Peroxide-Templated Assembly of a Trimetal Neodymium Complex Single-Molecule Magnet

Justin T. Miller, Yixin Ren, Sheng Li, Kui Tan, Gregory McCandless, Christine Jacob, Zheng Wu,

Communication

Peroxide-Templated Assembly of a Trimetal Neodymium Complex

Single-Molecule Magnet

†

Ching-Wu Chu, Bing Lv,* Michael C. Biewer, and Mihaela C. Stefan *

Cite This: https://dx.doi.org/10.1021/acs.inorgchem.0c00234

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sı Supporting Information

ABSTRACT: In this work, we present a trimetal neodymium complex with two notable qualities. First, the assembly of the complex

is templated by peroxide derived from atmospheric oxygen. Second, the bulk material behaves as a superparamagnet, implying that

the individual complexes are molecular magnets. Peroxide-templated assembly is possible because of the conﬂuence of the high

oxophilicity of neodymium along with the use of an azeotropic distillation synthesis method, which excludes water but admits

oxygen. SQUID magnetometry measurements show an extremely high magnetic susceptibility as well as a lack of remanence.

emplated assembly is a phenomenon in supermolecular

and coordination chemistry in which the templating

expedient azeotropic distillation method to remove water

during the synthesis. This method does not remove oxygen

from the reaction, and this quality, along with an emerging

water-free environment and the very oxophilic neodymium,

created conditions appropriate for atmosphere-sourced per-

oxide-templated assembly. A crystal structure was obtained in

which three neodymium atoms, each coordinated to one of the

tetradentate ligands, arrange around a central peroxide ion.

While this complex lacked the desired catalytic activity, the

structure was suﬃciently interesting to merit some further

study. Superconducting quantum interference device magneto-

metry (SQUID) measurements revealed surprisingly high

magnetic susceptibility, which is suggestive of superparamag-

netism. Because superparamagnetism is the result of an

ensemble of noninteracting ferromagnets, this, in turn, suggests

that the individual clusters represent single-molecule mag-

species has the principal eﬀect of determining the geometry of

another structure. Examples are commonly diﬀerentiated from

the more general structural eﬀects of reagents by virtue of

having a “small” impact on the chemistry of the system

noncovalent interactions) and the formation of similar

(

systems of diﬀering geometry in the absence of the template.

Templated assembly has been studied for several decades for

the preparation of mechanically interlinked structures such as

1,2

rotaxanes and catenanes. Much of the early work in the ﬁeld

focused on the interactions of metal cations with dipoles in

organic molecules. More recent attention has been paid to

the use of anion templates because of their advantages in

−8

speciﬁcity and available ion geometries.

In some cases,

templating anions can be derived from gases absorbed from the

atmosphere, such as carbonate formed from CO or the rarer

−11

nets.

Bis(2-pyridinal)ethylenediimine was synthesized using a

example of peroxide derived from oxygen.

This latter case,

which is the topic of this work, requires very oxophilic metals

and the strict exclusion of moisture.

previously reported modiﬁed method. 2-Pyrdinecarboxalde-

hyde and diethyl ether were introduced to a dry round-bottom

ﬂask, and ethylenediamine was then added dropwise.

Anhydrous magnesium sulfate was added to the ﬂask to

remove the water byproduct. The reaction mixture was stirred

at room temperature for 4 h. A precipitate formed and was

removed by ﬁltration and recrystallized in a hexane/diethyl

ether mixture, yielding a yellow solid. NMR and crystallo-

graphic characterization can be found in the Supporting

Information and are in good agreement with previously

Anion-templated assembly is useful in the synthesis of

complexes containing multiple metal centers, such as caging

12,13

complexes, and has gained attention more recently.

Such

structures are useful for applications in single-molecule

magnets, where a large number of metal centers aid the

14,15

synthesis of high-spin complexes.

Many of the reported

lanthanide single-molecule magnets feature a central anion

caged by a cluster of lanthanides in this manner.

We report here an unexpected complex that arose from

eﬀorts to synthesize a neodymium complex with a bis(2-

pyridinal)ethylenediimine ligand, which we had hoped would

be useful for diene polymerization. The neodymium feedstock

to produce this catalyst was neodymium chloride hexahydrate,

and earlier eﬀorts had revealed that the water of crystallization

would problematically remain bound to the neodymium atom

and prevent formation of the complex. While anhydrous

neodymium chloride is available and conventional water-free

methods would do just ﬁne, we instead chose to use an

18,19

reported results.

Received: January 28, 2020

XXXX American Chemical Society

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Figure 1. ORTEP diagram for [Nd Cl (L) (O) ]Cl.

Communication

The neodymium complex [Nd Cl (L) (O) ]Cl was synthe-

sized by azeotropic distillation. A molar ratio of 1:2 for NdCl₃·

H O and bis(2-pyridinal)ethylenediamine was used. NdCl ·

2 3

H O (1.00 g, 2.7 mmol) was dissolved in 20 mL of absolute

ethanol. Bis(2-pyridinal)ethylenediamine (1.337 g, 5.4 mmol)

was dissolved in 40 mL of toluene and then introduced to the

ﬂask. A distillation apparatus was assembled, and the reaction

mixture was heated. The ternary azeotrope consisting of

toluene, ethanol, and water started to boil oﬀ at 74.4 °C. After

complete removal of the water of crystallization, the ligands

were coordinated to the metal center. The binary ethanol/

toluene azeotrope was then removed at a temperature of 76.7

°C, followed by toluene at its boiling point of 110 °C. This

method expediently produces a water-free environment

without the need for an air-free technique and, importantly,

allows the reaction to be exposed to oxygen. After complete

removal of all of the solvent, a brown solid formed. This solid

was redissolved in chloroform, and continued exposure to air

prompted the precipitation of a light-brown solid, which can be

recovered by centrifugation with chloroform washing. The

precipitate was puriﬁed by a recrystallization method in

methanol/pentane. After 3 weeks, single crystals were obtained

via a slow vapor diﬀusion method with methanol and pentane

at 0 °C. Notably, if far less exposure to oxygen is permitted,

this reaction instead yields a monometallic [NdCl(L)(H O) ]-

Cl ·2(H O) complex [L = bis(2-pyridinal)ethylenediamine],

more information for which can be found in the Supporting

Information.

Single-crystal X-ray diﬀraction data were collected on a

Bruker D8 Quest Kappa single-crystal X-ray diﬀractometer,

equipped with an IμS microfocus source (Mo Kα, λ = 0.71073

Å), a PHOTON 100 CMOS detector, and an Oxford

Cryostreams cryostat at 100 K. The intrinsic method was

used for the initial structure model. SHELXL-2014 was used

to reﬁne all non-hydrogen atoms.

The crystal structure of the complex, shown in Figure 1,

consists of a triangular array of neodymium atoms coordinating

a peroxide anion. The atomic positions and thermal parameters

are in Table S8. The peroxide anion is coordinated to three

neodymium ions in a μ -η :η :η manner. The structure has 3-

fold rotational symmetry about the axis of the peroxide ion,

and the center of the O−O bond lies in the plane formed by

the three neodymium atoms. The Nd−O bond lengths are

.467 Å, and the O−O bond length is 1.512 Å. Each of the

three neodymium atoms is coordinated to a single bis(2-

pyridinal)ethylenediimine ligand through its four nitrogen

atoms, and the neodymium atoms are additionally bridged by a

set of three chloride anions. Each neodymium atom is eight-

coordinate. Identiﬁcation of the peroxide anion as such (as

opposed to other dioxygen species) was conﬁrmed by X-ray

crystallography and Fourier transform infrared (FT-IR)

spectroscopy. The observed long O−O distance of

,22

.515(12) Å is consistent with previously reported values

−

In the FT-IR spectrum (Figure 2), the week band at 850 cm

was attributed to the O−O bond vibration, which is consistent

with previous reports. Interestingly, this same band is absent

in the Raman spectrum (Figure S4), where it is usually

observed in most peroxide compounds. Selection rules would

suggest that this peak should be weak or absent in IR but

strong in Raman.

Figure 2. IR spectrum of [Nd Cl (L) (O) ]Cl.

X-ray photoelectron spectroscopy (XPS) was conducted on

the material, and the survey conﬁrmed the presence of the

expected elements (Figure S5). The anticipated diﬀerences

between the observed energies of peroxide oxygen atoms

versus other dioxygen species are unfortunately minimal, so the

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

these data are shown in Figure 3 a. The measured magnetic

Communication

XPS data are of limited use in support of that assignment,

especially relative to crystallographic and vibrational data.

In addition to the spectroscopic assignment of peroxide,

complex with Nd³⁺ions could form small magnetic clusters

without long-range order in this crystal structure, which has a

large interatomic Nd distance and superexchange interaction

[

Nd Cl (L) (O) ]Cl was tested for the peroxide species using

of Nd ions via oxygen atoms and organic ligands seem

infeasible.

Whatman starch iodide paper. The complex was dissolved in

methanol, and then a small drop of solution was transferred

onto a test strip and allowed to air-dry. A yellow color emerged

on the test strip, indicating the presence of peroxide species

within the complex.

To further prove that signiﬁcant susceptibility is indeed

coming from superparamagnetism rather than ferromagnetism

(ferrimagnetism), an isothermal magnetic-ﬁeld-dependent M−

H curve was measured for this complex at 5 K and up to 5 T.

The data are shown in Figure 3b. A clear nonlinear ﬁeld-

dependent M−H curve (at even very small magnetic ﬁelds)

and the absence of hysteresis within the equipment resolution

are observed and further rule out the existence of typical

paramagnetism and ferromagnetism (ferrimagnetism) in this

system. The relaxation moment is negligible down to 5 K over

our consequential measurements, implying a blocking temper-

ature far lower than 5 K. This is also consistent with the M−T

measurement. For a superparamagnetic material, it is notable

that hysteresis loss is negligible above the blocking temperature

T , i.e., a paramagnetic-like M−H behavior above T .

A similar motif of neodymium atoms caging peroxide has

appeared before in the work of Roitershtein. However, our

example has relatively longer Nd−O bonds and relatively

shorter O−O bonds. Dysprosium and gadolinium complexes

with hydroxo ions taking the place of the peroxide have also

4,25

been reported.

These compounds have been of interest for

their magnetic properties, so SQUID measurements were

taken.

The magnetic susceptibility of the complex was measured

over a temperature range from 5 to 300 K with a 1 T ﬁeld, and

The absence of hysteresis in the M−H curve further suggests

that the blocking temperature T is much lower than 5 K.

Additional alternating-current (ac) susceptibility measure-

ments are shown in the top-left inset of Figure 3b, where the

real components of the magnetic susceptibility at two diﬀerent

frequencies (one at 100 Hz and another at 1000 Hz) are

overlapping with each other under the ac magnetic ﬁeld of an

amplitude of 1000 Oe. This provides further clear evidence

that T is much lower than 5 K (the lowest temperature in the

ac susceptibility measurements). Therefore, we can use the

Langevin function with the temperature range above T to

quantitatively describe the M−H behavior at 5 K and calculate

the associated cluster density and magnetic moment of the

clusters in our compound.

The Langevin function used for our ﬁtting is

i μ Hμy

k T

M(H, T) = nμLj

z = nμÅcothj

z −

k T

ÇÅ

k T

μ HμÑ

{

ÖÑ

where n is the cluster density, μ is the magnetic permeability

of vacuum, k is the Boltzmann constant, and μ is the magnetic

moment of the cluster. The Langevin equation ﬁts the M−H

curve well, as seen in the inset in Figure 3b (the red line is the

ﬁtting curve), and further conﬁrms the existence of super-

paramagnetism in this system. The ﬁtting yields the magnetic

moment of the cluster at 3.49 μ , and the cluster density is 1.25

10 /cm . The magnetic moment of Nd is consistent with

the free Nd ion moment (3.62 μ_B). Additionally, the room-

temperature χT product is 66.05 cm ·K/mol, which is close to

the theoretical value of the χT product of three free Nd ions

Figure 3. (a) Temperature-dependent magnetic susceptibility data at

a magnetic ﬁeld of 1 T from 300 to 5 K. Inset: Real component of ac

susceptibility at 100 and 1000 Hz. (b) Isothermal magnetic-ﬁeld-

dependent susceptibility data at 5 K and up to 5 T. Inset: Langevin

ﬁtting results, where the red line is the ﬁtting curve.

(

61.5 cm ·K/mol).

Superparamagnetism arises from an ensemble of non-

interacting ferromagnets. The crystal structure of this complex

has tight clusters of three neodymium atoms, with the clusters

spaced widely apart. The most reasonable cause for the

observed superparamagnetic behavior is that the clusters each

behave ferromagnetically but are spaced too far apart to

interact, thus leading to superparamagnetic behavior in the

solid crystal material.

susceptibility shows a paramagnetic-like behavior, and the data

in both zero-ﬁeld-cooled and ﬁeld-cooled conditions are

completely overlapping with each other over the measured

temperature range. This excludes the possibility of ferromag-

netic or antiferromagnetic long-range orders in this system.

However, at room temperature (300 K), the magnetic

−

CONCLUSION

susceptibility is 3 × 10 , which is 2 orders magnitude larger

than the typical paramagnetic materials. Therefore, we suggest

the presence of superparamagnetism in this system because a

■

We have shown that anion-templated assembly via atmos-

pheric oxygen is a viable technique for the synthesis of large

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Complete contact information is available at:

Communication

metal clusters when suﬃciently oxophilic metals and oxygen-

tolerant chemistry are used. Even if the chemistry is not

moisture-tolerant, the complexes can still be easily reached

without conventional air-and-water-free techniques by using

the simple azeotropic distillation method, which excludes water

well while still allowing the reaction to be exposed to

atmospheric oxygen.

In addition, we have shown that such clusters may be of use

because of their magnetic properties. For suﬃciently large

clusters with the metal centers localized toward the center,

each cluster can act as a separate magnetic domain that ﬂips

easily and independently of nearby clusters. This produces

superparamagnetic behavior on a much smaller scale than the

more common approach of magnetic nanoparticles and still

occurs even in a crystal. This may be of use in itself, or the

implication of very small magnetic domains in similar materials

may be of use for high-density data storage.

Zheng Wu − Texas Center for Superconductivity and

Department of Physics, University of Houston, Houston, Texas

77004, United States

Ching-Wu Chu − Texas Center for Superconductivity and

Department of Physics, University of Houston, Houston, Texas

77004, United States; orcid.org/0000-0003-3955-7095

Michael C. Biewer − Department of Chemistry and

Biochemistry, The University of Texas at Dallas, Richardson,

Texas 75080, United States

https://pubs.acs.org/10.1021/acs.inorgchem.0c00234

Author Contributions

These authors contributed equally.

†

Notes

The authors declare no competing ﬁnancial interest.

ASSOCIATED CONTENT

ACKNOWLEDGMENTS

■

We gratefully acknowledgeﬁnancial support from the National

Science Foundation (Grant CHE-1566059), Welch Founda-

tion (Grant AT-1740), and Air Force Oﬃce of Scientiﬁc

Research (Grants FA9550-15-1-0236 and FA9550-19-1-0037).

M.C.S. acknowledges generous ﬁnancial support from the

Eugene McDermott Foundation. This project is also partially

funded by The University of Texas at Dallas Oﬃce of Research

through the Core Facility Voucher Program. We thank Yves

Chabal and the Materials and Surface Technology Resource

for XPS and Raman spectroscopic data collection. The XPS

and Raman spectroscopic characterization and analysis were

supported by the U.S. Department of Energy, Oﬃce of

Science, Basic Energy Sciences, under Award DE-SC0019902.

sı Supporting Information

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c00234.

Additional crystallographic and spectroscopic informa-

tion (PDF )

Accession Codes

CCDC 1974941 −1974943 contain the supplementary crys-

tallographic data for this paper. These data can be obtained

free of charge via www.ccdc.cam.ac.uk/data_request/cif , or by

emailing data_request@ccdc.cam.ac.uk , or by contacting The

Cambridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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■

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