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

Abstract

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 M^IIM^II+M^IIIM^III⇔2M^IIM^III 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.