Ion Pairing and Salt Structure in Organic Salts through Diffusion, Overhauser, DFT and X-ray Methods

Pulsed gradient spin-echo (PGSE) diffusion characteristics for a) the new [brucinium][X] salts 6 a–f [a: X=BF4−; b: X=PF6−; c: X=MeSO3−, d: X=CF3SO3−; e: X=BArF−; f: X=PtCl3(C2H4)−], b) 4-tert-butyl-N-benzyl analogue, 7 and c) the aryl carbocations (p-R-C6H4)2CH 9 a (R=CH3O) and 9 b (R=(CH3)2N), (p-CH3O-C6H4)xCPh3−x+10 a–c (x=1–3, respectively) and (p-R-C6H4)3C+11 (R=(CH3)2N) and 12 (R=H) all in several different solvents, are reported. The solvent dependence suggests strong ion pairing in CDCl3, intermediate ion pairing in CD2Cl2 and little ion pairing in [D6]acetone. 1H, 19F HOESY NMR spectra (HOESY: heteronuclear Overhauser effect spectroscopy) for 6 and 7 reveal a specific approach of the anion with respect to the brucinium cation plus subtle changes, which are related to the anion itself. Further, for carbocations 9–12, (all as BF4− salts) based on the NOE results, one finds marked changes in the relative positions of the BF4− anion. In these aryl cationic species the anion can be located either a) very close to the carbonium ion carbon b) in an intermediate position or c) proximate to the N or O atom of the p-substituent and remote from the formally positive C atom. This represents the first example of such a positional dependence of an anion on the structure of the carbocation. DFT calculations support the experimental HOESY results. The solid-state structures for 6 c and the novel Zeise's salt derivative, [brucinium][PtCl3(C2H4)], 6 f, are reported. Analysis of 195Pt NMR and other NMR measurements suggest that the η2-C2H4 bonding to the platinum centre in 6 f is very similar to that found in K[PtCl3(C2H4)]. Field dependent T1 measurements on [brucinium][PtCl3(C2H4)] and K[PtCl3(C2H4)], are reported and suggested to be useful in recognizing aggregation effects.