Rapid electrochemically induced linkage isomerism in a ruthenium(II) polypyridyl complex

Article abstract of DOI:10.1039/b500946d

Rapid and complete switching between the N₆ and the N ₅O donor set induced by changing the metal oxidation state has been observed for a new structural motif based on a ruthenium(II) polypyridyl complex. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b500946d

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Rapid electrochemically induced linkage isomerism in a ruthenium(II)
polypyridyl complex{
Olof Johansson*a and Reiner Lomoth*b
Received (in Cambridge, UK) 21st January 2005, Accepted 10th February 2005
First published as an Advance Article on the web 23rd February 2005
DOI: 10.1039/b500946d
Rapid and complete switching between the N6 and the N5O
donor set induced by changing the metal oxidation state has
been observed for a new structural motif based on a
ruthenium(II) polypyridyl complex.
The idea of building switches and memories on a molecular level
has attracted great interest, as it would provide the basis for
molecular electronic devices.1 A principle requirement for any
switch or memory is bistability,2 i.e. the system has to exist in two
Scheme 1
stable states that can be reversibly interconverted by an external
[RuIIN6(OH)]2+ complex 1.7 The 1H NMR spectrum of 1 in
trigger. Coordination compounds featuring electrochemically
induced linkage isomerism are interesting candidates as molecular
memories.3 In these compounds, the reversible rearrangement of
CD3CN revealed 23 signals in the aromatic region in accordance
with the low symmetry of the complex (see ESI{). With the aid of
COSY and NOESY techniques, all proton signals could be
1
unambiguously assigned. The H chemical shifts for the isolated
for example an ambidentate ligand triggered by a metal-centered
redox process results in a hysteresis-like bistable behavior, evident
from a displacement of the voltammetric reduction peak compared
pyridine are included in Table 1.
to the oxidation peak. At any potential in between the two redox
The redox properties of
1 were investigated by cyclic
processes the state of the system depends on the electrochemical
history of the system. Switching between the two states, that is
writing or erasing the memory, is accomplished by a temporarily
applied potential outside the range of bistability. Several such
systems have been described, in particular for copper-based
complexes where the oxidation states +1 and +2 have different
geometric preferences leading to reversible monomer/dimer inter-
conversions,4 and [Ru(NH3)5(sulfoxide)]3+/2+ complexes5 where the
ambidentate sulfoxide ligand is S-bound to RuII but isomerizes to
O-bound after oxidation. Recently, also ruthenium(II) polypyridyl
complexes containing sulfoxide ligands have been shown to
undergo reversible linkage isomerization,6 in which the favourable
electrochromic behavior of these complexes offers a simple way to
differentiate the redox states, i.e. to read the memory. However,
most examples of bistability based on linkage isomerism in Ru
complexes suffer from slow isomerization reactions (on the second
timescale) and not in all systems conversions between the isomers
are complete. These drawbacks would impede rapid writing and
erasing and complicate reading of a memory. Here, we report
linkage isomerism in a bistridentate heteroleptic ruthenium(II)
polypyridyl complex containing the 1-[6-(2,29-bipyridyl)]-1-(2-
pyridyl)-ethanol ligand, where the electrochemically induced
switching between a N6 donor set and the N5O analogue is rapid
and complete in both directions (Scheme 1).
voltammetry, controlled potential electrolysis, and UV-Vis spectro-
electrochemistry. In the cyclic voltammogram at low sweep rate
(0.100 V s21, CH3CN), an irreversible anodic wave at Ep,a 5 0.79 V
(vs. Fc+/0) was observed and an irreversible cathodic wave at
Ep,c 5 0.34 V was found on the reverse scan (Fig. 1). Both waves
can be attributed to one-electron processes according to the
coulometric data for exhaustive electrolysis at 0.92 V and re-
reduction at 20.080 V. The UV-Vis spectroelectrochemical
changes are shown in Fig. 2. Based on the spectral changes upon
oxidation, in particular the bleach of the MLCT band at 480 nm,
the anodic wave can be assigned to a metal-centered (RuII
A
RuIII) oxidation. All spectral changes were fully reversed by re-
reduction at 20.080 V.
The electrochemical behavior of 1 can be rationalized in terms
of a reversible follow-up reaction that is triggered by the RuII
RuIII oxidation, resulting in a lowered potential for the RuIII
RuII reduction. A linkage isomerization reaction that switches
from the N6 to the N5O donor set upon oxidation, and back to the
N6 isomer upon re-reduction, could account for the considerable
A
A
Table 1 Electrochemical data, UV-Vis data and 1H NMR shifts for 1
and 2 (PF6 salts)
2
EK/V (DEp/mV)a
Absorptionb
1H NMR (CD3CN)c
Reaction of RuII(ttpy)(DMSO)Cl2 (ttpy is 49-tolyl-2,29:69,20-ter-
pyridine) and one equivalent of 1-[6-(2,29-bipyridyl)]-1-(2-pyridy-
l)-ethanol in a refluxing ethanol/water mixture produced the red
l
max/nm
Ru2+/1+d
Ru3+/2+
(e/M21 cm21
) H3 H4 H5 H6
1
2
a
c
21.59 (71)
0.79 (2)e 482 (14400)
8.26 7.68 6.87 7.40
6.95 7.07 6.95 8.35
21.90 (73) 20.10 (73) 518 (14200)f
CH3CN, (n-C4H9)4NPF6 (0.1 M), 0.1 V s21, vs. Fc+/0
300 MHz, 298 K. Reduction of the ttpy ligand, see Ref. 15.
.
MeOH.
b
{ Electronic supplementary information (ESI) available: Experimental
details and Figs. S1–S5. See http://www.rsc.org/suppdata/cc/b5/b500946d/
*Olof.Johansson@fki.uu.se (Olof Johansson)
d
e
f
Anodic peak potential. e obtained by in situ preparation.
reiner.lomoth@fki.uu.se (Reiner Lomoth)
1578 | Chem. Commun., 2005, 1578–1580
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Fig. 1 Cyclic voltammograms (0.100 V s21) of 1 (a, 1 mM) and 2 (b,
0.8 mM) in CH3CN with 0.1 M (n-C4H9)4NPF6.
Fig. 3 Cyclic voltammograms of 1 (5 mM) in CH3CN with 0.5 M
(n-C4H9)4NPF6 and digital simulations with the parameters given in
Scheme 2.
Similar homogenous rate constants, k1 5 600 s21 ([RuIIIN6(OH)]3+
A [RuIIIN5O(H)]3+) and k2 5 500 s21 ([RuIIN5O(H)]2+
A
[RuIIN6(OH)]2+), respectively, were obtained for the two isomer-
ization reactions.10 For the reverse reactions of both chemical steps
a product of k21 6 k22 5 0.03 s22 could be obtained from the
thermodynamics of the square scheme. Both rate constants are
negligibly small compared to the forward reactions, as the
isomerization equilibria are basically completely on the side of
the N6 and N5O isomers in the RuII and RuIII states, respectively.
This is evidenced by the absence of any peaks for oxidation of the
N5O isomer and re-reduction of the N6 isomer in CVs at
sufficiently low scan rates (Fig. 1), and by the 1H NMR spectrum
of 1 which shows no detectable signals from the [RuIIN5O(H)]2+
isomer.11
Fig. 2 Electronic spectrum of 1 (y1.6 6 1025 M) in CH3CN with 0.1 M
(n-C4H9)4NPF6 and spectral changes upon electro-oxidation at 0.92 V.
Further support for the suggested N6 to N5O linkage isomeriza-
tion was obtained from the successful preparation of the
[RuIIN5O]+ complex 2 (inset Fig. 4), isolated after treatment of a
solution of 1in DMF with potassium tert-butoxide and subsequent
precipitation with diethylether. The 1H NMR spectrum of 2
(CD3CN) showed the expected changes for the non-coordinating
pyridine (Table 1). The H6 proton in 1 appears at 7.40 ppm
reflecting a position in a shielding region above the orthogonal
terpyridine ligand.12 In 2, in contrast, H6 resonances at 8.35 ppm.
The effect on H3 is even larger (Dd 5 21.3), suggesting a
conformation in 2 where H3 is located over the terpyridine ligand.
The kinetics of the base-induced [RuIIN6(OH)]2+ A [RuIIN5O]+
isomerization was followed spectrophotometrically after addition
of sodium methoxide to a methanolic solution of 1 (see ESI{ for
kinetic data). The solution gradually changed from an orange to a
purple colour, indicating a change from the N6 to the N5O donor
set, and showed isosbestic points at 497, 419, and 346 nm (Fig. 4).
The red-shift of the 1MLCT band is expected due to stabilization
of the RuIII(ttpy2) and RuIII(bpy2) excited states by the alkoxide
Scheme 2
separation of the two RuIII/II redox couples (Scheme 2). At high
sweep rates (v ¢ 50 V s21), the reverse peak for the RuIII/II couple
in 1 started to appear suggesting that the re-reduction can compete
with isomerization (Fig. 3). This peak became more pronounced
with increasing sweep rate and at values of v . 1000 V s21, the
cathodic wave attributed to the RuIII A RuII reduction of the
isomer 2-H+ almost completely disappeared. At the highest scan
rates a minor anodic peak at ca. 0.5 V also emerges (Fig. 3 and
ESI{) that can be attributed to the oxidation of the unstable
[RuIIN5O(H)]2+ isomer. The rate constants of the isomerization
reactions were obtained from digital simulations8 of the cyclic
voltammograms (Fig. 3) based on an ECEC mechanism according
to Scheme 2. The simulations with the parameters from Scheme 2
consistently reproduced the experimental voltammograms over the
whole range of sweep rates between 50 and 1700 V s21 (see ESI{).9
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˚
We thank Professor Bjo¨rn Akermark, Professor Licheng Sun,
and Professor Leif Hammarstro¨m for valuable discussions. This
work was financially supported by grants from the Knut and Alice
Wallenberg Foundation, the Swedish Research Council, the
Swedish Energy Agency, and the Swedish Foundation for
Strategic Research (SSF).
Olof Johansson*a and Reiner Lomoth*b
aDepartment of Chemistry, Organic Chemistry, BMC, Uppsala
University, Box 599, 75124 Uppsala, Sweden.
E-mail: Olof.Johansson@fki.uu.se; Fax: +46 18 471 3818
bDepartment of Physical Chemistry, BMC, Uppsala University, Box
579, 75123 Uppsala, Sweden. E-mail: reiner.lomoth@fki.uu.se;
Fax: +46 18 471 3654
Notes and references
Fig. 4 Spectral changes of 1 (2 6 1025 M) in MeOH containing
250 equiv. NaOMe at 292.5 K. Arrows indicate increasing reaction time
from 0 to160 min. Inset: Structure and proton numbering of complex 2.
1 A. P. De Silva, H. Q. N. Gunaratne, T. Gunnlaugsson, A. J. M. Huxley,
C. P. McCoy, J. T. Rademacher and T. E. Rice, Chem. Rev., 1997, 97,
1515; J.-P. Launay and C. Coudret, Ann. N. Y. Acad. Sci., 1998, 852,
116; M. D. Ward, Chem. Soc. Rev., 1995, 24, 121; L. Fabbrizzi,
M. Licchelli and P. Pallavicini, Acc. Chem. Res., 1999, 32, 846.
2 O. Kahn and J.-P. Launay, Chemtronics, 1988, 3, 140; U. Ko¨lle, Angew.
Chem., Int. Ed., 1991, 30, 956.
ligand.13 The rate of the forward reaction 1 A 2 is independent of
the base concentration with a first order rate constant of kiso
5
(7.5 ¡ 0.3) 6 1024 s21 (at 295 K).14 This indicates that a rate
limiting isomerization step is preceding a rapid deprotonation
reaction. The electrochemical and 1H NMR data show that in the
absence of base only an undetectable fraction could reside in the
N5O form, but in the presence of base subsequent deprotonation
completely shifts the reaction to the product 2. The reversibility of
the process was confirmed by the addition of acid (p-TsOH) that
instantaneously regenerated the starting spectrum of the
[RuIIN6(OH)]2+ complex.
3 M. Sano, Molecular Machines and Motors, Springer-Verlag, Berlin,
2001, pp 117–139; V. Amendola, L. Fabbrizzi and P. Pallavicini, Coord.
Chem. Rev., 2001, 216–217, 435.
4 V. Amendola, C. Mangano, P. Pallavicini and M. Zema, Inorg. Chem.,
2003, 42, 6056; V. Amendola, L. Fabbrizzi, L. Gianelli, C. Maggi,
C. Mangano, P. Pallavicini and M. Zema, Inorg. Chem., 2001, 40, 3579;
V. Amendola, L. Fabbrizzi, L. Linati, C. Mangano, P. Pallavicini,
V. Pedrazzini and M. Zema, Chem. Eur. J., 1999, 5, 3679; K. T. Potts,
M. Keshavarz-K, F. S. Tham, H. D. Abrun˜a and C. R. Arana, Inorg.
Chem., 1993, 32, 4422.
5 M. Sano and H. Taube, Inorg. Chem., 1994, 33, 705; A. Yeh, N. Scott
and H. Taube, Inorg. Chem., 1982, 21, 2542; A. Tomita and M. Sano,
Inorg. Chem., 2000, 39, 200.
6 J. J. Rack, J. R. Winkler and H. B. Gray, J. Am. Chem. Soc., 2001, 123,
2432; J. J. Rack and N. V. Mockus, Inorg. Chem., 2003, 42, 5792;
J. J. Rack, A. A. Rachford and A. M. Shelker, Inorg. Chem., 2003, 42,
7357; C. Sens, M. Rodr´ıguez, I. Romero, A. Llobet, T. Parella,
B. P. Sullivan and J. Benet-Buchholz, Inorg. Chem., 2003, 42, 2040.
7 M. Abrahamsson, H. Wolpher, O. Johansson, J. Larsson, M. Kritikos,
In the cyclic voltammogram of 2 (Fig. 1), a reversible redox
couple was observed at 20.10 V (DEp 5 73 mV at v 5 0.100 V s21
)
assigned to the RuIII/II redox couple in accordance with previous
results in ruthenium(II) polypyridyl complexes with oxygen
donors.13 The substantially lower potential of the RuIII/II couple
in the deprotonated complex 2 as compared to the electrogener-
ated isomer (2-H+) indicates that the electrochemically induced
isomerization does not involve deprotonation of the complex and
it can be anticipated that the electrogenerated isomer is protonated
at the pyridyl moiety.
˚
L. Eriksson, P.-O. Norrby, J. Bergquist, L. Sun, B. Akermark and
L. Hammarstro¨m, Inorg. Chem., submitted.
8 The electrochemical simulation package (ESP v. 2.4) was provided by
Dr Carlo Nervi, University of Torino, and can be retrieved free of
charge from http://www.lem.ch.unito.it.
9 For the heterogenous electron transfer reactions the simulations require
a significantly lower value of the standard rate constant k0 for the N5O
isomer to account for the larger scan rate dependency of the cathodic
peak potential associated with the RuIII/II couple at EK 5 0.35 V.
10 The relative high value of k2 explains the absence of a voltammetric
peak, except at the highest scan rates applied, arising from the oxidation
of the N5O isomer [RuIIN5O(H)]2+.
In summary, we have shown that the ruthenium(II) bistridentate
complex 1 containing the ambidentate ligand 1-[6-(2,29-bipyr-
idyl)]-1-(2-pyridyl)-ethanol rearranges reversibly from the N6 to the
N5O analogues 2-H+ (RuIII) and 2 (RuII) upon oxidation or
treatment with base, respectively. The electrochemically induced
process is rapid in both directions and both interconversions
between the isomers are complete. Due to the tridentate nature of
the ambidentate ligand the isomerization is an intramolecular
process in contrast to those systems where bimolecular steps such
as dimerization or re-binding of a monodentate ligand are involved
that would hamper isomerization in surface confined or solid state
systems. Both isomers can be easily distinguished owing to the
favourable electrochromic properties of the ruthenium complex or,
alternatively, the state of the system could be inferred from its
electrochemical response by sufficiently fast voltammetric reading
that avoids isomerization. With these favourable properties
complex 1 represents an interesting new structural motif for
linkage isomerism and variations of this motif should be explored
in view of factors that affect the rates of isomerization.
11 Upper limits of k21 ¡ 10 s21 and k22 ¡ 2 s21 follow from the
simulated voltammograms at slow scan rates (see ESI{).
12 E. C. Constable, A. M. W. Cargill Thompson, D. A. Tocher and M. A.
M. Daniels, New J. Chem., 1992, 16, 855.
13 A. Dovletoglou, S. A. Adeyemi and T. J. Meyer, Inorg. Chem., 1996, 35,
4120; B. M. Holligan, J. C. Jeffery, M. K. Norgett, E. Schatz and
M. D. Ward, J. Chem. Soc. Dalton Trans., 1992, 3345; T. Norrby,
˚
A. Bo¨rje, B. Akermark, L. Hammarstro¨m, J. Alsins, K. Lashgari,
R. Norrestam, J. Ma˚rtensson and G. Stenhagen, Inorg. Chem., 1997, 36,
5850.
14 From the temperature dependency of the isomerization rate constant an
activation energy of 79 kJ mol21 was obtained. See ESI{.
15 Reduction of the ttpy ligand occurs at significantly less negative
potential than reduction of the bpy ligand. See e.g.: O. Johansson,
M. Borgstro¨ m, R. Lomoth, M. Palmblad, J. Bergquist,
˚
L. Hammarstro¨m, L. Sun and B. Akermark, Inorg. Chem., 2003, 42,
2908.
1580 | Chem. Commun., 2005, 1578–1580
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