A “Pretender” Croconate-Bridged Macrocyclic Tetraruthenium Complex: Sizable Redox Potential Splittings despite Electronically Insulated Divinylphenylene Diruthenium Entities

Careful optimization of the reaction conditions provided access to the particularly small tetraruthenium macrocycle (2)Ru(2)Ph-Croc, which is composed out of two redox-active divinylphenylene-bridged diruthenium entities {Ru}-1,4-CH=CH-C(6)H(4)-CH=CH-{Ru} (Ru(2)Ph; {Ru} = Ru(CO)Cl(P(i)Pr(3))(2)) and two likewise redox-active and potentially non-innocent croconate linkers. According to single X-ray diffraction analysis, the central cavity of (2)Ru(2)Ph-Croc is shielded by the bulky P(i)Pr(3) ligands, which come into close contact. Cyclic voltammetry revealed two pairs of split anodic waves in the weakly ion pairing CH(2)Cl(2)/NBu(4)BAr(F)(24) (BAr(F)(24) = [B{C(6)H(3)(CF(3))(2)-3,5}(4)](−) electrolyte, while the third and fourth waves fall together in CH(2)Cl(2)/NBu(4)PF(6). The various oxidized forms were electrogenerated and scrutinized by IR and UV/Vis/NIR spectroscopy. This allowed us to assign the individual oxidations to the metal-organic Ru(2)Ph entities within (2)Ru(2)Ph-Croc, while the croconate ligands remain largely uninvolved. The lack of specific NIR bands that could be assigned to intervalence charge transfer (IVCT) in the mono- and trications indicates that these mixed-valent species are strictly charge-localized. (2)Ru(2)Ph-Croc is hence an exemplary case, where stepwise IR band shifts and quite sizable redox splittings between consecutive one-electron oxidations would, on first sight, point to electronic coupling, but are exclusively due to electrostatic and inductive effects. This makes (2)Ru(2)Ph-Croc a true “pretender”.