Vicinal tricarbonyls as redox-controlled molecular rotors

Article abstract of DOI:10.1016/S0040-4039(00)01632-4

Using electrochemistry, spectroelectrochemistry and density functional computational methods, we have demonstrated that tricarbonyl amide 1(ox) undergoes reversible conformational switching upon reduction to the corresponding radical anion 1(rad-). (C) 20

Full text of DOI:10.1016/S0040-4039(00)01632-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 9489–9492
Vicinal tricarbonyls as redox-controlled molecular rotors†
Trent H. Galow, Alejandro O. Cuello and Vincent M. Rotello*
Department of Chemistry, University of Massachusetts, Amherst, MA 01003, USA
Received 8 August 2000; revised 18 September 2000; accepted 21 September 2000
Abstract
Using electrochemistry, spectroelectrochemistry and density functional computational methods, we
have demonstrated that tricarbonyl amide 1ox undergoes reversible conformational switching upon
reduction to the corresponding radical anion 1rad−. © 2000 Elsevier Science Ltd. All rights reserved.
Molecular devices provide the ultimate goal in the miniaturization of mechanical systems. In
recent investigations, chemical and photochemical stimuli have been used to control the
operation of a wide variety of these molecular machines, including shuttles,1 rotors,2 and
switches.3 Application of these systems to pragmatic electromechanical devices, however, is
complicated by the requirement for electronic input from the computer interface to be converted
to either chemical or photonic stimuli for device actuation.
One way to greatly simplify the fabrication and application of multi-scale systems based on
molecular devices is through the use of electrochemically-activated devices.4 With these devices,
electronic input is used directly, allowing direct interfacing to computer systems.5 This simplicity
of operation makes redox control of molecular structure an obvious goal in device fabrication;
a target complicated by the relative lack of well-behaved acyclic redox systems. To expand the
diversity of structural motifs available for electronically-controlled devices, we have explored the
redox chemistry of vicinal tricarbonyls. These systems can be synthesized with a wide variety of
structures, making them potentially versatile components for device fabrication.6,7 We report
here the use of electrochemical, spectroelectrochemical and density functional (DFT) computa-
tional methods to demonstrate redox-controlled conformational switching of aryl-substituted
tricarbonyl 1ox (Scheme 1).
For our investigations, we used aryl tricarbonyl 1ox. In this system, the aryl substituent was
selected to provide potential electron delocalization and stabilization, and the amide functional-
ity to inhibit hydration.8 Tricarbonyl 1ox was synthesized starting from the commercially
available b-keto ester as shown in Scheme 2.9
* Corresponding author.
†
Dedicated to Professor Harry H. Wasserman: a mentor, scholar, and colleague.
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01632-4

9490
Scheme 1. Redox chemistry and numbering of tricarbonyl 1. h represents the dihedral angle between neighboring
carbonyls
Scheme 2. Synthesis of tricarbonyl 1ox
As expected, tricarbonyl 1ox is a well-behaved redox unit. Simultaneous electrochemistry and
EPR (SEEPR) demonstrated the clean conversion of 1ox to 1rad (Fig. 1a). In contrast to cyclic
−
tricarbonyl analogs, however, the reduction of 1ox to 1rad possesses a very large peak potential
−
difference (DEp=ꢀ400 mV, compared to ferrocene, DEp=ꢀ90 mV10) (Fig. 1b).11 This large
DEp is diagnostic of dramatic structural reorganization upon reduction, a required feature for
redox-activated devices.12
To establish the nature of the structural reorganization that occurs upon reduction of 1ox, we
performed B3LYP-DFT calculations for 1ox and 1rad−. In recent studies, we have demonstrated
the ability of these calculations to accurately predict not only the energetics of radical anion
systems,13 but also fundamental electronic properties such as hyperfine splittings (Fig. 1a).14
Geometry optimization studies of 1ox showed that the carbonyl groups are arranged in a helical
conformation resulting from electrostatic repulsions between the oxygen atoms (h1=89°; h2=
83°). Calculations on 1rad−, however, indicated that major conformational restructuring takes
place after reduction of 1ox. After electron transfer, the central (O(2)) carbonyl unit rearranges
into an antiperiplanar position (trans relative to O(1)), and coplanar with the aromatic moiety
(h1=179°; h2=−77°). Calculations show that this conformation of 1rad− is stabilized by ꢀ20 kcal
mol−1 relative to the helical conformation adopted by 1ox. This stabilization clearly arises from
an increased energetic benefit from electron delocalization: the electrostatic potential maps
reveal that the one-electron reduction greatly increases the negative potential of both the oxygen
atoms and phenyl ring. This enhanced delocalization is also supported by the SEEPR spectrum,
where extensive electron spin density in the phenyl ring of 1rad is demonstrated by large
−
hyperfine couplings arising from the aromatic protons (Fig. 2).

9491
Figure 1. (a) SEEPR of 1rad in CH2Cl2. 0.1 M TBAP electrolyte. [1ox]=1×10−3 M. (b) Cyclic voltammogram of 1ox
−
in CH2Cl2. 0.1 M TBAP electrolyte, 296 K, 275 mV/s scan rate. [1ox]=1×10−3 M. E1/2=1000 mV; relative to ferrocene
Figure 2. Electrostatic potential maps derived from B3LYP/6-31G*//UHF/3-21G* calculations for tricarbonyl 1ox in
(a) the oxidized (1ox) and (b) the radical anion (1rad−) forms. Clearly noticeable is the conformational rearrangement
that occurs after reduction
In summary, we have used electrochemical techniques to demonstrate redox-controlled
conformational switching of tricarbonyl 1ox. Through computational methods, we established
that this switching arises from changes in the conformational preferences of the carbonyl groups
upon reduction of 1ox to 1rad−. The application of this switching process to the creation of
effective molecular devices is currently underway, and will be reported in due course.

9492
Acknowledgements
This research was supported by the National Science Foundation (CHE-9905492) and the
Petroleum Research Fund of the ACS (PRF 33137-AC4,5). V.M.R. acknowledges support from
the Alfred P. Sloan Foundation, the Camille and Henry Dreyfus Foundation, and is a Cottrell
Scholar of Research Corporation.
References
1. Cao, J. G.; Fyfe, M. C. T.; Stoddart, J. F.; Cousins, G. R. L.; Glink, P. T. J. Org. Chem. 2000, 65, 1937–1946.
Balzani, V.; Becher, J.; Credi, A.; Nielsen, M. B.; Raymo, F. M.; Stoddart, J. F.; Talarico, A. M.; Venturi, M.
J. Org. Chem. 2000, 65, 1947–1956. Bissell, R. A.; Cordova, E.; Kaifer, A. E.; Stoddart, J. F. Nature 1994, 369,
133–137.
2. Schoevaars, A. M.; Kruizinga, W.; Zijlstra, R. W. J.; Veldman, N.; Spek, A. L.; Feringa, B. L. J. Org. Chem.
1997, 62, 4943–4948.
3. Balzani, V.; Credi, A.; Langford, S. J.; Raymo, F. M.; Stoddart, J. F.; Venturi, M. J. Am. Chem. Soc. 2000, 122,
3542–3543. Boal, A. K.; Ilhan, F.; DeRouchey, J. E.; Thurn-Albrecht, T.; Russell, T. P.; Rotello, V. M. Nature
2000, 404, 746–748. de Silva, A. P.; Dixon, I. M.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Maxwell, P. R. S.;
Rice, T. E. J. Am. Chem. Soc. 1999, 121, 1393–1394. de Silva, A. P.; McClenaghan, N. D. J. Am. Chem. Soc.
2000, 122, 3965–3966. Deans, R.; Niemz, A.; Breinlinger, E. C.; Rotello, V. M. J. Am. Chem. Soc. 1997, 119,
10863–10864. Fabbrizzi, L.; Licchelli, M.; Pallavicini, P. Acc. Chem. Res. 1999, 32, 846–853. Galow, T. H.; Ilhan,
F.; Cooke, G.; Rotello, V. M. J. Am. Chem. Soc. 2000, 122, 3595–3598. Kern, J. M.; Raehm, L.; Sauvage, J. P.;
Divisia-Blohorn, B.; Vidal, P. L. Inorg. Chem. 2000, 39, 1555–1560. Pina, F.; Melo, M. J.; Maestri, M.; Passaniti,
P.; Balzani, V. J. Am. Chem. Soc. 2000, 122, 4496–4498. Wytko, J. A.; Boudon, C.; Weiss, J.; Gross, M. Inorg.
Chem. 1996, 35, 4469–4477. Zahn, S.; Canary, J. W. Science 2000, 288, 1404–1407. Rathore, R.; Le Magueres,
P.; Lindeman, S. V.; Kochi, J. K. Angew. Chem., Int. Ed. 2000, 39, 809–901. Gobbi, L.; Seiler, P.; Diederich, F.
Angew. Chem., Int. Ed. 1999, 38, 674–678.
4. Mirzoian, A.; Kaifer, A. E. Chem. Commun. 1999, 1603–1604. Abel, E.; Castro, R.; McRobbie, I. M.; Barbour,
L.; Atwood, J. L.; Kaifer, A. E.; Gokel, G. W. Supramol. Chem. 1998, 9, 199–202. Bissell, R. A.; Cordova, E.;
Kaifer, A. E.; Stoddart, J. F. Nature 1994, 369, 133–137. Kaifer, A. E. Acc. Chem. Res. 1999, 32, 62–71.
5. Collier, C. P.; Wong, E. W.; Belohradsky, M.; Raymo, F. M.; Stoddart, J. F.; Kuekes, P. J.; Williams, R. S.;
Heath, J. R. Science 1999, 285, 391–394.
6. Rubin, M. B.; Gleiter, R. Chem. Rev. 2000, 100, 1121–1164.
7. Wasserman, H. H.; Baldino, C. M.; Coats, S. J. J. Org. Chem. 1995, 60, 8231–8235. Wasserman, H. H.; Baldino,
C. M. Bioorg. Med. Chem. Lett. 1995, 5, 3033–3038. Wasserman, H. H.; Vu, C. B.; Cook, J. D. J. Braz. Chem.
Soc. 1996, 7, 319–326. Wasserman, H. H.; Rotello, V. M.; Frechette, R.; DeSimone, R. W.; Yoo, J. U.; Baldino,
C. M. Tetrahedron 1997, 53, 8731–8738. Wasserman, H. H.; Chen, J. H.; Xia, M. D. J. Am. Chem. Soc. 1999,
121, 1401–1402. Wasserman, H. H.; Lee, K.; Xia, M. D. Tetrahedron Lett. 2000, 41, 2511–2514.
8. Wasserman, H. H.; Rotello, V. M.; Williams, D. R.; Benbow, J. W. J. Org. Chem. 1989, 54, 2785.
9. Mamberg, W.-D.; Vob, J.; Weinschneider, S. Liebigs Ann. Chem. 1983, 1694–1711.
10. No compensation was made for the relatively high resistance of the solvent, resulting in a larger DEp for
ferrocene than is observed in aqueous solvents.
11. Doane, L. M.; Fatiadi, A. J. Angew. Chem., Int. Ed. Engl. 1982, 21, 635–636.
12. Tricarbonyl 1 and ferrocene possessed similar DEp versus scan rate behavior, indicating that conformational
switching is very rapid in the tricarbonyl system.
13. Cuello, A. O.; McIntosh, C. M.; Rotello, V. M. J. Am. Chem. Soc. 2000, 122, 3517–3521.
14. Niemz, A.; Rotello, V. M. J. Am. Chem. Soc. 1997, 119, 6833–6836.
.