An accurate methodology to identify the level of aggregation in solution by PGSE NMR measurements: The case of half-sandwich diamino ruthenium(II) salts

The utility of PGSE NMR measurements in determining hydrodynamic radii (r(H)) and volumes (V(H)) of small- and medium-size molecules (3 angstrom < r(H) < 6 angstrom) was evaluated by performing measurements for a variety of pure deuterated solvents and their solutions containing the internal standard TMSS [tetrakis(trimethylsilyl)silane] also in the presence of a variable concentration of 3BPh(4). It was found that accurate rH and VH values can be obtained by introducing in the Stokes-Einstein equation (D(t) = kT/c pi eta r(H)) not only the correct values for temperature (T) and viscosity (eta) but, particularly, that for the C factor. PGSE NMR measurements were then applied to an investigation of the aggregation tendency of complexes [Ru(eta(6)-cymene)(R(1)R(2)NCH(2)CH(2)NR(1)R(2))Cl]X (R(1) = R(2) = H, 1; R(1) = H, R(2) = H, 2; R(1) = R(2) = Me, 3; X(-) = PF(6)(-) or BPh(4)(-)) in both protic and aprotic solvents with a relative permittivity (epsilon(r)) ranging from 4.81 (chloroform-d) to 32.66 (methanol-d(4)). Compounds I and 2 exhibited a remarkable tendency to aggregate through intercationic N-H(...)Cl and cation/anion N-H(...)FPF(5)(-) hydrogen bonds. In addition to ion pairs, ion triples and ion quadruples were also observed in solution. Compound 3, having no N-H moiety, showed less tendency to aggregate than I and 2, even though it also afforded ion quadruples in apolar and aprotic solvents. Relative anion-cation orientations and arene conformations were investigated by means of (1)H-NOESY and (19)F,(1)H-HOESY NMR spectroscopy. The relative anion-cation position was well-defined, especially for compounds bearing the PF(6)(-) counterion, and was modulated by the nature of the N,N ligand. A progressive slackening of the contact aggregates was observed in the series 1-3 that led to a higher mobility of the anion, as indicated by the observation of less specific interionic NOEs.