Gas-Phase Infrared Ion Spectroscopy Characterization of Cu(II/I)Cyclam and Cu(II/I)2,2′-Bipyridine Redox Pairs

We report the fingerprint IR spectra of mass-isolated gaseous coordination complexes of 2,2'-bipyridine (bpy) and 1,4,8,11-tetra-azacyclotetrade-cane (cyclam) with a copper ion in its I and II oxidation states. Experiments are carried out in a quadrupole ion trap (QIT) mass spectrometer coupled to the FELIX infrared free-electron laser. Dications are prepared using electrospray ionization (ESI), while monocations are generated by charge reduction of the dication using electron transfer-reduction (ETR) in the QT. Interestingly, [Cu(bpy)(2)](+) can also be generated directly using ESI, so that its geometries as produced from ETR and ESI can be compared. The effects of charge reduction on the IR spectra are investigated by comparing the experimental spectra with the IR spectra modeled by density functional theory. Reduction of Cu(II) to the closed-shell Cu(I) ion retains the square-planar geometry of the Cu-cyclam complex. In contrast, for the bis-bpy complex with Cu, charge reduction induces a conversion from a near-square planar to a tetrahedral geometry. The geometry of [Cu(bpy)(2)](+) is identical to that of the complex generated directly from ESI as a native structure, which indicates that the ETR product ion thermalizes. For [Cu(cyclam)](+), however, the square-planar geometry of the 2+ complex is retained upon charge reduction, although a (distorted) tetrahedral geometry was predicted to be lower in energy. These differences are attributed to different barriers to rearrangement.