Homometallic DyIII Complexes of Varying Nuclearity from 2 to 21: Synthesis, Structure, and Magnetism

Article abstract of DOI:10.1002/chem.201700471

The synthesis, structure, and magnetic properties of four Dy^III coordination compounds isolated as [Dy₂(LH₂)₂(μ₂-η¹:η¹-Piv)]Cl?2 MeOH?H₂O (1), [Dy₄(LH)₂(μ₃-OH)₂(Piv)₄(MeOH)₂]?4 MeOH?2 H₂O (2), [Dy₆(LH₂)₃(tfa)₃(O₃PtBu)(Cl)₃]Cl₄?15.5 H₂O?4 MeOH?5 CHCl₃ (3) and [Dy₂₁(L)₇(LH)₇(tfa)₇]Cl₇?15 H₂O?7 MeOH?12 CHCl₃ (4) are reported (Piv=pivalate, tfa=1,1,1-trifluoroacetylacetone, O₃PtBu=tert-butylphosphonate). Among these, 3 displays an equilateral triangle topology with a side length of 9.541 ? and a rare pentagonal-bipyramidal Dy³⁺ environment, whereas complex 4 exhibits a single-stranded nanowheel structure with the highest nuclearity known for a homometallic lanthanide cluster structure. A tentative model of the dc magnetic susceptibility and the low-temperature magnetization of compounds 1 and 2 indicates that the former exhibits weak ferromagnetic intramolecular exchange interaction between the Dy³⁺ ions, whereas in the latter both intramolecular ferromagnetic and antiferromagnetic magnetic exchange interactions are present. Compounds 1, 3, and 4 exhibit frequency-dependent ac signals below 15 K at zero bias field, but without exhibiting any maximum above 2 K at frequencies up to 1400 Hz. The observed slow relaxation of the magnetization suggests that these compounds could exhibit single molecule magnet (SMM) behavior with either a thermal energy barrier for the reversal of the magnetization that is not high enough to block the magnetization above 2 K, or there exists quantum tunneling of the magnetization (QTM).

Full text of DOI:10.1002/chem.201700471

A Journal of
Accepted Article
Title: Homometallic DyIII Complexes of Varying Nuclearity from 2 to 21:
Synthesis, Structure and Magnetism
Authors: Vadapalli Chandrasekhar, Sourav Biswas, Sourav Das,
Joydev Acharya, Vierandra Kumar, Jan Leusen, Juan Manuel
Herrera, Paul Koegerler, and Enrique Colacio
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10.1002/chem.201700471
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Homometallic DyIII Complexes of Varying Nuclearity from 2 to 21: Synthesis,
Structure and Magnetism
Sourav Biswas, a Sourav Das,b Joydev Acharya,a Vierandra Kumar,a Jan van Leusen, c Paul
Kögerler*,c Juan Manuel Herrera,d Enrique Colacio*,d and Vadapalli Chandrasekhar*a,e
aDepartment of Chemistry, Indian Institute of Technology Kanpur, Kanpur-208016, India.
bDepartment of Chemistry, Institute of Infrastructure Technology Research and Management,
Ahmedabad-380026, India.
cInstitut für Anorganische Chemie, RWTH Aachen University, D-52074 Aachen, Germany.
dDepartamento de Química Inorganica, Facultad de Ciencias, Universidad de Granada, 18071
Granada, Spain
eNational Institute of Science Education and Research, Institute of Physics Campus, Sachivalaya
Marg, PO: Sainik School, Bhubaneswar - 751 005, India
AUTHOR EMAIL ADDRESS: vc@iitk.ac.in; paul.koegerler@ac.rwth-aachen.de;
ecolacio@ugr.es
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Abstract
The synthesis, structure and magnetic properties of the four Dy(III) coordination compounds
[Dy2(LH2)2(µ2-η1:η1-Piv)]Cl·2MeOH·H2O
[Dy4(LH)2(µ3-
isolated
as
(1),
OH)2(Piv)4(MeOH)2]·4MeOH·2H2O(2), [Dy6(LH2)3(tfa)3(O3PtBu)(Cl)3]Cl4·15.5H2O·4MeOH·
5CHCl3 (3) and [Dy21(L)7(LH)7(tfa)7]Cl7·15H2O·7MeOH·12CHCl3 (4) are reported (Piv =
pivalate, tfa = 1,1,1-trifluoro-acetylacetone, O3PtBu = tert-butylphosphonate). These compounds
are accomplished through fine interplay between keto-enol tautomerism of a multidentate
flexible
ligand,
6-((bis(2-hydroxyethyl)amino)methyl)-N'-((8-hydroxyquinolin-2-
yl)methylene)picolinohydrazide (LH4), triggered by stepwise deprotonation. Some of these
polynuclear coordination complexes exhibit aesthetic and unprecedented structural features.
Thus, complex 3 displays an equilateral triangle topology with side length of 9.541 Å and a rare
pentagonal-bipyramidal Dy3+ environment, while complex 4 exhibits a single-stranded
nanowheel structure with highest nuclearity known for a homometallic lanthanide cluster
structure. A tentative model of the dc magnetic susceptibility and the low-temperature
magnetization of compounds 1 and 2 indicates that the former exhibits weak ferromagnetic
intramolecular exchange interaction between the Dy3+ ions, whereas in the latter both
intramolecular ferromagnetic and antiferromagnetic magnetic exchange interactions are present.
Compounds 1, 3 and 4 exhibit frequency-dependent ac signals below 15 K at zero bias field, but
without exhibiting any maximum above 2 K at frequencies up to 1400 Hz. The observed slow
relaxation of the magnetization suggests that these compounds could exhibit SMM behavior with
either a thermal energy barrier for the reversal of the magnetization that is not high enough to
block the magnetization above 2 K or there exists quantum tunneling of the magnetization
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(QTM). The very small energy separation between the ground and first excited states extracted
for 1 and 2 from the magnetic data are in good agreement with the above hypothesis.
Keyword: polynuclear lanthanide compound, hexanuclear assembly, Schiff base, butterfly
complex, equilateral triangle, macrocycle, anisotropy, energy barrier, ac susceptibility, slow
magnetic relaxation.
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Introduction
Synthesis and studies of molecular magnets have attracted considerable interest for about two
decades, particularly since the discovery of single-molecule magnet (SMM) behavior in
[MnIV MnIII (µ3-O)12(O2CMe)16(OH2)4]·2MeCO2H·4H2O.[1] A wide range of approaches have
4
8
been explored in order to identify novel members of the family of single-molecule magnets.[2]
The types of complexes investigated include 3d/4f complexes,[3] polynuclear transition metal
complexes[4] as well as homonuclear 4f complexes[2a-c,5]. Recently heavier transition metal ion[6]
as well as actinide[7] complexes are also being investigated. All of these strategies aim to achieve
a high ground-state spin for the complex along with a significant magnetic anisotropy. Both
theoretical and experimental investigations have revealed that the energy barrier of the double-
well potential in SMMs is governed by both the total spin of the complex along with the
magnetic anisotropy.[8] Among lanthanides, Dy3+ has been a natural choice of investigation since
it is a Kramer’s ion and can possess large single-ion magnetic anisotropy.[8] Accordingly, several
DyIII complexes have and continue to be investigated.[2a,b,5] One of the points of interest for a
synthetic chemist has been the capability to modulate the nuclearity of complexes by a subtle
control exercised by the choice of ligands. Most often, it has been noticed that one type of ligand
generally affords a particular type of complex possessing a certain nuclearity and topology.[9] In
this regard, we have been interested to investigate and ascertain if a given ligand under slightly
varying conditions can enable the assembly of complexes with varying nuclearity and topology.
Towards this end, we have designed a new multidentate, flexible, aroyl hydrazone-based Schiff
base
ligand,
6-((bis(2-hydroxyethyl)amino)methyl)-N'-((8-hydroxyquinolin-2-
yl)methylene)picolinohydrazide (LH4), with which we have been able to assemble the
coordination compounds [Dy2(LH2)2(µ2-η1:η1-Piv)]Cl·2MeOH·H2O (1), [Dy4(LH)2(µ3-
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OH)2(Piv)4(MeOH)2]·4MeOH·2H2O
(2),
[Dy6(LH2)3(tfa)3(O3PtBu)(Cl)3]Cl4·15.5H2O·4MeOH·5CHCl3
(3),
and
[Dy21(L)7(LH)7(tfa)7]Cl7·15H2O·7MeOH·12CHCl3 (4) (Scheme 1). Interestingly, depending on
the reaction conditions, the metal nuclearity of the obtained complexes varies from 2 to 21.
Herein, we report the synthesis, structure and magnetism of the complexes 1−4.
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Scheme 1. Formation of Dy2, Dy4, Dy6 and Dy21 assemblies using LH4.
Result and Discussion
Synthetic Aspects
We have been involved in the design and utilization of various types of ligand families for the
[2e,5a,j,f,9i]
preparation of 3d/4f[3g-i,9d,f] and 4f[2e,5a-d,f,i,j,9a,g] complexes. From our experience
and
those of others[2f,10] we have observed that flexible ligands that have multiple options to bind to a
metal ion are often suitable to generate polynuclear complexes. Among the ligands that we have
investigated, aroyl hydrazone ligands have been found to possess the following beneficial
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features. Firstly, these ligands can exhibit keto−enol tautomerism whereby the enolate/neutral
forms can be involved in binding. Secondly, the presence of C–C bond rotation in this family of
ligands allows the presence of conformational isomers of different coordinating capabilities.[9g,
10b-d]
Accordingly, we designed and synthesized a new multidentate flexible aroyl hydrazone
based Schiff base ligand, LH4, as shown in Scheme 2.
Scheme 2. Synthesis of LH4.
The ligand LH4 contains nine divergent coordination sites that can be partitioned into two
pockets in five different ways depending upon the deprotonation level. The observed
coordination behavior in the current instance can be summarized for the ligand in three different
possibilities. First, at the double deprotonation level the ligand can behave in two different ways:
in the enol form, [LH2]2– generates two tetradentate pockets with identical O2N2 coordination
environments (A1 and A2), while in the keto form, due to C−C bond rotation, two pockets N3O2
(K2) and O2N2 (K1) result, which are oriented in opposite directions. Second, at the triple
deprotonation level, [LH]3− (enol form), pockets P1 and P2 are tetradentate (O2N2) and
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pentadentate (O3N2), respectively, and are oriented in the same direction, while pockets C1
(O2N2) and C2 (N3O2) are projected in the opposite direction as a result of C−C bond rotation.
Third, at the tetra-deprotonation level, an unprecedented N−N bond rotation enables the
formation of two asymmetric pockets, T1 (ON2) and T2 (O3N2), which are oriented nearly
perpendicular to each other. These coordination modes and the nuclearities of the different types
of the resulting complexes are summarized in Scheme 3.
Scheme 3. LH4 showing two coordination pockets in various conformations.
The reaction of LH4, DyCl3 and pivalic acid in the presence of Et3N in a stoichiometric ratio of
2:2:1:6 afforded a dinuclear complex, isolated as [Dy2(LH2)2(µ2-η1:η1-Piv)]Cl·2MeOH·2H2O (1).
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On the other hand, the reaction of LH4, Dy(NO3)3·6H2O and pivalic acid in the presence of Et3N
with a stoichiometric ratio of 1:2:2:6 under refluxing conditions fostered the formation of a
charge-neutral
homometallic
tetranuclear
complex,
[Dy4(LH)2(µ3-
OH)2(Piv)2(MeOH)]·4MeOH·H2O (2) (Scheme 1). Changing the co-ligand from pivalic acid to
1,1,1-trifluoroacetyl acetone and utilizing tert-butyl phosphonic acid afforded the hexanuclear
complex [Dy6(LH2)3(tfa)3(O3PtBu)(Cl)3]Cl4·15.5H2O·4MeOH·5CHCl3 (3). A further change in
stoichiometry and reaction conditions (see experimental section) yielded a macrocyclic complex,
isolated as [Dy21(L)7(LH)7(tfa)7]Cl7·15H2O·7MeOH·12CHCl3 (4). Note that apart from the role
played by the primary ligand in the formation of the complexes, the co-ligands, namely pivalic
acid (for 1 and 2), trifluoroacetyl acetone and tert-butyl phosphonic acid (for 3) and
trifluoroacetyl acetone acid (for 4) assist the formation of the molecular complexes by providing
the additional binding that is needed to meet the Dy3+ coordination requirements. In addition, the
crucial capping coordination role of the phosphonate ligand in the formation of 3 is readily
noted. It seems likely that the coordination flexibility of the ligand (as a result of variable de-
protonation ability, keto-enol tautomerism, rotation around C-C and N−N bond) along with the
subtle influence of the co-ligand(s) allows the modulation of nuclearity among the homometallic
complexes. While the variation of nuclearity of the complexes was achieved through trial and
error, we hope that such endeavors are a step forward towards rational synthetic pathways for
achieving desired nuclearities.
In order to check the structural integrity of these complexes in solution, we have carried out ESI-
MS studies. These reveal for 1, 2, and 3 respectively, peaks at m/z = 1139.191, 1902.299 and
1474.066 corresponding to the species [C47H51Dy2N10O10 –Piv –H+]+, [C64H86Dy4N10O20
–
2MeOH +H+]+ and [C82H81Cl3Dy6F9N15O21P –2Cl– +3H2O +2MeOH –4H+]2+. This indicates that
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these complexes remain partially intact in solution. In contrast, 4 seems to fragment completely
in solution. A perspective view of the ESI-MS spectra of complex 1 is given in Figure 1 and that
of 2 and 3 are compiled in the Supporting Information (Figures S1 and S2).
a)
b)
c)
Figure 1. a) Full range ESI-MS spectrum of complex 1, b) experimental and c) simulated mass
spectrometry pattern of the species, [C47H51Dy2N10O10 –Piv –H+]+.
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X-ray Crystallography
Single-crystal X-ray diffraction data reveals that 1 comprises of a monocationic dinuclear
complex, [Dy2(LH2)2(µ2-η1,η1-Piv)]+ and a chloride counter anion. Complex 1 crystallized in the
monoclinic space group P21/c (Z = 4). A perspective view of 1 is given in Figure 2 and the
selected bond parameters are summarized in Table S1 (Supporting Information).
Figure 2. Molecular structure of the {Dy2} complex in 1 (solvent molecules, selected hydrogen
and chloride have been omitted for the sake of clarity).
1 is formed by the coordination action of two doubly deprotonated ligands [LH2]2–. Within 1,
each of the two [LH2]2– bind two Dy3+, simultaneously utilizing their tetradentate pockets (O2N2)
along with bridging enolate oxygen. While one arm of the diethanolamine of the pocket A2 binds
to Dy3+ in a monodentate fashion, the other arm does not participate in coordination (Figure2).
Apart from the binding provided by the [LH2]2–, the dinuclear framework is further strengthened
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by a bridging pivalate ligand. The coordination modes of the ligands involved in the assembly of
1 are given in Figure 3.
µ2–η1:η1:η1:η2:η1:η1:η1
µ2–η1:η1
Figure 3. Binding modes of the [LH3]2– and pivalate ligand in 1.
Note that in the formation of 1, the ligand has exclusively utilized the enol form to generate the
Dy2O2 core. Both the Dy3+in 1 are nine-coordinated and contain a 5O,4N coordination
environment with a distorted monocapped square antiprism geometry (Figure 4).
Figure 4. Distorted monocapped square antiprism coordination environment around Dy3+ in 1.
A few comments about the metric parameters found in 1. The Dy–N bond lengths fall in the
range of 2.506–2.859(5) Å with the longest bond for Dy–Ndiethanolamine. The average Dy–OCH2OH
bond length is 2.437(5) Å which is slightly longer than the average Dy–Ohydrazone bond length,
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2.408(6) Å, which in turn is longer than the average Dy–Opivalate [2.281(7) Å] and Dy–O8HQ
[2.319(5) Å] bond lengths. The Dy1–O1–Dy2 [113.93(2)°] and the Dy1–O6–Dy2 [113.61(2)°]
bond angles are similar and are comparable to those found in the literature.[9,10]
Complex 2 is neutral, tetrauclear and crystallized in the triclinic system (P-1, Z = 1). For 2, the
asymmetric unit contains one half of the entire molecule, viz, [Dy2(LH)(OH)(Piv)2(MeOH)]
(Figure 5a) and the complete molecule is generated by an inversion element which resides at the
center of the molecule. A perspective view of the complex 2 is given in Figure 5b (see Table S2
for selected bond lengths and angles).
b)
a)
Figure 5. a) Asymmetric unit of 2. b) Molecular structure of 2 (solvent molecules and selected
hydrogen have been omitted for clarity).
The tetranuclear assembly in 2 is made possible by the involvement of two triply deprotonated
ligands, [LH]3–, which are arranged in an almost antiparallel fashion (Figure 5). Among the nine
divergent coordination sites of LH4, eight are engaged in binding with Dy3+ while the other viz,
hydrazone N does not participate in coordination. Within 2, each of the two ligands in their triply
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deprotonated form hold two Dy3+ simultaneously in the two asymmetric pockets: one of them is
tetradentate (P1, 2O, 2N), whereas the other one is pentadentate (3O, 2N). Both the Dy3+ centers
within the pockets are connected to each other by a bridging hydrazine O to generate a dimeric
subunit, [Dy2(LH)]3– that is connected to a similar dimeric unit by a bridging ethanolamine
amine, giving rise to the tetranuclear assembly (Figure 5b). In addition to the binding provided
by the [LH]3–, the tetranuclear assembly is further stabilized by two exogenous µ3-OH ligands
and four monodentate pivalate ligands. Overall, each ligand has adopted a µ3-
η1:η1:η1:η2:η1:η1:η1:η2coordination mode (Figure 6), and along with the further assistance from
two µ3-OH and four pivalate ligands the hexacationic tetranuclear core, [Dy4(O)4(OH)2]6+, is
assembled, featuring a butterfly-shaped topology (Figure 7).
µ3 – η1: η1: η1: η2: η1: η1: η1: η2
µ3 – η1
µ3
Figure 6. Various coordination modes of the ligands involved in the formation of 2.
The butterfly-shaped Dy4 core is composed of four coplanar dysprosium ions: Dy1 and Dy1*
define the body while Dy2 and Dy2* represent the two wings (Figure7). Further inspection of the
Dy4 core of 2 reveals some interesting features. Considering the involvement of the two ligands,
the tetranuclear core can be seen as a face-sharing lacunary dicubane structure with two missing
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vertices. The central core, [Dy4(µ3–OH)2]10+ can be divided into a pair of edge sharing isosceles
triangles, whose one side is capped by µ3-OH (Figure 7).
Figure 7: Butterfly shaped core structure in 2.
All Dy3+ in 2 are eight-coordinated and can be grouped into two categories in terms of the
coordination environments and geometry: Dy1 and Dy1*, in a distorted dodecahedron geometry
are surrounded by a 6O,2N environment which is provided by [LH]3–, pivalate and µ3-OH
ligands. On the other hand, Dy2 and Dy2* possess a distorted triangular dodecahedron geometry
in a 6O, 2N coordination environment with contribution from [LH]3–, pivalate, µ3-OH and
solvent MeOH (Figure 8). Metric parameters in 2: The Dy–N bond lengths fall in the range of
2.516–2.604(5) Å, consistent with those found in the literature.[9,10] The average Dy–O bond
length is 2.345(5) Å with the largest bond distance being 2.417 Å encountered in Dy–OMeOH. The
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Dy–OHµ3–Dy bond angles lie in wider range (103.11–110.04(2)°) than that of the Dy–Oµ2–Dy
and Dy–Oethanolamine–Dy bond angles, which are almost identical, ~110.5(2)°.
b)
a)
Figure 8. a) Distorted dodecahedron geometry around Dy1. b) distorted triangular dodecahedron
geometry around Dy2 in 2.
The molecular structure of 3 represents a homometallic hexanuclear complex with four chloride
counter anions. 3 crystallized in the trigonal space group P–3c1 with Z = 2. The asymmetric unit
consists of one third of the entire molecular unit, namely [Dy2(LH2)(tfa)(OPtBu)(Cl)]Cl2 (Figure
9a) and the full molecule is generated as a result of a C3 element which passes through the
middle of the complex 3. The molecular structure of 3 is given in Figure 9b and selected bond
lengths and angles are summarized in Supporting Information (Figure S3).
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a)
b)
Figure 9. a) Asymmetric unit of 3. b) Molecular structure of 3 (solvent molecules, selected
chlorides and hydrogens have been omitted for clarity).
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The formation of 3 involves three doubly deprotonated ligands, [LH2]2–, each of which are
nonadentate and utilizes all the available coordinating centers. Within the complex 3, it has been
found that each ligand in a doubly deprotonated keto form accommodates two Dy3+ in its two
asymmetric pockets [one of these is tetradentate (2O, 2N) where the other one is pentadentate
(2O, 3N)] to generate a dimeric subunit [Dy2(LH2)] in which the two metal centers are secluded
(not attached through a single-atom bridge) unlike the former two complexes, vide supra. The
dimeric subunit thus formed is further bolstered by a chloride and tfa ligands: while the former is
coordinated to Dy1, the latter is chelated to Dy2. Finally, three such dimeric subunits are stitched
together by bridging oxygen atoms of [LH2]2– to afford a homometallic hexanuclear assembly
with a topology of a perfect equilateral triangle. Apart from the binding provided by [LH2]2–, the
formation of 3 is also made possible by a tertiary butyl phosphonate ligand which fits into the
center of 3 by acting as a tridentate ligand to Dy1, Dy1* and Dy1**. The unique binding modes
of the various ligands involved in 3 are given in Figure 10.
µ4 – η2: η1: η1: η1: η1: η1: η1: η1: η2:η2
η2
µ3 – η1: η1: η1
Figure 10. Various coordination modes of the ligands involved in the formation of 3.
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The hexanuclear core [Dy6(O)6(N2)6(O3PtBu)(Cl)3]7+ comprises of six coplanar Dy3+ and
remarkably represents a regular equilateral triangle with a side length of 9.541Å (Figure 11a).
Each side of the triangle contains three Dy3+ in which two metal centers being doubly bridged by
O of 8-hydroxyquinoline and O of diethanolamine of [LH2]2– is segregated from the third Dy3+.
Interestingly, the current structural topology is quite distinct and stands unique in the
homometallic hexanuclear Dy6 family.[11] Furthermore, if we exclude the ligands, the
hexametallic framework can be expanded into two overlapping equilateral triangles with side
lengths of 9.541 Å and 6.077 Å each (Figure 11b).
a)
b)
Figure 11. a) View of the central Dy6 core; b) Arrangement of the Dy6 unit in two overlapped
equilateral triangles in 3.
Overall, 3 comprises two crystallographically independent Dy3+ and can be categorized in two
geometries: Dy2, Dy2* and Dy2** are eight-coordinated in a 5O,3N environment and possess a
distorted square antiprism geometry (Figure 12a) while Dy1, Dy1* and Dy1** contain a 4O, 2N,
Cl environment in a distorted pentagonal bi-pyramidal geometry (Figure 12b). Although known,
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DyIII complexes with a pentagonal bipyramidal ligand environment are exceedingly rare[2a,b] and
are unique in the hexanuclear family.[11]
b)
a)
Figure 12. a) Distorted square antiprism geometry around Dy2; b) distorted pentagonal
bipyramidal geometry around Dy1 in 3.
The Dy–N bond lengths fall in the range of 2.490–2.591(8) Å with the largest distance for Dy–
Ndiethanolamine. The Dy–µ2Odiethanolamine bond length is ~2.251(8) Å, which is shorter than the Dy–
Odiethanolamine [2.360(6)Å] but larger than Dy–Ophosphonate [2.169(6)Å]. The Dy–Ohydrazone and Dy–
Cl bond lengths are 2.342(8) Å and 2.619(8) Å, respectively, and these values are consistent with
the literature precedents.[11,9] The Dy–µ2-Odiethanolamine–Dy bond angle is 112.77(2)°, significantly
larger than Dy– µ2-O8HQ–Dy, 103.95(2)°.
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Compound 4 is heptacationic and crystallized in the triclinic system in space group P-1 with Z =
2. The asymmetric unit contains a full molecule viz. [Dy21(L)7(LH)7(tfa)7]Cl7 (Figure 13) whose
selected metric parameters are sorted in the Supporting Information (Table S4).
Figure 13. Molecular structure of 4 (solvent molecules, selected chlorides and selected
hydrogens have been omitted for clarity).
4 can be described as a single-stranded nanowheel, containing seven (LH3–), seven (L4–), and
seven 1,1,1-trifluoroacetylacetonate ligands. Interestingly, both [LH]3– and [L]4– are present in
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their enolate forms holding two Dy3+ in their two pockets in a segregated manner. This capability
of the ligands is made possible by a 180° rotation across the C−C bond (displayed by [LH]3−)
and a 90° rotation across the hydrazine N−N bond (exhibited by [L]4−) (Scheme 3). While bond
rotation across the hydrazone C and benzene C is well known in the lanthanide families,
hydrazine N−N bond rotation is very rare.[10b-d] The varying coordination modes of the ligands
and co-ligands, as present in in 4 are shown in Figure 14.
µ3 – η2: η1: η1: η1: η1: η1: η1: η1: η1
µ5 – η2: η2: η1: η1: η1: η1: η1: η2
η2
Figure 14. Various coordination modes of the ligands in 4.
The formation of 4 can be understood in the following way: 1) the concerted coordination action
of a [L]4−, [LH]3− and 1,1,1-trifluoroacetylacetonate enables the formation of a trinuclear subunit
of metallic core {Dy3}; 2) seven such subunits are doubly bridged on either side by the O of
phenolate and O of the ethanolamine ligands to ultimately furnish a giant Dy21 nanowheel.
Within the Dy3 subunit, while two adjacent Dy3+ are doubly bridged by the O of the
ethanolamine ligands, the third Dy3+stands isolated.
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Figure 15. Dy21 core in 4 highlighting the connection of the Dy3O4 units via hydrazine nitrogen
on either side.
The core of 4 comprises seven non-planar Dy3O4 subunits, which in turn represent spirocycles
with a Dy3+ being located at each spirocyclic node (Figure 15). Within this subunit, intermetallic
distances have been found to vary in a narrow range, 3.74−3.81(6) Å, whereas Dy−O−Dy bond
angles lie in a much wider range, 108.2−113.69(2)°. Remarkably, seven such units are
interlinked via hydrazine nitrogen atoms to furnish a 45-membered non-planar macrocycle when
considering the shortest Dy−O−Dy pathways (Figure 15). Apart from these features, the space-
filling model of 4 reveals that the internal diameter, external diameter and the width are 0.96 nm,
3.3 nm and 1.2 nm respectively, suggesting that this nanowheel falls in the smallest nanoparticle
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domain (Figure 16). It is of interest to mention that high-nuclear lanthanide complexes that are
SMMs are still quite sparse: previously Dy22, Dy24, Dy26, Dy27, Dy28 and Dy30 complexes have
been reported, which were prepared using an anion-template synthesis.[12] Moreover, single-
stranded high nuclear lanthanide nanowheels are very rare with an example containing Gd18
being the largest known so far.[13] The present Dy21 complex is one of a kind; it is devoid of
oxide/hydroxide ligands and is also prepared from a non-template synthetic strategy unlike the
previous instances.[12]
b)
a)
Figure 16. Space filling model of 4. The inner and outer diameters (a) and the width (b) of the
nanowheel are shown.
4 consists of three crystallographically independent lanthanide centers which can be grouped into
two categories depending on their geometry: 1) distorted triangular dodecahedron (4N,4O)
(Figure 17a) and 2) and distorted square-antiprism geometry (2N,6O (Figure 17b) 3N, 5O
(Figure 17c).
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b)
b)
c)
c)
a)
a)
Figure 17. The Dy3+ coordination environments in 4: a) the distorted triangular dodecahedron
4N,4O environment; b) the distorted square anti-prism 2N,6O environment; c) the distorted
square anti-prism 3N,5O environment.
Several intramolecular and intermolecular interactions are persistent in complexes 1−4, and
some of these led to remarkable supramolecular motifs that are described in the Supporting
Information (Figures S3, S4).
In spite of the variation of nuclearity and local differences in the coordination modes of the
various ligands, the metric parameters found in complexes 1−4 are grossly similar as
summarized in Table 1, indicating the flexible nature of the ligands that can accommodate the
formation of complexes with varying nuclearities without causing undue steric strain.
Table 1: Comparison of bond distances (Å) and angles (°) in 1, 2, 3 and 4.
Parameters
Dy–O8HQ
1
2
3
4
2.319(4)
2.323(5)
2.379(6)
2.357(7)
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Dy–Ohydrazone
2.408(5)
2.437(4)
2.553(6)
2.849(6)
113.77(2)
2.384(5)
2.393(5)
2.529(6)
2.604(6)
110.45(2)
110.62(2)
2.342(6)
2.360(6)
2.490(8)
2.591(8)
2.324(7)
Dy–Odiethanolamine
2.334(7)
2.479(5)
2.589(5)
Dy–NPy
Dy–Ndiethanolamine
Dy–Ohydrazone–Dy
Dy–Odiethanolamine–Dy
112.77(2)
113.06(2)
Magnetic studies
Results of ac and dc susceptibility measurements of 1 are shown in Figure 18. The dc data are
represented by χmT vs. T and Mm vs. B plots. At 290 K, χmT is 28.06 cm3 K mol–1, i.e. close to the
upper limit of the range 26.02–28.10 cm3 K mol–1 expected14 for two non-interacting Dy3+
centers. Upon lowering the temperature, χmT is almost constant down to 50 K (27.62 cm3 K mol–
1), decreases to a minimum at 6.5 K (26.06 cm3 K mol–1) and increases to 26.38 cm3 K mol–1 at
2.0 K. Since the decrease of the χmT values is small, the exchange interaction between both Dy3+
centers is potentially weak, and presumably of ferromagnetic nature since the thermal
depopulation of the mJ sublevels (that are split into Kramers’ doublets due to the ligand field)
usually causes a sharper decline of χmT. The observation of the minimum at low temperatures
may hint at intermolecular interactions such as packing effects. The molar magnetization Mm as a
function of the applied field B is linear up to approximately 0.5 T, and reaches 11.0 NA µB at 5 T.
Although the magnetization hints at saturation for B > 2 T, the small but distinct slope of the Mm
vs. B data shows that the compound is not saturated at 5 T, and the saturation magnetization is
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thus potentially larger as well known for lanthanide centers.14 Since 11.0 NA µB is above the
saturation magnetization of a single free Dy3+ ion (gJ J NA µB = 10 NA µB), the exchange
interactions are weak, and may be antiferromagnetic or ferromagnetic.
Figure 18. Left: temperature dependence of χmT of 1 at 0.1 Tesla; inset: molar magnetization Mm
vs. applied field B at 2.0 K: experimental data (open circles), least-squares fit (solid lines). Right:
Cole-Cole plot of in-phase χm' vs. out-of-phase χm'' ac magnetic susceptibility data at zero bias
field.
In a tentative approach, the dc data were modeled employing the “full” model Hamiltonian using
the computational framework CONDON.15 The primary challenge in such an approximate model
concerns the treatment of the Dy3+ ligand fields, where in compound 1 the 5O,4N coordination
geometries are very similar but strictly speaking without any symmetry elements (Figure 4). This
would mean that a total of 27 ligand field parameters per Dy3+ site would be needed, inevitably
resulting in over-parameterization in lieu of additional experimental data on the magnetic states
(e.g. derived from inelastic neutron scattering, electron absorption spectroscopy etc.). As such,
we are forced to approximate the actual C1-symmetric ligand fields by a higher-symmetry ligand
field for both DyO5N4 environments. The rationale behind this approximation is that the number
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of independent and non-zero ligand field parameters is drastically reduced from 27 if higher-
symmetry elements are present; moreover, small geometric deviations from an ideal geometry of
a given point group are known to result only in marginal or small changes in the (otherwise
vanishing) parameters.
The site symmetry approximation is derived from continuous shape measures (CShM; derived
via SHAPE16) that are weighed against the symmetry-related number of non-vanishing ligand
field parameters. There are two approximate geometries describing the structure of the two Dy3+
environments in 1: capped square antiprism (C4v) and tri-capped trigonal prism (D3h). The best
match of these two are spherical capped square antiprisms with CShM of 3.318 and 3.063,
followed by spherical tri-capped trigonal prisms with moderately worse CShM (3.788 and
3.868), tri-capped trigonal prisms (4.095, 4.352), and capped square antiprisms (4.149, 4.332).
By comparison of the magnitudes of the CShM, the smallest differences of CShM of both sites
and the number of non-zero ligand field parameters (C4v: 5, D3h: 4), we arrive at the D3h
approximation as the best compromise for 1. To emphasize the exploratory nature of this
approach, we have detailed the least-squares fitting (that otherwise employs standard parameters
for discrete Dy3+ ions17) in the Supporting Information. The key results from the least-squares fit,
which reproduces the temperature-dependent low-field susceptibility and the field-dependent
low-temperature magnetization data of 1 relatively well (Figure 18), are the following: J is
positive (+0.25±0.04 cm–1) and therefore represents a ferromagnetic exchange interaction of
medium strength regarding lanthanide–lanthanide interactions. Intermolecular interactions are
close to negligible. The total ligand field splitting of the ground state (6H15/2) with its 2J+1
substates amounts to ca. 170 cm–1 with the first excited substate at 9.7 cm–1 above the ground
state.
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The magnetic ac susceptibility data are shown in Figure 18 as Cole-Cole plot and reveal slow
relaxation at zero bias field for T ≤ 10 K. However, the curvatures of the χm'' vs. χm' data for each
temperature are too small for fitting a generalized Debye expression.18 To enhance the
curvatures, frequencies significantly exceeding what is possible with our available experimental
setup would have to be applied. It should be noted that the mixture of wavefunctions in the
ground state (60% mJ = ±5/2, 40% mJ = ±7/2 states according to the tentative model fit) that
favors the fast QTM relaxation process, as well as the small energy gap between the ground and
first excited states, could explain why no maxima are observed in the χm'' vs. χm' plots above 2 K
for compound 1.
Solely dc susceptibility data of 2 are shown in Figure 19, since the ac data did not reveal any out-
of-phase component. The χmT vs. T data show a value of 52.97 cm3 K mol–1 at 290 K. This value
lies within the range 52.04–56.20 cm3 K mol–1 expected14 for four non-interacting Dy3+ centers.
Upon cooling compound 2, χmT continuously increases to a maximum of 55.50 cm3 K mol–1 at
40 K, subsequently decreases to a minimum of 53.18 cm3 K mol–1 at 4.0 K, and finally increases
to 53.90 cm3 K mol–1 at 2.0 K. The observation of both maximum and minimum of the detected
magnitude reveals the presence of both weak antiferromagnetic as well as ferromagnetic
exchange interactions in 2. In addition to intramolecular exchange interactions, the minimum at
low temperatures may hint at intermolecular interactions. The molar magnetization Mm is a linear
function of B up to approximately 0.5 T, and reaches 22.7 NA µB at 5 T. As for 2, Mm hints at
saturation for B > 2 T, but a small and distinct slope of the Mm vs. B data is observed, and the
saturation magnetization is thus potentially larger. Due to the value of 22.7 NA µB at 5 T, the
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