7.39–7.45 (m, 4H, C^N^C and C6H4), 7.30 (dt,
J = 7.5, 1.3 Hz, 2H, C^N^C), 6.67 (d, J = 8.6 Hz, 2H, C6H4), 3.84 ppm
(s, 2H, NH2); positive EI-MS: m/z: 542 [M]+; IR (KBr): n˜ = 2143 cmꢁ1
ꢀ
n(C C); elemental analysis (%) calcd for C25H17N2Au·0.5H2O
Figure 3. Normalized emission spectra of 1 (c), 4 (a), 5 (g)
in degassed CH2Cl2 and 1 in the solid state (thin film, d) at 298 K.
(found): C 54.45 (54.59), H 3.27 (3.13), N 5.08 (5.04). 5: Yield:
85%. 1H NMR (400 MHz, CH2Cl2, 298 K, Me4Si): d = 8.18 (d, J =
2.0 Hz, 2H, C^N^C), 7.86 (t, J = 8.0 Hz, 1H, C^N^C), 7.60 (d, J =
7.6 Hz, 2H, C6H5), 7.56 (d, J = 8.0 Hz, 2H, C^N^C), 7.46 (d, J = 8 Hz,
2H, C^N^C), 7.35 (m, 5H, C^N^C and C6H5), 1.39 (s, 9H, tBu),
1.53 ppm (s, 18H, tBu); positive EI-MS: m/z: 640 [M]+; IR (KBr): n˜ =
about 1300 cmꢁ1 are in line with the C C and C N stretching
frequencies of the tridentate ligand, indicative of the involve-
ment of the tridentate ligand in the excited state origin. The
luminescence is assigned as originating from a metal-pertur-
=
=
2149 cmꢁ1 n(C C); elemental analysis calcd for C33H32NAu·0.5H2O
(found): C 61.11 (61.02), H 5.09 (5.08), N 2.16 (2.17).
ꢀ
3
Crystal data for 1: C25H16AuN, Mr = 527.35, crystal dimensions
0.4 ꢁ 0.2 ꢁ 0.2 mm, orthorhombic, space group P212121, a = 6.735(1),
bed IL [p-p*] state of the tridentate C^N^C ligand. Similar
to the low-energy absorption band in the electronic absorp-
tion studies, 5 emits at lower energy than 1, since the electron-
rich tert-butyl groups on the phenyl rings of the C^N^C ligand
in 5 lead to a higher energy phenyl-localized HOMO and
hence a lower energy 3[p-p*] excited state. In contrast to 1–3
and 5, which show a vibronic-structured emission band in
dichloromethane, 4 exhibits a structureless emission band at
611 nm even in dilute solution (5 ꢁ 10ꢁ6 moldmꢁ3; Figure 3).
b = 14.265(3), c = 19.583(4) ꢀ, V= 1881.4(6) ꢀ3, Z = 4, 1calcd
=
1.862 gcmꢁ3, m(MoKa) = 7.827 mmꢁ1, F(000) = 1008, T= 253 K. Final
R = 0.0262, wR = 0.0555 with I > 2s(I); R = 0.0339, wR = 0.0569 for
all data; GOF = 0.925 for 244 parameters and a total of 10500
reflections, of which 3283 were independent (Rint = 0.0377). MAR
diffractometer, MoKa radiation (l = 0.71073 ꢀ); collection range
2qmax = 50.628 with 38 oscillation step of f, 300 s exposure time, and
scanner distance of 120 mm. 54 images were collected. CCDC-260767
(1) 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.
ꢀ
On the basis of the energetically higher lying p(C C-C6H4-
NH2-p) orbital, the lower energy emission band in 4 is
3
ꢀ
tentatively assigned as derived from an LLCT [p(C C-C6H4-
NH2-p)!p*(C^N^C)] excited state. It is noteworthy that the
emission spectra of 1–5 in the solid state show a low-energy
structureless band at 550–585 nm (Figure 3). The red shift of
the solid-state emission relative to that in solution is
attributed to dimeric or excimeric emission arising from the
Received: January 22, 2005
Published online: April 14, 2005
Keywords: alkynyl ligands · gold · luminescence
.
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