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)(ALPHA-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 E CE 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 high er temperatures it undergoes an electron-transfer reaction with the pa rent complexes producing the dimer [Mn(CO)(3)(iPr-DAB)](2) (the second C step in the overall ECEC sequence). The rate of this reaction decre ases 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-D AB)](./-) 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 lifetim e 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 t emperatures, 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-coord inate anion [Mn(CO)(3)(iPr-DAB)](-) via dissociation of X-. from the o ne-electron-reduced intermediate [Mn(X)(CO)(3)(iPr-DAB)](-) detectable by cyclic voltammetry for X = Me. Oxidation of the five-coordinate an ion [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 prese nce 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)(alpha-diimine)](-) can be regarded as strongly pi-delocalized compl exes with the negative charge equally distributed over the alpha-diimi ne and CO ligands. The intriguing mechanism of their photochemical for mation from fac-[Mn(Br)(CO)(3)(alpha-diimine)] at low temperatures has been rectified on the basis of this (spectro)electrochemical study.