Ethylene binding in mono- and binuclear Cu I complexes with tetradentate pyridinophane ligands.

Herein we report a series of Cu I complexes supported by tetradentate R N4 pyridinophane ligands that coordinate to ethylene forming either mononuclear complexes with ethylene coordinated in an η 2 -mode or a binuclear complex where ethylene binds to two Cu atoms in a μ-η 2 -η 2 -mode, depending on the steric effects of the R N4 ligand and the reaction conditions. In the binuclear complex with bridging ethylene, the CC bond is significantly elongated, with a bond length of 1.444(8) Å according to X-ray diffraction analysis. This complex represents the only examination a μ-η 2 -η 2 -coordinated Cu-olefin complex reported to date, featuring one of the longest reported CC bonds. The spectroscopic characterization, structure, electrochemical properties and solution behavior are analyzed in this study. Coordination of ethylene was found to be reversible in these complexes and more favored in less sterically hindered R N4 ligands, so that ethylene binding is observed in a coordinating solvent (MeCN) environment. In the case of the Me N4 ligand, the ethylene complex is photoluminescent in the solid state. The ethylene binding modes in mono- and binuclear complexes are elucidated through Natural Bond Orbital and QTAIM analyses.