Citation: Singh, A.P.; Brantley, C.L.;
Lee, K.H.K.; Abboud, K.A.; Peralta,
J.E.; Christou, G. Structural and
Magnetic Analysis of a Family of
Structurally Related Iron(III)-Oxo
Clusters of Metal Nuclearity Fe
8
,
Fe
12
Ca
4
, and Fe
12
La
4
. Chemistry 2023,
5, 1599–1620. https://doi.org/
10.3390/chemistry5030110
Academic Editors: Zoi Lada and
Konstantis Konidaris
Received: 1 July 2023
Revised: 18 July 2023
Accepted: 19 July 2023
Published: 24 July 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
Article
Structural and Magnetic Analysis of a Family of Structurally
Related Iron(III)-Oxo Clusters of Metal Nuclearity Fe
8
, Fe
12
Ca
4
,
and Fe
12
La
4
†
Alok P. Singh
1
, ChristiAnna L. Brantley
1
, Kenneth Hong Kit Lee
1
, Khalil A. Abboud
1
, Juan E. Peralta
2
and George Christou
1,
*
1
Department of Chemistry, University of Florida, Gainesville, FL 32611, USA
2
Department of Physics and Science of Advanced Materials, Central Michigan University,
Mount Pleasant, MI 48859, USA
* Correspondence: christou@chem.ufl.edu; Tel.: +1-352-392-8314
†
Dedicated to Prof. Spyros P. Perlepes on the occasion of his 70th birthday: A wonderful friend, excellent
researcher, great teacher, and true lover of inorganic chemistry and football!
Abstract: The synthesis, crystal structure, and magnetic characterization are reported for three new struc-
turally related iron(III) compounds (NHEt
3
)[Fe
8
O
5
(OH)
5
(O
2
PPh
2
)
10
](1), [Fe
12
Ca
4
O
10
(O
2
CPh)
10
(hmp)
4
]
(2), and [Fe
12
La
4
O
10
(OH)
4
(tbb)
24
](3), where hmpH is 2-(hydroxymethyl)pyridine and tbbH is 4-
t
Bu-benzoic acid. 1 was obtained from the reaction of Fe(NO
3
)
3
·9H
2
O, diphenylphosphinic acid
(Ph
2
PO
2
H), and NEt
3
in a 1:4:16 molar ratio in MeCN at 50
◦
C; 2 was obtained from the reaction of
[Fe
3
O(O
2
CPh)
6
(H
2
O)
3
](NO
3
), Ca(NO
3
)
2
, and NEt
3
in a 1:1:4:2 ratio at 130
◦
C; and 3 was obtained
from the reaction of Fe(NO
3
)
3
·9H
2
O, La(NO
3
)
3
·6H
2
O, 4-
t
Bu-benzoic acid, and NEt
3
in a 1:1:4:4 ratio
in PhCN at 140
◦
C. The core of 1 consists of two {Fe
4
(μ
3
-O)
2
}
8+
butterfly units stacked on top of each
other and bridged by O
2−
and HO
−
ions. The cores of 2 and 3 also contain two stacked butterfly units,
plus four additional Fe atoms, two at each end, and four M atoms (M = Ca
2+
(2); La
3+
(3)) on the sides.
Variable-temperature (T) and solid-state dc and ac magnetization (M) data collected in the 1.8–300 K
range revealed that 1 has an S = 0 ground state, 2 has a χ
M
T value at low T consistent with the central
Fe
8
in a local S = 0 ground state and the two Fe
3+
ions in each end-pair to be non-interacting, whereas
3 has a χ
M
T value at low T consistent with these end-pairs each being ferromagnetically coupled with
S = 5 ground states, plus intermolecular ferromagnetic interactions. These conclusions were reached
from complementing the experimental studies with the calculation of the various Fe
2
pairwise J
ij
exchange couplings by DFT computations and by using a magnetostructural correlation (MSC) for
polynuclear Fe
3+
/O complexes, as well as a structural analysis of the intermolecular contacts in the
crystal packing of 3.
Keywords: iron-oxo clusters; molecular magnetism; pseudo-carboxylates; exchange interactions;
density functional theory; magnetostructural correlation
1. Introduction
The chemistry of Fe
3+
/oxo complexes attracts considerable attention owing to its
relevance to a wide range of areas including molecular magnetism [1], bioinorganic chem-
istry [2], catalysis [3,4], and materials science. Many Fe
3+
/oxo/carboxylate complexes
spanning various nuclearities have been synthesized over the years from Fe
2
[5–11] up to
hexameric [Fe
28
]
6
nanocages [12,13]. Dinuclear Fe
3+
complexes serve as model systems to
understand magnetic exchange couplings via magnetostructural correlations (MSCs) and
as synthetic analogues of di-iron biomolecules such as ribonucleotide reductase [14–16],
methane monooxygenase [14,15,17–20], hemerythrin [21–23], and others [24–26]. Higher
nuclearity Fe
3+
/oxo clusters are highly desired and very useful for studies of interesting
magnetic effects such as spin frustration, and even as models of intermediates in the growth
Chemistry 2023, 5, 1599–1620. https://doi.org/10.3390/chemistry5030110 https://www.mdpi.com/journal/chemistry