Spectroscopic and Structural Characterization of Mn(III)-Alkylperoxo Complexes Supported by Pentadentate Amide-Containing Ligands

Abstract

Manganese-alkylperoxo species have been proposed as important intermediates in certain enzymatic pathways and are presumed to play a key role in catalytic substrate oxidation cycles involving manganese catalysts and peroxide oxidants. However, structural and spectroscopic understanding of these intermediates is very limited, with only one series of synthetic Mn-alkylperoxo complexes having been reported. In the present study, we describe the formation and properties of two new Mn-alkylperoxo complexes, namely, [Mn(OO Bu)(dpaq)] and [Mn(OO Bu)(dpaq)], which utilize the anionic, amide-containing pentadentate dpaq ligand platform. These complexes were generated by reacting the corresponding Mn precursors with a large excess of BuOOH at -15 °C in MeCN. In both cases, the corresponding mononuclear Mn-hydroxo complexes [Mn(OH)(dpaq)] and [Mn(OH)(dpaq)] are observed as intermediates en route to the Mn-alkylperoxo adducts. These new Mn-alkylperoxo complexes were characterized by electronic absorption, infrared, and Mn K-edge X-ray absorption spectroscopies. Complementary density functional theory calculations were also performed to gain insight into their bonding and structural properties. Compared to previously reported Mn-alkylperoxo adducts, the Mn centers in these complexes exhibit significantly altered primary coordination spheres, with a strongly donating anionic amide nitrogen located trans to the alkylperoxo moiety. This results in Mn-alkylperoxo bonding that is dominated by σ-interactions between the alkylperoxo π*(O-O) orbital and the Mn d orbital.

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

“…+ and [Mn III (OOCm)( 6Me dpaq)] + show features at 875 cm À1 and 861 cmÀ1 , respectively, that are absent in the FT-IR spectrum of [Mn III (OH)( 6Me dpaq)] + (Fig.7). These features have energies similar to those of O-O vibrations reported for Mn III -alkylperoxo complexes (872-895 cm À1 ; TableS2†) 15,16,26. To evaluate this assignment, we prepared [Mn III ( 18 O 18 O t Bu)( 6Me dpaq)] + , which was conrmed by ESI-MS experiments (Fig.…”

“…26 The observation of tBuOOc in EPR spectra of the complexes following their thermal decay provided support for a decay pathway involving Mn-O bond homolysis. 26 While these data suggest differences in decay pathways for the thiolate-versus non-thiolate-ligated complexes, the large excess of t BuOOH (ca. 100 equivalents) required to form the [Mn III (OO t Bu)(dpaq)] + and [Mn III (OO t Bu)(dpaq 2Me )] + complexes made a complete analysis of their reactivity and decay pathways unfeasible.…”

“…2.8 16 O sample. 26 A normal coordinate analysis of high-spin Fe IIIalkylperoxo adducts revealed coupling of O-O and t-butyl C-C vibrations that gave rise to unusual isotope shi patterns, 43 and vibrational coupling has also been observed for Fe III -hydroperoxo adducts. 24,44 + .…”

“…We previously generated a pair of Mn III -alkylperoxo complexes with ligands lacking thiolate ligation. 26 These Mn IIIalkylperoxo complexes were supported by the pentadentate dpaq and dpaq 2Me ligands, both of which feature strongly donating amide groups trans to the alkylperoxo moiety (Fig. 1 )] + were unstable (t 1/2 ¼ 3200 and 3600 s, respectively, for 2 mM solution in CH 3 CN at À15 C) but were characterized by electronic absorption, Mn K-edge Xray absorption, and FT-IR spectroscopies.…”

“…We previously generated a pair of Mn III –alkylperoxo complexes with ligands lacking thiolate ligation. 26 These Mn III –alkylperoxo complexes were supported by the pentadentate dpaq and dpaq 2Me ligands, both of which feature strongly donating amide groups trans to the alkylperoxo moiety ( Fig. 1 , right; dpaq = 2-[bis(pyridin-2-ylmethyl)]amino- N -quinolin-8-yl-acetamidate, dpaq 2Me = 2-[bis(pyridin-2-ylmethyl)]amino- N -2-methyl-quinolin-8-yl-acetamidate).…”