Constitutional Isomers of Macrocyclic Tetraruthenium Complexes with Vastly Different Spectroscopic and Electrochemical Properties

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

Three-dimensional molecular barrels Ru₆-4 and Ru₆-5 were synthesized in high yields from dinuclear ruthenium-vinyl clamps and tritopic triphenylamine-derived carboxylate linkers and characterized by multinuclear NMR spectroscopy including ¹H-¹H COSY and ¹H DOSY measurements, high-resolution electrospray ionization mass spectrometry, and X-ray crystallography. The metal frameworks of the cages adopt the shape of twisted trigonal prisms, and they crystallize as racemic mixtures of interdigitating Δ- and Λ-enantiomers with a tight columnar packing in Ru₆-4. Electrochemical studies and redox titrations revealed that the cages are able to release up to 11 electrons on the voltammetric timescale and that their cage structures persist up to the hexacation level. IR and UV-vis-near-infrared spectroelectrochemical studies confirm substituent-dependent intramolecular electronic communication within the π-conjugated 1,3-divinylphenylene backbone in the tricationic states, where all three divinylphenylene-bridged diruthenium clamps are present in mixed-valent radical cation states. The formation of 1:3 charge-transfer salts with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane as the electron acceptor is also demonstrated.

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

We report on the tetraruthenium macrocycles Ru4-5 and -6 with a π-conjugated pyrene-appended 5,8-divinylquinoxaline ligand and either isophthalate or thiophenyl-2,5-dicarboxylate linkers and their charge-transfer salts formed by oxidation with two equivalents of F4TCNQ. Both macrocyclic complexes were characterized by NMR spectroscopy, mass spectrometry, cyclic and square-wave voltammetry, and by IR, UV–vis–NIR, and EPR spectroscopy in their various oxidation states.

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 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/NBu₄BAr^F₂₄ (BAr^F₂₄ = [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”.

Redox-Induced Hydrogen Bond Reorientation Mimicking Electronic Coupling in Mixed-Valent Diruthenium and Macrocyclic Tetraruthenium Complexes

We present the coordination-driven self-assembly of three tetranuclear metallacycles containing intracyclic NH₂, OH, or OMe functionalities through the combination of various isophthalic acid building blocks with a divinylphenylene diruthenium complex. All new complexes of this study were characterized by means of nuclear magnetic resonance spectroscopy, ultrahigh-resolution ESI mass spectrometry, cyclic and square wave voltammetry and, in two cases, X-ray diffraction. The hydroxy functionalized macrocycle 4-BOH and the corresponding half-cycle 2-OH stand out, as their intracyclic OH···O hydrogen bonds stabilize their mixed-valent one- (2-OH, 4-BOH) and three-electron-oxidized states (4-BOH). Despite sizable redox splittings between all one-electron waves, the mixed-valent monocations and trications do not exhibit any intervalence charge-transfer band, assignable to through-bond electronic coupling, but nevertheless display distinct IR band shifts of their charge-sensitive Ru(CO) tags. We ascribe these seemingly contradicting observations to a redox-induced shuffling of the OH···O hydrogen bond(s) to the remaining, more electron-rich, reduced redox site.

Ring size matters: supramolecular isomerism in self-assembled redox-active tetra- and hexaruthenium macrocycles

Self-assembly of a divinylthiophene-bridged diruthenium complex with 2,5-furandicarboxylate leads to the formation of two macrocyclic structures that differ solely with regard to their respective nuclearities. Both supramolecular isomers were fully characterized and the conversion of the hexa- to the tetraruthenium macrocycle was followed by NMR monitoring.

Tetraruthenium Metallamacrocycles with Potentially Coordinating Appended Functionalities

We present four new tetraruthenium macrocycles built from two 1,4-divinylphenylene diruthenium and two isophthalic acid building blocks with peripheral, potentially mono- or tridentate donor functions attached to the isophthalic linkers. These macrocycles are characterized by multinuclear NMR spectroscopy, mass spectrometry and, in the case of the thioacetyl-appended complex 4, by X-ray crystallography. Cyclic and square wave voltammetry establish that the macrocycles can be oxidized in four consecutive redox steps that come as two pairs of two closely spaced one-electron waves. Spectroscopic changes observed during IR and UV/Vis/NIR spectroelectrochemical experiments (NIR = near infrared) show that the isophthalate linkers insulate the electroactive divinylphenylene diruthenium moieties against each other. The macrocycles exhibit nevertheless pronounced polyelectrochromism with highly intense absorptions in the Vis (2+/4+ states) and the NIR (2+ states) with extinction coefficients of up to >100,000 M−1·cm−1. The strong absorptivity enhancement with respect to the individual divinylphenylene diruthenium building blocks is attributed to conformational restrictions imposed by the macrocycle backbone. Moreover, the di- and tetracations of these macrocycles are paramagnetic as revealed by EPR spectroscopy.