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 (15)N-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 delta(H) and delta(N) and the scalar coupling constants J(N,N), J(N,H), J(H,N) of the (15)NH(15)N 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 delta(H) and J(N,N). The NMR spectroscopic parameters of DMANH(+) and of 4-nitro-DMANH(+) are independent of the anion in the case of CD(3)CN, which indicates ion-pair dissociation in this solvent. By contrast, studies using CD(2)Cl(2), [D(8)]toluene as well as the freon mixture CDF(3)/CDF(2)Cl, 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[{C(6)H(3)(CF(3))(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.