Metal-to-Ligand Charge Transfer Induced Valence Tautomeric Forms of Non-Innocent 2,2'-Azobis(benzothiazole) in Ruthenium Frameworks.

The impact of metal-to-ligand charge transfer towards the redox noninnocence of 2,2'-azobis(benzothiazole) (abbt) has been highlighted on coordination to {RuII (acac)2 } (acac=2,4-pentanedionato). It led to the authentication of a series of mononuclear and dinuclear complexes incorporating variable oxidation states of abbt (abbt0/.-/2- ). Mononuclear 1 was identified as [RuIII (abbt.- )], a MLCT excited state of [RuII (abbt)]. Dinuclear 2 was however recognized as two discrete redox isomers: (i) radical bridged mixed-valent meso-[Ru2.5 (μ-abbt.- )Ru2.5 ] (2a) and (ii) dianionic ligand bridged isovalent meso-[RuIII (μ-abbt2- )RuIII ] (2b), demonstrating unprecedented structural confirmation of valence tautomerism in azo-based ligand systems. A crystal structure of [2]ClO4 validated the formation of [RuIII (μ-abbt.- ) RuIII ]ClO4 . Analysis of electronic structural forms of 1 and 2 in accessible redox states via spectroelectrochemistry and DFT revealed their electron reservoir feature.