Electrochemical and IR/UV−Vis Spectroelectrochemical Studies of fac -[Mn(X)(CO) 3 (iPr-DAB)] n ( n = 0, X = Br, Me, Bz; n = +1, X = THF, MeCN, nPrCN, P(OMe) 3 ; iPr-DAB = 1,4-Diisopropyl-1,4-diaza-1,3-butadiene) at Variable Temperatures: Relation between Electrochemical and Photochemical Generation of [Mn(CO) 3 (α-diimine)] -

The title complexes [Mn(X)(CO) 3 (iPr-DAB)] n ( n = 0, X = Br; n = +1, X = donor solvent) undergo a two-electron reduction according to an ECE sequence. The chemical step (C) involves prompt dissociation of the X ligand from the primary one-electron reduction product, followed by instantaneous one-electron reduction of the five-coordinate transient [Mn(CO) 3 (iPr-DAB)] • producing the anion [Mn(CO) 3 (iPr-DAB)] - . The latter complex remains rather stable at T < 190 K, whereas at higher temperatures it undergoes an electron-transfer reaction with the parent complexes producing the dimer [Mn(CO) 3 (iPr-DAB)] 2 (the second C step in the overall ECEC sequence). The rate of this reaction decreases in the order THF > MeCN > Br. The driving force for this behavior is the more positive E 1/2 value of the redox couple [Mn(CO) 3 (iPr-DAB)] • / - relative to those of [Mn(Br)(CO) 3 (iPr-DAB)] 0/ • - and [Mn(X)(CO) 3 (iPr-DAB)] +/ • (X = donor solvent) and a very short lifetime of the primary reduction products. In contrast, the ligand P(OMe) 3 in [Mn{P(OMe) 3 }(CO) 3 (iPr-DAB)] • is bound rather firmly at low temperatures, where the ECE sequence to [Mn(CO) 3 (iPr-DAB)] - via [Mn(CO) 3 (iPr-DAB)] • is only a minor route. The reduction of [Mn(X)(CO) 3 (iPr-DAB)] (X = Me, Bz) at room temperature affords the five-coordinate anion [Mn(CO) 3 (iPr-DAB)] - via dissociation of X • from the one-electron-reduced intermediate [Mn(X)(CO) 3 (iPr-DAB)] • - detectable by cyclic voltammetry for X = Me. Oxidation of the five-coordinate anion [Mn(CO) 3 (iPr-DAB)] - produces the dimer [Mn(CO) 3 (iPr-DAB)] 2, following the reverse ECE(C) sequence involved in the reduction path. The direct dimerization of the radicals primarily formed, [Mn(CO) 3 (iPr-DAB)] •, is probably only a minor alternative route. In the presence of excess P(OMe) 3, the principal oxidation product is the cation [Mn{P(OMe) 3 }(CO) 3 (iPr-DAB)] + . The five-coordinate anions [Mn(CO) 3 (α-diimine)] - can be regarded as strongly π-delocalized complexes with the negative charge equally distributed over the α-diimine and CO ligands. The intriguing mechanism of their photochemical formation from fac -[Mn(Br)(CO) 3 (α-diimine)] at low temperatures has been rectified on the basis of this (spectro)electrochemical study.