From ion pairs to ion triples through a hydrogen bonding-driven aggregative process

Complexes [Ru(arene)(kappa(3)-dpk-OR]X (dpk = 2,2'-dipyridylketone) were synthesized, and their interionic structure was investigated through an integrated approach based on diffusion and NOE NMR experiments and X-ray single-crystal studies. PGSE NMR results indicated that the highest aggregation tendency occurred for complex 2BF(4) (arene = p-cymene and R = OH) that showed aggregation numbers consistent with the main presence of 2(2)BF(4)(+) ion triples (N+ = 1.9 and N- = 1.1, in CD2Cl2 at 0.5 mM). X-ray investigations indicated that a [1 x 1] network of HBs is present involving the two OH moieties of the two cationic fragments belonging to the two independent ion pairs of the asymmetric unit. This dication is likely the central moiety of 2(2)BF(4)(+) ion triples. According to F-19,H-1-HOESY NMR interionic studies, the counterion was close to two pyridyl rings belonging to two different cations undergoing a pi - pi-rstacking interaction in CD2Cl2 exactly as observed in the solid state. All of the other complexes having an alyphatic OR-tail showed a much smaller aggregation tendency, leading to ion pairs even when OR = OCH2CH2OH. In the latter case, X-ray studies showed that the terminal OH underwent an intracationic HB with the oxygen atom coordinated at the ruthenium.