4,4‘-Dithiodipyridine as a Bridging Ligand in Osmium and Ruthenium Complexes:  The Electron Conductor Ability of the −S−S− Bridge

• Published in: Inorganic chemistry Vol. 42; no. 21; pp. 6898 - 6906
• Main Authors: Silva, Welter C, Lima, Joacy B, Moreira, Icaro S, Neto, Antônio M, Gandra, Flávio C. G, Ferreira, Antônio G, McGarvey, Bruce R, Franco, Douglas W
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 20.10.2003
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic034630o

The compounds [Ru(NH3)5(dtdp)](TFMS)3, [Os(NH3)5(dtdp)](TFMS)3, [(NH3)5Os(dtdp)Os(NH3)5](TFMS)6, [(NH3)5Os(dtdp)Ru(NH3)5](TFMS)3(PF6)2, and [(NH3)5Os(dtdp)Fe(CN)5] (dtdp = 4,4‘-dithiodipyridine, TFMS = trifluoromethanesulfonate) have been synthesized and characterized by elemental analysis, cyclic voltammetry, electronic, vibrational, EPR, and 1H NMR spectroscopies. Changes in the electronic and voltammetric spectra of the ion complex [Os(NH3)5(dtdp)]3+ as a function of the solution pH enable us to calculate the pK a for the [Os(NH3)5(dtdpH)]4+ and [Os(NH3)5(dtdpH)]3+ acids as 3.5 and 5.5, respectively. The comparison of the above pK a data with that for the free ligand (pK 1 = 4.8) provides evidence for the −S−S− bridge efficiency as an electron conductor between the two pyridine rings. The symmetric complex, [(NH3)5Os(dtdp)Os(NH3)5]6+, is found to exist in two geometric forms, and the most abundant form (most probably trans) has a strong conductivity through the −S−S− bridge, as is shown by EPR, which finds it to have an S = 1 spin state with a spin−spin interaction parameter of 150−200 G both in the solid sate and in frozen solution. Further the NMR of the same complex shows a large displacement of unpaired spin into the π orbitals of the dttp ligand relative to that found in [Os(NH3)5(dtdp)]3+. The comproportionation constant, K c = 2.0 × 105, for the equilibrium equation [OsIIOsII] + [OsIIIOsIII] ⇌ 2[OsIIOsIII] and the near-infrared band energy for the mixed-valence species (MMCT), [(NH3)5Os(dtdp)Os(NH3)5]5+ (λMMCT = 1665 nm, ε = 3.5 × 103 M-1 cm-1, Δν̄1/2 = 3.7 × 103 cm-1, α = 0.13, and H AB = 7.8 × 102 cm-1), are quite indicative of strong electron delocalization between the two osmium centers. The electrochemical and spectroscopic data for the unsymmetrical binuclear complexes [(NH3)5OsIII(dtdp)RuII(NH3)5]5+ (λMMCT = 965 nm, ε = 2.2 × 102 M-1 cm-1, Δν̄1/2 = 3.0 × 103 cm-1, and H AB = 2.2 × 102 cm-1) and [(NH3)5OsIII(dtdp)FeII(CN)5] (λMMCT = 790 nm, ε = 7.5 × 10 M-1 cm-1, Δν̄1/2 = 5.4 × 103 cm-1, and H AB = 2.0 × 102 cm-1) also suggest a considerable electron delocalization through the S−S bridge. As indicated by a comparison of K c and energy of the MMCT process in the iron, ruthenium, and osmium complexes, the electron delocalization between the two metal centers increases in the following order:  Fe < Ru < Os.