Ligand Influence on Structural Properties and Reactivity of Bis(μ‐oxo)dimanganese(III,IV) Species and Comparison of Reactivity with Terminal Mn^IV‐oxo Complexes

Abstract

The ligand influence on structure and reactivity of a pair of bis (μ-oxo)dimanganese(III,IV) complexes was investigated. The pentadentate N4py and DMM N4py ligands were employed to assemble the [Mn III Mn IV (μ-O) 2 (N4py) 2 ] 3 + (1) and [Mn III Mn IV (μ-O) 2 ( DMM N4py) 2 ] 3 + (2) species by H 2 O 2 oxidation of the corresponding Mn II complexes. The electronic absorption and EPR properties of 1 and 2 are nearly identical. Analysis of the EXAFS region of the Mn K-edge XAS spectra of 1 and 2 reveals comparable Mn-ligand and MnÀ Mn distances. Molecular structures of 1 and 2 from DFT calculations showed only minor variations in Mn-ligand bond lengths between these complexes. Despite these structural similarities, a ligand influence was observed on the Mn III Mn IV to Mn III Mn III reduction potential. Nonetheless, the oxidation of 2,4-di-tert-butylphenol by 1 and 2 proceeded at essentially identical rates. In contrast, reactivity comparison between 2 and the corresponding monomeric Mn IV -oxo species, [Mn IV (O)( DMM N4py)] 2 + , showed that the latter complex is a vastly superior oxidant for OÀ H bond oxidation, showing a rate enhancement of 10 5 at parity of substrate and temperature.

Enhanced Understanding of Structure–Function Relationships for Oxomanganese(IV) Complexes

“…The complex signal of [Mn IV (O)(N3pyQ)] 2+ at g eff = 2.0 shows at least 14 lines, with a hyperfine splitting of a = 7.8 mT. A large number of hyperfine lines and the g and a values are consistent with reports for bis(μ-oxo)dimanganese(III,IV) complexes with similar ligands ( g = 2.0 and a = 7.7 mT) . Notably, bis(μ-oxo)dimanganese(III,IV) complexes have been observed as decay products of similar Mn IV -oxo species …”

“…A large number of hyperfine lines and the g and a values are consistent with reports for bis(μ-oxo)dimanganese(III,IV) complexes with similar ligands (g = 2.0 and a = 7.7 mT). 61 Notably, bis(μ-oxo)dimanganese-(III,IV) complexes have been observed as decay products of similar Mn IV -oxo species. 25 The EPR spectrum of [Mn IV (O)(N4py Me2 )] 2+ shows a positive signal with prominent hyperfine splitting at g = 4.6, a derivative at g = 3.1, and negative signals at g = 1.9 and 1.6.…”

A tetranuclear Mn-diamond core complex as a functional mimic of both catechol oxidase and phenoxazinone synthase enzymes

“…, Mn–O distance 1.80–1.94 Å, Mn⋯Mn distance 2.59–2.91 Å, and bond angle of Mn–O–Mn ranging between 92.5 and 102.3°). 40 The distances of ligands bound to Mn( iii ) were Mn1–O5 1 1.841(4) Å, Mn1–N1 2.116(4) Å, Mn1–N2 2.089(2) Å, and Mn1–N5 2.075(4) Å. The short metal–metal distance together with appropriately short Mn( iii )–O bond lengths indicate the existence of a diamond core at the center.…”

Model Dimeric Manganese(IV) Complexes Featuring Terminal Tris-hydroxotetraazaadamantane and Various Bridging Ligands

“…Several studies have recently shown that just like their Fe IV O analogues nonheme Mn IV -oxo compounds supported by neutral pentadentate ligands also activate strong C–H bonds. , It has been suggested that the enhanced hydrogen atom abstraction (HAT) reactivity of these Mn IV O species originate from the synergistic effects of two-level reactivity involving the 4 B 1 and 4 E levels electronic states, the π-bonding interactions of substrates with the Mn IV O group, and the unusually high redox potential of the Mn III/IV couple . Interestingly, the HAT reactivity was not limited to mononuclear compounds but also observed for some dinuclear complexes . Furthermore, the presence of terminal oxo ligands is not even a prerequisite for manganese­(IV) species to exhibit biomimetic catalytic activity .…”

“…9 Interestingly, the HAT reactivity was not limited to mononuclear compounds but also observed for some dinuclear complexes. 10 ligands is not even a prerequisite for manganese(IV) species to exhibit biomimetic catalytic activity. 11 One notable series of such compounds are the radical-supported Mn IV 2 (L • ) 2 (L •• ) and Mn IV L(L • ) 2 where L are o-aminophenols derived from aniline and m-phenylenediamine, which catalyze the oxidation of catechol to quinone and the oxidative coupling of C−C bonds of hindered phenols using oxygen as the sole oxidant.…”