Oximato-Based Ligands in 3 d/4 f-Metal Cluster Chemistry: A Family of {Cu3Ln} Complexes with a "propeller"-like Topology and Single-Molecule Magnetic Behavior

Article abstract of DOI:10.1021/acs.inorgchem.8b02495

The organic chelating and bridging ligands 9,10-phenanthrenedione-9-oxime (phenoxH) and 9,10-phenanthrenedione-9,10-dioxime (phendoxH₂) were synthesized and subsequently employed for the first time in heterometallic 3d/4f-metal cluster chemistry. The general reaction between CuCl₂·2H₂O, LnCl₃·6H₂O, phenoxH, and NEt₃ in a 1:2:2:4 molar ratio, in a solvent mixture comprising MeCN and MeOH, afforded brown crystals of a new family of [Cu₃LnCl₃(phenox)₆(MeOH)₃] clusters (Ln = Gd (1), Tb (2), Dy (3)) that possess an unprecedented [Cu₃Ln(μ-NO)₆]³⁺ "propeller"-like core. Complexes 1-3 are the first {Cu₃Ln} clusters in which the outer Cu^II and the central Ln^III atoms are solely bridged by diatomic oximato bridges. The {Cu-N-O-Ln} bridging units are very distorted with torsion angles spanning the range 35.5-48.9° and 25.2-55.6° in 1 and 2, respectively. As a result, complexes 1-3 are antiferromagnetically coupled, in agreement with previously reported magnetostructural criteria for oximato-bridged Cu/Ln complexes. The magnetic susceptibility data for all complexes were nicely fit to an isotropic spin Hamiltonian (for 1) or a Hamiltonian that accounts for the spin of the Cu^II atoms, the spin component of the Ln^III, the spin-orbit coupling (λ), an axial ligand-field component around the Ln^III atoms (Δ), and the Zeeman effect (for the anisotropic 2 and 3). The resulting fit parameters were J = -1.34 cm^-1 and g = 2.10 (1), J = -1.42 cm^-1, g_Cu = 2.10, and Δ = -26.3 cm^-1 (2), and J = -1.70 cm^-1, g_Cu = 2.05, and Δ = -38.1 cm^-1 (3). The reported fitting procedure, implemented in the PHI program, is here used for the first time. Even if this method is only valid in high-symmetry Ln environments, when it is properly used allows a very simple and efficient method to obtain the exchange parameters. In light of the negative anisotropy, compounds 2 and 3 were found to exhibit frequency-dependent tails of out-of-phase signals in the presence of a small external dc field, characteristic of the slow magnetization relaxation of a single-molecule magnet. By using the Kramers-Kronig equations, the effective energy barriers (U_eff) were derived and reported as U_eff = 10.1 and 5.4 cm^-1 for 2 and 3, respectively.

Full text of DOI:10.1021/acs.inorgchem.8b02495

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Oximato-Based Ligands in 3d/4f‑Metal Cluster Chemistry: A Family
of {Cu3Ln} Complexes with a “Propeller”-like Topology and Single-
Molecule Magnetic Behavior
Anne Worrell,† Di Sun,‡ Julia Mayans,§ Christos Lampropoulos,∥ Albert Escuer,§
and Theocharis C. Stamatatos*,†
†Department of Chemistry, 1812 Sir Isaac Brock Way, Brock University, L2S 3A1 St. Catharines, Ontario, Canada
‡Key Lab of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong
University, Jinan 250100, P. R. China
§
2
̀
̀
Departament de Quimica Inorganica i Organica and Institut de Nanociencia i Nanotecnologia (IN UB), Universitat de Barcelona,
́
Martí i Franques 1-11, 08028 Barcelona, Spain
∥Department of Chemistry, University of North Florida, 1 UNF Drive, Jacksonville, Florida 32224, United States
S
* Supporting Information
ABSTRACT: The organic chelating and bridging ligands 9,10-phenan-
threnedione-9-oxime (phenoxH) and 9,10-phenanthrenedione-9,10-dioxime
(phendoxH2) were synthesized and subsequently employed for the first time in
heterometallic 3d/4f-metal cluster chemistry. The general reaction between
CuCl2·2H2O, LnCl3·6H2O, phenoxH, and NEt3 in a 1:2:2:4 molar ratio, in a
solvent mixture comprising MeCN and MeOH, afforded brown crystals of a new
family of [Cu3LnCl3(phenox)6(MeOH)3] clusters (Ln = Gd (1), Tb (2), Dy (3))
that possess an unprecedented [Cu3Ln(μ-NO)6]3+ “propeller”-like core.
Complexes 1−3 are the first {Cu3Ln} clusters in which the outer CuII and the
central LnIII atoms are solely bridged by diatomic oximato bridges. The {Cu-N-O-
Ln} bridging units are very distorted with torsion angles spanning the range 35.5−
48.9° and 25.2−55.6° in 1 and 2, respectively. As a result, complexes 1−3 are
antiferromagnetically coupled, in agreement with previously reported magneto-
structural criteria for oximato-bridged Cu/Ln complexes. The magnetic susceptibility data for all complexes were nicely fit to an
isotropic spin Hamiltonian (for 1) or a Hamiltonian that accounts for the spin of the CuII atoms, the spin component of the
LnIII, the spin−orbit coupling (λ), an axial ligand-field component around the LnIII atoms (Δ), and the Zeeman effect (for the
anisotropic 2 and 3). The resulting fit parameters were J = −1.34 cm−1 and g = 2.10 (1), J = −1.42 cm−1, gCu = 2.10, and Δ =
−26.3 cm−1 (2), and J = −1.70 cm−1, gCu = 2.05, and Δ = −38.1 cm−1 (3). The reported fitting procedure, implemented in the
PHI program, is here used for the first time. Even if this method is only valid in high-symmetry Ln environments, when it is
properly used allows a very simple and efficient method to obtain the exchange parameters. In light of the negative anisotropy,
compounds 2 and 3 were found to exhibit frequency-dependent tails of out-of-phase signals in the presence of a small external
dc field, characteristic of the slow magnetization relaxation of a single-molecule magnet. By using the Kramers−Kronig
equations, the effective energy barriers (Ueff) were derived and reported as Ueff = 10.1 and 5.4 cm−1 for 2 and 3, respectively.
Other factors are also reported to affect the sign and
magnitude of the CuII···LnIII magnetic exchange interactions,
such as the type of the bridging ligand(s), the degree of
planarity of the bridging core, and the hinge angle.3
INTRODUCTION
■
The interest in polynuclear CuII/LnIII metal complexes
(coordination clusters or simply metal clusters) mainly stems
from the combined ability of these metal ions to stabilize high-
nuclearity structures with architecturally beautiful topologies
and occasionally fascinating magnetic properties.1 Ferromag-
netic exchange interactions are frequently observed in many
CuII/GdIII complexes owing to the orthogonality of the d- and
f-orbitals and consequently the efficient electron transfer from
the singly occupied 3d CuII orbital to an empty 5d GdIII
orbital.1,2 The CuII···LnIII (Ln = Tb or Dy) interactions can
also be ferromagnetic, but these are more rarely observed due
to the concurrent presence of first-order angular momentum.3
In isotropic CuII/GdIII compounds, weak and ferromagnetic
exchange interactions can lead to large spin ground states and
subsequently to an appreciable magnetocaloric effect (MCE),
i.e., the magnetic material’s thermal response upon exposure to
a magnetic field change.4 These compounds can thus act as
molecular magnetic refrigerants.5 In contrast, the presence of
Received: September 2, 2018
© XXXX American Chemical Society
A
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Inorganic Chemistry
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4000−400 cm−1 range. Elemental analyses (C, H, and N) were
performed on a PerkinElmer 2400 Series II Analyzer. Electrospray
ionization (ESI) mass spectra (MS) were taken on a Bruker HCT
Ultra mass spectrometer from MeOH solutions of both organic
ligands. NMR spectra were obtained on a Bruker Avance DPX-400
MHz instrument and are referenced to the residual proton signal of
significant magnetic anisotropy, as reflected in a large and
negative zero-field splitting parameter, D, renders some of
these CuII/LnIII (Ln = Tb and Dy) analogues suitable
candidates for the observation of single-molecule magnetic
properties. Single-molecule magnets (SMMs) are molecular
species that retain their magnetization in the absence of an
applied field.6 Experimentally, SMMs show superparamagnet-
like properties and exhibit slow relaxation of their magnet-
ization that is usually detected from the appearance of
frequency-dependent out-of-phase alternating-current (ac)
signals.
1
the deuterated solvent for H spectra according to published values.
Magnetic susceptibility measurements were carried out on polycrystal-
line samples of 1−3 with an MPMS5 Quantum Design susceptometer
operating in the temperature range 30−300 K under a magnetic field
of 0.3 T and a small field of 0.03 T in the 30−2 K region to avoid
saturation effects at low temperatures. Direct current (dc) magnetic
susceptibility data were fit with the program PHI.13 The quality of the
fits was parametrized as the R = (χMTexp − χMTcalc)2/(χMTexp)2 factor.
Pascal’s constants were used to estimate the diamagnetic correction,14
which was subtracted from the experimental susceptibility to give the
molar paramagnetic susceptibility (χM) for each complex.
For the synthesis of new CuII/LnIII clusters, the choice of the
primary bridging/chelating organic ligand remains one of the
most appealing challenges. From an extensive literature survey,
it becomes apparent that alkoxido-based organic chelates have
dominated the field and contributed the most to the synthesis
of CuII/LnIII clusters with interesting structural and magnetic
properties.7 Our group has had a longstanding interest in the
synthesis and use of new mono- and dioximato chelates as a
means of isolating heterometallic complexes with unprece-
dented topologies and nontrivial physicochemical properties
(i.e., magnetic, optical, and catalytical).8 To this end, we
recently reported a series of new CuII/LnIII clusters of different
nuclearities (i.e., {Cu6Ln12}9 and {Cu6Ln2}10) and topologies
from the use of 2,6-diacetylpyridine dioxime (dapdoH2) and
acenaphthenequinone dioxime (acndH2), respectively
(Scheme 1).
Synthesis of phenoxH. To an orange suspension of 9,10-
phenanthrenedione (2.08 g, 10.0 mmol) in EtOH (50 mL) was added
a solution of NH2OH·HCl (0.65 g, 10.0 mmol) and pyridine (0.81
mL, 10.0 mmol) in the same solvent (20 mL). The resulting
suspension was refluxed for 30 min, during which time the solids
dissolved and the solution turned to clear orange. The resulting
solution was cooled, filtered, and left for slow evaporation at room
temperature. The next day, an orange microcrystalline solid of the
titled mono-oxime was formed, which was collected by fitration,
washed with cold EtOH (2 × 5 mL), and dried under vacuum for 24
1
h. The yield was 96%. M.p.: 156−159 °C. H NMR (CDCl3, 400
MHz) δ (ppm): 7.48−7.58 (s, 3H), 7.80−7.84 (m, 1H), 8.13 (d, J =
7.8 Hz, 1H), 8.21 (d, J = 8.1 Hz, 1H), 8.37−8.45 (m, 2H), 12.15 (m,
1H). Elemental analysis (%) calcd for C14H9NO2: C 75.33, H 4.06, N
6.27; found C 77.25, H 4.01, N 6.42. Positive ESI-MS (m/z): 224 (M
− H+), 246 (M − Na+), 262 (M − K+).
Scheme 1. Structural Formulae and Abbreviations of the
Organic Chelating/Bridging Ligands Discussed in the Text
Synthesis of phendoxH2. To an orange suspension of 9,10-
phenanthrenedione (2.08 g, 10.0 mmol) in EtOH (50 mL) was added
a solution of NH2OH·HCl (1.63 g, 25.0 mmol) and pyridine (2.03
mL, 25.0 mmol) in the same solvent (30 mL). The resulting orange
suspension was refluxed for 22 h, during which time the solids
dissolved and the solution turned to clear yellow. The resulting
solution was cooled, filtered, and left for slow evaporation at room
temperature. The next day, a yellow microcrystalline solid of the titled
dioxime was formed, which was collected by fitration, washed with
cold EtOH (2 × 5 mL), and dried under vacuum for 24 h. The yield
1
was 92%. M.p.: 156−158 °C. H NMR (DMSO-d6, 400 MHz) δ
(ppm): 11.91−12.69 (m, 2H), 8.70 (2H, dd, J = 1.5 Hz), 8.30 (2H,
dd, J = 1.5 Hz), 7.43 (2H, dd, J = 4.5 Hz). Elemental analysis (%)
calcd for C14H10N2O2: C 70.58, H 4.23, N 11.76; found C 70.46, H
4.13, N 11.92. Positive ESI-MS (m/z): 239 (M − H+), 261 (M −
Na+).
The acndH2 ligand inspired us to move this research forward
and synthesize the structurally similar mono- and dioxime
ligands, 9,10-phenanthrenedione-9-oxime (phenoxH) and
9,10-phenanthrenedione-9,10-dioxime (phendoxH2, Scheme
1). This first step has been successful, and therefore, we
decided to systematically explore the coordination capabilities
of phenoxH and phendoxH2 in CuII/LnIII chemistry. Herein,
we report the synthesis, structures, and detailed magnetic
characterization of a new family of [Cu3LnCl3(phenox)6(Me-
OH)3] complexes (Ln = Gd (1), Tb (2), Dy (3)) bearing an
unprecedented [Cu3Ln(μ-NO)6]3+ “propeller”-like core and
exhibiting some interesting magnetic properties.
Synthesis of [Cu3LnCl3(phenox)6(MeOH)3] (Ln = Gd (1), Tb
(2), Dy (3)). All complexes were prepared in the same manner using
the corresponding lanthanide(III) chloride salts. To a stirred, orange
solution of phenoxH (0.05 g, 0.20 mmol) and NEt3 (56 μL, 0.40
mmol) in a solvent mixture comprising MeOH/MeCN (20 mL, 5:1
v/v) were added together solids CuCl2·2H2O (0.02 g, 0.10 mmol)
and LnCl3·6H2O [0.07 g (Ln = Gd and Tb), 0.08 g (Ln = Dy), 0.20
mmol]. The resulting dark red solution was stirred for 40 min, during
which time all solids dissolved and the solution turned to dark brown.
The resulting solution was filtered, and the filtrate was left to
evaporate slowly at room temperature. After 7−10 days (depending
on the Ln), brown crystals of complexes 1−3 were formed; the
crystals were collected by filtration, washed with cold MeOH (2 × 1
mL) and MeCN (2 × 2 mL), and dried in air. The yields were 40%
(1), 46% (2), 30% (3). Anal. Calcd for the lattice solvate-free
C87H60N6Cu3GdO15Cl3 (1): C, 55.47; H, 3.21; N, 4.46%. Found: C,
55.68; H, 3.47; N, 4.24%. Anal. Calcd for the lattice solvate-free
C87H60N6Cu3TbO15Cl3 (2): C, 55.42; H, 3.21; N, 4.46%. Found: C,
55.55; H, 3.47; N, 4.21%. Anal. Calcd for C87H60N6Cu3DyO15Cl3 (3):
C, 55.32; H, 3.20; N, 4.45%. Found: C, 55.43; H, 3.32; N, 4.36%.
EXPERIMENTAL SECTION
■
Synthesis and Physical Measurements. All manipulations were
performed under aerobic conditions using materials (reagent grade)
and solvents as received unless otherwise noted. The organic ligands
phenoxH and phendoxH2 were prepared and characterized according
to modified literature methods described elsewhere.11,12 Infrared
spectra were recorded in the solid state on a Bruker’s FT-IR
spectrometer (ALPHA’s Platinum ATR single reflection) in the
B
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Table 1. Crystal Data and Structural Refinement Parameters for Compounds 1 and 2
1
2·4MeCN
empirical formula
FW/g mol−1
temp/K
crystal system
space group
a/Å
b/Å
c/Å
α/deg
C
86.25H57.75N6O15Cu3GdCl3
C89H63N7O15Cu3TbCl3
1926.35
173(1)
monoclinic
P21/c
24.0242(3)
17.4143(2)
20.3222(2)
90
1872.35
100(2)
triclinic
P1
̅
14.1414(15)
15.3887(17)
22.197(2)
98.412(3)
β/deg
γ/deg
volume/Å3
Z
91.374(3)
113.192(3)
4374.5(8)
104.287(1)
90
8239.1(2)
4
2
ρcalc/g cm−3
μ/mm−1
F(000)
1.421
1.623
1878
1.553
6.488
3876
radiation
index ranges
Mo Kα (λ = 0.71073)
−16 ≤ h ≤ 16
−18 ≤ k ≤ 18
−26 ≤ l ≤ 26
169848
Cu Kα (λ = 1.54184)
−28 ≤ h ≤ 27
−20 ≤ k ≤ 20
−18 ≤ l ≤ 24
54165
reflns collected
data/restraints/parameters
goodness-of-fit on F2
final R indexes [I ≥ 2σ(I)]
15414/2353/1040
1.067
R1 = 0.0518
wR2 = 0.1075
R1 = 0.0987
wR2 = 0.1215
14579/9/1076
0.911
R1 = 0.0434
wR2 = 0.0894
R1 = 0.0606
wR2 = 0.1041
ab
,
final R indexes [all data]
(Δρ)max,min/e Å−3
0.776 and −0.562
0.858 and −0.699
a
b
R1 = ∑(||Fo| − |Fc||)/∑|Fo|. wR2 = [∑[w(Fo − Fc2)2]/∑[w(Fo )2]]1/2, w = 1/[σ2(Fo ) + (ap)2 + bp], where p = [max(Fo , 0) + 2Fc2]/3.
2
2
2
2
The IR spectra and the associated bands of all complexes 1−3 are
included in subsequent refinement cycles in riding-motion approx-
imation with isotropic thermal displacement parameters (Uiso) fixed at
1.2 or 1.5 × Ueq of the relative atom. Substantial electron density was
found on the data of compounds 1 and 2, most likely due to
additional disordered solvate molecules occupying the spaces
originated by the close packing of the complexes. Our efforts to
properly locate, model, and refine these residues were unsuccessful,
and the investigation for the total potential solvent area using the
software package PLATON confirmed clearly the existence of cavities
with potential solvent accessible void volume. Consequently, the
original data sets were treated with the program SQUEEZE,22 a part
of the PLATON package of crystallographic software, which
calculates the contribution of the smeared electron density in the
lattice voids and adds this to the calculated structure factors from the
structural model when refining against the .hkl file. Unit cell
parameters, structure solution, and refinement details for 1 and 2
are summarized in Table 1.
shown in Figure S1; the superposition of the IR bands of 1−3 further
demonstrates their similar solid-state structures. The chemical and
structural identities of 1 and 2 were also confirmed by single-crystal X-
ray diffraction studies. In the case of 3, we were not able to grow
single crystals of sufficient size, and thus, the acquisition of an X-ray
data set was not possible. Regardless, we have confirmed the identity
of 3 by (i) IR spectroscopic comparison with the authentic, single-
crystalline samples of 1 and 2, and (ii) CHN elemental analyses.
X-ray Crystallography. A crystal of complex 1 was selected and
mounted on MiteGen dual thickness micromounts using inert oil.15
Diffraction data for 1 were collected on a D8 VENTURE
diffractometer equipped with a multilayer mirror monochromator
and a Mo Kα microfocus sealed tube (λ = 0.71073 Å). Images were
processed with the software SAINT+,16a,b and absorption effects were
corrected with the multiscan method implemented in SADABS.16c
The structure was solved using the Bruker SHELXTL inside the
APEX-III software package and refined using the SHELXLE and
PLATON programs.17 Single-crystal X-ray diffraction data of 2 were
collected on a Rigaku Oxford Diffraction XtaLAB Synergy
diffractometer equipped with a HyPix-6000HE area detector at 173
K using Cu Kα (λ = 1.54184 Å) from a PhotonJet microfocus X-ray
source. The structure was solved using the charge-flipping algorithm,
as implemented in the program SUPERFLIP,18 and refined by full-
RESULTS AND DISCUSSION
■
Synthetic Comments. The reaction between stoichio-
metric amounts of 9,10-phenanthrenedione, NH2OH·HCl, and
pyridine as a base in refluxing EtOH has successfully led us to
an orange microcrystalline solid of the targeted mono-oxime
ligand, phenoxH, in quantitative yields. In contrast, the same
reaction, but in an excess of NH2OH·HCl and pyridine,
yielded a yellow microcrystalline solid in yields as high as 92%,
which was characterized as the dioxime phendoxH2 ligand. In
addition to the reported data in the experimental synthetic
section, the IR and ESI-MS spectra of both phenoxH and
phendoxH2 are shown in Figures S2 and S3, respectively. As
expected, the amounts of NH2OH·HCl and pyridine were
found to be decisive for the clean synthesis of phenoxH
2
matrix least-squares techniques against F0 using the SHELXL
program19 through the OLEX2 interface.20 Both structures were
examined using the Addsym subroutine of PLATON21 to ensure that
no additional symmetry could be applied to the models.
The non-hydrogen atoms of both crystal structures were
successfully refined using anisotropic displacement parameters, and
hydrogen atoms bonded to the carbon of the ligands and those of the
hydroxyl groups were placed at their idealized positions using
appropriate HFIX instructions in SHELXL. All of these atoms were
C
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(orange solid) and phendoxH2 (yellow solid). Furthermore,
the reported refluxing times (0.5 and 22 h for phenoxH and
phendoxH2, respectively) were also proved important for the
high-purity and high-yield syntheses of the two ligands. The
targeted organic molecules can also be formed in solvent
MeOH, but the yields are slightly lower than the reported
ones.
The organic chelates phenoxH and phendoxH2 have been
previously used in homometallic 3d-metal chemistry by Li,
Chen, and Liang, yielding antiferromagnetically coupled {Ni2},
{Mn3}, {Mn6}, and {Mn8} complexes.23 The employment of
these groups in heterometallic 3d/4f-metal cluster chemistry
has not been previously documented, and therefore, we
decided to investigate the general CuII/LnIII/phenoxH or
phendoxH2 reaction systems as a means of obtaining cluster
compounds with unique structural characteristics and interest-
ing magnetic properties (high-spin molecules and/or SMMs).
To this end, the general reaction between CuCl2·2H2O, LnCl3·
6H2O, phenoxH, and NEt3 in a 1:2:2:4 molar ratio, in a solvent
mixture comprising MeCN and MeOH, afforded brown
crystals of a new family of [Cu3LnCl3(phenox)6(MeOH)3]
clusters (Ln = Gd (1), Tb (2), Dy (3)) in yields of 30−46%
depending on the 4f-metal ion. The general formation of 1−3
is summarized in stoichiometric eq 1.
constant formation of dark-colored (green and brown)
precipitates from various different reaction systems, in the
presence or absence of ancillary bridging ligands (i.e.,
carboxylates, azides, β-diketones, etc.), was attributed to the
instability/decomposition of phendoxH2 in the presence of
hard Lewis acids (i.e., lanthanides) and presumably its metal-
assisted hydrolysis into the corresponding mono-oxime
(phenoxH) or diketone (9,10-phenanthrenedione).
Description of Structures. Complexes 1−3 are very
similar to each other and differ only in the lanthanide ion
present and the number of lattice solvate molecules (Figure 1
3CuCl2·2H2O + LnCl3·6H2O + 6phenoxH + 6NEt3
+ 3MeOH ⎯M⎯⎯e⎯O⎯⎯→H [Cu3LnCl3(phenox)6(MeOH)3]
MeCN
+ 6Et3N·HCl + 12H2O
(Ln = Gd (1), Tb (2), Dy (3))
(1)
Complexes 1−3 are very stable under the prevailing basic
conditions, and their identities are not affected by the nature of
the external base. The same products, but in lower yields, were
obtained when using NMe3, NPr3, Me4NOH, or other similar
organic bases. In contrast, there are some important synthetic
parameters which were found to affect the formation and/or
crystallinity of the reported compounds. First, the CuII:LnIII
molar ratio has to always be 1:2. When the stoichiometric 3:1
molar ratio of CuII:LnIII was adapted (as a part of our efforts to
optimize the conditions for a more rational synthesis of the
{Cu3Ln} clusters), CuII-only products (monomers and dimers)
were formed and structurally characterized.24 It appears that
the excess of CuII over the LnIII ions in solution facilitates
Figure 1. Labeled representation of the molecular structure of 2 (top)
and its [Cu3Tb(μ-NO)6]3+ “propeller”-like core (bottom). Color
scheme: CuII, cyan; TbIII, yellow; Cl, purple; O, red; N, green; C,
gray. H atoms are omitted for clarity.
eventually the formation of relatively stable {CuII /phenox}
and Figure S4). Complex 2 will be described in detail as a
representative example. Selected interatomic distances and
angles for the structurally characterized complexes 1 and 2 are
listed in Table S1 and Table 2, respectively.
x
complexes in the solid state. Second, the employment of
−
different CuX2 and LnX3 starting materials (i.e., X− = NO3 ,
−
−
−
ClO4 , CF3SO3 , and RCO2 ) did not lead us to any new
cluster compounds; these reactions afforded dark red/brown
insoluble precipitates which we were unable to crystallize and
eventually determine their crystal structures. Lastly, the
identity and crystallinity of 1−3 are strongly affected by the
reaction solvent mixture of MeCN and MeOH. The same
reactions in pure MeCN gave brown insoluble solids which
had different IR spectra than those of the {Cu3Ln} clusters.
When the reactions were performed in MeOH only, brown
microcrystalline materials were formed in low yields; these
were identified as {Cu3Ln} from IR spectroscopic studies and
elemental analyses.
The molecular structure of 2 (Figure 1, top) consists of a
central TbIII atom surrounded by three CuII atoms in a
“propeller”-like conformation. Although a similar metal
topology has been previously reported,25 complexes 1−3 are
the first oximato-bridged {Cu3Ln} clusters. Six deprotonated
phenox− ligands serve to bridge the TbIII atom with the outer
CuII atoms through their oximato groups. The carbonyl O
atoms of the phenox− ligands act terminally to the CuII atoms,
and together with the oximato N atoms, they form stable five-
membered chelate rings. The phenox− ligands are thus
bridging in an overall η1:η1:η1:μ mode, contributing to the
formation of the [Cu3Tb(μ-NO)6]3+ core (Figure 1, bottom).
The {Cu3Tb} core has a virtual D3 symmetry. Peripheral
ligation about the core is further provided by three terminally
Unfortunately, all of our synthetic attempts to prepare CuII/
LnIII complexes with the dioximato ligand phendoxH2 have
failed to produce any crystalline material as of today. The
D
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Table 2. Selected Interatomic Distances (Å) and Angles
(deg) for Complex 2
Tb1−O1
Tb1−O3
Tb1−O5
Tb1−O8
Tb1−O9
Tb1−O10
Tb1−O13
Tb1−O14
Tb1−O15
Cu2−O6
Cu2−O7
Cu2−N5
Cu2−N5−O8−Tb1
Cu2−N6−O5−Tb1
Cu3−N1−O1−Tb1
2.483(3)
2.355(3)
2.491(3)
2.346(3)
2.448(3)
2.370(3)
2.419(3)
2.411(3)
2.424(3)
2.012(3)
1.991(3)
1.976(4)
36.5(6)
Cu2−N6
Cu2−Cl10
Cu3−O2
Cu3−O4
Cu3−N1
Cu3−N2
Cu3−Cl8
Cu4−O11
Cu4−O12
Cu4−N3
Cu4−N4
Cu4−Cl9
Cu3−N2−O3−Tb1
Cu4−N3−O10−Tb1
Cu4−N4−O9−Tb1
1.971(4)
2.326(1)
2.017(3)
1.999(3)
1.965(4)
1.998(3)
2.366(1)
2.042(3)
2.005(3)
1.994(4)
1.967(4)
2.327(2)
48.3(5)
Figure 2. Spherical tricapped trigonal prismatic coordination
geometries of Gd1 (left) and Tb1 (right) atoms in the structures of
1 and 2, respectively. Points connected by the black thin lines define
the vertices of the ideal polyhedron.
35.5(5)
37.6(5)
46.3(5)
48.9(5)
bound Cl− ions, each on the CuII atoms, and three terminal
MeOH molecules on the central TbIII atom. The Cl− ions are
hydrogen-bonded to the MeOH groups, thus enhancing the
overall stability and crystallinity of 2. Furthermore, the three
CuII atoms seem to occupy the vertices of a distorted
equilateral triangle (Cu2···Cu3 = 7.090 Å, Cu3···Cu4 =
7.137 Å, and Cu2···Cu4 = 7.296 Å; Cu2−Cu4−Cu3 = 58.8°,
Cu3−Cu2−Cu4 = 59.5°, and Cu4−Cu3−Cu2 = 61.7°), while
the central TbIII atom is displaced 1.135 Å above the Cu3
plane. Finally, the CuII atoms are not directly linked to each
other but only through the long (NO)-Ln-(NO) pathway;
hence, there are no significant interactions to expect between
the 3d-metal ions. From a supramolecular perspective, both
complexes 1 and 2 exhibit some weak intermolecular π−π
stacking interactions between the aromatic rings of neighbor-
ing phenox− ligands (Figure S5).
Figure 3. χMT versus T plots for complexes 1 (black circles), 2 (blue
squares), and 3 (red diamonds). The solid lines are the fits of the
data; see the text for the spin Hamiltonian and the corresponding fit
parameters. (inset) J-coupling scheme employed for the determi-
nation of the fit parameters.
All CuII atoms are five-coordinate with very distorted
geometries, as confirmed by their trigonality indices, τ,26 which
gave values of 0.47, 0.57, and 0.67 for Cu2, Cu3, and Cu4,
respectively (where τ is 0 and 1 for perfect square pyramidal
and trigonal bipyramidal geometries, respectively). In all cases,
the Cl− ions occupy one of the apical positions. The central
TbIII atom, as well as the GdIII atom in 1, is nine-coordinate,
surrounded by nine O atoms, and it possesses a spherical
tricapped trigonal prismatic geometry. This was confirmed by
the so-called continuous shape measures (CShM) approach of
the SHAPE program,27,28 which allows one to numerically
evaluate by how much a particular polyhedron deviates from
the ideal shape. The best fit was obtained for the spherical
tricapped trigonal prism (CShM values = 0.30 for GdIII and
0.36 for TbIII; Figure 2 and Table S2). Values of CShM
between 0.1 and 3 usually correspond to a not negligible, but
still small, distortion from ideal geometry. We have recently
reported a symmetric {Dy2} complex with the DyIII atoms
possessing a unique spherical tricapped trigonal prismatic
geometry.29 As a result of that unusual polyhedron, a large
axiality and consequently an appreciable energy barrier for the
magnetization reversal was observed.
9.00 cm3·K·mol−1 expected for one GdIII (S = 7/2, L = 0) and
three CuII noninteracting atoms. On cooling, the χMT product
decreases monotonically down to a final value of 3.24 cm3·K·
mol−1 at 2 K, suggesting an overall antiferromagnetic behavior.
Complexes 2 and 3 exhibit room temperature χMT values of
12.85 and 14.67 cm3·K·mol−1, respectively. These are in
agreement with the expected χMT values of three CuII and one
TbIII (7F6 free ion; S = 3; L = 3; gJ = 3/2) or DyIII (6H15/2 free
ion; S = 5/2; L = 5; gJ = 4/3) atoms of 12.92 and 15.30 cm3·K·
mol−1, respectively. Upon decreasing temperature, the χMT
products continuously decrease down to 4.15 and 5.66 cm3·K·
mol−1 at 2 K.
The CuII atoms in 1−3 are not directly linked by any bridge,
and neglecting the CuII···CuII interactions, the only effective
superexchange pathways are those mediated by the double
diatomic oximato bridges between the CuII and LnIII atoms
(1J-model, inset of Figure 3). On the basis of this simple, but
effective, model, the experimental data were fit on the basis of
the proposal reported by Lloret and co-workers for highly axial
symmetric lanthanide environments.30 The applied Hamil-
̂
̂
Solid-State Magnetic Susceptibility Studies. Solid-
state direct-current (dc) magnetic susceptibility (χM) data
were collected on air-dried and analytically pure samples of 1−
3 in the 2.0−300 K temperature range and are plotted as χMT
versus T in Figure 3. The room temperature χMT value of
complex 1 is 9.14 cm3·K·mol−1, close to the spin-only value of
tonian is shown in eq 2, where S1‑3 and S are the spin operators
of the three CuII and LnIII atoms, respectively. Furthermore,
the J coupling constant describes the CuII···LnIII interactions, λ
is the spin−orbit coupling parameter, Δ parametrizes the axial
zero-field splitting of the LnIII atom, and κ is an orbital
reduction parameter. The first term of the Hamiltonian gives
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the interaction between the spin of the CuII atoms and the spin
component of the LnIII. The second term describes the spin−
orbit coupling, the third term accounts for an axial ligand-field
component around the LnIII atoms, and the last term is the
Zeeman effect.30
obtained J values and subsequently the antiferromagnetic
behavior of 1−3 agree well with previously reported oximato-
bridged Cu/Ln complexes.31 This is rationalized in terms of
the degree of distortion of the {Cu-N-O-Ln} magnetic
exchange pathways. For planar {Cu-N-O-Gd} units, the
interactions are expected to be ferromagnetic, but for large
Cu−N−O−Gd torsion angles, such as those of complexes 1−
3, the coupling switches to antiferromagnetic.32
̂
̂
̂
̂
̂
̂
̂
̂
̂
H = −2J(S1·S + S2·S + S3·S) + λLS
2
̂
̂
̂
+ Δ[Lz − L(L + 1)/3] + βH(−κL + 2S)
(2)
The magnetization (M) versus field (H) plots of 2 and 3
show a similar characteristic shape, with a fast increase of M in
the 0−1 T field range and a constant increase of M in larger
fields to reach a nonsaturated value of 4.66 and 5.29 NμB at the
maximum applied field of 5 T (Figure 4). It is remarkable that
the magnetization plots of 2 and 3 can be reproduced in values
and shapes with the J and Δ parameters obtained by the
magnetic susceptibility fits. It is important to mention at this
point that this fitting procedure, implemented in the program
PHI, is here used for the first time. Even if this method is only
valid in highly symmetric environments, when it is properly
used, it allows a very simple and efficient method to elucidate
the exchange parameters, avoiding more detailed, but
complicated, methods. However, the reported procedure can
be only used in very specific and selected examples. It is also
worthy to mention that all previously reported {Cu3Ln}
complexes with a “propeller”-like metal topology have the same
[Cu3Ln(μ-OR)6]3+ cores,25 where R belongs to the scaffold of
different macrocyclic and alkoxide-based chelates. In all of
these alkoxido-bridged {Cu3Ln} complexes, the metal ions are
In the case of {Cu3Gd} (1), the above Hamiltonian reduces to
the conventional isotropic spin Hamiltonian of eq 3.
̂
̂
̂
̂
̂
̂
̂
H = −2J(S1·S + S2·S + S3·S)
(3)
A very good fit of the experimental data for 1 using eq 3 was
obtained (solid line in Figure 3), and the corresponding best-fit
parameters were J = −1.34 cm−1 and g = 2.10 (R = 6.3 × 10−4).
The resulting antiferromagnetic interactions between the CuIII
and GdIII atoms lead to a rational (using the “spin-up”/“spin-
down” vector scheme) ground state spin value of S = 2. This is
also in excellent agreement with the recorded magnetization
data of 1 at 2 K (Figure 4), which show a regular increase that
ferromagnetically coupled and the compounds exhibit SMM
25
properties depending on the LnIII. In addition, Tang, Clerac,
́
and co-workers have reported {MIIIDy3} (MIII = Fe and Co)
complexes with a three-blade “propeller” conformation, where
the 3d-metal ions occupy the central position and they are
bridged to the external DyIII ions through three dithiooxalato
ligands.33 The long magnetic pathways induced very weak
intramolecular magnetic interactions between the spin carriers,
while both compounds exhibited SMM properties with fast
relaxation of their magnetization.
In light of the negative anisotropy of compounds 2 and 3,
alternating current (ac) magnetic susceptibility studies were
performed in the 1.8−7.0 K range using a 4.0 Oe ac field
oscillating at frequencies in the 10−1500 Hz range. The ac
measurements at zero external dc field revealed very weak tails
of out-of-phase (χ″M) signals for both 2 and 3, which were
enhanced under a transverse field of 1000 G, indicating the
presence of quantum tunneling of magnetization. Thus, the ac
measurements as a function of temperature and frequency were
performed under an external dc field of 1000 G. The χ″M
versus T plots for 2 and 3 are presented in Figure 5, and they
both show the absence of resolved peak maxima due to the
blocking of the magnetization. However, 2 and 3 show well-
defined frequency-dependent tails of peaks below ∼5 K, which
are indicative of the slow relaxation of magnetization of an
SMM.
Assuming that the relaxation process has only one
characteristic time corresponding to a Debye relaxation
process, the SMM parameters can be elucidated through the
Arrhenius law: τ(T) = exp(Ea/kBT). By using the Kramers−
Kronig equations,34 which combine the real and imaginary
parts of a function and relate χ″ with χT and χS, where χT and
χS are the isothermal and adiabatic susceptibilities, respectively,
the eq 4 can be derived.
Figure 4. Magnetization (M) versus field (H) plots for complexes 1
(black circles), 2 (blue squares), and 3 (red diamonds). Solid lines
show the simulation of M with the values obtained from the magnetic
susceptibility fits.
tends to a value of 4.32 NμB under the maximum applied field
of 5 T. For the anisotropic complexes 2 and 3, the
experimental data were perfectly fit to the Hamiltonian of eq
2. The best-fit parameters were J = −1.42 cm−1, gCu = 2.10, and
Δ = −26.3 cm−1 (R = 4.0 × 10−5) for 2, and J = −1.70 cm−1,
gCu = 2.05, and Δ = −38.1 cm−1 (R = 4.0 × 10−5) for 3. The
relatively small value of gCu for 3 is due to the fixed orbital
reduction parameter, κ, in the fitting Hamiltonian. The κ
parameter could range between 0.95 and 1 as a function of the
covalency. To avoid overparametrization, we have fixed the
value of κ as 1. A lower κ value will be compensated with a
larger gCu. Indeed, an alternative good fitting of the
experimental data of 3 with κ = 0.985 gives the following
best-fit parameters: J = −1.60 cm−1, gCu = 2.10, and Δ = −33.0
cm−1. The negative sign of Δ suggests that the superexchange
interactions take place between the three SCu = 1/2 and the
lanthanide’s MJ = 6 (for Tb) or 15/2 (for Dy) levels.30 The
F
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Article
Figure 6. Debye plots of complexes 2 (left) and 3 (right) for the
frequencies 1500 (orange), 1000 (green), and 500 (blue) Hz. The
solid lines correspond to the fit of the data by applying eq 4; see the
text for the fit parameters.
distortions of the {Cu-N-O-Ln} bridging units, the reported
compounds exhibit antiferromagnetic exchange interactions.
However, the presence of magnetic anisotropy has contributed
to the onset of slow magnetization relaxation for the {Cu3Tb}
and {Cu3Dy} analogues, albeit with small energy barriers of
10.1 and 5.4 cm−1, respectively. A new fitting method,
implemented in the program PHI, is here used for the first
time and allowed for the determination of the magnetic
exchange parameters in highly symmetric lanthanide environ-
ments. A very good fit of the magnetic susceptibility data gave
the parameters: J = −1.34 cm−1 and g = 2.10 (for 1), J = −1.42
cm−1, gCu = 2.10, and Δ = −26.3 cm−1 (for 2), and J = −1.70
cm−1, gCu = 2.05, and Δ = −38.1 cm−1 (for 3), thus confirming
the antiferromagnetic interactions between the CuII and LnIII
atoms and the presence of negative magnetic anisotropy. Our
synthetic efforts are currently oriented toward the accomplish-
ment of two short-term research objectives: (i) the deliberate
replacement of the outer CuII atoms by other divalent 3d-metal
ions (i.e., NiII and ZnII) without disturbing the “propeller”-like
core, and (ii) the chemical manipulation of the lanthanides’
polyhedra by removing the coordinated MeOH molecules
and/or substituting them with anionic groups.
Figure 5. Temperature dependence of the out-of-phase (χM″) ac
magnetic susceptibility under an applied dc field of 1000 G for
complexes 2 (top) and 3 (bottom) at the indicated frequencies. Solid
lines are guides for the eye only.
ln(χ″/χ′) = ln(ωτ0) + U /kBT
(4)
eff
In eq 4, ω is the angular frequency, τ0 is the pre-exponential
factor, which provides a quantitative measure of the attempt
time of relaxation from the thermal phonon bath and its value
is usually within the 10−6−10−10 s range, Ueff is the effective
energy barrier for the magnetization reversal, and kB is the
Boltzmann’s constant. Equation 4 is a valuable tool in the
absence of out-of-phase peak maxima from the χ″M versus T
plots and allows one to determine the important SMM
parameters, Ueff and τ0. On the basis of eq 4, the best-fit
parameters obtained for complexes 2 and 3 (Figure 6) were
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b02495.
■
S
U
eff = 10.1 cm−1 and τ0 = 1.7 × 10−6 s, and Ueff = 5.4 cm−1 and
τ0 = 5.1 × 10−6 s, respectively. The resulting energy barriers are
small and much lower than the energy gap between the ground
and the first excited states, and thus an Orbach process must
be excluded as an efficient magnetization relaxation process for
complexes 2 and 3.35
Various structural and spectroscopic figures (PDF)
Accession Codes
CCDC 1864722 and 1864723 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
CONCLUSIONS
■
In conclusion, we have reported an unprecedented family of
{Cu3Ln} complexes with a “propeller”-like topology, in which
the outer CuII and central LnIII atoms are exclusively bridged
by the diatomic oximato groups of six deprotonated phenoxH
(9,10-phenanthrenedione-9-oxime) ligands. Due to the large
AUTHOR INFORMATION
Corresponding Author
*E-mail: tstamatatos@brocku.ca.
■
G
DOI: 10.1021/acs.inorgchem.8b02495
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Inorganic Chemistry
Article
Bridged MIILnIII Complexes Derived from N,N′-Ethylenebis(3-
ethoxysalicylaldiimine) (M = Cu or Ni; Ln = Ce-Yb): Observation
of Surprisingly Strong Exchange Interactions. Inorg. Chem. 2005, 44,
3524−3536.
ORCID
Di Sun: 0000-0001-5966-1207
Christos Lampropoulos: 0000-0001-9065-1453
Theocharis C. Stamatatos: 0000-0002-9798-9331
(4) (a) Evangelisti, M. In Molecular Magnets: Physics and
́
́
Applications; Bartolome, J., Luis, F., Fernandez, J. F., Eds.; Springer:
Berlin, 2014; pp 365−387. (b) Evangelisti, M.; Luis, F.; de Jongh, L.
J.; Affronte, M. J. Magnetothermal properties of molecule-based
materials. J. Mater. Chem. 2006, 16, 2534−2549.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(5) For example, see: (a) Evangelisti, M.; Brechin, E. K. Recipes for
enhanced molecular cooling. Dalton Trans. 2010, 39, 4672−4676.
(b) Evangelisti, M.; Roubeau, O.; Palacios, E.; Camon, A.; Hooper, T.
This work was supported by NSERC-DG, ERA, Brock
University (Chancellor’s Chair for Research Excellence) to
Th.C.S. C.L. acknowledges support from the National Science
Foundation (NSF 1429428 and 1626332) and the CCSA
Research Corporation for Science Advancement. A.E and J.M
thank the Ministerio de Economia y Competitividad (Project
CTQ2015-63614-P) for the financial support. This work was
also funded by the National Natural Science Foundation of
China (Grant Nos. 21822107 and 21571115).
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N.; Brechin, E. K.; Alonso, J. J. Cryogenic Magnetocaloric Effect in a
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Hooper, T.; Brechin, E. K.; Evangelisti, M.; Murray, K. S. Molecular
coolers: The case for [CuII GdIII4]. Chem. Sci. 2011, 2, 1166−1169.
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DOI: 10.1021/acs.inorgchem.8b02495
Inorg. Chem. XXXX, XXX, XXX−XXX