Self-assembly of discrete metallamacrocycles employing half-sandwich octahedral diruthenium(II) building units and imidazole-based ligands

Equimolar combination of a series of binuclear half-sandwich p-cymene ruthenium(II) building units [Ru2(μ-η4-C2O4)(MeOH)2(η6-p-cymene)2](OTf)2[1a](OTf)2, [Ru2(μ-η4-N,N′-diphenyloxamidato)(MeOH)2(η6-p-cymene)2](OTf)2[1b](OTf)2 and [Ru2(μ-η4-C6H2O4)(MeOH)2(η6-p-cymene)2](OTf)2[1c](OTf)2 separately with imidazole-based ditopic ligands (L1–L2) in methanol yielded a series of tetranuclear metallamacrocycles [2–7](OTf)4, respectively [L1 = 1,4-bis(imidazole-1-yl)benzene; L2 = 4,4′-bis(imidazole-1-yl)biphenyl; OTf− = O3SCF3−]. Similarly, the reaction of [Ru2(μ-η4-C2O4)(MeOH)2(η6-p-cymene)2](OTf)2[1a](OTf)2 with a triazine-based tritopic ligand 1,3,5-tris(imidazole-1-yl)triazine (L3) in 3:2 M ratio afforded an unexpected tetranuclear macrocycle [8](OTf)4 instead of an expected trigonal prismatic cage [8a](OTf)6. All the self-assembled macrocycles [2−8](OTf)4 were isolated in moderate to high yields and were fully characterized by multinuclear [1H, 19F] NMR, IR and electrospray ionization mass spectrometry (ESI-MS). In addition, X-ray diffraction study on the single crystals of [3](OTf)4 and [8](OTf)4 also indicated the formation [2 + 2] self-assembled macrocycles. Despite the possibility of formation of different conformational isomeric macrocycles (syn- and anti) and polymeric product due to free rotation of ligand sites of imidazole linkers, the selective formation of single conformational isomer (anti) as the only product is quite interesting. Furthermore, the photo- and electrochemical properties of these assemblies have been studied using UV/Vis absorption and cyclic voltammetry analysis.