Synthesis of Asymmetric Variants in a Dinucleating Ligand Family and Application for Dinuclear Copper(II) Complexes

We developed a dinucleating ligand system with two identical tetradentate coordination compartments and varying terminal donors. Here, we report the synthesis of two differently asymmetric dinucleating ligands: the ligand selma (=4,7,10‐trimethyl‐1,1,10‐tris(2‐pyridylmethyl)‐1,4,7,10‐tetraazadecane) with one N 3 ‐ and one N 4 ‐ and the ligand susan (=4,7‐dimethyl‐1,1‐bis(6‐methylpyridine‐2‐yl‐methyl)‐10,10‐bis(2‐pyridylmethyl)‐1,4,7,10‐tetraazadecane) with two different N 4 ‐coordination compartments. The initial synthesis of the ligand selma based on extending dipicolylamine with 2‐(methylamino)ethyl units in repeated reductive aminations. Synthesis of 1‐trifluoroacetyl‐4,7‐dimethyl‐1,4,7,10‐tetraazadecane allowed the synthesis of both ligands selma and susan . To study the influence of the terminal donors, the Cu II chloride complexes of selma and susan and the two symmetric susan and susan 6−Me were synthesized. The introduction of the 6‐methyl substituents on the pyridines increases the bond lengths, reduces the electron donation, and shifts the reduction potentials anodically. The large 6‐methyl substituents enforce a meridional coordination of the dipicolylamine subunits, while it is mainly facial without the 6‐methyl substituents. Analysis of the solid‐state structures and the d‐d transitions in solution demonstrate a flat potential energy surface for the 5‐coordinate polyhedra. Analysis of d‐d transitions and reduction potentials indicate that the properties of the Cu II Cu II complexes are additive for but cooperative for selma reflecting the difference of these asymmetric dinucleating ligands.