Reduction of [Re(X)(CO)(3)(R'-DAB)] (X=Otf(-), Br-; DAB equals diazabutadiene; R'=iPr, pTol, pAn) and [Re(R)(CO)(3)(iPr-DAB)] (R=Me, Et, Bz) complexes: A comparative (spectro)electrochemical study at variable temperatures

This article describes the reduction routes of the complexes [Re(X)(CO)(3)(R'-DAB)] (X = Br-, Otf(-); R' = pAn, pTol, iPr) and the metal-alkyl complexes [Re(R)(CO)(3)(iPr-DAB)] (R = Me, Et, Bz), using data obtained from IR and UV-vis spectroelectrochemical measurements and cyclic voltammetry at variable temperatures. Oneelectron reduction of the complexes [Re(Br)(CO)(3)(R'-DAB)] in nPrCN results in the establishment of an equilibrium between the radical anions [Re(Br)(CO)3(R'-DAB)](.-) and the solvento radical [Re(nPrCN)(CO)(3)(R'-DAB)](.). The ratio between these radical species could be tuned by changing the character of the R' substituents on the R'-DAB ligand and by variation of the temperature. The radicals could be reduced in a subsequent one-electron step to give the five-coordinate anions [Re(CO)(3)(R'-DAB)](-) together with the six-coordinate anions [Re(nPrCN)(CO)(3)(R'-DAB)](-) as the minor products. The bonding properties of the five-coordinate anions are discussed on the basis of a qualitative MO scheme and compared with those of the related complex [Mn(CO)(3)(iPr-DAB)](-). The main feature of the [Re(CO)(3)(R'-DAB)](-) complexes is a strong delocalization of their HOMOs, which rules out application of the localized-valence concept in this case. The complexes [Re(R)(CO)(3)(iPr-DAB)] were found to be reduced in an electrochemically and chemically reversible one-electron step to give radical anionic products, tentatively formulated as {[Re(CO)(3)(iPr-DAB)](-)... R(.)}. The radical nature of the products has been confirmed by ESR spectroscopy whereas the IR and UV-vis data correspond with the presence of the [Re(CO)(3)(iPr-DAB)](-) moiety.