Synthesis of a molecular tripod to anchor molecular coordination compounds to semiconductor nanoparticles

Article abstract of DOI:10.1016/S0040-4039(00)01187-4

A 1,3,5,7-tetraphenyladamantane derivative having three arms terminating with carboxylic acids and a fourth substituted with a 9,10-phenanthrolinyl ligand was prepared as a new kind of molecular linkage to study dynamics of electron injection at the interface of TiO₂ nanoparticles. (C) 2000 Published by Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01187-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 7419–7421
Synthesis of a molecular tripod to anchor molecular
coordination compounds to semiconductor nanoparticles
Wenzhuo Guo, Elena Galoppini,* Gunilla Rydja and Gianluca Pardi
Rutgers University, Chemistry Department, 73 Warren Street, Newark, NJ 07102, USA
Received 24 May 2000; accepted 15 July 2000
Abstract
A 1,3,5,7-tetraphenyladamantane derivative having three arms terminating with carboxylic acids and a
fourth substituted with a 9,10-phenanthrolinyl ligand was prepared as a new kind of molecular linkage to
study dynamics of electron injection at the interface of TiO2 nanoparticles. © 2000 Published by Elsevier
Science Ltd.
Solar cells that convert sunlight into electricity by sensitization of wide bandgap n-type
semiconductor nanoparticles with dyes, usually Ru(II) polypyridyl coordination compounds,
have been studied since the early 1970s.1 The dyes are anchored to metal oxide semiconductor
surfaces, for instance TiO2, through the reaction of one or two carboxylic acid groups present
on a bpy ligand with surface hydroxyl groups.2 A highly promising and yet relatively unexplored
area in this field is the development of new rigid linkages to study dynamics of remote electron
injection at the interface of semiconductors nanoparticles.1a,3 We have now developed new
molecular linkages that act as rigid scaffolds for the dye and hold it at fixed distance with
respect to the semiconductor surface. These are molecules having the shape of a tripod, with
three rigid-rod arms terminating with carboxylic acids and a fourth substituted with a metal-
binding ligand, such as phenanthroline or bipyridine. Structural, optical and electrochemical
properties of the tripods will be tuned by changing one or more of their three components: core,
arms and sensitizer.
In this paper we describe the synthesis of the first model compound, 6 (Scheme 1). The
starting material, tetraiodide 1, was readily prepared from 1-bromoadamantane, following a
clever two-step procedure developed by Mathias and coworkers.4 Sonogashira cross-coupling5 of
* Corresponding author.
0040-4039/00/$ - see front matter © 2000 Published by Elsevier Science Ltd.
PII: S0040-4039(00)01187-4

7420
1 with trimethylsilylacetylene at room temperature produced a mixture of monosubstituted 2†
together with the disubstituted product and unreacted 1. Since separation by silica gel column
chromatography at this stage was lengthy, the two following steps (carboxylation and esterifica-
tion) were performed directly on the mixture and the esters were easily separated to afford 4† in
18–20% yield from 1.6 The alkynyl group was deprotected with tetrabutylammoniumfluoride
(TBAF), followed by a Suzuki-type coupling7 with 5-bromo-9,10-phenanthroline to yield 6a†, a
solid material that is soluble in numerous organic solvents.8
Scheme 1. Reagents and conditions: (a) Me3SiCꢀCH (1.5 equiv.), Cl2Pd(PPh3)2, CuBr, (i-Pr)2NH, benzene, rt, 30%. (b)
1. t-BuLi; 2. CO2. (c) H+, H2O. (d) DCC, DMAP, EtOH (R=Et) or CH2N2 (R=Me). (e) TBAF, CH3CN, rt, 95%.
(f) 1. (Me3Si)2NLi, −78°C; 2. 9-BBN, −78°C; 3. Pd(PPh3)4, THF, reflux, 30%. (g) KOH, EtOH, 90%
1
† Selected data for 2: H NMR lH (500 MHz, CDCl3): 7.65 (d, J=8.5, 6 H), 7.44 (d, J=8.5, 2 H), 7.34 (d, J=8.5,
2 H), 7.17 (d, J=8.5, 6 H), 2.05 and 2.04 (two s, 12 H, CH
149.04, 148.45 (PhI), 137.46 (PhI), 132.05, 127.13 (PhI), 124.91, 121.13, 104.85 (PhCꢀC
91.68 (CꢁI), 46.64 (CH2 and CꢁPhꢁI), 39.17 (CꢁPhꢁCꢀC), 39.01 (C
H2), 0.00 (SiMe3). 4: 1H NMR lH (500 MHz,
CDCl3): 8.03 (d, J=8.5, 6 H), 7.54 (d, J=8.5, 6 H), 7.46 (d, J=8.0, 2 H), 7.40 (d, J=8.0, 2 H), 4.37 (q, J=7.0,
OCH2CH3, 6 H), 2.19 and 2.17 (two s, 12 H, CH2), 1.39 (t, J=7.0, OCH2CH3
, 9 H), 0.25 (s, 9 H, SiMe3); 13C NMR
lC (125 MHz, CDCl3): 166.27 (COOEt), 153.65 (CꢁCOOEt), 148.99, 132.00, 129.68 (PhCOOEt), 128.53 (PhCOOEt),
124.94 (PhCOOEt), 124.79, 121.11 (CꢁCꢀCꢁSiMe3), 104.82 (CꢀCSiMe3), 93.98 (CꢀCSiMe3), 60.77 (OCH2CH3), 46.52
(CH2 and CꢁPhꢁCOOEt), 39.46 (CH2), 39.15 (CꢁPhꢁCꢀC), 14.33 (OCH2CH3), 0.43 (SiMe3). 6a: H NMR lH (500
MHz, CDCl3): 9.25 (m, 1 H), 9.21 (m, 1 H), 8.84 (m, 1 H), 8.25 (m, 1 H), 8.11 (m, 1 H), 8.05 (d, J=8.5, 6 H), 7.75
(m, 1 H), 7.65 (m, 3 H), 7.56 (m, 8 H), 4.38 (q, J=7.0, OCH2CH3, 6 H), 2.23 (broad s, 12 H, CH2), 1.40 (t, J=7,
OCH2CH3 OOEt), 153.59 (PhCOOEt), 150.85, 150.61, 149.62,
, 9 H); 13C NMR lC (125 MHz, CDCl3): 166.35 (C
146.03, 145.87, 135.74, 134.69, 131.89 (PhCꢀC), 130.58, 129.75 (PhCOOEt), 128.63 (PhCOOEt), 128.21, 127.99,
125.25 (PhCꢀC), 124.97 (PhCOOEt), 123.44, 123.32, 120.61, 119.91, 95.15 (CꢀCPhena), 85.79 (CꢀCPhen), 60.88
(OCH2CH3), 46.63 (CH2), 46.57 (CꢁPhꢁCOOEt), 39.53 (CH2), 39.34 (CꢁPhꢁCꢀC), 14.30 (OCH2CH3).
6
2), 0.24 (s, 9 H, SiMe3); 13C NMR lC (125 MHz, CDCl3):
6
SiMe3), 94.07 (PhC6 ꢀCSiMe3),
6
6
6
6
6
6
6
6
6
6
6
6
6
6
1
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6

7421
The Ru(bpy)2 complexes of both 6 and 7 were formed upon treatment with RuCl2(bpy)2·2H2O
and their photoelectrochemical studies will be published elsewhere. Although the yields of the
first and last step are modest (total yield from 1 is 5–7%), compound 6 was prepared in sufficient
quantities (100 mg scale) for the application of interest, and the same route was applied to other
substrates such as tetraphenylmethane derivatives. To the best of our knowledge, this is the first
example of a systematic synthesis aiming to differentiate one of the arms of tetrahedral
molecules, which have been recently employed in the synthesis of branched polymers,4,9
dendrimers and in crystallographic studies.10 In summary, the tripod-shaped molecules in
Scheme 1 may be of interest to researchers studying molecule/semiconductor systems as well as
other areas of materials chemistry.
Acknowledgements
This work was supported in part by a grant from the Petroleum Research Fund (ACS-PRF
35081-G5) and the Rutgers Research Council. E.G. thanks Professor Gerald Meyer for
insightful discussions and for providing the experimental procedure for 5-bromo-9,10-phenan-
throline. G.R. thanks the CHUST program sponsored by the Royal Institute of Chemical
Engineering, Stockholm, Sweden.
References
1. (a) Meyer, G. J. J. Chem. Educ. 1997, 74, 652. (b) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (c) Gra¨tzel,
M. Heterogeneous Photochemical Electron Transfer; CRC: Boca Raton, FL, 1989. (d) Hagfeldt, A.; Gra¨tzel, M.
Chem. Rev. 1995, 95, 49. (e) Kamat, P. V. Chem. Rev. 1993, 93, 267. (f) Gerischer, H. Photochem. Photobiol. 1972,
16, 243.
2. It is presumed that the carboxylic (or carboxylate) groups bind through ester bonds or non-covalent interactions.
3. (a) Stipkala, J. M.; Heimer, T. A.; Livi, K. J. T.; Meyer, G. J. Chem. Mater. 1997, 9, 2341. (b) Argazzi, R.;
Bignozzi, C. A.; Heimer, T. A.; Meyer, G. J. Inorg. Chem. 1997, 36, 2.
4. Eichert, V. R.; Mathias, L. J. Macromolecules 1994, 27, 7015.
5. Sonogashira, K. In Comprehensive Organic Synthesis; Trost, B. M., Ed. Pergamon: Oxford, UK, 1991; Vol. 3, pp.
551–561.
6. The separation of the mixture was difficult because of the presence of 1. When the reaction was repeated in the
presence of an excess of trimethylsilylacetylene, the crude contained mono-, di- and triethynylsubstituted
products, but not 1, and the separation was easy. In this case, however, the yield of 2 was lower. The di- and
trialkynyl derivatives of 1 are also of interest for our studies and have been isolated.
7. (a) Brown, H. C.; Bhat, N. G.; Srebnik, M. Tetrahedron Lett. 1988, 29, 2631. (b) Soderquist, T. A.; Matos, K.
Tetrahedron Lett. 1995, 36, 2401. (c) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457.
8. Sonogashira coupling was used to prepare 5-trimethylsilylethynyl-9,10-phenanthroline (Boatin, R. H.; Rapoport,
H. J. Org. Chem. 1986, 51, 5320), but repeated attempts to use these conditions with 5 and 5-bromo-9,10-phenan-
throline resulted exclusively in the dimerization of 5.
9. (a) Wilson, L. M.; Griffin, A. C. J. Mater. Chem. 1993, 3, 991. (b) Braun, D.; Keller, C.-C.; Roth, M. D.;
Schartel, B.; Voigt, M.; Wendorff, J. H. J. Prakt. Chem. 1997, 339, 708.
10. (a) Batten, S. R.; Robson, R. Angew. Chem., Int. Ed. Engl. 1998, 37, 1461 and references therein. (b) Galoppini,
E.; Gilardi, R. Chem. Commun. 1999, 173.
.