Analysis of Ion Pairing in Solid State and Solution in -Cymene Ruthenium Complexes.

The importance of ion pairing in different fields of chemistry is widely recognized. In this work, we have synthesized a set of cationic -cymene ruthenium complexes of general formula [(-cym)Ru(L')(κ-O^N-L)]X (-cym = -cymene; L' = -methylimidazole (MeIm), N-ethylpiperidylimidazole (EpipIm), 1,3,5-triaza-7-phosphaadamantane (PTA); L = 2-(1H-benzimidazol-2-yl)phenolato (L1), 2-(1,3-benzothiazol-2-yl)phenolato (L2); X = Cl, BF, OTf, BPh). X-ray diffraction studies on selected complexes revealed relatively strong anion-cation interactions in the solid state mainly based on N-H···X (X = Cl, F, O) and C-H···π interactions, also observed in the DFT-modeled complexes in the gas phase. Moreover, NMR studies showed that they exist as intimate ion pairs in solution and, remarkably, as head-to-tail quadruples in the particular case of the cation [(-cym)Ru(MeIm)(κ- O^N-L1)] ([]) with Cl and BPh as counteranions. Furthermore, a value of Δ = -2.9 kcal mol at 299 K has been estimated for the equilibrium {[]BPh···[]BPh} ⇆ 2{[]···BPh} in concentrated CDCl solutions. In addition, preliminary studies concerning the cytotoxic properties against HeLa cell lines of the derivatives suggested a positive effect derived from the presence of the lipophilic BPh anion and also from the NH group of the benzimidazolyl fragment.