Heptacoordinated nickel(II) as an ising-type anisotropic building unit: Illustration with a pentanuclear [(NiL)3{W(CN)8} 2] complex

Article abstract of DOI:10.1021/ic3027368

Heptacoordinated nickel(II) complexes characterized by substantial Ising-type single-ion anisotropy have been involved in the construction of two pentanuclear [Ni₃W₂] compounds by association with [W(CN)₈]^3-. For one of them, slow relaxation of magnetization was observed to occur concomitantly with antiferromagnetic ordering.

Full text of DOI:10.1021/ic3027368

Communication
pubs.acs.org/IC
Heptacoordinated Nickel(II) as an Ising-Type Anisotropic Building
Unit: Illustration with a Pentanuclear [(NiL)3{W(CN)8}2] Complex
Nayanmoni Gogoi,† Mehrez Thlijeni, Carine Duhayon, and Jean-Pascal Sutter*
LCC (Laboratoire de Chimie de Coordination), CNRS, 205 route de Narbonne, F-31077 Toulouse, France
́
Universite de Toulouse, UPS, INPT, LCC, F-31077 Toulouse, France
S
* Supporting Information
derivative (LN O R) with a Ni(NO3)2 salt (see the Supporting
Information, SI). The pentadentate ligand (R = Ph for 1 and R =
BiPh for 2) occupies five equatorial coordination sites of the NiII
ion. The coordination sphere is completed by two H2O
molecules for 19 or one MeOH molecule and one NO3− anion
for 2, located in axial positions. A view of the crystal structure for
210 is given in Figure 1. The N and O atoms in equatorial
positions are well located within a plane, and the distances to Ni
atom range from 2.070 to 2.414 Å, thus confirming bonding of
the ligand by all five atoms. The polyhedral shape around the Ni
centers was ascertained by a continuous-shape measures
analysis11 carried out with SHAPE.12 For 1 and 2, the
coordination sphere of the Ni atom adopts slightly distorted
pentagonal-bipyramidal (D5h) geometry (Table S2 in the SI).
The presence of labile ligands in axial positions makes 1 and 2
attractive building blocks for the construction of low-dimen-
sional heteronuclear compounds. Their reaction with [W-
(CN)8]3−, chosen because of its strong ferromagnetic interaction
with NiII,13 afforded the pentanuclear compounds
3
2
ABSTRACT: Heptacoordinated nickel(II) complexes
characterized by substantial Ising-type single-ion aniso-
tropy have been involved in the construction of two
pentanuclear [Ni3W2] compounds by association with
[W(CN)8]3−. For one of them, slow relaxation of
magnetization was observed to occur concomitantly with
antiferromagnetic ordering.
he paradigm of molecular (nano)magnets, known as single-
Tmolecule magnets (SMMs) and single-chain magnets, has
attracted tremendous interest in recent years because of its
putative relevance to high-density data storage materials. This
topic has also stimulated more fundamental questions with
relation to the properties and chemical constructions of such
materials. One topical challenge for chemists concerns control of
the magnetic anisotropy by design. Access to large anisotropy
appears to be essential to expect significant progress in the
performances of the materials.1 Recent reports suggest that
moving from octahedral to less common geometries such as
trigonal or pentagonal bipyramids can substantially increase the
anisotropy for CoII or FeII metal ions.2−7 In an extension of our
investigations on heptacoordinated complexes,2,8 we have
considered NiII and found that in D5h geometry this ion exhibits
substantial Ising-type anisotropy. Herein we report a series of
preliminary results on such complexes and on pentanuclear
heterometallic compounds involving heptacoordinated nickel
units in association with [W(CN)8]3−.
[{NiLN O Ph}3{W(CN)8}2(H2O)2]·2MeCN·12H2O (3) and
3
2
[{NiLN O BiPh}3{W(CN)8}2(H2O)2]·2MeCN·9H2O (4). The
molecular structure of 310 (Figure 2) consists of a cyano-bridged
pentanuclear cluster made up of two [W(CN)8] units and three
3
2
[NiLN O Ph] moieties in an almost linear arrangement. All of the
Ni ions exhibit heptacoordinated surroundings with the
3
2
pentadentate LN O Ph ligand in an equatorial position. The
coordination sphere for the terminal Ni1 units is completed by a
H2O ligand positioned trans to the cyano ligand. The mean
planes defined by one Ni atom and the coordinated atoms of
3
2
LN O Ph are almost parallel to each other with a deviation of
1.9(1)° for that of Ni1 and Ni2 and 3.8(1)° between the planes
containing Ni1 (Figure S4 in the SI). This neutral pentanuclear
compound crystallizes with 2 MeCN and 12 H2O molecules
located in the crystal lattice. Some of these H2O molecules
establish hydrogen bonds with N atoms of the cyano ligands and
act as bridges between the [Ni3W2] molecules, leading to 2D
networks (Figure S5 in the SI). We will see below that this has an
effect on the magnetic behavior found for 3. Compound 4
crystallizes in space group P21,10 and the arrangement of the
pentanuclear cluster (Figure 2) is reminiscent of that of 3.
However, the five constituting units form a bent molecule. As a
3
2
The heptacoordinated nickel complexes considered in this
study are depicted in Figure 1. These are readily obtained by the
reaction of a 2,6-diacetylpyridinebis(carboxylic acid hydrazone)
result, the angles between the {NiLN O BiPh} mean planes are
45.1(2)°, 40.6(1)°, and 3.5(2)° for {Ni1^Ni2}, {Ni1^Ni3}, and
{Ni2^Ni3}, respectively (Figure S6 in the SI). A quite important
3
2
Figure 1. (left) Sketch of the ligand system for 1 and 2. (right) Crystal
structure for the cationic unit [NiLN O BiPh(MeOH)(NO3)]+ of 2. For
3
2
Received: December 12, 2012
geometrical data see Figure S1 in the SI.
© XXXX American Chemical Society
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Inorganic Chemistry
Communication
high-frequency electron paramagnetic resonance, and theoretical
studies.15 Note that the easy axis of magnetization for these
complexes is along the apical positions.15 The temperature
dependence of χMT for 3 (Figure 4) confirms that ferromagnetic
○
Figure 4. χMT versus T behavior (blue ) for 3 recorded with Hdc = 1
kOe and the best fit (red ). Inset: M versus H at 2 K and H−T phase
diagram deduced from χM = f(T,H) data (Figure S9 in the SI).
interactions take place between the Ni and W centers. At 300 K,
the value for χMT is 4.73 cm3 mol−1 K−1, slightly above the value
of 4.42 anticipated for three S = 1 (g = 2.22) and two S = 1/2 (g =
2.0) centers without exchange interactions. This value
continuously increases as T is lowered to reach a maximum of
8.23 at 13 K. Below this temperature, χMT abruptly falls to reach
2.20 cm3 mol−1 K−1 at 2 K. The χM versus T behavior also exhibits
a maximum (Figure S9 in the SI), suggesting antiferromagnetic
ordering. The H−T phase diagram establishes TN at 3.6 K
(Figure 4). The propagation of the intermolecular interactions is
attributed to the hydrogen-bonding network formed with H2O
molecules.16 The field dependence of magnetization recorded at
2 K shows an S-shaped behavior characteristic for metamagnet-
ism. Saturation is not reached up to 50 kOe; this could be
attributed to magnetic anisotropy. The χMT behavior for 4
(Figure S10 in the SI) is similar except that the value reached at 2
K is larger (5.8 cm3 mol−1 K−1), and no maximum is seen for χM
versus T. Obviously, for 4 intermolecular interactions are
avoided. The magnetic susceptibilities have been computed by
exact calculations of the energy levels associated with the spin
Hamiltonian [H = −J(SNi1SW1 + SNi2SW1 + SNi2SW2 + SNi3SW2) +
Figure 2. Crystal structures for (top) 3 and (bottom) 4 (lattice solvent
molecules are not shown). Geometrical data can be found in the SI.
volume of the crystal (30%) is occupied by solvent molecules,
only some of which could be identified from X-ray data.
Therefore, the final refinement for the structure was done
without these molecules. Interestingly, because of the steric
demand of the biphenyl units, the positioning of the pentanuclear
clusters in the lattice allows one to exclude the possibility of H2O
acting as a bridge between them (Figure S7 in the SI). For both 3
and 4, the coordination spheres of the Ni centers have slightly
distorted pentagonal-bipyramidal shape, while W centers are in
distorted square-antiprism surroundings (Table S2 in the
SI).11,14
The magnetic behaviors for all compounds have been recorded
on crystalline samples mixed with grease and held in a gelatin
capsule. The field dependence of magnetization recorded
between 2 and 10 K for 2 revealed a behavior characteristic of
substantial magnetic anisotropy. As shown in Figure 3, the M =
f(H/T) curves do not superpose. These behaviors have been
modeled with the help of MAGPACK; best fits have been
obtained with D = −12.5 cm−1, E = 1.2 cm−1, and giso = 2.22.
These values well compare to that of 1 (D = −13.9 cm−1, E/D =
0.11, and giso = 2.26) deduced from combined magnetization,
2
DS2 + ∑igβHSi) through diagonalization of the full matrix with a
general program for axial and rhombic symmetries.17 The Ni−W
exchange JNiW, a mean D parameter, and intermolecular
interactions in the mean-field approximation, zJ′, have been
taken into account. The best fit to the experimental behavior
above 4 K for 3 yielded JNiW = 32 cm−1, D = −15 cm−1, zJ′ =
−0.17 cm−1, and g = 2.16 and that above 6 K for 4 yielded JNiW
=
17.2 cm−1, D = −5.0 cm−1, zJ′ = −0.34 cm−1, and g = 2.17. The
use of independent JNiW parameters for terminal and central Ni
centers did not improve the fits. Moreover, while D and zJ′ are
strongly correlated, both parameters were required to properly
reproduce the low-temperature behavior (Figures S9 and S10 in
the SI) . The strengths of the Ni−W interaction for 3 and 4
compare well to the reported values.13,18 This is in agreement
with localization of the two unpaired electrons in the s-type
2
2
2
orbitals (dz and dx −y ) of Ni as anticipated from the orbital
energy diagram for D5h symmetry. Moreover, the fittings suggest
a notably larger D for 3 compared to that of 4. The alternating-
current (ac) susceptibility behaviors for 3 and 4 have been
measured in the zero static field. For 3, slow relaxation of
magnetization is observed below 4 K (Figure 5), as indicated by
Figure 3. Field dependence of magnetization for 2.
B
dx.doi.org/10.1021/ic3027368 | Inorg. Chem. XXXX, XXX, XXX−XXX

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Communication
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the French Research Agency
(Agence Nationale de la Recherche; Grant ANR-09-BLAN-
0054-01).
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ASSOCIATED CONTENT
* Supporting Information
■
́
Lozano, P.; Guihery, N.; de Graaf, C.; Barra, A.-L.; Sutter, J.-P.; Mallah,
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T. Chem.Eur. J. 2013, 19, 950−956.
X-ray crystallographic data in CIF format, syntheses, crystal,
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material is available free of charge via the Internet at http://pubs.
acs.org. The atomic coordinates for 2−4 have also been
deposited with the Cambridge Crystallographic Data Centre.
The coordinates can be obtained, upon request, from the
Director, Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge CB2 1EZ, U.K., by references CCDC 892918,
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AUTHOR INFORMATION
Corresponding Author
*E-mail: sutter@lcc-toulouse.fr.
■
Present Address
†Department of Chemical Sciences, Tezpur University, Napaam,
Sonitpur, Assam 784 028, India.
C
dx.doi.org/10.1021/ic3027368 | Inorg. Chem. XXXX, XXX, XXX−XXX