Analysis of Ion Pairing in Solid State and Solution in p‑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 p-cymene ruthenium complexes of general formula [(p-cym)­Ru­(L′)­(κ²-O^N–L)]­X (p-cym = p-cymene; L′ = N-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^–, BF₄^–, OTf^–, BPh₄^–). 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 [(p-cym)­Ru­(MeIm)­(κ²- O^N–L1)]⁺ ([1]⁺) with Cl^– and BPh₄^– as counteranions. Furthermore, a value of ΔG = −2.9 kcal mol^–1 at 299 K has been estimated for the equilibrium {[1]­BPh₄···[1]­BPh₄} ⇆ 2­{[1]⁺···BPh₄^–} in concentrated CDCl₃ 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 BPh₄^– anion and also from the NH group of the benzimidazolyl fragment.