A trigonal-pyramidal Erbium(III) single-molecule magnet

Article abstract of DOI:10.1002/anie.201411190

Given the recent advent of mononuclear single-molecule magnets (SMMs), a rational approach based on lanthanides with axially elongated f-electron charge cloud (prolate) has only recently received attention. We report herein a new SMM, [Li(THF)₄[Er{N(SiMe₃)₂}₃Cl] 2 THF, which exhibits slow relaxation of the magnetization under zero dc field with an effective barrier to the reversal of magnetization (ΔE_eff/k_B=63.3 K) and magnetic hysteresis up to 3 K at a magnetic field sweep rate of 34.6 Oe s^-1. This work questions the theory that oblate or prolate lanthanides must be stabilized with the appropriate ligand framework in order for SMM behavior to be favored.

Full text of DOI:10.1002/anie.201411190

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201411190
German Edition: DOI: 10.1002/ange.201411190
Mononuclear Lanthanide SMMs
A Trigonal-Pyramidal Erbium(III) Single-Molecule Magnet**
Andrew J. Brown, Dawid Pinkowicz, Mohamed R. Saber, and Kim R. Dunbar*
Abstract: Given the recent advent of mononuclear single-
molecule magnets (SMMs), a rational approach based on
lanthanides with axially elongated f-electron charge cloud
pentamethylcyclopentadienide; COT= cyclooctatetraenide),
reported by Gao and co-workers, which exhibits two relax-
ation processes with energy barriers DE /k of 197 and 323 K
eff
B
[
6a]
(
prolate) has only recently received attention. We report herein
and a butterfly hysteresis loop as high as 5 K.
recently, Long and co-workers prepared [Er(COT) ] , which
More
ꢀ
a new SMM, [Li(THF) [Er{N(SiMe ) } Cl]·2THF, which
4
3
2
3
2
exhibits slow relaxation of the magnetization under zero dc
field with an effective barrier to the reversal of magnetization
exhibits enhanced SMM properties because the equatorial
nature of the ligands is more compatible with preserving the
prolate nature of the erbium(III) ion electron density and the
molecule is symmetrical. The compound exhibits a high
energy barrier of DE /k = 216 K with waist-restricted hys-
(
DE /k = 63.3 K) and magnetic hysteresis up to 3 K at
eff B
ꢀ
1
a magnetic field sweep rate of 34.6 Oes . This work questions
the theory that oblate or prolate lanthanides must be stabilized
with the appropriate ligand framework in order for SMM
behavior to be favored.
eff
B
teresis being observed at temperatures up to 10 K for a diluted
[
6b]
sample in a yttrium matrix. Murugesu and co-workers also
[7]
studied this system and derivatized the COT ligand with
trimethylsilyl appendages to study the effects of lowering the
L
anthanide-containing mononuclear single-molecule mag-
ꢀ
nets (SMMs) have received much attention in the last five
symmetry. Both [Er(COT’’)2] and [Er (COT’’) ] (COT’’ =
2
3
[1]
years, as they appear to be the best candidates in the field of
1,4-bis(trimethylsilyl)cyclooctatetraenyl dianion) were pre-
[6c]
molecular magnetism for application in high density data
pared, and it was found that the double-decker sandwich
[
2]
storage, molecular spintronics, and quantum processing.
compound exhibits DE /k = 335 K and hysteresis up to 14 K
eff
B
[
8]
The advantages of 4f elements stem from their inherent
anisotropy imparted by high ground state spin values and
strong spin–orbit coupling, producing an oblate or prolate
shape for the 4f electron density, which can be stabilized by
in solution.
Along with the rare-earth organometallic sandwich com-
pounds, there are also low-coordinate species of the type
[
9]
[Ln{N(SiMe ) } ] that have been studied for catalysis. The
3
2 3
[
3]
a ligand field of the appropriate symmetry.
bis(trimethylsilyl)amide ligand is known to enforce a trigonal
[
10]
By using geometric design principles to minimize elec-
tronic repulsions between the electron densities of the
lanthanide ions and the ligands, researchers have prepared
SMMs with extremely high barriers and magnetic hysteresis
planar geometry in transition metal complexes,
but the
lanthanide analogues are distorted towards a trigonal pyr-
amidal structure due to agostic interactions between the
lanthanide center with the b-SiꢀC bond of the ligand, as well
[
4]
as compared to previous examples. While most examples of
the rare-earth SMMs are based on the oblate terbium and
as bonding considerations involving the d-orbitals of the
[11]
lanthanide ion. Tang and co-workers recently reported the
magnetic properties of [Er{N(SiMe ) } ], the first example of
[5]
dysprosium ions (and particularly the latter ) more recent
work has revealed that prolate ions can also engender SMM
behavior, with the erbium(III) ion being the main choice for
3
2 3
an equatorially coordinated mononuclear lanthanide SMM;
the molecule exhibits a blocking temperature of about 13 K
based on the c’’(T) maximum at a frequency of 1488 Hz and
a barrier to the reversal of magnetization DE /k = 122 K
[
6]
such systems. The first reported erbium(III) mononuclear
SMM is the organometallic complex [(Cp*)Er(COT)] (Cp* =
eff
12]
B
[
with hysteresis reported to occur at 1.9 K.
They also
reported [Er(NHPhiPr ) (THF) ], which exhibits SMM
[
+]
2
3
2
[
*] A. J. Brown, Dr. M. R. Saber, Prof. K. R. Dunbar
Department of Chemistry, Texas A&M University
College Station, TX 77843 (USA)
behavior only in the presence of a dc field.
Recently we also prepared [Er{N(SiMe ) } ] for the
3
2 3
E-mail: dunbar@chem.tamu.edu
Homepage: http://www.chem.tamu.edu/rgroup/dunbar/
aforementioned reasons, but during the course of this work,
the results of Tang and co-workers appeared. We therefore
decided to pursue the properties of a related compound with
a bridging chloride that we isolated from one of the syntheses
under specific conditions (see note in synthesis section of
Supporting Information for 1). Herein we report the prepa-
ration, crystal structure, and magnetic characterization of
Dr. D. Pinkowicz
Faculty of Chemistry, Jagiellonian University
Ingardena 3, 30-060 Krakꢀw (Poland)
+
[
] Permenant address: Department of Chemistry
Fayoum University, Fayoum 63514 (Egypt)
[
**] This material is based on work supported by the US Department of
[
Li(THF) ][Er{N(SiMe ) } Cl]·2THF (1). The effects of solid-
4 3 2 3
Energy, Materials Sciences Division, under Grant No. DE-
state dilution with the yttrium complex are also reported.
SC0012582. D.P. gratefully acknowledges the financial support of
the EC REA within the Marie Curie International Outgoing Fellow-
ship, project MultiCyChem (grant agreement no. PIOF-GA-2011-
[
13]
While analogues with other lanthanide ions exist,
the
erbium(III) congener was not reported previously. Interest-
298569).
ingly, compound 1 exhibits improved magnetic properties as
compared with [Er(NHPhiPr ) (THF) ] despite the presence
of a rigorously axial and negatively charged chloride. These
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201411190.
2
3
2
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Angewandte
Communications
results are not easily understood in the framework of the
simple oblate versus prolate theory proposed for engendering
Table 1: Comparison of mononuclear erbium(III) single-molecule mag-
nets.
[
3]
mononuclear lanthanide SMMs, a point that was recently
Compound
U /k
[K]
Hysteresis
[K]
dc
field?
Source
eff
B
III
[14]
raised about the properties of another Er compound.
The ligand LiN(SiMe ) was synthesized by reaction of
3
2
1
1’
66.4
55.1
122
25
3
3
1.9
–
5
10–12
–
8
No
No
No
Yes
No
No
Yes
No
No
[a]
[a]
excess nBuLi with HN(SiMe ) followed by recrystallization
3
2
[
b]
from a THF/hexanes mixture to yield the dimeric species
[Er{N(SiMe ) } ]
[Er(NHPhiPr
[12]
[12]
[6a]
[6b,7]
[14]
[6c]
[8]
3
2 3
[15]
[
{Li(THF)m-N(SiMe )} ], as confirmed by X-ray crystallog-
2
)
3
(THF)
2
]
3
2
[
[
[
[
[
(Cp*)Er(COT)]
Er(COT)2]
Er(trensal)]
Er(COT’’)2]
Er (COT’’) ]
197, 323
216–286
raphy. The ligand was reacted with a stoichiometric amount of
ErCl in THF, the solution was reduced to a paste and then
3
[c]
77.7
finally extracted with pentane, which led to the formation of
light pink crystals of 1 (Figure 1) upon concentration and
cooling down to ꢀ158C.
187
335
14
2
3
[a] The current work. [b] Higher-temperature magnetic hysteresis mea-
surement not attempted/reported. [c] Calculated.
ꢀ1
decreases to reach 9.55 emumol K at 2 K, which is attrib-
uted to the depopulation of mJ sublevels and magnetic
anisotropy. The dynamic properties of 1 were investigated
using ac magnetic susceptibility measurements in the fre-
quency range 1–1500 Hz. The out-of-phase ac magnetic
susceptibility versus frequency plot (Figure 2) recorded in
the 1.8–11 K range under a zero applied dc magnetic field
reveals slow magnetic relaxation behavior typical of SMMs
(see the Supporting Information, Figure S2 for in-phase and
out-of-phase ac susceptibility as a function of temperature).
The relaxation times obtained from fitting the ac magnetic
[
16]
susceptibility data using the generalized Debye model were
plotted vs. 1/T to give an Arrhenius plot with two obvious
regimes: a temperature-dependent regime above approxi-
mately 5 K and a temperature-independent regime below
5
K. The selected high-temperature linear region (7.5–11 K)
Figure 1. Crystal structure of [Li(THF) ][Er{N(SiMe ) } Cl]·2THF (1)
4
3 2 3
of the Arrhenius plot was used to calculate the thermal energy
barrier to the magnetization reversal DE /k = 63.3 K and
showing the molecular trigonal pyramidal geometry of the Er center
+
and the [Li(THF)4] cation. Hydrogen atoms and solvent THF mole-
eff
B
ꢀ
7
cules omitted for clarity.
t0 = 1.07 ꢁ 10 s. The low-temperature regime is expected to
be controlled by quantum tunneling effects (direct relaxation
process). The Cole–Cole plot (Supporting Information, Fig-
ure S3) suggests the existence of only one thermal relaxation
process, as evidenced by the semicircle overlay of c’’ and c’
Compound 1 crystallizes in the monoclinic space group
P2 /n (Supporting Information, Table S1). Selected bond
1
distances and angles are provided in the Supporting Informa-
tion, Tables S2,S3. The Er1ꢀN bond distances are in the
2
.231–2.251 ꢀ range, which is slightly longer than reported for
[
11]
[
Er{N(SiMe ) } ],
and the Er1ꢀCl bond distance is
3
2 3
2
.528(2) ꢀ. The N-Er1-N angles are highly flattened with an
average N-Er-N angle of 116.08, which is closer to ideal
trigonal-planar angles than [Er{N(SiMe ) } ] (113.48); the
3
2 3
average value of the Cl-Er-N angles is 101.78. The closest Erꢀ
Er distance is 11.175(2) ꢀ, which justifies the assumption that
the magnetic intermolecular dipole–dipole interactions are
very weak.
The static magnetic properties of 1 were measured on
a sample of crushed crystals using a MPMS SQUID magneto-
meter under a 1000 Oe dc field. The temperature dependence
of the dc magnetic susceptibility of 1 is very similar to other
Er compounds (see references in Table 1). The room-temper-
Figure 2. Imaginary component of the ac magnetic susceptibility data
plotted versus frequency for 1. Lines represent fitting of the experimen-
tal data at different temperatures using the generalized Debye model.
Inset: Arrhenius plot of ln(1/t) vs T . Fitting of the thermal regime is
represented by linear fit of selected blue data points.
ꢀ1
ature cT value of 11.47 emumol K (Supporting Informa-
III
tion, Figure S1) is consistent with an isolated Er center (J =
ꢀ1
1
5/2, g = 1.2 with the expected cT value of 11.48 emu
ꢀ
1
mol K). At lower temperatures, the cT value slowly
2
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and a values below 0.29. On the other hand, because the
Arrhenius plot does exhibit some curvature below 6.3 K,
perhaps another relaxation pathway is also operative. Data in
the entire 1.8–11 K temperature range were analyzed by the
[17]
following equation:
t0 1 ¼ AT þ C þ BT þ t0ꢀ1 expðꢀDEeff=k TÞ
ꢀ
n
ð1Þ
B
n
ꢀ1
where AT+ C, BT , and t0 exp(ꢀDE /k T) represent
eff
B
direct, Raman, and Orbach relaxation processes, respectively.
To fit the Arrhenius plot to Equation (1), the values of t and
0
DE /kB were fixed based on the linear fit to the thermal
eff
regime (7.0–9.5 K; red dotted line, Figure 3). The values of
the A and C parameters were also fixed based on the linear fit
to the t vs T dependence (direct process; gray dotted curve,
Figure 4. Molecular structure of [Er0.1Y0.9{(Me Si) N} (m-Cl){Li-
3
2
3
.
(
THF) }] pentane (1’). Hydrogen atoms and pentane molecules were
3
ꢀ
1
1
omitted for the sake of clarity.
Cl-Er-N angles up to 88, as well as a 0.14 ꢀ difference in the
ErꢀCl bond distance compared to 1. Ac measurements were
performed on crushed crystals of the diluted sample, 1’,
revealing the retention of the out-of-phase signal with partial
suppression of the quantum tunneling regime (Figure 5;
Supporting Information, Figure S4). These results support
the molecular nature of the slow magnetic relaxation in 1. The
ac data (Figure 5) were fitted using a generalized Debye
[
16]
model. The resulting relaxation times were plotted vs. 1/T
and the data reveal that there is a temperature-dependent
regime above 3.8 K and a temperature-independent regime
below 3.8 K. The high-temperature region (above 3.8 K) was
fitted using the Arrhenius law, which resulted in an estimated
effective energy barrier to the magnetization reversal of DE /
eff
ꢀ
7
k = 55.8 K and pre-exponential factor t = 2.84 ꢁ 10 s. The
B
0
estimated effective energy barriers for 1 and 1’ are different
by about 7–8 K. We attribute this difference to the slightly
altered coordination sphere of the two analogues, as well as
the selection of points used to ascertain the energy barrier.
The low-temperature regime for 1’ is most likely controlled by
Figure 3. Fitting of Arrhenius plot based on Equation (1) and relaxa-
tion times obtained by simultaneous fitting of c’ and c’’ versus n plots
(
Figure 2; Supporting Information, Figure S3). The red dotted line
represents fitting of the Orbach/thermal regime; the gray dotted line
represents fitting of the low-temperature regime. The variables
obtained from these two fits were then fixed in the fitting of the full
temperature regime (blue line), the curved part of which represents
the Raman relaxation regime.
Figure 3) in the range of 1.8–3.1 K. With these restraints, the
temperature range 1.8–9.5 K was fitted using Equation (1)
assuming n = 5, 7, or 9 and letting the B parameter to vary
freely. The best fit was obtained for n = 5 and B =
ꢀ
1
ꢀ5
0
.00585 s
K
(Figure 3). Thus we conclude that the curva-
ture of the Arrhenius plot is due to Raman relaxation.
Quantum tunneling is enhanced by weak intermolecular
dipole–dipole interactions that can be suppressed by dilution
in a diamagnetic matrix or by applying external dc field. A
1
0% diluted sample was prepared by combining an approx-
imate 1:10 mass ratio of [Li(THF) ][Er{N(SiMe ) } Cl]·2THF
4
3 2 3
with the corresponding yttrium analogue in a THF/n-pentane
mixture. Placing the solution in the freezer at ꢀ158C led to
crystals of [Er Y {(Me Si) N} (m-Cl){Li(THF) }]·pentane
0
.1 0.9
3
2
3
3
Figure 5. Imaginary component of the ac susceptibility plotted versus
frequency for 1’. Lines represent fitting of the experimental data at
different temperatures using the generalized Debye model. Inset:
Arrhenius plot of ln(1/t) vs T . The blue line is the best fit of the
thermal regime (blue data points).
(1’). Single-crystal XRD structural analysis revealed a slightly
different crystal structure than what was observed for 1 in that
+
ꢀ
the [Li(THF) ] cation is connected to the Cl bridge
(
ꢀ1
3
[
18]
Figure 4). This difference leads to a slight change in the
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quantum tunneling of the magnetization. When the ac
measurements of 1’ are performed under a 1000 Oe static dc
field, a further suppression of the quantum tunneling is
observed as expected (Supporting Information, Figure S5).
The ac data indicate very slow magnetic relaxation at
temperatures below 3.1 K, on the order of 100 s; therefore,
hysteresis measurements were performed at a conventional
ions in lower symmetry ligand environments other than those
that are strictly limited to the oblate crystal-field geometry
are prospects for interesting SMM behavior.
In summary, the first trigonal pyramidal erbium SMM is
reported, the magnetic data of which indicate that strictly
prolate f-electron density is not required to stabilize a crystal
field that favors SMM behavior. The relaxation dynamics are
interesting in that the presence of the axial chloride ligand
does not quench slow relaxation of the magnetization at zero
dc field. While the ligand framework may primarily stabilize
ꢀ1
sweep rate of 34.6 Oes . The shape of the hysteresis loop at
.8 K (Figure 6) for 1 is best described as waist-restricted or
1
the m = 15/2 ground state, it is noteworthy that this molecule
J
exhibits strikingly different magnetic behavior than the
trigonal bipyramidal compound [Er(NHPhiPr ) (THF) ].
2
3
2
This work provides a good backdrop for future theoretical
studies as it hints that simple models are not entirely adequate
for accurate prediction of slow magnetic relaxation in
lanthanide-based SMMs, a conclusion also supported by the
[19]
study of Dy-based SMMs.
Additional experimental and
theoretical studies are underway to explain the origins of
zero-field SMM behavior for 1. Importantly, the axial chloride
poises this molecule to be a convenient precursor for the
preparation of a family of derivatives and even the possibility
of device applications by attaching the molecule to surfaces.
Figure 6. Magnetic hysteresis loop for 1 collected at 1.8 K and with
ꢀ
1
3
4.6 Oes sweep rate.
Experimental Section
Single-crystal X-ray data were collected at 110 K on a Bruker APEX
II diffractometer equipped with a CCD detector. The data sets were
recorded as w-scans at 0.38 step width. Integration was performed
with the Bruker SAINT software package and absorption corrections
were empirically applied using SADABS. The crystal structures were
III
butterfly-like, similar to what was observed for other Er
SMMs.
[6b,8]
A tunneling feature (reflection point) is observed
at both negative and positive fields. The yttrium dilution
sample exhibits sharper hysteresis out to 3 K (Supporting
Information, Figure S6).
The magnetic parameters for compound 1 compared with
other known mononuclear erbium(III) SMMs are compiled in
Table 1. It is interesting at this stage to point out that the
magnetic properties of the trigonal bipyramidal molecule
[20]
refined using the SHELXL suite of programs and the graphical
[
21]
interface Olex2.
Images of the crystal structure were rendered
using the crystal structure visualization software DIAMOND or
[
22]
CCDC Mercury.
All of the structures were solved by direct
methods. Any remaining non-hydrogen atoms were located by
alternating cycles of least squares refinements and difference Fourier
maps. All hydrogen atoms were placed at calculated positions (riding
model). Anisotropic thermal parameters were added for all non-
hydrogen atoms. A summary of pertinent information relating to unit
cell parameters, data collection, and refinement statistics is provided
in the Supporting Information. CCDC 1051281 (1) and
CCDC 1051282 (1’) contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Magnetic susceptibility and magnetization measurements were
collected using a Quantum Design MPMS-XL SQUID magneto-
meter. To prevent torqueing, the samples for magnetic measurements
were prepared using crushed crystals immobilized in eicosane matrix
sealed in quartz tubes. The dc magnetic susceptibility measurements
were performed at an applied field of 1000 Oe over the temperature
range 2–300 K. The ac magnetic susceptibility measurements were
performed in a 5 Oe oscillating field at operating frequencies of 1–
[
(
Er(NHPhiPr ) (THF) ] were compared to those of [Er{N-
2 3 2
SiMe ) } ] by Tang et al., and it was found that the former
3 2 3
compound exhibits fast quantum tunneling, which is presum-
ably due to increased transverse anisotropy owing to the
presence of the two axial THF molecules. The compound
exhibits out-of-phase signals in the c’’ versus n plots, albeit
with the necessity of applying a dc field. Given these findings,
we reasoned that the presence of the axial chloride ligand in
[
12]
1
and 1’ might destroy the dynamic susceptibility properties
but the effect of the chloride appears to only moderately
destabilize the ground state, leading to a smaller effective
energy barrier as compared to [Er{N(SiMe ) } ].
3
2 3
Compound 1 is the first mononuclear erbium SMM with
a negatively charged axial ligand that exhibits out-of-phase
signals in the ac susceptibility data in the absence of an
applied static dc field. Since other erbium(III) SMMs
containing an axial ligand require the application of an
external field to observe slow relaxation, the observed
behavior of 1 was not anticipated. Nevertheless, the lower
barrier observed for this new compound as compared to the
1500 Hz. The data were corrected for diamagnetic contributions of
the sample holder and as calculated from the Pascal constants.
Further synthetic details are given in the Supporting Information.
Keywords: erbium · magnetic hysteresis · mononuclear ·
prolate lanthanides · single-molecule magnets
[3]
trigonal [Er{N(SiMe ) } ] complex is entirely reasonable.
3
2 3
The current findings support the hypothesis that erbium(III)
4
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[18] Compound 1 also crystallizes in the P2 /c space group with
1
exactly the same coordination sphere as found for compound 1’,
but the X-ray data were poor. For details and discussion, see the
Supporting Information, Figures S7–S9.
[
[
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Received: November 18, 2014
Revised: January 14, 2015
1580 – 1584.
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Mononuclear Lanthanide SMMs
Riding the wave of erbium SMMs: The
first trigonal pyramidal mononuclear
erbium(III) single-molecule magnet
(SMM) was synthesized and its magnetic
properties investigated. Despite contain-
A. J. Brown, D. Pinkowicz, M. R. Saber,
K. R. Dunbar*
&&&& — &&&&
ꢀ
A Trigonal-Pyramidal Erbium(III) Single-
Molecule Magnet
ing an axial Cl ligand, which is expected
to reduce the prolate nature of the
erbium(III) ion, the molecule exhibits out-
of-phase signals in the ac susceptibility
data in the absence of an external field
and hysteresis behavior up to 3 K.
6
www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
These are not the final page numbers!