Seven-coordinate LnIIIcomplexes assembled from a bulky : Mes acacH ligand: Their synthesis, structure, photoluminescence and SMM behaviour

Article abstract of DOI:10.1039/d0dt03054f

The reaction of a bulky acetyl acetone ligand 1,3-dimesitylpropane-1,3-dione (MesacacH) with hydrated lanthanide chlorides in the presence of tetramethylammonium hydroxide afforded a new family of neutral mononuclear LnIII complexes [Ln(Mesacac)3(DMF)] (Ln = Dy (1); Tb (2); Y0.91Dy0.09 (3); and Er (4)). The molecular structures of these complexes were confirmed by single crystal X-ray diffraction studies. The coordination geometries of the LnIII centre were analysed by SHAPE analysis which revealed a capped octahedral geometry in 1-4. Photoluminescence studies showed ligand-sensitized green emissions for 2 with an appreciable quantum yield of 0.83%. Static (dc) and dynamic (ac) magnetic studies of complexes 1 and 3 were performed. The dynamic magnetic study revealed that complex 1 exhibits zero-field slow relaxation of the magnetization without showing a clear maximum in the out-of-phase ac susceptibility plots. However, magnetic dilution of 1 with the YIII metal ion (complex 3) and/or the application of a dc magnetic field induces a strong frequency dependence of the ac susceptibility signals with χ′′M peaks in the 3-10 K temperature range, thus supporting field-induced SMM behaviour of 1. The relaxation process takes place through a combination of the Orbach and Raman mechanisms. The fitting of the temperature dependence of the relaxation time to the equation τ-1 = τ0-1 exp(-Ueff/kBT) + BTn, allows the extraction of the effective energy barrier Ueff/kB = 70 K (48.7 cm-1) and pre-exponential parameter of τ0 = 2.7 × 10-7 s for the Orbach mechanism (first term) and the parameters B = 0.04 s-1 K-n and n = 6.11, for the Raman mechanism (second term).

Full text of DOI:10.1039/d0dt03054f

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10.1039/D0DT03054F.
Volume 46
Number 10
14 March 2017
Pages 3073-3412
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DOI: 10.1039/D0DT03054F
Seven-coordinate LnIII Complexes Assembled from a Bulky MesacacH
Ligand: Synthesis, Structure, Photoluminescence and SMM
behaviour
Pankaj Kalita,a,b Prakash Nayak,a Naushad Ahmed,b Juan Manuel Herrera,c Krishnan
Venkatasubbaiah,a* Enrique Colacio,c* and Vadapalli Chandrasekhar*b,d
aSchool of Chemical Sciences, National Institute of Science Education and Research
Bhubaneswar, HBNI, Jatni, Khurda - 752050, Odisha, India
bTata Institute of Fundamental Research Hyderabad, Gopanpally, Hyderabad-500 046, India
cDepartamento de Química Inorgánica, Facultad de Ciencias, Universidad de Granada, Avenida
de Fuentenueva s/n, 18071 Granada, Spain
dDepartment of Chemistry, Indian Institute of Technology Kanpur, Kanpur-208016, India
AUTHOR EMAIL ADDRESSES: vc@iitk.ac.in; vc@tifrh.res.in; krishv@niser.ac.in
ecolacio@ugr.es

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Abstract
The reaction of a bulky acetyl acetone ligand, 1,3-dimesitylpropane-1, 3-dione (MesacacH) with
hydrated lanthanide chlorides in the presence of tetramethylammonium hydroxide afforded a new
family of neutral mononuclear LnIII complexes [Ln(Mesacac)3(DMF)] (Ln = Dy; (1), Tb; (2),
Y0.91Dy0.09; (3), and Er; (4)). The molecular structures of these complexes were confirmed by
single crystal X-ray diffraction studies. The coordination geometries of the LnIII centre were
analysed by SHAPE analysis which revealed a capped octahedron geometry in 1–4.
Photoluminescence studies show ligand-sensitized green emissions for 2 with an appreciable
quantum yield of 0.83%. Static (dc) and dynamic (ac) magnetic studies were performed on the
complexes 1 and 3. The dynamic magnetic study reveals that complex 1 exhibits zero-field slow
relaxation of the magnetization without showing a clear maximum in the out-of-phase ac
susceptibility plots. However, magnetic dilution of 1 with the YIII metal ion (complex 3) and/or
the application of a dc magnetic field induces a strong frequency dependence of the ac
susceptibility signals with ”M peaks in the 3 -10 K temperature range, thus supporting field-
induced SMM behaviour in 1. The relaxation process takes places through a combination of
–
Orbach and Raman mechanisms. The fitting of the relaxation times to the equation τ–1 = BTn + τ0
1exp(–Ueff /kBT), allows the extraction of an effective energy barrier, Ueff/kB = 70 K (48.7 cm–1) and
pre-exponential parameter of τ0 = 2.7 x 10-7 s for the Orbach mechanism and the parameters B =
0.04 s-1 K-n and n = 6.11, for the Raman mechanism.
Introduction
Single-molecule- and single-ion magnets (SMMs and SIMs) have been receiving a lot of attention
in recent years.1 These are molecular systems that once magnetized, retain their magnetization for
2

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a sufficiently longer period of time, below certain critical temperatures.2 While most of the initial
efforts were devoted to mainly polynuclear 3d complexes3, soon this phenomenon was also
observed in heterometallic 3d/4f complexes.4 In a seminal discovery in 2003, slow magnetic
relaxation and magnetic hysteresis was observed for the first time in a mononuclear
bis(phthalocyanine)TbIII complex, [TBA][Tb(Pc)2]– (TBA = tetrabutylammonium).5 This
discovery opened up a new direction involving the utilization of 4f-ions in the construction of
molecular magnets. Recent advancements in this field reveal that mononuclear complexes could
exhibit promising SMM behaviour provided the following two criteria are met: (a) the complex
should have a bistable spin ground state with high magnitude of |mJ| i.e. an Ising ground state, and
should possess (b) well isolated excited states with magnetic moments co-linear with the ground
state.6 The 4f-ions are considered as suitable candidates for the design and assembly of molecular
magnets due to the fact that many of them possess a fairly large ground state spin, S, and also in
addition, have intrinsic magnetic anisotropy arising from a large unquenched orbital angular
momentum and strong spin-orbit coupling. One important drawback of some of the 4f complexes,
however, is the significant contribution of temperature independent zero field quantum tunnelling
mechanism (QTM) that causes a fast magnetization reversal.7 Indeed, such relaxation mechanisms,
short cut the thermal barrier to the effective barrier (Ueff) for the reversal of magnetization.
However, the QTM process is formally forbidden in the case of 4f-ions having half integer spins
(Kramers ions) in the strict absence of an external field whereas in the case of integer spins (non-
Kramers ions) it can be present in a significant way.8 One of the ways of effectively reducing these
effects and having some control on such deleterious relaxation mechanisms is to use strategic
designs of ligand fields which can provide appropriate coordination environment and local
symmetry to the 4f complex.9 In the literature, low coordination numbers around the LnIII centre
3

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are reported to be capable of inducing axiality as well as large crystal field splitting of the crystal
field doublets.6a, 10However, at the same time, low coordinate lanthanide complexes are extremely
sensitive towards air and moisture which limits their practical applicability. Recently outstanding
examples
involving
organometallic
lanthanide
complexes,
[Dy(Cpttt)2][B(C6F5)4]
t
t
(Cptttꢀ=ꢀ{C5H2 Bu3-1,2,4} and Buꢀ=ꢀC(CH3)3) and [(η5-Cp*)Dy(η5-CpiPr5)][B(C6F5)4] have been
reported with Ueff values of 1223 cm-1 and 1541 cm-1 and blocking temperatures of 60 K and 80
K.11 However, these complexes also are extremely air-sensitive. Among the air-and moisture-
stable mononuclear complexes important examples include seven-coordinate pentagonal
bipyramidal DyIII complexes with high energy barriers and high blocking temperatures.12
In the literature, a diverse range of ligand systems such as macrocyclic ligands,13 organometallic
ligands,14 and chelating ligands15 are known to produce monometallic complexes. We also have
been using various kinds of multidentate ligands for preparing both mono- and dinuclear
lanthanide complexes that have been shown to possess interesting magnetic properties.16 In this
quest, we wished to explore the well-known β-diketonato family for the synthesis of mononuclear
lanthanide complexes. In the literature, there have been precedents on the use of such ligands (Fig.
1) to afford mononuclear complexes of the type [Ln(β-diketonato)3(AB)], (where AB can be two
monodentate ligands or a bidentate chelating ligand) and [Ln(β-diketonato)4] .17 However, in both
these instances, the lanthanide ions are eight-coordinate. In fact, there has been only one report of
a seven-coordinate lanthanide complex with the β-diketone ligand (EIFD see Fig. 1).18 Other
mononuclear LnIII complexes with less than three β-diketonate ligands and β-diketonate as co-
ligands are also known.19 Interestingly, in all the mononuclear DyIII complexes obtained from the
EIFD ligand, coordination symmetry (C3V) and the ligand field around the metal ion effectively
suppresses QTM resulting SMM behaviour.18 We were intrigued by the possibility of increasing
4

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the steric encumbrance around the acetyl acetonate ligand to check if such a ligand design can lead
to a decrease in the coordination number around the lanthanide ions.
F
F
F
F
F
F
F
F
F
O
O
O
O
O
O
BTFA
HFAC
ACAC
N
F
F
F
F
F
F
O
O
O
O
EIFD
TFI
Fig. 1. Selected β-diketonate ligands utilized for the synthesis of mononuclear LnIII SIMs/SMMs.
An important reason for the interest in β-diketonate LnIII complexes is their photophysical
properties.20 Many lanthanide complexes, particularly those involving EuIII and TbIII ions show
interesting photoluminescence and this property can be tuned by functionalization of the ligand
such that the energy transfer process can be accentuated.20e Also, mononuclear LnIII complexes
with dual magnetic and luminescence properties are of considerable interest.21 Accordingly, we
have prepared a sterically bulky and flexible acetylacetone ligand, MesacacH by the Friedel-Crafts
acylation reaction of malonyl chloride with mesitylene and utilized this ligand to synthesize a new
family of seven-coordinate mononuclear lanthanide complexes [Ln(Mesacac)3(DMF)] (Ln = Dy;
(1), Tb; (2), Y0.91Dy0.09; (3), and Er; (4)) (Scheme 1). Herein, we report the syntheses, structure,
theoretical calculations, photophysical and magnetic properties of these complexes.
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N
O
O
O
(i) LnCl3.6H2O
(ii) Me4NOH.5H2O
Ln
O
O
O
EtOH/CHCl3
DMF
O
O
O
MesacacH
(Ln = Dy (1), Tb (2), Y0.91Dy0.09 (3), Er(4))
Scheme 1. Synthesis of the mononuclear complexes 1–4.
Results and discussion
Single crystal X-ray analysis
The molecular structures of complexes 1–4 were confirmed by single crystal X-ray studies. The
mononuclear complexes 1–4 are neutral, and isostructural. The complexes 1, 2, and 4 crystallized
in the triclinic P-1 space group (Z = 2) while the complex 3 crystallizes in the monoclinic P21/c
space group (Z = 4). These seven coordinate complexes are formed by the coordination action of
three monoanionic Mesacac– ligands and a DMF solvent molecule. In view of the overall structural
similarity, we have chosen complex 1 as a representative example to elucidate the common
structural features present in them. A perspective view of the molecular structure of 1 is shown in
Fig. 2, while those of 2–4 are given in the Electronic Supporting Information (Figs. S4–S6).
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Fig. 2. Molecular structure of complex 1 (left) and the capped octahedron coordination geometry
around DyIII centre (right). Colour codes: C = grey; N = blue; O = red; Dy = orange (H-atoms are
removed for clarity).
The seven coordinate central DyIII in 1 is coordinated to six oxygen atoms from three
Mesacac– ligands in a κ2O chelating coordination mode and one oxygen atom from DMF. The
cumulative coordination action of all the ligands resulted in a distorted C3v-capped octahedron
geometry around the DyIII centre as confirmed by SHAPE analysis (see Table S1).22 The average
Dy−O distance for the Mesacac– ligands is 2.2881(16) Å23 which is comparatively shorter than the
Dy−ODMF distance 2.3833(17) Å (see Table 1). This is primarily due to a strong electrostatic
interaction of the highly charged DyIII ion with the anionic ligands. The selected bond distances
and -angle parameters of 1 are given in Table 1 while the remaining parameters are given in Table
S2. The coordination geometry around the DyIII in 1 can also be viewed as four triangular O–Dy–
O planes (P1, O1–Dy1–O2; P2, O3–Dy1–O4; P3, O5–Dy1–O7; and P4, O6–Dy1–O7) which
approaches ~90° relative to an imaginary xy plane (P5) that bisects the central DyIII ion (see Fig.
7

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3 (left)). This implies that the donor O atoms are necessarily be more above/below this plane and
thus stabilize the desired mJ = 15/2 ground state of an oblate shape DyIII ion.
Another structurally interesting aspect of 1–4 is the presence of Λ-or Δ-helical chirality
arising solely from the chelation of the three Mesacac ligands.24 Complexes 1, 2 and 4 are roughly
the Λ isomers, (the centrosymmetric unit cell does mean that both Λ and Δ are present in equal
number in the unit cell) whereas complex 3 has both Λ and Δ isomers in the asymmetric unit (see
Figs. S7-S10). Analysis of the crystal structures reveals the presence of sandwich-type pi-pi
stacking between two mes rings, with the inversion centre of the P–1 unit cell being sited along
the ring centroid vector. For complexes 1 and 2 the two mes rings are involved in pi-pi stacking
with a centroid∙∙∙∙∙centroid distance of 4.120(3) Å and 4.113(2) Å respectively (Figs. S11 and
S112). Conversely the structure of 4 does not feature this sandwich-type pi-pi stacking and instead
there appears to be T-shaped pi stacking between Er1A and Er1B units, C–H∙∙∙∙∙C6-centroid
distance is 3.048 (Fig. S13 in ESI). Further, packing diagrams of 1 and 3 reveals that the metal
centres are physically far apart with the shortest intermolecular DyIII∙∙∙∙∙DyIII distance being 9.539
Å (Fig. 3 (right)) and 10.437 Å (Fig. S14) respectively. Powder X-ray diffraction studies were
performed to check the bulk phase purity of complexes 1 and 3. The experimental patterns were
compared with the simulated patterns obtained from the single crystal X-ray diffraction data (Fig.
S15 see ESI). The differences in intensities of the powder patterns are due to the preferred
orientation of the crystalline powdered samples.
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Fig. 3. (left) Illustration of all the four O−Dy−O planes and an imaginary xy plane around the DyIII
ion (The dihedral angle between P1 (O1–Dy1–O2) and P4 (O6–Dy1–O7) is 82.112(48)º and
between P2 (O3–Dy1–O4) and P3 (O5–Dy1–O7) is 82.042(48)º); (right) the solid-state crystal
packing diagram of complex 1.
Table 1. Selected bond lengths (Å) and angles (°) of complex 1.
Bond lengths (Å)
Bond angles (°)
O1−Dy1−O4
135.00(6)
82.21(6)
77.04(6)
74.52(6)
74.85(6)
86.69(7)
149.84(6)
87.88(6)
111.96(7)
74.07(6)
155.75(7)
76.53(6)
134.24(6)
148.83(6)
123.04(6)
105.50(6)
76.16(6)
O1−Dy1−O6
O1−Dy1−O7
O2−Dy1−O1
O2−Dy1−O4
O2−Dy1−O6
O2−Dy1−O7
O3−Dy1−O1
O3−Dy1−O2
O3−Dy1−O4
O3−Dy1−O6
O3−Dy1−O7
O4−Dy12O7
O5−Dy1−O1
O5−Dy1−O2
O5−Dy1−O3
O5−Dy1−O4
Dy1−O1
2.2920(15)
2.2780(17)
2.2755(17)
2.3068(15)
2.2725(16)
2.3035(17)
2.3839(17)
Dy1−O2
Dy1−O3
Dy1−O4
Dy1−O5
Dy1−O6
Dy1−O7
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O5−Dy1−O6
74.18(6)
78.92(6)
127.69(6)
79.69(7)
O5−Dy1−O7
O6−Dy1−O4
O6−Dy1−O7
Photophysical properties.
Electronic spectra of the complexes. The UV–vis absorption spectra of complexes 1, 2, and 4 and
MesacacH were recorded in the DMF solvent (c = 1 × 10–5 M) at 298 K (Fig. 4). The absorption
properties of MesacacH and complexes 1, 2, and 4 are summarized in Table 2.
Table 2. Absorption properties of MesacacH and the complexes in DMF at 298 K
Compound
Absorbance λ [nm] (εmax [1 x 103 M-1 cm-1]) in solution
MesacacH
300 (29.93)
305 (53.30)
307 (35.70)
306 (51.63)
1
2
4
The ligand (MesacacH) displays only one absorption maxima at 300 nm corresponding to the spin-
allowed singlet π → π* transition. In all the complexes this band is shifted to slightly higher wave
numbers (Table 2) consistent with coordination of the ligand to the metal centres (Fig. 4). The
molar absorption coefficients are calculated and presented in Table 2. The high molar absorption
coefficients in the complexes suggest that the ligand could be involved for sensitization of the
lanthanide luminescence.
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Fig. 4. Absorption spectra of free ligand, MesacacH (left) and normalized absorption spectra of
complexes 1, 2, and 4 (right) in DMF.
The emission spectrum of free ligand is shown in Fig. S16 (ESI). The free ligand exhibits four
emission bands at 360 nm, 407 nm, 492 nm, 547 nm respectively. Among all the complexes
studied, only the TbIII derivative showed strong metal-centred green luminescence in 5 x 10-4 M
solution in DMF solvent media (Fig. S17). The weak luminesce spectrum of the DyIII ion is given
in Fig. S18. Upon excitation at the absorption maximum (λex = 307 nm) the complex exhibits sharp
emission bands at 493 nm, 548 nm, 586 nm, and 621 nm (Fig. 5). These are characteristic of TbIII
emission resulting from the deactivation of 5D4 excited state to 7FJ ground state (J = 6, 5, 4, 3).25
5
7
Among the emission peaks, the most intense emission at 548 nm corresponds to the D4 → F5
transition. In view of the strong emission characteristics of the TbIII derivative, relative
luminescence quantum efficiencies, in the solution-state, of complex 2 were calculated (Table 3)
by comparing the emission intensities of the standard sample (Quinine sulfate monohydrate in 0.5
M H2SO4) and the unknown sample according to the following equation
Φunk = Φstd(Iunk/Istd)(Astd/Aunk)(ηunk/ηstd)2
Where Φunk and Φstd are the luminescence quantum yields of the unknown sample and the standard
sample, respectively, and Iunk and Istd are the integrated emission intensities of the unknown sample
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and standard sample solution, respectively. Aunk and Astd are the absorbances of the unknown
sample and standard sample solution at their excitation wavelengths, respectively. The ηunk and
ηstd terms represent the refractive indices of the corresponding solvents (pure solvents were
assumed). The absolute quantum yields of complex 2 were measured on a Spex-Fluorolog FL22
spectrofluorimeter and the results are tabulated in Table S3. The observed relative quantum yield
(0.83 %) is comparable with that of the quantum yield calculated using the absolute method (0.74
%).
Fig. 5. Emission spectrum of the TbIII complex 2 (excitation at 307 nm; DMF solution 5 x 10-4
M) at room temperature.
Table 3. Relative quantum yield of complex 2.
Complex
Iunk
Istd
Aunk
Astd
ηunk
ηstd
Φstd
Φunk
% Φunk
(DMF) (water)
2 (Tb)
123085.2 9323545.5 0.0693 0.0693 1.4305 1.33
0.546
0.00833 0.830
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Magnetic properties
We have performed preliminary ac measurements on complexes 1–4 and only compounds 1 and
its magnetically diluted analogue 3 exhibited out-of-phase signals under zero dc field. Moreover,
complexes 2 and 4 do not show any out-of-phase signal even in the presence of an applied dc field
and/or magnetic dilution with YIII diamagnetic metal ion. In view of this, we undertook only a
complete dc and ac magnetic study of the DyIII complex 1 and its diluted derivative 3.
Direct current (dc) magnetic susceptibility measurements were carried out on a polycrystalline
sample of 1 in the 290-2 K temperature range under an applied magnetic field of 1000 Oe. The
temperature dependence of the χMT product for 1 (χM is the molar magnetic susceptibility per
mononuclear DyIII unit) is given in Fig. 6. The χMT value at room temperature (13.75 cm3 mol-1 K)
is close to that expected for an isolated DyIII ion in the free ion approximation (14.18 cm3 mol-1
K). Upon cooling, χMT steadily decreases to reach a value of 12.46 cm3 mol-1 K at 2 K. This
behaviour is mainly due to the depopulation of the mJ sublevels of the DyIII ions, which arise from
the splitting of the 6H15/2 ground term by the ligand field, as well as Zeeman effects and possible
intermolecular dipolar interactions (which must be very small due to the large Dy·····Dy distance
of 9.538 Å). The field dependence of the magnetization at T = 2 K (Fig. 6 inset) exhibits a fast
increase of the magnetization up to ~ 1 T and then a very slow increase to reach almost saturation
at 7 T, thus indicating a well isolated ground state. The magnetization values at the highest applied
dc magnetic field of 5 T (M = 5.35 NB) are lower than that calculated for an isolated DyIII ion in
the free-ion approximation (M = 10 NB), which is due to crystal-field effects giving rise to
significant magnetic anisotropy.
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Fig. 6. Temperature dependence of the χMT product and field dependence of the magnetization
(inset) for compound 1. The solid magenta lines represent the SINGLE_ANISO computed data.
It should be noted that low-symmetry DyIII complexes, like 1, generally exhibit a MJ = ±15/2 axial
ground Kramers doublet. Assuming the axiality of the ground state, we have calculated the
direction of the anisotropy axes of the DyIII ions by using the electrostatic method by Chilton et al.
(Fig. S19).26 The result of this calculation indicates that the anisotropy axis is oriented almost
perpendicular to the barycentre of the triangle defined by the coordinated oxygen atoms of the
Mesacac– ligands, O3,O2 and O5, which present the shortest Dy-O bond distances in the 2.273-
2.278Å range (with Dy-O-Dy angles within this triangle in the 105.49-123.09° range) and are
above the Dy center. In order to reduce the repulsion between the oblate f electron density of the
MJ =15/2 DyIII center and the nearest coordinated oxygen atoms, the electron density disc is
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accommodated almost parallel to the plane of the triangle.27 Hence, the resulting magnetic moment
is perpendicular to the electron density disc and consequently almost perpendicular to the triangle
barycentre. The orientation of the anisotropy axis is almost coincident with that obtained from
theoretical calculations (see below).
Preliminary temperature dependent dynamic ac magnetic susceptibility measurements at a
frequency of 1400 Hz were carried out to know if 1 exhibited slow relaxation of the magnetization
and SIM behaviour. At zero dc field, complex 1 shows an out-of-phase (χ"M) signal below 15 K
with an intense tail below 5 K. This tail, which is due to fast resonant zero-field quantum tunneling
of the magnetization (QTM), avoids the observation of a clear peak around 5 K. This behaviour
can be due to the existence of dipolar and/or hyperfine interactions opening new relaxation
pathways for QTM process. When the ac measurements were carried out in the presence of a small
external field of 1000 Oe (this is the field leading to the slower relaxation) compound 1 showed a
well-defined out-of-phase (χ"M) signal with a maximum at 10 K (Fig. 7, right). This result clearly
points out that the application of a small dc field is enough to suppress QTM.
Fig. 7. Temperature dependence of the out-of-phase χ"M component of the ac susceptibility for 1
at 0.1 T (Left). Temperature dependence of the out-of-phase χ"M for 1 and 3 at 1400 Hz and under
zero and 0.1 T magnetic field (right).
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In order to know the effect of the magnetic dilution on the slow relaxation of the magnetization,
we prepared a magnetic diluted sample of 1 by substituting DyIII ions by YIII to achieve a DyIII/YIII
molar ratio 1/9 (this ratio was estimated from the comparison of the room temperature
susceptibility and magnetization data for 1 and those for the diluted complex 3 (Fig. S20). The
temperature dependence of the out-of-phase (χ"M) signal at 1400 Hz and under zero field for 3
shows a clear maximum centred at 10 K and a low intensity tail below 5 K, thus pointing out that
the QTM has been almost completely suppressed. Moreover, when the results for 1 at 0.1 T are
compared to those of 3 at zero field, one realizes that the effect of the dilution appears to be a little
bit less effective in suppressing QTM than the effect of the field (Fig. 7, right). As expected, at 0.1
T the results for the 1 and 3 are coincident. Considering this, the complete set of ac measurements
on complex 1 were carried out under a field of 0.1 T. The results show a strong frequency
dependence of the ac susceptibility signals with maxima in the 10 (1400 Hz)-3 K (1 Hz)
temperature range.
The relaxation times (τ) for 1 were extracted from the fitting of the frequency dependence of χ"M
at each temperature to the generalized Debye model (Fig. 8). The fit of the relaxation times to the
Arrhenius equation in the 8–10 K temperature range afforded the following values of the effective
energy barrier for the reversal of the magnetization and the pre-exponential factor, Ueff = 70(3) K
and τo = 2.7 x 10–7 s respectively. The deviation of the data from the Arrhenius law below 8 K is a
clear sign that either the magnetic relaxation does not takes place through an over barrier Orbach
relaxation process or there is coexistence of several competing relaxation processes. In the studied
temperature range (T > 4 K) and at 0.1 T direct and QTM relaxation processes for 1 are almost
negligible, and therefore the relaxation times were fitted to the following equation:
–1
τ–1 = BTn + τ0 exp(–Ueff /kBT)
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which indicates that Raman (first term) and Orbach (second term) processes contribute
concurrently to the relaxation of the magnetization. It should be noted that all attempts to fit the
data to this equation were unsuccessful. However, fixing the Ueff and 0 values to those extracted
from the Arrhenius plot for the Orbach process, a very good fit was obtained with B = 0.04 s-1 K-n
and n = 6.1. It is worth mentioning that n = 9 is expected for Kramers ions like DyIII.28 Nevertheless,
values between n = 2 to 7 are also are realistic when both acoustic and optical phonons are active.29
Similar values have been previously reported for other DyIII containing complexes.30
Fig. 8. Frequency dependence of the ''M at different temperatures for 1 at 0.1 T. (Inset)
Temperature dependence of the relaxation time τ for complex 1. The red line represents the best
fits of the experimental data to the Arrhenius equation whereas the green line corresponds to the
best fit to Raman relaxation process.
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The ongoing and previous results17a, 31 show that a good approach for obtaining DyIII SIMs is that
of using tris(β-diketonates) DyIII complexes, already formed or prepared in situ, with neutral
monodentate, bidentate or bis(bidentante) ligands (such as water, DMF, pyridine, 2, 2'-bipyridine
derivatives, bis-bipyrimidine, etc). Thus, the propeller- and T-shaped distributions of the three β-
diketonate ligands around the Dy3+ ion seem to assure the SIM behaviour. The former distribution
occurs, like in 1, when, in addition to the three β-diketonates), there exists a neutral monodentate
ligand coordinated to the Dy3+ ion,17a, 18 while the second arrangement of β-diketonates ligands
takes place when two monodentate or one bidentate neutral ligands are additionally coordinated
to the Dy3+ ion.31
Computational study
To shed light on the experimentally observed magnetic behaviour of complex 1 and to get more
insight about its electronic structure, we have performed CASSCF based ab initio calculations on
the single crystal X-ray structure using MOLCAS 8.2 suite.32 In order to predict the slow magnetic
relaxation behaviours of TbIII and ErIII analogues, we have also included complexes 2 and 4 in our
calculation. The basis functions for all the atoms used in our calculation were taken from ANO-
RCC library available in the MOLCAS 8.2 package (Table S4 in ESI). The computed energies of
the eight low lying KDs for complexes 1 and 4 span from zero to 546 cm-1 and 390 cm-1
respectively while energies for the low lying non KDs span up to 460 cm-1 in complex 2.
The computed g-tensors of the ground state KD (gxx = 0.015, gyy = 0.044 and gzz = 19.756) indicates
the presence of small but significant transverse anisotropy (gxx = 0.015, gyy = 0.044) which result
the deviation from the pure Ising nature (gzz = 20)12c in complex 1 (Table 4). We have also analysed
the composition of mJ states which further reveals that the ground state mJ level (±15/2) is only
79% pure and admixed with the ±11/2 (10.0%) and with other excited states in case of complex 1
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(Table S9 in ESI). The computed ground state g-tensors (gxx = 0.311, gyy = 0.563 and gzz = 15.156)
for complex 4 suggest the presence of significant amount of transverse anisotropy associated with
ground state KD which may trigger the fast magnetization relaxation between ground state KD
itself through QTM. Further the computed gzz value of 15.483 for the ground state non KD in
complex 2 also shows the lack of Ising nature33 which may help the fast magnetic relaxation
between the ground state pseudo doublet itself. The wave function analysis for 2 and 4 (Table S10
and S11 in ESI) also show that the ground ± mJ state is only 55.3 % and 54.8 % pure and strongly
admixed with excited states which further confirm the lack of axial-ligand field to provide axial
anisotropy in these complexes.
Further to get more insight about the axial behaviour and the probability of QTM process between
the ground-state KDs in complexes 1 and 4, we have also computed the crystal field parameters
(CF). In the presence of negligible or small hyperfine and intermolecular interactions, the
probability for the magnetic relaxation through QTM process can be best described by the CF
∑∑푞
퐵푞푘푂푞푘
(where
푘 = ―푞
parameters.34 We have used corresponding crystal-field Hamiltonian
퐻퐶퐹
=
푞
퐵푘푞
푂푘
and
are the crystal field parameters and Stevens’ operators respectively) implemented in
SINGLE_ANISO module to compute CF parameters. The computed axial 02 crystal field values
퐵
푞
for TbIII, DyIII and ErIII are found to be lower compared to the non-axial term
indicating the
퐵2
lack of axial-ligand field which result the first excited state being located at around ≈ 44 cm-1 in
all the complexes 1, 2, and 4 (Table S12).
Table 4. Computed Energy (cm-1) and the Associated g-Tensors of the Low-lying KDs generated
from the 6H15/2 State of Complex 1. Here  represent the angle between gzz axis of the ground and
corresponding excited KDs.
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 ( angle)
±mJ states (from 21 sextets only)
gxx; gyy; gzz
0.0
43.793
145.589
185.346
246.597
286.829
369.999
535.926
0.015; 0.044; 19.756
0.079; 0.198; 19.334
2.404; 4.356; 13.281
8.007; 7.434; 0.323
2.015; 4.562; 12.630
0.455; 0.744; 17.653
0.120; 0.256; 19.088
0.003; 0.008; 19.881
0.0
100.1
29.3
19.4
79.7
125.8
57.7
76.0
±mJ states
(from 21 sextets+ 108 Quartets only)
 ( angle)
gxx; gyy; gzz
0.0
44.462
0.010; 0.035; 19.666
0.080; 0.210; 19.252
2.478; 4.487; 13.119
8.1446; 7.237; 0.316
1.801; 4.644; 12.264
0.598; 1.0473; 17.350
0.123; 0.2511; 19.000
0.003; 0.007; 19.796
0.0
100.1
27.8
18.8
79.3
125.7
57.8
75.9
144.579
183.828
246.009
282.214
363.743
527.444
The average computed LoProp charge density (Table S13 in ESI) on the oxygen donor
atoms (belongs to coordinated Mesacac– ligands) was found to be -0.72 while the charge density on
oxygen donor atom of coordinated DMF ligand found to be -0.65 in all the cases. As usual, the
direction of the ground state anisotropy axis on oblate TbIII and DyIII ion in complexes 1 and 2 is
found to be oriented in between the Ln–O bonds of Mesacac– ligands (Figs. 9 and S21). The
anisotropy axes on oblate DyIII and TbIII ions in complexes 1 and 2 are orientated by an angle of
16.9˚ from each other while the anisotropy axis on prolate ErIII ion in complex 4 is orientated by
an angle of 125.9˚ (from TbIII) and 112.3˚ (from DyIII) and found to be almost perpendicular on
them (Fig. S21).
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Fig. 9. The orientation of ground state KDs anisotropy axis gzz of DyIII ion (left) and the plausible
magnetic relaxation mechanism (right) in complex 1. The red dotted arrows show the magnetic
relaxation via QTM (between ground KDs) and TA-QTM (between excited state KDs)
respectively. The green and sky blue arrows represent the magnetic relaxation through Orbach and
Orbach/Raman processes. The purple characters represent the composition of ± mJ states of KDs.
The computed quantum tunneling of magnetization (QTM) value of 0.77E-02 μB between
ground state KD in complex 1 is found to be slightly higher due to the significant admixing with
excited states. The large thermally assisted quantum tunnelling (TA-QTM) value of 0.15 μB
between the first excited KD which is located at only 44 cm-1 from the ground state KD, suggest
that the magnetic relaxation should occur ideally from this state with Ucal = 44 cm-1 (63.3 K). The
predicted very low energy barrier for spin reversal along with the large values of Orbach (0.36 μB)
and Orbach/Raman (0.15 μB) respectively for magnetic relaxation probability for complex 1 is
consistent with our experimental observation for dynamic magnetic property of magnetically
diluted sample of 3.
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The plausible magnetic relaxation mechanism for complexes 2 and 4 is given in Fig. S22
in ESI. The large tunnel splitting value 2.38 cm-1 between the ground state non KD for TbIII in
complex 2 and large QTM value of 0.14 μB between ground sate KD for ErIII in complex 4
respectively suggest the spin relaxation will occur within the ground state itself and hence there
will be no typical SMM behaviour for these complexes.
Summary
In conclusion, we have successfully prepared a new series of neutral mononuclear LnIII complexes
by utilizing a flexible and bulky bidentate acetyl acetone ligand (MesacacH). The solid-state
structures of these complexes show seven-coordinate capped octahedron/trigonal prism geometry
for the LnIII ions. Interestingly, the TbIII (2) derivative showed ligand sensitized green
luminescence characteristic of the LnIII metal centre. Dynamic ac susceptibility measurements
were performed on DyIII (1) derivative (complexes 2 and 4 do not exhibit slow relaxation below 2
K) which show well-defined peaks in the frequency dependence out-of-phase susceptibility
characteristics of SMM behaviour under a biased field of 1000 Oe with an anisotropic energy
barrier of 70 K (τ0 = 2.7 x 10-7 s). In order to know the effect of the magnetic dilution on the slow
relaxation of the magnetization, dynamic ac susceptibility measurements were performed on a
diluted sample YIII0.91DyIII
(3). Interestingly, in the absence of dc field, complex 3 revealed
0.09
SMM behaviour. Theoretical analysis was carried out to explain the observed magnetic behaviour
and the results match well with the experimental observations.
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Experimental Section
Reagents and general procedures
Solvents and other general reagents used in this work were received from commercial sources and
were used without any further purification. Mesitylene and malonyl chloride were obtained from
TCI chemicals (India) Pvt. Ltd. Tetramethylammonium hydroxide, aluminium chloride, and
carbon disulphide were obtained from Spectrochem Chemicals India. LnCl3·6H2O (Ln = Y, Gd,
Dy, Tb, Er) were obtained from Sigma Aldrich Chemical Co. (U.S.A.) and used as obtained. The
ligand, 1,3-dimesitylpropane-1,3-dione (MesacacH) was prepared by a previously reported
procedure and well characterized (see Supporting Information).35
General synthetic procedure for the preparation of complexes 1–4
A general synthetic protocol was used for the preparation of the metal complexes as follows:
MesacacH (3 eq.) was taken in ethanol/chloroform solvent mixture (20 mL) and to it was added 1
mL of DMF along with LnCl3·6H2O (1 eq.). To this solution tetramethyl ammonium hydroxide
was added (3 eq.). The resulting reaction mixture was heated under reflux for a period of 6 h and
brought to ambient temperature. The volume of the solution was reduced to 10 mL, filtered, and
kept undisturbed for crystallization under ambient conditions. Slow evaporation of the solvent
afforded colourless (for 1–3) and pink (4), block-shaped crystals suitable for single crystal X-ray
analysis after one week. The stoichiometry of the reactants involved in each reaction, yield of the
products, and their characterization data are provided below:
[Dy(Mesacac)3(DMF)] (1). MesacacH (0.196 g, 0.636 mmol), DyCl3·6H2O (0.080 g, 0.212 mmol),
and Me4NOH.5H2O (0.115 g, 0.634 mmol) were used. Yield: 0.132 g, 51% (based on Dy). M.P.:
>250 ℃. IR (KBr ν/cm-1): 3445(br), 2998(w), 2949(w), 2919(m), 2857(w), 2733(w), 1660(s),
1611(s), 1554(s), 1505(s), 1472(m), 1401(s), 1370(s), 1301(m), 1162(m), 1107(m), 1054(m),
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1032(w), 958(w), 848(m), 799(w), 777(w), 726(m), 679(w). Anal. Calcd for C66H76Dy1N1O7
(1157.83): C, 68.47; H, 6.62; N, 1.21. Found: C, 68.39; H, 6.55; N, 1.17.
[Tb(Mesacac)3(DMF)] (2). MesacacH (0.196 g, 0.636 mmol), TbCl3·6H2O (0.079 g, 0.212 mmol),
and Me4NOH.5H2O (0.115 g, 0.634 mmol) were used. Yield: 0.118 g, 48% (based on Tb). M.P.:
>250 ℃. IR (KBr ν/cm-1): 3443(br), 2998(w), 2949(m), 2917(m), 2857(w), 2731(w), 1660(s),
1611(s), 1554(s), 1505(s), 1472(m), 1401(s), 1386(s), 1301(m), 1162(m), 1107(m), 1054(m),
1030(w), 958(w), 848(m), 797(w), 777(w), 728(m), 679(w). Anal. Calcd for C66H76 Tb1N1O7
(1154.26): C, 68.68; H, 6.64; N, 1.21. Found: C, 68.52; H, 6.37; N, 1.13.
[Dy0.09Y0.91(
Mesacac)3(DMF)] (3). MesacacH (0.196 g, 0.636 mmol), DyCl3·6H2O (0.008 g, 0.021
mmol), YCl3·6H2O (0.058 g, 0.191 mmol) and Me4NOH.5H2O (0.115 g, 0.634 mmol) were used.
Yield: 0.112 g, 54% (based on Y). Anal. Calcd for C66H76N1O7Dy0.09Y0.91(1090.81): C, 72.60; H,
7.01; N, 1.28. Found: C, 72.56; H, 7.12; N, 1.16.
[Er(Mesacac)3(DMF)] (4). MesacacH (0.196 g, 0.636 mmol), ErCl3·6H2O (0.081 g, 0.212 mmol),
and Me4NOH.5H2O (0.115 g, 0.634 mmol) were used. Yield: 0.127 g, 57% (based on Er). M.P.:
>250 ℃. IR (KBr ν/cm-1): 3445(br), 3006(w), 2949(w), 2917(m), 2855(w), 2731(w), 1662(s),
1613(s), 1556(s), 1505(s), 1472(m), 1405(s), 1370(s), 1301(m), 1162(m), 1107(m), 1054(m),
1032(w), 958(w), 848(m), 797(w), 777(w), 728(m), 681(w). Anal. Calcd for C66H76Er1N1O7
(1162.59): C, 68.19; H, 6.59; N, 1.20. Found: C, 68.07; H, 6.49; N, 1.09.
Instrumentation
Melting points were measured using a StuartTM SMP10 melting point apparatus and are
uncorrected. 1H NMR and 13C{1H} NMR spectra were recorded on a Bruker Ascend-300 (1H: 300
13
MHz; C{1H}: 75 MHz) and were referenced to the resonances of the solvent used. IR spectra
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were recorded on a PerkinElmer FT-IR spectrometer. Mass spectra were recorded with a Bruker
micrOTOF-Q II spectrometer. Elemental analyses of the compounds were obtained from a Euro
Vector EA instrument (CHNS-O, model EuroEA3000). Powder X-ray diffraction data of the
complexes were collected with a Bruker D8 Advance X-ray powder diffractometer using Cu Kα
radiation (λ = 1.5418 Å).
Photophysical characterization
Absorption data were measured on an Agilent Cary 60 UV-Vis spectrophotometer at room
temperature. Photoluminescence (PL) spectra were recorded on a Spex-Fluorolog FL22
spectrofluorimeter. The latter was equipped with a double grating 0.22 m Spex 1680
monochromator and a 450 W Xe lamp as the excitation source and a Hamamatsu R928P
photomultiplier tube detector. Corrections were applied to the emission and excitation spectra
regarding source intensity (lamp and grating) by using standard correction curves.
Magnetic measurements
Variable-temperature (2−300 K) magnetic susceptibility measurements on polycrystalline samples
of 1 and 3 under an applied field of 1000 Oe were carried out with a Quantum Design SQUID
MPMS XL-5 device. Alternating-current (ac) susceptibility measurements under different applied
static fields were performed using an oscillating ac field of 3.5 Oe and ac frequencies ranging from
1 to 1500 Hz. The experimental susceptibilities were corrected for the sample holder and
diamagnetism of the constituent atoms using Pascal’s tables. A pellet of the sample cut into very
small pieces was placed in the sample holder to prevent any torquing of the microcrystals.
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X-ray crystallography
Single-crystal X-ray structural studies of 1, 2, and 4 were performed on a Bruker Apex II CCD
diffractometer equipped with graphite-monochromated Mo Kα radiation (λα = 0.71073 Å) at 296
K. The SMART and SAINT software package36 was used for collecting frames of data, indexing
reflections, determining lattice parameters, integration of the intensity of reflections and scaling.
A multi-scan absorption correction was performed using SADABS.37 Space groups were
determined using XPREP implemented in APEX2.38 The single-crystal X-ray diffraction data of
compound 3 was collected on a Rigaku Xtal LAB X-ray Diffractometer system equipped with a
CCD area detector and operated at 30 W power (50 kV, 0.6 mA) to generate MoKα radiation (λ =
0.71073 Å) at 120 K. Data were integrated using CrysAlisPro software with a narrow frame
algorithm.39 Data were subsequently corrected for absorption by the program SCALE3 ABSPACK
scaling algorithm. All the structures were solved with the ShelXT40 structure solution program
using Intrinsic Phasing and refined with the ShelXL41 refinement package using Least Squares
minimisation in the Olex-242 software. All the non-hydrogen atoms were refined with anisotropic
thermal parameters. Hydrogen atoms were placed in either geometrically calculated positions or
found in the Fourier difference map and included in the refinement process using riding model.
All the complexes crystallize out without any co-crystallized solvent molecule(s). For compound
3, the Dy population was determined by positional disorder treatments. All the mean plane
analyses and crystallographic figures have been generated using DIAMOND software (version
3.2).43 The crystal data and refinement parameters for 1–4 are summarized in Table 5. More details
on the crystallographic data are given in the X-ray crystallographic files in CIF format.
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Table 5. Details of the data collection and refinement parameters for compounds 1–4
1
2
3
4
Chemical
formula
C66H76Dy1N1O7
C66H76Tb1N1O7
C66H76Dy0.09N1O7Y0.91
C66H76Er1N1O7
Mr
1157.77
1154.19
1090.81
1162.53
Crystal
system,
Triclinic,
Triclinic,
Monoclinic,
Triclinic,
P-1
P-1
P21/c
P-1
space group
Temperature
(K)
296(2)
296(2)
120.02(10)
296(2)
a, b, c (Å)
α, β, γ (°)
V (Å3)
12.2208(4)
13.7998(4)
19.7289(7)
12.2338(3)
13.8401(3)
19.6751(5)
12.5954(4)
23.3723(7)
20.4784(6)
12.5323(2)
19.8358(3)
25.8063(4)
94.577(2)
99.474(2)
105.841(2)
94.6660(10)
99.1700(10)
106.0000(10)
90
99.867(3)
90
106.5780(10)
94.7510(10)
91.3860(10)
3129.76(18)
3133.69(13)
5939.3(3)
6119.67(17)
Z
2
2
4
4
Radiation
type
MoKα (λ =
0.71073)
MoKα (λ =
0.71073)
MoKα (λ = 0.71073)
MoKα (λ =
0.71073)
µ (mm−1)
1.243
1.178
1.059
1.422
Crystal size
(mm)
0.3 × 0.15 ×
0.09
0.3 × 0.24 × 0.2
0.25 × 0.16 × 0.1
0.2 × 0.17 ×
0.15
Reflections
collected
72393
54629
72075
94211
GOOF on F2
1.013
1.029
1.072
1.009
Independent
reflections
[Rint]
19101 [Rint
0.0456]
=
19073 [Rint
0.0286]
=
13977 [Rint = 0.0895]
24124 [Rint
0.0492]
=
Data/restrain
ts/parameters
19101/0/696
19073/0/696
13977/0/697
24124/89/1439
Δρmax, Δρmin
(e Å−3)
0.65, -0.68
1.229
0.81, -0.57
1.223
0.54, -0.69
1.220
0.65, -0.47
1.262
Density
(ρcalc, g/cm3)
Completenes
99% (30.60)
99% (30.53)
99% (25.24)
99% (26.06)
s to 
-17 ≤ h ≤ 17,
-18 ≤ k ≤ 19,
-28 ≤ l ≤ 28
-17 ≤ h ≤ 17,
-19 ≤ k ≤ 19,
-28 ≤ l ≤ 28
-14 ≤ h ≤ 17,
-31 ≤ k ≤ 30,
-27 ≤ l ≤ 26
-15 ≤ h ≤ 15,
-20 ≤ k ≤ 24,
-31 ≤ l ≤ 31
Limiting
indices
2θ range (°)
F (000)
3.094 to 61.214
1202.0
3.506 to 61.066
1200.0
5.058 to 57.982
2306.0
2.14 to 52.12
2412.0
Final R
indices
R1 = 0.0365,
wR2 = 0.0789
R1 = 0.0306,
wR2 = 0.0704
R1 = 0.0624,
wR2 = 0.1112
R1 = 0.0360,
wR2 = 0.0750
[I>2 (I)]
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R indices (all
data)
R1 = 0.0543,
wR2 = 0.0866
2023655
R1 = 0.0418,
wR2 = 0.0757
2023662
R1 = 0.1049,
wR2 = 0.1178
2023663
R1 = 0.0611,
wR2 = 0.0845
CCDC
2023665
Number
Computational details
We have used MOLCAS 8.2 program for ab initio calculations.32 The CAS (8, 7), (9, 7) and (11,
7) method have been used to compute spin-free energies of DyIII, TbIII, and ErIII metal ions
respectively by considering 9, 8, and 11 active electrons spanning in seven f-orbitals in their
respective complexes 1, 2 and 4. Under the configuration integration method 21 sextets and 224
quartets (for Dy), 7 septets and 140 quintets (for Tb), and 11 quartets and 112 doublets (for Er)
excited states were computed. The spin-orbit restricted active space state interaction (SO-RASSI)
was used to mix the limited number of computed spin-free states to obtain spin-orbit states.44
Further in the final and SINGLE_ANISO module we have computed the g-tensors, crystal field
parameter, dc magnetic properties, energy spectrum and the mechanism of magnetic relaxation.45
Atomic charges were calculated with the LoProp program46 implemented in MOLCAS 8.2.
Acknowledgements
V. C. is thankful to the Department of Science and Technology, New Delhi, India, for a National
J. C. Bose Fellowship. P. K. is thankful to the National Institute of Science Education and
Research, Bhubaneswar and Tata Institute of Fundamental Research, Hyderabad for doctoral and
postdoctoral fellowships, respectively. P. N. is thankful to the National Institute of Science
Education and Research, Bhubaneswar for doctoral fellowship. N.A is thankful to Tata Institute of
Fundamental Research, Hyderabad for postdoctoral fellowship. We thank Prof. G. Rajaraman,
Indian Institute of Technology Bombay, India for his help in computational studies. Financial
support from Ministerio de Economía y Competitividad (MINECO) for Projects CTQ2014-56312-
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P and PGC2018-102052-B-C21, the Junta de Andalucía (FQM-195 and FEDER project
A‐FQM‐172‐UGR18 and the University of Granada.
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