¹⁵N chemical shifts of a number of polyazaindenes: Correlation with charge separated resonance contributing structures

Abstract

The "N chemical shifts of a number of polyazaindenes have been determined, and have been correlated with the degree of contribution to the ground state of those resonance structures which place a partial positive charge on the bridge-bead nitrogen.The study of the chemistry and spectroscopic properties of polyazaindenes have interested us for some time.' A recent publication2 has described the results of some I4N NMR studies of pyrrocolines with additional nitrogens in the 5-membered ring ( We have now obtained the 15N spectra of a number of imidazo[ 1,2-a]pyridines with additional nitrogens in the 6-membered ring C, and have re-examined, by means of 15N spectroscopy, some of the compounds already described by Stefaniak.2 It is the purpose of this report to interrelate these results and to correlate the "N chemical shifts with respect to the major resonance contributing structures A and B, respectively. These resonance contributing structures were suggested on ' H NMR and chemical reactivity grounds some time ago.' A B Table 1 gives the 15N chemical shifts obtained in this study, as well as those reported in Ref. 2. Among all of these compounds N-4, the bridge-atom, is always the most shielded nitrogen, except in some instances where it is directly bonded to another nitrogen atom (8, 9, 11). The contribution of resonance structures such as A or B clearly places a partial positive charge on the bridge nitrogen and a partial negative charge on N-1 or N-2, respectively.* Author to whom correspondence should be addressed. the 'sole' resonance contributing structure to the ground state, the N-4 chemical shift would be similar to that of I5N in N-alkylpyridinium ions (this, of course, is only a qualitative comparison). Conversely, the more deshielded a particular N-4 with respect to other polyazaindenes, the less this resonance structure

Palladium(II) chloride complexes with 1,2,4-triazolo[1,5-a]pyrimidines: X-ray, 15N–1H NMR and 15N CP MAS studies

“…26-31 The 15 N NMR spectrum of tp was also published but the assignment of resonances was the subject of literature controversies. [32][33][34][35][36] Recently we have assigned those signals using the 15 N-1 H HETCOR method. 37 Selective decoupling techniques for 1 H, 13 C and 15 N NMR measurements of dmtp, dptp and dbtp were also applied.…”

“…N(1) and N(4). [32][33][34][35][36] The discussed palladium() complexes decompose in d 6dmso, therefore we have performed NMR measurements in CDCl 3 (2, 4a-4c) or in the solid phase (1,3), the data being collected in Tables 3-5. For comparison, we have measured also the 13 C and 15 N CP MAS spectra for the ligands tp and dptp (Tables 4, 5).…”