A new class of near-infrared electrochromic oxamide-based dinuclear ruthenium complexes

Article abstract of DOI:10.1021/ol048021b

(Chemical equation presented) We report the synthesis of a new class of symmetric and unsymmetric oxamide-based dinuclear ruthenium complexes. These complexes were characterized by NMR, ESI-MS, and electrochemical methods. Spectroelectrochemical analysis of the complexes showed broad absorptions in the NIR region for the mixed-valence state of the complexes. The introduction of a chiral group into the bridging ligand produced an optically active complex that was studied using circular dichroism.

Full text of DOI:10.1021/ol048021b

ORGANIC
LETTERS
2004
Vol. 6, No. 24
4519-4522
A New Class of Near-Infrared
Electrochromic Oxamide-Based
Dinuclear Ruthenium Complexes
Majid F. Rastegar, Erin K. Todd, Hongding Tang, and Zhi Yuan Wang*
Department of Chemistry, Carleton UniVersity, 1125 Colonel By DriVe, Ottawa,
Ontario, Canada K1S 5B6
wang@ccs.carleton.ca
Received September 28, 2004
ABSTRACT
We report the synthesis of a new class of symmetric and unsymmetric oxamide-based dinuclear ruthenium complexes. These complexes were
characterized by NMR, ESI-MS, and electrochemical methods. Spectroelectrochemical analysis of the complexes showed broad absorptions
in the NIR region for the mixed-valence state of the complexes. The introduction of a chiral group into the bridging ligand produced an
optically active complex that was studied using circular dichroism.
Homo- and heteropolynuclear metal complexes are of interest
for studying the electron-transfer phenomena1 and for ap-
plications in molecular electronics,2 solar cells,3 and as optical
attenuators.4 Ruthenium complexes containing bipyridine
ligands have attracted a great deal of interest in light of their
interesting luminescent and electrochemical properties. The
work by Kaim et al. showed that dinuclear ruthenium
complexes with 1,2-dicarbonylhydrazido (DCH) ligands
(Figure 1) are highly electrochromic.5 DCH-Ru complexes
in the mixed-valence state (RuII/RuIII) display intense near-
infrared (NIR) absorptions between 1000 and 2000 nm due
to intervalence charge transfer (IVCT) transitions between
the two metallic nuclei. The RuII/RuII state does not show
any absorption in the NIR region, while the RuIII/RuIII state
exhibits an absorption at around 800 nm. Extensive studies
on dinuclear ruthenium complexes with DCH bridging
ligands have been carried out in our group in the past few
years.4,6 Several synthetic routes to various ligands have been
established, and a series of DCH-Ru complexes having
(1) (a) Creutz, C.; Taube, H. J. Am. Chem. Soc. 1969, 91, 3988. (b)
Taube, H. Angew. Chem. 1984, 96, 315. (c) Taube, H. Angew. Chem. 1984,
96, 329.
(2) Mikkelson, K. V.; Ranter, M. A. Chem. ReV. 1987, 87, 113.
(3) (a) Subramanian, V.; Evans, D. G. J. Phy. Chem. 2004, 108, 1085.
(b) Marin, V.; Holder, E.; Schubert, U. S. J. Polym. Sci., Part A: Polym.
Chem. 2004, 42, 374.
(4) (a) Qi, Y.; Desjardins, P.; Wang, Z. Y. J. Opt. A: Pure Appl. Opt.
2002, 4, S273. (b) Qi, Y.; Desjardins, P.; Meng, X. S.; Wang, Z. Y. Opt.
Mater. 2003, 21, 255. (c) Qi, Y.; Wang, Z. Y. Macromolecules 2003, 36,
3146. (d) Wang, Z. Y.; Zhang, J.; Wu, X.; Birau, M.; Yu, G.; Yu, H.; Qi,
Y.; Desjardins, P.; Meng, X. S.; Gao, J. P.; Todd, E.; Song, N. H.; Bai, Y.
W.; Beaudin, A. M. R.; LeClair, G. Pure Appl. Chem. 2004, 76, 1435.
(5) (a) Kasack, V.; Kaim, W.; Binder, H.; Jordanov, J.; Roth, E. Inorg.
Chem. 1995, 34, 1924. (b) Kaim, W.; Kasack, V.; Binder, H.; Roth, E,
Jordanov, J. J. Angew. Chem., Int. Ed. Engl. 1988, 27, 1174. (c) Kaim, W.;
Kasack, V. Inorg. Chem. 1990, 29, 4696.
Figure 1. DCH-Ru and OXA-Ru complexes.
10.1021/ol048021b CCC: $27.50
© 2004 American Chemical Society
Published on Web 11/03/2004

Scheme 1
Figure 2. ORTEP structure of 2f.
various donor and acceptor groups have been synthesized.
In addition, linear and cross-linked polymers containing the
DCH-Ru moieties have been prepared.
allowed for the isolation of pure OXA-Ru complexes in
their RuII/RuII state.
DCH and OXA ligands contain the CO-N-N-CO and
N-CO-CO-N moieties, respectively, and are structural
isomers. Accordingly, the ruthenium complexes based on the
OXA ligand or OXA-Ru complexes as shown in Figure 1,
should be considered as isomers to DCH-Ru complexes.
Since DCH and OXA ligands can be synthesized in a number
of different ways and can have different functional groups
for further manipulation, it is necessary to have access to
the OXA-Ru complexes for studying and comparing the
electrical and optical properties for potential applications.
However, surprisingly enough, OXA-Ru complexes have
not been reported and are still unknown in the literature to
date. We report herein the synthesis and characterization of
a new class of dinuclear ruthenium complexes based on the
oxamide ligands.
For DCH-Ru complexes, the NIR absorption depends on
the electronic environment of the two metal cations, which
is strongly influenced by the substituents on the ligands.
Changing the electron-withdrawing or electron-donating
ability of the ligands will affect the HOMO/LUMO energy
levels and thus cause changes in the NIR λmax values.
Accordingly, a series of OXA-Ru complexes containing
electron-donating and electron-withdrawing groups on the
oxamide nitrogen atoms were prepared (Scheme 1).
The symmetric ligands (1a-h) were synthesized by
reaction of oxalyl chloride with the corresponding amines.
Unsymmetric ligands 1i and 1j were prepared in two steps,
first by refluxing aniline or 1-naphthylamine in diethyl
oxalate to give monsubstituted compounds that were sub-
sequently reacted with methylamine and (S)-(-)-R-methyl-
benzylamine, respectively. In all cases, the ligands were
isolated in yields greater than 80%. To make complexes 2a-
j, a mixture of ruthenium bis(bipyridine)dichloride and the
corresponding ligands were heated to reflux in the presence
of base under a nitrogen atmosphere. The purple complexes
were isolated upon addition of ammonium hexafluorophos-
phate and then passed through an alumina-gel column (eluted
with a mixture of acetonitrile and toluene, 1:1 v/v), which
Figure 2 shows the ORTEP crystal structure of complex
2f (as one of the stereoisomers).7 The crystals were grown
by dissolving 2f in acetone and slow evaporation of the
solvent at room temperature. The C-N-Cph-Cph torsion
angle is 69.2°, showing the weak resonance effect. Two fused
five-membered cyclic rings are planar, similar to the DCH-
Ru series.5 There is also π-π interaction between the
bipyridine groups within the unit cell.
Studies on the stereochemistry of mononuclear8 and
dinuclear ruthenium9 complexes have attracted much interest
in the area of supramolecular chemistry. In dinuclear
ruthenium complexes, the metal centers can possess right-
or left-handed chirality (∆ or Λ), respectively. The simplest
case for chiral OXA-Ru complexes, where the complex
possesses symmetric bridging and identical terminal units,
is shown in Figure 3 with meso (∆Λ) and two enantiomeric
isomers (∆∆, ΛΛ). However, by lowering the symmetry in
this system through altering the substituents on the bridging
and terminal units, it is possible to have a large number of
(7) Measurement was made on a Bruker SMART 1K CCD diffractometer
with graphite monochromated Mo KR (0.71073 Å) radiation. Data were
collected at the temperature 207(2) K, and the structure was solved by direct
methods and expended using Fourier techniques using SHELXTL. All non-
hydrogen atoms were refined anisotropically. Hydrogen atoms were included
in idealized positions and not refined. Data were merged and corrected for
beam inhomogeneous and absorption using SADSAB. Crystal size: 0.52
× 0.44 × 0.20 mm. Monoclinic, P2/n. Cell dimensions a ) 11.413(3) Å;
b ) 12.631(3) Å; c ) 22.655(5) Å; R ) 90°; â ) 95.192(4)°; γ ) 90°. V
) 3252.6(13) Å3 θmax)26.02°, F ) 1.502 g/cm3; R1 ) 0.0930 (for
,
reflections with I > 2σ(I)), wR2 ) 0.1559 (for all reflections).
(8) (a) Balzani, V.; Juris, A. Coord. Chem. ReV. 2001, 211, 97. (b) Furue,
M.; Ishibashi, M.; Satoh, A.; Oguni, T.; Maruyama, K.; Sumi, K. Coord.
Chem. ReV. 2000, 208, 103. (c) Tyson, D. S.; Luman, C. R.; Zhou, X. L.;
Castellano, F. N. Inorg. Chem. 2001, 40, 4063. (d) Walters, K. A.; Trouillet,
L.; Guillerez, S.; Schanze, K. S. Inorg. Chem. 2000, 39, 5496. (e) Hu, Y.
Z.; Tsukiji, S.; Shinkai, S.; Oishi, S.; Hamachi, I. J. Am. Chem. Soc. 2000,
122, 241. (f) Luo, J.; Reddy, K. B.; Salameh, A. S.; Wishart, J. F.; Isied,
S. S. Inorg. Chem. 2000, 39, 2321. (g) Stemp, E. D. A.; Holmlin, R. E.;
Barton, J. K. Inorg. Chim. Acta 2000, 297, 88. (h) Furue, M.; Maruyama,
K.; Kanematsu, Y.; Kushida, T.; Kamachi, M. Coord. Chem. ReV. 1994,
132, 201.
(9) (a) Patterson, B. T.; Foley, F. M.; Richards, D.; Keene, F. R. J. Chem.
Soc., Dalton Trans. 2003, 4, 709. (b) Keene, F. R. Chem. Soc. ReV. 1998,
27, 185. (c) Fletcher, N. C.; Junk, P. C.; Reitsma, D. A.; Keene, F. R. J.
Chem. Soc., Dalton Trans. 1998, 133. (d) Fletcher, N. C.; Keene, F. R. J.
Chem. Soc., Dalton Trans. 1999, 683. (e) Patterson, B. T.; Keene, F. R.
Inorg. Chem. 1998, 37, 645.
(6) Qi, Y.; Desjardins, P.; Birau, M,; Wu, X.; Wang, Z. Y. Chin. J. Polym.
Sci. 2003, 21, 147.
4520
Org. Lett., Vol. 6, No. 24, 2004

Table 1. Electrochemical Data for OXA-Ru Complexesa
complex 1EPa 1EPc 1E1/2
2EPa
2EPc
2E1/2
∆E1/2
2a
2b
2c
2d
533
659
836
513
576
719
904
595
555
689
870
554
961
901
1033
973
997
937
442
248
708
881b
1012
1035
1077
918
796
971
1080 1046
1103 1069
1164 1121
1014
1091 1057
1125 1093
752
926
198
174
420
412
414
400
424
442
2e
2f
2g
2h
2i
599
625
670
527
601
632
652
688
745
605
665
669
626
657
707
566
633
651
966
1022
1061
2j
Figure 3. Stereoisomers of symmetric OXA-Ru complexes.
a Electrochemical data (in mV) are corrected to ferrocene vs SCE.
b Additional couple due to oxidation of the dimethylaniline group.
mixtures of diastereomers, enantiomers, geometrical isomers,
and even conformational isomers.9 Following separation of
the rac and meso forms of an azobis(2-pyridine)-bridged
dinuclear ruthenium complex by cation exchange chroma-
tography, it was shown by Keene and co-workers that the
ruthenium centers in these systems communicated in unique
manners.10 For example, the MLCT absorptions and redox
couples were slightly shifted for the rac and meso forms of
these complexes.
it was anticipated that these complexes would behave in an
analogous fashion. Indeed, complexes 2a,b,e-j underwent
two reversible oxidation steps as anticipated. The half wave
potentials for the first and second oxidation steps range from
555 to 707 mV and 937 to 1121 mV, respectively. In all of
these cases, there is a potential window of 400-442 mV
between the first and second oxidation steps with the
exception of 2b where the difference is only 248 mV. For
complex 2c without substitutions on the nitrogen atoms, there
was only one oxidation step observed. The electrochemical
data for OXA-Ru complexes is summarized in Table 1.
Figure 4 shows the cyclic voltammogram of complex 2i with
In principle, each diastereomer that is present will also
1
have its own unique NMR spectrum. H and 13C NMR
analyses of these OXA-Ru complexes did indicate the
1
presence of isomers in solution. For example, the H NMR
spectrum of 2e shows two resonance peaks for the methyl
group, one corresponding to the pair of enantiomers and the
other to the meso compound. For complex 2a, only one
singlet peak for the two methyl groups was observed at 2.15
1
ppm in the H NMR spectrum, whereas in the 13C NMR
spectrum there are two peaks at 32.1 and 32.3 ppm for the
methyl carbons and two peaks for the carbonyl carbons at
170.7 and 170.8 ppm. Considering the point groups of these
systems, it is expected that each diastereomer will provide
the 22 resonance peaks in the NMR spectrum. However, due
to signal overlapping, only 34 peaks were present in the 13
C
NMR spectrum. For the unsymmetrical complexes, there are
at least four stereoisomers present without the meso isomer.
The 13C NMR spectrum of complex 2i showed two methyl
carbons and four distinct carbonyl carbons. By introducing
a chiral group into the ligand, the situation becomes even
more complicated. For complex 2j, at least seven distinct
Figure 4. Cyclic voltammogram of 2i in acetonitrile containing
0.1 M TBAP using a 50 mV/s scan rate (five cycles).
1
sets of resonance peaks were observed in the H and 13C
NMR spectra. There are significant differences in the
resonance intensities, and it is not possible to determine if
these are due to overlapping peaks or higher percentages of
certain isomers being formed.
Cyclic voltammetry (CV) was utilized to investigate the
electrochemical properties of OXA-Ru complexes. Since
the isomeric DCH-Ru complexes have been shown to
undergo two successive one-electron oxidation steps corre-
sponding to formation of the RuII/RuIII and RuIII/RuII states,
reversible redox cycles over time, indicating that it would
be suitable for use as an electrochemical switch.
The spectroelectrochemical properties of the complexes
were also examined in order to determine whether the OXA-
Ru systems possessed similar electrochromic properties as
the DCH-Ru complexes. Figure 5 shows the UV/vis-NIR
spectra of complex 2f. The spectra were recorded using an
OTTLE cell by applying different potentials. Complexes in
the RuII/RuII state display two intense absorptions around
370 and 506 nm associated with the MLCT transitions.
(10) Kelso, L. S.; Reitsma, D. A.; Keene, F. R. Inorg. Chem. 1996, 35,
5144.
Org. Lett., Vol. 6, No. 24, 2004
4521

Figure 5. UV/vis-NIR spectra of 2f in three different oxidation
states in acetonitrile containing 0.1 M TBAPF6.
Figure 6. CD spectra of 1j and 2j in acetonitrile
Oxidation to the RuII/RuIII state results in the appearance of
a strong and broad IVCT band centered in the NIR region.
Complex 2b had the most blue-shifted NIR absorption with
the λmax at 1393 nm, followed by complex 2d at 1474 nm.
The rest of the complexes exhibited λmax values around 1650
nm. Upon further oxidation to the RuIII/RuIII state, the intense
NIR absorptions disappear and broad peaks appear around
700 nm. At this time, the complexes also precipitated in the
OTTLE cell in acetonitrile solutions. These results are in
contrast to the DCH-Ru complexes in which a distinct peak
typically appeared at 800 nm when the complexes were
oxidized to the RuIII/RuIII states. These peaks were assigned
to LMCT transitions, which manifest differently in the
OXA-Ru complexes. Interestingly, complex 2c, which
showed only one oxidation step by CV, did not show any
NIR absorptions upon oxidation in the OTTLE cell. This
indicates that the two ruthenium centers were oxidized at
the same potential, and therefore a mixed-valence complex
was not produced. Thus, hydrogen as the substituents on the
nitrogen atoms of the oxamide group results in loss of
communication between the metal centers.
one intense negative peak at 234 nm with a molar ellipticity
of -56115 deg cm2 dmol-1. Complex 2j has its most intense
peak at 302 nm with a molar ellipticity of -53 500 deg cm2
dmol-1.
The other enantiomers of ligand 1j and complex 2j were
also synthesized using (R)-R-methylbenzylamine and, as
expected, possessed CD spectra that were opposite in sign
to those shown in Figure 6.
In summary, the OXA-Ru complexes are a new class of
NIR electrochromic compounds, which are useful for study-
ing the electron-transfer process and for potential applications
in NIR optical attenuation and chiroptical switching.
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council of Canada for financial
support.
Supporting Information Available: General synthetic
1
procedure, H and 13C NMR spectra of complexes, and a
CIF file of the crystal structure of complex 2f. This material
is available free of charge via the Internet at http://pubs.acs.org.
The chiroptical properties of ligand 1j and complex 2j
were examined using circular dichroism (CD) in acetonitrile
(Figure 6). The ligand bearing one chiral center exhibited
OL048021B
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Org. Lett., Vol. 6, No. 24, 2004