Flint, Kate L., Collins, J. Grant, Bradley, Siobhan J., Smith, Trevor A., Sumby, Christopher J., and Keene, F. Richard (2019) Synthesis and characterisation of helicate and mesocate forms of a double-stranded diruthenium(II) complex of a di(terpyridine) ligand. Australian Journal of Chemistry, 72 (10). pp. 762-768.

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Abstract

The diruthenium(II) complex involving the di(terpyridine) ligand 1,2-bis{5-(5′′-methyl-2,2′:6′,2′′-terpyridinyl)}ethane was synthesised by heating an equimolar ratio of RuCl3 and the ligand under reflux conditions in ethylene glycol for 3 days, realising double-stranded helicate and mesocate forms which were chromatographically separated. The two species were obtained in relatively low yield (each ~7-9%) from the reaction mixture. X-ray structural studies revealed differences in the cavity sizes of the two structures, with the helicate structure having a significantly smaller cavity. Furthermore, the helicate and mesocate structures pack with notably different arrangements of the structures with the helicate having large solvent and anion filled pores. 1D/2D NMR studies revealed rigidity in the mesocate structure relative to that of the helicate, such that the –CH2CH2- signal was split in the former and appeared as a singlet in the latter. In a manner analogous to the behaviour of the parent [Ru(tpy)2]2+ coordination moiety (tpy = 2,2′:6′,2′′-terpyridine), photophysical studies indicated that both the helicate and mesocate forms were non-emissive at ~610 nm at room temperature, but at 77 K in n-butyronitrile, both isomers showed emission at ~610 nm (λex = 472 nm). However, the temporal emission characteristics were very different: time-resolved studies showed the emission of the helicate species decayed with a dominant emission lifetime of ~10 μs (similar to the emissive properties of free [Ru(tpy)2]2+ under the same conditions), whereas for the mesocate the emission lifetime was at least three orders of magnitude lower (~ 4 ns).

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