Kinetics, mechanisms, and catalysis of oxygen atom transfer reactions of S-oxide and pyridine N-oxide substrates with molybdenum(IV,VI) complexes: relevance to molybdoenzymes

Abstract

before, most recently in those of the respective tetramethylammonium salts.15Crystal structure determinations of the remaining complexes of the system, with 5 to 8 mol of hydrogen fluoride per mol of pyridine, will most probably reveal these as pyridinium poly(hydrogen fluorides), too. Anions H4F5" and, in one instance each, H5F6' and H7F8" are already established species in the solid state.6,15

Mechanistic Studies of Rhodobacter sphaeroides Me2SO Reductase

“…With this activation energy, which approximates the enthalpy of activation, ⌬H ‡ in solution at room temperature (as described under "Experimental Procedures"), the entropy of activation, ⌬S ‡ , for the second phase of the reaction of reduced Me 2 SO reductase with Me 2 SO is calculated to be 6.5 cal/K⅐mol. The values of ⌬H ‡ and ⌬S ‡ for the reduction of Me 2 SO are comparable with those reported by Cardonna et al (21) for the reduction of the substrates 3-fluoropyridine N-oxide and (R F ) 2 SO (R F ϭ p-C 6 H 4 F) by molybdenum model complexes of the type Mo(IV)O(L-NS 2 ) (L-SN 2 ϭ 2,6-bis(2,2-diphenyl-2-mercaptoethyl)pyridine(2Ϫ)) (⌬H ‡ Ϸ 23 kcal/mol and ⌬S ‡ from 2.6 to 7.2 cal/K⅐mol). These workers suggest that the small activation entropies observed imply a strong similarity between reactant species Mo(IV)O(L-NS 2 )(XO) (where X ϭ 3-Fpy or (R F ) 2 S) and the transition state, with little formal Mo-OX interaction in the transition state.…”

“…These workers suggest that the small activation entropies observed imply a strong similarity between reactant species Mo(IV)O(L-NS 2 )(XO) (where X ϭ 3-Fpy or (R F ) 2 S) and the transition state, with little formal Mo-OX interaction in the transition state. It is thus unlikely that there is significant oxidation to Mo(VI) as the model system attains the transition state (21). The similar activation parameters obtained for the enzyme reaction suggest a comparable conclusion for the breakdown of the E red ⅐Me 2 SO complex in the case of the enzyme-catalyzed reaction.…”

Active transition metal oxo and hydroxo moieties in nature's redox, enzymes and their synthetic models: Structure and reactivity relationships

“…For example, Holm and coworkers demonstrated oxygen transfer between a synthetic Mo(VI) complex and a triaryl phosphine substrate (Fig. 12) [94]. In DMF solution, the synthetic Mo VI O 2 (L-NS 2 ) complex (L-NS 2 = 2,6-bis(2,2-diphenyl-2-mercaptoethyl)pyridine) can transfer oxygen from the Mo VI O unit to (p-FC 6 H 4 ) 3 P as it is reduced back to the Mo IV form.…”

“…Using the same [Mo VI O 2 (mnt) 2 ] 2− complex, Catalytic oxygen transfer by molybdenum complex. (Figure wasmodified from Ref [94],. with permission of the copyright holders.…”

Preparation, Characterization, and Aquation Kinetics of Pyridine N‐Oxide Complexes of Chromium(III)

“…Often, although not always, [6] such reactions involve oxygen-atom transfer [7][8][9] with pyridine N-oxide playing a role of either oxygen-atom donor [2,[7][8][9][10][11][12] or a promoter of oxygen transfer from active donors to the substrate. [13][14][15][16] Metal catalysis is frequently involved [17] in such processes, which makes mechanistic studies of transition-metal complexes of pyridine N-oxides an important area of investigation.…”