Homo- and heterobimetallic complexes of ruthenium(II) and osmium(II) with pyrazole-3,5-dicarboxylic acid and 2,2′-bipyridine as co-ligands. Synthesis, structure, stereochemistry, spectroscopy and redox activities

Mononuclear, homodinuclear and heterodinuclear ruthenium(II) and osmium(II) complexes of composition [(bpy)2Ru(H2pzdc)](ClO4)·3H2O (1), [(bpy)2Os(H2pzdc)](ClO4)·3H2O (2), [(bpy)2Ru(pzdc)Ru(bpy)2](ClO4)·H2O (3), [(bpy)2Os(pzdc)Os(bpy)2](ClO4)·H2O (4) and [(bpy)2Ru(pzdc)Os(bpy)2](ClO4)·H2O (5) have been synthesized with pyrazole-3,5-dicarboxylic acid (H3pzdc) and 2,2′-bipyridine (bpy). Complexes 3–5 isolated solely in the homochiral forms have been characterized by their 1H and {1H1H} COSY NMR spectra. The crystal structures determined for 3 and 5 have revealed that the two compounds are isostructural and the diruthenium compound 3 is present in the rac(ΛΛ/ΔΔ) form, while the osmiumruthenium compound 5 is present as the enantiomeric pair (ΛΛ and ΔΔ). The metal-centered oxidation and bipyridine-centred reduction in 1–5 take place reversibly. As compared with the diruthenium complex 3, the stepwise oxidation of the metal centers in the diosmium complex 4 take place at less positive potentials by 0.45 V. The equilibrium constants for the comproportionation reaction MIIMII+MIIIMIII⇌2MIIMIII are found to be almost equal (ca. 6×102) for the diruthenium and diosmium species. From pH-dependent redox potential measurements the acid dissociation constants for the mononuclear complexes 1 and 2 have been determined. The luminescence spectral behavior of the complexes have been investigated in methanol–ethanol (1:4) solution (at 300 K) and in glassy state (77 K) and the quantum yields for the ruthenium(II) complexes are reported.