Proton Sponge Zwitterions with Positively and Negatively Charged Hydrogen Bonds: Extremely Polar Organic Compounds and Interplay of [NHN]+ and [NHN]− Properties

Six previously unknown zwitterions with positively and negatively charged [NHN] hydrogen bonds were synthesized by acylation of 4,5-bis­(dimethylamino)-1-tosylamino-8-aminonaphthalene with subsequent alkaline treatment of the resulting 8-acylamino derivatives. Using NMR and XRD measurements in conjunction with quantum chemical DFT/PBE1PBE/6-311++G­(d,p) calculations, it was shown that the negatively charged [NHN]− bond in such compounds commonly differs from the [NHN]+ bond by significantly lower linearity, higher asymmetry, and moderate to strong paramagnetic shift of the chelated NH proton signal. Among other remarkable findings, the most important are (1) unusually high polarity (μ = 21–26 D) of the obtained zwitterions, (2) sharp difference in structures of the solid 1,8-bis­(tosylated) zwitterion (BTZ) grown from MeCN or DMF, and (3) registration for one of the stereoisomers of BTZ with the record short [NHN]− hydrogen bridge (N···N = 2.510 Å) almost reaching the theoretical limit (2.50 Å) for the [NHN]+ hydrogen bond.