Modulation of coordination chemistry in copper(I) complexes supported by bis[2-(2-pyridyl)ethyl]amine-based tridentate ligands

Structure and physicochemical properties of copper(I) complexes of the tridentate ligands L-2 (N,N-bis[2-(6-methylpyridin-2-yl)ethyl]phenethylamine) and L-3 (N,N-bis[2-(2-pyridyl)ethyl]-beta -methylphenethylamine) have been examined to obtain deeper insights into modulation of the coordination chemistry of copper(I) complexes. [Cu-I-(L-2)(CH3CN)](ClO4) (2 . CH3CN) has a distorted tetrahedral geometry, which consists of three nitrogen atoms of the ligand and one nitrogen atom of the bound CH3CN. Steric repulsion between the 6-methyl group on the pyridine nucleus of L-2 and the metal ion of the complex prevents the cuprous complex from adaptation to a three-coordinate geometry which must have a shorter Cu-N(pyridine) distance (similar to1.88 Angstrom). Thus, the four-coordinate copper(I) complex (2 . CH3CN) with a longer Cu-N bond (1.98 similar to2.13 Angstrom) becomes favorable, resulting in rather strong binding of CH3CN to the metal ion, In [Cu-I(L-3)] (ClO4) (3), there is a Cu-I-pi interaction between the cuprous ion and the phenyl group of the ligand sidearm. Such a copper(I)-arene interaction is essentially weak, but is significantly stabilized in complex 3. The methyl group at the benzylic position of L-3 reduces the degree of freedom of sidearm rotation to make the phenyl group stick on the cuprous ion, Thus, the reactivity of the copper(I) complexes of L-2 and L-3 toward dioxygen is significantly diminished, showing sharp contrast to the high reactivity of the copper(I) complex supported by a similar tridentate ligand L-1 (N,N-bis[2-(2-pyridiyl)ethyl]phenethylamine).