Tetranuclear Lanthanide(III) Complexes Containing a Square-Grid Core: Synthesis, Structure, and Magnetism

Article abstract of DOI:10.1002/ejic.201600744

The reactions of Ln(NO₃)₃·5H₂O (Ln = Dy³⁺, Tb³⁺, Ho³⁺, Er³⁺) and a multidentate flexible ligand, (E)-N′-[2-hydroxy-3-(hydroxymethyl)-5-methylbenzylidene]-6-(hydroxymethyl)picolinohydrazide (LH₄), in the presence of Et₃N in a 1:1:3 molar ratio afforded a series of complexes [Ln₄(LH₂)₄(μ₂-OH)₄]·xCH₃OH·yH₂O (Dy³⁺, x = 2, y = 2; Tb³⁺, x = 4, y = 5; Ho³⁺, x = 0, y = 13; Er³⁺, x = 4, y = 6). X-ray diffraction analysis revealed that all the complexes are neutral and possess a distorted [2 × 2] square-grid core [Ln₄(μ₂-O)₄(μ₂-OH)₄] anchored by the concerted coordination of four doubly deprotonated ligands, (LH₂)^2–, and four μ₂-OH groups. All the Ln centers adopt a distorted triangular dodecahedral coordination geometry. An ac magnetic susceptibility study revealed undulations of the out-of-phase (χ_M′′) component above 2 K for 1 with a quantum tunnelling of magnetization tail at zero dc field. Surprisingly, a field-induced temperature dependence of the ac frequencies at which the χ_M′′ maxima occur implies that the slow relaxation is a result of a mixed contribution from more than one Dy³⁺center.

Full text of DOI:10.1002/ejic.201600744

DOI: 10.1002/ejic.201600744
Full Paper
Lanthanide Square Grids
Tetranuclear Lanthanide(III) Complexes Containing a Square-
Grid Core: Synthesis, Structure, and Magnetism
Sourav Biswas,[a] Sourav Das,[b] Sakiat Hossain,[a] Arun Kumar Bar,[c,d] Jean-Pascal Sutter,*[c,d]
and Vadapalli Chandrasekhar*[a,e]
Abstract: The reactions of Ln(NO3)3·5H2O (Ln = Dy3+, Tb3+
Ho3+, Er3+) and a multidentate flexible ligand, (E)-N′-[2-hydroxy- torted triangular dodecahedral coordination geometry. An ac
3-(hydroxymethyl)-5-methylbenzylidene]-6-(hydroxymethyl)pic- magnetic susceptibility study revealed undulations of the out-
olinohydrazide (LH4), in the presence of Et3N in a 1:1:3 molar of-phase (ꢀM′′) component above 2 K for 1 with a quantum
ratio afforded
,
(LH2)2–, and four μ2-OH groups. All the Ln centers adopt a dis-
a
series of complexes [Ln4(LH2)4(μ2-OH)4]· tunnelling of magnetization tail at zero dc field. Surprisingly, a
xCH3OH·yH2O (Dy3+, x = 2, y = 2; Tb3+, x = 4, y = 5; Ho3+, x = 0, field-induced temperature dependence of the ac frequencies at
y = 13; Er3+, x = 4, y = 6). X-ray diffraction analysis revealed that which the ꢀM′′ maxima occur implies that the slow relaxation
all the complexes are neutral and possess a distorted [2 × 2] is a result of a mixed contribution from more than one Dy3+
square-grid core [Ln4(μ2-O)4(μ2-OH)4] anchored by the con- center.
certed coordination of four doubly deprotonated ligands,
Introduction
below a certain temperature (called the blocking temperature,
TB). The super-paramagnet-like behavior of such systems seems
to arise from a large ground-state spin (S) in combination with
a high uniaxial magnetic anisotropy (D) leading to an energy
barrier (U) to magnetization reversal. U can be defined as |D|S2
for integer values of S and |D|(S2 – 1/4) for half-integer values
of S.[1b,6] With this in mind, several strategies have been
adopted by synthetic chemists to assemble these materials.
These include the preparation of polynuclear 3d,[7] 3d/4f,[8] and
4f compounds.[9] Among such systems, the most fascinating be-
haviors are exhibited by Ln-based complexes, particularly those
Since the seminal finding that the dodecanuclear complex
[Mn12O12(OAc)16(H2O)4]·2HOAc·4H2O[1] acts as a single-mol-
ecule magnet, tremendous efforts are being made in the field
of molecular magnets to improve their characteristics. The ex-
citement and interest in this field have arisen for several rea-
sons. For example, at a fundamental level, these compounds
and the phenomenon they exhibit are of relevance to physicists
interested in quantum phenomenon such as quantum compu-
tation[2] and quantum tunneling.[3] Moreover, these systems are
projected to have several applications ranging from high-den-
sity information storage[3c,4] to magnetic refrigeration.[5]
involving Dy3+, Tb3+, Ho3+, and Er3+ [10]
These ions have a large
.
D parameter because of their diffuse valance orbitals and con-
sequent significant unquenched orbital angular momentum,
which renders strong spin–orbital coupling (SOC). Recent re-
ports describing systems such as [Dy4K2O(OtBu)12],[11]
[Dy5O(OiPr)13],[12] [Cp2Dy(thf)(μ-Cl)][13] (Cp = cyclopentadiene),
Molecular magnets, such as single-molecule magnets
(SMMs), single-ion magnets (SIMs), and single-chain magnets
(SCMs), are characterized by a slow relaxation of magnetization
and [(Cp*2Ln)2(μ-bpym)]+ (Cp*
dienyl; bpym = 2,2′-bipyrimidine; Ln = Gd3+, Tb3+ [14]
=
pentamethylcyclopenta-
have moti-
)
[a] Department of Chemistry, Indian Institute of Technology Kanpur,
Kanpur 208016, India
vated synthetic chemists to prepare new families of lanthanide
complexes with the hope of achieving a new family of
SMMs.
E-mail: vc@iitk.ac.in
vc@niser.ac.in
[b] Department of Chemistry, Institute of Infrastructure Technology Research
and Management,
Near Khokhara Circle, Maninagar East, Ahmedabad 380026, India
[c] CNRS, LCC (Laboratoire de Chimie de Coordination),
205 route de Narbonne, 31077 Toulouse, France
E-mail: jean-pascal.sutter@lcc-toulouse.fr
We have been working for some time on 3d/4f[8i,8m,15] and
4f[16] complexes. Interesting SMM behavior has been observed,
in particular, in the Ln4 family of complexes [{(LH)2Ln4}-
(μ2-O)4](H2O)8 {Ln = Dy3+, Ho3+; LH3 = (6-hydroxymethyl)-N′-[(8-
hydroxyquinolin-2-yl)methylene]picolinohydrazide},[16a] which
possess rhombus-shaped geometry (Scheme 1, a). Notably, we
observed two relaxation time constants in such complexes and
these were attributed to differences in geometry at the Dy3+
ions. In addition, we also prepared Ln4 complexes containing
two dinuclear sub-units (Scheme 1, b).[17] Very recently we also
[d] Université de Toulouse, UPS, INPT, LCC,
31077 Toulouse Cedex 4, France
[e] National Institute of Science Education and Research,
Jatni Campus, PO: Bhimpur-Padanpur, Bhubaneswar 752050, Orissa, India
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejic.201600744.
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reported planar square-grid complexes, but none of these Results and Discussion
showed SMM behavior.[17,18] These results as well as the fact
Aroyl hydrazone ligands are proving to be quite versatile in
affording polynuclear lanthanide complexes.[19] The features
that can be modulated in this ligand system are the compart-
mental coordination pockets, keto–enol tautomerism, which
can allow either the keto or the enolate form to be involved in
binding, the hydrazone nitrogen atoms and other coordinating
groups that can be incorporated at the periphery of the ligand.
In addition, such ligands seem to possess coordination flexibil-
ity to suit the needs of the interacting metal ions. In previous
work we exploited these concepts to construct two families of
Ln4 assemblies: 1) rhombus-shaped complexes, in which the
enolate form of the ligand (6-hydroxymethyl)-N′-[(8-hydroxy-
quinolin-2-yl)methylene]picolinohydrazide is involved in bind-
ing (Scheme 1, a) and 2) a tetranuclear complex containing two
dinuclear sub-units (Scheme 1, b). In the latter case, the ligand
[N′-(2-hydroxy-3-methoxybenzylidene)-6-(hydroxymethyl)picol-
inohydrazide] is involved in linking the two dinuclear units by
involving both the keto and enol forms of the ligand, a feature
that also attests to the conformational flexibility of such li-
gands.[17]
that planar square-shaped Ln4 cores are quite rare motivated
us to examine whether new synthetic strategies could lead to
new SMM families of tetranuclear lanthanide complexes con-
taining planar (square) cores.
It is worth mentioning that four homometallic Ln4 [2 × 2]
square-grid complexes have been reported in the litera-
ture[18,20] that exploit similar design features. With this in mind,
we synthesized a new aroyl hydrazone based ligand, (E)-N′-
[2-hydroxy-3-(hydroxymethyl)-5-methylbenzylidene]-6-
(hydroxymethyl)picolinohydrazide (LH4), following a two-step
synthetic protocol (Scheme 2). The multidentate ligand LH4
consists of divergent coordinating sites such as a pyridyl and
imino N atom, which can effectively bind to lanthanide centers,
a hydrazone oxygen, which can either undergo enolization or
retain its keto form, a phenolic OH and a pendant CH2OH,
which can proliferate the nuclearity of the lanthanide com-
plexes depending upon the degree of their deprotonation.
The reaction of Ln(NO3)3·5H2O and LH4 in methanol in a 1:1
stoichiometry in the presence of 3 equiv. of triethylamine led
to the formation of neutral homometallic Ln4 complexes
[Dy4(LH2)4(μ2-OH)4]·2CH3OH·2H2O (1), [Tb4(LH2)4(μ2-OH)4]·
4CH3OH·5H2O (2) [Ho4(LH2)4(μ2-OH)4]·13H2O (3), and
[Er4(LH2)4(μ2-OH)4]·4CH3OH·6H2O (4; Scheme 3). The organiza-
tion of the coordinating sites in the enolized form of the ligand
leads to two effective ONO coordination pockets each of which
holds one lanthanide center (Scheme 4, a).
Scheme 1. a) Rhombus-shaped Ln4 complexes.[16a] b) Ln4 complexes in which
both the keto and enol forms of the ligand are involved in binding.[17]
We report herein a new multidentate aroyl hydrazone ligand,
(E)-N′-[2-hydroxy-3-(hydroxymethyl)-5-methylbenzylidene]-6-
(hydroxymethyl)picolinohydrazide (LH4; see Scheme 2), which,
upon reaction with Ln(NO3)3·5H2O (Ln = Dy3+, Tb3+, Ho3+, Er3+
)
afforded
[Ln4(LH2)4(μ2-OH)4]·xCH3OH·yH2O [Dy3+, x = 2, y = 2 (1); Tb3+
x = 4, y = 5 (2); Ho3+, x = 0, y = 13 (3); Er3+, x = 4, y = 6
a
series of distorted square-grid complexes
,
To investigate the structural integrity of the complexes in
(4)]. The synthesis, structures, and magnetic behavior of these solution, we carried out ESI-MS analyses (see the Exp. Sect.).
complexes are discussed herein.
The appearance of prominent peaks at around 1000.60, 979.07,
Scheme 2. Synthesis of LH4.
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Figure 1. Full-range ESI-MS spectrum of complex 1 (top). Experimental (bot-
tom left) and simulated (bottom right) isotopic distribution of the ionized
species [C64H62Dy4N12O18 + 2MeOH]2+
.
Scheme 3. Synthesis of tetrametallic Ln4 complexes 1–4.
X-ray Crystal Structures of 1–4
Complexes 1–4 crystallize in the triclinic system with space
group P1¯. All these complexes are neutral and possess the same
square-grid structural topology (Figure 2). In view of this, only
the structure of 1 is discussed as a representative example. The
perspective view of the molecular structure 1 is depicted in
Figure 2, and the structures of 2–4 are given in the Supporting
Information (Figures S4–S6). Selected bond lengths and angles
of 1 are given in Table 1 and those of 2–4 are given in
Tables S1–S3.
The formation of the tetranuclear complexes can be de-
scribed in the following way. Each ligand in its dianionic form
(LH2)2– adopts a μ2-η1:η1:η2:η1:η1 coordination mode (Scheme 4,
b) to accommodate two Dy3+ centers in its two tridentate coor-
dinating pockets. Importantly, the ligands coordinate solely in
their enol form.
Scheme 4. a) Two coordinating pockets of [LH2]2– in its enolized form and
b) binding mode of the ligand in its enolized form with Dy3+ ions.
996.08, and 1010.10 can be attributed to the dicationic species
The cumulative coordinative action of four enolized di-
anionic (LH2)2– ligands leads to the construction of the square-
grid core that incorporates Dy3+ ions at the corners. In addition
[C64H62Dy4N12O18 + 2MeOH]2+, [C64H62Tb4N12O18 + 2H2O]2+
,
[C64H62Ho4N12O18 + H2O + MeOH]2+, and [C64H62Er4N12O18
+
2MeOH]2+, respectively, due to the loss of two OH groups from to the binding provided by (LH2)2–, the four Dy3+ centers are
each of the complexes. These results suggest the existence of also tightly held together by four μ-OH groups, which contrib-
tetranuclear units in solution for all the complexes. The ESI-MS ute additional strength to the cluster. Notably, the ligands in-
spectrum of complex 1 is shown in Figure 1 as a representative volved in holding each pair of this square grid project upwards
example, those of 2–4 are given in the Supporting Information in a slightly tilted manner with respect to each other such that
(Figures S1–S3).
the steric congestion is minimized.
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Table 1. Selected bond lengths and bond angles for 1.
Bond lengths [Å] around Dy3+ Bond angles [°] around Dy3+
Bond angles [°] around Dy3+
Bond angles [°] around Dy3+
Dy1–O9
Dy1–O1
Dy1–O6
Dy1–O7
Dy1–O12
Dy1–O17
Dy1–N6
Dy1–N1
Dy1–Dy3
Dy1–Dy2
Dy2–O20
Dy2–O4
Dy2–O5
Dy2–O10
Dy2–O6
Dy2–O17
Dy2–N4
Dy2–N12
Dy2–Dy4
Dy3–O11
Dy3–O1
Dy3–O7
Dy3–O8
Dy3–O3
Dy3–O16
Dy3–N7
Dy3–N3
Dy3–Dy4
Dy4–O18
Dy4–O4
Dy4–O8
Dy4–O5
Dy4–O3
Dy4–O19
Dy4–N10
Dy4–N9
2.167(7)
2.271(6)
2.318(7)
2.380(6)
2.408(6)
2.491(7)
2.522(8)
2.535(8)
3.8231(7)
3.9271(8)
2.204(7)
2.286(6)
2.352(7)
2.358(8)
2.384(7)
2.411(7)
2.483(9)
2.560(9)
3.8375(8)
2.192(7)
2.319(6)
2.333(6)
2.344(7)
2.363(7)
2.397(8)
2.497(8)
2.544(8)
3.8684(8)
2.231(8)
2.248(6)
2.337(7)
2.356(7)
2.393(8)
2.426(7)
2.516(10)
2.573(9)
O7–Dy1–N6
143.8(2)
81.9(2)
134.1(2)
74.4(3)
132.3(2)
131.4(3)
64.2(2)
O10–Dy2–Dy4
O6–Dy2–Dy4
O17–Dy2–Dy4
N4–Dy2–Dy4
N12–Dy2–Dy4
O20–Dy2–Dy1
O4–Dy2–Dy1
O5–Dy2–Dy1
O10–Dy2–Dy1
O6–Dy2–Dy1
O17–Dy2–Dy1
N4–Dy2–Dy1
N12–Dy2–Dy1
Dy4–Dy2–Dy1
O11–Dy3–O1
O11–Dy3–O7
O1–Dy3–O7
O11–Dy3–O8
O1–Dy3–O8
O7–Dy3–O8
O11–Dy3–O3
O1–Dy3–O3
O7–Dy3–O3
O8–Dy3–O3
O11–Dy3–O16
O1–Dy3–O16
O7–Dy3–O16
O8–Dy3–O16
O3–Dy3–O16
O11–Dy3–N7
O1–Dy3–N7
O7–Dy3–N7
O8–Dy3–N7
O3–Dy3–N7
O16–Dy3–N7
O11–Dy3–N3
O1–Dy3–N3
O7–Dy3–N3
O8–Dy3–N3
O3–Dy3–N3
O16–Dy3–N3
N7–Dy3–N3
95.6(2)
O3–Dy4–O19
O18–Dy4–N10
O4–Dy4–N10
O8–Dy4–N10
O5–Dy4–N10
O3–Dy4–N10
O19–Dy4–N10
O18–Dy4–N9
O4–Dy4–N9
87.5(3)
78.6(3)
131.2(3)
134.8(3)
65.0(3)
80.7(3)
63.6(3)
71.0(3)
97.0(3)
O12–Dy1–N6
O17–Dy1–N6
O9–Dy1–N1
O1–Dy1–N1
O6–Dy1–N1
104.87(15)
74.26(14)
109.57(17)
97.7(3)
83.8(2)
O7–Dy1–N1
76.51(16)
112.6(2)
162.6(2)
32.83(16)
37.45(16)
97.5(2)
115.1(3)
89.836(15)
157.2(2)
134.6(2)
68.2(2)
80.8(2)
79.1(2)
136.7(2)
93.7(3)
89.5(2)
O12–Dy1–N1
O17–Dy1–N1
N6–Dy1–N1
63.4(2)
79.9(2)
131.1(2)
101.47(17)
33.98(16)
106.74(15)
35.39(15)
154.93(17)
71.98(14)
122.58(16)
98.56(17)
168.56(17)
82.60(18)
33.90(16)
107.14(18)
81.29(18)
36.06(16)
98.07(19)
108.6(2)
89.140(14)
155.9(3)
135.3(3)
66.6(2)
O8–Dy4–N9
O5–Dy4–N9
O3–Dy4–N9
62.9(3)
O9–Dy1–Dy3
O1–Dy1–Dy3
O6–Dy1–Dy3
O7–Dy1–Dy3
O12–Dy1–Dy3
O17–Dy1–Dy3
N6–Dy1–Dy3
N1–Dy1–Dy3
O9–Dy1–Dy2
O1–Dy1–Dy2
O6–Dy1–Dy2
O7–Dy1–Dy2
O12–Dy1–Dy2
O17–Dy1–Dy2
N6–Dy1–Dy2
N1–Dy1–Dy2
Dy3–Dy1–Dy2
O20–Dy2–O4
O20–Dy2–O5
O4–Dy2–O5
O20–Dy2–O10
O4–Dy2–O10
O5–Dy2–O10
O20–Dy2–O6
O4–Dy2–O6
O5–Dy2–O6
O10–Dy2–O6
O20–Dy2–O17
O4–Dy2–O17
O5–Dy2–O17
O10–Dy2–O17
O6–Dy2–O17
O20–Dy2–N4
O4–Dy2–N4
143.5(3)
131.5(3)
76.4(3)
127.7(3)
97.2(2)
O19–Dy4–N9
N10–Dy4–N9
O18–Dy4–Dy2
O4–Dy4–Dy2
O8–Dy4–Dy2
O5–Dy4–Dy2
O3–Dy4–Dy2
O19–Dy4–Dy2
N10–Dy4–Dy2
N9–Dy4–Dy2
O18–Dy4–Dy3
O4–Dy4–Dy3
O8–Dy4–Dy3
O5–Dy4–Dy3
O3–Dy4–Dy3
O19–Dy4–Dy3
N10–Dy4–Dy3
N9–Dy4–Dy3
Dy2–Dy4–Dy3
Dy2–O17–Dy1
Dy3–O3–Dy4
Dy1–O6–Dy2
N(8)–N9–Dy4
N(5)–N6–Dy1
N(2)–N3–Dy3
Dy4–O8–Dy3
Dy4–O4–Dy2
Dy1–O1–Dy3
Dy3–O7–Dy1
Dy2–O5–Dy4
N11–N12–Dy2
Dy1–Dy3–Dy4
O18–Dy4–O4
O18–Dy4–O8
O4–Dy4–O8
O18–Dy4–O5
O4–Dy4–O5
O8–Dy4–O5
O18–Dy4–O3
O4–Dy4–O3
O8–Dy4–O3
O5–Dy4–O3
O18–Dy4–O19
O4–Dy4–O19
O8–Dy4–O19
O5–Dy4–O19
32.48(15)
105.39(16)
35.37(17)
75.15(16)
157.39(18)
98.7(2)
126.0(2)
167.9(2)
78.28(17)
34.34(17)
110.6(2)
35.33(17)
83.99(19)
110.2(3)
96.9(2)
89.800(15)
106.5(2)
108.8(3)
113.3(3)
117.5(6)
117.4(6)
117.4(5)
111.4(3)
115.6(3)
112.8(3)
108.4(3)
109.2(3)
116.8(7)
90.933(15)
102.5(3)
133.6(3)
79.1(2)
81.8(3)
70.0(2)
91.3(3)
93.2(3)
81.2(2)
129.4(2)
160.5(2)
76.3(3)
85.5(2)
135.4(3)
65.3(3)
135.2(3)
64.3(3)
71.2(2)
131.6(2)
63.6(2)
137.1(3)
79.8(3)
94.9(3)
99.7(3)
80.4(3)
77.6(3)
78.3(2)
137.7(2)
129.9(3)
91.7(3)
81.1(2)
81.3(3)
159.7(3)
70.3(2)
86.4(3)
82.6(2)
128.8(3)
65.1(3)
65.1(3)
134.6(3)
71.6(3)
129.6(3)
63.8(3)
80.6(3)
138.5(3)
83.3(3)
137.4(3)
163.5(2)
31.88(15)
35.44(17)
Bond angles [°] around Dy3+
O9–Dy1–O1
O9–Dy1–O6
O1–Dy1–O6
O9–Dy1–O7
O1–Dy1–O7
O6–Dy1–O7
O9–Dy1–O12
O1–Dy1–O12
O6–Dy1–O12
O7–Dy1–O12
O9–Dy1–O17
O1–Dy1–O17
O6–Dy1–O17
O7–Dy1–O17
O12–Dy1–O17
O9–Dy1–N6
O1–Dy1–N6
O6–Dy1–N6
103.8(3)
136.3(2)
83.4(2)
84.2(2)
68.2(2)
136.2(2)
90.6(3)
160.9(2)
77.6(2)
126.7(2)
152.2(2)
85.9(2)
69.9(2)
75.2(2)
87.1(2)
72.7(3)
90.4(2)
64.2(2)
84.0(3)
133.5(3)
168.08(18)
33.19(15)
36.20(16)
104.15(16)
78.18(16)
93.65(17)
115.61(19)
98.56(17)
87.4(2)
82.08(17)
111.15(18)
34.22(16)
35.85(18)
163.54(17)
99.5(2)
O11–Dy3–Dy1
O1–Dy3–Dy1
O7–Dy3–Dy1
O8–Dy3–Dy1
O3–Dy3–Dy1
O16–Dy3–Dy1
N7–Dy3–Dy1
N3–Dy3–Dy1
O11–Dy3–Dy4
O1–Dy3–Dy4
O7–Dy3–Dy4
O8–Dy3–Dy4
O3–Dy3–Dy4
O16–Dy3–Dy4
N7–Dy3–Dy4
N3–Dy3–Dy4
O5–Dy2–N4
O10–Dy2–N4
O6–Dy2–N4
O17–Dy2–N4
O20–Dy2–N12
O4–Dy2–N12
O5–Dy2–N12
O10–Dy2–N12
O6–Dy2–N12
O17–Dy2–N12
N4–Dy2–N12
O20–Dy2–Dy4
O4–Dy2–Dy4
O5–Dy2–Dy4
80.5(3)
67.1(2)
137.4(2)
156.5(3)
83.1(2)
69.7(2)
80.9(3)
93.0(3)
160.2(2)
81.3(2)
111.0(2)
128.5(2)
The [Dy4(μ2-O)4(μ2-OH)4] core consists of four four-mem- dination geometry and are coordinated by six oxygen and two
bered Dy2O2 sub-units (Figure 2, bottom), analogous to the nitrogen atoms (Figure 3, top). In spite of having the same coor-
cores reported previously by us[18] as well as Murugesu,[20c] dination environment around the Dy3+ ions, all the Dy–Npy dis-
Thompson,[20b] and Tang[20a] and their co-workers. Each of the tances fall in the range 2.476–2.539 Å whereas the Dy–Ophen
Dy3+ centers possesses a distorted trigonal dodecahedron coor- distances range from 2.164–2.238 Å.
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The Dy–μ-OH–Dy bond angles vary over a wider range
(108.6–115.9°) than the Dy–μ-Ohydrazone–Dy bond angles (109.1–
111.5°), which may account for the slight distortion in the
square grid. In addition to this, the Dy–Dy distances in the
square grid lie in the range 3.82–3.93 Å (Figure 3, bottom).
Precedents involving Ln4 complexes containing a square-grid
geometry (Scheme 5) involving mainly symmetric dihydrazide
ligands have been reported in the literature.[18,20a,20b] We have
explored aroyl hydrazone ligands.[19a,19b] The structural features
of three known tetranuclear Dy3+ square-grid complexes are
summarized in Table 2 and compared with those of complex 1.
Of the three literature precedents, one has four μ-OH ligands,
Figure 2. Top: ORTEP diagram of 1. Thermal ellipsoids are drawn at the 40 %
probability level. Hydrogen atoms, solvent molecules and the disordered
parts of the ligands have been omitted for clarity. Bottom: [2 × 2] square-
grid core of complex 1.
Figure 3. Distorted triangular dodecahedral geometry around the Dy3+ ion
(top) and the distorted square-grid core highlighting the distances between
consecutive metal centers (bottom) of 1.
Scheme 5. Square-grid Dy4 complexes reported in the literature.[18,20a,20b]
5 © 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Table 2. Comparison of bond lengths and angles in the square-grid core of Ln4 complexes.
Ligand
Dy–Npy [Å]
Dy–Ophen [Å] Dy–Nimmino [Å] Dy–Dy [Å]
Dy–O–Dy [°]
[Dy4(HL)4(MeOH)4][20b]
LH4 = N′,N′′-(butane-2,3-diylidene)bis(2-hydroxy-3-methoxybenzohydrazide)
2.27–2.37
2.45–2.49
2.52–2.53
2.47–2.59
2.51–2.56
3.82–3.87
3.76–3.79
3.78–3.82
3.82–3.92
107.2–109.9
107.3–108.4
107.2–110.8
109.1–111.5
[21]
[Dy4(L)4(OH)4]Cl2
2.540–2.56
2.48–2.576
2.47–2.53
LH2 = 2-[1-(pyridin-2-yl)ethylidene]hydrazinyl hydrazide
[19]
[Dy4(LH)4(μ2-OH)3(μ2-OMe)]·4NO3
LH2 = 6-(hydroxymethyl)-N′-[1-(pyridin-2-yl)ethylidene]picolinohydrazide
[Dy4(LH2)4(μ2-OH)4] (1)
2.16–2.23
LH4 = (E)-N′-[2-hydroxy-3-(hydroxymethyl)-5-methylbenzylidene]-6-(hydroxy-
methyl)picolinohydrazide
(this work)
similarly to the present instance,[20c] another has three μ-OH
Figure 4 shows the plots of ꢀM′′ versus T for frequencies be-
and one μ-OMe ligand,[18] and in the last example,[20a] phenol- tween 28 and 1500 Hz and ꢀM′′ versus frequency for tempera-
ate oxygen atoms serve the purpose of bridging the lanthanide tures between 2 and 6 K. Clearly, these plots are not consistent
ions (Scheme 5, b). Also, the distortion observed in the square with the behavior expected for an SMM. In the former, rather
grid in complex 1 is larger than in two of the literature prece- than a displacement of the maximum of ꢀM′′ to a higher T with
dents,[20a,20c] (Table 2).
increasing frequency, the shape of the ꢀM′′ versus T curves
change. In particular, the undulations seen in the curves ob-
tained at 28, 50, and 100 Hz suggest that the overall ꢀM′′ signal
results from several contributions with similar temperature
maxima.
Magnetic Studies
The temperature dependence of ꢀMT (ꢀM is the molar suscepti-
bility) and the magnetization versus field at 2 K have been in-
vestigated for 1–4 (see Figure S7 in the Supporting Informa-
tion). All exhibit very similar behavior, therefore only the Dy3+
homologue 1 is described in detail below (further information
can be found in Figures S8–S10 in the Supporting Information).
For 1, the value for ꢀMT at 300 K was found to be
56.0 cm3 K mol–1 (see Figure S7, left), in good agreement with
the expected contribution of four Dy3+ ions (i.e.,
56.7 cm3 K mol–1) in the absence of exchange interactions. This
value diminishes as T is lowered to reach 43.9 cm3 K mol–1 at
2 K, behavior that can be ascribed mainly to the crystal field
effect on Dy3+ [21]
The rather large ꢀMT observed at 2 K is indica-
.
tive of very weak, if any, exchange interactions between the
Ln3+ centers. Similar behavior was found for complexes 2–4 (see
Figure S7).
The field dependence of the magnetization observed at low
T for all complexes (see Figure S7, right) is characterized by a
rapid increase at weak fields followed by a more gradual but
steady increase at higher fields. The values reached for a field
of 5 T at 2 K are 20.9 μB (1), 18.2 μB (2), 20.1 μB (3), and 17.5 μB
(4).
The slow dynamics of the relaxation of magnetization were
examined for all complexes by ac susceptibility measurements.
An out-of-phase (ꢀM′′) signal was found for 1 below 15 K in the
absence of an applied dc field, but the tail characteristic of
quantum tunneling of magnetization (QTM) was observed. This
QTM was reduced in the presence of a dc magnetic field, but
the shape of the ꢀM′′ versus T curve broadened as HDC increased
(see Figure S11 in the Supporting Information). The signal ob-
tained at HDC = 3 kOe appears to be the best compromise be-
tween reduced QTM and peak broadening, and therefore a full
data set for different frequencies was collected with this applied
field.
Figure 4. ꢀM′′ vs. T plots for different frequencies (top) and ꢀM′′ vs. frequency
for temperatures between 2.4 and 5.9 K (bottom). The susceptibility data
were recorded at HDC = 3 kOe.
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For frequencies of 200 Hz and above, this leads to a broad Experimental Section
signal, the intensity of which increases with frequency and the
Solvents and other general reagents used in this work were purified
according to standard procedures.[24] Pyridine-2,6-dicarboxylic
acid, sodium borohydride, 2,6-bis(hydroxymethyl)-4-methylphenol,
activated manganese(IV) dioxide (MnO2), Dy(NO3)3·5H2O,
Tb(NO3)3·5H2O, Ho(NO3)3·5H2O, and Er(NO3)3·5H2O were obtained
from Sigma Aldrich Chemical Co. and were used as received. Hydra-
zine hydrate (80 %) and sodium sulfate (anhydrous) were obtained
from SD Fine Chemicals, Mumbai, India, and were used as such.
Methyl 6-(hydroxymethyl)picolinate, 6-(hydroxymethyl)picolino-
hydrazide,[17] and 6-formyl-2-(hydroxymethyl)-4-methylphenol[8h]
were prepared according to literature procedures.
maximum is slightly shifted to higher T. Such behavior could
be rationalized by considering independent contributions from
more than one (if not four) of the crystallographically independ-
ent Dy3+ centers[16a,17] with ꢀM′′ maxima occurring at a similar
temperature, hence resulting in the overlap of individual sig-
nals. In no case can this shift in T be considered as a shift of a
given maximum, as is usually seen for SMMs. This is supported
by the plot of ꢀM′′ versus frequency (Figure 4, bottom), which
does not show the displacement of the maximum towards
higher frequency as a function of temperature, at least not in
the frequency domain (<1500 Hz) that we can investigate. In
such a situation, the extraction of any information regarding
the energy barrier for spin reversal does not make sense. Ab
initio calculations in combination with spectroscopic investiga-
tions could provide an insight into the actual relaxation proc-
esses operating in this complex.[22] However, the overall modest
performances exhibited by the complex do not justify such de-
manding investigations, moreover poly-Ln complexes showing
independent multi-relaxation features are well docu-
mented.[10a,10f,10j,16a,17,22,23]
Instrumentation: Melting points were measured with a JSGW melt-
ing-point apparatus. IR spectra were recorded as KBr pellets with a
Bruker Vector 22 FT IR spectrophotometer operating at 400–
4000 cm–1. Elemental analyses of the compounds were performed
with a Thermoquest CE CHNS-O instrument, EA/110 model. Electro-
spray ionization mass spectrometry (ESI-MS) was carried out in
methanol with a Micromass Quattro II triple quadrupole mass spec-
trometer with an applied cone voltage of 5 V and a capillary voltage
of 3.5 kV. 1H NMR spectra were recorded in CD3OD solutions with
a JEOL JNM LAMBDA 400 spectrometer operating at 500 MHz.
Chemical shifts are reported in parts per million (ppm) and are
referenced to internal tetramethylsilane (1H). Powder X-ray diffrac-
tion (PXRD) patterns were recorded with a Bruker D8 advance dif-
fractometer equipped with nickel-filtered Cu-Kα radiation. The pow-
der X-ray diffraction (PXRD) patterns of complexes 1–4 are in good
agreement with the simulated patterns (see Figures S11–S14 in the
Supporting Information). The difference in intensities could be due
to the preferred orientation in the powder samples.
For the Tb4 (2) and Ho4 (3) derivatives, the ac susceptibly
data did not show a signal deviating from zero for ꢀM′′ even
when a dc field was applied. For the Er4 complex (4), only the
onset of the signal for ꢀM′′ was found above 2 K, even in the
presence of an applied dc field (see Figure S10 in the Support-
ing Information).
Magnetic Measurements: Magnetic measurements on all samples
were carried out with a Quantum Design MPMS 5S SQUID magne-
tometer in the temperature range 2–300 K. The measurements were
performed on polycrystalline samples isolated from crystallization
medium just before SQUID studies. The crystalline powders were
mixed with grease and placed in gelatine capsules. The magnetic
Conclusions
We have reported herein the synthesis and structural and mag- susceptibilities were measured in an applied field of 1 kOe. The
molar susceptibilities (ꢀM) were corrected for the sample holder,
grease, and for the diamagnetic contributions of all atoms by using
Pascal's tables.[25] Field-dependence magnetizations have been re-
corded at 2 K. The ac susceptibilities were collected for HAC = 3 Oe
and frequencies up to 1500 Hz. All data are plotted for a Ln4 molec-
ular complex.
netic characterization of homometallic tetranuclear lanthanide
(Ln4) complexes [Ln = Dy3+ (1), Tb3+ (2), Ho3+ (3), Er3+ (4)]. The
distorted [2 × 2] square-grid core was assembled by employing
a multidentate aroyl hydrazone based ligand that coordinates
exclusively in the enol form to hold one lanthanide center in
each of the two distinct pockets with the core further strength-
ened by four μ2-OH ligands. Regarding the magnetic behavior
of these complexes and especially the ac magnetic susceptibil-
ity, slow relaxation of the magnetization was found for the Dy3+
and Er3+ derivatives at low temperature. However, the out-of-
phase component suggests that several magnetic sites contrib-
ute concomitantly to the observed signal. Such behavior is of-
X-ray Crystallography: The crystal data of all the compounds were
collected with a Bruker SMART CCD diffractometer (Mo-Kα radiation,
λ = 0.71073 Å). The SMART[26] program was used to collect frames
of data, indexing reflections, and determining lattice parameters,
SAINT[26] for integration of the intensities of reflections and scaling,
SADABS[27] for absorption correction, and SHELXTL[28] for space-
group determination and least-squares refinements on F2. Structure
ten found in polynuclear Ln3+ compounds and results from the solution and refinement were performed by using the SHELXT and
SHELXL-2014/7 programs in the WinGX software package.[29] The
best-fit models for the data sets were satisfactorily good. In spite of
our best efforts to obtain the best quality data, several atoms in
each of the structures incorporate thermal disorder, especially the
phenyl and pyridyl rings, and the terminal CH2OH and methyl
groups in the main residues. In addition to these, the interstitial
solvent molecules are highly disordered. Hence, we employed sev-
eral restraints/constraints to achieve refinement stability. In the
cases of very large thermal displacements, we partitioned the elec-
tron densities of the corresponding atoms into two positions and
some of these atoms were refined isotropically. We could not assign
structural differences at the Ln3+ centers that modulate the in-
dividual SMM characteristics. In the present case, all four inde-
pendent Ln3+ have similar coordination spheres and therefore
are likely to exhibit similar characteristics for the relaxation dy-
namics of their magnetization. As a result, their magnetic signa-
tures occur within a very narrow temperature domain. An open
question remains the possible role of exchange coupling; unfor-
tunately it proved impossible to obtain the related Gd3+ deriva-
tive that would have revealed its occurrence in these com-
pounds.
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all the interstitial solvent molecules due to high disorder and weak
residual Q peaks and hence they were squeezed out with the help
of the PLATON/SQUEEZE program.[30] The void volumes and the
possible squeezed out electron counts have been included in the
corresponding CIFs. The treatment of disorder and refinement of
the structures as well as the plausible origins of the alerts in the
checkCIFs of the complexes are described in detail in the Support-
ing Information. The CIFs of all the complexes are provided in the
Supporting Information. Crystal data and refinement parameters
are summarized in Table 3. The ORTEP diagrams of complexes 1–4
are portrayed in Figure 2 and Figures S4–S6. In the case of the
positionally disordered solvents/main residue atoms partitioned at
two or three positions, only the positions of highest occupancy are
shown in the ORTEP diagrams for clarity (Figure 2, a and Figures S4–
S6).
H, Py-H), 8.08 (d, 1 H, Py-H), 8.56 (s, 1 H, imino-H) ppm. C16H17N3O4
(315.33): calcd. C 60.94, H 5.43, N 13.33; found C 61.57, H 5.77, N
12.10. MS (ESI): m/z = 316.12 [M + H]+.
General Synthetic Procedure for the Preparation of the Com-
plexes 1–4: All the metal complexes 1–4 were prepared by follow-
ing the same procedure as outlined here. Ln(NO3)3·5H2O
(0.13 mmol) was added to a stirring solution of LH4 (0.041 g,
0.13 mmol) in methanol (10 mL) to form an intense yellow solution.
After 15 min of stirring at room temperature, triethylamine
(0.051 mL, 0.39 mmol) was added dropwise and the resulting solu-
tion was stirred for a further 12 h at room temperature. Then the
solution was filtered and kept for crystallization. Slow evaporation
of the mother liquor for about 9 d led to yellow crystals that were
suitable for X-ray diffraction. Specific details of each reaction and
the characterization data of the complexes obtained are given be-
low.
CCDC 1457003 (for 1), 1457004 (for 2), 1457005 (for 3) and 1457006
(for 4) contain the supplementary crystallographic data for this pa-
per. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre.
[Dy4(LH2)4(μ2-OH)4]·4CH3OH·4H2O (1): Quantities: LH4 (0.041 g,
0.13 mmol), Dy(NO3)3·5H2O (0.057 g, 0.13 mmol), Et3N (0.051 mL,
0.39 mmol), yield 0.038 g, 51.47 % (based on Dy3+), m.p. 200 °C
(decomp.). IR (KBr): ν = 3387 (br), 2923 (w), 1616 (s), 1596 (s), 1564
˜
(E)-N′-[2-Hydroxy-3-(hydroxymethyl)-5-methylbenzylidene]-6-
(hydroxymethyl)picolinohydrazide (LH4): An ethanolic solution of
6-formyl-2-(hydroxymethyl)-4-methylphenol (1.10 g, 6.56 mmol)
was added dropwise to a stirred solution of 6-(hydroxymethyl)pico-
linohydrazide (1.09 g, 6.56 mmol) in ethanol (60 mL) and the result-
ing solution was heated at reflux for 4 h. Then the solution was
concentrated in vacuo to 20 mL and kept in a refrigerator at 0 °C
overnight. A white precipitate was obtained that was filtered,
washed with cold ethanol and diethyl ether, and dried, yield 1.61 g
(s), 1445 (w), 1384 (s), 1354 (s), 1263 (m), 1224 (m), 1185 (m), 1051
(w), 1015 (w), 849 (w), 818 (m), 757 (w) cm–1. MS (ESI): m/z = 1000.60
[C64H62Dy4N12O18 + 2MeOH]2+. C70H100Dy4N12O32 (1·4CH3OH·4H2O;
2271.6): calcd. C 37.01, H 4.44, N 7.4; found C 37.05, H 3.98, N 7.61.
[Tb4(LH2)4(μ2-OH)4]·7CH3OH·3H2O (2): Quantities: LH4 (0.041 g,
0.13 mmol), Tb(NO3)3·5H2O (0.056 g, 0.13 mmol), Et3N (0.051 mL,
0.39 mmol), yield 0.032 g, 44.05 % (based on Tb3+), m.p. 200 °C
(decomp.). IR (KBr): ν = 3391 (br), 2936 (w), 1616 (s), 1595 (s), 1564
˜
(76.92 %), m.p. 138 °C. FTIR (KBr): ν = 3336 (br), 3206 (br), 3054 (m),
(w), 1443 (m), 1384 (m), 1354 (s), 1263 (w), 1225 (w), 1184 (w), 1050
˜
2918 (m), 2873 (m), 1666 (s), 1587 (s), 1539 (s), 1477 (w), 1429 (w),
(w), 1015 (w), 868 (w), 848 (m), 757 (w) cm–1. MS (ESI): m/z = 979.07
1372 (w), 1339 (w), 1293 (w) cm–1. H NMR (500 MHz, CD3OD): δ = [C64H62Tb4N12O18 + 2H2O]2+. C71H98N12O30Tb4 ([Tb4(LH2)4-
1
2.34 (s, 3 H, Ar-Me), 4.60 (s, 2 H, Py-CH2OH), 4.81 (s, 2 H, Ar-CH2OH),
7.09 (s, 1 H, Ar-H), 7.26 (s, 1 H, Ar-H), 7.63 (d, 1 H, Py-H), 7.98 (t, 1
(μ2-OH)4]·7CH3OH·3H2O; 2235.3): calcd. C 38.15, H 4.42, N 7.52;
found C 38.13, H 3.79, N 7.63.
Table 3. Crystal data and structure refinement parameters of 1–4.
1
2
3
4
Formula
C66H55Dy4N12O24
2050.22
C68H53N12O29Tb4
2137.95
C64H55Ho4N12O33
2179.92
C66H56Er4N12O27
2118.27
M [g mol–1
]
Crystal system
Space group
a [Å]
b [Å]
c [Å]
triclinic
triclinic
triclinic
triclinic
P1
P1
P1
P1
¯
¯
¯
¯
14.131(5)
15.962(5)
19.114(5)
75.690(5)
79.916(5)
88.508(5)
4112(2)
13.677(5)
15.956(5)
19.067(5)
79.247(5)
77.702(5)
89.830(5)
3991(2)
14.108(5)
16.007(5)
18.684(5)
76.376(5)
80.418(5)
89.088(5)
4042(2)
13.971(5)
16.142(5)
18.797(5)
75.333(5)
81.295(5)
89.067(5)
4053(2)
α [°]
ꢀ [°]
γ [°]
V [Å3]
Z
2
2
2
2
ρcalcd. [g cm–3
]
1.656
3.667
1982
0.061 × 0.023 × 0.02
1.9 to 28.32
–18 ≤ h ≤ 18
–20 ≤ k ≤ 21
–25 ≤ l ≤ 24
34017
1.725
3.48
2074
0.08 × 0.04 × 0.01
2.01 to 24.88
–13 ≤ h ≤ 16
–18 ≤ k ≤ 18
–20 ≤ l ≤ 22
81049
1.791
3.962
2110
0.12 × 0.07 × 0.059
4.08 to 25.02
–16 ≤ h ≤ 16
–19 ≤ k ≤ 18
–17 ≤ l ≤ 22
20950
1.736
4.180
2048
0.07 × 0.03 × 0.02
1.9 to 28.4
–18 ≤ h ≤ 18
–21 ≤ k ≤ 21
–25 ≤ l ≤ 25
61397
μ [mm–1
F(000)
]
Crystal size [mm3]
θ range [°]
Limiting indices
Reflns. collected
Independent reflns.
Completeness to θ [%] 99
20297 [R(int) = 0.0526]
14282 [R(int) = 0.021]
99.7
13942 [R(int) = 0.0418]
97.6
14230 [R(int) = 0.0819]
99.7
Refinement method
full-matrix least-squares on F2
Goodness-of-fit on F2
Final R[a,b] indices
[I > 2θ(I)]
0.86
R1 = 0.0695
wR2 = 0.1830
0.859
R1 = 0.0532
wR2 = 0.1427
1.059
R1 = 0.0615
wR2 = 0.1685
0.95
R1 = 0.0792
wR2 = 0.1853
[a] R1 = Σ|Fo – Fc|/Σ|Fc|. [b] wR2 = {Σ[w(Fo – Fc2)2]/Σ[w(Fo2)2]}1/2
.
2
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0.39 mmol), yield 0.04 g, 57.06 % (based on Ho3+), m.p. 200 °C (de-
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comp.). IR (KBr): ν = 3388 (br), 2938 (w), 1617 (s), 1590 (s), 1561 (w),
˜
1443 (m), 1385 (m), 1354 (s), 1259 (w), 1224 (w), 1184 (w), 1050 (w),
1012 (w), 867 (w), 848 (m), 755 (w) cm–1. MS (ESI): m/z =
996.08 [C64H62Ho4N12O18 + H2O + MeOH]2+. C65H84Ho4N12O29
([Ho4(LH2)4(μ2-OH)4]·CH3OH·8H2O; 2157.15): calcd. C 36.19, H 3.92,
N 7.79; found C 36.01, H 3.47, N 7.97.
[Er4(LH2)4(μ2-OH)4]·4CH3OH·5H2O (4): Quantities: LH4 (0.041 g,
0.13 mmol), Ho(NO3)3·5H2O (0.057 g, 0.13 mmol), Et3N (0.051 mL,
0.39 mmol), yield 0.042 g, 58.51 % (based on Er3+), m.p. 200 °C
(decomp.). IR (KBr): ν = 3388 (br), 2921 (w), 1617 (s), 1597 (s), 1546
˜
[8]
(w), 1445 (m), 1384 (m), 1355 (s), 1263 (w), 1224 (w), 1186 (w), 1050
(w), 1016 (w), 869 (w), 850 (m), 757 (w) cm–1. MS (ESI): m/z = 1010.10
[C64H62Er4N12O18 + 2MeOH]2+. C68H90Er4N12O29 ([Er4(LH2)4-
(μ2-OH)4]·4CH3OH·5H2O; 2208.54): calcd. C 36.98, H 4.11, N 7.61;
found C 37.01, H 3.97, N 7.84.
Acknowledgments
The authors are thankful to the Department of Science and
Technology (DST), New Delhi, for financial support, V. C. is
thankful for a J. C. Bose National Fellowship. DST is also thanked
for support through the Single-Crystal CCD X-ray Diffractometer
facility at IIT-Kanpur. S. B. and S. D. thank the Council of Scien-
tific and Industrial Research (CSIR), India for a Senior Research
Fellowship. A. K. B. and J.-P. S. acknowledge the Indo-French
Center for the Promotion of Advanced Research (CEFIPRA/
IFCPAR) for support.
Keywords: Lanthanides · Single-molecule magnets ·
Magnetic properties · Quantum tunneling
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Received: July 11, 2016
Published Online: ■
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Full Paper
Lanthanide Square Grids
The reactions of multidentate aroyl
hydrazone based ligands (LH4) with
Ln(NO3)3 in the presence of triethyl-
amine has afforded a series of tetra-
nuclear complexes [Ln4(LH2)4(μ2-OH)4]·
xCH3OH·yH2O (Ln = Dy3+, x = 2, y = 2;
Ln = Tb3+, x = 4, y = 5; Ln = Ho3+, x =
0, y = 13; Ln = Er3+, x = 4, y = 6). De-
tailed analysis of the magnetic data re-
vealed that the Dy3+ analogue is a sin-
gle-molecule magnet.
S. Biswas, S. Das, S. Hossain,
A. K. Bar, J.-P. Sutter,*
V. Chandrasekhar* .......................... 1–11
Tetranuclear Lanthanide(III) Com-
plexes Containing
a Square-Grid
Core: Synthesis, Structure, and Mag-
netism
DOI: 10.1002/ejic.201600744
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