Hydrogen-1 nuclear magnetic resonance study of the self-association of 1,10-phenanthroline, 2,2′-bipyridyl, and their zinc(II) complexes

Abstract

The ¹H n.m.r. spectra of 1,10-phenanthroline (phen) and of 2,2 -bipyridyl (bipy) are very concentration dependent in dilute aqueous solution. The concentration dependence of the chemical shifts is consistent with the isodesmic model of indefinite non-co-operative stacking; for phenanthroline K= 23.6 ± 1.8, and for bipyridyl K= 7.4 ± 1.6 dm³ mol^–1. In solution in 25% v/v methanol in water self-association of phenanthroline is much weaker (K= 5.2 ± 1.0 dm³ mol^–1). No self-association is observed in organic solvents such as dioxan and chloroform. The ¹H n.m.r. spectra of [Zn(phen)]²⁺ and of [Zn(bipy)]²⁺ vary much less with concentration than do the spectra of the free ligands; the stability constants for the self-association of [Zn(phen)]²⁺(K= 1.1 ± 0.2 dm³ mol^–1) and of [Zn(bipy)]²⁺(K ca. 0.3 dm³ mol^–1) are much lower than for phenanthroline and bipyridyl.