Synthesis and Characterisation of Helicate and Mesocate Forms of a Double-Stranded Diruthenium(ii) Complex of a Di(terpyridine) Ligand

A 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 similar to 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 forms 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 similar to 610 nm at room temperature, but at 77 K in n-butyronitrile, both isomers showed emission at similar to 610 nm (lambda(ex) 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 similar to 10 mu s (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 (similar to 4 ns).