An Accurate Methodology to Identify the Level of Aggregation in Solution by PGSE NMR Measurements: The Case of Half-Sandwich Diamino Ruthenium(II) Salts
作者:Daniele Zuccaccia、Alceo Macchioni
DOI:10.1021/om050145k
日期:2005.7.1
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)(R1R2NCH2CH2NR1R2)Cl]X (R-1 = R-2 = H, 1; R-1 = H, R-2 = H, 2; R-1 = R-2 = Me, 3; X- = PF6- or BPh4-) 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-(HCl)-Cl-... and cation/anion N-(HFPF5-)-F-... 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 H-1-NOESY and F-19,H-1-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.