Base Hydrolysis of Methyltrioxorhenium. The Mechanism Revised and Extended: A Novel Application of Electrospray Mass Spectrometry

Article abstract of DOI:10.1021/ic980447x

The full kinetic pH profile for the base-promoted decomposition of MTO to CH₄ and ReO₄^- was examined with the inclusion of new data at pH 7-10. Spectroscopic and kinetic data gave evidence for mono- and dihydroxo complexes: MTO(OH^-) and MTO(OH^-)₂. Parallel unimolecular eliminations of methane from these species account for the rate-pH profile; the respective rate constants are MTO(OH^-), k = 4.56 × 10^-5 s^-1 and MTO(OH^-)₂, k = 2.29 × 10^-4 s^-1 at 25 °C. Some kinetic data were acquired with electrospray mass spectrometry to monitor the buildup in the concentration of perrhenate ions. Reliable signals for each of the isotopomers of ReO₄^- were obtained by this method with initial MTO concentrations of 160 μM.

Full text of DOI:10.1021/ic980447x

Inorg. Chem. 1998, 37, 4621-4624
4621
Base Hydrolysis of Methyltrioxorhenium. The Mechanism Revised and Extended: A Novel
Application of Electrospray Mass Spectrometry
James H. Espenson,* Haisong Tan, Sahana Mollah, R. S. Houk, and Matthew D. Eager
The Ames Laboratory and Department of Chemistry, Iowa State University, Ames, Iowa 50011
ReceiVed April 20, 1998
The full kinetic pH profile for the base-promoted decomposition of MTO to CH4 and ReO4- was examined with
the inclusion of new data at pH 7-10. Spectroscopic and kinetic data gave evidence for mono- and dihydroxo
complexes: MTO(OH-) and MTO(OH-)2. Parallel unimolecular eliminations of methane from these species
account for the rate-pH profile; the respective rate constants are MTO(OH-), k ) 4.56 × 10-5 s-1 and
MTO(OH-)2, k ) 2.29 × 10-4 s-1 at 25 °C. Some kinetic data were acquired with electrospray mass spectrometry
to monitor the buildup in the concentration of perrhenate ions. Reliable signals for each of the isotopomers of
-
ReO4 were obtained by this method with initial MTO concentrations of 160 µM.
Introduction
The base hydrolysis kinetics of methyltrioxorhenium (CH3-
ReO3, abbreviated as MTO) was reported by two groups of
workers in 1996.1,2 Both groups, agreeing that the exclusive
products are methane and perrhenate ions, wrote this chemical
equation for the net reaction:
-
CH3ReO3 + OH- ) CH4 + ReO4
(1)
The pH ranges of the two investigations were, however,
widely separated, approximately 3-61 and 11-14.2 Neither
set of data can correctly be extrapolated to the intermediate pH
region while also supporting the independent claims as to
mechanism. The dilemma in the kinetics is clearly apparent
from the two pieces of the experimental pH profile, shown in
Figure 1. Both sets of data are shown as solid lines within the
pH region actually measured, using the analytical expressions
for the rate constants to construct these curves. Additionally,
the rate law from each set of experiments was then extrapolated
toward neutral pH, outside the range of measurement, by the
mathematical forms given in the respective reports. The
extrapolated curves diverge widely. Both interpretations cannot
be as stated: the rate constants thus assigned could not be
reconciled with one another, in that they differ by more than 4
orders of magnitude. Expressed as the value of k in the equation
v ) k[MTO][OH-], the respective values of k are 8.6 × 102 L
Figure 1. The pH profiles for two sets of kinetic data for the OH--
induced decomposition of MTO. One set used data for pH 2.97-5.70,
extrapolated to 25 °C,1 the other set refers to measurements over the
range 11.7-13.8. Each is shown as a bold line. Each set of data was
extrapolated by the chemical and algebraic model given in the respective
publication, with the extrapolated portions shown as dashed lines.
the determination of the molecular weights and structures of
large biological molecules.3,4 This technique has also been used
for the determination of many inorganic species, from ion
clusters to bare metal ions.5-10 To date, applications of the ES-
MS method to kinetics problems have, however, been relatively
rare.7
mol-1 s-1 (extrapolated to 298 K)1 vs 2.7 × 10-2 L mol-1 s-1 2
.
Neither set of data necessarily disagrees with the other, however,
if a more refined model can be formulated on the basis of
kinetics in the intermediate pH range. The goal of this study
has been to obtain kinetic data that cover the full pH range and
to formulate a single mechanism to describe all the data.
The reaction kinetics has now been studied at intermediate
pH values, with UV-vis and electrospray mass spectrometric
(ES-MS) techniques. The ES-MS technique is widely used for
(4) Smith, R. D.; Loo, J. A.; Edmonds, C. G.; Barinaga, C. J.; Udseth, H.
R. Anal. Chem. 1990, 62, 882.
(5) Colton, R.; D’Agostino, A.; Traeger, J. C. Mass Spectrom. ReV. 1995,
14, 79.
(6) Corr, J. J.; Anacleto, J. F. Anal. Chem. 1996, 68, 2155.
(7) Charbonniere, L. J.; Williams, A. F.; Frey, U.; Merbach, A. E.;
Kamalaprija, P.; Schaad, O. J. Am. Chem. Soc. 1997, 119, 2488-
2496.
(8) Blades, A. T.; Jayaweera, P.; Ikonomou, M. G.; Kebarle, P. J. Chem.
Phys. 1990, 92, 5900.
(1) Laurenczy, G.; Luka´cs, F.; Roulet, R.; Herrmann, W. A.; Fischer, R.
W. Organometallics 1996, 15, 848-851.
(2) Abu-Omar, M.; Hansen, P. J.; Espenson, J. H. J. Am. Chem. Soc. 1996,
118, 4966-4974.
(3) Fenn, J. B.; Mann, M.; Meng, C. K.; Wong, S. F.; Whitehouse, C. M.
Science 1989, 246, 46.
(9) Blades, A. T.; Ho, Y.; Kebarle, P. J. J. Phys. Chem. 1996, 100, 2443.
(10) Klassen, J. S.; Kebarle, P. J. J. Am. Chem. Soc. 1996, 118, 12437.
S0020-1669(98)00447-9 CCC: $15.00 © 1998 American Chemical Society
Published on Web 08/13/1998

4622 Inorganic Chemistry, Vol. 37, No. 18, 1998
Table 1. Typical ES-MS Operating Conditions
Espenson et al.
sample flow rate
17 µL min-1
nitrogen, 280 kPa
nitrogen (ultrapure carrier
grade), 550 kPa, 45 °C
-4400 V
nebulizer gas, pressure
curtain gas, back-pressure,
temperature
ionization needle voltage (VISV
interface plate voltage (VIN)
)
-450 V
orifice plate voltage (VOR
)
-130 V
RF only quadrupole voltage (VRO
mass analyzer quadrupole
voltage (VRI)
)
-100 V
-95 V
CEM detector voltage
operating pressure of
quadrupole chamber
2500 V
4.0 mPa
Experimental Section
Materials. The solutions for kinetics were prepared using high-
purity water obtained by passing laboratory distilled water through a
Milllipore-Q water purification system. HPLC-grade acetonitrile (Fisher
Scientific) was used in the ES-MS measurements. MTO, sodium
perrhenate, sodium p-toluenesulfonate () tosylate), sodium carbonate,
and potassium phosphate were used as received.
Figure 2. Typical plot of the signal ratio for ReO4-/TsO- vs time
obtained in an ES-MS experiment. The initial concentration of MTO
was 160 µM and the pH was buffered at 8.0. The curve shown
corresponds to one of the two isotopes of rhenium. The curves through
the points are the first-order kinetic fits.
Instrumentation. UV-vis spectra were obtained from a Shimadzu
UV-3101PC instrument. A Perkin-Elmer SCIEX API-1 mass spec-
trometer was used for the ES-MS study. Typical conditions used in
the operation of the ES-MS instrumentation are summarized in Table
1. The voltages were optimized to maximize the signal for the species
of interest. Peak hopping data were collected using a 100-ms dwell
time. Spectral scans were collected by adding 10 consecutive scans
together using a dwell time of 10 ms per 0.1 amu.
ES-MS has been used for the determination of many inorganic
ions both qualitatively and quantitatively.11 We have found that
the perrhenate anion gives a clean mass spectrum in 1:1 CH3-
CN:H2O solution under ES-MS conditions. The only two
significant peaks are those for the two isotopes, 185Re and 187Re,
with natural abundances of 37.07 and 62.93%, respectively.12
Kinetics. All of the kinetics determinations, by the UV and ES-
MS methods, were carried out in aqueous solution at 25.0 ( 0.5 °C. A
5 cm quartz cuvette was used, and the starting concentration of MTO
was 1.60 × 10-4 M (40 ppm). Other concentrations were 3.09 × 10-5
M (6 ppm) sodium tosylate, and 1-20 mM buffer solution. The
solution was diluted with an equal volume of acetonitrile just before
the ES-MS measurement. The addition of an organic solvent greatly
enhances the ion signal in ES-MS. The actual kinetics data apply,
however, to a strictly aqueous medium. The buffers used for the
different pH ranges were 4.5-6.0, HOAc/NaOAc; 6.5-8.0, KH2PO2/
K2HPO4; and 8.0-10.5, NaHCO3/Na2CO3. Above pH 10.5, NaOH
alone determined the pH. The ionic strength change proved to have
little influence on the rate constants.
-
Thus ReO4 shows peaks at m/z 249 and 251 in a relative
abundance of 1:1.7. Using the tosylate anion (m/z 171) as the
internal standard for concentration, we were able to monitor
the increase in intensity of each of the perrhenate peaks with
time as the reaction proceeded at pH 7-10.
Figure 2 shows the change in the perrhenate signal at m/z
251, relative to tosylate, as a function of time. The time to
collect the mass spectral data was ∼100 s, which is much
smaller than the reaction time, typically ∼104 s. The intensity-
time data gave excellent fits to first-order kinetics. The errors
shown represent the standard deviations estimated from counting
statistics based on total counts observed for each species.
To determine the absolute conversion in the decomposition
reaction, the method of standard additions was applied. Under
the same ES-MS conditions, increments of a sodium perrhenate
solution were added to the reaction vessel at concentrations
4-16 ppm in total. This was done to spent reaction solutions
at pH 7 and 10. Figure S1 (Supporting Information) shows
this result at pH 10. There is a nearly linear increase of the
intensity ratio with the concentration of the added perrhenate
ions. If this line is extrapolated linearly to the abscissa, its
intercept corresponds to a value of 22 ppm of ReO4-. This is
only slightly higher than that of the nominal value of 20 ppm,
which could be due to the volume contraction upon mixing water
and acetonitrile.
Results
Kinetics. As will be shown, data in the intermediate pH
region do define an approximate plateau between the limiting
values of the earlier studies, although the rate constants are not
those that would have been extrapolated from either of the
previous experimental studies. We can now propose a mech-
anism to account for the kinetic data over the entire pH range.
The decomposition of MTO to perrhenate ions was followed
at several UV wavelengths. Some reactions were followed by
monitoring the absorbance as a function of time at a fixed
wavelength; the readings would rise or fall with time depending
on the relative values of the molar absorptivities of MTO and
ReO4- at the chosen wavelength. Other experiments were based
on repetitive UV scans over time. In one treatment, these files
were analyzed by extracting absorbance values at a given
wavelength. In both of these cases the rate constant was
obtained by nonlinear least-squares fits to a first-order rate
equation: Abst ) Abs∞ + (Abs0 - Abs∞) × exp(-kt). The
single-wavelength data were of greater precision, however, and
were used in this analysis.
pH Variation. The various buffers listed previously were
used to set [OH-] in different experiments in the pH range
5-11. Since MTO was present in the range (1-2) × 10-4 M,
much lower than the buffer capacity, the pH could be taken as
nearly constant in each experiment. The new kinetic data, from
UV and ES-MS, along with the experimental data at low pH1
and high pH,2 were used to create a new pH profile, given in
Figure 3.
(11) Guizdala, A. B., III; Johnson, S. K.; Mollah, S.; Houk, R. S. Anal.
Atom. Sepctro. 1997, 12, 503.
(12) Friedlander, G.; Kennedy, J. W. Nuclear and Radiochemistry; John
Wiley & Sons: New York, 1955; p 434.

Base Hydrolysis of Methyltrioxorhenium
Inorganic Chemistry, Vol. 37, No. 18, 1998 4623
parameters were deduced without reference to values reported
previously. The fitted values of the parameters are
k1 ) (4.56 ( 0.20) × 10-5 s-1
K1 ) (2.5 ( 0.9) × 107 L mol-1
k2 ) (2.29 ( 0.03) × 10-4 s-1
K2 ) 73.1 ( 5.6 L mol-1
Let us now examine what comparisons can be made on the
basis of these values. The value of K1 affords pKa1 ) 6.70,
perhaps acceptably close to the reported 7.5, given the different
methods employed. From K2 and pKw ) 14, pKa2 ) 12.2 for
MTO(OH-) + H2O ) MTO(OH-)2 + H+; this compares
favorably with the reported value,2 pKa ) 11.9 (kinetics) and
11.7 (UV titration) under alkaline conditions. Of course the
symbolism used in this work is different than that in ref 2, and
this constant is not labeled the same there as it appears here.
From the calculated product k1K1, we have the apparent
bimolecular rate constant for the reaction of MTO + OH- in
the acidic region: 1.03 × 103 L mol-1 s-1, compared to 0.86
× 103 L mol-1 s-1, calculated from the kinetic data and the
value of K1. The difference is not large and can be traced to
the ways in which pKa1 was treated.
What is one to make of the unimolecular constants cited
above: k1 ) (4.56 ( 0.20) × 10-5 s-1 and k2 ) (2.29 ( 0.03)
× 10-4 s-1? The two are similar to one another, being separated
by a factor of only 5. In contrast, MTO itself shows no tendency
to produce methane. These findings imply that the reaction
occurs by the elimination of methane from the methyl group
and the proton of the coordinated hydroxide. Whether one OH-
group or two are coordinated to rhenium makes rather little
difference; at most, the second hydroxide facilitates methane
release by making the incipient perrhenate ion into a better
leaving group.
Figure 3. Variation of the observed rate constant with pH, including
the new values at intermediate pH, shown as O (ES-MS) and × (UV-
vis). The solid curve is the fit of all data to eq 6. The inset expands the
middle region.
Interpretation and Discussion
Mechanisms. The pH profile shows two rising portions and
two plateaus, as if the data represented the titration curves for
the formation of two intermediates. Indeed, this concept
provides the basis for one of the mechanisms that can be drawn
to account for the pH profile. The acidity of MTO (pKa 7.5)13,14
has been recognized, which we find convenient to write as the
complexation of OH-:
K1
\
CH3ReO3 + OH- y
z CH3ReO3(OH-)
(2)
If the second, high-pH plateau is to be accounted for in an
analogous manner, then a second equilibrium, eq 3, must be
written:
Independent evidence has been obtained for each of the
hydroxo-MTO anions. The bright yellow [MTO(OH)2]- was
detected spectrophotometrically and is also evident from pH
measurements.13,14 The UV-vis spectra of MTO remains the
same between pH 1 and 6, whereas it changes above pH ∼6
and ∼11. These findings and the shape of the pH titration curve
provide support for the species [MTO(OH)]- and [MTO(OH)2]2-.
The electrospray conditions that produced strong signals for
perrhenate and tosylate ions did not show the presence of
[MTO(OH)]- or [MTO(OH)2]2-. This at first appeared alarm-
ing, since [MTO(OH)]- should be present at a significant
concentration at pH 8-11 according to our analysis, Figure S2
(Supporting Information). When the voltage difference between
the skimmer and the RF-only quadrupole rods was reduced from
30 to 3 V, weak peaks of [MTO(OH)‚4H2O]- became evident
in the spectrum at pH 11 in a carbonate buffer. Apparently,
collision conditions that are energetic enough to strip solvent
molecules from [MTO(OH)‚4H2O]- also cause it to dispropor-
tionate. This solvated anion would have been missed had the
measurements been restricted to a single collision energy,
illustrating the importance of examining the effect of collision
conditions on electrospray spectra of inorganic and organome-
tallic ions before concluding the certain species are absent from
the sample, as noted by others.5,8-10
K2
\
CH3ReO3(OH-) + OH- y
z CH3ReO3(OH-)2
(3)
If this is the model, then each species must react indepen-
dently at the unimolecular rate given by its plateau. The
chemical equations are as shown in eqs 4 and 5
k1
CH3ReO3(OH-)
98 CH4 + ReO4
(4)
(5)
-
k1
9
CH3ReO3(OH-)2
8 CH4 + ReO4- + OH-
The smooth curve in Figure 3 is the pH profile for the fit of
the rate constants to this model, as expressed in eq 6
k1K1[OH-] + k2K1K2[OH-]2
1 + K1[OH-] + K1K2[OH-]2
k )
(6)
In the low-pH region, the rate is k1K1[OH-], in agreement with
the form reported.1 In the limit of very high pH, the rate will
be given by k2K2[OH-]/(1 + K2[OH-]), also consistent with
the form reported.2
The data over the entire pH range were fitted by eq 6 using
the method of nonlinear least-squares regression, and the four
The secondary solvation of this anion was found under these
conditions, because of the low energy used. A further finding
emerged: an additional peak corresponding in mass to
“MeReO5-” was seen but only in carbonate buffer. The point
(13) Herrmann, W. A.; Fischer, R. W.; Scherer, W. AdV. Mater. 1992, 4,
653.
(14) Herrmann, W. A.; Fischer, R. W. J. Am. Chem. Soc. 1995, 117, 3223.

4624 Inorganic Chemistry, Vol. 37, No. 18, 1998
Espenson et al.
was not investigated further; perhaps in alkaline solution MTO
catalyzes the conversion of CO3 to CO2.
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.
2-
Figure S2 shows the constructed speciation diagram for MTO
and its hydroxo complexes, calculated from the best-fit values
of K1 and K2. Recognition of the species present over the entire
range allows the formulation of a satisfactory mechanism.
This appears to be one of the early applications, and perhaps
the first, of the ES-MS technique to a kinetics problem in
inorganic or organometallic chemistry. The exceptional sen-
sitivity of the method should commend it for many applications.
Supporting Information Available: Two figures depict the
calibration of the MS-ES data by standard additions and the speciation
of MTO as a function of hydroxide ion concentration (2 pages).
Ordering information is given on any current masthead page.
IC980447X