C. Zuccaccia et al., Solution structure investigation of Ru(II) complex ion pairs: QuantitativeNOE measurements and determination of average interionic distances, J AM CHEM S, 123(44), 2001, pp. 11020-11028
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 ch
loride-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 (heteronuclea
r 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 unsymmet
rical counteranions, the aliphatic chain points away from the metal center
as indicated by the absence of NOE between the terminal Me group and any ca
tionic protons. An estimation of the average interionic distances in soluti
on was obtained by the quantification of the NOE build-up versus the mixing
time under the assumption that the interionic and intramolecular correlati
on times (tau (c)) are the same. Such an assumption was checked by the expe
rimental measurements of tau (c) from both the dipolar contribution to the
carbon-13 longitudinal relaxation time (T-l(DD)) the comparison of the intr
amolecular 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 comp
ared with those obtained for the neutral trans- [Ru(COMe){(PZ(2))BH2}(CO)(P
Me3)(2)] complex (2), isosteric with the cation of 1. They were significant
ly 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 accu
rate. By using such interionic distances, it was possible to verify that th
e counteranion in complex 1b also orients the BMe group far away from the m
etal center.