Solution structure investigation of Ru(II) complex ion pairs: Quantitative NOE measurements and determination of average interionic distances

The structure of the Ru(II) ion pairs trans-[Ru(COMe){(pz(2))CH2}(CO)(PMe3)(2)]X (X- = BPh4-, 1a; BPh3Me-, 1b; BPh3(n-Bu)(-), 1c; BPh3(n-Hex)(-), 1d; B(3, 5-(CF3)(2)(C6H3))(4)(-), 1e; PF6-, 1f; and BF4-, 1g; pz = pyrazol-1-yl-ring) was investigated in solution from both a qualitative (chloroform-d, methylene chloride-d(2), nithromethane-d(3)) and quantitative (methylene chloride-d(2)) point of view by performing 1D- and 2D-NOE NMR experiments. In particular, the relative anion-cation localization (interionic structure) was qualitatively determined by H-1-NOESY and F-19, H-1-HOESY (heteronuclear Overhauser effect spectroscopy) NMR experiments. The counteranion locates close to the peripheral protons of the bispyrazolyl ligand independent of its nature and that of the solvent. In complexes Ic and Id bearing unsymmetrical counteranions, the aliphatic chain points away from the metal center as indicated by the absence of NOE between the terminal Me group and any cationic protons. An estimation of the average interionic distances in solution was obtained by the quantification of the NOE build-up versus the mixing time under the assumption that the interionic and intramolecular correlation times (tau (c)) are the same. Such an assumption was checked by the experimental measurements of tau (c) from both the dipolar contribution to the carbon-13 longitudinal relaxation time (T-l(DD)) the comparison of the intramolecular and interionic cross relaxation rate constant (sigma) dependence on the temperature. Both the methodologies indicate that anion and cation have comparable r, values. The determined correlation time values were compared with those obtained for the neutral trans- [Ru(COMe){(PZ(2))BH2}(CO)(PMe3)(2)] complex (2), isosteric with the cation of 1. They were significantly shorter (similar to3.8 times), indicating that the main contribution to dipolar relaxation processes comes from the overall ion pair rotation. As a consequence, the determined average interionic distances appear to be accurate. By using such interionic distances, it was possible to verify that the counteranion in complex 1b also orients the BMe group far away from the metal center.