Mixed-Valent Ruthenocene-Vinylruthenium Conjugates: Valence Delocalization Despite Chemically Different Redox Sites.

Ruthenocene-vinylruthenium conjugates Rc/Rc*-CH═CH-Ru(CO)(L)(P iPr3)2 (Rc = (η5-C5H5)Ru(η5-C5H4); Rc* = (η5-C5Me5)Ru(η5-C5H4); L = Cl or κ O, O' -acetylacetonato) have been prepared and investigated in their neutral, mono-, and dioxidized states by cyclic voltammetry, IR and UV/vis/NIR spectroelectrochemistry, and EPR spectroscopy. Their corresponding radical cations are (almost) completely delocalized mixed-valent systems as indicated by the low half-widths, the absence of solvatochromism, and the low-energy cutoff of their IVCT bands in the near-infrared (NIR) and their IR and EPR spectroscopic signatures. The degree of electronic coupling even exceeds that of their ferrocene analogs despite comparable differences between the intrinsic half-wave potentials of the vinylruthenium and the metallocenyl entities and substantially smaller half-wave potential splittings, Δ E1/2, in the ruthenocene congeners. All experimental results are backed by quantum chemical calculations.

Cobaltocenylidene: A Mesoionic Metalloceno Carbene, Stabilized in a Gold(III) Complex.

Oxidative addition of cobaltoceniumdiazonium bis(hexafluoridophosphate) with (pseudo)halide aurates gave gold(III) complexes containing zwitterionic cobaltoceniumide as a ligand. Its selenium derivative, cobaltoceniumselenolate, was obtained by an electrophilic aromatic substitution reaction of iodocobaltocenium iodide with Na2 Se. Spectroscopic and structural data in combination with DFT calculations showed that this cobaltocenylidene species is a mesoionic carbene quite different from common N-heterocyclic carbenes. Its ligand properties (TEP, singlet-triplet gap, nucleophilicity, π-acidity, Brønsted basicity) are in part comparable to those of cyclic (amino)(alkyl/aryl)carbenes. Electrochemistry data showed that the mesoionic cobaltoceniumides are more electron-rich than their parent ferrocenes. The reversible reduction of the tricyanido gold complex appears 50 mV negative of the cobaltocenium/cobaltocene couple, whereas that of the selenide derivative is shifted cathodically by 550 mV.

Manipulation and Assessment of Charge and Spin Delocalization in Mixed-Valent Triarylamine-Vinylruthenium Conjugates.

Triarylamine-vinylruthenium conjugates (4-RC6H4)2N{C6H4-4-CH═CHRu(CO)Cl(PiPr3)2}, with R = CHO (1-CHO), C(═O)Me (1-Ac), COOMe (1-E), and Me (1-Me), have been prepared and investigated in their neutral, mono- and dioxidized states by cyclic voltammetry, IR, and UV/vis/near-infrared spectroelectrochemistry, electron paramagnetic resonance spectroscopy, and quantum-chemical calculations. Electron-withdrawing substituents at the triarylamine moiety shift the charge and spin density toward the more electron-rich vinylruthenium site in comparison to the 4-OMe-substituted triarylamine-vinylruthenium conjugate 1-OMe. A more asymmetric charge distribution changes the intense vibrationally structured intervalence charge-transfer (IVCT) band of completely delocalized, mixed-valent (MV) 1-OMe+ to a weaker, highly asymmetric, nonsolvatochromic band with significantly smaller bandwidth at the low-energy side. The temperature dependence of the IVCT band of the formyl derivative 1-CHO+ proves that vibrational coupling of the IVCT transition to a symmetrical vibration is the underlying reason for band skewing. All of our results indicate that the MV radical cations remain electronically strongly coupled despite an increasingly stronger bias of the highest occupied molecular orbital to the vinylruthenium entity. Moreover, the dications of these complexes were found to be paramagnetic, which makes them rare examples of compounds that combine strong electronic coupling in the cationic MV state with paramagnetism of the dications.

Ferrocene- and Biferrocene-Containing Macrocycles towards Single-Molecule Electronics.

Cyclic multiredox centered systems are currently of great interest, with new compounds being reported and developments made in understanding their behavior. Efficient, elegant, and high-yielding (for macrocyclic species) synthetic routes to two novel alkynyl-conjugated multiple ferrocene- and biferrocene-containing cyclic compounds are presented. The electronic interactions between the individual ferrocene units have been investigated through electrochemistry, spectroelectrochemistry, density functional theory (DFT), and crystallography to understand the effect of cyclization on the electronic properties and structure.

Redox-Rich Metallocene Tetrazene Complexes: Synthesis, Structure, Electrochemistry, and Catalysis.

Thermal or photochemical metal-centered cycloaddition reactions of azidocobaltocenium hexafluoridophosphate or azidoferrocene with (cyclooctadiene)(cyclopentadienyl)cobalt(I) afforded the first metallocenyl-substituted tetrazene cyclopentadienyl cobalt complexes together with azocobaltocenium or azoferrocene as side products. The trimetallic CpCo compounds are highly conjugated, colored, and redox-active metallo-aromatic compounds, as shown by their spectroscopic, structural, and electrochemical properties. The CpCo-tetrazenido complex with two terminally appended cobaltocene units catalyzes electrochemical proton reduction from acetic acid at a mild overpotential (0.35 V). Replacing cobaltocene with ferrocene moieties rendered the complex inactive toward catalysis.