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 azocoupled bmpd(L4) [bmpd = (E)-1,2-bis(1-methyl-1H-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(mu-L4(center dot-))Ru-2.5(acac)(2) [rac, Delta Delta/Lambda Lambda (2a)Imeso, Delta Lambda (2b)] complexes, respectively. It also develops further one-step intramolecular electron transfer induced L4(center dot-) bridged isovalent higher analogue [(acac)(2)Ru-III(mu-L4(center dot-))Ru-III(acac)(2)]ClO4 in diastereomeric forms, rac-[2a]ClO4/meso-[2b]ClO4. On the contrary, under identical reaction conditions electronically and sterically permuted bimpd [L5, (E)- 1,2-bis(4-iodo- 1-methyl- 1H-pyrazol- 3-y1) diazene)] delivered mononuclear Ru-II(acac)(2)(L5) (3) as an exclusive product. Further, the generation of unprecedented heterotrinuclear complex [(acac)(2)Ru-II(mu-L4)Ag-I(mu-L4)Ru-II(acac)(2)]ClO4 ([4]ClO4) involving unreduced L4 via the reaction of 1 and AgClO4 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 center dot-, 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.

Automated summary

This article investigates the stabilization of metal-to-ligand charge transfer excited states on coordination of azocoupled bmpd(L4) to Ru(acac)(2) unit. The study includes mononuclear Ru-II(acac)(2)(L4) and diastereomeric dinuclear (acac)(2)Ru-2.5(mu-L4(center dot-))Ru-2.5(acac)(2) complexes, as well as the development of L4(center dot-) bridged isovalent higher analogue and the exclusive product formed by electronically and sterically permuted bimpd.