Structural, photophysical, and catalytic properties of Au(I) complexes with 4-substituted pyridines

Article abstract of DOI:10.1021/ic701872f

Ionic gold(I) complexes with general formula of [Au(Py)₂] [AuCl₂] and [Au(Py)₂][PF₆] (Py = 4-substituted pyridines) have been synthesized. Structures of five Au(I) complexes and a Ag(I) complex were determined by single crystal X-ray diffraction. Evidence for cationic aggregation of [Au(Py)₂][PF₆] complexes in solution was obtained by conductivity measurements and by the isosbestic point observed from variable temperature UV-visible absorption spectra. All compounds were luminous in the solid state. Calculations employing density functional theory were performed to shed light on the nature of the electronic transitions. While the [Au(4-dmapy)]₂[AuCl₂] (4-dmapy = 4-dimethylaminopyridine) and [Au(4-pic)₂][AuCl₂] (4-pic = 4-picoline) emissions were found to be mainly ligand in nature, their [PF ₆]^- counterparts involved a Au...Au-interaction imbedded in the highest occupied molecular orbital. [Au(4-dmapy) ₂][AuCl₂] was found to be an efficient catalyst for Suzuki cross-coupling of aryl bromide and phenylboronic acid.

Full text of DOI:10.1021/ic701872f

Inorg. Chem. 2008, 47, 2543-2551
Structural, Photophysical, and Catalytic Properties of Au(I) Complexes
with 4-Substituted Pyridines
J. C. Y. Lin,† S. S. Tang,‡ C. Sekhar Vasam,† W. C. You,‡ T. W. Ho,† C. H. Huang,† B. J. Sun,†
C. Y. Huang,† C. S. Lee,† W. S. Hwang,† A. H. H. Chang,*,† and Ivan J. B. Lin*,†
Department of Chemistry, National Dong Hwa UniVersity, Hualien, Taiwan, 97401, and
Department of Chemistry, Fu Jen Catholic UniVersity, Taipei, Taiwan, 24205
Received September 21, 2007
Ionic gold(I) complexes with general formula of [Au(Py)2][AuCl2] and [Au(Py)2][PF6] (Py ) 4-substituted pyridines)
have been synthesized. Structures of five Au(I) complexes and a Ag(I) complex were determined by single crystal
X-ray diffraction. Evidence for cationic aggregation of [Au(py)2][PF6] complexes in solution was obtained by conductivity
measurements and by the isosbestic point observed from variable temperature UV–visible absorption spectra. All
compounds were luminous in the solid state. Calculations employing density functional theory were performed to
shed light on the nature of the electronic transitions. While the [Au(4-dmapy)2][AuCl2] (4-dmapy ) 4-dimethylami-
nopyridine) and [Au(4-pic)2][AuCl2] (4-pic ) 4-picoline) emissions were found to be mainly ligand in nature, their
[PF6]- counterparts involved a Au · · · Au-interaction imbedded in the highest occupied molecular orbital. [Au(4-
dmapy)2][AuCl2] was found to be an efficient catalyst for Suzuki cross-coupling of aryl bromide and phenylboronic
acid.
Introduction
shell.2 Because of a similarity in magnitude with hydrogen
bonding, it could play a key role in molecular aggregation
in both solid state and solution. While the role of aurophi-
licity in the solid-state aggregations is well established, less
has been studied in solution.3 Aurophilic interaction is also
known to contribute to the intriguing luminescent behavior
of Au(I) complexes.4 Yet, its nature has not been completely
understood.
The significance of Au(I) complexes is manifested in
several fascinating properties, among which the aurophilic
attractions and photophysical properties have been intensively
studied.1 Aurophilicity, often attributed to relativistic effects,
is an attraction between the Au(I) ions with their closed d10
* To whom correspondence should be addressed. E-mail: ijblin@
mail.ndhu.edu.tw (I.J.B.L), hhchang@mail.ndhu.edu.tw (A.H.H.C.). Tel:
886-3-863-3599. Fax: 886-3-863-3570.
†
National Dong Hwa University.
Fu Jen Catholic University.
‡
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G.; Stang, P. J. J. Am. Chem. Soc. 2006, 128, 12668. (b) Yu, S.-Y.;
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10.1021/ic701872f CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 7, 2008 2543
Published on Web 03/05/2008

Lin et al.
Scheme 1. Au(I) Complexes with Different Substituted Pyridines
Au(I) complexes with ligands of phosphines and thiolates
have been the topic of many investigations; complexes of
nitrogen donor pyridine (Py) ligands have been less scruti-
nized,5 owing to perhaps their thermal/photo instability.
Nevertheless, there are several known applications, for
example, the antiarthritic activity of Au(I)-Py complexes6
and the use of pyridine ligands to facilitate the extraction of
gold in mining.7 Recently, Au(I)-Py compounds have been
considered as an emerging class of luminescent materials,5i,j
and utilized as catalysts in olefin aziridination,5k carbene
insertion into benzene,5k cycloisomerization of ꢀ-hydro-
xyallenes,5g and alcohol oxidation.5h
In this work, Au(I) complexes with four 4-substituted
pyridines were synthesized; their solid state supramolecular
structures, solution aggregation behavior, and photophysical
properties were examined. Density functional theory (DFT)
calculations were performed to shed light on the nature of
bonding and of electronic transitions. As far as we know,
only one similar study on Au-Py complexes has appeared
recently.5j This investigation certainly highlights the role of
aurophilicity in the stability and reactivities of Au(I)-Py
compounds. In view of the “gold rush” in various branches
of modern science,8 we also report the utilization of a
Au(I)-Py complex as catalyst for the Suzuki cross-coupling
reaction. The abbreviations for the Py ligands and Au(I)-Py
complexes are given in Scheme 1.
on a Shimadzu UV-2101PC spectrophotometer. Emission, excita-
tion, and lifetime spectra were acquired on an Aminco Bowman
AD2 luminescent spectrofluorometer.
Single crystal X-ray data of [Au(4-dmapy)2][AuCl2], [Au(4-
dmapy)2][PF6] and [Au(4-phpy)2][PF6] were collected on a Bruker
SMART APEX diffractometer, and those of [Au(4-pic)2][AuCl2]
and [Au(4-pic)2][PF6] on a Siemens P4 diffractometer. All the
structures were solved and refined by employing SHELXL 97; non
hydrogen atoms were refined anisotropically. Hydrogen atoms were
placed in calculated positions. The crystal data and experimental
details are given in the Supporting Information Table S1.
Theoretical Method. The calculations of density functional
B3LYP9a,b with LanL2DZ9c basis sets were carried out for free
4-dmapy and ion-pairs, [Au(4-dmapy)2][AuCl2] and [Au(4-
pic)2][AuCl2], mononuclear [Au(4-dmapy)2]+, [Au(4-pic)2]+, and
Experimental Section
AuCl2-, and dinuclear [(Au(4-dmapy)2)2]2+ and [(Au(4-pic)2)2]2+
.
General Information. Elemental analysis was carried out at the
Note that with basis set LanL2DZ, the ab initio effective core
potentials were employed to replace the core electrons of Au, in
which mass-velocity and Darwin relativistic effects have been
incorporated. The Gaussian 98 program10 was utilized in the ab
initio electronic structure calculations.
1
Taiwan Instrumentation Center. H NMR spectra were recorded
on a Bruker AC-F300 spectrometer; UV–vis spectra were obtained
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Patent 4098887, 1978. (m) Kristjansdottir, S. S.; Thompson, J. S. U.S.
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1111.
[Au(4-dmapy)2][AuCl2] (Ia). 4-dmapy (125 mg, 1.02 mmol)
was added to a dichloromethane solution (10 mL) of Au(SMe2)Cl
(300 mg, 1.02 mmol) and was stirred for 2 h at room temperature
in the dark. After removing the solvent under vacuum, the residue
was washed with ether to give a light yellow crude product.
Recrystallization by slow diffusion of ether into the CH3CN solution
produced light yellow crystals of [Au(4-dmapy)2][AuCl2]. Yield:
85%. Mp: 142 °C. Anal. Calcd. for C14H20N4Au2Cl2: C ) 23.71;
H ) 2.84; N ) 7.90. Found: C ) 23.69; H ) 2.83; N ) 7.66.
1
FAB/MS: 441.2 m/z ) [Au(4-dmapy)2]+. H NMR (DMSO-d6):
8.20 (d, 4H, J ) 7.2 Hz, o-H of py); 6.81 (d, 4H, J ) 7.4 Hz, m-H
of py); 3.07 ppm (s, 12H, N(CH3)2).
Complexes IIa-IVa were similarly prepared. Data for each
complex are given below.
[Au(4-pic)2][AuCl2] (IIa). Yield: 65%. Mp: 130 °C. Anal. Calcd.
for C12H14N2Au2Cl2: C ) 22.14; H ) 2.17; N ) 4.30. Found: C )
22.06; H ) 2.20; N ) 4.09. FAB/MS: 383 m/z ) [Au(4-pic)2]+.
1H NMR (DMSO-d6): 8.72 (d, 4H, J ) 6.1 Hz, o-H of py); 7.66
(d, 4H, J ) 5.6 Hz, m-H of py); 2.40 ppm (s, 6H, CH3).
[Au(4-phpy)2][AuCl2] (IIIa). Yield: 55%. Mp: 131 °C. Anal.
Calcd. for C22H18N2Au2Cl2: C ) 34.09; H ) 2.34; N ) 3.61. Found:
C ) 33.99; H ) 2.32; N ) 3.65. FAB/MS: 507.2 m/z ) [Au(4-
(6) (a) Rhodes, M. D.; Sandler, P. J.; Scawen, M. D.; Silver, S. J. Inorg.
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Pittsburgh, PA, 1998.
2544 Inorganic Chemistry, Vol. 47, No. 7, 2008

Properties of Au(I) Complexes with 4-Substituted Pyridines
1
phpy)2]+. H NMR (DMSO-d6): 8.62 (d, 4H, J ) 6.0 Hz, o-H of
py); 7.80 (d, 4H, J ) 6.9 Hz, m-H of py); 7.72–7.47 ppm (m, 10H,
Ph).
[Au(4-t-butylpy)2][AuCl2] (IVa). Yield: 58%. Mp: 142 °C. Anal.
Calcd. for C18H26N2Au2Cl2: C ) 29.40; H ) 3.56; N ) 3.81. Found:
C ) 29.57; H ) 3.65; N ) 3.73. FAB/MS: 467 m/z ) [Au(4-
1
tbutylpy)2]+. H NMR(CDCl3): 8.77 (d, 4H, J ) 7.0 Hz, o-H of
py); 7.50 (d, 4H, J ) 6.8 Hz, m-H of py); 1.36 ppm (s, 18H,
C(CH3)3).
[Au(4-dmapy)2][PF6] (Ib). 4-dmapy (250 mg, 2.04 mmol) was
added to a dichloromethane solution (10 mL) of Au(SMe2)Cl (300
mg, 1.02 mmol), followed by a EtOH (5 mL) solution of NH4PF6
(166 mg, 1.02 mmol) in the dark. The resultant solution was stirred
for 2 h at room temperature. The solvent was then removed under
vacuum to give a light yellow residue, which was washed with
CH3CN/ether and recrystallized from CH3CN/EtOH. Yield: 91%.
Mp: 181 °C. Anal. Calcd. for C14H20N4AuPF6: C ) 28.68; H )
3.44; N ) 9.56. Found: C ) 29.09; H ) 3.52; N ) 10.01. FAB/
1
MS: 441.2 m/z ) [Au(4-dmapy)2]+. H NMR (DMSO-d6): 8.20
(d, 4H, J ) 7.3 Hz, o-H of py); 6.81 (d, 4H, J ) 7.4 Hz, m-H of
py); 3.07 ppm (s, 12H, N(CH3)2).
Compounds IIb-IVb were prepared in the same manner as Ib.
Data for IIb-IVb are given below.
[Au(4-pic)2][PF6] (IIb). Yield: 87%. Mp: 139 °C (dec without
melting). Anal. Calcd. for C12H14N2AuPF6: C ) 27.29; H ) 2.61;
N ) 5.30. Found: C ) 27.49; H ) 2.60; N ) 5.17. FAB/MS: 383
Figure 1. (a) ORTEP diagram of Ia (50% probability ellipsoids), (b) crystal
packing diagram of Ia with a [+ - + - · · · ] pattern (the hydrogen atoms
have been omitted for clarity). Selected bond lengths (Å) and angles (deg):
Au(1)-Au(2), 3.2005(5); Au(1)-N(2), 2.022(3); N(2)-C(6), 1.345(5);
N(2)-C(4), 1.348(6); N(1)-C(12), 1.357(5); Au(2)-Cl(2), 2.2659(16);
N(2)-Au(1)-(N2′),177.7(2);C(4)-N(2)-C(6),116.3(4);Cl(2)-Au(2)-Cl(2′),
179.73(7). Au(1)-Au(2)-Au(1), 180.0.
1
m/z ) [Au(4-pic)2]+. H NMR (DMSO-d6): 8.72 (d, 4H, J ) 5.5
Hz, o-H of py); 7.66 (d, 4H, J ) 4.9 Hz, m-H of py); 2.40 ppm (s,
6H, CH3).
[Au(4-phpy)2][PF6] (IIIb). Yield: 84%. Mp: 174 °C. Anal.
Calcd. for C22H18N2AuPF6: C ) 40.51; H ) 2.78; N ) 4.29. Found:
C ) 40.51; H ) 2.84; N ) 4.12. FAB/MS: 507.2 m/z ) [Au(4-
plexes, Ia, Ib, IIa, IIb, and IIIb, were examined by X-ray
crystallography.
1
phpy)2]+. H NMR (DMSO-d6): 8.94 (d, 4H, J ) 6.1 Hz, o-H of
py); 8.20 (d, 4H, J ) 6.4 Hz, m-H of py); 7.98–7.53 ppm (m, 10H,
Ph).
Crystal Structures. [Au(4-dmapy)2][AuCl2] Ia. As seen
in Figure 1, compound Ia forms an ion-pair (+ -),
comprising a linear [Au(4-dampy)2]+ cation and a linear
[AuCl2]- anion with normal Au-N and Au-Cl bond lengths
(Figure 1caption).5a–e The two pyridyl rings are twisted by
about 79°. The short C(12)-N(1) distance (ca. 1.36 Å) is
consistent with the delocalization of the amine lone pair
together with the electrons of the pyridine ring.11 The cation
and anion are associated perpendicularly through a Au · · ·Au
contact of about 3.20 Å.1 Neighboring ion-pairs are further
linked through a Au · · · Au contact of about 3.27 Å to
generate a linear polymer with alternating cations and anions.
The similar [+ - + - · · ·] packing pattern has been
previously reported.5c,g,r The discrete trinuclear {[AuL2]+-
[LAuCl][AuCl2]-} (L ) 2-aminopyridine),5a and tetranuclear
{[AuX2][AuL2]2[AuX2]} (L ) unsubstituted pyridine, X )
Cl, Br, I; L ) 3-bromopyridine, X ) Cl) compounds5b,c are
also known. Secondary interactions of C-H· · ·Cl, C-H· · ·Au,
and C-H · · · π are responsible for holding the polynuclear
chains in adjacent positions in the supramolecular assembly
(see Supporting Information) as observed by other research-
ers.12
[Au(4-t-butylpy)2][PF6] (IVb). Yield: 81%. Mp: 150 °C. Anal.
Calcd. for C18H26N2AuPF6: C ) 35.31; H ) 4.28; N ) 4.57. Found:
C ) 35.25; H ) 4.30; N ) 4.74. FAB/MS: 467 m/z ) [Au(4-
1
tbutylpy)2]+. H NMR (DMSO-d6): 8.77 (d, 4H, J ) 6.6 Hz, o-H
of py); 7.74 (d, 4H, J ) 6.7 Hz, m-H of py); 1.32 ppm (s, 18H,
C(CH3)3).
General Procedure for the Suzuki Cross-Coupling Reac-
tion. Phenylboronic acid (90 mg, 0.70 mmol), p-bromotoluene (100
mg, 0.58 mmol), [Au(4-dmapy)2][AuCl2] (21 mg, 0.03 mmol), and
K2CO3 (120 mg, 0.87 mmol) were mixed in dimethylformamide
(DMF, 1 mL). Reactions were carried out at both 130 and 25 °C
under both aerobic and N2 atmospheric conditions. Yields and
conversions were determined by gas chromatography (GC) analysis.
Results and Discussion
Complexes of Ia-IVa were obtained by the reaction of
one equivalent of Au(SMe2)Cl in CH2Cl2 with one equivalent
of 4-dampy, 4-pic, 4-phpy and 4-tbupy, respectively, followed
by recrystallization from CH3CN/Et2O. On the other hand,
Ib-IVb were prepared by the reaction of pyridines with
Au(SMe2)Cl in 2:1 molar ratios in CH2Cl2 followed by the
addition of 1 molar of NH4PF6 in EtOH. Complexes I-IV
were moderately stable in the solid state.
[Au(4-dmapy)2][PF6] Ib. The molecular cations of Ib
have linearly coordinated Au(I) centers with essentially
coplanar pyridines (Figure 2). The cations are packed in [+
The complexes were characterized by elemental analyses,
conductivity measurements, 1H NMR, FAB Mass, and
electronic spectroscopies. In particular, five Au(I)-Py com-
(11) Su, C. -C.; Chiu, S. Y. Polyhedron 1996, 15, 2623.
Inorganic Chemistry, Vol. 47, No. 7, 2008 2545

Lin et al.
Figure 3. (a) ORTEP diagram of IIa (50% probability ellipsoids), (b)
crystal packing diagram of IIa with a [- + -] [+] pattern (the hydrogen
atoms have been omitted for clarity). Selected bond lengths (Å) and angles
(deg): Au(2)-Au(3), 3.245(66); Au(2)-N(2), 2.068(48); Au(5)-N(3),
2.066(46); Au(3)-Cl(1), 2.282(12); Au(3)-Cl(2), 2.298(13); N(2)-C(7),
1.350(48); N(2)-C(8), 1.337(53); N(3)-C(13), 1.311(49); N(3)-C(14),
1.351(4); N(2)-Au(2)-N(2′), 180.00(9); N(3)-Au(5)-N(3′), 179.99(1);
Cl(1)-Au(3)-Cl(2),177.42(4);C(7)-N(2)-C(8),117.72(2);C(13)-N(3)-C(14),
118.64(3).
Figure 2. (a) ORTEP diagram of Ib (50% probability ellipsoids), (b) crystal
packing diagram of [Au(4-dmapy)2]+ with a [+ + +][+] pattern (the [PF6]-
ion and hydrogen atoms have been omitted for clarity). Selected bond lengths
(Å) and angles (deg): Au(1)-N(1), 2.007(5); Au(1)-N(3), 2.012(5);
N(1)-C(1),1.336(7);N(1)-C(5),1.354(7);N(3)-C(8),1.348(7);N(3)-C(12),
1.340(7); N(2)-C(3), 1.349(7) ; N(4)-C(10), 1.329(7); N(1)-Au(1)-(N3),
177.10(17).
and the reported complex of 2-picoline gives a neutral [Au(2-
pic)Cl].5d In the set of [- + -], the [Au(4-pic)2]+ cation is
linked with two [AuCl2]- anions through Au · · ·Au distances
of about 3.25 Å and 3.26 Å with linear Au(3)-Au(2)-Au(3′).
Between the neighboring [- + -] and [+], the [AuCl2]-
anion forms two short Au-Cl · · ·HC contacts with the ortho
hydrogens of the pyridyl ring to become a 1-D array (Figure
4). The cations align into a perfect straight line with a
Au· · ·Au distance around 4.06 Å. These 1-D arrays turn into
a 3-D network via the Au-Cl· · ·HC hydrogen bonding (2.94
Å, 138°) utilizing the protons of the methyl substituents on
pyridine (Supporting Information).
+ +] and [+] arrangements. Within the [+ + +] unit, the
cations are lined up parallel, in which a Au · · · Au contact of
about 3.49 Å1 and a pyridine-pyridine π · · · π interaction
distance of about 3.50 Å are observed.13 The isolated cation
[+] sits between adjacent [+ + +] sets with a Au · · · Au
distance of around 5.40 Å (Figure 2). A [PF6]- ion bridges
between the [+ + +] and [+] sets with C-H · · · F distances
between 2.50 and 2.60 Å (Supporting Information).14 Note
that the crystal lattice also contained disordered CH3CN
solvent molecules.
[Au(4-pic)2][PF6] IIb. Six [Au(4-pic)2]+ cations are as-
sociated via aurophilic interactions to yield a [+ + + + +
+]patternasseeninFigure4.TheAu(3)-Au(2),Au(2)-Au(1),
and Au(1)-Au(1′) lengths are about 3.32, 3.29, and 3.35 Å,
respectively.15 The 3.55–3.65 Å separation between the two
nearest pyridyl rings suggests π · · · π interactions.13 Short
CH · · · F distances (2.5–2.6 Å) are generated between ring
hydrogens and [PF6]- (Supporting Information).14 As a result,
the Au skeleton is built of 1-D zigzagging Au chains in a
unit of six Au’s, in which the adjacent units are separated
by 5.78Å. For comparison, the polymeric [Au(3-pic)2]+n with
[SbF6]- counter-ions5d forms a 1-D straight chain with
Au · · · Au contacts of 3.40 Å.
The crystal structure of [Au(4-phpy)2][PF6] IIIb was also
determined (Supporting Information). Unlike the [PF6]-
complexes Ib and IIb, no Au· · · Au interactions were
observed, likely because of the bulky 4-phpy ligands.
Molar Conductance. Molar conductivity (ΛM) measure-
ments were carried out at room temperature in CH3CN with
[Ag(4-dmapy)2][PF6] was also prepared and crystallo-
graphically characterized (Supporting Information). It appears
that a mere change of Au(I) by Ag(I) has tilted the delicate
balance among “metalophilicity” and ring π-π interactions.
A different packing is assembled, in which the Ag centers
line up perfectly with a separation of about 4.44 Å, with
apparently no Ag · · ·Ag interaction. This result would suggest
that the Ag · · · Ag interaction is smaller than the Au · · · Au
interaction.
[Au(4-pic)2][AuCl2] IIa. The crystal of IIa adopts a [-
+ -] [+] ion-pair packing (Figure 3) while the known
[Au(3-pic)2][AuCl2] assumes a [+ - + - · · ·] arrangement
(12) (a) Aullon, G.; Bellamy, D.; Brammer, L.; Bruton, E. A.; Orpen, A. G.
Chem. Commun. 1998, 653. (b) Bardaji, M.; Jones, P. G.; Laguna,
A.; Villacampa, M. D.; Villaverde, N. Dalton Trans. 2003, 4529. (c)
Tzeng, B. C.; Yeh, H. T.; Wu, Y. L.; Kuo, J. H.; Lee, G. H.; Peng,
S. M. Inorg. Chem. 2006, 45, 591. (d) Nishio, M. CrystEngComm
2004, 130.
(13) Janiak, C. A. Dalton Trans. 2000, 3885.
(14) (a) Grepioni, F.; Cojazzi, G.; Draper, S. M.; Scully, N.; Braga, D.
Organometallics 1998, 17, 296. (b) Xu, C.; Anderson, G. K.; Brammer,
L.; Braddock-Wilking, J.; Rath, N. P. Organometallics 1996, 15, 3972.
(15) Bondi, A. J. Phys. Chem. 1964, 68, 441.
2546 Inorganic Chemistry, Vol. 47, No. 7, 2008

Properties of Au(I) Complexes with 4-Substituted Pyridines
Figure 5. Molar conductivity of a CH3CN solution of Ib, as a function of
the square root of the concentration.
at a concentration of less than 2.0 × 10-3 M, the ΛM vs c1/2
should be linear and, owing to the interionic force, the ΛM
decrease slightly with increasing concentration. The plot of
ΛM vs c1/2 for Ib in Figure 5 shows a linear decrease in ΛM
with increasing concentration in the range of 6.0 × 10-5-9.0
× 10-5 M, as expected for a 1:1 strong electrolyte. However,
for a further elevation from 1.0 × 10-4 to 2.0 × 10-4 M,
instead of a continuing decrease in ΛM, an unexpected
increase is observed. The phenomenon argues against a
simple 1:1 electrolyte and favors possible formation of an
electrolyte higher than 1:1. As stated earlier, in the solid state,
molecular cations of Ib aggregate via aurophilicity. In
solution, this interaction may also occur such that cations of
Ib associate to produce the 1:2 electrolyte [(Au(4-
dmapy)2)2][PF6]2. Aggregation of cationic Au(I) in solution
has been reported.3f,g For an equilibrium between the
monocation and dication (eq 1), a higher Ib concentration
will favor the formation of more dicationic species. Present
data indicate that at a concentration as low as 1.0 × 10-4
M, a substantial amount of dicationic species is formed.
Further study of the equilibrium between the monocation
and dication by UV–vis spectrometry are reported in the
section that follows.
Figure 4. (a) ORTEP diagram of IIb (50% probability ellipsoids), (b)
crystal packing diagram of [Au(4-pic)2]+ with a [+ + + + + +] pattern
(the [PF6]- ion and hydrogen atoms have been omitted for clarity). Selected
bond lengths (Å) and angles (deg): (Au(1)-Au(2), 3.2897(6); Au(1)-Au(1′)
3.3463(9); Au(2)-Au(3), 3.3242(6)); Au(1)-N(1), 2.011(11); Au(1)-N(2),
2.016(11); N(1)-C(1), 1.357(17); N(1)-C(2), 1.336(17); N(2)-C(7),
1.334(16);N(2)-C(8),1.340(16);N(1)-Au(1)-(N2),177.8(3);C(1)-N(1)-C(2),
117.6(13); C(8)-N(2)-C(7), 116.7(12); Au(2)-Au(1)-Au(1′), 166.47(3);
Au(1)-Au(2)-Au(3), 166.42(2).
Table 1. Molar Conductivities of Au(I)-Py Complexes in CH3CN
complex
ΛM, ohm-1 cm2 mol-1
Ia
[Au(4-dmapy)2][AuCl2]
[Au(4-pic)2][AuCl2]
[Au(4-phpy)2][AuCl2]
[Au(4-tbupy)2][AuCl2]
[Au(4-dmapy)2][PF6]
[Au(4-pic)2][PF6]
48
74
59
IIa
IIIa
IVa
Ib
IIb
IIIb
IVb
64
205
205
200
176
[Au(4-phpy)2][PF6]
[Au(4-tbupy)2][PF6]
concentrations of about 1.0 × 10-3 M, as given in Table 1.
The ΛM of I-IVa are in the range of 48–74 ohm-1 cm2
mol-1, smaller than the 120–160 ohm-1 cm2 mol-1 expected
for 1:1 electrolytes as reported in the literature.16 This is
likely due to (i) ion-pair formation promoted by the Au · · ·Au
interactions and (ii) the possible equilibrium between
[Au(py)Cl] and [Au(py)2][AuCl2] in solution.5a–d,17 Both
behaviors might decrease the concentrations of ions as
expected for a strong electrolyte.
On the other hand, the ΛM values of 176–205 ohm-1 cm2
mol-1 found for complexes I-IVb are higher than those for
1:1 electrolytes but lower than the 220–300 ohm-1 cm2 mol-1
expected for 1:2 electrolytes.16 With a large noncoordinating
[PF6]- anion, I-IVb are expected to behave as strong
electrolytes. Typically for a strong 1:1 electrolyte in solution
2[AuPy2][PF6] h [(AuPy2)2][PF6]2
monocationic
dicationic
K ) [dication]/[monocation]2
(1)
Photophysical Properties. Absorption Spectra. In solu-
tion, the electronic absorption spectra of all complexes exhibit
a high energy (HE) band and a low energy (LE) band in the
UV region. The relative intensity of these two bands is
temperature and concentration dependent. The temperature
dependent UV–vis absorption spectrum of Ib is shown in
Figure 6a. At 40 °C, a HE band appears at 257 nm and a
LE band of slightly higher intensity at 292 nm. As the
temperature is lowered to 35 °C, 32 °C, 28 °C, and then 23
°C, a significant increase in the intensity of the LE band
and the presence of an isosbestic point are observed. The
latter indicates that two species are in equilibrium. As stated
earlier, the two equilibrium species in solution are likely
[AuPy2]+ and [(AuPy2)2]2+. We propose that the HE band
can be attributed to mononuclear [AuPy2]+ while the LE band
(16) Geary, W. J. Coord. Chem. ReV. 1971, 7, 81.
(17) (a) Guy, J. J.; Jones, P. G.; Mays, M. J.; Sheldrick, G. M. J. Chem.
Soc., Dalton Trans. 1977, 8. (d) Ahrland, S.; Noren, B.; Oskarsson,
A. Inorg. Chem. 1985, 24, 1330. (e) Lock, C. J. L.; Wang, Z. Acta
Crystallogr. 1993, C49, 1330. (h) Ahrens, B.; Jones, P. G.; Fischer,
A. K. Eur. J. Inorg. Chem, 1999, 1103.
Inorganic Chemistry, Vol. 47, No. 7, 2008 2547

Lin et al.
Figure 6. (a) Temperature dependent absorption spectrum of Ib: (i) 40 °C, (ii) 35 °C, (iii) 32 °C, (iv) 28 °C, (v) 23 °C. (b) Concentration dependent
absorption spectra of Ib: (i) 6.60 × 10-6 M, (ii) 1.74 × 10-5 M, (iii) 1.09 × 10-4 M, (iv) 1.90 × 10-4 M, (v) 2.72 × 10-4 M.
Table 2. Emission Spectral Data for Au(I)-Py Complexes
compound
excitation
emission λmax (fine-structure), nm
ligand emission
Ia
[Au(4-dampy)2][AuCl2]
[Au(4-pic)2][AuCl2]
[Au(4-phpy)2][AuCl2]
[Au(4-t-butylpy)2][AuCl2]
[Au(4-dampy)2][PF6]
[Au(4-pic)2][PF6]
360
360
360
320
360
360
360
320
∼499 (439, 468, 499, 534) 610 (sh)
∼495 (462, 495, 530)
∼500 (438, 468, 500, 536)
∼446 (446, 476, 510)
405, 520 (sh)
335
380
470
385
IIa
IIIa
IVa
Ib
IIb
IIIb
IVb
430
[Au(4-phpy)2][PF6]
[Au(4-t-butylpy)2][PF6]
∼500 (437, 466, 500, 538)
442, 472, 505 (sh)
results from the dinuclear [(AuPy2)2]2+, as observed in other
lowest unoccupied molecular orbital (LUMO) gap for the
dinuclear species than the one in the mononuclear species.
This appears to be consistent with the assignment of LE and
HE bands in the absorption spectra with contributions from
the dinuclear and mononuclear species, respectively, assum-
ing that the absorption of a photon results in the promotion
of an electron from the HOMO to the LUMO. An isolated
[(Au(4-dmapy)2)2]2+ is not stable, where the monomers prefer
to stay apart in vacuum. However, two [PF6]- anions can
stabilize the dicationic [(Au(4-dmapy)2)2]2+, as the present
level of theory indicates. Thus, it is reasonable to anticipate
that at low enough concentrations the solution would be too
dilute to sustain even the simplest aggregated species, [(Au(4-
dmapy)2)2]2+. The scenario seems to be demonstrated nicely
systems.3f,g
Figure 6b gives the concentration dependent absorption
spectra of Ib. An elevation of concentration from 6.60 ×
10-6 to 2.72 × 10-4 M results in a drastic growth in the
intensity of the LE band compared with the HE band.
Notably, the LE absorption is not observed in the two most
dilute solutions of 6.60 × 10-6 and 1.74 × 10-5 M. The
nonlinear growth in the intensity of the absorption band upon
increasing the concentration is consistent with the occurrence
of molecular aggregation. Assuming an equilibrium between
mononuclear and dinuclear species, eq 2 can be used to
estimate the equilibrium parameters,
[M]o/A2 ) 1/(ε2Kb)1/2 + 2A21/2/b
(2)
at the concentrations of 6.60 × 10-6 M and 1.74 × 10-5
M
where [M]o is the initial concentration of the mononuclear
species, K is the equilibrium constant, b is the cell path
length, A2 is the absorbance of the dinuclear species, and ε2
is the extinction coefficient of the dinuclear species. The plot
of [M]o/A21/2 versus A21/2 for Ib is a straight line, supporting
the proposed equilibrium, in which ε2 is estimated to be 7.12
× 104 M-1 cm-1 with equilibrium constant K ) 8.0 × 103.
A plot of log K versus 1/T yields the association enthalpy
and entropy of –14 ( 5 kcal mol-1 and –10 ( 8 cal K-1
mol-1 from the slope and intercept, respectively. It has been
reported that the Au · · ·Au interaction is typically in the range
of 6–11 kcal/mol;3 the slightly higher enthalpy observed in
this work suggests that other interactions such as ring π-π
interactions may also contribute to the associative process;
the negative entropy is consistent with an associative process.
The magnitudes of equilibrium constants and thermodynamic
parameters derived for Ib are in line with aurophilic
aggregation, as stated by Schmidbaur.18
in Figure 6b, where the LE band is practically nonexistent.
Similarly, the electronic absorption spectra of Ia in solution
are also concentration and temperature dependent (Supporting
Information). At a concentration as low as 7.16 × 10-6 M,
equally weak HE (261 nm) and LE (283 nm) bands were
observed. Upon increasing concentration to 3.80 × 10-5 M,
the LE band intensity increases more substantially than that
of the HE band. However, unlike Ib, no isosbestic point is
observed upon changing the temperature between 40 and 22
°C. This result indicates that the presence of more than two
species, such as the neutral form, [+][-], [+ -], [+ - +
(18) (a) Mathieson, T.; Schier, A.; Schmidbaur, H. Dalton Trans. 2001,
1196. (b) Hunks, W. J.; Jennings, M. C.; Puddephatt, R. J. Inorg.
Chem. 2000, 39, 2699. (c) Ho, S. Y.; Cheng, E. C. C.; Tiekink,
E. R. T.; Yam, V. W. W. Inorg. Chem. 2006, 45, 8165. (d) White-
Morris, R. L.; Olmstead, M. M.; Attar, S.; Balch, A. L. Inorg. Chem.
2005, 44, 5021. (e) Su, H. C.; Fadhel, O.; Yang, C. J.; Cho, T. Y.;
Fave, C.; Hissler, M.; Wu, C. C.; Reau, R. J. Am. Chem. Soc. 2006,
128, 983. (f) Siemeling, U.; Rother, D.; Bruhn, C.; Fink, H.; Weidner,
T.; Trager, F.; Rothenberger, A.; Fenske, D.; Priebe, A.; Maurer, J.;
Winter, R. J. Am. Chem. Soc. 2005, 127, 1102. (g) Chao, H. Y.; Lu,
W.; Li, Y.; Chan, M. C. W.; Che, C. M.; Cheung, K. K.; Zhu, N.
J. Am. Chem. Soc. 2002, 124, 14696. (h) White-Morris, R. L.;
Olmstead, M. M.; Jiang, F.; Tinti, D. S.; Balch, A. L. J. Am. Chem.
Soc. 2002, 124, 2327.
Density functional B3LYP/LanL2DZ calculations9 were
performed for mononuclear [Au(4-dmapy)2]+ and dinuclear
[(Au(4-dmapy)2)2]2+, for which we assumed the arrangements
as in the crystal of the Ib compound. The computations
predict a smaller highest occupied molecular orbital (HOMO)–
2548 Inorganic Chemistry, Vol. 47, No. 7, 2008

Properties of Au(I) Complexes with 4-Substituted Pyridines
spacings are in the range of 1400–1600 cm-1, in good
agreement with the skeletal vibration frequencies (CdC and
CdN) of the pyridyl rings.19
It would be tempting to assign the 610 nm shoulder to
the Au · · ·Au interaction.20 However, the HOMO and LUMO
of the Ia molecule involve only a single Au atom, which
implies that neither HOMO nor LUMO exhibits the auro-
philicity. The highest occupied molecular orbital with
evidence of a Au · · · Au interaction is buried down in the
third HOMO (THOMO), while the LUMO with some sign
of aurophilicity does not appear until the sixth LUMO. This
signifies that for the Ia compound, aurophilicity would not
influence the spectra directly, even though it appears in the
crystal structure obtained in the current work. That is, the
present level of calculation does not support that the emission
band is the result of aurophilicity.
The situation persists for system of IIa. Namely, the
-
HOMO of ion-pair IIa is again the HOMO of AuCl2 , and
the LUMO of IIa is the [Au(4-pic)2]+ LUMO. The absence
of a Au· · · Au interaction in both HOMO and LUMO is an
indication that aurophilicity also does not play a vital role
in the spectra of the IIa compound.
Figure 7. Emission spectra of crystalline Ia, Ib, IIa, and IIb; (i) excitation,
(ii) emission.
In the systems with [PF6]- as counteranion, the circum-
stances differ, however. Distinct from the substantial hy-
bridization between the orbitals of the [AuCl2]- anion and
those of the Au(I)-Py complex in Ia and IIa, the calculations
show that [PF6]- plays a rather passive role and does not
participate in forming molecular orbitals with energies in
the proximity of the HOMO and LUMO of the systems. As
illustrated in Figure 9, for the IIb compound, the HOMO
and LUMO of [(Au(4-pic)2)2]2+ are simply composed of the
HOMO and LUMO of two [Au(4-pic)2]+, respectively. While
the [(Au(4-pic)2)2]2+ LUMO is associated with 4-pic, it can
be easily seen that the HOMO almost entirely stems from
two Au atoms, thus indicating that the Au · · · Au interaction
plays a role. In contrast to the IIa compound, the property
of aurophilicity could deeply affect the spectra of IIb through
the ground-state HOMO.
Ib bears a resemblance to IIb in the nature of its HOMO
and LUMO. The ground state [(Au(4-dmapy)2)2]2+ HOMO
arises from the HOMOs of two [Au(4-dmapy)2]+/[Au(4-
dmapy)2]+ and likewise its LUMO, the LUMOs of mono-
mers. In the HOMO of [(Au(4-dmapy)2)2]2+, considerable
ligand character makes the Au · · · Au interaction less domi-
nant than in [(Au(4-pic)2)2]2+; however, it nevertheless
remains significant. Thus, one would anticipate that via the
ground-state HOMO, aurophilicity could exert influence on
the Ib spectra.
-], [- + + -], or others, with or without Au · · · Au
interactions, is likely.
As indicated by the B3LYP/LanL2DZ calculations, in
contrast to the dinuclear [(Au(4-dmapy)2)2]2+, the Ia ion-
pair could exist comfortably and, in fact, is energetically
more stable than two separated ions. The extent of the
concentration dependence is not as prominent in the absorp-
tion spectra of the Ia solution as in the Ib counterpart. That
the LE band lingers even at the lowest concentration
measured, 7.16 × 10-6 M, could be attributed to the Ia ion-
pair, that is, [+ -].
Emission Spectra. Compounds I-IV are luminescent in
the solid state at room temperature with long life times in
the tens of microseconds. On the other hand, [Ag(4-
dmapy)2][PF6], an analogous silver compound, is nonlumi-
nescent. Features of solid-state emission spectra of I-IV at
room temperature are summarized in Table 2. As seen in
Figure 7, Ia and IIa both show a structured band at λmax
)
500 nm. In Ia, the structured band is followed by a broad
shoulder at 610 nm. Their [PF6]- counterparts, Ib and IIb,
give an intense and featureless band of λmax at 405 and 430
nm, respectively, with the former tailed by several very weak
peaks.
Figure 8 discloses that the HOMO of ion-pair Ia is the
HOMO of the [AuCl2]- species, while the LUMO is
practically the LUMO of the [Au(4-dmapy)2]+ entity. The
Ia HOMO is a heavily hybridized orbital involving the metal
and Cl-; the LUMO on the other hand, predominately
belongs to the 4-dmapy ligand, with a minor contribution
from the metal. On the basis of the theoretical prediction at
the present level, the observed phosphorescence may be
pictured as a consequence of the transition of an electron
from the [Au(4-dmapy)2]+ to its counterion [AuCl2]-. Thus,
the spectra is expected to be ligand (4-dmapy) related, which
is in line with the observation from Figure 7 that the vibronic
Interestingly, Ia and IIa, for which the HOMO and LUMO
reveal no sign of the Au · · · Au interaction, both yield
structured emission spectra, while the featureless spectra of
Ib and IIb coincides with a Au · · · Au interaction embedded
HOMO. It would be logical to correlate the striking feature-
(19) Tzeng, B. C.; Liao, J. H.; Lee, G. H.; Peng, S. M. Inorg. Chim. Acta
2004, 357, 1405.
(20) (a) Wang, H. M. J.; Vasam, C. S.; Tsai, T. Y. R.; Chen, S. H.; Chang,
A. H. H.; Lin, I. J. B. Organometallics 2005, 24, 486. (b) Wang,
H. M. J.; Chen, C. Y. L.; Lin, I. J. B. Organometallics 1999, 18, 1216.
Inorganic Chemistry, Vol. 47, No. 7, 2008 2549

Lin et al.
Figure 8. B3LYP/LanL2DZ molecular orbitals of the Ia ion pair, [AuCl2]-, and [Au(4-dmapy)2]+.
Figure 9. B3LYP/LanL2DZ molecular orbitals of [(Au(4-pic)2)2]2+and [Au(4-pic)2]+.
Scheme 2. Suzuki Cross-Coupling of p-Bromo Toluene with
Phenylboronic Acid
less emissions of Ib and IIb to a Au· · · Au interaction in
their HOMOs.
Catalysis. The richness of homogeneous and heteroge-
neous gold catalysis has only been realized quite recently.
The use of Au(I) complexes as potential catalysts for Suzuki
cross-coupling reactions was studied by Corma et al.,21 in
which the electronic effects of ligands were examined. Our
preliminary results employing one of the Au(I)-Pys in
catalyzing the Suzuki cross-coupling reaction are presented
as follows.
Table 3. Yields of Suzuki Cross-Coupling Reactions under Various
Conditions
130 °C
rt
catalyst
air
N2
air
N2
[Au(4-dmapy)2][AuCl2] Ia. This compound was utilized
as a catalyst in the coupling of p-bromo toluene with
phenylboronic acid. Reactions were carried out at 130 and
25 °C (room temperature, rt) in DMF under both aerobic
and N2 atmospheric conditions with base K2CO3 (Scheme
2). As tabulated in Table 3, under aerobic condition the cross-
coupling product, P-phenyl toluene, was generated in a GC
yield of 90% at 130 °C (6 h) and 52% at rt (22 h). On the
other hand, under a N2 atmosphere and a prolonged reaction
time of 22 h, P-phenyl toluene was obtained with 95% yield
at 130 °C and a mere about 5% yield at rt. Apparently,
reactions under air at high temperature gave a better cross-
coupling product yield with catalyst Ia. As far as we know,
Ia
90 (6)a
95(22)
52 (22)
<5 (22)
a Experimental: p-bromo toluene (0.30 mmol); phenylboronic acid (0.36
mmol); [Au(4-dmapy)2][AuCl2] (5% mol); K2CO3 (0.45 mmol); DMF (1
mL); GC yield; a: Yield % (h).
no Au(I) catalyst has been reported to activate the cross-
coupling between phenyl bromide and phenylboronic acid.
However, the reaction was known to occur when the more
reactive phenyl iodide was used (85% yield at 130 °C,
22 h).21a Corma et al.21b,c have attempted cross-coupling
between phenyl bromide and boronic acid catalyzed with
Au(I)/Au(III) complexes, which in turn yielded the homo-
coupling product of boronic acid.
Conclusion
(21) (a) Corma, A.; Gutiérrez-Puebla, E.; Iglesias, M.; Monge, A.; Pérez-
Ferreras, S.; Sánchez, F. AdV. Synth. Catal. 2006, 348, 1899. (b)
González-Arellano, C.; Corma, A.; Iglesias, M.; Sánchez, F. J. Catal.
2006, 238, 497. (c) González-Arellano, C.; Corma, A.; Iglesias, M.;
Sánchez, F. Chem. Commun. 2005, 1990.
Eight Au(I) complexes of 4-substituted pyridines were
synthesized. With Au · · · Au interactions, the [Au(4-
dmapy)2][AuCl2] and [Au(4-pic)2][AuCl2] complexes were
2550 Inorganic Chemistry, Vol. 47, No. 7, 2008

Properties of Au(I) Complexes with 4-Substituted Pyridines
-
found to adopt [+ - + - · · ·] and [- + -] [+] crystal
packing patterns, respectively. Their [PF6]- counterparts,
[Au(4-dmapy)2][PF6] and [Au(4-pic)2][PF6] complexes, how-
ever, crystallize in triset [+ + +] and hexa-set [+ + + +
+ +], respectively, also with the aid of aurophilic attractions.
While the aurophilic interaction acts continuously along the
infinite 1-D polymer of [Au(4-dmapy)2][AuCl2], it persists
in discrete units of 3, 3, and 6 Au atoms for the crystals of
[Au(4-pic)2][AuCl2], [Au(4-dmapy)2][PF6], and [Au(4-
pic)2][PF6], respectively.
In solution, both the conductivity measurements and the
absorption spectroscopic studies indicate molecular aggrega-
tion at high concentration. An equilibrium between the
mononuclear [AuPy2]+ and the dinuclear [(AuPy2)2]2+ is
proposed for the [PF6]- salts. The equilibrium constant and
thermodynamic parameters of [Au(4-dmapy)2][PF6] are
estimated based on the concentration-dependent and tem-
perature-dependent absorption spectra, respectively. DFT
calculations on the energetics of [Au(4-dmapy)2]+, [(Au(4-
dmapy)2)2]2+, and [Au(4-dmapy)2][AuCl2] ion-pair support
the molecular aggregation in solution.
nuclear [Au(4-dmapy)2]+, [Au(4-pic)2]+, and AuCl2 , and
dinuclear [(Au(4-dmapy)2)2]2+ and [(Au(4-pic)2)2]2+ indicates
that the emissions of [Au(4-dmapy)2][AuCl2] and [Au(4-
pic)2][AuCl2] are mainly ligand in nature, whereas those from
[Au(4-dmapy)2][PF6] and [Au(4-pic)2][PF6] crystals involve
aurophilic interactions.
Preliminary results show that contrary to the known Au(I)
catalysts, Ia is effective in catalyzing the cross-coupling of
aryl bromides with phenylboronic acid. While the use of Au
compounds as catalysts is still in infancy, our work could
encourage further efforts on the utilization of Au(I) com-
plexes in catalysis.
Acknowledgment. The authors wish to thank National
Science Council of Taiwan for the financial support (NSC
95-2113-M-259-012, NSC 95-2113-M-259-005), Prof. F. E.
Budenholzer at Fu Jen Catholic University for the proof
reading of the manuscript, and National Center for High-
performance Computer of Taiwan for computer resources.
Supporting Information Available: Figures, tables, and crystal-
lographic data for the compounds (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
All the compounds are luminescent in the solid state.
Analysis of the DFT derived molecular orbitals for ion-pairs,
[Au(4-dmapy)2][AuCl2] and [Au(4-pic)2][AuCl2], mono-
IC701872F
Inorganic Chemistry, Vol. 47, No. 7, 2008 2551