Heptanuclear Antiferromagnetic Fe(III)–d‑(-)-Quinato Assemblies with an <i>S</i> =
3/2 Ground StatepH-Specific Synthetic Chemistry, Spectroscopic,
Structural, and Magnetic Susceptibility Studies
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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) [Fe<sub>3</sub>O(CH<sub>3</sub>COO)<sub>6</sub>(H<sub>2</sub>O)<sub>3</sub>]·(NO<sub>3</sub>)·4H<sub>2</sub>O 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, [Fe<sub>7</sub>O<sub>3</sub>(OH)<sub>3</sub>(C<sub>7</sub>H<sub>10</sub>O<sub>6</sub>)<sub>6</sub>]·20.5H<sub>2</sub>O
(<b>1</b>) and (NH<sub>4</sub>)[Fe<sub>7</sub>(OH)<sub>6</sub>(C<sub>7</sub>H<sub>10</sub>O<sub>6</sub>)<sub>6</sub>]·(SO<sub>4</sub>)<sub>2</sub>·18H<sub>2</sub>O (<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