Proton-deuteron exchange reactions of methyltrioxorhenium(VII) in organic solvents

Article abstract of DOI:10.1021/om049029s

Rate constants for the proton/deuteron (H/D) exchange of methyltrioxorhenium(VII) (MTO) were determined in four organic solvents by means of time course ¹H NMR measurements. Exchange between MTO and a deuterium donor proceeds only in the presence of a Lewis base; the reaction does not take place in neutral or acidified solutions. The kinetics depends on the bases, deuterium donors, and solvents. The observed pseudo-first-order rate constants (k_ψ) are negligibly small when coordinating bases are used but increase with noncoordinating bases. A direct relationship was found between k_ψ and the pK_a values of the conjugate acids of the bases. With phenol-d₆ as a deuterium donor (D) and 2,6-lutidine as a base (B), the rate law varies with solvents: k_ψ = k_a[D] [B] in benzene-d₆ (k_a = 1.95 × 10^-3 kg² mol^-2 s^-1); k _ψ = k_aκ[D][B]/(1 + κ[D]) in chloroform-d (k_ψ = 1.41 × 10^-2 kg mol^-1 s ^-1, κ = 0.79 kg mol^-1); k_ψ = k _a[B] in acetone-d₆ and acetonitrile-d₃ (k _a = 1.22 × 10^-2 and 3.87 × 10^-2 kg mol^-1 s^-1, respectively) at 24.8°C. The same treatment applies to the methanol-d₄/lutidine system in benzene-d₆, once allowance is made for formation of an MTO-methanol adduct. Reasonably rapid exchange under mild conditions reflects the acidic character of MTO, which may arise from its base-promoted keto/enol tautomerization. Three consecutive steps are suggested for the reaction mechanism: attack of a base on an MTO methyl proton, isomerization to the enol intermediate (I₂), and H/D exchange between I₂ and a deuterium donor.

Full text of DOI:10.1021/om049029s

2780
Organometallics 2005, 24, 2780-2788
Proton-Deuteron Exchange Reactions of
Methyltrioxorhenium(VII) in Organic Solvents
Mitsuru Matsumoto and James H. Espenson*
Department of Chemistry, Iowa State University of Science and Technology, Ames, Iowa 50011
Received December 8, 2004
Rate constants for the proton/deuteron (H/D) exchange of methyltrioxorhenium(VII) (MTO)
were determined in four organic solvents by means of time course 1H NMR measurements.
Exchange between MTO and a deuterium donor proceeds only in the presence of a Lewis
base; the reaction does not take place in neutral or acidified solutions. The kinetics depends
on the bases, deuterium donors, and solvents. The observed pseudo-first-order rate constants
(kψ) are negligibly small when coordinating bases are used but increase with noncoordinating
bases. A direct relationship was found between kψ and the pKa values of the conjugate acids
of the bases. With phenol-d6 as a deuterium donor (D) and 2,6-lutidine as a base (B), the
rate law varies with solvents: kψ ) ka[D][B] in benzene-d6 (ka ) 1.95 × 10-3 kg2 mol-2 s-1);
kψ ) kaκ[D][B]/(1 + κ[D]) in chloroform-d (ka ) 1.41 × 10-2 kg mol-1 s-1, κ ) 0.79 kg mol-1);
kψ ) ka[B] in acetone-d6 and acetonitrile-d3 (ka ) 1.22 × 10-2 and 3.87 × 10-2 kg mol-1 s-1,
respectively) at 24.8 °C. The same treatment applies to the methanol-d4/lutidine system in
benzene-d6, once allowance is made for formation of an MTO-methanol adduct. Reasonably
rapid exchange under mild conditions reflects the acidic character of MTO, which may arise
from its base-promoted keto/enol tautomerization. Three consecutive steps are suggested
for the reaction mechanism: attack of a base on an MTO methyl proton, isomerization to
the enol intermediate (I2), and H/D exchange between I2 and a deuterium donor.
Scheme 1. H/D Exchange Reactions of MTO
Introduction
Methyltrioxorhenium(VII), CH3ReO3 or MTO, cata-
lyzes reactions of hydrogen peroxide, the mechanisms
of which have been extensively studied.1-5 Whereas no
H/D exchange of the CH3 group was found in deuteri-
oxide media,1 partially deuterated MTO (CDH2ReO3,
MTO-d: Scheme 1) was detected by 1H NMR spectros-
copy in nitromethane-d3 solution in the presence of
pyridine at 296 K.6 The mechanism was not resolved,
but the observation of exchange under mild conditions
could imply the indirect involvement of the enol form
of MTO, CH2dRe(O)2OH. In Scheme 1 the reaction steps
are not intended to describe unimolecular reactions but
involve concentrations of heterocyclic bases and deute-
rium donors, as will be described.
enol form of MTO by itself is out of range as an
accessible intermediate, with pKke ca. 15-16. The
estimate of pKke was made by approximating ∆G° ≈
∆H°, because ∆S° can be taken as ∼0 for a reaction with
nearly identical species on each side of the chemical
equation. In the solid state the enol form of MTO was
generated by UV irradiation and identified by its IR
spectrum at 14 K in a solid argon matrix.7,8 In solution,
although the enol form of MTO has not been detected,
it was reasonably proposed as the active intermediate
in olefin metathesis.9 We have suggested the involve-
ment of enol MTO as the reactive intermediate in the
synthesis of a complex in which the resulting Re-CH2-
pyridine unit was interpreted as the consequence of
nucleophilic attack of pyridine at the methylene group
of the enol.10
The energy difference between the keto and enol
forms of MTO has been estimated to be 89 kJ mol-1 by
a density functional theory calculation.7 Clearly, the
* To whom correspondence should be addressed. E-mail: espenson@
iastate.edu.
(1) Espenson, J. H.; Abu-Omar, M. M. Adv. Chem. Ser. 1997, No.
253, 99-134.
(2) Li, M.; Espenson, J. H. J. Mol. Catal. A 2004, 208, 123-128.
(3) Bouh, A. O.; Espenson, J. H. J. Mol. Catal. A 2003, 200, 43-47.
(4) Gisdakis, P.; Antonczak, S.; Kostlmeier, S.; Herrmann, W. A.;
Rosch, N. Angew. Chem., Int. Ed. 1998, 37, 2211-2214.
(5) Vassell, K. A.; Espenson, J. H. Inorg. Chem. 1994, 33, 5491-
5498.
(6) Wang, W.-D.; Espenson, J. H. J. Am. Chem. Soc. 1998, 120,
11335-11341.
(7) Morris, L. J.; Downs, A. J.; Greene, T. M.; McGrady, G. S.;
Herrmann, W. A.; Sirsch, P.; Scherer, W.; Gropen, O. Organometallics
2001, 20, 2344-2352.
The carbonyl group of a ketone increases the acidity
of the R-proton, because the enol tautomer is stabilized
(8) Morris, L. J.; Downs, A. J.; Greene, T. M.; McGrady, G. S.;
Herrmann, W. A.; Sirsch, P.; Gropen, O.; Scherer, W. Chem. Commun.
2000, 67-68.
(9) Hoffman, D. M. Comprehensive Organometallic Chemistry II;
Casey, C. P., Ed.; Pergamon: Oxford, U.K., 1995; Vol. 6, pp 231-255.
(10) Zhang, C.; Guzei, I. A.; Espenson, J. H. Organometallics 2000,
19, 5257-5259.
10.1021/om049029s CCC: $30.25 © 2005 American Chemical Society
Publication on Web 04/22/2005

H-D Exchange Reactions of Methyltrioxorhenium(II)
Organometallics, Vol. 24, No. 11, 2005 2781
Chart 1. Structures of the N-Heterocyclic Bases
by conjugation.11 The keto/enol equilibrium constants
of carbonyl compounds are generally small, pKke 6-9;
therefore, the presence of acid or base is needed to
observe H/D exchange.11 Even though MTO is acidic,
pKa ca. 7.5 at 25 °C,12-14 bases cause the rapid and
irreversible decomposition of MTO to perrhenate ions
and methane.15 We have yet to realize exchange in
acidic aqueous solution.16,17
For metal-bonded alkyl groups, various H/D exchange
reactions have been reported;18 however, the role of
metal-carbon double-bond formation is varied. Slow
deuterium incorporation to a metal-bonded alkyl group
has been observed for Mo19 and W20-22 complexes, where
the reaction proceeds via the formation of a metal-
carbon double bond and requires high temperature and
extended times. In turn, fast H/D exchange reactions
are found in alkyl complexes of Os,23 Ta,24 and Pt,25-27
where the participation of metal-carbon double bonds
is insignificant. As for rhenium complexes, very slow
H/D exchange was observed in the methylene proton of
Re2(µ-CH2)(µ-O)O2Me4 and the reaction is decelerated
by the addition of acid.28
Figure 1. 1H NMR spectra of MTO-dn (n ) 0-2) after
7 h at 323 K. Concentrations: 0.05 mol kg-1 of MTO,
1.0 mol kg-1 of CD3OD, 0.6 mol kg-1 of pyridine-d5.
coordinates extensively to the rhenium center will also
be discussed.
Experimental Section
MTO was synthesized by a literature method.29 Other
chemicals were purchased from commercial sources and used
without further purification. Throughout this study, we used
molality as the concentration unit to enable facile sample
preparation and to avoid wasting deuterated reagents. A
Bruker DRX-400 MHz spectrometer was used to record
1H NMR spectra at 298.0(1) K. The 1H chemical shifts of MTO
1
were measured relative to the residual H resonances of the
deuterated solvents.
Rate constants were calculated from H NMR time course
1
experiments; the signal intensities were used instead of peak
areas, because the resonances of MTO and MTO-dn species
overlap; see Figure 1. The intensities of the singlet of MTO,
the center peak of the MTO-d triplet, and the center peak of
the MTO-d2 quintet were used for the determinations. For
reactions with half-lives greater than 30 min, averaged spectra
from 16 scans were used; for faster reactions, only a single-
scan spectrum was used.
This article reports studies of the H/D exchange of
MTO catalyzed by weakly coordinating N-heterocyclic
bases (Chart 1). In the presence of an excess amount of
deuterium donor, exchange proceeds to completion in
three sequential steps. The kinetics depends on the
solvents, bases, and deuterium donors used. Nonethe-
less, a general mechanism can be assigned; the near
absence of exchange when one uses a Lewis base that
The rate constants for the three steps of H/D exchange were
calculated from eqs 1-3,30 in which A, B, and C correspond to
the peak intensities of MTO-dn, n ) 0, 1, 2. The observed rate
(11) Lowry, T. H.; Richardson, K. S. Mechanism and Theory in
Organic Chemistry, 3rd ed.; HarperCollins: New York, 1987.
(12) Herrmann, W. A.; Fischer, R. W.; Scherer, W. Adv. Mater. 1992,
4, 653-658.
A ) A0 exp(-kψ01t)
(1)
(2)
kψ01A0
(13) Espenson, J. H.; Tan, H.; Mollah, S.; Houk, R. S.; Eager, M. D.
Inorg. Chem. 1998, 37, 4621-4624.
B )
{exp(-kψ01t) - exp(-kψ12t)}
kψ12 - kψ
01
(14) Herrmann, W. A.; Fischer, R. W. J. Am. Chem. Soc. 1995, 117,
3223-3230.
(15) Herrmann, W. A.; Fischer, R. W.; Rauch, M. U.; Scherer, W. J.
Mol. Catal. 1994, 86, 243-266.
kψ01kψ12A0
(kψ12 - kψ01)(kψ23 - kψ
C )
exp(-kψ01t) +
(16) Abu-Omar, M. M.; Hansen, P. J.; Espenson, J. H. J. Am. Chem.
Soc. 1996, 118, 4966-4974.
01
)
(17) Brittingham, K. A.; Espenson, J. H. Inorg. Chem. 1999, 38,
744-750.
kψ01kψ12A0
(kψ01 - kψ12)(kψ23 - kψ
exp(-kψ12t) +
(18) Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507-514.
(19) Wada, K.; Pamplin, C. B.; Legzdins, P.; Patrick, B. O.; Tsyba,
I.; Bau, R. J. Am. Chem. Soc. 2003, 125, 7035-7048.
(20) Adams, C. S.; Legzdins, P.; McNeil, W. S. Organometallics 2001,
20, 4939-4955.
12
)
kψ01kψ12A0
(kψ01 - kψ23)(kψ12 - kψ
)
exp(-kψ23t) (3)
23
(21) Adams, C. S.; Legzdins, P.; Tran, E. J. Am. Chem. Soc. 2001,
123, 612-624.
(22) Tran, E.; Legzdins, P. J. Am. Chem. Soc. 1997, 119, 5071-5072.
(23) Hamilton, D. H.; Shapley, J. R. Organometallics 2000, 19, 761-
769.
constant for the loss of MTO, kψ01, was calculated directly from
the decrease of MTO signal intensity. The second-step rate
constant, kψ12, was calculated from the rise and fall of the
MTO-d signal intensity with kψ01 fixed at its established value.
Likewise, the third-step rate constant kψ23 was calculated from
(24) Fryzuk, M. D.; Johnson, S. A.; Rettig, S. J. J. Am. Chem. Soc.
2001, 123, 1602-1612.
(25) Prokopchuk, E. M.; Puddephatt, R. J. Organometallics 2003,
22, 787-796.
(26) Reinartz, S.; White, P. S.; Brookhart, M.; Templeton, J. L.
Organometallics 2000, 19, 3854-3866.
(29) Herrmann, W. A.; Kratzer, R. M.; Fischer, R. W. Angew. Chem.,
Int. Ed. Engl. 1997, 36, 2652-2654.
(30) Capellos, C.; Bielski, B. H. J. Kinetic Systems: Mathematical
Description of Chemical Kinetics in Solution; Wiley-Interscience: New
York, 1972.
(27) Shilov, A. E.; Shteinman, A. A. Coord. Chem. Rev. 1977, 24,
97-143.
(28) Hoffman, D. M.; Wierda, D. A. J. Am. Chem. Soc. 1990, 112,
7056-7057.

2782 Organometallics, Vol. 24, No. 11, 2005
Matsumoto and Espenson
Table 1. Equilibrium Constants at 298.0 K
between MTO and Lewis Bases in Benzene-d6
a
L
KML/kg mol-1
pKa
δML
4-picoline
pyridine-d5
quinoline
2-picoline
methanol-d4
MeQ
1710(240)
425(22)
37(5)
6.03
5.23
4.82
5.95
15.5
5.42
7.47
6.72
9.99
1.607
1.594
1.648b
1.592
1.686b
(1.59)
(1.59)
(1.59)
(1.59)
29.5(2)
1.01(6)
0.18c
Col
0.16c
Lut
phenol-d6
0.12c
0.054c
a Dissociation constant of conjugated acid of heterocyclic base
b
LH+, methanol, and phenol in water.35-37 δobs did not approach
a plateau at a high concentration of L. c Calculated from eq 8 with
δML fixed at 1.59 (see text).
equilibrium effects. Neutral and acidified samples in
D2O gradually turned deep blue and finally deposited
a dark blue gelatinous precipitate, an oligomer of
[MeReV(O)2(H2O)2].15,31,32
Adduct Formation in Benzene-d6. MTO forms
stable adducts with coordinating Lewis bases,
MTO‚L33,34 (eq 4), as had been studied previously in
nitromethane-d3.6 The same procedure was followed to
MTO + L ) MTO‚L KML
(4)
evaluate KML in benzene-d6. The notation L designates
a Lewis base that coordinates to rhenium; the symbol
B will be reserved for a Brønsted base that promotes
H/D exchange. Addition of Lewis bases to MTO causes
the chemical shift of MTO to move downfield in benzene-
d6. The MTO singlet remains sharp at any L/MTO ratio,
owing to the high rate of eq 4. Thus, the observed
chemical shift, δobs, is a concentration-weighted average
of the chemical shifts of MTO with δMTO and MTO‚L
with δML. The values of KML were calculated from eqs
5-7, in which the subscript T indicates the total
concentration.
Figure 2. 1H NMR signal intensities (arbitrary units) vs
time for (a) MTO, (b) MTO-d, and (c) MTO-d2 in benzene-
d6 at 298.0 K. Concentrations: 0.0175 mol kg-1 of MTO,
1.35 mol kg-1 of C6D5OD, 0.312 mol kg-1 of Lut. Curve
01
fittings from eqs 1-3 correspond to (a) kψ ) 9.05 ×
12
23
10-4 s-1, (b) kψ ) 5.77 × 10-4 s-1, and (c) kψ ) 2.47 ×
10-4 s-1, with each rate constant relying on its predecessor-
(s), as explained in the text.
01
the rise and fall of the MTO-d2 signal intensity with both kψ
and kψ12 fixed. Pseudo-first-order conditions were maintained
by using an excess amount of deuterium donor D over MTO;
generally [D] > 60[MTO]. This inequality also disallows
contributions from the reverse exchange processes. Typical
data are presented in Figure 2.
δMTO + KML[L]δML
δobs
)
(5)
1 + KML[L]
Results
ê2 + 4KML[L]T - ê
2KML
x
[L] )
(6)
(7)
Preliminary Measurements. Suitable conditions
were explored by recording the growth of the MTO-d
triplet, which lies 6-7 Hz upfield of the MTO singlet.
Exchange was not observed in neat deuterated organic
solvents or in D2O. A deuterium donor, methanol-d4,
was added to the sample solutions, and then they were
divided into three portions. The first portion was used
as is; trifluoroacetic acid-d was added to the second and
pyridine-d5 to the third. After 3 h at room temperature,
a new triplet near the MTO singlet grew slowly only in
the third sample. No H/D exchange was detected in
neutral or acidified samples even at 323 K. A base is
clearly required, but pyridine, which coordinates strongly
to rhenium, proved to be a poor choice of base catalyst
compared to those depicted in Chart 1.6
ê ) 1 + KML[MTO]T - KML[L]T
This treatment required a series of experiments with
constant [MTO]T; in this work it was maintained to
within <4% during each titration. When L is a weak
Brønsted base (i.e., pKa(LH+) >10) or its donor atom is
sterically hindered, KML is too small to be determined
from eqs 5-7 because the small extent of reaction
prevents the resolution of KMLδML into its component
values. In such cases, KML was calculated by assigning
δML as 1.59, a value found for bases that coordinate more
strongly; this is a valid approach in that the value of
δML had been shown to be largely independent of the
choice of L. Therefore, the denominator in eq 5 ap-
Most basic samples eventually became yellow, per-
haps indicating a trace of MTO decomposition,12,13,15,31
but the decomposition product(s) showed no kinetic or
(32) Espenson, J. H.; Yiu, D. T. Y. Inorg. Chem. 2000, 39, 4113-
4118.
(33) Herrmann, W. A. J. Organomet. Chem. 1995, 500, 149-174.
(34) Herrmann, W. A.; Ku¨hn, F. E. Acc. Chem. Res. 1997, 30, 169-
180.
(31) Laurenczy, G.; Lukacs, F.; Roulet, R.; Herrmann, W. A.; Fischer,
R. W. Organometallics 1996, 15, 848-851.

H-D Exchange Reactions of Methyltrioxorhenium(II)
Organometallics, Vol. 24, No. 11, 2005 2783
Figure 3. Pseudo-first-order rate constants, kψ01, for H/D
Figure 4. Pseudo-first-order rate constants for H/D ex-
exchange of MTO as a function of [B] and [D] in benzene-
change of MTO as a function of [Lut] with CD3OD in
01
d6. The solid lines show kψ with D ) C6D5OD and the
benzene-d6. Open circle, [CD3OD] ) 1.0 ( 0.1 mol kg-1
solid circle (five points), [CD3OD] ) 0.31-1.41 mol kg-1
;
.
indicated base catalysts. The dashed line refers to the
system o-cresol-d8/Lut. Solid symbols, [D] ) 1.0 mol kg-1
;
open symbols, [D] ) 0.5-1.4 mol kg-1
.
proaches unity (1 . KML[L]), giving eq 8. Table 1 lists
the values of KML
.
δobs ) δMTO + KML[L]δML
(8)
Measurements of the H/D Exchange Rate Con-
stants. Preliminary measurements showed that
MTO‚L exchanges H and D on methyl group little if at
all. To focus on MTO itself, therefore, the use of
sterically hindered, weakly coordinating bases was
essential. Exchange between MTO and phenol-d6 in
benzene-d6 was investigated by using the three bases
shown in Chart 1. In benzene-d6 other deuterium donors
such as o-cresol-d8 and methanol-d4 with Lut as the base
were used. (Because phenol is a weak acid, the extent
of proton transfer from phenol-d6, pKa ) 9.99, to Lut,
pKa(LutH+) ) 6.82, is negligible.) Rate constants were
also determined in chloroform-d, acetone-d6, and aceto-
nitrile-d3 with phenol-d6 and Lut. Additional measure-
ments were made in acetone-d6 with phenol-d6 and
MeQ.
Figure 5. Illustrating kinetic saturation with respect to
[C6D5OD] for B ) Lut (0.13-0.28 mol kg-1) in chloroform-d
and B ) MeQ (0.19-0.38 mol kg-1) in acetone-d6 at
298.0 K. The ordinate shows kψ01/[B], each reaction being
first-order with respect to [B].
to [Lut] was found. The rate constant is described by
eq 10: kb/10-3 s-1 ) 1.26(10) and Kc ) 0.83(10) kg mol-1
.
Reactions in Benzene-d6. The observed rate con-
stants for the first stage of H/D exchange, kψ01, were
directly proportional to both phenol-d6 and base con-
centrations (eq 9). In eq 9, the deuterium donor and base
kbKc[Lut]
01
kψ
)
(10)
1 + Kc[Lut]
kψ01 ) kaκ[D][B]
(9)
Reactions in Other Solvents. Experiments were
carried out in chloroform-d, acetone-d6, and acetonitrile-
d3, with D ) phenol-d6 and B ) Lut, MeQ. In each of
are designated D and B, and the overall third-order rate
constant is designated as kaκ for the sake of consistency
with subsequent notation. Values of kaκ/10-3 kg2 mol-2
s-1 with phenol-d6 as the deuterium donor are as
follows: MeQ, 0.315(7); Lut, 1.96(4); and Col, 7.42(15).
The o-cresol-d8/Lut pair also follows eq 9, with kaκ/10-3
kg2 mol-2 s-1 ) 1.18(4). The results are depicted in
Figure 3.
01
the cases kψ was directly proportional to the base
concentration. The exchange rates show kinetic satura-
tion with respect to [D] in two systems: B ) Lut in
chloroform-d and B ) MeQ in acetone-d6 (Figure 5).
With B ) Lut, the rates are independent of [D] in
acetonitrile-d3 and in acetone-d6 (data not shown). The
general equation is given by eq 11, the limiting form
01
kψ ) k1[B] being realized when 1 , κ[D]. Table 2
01
With D ) methanol-d4 and B ) Lut, values of kψ
summarizes the pertinent kinetic equations and nu-
merical values.
are independent of methanol-d4 concentration to 1.5 mol
kg-1 but increase slightly at 1.5-4 mol kg-1, where the
medium is in effect a mixed solvent. Data at methanol-
d4 concentrations >1.5 mol kg-1 were therefore ignored.
As shown in Figure 4, kinetic saturation with respect
kaκ[D][B]
01
kψ
)
(11)
1 + κ[D]

2784 Organometallics, Vol. 24, No. 11, 2005
Matsumoto and Espenson
Table 2. Summary of Rate Laws and Rate Constants for H/D Exchange of MTO at 298.0 K in Different
Solvents, Deuterium Donors, and Bases
01
solvent
donor/base
kψ
eq
kinetic paramsa
C6D6
C6D5OD/MeQ
C6D5OD/Lut
C6D5OD/Col
o-cresol-d8/Lut
CD3OD/Lut
C6D5OD/Lut
C6D5OD/Lut
C6D5OD/MeQ
C6D5OD/Lut
kaκ[B][D]
9b
K1K2k3 ) 0.315(7)
C6D6
kaκ[B][D]
9b
K1K2k3 ) 1.96(4)
C6D6
kaκ[B][D]
kaκ[B][D]
(kbKc[B])/(1 + Kc[B])
(kaκ[D][B])/(1 + κ[D])
ka[B]
(kaκ[D][B])/(1 + κ[D])
ka[B]
9b
K1K2k3 ) 7.42(15)
C6D6
9b
K1K2k3 ) 1.18(4)
C6D6
10
11
11d
11d
11d
k3 ) 1.26(10),c K1K2 ) 0.83(10)c
K1k2 ) 14.1(18), k3/k-2 ) 0.79(17)
K1k2 ) 12.2(3)
CDCl3
(CD3)2CO
(CD3)2CO
CD3CN
K1k2 ) 3.1(5), k3/k-2 ) 1.22(44)
K1k2 )38.7(13)
a The parameters are assigned according to eq 16, unless otherwise noted. K1is given in units of kg mol-1, k2 and k-2 in units of
10-3 s-1, and k3 in units of 10-3 kg mol-1 s-1
. .
b In the limit 1 . κ[C6D5OD]; κ ) k3/k-2 c k3 and K1K2 are assigned according to eq 20.d In
the limit 1 , κ[C6D5OD].
Table 3. Secondary Kinetic Isotope Effects for the
H/D Exchange of MTO in Different Solvents with
D ) C6D5OD and B ) Luta
Scheme 2. Structures Suggested for
Intermediates
solvent
k01:k12:k23
C6D6
1:0.89(3):0.87(6)
1:0.90(10):0.71(10)
1:0.90(3):0.75(7)
1:0.88(4):0.54(4)
CDCl3
(CD3)2CO
CD3CN
a kmn corresponds to the relative rate constants for MTO-dm to
MTO-dn.
enol tautomer as I2. Neither intermediate, however, has
been detected, as both species remain at very low
concentrations throughout. The hypothetical structures
are given in Scheme 2, and H/D exchange is described
by eqs 13-15. Equation 13 represents coordination of
Secondary Kinetic Isotope Effects (KIE). As the
H/D exchange reactions were studied under conditions
in which the deuterium donor was used in excess, only
the exchange of H by D was observed; therefore, primary
kinetic isotope effects were not available. The experi-
mental rate constants for each of the three steps were
used to estimate secondary isotope effects. The rate
constants were normalized by dividing the number of
C-H bonds in MTO-dn by 3 - n; thus, k01 ) kψ01/3,
k12 ) kψ12/2, k23 ) kψ23. The secondary KIE values for
H/D exchange of MTO and phenol-d6 with Lut in the
four solvents are listed in Table 3. A direct comparison
in this manner may include secondary equilibrium
isotope effects (EIE) as well as KIE owing to the complex
rate laws. Although H/D exchange grows slower at each
step, the precision of the data does not allow a more
penetrating analysis.
MTO + B ) I1 K1
I1 ) I2 k2/k-2
(13)
(14)
I2 + C6D5OD ) MTO-d + B + C6D5OH k3 (15)
a base to the methyl proton of MTO. This step does not
involve apparent bond breaking/forming; therefore, no
significant energy barrier is expected. We will treat this
step as a preequilibrium process. Equation 14 is the
keto/enol tautomerization of MTO promoted by a base.
This step presents a large structural change and thus
will have a significant energy barrier. Equation 15 is
the actual H/D exchange reaction between I2 and a
deuterium donor. Taking eq 13 as the preequilibrium
process and applying the steady-state approximation for
I2, we can write the rate law as eq 16, where D is phenol-
Discussion
General Reaction Scheme. The rate of exchange
depends on the bases, deuterium donors, and solvents
chosen. The reactions in eqs 12a and 12b can explain
ka[D][B][MTO]
vex
)
)
MTO + B ) I
(12a)
1 + [D]
I + C6D5OD ) MTO-d + B + C6D5OH (12b)
K1k2(k3/k-2)[D][B][MTO]
1 + (k3/k-2)[D]
(16)
the kinetic data, other than that with methanol-d4,
discussion of which is being deferred to the next section.
Equation 12a represents the formation of intermediate
I; note that I is distinct from the trigonal-bipyramidal
species MTO‚L with a Re-N interaction. Whereas I
participates directly in exchange, MTO‚L lies off the
pathway and, to the extent it forms, it inhibits exchange.
As the MTO‚L complexes do not undergo appreciable
H/D exchange, it can be concluded that the interaction
in I occurs not at rhenium but at a methyl proton. In
our suggestion, the interaction between MTO and a base
induces the keto/enol tautomerization of I, and the
initially formed intermediate is assigned as I1 and the
d6 or o-cresol-d8 and B the base. Comparison of this form
with eq 11 shows that ka ) K1k2 and κ ) k3/k-2. The
variations among the experimental rate laws can be
interpreted in terms of the limiting forms that are
realized with different relative values of the constants
in the denominator in eq 16. In benzene-d6, the
condition k-2 . k3[D] allows simplification to vex
)
kaκ[D][B][MTO]; therefore, kaκ ) K1k2(k3/k-2) ) K1K2k3.
In chloroform-d with phenol-d6 and Lut, or in acetone-
d6 with phenol-d6 and MeQ, eq 16 can be applied as is.
In acetone-d6 and acetonitrile-d3 with phenol-d6 and

H-D Exchange Reactions of Methyltrioxorhenium(II)
Organometallics, Vol. 24, No. 11, 2005 2785
Lut, the condition k-2 , k3[D] leads to a simplified
equation, vex ) ka[B][MTO]; ka ) K1k2. The observed rate
and equilibrium constants under different conditions are
listed in Table 2 with the appropriate rate laws.
Modified Reaction Scheme for Methanol-d4. It
is only reasonable to try to accommodate the data for
D ) methanol-d4 and D ) phenol-d6 with the same
model, eqs 13-15. There is one difference, however, that
arises because methanol-d4 is a considerably weaker
Brønsted acid than phenol-d6. Presumably, therefore,
k3(phenol-d6) . k3(methanol-d4). As a consequence, the
interaction between Lut and MTO in eq 14 can be
treated as a prior equilibrium. Were that the only factor,
the rate law would be expressed as
K1K2k3[Lut][CD3OD][MTO]
Figure 6. Plots of log(K1K2k3) against the aqueous pKa
values of the protonated forms of the heterocyclic bases.
vex
)
(17)
1 + K1K2[B]
Additionally, however, further interactions must be
considered. The nitrogen bases coordinate to the Re
center of MTO, and methanol-d4 may do so as well
despite its weaker Lewis basicity (eq 18a). One must
some have been reported.38-40 The equilibrium con-
stants KML are sensitive to steric and electronic effects.
Steric effects aside, stronger Brønsted bases are also
better Lewis bases: KML ) 420 kg mol-1 for pyridine-
d5 (PyH+, pKa ) 5.2) and 1700 kg mol-1 for 4-picoline
CH (O) Re‚‚‚O(D)CD
3
CH3ReO3 + CD3OD )
K18a
3
3
(PicH+, pKa ) 6.03). Steric hindrance reduces KML
;
R
values are 30 kg mol-1 for 2-picoline and 0.12 kg mol-1
for Lut. When steric effects predominate, KML becomes
independent of basicity toward protons: values of pKa
(LH+) increase as pyridine < 2-picoline < Lut < Col,
whereas KML values decrease as pyridine > 2-picoline
> Lut ≈ Col.
(18a)
CH3ReO3 + CD3OD )
O3ReC(H)2H‚‚‚O(D)CD3
K18b (18b)
â
also consider a separate interaction with the methyl
proton of MTO (eq 18b). The two equilibria must be
analyzed together, because the measurements do not
distinguish them. In other words, the value of KML for
methanol-d4 given in Table 1 represents the sum KML
) K18a + K18b. Both R and â are dead-end intermediates,
off the reaction pathway: R because we have shown that
five-coordinated derivatives of MTO do not exchange; â
because it is competitive with the formation of reactive
intermediate I1 (eq 13). Thus, the expression for the
reaction rate becomes more complex:
Rhenium-oxygen bonds in MTO‚L adducts are weaker
than those in MTO on the basis of structural data, which
give ∆d(Re-O) ) 5-7 pm.15,33 Infrared spectroscopy
also shows weaker rhenium-oxygen bonds with O- or
N-donating ligands.40 Certain coordinating solvents
such as THF-d8 and DMSO-d6 appear to form MTO‚L
adducts and decelerate exchange. The donor numbers
of these solvents are 29.8 kcal mol-1 for DMSO and
20 kcal mol-1 for THF.41
Base Effects. Reaction 14 in benzene-d6 represents
O-H‚‚‚N interactions because the bases do not coordi-
nate appreciably to rhenium. The correlation between
values of ka ()K1K2k3) with the pKa values of BH+ is
shown in Figure 6; linear fitting gives a slope of
+0.77(5). Because the value is large but less than unity,
it can be inferred that intermediate I2 contains a
substantially weakened O-H bond, but not fully depro-
tonated [CH2dReO3]-. Given that a weak base such as
MeQ forms I2, partially deprotonated MTO must be
substantially stabilized by tautomerization, which plau-
sibly affects the reactivity of I2.
K1K2k3[Lut][CD3OD][MTO]T
vex
)
(1 + K1K2[Lut])(1 + KML[CD3OD])
K1K2k3[Lut][CD3OD][MTO]T
)
1 + K1K2[Lut] + KML[CD3OD] + K1K2KML[Lut][CD3OD]
(19)
Because the rate was found to be independent of
[CD3OD], further simplification is required. The first
two terms of the denominator are neglected for the sake
of consistency with the experimental data, which gives
Solvent Effects. The kinetics of exchange are sensi-
tive to solvent, the major difference among them being
K1K2k3[Lut][MTO]T
vex
)
(20)
KML + K1K2KML[Lut]
(35) Smith, R. M.; Martell, A. E. Critical Stability Constants;
Plenum: New York, 1974.
(36) Lide, D. R., Ed. CRC Handbook of Chemistry and Physics, 81st
ed.; CRC Press: Boca Raton, FL, 2000.
To reconcile eqs 10 and 20, the value of KML needs to
be within experimental error of unity. Indeed, from
Table 1, the value is 1.01 ( 0.06 kg mol-1. Thus entirely
by coincidence, eq 10 is a correct phenomenological
description of the kinetics, for which eq 20 provides the
full mathematical form.
(37) Averaged pKa values at ca. 298 K reported after 1980; data from
Beilstein Commander.
(38) Herrmann,W.A.;Kuchler,J.G.;Weichselbaumer,G.;Herdtweck,
E.; Kiprof, P. J. Organomet. Chem. 1989, 372, 351-370.
(39) Herrmann, W. A.; Kuchler, J. G.; Kiprof, P.; Riede, J. J.
Organomet. Chem. 1990, 395, 55-67.
(40) Herrmann, W. A.; Correia, J. D. G.; Rauch, M. U.; Artus, G. R.
J.; Ku¨hn, F. E. J. Mol. Catal. A 1997, 118, 33-45.
(41) Marcus, Y. Ion Solvation; Wiley: Chichester, U.K., 1985.
Coordination Effects. The trigonal-bipyramidal
species MTO‚L are well-known; crystal structures of

2786 Organometallics, Vol. 24, No. 11, 2005
Matsumoto and Espenson
their polarity; dielectric constants are as follows: ben-
zene, 2.28; chloroform, 4.72; acetone, 20.7; acetonitrile,
35.95.36 The rate laws with D ) phenol-d6 and B ) Lut
in each solvent were interpreted according to the
relative values of terms in the denominator of eq 16:
i.e., k-2 . k3[D] in benzene-d6; k-2 ≈ k3[D] in chloroform-
d; k-2 , k3[D] in acetone-d6 or acetonitrile-d3. Although
the solvent dependence of the rate law does not allow a
rigorous comparison of the rate constants among all
solvents, the general trend indicates the accelerating
effect of polar solvents. The faster reactions observed
in more polar solvents suggest the intervention of
charged or polarized intermediates. In a polar solvent
such as acetonitrile-d3, enolate-MTO might be produced
by the deprotonation of MTO, whereas the partial
deprotonation of MTO or the formation of a strong ion
pair {[CH2dReO3]-‚[HB]+} might be favored in nonpolar
solvents because charged species are substantially
destabilized by the lack of solvation.15,41,43
Solvent effects on the basicity of the bases toward a
proton in MTO should also be considered.42 The ratio
of the equilibrium constants for the deprotonation
process in benzene-d6 is K1K2(Lut)/K1K2(MeQ) ) 4.4.
The ratio of the rate constants for the deprotonation
process in acetone-d6 is K1k2(Lut)/K1k2(MeQ) ) 3.9. (The
ratio of K1k2 strongly reflects the deprotonation process,
because acid/base neutralization reaction is rapid.) This
ratio can be compared with the ratio of Ka in water:
Ka(LutH+)/Ka(MeQH+) ) 6. The rough agreement indi-
cates solvent effects on the basicity of bases are rela-
tively small and the partial proton extraction from MTO
occurs in both benzene-d6 and acetone-d6.
Donor Effects. The H/D exchange rate diminishes
with increasing pKa of the deuterium donor: phenol
(9.99) > cresol (10.29) . methanol (15) . acetone (19).
Acetone-d6, a potential deuterium donor, did not ex-
change.
One might suspect that high concentrations of the
deuterium donors would alter the dielectric constant of
benzene-d6 solutions. The activity coefficient of the
proton and deuteron in a nonpolar solvent is likely to
be increased by the addition of polar reagents such as
phenol-d6 (ꢀ ) 12). To explore this matter, isodielectric
measurements were carried out by adding benzyl alco-
hol (ꢀ ) 10) at a concentration, together with phenol-
d6, such that [benzyl alcohol] + [phenol-d6] ) 1.3 mol
kg-1. The experiment failed, however, because benzyl
alcohol itself seems to coordinate to MTO, thus reducing
the H/D exchange reactivity.
their experimental errors, this can be taken as good
agreement. Thus, k3 directly reflects the heterolytic
O-D bond energy of phenol-d6 and o-cresol-d8.
The value K1K2k3 for D ) methanol-d4 can be also
calculated from the separately estimated K1K2 and k3.
The value 103K1K2k3(methanol-d4/Lut) ) 1.0 kg2 mol-2
s-1 is close to 103K1K2k3(o-cresol-d8/Lut) ) 1.2 kg2 mol-2
s-1, despite their pKa difference. Because methanol-d4
is as polar (ꢀ ) 32.6) as acetonitrile, methanol-d4 in the
bulk, which is 60 times in excess of MTO, may stabilize
the intermediates by specific solvation.
Reaction Coordinate Diagram. In this study, we
found that H/D exchange depends on the solvents, bases,
and deuterium donors. The formation of partially de-
protonated MTO as the intermediate is facilitated by a
strong Brønsted base. The pKa of a deuterium donor will
directly affect the activation barrier of H/D exchange.
The solvent may affect both the stability of the inter-
mediate and the activation barrier for H/D exchange.
Because H/D exchange directly reflects the pKa of the
deuterium donor, a lower activation barrier is expected
in polar solvents compared with that in nonpolar
solvents.
A reaction coordinate diagram of H/D exchange for
MTO is illustrated in Scheme 3; obviously it is nearly
symmetric for H/D exchange. In the beginning MTO
exists exclusively in the keto form because of the large
energy difference between keto- and enol-MTO. A base
present in solution interacts with MTO at the Re center
or at the methyl proton. Without steric hindrance, the
base interacts with the Re center of MTO to form a
trigonal-bipyramidal compound, which prevents keto/
enol tautomerization and, therefore, H/D exchange. A
base with a steric barrier initially interacts with MTO
at the methyl proton to form intermediate I1 with the
weakened C-H bond. This partial deprotonation of
MTO facilitates its keto/enol tautomerization. The
base-promoted enol isomer I2 or the strong ion pair
{[CH2dReO3]-‚[BH]+} is a reactive intermediate for
subsequent H/D exchange. The H/D exchange takes
place via an interaction between I2 and the deuterium
donor to form the activated complex. In Scheme 3 we
depicted the activated complex, which still requires the
old O-H/O-D bonds to break and the new O-D/O-H
bonds to form. Clearly, the heterolytic O-D bond energy
of a deuterium donor has a strong influence in this
process; however, the stability and reactivity of I2 also
depend on the nature of a base and a solvent. The
sequence as written contains several rapid steps that
have been postulated; not every one was capable of
independent verification. The deuterated intermediate
I2-d produces deuterated MTO, by the dissociation of
base. With an excess deuterium donor the reaction
sequence will be repeated until the three protons of
MTO are exchanged.
The rate constant difference between reactions with
phenol-d6 and o-cresol-d8 shows the absence of medium
effects, whereas the reaction with methanol-d4 implies
otherwise. The third-order rate constants measured
with D ) phenol-d6 and o-cresol-d8 in benzene-d6
represent the product K1K2k3. Because the K1K2 step is
common to both, the data reflect the relative values of
k3. The ratio of the rate constants with phenol-d6 and
o-cresol-d8 was 1.66, which roughly agrees with the Ka
ratio between phenol and o-cresol: Ka(phenol)/Ka(cresol)
) 1.99. Considering the use of aqueous pKa values and
In acetonitrile-d3, a noncoordinating polar solvent, the
deprotonated intermediate I2 can be stabilized by sol-
vation. This enolate-MTO facilitates the access to the
transition state and therefore lowers the activation
barrier for H/D exchange. Consequently, the formation
of I2 becomes the rate-determining step for B ) Lut, in
acetonitrile-d3 or in acetone-d6. On the other hand, a
weaker base cannot produce the reactive enolate-MTO;
(42) Popovych, O.; Tomkins, R. P. T. Nonaqueous Solution Chem-
istry; Wiley-Interscience: New York, 1981.
(43) Reichardt, C. Solvents and Solvent Effects in Organic Chemistry,
2nd ed.; VCH: Weinheim, Germany, 1988.

H-D Exchange Reactions of Methyltrioxorhenium(II)
Organometallics, Vol. 24, No. 11, 2005 2787
Scheme 3. Reaction Coordinate Diagram for H/D Exchange of MTO
therefore, the higher energy barrier remains the sub-
sequent H/D exchange step as for B ) MeQ in acetone-
d6.
Absence of Acid Catalytic Reaction. Acids and
bases catalyze the H/D exchange of ketones, as shown
in eqs 21 and 22.7 As for MTO, even pyridine (the pKa
compared to the oxygens in the other compounds.48 The
three oxo ligands in MTO donate electron density to the
rhenium center to compensate for electron deficiency at
the metal center, which disfavors the cationic interme-
diate CH3ReO3H+. In base catalysis, on the other hand,
nucleophilic attack on the methyl proton forms a
negatively charged intermediate, which can distribute
electron density to the rhenium-oxygen moiety, stabi-
lizing the enol. The perrhenate ion also has a rhenium-
oxygen bond length near those of L-ReO3 complexes,49,50
but its 17O NMR resonance appears at a higher field.48
The distribution of the negative charge over four oxygen
atoms is suggested for this phenomenon.51
The absence of H/D exchange for the trigonal-bi-
pyramidal species MTO‚L also implies the importance
of the electronic deficiency of the -ReO3 moiety for the
keto/enol tautomerization of MTO. The formation of
MTO‚L eases the electronic deficiency of the Re center
prior to the deprotonation; hence, the deprotonation
loses the advantage of stabilizing a negatively charged
intermediate for MTO‚L. As a result, the MTO‚L forma-
tion inhibits the access to keto/enol tautomerization.
Kinetic Isotope Effects. Secondary kinetic isotope
effects are usually small, kH/kD ) 0.7-1.5, because C-H
bond cleavage is not directly involved in a reaction.52
The sources of the secondary KIE are the steric differ-
of PyH+ is 5.23) catalyzes slow H/D exchange of MTO
and the bases in Chart 1 do so more efficiently, whereas
trifluoroacetic acid does not.
The lack of catalytic ability of acids might be rational-
ized by the characteristic rhenium-oxygen bond in
MTO. Despite the fact that members of the L-ReO3
family (L ) η5-C5Me5, HB(pz)3, σ-Mes, CH3) have quite
similar rhenium-oxygen bond lengths,44-47 the oxygens
in MTO in particular have a low electron density
(48) Herrmann, W. A.; Kiprof, P.; Rypdal, K.; Tremmel, J.; Blom,
R.; Alberto, R.; Behm, J.; Albach, R. W.; Bock, H.; Solouki, B.; Mink,
J.; Lichtenberger, D.; Gruhn, N. E. J. Am. Chem. Soc. 1991, 113, 6527-
6537.
(44) Herrmann, W. A.; Ladwig, M.; Kiprof, P.; Riede, J. J. Organo-
met. Chem. 1989, 371, C13-C17.
(45) Okuda, J.; Herdtweck, E.; Herrmann, W. A. Inorg. Chem. 1988,
27, 1254-1257.
(46) Herrmann, W. A.; Okuda, J. J. Mol. Catal. 1987, 41, 109-122.
(47) Herrmann, W. A.; Romao, C.; Fischer, R. W.; Kiprof, P.; De
Meric de Bellefon, C. Angew. Chem., Int. Ed. Engl. 1991, 30, 185-
187.
(49) Lock, C. J. L.; Turner, G. Acta Crystallogr., Sect. B 1975, B31,
1764-1765.
(50) Krebs, B.; Hasse, K. D. Acta Crystallogr., Sect. B 1976, B32,
1334-1337.
(51) Herrmann, W. A.; Ku¨hn, F. E.; Roesky, P. W. J. Organomet.
Chem. 1995 485, 243-251.
(52) Carey, F. A.; Sundberg, R. J. Advanced Organic Chemistry, 4th
ed.; Kluwer Academic/Plenum: New York, 2000.

2788 Organometallics, Vol. 24, No. 11, 2005
Matsumoto and Espenson
ences of a C-H(D) bond, the differences in hyper-
conjugative stabilization arising from C-H(D) bond
strengths, and the electronegativity differences of H
and D.
In our study, large errors of the KIE values and the
complex rate laws prevent a detailed quantitative
analysis of isotope effects. However, the general trend
is found to be k01 > k12 > k23; that is, the replacement
of a proton in MTO with a deuteron decelerates H/D
exchange. From this trend, we infer that the heavier
deuterium atom hinders the twist motion of sp3 to sp2
during tautomerization. Stabilization from hypercon-
jugation may be reduced because a proton is slightly
more electronegative than a deuteron.53,54 This analysis
of the KIE also supports the involvement of the enol-
MTO intermediate.
Acknowledgment. This research was supported
by the National Science Foundation (Grant No.
CHE-020409). Some experiments were conducted with
the use of the facilities of the Ames Laboratory, which
is operated by Iowa State University of Science and
Technology under contract W-7405-Eng-82 with the U.S.
Department of Energy.
(53) Alston, W. C., II; Haley, K.; Kanski, R.; Murray, C. J.; Pranata,
J. J. Am. Chem. Soc. 1996, 118, 6562-6569.
(54) Kovach, I. M.; Hogg, J. L.; Raben, T.; Halbert, K.; Rodgers, J.;
Schowen, R. L. J. Am. Chem. Soc. 1980, 102, 1991-1999.
OM049029S