CHEMISTRY OF IRON WITH DIPICOLINIC ACID .1. MONONUCLEAR COMPLEXES OF IRON(II) OR IRON(III)

The synthesis, characterization, and properties of dipicolinic acid (d ipicH(2)) derivatives are described. Neutralization of hot aqueous con centrated solutions of dipicH(2) showed successive gelifications-lique faction processes. Isolation of crystalline-phase [(dipicH)(dipicH(2)) ]Na . 3H(2)O (1) revealed an intricate network of interactions among t he acid, the sodium cation, and water molecules. [(dipic)Fe-II(OH2)(3) ] (2) was prepared in nearly quantitative yield by reaction of dipic(2 -) with an excess of ammonium iron(II) sulfate hexahydrate. 2 reacts w ith bases (cation(+))OH- to give [(dipic)(2)Fe-II](cation)(2) in high yields (3: cation = NProp(4)(+)). [(dipic)(2)Fe-II](2-) was solvatochr omic, due to interactions between the dipic oxygen lone pairs and solv ents. Air or bromine water oxidation of 3 yielded quantitatively the i ron(III) complexes [(dipic)(2)Fe-III](cation) (4: cation = Na+). The c rystal structures of 1-4 have been determined. Comparison of the geome tries of 3 and 4 showed that the iron-nitrogen bond lengths are insens itive to iron oxidation states whereas the iron-oxygen distances decre ase by 0.15 Angstrom upon iron oxidation. This allowed the calculation of the intervalence band energy in dipic mixed-valence systems. 1 cry stallized in the triclinic space group <P(1)over bar> with cell consta nts a = 6.882(1) Angstrom, b = 11.125(2)Angstrom, c = 11.171(2) Angstr om, alpha = 82.65 (4)degrees, beta = 82.13(4)degrees, gamma = 87.38(6) degrees, V = 844(3) Angstrom(3), and Z = 2. 2 was found in the monocli nic space group P2(1)/c with alpha = 12.410(4) Angstrom, b = 9.314(4) Angstrom, c = 8.746(3) Angstrom, beta = 101.44(4)degrees, V = 990(1) A ngstrom(3) and Z = 4. 3 . 7.25H(2)O occurred in the monoclinic space g roup P2(1)/c with a = 15.320(9) Angstrom, b = 33.83(2) Angstrom, c = 1 7.24(1) Angstrom, beta = 91.55(4)degrees, V = 8932(9) Angstrom(3), and with Z = 8. 4 . H2O crystallized in the monoclinic space group Cc wit h a = 14.95(1) Angstrom, b = 12.45(3) Angstrom, c = 8.663(5) Angstrom, beta = 92.99(4)degrees, V = 609(4) Angstrom(3), and Z = 4.