On the metal-ligand bonding in dinuclear complexes with redox-active guanidine ligands.

Coordination compounds with redox-active ligands are currently intensively studied. Within this research theme, redox-active guanidines have been established as a new, eminent class of redox-active ligands. In this work the variation of metal-guanidine bonding in dinuclear transition metal complexes with bridging redox-active tetrakisguanidine ligands is analysed. A series of dinuclear complexes with different metals (Mn, Fe, Co, Ni, Cu and Zn) is synthesized, using either newly prepared redox-active tetrakisguanidino-dioxine or previously reported tetrakisguanidino-benzene ligands. The discussion of the bond properties in this work is predominantly based on the trends of structural parameters, derived from determination of single-crystal structures by X-ray diffraction and quantum chemical calculations. In addition, the trends in the redox potentials and magnetometric (SQUID) measurements on some of the complexes are included. Due to their combined σ- and π-electron donor capability, redox-active guanidine ligands are weak-field ligands; the σ- and π-bonding contributions vary with the metal. The results highlight the peculiarity of copper-guanidine bonding with a high π-bond contribution to metal-guanidine bonding, enabled by structural distortion of the coordination mode from tetrahedral in the direction of square-planar, short copper-guanidine bonds and minor displacement of the copper atoms from the ligand aromatic plane.

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.

Synthesis of Substituted Trinaphthylenes.

A short synthesis of six trinaphthylenes is reported. The cyclotrinaphthylenes carry six alkoxy groups, and derivatives featuring OHex, OBu, OiPr, OPr, OEt, and OMe substituents can be obtained by an ordinary Ni(COD)2-promoted, Yamamoto-type coupling reaction. Cyclotrimerization yields range from 38% to 65%. Dependent upon their structure, the cyclotrinaphthylenes assume different packing patterns, according to single-crystal X-ray structure determination. The crystal structures of such trinaphthylenes were hitherto undescribed.