REACTIVITY OF DIOXYGEN WITH MANGANESE SCHIFF-BASE COMPLEXES - A CHEMICAL AND ELECTROCHEMICAL INVESTIGATION

The bis(mu-oxo) dimers [(X-SALPRN)Mn-IV(mu-O)](2), where X = 5-Cl, 1c, 3,5-Cl-2, 2c, and 5-CH3O, 3c, and SALPRN is 1,3-bis(salicylideneamino)propane, were prepared by two routes: oxidation of (X-SALPRN)Mn-II, X = 5-Cl, 1b, 3,5-Cl-2, 2b, and 5-CH3O, 3b, with O-2 in acetonitrile or bulk electrochemical reduction of the Mn(III) salt, [(X-SALPRN)Mn-III]PF6, X = 5-Cl, 1a, 3,5-Cl-2, 2a, and 5-CH3O, 3a, in the presence of dioxygen in 0.2 M TBAPF(6)/CH3 CN. Reaction of a mixture of 2b and 3b with oxygen produced 2c, 3c, and the mixed-ligand complex [(3,5-Cl-2-SALPRN)Mn-IV(5-CH3O-SALPRN)Mn-IV(mu-O)(2)], 4. The ternary mixture also formed when a CH3CN solution of 2a and 3a was electrolyzed at a potential sufficient to produce the Mn(II) form of both complexes. Electrolysis of the same mixture at a potential where essentially only 2b was reduced produced 2c, a trace of 4, and an insoluble green solid that appeared to be polymeric. The data from cyclic voltammetry, rotating disk electrode voltammetry, and bulk electrosynthesis experiments suggested an ECC reaction pathway leading to formation of the bis(mu-oxo) dimers. The chemical steps in the ECC pathway were the same as those when 1b, 2b, or 3b was oxidized in the absence of external reducing equivalents. Bulk electrolysis with H2O present completely inhibited bis(mu-oxo) dimer formation and resulted in the precipitation of a green solid insoluble in common organic solvents.