Infrared Ion Spectroscopyof Gaseous [Cu(2,2′-Bipyridine)₃]²⁺:Investigation of Jahn–Teller ElongationVersus Compression

Symmetry breaking is ubiquitous in chemical transformations and affects various physicochemical properties of materials and molecules; Jahn–Teller (JT) distortion of hexa-coordinated transition-metal–ligand complexes falls within this paradigm. An uneven occupancy of degenerate 3d-orbitals forces the complex to adopt an axially elongated or compressed geometry, lowering the symmetry of the system and lifting the degeneracy. Coordination complexes of Cu²⁺ are known to exhibit axial elongation, while compression is far less common, although this may be due to the lack of rigorous experimental verification. Here, we present the gas-phase vibrational spectrum of the archetypal [Cu(2,2′-bipyridine)₃]²⁺ ionic complex, obtained by infrared multiple-photon dissociation (IRMPD) spectroscopy using the widely tunable IR free-electron laser FELIX. Predicted vibrational spectra at the density functional level of theory are nearlybut not entirelyidentical for the two JT-distorted geometries. We compare experimental and theoretical spectra and address the question of an axially elongated or compressed geometry of the complex, or a mixture thereof, in the gaseous ion population.