Sequential synthesis of rigid multi-bipyridine ligands bearing diethynyl/phenyl solubilizing fragments

Article abstract of DOI:10.1016/S0040-4039(00)00498-6

Reliable and practical synthetic routes for the construction of multitopic bipyridine ligands are presented. The first series contains the chelating fragments connected via an ethynyl function and the second series is built from an alternation of ethynyl/phenyl/bipyridine modules. The synthetic protocol is based on sequential Pd-promoted cross-coupling reactions between selected bis-bpy or ethynyl/phenyl/bpy intermediates of increasing size. The ligands bearing functionalized 1,4-diethynyl-2,5- di(dodecyloxy)benzene subunits are soluble in chlorinated solvents. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00498-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 3837–3841
Sequential synthesis of rigid multi-bipyridine ligands bearing
diethynyl/phenyl solubilizing fragments
Abderrahim Khatyr and Raymond Ziessel ∗
Laboratoire de Chimie, d’Electronique et de Photonique Moléculaires, associé au CNRS, Ecole Chimie, Polymères,
Matériaux, 25 rue Becquerel, 67087 Strasbourg, Cédex 02, France
Received 15 February 2000; accepted 18 March 2000
Abstract
Reliable and practical synthetic routes for the construction of multitopic bipyridine ligands are presented. The
first series contains the chelating fragments connected via an ethynyl function and the second series is built from an
alternation of ethynyl/phenyl/bipyridine modules. The synthetic protocol is based on sequential Pd-promoted cross-
coupling reactions between selected bis-bpy or ethynyl/phenyl/bpy intermediates of increasing size. The ligands
bearing functionalized 1,4-diethynyl-2,5-di(dodecyloxy)benzene subunits are soluble in chlorinated solvents.
© 2000 Elsevier Science Ltd. All rights reserved.
Tremendous innovations in synthetic methodology, together with allied advances in technology, pro-
vide new opportunities for the chemist to explore classical problems and to apply the results to newly-
emerging disciplines. A good example of this progressive advancement in scientific knowledge concerns
the design of materials for use in molecular-scale electronic devices.1 Following the realization that
many simple transition metal complexes with 2,20-bipyridine (bpy) ligands exhibit outstanding redox
and photochemical properties,2 strenuous efforts have been made to incorporate such complexes into
highly-organized molecular arrays. Subsequent examination by fast laser spectroscopy reveals that such
supramolecular systems facilitate rapid energy and/or electron transfer between the individual compo-
nents. With sophisticated spectroscopic tools it becomes possible to delve deeply into the information
transfer mechanism, thereby stimulating rational design of advanced molecular systems. In turn, it
leads to interesting synthetic and analytical problems and leads to an exciting iteration of ideas and
methodology.3
As regards the construction of linear polynuclear metal complexes having discrete units linked by
short, highly-conductive wires, we and others have identified polyynes as being particularly attrac-
tive molecular-scale connectors.3,4 The interest in such multinuclear complexes is stimulated by the
recent structural determinations of photosynthetic light-harvesting complexes and by the realization
that these natural antennaes effectively delocalize the photon over a large number of spatially remote,
∗
Corresponding author. Fax: 33 3 88 13 68 95; e-mail: ziessel@chimie.u-strasbg.fr (R. Ziessel)
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00498-6
tetl 16777

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but chemically identical pigments.5 Clear proof that triplet energy migration occurs between terminal
metal fragments in artificial systems has not yet been clearly identified despite some hints deduced from
transient depolarized measurements.2
Here we wish to report a synthetic strategy which allows the preparation of stable and quasi-
linear multitopic bipyridine ligands which are auspicious for the study of photon shuttling between
chromophoric units. One of the most straightforward methods for the synthesis of arylalkynes, conjugated
enynes and pyridylalkynes is the palladium-promoted coupling of terminal alkynes with aryl, alkenyl or
pyridyl halides which was pioneered by Sonogashira et al.6 We have previously found that this reaction
is extremely versatile and highly effective when run in the presence of an inordinately wide variety of
available oligopyridines.7 Here we discovered that mono-functionalization of 5,50-dibromo-bpy with 5-
ethynyl-bpy is effective when the reaction is carried out under specific conditions in the presence of the
acetylenic derivative (1 equiv.) as outlined in Scheme 1. The side product assigned as ligand 1 (≤10%)8
results from a double cross-coupling reaction. The key intermediate 2 was converted to the ethynyl-
grafted compound 3 under similar experimental conditions.
Scheme 1. (i) [Pd(PPh3)4] (6 mol%), benzene, iPr2NH, 80°C; (ii) TMSC≡CH, [Pd(PPh3)2Cl2] (6 mol%), CuI (10 mol%), THF,
iPr2NH, rt; (iii) K2CO3, CH3OH, rt; (iv) [Pd(PPh3)4] (6 mol%), benzene, iPr2NH, 80°C
Both intermediates 2 and 3 were allowed to react together to give the segmented octapyridine ligand 4
in a modest yield. Furthermore, the reaction of compound 3 with 5,50-dibromo-bpy (0.5 equiv.) afforded
the segmented decapyridine ligand 5.9 Unfortunately, an emerging problem on increasing the size of the
system is the drop of solubility as observed for ligands 4 and 5. Therefore it is desirable to design soluble
multicomponent molecules of nanometric dimension without loss of the effectiveness of the electronic
conductivity. Solubility and tunability can be achieved by spanning two bpy subunits with substituted
diethynylphenyl subunits functionalized with two dodecyloxy moieties.
Here we show that this strategy is succesful in generating a family of rigid and multitopic ligands (2 to
5 chelating centers), where the edge-to-edge distance between the two external pyridine rings vary from
∼21–72 Å. The adopted synthetic protocol utilizes the pivotal intermediates 7 and 8, each of which being
readily prepared from synthon 6 by a selective or complete deprotection reaction using NaOH as base.10

3839
With these two intermediates in hand it was simple to prepare the soluble bis-bpy ligand 9 by
double cross-coupling of derivative 8 with 5-bromo-bpy. Furthermore, access to the pivotal mono-bpy
intermediate 10 allows us to synthesize, under mild conditions, the target tris-bpy ligand 11 in fair yield
(Scheme 2).
Scheme 2. (i) nPrNH2, [Pd0(PPh3)4] (6 mol%), 70°C, 84% for 9, 91% for 10 and 80% for 11; (ii) NaOH (excess), benzene, 87%
This rational design has been extended to the construction of larger multitopic ligands. Here, the
pivotal intermediates 12 and 14 were obtained as shown in Scheme 3 and allowed to react, under Pd-
catalyzed conditions, with starting material 8 (0.5 equiv.) and 5,50-dibromo-bpy to produce the tetra- and
penta-bpy ligands 13 and 15, respectively. It is worth pointing out that during the synthesis of derivative
12, a side product assigned as the tritopic ligand 11 is formed by an inescapable double cross-coupling
reaction. These desired products obtained in satisfactory yields were soluble in most chlorinated solvents,
photostable and characterized by 1H, 13C{1H} NMR, FAB+-MS, UV–vis and IR spectroscopies. All data
were consistent with the proposed structures (see Table 1 for selected analytical data). A typical NMR
spectrum for compound 13 is given in Ref. 11.
In summary, we have presented a logical synthetic protocol for the synthesis of soluble segmented
bipyridine-based ligands. At each stage of the procedure, key intermediates were prepared by either a
selective or a complete deprotection reaction. One set of reaction conditions based on Pd(0)-catalyzed
cross-coupling reactions is suitable for the entire iterative synthetic sequence. The ready availability
of the reagents, the overall simplicity of the procedure, the use of mild reaction conditions, and the
reasonable yields obtained, suggest that this methodology will be useful for the preparation of even
more sophisticated scaffolds. The chemical- and photo-stability of these rigid-rod conjugated oligomers
makes them very attractive for the construction of luminophoric d-block metal complexes where each
metal subunit could act as a relay station in photon shuttling processes. We are currently exploring the
coordinative properties of these multitopic ligands, to offer further insights into the chemistry of these
interesting systems.12

3840
n
n
Scheme 3. (i) PrNH2, [Pd0(PPh3)4] (6 mol%), 70°C, 76% for 12, 76% for 13 and 71% for 15; (ii) PrNH2, [Pd0(PPh3)4] (6
mol%), 70°C, 85%; (iii) NaOH (excess), benzene, 80°C, 84% for 14
Table 1
Selected data for the novel poly-bipyridine ligands
Acknowledgements
We thank Dr. Muriel Hissler for the synthesis of intermediates 2 and 3 in the earlier stage of this project
and the ECPM and CNRS for continuous financial support.

3841
References
1. Molecular Elecronics and Molecular Electronic Devices; Sienicki, K., Ed.; CRC Press: USA, 1993; Vol. 1. Introduction to
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2. Harriman, A.; Ziessel, R. J. Chem. Soc., Chem. Commun. 1996, 1707–1716.
3. Ziessel, R.; Hissler, M.; El-ghayoury, A., Harriman A. Coord. Chem. Reviews 1998, 180, 1251–1298.
4. Maulding, D. R.; Roberts, B. G. J. Org. Chem. 1969, 34, 1734–1736; Ley, K. D.; Li, Y.; Johson, J. V.; Powell, D. H.;
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7. Ziessel, R.; Suffert, J.; Youinou, M.-T. J. Org. Chem. 1996, 61, 6535–6546.
8. Grosshenny, V.; Romero, F. M.; Ziessel, R. J. Org. Chem. 1997, 62, 1491–1500.
9. Ligand 4: mp >300°C; FAB+ m/z 691.2 [M+H]+; FT-IR (KBr, cm−1) 3049 (m), 2977 (m), 2200 (w, νC≡C), 1589 (m), 1569
(m), 1542 (m), 1465 (s), 1434 (m), 1368 (m), 1022 (m), 840 (s), 796 (s), 738 (s). Anal. calcd for C46H26N8: (Mr=690.773);
C 79.98, H 3.79, N 16.22; found: C 79.61, H 3.72, N 15.86; Ligand 5: mp >300°C; FAB+ m/z 869.4 [M+H]+; FT-IR (KBr,
cm−1) 3050 (m), 2979 (m), 2216 (w, νC≡C), 1590 (m), 1467 (s), 1439 (m), 1375 (m), 1032 (m), 799 (s), 742 (s). Anal. calcd
for C58H32N10: (Mr=868.968); C 80.17, H 3.71, N 16.12; found: C 79.80, H 3.49, N 15.89.
10. Khatyr, A.; Ziessel, R. Tetrahedron Lett. 1999, 40, 5515–5518.
1
11. H NMR (CDCl3, 20°C) for ligand 13: δ 8.83 (s, 6H), 8.69 (d, 3JH–H=4.8 Hz, 2H), 8.44 (m, 8H), 7.94 (dd, 3JH–H=8.4 Hz,
3
4
3
4
4JH–H=1.9 Hz, 6H), 7.84 (td, JH–H=7.5 Hz, JH–H=1.6 Hz, 2H), 7.33 (td, JH–H=7.5 Hz, JH–H=1.9 Hz, 2H), 7.07 (s, Ph,
6H), 4.07 (t, 3JH–H=6.2 Hz, OCH2, 12H), 1.89 (m, CH2, 12H), 1.24 (m, CH2, 108H), 0.87 (m, CH3, 18H).
12. Ziessel, R. Synthesis 1999, 1839–1865.