Bonding and structures of copper-aminopyridine complexes - High-resolution electron spectroscopy and ab initio calculations

Copper complexes of x-aminopyridine (x = 2, 3, 4) were prepared in a laser vaporization supersonic molecular beam source and identified using laser photoionization time-of-flight mass spectrometry. These complexes were studied by pulsed-field ionization zero electron kinetic energy (ZEKE) spectroscopy and second-order Møller-Plesset perturbation theory. Three structures formed by Cu binding to the pyridine nitrogen (σ_α), the amino nitrogen (σ_β), and the pyridine ring (π) were considered by the theoretical calculations, but only the σ _α structures with Cu binding to the pyridine nitrogen were confirmed by the spectroscopic measurements. Adiabatic ionization energies and metal-ligand and ligand-based vibrational frequencies of the σ_α complexes were measured from the ZEKE spectra, and the metal-ligand bond energies of the neutral and ionized complexes were predicted by the theory. The ionization energies of the Cu complexes are about 20 000 cm^-1 lower than that of bare Cu atom. This ionization energy shift is the result of the stronger Cu⁺-ligand bonding because of the additional charge-dipole interaction in the ion. Although the three complexes are formed by Cu coordination to the pyridine nitrogen atom, the position of the amino group affects the metal-ligand bonding strengths in both neutral and ionized species. These effects include the structural resonance and hydrogen bonding in the neutral complexes and the electric dipole moment and bidentate bonding in the ions.