Vibrational Spectra of Dipyrido[3,2-a:2′,3′-c]phenazine and Its Radical Anion Analyzed by Ab Initio Calculations and Deuteration Studies

The ligand dipyrido[3,2-a:2′,3′-c]phenazine (dppz) and its deuterated analogues, d4-dppz, d6-dppz, d10-dppz, and [ReCl(CO)3] complexes with these ligands have been synthesized. Using DFT calculations it is possible to calculate a geometry of the ligand such that it matches the crystallographic data for a variety of dppz complexes. B3LYP/6-31G(d) frequency calculations correspond closely to the experimental IR and Raman data. Analysis of the calculated normal modes of vibration reveals the presence of a number of modes that are localized to the ring sections of the dppz-framework. Modes 79, 78, 77, 50, and 49, which lie at 1602, 1586, 1576, 1071, and 1034 cm-1, respectively, in the experimental spectra of dppz correspond to phenanthroline-based modes. Modes 68 and 67, lying at 1414 and 1402 cm-1 in dppz are phenazine-based vibrations. These provide an insight into the nature of the MLCT transitions for metal complexes with dppz. The preferential enhancement of phenanthroline-based modes in the resonance Raman spectra of these complexes strongly suggests the π* accepting MO is of phenanthroline character. DFT calculations (B3LYP/6-31G(d)) on the radical anion reveal the elongation of bonds about the pyrimidine ring. The frequency calculations also reveal significant changes in the vibrational spectra for dppz•- from the neutral ligand. Experimental resonance Raman data for the electrochemically reduced [ReCl(CO)3(dppz)]- and the deuterated analogues show distinct isotope shifts that may be correlated to results obtained from the calculations. The characteristic bands observed in these spectra are phenanthroline-based. They may be correlated with time-resolved resonance Raman spectra of Ru(II) and Re(I) complexes.