Oxidation of Naphthalene with a Manganese(IV) Bis(hydroxo) Complex in the Presence of Acid

Abstract

Naphthalene oxidation by metal-oxygen intermediates is one of difficult reactions in environmental and biological chemistry. Herein, we report that a MnIV-bis(hydroxo) complex, which was fully characterized by various physicochemical methods, such as UV-vis, ESI-MS, EPR, X-ray and XAS, shows the naphthalene oxidation in the presence of acid to afford 1,4-naphthoquinone. Redox titration of the MnIV-bis(hydroxo) complex exhibits one electron reduction potential of 1.09 V, which is the most positive potential for the previously reported nonheme MnIV-bis(hydroxo) species as well as MnIV-oxo analogues. Kinetic studies including kinetic isotope effect suggest that the naphthalene oxidation by the MnIV-bis(hydroxo) complex in the acid-promoted reaction occurs via a rate-determining electron transfer process.

Structure and Reactivity of Nonporphyrinic Terminal Manganese(IV)–Hydroxide Complexes in the Oxidative Electrophilic Reaction

“…Notably, 2 is the first crystallographically characterized mononuclear nonporphyrinic manganese­(IV) complex with a single terminal hydroxide ligand. The Mn–OH bond length in 2 [1.798(3) Å] is similar to those of the nonporphyrinic manganese­(IV)–bishydroxide complexes, [Mn IV ( H,Me Pytacn)­(OH) 2 ] 2+ (1.799 Å), [Mn IV (Me 2 EBC)­(OH) 2 ] 2+ (1.811 Å), and [Mn IV (TBDAP)­(OH) 2 ] 2+ (1.806 Å), but shorter than that of a porphyrinic Mn­(IV)–(OH) complex, [Mn IV (OH)­(ttppc)] (1.881 Å). ,,, From the electron density map, in addition, we confirmed the presence of the H atom of the hydroxide (see Figure S4 for the refinement of the H atom of the hydroxo moiety). Thus, taken together, these structural and spectroscopic data clearly demonstrate that 2 is a manganese­(IV)–hydroxide complex.…”

“…These isotope labeling experiments support the presence of a hydroxide ligand in 2 . The X-band EPR spectrum of 2 , measured in a frozen acetone/CH 3 CH 2 OH solvent [3:1 (v/v)] at 113 K, indicates that 2 is of an S = 3 / 2 ground state with g values of 4.42 and 2.06 (Figure a, inset), which are comparable to those of [Mn IV (TBDAP)­(OH) 2 ] 2+ [TBDAP = N , N -di- tert -butyl-2,11-diaza­[3.3]­(2,6)­pyridinophane] ( g = 4.32 and 1.99) . Moreover, the signal centered at g = 2.06 exhibits a six-line hyperfine splitting with an A ( 55 Mn) of 94 G, which is similar to that of the reported S = 3 / 2 Mn­(IV). , …”

“…Fundamental biological oxidation processes such as the lipoxygenase-catalyzed reaction, metal-mediated O 2 activation, and oxygen evolution involve many critical reactive metal–oxygen intermediates. − Among the oxygen-coordinating metal complexes, high-valent manganese–hydroxide [Mn n + –(OH), where n ≥ 3] adducts have been assessed as reactive species in enzymatic oxidation reactions. − For example, Mn III –(OH) complexes have been employed as an active oxidant in lipoxygenases, which activate the C–H bond of polyunsaturated fatty acids by hydrogen atom abstraction. − Mn IV –(OH) species have often been proposed as intermediates in catalytic O 2 activation or O 2 production. − , …”

“…In nonporphyrinic systems, there are a few examples of Mn IV –(OH) intermediates, whose geometric information was obtained by either X-ray absorption spectroscopy or density functional theory (DFT) calculations. − However, only the weak O–H bond activation reaction has been reported for the nonporphyrinic Mn IV –(OH) species to date . Very recently, it has been reported that a nonporphyrinic manganese­(IV)–bishydroxide [Mn IV –(OH) 2 ] complex works as a strong oxidant for naphthalene oxidation with the aid of acid …”

Controlling the Reactivity of a Metal-Hydroxo Adduct with a Hydrogen Bond

“…Metal–oxygen adducts (i.e., metal-oxo, -peroxo, and -hydroxo species) feature prominently in the proposed mechanisms of a variety of metalloenzymes and small-molecule, synthetic catalysts. − In many cases, these metal–oxygen species are involved in critical substrate oxidation steps in the catalytic cycle. While it is now well established that high-valent metal-oxo species can be involved in such reactions, there are increasing examples of mid- and high-valent metal-hydroxo species that can effect substrate oxidation reactions. − Two metalloenzymes that rely on midvalent metal­(III)-hydroxo adducts to perform their function are manganese superoxide dismutase (MnSOD) and manganese lipoxygenase (MnLOX). MnSOD regulates the levels of reactive oxygen species in the cell by catalyzing the disproportionation of superoxide to hydrogen peroxide and dioxygen. − The MnLOX enzyme catalyzes the oxidation of polyunsaturated fatty acids into their hydroperoxides, which are further metabolized into biologically active oxylipins such as a leukotrienes and jasmonates.…”