Phenanthroline dipyrromethene conjugates: potential building blocks for the construction of novel supramolecular architectures

Article abstract of DOI:10.1016/j.tetlet.2010.03.107

Novel tritopic ligands, based on dipyrromethenes and the 1,10-phenanthroline nucleus, as well as BF₂ complexes derived thereof are described. While BODIPY dyes 9-12 have been successfully prepared, the structural features of the free ligands 7

Full text of DOI:10.1016/j.tetlet.2010.03.107

Tetrahedron Letters 51 (2010) 2917–2919
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Tetrahedron Letters
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Phenanthroline dipyrromethene conjugates: potential building blocks
for the construction of novel supramolecular architectures
*
Antonio Garrido Montalban , Antonio J. Herrera, Jes Johannsen
Department of Chemistry, Imperial College of Science, Technology and Medicine, South Kensington, London SW7 2AY, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel tritopic ligands, based on dipyrromethenes and the 1,10-phenanthroline nucleus, as well as BF2
complexes derived thereof are described. While BODIPY dyes 9–12 have been successfully prepared,
the structural features of the free ligands 7 and 8 may render them useful as precursors for the elabora-
tion of novel supramolecular architectures.
Received 16 March 2010
Revised 25 March 2010
Accepted 25 March 2010
Available online 30 March 2010
Ó 2010 Elsevier Ltd. All rights reserved.
The self-assembly of suitable tectons using the coordination
motif is now recognized as a highly efficient strategy for the con-
struction of supramolecular architectures.1 The incorporation of
transition metal centers into such species offers access to potential
host molecules with electron transfer, magnetic, and/or optical
properties.2 From the many molecular polygons described thus
far, triangular structures were scarce and little studied,3 at the time
we initiated our work. More recently, however, the synthesis of
multimetallic supramolecular triangles has met more success.4
Our efforts in this field are aimed toward the design and synthesis
of such metal-based macrocycles and, as a first step, we report the
synthesis of novel rigid angular building blocks with a prepro-
grammed 60° angle based on the dipyrromethene-(dipyrrin)5 and
1,10-phenanthroline-ligands.6
Meso-aryl-dipyrrins can be prepared through oxidation of the
corresponding dipyrromethanes, which in turn are accessible via
the acid-catalyzed condensation of arylaldehydes and pyrrole,7
or directly from the latter and arylacyl chlorides.8 However, when
excess pyrrole was reacted with phenanthrolinedicarboxaldehyde
19 or its diacyl chloride derivative 2,9 under a variety of condi-
tions (vide infra), multicomponent mixtures, from which the
expected bis-adducts could not be isolated, were obtained. We
therefore turned our attention to less reactive pyrrole deriva-
tives with improved organic solubility. This approach was
successfully applied by us earlier for the synthesis of novel
C2v-symmetrical boron-complexes derived from dipyrromethene-
2,10-dicarboxylates which were in turn prepared from the
corresponding dipyrromethane derivatives (Fig. 1).10 Thus, while
no reaction between acyl chloride 2 with 311 or 412 occurred,
treatment of trisubstituted pyrrole 3 with dialdehyde 1 in a 4:1
molar ratio in TFA gave the desired bis(dipyrromethane) 5 in
65% yield (Scheme 1). Similarly, reaction of 1 with an excess of
ethyl pyrrole ester 4 in TFA at room temperature gave the bis-
adduct 6 in an analogous manner (66%).13 However, with a
co-solvent (MeOH or CH2Cl2) or other acid catalysts (p-TsOH or
BF3ÁOEt2) lower yields or no reaction was observed.
Oxidation of 5 and 6 (Scheme 2) to their corresponding dipyrro-
methene derivatives 7 (98%) and 8 (92%) occurred readily with
DDQ in dry CH2Cl2. In contrast to their precursors, 7 and 8 appear
to be hydrogen bonded to a molecule of water as indicated from
their proton NMR and mass spectra.
In order to probe the coordination properties of our novel li-
gands 7 and 8, we decided to synthesize BF2 complexes (BODIPY
dyes) since they are known to exhibit rich electro- and photo-
chemical properties.14 Thus, reaction of the tritopic ligands 7
and 8 with an excess of BF3ÁOEt2 in dry toluene in the presence
of Et3N at ambient temperature resulted in the formation of both,
the mono- and di-nuclear complexes 915 (18%), 10 (24%), and 11
(52%), 1216 (65%), respectively (Scheme 3). As expected, com-
plexes 9–12 display diastereotopic methylene signals in their pro-
ton- as well as two distinct resonances (dq) in their 19F NMR
spectra, consistent with restricted rotation along the
r-bonds
connecting the dipyrrin and phenanthroline fragments. In addi-
tion, 9 and 10 appear also to complex a molecule of water (vide
supra).
In summary, the synthesis and initial complexation studies of
novel angular building blocks, based on the dipyrrin- and 1,10-
phenanthroline-nuclei, have been carried out successfully. These
rigid multidentate ligands may prove useful for the elaboration
of coordinatively linked supramolecular triangles. In addition, it
is expected that complexes derived thereof should have rich and
diverse physicochemical properties. In fact, preliminary results
show the red colored complexes 9 (kmax = 577 nm), 10 (kmax
=
575 nm), 11 (kmax = 580 nm), and 12 (kmax = 579 nm) to be distinc-
tively fluorescent (Fig. 2) and such work will be reported in due
course.
* Corresponding author.
E-mail address: amontalban@arenapharm.com (A.G. Montalban).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.03.107

2918
A. G. Montalban et al. / Tetrahedron Letters 51 (2010) 2917–2919
Figure 1.
Scheme 1.
Figure 2. Fluorescence spectra of complexes 9–12.
Acknowledgments
We thank Professor A. G. M. Barrett, the Erasmus Scheme (J.J.),
and the Spanish Government (A.J.H.) for generous support of our
studies.
References and notes
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2. Jones, C. J. Chem. Soc. Rev. 1998, 27, 289.
3. Leininger, S.; Olenyuk, B.; Stang, P. J. Chem. Rev. 2000, 100, 853.
4. Northrop, B. H.; Zheng, Y.-R.; Chi, K.-W.; Stang, P. J. Acc. Chem. Res. 2009, 42,
1554. and references cited therein.
5. Thompson, A.; Dolphin, D. J. Org. Chem. 2000, 65, 7870. and references cited
therein.
6. Sammes, P. G.; Yahioglu, G. Chem. Soc. Rev. 1994, 327.
Scheme 2.
7. Brückner, C.; Sternberg, E. D.; Boyle, R. W.; Dolphin, D. Chem. Commun. 1997,
1689. and references cited therein.
8. Chen, J.; Burghart, A.; Derecskei-Kovacs, A.; Burgess, K. J. Org. Chem. 2000, 65,
2900. and references cited therein.
9. Chandler, C. J.; Daday, L. W.; Reiss, J. A. J. Heterocycl. Chem. 1981, 18, 599.
10. Garrido Montalban, A.; Herrera, A. J.; Johannsen, J.; Beck, J.; Godet, T.; Vrettou,
M.; White, A. J. P.; Williams, D. J. Tetrahedron Lett. 2002, 43, 1751. and
references cited therein.
11. Lash, T. D.; Bellettini, J. R.; Bastian, J. A.; Couch, K. B. Synthesis 1994, 170.
12. Sessler, J. L.; Mozaffari, A.; Johnson, M. R. Org. Synth. 1992, 70, 68.
13. Synthetic procedure for the preparation of bis-dipyrromethanes 5 and 6: To a
solution of dialdehyde 1 (0.48 g, 2.0 mmol) in TFA (20 mL) benzyl pyrrole ester
3 (2.1 g, 8.2 mmol) was added and the mixture was stirred at 20 °C for 20 min
under N2. The reaction mixture was poured onto ice and water (100 mL),
neutralized with 2.5 M NaOH, and extracted with CHCl3 (3 Â 20 mL). The
combined organics were dried (Na2SO4), rotary evaporated, and the crude
residue was redissolved in the minimum amount of CHCl3. Precipitation upon
the addition of EtOAc followed by filtration and drying under vacuum gave
bis(dipyrromethane) 5 (1.6 g, 65%) as a pale yellow solid. The same reaction as
mentioned above with ethyl pyrrole ester
4 (1.6 g, 8.2 mmol) gave
bis(dipyrromethane) 6 (1.3 g, 66%) as a white solid.
14. (a) Beer, G.; Niederalt, C.; Grimme, S.; Daub, J. Angew. Chem., Int. Ed. 2000, 39,
3252; (b) Burghart, A.; Thoresen, L. H.; Chen, J.; Burgess, K.; Bergström, F.;
Scheme 3.

A. G. Montalban et al. / Tetrahedron Letters 51 (2010) 2917–2919
2919
Johansson, L. B.-Å. Chem. Commun. 2000, 2203; (c) Rurack, K.;
Kollmannsberger, M.; Daub, J. Angew. Chem., Int. Ed. 2001, 40, 385; (d)
Imahori, H.; Norieda, H.; Yamada, H.; Nishimura, Y.; Yamazaki, I.; Sakata, Y.;
Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 100.
0.23 (t, J = 28.0 Hz); 19F NMR (CDCl3, 235 MHz) d À142.7 (dq, J = 28.0 and
96.6 Hz), À148.7 (dq, J = 28.0 and 96.6 Hz); HRMS (FAB) calcd for
C78H75BF2N6O8: [M+] 1272.5708, found [M+] 1272.5736.
16. Selected data for 12: 1H NMR (CDCl3, 300 MHz) d 0.51 (t, J = 7.4 Hz, 12H),
1.01 (t, J = 7.5 Hz, 12H), 1.31 (m, 4H), 1.42 (t, J = 7.1 Hz, 16H), 2.36 (m, 8H),
4.44 (m, 8H), 7.91 (d, J = 8.2 Hz, 2H), 8.13 (s, 2H), 8.51 (d, J = 8.2 Hz, 2H);
11B NMR (CDCl3, 125 MHz) d 0.02 (t, J = 27.7 Hz); 19F NMR (CDCl3, 235 MHz)
d À142.2 (dq, J = 27.7 and 96.4 Hz), À149.7 (dq, J = 27.7 and 96.4 Hz); HRMS
(FAB) calcd for C58H67B2F4N6O8: [M+H]+ 1073.5142, found: [M+H]+
1073.5167.
15. Selected data for 9: 1H NMR (CDCl3, 300 MHz) d 0.53 (t, J = 7.4 Hz, 6H), 0.68 (t,
J = 7.4 Hz, 6H), 0.96 (t, J = 7.4 Hz, 6H), 1.10 (t, J = 7.4 Hz, 6H), 1.31 (m, 2H), 1.61
(m, 2H), 2.12 (m, 4H), 2.37 (m, 4H), 2.70 (m, 4H), 5.21 and 5.27 (ABq,
J = 12.5 Hz, 4H), 5.47 (s, 4H), 6.54 (s, 1H), 7.25–7.44 (m, 16H), 7.50–7.60 (m,
4H), 7.62 (d, J = 8.4 Hz, 1H), 7.80 (d, J = 8.2 Hz, 1H), 7.99 (s, 2H), 8.32 (d,
J = 8.4 Hz, 1H), 8.43 (d, J = 8.2 Hz, 1H), 9.02 (s, 2H); 11B NMR (CDCl3, 125 MHz) d