Structure and magnetic characteristics of an oxalate-bridged U(IV)-Mn(II) three-dimensional network

Article abstract of DOI:10.1021/ic9911825

An oxalate-bridged 3d - 5f compound, K₂MnU(C₂O₄)₄ 9H₂O, is the product of the reaction of K₄U(C₂O₄)₄ with a Mn(II) salt. Single-crystal structure analysis shows a three-dimensional network with a diamond-type architecture. The magnetic properties have been examined.

Full text of DOI:10.1021/ic9911825

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Inorg. Chem. 2000, 39, 1626-1627
Structure and Magnetic Characteristics of an Oxalate-Bridged U(IV)-Mn(II) Three-Dimensional Network
Klaus P. Mo1rtl,† Jean-Pascal Sutter,*,†,‡ Ste´phane Golhen,§ Lahce`ne Ouahab,§,| and Olivier Kahn†,
Laboratoire des Sciences Mole´culaires, Institut de Chimie de la Matie`re Condense´e de Bordeaux, UPR CNRS No 9048,
F-33608 Pessac, France, and Groupe Mate´riaux Mole´culaires, Laboratoire de Chimie du Solide et Inorganique Mole´culaire,
UMR CNRS No 6511, Universite´ de Rennes 1, F-35042 Rennes, France
ReceiVed October 7, 1999
The construction of extended solids from molecular building
blocks may afford materials with controlled and tunable properties.
Our interest in this field concerns molecule-based magnetic
materials. In such materials a three-dimensional structure, in which
each paramagnetic molecular subunit has interactions with its
neighbors, is a prerequisite for magnetic order to occur at higher
temperatures.1-4
ion is in interaction with another spin carrier. In this report we
describe a compound involving the 5f 5 U(IV) and 3d5 Mn(II)
ions.
One of the challenges for a chemist in this field of research is
to control the topology in order to obtain interactions between
spin carriers in all three directions of space. The high coordination
number usually exhibited by the actinide ions is promising with
respect to the construction of extended structures. Several ligands
have been involved in the construction of extended structures
where two paramagnetic ions are interacting. Among them, the
oxalate anion is one of the most versatile. The assembling of two
metal ions by means of oxalate bridges most often leads to a
two-dimensional honeycomb-like structure.18-23 Examples of 3D
polymers involving this ligand could be obtained only in the
presence of a chiral cation.23-27 Surprisingly, all examples reported
so far are based on trisoxalato transition metal building blocks,
whereas tetrafunctional synthons, with tetrahedrally arranged
linkers, are well-known to assemble in diamond-like networks.28
We now report that the partial substitution of potassium in K4U-
(C2O4)4 by Mn(II) ions spontaneously affords a three-dimensional
network.
The search for magnets with large coercive fields (hard
magnets) makes the f-block ions desirable paramagnetic elements
because of their large and anisotropic magnetic moments. In solid-
state chemistry lanthanides and actinides are already used for the
design of such materials.5-9 Since the discovery of ferromagnetic
coupling in Cu(II)-Gd(III) complexes,10 molecular compounds
of the 4f elements have been intensively studied.11-17 In contrast,
not much is known about the magnetic properties of molecular
compounds involving 5f ions, although for these ions the f
electrons are recognized to be less shielded than they are for the
lanthanide ions. Consequently, interesting magnetic behaviors
should be anticipated for compounds in which a paramagnetic 5f
The reaction of the tetraoxalato uranate compound, K4U(C2O4)4,
with Mn(II) in H2O leads to well-shaped single crystals of K2-
MnU(C2O4)4‚9H2O suitable for X-ray diffraction analysis.29 As
a result of the reaction, an U(IV) ion is linked to four Mn(II)
ions via each of its oxalate ligands, as shown in Figure 1. The
coordination sphere of the Mn(II) ion also consists of four
oxalate-U linkages. Hence, every metal ion is linked four times
by oxalate bridges to the other metal ion. In this architecture the
Mn(II) sites are surrounded by eight oxygen atoms. This
comparatively high coordination number of 8 for Mn(II), though
unusual, has already been observed in other compounds.30,31 The
U(IV) ion is 9-coordinate, with four oxalate ligands and one H2O
molecule. The resulting 3D network shows a diamond-like
topology, and the charge neutrality of the compound is provided
* To whom correspondence should be addressed. Fax: (+33) 5-56-84-
26-49.
† Institut de Chimie de la Matie`re Condense´e de Bordeaux.
‡ E-mail: jpsutter@icmcb.u-bordeaux.fr.
§ Laboratoire de Chimie du Solide et Inorganique Mole´culaire.
| Author for enquiries relating to the crystallographic study.
This paper is dedicated to the memory of Prof. O. Kahn who passed
away suddenly on December 8, 1999.
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10.1021/ic9911825 CCC: $19.00 © 2000 American Chemical Society
Published on Web 03/31/2000

Communications
Inorganic Chemistry, Vol. 39, No. 8, 2000 1627
Figure 2. View of the {U(ox)4Mn}2- network shown in the [010]
Figure 1. View of the coordination sphere of U(IV) within K2MnU-
(C2O4)4‚9H2O. Selected bond lengths and distances (Å): U-O, 2.375-
(4)-2.456(4); U-O(17), 2.542(5); Mn-O, 2.283(4)-2.416(4); U-Mn,
6.102(3)-6.193(3).
projection. K+ and H2O are omitted for clarity.
independent value as T approaches absolute zero.33,34 The product
of the magnetic susceptibility with temperature, øMT, is expected
to decrease monotonically down to zero as T is lowered to a very
low temperature. In the present case, the magnetic properties can
be interpreted as the sum of the U(IV) and Mn(II) contributions.
At room temperature øMT is equal to 5.19 cm3 K mol-1. As T is
lowered, øMT slowly decreases to reach 4.2 cm3 K mol-1 at 2 K.
The high-temperature value of øMT is consistent with the presence
of Mn(II) and U(IV). For the latter ion, values on the order of 1
cm3 K mol-1 at room temperature are usually observed in
molecular compounds.33,34 At 2 K the value of øMT is 4.2 cm3 K
mol-1, close to what is expected for the contribution of Mn(II)
only. The field dependence of the magnetization of K2MnU-
(C2O4)4‚7H2O was recorded at 2 K. The experimental data agree
almost perfectly with the theoretical behavior for Mn(II) calculated
by K+ cations. Each potassium cation interacts with four H2O
molecules and four O atoms of two M(C2O4)4 units, giving rise
to {K-Mn} and {K-U} chains. A view of the network is shown
in Figure 2, pointing out the square-shaped channels running along
the b direction.
The magnetic susceptibility of a polycrystalline sample of K2-
MnU(C2O4)4‚7H2O32 was investigated in the temperature range
2-300 K, with an applied field of 1 kOe. The resulting øMT versus
T plot (øM stands for the molar magnetic susceptibility and T for
the temperature) is reported in Figure S1 (see Supporting
Information). The magnetic properties of most mononuclear U(IV)
compounds are characterized by a temperature-independent
paramagnetism resulting from the coupling between a nonmag-
netic ground state at the first order and low-lying excited states
through the Zeeman perturbation. The magnetic susceptibility
usually increases as T is lowered, and it tends to a temperature-
5
with the Brillouin function for a /2 spin (inset of Figure S1),
which confirms that the local ground state of the U(IV) site should
be nonmagnetic.
The use of a tetrafunctionalized supramolecular synthon like
{U(C2O4)4}4- appears to be a valuable approach for the construc-
tion of 3D networks of paramagnetic units. However, in the
reported compound the magnetic coupling is too weak to become
apparent. A modification of either the spin carrier or the bridging
ligand surrounding the U(IV) ion might enhance these interactions.
(29) Crystal structure analysis: C8H18O25K2MnU; M ) 885.39 g mol-1
;
monoclinic, space group P21 (no. 4); a ) 11.360(6) Å, b ) 8.9614(17)
Å, c ) 11.405(9) Å, â ) 97.22(9)°, V ) 1151.8(11) Å3; Fcal ) 2.521 g
cm-3 for Z ) 2. Cell dimensions and orientation matrix for data collection
were obtained at 273 K from least-squares refinement with 11.956 g θ
g 9.323. A green crystal was measured on an Enraf-Nonius CAD-4
diffractometer (Mo KR radiation, λ ) 0.710 73 Å) by using θ-2θ scan
(2θmax ) 30°), F(000) ) 820. From a total of 3695 reflections measured,
3537 were independent reflections and 3410 reflections were observed
with F2 > 2σ(F2). After a semiempirical ψ-scan36 absorption correction
(Tmin ) 0.769 727; Tmax ) 0.998 630; µ(Mo KR) ) 8.045 mm-1). From
the systematic extinctions, the possible space groups are P21 or P21/m.
All attempts with the centrosymmetric space group P21/m lead to strong
disorder, giving rise to aberrant results. The noncentrosymmetric space
group P21 was then used. The structure solution was found by direct
methods with SHELXS-86 37 and refined with SHELXL-97 38 by least
Experimental section
K4U(C2O4)4 is synthesized in situ from U(C2O4)2‚6H2O (0.2
g, 0.4 mmol) and potassium oxalate (0.15 g, 0.8 mmol) in 12
mL of water at 80 °C.35 To the hot, green solution, Mn(ClO4)2‚
6H2O (0.15 g, 0.4 mmol) dissolved in 2 mL of water is added.
The reaction mixture is allowed to cool, and ethanol is added by
slow diffusion. After 3 days green crystals were obtained that
were manually separated from precipitated white manganese
oxalate and light-green uranium oxalate. Anal. Calcd for C8O16K2-
MnU‚7H2O: C, 11.31; H, 1.66; K, 9.20; Mn, 6.47; U, 28.02.
Found: C, 12.16; H, 1.62; K, 9.32; Mn, 6.58; U, 28.24. IR (KBr,
cm-1): ν ) 3467 (s), 1654 (s), 1455 (m),1440 (m), 1303 (m),
907 (w), 800 (m), 485 (m).
2
squares against Fo ; 334 parameters. The crystal structure was solved in
both P21 and P21/m; however, best results were obtained with the P21
space group. The refinement of variables with anisotropic thermal
parameters, with all reflections, gave R ) 0.0219 and wR ) 0.0548 and
a residual electron density of 1.40 (0.87 from U1) and -1.87 (1.50 from
O2).
(30) Rochon, F. D.; Melanson, R.; Andruh, M. Inorg. Chem. 1996, 35, 6086-
6092.
(31) Hagen, K. S. Angew. Chem., Int. Ed. Engl. 1992, 31, 764-766.
(32) After drying the crystalline compound in vacuo at room temperature,
two water molecules are removed, as found by the elemental analysis.
(33) Boudreaux, E. A.; Mulay, L. N. Theory and Applications of Molecular
Paramagnetism; Wiley-Interscience: New York, 1976; pp 317-348.
(34) Aderhold, C.; Baumga¨rtner, F.; Dornberger, E.; Kanellakopulos, B. Z.
Naturforsch. 1978, 33a, 1268-1280.
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Dalton Trans. 1983, 571-581.
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A 1968, A24, 351.
Acknowledgment. The TMR Research Network ERB 4061PL-
97-0197 of the European Union, entitled “Molecular Magne-
tism: From Materials toward DeVices”, is gratefully acknowl-
edged for financial support through a grant to K.P.M.
Supporting Information Available: Experimental øMT vs T and field
dependence of the magnetization for K2MnU(C2O4)4‚7H2O. Listings of
crystal data, atomic coordinates, and bond distances and angles in CIF
format. This material is available free of charge via the Internet at
http://pubs.acs.org.
(37) Sheldrick, G. M. SHELXS-86: Program for the Solution of Crystal
Structure; University of Go¨ttingen: Go¨ttingen, Germany, 1986.
(38) Sheldrick, G. M. SHELXL-97: Program for the Refinement of Crystal
Structure; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
IC9911825