Design and synthesis of 4,4′-π-conjugated[2,2′]-bipyridines: A versatile class of tunable chromophores and fluorophores

Article abstract of DOI:10.1039/b106096c

A series of 4,4′-disubstituted[2,2′]-bipyridines, featuring π-conjugated substituents such as donor- (acceptor-) substituted styryl, thienylvinyl, phenylimino and phenylazo groups, have been synthesized. X-Ray structures are provided for three ligands con

Full text of DOI:10.1039/b106096c

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Design and synthesis of 4,40-p-conjugated[2,20]-bipyridines:
a versatile class of tunable chromophores and ¯uorophores
Olivier Maury,a Jean-Paul Guegan,a Thierry Renouard,a Adam Hilton,a Philippe Dupau,a
Nicolas Sandon,a Loõc Toupetb and Hubert Le Bozec*a
a
OrganomeÂtalliques et Catalyse: Chimie et Electrochimie MoleÂculaires (CNRS UMR 6509),
Institut de Chimie de Rennes, Universite de Rennes 1, Campus de Beaulieu, 35042 Rennes cedex,
France. E-mail: lebozec@univ-rennes1.fr; Fax: +33 2 99 28 69 39
Groupe de la MatieÁre CondenseÂe et MateÂriaux (CNRS UMR 6626), Universite de Rennes 1,
b
Campus de Beaulieu, 35042 Rennes cedex, France
Received (in Strasbourg, France) 10th July 2001, Accepted 3rd September 2001
First published as an Advance Article on the web
A series of 4,40-disubstituted[2,20]-bipyridines, featuring p-conjugated substituents such as donor- (acceptor-)
substituted styryl, thienylvinyl, phenylimino and phenylazo groups, have been synthesized. X-Ray structures
are provided for three ligands containing ±C=C±, ±C=N± and ±N=N± linkers, respectively. These
chromophores display good to excellent thermal stabilities with decomposition temperatures of up to 350 ꢀC.
The strong in¯uence of the nature of the endgroups and p-linkers on the optical properties is discussed. The
stepwise protonation of 4,40-dibutylaminostyryl-[2,20]-bipyridine and its coordination behavior to di€erent
metallizc moieties [Zn(II), Hg(II), Pd(II), Re(I), Re(VII)] have also been investigated. It is found that the
absorption and emission maxima can be easily tuned by these exogenous additives over a wide range of
wavelengths (360 < labs < 560 nm; 482 < lem < 646 nm).
Much attention is currently devoted to the search for new
molecular materials for optoelectronics due to their potential
applications in photonic1 and=or light-emitting devices.2
Recent recognition of the great potential of p-conjugated
molecules and oligomers in light-emitting diodes has triggered
the emergence of a new ®eld of research. Furthermore, exten-
sive e€orts have recently been concentrated on the synthesis of
macroscopic assemblies, such as polymers or dendrimers, fea-
turing p-conjugated push-pull nonlinear optical (NLO) chro-
mophores.3 This rapid development explains the increasing
e€orts from research laboratories to design new chemically and
thermally stable molecules presenting nonlinear optical and=or
luminescent properties. It is worth pointing out that for both
cases the control of the molecular geometry as well as the
electronic structureÐwhich are interconnectedÐis of crucial
importance to optimize these physical properties.4 For exam-
ple, the transparency=eciency trade-o€ of NLO-phores and
the light absorption=emission properties can be tuned by
rational modi®cations of the p-conjugated backbone.5
changes in the ligands or metals. This paper reports the detailed
syntheses, characterization, thermal and optical properties of
di€erent 4,40-disubstituted-[2,20]-bipyridines featuring a series
of p-conjugated moieties (Chart 1). We have focused our study
on p-conjugated substituents belonging to the family of p-
phenylenevinylene, such as p-donor- (or acceptor-) substituted
styryl, thienylvinyl, phenylazo and phenylimino groups. We
show how simple modi®cation of either the end substituent or
the p-linker enables the generation of tunable chromophores.
In the last part of the paper we discuss the stepwise protona-
tion and coordination study using di€erent metallic fragments
[Zn(II), Hg(II), Pd(II), Re(I), Re(VII)] of 4,40-bis(dibutylamino-
styryl)-[2,20]-bipyridine, which gives rise to ¯uorophores that
span a wide range of wavelengths in the visible spectrum.
Our research group has been involved for the past ten
years in the study of the NLO properties of bipyridyl metal
complexes.6 We have previously shown that ligands such as
4,40-bis(dialkylaminostyryl)-[2,20]-bipyridines are excellent
building blocks for the construction of either dipolar com-
pounds7 or non-dipolar metal complexes of D3 and D2d
8
9
symmetry. In these systems the NLO response is dictated by the
intense intraligand charge-transfer (ICT) transition from
the donor amino group to the acceptor pyridine ring, and the
role of the metal fragment is that of an inductive acceptor.
Another feature that these chromophores (ligands and
complexes) can o€er is their optical properties.10±12 Moreover,
the wide range of 4,40-disubstituted[2,20]-bipyridines available
makes them attractive candidates for ¯uorescent applications
and their synthetic ¯exibility may allow ®ne-tuning of the
optical (absorption and emission) properties through simple
Chart 1
Results and discussion
Syntheses
Several methodologies for the preparation of the target bi-
pyridyl ligands have been developed. Two di€erent routes were
DOI: 10.1039/b106096c
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Scheme 1 Synthesis of bipyridines 1 and 2 featuring electron-donating substituents.
used for the synthesis of 4,40-p-substituted styryl-[2,20]-bi-
condensation of 5 with the appropriate phosphonates18 gave 3
pyridines 1±4 (Chart 2), depending on the nature of
the substituent Z. The synthesis of bipyridines 1 and 2 has
already been described,13 by deprotonation of 4,40-dimethyl-
[2,20]-bipyridine followed by addition of the corresponding
aldehydes (Scheme 1, method A). Since this methodology
a€orded bipyridine 2 in poor yield (17%), an alternative one
pot procedure was employed (Scheme 1, method B): the use of
tBuOK in dimethylformamide at room temperature14 produced
2 cleanly and in reasonable yield (60%).
and 4 in 20 and 49% yield, respectively (Scheme 3). It is note-
worthy that the presence of the long octyl chain on the sulfone
moieties of 3 results in enhanced solubility as compared to the
very poor solubility of 4 containing nitro acceptor end groups.
Scheme 2 Synthesis of 4,40-diformyl-[2,20]-bipyridine 5.
Chart 2
The synthesis of 3 and 4 featuring electron-withdrawing
substituents (Z ˆ ±SO2Oct, ±NO2), was achieved with 4,40-
diformyl-[2,20]-bipyridine 5 as the building block. A new
convenient two-step synthesis of the dicarboxaldehyde
derivative 5 was ®rst developed,15 contrasting with the
previous multistep procedure reported in the literature.16
The ®rst step involved the straightforward enamination of
4,40-dimethyl-[2,20]-bipyridine with the Bredereck reagent tert-
butoxybis(dimethylamino)methane, giving rise to 6 in quanti-
tative yield (Scheme 2). Further oxidation of the enamine
groups with sodium periodate17 in THF at room temperature
furnished 5 in an overall 80% yield. A Wadworth±Emmons
Scheme 3 Synthesis of bipyridines 3 and 4 featuring electron-with-
drawing substituents.
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Compound 7 (Chart 2) containing a thienyl conjugated
bridge was prepared in high yield (91%) by following the
synthetic sequence shown in Scheme 1 (method A) from 5-
dibutylaminothiophene-2-carboxaldehyde.19 According to the
1H NMR spectrum, a mixture of Z and E isomers was formed
but a slow isomerization occurred in solution and=or in the
solid state to give, after several days, the thermodynamically
more stable E isomer. The 4,40-dialkenyl-[2,20]-bipyridines were
characterized by high resolution mass spectrometry (HRMS),
as well as by 1H and 13C NMR (see Experimental). All the
1
double bond linkages were con®rmed to be trans by H NMR
analysis, based on the determination of the coupling constant
JCH=CH ꢁ 16 Hz.20
The imino-containing bipyridine 8 was synthesized in 95%
yield by a Schi€ base condensation reaction between N, N-di-
ethyl-1,4-phenylenediamine and 5 in dichloromethane at room
temperature21 (Scheme 4). This compound was unambiguously
characterized by the presence of strongly deshielded signals at
8.71 and 151.26 ppm, corresponding to the N(8)=C(7)H proton
and carbon, respectively. In contrast to 1±4, compound 8 is
rather acid=moisture sensitive, and we could rapidly regenerate
the starting products upon hydrolysis in the presence of traces
of acid. All attempts to isolate the ``reverse'' imino-bipyridine
(in this case X=Y represents a C=N double bond) from the
reaction between 4,40-diamino-[2,20]-bipyridine and diethylami-
nobenzaldehyde failed, even under severe experimental condi-
tions, con®rming the already observed deactivation of the amino
and carbonyl functions in the presence of acceptor and donor
groups, respectively.22 The azo-containing bipyridyl ligand 9
was synthesized by diazotation of 4,40-diamino-[2,20]-bipyridine
10 with sodium nitrite and subsequent coupling with N, N-dibu-
tylaniline (Scheme 5). After chromatographic work-up,
the product was isolated with a modest 25% yield as a
dark red microcrystalline powder. All attempts to increase
the yield, by using anhydrous conditions described in the litera-
ture for the synthesis of various diazonium salts (CF3COOH±
Scheme 5 Synthesis of the azo-containing bipyridine 9.
Crystallographic structures
The structures of compounds 1, 8 and 9 were determined by X-
ray di€raction; selected bond lengths and angles, crystal data
and re®nement parameters are listed in Tables 1 and 2,
respectively. In contrast to 8 and 9, 1 crystallizes as a dimer in
which the two molecules are roughly identical with no over-
lapping of the aromatic rings. In this latter case, only one
molecule will be described for clarity (Fig. 1). In all cases, the
bipyridines adopt the classical transoid arrangement due to
the repulsion of the nitrogen lone pairs, with a symmetry
center in the middle of the C2±C20 bond for 8 and 9 (Fig. 2 and
3) and a pseudo-symmetry center for 1.27 The E con®guration
of the X7=Y8 double bonds and the s-trans conformation
between the X7=Y8 and C4±C5 bonds are con®rmed, what-
ever the nature of the X7=Y8 linkage (C=C, C=N, N=N). The
planarity in the solid state can be estimated by measuring
the twist angle t between the phenyl and the pyridyl rings
(Table 3). Assuming that the C4±X7±Y8±C9 atoms are
coplanar, t is the algebric sum of the dihedral angles C3±C4±
X7±Y8 and X7±Y8±C9±C10. In the three structures, t is
found to be between 8.8 and 18.7ꢀ, clearly indicating the good
PenONO in CH3CN at
10 ꢀC), failed.23 The 4,40-diamino-
[2,20]-bipyridine precursor 10 was quantitatively prepared upon
reduction of 4,40-dinitro-[2,20]-bipyridine-[1,10]-dioxide24 by
hydrazine hydrate in the presence of Pd=C.25 This mild and
ecient method contrasts favorably with the known, low-yield
procedure (20%) using Fe=AcOH as reducing agent.26 Com-
pound 9 was characterized by 1H and 13C NMR, and the trans
con®guration of the azo as well as the imino linkages were
evidenced by an X-ray di€raction analysis (vide supra).
Table 1 Selected bond lengths and angles for 1, 8 and 9
8
9
Bond
lengths=A
1
(X7 ˆ C7,
(X7 ˆ N7,
+
(X7 ˆ C7, Y8 ˆ C8)
Y8 ˆ N8)
Y8 ˆ N8)
C2±C20
N1±C2
C2±C3
1.488 (8)
1.482 (7)
1.343 (4)
1.382 (4)
1.381 (5)
1.465 (5)
1.267 (4)
1.414 (4)
1.386 (5)
1.364 (5)
1.395 (5)
1.492 (4)
1.338 (3)
1.383 (3)
1.392 (3)
1.424 (3)
1.261 (2)
1.394 (3)
1.389 (3)
1.367 (3)
1.411 (3)
1.366 (13)
1.406 (13)
1.384 (15)
1.461 (14)
1.312 (14)
1.478 (14)
1.376 (16)
1.404 (15)
1.389 (15)
1.311 (12)
1.369 (14)
1.407 (15)
1.472 (15)
1.306 (14)
1.454 (15)
1.393 (15)
1.350 (14)
1.358 (14)
C3±C4
C4±X7
X7±Y8
Y8±C9
C9±C10
C10±C11
C11±C12
Angles=ꢀ
C5±C4±X7
C4±X7±Y8
X7±Y8±C9
118.9 (10)
124.1 (11)
126.8 (11)
119.4 (10)
128.7 (10)
129.2 (10)
121.5 (4)
120.1 (4)
121.0 (3)
116.6 (2)
113.40 (19)
114.63 (19)
Scheme 4 Synthesis of the imino containing bipyridine 8.
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Table 2 Selected crystallographic and data collection parameters for
1, 8 and 9
Optical properties
In¯uence of the substituents. 4,40-Substituted styryl-[2,20]-
bipyridines 1±3 show di€erent UV-visible spectra, depending
on the nature of the substituent Z. The optical spectra of
``push-pull'' molecules 1 and 2 are characterized by an
intense, structureless and broad absorption band at 400 and
337 nm, respectively, which can be assigned to an intraligand
charge-transfer transition (ICT).13 In contrast, bipyridine 3
featuring the electron-withdrawing group SO2Oct can be
considered as a ``pull-pull'' molecule, and thus exhibits
maxima in the UV region at 275 and 300 nm assigned to
p±p* transitions. Photoluminescence is observed for the three
compounds in dichloromethane solution at room
temperature, and the emission maxima are also strongly
dependent on the nature of Z (Table 4 and Fig. 4). A blue
emission is observed at lem ˆ 363 nm for 3, whereas 1 and 2
each display an emission band in the visible, that of 1 being
as expected red-shifted with respect to that 2 (1, lem ˆ 497
nm; 2, lem ˆ 411 nm).
1
8
9
Formula
M=g
Crystal system
C
80H76N8
C16H18N3
252.33
Monoclinic
P 21=c
7.309 (3)
7.698 (5)
25.101 (7)
90
92.13 (2)
90
1411.3 (12)
4
0.71069
0.72
293
3264
872
0.2724
0.0633
0.2056
0.1429
C19H25N4
309.43
Monoclinic
P 21=c
10.145 (4)
10.820 (4)
16.051 (9)
90
96.949 (5)
90
1749.0 (14)
4
0.71069
0.71
293
3813
1802
0.1568
0.0521
0.1732
0.1378
1149.49
Triclinic
P1
Space group
+
a=A
+
11.915 (3)
12.050 (4)
13.544 (2)
105.67 (2)
99.141 (10)
93.40 (2)
1837.9 (8)
1
0.71069
0.65
293
8401
3534
b=A
+
c=A
a=ꢀ
b=ꢀ
g=ꢀ
+
U=A3
Z
+
l(Mo-Ka)=A
m=cm
T=K
1
Re¯ections measured
Re¯ections [I > 2s(I)]
R1 (all data)
R1 [I > 2s(I)]
wR2 (all data)
0.2044
0.0776
0.3179
0.2287
Solvatochromic measurements were performed for com-
pound 1 by using seven di€erent solvents (Table 5). The cor-
relation between the absorption and emission energies vs.
Reichardt's ET polarity scale28 is presented in Fig. 5. A small
positive solvatochromism (i.e., bathochromism) of the
absorption band is observed, which is consistent with an ICT
transition. These results compare well with the optical spectra
of the related 4,40-diethylaminostyryl-[2,20]-bipyridine10a and
those of chromophores like bis(3,8-p-substituted-phenyl-
ethynyl)phenanthroline11 or bis(6,60-p-substituted-styryl)-
[3,30]-bipyridine.12
wR2 [I > 2s(I)]
planarity of the p-conjugated systems. Moreover, for the three
compounds, the C4±X7 and Y8±C9 bond lengths fall between
those of classical single and double bonds, indicating for
the three structures
p-conjugated backbone.
a good delocalization along the
Fig. 1 ORTEP drawing of compound 1; for clarity only one molecule is represented.
Fig. 2 ORTEP drawing of compound 8.
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Fig. 3 ORTEP drawing of compound 9.
Table 5 Solvatochromic behavior of 1 and 7
Table 3 Twist angle t between the phenyl and pyridyl rings for 1, 8
and 9
1
7
Dihedral angle/ꢀ
C4±X7±Y8±C9
Solvent
lmax=nm
lem=nm
l
max=nm
lem=nm
C3±C4±X7±Y8
X7±Y8±C9±C10
t=ꢀ
Toluene
THF
Acetone
CH2Cl2
DMF
392
393
395
399
403
408
403
440
479
497
497
513
525
505
429
432
441
443
446
451
444
496
513
521
532
531
541
540
1
177.0
180.0
179.6
179.4
7.3
16.8
5.8
4.2
1.9
12.8
5.3
11.5
18.7
18.6
8.8
8
9
3.5
DMSO
EtOH
Table 4 Optical data of the studied compounds in dichloromethane
conjugated systems can contain a X=Y double bond and an
aromatic fragment that can be independently modi®ed, two
classes of chromophores were considered for comparison.
The ®rst class features a C=C double bond, which can either
be directly linked to the dialkylmino group (6) or bonded to
an aromatic ring,=phenyl (1) or thienyl (7). In the second
class of chromophores, the p-conjugated bridge contains an
X=Y double bond, which can be either C=C (1), N=C (8) or
N=N ( 9), associated with a phenyl group. The absorption
and emission data for these di€erent chromophores are
gathered in Table 4 and Fig. 6. An elongation of the
conjugated bridge, as when going from 6 to 1, induces a
red shift of the ICT band (Dlmax ˆ 58 nm).30 Moreover,
the substitution of the phenyl ring in 1 for a thiophene
ring in 7 results in a further substantial bathochromic
shift by 42 nm.31 This behavior is in accord with other
studies on push-pull thiophene stilbene derivatives,32 and
can be explained by (i) the lower aromatic stabilization
energy of the thienyl vs. the phenyl moieties (28 vs. 36 kcal
Stokes
lem=nm shift=nm FF
1
1
Ligand
l
max=nm e=L mol
cm
1
2
3
6
7
8
9
401
337
65000
34000
Ð
38000
69000
53000
64000
497
411
363
449
528
Ð
96
74
88
104
85
Ð
Ð
0.23a
0.01b
0.03b
0.015b
0.015a
Ð
275, 300c
345
443
433
473
Ð
Ð
a
Relative quantum yield determined using ¯uorescein as standard.
Relative quantum yield determined using quinine sulfate as
b
c
standard. Not an ICT band.
In¯uence of the p-conjugated system. The e€ect of the p-
linker (between the pyridyl and donor endgroups) on the
optical properties has been studied by using the same
donor substituent,
a
dialkylamino group.29 As the p-
Fig. 4 Emission spectra of bipyridines 1±3, 6 and 7 in dichloro-
methane. Intensities of the spectra have been normalized to 1.
Fig. 5 Energy maxima vs. the Reichardt solvent parameter for 1 and 7
in absorption (®lled symbols) and emission (open symbols), respectively.
New J. Chem., 2001, 25, 1553±1566
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Fig. 6 In¯uence of the p-linker on the absorption properties.
Fig. 7 Thermogravimetric analysis of 1.
mol 1), which allows a better delocalization,33 and (ii) its
auxiliary donor property, which increases the intra-
molecular charge transfer.34 These di€erent e€ects can also
be observed in the luminescence spectra of 6, 1 and 7, where
the emission maxima are found at 449, 497 and 528 nm,
respectively. Moreover, the nature of the linker has a
dramatic in¯uence on the quantum yield (FF) of the
chromophores (Table 4). For instance, a considerable drop
in ¯uorescence quantum eciency is observed upon
replacement of the phenyl by thienyl ring (FF ˆ 23% for
1 and 1.5% for 7). The incorporation of nitrogen atoms
As an example, the TGA of compound 1 is reported in Fig. 7:
no loss of weight occurs up to 350 ꢀC and the thermal decom-
position temperatures Td and Td are found at 355 and 380 ꢀC,
5
10
respectively. It should be noted that the UV-visible and 1H
NMR spectra of 1 remain unchanged after 60 min exposure at
310 ꢀC under nitrogen, con®rming the high thermal stability of
this chromophore. The Td values depend strongly on the nature
of the p-conjugated groups. Thermal decomposition of the
styryl bipyridine compounds 1±3 does not occur below 260 ꢀC,
the more stable being the push-pull chromophores 1 and 2
into the double bond of the transmitter induces
a
(Td ꢁ 350 ꢀC). However, the introduction of a thienyl ring in
signi®cant shift of the ICT transition to lower energy. Thus,
compound 8 (C=N) is red-shifted by 33 nm relative to 1, and
furthermore 9 (N=N) is red-shifted by 38 nm relative to 8.
Similar behavior has been observed for other organic or
organometallic chromophores. For example, a bathochromic
shift of 40 nm between 4-dimethylamino-40-dimethyl-
5
place of a phenyl leads to a decrease of the Td value (275 ꢀC).32c
Moreover, compound 9 bearing an azo bridge has a relatively
5
low decomposition temperature (Td ˆ 205 ꢀC), whereas for 8,
5
decomposition occurs prior to melting. Thus, the styryl bi-
pyridine derivatives appear to be the most thermally robust
chromophores, comparable to the highest thermal stability
reported in the literature,36 which makes them good candidates
for incorporation into a host matrix.
sulfonylazobenzene
and
the
corresponding
stilbene
derivative,18b and a red shift of 20 nm occurred on changing
from a stilbene to an iminobenzene ruthenium complex.22
According to theoretical studies,35 this signi®cant batho-
chromic shift seems to be due to the higher electronegativity
of the nitrogen, which induces a supplementary dipole
moment enforcing the overall molecular charge transfer.
Finally it is worth noting that, in contrast to 1, compounds 8
and 9 do not display any emission in solution at room
temperature.
Modulation of the optical properties
Stepwise protonation of 1 with p-toluenesulfonic acid. As
1 contains
the 4,40-bis(dibutylaminostyryl)-[2,20]-bipyridine
four potentially basic sitesÐthe imino groups of the two
pyridyl rings and the amino functions of the two
dialkylamino substituentsÐa stepwise protonation was
carried out by using p-toluenesulfonic acid (PTSA). The
reaction was followed by UV-visible and emission
spectroscopy (Fig. 8 and Table 7). The addition of one
equivalent of PTSA to a dichloromethane solution of 1
promotes a rapid color change, from yellow to deep red. The
Thermal stability
Chemical and thermal stability are important requirements
for practical applications, in particular for the use of chromo-
phore dopants in a polymeric or inorganic matrix. Except for
8, the current family of chromophores is chemically inert
toward air or water as the molecules were found to remain
unchanged over several months in an open ¯ask. The deter-
UV-visible spectrum shows
a
one-half decrease in
mination of their thermal decomposition temperatures Td (5%
loss of weight) and Td (10% loss of weight) was investigated by
thermogravimetric analysis (TGA) under nitrogen (Table 6).
5
10
Table 6 Melting points and thermal stabilities of compounds 1±3 and
6±9
Ligand
Mp=ꢀC
Td =ꢀC
Td =ꢀC
5
10
1
2
3
6
7
8
9
147
146
Ð
185
144
Decomp.
138
355
350
195
205
275
Ð
380
370
260
230
310
Ð
‡
Fig. 8 UV-visible spectra of 1 and 1ÁxH (x ˆ 1, 2 and 4) in dichloro-
205
250
methane.
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two dialkylamino donor end groups inhibits charge transfer
Table 7 Optical data for compounds 1 and 1ÁxH in dichloro-
and raises the energy of the transition.39 All these
protonation reactions are reversible and addition of an
aqueous solution of NaHCO3 reforms the starting bipyridine
1. Thus, it appears that the absorption and emission
properties of a given chromophore can be tuned over a large
range (360 < lmax < 510 nm; 482 < lem < 632 nm) simply by
adding a controlled amount of acid.
methane
PTSA
(equiv.)
lmax=nm
1
1
)
(e  10 3=L mol
cm
lem=nm
0
1
2
4
400 (65)
400 (38)
497
497
311 (16)
311 (32)
360 (37)
510 (33)
510 (42)
632
632
482
Formation of a 1 : 1 adduct between 1 and tetrachloro-
catechol. Crystal engineering of 1 : 1 or 1 : 2 adducts between
substituted pyridines and phenol derivatives has been
eciently used in nonlinear optics to enhance the
hyperpolarizability b of the neutral molecules by acid±base
proton transfer.40 Depending on the values of the DpKa
between the acid and the base, two types of interactions can be
observed, either adducts linked by O±HÁÁÁN hydrogen bonds
the intensity of the charge transfer band at 400 nm band and
the appearance of two new absorption bands at 510 and 311
nm, which can be assigned to ICT and p±p* transitions,
respectively.37 The presence of two CT bands in the visible
region suggests that the protonation occurs only on one
‡
‡
pyridinic site of 1, giving rise to 1ÁH (Scheme 6). The
or ionic adducts linked by N ±HÁÁÁO interactions.41 Both
addition of
a
second equivalent of PTSA causes the
situations have already been described in the literature
depending on the nature of the substituents on the pyridine or
phenol derivatives.42 With the goal of understanding how such
types of adducts can modulate the optical properties of our
chromophores, we have selected as a model system the
disappearance of the band at 400 nm whereas the two
intense transitions at 510 and 311 nm are still observed, as
expected by the selective formation of the bipyridinium salt
‡
1Á2H . The large bathochromic shift (Dl ˆ 110 nm) is
consistent with the strong electron-accepting nature of the
pyridinium ring compared with pyridine. This behavior can
be compared with that of the aminostilbazole series, which
exhibits a red shift of ca. 110 nm after methylation of the
pyridinic site.30c,38 Similarly, a bathochromic shift of the
interaction between 1 and a 1,2-benzenediol derivative,
tetrachlorocatechol. Addition of one equivalent of this diol to
a dichloromethane solution of 1 promotes an instantaneous
color change, from yellow to yellow±brown. In the NMR
spectrum, the phenol protons show a strong low-®eld shift
with respect to those of the free catechol (Dd ꢁ 2). The
aromatic region gives rise to signals that are diagnostic of a
symmetric 1:1 adduct (Scheme 7): a shielding of ca. 0.2 and 0.4
ppm is observed for the H6 and H3 protons, respectively, in
accordance with a conformational transoid±cisoid change of 1
upon interaction with the 1,2-diol. In the UV-visible spectrum,
the charge transfer band is red-shifted by 13 nm (lmax ˆ 414
emission (Dlem ˆ 135 nm) is observed, yielding
a red
¯uorescence at 632 nm for 1Á2H . The addition of two
‡
‡
further equivalents of PTSA to 1Á2H produces a dramatic
color change of the solution, from deep red to colorless, and
the corresponding absorption spectrum displays only one
maximum at 360 nm (Fig. 8). A profound blue shift of the
emission is also observed: excitation at 360 nm results in one
weak emission at 482 nm. Such a blue shift is consistent with
nm) relative to 1.43 This small bathochromic shift, as
‡
‡
the production of 1Á4H in which the protonation of the
compared to 1Á2H
(Dl ˆ 110 nm), suggests that the
Scheme 6 Stepwise protonation of 1.
New J. Chem., 2001, 25, 1553±1566
1559

coupling constant (3JH7±H8 ꢁ 16 Hz) for 1a±f, showing that
View Online
there is no isomerization of the C=C linker upon complexa-
tion.44 It should be noted that these complexes adopt di€erent
geometries such as tetrahedral for the Hg(II) and Zn(II) in 1a±c,
square planar for the Pd(II) in 1d and fac-octahedral for the
Re(I) complex 1e.7b,10a The symmetric structure of the Re(VII)
dimer 1f is re¯ected in its 1H and 13C NMR spectra, which
exhibit the pattern of three C±H signals for the aromatic
region of the bipyridyl moieties. Moreover, the IR spectrum
1
reveals the presence of a strong nReˆO vibration at 911 cm
and a weaker one at 967 cm 1, as usually observed for
related RO±ReO3 complexes (R ˆ K, SnMe3).45 From these
spectroscopic data, we can assume that both metallic
centers are equivalent in 1f and adopt a facial octahedral
geometry (Chart 3).
A similar geometry has already
been described for a Mo(VI) dioxo-m-oxo dimer featuring a
4,40-di-tert-butyl-[2,20]-bipyridine ligand.46 The thermal stabi-
lity of complexes 1a±f was determined by thermogravimetric
analysis and the TGA data are collected in Table 8. It is
noteworthy that, except for the Hg(II) and Re(VII) derivatives,
these complexes display high thermal stability comparable to
that of the ligand precursor 1.
The complexation induces a substantial bathochromic shift of
the absorption bands, which is sensitive to the nature of the
metallic moiety (Table 8). For instance, in the case of 1b, a red
shift of ca. 60 nm in the absorption maximum is observed. For a
given (1)MCl2 structure, this red shift can be tuned by the Lewis
acidity of the metal ion and, as expected, Hg(II) complex 1a has a
smaller shift in absorption (DlICT ˆ 37 nm) than Zn(II)-
containing 1b (DlICT ˆ 58 nm). However, whereas Pd(II) is a
weaker Lewis acid than Zn(II),47 a more pronounced red shift is
observed for 1d (DlICT ˆ 80 nm). Thus, it appears that other
factors such as the geometry of the complexes can play a non-
negligible role: in contrast to the Zn(II) and Hg(II) complexes,
which adopt a tetrahedral geometry, the d8-Pd(II) complex is
square planar and thus, the two chloride ligands lie trans to the
pyridyl rings. The in¯uence of the ancillary ligands as well as of
the oxidation state on the optical properties is clearly shown by
comparing the optical spectra of the two Re(I) and Re(VII)
complexes 1e and 1f; the absorption maximum shifts from 473
nm for 1e to 559 nm for 1f. Photoluminescence is also observed
for all the complexes in dichloromethane solution (Table 8).
A substantial red shift of the emission band, from 120 to 150 nm,
is found when compared to that of the free ligand 1. However, it
should be noted that the modulation of the emission wavelength
upon varying the metallic moiety occurs in a more narrow range
(Dlem ꢁ 35 nm) than for the absorption band (Dlabs ꢁ 120 nm).
Finally, complexation of 1 can also in¯uence profoundly the
¯uorescence quantum yield. For example, whereas the Hg(II)
derivative 1a emits with a ¯uorescence intensity similar to that of
the ligand (FF ˆ 21%), the quantum yields for the Zn(II) com-
plexes 1b±c are more than threefold enhanced (FF ꢁ 70%)
(Fig. 9). Such a chelation-enhanced ¯uorescence much pre-
cedent with the Zn(II) ion,48 and the use of polydentate ligands
like substituted sulfonamidoquinolines, dipicolylamines, and
macrocyclic tetraamines has recently been reported to give
ecient zinc(II) ¯uorophores for biological applications.49
Scheme 7 Formation of a 1 : 1 adduct between 1 and tetrachloro-
catechol.
interaction can be best described by the formation of a neutral
adduct in which two O±HÁÁÁN hydrogen bonds are formed.
Modulation of the electronic properties by complexation.
Bipyridyl metal complexes represent an important class of
chromophores as they o€er a large range of metals with
di€erent oxidation states and ligands that can give rise to
tunable optical properties. For the design of new dipolar
chromophores for nonlinear optics, we have recently
developed the synthesis of push-pull complexes containing
bipyridyl ligands and acceptor organometallic fragments
such as MX2 (M ˆ Zn, Hg) and Re(CO)3X.7b The results
obtained with such systems revealed an enhancement of the
molecular hyperpolarizabilities and a bathochromic shift of
the charge transfer band, which were sensitive to the Lewis
acidity of the organometallic fragment. The goal of the
present study is to explore how the optical properties of
bipyridyl ligands such as 1 can be tuned by varying the
metal and its oxidation state.
The Hg(II), Zn(II) and Pd(II) complexes 1a±d (Chart 3)
were readily prepared upon room temperature treatment
of 1 with HgCl2 , ZnCl2 ZnOAc2 , and PdCl2 , respectively.
The rhenium(I) complex 1e was classically obtained from
Re(CO)5Cl and 1 in re¯uxing toluene,7b,10a whereas the
violet rhenium(VII) compound 1f was synthesized by reacting 1
with Re2O7 (0.5 equiv.) in dichloromethane at room tem-
perature. All these complexes were fully characterized by
means of 1H and 13C NMR, UV-visible and ¯uorescence
spectroscopy (Table 8) and gave satisfactory microanalyses
(see Experimental). The E con®guration of the double bond is
clearly established on the basis of the strong ole®nic proton
Conclusions
In this study we have described the syntheses, thermal sta-
bilities and optical spectra for a versatile family of 4,40-p-
conjugated[2,20]-bipyridines. Tuning of the electronic
absorption properties is made possible by simple modi®ca-
tion of the endgroup and p-linker. For the 4,40-p-substituted
styryl-[2,20]-bipyridine series, the bathochromic shift is con-
sistent with relative values of the donor strengths in the
order NBu2 > OOct > SO2Oct. With a given donor group
such dialkylamino,
a substantial red shift of the ICT
1560
New J. Chem., 2001, 25, 1553±1566

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Chart 3
Table 8 Optical data (in dichloromethane) and thermal stabilities of complexes 1a±1f
1
1
Complex
lmax=nm
e=L mol
cm
lem=nm
Stokes shift=nm
FF
Td±Td =ꢀC
10
(1)HgCl2
(1)ZnCl2
1a
1b
1c
1d
1e
1f
438
459
444
481±383
473±381
559
51000
62000
60000
63000±26000
67000±30000
80000
615
624
610
622
630
646
177
164
166
141
157
87
0.21
0.74
0.65
Ð
Ð
Ð
192±206
392±399
353±367
312±321
382±392
228±360
(1)Zn(OAc)2
(1)PdCl2
(1)ReCl(CO)3
[(1)ReO3]2-m2-O
transition occurs in the following order: phenylazo > thie-
nylvinyl > phenylimino > styryl > vinyl. The emission
energy and ¯uorescence eciency are also dictated by the
nature of the end substituents and p-conjugated systems:
substitution of a phenyl by a thienyl ring results in a red
shift in emission wavelength and substantial decrease in
¯uorescence quantum yield, whereas introduction of nitrogen
atoms in the p-linker produces an extinction of the ¯uor-
escence. Finally, the optical properties of a given chromo-
phore show a ®ne degree of tunability upon either stepwise
protonation or metal coordination. Notably, depending on
the controlled amount of protons but also on the Lewis
acidity of the metallic fragment and=or the geometry of the
resulting complex, further modulation can be achieved
over a large range of absorption and emission wavelengths
(360 < labs < 560 nm; 482 < lem < 646 nm).
New J. Chem., 2001, 25, 1553±1566
1561

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spectrometer. Fluorescence quantum yields were measured on
non-degassed samples at room temperature. Solutions of
¯uorescein in NaOH (1 M) or of quinine sulfate in H2SO4
(1 M) were used as standards for the quantum yield mea-
surement (FF ˆ 0.90 and FF ˆ 0.546, respectively). Refractive
index corrections have been performed.50
Crystal structure analysis
The samples were studied on an CAD4 NONIUS automatic
di€ractometer with graphite monochromated Mo-Ka radia-
tion.51 The cell parameters were obtained by ®tting a set of 25
high-theta re¯ections. After Lorentz and polarization correc-
tions,52 the structure was solved with SIR-97,53 which revealed
the non-hydrogen atoms of the structure. After anisotropic
re®nement all the hydrogen atoms were found with a Fourier
di€erence synthesis. The whole structure was re®ned with
SHELXL9754 by the full-matrix least-squares techniques [use of
F magnitude; x, y, z, (ij for C, N and O atoms, x, y, z in riding
mode for H atoms)]. ORTEP views were realized with PLA-
TON98.55 All the calculations were performed on a Silicon
Graphics Indy computer.
Fig. 9 Comparison of the emission spectra of 1 and 1b in dichloro-
methane at the same concentration.
Experimental
General procedures
All reactions were routinely performed under argon using
Schlenk techniques. NMR spectra (1H, 13C, 31P) were recorded
at room temperature on a Bruker DPX 200 (operating at
200.12 MHz for 1H) or on a Bruker AC 300 (operating at
300.13 MHz for 1H) spectrometer. NMR data are listed in
parts per million (ppm) and are reported relative to tetra-
methylsilane (1H, 13C), residual solvent peaks being used as
internal standard (CD2Cl2 1H: 5.25 ppm; 13C: 53.45 ppm).
CCDC reference numbers 166922±166924. See http:== www.
rsc.org=suppdata=nj=b1=b106096c= for crystallographic data
in CIF or other electronic format.
Chemicals
p-Dibutylaminobenzaldehyde was synthesized on the 15 g scale,
in a one-step reaction using the classical Vilsmeier±Haack for-
mylation. 5-Dibutylaminothiophene-2-carbaldehyde19 was
synthesized in 70% yield using dimethylsulfoxide as solvent
instead of water and the puri®cation of the crude product was
carried out by distillation under reduced pressure (125 ꢀC under
0.05 mm Hg). The synthesis of 1-methyl-4-(octane-1-sulfo-
nyl)benzene,56 and 4-octyloxybenzaldehyde57 were carried out
using published methods. 4,40-Dinitro-[2,20]-bipyridine-[1,10]-
dioxyde was classically synthesized.24 (Caution! During this
nitration reaction a violent explosion occurred.) N,N-Diethyl-
1,4-phenylenediamine (ACROS) was distilled just before used
under reduced pressure. NaIO4 was purchased from Aldrich in a
5 g bottle.58 Tetrachloro-1,2-benzenediol was purchased from
Lancaster and puri®ed by recrystallization from hot toluene.
THF was distilled over Na±benzophenone, DMF was distilled
prior to use, toluene was distilled over Na and CH2Cl2 was
distilled over P2O5 .
1
Complete assignment of the H and 13C spectra required 2D
experiments [COSY, H±C correlation (HMQC and HMBC
sequences)]. The atom numbering for bipyridyl ligands and
related complexes is depicted Fig. 10.
UV-visible spectra were recorded on a Kontron UVIKON
941 spectrophotometer in dilute dichloromethane solution (ca.
10 5 mol L 1). Infra-red spectra were recorded in KBr pellets
using a Nicolet 205 FTIR. Thermal stability was measured by
means of a TA Instrument TGA 2050 thermogravimetric
analyzer, decomposition temperature at 5 and 10% weight lost
are called Td and Td respectively, and below are reported
5
10
quoted to the 5 ꢀC inferior. The heating program used was the
following (i) isothermal at 50 ꢀC for 15 min, (ii) a temperature
1
ramp of 10 ꢀC min
up to 600 ꢀC. Melting points were
measured using a TA Instrument DSC 2010 di€erential scan-
ning calorimeter. High resolution mass spectrometry mea-
surements (FAB or EI) were performed at the Centre Regional
de Mesures Physiques de l'Ouest (Rennes, France) and ele-
mental analysis by the Service Central d'Analyse du CNRS
(Solaize, France).
Syntheses
4,40-Bis(dibutylaminostyryl)-[2,20]-bipyridine (1). The bi-
pyridyl ligand 4,40-bis(dibutylaminostyryl)-[2,20]-bipyridine
was synthesized using a previously reported procedure.13b
Single crystals suitable for X-ray di€raction analysis were
obtained by slow di€usion of pentane in a dichloromethane
solution. 13C NMR (CD2Cl2): d 156.08 (C2), 148.31 (C12),
146.22 (C4), 148.90 (C6), 133.02 (C8), 128.14 (C10), 122.79
(C9), 120.16 (C7), 119.84 (C5), 117.10 (C3), 111.14 (C11),
50.33 (C13), 29.08 (C14), 19.97 (C15), 13.47 (C16).
Fluorescence analysis
Fluorescence experiments were performed in dilute dichloro-
6
methane solution (ca. 10 5±10
mol L 1) using a PTI
4,40-Bis( p-octyloxystyryl)-[2,20]-bipyridine (2). In a Schlenk
¯ask, 4,40-dimethyl-[2,20]-bipyridine (1 g, 5.42 mmol) and 4-
octyloxybenzaldehyde (2.53 g, 10.84 mmol, 2 equiv.) were
t
dissolved in dimethylformamide (5 mL) and BuOK (1.51 g,
13.55 mmol) was slowly added at room temperature. The
solution, which immediately turned brown, was then stirred
for 2 h. Addition of water led to the formation of a pale
precipitate, which was ®ltered o€ by centrifugation (15 min,
14000 pm). The crude solid was dissolved in dichloro-
methane, washed with water, dried over MgSO4 , and the
solvent was removed under vacuum. Recrystallization from a
Fig. 10 Generic atom numbering of the p-substituted styryl-[2.20]-
bipyridyl ligands and related complexes. The labels are used for the
assignment of the 1H and 13C NMR signals.
1562
New J. Chem., 2001, 25, 1553±1566

0
0
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dichloromethane±pentane mixture gave
2
as
a
white
4,4 -Bis( p-nitrostyryl)-[2,2 ]-bipyridine (4). In a Schlenk ¯ask,
4-nitrobenzyl-diethylphosphonate (0.6 g, 2 mmol) was dissolved
in THF (50 mL) and cooled to 0 ꢀC. nBuLi (1.6 M in hexane, 1.3
mL, 2.1 mmol) was added dropwise by means of a syringe and
the solution was stirred for 1 h at room temperature. A THF
solution (50 mL) of 5 (0.21 g, 1 mmol) was then slowly added
and the mixture was heated under re¯ux for 3 h. After cooling
to room temperature, the solution was hydrolyzed upon
addition of water (40 mL). The orange precipitate was ®ltered
o€, washed several times with diethyl ether and dried in vacuo.
Compound 4 was obtained as an orange microcrystalline
microcrystalline powder (2 g, 60%). 13C NMR (CD2Cl2):
d 160.28 (C4), 159.90 (C2), 149.88 (C6), 146.41 (C12), 133.16
(C8), 129.25 (C9), 128.82 (C10), 124.22 (C7), 121.08 (C5),
118.22 (C3), 115.20 (C11), 68.61 (C13), 32.28 (C14), 29.81
(C15), 29.70 (C16, C17), 26.46 (C18), 23.12 (C19), 14.32 (C20).
1-(Bromomethyl)-4-(octane-1-sulfonyl)benzene.
A solution
containing 1-methyl-4-(octane-1-sulfonyl)benzene (2 g, 7.4
mmol), N-bromosuccinimide (1.5 g, 8.4 mmol) and
azoisobutyronitrile (0.1 g, 0.6 mmol) in tetrachloromethane was
heated at 76 ꢀC for 17 h. After cooling to room temperature, the
solvent was removed under vacuum. The residue was dissolved
in dichloromethane, washed with water and dried over MgSO4 .
The solvent was removed and the resulting oil was puri®ed by
column chromatography (silica gel, CH2Cl2±pentane 1:1) to
powder (0.22 g, 49% yield). H NMR (DMSO-d6): d 8.75 (d,
1
2H, J ˆ 5.0 Hz, H6); 8.64 (s, 2H, H3); 8.28 (d, 4H, J ˆ 8.4 Hz,
H11); 8.00 (d, 4H, J ˆ 8.4 Hz, H10); 7.81 (d, 2H, J ˆ 16.2 Hz,
H8); 7.75 (d, 2H, J ˆ 5.0 Hz, H5); 7.70 (d, 2H, J ˆ 16.2 Hz,
H7). UV-visible (acetone): lmax ˆ 339 nm. IR (KBr):
nN=O ˆ 1342 cm 1. HRMS (FAB): calc. for C26H18N4O2
1
give the title product as a brown oil (2.3 g, 90%). H NMR
‡
[M ‡ H] 450.1328; found 450.1339.
(CDCl3): d 7.85 (d, J ˆ 7.8 Hz, 2H, ±C6H4±), 7.55 (d, J ˆ 7.8
Hz, 2H, ±C6H4±), 4.48 (s, 2H, ±CH2Br), 3.03 (t, J ˆ 7.9 Hz, 2H,
SO2CH2), 1.66 (m, 2H, SO2CH2CH2), 1.2 [m, 10H, ±(CH2)5 ±],
0.83 (t, J ˆ 5.8 Hz, ±CH2CH3).
4,40-Diformyl-[2,20]-bipyridine (5). In a Schlenk ¯ask, 4,40-
bis(N,N-dimethylaminovinyl)-[2,20]-bipyridine 6 (1.7 g, 5.8
mmol, 1 equiv.) was dissolved in THF (180 mL) and an
aqueous solution of NaIO4 (10 g, 46.7 mmol, 8 equiv.) was
added dropwise; the orange mixture turned white when
stirred at 40 ꢀC for 18 h. After cooling to room temperature,
the solvent was removed under vacuum. The product was
dissolved in dichloromethane (40 mL) and washed with
water (2 Â 50 mL). The organic phase was dried over
MgSO4 , ®ltered, and the solvent then removed under
vacuum. After precipitation from dichloromethane±pentane,
5 was ®nally obtained as a pale-yellow microcrystalline
powder (1.45 g, 80% yield). 1H NMR (CDCl3): d 10.19 (s,
1-[(Diethylphosphonate)methyl]-4-(octane-1-sulfonyl)-
benzene. Triethylphosphite (1 mL) was added dropwise to 1-
(bromomethyl)-4-(octane-1-sulfonyl)benzene (1.84 g, 5.3 mmol)
at 0 ꢀC. After heating under re¯ux overnight, the excess
triethylphosphite was evaporated and the crude brown oil
was puri®ed by column chromatography (silica gel, heptane±
ether±dichloromethane 55:40:5). The title product was
1
recovered as a colorless oil (1.3 g, 62%). H NMR (CDCl3):
d
7.81 (d, J ˆ 7.8 Hz, 2H, ±C6H4±), 7.46 (d, J ˆ
4
7.8 Hz, 2H, ±C6H4±), 4.11±3.94 (m, 4H, O±CH2), 3.19 (d,
JH±P ˆ 22.4 Hz, 2H, PCH2), 3.03 (t, J ˆ 7.9 Hz, 2H, SCH2),
1.66 (m, 2H, SCH2CH2), 1.23 (t, J ˆ 7.0 Hz, OCH2CH2), 1.2
[m, 10H, ±(CH2)5±], 0.83 (t, J ˆ 5.8 Hz, ±CH2CH3). 13C
NMR (CDCl3): d 138.4 (d, JP±C ˆ 8.8 Hz, Cipso or Cpara),
137.7 (d, JP±C ˆ 3.6 Hz, Cipso or Cpara), 130.6 (d, JP±C ˆ 6.5
Hz, Cortho or Cmeta , 128.2 (d, JP±C ˆ 2.5 Hz, Cortho or Cmeta),
62.3 (d, JP±C ˆ 6.8 Hz, OCH2CH3), 16.3 (d, JP±C ˆ 6.0 Hz,
OCH2CH3), 33.8 [d, JP±C ˆ 137.4 Hz, PhCH2P(O)(OEt)2),
56.3, 31.6, 28.9, 28.8, 28.2, 22.6, 22.5, 14.0 (s, OOCt). 31P
NMR (CDCl3) d 24.8. HRMS (EI): calc. for C19H33O5PS
404.1786; found 404.1798.
2H, CHO), 8.92 (d, J ˆ 5 Hz, 2H, H6), 8.85 (d, J ˆ 1.5 Hz,
2H, H3), 7.75 (dd, 3J ˆ 5 Hz, 4J ˆ 1.5 Hz, 2H, H5). 13C
NMR (CDCl3): d 157.4 (C2), 157.0 (CHO), 150.7 (C6), 142.8
(C4), 121.7 (C5), 121.0 (C3). Mp ˆ 192 ꢀC. IR (KBr)
1
nC=O ˆ 1704 cm
.
4,40-Bis(N,N-dimethylaminovinyl)-[2,20]-bipyridine (6). To a
solution of 4,40-dimethyl-[2,20]-bipyridine (1.57 g, 8.5 mmol, 1
equiv.) in DMF (15 mL) was added under argon the Bredereck
reagent [tert-butoxybis(diethylamino)methane; (8.1 mL, 39.1
mmol, 4.6 equiv.]. The mixture was then heated at 140 ꢀC for
18 h. After cooling to room temperature, the pale orange
mixture was hydrolyzed by addition of water (30 mL) and
extracted with dichloromethane (4 Â 30 mL). The organic
phase was dried over MgSO4 and the solvent removed under
vacuum. The resulting orange solid was recrystallized from
dichloromethane±pentane to give 6 as a sa€ron-yellow powder
4,40-Bis( p-octylsulfonylstyryl)-[2,20]-bipyridine (3). In
a
Schlenk ¯ask, 1-[(diethylphosphonate)methyl]-4-(octane-1-
sulfonyl)benzene (1.13 g, 2.8 mmol) was dissolved in THF
(50 mL) and cooled to 0 ꢀC. nBuLi (1.6 M in hexane, 2 mL,
3 mmol) was added dropwise by means of a syringe and the
solution was stirred for 1 h at room temperature. A THF
solution (50 mL) of 5 (0.3 g, 1.4 mmol) was then slowly
added and the mixture was heated for 3 h under re¯ux. After
cooling to room temperature, the solution was hydrolyzed
upon addition of water (40 mL). The organic phases were
dried over MgSO4 , ®ltered, and the solvent was removed
under vacuum. After recrystallization from ethanol,
compound 3 was obtained as an orange microcrystalline
powder (0.2 g, 20% yield). 1H NMR (CD2Cl2): d 8.65 (d,
J ˆ 5 Hz, 2H, H6), 8.60 (br s 2H, H3), 7.86 (d, J ˆ 8.5 Hz,
4H, H11), 7.73 (d, J ˆ 8.5 Hz, 4H, H10), 7.49 (d, J ˆ 16 Hz,
2H, H8), 7.43 (dd, 3J ˆ 5 Hz, 4J ˆ 1.5 Hz, 2H, H5), 7.28 (d,
J ˆ 16 Hz, 2H, H7), 3.06 (br t, 8H, H13), 1.61 (m, 8H, H14),
1.20 (m, 20H, H15±19), 0.81 (br t, 12H, H20). 13C NMR
(CD2Cl2) : d 156.17 (C2), 149.50 (C6), 144.57 (C4), 141.29
(C9), 138.50 (C12), 131.00 (C8), 129.66 (C7), 128.40 (C11),
127.35 (C10), 121.15 (C5), 118.14 (C3), 56.06 (C13), 31.46
(C14), 28.73 (C15), 28.70 (C16) 28.02 (C17), 22.51 (C18),
22.37 (C19), 23.62 (C20). HRMS (FAB): calc. for
1
(2.4 g, 95% yield). H NMR (CDCl3): d 8.30 (d, J ˆ 5.2 Hz,
4
2H, H6), 8.10 (d, J ˆ 1.8 Hz, 2H, H3), 7.15 (d, J ˆ 13.6 Hz,
3
4
2H, H8), 6.91 (dd, J ˆ 5.2 Hz, J ˆ 1.8 Hz, 2H, H5), 5.02 (d,
J ˆ 13.6 Hz, 2H, H7), 2.83 (s, 12H, CH3). 13C NMR (CDCl3):
d 156.2 (C2), 148.8 (C6), 148.7 (C4), 143.4 (C8), 118.0 (C5),
114.7 (C3), 94.5 (C7), 40.6 (C9). HRMS (FAB): calc. for
‡
C18H22N4 [M ‡ H] 294.1844; found 294.1862.
4.40-Bis(dibutylaminothienylvinyl)-[2,20]-bipyridine (7). To a
THF solution (20 mL) of diisopropylamine (1 mL, 7 mmol)
at
20 ꢀC was added nBuLi (1.6 M in hexane, 5 mL, 6.5
mmol) via syringe. The solution was then stirred for 15 min
and a solution of 4,40-dimethyl-[2,20]-bipyridine (0.58 g, 3.1
mmol) in THF (30 mL) was added dropwise at 20 ꢀC. The
red mixture was stirred for 2 h. A solution of 5-(N,N-
dibutylaminothiophene-2-carbaldehyde (1.5 g, 6.2 mmol) in
THF (20 mL) was slowly added at
yellow±green solution was stirred for 2 h at
then at room temperature overnight. After hydrolysis and
extraction with dichloromethane, the organic layers were
dried over MgSO4 , ®ltered and the solvent was removed
20 ꢀC. The resulting
20 ꢀC and
‡
C42H53N2O4S2 [M ‡ H] 713.3447; found 713.3430.
New J. Chem., 2001, 25, 1553±1566
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0
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under vacuum. The resulting yellow oil was dissolved in
toluene (150 mL). After addition of a catalytic amount of
pyridinium p-toluenesulfonate salt (0.1 g, 0.4 mmol), the red
mixture was stirred under re¯ux for 4 h in a Dean±Stark
apparatus. After evaporation of the toluene in vacuo, the
residue was dissolved in dichloromethane and washed with a
saturated aqueous solution of NaHCO3 . The organic layer
was dried over MgSO4 , ®ltered and the solvent removed.
After recrystallisation in ethyl acetate, compound 7 was
obtained as a dark-red microcrystalline powder (1.77 g,
91%). 1H NMR (CD2Cl2): d 8.43 (d, J ˆ 5.2 Hz, 2H, H6),
8.33 (br s, 2H, H3), 7.39 (d, J ˆ 15.8 Hz, 2H, H8), 7.16 (dd,
3J ˆ 5.2 Hz, 4J ˆ 1.6 Hz, 2H, H5), 6.79 (d, J ˆ 4 Hz, 2H,
H10), 6.35 (d, J ˆ 15.8 Hz, 2H, H7), 5.65 (d, J ˆ 4 Hz, 2H,
H11), 3.21 (t, 8H, J ˆ 7.5 Hz, H13), 1.57 (m, 8H, H14), 1.30
(m, 8H, H15), 0.90 (t, 12H, H16). 13C NMR (CD2Cl2):
d 158.95 (C12), 156.32 (C2), 149.22 (C6), 146.55 (C4), 130.94
(C10), 127.45 (C8), 124.06 (C9), 119.75 (C5), 117.95 (C7),
116.77 (C3), 100.68 (C11), 53.26 (C13), 29.22 (C14), 20.28
(C15), 13.74 (C16). HRMS (FAB): calc. for C38H50N4S2
4,4 -Diamino-[2,2 ]-bipyridine (10). In a typical experiment,
4,40-dinitro-[2,20]-bipyridine-[1,10]-dioxide (3 g, 10.8 mmol)
was dissolved in ethanol (100 mL) and 10% palladium on
carbon was added (0.7 g).
A solution of hydrazine
monohydrate (2.5 mL, 54 mmol) in ethanol (20 mL) was
added dropwise over a period of 1 h and the reaction
mixture was re¯uxed for 8 h. The mixture was then ®ltered
hot and washed with cold diethyl ether. The ®ltrate was
evaporated under vacuum and the residue recrystallized from
ethanol to yield 10 (2.01 g, 100%) as a yellow solid. 1H
NMR (DMSO-d6): d 7.89 (d, 2H, J ˆ 5.5 Hz, H6), 7.45 (d,
4
2H, J ˆ 2.2 Hz, H3), 6.42 (dd, 2H, J ˆ 5.5, J ˆ 2.2 Hz, H5),
5.92 (s, 4H, NH2).
Typical procedure for successive additions of p-toluenesulfonic
acid to 1. In a Schlenk ¯ask, 4,40-bis(dibutylaminostyryl)-[2,20]-
bipyridine (36 mg, 58.6 mmol) was dissolved in dichloromethane
(100 mL) at room temperature. p-Toluenesulfonic acid (PTSA)
was added, 1 equiv. at a time (1 equiv. ˆ 11.2 mg, 58.6 mmol) and
the mixture was stirred for 3 h, due to the low solubility of PTSA
in CH2Cl2 . A 1 mL aliquot was diluted in order to follow the
reaction by UV-visible and ¯uorescence spectroscopy. The
initially yellow solution turned deep-red after addition of 1
and 2 equiv. of PTSA. After further addition of acid (4
equiv.), the mixture became colorless.
‡
[M ‡ H] 627.3555; found 627.3556.
4,40-Bis(diethylaminophenylimino)-[2,20]-bipyridine (8). 4,40-
Diformyl-[2,20]-bipyridine (70 mg, 0.33 mmol) was dissolved
in 10 mL of dichloromethane. N,N-Diethyl-1,4-phenyl-
enediamine (0.12 mL, 0.69 mmol) and anhydrous magnesium
sulfate (1 g) were added and the mixture was stirred for 2 h.
The reaction mixture was ®ltered, the solvent evaporated, the
resulting solid dissolved in dichloromethane and puri®ed by
precipitation with pentane to yield 8 as a yellow powder
(0.24 g, 95%). Single crystals suitable for X-ray di€raction
analysis were obtained by slow evaporation of a dichlo-
romethane solution. 1H NMR (CD2Cl2): d 8.78 (br s, 2H,
H3), 8.74 (d, J ˆ 5.0 Hz, 2H, H6), 8.61 (s, 2H, H7), 7.79 (dd,
3J ˆ 5.0 Hz, 4J ˆ 1.5 Hz, 2H, H5), 7.33 (d, J ˆ 9.0 Hz, 4H,
H10), 6.69 (d, J ˆ 9.0 Hz, 4H, H11), 3.38 (q, J ˆ 7.0 Hz, 8H,
H13), 1.16 (t, J ˆ 7.0 Hz, 12H, H14). 13C NMR (CD2Cl2): d
157.0 (C2), 151.7 (C7), 150.1 (C6), 148.2 (C12), 145.5 (C4),
138.5 (C9), 123.7 (C11), 121.7 (C5), 120.1 (C3), 112.1 (C10),
44.9 (C13), 12.8 (C16). HRMS (FAB): calc. for C32H37N6
[4,40-Bis(dibutylaminostyryl)-[2,20]-bipyridine][tetra-chloro-
1,2-benzenediol]. In a Schlenk ¯ask, 4,40-bis(dibutylamino
styryl)-[2,20]-bipyridine (100 mg, 0.163 mol) and tetrachloro-
1,2-benzenediol (40 mg, 0.163 mmol) were dissolved in
dichloromethane (30 mL) at room temperature. The solution
immediately turned yellow±brown. After 1 h of stirring, the
solvent was evaporated and the product precipitated from
dichloromethane±pentane to yield the title product as a
brown powder (0.14 g, 100%). 1H NMR (CD2Cl2): d 8.30
(d, J ˆ 4.5 Hz, 2H, H6), 8.04 (br s, 2H, H3), 7.5 (br s, 2H,
OH), 7.31 (d, J ˆ 8.5 Hz, 4H, H10), 7.20 (d, J ˆ 16 Hz, 2H,
H8), 7.12 (masked, 2H, H5), 6.72 (d, J ˆ 16.2 Hz, 2H, H7),
6.58 (d, J ˆ 8.5 Hz, 4H, H11), 3.25 (t, J ˆ 7 Hz, 8H, H13),
1.5 (m, 8H, H14), 1.30 (m, 8H, H15), 0.92 (t, J ˆ 7.2 Hz,
12H, H16). 13C NMR (CD2Cl2): d 154.98 (C2), 148.62 (C12),
148.60 (C6), 147.75 (C4), 134.53 (C8), 129.71 (C10), 122.79
(C9), 120.42 (C5), 119.69 (C7), 118.21 (C3), 111.49 (C11),
50.70 (C13), 29.41 (C14), 20.30 (C15), 13.79 (C16), 144.32,
128.15, 121.90 [Cl4C6(OH)2]. UV-visible (CH2Cl2): lmax ˆ 414
nm. Anal. calc. (found) for C48H56N4Cl4O2 Á 0.5CH2Cl2 : C
64.35 (63.83), H 6.35 (6.41), N 6.19 (5.96%).
‡
[M ‡ H] 505.3079; found 505.3088.
4,40-Bis(dibutylaminophenylazo)-[2,20]-bipyridine (9). 4,40-Di-
amino-[2,20]-bipyridine (0.5 g, 2.6 mmol) was dissolved in 10 mL
of sulfuric acid (4 N) under argon. The solution was cooled to
0 ꢀC and sodium nitrite (0.38 g, 5.5 mmol), dissolved in the
minimum amount of water, was added with the temperature
kept between 0 and 5 ꢀC. After 30 min of stirring, N,N-
dibutylaniline (1.25 g, 5.5 mmol) in THF (10 mL) was added
dropwise to the diazonium salt solution. The solution was
then allowed to warm to room temperature and the excess
acid was neutralized by an aqueous solution of potassium
hydroxide (4 N). The reaction mixture was ®ltered and
extracted with dichloromethane. The organic layer was dried
over magnesium sulfate and concentrated under reduced
pressure. The crude product was then puri®ed by silica gel
column chromatography (dichloromethane±ether 1:1) to yield
9 as a dark-red solid (0.25 g, 25%). Orange crystals suitable
for an X-ray di€raction study were obtained by slow
evaporation of a dichloromethane-d2 solution in an NMR
Typical procedure for the complexation of 1. In a Schlenk
¯ask, 4,40-bis(dibutylaminostyryl)-[2,20]-bipyridine (1) and
the corresponding metallic salt [HgCl2 , ZnCl2 , Zn(OAc)2 ,
PdCl2 , Re2O7] were dissolved in dichloromethane (10 mL)
at room temperature. The yellow solution turned from
orange to deep violet depending on the nature of the metallic
moiety. The solution was then concentrated in vacuo and
microcrystalline product obtained from dichloro-methane±
pentane.
(1)HgCl2 (1a). Orange microcrystalline powder (80% yield).
1H NMR (CD2Cl2): d 8.19 (d, J ˆ 5.5 Hz, 2H, H6), 8.11 (br s,
2H, H3), 7.48 (d, J ˆ 15.8 Hz, 2H, H8), 7.48 (d, J ˆ 9 Hz, 4H,
H10), 7.26 (d, J ˆ 5.5 Hz, 2H, H5), 6.76 (d, J ˆ 15.8 Hz, 2H,
H7), 6.65 (d, J ˆ 9 Hz, 4H, H11), 3.31 (t, J ˆ 7.4 Hz, 8H,
H13), 1.56 (m, 8H, H14), 1.36 (m, 8H, H15), 0.96 (t, J ˆ 7.2
Hz, 12H, H16). 13C NMR (CD2Cl2): d 150.07 (C2), 149.46
(C12), 148.76 (C4), 148.76 (C6), 137.31 (C8), 129.43 (C10),
122.31 (C9), 122.06 (C5), 118.54 (C3), 117.93 (C7), 111.50
(C11), 50.74 (C13), 29.43 (C14), 20.30 (C15), 13.79 (C16).
1
3
5
tube. H NMR (CD2Cl2): d 8.76 (dd, 2H, J ˆ 5.2 Hz, J ˆ 0.6
4
5
Hz, H6), 8.71 (dd, 2H, J ˆ 2.0 Hz, J ˆ 0.6 Hz, H3), 7.88 (d,
3
4
4H, J ˆ 9.2 Hz, H11), 7.63 (dd, 2H, J ˆ 5.2 Hz, J ˆ 2.0 Hz,
H5), 6.71 (d, 4H, J ˆ 9.2 Hz, H10), 3.38 (t, 8H, J ˆ 7.6 Hz,
H13), 1.5 (m, 8H, H14), 1.30 (m, 8H, H15), 0.96 (t, 12H,
J ˆ 7.2 Hz, H16). 13C NMR (CD2Cl2): d 159.3 (C4), 157.7
(C2), 151.8 (C12), 150.4 (C6), 143.0 (C9), 126.2 (C11), 116.2
(C5), 113.2 (C3), 111.2 (C10), 51.0 (C13), 29.4 (C14), 20.3
(C15), 13.7 (C16). HRMS (FAB): calc. for C38H51N8
Anal. calc. (found) for C42H54N4Cl2HgÁH2O:
C 55.78
‡
(55.64), H 6.24 (5.87), N 6.19 (6.30%).
[M‡ H] 619.4237; found 619.4255.
1564
New J. Chem., 2001, 25, 1553±1566

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(1)ZnCl2 (1b). Orange microcrystalline powder (65% yield).
1H NMR (CD2Cl2): d 8.20 (d, J ˆ 5.5 Hz, 2H, H6), 8.11 (br
s, 2H, H3), 7.60 (d, J ˆ 16 Hz, 2H, H8), 7.53 (d, J ˆ 9 Hz,
4H, H10), 7.18 (d, J ˆ 5.5 Hz, 2H, H5), 6.70 (d, J ˆ 16 Hz,
2H, H7), 6.67 (d, J ˆ 9 Hz, 4H, H11), 3.33 (t, J ˆ 7Hz, 8H,
H13), 1.60 (m, 8H, H14), 1.40 (m, 8H, H15), 0.97 (t, J ˆ 7.2
Hz, 12H, H16). 13C NMR (CD2Cl2): d 150.95 (C2), 149.56
(C12), 148.34 (C4), 147.45 (C6), 138.38 (C8), 129.68 (C10),
122.19 (C9), 122.50 (C5), 116.80 (C3), 117.25 (C7), 111.42
(C11), 50.72 (C13), 29.39 (C14), 20.27 (C15), 13.79 (C16).
Anal. calc. (found) for C42H54N4Cl2ZnÁH2O: C 65.77 (65.75),
H 7.36 (7.00), N 7.31 (7.40%).
Acknowledgements
The authors would like to thank the Region Bretagne and
France Telecom CNET for ®nancial support. We gratefully
acknowledge Brigitte Corre and Thomas Le Bouder for tech-
nical support.
References and notes
1
(a) Molecular Nonlinear Optics, Materials, Physics and Devices, ed.
J. Zyss, Academic Press, New York, 1994; (b) D. M. Burland,
Chem. Rev., 1994, 94, 1.
(1)Zn(OCOCH3)2 (1c). Orange microcrystalline powder (65%
yield). 1H NMR (CD2Cl2): d 8.51 (d, J ˆ 5.5 Hz, 2H, H6), 8.05
(br s, 2H, H3), 7.38 (d, J ˆ 16 Hz, 2H, H8), 7.44 (d, J ˆ 9 Hz,
4H, H10), 7.30 (d, J ˆ 5.5 Hz, 2H, H5), 6.73 (d, J ˆ 16 Hz, 2H,
H7), 6.64 (d, J ˆ 9 Hz, 4H, H11), 3.30 (t, J ˆ 7Hz, 8H, H13),
1.58 (m, 8H, H14), 1.35 (m, 8H, H15), 0.95 (t, J ˆ 7.2 Hz,
12H, H16), 1.99 (s, 6H, OCOCH3). 13C NMR (CD2Cl2): d
150.48 (C2), 149.37 (C12), 149.24 (C4), 148.82 (C6), 137.08
(C8), 129.26 (C10), 122.33 (C9), 121.89 (C5), 118.28 (C7),
117.31 (C3), 111.47 (C11), 50.70 (C13), 29.40 (C14), 20.29
(C15), 13.79 (C16), 179.80, 21.95 (OCOCH3). Anal. calc.
(found) for C42H54N4Cl2ZnÁH2O: C 67.68 (67.84), H 7.65
(7.48), N 6.86 (7.01%).
2
(a) J. L. Segura and N. Martin, J. Mater. Chem., 2000, 10, 2403; (b)
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6
7
H. Le Bozec and T. Renouard, Eur. J. Inorg. Chem., 2000, 229.
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and J. Zyss, Chem. Commun., 1999, 2521; (b) M. Bourgault,
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(1)PdCl2 (1d). Red±orange microcrystalline powder (90%
yield). 1H NMR (CD2Cl2): d 8.93 (d, J ˆ 6.2 Hz, 2H, H6),
7.96 (br s, 2H, H3), 7.46 (d, J ˆ 16 Hz, 2H, H8), 7.46 (d,
8
3
4
J ˆ 8.9 Hz, 4H, H10), 7.34 (dd, J ˆ 6.2 Hz, J ˆ 1.5 Hz, 2H,
H5), 6.84 (d, J ˆ 16 Hz, 2H, H7), 6.63 (d, J ˆ 8.9 Hz, 4H,
H11), 3.30 (t, J ˆ 7.5 Hz, 8H, H13), 1.56 (m, 8H, H14), 1.34
(m, 8H, H15), 0.94 (tr, J ˆ 7.2 Hz, 12H, H16). 13C NMR
(CD2Cl2): d 156.21 (C2), 149.89 (C12), 149.76 (C4), 149.60
(C6), 138.28 (C8), 129.64 (C10), 121.99 (C9), 121.45 (C5),
118.40 (C3), 117.33 (C7), 111.52 (C11), 50.74 (C13), 29.40
(C14), 20.28 (C15), 13.76 (C16). Anal. calc. (found) for
C42H54N4Cl2PdÁH2O: C 62.26 (62.19), H 6.97 (6.78), N 6.91
(7.34%).
9
10 (a) R. Ziessel, A. Juris and M. Venturi, Inorg. Chem., 1998, 37,
5061; (b) N. Armaroli, L. De Cola, V. Balzani, J.-P. Sauvage,
C. O. Dietrich-Buchecker, J.-M. Kern and A. Bailal, J. Chem.,
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11 H. S. Joshi, R. Jamshidi and Y. Tor, Angew. Chem., Int. Ed., 1999,
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12 A.-J. Attias, P. Hapiot, V. Witgens and P. Valat, Chem. Mater.,
2000, 12, 461.
(1)Re(CO)3Cl (1e). Re(CO)5Cl (88 mg, 24 mmol) and 1 (150
mg, 24 mmol, 1 equiv.) were stirred overnight in re¯uxing
toluene (5 mL). The red solution was concentrated and the
complex was precipitated upon addition of pentane (20 mL).
After recrystallization from dichoromethane±pentane 1e was
obtained as a red microcrystalline powder (130 mg, 57%).
1H NMR (CD2Cl2): d (d, J ˆ 5.9 Hz, 2H, H6), 8.05 (br s,
2H, H3), 7.56 (d, J ˆ 8.8 Hz, 4H, H10), 7.29 (d, J ˆ 16.3 Hz,
2H, H8), 6.79 (d, J ˆ 5.9 Hz, 2H, H5), 6.69 (d, J ˆ 8.8 Hz,
4H, H11), 6.52 (d, J ˆ 16.3 Hz, 2H, H7), 3.31 (t, J ˆ 7.7 Hz,
8H, H13), 1.58 (m, 8H, H14), 1.3 (m, 8H, H15), 0.94 (t,
J ˆ 7.2 Hz, 12H, H16). 13C NMR (CD2Cl2 53.45): d 198.3
(CO equatorial), 191.0 (CO axial), 155.7 (C2), 151.5 (C6),
149.5 (C12), 149.0 (C4), 137.8 (C8), 129.8 (C10), 123.0 (C5),
122.5 (C9), 118.8 (C3), 117.3 (C7), 111.7 (C11), 50.8 (C13),
29.4 (C14), 20.3 (C15), 13.8 (C16). Anal. calc. (found) for
C45H54N4O3ClRe: C 58.71 (58.25), H 5.91 (5.79), N 6.09
(5.95%).
[(1)ReO3]2-m2-O (1f). Deep purple microcrystalline powder.
(80% yield). 1H NMR (CD2Cl2): d 8.28 (d, J ˆ 5.6 Hz, 2H,
H6), 8.23 (br s, 2H, H3), 7.36 (d, J ˆ 8.8 Hz, 4H, H10), 7.37
(d, J ˆ 16 Hz, 2H, H8), 7.22 (d, J ˆ 5.6 Hz, 2H, H5), 6.64 (d,
J ˆ 16 Hz, 2H, H7), 6.54 (d, J ˆ 8.8 Hz, 4H, H11), 3.2 (t,
J ˆ 6.7 Hz, 8H, H13), 1.5 (m, 8H, H14), 1.2 (br m, 8H,
H15), 0.90 (t, J ˆ 7.2 Hz, 12H, H16).13C NMR (CD2Cl2): d
151.72 (C4), 149.62 (C12), 147.10 (C2), 144.95 (C6), 138.69
(C8), 129.98 (C10), 122.32 (C9), 121.14 (C5), 118.32 (C3),
117.77 (C7), 111.48 (C11), 50.74 (C13), 29.39 (C14), 20.26
(C15), 13.76 (C16). IR (KBr): n(ReˆO) ˆ 911.5 (vs), 967 (w)
cm 1. Anal. calc. (found) for C84H108N8O7Re2ÁCH2Cl2 : C
56.74 (57.20), H 6.16 (6.46), N 6.41 (6.23%).
13 (a) A. Juris, S. Campagna, I. Bidd, J.-M. Lehn and R. Ziessel,
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14 A.-J. Attias, C. Cavally, B. Bloch, N. Guillou and C. Noel, Chem.
Mater., 1999, 11, 2057.
15 For a preliminary communication see: P. Dupau, T. Renouard
and H. Le Bozec, Tetrahedron Lett., 1996, 37, 7503.
16 (a) O. Kocian, R. J. Mortimer and P. D. Beer, Tetrahedron Lett.,
1990, 31, 5069; (b) M. Harding, U. Koert, J.-M. Lehn,
A. Marquis-Rigault, C. Piguet and J. Siegel, Helv. Chim. Acta,
1992, 74, 594.
17 M. G. Vetelino and J. W. Coe, Tetrahedron Lett., 1994, 35, 219.
18 (a) For an analogous phosphonate synthesis see: S. Nijhuis,
G. L. A. Rikken, E. E. Havinga, W. Ten Hoeve, H. Wynberg and
E. W. Meijer, J. Chem. Soc., Chem. Commun., 1990, 1093;
(b) A. Ulman, C. S. Willand, W. Kohler, D. R. Robello,
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19 D. Prim, G. Kirsch and J.-F. Nicoud, Synth. Lett., 1998, 383.
20 In the case of 2, acidic impurities in deuterated chloroform are
able to promote the isomerization of the double bond. Such
phenomena are not observed in deuterated dichloromethane.
21 For analogous synthetic procedures of imino-containing dyes see:
S.-S. Chou, D.-J. Sun, H.-C. Lin and P.-K. Yang, Chem. Com-
mun., 1996, 1045.
22 (a) I. R. Whittall, M. G. Humphrey, A. Persoons and
S. Houbrechts, Organometallics, 1996, 15, 1935; (b) I. R. Whittall,
M. G. Humphrey, S. Houbrechts, A. Persoons and D. C. Hockless,
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23 C. Colas and M. Goelner, Eur. J. Org. Chem., 1999, 1357.
24 P. Wehman, G. C. Dol, E. R. Morrman, P. C. J. Kramer,
P. W. N. M. V an Leeuwen, J. Fraanje and K. Goubitz, Organo-
metallics, 1994, 13, 4856.
25 H. Camren, M.-Y. Chang, L. Zen and E. McGuire, Synth. Com-
mun., 1996, 26, 1247.
New J. Chem., 2001, 25, 1553±1566
1565

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26 G. Maerker and F. H. Case, J. Am. Chem. Soc., 1958, 80, 2745.
27 For other molecular structures of 2,20-bipyridines see: (a)
M. H. Chisholm, J. C. Hu€man, I. P. Rothwell, P. G. Bradley, N.
Kress and W. H. Woodru€, J. Am. Chem. Soc., 1981, 103, 4945;
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Ber., 1992, 125, 453.
39 M. Maus and K. Rurack, New J. Chem., 2000, 24, 677.
40 C. C. Evans, M. Bagieu-Beucher, R. Masse and J.-F. Nicoud,
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1566
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