pH-Specific Synthetic Chemistry and Solution Studies in the Binary System of Iron(III) with the α-Hydroxycarboxylate Substrate Quinic Acid: Potential Relevance to Iron Chemistry in Plant Fluids

Iron is an essential metal ion in plant growth and development. Mobilization and further use of that metal by cellular structures in metabolic pathways entails the existence of soluble forms complexed with indigenous organic substrates, such as the low molecular mass d-(-)-quinic acid. In an effort to understand the relevant aqueous chemistry involving well-defined forms of iron, research efforts were carried out on the binary Fe(III)-quinic acid system. pH-specific reactions of FeCl3·6H2O with quinic acid in a molar ratio 1:3 led to the isolation of the mononuclear Fe(III)-quinate complexes, K[Fe(C7H11O6)3]·(OH)·3H2O (1), (NH4)[Fe(C7H11O6)3]·(OH) (2), and Na[Fe(C7H11O6)3]·(OH)·8H2O (3). Compounds 1−3 were characterized by analytical, spectroscopic techniques (UV/vis, FT-IR, Electron Paramagnetic Resonance (EPR), and Mössbauer spectroscopy), cyclic voltammetry, and magnetic susceptibility measurements. Compound 1 crystallizes in P213, with a = 15.1693(9) Å, V = 3490.6(4) Å3, and Z = 4. Compound 2 crystallizes in P213, with a = 15.2831(9) Å, V = 3569.7(4) Å3, and Z = 4. Compound 3 crystallizes in P213, with a = 15.6019(14) Å, V = 3797.8(6) Å3, and Z = 4. The X-ray crystal structures of 1−3 reveal a mononuclear Fe(III) ion bound by three quinates in an octahedral fashion. Each quinate ligand binds Fe(III) through the α-hydroxycarboxylate group as a singly deprotonated moiety, retaining the alcoholic hydrogen. EPR measurements in solution suggest that 1 dissociates, releasing free quinate. Solution speciation studies of the binary system (a) unravel the aqueous species distribution as a function of pH and reagent molar ratio, and (b) corroborate the EPR results proposing the existence of a neutral Fe(III)-quinate complex form. The collective physicochemical properties of 1−3 formulate a well-defined profile for the Fe(III) assembly in aqueous media and project structural features consistent with solubilized Fe(III)-hydroxycarboxylate binary forms potentially mobilized into plant (bio)chemical processes.