Structural Changes upon Reduction of Dipyrido[2,3-a:3‘,2‘-c]phenazine Probed by Vibrational Spectroscopy, ab Initio Calculations, and Deuteration Studies

A series of bridging ligands, dipyrido[2,3-a:3‘,2‘-c]phenazine (ppb), dipyrido[2,3-a:3‘,2‘-c]-6,7-dichlorophenazine
(ppbCl2), and dipyrido[2,3-a:3‘,2‘-c]-6,7-dimethylphenazine (ppbMe2), and their binuclear copper(I) complexes have
been synthesized, and their spectral properties were measured. The single-crystal structure of the complex,
[(PPh3)2Cu(μ-ppbCl2)Cu(PPh3)2](BF4)2 in the monoclinic space group P21/c, 18.2590(1), 21.1833(3), 23.2960(3) Å
with Z = 4 is reported. The copper(I) complexes are deeply colored through MLCT transitions in the visible region.
The vibrational spectra of the ligands have been modeled using ab initio hybrid density functional theory (DFT)
methods (B3LYP/6-31G(d)) and compared to experimental FT-Raman and IR data. The DFT calculations are used
to interpret the resonance Raman spectra, and thus the electronic spectra, of the complexes. The preferential
enhancement of modes associated with the phenanthroline section of the ligands with blue excitation (λexc = 457.9
nm) over phenazine-based modes with redder excitation (λexc = 514.5 and 632.8 nm) suggests the 2 MLCT
transitions terminated on different unoccupied MOs are present under the visible absorption envelope. The radical
anion species of the ligands are prepared by the electrochemical reduction of the binuclear copper(I) complexes;
no evidence of dechelation prevalent in other copper(I) complexes is observed. The resonance Raman spectra of
the reduced complexes are dramatically different from those of the parent species. Across the series common
bands are observed at about 1590 and 1570 cm-1 which do not shift with reduction but are altered in intensity.
The normal-mode analysis of the radical anion species suggests that these normal modes primarily involve bond
length distortions that are unaffected by reduction.