Thermal expansion and magnetic properties of benzoquinone-bridged dinuclear rare-earth complexes

Article abstract of DOI:10.1039/c7dt02565c

The synthesis and structural characterization of two benzoquinone-bridged dinuclear rare-earth complexes [BQ(MCl₂·THF₃)₂] (BQ = 2,5-bisoxide-1,4-benzoquinone; M = Y (1), Dy (2)) are described. Of these reported metal complexes, the dysprosium analogue 2 is the first discrete bridged dinuclear lanthanide complex in which both metal centres reside in pentagonal bipyramidal environments. Interestingly, both complexes undergo significant thermal expansion upon heating from 120 K to 293 K as illustrated by single-crystal X-ray and powder diffraction experiments. AC magnetic susceptibility measurements reveal that 2 does not show the slow relation of magnetization in zero dc field. The absent of single-molecule behaviour in 2 arises from the rotation of the principal magnetic axis as compared to the pseudo-C₅ axis of the pentagonal bipyramidal environment as suggested by ab initio calculations. The cyclic voltammetry and chemical reduction experiments demonstrated that complexes 1 and 2 can be reduced to radical species containing [BQ^3-]. This study establishes efficient synthetic strategy to make bridged redox-active multinuclear lanthanide complexes with a pentagonal bipyramidal coordination environment that are potential precursors for single-molecule magnets.

Full text of DOI:10.1039/c7dt02565c

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Mansikkamäki, M. Lahtinen, F. Guo, E. Kalenius, R. Layfield and L. F. Chibotaru, Dalton Trans., 2017, DOI:
10.1039/C7DT02565C.
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Thermal expansion and magnetic properties of benzoquinone-
bridged dinuclear rare-earth complexes
Jani O. Moilanen,*a Akseli Mansikkamäki,a Manu Lahtinen,a Fu-Sheng Guo,b Elina Kalenius,a
Received 00th January 20xx,
Accepted 00th January 20xx
Richard A. Layfield,b and Liviu F. Chibotaruc
DOI: 10.1039/x0xx00000x
The synthesis and structural characterization of two benzoquinone-bridged dinuclear rare-earth complexes [BQ(MCl2·THF3)2]
(BQ = 2,5-bisoxide-1,4-benzoquinone; M = Y (1), Dy (2)) are described. Of these reported metal complexes, the dysprosium
analogue 2 is the first discrete bridged dinuclear lanthanide complex in which both metal centres reside in pentagonal
bipyramidal environments. Interestingly, both complexes undergo significant thermal expansion upon heating from 120 K
to 293 K as illustrated by single-crystal X-ray and powder diffraction experiments. AC magnetic susceptibility measurements
reveal that 2 does not show the slow relation of magnetization in zero dc field. The absent of single-molecule behaviour in
2 arises from the rotation of the principal magnetic axis as compared to the pseudo-C5 axis of the pentagonal bipyramidal
environment as suggested by ab initio calculations. The cyclic voltammetry and chemical reduction experiments
demonstrated that complexes 1 and 2 can be reduced to radical species containing [BQ3•-]. This study establishes efficient
synthetic strategy to make bridged redox-active multinuclear lanthanide complexes with a pentagonal bipyramidal
coordination environment that are potential precursors for single-molecule magnets.
www.rsc.org/
provide an excellent opportunity to synthesize bridged
dinuclear rare-earth complexes and scope their redox
properties.
Introduction
The redox active ligand 2,5-dihydroxy-1,4-benzoquinone
(DHBQ) can function as a bridging unit between two metal
centres.1 Its ability to bind transition metals has been utilized to
synthesize a plethora of dinuclear transition metal complexes,1,2
magnetically ordered metal-organic frameworks3 and cage
complexes that are potential anticancer agents.4 In contrast to
the rich transition metal chemistry of DHBQ, the rare-earth
chemistry of DHBQ is less developed. Only a couple of the
reported structures are discrete dinuclear complexes and all
others are coordination polymers.5,6 The lack of discrete
dinuclear rare earth complexes of DHBQ can be partly explained
by the fact that most of the previous studies have focused on
the development of extended coordination frameworks
suitable for solid oxide fuel cells, gas hydrates and three-
dimensional magnetic materials.5b,6e-g The redox activity of
DHBQ, and its two coordinating pockets with hard donor atoms
Previous studies focused on dinuclear lanthanide complexes
with pentagonal bipyramidal symmetry around the lanthanide
centres have shown that these complexes possess intriguing
magnetic properties, such as single-molecule magnetism.7 In all
four such complexes the metal centres are bridged by two Schiff
base ligands. Also, carboxylate groups or chloride ions have
been used as a bridging unit in three reported 1D-chain
lanthanide complexes that show single-molecule magnetic
behaviour and feature pentagonal bipyramidal environments
around the lanthanide centres.8 Thus, the range of ligands to
bridge between pentagonal bipyramidal lanthanide centres is
still rather limited. Particularly, interesting bridging units are
redox-active ligands because they serve as building blocks for
the radical bridged complexes. Indeed, the redox-active ligands
have been used to synthesize diamagnetic and radical bridged
dinuclear lanthanide complexes that show single-molecule
magnet behaviour.9 However, to the best of our knowledge, no
dinuclear lanthanide complex with local pseudo-D5h
coordination environments around the lanthanide ions
incorporating a single bridging redox-active ligand has been
previously reported.
a.University of Jyväskylä, Department of Chemistry, Nanoscience Centre, P.O. Box
35, FI-40014, University of Jyväskylä, Finland. E-mail: jani.o.moilanen@jyu.fi
b.The University of Manchester, School of Chemistry, Oxford Road, M13 9PL,
Manchester, UK.
c. Theory of Nanomaterials Group, and INPAC-Institute of Nanoscale Physics and
Chemistry Katholieke Universiteit Leuven, Celestijnenlaan 200F, 3001 Leuven,
Belgium.
Herein, we report the synthesis, structural characterization,
redox and magnetic properties of two new benzoquinone-
bridged dinuclear rare-earth complexes [BQ(MCl2·THF3)2] (M =
d.†Electronic Supplementary Information (ESI) available: Figures S1-S15, Tables S1-
S5 and crystallographic data in CIF format. CCDC 1557623, 1557624 and
1557625. For ESI and crystallographic data in CIF format see
DOI: 10.1039/x0xx00000x
Y (1
), Dy (
2
)) of which the dysprosium analogue represents the
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first example of pentagonal bipyramidal dinuclear lanthanide dichloromethane, dimethoxyethane, hexanes, tetVraiehwyAdrtircolefOunrlainne
DOI: 10.1039/C7DT02565C
complex containing one redox active ligand as a bridging unit.
and toluene).
Cyclic voltammetry
Experimental section
Cyclic voltammetry measurements were carried out utilizing a
Gamry Reference 600 potentiostat. Potentials were scanned
with respect to the quasi-reference electrode in a single
compartment cell fitted with Pt wire electrodes and referenced
to the Fc/Fc+ couple of ferrocene at 0.547 V vs SCE. All
measurements were made under argon atmosphere using the
scan rate 200 mV s-1 in freshly distilled and degassed THF which
was 0.1 M in [(n-Bu)4N]+[PF6]- supporting electrolyte at 293 K.
General considerations
The synthesis and manipulation of all compounds was carried
out employing standard Schlenk and glovebox techniques in an
inert atmosphere of argon. Solvents were dried by refluxing
them either over CaH2 (dichloromethane, bromobenzene) or
CaCl2 (chloroform) or sodium without (hexanes, toluene) and
with benzophenone (tetrahydrofuran, dimethoxyethane) at
least three days. Solvents were then distilled, degassed and
stored over activated 4 Å molecular sieves. Acetonitrile was
dried by distilling it first over P2O5 and then refluxing it over
CaH2 at least three days before being distilled, degassed and
stored over activated 3 Å molecular sieves. Trimethylsilyl
chloride, triethylamine and tetrakis(dimethylamino)ethylene
(TDAE) were distilled prior to use. Anhydrous MCl3 (RE = Y and
Dy) (Strem), tetrabutylammonium hexafluorophosphate
(Sigma-Aldrich) and 2,5-dihydroxy-1,4-benzoquinone (Sigma-
Aldrich) were used as received. THF-d8 was degassed and
refluxed over sodium for two days after which it was vacuum
transferred into an ampoule and stored in the glovebox.
No precise molarity of analytes (1 and 2) could be determined
due to their poor solubility to THF.
Magnetic measurements
Magnetic experiments were performed on a Quantum Design
MPMS-XL SQUID magnetometer equipped with a 7 T magnet.
Variable-temperature magnetic susceptibility was measured
with an external magnetic field of 10000 Oe in the temperature
range of 2–300 K and the frequency dependent ac susceptibility
was measured with an oscillating field of 1.55 Oe. Finely ground
microcrystalline powders of samples were immobilized in
eicosane matrix inside a NMR tube sealed by flame torch
Single crystal X-ray crystallography and powder diffraction
NMR, IR, ICP, EA and mass spectrometry
The single crystals of
1-3 were measured with Agilent
1H NMR and 13C NMR spectra were measured on a Bruker
Avance III 300 MHz and Bruker Avance DRX 500 MHz NMR
spectrometers. FTIR spectra were acquired on a Bruker ALPHA
platinum single reflection diamond ATR-FTIR spectrometer. ICP
atomic emission spectroscopy was carried out by Perkin Elmer
Optima 8300 DV ICP spectrometer and elemental analyses were
performed at Elementar Vario EL III elemental analyzer.
Technologies SuperNova diffractometer equipped with a
multilayer optics monochromatic dual source (Cu and Mo) Atlas
detector using CuKα (λ = 1.5418 Å) radiation at 120 K and 293 K.
CrysalisPro was used for data acquisition, reduction and
analytical face-index absorption correction.10 the crystal
structures were solved by the SHELXS11 program using direct
methods and refined by the SHELXL12 program employing least
squares minimization. Olex2 program was used throughout the
solving and refining procedures.13
ESI-MS experiments for complexes 1 and 2 were performed
on ABSciex QSTAR Elite ESI-Q-TOF mass spectrometer equipped
with an API 200 TurboIonSpray ESI source from ABSciex (former
MDS Sciex) in Concord, Ontario (Canada). The sample solutions
were prepared in dry THF or THF/MeCN. The samples were
injected into the ESI source with a flow rate of 5 µl/min. The
parameters were optimized to get maximum abundance of the
ions under study. Room-temperature nitrogen was used for
nebulization. The measurements and data handling were
accomplished with Analyst® QS 2.0 Software. Mass spectra
were externally calibrated by ESI Tuning mix (Agilent
Technologies). The compositions of the ions were verified by
comparing experimental m/z values and isotopic patterns with
the theoretical ones.
Powder diffraction data of complexes
1 and 2 were
measured with PANalytical X´Pert PRO MPD diffractometer in
Bragg–Brentano geometry and Johansson monochromator to
produce pure CuKα1 radiation (1.5406 Å; 45kV, 40mA). Each
sample was prepared in glove box between two Kapton foils to
protect the sample from air and atmospheric moisture. The
data was recorded from a spinning sample by X´Celerator
detector using continuous scanning mode in 2θ range of 6–70°
with a step size of 0.017° and counting times of 60 s per step.
The diffraction data were analysed using program PANalytical
HighScore Plus v. 4.5.
EPR
Computational details
The solid state X-band EPR spectrum of 1•- was acquired on
Magnettech MiniScope MS 200 spectrometer at 293 K using
following parameters: ν = 9.3905 GHz, centre field = 334.964
mT, spectrum width = 100 mT, modulation amplitude = 2 mT
and microwave attenuation = 30 dB. No solution state EPR
spectrum was recorded for 1•- due to its insolubility to tested
organic solvents (acetonitrile, bromobenzene, chloroform,
The multireference ab initio calculations on
2 were performed
using the standard CASSCF/SO-RASSI methodology as
implemented in the Molcas quantum chemistry code version
8.2.14 The geometry of
2 was extracted from the crystal
structure and used without any further optimization. Two
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separate calculations were carried out were the other Dy ion 300 K, δ/ppm) = 0.85, 113.24, 157.72, 184.80. IR (ν/cm–1): 2961
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DOI: 10.1039/C7DT02565C
was replaced by an Y ion. Roos’ ANO-RCC basis sets were (w), 1655 (m), 1585 (s), 1408 (w), 1368 (m), 1254 (m), 1194 (s),
employed throughout.15 A VTZP quality basis set corresponding 901 (m), 830 (s), 787 (m), 752 (s), 693 (m), 641 (m), 554 (m).
to a [9s8p6d4f3g2h] contraction was used for the Dy ions and
VDZP quality basis sets corresponding to [2s1p], [3s2p1d], Synthesis of [BQ(YCl2·THF3)2], 1. The suspension of 2,5-
[3s2p1d], [4s3p1d], and [6s5p3d1f] contractions for H, C, O, Cl, bis[(trimethylsilyl)oxy]-2,5-cyclohexadiene-1,4-dione (400 mg,
and Y, respectively, were used for the other atoms. Scalar 1.4 mmol) and YCl3 (540 mg, 2.8 mmol) in THF (35 ml) was
relativistic effects were treated using the exact two component stirred 1 hour at 55°C. Temperature was increased to 66°C and
(X2C) transformation.16 State-averaged CASSCF calculations17 the stirring was continued 1.5 hours. The mixture was filtered
correlating all nine 4f electrons in the seven 4f orbitals was hot. The remaining red residues was washed with hexanes (2 ×
performed. All 21 sextet, 224 quartet and 490 doublet roots 10 ml) and dried under vacuum to afford
1 as a red
were solved. The spin-free states obtained as solutions to the microcrystalline solid. Yield: 851 mg (68 %). EA analysis
CASSCF calculations were than mixed by spin-orbit coupling (C30H50Cl4O10Y2): calc.: C: 40.47, H: 5.66; found C: 40.10; H: 5.34.
using the restricted active space state interaction (SO-RASSI) 1H NMR (300 MHz, THF-d8, 300 K, δ/ppm) = 1.78 (m, THF), 3.62
method.18 All spin sextets, 128 spin quartets and 130 spin (m, THF), 5.24 (s, 2H, BQ). 13C NMR (125 MHz, THF-d8, 300 K,
doublets corresponding to an energy cut-off of 50,000 cm–1 δ/ppm)
were included in the SO-RASSI procedure. The local magnetic [BQ(YCl·THF2)2]2+ (C22H34O8Cl2Y2)2+: m/z 336.9868; found: m/z
properties (
tensor, crystal field parameters and transition 336.9858. IR (ν/cm–1): 2980 (w), 2953 (w), 2886 (w), 1545 (s),
=
98.82, 184.88. ESI-Q-TOF-MS calc. for
g
magnetic moments) were then extracted from the SO-RASSI 1399 (m), 1379 (m), 1286 (w), 1273 (w), 1254 (m), 1018 (m), 915
wave functions using the SINGLE_ANISO routine.19 The (w), 867 (m), 816 (m), 770 (w), 673 (w), 504 (w), 467 (m).
exchange and dipolar interactions between the Dy ion were
calculated with the POLY_ANISO routine.20 The sixteen lowest Synthesis of [BQ(DyCl2·THF3)2], 2
states corresponding to the crystal-field split states of the 6H15/2 same synthetic procedure as for
.
2
was synthesized using the
employing 2,5-
1
multiplet were taken into account in the exact diagonalization bis[(trimethylsilyl)oxy]-2,5-cyclohexadiene-1,4-dione (200 mg,
of the exchange Hamiltonian. The exchange interaction was 0.7 mmol), DyCl3 (369 mg, 1.37 mmol) and THF (20 ml). Yield:
modelled using the Lines model21 and the exchange parameter 491 mg (68 %). EA analysis (C30H50Cl4O10Dy2): calc.: C: 34.73, H:
was determined by fitting the calculated susceptibility to the 4.86; found C: 34.28; H: 4.66. ESI-Q-TOF-MS calc. for
experimental data by scanning the exchange parameter with [BQ(DyCl·THF2)2]2+ (C22H34O8Cl2Dy2)2+: m/z 411.0087; found:
0.001 cm–1 increments.
m/z 411.0336. IR (ν/cm–1): 2980 (w), 2950 (w), 2886 (w), 1543
(s), 1399 (m), 1379 (m), 1295 (w), 1254 (m), 1018 (m), 916 (w),
867 (m), 816 (m), 770 (w), 673 (w), 504 (w), 467 (m).
Synthesis of studied systems
Synthesis of 2,5-bis[(trimethylsilyl)oxy]-2,5-cyclohexadiene-
1,4-dione, 3. Et3N (4.3 g, 42.8 mmol) in THF (8 ml) was added
dropwise to the brown solution of 2,5-dihydroxy-1,4-
benzoquinone (2.1 g, 15.0 mmol) in THF (30 ml). The solution
was stirred a hour at the ambient temperature during which a
dark red precipitate formed. The precipitate was dissolved in
hot THF and the solution was filtered hot. The storage of the
solution at -20° afforded red crystals. Crystals were isolated by
filtration and washed with hexanes (2 × 10 ml) to afford [2,5-
bisoxide-1,4-benzoquinone][Et3NH]2 in yield of 4.7 g that was
used in the next step without further characterization or
purification.
Doped sample Dy@1. The doped sample was obtained by
reacting
2,5-bis[(trimethylsilyl)oxy]-2,5-cyclohexadiene-1,4-
dione (202 mg, 0.71 mmol), YCl3 (258 mg, 1.32 mmol) and DyCl3
(19 mg, 0.07 mmol) in THF (35 ml) and following the same
synthetic procedure as for 1. The purity of doped sample was
ensured by powder diffraction and IR experiments (Figure S13
and Figure S12). The accurate dysprosium/yttrium ratio was
determined by ICP atomic emission spectroscopy to be 5.1 ± 0.2
%.
Chemical reduction of 1. The red suspension of 1 (100 mg, 0.11
mmol) and TDAE (22 mg, 0.11 mmol) was stirred in THF (4 ml)
overnight during which a green precipitate formed. The green
mixture was filtered and the remaining residue was washed
with hexanes (2 × 3 ml) to yield a green solid as 56 mg yield. The
green solid was characterized by ATR-IR and EPR spectrometers
to contain radical species (Figure S5 and Figure S6). IR (ν/cm–1):
2980 (w), 2880 (w), 1656 (w), 1530 (m), 1468 (m), 1397 (w),
1374 (w), 1250 (w), 1205 (w), 1168 (w), 1020 (w), 875 (m), 773
(w), 497 (w), 454 (w).
Trimethylsilyl chloride (1.56 ml, 12.3 mmol) was slowly
added
to
the
suspension
of
[2,5-bisoxide-1,4-
benzoquinone][Et3NH]2 (2.0 g, 5.8 mmol) in Et2O (40 ml). The
reaction gave immediately a yellow solution and white
precipitate. The solution was filtered and the remaining white
residue was washed with Et2O (2 × 5ml). The combined filtrates
were evaporated to dryness to give a yellow solid. The solid was
dissolved in hot hexanes, filtered hot and stored in the fridge (-
20°) overnight to afford 2,5-bis[(trimethylsilyl)oxy]-2,5-
cyclohexadiene-1,4-dione as yellow crystals. Yield: 1.33 g (80%).
EA analysis (C12H20O4Si2): calc.: C: 50.67; H: 7.09; found C: 50.14;
Chemical reduction of 2. The red suspension of
2 (80 mg, 0.08
mmol) and TDAE (28 mg, 0.14 mmol) was stirred in THF (4 ml)
overnight during which a green precipitate formed. The green
mixture was filtered and the remaining residue was washed
1
H: 7.13. H NMR (300 MHz, THF-d8, 300 K, δ/ppm) = 0.28 (s,
18H, TMS), 5.96 (s, 2H, Ar-H), 5.24. 13C NMR (75 MHz, THF-d8,
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with hexanes (2 × 3 ml) to yield a green solid as 32 mg yield. The preliminary comparisons were made with powder diffraction
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DOI: 10.1039/C7DT02565C
green solid was characterized by ATR-IR to contain radical data recorded at 293 K (Figure 2, Figure S3 and Figure S4). When
species (Figure S7). IR (ν/cm–1): 2980 (w), 2890 (w), 1657 (w), additional single crystal data was acquired at 293 K, we were
1542 (m), 1466 (m), 1399 (w), 1378 (w), 1255 (w), 1208 (w), able to show that the room temperature single crystal data is
1167 (w), 1019 (w), 868 (m), 817 (w), 771 (w), 673 (w), 502 (w), actually identical with the measured powder diffraction data at
467 (w).
293 K as shown in Figure 2 for 2 and in Figure S3 for 1. This
confirms that the bulk powders and single crystals are
structurally consistent with each other. The thermal expansion
Results and discussion
in complexes
1 and 2 presumably originates from the
herringbone arrangement of the complexes in which each
discrete complex interacts with neighbouring complexes mainly
through weak dispersion force that can be modulated via
thermal energy (Figure 1c, Figure S1, Figure S2 and Table S2). A
similar type of thermal expansion has been observed for
framework materials in which metallophilic interactions
(M+···M+) (M = Cu, Ag, Au) are largely responsible for the
thermal expansions of the materials.22
To obtain targeted complexes [BQ(MCl2·THF3)2] (M = Y (1), Dy
(
2
), BQ = 2,5-bisoxide-1,4-benzoquinone) we reacted two
equivalents of anhydrous MCl3 with 2,5-bis[(trimethylsilyl)oxy]-
2,5-cyclohexadiene-1,4-dione in hot THF (Scheme 1). The
reaction yielded the dinuclear bridged complexes and as
3
1
2
analytically pure red crystalline solids that are sparingly soluble
in THF and insoluble in other common organic solvents.
Complexes,
the crystallographic 2-fold rotation axis passing through the
centre of the BQ ligand. In , the symmetry-related seven-
1 and 2 crystallize in the P21/n space group with
2
coordinate Dy3+ ions occupy pentagonal bipyramidal
environments with a nearly linear Cl1-Dy1-Cl2 angle 175.87(8)°
(Figure 1, Figure S1 and Table S1). The O-Dy-Cl angles vary from
81.93(17)° to 98.76(17)°. The equatorial Dy-O bonds to the
coordinated THF ligands and the BQ ligand are in the range
2.388(6)–2.427(6) Å and 2.329(6)–2.338(6) Å, respectively. The
latter bonds are slightly shorter, most likely due to the stronger
electrostatic bonding. The Cl1 and Cl2 ligands are axially
coordinated to dysprosium, with Dy-Cl distance of 2.603(2) Å
and 2.587(2) Å, respectively. The shortest intermolecular
distance between two Dy3+ ions is 7.7950(8) Å and the
intramolecular distance between Dy1 and Dy2 is 8.5531(8) Å.
a)
b)
Compound
1
shows very similar geometrical parameters to
2
(Figure S2 and Table S1).
Comparison of the powder X-ray diffraction patterns of
1
and
undergo
Anisotropic expansion of the unit cells on both
2
measured at 120 K to 293 K reveals that both structures
significant anisotropic thermal expansion.
and are best
a
c)
1
2
seen along the b-axis, which elongates by almost 9% (the b-axis
changes from 14.88 to 16.15 Å, and 15.00 to 16.15 Å for 1 and
2, respectively). In comparison, the a- and c-axes show 1-2%
expansion. This expansion is evidenced also in unit cell volumes
that increase from ~1900 to ~2050 Å3. To visualize the unit cell
transformation and thermal expansion, the overlay of the
packing of the low (120 K) and high (293 K) temperature crystal
structures are shown in Figures 1c and S2. Both figures clearly
show the elongation of intermolecular distance between
complexes along b-axis of crystal structures. Due to the
substantial changes in the unit cell parameters of
1
and
2, their
calculated diffraction patterns at 120
K differed when
Figure 1 (a) Crystal structure of 2. Thermal ellipsoids are drawn at 50% probability level
and hydrogen atoms are omitted for clarity (black = C; red = O; Green = chlorine;
turquoise = dysprosium). (b) The pentagonal bipyramidal environment around Dy3+ ions
illustrated by polyhedra; side view (left) and top view (right). (c) The overlay picture of
low (120 K, green) and high (293 K, blue) temperature structures along a-axis. Selected
bond lengths and angles are given in Table S1.
Scheme 1 The synthesis of 1 and 2.
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observed within the accessible redox window of the solvent for
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DOI: 10.1039/C7DT02565C
and 2, although it is known that the transition metal
1
complexes of [BQ2-] ligand can be reduced sequentially in two
separate, one electron redox processes to a radical trianion
[BQ3•-] and then to the tetraanion [BQ4-].1 While we cannot
unambiguously assign the observed -1/0 wave of
0.21 V vs SCE to a one electron reduction processes from the CV
1 and 2 at -
results, the chemical reduction studies of
1 and 2 afforded
complexes which contained the radical trianion [BQ3•-] (see
below).
In addition to CV measurements, the electron acceptor
ability of the complexes was tested by reacting
1 and 2 with
tetrakis(dimethylamino)ethylene (TDAE), in THF. During the
reduction, a clear colour change occurred: an initially red
suspension turned to a green solid. Evidence for the formation
of a radical anion were acquired by performing X-band EPR
spectroscopy on the powdered sample of the reduced complex
1•-. As shown in Figure S5, the EPR spectrum of 1•- displays an
intensive single peak with giso value of 2.003 G, which is typical
for an organic radical with a doublet ground state. Moreover, a
new absorption band of C=O stretch (≈ 1470 and 2•- indicating
the formation of trianion [BQ3•-] as shown by previous studies
(Figure S6 and Figure S7).2a,c,23 Unfortunately, both reduced
complexes were completely insoluble in all common organic
solvents, which prevented characterization of their solid-state
structures by single-crystal X-ray diffraction. However, the
spectroscopic evidence and cyclic voltammetry measurements
Figure 2 Calculated (red 120 K and green 293 K) and measured (blue 293 K) powder
patterns from 10° to 35° for 2. The full pattern (from 5° to 70°) and the data for 1 is given
in supporting info (Figure S3 and S4).
The electrochemical properties of complexes 1 and 2 were
studied by cyclic voltammetry (CV) using a Pt electrode and [(n-
Bu)4N]+[PF6]- as a supporting electrolyte in THF solutions. Both
complexes showed one irreversible reduction process with
similar E1/2 values of -0.21 V and -0.21 V vs SCE for
1 and 2,
respectively (Figure 3).‡ The result indicates that the nature of
the rare earth ion has a very little effect on the observed
reduction potential of the bridging ligand. This finding is in a
good agreement with previous cyclic voltammetry studies
carried out on redox-active bridged dinuclear rare-earth
complexes.9a Interestingly, no other reduction processes were
clearly demonstrate the redox activity of complexes
Variable-temperature DC magnetic susceptibility
1 and 2.
measurement was performed on a microcrystalline sample of 2
in a field of H = 1 T and in the temperature range 2-300 K. The
measured room-temperature χMT value of 28.47 cm3 K mol-1 is
in good agreement with the theoretical value of 28.34 cm3 K
mol-1 for two non-interacting Dy3+ ions (6H15/2, g = 4/3). Upon
cooling, χMT slowly decreases down to 25 K, and then rapidly
reaches the minimum value of 10.41 cm3 K mol-1 at 2 K (Figure
Figure 3 Cyclic voltammograms for 1 (red) and 2 (green) in THF solutions containing
0.1 M [(n-Bu)4N]+[PF6]- at Pt electrode with scan rate 200 mV s-1. Peak potentials and
currents are given in Table S3.
Figure 4 Experimental (green circles) and calculated (green solid line) plot of χMT as
function of temperature in the field 1 T. Plots of M as function of magnetic field at 1.8 K
(red) and 5 K (blue) (inset, dots = experimental data, solid lines = calculated data).
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4). The inset of Figure 3 shows isothermal magnetization (M vs. systems the ground state QTM is suppressed by relatively
View Article Online
3+
H) plot for 2 at 2 K and 5 K. At both temperatures, the strong exchange interaction between DtOhIe: 10D.1y039/Cio7DnTs,025w6i5tCh
magnetization quickly increases to 2.0 T, and then rises exchange parameter magnitudes of the order 3–7 cm–1
gradually to the saturation value of 11.06 µB at 7 T.
Precise control of the local pseudo-D5d crystal field model, an effective intramolecular exchange parameter was
symmetry around lanthanide metal centres has also been determined for by fitting an ab initio calculated susceptibility
acquired by the Lines model.7a,b Therefore, following the Lines
2
applied to design single-ion dysprosium complexes that to the experimental data (see computational details). Owing to
function as single-molecule magnets (SMMs).24 The strong the large distance between the Dy3+ ions, the exchange
magnetic anisotropy of Dy3+ ion in axial environments is parameter is weak (J = –0.142 cm–1) and, thus, the exchange
enhanced by crystal fields with strong negatively charged axial interaction is not strong enough to block the QTM process. It
donors but weakly charged equatorial donors. For example, should be noted that the agreement between the experimental
[Dy-(Cy3PO)2(H2O)5]Br3·2(Cy3PO)·2H2O·2EtOH
(Cy3PO
=
and calculated susceptibility and isothermal magnetization
tricyclohexyl phosphine oxide) complex displays open plots are not perfect (Figure 4). The deviations from experiment
hysteresis loops up to 20 K24e and [Dy(OtBu)2(pyridine)5][BPh4] most likely arise from the electron correlation outside the 4f
shows the second highest effective barrier for thermally orbital space, which has been neglected in the CASSCF
activated relaxation of magnetization ever reported.24d Thus, calculations. This correlation affects the crystal-field splitting of
6
we were interested to see if complex
2
and its diluted analogue the Dy3+ ground H15/2 multiplet and, consequently, influences
Dy@1 would display SMM behaviour. AC magnetic the rate of thermal depopulation of excited levels as the
susceptibility measurements in zero dc field did not show any temperature is lowered as well as the degree of mixing of states
evidence of slow relaxation of the magnetization neither for
2
due to the Zeeman interaction.25
nor for Dy@1. To better understand this lack of SMM
behaviour, quantum chemical calculations were carried out for
Conclusions
2
.
Ab initio calculations at CASSCF/SO-RASSI level (see
computational details) show that the principal magnetic axes of
the Dy3+ ions in
are not coincident with the pseudo-C5 axes of
In conclusions, we have reported the synthesis of two discrete
benzoquinone-bridged dinuclear rare-earth complexes
2
[BQ(MCl2·THF3)2]
1 and 2 (M = Y (1), Dy (2), BQ = 2,5-bisoxide-
the pentagonal bipyramidal coordination spheres, but are in
fact rotated some 90° and lie in the molecular plane (Figure 5).
This leads to non-negligible transverse components in the g
tensor of the ground Kramers doublet (Table S4), which enables
fast quantum tunnelling of magnetization (QTM). Rotation of
the principal magnetic axes demonstrates that, although from a
purely geometric point of view the ions have pentagonal
bipyramidal coordination environments, the O-donor atoms of
the [BQ]2– ligand provides a stronger, equatorial crystal field
than the THF ligands or the Cl ions. Thus, from a magnetic point
of view, the coordination environment has pseudo-C2v
symmetry with the principal axis in the molecular plane.
In principle, the ground state QTM could be supressed by
sufficiently strong exchange interaction between the Dy3+ ions.
SMM behaviour has indeed been observed in dinuclear
dysprosium complexes which have similar coordination spheres
around the Dy3+ as in
2, that is, Cl and O atoms occupying the
axial and equatorial sites of Dy3+ ions, respectively.7 In these
1,4-benzoquinone) in which the rare earth ions reside in
pentagonal bipyramidal environments. The complexes are
redox active as shown by cyclic voltammetry studies and the
reduction of complexes with TDAE and subsequent
characterization of
1 by EPR spectroscopy. The neutral
complexes also undergo significant anisotropic thermal
expansion when heated from 120 K to 293 K. AC magnetic
susceptibility measurements revealed that complex
2 does not
show any slow relaxation of the magnetization. The lack of SMM
behaviour results from the rotation of the principal magnetic
axis as compared to the pseudo-C5 axis of the pentagonal
bipyramidal environment, and from the lack of sufficiently
strong intramolecular exchange interaction to block the ground
state QTM, as shown by quantum chemical calculations. It
should be possible to modulate the crystal field of complex 2 by
substituting stronger and weaker donor atoms to axial and
equatorial positions, respectively, as well as increase the
solubility of the reduced species by introducing either alkyl or
aryl substituents to the ligand framework. Thus,
2 can be
considered to be a blueprint compound for bridged redox active
multinuclear lanthanide SMMs that adhere to an SMM design
criteria i.e. an ideal geometry coupled with bridging organic
radical ligand.24d
Conflicts of interest
There are no conflicts to declare.
Figure 5 The principal magnetic axis of the ground Kramers doublet (red) of the two Dy3+
ions in 2.
6 | J. Name., 2012, 00, 1-3
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10 CrysalisPro
program,
version
1.71.38.43,
Agilent
Acknowledgements
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Technologies, Oxford, 2012.
11 G. M. Sheldrick, Acta Cryst. 2008, A64 DOI: 10.1039/C7DT02565C
12 G. M. Sheldrick, Acta Cryst. 2015, C71, 3.
13 O.V. Dolomanov, L. J. Bourhis, R. J. Gildea, J. A. K. Howard and
H. Puschmann, J. Appl. Cryst, 2009, 42, 339.
14 F. Aquilante, J. Autschbach, R. K. Carlson, L. F. Chibotaru, M.
G. Delcey, L. De Vico, I. Fdez. Galván, N. Ferré, L. M. Frutos, L.
Gagliardi, M. Garavelli, A. Giussani, C. E. Hoyer, G. Li Manni, H.
Lischka, D. Ma, P. Å. Malmqvist, T. Müller, A. Nenov, M.
Olivucci, T. B. Pedersen, D. Peng, F. Plasser, B. Pritchard, M.
Reiher, I. Rivalta, I. Schapiro, J. Segarra-Martí, M. Stenrup, D.
G. Truhlar, L. Ungur, A. Valentini, S. Vancoillie, V. Veryazov, V.
P. Vysotskiy, O. Weingart, F. Zapata and R. Lindh, J. Comp.
Chem., 2016, 37, 506.
, 112.
JOM, AM, ML and EK thank Academy of Finland (project
numbers 285855 (JM), 282499 (AM), 277250 (ML), 284562 and
278743 (EK)), FSG thanks EC for an MSC Fellowship and RAL
thanks the ERC for the Consolidator Grant ‘RadMag’. Prof. H. M.
Tuononen (University of Jyväskylä) is acknowledged by
providing computational resources for the project. We also
thank Laboratory Technician Elina Hautakangas (University of
Jyväskylä) and Dr. Siiri Perämäki for elemental analyses and ICP
measurements, respectively.
Notes and references
‡ In addition to the -1/0 reduction wave, one irreversible
oxidation process was observed for both complexes (Table S3 and
Figure S8).
15 (a) K. Andersson, P. Å. Malmqvist and B. O. Roos, J. Chem.
Phys., 1992, 96, 1218. (b) B. O. Roos, R. Lindh, P. Å. Malmqvist,
V. Veryazov, and P.-O. Widmark, J. Phys. Chem. A, 2004, 108
2851. (c) B. O. Roos, R. Lindh, P. Å. Malmqvist, V. Veryazov, P.-
,
O. Widmark and A. C. Borin, J. Chem. Phys. A, 2008, 112
11431.
,
,
1
2
S. Kitagawa and S. Kawata, Coord. Chem. Rev., 2002, 224, 11.
For some examples, see: (a) K. S. Min, A. G. DiPasquale, A. L.
Rheingold, H. S. White and J. S. Miller, J. Am. Chem. Soc., 2009,
131, 6229. (b) C. Carbonera, A. Dei, J.-F. Létard, C. Sangregorio
and L. Sorace, Angew. Chem. Int. Ed. 2004, 43, 3135. (c) J. Tao,
H. Maruyama and O. Sato, J. Am. Chem. Soc. 2006, 128, 1790.
(d) W. Cao, X. Zhu, P. Liu, S. Su, S. Hu, Y. Wen, X. Wua and T.
Sheng, Dalton Trans., 2017, 46, 3435.
(a) I.-R. Jeon, B. Negru, R. P. Van Duyne and T. D. Harris, J. Am.
Chem. Soc., 2015, 137, 15699. (b) L. E. Darago, M. L. Aubrey,
C. J. Yu, M. I. Gonzalez and J. R. Long, J. Am. Chem. Soc., 2015,
137, 15703.
16 (a) W. Kutzelnigg and W. Liu, J. Chem. Phys., 2005, 123
241102. (b) M. Filatov, J. Chem. Phys., 2006, 125, 107101. (c)
P. Daoling and M. Reiher, Theor. Chem. Acc., 2012, 131, 1.
17 (a) B. O. Roos, The Complete Active Space Self-Consistent Field
Method and Its Applications in Electronic Structure
Calculations in Advances in Chemical Physics: Ab Initio
Methods in Quantum Chemistry II, Vol. 69, K. P. Lawley (Ed.),
Wiley, New York, 1987, 399–455. (b) B. O. Roos, R. Lindh, P. Å.
3
Malmqvist,
V.
Veryazov
and
P.-O.
Widmark,
Multiconfigurational Quantum Chemistry, Wiley, Hoboken,
2016.
4
5
B. Therrien, G. Süss-Fink, P. Govindaswamy, A. K. Renfrew
and P. J. Dyson, Angew. Chem. Int. Ed., 2008, 120, 3833.
(a) M. A. Subhan, R. Kawahata, H. Nakata, A. Fuyuhiro, T.
Tsukuda and S. Kaizaki, Inorg. Chim. Acta, 2004, 357, 3159. (b)
B. F. Abrahams, J. Coleiro, K. Ha, B. F. Hoskins, S. D. Orchard,
R. Robson, J. Chem. Soc. Dalton Trans., 2002, 1586.
(a) P. E. Riley, S. F. Haddad and K. N. Raymond, Inorg. Chem.,
1983, 22, 3090. (b) C. Robl, Mater. Res. Bull., 1987, 22, 1483.
(c) G. Zucchi, P. Thuery and M. Ephritikhine, CSD
Communication (Private Communication), 2012. (d) K.
Nakabayashi and S. Ohkoshi, Acta Cryst., 2010, E66, m1300.
(e) B. F. Abrahams, J. Coleiro, B. F. Hoskins and R. Robson,
Chem. Commun., 1996, 603. (f) T. Demars, M. Boltoeva, N.
Vigier, J. Maynadié, J. Ravaux, C. Genre and D. Meyer, Eur. J.
Inorg. Chem., 2012, 3875. (g) K. Nakabayashi and S. Ohkoshi,
Inorg. Chem., 2009, 48, 8647.
18 P. Å. Malmqvist, B. O. Roos and B. Schimmelpfennig, Chem.
Phys. Lett., 2002, 357, 230.
19 L. F. Chibotaru and L. Ungur, J. Chem. Phys., 2012, 137
,
064112.
20 (a) L. F. Chibotaru, L. Ungur and A. Soncini, Angew. Chem. Int.
Ed., 2008, 47, 4126. (b) L. Ungur, W. Van den Heuvel and L. F.
Chibotaru, New. J. Chem., 2009, 33, 1224.
6
21 M. E. Lines, J. Chem. Phys., 1971, 55, 2977.
22 (a) A. L. Goodwin, Ma. Calleja, M. J. Conterio, M. T. Dove, J. S.
O. Evans, D. A. Keen, L. Peters and M. G. Tucker, Science, 2008,
319, 794. (b) J. L. Korčok, M. J. Katz and D. B. Leznoff, J. Am.
Chem. Soc. 2009, 131, 4866. (c) J. S. Ovens and D. B. Leznoff,
Inorg.
Chem.,
article
ASAP,
DOI:10.1021/acs.inorgchem.6b03153
23 K. S. Min, A. G. DiPasquale, J. A. Golen, A. L. Rheingold and J.
S. Miller, J. Am. Chem. Soc., 2007, 129 , 2360.
7
(a) Y.-N. Guo, G.-F. Xu, W. Wernsdorfer, L. Ungur, Y. Guo, J.
Tang, H.-J. Zhang, L. F. Chibotaru and A. K. Powell, J. Am.
Chem. Soc., 2011, 133, 11948. (b) Q. Chen, F. Ma, Y.-S. Meng,
24 (a) J.-L. Liu, Y.-C. Chen, Y.-Z. Zheng, W.-Q. Lin, L. U., W.
Wernsdorfer, L. F. Chibotaru and M.-L. Tong, Chem. Sci.,
2013,
4
, 3310. (b) S. K. Gupta, T. Rajeshkumar, G Rajaraman
, 5181. (c) J. Liu, Y.-C.
H.-L. Sun, Y.-Q. Zhang and S. Gao, Inorg. Chem., 2016, 55
,
and R. Murugavel, Chem. Sci., 2016,
7
12904. (c) Y. Jiang, G. Brunet, R. J. Holmberg, F. Habib, I.
Korobkov and M. Murugesu, Dalton Trans., 2016, 45, 16709.
(d) D. N. Woodruff, R. E. P. Winpenny and R. A. Layfield, Chem.
Rev. 2013, 113, 5110. (e) R. A. Layfield, Organometallics, 2014,
33, 1084.
(a) X.-C. Huang, M. Zhang, D. Wu, D. Shao, X.-H. Zhao, W.
Huanga and X.-Y. Wang, Dalton Trans., 2015, 44, 20834. (b)
G.-J. Zhou, Y.-S. Ding and Y.-Z. Zheng, Dalton Trans., 2017, 46
3100. (c) S. She, B. Liu, Y. Yang, Z. Ba, L. Gong, B. Wang and S.
Min, Inorg. Chem. Commun., 2015, 61, 132.
(a) S. Demir, M. Nippe, M. I. Gonzaleza and J. R. Long, Chem.
Sci., 2014, 5, 4701. (b) F.-S. Guo and R. A. Layfield, Chem.
Chen, J.-L. Liu, V. Vieru, L. Ungur, J.-H. Jia, L. F. Chibotaru, Y.
Lan, W. Wernsdorfer, S. Gao, X.-M. Chen and M.-L. Tong, J.
Am. Chem. Soc. 2016, 138, 5441. (d) Y.-S. Ding, N. F. Chilton,
R. P. Winpenny and Y.-Z. Zheng, Angew.Chem. Int.Ed., 2016,
55, 16071. (e) Y.-C. Chen, J.-L. Liu, L. Ungur, J. Liu, Q.-W. Li,
L.-F. Wang, Z.-P. Ni, L. F. Chibotaru, X.-M. Chen, M.-L. Tong, J.
Am. Chem. Soc. 2016, 138, 2829. (f) Y.-C. Chen, J.-L. Liu, Y.
Lan, Z.-Q. Zhong, A. Mansikkamäki, L. Ungur, Q.-W. Li, J. -H.
Jia, L. F. Chibotaru, J. -B. Han, W. Wernsdorfer, X.-M. Chen
and M.-L. Tong, Chem. Eur. J., 2017, 23, 1.
8
9
,
25 L. Ungur and L. F. Chibotaru, Chem. Eur. J. 2017, 23, 3708.
Commun., 2017, 53, 3130. (c) B. S. Dolinar, S. Gómez-Coca, D.
I. Alexandropoulos and K. R. Dunbar, Chem. Commun., 2017,
53, 2283. (d) S. Demir, J. M. Zadrozny, M. Nippe and J. R. Long,
J. Am. Chem. Soc., 2012, 134, 18546.
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DOI: 10.1039/C7DT02565C
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TOC
The benzoquinone-bridged dinuclear rare-earth complexes [BQ(MCl2·THF3)2] (M = Y or Dy) possessing pentagonal bipyramidal
environment around metal centers undergo significant thermal expansion as revealed by single-crystal X-ray and powder
diffraction experiments.
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