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REACTIVITY OF TRANSITION-METAL-ACTIVATED OXYGEN

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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REACTIVITY OF TRANSITION-METAL-ACTIVATED OXYGEN

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  1. REACTIVITY OF TRANSITION-METAL-ACTIVATED OXYGEN ANDREJA BAKAC AMES LABORATORY, IOWA STATE UNIVERSITY

  2. 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)

  3. (P)FeIIIOOH (P)FeVO / (P•+)FeIVO Cytochrome P450 Both reactive in substrate oxidations? Epoxidation vs. hydroxylation?

  4. SIMPLE INORGANIC ANALOGS (N4)(H2O)MIIIOOH2+ (M = Rh, Co, Cr) CraqOOH2+

  5. (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+]

  6. Some not-so-standard chemistry (NH3)4(H2O)RhOOH2+ + Br- Expect Rate = k[Br-][H+][RhOOH2+]

  7. 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)

  8. (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)

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

  10. -d[RhOOH2+]/dt = Br2 + (NH3)4(H2O)RhOOH2+ , kinetics

  11. 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

  12. 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

  13. 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]

  14. LCr(18O)(16O)+ + PAr3 LCrIII + 16OPAr3 LCr(O)2+ + PAr3LCrIII + OPAr3 PPh3, k = 4.4 × 105 M-1 s-1

  15. LCr(O)2+ + PAr3 LCrIV + PAr3•+ HOPAr3• + LCrIV OPAr3 + LCrIII + H+ PAr3•+ + H2O HOPAr3• + H+ LCr(O)2+ + PAr3LCrIII + OPAr3, mechanism Electron transfer

  16. LCrOOH2+ + PAr3 Competitionwith LCrOOH2+ LCr(O)2+

  17. L1CrOOH2+ + PPh3 + H+ L1CrIII + OPPh3 LCrOOH2+ + PAr3 Mechanism

  18. 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?

  19. Acknowledgement Dr. Oleg Pestovsky Dr. Kelemu Lemma U.S. Department of Energy U.S. National Science Foundation

  20. Rh(H2O)3+ + Br- + O2 HOBr + RhOOH2+ WHY SO FAST? HOBr + H2O2O2 + Br- + H+ + H2O (2-5) 104 M-1 s-1

  21. 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

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