Spectroscopic, Electrochemical, and Computational Aspects of the Charge Distribution in Ru(acac)2(R-o-benzoquinonediimine) Complexes

The syntheses and properties are reported for five Ru(acac)2(R-bqdi) species where acac is acetylacetonate, and R-bqdi is the non-innocent ligand ortho-benzoquinonediimine substituted with R = H (1), 4,5-dimethyl (2), 4-Cl (3), or 4-NO2 (4), and N,N′′-dimethylsulfonyl (5). Their identities and purities were confirmed by NMR, mass spectra, IR and analytical data. The large degree of metal-to-ligand π-back-donation was analyzed by spectroscopic (UV/visible, IR, Raman) and electrochemical data, supported by molecular orbital composition computations using density functional theory (DFT), with the polarizable continuum model (PCM) to mimic the presence of solvent, and prediction of electronic spectra using time-dependent DFT methods. Extended charge decomposition analysis (ECDA) and natural population analysis (NPA) both produced a detailed picture of the bonding between the non-innocent bqdi ligand and the metal center, allowing correlations to be drawn between the nature of the R substituents and the quantitative extent of π-back-donation and σ-forward donation. In conclusion, the issue of whether these species are best regarded as RuII(quinonediimine) or coupled RuIII(semiquinonediiminate) species is discussed.