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

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′-(NO 2)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 situ UV/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 {PtCl 2} 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 NO 2-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. This journal is © The Royal Society of Chemistry.