Heptanuclear antiferromagnetic Fe(III)-d-(-)-quinato assemblies with an S = 3/2 ground state - PH-specific synthetic chemistry, spectroscopic, structural, and magnetic susceptibility studies

Article abstract of DOI:10.1021/ic401036e

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)·4H ₂O 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(C₇H₁₀O ₆)₆]·20.5H₂O (1) and (NH ₄)[Fe₇(OH)₆(C₇H₁₀O ₆)₆]·(SO₄)₂·18H ₂O (2). Compounds 1 and 2 were characterized by analytical, spectroscopic (UV-vis, FT-IR, EPR, and Moessbauer) 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. Moessbauer 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.

Full text of DOI:10.1021/ic401036e

Article
pubs.acs.org/IC
Heptanuclear Antiferromagnetic Fe(III)−D‑(-)-Quinato Assemblies
with an S = 3/2 Ground StatepH-Specific Synthetic Chemistry,
Spectroscopic, Structural, and Magnetic Susceptibility Studies
M. Menelaou,†,# E. Vournari,† V. Psycharis,‡ C. P. Raptopoulou,‡ A. Terzis,‡ V. Tangoulis,§ Y. Sanakis,‡
C. Mateescu,∥ and A. Salifoglou*,†
†Laboratory of Inorganic Chemistry, Department of Chemical Engineering, Aristotle University of Thessaloniki, Thessaloniki 54124,
Greece
‡Institute of Advanced Materials, Physicochemical Processes, Nanotechnology and Microsystems, Department of Materials Science,
NCSR “Demokritos”, Aghia Paraskevi, 15310 Attiki, Greece
§Department of Chemistry, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece
∥Banat’s University of Agricultural Sciences and Veterinary Medicine from Timisoara, Timisoara 300645, Romania
S
* Supporting Information
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 (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 Mossbauer) 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. Mossbauer 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.
as chelates. In this regard, organic ligands bearing O-, N-, and S-
containing anchors are excellent candidates, interacting with
transition metal ions and leading to the isolation of polynuclear
clusters. As a result, hydroxido-, alkoxido-, and/or phenoxido-
type O-ligands have been widely used in the synthesis of
polynuclear species with intriguing lattice architecture (wheel-
like structures,14−17 diamond-like structures18) and distinctly
remarkable magnetic properties.
INTRODUCTION
■
Over the last decades, research activities have focused on the
synthesis and characterization of polynuclear cluster com-
pounds of magnetic transition metal ions,1 such as
manganese2−4 and iron.5,6 The main reasons for this attractive
research interest are due to the potential applications of the
arising materials as molecular magnets (e.g., SMMs), exhibiting
hysteresis and slow magnetic relaxation behavior.7−9 Iron is an
important transition metal ion found in many biological
systems in all living organisms.10−13 It is ubiquitous in the
environment and participates in a wide range of material
processes involving alloys, catalysts, and dye-casting. Metal ions
such as iron are mobilized in various abiotic systems through
interactions with appropriately structured organic ligands acting
A very efficient low molecular mass O-chelator ligand is D-
(-)-quinic acid, 1α,3α,4α,5β-tetrahydroxy-1-cyclohexane car-
boxylic acid, encountered as a vital binder in plant cellular
Received: April 25, 2013
Published: November 22, 2013
© 2013 American Chemical Society
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physiology. It is a precursor of shikimic acid,19 which is
involved in the biosynthesis of many natural products including
essential amino acids and folic acid.20,21 It is a multifunctional
binder with versatile coordination modes containing important
features: (a) a carboxylate moiety, known to promote metal ion
binding, (b) one alcoholic group in a position α- to the
carboxylate group, and (c) three additional alcoholic groups
relevant to polyol functionalities. These structural attributes are
very essential in promoting diverse metal-ion-binding chem-
istries in aqueous media. In this regard, the paucity of well-
defined and characterized binary multinuclear M(II,III)−
quinato species in aqueous media with unique physicochemical
properties, such as Fe(III)−quinic acid species, has initiated
research efforts in our laboratories, targeting synthetic and
solution studies. To this end, exploration of the binary
Fe(II,III)−D-(-)-quinic acid system aims at (a) unravelling
key aspects of the structural speciation of the aforementioned
system, (b) pursuing variable synthetic strategies toward the
synthesis of novel binary/ternary materials, and (c) delineating
reactivity and physicochemical properties in the solid state and
in solution, thereby shedding light on the magnetic properties
and establishing magnetostructural correlations in large binary
ferric−ligand assemblies. To this end, we herein report on the
pH-specific synthesis and isolation of the first two binary
heptanuclear Fe(III)−quinato species with an antiferromag-
netic ground state S = 3/2, bearing distinct lattice architecture
and spectroscopic properties, and shedding light onto
intertwined factors correlating aqueous solution chemical
reactivity and solid state properties in iron−oxido assemblies.
M in supporting electrolyte. Purified argon was used to purge the
solutions prior to the electrochemical measurements.
Synthesis of [Fe3O(CH3COO)6(H2O)3]·(NO3)·4H2O. The synthesis
and isolation was carried out according to the literature.22
Synthesis of [Fe7O3(OH)3(C7H10O6)6]·20.5H2O (1). A quantity of
[Fe3O(CH3COO)6(H2O)3]·(NO3)·4H2O (0.15 g, 0.21 mmol) was
placed in a flask and dissolved in 4 mL of H2O. To that solution, D-
(-)-quinic acid (0.12 g, 0.63 mmol) was added slowly and under
continuous stirring, and the pH of the solution was 2.5. The resulting
reaction solution was stirred for 1 h at room temperature, with the
color turning brownish and staying on as such. Subsequently, the
reaction mixture was placed at 4 °C and left undisturbed. A few
months later, brownish crystals appeared at the bottom of the flask.
The crystals were isolated by filtration and dried in vacuo. The yield
was 0.10 g (∼56%). Formula: C42H104Fe7O62.50. Mr = 2000.20. Anal.
Calcd for (1) [Fe7O3(OH)3(C7H10O6)6]·20.5H2O: C, 25.19; H, 5.20.
Found: C, 25.03; H, 5.30%.
Synthesis of (NH4)[Fe7(OH)6(C7H10O6)6]·(SO4)2·18H2O (2). A
quantity of Mohr’s salt ((NH4)2Fe(SO4)2·6H2O) (0.20 g, 0.50
mmol) was placed in a flask and dissolved in 4 mL of H2O. To that
solution, D-(-)-quinic acid (0.29 g, 1.5 mmol) was added slowly and
under continuous stirring. Aqueous ammonia was then added slowly
to adjust the pH to a final value of 7.5. The color of the solution was
greenish and stayed on as such. Subsequently, ethanol was added and
the reaction mixture was placed at 4 °C. A few weeks later, crystals
appeared at the bottom of the flask. The crystals were isolated by
filtration and dried in vacuo. The yield was 0.12 g (∼78%). Formula:
C42H106Fe7NO68S2. Mr = 2168.35. Anal. Calcd for (2) (NH4)-
[Fe7(OH)6(C7H10O6)6]·(SO4)2·18H2O: C, 23.24; H, 4.89; N, 0.64.
Found: C, 23.23; H, 5.05; N, 0.72%.
X-ray Crystal Structure Determination. Single crystals of 1 and
2 were grown from aqueous solutions according to the described
synthetic procedures. A single crystal of 1 with dimensions 0.44 × 0.21
× 0.14 mm was mounted on a capillary. A single crystal of 2 with
dimensions 0.36 × 0.13 × 0.10 mm was taken directly from the
mother liquid and immediately cooled to −113 °C. Diffraction
measurements for 1 and 2 were made on a Rigaku R-AXIS SPIDER
Image Plate diffractometer using graphite monochromated Mo Kα (for
1) and Cu Kα (for 2) radiation. Data collection (ω-scans) and
processing (cell refinement, data reduction, and empirical absorption
correction) were performed using the CrystalClear program package.23
Important crystallographic data for both structures are listed in Table
1. Further experimental crystallographic details: for 1, 2θmax= 50°;
reflections collected/unique, 33383/16508 [Rint = 0.0289]/16508;
1115 parameters refined; Δ/σ = 0.005; (Δρ)max/(Δρ)min = 0.575/−
1.151 e/Å3; R/Rw (for all data), 0.0548/0.1491. For 2, 2θmax = 130°;
reflections collected/unique/used, 35313/2392 [Rint = 0.0351]/2392;
216 parameters refined; Δ/σ = 0.000; (Δρ)max/(Δρ)min = 0.607/−
0.509 e/Å3; R/Rw (for all data), 0.0494/0.1320.
EXPERIMENTAL SECTION
■
Materials and Methods. All experiments were carried out in
aqueous media under aerobic conditions. Nanopure-quality water was
used for all reactions. Fe(NO3)3·9H2O and D-(-)-quinic acid (Fluka),
(NH4)2Fe(SO4)2·6H2O (Merck), CH3COONa·3H2O (Mallinckrodt),
and ammonia (Aldrich) were used in this investigation and obtained
from commercial suppliers.
Physical Measurements. FT-infrared spectra were recorded on a
Perkin-Elmer 1760X FT-infrared spectrometer. UV−vis measurements
were carried out on a Hitachi U2001 spectrophotometer in the range
from 190 to 1000 nm. A ThermoFinnigan Flash EA 1112 CHNS
elemental analyzer was used for the simultaneous determination of
carbon, hydrogen, and nitrogen (%). A TA Instruments (model Q
600) system was used to run the simultaneous TGA-DTG
experiments.
The structures of 1 and 2 were solved by direct methods using
SHELXS-9724 and refined by full-matrix least-squares techniques on F2
with SHELXL-97.25 All hydrogen atoms in 1 and 2 were located by
difference maps and were refined isotropically or were introduced at
calculated positions as riding on bonded atoms. No hydrogen atoms
for the lattice water molecules were included in the refinement. All
non-hydrogen atoms in 1 and 2 were refined anisotropically. The
lattice water molecules were treated as follows: in 1 there are 52
crystallographically independent sites in the asymmetric unit assigned
as oxygen atoms; only 10 of them were refined anisotropically with full
occupancy, whereas the rest of them were refined with occupancy
factors fixed at less than 0.5 (only four of which were refined
anisotropically). In 2, one of the lattice water molecules was refined
anisotropically with full occupancy, and two were found disordered
over two positions and refined anisotropically with occupancy factors
fixed at 0.5. The rest were refined isotropically with occupancy factors
fixed at less than 0.5.
The EPR spectra of 1 and 2 in the solid state and in aqueous
solutions were recorded on a Bruker ER 200D-SRC X-band
spectrometer, equipped with an Oxford ESR 9 cryostat at 9.61 GHz,
10 dB (2 mW), and 4 K. Magnetic susceptibility data were collected on
powdered samples of 1 and 2 with a Quantum Design SQUID
susceptometer in the 2−300 K temperature range, under various
applied magnetic fields. Magnetization measurements were carried out
at three different temperatures in the field range of 0−5 T.
Zero-field Mossbauer spectra from polycrystalline samples of 1 and
̈
2 were recorded with the conventional constant acceleration method,
using a 57Co/Rh source and an Oxford cryostat. Samples for
Mossbauer spectroscopy were prepared by grinding single crystals in
̈
liquid nitrogen.
Electrochemical measurements were carried out with a model
PGSTAT30 potensiostat−galvanostat from Autolab Electrochemical
Instruments. The entire system was under computer control and
supported by the appropriate computer software GPES, running on
Windows XP. The employed electrochemical cell had platinum (disk)
working and auxiliary (wire) electrodes. A Ag/AgCl electrode was used
as reference electrode. KNO3 was used as a supporting electrolyte.
Normal concentrations used were 1−6 mM in electroanalyte and 0.1
S y n t h e s i s . Tr i n u c l e a r F e ( II I)−a c e t a t e , [ F e 3 O-
(CH3COO)6(H2O)3]·(NO3)·4H2O, used as a starting material for
the synthesis of 1, was isolated at very acidic pH in a crystalline form.22
Its identity was proven through: (a) elemental analysis and FT-IR
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Inorganic Chemistry
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ligands bound to iron ions, and (b) X-ray cell determination of single
crystals of each compound (vide infra). Both compounds are soluble in
water, but insoluble in organic solvents. They are both stable in the air
at room temperature for long periods of time.
Table 1. Summary of Crystal, Intensity Collection, and
Refinement Data for [Fe7O3(OH)3(C7H10O6)6]·20.5 H2O
(1) and (NH4)[Fe7(OH)6(C7H10O6)6]·(SO4)2·18H2O (2)
X-ray Crystallographic Structures. The X-ray crystal structure of
1 reveals the presence of a discrete seven iron assembly held by six
quinato ligands [C7H10O6]2− as well as oxido- and hydroxido-bridges
in the lattice. The seven-iron assembly is planar with the largest
deviation being 0.035 Å for Fe(2). The DIAMOND structure for 1 is
shown in Figure 1, and a list of selected bond distances and angles is
1
2
formula
C42H104Fe7O62.50
2000.20
C42H106Fe7NO68S2
2168.35
formula weight
temperature, K
wavelength
space group
a (Å)
293 (2)
160 (2)
Mo Ka 0.71073
P212121
Cu Ka 1.54178
P6322
11.1003(2)
21.0919(5)
40.3401(8)
9444.7(3)
90.00
16.2964(3)
16.2964(3)
18.6865(3)
4297.8(1)
90.00
b (Å)
c (Å)
V, (Å3)
α (deg)
β (deg)
90.00
90.00
γ (deg)
90.00
120.00
Z
4
2
Dcalcd (Mg m−3
)
1.407
1.676
abs. coeff. (μ), mm−1
1.144
10.733
range of h,k,l
−11 ≤ h ≤ 13
−23 ≤ k ≤ 25
−47 ≤ l ≤ 47
1.055
−18 ≤ h ≤ 18
−19 ≤ k ≤ 19
−16 ≤ l ≤ 20
1.068
goodness-of-fit on F2
a
b
b
R
R = 0.0490
R = 0.0446
a
b
b
Rw
Rw = 0.1435
Rw = 0.1273
a
R values are based on F values, Rw values are based on F2.
∑ || F | − |F ||
o
c
R =
, Rw
=
[w(F 2 − F 2)2]/
[(wF 2)2]
∑
∑
o
c
o
∑ (|F |)
o
Figure 1. DIAMOND structure of the [Fe7(OH)3(O)3(C7H10O6)6]
assembly in 1, with the atom labeling scheme. Hydrogen atoms have
been omitted for clarity.
b
c
For 14897 (1) and 2168 (2) reflections with I > 2σ(I) w = 1/
[σ2(Fo ) + (aP)2 + bP] where P = (Max (Fo ,0) +2Fc2)/3
2
2
spectroscopy, and (b) X-ray unit cell determination of one of the
isolated single crystals of the respective compound. Subsequently,
compound 1 was synthesized through a reaction between trinuclear
Fe(III) acetate and D-(-)-quinic acid in aqueous media at pH 2.5 and
isolated in pure crystalline form. Elemental analyses of the isolated
brownish crystalline material suggested a molecular formulation
consistent with [Fe7O3(OH)3(C7H10O6)6]·20.5H2O. In this sense,
the general stoichiometric reaction leading to the formation of 1 is
shown below (Reaction 1):
provided in Table 2 (Supporting Information, Table S1). Compound
1 crystallizes in the orthorhombic space group P212121 with four
molecules per unit cell. Each iron center is six-coordinate, with the
geometry being distorted octahedral. Bond valence calculations for all
Fe atoms give values in the range 2.81−3.08, indicating a +3 oxidation
state for all iron atoms in 1. This is further supported by Mossbauer
̈
spectroscopic studies (Supporting Information, Figure S6A). The
central iron Fe(1) is coordinated to six oxygen atoms. Each oxygen
acts either as an oxido bridge (O(62), O(64), O(66)) or a hydroxido
bridge (O(61), O(63), O(65)), respectively, thereby linking the
central iron to the remaining six iron centers (Fe(2)−Fe(7)). The Fe−
Ooxido and Fe−Ohydroxido bond distances are in the range 1.974(3)−
2.035(3) and 2.053(3)−2.104(3) Å, respectively. The weakening of
Fe−Ohydroxido distances with respect to Fe−Ooxido are consistent with
the assignment of O(62), O(64), O(66) atoms as oxido and those of
O(61), O(63), O(65) as hydroxido. The bond valence sum values for
the oxido atoms are in the range of 1.20−1.31 and those for hydroxido
are in the range of 1.44−1.62. All six oxido and hydroxido entities act
as triple bridges, thereby generating abutting Fe2O2 and Fe2O(OH)
cores. Therefore, six four-membered metallacyclic rings appear in the
form Fecentral−(μ3−OH)−Feperipheral−(μ3−O) with Fecentral···Feperipheral
distances in the range of 3.074−3.165 Å. Concurrently, six other
peripheral Fe2O2 cores emerge from the participation of the
deprotonated alkoxido terminal of the quinato ligand in the vicinity
of the iron-oxido core(s) with Feperipheral···Feperipheral distances in the
range of 3.061−3.155 Å. The Fe−Oalkoxido bond distances are in the
range of 1.961(3)−2.001(3) Å. The remaining four coordination sites
in those cores are occupied by oxygen atoms provided by adjacently
located pairs of deprotonated quinato ligands. In particular, each
quinato ligand in 1 is doubly deprotonated and coordinated to two
adjacent iron atoms in each core, with the deprotonated sites being:
(a) the carboxylato group and (b) the α-alkoxido group. Specifically,
7[Fe3O(CH3COO)6(H2O)3]·(NO3)·4H2O + 18C7H12O + 23.5H2O
6
⎯⎯⎯⎯⎯→ 3[Fe7O3(OH)3(C7H10O )6]·20.5H2O + 7HNO3
6
pH2.5
+ 42CH3COOH
(Reaction 1)
The synthesis of compound 2 was pursued through an expedient
synthetic procedure between ferrous iron ions (Mohr’s salt) and D-
(-)-quinic acid in aqueous media. The pH at which the reaction was
run was 7.5 (2). Adjustment of pH was achieved through addition of
aqueous ammonia, which served as a base, raising the pH of the
reaction mixture. Elemental analysis of the isolated crystalline product
suggested the molecular formulation (NH4)[Fe7(OH)6(C7H10O6)6]·
(SO4)2·18H2O (2). The general stoichiometric reaction leading to the
formation of compound 2 is shown below (Reaction 2):
7[(NH4)2Fe(SO )2·6H2O] + 6C7H12O + 1.75O2 + 11NH3
4
6
+ 13.5H2O
⎯⎯⎯⎯⎯→ (NH4)[Fe7(OH)6(C7H10O )6]·(SO )2·18H2O + 12(NH4)2SO
6
4
4
pH7.5
+ 35H2O
(Reaction2)
Further confirmation on the structural identity of 1 and 2 came
from (a) FT-IR spectroscopy, confirming the presence of quinato
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Table 2. Selected Bond Lengths [Å] and Angles [deg] for 1
distances
Fe(1)−O(62)
Fe(1)−O(64)
Fe(1)−O(66)
Fe(1)−O(61)
Fe(1)−O(63)
Fe(1)−O(65)
Fe(2)−O(1)
Fe(2)−O(52)
Fe(2)−O(51)
Fe(2)−O(2)
Fe(2)−O(66)
Fe(2)−O(61)
Fe(3)−O(2)
Fe(3)−O(12)
1.974(3)
1.976(3)
2.022(3)
2.090(3)
2.094(3)
2.104(3)
1.983(3)
1.989(3)
1.992(4)
2.001(3)
2.035(3)
2.053(3)
1.973(3)
1.974(3)
Fe(3)−O(62)
Fe(3)−O(4)
Fe(3)−O(14)
Fe(3)−O(61)
Fe(4)−O(21)
Fe(4)−O(11)
Fe(4)−O(62)
Fe(4)−O(12)
Fe(4)−O(22)
Fe(4)−O(63)
Fe(5)−O(32)
Fe(5)−O(22)
Fe(5)−O(64)
Fe(5)−O(24)
1.990(3)
2.040(3)
2.069(3)
2.093(3)
1.988(4)
1.997(4)
1.997(3)
1.997(3)
2.012(3)
2.078(3)
1.961(3)
1.967(3)
1.987(3)
2.029(3)
Fe(5)−O(34)
Fe(5)−O(63)
Fe(6)−O(41)
Fe(6)−O(31)
Fe(6)−O(32)
Fe(6)−O(42)
Fe(6)−O(64)
Fe(6)−O(65)
Fe(7)−O(52)
Fe(7)−O(42)
Fe(7)−O(44)
Fe(7)−O(54)
Fe(7)−O(66)
Fe(7)−O(65)
2.062(3)
2.089(3)
1.987(4)
1.993(4)
2.000(3)
2.000(3)
2.019(3)
2.084(3)
1.972(3)
1.972(3)
2.022(3)
2.026(4)
2.035(3)
2.103(3)
angles
O(62)−Fe(1)−O(61)
O(66)−Fe(1)−O(61)
O(62)−Fe(1)−O(63)
O(64)−Fe(1)−O(63)
O(64)−Fe(1)−O(65)
O(66)−Fe(1)−O(65)
O(66)−Fe(2)−O(61)
O(62)−Fe(3)−O(61)
O(62)−Fe(4)−O(63)
O(64)−Fe(5)−O(63)
O(64)−Fe(6)−O(65)
O(66)−Fe(7)−O(65)
81.6(1)
80.2(1)
81.9(1)
80.1(1)
82.0(1)
80.0(1)
80.7(1)
81.2(1)
81.7(1)
80.0(1)
81.5(1)
79.7(1)
Fe(2)−O(61)−Fe(1)
Fe(2)−O(61)−Fe(3)
Fe(1)−O(61)−Fe(3)
Fe(1)−O(62)−Fe(3)
Fe(1)−O(62)−Fe(4)
Fe(3)−O(62)−Fe(4)
Fe(4)−O(63)−Fe(5)
Fe(4)−O(63)−Fe(1)
Fe(5)−O(63)−Fe(1)
Fe(1)−O(64)−Fe(5)
Fe(1)−O(64)−Fe(6)
98.1(1)
98.5(1)
95.0(1)
102.2(1)
101.4(1)
100.3(1)
97.8(2)
94.9(1)
96.2(1)
103.6(2)
101.4(2)
Fe(6)−O(65)−Fe(7)
Fe(6)−O(65)−Fe(1)
Fe(7)−O(65)−Fe(1)
Fe(1)−O(66)−Fe(7)
Fe(1)−O(66)−Fe(2)
Fe(7)−O(66)−Fe(2)
Fe(7)−O(42)−Fe(6)
Fe(3)−O(2)−Fe(2)
Fe(5)−O(22)−Fe(4)
Fe(5)−O(32)−Fe(6)
Fe(5)−O(64)−Fe(6)
97.8(1)
95.1(1)
97.6(1)
102.6(1)
100.9(1)
99.2(1)
105.2(2)
104.4(2)
104.2(1)
101.3(2)
99.7(1)
Figure 2. Small segment of the helical chains in the structure of 1 due to hydrogen bonds (dashed lines).
the mode of coordination of each quinato ligand projects binding of
the α-hydroxycarboxylic acid moiety to one iron center, with the first
alcoholic moiety spanning over and binding to the second iron center.
The alcoholic moiety of each quinato ligand bound to the iron center
in every dinuclear unit remains protonated with bond distances in the
range of 2.022(3)−2.040(3) Å. The Fe−Ocarboxylato bond distances are
in the narrow range of 1.983(3)−1.997(4) Å. The second carboxylato
oxygen and the other two alcoholic groups of all six quinato ligands are
not bound to iron centers and thus stand away from the actual Fe7
core assembly. To this end, five-membered metallacyclic rings form
through the carboxylato and α-hydroxido oxygens, while six-
membered metallacyclic rings arise through the α-hydroxido and the
alcoholato oxygens, thereby rendering the arising dinuclear units and
the Fe7 species quite stable. Consequently, three of the six iron centers
(Fe(2), Fe(4), Fe(6)) in 1 have their four remaining coordination sites
occupied through formation of two five-membered metallacyclic rings
formed by two adjacently located quinic acids. The three remaining
iron centers (Fe(3), Fe(5), Fe(7)) fulfill their octahedral coordination
site requirements through formation of two six-membered metal-
lacyclic rings formed by two adjacently located quinic acids.
The heptanuclear clusters are held together through strong
intermolecular hydrogen bonds [O6···O45 (2 − x, −0.5 + y, 0.5 −
o
z) = 2.765 Å, H6O···O45 = 2.062 Å, O6−H6O···O45 = 143.7
;
O56···O15 (2 − x, 0.5 + y, 0.5 − z) = 2.712 Å, H56O···O15 = 1.917 Å,
o
O56−H56O···O15 = 162.9 ], thus forming helical chains parallel to
the b axis; the pitch of the helix calculated as the Fecentral···Fecentral
interatomic distance is 15.172 Å, and the angle between the mean
o
planes of adjacent Fe7 entities is 65.42 (Figure 2). These helices are
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further linked through water molecules of crystallization, which are
present in the lattice of 1, thereby generating an overall 3D network of
hydrogen bonds bestowing substantial stability in the arisen crystal
lattice (Supporting Information, Table S2).
The X-ray crystal structure of 2 reveals the presence of discrete
seven-iron assemblies held by six quinato ligands [C7H10O6]2− and six
hydroxido-bridges. Moreover, sulfate and ammonium ions are present
in the lattice of 2. The DIAMOND structure for the cationic assembly
[Fe7(OH)6(C7H10O6)6]3+ in 2 is shown in Figure 3. A list of selected
bond distances and angles for 2 is given in Table 3. Compound 2
crystallizes in the hexagonal crystal system and in the space group
P6322, with two molecules per unit cell. Fe(1) occupies the 2c Wyckoff
position, which possesses 3.2 symmetry (the 2-fold axes are parallel to
the tertiary directions of the hexagonal cell which pass through the
(0,0,1/4) point). Fe(2) and Fe(3) lie on the aforementioned axes
(occupy 6h Wyckoff positions) and as a consequence the whole Fe7
entity is strictly planar. Bond valence calculations for the three
symmetry independent iron atoms give values close to 3, indicating a
+3 oxidation state for all iron atoms in 2. This is further supported by
Mossbauer spectroscopic studies (Supporting Information, Figure
̈
S6B). The sulfate anion also sits on the same 3-fold axis, which passes
through the sulfur and O(11) atom; the sulfate anion is capping the
Fe7 plane at a distance of 4.148 Å (for S) and 3.664 Å (for O(12)).
Every iron center in 2 is six coordinate, with the geometry being
distorted octahedral. The central iron atom (Fe(1)) is coordinated to
six oxygen atoms that act as triply bridging hydroxido moieties (O(7))
with bond distances at 2.057(3) Å. They each coordinate to the central
iron in a way that six internal four-membered metallacyclic rings arise
forming Fe(1)central−(μ3−OH)−Feperipheral−(μ3−OH) cores with
Fecentral···Feperipheral distances in the range of 3.147−3.234 Å and
Feperipheral−Ohydroxido bond distances at 2.070(3) and 2.144(3) Å for
Fe(2)−O(7) and Fe(3)−O(7), respectively. These bond distance
values indicate a weakening of Fe(III) bonds with hydroxido oxygen
atoms, a fact which is reflected in the values of the bond valence sum
calculated for O(7), which is 1.23, and is consistent with the
assignment of these oxygen atoms as hydroxido entities. All six
coordination sites around the iron centers are occupied by the triply
bound hydroxido bridge oxygens. Concurrently, six other peripheral
Fe(1)−(μ3−OH)−Fe−(μ2−O) cores emerge from the participation of
the deprotonated alkoxido terminal of the quinato ligand in the vicinity
of the iron−hydroxido core(s) with Feperipheral···Feperipheral distances at
≈3.19 Å. The Fe−Oalkoxido bond distances are 1.974(3) and 1.997(3) Å
to Fe(2) and Fe(3), respectively. The remaining four coordination
sites in those external cores are occupied by oxygen atoms provided by
adjacently located pairs of deprotonated quinato ligands. In particular,
each quinato ligand is doubly deprotonated and coordinated to two
adjacent iron atoms in each core, with the deprotonated sites being (a)
the carboxylato group and (b) the α-alkoxido group. Specifically, the
Figure 3. DIAMOND structure of the cationic assembly
[Fe7(OH)6(C7H10O6)6]3+ in 2, with the atom labeling scheme. (A)
indicates the five-membered and (B) the six membered metallacyclic
rings in the heptanuclear core. Hydrogen atoms have been omitted for
clarity. Primed atoms are generated by symmetry: (′) 1 − y, 1 − x, 0.5
− z; (″) −x + y, 1 − x, z; (‴) x, 1 + x − y, 0.5 − z; (*) −x + y, y, 0.5 −
z, (**)1 − y, 1 + x − y, z.
Table 3. Selected Bond Lengths [Å] and Angles [deg] for 2
distances
Fe(1)−O(7′)
Fe(1)−O(7)
Fe(1)−O(7″)
Fe(1)−O(7‴)
Fe(1)−O(7*)
Fe(1)−O(7**)
2.057(3)
2.057(3)
2.057(3)
2.057(3)
2.057(3)
2.057(3)
Fe(2)−O(2″)
Fe(2)−O(2‴)
Fe(2)−O(1″)
Fe(2)−O(1‴)
Fe(2)−O(7)
Fe(2)−O(7′)
1.974(3)
1.974(3)
1.979(3)
1.979(3)
2.070(3)
2.070(3)
Fe(3)−O(4‴)
Fe(3)−O(4)
Fe(3)−O(2‴)
Fe(3)−O(2)
Fe(3)−O(7)
Fe(3)−O(7‴)
1.987(3)
1.987(3)
1.997(3)
1.997(3)
2.144(3)
2.144(3)
angles
O(7′)−Fe(1)−O(7)
O(7′)−Fe(1)−O(7″)
O(7)−Fe(1)−O(7″)
O(7′)−Fe(1)−O(7‴)
O(7)−Fe(1)−O(7‴)
O(7″)−Fe(1)−O(7‴)
O(7′)−Fe(1)−O(7*)
O(7)−Fe(1)−O(7*)
O(7″)−Fe(1)−O(7*)
O(7‴)−Fe(1)−O(7*)
O(7′)−Fe(1)−O(7**)
O(7)−Fe(1)−O(7**)
O(7″)−Fe(1)−O(7**)
O(7‴)−Fe(1)−O(7**)
O(7*)−Fe(1)−O(7**)
80.9(2)
81.4(2)
98.9(1)
98.9(1)
81.4(2)
179.7(2)
98.9(1)
179.7(2)
80.9(2)
98.9(1)
179.7(2)
98.9(1)
98.9(1)
80.9(2)
81.4(2)
O(2″)−Fe(2)−O(2‴)
O(2″)−Fe(2)−O(1″)
O(2‴)−Fe(2)−O(1″)
O(2″)−Fe(2)−O(1‴)
O(2‴)−Fe(2)−O(1‴)
O(1″)−Fe(2)−O(1‴)
O(2″)−Fe(2)−O(7)
O(2‴)−Fe(2)−O(7)
O(1″)−Fe(2)−O(7)
O(1‴)−Fe(2)−O(7)
O(2″)−Fe(2)−O(7′)
O(2‴)−Fe(2)−O(7′)
O(1″)−Fe(2)−O(7′)
O(1‴)−Fe(2)−O(7′)
O(7)−Fe(2)−O(7′)
176.2(2)
80.2(1)
97.5(1)
97.5(1)
80.2(1)
103.8(2)
104.8(1)
78.2(1)
92.4(1)
154.4(1)
78.2(1)
104.8(1)
154.4(1)
92.4(1)
80.3(2)
O(4‴)−Fe(3)−O(4)
O(4‴)−Fe(3)−O(2‴)
O(4)−Fe(3)−O(2‴)
O(4‴)−Fe(3)−O(2)
O(4)−Fe(3)−O(2)
O(2‴)−Fe(3)−O(2)
O(4‴)−Fe(3)−O(7)
O(4)−Fe(3)−O(7)
O(2‴)−Fe(3)−O(7)
O(2)−Fe(3)−O(7)
O(4‴)−Fe(3)−O(7‴)
O(4)−Fe(3)−O(7‴)
O(2‴)−Fe(3)−O(7‴)
O(2)−Fe(3)−O(7‴)
O(7)−Fe(3)−O(7‴)
106.0(2)
88.0(1)
93.2(1)
93.2(1)
88.0(1)
178.1(2)
157.3(1)
91.0(1)
75.9(1)
102.5(1)
91.0(1)
157.3(1)
102.5(1)
75.9(1)
77.4(2)
a
Symmetry operations: (′) 1 − y, 1 − x, 0.5 − z; (″) −x + y, 1 − x, z; (‴) x, x − y + 1, 0.5 − z; (*) −x + y, y, 0.5 − z; (**) 1 − y, x − y + 1, z.
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Figure 4. Small fragment of the 2D network in the structure of 2 due to hydrogen bonds (dashed lines).
mode of coordination of each quinato ligand projects binding of the α-
hydroxycarboxylic acid moiety to one iron center, with the first
terminal alcoholic moiety spanning over and binding to the second
iron center. The alcoholic moiety of each quinato ligand bound to iron
center in every dinuclear unit remains protonated with distances at
1.987(3) Å. The Fe−Ocarboxylato bond distances are 1.979(3) Å. The
second carboxylato oxygen and the other two alcoholic groups of all
six quinato ligands refrain from binding, thereby dangling away from
the actual Fe7 core assembly. The total number of quinato binders
bound to the Fe7 assembly is six. Three Fe(III) ions (Fe(2)) in 2 have
the same coordination environment with (a) two coordination sites
occupied by two triply bridging hydroxido moieties (O(7)) and (b)
the other four coordination sites being occupied by two abutting
quinic acids promoting formation of two discrete five-membered
metallacyclic rings of the type O(2)−C(2)−C(1)−O(1)−Fe(2)
(Figure 3). Finally, the remaining three Fe(III) ions (Fe(3)) present
in the lattice of 2 have the same coordination environment yet
different from that of the aforementioned three iron ions (Fe(2)). In
particular, two of the six coordination sites are occupied by two
hydroxido bridges (O(7)), while the other four sites are taken up by
two quinic acids forming two discrete six-membered metallacyclic
rings of the type O(2)−C(2)−C(3)−C(4)−O(4)−Fe(3) (Figure 3).
The positive charge of the heptanuclear iron core is counteracted by
the presence of two sulfate ions. An additional ammonium counterion
and 18 molecules of water of crystallization are present in the lattice of
2.
(1.856(2)−2.011(2) Å) (4),26 [FeIII7(μ3−O)3(phdeaH)3(μ−
O2CCMe3)6(η1−O2CCMe3)3(H2O)3] (1.866(2)−2.003(2) Å) (5)26
where teaH3 = triethanolamine, bdeaH2=N-butyldiethanolamine and
p h d e a
=
p h e n y l d i e t h a n o l a m i n e , r e s p e c t i v e l y ,
[FeIII7O3(O2CCMe3)9(bheapH)3(H2O)3] (1.852(5)−2.102(5) Å)
(6),27 [FeIII7O3(O2CCMe3)9(teaH)3(H2O)3] (1.852(6)−2.110(6) Å)
(7),27 where bheapH3 = 1-[N,N-bis(2-hydroxyethyl)-amino]-2-prop-
anol. Heptanuclear mixed Fe(II,III) species have been reported,
including [FeIIIFeII (MeO)6(HL)6]Cl3 (2.041(3)−2.367(4) Å) (8),28
6
where H2L = 3-methoxy-2-salicylideneamino-1-ethanol, and
[FeIIFeIII6(tea)6](ClO4)2 (2.123(3)−2.181(4) Å) (9),29 and
[FeII FeIII4(C4H9NO2)6Cl6]·2CH3CN·H2O (1.994(5)−2.203(5) Å)
3
(10).30
The quinato ligand plays the role of an efficient metal ion chelator
(mode I and mode II in Scheme 1), capable of effectively formulating
Scheme 1
The heptanuclear cations are held together through hydrogen
bonds [O6···O5 (−1 − x + y, −x, z) = 3.020 Å, H6O···O5 = 2.460 Å,
o
O6−H6O···O5 = 125.0 ], thus creating a 2D network which extends
parallel to the ab plane (Figure 4). The sulfate and ammonium
counterions as well as the water molecules of crystallization are also
involved in hydrogen bonding interactions, thus generating an overall
3D framework in the lattice of 2 (Supporting Information, Table S3).
Variable coordination modes of numerous carboxylate-bearing
ligands were previously observed, leading to the formation of
heptanuclear Fe(III) species. The Fe−O distances are in the range
from 1.961(3) to 2.104(3) Å found in 1 and in the range from
1.974(3) to 2.144(3) Å found in 2. Examples of heptanuclear Fe(III)
complexes include the following species: [FeIII7(μ3−O)3(teaH)3(μ−
O2CCMe3)6(η1−O2CCMe3)3(H2O)3] (1.892(2)−2.002(2) Å) (3),26
[FeIII7(μ3−O)3(bdea)3(μ−O2CCMe3)6(η1−O2CCMe3)3(H2O)3]
the coordination environment around iron ions in 1 and 2. This
coordination ability has also been observed in the case of (di-,
multi)valent transition metal ions, with representative examples being
K[Co(C7H11O6)3]·3CH3CH2OH (11),31 [Co(C7H11O6)2(H2O)2]·
3H2O (12),31 Na[Ni(C7H11O6)3]·2.75H2O (13),32 [Cu(C7H10O6)-
(H2O)]2(H2O)2 (14),33 {[CuCl(C7H11O6)(H2O)]·H2O}n (15),34
[Zn(C7H11O6)2] (16),35 [Cd(C7H11O6)2]·H2O (17),35
[Co2(C7H11O6)4]n·nH2O·(18),36 [Zn3(C7H11O6)6]n·nH2O (19),36
and [Pt(C6H14N2)(C7H10O6)] (20).37 Only three mononuclear
Fe(III)−quinato species have been reported: (Cat)[Fe(C7H11O6)3]·
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+
(OH)·yH2O where (Cat)/y = NH4 /0 (21), K+/3 (22), Na+/8 (23),
respectively.38
The angles were in the range of 77.6(1)−107.5(1) ° for 1 and
75.9(1)−106.0(1) ° for 2, respectively, and hence varied broadly
around the ideal octahedral angle of 90°. The aforementioned angle
values were similar to those observed in 3−10. Moreover, the angles
were similar to the ones in the three mononuclear Fe(III)−quinate
species: 78.08(7)−105.23(8) ° (21), 78.51(1)−104.91(1) ° (22),
79.10(2)−106.46(2) ° (23).
Electronic Spectroscopy. The UV−vis spectra of 1 and 2 were
recorded in H2O (Supporting Information, Figures S1 and S2). The
spectra showed a band at λmax = 316 nm (ε ∼18620 M−1 cm−1) for 1
and λmax = 325 nm (ε ∼ 6089 M−1 cm−1) for 2, respectively, with the
latter feature rising well into the ultraviolet region. Both bands have
been attributed to the presence of a quinato oxygen to Fe(III) Ligand
to Metal Charge Transfer (LMCT). The spectra were featureless
beyond 400 nm.
Infrared Spectroscopy. The FT-IR spectra of 1 and 2 in KBr
exhibit strong absorptions for the carbonyl carboxylato groups in both
the antisymmetric and symmetric vibration regions. The antisymmetric
stretching vibrations νas(COO−) appear in the range from 1623 to
1584 cm−1 (1) and around 1628 cm−1 (2), whereas the corresponding
symmetric stretches νs(COO−) appear in the range from 1429 to 1381
cm−1 (1) and around 1404 cm−1 (2). The frequencies for the carbonyl
stretches in 1 and 2 are shifted to lower values in comparison to those
of free D-(-)-quinic acid, indicating changes in the vibrational status of
the ligand upon coordination to the metal ion.39
Figure 5. Temperature dependence of the magnetic susceptibility of 1
(solid stars) and 2 (open stars), in the form of χMT vs T, in the
temperature range of 2.0−300 K using an external magnetic field of 0.5
T. Solid line represents the fitting results (see text).
because the temperature dependence of χT reflects just a few low-lying
spin states. Hence, the simplified magnetic model is described in
Scheme 2
Thermal Studies. The thermal decomposition of both 1 and 2 was
studied by TGA-DTG under an atmosphere of oxygen (Supporting
Information, Figures S3 and S4). The TGA diagram shows an initial
process reaching a temperature of 235 °C, involving loss of water
molecules in 1. The observed total weight loss of water amounts to
44%, a value close to the calculated value of 46% based on the
molecular formula of 1. Additional steps are observed in the ensuing
thermal process up to 470 οC, corresponding to the decomposition of
the organic moiety bound to Fe(III). Beyond that temperature, a
stable line is observed consistent with the formation of Fe2O3,
amounting to ∼28% in very good agreement with the calculated value
of 28% based on the molecular formula of 1. The stable behavior on
the thermal decomposition of 1 extends out to 800 °C.
In much the same fashion, the TGA diagram of 2 shows an initial
process reflecting the loss of water molecules. The release of all water
molecules up until ∼251 °C corresponds to an observed weight loss of
43%. Additional steps are observed owing to the decomposition of the
organic moiety in 2. The total weight loss due to the decomposition of
the organic composition of 2 is 45.7% and is reached at approximately
935 °C, signifying an endothermic process.
Scheme 2 and is given in eq 1. The susceptibility data were fitted by
the following zero-field Hamiltonian:
6
̂ ̂ ̂ ̂ ̂ ̂ ̂ ̂ ̂
Si·Si+1 + S7·S2) − 2J2(S2 + S3 + S4 + S5 + S6
H = −2J (
∑
1
i=2
Cyclic Voltammetry. The cyclic voltammetry of 1 and 2 was
studied in aqueous solutions in the presence of KNO3 as a supporting
electrolyte. The cyclic voltammogram of both compounds projects an
̂
̂
+ S7)·S1
(1)
It was possible to apply the well-known ITO method, where the total
dimension of the energy matrix (279936 × 279936) can be reduced
(point symmetry reasons) to S-block matrixes, with the maximum size
being (3150 × 3150).40 The best fits for 1 and 2 (solid lines in Figure
5) are given by the following parameters J1 = −8.8(2)/−7.4(2) cm−1,
J2 = −2.3(2)/1.6(2) cm−1 and g = 1.98(1)/1.98(1).
ill-defined electrochemical behavior with an irreversible wave at Epc
=
−0.70 V (1) and Epc = −0.72 V (2) versus Ag/AgCl, respectively. The
irreversible reduction wave likely reflects complex process(es)
involving the Fe(III)/Fe(II) redox couple in 1 and 2, with concurrent
coordination sphere changes. Attempts to pursue the redox chemistry
of 1 and 2 at low reduction potentials are currently ongoing.
Focusing on diferric complexes, Gorun and Lippard had previously
reported an exponential relationship between the exchange constant
and a parameter related to the Fe−O distances in oxido-bridged
complexes.41 The Fe−O−Fe angle was found to have only a second-
Magnetic Susceptibility Studies. The temperature dependence
of χMT (χM being the magnetic susceptibility for seven Fe(III) ions)
for complex 1 (solid stars) and 2 (open stars) is shown in Figure 5.
The χMT value is 19.35/17.55 cm3 mol−1 K at 300 K, significantly
lower than the value expected for seven isolated ions of Fe(III), which
is 7 × 4.38 cm3 mol−1 K. From 300 K down to 10 K, there is a smooth
linear decrease, while below that temperature and toward 2 K, a more
pronounced decrease is observed, reaching the value of 2.0 cm3 mol−1
K for both compounds. In order to avoid overparameterization, the
same magnetic model was used to fit the susceptibility data of 1 and 2
assuming a 3-fold symmetry. Although in the case of 1, there is a slight
deviation from this symmetry, influencing the exchange integrals, the
effects are expected to be weak. On the other hand, the available data
do not allow determination of many strong exchange constants
order effect.42 Weihe and Gudel43 used a formulation based on the
̈
angular overlap model (AOM) to derive an expression for J as a
function of both the Fe−O−Fe angle (φ) and the Fe−O distance (r).
Despite the extensive literature devoted to magnetostructural
correlations in dinuclear iron(III) compounds, very little has been
reported on polynuclear complexes. The exchange becomes more
strongly antiferromagnetic (i.e., J has a larger negative value) as the
Fe−O distance decreases and the Fe−O−Fe angle increases. This
behavior has been noted by Canada-Vilalta et al.,44 where it can be
̃
seen that the angular dependence is accentuated as r decreases, with
the radial dependence being more important at wider angles. While the
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Figure 6. Full energy spectrum of compound 1 (A) and 2 (B) along with a more detailed view of the low lying states.
observation of a radial dependence was expected and is in accord with
previous studies on dinuclear Fe(III) compounds, the strong angular
dependence is surprising and unprecedented. We believe that this is
the case for the small exchange interaction values for both complexes
since the Fe−O values are very high. Especially in the case of 2, the J2
interaction involves Fe(III) ions with an Fe−O distance of around
2.05 Å, which is the upper limit. A careful inspection of literature
data44 also shows that the values obtained in this work are of the same
magnitude. In this sense, it may be that the value of J2 in complex 2 is
overestimated, but still denotes the trend of a very small
antiferromagnetic interaction, which probablydue to the topology
of spins (triangles)could yield frustration effects.
numerically using the ITO method45 and corresponds to the
interactions of the peripheral Fe(III) ions (α is an additional quantum
number for labeling of the different spin levels with the same SR value
as mentioned above).46 The second part is a simple interpretation of
Kambe’s method47 describing the interaction of the central Fe(III) ion
with the outer ones. The energy function is the following:
E(S, SR , a) = −2J ΕR(SR , α) − J2[S(S + 1) − SR(SR + 1)
1
− S7(S7 + 1)]
(2)
In Figure S5 (Supporting Information), the total spin of the ground
state based on eq 2 as a function of the term J2/|J1| (where J1 was
assumed to be antiferromagnetic) is given. For the specific case, the
values J2/|J1| are −0.26/−0.22, pointing to a ground state of S = 3/2
(inset of Figure S5).26
Isothermal magnetization curves at T = 2 K in an applied field range
of 0−5 T are shown in Figure 7 for complex 1 (solid stars) and 2
(open stars). It must be pointed out that in both cases there is a clear
contribution of the excited (S = 5/2) state to the ground (S = 3/2)
state, in view of the fact that the magnetization curve exhibits a linear
increase at high magnetic fields with no saturation values.
According to these values, the ground state is S = 3/2 for both 1 and
2, while the first excited state is an S = 5/2, standing 4.0(1)/1.0(1)
cm−1 higher (Figure 6). These are the first examples of heptanuclear
Fe(III) complexes with an S = 3/2 ground state, since usually the
ground state of antiferromagnetically coupled rings of this type is an S
= 5/2.26,27 In order to rationalize our findings, a theoretical calculation
of the energy function based on eq 1 was carried out and is shown in
eq 2, where the spin-coupling scheme is SR = S2 + S3 + S4 + S5 + S6 +
S7 and S = SR + S1. The first part of this equation was calculated
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Figure 7. Magnetization of 1 (solid stars) and 2 (open stars), in the
form of M/NμB vs H/T, at 2 K and in the field range 0−5 T.
EPR Spectroscopy. X-Band EPR measurements of 1 and 2 were
carried out in powder samples as well as in frozen solutions in water
and are shown in Figures 8 and 9 for 1 and Figures 10 and 11 for 2,
Figure 9. Powder X-Band EPR spectrum and frozen water solution X-
Band EPR spectrum of 1 in the field range of 0−5000 G, recorded at
4.2 K.
Figure 8. Temperature dependence of the powder X-band EPR
spectrum of 1 in the field range of 0−6000 G.
Figure 10. Temperature dependence of the powder X-band EPR
spectrum of 2 in the field range of 0−6000 G.
respectively. The systems retain their structure in solution, where a
typical rhombic signal appears at low magnetic fields with prominent
features at ≈1000 and 1600 G for both compounds (Figure 9 and 11,
respectively). The EPR spectra of finely ground polycrystalline
powders of 1 and 2 exhibited similar features and temperature
dependences over the range of 295 K down to 2.5 K, although there
were some differences in detail as shown in Figure 8 (1) and Figure 10
(2). A single isotropic broad line at g ∼ 2.0 and peak-to peak derivative
width ∼600 G was observed at 295 K for both complexes, while for
complex 2, a second strong rhombic signal appears with prominent
features at ≈1000 and 1600 G. As the temperature decreases, the
resonance intensity of the central g ∼ 2 signal decreases. Below 100 K,
signals with narrower resonances could be discerned at lower magnetic
fields in the case of 1 that become better resolved as the temperature
decreases toward 30 K and below, while in the case of 2 no new signal
emerges. The g ∼ 2 resonance could be observed also at 2 K for 1,
while it disappeared in the case of 2. The broad resonance observed at
and below 295 K is attributed to resonances within the combined spin
manifold of the seven Fe(III) ions. As shown by the magnetic
susceptibility measurements, only some of the expected states are
populated even at 295 K, and even those are swiftly depopulated as
temperature decreases. Transitions between the spin levels within the
multiplicity of spin states arising from the interactions of the spins of
the individual ions are not observed, as they will be averaged out by
the fluctuations of the direction of magnetization of the seven Fe(III)
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and overall structural integrity and (b) variable OH:O2− ratio.
Concomitantly, the analytical, spectroscopic, and structural
characterization of the isolated compounds 1 and 2: (a)
reflected their physical and chemical properties, dictated by the
reaction conditions (pH and the employed trivalent or divalent
form of precursor iron species), (b) revealed a considerable
number of lattice structural details related to the reactivity of
iron toward D-(-)-quinic acid, and (c) denoted the diversified
chemical basis for the synthesis of new potential multinuclear
M(II,III)−quinato species assemblies. Worth noting in the
structures of 1 and 2 was (a) the oxidation state of iron with
bond valence calculations, suggesting the existence of Fe(III)
centers in both 1 and 2, a contention further supported by
Mossbauer spectroscopic studies, and (b) the state of
̈
deprotonation of the quinato ligand bound to Fe(III) centers.
In the aforementioned metal−quinato assemblies, the alcoholic
group adjacent to the carboxylic acid moiety does not retain its
proton.33,37 It appears that in the presence of various metal ions
and depending on the experimental conditions in each
examined binary or ternary system, the α-alcoholic group can
lose or retain its proton following binding of the quinate to
metal ions. A similar behavior for the α-alcoholic group was
observed in the case of another α-hydroxycarboxylic acid, citric
acid, which had been widely studied with various metal
ions,48,49 also leading to the formation of five-membered
metallacyclic rings in diverse mononuclear and oligonuclear
assemblies. Moreover, in both 1 and 2, one of the terminal
alcoholic moieties of each quinato binder is employed in the
coordination to an adjacently located Fe(III) ion, thereby
satisfying the octahedral coordination requirements of each
Figure 11. Powder X-Band EPR spectrum of 2 in the field range of 0−
5000 G. Frozen water solution X-Band EPR spectrum in the field
range of 0−5000 G, recorded at 4.2 K.
cluster. Thus, the resonance arises from dipolar interactions between
the seven Fe(III) ions, mediated by what may be described as a
process of exchange narrowing. The temperature dependence of this
resonance is then readily explained by the depopulation of the higher
spin states and the decrease in the dipolar broadening effects as that
depopulation takes place. An S = 3/2 state is populated at 4 K for both
complexes, with gxy = 4.0 and gz = 2 for 1, whereas for 2 only the g = 4
is resolved. Also, additional features appear at lower magnetic fields for
both compounds denoting population of the excited S = 5/2 state even
at 4 K, which has been already suggested from the magnetization
measurements.
Fe(III) center (also supported by Mossbauer spectroscopy).
̈
Thus, each hydroxycarboxylato binder is coordinated to two
distinct iron sites in 1 and 2. Involvement of one of the
terminal alcoholic moieties in the formation of dinuclear
species in binary systems has been previously observed, leading
to a 1:2 metal-to-ligand ratio compounds in Mn(II),50 Co(II),36
and trinuclear species in Zn(II);36 thus, suggesting that the
presence of the terminal alcohols in D-(-)-quinic acid can be a
key factor in the synthesis of polynuclear clusters either from
binary or ternary systems, with structural differences and/or
similarities compared to the aforementioned characterized
materials.
DISCUSSION
■
Structural Diversity in Iron−Quinato Chemistry. The
synthesis of the first heptanuclear compounds 1 and 2 between
Fe(III) and D-(-)-quinic acid exemplifies the affinity of the
aforementioned hydroxylcarboxylate binder toward transition
metal ions, such as Fe(III). Undoubtedly, the nature of the iron
reagent used to synthesize 1 and 2 was crucial in promoting the
investigated reactions and ultimately leading to the isolation of
discrete crystalline products in a suitable form for further
characterization. Compound 1 arose from a trinuclear Fe(III)
compound (trinuclear basic iron acetate)22 reacting with D-
(-)-quinic acid at acidic pH. On the other hand, compound 2
was synthesized and isolated at basic pH, with the base used to
raise the pH being ammonia. In this latter case, a mononuclear
Fe(II) compound (Mohr’s salt) reacted with D-(-)-quinic acid
and led to the isolation of 2, with the (a) initially employed
Fe(II) oxidized to Fe(III) and (b) the sulfate anions proving to
be a crucial factor ultimately counterbalancing efficiently the
positively charged species (cationic cluster and ammonium
ions) and leading to the isolation of 2. It appears, therefore, that
the variable nature of the starting iron source, namely, the
trinuclear iron(III) acetate or Mohr’s salt instead of iron(III)
nitrate, was crucial in the synthesis and isolation of the
polynuclear clusters 1 and 2 bearing (a) the same nuclearity
Further physicochemical characterization of the two clusters
in the solid state and in solution reveals that the overall
structure of the two species is retained in solution as evidenced
by EPR measurements. Interestingly, magnetic susceptibility
measurements on the two species, supported by EPR data,
showed that magnetic interactions among the Fe(III) centers
can be simulated through a discrete model exemplified by
similar coupling constants. Employment of such a model
revealed that 1 and 2 are the first two examples of
antiferromagnetic heptanuclear Fe(III) clusters with an S =
3/2 ground state.26,27 It appears, therefore, that synthetic
exploration of binary M(II,III)−quinato and further M(II,III)−
hydroxycarboxylato systems holds promise toward oligo and
polynuclear cluster assemblies, of variable yet specified
magnetostructural correlations, when the aforementioned
factors are taken into consideration.
Structure−Lattice Architecture and Dimensionality
Correlations. The binary Fe(III)−D-(-)-quinic acid system
possesses considerable diversity toward assemblies of specific
lattice architectures in the solid state. This diversity is the result
of the reactivity characteristics of the binary system promoting
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well-defined lattices of their own composition, properties, and
structural uniqueness. Previously described binary Fe(III)−
quinato species38 differentiate themselves from 1 and 2 in that
they are mononuclear, thereby presenting their own unique (a)
factors affecting their synthesis and isolation, pH, and reagent
molar ratio and (b) lattice assembly and nature. Despite the
variable metal/ligand ratio employed in the synthesis of the
mononuclear and heptanuclear species, the (a) binding mode
of the quinato ligand to iron through the carboxylato and
alkoxido groups and (b) the coordination number around
Fe(III) persists, whereas the remaining polyols enter the
coordination sphere of Fe(III) only when the nuclearity of the
assembled species increases from 1 to 7. The emergence of
such heptanuclear assemblies does not appear to be influenced
by the distinctly different charge of the two species (zero for 1
and +3 for 2). With that in mind, the arisen Fe7 cluster
assemblies link themselves into helices which upon interjection
of lattice water molecules further assemble a 3D lattice
architecture (1), while the concurrent presence of (a) water
Compounds 1 and 2 represent the first examples of
heptanuclear Fe7-O clusters exhibiting an overall antiferro-
magnetic behavior with similar exchange interactions and a
ground state S = 3/2. Further perusal of the above factors
dictating mechanistic details into the fundamental assembly of
multinuclear species in this family is linked to the development
of knowledge useful in the synthesis of novel materials with
defined physicochemical properties and is currently under
investigation in our lab.
ASSOCIATED CONTENT
* Supporting Information
■
S
X-ray crystal crystallographic files, in CIF format, (CCDC
930734 (1) and 930735 (2)), and listings of positional and
thermal parameters and H-bond distances and angles for 1 and
2. The material is available free of charge via Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: salif@auth.gr. Fax: +30-2310-996-196. Tel.: +30-2310-
996-179.
+
■
molecules of crystallization and (b) cations (NH4 ) and anions
(SO42−) raise the dimensionality of the lattice in 2 from 2D to
3D, all through hydrogen bonding interactions.
Collectively, maintenance of the coordination binding mode
of quinate to Fe(III) gives rise to the same nuclearity Fe7
cluster assemblies in 1 and 2, where the absence or presence of
charge promote distinctly different lattice architectures of the
same ultimate dimensionality (3D). Auxiliary entities in the
respective lattices are linked to lattice dimensionality with (a)
lattice water molecules of crystallization being present as a
common moiety in both lattices, regardless of the Fe7 assembly
charge (1,2), (b) cations and anions contributing to electro-
neutrality (2), and (c) both water molecules and charged
counterions promoting higher lattice dimensionality through
hydrogen bonding. It is likely that careful consideration of such
factors into the assembly of distinct lattice architecture and
dimensionality materials, in binary and ternary Fe(II,III)−
hydroxycarboxylato systems, may give rise to new materials
with distinct spectroscopic and magnetic properties. Research
efforts in this direction are currently ongoing in our
laboratories.
Present Address
#Department of Chemistry, Aristotle University of Thessaloni-
ki, Thessaloniki 54124, Greece
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by a ‘‘PENED” grant co-financed by
the European Union European Social Fund (75%) and the
Greek Ministry of Development-GSRT (25%).
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