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.

Electrochemical and spectroelectrochemical (UV–Vis, IR, EPR) of pd (pd = 1,10-phenanthroline-5,6-dione), Pt(N,N′-pd)Cl2, Pd(N,N′-pd)Cl2, [Ru(bpy)2(N,N′-pd)]Cl2 (bpy = 2,2′-bipyridine) and Pt(O,O′-pd)(PPh3)2, where N,N′ and O,O′ refers to coordination of pd to the metal centre via N and O atoms, respectively, reveals that the electron transfer processes between +0.5 and −1.25 V all occur at the pd ligand in agreement with DFT calculations. The two CO groups carry a significant amount of the negative charge in mono-reduced pd1−. The mode of coordination of pd has a greater influence on its redox chemistry than the metal centre or the ancillary ligands.Graphical abstractThe redox chemistry of 1,10-phenanthroline-5,6-dione and N,N′- and O,O′-bound complexes of Pt, Pd and Ru is presented. Stable redox states are characterised by UV–Vis, EPR and IR spectroscopies.Highlights► Redox studies of 1,10-phenanthroline-5,6-dione and its Pt, Pd and Ru complexes. ► N,N′-bound complexes undergo facile reversible one-electron reduction. ► O,O′-bound complex has reversible one-electron oxidation at low temperature. ► EPR, UV–Vis and DFT results indicate CO groups electronically important.

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.