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

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 chemical reactivity Stability: from transients to stable compounds (crystal structure) TRANSITION METAL HYDROPEROXIDES

Cytochrome P450 Both reactive in substrate oxidations? Epoxidation vs. hydroxylation? (P)Fe V O / (P + )Fe IV O(P)Fe III OOH

(N 4 )(H 2 O)M III OOH 2+ (M = Rh, Co, Cr) Cr aq OOH 2+ SIMPLE INORGANIC ANALOGS

Some standard chemistry O-ATOM TRANSFER (NH 3 ) 4 (H 2 O)RhOOH 2+ + PPh 3 OPPh 3 + (NH 3 ) 4 Rh(H 2 O) O labeling:100% O-transfer Rate = 8.8 × 10 3 [RhOOH 2+ ][PPh 3 ][H + ] Nucleophilic attack at oxygen

Some not-so-standard chemistry (NH 3 ) 4 (H 2 O)RhOOH 2+ + Br - Expect Rate = k[Br - ][H + ][RhOOH 2+ ]

Experiment -- High [H + ] (0.2 – 1 M), high [Br - ] (0.1 M) -- Low [H + ] ( M), low [Br - ] ( M) Br 2 /Br 3 - not produced O 2 is generated k = 3.8 M -2 s -1 Br 2 /Br 3 - produced 266 nm (Br 3 - )  = 4.09 × 10 4 M -1 cm -1 k = 1.8 M -2 s -1

Hypothesis (3) (1) RhOOH 2+ + Br - HOBr + RhOH 2+ (2) HOBr + Br - + H + Br 2 + H 2 O (4) Br 2 + RhOOH 2+  products Speed up (1), slow (4)  facilitate formation of Br 2 /Br 3 -

Direct look at Br 2 /(NH 3 ) 4 (H 2 O)RhOOH 2+

Br 2 + (NH 3 ) 4 (H 2 O)RhOOH 2+, kinetics -d[RhOOH 2+ ]/dt =

HOBr is reactive form Br 2 + H 2 O HOBr + Br - + H + K = 6 × M 2 HOBr + RhOOH 2+ Rh(H 2 O) 3+ + Br - + O 2 -d[RhOOH 2+ ]/dt = k

RhOOH 2+ + Br - HOBr + RhOH 2+ k = 1.8 M -2 s -1 Explains products, kinetic dependencies, and f(2) between extremes (NH 3 ) 4 (H 2 O)RhOOH 2+ + Br -, mechanism

Some unexpected chemistry Sequential stopped-flow - generate LCrOOH 2+ from LCrOO 2+ + Ru II - allow formation of LCr(O) mix with PAr 3, monitor kinetics at 470 nm LCr(O) PAr 3  LCr III + OPAr 3 Rate = k[LCr(O) 2 + ][PAr 3 ]

LCr(O) PAr 3  LCr III + OPAr 3 PPh 3, k = 4.4 × 10 5 M -1 s -1 LCr( 18 O)( 16 O) + + PAr 3 LCr III + 16 OPAr 3 Not O-atom transfer

Electron transfer LCr(O) PAr 3 LCr IV + PAr 3 + PAr H 2 OHOPAr 3 + H + HOPAr 3 + LCr IV OPAr 3 + LCr III + H + LCr(O) PAr 3  LCr III + OPAr 3, mechanism

Competition with LCrOOH 2+  LCr(O) 2 + LCrOOH 2+ + PAr 3

L 1 CrOOH 2+ + PPh 3 + H + L 1 Cr III + OPPh 3 OXYGEN ATOM TRANSFER Mechanism

LCr(O) 2 + and LCrOOH 2+ react with PPh 3 LCr(O) 2 + Electron transfer, k = 4.4 × 10 5 M -1 s -1 LCrOOH 2+ O-atom transfer, H + - catalyzed, k = 850 M -2 s -1 Hints about P450-OOH reactivity? SUMMARY -- LCrOOH 2+  LCr V (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 + H 2 O 2  O 2 + Br - + H + + H 2 O (2-5)  10 4 M -1 s -1 HOBr + RhOOH 2+ Rh(H 2 O) 3+ + Br - + O 2

O 2 + 2e - + 2H + H 2 O 2 E = 0.78 V (pH 0)Cr aq 3+ + O 2 + 2e - + H + CrOOH 2+ E = 0.65 V (pH 0) Thermodynamics: small advantage for Cr aq OOH 2+ COORDINATION FACILITATES OXIDATION & REDUCTION 2-electron reduction of HOBr, thermodynamics Cr aq OOH 2+ + HOBr, k = 10 7 M -1 s -1