Influence of Quinoidal Distortion on the Electronic Properties of Oxidized Divinylarylene-Bridged Diruthenium Complexes

We present four new divinylarylene-bridged diruthenium complexes of the type {Ru­(CO)­Cl­(P i Pr3)2}2(μ-CHCH–Ar–CHCH) with naphthalene-1,4-diyl (Ru 2 NA), 2,1,3-benzothiadiazole (Ru 2 BTD), bis­(benzothiadiazolyl)­ethene (Ru 2 v BTD 2 ), or bis­(benzothiadiazolyl)­ethyne (Ru 2 e BTD 2 ) as the arylene units. The radical cations of the complexes with shorter arylene bridges are intrinsically valence-delocalized on the IR time scale. In contrast to phenylene-bridged [{Ru­(CO)­Cl­(P i Pr3)2}2(μ-CHCH–C6H4-1,4-CHCH)]2+ (Ru 2 -p-phen 2+ ), the dioxidized forms of complexes Ru 2 BTD and Ru 2 NA are electron paramagnetic resonance (EPR) silent. Replacing the E-stilbenediyl linker of the complex {Ru­(CO)­Cl­(P i Pr3)2}2(μ-CHCH–C6H4–CHCH–C6H4–CHCH) (Ru 2 v p-phen 2 ) by vBTD2 or eBTD2 enhances electronic coupling and charge delocalization of their one-electron-oxidized mixed-valent forms. While the latter radical cations are still charge-localized species on the IR time scale, they are fully delocalized on the slower EPR time scale. As a token of a quinoidally distorted bridge, Ru 2 e BTD 2 2+ features a characteristic IR stretch of a cumulenic CCCC increment. For compound Ru 2 v BTD 2 , rearrangement to a quinoidal structure is manifested through potential inversion (i.e., the half-wave potential of the second oxidation is lower than that of the 0/1+ redox couple) in the CH2Cl2/NBu4BArF electrolyte. The BTD complexes are intensely colored in their neutral states and exhibit strong NIR absorptions of their radical cations as well as of their dioxidized forms.