A New Totally Flat N(sp2)C(sp2)N(sp2) Pincer Palladacycle: Synthesis and Photoluminescent Properties

Article abstract of DOI:10.1021/ic0346679

The Sonogashira coupling of 2-bromopyridine with 8-quinolinyl-acetylene affords 2-pyridinyl-8-quinolinyl-acetylene (1) in high yields. The chloropalladation of 1 with Li₂PdCl₄ in methanol at room temperature affords the pincer palladacycle (C₅H ₄N-2-C=C(Cl)-8-C₉H₆κN,κC,κ N)PdCl (2) in 63% yield. The X-ray molecular structure of (2) shows that it is totally flat and that it is associated in pairs though π-stacking between alternate pyridine-quinoline moieties (3.448 A). The pairs are also connected by π-stacking with an interpair distance of 3.452 A between quinoline-quinoline moieties. A very low fluorescence emission was also revealed by the pincer palladacycle 2 in both solution and the solid state, which has been ascribed to an excimeric emission due to the particular structure (rigid and totally flat) of 2 in the solid state.

Full text of DOI:10.1021/ic0346679

Inorg. Chem. 2004, 43, 530−536
A New Totally Flat N(sp2)C(sp2)N(sp2) Pincer Palladacycle: Synthesis
and Photoluminescent Properties
Crestina S. Consorti,† Gunter Ebeling,† Fabiano Rodembusch,‡ Valter Stefani,‡ Paolo R. Livotto,§
Frank Rominger,| Frank H. Quina, Chang Yihwa, and Jairton Dupont†,*
Laboratory of Molecular Catalysis, Laboratory of New Organic Materials, and Laboratory of
Theoretical Chemistry, Institute of ChemistrysUFRGS AV. Bento Gonc¸alVes,
9500 Porto Alegre 91501-970 RS Brazil, Instituto de Qu´ımica, UniVersidade de Sa˜o Paulo,
Caixa Postal 26.077, Sa˜o Paulo 05599-970, Brazil, and Organisch-Chemisches Institut der
Ruprecht-Karls-UniVersita¨t Heidelberg, Im Neuenheimer Feld 270, D-69120 Heidelberg, Germany
Received June 13, 2003
The Sonogashira coupling of 2-bromopyridine with 8-quinolinyl-acetylene affords 2-pyridinyl-8-quinolinyl-acetylene
(1) in high yields. The chloropalladation of 1 with Li2PdCl4 in methanol at room temperature affords the pincer
palladacycle (C H N-2-CdC(Cl)-8-C H κN,κC,κN)PdCl (2) in 63% yield. The X-ray molecular structure of (2) shows
5
4
9 6
that it is totally flat and that it is associated in pairs though π-stacking between alternate pyridine−quinoline moieties
(3.448 Å). The pairs are also connected by π-stacking with an interpair distance of 3.452 Å between quinoline−
quinoline moieties. A very low fluorescence emission was also revealed by the pincer palladacycle 2 in both solution
and the solid state, which has been ascribed to an excimeric emission due to the particular structure (rigid and
totally flat) of 2 in the solid state.
Introduction
confers to this class of compounds a plethora of applications.3
These compounds have been used as efficient catalyst
precursors for C-C coupling reactions,4 and as “molecular”
materials for crystalline switches, sensors,5 etc. In particular,
light-emitting NNC and NCN pincer type palladacycles
(Figure 1), obtained mainly from the metalation of 6-aryl-
2,2′-bipyridines ligands, are potential candidates for optical
luminescent devices.6 However, with rare exceptions, reason-
able luminescence of these compounds is only observed in
a glass matrix at 77 K. The solid state luminescence at room
temperature has been reported only recently for the nearly
flat palladated 6-carboxyl-2,2′-bipyridine compound.6a The
observed solid state luminescence has been ascribed to an
excimeric emission. The C-H bond activation methods used,
Pincer type palladacycles are one of the most popular and
investigated classes of cyclometalated compounds.1,2 The
possibility of fine-tuning the steric and electronic properties
of the metal center by simply changing the donor groups or
their substituents in the anionic six-electron donor ligand
* To whom correspondence should be addressed. E-mail: dupont@
iq.ufrgs.br.
† Laboratory of Molecular Catalysis.
‡ Laboratory of New Organic Materials.
§ Laboratory of Theoretical Chemistry.
| Organisch-Chemisches Institut der Ruprecht-Karls-Universita¨t Heidel-
berg.
Instituto de Qu´ımica, Universidade de Sa˜o Paulo.
(1) (a) van der Boom, M. E.; Milstein, D. Chem. ReV. 2003, 103, 1759-
1792. (b) Albrecht, M.; van Koten, G. Angew. Chem., Int. Ed. 2001,
40, 3750-3781. (c) Dupont, J.; Pfeffer, M.; Spencer, J. Eur. J. Inorg.
Chem. 2001, 1917-1927. (d) Steenwinkel, P.; Gossage, R. A.; van
Koten, G. Chem. Eur. J. 1998, 4, 759-762.
(2) For recent examples of PCP, SCS, and NCN pincer palladacycles
see: (a) Bergbreiter, D. E.; Osburn, P. L.; Liu, Y. S. J. Am. Chem.
Soc. 1999, 121, 9531-9538. (b) Zim, D.; Gruber, A. S.; Ebeling, G.;
Dupont, J.; Monteiro, A. L. Org. Lett. 2000, 2, 2881-2884. (c)
Morales-Morales, D.; Redon, R.; Yung, C.; Jensen, C. M. Chem.
Commun. 2000, 1619-1620. (d) Beletskaya, I. P.; Chuchurjukin, A.
V.; Dijkstra, H. P.; van Klink, G. P. M.; van Koten, G. Tetrahedron
Lett. 2000, 41, 1075-1079. (e) Bedford, R. B.; Draper, S. M.; Scully,
P. N.; Welch, S. L. New J. Chem. 2000, 24, 745-747. (f) Dijkstra,
H. P.; Ronde, N.; van Klink, G. P. M.; Vogt, D.; van Koten, G. AdV.
Synth. Catal. 2003, 345, 364-369.
(3) Rietveld, M. H. P.; Grove, D. M.; van Koten, G. New J. Chem. 1997,
21, 751-771.
(4) Singleton, T. J. Tetrahedron 2003, 59, 1837-1857.
(5) Albrecht, M.; van Koten, G. Angew. Chem., Int. Ed. 2001, 40, 3750-
3781.
(6) See for examples: (a) Neve, F.; Crispini, A.; Di Pietro, C.; Campagna,
S. Organometallics 2002, 21, 3511-3518. (b) Song, D. T.; Wu, Q.
G.; Hook, A.; Kozin, I.; Wang, S. N. Organometallics 2001, 20, 4683-
4689. (c) Wu, Q. G.; Hook, A.; Wang, S. N. Angew. Chem., Int. Ed.
2000, 39, 3933. (d) Neve, F.; Crispini, A.; Campagna, S. Inorg. Chem.
1997, 36, 6150-6156. (e) Wakatsuki, Y.; Yamazaki, H.; Grusth, P.
A.; Santhanam, M.; Kutal, C. J. Am. Chem. Soc. 1985, 107, 8153-
8159.
530 Inorganic Chemistry, Vol. 43, No. 2, 2004
10.1021/ic0346679 CCC: $27.50 © 2004 American Chemical Society
Published on Web 12/17/2003

New Totally Flat Pincer Palladacycle
exposures of 0.3° in ω were taken. An absorption correction was
applied using SADABS9 based on the Laue symmetry of the
reciprocal space; the data were corrected for Lorentz and polariza-
tion effects. The structure was solved by direct methods and
expanded using Fourier techniques, all non-hydrogen atoms were
refined with anisotropic displacement parameters, the hydrogen
atoms could be located in the Fourier map, and all hydrogen atoms
were considered at calculated positions. Half a molecule of CH2-
Cl2 was located per one molecule of the palladium compound
(special position on a 2-fold symmetry axis). The full-matrix least-
squares refinement against F2 converged. All calculations were
performed using the SHELXTL crystallographic software package
of Bruker.10
Figure 1. Examples of pincer NCN and NNC pincer palladacycles that
display luminescent properties.
in most of the cases, for the preparation of these potential
light-emitting palladacycles are almost restricted to those
containing Pd-aryl bonds.
Synthesis of 8-Quinolinyl-acetylene. A mixture of 8-bromo-
quinoline (3.00 g, 14.4 mmol), trimethylsilyl-acetylene (1.86 g, 19.0
mmol), Pd(PPh3)2Cl2 (20 mg), cuprous iodide (20 mg), and
triphenylphosphine (70 mg) was suspended in triethylamine (20
mL), and the resulting suspension was stirred and heated at 90 °C
for 1.5 h. The reaction mixture was quenched with aqueous sodium
carbonate solution (10%, 20 mL) and was extracted with ether
(2 × 15 mL). The solvent of the combined organic extract was
evaporated under reduced pressure, and the resulting oily residue,
redissolved in methanol (20 mL), was treated with potassium
fluoride (1.74 g, 30 mmol) and stirred at room temperature for 1.5
h. Evaporation of the solvent and chromatographic purification
(silica gel column, hexanes/EtOAc ) 1/1) afforded a brown oil
(1.20 g, 54% yield), that solidified in the refrigerator. CG-MS (m/
z, rel int, peak): 153, 100, M+•; 126, 30, [M - HCN]+•. 1H NMR
(CDCl3): δ 9.06 (dd, 1H, CH arom, 3JHH ) 4.3 Hz and 4JHH ) 1.7
We have recently reported that the trans-chloropalladation
of heterosubstituted alkynes is an effective method for
preparing various types of pincer palladacycles.7 We envis-
aged that this method could be used for the construction of
palladacycles containing pyridine-type ligands. We wish to
report that this method can indeed be used for the construc-
tion of a “rigid” and totally flat compound containing
pyridine and quinoline ligands that displays luminescence
properties in both solution and the solid state at room
temperature.
Experimental Section
General Methods. All reactions involving organometallic
compounds were carried out under argon or nitrogen atmosphere
in oven dried Schlenk tubes. Solvents were dried with suitable
drying agents and distilled under argon prior to use. The alkyne 5
was prepared in 85% yield using a published method.8 All the other
chemicals were purchased from commercial sources (Acros or
Aldrich) and used without further purification. Elemental analyses
were performed by the Analytical Central Service of IQ-USP
(Brazil). NMR spectra were recorded on a Varian Inova 300
spectrometer. Infrared spectra were performed on a Bomem B-102
spectrometer. Gas chromatography analyses were performed with
a Hewlett-Packard-5890 gas chromatograph with a FID and 30 m
capillary column with a dimethylpolysiloxane stationary phase.
Mass spectra were obtained using a GC/MS Shimadzu QP-5050
(EI, 70 eV). UV-vis absorption spectra were taken on a Cary 50
Varian spectrophotometer or a Shimadzu, model UV-1601PC. Solid
state and solution fluorescence spectra were measured with a Hitachi
spectrophotometer model F-4500. The experiments in solution were
performed at room temperature in a concentration of 10-5 M.
X-ray Structures Analyses of 2. Crystals of 2 were first prepared
by slow diffusion of hexane into an acetone solution. However,
these crystals suffered from loss of solvent and were recrystallized
from CH2Cl2 by very slow solvent evaporation. A crystal was
mounted on a glass fiber with perfluoropolyether. The measurement
was made on a Bruker SMART-CCD diffractometer with graphite
monochromated Mo KR radiation. Frames corresponding to a sphere
of data were collected using the ω-scan technique, and 20 s
3
4
Hz); 8.15 (dd, 1H, CH arom, JHH ) 8.3 Hz and JHH ) 1.7 Hz);
3
4
7.96 (dd, 1H, CH arom, JHH ) 7.2 Hz and JHH ) 1.5 Hz); 7.81
3
4
(dd, 1H, CH arom, JHH ) 8.3 Hz and JHH ) 1.5 Hz); 7.49 (dd,
1H, CH arom, JHH ) 8.3 Hz and JHH ) 7.2 Hz); 7.44 (dd, 1H,
CH arom, JHH ) 4.3 Hz and JHH ) 8.3 Hz); 3.62 (s, 1H,
CtCH). 13C{1H} NMR (CDCl3): δ 151.2, 136.4, 134.6, 128.9,
125.9 and 121.6 (CH arom); 148.3, 128.1 and 122.0 (C arom quat);
83.0 and 81.2 (CtC).
3
3
3
3
Synthesis of 8-Quinolinyl-2-pyridinyl-acetylene (1). A reseal-
able Schlenk tube was charged with 8-quinolinyl-acetylene (0.445
g, 2.90 mmol), 2-bromo-pyridine (0.460 g, 2.90 mmol), Pd(PPh3)4
(12 mg), cuprous iodide (12 mg), diethylamine (4 mL), and DMF
(100 µL). The resulting mixture was heated to 50 °C and stirred at
this temperature for 2 h. The reaction mixture was quenched with
aqueous sodium carbonate solution (10%, 10 mL) and extracted
with methylene chloride (2 × 10 mL), and the solvent of the
combined organic extract was evaporated under reduced pressure.
Chromatographic purification (silica gel column, EtOAc) of the
resulting residue afforded a pale brown solid (0.570 g, 85% yield).
Anal. Calcd for C16H10N2 (230.27): C, 83.46; H, 4.38; N, 12.17.
1
Found: C, 83.78; H, 4.55; N, 12.21. H NMR (CDCl3): δ 9.06
(dd, 1H, CH arom, 3JHH ) 5.0 Hz and 4JHH ) 1.7 Hz); 8.67-8.64
3
(m, 1H, CH arom); 8.17 (dd, 1H, CH arom, JHH ) 8.3 Hz and
3
4
4JHH ) 1.7 Hz); 8.07 (dd, 1H, CH arom, JHH ) 7.2 Hz and JHH
) 1.5 Hz); 7.84 (dd, 1H, CH arom, 3JHH ) 8.3 Hz and 4JHH ) 1.2
3
Hz); 7.71-7.68 (m, 2H, CH arom); 7.53 (dd, 1H, CH arom, JHH
(7) (a) Zanini, M. L.; Meneghetti, M. R.; Ebeling, G.; Livotto, P. L.;
Rominger, F.; Dupont, J. Inorg. Chim. Acta 2003, 350, 520-530. (b)
Zanini, M. L.; Meneghetti, M. R.; Ebeling, G.; Livotto, P. L.;
Rominger, F.; Dupont, J. Polyhedron 2003, 22, 1665-1671. (c)
Ebeling, G.; Meneghetti, M. R.; Rominger, F.; Dupont, J. Organo-
metallics 2002, 21, 3221-3227. (d) Dupont, J.; Basso, N. R.;
Meneghetti, M. R.; Konrath, R. A.; Burrow, R.; Horner, M. Organo-
metallics 1997, 16, 2386-2391.
) 8.3 Hz and 3JHH ) 7.2 Hz); 7.46 (dd, 1H, CH arom, 3JHH ) 8.3
Hz and JHH ) 5.0 Hz); 7.29-7.22 (m, 1H, CH arom). 13C{1H}
3
NMR (CDCl3): δ 151.1, 149.9, 136.5, 136.0, 134.6, 129.1, 127.4,
126.0, 122.8 and 121.7 (CH arom); 148.1, 143.4, 128.2 and 122.2
(C arom quat); 94.2 and 87.0 (CtC).
(8) Aiub, A. C.; Basso, N. R.; Casagrande, O. L.; Dupont, J.; Horner,
M.; Bortoluzzi, A. J.; MaichleMossmer, C. J. Coord. Chem. 1996,
40, 35-44.
(9) Sheldrick, G. M. SADABS; Bruker AXS, Inc.: Madison, WI, 1996.
(10) Sheldrick, G. M. SHELXTL V5.10; Bruker AXS, Inc.: Madison, WI,
1997.
Inorganic Chemistry, Vol. 43, No. 2, 2004 531

Consorti et al.
Synthesis of Palladacycle 2. A Li2PdCl4 solution was prepared
by dissolving PdCl2 (0.186 g, 1.05 mmol) and LiCl (0.106 g, 2.50
mmol) in methanol (7 mL) with gentle heating. A solution of
8-quinolinyl-2-pyridinyl-acetylene (0.235 g, 1.02 mmol) in methanol
(8 mL) was then added to the former at 0 °C. The resulting yellow
suspension was stirred for 1.5 h at room temperature. The volatiles
were removed under reduced pressure, the residue was taken up in
a minimum amount of CH2Cl2, and the resulting solution was
filtered through a pad of Celite. Precipitation with hexanes afforded
the desired compound as a yellow solid (0.267 g, 63% yield). Anal.
Calcd for C16H10Cl2N2Pd‚1/3(CH3)2CO: C, 47.82; H, 2.83; N, 6.56.
Found: C, 47.62; H, 2.78; N, 6.18. IR (KBr, cm-1): 1595 (νCdC).
methanol (50 mL) at room temperature. The orange precipitate
formed after 22 h stirring at room temperature was isolated by
filtration, and purification by column chromatography on silica
(CH2Cl2) affords a yellow solution. Concentration of this solution
to 3-5 mL and addition of hexane (20 mL) affords a yellow solid
that has been recovered by filtration washed with hexane (3 × 15
mL) and dried under reduced pressure (0.426 g, 70% yield). Mp:
148-150 °C (decomposition). Anal. Calcd for C13H9Cl2NPd: C,
43.80; H, 2.52; N, 3.93. Found: C, 43.50; H, 280; N, 3.70. IR
1
(KBr, cm-1): 1579 (νCdN). H NMR (CDCl3 + ꢀPy-d5)): δ 8.85
3
(m, 1H, o-Py, JHH) 5.6 Hz); 7.81 and 7.37 (2m, 8H, aromatic).
13C{1H} NMR (CDCl3): δ 178.1 and 120.2 (CdC); 163.7, 149.4,
139.7, 128.1, 127.4, 125.8, 121.9 (CH aromatic), 142.7, 118.2
(C quat aromatic).
3
1H NMR (CDCl3): δ 9.35 (dd, 1H, CH arom, JHH ) 4.8 Hz and
3
4
4JHH ) 1.2 Hz); 8.98 (ddd, 1H, CH arom, JHH ) 5.4 Hz, JHH
)
1.5 Hz and 5JHH ) 0.6 Hz); 8.70 (dd, 1H, CH arom, 3JHH ) 7.8 Hz
Computational Details. The density functional theory calcula-
tions were performed at the B3LYP level11 using the Gaussian 98
program package.12 The basis set for the palladium and chloro atoms
were the LANL2DZ with the corresponding effective core poten-
tial,13 and the DZ Dunning-Hay basis set complemented with
polarization and diffuse functions (with the exception of the
hydrogen atoms)14 was used for the remaining atoms. Initially, a
completely optimized geometry was obtained and an excitation
energies calculation was performed using the time dependent theory
(TD-DFT).15
Quantum Yield of Fluorescence. For quantum yield of
fluorescence, a Shimadzu UV-1601PC spectrophotometer and a
Hitachi spectrofluorometer model F-4500 were applied. The quan-
tum yield of fluorescence (φfl) was determined at 25 °C in spec-
troscopic grade ethanol solution (Merck). Quinine sulfate (Riedel)
in H2SO4 1 M (φfl ) 0.55)16 was used as quantum yield standard.
Laser Flash Photolysis. The laser flash photolysis experiments
were carried out with an Edinburgh Analytical Instruments LP900
laser flash photolysis system equipped with a Surelite I-10 Nd:
YAG laser for excitation and a 450 W high-pressure xenon lamp
for monitoring transient absorption. A solution of the Pd complex
in acetonitrile (with an absorbance of ca. 0.5 at 355 nm), contained
in standard 1 cm path length quartz fluorescence cuvette, was
excited with the third harmonic (5 ns halfwidth pulses at 355 nm)
4
3
and JHH ) 1.5 Hz); 8.30 (dd, 1H, CH arom, JHH ) 7.8 Hz and
4JHH ) 1.5 Hz); 7.76 (apparent dt, 1H, CH arom, JHH ) 7.8 Hz
3
4
3
and JHH ) 1.5 Hz); 7.77 (dd, 1H, CH arom, JHH ) 8.1 Hz and
4JHH ) 1.2 Hz); 7.58 (apparent t, 1H, CH arom, JHH ) 7.8 Hz);
3
3
3
7.45 (dd, 1H, CH arom, JHH ) 8.1 Hz and JHH ) 4.8 Hz); 7.45
(ddd, 1H, CH arom, 3JHH ) 7.8 Hz, 4JHH ) 1.5 Hz, and 5JHH ) 0.6
Hz); 7.17 (ddd, 1H, CH arom, 3JHH ) 7.8 Hz, 3JHH ) 5.4 Hz, and
4JHH ) 1.5 Hz). 13C{1H} NMR (CDCl3): δ 152.3, 151.5, 139.5,
138.3, 128.9, 128.6, 128.3, 122.6, 122.4 and 120.7 (CH arom);
165.84, 165.78, 156.0, 139.8, 129.8 and 118.4 (C arom quat and
CdC).
Synthesis of Phenyl-8-quinolinyl-acetylene (4). A resealable
Schlenk tube was charged with 8-quinolinyl-acetylene (0.230 g,
1.50 mmol), iodobenzene (0.408 g, 2.00 mmol), Pd(PPh3)4 (8 mg),
cuprous iodide (8 mg), diethylamine (4 mL), and DMF (100 µL).
The resulting mixture was heated to 50 °C and stirred at this
temperature for 1 h. The reaction mixture was quenched with
aqueous sodium carbonate solution (10%, 10 mL) and extracted
with ether (2 × 20 mL), and the solvent of the combined organic
extract was evaporated under reduced pressure. Chromatographic
purification (silica gel column, hexanes/EtOAc: 90/10) of the
resulting residue afforded a pale yellow liquid (0.290 g, 84% yield).
CG-MS (m/z, int rel, peak): 229, 100, M+•; 202, 10, [M -
HCN]+•.1H NMR (CDCl3): δ 9.08 (dd, 1H, CH arom, 3JHH ) 4.2
4
3
Hz and JHH ) 1.9 Hz); 8.18 (dd, 1H, CH arom, JHH ) 8.2 Hz
(11) (a) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. ReV. B 1988, 37, 785-
789. (b) Becke, A. D. Phys. ReV. A 1988, 38, 3098-3100.
(12) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels,
A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone,
V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.;
Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R.
L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara,
A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.;
Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle,
E. S.; Pople, J. A. Gaussian 98, revision A.5; Gaussian, Inc.:
Pittsburgh, PA, 1998.
(13) (a) Hay, P. J.; Wadt, W. R J. Chem. Phys. 1985, 82, 270-283. (b)
Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284-298. (c) Hay,
P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299-310.
(14) (a)Dunning, T. H., Jr.; Hay, P. J. In Modern Theoretical Chemistry;
Schaefer, H. F., III, Ed.; Plenum: New York, 1976; Vol. 3, p 1. (b)
Frisch, M. J.; Pople, J. A.; Binkley, J. S. J. Chem. Phys. 1984, 80,
3265-3269. (c) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.;
Schleyer, P. v. R. J. Comput. Chem. 1983, 4, 294-301.
4
3
and JHH ) 1.9 Hz); 8.03 (dd, 1H, CH arom., JHH ) 8.0 Hz and
4JHH ) 1.4 Hz); 7.82 (dd, 1H, CH arom, JHH ) 8.0 Hz and JHH
) 1.4 Hz); 7.76-7.70 (m, 2H, CH arom); 7.55 (apparent t, 1H,
CH arom, 3JHH ) 8.0 Hz); 7.47 (dd, 1H, CH arom, 3JHH ) 8.2 Hz
and 3JHH ) 4.2 Hz); 7.43-7.36 (m, 3H, CH arom). 13C{1H} NMR
(CDCl3): δ 151.4, 136.8, 134.4, 132.3, 128.7, 128.5, 126.4 and
121.9 (CH arom); 148.3, 128.5, 123.7 and 123.6 (C arom quat);
95.7 and 87.5 (CtC).
3
4
Attempted Chloropalladation of 4. A solution of acetylene 4
(0.162 g, 0.7 mmol) in methanol (15 mL) was added, at room
temperature, to a preformed solution of Li2PdCl4 (0.125 g (0.7
mmol) of PdCl2 and 0.075 g (1.8 mmol) of LiCl in methanol (30
mL)). The reaction mixture turns to dark brown almost instanta-
neously with the formation of a suspension. After 2 h of stirring at
room temperature a brown precipitate was formed that was
recovered by filtration, washed with methanol (2 × 15 mL), and
1
dried under reduced pressure (165 mg). H and 13C NMR spectra
showed that this solid is constituted of a complex mixture of
products, and attempts to purify it by column chromatography or
recrystallization were unsuccessful.
(15) (a) Casida, M. E.; Jamorski, C.; Casida, K. C.; Salahub, D. R. J. Chem.
Phys. 1998, 108, 4439-4449. (b) Stratmann, R. E.; Scuseria, G. E.;
Frisch, M. J. J. Chem. Phys. 1998, 109, 8218-8224. (c) Bauernschmitt,
R.; Ahlrichs, R. Chem. Phys. Lett. 1996, 256, 454-464.
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(b) Fery-Forgues, S.; Lavabre, D. J. J. Chem. Educ. 1999, 76, 1260-
1264.
Synthesis of Palladacycle 6. A solution of phenyl-2-pyridinyl-
acetylene (5) (0.304 g; 1.7 mmol) in methanol (10 mL) was added
dropwise to a brown solution of Li2PdCl4 (0.5 g; 1,7 mmol) in
532 Inorganic Chemistry, Vol. 43, No. 2, 2004

New Totally Flat Pincer Palladacycle
Scheme 1. Preparation of Alkyne 1 from 8-Bromoquinoline
Figure 2. Postulated intermediates in the chloropalladation of 1.
of the Nd:YAG laser. Solutions were stirred between each laser
shot, and 5 laser shots were averaged to obtain the transient
absorption decays. Lifetime measurements were performed at 20
°C on air-equilibrated solutions and on solutions deoxygenated by
exhaustive purging with argon in cuvettes capped with a rubber
septum. Solutions were monitored for laser-induced decomposition
by conventional UV-vis absorption spectroscopy (Hewlett-Packard
8452A diode array spectrometer) and replaced by fresh solution at
the first signs of decomposition. The standard exponential decay
routines of the Edinburgh Analytical Instruments LP900 system
software were used to analyze the decays of the transient and obtain
the lifetimes of the excited species. In all cases, a single exponential
adequately described the decay. Transient absorption spectra were
determined point-by-point, and in all cases, the lifetime of the
transient showed no significant variation with wavelength.
Figure 3. Possible palladacycles produced by the chloropalladation of 4.
Scheme 2
Results and Discussion
Preparation of the Ligand and Chloropalladation.
Initial attempts to prepare 2-pyridinyl-8-quinolinyl-acetylene
1 by Sonogashira17 coupling of the commercially available
2-pyridinyl-acetylene with 8-bromoquinoline under various
reaction conditions failed. However, the alkyne 1 was easily
prepared in two steps starting from 8-bromoquinoline as
presented in Scheme 1. Alkyne 1 was isolated in 65% overall
yield as a white powder and characterized by means of CHN
the CtC triple bond yielding intermediates such as 3 (Figure
2). Selective intermolecular chloride nucleophilic addition
on the activated CtC bond affords the more thermodynamic
stable five-membered palladacyclic ring followed by coor-
dination of the second donor group affording 2.
Modeling experiments suggest that the simultaneous
coordination of the quinoline nitrogen and the alkyne to the
Pd center to generate an intermediate like 3b is more
favorable than the formation of 3a. However, evidence
suggesting the preferential coordination of the pyridine
moiety rather than N-quinoline atom and formation of the
five-membered ring have been obtained from the reactions
of phenyl-8-quinolinyl-acetylene (4) and phenyl-2-pyridinyl-
acetylene (5) with Li2PdCl4 (Scheme 2).
The reaction of phenyl-8-quinolinyl-acetylene (4) affords
a mixture of unidentified compounds together with PdCl2
and metallic palladium whereas the same reaction using
phenyl-2-pyridinyl-acetylene (5) affords the chloro dimer
palladacycle 6. In the case of 4, the chloropalladation can
generate a six-membered ring palladacycle 7 or a five-
membered ring containing an exo-cyclic double bond 8
(Figure 3) depending on the regiochemistry of chloride
nucleophilic addition. These types of dimeric compounds are
apparently not stable since previous attempts using analogous
alkynes (possessing the homo propargyl skeleton) have
failed.7 In contrast the formation of five-membered palla-
dacycles containing the endocyclic double like in 6 is a
general case for the chloropalladation of heterosubstituted
alkynes.7
combustion analysis, CG-MS, IR, UV-vis, and 1H and 13
C
NMR (see Experimental Section).
The addition of alkyne 1 to a methanolic solution of Li2-
PdCl4 at room temperature affords almost instantaneously
and in high yields the pincer palladacycle 2 (eq 1). Palla-
dacycle 2 was isolated as a yellow crystalline solid and is
air and water stable, soluble in most of polar organic solvents
(dichloromethane, acetone, toluene, etc.), and insoluble in
nonpolar solvents such as hexane and diethyl ether. Com-
pound 2 has a relatively high thermal stability since it only
starts to decompose at temperatures above 140 °C.
The formation of 2 probably occurs through the coordina-
tion of only one donor group followed by coordination of
Crystal Structure of 2. The molecular structure of 2 has
been ascertained by means of an X-ray diffraction analysis.
(17) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975,
4467-4470.
Inorganic Chemistry, Vol. 43, No. 2, 2004 533

Consorti et al.
Figure 5. Diagram showing the π-π stacking arrangement (pyridine-
quinoline and quinoline-quinoline interactions) along the b axis.
Figure 4. Molecular structure of the complex (C5H4N-2-CdC(Cl)-8-
C9H6κN,κC,κN)PdCl (2). Selected bond lengths (Å) and angles (deg): Pd1-
C1 1.955(5), Pd1-N17 2.035(4), Pd1-N8 2.040(4), Pd1-Cl2 2.407(14),
C1-Pd1-N17 82.7(2), C1-Pd1-N8 82.13(19), N17-Pd1-N8 164.85-
(17), C1-Pd1-Cl2 178.64(15), N17-Pd1-Cl2 96.93(13), C7-N8-Pd1
127.5(4), C3-N8-Pd1 112.6(3), C16-N17-Pd1 127.4(4), C18-N17-
Pd1 112.5(3).
Table 1. Summary of the Crystal Data and Structure Refinement for 2
empirical formula
fw
C15H11Cl3N2Pd
450.02
temp
200(2) K
wavelength
cryst syst
space group
Z
0.71073 Å
monoclinic
C2/c
8
unit cell dimensions
a ) 17.6947(4) Å, R ) 90°
b ) 7.8438(2) Å, â ) 92.881(1)°
c ) 23.1986(5) Å, γ ) 90°
3215.75(13) Å3
1.86 g/cm3
Figure 6. UV-vis absorption spectra of 2.
V
trans positions with a bond angle of 165° showing an angular
deviation of 15° from an exact trans coordination. This
distortion from the ideal square-planar arrangement is the
result of the small C(1)-Pd(1)-N(17) and C(1)-Pd(1)-
N(8) bite angles in the two five-membered rings, of 83° and
82°, respectively. The σ Pd-C(vinyl) single bond length in
2 of 1.95 Å is of the same order of magnitude of those
observed in similar compounds, where the distances fall in
the range between 1.991 and 2.011 Å.18 The two five-
membered rings are almost perfectly flat.
The crystal structure packing of 2 consists of inversion
related pairs of molecules through aromatic π-π stacking
(Figure 5). The pairs are arranged in a head-to-tail manner
and involve the close interaction of pyridine and quinoline
moieties (3.448 Å) and a large Pd-Pd distance (5.004 Å).
The stacking along the b axis through quinoline-quinoline
interactions is 3.452 Å.
UV-Vis Characterization and Theoretical Calcula-
tions. The UV-vis spectra of complex 2 are presented in
Figure 6, and the absorption bands are summarized in
Table 2.
The complex has intense absorption bands at 263 nm
(CH2Cl2) and 266 nm (CH3CN) that are probably related to
the ligand-centered transition (π-π*). Moderate absorptions
were also observed between 385 and 437 nm with molar
D(calcd)
abs coeff
cryst shape
cryst size
θ range for data collection
index ranges
1.65 mm-1
needle
0.47 × 0.06 × 0.02 mm3
1.8-24.1°
-20 e h e 20
-9e k e 9
-26e l e 26
12149
reflns collected
indep reflns
obsd reflns
abs correction
max and min transm
refinement method
data/restraints/params
GOF on F2
final R indices (I > 2σ(I))
largest diff peak and hole
2541 (R(int) ) 0.0602)
1894 (I > 2σ(I))
semiempirical from equivalents
0.98 and 0.84
full-matrix least-squares on F2
2541/0/204
1.01
R1 ) 0.037, wR2 ) 0.079
0.70 and -0.56 e Å-3
An ORTEP drawing of the structure is shown in Figure 4
together with selected bond distances and angles. Crystal-
lographic data and details of the structure determination are
presented in Table 1. Tables of atomic coordinates, hydrogen
coordinates, complete list of bond distances and angles, and
anisotropic thermal parameters are supplied as Supporting
Information.
In compound 2, the Pd(II) center is coordinated in a
slightly distorted square-planar fashion by the pyridine N
donor groups, a C(sp2) vinyl atom of the anionic terdentate
ligand system, and a Cl atom. The C(vinyl)-Pd-Cl bond
angle is 179°, and the two N donor groups are also in mutual
(18) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson, D.
G.; Robin Taylor, R. J. Chem. Soc., Dalton Trans. 1989, S1-S83.
534 Inorganic Chemistry, Vol. 43, No. 2, 2004

New Totally Flat Pincer Palladacycle
Table 2. UV-Vis Absorption Data 2a
solvent
λambasx (nm)
dichloromethane
toluene
266 (2.9), 388 (1.0), 408m (1.5), 431 (1.3)
393 (0.8), 413m (1.3), 437 (1.2)
acetonitrile
DMA
263 (2.5), 385 (0.9), 404m (1.2), 426 (1.1)
389 (0.9), 409m (1.4), 432 (1.2)
a In parenthesis are presented the molar extinction coefficients values
(ꢀmax ) ×10-4 L‚mol-1‚cm-1).
Figure 8. Normalized steady-state fluorescence spectra of the Pd complex.
Table 3. Fluorescence Data of 2 in Solution
λ
max excitation
(nm)
λ
max emission
(nm)
Stokes shift
(nm)
solvent
CH2Cl2
toluene
acetonitrile
DMA
369
331
320
338
442
447
457
457
73
116
137
118
Figure 7. Normalized solid state and CH2Cl2 solution (10-5 mol‚L-1
fluorescence spectra of the ligand 1.
)
excitation wavelengths for fluorescence emission spectra. The
Pd complex fluorescence spectra are presented in Figure 8.
Relevant data obtained by fluorescence are listed in
Table 3.
extinction coefficients in the range 0.8 × 104 to 1.5 × 104
L‚mol-1‚cm-1. It can also be seen that the solvent polarity
does not present a fundamental role in the maximum
absorption wavelength. However, this small solvatochromic
shift is an indication of their charge-transfer nature. Of note
is that the bands at 350-450 nm have been ascribed to metal-
perturbed intraligand transitions for analogous cyclopalla-
dated CN aromatic compounds.19-21
In order to get some insight into the nature of these
transitions, we carried out molecular orbital calculations for
pincer palladacycle 2 using density functional theory. The
LUMO orbitals of 2 are basically π* (LUMO; LUMO + 1,
LUMO + 2, and LUMO + 3), mainly centered on the
quinoline-pyridine ligand with little palladium contribution,
and σ* (LUMO + 4). The (first) HOMO and HOMO - 2
orbitals are characteristically π while the remaining first
HOMOs are σ orbitals including the HOMO - 4 orbital with
remarkable metal d(z2) character. On the basis of the
TD-DFT calculations, the strongest transitions can be attrib-
uted, considering the main contribution, to HOMO - 2 f
LUMO (393.2 nm, oscillator strength (os) 0.19), HOMO f
LUMO + 1 (409.4 nm, os 0.02), and HOMO f LUMO
(480.8 nm, os 0.07) transitions with π f π* character.
Fluorescence Characterization. The 2-pyridinyl-8-quino-
linyl-acetylene 1 is an emitter in both solution and in the
solid state with emission maxima of 389 nm (Figure 7).
The Pd complex is also fluorescent in solution and in the
Values of maximum excitation fluorescence between 320
and 369 nm could be detected for the Pd complex. The
complex presented emission spectra in the blue region with
a maximum emission wavelength range of 442-457 nm. As
it can be seen, the solvent polarities affect the maximum
emission of fluorescence. The local environment can affect
the energies of both the ground and excited states. Polar
solvents can stabilize all electronic states, but the larger the
dipolar moment of the electronic state, the greater the
stabilization energy. In this way, transitions would be red-
shifted in polar solvents due to the high stabilization of these
orbitals, as can be detected for DMA compared with toluene
and CH2Cl2. Using acetonitrile as a solvent, a similar value
obtained with DMA could be observed. The luminescence
displayed by 2 in solution at room temperature is unprec-
edented for mononuclear palladacycles, which usually display
strong luminescence only in a rigid matrix at 77 K.6
Solid state fluorescence spectra of the Pd complex have
also been measured at room temperature and can be seen in
Figure 9. A single emission (501 nm) and excitation (322
nm) band could be seen for the complex with a Stokes shift
of 179 nm. This value in the solid state is higher than those
obtained in solution, indicating that the solvent plays a
fundamental role in the complex fluorescence emission
mechanism. Moreover, the different values (red-shifted)
obtained for the emission in solution and solid state suggest
that emission in the solid state can be attributable to
excimeric species. The excimeric emission can result from
metal-metal to ligand charge transfer (MMLCT) where the
donor orbital is derived from the metal-metal interaction
or π-π* emission resulting from the aromatic stacking
interactions between the metalated ligands.6 It is, however,
solid state when irradiated with UV light at room tempera-
abs
max
ture. The λ
obtained by UV-vis spectra were used as
(19) Ghedini, M.; Pucci, D.; Calogero, G.; Barigelletti, F. Chem. Phys.
Lett. 1997, 267, 341-344.
(20) Aiello, I.; Ghedini, M.; La Deda, M. J. Lumin. 2002, 96, 249-259.
(21) Ferna´ndez, S.; Fornie´s, J.; Gil, B.; Go´mez, J.; Lalinde, E. J. Chem.
Soc., Dalton Trans. 2003, 822-830.
Inorganic Chemistry, Vol. 43, No. 2, 2004 535

Consorti et al.
(400-800 nm) and decays exclusively back to ground state;
thus, the lifetime determined from decay of the transient (e.g.,
at 550 nm) is identical to that obtained by monitoring the
recovery of the ground-state absorption at 410 nm, and no
longer-lived species are observed. In air-equilibrated aceto-
nitrile, the decay of the transient is monoexponential at all
wavelengths monitored (400-800 nm), with a lifetime of
197 ( 5 ns. Upon purging the solution with argon to remove
oxygen, the lifetime of the transient increases to 2.0 ( 0.1
µs, indicating that it is the triplet state of the Pd complex. In
contrast, the intensity of the emission of the complex is only
marginally enhanced (by only a few percent) upon argon
purging of the solution, confirming that the observed
emission is inefficiently quenched by oxygen and is therefore
fluorescence rather than phosphorescence.
Figure 9. Normalized solid state fluorescence spectra of the Pd
complex 2.
In summary, we have shown that chloropalladation is a
simple and useful method for the construction of rigid and
totally flat pincer palladacycles that display luminescence
properties. The palladacycle 2 exhibits fluorescence at room
temperature in both the solid state and in solution. This
property is probably a result of excimeric transitions due to
the particular structure of this compound (rigid and totally
flat).
very difficult to unambiguously assign the contribution of
one of these interactions. In the case of 2, the long Pd-Pd
distance (5.004 Å) compared with other similar palladacycles
(3.27 Å observed in 6-phenyl-2,2′-bipyridine, for example)6
and the relatively short π-stacking (quinoline-quinoline
interactions ) 3.452 Å, see Figure 5) suggests that excimeric
π-π* may be responsible for the observed transition in the
solid state.
Palladacycle 2 is a very weak fluorescent emitter, and the
quantum yield of fluorescence for complex 2 (6.8 × 10-3)
is lower than of the ligand 1 (3.0 × 10-2), both relative to
quinine sulfate (φfl ) 0.55). Our assignment of the weak
emission of the Pd complex as being fluorescence rather than
room-temperature phosphorescence is confirmed by laser
flash photolysis experiments on the Pd complex in aceto-
nitrile solution. Upon excitation of the complex at 355 nm
(Nd:YAG laser, 5 ns pulse), a single transient was observed.
The transient absorbs across the entire visible spectral range
Acknowledgment. Financial support from FAPERGS,
and CNPq (Brazil), is gratefully acknowledged. Thanks are
also due to FAPESP and CAPES for fellowships.
Supporting Information Available: Tables of full crystal data,
atomic coordinates, calculated hydrogen coordinates, anisotropic
thermal parameters; a complete list of bond lengths and angles;
and Gaussian98 output files containing full optimized geometry
and excited state energy calculations. This material is available free
of charge via the Internet at http://pubs.acs.org.
IC0346679
536 Inorganic Chemistry, Vol. 43, No. 2, 2004