A rational protocol for the synthesis of arylated bipyridine ligands via a cycloaddition pathway

Article abstract of DOI:10.1021/jo7019866

(Chemical Equation Presented) A generic design principle for the preparation of a variety of substituted phenyl-polypyridine ligands is described. These ligands are readily prepared by a regioselective [4+2] cycloaddition between electron-deficient dienes, such as 2,6-disubstituted-1,3, 4-triazines, and ethynyl-arenes or ethynyl-alkanes. Exceptional reactivity is found with electron-rich dienophiles bearing ethynylgallate or ethynylphenyldibutylamino groups. Two regioisomers are formed, the meta being preferred due to favorable π-π interactions in the transition state, while the para isomers are formed in low yields in most cases. The use of tert-butylacetylene or N,N-dimethylamino-2-propyne, however, drives the reaction exclusively to the para isomer. Di-N,N-dibutylaminophenyl or isoquinoline ligands can also be produced in a single step by reverse Diels-Alder reactions. Cross-coupling reactions of iodo-substituted ligands or their platinum(II) complexes under Pd(0) catalysis gives branched ligands and complexes bearing paraffin chains, electron-donor or electron-acceptor groups. The use of a chloro-Pt(II) complex of an iodo-functionalized ligand allows both halogens to be replaced by ethynyl groups by using different catalysts. This methodology readily accommodates various functional groups and has been successfully extended to systems containing a variety of donor/acceptor frameworks. All ligands strongly absorb in the near-UV and luminesce in solution at rt with quantum yields ranging from 0 to 66%. Excited state lifetimes are in the nanosecond range and the solvent effects are in keeping with singlet excited states mixed with charge-transfer character. As deduced from spectroscopic and electrochemical studies, the di-n-butylamino derivatives are strong reductants in the excited state.

Full text of DOI:10.1021/jo7019866

A Rational Protocol for the Synthesis of Arylated Bipyridine
Ligands via a Cycloaddition Pathway
Ste´phane Diring,† Pascal Retailleau,‡ and Raymond Ziessel*,†
Laboratoire de Chimie Mole´culaire, Centre National de la Recherche Scientifique (CNRS),
EÄ cole de Chimie, Polyme`res, Mate´riaux (ECPM), 25 rue Becquerel,
67087 Strasbourg Cedex 02, France, and Laboratoire de Cristallochimie, ICSN-CNRS,
Baˆt 27-1 aVenue de la Terrasse, 91198 Gif-sur-YVette, Cedex, France
ziessel@chimie.u-strasbg.fr
ReceiVed September 14, 2007
A generic design principle for the preparation of a variety of substituted phenyl-polypyridine ligands is
described. These ligands are readily prepared by a regioselective [4+2] cycloaddition between electron-
deficient dienes, such as 2,6-disubstituted-1,3,4-triazines, and ethynyl-arenes or ethynyl-alkanes.
Exceptional reactivity is found with electron-rich dienophiles bearing ethynylgallate or ethynylphe-
nyldibutylamino groups. Two regioisomers are formed, the meta being preferred due to favorable π-π
interactions in the transition state, while the para isomers are formed in low yields in most cases. The
use of tert-butylacetylene or N,N-dimethylamino-2-propyne, however, drives the reaction exclusively to
the para isomer. Di-N,N-dibutylaminophenyl or isoquinoline ligands can also be produced in a single
step by reverse Diels-Alder reactions. Cross-coupling reactions of iodo-substituted ligands or their
platinum(II) complexes under Pd(0) catalysis gives branched ligands and complexes bearing paraffin
chains, electron-donor or electron-acceptor groups. The use of a chloro-Pt(II) complex of an iodo-
functionalized ligand allows both halogens to be replaced by ethynyl groups by using different catalysts.
This methodology readily accommodates various functional groups and has been successfully extended
to systems containing a variety of donor/acceptor frameworks. All ligands strongly absorb in the near-
UV and luminesce in solution at rt with quantum yields ranging from 0 to 66%. Excited state lifetimes
are in the nanosecond range and the solvent effects are in keeping with singlet excited states mixed with
charge-transfer character. As deduced from spectroscopic and electrochemical studies, the di-n-butylamino
derivatives are strong reductants in the excited state.
Introduction
molecular electronics, opto-electronics, chemical sensing and
analysis, and photovoltaics.5-8 The necessary ligands, however,
The construction of sophisticated ligands is a primary stimulus
for the production of novel transition metal complexes, some
of which adopt quite exotic structures.1-4 Many applications
of the resultant new materials are foreseen in areas such as
(3) (a) Ziessel, R.; Matt, D.; Toupet, L. Chem. Commun. 1995, 2033.
(b) Romero, F. M.; Ziessel, R.; Dupont-Gervais, A.; Van Dorsselaer, A.
Chem. Commun. 1996, 551. (c) El-ghayoury, A.; Douce, L.; Skoulios, A.;
Ziessel, R. Angew. Chem., Int. Ed. 1998, 37, 2205.
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Wong, E. W.; Belohradsky, M.; Raymo, F. M.; Stoddart, J. F.; Kuekes, P.
J.; Williams, R. S.; Heath, J. R. Science 1999, 285, 391.
† EÄ cole de Chimie, Polyme`res, Mate´riaux (ECPM).
‡ Laboratoire de Cristallochimie, ICSN-CNRS.
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10.1021/jo7019866 CCC: $37.00 © 2007 American Chemical Society
Published on Web 11/29/2007
J. Org. Chem. 2007, 72, 10181-10193
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Diring et al.
have commonly been obtained via Skraup,9 Kro¨nke,10 or
Friedla¨nder11 condensations, which usually provide symmetri-
cally substituted species.12 Development, more recently, of
methods for the synthesis of unsymmetrical oligopyridines
(bipyridines, terpyridines, quaterpyridines, quinquepyridines,
hexapyridines, and branched derivatives) has attracted consider-
able attention because of merits including the ready availability
of starting materials, good regioselectivity, and facile separation
of the products.13 While these [4+2] cycloaddition reactions of
electron-deficient dienes, such as 1,2,4,5-tetrazines or 1,2,4-
triazines, do require the use of dienophiles of complementary
electronic character, such as ynamines which guarantee sufficient
overlap of the frontier orbitals,14 it is known that simple
organotin alkynes, for example, offer good reactivity and new
possibilities for the synthesis of polypyridine ligands.13a
There is a general agreement that additional functionalization
of the periphery of a ligand may provide even more interesting
complexes and it has been recognized in particular that the use
of 6-aryl-2,2′-bipyridine and its derivatives leads to orthomet-
allation reactions with transition metal salts. Neutral Pt(II)
complexes with intriguing structural and spectroscopic properties
have been prepared on this basis.15 Some of these have found
applications in light emitting devices due to their good stability,
bright phosphorescence, and processability.16 The incorporation
of transition metals such as Pt(II) is crucial in order to relax
the spin selection rules for phosphorescence through spin-orbit
coupling.17 This has proven to be also important in the design
of light-emitting diodes,18 nonlinear optical materials,19 and
conductance switches.20
CHART 1
Essential for this purpose are robust and easily activated
molecules that react under mild conditions and exhibit favorable
regioselectivity when cross-coupling is envisaged. These re-
quirements may be met by complexation of 4′-iodo-6′-phenyl-
2,2′-bipyridine to Pt(II), enabling the linkage of an alkynyl group
either in place of the iodo-substituent using Pd(0) catalysis or
to the metal center using a Cu(I) cross-coupling reaction (Chart
1).
The use of tributyl(ethynyl)tin as a dienophile in high-
temperature [4+2] cycloadditions with electron-deficient het-
erodienes, such as 1,2,4-triazines,21 illustrates the potential of
such reactions for the construction of a variety of different
dissymmetrically substituted 2,2′-bipyridine ligands. Herein, we
report the successful extension of this chemistry by the use of
different ethynyl-substituted templates affording 3- or 4-aryl-
6-phenyl-2,2′-bipyridine ligands, readily converted to stable Pt-
(II) complexes. This chemistry generally produces good to
excellent yields, tolerates various functional groups (donors and
acceptors), and has been successfully extended to systems
containing polyaromatic frameworks. The viability of the
procedure outlined in Chart 1 has been demonstrated in several
cases.
Despite intense interest, progress in this field has been
hampered by the availability of conveniently prepared starting
materials of adequate stability. Conceptually, the preparation
of functionalized, potentially tridendate ligands by a retro Diels-
Alder reaction is a highly attractive method for the linking of
electron-donor or -acceptor units, paraffin chains for the
engineering of soft materials, and functional groups suitable for
the assembly and stabilization of highly organized edifices.
Results and Discussion
As part of our research program on the design and construc-
tion of novel ligands bearing various appended functions,22 we
planned to develop an efficient method for the synthesis of aryl-
substituted bipyridine ligands. Earlier work on LEGO systems
provided appropriate inspiration.13 Condensation of 2-cyanopy-
ridine with hydrazine gave the corresponding carboxamidrazone
which, by condensation with 2-phenylglyoxal, afforded the 2,6-
disubstituted-1,3,4-triazine 1 as major isomer, as unambiguously
confirmed by NMR spectroscopy (Scheme 1). As found earlier
with monosubstituted glyoxals, the formation of the 2,6-
disubstituted isomer is strongly favored over that of the 2,5-
disubstituted isomer owing to the greater nucleophilicity of the
N-NH2 fragment.23
Thermal condensation of 4-ethynyltoluene with the 6-phenyl-
2-pyridyl-1,3,4-triazine 1 provided compound 2a as the major
and more polar product, and 2b as the minor and less polar
product (Scheme 1). Unambiguous establishment of the isomeric
forms was provided by 1H NMR spectroscopy and X-ray
structure determinations on single crystals. Aromatic proton
resonances in the NMR spectra of 2a and 2b are shown in Figure
1a.
(6) Constable, E. C. In Metal and Ligand ReactiVity; VCH: Weinheim,
Germany, 1996.
(7) Bignozzi, C. A.; Schoonover, J. R.; Scandola, F. Prog. Inorg. Chem.
1997, 44, 1.
(8) Ziessel, R. F. J. Chem. Educ. 1997, 74, 673.
(9) (a) Skraup, Z. H. Ber. Dtsch. Chem., 1880, 13, 2086. (b) Manske, R.
H. F.; Kulka, M. In Organic Reactions; The Skraup Synthesis of Quinolines;
John Wiley & Sons: New York, 1953; Vol. 7, p 59.
(10) Kro¨hnke, F. Synthesis 1976, 1.
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In Organic reactions; The Friedla¨nder Synthesis of Quinolines; John Wiley
& Sons: New York, 1982; Vol. 28, p 37.
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Am. Chem. Soc. 2004, 126, 7639.
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Lee, S.-T. J. Am. Chem. Soc. 2004, 126, 4958.
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Chem. 1986, 90, 3722.
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N.; Mackay, K.; Fried, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990,
347, 539. (b) Holder, E.; Langeveld, B. M. W.; Schubert, U. S. AdV. Mater.
2005, 17, 1109.
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F. Chem. Commun. 2005, 802. (b) Goeb, S.; De Nicola, A.; Ziessel, R.;
Sabatini, C.; Barbieri, A.; Barigelletti, F. Inorg. Chem. 2006, 45, 1173. (c)
Harriman, A.; Izzet, G.; Goeb, S.; De Nicola, A.; Ziessel, R. Inorg. Chem.
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Synthesis of Arylated Bipyridine Ligands
SCHEME 1
Noteworthy is the AB quartet observed near δ 8 ppm (JAB
)
emitting complexes.29 Heating these stable ethynyl derivatives
in o-dichlorobenzene at 180 °C provided compounds 3a/b,
4a/b, and 5a/b in acceptable yields (Scheme 2). 1H NMR
spectroscopy showed that in each case the major product was
the meta isomer, as initially found with the ethynyltoluene
compounds 2a/b, but the less polar para isomers were also
isolated in most cases. The 1H NMR spectra (see Figure 1b-d)
also established the nature of each isomer. When the less
thermally stable 1-ethynylpyrene30 and 3-ethynylperylene31
derivatives were used in the reverse Diels-Alder condensation,
the yields were poor and only the meta isomers 6 and 7 were
isolated, with traces of the para isomers only detected by thin-
layer chromatography. Interestingly, by using 4-ethynyl-N,N-
dibutylaminophenyl32 the condensation provided the meta-
isomer 8a in relatively good yield and the para isomer 8b in
modest yield.
8.1 Hz, υ0δ ) 12.7 Hz) for H4, H5 of the meta isomer 2a,
whereas for the para isomer 2b, H3 and H5 are split into two
simple doublets (4J ) 1.5 Hz) at 8.50 and 8.09 ppm. Another
key feature is that the doublet at 8.61 ppm assigned to H3′ on
the outer pyridine ring of the para isomer 2b is shielded by
∼1.15 ppm for the meta isomer 2a, probably influenced by the
shielding cone of the tolyl ring. These differences, shown for
selected cases in Figure 1, are systematically observed for the
series of molecules depicted in Scheme 2.
To confirm the isomer assignments, the two first ligands were
characterized by single-crystal X-ray diffraction (Figure 2).
Briefly, in the solid state the nitrogen atoms of the bipyridine
moiety lie in a transoid position that minimizes repulsive
interactions between the neighboring lone pairs, a situation
frequently found in oligopyridine ligands.24 However, while the
two pyridine rings of the para isomer 2b are almost planar (tilt
angle of 5.35°), they are strongly tilted (by 56.94°) in the meta
isomer 2a as a consequence of the steric crowding induced by
the tolyl fragment. In line with this hypothesis is the large 53.90°
tilt angle of the tolyl relative to the central pyridine ring in the
meta isomer 2a, compared to the 30.92° tilt angle found in the
para structure 2b.
To further explore the scope of this synthesis, we examined
the reactivity of some other alkynes bearing electron-deficient
or bulky substituents on the remote end of the alkyne (Scheme
2). With electron-withdrawing fragments, the reaction proceeded
very slowly and no condensation occurred with 2-ethynyl-4,6-
dimethoxy-1,3,5-triazine.33 Here, this was due to the instability
of the dienophile. When 4-ethynyl-2,2′:6′,2′′-terpyridine,34 3,3-
dimethylbut-1-yne, or 1-dimethylamino-2-propyne was con-
densed at high temperature with 6-phenyl-2-pyridyl-1,3,4-
triazine, the reaction proceeded smoothly and afforded exclusively
the para isomers 9, 10, and 11, respectively.
To explore the limits of this inverse Diels-Alder reaction
we initially chose a variety of acetylenic derivatives such as
4-ethynyltrialkoxygallate,25 2-ethynyl-9,9-dimethylfluorene,26
and 3-ethynyl-9-methylcarbazole27 because (i) these compounds
could be efficiently prepared by Sonogashira protocols, (ii) they
are electron-rich dienophiles and thermally stable, and (iii) the
resulting compounds 3a/b to 5a/b might find useful applications
as liquid crystalline materials28 and optically tunable light
(28) Camerel, F.; Bonardi, L.; Schmutz, M.; Ziessel, R. J. Am. Chem.
Soc. 2006, 128, 4548.
(29) Montes, V. A.; Pohl, R.; Shinar, J.; Anzenbacher, P., Jr. Chem. Eur.
J. 2006, 12, 4523.
(24) Drew, M. G.; Hudson, M. J.; Iveson, P. B.; Russell, M. L.; Liljenzin,
J.-O.; Skalberg, M.; Spjuth, L.; Madic, C. J. Chem. Soc., Dalton Trans.
1998, 2973.
(25) Wu, J.; Watson, M.; Zhang, L.; Wang, Z.; Mu¨llen, K. J. Am. Chem.
Soc. 2004, 126, 177.
(30) Hissler, M.; Harriman, A.; Khatyr, A.; Ziessel, R. Chem. Eur. J.
1999, 5, 3366.
(31) Inouye, M.; Hyodo, Y.; Nakazumi, H. J. Org. Chem. 1999, 64, 2704.
(32) Traber, B.; Wolff, J.; Rominger, F.; Oeser, T.; Gleiter, R.; Wortmann,
R. Chem. Eur. J. 2004, 10, 1227.
(26) Yu, X.-F.; Lu, S.; Ye, C.; Li, T.; Liu, T.; Liu, S.; Fan, Q.; Chen, E.
Q.; Huang, V. Macromolecules 2006, 39, 1364.
(27) Sakamki, K.; Ohshita, J.; Kunai, A.; Nakao, H.; Adachi, H.; Okita,
K. Appl. Organomet. Chem. 2001, 15, 604.
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8419.
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62, 1491.
J. Org. Chem, Vol. 72, No. 26, 2007 10183

Diring et al.
FIGURE 1. 1H NMR spectra (aromatic resonances only) recorded in CD3OD for (a) 2a and 2b and in CDCl3 for (b) 3a and 3b, (c) 4a and 4b,
and (d) 5a and 5b.
10184 J. Org. Chem., Vol. 72, No. 26, 2007

Synthesis of Arylated Bipyridine Ligands
FIGURE 2. ORTEP view of compounds 2a (left) and 2b (right) (50% probability displacement ellipsoids), with all hydrogen atoms removed for
the sake of clarity.
SCHEME 2
In terms of mechanism, we hypothesize that the meta isomer
predominates due to favorable π-π interactions in the transition
state between the ethynylaryl and the external pyridine rings
(Figure 3). In the absence of such a stabilizing interaction, the
para isomer should be favored for steric reasons. In keeping
with such a hypothesis is the fact that the bulky 4-ethynyl-2,2′:
6′,2′′-terpyridine, and the nonaromatic 3,3′-dimethylbutyne and
1-dimethylamino-2-propyne substrates reacted with the triazine
1 to give exclusively the para isomers. This can be understood
assuming that the reaction partners adopt an orientation in the
transition state that minimizes unfavorable steric interaction
between the bulky terpyridine or tert-butyl fragment and the
pyridine substituent. The absence of favorable aromatic interac-
tions might also be in favor for the formation of the less
encumbered para isomer.
Interestingly, 1-dimethylamino-2-propyne underwent smooth
condensation to afford the para isomer 11 in acceptable yields
(Scheme 2). This result can be attributed to the fact that this
J. Org. Chem, Vol. 72, No. 26, 2007 10185

Diring et al.
FIGURE 3. Schematic representation of (a) the bimolecular contact between the triazine and ethynyl derivative and (b) the transition state.
(Representation obtained with energy minimization in Chem3D.)
SCHEME 3
stable and non-encumbered alkyne can easily approach the 2,5
positions of the triazine to induce the reverse-Diels-Alder
condensation. Such regioselectivity has previously been ob-
served with stannane derivatives.21
2b, the nitrogen atoms of the bipyridine moiety of 13 lie in a
transoid position, although the bipyridine unit is far from planar,
Subsequent work was focused on the use of soluble diacety-
lene derivatives, disubstituted acetylene derivatives, and benzyne
in the retro-Diels-Alder reaction. The use of 1,4-diethynyl-
2,5-dibutoxyphenyl35 produced a mixture of two major isomers,
the meta/meta derivative 12a and the meta/para derivative 12b
(Scheme 3). The para/para compound was present in very low
quantities and could not be thoroughly characterized. The
condensation of 1,2-di(4-N,N-dibutylaminophenyl)ethyne36 with
6-phenyl-2-pyridyl-1,3,4-triazine produced ligand 13 in 57%
yield. A similar reaction of benzyne prepared in situ from
anthranilic acid and isoamyl nitrite37 gave compound 14 in 20%
isolated yield.
The molecular structure of 13 was established by single-
crystal X-ray diffraction (Figure 4). As in compounds 2a and
FIGURE 4. ORTEP view of compound 13 (30% probability displace-
ment ellipsoids), with all hydrogen atoms removed for the sake of
clarity.
(35) Ziessel, R.; Diring, S.; Retailleau, P. Dalton Trans. 2006, 3285.
(36) Umezawa, H.; Okada, S.; Oikawa, S.; Matsuda, H.; Nakanishi, H.
J. Phys. Org. Chem. 2005, 18, 468.
10186 J. Org. Chem., Vol. 72, No. 26, 2007

Synthesis of Arylated Bipyridine Ligands
SCHEME 4
with a ring dihedral angle of 66.4°. This is presumably a
consequence of the proximity of a dibutylaminophenyl sub-
stituent on the tetrafunctionalized pyridine to the simple pyridine
substituent. The phenyl ring is also strongly tilted (dihedral angle
38.8°) with respect to the pyridine ring, this facilitating
aromatic-aromatic (CH‚‚‚π) interactions with both a tolyl group
(at 1 -x, -y, 2 - z) and a pyridine ring (at x, 1/2 - y, 1/2 + z)
of adjacent molecules. Tilting of both dibutylaminophenyl rings
with respect to the pyridine (dihedral angles of 49.6 (meta) and
48.7° (para)) is associated with aggregation of the butyl chains
within the lattice.
Condensation of the triazine 1 with tri-n-butyl(ethynyl)tin
provides compound 15a in 76% yield. Conversion of the
stannane derivative 15a to the iodo derivative 15b is straight-
forward by reaction with iodine in chloroform (89%). 15b is a
valuable intermediate for the preparation of various ethynyl-
substituted platforms such as 16, 17, and 18. By employing a
standard protocol ([Pd(PPh3)2Cl2] 6 mol % and CuI 10 mol %)
at rt, the stable cross-coupled compounds 19 to 22 (Scheme 4)
were isolated in acceptable yields (70%, 77%, 82%, and 39%,
respectively).
Unfortunately, attempts to complex ligand 20 with [Pt-
(DMSO)2Cl2]38 or K2PtCl4 in water/acetonitrile or water/THF
were unsuccessful and achieved only in very low yield for 19.
This unfavorable result is likely due to the inductive electron-
donating character of the ethynyl residue,39 rendering the phenyl
groups less acidic and more difficult to deprotonate by ortho-
metallation to the aromatic residue. Fortunately, the iodo-
containing ligand 15b afforded the desired complex 23 by
complexation with K2PtCl4 in water/acetonitrile despite our
concern that the aryl-iodo fragment might promote competitive
side reactions (Scheme 5). With this pivotal complex 23 in hand,
we were pleased to find it possible to selectively substitute the
platinum-chloro ligand by ethynyltolyl and the ethynyl derivative
16, leading respectively to the stable complexes 24 and 25 in
respectable yields (Scheme 5). This reaction took place smoothly
in the presence of CuI (6 mol %) at rt under anaerobic conditions
by using triethylamine to quench the nascent acid. In a further
demonstration of the versatility of these procedures, the iodo
group was found to be easily substituted by the ethynyl
derivative 16 under Sonogashira conditions, providing the highly
functionalized and unsymmetrically substituted platinum com-
plex 27. As well, simultaneous cross-coupling of both the iodo
and chloro groups could be performed by using [Pd(PPh3)4] and
ethynyltoluene (2 equiv) under anaerobic conditions and in the
absence of Cu(I), leading to the symmetrically substituted
derivative 26.
To characterize ligand 15b in its bound form and to establish
various substituent orientations in these Pt(II) complexes, single-
crystal structure determinations were performed on compounds
23 and 24 (Figure 5).
These studies revealed a distorted square-planar geometry of
the PtII center, with the tridentate ligand quasi-planar in all
cases.40 The Pt-C bond lengths are not unusual, although that
in 23 is significantly longer than those of 24. The platinum-
nitrogen bond lengths trans to the phenyl group (2.062-2.131
Å) are markedly longer than those trans to the chlorine or the
ethynyltoluyl ligands (1.941-1.966 Å). In the Pt-ethynyl-tolyl
derivative structure 24, the tolyl group is tilted out of the plane
of the Pt core by 76.3° but is quasi-parallel to the bc plane of
the unit cell, whereas the N2-Pt-C17/C18-C19 axe lie parallel
to the plane (1, 0, -1), with an aromatic ring plane separation
of ∼3.4 Å being indicative of π-π stacking.
Optical Properties. The absorption spectra (Figure 6) display
several well-resolved absorption bands in the 230-420 nm range
attributed to π-π* transitions of the phenyl and pyridine rings
and n-π transitions due to the presence of heteroatoms in the
framework.41,42 In the carbazole cases 5a/b the low-energy
absorption bands are bathochromically shifted by 22 nm with
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J. Org. Chem, Vol. 72, No. 26, 2007 10187

Diring et al.
SCHEME 5
respect to the fluorene molecules 4a/b (Table 1). This shift to
low energy is enhanced when a triple bond is present in the
para position of the dibutylaminophenyl-substituted compounds
(compare compounds 21 and 8b, Figure 6b). This effect reflects
a better delocalization in the ground state between the acceptor
fragment (bipyridine subunit) and the electron-donating frag-
ments such as N-methylcarbazole and N,N-dibutylaminophenyl.
This delocalization is clearly hindered when the carbazole or
phenyl rings are directly linked to the central pyridine ring in
the meta and para regioisomers due to steric effects responsible
for the twist of the phenyl ring with respect to the mean plane
(see X-ray structures in Figures 2 and 4). No particular
absorption differences are observed between the meta and the
para isomers within the same series. Note that when two
dibutylamino groups are present in compound 13, a 20 nm
hypsochromic shift is observed (Figure 6b), possibly due to the
destabilization of the S0 f S1 transition. In the isoquinoline
compounds 14 two absorption bands at 311 and 242 nm are
clearly evident, in keeping with previous data obtained on
similar ligands.43
for the meta versus the para isomers (Figure 7 and Table 1).
This is a general trend, the meta regioisomer always fluoresces
at lower energy than the para regioisomer and the latter always
exhibits a much higher fluorescence quantum yield versus the
meta. For instance, in the tolyl case 2b (para isomer) the
quantum yield reaches 38% whereas the meta case 2a is not
fluorescent at all. The excitation spectra measured at the lower
emission band match perfectly the absorption spectra, meaning
in each case that the fluorescence originates from a single
excited state (Figure 7).
With the exception of the pyrene and perylene cases, the
fluorescence spectra show very poor mirror symmetry with the
lowest energy absorption band and they indeed look very
different. For all the compounds, the emission spectra are similar
in shape and significantly red-shifted compared to the absorption
spectra, indicating a strong Stokes shift, which suggests a
reorganization occurring in the excited state (Table 1). The
presence of a triplet excited state is excluded on the basis of
the absence of any oxygen effect in the steady-state emission
and quantum yields. The very short excited state lifetimes that
were measured in some cases (vide infra) are in keeping with
such a statement.37 In fact, the broad and structureless absorption
bands are reminiscent of charge transfer (CT) transitions and it
is noteworthy that these ligands contain electron-donating groups
Most of these novel ligands are luminescent in solution at rt
with fluorescence maxima progressively shifted to lower energy
(43) Anton, M. F.; Moomaw, W. R. J. Chem. Phys. 1977, 66, 1808.
(44) Olmsted, J. J. Phys. Chem. 1979, 83, 2581.
10188 J. Org. Chem., Vol. 72, No. 26, 2007

Synthesis of Arylated Bipyridine Ligands
TABLE 1. Spectroscopic Data for the Compounds at 298 Ka
λabs
(nm)
ꢀ
λF
Stokes’
fwhm
b
c
compd
(M-1 cm-1
)
(nm) shift (cm-1
)
(cm-1
)
ΦF
2a
2b
3a
3b
4a
4b
5a
5b
6
289
264
306
285
318
318
319
320
349
450
350
356
315
283
304 sh
338 sh
328 sh
332
312
310
323
378
332
22900
43300
21000
29500
33000
41000
20500
19000
40100
34500
21900
28200
15100
15000
10600
17600
18500
28600
7400
362
451
435
410
372
438
402
417
470
487
461
374
346
358
422
424
468
429
357
440
480
391
4427
10507
9584
7056
4564
8516
6374
4672
945
8037
6397
5008
4101
4961
5889
6902
8752
8741
4246
8232
5621
4545
3333
4156
4476
4551
3580
4507
3308
3732
2196
3644
3443
3285
3869
3890
3701
3339
3492
4481
3234
3905
3326
3427
38
4.1
34.6
1.7
66
4.7
53
1.5
37
7.4
31.1
17
3.5
1.8
<1
5.9
<1
5.5
41.5
43
7
8a
8b
9
10
11
12a
12b
13
14
19
20
21
22
FIGURE 5. ORTEP view of compounds 23 and 24 (50% probability
displacement ellipsoids), with all hydrogen atoms removed for the sake
of clarity. Selected distances (Å) and angles (deg) for 23: Pt-N(1)
1.941, Pt-N(2) 2.062, Pt-C(16) 2.088, Pt-Cl 2.304, N(1)-Pt-N(2)
80.89, N(1)-Pt-C(16) 160.65, N(2)-Pt-Cl-179.15. Selected distances
(Å) and angles (deg) for 24: Pt-N(1) 2.093, Pt-N(2) 1.980, Pt-C(1)
1.983, Pt-C(17) 1.985, C(17)-C(18) 1.182, N(1)-Pt-N(2) 79.02,
N(1)-Pt-C(1) 160.83, N(2)-Pt-C(17) 175.75.
33000
31000
42700
7900
28.4
19
a Determined in dichloromethane solution. b fwhm measurements. c De-
termined in aerated dichloromethane solution (ca 10-6 M) with quinine
bisulfate dihydrate in 0.5 M H2SO4 solution as standard, which absorbs at
366 nm and emits at 456 nm with a known quantum yield of ΦF ) 0.546.43
All ΦF are corrected for changes in refractive index.
emitting state. The involvement of CT states is further confirmed
by studying the influence of solvent (Figure S2, Supporting
Information).
For the pyrene, perylene, and quinoline cases, the situation
looks quite different (Figure 8). The fluorescence spectrum
shows excellent mirror symmetry with the lowest energy
absorption and, in fact, this is confirmed by excitation spectra
obtained under similar experimental conditions. On this basis,
we assign the absorption and fluorescence peaks to spin-allowed
π,π* transitions. The lowest energy singlet state S0 f S1 is
responsible for this intense fluorescence emission band without
any perturbation of a CT state. In fact, the fluorescence of
compounds 6, 7, and 14 is dominated by the emission of the
pyrene, perylene, and isoquinoline fragments.
As would be expected from singlet excited states mixed with
some charge-transfer character, the excited state lifetimes lie
within the nanosecond time scale. Indeed, the fluorescence decay
profiles could be described by a single-exponential fit, with
fluorescence lifetimes in the range of 1 to 10 ns, in accordance
with a singlet excited state (Table 2). In fact, most of the
radiative rate constants lie within the 3 to 6 × 10-7 s-1 range
(Table 2). However, the nonradiative rate constants are signifi-
cantly higher than the radiative rate constants, mostly because
of the short excited state lifetime, which may be attributed to
the interaction of the emitting state with an energetically low-
lying localized state. At this stage of our investigation, we do
not have evidence for the formation of a triplet excited state
that could quench the fluorescence.
FIGURE 6. Absorption spectra of (a) 4a, 4b, 5a, and 5b and (b) 8a,
8b, 13, and 21 measured in CH2Cl2 at rt, at ca. 3.10-5 M.
(the alkyl and alkoxy chain, the tertiary amines, ...) and electron-
accepting fragments such as the bipyridine and the acetylenic
subunits. The latter groups promote charge transfer along the
molecular axis.45,46 The large Stokes shifts of 1000 to 10000
cm-1 are in keeping with a CT state and not a pure π-π*
Electrochemical data for the di-n-butylamino grafted mol-
ecules, derived from cyclic voltammetry in dichloromethane,
are gathered in Table 2. The prototypical p-toluyl molecules
2a and 2b are inactive within the electrochemical window used
(+1.8 V to -2.2 V versus SCE). As expected, ligands 8a, 8b,
and 21 display a reversible oxidation wave around +0.8 V,
(45) Harriman, A.; Rostron, S. A.; Khatyr, A.; Ziessel, R. Faraday
Discuss. 2006, 131, 377.
(46) Khatyr, A.; Ziessel, R. J. Org. Chem. 2000, 65, 7814.
n
results in keeping with the oxidation of Bu3N occurring at
J. Org. Chem, Vol. 72, No. 26, 2007 10189

Diring et al.
FIGURE 7. Normalized emission and excitation spectra measured in CH2Cl2 at rt: (a) 3a, 3b, and 20, (b) 8a, 8b, and 21, (c) 4a and 4b, and (d)
5a and 5b.
FIGURE 8. Normalized absorption (blue), excitation (green), and emission (red) spectra in CH2Cl2 of 6 (left) and 7 (right).
TABLE 2. Spectroscopic Data in Dichloromethane Solution at 298 K
E°(L+•/L) (V)
(∆Ep (mV)d)
E°(L+•/L*)
a
b
c
λabs
(nm)
ꢀ
λF
(nm)
ΦF
τF
(ns)
krb
knr
∆GES
(eV)
compds
(M-1 cm-1
)
(%)
(108 s-1
)
(108 s-1
)
(eV)e
8a
8b
21
350
356
378
21,900
28,200
42,700
487
461
480
7.4
31.1
28.4
2.6
6.3
4.7
28.5
49.4
60.4
3.6
1.1
1.5
2.92
3.00
2.91
+ 0.73 (70)
+ 0.83 (60)
+ 0.79 (75)
-2.19
-2.17
-2.12
a Determined in undegassed dichloromethane solution (ca. 10-6 M) with quinine bisulfate dihydrate in 0.5 M H2SO4 solution as standard, which absorbs
at 347 nm and emits at 448 nm with a known quantum yield of ΦF ) 0.546.44 All ΦF are corrected for changes in refractive index. b Calculated by using
the following equations: kr ) ΦF/τF, knr ) (1 - ΦF)/τF, assuming that the emitting state is produced with unit quantum efficiency. c Excited state energies
were estimated ((5%) by drawing a tangent on the high-energy side of the emission; λ (nm) × E (eV) ) 1239.8. d Potentials determined by cyclic voltammetry
in deoxygenated CH2Cl2 solution, containing 0.1 M TBAPF6, at a solute concentration of ca. 1 mM and at 20 °C. Potentials were standardized versus
ferrocene (Fc) as internal reference and converted to the SCE scale assuming that E1/2(Fc/Fc+) ) +0.38 V (∆Ep ) 70 mV) vs SCE. The error in half-wave
potentials is (10 mV. e Calculated excited state oxidation potential vs SCE, E°(L+/L*) ) E°(L+/L) - ∆GES. L accounts for the ligands
+0.69 V versus SCE.47 Note that di-n-butylamino derivatives
are more difficult to oxidize by 100 mV compared to the tertiary
amine. The meta regioisomer 8a is easier to oxidize than the
para isomer by 100 mV likely due to a better stabilization of
the radical cation generated after single electron oxidation.
Unlike certain oligopyridine adducts,48 there is no indication
of the reduction of the bipyridine-phenyl fragment within the
given electrochemical window.
10190 J. Org. Chem., Vol. 72, No. 26, 2007

Synthesis of Arylated Bipyridine Ligands
General Procedure 3 for the Preparation of Platinum
Complexes. The appropriate 6-phenyl-2,2′-bipyridine ligand and
K2PtCl4 were dissolved in a mixture of acetonitrile-water (v/v 2/1).
The mixture was refluxed for 18 to 24 h. The resulting orange
precipitate was centrifuged and successively washed with water,
acetonitrile, and diethyl ether. Single crystals for X-ray diffraction
were obtained by vapor diffusion of diethyl ether in a dimethyl
sulfoxide solution containing the chloroplatinum complex.
General Procedure 4 for the Copper-Promoted Coupling
Reaction. The chloroplatinum complex and the ethynyl derivative
were dissolved in the appropriate solvent (either CH2Cl2, DMF, or
CH2Cl2/DMF). Triethylamine was added and the mixture was
degassed by bubbling argon for 30 min, after which CuI was added
and the reaction mixture was stirred in the dark at room temperature
for 1 to 3 days. The residue was treated with water and extracted
with dichloromethane. The organic extracts were washed with water,
then with saturated brine and filtered through hygroscopic cotton
wool. The solvent was removed by rotary evaporation and the
residue was purified by column chromatography.
As all excited state energies lie in the 2.91-3.00 eV range,
calculation of the redox potential of the first excited states from
the measured oxidation potentials enables the conclusion to be
drawn that these excited states are strong reductants (Table 2).
Concluding Remarks
An interesting facet of this chemistry is the fact that various
highly functionalized polypyridine ligands suitable for transition
metal complexation can be easily prepared. The reverse Diels-
Alder reaction, effective with various ethynyl-substituted deriva-
tives, is regioselective when sterically hindered ethynyl sub-
strates are transformed. The introduction of functionalized
ethynyl groups opens the way to a new class of C∧N∧N-
tridendate ligands bearing various functionalities, some of which
include solubilizing chains, additional chromophoric residues,
and easily transformable functions. This methodology accom-
modates various functional groups and affords the anticipated
substituted aromatics in good to excellent yields. Unfortunately,
the ligands bearing ethynyl-substituted groups attached to the
central pyridine ring do not provide platinum complexes.
Fortunately, with the iodo function present on the ligand,
platinum complexation and cross-coupling with various ethynyl-
grafted residues is feasible and allows the introduction of
different functional groups on the central pyridine ring. Of
particular interest is the (iodo-ligand)-Pt-Cl complex for which
orthogonal substitution of the iodo or the chloro substituent can
be achieved by using the appropriate catalysts. In particular, it
can be used in the synthesis of hybrid complexes bearing the
donor or the acceptor at a predetermined position. The resulting
ligands display intriguing optical properties such as strong
absorption in the visible and strong fluorescence at rt due to a
mixed emissive state with a pronounced charge-transfer char-
acter. The use of these ligands to synthesize multimetallic
complexes pre-organized at different places and carrying various
redox and optical properties is currently in progress in our
laboratory.
3-(p-Tolyl)-6-phenyl-2,2′-bipyridine (2a): prepared, using pro-
cedure 1, from 6-phenyl-2-pyridyl-1,3,4-triazine 1 (300 mg, 1.28
mmol), p-tolylacetylene (330 µL, 2.70 mmol), and 1,2-dichloroben-
zene (1 mL); chromatography on silica gel eluting with petroleum
ether to dichloromethane to give 173 mg (42%) of 2a as a white
solid after precipitation in pentane; 1H NMR (CD3OD, 300 MHz)
δ 8.47 (ddd, 1H, J ) 4.9 Hz, 1.7 Hz, 0.9 Hz), 8.14-8.11 (m, 2H),
3
7.98 (ABsys, 2H, JAB ) 8.1 Hz, υ0δ ) 12.7 Hz), 7.78 (td, 1H, J
4
) 7.7 Hz, J ) 1.8 Hz), 7.52-7.40 (m, 4H), 7.36 (ddd, 1H, J )
7.6 Hz, 5.0 Hz, 1.2 Hz), 7.10-7.03 (m, 4H), 2.30 (s, 3H); 13C NMR
(CDCl3, 75 MHz) δ 158.9, 155.7, 155.6, 148.9, 139.4, 139.0, 136.7,
136.6, 136.0, 135.0, 129.2, 128.9, 128.7, 127.1, 124.9, 122.4, 119.8,
21.2; UV-vis (CH2Cl2) λ (nm) (ꢀ, M-1 cm-1) 288 (22800), 277
(22800); IR (KBr, cm-1) ν 3053 (w), 3024 (w), 2915 (w), 2854
(w), 1580 (m), 1565 (s), 1452 (s), 1423 (s), 1207 (w), 1092 (m),
804 (s), 756 (s); FAB+ m/z (nature of the peak, rel intensity) 323.1
([M + H]+, 100). Anal. Calcd for C23H18N2: C, 85.68; H, 5.63; N,
8.69. Found: C, 85.52; H, 5.32; N, 8.40.
4-(p-Tolyl)-6-phenyl-2,2′-bipyridine (2b): 55 mg (13%) of 2b
1
as a white solid after precipitation in pentane; H NMR (CD3OD,
3
300 MHz) δ 8.69-8.66 (m, 1H), 8.61 (dt, 1H, J ) 8.0 Hz, 1.0
4
Hz), 8.50 (d, 1H, J ) 1.5 Hz), 8.24-8.20 (m, 2H), 8.09 (d, 1H,
4J ) 1.5 Hz), 7.98 (td, 1H, 3J ) 7.7 Hz, 4J ) 1.8 Hz), 7.78 (d, 2H,
3J ) 8.3 Hz), 7.56-7.43 (m, 4H), 7.36 (d, 2H, 3J ) 7.9 Hz), 2.42
(s, 3H); 13C NMR (CDCl3, 75 MHz) δ 157.1, 156.5, 156.2, 150.1,
149.1, 139.6, 139.1, 136.8, 135.8, 129.7, 129.0, 128.8, 127.1, 123.8,
Experimental Section
General Procedure 1 for the Diels-Alder Reaction. A Schlenk
tube was charged with 6-phenyl-2-pyridyl-1,3,4-triazine 1, the
appropriate ethynyl compound, and o-dichlorobenzene. The mixture
was argon-degassed via at least three freeze-pump-thaw cycles,
heated to 180 °C, and finally stirred in the dark for 1 to 3 days.
The solvent was removed under high vacuum. The residue was
treated with water and extracted with dichloromethane. The organic
extracts were washed with water, then with saturated brine and
filtered through cotton wool. The solvent was removed by rotary
evaporation and the residue was purified by column chromatogra-
phy.
General Procedure 2 for the Sonogashira Cross-Coupling
Reaction. A Schlenk flask was charged with 4-iodo-6-phenyl-2,2′-
bipyridine, the appropriate ethynyl derivative, [Pd(PPh3)2Cl2] (10%
mol), tetrahydrofuran, and diisopropylamine (v/v, 7/1), except for
22 where triethylamine was used. The mixture was argon-degassed
for 30 min, then CuI (10 mol %) was introduced and the reaction
mixture stirred a room temperature for 18 h. The solvent was
evaporated, and the residue was treated with water and extracted
with dichloromethane. The organic extracts were washed with water,
then with saturated brine and filtered through hygroscopic cotton
wool. After rotary evaporation, the residue was purified by column
chromatography.
121.5, 118.3, 117.3, 21.3; UV-vis (CH2Cl2) λ (nm) (ꢀ, M-1 cm-1
)
311 (9500), 278 sh (37500), 264 (43300); IR (KBr, cm-1) ν 3034
(w), 2920 (w), 1598 (m), 1565 (s), 1470 (m), 1387 (s), 816 (s),
768 (s); FAB+ m/z (nature of the peak, rel intensity) 323.1 ([M +
H]+, 100). Anal. Calcd for C23H18N2: C, 85.68; H, 5.63; N, 8.69.
Found: C, 85.47; H, 5.47; N, 8.52.
3-(Perylen-3-yl)-6-phenyl-2,2′-bipyridine (7): prepared, using
procedure 1, from 5-phenyl-3-(2-pyridyl)-1,3,4-triazine (85 mg, 0.36
mmol), 3-ethynylperylene (100 mg, 0.36 mmol), and 1,2-dichlo-
robenzene (1 mL); chromatography on alumina eluting with
dichloromethane-petroleum ether (v/v 25/75 to 50/50), second
chromatography on silica gel eluting with dichloromethane to give
15 mg (9%) of 7 as an orange to brown powder; 1H NMR (CDCl3,
400 MHz) δ 8.33-8.31 (m, 1H), 8.22-8.13 (m, 6H), 7.92 (ABsys,
3
2H, JAB ) 8.0 Hz, υ0δ ) 6.8 Hz), 7.69 (d, 2H, J ) 8.0 Hz), 7.6
(d, 1H, 3J ) 8.0 Hz), 7.55-7.44 (m, 7H), 7.34-7.29 (m, 2H), 6.99
(ddd, 1H, J ) 7.5 Hz, 5.0 HZ, 1.0 Hz); 13C NMR (CDCl3, 100
MHz) δ 158.2, 156.6, 156.5, 149.0, 140.8, 139.1, 137.7, 135.9,
134.9, 133.7, 133.1, 131.5, 131.4, 131.3, 130.9, 129.3, 128.9, 128.7,
128.6, 128.0, 127.3, 126.8, 126.7, 125.9, 124.3, 122.5, 120.5, 120.4,
120.0, 119.7; UV-vis (CH2Cl2) λ (nm) (ꢀ, M-1 cm-1) 450 (34500),
423 (27000), 401 (13000), 293 sh (13100), 257 (48500); IR (KBr,
cm-1) ν 3049 (w), 1585 (m), 1563 (m), 1461 (m), 1420 (m), 1386
(m), 810 (s), 748 (s); FAB+ m/z (nature of the peak, rel intensity)
(47) Mann, C. K. Anal. Chem. 1961, 36, 2424.
(48) Goeb, S.; De Nicola, A.; Ziessel, R. J. Org. Chem. 2005, 70, 1518.
J. Org. Chem, Vol. 72, No. 26, 2007 10191

Diring et al.
plates; H NMR (CDCl3, 400 MHz) δ 8.71 (dt, 1H, J ) 3.8 Hz,
1
483.1 ([M + H]+, 100). Anal. Calcd for C36H22N2: C, 89.60; H,
4.60; N, 5.81. Found: C, 89.47; H, 4.38; N, 5.59.
3
4
1.0 Hz), 8.65 (d, 1H, J ) 8.0 Hz), 8.50 (d, 1H, J ) 1.5 Hz),
8.18-8.16 (m, 2H), 7.88-7.84 (m, 2H), 7.54-7.44 (m, 3H), 7.34
(ddd, 1H, J ) 7.4 Hz, 4.9 Hz, 1.1 Hz), 6.80 (s, 2H), 4.03-3.99
(m, 6H), 1.87-1.73 (m, 6H), 1.51-1.46 (m, 54H), 0.90-0.87 (m,
9H); 13C NMR (CDCl3, 100 MHz) δ 156.8, 156.0, 153.2, 149.2,
139.9, 139.0, 137.1, 133.4, 129.4, 128.9, 127.1, 124.1, 121.9, 121.5,
121.4, 116.8, 110.6, 94.3 (CtC), 86.5 (CtC), 73.7, 69.3, 32.1,
32.0, 30.5, 29.9, 29.8, 29.7, 29.6, 29.5, 29.4, 26.2, 22.8, 14.3; UV-
vis (CH2Cl2) λ (nm) (ꢀ, M-1 cm-1) 323 (31000), 286 (27300), 266
(31600), 254 (32900), 240 (35400); IR (KBr, cm-1) ν 3054 (w),
2919 (s), 2849 (s), 2208 (m, CtC), 1578 (m), 1500 (m), 1470 (m),
1394 (m), 1327 (m), 1239 (s), 1118 (s), 826 (m), 772 (m), 726
(m); FAB+ m/z (nature of the peak, rel intensity) 886.0 ([M + H]+,
100). Anal. Calcd for C60H88N2O3: C, 81.40; H, 10.02; N, 3.16.
Found: C, 81.67; H, 10.27; N, 3.33.
4-(tert-Butyl)-6-phenyl-2,2′-bipyridine (10): prepared, using
procedure 1, from 5-phenyl-3-(2-pyridyl)-1,3,4-triazine (200 mg,
0.85 mmol), 3,3-dimethylbut-1-yne (315 µL, 2.55 mmol), and 1,2-
dichlorobenzene (1 mL); chromatography on silica gel eluting with
1
dichloromethane to give 30 mg (12%) of 10 as a white solid; H
NMR (CDCl3, 400 MHz) δ 8.72-8.70 (m, 1H), 8.65 (d, 1H, 3J )
8.0 Hz), 8.44 (d, 1H, 4J ) 2.0 Hz), 8.15 (d, 2H, 3J ) 7.0 Hz), 7.83
3
4
4
(td, 1H, J ) 7.6 Hz, J ) 1.7 Hz), 7.78 (d, 1H, J ) 2.0 Hz),
7.54-7.42 (m, 3H), 7.31 (ddd, 1H, J ) 7.4 Hz, 4.9 Hz, 1.1 Hz),
1.46 (s, 9H); 13C NMR (CDCl3, 100 MHz) δ 161.9, 156.9, 156.8,
155.8, 149.1, 140.1, 136.9, 128.9, 128.8, 127.2, 123.7, 121.7, 117.8,
116.6, 35.4, 30.9; UV-vis (CH2Cl2) λ (nm) (ꢀ, M-1 cm-1) 306 sh
(11000), 283 (14900), 260 (19000), 248 sh (22000), 241 (23500);
IR (KBr, cm-1) ν 3064 (w), 2960 (m), 2867 (w), 1600 (m), 1583
(m), 1542 (m), 1400 (m), 1247 (m), 1026 (m), 875 (m), 773 (s),
694 (s); FAB+ m/z (nature of the peak, rel intensity) 289.1 ([M +
H]+, 100). Anal. Calcd for C20H20N2: C, 83.30; H, 6.99; N, 9.71.
Found: C, 82.99; H, 6.47; N, 9.52.
(4-Iodo-6-phenyl-2,2′-bipyridine)chloroplatinum (23): pre-
pared, using procedure 3, from 4-iodo-6-phenyl-2,2′-bipyridine 15b
(200 mg, 056 mmol), K2PtCl4 (255 mg, 0.61 mmol), acetonitrile
(16 mL), and H2O (8 mL); 229 mg (70%) of 23 obtained as an
orange powder; 1H NMR ((CD3)2SO), 400 MHz) δ 8.81 (d, 1H, 3J
) 4.0 Hz), 8.55-8.50 (m, 2H), 8.32-8.24 (m, 2H), 8.88 (t, 1H, 3J
) 6.5 Hz), 7.58 (d, 1H, 3J ) 7.5 Hz), 7.43 (d with 195Pt satellites,
4-(N,N-Dimethylaminomethyl)-6-phenyl-2,2′-bipyridine (11):
prepared, using procedure 1, from 5-phenyl-3-(2-pyridyl)-1,3,4-
triazine (200 mg, 0.85 mmol), N,N-dimethylprop-2-yn-1-amine (200
µL, 1.85 mmol), and 1,2-dichlorobenzene (1 mL); chromatography
on aluminum oxide eluting with dichloromethane-petroleum ether
(v/v 50/50 to 75/25) to give 64 mg (26%) of 11 as a light yellow
solid; 1H NMR (CDCl3, 400 MHz) δ 8.69 (dt, 1H, J ) 4.0 Hz, 1.0
3
3
4
1H, J ) 7.5 Hz), 7.14 (td, 1H, J ) 7.3 Hz, J ) 1.3 Hz), 7.05
(td, 1H, J ) 7.5 Hz, J ) 1.0 Hz); 13C NMR ((CD3)2SO, 100
MHz) δ 165.2, 155.6, 154.6, 148.1, 145.8, 142.7, 140.4, 134.3,
130.6, 128.5, 127.9, 127.8, 125.3, 124.3, 123.9, 109.1; UV-vis
(CH2Cl2) λ (nm) (ꢀ, M-1 cm-1) 438 (3500), 369 (7700), 334
3
4
3
Hz), 8.65 (d, 1H, J ) 8.0 Hz), 8.30 (s, 1H), 8.20-8.17 (m, 2H),
(15200), 283 (35600), 270 sh (27800), 243 (30200); IR (KBr, cm-1
)
3
4
7.84 (td, 1H, J ) 7.8 Hz, J ) 2.0 Hz), 7.83 (s, 1H), 7.52-7.41
(m, 3H), 7.31 (ddd, 1H, J ) 7.4 Hz, 4.9 Hz, 1.1 Hz), 3.59 (s, 2H),
2.32 (s, 6H); 13C NMR (CDCl3, 100 MHz) δ 156.8, 156.6, 155.9,
150.1, 149.2, 139.5, 137.0, 129.1, 128.8, 127.2, 123.8, 121.6, 120.6,
119.8, 63.7, 45.8; UV-vis (CH2Cl2) λ (nm) (ꢀ, M-1 cm-1) 304
(10600), 280 (14000), 261 (18600), 248 sh (20200), 240 (21700);
IR (KBr, cm-1) ν 3059 (w), 2973 (w), 2942 (w), 2813 (m), 2765
(m), 1583 (m), 1556 (m), 1406 (s), 1260 (m), 1043 (m), 885 (m),
774 (s), 735 (s); FAB+ m/z (nature of the peak, rel intensity) 290.1
([M + H]+, 100). Anal. Calcd for C19H19N3: C, 78.86; H, 6.62; N,
14.52. Found: C, 78.59; H, 6.42; N, 14.35.
ν 3057 (w), 1582 (m), 1532 (m), 1475 (m), 1405 (m), 1237 (m),
772 (s); FAB+ m/z (nature of the peak, rel intensity) 553.0, 552.0,
551.0 ([M - Cl]+, 60, 100, 80). Anal. Calcd for C16H10ClIN2Pt:
C, 32.70; H, 1.72; N, 4.77. Found: C, 32.51; H, 1.47; N, 4.62.
(2-p-Tolylethynyl)(4-iodo-6-phenyl-2,2′-bipyridine)platinum
(24): prepared, using procedure 4, from (4-iodo-6-phenyl-2,2′-
bipyridine)chloroplatinum 23 (130 mg, 0.22 mmol), p-tolylacetylene
(150 µL, 1.18 mmol), CuI (4 mg, 0.02 mmol), DMF (25 mL), and
triethylamine (5 mL); chromatography on aluminum oxide eluting
with dichloromethane to give 103 mg (69%) of 24 as a dark red
powder; 1H NMR ((CD3)2SO), 400 MHz) δ 8.98 (d, 1H, 3J ) 5.3
Hz), 8.56 (s, 1H), 8.47 (d, 1H, 3J ) 8.0 Hz), 8.31 (s, 1H), 8.28 (td,
4-(2-p-Tolylethynyl)-6-phenyl-2,2′-bipyridine (19): prepared,
using procedure 2, from 4-iodo-6-phenyl-2,2′-bipyridine 15b (400
mg, 1.11 mmol), p-tolylacetylene (200 µL, 1.58 mmol), [Pd(PPh3)2-
Cl2] (88 mg, 0.12 mmol), CuI (21 mg, 0.11 mmol), THF (35 mL),
3
4
3
1H, J ) 7.8 Hz, J ) 1.5 Hz), 7.82 (t, 1H, J ) 6.4 Hz), 7.71 (d
3
3
with 195Pt satellites, 1H, J ) 7.2 Hz), 7.60 (d, 1H, J ) 7.6 Hz),
7.19 (Absys, 4H, JAB ) 8.0 Hz, υ0δ ) 71.3 Hz), 7.14-7.11 (m,
1H), 7.04 (t, 1H, 3J ) 7.6 Hz), 2.30 (s, 3H); 13C NMR ((CD3)2SO,
75 MHz) δ 164.1, 156.3, 154.6, 150.7, 145.9, 142.7, 139.8, 137.5,
133.9, 130.9, 130.8, 128.7, 128.6, 128.5, 127.6, 127.5, 125.8, 125.3,
124.4, 123.4, 109.1, 106.6 (CtC), 105.2 (CtC), 20.7; UV-vis
(CH2Cl2) λ (nm) (ꢀ, M-1 cm-1) 470 (6300), 450 (6400), 377 (9600),
339 (14400), 284 (46000); IR (KBr, cm-1) ν 3039 (w), 3012 (w),
2917 (w), 2850 (w), 2094 (m, CtC), 1603 (m), 1578 (m), 1537
(m), 1500 (m), 1403 (m), 1240 (m), 1096 (m), 810 (s), 771 (s),
715 (s); FAB+ m/z (nature of the peak, rel intensity) 669.1, 668.1,
667.1 ([M + H]+, 80, 100, 75), 540.1 ([M - I]+, 30). Anal. Calcd
for C25H17IN2Pt: C, 44.99; H, 2.57; N, 4.20. Found: C, 44.68; H,
2.33; N, 3.87.
i
and Pr2NH (5 mL); chromatography on silica gel eluting with
dichloromethane-petroleum ether (v/v 50/50 to 1/0); recrystaliza-
tion from dichloromethane-petroleum ether to afford 270 mg (70%)
of 19 as white crystals; 1H NMR (CDCl3, 300 MHz) δ 8.72 (ddd,
3
1H, J ) 4.8 Hz, 1.7 Hz, 0.8 Hz), 8.64 (d, 1H, J ) 7.9 Hz), 8.51
4
4
(d, 1H, J ) 1.3 Hz), 8.19-8.15 (m, 2H), 7.87 (d, 1H, J ) 1.3
Hz), 7.86 (td, 1H, 3J ) 7.7 Hz, 4J ) 1.8 Hz), 7.56-7.43 (m, 5H),
7.34 (ddd, 1H, J ) 7.5 Hz, 4.8 Hz, 1.1 Hz), 7.20 (d, 2H, 3J ) 7.9
Hz), 2.40 (s, 3H); 13C NMR (CDCl3, 75 MHz) δ 156.8, 156.0,
155.9, 149.2, 139.5, 139.0, 137.0, 133.5, 132.0, 129.4, 129.3, 128.9,
127.1, 124.1, 122.1, 121.5, 121.4, 119.4, 94.0 (CtC), 87.1 (Ct
C), 21.7; UV-vis (CH2Cl2) λ (nm) (ꢀ, M-1 cm-1) 334 sh (7300),
310 (32800), 294 (37900), 264 (33000), 237 (27100); IR (KBr,
cm-1) ν 3059 (w), 2960 (w), 2935 (w), 2213 (m, CtC), 1585 (m),
1538 (m), 1473 (m), 1393 (m), 1245 (m), 87 (m), 812 (s), 784 (s);
FAB+ m/z (nature of the peak, rel intensity) 347.2 ([M + H]+,
100). Anal. Calcd for C25H18N2: C, 86.68; H, 5.24; N, 8.09.
Found: C, 86.52; H, 5.18; N, 7.92.
(2-p-Tolylethynyl)(4-(2-p-tolylethynyl)-6-phenyl-2,2′-bipyridine)-
platinum (26): 4-Iodo-6-phenyl-2,2′-bipyridine)chloroplatinum 23
(80 mg, 0.136 mmol) and p-tolylacetylene (80 µL, 0.63 mmol) were
charged into a Schlenk tube with DMF (10 mL) and triethylamine
(2 mL). The mixture was degassed by bubbling argon for 30 min,
then [Pd(PPh3)4] (34 mg, 0.029 mmol) was introduced and the
reaction mixture stirred at room temperature for 42 h. The solvent
was removed and the residue was treated with water and extracted
with dichloromethane. The organic extracts were washed several
times with water, then with saturated brine and filtered through
hygroscopic cotton wool. After rotary evaporation the residue was
purified by column chromatography on silica gel eluting with
dichloromethane-petroleum ether (v/v 75/25 to 1/0). The residue
4-(2-(3,4,5-Tris(dodecyloxy)phenyl)ethynyl)-6-phenyl-2,2′-bi-
pyridine (20): prepared, using procedure 2, from 4-iodo-6-phenyl-
2,2′-bipyridine 15b (215 mg, 0.60 mmol), 3,4,5-tris(dodecyloxy)-
5-ethynylbenzene (400 mg, 0.61 mmol), Pd(PPh3)2Cl2 (43 mg, 0.061
i
mmol), CuI (12 mg, 0.063 mmol), THF (35 mL), and Pr2NH (5
mL); chromatography on silica gel eluting with dichloromethane-
petroleum ether (v/v 25/75 to 80/20); reprecipitation from dichlo-
romethane-methanol to afford 410 mg (77%) of 20 as light yellow
10192 J. Org. Chem., Vol. 72, No. 26, 2007

Synthesis of Arylated Bipyridine Ligands
3
was reprecipitated in dichloromethane-cyclohexane to give 26 mg
(m, 2H), 7.14 (d, 1H, J ) 7.7 Hz), 7.08-7.04 (m, 3H), 6.90 (td,
3 4
1
(30%) of 26 as a dark red solid; H NMR (CDCl3, 400 MHz) δ
1H, J ) 7.5 Hz, J ) 1.1 Hz), 6.69 (s, 2H), 4.05-3.95 (m, 6H),
2.34 (s, 3H), 1.87-1.74 (m, 6H), 1.51-1.28 (m, 54H), 0.89 (t,
9.03 (dd, 1H, 3J ) 5.5 Hz, 4J ) 1.0 Hz), 7.94-7.89 (m, 2H), 7.79
(d, 1H, 3J ) 8.0 Hz), 7.50-7.40 (m, 7H), 7.29-7.26 (m, 1H), 7.22
(d, 2H, 3J ) 8.0 Hz), 7.14 (td, 1H, 3J ) 7.4 Hz, 4J ) 1.3 Hz), 7.08
(d, 2H, 3J ) 8.0 Hz), 7.02 (td, 1H, 3J ) 7.5 Hz, 4J ) 1.5 Hz), 2.42
(s, 3H), 2.35 (s, 3H); 13C NMR (CDCl3, 100 MHz) δ 165.0, 157.8,
154.4, 151.6, 146.4, 142.7, 140.3, 138.7, 138.6, 134.8, 134.1, 132.2,
131.8, 131.5, 129.5, 128.8, 127.6, 126.1, 124.6, 123.7, 123.0, 120.2,
120.0, 118.8, 106.9, 104.6, 97.3, 86.9, 21.9, 21.5; UV-vis (CH2-
Cl2) λ (nm) (ꢀ, M-1 cm-1) 478 (10600), 454 (10500), 374sh
9H, J ) 6.7 Hz); 13C NMR (CDCl3, 75 MHz) δ 164.7, 157.6,
3
154.3, 153.3, 151.2, 146.4, 142.7, 140.5, 138.5, 138.4, 134.8, 133.8,
131.8, 131.3, 128.8, 127.3, 126.1, 124.6, 123.6, 123.2, 120.1, 119.9,
116.1, 110.8, 106.8, 104.9, 97.7, 86.4, 73.8, 69.4, 32.1, 30.6, 29.9,
29.8, 29.7, 29.6, 29.5, 26.3, 22.8, 21.5, 14.3; UV-vis (CH2Cl2) λ
(nm) (ꢀ, M-1 cm-1) 478 (10900), 455 (10700), 361 (28100), 346
(28400), 283 (55100); IR (KBr, cm-1) ν 3042 (w), 2919 (s), 2851
(s), 2207 (m, CtC), 2102 (m, CtC), 1601 (m), 1572 (m), 1504
(m), 1466 (m), 1364 (m), 1235 (s), 1116 (s), 1020 (m), 816 (m),
777 (m), 723 (m); FAB+ m/z (nature of the peak, rel intensity)
1196.0, 1195.0, 1194.0 ([M + H]+, 100, 95, 55), 1080.0 ([M -
(CtC-tol)]+, 30). Anal. Calcd for C69H94N2O3Pt: C, 69.38; H, 7.93;
N, 2.35. Found: C, 69.19; H, 7.78; N, 2.07.
(15900), 339 (37200), 282 (51800), 249 (37000); IR (KBr, cm-1
)
ν 3050 (w), 2209 (w, CtC), 2088 (m, CtC), 1596 (m), 1500 (m),
1400 (m), 1237 (m), 1018 (w), 862 (m), 814 (s), 778 (s); FAB+
m/z (nature of the peak, rel intensity) 657.1, 656.1, 655.1 ([M +
H]+, 85, 100, 70), 540.0 ([M - (CtC-tol)]+, 35). Anal. Calcd for
C34H24N2Pt: C, 62.28; H, 3.69; N, 4.27. Found: C, 61.92; H, 3.44;
N, 3.89.
Acknowledgment. The authors thank the CNRS, the Min-
iste`re de la Recherche Scientifique, and the ANR FCP-OLEDS
No. 05-BLAN-0004-01 for financial support. We are also
indebted to Professor Jack Harrowfield (ISIS Strasbourg) for
commenting on the manuscript prior to submission.
(2-p-Tolylethynyl)(4-(2-(3,4,5-tris(dodecyloxy)phenyl)ethynyl)-
6-phenyl-2,2′-bipyridine)platinum (27): (2-p-Tolylethynyl)(4-
iodo-6-phenyl-2,2′-bipyridine)platinum 24 (75 mg, 0.11 mmol) and
3,4,5-tris(dodecyloxy)-5-ethynylbenzene (98 mg, 0.15 mmol) were
charged into a Schlenk tube with piperidine (15 mL). The mixture
was degassed by bubbling argon for 30 min, then [Pd(PPh3)4] (15
mg, 0.013 mmol) was introduced and the reaction mixture stirred
at room temperature for 18 h. The solvent was evaporated and the
residue purified by column chromatography on silica gel eluting
with dichloromethane-petroleum ether (v/v 70/30). The product
was reprecipitated in dichloromethane-methanol to give 62 mg
(47%) of 27 as a red solid; 1H NMR (CDCl3, 300 MHz) δ 8.88 (d,
Supporting Information Available: General experimental
procedure, reagents, materials, and experimental details for com-
pounds 3, 4, 5, 6, 8, 9, 12, 13, 14, 15, 21, 22, and 25, as well as
CCDC numbers for X-ray structures of compounds 2a, 2b, 13, 23,
and 24. This material is available free of charge via the Internet at
http://pubs.acs.org.
3
3
1H, J ) 5.1 Hz), 7.85 (dd with 195Pt satellites, 1H, J ) 7.3 Hz,
4J ) 0.9 Hz), 7.79-7.69 (m, 2H), 7.45-7.41 (m, 3H), 7.31-7.23
JO7019866
J. Org. Chem, Vol. 72, No. 26, 2007 10193