Correlation Between Structural, Spectroscopic, and
Reactivity Properties Within a Series of Structurally Analogous Metastable
Manganese(III)–Alkylperoxo Complexes
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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)–(η<sup>1</sup>-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···N<sup>py,quin</sup> distances), spectroscopic (E(π<sub>v</sub>*(O–O)
→ Mn CT band), ν<sub>O–O</sub>), and kinetic (Δ<i>H</i><sup>⧧</sup> and Δ<i>S</i><sup>⧧</sup>) 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 π<sub>v</sub>*(O–O) orbital. The reported work represents one of very few
examples of experimentally verified relationships between structure
and function