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 L² (N,N-bis[2-(6-methylpyridin-2-yl)ethyl]phenethylamine) and L³ (N,N-bis[2-(2-pyridyl)ethyl]-β-methylphenethylamine) have been examined to obtain deeper insights into modulation of the coordination chemistry of copper(I) complexes. [Cu^I(L²)(CH₃CN)](CIO₄) (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 L² 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 (@O1.88 Å). Thus, the four-coordinate copper(I) complex (2·CH₃CN) with a longer Cu-N bond ([email protected] Å) becomes favorable, resulting in rather strong binding of CH₃CN to the metal ion. In [Cu^I(L³)](CIO₄) (3), there is a Cu^I-π 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 L³ 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 L² and L³ toward dioxygen is significantly diminished, showing sharp contrast to the high reactivity of the copper(I) complex supported by a similar tridentate ligand L¹ (N,N-bis[2-(2-pyridiyl)ethyl] phenethylamine).