Lewis-Acid-Promoted
Stoichiometric and Catalytic Oxidations
by Manganese Complexes Having Cross-Bridged Cyclam Ligand: A Comprehensive
Study
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Abstract
Redox-inactive metal ions have been
recognized to be able to participate
in redox metal-ion-mediated biological and chemical oxidative events;
however, their roles are still elusive. This work presents how the
redox-inactive metal ions affect the oxidative reactivity of a well-investigated
manganese(II) with its corresponding manganese(IV) complexes having
cross-bridged cyclam ligand. In dry acetone, the presence of these
metal ions can greatly accelerate stoichiometric oxidations of triphenylphosphine
and sulfides by the manganese(IV) complexes through electron transfer
or catalytic sulfoxidations by the corresponding manganese(II) complexes
with PhIO. Significantly, the rate enhancements are highly Lewis-acid
strength dependent on added metal ions. These metal ions like Al<sup>3+</sup> can also promote the thermodynamic driving force of the
Mn<sup>IV</sup>–OH moiety to facilitate its hydrogen abstraction
from ethylbenzene having a BDE<sub>CH</sub> value of 85 kcal/mol,
while it is experimentally limited to 80 kcal/mol for Mn<sup>IV</sup>–OH alone. Adding Al<sup>3+</sup> may also improve the manganese(II)-catalyzed
olefin epoxidation with PhIO. However, compared with those in electron
transfer, improvements in hydrogen abstraction and electron transfer
are minor. The existence of the interaction between Lewis acid and
the manganese(IV) species was evidenced by the blue shift of the characteristic
absorbance of the manganese(IV) species from 554 to 537 nm and by
converting its EPR signal at <i>g</i> = 2.01 into a hyperfine
6-line signal upon adding Al<sup>3+</sup> (<i>I</i> = 5/2).
Cyclic voltammograms of the manganese(IV) complexes reveal that adding
Lewis acid would substantially shift its potential to the positive
direction, thus enhancing its oxidizing capability