Pentanuclear [2.2] spirocyclic lanthanide(iii) complexes: slow magnetic relaxation of the DyIII analogue

Article abstract of DOI:10.1039/c5dt03060a

The reaction of LnCl₃·6H₂O (Ln = Dy³⁺, Tb³⁺ and Ho³⁺) with the multisite coordinating ligand N′-(2-hydroxy-3-(hydroxymethyl)-5-methylbenzylidene)acetohydrazide (LH₃) in the presence of pivalic acid (PivH) leads to the formation of three isostructural homometallic pentanuclear complexes, [Dy₅(LH)₄(η¹-Piv)(η²-Piv)₃(μ₂-η²η¹Piv)₂(H₂O)]·Cl·9·5H₂O·5MeOH (1), [Tb₅(LH)₄(η¹-Piv)(η²-Piv)₃(μ₂-η²η¹Piv)₂(H₂O)]·Cl·10.5H₂O·2MeOH·2CHCl₃ (2) and [Ho₅(LH)₄(η¹-Piv)(η²-Piv)₃(μ₂-η²η¹Piv)₂(H₂O)]·Cl·14.5H₂O·2CHCl₃ (3). 1-3 are monocationic and are comprised of four doubly deprotonated [LH]^2- ligands along with six pivalate ions. These complexes possess a [2.2] spirocyclic topology formed by the fusion of two triangles of Ln^III ions at a common vertex. The magneto chemical analysis reveals the presence of antiferromagnetic exchange interactions at low temperature, and the Dy^III complex 1 gives an out-of-phase signal with a small curvature in alternating current (ac) magnetic susceptibility measurement. Application of a 3000 G static field during ac measurement intensifies the signals, revealing a second slow relaxation process in the Dy^III analogue.

Full text of DOI:10.1039/c5dt03060a

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Page 1 of 32
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DOI: 10.1039/C5DT03060A
Pentanuclear [2.2] Spirocyclic Lanthanide (III) Complexes: Slow
III
Magnetic Relaxation of the Dy Analogue
a
a,b
c
c
Sourav Biswas, Sourav Das,
Jan van Leusen, Paul Kögerler* and Vadapalli
a,d
Chandrasekhar*
a
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpurꢀ208016, India.
http://www.iitk.ac.in
b
Department of Emerging Materials Science, DGIST, Daegu 711ꢀ873, Korea
c
Institut für Anorganische Chemie, RWTH Aachen University, Dꢀ52074 Aachen, Germany.
http://www.ac.rwthꢀaachen.de
d
National Institute of Science Education and Research, Institute of Physics Campus,
Sachivalaya Marg, PO: Sainik School, Bhubaneswar ꢀ 751 005, India.
http://www.niser.ac.in
AUTHOR EMAIL ADDRESS: vc@iitk.ac.in; paul.koegerler@ac.rwthꢀaachen.de

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Abstract
The reaction of LnCl ·6H O (Ln = Dy , Tb and Ho ) with the multisite coordinating ligand
3
+
3+
3+
3
2
N'ꢀ(2ꢀhydroxyꢀ3ꢀ(hydroxymethyl)ꢀ5ꢀmethylbenzylidene)acetohydrazide (LH ) in the presence
3
pivalic acid (PivH) leads to the formation of three isostructural homometallic pentanuclear
1
2
2 1
complexes,
[Dy (LH) (η –Piv)(η –Piv) (ꢀ –η η Piv) (H O)]·Cl·9.5H O·5MeOH
(1),
5
4
3
2
2
2
2
1
2
2 1
[
[
Tb (LH) (η –Piv)(η –Piv) (ꢀ –η η Piv) (H O)]·Cl·10.5H O·2MeOH·2CHCl
3
(2)
and
5
4
3
2
2
2
2
1
2
2 1
Ho (LH) (η –Piv)(η –Piv) (ꢀ – η η Piv) (H O)]·Cl·14.5H O·2CHCl (3). 1–3 are monocationic
5
4
3
2
2
2
2
3
2−
and are comprised of four doubly deprotonated [LH] ligands along with six pivalate ions. These
III
complexes possess a [2.2] spirocyclic topology formed by the fusion of two triangles of Ln ions
at a common vertex. The magneto chemical analysis reveals the presence of antiferromagnetic
III
exchange interactions at low temperature, and the Dy complexs 1 gives an outꢀofꢀphase signal
with a small curvature in alternating current (ac) magnetic susceptibility measurements.
Application of a 3000 G static field during ac measurements intensifies the signals, revealing a
III
second slow relaxation process in Dy analogue.
Keywords: pentanuclear lanthanide complexes, [2.2] spirocyclic complexes, singleꢀmolecule
magnets, ac susceptibility, slow magnetic relaxation, Schiff base ligands.

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Introduction
The synthesis of polynuclear clusters of 4f metal ions is of interest because of their potential
1
2
3
utility in singleꢀmolecule magnets (SMMs), catalysis , luminescence , medical imaging ,
4
5
6
7
magnetic refrigeration , highꢀdensity data storage , spintronics and quantum computing . SMMs
are characterized by a slow relaxation of magnetization below the blocking temperature, TB.
8
,9
Magnetic phenomena have been thoroughly explored in polynuclear 3d metal complexes and
10
heterometallic 3d/4f complexes , and recent years have seen the emergence of lanthanide
3
+
3+
complexes, particularly those involving Dy or Tb . Here, the zeroꢀfield splitting of the m subꢀ
J
states belonging to the J ground state produces the thermal energy barriers. Interest in lanthanide
complex SMMs has been fueled by the seminal discovery by Winpenny and coꢀworkers of
t
i
complexes [Dy K O(O Bu) ] and [Dy O(O Pr) ], possessing exceptionally high energy barriers
4
2
12
5
13
1
1
12
13
for magnetization reversal. Homometallic 4f complexes of varying nuclearities (Ln , Ln ,
2
1
4
15
16
17
18
19
20
21
22
Ln , Ln , Ln , Ln , Ln , Ln , Ln , Ln
10
and Ln11 ) are now documented in
3
4
5
6
7
8
9
2
3
24
literature, and we have reported new SMMs involving 3d/4f and 4f metal ions. The five
examples of pentanuclear lanthanide complexes known displaying three distinct structural
1
6a,16b,16c
16e
16d
topologies: pyramidal
, butterfly
and trigonal bipyramidal . These pentanuclear
i
complexes evince SMM behavior including Winpenny’s [Dy O(O Pr) ] complex, which showed
5
13
1
6a
a slow relaxation of magnetization with a 528 ± 11 K thermal energy barrier . To pursue new
polynuclear lanthanide complexes, we have designed a new linear alkyl hydrazoneꢀbased
multidentate
Schiff
base
ligand,
N'ꢀ(2ꢀhydroxyꢀ3ꢀ(hydroxymethyl)ꢀ5ꢀ
3
+
3+
methylbenzylidene)acetohydrazide (LH ); upon reaction with LnCl ·6H O (Ln = Dy , Tb and
3
3
2
3
+
1
Ho ), this afforded homometallic pentanuclear [2.2] spirocyclic complexes 1–3, [Ln (LH) (η –
5
4
2
2
1
3+
3+
Piv)(η –Piv) (ꢀ –η η Piv) (H O)]·Cl·xH O·yMeOH·zCHCl (Dy , x = 9.5, y = 5, z = 0; Tb , x
3
2
2
2
2
3

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3
+
3+
=
10.5, y = 2, z = 2 and Ho , x = 14.5, y = 0, z = 2). In these complexes two triangles of Ln
ions are fused through a common vertex. The synthesis, structure and magnetic properties of
these complexes are discussed herein.
Results and Discussion
Synthesis
Ligand design is a crucial element in modulating nuclearity and structural topology of lanthanide
complexes. Recently, we have prepared rhombusꢀshaped Ln complexes, [Ln (LH) (ꢀ –
4
4
2
2
3
+
3+ 25
O) (H O) ] (Ln = Dy and Ho ), incorporating an aroyl hydrazoneꢀbased Schiff base ligand
4
2
8
(6ꢀhydroxymethyl)ꢀN′ꢀ((8ꢀhydroxyquinolinꢀ2ꢀyl)ꢀmethylene)picolinohydrazide (Scheme 1a).
a)
b)
2
5
Scheme 1. a) Synthesis of rhombusꢀshaped tetranuclear lanthanide complexes b) Synthesis of
27
cubaneꢀshaped tetranuclear lanthanide complexes .

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Motivated by this work, as well as by the use of other hydrazoneꢀbased Schiff base ligands in
1
4c,15d,17d,24
recent literature
hydrazoneꢀbased ligand, N'ꢀ(2ꢀhydroxyꢀ3ꢀ(hydroxymethyl)ꢀ5ꢀmethylbenzylidene)acetohydrazide
LH ). This was produced using a twoꢀstep synthetic protocol that involved the preparation of
, we have prepared a multisite coordinating semiꢀflexible alkyl
(
3
B1, which subsequently undergoes a condensation reaction with B2 to give LH (Scheme 2). The
3
semi flexible ligand, LH provides five divergent coordinating sites: an alkyl hydrazone oxygen,
3
an imine N, an amide N, a phenolic O and a flexible –CH OH arm. The latter is a crucial element
2
to the formation of the coordination geometry, as recently shown by us in the preparation of
2
7
cubaneꢀshaped tetranuclear lanthanide complexes (Scheme 1b). The pivalic acid coꢀligands
help to saturate the primary coordination spheres of the metals by bridging the metal centers, and
confer lipophilicity to the complexes.
O
O
MeOH, 6h
N H ·H O
2
4
2
NH NH2
OEt
B1
O
MeOH, 3h
refluxed
NH NH2
OH OH
O
OH OH
N
O
NH
B2
Scheme 2. Synthesis of the ligand LH3.
The reaction of LH , LnCl ·6H O, and pivalic acid in the stoichiometric ratio of 4:5:6 in the
3
3
2
3
+
1
presence of 4 equivalents of triethylamine in methanol afforded pentanuclear Ln [Ln (LH) (η –
5
4
2
2
1
3+
Piv)(η –Piv) (ꢀ –η η Piv) (H O)]·Cl·xH O·yMeOH·zCHCl [compound 1, Ln = Dy , x = 9.5,
3
2
2
2
2
3
3
+
3+
y = 5, z = 0; compound 2, Ln = Tb , x = 10.5, y = 2, z = 2 and compound 3, Ln = Ho , x =
14.5, y = 0, z = 2)] (Scheme 3).

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LnCl ·6H O
3
2
O
OH
N
NH
O
HO
OH
Et N, 12hrs MeOH/CHCl (1:1)
3
3
O
O
H
O
H
N
H
N
H
N
N
O
O
O
O
Ln
O
O
O
O
O
Ln
O
O
Ln
x = H O
2
O
O
y = MeOH
z = CHCl3
O
O
O
O
Ln
O
O
O
Ln
O
H
O
N
N
H
N
N
Ln = Dy3+, x = 9.5, y = 5 and z = 0(1)
Ln = Tb3+, x = 10.5, y = 2 and z = 2(2)
Ln = Ho3+, x = 14.5 y = 0 and z = 2(3)
1
2
2
Scheme
3.
Synthesis
of
[Ln (LH) (η –Piv)(η –Piv) (ꢀ –η
5
4
3
2
1
η Piv) (H O)]·Cl·xH O·yMeOH·zCHCl (Dy, x = 9.5, y = 5, z = 0; Tb, x = 10.5, y = 2, z = 2 and
2
2
2
3
Ho, x = 14.5, y = 0, z = 2).
Xꢀray Crystal Structures
Singleꢀcrystal Xꢀray diffraction measurements reveal that compounds 1–3 are isostructural, and
crystallize in the tetragonal space group Iꢁ4 with Z = 4. The complexes are monocationic, with

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III
their charges each balanced by a chloride anion, and possess a [2.2] spirocycle of Ln ions
containing two fused triangular motifs. Complex 1 has been chosen as a representative example
to illustrate the common structural features of these clusters. Selected bond parameters of 1 are
summarized in Table 1. The molecular structure and selected bond parameters of the other
complexes (2 and 3) are presented in the ESI (Figures S1–S2 and Tables S1–S2). A perspective
view of the molecular structure of 1 is depicted in Figure 1.
Figure 1. Molecular structure of 1 (selected hydrogen atoms, chloride and the solvent molecules
have been omitted for clarity).

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Table 1. Selected bond distances (Å) and bond angles (deg) parameters for 1.
Selected bond length
around Dy1
Dy(3)ꢀO(8)
Dy(3)ꢀO(9)
Dy(3)ꢀO(11)
2.321(6)
2.330(6)
2.351(6)
2.351(6)
2.351(6)
2.353(6)
2.381(7)
2.414(6)
Dy(5)ꢀO(4)
Dy(5)ꢀO(10)
Dy(5)ꢀO(18)
Dy(5)ꢀN(7)
2.387(6)
2.409(6)
2.456(9)
2.508(9)
Dy(1)ꢀO(11)
Dy(1)ꢀO(12)
Dy(1)ꢀO(20)
Dy(1)ꢀO(9)
Dy(1)ꢀO(19)
Dy(1)ꢀO(21)
Dy(1)ꢀO(1)
Dy(1)ꢀN(1)
Dy(1)ꢀO(22)
2.310(6) Dy(3)ꢀO(4)
2.396(7) Dy(3)ꢀO(4)
2.409(7) Dy(3)ꢀO(1)
2.417(6) Dy(3)ꢀO(2)
2.442(7) Dy(3)ꢀO(5)
2.447(6)
Selected Bond angles around
Dy
Dy(2)ꢀO(1)ꢀDy(1)
97.9(2)
94.3(2)
97.2(2)
95.2(2)
96.9(2)
99.5(2)
95.3(2)
98.4(2)
95.9(2)
94.5(2)
96.18(9)
96.0(2)
95.4(2)
96.1(2)
98.3(2)
95.4(3)
96.4(2)
96.7(2)
2.457(6) Selected bond length around Dy(3)ꢀO(1)ꢀDy(1)
2.519(8)
2.521(6)
Dy4
Dy(5)ꢀO(2)ꢀDy(3)
Dy(3)ꢀO(4)ꢀDy(5)
Dy(3)ꢀO(4)ꢀDy(4)
Dy(5)ꢀO(4)ꢀDy(4)
Dy(2)ꢀO(5)ꢀDy(3)
Dy(3)ꢀO(8)ꢀDy(4)
Dy(3)ꢀO(9)ꢀDy(1)
Dy(3)ꢀO(9)ꢀDy(2)
Dy(1)ꢀO(9)ꢀDy(2)
Dy(3)ꢀO(10)ꢀDy(5)
Dy(3)ꢀO(10)ꢀDy(4)
Dy(5)ꢀO(10)ꢀDy(4)
Dy(4)ꢀO(15)
Dy(4)ꢀO(8)
Dy(4)ꢀO(7)
Dy(4)ꢀO(4)
2.312(7)
2.373(6)
2.384(7)
2.396(6)
2.411(7)
2.494(8)
2.500(6)
2.505(8)
2.666(9)
Selected bond length
around Dy2
Dy(2)ꢀO(24)
Dy(2)ꢀO(5)
Dy(2)ꢀO(22)
Dy(2)ꢀO(1)
Dy(2)ꢀO(6)
Dy(2)ꢀO(9)
Dy(2)ꢀN(5)
Dy(2)ꢀO(23)
2.320(7) Dy(4)ꢀO(30)
2.324(6) Dy(4)ꢀN(4)
2.325(6) Dy(4)ꢀO(10)
2.333(6) Dy(4)ꢀO(14)
2.402(7) Dy(4)ꢀO(13)
2.439(6)
2.450(8) Selected bond length around Dy(1)ꢀO(11)ꢀDy(3)
2.454(7)
Dy5
Dy(5)ꢀO(13)ꢀDy(4)
Dy(2)ꢀO(22)ꢀDy(1)
Dy(2)ꢀO(1)ꢀDy(3)
Selected bond length
around Dy3
Dy(5)ꢀO(13)
Dy(5)ꢀO(2)
Dy(5)ꢀO(17)
2.258(7)
2.286(6)
2.356(11)
2.381(7)
Dy(3)ꢀO(10)
2.301(6) Dy(5)ꢀO(3)
The pentanuclear complex 1 is formed by the concerted coordination action of the four doubly
2
−
deprotonated ligands [LH] . Four out of five potential coordinating sites of the ligand
coordinated to the metal centers: a phenolate oxygen which functions as a bridging ligand
3
+
between two Dy centers, a deprotonated hydroxymethyl arm which functions as a ꢀ –capping
3
III
3+
ligand among three Dy centers, an imine nitrogen binding a Dy center, and an acetohydrazide

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3
+
2−
oxygen coordinating a Dy center. The framework provided by the ligand [LH] is bolstered by
1
the coordination of six pivalate ligands: one is η –coordinated to Dy4, while three others chelate
2
Dy1, Dy2 and Dy5 in η fashion. The remaining two pivalate ligands exhibit both bridging and
2
1
chelating modes (ꢀ –η :η ) in their coordination that links Dy1 to Dy2 and Dy4 to Dy5. The
2
2
ꢀ
pentanuclear [2.2] spirocyclic core is thus constructed by the coordination action of four [LH]
3
2
1
1
(
ꢀ –η :η :η :η ) and six pivalate ligands. The coordination modes of the ligands are depicted in
4
Figure 2.
Dy
O
N NH
O
O
O
O
O
O
O
O
Dy
Dy
Dy
Dy
Dy
Dy
Dy
3
2
1
1
1
2
2
1
ꢀ4
–η : η : η : η
η
η
ꢀ –η : η
2
2
−
3+
Figure 2. Binding modes of [LH] and the pivalate ligands with Dy ions in 1.
2
1
5+
The pentacationic pentanuclear core, [Dy (ꢀ –O) (ꢀ –O) (ꢀ –η η Piv) ] consists of two Dy3
5
3
4
2
4
2
2
triangles which are interconnected by sharing a common vertex, Dy3. The remaining four
vertices of the two triangles are occupied by Dy1, Dy2, Dy4 and Dy5 (Figure 3a). The edges of
2
−
the triangles are formed by the pivalate ligands as well as phenolate oxygens of the ligand [LH]
while the faces of the triangle are effectively capped by ꢀ –O, emanating from the flexible
3
pendant hydroxymethyl arms (Figure 3a). The capping ꢀ oxygen atoms lie an average of
3
…
~
1.219 Å away from the two triangular planes. The triangles are nearly equilateral, with Dy Dy

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⁰
⁰
vertex lengths of 3.50–3.651 Å and Dy–Dy–Dy angles that range from 58.1 – 62.2 . The two
⁰
triangles are twisted at the Dy3 [2.2] spirocyclic node by a dihedral angle 55.85 (Figure 3b).
b)
a)
Figure 3. a) [2.2] spirocyclic core of the complex 1 b) [2.2] spirocyclic core showing the
dihedral angle between the two triangular motifs along with the distance between the
3
+
Dy centers.
The five crystallographically independent lanthanide centers present in 1 can be classified into
three coordination geometry types (all distorted): triangular dodecahedron (Dy1, Dy5), square
antiprism (Dy3), and monoꢀcapped squareꢀantiꢀprism (Dy2, Dy4) (Figure 4). A minor variation
exists in the coordination environment around Dy2 and Dy4: unlike the rest of the dysprosium
centers, Dy4 is coordinated by a water molecule, which is engaged in strong hydrogen bonding
1
…
with the oxygen atom of the η ꢀpivalate ligand (Figure 1) (D–H A distance 1.746 Å and angle
⁰
1
2
1
54.41 ) accounting for the η binding mode rather than the anticipated η coordination mode of
the pivalate ligand. The Dy–Ophenoxy bond lengths fall the range of ~2.286–2.414 Å which are
slightly shorter than the Dy–Oalkoxy bond lengths (~2.330–2.457 Å). These are in turn shorter

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than many Dy–O bond distances that range from ~2.258 to ~2.660 Å. Bond lengths involving
piv
⁰
the coordinated imine nitrogens lie in the range of ~2.449–2.519 Å. The Dy–Ophen–Dy (95.31 –
⁰
⁰
9
8.36 ) and Dy–O –Dy angles (95.36–96.36 ) are in a similar range.
piv
c)
a)
b)
Figure 4. Local geometry around the three types of dysprosium centres in 1: a) Dy1 and Dy5
possessing a distorted triangular dodecahedral geometry, b) Dy3 possessing a distorted square
antiprism geometry and c) Dy2 and Dy4 possessing a distorted monocapped square antiprism.
The [2.2] spirocyclic topology observed in complexes 1ꢀ3 is quite distinct from the
1
6a,16b,16c
16e
16f
pyramidal
, butterfly
or trigonal bipyramidal shaped homometallic Ln clusters
5
reported previously (Figure S9). A comparison of the geometry around the metal centers, the
structural topology around the metal ions and the SMM properties of the pentanuclear lanthanide
families are summarized in Table 2.

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Table 2: Structural and magnetic features of pentanuclear lanthanide assemblies.
Compound
Core topology
Coordination
numbers (local
geometries around
Ln(III) centers)
Eight coordinated
(distorted squareꢀ
antiprism)
Magnetic
properties
Refe
renc
es
[
(
Dy (ꢀ –OH) (Acc)
6
Trigonal
bipyramidal
SMM
16d
5
3
6
H O) ]·Cl
U = 1.91 K
2
10
9
eff
ꢀ6
Acc = 1ꢀamino
cyclohexaneꢀlꢀ
carboxylic
τ = 1.01 X10 s
0
acid.
i
[
Dy O(O Pr) ]
Square pyramidal
Square pyramidal
Six coordinated
(octahedral)
SMM
U = 528±11 K,
16a
5
13
i
Pr = isopropyl
eff
46.6±0.7 K
ꢀ
10
τ0 = 4.7 X10 s,
ꢀ
6
3.8 X10 s
[
Dy (OH) (αꢀ
Eight coordinated
SMM
5
5
16b
AA) (Ph acac) ]
4
2
6
αꢀAA = DꢀPhGly
[
(
Dy (ꢀ –OH) (opch)
6
Butterfly
Eight coordinated
(distorted
dodecahedral), nine
coordinated
SMM
U = 8.1 ; 37.9 K
τ = 1.7 X10 s;
16e
5
3
3
H O) ]
2
3
eff
ꢀ
5
H opch = oꢀvanillin
pyrazine acylhydrazone
2
0
ꢀ8
9.7 X 10 s
(distorted
monoꢀcapped
squareꢀ
antiprismatic) and
nine coordinated
(distorted
triplecapped
prismatic)
[
(
Dy (ꢀ –OH) (opch)
6
Butterfly
Eight coordinated
(distorted
dodecahedral), nine τ = 3.2 X 10 s
SMM
U = 197K ;
eff
16e
5
3
3
H O) ](ClO )
2
3
4 2
ꢀ9
H opch = oꢀvanillin
2
0
pyrazine acylhydrazone
coordinated
(distorted
monoꢀcapped

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squareꢀ
antiprismatic) and
nine coordinated
(distorted
triplecapped
prismatic)
1
2
[
Dy (LH) (η –Piv)( η –
[2.2] spirocycle
Eight coordinated
(distorted
triangulardodecahe
dral), eight
Slow relaxation This
work
5
4
2
Piv) (ꢀ –η
3
2
1
η Piv) (H O)]·Cl
2
2
coordinated
(distorted squareꢀ
antiprism) and nine
coordinated (monoꢀ
cappedꢀsquareꢀ
antiprism)
Thermogravimetric study of complex 1–3
Thermogravimetric analysis reveals that all the complexes 1–3 exhibit almost a similar
decomposition pattern towards heat treatment involving a twoꢀstep weight loss process (Figures
S10ꢀS12). A small weight loss in the region of 60–120 °C was observed for all the complexes
which is in part due to the loss of solvents of crystallization. In all the cases, some of the solvent
molecules of crystallization are lost rapidly as the crystals are brought outside the mother liquor
at room temperature. In the second step, above 300 °C, rapid weight loss of all the complexes
was observed, confirming the decomposition of the complexes.

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Magnetic Studies
Direct current (dc) magnetic susceptibility data for 1–3 are presented as χ T vs. T curves at B =
m
0
.1 Tesla, and molar magnetization M vs. applied field B diagrams at T = 2 K in Figure 5. For
m
III
3
–1
the Dy analogue 1, χ T reaches a value of 64.9 cm K mol at 290 K which is slightly below
m
3
–1
27a
3+ 6
the range 65.1–70.3 cm K mol expected for five nonꢀinteracting Dy ( H , J = 15/2, g =
1
5/2
J
4
/3) centers. The decrease of χ T with lowering temperature is gradual down to ~100 K, and then
m
more precipitous (approximate slope). This behavior and the low value of χ T at 290 K have
m
their origin in the ligand field effect (i.e. depopulation of the m sublevels) and potential
J
antiferromagnetic exchange interactions within the compound (note that the distinct deviation
from a linear behavior in χ T is already introduced at higher temperatures T ≈ 200 K). Analysis
m
of the dc magnetic susceptibility data for 2 and 3 reveals a similar behavior, and the χ T values
m
at 290 K are also slightly lower than those expected for the respective number of nonꢀinteracting
3
–1
27a
lanthanide centers. For 2, χ T reaches 58.1 cm K mol at 290 K (expected
58.2–
m
3
–1
III
7
6
6
0.1 cm K mol for five nonꢀinteracting Tb ( F , J = 6, g = 3/2) centers); for 3, χ T is
6
J
m
3
–1
27a
3
–1
III 5
6.1 cm K mol at 290 K (expected 66.3–69.0 cm K mol for five nonꢀinteracting Ho ( I ,
8
J = 8, g = 5/4) centers).
J
At 2 K, the molar magnetizations M of 1–3 (Figure 5, inset) indicate saturation and thus hint
m
III
roughly at the ground state of the Ln centers in each compound expected due to ligand field
effects on the spinꢀorbit ground term, since the weak exchange interactions of lanthanides at zero
field are eliminated by the applied maximum fields: all of the extrapolated saturation values are
roughly right in the center between the minimum magnetization of corresponding five nonꢀ
III
interacting Ln centers (5g m
N ꢀ ) and the maximum magnetization of such centers
A B
J
J,min

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III
(
5g J N ꢀ ). This indicates that neither all Ln centers of a compound are characterized by a
J
A
B
ground state of minimal mJ,min (±1/2 for 1, 0 for 2, 3) nor of the maximum m = J.
J
Figure 5. Temperature dependence of χ T of 1–3 at 0.1 Tesla; inset: molar magnetization M as
m
m
function of the applied field B at 2 K.
A comprehensive model of compounds 1–3 based exclusively on the magnetic susceptibility data
is not feasible because the description of the compounds should include at least two different
lanthanide sites (approximately D and D symmetric, eightꢀ or nineꢀfold coordinated centers)
4
d
3h
and three different exchange pathways. Without a simpler model system such as {Gd }, or
5
complementary data such as from inelastic neutron scattering, degenerate preliminary solutions
2
7b,27c
modeled by the computational framework CONDON 2.0
cannot be ruled out. One feature
common among all of these preliminary solutions provides qualitative insight into χ T and M
m
m
behavior presented in Figure 5: all calculations reveal very weak ferromagnetic exchange

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interactions between the two outer lanthanides of each triangle (i.e. Ln1⋯Ln2 and Ln4⋯Ln5,
–
1
Figure 3b), and either antiferromagnetic (≈ –0.3 cm ) or almost nonexistent exchange
interactions for the remaining pathways (Ln1⋯Ln3, Ln2⋯Ln3, Ln3⋯Ln4, and Ln3⋯Ln5). Note
that although the bridging ligands are similar, and thus exchange interactions might be assumed
to be similar as well, the various distances between the Ln pairs and the presence of three
different site geometries, and thus ligand fields, explain the difference in the exchange
interaction parameters.
Figure 6. Left: inꢀphase magnetic susceptibility χ ′ vs. T; right: outꢀofꢀphase magnetic
m
susceptibility χ ′′ vs. T of 1 at different frequencies f at zero dc bias field; insets: at 3000 G dc
m
bias field (dashed lines are guides for the eye).
Alternating current (ac) molar magnetic susceptibility measurements on complexes 1–3 revealed
III
outꢀofꢀphase signals for Dy analogue 1, but not for 2 and 3, at zero dc field. The very small
curvatures within the Argand plane for complex 1 (Figure S3) reveal inꢀphase signals χ ′ that are
m
almost independent from the applied frequency (Figure 6). The application of external static
fields of up to 4000 G slightly shifted χ ′, but did not produce significantly greater curvatures for
m

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1
6b
frequencies greater than 100 Hz. Similar behavior was observed by Thielemann et al. who
ascribed it to a blocking temperature which lies significantly below the range explored by the
magnetic measurements. Optimal slow relaxation magnetic measurements of complex 1 with
respect to the experimental options at hand were obtained using a 3000 G static field, and a
frequency range of 0.03 Hz–111 Hz (Figure 7, insets Figure 6. The inꢀphase (χ ′) and outꢀofꢀ
m
2
8
phase (χ ′′) ac susceptibility components can be fitted to a ColeꢀCole equation for each
m
temperature (Figure 7, solid lines; Figure S3). To determine the average relaxation times of the
magnetization under the optimized conditions, the temperatureꢀdependent fit parameters have
been analyzed using an Arrhenius expression (τ = τ ⋅exp(ꢁU/k T)). This results in an effective
0
B
–
1
–2
energy barrier ꢁU = (5.2 ± 0.5) cm and a time constant of τ = (2.6 ± 0.6)×10 s (Figure S5).
0
In the generalized Debye model, the distribution width of τ is parametrized by the scalar α. The
nonzero mean value of α = 0.45 ± 0.08 reveals that several relaxation processes are active in this
system. The time constant τ is anomalously large for SMM behavior, i.e. the entirely
0
’
phenomenological parameter τ does not fall within the typical SMM range, and indicates that
0
rather a secondary relaxation process was observed in the presence of a bias field instead of the
relaxation process indicated at zero field. This is supported by the occurrence of the minima in
the ColeꢀCole curves for higher frequencies (lower χ ′ values) hinting at further subsequent
m
semiꢀcircles which we could not enhance with the experimental setꢀup at hand. Since some or all
exchange interactions within lanthanide compounds are overridden by the application of an
external field of 3000 G, the observed process may be connected to a forced alignment of the
momenta albeit further evidence is needed to proof this hypothesis.

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Figure 7. Normalized ColeꢀCole plot of 1 at different temperatures and 3000 G dc field (solid
circles), frequencies range from 0.03–111 Hz, χ is the isothermal susceptibility in the limit of
T
lowest frequencies; fit to ColeꢀCole equation (solid lines).
Conclusions
In summary, a series of isostructural homometallic pentanuclear Ln complexes that possess an
5
unprecedented [2.2] spirocyclic topology were synthesized. The frameworks of these complexes
are comprised of the multidentate Schiff base ligand, N'ꢀ(2ꢀhydroxyꢀ3ꢀ(hydroxymethyl)ꢀ5ꢀ
methylbenzylidene)acetohydrazide, along with several bridging pivalate groups. The lanthanide
centers present in the pentanuclear assembly can be grouped into three types based on their local
coordination geometry: an eightꢀcoordinate lanthanide in a distorted triangular dodecahedral
geometry, an eightꢀcoordinate lanthanide in a distorted squareꢀantiprism geometry and a nineꢀ
coordinate lanthanide in a monoꢀcapped square antiꢀprism geometry. Variableꢀtemperature dc
and ac magnetic susceptibility measurements reveal weak antiferromagnetic exchange
interactions among the lanthanide centers of complexes 1ꢀ3. Compound 1 exhibited temperatureꢀ

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dependent outꢀofꢀphase ac molar magnetic susceptibility signals with small curvatures at zero dc
field. The application of a static 3000 G field intensified the ac magnetic susceptibility
III
component, and an Arrhenius analysis confirmed slow magnetic relaxation of the Dy analogue.
Experimental Section
Solvents and other general reagents used in this work were purified according to standard
2
9
procedures.
2,6ꢀBis(hydroxymethyl)ꢀ4ꢀmethylphenol, activated manganese (IV) dioxide
(
MnO ), DyCl ·6H O, TbCl ·6H O and HoCl ·6H O were obtained from Sigma Aldrich
2
3
2
3
2
3
2
Chemical Co. and were used as received. Acetyl hydrazide and 2ꢀ(hydroxymethyl)ꢀ6ꢀ
2
3e
carbaldehydeꢀ4ꢀmethylphenol were prepared according to the literature procedure . Hydrazine
hydrate (80%), pivalic acid and sodium sulphate (anhydrous) were obtained from S.D. Fine
Chemicals, Mumbai, India and were used as such.
Instrumentation. Melting points were measured using a JSGW melting point apparatus and are
uncorrected. IR spectra were recorded as KBr pellets on a Bruker Vector 22 FT IR
ꢀ1
spectrophotometer operating at 400–4000 cm . Elemental analyses of the compounds were
1
obtained from Thermoquest CE instruments CHNSꢀO, EA/110 model. H NMR spectra were
recorded in CD OD solutions on a JEOL JNM LAMBDA 400 model spectrometer operating at
3
500·0 MHz, Chemical shifts are reported in parts per million (ppm) and are referenced with
1
respect to internal tetramethylsilane ( H). Powder Xꢀray diffraction (PXRD) patterns were
recorded with a Bruker D8 advance diffractometer equipped with nickelꢀfiltered Cu Kα
radiation. Powder Xꢀray diffraction (PXRD) patterns of complexes 1–5 were in good agreement

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with the simulated patterns (ESI, Figures S6–S8). The difference in intensities could be due to
the preferred orientation in the powder samples.
Magnetic Measurements. Magnetic susceptibility data of 1–3 were recorded using a Quantum
Design MPMSꢀ5XL SQUID magnetometer for static field (DC) and dynamic field (AC)
measurements. The polycrystalline samples were compacted and immobilized into PTFE
capsules. DC susceptibility data were acquired as a function of the field (0.1−5.0 T) and
temperature (2−290 K). AC susceptibility data were measured at zero field and in presence of
various static fields in the frequency range 0.03−1000 Hz (T = 1.8−50 K, B = 3 G, B = 0–
ac
dc
4000 G). All data were corrected for the contribution of the sample holder (PTFE capsule) and
–
the diamagnetic contributions of compound 1–3 calculated from tabulated values (–1.25×10
3
3
–1
–3
3
–1
–3
3
–1
cm mol , –1.37×10 cm mol and –1.32×10 cm mol , respectively).
Xꢀray Crystallography. The crystal data for the compounds have been collected on a Bruker
3
0a
SMART CCD diffractometer (MoKα radiation, λ = 0.71073 Å). The program SMART was
used for collecting frames of data, indexing reflections, and determining lattice parameters,
3
0a
30b
SAINT for integration of the intensity of reflections and scaling, SADABS for absorption
3
0c,30d
correction, and SHELXTL
for space group and structure determination and leastꢀsquares
2
refinements on F . All the structures were solved by direct methods using the program SHELXSꢀ
3
0e
2
30e .
97
and refined by fullꢀmatrix leastꢀsquares methods against F with SHELXLꢀ97 Hydrogen
atoms were fixed at calculated positions and their positions were refined by a riding model. All
the nonꢀhydrogen atoms were refined with anisotropic displacement parameters. The lattice
solvent molecules of the complexes 1–3 cannot be modeled satisfactorily due to the presence of

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3
0f,30g
very high disorder. PLATON/SQUEEZE
routine was utilized to remove those severely
disordered solvents molecules. The total electron count thus squeezed is 396, 646 and 780
respectively per unit cell which corresponds to 99, 161 and 195 electrons per molecule (Z = 4).
These electron counts can be assigned to 5MeOH, H O (expected 100) for 1, 2CHCl , 2MeOH,
2
3
H O (expected 162) for 2 and 2CHCl , 8H O (expected 196) for 3. The crystallographic figures
2
3
2
3
0h
have been generated using Diamond 3.1e software . The crystal data and the cell parameters for
compounds 1ꢀ3 are summarized in Table 3. Crystallographic data (excluding structure factors)
for the structures in this paper have been deposited with the Cambridge Crystallographic Data
Centre as supplementary publication nos. CCDC 1031235–1031237. Copies of the data can be
obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K.:
http://www.ccdc.cam.ac.uk/cgiꢀbin/catreq.cgi , eꢀmail: data_request@ccdc.cam.ac.uk, or fax:
+44 1223 336033.
Table 3.Crystal data and structure refinement parameters of 1ꢀ3.
1
2
3
formula
C148 H206 Cl N O
Tb10
C148 H208 Cl Ho N
O63
2
16 69
2
10 16
C1 H208 Cl Dy N
10 16
48
2
O67
M/g
4979.22
Tetragonal
Iꢀ4
40.368(2)
40.368(2)
11.922(7)
90
19429(2)
4
1.702
4967.45
Tetragonal
Iꢀ4
40.479(5)
40.479(5)
11.943(5)
90
19569(10)
4
1.688
4939.52
Tetragonal
Iꢀ4
40.446(5)
40.446(5)
11.940(5)
90
19532(9)
4
1.680
crystal system
space group
a/Å
b/Å
c/Å
α=β=γ (°)
3
V/Å
Z
ꢀ3
ρ /g cm
c
ꢀ1
ꢀ
/mm
F(000)
cryst size (mm )
3.905
3.674
4
.107
9664.0
0.15 × 0.11 × 0.06
9752.0
0.09 × 0.078 × 0.023
9768.0
0.12 × 0.07 × 0.06
3

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θ range (deg)
limiting indices
2.26 to 26.80
ꢀ43<=h<=51
2.25 to 28.04
2.25 to 20.39
ꢀ51<=h<=50
ꢀ51<=k<=51
ꢀ15<=l<=12
68807
ꢀ49<=h<=29
ꢀ48<=k<=49
14<=l<=14
53438
ꢀ44<=k<=51
ꢀ15<=l<=14
reflns collected
ind reflns
76684
21174 [R(int) =0.0722]
21266 [R(int) =0.1230] 18181[R(int) =0.0923]
completeness to θ (%) 99.9
99.8
100.0
refinement method
Fullꢀmatrix leastꢀsquares Fullꢀmatrix leastꢀ
Fullꢀmatrix leastꢀ
2
2
2
on F
1174 / 1 / 1068
squares on F
squares on F
18181 / 1 / 1081
data/restraints/params
21266 / 22 / 1101
2
2
goodnessꢀofꢀfit on F
1.029
1.028
1.027
Final R indices
R1 = 0.0501
R1 = 0.0565
R1 = 0.0676
[
I > 2θ(I)]
wR = 0.1065
wR = 0.1359
wR = 0.1536
2
2
2
R indices (all data)
R1 = 0.0671
R1 = 0.0828
R1 = 0.1009
wR = 0.1123
wR = 0.1532
wR = 0.1799
2
2
2
Synthesis
N'ꢀ(2ꢀhydroxyꢀ3ꢀ(hydroxymethyl)ꢀ5ꢀmethylbenzylidene)acetohydrazide (LH3)
To a stirred solution of 2ꢀ(hydroxymethyl)ꢀ6ꢀcarbaldehydeꢀ4ꢀmethylphenol (1.50 g, 9.02 mmol)
(B2) in 40 mL ethanol, acetyl hydrazide (0.66 g, 9.02 mmol) (B1) was added drop wise over a
period of 15 minutes and the resultant yellow colored solution was refluxed for 5 hrs. Then, the
yellow solution was concentrated in vacuo to 15 mL and kept in a refrigerator at 0 °C for
overnight. A light yellow colored heavy precipitate was obtained which was filtered and washed
with cold ethanol as well as diethyl ether before being dried. Yield: 1.6 g (79.8 %). Mp: 180 °C.
−
1
FTꢀIR (KBr) cm : 3398 ν(O−H); 3181 ν(N−H); 1661 ν(C＝O); 1624 ν(C＝N)imine; 1516
1
ν(C＝N)py. H NMR (CD OD, δ, ppm, 500MHz ): 2.05 (s, 3H, –CH3acetyl), 2.27 (s, 3H, –CH3),
3
4
.55 (s, 2H, –CH OH), 5.46 (s, 1H, –OHphen), 7.02 (s, 1H, Ar–H), 7.22 (s, 1H, Ar–H), 8.06 (s,
2
1
H, –NH), 8.17 (s, 1H, imine). Anal. Calcd for C H N O : C, 59.45; H, 6.35; N, 12.60 Found:
1
1
14
2
3
+
C, 58.77; H, 6.01; N, 12.19. ESIꢀMS, m/z: (M+H) . 223.09.

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General Synthetic Procedure for the Preparation of the Complexes 1−3.
All the pentanuclear complexes (1−3) have been synthesized according to the following
procedure. LH (0.04 g, 0.18 mmol) was dissolved in 40 mL methanol. To this solution, under
3
stirring, LnCl ·6H O (0.23mmol) was added and the reaction mixture was stirred for 20 minutes
3
2
at room temperature. At this stage, triethylamine (0.096 mL, 0.73 mmol) was added drop wise
and the stirring was continued for a further 10 minutes and pivalic acid (0.028 g, 0.27 mmol)
was added drop wise to the mixture. The resulting yellow colored solution was continued to stir
for 12 hours at room temperature. Then, the solution was completely evaporated in vacuo to
afford a light yellow colored solid mass which was washed 2–3 times with diethyl ether and
dried. The solid mass was reꢀdissolved in MeOH/CHCl (1:1) and kept for crystallization. After
3
about 12 days, needleꢀshaped yellow colored crystals suitable for Xꢀray crystallography were
obtained by slow evaporation from the solvent mixture. Specific details of each reaction and the
characterization data of the products obtained are given below.
1
2
2
1
[
Dy (LH) (η –Piv)( η –Piv) (ꢀ –η η Piv) (H O)]·Cl·9.5H O·5MeOH (1)
5 4 3 2 2 2 2
Quantities: LH (0.04 g, 0.18 mmol), DyCl ·6H O (0.087 g, 0.23 mmol), Et N (0.096 mL, 0.73
3
3
2
3
3
+
mmol), PivH (0.028 g, 0.27 mmol). Yield: 0.076 g, 63.3 % (based on Dy ). Mp: 200 °C (d). IR
–
1
(
KBr) (cm ): 3433 (b), 2978 (s), 2950 (s), 2604 (s), 2497(s), 2343 (w), 1620 (s), 1574 (s), 1482
(s), 1429 (s), 1397 (s), 1309 (w), 1262 (w), 1228 (w), 1172 (w), 1072 (w), 1037 (s), 897 (w), 851
(
w), 808 (s), 608 (w). Anal. Calcd. for C H Cl Dy N O
(2663.80): C, 35.62; H, 4.62 N,
7
9
122
5
8
39.5
4.21. Found: C, 34.96; H, 4.33 N, 4.09.

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1
2
2
1
[
Tb (LH) (η –Piv)( η –Piv) (ꢀ – η η Piv) (H O)]·Cl·10.5H O·2MeOH·2CHCl (2)
5
4
3
2
2
2
2
3
Quantities: LH (0.04 g, 0.18 mmol), TbCl ·6H O (0.085 g, 0.23 mmol), Et N (0.096 mL, 0.73
3
3
2
3
3+
mmol), PivH (0.028 g, 0.27 mmol). Yield: 0.058 g, 46.03 % (based on Tb ). Mp: 200 °C (d). IR
–
1
(
KBr) (cm ): 3441 (b), 2971 (s), 2930 (s), 2601 (s), 2499(s), 2339 (w), 1610 (s), 1570 (s),
477(s), 1425 (s), 1393 (s), 1302 (w), 1252 (w), 1235 (w), 1178 (w), 1071 (w), 1032 (s), 895
w), 854 (w), 803 (s), 610 (w). Anal. Calcd.for C H Cl N O Tb (2804.55): C, 33.40; H,
1
(
78
112
7
8
37.5
5
4.03; N, 4.00. Found: C, 32.93; H, 4.10 N, 4.04
1
2
2
1
[
Ho (LH) (η –Piv)( η –Piv) (ꢀ – η η Piv) (H O)]·Cl·14.5H O·2CHCl (3)
5 4 3 2 2 2 2 3
Quantities: LH (0.04 g, 0.18 mmol), HoCl ·6H O (0.087 g, 0.23 mmol), Et N (0.096 mL, 0.73
3
3
2
3
3
+
mmol), PivH (0.028 g, 0.27 mmol). Yield: 0.062 g, 48.4 % (based on Ho ). Mp: 200 °C (d). IR
–
1
(
KBr) (cm ): 3447 (b), 2965 (s), 2933 (s), 2607 (s), 2497(s), 2331 (w), 1603 (s), 1579 (s),
475(s), 1424 (s), 1390 (s), 1297 (w), 1251 (w), 1233 (w), 1172 (w), 1070 (w), 1029 (s), 899
w), 844 (w), 798 (s), 611 (w). Anal. Calcd. for C H Cl Ho N O (2848.60): C, 32.04; H,
1
(
76
118
7
5
8
39.5
4.18; N, 3.93. Found: C, 31.73; H, 4.23; N, 3.61.
Electronic Supplementary Information (ESI): Molecular structure of 2, 3 (Figures S1ꢀS2), list
of bond length and bond angles (Tables S1ꢀS2), ColeꢀCole plot at zero dc field (Figure S3), χm′
–
1
vs. T plot at 3000 G (Figure S4), χ ′′ vs. T at 3000 G (Figure S5), τ vs. T (Figure S6) PXRD
m
(Figure S6ꢀS8), reported Ln5 complexes (Figure S9) and TGA (Figure S10ꢀS12).
Acknowledgements: We thank the Department of Science and Technology (DST), India, for
financial support, including support for a Single Crystal CCD Xꢀray Diffractometer facility at

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IITꢀKanpur. V.C. is grateful to the DST for a J. C. Bose fellowship. S.B. thanks Council of
Scientific and Industrial Research, India for Senior Research Fellowship.
References:
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and K. Binnemans, Eur. J. Org. Chem., 2005, 1810; (c) F. Pohlki, S. Doye, Chem. Soc. Rev.,
(
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003, 32, 104; (d) X. Yu, S.Y. Seo and M. J. Tobin, J. Am. Chem. Soc., 2007, 129, 7244.
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