REACTIVITY OF

TRANSITION-METAL-ACTIVATED OXYGEN

ANDREJA BAKAC

AMES LABORATORY, IOWA STATE UNIVERSITY

LMOOHn+

Intermediates in metal-mediated oxidations by O2 and H2O2

Some are well characterizedO-O bond length O-O stretching frequencychemical reactivity

Stability: from transients to stable compounds (crystal structure)

TRANSITION METAL HYDROPEROXIDES

Cytochrome P450

Both reactive in substrate oxidations?

Epoxidation vs. hydroxylation?

fast ,H

(P)FeVO / (P•+)FeIVO(P)FeIIIOOH

(N4)(H2O)MIIIOOH2+ (M = Rh, Co, Cr)

CraqOOH2+

SIMPLE INORGANIC ANALOGS

N

N

N

NOOH

2+

N

N

N

NO

Co

H

H

H

OH2

OH

H

Rh

NH3

NH3H3N

H3NOH

OH2

OHH

H HOH2

Cr

2+2+

Some standard chemistry

O-ATOM TRANSFER

(NH3)4(H2O)RhOOH2+ + PPh3 H OPPh3 + (NH3)4Rh(H2O)2

3+

18O labeling:100% O-transfer

Rate = 8.8 × 103 [RhOOH2+][PPh3][H+]

Nucleophilic attack at oxygen

2.5

3.0

3.5

4.0

4.5

-1.0 -0.5 0.0 0.5 1.0 1.5 2.0

log

k

3

CH3

H

F

Cl

CF3

= -0.6

30% AN, = 0.1 M

25 oC

Some not-so-standard chemistry

(NH3)4(H2O)RhOOH2+ + Br-

Expect

RhOOH2+ + Br- H+

RhOH2+ + HOBrBr-

Br2/Br3-

Rate = k[Br-][H+][RhOOH2+]

Experiment

-- High [H+] (0.2 – 1 M), high [Br-] (0.1 M)

-- Low [H+] (0.01 - 0.1 M), low [Br-] (10-3 - 10-2 M)

0.00

0.05

0.10

0.15

0.20

0.25

0 200 400 600 800 1000

Ab

s2

40

t/s

[H+] = 0.09 M, = 0.1 M

[A4RhOOH2+] = 0.06 mM

[Br-] = 10 mM

Br2/Br3- not produced

O2 is generated k = 3.8 M-2 s-1

0.05

0.10

0.15

0.20

0.25

0 50 100 150

Ab

s 26

6

t/s

0.025 mM A4RhOOH2+

20 mM Br-, = 1 M

[H+] = 0.98 M, 25 oC

Br2/Br3- produced

266 nm (Br3-)

= 4.09 × 104 M-1 cm-1

k = 1.8 M-2 s-1

Hypothesis

(3)

(1) RhOOH2+ + Br- H

HOBr + RhOH2+

(2) HOBr + Br- + H+

97

106.1 10

Br2 + H2O

(4) Br2 + RhOOH2+ products

Speed up (1), slow (4) facilitate formation of Br2/Br3-

Br2 + Br- Br3-

unreactive

Direct look at Br2/(NH3)4(H2O)RhOOH2+

0.15

0.2

0.25

0.3

0.35

0 2 4 6 8 10

Abs

238

t/s

[A4RhOOH2+] = 0.1 mM

[Br2] = 2.11 mM

[Br-] = 3.03 mM

[H+]= 1 M

Br2 + (NH3)4(H2O)RhOOH2+ , kinetics

-d[RhOOH2+]/dt = ]Br][H[

]Br][RhOOH[k 2

2app

0

5

10

15

20

25

0 5 10 15

k obs/1

03 [

Br 2

]

1/103 [H+][Br-]

slope = 1.64 (3) M s-1

HOBr is reactive form

Br2 + H2O 1010

6.1

97 HOBr + Br- + H+ K = 6 × 10-9 M2

HOBr + RhOOH2+ 8103 Rh(H2O)3+ + Br- + O2

-d[RhOOH2+]/dt = ]Br][H[

]Br][RhOOH[ k K 2

2

k

RhOOH2+ + Br- H

HOBr + RhOH2+ k = 1.8 M-2 s-1

HOBr

Br-, H+

Br2/Br3-

RhOOH2+

+ H2O

Rh(H2O)3+ + O2 + Br-

Explains products, kinetic dependencies, and f(2) between extremes

(NH3)4(H2O)RhOOH2+ + Br-, mechanism

Some unexpected chemistry

N

N

N

NOOH

2+

0.31 s-1 0.51 s-1LCrV(O)2

+ L'CrIII

HH

H HOH2

Cr

Sequential stopped-flow

- generate LCrOOH2+ from LCrOO2+ + RuII

- allow formation of LCr(O)2+

- mix with PAr3, monitor kinetics at 470 nm

LCr(O)2+ + PAr3 LCrIII + OPAr3

Rate = k[LCr(O)2+][PAr3]

LCr(O)2+ + PAr3 LCrIII + OPAr3

PPh3, k = 4.4 × 105 M-1 s-1

4

5

6

7

-1 0 1 2

log(

k)

3

H

F

Cl

CF3

CH3

= - 0.69

LCr(18O)(16O)+ + PAr3 OH 162 LCrIII + 16OPAr3

Not O-atom transfer

Electron transfer

LCr(O)2+ + PAr3 LCrIV + PAr3

•+

PAr3•+ + H2O HOPAr3

• + H+

HOPAr3• + LCrIV OPAr3 + LCrIII + H+

LCr(O)2+ + PAr3 LCrIII + OPAr3, mechanism

Competition with LCrOOH2+ LCr(O)2+

LCrOOH2+ + PAr3

LCrOO2+ LCrOOH2+

0.31 s-1

LCrV(O)2+

PPh3, H+

LCrIII + OPPh3

2 Ru(NH3)5py2+

LCrIII

PPh3LCrIII + OPPh3

< 9%

Ru(NH3)5py2+

LCrOOH2+ + PAr3

L1CrOOH2+ + PPh3 + H+ L1CrIII + OPPh3

OXYGEN ATOM TRANSFER

Mechanism

1

1.5

2

2.5

3

3.5

4

-0.5 0 0.5 1 1.5 2

3

H FCl

CF3

log(k)

= - 0.24

LCr(O)2+ and LCrOOH2+ react with PPh3

LCr(O)2+ Electron transfer, k = 4.4 × 105 M-1 s-1

LCrOOH2+ O-atom transfer, H+- catalyzed, k = 850 M-2 s-

1

Hints about P450-OOH reactivity?

SUMMARY

-- LCrOOH2+ LCrV(O)2+ unusual

-- Reactivity not outstanding & requires H+

Acknowledgement

Dr. Oleg Pestovsky

Dr. Kelemu Lemma

U.S. Department of Energy

U.S. National Science Foundation

WHY SO FAST?

HOBr + H2O2 O2 + Br- + H+ + H2O (2-5) 104 M-1 s-1

HOBr + RhOOH2+ 8103 Rh(H2O)3+ + Br- + O2

O2 + 2e- + 2H+ H2O2 E = 0.78 V (pH 0)

Craq3+ + O2 + 2e- + H+ CrOOH2+ E = 0.65 V (pH 0)

Thermodynamics: small advantage for CraqOOH2+

COORDINATION FACILITATES OXIDATION & REDUCTION

2-electron reduction of HOBr, thermodynamics

CraqOOH2+ + HOBr, k = 107 M-1 s-1