Tetranuclear polybipyridyl complexes of RuII and MnII, their electro- and photo-induced transformation into di-µ-oxo MnIIIMnIV hexanuclear complexes

Abstract

Three heterotetranuclear complexes, [{Ru^II(bpy)₂(L_n)}₃Mn^II]⁸⁺ (bpy = 2,2′-bipyridine, n = 2, 4, 6), in which a Mn^II-tris-bipyridine-like centre is covalently linked to three Ru^II–tris-bipyridine-like moieties using bridging bis-bipyridine L_n ligands, have been synthesised and characterised. The electrochemical, photophysical and photochemical properties of these complexes have been investigated in CH₃CN. The cyclic voltammograms of the three complexes exhibit two successive very close one-electron metal-centred oxidation processes in the positive potential region. The first, which is irreversible, corresponds to the Mn^II/Mn^III redox system (E_pa ≈0.82 V vs Ag/Ag⁺ 0.01 M in CH₃CN–0.1 M Bu₄NClO₄), whereas the second which is, reversible, is associated with the Ru^II/Ru^III redox couple (E_1/2 ≈0.91 V). In the negative potential region, three successive reversible four electron systems are observed, corresponding to ligand-based reduction processes. The three stable dimeric oxidized forms of the complexes, [Mn₂^III,IVO₂{Ru^II(bpy)₂(L_n)}₄]¹¹⁺, [Mn₂^IV,IVO₂{Ru^II(bpy)₂(L_n)}₄]¹²⁺ and [Mn₂^IV,IVO₂{Ru^III(bpy)₂(L_n)}₄]¹⁶⁺ are obtained in fairly good yields by sequential electrolyses after consumption of respectively 1.5, 0.5 and 3 electrons per molecule of initial tetranuclear complexes. The formation of the di-μ-oxo binuclear complexes are the result of the instability of the {[Ru^II(bpy)₂(L_n)]₃Mn^III}⁹⁺ species, which react with residual water, via a disproportionation reaction and the release of one ligand, [Ru^II(bpy)₂(L_n)]²⁺. A quantitative yield can be obtained for these reactions if the electrochemical oxidations are performed in the presence of an added external base like 2,6-dimethylpyridine. Photophysical properties of these compounds have been investigated showing that the luminescence of the Ru^II-tris-bipyridine-like moieties is little affected by the presence of manganese within the tetranuclear complexes. A slight quenching of the excited states of the ruthenium moieties, which occurs by an intramolecular process, has been observed. Measurements made at low concentration (<1 × 10⁻⁵ M) indicate that some decoordination of Mn²⁺ arises in 1a–c. These measurements allow the calculation of the association constants for these complexes. Finally, photoinduced oxidation of the tetranuclear complexes has been performed by continuous photolysis experiments in the presence of a large excess of a diazonium salt, acting as a sacrificial oxidant. The three successive oxidation processes, Mn^II → Mn^IIIMn^IV, Mn^IIIMn^IV → Mn^IVMn^IV and Ru^II → Ru^III are thus obtained, the addition of 2,6-dimethylpyridine in the medium giving an essentially quantitative yield for the two first photo-induced oxidation steps as found for electrochemical oxidation.