Intramolecular metal-ligand electron transfer triggered by co-ligand substitution.

The possibility of directed stimulation of intramolecular electron transfer between a metal and a redox-active ligand in a molecular coordination compound is the key to its application in molecular catalysis and other research themes. Although the stimulation by a substitution reaction of the co-ligands is often postulated as key step in catalytic cycles using redox-active ligands as electron reservoirs, there are only a few explicit examples for such reactions. Herein we report the synthesis of the first dicationic and dinuclear CuI complexes featuring the oxidized form of a redox-active tetrakisguanidine ligand (1,2,4,5-tetrakis(tetramethylguanidino)benzene 1) as a bridging ligand and two neutral co-ligands L (acetonitrile or pyridine), [1{Cu(Cl)L}2]2+. An intramolecular electron transfer between the copper atom and the tetrakisguanidine ligand 1, leading to a dinuclear CuII complex with the reduced form of the tetrakisguanidine ligand 1, is triggered by substitution of the neutral co-ligands L.

Counter-ligand control of the electronic structure in dinuclear copper-tetrakisguanidine complexes.

The redox-active GFA (Guanidino-Functionalized Aromatic compound) 1,4,5,8-tetrakis(tetramethylguanidino)-naphthalene (6) is used to synthesize new dinuclear copper complexes of the formula [6(CuX2)2] with different electronic structures. With X = OAc, a dinuclear Cu(II) complex of the neutral GFA is obtained (electronic structure [Cu(II)-GFA-Cu(II)], two unpaired electrons), and with X = Br a diamagnetic dinuclear Cu(I) complex of the dicationic GFA (electronic structure [Cu(I)-GFA(2+)-Cu(I)], closed-shell singlet state). The different electronic structures lead to significant differences in the optical, structural and magnetic properties of the complexes. Furthermore, the complex [6(CuI)2](2+) (electronic structure [Cu(I)-GFA(2+)-Cu(I)], closed-shell singlet state) is synthesized by reaction of 6(2+) with two equivalents of CuI. Slow decomposition of this complex in solution leads to the fluorescent dye 2,7-bis(dimethylamino)-1,3,6,8-tetraazapyrene. In an improved synthesis of this tetraazapyrene, 6 is reacted with CuBr in the presence of dioxygen. Quantum chemical calculations show that the addition of counter-ligands to the trigonal planar Cu(I) atoms of [6(CuI)2](2+) favors or disfavors one of the electronic structures, depending on the nature of the counter-ligand.

What Makes a Strong Organic Electron Donor (or Acceptor)?

Organic electron donors are of importance for a number of applications. However, the factors that are essential for a directed design of compounds with desired reduction power are not clear. Here, we analyze these factors in detail. The intrinsic reduction power, which neglects the environment, has to be separated from extrinsic (e.g., solvent) effects. This power could be quantified by the gas-phase ionization energy. The experimentally obtained redox potentials in solution and the calculated ionization energies in a solvent (modeled with the conductor-like screening model (COSMO)) include both intrinsic and extrinsic factors. An increase in the conjugated π-system of organic electron donors leads to an increase in the intrinsic reduction power, but also decreases the solvent stabilization. Hence, intrinsic and extrinsic effects compete against each other; generally the extrinsic effects dominate. We suggest a simple relationship between the redox potential in solution and the gas-phase ionization energy and the volume of an organic electron donor. We finally arrive at formulas that allow for an estimate of the (gas-phase) ionization energy of an electron donor or the (gas-phase) electron affinity of an electron acceptor from the measured redox potentials in solution. The formulas could be used for neutral organic molecules with no or only small static dipole moment and relatively uniform charge distribution after oxidation/reduction.

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.