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CH‐π Interactions in 1‐ n ‐Butyl‐3‐methylimidazolium Tetraphenylborate Molten Salt: Solid and Solution Structures
Author(s) -
Dupont Jairton,
Suarez Paulo A. Z.,
De Souza Roberto F.,
Burrow Robert A.,
Kintzinger JeanPierre
Publication year - 2000
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20000703)6:13<2377::aid-chem2377>3.0.co;2-l
Subject(s) - tetraphenylborate , chemistry , ionic liquid , nmr spectra database , proton nmr , ion , proton , crystallography , ionic bonding , salt (chemistry) , chemical shift , solvent , two dimensional nuclear magnetic resonance spectroscopy , inorganic chemistry , spectral line , stereochemistry , organic chemistry , catalysis , physics , quantum mechanics , astronomy
The crystal structure of 1‐ n ‐butyl‐3‐methylimidazolium tetraphenylborate molten salt ( 1 ) shows CH‐π interactions between the hydrogens of the imidazolium cation and the phenyl rings of the tetraphenylborate anion. The imidazolium ring is surrounded by three tetraphenylborate anions that are connected with the same cation by CH‐π (phenyl rings) interactions. The nearest inter‐ion interaction is found between the N‐CHN proton of the cation and the B‐phenyl centroid (2.349 Å) with a nearly T‐shaped geometry. The inter‐ionic solution structure of 1 has been investigated by the detection of inter‐ionic contacts in 1 H NOESY NMR spectra between the protons of the cation and the anion. The 1 H‐NMR spectra of molten salt 1 is almost independent of its concentration in [D 6 ]DMSO solution, the imidazolium proton chemical shifts are in the expected region and there are no observable NOE effects between the protons of the cation with those of the anion, indicating that 1 behaves in [D 6 ]DMSO as a solvent‐separated ion pair. In CDCl 3 the 1 H‐NMR spectra of 1 are concentration dependent and all the imidazolium protons are shielded as compared with those observed in [D 6 ]DMSO. Moreover, the 1 H NOESY NMR spectra show all the peaks affected by the interaction between the protons of the imidazolium cation and those of the anion, indicating that in CDCl 3 1 possesses a contact ion pair structure. The NCHN proton of the cation exhibits the greatest shielding (up to −4.5 ppm), an indication of the existence of CH‐π interactions, even in solution. The calculated distance of this proton to the phenyl centroid is 2.3 Å for a CH‐π angle of 180°. The apparent volumes for the cation and anion, calculated from the measured 13 C‐NMR relaxation times, increase from 38 and 140 Å 3 in [D 6 ]DMSO to 360 and 600 Å 3 in CDCl 3 , respectively; this indicates the formation of floating aggregates of the type ( 1 ) n in CDCl 3 via weak hydrogen bonds, with increasing concentration.