Lewis-Acid-Promoted Stoichiometric and Catalytic Oxidations by Manganese Complexes Having Cross-Bridged Cyclam Ligand: A Comprehensive Study

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(3+) can also promote the thermodynamic driving force of the Mn(IV)-OH moiety to facilitate its hydrogen abstraction from ethylbenzene having a BDE(CH) value of 85 kcal/mol, while it is experimentally limited to 80 kcal/mol for Mn(IV)-OH alone. Adding Al(3+) 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 g = 2.01 into a hyperfine 6-line signal upon adding Al(3+) (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.

Crystal structures of two cross-bridged chromium(III) tetraazamacrocycles

“…Ethylene cross-bridged tetraazamacrocycles were introduced to coordination chemists in 1990 by Weisman and Wong (Weisman et al, 1990). Since then, their transition metal complexes have become important to the fields of oxidation catalysis (Yin et al, 2007;Dong et al, 2013), medical/biological imaging (Boswell et al, 2004;Sprague et al, 2007;Silversides et al, 2011) and chemokine receptor antagonism (Lewis et al, 2005;Valks et al, 2006;Smith et al, 2012) due to the combination of restricted macrocycle configuration and kinetic inertness inherent to these ligands.Chromium(III) complexes have played an important role in characterizing new ligands due to their relative kinetic inertness (Cotton & Wilkinson, 1988). Yet, to date, only one report of the chromium coordination chemistry of these macrobicyclic ligands has appeared in the literature (Maples et al, 2009).…”

Strong Inhibition of O-Atom Transfer Reactivity for MnIV(O)(π-Radical-Cation)(Lewis Acid) versus MnV(O) Porphyrinoid Complexes

“…26 Significant rate enhancements were generated by addition of LAs to MnO 4 − in the oxidation of alcohol 27 and alkane 28 substrates. Yin and coworkers 29 have also reported on stoichiometric and catalytic rate enhancements for OAT produced by addition of Lewis acids to manganese cross-bridged cyclam complexes. Fukuzumi, Nam, and coworkers 30 found that there is a 2,200-fold rate increase in OAT for [Mn IV (O)(N4Py)] 2+ to sulfide substrates upon binding of Sc III ions.…”

Designation of highly efficient catalysts for one pot conversion of glycerol to lactic acid

“…In addition, the shifts of νas Mo = O occurred from 965 cm −1 of HPMo to 942 cm −1 corresponding to Ag 1 PMo, Ag 2 PMo, and Ag 3 PMo, respectively. This was proposed to be the interaction between Ag and terminal oxygen of Mo = O leading to some blue shifts of the IR band of νas Mo = O, which was the main contribution to acceleration of electron transferring in oxidation 37 . Such blue shifts were correlative to the amount of Ag with the order of Ag 3 PMo (23 cm −1 ) >Ag 2 PMo (21 cm −1 ) >Ag 1 PMo (15 cm −1 ).…”