REDOX PROPERTIES OF [MN(CO)3(DBCAT)]- (DBCAT=3,5-DI-TERT-BUTYLCATECHOLATE) - FORMATION AND CHARACTERIZATION OF A 4-MEMBERED REDOX SERIES

The [Mn(CO)3(3,5-di-tert-butylcatecholate)]- complex undergoes a metal-localized one-electron reduction (E1/2 = -2.39 V vs Fc/Fc+ in THF) and two successive one-electron oxidations (E1/2 = -0.70, +0.23 V vs Fc/Fc+ in THF) that are ligand localized. All redox steps are electrochemically reversible or quasireversible. The oxidations are followed by rapid coordination or a sixth ligand, i.e. THF, PPh3, or CO in CO-saturated CH2Cl2 solution. In argon-saturated CH2Cl2, a CO ligand disproportionation of the oxidation products takes place. In effect, the existence of a four-membered redox series was established: Mn(CO)3-(L)(quinone)]+, [Mn(CO)3(L)(semiquinone)], [Mn(CO)3(catecholate)]-, and [Mn(CO)3(catecholate)]2-; L = CO, THF, or PPh3. The spectral properties (EPR, IR, and UV-vis) of the neutral, monoanionic, and dianionic species are compared. The EPR spectra of the ligand- and metal-localized radicals [Mn(CO)3(L)(semiquinone)] and [Mn(CO)3(catecholate)]2- are markedly different: a(Mn) = 3.7 G (L = THF), 6.9 G (L = CO), 9.9 G (L = PPh3); a(Mn) = 53.4 G. The IR nu(CO) frequencies significantly increase from the dianion to the neutral species, reflecting the change in the Mn oxidation state in the mono-/dianion couple and the decrease in pi-donation upon the oxidation of the catecholate to the semiquinone radical-anionic ligand.