Bond activation by electron transfer in indenyl ruthenium(II) complexes. The electrochemical reduction of [Ru(η5-C9H 7)Cl(L)2] and [Ru(η5-C9H 7)(L)2]+, L2 = COD, L = PPh 3

Article abstract of DOI:10.1021/om0300878

The reduction of half-sandwich indenyl complexes of general formula [Ru(η⁵-C₉H₇)Cl(L)₂] (L = PPh₃, 1; L₂ = 1,5-cyclooctadiene, 2) and [Ru(η ⁵-C₉H₇)(L)₂]⁺ has been carried out in order to investigate the effects of electron transfer on the structural and chemical properties of the complexes. The reduction of these complexes proceeds by irreversible bielectronic processes. In the case of the metal halide complexes, the first electron transfer generates a 19-electron radical anion, which undergoes Ru-Cl bond cleavage to form a 17-electron Ru(η⁵-C₉H₇)(L)₂ radical, which is in turn reduced at a less negative potential. Therefore the overall process proceeds according to a ECE mechanism, characterized by two electron transfers separated by a chemical reaction. The cationic [Ru(η⁵-C ₉H₇)(L)₂]⁺ complexes were generated in situ by chloride abstraction, upon reacting complex 1 or 2 with AgBF ₄ or AgPF₆; both [Ru(η⁵-C₉H ₇)(PPh₃)₂]⁺ (1a⁺) and [Ru(η⁵-C₉H₇)COD]⁺ (2a ⁺) undergo a monoelectronic reductive process forming the radical intermediates, which rapidly dimerize. The [Ru(η⁵C ₉H₇)COD]^. radical is sufficiently stable to be detected by cyclic voltammetry and partially generates the 18-electron anion [Ru(η⁵-C₉H₇)COD]^-.

Full text of DOI:10.1021/om0300878

3478
Organometallics 2003, 22, 3478-3484
Bon d Activa tion by Electr on Tr a n sfer in In d en yl
Ru th en iu m (II) Com p lexes. Th e Electr och em ica l
Red u ction of [Ru (η5-C9H7)Cl(L)2] a n d [Ru (η5-C9H7)(L)2]+,
§
L2 ) COD, L ) P P h 3
Saverio Santi* and Lorenza Broccardo
Dipartimento di Chimica Fisica, Universita` degli Studi di Padova, Via Loredan 2, 35131,
Padova, Italy
Mauro Bassetti
Istituto CNR di Metodologie Chimiche, and Dipartimento di Chimica,
Universita` “La Sapienza”, 00185 Roma, Italy
Patricia Alvarez
Departamento de Quı´mica Orga´nica e Inorga´nica, Instituto de Qu´ımica Organometa´lica
“Enrique Moles” (Unidad Asociada al CSIC), Facultad de Qu´ımica, Universidad de Oviedo,
33006 Oviedo, Spain
Received February 5, 2003
The reduction of half-sandwich indenyl complexes of general formula [Ru(η5-C9H7)Cl(L)2]
(L ) PPh3, 1; L2 ) 1,5-cyclooctadiene, 2) and [Ru(η5-C9H7)(L)2]+ has been carried out in
order to investigate the effects of electron transfer on the structural and chemical properties
of the complexes. The reduction of these complexes proceeds by irreversible bielectronic
processes. In the case of the metal halide complexes, the first electron transfer generates a
19-electron radical anion, which undergoes Ru-Cl bond cleavage to form a 17-electron Ru-
(η5-C9H7)(L)2 radical, which is in turn reduced at a less negative potential. Therefore the
overall process proceeds according to a ECE mechanism, characterized by two electron
transfers separated by a chemical reaction. The cationic [Ru(η5-C9H7)(L)2]+ complexes were
generated in situ by chloride abstraction, upon reacting complex 1 or 2 with AgBF4 or AgPF6;
both [Ru(η5-C9H7)(PPh3)2]+ (1a +) and [Ru(η5-C9H7)COD]+ (2a +) undergo a monoelectronic
reductive process forming the radical intermediates, which rapidly dimerize. The [Ru(η5-
C9H7)COD]• radical is sufficiently stable to be detected by cyclic voltammetry and partially
generates the 18-electron anion [Ru(η5-C9H7)COD]-.
In tr od u ction
have been shown to be active in a variety of chemical
processes. Stoichiometric reactions include phosphine
substitution as in the case of complex [Ru(η5-C9H7)Cl-
(PPh3)2],5 insertion of alkynes into the Ru-H bond,6 and
activation of terminal alkynes to form vinylidene and
allenylidene complexes.7 Catalytic processes involve
isomerization of allylic alcohols to their saturated al-
dehydes or ketones,8 dimerization of terminal alkynes,9
[2+2] and [2+4] cycloaddition reactions of dienes with
alkynes, as in the case of complex [Ru(η5-C9H7)Cl-
Half-sandwich indenyl complexes of transition metals
are often characterized by greater reactivity with re-
spect to their cyclopentadienyl analogues, in either
stoichiometric1 or catalytic reactions.2 This behavior is
ascribed to the particular properties of the polyene
ligand, which are know as “indenyl effect”3 or “extra-
indenyl effect”.4 Indenyl complexes of ruthenium(II)
§ This paper is dedicated to Prof. Alberto Ceccon on the occasion of
his 70th birthday.
* Corresponding author. E-mail: saverio.santi@unipd.it.
(1) Frankom, T. M.; Green, J . C.; Nagy, A.; Kakkar, A. K.; Marder,
T. B. Organometallics 1993, 12, 3688. (b) Rerek, M. E.; J i, L.-N.; Basolo,
F. J . Chem. Soc., Chem. Commun. 1983, 1208. (c) Hart-Davis, A. J .;
White, C.; Mawby, R. J . Inorg. Chim. Acta 1970, 4, 441. (d) Hart-Davis,
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10.1021/om0300878 CCC: $25.00 © 2003 American Chemical Society
Publication on Web 07/22/2003

Electron Transfer in Indenyl Ru(II) Complexes
Organometallics, Vol. 22, No. 17, 2003 3479
(COD)],10a and anti-Markovnikov addition of water in
aqueous media catalyzed by complex 1.11 Despite this
extensive chemistry, information about the electrochemi-
cal features of these complexes is scarce,5 as well as
about the reactivity of chemically or electrochemically
generated species of different oxidation state.
Sch em e 1
It is known that organometallic radicals with 17 or
19 electrons on the metal generally show greatly
enhanced reactivity in comparison to their 18-electron
analogues.12 Electrochemical activation generates more
reactive species by raising the ground state energy of
the molecules and by altering the energy barrier of the
reaction. These species, which can be obtained by
photolysis of metal-metal bonded dimeric complexes or
by electron transfer on the 18-electron monomers, are
frequent intermediates in several organometallic pro-
cesses, such as atom abstraction, ligand substitution,
and disproportionation reactions.13
In particular, studies on the redox properties of
monomeric metal cyclopentadienyl halides and on their
applications as precursors of radical intermediates have
been relatively scarce,14a,15 although more information
is available about the corresponding dimeric species.14
Moreover, despite the high interest in the electron
transfer correlated to the indenyl effect,16 the reductions
of indenyl organometallic halides have not been inves-
tigated.
lustrated in Scheme 1, viz., [RuCl(η5-C9H7)(PPh3)2] (1)
and [RuCl(η5-C9H7)COD] (2, COD ) 1,5-cyclooctadiene)
and of the corresponding cations [Ru(η5-C9H7)(PPh3)2]
(1+) and [Ru(η5-C9H7)COD] (2+). The bis-phosphine
cationic complex 1+ is the postulated intermediate in
the preparation of ruthenium vinylidene complexes from
the reaction of terminal alkynes with complex 1.7
Our aims have been (i) to generate 17- and 19-electron
species, (ii) to intercept ruthenium complexes in low
oxidation state, and (iii) to identify which bonds are
activated by electron transfer. The presence of ligands
with different donor properties, such as PPh3 and COD,
affects the electron density on the metal and is therefore
expected to influence the electrochemical features of the
complexes. Since the stoichiometric and catalytic reac-
tivity strongly depends on the ability to create coordina-
tion vacancy at the metal, it is of great interest to assess
the structural modifications of the coordination shell
induced by electron transfer.
In this context we report on the electrochemical
reduction of two ruthenium- indenyl complexes il-
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der Maelen, J . F.; Bassetti, M. Organometallics 2001, 20, 3762. (b)
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H. J . Organomet. Chem. 1988, 356, 315. (e) Neto, C. C.; Kim, S.; Meng,
Q.; Sweigart, D. A. J . Am. Chem. Soc. 1993, 115, 2077. (f) Kochi, J . K.
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D. J .; Amatore, C.; Kochi, J . K. Inorg. Chem. 1986, 25, 7. (e)
Organometallic Radical Processes; Trogler, W. C., Ed.; J ournal of
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Am. Chem. Soc. 2001, 123, 10595. (c) Amatore, C.; Ceccon, A.; Santi,
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Exp er im en ta l Section
Gen er a l P r oced u r es. The complexes [RuCl(η5-C9H7)-
(PPh3)2] (1)17 and [RuCl(η5-C9H7)COD]10b,18 (2) were prepared
according to published procedures. Complexes [Ru(η5-C9H7)-
(PPh3)2]+ (1a +) and [Ru(η5-C9H7)COD]+ (2a +) were prepared
in situ, by reacting complex 1 or 2 in THF with an equimolar
amount of AgBF4. Electrochemical measurements were per-
formed on clear solutions after removing a pale yellow
precipitate by centrifugation. A conductivity measurement
(J ENWAY PCM 3 conductivity meter) on a THF solution of
the cation 2a + (1 mM) indicates that the salt behaves as a 1:1
electrolyte (Λm )123 Ω-1 cm2 mol-1).
The cationic complex 2a + has been fully characterized as
the corresponding pyridine or acetonitrile (L) adducts [Ru(η5-
C9H7)(L)COD][BF4].10a [Ru(η5-C9H7)(PPh3)2]+ (1a +) has been
characterized as hexafluorophosphate salt of the corresponding
nitrile adducts [Ru(η5-C9H7)(RCN)(PPh3)2][PF6] (R ) Me, Et,
Ph).18
Electr och em ica l Ap p a r a tu s a n d P r oced u r e. All com-
plexes’ manipulations were performed in an oxygen- and
moisture-free atmosphere; tetrahydrofuran (THF) was purified
by distillation from Na/benzophenone under argon atmosphere
and then deoxygenated with vacuum line techniques just
before use. Ferrocene was purified by crystallization before use.
Supporting electrolyte was prepared by exchange reaction of
NaBF4 and n-Bu4NHSO4 in aqueous solution; the precipitate
(17) Oro, L. A.; Ciriano, M. A.; Campo, M.; Foces-Foces, C.; Cano,
F. H. J . Organomet. Chem. 1985, 289, 117.
(18) Cadierno, V.; Gamasa, M. P.; Gimeno, J . Organometallics 1999,
18, 2821.

3480 Organometallics, Vol. 22, No. 17, 2003
Santi et al.
was dissolved in CH2Cl2, dried with Na2SO4, filtered, and dried
by evaporation. The salt n-Bu4NBF4 was then purified by
recrystallization from ethyl acetate/petroleum ether. PPh3 was
purified by recrystallization from hexane. COD (Aldrich, 99%)
was distilled prior to use. AgBF4, AgPF6 (Aldrich, 99.99%), and
Bu4NCl (Fluka, puriss.) were used without any further puri-
fication. Cyclic voltammetry experiments were performed in
an airtight three-electrode cell connected to a vacuum/argon
line. The reference electrode was a SCE (Taccussel ECS C10)
separated from the solution by a bridge compartment filled
with the same solvent/supporting electrolyte solution used in
the cell. The counter electrode was a platinum spiral with ca.
1 cm2 apparent surface area. The working electrodes were
disks obtained from cross section of gold wires of different
diameter (0.5, 0.125, 0.025 mm) sealed in glass. Between each
CV scan the working electrode was polished on alumina
according to standard procedures and sonicated before use. The
currents and potentials were recorded on a Lecroy 9310L
oscilloscope. The potentiostat was home-built with a positive
feedback loop for compensation of ohmic drop.19 All potential
values are reported versus SCE.
Absolu te Electr on Stoich iom etr y Deter m in a tion .20 The
chronoamperometric diffusion current at 0.5 mm diameter disk
electrode for a step duration of 0.2 s and the steady state
current at 12.5 µm radius gold microdisk electrode (potential
scan rate 10 mV s-1) were measured for 3 mM solutions of 1
and 2 and for the standard ferrocene [D ) (1.3 ( 0.2) × 10-5
cm2 s-1] under identical conditions, i.e., THF/0.2 M n-Bu4NBF4.
These data allowed determination of the absolute consumption
of electrons and of the diffusion coefficients.
F igu r e 1. Cyclic voltammetric scans in THF/0.2 M n-Bu4-
NBF4 at a gold disk electrode of 0.5 mm diameter, T ) 20
°C. (a) Reduction (solid line) and oxidation (dotted line) of
3 mM [RuCl(η5-C9H7)(PPh3)2] (1), v ) 0.5 V s-1; (b)
reduction of 3 mM 1, v ) 200 V s-1; (c) reduction of 3 mM
[Ru(η5-C9H7)(PPh3)2]+ (1a +) (solid line) and after addition
Resu lts a n d Discu ssion
Electr och em ica l Red u ction of [Ru Cl(η5-C9H7)-
(P P h 3)2] (1). The electrochemical reduction of 1 at 0.5
V s-1 (Figure 1a) occurs at Ep ) -1.81 V as an
irreversible process (I) over all the potential scan rates
investigated (v ) 0.05-100 V s-1). After scan reversal,
three oxidation waves appear: II (Ep ) -0.52 V), III
(Ep ) 0.29 V), and IV (E° ) 0.69 V). The last one is
chemically and electrochemically reversible (V). A previ-
ous work assigned IV to the one-electron oxidation of
complex 1.5 II and III are not recorded in the anodic
scan, which suggests to ascribe them to the oxidation
of species formed in solution after reduction of the
of 1 equiv of Bu4NCl (dotted line), v ) 0.5 V s-1
.
complex. At high scan rate (Figure 1b, v ) 200 V s-1
)
the wave II has disappeared but the wave III is still
present.
The analysis of the reduction wave shows that the
current function (ip v-1/2 M-1) changes by varying the
scan rate: in fact, it decreases by a factor of 1.7 on going
from low scan rate (v ) 0.05 V s-1) to high scan rate
values (v g 10 V s-1, see Figure 2a). Determination of
the absolute electron stoichiometry20 confirmed that the
reduction wave tends to become overall bielectronic at
sufficiently low scan rate (napp ) 1.60 ( 0.15 at v ≈ 0.1
V s-1, see Figure 2a). In the general case in which the
electron transfer is Nernstian at low scan rate and
irreversible at high scan rate, the current function
normalized to its value at the highest scan rate is
expected to reach the maximum factor of 0.446/
F igu r e 2. (a) Variation of the current function ip v-1/2 M-1
normalized to its value at high scan rate v g 100 V s-1
and (b) variation of the peak potential Ep for the reduction
wave of 3 mM [RuCl(η5-C9H7)(PPh3)2] (1) at a gold disk
electrode diameter of 0.5 mm, 0.05 e v e 0.5, and 0.125
mm, 1 e v e 100, T ) 20 °C. Abscises in logarithmic scale.
irreversible electron transfer (nR ) 1), justifies a de-
crease of the current function by a factor 1.22,
Ep ) (1.15 RT/RnRF) log v + constant
(1)
0.495(R)1/2 21a
The estimated value of R ) 0.54 (coef-
.
a value definitely less than that observed in our
experiments.21b This evidence indicates that the experi-
ficient of the electron transfer) obtained from eq 1, which
describes the variation of the peak potential for a totally
(21) (a) Bard, A. J .; Faulkner, L. R. Electrochemical Methods, 2nd.
ed.; Wiley: New York, 2001. (b) To verify if the observed trend should
be ascribed to a slow electron transfer, we calculated R considering
the slope d(Ep)/d(log v) in the range where the global electronicity
remains constant (v g 10 V s-1, Figure 2b for 1 and 5b for 2).
(19) Amatore, C.; Lefrou, C.; Pflu¨ger, F. J . Electroanal. Chem. 1989,
43, 270.
(20) Amatore, C.; Azzabi, M.; Calas, P.; J utand, A.; Lefrou, C.; Rollin,
J . J . Electroanal. Chem. 1990, 288, 45.

Electron Transfer in Indenyl Ru(II) Complexes
Organometallics, Vol. 22, No. 17, 2003 3481
mental trend does not depend only on a single and slow
electron transfer.
One explanation for the decrease of the current
function could be to assume that the reduction of 1 in
THF occurs via a CE mechanism in which the chemical
step (C) of eq 2 precedes the electron transfer (E) of eq
3, which could occur at sufficiently low scan rate.
[RuCl(η5-C9H7)(PPh3)2] yk1z
k-1
[RuCl(η5-C9H7)(PPh3)] + PPh3 (2)
[RuCl(η5-C9H7)(PPh3)]+ e- h
[RuCl(η5-C9H7)(PPh3)]- (3)
F igu r e 3. (a) Variation of the current function ip v-1/2 M-1
normalized to its value at high scan rate v g 100 V s-1 of
3 mM [RuCl(η5-C9H7)(PPh3)2] (1) in the presence of Bu4-
NCl, CCl- ) 0 mM (open squares), 3 mM (open diamonds),
12 mM (open circles), and 24 mM (×’s). (b) Variation of
the current function obtained in a plot of ip v-1/2 M-1 vs
log(v × CCl-). Abscises in logarithmic scale.
A kinetic study on phosphine substitution for complex
1 in THF has shown the existence of the equilibrium
(2) as a step of a dissociative mechanism, in which the
release of PPh3 from ruthenium to form a 16-electron
intermediate is rate detemining (k1 ) 2.04 × 10-4 s-1
,
THF, 30 °C) (eq 2).5 As far as the reduction mechanism
is concerned, the slow rate of the dissociative step
reaction k1 causes the contribution of a CE sequence to
become negligible in the global reduction process. In
fact, we assessed by digital simulation that such a
mechanism would be relevant only for a chemical
the effect of chloride we conclude that the rate-
determining chemical step is the chloride dissociation
from [RuCl(η5-C9H7)(PPh3)2]-• (1-•), which is retarded
by free Cl- in solution, preceded by a very fast reaction
which may be a η5 to η3 ring-slippage (eq 5). These
results allow the identification of the 17-electron radical
[Ru(η5-C9H7)(PPh3)2]• (1•) as the species responsible for
the second electron transfer at a more positive potential
than that of the neutral complex 1. Equations 4-6
depict the proposed mechanism.
At high scan rate, when the wave I is monoelectronic
and still irreversible, the intensity of wave III is
unchanged (Figure 1b). This indicates that it corre-
sponds to the oxidation of the slipped 17-electron species
RuCl(η3-C9H7)(PPh3)2]•-, which regenerates the starting
complex 1.
reaction with k1 > 0.01 s-1
.
On the other hand, an ECE mechanism explains the
current function shape: the first heterogeneous electron
transfer (E) generates an unstable 19-electron species
in which a ligand rapidly decoordinates from ruthenium,
via loss of Cl- or PPh3, or η5 to η3 ring-slippage in the
chemical step (C). At the electrode-solution interface
now exists a 17-electron molecule whose reduction is
easier than that of 1; this species undergoes a second
electron transfer (E), generating a bielectronic wave.
To identify the nature of the chemical step, we tested
the effect of the complex concentration on the current
function and of different amounts of Cl- and PPh3. Since
no concentration dependence of the current function up
to 8 mM 1 was observed, the existence of a second-order
chemical reaction involving 1, such as a disproportion-
ation reaction,22 can be excluded.
[RuCl(η5-C9H7)L2] + e- h [RuCl(η5-C9H7)L2]•-
[RuCl(η5-C9H7)L2]•- f [RuCl(η3-C9H7)L2]•-
[Ru(η5-C9H7)L2]• + Cl- (5)
[Ru(η5-C9H7)L2]• + e- h [Ru(η5-C9H7)L2]-
(4)
h
Similar results were obtained from CV experiments
at different scan rate carried out on a solution of
complex 1 containing up to 10 equiv of PPh3. The
phosphine concentration does not affect the reduction
of 1 since no effect is detectable on the current function,
and it can be excluded that the chemical reaction is the
phosphine dissociation.
(6)
1: L2 ) (PPh3)2
2: L2 ) COD
In contrast, the addition in solution of similar amounts
of n-Bu4NCl causes a decrease of the current function
proportional to the concentration of Cl- (Figure 3a). A
common trend is obtained in a plot of ip v-1/2 M-1 versus
log(v × CCl-) (Figure 3b), which confirms the presence
of a second-order back reaction involving the Cl- anion.
Concerning the η5 to η3 ring-slippage process, it is very
typical in 18-electron indenyl complexes3 and much
faster in the corresponding 19-electron radicals.16 On
the basis of the chemical irreversibility of the wave I
up to the scan rate investigated (v ) 500 V s-1) and of
Another consequence of the addition of n-Bu4NCl is
the decrease of the intensity of II. The wave II is
detectable only when the reduction wave of 1 is tra-
versed and disappears at high scan rate (v g 200 V s-1).
It corresponds to the oxidation wave of the indenyl
anion,16c as demonstrated by comparison with an au-
thentic sample of potassium indenyde and previously
observed in analogous experimental conditions for com-
plex Rh(η5-C9H7)(COD). Therefore, it corresponds to the
oxidation of the indenyl anion produced by decomposi-
tion of 1-. The intensity of II increases if the scan
direction is reversed at more negative potential, i.e.,
when the chemical reaction has more time to produce
(22) Amatore, C.; Saveant, J . M. J . Electroanal. Chem. 1977, 85,
27.

3482 Organometallics, Vol. 22, No. 17, 2003
Santi et al.
the oxidizable species. By this evidence, we conclude
that [Ru(η5-C9H7)(PPh3)2]- undergoes a partial degrada-
tion which produces the indenyl anion and the fragment
Ru(PPh3)2, which was not detected. The addition of Cl-
in solution inhibits the formation of [Ru(η5-C9H7)-
(PPh3)2]-, and consequently the intensity of wave II
decreases due to the indenyl anion.
E lect r och em ica l R ed u ct ion of [R u (η5-C9H 7)
(P P h 3)2]+ (1a +). To determine the redox potential of 1a •,
i.e., the key intermediate of the mechanism proposed
above, 1a + was generated inside the electrochemical
cell. The CV shows a new chemically irreversible
cathodic wave (VI) at -1.5 V, i.e., 310 mV before the
peak potential assigned to the reduction of 1 (Figure
1c), which corresponds to the reduction of 1a +. By
increasing the scan rate from 0.05 V s-1 to 20 V s-1
,
the current function remains constant, as expected for
a process in which the electronicity is unchanged. At
the same time, Ep varies linearly with log(v), the slope
d(Ep)/d(log v) being 21 mV, which corresponds, within
experimental error, to the value predicted for a radical-
radical dimerization (DIM1 mechanism, 19.7 mV).23 The
formation of a new indenyl-containing species after the
reduction of the cation is supported by (i) the disap-
pearance of wave II, due to the oxidation of the indenyl
anion, and by (ii) the appearance of a new anodic wave
VII at Ep ) 0.36 V. This experimental evidence suggests
the formulation of the following DIM1 mechanism for
the electrochemical reduction of [Ru(η5-C9H7)(PPh3)2]+
(eqs 7, 8):
F igu r e 4. Cyclic voltammetric scans in THF/0.2 M n-Bu4-
NBF4 at a gold disk electrode of 0.5 mm diameter for v )
0.5 V s-1, 0.125 mm for v ) 500 V s-1, T ) 20 °C. (a)
Reduction at v ) 0.5 V s-1 with the potential scan reversed
at -1.82 and -2.9 V (solid line) and oxidation (dotted line)
of 3 mM [RuCl(η5-C9H7)COD] (2); (b) reduction of 3 mM 2
at v ) 500 V s-1; (c) reduction (solid line) and oxidation
(dotted line) of 3 mM [Ru(η5-C9H7)COD]+ (2a +); (d) reduc-
tion of 3 mM [Ru(η5-C9H7)COD]+ (2a +) (dotted line) with
the potential scan reversed at -2.9 V (dotted line) and after
addition of 1 equiv of Bu4NCl (solid line).
[Ru(η5-C9H7)(PPh3)2] + + e- h
[Ru(η5-C9H7)(PPh3)2]• (7)
2[Ru(η5-C9H7) (PPh3)2]• h
[Ru(η5-C9H7)(PPh3)2]2 (8)
reversal, four oxidation waves: II (Ep ) -0.90 V), III
(Ep ) 0.43 V), IV (Ep ) 0.61 V), and V (Ep ) 1.06 V),
the last one being in part chemically reversible (VI)
(Figure 4a). If the anodic scan starts from -1.1 V, only
the waves V and its cathodic counterpart VI are
detected, indicating that they correspond to the partially
reversible oxidation of 2. Therefore, the wave I repre-
sents the reduction of 2, whereas II, III, and IV
correspond to the oxidation of species generated in
solution after the reduction of the complex 2.
The wave II is present even at high scan rate (v )
500 V s-1, Figure 4b) when the wave I is monoelectronic.
Hence, it corresponds to the oxidation of 2•- in the scan
reversal, demonstrating that reduction of 2 is partially
chemically reversible. The current function of the wave
I decreases in the range of the scan rate explored (0.05-
100 V s-1) (Figure 5a). The value R ) 0.40 ( 0.01 allows
the prediction of a maximum decrease by a factor 1.42
when the trend is due only to a slow electron transfer.21
In contrast, the experimental leap is about 1.75, which
suggests a more complex mechanism.
The 16-electron cation 1a + undergoes a single electron
transfer, generating the radical 1a • (eq 7), which rapidly
dimerizes (eq 8) forming a 34-electron species (wave VII
at 0.36 V). Therefore the radical 1a • dimerizes when
produced by reduction of the cation, whereas it is
reduced to the anion when formed by chloride dissocia-
tion from 1•-. This is due to the fact that the radical, in
the latter case, is produced at the reduction potential
of the neutral complex 1 and is therefore reduced as
soon as it is formed. The dimerization process is
disfavored since at this potential the redox couple 1a •/
1a - can exist only in the reduced form. Alternative
dimerization mechanisms involving 1a • and 1a - (DIM2),
and 1a + and 1a - (DIM3), were ruled out on the basis of
the slope d(Ep)/d(log v), which should be 29.6 and 14.8
mV, respectively.23
Addition of 1 equiv of n-Bu4NCl to the solution of the
cation restores the starting neutral species, and the
waves (I and IV/V) characteristic of 1 are reestablished
(Figure 1c).
Determination of absolute electronic stoichiometry20
confirms that at low scan rate more than one electron
is involved. The value napp ) 1.61 ( 0.07 at v ≈ 0.12 V
s-1 is in agreement with the current function value at
the same scan rate (Figure 5a).
Electr och em ica l Red u ction of Ru Cl(η5-C9H7)-
COD (2). The voltammetric investigation carried out
on complex 2 reveals at scan rate v ) 0.5 V s-1
a
reduction wave (I) at -1.42 V and, after the scan
This case also supports the existence of an ECE
mechanism in which the chemical step can be either
(23) Andrieux, C. P.; Nadjo, L.; Save´ant, J . M. J . Electroanal. Chem.
1973, 42, 223.

Electron Transfer in Indenyl Ru(II) Complexes
Organometallics, Vol. 22, No. 17, 2003 3483
Sch em e 2
of
the
slipped
17-electron
species
RuCl-
(η3-C9H7)COD]•-, which regenerates the neutral complex
2.
F igu r e 5. (a) Variation of the current function ip v-1/2 M-1
normalized to its value at high scan rate v g 100 V s-1
and (b) variation of the peak potential Ep for the reduction
wave of 3 mM [RuCl(η5-C9H7)COD] (2) at a gold disk
electrode diameter of 0.5 mm, 0.05 e v e 0.5, and 0.125
mm, 1 e v e 200, T ) 20 °C. Abscises in logarithmic scale.
A previous work showed that free COD coordinates
[Rh(η3-η5-C9H7)COD]•- electrochemically generated from
Rh(η5-C9H7)COD, producing C9H7- and Rh(COD)2.16c In
the case of [RuCl(η5-C9H7)COD]•-, such a mechanism
can be ruled out on the basis of the lack of effects upon
addition of free COD.
Electr och em ical Redu ction of [Ru (η5-C9H7)COD]+
(2a +). The cathodic scan shows two waves of similar
potential values (VII, Ep ) -1.01 V; VIII, Ep ) -1.08
V), which are 390 mV less negative than that of 2
(Figure 4c). VII can be assigned to the reduction of the
16-electron cation 2a + to give the neutral radical 2a •
via the 17-electron radical Ru(η5-C9H7)COD•. The latter
is a 17-electron species, and it is reasonable to suppose
that its reduction corresponds to the second wave VIII
at more negative potential and the overall reduction of
2a + proceeds by two subsequent electron transfers (EE
mechanism). The fact that the second peak is not so
intense as the first indicates the existence of a chemical
reaction which subtracts the radical and at the same
time shifts the wave at a more positive potential, close
to the first wave, in analogy with what is observed for
1a +. The formation of new species in solution is sup-
ported by the presence of waves at ca. 2.3 V (Figure 4d),
which are not observable in the neutral complex. The
oxidation wave IV, since it is detected even in the anodic
scan (Figure 4c), corresponds to the oxidation of 2a +.
Moreover, by comparison with the cathodic voltammo-
gram at low scan rate in Figure 4a, it is possible to
observe that the wave IV is present already before the
ionization of 2, indicating that the cation is formed by
oxidation of 2a - in the scan reversal after the reduction
of 2.
F igu r e 6. (a) Variation of the current function ip v-1/2 M-1
normalized to its value at high scan rate v g 100 V s-1 of
3 mM [RuCl(η5-C9H7)COD] (2) in the presence of Bu4NCl,
-
CCl ) 0 mM (open squares), 6 mM (open diamonds), 12
mM (open circles), and 39 mM (×’s). (b) Variation of
the current function obtained in a plot of ip v-1/2 M-1 vs
log(v × CCl-). Abscises in logarithmic scale.
Ru-Cl or Ru-COD dissociation. No effect was observed,
either on the current function or on the voltammogram
shape, in the presence of free COD (3-24 mM). On the
other hand, the CVs obtained for solutions containing
different amounts of n-Bu4NCl showed a decrease of the
current function proportional to the concentration of Cl-
(Figure 6a), and a common trend is obtained when ip
v-1/2 M-1 is plotted versus log(v × CCl-) (Figure 6b). This
indicates that the rate-determining step is the Ru-Cl
cleavage, which is retarded when the chloride salt is
added in solution, preceded by a η5 to η3 ring-slippage,
analogous but slower than that occurring at the level
of 1•- (eq 5) since 2•- shows partial chemical reversibility
(wave II). These results suggest that the 17-electron
radical [Ru(η5-C9H7)(COD)]• is responsible for the second
electron transfer at a more positive potential than the
starting complex 2 (eq 6). As in the oxidation of complex
1, at high scan rate (500 V s-1) the wave III is still
present, indicating that it corresponds to the oxidation
Addition of 1 equiv of n-Bu4NCl to the solution of the
cationic complex causes a better definition of VII, the
disappearance of VIII, and the formation of the waves
characteristic of the neutral complex (Figure 4d). These
results can be explained by the mechanism reported in
Scheme 2.
Since the reduction of 2a + is still observed even after
the addition of n-Bu4NCl, 2a + does not react with Cl-.
The equilibrium K1 is proved by the disappearance of
the wave VIII relative to 2a •, which recombines with
Cl-, generating the radical anion 2•-. The latter radical
anion is not stable at the potential at which it is formed
and hence oxidizes to regenerate the neutral complex 2
and wave I reappears.
Con clu sion s
This work represents the first report on the electro-
chemical reduction of indenyl halide complexes and of

3484 Organometallics, Vol. 22, No. 17, 2003
Santi et al.
the corresponding cationic species obtained by halide
abstraction. The study indicates that the electrochemi-
cal reduction of both complexes [RuCl(η5-C9H7)(PPh3)2]
(1) and [RuCl(η5-C9H7)COD] (2) proceeds by an ECE
mechanism, in which the chemical step is the Ru-Cl
bond dissociation. This bond, under ordinary chemical
conditions, can be cleaved only in the presence of strong
donor ligands such as PMe3 or a bidentate chelating
phosphine (dppm),5 or in a solution of strongly coordi-
nating solvents such as acetonitrile and methanol in the
presence of NaPF6, or by the action of a precipitating
agent (Ag+).
reduction of the corresponding cation 2a +. In fact, the
reduction of 2a +, generated in situ by chloride abstrac-
tion, proceeds by two subsequent electron transfers (EE
mechanism) via the 17-electron radical Ru(η5-C9H7)-
COD•, in which the metal is in the oxidation state I. On
the contrary, the analogous radical Ru(η5-C9H7)(PPh3)3
•
was not detected as intermediate of the reduction of 1a +
due to a fast radical-radical dimerization via the DIM1
mechanism.
Ack n ow led gm en t. This work was supported by the
Ministero dell’Universita` e della Ricerca Scientifica
e
Tecnologica MURST (COFIN 99, project code
The reduction of complex 1 and 2 generates the 17-
9903198953) and by the CNR (Roma) through its Centro
di Studi sugli Stati Molecolari, Radicalici ed Eccitati
(Padova).
•
electron radical Ru(η5-C9H7)L2 and the corresponding
18-electron anion Ru(η5-C9H7)L2-, in which the metal
is formally Ru(0). The radical Ru(η5-C9H7)COD• is
sufficiently stable to be observed when generated by
OM0300878