Multiple one-electron oxidation and reduction of trinuclear bis(2,4-pentanedionato)ruthenium complexes with substituted diquinoxalino[2,3-a:2′,3′-c]phenazine ligands

Add time:09/29/2019    Source:infona.pl

The complexes (μ 3 -L 1 /L 2 )[Ru(acac) 2 ] 3 , acac − =2,4-pentanedionato, L 1 =2,3,8,9,14,15-hexachlorodiquinoxalino[2,3-a:2′,3′-c]phenazine and L 2 =2,3,8,9,14,15- hexamethyldiquinoxalino[2,3-a:2′,3′-c]phenazine, undergo stepwise one-electron oxidation involving a total of three electrons and stepwise one-electron reduction with three (L 2 ) or four electrons (L 1 ). All reversibly accessible states were characterized by UV–Vis–NIR spectroelectrochemistry. Oxidation leads to mixed-valent intermediates {(μ 3 -L)[Ru(acac) 2 ] 3 } + and {(μ 3 -L)[Ru(acac) 2 ] 3 } 2+ of which the Ru III Ru II Ru II combinations exhibit higher comproportionation constants K c than the Ru III Ru III Ru II states – in contrast to a previous report for the unsubstituted parent systems {(μ 3 -L 3 )[Ru(acac) 2 ] 3 } +/2+ , L 3 =diquinoxalino[2,3-a:2′,3′-c]phenazine. No conspicuous inter-valence charge transfer absorptions were observed for the mixed-valent intermediates in the visible to near-infrared regions. The monocations and monoanions were characterized by EPR spectroscopy, revealing rhombic ruthenium(III) type signals for the former. Electron addition produces ruthenium(II) complexes of the reduced forms of the ligands L, a high resolution EPR spectrum with 14 N and 35,37 Cl hyperfine coupling and negligible g anisotropy was found for {(μ 3 -L 1 )[Ru(acac) 2 ] 3 } − . DFT calculations of (μ 3 -L 1 )[Ru(acac) 2 ] 3 confirm several ligand-centered low-lying unoccupied MOs for reduction and several metal-based high-lying occupied MOs for electron withdrawal, resulting in low-energy metal-to-ligand charge transfer (MLCT) transitions.

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