MTO/H2O2

15 °C, CH3CN(aq)

Rate-Enhancing Roles of Water Molecules in Methyltrioxorhenium-Catalyzed Olefin Epoxidation by Hydrogen Peroxide

Bryan R. Goldsmith, Taeho Hwang, Stefan Seritan, Baron Peters, Susannah L. ScottDepartment of Chemical Engineering, University of California, Santa Barbara, CA, 93106-5080

Department of Chemistry & Biochemistry, University of California, Santa Barbara, CA 93106-9510

AcknowledgmentsThis work was funded by the Catalysis Science Initiative of the U.S. Department of Energy, Basic Energy

Sciences (DE-FG02-03ER15467). B.R.G. acknowledges the PIRE-ECCI program (NSF-OISE 0530268) for a

graduate fellowship. T.H. is grateful to Air Products for a graduate fellowship.

References

Final Remarks

[1] Saavedra, J.; Doan, H. A.; Pursell, C. J.; Grabow, L. C.; Chandler, B. D., Science 2014, 345, 1599.

[2] Jee, J.-E.; Comas-Vives, A.; Dinoi, C.; Ujaque, G.; van Eldik, R.; Lledós, A.; Poli, R., Inorg. Chem. 2007, 46, 4103.

[3] Beattie, I. R.; Jones, P. J., Inorg. Chem. 1979, 18, 2318

[4] Herrmann, W. A.; Fischer, R. W.; Correia, J. D. G., J. Mol. Catal. 1994, 94, 213.

[5] Espenson, J. H., Chem. Comm. 1999, 479.

MTO-catalyzed olefin epoxidation

Active catalyst for many reactions

Primarily for olefin metathesis and

oxygen atom transfer reactions

First organometallic Re oxide synthesized[3]

Level of theory; ωB97X-D/aug;def2-TZVP

Explicit/Implicit solvent; Conducting-type polarized continuum model (CPCM, solvent=acetonitrile)

Enthalpic interactions due to explicit waters present in each step (either 0, 1, 2, 3 H2O)

Wertz entropy correction used to account for structural correlations due to solvation in acetonitrile

Apparent Free energies; include zero point, rotation, translation, vibrations, proton-tunneling, and [H2O].

Methyltrioxorhenium (CH3ReO3, MTO)

The H2O-dependence of MTO-catalyzed

epoxidation has not been explained

We use experimental kinetic measurements, kinetic simulations,

and DFT to explore the participation of water in MTO-catalyzed olefin epoxidation.

We address:

(i) The relative magnitudes of the rate enhancements due to water, in the epoxide formation steps vs. the peroxide

activation steps;

(ii) The relative importance of water-catalyzed and uncatalyzed pathways;

(iii) The roles and precise numbers of water molecules involved in each step of the epoxidation catalytic cycle; and

(iv) The extent to which DFT calculations, despite limitations in chemical accuracy, can predict and explain the observed

kinetics of the complex solution-phase reaction involving multiple proton transfers

Elucidating the origin of dramatic solvent effects is critical to being able to model solution-phase reactions

The remarkable water-dependence

has yet to be explained

B. R. Goldsmith, T. Hwang, S. Seritan, B. Peters, and S. L. Scott, J. Am. Chem. Soc. 137, 9604 (2015)

T. Hwang, B. R. Goldsmith, B. Peters, and S. L. Scott, Inorg. Chem. 52, 13904 (2013)

+

Methyltrioxorhenium activates H2O2 for oxygen atom transfer.[4,5]

Experiments find that water accelerates the epoxidation steps only weakly

k4app = k4 + k4w[H2O]0

k4 = (4.0 ± 0.4) x 10-2 M-1 s-1

k4w = (1.38 ± 0.09) x 10-2 M-2 s-1

(i) k4app ≈ 10 k3app for a given [H2O]

(ii) similar, weak water dependences

k3app = k3 + k3w [H2O]0

k3 = (5.3 ± 0.6) x 10-3 M-1 s-1

k3w = (1.14 ± 0.39) x 10-3 M-2 s-1

0.0

0.2

0.4

0.6

0.8

1.0

0

2

4

6

8

10

0 1 2 3 4 5

10

2 k

3a

pp /

M-1

s-1

10

2 k4

ap

p / M-1 s

-1

[H2O]

0 / M

[H2O] / M

Transition state description Experiment DGⱡ Calculated2 2 4 8

A + C6H10 → [A-C6H10]‡

82118 118 118

A + H2O + C6H10 → [A-C6H10-H2O]‡ 116 115 114

B + C6H10 → [B-C6H10]‡

77

96 96 96

B + C6H10 → [B´-C6H10]‡ + H2O 99 100 102

B + H2O + C6H10 → [B-C6H10-H2O]‡ 97 96 94

DFT corroborates with experiments that water accelerates

the epoxidation steps only weakly

Free energy barriers are in kJ mol-1 and reported at 15 °C

0

1

2

3

4

0

20

40

60

80

100

120

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

10

3 k

slo

w / s

-1 10

3 kfa

st / s

-1

[H2O] / M

Strong water acceleration for regeneration of peroxo complexes A and B

0H2O

1H2O

2H2O

DHⱡ2,experiment

DHⱡ1,experiment

(A→ B)

(MTO→A)

0H2O 1H2O 2H2O 3H2O

DH

ⱡ/

kJ m

ol-1

0H2O 1H2O 2H2O 3H2OD

Sⱡ/

J m

ol-1

K-1

DSⱡ2,experiment

DSⱡ1,experiment

(A→ B)

(MTO→A)

On average two waters participate in rate-determining steps

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.4 0.6 0.8 1.0

k1

,n / k

1,0

n, deuterium atom fraction

k2,n / k

2,0

m = 2

f1* = 0.66 ± 0.01

m = 2

f2* = 0.66 ± 0.01

Have described the full catalytic cycle and can

quantitatively explain the effects of water

Strong water acceleration for regeneration of

active peroxo-complexes

Weak water acceleration in epoxidation steps

Experimental measurements confirm

water catalysis

Current modeling procedure adequately

described complicated inorganic reaction

in non-ideal solution

Water may also be an important component

of the mechanism in other metal-catalyzed

reactions for which proton transfer is a key step

H2O strongly affects the rates at which

the peroxo complexes are regenerated

Methyltrioxorhenium-catalyzed epoxidation by H2O2 shows strong non-linear water acceleration effects,

even though no step in the catalytic cycle explicitly requires water as a reactant.

[1-2]

ReO

CH

Water dependence of the measured pseudo-first-order rate

constants kfast (blue points) and kslow (red points) for the

reaction of 1.0 mM MTO with 49.1 mM H2O2, in aqueous

CH3CN at 25.0 °C. The curvefits (lines) correspond to a

second-order dependence on [H2O] (for kfast) and a mixed

first- and second-order dependence on [H2O] (for kslow).

Computed free energies for formation of the peroxo complexes

of MTO. Transition states are identified by a hyphen between

the reactant and product states of the Re complex. Solid green,

red, and blue lines show free energies with 0H2O, 1H2O, and

2H2O-assisted transition states.

Computed apparent activation parameters for the formation of the peroxo complexes A (blue circles) and B (black squares),

via transition states involving different numbers of water molecules, compared to experimental values (dashed lines)

measured at 25.0 °C with 2.0 M H2O.

Comparison of computed and experimental kinetic isotope effects for the

sequential formation of peroxo complexes A and B from MTO, at 25.0 °C

* * * 1

0/ (1 )(1 ) (1 ) m m

nk k n n n n n n2 2 2H O H O

Parameter Calculated Experimental

1H2O/D2O 2H2O/D2O m = 1 m = 2

k1 / kD1 6.05 3.91 3.77 3.54

1* 0.406 0.635 0.515 0.656

k2 / kD2 3.26 2.91 3.74 3.51

2* 0.554 0.700 0.517 0.658

(MTO→A)

(A→ B)

Open points = simulated

Closed points = measured

dimensionless time