Tuning the Relative Stability and Reactivity of Manganese Dioxygen and Peroxo Intermediates via Systematic Ligand Modification

Abstract

CONSPECTUS Many fundamental processes of life depend on the chemical energy stored in the O–O bond of dioxygen (O2), the majority of which is derived from photosynthetic H2O oxidation. Key steps in these processes involve Mn-, Fe-, or Cu-promoted formation or cleavage of O–O and O–H bonds, the mechanisms of which are not fully understood, especially with Mn. Metal–peroxo and high-valent metal–oxo species are proposed to be involved as intermediates. The metal ion properties that favor O–O and O–H bond formation versus cleavage have yet to be systematically explored. Herein we examine the O2 reactivity of a series of structurally related Mn(II) complexes and show that several metastable intermediates are observed, the relative stabilities of which depend on subtle differences in ligand architecture. We show that in contrast to Fe and Cu complexes, O2 binds irreversibly to Mn(II). By crystallizing an entire series of the first reported examples of Mn(III)–OOR peroxos as well as an O2-derived binuclear trans-μ-1,2-bridged Mn(III)–peroxo with varying degrees of O–O bond activation, we demonstrate that there are distinct correlations between spectroscopic, structural, and reactivity properties. Rate-limiting O–O bond cleavage is shown to afford a reactive species capable of abstracting H atoms from 2,4-tBu2-PhOH or 1,4-cyclohexadiene, depending on the ligand substituents. The weakly coordinated N-heterocycle Mn⋯Npy,quino distance is shown to correlate with the peroxo O–O bond length and modulate the π overlap between the filled πv∗false(normalO−normalOfalse) and Mn dxz orbitals. We also show that there is a strong correlation between the peroxo → Mn charge transfer (CT) band and the peroxo O–O bond length. The energy difference between the CT bands associated with the peroxos possessing the shortest and longest O–O bonds shows that these distances are spectroscopically distinguishable. We show that we can use this spectroscopic parameter to estimate the O–O bond length, and thus the degree of O–O bond activation, in intermediates for which there is no crystal structure, as long as the ligand environment is approximately the same.

Oxygen activation by mononuclear Mn, Co, and Ni centers in biology and synthetic complexes

“…1) generated by treatment of a five-coordinate Mn(II)-thiolate complex with O2. 5,6 This species quickly reacts with a second equivalent of Mn(II) to afford a dimanganese(III) complex bridged by a trans-μ-1,2-peroxide ligand. Lang et al succeeded in obtaining a crystal structure of a Mn(III) complex (2) featuring a superoxide ligand in an end-on (η 1 ) binding mode ( Fig.…”

“…Scheme 1. Dioxygen reactivity of mononuclear Mn species (1) and X-ray crystal structure (2) of superoxomanganese complexes prepared by the Kovacs 5,6 and Lang 7 groups, respectively Synthetic nonheme Mn(III) complexes with side-on (η 2 ) peroxide ligands-first reported by Kitajima and Morooka in 1994 8 -are generally prepared by treatment of Mn(II) precursors with superoxide or H2O2 (Scheme 1). 9,10 The Borovik group, however, was able to derive a peroxomanganese(III) complex (3; Fig.…”

C–H activation and nucleophilic substitution in a photochemically generated high valent iron complex

“… 22 These complexes have provided seminal insights into pivotal biochemical transformations using oxidants such as O 2 and H 2 O 2 . 1 , 23 , 24 In particular, the (TAML)Fe system developed by Collins and coworkers is such a potent oxidant that it has been used for pulp bleaching, degradation of recalcitrant pollutants, water oxidation, and even the destruction of explosives in water. 15 , 25 – 28 …”

Understanding the Catalase-Like Activity of a Bioinspired Manganese(II) Complex with a Pentadentate NSNSN Ligand Framework. A Computational Insight into the Mechanism

“…[8][9][10][11][12] Although Mn-polyamine complexes show very high SOD-mimetic activity, 13 certain Mn-porphyrin and Mn-salen complexes exhibit dual SOD and CAT activity. [14][15][16][17] Additional functions of manganese complexes are known such as oxidation, [18][19][20] reduction, 21 or sulfur-oxygenation. 22 In a recent paper by Britovsek, Bonchio and coworkers, 23 .…”