Diruthenium Complexes with Bridging Diethynyl Polyaromatic Ligands: Synthesis, Spectroelectrochemistry, and Theoretical Calculations

This work describes syntheses and
electrochemical, spectroscopic,
and bonding properties in a new series of dinuclear ruthenium­(II)
complexes bridged by polyaromatic (biphenyl, fluorene, phenanthrene,
and pyrene) alkynyl ligands. Longitudinal expansion of the π-conjugated
polyaromatic core of the bridging ligands caused a reduced potential
difference between the anodic steps and reinforced their bridge-localized
nature, as evidenced by UV/vis/near-IR and IR spectroelectrochemical
data combined with DFT and TDDFT calculations. Importantly, the intricate
multiple IR ν­(CC) absorption bands for the singly oxidized
states imply a thermal population of a range of conformers (rotamers)
with distinct electronic character. This behavior was demonstrated
with more accurate DFT calculations of selected nontruncated 1e– oxidized complexes in three different conformations.
The combined experimental and theoretical data reveal that thermally
populated rotamers featuring various mutual orientations of the ligated
metal termini and the bridging diethynyl polyaromatic moieties have
a significant impact on the electronic absorption and ν­(CC)
wavenumbers of the singly oxidized systems.