Issue 35, 2010

Counteranion-dependent mechanisms of intramolecular proton transfer in aprotic solution

Abstract

Using the freon mixture CDF3/CDClF2 as solvent we have been able to measure the 1H and 15N NMR spectra of the doubly 15N labeled 2,2′-bipyridinium cation (BpyH+) at temperatures down to 115 K. The obtained NMR parameters strongly depend on the type of counteranions indicating the formation of ion pairs. In the case of the bulky poorly coordinating tetrakis[3,5-bis(trifluoromethyl)phenyl]-borate as the counteranion a strong intramolecular NHN hydrogen bond was observed in BpyH+ exhibiting a degenerate intramolecular proton transfer which is of the order of 106 s−1 even at 120 K. By contrast, the weak hydrogen bond acceptor tetrafluoroborate favors a weak intermolecular FHN interaction and quenches the intramolecular proton transfer. The intramolecular proton transfer requires in this case a dissociation of the ion pair which is hindered by the Coulomb interaction. A slow intramolecular proton transfer was observed in the case of dichloroacetate which forms a strong intermolecular OHN hydrogen bond to BpyH+. The mechanism of this transfer presumably involves a preliminary intermolecular proton transfer from nitrogen towards oxygen followed by a hydrogen bond switch to the neighboring nitrogen to which the proton is then transferred.

Graphical abstract: Counteranion-dependent mechanisms of intramolecular proton transfer in aprotic solution

Supplementary files

Article information

Article type
Paper
Submitted
19 Mar 2010
Accepted
28 Apr 2010
First published
25 Jun 2010

Phys. Chem. Chem. Phys., 2010,12, 10373-10379

Counteranion-dependent mechanisms of intramolecular proton transfer in aprotic solution

S. B. Lesnichin, P. M. Tolstoy, H. Limbach and I. G. Shenderovich, Phys. Chem. Chem. Phys., 2010, 12, 10373 DOI: 10.1039/C004499G

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