Structural characterization of ruthenium-dioxolene complexes with Ru II-SQ and RuII-cat frameworks

Article abstract of DOI:10.1246/cl.2005.1562

The structural and electronic properties of [Ru^II(trpy)-(ClSQ) (PPh₃)]⁺ (trpy = 2,2′:6′,2″-terpyridine; ClSQ = 4-chlorobenzosemiquinonate) and [Ru^II(trpy)(ClCat)(PPh ₃)] (ClCat = 4-chlorocatecholate)

Full text of DOI:10.1246/cl.2005.1562

1562
Chemistry Letters Vol.34, No.11 (2005)
Structural Characterization of Ruthenium–Dioxolene Complexes
with RuII–SQ and RuII–Cat Frameworks
Tetsuaki Fujihara, Rei Okamura, and Koji Tanakaꢀ
Institute for Molecular Science and CREST, Japan Science and Technology Agency (JST),
5-1 Higashiyama, Myodaiji, Okazaki 444-8787
(Received August 3, 2005; CL-051008)
The structural and electronic properties of [RuII(trpy)-
and ESR spectra.6 Furthermore, an ammine complex, [Ru(trpy)-
(Bu2SQ)(NH3)](ClO4) (Bu2SQ = 3,5-di-tert-butylbenzosemi-
quinonate), has an ability to oxidize MeOH and i-PrOH catalyti-
cally under very mild conditions such as the electrolysis at 0 V
(vs SCE) in CH2Cl2.7
To our surprise, among various structural reports on the
ruthenium–dioxolene system,4–6,8 structural comparison of di-
oxolene ligands in different oxidation states has never been
made within the same metal–ligand system. It should be noted,
however, that the carbon–oxygen bond lengths of dioxolenes
have been widely used to evaluate the oxidation state in some
metal–dioxolene systems.3 We now report, for the first time,
on the structural difference in the C–O bond length between
the RuII–SQ and RuII–Cat systems having the same metal–
ligand framework; [Ru(trpy)(ClSQ)(PPh3)](ClO4) ((1)[ClO4])
(ClSQ = 4-chlorobenzosemiquinonate) and [Ru(trpy)(ClCat)-
(PPh3)] (2) (ClCat = 4-chlorocatecholate).
The ruthenium–dioxolene complex [1]þ was prepared by
the reaction of cis-Ru(trpy)(PPh3)Cl2 with ClCatH2 in the pres-
ence of 2 equivalent of NaOMe in MeOH followed by the oxi-
dation with AgClO4.9 The reduction of [1]þ in THF with an
excess amount of an aqueous sodium dithionite solution gave
a reduced complex 2. The X-ray photoelectron spectroscopy is
one of the most effective method to estimate the oxidation state
of Ru.10 The Ru(3d5=2) binding energies for [1]þ and 2 are
observed at 280.7 and 280.3 eV, respectively, revealing that
the oxidation states of [1]þ and 2 are Ru(II).11
The structures of [1]þ and 2 are essentially the same, that is,
each complex has a distorted octahedral geometry with three
nitrogen atoms of trpy, two oxygen atoms of the dioxolene
ligand, and one phosphorous atom of the triphenylphosphine
ligand (Figure 1).12 The Ru–P(PPh3) bond length of [1]þ
(ClSQ)(PPh3)]þ (trpy = 2,20:60,200-terpyridine; ClSQ = 4-
chlorobenzosemiquinonate) and [RuII(trpy)(ClCat)(PPh3)]
(ClCat = 4-chlorocatecholate) have been characterized by
X-ray diffractometry, UV–vis spectroscopy, ESR, and electro-
chemistry.
Metal complexes that absorb photons strongly in the near-
infrared (NIR) region are of considerable interest for chemical
and material science because of their potentials in applications
such as optical data processing.1 Ruthenium–dioxolene com-
plexes that display strong metal-to-ligand charge transition
(MLCT) in the NIR region have been applied to redox-switching
devices since their MLCT bands undergo significant changes de-
pending on the oxidation state of the ruthenium–dioxolene
frameworks.2 The characteristic NIR absorption in the ruthe-
nium–dioxolene framework is regulated by the balance between
dꢀ orbital energy of ruthenium and ꢀꢀ orbital energy of the
dioxolene ligands, that is charge distribution.3–5 Accordingly,
the actual electronic structures of Ru–dioxolene complexes are
often ambiguous because of close energy levels of metal-
centered d orbital and ligand-based ꢀꢀ orbital.
Recently, we have shown that proton dissociation of a
Ru–OH2 group in [Ru(trpy)(dioxolene)(OH2)]2þ (trpy =
2,20:60,200-terpyridine, dioxolene = 3,5-di-tert-butylbenzosemi-
quinonate and 4-chlorobenzosemiquinonate) results in electron
transfer from the group to the Ru–dioxolene framework to
form the correspondent Ru–oxyl radical complex [Ru(trpy)-
(dioxolene)(Oꢁꢂ)], in which the actual electronic structure of
the Ru–dioxolene framework is determined by C–O bond
lengths of the dioxolene ligand, binding energy of Ru 3d orbital,
ꢀ
ꢀ
(2.314(1) A) is similar to that of 2 (2.307(2) A). The bond lengths
of three Ru–N(trpy) in [1]þ (2.059(4), 1.965(4), and 2.081(5) A,
ꢀ
respectively) are also comparable to those of 2 (2.059(6),
ꢀ
1.931(6), and 2.066(7) A, respectively).
The two C–O lengths of [1]þ (1.289(7) and 1.304(7) A) are
ꢀ
ꢀ
apparently shorter than those of 2 (1.320(10) and 1.343(9) A), re-
flecting the difference in the oxidation state at the dioxolene unit
between [1]þ and 2. The former is in the range of the RuII–SQ
4–6,8
ꢀ
frameworks (ca. 1.29 A),
and the latter is comparable to
those of Ru(Cpꢀ)(NO)(2,3-naphthalenediolate) (1.342(9) and
ꢀ
1.345(9) A). The difference of the C–O bond length between
the SQ and Cat states is also comparable to those of Co(bpy)-
13
ꢀ
ꢀ
(Bu2SQ)(Bu2Cat) (1.297(9) A for Bu2SQ and 1.358(10) A for
Bu2Cat: Bu2Cat = 3,5-di-tert-butylcatecholate).14 The differ-
ence of the Ru–O(dioxolene) bond lengths between [1]þ
ꢀ
ꢀ
(2.092(4) and 2.082(4) A) and 2 (2.102(5) and 2.120(6) A) also
reflects the change in the oxidation state at the dioxolene moiety.
The complex [1]þ exhibited a sharp isotropic ESR signal at
Figure 1. ORTEP drawing of 2. Thermal ellipsoids are drawn
at the 50% probability level. Hydrogen atoms are omitted for
clarity.
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.11 (2005)
1563
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3
4
Figure 2. UV–vis spectra of the ruthenium–dioxolene com-
plexes in CH2Cl2 at room temperature under N2.9 Solid line is
[1]þ (8:1 ꢅ 10ꢂ5 mol dmꢂ3) and dashed line 2 (7:7 ꢅ 10ꢂ5
mol dmꢂ3).
5
S. Bhattacharya, S. R. Boone, G. A. Fox, and C. G. Pierpont,
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C. G. Pierpont, Inorg. Chem., 33, 6038 (1994); S. Bhattacharya
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J. Am. Chem. Soc., 125, 6729 (2003).
g ¼ 2:00 in CH2Cl2 at room temperature, whereas the catechol-
ate complex 2 was found to be diamagnetic as expected. The
complex [1]þ exhibited a strong absorption band at 832 nm
(Figure 2),9 whose transition energy is similar to those reported
so far for Ru(trpy)(ClSQ)(OAc) (885 nm)6 and other RuII–SQ
complexes (750–950 nm).4–8 Thus, the CT band of [1]þ is as-
signed to the MLCT transition from the dꢀ orbital of ruthe-
nium(II) to the ꢀꢀ orbital of the dioxolene ligand. On the other
hand, the catecholate complex 2 exhibited an absorption band
at 523 nm (Figure 2),9 which must be viewed as related to the
maximum absorption length reported for cis-Ru(trpy)(PPh3)Cl2
(530 nm).15 Accordingly, the absorption band is assigned to
the MLCT transition form the dꢀ orbital of ruthenium(II) to
the ꢀꢀ orbital of terpyridine ligand. The spectroscopic results
strongly indicate that [1]þ and 2 have the RuII–SQ and RuII–
Cat frameworks, respectively.
The complexes [1]þ and 2 both exhibited two redox couples
at E1=2 ¼ ꢂ0:09 and þ0:76 V vs SCE, respectively. As expected
from the RuII–SQ and the RuII–Cat frameworks of [1]þ and 2,
respectively, the equilibrium electrode potential of [1]þ was
located at a potential between the two redox couples (þ0:34
V), and that of 2 was positioned at the potential lower than the
two redox couples (ꢂ0:22 V). As a result, the redox couples
observed at ꢂ0:09 and þ0:76 V are reasonably assigned to the
Cat/SQ and RuII/RuIII processes, respectively. In addition,
electrochemical reduction of [1]þ in CH2Cl2 at ꢂ0:4 V gave a
reduced species whose UV–vis absorption property is quite
similar to that of 2. By selecting the chloro-substituted dioxolene
as the ligand, the redox potential based on the Cat/SQ couple
was controlled in the region where both the reduced and the
oxidized forms of the complex are stable enough to be isolated
in their crystalline form.
6
7
8
T. Hino, T. Wada, T. Fujihara, and K. Tanaka, Chem. Lett., 33,
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Wada, T. Fujihara, M. Tomori, D. Ooyama, and K. Tanaka,
Bull. Chem. Soc. Jpn., 77, 741 (2004).
9
For [1](ClO4): Yield 40%. Found: C, 54.91; H, 4.11; N, 4.53%.
.
Calcd for C39H29N3O6Cl2PRu (C4H10O2): C, 55.61; H, 4.23;
N, 4.52%. UV–vis: ꢁmax/nm (CH2Cl2) 274 ("/dm3 molꢂ1
cmꢂ1 21700), 313 (25900), 479 (4270), 832 (10000). ESR
(CH2Cl2): g ¼ 2:00. ESI MS (CH2Cl2): m=z 739 (½M ꢂ
ClO4ꢃþ). For 2: Yield 65%. Found: C, 63.36; H, 4.38; N,
5.38%. Calcd for C39H29N3O2ClPRu: C, 63.37; H, 3.95; N,
5.68%. UV–vis: ꢁmax/nm (CH2Cl2) 280 ("/dm3 molꢂ1 cmꢂ1
22200), 314 (27100), 523 (5250). ESI MS (CH2Cl2): m=z
739 ([M]þ).
10 S. Srivastava, Appl. Spectrosc. Rev., 22, 401 (1986); R. D.
Feltham and P. Brant, J. Am. Chem. Soc., 104, 641 (1982).
11 X-ray photoelectron spectra (XPS) were measured with an
ESCALAB 220i-XL. Mg Kꢂ radiation (1253.6 eV) was
used as the X-ray excitation source. The samples were depos-
ited on gold foil from CH2Cl2 solution. The C 1s peak was
calibrated as the value of 284.6 eV and used as the internal
reference.
.
12 Crystal data: For [1](ClO4) (C4H10O2): C43H39N3O8Cl2PRu,
Mr ¼ 928:75, T ¼ 173 K, monoclinic, space group P21=c
ꢀ
(No. 14), a ¼ 12:566ð1Þ, b ¼ 16:937ð1Þ, c ¼ 20:062ð2Þ A,
ꢄ
ꢀ 3
ꢃ ¼ 110:617ð3Þ , V ¼ 3996:2ð5Þ A , Z ¼ 4, ꢄ(Mo Kꢂ) =
6.25 cmꢂ1
,
Observed reflections 6368 (I > 3ꢅðIÞ), R1,
In conclusion, we have succeeded for the first time to char-
acterize the structural features of the RuII–SQ and RuII–Cat
frameworks arising from a unique ruthenium–dioxolene system.
wR2 ¼ 0:063, 0.192. For 2: C39H29N3O2ClPRu, Mr ¼
739:17, T ¼ 173 K, monoclinic, space group C2=c (No. 15),
ꢀ
a ¼ 32:620ð3Þ, b ¼ 8:9988ð7Þ, c ¼ 26:998ð2Þ A, ꢃ ¼
102:927ð3Þꢄ, V ¼ 7724ð1Þ A , Z ¼ 8, ꢄ(Mo Kꢂ) = 5.51
ꢀ 3
cmꢂ1, Observed reflections 5501 (I > 3ꢅðIÞ), R1, wR2 ¼
0:067, 0.217. CCDC Nos. 245947 and 245948.
13 K. Yang, J. A. Martin, S. G. Bott, and M. G. Richmond, Inorg.
Chim. Acta, 254, 19 (1997).
We are thankful to Prof. T. Yokoyama and Dr. T. Nakagawa
(Institute for Molecular Science) for the measurements of XPS.
References and Notes
1
2
A. Tsuda and A. Osuka, Science, 293, 79 (2001); L. Blacke,
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4951 (1980).
15 B. P. Sullivan, J. M. Calvert, and T. J. Meyer, Inorg. Chem., 19,
1404 (1980).
´
J. Garcıa-Can˜adas, A. P. Mecham, L. M. Peter, and M. D.
Published on the web (Advance View) October 22, 2005; DOI 10.1246/cl.2005.1562