Tautomeric pyridines. Part 22. The effect of intramolecular hydrogen bonding on the tautomeric structure of 4-amino-1,10-phenanthrolines

Abstract

The tautomeric equilibria of 4-amino-1,10-phenanthroline, 4-amino-2,2′-bipyridyl, and certain of their N-substituted derivatives are assessed from pK_a and u.v. spectral data. Comparison of pK_T values for 4-amino-, and 4-anilino-1,10-phenanthroline with those for the corresponding pyridine derivatives indicates that in the former series a combination of intramolecular hydrogen bonding and destabilisation by lone pairs in close proximity reduces the predominance of the ‘amino’ tautomer by a factor of ca. 100.