860401-89-4Relevant academic research and scientific papers
An accurate methodology to identify the level of aggregation in solution by PGSE NMR measurements: The case of half-sandwich diamino ruthenium(II) salts
Zuccaccia, Daniele,Macchioni, Alceo
, p. 3476 - 3486 (2005)
The utility of PGSE NMR measurements in determining hydrodynamic radii (rH) and volumes (VH) of small- and medium-size molecules (3 A H 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 r H 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)(R1R2NCH2CH2NR 1R2)Cl]X (R1 = R2 = H, 1; R 1 = H, R2 = H, 2; R1 = R2 = Me, 3; X- = PF6- or BPh11) in both protic and aprotic solvents with a relative permittivity (εr) ranging from 4.81 (chloroform-d) to 32.66 (methanol-d 4). Compounds 1 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 1 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 1H-NOESY and 19F, 1H-HOESY NMR spectroscopy. The relative anion-cation position was well-defined, especially for compounds bearing the PF6- 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.
