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Inorganica Chimica Acta 297 (2000) 389 – 399
New hexanuclear and octanuclear iron(III) oxide clusters:
octahedral [Fe
6
O
2
]
14 +
species and core isomerism in [Fe
8
O
4
]
16 +
complexes
Euan K. Brechin
a
, Michael J. Knapp
b
, John C. Huffman
a
, David N. Hendrickson
b,
*
1
,
George Christou
a,
*
2
a
Department of Chemistry and Molecular Structure Center, Indiana Uniersity, Bloomington, IN 47405 -7102, USA
b
Department of Chemistry, Uniersity of California at San Diego, La Jolla, CA 92093 -0358, USA
Received 1 July 1999; accepted 20 August 1999
Abstract
The synthesis, X-ray crystal structures and magnetic properties of two new classes of hexanuclear and octanuclear iron(III)
clusters are reported. The reaction of hmpH (2-(hydroxymethyl)pyridine) with [Fe
3
O(O
2
CR)
6
(H
2
O)
3
](NO
3
) (R =Bu
t
, Ph) gives
[Fe
6
O
2
(O
2
CR)
6
(hmp)
6
](NO
3
)
2
containing a Fe
III
6
octahedron with two opposite faces bridged by
3
-O
2 -
ions. Similar reactions
employing FeCl
3
/NaO
2
CR (R =Me, Ph) give [Fe
8
O
4
(O
2
CPh)
11
(hmp)
5
] and [Fe
8
O
4
(O
2
CMe)
12
(hmp)
4
]. Both contain a similar
[Fe
8
(
3
-O)
4
] core that can be described as four triangular [Fe
3
O] units joined together by edge or vertex sharing or, alternatively,
as two [Fe
3
O] units bridged by a [Fe
2
O
2
] rhombus; however, the dispositions of the [Fe
3
O] units about the [Fe
2
O
2
] central unit
differ in the two complexes and can be described as syn and anti core isomers. The Fe
6
and Fe
8
complexes have been found to
have S =0 ground states when studied by variable-temperature, solid-state magnetic susceptibility measurements, consistent with
the expected antiferromagnetic exchange interactions between the Fe(III) ions. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Iron complexes; Polynuclear complexes; Magnetism; Crystal structures
1. Introduction
Polynuclear ironoxo complexes have been of inter-
est from a number of viewpoints. From a biological
point of view, ironoxo clusters of various sizes have
represented model systems for the build up of the
ironoxo core of the iron storage protein ferritin [1,2],
or for biomineralization processes that form a variety
of ironoxo minerals such as ferrihydrite, goethite, etc.
[3]. Ironoxo clusters have also been of interest from a
materials point of view; discrete molecular species offer
the potential of acting as precursors to molecule-based
magnetic materials and even of functioning as nano-
scale magnetic particles themselves [4]. As a result of
these and other stimuli, a wide variety of such clusters
with aesthetically pleasing structures and interesting
magnetic properties have been prepared and studied
over the last 15 years or so [5–19].
The continuing development of the above areas re-
quires the discovery of new examples on a regular basis,
and we have thus been investigating new synthetic
procedures capable of allowing access to new structural
types. One approach has been to use appropriately
chosen chelates to encourage aggregation while ensur-
ing discrete products that are soluble and can be
purified and crystallized. In this paper we describe the
use of the anion of 2-(hydroxymethyl)pyridine (hmp
-
)
in reaction systems that lead to Fe
6
and Fe
8
products
that represent new structural types in ironoxo cluster
chemistry, and show that syn, anti core isomerism is
possible for the Fe
8
species.
1
* Corresponding author.
2
* Corresponding author. Tel./fax: +1-812-855-2399.
0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII:S0020-1693(99)00377-1