High-resolution electron spectroscopy and molecular structures of Cu-(2,2′-bipyridine) and Cu-(4,4′-bipyridine)

Copper complexes of 2,2′-bipyridine (22BIPY) and 4,4′- bipyridine (44BIPY) were prepared in a laser-vaporization supersonic molecular beam source and identified by laser photoionization time-of-flight mass spectrometry. Electronic spectra and molecular structures were studied with pulsed-field ionization zero electron kinetic energy (ZEKE) electron spectroscopy, density functional theory (DFT) and second-order Møller-Plesset perturbation (MP2) calculations, and spectral simulations. Adiabatic ionization energies and metal-ligand and ligand-based vibrational frequencies of Cu-22BIPY and Cu-44BIPY were measured from the ZEKE spectra. Ground electronic states and molecular structures of the two complexes were determined by comparing the spectroscopic measurements with the theoretical calculations. The ground state of Cu-22BIPY (²B₁, C _2v) has a planar bidentate structure with Cu binding to two nitrogen atoms and two pyridine molecules in the cis configuration. The ground state of Cu-44BIPY (²A, C₂) has a monodentate structure with Cu binding to one nitrogen and two pyridines in a twisted configuration. The ionization energy of Cu-22BIPY is considerably lower and its bond energy is much higher than that of Cu-44BIPY. The different ionization and dissociation energies are attributed to the distinct metal binding modes of the two complexes. It has been found that the DFT calculations yield the correct structures for the Cu-22BIPY complex, whereas the MP2 calculations produce the best structures for the Cu-44BIPY complex.