Use of Hydrophobic Ligands for the Stabilization of Low-Valent Transition Metal Complexes. 1. The Effect of N-Methylation of Linear Tetraazaalkane Ligands on the Properties of Their Copper Complexes

The electrochemical reduction of the Cu(II) complexes with the ligands 2,5,8,11-tetramethyl-2,5,8,11-tetraazadodecane, 2,5,9,12-tetramethyl-2,5,9,12-tetraazatridecane, and 2,6,9,13-tetramethyl-2,6,9,13-tetraazatetradecane in deaerated aqueous solutions yields the corresponding thermodynamically stable copper(I) complexes. The same complexes are obtained also via the comproportionation reaction of CuL^{i 2+} and Cu⁰ in the presence of excess ligand. ΔS⁰ values of the reduction processes were determined. The basicity constants of Lⁱ and the stability constants of their Cu(II) complexes were determined potentiometrically. The ESR spectra of CuL^{i 2+} point out that these complexes are not significantly tetrahedrally distorted. The binding constants of axial singly charged anions to the CuL^{i 2+} complexes were determined. Analysis of the data points out that the thermodynamic stabilization of the monovalent copper complexes via N-methylation is mainly due to the destabilization of the CuL^{i 2+} complexes. It is concluded that a major factor contributing to the observed effects is the hydrophobic nature of the tertiary-amine ligands.