Electrochemical and DFT studies of the oxidative decomposition of the trihydride complexes Cp*M(dppe)H3 (M = Mo, W) in acetonitrile

Article abstract of DOI:10.1039/b518343j

A detailed electrochemical study of the oxidative decomposition of the trihydride complexes Cp*M(dppe)H₃ (M = Mo, W) in acetonitrile is presented. For the Mo complex, the decomposition occurs by four different pathways involving classical and non-classical tautomers, whereas only the classical form is accessible for the W derivative. Each of the decomposition pathways has been quantitatively assessed by analyses of the linear sweep voltammograms. In addition to the previously established (B. Pleune, D. Morales, R. Meunier-Prest, P. Richard, E. Collange, J. C. Fettinger and R. Poli, J. Am. Chem. Soc., 1999, 121, 2209-2225) deprotonation, disproportionation, and H ₂ reductive elimination occurring via the non-classical tautomer of the 17-electron complex [Cp*Mo(dppe)H₃]⁺ (obtained by oxidation at E_1/2 = -0.33 V vs. Ag/AgCl), a new decomposition pathway from the more stable classical tautomer has been identified following a second oxidation process. In addition, the oxidatively induced H₂ reductive elimination, previously evidenced only in THF or CH₂Cl ₂, has been quantitatively assessed in MeCN. This process occurs preferentially by an associative mechanism (k = 0.020(4) M^-1s ^-1) via the 19-electron [Cp*Mo(dppe)H(H₂)(MeCN)] ⁺ intermediate and is therefore in direct competition with the disproportionation mechanism. The resulting 17-electron [Cp*Mo(dppe) H(MeCN)]⁺ product is further oxidized at ca. 0.2 V. The oxidation of [Cp*Mo(dppe)H₃]⁺ occurs at ca. 1.0 V and the resulting 16-electron [Cp*Mo(dppe)H₃]²⁺ complex immediately delivers a proton to the starting material, giving [Cp*Mo(dppe)H₄]⁺ and [Cp*Mo(dppe)H ₂]⁺. The latter coordinates MeCN in a rate determining step to afford [Cp*Mo(dppe)H₂(MeCN)]⁺. The mechanistic details are consistent with studies at different scan rates and different MeCN concentrations, and are backed up by DFT calculations. the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2006.

Full text of DOI:10.1039/b518343j

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Electrochemical and DFT studies of the oxidative decomposition of the
trihydride complexes Cp*M(dppe)H3 (M = Mo, W) in acetonitrilew
Rinaldo Poli,*a Miguel Baya,a Rita Meunier-Prestb and Suzanne Raveaub
Received (in Montpellier, France) 4th January 2006, Accepted 3rd March 2006
First published as an Advance Article on the web 20th March 2006
DOI: 10.1039/b518343j
A detailed electrochemical study of the oxidative decomposition of the trihydride complexes
Cp*M(dppe)H3 (M = Mo, W) in acetonitrile is presented. For the Mo complex, the
decomposition occurs by four different pathways involving classical and non-classical tautomers,
whereas only the classical form is accessible for the W derivative. Each of the decomposition
pathways has been quantitatively assessed by analyses of the linear sweep voltammograms. In
addition to the previously established (B. Pleune, D. Morales, R. Meunier-Prest, P. Richard, E.
Collange, J. C. Fettinger and R. Poli, J. Am. Chem. Soc., 1999, 121, 2209–2225) deprotonation,
disproportionation, and H2 reductive elimination occurring via the non-classical tautomer of the
17-electron complex [Cp*Mo(dppe)H3]1 (obtained by oxidation at E1/2 = ꢀ0.33 V vs. Ag/AgCl),
a new decomposition pathway from the more stable classical tautomer has been identified
following a second oxidation process. In addition, the oxidatively induced H2 reductive
elimination, previously evidenced only in THF or CH2Cl2, has been quantitatively assessed in
MeCN. This process occurs preferentially by an associative mechanism (k = 0.020(4) Mꢀ1sꢀ1) via
the 19-electron [Cp*Mo(dppe)H(H2)(MeCN)]1 intermediate and is therefore in direct competition
with the disproportionation mechanism. The resulting 17-electron [Cp*Mo(dppe)H(MeCN)]1
product is further oxidized at ca. 0.2 V. The oxidation of [Cp*Mo(dppe)H3]1 occurs at ca. 1.0 V
and the resulting 16-electron [Cp*Mo(dppe)H3]21 complex immediately delivers a proton to the
starting material, giving [Cp*Mo(dppe)H4]1 and [Cp*Mo(dppe)H2]1. The latter coordinates
MeCN in a rate determining step to afford [Cp*Mo(dppe)H2(MeCN)]1. The mechanistic details
are consistent with studies at different scan rates and different MeCN concentrations, and are
backed up by DFT calculations.
pathways depending on conditions, all of them occurring via
the non-classical tautomer, [Cp*Mo(dppe)H(H)2]1ꢁ. These
Introduction
Transition metal hydride complexes have attracted consider-
consist of (i) substitution of H2 with a solvent molecule (only
able attention both for practical and fundamental reasons
evidenced in THF or CH2Cl2); (ii) solvent assisted dispropor-
such as hydrogen storage,1 industrial catalysis,2 the study of
tionation, prevalent in MeCN; and (iii) deprotonation by
coordinated H2 ligands (so called non-classical hydride com-
excess Cp*M(dppe)H3. The last two mechanisms have been
plexes),3–7 the existence of stable paramagnetic transition
elucidated by analyses of the cyclic and thin layer voltammo-
metal mono- and polyhydride complexes,8 and so forth. We
grams of the first oxidation step of Cp*M(dppe)H3 in MeCN.
are interested in the kinetic stabilization of paramagnetic
We present in this paper a more detailed investigation over a
complexes and their further transformation to highly unsatu-
larger range of potentials covering the first and the second
rated reactive species. In this context, it is of fundamental
oxidation stages. This study reveals an additional electroche-
importance to have a complete knowledge of the available
mical process that can be linked to the oxidatively induced H2
decomposition mechanisms and to understand how they can
substitution by MeCN, allowing its quantitative assessment.
be selectively affected by the ligand environment and other
Furthermore, an additional pathway involves the more stable
external parameters. In a previous paper,9 we have shown that
classical form of the 17-electron trihydride complex, following
[Cp*Mo(dppe)H3]1 decomposes according to three different
ꢁ
its further oxidation.
a
´
Laboratoire de Chimie de Coordination, UPR CNRS 8241, lie par
Results and discussion
`
´
`
convention a l’Universite Paul Sabatier et a l’Institut National
Polytechnique de Toulouse, 205 Route de Narbonne, 31077 Toulouse,
France. E-mail: poli@lcc-toulouse.fr; Fax: þ33 (0)561553003;
Tel: þ33 (0)561333195
This article mostly focuses on the complex electrochemical
behavior of the molybdenum compound Cp*Mo(dppe)H3.
The related tungsten complex Cp*W(dppe)H3 behaves in a
much simpler fashion. Its study has helped on many occasions
to clarify obscure issues related to the behavior of the molyb-
denum system. To simplify the notation, the Cp*M(dppe)
b
`
`
´
Laboratoire de Synthese et d’Electrosynthese Organometalliques,
´
UMR CNRS 5188, Universite de Bourgogne, Faculte des Sciences
Mirande, 9 avenue Alain Savary, 21078 Dijon, France
w Electronic supplementary information (ESI) available: Cartesian
coordinates of all DFT optimized geometries. See 10.1039/b518343j
´
ꢂc
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moiety (M = Mo, W), is abbreviated as [M] except in the
section titles, e.g. complex Cp*Mo(dppe)H3 will be abbre-
viated as [Mo]H3.
to a switching potential of 0.5 V, the C/C0 system is not
observed on the reverse and second scans. On the other hand,
peak B is present on the very first scan. Neither its relative
intensity nor its irreversibility in subsequent multiple scans is
affected by the occurrence of process D. In comparison with
complex [Mo]H3, the behavior of [W]H3 in MeCN is much
simpler, see Fig. 2. Peak B is quasi non-existent and system
F/F0 is not observed whatever the sweep rate in the potential
range studied (from ꢀ1 V to þ1.5 V vs. Ag/AgCl). As for the
Voltammetric study of Cp*M(dppe)H3 (M = Mo, W)
General features. The voltammogram of [Mo]H3 in MeCN
presents five redox systems over the potential range from ꢀ1 V
to þ1.5 V vs. Ag/AgCl, see Fig. 1. These consist of a system
A/A0 which corresponds to the first oxidation of [Mo]H3
(E1A = ꢀ0.34 V), an irreversible peak B, a reversible system
C/C0 (E1C = 0.336 V), an irreversible peak D and a reversible
system F/F0 (E1F = 1.2 V).
analogous molybdenum system, the C/C0 process (E1C
=
0.225 V) is not observed when the scan is reversed before the
occurrence of process D.
Our previous investigations9 have shown that the greater
electron richness of W relative to Mo disfavors the chemical
The A/A0 system has been previously investigated in detail
in MeCN solution at various scan rates and concentrations,
both by cyclic voltammetry and by thin-layer cyclic voltam-
metry.9 A solvent-assisted disproportionation and a deproto-
1
ꢁ
decomposition of WH3 which could in fact be isolated from
a CH3CN–Et2O solution. Its X-ray structure shows a classical
trihydride geometry (as opposed to a non-classical monohy-
dride(dihydrogen) structure, [W](H)(H2)1ꢁ). The stability of
1
ꢁ
nation of the one-electron oxidation product [Mo]H3
by
1
ꢁ
residual unoxidized [Mo]H3 take place competitively. The
disproportionation rate constant was obtained by analyzing
[W]H3 in MeCN suggests that peak D corresponds to the
oxidation of [W]H3 to [W]H321. This is confirmed by an
1
ꢁ
the cyclic voltammetry results (kdisp = 3.98(9) ꢃ 103 Mꢀ1sꢀ1
)
1
ꢁ
independent cyclic voltammetric study of complex [W]H3
,
while the deprotonation rate constant was derived from the
thin-layer experiments (kdep = 2.8(2) ꢃ 102 Mꢀ1sꢀ1). In the
previous paper,9 we have also shown that the stoichiometric
vide infra. Thus, the similarity of peak D for the Mo and W
systems (cf. Fig. 1 and Fig. 2) indicates that the oxidized Mo
1
system, [Mo]H3 ꢁ, also adopts a classical trihydride structure
and is oxidized irreversibly to species [Mo]H321 at peak D. An
internal electronic rearrangement from the classical MoV
trihydride to a non-classical MoIII hydride–dihydrogen com-
plex would be expected to lead to more dramatic differences in
the cyclic voltammogram, as shown in previous studies.10
chemical oxidation of [Mo]H3 in MeCN leads to a mixture of
21
[Mo]H2(MeCN)1 and [Mo]H(MeCN)2
.
An independent electrochemical investigation of these two
products in MeCN helps clarify some of the features of the
complex voltammetric behavior of MoH3. Both complexes
exhibit reversible oxidation processes and, as expected,
DFT calculations (vide infra) agree with a classical form for
1
[Mo]H2(MeCN)1 is easier to oxidize (E1 = 0.336 V) than
the Mo complex, at least at the oxidation state level [Mo]H3
.
21
[Mo]H(MeCN)2
(E1 = 1.2 V). By superposition of the
When the scan rate is increased, the voltammogram of
[Mo]H3 evolves (e.g. see Fig. 3 at v = 8 V sꢀ1) to become
similar to that of [W]H3, i.e. peaks B and F/F0 disappear while
the current of the cathodic peak A0 increases. At the same
time, peak D shifts towards more positive values. To summar-
ize the observations made so far, we can identify two inde-
pendent phenomena. The first one, prevalent at high scan
rates, is the development of process C/C0 (reversible oxidation
voltammograms recorded under the same experimental con-
ditions, the C/C0 couple in Fig. 1 can be assigned to the
reversible oxidation of [Mo]H2(MeCN)1, while F/F0 corre-
21
sponds to that of [Mo]H(MeCN)2
. Species [Mo]H2
(MeCN)1 is formed only after process D has taken place,
because when the voltammetric measurement is carried out up
21
of [Mo]H2(MeCN)1) after the formation of [Mo]H3 at D.
Fig. 1 Two-cycle cyclic voltammogram of [Mo]H3, 3.7 ꢃ 10ꢀ4 M in
MeCN containing 0.2 M Bu4NPF6 at a scan rate of 0.2 V sꢀ1
.
Superposition of the voltammograms of (--) [Cp*MoH2(dppe)
(MeCN)]1 and (— ꢄ ꢄ —) [Cp*MoH(dppe)(MeCN)2]21 under the same
conditions.
Fig. 2 Two-cycle cyclic voltammogram of [W]H3, 2 ꢃ 10ꢀ4 M in
MeCN containing 0.2 M Bu4NPF6 at a scan rate of 0.6 V sꢀ1
.
ꢂc
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Scheme 1
In order to confirm the involvement of MeCN in the EECE
mechanism of Scheme 1, we have studied the electrochemical
response of [Mo]H3 in various THF–MeCN mixtures. In pure
THF, the oxidation of [Mo]H3 induces a slow dihydrogen
reductive elimination (k = 0.0016 minꢀ1)9 that cannot be seen
on the cyclic voltammetry time scale. In addition, the decom-
position modes by disproportionation and proton transfer9
are also not observable in this solvent at the scan rates used for
this study. Consequently, the voltammogram presents only the
reversible system A/A0 corresponding to the first oxidation
followed by the second oxidation (peak D), see Fig. 4, part a.
As a matter of fact, a low intensity feature also appears at a
potential close to that characteristic of the reversible [Mo]H2
(MeCN)1 oxidation, but peaks C/C0 appear as a well defined
reversible wave only after MeCN is gradually added to the
solution. We tentatively attribute this additional peak to a
trapping process by the THF solvent according to Scheme 1,
Fig. 3 Two-cycle cyclic voltammogram of [Mo]H3, 4 ꢃ 10ꢀ4 M in
MeCN containing 0.2 M Bu4NPF6 at a scan rate of 8 V sꢀ1
.
The second one, most visible at slow scan rates, consists of the
development of processes B and F/F0, which may or may not
be related to each other. We shall examine the first pheno-
menon in the next section and will return to processes B and
F/F0 in the following one.
Analysis of the A/A0, C/C0, and D processes. As seen above,
the C/C0 system is observed only after the second oxidation
process leading to [Mo]H321. In addition, an analysis of Fig. 2
and Fig. 3 shows that, upon running two consecutive cycles,
the height of peak A in the second scan is reduced to one half
that of the first scan for the W compound while a smaller but
nevertheless significant decrease is observed for the Mo com-
pound (A/C = 1 for W; 3 for Mo; this ratio becomes 1 also for
the Mo compound at slower scan rates, see Fig. 1). From these
21
observations, we conclude that the [M]H3 complex formed
in the second oxidation stage is involved in a chemical reaction
leading to [M]H2(MeCN)1 (this is in accordance with the
irreversibility of system D and with its potential shift as the
scan rate increases) and that two [M]H3 molecules have been
involved in the global mechanism, see Scheme 1. The overall
scheme corresponds therefore to an EECE process. The
chemical step must obviously be slower for the Mo than for
the W system, since the height of peaks A and A0 in the second
scan is reduced to one half that in the first scan only at slow
scan rates (cf. Fig. 1 and Fig. 3).
The proposed deprotonation of the doubly oxidized com-
plex by the starting compound has ample precedent in the
literature8 and is in accordance with the known acidity in-
crease11 on going from the 18- to the 17- and further to 16-
electron hydride complexes. According to our previous che-
1
mical study,9 complex [M]H4 evolves to generate additional
[M]H2(MeCN)1 by the replacement of two hydride ligands
(reductively eliminating to afford H2) by a solvent molecule.
This reaction, however, is slow on the time scale of the cyclic
voltammetric experiment and consequently does not contri-
bute to the shape of the cyclic voltammetric response. Further-
more, [M]H41 is not electroactive in the potential range ꢀ1 to
þ1.5 V (vs. Ag/AgCl) and therefore no kinetic data on this
chemical step would be accessible from the electrochemical
experiments.
Fig. 4 Two-cycle cyclic voltammogram of [Mo]H3, 1.5 ꢃ 10ꢀ4 M in
THF–MeCN mixtures containing 0.2 M Bu4NPF6 at a scan rate of
1 V sꢀ1; (a): CMeCN = 0 M, (b): CMeCN = 3.2 M.
ꢂc
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but the resulting [M]H2(THF)1 product is presumably less
stable. Peak B is completely absent in pure THF.
The A/A0 peak potential values do not depend on the
MeCN concentration up to ca. 1.7 M, demonstrating that
no thermodynamically favorable interaction with the solvent
occurs after the first oxidation process. At higher MeCN
concentrations, the whole A/A0 system starts to shift toward
less positive potentials with a slope of ꢀ200 mV/log CMeCN
.
This shift is probably caused by solvation effects, as has
previously been shown for other similar cation–solvent inter-
actions.12 The slight A/A0 current increase on going from THF
to MeCN may be attributed to the low dielectric constant of
THF (eTHF = 7.4) compared to that of MeCN (eMeCN =
37).13 This makes the THF solutions quite resistive inducing a
bigger gap between the anodic and cathodic peaks.
Fig. 5 Variations of EpD as a function of log CMeCN extracted from
the experimental voltammograms of [Mo]H3, 1.5 ꢃ 10ꢀ4 M in
THF–MeCN mixtures containing 0.2 M Bu4NPF6 at different scan
The C/C0 couple appears immediately at the lowest CMeCN
used (i.e. 0.038 M). Upon further increasing CMeCN, the
current and the peak potentials of these peaks do not further
change. This is due to the large excess of MeCN compared to
rates: (’) 0.2 V sꢀ1, (þ) 0.1 V sꢀ1, (D) 0.06 V sꢀ1, (J) 0.04 V sꢀ1
.
21
[Mo]H3 and to the fast chemical transformation leading to
[Mo]H2(MeCN)1. Finally, the solvent addition to [Mo]H3
Table 1 Slopes of the variations of the peak potential of process D
for compound [Mo]H3 as a function of log CMeCN (Fig. 5)
21
has an influence on the evolution of peak D with the MeCN
concentration.14 For a mechanism of the type shown in eqn (1)
and (2), (S = redox inactive reagent; nX and nS = stoichio-
metric coefficients), the theory shows that the peak potential
should vary linearly with the logarithm of the S concentration
according to eqn (3)15 where rX and rS are the reaction orders.
v/V sꢀ1
(qEpD/qlog CMeCN)/mV
Regression coefficient
0.2
0.1
0.06
0.04
34 (8)
32 (4)
29 (4)
25 (4)
0.88
0.97
0.93
0.94
Any such rearrangement would consist of a (reversible) first
order process and would not affect the overall electrochemical
21
X ꢀ neꢀ - Xn1
nX Xn1 þ nS S - Products
(1)
(2)
and kinetic behavior. The relative energy of the two [Mo]H3
isomers will be analyzed later in the DFT section.
@Ep
rS
60 T
¼
ð3Þ
@ log CS ðrX þ 1Þ n 302
Cyclic voltammetry of [Cp*W(dppe)H3]1ꢁ. Further confir-
mation of the mechanism shown in Scheme 1 comes from the
1
A plot of the peak potential EpD as a function of log CMeCN
is shown in Fig. 5. For each scan rate, peak D shifts towards
more positive values as CMeCN increases with a slope of
approximately 30 mV per log unit (Fig. 5 and Table 1). This
is consistent with rX = rS = 1 (eqn (3), n = 1), i.e. the
chemical reaction is first order in both [Mo]H321 and MeCN,
in agreement with the formation of [Mo]H2(MeCN)1 as
postulated in Scheme 1. This observation excludes that the
chemical process consists of an irreversible and rate determin-
ing proton transfer followed by a fast solvent coordination
(Scheme 2, path (a)). However, either a rate determining
solvent coordination followed by proton transfer (path (b))
or a fast (reversible or irreversible) proton transfer followed by
an irreversible and rate determining solvent coordination
(path (c)) would be compatible with the results.
cyclic voltammogram of complex [W]H3 ꢁ. The molybdenum
analogue, as stated above, is not a stable compound and
therefore cannot be studied under analogous conditions. The
compound shows the expected reversible A0/A process in the
reductive direction and the irreversible peak D in the oxidative
direction. When the scan is extended on the positive potential
side to þ0.8 V to encompass process C/C0, see Fig. 6(a), no
significant redox activity is observed at the C/C0 potential,
confirming that complex [W]H2(MeCN)1 does not form
An additional dichotomy in relation to the proton transfer
21
process must be considered. When complex [Mo]H3 (or its
MeCN adduct) furnishes its proton to [Mo]H3, it may adopt
either a classical or a non-classical form. It has been shown in
a number of cases that the kinetic acidity is greater for a
dihydrogen complex even when the thermodynamic acidity is
greater for the dihydride tautomer.16–18 Thus, we must con-
21
sider the possibility that the [Mo]H3 species delivers the
proton only after an internal rearrangement to the non-
classical form, even if this may be thermally less favored.
Scheme 2
ꢂc
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1
at lower concentrations from [W]H3 during the reductive
ꢁ
21
scan (peak A0). Therefore, the reaction between [W]H3
(produced at D) and [W]H3 (produced at A0), leading to
[W]H2(MeCN)1 according to Scheme 1, occurs to a lower
extent. A continuation of the same experiment, but switching
again the potential scan at þ0.8 V, allows a recovery of the
reversible A/A0 process intensity, while the C/C0 peaks slowly
fade away as shown in Fig. 6(c). Given the close correspon-
dence of the peak potentials for all processes of the Mo and W
complexes, the behavior established for the W complex may
safely be extrapolated to the Mo analogue. Therefore, the
combination of the experiments in mixed THF–MeCN solvent
for [Mo]H3 (Fig. 4) and the voltammograms of [W]H3 (Fig. 2)
1
ꢁ
and [W]H3 (Fig. 6) in pure MeCN establish unambiguously
that complex [M]H2(MeCN)1, responsible for process C/C0, is
only generated upon the simultaneous presence of [M]H3,
[M]H321, and MeCN.
Analysis of process B. As shown previously, process B
becomes predominant only at low scan rates and its presence
is independent from the EECE process examined in the
previous section. In order to understand the origin of this
process, more detailed voltammetric investigations were car-
ried out up to a switching potential of 0.5 V vs. Ag/AgCl. The
investigations carried out in mixed THF–MeCN solvent show
that the intensity of this peak increases proportionally with
C
MeCN. Therefore, the compound oxidized in B must result
1
from the reaction of MeCN with [Mo]H3 ꢁ, i.e. we have a
chemical–electrochemical (CE) mechanism, the chemical reac-
tion preceding the electrochemical one being a solvent addi-
tion process. We can consider three possibilities for the nature
of this chemical process. The first one (pathway (a) in Scheme
3) is the direct addition of MeCN to the classical 17-electron
1
Fig. 6 Multiple-cycle cyclic voltammogram of [W]H3 ꢁ, 1.4 ꢃ 10ꢀ3
1
1
[Mo]H3 ꢁ complex leading to a 19-electron [Mo](MeCN)H3
ꢁ
M in MeCN containing 0.2 M Bu4NPF6 at a scan rate of 50 V sꢀ1; (a):
scans 1–3 between 0 and ꢀ1 V, then scans 4–5 between ꢀ1 and þ0.8 V,
(b): scans 5–6 between ꢀ1 and þ0.8, then scans 7–9 between ꢀ1 and
þ1.5 V; (c): scans 9–12 between ꢀ1 and þ0.8 V.
product which would then be oxidized at EB. There are
literature reports underlining solvent addition equilibria to
17-electron complexes, to afford ‘‘19-electron’’ species.19–23 As
we shall see below, the computational studies do not support
this mechanism. The second one (pathway (b) in Scheme 3) is
1
rapidly upon spontaneous decomposition of [W]H3 ꢁ. Further
the addition of MeCN to the 17-electron complex after
rearrangement to the non-classical isomer, [Mo]H(H2)1
,
ꢁ
extension of the scan to þ1.5 V gives rise to the appearance of
the C/C0 peaks at low intensity after transit over the irrever-
sible D peak, and a slight decrease of the A/A0 peak intensities,
see Fig. 6(b). However, peaks C and C0 do not reach the same
intensity as peak A0, a very different result from that of the
experiment carried out on [W]H3 (Fig. 2). The obvious reason
for this outcome is that complex [W]H3 is not present in the
bulk of the solution; it is only generated in the diffusion layer
leading to the 19-electron [Mo](MeCN)H(H2)1 product.
Note that this is the same proposed intermediate for the
disproportionation mechanism, which requires that this com-
pound is oxidized at a lower potential than EA. Therefore, this
intermediate cannot be responsible for peak B. However, H2
loss may take place competitively with the disproportionation
ꢁ
process, leading to the 17-electron [Mo](MeCN)H1 complex,
ꢁ
Scheme 3
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Fig. 7 Cyclic voltammetry of [W]H3, 2 ꢃ 10ꢀ4 M in MeCN þ 0.2 M
Fig. 8 Intensity ratio of peaks B and A (iaB/iaA) for [Mo]H3 as a
function of log(v/CMeCN). CMeCN = (ꢃ) 19 M, (m) 6.5 M, (K) 5.4 M,
(J) 3.2 M, (’) 1.7 M, (&) 1.4 M. The 3 values near iaB/iaA = 0.5 are
reported in Table 2.
Bu4NPF6, v = 0.016 V sꢀ1
.
which may be the species responsible for peak B. The overall
1
process from [Mo]H(H2)1 to [Mo](MeCN)H is an associa-
tive ligand substitution. The third possibility (pathway (c) in
Scheme 3) is the same ligand substitution process as in path-
way (b), but proceeding via a dissociative mechanism involving
the 15-electron [Mo]H1 intermediate. The combination of the
experimental and computational evidence, vide infra, agrees
best with pathway (b).
ꢁ
ꢁ
fast scan rates and/or low MeCN concentrations. The inverse
dependence on v and CMeCN clearly shows that the chemical
step has a first order dependence on the acetonitrile concen-
tration. This can be easily rationalized on the basis of path-
ways (a) and (b), where the slow step is likely to be the MeCN
addition to the 17-electron classical or non-classical trihydride
complex, respectively. On the other hand, the H2 elimination
in pathway c precedes the MeCN coordination and thus
probably represents the reaction slow step. Therefore, a first
order dependence on MeCN is not anticipated for pathway c.
As mentioned above, process A/A0 is fully reversible and no
peak B is observed when MeCN is not present (r = 0). For a
given scan rate, the time available for the chemical reaction to
A slow-scan voltammetric investigation of the W compound
reveals a small peak at 0.104 V (see Fig. 7). This peak is in the
same region as peak B for the Mo system (cf. Fig. 1), but is
visible only at very small intensity and only at very slow scan
rates. For the Mo complex, on the other hand, the intensity of
the B peak may become as large as that of the A peak (vide
infra). Thus, if path (a) of Scheme 3 takes place, we must
assume that the reason for its low intensity for the W system is
a slower MeCN addition rate or a less favorable solvent
occur corresponds to the time required to go from EpA to EpB
,
as shown in eqnꢁ(4). At any time, the [Mo]H3(MeCN)1 or
[Mo]H(MeCN)1 concentration is described by eqn (5) where
kobs is the pseudo-first order forward rate constant of the
chemical process (see Scheme 3). The scan rate for which
r = 0.5, i.e. when half the amount of the electrogenerated
ꢁ
addition equilibrium leading to classical [W]H3(MeCN)1
.
ꢁ
These propositions do not have an easy rationalization (see
also the DFT results below). If the decomposition follows
instead path (b) or (c) of Scheme 3, on the other hand, it is
relatively easy to explain the lower intensity for the W system
on the basis of the different position of the classical–non-
classical equilibrium (more heavily shifted toward the classical
isomer for the tungsten system). Note that peak B is not
clearly visible at significant intensity for the voltammogram
1
ꢁ
[Mo]H3 reacts with MeCN, provides the reaction half-life
from eqn (6) and the chemical rate constant from eqn (7).
According to Fig. 8, there are three points in the interval 0.45
o r o 0.55. They are collected in Table 2. The mean rate
constant thus obtained is k = 0.020(4) Mꢀ1 sꢀ1
.
1
ꢁ
of complex [W]H3 (Fig. 6), proving that its low intensity has
a thermodynamic origin. For the Mo complex, on the other
hand, peak B increases as the scan rate decreases (compare
Fig. 1 and Fig. 3), indicating a kinetic control.
ꢀ
ꢀ
ꢀ
ꢀ
EpA ꢀ EpB
DEp
v
t ¼
¼
ð4Þ
ð5Þ
v
iaB
r ¼
¼ expðꢀk0f2tÞ
iaA
The relative intensity of peak B compared to that of A for
the Mo system has been quantitatively studied as a function of
the scan rate and of the acetonitrile concentration. The
intensity of peaks A (iaA) and B (iaB) are proportiꢁonal to the
amounts of [Mo]H3 anꢁd of the [Mo]H3(MeCN)1 (pathway
(a)) or [Mo]H(MeCN)1 (pathways (b) and (c)) complex being
generated in the chemical step. Thus, the fraction r of
r = 0.5 = exp(ꢀk0f2t1/2
)
(6)
ln 2
k0f2
¼
ð7Þ
t1=2
The electrochemical process B is irreversible (no cathodic
peak is observed in the reverse scan). The shift of the peak
potential as a function of the scan rate and [Mo]H3 concen-
tration confirms that this process is followed by a rapid
chemical reaction. The potential EpB is not affected by the
1
ꢁ
[Mo]H3 which is converted to the solvent adduct is given
by iaB/iaA. Fig. 8 presents the variation of r as a function of log
(v/CMeCN). The curve has a sigmoidal form varying from 1 for
small scan rates and/or large MeCN concentrations to 0 for
ꢂc
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1
Table 2 Experimental values of the rate constant of the reaction of [Mo]H3 with MeCN (Scheme 3), obtained from Fig. 8 when 0.45 o r o
ꢁ
0.55
Log (v/CS)
v/V sꢀ1
CMeCN/M
DEp/V
t1/2 eqn. (10)
kobs/ sꢀ1 eqn. (14)
k/Mꢀ1 sꢀ1 eqn. (12)
ꢀ2.376
ꢀ2.279
ꢀ2.21
0.08
0.1
0.04
19
19
6.5
0.444
0.446
0.436
5.55
4.46
10.9
0.305
0.38
0.155
Mean value
0.016
0.020
0.024
0.020(4)
concentration but varies linearly with log v with a slope of
approximately 30 mV per log v unit (Fig. 9). This is character-
istic of a charge transfer followed by a first order chemical
reaction.24 The follow-up processes for this decomposition
pathway could either be an H2 reductive elimination and
replacement by a solvent molecule (in case of pathway (a)),
or a simple solvent addition (pathway (b)). In each case, the
essential mechanistic interpretation of our previously reported
results, all three decomposition pathways (II)–(IV) are ac-
1
ꢁ
cessed via the rearrangement of [Mo]H3 to its non-classical
tautomer, [Mo]H(H2)1ꢁ, which then evolves in different ways
depending on conditions. The newly determined rate of the
dihydrogen dissociation process (pathway (IV); average kobs
=
0.34 sꢀ1 in the pure solvent; k = 0.020(4) Mꢀ1sꢀ1) is much
faster than the pseudo-first order rates in THF (kobs = 2.7(2)
ꢃ 10ꢀ4 sꢀ1 at 0 1C) and CH2Cl2 (kobs = 2.1(1) ꢃ 10ꢀ3 sꢀ1 at
0 1C).9 The dependence of the rate on the solvent nature is in
line with the higher nucleophilicity of MeCN relative to
CH2Cl2 and THF and with the associative nature of this
process. It is to be noted that the disproportionation and
reductive elimination pathways (III) and (IV) share the com-
mon intermediate [Mo](S)H(H2)1ꢁ. For S = MeCN, the
21
process leads to the formation of complex [M]H(MeCN)2
which is responsible for the electrochemical processes F/F0.
The complete scheme. By analysis of the voltammogram
over a large range of potentials, we have found a new oxidative
decomposition pathway for [Mo]H3 and [W]H3 in MeCN.
Furthermore, the oxidatively induced H2 reductive elimina-
tion, which was previously evidenced only in THF and
CH2Cl2, has been linked to the previously undetected peak
B. The detailed analysis of this electrochemical process has
allowed us to quantify this decomposition pathway in MeCN.
As already mentioned, the DFT calculations provide evidence
in favor of pathway (b) or (c) of Scheme 3 and against pathway
(a), whereas the experimentally determined first order depen-
dence on CMeCN for the rate of formation of the electroactive
disproportionation reaction (kdisp = 3.98(9) ꢃ 103 Mꢀ1sꢀ1 9
)
is ca. 5 orders of magnitude faster than the H2 replacement by
MeCN, explaining why the direct H2 elimination was not
previously evident in this solvent. In the current experiment,
it is highlighted only thanks to the low concentration of
1
ꢁ
[Mo]H3 relative to the MeCN solvent in the diffusion layer,
especially for the experiments carried out at slower scan rates.
The 18-electron [Mo]H(H2)S21 species is a proposed inter-
mediate of the disproportionation pathway and evolves to the
other observed product [Mo]HS221, responsible for peak F, by
replacement of H2 with a solvent molecule. The deprotonation
pathway (II) originates from the high kinetic acidity of the
dihydrogen ligand in complex [Mo]H(H2)1ꢁ. This is therefore
deprotonated when a strong base (e.g. the unoxidized trihy-
dride complex [Mo]H3) is present, leading to the 17-electron
species [Mo]H(MeCN)1 favors pathways (a) and (b) over
ꢁ
pathway (c). The common denominator of the two results is
pathway (b). Therefore, the complete scheme shown in Scheme
4 is based on this mechanistic hypothesis.
Pathway (I) is the newly discovered decomposition path,
which takes place after further oxidation of the 17-electron
1
classical trihydride [Mo]H3 ꢁ. The previously established
pathways (deprotonation, disproportionation, and H2 elimi-
nation) are indicated as (II)–(IV). To briefly summarize the
ꢁ
species [Mo]H2 . The latter is presumably further oxidized by
1
ꢁ
[Mo]H3 (since its oxidation potential is likely to be lower
than EA) and trapped by a solvent molecule, yielding the
observed product [Mo]H2S1 which is responsible for peak
C. The necessary rearrangement to the non-classical structure
as a prerequisite for pathways (II), (III) and (IV) nicely
1
explains the stability of the tungsten analogue [W]H3 ꢁ, since
its more basic character relative to molybdenum makes the
rearrangement to the dihydrogen complex energetically more
difficult. The results of computational studies shown in the
next few sections serve to back up the above assumptions and
to verify the self-consistency of the mechanism proposed in
Scheme 4.
Geometry optimization of key intermediates
The geometry optimizations were carried out by DFT methods
with the B3LYP functional and the standard LANL2DZ basis
set, which was proven suitable for organometallic compounds
of the type studied here, affording results within o5 kcal
Fig. 9 Variations of EpB as a function of log v extracted from the
experimental voltammograms of [Mo]H3 in MeCN þ 0.2 M Bu4NPF6.
ꢂc
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Scheme 4
molꢀ1 of the experiment.25,26 Most calculations were carried
out on model systems where dpe (1,2-diphosphinoethane,
H2PCH2CH2PH2) was used in place of dppe and Cp in place
of Cp*. A larger calculation which included the real dppe
ligand, but with the Cp* ligand still modeled by Cp, was
carried out for the 17-electron trihydride system only. The
relevant optimized parameters of the most stable structures are
shown in the various Figures. The atomic coordinates for all
optimized structures are available as ESIw. Some of the species
described here [notably the neutral CpMoH3(dpe) and
CpMoH(H2)(dpe) complexes, and the protonated complex
CpMoH4(dpe)1] have also been calculated at a very similar
level of theory as part of a different study, focused on hydro-
gen bonding and proton transfer.27 They have been recalcu-
lated in the present study in order to compare their relative
energy with those of the new calculations at the same theory
level. The results do not differ significantly from those of the
other study. Key calculations have also been carried out with
the PCM,28 to simulate the effect of the MeCN solvent. As will
be shown, this affects the processes where a charge redistribu-
tion is involved.
The optimized geometries for the 17-electron C and NC
species are rather similar to each other and to those previously
found experimentally (L2 = dppe) and by DFT optimization
1
9
ꢁ
(L2 = dpe) on the classical tungsten analogue, CpWH3L2
.
The most relevant geometric parameters are shown in Fig. 10.
As expected, the Mo–H distances for the two H atoms that
participate in the rearrangement lengthen on going from the
classical isomer to the transition state, while the H–H bond
starts to form. The other Mo–H bond, on the other hand,
becomes shorter during the rearrangement.
The transition state for this C/NC tautomerization process
(TS) has also been optimized. This point lies higher than the
classical minimum by only 3.2 kcal molꢀ1. Therefore, it can be
predicted that the interconversion between the two isomers
will be very rapid and will not limit the rates of the H2
elimination, deprotonation, and disproportionation processes.
This is consistent with the previous experimental findings. In
terms of free energy, the NC geometry is estimated 1.3 kcal
molꢀ1 higher than the C geometry, whereas TS is 1.9 kcal
molꢀ1 higher. If the calculated energetic parameters are as-
sumed to be valid also for the real Cp*–dppe system, we can
estimate that ca. 90% of the 17-electron trihydride adopts the
classical form at equilibrium, while the rate of transformation
1
The 17-electron CpMoH3(dpe)1 and CpMoH3(dppe)1 sys-
tems. One key question for this investigation is the energy
difference between the classical (C) and non-classical (NC)
tautomers of the 17-electron trihydride species obtained by
oxidation of compound [Mo]H3. The results for the simpler
model systems are shown in Fig. 10. Both C and NC structures
are localized as distinct minima on the potential energy sur-
face. In line with expectations (see above), the C form is found
to be the more stable one, although by only 1.9 kcal molꢀ1
relative to the NC isomer. An analogous calculation on the
two tautomers for the 18-electron neutral species shows an
energy difference of 5.7 kcal molꢀ1 in favor of the C form. This
is in line with the expected relative stabilization of the classical
form as the metal center becomes electron-richer.29,30
of MoH3 to the non-classical isomer MoH(H2)1 would be
ꢁ
ꢁ
ꢁ
ꢁ
>1011 ꢀ1. Therefore, this rearrangement does not limit the
s
rate of the decomposition processes involving the non-classical
intermediate. Note also that the energy differences do not
change significantly on going from the gas phase to the MeCN
conditions (Fig. 10).
A second classical local minimum (C0) was found at a higher
energy (3.6 kcal molꢀ1). Its geometry can be best described as
a pseudo-pentagonal bipyramid, the center of the Cp ring and
one P donor occupying the axial positions and one equatorial
position (between one P and one H ligand) being vacant. The
transition state between the two classical tautomers (TS0) is
located 5.0 kcal molꢀ1 above the absolute minimum. Its
structure is approximately midway between those of C and
ꢂc
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Fig. 10 B3LYP/LANL2DZ-optimized geometries and gas-phase relative energies (MeCN solution values in parentheses) in kcal molꢀ1 for
[CpMoH3(H2PCH2CH2PH2)]1
.
ꢁ
C0, the three H ligands and the equatorial P atom being close
to a square planar arrangement. It is easy to see how the
interconversion of isomers C and C0 allows the rapid exchange
of the inequivalent hydride ligands, explaining the shape of the
experimental EPR spectrum of complex [Mo]H31, which
shows coupling to three equivalent H nuclei.
stable classical form can be easily viewed as the coming
together of the two H ligands opposite to the equatorial P
atom, while all other ligands remain essentially unchanged.
This interconversion, therefore, involves only minor ligand
motion and should take place, like that of the two 17-electron
isomers examined above, with a minimal activation barrier. A
The calculation on the larger dppe system did not change
the picture dramatically. The classical C structure still results
the most stable one, with the non-classical form C0 now being
higher by 5.2 kcal molꢀ1 and the transition state NC by
5.3 kcal molꢀ1
.
The 16-electron CpMoH3(dpe)21 system. This dicationic
species is a model for the double oxidation product of pathway
(I) in Scheme 4. As mentioned above, an increase in metal
oxidation state should favor the non-classical tautomer by way
of a less effective back-donation from the metal center to H2.
Indeed, this expectation is verified by the computational
results. The relevant energetic and structural information is
presented in Fig. 11. The situation is further complicated here
by the possibility for each of the forms (classical and non-
classical) to adopt either the singlet or the triplet spin state, as
well as by additional stereoisomerism. Several species are
found at comparable energy. The most stable classical struc-
ture >Fig. 11(a)] is a singlet species whose geometry is very
close to that of the higher energy classical tautomer of the 17-
electron monocation (C0) in Fig. 10. The stereoisomer having
the open equatorial site between two H atoms is found
10.2 kcal molꢀ1 higher in energy. All attempts to find a local
minimum for a classical structure in the triplet state led to the
non-classical isomers.
Three different non-classical species (one singlet and two
triplets) are found to be close in energy to the classical species
discussed above. They are all slightly less stable than the
classical one in the gas phase. Of these, the singlet [Fig.
11(b)] is slightly more stable in MeCN solution. This species
has a geometry based on a distorted pseudo-octahedron with
an unfilled equatorial position. Its formation from the most
Fig. 11 B3LYP/LANL2DZ-optimized geometries and relative
gas phase (MeCN solution) energies in kcal molꢀ1 for
[CpMoH3(H2PCH2CH2PH2)]21
.
ꢂc
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second non-classical singlet stereoisomer having the open site
trans to the equatorial P atom is located 4.8 kcal molꢀ1 higher
than the most stable classical structure. The two triplet
geometries can be described as four-legged piano stools having
cisoid (P trans to P, H trans to H2) and transoid configura-
tions, respectively. The transoid geometry is very similar to
that of the non-classical CpMoH(H2)(dpe)1 [compare Fig. 10
and Fig. 11(d)], the terminal Mo–H being shorter and all other
distances longer in the more oxidized ion. The interconversion
between these triplet isomers and the singlet may be more
difficult than suggested by their mere energy difference, since a
spin pairing process must take place. It is not clear-cut to
establish which of these structures is the real ground state,
because the energy changes caused by the computational level
and the quality of the model are probably comparable or
greater than the calculated energy differences.
Other intermediates and products of the proton transfer of
pathway I. Complex CpMoH4(dpe)1, a model for the product
of proton addition to [Mo]H3, adopts a distorted pseudo-
pentagonal bipyramidal geometry, as found in a parallel study
for the same system27 and also analogous to that previously
calculated9 for the related W system (see Fig. 12(a)). This also
corresponds to the experimentally determined structure of
1 31
complex [W]H4
21
[Mo]H3
.
As discussed above, intermediate
(or its non-classical tautomer [Mo]H(H2)21) of
pathway (I) may either lose a proton or coordinate a MeCN
molecule as the first step. The model system for the proton loss
product, the 16-electron complex CpMoH2(dpe)1, adopts
either a singlet or a triplet configuration like its protonated
precursor examined above. In this case, the most stable
structures are the triplets (Fig. 12) whereas the singlet (not
shown) is found 6.3 kcal molꢀ1 higher in energy. No non-
classical minima were located for this formulation.
Fig. 12 B3LYP/LANL2DZ-optimized geometries and gas phase
(MeCN solution) relative energies in kcal molꢀ1 of complexes
[CpMoH4(H2PCH2CH2PH2)]1 (a) and [CpMoH2(H2PCH2CH2PH2)]1 (b).
The model for the MeCN addition product is most stable in
the classical trihydride form, CpMoH3(dpe)(MeCN)21. The
MeCN ligand occupies an equatorial position in a distorted
pseudo-pentagonal geometry, adjacent to one H ligand and
the equatorial P donor [see Fig. 13(a)]. This geometry can be
seen as deriving from the straightforward addition of MeCN
to the most stable geometry of CpMoH3(dpe)21 (Fig. 11). All
Mo-ligand distances remain essentially unaltered upon MeCN
addition. Three geometrically related non-classical structures,
however, are only slightly higher in energy. These are distorted
pseudo-octahedral structures with the Cp center and one P
donor in the axial positions and may be distinguished by the
ligand occupying the equatorial position trans to the second P
donor: H2 (b), þ2.6 kcal molꢀ1; H (c), þ3.0 kcal molꢀ1; and
MeCN (d), þ4.2 kcal molꢀ1. These structures will be referred
to below as isomers a, b, c and d according to the order they
are presented in Fig. 13. The non-classical isomers b and c may
be obtained from the most stable classical isomer a with
minimal molecular rearrangements, leading to the hypothesis
that they probably establish a rapid equilibrium. Isomer d, on
the other hand, cannot be readily obtained by intramolecular
motions from any of the other isomers. Given that the most
stable 16-electron trihydride systems are the singlet classical
and non-classical ones in Fig. 11(a) and (b), the straightfor-
ward MeCN addition to these species may be anticipated to
lead preferentially to isomers a and b of the solvated product.
Isomer c may be, as mentioned above, in rapid equilibrium
with a and b, or could be directly obtained from the precursor
in Fig. 11(c) after spin pairing. Isomer d, on the other hand,
may readily be obtained, after spin pairing, only from the
precursor in Fig. 11(d).
Finally, the CpMoH2(dpe)(MeCN)1 system adopts a dis-
torted pseudo-octahedral geometry with the two hydrido
ligands either trans or cis with respect to each other. The cis
isomer is the most stable one by a fair amount (see Fig. 14),
probably a consequence of the electronic destabilization in-
troduced by the two mutually trans hydrides. A comparison
with the geometric parameters of the CpMoH3(dpe)
(MeCN)21 isomers (classical and non-classical) in Fig. 13
shows that the loss of a proton has the effect of shortening
considerably the Mo–P, Mo–N and Mo–CNT bonds while the
Mo–H bonds are lengthened.
1
ꢁ
Addition of MeCN to classical CpMo(dpe)H3
. As
discussed above, one possibility envisaged for the electroche-
mical process is oxidation of a 19-electron species,
[Mo]H3(MeCN)1ꢁ, deriving from the solvent addition to
B
ꢂc
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17-electron precursor had been calculated earlier at the same
level of theory, the geometry being essentially identical to that
of the classical Mo analogue.9 The optimized geometry of the
tungsten MeCN adduct differs substantially from that of the
molybdenum counterpart, as shown in Fig. 15(b). The acet-
onitrile ligand is now at a regular bonding distance from the
metal center, while the Cp ring has slipped off to an essentially
Z3 coordination mode, the two longer W–C distances not
shown in the figure being 2.849 and 2.903 A. The complex is
essentially isoenergetic with the separate fragments (0.6 kcal
molꢀ1 less stable). When the Mo system was optimized start-
ing from the optimized geometry of the W system, the mini-
mum of Fig. 15(a) was again obtained.
This result disagrees with the hypothesis that this 19-elec-
tron complex is the source of peak B, because the experiment
shows a slow and quantitative formation of the electroactive
species for the Mo system, and a thermodynamically unfavor-
able one for the W system. The minima of Fig. 15 are not
energetically very favorable and should be accessed quite
rapidly from the individual components. It seems safe to
conclude that this mechanistic hypothesis is wrong. As will
be shown below, the alternative hypothesis (Scheme 3, path-
way (b)) is in much greater harmony with the computational
results.
Associative
substitution
of
H2
by
MeCN
in
Fig. 13 B3LYP/LANL2DZ-optimized geometries and relative gas
phase (MeCN solution) energies in kcal molꢀ1 for [CpMoH3
CpMo(dpe)H(H2)1ꢁ. Three different minima were located for
the non-classical tautomer of the ‘‘19-electron’’ molybdenum
complex, [Mo]H(H2)(MeCN)1ꢁ, starting from each of the
optimized non-classical isomers of the 18-electron dication
(isomers b, c and d) of Fig. 13. Addition of one electron to
isomer b (Z2-H2 ligand trans to P) causes the spontaneous
departure of the H2 ligand and convergence to the structure
illustrated in Fig. 16(a), which corresponds to a van der Waals
(H2PCH2CH2PH2)(MeCN)]21
.
1
ꢁ
[Mo]H3 (see Scheme 3, pathway (a)). Addition of MeCN to
the classical structure leads to a stable minimum with a rather
odd geometry, see Fig. 15(a). This exhibits a very long Moꢄ ꢄ ꢄN
contact (4.158 A), which is indicative of
a charge–
dipole interaction rather than a bonding interaction. The
geometry of the metal fragment corresponds to the higheꢁr
energy classical isomer of 17-electron [CpMoH3(dpe)]1
species (C0 in Fig. 10). In this structure, the Cp ring binds
the metal in an essentially Z5 fashion (the individual Mo–C
distances are in the 2.367–2.442 A range). This minimum is
located at 8.2 (gas phase) or 1.4 (MeCN solution) kcal molꢀ1
below the combination of the separate 17-electron trihydride
and MeCN.
complex of CpMoH(dpe)(MeCN)1 and H2. Its energy is
ꢁ
essentially the same as the sum of the energies of the ꢁseparately
optimized species. The CpMoH(dpe)(MeCN)1 moiety
adopts a trans four-legged piano stool geometry which is
closely related, also in terms of boꢁnd distances, to that of
the isoelectronic CpMoH(H2)(dpe)1 shown in Fig. 10. The
Cp ligand is again essentially symmetrically bonded to the
metal atom (2.361–2.417 A). On the other hand, adding an
electron to isomers c and d (Z2-H2 trans to MeCN and H,
respectively) yields geometries where the H2 ligand remains
bonded to the metal atom while the Cp ring becomes asym-
metrically bonded. The geometry of the MoH(H2)(dpe)
(MeCN) moiety remains essentially unchanged from that
shown in the corresponding dication, except for minor
The [W]H3 complex also displays the same electrochemical
process B, though at much reduced intensity (Fig. 7). The
Fig. 14 B3LYP/LANL2DZ-optimized geometries and relative gas
phase (MeCN solution) energies in kcal molꢀ1 of complex
[CpMoH2(H2PCH2CH2PH2)(MeCN)]1.
Fig. 15 B3LYP/LANL2DZ-optimized geometries of complexes
ꢁ
[CpMH3(MeCN)(H2PCH2CH2PH2)]1 (a): M = Mo; (b): M = W.
ꢂc
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Fig. 16 B3LYP/LANL2DZ-optimized geometries and gas phase
(MeCN solution) energies in kcal molꢀ1, relative to the classical isomeꢁr
of Fig. 15(a), of various [CpMoH(H)2(MeCN)(H2PCH2CH2PH2)]1
species.
Fig. 17 B3LYP/LANL2DZ-optimized geometries and relative gas
phase (MeCN solution) energies in kcal molꢀ1 for the transformation
variation in the bond distances [cf. Fig. 13(c) with Fig. 16(b)
and Fig. 13(d) with Fig. 16(c)]. The Mo–C distances, on the
other hand, span a large range in each case, the longer
distances not shown in the figure being 2.557, 2.817 and
2.634 A for the isomer in Fig. 16(b) and 2.566, 2.859 and
2.679 A for the isomer in Fig. 16(c). These local minima are at
higher energy relative to the classical isomer, but still lower
of [CpMoH(H2)(H2PCH2CH2PH2)]1 and MeCN to [CpMoH
ꢁ
(MeCN)(H2PCH2CH2PH2)]1 and H2.
ꢁ
slightly longer in the higher-spin system as expected, whereas
the Mo–H distance is exactly the same for both systems.
than the sum of free CpMoH(H2)(dpe)1 and MeCN. The
ꢁ
geometry of the reaction product of the ligand substitution,
namely the 17-electron [Mo]H(MeCN)1ꢁ, is identical to that
obtained by the spontaneous expulsion of H2 starting from 19-
Further mechanistic considerations. Both possible pathways
for the transformation of Scheme 2 (proton transfer followed
by MeCN coordination, or vice versa) are examined again in
Scheme 5 in terms of the calculated total energies. The direct
electron [Mo]H(H2)(MeCN)1 (Fig. 16(a)). Thus, it is possible
ꢁ
21
to predict that the associative exchange of H2 with MeCN
(pathway (b) of Scheme 3) will occur readily. The energy
profile of this exchange is shown in Fig. 17.
and irreversible proton transfer from [Mo]H3 to [Mo]H3
(first step for pathways (a) and (c) of Scheme 2) is calculated as
the more exoergic possibility for the model system, being
downhill by 45.7 kcal molꢀ1 in MeCN solution (123.0 kcal
molꢀ1 in the gas phase). Note that the PCM has a very large
effect on this energy, since the process transforms a neutral
and a doubly charged species into two singly charged species.
A doubly charged species is expected to be far most stabilized
by solvation than the sum of two singly charged species. Only
the MeCN solution values will be further discussed. The
subsequent MeCN coordination to the CpMoH2(dpe)1 inter-
mediate takes the system further down in energy by 26.5 kcal
molꢀ1, for a total energy difference of 72.2 kcal molꢀ1. The
alternative pathway ((b) of Scheme 2) involves acetonitrile
coordination to the [Mo]H321 complex for a gain of 40.9 kcal
molꢀ1, followed by the proton transfer process which is
exoergic by 31.3 kcal molꢀ1. The calculations suggest that
the most probable first step of the process is proton transfer
from [Mo]H321 to [Mo]H3. Since the kinetic data exclude that
this is the rate determining step (vide supra), the remaining
possibility is that illustrated in pathway (c), i.e. a fast proton
The 15-electron CpMo(dpe)H1 system. The alternative pro-
position forꢁ the exchange of H2 with acetonitrile from
[Mo]H(H2)1 involves a dissociative mechanism via the 15-
electron [Mo]H1 intermediate. The latter could in principle
adopt either a doublet or a quartet state,32,33 since the orbital
left vacant by the H2 dissociation could serve to host an
electron following the unpairing of a metal lone pair. The
calculations show that this highly unsaturated intermediate is
more stable as a spin quartet, and that the energy required for
the H2 dissociation from the [Mo]H(H2)1 precursor is only
ꢁ
11.4 (gas phase or 11.7 (MeCN solution) kcal molꢀ1, see Fig.
17. Thus, this substitution pathway is predicted by the calcula-
tion to be relatively facile. However, this mechanism remains
less favorable than the associative mechanism examined
above, at least in the absence of significant steric bulk. The
geometries of the two 15-electron intermediates are quite close
to each other. Both the Mo–CNT and Mo–P distances are
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by 12.0 kcal molꢀ1 in MeCN solution. Equating this value to
the standard free energy, with all related approximations as
discussed above, yields an acid dissociation constant for
[Mo]H3S21 which is ca. 9 orders of magnitude smaller than
21
that of [Mo]H3
.
The computational results presented here also allow us to
comment further upon the previously discussed pathways.9
The primary product of pathway (II) in Scheme 4, i.e. the
deprotonation product of [M]H(H2)1ꢁ, is the neutral 17-
electron species [M]H2ꢁ. Previous studies on monohydride
complexes MH8,11,22,36 have shown that the 17-electron pro-
duct of formal H atom abstraction, Mꢁ, is much easier to
oxidize than its 18-electron MH precursor (by about ꢀ1.3 V).
This oxidation is probably preceded by solvent coordination.
Although we have no direct experimental evidence, the same
evolution can bꢁe postulated for the trihydride compound, i.e.
complex [M]H2 is immediately oxidized under the condition
in which it forms. This leads to the formation of complex
[M]H2S1 which is responsible for wave C. The deprotonation
pathway is very slow in MeCN (it was only detected by a thin-
layer CV study at scan rates oca. 10 mV sꢀ1).9 This is the
reason why the presence of wave C during the first scan is only
observed at very slow scan rates.
Scheme 5
transfer followed by a rate determining MeCN coordination.
In other words, after the second oxidation step, the hydride
compound becomes so acidic that the deprotonation process
becomes a very favorable process. Therefore, it is likely to be
quantitative rather than an equilibrium process. It is in fact
quite possible that the proton is transferred in a first rapid
process from [Mo]H321 to the surrounding MeCN solvent and
subsequently delivered from MeCNH1 to the [Mo]H3 base.
On the basis of the theoretical calculations shown above, it
A comparison of the oxidation potentials of the different 18-
electron complexes [Mo]H3, [Mo]H2S1 and [Mo]HS221 (peaks
A, C and F, respectively) shows that the nature of the ligands
strongly affects the ability of the complexes to be oxidized: the
oxidation potential increases in the order E1([M]H3) o
E1([M]H2S1) o E1([M]HS221). This trend reflects the greater
donating power of a Hꢀ ligand relative to MeCN. The
is possible to speculate further. The rearrangement of
to its non-classical isomer [Mo]H(H2)21 [Fig.
21
[Mo]H3
13(b)] is likely to be very fast and both isomers are calculated
as essentially isoenergetic. Furthermore, although the pKa
values of tautomeric classical and non-classical polyhydrides
are usually rather similar,34 non-classical hydride complexes
are often kinetically more acidic than their classical counter-
parts.17,34,35 Therefore, it seems quite likely that the proton
transfer occurs via the [Mo]H(H2)21 complex and the involve-
ment of this rapid interconversion is included in path (I) of
potential for the oxidation of [Mo]H2 (oEA) and [Mo]HS1
ꢁ
ꢁ
(EB) follow the same trend. A change of metal from Mo to W
does not greatly alter the oxidation potential of waves A, B, C
and D (wave F was never observed for the W system). Since
the disproportionation pathway occurs only upon rearrange-
ment to the non-classical structure, the oxidation of the non-
classical solvent adduct [MoH(H2)S]1 must lie at a less
ꢁ
Scheme 4. Furthermore, the electrochemical mechanistic study
or [Mo]H(H2)21 coordinate
positive potential than process A. This potential (which cannot
be measured directly) can be calculated from eqn (8), since the
disproportionation equilibrium constant (KDisp = >1010) is
estimated from our previous study.9 The resulting value is
more negative than wave A (by >600 mV), as expected.
21
excludes that either [Mo]H3
MeCN in a rate determining step before being deprotonated.
According to the calculations, these processes would lead
straightforwardly to isomers a and b, respectively, of system
[Mo]H3(MeCN)21 (Fig. 13) with consequent H2 elimination
and generation of species [Mo]H(MeCN)221, which is respon-
sible for process F. The lack of such competitive solvent
addition processes rationalizes why, upon fast scan rate vol-
tammetry beyond the second oxidation process D (i.e. when
insufficient time is allowed for pathways (II) and (III) to
occur), peak F is not observed in the voltammogram.
The striking acidity difference between [Mo]H(H2)21 (path-
way (I), only deprotonation leading to process C) and
[Mo]H(H2)S21 (pathway (III), only H2 elimination leading
to process F) must be related to the buffering effect of the
acetonitrile coordination on the H2 ligand acidity. Conversely,
the more saturated configuration of the acetonitrile adduct
certainly favors the H2 elimination. A rough estimate of the
2:3RT
Eꢅ
¼ EMꢅ H =½MH ꢆþꢁ
ꢀ
log KDisp ð8Þ
2þ
½MHðH2ÞSꢆ þꢁ=½MHðH2ÞSꢆ
3
3
F
Conclusions
Our work constitutes a complete study of the decomposition
mechanism of the 17-electron trihydride complexes
[Cp*MH3(dppe)]1 (M = Mo, W) in the presence of acetoni-
ꢁ
trile. Even though quite complex, the complete picture in
Scheme 4 satisfactorily rationalizes all the observed chemical
and electrochemical behavior while being compatible with the
results of the theoretical calculations. The present study has
highlighted the following new aspects of paramagnetic hydride
chemistry.
acidity decrease is possible on the basis of the computational
21
results. The proton transfer process from [Mo]H(H)2
1
to
[Mo]H2S1 yielding [Mo]H2 and [Mo]H(H)2S21 is exoergic
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The double oxidation of the title trihydride complex induces
an immediate proton transfer to the starting complex, fol-
lowed by rate determining solvent coordination.
other atoms were treated by the LANL2DZ basis set as for the
simpler model. The calculations in the MeCN medium (e =
36.64) were carried out on the gas-phase optimized structures
with the polarizable continuum model (PCM),28 using stan-
dard options of PCM and cavity keywords.37 All geometries
were fully optimized without symmetry restrictions and were
characterized as local minima or transition states of the
potential energy surface (PES) by inspection of the sign of
the second derivatives. The mean value of the S2 operator was
always very close to the theoretical value of 0.75 for spin
doublets, 2 for spin triplets, and 3.75 for spin quartets
indicating minor amounts of spin contamination. In all cases,
energies are given in kcal molꢀ1. The reported energy values
do not include a correction for the zero-point vibrational
energy (ZPVE), whereas the reported free energy values
include ZPVE, PV and TS contributions at 298 k.
The oxidatively induced H2 reductive elimination from a
polyhydride complex has been quantitatively assessed in
MeCN. This process occurs via solvent coordination and
competes with disproportionation and slow proton transfer.
From the computational point of view, it seems possible that
an alternative dissociative pathway via a 15-electron species,
which would adopt a spin quartet ground state, could take
place for bulkier systems.
The addition of an acetonitrile molecule is shown to dra-
matically reduce the acidity of the oxidized hydride complex.
Continuing work in this area must address the effect of the
metal oxidation on the fundamental properties (polarity,
polarizability, homolytic bond strength) of the M–H bond(s),
an area where there is still precious little information,8 in order
to develop new molecules for catalytic and electrocatalytic
applications by rational design.
Acknowledgements
We are grateful to the European Commission for sponsoring
this research through the HYDROCHEM Research Training
Network (Contract no. HPRN-CT-2002-00176) and CINES
for granting us free computer time. MB thanks the Spanish
Experimental
General
Ministerio de Educacion y Ciencia for a post-doctoral fellow-
´
Compounds Cp*M(dppe)H3 (M
= Mo, W), [Cp*Mo
ship.
(dppe)H2(MeCN)]1 and [Cp*Mo(dppe)H(MeCN)2]21 have
been prepared as previously described.31 All manipulations
were carried out under an inert atmosphere of dinitrogen or
argon by the use of Schlenk line or glove box techniques. THF
was distilled under dinitrogen prior to use. MeCN (HPLC
grade) was carefully dried by percolation on neutral alumina
that had been activated for 8 h under vacuum at 220 1C.
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