Inter- and Intramolecular Electron Transfer in Copper Complexes: Electronic Entatic State with Redox-Active Guanidine Ligands.

Fast and efficient electron transfer in blue copper proteins is realized by a structural harmonization between the CuI and CuII complex pair ("entatic state" model). Herein, we present now a CuI /CuII complex pair with redox-active guanidine ligands showing almost perfect match between both redox states. By modifying the ligand electron donor strength, the redox chemistry of the copper complex can be controlled to be either metal-centered or to cross the borderline to ligand-centered. This work is the first systematic study of complexes with redox-active ligands within the concept of the entatic state.

Homo- and Heterobinuclear Cu and Pd Complexes with a Bridging Redox-Active Bisguanidino-Substituted Dioxolene Ligand: Electronic Structure and Metal-Ligand Electron-Transfer.

A new redox-active 4,5-bisguanidino-substituted o-benzoquinone ligand L is synthesized, which allows rational access to heterobinuclear complexes through the sequential coordination of two metals. In the examples discussed in this work, mononuclear Cu and Pd complexes are prepared in a first coordination step, and these complexes are then used as precursors to homobinuclear [CuII -L0 -CuII ] and heterobinuclear [PdII -L0 -CuII ] complexes. In the heterobinuclear complex, the PdII is coordinated by the softer bisguanidine side of L and the CuII by the harder dioxolene side (in line with the HSAB concept). The heterobinuclear complex is in a temperature-dependent equilibrium with its dimer, with two unsymmetrical Cu-Cl-Cu bridges. The redox-chemistry of the [CuII -L-CuII ] and [PdII -L-CuII ] complexes is studied. One-electron oxidation of both complexes was found to be quasi-reversible in CV experiments, and chemical one-electron oxidation was achieved with NO+ (SbF6- ). In the case of the homobinuclear complex [L(CuCl2 )2 ]+ , intramolecular ligand-metal electron-transfer, triggered by coordination of a CH3 CN solvent molecule, leads to a temperature-dependent equilibrium between the form [CuII -L0 -CuIII ] at low temperatures (with CH3 CN coordinated to the CuIII atom) and [CuII -L⋅+ -CuII ] at higher temperatures (without CH3 CN).

Copper Complexes of New Redox-Active 4,5-Bisguanidino-Substituted Benzodioxole Ligands: Control of the Electronic Structure by Counter-Ligands, Solvent, and Temperature.

Herein, we analyze the possibility of controlling the electronic structure of mononuclear copper complexes featuring new redox-active 4,5-bisguanidino-substituted benzodioxole ligands. The nature of the guanidino groups, the anionic counter-ligands, the applied solvent (polarity), and the temperature are the parameters that decide if a CuII complex with neutral ligand unit or a CuI complex with radical monocationic ligand unit is the adequate description. Under special conditions, a temperature-dependent equilibrium of the two valence tautomeric forms (CuII /neutral ligand and CuI /radical monocationic ligand) is achieved. Removal of a ligand-centered electron from a paramagnetic CuII complex with a neutral ligand unit leads to a diamagnetic CuI complex with a dicationic ligand unit through a redox-induced electron-transfer (RIET) process.

Substituted Tetraaza- and Hexaazahexacenes and their N,N'-Dihydro Derivatives: Syntheses, Properties, and Structures.

The palladium-catalyzed coupling of a substituted o-diaminoanthracene and a substituted o-diaminophenazine to substituted 2,3-dichloroquinoxalines furnishes 10 differently substituted N,N'-dihydrotetraaza- or -hexaazahexacenes with the quinoxaline group of the azaacenes carrying fluorine, chlorine, or nitro groups. The N,N'-dihydrotetraazahexacenes with hydrogen, chlorine, and fluorine subtituents are oxidized to azaacenes, whereas only the parent N,N'-dihydrohexaazahexacenes, with hydrogen substituents, are oxidized by MnO2. The resultant azaacenes are characterized by their optical and spectroscopic data. In addition, single-crystal X-ray structures have been obtained for the parent tetraazahexacenes and their difluoro-substituted derivatives. The di- and tetrachloro derivatives of the N,N'-dihydrohexaazahexacene have also been structurally characterized.

Tetracyanoquinodimethane reduction by complexed guanidinyl-functionalized aromatic compounds.

In this work, we report on the reduction of tetracyanoquinodimethane (TCNQ) with dicationic complexes of guanidinyl-functionalized aromatic (GFA) electron donors. In contrast to reduction with free GFAs, milder reduction conditions were achieved, and this led to semiconducting materials with extended TCNQ π stacking. The charge on the TCNQ units was estimated from the structural data obtained by single-crystal X-ray diffraction analysis and from IR spectroscopic data. The electrical conductivity was studied and the activation energy of the semiconducting materials was estimated from the temperature dependence of the conductivity.