An Accurate Methodology to Identify the Level of Aggregation in Solution by PGSE NMR Measurements: The Case of Half-Sandwich Diamino Ruthenium(II) Salts

Abstract

The utility of PGSE NMR measurements in determining hydrodynamic radii (rH) and volumes (VH) of small- and medium-size molecules (3 A < rH < 6 A) 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 3BPh4. It was found that accurate rH and VH values can be obtained by introducing in the Stokes−Einstein equation (Dt = kT/cπηrH) not only the correct values for temperature (T) and viscosity (η) but, particularly, that for the c factor. PGSE NMR measurements were then applied to an investigation of the aggregation tendency of complexes [Ru(η6-cymene)(R1R2NCH2CH2NR1R2)Cl]X (R1 = R2 = H, 1; R1 = H, R2 = H, 2; R1 = R2 = Me, 3; X- = PF6- or BPh4-) in both protic and aprotic solvents with a relative permittivity (er) ranging from 4.81 (chloroform-d) to 32.66 (methanol-d4). Compounds 1 and 2 exhibited a remarkable tendency to aggregate through i...