Catalysis by methyltrioxorhenium(VII): Reduction of hydronium ions by europium(II) and reduction of perchlorate ions by europium(II) and chromium(II)

Article abstract of DOI:10.1021/ic048669k

The title reactions occur stepwise, the first and fastest being MeReO ₃ + Eu²⁺ → Re(VI) + Eu³⁺ (k₂₉₈ = 2.7 × 10⁴ L mol^-1 s^-1), followed by rapid reduction of Re(VI) by Eu²⁺ to MeReO₂. The latter species is reduced by a third Eu²⁺ to Re(IV), a metastable species characterized by an intense charge transfer band, ε₄₁₀ = 910 L mol^-1 cm^-1 at pH 1; the rate constant for its formation is 61.3 L mol^-1 s^-1 independent of [H+]. Yet another reduction step occurs, during which hydrogen is evolved at a rate v = k[Re(IV)][Eu²⁺][H⁺]^-11, with k = 2.56 s ^-1 at μ = 0.33 mol L^-1. The 410 nm Re(IV) species bears no ionic charge on the basis of the kinetic salt effect. We attribute hydrogen evolution to a reaction between H-Re^VO and H₃O ⁺, where the hydrido complex arises from the unimolecular rearrangement of Re^III-OH in a reaction that cannot be detected directly. Chromium(II) ions do not evolve H₂, despite E°_Cr ～ E°_Eu. We attribute this lack of reactivity to the Re(IV) intermediate being captured as [Re^IV-O- Cr^III]²⁺, with both metals having substitutionally inert d³ electronic configurations. Hydrogen evolution occurs in chloride or triflate media; with perchlorate present, MeReO₂ reduces perchlorate to chloride, as reported previously [Abu-Omar, M. M.; Espenson, J. H. Inorg. Chem. 1995, 34, 6239-6240].

Full text of DOI:10.1021/ic048669k

Inorg. Chem. 2005, 44, 489−495
Catalysis by Methyltrioxorhenium(VII): Reduction of Hydronium Ions by
Europium(II) and Reduction of Perchlorate Ions by Europium(II) and
Chromium(II)
Yang Cai and James H. Espenson*
Ames Laboratory and Department of Chemistry, Iowa State UniVersity, Ames, Iowa 50011
Received September 22, 2004
+
Eu3 (k298 104
) 2.7 ×
+
The title reactions occur stepwise, the first and fastest being MeReO
3
+
Eu2
f
Re(VI)
+
L mol s-1), followed by rapid reduction of Re(VI) by Eu2 to MeReO
-1
+
. The latter species is reduced by a third
2
Eu2 to Re(IV), a metastable species characterized by an intense charge transfer band,
+
ꢀ
)
910 L mol- cm-
1
1
410
at pH 1; the rate constant for its formation is 61.3 L mol-1 s- , independent of [H ]. Yet another reduction step
1
+
k[Re(IV)][Eu2 ][H ] 1, with k
+
+ -
)
2.56 s- at µ ) 0.33 mol
1
occurs, during which hydrogen is evolved at a rate v
1
)
L- . The 410 nm Re(IV) species bears no ionic charge on the basis of the kinetic salt effect. We attribute hydrogen
V
+
evolution to a reaction between H
rearrangement of ReIII
−
Re O and H
3
O , where the hydrido complex arises from the unimolecular
−
2
OH in a reaction that cannot be detected directly. Chromium(II) ions do not evolve H ,
despite E
°
Cr
∼
E°
Eu. We attribute this lack of reactivity to the Re(IV) intermediate being captured as [ReIV−
O
−
Cr ] +, with both metals having substitutionally inert d electronic configurations. Hydrogen evolution occurs in
III 2
3
chloride or triflate media; with perchlorate present, MeReO
2
reduces perchlorate to chloride, as reported previously
6240].
[
Abu-Omar, M. M.; Espenson, J. H. Inorg. Chem. 1995, 34, 6239
−
2+
Introduction
-2.3 V.3,4 Although V ion can undergo a two electron
2
+
2+
change to produce VO , the reduction potential for VO /
+
3 2
Many metal ions that lie below H O /H in the electro-
2+
V
is 0.08 V, which means reaction 2 is thermodynamically
chemical series survive for lengthy periods in acidic solution
because of substantial kinetic barriers. Among them are
unfavorable.
2+
2+
2+
hydrated Cr , Eu , and V ions which remain indefinitely
unchanged provided the solutions are acidic and oxygen is
rigorously excluded, despite the fact that these species are
thermodynamically unstable toward the formation of M(III)
and dihydrogen, shown as eq 1.
2
+
2+
V
+ H O f VO + H2
(2)
2
Thus, any actual process for proton reduction must avoid
the intermediacy of hydrogen atoms. Hydrogen evolution
reactions from a metal or a metal complex have one common
feature, an intermediate hydridometal species. Its formation
will lower the energy barrier for the production of H
can react directly with H
atomic hydrogen. Thus, hydrogen gas can be generated
directly by a bimolecular reaction pathway or by a proton-
hydride reaction pathway.
What we believe was the first example of a homoge-
neously catalyzed, nonphotochemical process for proton
2
+
+
3+
2
M
+ 2H f 2M + H2
(1)
2
as it
+
3
O
and avoid the formation of
The driving forces for these reactions are ∆E° ) 0.415,
.380, and 0.230 V for Cr , Eu , and V respectively.
2+
2+
2+
1,2
0
5,6
Because the reactions are spontaneous, a kinetic barrier is
responsible for the lack of reactivity. These metal ions are
strong single-electron reductants, so the chemistry of dihy-
drogen formation would entail intermediate formation of Haq
which is not thermodynamically feasible, E° for H /H being
6
,7
•
,
+
•
(
3) Swallow, A. J. Radiation Chemistry: An Introduction; Longman:
London, 1973.
(4) Schwarz, H. A. J. Chem. Educ. 1981, 58, 101-5.
(5) Evans, J.; Norton, J. R. J. Am. Chem. Soc. 1974, 96, 7577.
(6) Chao, T.-H.; Espenson, J. H. J. Am. Chem. Soc. 1978, 100, 129.
(7) Ryan, D. A.; Espenson, J. H. Inorg. Chem. 1981, 20, 4401.
*
To whom correspondence should be addressed. E-mail: espenson@
iastate.edu.
(
(
1) Weaver, M. J. J. Am. Chem. Soc. 1979, 101, 1131-1137.
2) Weaver, M. J.; Yee, E. L. Inorg. Chem. 1980, 19, 1936-1945.
1
0.1021/ic048669k CCC: $30.25
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 3, 2005 489
Published on Web 12/21/2004

Cai and Espenson
reduction was reported in 1986. Molecular hydrogen is
evolved rapidly from acidic solutions of chromium(II),
europium(II), or vanadium(II) ions in the presence of trace
concentrations of a catalyst, the cobalt(II) macrocycle Co-
8
(
dmgBF
2
)
2
, 1. Further, in 1988, Gould et al. noted that
-
ReO
4
+
catalyzes what was presumed to be the reduction of
H O to H by vitamin B12s, cobalt(I)alamin, the cobalt(I)
3 2
form of B12. Further studies were not carried out because
of the difficulties in measuring the yield of H and in
9
2
determining the oxidation state of the rhenium species
involved during the reaction process.9
Recently, we observed that upon mixing MeReO
3
(MTO)
2
+
with Eu in acidic aqueous solution H
2
is evolved fairly
rapidly when perchlorate ions are absent. Even though most
of our experiments have been carried out in chloride media,
triflate solutions also evolve H
2
. It is also important to note
Figure 1. A recording gas microvolumeter system for hydrogen evolution.
Top: the microvolumeter apparatus. Bottom: the circuit diagram. R,
resistance; E, battery.
is not evolved when Cr2 is used instead of Eu .
+
2+
that H
2
Because these observations are unprecedented, we have
undertaken a study of the Eu2 reaction to understand,
principally by stoichiometry and kinetics, the mechanism of
catalyzed hydrogen formation and also the reasons why Cr
does not behave in the same way.
+
Experimental Section
2+
Materials. High-purity water was obtained by passing laboratory
distilled water through a Millipore-Q water purification system.
Methyltrioxorhenium(VII), CH
from sodium perrhenate and tetramethyltin (Strem). Europium-
3 3
ReO or MTO, was synthesized
16
(
(
III) solutions were made by dissolving europium(III) oxide
99.97%) in hydrochloric acid or triflic acid. Solutions of euro-
pium(II) were prepared from europium(III) solutions either by
reduction with amalgamated zinc or by electrochemical reduction
at a mercury cathode. Europium(II) solutions were stored in 25
mL Pyrex bottles under a positive pressure of argon. The concentra-
tions of europium(II) were measured by spectrophotometry at 321
-
1
-1
nm, where its molar absorptivity is 587 L mol cm . The acid
concentration of electrochemically prepared Eu(II) solutions was
titrated by sodium hydroxide after the solution had been passed
through a column of Dowex 50W-X8 H cation-exchange resin
and rinsed with purified water.
2
+
+
The Eu -H reaction occurs in several steps. The kinetics
of three of the steps could be measured by different methods,
although to do so required the use of different absolute and
relative concentrations of Eu2 and MeReO
In the course of this study, we also found that when
perchlorate ions are present in the solution, they are reduced
2
to chloride ions before H can be formed. Perchlorate has a
powerful thermodynamic tendency for reduction in dilute
aqueous solution, although reduction is often very slow. In
this study, Eu2 aq or Cr aq were used to reduce MeReO
+
+
3
.
Chromium(II) solutions were prepared by reduction of aqueous
chromium(III) chloride with amalgamated zinc. Chromium(III) was
-
1
determined spectrophotometrically at 408 nm, ꢀ ) 15.8 L mol
-
1 17
cm
.
Most other chemicals were reagent grade and obtained
commercially: hydrochloric acid, triflic acid, lithium chloride (used
to maintain ionic strength), and sodium perchlorate. The acid
concentration was analyzed by titration with sodium hydroxide to
a phenolphthalein endpoint.
Instrumentation. UV-vis spectra and kinetic data were obtained
with Shimadzu model 3101 and OLIS RSM stopped-flow spectro-
photometers. Temperature was controlled at 25.0 ( 0.2 °C by
circulating thermostated water in the stopped flow instrument and
by an electronic (Peltier) cell holder in the spectrophotometer.
Hydrogen was identified by gas chromatography. A Gow-Mac GC
+
2+
3
to
MeReO
2
(H
2
O)
n
2 3 2
(n ) 2, presumably, because MeReO (PAr )
10
has been isolated and characterized ), which then rapidly
reduces perchlorate ions.
The chemistry of MeReO has been extensively reviewed,
3
particularly in the context of its use as a catalyst for reactions
of hydrogen peroxide.11-15
3
50 chromatograph with a thermal conductivity detector was used
(
(
8) Connolly, P.; Espenson, J. H. Inorg. Chem. 1986, 25, 2684-2688.
9) Gould, E. S. Inorg. Chem. 1988, 27, 1868-1871. See also the
following reference, which reports perchlorate reduction catalyzed by
perrhenate ions: Haight, G. P., Jr.; Swift, A. C.; Scott, R. Acta Chem.
Scand., Ser. A 1979, A33, 47-52.
with argon as the carrier gas and molecular sieve 13x as the
stationary phase.
2
The rate of H formation was determined volumetrically with a
recording microvolumeter apparatus, modified from the one previ-
ously reported, as diagrammed in Figure 1.18 The top part represents
the glass apparatus in which mercury in the glass capillary tube is
(
10) Herrmann, W. A.; Roesky, P. W.; Wang, M.; Scherer, W. Organo-
metallics 1994, 13, 4531-5.
(
(
(
11) Herrmann, W. A.; Kuehn, F. E. Acc. Chem. Res. 1997, 30, 169-180.
12) Kuhn, F. E.; Herrmann, W. A. Chemtracts 2001, 14, 59-83.
13) Owens, G. S.; Arias, J.; Abu-Omar, M. M. Catal. Today 2000, 55,
3
17-363.
14) Espenson, J. H.; Abu-Omar, M. M. AdV. Chem. Ser. 1997, 253, 99-
34.
15) Espenson, J. H. Chem. Commun. 1999, 479-488.
(16) Herrmann, W. A.; Kratzer, R. M. Angew. Chem., Int. Ed. Engl. 1997,
36, 2652.
(17) Leslie, J. P.; Espenson, J. H. J. Am. Chem. Soc. 1976, 98, 4839.
(18) Davis, D. D.; Stevenson, K. L. J. Chem. Educ. 1977, 54, 394-395.
(
(
1
490 Inorganic Chemistry, Vol. 44, No. 3, 2005

Catalysis by Methyltrioxorhenium(VII)
Figure 2. Visual changes in acidic solution, from left to right: 12 mmol
Figure 3. Spectral changes illustrating the formation of L410 from the
-
1
2+
-1
2+
-1
L
Eu ; 12 mmol L Eu with 0.3 mmol L MeReO3 after 1 min; 30
2+
-1
reaction between MeReO3 and Eu in acid: (a) 0.4 mmol L MeReO3;
-
1
2+
-1
min later, showing hydrogen evolution; 12 mmol L Cr ; 12 mmol L
Cr with 0.3 mmol L MeReO3 after 1 min; 1 h later. All solutions contain
-1
2+
-1
-1
2+
(
b) 1 mmol L Eu ; (c) 0.2 mmol L MeReO3 with 1 mmol L Eu
2
+
-1
-1
+
at 0.14 mol L H .
-
1
1
20 mmol L HCl.
pushed into the reservoir as the evolved hydrogen enters through
the three-way stopcock at the right. A 30-gauge Nichrome-80
resistance wire passes concentrically through the length of the
volumeter tube. A simple circuit is applied to the wire, which is
shown in the bottom of the diagram. As the mercury moves through
the glass tube, the length of unshorted wire changes in proportion
to the change of the gas volume. Thus, the voltage changes linearly
with the volume of the gas, which is recorded by a HP 3457A
multimeter. A VICI Pressure-Lok gas syringe was used to calibrate
the volume against voltage, Figure S-1. In order to get accurate
volume readings, the dead volumes in the reaction vessel and the
connecting tubing were made as small as possible. A certain
“
inertia” toward initiating the movement of the mercury was noted.
Figure 4. Absorbance decreases at 2 min time intervals during the reaction
-
1
2+
-1
-1
+
Thus, the evolved H did not appear immediately at the onset of
the reaction. The same phenomenon was noted when gas was
injected to calibrate the apparatus.
2
of 0.93 mmol L Eu with 0.2 mmol L MeReO3 at 0.14 mol L H .
-
1
2+
Inset: A control experiment with 3.2 mmol L Eu in acidic solution for
2 h, showing the stable absorbance in the absence of MeReO3
1
.
Chloride ion concentrations from the reduction of perchlorate
were determined quantitatively by ion chromatography using
potentiometry.19 In the experiments, a Weiss Research CL3005
combination chloride ion selective electrode and a Hanna Instru-
ments HI 1131 combination pH electrode were connected to a
Hanna Instruments pH302 pH-meter. The electrodes were calibrated
before use with standard NaCl solutions and standard buffers,
respectively.
Table 1. Quantitative Measurements of Hydrogen Evolution by a
Microvolumeter Apparatus
+ a
[MTO]a
VH /mLb
% H2c
[
Eu2
]
E/mV
2
6
6
6
6
5
4
4
0.2
0.2
0.2
0.2
0.2
0.2
0.2
1.196
1.131
1.121
1.183
0.958
0.852
0.848
1.434
1.330
1.314
1.413
1.053
0.883
0.877
97.5
90.3
89.3
96.0
85.8
90.0
89.5
Results
a
Units: mmol L-1. b From a 20 mL reaction vessel. c Calculated on the
Hydrogen Evolution from Acidic Europium(II) Solu-
2+
basis of eq 3, with limiting Eu , from the amount of H2 measured.
-
1
tions Containing MeReO
3
. When 0.3 mmol L MeReO
3
-
1
2+
and 12 mmol L Eu solutions were mixed in anaerobic
20 mmol L HCl, a yellow color was immediately seen.
stable until near the very end of the hydrogen evolution
period, Figure 4.
-
1
1
As a control experiment, an acidic Eu2 solution was
+
Following that, copious bubbles were generated, increasing
2+
over 30-60 min. However, when Cr was used instead,
we only observed a color change from pale blue to yellow-
green, and no bubbles formed over 10 h. The phenomena
are depicted in Figure 2.
sealed in a cuvette; no bubbles were observed for 12 h, and
2
+
the absorbance of Eu remained unchanged, as shown in
the inset to Figure 4. But with MeReO in the solution and
3
-
1
+
0.2 mol L H , the reaction finished in ca. 20 min. The
bubbles evolved were shown to be H by GC. The buildup
of H gas was measured quantitatively by the microvolumeter
apparatus. Quantitative measurements showed that the yield
of H is generally about 90%, Table 1. The yield was
calculated under the assumption that eq 3 represents the
Immediately upon mixing solutions of MeReO
3
and excess
2
2+
2+
Eu , two species are evident in the UV-vis spectrum, Eu
and one we call L410, named for the yellow intermediate
with a maximum absorbance at 410 nm, with the spectrum
shown in Figure 3. The peak at 321 nm of Eu2+ immediately
begins to decrease in intensity while the L410 band remains
2
2
2+
correct stoichiometry. Note that Eu is the limiting reagent,
because excess HCl was used and MTO is a catalyst. The
shortfall from 100% is taken to reflect the previously
(19) Lente, G.; Espenson, J. H. Int. J. Chem. Kinet. 2004, 36, 449-455.
Inorganic Chemistry, Vol. 44, No. 3, 2005 491

Cai and Espenson
Figure 5. Spectrophotometric titration of MeReO3 with Eu2+. Condi-
Figure 6. A typical kinetic trace for L410 formation monitored at 410
-
1
-1
+
-1
-1
2+
-1
+
tions: 0.2 mmol L MeReO3, 0.60 mol L H , 1-cm cell at 25 °C. The
nm, with 0.2 mmol L MeReO3, 2.12 mmol L Eu in 0.6 mol L
H
2
+
-1
endpoint corresponds to a reactant ratio 3:1 [Eu ]/[MeReO3].
at 25 °C. The plot is fitted to first-order kinetics, giving kobs ) 0.118 s
.
Inset: linear variation of kobs with [Eu2 ].
+
mentioned inertia in moving mercury at the start to obtain
the first voltammetric response.
MeReO3
2
+
+
3+
2
Eu + 2H
8 2Eu + H2
(3)
Formation of the Intermediate L410. Figures 5 and S-2
present the results of a spectrophotometric titration of
2+
2+
MeReO by Eu . With <2 equiv Eu , no L410 was formed.
3
This shows that the first two reduction steps occur more
rapidly than the next. The Re(VI) species never accumulates;
V
20
MeRe O
2
is known not to absorb at 410 nm. MeReO
2
is
-
+
formed irreversibly because E° for MeReO
MeReO + H
3
+ 2e +2H
O is -0.05 V. With one additional Eu ,
2
+
)
2
2
2
+
the L410 peak appeared, and the titration arrived at the
Figure 7. Experiments showing hydrogen evolution from Eu acidic
-1
2+
solution catalyzed by MeReO3. Conditions: 12.8 mmol L Eu , 0.22 mol
H , and µ ) 0.29 mol L . (a) The control experiment without MeReO3.
b) Experiment with 0.193 mmol L MeReO3. The time lag in hydrogen
endpoint. Thus, the spectrophotometric titration yields a ratio
-1
+
-1
L
2
+
[
Eu ]/[MeReO
3
] of 3 to form L410, consistent with the
-1
(
stoichiometry of eq 4.
evolution is the “inertia” in moving the Hg plug at the beginning.
2
+
+
3+
MeReO + 3 Eu + 2H ) L410 + 3 Eu + H O (4)
to exclude from the instrument. Nonetheless, two determina-
3
2
-
1
2+
tions with 0.4 mmol L Eu and excess MeReO
3
were
Because Eu2 is a one-electron reducing agent, the overall
:1 stoichiometry in the L410 formation strongly suggests
that L410 is a Re(IV) species. We formulate the sequence
of reactions between MeReO
+
successful. The reaction reached completion in 0.1 s with a
3
4
-1 -1
second-order constant k
Figures S-3 and S-4.
1
of 2.7 × 10 L mol s ; see
and Eu2 as follows.
+
3
The rate-controlling step for L410 formation is given in
eq 7. This reaction was followed by monitoring the increase
in the absorbance of this species at 410 nm, shown in Figure
2+
VI
3+
MeReO + Eu f Re + Eu
(k1)
(5)
3
6
. The reaction obeyed first-order kinetics in the presence
VI
2+
+
Re + Eu + 2H
f
2
+
of excess Eu ; see Figures S-5 and S-6. The second-order
rate constant k
3+
MeReO + Eu + H O (fast) (6)
-1 -1
2
2
is 61.3 ( 0.2 L mol
s
at 25 °C. In
3
3
addition, variation of acid concentration did not alter k . The
kinetic data are listed in Table S-1.
2+
3+
MeReO + Eu f L410 + Eu
(k3)
(7)
2
Determination of the Rate Law for Hydrogen Evolu-
tion. The hydrogen gas was collected by the microvolumeter
apparatus, and a full time course kinetic method was used
to study the dependence of the rate on the concentrations of
the species in the rate law. All the experiments were studied
In these reactions, MeReO
electron changes in which the oxidation state of rhenium
changes from VII to IV. We assume the first two steps, eqs
3
undergoes sequential one-
5
-6, are very fast because of the large driving force between
2+
3
Eu and MeReO . Stopped-flow experiments were carried
2+
with excess acid and a limiting concentration of Eu . Figure
out to follow the progress of reaction 5. The concentrations
7
gives a typical kinetic trace of gas buildup with MeReO
3
+
needed to be MeReO
MeReO
easily oxidized by O
3
. Eu2 to avoid reduction beyond
-
1
as catalyst. When 0.193 mmol L MeReO
1
3
reacted with
2
+
2
. This was technically difficult because Eu is so
-
1
2+
-1
2.8 mmol L Eu in 0.22 mol L anaerobic HCl, the
2
, the last traces of which are difficult
reaction followed first-order kinetics. The kinetic data are
summarized in Tables S-2 and S-3. The reaction is first-
(
20) Abu-Omar, M. M.; Espenson, J. H. Inorg. Chem. 1995, 34, 6239-
2
+
6240.
order with respect to both [Re] and [Eu ].
492 Inorganic Chemistry, Vol. 44, No. 3, 2005

Catalysis by Methyltrioxorhenium(VII)
+
Figure 8. The variation of kobs with [H ] for hydrogen evolution from
2
+
-1
acidic Eu solution by MeReO3 as catalyst. Conditions: 0.21 mmol L
MeReO3, 6-9 mmol L-1 Eu at µ ) 0.33 mol L and 25 °C. The curve
2+
-1
+
-1
Figure 10. The effect of variable ionic strength on the rate constants for
the hydrogen evolution, as plotted according to the Bronsted-Debye-
H u¨ ckel equation.
through the points is the least-squares fit to kobs ) a × [H ] . Inset: plot
+
-1
of kobs vs [H ]
.
-
L410A at 410 nm, the average molar absorptivity jꢀ at any
pH should obey eq 9:
+
ꢀAH[H ] + K ‚ꢀ
a
A
jꢀ )
(9)
+
K + [H ]
a
+
+ -1
Only when K , [H ] is jꢀ linearly dependent on [H ] ,
a
in agreement with the experimental results shown in the inset
to Figure 9. Thus, K is much smaller than 0.1, and ꢀAH is
a
-1
-1
a
about 550 L mol cm . Since K falls well below the range
of acid concentrations examined, rhenium(IV) exists almost
Figure 9. The change in the absorption spectrum of L410 with acid
+
concentration. Reading downward at 410 nm, [H ] is 0.10, 0.16, 0.20, 0.30,
completely in its protonated form L410H throughout this
-
1
+ -1
+
0
.40, 0.50, and 0.60 mol L . Inset: plot of jꢀ410 versus [H ] .
range of [H ]. It is clear therefore that in eq 8 k
8 4 a
) k K ,
where k
4
represents the second-order rate constant for the
-
1
In addition, at a constant ionic strength of 0.33 mol L ,
-
2+
reaction between L410A and Eu .
Variation of Ionic Strength for Hydrogen Formation.
Kinetic salt effects on hydrogen evolution were examined
in the ionic strength range 0.118-0.297 mol L by adjusting
the LiCl concentration. In these experiments, the acid
concentration was kept constant at 0.10 mol L with variable
the reaction was retarded by raising the concentration of acid,
Table S-3. Figure 8 shows that kobs is inversely dependent
+
on [H ]. From these kinetic results, the rate law can be
-1
-
1
-1
expressed by eq 8, with k ) 2.56 s at µ ) 0.33 mol L .
8
-1
2
+
[
Re][Eu ]
-4
-1
concentrations of 2.16-3.54 × 10 mol L MeReO
3
and
V ) k8
(8)
+
-3
-1
2+
[
H ]
5.00-5.66 × 10 mol L Eu . The results are summarized
in Table S-4. On the basis of the rate dependence studies,
the pseudo-first-order rate constant is independent of Eu2
+
Investigation of the Acid Dissociation Constant for
L410. As described above, the rate of hydrogen formation
is inhibited by acid concentration. In addition, the UV-vis
spectrum of L410 changes with different acid concentrations.
This indicates that there is a protonation step before the rate-
controlling hydrogen evolution step, and implicates two Re-
and first-order with respect to MeReO
is a catalyst, its concentration remains constant; therefore,
the second-order constant kobs/[Re] is the form that normal-
izes the data to unit catalyst concentration. The plot of log-
3 3
. Because MeReO
T
1
/2
1/2
(kobs/[Re] ) against 0.509µ /(1 + µ ), which is the form
T
(
IV) species that absorb at 410 nm, partners in an acid-
suggested by the Bronsted-Debye-H u¨ ckel equation, is
presented in Figure 10, which shows the retarding effect of
higher ionic strength; its slope is -2.3 ( 0.2.
base equilibrium. The acid form is a neutral Re(IV) species,
L410H, and its conjugate base is a Re(IV) anion, L410A .
The equilibrium can be expressed by L410H ) H
L410A (K
-
+
2+
2+
+
2
The Failure of Cr To Generate H . Cr does not
-
a
).
evolve H with MTO as the attempted catalyst. Experiments
2
2+
2+
To determine K
a
, we studied the acid effect on the
were carried out with Cr and Eu used together. The order
of addition of the two was varied. In both cases, hydrogen
evolution occurs much more slowly, and thus imprecisely,
spectrum of L410. Figure 9 shows the UV-vis spectrum of
L410 at different pH values. The absorbance at 410 nm
decreases with increasing acid concentration. If ꢀAH and ꢀ
designate the respective molar absorptivities of L410H and
2
+
A
especially when Cr is the first metal ion added. The data
are shown in Figure 11.
Inorganic Chemistry, Vol. 44, No. 3, 2005 493

Cai and Espenson
consumed. The data were fitted to a linear equation and the
-
4
-1
pseudo-zero-order rate constant kobs is 1.10 × 10 mol L
-1
s . The same method was applied to the kinetic studies with
varying concentrations of the three reactants. The data are
summarized in Table S-6. The reaction rates are independent
2
+
-
of [Eu ] and directly proportional to [Re]
T 4
and [ClO ], as
shown in Figures S-9 and S-10. This implies the rate law
-
1
-1
given in eq 11, with k11 ) 43.2 ( 0.7 L mol
s
at 25 °C.
2
+
d[Eu ]
-
-
) k [Re] [ClO ]
(11)
1
1
T
4
dt
The kinetics is drastically altered by the order of mixing
of M2 (M ) Eu or Cr), MeReO
+
, and ClO . When MeReO
-
3 4 3
Figure 11. Plots of hydrogen evolution with both Cr +aq and Eu2+
2
.
was the last reagent added, perchlorate reduction is complete
within 10 s. On the other hand, When perchlorate ions are
the last species added, the reaction is much slower, and takes
100 s or longer, as depicted in Figure S-11.
+
2+
2+
Conditions: [H ] ) 0.11M, µ ) 0.3 M, 25 °C. Case 1 (O): 3.3 mM Eu
mixed with 0.19 mM MTO, and then 3 mM Cr2+. Case 2 (9): 3 mM Cr
2
+
mixed with 0.19 mM MTO, and then 3.3 mM Eu . Case 3 (2):
comparison experiment, 3.3 mM Eu mixed with 0.19 mM MTO, and
then another 3.3 mM Eu . Data from case 3 are fit to first-order kinetics,
ignoring the time lag.
a
2
+
2
+
Discussion
Mechanism of Hydrogen Evolution. In two separately
was reduced by Eu2 to
+
measured kinetic steps, MeReO
the Re(IV) species L410
3
2
Eu2+
k1
Eu2+
MeReO3
8 MeReO2
8 L410
(12)
k3
4
-1
) 61.3 L mol-1 s-1
with k
1
) 2.7 × 10 L mol s-1 and k
3
in aqueous solution at 25 °C. Evolution of hydrogen occurs
2
+
+ -1
-1
at a rate k
)
8
[L410H][Eu ][H ] , k
0.33 mol L , implicating the reactant as the anionic
species of Re(IV), L410A , the minority Re(IV) species.
8
4 a
) k K ) 2.56 s at µ
-
1
-
The effect of ionic strength on k
the charge on the predominant Re(IV) species, L410H.
4
was used to determine
Figure 12. A typical kinetic trace for perchlorate reduction. Conditions:
-
1
2+
-1
-
-1
1
.55 mmol L Eu , 30 mmol L ClO4 , and 0.10 mmol L MeReO3,
-
1
+
-1
1/2
added last at 0.55 mol L
H
and µ ) 0.8 mol L at 25 °C.
According to the equation log k ) log k° + 2Az
z
A B
× [µ /
1
/2
2+
(
1 + µ )], the salt effect on the reaction between Eu and
Stoichiometry of Perchlorate Reduction. Measurement
-
L410A will agree only if, as postulated, the active form of
rhenium is the minority conjugate base with an ionic charge
of -1. An approach that may be preferable (because it
employs the same concept as a net chemical equation) is
of chloride ion concentration in acidic solutions showed that
-
2+
Cl aq was formed quantitatively by the reaction of Eu ,
MeReO , and sodium perchlorate in HOTf (trifluoromethane-
3
sulfonic acid). The same results were found for reactions of
21
that of the net actiVation process, defined as the reaction
in which the transition state is formed from the predominant
species. In the case at hand, it is
+
MeReO
3
and Cr2 in perchloric acid solution. These sto-
ichiometric determinations (Table S5) established that the
2+
2+
overall stoichiometry to be 8.1((0.6):1 Eu (or Cr ) per
Cl as expected from eq 10.
-
2+
+ q
+
L410H + Eu ) [ ] + H
(13)
MeReO3
for which ∆(z2)q ) 12 + 12 - 22 ) -2, under the
presumption that L410H is uncharged. The Bronsted-
Debye-H u¨ ckel equation states the following: log k ) log
2
+
2+
-
+
8
Eu /Cr + ClO4 + 8H
8
3+
3+
-
8
Eu /Cr + Cl + 4H O (10)
2
2
q
1/2
1/2
k° + ∆(z ) × A[µ /(1 + µ )], where A ) 0.509. Because
Kinetics of Perchlorate Reduction. A repetitive scan
stopped-flow experiment was performed with 1.55 mmol L
, 30 mmol L ClO , and 550
4
mmol L HCl at 25 °C with µ ) 0.8 mol L ; see Figures
2 q
this value of ∆(z ) matches the slope of Figure 10, -2.3,
-
1
the data confirm that L410H is uncharged.
With these features in mind, along with our presumption
2
+
-1
-1
-
Eu , 0.1 mmol L MeReO
3
-
1
-1
+
2
that H formation will arise from the reaction between H
1
2, S-7, and S-8. Some initial curvature is apparent, which
by the deliberate introduction of some O was traced to
residual oxygen in the instrument system. The graph of Eu
versus time with deliberate O is presented in Figure S12.
After the initial curvature in Figure 12, the absorbance
V
and a H-Re O species, we can suggest a chemical mech-
anism. The version shown in Scheme 1 agrees with all the
experimental data. Coordinated solvent molecules are not
shown for sake of simplicity. Three fast reactions are intro-
2
2+
2
2
+
readings lay on a straight line until all the Eu was
(21) Daugherty, N. A.; Newton, T. W. J. Phys. Chem. 1964, 68, 612.
494 Inorganic Chemistry, Vol. 44, No. 3, 2005

Catalysis by Methyltrioxorhenium(VII)
Scheme 1. Mechanism Proposed for Hydrogen Evolution
Perchlorate Reduction. Perchlorate has a powerful ther-
modynamic tendency for reduction in dilute aqueous solu-
-
-
4 3
tion: E°(ClO /ClO ) ) 1.19 V. Although this reduction
is thermodynamically favorable, it is often very slow. To
our knowledge, the only exception to sluggish reduction of
perchlorate is the fast reaction provided by a system
2
0
containing MTO and H
)
3
PO
2
. These components react (k298
-
1
-1 20
0.028 L mol s ) to form MeReO (H O) (n ) 2,
2
2
n
presumably, because MeReO
2
(PAr
3
)
2
has been isolated and
1
0
-
characterized ) (MDO), following which it reduces ClO
4
-
1
-1
rapidly, k298 ) 7.3 L mol
s
at 25 °C, which is by far the
highest rate constant (see Table S-7) recorded for perchlorate
2
0
reduction.
We recall the case of the reaction between H
3
PO
2
and
MeReO
3
1
, which generates H PO and MeReO (k ) 0.028
3 3 2
-
-1 20
L mol s ). The subsequent reaction between MeReO
and ClO
2
duced in the last part of the scheme. The most interesting of
-
III
results in reduction of perchlorate to chloride ions
them is the unimolecular conversion of Re OH into H-
4
V
in four steps. The first of these is rate-controlling, with k14
Re O, which is unfortunately not amenable to direct study.
-
1
-1
2+
)
7.3 L mol
s
at 25 °C:
Failure of Cr To Evolve Hydrogen. Mixing 0.3 mmol
L- MeReO
1
and 12 mmol L Cr generates an immediate
-1
2+
3
-
-
MeReO + ClO f MeReO + ClO
(14)
2
4
3
3
intense yellow-green color; see Figure 2. This species also
has an absorption maximum at 410 nm, and we designate it
as L410(Cr). A spectrophotometric titration showed that
The difference in the present case is that Eu2 or Cr2+
+
ions serve as the initial reducing agent in reactions that are
2
+
L410Cr formation required a 3:1 consumption ratio of Cr
much faster then eq 14. We find k ) k /4 ) 5.3 ( 0.1 L
1
4
11
2+
-1 -1
to MeReO
3
, just as for Eu . Clearly, L410(Cr) is also a
mol
s
at 25 °C, which agrees with the previous value. It
2
2
rhenium(IV) species. The species L410(Cr) has a different
had been shown in the earlier work that further reduction
2+
-
4
-1 -1
molar absorptivity from L410H from Eu reduction. At 0.1
of ClO3 occurs more rapidly, k ) 3.8 × 10 L mol s :
15
-1
mol L acid concentration, the respective molar absorptivi-
ties at 410 nm of L410H and L410(Cr) are 910 and 1200 L
-
-
MeReO + ClO f MeReO + ClO
(15)
2
3
3
2
-
1
-1
mol cm .
2+
Perchlorate reduction occurs much more slowly when Eu
Hydrogen was, however, not evolved over 10 h, during
which time L410(Cr) persists unchanged. Because the
reactions in Scheme 1 beyond L410 formation are based on
-
and MeReO
3
are mixed prior to the addition of ClO
4
, Figure
S-11. This is because the needed MeReO
2
has been converted
to L410, which is not active toward perchlorate reduction.
2
rhenium, the cause for the lack of H formation must lie
Acknowledgment. This research was supported by the
U. S. Department of Energy, Office of Basic Energy
Sciences, Division of Chemical Sciences, under Contract
W-7405-Eng-82 with Iowa State University of Science and
Technology. We are grateful to Masoud Nabavizadeh and
Patrick Huston for exploring the initial interactions between
elsewhere. Consider the structural formula of 2, which we
suggest may be the structure of L410(Cr) as it is immediately
formed. This assumes that the k step proceeds by an inner-
4
sphere mechanism.
2
+
M
3
and MeReO , to Professor Henry Taube for helpful
discussion and comments, and to Professor Edwin S. Gould
for calling our attention to the citation in ref 9.
Species L410(Cr), as 2, represents a derivative of Re(IV)
Supporting Information Available: Tables and plots of kinetic
data to illustrate agreement to selected mathematical forms and to
evaluate numerical parameters. This material is available free of
charge via the Internet at http://pubs.acs.org.
and Cr(III) after electron transfer. Both metals in these
3
oxidation states will have d electronic configurations, and
IV
as such will be slow to hydrolyze at either the Re -O or
III
Cr -O bonds. One need only postulate that the next needed
IC048669K
2+
reaction, between L410(Cr) and Cr , is slow (perhaps owing
to electrostatic repulsion) to understand why the reaction
proceeds no further.
(
22) Abu-Omar, M. M.; Appelman, E. H.; Espenson, J. H. Inorg. Chem.
1996, 35, 7751-7757.
Inorganic Chemistry, Vol. 44, No. 3, 2005 495