Synthesis and characterization of 2,6-di(quinolin-8-yl)pyridines. New ligands for bistridentate RuII complexes with microsecond luminescent lifetimes

Article abstract of DOI:10.1021/jo7015373

(Chemical Equation Presented) The synthesis of 4-substituted and 4-aryl-substituted 2,6-di-(quinolin-8-yl)pyridines is described. The tridentate ligands were prepared via a palladium-catalyzed Suzuki-Miyaura cross-coupling reaction or via a one-step ring-forming reaction generating the central pyridine ring. X-ray crystal structures and ¹H NMR shifts are discussed and compared to the corresponding data for a Ru^II bistridentate complex. Intramolecular stacking of two quinoline units in the Ru^II complex is suggested by ¹H NMR data and also observed in the X-ray structure.

Full text of DOI:10.1021/jo7015373

construction of multicomponent metal-containing systems for
vectorial electron and energy transfer. However, the luminescent
properties of tpy-based ruthenium(II) complexes are generally
poor, and [Ru(tpy)2]2+ has an excited-state lifetime of only 0.25
ns at room temperature,5 which has limited their use in many
applications. Therefore, the development of novel tridentate
nitrogen-containing ligands that result in highly luminescent
bistridentate ruthenium(II) complexes with long excited-state
lifetimes would considerably expand the use of this class of
complexes in many research fields. Along these lines, we
recently reported the synthesis of 2,6-di(quinolin-8-yl)pyridine
Synthesis and Characterization of
2,6-Di(quinolin-8-yl)pyridines. New Ligands for
Bistridentate RuII Complexes with Microsecond
Luminescent Lifetimes
Michael Ja¨ger,† Lars Eriksson,‡ Jonas Bergquist,§ and
Olof Johansson*,†
Department of Photochemistry and Molecular Science, Uppsala
UniVersity, Box 523, 751 20 Uppsala, Sweden, Department of
Physical, Inorganic and Structural Chemistry, Stockholm
UniVersity, 106 91 Stockholm, Sweden, and Department of
Physical and Analytical Chemistry, Uppsala UniVersity,
Box 599, 751 24 Uppsala, Sweden
(1) and the corresponding ruthenium(II) complex [Ru(1)2]2+
,
which has a remarkable 3 µs excited-state lifetime at room
temperature.6 The tridentate ligand provides a larger bite angle
than tpy resulting in an increase in the ligand field splitting.
Consequently, the normally rapid activated decay of the metal-
to-ligand charge transfer (MLCT) state via the metal-centered
(MC) state in [Ru(tpy)2]2+ complexes is slowed down in [Ru-
(1)2]2+ resulting in favorable properties.
olof.johansson@fotomol.uu.se
ReceiVed July 13, 2007
To have readily accessible ligands for future preparation of
linear multiunit assemblies based on the RuII bistridentate motif,
we were interested in developing synthetic routes to substituted
2,6-di(quinolin-8-yl)pyridyl ligands. Herein, we present the
synthesis of a range of functionalized 2,6-di(quinolin-8-yl)-
pyridyl ligands prepared via the Pd-catalyzed coupling strategy
and via a one-step ring-forming methodology generating the
central pyridine ring.7
2,6-Di(quinolin-8-yl)pyridines. Our initial strategy toward
2,6-di(quinolin-8-yl)pyridines was based on the Stille-type
carbon-carbon bond-forming reaction which has previously
been widely used for the preparation of functionalized oligopy-
ridyl ligands.8 Reacting 8-(tri-n-butyltin)quinoline9 and 2,6-
dibromopyridine with 5 mol % of Pd(PPh3)4 using different
conditions (e.g., refluxing toluene, THF at 140 °C using
microwave heating) resulted in low yields, less than 30%, and
various byproducts. Instead, we focused on the Suzuki-Miyaura
The synthesis of 4-substituted and 4-aryl-substituted 2,6-di-
(quinolin-8-yl)pyridines is described. The tridentate ligands
were prepared via a palladium-catalyzed Suzuki-Miyaura
cross-coupling reaction or via a one-step ring-forming
reaction generating the central pyridine ring. X-ray crystal
1
structures and H NMR shifts are discussed and compared
to the corresponding data for a RuII bistridentate complex.
Intramolecular stacking of two quinoline units in the RuII
complex is suggested by 1H NMR data and also observed in
the X-ray structure.
(3) (a) Ziessel, R.; Hissler, M.; El-ghayoury, A.; Harriman, A. Coord.
Chem. ReV. 1998, 178-180, 1251-1298. (b) Barigelletti, F.; Flamigni, L.
Chem. Soc. ReV. 2000, 29, 1-12. (c) Amini, A.; Bates, K.; Benniston, A.
C.; Lawrie, D. J.; Soubeyrand-Lenoir, E. Tetrahedron Lett. 2003, 44, 8245-
8247. (d) Benniston, A. C.; Harriman, A.; Li, P.; Patel, P. V.; Sams, C. A.
J. Org. Chem. 2006, 71, 3481-3493.
(4) (a) Andres, P. R.; Schubert, U. S. AdV. Mater. 2004, 16, 1043-
1068. (b) Schubert, U. S.; Hofmeier, H.; Newkome, G. R. Modern
Terpyridine Chemistry; Wiley-VCH: Weinheim, 2006; pp 131-170.
(5) Winkler, J. R.; Netzel, T. L.; Creutz, C.; Sutin, N. J. Am. Chem.
Soc. 1987, 109, 2381-2392.
(6) Abrahamsson, M.; Ja¨ger, M.; O¨ sterman, T.; Eriksson, L.; Persson,
P.; Becker, H.-C.; Johansson, O.; Hammarstro¨m, L. J. Am. Chem. Soc. 2006,
128, 12616-12617.
(7) (a) Vaduvescu, S.; Potvin, P. G. Inorg. Chem. 2002, 41, 4081-4083.
(b) Smith, C. B.; Raston, C. L.; Sobolev, A. N. Green Chem. 2005, 7, 650-
654. (c) Wang, J.; Hanan, G. S. Synlett 2005, 1251-1254. (d) Hwang, S.-
H.; Moorefield, C. N.; Wang, P.; Fronczek, F. R.; Courtney, B. H.;
Newkome, G. R. Dalton Trans. 2006, 3518-3522. (e) Winter, A.; van den
Berg, A. M. J.; Hoogenboom, R.; Kickelbick, G.; Schubert, U. S. Synthesis
2006, 2873-2878.
Photoactive polypyridyl ruthenium(II) complexes based on
tridentate nitrogen-containing heterocycles such as 2,2′:6′,2′′-
terpyridine (tpy) continue to attract wide interest1 for their
possible use in, e.g., artificial photosynthetic systems,2 in
molecular photonic devices,3 and in metallo-supramolecular
polymers.4 Due to the symmetry of the 4′-substituted 2,2′:6′,2′′-
terpyridines, the resulting [Ru(tpy)2]2+ complexes are achiral,
which makes these ligands ideal building blocks for the
† Department of Photochemistry and Molecular Science, Uppsala University.
‡ Department of Physical, Inorganic and Structural Chemistry, Stockholm
University.
§ Department of Physical and Analytical Chemistry, Uppsala University.
(1) (a) Sauvage, J.-P.; Collin, J.-P.; Chambron, J.-C.; Guillerez, S.;
Coudret, C.; Balzani, V.; Barigelletti, F.; De Cola, L.; Flamigni, L. Chem.
ReV. 1994, 94, 993-1019. (b) Medlycott, E. A.; Hanan, G. S. Chem. Soc.
ReV. 2005, 34, 133-142. (c) Medlycott, E. A.; Hanan, G. S. Coord. Chem.
ReV. 2006, 250, 1763-1782.
(8) (a) Hanan, G. S.; Lehn, J.-M.; Kyritsakas, N.; Fischer, J. J. Chem.
Soc., Chem. Commun. 1995, 765-766. (b) Lehmann, U.; Henze, O.;
Schlu¨ter, A. D. Chem. Eur. J. 1999, 5, 854-859. (c) Fallahpour, R.-A.;
Neuburger, M.; Zehnder, M. New J. Chem. 1999, 53-61. (d) Fallahpour,
R.-A. Synthesis 2000, 1138-1142. (e) Heller, M.; Schubert, U. S. Synlett
2002, 751-754.
(2) (a) Baranoff, E.; Collin, J.-P.; Flamigni, L.; Sauvage, J.-P. Chem.
Soc. ReV. 2004, 33, 147-155, and references cited therein. (b) Laine´, P.;
Bedioui, F.; Amouyal, E.; Albin, V.; Berruyer-Penaud, F. Chem. Eur. J.
2002, 8, 3162-3176. (c) Laine´, P.; Bedioui, F.; Ochsenbein, P.; Marvaud,
V.; Bonin, M.; Amouyal, E. J. Am. Chem. Soc. 2002, 124, 1364-1377. (d)
Borgstro¨m, M.; Ott, S.; Lomoth, R.; Bergquist, J.; Hammarstro¨m, L.;
Johansson, O. Inorg. Chem. 2006, 45, 4820-4829.
(9) DiMauro, E. F.; Mamai, A.; Kozlowski, M. C. Organometallics 2003,
22, 850-855.
10.1021/jo7015373 CCC: $37.00 © 2007 American Chemical Society
Published on Web 11/15/2007
J. Org. Chem. 2007, 72, 10227-10230
10227

SCHEME 1. Pd-Catalyzed Coupling of Quinoline-8-boronic
Acid and 2,6-Dihalopyridines
FIGURE 1. ORTEP view of 1 at 50% probability level.
The X-ray crystal structures of 1 (Figure 1) and the amino-
substituted 5 (Supporting Information) confirmed the proposed
structures. Compound 1 is twisted with torsion angles between
the central pyridine and quinoline units of -47.0° (N1-C2-
C7-C8) and 46.4° (N1-C6-C17-C18), respectively. This is
distinct from structures of 2,2′:6′,2′′-terpyridines which usually
show angles between the planes of the central pyridyl ring and
the two terminal rings smaller than 10-12°.14,17 The interannular
C-C bond lengths in 1 are 1.496(3) and 1.483(3) Å, respec-
tively. The crystal packing is typical for aromatic molecules in
a pseudoherringbone pattern. It does not show any short
intermolecular interactions between the centers of gravity of
different rings that are indicative of π-π interactions in the
lattice (Supporting Information). In addition, a geometry
optimization calculation was performed with MOPAC-618 using
the AM1-Hamiltonian regarding the observed torsion angles.
The calculated minimum (-55°) is close to the experimental
value observed in the X-ray crystal structure and gives support
for a twisted structure also in solution. However, the torsion
angle can vary between -90° to -45° with only 2 kcal mol-1
changes in the heat of formation (Supporting Information).
cross-coupling reaction using a catalyst system composed of
Pd and commercially available 2-dicyclohexylphosphino-2′,6′-
dimethoxybiphenyl (P′), which recently has been used to
effectively couple a range of heterocyclic substrates.10 Reacting
quinoline-8-boronic acid and 2,6-dibromopyridine with 1 mol
% of Pd(dba)2 and 2 mol % of P′ in toluene at 110 °C gave
compound 1 in 80% isolated yield (Scheme 1).6 Using 2,6-
dichloroisonicotinic ethyl ester,11 2,6-dichloro-4-hydroxy-
methylpyridine,8e or 2,6-dibromo-4-nitropyridine12 furnished
2-4 in moderate to excellent yields (Scheme 1). The ester-
substituted 2 was obtained in 90% yield after recrystallization
from EtOH. Using standard functional group manipulations,
compound 4 was reduced to give the amino-functionalized 5,
which was subsequently converted to the bromo-functionalized
ligand 6 by treatment with tert-butyl nitrite-copper(II) bro-
mide.13
In the 1H NMR spectra (in CDCl3) of the symmetric ligands
1-6, the lowest field resonance in all ligands is H2′ adjacent to
the nitrogen of the quinoline ring. It is interesting to compare
the proton resonances of the central pyridine of the 2,6-di-
(quinolin-8-yl)pyridyl ligands to those of 2,2′:6′,2′′-terpyridines.
The H4 proton in 1 (at 7.96 ppm) is close to that of tpy (7.97
ppm),14 whereas the H3 protons in 1 (at 8.13 ppm) are found to
be upfield shifted compared to tpy (8.46 ppm). In the latter
compounds, the H3 protons are deshielded by the nearby
pyridines in the presumed transoid planar arrangement.14 Except
for the expected difference in electron-accepting property of
the 8-quinolinyl compared to 2-pyridyl substituents, we believe
that the favored conformation in 1 deviates from a planar
arrangement of the aromatic ring-systems resulting in a less
deshielded environment of the H3 protons in 1 compared to tpy.
Similar shifts of the H3 protons when compared to the
corresponding substituted tpy ligands8c,d,15,16 (0.1-0.6 ppm) are
observed in all ligands 1-6. Support for such twisted arrange-
ments comes from the observed solid-state structures.
The solid-state structure of 5 shows considerably larger and
less symmetric torsional angles between the central pyridine
and quinoline units, -98.5° and 60.5°, respectively (Supporting
Information). The structure shows no unusual π-π interactions
in the lattice; however, hydrogen-bonded dimers are evident
which can explain the increased torsional angles8c as compared
to 1. These dimers then pack in a typical pattern for aromatic
compounds.
Structural Characterization of [Ru(1)2]2+. Upon coordina-
tion of 1 to RuII forming [Ru(1)2]2+,6 the proton resonances
change significantly as shown in Figure 2 (both the free ligand
and the complex were recorded in CD3CN), and the assignment
was accomplished by two-dimensional NMR techniques (COSY,
NOESY). The downfield shift for the H4 proton upon coordina-
tion is similar to that of [Ru(tpy)2]2+ 19 However, the H3 protons
.
are upfield shifted (∆δ ) -0.2 ppm), which is opposite to that
observed in bisterpyridine ruthenium(II) complexes (usually
downfield shifted by ∆δ g +0.3 ppm upon coordination due
to the electron-withdrawing effect of the metal ion).19,20 In [Ru-
(tpy)2]2+, the two tridentate ligands are close to planar and
(10) (a) Walker, S. D.; Barder, T. E.; Martinelli, J. R.; Buchwald, S. L.
Angew. Chem., Int. Ed. 2004, 43, 1871-1876. (b) Barder, T. E.; Walker,
S. D.; Martinelli, J. R.; Buchwald, S. L. J. Am. Chem. Soc. 2005, 127,
4685-4696.
(11) Prepared as 2,6-dibromoisonicotinic ethylester (ref 8d). 1H NMR
identical to the material prepared previously (Tutonda, M.; Vanderzande,
D.; Hendrickx, M.; Hoornaert, G. Tetrahedron 1990, 46, 5715-5732).
(12) Neumann, U.; Vo¨gtle, F. Chem. Ber. 1989, 122, 589-591.
(13) Doyle, M. P.; Siegfried, B.; Dellaria, J. F., Jr. J. Org. Chem. 1977,
42, 2426-2431.
(17) (a) Constable, E. C.; Lewis, J.; Liptrot, M. C.; Raithby, P. R. Inorg.
Chim. Acta 1990, 178, 47-54. (b) Fallahpour, R.-A.; Neuburger, M.;
Zehnder, M. Polyhedron 1999, 18, 2445-2454. (c) Eryazici, I.; Moorefield,
C. N.; Durmus, S.; Newkome, G. R. J. Org. Chem. 2006, 71, 1009-1014.
(18) Stewart, J. J. P. MOPAC-6 public domain version, Frank J. Seiler
Research Laboratory, United States Air Force Academy, CO, 1990.
(19) 1H NMR spectra in CD3CN: Schubert, U. S.; Hofmeier, H.;
Newkome, G. R. Modern Terpyridine Chemistry; Wiley-VCH: Weinheim,
2006; p 42.
(14) Constable, E. C.; Cargill Thompson, A. M. W.; Tocher, D. A.;
Daniels, M. A. M. New J. Chem. 1992, 16, 855-867.
(15) Cuenoud, B.; Schepartz, A. Proc. Natl. Acad. Sci. U.S.A., 1993,
90, 1154-1159.
(16) Pe´chy, P.; Rotzinger, F. P.; Nazeeruddin, M. K.; Kohle, O.;
Zakeeruddin, S. M.; Humphry-Baker, R.; Gra¨tzel, M. J. Chem. Soc., Chem.
Commun. 1995, 65-66.
(20) See also Constable et al. in ref 14. However, the free ligands were
recorded in CDCl3 and the RuII complexes in CD3COCD3.
10228 J. Org. Chem., Vol. 72, No. 26, 2007

SCHEME 2. One-Step Synthesis of 4-Aryl-Substituted
2,6-Di(quinolin-8-yl)pyridines
second crop was obtained by chromatography to give a total
yield of 35%. The intermediate enone precipitates as a pale
yellow solid during the reaction and was isolated in a separate
reaction of equimolar 4-bromobenzaldehyde and 8-acetylquino-
line in 83% yield. The MS spectrum showed the expected peak
at m/z ) 338 [M + H+]+ and 1H NMR signals from the vinylic
protons at around 7.6 ppm. Overlapping resonances of the
vinylic protons precluded a definite assignment, but tentatively
we assign it to the trans isomer.27
1
FIGURE 2. Ball and stick representation of [Ru(1)2][PF6]2 and H
NMR spectra of 1 (top) and [Ru(1)2][PF6]2 (bottom) in CD3CN.
orthogonal to each other.21 Instead, the recently reported X-ray
crystal structure of [Ru(1)2]2+ (Figure 2)6,22 shows enforced
dihedral angles between the pyridine and the quinoline least-
square planes (35 and 39°, respectively). This leads to quasi-
planar arrangements of two quinoline units, one from each
tridentate ligand, with an interplanar distance less than 3.5 Å.23
The different magnetic environment for the H3 protons in [Ru-
(1)2]2+ relative to [Ru(tpy)2]2+ most likely explains the shift
differences.24 For the quinoline units, the most dramatic change
is that of H2′ which experiences a large upfield shift (∆δ )
-0.9 ppm) due to its position above the other ligand. Similar
effects of the protons adjacent to the nitrogens of the terminal
pyridine rings of 2,2′:6′,2′′-terpyridines when coordinated to RuII
are well-known.14,19 All other protons of the quinolines are also
significantly upfield shifted which suggest intramolecular stack-
ing25 of the quinoline units in agreement with the X-ray crystal
structure.
4-Aryl-Substituted 2,6-Di(quinolin-8-yl)pyridines. Having
the substituted 2,6-di(quinolin-8-yl)pyridyl ligands in hand, we
also targeted the introduction of 4-aryl substituents on the central
pyridine since such substituents often have a profound effect
on the photophysical properties of the corresponding bisterpy-
ridine RuII complexes,1 and this is a well-known strategy to
increase the donor (D)-acceptor (A) distance in D-chro-
mophore-A triads.2a Hanan and co-workers recently reported a
simple one-step procedure for the preparation of a variety of
4′-aryl substituted tpy ligands,7c and we were interested in using
a similar protocol to have access to 4-aryl-substituted 2,6-di-
(quinolin-8-yl)pyridyl ligands. The reaction of 4-bromobenzal-
dehyde with 2 equiv of 8-acetylquinoline26 in a basic aqueous
ethanolic solution of ammonia at 40 °C afforded 7 (Scheme 2),
which solidified from solution and was purified by recrystal-
lization from EtOH to give an isolated yield of 20-25%. A
The analogous reaction using 4-methylbenzaldehyde led to
a lower isolated yield of 8. In an attempt to optimize this
reaction, it was noted that a higher overall yield was obtained
by increasing the temperature to 60 °C and by addition of some
CHCl3 to improve the solubility of the intermediate enone.
However, careful analysis of the reaction mixture revealed a
mixture of two cyclized products, the desired 2,6-di(quinolin-
8-yl)-4-(p-tolyl)pyridyl ligand 8 as well as the isomer 2,4-di-
(quinolin-8-yl)-6-(p-tolyl)pyridine. These were separated by
careful column chromatography to give 8 and the isomer in up
to 30% and 15% isolated yields, respectively. A higher yield
in the synthesis of the bromo-substituted 7 was also found when
the reaction was performed at 60 °C but with an increased
amount of the undesired isomer. Since the yields of the desired
isomers were not significantly improved and that the separations
are tedious, subsequent reactions were run using the optimal
conditions at 40 °C. Next, 4-cyanobenzaldehyde and 2,5-
dimethoxybenzaldehyde, respectively, were used as substrates
to give the substituted ligands 9 and 10 in moderate yields
(Scheme 2). Compound 10 containing a viable quinone precursor
is particularly interesting for the preparation of linear bistri-
dentate RuII electron donor-acceptor assemblies based on the
1
2,6-di(quinolin-8-yl)pyridyl ligands. The H NMR spectra of
the 4-aryl substituted ligands 7-10 follow the same pattern as
for 1-6 with the H3 protons considerably upfield shifted as
compared to the corresponding 4′-aryl-substituted terpyridines.
Given the recent interest in rigid ditopic bridging ligands
based on the tridentate motif,28 the synthesis of the “back-to-
back” bridging ligand 11 was also pursued. Nickel-catalyzed
homocoupling29 of 7 gave an inseparable mixture of products,
and instead, we adopted the recently published procedure for
Pd-catalyzeddimerizationofbromophenyl-substitutedterpyridines28c
to give the bridging ligand 11 in 78% yield (Scheme 3).
(21) Lashgari, K.; Kritikos, M.; Norrestam, R.; Norrby, T. Acta Crys-
tallogr. 1999, C 55, 64-67.
(22) [Ru(1)2][PF6]2 was originally described in the space group P1 (ref
6). As kindly pointed out by Professor Clemente Dore Augusto, the complex
could be described in the centrosymmetric space group P-1.
(23) The quinolines are somewhat displaced, and the two pairs of stacked
quinolines in the structure are slightly different. The other pair (not
highlighted) is somewhat more tilted. However, the average distance between
centers of gravity in the benzene and pyridine rings in one quinoline unit
and the benzene ring of the other quinoline is 3.6 Å.
(27) Korall, P.; Bo¨rje, A.; Norrby, P.-O.; Åkermark, B. Acta Chem.
Scand. 1997, 51, 760-766.
(28) (a) Vaduvescu, S.; Potvin, P. G. Eur. J. Inorg. Chem. 2004, 1763-
1769. (b) Yuan, S.-C.; Chen, H.-B.; Zhang, Y.; Pei, J. Org. Lett. 2006, 8,
5701-5704. (c) She Han, F.; Higuchi, M.; Kurth, D. G. Org. Lett. 2007, 9,
559-562. (d) Winter, A.; Egbe, D. A. M.; Schubert, U. S. Org. Lett. 2007,
9, 2345-2348.
(24) Electrostatic contributions cannot be ruled out.
(25) (a) Martin, C. B.; Mulla, H. R.; Willis, P. G.; Cammers-Goodwin,
A. J. Org. Chem. 1999, 64, 7802-7806. (b) Rathore, R.; Abdelwahed, S.
H.; Guzei, I. A. J. Am. Chem. Soc. 2003, 125, 8712-8713.
(26) Taylor, R. J. Chem. Soc. B 1971, 2382-2387.
(29) Constable, E. C.; Ward, M. D. J. Chem. Soc., Dalton Trans. 1990,
1405-1409.
J. Org. Chem, Vol. 72, No. 26, 2007 10229

SCHEME 3. Carbon-Carbon Homocoupling of
Bromophenyl-Substituted 7
0.81 mmol) were dissolved in EtOH (1 mL). A solution of
potassium hydroxide (0.091 g, 1.62 mmol, 85%) in aqueous
ammonia (1 mL, 28%) was added. The reaction mixture was
warmed at 40 °C overnight. The solid was filtered off and
recrystallized twice from EtOH to afford 7 as an off-white powder
(0.087 g, 22%). A second crop (0.051 g, 13%) was obtained from
column chromatography (silica gel, CH2Cl2/2.5-3.5% MeOH) of
1
the combined filtrates. H NMR (CDCl3): δ 9.01 (2H, dd, J )
4.2, 1.8 Hz), 8.32 (2H, s), 8.31 (2H, dd, J ) 7.2, 1.5 Hz), 8.26
(2H, dd, J ) 8.3, 1.8 Hz), 7.91 (2H, dd, J ) 8.2, 1.5 Hz), 7.73-
7.69 (2H, m, AA′ part of AA′BB′), 7.69 (2H, dd, J ) 8.2, 7.2 Hz),
7.64-7.60 (2H, m, BB′ part of AA′BB′), 7.47 (2H, dd, J ) 8.3,
4.2 Hz). 13C NMR (CDCl3): δ 157.5, 150.4, 146.0, 145.9, 139.2,
138.2, 136.5, 132.1, 131.8, 129.2, 128.7 (two overlapping signals),
126.4, 123.3, 123.0, 120.8. HRMS (ESI) m/z 488.0754 ([M +
H+]+), calcd for C29H19BrN3 488.0762.
4,4′-Di[2,6-di(quinolin-8-yl)pyridin-4-yl]biphenyl (11). Com-
pound 7 (0.362 g, 0.74 mmol), bis(neopentylglycolato)diboron
(0.087 g, 0.39 mmol), Pd(dba)2 (0.021 g, 0.04 mmol), 2-dicyclo-
hexylphosphino-2′,6′-dimethoxybiphenyl (0.032 g, 0.08 mmol), and
potassium carbonate (0.310 g, 2.25 mmol) were suspended in dry
DMF (10 mL). The mixture was heated at 80 °C under an argon
atmosphere for 5 h. After cooling, H2O and CH2Cl2 were added,
the organic phase was separated, and the aqueous layer was
extracted with additional CH2Cl2. The combined organic fractions
were combined and concentrated in vacuo, and the obtained solid
was washed with hot EtOH (0.245 g, 78%). 1H NMR (CDCl3): δ
9.03 (4H, dd, J ) 4.2, 1.9 Hz), 8.40 (4H, s), 8.32 (4H, dd, J ) 7.2,
1.5 Hz), 8.24 (4H, dd, J ) 8.3, 1.9 Hz), 7.95-7.92 (4H, m, AA′
part of AA′MM′), 7.90 (4H, dd, J ) 8.2, 1.5 Hz), 7.80-7.77 (4H,
m, MM′ part of AA′MM′), 7.69 (4H, dd, J ) 8.2, 7.2 Hz), 7.45
(4H, dd, J ) 8.3, 4.2 Hz). 13C NMR (CDCl3): δ 157.5, 150.5,
146.7, 146.2, 140.8, 139.6, 138.6, 136.5, 131.7, 128.8, 128.7, 128.2,
127.6, 126.7, 123.8, 121.1. HRMS (ESI) m/z 817.3071 ([M +
H+]+), calcd for C58H37N6 817.3080.
In conclusion, a family of 2,6-di(quinolin-8-yl)pyridines has
been synthesized. These ligands are distinct from the related
terpyridines in that a larger bite angle is provided upon metal
coordination. Work is now in progress to study the photophysical
properties of a variety of 2,6-di(quinolin-8-yl)pyridyl-based RuII
complexes which will be the subject of a forthcoming publica-
tion, and we believe that this class of ligands has great potential
for future research involving luminescent RuII complexes with
long excited-state lifetimes.
Experimental Section
Typical Procedure for the Pd-Catalyzed Coupling (1-4):
4-Ethylcarboxy-2,6-di(quinolin-8-yl)pyridine (2). Quinoline-8-
boronic acid (0.350 g, 2.02 mmol), 2,6-dichloroisonicotinic ethy-
lester (0.220 g, 1.00 mmol), Pd(dba)2 (0.006 g, 0.01 mmol, 0.5%),
2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl (0.008 g, 0.02
mmol, 1%), and finely ground potassium phosphate (1.060 g, 5.00
mmol) were suspended in dry toluene (7 mL). The mixture was
purged with argon and heated at 110 °C over night. The dark yellow
reaction mixture was cooled to room temperature, poured into Et2O
(150 mL), and allowed to stir for 30 min. The mixture was filtered
and concentrated in vacuo and the remaining solid purified by
recrystallization from EtOH to give 2 as a white solid (0.364 g,
1
90%). H NMR (CDCl3): δ 9.02 (2H, dd, J ) 4.2, 1.8 Hz), 8.67
Acknowledgment. This work was financially supported by
the Swedish Energy Agency, the Knut and Alice Wallenberg
Foundation, and NEST-STRP, SOLAR-H (EU Contract 516510).
(2H, s), 8.26 (2H, dd, J ) 7.2, 1.5 Hz), 8.24 (2H, dd, J ) 8.3, 1.8
Hz), 7.90 (2H, dd, J ) 8.2, 1.5 Hz), 7.67 (2H, dd, J ) 8.2, 7.2
Hz), 7.46 (2H, dd, J ) 8.3, 4.2 Hz), 4.46 (2H, q, J ) 7.1 Hz), 1.41
(3H, t, J ) 7.1 Hz). 13C NMR (CDCl3): δ 166.1, 157.9, 150.6,
146.0, 138.8, 137.0, 136.6, 131.7, 129.0, 128.8, 126.7, 124.9, 121.3,
61.6, 14.4. MS (ESI): m/z 406 ([M + H+]+). Anal. Calcd for
C26H19N3O2: C, 77.02; H, 4.72; N, 10.36. Found: C, 76.81; H,
4.80; N, 10.15.
Supporting Information Available: Experimental details and
characterizations for compounds 3-6 and 8-10, 1H NMR and 13
C
spectra of compounds 2-11 and 2,4-di(quinolin-8-yl)-6-(p-tolyl)-
pyridine, crystallographic data and CIF files of 1 and 5, and
geometry optimization of 1. This material is available free of charge
via the Internet at http://pubs.acs.org.
Typical Procedure for the Ring Cyclization Reaction (7-
10): 4-(p-Bromophenyl)-2,6-di(quinolin-8-yl)pyridine(7).8-Acetylquin-
oline (0.280 g, 1.64 mmol) and 4-bromobenzaldehyde (0.150 g,
JO7015373
10230 J. Org. Chem., Vol. 72, No. 26, 2007