Density functional theory interpretation of the 1H photo-chemically induced dynamic nuclear polarization enhancements characterizing photoreduced polyazaaromatic Ru(II) coordination complexes.

The unprotonated and protonated monoreduced forms of the polyazaaromatic Ru(II) coordination complexes Ru(tap)(3) and Ru(tap)(2)(phen) (tap = 1,4,5,8-tetraazaphenanthrene ; phen = 1,10-phenanthroline), that is, Ru(tap)(3), Ru(tap)(2)(phen), Ru(tap)(2)(tap-H), and Ru(tap)(tap-H)(phen), were studied by Density Functional Theory (DFT). The electron spin density of these radical cations, the isotropic Fermi-contact, and the anisotropic dipolar contributions to the hyperfine coupling constants of the H nuclei were calculated in vacuo and using a continuum model for water solvation. For Ru(tap)(2)(phen), as well as for its protonated form, the DFT results show that the unpaired electron is not localized on the phen ligand. For both Ru(tap)(3) and Ru(tap)(2)(phen), they reveal high electron spin density in the vicinity of tap H-2 and tap H-7 (the H atoms in the ortho position of the tap non-chelating N atoms). These results are in full agreement with recent steady-state (1)H photo-Chemically Induced Dynamic Nuclear Polarization (photo-CIDNP) measurements. The DFT calculations performed for the protonated species also predict major (1)H photo-CIDNP enhancements at these positions. Interestingly, they indicate significantly different polarization for tap H-9,10, suggesting that the occurrence of a photoinduced electron transfer with protonation of the reduced species might be detected by high-precision photo-CIDNP experiments.