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) Å3, and Z = 2. Compound 2 crystallizes in the triclinic space group P1, 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) Å3, 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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry