Binuclear metal carbonyl dab complexes : II. The syntheses and coordination properties of Mn(CO) 5M′(CO) 3DAB (M′ = Mn, Re; DAB = 1,4-diazabutadiene)

Abstract

Mn(CO) 5M′(CO) 3DAB complexes (M′ = Mn, Re; DAB = R 1N=C(R 2)-C(R′ 2)=NR 1) can be easily obtained from the reaction between Mn(CO) 5 − and M′(CO) 3X(DAB) (M′ = Mn, Re; X = Cl, Br, I). The complexes are formed by a nucleophilic mechanism, while a redistribution is responsible for the formation of a small amount of Mn 2(CO) 10. A diastereotopic effect can be observed in the 1H and 13C NMR spectra of complexes having isopropyl groups attached to the DAB ligand skeleton. A comparison is made with mononuclear complexes of the same symmetry, and the chemical shift differences for the methyl groups strongly depend on the substituent on the central metal responsible for the asymmetry. The low temperature enhancement of the σ → σ ★ transition localised on the metal—metal bond, which is normally observed for this type of compounds, was not observed for the Mn(CO) 5M′(CO) 3(DAB) complexes. The metal—metal bond can be activated by irradiating at the wave lengths associated with the CT transitions between the metal and the DAB ligand. Metal—metal bond cleavage occurs and Mn 2(CO) 10 is formed.