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

The synthesis, characterization, and properties of dipicolinic acid (dipicH(2)) derivatives are described. Neutralization of hot aqueous concentrated solutions of dipicH(2) showed successive gelifications-liquefaction processes. Isolation of crystalline-phase [(dipicH)(dipicH(2))]Na . 3H(2)O (1) revealed an intricate network of interactions among the 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 with bases (cation(+))OH- to give [(dipic)(2)Fe-II](cation)(2) in high yields (3: cation = NProp(4)(+)). [(dipic)(2)Fe-II](2-) was solvatochromic, due to interactions between the dipic oxygen lone pairs and solvents. Air or bromine water oxidation of 3 yielded quantitatively the iron(III) complexes [(dipic)(2)Fe-III](cation) (4: cation = Na+). The crystal structures of 1-4 have been determined. Comparison of the geometries of 3 and 4 showed that the iron-nitrogen bond lengths are insensitive to iron oxidation states whereas the iron-oxygen distances decrease by 0.15 Angstrom upon iron oxidation. This allowed the calculation of the intervalence band energy in dipic mixed-valence systems. 1 crystallized in the triclinic space group <P(1)over bar> with cell constants a = 6.882(1) Angstrom, b = 11.125(2)Angstrom, c = 11.171(2) Angstrom, 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 monoclinic 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) Angstrom(3) and Z = 4. 3 . 7.25H(2)O occurred in the monoclinic space group P2(1)/c with a = 15.320(9) Angstrom, b = 33.83(2) Angstrom, c = 17.24(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 with 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.