Sensitive oxidation state ambivalence in unsymmetrical three-center (M/Q/M) systems [(acac)2Ru(μ-Q)Ru(acac)2]n, Q = 1,10-phenanthroline-5,6-dione or 1,10-phenanthroline-5,6-diimine (n = +, 0, -, 2-)

Article abstract of DOI:10.1021/ic048309x

The new redox systems [(acac)₂Ru(μ-Q¹)Ru(acac) ₂]ⁿ (1ⁿ) and [(acac)₂Ru(μ-Q ²)Ru(acac)₂]ⁿ (2ⁿ) with Q1 = 1,10-phenanthroline-5,6-dione and Q² = 1,10-phenanthroline-5,6- diimine were studied for n = +, 0, -, and 2- using UV-Vis-NIR spectroelectrochemistry and, in part, EPR and susceptometry. The ligands can bind the first metal (left) through the phenanthroline nitrogen atoms and the second metal (right) at the o-quinonoid chelate site. The neutral compounds are already different: Compound 1 is formulated as a Ru^II(μ-Q ¹)^.- Ru^III species with partially coupled semiquinone and ruthenium(III) centers. In contrast, a Ru^III(μ- Q²)^2- Ru^III structure is assigned to 2, which shows a weak antiferromagnetic spin-spin interaction (J = -1.14 cm^-1) and displays an intense half-field signal in the EPR spectrum. The one-electron reduced forms are also differently formulated as Ru^II(μ-Q ¹)^2- Ru^III for 1^- with a Ru ^III-typical EPR response and as Ru^II(μ-Q ²)^.- Ru^II for 2^- with a radical-type EPR signal at g = 2.0020. In contrast, both 1^2- and 2^2- can only be described as Ru^II(μ-Q)²-Ru^II species. The monooxidized forms 1⁺ and 2⁺ show very similar spectroscopy, including a Ru^III-type EPR signal. Although no unambiguous assignment was possible here for the alternatives Ru ^II(μ-Q)⁰Ru^III, Ru^III(μ-Q) ^2-Ru^IV or Ru^III(μ-Q)^.-Ru ^III, the last description is favored. The reasons for identical or different oxidation state combinations are discussed.

Full text of DOI:10.1021/ic048309x

Inorg. Chem. 2005, 44, 3210−3214
Sensitive Oxidation State Ambivalence in Unsymmetrical Three-Center
(M/Q/M) Systems [(acac)2Ru(
-Q)Ru(acac)2]n, Q
1,10-Phenanthroline-5,6-dione or 1,10-Phenanthroline-5,6-diimine (n
, 0, , 2
µ
)
)
+
−
−)
Sandeep Ghumaan,† Biprajit Sarkar,‡ Srikanta Patra,† Joris van Slageren,§ Jan Fiedler,|
Wolfgang Kaim,*,‡ and Goutam Kumar Lahiri*,†
Department of Chemistry, Indian Institute of Technology - Bombay, Powai, Mumbai, 400076,
India, Institut fu¨r Anorganische Chemie and 1. Physikalisches Institut, UniVersita¨t Stuttgart,
Pfaffenwaldring 55, D-70550 Stuttgart, Germany, and J. HeyroVsky´ Institute of Physical
Chemistry, Academy of Sciences of the Czech Republic, DolejsˇkoVa 3, CZ - 18223 Prague,
Czech Republic
Received December 1, 2004
The new redox systems [(acac)2Ru(
phenanthroline-5,6-dione and Q2
1,10-phenanthroline-5,6-diimine were studied for n ) +, 0,
UV Vis
µ
-Q1)Ru(acac)2]n (1n) and [(acac)2Ru(
µ
-Q2)Ru(acac)2]n (2n) with Q1
, and 2−
)
1,10-
using
)
−
−
−
NIR spectroelectrochemistry and, in part, EPR and susceptometry. The ligands can bind the first metal
(left) through the phenanthroline nitrogen atoms and the second metal (right) at the o-quinonoid chelate site. The
neutral compounds are already different: Compound 1 is formulated as a RuII(
µ
-Q1)•- RuIII species with partially
-
coupled semiquinone and ruthenium(III) centers. In contrast, a RuIII(
µ
-Q2)2 RuIII structure is assigned to 2, which
shows a weak antiferromagnetic spin
−
spin interaction (J ) −1.14 cm-1) and displays an intense half-field signal
in the EPR spectrum. The one-electron reduced forms are also differently formulated as RuII(
µ
-Q1)2 RuIII for 1-
-
with a RuIII-typical EPR response and as RuII(
µ
-Q2)•- RuII for 2- with a radical-type EPR signal at g
contrast, both 12 and 22 can only be described as RuII( -Q)2-RuII species. The monooxidized forms 1+ and 2+
show very similar spectroscopy, including a RuIII-type EPR signal. Although no unambiguous assignment was
possible here for the alternatives RuII( -Q)0RuIII, RuIII( -Q)2-RuIV or RuIII( -Q)•-RuIII, the last description is favored.
The reasons for identical or different oxidation state combinations are discussed.
) 2.0020. In
-
-
µ
µ
µ
µ
Introduction
Mononuclear and dinuclear3,4 paramagnetic states within such
redox systems, ranging from clear radical complexes3,5 via
mixed systems6 to predominantly metal-centered species,7
Ruthenium complexes of “non-innocent”1 quinonoid ligands
Q have been studied extensively2-8 because they offer to
combine a substitutionally inert but electron-transfer active
transition metal with a redox-active chelate ligand with three
viable oxidation states Q (quinone), Q•- (semiquinone
radical), or Q2- (catecholate or hydroquinone dianion).
(2) (a) Haga, M.; Dodsworth, E. S.; Lever, A. B. P. Inorg. Chem. 1986,
25, 447. (b) Masui, H.; Lever, A. B. P.; Auburn, P. Inorg. Chem.
1991, 30, 2402. (c) Ebadi, M.; Lever, A. B. P. Inorg. Chem. 1999,
38, 467. (d) Lever, A. B. P.; Auburn, P. R.; Dodsworth, E. S.; Haga,
M.; Liu, W.; Melnik, M.; Nevin, W. A. J. Am. Chem. Soc. 1988, 110,
8076. (e) Auburn, P. R.; Dodsworth, E. S.; Haga, M.; Liu, W.; Nevin,
W. A.; Lever, A. B. P. Inorg. Chem. 1991, 30, 3502. (f) Bag, N.;
Pramanik, A.; Lahiri, G. K.; Chakravorty, A. Inorg. Chem. 1992, 31,
40. (g) Bag, N.; Lahiri, G. K.; Basu, P.; Chakravorty, A. J. Chem.
Soc., Dalton Trans. 1992, 113.
(3) (a) Ernst, S.; Ha¨nel, P.; Jordanov, J.; Kaim, W.; Kasack, V.; Roth, E.
J. Am. Chem. Soc. 1989, 111, 1733. (b) Ernst, S.; Kasack, V.;
Bessenbacher, C.; Kaim, W. Z. Naturforsch. 1987, 42b, 425.
(4) (a) Dei, A.; Gatteschi, D.; Pardi, L. Inorg. Chem. 1990, 29, 1443. (b)
Kasack, V.; Kaim, W.; Binder, H.; Jordanov, J.; Roth, E. Inorg. Chem.
1995, 34, 1924. (c) Kno¨dler, A.; Fiedler, J.; Kaim, W. Polyhedron
2004, 23, 701. (d) Patra, S.; Sarkar, B:, Ghumaan, S.; Fiedler, J.; Zalis,
S.; Kaim, W.; Lahiri, G. K. Dalton Trans. 2004, 750.
* To whom correspondence should be addressed. E-mail: kaim@iac.uni-
stuttgart.de (W.K.); lahiri@chem.iitb.ac.in (G.K.L.).
† Indian Institute of Technology Bombay.
‡ Institut fu¨r Anorganische Chemie, Universita¨t Stuttgart.
§ 1. Physikalisches Institut, Universita¨t Stuttgart.
| Academy of Sciences of the Czech Republic.
(1) (a) Jørgensen, C. K. Oxidation Numbers and Oxidation States;
Springer: Berlin, 1969; p 213. (b) Ward, M. D.; McCleverty, J. A. J.
Chem. Soc., Dalton Trans. 2002, 275. (c) Herebian, D.; Bothe, E.;
Bill, E.; Weyhermu¨ller, T.; Wieghardt, K. J. Am. Chem. Soc. 2001,
123, 10012.
3210 Inorganic Chemistry, Vol. 44, No. 9, 2005
10.1021/ic048309x CCC: $30.25
© 2005 American Chemical Society
Published on Web 04/09/2005

Oxidation State AmbiWalence in Three-Center (M/Q/M) Systems
have been investigated mainly by spectroelectrochemistry
and EPR.
Because of symmetrically bis-chelating bridging ligands
such as 4,7-phenanthroline-5,6-dione (µ-Q3)3a or 1,4,5,8-
tetraoxonaphthalene,4a the dinuclear complexes investigated
were usually symmetrical, leading to interesting alternatives
Run(µ-Q)mRuk for the oxidation state combinations.4 In
contrast, the isomeric form of Q3, 1,10-phenanthroline-5,6-
dione ) Q1, has become a popular, commercially available
ligand9-11 or precursor of ligands11 that offers two distinctly
different chelating sites, an o-quinone function and an
R-diimine binding site. Calculations and experiments identify
the o-quinone side as the location of primary electron uptake,
R-diones being much better π acceptors than aromatic
R-diimines. Dinuclear complexes of Q1 in different oxidation
states have been reported;9d-g however, the coordination
chemistry with the o-quinonediimine analogue 1,10-phenan-
throline-5,6-diimine ) Q2 is much less developed.12
In this paper, we describe the syntheses and, for some
states, surprisingly different electrochemical, spectroscopic
(UV-Vis-NIR, EPR), and magnetic properties of the
dinuclear systems [(acac)2Ru(µ-Q1)Ru(acac)2]n (1n) and
[(acac)2Ru(µ-Q2)Ru(acac)2]n (2n), n ) +,0, -, 2-; acac- )
acetylacetonate ) 2,4-pentanedionate(1-).
Experimental Section
The starting complex Ru(acac)2(CH3CN)213 and the ligands 1,10-
phenanthroline-5,6-dione (Q1)14 and 5,6-diamino-1,10-phenanthro-
line (H2Q2)15 were prepared according to the reported procedures.
Other chemicals and solvents were reagent grade and used as
received. For spectroscopic and electrochemical studies HPLC-grade
solvents were used.
UV-Vis-NIR spectroelectrochemical studies were performed
in CH3CN/0.1 M Bu4NPF6 at 298 K using an optically transparent
thin layer electrode (OTTLE) cell16 mounted in the sample
compartment of a Bruins Instruments Omega 10 spectrophotometer.
FT-IR spectra were taken on a Nicolet spectrophotometer with
samples prepared as KBr pellets. Solution electrical conductivity
(5) Waldho¨r, E.; Schwederski, B.; Kaim, W. J. Chem. Soc., Perkin Trans.
2 1993, 2109.
(6) Patra, S.; Sarkar, B.; Mobin, S. M.; Kaim, W.; Lahiri, G. K. Inorg.
Chem. 2003, 42, 6469.
(7) Salmonsen, R. B.; Abelleira, A.; Clarke, M. J. Inorg. Chem. 1984,
23, 387.
1
was checked using a Systronic 305 conductivity bridge. H NMR
(8) (a) Bhattacharya, S.; Boone, S. R.; Fox, G. A.; Pierpont, C. G. J. Am.
Chem. Soc. 1990, 112, 1088. (b) Bhattacharya, S.; Pierpont, C. G.
Inorg. Chem. 1991, 30, 1511. (c) Haga, M.; Dodsworth, E. S.; Lever,
A. B. P.; Boone, S. R.; Pierpont, C. G. J. Am. Chem. Soc. 1986, 108,
7413. (d) Boone, S. R.; Pierpont, C. G. Inorg. Chem. 1987, 26, 1769.
(e) Bhattacharya, S.; Pierpont, C. G. Inorg. Chem. 1994, 33, 6038. (f)
Cheng, H.-Y.; Lin, C.-C.; Tzeng, B.-C.; Peng, S.-M. J. Chin. Chem.
Soc. 1994, 41, 775. (g) Mitra, K. N.; Majumdar, P.; Peng, S.-M.;
Castin˜eiras, A.; Goswami, S. Chem. Commun. 1997, 1267. (h) Mitra,
K. N.; Choudhury, S.; Castin˜eiras, A.; Goswami, S. J. Chem. Soc.,
Dalton Trans. 1998, 2901. (i) Das, C.; Kamar, K. K.; Ghosh, A. K.;
Majumdar, P.; Hung, C.-H.; Goswami, S. New J. Chem. 2002, 26,
1409.
(9) (a) Goss, C. A.; Abrun˜a, H. D. Inorg. Chem. 1985, 24, 4263. (b) Bock,
H.; Ha¨nel, P. Z. Naturforsch. 1992, 47b, 288. (c) Klein, A.; Kaim,
W.; Waldho¨r, E.; Hausen, H.-D. J. Chem. Soc., Perkin Trans. 2 1995,
2121. (d) Paw, W.; Connick, W. B.; Eisenberg, R. Inorg. Chem. 1998,
37, 3919. (e) Calderazzo, F.; Pampaloni, G.; Passarelli, V. Inorg. Chim.
Acta 2002, 330, 136. (f) Shavaleev, N. M.; Moorcraft, L. P.; Pope, S.
J. A.; Bell, Z. R.; Faulkner, S.; Ward, M. D. Chem. Commun. 2003,
1134. (g) Fujihara, T.; Okamura, R.; Wada, T.; Tanaka, K. Dalton
Trans. 2003, 3221. (h) Calderazzo, F.; Marchetti, F.; Pampaloni, G.;
Passarelli, V. J. Chem. Soc., Dalton Trans. 1999, 4389. (i) Hill, P. L.;
Lee, L. Y.; Younkin, T. R.; Orth, S. D.; McElwee-White, L. Inorg.
Chem. 1997, 36, 5655. (j) Fox, G. A.; Bhattacharya, S.; Pierpont, C.
G. Inorg. Chem. 1991, 30, 2895. (k) Paw, W.; Eisenberg, R. Inorg.
Chem. 1997, 36, 2287. (l) Girgis, A. Y.; Sohn, Y. S.; Balch, A. L.
Inorg. Chem. 1975, 14, 2327. (m) Lei, Y.; Shi, C.; Anson, F. C. Inorg.
Chem. 1996, 35, 3044.
spectra were obtained with a 300 MHz Varian FT spectrometer.
The EPR measurements were made in a two-electrode capillary
tube17 with an X-band Bruker system ESP300, equipped with a
Bruker ER035M gaussmeter and a HP 5350B microwave counter.
Cyclic voltammetric, differential pulse voltammetric, and coulo-
metric measurements were carried out using a PAR model 273A
electrochemistry system. Platinum wire working and auxiliary
electrodes and an aqueous saturated calomel reference electrode
(SCE) were used in a three-electrode configuration. The supporting
electrolyte was Et4NClO4, and the solute concentration was ∼10-3
M. The half-wave potential E° was set equal to 0.5(Epa + Epc),
298
where Epa and Epc are anodic and cathodic cyclic voltammetric peak
potentials, respectively. A platinum wire-gauze working electrode
was used in coulometric experiments. All experiments were carried
out under a dinitrogen atmosphere, and no correction was made
for junction potentials. The elemental analysis was carried out with
a Perkin-Elmer 240C elemental analyzer. Electrospray mass spectra
were recorded on a Micromass Q-ToF mass spectrometer. The
magnetic susceptibility of the complexes 1 and 2 as a function of
temperature was recorded from 1.8 to 300 K using a 0.1 T applied
field on a Quantum Design MPMS XL7 SQUID magnetometer.
The data were corrected for diamagnetic contributions to the
magnetic susceptibility using Pascal’s constants, for the diamagnetic
contribution of the sample holder, and for temperature-independent
paramagnetism (TIP).
(10) (a) Ackermann, M. N.; Interrante, L. V. Inorg. Chem. 1984, 23, 3904.
(b) Friedman, A. E.; Chambron, J. C.; Sauvage, J.-P.; Turro, N. J.;
Barton, J. K. J. Am. Chem. Soc. 1990, 112, 4960.
(11) (a) Berger, S.; Scheiring, T.; Fiedler, J.; Kaim, W. Inorg. Chem. 2004,
43, 1530. (b) Chen, J.; MacDonnell, F. M. Chem. Commun. 1999,
2529. (c) Shukla, A.; Das, A. Polyhedron 2000, 19, 2605. (d) Kim,
M.-J.; Macdonnell, F. M.; Gimon-Kinsel, M. E.; Du Bois, T.;
Asgharian, N.; Griener, J. C. Angew. Chem. Int. Ed. 2000, 39, 615.
(e) Macdonnell, F. M.; Bodige, S. Inorg. Chem. 1996, 35, 5758. (f)
Rice, C. R.; Guerrero, A.; Bell, Z. R.; Paul, R. L.; Motson, G. R.;
Jeffery, J. C.; Ward, M. D. New J. Chem. 2001, 25, 185.
(12) (a) Vickery, K. A.; Bonin, A. M.; Vagg, R. S.; Williams, P. A. In
Structure, Motion, Interaction and Expression of Biological Macro-
molecules; Sarma, R. H., Sarma, M. H., Eds.; Adenine Press:
Schenectady, 1998. (b) Fletcher, N. C.; Robinson, T. C.; Behrendt,
A.; Jeffery, J. C.; Reeves, Z. R.; Ward, M. D. J. Chem. Soc., Dalton
Trans. 1999, 2999.
Synthesis of [(acac)2Ru(Q1)Ru(acac)2] (1). The starting complex
Ru(acac)2(CH3CN)2 (100 mg, 0.26 mmol) and the ligand Q1 (27.5
mg, 0.13 mmol) were dissolved in 20 mL of ethanol, and the
(13) Kobayashi, T.; Nishina, Y.; Shimizu, K.; Satoˆ, G. P. Chem. Lett. 1988,
1137.
(14) Yamada, M.; Tanaka, Y.; Yoshimoto, Y.; Kuroda, S.; Shimao, I. Bull.
Chem. Soc. Jpn. 1992, 6, 1006.
(15) Bodige, S.; MacDonnell, F. M. Tetrahedron Lett. 1997, 38, 8159.
(16) Krejcik, M.; Danek, M.; Hartl, F. J. Electroanal. Chem. 1991, 317,
179.
(17) Kaim, W.; Ernst, S.; Kasack, V. J. Am. Chem. Soc. 1990, 112, 173.
Inorganic Chemistry, Vol. 44, No. 9, 2005 3211

Ghumaan et al.
mixture was heated to reflux for 10 h under a dinitrogen atmosphere.
The initial orange color of the solution gradually changed to brown.
The solvent was removed under reduced pressure, and the residue
was purified using a silica gel column. Initially, a red compound
corresponding to Ru(acac)3 was eluted by CH2Cl2-CH3CN (15:
1), followed by a brown compound corresponding to 1, which was
eluted with CH2Cl2-CH3CN (1:5). Evaporation of the solvent
mixture yielded the pure compound 1. Yield: 53 mg (50%). Anal.
Calcd (Found) for 1: C, 47.52 (47.19); H, 4.24 (3.99); N, 3.46
(3.05). The positive ion electrospray mass spectrum of 1 in CH2-
Cl2 exhibited signals centered at m/z values of 810.13, 711.07, and
510.09 (Figure S1a, Supporting Information), corresponding to {1}+
(calculated molecular mass, 810.03), {1-acac}+ (calc. 710.98) and
{1-Ru(acac)2}+ (calc. 510.03), respectively.
Synthesis of [(acac)2Ru(Q2)Ru(acac)2] (2). The starting complex
Ru(acac)2(CH3CN)2 (100 mg, 0.26 mmol), the ligand H2Q2 (27.3
mg, 0.13 mmol), and sodium acetate (21 mg, 0.26 mmol) were
dissolved in 20 mL of ethanol, and the mixture was heated to reflux
for 10 h. The purple solution thus formed was purified using a
silica gel column. Initially, a red compound corresponding to Ru-
(acac)3 was eluted with CH2Cl2-CH3CN (15:1). With CH3CN-
MeOH (1:20), a red compound corresponding to 2 was eluted.
Evaporation of the solvent mixture yielded the pure complex 2.
Yield: 58 mg (55%). Anal. Calcd (Found) for 2: C, 47.64 (47.09);
H, 4.50 (4.84); N, 6.94 (7.08). The positive ion electrospray mass
spectrum in dichloromethane of 2 exhibited signals centered at m/z
values of 807.08 and 708.03 (Figure S1b, Supporting Information),
corresponding to {2}+ (calculated molecular mass, 808.06) and {2-
acac}+ (calc. 709.02), respectively.
Figure 1. Cyclic voltammograms (s) and differential pulse voltammo-
grams (- - -) of (a) 1 and (b) 2 in CH3CN/0.1 M Et4NClO4.
Table 1. Redox Potentialsa for Compounds 1 and 2
E°/V(∆Ep/mV)
compound
ox 2
ox 1
red 1
red 2
1
2
1.44 (190)
1.43 (150)
-0.02 (90)
0.41 (80)
-0.37 (90)
-0.17 (90)
-1.51 (100)
-1.21 (90)
Results and Discussion
a In CH3CN/0.1 M Et4NClO4, potentials vs SCE.
The neutral complexes [(acac)2Ru(µ-Q)Ru(acac)2] were
obtained by reacting the ruthenium(II) precursor compound
Ru(acac)2(CH3CN)2 with either Q1 or H2Q2. Obviously, the
latter reaction involved an oxidative deprotonation step,
facilitated by the coordination of two imine-affine ruthenium
atoms. The compounds 1 and 2 were identified by analysis
and electrospray mass spectroscopy (Figure S1, Supporting
Information); they were further investigated using SQUID
susceptometry, EPR, cyclic voltammetry (Figure 1, Table
1), and absorption spectroscopy in various oxidation states
(UV-Vis-NIR spectroelectrochemistry, Figure 2, Table 2).
of the residual paramagnetism will require more detailed
investigations. Alternative formulations such as RuIII(µ-Q1)•--
RuII (RuIII at the R-diimine site), RuII(µ-Q1)0RuII, or RuIII-
(µ-Q1)2-RuIII can be ruled out, considering the magnetic and
spectroscopic results. The range between the one-electron
oxidation and reduction potentials of 1 is rather small at only
about 350 mV (Figure 1, Table 1).
In contrast, the difference between oxidation and reduction
potentials amounts to 580 mV for 2 (Figure 1, Table 1). This
compound exhibits a temperature-dependent magnetic be-
havior between 1.8 and 300 K (Figure S3, Supporting
Information), which can be reproduced assuming two anti-
ferromagnetically coupled spins (J ) -1.14 ( 0.02 cm-1).
Compound 2 also displays a very strong RuIII-type EPR
spectrum with g1 ) 2.36, g2 ) 2.21, and g3 ) 1.82,
complemented by an intense half-field signal at g ) 4.28
(Figure 3). The absorption spectrum shows major long-
wavelength bands at 615 and 418 nm (Figure S4, Supporting
Information, Table 2).
1
The absence of conventional H NMR spectra for the
neutral reaction products indicated the presence of oxidation
states different than the diamagnetic RuII(µ-Q)0RuII config-
uration.
Complex 1 exhibits a weak, broad EPR signal in the g )
2 region and a small, temperature-independent øT value
(Figure S2, Supporting Information), corresponding to about
0.6 unpaired electron equivalents per molecule. Absorption
bands at 780, 497, and 405 nm are observed (see Figure 2
and Table 2), in agreement with the presence of the anion
radical of Q1 (λmax ) 747sh, 566, 404 nm11). The results
suggest a RuII(µ-Q1)•- RuIII formulation for 1 with partially
coupled spins at the RuIII/semiquinone coordination site. Such
RuIII/o-semiquinone antiferromagnetic spin-spin coupling
has been observed before,6 the oxidation state assignment
being supported by structural data. The situation is more
complicated for 1 which formally involves a RuIIRuIII mixed-
valent arrangement bridged by an anion radical; the origin
We associate these very different results for 2 in contrast
to 1 with an oxidation state situation RuIII(µ-Q2)2-RuIII,
assigning the half-field transition in the EPR with the triplet
state available for a diruthenium(III) complex (estimated
Ru‚‚‚Ru distance at about 8 Å9f,g). The absorption bands are
then attributed to ligand-to-metal charge transfer (LMCT)
transitions and to intraligand charge-transfer transitions
between the occupied quinone-centered MO (b1) and the
3212 Inorganic Chemistry, Vol. 44, No. 9, 2005

Oxidation State AmbiWalence in Three-Center (M/Q/M) Systems
differs considerably: It exhibits a narrow (∆Hpp ) 0.8 mT)
unresolved radical-type EPR signal at giso ) 2.0020 (Figure
3), and the absorptions are also quite different at 640(sh),
552, 440, and 415 nm. On the basis of the EPR evidence
we formulate 2- as a RuII(µ-Q2)•- RuII species.3a The
absorption bands would thus be assigned to MLCT and
internal semiquinone transitions.
Whereas the transition from 1 to 1- would thus only
involve a ligand reduction from the semiquinone to the
catecholate form, the conversion of 2 to 2- as elucidated
here implies the oxidation of the ligand to the semiquinone
form, set off by double metal reduction. Such seemingly
paradoxical redox rearrangements were observed previously
by Pierpont and co-workers for dioxolene compounds of
vanadium where the reduction of VIII(Q•-)3 led to an
oxidation of the metal in VV(Q2-)3.18 The reason for the
unusual triple oxidation state change observed for 2 f 2-
lies in the strong affinity between ruthenium(II) and imine
nitrogen centers.19
Simple one-electron reduction from either 1- or 2- leads
to the fully reduced EPR-silent dianionic species which can
only be formulated as RuII(µ-Q)2- RuII. Protonation of the
reduced forms at the metal-coordinated amido function is
unlikely because rigorously dried solvent and electrolyte were
used. In addition, the full reversibility of the reduction steps
both in cyclic voltammetry and during the OTTLE reduction/
reoxidation cycles suggests that no chemical complication
is involved. Nevertheless, the differences between the
catecholate and the much more electron-rich 1,2-diamido-
arene ligand are evident from the distinct spectral variations
as summarized in Table 2, possibly signifying energetically
different intraligand charge transfer transitions between the
now fully occupied quinone-centered MO (b1) and the
unoccupied phenanthroline-based MOs b1(ψ) and a2(ø)
(Scheme 1).11
Oxidation of 1 to 1+ (or of 2 to 2+) produces rather similar
results, viz., the appearance of RuIII-type EPR signals at g1
) 2.354 (2.356), g2 ) 2.212 (2.204), and g3 ) 1.832 (1.831),
respectively (Figure 3). The absorption spectra are dominated
in both cases by an intense band around 600 nm (Figure 2,
Table 2); we thus assume a very similar electronic config-
uration for 1+ and 2+. However, all three reasonable
oxidation state combinations RuII(µ-Q)0RuIII, RuIII(µ-Q)•-
RuIII, and RuIII(µ-Q)2- RuIV would be compatible with a RuIII-
type EPR signal. On the basis of the above argument about
strongly different intraligand transitions of (Q1)2- and (Q2)2-
we disfavor the third alternative, and a bridging quinone in
the presence of (acac)2RuII seems unlikely. We therefore
consider the three-spin situation RuIII(µ-Q)•- RuIII with
coupled spins at the RuIII/semiquinone coordination site6 (as
for 1), an assignment that is supported by similar absorption
maxima of 1+ (920sh, 615, 418 nm), (Q1)•- (747sh, 566,
404 nm),11 and 1 (780, 497, 405 nm). In contrast to 1, the
Figure 2. Spectroelectrochemical (OTTLE) response of a 3.5 mM solution
of compound 1n in CH3CN/0.1 M Bu4NPF6 during potentiostatic scans
within one oxidation and two reduction peaks.
unoccupied phenanthroline-based MOs b1(ψ) and a2(ø)
(Scheme 1).9,11
Although there is a well-established stronger preference
of quinone molecules for the reduced state relative to
corresponding quinoneimines, the differences between the
dinuclear complexes 1 and 2 point into the opposite direction,
reflecting the strong coordination between the RuIII centers
and the more basic diamido ligand (Q2)2- in contrast to
(Q1)2-. This effect, attributed to enhanced covalent bonding
contributions, is confirmed by the ca. 0.3 V positive shift
for the redox potentials of 2 (Table 1).
One-electron reduction of 1 produces a species 1- with a
RuIII-type EPR response (g1 ) 2.39, g2 ) 2.11, g3 ) 1.95)
and long-wavelength LMCT and IVCT absorptions at 830,
600, 517, and 410 nm (Figure 2, Table 2). These results are
compatible with a RuII(µ-Q1)2- RuIII formulation, involving
a reduction of the bridging ligand and a metal-metal mixed-
valent situation in a rather unsymmetrical setting (R-diimine
bonded RuII). The strong asymmetry is responsible for high-
energy IVCT transitions. Again, the formally analogous 2-
(18) (a) Pierpont, C. G.; Lange, C. W. Prog. Inorg. Chem. 1994, 41, 331.
(b) Pierpont, C. G. Coord. Chem. ReV. 2001, 219-221, 415.
(19) (a) Taube, H. Angew. Chem. 1984, 96, 315; Taube, H. Angew. Chem.
Int. Ed. 1984, 23, 329. (b) Evans, I. P.; Everett, G. W.; Sargeson, A.
M. J. Am. Chem. Soc. 1976, 98, 8041.
Inorganic Chemistry, Vol. 44, No. 9, 2005 3213

Ghumaan et al.
Table 2. UV-VIS-NIR Absorption Dataa for Complexes [(acac)2Ru(µ-Q)Ru(acac)2]n from Spectroelectrochemistryb
1n
2n
n ) +1
n ) 0
n ) -1
n ) -2
920sh, 615 (7200), 418 (7930)
650sh, 573 (8680)
780 (2800), 497 (5690), 405 (8110)
830 (3700), 600 (5820), 517 (7580), 410 (9470)
576 (13220), 480sh, 435 (11780)
554 (7950), 530sh, 455 (9780), 398sh
640sh, 552 (10230), 440sh, 415 (8250)
780 (5830), 585sh, 508 (11100), 423sh
a Absorption maxima in nanometers, molar extinction coefficients in M-1 cm-1 (in parentheses). b In CH3CN/0.1 M Bu4NPF6.
Scheme 2
serve to ascertain the oxidation state distribution even within
an asymmetric metal/ligand/metal system with three redox
active components (Scheme 2):
Starting from the unequivocal RuII(µ-Q)2-RuII formulation
for both 12- and 22-, the stepwise oxidation proceeds
straightforwardly for 1n via the O,O-bonded ruthenium ion
and the bridging ligand to the N,N-coordinated metal center.
For system 2n, however, the first oxidation of the dianionic
form involves the bridging ligand. As outlined above, the
step from 2- to 2 implies an unusual triple oxidation state
change, an oxidation of both metal ions offset by the one-
electron reduction of the bridge. Conversion to 2+ involves
again ligand-based oxidation. Both the lower coordination
asymmetry (all N donor atoms) and the more covalent metal-
donor bonding for the quinonediimine bridging system (Q2)k
vs (Q1)k are responsible for these remarkably different
electron pathways.
Figure 3. EPR spectra of redox system 2n in CH2Cl2/0.1 M Bu4NPF6 at
4 K.
Scheme 1
Acknowledgment. Financial support received from the
Department of Science and Technology and Council of
Scientific and Industrial Research, New Delhi (India), the
DAAD, the DFG and the FCI (Germany) is gratefully
acknowledged.
presence of a second oxidized metal center (RuIII) in 1+
allows for a more intense LMCT band in the visible range
at 615 nm. It has to be reiterated here that the notorious
strong orbital mixing between ruthenium and o-quinonoid
ligands2-8 can lead to a high degree of covalent bonding
that may render conventional oxidation state assignments
difficult. However, we have shown in this report how
combinations of spectroscopic methods can in favorable cases
Supporting Information Available: Electrospray mass spectra
of 1 and 2 (Figure S1), øT vs T diagrams for 1 (Figure S2) and 2
(Figure S3) and spectroelectrochemical response of compound 2
(Figure S4). This material is available free of charge via the Internet
at http://pubs.acs.org.
IC048309X
3214 Inorganic Chemistry, Vol. 44, No. 9, 2005