Symmetrization of Cationic Hydrogen Bridges of Protonated Sponges Induced by Solvent and Counteranion Interactions as Revealed by NMR Spectroscopy

Abstract

The properties of the intramolecular hydrogen bonds of doubly 15N‐labeled protonated sponges of the 1,8‐bis(dimethylamino)naphthalene (DMANH+) type have been studied as a function of the solvent, counteranion, and temperature using low‐temperature NMR spectroscopy. Information about the hydrogen‐bond symmetries was obtained by the analysis of the chemical shifts δH and δN and the scalar coupling constants J(N,N), J(N,H), J(H,N) of the 15NH15N hydrogen bonds. Whereas the individual couplings J(N,H) and J(H,N) were averaged by a fast intramolecular proton tautomerism between two forms, it is shown that the sum |J(N,H)+J(H,N)| generally represents a measure of the hydrogen‐bond strength in a similar way to δH and J(N,N). The NMR spectroscopic parameters of DMANH+ and of 4‐nitro‐DMANH+ are independent of the anion in the case of CD3CN, which indicates ion‐pair dissociation in this solvent. By contrast, studies using CD2Cl2, [D8]toluene as well as the freon mixture CDF3/CDF2Cl, which is liquid down to 100 K, revealed an influence of temperature and of the counteranions. Whereas a small counteranion such as trifluoroacetate perturbed the hydrogen bond, the large noncoordinating anion tetrakis[3,5‐bis(trifluoromethyl)phenyl]borate B[{C6H3(CF3)2}4]− (BARF−), which exhibits a delocalized charge, made the hydrogen bond more symmetric. Lowering the temperature led to a similar symmetrization, an effect that is discussed in terms of solvent ordering at low temperature and differential solvent order/disorder at high temperatures. By contrast, toluene molecules that are ordered around the cation led to typical high‐field shifts of the hydrogen‐bonded proton as well as of those bound to carbon, an effect that is absent in the case of neutral NHN chelates.

The specificity of the [NHN]+ hydrogen bonds in protonated naphthalene proton sponges

“…As far as 15 N resonance data, which yields important information, is concerned, one can be found in papers by Limbach et al [28].…”

15N–15N spin–spin coupling constants through intermolecular hydrogen bonds in the solid state

“…The actual position of the NH (NH bond length and NHN bond angle) should be intermediate between that determined by X-ray crystallography (a well-known problem resulting in abnormally short X-H distances) [26] and the one calculated for the gas phase. As studied for 3, the effect of anions on the 2h J NN present in proton sponges has been examined by Limbach et al [6]. Strong coordination of the anion with the proton of the HB was found to weaken the HB, which becomes more asymmetric, and 2h J NN decreases; in contrast, anions that are less solvating will increase the observed coupling.…”

“…In small molecules the 2h J NN couplings in solution, ranging from 16.5 Hz to 5.3 Hz, have primarily been determined for intramolecular hydrogen bonds [6,7]. The only contribution concerning intermolecular hydrogen bonds is due to Sijbesma et al dealing with ureidopyrimidinones that exhibit 2h J NN couplings with values of about 5 Hz that, in some cases appeared to be temperature dependent [8].…”

Hydrogen Motion in Proton Sponge Cations: A Theoretical Study

“…[6][7][8] Many experimental studies have considered the bonding situation in proton sponge cations. NMR [8][9][10][11] and IR spectroscopy, [12][13][14][15][16][17] and X-ray diffraction [12][13][14][15] are typical methods used for the examination of such systems. Nevertheless, it is very difficult to draw conclusions regarding the hydrogen dynamic behavior from purely experimental data.…”