Heptanuclear Antiferromagnetic Fe(III)–d‑(-)-Quinato Assemblies with an S = 3/2 Ground StatepH-Specific Synthetic Chemistry, Spectroscopic, Structural, and Magnetic Susceptibility Studies

Iron is an essential metal ion with numerous roles in biological systems and advanced abiotic materials. d-(-)-Quinic acid is a cellular metal ion chelator, capable of promoting reactions with metal M­(II,III) ions under pH-specific conditions. In an effort to comprehend the chemical reactivity of well-defined forms of Fe­(III)/Fe­(II) toward α-hydroxycarboxylic acids, pH-specific reactions of: (a) [Fe3O­(CH3COO)6(H2O)3]·(NO3)·4H2O with d-(-)-quinic acid in a molar ratio 1:3 at pH 2.5 and (b) Mohr’s salt with d-(-)-quinic acid in a molar ratio 1:3 at pH 7.5, respectively, led to the isolation of the first two heptanuclear Fe­(III)–quinato complexes, [Fe7O3(OH)3(C7H10O6)6]·20.5H2O (1) and (NH4)­[Fe7(OH)6(C7H10O6)6]·(SO4)2·18H2O (2). Compounds 1 and 2 were characterized by analytical, spectroscopic (UV–vis, FT-IR, EPR, and Mössbauer) techniques, CV, TGA-DTG, and magnetic susceptibility measurements. The X-ray structures of 1 and 2 reveal heptanuclear assemblies of six Fe­(III) ions bound by six doubly deprotonated quinates and one Fe­(III) ion bound by oxido- and hydroxido-bridges (1), and hydroxido-bridges (2), all in an octahedral fashion. Mössbauer spectroscopy on 1 and 2 suggests the presence of Fe­(III) ions in an all-oxygen environment. EPR measurements indicate that 1 and 2 retain their structure in solution, while magnetic measurements reveal an overall antiferromagnetic behavior with a ground state S = 3/2. The collective physicochemical properties of 1 and 2 suggest that the (a) nature of the ligand, (b) precursor form of iron, (c) pH, and (d) molecular stoichiometry are key factors influencing the chemical reactivity of the binary Fe­(II,III)-hydroxy­carboxylato systems, their aqueous speciation, and ultimately through variably emerging hydrogen bonding interactions, the assembly of multinuclear Fe­(III)–hydroxy­carboxylato clusters with distinct lattice architectures of specific dimensionality (2D–3D) and magnetic signature.