Synthesis, structures, and magnetic properties of a family of 3d-4f [Na2Fe6Ln2] complexes (Ln = Y, Gd and Dy): Effect of ligands on the connection of inorganic subunits

Article abstract of DOI:10.1039/c2dt31050c

A family of 3d-4f heterometallic compounds [Na₂Fe ^III₆Dy^III₂(N₃) ₄(HL)₄(CH₃O)₄(PhCO₂) ₆] (1, H₄L = 2-{[(2-hydrox

Full text of DOI:10.1039/c2dt31050c

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PAPER
Synthesis, structures, and magnetic properties of a family of 3d–4f
[Na2Fe6Ln2] complexes (Ln = Y, Gd and Dy): effect of ligands on the
connection of inorganic subunits†
Qi Zhou,a Fen Yang,a Dan Liu,b Yu Peng,a Guanghua Li,*a Zhan Shi*a and Shouhua Fenga
Received 13th January 2012, Accepted 16th October 2012
DOI: 10.1039/c2dt31050c
A family of 3d–4f heterometallic compounds [Na2FeIII6DyIII2(N3)4(HL)4(CH3O)4(PhCO2)6] (1, H4L =
2-{[(2-hydroxy-3-methoxyphenyl)methylene]amino}-2-(hydroxymethyl)-1,3-propanediol), [Na2FeIII
-
6
DyIII2(N3)4(L′)4(CH3O)4(PhCO2)6(H2O)] (2, H3L′ = (E)-2-ethyl-2-(2-hydroxy-3-methoxybenzyl-
ideneamino)propane-1,3-diol), [Na2FeIII6DyIII2(N3)4(L′)4(CH3O)4(ButCO2)6] (3) [Na2FeIII6YIII2(N3)4-
(L′)4(CH3O)4(PhCO2)6(H2O)] (4), and [Na2FeIII6GdIII2(N3)4(L′)4(CH3O)4-(PhCO2)6(CH3OH)2] (5) have
been prepared using Schiff-base ligands, trinuclear iron precursor complexes, azides and lanthanide
nitrates as reactants. In compounds 1 and 2, the structure of the [Na2FeIII6DyIII2] cluster forms a couple of
cis,trans-isomers with substitution of methyl for a free hydroxyl group which belongs to the Schiff-base
ligand. When the pivalates are employed instead of bulkier benzoates, the trans-[Na2FeIII6DyIII2] clusters
act as network nodes in the formation of rhombic grid-like layered structures in compound 3. Compounds
2, 4 and 5 have similar metallic cores, only with different crystal solvent molecules. The magnetic
measurements on all the compounds indicate dominant antiferromagnetic interactions between the metal
centers.
extensively employed, it is still an enormous challenge for
researchers to rationally design, precisely control, and effectively
re-assemble novel structures.
Introduction
In recent years, the synthesis of spin coupled polynuclear para-
magnetic clusters (PMCs) has attracted much attention due to
Among the large variety of chelating ligands, polyhydroxy
Schiff-base ligands are rightly considered as one of the most
important ones.5a,d,6 Because the soft-donor nitrogen tends to
bind to transition-metal ions and the harder oxygen favors
binding to oxophilic lanthanide ions, simultaneously, the depro-
tonated hydroxyethyl portions of these ligands are outstanding
bridging groups.7–10 Recognizing that Schiff base ligands are a
good way of connecting metal centers together for forming poly-
nuclearities, we have obtained the octa- and hexadecanuclear
manganese clusters11 by using quinquedentate Schiff base H4L
(H4L, 2-{[(2-hydroxy-3-methoxyphenyl)methylene]amino}-2-
(hydroxymethyl)-1,3-propanediol), and we found an interesting
phenomenon in those compounds. In comparison with tripodal
alcohol ligands in which all the hydroxyl groups coordinate with
metal ions,12 the tripodal alcohol unit of H4L has one or two
hydroxyl groups which do not coordinate but form intermolecu-
lar hydrogen bonds to solvent molecules or neighboring clusters.
Since the reduction of alcohol groups has been proved to show a
potent effect on the structures of PMCs,9b,13 we employ both
H4L and a new ligand H3L′ (H3L′, (E)-2-ethyl-2-(2-hydroxy-3-
their potential to act as single-molecule magnets (SMMs).1,2
A
great number of 3d, 3d–4f and 4f polynuclear clusters have been
synthesized and their magnetic properties have been widely
studied, mainly because of their fascinating structures and inter-
esting properties.3 Among those PMCs, 3d–4f clusters are more
important, because they may combine the large magnetic aniso-
tropy of lanthanide ions with the high spin-states of transition ions.4
To assist the formation of polynuclear clusters, chelating
ligands have been widely used, which control the structures of
metal cores and prevent metal ions from forming a high dimen-
sional framework.5 Although chelating ligands have been
aState Key Laboratory of Inorganic Synthesis and Preparative
Chemistry, College of Chemistry, Jilin University, Changchun 130012,
P.R. China. E-mail: zshi@mail.jlu.edu.cn, leegh@jlu.edu.cn;
Fax: +86 431 85168624
bState Key Laboratory of Rare Earth Resource Utilization, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun 130022, China
†Electronic supplementary information (ESI) available: Crystallographic
details in CIF format, selected bond lengths (Tables S1–S5), χMT vs.
methoxybenzylideneamino)propane-1,3-diol,
shown
in
T plot at 1000 Oe without the magnetic contribution of the [NaFeIII
]
3
Scheme 1) to investigate the effect of hydroxyl groups on the
structure of polynuclear polymers.
units for 2 and 5 (Fig. S1), and XRPD patterns (Fig. S2–S6). CCDC
857518, 857519, 857520, 874772 and 874771 for compounds 1, 2, 3, 4
and 5, respectively. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c2dt31050c
In the present paper, we obtained a family of heterometallic
Fe–Ln complexes of formulae [Na2FeIII6DyIII2(N3)4(HL)4-
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C98H110Dy2Fe6N16Na2O34: C, 42.61; H, 4.01; N, 8.11. Found:
C, 42.21; H, 4.15; N, 8.06.
[Na2FeIII6DyIII2(N3)4(L′)4(CH3O)4(ButCO2)6](CH3CN)(H2O)5 (3)
This was prepared in a similar way to 2, but using [FeIII3O-
(ButCO2)6(H2O)3](ButCO2) instead of [FeIII3O(PhCO2)6(H2O)3]-
(PhCO2), in 21% yield. IR (KBr, cm−1): 3449(w), 2967(w),
2071(s), 1623(s), 1558(s), 1425(m), 1292(w), 1220(m),
1081(m), 736(m). Elem anal. calcd (%) for C88H143Dy2Fe6N17-
Na2O37: C, 38.61; H, 5.26; N, 8.69. Found: C, 38.99; H, 5.10;
N, 9.06.
Scheme 1 Schiff base ligands H4L (left) and H3L′ (right).
(CH3O)4(PhCO2)6(CH3OH)2](CH3OH)3(CHCN)2(H2O)3 (1),
[Na2FeIII6DyIII2(N3)4(L′)4(CH3O)4(PhCO2)6(H2O)](H2O)2 (2)
and [Na2FeIII6DyIII (N3)4(L′)4(CH3O)4(ButCO2)6](CH3CN) (3)
2
using H4L and H3L′ respectively. It should be mentioned here
that although the Fe–Ln clusters have been widely repor-
ted,5d,6c,7,14 compounds 1 and 2 are the first examples to include
two alternate [NaFeIII3] units and one [DyIII2] unit. Furthermore,
the [Na2FeIII6DyIII2] cores of 1 and 2 form a couple of cis,trans-
isomers. In compound 3, the trans-[Na2FeIII6DyIII2] clusters act
as network nodes in the formation of rhombic grid-like layered
structures. The magnetic measurements on all the compounds
indicate dominant antiferromagnetic interactions between the
metal centers. In order to probe the magnetic behaviour of 3d–4f
systems, [Na2FeIII6YIII2(N3)4 (L′)4(CH3O)4(PhCO2)6(H2O)]-
[Na2FeIII6YIII2(N3)4(L′)4(CH3O)4(PhCO2)6(H2O)](H2O) (4)
This was prepared in
a similar way to 2, but using
Y(NO3)3·6H2O(1 mmol, 0.383 g) instead of Dy(NO3)3·6H2O in
24% yield. IR (KBr, cm−1): 3363(w), 2943(w), 2058(s),
1623(s), 1548(m), 1398(s), 1309(w), 1218(w), 1068(m), 724(m).
Elem anal. Calcd (%) for C98H110Fe6N16Na2O34Y2: C, 45.01; H,
4.24; N, 8.57. Found: C, 44.89; H, 4.35; N, 8.48.
[Na2FeIII6GdIII2(N3)4(L′)4(CH3O)4(PhCO2)6(CH3OH)2]-
(H2O)6 (5)
(H2O) (4) and [Na2FeIII6GdIII (N3)4(L′)4(CH3O)4(PhCO2)6-
2
(CH3OH)2](H2O)6 (5) were synthesized to turn off the lanthanide
anisotropy and the 4f contribution.
This was prepared in
a similar way to 2, but using
Gd(NO3)3·6H2O(1 mmol, 0.453 g) instead of Dy(NO3)3·6H2O
in 26% yield. IR (KBr, cm−1): 3363(w), 2958(w), 2059(s),
1624(s), 1549(m), 1413(s), 1309(w), 1208(w), 1083(m), 724(m).
Elem anal. calcd (%) for C100H126Fe6Gd2N16Na2O40: C, 41.59;
H, 4.40; N, 7.76. Found: C, 41.63; H, 4.42; N, 7.79.
Experimental section
All reagents and solvents were commercially available and were
used without further purification. The ligands H4L and H3L′
were synthesized by condensation of o-vanillin and tris(hydroxy-
methyl)aminomethane (or 2-amino-2-ethyl-1,3-propanediol)
(1 : 1) in hot methanol solution. [FeIII3O(RCO2)6(H2O)3](RCO2)
complexes were prepared using the procedures reported
previously.15
Physical measurements
Elemental analyses of C, H, and N were carried out on a Perkin-
Elmer 240C elemental analyzer. IR spectra as KBr pellets were
recorded using a reflectance technique over the range of
4000–400 cm−1 on a Magna 750 FT-IR spectrophotometer.
X-ray powder diffraction (XRPD) patterns were taken on a
Rigaku D/max 2550 X-ray Powder Diffractometer. The measure-
ments of variable-temperature magnetic susceptibility and field
dependence of magnetization were obtained with a Quantum
Design MPMS-XL5 SQUID magnetometer. The sample was
embedded in solid eicosane to prevent torquing. Alternating
current magnetic susceptibility measurements were performed in
an oscillating ac field of 3.0 G and a zero dc field. The oscil-
lation frequencies were in the 10–1000 Hz range. Pascal’s con-
stants were used to estimate the diamagnetic corrections, which
were subtracted from the experimental susceptibilities to give the
molar paramagnetic susceptibilities (χM).
[Na2FeIII6DyIII2(N3)4(HL)4(CH3O)4(PhCO2)6(CH3OH)2]-
(CH3OH)3(CH3CN)2(H2O)3 (1)
A mixture of Dy(NO3)3·6H2O (1 mmol, 0.455 g), H4L
(0.6 mmol, 0.153 g) and NaN3 (3 mmol, 0.195 g) in MeCN
(10 mL) and MeOH (15 mL) was stirred for 30 min, followed by
the addition of 0.3 mmol of [FeIII3O(PhCO2)6(H2O)3](PhCO2).
The resulting dark solution was stirred for 2 h and then filtered.
Subsequent slow diffusion of ether into the deep-brown filtrate
yielded dark black crystals of 1 after two weeks. Yield: ca. 20%
based on Fe. IR (KBr, cm−1): 3357(w), 2925(w), 2061(s),
1619(s), 1554(m), 1398(s), 1304(w), 1216(w), 1024(m), 721(m).
Elem anal. calcd (%) for C103H130Dy2Fe6N18Na2O44: C, 40.82;
H, 4.32; N, 8.32. Found: C, 40.86; H, 4.22; N, 8.50.
X-ray crystallography
[Na2FeIII6DyIII2(N3)4(L′)4(CH3O)4(PhCO2)6(H2O)](H2O) (2)
Suitable single crystals for 1–5 were glued onto a glass fiber.
Single-crystal structure determination of 1 and 5 was carried out
on a Rigaku RAXIS-RAPID diffractometer equipped with
graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at
293 K. Integrated intensity was obtained with SAINT+ and was
A
similar preparation route was conducted using H3L′
(0.6 mmol, 0.152 g) instead of H4L in 24% yield. IR (KBr,
cm−1): 3453(w), 2929(w), 2059(s), 1621(s), 1554(m), 1401(s),
1311(w), 1208(w), 1041(m), 721(m). Elem anal. calcd (%) for
Dalton Trans.
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corrected for absorption using SADABS. Diffraction intensity
data for 2, 3 and 4 were collected with a Bruker Smart CCD dif-
fractometer equipped with graphite-monochromated Mo-Kα
radiation (λ = 0.71073 Å) at 293 K. The intensity data sets were
collected with the ω-scan technique and reduced by CrystalClear
software. The structures of the compounds were solved by direct
methods and refined with the full-matrix leastsquares technique
using the program SHELXTL.16 The location of metal atoms
was easily determined, and O, N, and C atoms were sub-
sequently determined from the difference Fourier maps. Aniso-
tropic thermal parameters were assigned to all non-hydrogen
atoms. The disordered atoms were refined with constrained
dimensions. The hydrogen atoms were set in calculated posi-
tions. Compounds 1 and 3 were treated with SQUEEZE which
helped determine solvent molecules. The crystal data, data col-
lection and refinement parameters for complexes 1–5 are listed
in Table 1, and selected bond lengths for complexes 1–5 are
listed in Tables S1–S5 in the ESI.†
Results and discussion
Syntheses
The reaction of preformed trinuclear iron complexes with rare
earth ions represents a commonly employed and successful route
to a wide range of Fe-4f clusters, including Fe2Ln2,17 Fe4Ln2,6c
14c
Fe5Ln8
and Fe12Ln4.14g In the present work, the reaction of
trinuclear iron complexes with Ln(NO3)3·6H2O and the Schiff
base ligands in the presence of NaN3 in a 1 : 3 : 2 : 10 ratio in an
acetonitrile–methanol (2 : 3) solvent followed by slow diffusion
of ether into the mixture led to the dark black crystals. The same
product can also be obtained from reactions with different
reagent ratios, but in lower yields. Trinuclear complexes of other
transition metal ions were also employed in place of [Fe3O-
(RCO2)6(H2O)3](RCO2). We obtained a known octanuclear
manganese cluster11 when [Mn3O(O2CR)6(H2O)3] complexes
were used as the starting materials; and no crystals could be
formed by using Cr3 or Co3. In order to discuss the magnetic be-
haviour of 3d–4f systems, we tried to use GdIII or YIII instead of
DyIII. Compounds 4 and 5 were obtained, all of which were in
the chair conformation. But all attempts to substitute for Dy ions
were unsuccessful in the reactions of compounds 1 and 3. In
order to investigate the effect of ligands on the structure, a
ligand H3L′′ (H3L′′, (E)-2-(2-hydroxy-3-methoxybenzylidene-
amino)-2-methylpropane-1,3-diol (shown in Scheme S1†) was
also employed, but unfortunately our efforts resulted in failure.
This may be due to an absence of the methyl or the hydroxyl,
and the compound is more difficult to crystallize under these
conditions.
Crystal structure
Compound 1 crystallizes in the monoclinic space group P21/c
and the structure is shown in Fig. 1a. The [Na2FeIII6DyIII
]
2
cluster of 1 contains two outer [NaFeIII3] units and one central
[DyIII2] unit (Fig. 1b). In the first [NaFeIII3] unit, three Fe ions
and one Na ion form a defect double-cubane. μ3-O33 bridges
three Fe ions (Fe1, Fe2, Fe3) forming an isosceles triangle with
the Fe–Fe distances of 3.166(7) Å, 3.160(2) Å, 3.250(2) Å. Fe1
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Fig. 2 (a) Molecular structure of compound 2. (b) The structure of the
[NaFeIII3] unit and the [DyIII2] unit in compound 2. (c) The metallic core
of compound 2.
Fig. 1 (a) Molecular structure of compound 1. (b) The structure of the
[NaFeIII3] unit and the [DyIII2] unit in compound 1. (c) The metallic core
of compound 1. The C–H hydrogen atoms and minor disordered com-
ponents have been omitted for clarity.
linked through [DyIII2] units to form a novel cis-[Na2FeIII6DyIII
]
2
cluster, and the metal atoms are organized in the boat confor-
mation along the backbone of the cluster (Fig. 1c). This structure
of boat conformation is not common, with only a few similar
small clusters such as the complexes [Fe2Ln2],17 [Mn4]18 and
[Mn2Ln3].19
and Fe2 adopt distorted octahedral geometries with the coordi-
nation spheres completed by two HL3− ligands and two terminal
azido ligands. Fe3 also adopts distorted octahedral geometries
with the coordination spheres completed by four ethanolic
O from the two Schiff base ligands and one bridging benzoate.
The Na1 is linked to the Fe1 and Fe2 via μ3-O35 which bridges
the atoms forming another isosceles triangle with the Fe–Na dis-
tances of 3.527(6) Å, 3.490(5) Å. Na1 is considered six-coordi-
nate with the coordination spheres completed by two phenol
O and two methoxy O from the two Schiff base ligands and one
methanol molecule. The dihedral angle between the two tri-
angles is 147.98°. The μ3-O33 and μ3-O35 atoms from two
deprotonated methanol molecules sit 1.04 Å and 1.07 Å above
the triangular planes, respectively. The Dy1 is linked to the Fe3
via two ethanolic O (O3, O8) from the two Schiff base ligands
and one bridging benzoate to construct the unit, and the Dy1 and
Fe3 ions are separated by a distance of 3.337(2) Å. The struc-
tures of the two units are very similar, with some slight differ-
ences in triangles and distances. In the other [NaFeIII3] unit, the
Fe–Fe distances in the triangle are 3.171(8) Å, 3.167(4) Å, 3.222(1)
Å, and the Fe–Na distances in another triangle are 3.483(3) Å,
3.417(1) Å. The dihedral angle between the two triangles is
150.92°. The dysprosium and iron ions are separated by a dis-
tance of 3.290(5) Å. In the [DyIII2] unit, the distance between
Dy1 and Dy2 is 3.914(7) Å. As the two Dy ions are bridged by
one benzoate in a μ2-η1:η2-fashion and three in a μ2-η1:η1-
fashion, Dy1 is considered eight-coordinate, Dy2 is considered
seven-coordinate. Interestingly, the two [NaFeIII3] units are
Compounds 2, 4 and 5 have similar metallic cores, only with
different crystal solvent molecules. As an example, the structure
of compound 2 will be described (Fig. 2a). The [Na2FeIII6DyIII
]
2
cluster of 2 also contains two outer [NaFeIII3] units and one
central [DyIII2] unit (Fig. 2b). It is very similar to the unit of
compound 1. The differences are that the L′3− ligands take the
place of the HL3− ligands, and the coordination sphere of the
Na+ ion is completed by one H2O molecule replacement of the
methanol molecule. In the unit, the Fe–Fe distances in the first
triangle are 3.176(2) Å, 3.167(9) Å, 3.246(0) Å, and the Fe–Na
distances in the second triangle are 3.439(7) Å, 3.501(8) Å. The
dihedral angle between the two triangles is 152.88°. The dyspro-
sium and iron ions are separated by a distance of 3.380(4) Å. As
the [DyIII2] unit is bridged by four benzoates in a μ2-η1:η1-
fashion, all the dysprosium ions of compound 2 are considered
seven-coordinate, and the distance between the two Dy ions is
4.046(2) Å. Interestingly, complex 2 is centrosymmetric. The
two outer [NaFeIII3] units and the central [DyIII2] unit form a
trans-[Na2FeIII6DyIII2] cluster, and the metal atoms are organized
in the chair conformation along the backbone of the cluster
(Fig. 2c). There are only a few similar clusters which have been
obtained, for example the complexes [Mn4Dy6]20 and
[Fe4Ln2].6c Furthermore, to the best of our knowledge, the boat
conformations of those metallic cores have never been reported.
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solvent molecules in compound 2. (2) Since a methyl group is
considered to be bulkier than a hydroxyl group, the steric
hindrance of methyls may lead two [NaFeIII3] units to form a
trans-[Na2FeIII6DyIII2] cluster. When [FeIII3O(ButCO2)6(H2O)3]-
(ButCO2) is used instead of [FeIII3O(PhCO2)6(H2O)3](PhCO2) in
the reaction of compound 2, compound 3 is obtained. The minor
steric hindrance of pivalates may be responsible for the for-
mation of rhombic grid-like layered structures in compound 3.
Magnetic properties
Solid-state, variable-temperature dc magnetic susceptibility
measurements for the compounds have been carried out in an
applied magnetic field of 1000 Oe in the temperature range
2–300 K. The plots χMT vs. T are shown in Fig. 4, where χM is
the molar magnetic susceptibility. For compound 1, the room
temperature χMT value is 47.7 cm3 K mol−1, and decreases stea-
dily to 33.2 cm3 K mol−1 at 50 K and then to 22.5 cm3 K mol−1
at 2 K. Complexes 2 and 3 behave in a similar fashion to 1. For
compound 2, the χMT value of 42.4 cm3 K mol−1 at 300 K
decreases with decreasing temperature to 32.3 cm3 K mol−1 at
50 K, and then to 22.4 cm3 K mol−1 at 2 K. For compound 3,
the χMT value of 46.1 cm3 K mol−1 at 300 K decreases with
decreasing temperature to 34.1 cm3 K mol−1 at 50 K, and then to
23.1 cm3 K mol−1 at 2 K. The overall behavior of χMT is consist-
ent with antiferromagnetic exchange interactions between the
constituent metal atoms, and the thermal depopulation of the
Stark sublevels of the anisotropic DyIII ion may also participate
in it. The experimental data of all compounds can be fitted satis-
factorily to the Curie–Weiss law down to 100 K (Fig. S1–S3†),
leading to the following Curie and Weiss constants: C =
53.96 cm3 K mol−1 and θ = −41.31 K for 1; C = 46.73 cm3 K
mol−1 and θ = −30.11 K for 2; C = 51.47 cm3 K mol−1 and θ =
−34.36 K for 3, respectively. The Curie constants are in rela-
tively good agreement with the value of 54.59 cm3 K mol−1
expected for six FeIII (S = 5/2, g = 2, and C = 4.375 cm3 K
Fig. 3 (a) Molecular structure of the [Na2FeIII6DyIII2] SBU in com-
pound 3. (b) The structure of the [NaFeIII3] unit and the [DyIII2] unit in
compound 3. (c) Polyhedral 2D structure (left) and view of the (4,4) grid
(right).
Compound 3 crystallizes in the monoclinic space group P21/c
and the structure is shown in Fig. 3a. The [Na2FeIII6DyIII
]
2
cluster of 3 also contains two outer [NaFeIII3] units and one
central [DyIII2] unit (Fig. 3b). Complex 3 is also centrosym-
metric and the trans-[Na2FeIII6DyIII2] clusters of complexes 3
and 2 are almost the same. The differences lie in that the brid-
ging pivalates take the place of bridging benzoates, and the
6
mol−1) and two DyIII (S = 5/2, L = 5, H15/2, g = 4/3 and
C = 14.17 cm3 K mol−1) noninteracting ions. The negative θ
coordination sphere of the Na+ ion is completed by one μ1,3
-
azide of a neighboring cluster replacement of the H2O molecule.
Each cluster is then connected to four others via azide ligands
resulting in a 2D (4,4) grid framework. Up to now, azide ions
have been employed frequently to assemble 3d polynuclear para-
magnetic clusters as the available building blocks into multi-
dimensional network systems.21 However, to the best of our
knowledge, it is the only example known where an azido has
been used to bridge discrete Fe–Ln clusters in a stepwise manner
to form a polymer. Thus, compound 3 exhibits a perfect
rhombic-grid with rhombus dimensions of 22.00 × 22.00 Å
represented by a rhombic grid model (Fig. 3b).
We have obtained three complexes with [Na2FeIII6DyIII2] clus-
ters via different ligands. The [Na2FeIII6DyIII2] clusters of com-
pounds 1 and 2 form a couple of cis,trans-isomers with similar
synthetic methods. There are two possible reasons for the differ-
ence: (1) Each HL3− ligand in 1 possesses a free alcohol group
that is involved in the formation of hydrogen bonds to solvent
molecules. As a methyl group takes the place of a hydroxyl,
there is no free alcohol group that can form hydrogen bonds to
Fig. 4 Temperature dependence of the χMT product for compounds
1–5 at 1000 Oe. The red line is a simulation of the experimental data
for 4.
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indicates antiferromagnetic interactions between the spins within
the cluster.
It is generally known that the 4f contribution can be removed
by using Y ions, and the lanthanide anisotropy can be removed
by using Gd ions.22 Hence, compounds 4 and 5 which contain Y
ions and Gd ions have been prepared. For compound 4, the χMT
value of 19.9 cm3 K mol−1 at 300 K decreases with decreasing
temperature to 2.4 cm3 K mol−1 at 2 K. The experimental data
can be fitted satisfactorily to the Curie–Weiss law down to
100 K (Fig. S4†) with a Weiss constant (θ) of −153.5 K and a
Curie constant (C) of 30.4 cm3 K mol−1. The Curie constant is
in relatively good agreement with the value of 26.3 cm3 K mol−1
expected for six FeIII. The negative θ indicates antiferromagnetic
interactions between the spins within the cluster. While the 4f
contributions are removed by using Y ions in compound 4, the
magnetic behaviours are dominated by the [NaFeIII3] unit. The
temperature dependence of the magnetic susceptibility can be
analyzed by assuming isotropic exchange with the spin-only
Fig. 5 χMT vs. T plot at 1000 Oe without the magnetic contribution of
the [NaFeIII3] units for 5 (a) and 2 (b).
Hamiltonian for trinuclear iron complexes H = −2J1(S1·S2
+
S1·S3) − 2J2(S2·S3). The energies of the spin states as functions
of the exchange integrals J1 and J2 were derived by using the
Kambe23 vector coupling approach model. A fit to the experi-
mental data gives J1 = −5.01 cm−1, J2 = −8.74 cm−1, and g =
2.08 with R = 1.48 × 10−3 (R = ∑(χobsT − χcalcT)2/∑(χobsT)2).
The absolute value of J is significantly less than the magnitude
of the antiferromagnetic coupling constants (−45.4 cm−1 to
−80.1 cm−1) found for the [Fe3O]7+ cores,24 but similar to the
values (−4.2 cm−1) observed for the [Fe4O2]8+ cores14e which
are in the 3d–4f system. Since the exchange integrals are closely
related to the distances between μ3-O and Fe,25 this difference
may be due to the fact that the distances between μ3-O and Fe in
compound 4 are about 2.10 Å, which are obviously longer than
those in the [Fe3O]7+ compounds, but close to those in the
[Fe4O2]8+ compounds.
Field-dependence measurements of the magnetization up to
5 T were performed at 2 K for compounds 1–5, and are shown in
Fig. S6.† For compounds 1, 2 and 3, the values of the magneti-
zation at 5 T are 23.8, 21.2 and 22.6 μB, respectively. All of
them are far lower than the value expected if all the spins are
ferromagnetically aligned. The lack of saturation on the M vs. H
data also supports the presence of antiferromagnetic interactions
and the intrinsic magnetic anisotropy of the DyIII ions. For com-
pound 4, the magnetization at 2 K shows a gradual increase
without clear saturation up to 5 T, where it reaches 4.3 μB. For
compound 5, the value of the magnetization at 5 T is 17.6 μB.
It is in relatively good agreement with the value expected for the
two [NaFeIII3] units and two GdIII. The ac susceptibility
measurements were carried out in a zero dc field with a 3.0 G ac
field oscillating at frequencies 10, 50, 100, 1000 Hz. The out-of-
phase signals of the complexes show no peaks down to 2.0 K,
and no frequency-dependence of out-of-phase signals was
observed. The results indicate that all the complexes are not
SMMs in the temperature range studied.
For compound 5, the χMT value of 34.1 cm3 K mol−1 at
300 K decreases with decreasing temperature, reaching a
minimum of 19.8 cm3 K mol−1 at 5 K, and then increases to
20.8 cm3 K mol−1 at 2 K. The experimental data can be fitted
satisfactorily to a Curie–Weiss law down to 100 K (Fig. S5†)
with a Weiss constant (θ) of −44.9 K and a Curie constant (C)
of 39.1 cm3 K mol−1. The Curie constant is in relatively good
agreement with the value of 42.0 cm3 K mol−1 expected for six
FeIII and two GdIII. The negative θ indicates that antiferromagnetic
interactions between the spins within the cluster are dominant.
Nevertheless, the increase of the low temperature χMT value
suggests the presence of ferromagnetic interactions in compound
5. Although both 3d–4f and 4f–4f interactions are weak,5d,14f,26
this is probably ascribed to the Fe–Gd and Gd–Gd interactions.
For compound 5, the lanthanide anisotropy has been removed
by using GdIII ions. We can subtract the magnetic contribution
of [NaFeIII3] units found in compound 4 to investigate the inter-
actions of Fe–Gd and Gd–Gd. The profile of the subtracted χMT
vs. T curve (Fig. 5a) suggests an obviously ferromagnetic behav-
iour. For compound 2, the χMT vs. T curve with the contribution
of [NaFeIII3] units subtracted from the data (Fig. 5b) is plotted.
The decrease in χMT is probably ascribed to the interactions of
Fe–Dy and Dy–Dy, but the progressive depopulation of excited
Stark sublevels and magnetic anisotropy may be more important
in the complexes which contain DyIII ions.27
Although the structures of compounds 1, 2 and 3 are different,
their magnetic behaviors are similar. It may be the result of the
weak magnetic interactions between the two central LnIII ions
and the LnIII–FeIII interactions, and the magnetic behaviors are
mainly due to [NaFeIII3] units.
Conclusions
We have shown that the Schiff-base ligands with different
numbers of hydroxyl groups can provide access to two unusual
polynuclear 3d–4f compounds 1 and 2, in which the [Na2FeIII
-
6
DyIII2] cores form a couple of cis,trans-isomers. Furthermore,
the trans-[Na2FeIII6DyIII2] clusters in compound 3 act as
network nodes in the formation of a 2D rhombic grid-like
layered structure, when the pivalates take the place of bulkier
benzoates. In order to investigate the magnetic behaviour of 3d–
4f systems, compounds 4 and 5 which differ in the lanthanide
have also been prepared. The magnetic properties of these com-
pounds are dominated by antiferromagnetic interactions between
Dalton Trans.
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[NaFeIII3] units, and the interactions between LnIII–FeIII and
LnIII–LnIII also make some contribution. To some extent, this
research opens up a promising pathway to investigate the effect
of organic ligands on the structure of polynuclear polymers.
Though all the compounds are not SMMs themselves, the suc-
cessful synthesis of the compounds may suggest new methods to
construct novel 3d–4f magnetic materials.
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. 20971054, 21131002 and 90922034)
and Specialized Research Fund for the Doctoral Program of
Higher Education (no. 20110061110015).
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Dalton Trans.