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 Cu complex with neutral ligand unit or a Cu complex with radical monocationic ligand unit is the adequate description. Under special conditions, a temperature-dependent equilibrium of the two valence tautomeric forms (Cu /neutral ligand and Cu /radical monocationic ligand) is achieved. Removal of a ligand-centered electron from a paramagnetic Cu complex with a neutral ligand unit leads to a diamagnetic Cu complex with a dicationic ligand unit through a redox-induced electron-transfer (RIET) process.

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Fast and efficient electron transfer in blue copper proteins is realized by a structural harmonization between the Cu and Cu complex pair ("entatic state" model). Herein, we present now a Cu /Cu 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.

Intramolecular electron transfer (IET) processes between the metal and a redox-active ligand in a transition-metal complex are currently intensively studied, as they represent key steps of some redox-catalytic reactions in synthetic chemistry and of some enzymatic redox reactions in biological systems. Using a combination of experimental results and quantum chemical calculations, we herein examine in detail the role of [a] 3661 cal analysis highlights the effect of the solvent polarity on the electronic structure of these copper-bisguanidine complexes. Moreover, a direct correlation between the copper coordination mode and the electronic structure is established. Scheme 2. Lewis representations of the two redox-active bisguanidine ligands relevant for this work. Results and Discussion Synthesis and Solid-State CharacterizationThe redox-active ligand 2 Et was synthesized as previously reported from benzodioxan according to a three-step procedure Eur.

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.

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 [Cu -L -Cu ] and heterobinuclear [Pd -L -Cu ] complexes. In the heterobinuclear complex, the Pd is coordinated by the softer bisguanidine side of L and the Cu 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 [Cu -L-Cu ] and [Pd -L-Cu ] 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 (SbF ). In the case of the homobinuclear complex [L(CuCl ) ] , intramolecular ligand-metal electron-transfer, triggered by coordination of a CH CN solvent molecule, leads to a temperature-dependent equilibrium between the form [Cu -L -Cu ] at low temperatures (with CH CN coordinated to the Cu atom) and [Cu -L -Cu ] at higher temperatures (without CH CN).

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.

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