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)(2)Ru(H(2)pzdc)](ClO4).3H(2)O (1), [(bpy)(2)Os(H(2)Pzdc)](ClO4).3H(2)O (2), [(bpy)(2)Ru(pzdc)Ru(bpy)(2)](ClO4).H2O (3), [(bpy)(2)Os(pzdc)Os(bpy)(2)](ClO4).H2O (4) and [(bpy)(2)Ru(pzdc)Os(bpy)(2)](ClO4).H2O (5) have been synthesized with pyrazole-3,5-dicarboxylic acid (H(3)pzdc) and 2,2'-bipyridine (bpy). Complexes 3-5 isolated solely in the homochiral forms have been characterized by their H-1 and {H-1-H-1} 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(LambdaLambda/DeltaDelta) form, while the osmium-ruthenium compound 5 is present as the enantiomeric pair (LambdaLambda and DeltaDelta). 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 (MMII)-M-II+(MMIII)-M-III reversible arrow 2M(II)M(III) are found to be almost equal (ca. 6 x 10(2)) 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. (C) 2002 Elsevier Science B.V. All rights reserved.