Synthesis, pH-Dependent Structural Characterization, and Solution Behavior of Aqueous Aluminum and Gallium Citrate Complexes

• Published in: Inorganic chemistry 2001-04, Vol.40 (8), p.1734-1744
• Main Authors: Matzapetakis, M, Kourgiantakis, M, Dakanali, M, Raptopoulou, C. P, Terzis, A, Lakatos, A, Kiss, T, Banyai, I, Iordanidis, L, Mavromoustakos, T, Salifoglou, A
• Format: Article
• Language: English
• Subjects: Aluminum - pharmacokinetics; Aluminum Compounds - chemical synthesis; Aluminum Compounds - chemistry; Citrates - chemical synthesis; Citrates - chemistry; Crystallography, X-Ray; Gallium - chemistry; Gallium - pharmacokinetics; Hydrogen-Ion Concentration; Kinetics; Magnetic Resonance Spectroscopy; Molecular Structure; Molecular Weight; Solutions; Spectroscopy, Fourier Transform Infrared; Water - chemistry
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic000461l

Reactions of Al(III) and Ga(III) with citric acid in aqueous solutions, yielded the complexes (NH4)5{M(C6H4O7)2}·2H2O (M(III) = Al (1), Ga (2)) at alkaline pH, and the complexes (Cat)4{M(C6H5O7)(C6H4O7)}·nH2O (M(III) = Al (3), Ga (4), Cat. = NH4 +, n = 3; M(III) = Al (5), Ga (6), Cat. = K+, n = 4) at acidic pH. All compounds were characterized by spectroscopic (FT-IR, 1H, 13C, and 27Al NMR, 13C-MAS NMR) and X-ray techniques. Complex 1 crystallizes in space group P1̄, with a = 9.638(5) Å, b = 9.715(5) Å, c = 7.237(4) Å, α = 90.96(1)°, β = 105.72(1)°, γ = 119.74(1)°, V = 557.1(3) Å3, and Z = 1. Complex 2 crystallizes in space group P1̄, with a = 9.659(6) Å, b = 9.762(7) Å, c = 7.258(5) Å, α = 90.95(2)°, β = 105.86(2)°, γ = 119.28(1)°, V = 564.9(7) Å3, and Z = 1. Complex 3 crystallizes in space group I2/a, with a = 19.347(3) Å, b = 9.857(1) Å, c = 23.412(4) Å, β = 100.549(5)°, V = 4389(1) Å3, and Z = 8. Complex 4 crystallizes in space group I2/a, with a = 19.275(1) Å, b = 9.9697(6) Å, c = 23.476(1) Å, β = 100.694(2)°, V = 4432.8(5) Å3, and Z = 8. Complex 5 crystallizes in space group P1̄, with a = 7.316(1) Å, b = 9.454(2) Å, c = 9.569(2) Å, α = 64.218(4)°, β = 69.872(3)°, γ = 69.985(4)°, V = 544.9(2) Å3, and Z = 1. Complex 6 crystallizes in space group P1̄, with a = 7.3242(2) Å, b = 9.4363(5) Å, c = 9.6435(5) Å, α = 63.751(2)°, β = 70.091(2)°, γ = 69.941(2)°, V = 547.22(4) Å3, and Z = 1. The crystal structures of 1−6 reveal mononuclear octahedral complexes of Al(III) (or Ga(III)) bound to two citrates. Solution NMR, on both 4− and 5− species, reveals rapid intramolecular exchange of the bound and unbound terminal carboxylates. Upon dissolution in water, the complexes, through a complicated reaction cascade, transform to oligonuclear 1:1 species that, in agreement with previous studies, represent the thermodynamically stable state in solution. The data provide, for the first time, structural details of low MW, mononuclear complexes of Al(III) (or Ga(III)) with citrate that are dictated, among other factors, by pH. The properties of 1−6 may provide clues relevant to their biological association with humans.