[Cp*Ru(s-indacene)RuCp*] and [Cp*Ru(s-indacene)RuCp*]+: Experimental and theoretical findings concerning the electronic structure of neutral and mixed valence organometallic systems

Article abstract of DOI:10.1016/j.poly.2009.12.002

The reaction of 2,6-diethyl-4,8-dimethyl-s-indacenyl-dilithium (Li₂Ic′) with [Cp*RuCl]₄ gives the organometallic binuclear bis-pentamethylcyclopentadienyl-ruthenium-s-indacene complex, [{Cp*Ru}₂Ic′] (1, Ic′ = 2,4-diethyl-4,8-dimethyl-s-indacene), in high yields. The subsequent oxidation of 1 with a ferricinium salt ([Fc]⁺[BF₄]^-) gives the mixed valence compound [{Cp*Ru}₂Ic′]⁺[BF₄]^- (1⁺). Compound 1 was structurally characterized by X-ray crystallography, finding that both {Cp*Ru} fragments are coordinated to opposite sites of the Ic′ ligand. The structural and electronic features of 1 and 1⁺ have been rationalized by Density Functional Theory (DFT) calculations, which suggest that both metallic centers get closer to the Ic′ and subtle electronic reorganizations occurs when chemical oxidation takes place. Cyclic voltammetry and ESR experiments suggest a high electronic interaction between the metallic centers mediated by the Ic′ bridging ligand. Time dependent DFT (TD-DFT) calculations were carried out to understand and assign the intervalence band present in the mixed-valent specie (1⁺). The main achievement of this article is to feature the relationship of the experimental data with the computational results obtained with the Amsterdam Density Functional package (ADF). Both experimental and theoretical facts demonstrate that the mixed valence system (1⁺) is a delocalized one, and it can be classified as a Class III system according to the Robin & Day classification.

Full text of DOI:10.1016/j.poly.2009.12.002

Polyhedron 29 (2010) 1137–1143
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[Cp*Ru(s-indacene)RuCp*] and [Cp*Ru(s-indacene)RuCp*]+: Experimental and
theoretical findings concerning the electronic structure of neutral and mixed
valence organometallic systems
D. Mac-Leod Carey b, C. Morales-Verdejo a, A. Muñoz-Castro b, F. Burgos c, D. Abril d, C. Adams a, E. Molins e,
b
f
O. Cador f, I. Chávez a, J.M. Manríquez a, , R. Arratia-Pérez , J.Y. Saillard
*
a Facultad de Química, Pontificia Universidad Católica de Chile, Avda. Vicuña Mackenna 4860, Santiago, Chile
b Departamento de Ciencias Químicas, Universidad Andrés Bello, República 275, Santiago, Chile
c Facultad de Ciencias Forestales, Universidad Austral de Chile, Avda. Los Laureles s/n, Valdivia, Chile
d Instituto de Ciencias Básicas, Universidad Católica del Maule, Avda. San Miguel 3605, Talca, Chile
e Institut de Ciencia de Materials de Barcelona (CSIC), Campus UAB, 08193 Bellaterra, Spain
f Laboratoire de Chimie du Solide et Inorganique Moléculaire UMR CNRS 6511, Institut de Chimie de Rennes, Université de Rennes 1, 35042 Rennes Cedex, France
a r t i c l e i n f o
a b s t r a c t
The reaction of 2,6-diethyl-4,8-dimethyl-s-indacenyl-dilithium (Li2Ic0) with [Cp*RuCl]4 gives the organo-
metallic binuclear bis-pentamethylcyclopentadienyl-ruthenium-s-indacene complex, [{Cp*Ru}2Ic0] (1,
Ic0 = 2,4-diethyl-4,8-dimethyl-s-indacene), in high yields. The subsequent oxidation of 1 with a ferricini-
um salt ([Fc]+[BF4]ꢀ) gives the mixed valence compound [{Cp*Ru}2Ic0]+[BF4]ꢀ (1+). Compound 1 was struc-
turally characterized by X-ray crystallography, finding that both {Cp*Ru} fragments are coordinated to
opposite sites of the Ic0 ligand. The structural and electronic features of 1 and 1+ have been rationalized
by Density Functional Theory (DFT) calculations, which suggest that both metallic centers get closer to
the Ic0 and subtle electronic reorganizations occurs when chemical oxidation takes place. Cyclic voltam-
metry and ESR experiments suggest a high electronic interaction between the metallic centers mediated
by the Ic0 bridging ligand. Time dependent DFT (TD-DFT) calculations were carried out to understand and
assign the intervalence band present in the mixed-valent specie (1+). The main achievement of this article
is to feature the relationship of the experimental data with the computational results obtained with the
Amsterdam Density Functional package (ADF). Both experimental and theoretical facts demonstrate that
the mixed valence system (1+) is a delocalized one, and it can be classified as a Class III system according
to the Robin & Day classification.
Article history:
Received 24 June 2009
Accepted 2 December 2009
Available online 5 December 2009
Keywords:
s-Indacene
Ruthenium
Organometallic
Electron transfer
Mixed valence
DFT
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
Cp*MII– (M = Fe, Ru, Co, Ni) and (COD)RhI-metallic fragments [9]
has been reported.
Organometallic polynuclear compounds bridged by spacer
ligands provide an interesting research area, due to the possibility
of dealing with metallic centers which may communicate depend-
ing on the nature of the ligand, and of the chosen metallic fragments
[1,2]. In the particular case of s- and as-indacene spacer molecules,
several articles have been published describing the synthetic proce-
dure using metallic fragments such as Cp*MII– (M = Fe, Co, Ni)
(Cp* = pentamethylcyclopentadienyl) [3], (CO)3MI (M = Fe,Co)
[4,5], (COD)RhI– (COD = 1,5-cyclooctadiene) [6], amongst others
[7,8] to produce homobimetallic systems. Also a recent stepwise
procedure to synthesize heterobimetallic systems containing
To date, the only known theoretical calculations describing the
electronic structure of anti-[{CpM}2(Ic)] diamagnetic complexes
(Cp = cyclopentadienyl; M = Fe, Co, Ni; Ic = s-indacene) were done
at the extended Hückel level [10].
It has been experimentally demonstrated that these type of sys-
tems present a high degree of interaction between the metallic
centers, but a detailed study which may explain the electronic
mechanism that allows this interaction, has not been reported
for anti-[{CpM}2(Ic)] systems, i.e. both CpM fragments at opposite
sides of the Ic ligand.
The first concern of this article is to report the synthesis of a
ruthenium binuclear organometallic system anti-[{Cp*Ru}2Ic0]
(1, Ic0 = 2,4-diethyl-4,8-dimethyl-s-indacene) and its correspond-
ing mixed-valent derivative anti-[{Cp*Ru}2Ic0]+[BF4]ꢀ (1+), together
with the structural characterization by means of single-crystal
* Corresponding author. Tel.: +56 2 6864393.
E-mail address: jmanriqm@uc.cl (J.M. Manríquez).
0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2009.12.002

1138
D. Mac-Leod Carey et al. / Polyhedron 29 (2010) 1137–1143
X-ray diffraction, electrochemical analysis, spectroscopical studies
and theoretical calculations at DFT level of theory. The degree of
communication that exists between the two metallic centers
through the Ic0 bridging ligand was studied, engaging all the exper-
imental information with the theoretical analysis in order to ratio-
nalize and understand which are the main features that enable the
electronic delocalization in the mixed valence system.
2. Experimental
2.1. General experimental details
All manipulations were carried out under an atmosphere of
nitrogen using standard Schlenck techniques or in a Vacuum
Atmosphere glove box under nitrogen. Solvents used were pre-
dried and distilled with standard techniques using appropriate
drying agents [11]. The following compounds were prepared
according literature methods: 1,5-dihydro-2,6-diethyl-4,8-di-
methyl-s-indacene (Ic0H2) [12], [Cp*RuCl]4 [13]. NMR samples were
made up in the dry box, and the sample tubes were sealed with
septa. 1H and 13C{1H} NMR spectra were recorded with a Bruker
Avance 400 MHz instrument by using C6D6 as solvent at 25 °C,
the spectra were referenced internally, using the residual protio
solvent resonances, to tetramethylsilane (d = 0 ppm). Mass spectra
were measured with a Hewlett–Packard HP 5989A in the electron
impact mode (70 eV). Elemental analyses (C and H) were made
with a Fisons EA 1108 microanalyzer. Cyclic voltammetry experi-
ments were performed in an airtight three-electrode cell con-
nected to an argon line. The reference electrode was an SCE
(saturated calomel electrode, SCE versus Fc/Fc+ couple is
430 mV). The counter electrode was a platinum wire, and the
working electrode was a platinum disc with diameter of ca.
3 mm. The currents and potentials were recorded with a Pentium
II 350 MHz processor, with a BAS CV-50 Voltammetric Analyzer
Potentiometer. Each electrochemical experiment employed the
first fraction of distilled CH2Cl2, which was collected over phospho-
rus pentoxide and heated at reflux once again. The supporting elec-
Scheme 1. Labelling scheme for Table 1 and NMR assignation.
C(2,6), ,
3JH–H = 7.48 Hz); 1.08 (t, 6H, CH3–CH2–C(2,6) 3JH–H = 7.48 Hz).
13C{1H} NMR (C6D6, d ppm, 293 K): 11.16 (10C, CH3–C(Cp*)); 81.46
0
(10C, CH3–Cq(Cp*)); 63.45 (4C, CH(1,3,5,7-Ic )); 15.73 (2C, CH3–
0
C(4,8-Ic )); 14.80 (2C, CH3–CH2–C(2,6)); 22.82 (2C, CH3–CH2–C(2,6));
0
100.34 (2C, CH3–CH2–Cq(2,6)); 120.28 (2C, CH3–Cq(4,8-Ic )); 99.01
(4C, Cq(9,10,11,12-Ic )). MS (FAB): m/z 710 (MÅ+, 100%), 695 (MÅ+ꢀCH3)
0
(Scheme 1).
2.2.2. anti-[Cp*Ru–Ic0–RuCp*]+[BF4]ꢀ, 1+
[FeCp2]+[BF4]ꢀ (63 mg, 0.231 mmol) and 1 (180 mg, 0.254 mmol)
were dissolved in tetrahydrofuran (15 mL) the mixture was stirred
at room temperature by 4 h resulting in the formation of a brown
precipitate, the mixture was filtered and the insoluble material
was washed with diethyl ether to remove all the ferrocene and dried
in vacuo. The solid was recrystallized by slow diffusion of diethyl
ether into a concentrated CH2Cl2 solution of 2. Brown crystals were
observed within 24 h. The crystals were filtered, washed with
diethyl ether and dried in vacuo. Yield 174 mg, 86%.
Anal. Calc. for C38H50Ru2BF4 (795.52 g molꢀ1): C, 57.36; H, 6.33.
Found: C, 57.30; H. 6.10. MS (FAB): m/z 798 (MÅ+, 100%), 695 (MÅ+–
CH3).
trolyte,
tetrabutylammonium
tetrafluoroborate
(TBATFB,
[NBu4]+[BF4,]ꢀ), was dried under vacuum for 3 h prior use. ESR
spectra were recorded on a Brucker ER 200 instrument with fre-
quency meter EIP.
2.3. Crystal structure determination
Suitable crystals of 1 for X-ray diffraction were obtained from a
toluene/pentane solution. A single crystal of approximate dimen-
sions 0.43 ꢁ 0.21 ꢁ 0.19 was used for the cell determination and
data collection. Lattice constants were determined by least-squares
fitting of the setting angles of 25 reflections. Data were recorded
2.2. Synthesis of the binuclear systems
2.2.1. anti-[Cp*Ru–Ic0–RuCp*], 1
A solution of Ic0H2 (0.50 g, 2.10 mmol) in tetrahydrofuran (THF,
15 mL) was cooled to ꢀ80 °C. A hexane solution of n-butyllithium
(n-BuLi, 1.6 mol Lꢀ1, 2.63 mL, 4.20 mmol) was then added drop wi-
sely over the Ic0H2 solution. The reaction mixture was allowed to
warm to room temperature and stirred by 30 min, producing the
dilithiated salt of Ic0 (Ic0Li2). The mixture was then cooled to
ꢀ80 °C, and a solution of the ruthenium tetramer ([Cp*RuCl]4,
1.14 g, 1.05 mmol) in tetrahydrofuran (20 mL) was added via syr-
inge. Then, the solution was allowed to warm to room temperature
and stirred by 2 h, the solvent was removed until dryness. The solid
residue was extracted with toluene, filtered to eliminate the solid
lithium chloride formed, washed 3 times with toluene, and dried
under vacuum. A mixture of the binuclear and mononuclear spe-
cies is obtained. This mixture was washed with pentane in order
to remove the mononuclear specie. The binuclear compound was
obtained then as green crystals unstables to air. Yield 0.78 g, 53%.
Anal. Calc. for C38H50Ru2 (708.53 g molꢀ1): C, 64.36; H, 7.11.
Found: C, 64.04; H. 6.75.
using the
corrected for Lorentz polarization and absorption effects (empirical
scan, maximum and minimum transmission were 0.8405 and
x
ꢀ2h scan mode up to h of 26.98°. All intensities were
w
0.6862, respectively). The structure was solved by direct methods
using SHELXS-86 [14] and refined by full-matrix least squares meth-
ods on F2 over the complete set of data using SHELXL-97 [15]. Aniso-
tropic thermal parameters were refined for the non-hydrogen
atoms, and the hydrogen atoms were introduced in calculated
positions. Crystal data, data collection and refinement details are
given in Supplementary data. All X-ray diffraction data were col-
lected at 294(2) K.
3. Computational details
Density Functional Theory (DFT) calculations were carried out
by using the Amsterdam Density Functional (ADF) package [16].
Geometry optimizations were done via the analytical energy gradi-
ent method implemented by Verluis and Ziegler employing the
1H NMR (C6D6, d ppm, 293 K): 4.92 (s, 4H, CH(1,3,5,7-Ic )); 1.56 (s,
30H, CH3–C(Cp*)); 2.45 (s, 6H, CH3–C(4,8-Ic )); 2.29 (q, 4H, CH3–CH2–
0
0

D. Mac-Leod Carey et al. / Polyhedron 29 (2010) 1137–1143
1139
Scheme 2. Synthetic route to 1 and 1+ complexes.
Local Density Approximation (LDA) within the Vosko, Wilk and
Nusair parametrization (VWN) for the local exchange correlation
[17], nonlocal corrections proposed by Perdew–Wang in 1991
[18] were incorporated into the exchange and correlation function-
als, and were treated by a fully self-consistent method. Scalar rel-
ativistic effects were taking into account via the Zero Order Regular
Approximation (ZORA) Hamiltonian [19,20]. For the calculations
we used the standard ADF IV basis set [21], which is an uncon-
tracted triple-f STO basis set for H 1s, C 2s, Ru 4d; augmented with
one 3d single polarization function for C atoms, a 2p single polar-
ization function for H atoms and a 5p single polarization function
for Ru atoms. Spin-unrestricted calculations were performed for
the open-shell systems. The molecular structure described here
have been characterized as the energy minima through calcula-
tions of normal mode vibrational frequencies, which were per-
formed at the optimized geometry via second derivatives of the
total energy with respect to the internal coordinates as imple-
mented in the ADF code [22,23]. Some imaginary frequencies were
founded around ꢀ60 cmꢀ1 (60i cmꢀ1) for 1 and 2 which can be
attributed to the free rotation of the Cp* over the metal atom.
The TD-DFT calculations have been performed on the optimized
geometries with the ADF-RESPONSE module [24] which is an
extension of the ADF package. We employ for these response calcu-
lations the exchange-correlation model potential SAOP [25,26],
which is constructed with a statistical average of different model
potential for the occupied orbitals. SAOP was specially designed
for the calculation of optical properties.
4. Results and discussion
4.1. Synthesis and characterization
Our synthetic strategy for the preparation of the bimetallic
complexes incorporating the Ic0 ligand was salt elimination, using
the dianion of the bridging ligand. Several substituted s-indacene
dianions has been employed successfully in the synthesis of sev-
eral homobimetallic complexes as [(Cp*M)2Ic] (M = Fe, Co, Ni) [3],
[(RhCOD)2Ic] (COD is 1,5-cyclooctadiene) [7], whereas other
Fig. 1. Molecular representation in the crystal structure of [{Cp*Ru}2Ic0] (1). Displacement ellipsoides drawn at 40% level. Hydrogen atoms not shown, for the sake of
simplicity.

1140
D. Mac-Leod Carey et al. / Polyhedron 29 (2010) 1137–1143
authors have employed the neutral 12
p
s-indacenes derivatives
in the synthesis of bimetallic complexes [{(CO)3M}2(Ic00)]
{M = Fe, Co; Ic00 = tetra-1,3,5,7-tert-buthyl-s-indacene} [4,5]. For
the preparation of the dianion, the dihydro-s-indacene was dou-
bly deprotonated with two equivalents of n-butyllithium, as
shown in Scheme 2. Once the dianion is prepared, it can be cou-
pled to the metallic fragment via salt elimination. The bimetallic
derivative 1 is chemically oxidized with ferricinium tetrafluoro-
borate, giving the mixed valence specie 1+. These two reactions
provide a high yield (50–85%) and NMR spectroscopy showed
only a single isomer to be present in 1.
Suitable crystals of 1 for single-crystal X-ray diffraction analysis
were grown by cooling a concentrated solution of toluene and pen-
tane diffusion. The structure determination revealed that, in the
solid state, the molecule is centrosymmetric and the Cp*Ru organo-
metallic fragments are located in an anti coordination mode to
opposite sides of the Ic0 ligand. This can be attributed to the steric
hindrance provided by the two Cp* moieties.
The Ortep representation showed in Fig. 1, evidences that both
metallic centers present a nearly perfect
g [5] hapticity with Cp*
Fig. 3. UV–Vis-NIR spectra of 1 and 1+ in THF solution (ꢂ1 ꢁ 10ꢀ5 M).
and Ic0 ligands. The Ic0 centroid distances to the ruthenium atoms
are 1.847 Å. Furthermore the distance between the Ru and Cp*
centroids are 1.780 Å, which provide evidence that ruthenium
has a stronger coordination to Cp* than the Ic0 ligand. These values
are comparable with those found for syn-[Cp*RuIc0RhCOD] [9]
which are 1.843 and 1.775 Å for the respective Ru–Ic0 and Ru–
Cp* centroids, and can be compared with 1.840 and 1.794 Å for
the respective Ru–Pentalenyl and Ru–Cp* centroids in the anti-
[Cp*Ru(Pentalenyl)RhCOD] [27] complex. The distance data related
to the coordination mode of the Ru atom in complex 1 it does not
present significant differences compared with those from
literature.
(Kc = 2.55 ꢁ 1013) suggesting a high stability for the intervalence
system 1+. This significative difference between those oxidation
potentials give us the indication of a strong communicationbetween
the two metallic centers through the s-indacene bridge [28–31].
This can be compared with the [Cp*Ru-Pentalene-RuCp*] reported
by Manríquez et al. [3] in which
DE = 500 mV. Allowing us to sug-
gest that the Ic0 ligand grants a stronger communication than
pentalene.
4.3. Electronic spectra
4.2. Redox behaviour of 1
The UV–Vis-NIR spectra of 1+ shows the presence of an interva-
lence band, between 680 and 1150 nm, with kmax at 898 and
901 nm in THF and DCM solution respectively, which was not ob-
served in the spectra of the neutral specie 1 (see Fig. 3). This band
shows us the unambiguous existence of a strong communication
between the two metallic Ru centers. If we compare this interva-
lence band with its analogous [Cp*Ru-Pentalene-RuCp*]+[BF4]ꢀ
(1219 nm in CD2Cl2) or its isoelectronic analogous [Cp*Fe-s-inda-
cene-FeCp*]+[BF4]ꢀ (1708 nm in CD2Cl2) [3], it can be deduced that,
It can be seen in Fig. 2 that the cyclic voltammogram presents two
quasireversible monoelectronic redox process, the first at E1/2
=
ꢀ388 mV corresponding to the [{Cp*Ru}2Ic0]0/[{Cp*Ru}2Ic0]1+ couple
and the other at E1/2 = +405 mV corresponding to [{Cp*Ru}2Ic0]1+
/
[{Cp*Ru}2Ic0]2+ couple, using as reference a saturated calomel elec-
trode (SCE). The difference of potential between the two oxidation
picks is
DE1/2 = 793 mV, giving a high comproportionation constant
Table 1
Bond lengths (Å) from the crystal structure of
1
compared with corresponding
calculated distancesa in the compounds I and I+.
1
I
I+
Ru–C1
Ru–C2
Ru–C3
Ru–C9
2.191(4)
2.210(4)
2.185(4)
2.246(4)
2.241(3)
1.847
2.162
2.147–2.172
1.780
1.430(6)
1.420(5)
1.434(5)
1.434(5)
1.420(5)
1.425
2.242
2.254
2.229
2.376
2.378
1.940
2.216
2.197–2.229
1.845
1.434
1.434
1.449
1.450
1.409
1.442
1.435–1.452
2.249
2.276
2.233
2.299
2.297
1.910
2.233
2.214–2.254
1.866
1.433
1.432
1.447
1.445
1.415
1.443
1.439–1.448
Ru–C10
Ru–Centroid a in Ic0
Ru–Cp* ring average
Ru–Cp* ring range
Ru–centroid in Cp*a
C2–C1
C2–C3
C1–C9
C3–C10
C4–C10
C–C Cp* average
C–C Cp* range
1.415–1.434
a
Fig. 2. Cyclic voltammogram of 1 3.0 mM in CH2Cl2 (0.1 [n-Bu4N]+[BF4]ꢀ; scan rate
Non-listed data are redundant distances. The atom labels are referred to
Scheme 1, which could be found in Section 2.
0.1 V sꢀ1).

D. Mac-Leod Carey et al. / Polyhedron 29 (2010) 1137–1143
1141
Fig. 4. EPR spectra of 1+ at 4 K.
Fig. 5. MO interaction diagram of a simplified model of 1, [{CpRu}2(s-Ic)].
in the case of 1+, the energy requirements to accomplish the elec-
tron transfer are bigger.
4.4. Electron paramagnetic resonance
Here, two possibilities for system 1+ can be attained, to be in
presence of a valence trapped system in which a combination of
two distinct Ru centers, a diamagnetic EPR silent RuII and the para-
magnetic counterparts RuIII which exhibits a high degree of g-ten-
sor anisotropy. In the other expected behaviour, 1+ exhibit two
equivalent Ru centers (valence detrapped), thus a low value of g-
tensor anisotropy is expected for a delocalized system [3].
The 4 K EPR spectra of 1+ presented in Fig. 4 shows a significantly
ˆ
Fig. 6. (a) LUMO, (b) HOMO of I at 0.03 e/Å3 and (c) spin density of I+ at 0.004 e/Å3.
reduced g-tensor anisotropy of
D
g = |g ꢀ g\| = 0.28 when com-
(D
g = 0.35) [3], which denotes a higher degree of delocalization for
k
pared with its isoelectronic analogous [{Cp*Fe}2(s-Indacene)]+[BF4]+
1+.

1142
D. Mac-Leod Carey et al. / Polyhedron 29 (2010) 1137–1143
Table 2
Calculated excitation energies (eV) and oscillator strength for I and I+ complexes.
Wavelength (nm)
Exp.
Energy (eV)
Oscillator strength (ꢁ103)
Symmetry
Composition (%)
Nature of the electronic transitions
Calc.
I
626a, 627b
435a, 436b
694
429
1.79
2.89
13.31
16.69
1Au
1Au
96.9
50.0
48.3
41.9
41.9
5ag ? 5au
1ag ? 5au
5ag ? 6au
5ag ? 6au
1ag ? 5au
333a, 329b
898a, 901b
382
3.25
635.96
1Au
I+
903
899
1.37
1.38
20.07
26.57
1Au
1Au
54.7
40.3
51.0
43.0
94.9
59.6
21.0
b 2au ? b 5ag
b 4ag ? b 5ag
b 4ag ? b 5ag
b 2au ? b 5ag
a 5ag ? a 5au
b 3au ? b 5au
821
472
1.51
2.63
6.11
32.13
1Au
1Au
483a, 486b
b 4au ? b 5au
a
From a THF solution.
From a DCM solution.
b
Orbital composition analysis (see Tables in Supplementary
data) shows that there exist several significant changes when the
oxidation is carried out, which must be taken into account. Here
we employ the unrestricted formalism for system I+. The SOMO
5. Density functional calculations
5.1. Molecular structure
(
a 5ag) in which resides the unpaired electron presents an ele-
As it can be seen in Table 1, the calculated geometrical param-
eters of I are in agreement with those obtained by X-ray diffraction
for 1. The most striking information is obtained in the analysis of
the Ru–Cp* and Ru–Ic0 calculated distances, comparing both the
neutral (I) and oxidized (I+) calculated species, in which the oxida-
tion causes that ruthenium atoms come closer to the Ic0 ligand,
thus increasing the interaction between them.
vated contribution from the Ic0 fragment compared with the HOMO
in the neutral molecule. Its unoccupied counterparts (b 5ag) also
present an elevated contribution from the Ic0 fragment. The
HOMO-1 (
tribution from the Ic0 fragment, the same is observed for HOMO-4
3ag). These facts suggest that when the oxidation is carried out,
a and b of 4au) presents a slightly increasing of the con-
(a
an increasing of the interaction between the ruthenium and the Ic0
ligand is observed, also Ic0 acts as a bridging electronic ligand,
allowing the exchange of the remanent electron between the two
metallic centers, and for that reason the electronic density over
the Ic0 ligand increases, and with it, the probability of finding the
electron over it, as can be observed at the spin density of the oxi-
dized system, Fig. 6c, with values of 25% for each Cp*Ru fragment
and 50% for Ic0 fragment.
5.2. Molecular orbital analyses
A simplified molecular orbital (MO) diagram of the parent mol-
ecule [{CpRu}2(Ic)] of C2h symmetry is depicted in Fig. 5. This MO
diagram exhibit the main features as that of its isoelectronic
[{CpFe}2(Ic)] [10] relative. The HOMO (2bg) is essentially related
to the non-bonding p4 HOMO of Ic2ꢀ with some CpRu admixture.
This HOMO is well separated from the six ‘‘t2g” metallic orbitals
spanning as 2 ꢁ ag + bg + au + 2 ꢁ bu. The LUMO (3bu) is a combina-
tion of a CpRu acceptor orbital with the Ic2ꢀ LUMO. It is also ener-
getically isolated. The significant computed HOMO–LUMO gap
(1.46 eV) is consistent with the closed-shell configuration observed
experimentally here for system 1.
5.3. Time dependent density functional calculations
The calculated spectra of neutral complex shows several transi-
tions, as is presented in Table 2. The calculated excitation at
382 nm presents the highest oscillator strength, which was as-
signed to a transition in which there exist a slight charge transfer
from the Ic to both Cp*Ru fragments. The transition calculated at
694 nm, was assigned to a MLCT.
In the calculated neutral full molecule, [{Cp*Ru}2(Ic0)] (I), the
main features of the previous analysis are retained. The plot of
the HOMO and LUMO isosurfaces of I and theꢀSpin Density of
the mixed-valent system, [{Cp*Ru}2(Ic0)]+[BF4] (I+), are pre-
sented in Fig. 6. The orbital composition analysis for I and I+ is
presented in a Table as in a graphical form at the Supplementary
data.
The calculated spectra for the oxidized complex (I+) shows the
presence of several transitions. Those in the range from 821 to
903 nm contributes to the intervalence band (experimentally ap-
pears as a broad band between 680 and 1150 nm). Those calcu-
lated at 899 and 903 nm transitions occur between orbitals
which present a large metallic character to an orbital which is
delocalized over the entire molecule, these can be seen as a MLCT
transition. In the case of the third calculated transition at 821 nm,
this transition occurs between orbitals which have an extensive
mixing of the Ic0 bridge and the metal centers, as it should be for
an intervalence band according to the description given by Launay
[32]. It must be noted that an intervalence band was not found in
the neutral complex (I).
As can be seen from Fig. 6b, HOMO of I is composed by the Ic0
(32%) and Ru–Cp* (34% each) fragments in an equitative way. It
must be noted the high participation of the Ic0 ligand into the
LUMO, Fig. 6a, suggesting that an hypothetical radical anion, which
will be isoelectronic with the syn-[{Fe(CO)3}2(Ic00)]+[BF4]ꢀ(4,5) can
be obtained, and the presence of large hyperfine coupling con-
stants shall indicate that the electron resides in an orbital with a
large ligand character as the observed by O’Hare, in which the
DFT calculations suggest a spin density of 32% over the Ic00 ligand.
Broken Symmetry calculations have been carried out in order to
ensure that we are in presence of a completely delocalized system.
Starting from a strongly asymmetric molecule we force the system
to be localized, however, always converge to a delocalized system.
6. Conclusions
This work describes the synthesis and full characterization
including X-ray structural analysis of a homobimetallic organome-

D. Mac-Leod Carey et al. / Polyhedron 29 (2010) 1137–1143
1143
tallic system entitled as [{Cp*Ru}2Ic0] (1) and the further oxidation
affording a stable mixed-valent cation [{Cp*Ru}2Ic0]+[BF4]ꢀ (1+)
which presents a strong electronic communication between metal
sites as suggested by cyclic voltammetry, spectroscopic and DFT
studies. This coupling results in a large electrochemical potential
separation between successive one-electron redox events. The
DFT calculations indicate that the oxidation of the neutral molecule
occurs in an orbital that comprises the entire molecule, and the
remanent electron is shared and delocalized between the two
metallic moieties and the Ic0 bridging ligand. The mixed-valent sys-
tem exhibit an intervalence charge transfer absorption band which
was assigned by means of TD-DFT calculations.
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The authors thank the financial support of: FONDECYT Grants
1060589, 1060588 and 1070345, Apoyo de tesis Doctoral CONICYT
Grants N° 23070215 and 24080034, UNAB Grants UNAB-DI-02-09/
R and UNAB-DI-09-09/I, PROJECT MILLENNIUM Grant No. P07-
006–F. Also we thank the following Fellowships: Beca Doctoral
CONICYT; Beca Doctoral UNAB.
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Appendix A. Supplementary data
CCDC 736938 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.poly.2009.12.002. Supplemen-
tary data associated with this article can be found, in the online
version, at doi:10.1016/j.poly.2009.12.002.
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