⁹⁹Ru NMR spectroscopy of organometallic and coordination complexes of ruthenium(II)

The ruthenium(II) organometallic and coordination complexes of type Ru(X)(Y)(CO)₂-(i-Pr-DAB) (i-Pr-DAB = 1,4-diisopropyl-1,4-diaza-1,3-butadiene; X, Y = Cl, Br, I, Me, Et, i-Pr, neo-Pe, Ph₃Sn, Me₃Sn, Ph₃Pb, Ph₃Ge, Mn(CO)₅, RuCp(CO)₂) have been analyzed by means of ⁹⁹Ru NMR spectroscopy. The ⁹⁹Ru NMR resonances for these compounds are generally sharp, allowing scalar couplings to group 14 nuclei to be observed. Correlation times τ_c have been determined from NOE factors and ¹³C T₁ values for selected compounds and were found to parallel the molecular weight of the compounds studied. The M - Ru - M′ and M - Ru - X complexes have ⁹⁹Ru NMR resonances in distinct chemical shift regions. In the series M - Ru - M′ a correlation between the inverse of the crystal field splitting energy ΔE and δ(⁹⁹Ru) exists. These complexes have a HOMO which is delocalized over the M - Ru - M′ σ system and a LUMO which has predominantly π*-DAB character; therefore, (ΔE)^-1 is dominant in the paramagnetic shielding of the ruthenium nucleus. The difference in the observed chemical shift regions of the M - Ru - M′ and M - Ru - X compounds can be attributed to differences in d-orbital radius 〈r^-3〉_d, which is in good agreement with previously performed DFT calculations. For the Ru(I)(R)(CO)₂(i-Pr-DAB) complexes, δ(⁹⁹Ru) follows the same trend as for a series of iron compounds, indicating an increase of the Ru - C bond length in the series Me < Et < neo-Pe < i-Pr. This finding emphasizes the power of transition-metal NMR spectroscopy in the structural analysis of transition-metal complexes. Furthermore, ⁹⁹Ru NMR provides a tool to identify and assign isomers in mixtures of Ru - polypyridyl complexes. The ⁹⁹Ru NMR resonances of coordination isomers of various triazole-containing ruthenium complexes are found at different frequencies, which can be used to unambiguously identify coordination isomers in mixtures by means of ⁹⁹Ru NMR spectroscopy. The δ(⁹⁹Ru) value is found to increase with increasing redox potential of the polypyridyl compounds.