Structure and Magnetic Properties of Lanthanide Compounds with the 3,6-Di(Tert-Butyl)-1,2-Benzoquinone Radical Anion

Article abstract of DOI:10.1134/S0022476619070102

Molecular and crystal structures of mononuclear lanthanide (Ln) compounds with the 3,6-di(tert-butyl)-1,2-benzoquinone (SQ) radical anion of the composition [Ln(SQ)₃(THF)₂] (Ln = Pr, Nd, Eu, Tb, Dy), [Ln(SQ)₃(Q)], (Ln = Gd

Full text of DOI:10.1134/S0022476619070102

ISSN 0022-4766, Journal of Structural Chemistry, 2019, Vol. 60, No. 7, pp. 1091-1100. © Pleiades Publishing, Inc., 2019.
Russian Text © The Author(s), 2019, published in Zhurnal Strukturnoi Khimii, 2019, Vol. 60, No. 7, pp. 1139-1148.
STRUCTURE AND MAGNETIC PROPERTIES
OF LANTHANIDE COMPOUNDS WITH THE
3,6-DI(TERT-BUTYL)-1,2-BENZOQUINONE RADICAL ANION
1 1
G. V. Romanenko *, S. V. Fokin ,
1,2 1
G. A. Letyagin , A. S. Bogomyakov ,
1
and V. I. Ovcharenko
Molecular and crystal structures of mononuclear lanthanide (Ln) compounds with the 3,6-di(tert-butyl)-1,2-
benzoquinone (SQ) radical anion of the composition [Ln(SQ)3(THF)2] (Ln = Pr, Nd, Eu, Tb, Dy),
IV
[Ln(SQ)3(Q)], (Ln = Gd, Eu; Q – 3,6-di(tert-butyl)-1,2-benzoquinone), [La(SQ)3(Q)(THF)], [Ce (SQ)4],
[NaLn(SQ)4(THF)3]⋅0.25C6H14 (Ln = Gd, Tb), [NaLn(SQ)4(Q)(THF)] (Ln = La, Pr) are determined. The
study of the magnetic properties of isolated heterospin solid phases reveals that intramolecular
antiferromagnetic exchange interactions between unpaired electrons of SQ moieties dominate in them. At
T ≤ 2 K [Dy(SQ)3(THF)2] exhibits the properties typical of a metamagnetic compound.
DOI: 10.1134/S0022476619070102
Keywords: lanthanides, o-semiquinolates, mononuclear compounds, sodium, mixed-metal compounds,
crystal structure, magnetic properties.
Semiquinolates of d elements is an actively studied class of coordination compounds in which the external action
can induce the effect of the intramolecular redox isomerism causing jumps in thermomagnetic curves [1-7]. A high sensitivity
of magnetic characteristics of these complexes to the external effect underlies the design of heterospin sensors based on them,
which arouses great interest in these objects [8-15]. It should be noted that 3d metal compounds with paramagnetic
derivatives of spatially hindered o-quinones have been well studied whereas information about semiquinolates of rare-earth
elements (REE), and especially, REE compounds with di(tert-butyl)-1,2-benzoquinone radical anion derivatives is extremely
scarce [16]. This is associated with that it is more difficult to investigate such compounds because under ambient conditions
they are kinetically less stable than similar complexes of d elements. On the other hand, for REE semiquinolates the effect of
redox isomerism was not detcted. So far available limited structural and magnetic measurement data for polynuclear REE
compounds with 3,5-di(tert-butyl)-butyl-1,2-benzoquinone derivatives [17, 18] or mononuclear compounds containing
various additional ligands with different charges [19-21] do not allow the determination of structural-magnetic correlations
characteristic of REE compounds with - 3,6-di(tert-butyl)-1,2-benzoquinone (SQ) radical anion derivatives. We managed to
synthesize a sufficiently representative series of mononuclear REE compounds containing only SQ as negatively charged
1
International Tomography Center, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia;
2
*romanenko@tomo.nsc.ru. Novosibirsk State University, Novosibirsk, Russia. Original article submitted January 24, 2019;
revised January 30, 2019; accepted January 31, 2019.
0022-4766/19/6007-1091 © 2019 by Pleiades Publishing, Ltd.
1091

counterions. In this work, we describe the structure and magnetic properties of [Ln(SQ)3(THF)2] (Ln = Pr, Nd, Eu, Tb, Dy),
IV
[Ln(SQ)3(Q)] (Ln = Gd, Eu; Q is 3,6-di-tert-butyl-1,2-benzoquinone), [La(SQ)3(Q)(THF)], [Ce (SQ)4], and also
[NaLn(SQ)4(THF)3]0.25C6H14 (Ln = Gd, Tb) and [NaLn(SQ)4(Q)(THF)] (Ln = La, Pr).
EXPERIMENTAL
All synthesis and growth procedures of single crystals and the sample selection for the X-ray crystallographic
analysis and magnetochemical measurements were performed in a MBraun box in the argon atmosphere.
[Eu(SQ)3(THF)2]⋅2/3THF. Q (0.50 g, 2.27 mmol) was dissolved in 10 mL of THF and an excess of metallic Eu was
added. The mixture was stirred for a day. The dark blue solution formed was filtered, the solvent excess was removed by the
argon flow over the solution surface (up to ∼1 mL). To the solution obtained 10 mL of hexane were added and the reaction
mixture was kept in a closed flask in a freezer at –22 °С for three days. The dark blue crystals formed were filtered off and
washed with cold hexane. Yield 41%. Calculated for desolvated C42H60O6Eu, %: C 62.1, H 7.4; found, %: C 61.6, H 7.4. The
solution of the compound is unstable in the air: in a few seconds it becomes green and decomposition products precipitate.
[Eu(SQ)3Q]. Q (0.50 g, 2.27 mmol) was dissolved in 10 mL of THF and an excess of metallic Eu was added. The
mixture was stirred for a day. The dark blue solution formed was filtered, more Q (0.17 g, 0.76 mmol) was added to the
filtrate, after which the solution became bluish green. The solution volume was decreased to ∼3 mL with blowing its surface
with an Ar flow. Then 10 mL of hexane were added and the mixture was kept in a closed flask in a freezer at –22 °С for
a day. Black elongated prismatic crystals were filtered off and washed with cold hexane. Yield 45%. Calculated for
C56H80O8Eu, %: C 65.1, H 7.8; found, %: C 65.5, H 8.0.
[Ln(SQ)3(THF)2] (Ln = Pr, Nd, Gd, Tb, Dy). To the Q (0.20 g, 0.9 mmol) solution in 10 mL of THF and excees of
metallic sodium was added. The mixture was stirred until the formation of a light yellow solution. To the filtered solution Q
(0.20 g, 0.9 mmol) was added; after its dissolution the color of the reaction mixture became dark blue and then the
Ln(NO3)3(H2O)6 (0.6 mmol) solution in 5 mL of THF was added. The white precipitate was filtered off and the solvent was
removed from the filtrate by an argon flow. The precipitate was dissolved in 10 mL of hexane, filtered, and kept in a closed
flask in a freezer at –22 °С for 10 h. The black-blue crystals formed were filtered off and washed with cold hexane. Yield 35-
62%. Unlike Pr, Nd, Gd, and Tb the Dy compound crystallizes as a [Dy(SQ)3(THF)2]⋅C6H14 solvate. A similar synthesis
procedure using La and Ce salts resulted in the isolation of compounds of the compositions [La(SQ)3Q(THF)] and [Ce(SQ)4]
into a solid phase with yields of 37% and 16% respectively (the last product was isolated with a Q admixture).
[Gd(SQ)3Q]. To the [Gd(SQ)3(THF)2] (0.30 g, 0.31 mmol) solution in 10 mL of THF a Q (0.068 g, 0.31 mmol)
solution in 2 mL of THF was added. The solvent was removed by an argon flow, the residue was dissolved in 15 mL of
hexane, the solution was filtered and kept in an open flask in a freezer at –22 °C for 12 h. The black crystals formed were
filtered off and quickly washed with cold hexane. Yield 31%. Calculated for C56H80O8Gd, %: C 64.8, H 7.8; found, %:
C 65.2, H 8.1.
[NaLn(SQ)4(THF)3]⋅0.25C6H14 (Ln = Gd, Tb). To the Q (0.20 g, 0.9 mmol) solution in 10 mL of THF and excess
of metallic sodium was added, the mixture was stirred until the solution became light yellow, and then filtered. Solid Q
(0.20 g, 0.9 mmol) was added to the filtrate. After its dissolution the color of the reaction mixture became dark blue, and then
the Ln(NO3)3(H2O)6 (0.45 mmol) solution in 5 mL of THF was added. The white precipitate formed was filtered off, the
solvent was removed with a argon flow. The residue was dissolved in 10 mL of hexane, filtered, and the filtrate was kept in
a closed flask in a freezer at –22 °С for 12 h. The black-blue crystals formed were filtered off and washed with cold hexane.
Yield 23-41%.
[Na(Q)(THF)Ln(SQ)4] (Ln = La, Pr). Compounds crystallize in a few days after the isolation of
[NaLa(SQ)4(THF)3]⋅0.25C6H14 and [Pr(SQ)3(THF)2] from the respective mother liquors left at room temperature.
X-ray crystallographic study. Arrays of reflections from single crystals were measured on Bruker AXS Smart
APEX II and APEX Duo diffractometers (absorption correction was applied using the SADABS program, version 2.10). The
1092

structures were solved by a direct method and refined by the full-matrix least-squares technique in the anisotropic
approximation for all non-hydrogen atoms. The positions of Н atoms were calculated geometrically and refined in the riding
model. All calculations on the structure solution and refinement were carried out using the SHELX program package. Full
information on the structures has been deposited with the Cambridge Crystallography Data Center (CCDC 1891301-1891312
and 1892250; deposit@ccdc.cam.ac.uk; http://www.ccdc.cam.ac.uk/data_request/cif). Crystallographic characteristics of the
compounds, details of the experiment, and selected bond lengths are summarized in Tables 1-3.
Magnetochemical study. Samples of the compounds were loaded into previously measured and tightly closed
quartz ampoules. The magnetic susceptibility (χ) of polycrystalline samples was measured on a SQUID MPMSXL
magnetometer (Quantum Design) in the temperature range 2-300 K at H = 5 kOe. Paramagnetic components of the magnetic
susceptibility were found with regard to the diamagnetic contribution estimated from Pascal constants. The effective
2 1/2
magnetic moment was calculated by the formula μeff = [3kχT/(NAμB )] , where NA, μB, and k are the Avogadro number, the
Bohr magneton, and the Boltzmann constant respectively.
TABLE 1. Crystallographic Characteristics, Details of the Experiment, and Selected Bond Lengths (Å) in [Ln(SQ)3(THF)2]
(Ln = Pr, Nd, Eu, Tb, Dy) Molecules
[Pr(SQ)3(THF)2]×
×C6H14
[Dy(SQ)3(THF)2]×
×C6H14
Compound
[Nd(SQ)3(THF)2] [Eu(SQ)3(THF)2] [Eu(SQ)3(THF)2] [Tb(SQ)3(THF)2]
Molecular
weight
984.09
949.34
957.06
957.06
964.02
1503.78
T, K
RT
RT
RT
240
240
240
Space group;
R-3c; 36
R-3c; 36
R-3c; 36
R-3c; 36
R-3c; 36
P21/c; 4
Z
a, Å
b, Å
28.200(3)
28.200(3)
73.651(7)
90
28.0457(5)
28.0457(5)
73.7711(16)
90
28.0138(5)
28.0138(5)
73.3201(14)
90
27.804(2)
27.804(2)
73.450(11)
90
27.8153(6)
27.8153(6)
72.986(4)
90
12.3972(5)
29.0441(12)
16.3528(7)
98.403(2)
c, Å
α, deg
β, deg
γ, deg
90
90
90
90
90
120
120
120
120
120
3
V, Å
50722(11)
1.160
50252(2)
1.129
49831(2)
1.148
49173(11)
1.164
48903(3)
1.178
5824.9(4)
1.202
3
Dc, g/cm
28.276
28.369
28.706
28.060
67.840
67.732
θmax, deg
I
hkl
67468 / 13840 100736 / 13901 88800 / 14160 138011 / 13248 139290 / 9828
74699 / 10441
(meas./indep.)
Rint
0.2891
0.1152
0.0940
0.0793
0.1027
0.0426
2915 / 592
5949 / 551
5261 / 550
6892 / 532
8349 / 542
9330 / 586
I
hkl
(I > 2σ ) /
I
N
GOOF
0.853
0.866
0.856
0.945
1.079
1.043
R1 / wR2
0.0739 / 0.1581 0.0437 / 0.1153 0.0452 / 0.1254 0.0435 / 0.1326 0.0448 / 0.1289 0.0300 / 0.0774
(I > 2σ )
I
R1 / wR2
0.37867 / 0.2777 0.1309 / 0.1418 0.1687 / 0.1560 0.1009 / 0.1505 0.0510 / 0.1325 0.0346 / 0.0805
Ln–O
2.396(7)-
2.445(7)
2.398(3)-
2.447(3)
2.354(3)-
2.400(3)
2.355(3)-
2.407(3)
2.326(2)-
2.350(3)
2.31821(2)-
2.351(2)
Ln–OTHF
C–O
2.555(7),
2.621(9)
2.594(4),
2.536(3)
2.556(4),
2.507(3)
2.491(3),
2.535(4)
2.463(3),
2.502(3)
2.461(2),
2.506(2)
1.259(11)-
307(13)
1.274(5)-
1.285(5)
1.266(6)-
1.287(5)
1.278(5)-
1.286(5)
1.271(4)-
1.288(5)
1.275(3)-
1.282(3)
1093

TABLE 2. Crystallographic Characteristics, Details of the Experiment, and Selected Bond Lengths (Å)
in [La(SQ)3(Q)(THF)], [Ln(SQ)3(Q)] (Ln = Gd, Eu), and [Ce(SQ)4] Molecules
Compound
[La(SQ)3(Q)(THF)]
[Eu(SQ)3(Q)]
[Gd(SQ)3(Q)]
[Ce(SQ)4]
Molecular weight
T, K
1092.21
RT
1033.16
RT
1038.45
240
1021.32
240
Space group; Z
a, Å
P-1; 2
P21/c; 4
23.2295(9)
13.4958(5)
19.5986(8)
114.060(2)
P21/c; 4
23.213(3)
13.4926(17)
19.621(3)
114.175(7)
P21/c; 4
11.0842(4)
14.3123(5)
20.0890(7)
81.7046(18)
76.5555(18)
78.6340(17)
23.0881(14)
13.4750(8)
19.7365(12)
113.893(2)
b, Å
c, Å
α, deg
β, deg
γ, deg
3
V, Å
3022.56(19)
1.200
5610.4(4)
1.223
5606.3(13)
1.231
5614.1(6)
1.208
3
Dc, g/cm
67.768
27.580
28.063
67.760
θmax, deg
I
hkl
(meas./indep.)
Rint
50051 / 10763
0.0559
48813 / 12898
0.1908
50089 / 13215
0.1013
47237 / 10028
0.0955
9941 / 632
1.057
5794 / 581
0.759
6092 / 581
0.789
8734 / 586
1.046
I
hkl
( > 2σ ) / N
I
GOOF
0.0378 / 0.1081
0.0461 / 0.1115
0.0466 / 0.0536
0.1263 / 0.0642
0.0470 / 0.0657
0.1193 / 0.0802
0.0613 / 0.1334
0.0738 / 0.1401
R1 / wR2 ( > 2σ )
I
R1 / wR2
Ln–O
Ln–OQ
C–O
2.476(2)-2.716(2)
2.330(2)-2.371(2)
2.556(2), 2.571(2)
1.271(4)-1.290(4)
1.221(4), 1.223(4)
2.319(2)-2.361(2)
2.547(3), 2.549(3)
1.276(4)-1.292(4)
1.226(4 1.232(4)
2.326(2)-2.367(2)
–
–
1.248(4)-1.283(4)
–
1.268(4)-1.293(4)
–
C–OQ
RESULTS AND DISCUSSION
Solutions of REE compounds with SQ are extremely unstable in air. Therefore, the synthesis of all compounds and
the growth of single crystals were performed in the inert atmosphere of the MBraun chamber. Crystals of the compound,
separated from the mother liquor, can be stored in air for several weeks. By the interaction of the solution of 3,6-di(tert-
butyl)-1,2-benzoquinone (Q) with metal we managed to synthesize [Eu(SQ)3(THF)2]. Other [Ln(SQ)3(THF)2], in which
Ln = Pr, Nd, Tb, and Dy, were obtained by the exchange reaction of respective Ln nitrates and Na semiquinolate [22] in dry
THF. Note that in the case of La and Ce, only compounds of the composition [La(SQ)3Q(THF)] and [Ce(SQ)4] were
extracted into the solid phase, which is likely to be due to their lower solubility as compared to that of [Ln(SQ)3(Solv) ].
n
Quinone Q in the composition of the compounds is formed in the reaction mixture because of the presence of a small amount
of water from crystal hydrates of initial Ln salts.
In [Ln(SQ)3(THF)2] molecules, in which Ln = Pr, Nd, Eu, Tb, Dy, the REE environment is a distorted square
antiprism (Fig. 1a) with three edges tightened by SQ ligands; the other two sites are occupied by O atoms of THF molecules.
The average Ln–OSQ distances are 2.423 Å, 2.416 Å, 2.372 Å, 2.346 Å, and 2.333 Å for Pr, Nd, Eu, Tb, and Dy respectively,
and decrease in the series, which is caused by a decrease in the radius of the lanthanide ion. The Ln–OTHF distances are
noticeably longer and are not equal to each other (Table 1). The average C–OSQ bond length of 1.28(1) Å is typical of
coordinated semiquinolate ligands [23].
1094

TABLE 3. Crystallographic Characteristics, Details of the Experiment, and Selected Bond Lengths (Å) in Na–Ln
Compounds
Compound [NaLa(SQ) (Q)(THF)] [NaPr(SQ) (Q)(THF)]
4 4
[NaGd(SQ)4(THF)3]⋅0.25C6H14 [NaTb(SQ)4(THF)3]⋅0.25C6H14
Molecular
weight
1335.50
1337.50
1299.29
1300.96
T, K
240
RT
240
240
Space group;
P-1; 2
P-1; 2
P-1; 2
P-1; 2
Z
a, Å
b, Å
13.5762(5)
13.7012(5)
20.7061(8)
87.188(2)
88.870(2)
86.567(2)
13.6244(10)
13.7213(11)
20.7012(15)
89.314(6)
13.8878(9)
22.8149(15)
25.2376(17)
102.332(4)
101.587(4)
106.934(3)
13.8475(7)
22.8239(12)
25.1587(13)
102.245(3)
101.620(3)
106.899(2)
c, Å
α, deg
β, deg
γ, deg
87.106(6)
86.580(6)
3
V, Å
3839.5(2)
1.155
3858.0(5)
1.151
7171.1(8)
1.203
7134.7(7)
1.211
3
Dc, g/cm
28.140
28.594
67.742
67.832
θmax, deg
I
57676 / 18419
0.0677
68524 / 19094
0.3093
134422 / 25598
0.0889
115825 / 25578
0.0814
hkl
(meas./indep.)
Rint
11939 / 784
4866 / 779
20309 / 1535
20985 / 1535
I
hkl
(I > 2σ ) /
I
N
GOOF
0.906
0.687
0.979
1.028
R1 / wR2
0.0427 / 0.0774
0.0544 / 0.0638
0.0398 / 0.1031
0.0412 / 0.1149
(I > 2σ )
I
R1 / wR2
0.0768 / 0.0918
0.2633 / 0.0981
0.0508 / 0.1080
0.0498 / 0.1194
Ln–O
C–OSQ
C–OQ
Na–O
2.448(2)-2.577(2)
1.272(3)-1.297(3)
1.221(3), 1.207(4)
2.278(3)-2.463(2)
2.812(2)
2.415(4)-2.509(4)
1.270(6)-1.305(6)
1.216(8), 1.224(8)
2.353(5)-2.458(6)
2.335(2)-2.466(2)
1.273(3)-1.291(4)
2.317(3)-2.446(3)
1.263(3)-1.297(4)
2.322(3)-2.527(4)
2.350(11)-2.766(4)
2.324(3)-2.508(5)
Fig. 1. Environment of the metal atom in [Ln(SQ)3(THF)2] (Ln = Pr, Nd, Eu, Tb, Dy; the structure of a molecule
of the Eu compound is given as an example) (а) and [Ln(SQ)3(Q)] (Ln = Gd, Eu; the structure of
a [Eu(SQ)3(Q)] molecule is shown) (b).
1095

In [Ln(SQ)3(Q)] (Ln = Gd, Eu) molecules, Q forms a five-membered chelate ring with metal, taking the sites of two
THF molecules in [Ln(SQ)3(THF)2] (Fig. 1b). The scatter of one-type distances is not large (Table 1), however, the Ln–OQ
distances are much longer (Table 2) than Ln–OSQ: the average values are 2.564 Å and 2.356 Å for the Eu compound and 2.553 Å
and 2.344 Å for the Gd compound. The C–OQ bond lengths are close to those of the coordinated C=O group [23]: the average
values in Eu and Gd compounds are 1.222 Å and 1.227 Å respectively.
4+
In [Ce(SQ)4] the Ce ion links four SQ ligands (Fig. 2а). The Ce–OSQ and C–OSQ distances are within 2.326(2)-
2.367(2) Å and 1.268(4-1.293(4) Å respectively.
The [La(SQ)3(Q)(THF)] compound in which yet another THF molecule is bonded to metal because of a larger La ion
radius and supplements its environment to the one-capped square antiprism is close in structure to [Ln(SQ)3(Q)] compounds
(Fig. 2b). The scatter of La–O distances is quite large: 2.476(2)-2.716(2) Å (Table 2, average 2.556 Å), with La–OTHF
(2.675(3) Å) being shorter than a number of distances to bidentate ligands. It was impossible to accurately determine the position
of quinone Q because six of all C–O distances are very close (average 1.274 Å, Table 2) while two to O atoms belonging to
different ligands are much shorter (1.248(4) Å and 1.251(4) Å).
It should be noted that in the compounds studied the metallocycles with Q are practically planar while those with SQ
are bent along the O⋯O line in most cases; for [Ce(SQ)4] the angle between the OCeO and OCCO planes reaches 23° in one of
the ligands.
In the molecules of mixed-metal Na–Ln compounds [NaLn(SQ)4(Q)(THF)] (Ln = La, Pr) and
[NaLn(SQ)4(THF)3]⋅0.25C6H14 (Ln = Gd, Tb) the environment of the Ln atom is a distorted square antiprism formed by O atoms
of four SQ out of which two are bidentate-cyclically bonded and the other two are the bridges between Ln and Na (Fig. 3). The
environment of Na atoms in [NaLn(SQ)4(Q)(THF)] (Ln = La, Pr) supplement quinone Q bidentately bonded to it and the THF
molecule while in hexane solvates [NaLn(SQ)4(THF)3] 0.25C6H14 (Ln = Gd, Tb) there are only THF molecules (Fig. 4).
The C–OSQ bond lengths are within 1.270(6)-1.305(6) Å, La–OSQ 2.448(2)-2.577(2) Å, Pr–OSQ 2.398(4)-2.537(4) Å
(Table 3). The environment of the Na atom is close to a distorted octahedron formed by two OQ atoms (C–OQ 1.207(4)-
1.224(8) Å, Na–OQ 2.415(6)-2.463(2) Å); OTHF (Na–OTHF 3,27(3)-2.279(7) Å) and two bridging OSQ (Na–OQ 2.353(5)-
2.462(2) Å).
In [NaLn(SQ)4(THF)3]⋅0.25C6H14 (Ln = Gd, Tb) compounds, in one of two crystallographically independent
molecules, the Na atom and one of THF molecules bonded to it are disordered over two positions Na2 and Na2′ with
a weight of 0.5 (Fig. 4).
It is convenient to begin the discussion of magnetic properties of the compounds from the consideration of the
3+
4+
1
dependence μeff(T) for [La(SQ)3Q(THF)] and [Ce(SQ)4] in which La and Ce ions are diamagnetic (ground state S0) and their
paramagnetism is governed only by spins of SQ ligands. Curves of the dependence μeff(T) for [La(SQ)3Q(THF)] and [Ce(SQ)4]
are depicted in Fig. 5.
Fig. 2. Environment of the metal atom in [Ce(SQ)4] (а) and [La(SQ)3(Q)(THF)] (b).
1096

Fig. 3. Environment of Na and Pr atoms in
[NaPr(SQ)4(Q)(THF)].
Fig. 4. Two crystallographically independent molecules of [NaLn(SQ)4(THF)3] (Ln = Gd, Tb).
Fig. 5. Dependence μeff(T) for [La(SQ)3Q(THF)] (1)
and [Ce(SQ)4] (2) complexes; theoretical curves (3).
1097

The μeff values are 2.63 μB и 2.04 μB at 300 K and decrease with decreasing temperature, coming to a plateau of
∼1.68 μB and ∼0.36 μB below 60 K for [La(SQ)3Q(THF)] and [Ce(SQ)4] respectively. High-temperature μeff values are lower
than theoretical purely spin values of 3.00 μB and 3.46 μB for three and four SQ ligands respectively. A decrease in μeff with
decreasing temperature indicates the occurrence of strong antiferromagnetic exchange interactions between spins of SQ
ligands. The value μeff ∼1.68 μB below 60 K for [La(SQ)3Q(THF)] is consistent with a theoretical purely spin value of 1.73 μB
for one paramagnetic center with spin S = 1/2 at the g factor of 2, which gives evidence of a doublet ground state. The
dependence μeff(T) is well described by an expression for a three-spin exchange claster (H = –2J12S1S2–2J23S2S3); optimal
–1
–1
values of J12 and J23 parameters are –100 cm and 9 cm respectively. Strong antiferromagnetic exchange interactions
between spins of the SQ ligands result in a singlet ground state of [Ce(SQ)4]. The value μeff ∼0.36 μB below 60 K is due to
a small amount of a paramagnetic impurity. The dependence μeff(T) was analyzed within the model of a four-spin exchange
cluster (H = –2J1(S1S2 + S2S3 + S3S4 + S1S4) – 2J2(S1S3 + S2S4)) with regard to an admixture of the monomer p. The optimal
–1 –1
exchange interaction parameters are J1 = –152 cm and J2 = 0.2 cm at g = 2 (fixed) and p = 1.0%.
The μeff values for [Ln(SQ)3(THF)2] (Ln = Pr, Nd, Eu, Tb, and Dy) (Fig. 6) at 300 K are 4.22 μB, 4.27 μB, 4.15 μB,
9.93 μB, and 11.09 μB for Pr, Nd, Eu, Tb, and Dy compounds respectively and are close to theoretical values of 4.67 μB,
3+
4.70 μB, 4.67 μB, 10.17 μB, and 11.06 μB for four not-interacting paramagnetic centers: one Ln ion and three SQ ligands.
A decrease in μeff with decreasing temperature is due to both antiferromagnetic exchange interactions between SQ spins and
3+
magnetic anisotropy typical of Ln ions.
Fig. 6. Dependence μeff(T) for [Ln(SQ)3(THF)2]:
Ln = Pr (1), Nd (2), Eu (3), Tb (4), Dy (5).
Fig. 7. Field dependence of the magnetization for
[Dy(SQ)3(THF)2].
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Fig. 8. Dependence μeff(T) for [NaGd(SQ)4(THF)3]
(1) and [NaTb(SQ)4(THF)3] (2).
For [Dy(SQ)3(THF)2] the dependence of the magnetization (M) on the strength of the applied magnetic field is
nonlinear at low temperatures (Fig. 7). At 2 K the dependence M(H) is characteristic of a metamagnetic compound: with
an increase in the magnetic field to ∼10 kOe the magnetization of [Dy(SQ)3(THF)2] almost linearly increases to 1 μB, and
above 10 kOe it abruptly increases and reaches saturation at ∼4.9 μB. At 3 K the magnetization increases with increasing
magnetic field strength, and above 25 kOe reaches saturation of ∼4.5 μB.
The dependence μeff(T) for mixed-metal [NaLn(SQ)4(THF)3] (Ln = Gd, Tb) is depicted in Fig. 8. The μeff values are
8.33 μB and 9.75 μB at 300 K and gradually decrease with decreasing temperature, reaching 7.43 μB and 7.23 μB at 2 K for Gd
and Tb compounds respectively. High-temperature μeff неvalues are much lower than theoretical 8.66 μB and 10.32 μB values
3+
for five non-interacting paramagnetic centers – the Ln ion and four SQ ligands, which indicates the presence of strong
antiferromagnetic exchange interactions between them. The value μeff ∼7.87 μB for [NaGd(SQ)4(THF)3] in the temperature
3+
range 30-5 K is close to the value of 7.94 μB that is characteristic of one Gd ion, which corresponds to antiferromagnetic
exchange interactions between spins of the SQ ligands. The experimental dependence μeff(T) is well described by
an expression taking into account only two exchange interactions between pairs of SQ ligands (H = –2J1⋅SSQ1SSQ2
–
–1 –1
2J2⋅SSQ3SSQ4). The optimal values of J1 and J2 parameters are –165 cm and –36 cm .
Thus, as a result of the study performed heterospin mononuclear compounds of lanthanides with paramagnetic
3,6-di(tert-butyl)-1,2-benzoquinone derivatives were synthesized and isolated as single crystals and their crystal structures
and magnetic properties were studied for the first time. By the example of La, Ce, and Gd compounds it is shown that
intramolecular antiferromagnetic exchange interactions between spins of SQ ligands are dominant. It is established that
[Dy(SQ)3(THF)2] is a metamagnetic compound.
FUNDING
The structural and magnetic measurements in this research was supported by 17-13-01022 and FASO (synthesis of
compounds).
CONFLICT OF INTERESTS
The authors declare that they have no conflict of interests.
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