Reduction of tris(benzene-1,2-dithiolate)molybdenum(VI) by hydroxide ions in dry tetrahydrofuran solution

Article abstract of DOI:10.1039/b106123m

Tris(benzene-1,2-dithiolate)molybdenum(VI) reacts rapidly and quantitatively with tetrabutylammonium hydroxide to yield the corresponding Mo(V) and Mo(IV) complexes and hydrogen peroxide; the reaction has been executed in dry tetrahydrofuran where the rea

Full text of DOI:10.1039/b106123m

View Article Online / Journal Homepage / Table of Contents for this issue
Reduction of tris(benzene-1,2-dithiolate)molybdenum(VI) by hydroxide
ions in dry tetrahydrofuran solution
Antonio Cervilla,*a Francisco Pérez-Plab and Elisa Llopisa
a Departamento de Química Inorgànica, ICMUV, PO Box 2085, Polígono La Coma, Paterna, Valencia,
Spain. E-mail: Antonio Cervilla@.uv.es; Fax: 34963983633
b Departamento de Química-Física, Universidad de Valencia, Dr. Moliner 50, 46100-Burjasot, Valencia,
Spain
Received (in Cambridge, UK) 10th July 2001, Accepted 4th October 2001
First published as an Advance Article on the web 25th October 2001
Tris(benzene-1,2-dithiolate)molybdenum(VI) reacts rapidly
and quantitatively with tetrabutylammonium hydroxide to
[Mo(bdt)3]22 complex. All these complexes have been thor-
oughly characterised elsewhere in solid5 and in solution.4
Because the whole reaction was complete in less than 20 s in
excess of base, both reduction processes have been studied by
stopped-flow spectrophotometry. A general treatment of spec-
trophotometric data has been used to investigate the kinetics of
the reaction. The solution absorption spectra of authenticated
yield the corresponding Mo( ) and Mo(IV) complexes and
V
hydrogen peroxide; the reaction has been executed in dry
tetrahydrofuran where the reaction rate shows a fair
dependence on complex and OH2 concentrations.
Tris(dithiolene) complexes of transition metals have been the
subject of considerable attention since they were first reported.1
While early interest focused primarily on the facile one-electron
redox reactions which these complexes undergo, more recently
the scope has expanded to areas ranging from bioinorganic
chemistry to materials science.2
Surprisingly little is known about the interaction of this class
of compounds with hydroxide ions. Kawashima et al.3 first
reported the reduction of neutral M(tdt)3 complexes (M = Re,
Mo, and W; tdt = toluene-3,4-dithiolate) dissolved in acetone
or THF by aqueous NaOH, but little more than the formation of
the corresponding one-electron reduced complex was de-
scribed. Confirmation of the occurrence of this reaction was
subsequently provided by Sellmann et al.4 who reacted
Mo(bdt)3 (bdt = benzene-1,2-dithiolate) with NaOH in metha-
nol. The reaction yields the [Mo(bdt)3]2 or [Mo(bdt)3]22
complex, depending upon the ratio in which NaOH and
Mo(bdt)3 are mixed, and it was thought to proceed by oxidation
of methanol to methanal, although this hypothesis has not been
tested.
Mo(VI), Mo( ), and Mo(IV) complexes were fitted to a Gaussian
V
function basis set within the 400–800 nm region.6 The
calculated optical densities ei(l) were used to analyse the
Mo(bdt)3–n-Bu4NOH system spectrum at each time and to
deduce the variation of molybdenum complex concentrations
with time.† In all cases the sums of the Mo complex
concentrations were equal to the initial Mo(bdt)3 concentration
and the resulting ci vs. time curves were subject to further
kinetic analysis by using a non-linear least-squares method.7
The monitoring of an equimolecular Mo(bdt)3–n-Bu4NOH
reaction system is presented in Fig. 1a. On the basis of
comparisons with authentic samples of Mo(bdt)3 and
[Mo(bdt)3]2, it was concluded that the latter complex is the only
molybdenum product formed. This was further supported by
HPLC which enabled the separation of Mo(bdt)3 from
[Mo(bdt)3]2 in a reaction mixture where no other peaks were
detected. Finally, GC–FID did not show peaks other than that
from the THF, indicating that this solvent is not involved in the
redox process.
All evidence indicates that no absorbing intermediates are
formed in appreciable quantities. First, as can be seen in Fig. 1,
repetitive UV–Vis scans show tight isosbestic behaviour.
Secondly, and more significantly, the kinetics treatment
We were surprised to find that addition of n-Bu4NOH to a
THF solution of neutral Mo(bdt)3 caused its conversion to the
monoanionic [Mo(bdt)3]2 complex and then to the dianionic
Fig. 1 Repetitive diode-array UV-scans from reaction mixtures in dry THF containing: (a) Mo(bdt)3 3.38 3 1025 mol dm23 and n-Bu4NOH 3.38 3 1025
mol dm23 (spectra taken every 0.2 s, shown every 1 s, T = 300.2 K). (b) Mo(bdt)32 1.16 3 1025 mol dm23, n-Bu4NOH 3.28 3 1024 mol dm23 (spectra
taken every 0.5 s, shown every 10 s, T = 301.2 K).
2332
Chem. Commun., 2001, 2332–2333
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b106123m

View Article Online
described here is capable of reproducing any of these experi-
mental spectra satisfactorily by using only the component
Gaussian functions of the Mo(bdt)3 and Mo(bdt)32 spectra.
were simulated and the concentration of each complex eval-
uated as a function of time. The disappearance rate of
[Mo(bdt)3]2 parallels the appearance rate of [Mo(bdt)3]22 for
all runs, but it should be noted that both rates become negligible
a few seconds after the reaction starts (ca. 15 s), and also that a
complete conversion does not take place even when an excess of
ca. 20 equivalents of base are initially present. These observa-
tions can be plausibly explained when the equilibrium shown in
reaction (2) is assumed.
The kinetics of the Mo(VI)–Mo(
V
) transformation were
investigated with Bu4NOH–Mo(bdt)3 mole ratios of 0.4–1.3. At
higher ratios, a further reduction of the formed Mo( ) complex
V
with excess n-Bu4NOH led to the Mo(IV) complex. Because of
this limitation on the ratio of reactants, the kinetics data were
treated in complete form, fitting the Mo(VI) and Mo( )
V
concentration vs. time curves to the integrated form of the
d[MoVI]/dt = 2 k[MoVI]p[OH2]q equation by using non-linear
regression methods. An excellent fit was obtained over a wide
range of conditions for p = 2, q = 1, and k = (5.0 0.2) 3 108
s21 mol22 dm6 at 27 °C. Reaction orders were also corroborated
by applying the initial rate method.
The second-order dependence on [Mo(bdt)3] suggests a two-
electron reduction of hydroxide ions to hydroperoxide ions that
were actually identified in separate experiments [reaction (1)].
2 Mo(bdt)32 + 3 OH2 " 2 Mo(bdt)322 + HO22 + H2O (2)
Support for the occurrence of this equilibrium comes from
the fact that final complex concentrations are dependent on the
initial Bu4NOH concentration (K = (1.2 0.3) 3 103 mol21
dm3 at 298 K). A first analysis of the kinetic data seems to
indicate a reversible process in which the rate of [Mo(bdt)3]2
reduction is first-order in both [Mo(bdt)3]2 and [Mo(bdt)3]22
concentrations, being the rate constants dependent on the
concentration of base.
2 Mo(bdt)3 + 3 OH2 ? 2 Mo(bdt)32 + HO22 + H2O (1)
In summary, on the basis of this and other works,3,4 it should
no longer be considered unusual to react a higher valent
molybdenum tris(dithiolene) compound with hydroxide ions to
produce a lower valent molybdenum tris(dithiolene) compound.
Moreover, such electron-transfer processes have also been
observed to occur with tungsten, e.g. W(bdt)3, and this will be
the basis of a later publication. Further investigations of the
reactivity of Mo(bdt)3 and related compounds in solvents other
than THF may clarify whether the observed reactivity is
relevant to that of molybdenum cofactor (Moco).2
The experiments were performed in biphasic systems by
dissolving Mo(bdt)3 in the organic phase and extracting the
2
produced H2O2 as HO2 in the aqueous basic phase. Thus,
when a 2.0 3 1023 M solution of Mo(bdt)3 in THF–toluene
mixture (70+30) was added to a 0.1 M aqueous solution of
NaOH over a period of 4 h, H2O2 could be detected and
spectrophotometrically analysed by reacting an aliquot of the
aqueous phase with a solution of TiOSO4 in 25% H2SO4–H2O.
The transmittance at 405 nm due to the yellow peroxotitanyl
cation indicated the presence of H2O2 in yields of 80–90%
based on the initial Mo(bdt)3 concentration, while no formation
of H2O2 was detected in the absence of Mo(bdt)3.
Notes and references
† The molybdenum complex concentrations were calculated by minimising
The second-order dependence on [Mo(bdt)3] is consistent
with a mechanism having a bimolecular Mo complex inter-
action as the rate-limiting step. Taking into account that the
current spectrophotometric results do not show that OH2 binds
to Mo through the oxygen, we suggest as a reasonable pathway
the association of one hydroxide ion with two neutral Mo(bdt)3
molecules by hydrogen bonding to the co-ordinated sulfur
atoms of the ligands.‡1,2 The m-hydroxo Mo(bdt)3…HO2
…Mo(bdt)3 species thus formed may pass through a highly
concerted transition state in which the interaction with another
OH2 ion would lead to a polar group-transfer reaction,8
dominated by HO…OH bond making and metal reduction, to
yield the [Mo(bdt)3]2 radical ( < g > = 2.006; < A > (95,97Mo)
= 26.6 3 1024 cm21 ) and H2O2‡3 which in basic solution
the following least-squares equation with respect to the concentration of the
Mo(VI
–
IV) complexes:
f(c1, c2,º, cn; l) =
(A(lk ) - l
ei(lk )ci )2
 Â
k =1
i =1
where A(l) is the reaction mixture absorbance at wavelength l, l is the
optical pathway, and ei(l) and ci, i ñ {1…3}, are the optical densities and
concentrations of Mo(bdt)3 , Mo(bdt)3 , and Mo(bdt)3
-
22
species. The
calculations were performed using the C++ Linux programs GssKin and
GssFit. Free copies of the program are supplied under request.
k1
‡ (1) Mo(bdt)3 + OH2
Mo(bdt)3…OH2 (fast)
[|
k-1
2
forms the HO2 ion. An Eyring plot of the rate constant k
k2
(2) Mo(bdt)3…OH2 + Mo(bdt)3 [| Mo(bdt)3…OH2…Mo(bdt)3 (fast)
determined at a series of temperatures gives an excellent
straight line from which DH≠ = 37.9 0.8 kJ mol21 and DS≠
= 2 39 3 J K21 mol21 are deduced. The DS≠ value is
consistent with the proposed associative transition-state.
As estimated from standard reduction potential data, the
overall reaction (1) must be exergonic at least in acetonitrile
k-2
k3
(3) Mo(bdt)3…OH2…Mo(bdt)3 + OH2
2Mo(bdt)32 + H2O2 (rls)
ææÆ
1 R. P. Burns and C. A. McAuliffe, Adv. Inorg. Chem. Radiochem., 1979,
22, 303; J. A. McCleverty, Progr. Inorg. Chem., 1968, 10, 49.
2 C. Collison, C. D. Garner and J. A. Joule, Chem. Soc. Rev., 1996, 25, 25;
P. Falaras, C. Mitsopoulou, D. Argyropoulos, E. Lyris, N. Psaroudakis, E.
Vrachnou and D. Katakis, Inorg. Chem., 1995, 34, 4536 and references
therein.
3 M. Kawashima, M. Koyama and T. Fujinaga, J. Inorg Nucl. Chem., 1976,
38, 801.
4 D. Sellmann and L. Zapf, Z. Naturforsch. B, 1985, 40B, 380.
5 A. Cervilla, E. Llopis, D. Marco and F. Pérez-Pla, Inorg. Chem., in
press.
6 P. Gans, Data Fitting in the Chemical Sciences, John Wiley & Sons,
Chichester, 1992, chap. 8.
7 F. F. Pérez Pla, J. F. Bea Redón and R. Valero, Chemometrics and
Intelligent Laboratory Systems, 2000, 53, 1.
8 J. A. McCleverty, Encyclopedia of Inorganic Chemistry, ed. R. B. King,
John Wiley & Sons, New York, 1994, p. 2304.
2
solution. In fact, the redox potential for the reduction of HO2
to OH2 ions has been reported to be 20.11 V vs. ENH, [1 M
(Bu4N)OH, pH 30.4],8 whereas the measured redox potential
for the Mo(bdt)3–Mo(bdt)32 pair was found to be +0.610 V by
cyclic voltammetry.5 Thus, reaction (1) lies indeed far to the
right.
However, the [Mo(bdt)3]12/22 reduction potential is con-
siderably more negative (20.040 V) than that of the
[Mo(bdt)3]0/12 couple, and thus the former reduction must have
a smaller driving force and a relatively slower reactivity. This
was confirmed by monitoring the reaction of (Bu4N)-
[(Mo(bdt)3] with Bu4NOH, which also proceeds cleanly with
well-defined isosbestic points upon addition of excess base
(Fig. 1b). By using the procedure indicated above, these spectra
Chem. Commun., 2001, 2332–2333
2333