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=BF 4 − ; b : X=PF 6 − ; c : X=MeSO 3 − , d : X=CF 3 SO 3 − ; e : X=BArF − ; f : X=PtCl 3 (C 2 H 4 ) − ], b) 4‐ tert ‐butyl‐ N ‐benzyl analogue, 7 and c) the aryl carbocations ( p ‐R‐C 6 H 4 ) 2 CH 9 a (R=CH 3 O) and 9 b (R=(CH 3 ) 2 N), ( p ‐CH 3 O‐C 6 H 4 ) x CPh 3− x + 10 a – c ( x =1–3, respectively) and ( p ‐R‐C 6 H 4 ) 3 C + 11 (R=(CH 3 ) 2 N) and 12 (R=H) all in several different solvents, are reported. The solvent dependence suggests strong ion pairing in CDCl 3 , intermediate ion pairing in CD 2 Cl 2 and little ion pairing in [D 6 ]acetone. 1 H, 19 F 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 BF 4 − salts) based on the NOE results, one finds marked changes in the relative positions of the BF 4 − 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][PtCl 3 (C 2 H 4 )], 6 f , are reported. Analysis of 195 Pt NMR and other NMR measurements suggest that the η 2 ‐C 2 H 4 bonding to the platinum centre in 6 f is very similar to that found in K[PtCl 3 (C 2 H 4 )]. Field dependent T 1 measurements on [brucinium][PtCl 3 (C 2 H 4 )] and K[PtCl 3 (C 2 H 4 )], are reported and suggested to be useful in recognizing aggregation effects.