Redox control of the ring-gliding motion in a Cu-complexed catenane: A process involving three distinct geometries

Full text of DOI:10.1021/ja962774e

11980
J. Am. Chem. Soc. 1996, 118, 11980-11981
Redox Control of the Ring-Gliding Motion in a
Cu-Complexed Catenane: A Process Involving
Three Distinct Geometries
Diego J. Ca´rdenas, Aude Livoreil, and Jean-Pierre Sauvage*
Laboratoire de Chimie Organo-Mine´rale
UA 422 au CNRS, Faculte´ de Chimie
UniVersite´ Louis Pasteur, 67000 Strasbourg, France
ReceiVed August 8, 1996
Figure 1. A three-configuration Cu(I) catenate whose general molec-
ular shape can be dramatically modified by oxidizing the central metal
(Cu(I) to Cu(II)) or reducing it back to the monovalent state. Each
ring of the [2]-catenate incorporates two different coordinating units:
the bidentate dpp unit (dpp ) 2,9-diphenyl-1,10-phenanthroline) is
symbolized by a U whereas the terpy fragment (2,2′:6′,2′′-terpyridine)
is indicated by a stylized W. Starting from the tetracoordinate
monovalent Cu complex (Cu(I)N4+; top left) and oxidizing it to the
divalent state (Cu(II)N42+), a thermodynamically unstable species is
obtained which should first rearrange to the pentacoordinate complex
ReVised Manuscript ReceiVed October 9, 1996
Molecules whose shape and physical properties can be
controlled and modified at will by using an external signal are
fascinating. Electrochemical triggering of molecular motions
between two forms has recently been reported for synthetic
systems, either on transition metal complexes whose oxidation
states are varied1-3 or using rotaxanes incorporating a control-
lable acceptor-donor complex.4 Proteins have also afforded
interesting examples of redox-controlled molecular motions, the
folding-unfolding process being switched by reducing or
oxidizing the heme moiety of the enzyme.5
2+
Cu(II)N5 by gliding of one ring (left) within the other and, finally,
to the hexacoordinate stage Cu(II)N62+ by rotation of the second cycle
2+
(right) within the first one. Cu(II)N6 is expected to be the thermo-
dynamically stable divalent complex. The double ring-gliding motion
Multistage systems seem to be uncommon, although they are
particularly challenging and promising in relation to photo- and
electrochemical devices aimed at important electronic functions
and information storage.6 We would like now to report that a
Cu-complexed [2]-catenane represents an example of such a
compound, with three distinct geometries, each stage corre-
sponding to a different coordination number (CN) of the central
complex (CN ) 4, 5, or 6). The principle of the three-stage
electrocontrollable catenate is represented in Figure 1.
It relies on the important differences of stereochemical
requirements for coordination of Cu(I) and Cu(II). For the
monovalent state the stability sequence is CN ) 4 > CN ) 5
> CN ) 6. On the contrary, divalent Cu is known to form
stable hexacoordinate complexes, with pentacoordinate systems
being less stable and tetrahedral Cu(II) species being even more
strongly disfavored.
+
following oxidation of Cu(I)N4 can be inverted by reducing Cu(II)-
2+
N6 to the monovalent state (Cu(I)N6+; top right), as represented on
the top line of the Figure.
The synthesis of the key catenate Cu(I)N4+PF6- derives from
the general three-dimensional template strategy which has been
proposed by our group some time ago for making catenanes.7
In this case, a one-pot two-ring formation approach was
employed. The starting fragments, the entwined Cu(I) complex
precursor, and the catenate formation reaction are depicted in
- 8
Figure 2. The reaction between complex Cu(1)2+BF4
and
-
Figure 2. Synthesis of the catenates Cu(I)N4+PF6 and Cu(II)-
5,5′′-di(3-bromo-1-propyl)-2,2′:6′,2′′-terpyridine (2) in DMF in
the presence of Cs2CO3 under high dilution conditions gave the
N62+(BF4-)2.
(1) Tomita, A.; Sano, M. Inorg. Chem. 1994, 33, 5825. Sano, M.; Taube,
H. J. Am. Chem. Soc. 1991, 113, 2327. Sano, M.; Taube, H. Inorg. Chem.
1994, 33, 705.
-
desired complex which was isolated as the PF6 salt in 21%
yield after chromatographic purification.9 The visible spectrum
of this deep red complex shows a metal-to-ligand charge transfer
(2) Livoreil, A.; Dietrich-Buchecker, C. O.; Sauvage, J.-P. J. Am. Chem.
Soc. 1994, 116, 9399.
(MLCT) absorption band λmax ) 439 nm (ꢀ ) 2570 mol-1
L
(3) Zelikovich, L.; Libman, J.; Shanzer, A. Nature 1995, 374, 790.
(4) Bissell, R. A.; Co´rdova, E.; Kaifer, A. E.; Stoddart, J. F. Nature 1994,
369, 133. Benniston, A. C.; Harriman, A. Angew. Chem., Int. Ed. Engl.
1993, 32, 1459. Benniston, A. C.; Harriman, A.; Lynch, V. M. Tetrahedron
Lett. 1994, 35, 1473.
(5) Bixler, J.; Bakker, G.; McLendon, G. J. Am. Chem. Soc. 1992, 114,
6938. Pascher, T.; Chesick, J. P.; Winkler, J. R.; Gray, H. Science 1996,
271, 1558.
(6) Parthenopoulos, D. A.; Rentzepis, P. M. Science 1989, 245, 843.
Willner, I.; Blonder, R.; Dagan, A. J. Am. Chem. Soc. 1994, 116, 3121.
Irie, M.; Miyatake, O.; Uchida, K. J. Am. Chem. Soc. 1992, 114, 8715.
Gilat, S. L.; Kawai, S. H.; Lehn, J. M. J. Chem. Soc., Chem. Commun.
1993, 1439.
cm-1, MeCN). Cyclic voltammetry (CV) of a MeCN solution
shows a reversible redox process at +0.63 V (vs SCE). Both
the CV data and the UV-vis spectrum are similar to those of
other related species.2,7 The reaction of Cu(I)N4+PF6 with
-
KCN in MeCN/H2O afforded the free catenand, which was
subsequently reacted with Cu(BF4)2 to give Cu(II)N62+(BF4
)
-
2
as a very pale green complex (Figure 2). The hexacoordinate
structure of this species was evidenced by UV-vis spectroscopy
and electrochemistry. A weak absorption appears at λmax
)
687 nm (ꢀ ) 100 mol-1 L cm-1, MeCN). The cyclic
voltammogram shows an irreversible reduction at -0.43 V (vs
SCE, MeCN). These data are similar to the ones obtained for
(7) Dietrich-Buchecker, C. O.; Sauvage, J.-P. Bioorganic Chemistry
Frontiers; Springer-Verlag: Berlin, 1991; Vol. 2, pp 197-248. Dietrich-
Buchecker, C.; Sauvage, J.-P. Tetrahedron 1990, 46, 503. Dietrich-
Buchecker, C. O.; Sauvage, J.-P. Tetrahedron Lett. 1983, 24, 5091. Dietrich-
Buchecker, C. O.; Sauvage, J.-P.; Kintzinger, J.-P. Tetrahedron Lett. 1983,
24, 5098.
(9) Experimental data for Cu(I)N4+PF6-: MS (FAB, m/z) 1418.4 (M
- PF6). Anal. Calcd: C, 69.12; H, 4.52; N, 8.96. Found: C, 69.13; H,
4.59; N, 9.21. See text for CV and UV-vis data.
-
(8) Cu(1)2+BF4 was prepared in quantitative yield by reaction of
Cu(MeCN)4BF4 with 1 (2 equiv) in MeCN/CH2Cl2 at 23 °C.
S0002-7863(96)02774-6 CCC: $12.00 © 1996 American Chemical Society

Communications to the Editor
J. Am. Chem. Soc., Vol. 118, No. 47, 1996 11981
the complex Cu(diMe-tpy)2(BF4)2 (diMe-tpy ) 5,5′′-dimethyl-
2,2′:6′,2′′-terpyridine).10
-
When a MeCN dark red solution of Cu(I)N4+PF6 was
oxidized by an excess of NO+BF4-, a green solution of Cu-
2+
(II)N4 was obtained. The CV is the same as for the starting
complex, and the visible absorption spectrum shows a band at
λmax ) 670 nm (ꢀ ) 810 mol-1 L cm-1, MeCN) typical of
these tetrahedral Cu(II) complexes.2 A decrease of the intensity
of this band was observed when monitoring it as a function of
time. This fact is due to the gliding motion of the rings to give
the penta- and hexacoordinate Cu(II) complexes, whose extinc-
tion coefficients are lower as compared to the one for Cu(II)-
N42+ (ca. 125 2 and 100, respectively). The rate of this process
is strongly dependent on the presence of water and other
cosolvents. Water seems to retard the process whereas DMF
accelerates the ring-gliding motion. The rate of the rearrange-
ment is roughly twice as large as that for the monoterpy-related
catenate,2 which is in accordance with the presence of two terpy
2+
moieties. The final product is Cu(II)N6 as indicated by the
final spectro- and electrochemical data. A similar behavior was
+
observed when a solution of Cu(I)N4 was electrochemically
-
Figure 3. Cyclic voltammogram of a solution of Cu(II)N62+(BF4
)
2
oxidized.
(MeCN; Bu4NBF4, 0.1 M; 200 mV s-1; reference electrode: SCE). (a)
2+
When either the Cu(II)N6 solution resulting from this
2+
Starting from +1 V, only one reduction wave is observed (Cu(II)N6
process or a solution prepared from a sample of isolated solid
Cu(II)N62+(BF4-)2 were electrochemically reduced at -1 V, the
tetracoordinate catenate was quantitatively obtained. The cycle
depicted in Figure 1 was thus completed. The changeover
process for the monovalent species is faster than the rearrange-
ment of the Cu(II) complexes, as previously observed for the
above-mentioned related catenate.2 In fact, the rate is compa-
to Cu(I)N6+). When coming back to positive potential, the three
possible species (CN ) 4, 5, and 6) formed by ring gliding are detected.
In the time scale of the experiment, the tetracoordinate Cu(I) complex
becomes the major one. (b) The same solution but starting from -1
2+
V. The reversible signal at +0.63 V corresponds to the Cu(II)N4
/
Cu(I)N4+ couple. The tetrahedral Cu(I) complex is formed during the
experiment from the reduced hexacoordinate species. The reduction
wave of Cu(II)N62+ is not due to the complex formed by ring rotation
(which is too slow) but to the starting species already existing in the
bulk solution.
rable to the CV time scale and three Cu species are detected
-
when a CV of a MeCN solution of Cu(II)N62+(BF4
) is
2
performed (Figure 3). The waves at +0.63 V and -0.41 V
correspond, respectively, to the tetra- and hexacoordinate
complexes mentioned above. Given that the topological
constraint reduces the number of possible species that may be
formed, and by analogy with the value found for the isolated
and previously reported structurally related catenate (-0.07 V),2
the wave at -0.05 V was assigned to the pentacoordinate couple.
The relative heights of the oxidation waves for the three
complexes depend on the scan rate and vary from 1:1:4 for CN
) 4, 5, and 6, respectively, at 100 mV s-1 to 1:4:1.5 at 2 V
s-1. Therefore, at high scan rates the pentacoordinate species
characterized by various techniques as transient species, but in
principle, the present system does not allow to stop motions at
this stage. Only three complexes of the six species of Figure 1
have been spectroscopically identified.
The overall cyclic process depicted in Figure 1 does not
consume or produce electrons; it describes the interconversion
between three molecular topographies (CN ) 4, 5, or 6), the
topological properties of the backbone being of course unmodi-
fied in the whole process.
+
Cu(I)N5 becomes the major one. Both Cu(I) and Cu(II)
Acknowledgment. We thank the CNRS and the European Union
for financial support. D.J.C. acknowledges the Ministerio de Educacio´n
y Ciencia (Spain) for a postdoctoral fellowship.
pentacoordinate complexes are intermediates which could be
(10) This compound was prepared by reacting Cu(BF4)2 with an excess
of 5,5′′-dimethyl-2,2′:6′,2′′-terpyridine in MeCN: λmax ) 693 nm (ꢀ ) 70
mol-1 L cm-1, MeCN); Ered ) -0.41 V (vs SCE, MeCN).
JA962774E