Synthesis, pH-dependent structural characterization, and solution behaviorof aqueous aluminum and gallium citrate complexes

Reactions of Al(III) and Ga(III) with citric acid in aqueous solutions, yie lded the complexes (NH4)(5){M(C6H4O7)(2)}. 2H(2)O (M(III) = Al(1), Ga (2)) at alkaline pH, and the complexes (Cat)(4){M(C6H4O7)(C6H4O7)}. nH(2)O (M(II I) = Al (3), Ga (4), Cat. = NH4(+), n = 3; M(III) = Al (5), Ga (6), Cat. = Kf, n = 4) at acidic pH. All compounds were characterized by spectroscopic (FT-IR, H-1,C-13, and Al-27 NMR, C-13-MAS NMR) and X-ray techniques. Comple x 1 crystallizes in space group P (1) over bar, with a 9.638(5) Angstrom, b = 9.715(5) Angstrom, c = 7.237(4) Angstrom, alpha = 90.96(1)degrees, beta = 105.22(1)degrees, gamma = 119.74(1)degrees, V = 557.1(3) Angstrom (3), an d Z = 1. Complex 2 crystallizes in space group P (1) over bar, with a = 9.6 59(6) Angstrom, b = 9.762(7) Angstrom, c = 7.258(5) Angstrom, alpha = 90.95 (2)degrees, beta = 105.86(2)degrees, gamma = 119.28(1)degrees, V = 564.9(7) Angstrom (3). and Z = 1. Complex 3 crystallizes in space group I2/a, with a = 19.347(3) Angstrom, b = 9.857(1) Angstrom, c = 23.412(4) Angstrom, beta = 100.549(5)degrees V = 4389(1) Angstrom (3), and Z = 8. Complex 4 crystal lizes in space group I2/a, with a = 19.275(1) Angstrom, b = 9.9697(6)Angstr om, c = 23.476(1) Angstrom, beta = 100.694(2)degrees, V = 4432.8(5).Angstro m (3), and Z = 8. Complex 5 crystallizes in space group P (1) over bar, wit h a = 7.316(1) Angstrom, b = 9.454(2) Angstrom. c = 9.569(2) Angstrom. alph a = 64.218(4)degrees, beta = 69.872(3)degrees, gamma = 69.985(4)degrees, V = 544.9(2) Angstrom (3), and Z = 1. Complex 6 crystallizes in space group P (1) over bar, with a = 7.3242(2) Angstrom, b = 9.4363(5) Angstrom, c 9.643 5(5) Angstrom. alpha = 63.751(2)degrees, beta = 70.091(2)degrees, gamma = 6 9.941(2)degrees V = 547.22(4) Angstrom (3), and Z = 1. The crystal structur es of 1-6 reveal mononuclear octahedral complexes of AI(III) (or Ga(III)) b ound to two citrates. Solution NMR, on both 4- and 5- species, reveals rapi d intramolecular exchange of the bound and unbound terminal carboxylates. U pon dissolution in water, the complexes. through a complicated reaction cas cade, transform to oligonuclear 1:1 species that, in agreement with previou s studies, represent the thermodynamically stable state in solution. The da ta provide, for the first time, structural details of low MW, mononuclear c omplexes of AI(III) (or Ga(III)) with citrate that are dictated, among othe r factors, by pH. The properties of 1-6 may provide clues relevant to their biological association with humans.