Interpretation of the Infrared Spectra of Organophosphorus Compounds

Abstract

The stability constants of ferric complexes with several substituted catechol (1,2-dihydroxybenzene) ligands in aque-ous solutions of low ionic strength have been determined at 27 °C in the pH range 2-11. These simple compounds are used as models of the catechol-containing iron transport compounds (siderophores) found in microorganisms. Enterobactin, the principal siderophore of enteric bacteria, is a tricatechol and, from the formation constants reported here, is estimated to have a formation constant with ferric ion which is greater than 1045. The stepwise formation constants, K", of the catechol ligands reported here are defined as [ML"]/[ML"_i][L], in units of L mol-1, where [L] is the concentration of the deprotonated cate-chol ligand. The constants were determined from potentiometric and spectroscopic data and were refined on pH values by weighted least squares. Qualitative examination of electron spin resonance spectra of some of the systems indicated some oxidation of the ligand by ferric ions at pH values as high as 4. The ligands studied included catechol (cat) (log K\ = 20.01, log A2 = 14.69, log A3 = 9.01); 4,5-dihydroxy-m-benzenedisulfonate (Tirón) (log A2 = 15.12, log A3 = 10.10); 4-nitrocatechol (neat) (log A] = 17.08, log A2 = 13.43, log A3 = 9.51); 3,4-dihydroxyphenylacetic acid (dhpa) (log K\ = 20.1, log A2 = 14.9, log A3 = 9.0); and 2,3-dihydroxybenzoic acid (dhba) (log At = 20.5). The acid dissociation constants, Aas, were determined also. For the catechol protons these follow: cat (pAa, = 9.22, pAa2 = 13.0); Tirón (pAa, = 7.70, pAaz = 12.63); neat (pAa, = 6.65, pAa2 = 10.80); dhpa (pAa, = 9.49, pAa2 = 13.7); and dhba (pAa, = 10.06, pAa2 = 13.1). In addition, carboxylate substit-uents of dhpa and dhba have pAas of 4.17 and 2.70, respectively. In acidic solution there is evidence of coordination by dhpa via the carboxylate group (which presumably does not involve chelation). Similar coordination by dhba does involve chelation (the same as salicylate) and the system is complicated by mixed-mode coordination of both catechol and salicylate type. In solution , exchange is slow between these two types of coordination following changes in pH.