Density Functional Theory Interpretation of the ¹H Photo-Chemically Induced Dynamic Nuclear Polarization Enhancements Characterizing Photoreduced Polyazaaromatic Ru(II) Coordination Complexes

Abstract

The unprotonated and protonated monoreduced forms of the polyazaaromatic Ru(II) coordination complexes [Ru(tap)(3)](2+) and [Ru(tap)(2)(phen)](2+) (tap = 1,4,5,8-tetraazaphenanthrene ; phen = 1,10-phenanthroline), that is, [Ru(tap)(3)](*+), [Ru(tap)(2)(phen)](*+), [Ru(tap)(2)(tap-H)](*2+), and [Ru(tap)(tap-H)(phen)](*2+), 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.

Spectro-electrochemical Studies on [Ru(TAP)2(dppz)]2+—Insights into the Mechanism of its Photosensitized Oxidation of Oligonucleotides

“…26 An isotope effect is also observed when [Ru(TAP)2(dppz)] 2+ oxidizes 5′-GMP 12 and CIDNP studies with other [Ru(TAP)2L 2+ complexes suggest that this process might proceed by PCET within the solvent cage resulting in the formation of Ru(TAP)(TAPH)(dppz)] 2+ (equation 4). [13][14][15][16] [Ru(TAP)2(dppz)]…”

“…More recently a series of detailed photo-CIDNP (chemically induced dynamic nuclear polarization) studies of the reduction of various [Ru(TAP)2(L)] 2+ with 5′-GMP and other reducing biomolecules have been reported by Kirsch-De Mesmaeker, Luhmer and coworkers. [13][14][15][16] With 5′-GMP as quencher, strongly enhanced absorption signals are found, predominantly for the 2,7-TAP protons (much weaker for the 3,6-TAP protons and absent for the 9,10-TAP protons) (see Figure 1 for the numbering of atoms in TAP), while a strong effect is also observed for the H8 of the GMP. This result is consistent with formation of the reduced metal complex and the oxidized nucleotide in the solvent cage; the CIDNP signal arises because of the competition between the back-electron transfer and the escape of the two radical species.…”

From atoms to biomolecules: a fruitful perspective

“…These calculations are based on the determination of the electronic structure of the mono-reduced form of Ru(II) complexes in gas phase and aqueous solution. Recently, some of us showed that the electron spin density and the isotropic Fermi contact contribution to the hyperfine interactions with the 1 H nuclei agree remarkably well with the observed 1 H photo-CIDNP enhancements [34]. Thus, combined photo-CIDNP experiments and DFT calculations open up new important perspectives for the study of polyazaaromatic Ru(II) complexes photoreactions.…”

Full assignment of the 1H NMR spectrum of heteroleptic polyazaaromatic Ru(II) complexes based on long‐range interligand NOEs

“…Similar distances are expected in the complex [Ru(tap) 2 (hat)] 2+ ; the corresponding hat protons are indicated in italic. See Figs and for atom numbering …”

“…Calculated distances between the 1 H nuclei of the phen and tap ligands in the complex [Ru(tap) 2 (phen)] 2+ are quoted in Table 1. [4,5] The shortest interligand distances are found between 1 H in ortho position of a chelating N atom, i.e. between phen H-2,9 and tap H-6 (~3.3 Å), then between phen H-2,9 and tap H-3 (~3.5 Å; see also Supporting Information).…”