Mixed-valent metals bridged by a radical ligand: Fact or fiction based on structure-oxidation state correlations

Article abstract of DOI:10.1021/ja077676f

Electron-rich Ru(acac)₂ (acac^- = 2,4-pentanedionato) binds to the π electron-deficient bis-chelate ligands L, L = 2,2′-azobispyridine (abpy) or azobis(5-chloropyrimidine) (abcp), with considerable transfer of negative charge. The compounds studied, (abpy)Ru(acac)₂ (1), meso(μ-abpy)[Ru(acac)₂] ₂ (2), rac-(μ-abpy)[Ru(acac)₂]₂ (3), and (μ-abcp)[Ru(acac)₂]₂ (4), were calculated by DFT to assess the degree of this metal-to-ligand electron shift. The calculated and experimental structures of 2 and 3 both yield about 1.35 A for the length of the central N-N bond which suggests a monoanion character of the bridging ligand. The NBO analysis confirms this interpretation, and TD-DFT calculations reproduce the observed intense long-wavelength absorptions. While mononuclear 1 is calculated with a lower net ruthenium-to-abpy charge shift as illustrated by the computed 1.30 A for d(N-N), compound 4 with the stronger π accepting abcp bridge is calculated with a slightly lengthened N-N distance relative to that of 2. The formulation of the dinuclear systems with monoanionic bridging ligands implies an obviously valence-averaged Ru^IIIRu ^II mixed-valent state for the neutral molecules. Mixed valency in conjunction with an anion radical bridging ligand had been discussed before in the discussion of MLCT excited states of symmetrically dinuclear coordination compounds. Whereas 1 still exhibits a conventional electrochemical and spectroelectrochemical behavior with metal centered oxidation and two ligand-based one-electron reduction waves, the two one-electron oxidation and two one-electron reduction processes for each of the dinuclear compounds Ru ^2.5(L^?-)Ru^2.5 reveal more unusual features via EPR and UV-vis-NIR spectroelectrochemistry. In spite of intense near-infrared absorptions, the EPR results show that the first reduction leads to Ru^II(L^?-)Ru^II species, with an increased metal contribution for system 4^?-. The second reduction to Ru^II(L^2-)Ru^II causes the disappearance of the NIR band. One-electron oxidation of the Ru ^2.5(L^?-)Ru^2.5 species produces a metal-centered spin for which the alternatives Ru^III(L ⁰)Ru^II or Ru^III(L^?-)Ru ^III can be formulated. The absence of NIR bands as common for mixed-valent species with intervalence charge transfer (IVCT) absorption favors the second alternative. The second one-electron oxidation is likely to produce a dication with Ru^III(L⁰)Ru^III formulation. The usefulness and limitations of the increasingly popular structure/oxidation state correlations for complexes with noninnocent ligands is being discussed.

Full text of DOI:10.1021/ja077676f

Published on Web 02/22/2008
Mixed-Valent Metals Bridged by a Radical Ligand: Fact or
Fiction Based on Structure-Oxidation State Correlations
Biprajit Sarkar,† Srikanta Patra,‡ Jan Fiedler,§ Raghavan B. Sunoj,*,‡
Deepa Janardanan,‡ Goutam Kumar Lahiri,*,‡ and Wolfgang Kaim*,†
Institut fu¨r Anorganische Chemie, UniVersita¨t Stuttgart, Pfaffenwaldring 55, D-70550 Stuttgart,
Germany, Department of Chemistry, Indian Institute of Technology, Bombay, Powai,
Mumbai-400076, India, and J. HeyroVsky´ Institute of Physical Chemistry, V.V.i., Academy of
Sciences of the Czech Republic, DolejsˇkoVa 3, CZ-18223 Prague, Czech Republic
Received October 5, 2007; E-mail: sunoj@chem.iitb.ac.in; lahiri@chem.iitb.ac.in; kaim@iac.uni-stuttgart.de
Abstract: Electron-rich Ru(acac)2 (acac- ) 2,4-pentanedionato) binds to the π electron-deficient bis-chelate
ligands L, L ) 2,2′-azobispyridine (abpy) or azobis(5-chloropyrimidine) (abcp), with considerable transfer
of negative charge. The compounds studied, (abpy)Ru(acac)2 (1), meso-(µ-abpy)[Ru(acac)2]2 (2), rac-(µ-
abpy)[Ru(acac)2]2 (3), and (µ-abcp)[Ru(acac)2]2 (4), were calculated by DFT to assess the degree of this
metal-to-ligand electron shift. The calculated and experimental structures of 2 and 3 both yield about 1.35
Å for the length of the central N-N bond which suggests a monoanion character of the bridging ligand.
The NBO analysis confirms this interpretation, and TD-DFT calculations reproduce the observed intense
long-wavelength absorptions. While mononuclear 1 is calculated with a lower net ruthenium-to-abpy charge
shift as illustrated by the computed 1.30 Å for d(N-N), compound 4 with the stronger π accepting abcp
bridge is calculated with a slightly lengthened N-N distance relative to that of 2. The formulation of the
dinuclear systems with monoanionic bridging ligands implies an obviously valence-averaged RuIIIRuII
mixed-valent state for the neutral molecules. Mixed valency in conjunction with an anion radical bridging
ligand had been discussed before in the discussion of MLCT excited states of symmetrically dinuclear
coordination compounds. Whereas 1 still exhibits a conventional electrochemical and spectroelectrochemical
behavior with metal centered oxidation and two ligand-based one-electron reduction waves, the two one-
electron oxidation and two one-electron reduction processes for each of the dinuclear compounds
Ru2.5(L•-)Ru2.5 reveal more unusual features via EPR and UV-vis-NIR spectroelectrochemistry. In spite
of intense near-infrared absorptions, the EPR results show that the first reduction leads to RuII(L•-)RuII
species, with an increased metal contribution for system 4•-. The second reduction to RuII(L2-)RuII causes
the disappearance of the NIR band. One-electron oxidation of the Ru2.5(L•-)Ru2.5 species produces a metal-
centered spin for which the alternatives RuIII(L0)RuII or RuIII(L•-)RuIII can be formulated. The absence of
NIR bands as common for mixed-valent species with intervalence charge transfer (IVCT) absorption favors
the second alternative. The second one-electron oxidation is likely to produce a dication with RuIII(L0)RuIII
formulation. The usefulness and limitations of the increasingly popular structure/oxidation state correlations
for complexes with noninnocent ligands is being discussed.
Scheme 1
Introduction
The use of structural parameters to establish oxidation states
of ligands and, by implication, of coordinating metals has
become a popular exercise for transition metal compounds with
potentially noninnocent ligands.1 The chelating o-quinone/o-
semiquinone/catecholate type redox system of Scheme 1 has
been most thoroughly studied,2,3 and statistically derived
guidelines to aid in structure-oxidation state correlations were
presented.2b,c
Scheme 2
In the absence of aromatic stabilization which can favor the
reduced form (as in Scheme 1) or the oxidized state (as in
Scheme 2, 2,2′-bipyridine (bpy) behaving as a noninnocent
ligand), the chelating 1,4-diazabutadiene/1,2-enediamido redox
systems (Scheme 3) were shown by computationally supported
structure analysis to function as noninnocent ligands not only
toward main group elements but also toward transition metals.4
† Universita¨t Stuttgart.
‡ Indian Institute of Technology.
§ J. Heyrovsky´ Institute of Physical Chemistry, v.v.i.
(1) (a) Ward, M. D.; McCleverty, J. A. J. Chem. Soc., Dalton Trans. 2002,
275. (b) Chaudhuri, P.; Verani, C. N.; Bill, E.; Bothe, E.; Weyhermu¨ller,
T.; Wieghardt, K. J. Am. Chem. Soc. 2001, 123, 2213.
9
3532
J. AM. CHEM. SOC. 2008, 130, 3532-3542
10.1021/ja077676f CCC: $40.75 © 2008 American Chemical Society

Mixed-Valent Metals Bridged by a Radical Ligand
A R T I C L E S
Scheme 3
Scheme 6
taken as evidence for a singly reduced, i.e., anion radical
bridging ligand, abpy•- in each case, which implies an average
oxidation state of 2.5 for the two equivalent metal centers. While
valence-localized diruthenium(III,II) or valence-delocalized
diruthenium(2.5,2.5) complexes bridged by diamagnetic mol-
ecules have been studied in great number,13 the bridging by an
anion radical had not been considered before for ground state
situations; however, it was implied for metal-to-ligand charge
transfer (MLCT) excited states of acceptor-bridged sym-
metrically dinuclear complexes (Scheme 6).14 Very recently,
Fabre, Bonvoisin, Launay et al. have proposed a similar situation
for a diruthenium complex containing a 1,4-dicyanamidobenzene
bridge.15
In the following we report studies (electrochemistry, magnetic
resonance, spectroscopy, DFT calculations) of the mononuclear
reference compound (abpy)Ru(acac)2 (1) and give a full
description of the rac and meso isomers (2, 3) of (acac)2Ru(µ-
abpy)Ru(acac)2, including calculations of geometries, charges,
and electronic transitions for the lowest singlet and triplet states.
An additional system, (acac)2Ru(µ-abcp)Ru(acac)2 (4), with the
better9,16 π acceptor 2,2′-azobis(5-chloropyrimidine) ) abcp,
is presented for which radical complexes of copper(I) and
rhenium(I)17 were structurally characterized by experiment and
theory.9,16 This and the abpy-bridged complexes 2 and 3 have
also been studied with respect to their stepwise electrochemical
oxidation and reduction within the series [(acac)2Ru(µ-BL)Ru-
(acac)2]n, n ) -2, -1, 0, +1, +2, by cyclic voltammetry, EPR,
and UV-vis-NIR spectroelectrochemistry. The latter studies
are useful to provide frontier MO information and to specify
differences based on bridging ligand variation (abpy/abcp) or
on configurational isomerism (rac/meso). The purpose of these
efforts is to confirm the hypothesis of radical ion bridged mixed
valent species and to understand the particular electronic
structures which are related to the postulated mixed-valent
excited-state configurations (Scheme 6).14 In a wider connection,
metal complexes of 1,2-diaryldiazenes such as abpy8,17,18 have
found interest as potentially DNA intercalating chemotherapeutic
agents19 and as photoresponsive species.20
Scheme 4
Scheme 5
A corresponding approach was described recently5a in detail
for the potentially fourfold metal-bridging6 TCNE0/•-/2- redox
system (TCNE ) tetracyanoethene, Scheme 4) which can
produce a variety of magnetic and nonmagnetic coordination
compounds.5,6
One of the simplest noninnocent ligand systems involves the
double bond/single bond conversion in Scheme 5 where E )
O represents the biologically important dioxygen species O2
Azo compounds (diazenes) also fall into the category of
Scheme 5 with E ) NR; enhanced stability and additional
potential for coordination are obtained when R is a 2-pyridyl,8
related heteroaryl,9 acyl,10 or imine11 substituent.
In the course of studying the coordination chemistry of 2,2′-
azobispyridine (abpy)8 we have recently obtained and structur-
ally characterized the two diastereoisomers, meso and rac forms,
of (acac)2Ru(µ-abpy)Ru(acac)2 (acac- ) acetylacetonato ) 2,4-
pentanedionato).12 The N-N bond lengths of about 1.36 Å were
n- 7
.
(2) (a) Pierpont, C. G.; Lange, C. W. Prog. Inorg. Chem. 1994, 41, 331. (b)
Pierpont, C. G. Coord. Chem. ReV. 2001, 219-221, 415. (c) Bhattacharya,
S.; Gupta, P.; Basuli, F.; Pierpont, C. G. Inorg. Chem. 2002, 41, 5810. (d)
Lever, A. B. P.; Gorelsky, S. L. Struct. Bonding 2004, 107, 77. (e) Mitra,
K. N.; Choudhury, S.; Castin˜eiras, A.; Goswami, S. J. Chem. Soc., Dalton
Trans. 1998, 2901.
(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)
Kokatam, S.; Weyhermu¨ller, T.; Bothe, E.; Chaudhuri, P.; Wieghardt, K.
Inorg. Chem. 2005, 44, 3709.
(4) (a) Greulich, S.; Kaim, W.; Stange, A.; Stoll, H.; Fiedler, J.; Zalis, S. Inorg.
Chem. 1996, 35, 3998 and literature cited. (b) Muresan, N.; Weyhermu¨ller,
T.; Wieghardt, K. Dalton Trans. 2007, 4390 and literature cited.
(5) (a) Miller, J. S. Angew. Chem. 2006, 118, 2570; Angew. Chem., Int. Ed.
2006, 45, 2508. (b) Bagnato, J. D.; Shum, W. W.; Strohmeier, M.; Grant,
D. M.; Arif, A. M.; Miller, J. S. Angew. Chem. 2006, 118, 5448; Angew.
Chem., Int. Ed. 2006, 45, 5322. (c) Manriquez, J. M.; Yee, G. T.; McLean,
R. S.; Epstein, A. J.; Miller, J. S. Science 1991, 252, 1415.
(6) Kaim, W.; Moscherosch, M. Coord. Chem. ReV. 1994, 129, 157.
(7) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. AdVanced
Inorganic Chemistry, 6th ed.; Wiley: New York, 1999; pp 468-470.
(8) Kaim, W. Coord. Chem. ReV. 2001, 219-221, 463.
(9) (a) Doslik, N.; Sixt, T.; Kaim, W. Angew. Chem. 1998, 110, 2521; Angew.
Chem., Int. Ed. 1998, 37, 2403. (b) 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.
(10) (a) Kasack, V.; Kaim, W.; Binder, H.; Jordanov, J.; Roth, E. Inorg. Chem.
1995, 34, 1924. (b) Kno¨dler, A.; Fiedler, J.; Kaim, W. Polyhedron 2004,
23, 701.
(11) Maji, S.; Sarkar, B.; Patra, S.; Fiedler, J.; Mobin, S. M.; Puranik, V. G.;
Kaim, W.; Lahiri, G. K. Inorg. Chem. 2006, 45, 1316.
Experimental Section
General. The precursor complex Ru(acac)2(CH3CN)221a and
the ligands 2,2′-azobispyridine (abpy)8 and 2,2′-azobis-(5-
(13) (a) Kaim, W.; Lahiri, G. K. Angew. Chem. 2007, 119, 1808; Angew. Chem.,
Int. Ed. 2007, 46, 1778. (b) Creutz, C. Prog. Inorg. Chem. 1983, 30, 1. (c)
Richardson, D. E.; Taube, H. Coord. Chem. ReV. 1984, 60, 107. (d)
Crutchley, R. J. AdV. Inorg. Chem. 1994, 41, 273. (e) Demadis, K. D.;
Hartshorn, D. C.; Meyer, T. J. Chem. ReV. 2001, 101, 2655.
(14) (a) Dattelbaum, D. M.; Hartshorn, C. M.; Meyer, T. J. J. Am. Chem. Soc.
2002, 124, 4938. (b) Kaim, W.; Kohlmann, S. Inorg. Chem. 1987, 26, 68.
(c) Ernst, S. D.; Kaim, W. Inorg. Chem. 1989, 28, 1520. (d) For excited
state mixed valency of mononuclear complexes with two equivalent charge
transfer active ligands, see: Plummer, E. A.; Zink, J. I. Inorg. Chem. 2006,
45, 6556.
(15) (a) Fabre, M.; Jaud, J.; Hliwa, M.; Launay, J.-P.; Bonvoisin, J. Inorg. Chem.
2006, 45, 9332. (b) Fabre, M.; Bonvoisin, J. J. Am. Chem. Soc. 2007, 129,
1434.
(16) Frantz, S.; Hartmann, H.; Doslik, N.; Wanner, M.; Kaim, W.; Ku¨mmerer,
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.
(17) (a) Hartmann, H.; Scheiring, T.; Fiedler, J.; Kaim, W. J. Organomet. Chem.
2000, 604, 267. (b) Frantz, S.; Fiedler, J.; Hartenbach, I.; Schleid, Th.;
Kaim, W. J. Organomet. Chem. 2004, 689, 3031.
(12) Sarkar, B.; Patra, S.; Fiedler, J.; Sunoj, R.; Janardanan, D.; Mobin, S. M.;
Niemeyer, M.; Lahiri, G. K.; Kaim, W. Angew. Chem. 2005, 117, 5800;
Angew. Chem., Int. Ed. 2005, 44, 5655.
9
J. AM. CHEM. SOC. VOL. 130, NO. 11, 2008 3533

A R T I C L E S
Sarkar et al.
chloropyrimidine)9 were prepared according to reported proce-
dures. The synthesis and separation of complexes 2 and 3 have
been reported elsewhere.12 Other chemicals and solvents were
reagent grade and used as received. For spectroscopic and
electrochemical studies HPLC grade solvents were used.
Table 1. Electrochemical and Spectroscopic Data of Complexes
1
2(meso)
3(rac)
4
Cyclic
Voltammetrya
-1.67
E1/2 (red 2)
-1.68
-1.17
0.46
n.o.
n.a.
-1.67
-1.03
0.05
-1.39
-0.76
0.22
E
E
E
1/2 (red 1)
1/2 (ox 1)
1/2 (ox 2)
-1.03
0.05
Instrumentation. UV-vis-NIR spectroelectrochemical stud-
ies were performed in CH3CN/0.1 M Bu4NPF6 at 298 K using
an optically transparent thin layer electrode (OTTLE) cell21b in
connection with a J&M TIDAS spectrophotometer. FT-IR
spectra were taken on a Nicolet spectrophotometer with samples
prepared as KBr pellets. 1H NMR spectra were obtained with a
300 MHz Varian FT spectrometer. The EPR measurements were
made in a two-electrode capillary tube18d with an X-band Bruker
system ESP300 spectrometer. Susceptibility was measured with
a Faraday balance (CAHN Instruments, serial no. 76240). Cyclic
voltammetric, differential pulse voltammetric, and coulometric
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 0.1 M NEt4ClO4, and the solute
concentration was ∼10-3 M. Caution: Perchlorates are poten-
tially explosive and need to be treated with special precaution.
0.89
0.89
1.02
Kc (0)b
1018.3
1018.3
1016.6
EPRc Anions
1.991
1.991
1.991
<0.05
1.991
g1
g2
g3
g1-g3
gav
1.999
1.999
1.999
<0.05
1.999
1.991
1.991
1.991
<0.05
1.991
2.035
1.990
1.953
0.082
1.993
d
EPRc Cations
2.347
2.151
1.810
0.537
g1
g2
g3
g1-g3
gav
2.190
2.190
1.914
0.276
2.102
2.250
2.140
1.984
0.266
2.128
2.321
2.182
1.843
0.478
2.125
d
2.114
UV-vis-NIRe
619 (11 700)
501sh
λ
λ
max(ꢀ) [2-] 570 (20 300)
464 (49 600)
620 (11 900)
510sh
422 (38 400)
327sh
272 (83 300)
1830 (3100)
625sh
445 (60 900)
413 (51 900)
345 (58 300)
323sh
273 (110 000) 267 (53 100)
1780 (2700)
272 (112 800)
1452 (11 100)
max(ꢀ) [-]
840 (13 200)
630 (10 200)
453sh
1185 (12 400) 1185 (10 900) 740sh
665 (6900)
452sh
The half-wave potential Eo was set equal to 0.5(Epa + Epc),
665 (7400)
450sh
518 (23 300)
298
where Epa and Epc are the anodic and cathodic cyclic voltam-
metric peak potentials, respectively. A platinum wire-gauze
working electrode was used in the coulometric experiments. 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.
389 (77 400)
275 (91 400)
530 (25 000)
410sh
332 (49 100)
270 (77 900)
670sh
577 (22 200)
457sh
330sh
394 (36 000)
268 (51 500)
848 (29 000)
430sh
340 (39 500)
270 (52 800)
396 (39800)
274 (79 600)
835 (29 000)
430sh
339 (43 400)
272 (67 600)
395 (46 200)
271 (101 200)
920 (30 000)
480 (16 600)
342 (53 400)
272 (86 500)
λ
λ
max(ꢀ) [0]
max(ꢀ) [+]
655 (22 900)
300sh
662 (25 800)
615sh
372sh
702 (22 100)
Synthesis of {Ru(acac)2(abpy)} (1). The precursor complex
[Ru(acac)2(CH3CN)2] (100 mg, 0.26 mmol) and the ligand abpy
(22 mg, 0.12 mmol) were combined in 20 mL of ethanol, and
the solution was heated at reflux for 4 h under a dinitrogen
atmosphere. The color of the solution gradually changed from
orange to dark brown. The solvent was removed, and the residue
was subjected to chromatography on a silica gel column. After
the initial elution of a red fraction corresponding to [Ru(acac)3]
with CH2Cl2/CH3CN (10:1), a second green fraction was eluted
with CH2Cl2/CH3CN (5:1) which corresponded to the diaster-
eomeric mixtures of 2 and 3. Finally, a third purple fraction
corresponding to monomeric complex 1 was eluted with CH2-
Cl2/CH3CN (3:1). The solid complex 1 was obtained via
evaporation of solvent under reduced pressure, yield of 25 mg
(20%). Anal. Calcd for C20H22N4O4Ru: C, 49.68; H, 4.59; N,
11.59. Found: C, 49.71; H, 4.62; N, 11.57. ESI MS (CH2Cl2):
396 (26 100)
305sh
269 (66 400)
847 (39 400)
404 (30 100)
286 (58 100)
287 (60 300)
max(ꢀ) [2+] n.d.
272 (44 600)
785 (26 300)
398 (23 100)
291 (40 000)
274 (53 900)
801 (29 200)
400 (25 000)
288 (47 700)
λ
a In CH3CN/0.1 M Bu4NPF6. Potentials in V vs Cp2Fe+/0
.
b Kc
)
+
E/59mV
d
2
2
10∆
.
c In CH3CN/0.1 M Bu4NPF6 at 4 K. gav ) x[(g1 + g2
2
g3 )/3]. e In CH3CN/0.1 M Bu4NPF6. λ in nm, ꢀ in M-1 cm-1
.
m/z ) 484.26 [M+] (calcd m/z, 484.07). 1H NMR {(CD3)2SO},
δ (J/Hz): 8.51 (d, J ) 6.3 Hz, H6), 8.47 (d, J ) 8.2 Hz, H3),
8.19 (d, J ) 6.4 Hz, H6′), 7.96 (t, J ) 6.7 and 8.1 Hz, H5),
7.84 (t, J ) 6.4 and 7.8 Hz, H4), 7.61 (d, J ) 8.1 Hz, H3′),
7.51 (m, H4′ and H5′), 5.51 (s, CH(acac)), 5.18 (s, CH(acac)),
2.32 (s, CH3(acac)), 1.73 (s, CH3(acac)), 1.71 (s, CH3(acac)),
1.61 (s, CH3(acac)).
Synthesis of {(µ-abcp)[Ru(acac)2]2} (4). The precursor
complex [Ru(acac)2(CH3CN)2] (100 mg, 0.26 mmol) and the
ligand abcp (34 mg, 0.13 mmol) were mixed in 20 mL of
ethanol, and the solution was heated to reflux for 7 h under a
dinitrogen atmosphere. The color of the solution changed from
orange to dark brown. The solvent was removed, and the residue
was subjected to chromatography on a silica gel column. Initially
a red compound corresponding to [Ru(acac)3] was eluted with
CH2Cl2/CH3CN (15:1). With CH2Cl2/CH3CN (4:1) a brown
band was eluted. After evaporation of the solvent under reduced
(18) (a) Kohlmann, S.; Ernst, S.; Kaim, W. Angew. Chem. 1985, 97, 698; Angew.
Chem., Int. Ed. Engl. 1985, 24, 684. (b) Ernst, S. D.; W. Kaim, W. Inorg.
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J.; Duboc, C. Inorg. Chim. Acta 2006, 359, 821. (d) Kaim, W.; Ernst, S.;
Kasack, V. J. Am. Chem. Soc. 1990, 112, 173. (e) Kaim, W.; Kohlmann,
S.; Jordanov, J.; Fenske, D. Z. Anorg. Allg. Chem. 1991, 598/599, 217. (f)
Krejcik, M.; Zalis, S.; Klima, J.; Sykora, D.; Matheis, W.; Klein, A.; Kaim,
W. Inorg. Chem. 1993, 32, 3362. (g) Fees, J.; Hausen, H.-D.; Kaim, W. Z.
Naturforsch. 1995, 50b, 15.
(19) Corral, E.; Hotze, A. C. G.; Tooke, D. M.; Spek, A. L.; Reedijk, J. Inorg.
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2007, 119, 2136; Angew. Chem., Int. Ed. 2007, 46, 2090. (b) Mustroph,
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T.; Nishihara, H. J. Am. Chem. Soc. 2005, 127, 490.
(21) (a) Kobayashi, T.; Nishina, Y.; Shimizu, K. G.; Sato, G. P. Chem. Lett.
1988, 1137. (b) Krejcik, M.; Danek, M.; Hartl, F. J. Electroanal. Chem.
1991, 317, 179.
1
pressure, 4 could be obtained as an analytically pure solid. H
NMR spectroscopy revealed a mixture of the meso and rac
diastereomers (2:1), but these could not be separated even after
careful chromatographic experiments. Yield: 40 mg (18%).
9
3534 J. AM. CHEM. SOC. VOL. 130, NO. 11, 2008

Mixed-Valent Metals Bridged by a Radical Ligand
A R T I C L E S
Figure 1. DFT calculated energy minimum structures of (abpy)Ru(acac)2 (1, left) and (abcp)Ru(acac)2 (right).
Anal. Calcd for C28H32Cl2N6O8Ru2: C, 39.40; H, 3.78; N, 9.84.
Found: C, 39.36; H, 3.81; N, 9.82. ESI MS (CH2Cl2): m/z )
854.02 [M+] (calcd m/z, 852.98). 1H NMR (meso) {(CD3)2SO},
δ (J/Hz): 8.60 (s), 8.59 (s), 8.21 (s), 8.20 (s), 5.60 (s, CH-
(acac)), 5.31 (s, CH(acac)), 2.32 (s, CH3(acac)), 2.13 (s, CH3-
(acac)), 2.06 (s, CH3(acac)), 1.97 (s, CH3(acac)). 1H NMR (rac)
{(CD3)2SO}, δ (J/Hz): 8.61 (s), 8.596 (s), 8.23 (s), 8.22 (s),
5.55 (s, CH(acac)), 5.21 (s, CH(acac)), 2.30 (s, CH3(acac)), 2.12
(s, CH3(acac)), 1.98 (s, CH3(acac)), 1.94 (s, CH3(acac)).
Computational Details. Full geometry optimizations of the
complexes were carried out using the density functional theory
method at the B3LYP level.22 All atoms except ruthenium were
assigned a 6-31G* basis set. The SDD basis set with effective
core potential was employed for the Ru atoms.23 Calculations
were performed with Gaussian98 and Gaussian03 program
packages.24 Optimized geometries were then used for the Natural
Bond Orbital (NBO) analysis with the NBO 3.1 program25 as
implemented in the Gaussian suite. Vertical electronic excita-
tions based on B3LYP optimized geometries were computed
using the time-dependent density functional theory (TD-DFT)
formalism26 with the B3LYP functional using the above
combination of basis sets. Visual inspection of key orbitals was
done with MOLDEN27 to assign the nature of various electronic
transitions. Molecular orbital compositions were analyzed using
the AOMIX program.28
conventional18b,d,f electron transfer behavior: The one-electron
oxidation produces a ruthenium(III) species as evident from the
typical29 EPR response for a low-spin 4d5 system (Table 1) with
g1,g2 > 2 and g3 < 2 in frozen solution while the first one-
electron reduction leads to a g ≈ 2.00 signal, signifying an
abpy•- radical anion complex30 of ruthenium(II).18d These
experimental results are supported by DFT calculations which
yield a metal centered HOMO (62% Ru) and an abpy centered
LUMO (74% abpy, Table S1). Accordingly, the observed
absorptions in the visible (Table 1) are assigned to MLCT
transitions (570, 464, 413 nm) and are reproduced by TD-DFT
calculations (516, 427, 402 nm; Table S2), the difference being
attributed to solvent effects. In comparison18b,f to [(abpy)Ru-
(bpy)2]2+ the redox potentials from cyclic voltammetry illustrate
that the replacement of the bpy acceptors by acac- donors causes
shifts to more negative potentials by 0.6-0.8 V for reduction
and by 1.1 V for the oxidation. Spectroelectrochemistry in the
UV-vis region shows the emergence of absorptions at lower
energies on reduction and oxidation, the assignment of which
as based on related systems18f will be discussed in connection
with the dinuclear species.
In the absence of single crystalline material, DFT calculations
of (abpy)Ru(acac)2 and of the analogous (abcp)Ru(acac)2 yield
twisted conformations as optimized geometries (Figure 1, Table
S3) with shorter Ru-Nazo than Ru-Npy bonds (Npy ) hetero-
cyclic N).18g,19 The NNCN torsional angles reflecting the
twisting between chelating and free heterocycles amount to 46.6°
(abpy complex 1) and 57.3° (abcp analogue) while the shorter
Ru-Nazo bonds illustrate the stronger d(Ru) f π* back-donation
to the stronger π accepting function.18 At about 1.30 Å the
calculated NdN bonds are rather long but still more similar to
those (e1.304 Å) of unreduced ruthenium(II),18g,19 molybdenum-
(0),18e copper(I),18e or rhenium(I)17 compounds than that of EPR-
certified copper(I) radical anion complexes of abcp (1.345(7)
Å)9 or abpy (1.317(1) Å)31 or calculated rhenium(I) complexes
of abpy or abcp (≈1.36 Å).16 These results suggest a strong
amount of π back-donation from one RuII(acac)2 complex
Results and Discussion
Mononuclear Reference Systems. For reference purposes
in relation to the dinuclear complexes 2 and 3 we isolated the
mononuclear (abpy)Ru(acac)2 (1). This complex exhibits
(22) (a) Becke, A. D. Phys. ReV. A 1988, 38, 3098. (b) Lee, C.; Yang, W.; Parr,
R. G. Phys. ReV. B 1988, 37, 785.
(23) (a) Andrae, D.; Ha¨uâermann, U.; Dolg, M.; Stoll, H.; Preuâ, H. Theor.
Chim. Acta 1990, 77, 123. (b) Fuentealba, P.; Preuss, H.; Stoll, H.;
Szentpa´ly, L. v. Chem. Phys. Lett. 1982, 89, 418.
(24) (a) Frisch, M. J. et al. Gaussian 98, revision A.11.4; Gaussian, Inc.:
Pittsburgh, PA, 2001 (b) Frisch, M. J. et al. Gaussian 03, revision C.02;
Gaussian, Inc.: Wallingford, CT, 2004.
(25) (a) Glendening, E. D.; Reed, A. E.; Carpenter, J. E.; Weinhold, F. NBO,
version 3.1. (b) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988,
88, 899.
(26) Casida, M. In Recent AdVances in Density Functional Methods; Chong,
D. P., Ed.; World Scientific Press: Singapore, 1995; Vol. I, p 155.
(27) Schaftenaar, G.; Noordik, J. H. J. Comput.-Aided Mol. Des. 2000, 14, 123.
(28) (a) Gorelsky, S. I. AOMIX program, http://www.sg-chem.net/. (b) Gorelsky,
S. I.; Lever, A. P. B. J. Organomet. Chem. 2001, 635, 187.
(29) Hariram, R.; Santra, B. K.; Lahiri, G. K. J. Organomet. Chem. 1997, 540,
155.
(30) Kaim, W. Coord. Chem. ReV. 1987, 76, 187.
(31) Roy, S.; Sarkar, B:, Lissner, F.; Schleid, T.; Lahiri, G. K.; Duboc, C.;
Fiedler, J.; Kaim, W. Unpublished results.
9
J. AM. CHEM. SOC. VOL. 130, NO. 11, 2008 3535

A R T I C L E S
Sarkar et al.
Figure 2. Experimentally determined configurations of complexes 2 (top, two crystallographically independent molecules found, ref 12) and 3 (bottom).
isomers of (µ-abpy)[Ru(acac)2]2 were performed for the lowest
singlet and triplet ground states, the values for the singlet forms
showing excellent agreement with the experimental parameters
for diamagnetic 2 and 3 (Table 2).12 The calculations for (µ-
abcp)[Ru(acac)2]2 gave slightly longer NN and shorter C-NN
and Ru-N bonds, confirming a more pronounced transfer of
negative charge to the bridge. From the excellent structural
agreements for 2 and 3 we conclude that the other calculated
properties, e.g., orbital energies and orbital compositions as well
as transition energies are also reproducing the electronic situation
in a proper way.
The experimental and calculated NN distances for the meso
and rac forms (2,3) of complex (µ-abpy)[Ru(acac)2]2 lie in the
range corresponding to the anion radical state, i.e., about
halfway33 between the values for unreduced azo NdN bonds16-19
and those of 2e-reduced NN, i.e., hydrazido(2-) bonds32
(Scheme 7).
Figure 3. Cyclic voltammogram of 4 in CH3CN/0.1 M Bu4NPF6 at 298 K
(100 mV/s scan rate).
fragment into the low lying π* orbitals of abpy or abcp yet not
sufficient for complete reduction.
Dinuclear Compounds: Structures. The two structurally
characterized12 isomers, meso (2) and rac (3), of (µ-abpy)[Ru-
(acac)2]2 (Figure 2) and the isomer mixture 4 of (µ-abcp)[Ru-
(acac)2]2 were characterized by 1H NMR and were subjected to
cyclic voltammetry (Table 1, Figure 3), EPR and UV-vis-
NIR spectroelectrochemistry (Table 1, Figures 4-7) and were
calculated using DFT and TD-DFT methodology (Tables 2-8,
S4-S10, Figures 8 and 9). Geometry optimizations for both
In particular, the structural response of azopyridine bis-chelate
ligands such as abpy and abcp on electron addition has been
(32) (a) Marabella, C. P.; Enemark, J. H.; Newton, W. E.; McDonald, J. W.
Inorg. Chem. 1982, 21, 623. (b) Ittel, S. D.; Ibers, J. A. Inorg. Chem. 1973,
12, 2290. (c) Mun˜iz, K.; Nieger, M. Angew. Chem. 2006, 118, 2363; Angew.
Chem., Int. Ed. 2006, 45, 2305.
(33) Shivakumar, M.; Pramanik, K.; Ghosh, P.; Chakravorty, A. Inorg. Chem.
1998, 37, 5968.
9
3536 J. AM. CHEM. SOC. VOL. 130, NO. 11, 2008

Mixed-Valent Metals Bridged by a Radical Ligand
A R T I C L E S
Figure 5. UV-vis-NIR spectroelectrochemical response of 1 on oxidation
(top) and reduction (two steps, bottom) in CH3CN/0.1 M Bu4NPF6.
of the CusN distances from about 2.10 Å for {(µ-abpy)[Cu-
(PPh3)2]2}(BF4)2 to ca. 2.05 Å for {(µ-abpy)[Cu(dppf)]2}(BF4)
also reflects the enhanced charge-based attraction between
cationic copper(I) and the now monoanionic ligand bridge.
X-band (9.5 GHz) and high-field EPR at 285 GHz have clearly
confirmed the abpy•-/(CuI)2 oxidation state situation for the one-
electron reduced paramagnetic complexes.9,31
Using the bis(diphosphinocopper(I)) complexes9,18e,31 as an
unambiguous standard, the amount of (ruthenium)-to-(azo
ligand) electron transfer which causes the structural changes as
observed experimentally (2,3) and as calculated for 2-4 (Table
2) corresponds to about one electron. Tables 3 and 4 contain
calculated structures, charges, and orbital occupancies which
confirm this notion.
Figure 4. EPR spectra of 2•-, 3•-, 2•+, and 3•+ at 4 K in CH3CN/0.1 M
Bu4NPF6 (from top to bottom).
well documented for dinuclear bis(phosphino)copper(I) com-
plexes: Whereas {(µ-abpy)[Cu(PPh3)2]2}(BF4)2 has a typical
azo ligand structure pattern with d(NdN) ) 1.25 Å and d(Cs
NN) ) 1.45 Å,18e the one-electron reduced species {(µ-abpy)-
[Cu(dppf)]2}(BF4), dppf ) 1,1′-bis(diphenylphosphino)fer-
rocene, has d(NdN) ) 1.32 Å and d(CsNN) ) 1.38 Å31 and
{(µ-abcp)[Cu(PPh3)2]2}(PF6) has d(NdN) ) 1.35 Å and d(Cs
NN) ) 1.36 Å (averaged/rounded values).9 These bond lengths
are not compatible, on the other hand, with an abpy2- formula-
tion which would imply a NsN single bond at >1.40 Å as
found for hydrazido(2-) complexes.11,32 While the increase of
d(NdN) and the decrease of d(CsNN) within the ligand are
expected for a reduced abpy or abcp molecule (Table 3)16 or
for the related mononucleating 2-phenylazopyridine radical
anion (d(NdN) ) 1.333(7) Å in a RuII complex33), the decrease
The DFT calculated values differ only slightly from the
experimental data which provides confidence in the computa-
tional results. Moreover, the bond distances vary only marginally
between the meso(2) and rac(3) configurated isomers, indicating
that the bond length structural manifestation of the metal-
9
J. AM. CHEM. SOC. VOL. 130, NO. 11, 2008 3537

A R T I C L E S
Sarkar et al.
Figure 7. UV-vis-NIR spectroelectrochemical response of 3 on stepwise
oxidation (top) and reduction (bottom) in CH3CN/0.1 M Bu4NPF6.
anion radical abpy•- helps to lower barriers for torsional
movements and thus allows greater susceptibility for twisting.
The validity of the calculations was further established by
TD-DFT computed transition energies for complexes 2-4
(Tables 6-8). The values obtained for intense transitions at
about 800 nm agree well with the observed band maxima at
ca. 900 nm (Table 1, cf. Figures 6, 7), suggesting the suitability
of the calculations for these compounds. According to TD-DFT
(Tables 6-8) the intense transitions are from filled Ru(acac)2-
based orbitals to azo π*/Ru-centered unoccupied MOs, indicat-
ing a mixed IVCT/MLCT type transition (IVCT ) intervalence
charge transfer); typical delocalized frontier MOs are depicted
Figure 6. UV-vis-NIR spectroelectrochemical response of 2 on stepwise
oxidation (top) and reduction (bottom) in CH3CN/0.1 M Bu4NPF6.
ligand-metal interaction is not too sensitive regarding this kind
of isomerism.
However, an interesting aspect is the experimental variability
of the twisting of the central ligand which is different between
the meso and rac isomers but also within the crystal of 2.12
This result and the lesser agreement with the calculated twisting
(Table 2) suggest a fairly shallow energy surface for this
structural mode, depending largely on intermolecular influences
(packing forces). The equilibration of NdN double bonds and
CsNN single bonds on reduction to a ligand describable as
9
3538 J. AM. CHEM. SOC. VOL. 130, NO. 11, 2008

Mixed-Valent Metals Bridged by a Radical Ligand
A R T I C L E S
Table 2. Essential Structure Parameters of Complexes from
Table 5. Kohn-Sham Orbital Energies and Corresponding Orbital
Compositions for Isomeric Complexes 2 (meso) and 3 (rac)
Obtained at the B3LYP/6-31G* Level with Optimized Singlet
Ground States
Experimenta and DFT Calculationsb
1
2d
3
4e
Bond Lengths (Å)
1.339 (1.363)
1.375/1.438 1.391 (1.398)
percentage
NsN
CsN(tN)
RusNazo
1.300
1.344 (1.372) 1.352
1.390 (1.390) 1.381
2.014 (1.959) 2.005
2.027 (2.008) 2.015
orbital energy
cm-
contributions
1
orbital
au
Ru
abpy
acac
1.963
2.041
2.029 (1.973)
2.021 (2.008)
c
RusNpy
Compound 2
-6500
173 (LUMO+2)
172 (LUMO+1)
171 (LUMO)
170 (HOMO)
169 (HOMO-1)
168 (HOMO-2)
-0.029
-0.031
-0.098
-0.162
-0.169
-0.174
3.9
4.8
32.1
72.2
64.7
67.0
71.9
85.6
62.1
4.8
5.1
9.9
24.2
9.6
5.9
23.0
30.2
23.0
Torsional Angles (deg)
-6900
CsNsNsC 1.6
0.0 (15.5/0.0) d 18.3 (26.1)
16.9
-21 600
-35 600
-37 100
-38 200
a Averaged values in parentheses, from ref 12. b For singlet ground states
obtained at the B3LYP/SDD/6-31G* level of calculations. c py: 2-pyridine
(abpy) or 2-(5-chloropyrimidine) (abcp). d Two crystallographically inde-
pendent molecules in the crystal (ref 12). e meso configuration.
Compound 3
-6700
173 (LUMO+2)
172 (LUMO+1)
171 (LUMO)
170 (HOMO)
169 (HOMO-1)
168 (HOMO-2)
-0.031
-0.032
-0.099
-0.160
-0.171
-0.176
3.6
4.5
33.6
70.9
65.5
68.0
66.2
84.0
60.3
6.2
5.2
8.8
30.3
11.5
6.1
22.9
29.4
23.2
Table 3. Selected Structure Parameters and NBO Results for the
-7100
Free and Coordinated abpy Liganda
-21 700
-35 100
-37 500
-38 500
parameter
abpy
abpy•-
abpy in complex 3
Bond Lengths (Å)
N2-N3
N3-C16
N4-C16
1.26
1.43
1.34
1.33
1.37
1.37
1.34 (1.37)b
1.39 (1.39)
1.36 (1.36)
Having established the approximate one-electron reduction
of the bridging ligand through experimental structure/oxidation
state correlations and through DFT calculations the question of
spin coupling needs to be addressed. The molecules 2-4 are
all diamagnetic as evident from susceptibility measurements;
Dihedral Angle (deg)
C15-N2-N3-C16
1.5
0.1
18.3 (26.1)
NBO analysisc
abpy
abpy•-
abpy in complex 3
Bond Order
N2-N3
1.78
1.35
1.30
1
they exhibit H NMR features without unusual shifts or line
Occupancy
1.90
0.16
broadening and no EPR signals.
π(N2-N3)
0.97
0.70
1.88
0.73
The interaction between a mixed-valent dimetal arrangement,
valence-averaged or not, and an anion radical ligand creates
the possibility of singlet or triplet configurations. While the
experimental evidence suggests the singlet state as the ground
state for 2-4, we used the DFT method to confirm this result
and to calculate the energy and properties of the triplet
configurations (Tables S5, S7-S10). The triplet states are
calculated to lie 1535 cm-1 (2) or 840 cm-1 (3) above the
corresponding singlet ground states, and their computed struc-
tures, especially the lengthened NN bonds,12 reflect decreased
NN bonding for the triplet following from a depopulated HOMO
(Table 4). The preference for a singlet ground state in such
situations may not always be so pronounced; strong interaction
between two abpy or abcp bridged metal centers must be favored
here because these “S-frame” ligands8 induce a rather short
metal-metal distance of less than 5 Å despite the molecular
nature of the bridge.12 Accordingly, when applying the con-
ventional formalism to calculate the comproportionation constant
Kc (RT ‚ ln Kc ) nF ‚ ∆E) to the isolated neutral states, one
arrives at large values of about 1017 (Table 1) which are not so
common in diruthenium mixed-valence chemistry.13
π*(N2-N3)
Charges
-0.18
-0.27
NPA (N2,N3)
Mulliken (N2,N3)
-0.29
-0.39
-0.19
-0.36
a Structure parameters are from B3LYP/6-31G* optimized geometries.
b Values in parentheses from experimental X-ray diffraction analysis. c NBO
analysis was performed at the B3LYP/6-31G* level using the NBO 3.1
program as implemented in Gaussian 03.
Table 4. Electron Occupancy of NdN π Orbitals Using NBO
Analysis at the B3LYP/SDD/6-31G* Level
compound
πN
d
π*
Nd
N
N
1
1.89
1.88
1.87
0.92
1.90
a
0.50
0.73
0.73
0.70
0.16
b
2 (singlet)
3 (singlet)
3 (triplet)
abpy
abpy•-
a LP1(N1,N2) ) 0.97. LP2(N1,N2) ) 0.70.
b
in Figure 8. Reduction and oxidation described below shed
further light on the composition of HOMO and LUMO.
Dinuclear Compounds: Two-Step Oxidation and Reduc-
tion. The opportunity of having two fully reversible one-electron
oxidation and reduction processes available for spectroelectro-
chemical studies (UV-vis-NIR, EPR) provides valuable ad-
ditional information on the electronic situation in complexes
2-4 and their neighboring charged states. Before spectroelec-
trochemical characterization one may note that the potentials
are practically identical for the two disastereoisomers 2 and 3
despite different solid-state structures (Figure 2);12 the potentials
are slightly anodically shifted for system 4 with the better π
acceptor bridge (Table 1).
The net transfer of one electron equivalent from the metal(s)
to the bridging ligand creates a mixed-valent situation with a
formally radical anionic spacer in the electronic ground state.
In principle, the mixed valency can result in an unsymmetrical
situation RuIII(L•-)RuII with different valences or in a sym-
metrical alternative Ru2.5(L•-)Ru2.5 with averaged net metal
valences. Such electronic arrangements have been referred to
before for metal-to-ligand charge transfer (MLCT) excited states
(Schemes 6 and 8) of symmetrically dinuclear complexes
bridged by π acceptor ligands;14a-c excited-state mixed valency
has recently received attention for mononuclear ruthenium
containing systems which involve noninnocent ligands.14d
For the EPR spectroscopically accessible monoanions and
monocations the shift and anisotropy of the g factors in a frozen
9
J. AM. CHEM. SOC. VOL. 130, NO. 11, 2008 3539

A R T I C L E S
Sarkar et al.
Table 6. Selected List of Vertical Excitations Computed at the TD-DFT/B3LYP//B3LYP/SDD/6-31G* Level for 2
excitation
energya
oscillator
strength
1
(103 cm-
)
ꢀc
ψo − ψvd
type of transition
6.950
0.0036
0.2146
0.0044
0.0051
0.0203
250
HOMO f LUMO (0.54)e
dπ (Ru1,Ru2), π(acac) f
π*(abpy), dπ (Ru1,Ru2)
dπ (Ru1,Ru2), π(acac) f
π*(abpy), dπ (Ru1,Ru2)
(1440)b
13.850
(722)
15070
310
HOMO-2 f LUMO (0.50)
HOMO-6 f LUMO (0.69)
HOMO-8 f LUMO (0.67)
HOMO f LUMO+2 (0.63)
20.610
(485)
22.430
(446)
23.070
π(acac) f π*(abpy), dπ (Ru1,Ru2)
360
π(acac), dπ (Ru1,Ru2) f
π*(abpy), dπ (Ru1,Ru2)
dπ (Ru1,Ru2), π(acac) f
π*(abpy), π*(acac)
1430
(434)
b
c
d
e
a Singlet excitation energies. Values in nm given in parentheses. ꢀ in /M-1 cm-1. Occupied and virtual orbitals. Transition coefficients.
Table 7. Selected List of Vertical Excitations Computed at the TD-DFT/B3LYP//B3LYP/SDD/6-31G* Level for 3
excitation
energya
oscillator
strength
1
(103 cm-
)
ꢀc
ψo − ψvd
type of transition
6.980
0.0065
0.1910
0.0090
0.0072
0.0061
0.0123
460
HOMO f LUMO (0.51)e
dπ (Ru1,Ru2), π(acac) f
π*(abpy), dπ (Ru1,Ru2)
dπ (Ru1,Ru2), π(acac) f
p*(abpy), dπ (Ru1,Ru2)
π(acac), π(abpy), dπ(Ru1,Ru2) f
π*(abpy), dπ (Ru1,Ru2)
π (acac), dπ (Ru1,Ru2) f
π*(abpy), dπ (Ru1,Ru2)
π (acac), dπ (Ru1,Ru2) f
π*(abpy), dπ (Ru1,Ru2)
dπ (Ru1,Ru2), π(acac) f
p*(abpy), π*(acac)
(1432)b
13.720
(729)
13410
630
HOMO-2 f LUMO (0.47)
HOMO-5 f LUMO (0.64)
HOMO-6 f LUMO (0.69)
HOMO-8 f LUMO (0.67)
HOMO f LUMO+2 (0.65)
18.420
(543)
20.590
(486)
22.360
(447)
22.640
(442)
510
430
860
b
c
d
e
a Singlet excitation energies. Values in nm given in parentheses. ꢀ in /M-1 cm-1. Occupied and virtual orbitals. Transition coefficients.
Table 8. Selected List of Vertical Excitations Computed at the TD-DFT/B3LYP//B3LYP/SDD/6-31G* Level for 4
excitation
energya
oscillator
strength
1
(103 cm-
)
ꢀc
170
ψo − ψvd
type of transition
6.170
(1621)b
0.0024
HOMO-2 f LUMO (0.46)e
HOMO-1 f LUMO (0.42)
dπ(Ru2), π(acac), dπ(Ru1) f
π*(abcp), dπ(Ru1,Ru2) dπ(Ru1),
π(acac), dπ(Ru2), π(abcp) f
π*(abcp), dπ(Ru1,Ru2)
dπ(Ru2), π(acac), dπ(Ru1) f
π*(abcp), dπ(Ru1,Ru2)
π(acac), π(abcp), dπ(Ru1) f
π*(abcp), dπ(Ru1,Ru2)
π(acac), dπ(Ru1) f π*(abcp),
dπ(Ru1,Ru2)
dπ(Ru1), dπ(Ru2), π(acac) f π*(abcp)
12.180
(821)
16.030
(624)
17.390
(575)
18.790
(532)
20.440
(489)
0.1757
0.0109
0.0080
0.0067
0.0090
12 340
770
HOMO-2 f LUMO (0.41)
HOMO-5 f LUMO (0.65)
HOMO-6 f LUMO (0.64)
HOMO f LUMO+1 (0.62)
560
470
630
HOMO-1 f LUMO+2 (0.43)
HOMO-1 f LUMO+1 (0.37)
dπ(Ru1), π(acac), dπ(Ru2),
π(abcp) f π*(abcp) dπ(Ru1), π(acac),
dπ(Ru2), π(abcp) f π*(abcp)
dπ(Ru1), π(acac), dπ(Ru2),
π(abcp) f π*(abcp)
π(acac), dπ (Ru2), π (abcp) f
π(abcp), dπ(Ru1,Ru2)
20.760
(482)
21.250
0.0062
0.0065
440
460
HOMO-1 f LUMO+1 (0.53)
HOMO-9 f LUMO (0.46)
(471)
b
c
d
e
a Singlet excitation energies. Values in nm given in parentheses. ꢀ in /M-1 cm-1. Occupied and virtual orbitals. Transition coefficients.
solution proved to be elucidating. Both 2•- and 3•- exhibit no
such g component splitting in the X band (9.5 GHz), and the
value of 1.991, very slightly below 2, is typical for ruthenium-
(II) complexes of anion radical ligands.18d The oxidation state
description RuII(L•-)RuII is thus considered most appropriate.
For 4•- the g factor components are clearly split (Table 1),
suggesting some detectable but minor contribution from the
alternative mixed-valent formulation Ru2.5(L2-)Ru2.5 (Scheme
9, valence averaging) with participation from the heavy metals
with their considerable spin-orbit coupling constant. However,
all three systems 2•--4•- are distinguished by intense NIR
absorptions which cannot be due to any IVCT transition but to
MLCT transitions from the ruthenium(II) donors to the still π
accepting anion radical bridge.11,34 Notably, this feature is absent
for the mononuclear compound 1•- and is hypsochromically
shifted for 4•- with the more pronounced metal participation
9
3540 J. AM. CHEM. SOC. VOL. 130, NO. 11, 2008

Mixed-Valent Metals Bridged by a Radical Ligand
A R T I C L E S
Figure 8. DFT calculated frontier orbitals for compounds 2 and 4.
Scheme 7
Scheme 8
possibly because of different structures as established already
for the neutral precursors (Figure 2).12 Otherwise, the UV/vis/
NIR spectroelectrochemical response of the isomers 2 and 3 is
small (Figures 6, 7). Unfortunately, the obviously metal centered
spin for the monocations is compatible with two alternative
descriptions, starting from the neutral forms: Oxidation of the
bridging ligand would produce a classical mixed-valent situation
Ru2.5(L0)Ru2.5 with an unreduced nonradical bridge while a
metal-based oxidation would create a three-spin35 arrangement
RuIII(L•-)RuIII (low-spin ruthenium(III)). Assuming dominant
metal-ligand antiferromagnetic interactions the resulting up-
down-up arrangement for the spins35 would leave one net
metal-centered spin, just as in the previous alternative. Support
Figure 9. DFT calculated MO energy diagrams for compounds 2 and 4.
as evident from the EPR results. Further reduction can only
lead to the RuII(L2-)RuII state which exhibits intense charge-
transfer bands in the visible around 600 and 430 nm, tentatively
attributed to MLCT or LLCT with π*(acac) as the target MO.
The more pronounced bridging ligand character of the LUMOs
has been calculated by DFT (cf. Table 5, Figure 8).
One-electron oxidation of compounds 2-4 produces EPR
spectra with pronounced g factor shifts and g anisotropies. The
latter, a very sensitive probe of the electronic structure, shows
a clear difference between the two diastereoisomers 2+ and 3+,
(35) Ye, S.; Sarkar, B.; Lissner, F.; Schleid, Th.; van Slageren, J.; Fiedler, J.;
Kaim, W. Angew. Chem. 2005, 117, 2140; Angew. Chem., Int. Ed. 2005,
44, 2103.
(34) Ernst, S.; Ha¨nel, P.; Jordanov, J.; Kaim, W.; Kasack, V.; Roth, E. J. Am.
Chem. Soc. 1989, 111, 1733.
9
J. AM. CHEM. SOC. VOL. 130, NO. 11, 2008 3541

A R T I C L E S
Scheme 9
Sarkar et al.
and computationally established amount of charge shift, corre-
sponding to about one electron, formally creates an unfamiliar
situation involving mixed-valency, expounded systematically in
1967,37 and a bridging ligand existing in anion radical form, as
first reviewed in 1987.30 In this way, a configuration used to
describe excited states (Scheme 6) has been obtained as a ground
state species, just as mononuclear MLCT excited state-type
configurations were established earlier38 for ground state species.
The consequences of this uncommon arrangement are structural
flexibility as indicated for both meso and rac diastereoisomers
studied with L ) abpy, intense long-wavelength absorptions
ascribed to mixed IVCT/MLCT transitions, and relatively small
singlet-triplet gaps as calculated by DFT. Reversible oxidation
and reduction processes with spectroelectrochemical character-
ization of the charged species confirm a more pronounced ligand
contribution to the LUMO than to the HOMO.
Viewed in connection with previously and currently practiced
structure/oxidation state correlations in coordination chemistry
involving noninnocently behaving ligands (Schemes 1-5) the
present results suggest that, at least for strongly mixing systems
such as ruthenium compounds or S-ligand containing com-
plexes,39 that approach can be seen as a formal exercise which
may provide some insight into the electronic structure without
warranting a too literal significance of the valence denomination.
for one of the alternatives from UV-vis-NIR spectroelectro-
chemistry appears in the form of the absence of detectable NIR
absorption which seems to rule out the former, mixed-valent
formulation. However, there are dinuclear and trinuclear mixed-
valent complexes of Ru(acac)2, bridged by neutral acceptor
ligands, which do not exhibit detectable IVCT absorption in
the NIR region.36 We thus cannot assign unambiguously an
oxidation state formulation to the monocationic states. On
second oxidation we assume the formation of RuIII(L0)RuIII
compounds with intense LMCT absorptions around 800 nm
(Figures 6 and 7).
Acknowledgment. Financial support from the Deutsche
Akademische Austauschdienst, Fonds der Chemischen Industrie,
Deutsche Forschungsgemeinschaft (Germany), the COST D35
program (EU) and from the Department of Science and
Technology (New Delhi, India) is gratefully acknowledged.
Computing facilities from the IIT Bombay computer center are
also acknowledged.
Conclusion
Using two familiar kinds of components in coordination
chemistry, viz., ruthenium(II) donors and “polypyridine”-type
acceptor ligands, we have been able to show the consequences
of excessive donor-to-acceptor transfer of negative charge in
corresponding dinuclear complexes (acac)2Ru(µ-L)Ru(acac)2,
L ) abpy or abcp. The experimentally (structural, spectroscopic)
Supporting Information Available: Complete ref 24; Tables
S1-S10 of DFT and TD-DFT calculation results. This material
is available free of charge via the Internet at http://pubs.acs.org.
JA077676F
(36) (a) Patra, S.; Sarkar, B.; Ghumaan, S.; Fiedler, J.; Kaim, W.; Lahiri, G. K.
Dalton Trans. 2004, 754. (b) Ghumaan, S.; Sarkar, B.; Patil, M. P.; Fiedler,
J.; Sunoj, R. B.; Kaim, W.; Lahiri, G. K. Polyhedron 2007, 26, 3409. (c)
Patra, S.; Sarkar, B.; Ghumaan, S.; Fiedler, J.; Kaim, W.; Lahiri, G. K.
Inorg. Chem. 2004, 43, 6108.
(37) Robin, M. B.; Day, P. AdV. Inorg. Chem. Radiochem. 1967, 10, 247.
(38) Kaim, W.; Reinhardt, R.; Sieger, M. Inorg. Chem. 1994, 33, 4453.
(39) Kapre, R. R.; Bothe, E.; Weyhermu¨ller, T.; DeBeer, George, S.; Muresan,
N.; Wieghardt, K. Inorg. Chem. 2007, 46, 7827.
9
3542 J. AM. CHEM. SOC. VOL. 130, NO. 11, 2008