1,4-Diaza-1,3-butadiene (a-diimine) ligands: their coordination modes and the reactivity of their metal complexes

Molecules containing the 1,4-diaza-1,3-butadiene skeleton have attracted much interest because of both their versatile coordination behavior and the interesting properties of their metal complexes. 1,4-diaza-1,3-butadienes (R-DAB) may be prepared by condensation reactions involving glyoxals, α-ketoaldehydes or α, β-–diketones with primary amines RNH2. It was concluded from nuclear magnetic resonance (NMR) spectra, dipole moments, and infrared (IR) spectra that the R-DAB molecule exists in solution in the E configuration at both C=N double bonds while the conformation of the central C–C bond is predominantly s-trans. So far stable complexes containing monodentate R–DAB have only been reported for the square planar d8–metals Pd11, Pt11, and Rh1. The reports of metal complexes with R–DAB ligands date back to 1953 when Krumholz described the synthesis of some ferrous complexes. The unusual stability and characteristic color being ascribed to the presence of π bonding between the metal and the nitrogen atoms. Factors, such as the substituents R and R, the metal atom, and the other ligands bonded to the metal atom, will influence the type of coordination of the R–DAB( R’,R) ligands. Just as 2, 2’-bipyridine, the R–DAB ligands can be readily converted to stable paramagnetic radical anions by potassium in 1,2-dirnethoxyethane or tetrahydrofuran (THF).