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REACTIVITY OF TRANSITION-METAL-ACTIVATED OXYGEN. ANDREJA BAKAC AMES LABORATORY, IOWA STATE UNIVERSITY. TRANSITION METAL HYDROPEROXIDES. LMOOH n+ Intermediates in metal-mediated oxidations by O 2 and H 2 O 2 Some are well characterized O-O bond length O-O stretching frequency

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slide1

REACTIVITY OF

TRANSITION-METAL-ACTIVATED OXYGEN

ANDREJA BAKAC

AMES LABORATORY, IOWA STATE UNIVERSITY

slide2

TRANSITION METAL HYDROPEROXIDES

LMOOHn+

Intermediates in metal-mediated oxidations by O2 and H2O2

Some are well characterized

O-O bond length

O-O stretching frequency

chemical reactivity

Stability: from transients to stable compounds (crystal structure)

slide3

(P)FeIIIOOH

(P)FeVO / (P•+)FeIVO

Cytochrome P450

Both reactive in substrate oxidations?

Epoxidation vs. hydroxylation?

slide4

SIMPLE INORGANIC ANALOGS

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

CraqOOH2+

slide5

(NH3)4(H2O)RhOOH2+ + PPh3

OPPh3 + (NH3)4Rh(H2O)23+

Nucleophilic attack at oxygen

Some standard chemistry

O-ATOM TRANSFER

18O labeling:100% O-transfer

Rate = 8.8 × 103 [RhOOH2+][PPh3][H+]

slide6

Some not-so-standard chemistry

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

Expect

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

slide7

Br2/Br3- produced

l 266 nm (Br3-)

e = 4.09 × 104 M-1 cm-1

k = 1.8 M-2 s-1

Br2/Br3- not produced

O2 is generated

k = 3.8 M-2 s-1

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)

slide8

(1) RhOOH2+ + Br-

HOBr + RhOH2+

(2) HOBr + Br- + H+

Br2 + H2O

products

(4) Br2 + RhOOH2+

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

Hypothesis

(3)

slide10

-d[RhOOH2+]/dt =

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

slide11

Br2 + H2O

HOBr + Br- + H+ K = 6 × 10-9 M2

-d[RhOOH2+]/dt =

HOBr is reactive form

HOBr + RhOOH2+

Rh(H2O)3+ + Br- + O2

k

slide12

RhOOH2+ + Br-

HOBr + RhOH2+

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

k = 1.8 M-2 s-1

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

slide13

Some unexpected chemistry

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]

slide14

LCr(18O)(16O)+ + PAr3

LCrIII + 16OPAr3

LCr(O)2+ + PAr3LCrIII + OPAr3

PPh3, k = 4.4 × 105 M-1 s-1

slide15

LCr(O)2+ + PAr3

LCrIV + PAr3•+

HOPAr3• + LCrIV

OPAr3 + LCrIII + H+

PAr3•+ + H2O

HOPAr3• + H+

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

Electron transfer

slide16

LCrOOH2+ + PAr3

Competitionwith LCrOOH2+ LCr(O)2+

slide17

L1CrOOH2+ + PPh3 + H+

L1CrIII + OPPh3

LCrOOH2+ + PAr3

Mechanism

slide18

SUMMARY

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?

slide19

Acknowledgement

Dr. Oleg Pestovsky

Dr. Kelemu Lemma

U.S. Department of Energy

U.S. National Science Foundation

slide20

Rh(H2O)3+ + Br- + O2

HOBr + RhOOH2+

WHY SO FAST?

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

slide21

O2 + 2e- + 2H+

H2O2 E = 0.78 V (pH 0)

Craq3+ + O2 + 2e- + H+

CrOOH2+ E = 0.65 V (pH 0)

2-electron reduction of HOBr, thermodynamics

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

Thermodynamics: small advantage for CraqOOH2+

COORDINATION FACILITATES OXIDATION & REDUCTION