³¹P and ³⁵Cl PGSE Diffusion Studies on Phosphine Ligands and Selected Organometallic Complexes. Solvent Dependence of Ion Pairing

Abstract

P and 35 Cl PGSE (pulsed gradient spin-echo) diffusion measurements on phosphine ligands and transition-metal complexes show that these nuclei can provide a useful complement to the existing 1 H and 19 F NMR forms of this experiment. The observed solvent dependence of the diffusion values in several palladium salts suggests that ion pairing is a major contributor in chloroform, important in methylene chloride, but small in acetone and methanol. A concentration dependence of the diffusion values is observed.

NMR spectroscopy and ion pairing: Measuring and understanding how ions interact

“…Specifically, one finds a substantial, multinuclear ( 1 H, 7 Li, 19 F, 31 P, 195 Pt) literature concerned with the use of pulsed field gradient NMR methods to measure diffusion constants [3][4][5][6], and the subject has been reviewed several times [7,8]. In the classical pulsed field gradient spin-echo (PGSE), Fig.…”

Low Temperature 1H‐, 19F‐, and 31P‐PGSE Diffusion Measurements. Applications to Cationic Alcohol Complexes

“…( Fig. 1, a) From our previous work [18] with 5 in CD 3 OD at ambient temperature (see Table 7), it is clear that the CF 3 SO À 3 anion in this compound moves independently of the cation. The ratio D(anion)/D(cation) for 5 at 299 K (1.8 ± 1.9) is identical to that observed for the same compound at 240 K ( Table 6), so that it can be concluded that, within this temperature range, there is no change in interionic interactions in 5.…”

“…In morepolar solvents, such as MeOH, the cation and anion are usually not associated and show different and mutually independent D-values. In CH 2 Cl 2 , we have observed an intermediate situation, where there is partial but usually not complete ion pairing [18] [19]. The translational properties of the cation will then be affected by the anion, and vice versa.…”

“…Through a separate analysis of the translational properties of cation and anion, it is possible to gain insight into whether these charged species move independently or as a single unit in solution. Solvents with a relatively low dielectric constant, such as CHCl 3 , favor the formation of tight ion pairs [18] [19]. In these solvents, both cation and anion have been found to show the same, relatively small, diffusion coefficient as they move together as a relatively large unit.…”

“…By means of these methods, the relative anion-dependent activity of the Ir phosphinooxazoline (PHOX) complexes 1 in the catalytic hydrogenation of olefins in CH 2 Cl 2 can be explained [6] H-PGSE diffusion measurements for the cation and anion, respectively. In addition, we have recently shown that 31 P is a reliable alternative to 1 H as a diffusion probe in organometallic complexes [18]. 31 P-Nuclei can be useful when no suitable…”