Single-molecule magnet behaviour in a tetranuclear DyIII complex formed from a novel tetrazine-centered hydrazone Schiff base ligand

Article abstract of DOI:10.1039/c6dt04413a

Two analogous tetranuclear lanthanide complexes have been synthesized with the general formula [Ln₄(vht)₄(MeOH)₈](NO₃)₄·aMeOH·bH₂O, where H₂vht = (3,6-bis(vanillidenehydrazinyl)-1,2,4,5-tetrazine) and Ln = Dy^III (1), Gd^III (2). These complexes are characterized by several techniques; including single-crystal X-ray diffraction, SQUID magnetometry and single-crystal micro-SQUID hysteresis loop measurements. Elucidation of the crystal structure of the complexes shows that the lanthanide ions are bridged by a tetrazine ring, a rare bridging moiety for lanthanide ions. Magnetic studies reveal that both 1 and 2 exhibit weak ferromagnetic exchange interactions between Ln ions, and 1 displaying Single-Molecule Magnet (SMM) behaviour with a magnetisation reversal barrier of U_eff = 158 K (τ₀ = 1.06 × 10^?7 s).

Full text of DOI:10.1039/c6dt04413a

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Single-molecule magnet behaviour in a
III
tetranuclear Dy complex formed from a novel
Cite this: DOI: 10.1039/c6dt04413a
tetrazine-centered hydrazone Schiff base ligand†
a
a
a
a
b
a
T. Lacelle, G. Brunet, A. Pialat, R. J. Holmberg, Y. Lan, B. Gabidullin,
a
b,c,d
a
I. Korobkov, W. Wernsdorfer
and M. Murugesu*
Two analogous tetranuclear lanthanide complexes have been synthesized with the general formula
[Ln
4
(vht)
4
8 3 4 2 2
(MeOH) ](NO ) ·aMeOH·bH O, where H vht = (3,6-bis(vanillidenehydrazinyl)-1,2,4,5-tetrazine)
III
III
and Ln = Dy (1), Gd (2). These complexes are characterized by several techniques; including single-
crystal X-ray diffraction, SQUID magnetometry and single-crystal micro-SQUID hysteresis loop measure-
ments. Elucidation of the crystal structure of the complexes shows that the lanthanide ions are bridged by a
tetrazine ring, a rare bridging moiety for lanthanide ions. Magnetic studies reveal that both 1 and 2 exhibit
Received 21st November 2016,
Accepted 14th January 2017
DOI: 10.1039/c6dt04413a
weak ferromagnetic exchange interactions between Ln ions, and 1 displaying Single-Molecule Magnet
= 1.06 × 10− s).
7
rsc.li/dalton
(SMM) behaviour with a magnetisation reversal barrier of Ueff = 158 K (τ
0
5
radical of a tetrazine, illustrating that tetrazines can be used
to promote strong magnetic exchange interactions.
Introduction
Tetrazines, particularly 1,2,4,5-tetrazines, have been employed
In order to implement tetrazines into our systems, we
in numerous fields of scientific research, ranging from coordi- resorted to a Schiff base motif in our ligand design. The
1
nation chemistry to energetic materials. Tetrazines have even chemistry of Schiff bases offers a wide variety of potential
been shown to have biological applications through their ligands through their high degree of tunability. The availability
ability to undergo rapid inverse electron-demand Diels–Alder of numerous keto and amino precursors presents a great
2
reactions. Primarily, interest in tetrazines stems from their opportunity for synthesizing imine compounds with varying
3
unique structural and electrochemical properties. The four structural properties. Careful selection of the precursors allows
N-atoms in the tetrazine ring result in a remarkably low-lying for the control of denticity, size/shape of binding sites, as well
π* orbital, resulting in tetrazines being strong π-acceptors as any desired moieties to be incorporated into the ligand.
and having low one electron reduction potentials (typically Thus, our research efforts revolved around implementing a
E > −1.3 V vs. ferrocene) toward the formation of radical tetrazine ring as a functional moiety of a Schiff base ligand,
1
anions. In addition, these N-donors enable tetrazines to be allowing for the design of polymetallic lanthanide complexes.
4
excellent bridging ligands in polymetallic complexes. Recent
Through the introduction of tetrazines into Schiff-base
results from Dunbar and co-workers on a tetrazine-based ligands, we are able to access a largely unexplored field. There
complex show remarkable magnetic properties with large has been only one previously reported structure composed of a
II
exchange coupling constants between Co ions and the anion Schiff base ligand containing a tetrazine moiety; Roberts and
4
co-workers reported Fe tetrahedral cages which underwent
post-assembly modification through inverse electron-demand
Diels–Alder reactions. Tetrazine-based bridging systems, par-
a
6
Department of Chemistry and Biomolecular Sciences, University of Ottawa,
0 Marie Curie, Ottawa, ON K1N 6N5, Canada. E-mail: m.murugesu@uottawa.ca
1
ticularly with lanthanides, are also rare in the literature. The
oxophilic nature of the lanthanide ions, combined with the
weak σ-donating character of tetrazines, presents a great chal-
lenge for synthesizing tetrazine-based lanthanide compounds.
To the best of our knowledge Ward and co-workers reported
the only two examples of tetrazine-bridged lanthanide com-
b
Institut Néel, CNRS and Université Grenoble Alpes, BP 166, 25 Avenue des Martyrs,
3
8042 Grenoble, France
c
Physikalisches Institut, Karlsruhe Institute of Technology (KIT), Wolfgang-Gaede Str. 1,
D-76131 Karlsruhe, Germany
d
Institut für Nanotechnologie, Karlsruhe Institute of Technology (KIT),
Hermann-von-Helmholtz Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany
†
Electronic supplementary information (ESI) available: Detailed experimental
7
plexes, which were found to act as near-infrared emitters. In
procedures, CIF files and supporting figures. CCDC 1518373–1518375. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c6dt04413a
order to isolate multinuclear complexes with new structural
topologies and interesting magnetic properties, we situated
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(
hydrazinyl)-tetrazine (4.00 g, 0.028 mol) in methanol (500 mL)
was added o-vanillin (9.47 g, 0.062 mol) (Scheme 1). The result-
ing reaction mixture was stirred under reflux over 18 h and
then was allowed to cool to room temperature. The red precipi-
tate was collected by filtration and washed with methanol.
Recrystallization from DMF yields the DMF solvate of H vht as
2
dark red needle-shaped crystals suitable for single-crystal X-ray
Scheme 1 Synthesis of H
2
vht.
diffraction. The crystalline material was collected by suction
filtration and dried under vacuum prior to its use as the ligand
1
(
5.85 g, 0.014 mol, 51%). H NMR (DMSO-d
6
, 400 MHz) δ 3.78
the tetrazine ring as an integral part of the ligand coordination (s, 6H, OCH ), 6.82 (t, J = 7.9 Hz, 2H, Ar), 6.96 (dd, J = 7.9 and
3
pockets, which contain donor atoms with higher Lewis basicity 1.4 Hz, 2H, Ar), 7.10 (dd, J = 7.9 and 1.4 Hz, 2H, Ar), 8.41 (s,
1
3
to enable the tetrazine to act as a bridge between metal ions 2H, NvCH), 10.73 (s, 2H, NH), 12.05 (s, 2H, OH); C (DMSO-
through chelation (Scheme 1).
d , 100 MHz) δ 160.0, 148.4, 147.0, 144.5, 120.8, 119.9, 119.5,
6
With these principles in mind, we focused our attention on 113.7, 56.3. IR (ATR, cm− ): 3207 (br), 2980 (br), 1569 (m), 1538
tetrazines as a bridging motif to probe the nature and strength (m), 1463 (m), 1417 (s), 1385 (m), 1366 (m), 1281 (w), 1247 (s),
of the magnetic interactions between lanthanide ions. The 1147 (m), 1094 (m), 1079 (m), 1043 (s), 982 (m), 940 (m), 885
large unquenched orbital angular momentum exhibited by (w), 856 (w), 833 (w), 781 (m), 736 (s), 633 (m), 585 (w), 565
1
III
lanthanide ions, such as Dy , gives rise to magnet-like behav- (m). Elemental analysis; Expected: C 52.68%, H 4.42%, N
ior in many lanthanide complexes in the form of slow relax- 27.30%; Found: C 51.96%, H 4.54%, N 27.35%.
ation of the magnetisation. Discrete molecules that demon-
4 4 8 3 4 2
Synthesis of [Dy (vht) (MeOH) ](NO ) ·8.07MeOH·0.65H O
strate this slow relaxation are referred to as Single-Molecule (1).
3 3 2
A room temperature suspension of Dy(NO ) ·6H O
Magnets (SMMs). These systems have vast potential appli- (114 mg, 0.25 mmol) and H vht (51 mg, 0.125 mmol) in
2
cations in several high-tech disciplines such as ultra-high MeOH was stirred for 5 min, after which NaN3 (16 mg,
density data storage, spintronic devices as well as next- 0.25 mmol) was added. The reaction mixture was stirred for
8
generation MRI contrast agents. Herein, we report a unique another 30 minutes, filtered, and the filtrate was left to stand
system consisting of two tetranuclear lanthanide complexes in a sealed vial. After 2 days, black block-shaped crystals suit-
III
III
based on anisotropic Dy (1) and isotropic Gd (2) metal ions. able for single-crystal X-ray diffraction were obtained. Yield =
Complex 1 exhibits slow relaxation of the magnetisation, making 33%. IR (ATR, cm− ): 3201 (br), 2941 (w), 2832 (w), 1603 (m),
1
1
the first tetrazine-bridged SMM under zero applied field.
1573 (m), 1524 (m), 1447 (m), 1397 (m), 1310 (m), 1283 (m),
1
9
239 (m), 1220 (s), 1169 (m), 1078 (m), 1055 (m), 1034 (m),
64 (m), 911 (w), 849 (m), 826 (w), 784 (w), 767 (w), 735 (s),
Experimental
Materials
2
657 (m), 627 (m). UV-VIS spectra of 1, 2 and H vht are depicted
in Fig. S1.† Elemental analysis; Expected: C 34.59%, H 4.27%,
N 16.49%; Found: C 34.21%, H 4.06%, N 16.97%.
All chemicals were of reagent grade, purchased from TCI, Alfa
Aesar and Strem Chemicals and were used without any further
purification.
Synthesis of [Gd
2). The same procedure as 1 was employed to synthesize 2,
however, the metal salt was replaced by Gd(NO ) ·6H O
4 4 8 3 4 2
(vht) (MeOH) ](NO ) ·8.19MeOH·0.91H O
(
3
3
2
(
113 mg, 0.25 mmol) to yield black block-shaped crystals suit-
Elemental analysis, IR, UV-VIS, NMR spectroscopy
able for single-crystal X-ray diffraction. Yield = 35%. IR (ATR,
cm ): 3211 (br), 2941 (w), 2832 (w), 1606 (m), 1585 (m),
1
1
9
6
−
1
Elemental analysis was performed using an Isotope Cube
elemental analyser. Infrared spectra were obtained with a
Varian 640 FTIR spectrometer equipped with an ATR in the
530 (m), 1447 (m), 1397 (m), 1316 (m), 1287 (m), 1239 (m),
220 (s), 1168 (m), 1114 (m), 1077 (m), 1056 (m), 1013 (m),
67 (m), 910 (w), 851 (m), 826 (w), 784 (w), 768 (w), 737 (s),
59 (m), 630 (m). Elemental analysis; Expected: C 34.77%,
1
−1
4
000 cm− to 600 cm range. UV-VIS spectra were measured
using an Agilent Cary 5000 UV-VIS-NIR spectrometer in the
range of 200 to 800 nm. All samples were measured in MeOH
using a standard 10 mm pathlength cuvette. NMR analyses
were carried out using a Bruker Avance 400 spectrometer
equipped with an automated sample holder and a 5 mm auto-
tuning broadband probe with Z gradient.
H 4.32%, N 16.56%; Found: C 34.36%, H 4.10%, N 16.74%.
Single-crystal X-ray diffraction analysis
The crystals were mounted on a thin glass fibre using paraffin
oil. Prior to data collection crystals were cooled to 200 K. Data
were collected on a Bruker AXS SMART single-crystal diffrac-
tometer equipped with a sealed Mo tube source (λ = 0.71073 Å)
APEX II CCD detector. Raw data collection and processing
Synthesis of (3,6-bis(vanillidenehydrazinyl)-1,2,4,5-tetrazine),
H vht
2
The precursor, 3,6-bis(hydrazinyl)-1,2,4,5-tetrazine, was syn- were performed with the APEX II software package from
1
0
thesized according to a previously reported procedure with BRUKER AXS. Semi empirical absorption corrections based
9
11
slight modifications (Scheme S1†). To a suspension of 3,6-bis on equivalent reflections were applied.
Direct methods
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yielded all non H-atoms, which were refined with anisotropic the encapsulation of lanthanide ions. The N–N bond lengths
thermal parameters. All hydrogen atoms were calculated of 1.32 Å and C–N bond lengths of 1.34 Å are in agreement
12
1
geometrically and were riding on their respective atoms. with previously reported tetrazine compounds. The H NMR
Crystallographic data for the ligand H vht, 1 and 2 are spectrum of H vht consists of four singlets and three phenyl
2
2
depicted in Table S1.† CCDC numbers are as follows: 1518373 multiplets. The full spectrum of H vht is depicted in Fig. S2.†
2
III
III
(
1), 1518374 (2), 1518375 (H
2
vht).
Coordination of the deprotonated ligand to Dy or Gd
yields two isostructural complexes, with the Dy complex (1)
III
Magnetic measurements
depicted in Fig. 2. The complexes crystallize in the triclinic Pˉ1
Magnetic susceptibility measurements were performed using a space group. For brevity, the Dy analogue will be described.
III
MPMS-XL7 Quantum Design SQUID magnetometer. Direct Each Dy ion adopts an ennea-coordinate motif, with three
current (dc) susceptibility data measurements were performed N-donors and six O-donors. For example, Fig. 2 shows that Dy1
at temperatures ranging from 1.9 to 300 K, and between is coordinated to two imine nitrogen atoms, N8 and N9, and
applied fields of −5 to 5 T. Measurements were performed on one nitrogen from the tetrazine ring, N11. The o-vanillin side
crushed polycrystalline samples with 24.6 and 23.4 mg for groups contribute oxygen atoms O1, O2, O3 and O5 to the
samples 1 and 2, respectively. Each sample was wrapped in a coordination sphere of Dy1. Two oxygen atoms from two
polyethylene membrane. Alternating current (ac) susceptibility methanol molecules, O15 and O16, make up the remainder of
measurements were performed under an oscillating ac field of the coordination sphere. Dy2 exhibits a similar coordination
3
.78 Oe and ac frequencies ranging from 0.1 to 1500 Hz. environment to Dy1. The comparison of thirteen reference
Magnetisation vs. field measurements were performed at
00 K in order to check for the presence of ferromagnetic
1
impurities, which were found to be absent. Magnetic data
were corrected for diamagnetic contributions using Pascal’s
constants.
Results and discussion
Structural details
In order to give a detailed description of the structural charac-
teristics of the free ligand, and to observe the effects of coordi-
nation, single-crystal X-ray diffraction was carried out on the
H vht ligand, in addition to NMR spectroscopy. The ligand
2
1
crystallizes in the monoclinic P2 /c space group with all of the
ligand atoms being nearly coplanar. The structure of the
ligand consists of one central tetrazine ring and two identical
hydrazone substituents derived from o-vanillin (Fig. 1). The
molecule exhibits centrosymmetry with the inversion center
located directly in the center of the tetrazine ring. The two
vanillidene moieties, in conjunction with the tetrazine ring,
form four coordination pockets. These pockets are comprised
of two large tridentate pockets consisting of N1, N4a and O1
and two smaller bidentate pockets formed by O1 and O2 from
the vanillidene moieties. The larger pockets are well-suited to
2
Fig. 1 Crystal structure of H vht (3,6-bis(vanillidenehydrazinyl)-1,2,4,5-
tetrazine). Solvent molecules and hydrogen atoms were omitted for
clarity. Symmetry equivalent atoms are denoted by an additional “a” in
the label. Colour code: red (O), blue (N). All unfilled vertices are carbon
atoms.
Fig. 2 Molecular structure (top), dinuclear subunit (middle) and coordi-
nation polyhedra (bottom) for complex 1. Colour code: yellow (Dy), red (O),
blue (N). Hydrogen atoms are omitted for clarity. All unfilled vertices are
carbon atoms.
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polyhedra with the coordination polyhedra of Dy1 and Dy2 by
1
3
SHAPE analysis (Table S2†) reveals that Dy1 possesses a dis-
torted spherical capped square antiprism (C4v) geometry while
Dy2 resembles a spherical tricapped trigonal prism (D3h
)
(Fig. 2). It is also important to note that the cationic complexes
are charge balanced by four nitrate anions, some of which
exhibit partial occupancies (N18 and N19). Moreover, a signifi-
cant amount of lattice solvent molecules can be found; most
notable is the presence of roughly 8 MeOH molecules (8.07
and 8.19, for 1 and 2, respectively), along with a smaller contri-
bution of water molecules (0.65 and 0.91 for 1 and 2, respect-
ively). Thus, while manipulating the crystals of 1 and 2, care
must be taken to prevent evaporation of the solvent molecules,
which causes visible cracking of the crystals.
Fig. 3 Bond distances of the central tetrazine moiety of 1 in Å. Colour
code: yellow (Dy), blue (N). Hydrogen atoms are omitted for clarity. All
unfilled vertices are carbon atoms.
While complexes 1 and 2 are tetranuclear overall, they are
composed of two dinuclear subunits (Fig. 2). These subunits
are linked by two ligand molecules and bound by the vanilli-
dene donor atoms. This bridging motif between subunits also
provides two bridging modes between Dy1 and Dy2 in the
form of phenoxide oxygen atoms, O1 and O3. With the two
phenoxide atoms and the tetrazine ring there is a total of three
ated by distances of 10.49 Å and 11.21 Å, and thus are unlikely
to experience any significant interaction between spin carriers.
Close inspection of the bridging between Ln ions within a
single dimer reveals the potential for three superexchange
pathways. Two of the pathways are mediated by phenoxide
moieties of the H vht ligand, in a similar fashion to previously
III
2
bridging motifs between the Dy centers.
1
4
reported Schiff-base compounds, while an additional bridg-
ing occurs through two nitrogen atoms (N11 and N12) of the
tetrazine ring. It is important to note that dipole–dipole inter-
In addition, the complex is also centrosymmetric, giving
rise to symmetry equivalent atoms between subunits. While
the dinuclear lanthanide complexes of Shavaleev and co-
III
III
actions will also likely contribute to the Ln –Ln coupling, in
addition to the superexchange interactions like in most cases
of lanthanide systems.
7
workers do possess a tetrazine bridge, the lanthanide ions are
bridged from across the tetrazine moiety. In the case of com-
III
plexes 1 and 2, the Ln ions are bridged by the tetrazine ring
The temperature dependence of the χT product displays the
presence of non-negligible ferromagnetic coupling between
spin carriers (Fig. 4). The room temperature values at 300 K
from the same side. This leads to an unprecedented bridging
motif for lanthanide-based systems. Another distinguishing
feature in our system is the large spatial separation between
the dinuclear subunits with a Dy1–Dy2a distance of 10.49 Å
and a Dy1–Dy1a distance of 11.21 Å. For comparison, the smal-
lest intermolecular Dy⋯Dy distance for complex 1 is 10.43 Å.
These distances are significantly larger than those of the
Dy1–Dy2 subunit distances of 3.91 Å. Statistical analysis for all
3
−1
are 55.81 and 30.81 cm K mol for 1 and 2, respectively,
which are in good agreement with the expected theoretical
3
−1
values of 56.68 and 31.52 cm K mol , for four non-inter-
III
6
acting lanthanide ions (Dy : H15/2, S = 5/2, L = 5, g = 4/3;
III
reported compounds with Dy ions bridged by moieties con-
sisting of one or two atoms shows that the average intra-
molecular Dy⋯Dy distance is 3.83 Å. The Dy1–Dy2 distance of
1
is then slightly above the average intramolecular distances
for similarly bridged compounds.
When comparing the free ligand to the coordinated tetra-
zine moiety, the latter exhibits a relatively large contortion of
the tetrazine ring. The bond distance of the nitrogen atoms
N11 and N12 in complex 1 is elongated to 1.36 Å while the
opposite nitrogen atoms, N13 and N14, have a shorter bond
III
distance of 1.27 Å (Fig. 3). The coordination of Dy is likely
the cause of this contortion, resulting in the ligand being
forced outward from the metal ions.
Static magnetic susceptibility
The analysis of the crystal structure of compounds 1–2
(
vide supra) suggests that the most likely intramolecular mag-
netic interactions would occur between Ln1 and Ln2 of each
subunit. This is due to the close proximity of these metal ions
Fig. 4 χT vs. T plot for 1 and 2 under applied dc fields of 1000 Oe. The
solid line shows the best fit obtained through the magnetic model
(3.91 Å). Conversely, the two dinuclear subunits are well separ- described in the text.
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III
8
Gd : S7/2, S = 7/2, L = 0, g = 2). Upon lowering the tempera-
ture, the χT values of 1 and 2 remain relatively constant down
to 12 K, before abruptly increasing to a maximum of 69.86
3
−1
3
−1
cm K mol for 1 and 33.73 cm K mol for 2 at 1.9 K. This
sharp increase is indicative of intramolecular ferromagnetic
coupling, which dominates the magnetic behaviour at low
temperatures. In order to further probe the magnetic exchange
interactions, we applied the Van Vleck equation to Kambe’s
vector coupling scheme using the isotropic spin Hamiltonian:
Ĥ ¼ ꢀ2JðŜaꢁŜb þ ŜaꢁŜbÞ
with S = S = 7/2, allowing us to reproduce the χT curve of
a
b
2
(Fig. 4). Due to the symmetry equivalence of the dimeric sub-
units and their spatial separation with respect to each other
we decided to model the exchange interactions of 2 with one
J parameter. The best-fit parameters obtained were J = 0.009(3)
−
1
cm and g = 1.982(1), testifying the weak ferromagnetic inter-
1
4
actions between lanthanide ions. As noted above, these inter-
actions likely originate from superexchange and dipole–dipole
interactions. It is also important to note that there is a greater
upturn in the χT product of 1, which suggests that the inter-
action in 1 could be significantly larger than that in 2.
Furthermore, we examined the field (H) dependence of the
magnetisation (M), which shows a rapid increase of the mag-
netisation at low fields up to 27.40 μ (1) and 25.49 μ (2) at
B
B
5
T and 2.0 and 1.9 K, respectively (Fig. S3†). This rapid increase
in magnetisation is expected for ferromagnetically coupled
1
5
−1
systems. The M vs. HT plots, at varying temperatures, show
magnetisation curves that are slightly deviated from one
another, suggesting the presence of non-negligible magneto-
anisotropy and/or low-lying excited states for compound 1
(Fig. S4†). On the other hand, the isotropic nature of 2 is con-
firmed through the superimposition of the analogous magne-
tisation curves at differing temperatures (Fig. S4†).
Fig. 5 Out-of-phase magnetic susceptibility (χ’’) vs. frequency (top) and
out-of-phase magnetic susceptibility vs. temperature (bottom) for 1.
The χ’’ vs. ν data for 1 were fitted using a generalized Debye model for a
single relaxation process.
Dynamic magnetic susceptibility
Due to the recent successes of discrete Dy-based complexes
1
6
displaying record SMM properties, we were prompted to
investigate the dynamics of the magnetisation of compound 1.
The frequency dependence of the ac susceptibility was investi- values obtained from the fits of χ″ vs. ν are summarized
gated under zero applied dc field (Fig. 5). The χ″ vs. ν data for in Table S3.† The shifting of peak maxima confirms slow
1
were fitted using a generalized Debye model for a single relaxation of the magnetisation. The relaxation time (τ) is
1
7
relaxation process:
derived from the frequency dependent measurements between
ꢀ
ꢁ
5 and 18 K and plotted as a function of 1/T (Fig. S5†). Above
πα
2
1
ꢀα
1
þ ð2πντÞ
sin
15 K, the relaxation follows a thermally activated mechanism,
χ′ðνÞ ¼ χ þ ðχ ꢀ χ Þ
ꢀ
ꢁ
S
T
S
πα
2
1
ꢀα
2ð1ꢀαÞ
eliciting an energy barrier of 158 K and a pre-exponential
1
þ 2ð2πντÞ
sin
þ ð2πντÞ
−
7
factor (τ ) of 1.06 × 10 s using the Arrhenius equation:
0
ꢀ
ꢁ
πα
0
τ = τ exp(Ueff/kT).
1
ꢀα
ð2πντÞ
cos
2
A graphical representation of χ″ vs. χ′ (Cole–Cole plot) for 1
was fitted using a generalized Debye model for a single relax-
ation process between 7 and 20 K (Fig. 6). The semi-circular
χ′′ðνÞ ¼ ðχ ꢀ χ Þ
ꢀ
ꢁ
T
S
πα
2
1
ꢀα
2ð1ꢀαÞ
1
þ 2ð2πντÞ
sin
þ ð2πντÞ
where χ
T
and χ
S
are the isothermal and adiabatic suscepti- plots give a moderately narrow distribution of α parameters
bilities, respectively, τ is the relaxation time and α depicts the ranging from 0.25 to 0.41, which is consistent with the α
distribution of relaxation times. From these data, we can also values obtained by fitting the frequency dependent data
observe frequency dependence in both the in-phase (χ′) and (Table S3†). The temperature dependence of the ac suscepti-
out-of-phase (χ″) signals. Selected data, including τ and α bility was also investigated under zero applied dc field (Fig. 5).
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Fig. 6 Cole–Cole plot for frequency dependent ac susceptibility data
of 1. Solid lines are the best fit to the generalized Debye model.
The temperature dependence of the ac susceptibility under
zero dc field reveals an out-of-phase signal (χ″) with observable
maxima. As the frequency is increased, the maxima are shifted
to higher temperatures, confirming slow relaxation of the mag-
netisation, characteristic of SMMs.
To further investigate the low temperature magnetic behav-
iour of complex 1, single-crystal relaxation measurements were
1
8
carried out on a micro-SQUID array. Below 0.5 K at a sweep
rate of 0.14 T s− , the M vs. H sweeps exhibited hysteretic be-
haviour and a small opening could be observed up to a temp-
1
Fig. 7 Single-crystal magnetic hysteresis loop measurements on
micro-SQUID array for 1 with varying sweep rates (top) and tempera-
a
erature of 4 K (Fig. 7). The width of the magnetic hysteresis tures (bottom).
loop of 1 shows strong temperature, and moderate sweep rate,
dependence. The S shape of the hysteresis loop and the step
position located between 0 and 0.2 T are reminiscent of pre-
viously reported weakly coupled lanthanide dimers. Such sig-
nature behaviours result from single-ion relaxation entangled
with the neighbouring Dy ion relaxation within the molecule
1
4b
via weak intramolecular-exchanged biased interactions.
Electrostatic modelling of the anisotropy axes in complex 1
1
9
was also carried out using Magellan. In the case of low
III
symmetry Dy
complexes the ground Kramers doublet
shows strong axiality and the g-tensor approaches that of the
2
0
j x y z
m = 15/2 levels, where g = g = 0 and g = 20. The electron
III
density distribution in Dy can be approximated by an oblate
spheroid, and by solving the electrostatic energy minimum
with respect to the crystal field potential, the orientation of the
magnetic anisotropy axes can be obtained. Carrying out this
electrostatic modelling reveals a near-collinear alignment of
III
the easy-axis vectors between Dy centers (Fig. 8), with an
angle of 7° between the Dy1 and Dy2 axes. The vectors lie
along the direction of the anionic phenoxide atoms for each
subunit, with deviations in the alignment of these vectors
attributed to slight differences in the coordination geometry
between the Dy1 and Dy2 ions. The centrosymmetry of the
complex results in the antiparallel alignment of the easy axis
vectors between the dinuclear subunits. Despite this antiferro-
j
Fig. 8 Anisotropy axes in the ground Kramers doublet (m = 15/2) for
each Dy ion in 1. Axes modelled using Magellan software. Colour code:
yellow (Dy), red (O), blue (N). Hydrogen atoms are omitted for clarity. All
III
magnetic alignment, the ferromagnetic interaction between unfilled vertices are carbon atoms.
Dalton Trans.
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Dy1 and Dy2 is shown to be dominant due to the compara-
tively short intramolecular distance of 3.91 Å.
8 (a) L. Bogani and W. Wernsdorfer, Nat. Mater., 2008, 7, 179;
(b) E. Coronado and M. Yamashita, Dalton Trans., 2016, 45,
16553; (c) Y. Wang, W. Li, S. Zhou, D. Kong, H. Yang and
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Conclusions
The structural and magnetic properties of two analogous tetra-
nuclear lanthanide complexes consisting of two dimeric sub-
units are presented and discussed. The synthesis of these com-
plexes was achieved using a new compartmental Schiff base
ligand incorporating a tetrazine ring. Both the Dy and Gd ana-
logues exhibit dominant ferromagnetic exchange interactions
2012, 51, 1606; (e) Y.-N. Guo, G.-F. Xu, P. Gamez, L. Zhao,
S.-Y. Lin, R. Deng, J. Tang and H.-J. Zhang, J. Am. Chem.
Soc., 2010, 132, 8538.
B. Rao, S. Dhokale, P. Rajamohanan and S. Hotha, Chem.
Commun., 2013, 49, 10808.
9
1
1
1
1
1
0 APEX Software Suite v.2012, Bruker AXS, Madison, WI,
005.
1 R. Blessing, Acta Crystallogr., Sect. A: Fundam. Crystallogr.,
995, 51, 33.
2 M. Schwach, H. Hausen and W. Kaim, Inorg. Chem., 1999,
8, 2242.
at low temperature. The {Ln } subunits of these tetranuclear
2
2
complexes are sufficiently separated to be treated as individual
dinuclear systems and can therefore be modelled as such. In
addition to exhibiting ferromagnetic exchange, compound 1
exhibits SMM behaviour with a sizeable magnetisation reversal
barrier of 158 K. Although the strength of the interaction is
weak, it is noteworthy that in this system dominant ferro-
magnetic interactions are observed. If such ferromagnetically
coupled systems are strongly coupled, high spin ground states
with large energy barriers can be expected. With this in mind
our ongoing studies focus on the reduction of the tetrazine
1
3
3 D. Casanova, M. Llunel, P. Alemany and S. Alvarez, Chem. –
Eur. J., 2005, 11, 1479.
4 (a) F. Habib, G. Brunet, V. Vieru, I. Korobkov, L. Chibotaru
and M. Murugesu, J. Am. Chem. Soc., 2013, 135, 13242;
(
b) F. Habib, P.-H. Lin, J. Long, I. Korobkov,
W. Wernsdorfer and M. Murugesu, J. Am. Chem. Soc., 2011,
33, 8830; (c) P.-H. Lin, W.-B. Sun, Y.-M. Tian, P.-F. Yan,
L. Ungur, L. Chibotaru and M. Murugesu, Dalton Trans.,
012, 41, 12349; (d) F. Yang, P. Yan, Q. Li, P. Chen and
III
III
ring in order to improve the strength of the Ln –Ln inter-
actions through radical exchange coupling.
1
2
G. Li, Eur. J. Inorg. Chem., 2012, 4287; (e) S.-Y. Lin, G.-F. Xu,
L. Zhao, Y.-N. Guo and J. Tang, Dalton Trans., 2011, 40,
Acknowledgements
8
213; (f) A. Gorczynski, M. Kubicki, D. Pinkowicz, R. Pelka,
V. Patroniak and R. Podgajny, Dalton Trans., 2015, 44,
6833; (g) Y.-L. Chien, M.-W. Chang, Y.-C. Tsai, G.-H. Lee,
W.-S. Sheu and E.-C. Yang, Polyhedron, 2015, 102, 8;
h) P. Bag, C. Rastogi, S. Biswas, S. Sivakumar, V. Mereacre
We would like to thank the University of Ottawa, NSERC, CFI,
and ORF for financial support.
1
(
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