Incorporation of spin-5/2 chain into 2D network with conformational pure e,a-cis-cyclohexane-1,4-dicarboxylato linker

Article abstract of DOI:10.1039/c2dt31613g

A two-dimensional coordination network, 2∞[Fe^III (OH)(e,a-cis-1,4-chdc)] (1,4-chdc: cyclohexane-1,4-dicarboxylate), featuring one-dimensional iron(iii)-hydroxy chains that are transversely bridged by 1,4-chdc in its pure e,a-cis conformation have been isolated. The magnetic study shows a typical spin-5/2 Heisenberg chain above 20 K, but canted antiferromagnetic ordering at lower temperatures.

Full text of DOI:10.1039/c2dt31613g

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Incorporation of spin-5/2 chain into 2D network with conformational pure
e,a-cis-cyclohexane-1,4-dicarboxylato linker†‡
Yan-Zhen Zheng*a,b and Xiao-Ming Chenb
Received 19th July 2012, Accepted 16th August 2012
DOI: 10.1039/c2dt31613g
2
∞[FeIII(μ-OH)(e,a-cis-1,4-chdc)] (1), magnetically behaves as a
2
A two-dimensional coordination network, ∞[FeIII (OH)(e,a-
typical spin-5/2 Heisenberg chain above 20 K. Below this
temperature, long-range canted antiferromagnetic ordering was
developed.
cis-1,4-chdc)] (1,4-chdc: cyclohexane-1,4-dicarboxylate),
featuring one-dimensional iron(III)-hydroxy chains that are
transversely bridged by 1,4-chdc in its pure e,a-cis confor-
mation have been isolated. The magnetic study shows a
typical spin-5/2 Heisenberg chain above 20 K, but canted
antiferromagnetic ordering at lower temperatures.
We prepared 1 from a conformational mixture of 1,4-chdcH2,
FeCl3, NaOH at a molecular ratio of 1 : 1 : 1.5 and relatively
mild hydrothermal conditions (160 °C for 2 days). The resulting
pure e,a-cis conformation of 1,4-chdc in 1 is somehow, not sur-
prising because in the three conformations of 1,4-chdcH2,
namely a,a-trans, e,e-trans, and e,a-cis the e,a-cis conformation
is the thermodynamically medium stable form, but only
1.4 kcal mol−1 above the most stable e,e-trans form, as shown in
Fig. 1. Therefore, most of the conformations of 1,4-chdc in solid
states adopt either the e,e-trans or e,a-cis forms,8,9 and only very
few cases of a,a-trans conformation are known.10
One-dimensional (1D) magnetic chains attract lots of research
interest in that many intriguing quantum phenomena such as
spin-Peierls transitions, Haldane gaps and tunnelling effects were
observed experimentally or theoretically in recent years.1 Among
these materials many of them are “regular magnets” with equal
dimensions of structure and magnetism. In fact, higher-dimen-
sional structures can effectively contain magnetic behaviour with
low-dimensional characteristics. This phenomenon has been
emerging in the field of magnetic metal–organic frameworks
(MMOFs),2 in which typical low-dimensional magnetism such
as slow-relaxation of single-molecule and single-chain magnets
have been observed.3,4
We have been endeavouring to obtain low-dimensional mag-
netism with MMOFs since 2006 when we successfully obtained
a single-chain magnet (SCM) by assembling an Ising-type
[–Co–(OCO)4–Co–]n chain into a 2D coordination network with
the trans-cyclohexane-1,2-dicarboxylato bridge.5 Later on, we
expanded this method to other dicarboxylato systems with differ-
ent metal centres such as Ni(II), Fe(II) and mixed-valent Fe(II)–
Fe(III).6 Most of these materials exhibit dynamic magnetism
related to the SCM behaviour, but being disturbed by peripheral
magnetic contacts.7
We have been able to isolate the conformational pure e,e-trans
form of 1,4-chdc in a mixed-valent iron(II,III) compound, [FeII-
FeIII(μ4-O)(e,e-trans-1,4-chdc)1.5 ∞
(2), at higher temperature
]
hydrothermal conditions (220 °C for 3 days).6d Therefore, with
both 1 and 2 the story of conformation separation by differentiat-
ing synthetic temperature is now complete, that is, higher temp-
erature leads to the most stable e,e-trans product whereas lower
temperatures prefer a less stable e,a-cis conformation. A similar
story can be found in cadmium products reported by Cao et al.8b
In fact, the maximum Gibbs free energy difference between
the conformations of 1,4-chdc is only 2.8 kcal mol−1, a value
that is comparable to several types of supramolecular interactions
such as hydrogen bonding, π–π stacking and metallophilicity.12
Once other peripheral factors are competing with the free energy
the resulting conformations are hardly predicted, depending
largely on a kinetic process. Therefore, many solid-state struc-
tures of 1,4-chdc comprise not just a single conformation but a
mixture of cis- and trans-1,4-chdc, instead.8,9 This phenomenon
has also been observed extensively in other cyclohexane-
carboxylate derivatives.13
As part of our on-going exploration of 1D magnetism in
MMOFs we report here a unique iron(III)-hydroxy chain trapped
into a 2D network by a e,a-cis-cyclohexane-1,4-dicarboxylate
(e,a-cis-1,4-chdc)
bridge.
The
resulting
compound,
aCentre for Applied Chemical Research, Frontier Institute of Science
and Technology, Xi’an Jiaotong University, Xi’an 710054, China.
E-mail: zheng.yanzhen@mail.xjtu.edu.cn
bMOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School
of Chemistry and Chemical Engineering, Sun Yat-Sen University,
Guangzhou 510275, China
†In memory of Ian J. Hewitt.
‡Electronic supplementary information (ESI) available: Experimental
details and additional magnetic data. CCDC 892309. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c2dt31613g
Fig. 1 Equilibria between the three conformations of 1,4-chdc. The
Gibbs free energy difference ΔG values are compared to the e,e-trans
form.11.
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The solid-state structure of 1 features layers (Fig. 2a) with
alternating iron(III)-hydroxy chains and e,a-cis-1,4-chdc linkers.
The compound condensed in the monoclinic space group P21/m.
Interestingly, the coordination geometry of the iron(III) ion is in a
compressed octahedron with two shorter axial oxygen vertexes
(Fe–O 1.956(2) Å) from the hydroxy groups and four carboxy-
lato-O atoms forming the equatorial plane with longer Fe–O
bond distance of av. 2.015(2) Å. Each iron(III) centre sits exactly
on the inversion centre, reflecting the “L”-shaped e,a-cis-1,4-
chdc ligand to other side. The two symmetry-equivalent e,a-cis-
1,4-chdcs coordinate to the iron(III) ions in a head-to-head
fashion like a “ ” shape. In this arrangement the adjacent
e,a-cis-1,4-chdc ligands along the crystallographic§ b-direction
are not parallel, but facing each other. Similar situation occurs to
other parts of the structure running along the b-axis, i.e. the
hydroxide groups in the [–Fe(III)–OH–]n chain are undulating
around the central iron(III) ions (Fe–O–Fe 123.4(2)°). Note that
the single-arm hydroxide indicates the adjacent iron(III) ions are
not central-symmetry related. The intrachain Fe⋯Fe separation
is 3.444 Å and the shortest interchain Fe⋯Fe distance is 8.569 Å.
These laminated layers are further stacked along the a-axis with a
shortest interlayer Fe⋯Fe distance of 8.010 Å (Fig. 2b).
where M = g[S(S + 1)]1/2 and u = coth(J/kBT) − kBT/J. In the
classical spin limit, the exchange energy J must be scaled fol-
lowing the usual procedure: J → JS(S + 1). A best fit between
20 and 320 K using eqn (2) results in J = −21.4(2) cm−1, g =
2.05(1) and R = 4.7 × 10−5, where R = Σ[(χobs − χcal)/(χobs)]2
indicates the fit quality. The large negative value J corresponds
to strong antiferromagnetic interactions through the hydroxy
syn–syn carboxylato bridge and the Landé g-factor close to a
value of 2 indicates a fairly isotropic magnetism, further confi-
rming the justifiability of using the Heisenberg fitting model.
The magnetic properties of 1 (Fig. 3) reflect the 1D [–Fe(III)–
OH–]n chain at higher temperatures. The χT product exhibits a
continuous decrease upon cooling to yield χT = 2.67 cm3 mol−1
K
at room temperature, a value much lower than the spin-only
value (4.38 cm3 mol−1 K) of the iron(III)-ion with half-filled 3d
orbitals, indicating strong antiferromagnetic interactions within
the chain.
Since the basic magnetic unit is represented by the syn–syn
carboxylato-bridged [–Fe(III)–OH–]n chain and the magnetic an-
isotropy of the high-spin d5 ions are virtually negligible, the
magnetic properties of the Fe(III) spin-5/2 chain in 1 were
modeled using the Hamiltonian in eqn (1),
þ1
X
~~
SiSiþ1
ˆ
H ¼ ꢀJ
ð1Þ
ꢀ1
which can be solved by the classical Fisher model,14 resulting in
eqn (2),
2
Fig. 3 Upper: temperature dependence of χT and χm (inset) of 1,
referred to one Fe(III) ion, at indicated applied fields. Red line: fitting
result using eqn (2). Lower: hysteresis loop of 1 measured at 1.9 K.
NðMβÞ ð1 ꢀ uÞ
χchain
¼
ð2Þ
3kBT ð1 þ uÞ
Fig. 2 The layered structure (a) and the crystal packing comprising two layers (b) in 1. Colour codes: iron, green; oxygen, red and carbon, light grey.
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Below 20 K, a kink was observed in the χT product, which is
a typical signal of the non-diamagnetic ground state. To further
elucidate this phenomenon the field-cooled magnetisation at
different external fields was measured, see the inset of Fig. 3.
The molar susceptibility curve at lower fields show a ramp
below 7 K, reaching a maximum of 0.5 cm3 mol−1 at 2 K. The
onset of a sharp increase at lower fields referred to a spin-canted
antiferromagnetism with long-range ordering, which was further
confirmed by the ac susceptibility data (Fig. S1‡). Both in-phase
and out-of-phase ac susceptibility peak at 3.2 K without any fre-
quency-dependent behaviour.
Moreover, a hysteresis loop was clearly recorded at 1.9 K with
a coercive field of 110 Oe and a remnant magnetisation (MR) of
0.04 μB (Fig. 3), showing the typical behaviour of a weak ferro-
magnet. The linear increase of the M vs. H plot at higher fields
without saturation confirms this behaviour (Fig. S2‡). The
equation, tanα = MR/MS, was therefore used to measure the
canting angle α by taking MS = gS = 5 μB. The resulting α =
0.5° indicates a very small canting angle between two spin orien-
tations. Compared to the Fe–Ohydroxy–Fe bond angle α does not
suggest a collinear alignment of the bond direction and the prin-
ciple magnetic axis.
Indeed, the isotropic nature of the iron(III) ion does not favour
the spin-canting behaviour originated from the Dzialoshinski–
Moriya (DM) interaction,15 which tends to orientate the parallel
spins into a perpendicular fashion. Without a significant mag-
netic anisotropy the occurrence of the canted antiferromagnetism
mainly due to the lack of inversion-centre between the adjacent
iron(III) ions. The asymmetric hydroxy-carboxylato bridge
between the nearing metal centres contributes the main source of
DM interaction.
Bearing a net magnetic moment from spin-canting does not
necessarily lead to long-range collective behaviour. The presence
of the magnetic ordering in 1 suggests that some superexchange-
coupling interaction propagating through the chemical bonds does
exist regardless of the far separation of the chains. We took the
risk of over-parameters to extract this interchain magnetic inter-
action by appending a mean-field approximation term to eqn (2):
χ = χchain/(1 − zJ′χchain/Ng2μB2), where zJ′ accounts for the inter-
chain magnetic interaction. The best fitting between 20 and 320 K
leads to J = −20.3(2) cm−1, zJ′ = −1.1(2) cm−1, g = 2.16(1) and
R = 6.8 × 10−6. Interestingly, the quantity of the zJ′ agrees well
with the reported value of an e,e-trans-1,4-chdc bridge between
two titanium(III) ions,16 indicating a potential superexchange-
coupling pathway through the σ-bonds.
References
§Crystal data for 1: C8H22FeO5, M = 234.02, monoclinic, space group
P21/m (No. 11), a = 8.010(2), b = 6.8872(18), c = 8.569(2) Å, β =
108.232(5)°, V = 449.0(2) Å3, Z = 2, Dc = 1.797 g cm–3, Mo Kα (λ =
0.71073 Å). Completeness = 99.2% (to θmax = 27°), T = 223(2) K, total
reflections: 2447, unique reflections: 1054, μ = 1.671 mm–1, 76 para-
meters, R1 = 0.0380 (I > 2σ), wR2 = 0.895 (all data) and S = 1.038.
Bruker SMART Apex CCD diffractometer. The structure was solved by
direct methods and all non-H atoms were subjected to anisotropic refine-
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In summary, by using mild hydrothermal synthesis the pure
e,a-cis conformation of the 1,4-chdc ligand can be integrated
into a 2D coordination polymer, linking a novel iron(III)-hydroxy
chain with spin-canted antiferromagnetism. We postulate that the
σ-bonding is the primary superexchange-coupling pathway of
weak inter-chain magnetic interactions, which leads to the estab-
lishment of long-range magnetic ordering below 4 K.
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YZZ is grateful for the financial support by the “Young 1000
Plan” program and the Starting Fund by XJTU.
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Dalton Trans.