Transition Metals Meet Scorpiand-like Ligands

Abstract

Scorpiand-like ligands combine the preorganization of the donor atoms of macrocycles and the degrees of freedom of the linear ligands. We prepared the complexes of several of these ligands with transition metal ions and made a crystallographic and water solution speciation studies. The analysis of the resulting crystal structures show that the ligands have the ability to accommodate several metal ions and that the coordination geometry is mostly determined by the ligand. Ligand 6-[3,7-diazaheptyl]-3,6,9–triaza-1-(2,6)-pyridinacyclodecaphane (L3) is an hexadentate ligand that affords a family of isostructural crystals with Cu(II), Mn(II), Ni(II) and Zn(II). The attempts to obtain Co(II) crystals afforded the Co(III) structures instead. Ligand 6-[4-(2-pyridyl)-3-azabutyl]-3,6,9-triaza-1(2,6)-pyridinacyclodecaphane (L2) is very similar to L3 and yields structures similar to it, but its behavior in solution is very different due to the different interaction with protons. Ligand 6-(2-aminoethyl)-3,6,9–triaza-1-(2,6)-pyridinacyclodecaphane (L1) is pentadentate and its complexes allow the metal to be more accessible from the solvent. A Zn(II) structure with L1 shows the species [ZnBrHL1]2+, which exists in a narrow pH range.