Novel 2-mercaptopyridine-ruthenium complex exhibiting electrochemically induced linkage isomerization switched on/off by protolysis

Article abstract of DOI:10.1021/ic701186k

A ruthenium complex [ruthenium bis(2,2′-bipyridine)(2- mercaptopyridine)(pyridine)](PF₆)₂ was crystallographically characterized from its deprotonated form and was electrochemically investigated. In the deprotonated complex, the 2-mercaptopyridine ligand coordinates to the Ru atom only by the S atom; therefore, the N atom of the 2-mercaptopyridine ligand can be protonated. In a CH₃CN solution, the complex shows a reversible redox couple attributed to Ru^III/II-S. The addition of a base to the CH ₃CN solution of the complex gives irreversible voltammograms, implying electrochemically induced linkage isomerization between Ru ^II-S and Ru^III-N. Analysis of the observed cyclic voltammograms gave the equilibrium and rate constants for linkage isomerization: K^II_NS = 1.2 × 10¹⁸, K^III _NS = 0.64, k^II_NS = 5 × 10 s^-1, k^II_SN = 4 × 10^-17 s^-1, k ^III_NS = 0.26 s^-1, and k^III _SN = 0.40 s^-1.

Full text of DOI:10.1021/ic701186k

Inorg. Chem. 2007, 46, 10455−10457
Novel 2-Mercaptopyridine−Ruthenium Complex Exhibiting
Electrochemically Induced Linkage Isomerization Switched On/Off by
Protolysis
Tomohiko Hamaguchi,* Kikujiro Ujimoto, and Isao Ando
Department of Chemistry, Faculty of Science, Fukuoka UniVersity, 8-19-1 Nanakuma, Jonan-ku,
Fukuoka 814-0180, Japan
Received June 18, 2007
A ruthenium complex [ruthenium bis(2,2
′
-bipyridine)(2-mercapto-
information. This is an important feature for memory devices.
Write denotes recording information by oxidization or
reduction of the metal center and retaining the information
as the difference in the coordination environment. Read
denotes getting the information back from the complex as
the difference in a property, e.g., UV-vis absorption.
For example, [RuII(NH3)5(DMSO)]2+ (DMSO ) dimethyl
sulfoxide) is one of the most famous molecules exhibiting
electrochemically induced linkage isomerization.4 In this
complex, RuII is coordinated by the S atom of DMSO. After
oxidation of RuII to RuIII, the complex isomerizes to the other
isomer in which RuIII is coordinated by the O atom. The
related complexes5,6 are suitable as candidates for a molecular
memory device. However, they have a problem; information
could be overwritten by an unintended external signal
because they have no mechanism to protect information like
the write-protect tab of a floppy disk. Some carboxamide-
ruthenium complexes6 could solve this problem, but only a
few such complexes have been reported so far.5g
Here, we report a new ruthenium complex, 1H2+, that has
a mechanism to protect information. 1H2+ is designed to
contain a RuII atom and 2-mercaptopyridine in thione form,7
which coordinates to the Ru atom only with the S atom. The
N atom of 2-mercaptopyridine is protonated and does not
coordinate to the Ru atom. Linkage isomerization from S-
to N-bound is difficult in oxidation of 1H2+ because
protonation of 2-mercaptopyridine prevents the N atom from
pyridine)(pyridine)](PF6)2 was crystallographically characterized from
its deprotonated form and was electrochemically investigated. In
the deprotonated complex, the 2-mercaptopyridine ligand coordi-
nates to the Ru atom only by the S atom; therefore, the N atom
of the 2-mercaptopyridine ligand can be protonated. In a CH3CN
solution, the complex shows a reversible redox couple attributed
/
II
to RuIII
complex gives irreversible voltammograms, implying electrochemi-
cally induced linkage isomerization between RuII S and RuIII
N.
Analysis of the observed cyclic voltammograms gave the equilib-
−S. The addition of a base to the CH3CN solution of the
−
−
rium and rate constants for linkage isomerization: KII
)
1.2
×
NS
17
1018, KIII
)
0.64, kII
)
)
5
×
10 s-1, kII
)
4
×
10- s-1, kIII
NS
NS
SN
NS
1
)
0.26 s-1, and kIII
0.40 s-
.
SN
Molecular devices,1,2 especially molecular memory,2 which
is interesting as an advanced information storage device, have
received a lot of attention. Molecular memory is able to store
information into one molecule and should help in increasing
the storage density. Such devices require molecular bista-
bility, i.e., a molecule with two or more stable states that
are reversibly exchanged by external stimuli. Transition-metal
complexes exhibiting electrochemically induced linkage
isomerization have been reported3 as model compounds for
molecular memory. They contain an ambidentate ligand
bound to a metal center and exhibit exchange of the coor-
dination atom for another atom on applying voltage sufficient
to oxidize or reduce the metal center. A different coordina-
tion environment produces different properties of the com-
plex; therefore, those complexes are able to write and read
(4) Scotto, N.; Yen, A.; Taube, H. Inorg. Chem. 1982, 21, 2542.
(5) (a) Giesbrecht, E.; Rojas, R. L. E.; Toma, H. E. J. Chem. Soc., Dalton
Trans. 1985, 2469. (b) Johansson, O.; Lomoth, R. Chem. Commun.
2005, 1578. (c) Tomita, A.; Sano, M. Inorg. Chem. 1994, 33, 5825.
(d) Tomita, A.; Sano, M. Inorg. Chem. 2000, 39, 200. (e) Sano, M.;
Taube, H. Inorg. Chem. 1994, 33, 705. (f) Silva, D. O.; Toma, H. E.
Can. J. Chem. 1994, 72, 1705. (g) Sens, C.; Rodr´ıguez, M.; Romero,
I.; Llobet, A.; Parella, T.; Sullivan, B. P.; Buchholz, J. B. Inorg. Chem.
2003, 42, 2040.
(6) (a) Fairlie, D. P.; Ilan, Y.; Taube, H. Inorg. Chem. 1997, 36, 1029.
(b) Chou, M. H.; Brunschwing, B. S.; Creutz, C.; Sutin, N.; Yeh, A.;
Chang, R. C.; Lin, C.-T. Inorg. Chem. 1992, 31, 5347.
(7) (a) Bajaj, H. C.; Das, A.; van Eldik, R. J. Chem. Soc., Dalton Trans.
1998, 1563. (b) Deeming, A. J.; Hardcastle, K. I.; Meah M. N.; Bates,
P. A.; Dawes, H. M.; Hursthouse, M. B. J. Chem. Soc., Dalton Trans.
1988, 227.
* To whom correspondence should be addressed. E-mail: thama@
fukuoka-u.ac.jp.
(1) (a) Gunaratne, H. Q. N.; Silva, A. P.; Gunnlaugsson, T.; Huxley, A.
J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. ReV. 1997,
97, 1515. (b) Licchelli, M.; Pallavicini, P.; Fabbrizzi, L. Acc. Chem.
Res. 1999, 32, 846. (c) Ward, M. D. Chem. Soc. ReV. 1995, 24, 121.
(2) (a) Carroll, R. L.; Gorman, C. B. Angew. Chem., Int. Ed. 2002, 41,
4378. (b) Fabbrizzi, L.; Pallavicini, P.; Amendola, V. Coord. Chem.
ReV. 2001, 216-217, 435.
(3) Toma, H. E.; Rocha, R. C. Croat. Chem. Acta 2001, 74, 499.
10.1021/ic701186k CCC: $37.00
Published on Web 11/09/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 25, 2007 10455

COMMUNICATION
Scheme 1. Electrochemically Induced Linkage Isomerization
Controlled by the Addition of a Base and an Acid
Figure 1. Molecular structure of the complex cation of 1+. Thermal
ellipsoids are drawn at the 50% probability level, and H atoms are omitted.
coordinating to the Ru atom. On the other hand, linkage
isomerization may proceed upon oxidation of 1+ (deproto-
nated form). Therefore, adding a base or an acid to the
solution of the complex should control electrochemically
induced linkage isomerization of 1H2+ (Scheme 1).
Figure 2. Cyclic voltammograms of 1H‚(PF6)2 in CH3CN (a and b) and
in Et3N/CH3CN (c and d). (a and c) Anodic scan starting at -1.50 V vs
Ag+/Ag. (b and d) Cathodic scan starting at 0.80 and 0.25 V vs Ag+/Ag
after oxidation at 0.80 and 0.25 V vs Ag+/Ag for 3 min, respectively. The
1H2+ was prepared as a PF6 salt and was characterized by
arrow shows the initial scan direction. The scan rate is 0.1 V s-1
.
1
elemental analysis, H NMR, and UV-vis absorption. We
could not obtain a suitable crystal of 1H2+ for single-crystal
X-ray analysis as a PF6 salt; however, we prepared a good
crystal of the deprotonated form (1‚BPh4) by recrystallization
in the presence of NaBPh4 and Et3N.8
Cyclic voltammograms of 1H2+ were examined in the
absence and presence of Et3N (Figures 2 and 2S in the
Supporting Information).9 In the absence of Et3N, the cyclic
voltammograms exhibited one reversible redox couple,
associated with RuIII/II at E1/2 ) 0.47 V vs Ag+/Ag (Figure
2a,b). The shape of the voltammogram was independent of
the scan direction and scan rate. These results imply that
the linkage isomerization occurs sparingly during the redox
reaction.
The ORTEP drawing of a 1+ cation reveals that 2-mer-
captopyridine coordinates to the Ru atom with the S atom
monodentately (Figure 1). These data indicate that 1H2+ has
the intended structure that we require. The UV-vis absorp-
tion spectrum of 1H‚(PF6)2 in CH3CN showed three absorp-
tion bands at 293, 354 (sh), and 454 nm. The first band is
assigned to a ligand-centered π-π* transition; the others
are assigned to metal-to-ligand charge-transfer transitions.
Upon the addition of Et3N as a base, the spectrum changed
with a set of isosbestic points at 335, 399, and 490 nm
(Figure S1a in the Supporting Information). This change was
completed with the addition of 1 equiv of a base. The initial
spectrum was recovered completely upon the addition of
excess CF3COOH as an acid (Figure S1b in the Supporting
Information). This confirms the reversible protolysis of 1H2+.
In the presence of Et3N, the voltammograms dramatically
changed depending on the scan direction and scan rate. With
the anodic scan starting at -1.50 V vs Ag+/Ag (Figure 2c),
one quasi-reversible redox couple, associated with RuIII/II at
E11/2 ) 0.01 V vs Ag+/Ag, was observed. A small cathodic
peak was observed at about -1 V vs Ag+/Ag. After oxidation
at 0.25 V for 3 min (Figure 2d), two quasi-reversible redox
couples (E11/2 ) 0.01 and E21/2 ) -1.07 V vs Ag+/Ag) were
observed with the cathodic scan starting at 0.25 V vs
Ag+/Ag. The ratios of ic1/ia1 and ic2/ia2 were dependent on
the scan rate (ic1 and ic2 are cathodic peak currents and ia1
(8) Crystal data for 1‚BPh4: C54H45BN6RuS, M ) 921.90, monoclinic,
C2/c, a ) 33.696(10) Å, b ) 9.513(3) Å, c ) 30.793(8) Å, â )
115.828(4)°, V ) 8885(4) Å3, Z ) 8, T ) 100 K, 11 939 collected
reflections and 5397 independent reflections, R1 ) 0.0675 and wR2
) 0.0891 for I > 2σ(I). The structure was determined in SHELXS-97
and refined using SHELXL-97.12 Selected bond distances: Ru1-S1
2.396(3) Å, Ru1-N1 2.056(8) Å, Ru1-N2 2.063(7) Å, Ru1-N3
2.032(7) Å, Ru1-N4 2.056(7) Å, Ru1-N5 2.098(8) Å Selected bond
angles: N1-Ru1-S1 173.0(2)°, N3-Ru1-N5 175.7(3)°, N4-Ru1-
N2 171.3(3)°.
(9) Cyclic voltammetry was performed on BAS100B/W (BAS) using three
electrode cells (glassy carbon as the working electrode, Pt coil as the
counter electrode, homemade Ag+/Ag as the reference electrode). The
medium was 0.1 mol dm-3 n-Bu4NPF6/CH3CN or 1 × 10-3 mol dm-3
Et3N/0.1 mol dm-3 n-Bu4NPF6/CH3CN. The concentration of the
complex was 1 × 10-3 mol dm-3. E1/2 of the ferrocenium/ferrocene
couple was 0.10 V vs Ag+/Ag.
10456 Inorganic Chemistry, Vol. 46, No. 25, 2007

COMMUNICATION
of kIIINS and kIIISN were also evaluated from the equation for
the chemical reaction preceding a reversible charge transfer
(case III) proposed by Nicholson and Shain.10 Assuming that
ik (the measured peak current) is ic1 of the voltammogram
in which the initial scan direction is cathodic and id (the
corresponding diffusion-controlled current in the absence of
a chemical reaction) is ia1 of the voltammogram in which
the initial scan direction is anodic, the evaluated values are
kIII ) 0.26 s-1 and kIII ) 0.40 s-1. The thermodynamic
Scheme 2. Redox Cycle Including Linkage Isomerization
and ia2 are anodic peak currents at the redox peak potentials
NS
SN
of E1 and E21/2, respectively). This dramatic change
1/2
cycle in Scheme 2 can be solved to obtain the equilibrium
suggests that electrochemically induced linkage isomerization
constant KIINS for the RuII-N/RuII-S reaction, based on KIII
NS
occurs between Ru-S and Ru-N linkage isomers in 1+. By
and assuming E° ) E1/2. In this way, KII ) 1.2 × 1018.
NS
analogy to the literature,5f,g the redox couples at E1 and
1/2
The rate constants of kII and kII are evaluated from the
NS
SN
E2 are assigned to RuIII/II-S and RuIII/II-N, respectively.
1/2
simulation of the cyclic voltammograms (Figure S4 in the
The difference in voltammetric behaviors in the absence and
presence of Et3N reveals that only the deprotonated form of
the complex shows linkage isomerization.
Supporting Information).11 Because kII is dependent on
SN
kIINS, simulation of the cyclic voltammogram was performed
upon fixing E11/2, E21/2, KIIINS, kIIINS, kIIISN, and KII to the
NS
To confirm this isomerization, the spectrum of the
electrolytic product of 1+ was measured by spectroelectro-
chemistry (Figure S3 in the Supporting Information). The
spectrum of 1+ did not change upon application of a potential
of -0.3 V vs Ag+/Ag. Upon oxidation at +0.35 V, bands
around 350, 500, and 600 nm disappeared, and a new band
appeared around 430 nm. When the oxidized complex was
reduced at -0.30 V, the spectrum was almost the same as
that at +0.35 V rather than that at the initial -0.30 V. This
irreversibility might show that the complex was decomposed
upon oxidation; however, the spectrum upon successive
reduction at -1.35 V was almost the same as that at the
initial -0.30 V. This shows that the complex recovered the
isolated state upon reduction. When the potential was held
at -0.30 V, the complex retained the same spectrum. The
observed hysteresis seems to suggest that 1+ isomerizes to
RuIII-N upon oxidation and recovers RuII-S upon reduction.
The reversibility of 1H2+ for deprotonation was also
confirmed by cyclic voltammetry. As mentioned above, 1H2+
did not isomerize in CH3CN, but it did after the addition of
1 equiv of Et3N. When excess CF3COOH was added into
the basic solution, this complex did not isomerize again. This
shows that electrochemically induced linkage isomerization
can be controlled reversibly by the addition of a base or an
acid.
evaluated values and changing kII as the only variable
NS
parameter. From this simulation, we obtain kII ) 5 × 10
NS
s-1 and kII ) 4 × 10-17 s-1. KII and KIII suggest that
SN
NS
NS
RuII favors hugely the S-bound isomer and RuIII does slightly
favor the N-bound isomer.
In this study, we demonstrated that the complex 1+ shows
linkage isomerization induced by the redox reaction from
the S-bound form to the N-bound form. However, the
protonated complex 1H2+ does not isomerize upon oxidation.
Thus, the present complex can be set to forbid or allow
isomerization by changing the pH of the solution and should
be a candidate for molecular memory equipped with a write-
protect tab.
Acknowledgment. The authors thank Shinetsu Igarashi
and Dr. Kenji Yoza (Bruker AXS K.K.) for the X-ray crystal
structure determination. This work was supported in part by
funds (Grant 075001) from the Central Research Institute
of Fukuoka University.
Supporting Information Available: Synthesis of 1H‚(PF6)2,
absorption spectra of 1H‚(PF6)2 upon the continuous addition of
Et3N and 1+ upon the continuous addition of CF3COOH, cyclic
voltammograms, absorption spectra of oxidized and reduced
1H‚(PF6)2 upon the addition of 1 equiv of Et3N, and crystallographic
details in CIF format. This material is available free of charge via
the Internet at http://pubs.acs.org.
Scheme 2 shows the redox cycle including linkage
isomerization. The equilibrium constants (KII and KIII
)
NS
NS
IC701186K
and rate constants (kIIINS and kIIISN) for linkage isomerization
were estimated from the cyclic voltammograms as follows.5f,g
The value of KIIINS can be obtained from the intercept of the
plot of ic1/ic2 against the reciprocal of the scan rates, V-1.
(10) Nicholson, R. S.; Shain, I. Anal. Chem. 1964, 36, 706.
(11) Digital simulation of proposed electrochemical mechanisms was
performed with DigiSim (BAS).
(12) Sheldrick, G. M. SHELXL-97; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
The obtained value of KIII is 0.64 ( 0.03 s-1. The values
NS
Inorganic Chemistry, Vol. 46, No. 25, 2007 10457