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

Abstract

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 (<b>1</b>) and (NH₄)­[Fe₇(OH)₆(C₇H₁₀O₆)₆]·(SO₄)₂·18H₂O (<b>2</b>). Compounds <b>1</b> and <b>2</b> 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 <b>1</b> and <b>2</b> 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 (<b>1</b>), and hydroxido-bridges (<b>2</b>), all in an octahedral fashion. Mössbauer spectroscopy on <b>1</b> and <b>2</b> suggests the presence of Fe­(III) ions in an all-oxygen environment. EPR measurements indicate that <b>1</b> and <b>2</b> retain their structure in solution, while magnetic measurements reveal an overall antiferromagnetic behavior with a ground state <i>S</i> = 3/2. The collective physicochemical properties of <b>1</b> and <b>2</b> 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