From Ion Pairs to Ion Triples through a Hydrogen Bonding-Driven Aggregative Process

Complexes [Ru(arene)(κ3-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 2BF4 (arene = p-cymene and R = OH) that showed aggregation numbers consistent
with the main presence of 22BF4+ ion triples (N+ = 1.9 and N- = 1.1, in CD2Cl2 at 0.5 mM). X-ray
investigations indicated that a [1 × 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 22BF4+ ion triples. According to 19F,1H-HOESY NMR interionic studies,
the counterion was close to two pyridyl rings belonging to two different cations undergoing a π−π-stacking 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.