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)]. 2H(2)O (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 P2(1)/c, with a = 8.777(1) Angstrom, b = 13.656(3) Angstrom, c = 9.162(2) Angstrom, beta = 113.62(2)degrees, V = 1006.2(6) Angstrom(3), and Z = 2. Compound 2 crystallizes in the triclinic Space group P (1) over bar, with a = 9.606(3) Angstrom, b = 9.914(3) Angstrom, c = 7.247(3) Angstrom, alpha = 91.05(1)degrees, beta = 205.60(1)degrees, gamma = 119.16(1)degrees, V = 571.3(3) Angstrom(3), 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 ease 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.