Manganese Citrate Chemistry:  Syntheses, Spectroscopic Studies, and Structural Characterizations of Novel Mononuclear, Water-Soluble Manganese Citrate Complexes

The first two mononuclear manganese citrate complexes, (NH₄)₄[Mn^II(C₆H₅O₇)₂] (1) and (NH₄)₅[Mn^III(C₆H₄O₇)₂]·2H₂O (2) were synthesized in aqueous solutions near physiological pH values. They were isolated in their pure crystalline forms and characterized by elemental analyses and spectroscopic techniques, including UV/visible, electron paramagnetic resonance, Fourier transformed infrared, and magnetic susceptibility measurements. Compound 1 crystallizes in the monoclinic space group P2₁/c, with a = 8.777(1) Å, b = 13.656(3) Å, c = 9.162(2) Å, β = 113.62(2)°, V = 1006.2(6) ų, and Z = 2. Compound 2 crystallizes in the triclinic space group P1̄, with a = 9.606(3) Å, b = 9.914(3) Å, c = 7.247(3) Å, α = 91.05(1)°, β = 105.60(1)°, γ = 119.16(1)°, V = 571.3(3) ų, and Z = 1. The X-ray crystal structures of 1 and 2 revealed that, in both cases, the manganese ion is six-coordinate and is bound by two citrate ligands in a distorted octahedral fashion. In the case of complex 1, the citrate ion binds to Mn²⁺ as a triply deprotonated ligand, retaining the central carbon hydroxyl hydrogen, whereas, in the case of compound 2, the citrate ligand coordinates to Mn³⁺ as a fully deprotonated entity. Compound 2 contains water molecules of crystallization in the unit cell which, through extensive hydrogen-bonding interactions, bestow considerable stability upon the Mn³⁺−citrate assembly. There are significant contributions to the stabilities of the assembled lattices in 1 and 2 arising from the ammonium counterions neutralizing the high anionic charges of the complexes. The EPR spectra attest to the presence of paramagnetic Mn²⁺ and Mn³⁺ species in the solid state. Corroborative evidence is obtained from the magnetic susceptibility measurements in the range 5−300 K. Complexes 1 and 2 present clear cases of mononuclear manganese citrate species relevant to manganese speciation in biological media and potentially related to the beneficial as well as toxic effects of manganese on humans.