Influence of Thiolate versus Alkoxide Ligands on the Stability of Crystallographically Characterized Mn(III)-Alkylperoxo Complexes

Abstract

The work described herein demonstrates the exquisite control that the inner coordination sphere of metalloenzymes and transition-metal complexes can have on reactivity. We report one of few crystallographically characterized Mn-peroxo complexes and show that the tight correlations between metrical and spectroscopic parameters, established previously by our group for thiolate-ligated RS-Mn(III)-OOR complexes, can be extended to include an alkoxide-ligated RO-Mn(III)-OOR complex. We show that the alkoxide-ligated RO-Mn(III)-OOR complex is an order of magnitude more stable (t 1/2 298 K = 6730 s, k obs 298 K = 1.03 × 10 −4 s −1 ) than its thiolate-ligated RS-Mn(III)-OOR derivative (t 1/2 293 K = 249 s, k 1 293 K = 2.78 × 10 −3 s −1 ). Electronic structure calculations provide insight regarding these differences in stability. The highest occupied orbital of the thiolate-ligated derivative possesses significant sulfur character and πbackdonation from the thiolate competes with π-backdonation from the peroxo π*(O−O). DFT-calculated Mulliken charges show that the Mn ion Lewis acidity of alkoxide-ligated RO-Mn(III)-OOR (+0.451) is greater than that of thiolate-ligated RS-Mn(III)-OOR (+0.306), thereby facilitating π-backdonation from the antibonding peroxo π*(O−O) orbital and increasing its stability. This helps to explain why the photosynthetic oxygen-evolving Mn complex, which catalyzes O−O bond formation as opposed to cleavage, incorporates O-and/or N-ligands as opposed to cys S-ligands.

Characterization and chemical reactivity of room-temperature-stable MnIII–alkylperoxo complexes

“… 16 Very recently, Kovacs et al reported a room-temperature stable Mn III –alkylperoxo complex supported by an alkoxide analogue of the N 4 S − ligands. 17 DFT computations for the [Mn III (OO t Bu)(N 4 O)] + and [Mn III (OO t Bu)(N 4 S)] + pair support the notion that the enhanced stability of the alkoxide-ligated complex arises from greater Lewis acidity of the Mn III center.…”

Homogeneous catalysis of dioxygen reduction by molecular Mn complexes

“…We note that the stoichiometric chemistry of Mn complexes with thiolate containing ligands and O 2 has been explored extensively by Kovacs and coworkers. [71][72][73][74][75] Upon exposure to dry air, both centers were oxidized by a single electron and a bridging hydroxide species was recovered. Mechanistic studies established that the dinuclear monothiol species united two separate reaction cycles, where either H 2 O 2 or H 2 O production was favored.…”

Electronic Structure and Reactivity of Dioxygen-Derived Aliphatic Thiolate-Ligated Fe-Peroxo and Fe(IV) Oxo Compounds

“…21,47 We also recently showed that aliphatic thiolates help to promote peroxo O−O bond cleavage. 48,49 Very few well-characterized iron-superoxo compounds have been reported, 21,50−53 and of those reported, only two incorporate a thiolate in the coordination sphere 21,50 and only one cleaves strong C−H bonds. 21 We recently showed that aliphatic thiolate-ligated [Fe II (S 2…”

“…Much of our understanding of the properties of heme and nonheme iron enzyme dioxygen intermediates such as Fe­(III)–superoxos, Fe­(III)–peroxos, and high-valent iron oxos comes from small-molecule chemistry. − Only a small number of these are derived from O 2 , ,− and despite the key role of thiolates in a wide variety of nonheme iron enzymes, only a small number of these contain thiolates in the coordination sphere. ,,, Key properties unique to aliphatic thiolate (RS – or Cys-S – ) ligands include an increase in the potency of high-valent iron oxo intermediates − and, as shown by our group, the potency of iron-superoxo compounds with respect to their ability to cleave strong C–H bonds. , We also recently showed that aliphatic thiolates help to promote peroxo O–O bond cleavage. , Very few well-characterized iron-superoxo compounds have been reported, ,− and of those reported, only two incorporate a thiolate in the coordination sphere , and only one cleaves strong C–H bonds . We recently showed that aliphatic thiolate-ligated [Fe II (S 2 Me2 N 3 (Pr,Pr))] ( 1 ) reacts with O 2 (Scheme ) to form an unprecedented example of an aliphatic thiolate-ligated iron superoxo intermediate, [Fe III (S 2 Me2 N 3 (Pr,Pr))­(O 2 )] ( 2 ), which is capable of cleaving strong C–H bonds (e.g., tetrahydrofuran (THF) (BDE­(C–H) = 92 kcal mol –1 )) at −73 °C with a k H / k D = 4.8 .…”