Tris(dipivaloylmethanato)europium induced shifts of proton resonances in π‐deficient nitrogen heterocycles

Tris(dipivaloylmethanato)europium [Eu(DPM) 3 ]induced shifts of the proton resonances of pyridine and its 2‐methyl, 3‐methyl, 2,3‐dimethyl and 2,5‐dimethyl derivatives, pyridazine, pyrimidine, pyrazine, quinoline, isoquinoline, 1,6‐, 1,7‐ and 1,8‐naphthyridine, cinnoline, phthalazine, quinazoline, quinoxaline, 1,4,5‐ and 1,4,6‐triazanaphthalene, pteridine, acridine, phenanthridine and 1, 10‐phenanthroline, have been measured for solutions containing 0·1, 0·3 and 0·5 molar equivalents of Eu(DPM) 3 . Differences in the behaviour of these heterocycles are discussed in terms of the steric relationship between the heterocycle and the complex. The 3 cos 2 ϕ term, present in the expression for calculating pseudo‐contact shifts, is shown to be important. The order of basicity of these heterocycles, as measured by their pK a values, cannot be used to predict the order of Eu(DPM) 3 induced chemical shifts.