Structural and Magnetization Studies of a New (μ-Oxo)bis(μ-carboxylato)dimanganese(III)
Complex with a Terminal Hydroxo Ligand
Montserrat Corbella,* Ramon Costa, and Joan Ribas
Departament de Quı ´mica Inorga `nica, Universitat de Barcelona, Diagonal, 647, 08028-Barcelona, Spain
Pascal H. Fries,* Jean-Marc Latour, and Lars O 2 hrstro 1 m
CEA/De ´partement de Recherche Fondamentale sur la Matie `re Condense ´e, Laboratoire SESAM/CC
(URA CNRS 1194), Centre d’Etudes Nucle ´aires de Grenoble. 38054-Grenoble Cedex 9, France
Xavier Solans and Vı ´ctor Rodrı ´guez
Departament de Cristal.lografia i Dipo `sits Minerals, Universitat de Barcelona, Martı ´ i Franque `s, s/n,
08028-Barcelona, Spain
ReceiVed July 27, 1995
X
The dinuclear Mn
III
complex [Mn
2
O(PhCOO)
2
(bpy)
2
(OH)(NO
3
)]‚H
2
O was prepared by controlled oxidation of
manganous nitrate with n-tetrabutylammonium permanganate in the presence of benzoic acid (PhCOOH) and
2,2′-bipyridine (bpy). Its structure was determined in a single-crystal X-ray diffraction experiment, and consists
of a triply-bridged [Mn
2
(µ-O)(µ-PhCOO)
2
]
2+
dinuclear core. Each manganese(III) ion bears a chelating bpy and
a terminal X anion (X ) OH
-
or NO
3
-
) completing a distorded octahedral coordination geometry. Although the
terminal anions are located in disordered positions, the analysis of bond lengths and steric considerations led us
to assign to the complex an asymmetric structure [(bpy)(OH)Mn
III
(µ-O)(µ-PhCOO)
2
Mn
III
(bpy)(ONO
2
)]. The
product, with a chemical formula C
34
H
29
Mn
2
N
5
O
10
, crystallizes in the monoclinic system, space group C2/c,
with a ) 16.607(4) Å, b ) 25.619(6) Å, c ) 9.796(3) Å, ) 100.15(3)°, and Z ) 4. Magnetic studies performed
on a series of related compounds have revealed a moderate ferro- or antiferromagnetic interaction and significant
zero-field splittings in line with the strong Jahn-Teller distortion of the high spin d
4
manganese(III) ions. In the
present work we interpreted the magnetic properties of the complex using a spin Hamiltonian which includes the
Heisenberg exchange, axial and rhombic ZFS, and an anisotropic Lande ´ factor, under the assumption of a pseudo-
C
2
symmetry of the dinuclear core. In order to determine the anisotropy parameters with enough accuracy, we
resorted to variable-temperature variable-field magnetization measurements over the 2-300 K range at fields of
0.5, 1.0, 2.5, and 5 T. The whole set of data was fit with a single set of parameters through diagonalization of
the complete spin Hamiltonian. The best fit values J )+1.0(4) cm
-1
, D )+4.5(5) cm
-1
, E ) 0, g
x
) g
y
)
1.96, and g
z
) 2.00 showed that a ferromagnetic interaction occurs between manganese(III) ions in a compressed
octahedral environment. A magnetostructural relationship linking the ferromagnetic behavior of the dinuclear
complex to the compression of the Mn(III) coordination sphere (and conversely of the antiferromagnetism to the
elongation) is then proposed. It is substantiated by theoretical molecular orbital calculations of the extended
Hu ¨ckel type.
Introduction
In the past two decades manganese has been implicated as
an essential cofactor of various enzymatic systems where it can
be either an enzyme activator or an essential part of the active
site.
1-4
In the latter case various nuclearities have been observed
from mononuclear sites as in superoxide dismutase
5
to the
tetranuclear site of oxygen evolution in photosystem II.
6
Nevertheless, a growing number of manganese proteins have
been shown to possess a dinuclear active site.
7
Some of them
are understood to operate at the Mn
II
Mn
II
level,
1,7
but in others
the function involves a change in the oxidation state of the pair.
Manganese catalase
8
is the prototype of these enzymes. The
first evidence that it contains a dimanganese unit came from
electronic absorption spectroscopy, which showed a spectrum
typical of the (µ-oxo)(µ-carboxylato)dimanganese(III) cores.
8a
Its active site has been shown to exist under several redox forms
Mn
II
Mn
II
, Mn
II
Mn
III
, Mn
III
Mn
III
, and Mn
III
Mn
IV
.
8b
A low
resolution X-ray structure determination of the oxidized Mn
III
-
Mn
III
protein from Thermus thermopilus has revealed that the
two metal ions are 3.6 Å apart.
8c
On the other hand, EXAFS X
Abstract published in AdVance ACS Abstracts, February 15, 1996.
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1857 Inorg. Chem. 1996, 35, 1857-1865
0020-1669/96/1335-1857$12.00/0 © 1996 American Chemical Society