On the electronic structure of nitro-substituted bipyridines and their platinum complexes

Abstract

We report the preparation and electrochemical studies of a systematic series of mono- and di-nitro-substituted 2,2′-bipyridine (bipy) compounds [x-NO₂-bipy (x = 3,4) and x,x′-(NO₂)₂-bipy (x,x′ = 3, 4, 5)] and their complexes with platinum(II), [Pt(x-NO₂-bipy)Cl₂] and [Pt(x,x′-(NO₂)₂-bipy)Cl₂]. The effect of the number and substitution pattern of the nitro groups on the low-lying acceptor molecular orbitals (involved in charge transfer transitions) is probed by in situUV/Vis/NIR and EPR spectroelectrochemical methods, supported by DFT calculations. The LUMOs of x-NO₂-bipy (x = 3–5) are largely localised on the NO₂-pyridyl moiety; this is also true of their {PtCl₂} complexes but with a small but significant shift of electron density from the nitro groups. The LUMOs of x,x′-(NO₂)₂-bipy with x = 3 and 5 are delocalised over both NO₂-pyridyl rings, but for 4,4′-(NO₂)₂-bipy is localised on a single NO₂-pyridyl ring. In all cases the LUMO of the [Pt(x,x′-(NO₂)₂-bipy)Cl₂] complexes is delocalised over both nitro-pyridyl rings. For all complexes, the 4(4′) derivatives allows greatest overlap with metal valence orbitals in the LUMO.