13C and 1H nuclear magnetic resonance study of solvent effects on tautomerism in 1-aryl-3-methyltriazenes

Abstract

13 C and 1H n.m.r. spectra of 1-p-tolyl-(II) and 1-p-nitrophenyl-3-methyltriazene (III) have been recorded in CDCl3 at variable temperatures and in [2H6]DMSO solutions at ambient temperature. Assignment of 13C and 1H signals to one or both of the tautomeric species (ArNN˙NHCH3 or ArNH˙NNCH3) has been carried out with the aid of gated decoupling and full 1H decoupling methods and by comparison of chemical shifts with those of the model compounds p-nitroaniline, 3,3-dimethyl-1-p-nitrophenyltriazene, and 3-methyl-1,5-di-(p-tolyl)-pentaza-1,4-diene (IV). The 13C spectrum of (II) in CDCl3 shows signals assigned to the conjugated tautomer (IIa), but not the unconjugated tautomer (IIb); additional signals in the 13C spectrum of (II) are shown to arise from the pentazadiene (IV), which is present as an impurity (ca. 10%) in commercial samples of (II). 1H and 13C spectra of (IV) have been completely assigned. The tautomeric equilibrium (IIa)⇄(IIb) is considerably slowed in [2H6]DMSO as solvent, so much so that two full sets of aromatic peaks are visible in the 13C spectrum of (II) in [2H6]DMSO at room temperature and the 1H spectrum clearly shows signals unequivocally assigned to the unconjugated tautomer (IIb). The equilibrium (IIa)⇄(IIb) is shown to be concentration dependent. For CDCl3 solutions of (II), resolution of 1H signals due to the minor tautomer (IIb) is only possible in dilute solutions, and the 13C signals of IIb are observed in the 62.8 MHz spectra of dilute solutions. For the first time, full 13C spectra of both tautomers of the p-nitrophenyltriazene (IIIa)⇄(IIIb) are observed at room temperature in [2H6]DMSO and the 1H spectra of (III) in [2H6]DMSO or in CDCl3 at room temperature also show the presence of both tautomers. The relative proportions of tautomers (IIIa) : (IIIb) was found to be 49 : 51 in [2H6]DMSO.