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

The first two mononuclear manganese citrate complexes, (NH<sub>4</sub>)<sub>4</sub>[Mn<sup>II</sup>(C<sub>6</sub>H<sub>5</sub>O<sub>7</sub>)<sub>2</sub>] (<b>1</b>) and (NH<sub>4</sub>)<sub>5</sub>[Mn<sup>III</sup>(C<sub>6</sub>H<sub>4</sub>O<sub>7</sub>)<sub>2</sub>]·2H<sub>2</sub>O (<b>2</b>) 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 <b>1</b> crystallizes in the monoclinic space group <i>P</i>2<sub>1</sub>/<i>c</i>, with <i>a</i> = 8.777(1) Å, <i>b</i> = 13.656(3) Å, <i>c</i> = 9.162(2) Å, β = 113.62(2)°, <i>V</i> = 1006.2(6) Å<sup>3</sup>, and <i>Z</i> = 2. Compound <b>2</b> crystallizes in the triclinic space group <i>P</i>1̄, with <i>a</i> = 9.606(3) Å, <i>b</i> = 9.914(3) Å, <i>c</i> = 7.247(3) Å, α = 91.05(1)°, β = 105.60(1)°, γ = 119.16(1)°, <i>V</i> = 571.3(3) Å<sup>3</sup>, and <i>Z</i> = 1. The X-ray crystal structures of <b>1</b> and <b>2</b> 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 <b>1</b>, the citrate ion binds to Mn<sup>2+</sup> as a triply deprotonated ligand, retaining the central carbon hydroxyl hydrogen, whereas, in the case of compound <b>2</b>, the citrate ligand coordinates to Mn<sup>3+</sup> as a fully deprotonated entity. Compound <b>2</b> contains water molecules of crystallization in the unit cell which, through extensive hydrogen-bonding interactions, bestow considerable stability upon the Mn<sup>3+</sup>−citrate assembly. There are significant contributions to the stabilities of the assembled lattices in <b>1</b> and <b>2</b> arising from the ammonium counterions neutralizing the high anionic charges of the complexes. The EPR spectra attest to the presence of paramagnetic Mn<sup>2+</sup> and Mn<sup>3+</sup> species in the solid state. Corroborative evidence is obtained from the magnetic susceptibility measurements in the range 5−300 K. Complexes <b>1</b> and <b>2</b> 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.