Interionic structure of ion pairs and ion quadruples of half-sandwich ruthenium(II) salts bearing alpha-diimine ligands

The interionic structure of complexes [Ru(eta(6)-Arene){(2-R-C6H4)N = C(Me)-C(Me)= N(2-R-C6H4)}Cl]X was investigated by an integrated experimental (PGSE diffusion and NOE NMR spectroscopy and X-ray single-crystal studies) and theoretical (DFT and ONIOM calculations) approach. PGSE NMR experiments indicated that ion pairing is the main aggregative process in CD2Cl2 and solvents with higher relative permittivity. They also showed that the tendency to ion pairing for isodielectric solvents is higher when the latter are protic. NOE interionic contacts were observed in 2-propanol-d(8) even for BARF(-) salts. Ion pairing was favored by more coordinating counterions and an increase in concentration. An equilibrium between ion pairs and ion quadruples was observed by PGSE measurements in chloroform-d and benzene-d(6). Such equilibrium is shifted toward ion quadruples by an increase in the concentration or when least coordinating counterions are used. For small fluorinated counterions, NOE studies located the anion in ion pairs above the plane containing the C = N imine moieties. ONIOM calculations found that this anion-cation orientation was at least 35.9 kJ/mol lower in energy than a second orientation with the anion close to cymene, which, in some cases, was observed in the solid state. NOE investigations on complexes with BPh4- counterion did not allow a single orientation capable of explaining the observed NOEs to be found. X-ray studies showed that one cation is surrounded by two anions. ONIOM calculations found that these two anion-cation orientations have similar energies. X-ray and NOE NMR data strongly suggest that ion quadruples with BPh4- anions are constituted by an alternation of cations and anions. Interionic NOE intensities are almost invariant on passing from ion pairs to ion quadruples with small fluorinated counterions. X-ray studies suggested at least four possible structures of ion quadruples differing in both disposition and orientation of the ionic moieties. Three structures considered by ONIOM calculations were similar in energy, but more stable than the separated ion pairs.