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

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.