Inner-Sphere Electron Transfer Induced Reversible Electron Reservoir Feature of Azoheteroarene Bridged Diruthenium Frameworks.

This article demonstrates the stabilization of ground- and redox-induced metal-to-ligand charge transfer excited states on coordination of azo-coupled bmpd(L4) [bmpd = ( E )-1,2-bis(1-methyl-1 H -pyrazol-3-yl)diazene; L4 = -N═N-] to the electron-rich {Ru(acac) 2 } (acac = acetylacetonate) unit in mononuclear Ru II (acac) 2 (L4) ( 1 ) and diastereomeric dinuclear (acac) 2 Ru 2.5 (μ-L4 •- )Ru 2.5 (acac) 2 [ rac , ΔΔ/ΛΛ ( 2a )/ meso , ΔΛ ( 2b )] complexes, respectively. It also develops further one-step intramolecular electron transfer induced L4 •- bridged isovalent higher analogue [(acac) 2 Ru III (μ-L4 •- )Ru III (acac) 2 ]ClO 4 in diastereomeric forms, rac -[ 2a ]ClO 4 / meso -[ 2b ]ClO 4 . On the contrary, under identical reaction conditions electronically and sterically permuted bimpd [L5, ( E )-1,2-bis(4-iodo-1-methyl-1 H -pyrazol-3-yl)diazene)] delivered mononuclear Ru II (acac) 2 (L5) ( 3 ) as an exclusive product. Further, the generation of unprecedented heterotrinuclear complex [(acac) 2 Ru II (μ-L4)Ag I (μ-L4)Ru II (acac) 2 ]ClO 4 ([ 4 ]ClO 4 ) involving unreduced L4 via the reaction of 1 and AgClO 4 revealed the absence of any inner-sphere electron transfer (IET) as in precursor 1 , which in turn reaffirmed an IET (at the interface of electron-rich Ru(acac) 2 and acceptor L4) mediated stabilization of 2 . Structural authentication of the complexes with special reference to the tunable azo distance (N═N, N-N •- , N-N 2- ) of L and their spectro-electrochemical events in accessible redox states including the reversible electron reservoir feature of 2 → 2 + /2 + → 2 were evaluated in conjunction with density functional theory/time-dependent density functional theory calculations. The varying extent of IET as a function of heteroaromatics appended to the azo group of L (L1 = abpy = 2,2'-azobipyridine, L2 = abbt = 2,2'-azobis(benzothiazole), L3 = abim = azobis(1-methylbenzimidazole), L4 and L5, Schemes 1 & 2) in the Ru(acac) 2 -derived respective molecular setup has been addressed.