Influence of the initial bonding mode of the hydrocarbyl bridge on the mechanisms and products of the electrochemical reduction of alkyne- and vinylidene dimolybdenum tris(μ-thiolate) complexes

Article abstract of DOI:10.1039/b614948k

The electrochemical reduction of isomeric complexes, [Mo₂Cp ₂(μ-SMe)₃(μ-η¹:η¹- HCCPh)]⁺ (1⁺) and [Mo₂Cp₂(μ-SMe) ₃(μ-η¹:η²-C*CHPh)] ⁺ (3⁺), where the hydrocarbyl bridges in a η¹:η¹- or a η¹:η² mode, has been studied by cyclic voltammetry and controlled-potential electrolysis in thf-[NBu₄][PF₆] and CH₂Cl ₂-[NBu₄][PF₆], in the absence and in the presence of acid. The binding mode of the CC fragment induces different electrochemical behaviour of the complexes in acid-free solutions since 1 ⁺ reduces in two diffusion-controlled one-electron steps while the first reduction of 3⁺ is characterized by slow electron transfer kinetics. Controlled-potential reduction of both 1⁺ and 3⁺ produces a mixture of the acetylide [Mo₂Cp₂(μ-SMe) ₃(μ-η¹:η²-CCPh)] (2) and alkylidyne complexes [Mo₂Cp₂(μ-SMe)₃(μ- η¹-CCH₂Ph)] (4). In the presence of acid, the electrochemical reduction of 1⁺ and of 3⁺ occurs according to ECE processes. The nature of the products formed by controlled-potential reduction of 1⁺ depends on the nature of the acid and of the solvent. The transient formation of a complex with a μ-alkenyl ligand, either [Mo₂Cp₂(μ-SMe)₃(μ-η¹: η²-CH*CHPh)] (7) or an isomer, is suggested by the oxidative electrochemistry of 7 and by its reaction with acids. In thf-[NBu ₄][PF₆] in the presence of an excess of acid (HBF ₄/Et₂O) and of phenylacetylene, electrolysis of 1 ⁺ gives rise to catalytic reduction of phenylacetylene to styrene. However, unidentified reactions limit the efficiency of this process. The reduction of 3⁺ in acidic medium produces the alkyl complex [Mo ₂Cp₂(μ-SMe)₃(μ-CH₂CH ₂Ph)] (6) through alkylidyne [Mo₂Cp₂(μ-SMe) ₃(μ-η¹-CCH₂Ph)] (4) and alkylidene [Mo₂Cp₂(μ-SMe)₃(μ-η¹- CHCH₂Ph)]⁺ (5⁺) intermediates. Some ethylbenzene was formed after reduction of 5⁺ in the presence of acid. These results show an effect of the binding mode of the hydrocarbyl bridge on the mechanism and products of the reduction of the corresponding complexes. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/b614948k

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Influence of the initial bonding mode of the hydrocarbyl bridge on the
mechanisms and products of the electrochemical reduction of alkyne- and
vinylidene dimolybdenum tris(l-thiolate) complexesw
´
Alan Le Goff, Christine Le Roy, Franc¸ ois Y. Petillon, Philippe Schollhammer and
Jean Talarmin*
Received (in Durham, UK) 13th October 2006, Accepted 28th November 2006
First published as an Advance Article on the web 2nd January 2007
DOI: 10.1039/b614948k
The electrochemical reduction of isomeric complexes, [Mo2Cp2(m-SMe)3(m-Z1:Z1-HCCPh)]+ (1+)
and [Mo2Cp2(m-SMe)3(m-Z1:Z2-CQCHPh)]+ (3+), where the hydrocarbyl bridges in a Z1:Z1- or a
Z1:Z2 mode, has been studied by cyclic voltammetry and controlled-potential electrolysis in
thf–[NBu4][PF6] and CH2Cl2–[NBu4][PF6], in the absence and in the presence of acid. The binding
mode of the CC fragment induces different electrochemical behaviour of the complexes in acid-
free solutions since 1+ reduces in two diffusion-controlled one-electron steps while the first
reduction of 3+ is characterized by slow electron transfer kinetics. Controlled-potential reduction
of both 1+ and 3+ produces a mixture of the acetylide [Mo2Cp2(m-SMe)3(m-Z1:Z2-CCPh)] (2) and
alkylidyne complexes [Mo2Cp2(m-SMe)3(m-Z1-CCH2Ph)] (4). In the presence of acid, the
electrochemical reduction of 1+ and of 3+ occurs according to ECE processes. The nature of the
products formed by controlled-potential reduction of 1+ depends on the nature of the acid and of
the solvent. The transient formation of a complex with a m-alkenyl ligand, either [Mo2Cp2
(m-SMe)3(m-Z1:Z2-CHQCHPh)] (7) or an isomer, is suggested by the oxidative electrochemistry of
7 and by its reaction with acids. In thf–[NBu4][PF6] in the presence of an excess of acid (HBF4/
Et2O) and of phenylacetylene, electrolysis of 1+ gives rise to catalytic reduction of
phenylacetylene to styrene. However, unidentified reactions limit the efficiency of this process. The
reduction of 3+ in acidic medium produces the alkyl complex [Mo2Cp2(m-SMe)3(m-CH2CH2Ph)]
(6) through alkylidyne [Mo2Cp2(m-SMe)3(m-Z1-CCH2Ph)] (4) and alkylidene [Mo2Cp2(m-SMe)3
(m-Z1-CHCH2Ph)]+ (5+) intermediates. Some ethylbenzene was formed after reduction of 5+ in
the presence of acid. These results show an effect of the binding mode of the hydrocarbyl bridge
on the mechanism and products of the reduction of the corresponding complexes.
as to how a {NN} ligand is progressively reduced by successive
Introduction
{H+/e} transfers at such sites. Using this approach for the
synthesis of {NN}-containing complexes, we demonstrated
previously that the HNQNPh ligand bound in a m-Z1:Z1
fashion to the {[Mo2Cp2(m-SMe)3]+} core could be cleaved
to aniline and ammonia by controlled-potential reduction of
the phenyldiazene complex in acidic medium.4
Di- or polynuclear complexes where the metal centres are
situated in a sulfur-rich environment are attractive models to
study the way unsaturated substrates are activated and re-
duced by nitrogenase enzymes.1 However, the presence of S
ligands in the coordination sphere of the metal centres of
model compounds still remains a major difficulty as far as
binding the dinitrogen molecule itself is concerned.1–3 This has
led to the ‘‘rocket fuel approach’’3 of chemical nitrogen
fixation where an {NN} moiety is introduced in the com-
pounds by using hydrazines. Although it does not solve the
central problem of N2 binding at a metal-sulfur centres, this
indirect approach is helpful in that it may provide information
Another indirect approach consists in investigating the
reduction of nitrogenase co-substrates by sulfur-containing
model complexes. Although the nitrogenases have not yet
disclosed all their secrets as to how the substrates are coordi-
nated and reduced at the active site,5 the FeMo cofactor,6 it
recently became possible to obtain information on the site
where the CRC bond is reduced7 within FeMo-co. On our
side of chemical modelling, several complexes bearing {CC}
bridging units at the {[Mo2Cp2(m-SMe)3]+} site are now
available.8 In particular, the synthesis of compounds where
isomeric alkyne and vinylidene bridging ligands are bound to
{[Mo2Cp2(m-SMe)3]+}8a in a respectively m-Z1:Z1 and m-Z1:Z2
fashion (Scheme 1) provides the opportunity to examine
whether the initial bonding mode of the substrate effects the
course of the electrochemical reduction at a single bimetallic
UMR CNRS 6521 ‘‘Chimie, Electrochimie Mole´culaires et Chimie
´
Analytique’’ UFR Sciences et Techniques, Universite de Bretagne
Occidentale, CS 93837, 29238 Brest-Cedex 3, France. E-mail:
Jean.Talarmin@univ-brest.fr; Fax: + 332 98 01 70 01; Tel: + 332
98 01 79 10
w Electronic supplementary information (ESI) available: Scheme S1.
See DOI: 10.1039/b614948k
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In this paper, we report on the electrochemical reduction of
[Mo2Cp2(m-SMe)3(m-Z1:Z1-HCCPh)]+, 1+, and [Mo2Cp2
(m-SMe)3(m-Z1:Z2-CQCHPh)]+,3+, both in the absence and
in the presence of acid.
Results and discussion
1
Electrochemical reduction of complexes 1+ and 3+ in the
absence of acid
The first part of this study focuses on the primary reduction of
1+ and 3+. The nature of the follow-up chemical step(s) that is
(are) revealed by the scan rate dependence of the peak current
ratio [(ipa/ip )
c red1 13
]
will be examined in Section 1.2. No
electron-transfer induced alkyne to vinylidene interconver-
sion14 was observable in the present case.
1.1 The primary reduction of 1+ and 3+. The cyclic vol-
tammetry (CV) of 1+ and 3+ in CH2Cl2–[NBu4][PF6] (Fig. 1)
shows that both complexes undergo two one-electron reduc-
tion steps (Table 1, Scheme 2) and irreversible or partially
reversible oxidation(s). Similar results are obtained in
thf–[NBu4][PF6].
Scheme 1 In this and the following schemes, the SMe groups of the
bridges and the Cp rings were omitted for clarity.
The reduction processes of the m-alkyne and m-vinylidene
species present similarities, both showing two one-electron
steps the first of which is observed at very close potentials
for 1+ and 3+ (Table 1, Scheme 2). However, there is also a
major difference between these processes. Indeed, both reduc-
tions of 1+ are electrochemically reversible (DEpred B 60 mV,
Table 1)13,15 which suggests that the initial geometry of the
complex is essentially retained16,17 in its one- and two-electron
reduced forms (Scheme 2). This is consistent with the sub-
stantial metal (d) character of the LUMO of [Mo2Cp2
(m-SPri)2(m-S)(m-Z1:Z1-PhCCPh)],11 a compound that is simi-
lar to 1+.
site. It is interesting to note that the m-Z1:Z2 coordination of
the vinylidene ligand is similar to the binding of the
isoelectronic N2 molecule discovered by Fryzuk et al.
in ([NPN]Ta)2(m-H)2(m-Z1:Z2-N2) [NPN
=
PhP(SiMe2
CH2NPh)2].1c,9
All the complexes in Scheme 1 were fully characterized
previously, and different aspects of their reactivity were re-
ported.8 While the coordination mode of the m-vinylidene
ligand in 3+ was established by a crystal structure determina-
tion, the assignment of the bonding mode of the alkyne in 1+
was based upon NMR spectroscopy,8a and on the crystal
structures of an analogue of 1+,10 and of a complex similar
to 1+, namely [Mo2Cp2(m-SPri)2(m-S)(m-Z1:Z1-PhCCPh)].11
The formation of the m-alkylidyne derivative [Mo2Cp2
(m-SMe)3(m-Z1-CCH2Ph)] (4) by treatment of 3+ with hydride,
and the protonation of 4 to the m-alkylidene complex
[Mo2Cp2(m-SMe)3(m-Z1-CHCH2Ph)]+ (5+) were also repor-
ted,8b as well as the chemical and electrochemical conversion
of 5+ to the m-alkyl [Mo2Cp2(m-SMe)3(m-CH2CH2Ph)] (6).8c
The kinetic analysis of the protonation of the m-alkylidyne
to m-alkylidene ligand will be the object of a forthcoming
paper.12
In contrast, the first reduction of 3+ is characterized by
slower electron transfer kinetics, as evidenced by the large
peak to peak separation18 (Table 1), and by the scan rate- and
red1
temperature dependence of DEp
[at 18 1C, DEpred1(3+)
increases from 90 mV for v = 0.02 V sꢁ1 to 350 mV for v =
2 V sꢁ1; for a common scan rate of 0.2 V sꢁ1, DEpred1 increases
from 170 mV to 770 mV upon lowering the temperature from
red1
18 to ꢁ38 1C]. The shift of Ep
to more negative potentials
as the temperature is lowered (Fig. 2) makes the first reduction
of 3+ overlap with the second one at ꢁ38 1C. It can also be
red2
seen from Fig. 2 that DEp is little affected by the tempera-
ture changes, what indicates that the second reduction step of
3+ is electrochemically more reversible than the first one.
Table 1 Redox dataa of the complexes as measured by cyclic voltammetry in CH2Cl2–[NBu4][PF6] (v = 0.2 V sꢁ1, vitreous carbon electrode;
potentials are in V/Fc)
red1
red2
ox1
E1/2
ox2
E1/2
Complex
E1/2
E1/2
[Mo2Cp2(m-SMe)3(m-Z1:Z2-CQCHPh)]+ (1+
[Mo2Cp2(m-SMe)3(m-Z1:Z2-CQCHPh)]+ (3+
)
)
ꢁ0.94 (60)
ꢁ1.84 (60)
0.4 (irr)
0.44
0.56 (irr)
0.7 (irr)
—
0.81 (irr)
0.17
0.9 (irr)
0.5 (irr)
0.43
ꢁ1.01 (170)
ꢁ1.48 (90)
[Mo2Cp2(m-SMe)3(m-Z1-CHCH2Ph)]+ (5+ b
)
ꢁ0.95
ꢁ1.72
[Mo2Cp2(m-SMe)3(m-Z1:Z2-CRCPh)] (2)
[Mo2Cp2(m-SMe)3(m-Z1-CCH2Ph)] (4)
[Mo2Cp2(m-SMe)3(m-CH2CH2Ph)] (6)b
[Mo2Cp2(m-SMe)3(m-Z1:Z2-CHQCHPh)] (7)
a
—
—
—
—
ꢁ0.62
ꢁ0.54
ꢁ0.68
ꢁ0.68
—
—
—
—
b
The figures in parentheses are the peak separation (DEp) in mV; irr: irreversible. Ref. 8c.
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Scheme 2
Slow electron transfer kinetics may be caused by concurrent
complexes at the potential of their first reduction (Eapplied
=
ꢁ1.2 V) were complete after the passage of ca. 1 F molꢁ1
starting material. CV of the catholyte shows the formation of
products with redox potentials consistent with those of the
structure changes if the modifications in bond lengths and
bond angles that accompany electron transfer create a large
activation barrier to the electrode reaction.16,17 In the present
case, EHMO calculations on a model of 3+ (SMe replaced by
SH bridges) indicate that the LUMO possesses Mo2–Cb and
Mo1–Ca antibonding characters (Scheme 3). Therefore, the
first one-electron reduction of 3+ might induce a reorientation
of the hydrocarbyl bridge, with an opening of the
Mo2–Ca–Cb angle and a lengthening of the Mo1–Ca and
Mo2–Cb bonds, that would result in an intermediate geometry
between those of 3+ and of the m-alkylidyne ligand in 4 that
were both characterized structurally.8
ox1
ox2
m-acetylide complex 2 (E1/2
= ꢁ0.62 V; E1/2
= 0.17 V)
and of the m-alkylidyne derivative [Mo2Cp2(m-SMe)3(m-Z1-
CCH2Ph)] (4) (E1/2ox1 = ꢁ0.54 V; Epox2 = 0.9 V). The nature
of the electrolysis products was confirmed by comparison of
the 1H NMR spectrum of the solid isolated from the catholyte
(after removal of the supporting electrolyte) with those of
authentic samples of 2 and 4.8 The 1H NMR spectrum showed
that the solid residue extracted from the catholyte consisted in
a mixture of 2 and 4 in a ca. 1 : 1 ratio in the case of 1+, and in
a 70 : 30 ratio in the case of 3+.
Two possibilities may be considered to rationalize these
results.
An H-atom transfer between two one-electron reduced
species (1 or 3) might produce [Mo2Cp2(m-SMe)3(m-Z1:Z2-
CCPh)] (2) and [Mo2Cp2(m-SMe)3(m-Z1-CCH2Ph)] (4) after
the passage of 1 F molꢁ1 of the cationic starting material.
H-atom transfer has been reported in the case of the one-
electron reduction of a m2-ethylidyne cation to m2-vinylidene
and m2-ethylidene neutral compounds19 and this was also
considered as a possible follow-up step in the electrochemical
reduction of the alkylidene complex [Mo2Cp2(m-SMe)3(m-Z1-
CHCH2Ph)]+ (5+).8c For complex 3+, the loss of a H atom
from 3 might also contribute to the formation of the acetylide
complex 2 in more than 50% yield, while an isomerization (1,2-
H shift) would be required in the case of the m-alkyne derivative
in order to generate [Mo2Cp2(m-SMe)3(m-Z1-CCH2Ph)] (4).
On the other hand, it has been shown that 2 is formed upon
treatment of 1+ or 3+ with a base8a (Scheme 1, top) while 4
resulted from the reaction of 3+ with Hꢁ.8b Therefore, the
formation of both 2 and 4 upon electrochemical reduction of
3+ could derive from a proton transfer between 3+ and 3.
Such a ‘‘father–son’’ acid–base reaction8c,20 (Scheme 4, eqn.
(1)) would occur if the increased electron density resulting
from the one-electron reduction of 3+ to 3 makes the latter
Scheme 3
The fact that the reduction of 3 to 3ꢁ is electrochemically
reversible suggests16 that the structure change occurs essen-
tially during the first reduction step.
From these results, it appears that the initial bonding mode
of the unsaturated hydrocarbyl bridge has an effect on the
primary electrochemical reduction of the complexes.
1.2 The follow-up chemical steps. The scan rate dependence
c red1
of the peak current ratio [(ipa/ip )
approaches 1 with
increasing v] indicates that the primary reduction of both 1+
and 3+ is followed by chemical steps (EC process).13,15 The
products of the follow-up reaction(s) are detected by the
presence of oxidation peaks around ꢁ0.6 V on the return
CV scan (Fig. 1). Controlled-potential electrolyses of the
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Fig. 1 Cyclic voltammetry of (A) [Mo2Cp2(m-SMe)3(m-Z1:Z1-HCCPh)]+ 1+ (1.3 mM), and (B) [Mo2Cp2(m-SMe)3(m-Z1:Z2-CQCHPh)]+ 3+
(0.9 mM) in CH2Cl2–[NBu4][PF6] (vitreous carbon electrode, v = 0.2 V sꢁ1).
Fig. 2 Temperature dependence of the cyclic voltammetry of [Mo2Cp2(m-SMe)3(m-Z1:Z2-CQCHPh)]+ 3+ (0.9 mM, CH2Cl2–[NBu4][PF6];
vitreous carbon electrode; v = 0.2 V sꢁ1). (A) (a) 18 1C, (b) ꢁ18 1C and (c) ꢁ25 1C; (B) (a) 18 1C, (d) ꢁ38 1C
Scheme 4 Possible reactions contributing to the formation of 2 and 4 upon reduction of 3+
.
sufficiently basic to deprotonate the former.21 The intervening
acid–base reaction would be followed by the reduction of 4+
at the electrode (Eapplied = ꢁ1.2 V) or by complex 2 (E1/2ox1(2)
= ꢁ0.62 V; E1/2red (4+) = ꢁ0.54 V, Scheme 4, eqn. (2)). In the
latter case, the overall process in Scheme 4 would be equiva-
lent to a H-atom transfer between 3+ and 3, and the ECE-type
mechanism would be completed by the one-electron reduction
of 2+ at the electrode. However, the scan rate dependence of
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the current function (ipc)red1/v1/2 does not provide conclusive
evidence for an ECE mechanism [(ipc)red1/v1/2 was found
essentially independent of scan rate]. At this point it should
be underlined that the increase in the current function at slow
scan rates that would typify an ECE process15 might be
obscured by the fact that the steps (the ‘‘father–son’’ reaction
(m-SMe)3(m-Z1:Z2-CHQCHPh)]+ (7+), we first looked at the
CV of complex 7, and examined its reaction with acids.
2.1.1 The electrochemical oxidation of 7. The m-vinyl com-
plex 78c undergoes two one- electron oxidation steps (Table 1)
that are more reversible in CH2Cl2–[NBu4][PF6] than in thf.
Controlled-potential oxidation of 7 in thf affords 7+ as the
major product along with a species reducible around ꢁ0.9 V,
after consumption of 1.3 F molꢁ1 7. Upon addition of a base
(2 equiv. Et3N) to the electrolyzed solution, the reduction peak
of 7+ decreases while the reduction at ꢁ0.9 V increases,
suggesting that the latter might be due to a product arising
from the deprotonation of 7+. In thf in the presence of a base
(Et3N or [NBu4]OH), a slight increase of the current function
and the electrochemical reduction of either 4+ or 2+) involved
c red1 8c
in the overall process have opposite effects on (ip )
.
In the case of the m-alkyne complex 1+, the acid–base
reaction corresponding to eqn. (1) in Scheme 4 would produce
2 and the protonated alkyne complex, that is not 4+ but most
probably a m-vinyl cation.22 Whether the formation of 4 results
from an isomerization or from a sequence of acid–base and
electron transfer reactions is not known. It has been shown8a
that 1+ isomerizes to 3+ (that is a precursor of 4) but in
CH2Cl2 this was observed on a much longer time scale (days)
than that of controlled-potential electrolysis (hour(s)). A
1+ - 3+ isomerization catalyzed by 1 could be envisaged,
but the reactions in Scheme SI-1 (ESIw) are not likely to take
place when acid is present, while 4 is also formed by electro-
lysis of 1+ under acidic conditions.
(ip ) )
a ox1/v1/2 [where (ipa ox1 is the first oxidation peak current of
7] at low v (Fig. 4) suggests the occurrence of an ECE process
where the intervening chemical step would consist in either a
slow reaction or an equilibrium that is little shifted away from
7+. The scan rate dependence of the current function and the
detection potential of the product are consistent with the
red1
formation of the m-alkyne complex 1+ (E1/2
= ꢁ0.91 V
in thf) upon oxidation of 7 (Scheme 5). The assignment of the
intervening chemical step as an acid–base equilibrium is also in
agreement with the formation of alkenyl species upon proto-
nation of alkyne ligands.22
2
Electrochemical reduction of 1+ and 3+ in acidic medium
2.1 Reduction of 1+ in the presence of acid and electro-
chemical oxidation of 7. The cyclic voltammetry of 1+ in
CH2Cl2–[NBu4][PF6] in the presence of acid (HBF4 ꢂ Et2O or
CF3CO2H, Fig. 3(A), curve (b)) is consistent with the occur-
rence of an ECE process, characterized by an increase in the
2.1.2 Reaction of 7 with acids. Treatment of 7 with acid
generated an intermediate that was detected by CV but could
not be isolated nor identified. The final product was shown to
depend on the nature of the acid and of the solvent used for
the reaction.
first reduction peak current and suppression of the second
c red1
reduction. This is confirmed by the profile of the plot [(ip )
/
v1/2] against v that deviates from linearity at slow scan rates.
The occurrence of an ECE mechanism in acidic medium
demonstrates that protonation of the one-electron reduced
complex (1) generates a species that is easier to reduce than
1+. There is ample precedent showing that protonation of
alkyne ligands produce alkenyl derivatives.22 Because the
protonation of 1 might afford the m-vinyl cation [Mo2Cp2
When a MeCN–[NBu4][PF6] solution of 7 was treated with
HBF4 ꢂ Et2O, an intermediate that oxidizes at Epox = ꢁ0.22 V
was formed. It eventually afforded [Mo2Cp2(m-SMe)3
(MeCN)2]+ (8+) that was identified by its two characteristic
oxidation processes.23 The fact that styrene was a co-product
of the reaction was evidenced by NMR spectroscopy. 1H
NMR study of the protonation of 7 by DCl in CD3CN showed
Fig. 3 Cyclic voltammetry of (A) [Mo2Cp2(m-SMe)3(m-Z1:Z1-HCCPh)]+ 1+ (0.6 mM) (a) before and (b) after addition of 2 equiv. CF3CO2H, and
(B) [Mo2Cp2(m-SMe)3(m-Z1:Z2-CQCHPh)]+ 3+ (1.2 mM) (a) before and (b) after addition of 1 equiv. HBF4 ꢂ Et2O (CH2Cl2–[NBu4][PF6], vitreous
carbon electrode, v = 0.2 V sꢁ1).
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Scheme 6
Scheme 5 (starting from 1+ in this case). Controlled-potential
electrolyses of 1+ in the presence of B2 equiv. of acid under
the same conditions as those in Scheme 7 (nature of the acid
and of the solvent) produced the complexes also obtained by
protonation of 7 (that is 8+, 9 or 10). Although it does not
prove that [Mo2Cp2(m-SMe)3(m-Z1:Z2-CHQCHPh)] (7) is an
intermediate of the ECE reduction of 1+ in acidic medium,
this appears as a reasonable possibility.
Fig. 4 Scan rate dependence of the current function for the first
oxidation of [Mo2Cp2(m-SMe)3(m-HCQCHPh)] 7 (0.6 mM) in the
absence (’) and in the presence (E) of [NBu4]OH in thf–[NBu4][PF6].
However, the reactions in Scheme 5 and Scheme 7 are not
sufficient to represent the entire reduction process. Indeed, the
m-alkylidyne complex [Mo2Cp2(m-SMe)3(m-Z1-CCH2Ph)] (4) is
always a co-product of the electrolyses (B30–50%). While it is
expected28 that 4 be obtained by reduction of 3+ under acidic
conditions (see below), there is no straightforward explanation
for its formation from 1+ both in the absence and in the
presence of acid. Monitoring of a solution of 1+ by RDE
voltammetry in acid-free thf–[NBu4][PF6] shows that substan-
tial (ca. 50%) isomerization to 3+ takes place within 2 h.29
Therefore, the formation of 4 might result, at least in thf, from
partial isomerization of 1+ to 3+ during the electrolysis.
Although interconversion of m-alkylidyne and m-alkenyl spe-
cies has been reported,30 the possibility of an equilibrium
between 7 and 4 can be ruled out in the present case on the
basis of (i) the formation of 7 (and no 4) as one of the products
obtained by electrolysis of 5+ (no acid, 0 1C),8c and (ii) the
formation of 4 (no 7) as one of the products of the reduction of
1+ in the absence of acid.
Scheme 5
the formation of only the (E)-PhCHCHD isomer, which shows
that H+ (or D+) attack is stereospecific and occurs on the
opposite side of the phenyl group, probably for steric reasons
(Scheme 6).
The transient formation of a m-alkenyl species upon electro-
chemical reduction of 1+ in acidic medium is a reasonable
possibility, but the intermediate might be different from
[Mo2Cp2(m-SMe)3(m-Z1:Z2-CHQCHPh)] (7) since 7 was pre-
pared by deprotonation of the alkylidene complex [Mo2Cp2
(m-SMe)3(m-Z1-CHCH2Ph)]+ (5+) at the b carbon atom,8c
while the intermediate results from the protonation of the
reduced alkyne complex [Mo2Cp2(m-SMe)3(m-Z1:Z1-CHCPh)]
(1). Conceivably, 1 may possess different sites for protonation.
These reactions might afford distinct m-alkenyl complexes [for
example with hydrogen atoms either in a cis (70) or trans (7)
disposition] that show different reactivity as far as isomeriza-
tion to the alkylidyne [Mo2Cp2(m-SMe)3(m-Z1-CCH2Ph)] (4) is
concerned.
Treatment of 7 with HBF4 ꢂ Et2O in CH2Cl2 produced
[Mo2Cp2(m-SMe)3(m-Cl)] (9)23 both in its neutral and cationic
forms as shown by rotating disc electrode (RDE) voltamme-
try, while the reaction with CF3CO2H in CH2Cl2 afforded
[Mo2Cp2(m-SMe)3(m-Z1:Z1-OCOCF3)] (10)24 essentially quan-
titatively.25 CV of a thf–[NBu4][PF6] solution of 7 in the
presence of 1 equiv. HBF4 ꢂ Et2O showed that most of the
starting material was still present after ca. 20 min stirring.
Partial oxidation to 7+ and the formation of an intermediate
ox
with Ep = ꢁ0.3 V were also noted. Addition of phenylace-
tylene (5 equiv.) to this solution resulted in the replacement of
the oxidation peaks of 7 and of the intermediate by the redox
processes of the m-alkyne complex 1+. These reactions are
summarized in Scheme 7, where [7-H]+ represents the detected
but uncharacterized intermediate.
Taken together, the reactions in Scheme 5 and Scheme 7
(eqn. (3)) suggest the possibility to achieve the catalytic
reduction of phenylacetylene to styrene by reducing 1+ in
thf, in the presence of an excess of acid and of PhCRCH.
Controlled-potential electrolysis of 1+ was thus performed at
ꢁ1.2 V in the presence of 10 equiv. HBF4 ꢂ Et2O and of a large
excess of PhCRCH (15–40 equiv.). The electrolysis was
complete after consumption of ca. 8 F molꢁ1 1+ while only
The release of an alkene molecule upon protonation of an
alkenyl complex had already been reported.26,27
2.1.3 The electrochemical reduction of 1+ in acidic medium.
As mentioned above, the reduction of 1+ in the presence of
acid follows an ECE process, consistent with the mechanism in
ꢀc
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Scheme 7 Protonation of 7 by CF3CO2H [reaction (1)] or HBF4 ꢂ Et2O [reactions (2)–(4)].
2 F molꢁ1 1+ were used in the absence of the unsaturated
substrate. Although the charge (11 F molꢁ1 1+) that was
expected under the present (acid limiting) conditions was not
reached, the difference with respect to the charge used when no
PhCRCH is present can be assigned to the reduction of this
substrate (Scheme 8).
with the reactivity of vinylidene compounds with acid,28
the reduction of 3+ in the presence of protons produced the
m-alkylidyne complex [Mo2Cp2(m-SMe)3(m-Z1-CCH2Ph)] (4)
ox1
that was detected by its first oxidation at E1/2
= ꢁ0.54 V
(Fig. 3(B), Scheme 9). Controlled-potential electrolysis
(Eapplied = ꢁ1.2 V, 1 equiv. HBF4 ꢂ Et2O in CH2Cl2–
[NBu4][PF6]) afforded the m-acetylide complex [Mo2Cp2
GC/MS analysis of the electrolyzed solution confirmed the
formation of about 3 mole of styrene per mole of 1+ (that is a
60% yield, since the maximum amount of styrene expected
under these acid-limiting conditions was 5 mol/mol 1+),
corresponding to a charge consumption of ca. 6 F molꢁ1
1+. The cumulated amount of styrene and phenylacetylene
found after electrolysis accounts for only ca. 50% of the
phenylacetylene initially present. The presence of 4 in the
catholyte once more underlines that uncontrolled reactions
perturbed the course of the reduction of 1+.
Despite the side reactions producing 4, the formation of
[Mo2Cp2(m-SMe)3(m-Z1:Z1-OCOCF3)], [Mo2Cp2(m-SMe)3(m-
Cl)] or [Mo2Cp2(m-SMe)3(MeCN)2]+ upon electrochemical
reduction of 1+ in acidic medium demonstrates that the initial
alkyne bridge was converted to a labile ligand and eventually
released, as confirmed by the detection of styrene after con-
trolled-potential electrolysis.
2.2 Electrochemical reduction of 3+ in the presence of acid.
The electrochemical reduction of the m-vinylidene complex
[Mo2Cp2(m-SMe)3(m-Z1:Z2-CQCHPh)]+ 3+ in the presence
of acid (HBF4 ꢂ Et2O) occurs according to an ECE mechanism,
like that of the m-alkyne isomer. This is evidenced by the
increase of the first reduction peak current (Fig. 3(B)) and by
the scan rate dependence of the associated current function
which deviates markedly from linearity at low v. Consistent
Scheme 8 The identity of the m-vinyl intermediate (7 or 70) is not
known.
ꢀc
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redox steps involved in this sequence all occur at less negative
red
potentials than the initial one (E1/2 = ꢁ1.01 V).
Controlled-potential electrolysis of a CH2Cl2–[NBu4][PF6]
solution of 2 in the presence of 4 equiv. HBF4 ꢂ Et2O (Eapplied
= ꢁ1.2 V) leads to the formation of the alkylidyne complex 4
essentially quantitatively25 after consumption of 4.6 F molꢁ1
3+. The excess charge passed with respect to that needed to
produce 4 is assigned to proton reduction (see above), but no
attempt was made to detect H2. A supplement of 3 equiv. acid
was added at this stage, and the electrolysis was resumed at
ꢁ1.2 V. Cyclic voltammetry of the catholyte obtained after the
passage of a further 2.5 F molꢁ1 3+ showed the presence of the
alkyl compound [Mo2Cp2(m-SMe)3(m-CH2CH2Ph)] (6)
(B60%) along with the chloro-bridged species [Mo2Cp2(m-
SMe)3(m-Cl)] (9) in ca. 40% yield.25
A controlled-potential electrolysis of a CH2Cl2–[NBu4][PF6]
solution of 2 was performed in the presence of a larger excess
of acid (about 10 equiv. HBF4 ꢂ Et2O) in an attempt to over-
come the limiting step of the process, that is the protonation of
Scheme 9
(m-SMe)3(m-Z1:Z2-CCPh)] (2) and the m-alkylidyne 4 in a 1 : 4
ratio after the transfer of 1.8 F molꢁ1 3+. Since 2 is known to
protonate readily to 3+,8a its presence (20%) after completion
of the electrolysis indicates that there has been a shortage of
H+ during the reduction process. This is assigned to a futile
proton reduction loop due to oxidation of 4 by H+,12 followed
by reduction of 4+ at the electrode. Controlled-potential
electrolysis in the presence of 2 equiv. acid produced 4 in
almost quantitative yield25 as expected, after the transfer of 2.1
F molꢁ1 3+. The identity of the products of the electrolyses
the alkylidyne complex 4. Under these conditions (Eapplied
=
ꢁ1.2 V; 5.4 F molꢁ1 3+), [Mo2Cp2(m-SMe)3(m-CH2CH2Ph)]
(6) was effectively obtained (ca. 40%25), but 4 (ca. 45%25) and
the chloro-bridged derivative 9 (ca. 15%25) were also detected
by CV of the catholyte.
Although the process in Scheme 10 is rendered non-quanti-
tative by the limiting protonation of [Mo2Cp2(m-SMe)3(m-Z1-
CCH2Ph)] (4) that is also responsible for a {H+/e} waste,
the electrochemical reduction of the CRC triple bond of a
m-alkynyl species (2) straight to the C–C single bond of a m-
alkyl complex (6) is unprecedented as far as we know. Treat-
ment of 6 with acid resulted in the oxidation to 6+ and in the
formation of [Mo2Cp2(m-SMe)3(m-Cl)] (9). GC/MS analysis of
the solution obtained by controlled-potential electrolysis of
1
was ascertained by comparison of the H NMR spectrum of
the solid extracted from the catholyte with those of authentic
samples of 2 and 4.
It has been shown that protonation of 4 by HBF4 ꢂ Et2O
produces the m-alkylidene complex [Mo2Cp2(m-SMe)3(m-Z1-
CHCH2Ph)]+ (5+)8b (along with 4+ as a by-product12), and
that the electrochemical reduction of 5+ in acidic medium
affords the m-alkyl complex [Mo2Cp2(m-SMe)3(m-CH2CH2Ph)]
(6) along with [Mo2Cp2(m-SMe)3(m-Cl)] (9).8c The latter was
previously shown to arise from the reaction of 6 and/or 7 with
HBF4 ꢂ Et2O in CH2Cl2.8c
the
m-alkylidene
complex
[Mo2Cp2(m-SMe)3(m-Z1-
CHCH2Ph)]+ (5+) in the presence of an excess of HBF4 ꢂ Et2O
(3 equiv, 3 F molꢁ1 5+) revealed the presence of both styrene
(0.24 mol per mol 5+) and ethylbenzene (0.28 mol per mol
5+), along with a mixture of 6 and 6+ in a combined 45%
yield, [Mo2Cp2(m-SMe)3(m-Cl)] (ca. 25%) and unidentified
products.25 While the formation of styrene might arise from
the protonation of the m-vinyl complex [Mo2Cp2(m-SMe)3
Therefore, by setting the electrolysis potential at ꢁ1.2 V, it
should be possible to generate 6 directly from the m-acetylide
complex 2 along the {4H+/4e} process in Scheme 10 since the
Scheme 10
ꢀc
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(m-Z1:Z2-CHQCHPh)] (7) that was previously shown to be
produced upon reduction of 5+,8c the formation of some
PhCH2CH3 suggests that the m-alkyl complex 6 may undergo
protonation at the a-carbon and release the hydrocarbyl group
as an alkane.
A parallel can be made between the results of the present
work and those of previous studies focused on the reduction of
{NQN} ligands bound at the same {[Mo2Cp2(m-SMe)3]+}
site. The electrochemical reduction of the organodiazene
complex [Mo2Cp2(m-SMe)3(m-Z1:Z1-HNQNR)]+ in acidic
medium resulted in the cleavage of the NQN bond (and the
release of ammonia plus an amine) when R = Ph, but not
when R = Me.4d This was assigned to the rearrangement of
the {NN} ligand from a m-Z1:Z1 to a m-Z1 coordination mode
at the hydrazido(1ꢁ) stage that was possible only for R = Ph.
When R = Me, the electrochemical reduction in acidic
medium of [Mo2Cp2(m-SMe)3(m-Z1-NQNHMe)]+, that is
the hydrazido(2ꢁ) isomer of the methyldiazene complex,
afforded NH3 and NH2Me,4d showing that the initial bonding
mode of the {NN} ligand affected the course of the reduction
process.
3
Concluding comments
3.1 On the effect of the initial {l-CC} binding mode upon
the reduction processes. The results presented above show that
the electrochemical reductions of the m-alkyne and m-vinyli-
dene complexes in the presence of acid result in successive
transformations of the hydrocarbyl bridge along two main
routes that depend, to some extent, upon the initial {m-CC}
binding mode (Scheme 11).
The reduction of the vinylidene isomer [Mo2Cp2(m-SMe)3
(m-Z1:Z2-CQCHPh)]+ 3+ consists in two consecutive ECE
processes separated by a difficult protonation. These {H+/e}
transfer steps afford an alkyl derivative via alkylidyne and
alkylidene intermediates. While protonation of vinylidene to
alkenyl is known,31 we obtained no evidence for protonation
of 3 at the a carbon atom, that would have generated
[Mo2Cp2(m-SMe)3(m-Z1:Z2-CHQCHPh)]+, a m-alkenyl com-
plex such as 7+.
In the case of the alkyne complex 1+, the initial ECE
process probably produces a m-alkenyl complex (7 or an
isomer, 70) that releases the hydrocarbyl unit as styrene upon
protonation. This is in keeping with the known reactivity of
alkyne and alkenyl compounds with acid.22,26,27 Under appro-
priate conditions (thf, PhCRCH), the starting material 1+
can be regenerated, so the conditions are met for a catalytic
reduction of phenylacetylene to styrene. Nevertheless, this
process suffers from severe restrictions due to unidentified side
reactions that generate [Mo2Cp2(m-SMe)3(m-Z1-CCH2Ph)] 4.
Therefore, while the initial binding mode of the hydrocarbyl
ligand appears to privilege one route over the other, there exist
ways through that connect both pathways.
3.2 Relevance to the reduction of the CRC bond by
nitrogenases? Recent studies provided unique information on
the bonding mode and the geometry of an intermediate
trapped by rapid freezing of the a-70Ala mutant Mo nitrogen-
ase turning over in the presence of propargyl alcohol
(HCRCCH2OH) as the substrate.7 The EPR-detection of
the substrate-derived intermediate identified as the {2H+/2e}-
reduced form of propargyl alcohol (that is allyl alcohol,
H2CQCHCH2OH) bound in a Z2 fashion to a single iron
atom of a [4Fe–4S] face of FeMo-co7b was dependent upon its
stabilization by hydrogen bonding with a-195His 7c
Although
.
this gives unprecedented data on the ultimate intermediate of
the reduction of a CRC bond by a-70Ala-modified Mo
nitrogenase, this does not preclude that the initial binding
mode of the alkyne substrate may be different.
The reactions occurring at the bimetallic-sulfur site (Scheme
11) are reminiscent to those proposed to illustrate the reduc-
tion of C2H2 by nitrogenase, on the bases of kinetic studies of
the enzyme and of the chemistry of mononuclear model
compounds.1b,32 Keeping in mind the limitations underlined
Scheme 11
ꢀc
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above, the {2H+/2e} reduction of an alkyne ligand bound in a
m-Z1:Z1 fashion to a bimetallic-sulfur site to a free alkene, and
the {4H+/4e} reduction of an isomeric m-Z1:Z2 vinylidene
ligand bound at the same site to an alkyl complex (and some
alkane) raise the question of whether the substrate specificity
shown by Mo-, V- and Fe-dependent nitrogenases might result
from a different coordination mode of the CRC substrate to
FeMo-co, FeV-co and FeFe-co.
HBF4 ꢂ Et2O before the electrolysis (Eapplied = ꢁ1.2 V) is
started. The catholyte was treated as above.
Controlled-potentiel reduction of [Mo2Cp2(l-SMe)3(l-g1:g1-
HCCPh)]+ (1+) in the presence of acid and phenylacetylene. 1+
(0.0084 g, 1.29 ꢃ 10ꢁ5 mol) was dissolved in 30 mL
thf–[NBu4][PF6] in the electrochemical cell wrapped in an
aluminium foil. After addition of 10 equivalents HBF4 ꢂ Et2O
and 15 equiv. (657 mg Lꢁ1) phenylacetylene, the solution was
electrolysed (graphite electrode) at ꢁ1.2 V vs. Fc/Fc+. The
electrolysis was complete after the passage of ca. 8 F molꢁ1 1+
(ca. 0.8 F molꢁ1 H+). The catholyte was filtered through a
SPE (DSC-18 500 mg) cartridge in order to remove the metal
products and the supporting electrolyte. The filtrate was
subjected to GC/MS analysis for phenylacetylene and styrene
that confirmed the presence of styrene (140 mg Lꢁ1) and of
phenylacetylene (215 mg Lꢁ1) after electrolysis. The amount of
styrene detected after electrolysis following the above proce-
dure accounted for about 60% of the maximum amount that
could form under these acid-limiting conditions.
Experimental
Methods and materials
All the experiments were carried out under an inert atmo-
sphere, using Schlenk techniques for the syntheses. Tetrahy-
drofuran (thf) and dichloromethane were purified as described
previously.4c Acetonitrile (Merck) was used as received. Fluor-
oboric acid (diethyl ether complex), phenylacetylene and tri-
fluoroacetic acid (Aldrich) were used as received. The
complexes
[Mo2Cp2(m-SMe)3(m-Z1:Z1-HCCPh)]+
(1+),
[Mo2Cp2(m-SMe)3(m-Z1:Z2-CCPh)] (2), [Mo2Cp2(m-SMe)3
(m-Z1:Z2-CQCHPh)]+ (3+), [Mo2Cp2(m-SMe)3(m-Z1-CCH2Ph)]
(4), [Mo2Cp2(m-SMe)3(m-Z1-CHCH2Ph)]+ (5+), [Mo2Cp2
(m-SMe)3(m-CH2CH2Ph)] (6) and [Mo2Cp2(m-SMe)3(m-Z1:
Z2-CHQCHPh)] (7) were prepared according to reported
procedures.8
Controlled-potentiel reduction of [Mo2Cp2(l-SMe)3(l-g1:g1-
CHCH2Ph)]+ (5+) in the presence of acid, and ethylbenzene
analysis. A solution of 5+ (14 mg, 2.15 10ꢁ5 mol) in 20 mL
CH2Cl2 in the presence of 3 equiv. HBF4 ꢂ Et2O was electro-
lyzed at ꢁ1.2 V. After the passage of ca. 3 F molꢁ1 5+, the
electrolysis was stopped, and 1 equiv. of acid was added to the
catholyte. After filtration through a SPE (DSC-18 500 mg)
cartridge a sample of the filtrate was subjected to GC-MS for
detection of styrene and of ethylbenzene. The amounts of
The preparation and the purification of the [NBu4][PF6]
supporting electrolyte were as described previously.4c Cyclic
voltammetric experiments were performed with a PGSTAT 12
driven by an AUTOLAB software. Controlled-potential elec-
trolyses were performed using Tacussel/Radiometer GCU
potentiostat and IG5-N integrator. The cell and electrodes
were as described previously.4c All the potentials (text, tables,
figures) are quoted against the ferrocene-ferrocenium couple;
ferrocene was added as an internal standard at the end of the
experiments.
The 1H NMR spectra (CDCl3 or C6D6) were recorded with
a Bruker AC 300 or AMX 400 spectrometer and were refer-
enced to SiMe4. GC-MS analysis was performed on a GC/MS
VARIAN 2100T Technology Ion Trap, equiped with a
Chrompack CpSil 8 CB capillary column (30 m ꢃ 0.25 mm
internal diameter).
styrene (ca. 24.5 mg Lꢁ1) and ethylbenzene (ca. 32.80 mg Lꢁ1
)
were quantified using calibrations plots.
EH calculations
Extended Huckel calculations were performed with the CA-
¨
CAO package developed by Mealli and Proserpio.33 Standard
atomic parameters were taken for H and C.34 The exponents
(z) and the valence shell ionization potential (Hii in eV) used
for Mo are the standard CACAO parameters,33 that is,
respectively, 1.956 and ꢁ8.34 for 5s, 1.921 and ꢁ5.24 for
5 p. The Hii value for 4d was ꢁ10.50; a linear combination of
two Slater-type orbitals (z1 = 4.54, c1 = 0.5899; z2A = 1.900,
c2 = 0.5899) was used to represent the atomic 4d orbitals.
The model used in the calculations for 3+ was built from the
crystallographically determined bond lengths and bond angles
of this complex8 (except C–H = 1.09 A) with the Me
substituents of the equatorial S atoms replaced by H.
Controlled-potential electrolyses of 1+ and 3+
In the absence of acid. In a typical experiment 10 mg (1.5 ꢃ
10ꢁ5 mol) of complex were dissolved in 15 mL of
CH2Cl2–[NBu4][PF6] under N2, and the potential of the Pt
cathode was set at ꢁ1.2 V. The electrolysis was complete after
the transfer of about 1 F molꢁ1 complex. The catholyte was
cannulated in a Schlenk flask under N2, and the solvent was
removed under reduced pressure. 10 mL pentane were then
added to the solid residue and the mixture was stirred for 10
min before being filtrated. The filtrate was taken down to
dryness, and the solid dried under vacuum. The solid residue
was dissolved in C6D6 or CDCl3 and the products formed by
Acknowledgements
The authors thank the CNRS (Centre National de la Re-
cherche Scientifique) and UBO (Universite de Bretagne
´
Occidentale) for financial support.
References
1
1 (a) F. Barriere, M. C. Durrant and C. J. Pickett, in Catalysts for
Nitrogen Fixation: Nitrogenases, Relevant Chemical Models, and
Commercial Processes, ed. B. E. Smith, R. L. Richards and W. E.
Newton, Kluwer Academic Publishers, The Netherlands, 2004,
ch. 7, pp. 161; (b) D. J. Evans, R. A. Henderson and B. E. Smith,
electrolysis were characterized by H NMR spectroscopy.
.
In the presence of HBF4 Et2O. The same conditions as
above were used except for the addition of 1 or 2 equiv.
ꢀc
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274 | New J. Chem., 2007, 31, 265–276

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a
in Bioinorganic Catalysis, ed. J. Reedijk and E. Bouwman, Marcel
Dekker, New York, 2nd edn, 1999, ch. 7, pp. 153; (c) B. A.
McKay and M. D. Fryzuk, Chem. Rev., 2004, 104, 385; (d) F.
Barriere, Coord. Chem. Rev., 2003, 236, 71; (e) D. Sellmann and J.
Sutter, J. Biol. Inorg. Chem., 1996, 1, 587; (f) D. Coucouvanis, J.
Biol. Inorg. Chem., 1996, 1, 594; (g) M. Hidai and Y. Mizobe,
Chem. Rev., 1995, 95, 1115; (h) E. A. MacLachlan and M. D.
Fryzuk, Organometallics, 2006, 25, 1530, and references
therein.
Ep ꢁ Epc. CV stands for cyclic voltammetry; v (V sꢁ1) is the scan
rate in CV experiments.
14 N. G. Connelly, W. E. Geiger, M. C. Lagunas, B. Metz, A. L.
Rieger, P. H. Rieger and M. J. Shaw, J. Am. Chem. Soc., 1995,
117, 12202.
15 (a) A. J. Bard and L. R. Faulkner, Electrochemical Methods.
Fundamentals and Applications, Wiley, New York, 1980, ch. 11,
pp. 429; (b) E. R. Brown and R. F. Large, in Techniques of
Chemistry Volume I-Physical Methods of Chemistry, Part IIA,
ed. A. Weissberger, Wiley, 1971, ch. 6, pp. 423.
16 Although Nernstian electron transfers are usually associated with
preservation of the structural integrity of the redox partners, and
slow electron transfers are taken as indicative of accompanying
structure change, this is not necessarily true, see: (a) ref. 17; (b) A.
M. Bond and K. B. Oldham, J. Phys. Chem., 1983, 87, 2492; (c) T.
Gennett, W. E. Geiger, B. Willett and F. C. Anson, J. Electroanal.
Chem., 1987, 222, 151; (d) A. Miedaner, R. C. Haltiwanger and D.
L. DuBois, Inorg. Chem., 1991, 30, 417; (e) A. J. Downard, L. R.
Hanton, D. A. McMorran and R. L. Paul, Inorg. Chem., 1993, 32,
6028, and references therein.
2 (a) D. Sellmann, B. Hautsch, A. Rosler and F. W. Heinemann,
¨
Angew. Chem., Int. Ed., 2001, 40, 1505; (b) D. Sellmann, B.
Hautsch, A. Rosler and F. W. Heinemann, Angew. Chem.,
¨
2001, 113, 1553, and references therein; (c) D. Sellmann, A. Hille,
A. Rossler, F. W. Heinemann, M. Moll, G. Brehm, S. Schneider,
¨
M. Reiher, B. A. Hess and W. Bauer, Chem.–Eur. J., 2004, 10,
819; (d) D. Sellmann, A. Hille, F. W. Heinemann, M. Moll, A.
¨
Rossler, J. Sutter, G. Brehm, M. Reiher, B. A. Hess and S.
Schneider, Inorg. Chim. Acta, 2003, 348, 194; (e) M. Reiher, O.
Salomon, D. Sellmann and B. A. Hess, Chem.–Eur. J., 2001, 7,
5195, and references therein.
3 C. J. Pickett, J. Biol. Inorg. Chem., 1996, 1, 601.
17 For leading references on structural changes accompanying elec-
tron transfer steps, see: (a) W. E. Geiger, Prog. Inorg. Chem.,
1985, 33, 275, and references therein; (b) D. H. Evans, K. M.
O’Connell, Electroanalytical Chemistry, ed. A. J. Bard, Marcel
Dekker, New York, 1986, vol. 14, pp. 113, and references therein.
4 (a) P. Schollhammer, E. Gue
W. Muir and D. S. Yufit, Organometallics, 1998, 17, 1922; (b) F.
Y. Petillon, P. Schollhammer, J. Talarmin and K. W. Muir, Inorg.
Chem., 1999, 38, 1954; (c) J.-Y. Cabon, C. Le Roy, K. W. Muir,
F. Y. Petillon, F. Quentel, P. Schollhammer and J. Talarmin,
Chem.–Eur. J., 2000, 6, 3033; (d) N. Le Grand, K. W. Muir, F. Y.
Petillon, C. J. Pickett, P. Schollhammer and J. Talarmin,
nin, F. Y. Petillon, J. Talarmin, K.
´ ´
´
´
18 Comparison of DEp for the first reduction of 1+ to that of 3+
,
and for the first and second reductions of 3+ provides an estimate
for the contribution of slow electron transfer kinetics (vs. un-
compensated solution resistance) in DEp, since all were measured
under the same experimental conditions.
´
Chem.–Eur. J., 2002, 8, 3115.
5 (a) L. C. Seefeldt, I. G. Dance and D. R. Dean, Biochemistry,
2004, 43, 1401; (b) D. C. Rees, Annu. Rev. Biochem., 2002, 71, 221;
(c) J. Christiansen, D. R. Dean and L. C. Seefeldt, Annu. Rev.
Plant Physiol. Plant Mol. Biol., 2001, 52, 269; (d) J. B. Howard
and D. C. Rees, Chem. Rev., 1996, 96, 2965; (e) B. K. Burgess and
D. J. Lowe, Chem. Rev., 1996, 96, 2983; (f) J. Kim and D. C. Rees,
Biochemistry, 1994, 33, 389; (g) B. K. Burgess, Chem. Rev., 1990,
90, 1377, and references therein.
6 (a) J. Kim and D. C. Rees, Science, 1992, 257, 1677; (b) J. Kim
and D. C. Rees, Science, 1993, 260, 792; (c) S. M. Mayer, B. M.
Lawson, C. A. Gormal, M. Roe and B. E. Smith, J. Mol. Biol.,
1999, 292, 871; (d) O. Einsle, F. A. Tezcan, S. L. Andrade, B.
Schmid, M. Yoshida, J. B. Howard and D. C. Rees, Science, 2002,
297, 1696.
7 (a) P. M. C. Benton, M. Laryukhin, S. M. Mayer, B. M. Hoff-
man, D. R. Dean and L. C. Seefeldt, Biochemistry, 2003, 42, 9102;
(b) H.-I. Lee, R. Y. Igarashi, M. Laryukhin, P. E. Doan, P. C.
Dos Santos, D. R. Dean, L. C. Seefeldt and B. M. Hoffman, J.
Am. Chem. Soc., 2004, 126, 9563; (c) R. Y. Igarashi, P. C. Dos
Santos, W. G. Niehaus, I. G. Dance, D. R. Dean and L. C.
Seefeldt, J. Biol. Chem., 2004, 279, 34770; (d) B. M. Barney, R. Y.
Igarashi, P. C. Dos Santos, D. R. Dean and L. C. Seefeldt, J. Biol.
Chem., 2004, 279, 53621; (e) H.-I. Lee, M. Sorlie, J. Christiansen,
T.-C. Yang, J. Chao, D. R. Dean, B. J. Hales and B. M. Hoffman,
J. Am. Chem. Soc., 2005, 127, 15880.
19 T. Aase, M. Tilset and V. D. Parker, Organometallics, 1989, 8,
1558.
20 Y. Alias, S. K. Ibrahim, M. A. Queiros, A. Fonseca, J. Talarmin,
F. Volant and C. J. Pickett, J. Chem. Soc., Dalton Trans., 1997,
4807.
21 Considering the fact that 3+ is likely to be a much stronger base
than 3, an electrolysis carried out at the potential of the second
reduction of 3+ should involve a proton transfer reaction between
3+ and 3ꢁ that would lead to the formation of 2 and 4 in 1 : 1
ratio. That this is effectively the case was shown by a differential
pulse polarogram of the catholyte obtained after completion of
the electrolysis at –1.8 V (n = 1 F molꢁ1 3+).
22 For examples of the protonation of alkyne to alkenyl ligands, see:
(a) M. Etienne, C. Carfagna, P. Lorente, R. Mathieu and D. de
Montauzon, Organometallics, 1999, 18, 3075; (b) C. P. Casey, J.
T. Brady, T. M. Boller, F. Weinhold and R. K. Hayashi, J. Am.
Chem. Soc., 1998, 120, 12500; (c) G. A. Acum, M. J. Mays, P. R.
Raithby and G. A. Solan, J. Organomet. Chem., 1995, 492, 65; (d)
L.-S. Wang and M. Cowie, Can. J. Chem., 1995, 73, 1058; (e) R.
A. Henderson, K. E. Oglieve and P. Salisbury, J. Chem. Soc.,
Dalton Trans., 1995, 2479; (f) M. J. Chetcuti, J. C. Gordon, K. A.
Green, P. E. Fanwick and D. Morgenstern, Organometallics,
1989, 8, 1790; (g) R. F. Gerlach, D. N. Duffy and M. D. Curtis,
Organometallics, 1983, 2, 1172.
8 (a) P. Schollhammer, N. Cabon, J.-F. Capon, F. Y. Pe
Talarmin and K. W. Muir, Organometallics, 2001, 20, 1230; (b) N.
Cabon, P. Schollhammer, F. Y. Petillon, J. Talarmin and K. W.
Muir, Organometallics, 2002, 21, 448; (c) N. Cabon, A. Le Goff,
C. Le Roy, F. Y. Petillon, P. Schollhammer, J. Talarmin, J. E.
´
tillon, J.
23 F. Barriere, Y. Le Mest, F. Y. Petillon, S. Poder-Guillou, P.
´
Schollhammer and J. Talarmin, J. Chem. Soc., Dalton Trans.,
1996, 3967.
´ ´
24 M. Le Henanf, C. Le Roy, K. W. Muir, F. Y. Petillon, P.
Schollhammer and J. Talarmin, Eur. J. Inorg. Chem., 2004, 1687.
25 The yields of the products formed upon controlled-potential
electrolyses were obtained by comparison of the CV peak current
of the products with that of the starting material before con-
trolled-potential electrolysis, assuming identical diffusion coeffi-
cients.
26 D. Glushove, G. Hogarth and M. H. Lavender, J. Organomet.
Chem., 1997, 528, 3.
27 D. S. Frohnapfel and J. L. Templeton, Coord. Chem. Rev., 2000,
206–207, 199, and references therein.
28 For examples of protonation of vinylidene to alkylidyne, see: (a)
G. M. Dawkins, M. Green, J. C. Jefferey, C. Sambale and F. G.
A. Stone, J. Chem. Soc., Dalton Trans., 1983, 499; (b) R. A.
Colborn, D. L. Davies, A. F. Dyke, A. Endesfelder, S. A. R.
Knox, A. G. Orpen and D. Plaas, J. Chem. Soc., Dalton Trans.,
1983, 2661; (c) K. R. Birdwhistell, T. L. Tonker and J. L.
Templeton, J. Am. Chem. Soc., 1985, 107, 4474; (d) A. Hills, D.
´
´
McGrady and K. W. Muir, Organometallics, 2005, 24, 6268.
9 (a) M. D. Fryzuk, S. A. Johnson, B. O. Patrick, A. Albinati, S. A.
Mason and T. F. Koetzle, J. Am. Chem. Soc., 2001, 123, 3960; (b)
M. D. Fryzuk, Chem. Rec., 2003, 3, 2.
10 The
crystal
structure
of
[Mo2Cp2(m-SMe)3(m-Z1:Z1-
HCCCO2Me)]+ shows that the coordination of the alkyne is as
illustrated in Scheme 1 for complex 1+; P. Schollhammer and W.
Solo-Ojo, unpublished results.
11 H. Adams, M. J. Morris, P. Mountford, R. Patel and S. E. Spey,
J. Chem. Soc., Dalton Trans., 2001, 2601.
12 A. Le Goff, C. Le Roy, F. Y. Petillon, P. Schollhammer, J.
´
Talarmin and R. A. Henderson, to be published.
13 The parameters ip and Ep are respectively the peak current and
a
,
the peak potential of a redox process; E1/2 = (Epa + Epc)/2; Ep
ipa and Epc, ipc are respectively the potential and the current of the
anodic and of the cathodic peak of a reversible process; DEp
=
ꢀc
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New J. Chem., 2007, 31, 265–276 | 275

View Article Online
L. Hughes, N. Kashef, N. A. N. D. A. Lemos, A. J. L. Pombeiro
and R. L. Richards, J. Chem. Soc., Dalton Trans., 1992, 1775; (e)
M. F. N. N. Carvalho, S. S. P. R. Almeida, A. J. L. Pombeiro and
R. A. Henderson, Organometallics, 1997, 16, 5441; (f) T. B. Wen,
Y. K. Cheung, J. Yao, W.-T. Wong, Z. Y. Zhou and G. Jia,
Organometallics, 2000, 19, 3803; (g) P. Gonzalez-Herrero, B.
31 For examples of protonation of a vinylidene to an alkenyl ligand,
see: (a) H. Werner, R. Weinand, W. Knaup, K. Peters and H. G.
von Schnering, Organometallics, 1991, 10, 3967; (b) M. Bamber,
G. C. Conole, R. J. Deeth, F. T. Froom and M. Green, J. Chem.
Soc., Dalton Trans., 1994, 3569.
32 (a) D. J. Lowe, K. Fisher and R. N. F. Thorneley, Biochem. J.,
1990, 272, 621; (b) G. A. Ashby, M. J. Dilworth and R. N. F.
Thorneley, Biochem. J., 1987, 247, 547.
Weberndorfer, K. Ilg, J. Wolf and H. Werner, Organometallics,
¨
2001, 20, 3672; (h) N. J. Beach, H. A. Jenkins and G. J. Spivak,
Organometallics, 2003, 22, 5179.
29 The isomerization process is much slower in CH2Cl2
33 C. Mealli and D. M. Proserpio, J. Chem. Educ., 1990, 67,
399.
8b
.
30 (a) C. P. Casey, S. R. Marder and P. J. Fagan, J. Am. Chem. Soc.,
1983, 105, 7197; (b) C. P. Casey, S. R. Marder and B. R. Adams,
J. Am. Chem. Soc., 1985, 107, 7700.
34 (a) R. Hoffmann, J. Chem. Phys., 1963, 39, 1397; (b) R. Hoff-
mann and W. N. Lipscomb, J. Chem. Phys., 1962, 36,
2179.
ꢀc
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276 | New J. Chem., 2007, 31, 265–276