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Energetic and Constitutional Hysteresis in Bistable Molecules.

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Energetic and Constitutional Hysteresis in Bistable Molecules
By Ulrich K d k *
Bistable or multistable systems, so-called flip-flops, which
remain in a state Z, until they are converted by a trigger
signal into a second state Z,, are the key building blocks of
all digital electronic devices. Taube and S a n ~ [have
' ~ recently
realized such a system at the molecular level in an elegant
way in the form of a dinuclear Ru-sulfide/Ru-sulfoxide complex.
In principle, any isomeric pair whose isomers are separated by a sufficiently high activation energy barrier constitutes
such a bistable chemical system. However, the interconversion of the isomers can involve complicated chemical operations, including even chemical degradation and synthesis
with the breaking and making of bonds; such conversions
can no longer be considered as being equivalent to electrical
triggering.
Molecules which photochemically isomerize and undergo
thermal reconversion approach more closely the ideal of a
triggerable multistable molecular system; like the norbornadiene/quadricyclene system [Eq. (a)], which, together with
similar compounds, has been studied intensively as a converter of light energy into storable thermal energy.12]A conversion catalyst, for example a suitable transition metal complex, can be used as trigger for the thermal reconversion.
Systems in which the conversion in both directions is triggered photochemically by light of suitable wavelengths already show a high degree of reversibility. In a series of Fe"
with relatively weak ligands which have been
investigated by Giitlich et al., the thermal conversion barrier
between a 'A- and a 'T-term is large enough in both directions to prevent a spontaneous conversion, at least at temperatures < 60 K. The reason for this is the considerably
larger ionic radius of a high-spin Fe" ion compared to the
low-spin form. On the other hand, high-spin and low-spin
forms can be intentionally interconverted by selective light
excitation [Eq. (b)].
nm)
.hv(350-650
hv (700nm)
' A , , [Fe(mt~)~]'+
'T,,[Fe(mtz)J''
mtz = 1-methyl-1H-tetrazole
At present, most examples of triggerable isomerizations
involve transition metal complexes in which the triggering is
brought about by an electrochemical oxidation or reduction
at the electrode. In some cases the process is not only completely reversible, but also selectively and specifically triggerable by applying a suitable potential.
[*I
Prof. Dr. U. Kolle
Institut fur Anorganische Chemie der Technischen Hochschule
Templergraben 55, W-5100Aachen (FRG)
956
0 VCH Verlagsgesellschafi mbH, W-6940 Weinheim, 1991
For a bistable system in the above sense, however, not
only the comparatively trivial recharging is necessary but
also a concomitant chemical reaction, often an isomerization. Such a system (Scheme 1) is characterized by (at
4
z,+
z;-
Scheme 1
least) two redox potentials E , and E, and, in each case, two
chemical reactions with the rate constants k , and k , . The
redox pairs Z,o,/Z;,, are in principle reversibly interconvertible; however, if the subsequent chemical reaction is sufficiently rapid, the electron transfer appears irreversible.
Since Z, and Zy- differ in chemical constitution, E, and E,
can differ. In electroanalytical terminology this is designated
an ECEC system. In the cyclic voltammogram such a system
is characterized by a more or less strong displacement of the
reduction peak compared to the oxidation speak.
These systems are bistable in the above sense when the
chemical reaction can be separated from the electron transfer; i.e. the energy minima ZY- and Z,, which are reached
after the electron transfer, must be separated by a small
activation barrier from the stable configuration Zy- or Z,
for each charge state (see Scheme 2). In organotransition-
"rtn
I
Scheme 2. rrl = electrochemicalreaction coordinate, rlh= thermal reaction coordinate.
metal complexes which obey the 18-electron rule, the constitution often changes upon varying the charge of the complex. As long as this process is reproducible and reversible a
chemical bistable system can exist.
Many compounds in which reversible bond rearrangements take place during a charge transfer have long been
known, and redox isomerizations have recently been reviewed in detail.I4I However, the possibility of isomerization
does not appear to suffice for separate redox potentials in
reduction and oxidation; rather, examples are known in
which the redox transition takes place reversibly electrochemically despite drastic changes in the molecular structure.[']
Examples of compounds in which the reduction potential
and oxidation potential differ greatly, and in which reversible bond breaking and bond formation take place dur-
0570-0833/91lO808-0956$ 3 . 5 0 + ,2510
Angew. Chem. I n l . Ed. Engl. 30 (1991) No. 8
ing the course of the charge transfer are dinuclear carbonyl
derivatives with metal-metal bonding ([{Mn(CO>,),],
[{Co(CO),},], [{CpFe(CO),},] etc.16]), for which reductive
cleavage and oxidative coupling of the metal-metal bond are
characteristic reactions. Closed cycles in the ECEC scheme
are realized in particular when the metal centers are linked
by suitable substituents (e.g. fulvaIenes).['' In sandwich complexes, the formation and breakage of peripheral C-C
bonds, concomitantly in some cases with isomerization of
the n-cyclic ring ligands during the charge transfer, is a frequently observed reaction.18]But also purely intramolecular
redox isomerizations with closed cycles have in the meantime
been well documented. Examples are the cis-trans- and facmer-isomerizations of octahedral carbonyls [ML4(C0),] and
[ML,L',(CO),], respectively, of the chromium and manganese groups, the cleavage of a metal-metal bond in dinuclear, anionically bridged transition metal complexes during
a two-electron reduction, and the opening of clusters by the
uptake of electrons.
In some sandwich complexes which can change their oxidation states in two-electron steps, a change in the hapticity
and thus an isomerization occurs in accord with the equivalence of one double bond to two complex electrons in the
valence electron count,191for example by bending of an q6arene to give an q4-cyclohexatriene ligand. So-called
pseudo-triple-decker complexes alter the conformation of
the cyclic n-bridge ligand during the charge transfer.14'
The reversible rearrangement of bonds in the redox isomerization of the diruthenium complex [(CpRu),(cot)]
(Scheme 3) is one of the examples that has been studied in
detail.["] Two [CpRu] units are linked via a cyclooctatetraene unit to give a neutral molecule in which two Catoms are pentacoordinated. Upon oxidation to the dication
a C-C bond is opened and a so-called flyover complex is
formed. A detailed insight into the reaction mechanism is
shows that the isomerization takes place at the stage of the
monocation at room temperature (indeed the generation of
Z:+ at low temperature leads to another product). A similar
sequence is found for the reduction of the stable and likewise
structurally characterized dication Z: +,with the exception,
that here, reduction to the flyover neutral complex must take
place first in order to trigger the isomerization to Z, .
Finally, in the area of classical complex chemistry, redox
isomerization can be expected wherever an ambident ligand
changes its coordination site with changing oxidation state
of the central metal. The few well-known examples are all
from the coordination chemistry of Ru, since both Ru" as
well as Ru"' are coordinated exclusively octahedrally, and in
addition, both oxidation states are inert to substitution.
5+
0.57
v
Scheme 4.
1
2t
e
zl+ l
+
RuCp
RuCp
Scheme 3
provided in this case by cyclic voltammetry. Firstly, the system is characterized by a chemically irreversible two-electron
oxidation and reduction with potentials of 0.02 and
- 0.26 V (glassy carbon electrode), due to the fact that
E; < El and E, x E;. Below room temperature the isomerization Z
: -+ ZT is slow. The oxidation of Z, gradually becomes a reversible one-electron oxidation and a second oxidation step can be observed at 0.4 V (Z:/'+).This clearly
Angew. Chrm. Int. Ed. Engl. 30 (1991) No. 8
0 VCH Verlagsgese1ischaft mbH,
In sulfoxide complexes the transition Ru" + Ru"' can lead
to a bond isomerization from Ru-SO to Ru-OS. This fact is
exploited by Taube et aI.['] The authors have incorporated
an S-function and an SO-function into a macrocycle
and complexed both groups to a [Ru(NH,),]-unit
(Scheme4). Starting from a complex in which both R u
atoms have the oxidation state + 11, firstly the Ru-S group
and subsequently, at a more positive potential, the Ru-SO
function is oxidized. The isomerization to the Ru-OS isomer
takes place during the second oxidation step. The Ru3+/'+
potential of the Ru-OS group now lies at a more negative
value than that of the Ru-S function, so that Ru"'-S is reduced prior to Ru"'4S during the back-reduction. Thus, by
selective redox triggering both the Ru-OS as well as the
Ru-SO isomer can be generated, depending upon whether
one starts from the completely oxidized or reduced complex
with Ru-S as Ru" or Ru"' center. As the authors remark,
similar cases can be expected in coordination chemistry in
the case of ambident ligands capable of bond isomerization
(NO;, NCS-, CN-, R,SO).
A broad spectrum of applications are conceivable for molecules which can be triggered selectively between different
states: from micro- and macroscopic display systems to
W-6940 Weinheim, 1991
0S70-0833/9l/0808-09S7$3.SO+.ZS/O
957
molecular information storage devices. The results obtained
by Taube et al. disclose, in particular, a strategy for the
planned search for novel systems with molecular hysteresis,
which could satisfactorily meet practical requirements.
German versions: Angew. Chem. 103 (1991) 970
111 M. Sano, H. Taube, J. Am. Chem. SOC.1 f 3 (1991) 2327.
[2] P. S . Mariano, T. L. Rose, A. A. Leone. L. Fisher in R. R. Hautala, R. B.
King, C. Kutal (Eds.): Solar Energy, Clifton, New York 1979, p. 299ff.
[3] P. Poganiuch, S. Decurtins, P. Giitlich, J. Am. Chem. SOC.112 (1990) 3270,
P. Giitlich, P. Poganiuch, Angew. Chem. 103 (1991) 1015; Angew. Chem.
30 (1991) 975.
[4] W. E. Geiger, (Stuctural Consequences of Electron Transfer) Piog. Inorg.
Chem. 33 (1985) 275.
[5] See, for example, [((C,H,)Rh(CO)(PPh,)},]: N. G. Connelly, A. R. Lucy,
J. D. Payne, A. M. R. Galas, W. E. Geiger, J. Chem. SOC.Dallon Trans.
1983, 1879.
[6] N. G. Connelly, W. E. Geiger, Adv. Organomet. Chem. 23 (1983) 18.
[7] R. Moulton, T. W Weidman, K. P. C. Vollhardt, A. J. Bard, Inorg. Chem.
25 (1986) 1846.
[S] N. El Murr, J. E. Sheats, W. E. Geiger, J. D. L. Holloway, Inorg. Chem. 18
(1979) 1443; U. Kolle, T.4. Ding, H. Keller, B. L. Ramakrishna, E.
Raabe, C. Kriiger, G. Raabe, J. Fleischhauer, Chem. Ber. 123 (1990) 227,
and references cited therein.
[9] R. G. Finke, R. H. Voegeli, E. D. Laganis, V. Boekelheide, Organometallies 2 (1983) 347: W. J. Bowyer, W.E. Geiger in [4], p. 321; U. Kolle, B.
Fuss, Chem. Ber. 119 (1986) 116.
[lo] W. E. Geiger, A. Salzer, J. Edwin, W
. von Philipsborn, U. Piantini, A. L.
Rheingold, J. Am. Chem. SOC.112 (1990) 7113.
Dyes for Visual Distinction between Enantiomers :
Crown Ethers as Optical Sensors for Chiral Compounds**
By Fritz Vogtle* and Peter Knops
The discovery of crown ethers by Pedersen and the study
of their complexation properties heralded the blossoming of
the supramolecular chemistry which deals with host-guest
interactions.‘‘’ Crown ethers enable not only the selective
complexation of cations and neutral organic molecules; with
chiral crown ethers as host compounds, the (enantio)selective complexation of optically active guests can be
achieved.I2]
By combining a crown ether with chromophoric units a
“chromoionophore” is created. With these crown-dyes the
selective cation complexation is made visible by a color
change in the same molecule.[31Figure 1 gives an idea of the
significant effects which can be achieved with a crown ether
coupled to a quinonimine.
&
Now the fascinating challenge of making the enantioselective complexation of chiral guests visible through a color
change was recognized, i.e. of being able to see the enantiomeric differentiation of chiral guests by an optically active
host molecule in order to draw conclusions about the absolute configuration of the guest molecule. Figure 2 illustrates
the concept: through complexation of the chiral guest 1 with
the enantiomerically pure chromoionophore 2, different
shifts for the absorption maximum are induced for the resulting complexes depending on the absolute configuration
of the guest 1. In the ideal case, these can be observed with
the naked eye.
*
500
600
700
A [nm]
Fig. 1. Selective ion detection by a chromoionophore of the neutral quinonimine type as a result of cation-selective absorption of light.
[*] Prof. Dr. F. Vogtle, Dr. P. Knops
Institut fur Organische Chemie und Biochemie der Universitat
Gerhard Domagk-Strasse 1, W-5300 Bonn 1 (FRG)
958
0 VCH Verlagsgesellschafl mbH, W-6940 Weinheim. 1991
Fig. 2. Mode of action of chiral chromoionophores (schematic) for the visual
recognition and differentiation of enantiomeric guest compounds.
0570-0833/91~080S-OYS8$3.50(0+
.2S/O
Angew. Chem. Int. Ed. Engl. 30 (1991) No. 8
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