Singlet diradical complexes of ruthenium and osmium: Geometrical and electronic structures and their unexpected changes on oxidation

Article abstract of DOI:10.1021/ic702301a

Reaction of HL, HL^a (2-[(2-N-phenylamino)phenylazo]pyridine), HL^b (2-[{2-N-(4-methylphenyl)amino}phenylazo]pyridine), or HL ^c (2-[{2-N-(4-chlorophenyl)amino}phenylazo]pyridine), with KRuO ₄ or OsO₄ and PPh₃ under exhaustive deoxygenation (PPh₃ → OPPh₃) yields diamagnetic compounds ML₂. Crystal structure determination for M(L ^a)₂ indicates the radical dianion state, L ^?2-, for the ligands as evident from the typical N-N bond length of about 1.33 A for a one-electron reduced azo function. The resulting spin-coupled complexes, M^IV(L^?2-)₂, can be oxidized in two reversible one-electron steps, as probed by cyclic voltammetry and UV-vis-NIR spectroelectrochemistry. The paramagnetic intermediates, [M(L^a)₂]⁺, are distinguished by intense NIR absorption, largely metal-centered spin as revealed by EPR, and, in the case of [Os(L^a)₂]I₃, by crystallographically determined shortening of the N=N bond to about 1.30 A, corresponding to a coordinated unreduced azo function. Thus, oxidation of the complex M ^IV(L^?2_)₂ involves partial reduction of the metal in [M^III(L^-)₂]⁺ because intramolecular double electron transfer is offsetting the external charge removal. Density-functional theory calculations were employed to confirm the structural features and to support the spectroscopic assignments.

Full text of DOI:10.1021/ic702301a

Inorg. Chem. 2008, 47, 1625-1633
Singlet Diradical Complexes of Ruthenium and Osmium: Geometrical
and Electronic Structures and their Unexpected Changes on Oxidation
Subhas Samanta,† Priti Singh,‡ Jan Fiedler,§ Stanislav Záliš,§ Wolfgang Kaim,*,‡
and Sreebrata Goswami*,†
Department of Inorganic Chemistry, Indian Association for the CultiVation of Science, JadaVpur,
Kolkata 700 032, India, Institut für Anorganische Chemie, UniVersität Stuttgart, Pfaffenwaldring 55,
D-70550 Stuttgart, Germany, and J. HeyroVský Institute of Physical Chemistry, V.V.i., Academy of
Sciences of the Czech Republic, DolejškoVa 3, CZ-18223 Prague, Czech Republic
Received November 22, 2007
Reaction of HL, HLa (2-[(2-N-phenylamino)phenylazo]pyridine), HLb (2-[{2-N-(4-methylphenyl)amino}phenylazo]pyridine),
or HLc (2-[{2-N-(4-chlorophenyl)amino}phenylazo]pyridine), with KRuO4 or OsO4 and PPh3 under exhaustive
deoxygenation (PPh3 f OPPh3) yields diamagnetic compounds ML2. Crystal structure determination for M(La)2
indicates the radical dianion state, L•2-, for the ligands as evident from the typical N-N bond length of about 1.33
Å for a one-electron reduced azo function. The resulting spin-coupled complexes, MIV(L•2-)2, can be oxidized in
two reversible one-electron steps, as probed by cyclic voltammetry and UV-vis-NIR spectroelectrochemistry.
The paramagnetic intermediates, [M(La)2]+, are distinguished by intense NIR absorption, largely metal-centered
spin as revealed by EPR, and, in the case of [Os(La)2]I3, by crystallographically determined shortening of the NdN
bond to about 1.30 Å, corresponding to a coordinated unreduced azo function. Thus, oxidation of the complex
MIV(L•2-)2 involves partial reduction of the metal in [MIII(L-)2]+ because intramolecular double electron transfer is
offsetting the external charge removal. Density-functional theory calculations were employed to confirm the structural
features and to support the spectroscopic assignments.
Scheme 1
Introduction
Ruthenium and osmium complexes with unsaturated
nitrogen ligands have lately been among the most researched
kinds of coordination compounds because the dπ(M)-π*-
(ligand) interaction can effect interesting and well-accessible
intramolecular charge and electron transfer processes. Energy-
converting photosensitizers1,2 of the “Ru-bpy” type ([Ru-
(bpy)3]2+) as well as mixed-valent compounds such as the
Creutz-Taube ion (1, Scheme 1)3 have thus found their way
into standard textbooks.4,5 Taking advantage of the kinetic
stability of Ru-N and Os-N bonds and of the redox capacity
of these group 8 elements, we can now describe conceptually
simple ML2 compounds, HL ) 2-[(2-N-phenylamino)pheny-
lazo]pyridines (Chart 1), with bridging transition metal centers
M ) Ru or Os and two electroactive peripheral conjugated
tridentate ligands Ln-. Therefore, arrangements 2 and 3 are
complementary to systems such as 1 where a π-conjugated
central ligand bridges two electroactive metal sites.
* To whom correspondence should be addressed. E-mail: icsg@iacs.res.in
(S.G.), kaim@iac.uni-stuttgart.de (W.K.).
†
Indian Association for the Cultivation of Science.
Universität Stuttgart.
Academy of Sciences of the Czech Republic.
‡
§
(1) (a) Grätzel, M. Inorg. Chem. 2005, 44, 6841. (b) Alstrum-Acevedo,
J. H.; Brennaman, M. K.; Meyer, T. J. Inorg. Chem. 2005, 44, 6802.
(2) Wolpher, H.; Sinha, S.; Pan, J.; Johansson, A.; Lundqvist, M. J.;
Persson, P.; Lomoth, R.; Bergquist, J.; Sun, L.; Sundström, V.;
Åkermark, B.; Polivka, T. Inorg. Chem. 2007, 46, 638.
The coordination chemistry of the tridentate azoaromatic
ligands (HL, Chart 1) has been investigated intensely in our
(3) Creutz, C. Prog. Inorg. Chem. 1983, 30, 1.
(5) (a) Shriver, D. F.; Atkins, P. W. Inorg. Chem., 3rd Ed., Oxford
University Press, Oxford, 1999, p 461. (b) Porterfield, W. W. Inorganic
Chemistry; 2nd Ed., Academic Press, San Diego: 1993, pp 870–873.
(4) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. AdVanced
Inorganic Chemistry, 6th Ed.; Wiley, New York: 1999, pp 1017–1018.
10.1021/ic702301a CCC: $40.75
Published on Web 02/05/2008
2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 5, 2008 1625

Samanta et al.
Chart 1
has revealed that such synthetic methodology involving
complete oxo abstraction and subsequent ligand coordination
is not commonly7 applied. It has been possible to study the
neutral parent ML2 molecules and the mono- and dioxidized
forms partially by single-crystal structure analysis (RuL2,
OsL2, and [OsL2]I3). These studies, including EPR and
UV-vis-NIR spectroelectrochemistry, have allowed us to
experimentally establish oxidation states and to compare
these results with density-funtional theory (DFT) calculation
data.
Experimental Section
Materials. The high-valent metal oxides K[RuO4] and OsO4 were
reagents from Aldrich, and the solvents used were obtained from
Qualigens and MERCK (India). The ligands HLa-HLc were
prepared by following the reported6 procedure. Tetrabutylammo-
nium perchlorate (TEAP) was prepared and recrystallized as
reported earlier.12 Caution! OsO4 and perchlorate salts haVe to
be handled with care and appropriate safety precautions. All other
chemicals were of reagent grade and were used as received.
Instrumentation. UV-vis-NIR absorption spectra were re-
corded on a Perkin-Elmer Lambda 950 UV/vis spectrophotometer
and on a J&M TIDAS instrument. 1H NMR spectra were taken on
a Bruker Avance DPX 300 spectrometer, and SiMe4 was used as
the internal standard. Infrared spectra were obtained using a Perkin-
Elmer 783 spectrophotometer. Cyclic voltammetry was carried out
in 0.1 M Bu4NClO4 solutions using a three-electrode configuration
(glassy carbon working electrode, Pt counter electrode, Ag/AgCl
reference) and a PC-controlled PAR model 273A electrochemistry
system. The E1/2 for the ferrocenium-ferrocene couple under our
experimental condition was 0.39 V. A Perkin-Elmer 240C elemental
analyzer was used to collect microanalytical data (C, H, and N).
ESI mass spectra were recorded on a micro mass Q-TOF mass
spectrometer (serial No. YA 263). Spectroelectrochemistry was
performed using an optically transparent thin-layer electrode
(OTTLE) cell.13a EPR spectra in the X-band were recorded with a
Bruker System EMX. A two-electrode capillary served to generate
intermediates for X-band EPR studies.13b
laboratory6 during the recent years. Several novel chemical
reactions and the isolation of interesting systems with unusual
composition have been the focus of investigation. Our studies
on these complexes have indicated that such azoaromatics
can behave in a redox noninnocent way and display rich
electrochemistry upon coordination. Consequently, the as-
signment of electronic structures of these complexes is an
important issue in view of different electronic structure
possibilities. We became more interested in this area after
the successful isolation of a series of molybdenum com-
plexes7 with uncharacteristically long N-N bonds of the
coordinated azo functionality. Subsequently, we have pro-
vided8 extended evidence in favor of the existence of singlet
diradical complexes of the group 6 metal elements. The topic
of organic radicals coordinated to transition metal(s) is
relevant to the fields of bioinorganic chemistry9 and inorganic
syntheses. Moreover, singlet diradicals are difficult to
characterize because of their diamagnetic nature that may
potentially cause misinterpretation10 of the electronic struc-
tures of such systems.
In the present paper we report the synthesis and charac-
terization of a number of ML2 complexes (M ) Ru and Os,
Scheme 1). Studies of the chemical reactivity of some mixed-
ligand complexes of the reference metal ions and ligands
have recently been reported11 by us. The isolation of simple
bis-ligated complexes of the above type has now been
Synthesis. Syntheses of the complexes were carried out by the
reaction of high-valent metal oxo compounds [MO4]0/- (M ) Ru
or Os) with the corresponding ligands in refluxing 2-methoxyethanol
or methanol. The complexes of general formula ML2 were obtained
as the major products along with some unidentified minor products
that are not considered here.
achieved from high-valent metal oxo compounds, [MO4]0/-
,
in one step, using PPh3 as reducing agent. A literature survey
Synthesis of RuL2. Ru(La)2 (2a): A mixture of 120 mg (0.58
mmol) of KRuO4, 320 mg (1.17 mmol) of HLa, and 620 mg (2.5
mmol) of PPh3 in 30 mL of 2-methoxyethanol was heated at reflux
for 25 h. During this period, the color of the solution changed from
red to greenish brown. The crude mass thus obtained by evaporation
of the solvent was dissolved in the minimum volume of dichlo-
romethane and was loaded on a preparative silica gel TLC plate
for purification. Benzene was used as the eluent. A greenish brown
zone that moved ahead of the unreacted red ligand band was
collected, and the solvent was evaporated under vacuum. The
residue was crystallized by slow evaporation of a dichloromethane-
(6) (a) Saha, A.; Ghosh, A. K.; Majumder, P.; Mitra, K. N.; Mondal, S.;
Rajak, K. K.; Falvello, L. R.; Goswami, S. Organometallics 1999,
18, 3772. (b) Saha, A.; Majumder, P.; Goswami, S. J. Chem. Soc.,
Dalton Trans. 2000, 11, 1703. (c) Saha, A.; Majumder, P.; Peng, S.-
M.; Goswami, S. Eur. J. Inorg. Chem. 2000, 12, 2631. (d) Kamar,
K. K.; Saha, A.; Castiñeiras, A.; Hung, C.-H.; Goswami, S. Inorg.
Chem. 2002, 41, 4531.
(7) Sanyal, A.; Banerjee, P.; Lee, G.-H.; Peng, S.-M.; Hung, C.-H.;
Goswami, S. Inorg. Chem. 2004, 43, 7456.
(8) Sanyal, A.; Chatterjee, S.; Castin˜eiras, A.; Sarkar, B.; Singh, P.; Fiedler,
J.; Záliš, S.; Kaim, W.; Goswami, S. Inorg. Chem. 2007, 46, 8584.
(9) (a) Jadzewski, B. A.; Tolman, W. B. Coord. Chem. ReV. 2000,
200–202, 633. (b) Stubbe, J.; van der Donk, W. A. Chem. ReV. 1998,
98, 705. (c) Kaim, W. Dalton Trans. 2003, 761.
(10) (a) Bachler, V.; Olbrich, G.; Neese, F.; Wieghardt, K. Inorg. Chem.
2002, 41, 4179. (b) Herebian, D.; Bothe, E.; Bill, E.; Weyhermüller,
T.; Wieghardt, K. J. Am. Chem. Soc. 2001, 123, 10012. (c) Chaudhury,
P.; Verani, C. N.; Bill, E.; Bothe, E.; Weyhermüller, T.; Wieghardt,
K. J. Am. Chem. Soc. 2001, 123, 2213. (d) Bill, E.; Bothe, E.;
Chaudhury, P.; Chlopek, K.; Herebian, D.; Kokatam, S.; Roy, K.;
Weyhermuller, T.; Neese, F.; Wieghardt, K. Chem.—Eur. J. 2005,
11, 204.
(11) (a) Das, C.; Ghosh, A. K.; Hung, C.-H.; Lee, G.-H.; Peng, S.-M.;
Goswami, S. Inorg. Chem. 2002, 41, 7125. (b) Das, C.; Peng, S.-M.;
Lee, G.-H.; Goswami, S. New J. Chem. 2002, 26, 222.
(12) Goswami, S.; Mukherjee, R. N.; Chakravarty, A. Inorg. Chem. 1983,
22, 2825.
(13) (a) Krejcik, M.; Danek, M.; Hartl, F. J. Electroanal. Chem. 1991, 317,
179. (b) Kaim, W.; Ernst, S.; Kasack, V. J. Am. Chem. Soc. 1990,
112, 173.
1626 Inorganic Chemistry, Vol. 47, No. 5, 2008

Singlet Diradical Complexes of Ruthenium and Osmium
Table 1. Crystallographic Data of 2a, 3a, and [3a]I3
2a
3a
[3a]I3
C34H26N8I3Os
1117.56
293(2)
empirical formula
molecular mass
temperature (K)
cryst syst
space group
a (Å)
C34H26N8Ru
647.70
293(2)
monoclinic
P21/n
10.3999(13)
16.029(2)
16.665(2)
90
90.256(2)
90
2778.0(6)
4
C34H26N8Os
736.83
293(2)
monoclinic
P21/n
10.3341(10)
16.0851(15)
16.7074(16)
90
90.453(2)
90
2777.1(5)
4
triclinic
P1
j
12.2467(13)
13.1910(2)
13.6083(15)
114.751(13)
103.044(9)
107.607(11)
1738.1(5)
2
b (Å)
c (Å)
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
Z
Dcalcd (g/cm3)
cryst dimens (mm)
θ range for data coll (deg)
GOF
1.549
1.762
2.135
0.15 × 0.16 × 0.19
2.3-28.3
1.086
0.13 × 0.15 × 0.17
2.3-28.4
0.976
0.19 × 0.21 × 0.25
3.1-32.7
1.033
reflns collected
unique reflns.
largest diff. between peak
18325
6872
18226
6757
23183
11302
and hole (eÅ-3
)
1.36, -1.56
R1 ) 0.0578, wR2 ) 0.1779
1.80, -1.02
R1 ) 0.0373, wR2 ) 0.0817
2.65, -1.48
R1 ) 0.0385, wR2 ) 0.1041
final R indices [I 2σ(I)]
acetonitrile solution mixture. Yield: 60%. IR (KBr, cm-1): 1591
[ν(CdN)], 1135 [ν(NdN)]. ESI-MS, m/z: 649 [MH]+. Anal. Calcd
for C34H26N8Ru: C, 63.05; H, 4.05; N, 17.30. Found: C, 63.09; H,
4.03; N, 17.35.
Other substituted complexes (2b and 2c) were similarly synthe-
sized using the HLb and HLc ligands in place of HLa. Their yields
and characterization data are as follows.
rotary evaporator. The crude product thus obtained was thoroughly
washed with hexane and was purified through crystallization by
slow diffusion of a dichloromethane solution into hexane. Yield:
94% ESI-MS, m/z: 648 [M]+. IR (KBr, cm-1): 1593 [ν(CdN)],
1220 [ν(NdN)]. Anal. Calcd for C34H30I3N8Ru: C, 39.55; H, 2.92;
N, 10.85. Found: C, 56.51; H, 2.86; N, 10.89.
Regeneration of [2a] from [2a]I3 can be achieved by the addition
of a dilute aqueous solution of N2H4 to an acetonitrile solution of
[2a]I3. The spectra and other properties of [2a] thus obtained match
exactly with the authentic sample.
[Os(La)2]I3. Synthesis of [Os(La)2]I3 was made by following an
identical procedure as above using 3a in place of 2a. Yield: 99%.
ESI-MS, m/z 737 [M]+. IR (KBr, cm-1): 1593 [ν(CdN)], 1218
[ν(NdN)]. Anal. Calcd for C34H26I3N8Os: C, 36.54; H, 2.34; N,
10.03. Found: C, 36.50; H, 2.32; N, 9.95.
Ru(Lb)2 (2b) Yield: 55%. IR (KBr, cm-1): 1591 [ν(CdN)], 1135
[ν(NdN)]. ESI-MS, m/z: 676 [MH]+. Anal. Calcd for C36H30N8Ru:
C, 63.89; H, 4.47; N, 16.56. Found: C, 64.05; H, 4.49; N, 16.51.
Ru(Lc)2 (2c) Yield: 53%. IR (KBr, cm-1): 1583 [ν(CdN)], 1128
[ν(NdN)]. ESI-MS, m/z: 718 [MH]+. Anal. Calcd for
C34H24Cl2N8Ru: C, 56.98; H, 3.37; N, 15.63. Found: C, 57.03; H,
3.39; N, 15.69.
Synthesis of OsL2. Os(La)2 (3a): To a 40 mL methanolic solution
of 2.15 g (7.87 mmol) of HLa, 4.10 g (15.74 mmol) of PPh3 and
1.0 g (3.93 mmol) of solid OsO4 were added, and the mixture was
refluxed for 4 h. The red solution became dark greenish brown
during this period. The crude mass obtained by evaporation of the
solvent was dissolved in the minimum amount of dichloromethane
and was loaded on a preparative alumina TLC plate for purification.
Toluene was used as eluent. A green band that moved ahead of
the unreacted ligand red band was collected. Final purification of
the compound was made by crystallization of a dichloromethane
solution of the compound using hexane. Yield: 70%. IR (KBr,
cm-1): 1593 [ν(CdN)], 1135 [ν(NdN)]. ESI-MS, m/z 738 [MH]+.
Anal. Calcd for C34H26N8Os: C, 55.42; H, 3.56; N, 15.20. Found:
C, 55.45; H, 3.52; N, 15.16.
Other substituted complexes (3b and 3c) were synthesized
similarly using the HLb and HLc ligands in place of HLa. Their
yields and characterization data are as follows.
Os(Lb)2 (3b). Yield: 60%. IR (KBr, cm-1): 1600 [ν(CdN)], 1135
[ν(NdN)]. ESI-MS, m/z: 766 [MH]+. Anal. Calcd for C36H30N8Os:
C, 56.52; H, 3.95; N, 14.65. Found: C, 56.60; H, 3.99; N, 14.56.
Os(Lc)2 (3c). Yield: 63%. IR (KBr, cm-1): 1593 [ν(CdN)], 1135
[ν(NdN)]. ESI-MS, m/z: 807 [MH]+. Anal. Calcd for
C34H24N8Cl2Os: C, 50.68; H, 3.00; N, 13.91. Found: C, 50.63; H,
2.98; N, 13.94.
Regeneration of [3a] from [3a]I3 was achieved following a
similar procedure as described above.
Crystallography. Crystallographic data for the compounds 2a,
3a and [3a]I3 are collected in Table 1. Suitable X-ray quality crystals
of these are obtained as follows: 2a, by slow evaporation of a
dichloromethane-acetonitrile solution of the compound; 3a, by
slow diffusion of a dichloromethane solution of the compound into
hexane; and [3a]I3, by slow evaporation of a dichloromethane-hexane
solution of the compound.
All data were collected on a Bruker SMART APEX diffractometer,
equipped with graphite monochromated Mo KR radiation (λ ) 0.71073
Å), and were corrected for Lorentz-polarization effects. 2a: A total of
18325 reflections was collected, of which 6872 were unique (Rint
)
0.067), satisfying the (I > 2σ(I)) criterion, and were used in subsequent
analysis. 3a: A total of 18226 reflections was collected, of which 6757
were unique (Rint ) 0.048). [3a]I3: A total of 23183 reflections were
collected, of which 11302 were unique (Rint ) 0.039).
The structures were solved by employing the SHELXS-97
program package14a and were refined by full-matrix least-squares
based on F2 (SHELXL-97).14b All hydrogen atoms were added in
calculated positions.
DFT Calculations. The electronic structure calculations on
[M(La)2] complexes and their oxidized form have been done by
Isolation of [M(La)2]I3. [Ru(La)2]I3. To a solution of 50 mg
(0.07 mmol) of 2a in 10 mL of dichloromethane, a solution of 25
mg (0.09 mmol) of iodine in the same solvent was added dropwise.
The solution turned brown, and the solvent was evaporated in a
(14) (a) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467. (b)
Sheldrick, G. M., SHELXL 97. Program for the refinement of crystal
structures; University of Göttingen; Göttingen, Germany, 1997..
Inorganic Chemistry, Vol. 47, No. 5, 2008 1627

Samanta et al.
DFT methods using the Gaussian 0315 program package. Spectral
calculations on the neutral complexes were done by time-dependent
DFT (TD-DFT).
complete in about 4 h. In comparison, the preparations of
the ruthenium complexes need 25 h of reflux of the reaction
mixtures in high-boiling 2-methoxyethanol. This is not
unexpected because OsO4 is known to be stronger oxidizing
and faster reacting than KRuO4.
The present synthetic strategy is based on the reduction
of the metal ions via complete oxo abstraction by PPh3 from
high-valent metal oxide precursors. This methodology was
developed recently by us for one-step syntheses7 of a series
of MoL2 complexes from the reaction of ammonium hep-
tamolybdate and HL in presence of PPh3. It may be relevant
to note here that the reactions of HL with chloride or bromide
salts of the above metal ions (RuCl3 or [OsBr6]2-) led to
incomplete substitution11 of halides, resulting in different
mixed ligand halide containing metal complexes.
Either the hybrid Becke’s three parameter functional with the
Lee, Yang, and Parr correlation functional (B3LYP)16 or the hybrid
functional of Perdew, Burke and Ernzerhof17 (PBE0) were used
together with 6–31G* polarized double-ꢁ basis sets18 for C, N, and
H atoms and effective core pseudopotentials and corresponding
optimized sets of basis functions for Ru and Os atoms.19 The
conductor-like polarizable continuum model20 (CPCM) was used
for modeling of the solvent influence in TD-DFT calculations.
Calculations were performed without any symmetry constraints.
For radical anions, a spin-unrestricted Kohn–Sham formalism was
used. For analysis of singlet diradicals, a symmetry breaking
approach21,22 within DFT should be used. Therefore, the calcula-
tions on ground-state singlet states were performed using either
spin-restricted or spin-unrestricted (in G03 combined with GUESS
) MIX). The lowest excited states were calculated using the TD-
DFT method.
The complexes gave satisfactory elemental analyses (cf.,
Experimental Section). Electrospray mass spectra of the
complexes corroborate with their formulation as ML2 (M )
Ru and Os). For example, the complex Ru(La)2 showed an
intense peak due to the molecular ion [2a + H+]+ at m/z )
649 amu, and that for the osmium congener appeared at m/z
) 738 amu. Notably, the experimental spectral features of
the complexes correspond very well to the simulated isotopic
pattern for the given formulation. A representative spectrum,
that of [3a + H+]+ along with the simulated spectrum, is
submitted in the Supporting Information (Figure S1).
The complexes 2 and 3 possess S ) 0 ground states, as
determined by magnetic susceptibility measurements at room
Results and Discussion
Synthesis. Three ligands, HLa-HLc, differing with respect
to substitution6a on the aminophenyl ring, have been used
in this work is listed in Chart 1.
The ligands offer three very different kinds of N-
coordination atoms, namely, a pyridyl-N (N1), a reducible
azo function containing N2, and a diarylamido-N (N3, after
deprotonation). Pyridyl-N and azo-N are π accepting,
whereas reduced azo-N and diarylamido-N are π donating.
Reactions of KRuO4 (1 mmol) with the HL ligands (2
mmol) (HLa-HLc) in boiling 2-methoxyethanol in the
presence of PPh3 (4 mmol) produced new greenish brown
RuL2 complexes (2a-c) in 50–60% yield. Similar reactions
using OsO4 yielded the corresponding osmium complexes
OsL2 (3a-c) in 60–70% yield. The reactions of OsO4 with
HL are more facile, occurring in methanol and being
1
temperature. They display sharp H NMR signals in the
normal range6 for diamagnetic compounds. The NMR spectra
reveal that the ligands in ML2 are magnetically equivalent
1
on the NMR time scale. The H NMR spectrum of 2a is
submitted in the Supporting Information (Figure S2).
X-ray Crystal Structures. M(La)2 {M ) Ru (2a) and
M ) Os (3a)}. Single-crystal X-ray structure determination
(Table 1) of the two representative ruthenium and osmium
complexes 2a and 3a revealed similar coordination and
geometry and are discussed together for comparison. The
ORTEP representations with atom numbering schemes of
2a and 3a are depicted in Figures 1 and 2, respectively. The
two deprotonated tridentate ligands are coordinated by the
respective metal ion using pairs of pyridyl-N, azo-N, and
diarylanilido N atoms. The configuration is bis-meridional,
and the relative orientations of the aforesaid pairs of
coordinated atoms are cis, trans, and cis, respectively. Similar
geometry and bonding was noted before7,8 for the corre-
sponding group 6 metal complexes [MIVL2] (M ) Cr, Mo,
and W). Notably, the average of the chelate bite angles,
Npyridyl-M-Nazo, of 76.6° is systematically smaller than that
of Nazo-M-Namide (79.5°), underlining the asymmetry of the
mer-tridentate ligands. The X-ray crystallographic analysis
thus reveals considerably distorted octahedral structures with
trans positioned azo-N, cis positioned 2-pyridyl-N, and cis-
oriented anilido nitrogen atoms. The Nazo-M-Nazo angle is
close to 170°; however, the other two trans angles are smaller
at about 155° because of the overarching bite of the mer-
tridentate ligands. Table 2 shows that DFT calculations
reasonably reproduce the structures of the systems studied.
(15) Gaussian 03, Revision C.02,Frisch, M. J.; Trucks, G. W.; Schlegel,
H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery,
J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.;
Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.;
Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M,;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.;
Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.;
Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi,
R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth,
G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck,
A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul,
A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko,
A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.;
Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.;
Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.;
Pople, J. A. Gaussian, Inc., Wallingford, Connecticut, 2004.
(16) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(17) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77,
3865.
(18) (a) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213.
(b) Rassolov, V. A.; Pople, J. A.; Ratner, M. A.; Windus, T. L.
J. Chem. Phys. 1998, 109, 1223.
(19) Andrae, D.; Häussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor.
Chim. Acta 1990, 77, 123.
(20) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. J. Comput. Chem. 2003,
24, 669.
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(22) (a) Noodleman, L. J. Chem. Phys. 1981, 74, 5737. (b) Lahti, P. M.;
Ichimura, A. S.; Sanborn, J. A. J. Phys. Chem. A 2001, 105, 251.
1628 Inorganic Chemistry, Vol. 47, No. 5, 2008

Singlet Diradical Complexes of Ruthenium and Osmium
characterized (RuII, OsII)25 and as independently verified
through EPR based spin distribution analysis (CuI com-
plexes)26 have about 1.35 Å, whereas the two-electron
reduced hydrazido(2-) ligands have single bonds23,27 with
d(N-N) > 1.40 Å. Similar structure/oxidation state correla-
tions are known for the complexes of the O2n- ligands28 and
for o-quinone-type (1,2-dioxolene) ligands.29
In the present case, the nonoxidized ML2 molecules have
d(N-N) ≈ 1.33 Å (Table 2), clearly suggesting the radical
dianion form of the ligand (one charge from deprotonation
at N3 and another from one-electron reduction centered on
the azo function), and, by implication, the +IV state for the
metals, M ) Ru or Os,: MIV(L•2-)2. The same conclusion as
derived here for M ) Ru and Os was obtained8 for M ) Cr,
Mo, and W with d(N-N) ranging from 1.339(2) to 1.373(3)
Å. All of these diamagnetic molecules may thus be viewed
as singlet diradical10,30,31 species, a description that has
recently been propagated, e.g., for bis(semiquinone) and
ruthenium trithiolate complexes. In the present situation the
electrons from a d4 configuration at the metals couple with
the singly occupied ligand molecular orbitals (MOs) to create
a spin-paired entity, as is confirmed by the DFT calculations
(cf. below).
Figure 1. ORTEP representation and atom numbering scheme for [Ru(La)2]
(2a).
The metal-nitrogen bonds in these complexes are unex-
ceptional, lying in the range 1.95-2.07 Å. The effects of
-NdN- bond elongation of the coordinated ligands in the
present complexes are reflected by the lowering of vibrational
frequencies νNdN as compared to uncoordinated HL. The
νNdN band in free HL appears around 1380 cm-1, whereas
those in the present complexes are considerably lower,32
appearing near 1135 cm-1.
The one-electron oxidized form of the osmium complex
could be crystallized as triiodide, allowing us to establish
the N-N bond lengths in the ligands (cf. below) at close to
1.30 Å (average, cf. Table 2). The ORTEP representation
with atom numbering scheme of the compound is displayed
in Figure 3. The coordination geometry of the oxidized
complex is similar to that of the parent complex, [3a].
However, the most striking feature of this structure is the
notable contraction of the N-N bond length. Accordingly
(25) (a) Shivakumar, M.; Pramanik, K.; Ghosh, P.; Chakravorty, A. J. Chem.
Soc. Chem. Commun. 1998, 19, 2103. (b) Shivakumar, M.; Pramanik,
K.; Ghosh, P.; Chakravorty, A. Inorg. Chem. 1998, 37, 5968. (c)
Pramanik, K.; Shivakumar, M.; Ghosh, P.; Chakravorty, A. Inorg.
Chem. 2000, 39, 195.
Figure 2. ORTEP representation and atom numbering scheme for [Os(La)2]
(3a).
(26) (a) Doslik, N.; Sixt, T.; Kaim, W. Angew. Chem., Int. Ed. 1998, 37,
2403. (b) Kaim, W.; Doslik, N.; Frantz, S.; Sixt, T.; Wanner, M.;
Baumann, F.; Denninger, G.; Kümmerer, H.-J.; Duboc-Toia, C.;
Fiedler, J.; Záliš, S. J. Mol. Struct. 2003, 656, 183.
The N-N distance is an excellent indicator of the charge
on an azo function.23 From the ca. 1.24 Å for free ligands,
the coordination by back-donating metals may shift this value
to about 1.25–1.30 Å. Real one-electron reduced (i.e., anion
radical) ligands as calculated (ReI, Ru2.5)24 and structurally
(27) (a) Chan, D.; Cronin, L.; Duckett, S. B.; Hupfield, P.; Perutz, R. N.
New J. Chem. 1998, 22, 511. (b) Morino, Y.; Iijima, T.; Murata, Y.
Bull. Chem. Soc. Jpn. 1960, 33, 46. (c) Marabella, C. P.; Enemark,
J. H.; Newton, W. E.; McDonald, J. W. Inorg. Chem. 1982, 21, 623.
(d) Ittel, S. D.; Ibers, J. A. Inorg. Chem. 1973, 12, 2290. (e) Muñiz,
K.; Nieger, M. Angew. Chem., Int. Ed. 2006, 45, 2305.
(28) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. AdVanced
Inorganic Chemistry, 6th ed., Wiley, New York, 1999, pp. 468–471.
(29) Bhattacharya, S.; Gupta, P.; Basuli, F.; Pierpont, G. C. Inorg. Chem.
2002, 41, 5810.
(23) Kaim, W. Coord. Chem. ReV. 2001, 219–221, 463.
(24) (a) Frantz, S.; Hartmann, H.; Doslik, N.; Wanner, M.; Kaim, W.;
Kümmerer, H. J.; Denninger, G.; Barra, A.-L.; Duboc-Toia, C.; Fiedler,
J.; Ciofini, I.; Urban, C.; Kaupp, M. J. Am. Chem. Soc. 2002, 124,
10563. (b) Sarkar, B.; Patra, S.; Fiedler, J.; Sunoj, R. B.; Janardanan,
D.; Mobin, S. M.; Niemeyer, M.; Lahiri, G. K.; Kaim, W. Angew.
Chem., Int. Ed. 2005, 44, 5655.
(30) Herebian, D.; Wieghardt, K. E.; Neese, F. J. Am. Chem. Soc. 2003,
125, 10997.
(31) Grapperhaus, C. A.; Kozlowski, P. M.; Kumar, D.; Frye, H. N.; Venna,
K. B.; Poturovic, S. Angew. Chem., Int. Ed. 2007, 46, 4085.
(32) Itoh, T.; McCreery, R. L. J. Am. Chem. Soc. 2002, 124, 10894.
Inorganic Chemistry, Vol. 47, No. 5, 2008 1629

Samanta et al.
Table 2. Selected Experimental Bond Lengths (Å) and trans Angles (°) of [M(La)2]0/+ Complexes
(2a) (3a)
[3a]I3
bond
Exp.
Calc.
Exp.
Calc.
Exp.
Calc.
M-N1
2.075(5)
1.966(5)
2.056(5)
2.041(4)
1.960(4)
2.079(5)
1.324(7)
1.327(6)
1.378(7)
1.375(7)
1.357(8)
1.320(7)
170.21(19)
154.47(17)
155.70(18)
2.067
1.968
2.095
2.067
1.968
2.095
1.307
1.307
1.367
1.368
1.343
1.342
2.047(3)
1.967(3)
2.074(3)
2.058(3)
1.955(3)
2.036(3)
1.334(4)
1.341(5)
1.393(6)
1.382(6)
1.370(5)
1.358(5)
168.76(13)
154.48(12)
152.08(12)
2.064
1.976
2.097
2.063
1.976
2.097
1.312
1.313
1.381
1.381
1.356
1.355
2.062(4)
1.973(5)
1.999(4)
2.077(5)
1.973(5)
1.995(4)
1.303(7)
1.310(7)
1.394(7)
1.398(7)
1.378(8)
1.386(8)
175.80(19)
153.77(18)
154.2(2)
2.081
1.988
2.038
2.088
1.996
2.026
1.299
1.295
1.376
1.378
1.413
1.414
176.2
154.6
155.1
M-N3
M-N4
M-N5
M-N7
M-N8
N2-N3
N6-N7
N3-C6
N7-C23
N4-C11
N8-C28
N3-M-N7
N1-M-N4
N5-M-N8
172.5
155.5
155.6
167.3
152.8
152.7
that are perpendicularly oriented; such a description is
considered to be less plausible, as will also be discussed
below in connection with EPR (metal-centered spin) and
UV-vis-NIR results (NIR absorption).The compounds
[2a]I3 and [3a]I3 both are 1:1 electrolytes in acetonitrile. Their
elemental analyses as well as ESI-mass spectra are consistent
with their formulations. The magnetic moments at 298 K of
the solid samples of [2a]I3 and [3a]I3 are 1.68 and 1.55 µB,
respectively, signifying the presence of one unpaired electron.
The fact that oxidation of the complex would cause a
reduction of the metal seems paradoxical but has been
similarly observed in quinone and TCNX chemistry; the
combined oxidation state change of more than one compo-
nent (ligand or metal) is capable of offsetting a counterin-
tuitive electron transfer of the single center (metal or ligand).
Thus, the reduction of tris(o-semiquinonato)vanadium(III)
produced34 a tris(catecholato)vanadium(V) anion, and the
reduction of {(µ4-TCNE2-)[Ru2.5(NH3)5]4}8+ gives35 (µ4-
TCNE•-)[RuII(NH3)5]4}7+.
Figure 3. ORTEP representation and atom numbering scheme for [Os-
(La)2]+ (3a+) in [3a]I3
The Electronic Structures of [M(La)2]0,+ (M ) Ru
and Os). All neutral complexes examined are diamagnetic
species; therefore, the geometry optimizations were per-
formed for singlet ground states. Calculations were done
using both the restricted and the unrestricted approach. The
structures of all systems are better described by using the
PBE0 hybrid functional than by using B3LYP; the results
depend only slightly on the approach used. Calculated
selected bond lengths and angles are compared with experi-
mental ones in Table 2. The calculated structural parameters
within both neutral complexes are in very good agreement
with the experimental structure; the variation of intraligand
bond lengths due to the central atom and the charge variations
are also well described by the calculations. A previously
published8 geometry optimization of the uncoordinated
protonated (2-phenyl)-substituted ligand radical anion gave
N-N, N2-C12, and N1-C7 bond lengths of 1.317, 1.365,
and 1.397 Å, respectively. These values are quite close to
Table 3. Cyclic Voltammetry Dataa of [ML2] 2a-c and 3a-c
reduction -E ,b
V (∆Ep, mV)
½
b
compound
oxidation E V (∆Ep, mV)
½,
[Ru(La)2] (2a)
[Ru(Lb)2] (2b)
[Ru(Lc)2] (2c)
[Os(La)2] (3a)
[Os(Lb)2] (3b)
[Os(Lc)2] (3c)
0.04 (70), 0.76 (80)
0.01 (70), 0.73 (75)
0.11 (80), 0.78 (80)
-0.03 (75), 0.62(80)
-0.06 (70), 0.58 (75)
-0.01 (75), 0.64 (80)
1.15 (70)
1.13(70)
1.06 (65)
1.13 (75)
1.08 (70)
1.11 (75)
a In dichloromethane solution, supporting electrolyte Bu4NClO4 (0.1 M),
reference electrode SCE. b E ) 0.5(Epa + Epc) where Epa and Epc are anodic
½
and cathodic peak potentials, respectively; ∆Ep ) Epa - Epc; scan rate )
50 mV s-1
.
the νNdN band in [2a]I3 and [3a]I3 appears at a higher
frequency (near 1220 cm-1) than that (1135 cm-1) in the
corresponding reduced complex.
This corresponds to monoanionic ligands with unreduced
azo functions coordinated to a transition metal,6,23,33 and as
a consequence, one may surmise the formulation [MIII(L-)2]+.
The alternative resonance forms [(L-)MIV(L•2-)]+
T
[(L•2-)MIV(L-)]+ would have to assume valence delocaliza-
tion between one radical dianion and one monoanion ligand
(34) (a) Pierpont, C. G.; Lange, C. W. Prog. Inorg. Chem. 1994, 41, 331.
(b) Cass, M. E.; Gordon, N. R.; Pierpont, C. G. Inorg. Chem. 1986,
25, 3962.
(35) (a) Moscherosch, M.; Waldhör, E.; Binder, H.; Kaim, W.; Fiedler, J.
Inorg. Chem. 1995, 34, 4326. (b) Waldhör, E.; Kaim, W.; Lawson,
M.; Jordanov, J. Inorg. Chem. 1997, 36, 3248.
(33) Hartmann, H.; Scheiring, T.; Fiedler, J.; Kaim, W. J. Organomet.
Chem. 2000, 604, 267.
1630 Inorganic Chemistry, Vol. 47, No. 5, 2008

Singlet Diradical Complexes of Ruthenium and Osmium
Figure 5. EPR Spectrum of electrogenerated [Ru(La)2]+ (2a+) in CH2Cl2/
0.1 M Bu4NPF6 at 4K.
Figure 4. Cyclic voltammograms of the complexes Ru(La)2 (blue) and
Os(La)2 (pink) in CH2Cl2/0.1 M Bu4NClO4.
Table 4. EPR Dataa of Monocation Intermediates [M(La)2]+
M ) Ru
M ) Os
g1
g2
g3
gav
2.148
2.113
1.965
2.077
0.075
0.183
1200
2.386
2.283
1.848
2.185
0.183
0.538
3000
b
c
gav - ge
∆g ) g1 - g3
d
(cm-1
)
a From oxidation in CH2Cl2/0.1 M Bu4NPF6, recording at 4 K. b Isotropic
values at 295 K: 2.076 ([Ru(La)2]+), 2.180 [Os(La)2]+). c ge ) 2.0023 (free
d
electron value). Spin–orbit coupling constants for M3+ (from ref 36).
the ones calculated for the neutral [M(L)2] complexes, which
indicates the radical anion ligand character of the bound
ligands. The elongation of the N-N bond length due to the
reduction of the ligand is comparable with the elongation
caused by metal coordination. The DFT calculated charges
on the central metal atoms are 1.07 and 1.35 for the Ru and
Os systems, respectively. In agreement with the experiment,
the DFT calculations as listed in Table 2 predict a contraction
of the N-N bonds following the oxidation of [Os(La)2]
together with other structure variations: elongation of M-N1
and M-N5 distances and a remarkable shortening of the
M-N4 and M-N8 bond lengths.
The energies and compositions of the DFT calculated
frontier orbitals are similar for both [RuL2] and [OsL2]
complexes; they are listed in the Supporting Information
(Tables S1 and S2). The MO representations show the highest
occupied MO (HOMO) to be formed by a combination of d
metal orbitals (27% and 25% for Ru and Os complexes,
respectively) and π ligand orbitals. Closely lying occupied
HOMO-1 is formed by π ligand orbitals, and the next three
lower lying occupied orbitals have larger contributions (over
46%) from the corresponding central metal atom. The almost
degenerate set of lowest lying unoccupied MOs (LUMOs)
is, to a large extent, delocalized over the π system of the
ligands; d orbitals contribute to this orbital only by 14–21%.
The higher unoccupied MOs also have ligand character.
Electrochemistry. Cyclic voltammetry of RuL2 (2a-c)
complexes in CH2Cl2 exhibits two reversible oxidation waves
and one irreversible cathodic process (Table 3 and Figure
Figure 6. EPR Spectrum of electrogenerated [Os(La)2]+ (3a+) in CH2Cl2/
0.1 M Bu4NPF6 at 4K.
4). One-electron stoichiometry of the reversible processes
is confirmed by exhaustive electrolyses of the representative
compound 2a at 0.25 and 1.00 V, respectively. The redox
potentials are found to be sensitive to the nature of substitu-
tion on the amino phenyl ring. Thus, a gradual shift of the
reduction potential to more positive values is observed upon
moving from 2b (0.01 and 0.73 V) via 2a (0.04 and 0.76 V)
to 2c (0.11 and 0.78 V). The first oxidation potential is low,
and accordingly, we have succeeded in isolating a represen-
tative cationic complex [2a]I3 in the pure state, using iodine
as the oxidizing agent. The osmium analogue is more
crystalline (vide supra), and its X-ray structure has been
solved. Notably, the reduced complexes [2a] and [3a] can
be regenerated quantitatively from [2a]I3 and [3a]I3, respec-
tively, by the use of hydrazine as reducing agent. Our
attempts to isolate the dicationic complex [2a]2+ have so far
been unsuccessful. However, characterization of both [2a]+
and [2a]2+ was achieved by spectroelectrochemical measure-
ments (cf. below).
The osmium analogues OsL2 (3a-c) show similar redox
behavior (Table 3). Examination of the data in Table 3
indicates that the osmium complexes are oxidized more easily
than the ruthenium congeners by 100-150 mV, an expected
Inorganic Chemistry, Vol. 47, No. 5, 2008 1631

Samanta et al.
Figure 7. DFT calculated spin densities for [Os(La)2]+. The blue and green
areas correspond to regions of positive and negative spin density, respectively.
result. Chemical oxidation of 3a by iodine followed by
crystallization thus produced X-ray quality crystals of [3a]I3.
EPR Spectroscopy. The two monocations 2a+ and 3a+
show a similar splitting pattern of the g components in frozen
solution (Table 4, Figures 5 and 6). Two close lying
components g1,g2 > 2 are complemented by g3 < 2. Taken
together with gav > 2, this is typical for a low-spin d5
situation (RuIII and OsIII) with not-too-distorted octahedral
ligation. Clearly, the deviation of g factor components and
of the average g (gav) from g(electron) [ge] ) 2.0023 is larger
for the osmium alternative because of the much higher
spin–orbit coupling constant of the 5d element;36 remark-
ably, both the total g anisotropy ∆g ) g1 - g3 and the
deviation gav - ge are about 2.5 times as large for the Os
system, in accordance with the similar ratio between (Os3+)
) 3000 cm-1 and (Ru3+) ) 1200 cm-1.36
In absolute terms, the values from Table 4 are intermediate
between those of MII containing radicals and of true MIII
systems, that is, those without significant metal–ligand orbital
mixing.37 The data for the ruthenium derivative correspond
to values that have been reported for complexes
[(Q)Ru(acac)2]n, Q ) o-imino(thio)quinone, n ) - or +,
and which have been thoroughly analyzed in a theoretical
study.38 Drawing similar conclusions, we assume largely
metal-centered spin corresponding to MIII systems, [MIII-
(L-)2]+, with, however, very significant participation of the
potentially noninnocent ligands L–/•2-. These conclusions are
qualitatively supported by DFT (PBE0) calculations of
[Os(La)2]+, which gives a spin density of 0.530 for Os; the
remaining spin density is distributed over the ligand system
(Figure 7).
Figure 8. UV-vis-NIR Spectroelectrochemical response of Os(La)2 (3a)
in CH2Cl2/0.1 M Bu4NPF6 during the first (top) and the second oxidation
(bottom).
UV-vis-NIR Spectroelectrochemistry. The absorption
spectral changes upon stepwise reversible one-electron
oxidation of ML2 complexes are shown in Figure 8 for
the osmium compound with La, the data are summarized
in Table 5.
Neutral and dicationic forms exhibit rather similar low-
energy absorption spectra with a weaker band around 1200
(Ru) or 1000 nm (Os) and another one at 660/530 (Ru) or
600 nm (Os). The EPR active intermediates show long-
wavelength shifted absorptions at about 1800 and 800/700
nm (Ru) or 1320 and 640 nm (Os). The assignment of these
bands is based on structural and EPR information for the
oxidation state distribution in the ground-state and on
the results of TD-DFT calculations. As shown in Table 5,
the TD-DFT calculations qualitatively describe the main
features of the absorption spectra for the neutral Ru and Os
complexes quite well. The band around 1000 nm is assigned
to involve HOMO f LUMO and HOMO f LUMO + 1
excitations with a predicted shift of this band to longer
wavelengths when going from Os to Ru. The next features
within the spectra around 600 nm can be assigned to HOMO
- 1 f LUMO and HOMO - 1 f LUMO + 1 excitations.
The intense broad band at shorter wavelength is composed
(36) Weil, J. A.; Bolton, J. R. Electron Paramagnetic Resonance, 2nd Ed.,
Wiley Interscience, New York, 2007.
(37) Patra, S.; Sarkar, B.; Mobin, S. M.; Kaim, W.; Lahiri, G. K. Inorg.
Chem. 2003, 42, 6469.
(38) Remenyi, C.; Kaupp, M. J. Am. Chem. Soc. 2005, 127, 11399.
1632 Inorganic Chemistry, Vol. 47, No. 5, 2008

Singlet Diradical Complexes of Ruthenium and Osmium
Table 5. Absorption Maxima for [ML2]n+ from UV-vis-NIR Spectroelectrochemistry and TD-DFT Calculated Lowest-Lying Transitions for [M(La)2]
complexesa
complex
λ )
max/nm (ꢀ/M-1 cm-1
[Ru(La)2]
[Ru(La)2]b
1150 (1575), 675 (8500), 555 (7730), 450 (15 770), 410 (13 330), 315 (22 965), 245 (26 220)
1074 (0.015), 1025 (0.024), 652 (0.091), 630 (0.138), 480 (0.105), 449 (0.040), 416 (0.073), 395 (0.199), 372 (0.126), 361 (0.206),
358 (0.128), 349 (0.098), 309 (0.073)
1155 (1410), 680 (7290), 556 (6610), 452 (13 080), 413 (11 680), 313 (20 270), 230 (27 445)
1155 (1055), 678 (5380), 550 (5395), 445 (10 830), 405sh, 315sh, 280sh, 250 (31 145)
1800 (1490), 790sh, 708 (7160), 496sh, 420sh, 355sh, 299sh
[Ru(Lb)2]
[Ru(Lc)2]
[Ru(La)2]+
[Ru(La)2]2+ 1290 (2700), 1125sh, 649sh, 510sh, 456 (12 510), 367 (24 120)
[Os(La)2]
[Os(La)2]b
Os(Lb)2]
1003 (1280), 630 (9680), 435sh, 395sh, 345sh, 305 (24 790), 260 (24 470)
978 (0.026), 635 (0.076), 604 (0.120), 428 (0.093), 389 (0.208), 363 (0.130), 359 (0.138), 352 (0.136), 351 (0.136), 351 (0.186), 349 (0.142)
1050 (1020), 630 (8640), 430sh, 400sh, 310sh, 255 (29 125)
1055 (960), 635 (7520), 435sh, 395sh, 310sh, 250 (27 430)
1320 (1750), 710sh, 639 (7970), 401 (21 180), 360sh, 313(21 080)
[Os(Lc)2]
[Os(La)2]+
[Os(La)2]2+ 1003 (2170), 590sh, 462 (18 520), 388 (25 140)
a In CH2Cl2/0.1 M Bu4NPF6. b TD DFT-calculated transitions, oscillator strengths in parenthesis. Only transitions with oscillator strength larger than 0.01
are presented.
Scheme 2
azo-containing ligands, we have synthesized new singlet
diradical complexes of RuIV or OsIV with the corresponding
dianion radical species. Experimental (X-ray diffraction) and
theoretical studies (DFT) support this interpretation that could
be extended to the oxidized states formed via two one-
electron steps (Scheme 2). Structural effects (N-N bond
shortening), EPR results (metal-centered spin), and spectro-
electrochemical information in conjunction with DFT cal-
culations confirm the oxidation of both ligands but reduction
of the metal in the first oxidation process.
Acknowledgment. Financial support from the Department
of Science and Technology (DST)(Project SR/S1/IC-24/
2006) and the Council of Scientific and Industrial Research,
of many intense transitions, most of which have some metal-
New Delhi, from the Deutsche Forschungsgemeinschaft and
to-ligand charge transfer (MLCT) character.
Fonds der Chemischen Industrie (Frankfurt/Main), and from
The L•2-/MIV/L•2- singlet diradical ground-state can be
the Grant Agency of the Czech Republic (KAN100400702)
converted to excited states approximately described as *[L-/
and the Ministry of Education of the Czech Republic (COST
MIII/L•2-] in ligand-to-metal charge transfer (LMCT) pro-
Grants OC 139 and OC 140) is gratefully acknowledged.
cesses. The monocations, on the other hand, identified by
EPR and structure analysis as [L-/MIII/L-]+ species, may
be excited to *[L-/MIV/L•2-]+ in MLCT transitions. MLCT
transitions are also believed to cause the low-energy absorp-
Crystallography was performed at the DST-funded National
Single Crystal Diffractometer Facility at the Department of
Inorganic Chemistry, IACS. S. S. also thanks the Council
of Scientific and Industrial Research for his fellowship.
tion bands in [L-/MIV/L-]2+, leading to *[L-/MV/L•2- 2+ or
]
*[L•2-/MVI/L•2-]2+. Such excited states correspond to the
ground states of the ML2 complexes, M ) MoIV or WIV,
described earlier.8
Supporting Information Available: X-ray crystallographic files
in CIF format for 2a, 3a, and [3a]I3; figures of ESI-MS for 3a
1
(S1); H NMR of 2a (S2); and cyclic voltammograms of 2a and
Conclusion
3a in CH2Cl2/ 0.1 M Bu4NClO4 (S3) are provided. This material is
available free of charge via the Internet at http://pubs.acs.org.
Employing high-valent oxo-ruthenium or -osmium precur-
sors and PPh3 as reducing agent in the presence of tridentate
IC702301A
Inorganic Chemistry, Vol. 47, No. 5, 2008 1633