The redox series [M(bpy)2(Q)]n+, M = Ru or Os, Q = 3,5-Di-tert-butyl-N-phenyl-1,2-benzoquinonemonoimine. Isolation and a complete X and W band EPR study of the semiquinone states (n = 1)

Article abstract of DOI:10.1021/ic048407i

The complexes [M(bpy)₂(Q)](PF₆) (bpy = 2,2′-bipyridyl; M = Ru, Os; Q = 3,5-di-tert-butyl-N-phenyl-1,2- benzoquinonemonoimine) were isolated and studied by X and W band EPR in a dichloromethane solution at ambient temperatures and at 4 K. For M = Ru, the ¹⁴N hyperfine splitting confirms the Ru^II/semiquinone formulation, although at a > 1 mT, the ^99,101Ru satellite coupling is unusually high. W band EPR allowed us to determine the relatively small g anisotropy Δg = g₁ - g₃ = 0.0665 for the ruthenium complex. The osmium analogue exhibits a much higher difference Δg = 0.370, which is attributed not only to the larger spin-orbit coupling constant of Os versus that of Ru but also to a higher extent of metal contribution to the singly occupied molecular orbital. The difference ΔE between the oxidation and reduction potentials of the radical complexes is larger for the ruthenium compound (ΔE = 0.87 V) than for the osmium analogue (ΔE = 0.72), confirming the difference in metal/ligand interaction. The electrochemically generated states [M(bpy)₂(Q)]ⁿ⁺, n = 0, 1, 2, and 3, were also characterized using UV-vis-near-infrared spectroelectrochemistry.

Full text of DOI:10.1021/ic048407i

Inorg. Chem. 2005, 44, 2843−2847
+
The Redox Series [M(bpy)2(Q)]n , M
) Ru or Os, Q )
3,5-Di-tert-butyl-N-phenyl-1,2-benzoquinonemonoimine. Isolation and a
Complete X and W Band EPR Study of the Semiquinone States (n
) 1)
Shengfa Ye,† Biprajit Sarkar,† Carole Duboc,‡ Jan Fiedler,§ and Wolfgang Kaim*,†
Institut fu¨r Anorganische Chemie, UniVersita¨t Stuttgart, Pfaffenwaldring 55,
D-70550 Stuttgart, Germany, Grenoble High Magnetic Field Laboratory, MPI-CNRS,
25 AVenue des Martyrs, BP 166, F-38042 Grenoble Cedex 9, France, and J. HeyroVsky´ Institute
of Physical Chemistry, Academy of Sciences of the Czech Republic, DolejsˇkoVa 3,
CZ-18223 Prague, Czech Republic
Received November 12, 2004
The complexes [M(bpy)2(Q)](PF6) (bpy
benzoquinonemonoimine) were isolated and studied by X and W band EPR in a dichloromethane solution at
ambient temperatures and at 4 K. For M
Ru, the 14N hyperfine splitting confirms the RuII/semiquinone formulation,
although at a > 1 mT, the 99,101Ru satellite coupling is unusually high. W band EPR allowed us to determine the
relatively small g anisotropy g1 g3 0.0665 for the ruthenium complex. The osmium analogue exhibits
a much higher difference 0.370, which is attributed not only to the larger spin orbit coupling constant of Os
versus that of Ru but also to a higher extent of metal contribution to the singly occupied molecular orbital. The
difference E between the oxidation and reduction potentials of the radical complexes is larger for the ruthenium
compound ( 0.87 V) than for the osmium analogue ( 0.72), confirming the difference in metal/ligand
interaction. The electrochemically generated states [M(bpy)2(Q)]n+, n
0, 1, 2, and 3, were also characterized
using UV vis near-infrared spectroelectrochemistry.
) 2,2′-bipyridyl; M ) Ru, Os; Q ) 3,5-di-tert-butyl-N-phenyl-1,2-
)
∆g
)
−
)
∆g
)
−
∆
∆E
)
∆E )
)
−
−
Ruthenium complexes of “non-innocent” 1 1,2-dioxolene2
ligands Qn have long been studied1b,2-9 because of the
possibility to combine a redox active chelating ligand Q/Q•-/
transition metal. Mononuclear2-7 and dinuclear8,9 paramag-
netic species with quinonoid ligands ranging from clear
radical complexes3,5,8a to predominantly metal-centered sys-
tems7 have been investigated mainly by spectroelectrochem-
Q
2- with a substitutionally inert but electron-transfer active
* To whom correspondence should be addressed. E-mail: kaim@
(4) (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. (j) Connelly, N. G.; Manners, I.; Protheroe, J. R. C.; Whiteley,
M. W. J. Chem. Soc., Dalton Trans. 1984, 2713. (k) Bag, N.; Pramanik,
A.; Lahiri, G. K.; Chakravorty, A. Inorg. Chem. 1992, 31, 40. (l)
Kurihara, M.; Daniele, S.; Tsuge, K.; Sugimoto, H.; Tanaka, K. Bull.
Chem. Soc. Jpn. 1998, 71, 867. (m) Meacham, A. P.; Druce K. L.;
Bell, Z. R.; Ward, M. D.; Keister, J. B.; Lever, A. B. P. Inorg. Chem.
2003, 42, 7887.
iac.uni-stuttgart.de.
† Universita¨t Stuttgart.
‡ Grenoble High Magnetic Field Laboratory.
§ 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.
(2) Pierpont, C. G. Coord. Chem. ReV. 2001, 219-221, 415.
(3) (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. (h) Okamura, R.; Wada, T.; Aikawa,
K.; Nagata, T.; Tanaka, K. Inorg. Chem. 2004, 43, 7210. (i) Lever,
A. B. P.; Gorelsky, S. L. Struct. Bonding (Berlin) 2004, 107, 77.
(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.
10.1021/ic048407i CCC: $30.25
Published on Web 03/12/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 8, 2005 2843

Ye et al.
Chart 1
istry and electron paramagnetic resonance (EPR). However,
the EPR information was frequently found unsatisfactory
because of insufficient resolution, either for the hyperfine
structure or for the g tensor anisotropy. In particular, those
systems with a considerable metal contribution to the singly
occupied molecular orbital (SOMO) did show g anisotropy
in the X band (9.5 GHz) but no hyperfine splitting due to
broad lines.6,7,9 On the other hand, the small g anisotropies
expected for complexes with a primarily ligand-based SOMO
(anion radical complexes10) were often difficult to determine
accurately in the X band.3,5 Osmium analogues are less well-
documented than the ruthenium complexes.11 The EPR
spectroscopy of osmium-containing compounds is typically
affected by the high spin-orbit coupling constant of that 5d
element12,13 and by inherently rapid relaxation, leading to
broadened lines or even “EPR silence”. On the other hand,
if the line width allows for detection, the conditions for metal
hyperfine coupling are a little better for 189Os (I ) 3/2, 16.1%
natural abundance, Aiso ) 471.0 mT) than for the ruthenium
isotopes 99Ru (I ) 5/2, 12.7%, Aiso ) 62.94 mT) and 101Ru (I
were recorded on J & M Tidas Agilent 8453 and Bruins Instruments
Omega 10 spectrophotometers. Cyclic voltammetry was carried out
in 0.1 M Bu4NPF6 solutions using a three-electrode configuration
(glassy carbon electrode, Pt counter electrode, Ag/AgCl reference)
and a PAR 273 potentiostat and function generator. The ferrocene/
ferrocenium couple served as an internal reference. Spectroelec-
trochemical measurements were performed using an optically
transparent thin-layer electrode (OTTLE) cell15 for UV-vis-NIR
absorption spectra.
Syntheses. The ligand 2-anilino-4,6-di-tert-butylphenol (Chart
1) was prepared according to a published procedure.16
[Ru(bpy)2(Q)]PF6 and [Os(bpy)2(Q)]PF6. The complexes were
obtained in an analogous way. Solutions of 1.0 mmol of the ligand,
4 mL of 0.5 M NaOCH3, and 1.0 mmol of cis-M(bpy)2Cl2‚2H2O
(M ) Ru, Os) were heated to reflux in the presence of air in 25
mL of acetonitrile for 4 h. After cooling, 1.0 mmol of KPF6 was
added to precipitate the complexes at 4 °C. The precipitates were
collected by filtration, and after washing with cold acetonitrile, dark
microcrystalline materials were obtained in about 60% yield. Anal.
Calcd for C40H41F6N5ORuP: C, 56.27; H, 4.84; N, 8.20. Found:
C, 56.57; H, 4.41; N, 8.34%. Anal. Calcd for C40H41F6N5OOsP:
C, 50.95; H, 4.38; N, 7.43. Found: C, 50.41; H, 4.06; N, 7.11%.
5
) /2, 17.0%, Aiso ) 70.52 mT).13
In this report, we present a detailed EPR analysis of the
isolated complexes [M(bpy)2(Q)](PF6) (bpy ) 2,2′-bipyridyl)
at X and W band frequencies (95 GHz) in a dichloromethane
solution at ambient temperatures and at 4 K. We also report
UV-vis-near-infrared (NIR) spectroelectrochemical results
for the electrochemically generated states [M(bpy)2(Q)]n+,
n ) 0, 1, 2, and 3.
Results and Discussion
Experimental Section
Synthesis and Electrochemistry. The radical complexes
[M(bpy)2(Q)](PF6) were obtained from reactions between the
cis-M(bpy)2Cl2 precursors and 2-anilino-4,6-di-tert-butyl-
phenol. The oxidation equivalents required to obtain the
semioxidized complexes are believed to come from traces
of O2, which, in the reduced form, also acts as proton
acceptor. Identification of the isolated complexes by elemen-
tal analysis, EPR, and cyclic voltammetry in CH2Cl2/0.1 M
Bu4NPF6 showed that the isolated paramagnetic compounds
are intermediates with not too closely spaced one-electron
reduction and oxidation waves (Figure 1; Table 1). After
the first oxidation to [M(bpy)2(Q)]2+ at about -0.5 V, a
second oxidation to [M(bpy)2(Q)]3+ was observed in both
cases at rather high potentials; bpy-centered reduction
processes cannot be observed before the negative potential
limit of the CH2Cl2 solvent.
The second oxidation waves are associated with MII f
MIII processes,3 characteristically11,17 with a significantly
lower value for the osmium analogue. The difference ∆E
between the oxidation and reduction potentials of the
complexes [M(bpy)2(Q)](PF6) is slightly higher for the
ruthenium analogue (0.87 V vs 0.72 V), a phenomenon
familiar from related systems that signifies less metal/ligand
orbital mixing.11,17
Instrumentation. X band EPR spectra were recorded on a
Bruker System ESP 300 equipped with a Bruker ER035M gauss-
meter and an HP 5350B microwave counter. W band EPR spectra
were recorded using a multifrequency spectrometer.14 A Gunn diode
operating at 95 GHz was used as a radiation source. An InSb
bolometer (QMC Instruments) was used for detection. The main
magnetic field was provided by a superconducting magnet (Cryo-
genics Consultant), which generates fields up to 12 T. As a result
of different field sweep conditions, the absolute values of the g
components were obtained by calibrating the precisely measured g
anisotropy data with the isotropic g value from X band measure-
ments. Although this procedure does not account for the temperature
dependence of g, the values extracted are identical with those
obtained using an added standard. UV-vis-NIR absorption spectra
(7) Salmonsen, R. B.; Abelleira, A.; Clarke, M. J. Inorg. Chem. 1984,
23, 387.
(8) (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.
(9) (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. J. Chem. Soc., Dalton. Trans. 2004, 750.
(10) Kaim, W. Coord. Chem. ReV. 1987, 76, 187.
(11) (a) Haga, M.; Isobe, K.; Boone, S. R.; Pierpont, C. G. Inorg. Chem.
1990, 29, 3795. (b) Mitra, K. N.; Goswami, S. Inorg. Chem. 1997,
36, 1322.
(12) Kober, E. M.; Meyer. T. J. Inorg. Chem. 1984, 23, 3877.
(13) Weil, J. A.; Bolton, J. R.; Wertz, J. E. Electron Paramagnetic
Resonance; Wiley: New York, 1994.
(14) Barra, A.-L.; Brunel, L.-C.; Robert, J.-B. Chem. Phys. Lett. 1990, 165,
107.
(15) Krejcik, M.; Danek, M.; Hartl, F. J. Electroanal. Chem. 1991, 317,
179.
(16) Chaudhuri, P.; Verani, C. N.; Bill, E.; Bothe, E.; Weyhermu¨ller, T.;
Wieghardt, K. J. Am. Chem. Soc. 2001, 123, 2213.
(17) Kaim, W.; Kasack, V. Inorg. Chem. 1990, 29, 4696.
2844 Inorganic Chemistry, Vol. 44, No. 8, 2005

Redox Series [M(bpy)2(Q)]n+
Figure 1. Cyclic voltammograms of compounds [M(bpy)2(Q)](PF6) in
dichloromethane/0.1 M Bu4NPF6 at 100 mV/s scan rate: M ) Ru (top), M
) Os (bottom, preoxidized to [Os(bpy)2(Q)]2+).
Figure 2. UV-vis-spectroelectrochemical response of [Ru(bpy)2(Q)](PF6)
in dichloromethane/0.1 M Bu4NPF6 on one-electron reduction (top) and
on first (center) and second oxidations (bottom).
Table 1. Redox Potentialsa for Complexes [M(bpy)2(Q)]n
Ea
n
M ) Ru
M ) Os
π*(bpy)-targeted metal-to-ligand charge transfer (MLCT)
bands at about 530 and 370 nm for [M(bpy)2(Q)]0, semi-
quinone- and bpy-targeted MLCT bands at about 690 and
500 nm for [M(bpy)2(Q)]+, and quinone- and bpy-targeted
MLCT bands at about 590 and 410 nm for [Ru(bpy)2-
(Q)]2+.3b,i The bands of the [M(bpy)2(Q)]3+ species at ca.
500 nm are assigned to intraquinone ligand transitions.16
EPR Spectroscopy. Both [M(bpy)2(Q)](PF6) complexes
exhibit EPR signals in the dichloromethane solution at room
temperature. This result alone is evidence for a predominantly
ligand-based spin, as has been mentioned before.6 Significant
amounts of metal-centered spin would be expected to lead
to rapid relaxation and to large g anisotropy with concomitant
line broadening.8,10
3+/2+
2+/+
+/0
1.22
-0.42
-1.29
0.95
-0.51
-1.23
a From cyclic voltammetry at 100 mV/s scan rate in CH2Cl2/0.1 M
Bu4NPF6. Potentials in V vs that of [Fe(C5H5)2]+/0
.
Table 2. Electronic Spectra of Complexesa
λ
max (103 ꢀ)b
[Ru(bpy)2(Q)]3+
[Ru(bpy)2(Q)]2+
[Ru(bpy)2(Q)]+
[Ru(bpy)2(Q)]0
[Os(bpy)2(Q)]3+
[Os(bpy)2(Q)]2+
[Os(bpy)2(Q)]+
[Os(bpy)2(Q)]0
309(26.1), 437(8.1), 504(8.1), 591(sh)
281(31.1), 411(5.8), 595(13.4)
294(36.0), 350(9.5), 376(sh), 496(6.3), 686(7.4)
295(35.6), 374(9.8), 423(sh), 531(7.0), 607(sh)
286(17.0), 501(6.6)
287(22.1), 512(7.6)
292(27.1), 390(6.9), 465(sh), 534(5.5), 688(5.9)
295(29.9), 385(8.6), 542(6.8), 805(sh)
a From spectroelectrochemistry in CH2Cl2/0.1 M Bu4NPF6. b Wave-
lengths in nm, molar extinction coefficients in M-1 cm-1
Semiquinone complexes of ruthenium(II) were often found
to show only partially useful EPR spectra because of
unresolved hyperfine structure or detailed g tensor anisotropy
information in the X band.3 Osmium analogues are less well-
documented than the ruthenium complexes.11
Whereas [Os(bpy)2(Q)](PF6) exhibits only a broad un-
resolved line (∆Hpp ) 9 mT) at giso ) 1.982 in the X band
EPR experiment at room temperature, the ruthenium ana-
logue [Ru(bpy)2(Q)](PF6) gives a spectrum at giso ) 2.0049
(Figure 3, Table 3), a typical value for free semiquinone
radicals.18,19 Fortunately, the spectrum is sufficiently resolved
to determine the 14N and 99,101Ru hyperfine couplings.
At 0.78 mT, the magnitude of a(14N) is typical for
o-semiquinonemonoimine complexes.18 Remarkably, how-
.
Spectroelectrochemistry. The results from OTTLE spectro-
electrochemistry are listed in Table 2 and illustrated for the
ruthenium complex in Figure 2.
A one-electron reduction to [M(bpy)2(Q)] and two one-
electron oxidation processes to [M(bpy)2(Q)]2+ and [M(bpy)2-
(Q)]3+ could be observed. Although more bands could have
been expected for the osmium analogue as a result of the
effect of higher spin-orbit coupling allowing observable
triplet absorption features,11 both the osmium and the
ruthenium redox systems [M(bpy)2(Q)]n exhibit bands very
similar to those reported for the N-unsubstituted ruthenium
complex.3b,i This overall agreement justifies the adoption of
the reasonable assignments made by Masui, Lever, and
Auburn for the main long-wavelength transitions, that is,
ligand-to-ligand charge transfer (LLCT; Q2- f bpy) and
(18) Stegmann, H. B.; Ulmschneider, K. B.; Hieke, K.; Scheffler, K. J.
Organomet. Chem. 1976, 118, 259.
(19) Burghaus, O.; Plato, M.; Rohrer, M.; Mo¨bius, K.; MacMillan, F.;
Lubitz, W. J. Phys. Chem. 1993, 97, 7639.
Inorganic Chemistry, Vol. 44, No. 8, 2005 2845

Ye et al.
Figure 4. X band EPR spectrum of [Os(bpy)2(Q)](PF6) in dichloromethane
at 4 K.
Figure 3. X band EPR spectrum of [Ru(bpy)2(Q)](PF6) in dichloromethane
at 298 K: experimental (top) and simulated spectra (bottom).
Table 3. EPR Data of Complexes [M(bpy)2(Q)](PF6)a
M ) Ru
M ) Os
298 K
giso
2.0049b
1.982c
5 K
g1
g2
g3
2.0393d
2.0022d
1.9728d
0.0665
2.0048
2.160
1.913
1.790
0.370
1.960
∆g ) g1 - g3
gav
a In CH2Cl2. b a(14N) ) 0.78 mT; a(99Ru) ) 1.01 mT; a(101Ru) ) 1.13
mT. c ∆Hpp ) 9 mT. d From the W band spectrum.
Figure 5. W band EPR spectrum of [Ru(bpy)2(Q)](PF6) in dichloro-
methane/toluene (4:1) at 5 K.
ever, the metal hyperfine coupling is large enough to be
clearly detectable at the wings of the main signal. The values
of a(99Ru) ) 1.01 mT and a(101Ru) ) 1.13 mT are larger
than those observed for [Ru(bpy)2(abpy)]+ (0.77/0.86 mT;
abpy ) 2,2′-azobispyridine),20 [Ru(bpy)2(abcp)]+ [0.72 mT;
abcp ) 2,2′-azobis(5-chloropyrimidine)],21 [Ru(bpy)2(PQQ)]+
(0.50 mT; PQQ ) pyrroloquinolinequinone),22 [Ru(bpz)-
(CN)4]3- (0.514/0.458 mT; bpz ) 2,2′-bipyrazine),23 [Ru(mpz)-
(CN)5]3- (0.39/0.437 mT; mpz+ ) methylpyrazinium),24
[Ru(mpz)(NH3)5]2+ (0.58/0.65 mT),25 or [Ru(bpy)2(Q′)]
(0.225 mT; Q′ ) 5-methyl-2-oxido-1,4-benzosemiquinone).5
Even for [Ru(NO)(CN)5]3-, the calculated isotropic value
for a (101Ru) was below 1 mT.26 We attribute this large metal
hyperfine coupling to an efficient spin transfer (spin polar-
ization) from the o-semiquinoneimine ligand to the metal in
a chelate situation, this enhanced orbital overlap being
additionally favored by the well-established strength of the
RuII-N bond.27
here for [Os(bpy)2(Q)](PF6) because of the large line width.
Conversely, however, the g components could be detected
in frozen dichloromethane solution in the X band (Figure 4;
Table 3), confirmed also through W band measurements.
For the analogous [Ru(bpy)2(Q)](PF6), the W band studies
were essential to determine the g anisotropy (Figure 5; Table
3) because of the much smaller difference ∆g ) g1 - g3 )
0.0665 in comparison to ∆g ) 0.370 for the osmium
complex.
In relation to typical catecholatoruthenium(III) species (∆g
≈ 0.8)7 or extensively metal-ligand mixed systems (∆g ≈
0.25),6 the g anisotropy of [Ru(bpy)2(Q)](PF6) is so small
as to allow its labeling as a ruthenium(II)-semiquinoneimine
complex [RuII(bpy)2(Q•-)]+. However, the isotropic g value
of 2.0049 showing almost no difference to a metal-free
semiquinone is deceptive; the ∆g value of 0.0665 is still
much larger than the corresponding g anisotropies of about
0.006 for typical free semiquinones.19 Recently reported high-
field EPR studies29 of RuII-radical complexes have shown
∆g values between 0.01 and 0.04, that is, clearly lower than
the 0.0665 observed for [Ru(bpy)2(Q)]+. Thus, both the g
anisotropy as determined from W band EPR and the 99,101Ru
hyperfine coupling from high-resolution X band EPR suggest
a nonnegligible contribution from the metal to the SOMO.
The metal participation at the SOMO is obviously still
higher for [Os(bpy)2(Q)](PF6). Although the spin-orbit
coupling constants of osmium centers are only about 2-3
times higher than those of ruthenium analogues,12,13 the g
anisotropy of the osmium complex is more than 5 times as
large as that for [Ru(bpy)2(Q)](PF6). This disproportionate
Although the hyperfine coupling of 189Os should be better
detectable than the ruthenium values,28 it was not observed
(20) Krejcik, M.; Zalis, S.; Klima, J.; Sykora, D.; Matheis, W.; Klein, A.;
Kaim, W. Inorg. Chem. 1993, 32, 3362.
(21) Kaim, W.; Doslik, N.; Frantz, S.; Sixt, T.; Wanner, M.; Baumann, F.;
Denninger, G.; Ku¨mmerer, H.-J.; Duboc-Toia, C.; Fiedler, J.; Zalis,
S. J. Mol. Struct. 2003, 656, 183.
(22) Schwederski, B.; Kasack, V.; Kaim, W.; Roth, E.; Jordanov, J. Angew.
Chem. 1990, 102, 74; Angew. Chem., Int. Ed. Engl. 1990, 29, 78.
(23) Waldho¨r, E.; Poppe, J.; Kaim, W.; Cutin, E.; Garcia Posse, M.; Katz,
N. E. Inorg. Chem. 1995, 34, 3093.
(24) Waldho¨r, E.; Kaim, W.; Olabe, J. A.; Slep, L. D.; Fiedler, J. Inorg.
Chem. 1997, 36, 2969.
(25) Poppe, J.; Kaim, W.; Ben Altabef, A.; Katz, N. E. J. Chem. Soc.,
Perkin Trans. 2 1993, 2105.
(26) Frantz, S.; Sarkar, B.; Sieger, M.; Kaim, W.; Roncaroli, F.; Olabe, J.
A.; Zalis, S. Eur. J. Inorg. Chem. 2004, 2902.
(27) Taube, H. Angew. Chem. 1984, 96, 315; Angew. Chem., Int. Ed. Engl.
1984, 23, 329.
(28) Kaim, W.; Reinhardt, R.; Sieger, M. Inorg. Chem. 1994, 33, 4453.
(29) (a) Sarkar, B.; Frantz, S.; Kaim, W.; Duboc, C. J. Chem. Soc., Dalton
Trans. 2004, 3727. (b) Sarkar, B.; Kaim, W.; Fiedler, J.; Duboc, C. J.
Am. Chem. Soc. 2004, 126, 14706.
2846 Inorganic Chemistry, Vol. 44, No. 8, 2005

Redox Series [M(bpy)2(Q)]n+
increase suggests a stronger contribution from formulations
such as [OsIII(bpy)2(Q2-)]+, the higher oxidation state being
more stable for the 5d element system. The shifts of giso, g2,
and g3 to lower values < 2 indicate the presence of close-
lying excited states with nonzero angular orbital momen-
tum to the doublet ground state.10 However, such states
cannot yet be identified with certainty; high-level calculations
based on structural data will be necessary for such assess-
ments. It is remarkable that the formally related species
[Fe(cyclam)(Q)]+ was reported to exhibit a catecholate (o-
imidophenolate) ligand state with coordinated high-spin
iron(III).30 Apparently, the preference for the low-spin d6
metal configuration with the semiquinoneimine state of the
ligand is most pronounced for the ruthenium system, whereas
the heavier homologue (Os) tends toward higher metal
oxidation states, and the first row transition metal analogue
(Fe) readily adopts odd d-electron and even high-spin
configurations.
In summary, this report on redox systems [M(bpy)2(Q)]n+
(M ) Ru, Os) has not only confirmed the UV-vis spectro-
electrochemical results obtained by Lever and co-workers
for the N-unsubstituted quinoneimine ligand form (M )
Ru),3i it also describes the facile isolation of both radical
states and their comprehensive EPR analysis at two very
different frequencies with the result of a remarkably large
99,101Ru hyperfine coupling and an unusually pronounced g
anisotropy difference between the ruthenium and osmium
analogues.
Acknowledgment. We thank the DFG (Graduate College
“Magnetic Resonance”), the Fonds der Chemischen Industrie,
and the European Community (Access to Research Infra-
structure Action of the Improving Human Potential Program)
for continued support. J.F. acknowledges grants from the
Grant Agency of the Czech Republic (Grant 203/03/082) and
the Ministry of Education of the Czech Republic (Grant
COST OC D15.10).
(30) Chun, H.; Bill, E.; Bothe, E.; Weyhermu¨ller, T.; Wieghardt, K. Inorg.
Chem. 2002, 41, 5091.
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