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

Abstract

Careful optimization of the reaction conditions provided access to the particularly small tetraruthenium macrocycle 2Ru2Ph-Croc, which is composed out of two redox-active divinylphenylene-bridged diruthenium entities {Ru}-1,4-CH=CH-C6H4-CH=CH-{Ru} (Ru2Ph; {Ru} = Ru(CO)Cl(PiPr3)2) and two likewise redox-active and potentially non-innocent croconate linkers. According to single X-ray diffraction analysis, the central cavity of 2Ru2Ph-Croc is shielded by the bulky PiPr3 ligands, which come into close contact. Cyclic voltammetry revealed two pairs of split anodic waves in the weakly ion pairing CH2Cl2/NBu4BArF24 (BArF24 = [B{C6H3(CF3)2-3,5}4]- electrolyte, while the third and fourth waves fall together in CH2Cl2 / NBu4PF6. 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 Ru2Ph entities within 2Ru2Ph-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. 2Ru2Ph-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 2Ru2Ph-Croc a true “pretender”.

Reversible Multielectron Release from Redox-Active Three-Dimensional Molecular Barrels with Ruthenium-Alkenyl Moieties

“…53−60 The selective transformations of intermittently formed dicarboxylate-bridged tetra-or hexaruthenium metallamacrocycles into larger octa-or smaller tetraruthenium ones on prolonged warming provide recent examples in ruthenium-alkenyl chemistry that are related to the present work. 45,46,61 Attempts to obtain hexaruthenium cages by reacting the m-divinylphenylene-bridged diruthenium clamps Ru 2 -2 or Ru 2 -3 of Scheme 1 with the deprotonated planar 62, 63 1,3,5-trisubstituted triazinederived linker TTBA of Figure 1 were not met by success. To our delight, their reaction with the conformationally more flexible, propeller-shaped triphenylamine-4,4′,4″-tricarboxylate, which was obtained from the deprotonation of the parent tricarboxylic acid 1 with potassium carbonate, at 40 °C in methanol/dichloromethane yielded, after 30 h, hexaruthenium cages Ru 6 -4 or Ru 6 -5 in yields of 90 or 87%.…”

Tetraruthenium Macrocycles with Laterally Extended Bis(alkenyl)quinoxaline Ligands and Their F4TCNQ•− Salts

“…The reversible formation and degradation of oligonuclear and hexaruthenium metallamacrocycles from initially formed mixtures of complexes with varying nuclearities [15][16][17][18][19]. The reversible formation and degradation of oligonuclear complexes along the way to the thermodynamically most stable metallamacrocyclic or cage structure are highlighted by the selective transformation of intermittently formed tetra-or hexaruthenium metallamacrocycles into larger octa-or smaller tetraruthenium ones on prolonged warming, which is strongly accelerated in the presence of an excess of the respective carboxylate ligand [18][19][20]. Similar phenomena en route to supramolecular coordination compounds (SCCs) and interconversions between different types of defined metallamacrocyclic or cage architectures that originate from reversible ligand binding were also described for other complex/ligand combinations [21][22][23][24][25][26][27][28][29][30].…”