Correlation Between Structural, Spectroscopic, and Reactivity Properties Within a Series of Structurally Analogous Metastable Manganese(III)–Alkylperoxo Complexes

Abstract

Manganese–peroxos are proposed as key intermediates in a number of important biochemical and synthetic transformations. Our understanding of the structural, spectroscopic, and reactivity properties of these metastable species is limited, however, and correlations between these properties have yet to be established experimentally. Herein we report the crystallographic structures of a series of structurally related metastable Mn(III)–OOR compounds, and examine their spectroscopic and reactivity properties. The four reported Mn(III)–OOR compounds extend the number of known end-on Mn(III)–(η1-peroxos) to six. The ligand backbone is shown to alter the metal–ligand distances and modulate the electronic properties key to bonding and activation of the peroxo. The mechanism of thermal decay of these metastable species is examined via variable-temperature kinetics. Strong correlations between structural (O–O and Mn⋯Npy,quin distances), spectroscopic (E(πv*(O–O) → Mn CT band), νO–O), and kinetic (ΔH‡ and ΔS‡) parameters for these complexes provide compelling evidence for rate-limiting O–O bond cleavage. Products identified in the final reaction mixtures of Mn(III)–OOR decay are consistent with homolytic O–O bond scission. The N-heterocyclic amines and ligand backbone (Et vs Pr) are found to modulate structural and reactivity properties, and O–O bond activation is shown, both experimentally and theoretically, to track with metal ion Lewis acidity. The peroxo O–O bond is shown to gradually become more activated as the N-heterocyclic amines move closer to the metal ion causing a decrease in π-donation from the peroxo πv*(O–O) orbital. The reported work represents one of very few examples of experimentally verified relationships between structure and function.

X-ray Absorption and Emission Study of Dioxygen Activation by a Small-Molecule Manganese Complex

“…The long Mn–N py bonds in peroxo 2 should lead to a more Lewis acidic Mn(III) ion, and thus a stabilized peroxo O–O bond as previously shown. 19 Contraction of the Mn-pyridine distances, and thus decreasing Lewis acidity, should therefore lead to localization of more electron density in the peroxo π * orbitals, weakening the peroxo bond and ultimately promoting O–O bond scission. A schematic for the mechanism of decreasing the Lewis acidity of the Mn ion is shown in Figure 10, and illustrates the contributing factors and the resultant localization of negative charge into peroxo antibonding orbitals.…”

“…This is consistent with not only the thermal instability of 2 17 but also the established correlations between O–O bond length (and thus Mn–N-heterocycle bond length) and the activation parameters Δ H ‡ and Δ S ‡ . 19 …”

“…19 Variation in the peroxo bond length was found to correlate with the Mn(III)-N-heterocyclic amine distance, indicating that these ligands cis to the peroxo play an important role in mediating O–O bond activation. Theoretical investigation of these compounds revealed that the Mulliken charge, or Lewis acidity, of the Mn(III) ion can be tuned by the N-heterocyclic amine distance, and these changes affect the degree of peroxo bond activation.…”

Manganese and Iron Catalysts in Alkyd Paints and Coatings

“…Such adducts are wellknown for cobalt, manganese and iron complexes (see [8][9][10][11] for some representative examples of characterized metal-alkylperoxo species). These adducts may decompose to form peroxyl or alkoxy radicals (Reactions (8) and (9), respectively) [4]:…”

“…The rate of oligomerization of the substrate appeared to be best for Co II (2-EH) 2 , but again the data indicated that the addition of bpy to [Mn III (acac) 3 ] leads to a clear increase of reactivity compared to [Mn III (acac) 3 ]. During these oxidation processes, β-scission reactions also took place, leading to the formation of volatile aldehydes, such as pentanal and hexanal (Reaction (10) In addition, paint drying experiments were done using [Mn III (acac) 3 ] and bpy on various alkyd-based formulations [22]. It was shown that at relatively high levels of Mn (0.1 wt %) the alkyd-paint drying activity of [Mn III (acac) 3 ] was clearly better than that of Mn-bpy (Section 4.2.1) (surface dry time of 6 and 13 h, respectively) and slightly slower than when using Co II (EH) 2 used at 0.08 wt % Co (surface dry time of 5 h).…”