Iron (III) derivatives of 4,7-dihydroxy-1,10-phenanthroline

The chemistry of the iron (III) derivatives of 4,7-dihydroxy-l,10-phenanthroline has been studied in detail. Oxidation of the intensely red tris(4,7-dihydroxy-l,10-phenanthroline) iron (II) ion results in a grey compound, tris(4,7-dihydroxy-l,10-phenanthroline)iron(III), which is stable below pH 10. Above pH 10 the grey compound is partially converted into an amber compound in which the ratio of phenanthroline to iron is 2:1. The amber compound is the conjugate base of a purple 2:1 compound with pK(a) = 9.77. The visible absorption spectra of the three species at various pH values are reported. For 4,7-dihydroxy-1,10-phenanthroline pK(3), as determined by ultraviolet absorptometry, is 12.62 +/- 0.2.