Hexanuclear, heterometallic, Ni3Ln3 complexes possessing O-Capped homo- and heterometallic structural subunits: SMM behavior of the dysprosium analogue

Article abstract of DOI:10.1021/ic403090z

The reaction of hetero donor chelating mannich base ligand 6,6′-{(2-(dimethylamino)ethylazanediyl)bis(methylene)}bis(2-methoxy-4- methylphenol) with Ni(ClO₄)₂·6H₂O and lanthanide(III) salts [Dy(III) (1); Tb(III) (2); Gd (III) (3); Ho(III) (4); and Er(III) (5)] in the presence of triethylamine and pivalic acid afforded a series of heterometallic hexanuclear Ni(II)-Ln(III) coordination compounds, [Ni ₃Ln₃(μ₃-O)(μ₃-OH) ₃(L)₃(μ-OOCCMe₃)₃] ·(ClO₄)·wCH₃CN·xCH₂Cl ₂·yCH₃OH·zH₂O [for 1, w = 8, x = 3, y = 0, z = 5.5; for 2, w = 0, x = 5, y = 0, z = 6.5; for 3, w = 15, x = 18, y = 3, z = 7.5; for 4, w = 15, x = 20, y = 6, z = 9.5; and for 5, w = 0, x = 3, y = 2, z = 3]. The molecular structure of these complexes reveals the presence of a monocationic hexanuclear derivative containing one perchlorate counteranion. The asymmetric unit of each of the hexanuclear derivatives comprises the dinuclear motif [NiLn(L)(μ₃-O)(μ₃-OH)(μ-Piv)]. The cation contains three interlinked O-capped clusters: one Ln ^III₃O and three Ni^IILn^III ₂O. Each of the lanthanide centers is eight- coordinated (distorted trigonal-dodecahedron), while the nickel centers are hexacoordinate (distorted octahedral). The study of the magnetic properties of all compounds are reported and suggests single molecule magnet behavior for the Dy(III) derivative (1).

Full text of DOI:10.1021/ic403090z

Article
pubs.acs.org/IC
Hexanuclear, Heterometallic, Ni3Ln3 Complexes Possessing
O‑Capped Homo- and Heterometallic Structural Subunits: SMM
Behavior of the Dysprosium Analogue
Joydeb Goura,† Rogez Guillaume,‡ Eric Riviere,§ and Vadapalli Chandrasekhar*,†,∥
̀
†Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India
‡Institut de Physique et Chimie des Mater
Strasbourg Cedex 2, France
§Equipe de Chimie Inorganique, ICMMO (CNRS, UMR 8182, Universite
́
iaux de Strasbourg and Labex NIE, University of Strasbourg, CNRS UMR 7504, 67034
́
Paris-Sud), Orsay F-91405, France
∥National Institute of Science Education and Research, Institute of Physics Campus, Sachivalaya Marg, Sainik School, Bhubaneswar,
Orissa 751 005, India
S
* Supporting Information
ABSTRACT: The reaction of hetero donor chelating
mannich base ligand 6,6′-{(2-(dimethylamino)ethylazanediyl)-
bis(methylene)}bis(2-methoxy-4-methylphenol) with Ni-
(ClO4)2·6H2O and lanthanide(III) salts [Dy(III) (1); Tb(III)
(2); Gd (III) (3); Ho(III) (4); and Er(III) (5)] in the
presence of triethylamine and pivalic acid afforded a series of
heterometallic hexanuclear Ni(II)−Ln(III) coordination com-
pounds, [Ni3Ln3(μ3-O)(μ3-OH)3(L)3(μ-OOCCMe3)3]·
(ClO4)·wCH3CN·xCH2Cl2·yCH3OH·zH2O [for 1, w = 8, x
= 3, y = 0, z = 5.5; for 2, w = 0, x = 5, y = 0, z = 6.5; for 3, w =
15, x = 18, y = 3, z = 7.5; for 4, w = 15, x = 20, y = 6, z = 9.5;
and for 5, w = 0, x = 3, y = 2, z = 3]. The molecular structure
of these complexes reveals the presence of a monocationic hexanuclear derivative containing one perchlorate counteranion. The
asymmetric unit of each of the hexanuclear derivatives comprises the dinuclear motif [NiLn(L)(μ3-O)(μ3-OH)(μ-Piv)]. The
cation contains three interlinked O-capped clusters: one LnIII3O and three NiIILnIII2O. Each of the lanthanide centers is eight-
coordinated (distorted trigonal-dodecahedron), while the nickel centers are hexacoordinate (distorted octahedral). The study of
the magnetic properties of all compounds are reported and suggests single molecule magnet behavior for the Dy(III) derivative
(1).
those containing Co/Ln, Mn/Ln, and Ni/Ln compounds are of
interest because Ni(II) can contribute to magnetic anisotropy.7
From our lab, by using phosphorus-supported hydrazone
ligands, we have investigated new families of Co2Ln,15
Mn2Ln,16 CuLn,17 and Ni2Ln18 complexes, many of which
have been shown to be SMMs. Encouraged by these results and
considering the relative paucity of heterometallic 3d/4f
complexes containing Ni(II), we examined the possibility of
assembling polynuclear heterometallic complexes containing
Ni(II) and lanthanide metal ions. One of the key ingredients
for success in this area is the design of ligands that allow specific
binding of both transition metal and lanthanide metal ions at
the same time, allowing magnetic interactions between them
through an appropriate single-atom or multiatom bridges.
Another requirement is that the ligand through a multifunc-
tional property should allow assembly of polynuclear
complexes. Keeping these requirements in mind we prepared
INTRODUCTION
■
Since the discovery of the first single-molecule magnet (SMM),
[MnIV4MnIII8(μ3-O)12(O2C-Me)16(OH2)4]·2MeCO2H·4H2O,1
there has been considerable attention in this area. Several
efforts by synthetic chemists to assemble polynuclear transition
metal ion complexes have been motivated by this discovery
with an aim to achieve a high ground-state spin (S).2 The
realization that uniaxial magnetic anisotropy (D) is equally
important led to efforts to incorporate metal ions that would
show this property as a result of a structural distortion (such as
Jahn−Teller distortion for a Mn(III) in an octahedral
geometry)3 or a spin−orbit coupling (such as Co(II)).4
A
second paradigm in this area is reflected by an investigation in
the field of heterometallic 3d/4f complexes where two factors,
viz., S and D, can be optimized by taking advantage of the large
inherent anisotropy of certain lanthanide ions.5 Since the first
example, viz., a Cu−Gd complex,6 many 3d/4f heterometallic
complexes with varying nuclearities, transition metals, and
transition metal/lanthanide ion ratios have been prepared and
investigated with mixed success.7−14 Among such compounds,
Received: December 18, 2013
Published: July 22, 2014
© 2014 American Chemical Society
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Scheme 1. Synthesis of Ligand LH2
5CH2Cl2 and 5H2O; complex 3, 15CH3CN, 18CH2Cl2, 3CH3OH, and
2H2O; complex 4, 15CH3CN, 20CH2Cl2, 6CH3OH, and 5H2O; and
complex 5, 3CH2Cl2 and 2CH3OH) could not be modeled
satisfactorily due to the presence of heavy disorder and were removed
by using the SQUEEZE routine of PLATON. The total electron count
removed by this routine corresponded to 623 (for complex 1); 518
(for complex 2); 2023 (for complex 3); 2154 (for complex 4); and
643 (for complex 5) per unit cell. This corresponds to 311, 259, 1011,
1177, and 160 electrons per molecule (Z = 2 for complexes 1−4 and Z
= 4 for complex 5). All the mean plane analyses as well as molecular
drawings were obtained from DIAMOND (version 3.1).
the multidentate ligand 6,6′-{(2-(dimethylamino)-
ethylazanediyl)bis(methylene)}bis(2-methoxy-4-methylphe-
nol) and using it along with a coligand, pivalic acid, assembled a
new family of hexanuclear heterometallic complexes,
[Ni3Ln3(μ3-O)(μ3-OH)3(L)3(μ-OOCCMe3)3]·(ClO4)·
wCH3CN·xCH2Cl2·yCH3OH·zH2O [for 1, w = 8, x = 3, y = 0,
z = 5.5; for 2, w = 0, x = 5, y = 0, z = 6.5; for 3, w = 15, x = 18, y
= 3, z = 7.5; for 4, w = 15, x = 20, y = 6, z = 9.5; and for 5, w =
0, x = 3, y = 2, z = 3]. The synthesis, structure, and magnetism
of these complexes are discussed herein.
Syntheses. Preparation of 6,6′-{(2-(Dimethylamino)-
ethylazanediyl)bis(methylene)}bis(2-methoxy-4-methylphenol)
(LH2). The synthesis of LH2 was carried out by an adaptation of the
literature procedure.20 To an ethanolic (40 mL) stirred solution of 2-
methoxy-4-methylphenol (2.180 g, 15.78 mmol) were added N,N-
dimethylethylenediamine (0.695 g, 7.884 mmol), a 37% formaldehyde
solution (2.376 g, 79.12 mmol), and triethylamine (1.597 g, 15.78
mmol), and the solution was heated under reflux for 3 days (Scheme
1). After this, the solvent was stripped off the solution in vacuo, and
the resulting oil was kept in a refrigerator overnight and triturated with
petroleum ether and diethyl ether to afford a colorless solid, which was
filtered and dried in air affording LH2. Yield: 1.56 g (51%). Mp: 116
°C. IR (KBr), cm−1: 3424(br), 2986(w), 2939(w), 2833(s), 2781(w),
1589(s) 1500(s), 1466(s), 1381(s), 1358(s), 1325(w), 1283(s),
1262(s), 1225(s), 1157(s), 1128(s), 1106(s), 1082(s), 1045(s),
988(w), 977(w), 935(s), 895(w), 844(s), 814(s), 782(s), 748(w),
596(w), 554(w), 430(w). The 1H and 13C NMR data are given in the
Supporting Information (Figures S1, S2). ESI-MS (m/z): 389.2441
(M + H)+. Anal. Calcd for C22H32N2O4: C, 68.01.16; H, 8.30; N, 7.21.
Found: C, 67.72; H, 8.03; N, 6.96.
Preparation of the Hexanuclear Complexes 1−5. The general
synthetic protocol that was used for the preparation of the metal
complexes (1−5) is as follows: A methanolic solution (5 mL) of
Ln(NO3)3·nH2O (3 equiv) was added dropwise, under constant
stirring, to a 20 mL methanolic solution containing a mixture of LH2
(2 equiv) and triethylamine (8 equiv) with constant stirring. To this
reaction mixture was added solid Ni(ClO4)2·6H2O (3 equiv), and the
reaction mixture was stirred for a further 1.5 h. A deep green-colored
solution was obtained. At this stage pivalic acid (4 equiv) was added,
and reaction mixture stirred again for 12 h, and the solvent was
stripped off in vacuo, resulting in a green solid, which was washed with
diethyl ether (3 × 5 mL), redissolved in acetonitrile/dichloromethane
(1:1), and filtered. The filtrate was kept for crystallization under slow
evaporation at room temperature. After 1 week, green, block-shaped
crystals suitable for X-ray analysis were isolated. The quantity of the
reactants used in each reaction and the characterization data of the
compounds are given below.
EXPERIMENTAL SECTION
■
Reagents and General Procedures. Solvents and other general
reagents used in this work were purified according to standard
procedures.19 The following chemicals were used as obtained: 2-
methoxy-4-methyl phenol, N,N-dimethylethylenediamine, Gd(NO3)3·
6H2O, Tb(NO3)3·5H2O, Dy(NO3)3·5H2O, Ho(NO3)3·5H2O, and
Er(NO3)3·5H2O (Aldrich, USA); Ni(ClO4)2·6H2O (Alfa Asear, UK);
37% formaldehyde solution and triethylamine (S. D. Fine Chemicals,
Mumbai, India). The ligand 6,6′-{(2-(dimethylamino)ethylazanediyl)-
bis(methylene)}bis(2-methoxy-4-methylphenol) was prepared by
adapting a literature procedure.20
Instrumentation. Melting points were measured using a JSGW
melting point apparatus and are uncorrected. H NMR spectra were
1
recorded on a JEOL-JNM Lambda 400 model NMR spectrometer
operating at 500.0 MHz in CDCl3 solutions. Chemical shifts are
referenced with respect to tetramethylsilane. IR spectra were recorded
as KBr pellets on a Bruker Vector 22 FT IR spectrophotometer
operating from 400 to 4000 cm−1. Elemental analyses of these
compounds were obtained using a Thermoquest CE Instrument
CHNS-O, EA/110 model. ESI-MS spectra were recorded on a
Micromass Quattro II triple quadrupole mass spectrometer.
Magnetic measurements were performed using a Quantum Design
SQUID-VSM magnetometer. Magnetization measurements at differ-
ent fields at a given temperature confirm the absence of ferromagnetic
impurities. Data were corrected for the sample holder, and
diamagnetism was estimated from Pascal constants. The samples
were blocked in eicosane to prevent orientation under a magnetic field.
X-ray Crystallography. Single-crystal X-ray structural studies of
1−5 were performed on a CCD Bruker SMART APEX diffractometer
equipped with an Oxford Instruments low-temperature attachment.
Data were collected using graphite-monochromated Mo Kα radiation
(λα = 0.71073 Å). The crystals did not degrade/decompose during
data collection. Data collection, structure solution, and refinement
were performed using SMART, SAINT, and SHELXTL programs,
respectively.21a−f All the calculations for the data reduction were done
using the Bruker SADABS program. All the non-hydrogen atoms were
refined anisotropically using full-matrix least-squares procedures. All
the hydrogen atoms were included in idealized positions, and a riding
model was used. Large solvent-accessible voids remain in all the
structures. They are presumably filled with several disordered
molecules of dichloromethane, acetonitrile, methanol, and water,
which could not be modeled by the present analysis. Therefore, the
“PLATON/“SQUEEZE”21g,h program was used to remove those
disordered solvent molecules. The PLATON-SQUEEZE routine
reports that the total solvent-accessible void volumes (per unit cell)
are 3690 (for 1); 3983 (for 2); 3766 (for 3); 3819 (for 4); and 4135
(for 5) Å3. These volumes are nearly one-quarter of the unit cell
volume for each molecule, which is ∼12 500 Å3. The lattice solvent
molecules (complex 1, 8CH3CN, 3CH2Cl2, and H2O; complex 2,
[Ni3Dy3(μ3-O)(μ3-OH)3(L)3(μ-OOCCMe3)3]·(ClO4)·8CH3CN·3CH2Cl2·
5.5H2O (1). Ni(ClO4)2·6H2O (0.062 g, 0.169 mmol), Dy(NO3)3·
5H2O (0.074 mg, 0.169 mmol), LH2 (0.044 g, 0.113 mmol), Et3N
(0.046 g, 0.454 mmol), and pivalic acid (0.023 g, 0.223 mmol) were
used. Yield: 0.096 g, 57% (based on Dy). Mp: >240 °C. IR (KBr, ν/
cm−1): 3584(br), 2955(s), 2867(w), 2678(m), 2491(w), 1588(s),
1565(s), 1498(s), 1465(s), 1418(s), 1383(s), 1359(m), 1302(m),
1267(w), 1252(s), 1229(s), 1154(s), 1105(s), 1080(s), 1032(s),
978(w), 933(m), 905(w), 832(s), 783(w), 629(s), 598(s), 503(w),
469(s). Anal. Calcd for C100H161Cl7Dy3N14Ni3O31.5 (2975.17): C,
40.37; H, 5.45; N, 6.59. Found: C, 40.06; H, 5.23; N, 6.39.
[Ni3Tb3(μ3-O)(μ3-OH)3(L)3(μ-OOCCMe3)3]·(ClO4)·5CH2Cl2·6.5H2O
(2). Ni(ClO4)2·6H2O (0.062 g, 0.169 mmol), Tb(NO3)3·5H2O (0.074
mg, 0.170 mmol), LH2 (0.044 g, 0.113 mmol), Et3N (0.046 g, 0.454
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Scheme 2. Synthesis of Complexes 1−5
mmol), and pivalic acid (0.023 g, 0.223 mmol) were used. Yield: 0.090
g, 56% (based on Tb). Mp: >240 °C. IR (KBr, ν/cm−1): 3587(br),
2956(s), 2923(s), 2867(m), 2840(w), 1706(w), 1586(s), 1564(s),
1497(s) 1484(s), 1464(s), 1419(s), 1373 (s), 1359(s), 1340(s),
1302(m), 1268(s), 1251(m), 1229(s), 1154(s), 1103(s), 1092(s),
1080(s), 1032(s), 978(s), 934(s), 905(m), 831(s), 783(w), 629(s),
598(s), 571(w), 502(m), 468(s). Anal. Calcd for
C86H143Cl11Tb3N6Ni3O32.5 (2823.91): C, 36.58; H, 5.10; N, 2.98.
Found: C, 36.26; H, 4.86; N, 2.78.
[Ni3Gd3(μ3-O)(μ3-OH)3(L)3(μ-OOCCMe3)3]·(ClO4)·15CH3CN·
18CH2Cl2·3MeOH·7.5H2O (3). Ni(ClO4)2·6H2O (0.062 g, 0.169
mmol), Gd(NO3)3·6H2O (0.077 mg, 0.170 mmol), LH2(0.044 g,
0.113 mmol), Et3N (0.046 g, 0.454 mmol), and pivalic acid (0.023 g,
0.225 mmol) were used. Yield: 0.109 g, 41% (based on Gd). Mp: >240
°C. IR (KBr, ν/cm−1): 3507(br), 2956(s), 2922(s), 2868(s), 2841(m),
1706(w), 1559(s), 1497(s), 1483(s), 1466(s), 1416(s), 1374(s),
1359(s), 1280(m), 1264(s), 1227(s), 1192(w), 1155(s), 1101(s),
1081(s), 1029(s), 976(w), 947(m), 933(s), 889(w), 832(s), 787(s),
748(w), 624(s), 601(s), 568(m), 505(w), 467(s), 445(s). Anal. Calcd
for C132H228Cl37Gd3N21Ni3O36.5 (4652.93): C, 34.07; H, 4.94; N, 6.32.
Found: C, 33.82; H, 4.68; N, 6.09.
[Ni3Ho3(μ3-O)(μ3-OH)3(L)3(μ-OOCCMe3)3]·(ClO4)·15CH3CN·
20CH2Cl2·6MeOH·9.5H2O (4). Ni(ClO4)2·6H2O (0.062 g, 0.169
mmol), Ho(NO3)3·5H2O (0.75 mg, 0.170 mmol), LH2(0.044 g,
0.113 mmol), Et3N (0.046 g, 0.454 mmol), and pivalic acid (0.023 g,
0.223 mmol) were used. Yield: 0.112 g, 40% (based on Ho). Mp: >240
°C. IR (KBr, ν/cm−1): 3588(br), 2955(s), 2869(s), 2801(w),
2739(m), 2678(s), 2491(m), 1704(w), 1590(s), 1566(s), 1499(s),
1465(s), 1433(s), 1418(s), 1383(s), 1359(s), 1302(m), 1269(w),
1252(m), 1229(s), 1154(s), 1108(w), 1080(s), 1033(s), 979(m),
934(w), 906(w), 832(s), 783(w), 637(s), 597(s), 470(s). Anal. Calcd
for C137H248Cl41Ho3N21Ni3O41.5 (4977.99): C, 33.05; H, 5.02; N, 5.91.
Found: C, 32.76; H, 4.78; N, 5.72.
[Ni3Er3(μ3-O)(μ3-OH)3(L)3(μ-OOCCMe3)3]·(ClO4)·3CH2Cl2·2MeOH·
3H2O (5). Ni(ClO4)2·6H2O (0.062 g, 0.169 mmol), Er(NO3)3·5H2O
(0.075 mg, 0.169 mmol), LH2(0.044 g, 0.113 mmol), Et3N (0.046 g,
0.454 mmol), and pivalic acid (0.023 g, 0.223 mmol) were used. Yield:
0.086 g, 57% (based on Er). Mp: >240 °C. IR (KBr, ν/cm−1):
3585(br), 3501(br), 2924(s), 2955(s), 2855(s), 1705(w), 1590(s),
1565(s), 1500(s), 1483(s), 1463(s), 1433(s), 1418(s), 1373(s),
1358(s), 1300(w), 1258(w), 1228(s), 1155(s), 1105(s), 1080(s),
1030(s), 978(w), 935(m), 905(w), 832(s), 783(w), 643(w), 628(w),
600(s), 568(m), 507(w), 471(s). Anal. Calcd for
C86H140Cl7Er3N6Ni3O31 (2669.33): C, 38.54; H, 5.27; N, 3.14.
Found: C, 38.22; H, 5.04; N, 2.89.
RESULTS AND DISCUSSION
■
Synthetic Aspects. The multisite coordination ligand LH2
was prepared by a one-pot synthesis employing the Mannich
condensation of the corresponding phenol, amine, and
formaldehyde.20 The design of this ligand was based on the
observation in the literature that structurally similar ligands
possessing alkyl substituents (instead of the −OMe in the
current instance) afforded mononuclear complexes (Supporting
Information, Schemes S1−10). On the other hand, LH2
possesses six coordinating sites including potential bridging
phenolate oxygen. Also, although not strictly a compartmental
ligand, LH2, by virtue of possessing the −OMe groups, has
specific sites that can interact with lanthanide ions while leaving
other sites for interacting with transition metal ions. The use of
pivalic acid as a coligand was to allow consolidation of the
heterometallic framework through possible bridging coordina-
tion action. In accordance with the above design strategy, LH2
reacts with Ni(ClO4)2·6H2O, Ln(NO3)3·nH2O, NEt3, and
pivalic acid to afford heterometallic hexanuclear monocationic
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Table 1. Crystal Data and Structure Refinement Parameters of 1−5
1
2
3
4
5
formula
C162H240Cl2Dy6N12Ni6O61
C162H240Cl2N12Ni6O55Tb6
C162H240Cl2Gd6N12Ni6O63
C162H240Cl2Ho6N12Ni6O61
C81H120Cl
Er3N6Ni3O29
2355.13
fw
4729.70
100(2)
trigonal
4612.28
100(2)
trigonal
4730.20
100(2)
trigonal
4744.28
100(2)
trigonal
temp (K)
cryst syst
space group
unit cell dimens
100(2)
trigonal
P3c1
̅
25.048(5)
P3c1
̅
25.180(5)
P3c1
̅
25.212(5)
P3c1
̅
25.066(5)
P3c1
̅
24.964(5)
a = b
(Å)
c (Å)
22.985(5)
90
22.779(5)
90
22.745(5)
90
22.972(5)
90
23.036(5)
90
α = β
(deg)
γ (deg)
120
120
120
120
120
volume (Å3); Z
12489(4); 2
1.258
12508(4); 2
1.225
12521(4); 2
1.255
12500(4); 2
1.260
12433(4); 4
1.258
density (Mg m−3
)
abs coeff (mm−1
)
2.297
2.194
2.090
2.400
2.527
F(000)
4764
4656
4772
4776
4740
cryst size (mm)
0.22 × 0.20 × 0.18
0.22 × 0.20 × 0.18
0.18 × 0.16 × 0.14
0.20 × 0.18 × 0.16
0.22 × 0.20 ×
0.18
θ range (deg)
1.63 to 25.50
−30 ≤ h ≤ 29
−30 ≤ k ≤ 30
−27 ≤ l ≤ 12
65 619
2.41 to 25.50
−25 ≤ h ≤ 30
−30 ≤ k ≤ 30
−27 ≤ l ≤ 27
88 168
4.14 to 25.03
−30 ≤ h ≤ 23
−30 ≤ k ≤ 28
−26 ≤ l ≤ 27
63 177
4.09 to 25.02
−25 ≤ h ≤ 29
−29 ≤ k ≤ 29
−27 ≤ l ≤ 27
63 190
2.00 to 28.38
−33 ≤ h ≤ 33
−33≤ k ≤ 22
−30 ≤ l ≤ 30
109 357
limiting indices
reflns collected
unique reflns [Rint
]
7773 [0.0795]
100.0% (25.50°)
7773/13/388
7774 [0.0499]
99.9% (25.50°)
7774/13/379
7343 [0.1029]
99.5% (25.03°)
7343/12/391
7347 [0.0807]
99.5% (25.02°)
7347/7/370
10 367 [0.0604]
99.5% (28.38°)
10367/31/418
completeness to θ
data/restraints/
params
GOOF on F2
1.081
1.031
1.033
1.012
1.049
final R indices [I >
2σ(I)]
R1 = 0.0488, wR2 = 0.1385 R1 = 0.0364, wR2 = 0.0913 R1 = 0.0431, wR2 = 0.1154 R1 = 0.0527, wR2 = 0.1430 R1 = 0.0580, wR2
= 0.1421
R indices (all data)
R1 = 0.0588, wR2 = 0.1464 R1 = 0.0461, wR2 = 0.0954 R1 = 0.0511, wR2 = 0.1192 R1 = 0.0623, wR2 = 0.1478 R1 = 0.0715, wR2
= 0.1490
largest residual
2.933 and −0.822
2.085 and −0.889
2.106 and −0.732
2.720 and −1.309
3.839 and −1.957
peaks (e Å−3
)
complexes [Ni3Ln3(μ3-O)(μ3-OH)3(L)3(μ-OOCCMe3)3]·
(ClO4)·wCH3CN·xCH2Cl2·yCH3OH·zH2O [for 1, w = 8, x =
3, y = 0, z = 5.5; for 2, w = 0, x = 5, y = 0, z = 6.5; for 3, w = 15,
x = 18, y = 3, z = 7.5; for 4, w = 15, x = 20, y = 6, z = 9.5; and
for 5, w = 0, x = 3, y = 2, z = 3] (Scheme 2; see Experimental
Section for details of syntheses). These represent the first
examples of the Ni3/Ln3 family.
Chart 1. Asymmetric Unit of Complexes 1−5
Molecular Structures of Complexes 1−5. The molecular
structures of complexes 1−5 were determined by single-crystal
X-ray crystallography. All of these crystallize in the trigonal
system with the P3c1 space group. The crystallographic
̅
parameters of these compounds are given in Table 1. In view
of the structural similarities of these compounds the molecular
structure of 1 is described as a representative example. The
structural details of all other compounds are given in the
Supporting Information (Figures S3−S6).
The asymmetric unit of 1 contains one-third of the total
molecule, namely, [NiDy(L)(μ-Piv)(μ3-O)(μ3-OH)] (Chart 1
and Figure 1). The molecular structure of 1 is given in Figure 2
and Scheme 2. The molecular structure of 1 reveals that it is a
monocationic heterometallic hexanuclear complex containing
one counter perchlorate anion. The hexanuclear complex is
assembled by the cumulative coordination action of [L]2−,
[OH]−, [O]2−, and the pivalate ligand, the coordination modes
of all of which are summarized in Chart 2. Each [L]2− holds a
trinuclear Dy−Ni−Dy motif together. Thus, the methoxy group
binds to Dy (Dy1−O2, 2.614(4) Å; Dy1−O3, 2.646(3) Å),
while the phenolate bridges a Ni(II) and the Dy(III) [Ni1−O1,
Figure 1. Asymmetric unit of 1 (hydrogen atoms and solvent
molecules are omitted for clarity).
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Dy1−O7, 2.296(4) Å]. Three [OH]− ligands function in a μ-3
manner by bridging one nickel and two lanthanide centers
[Ni1−O6, 2.111(4) Å; Dy1−O6, 2.284(4) Å; Dy1−O6*,
2.426(3) Å]. Finally a lone [O]2− bridges the three lanthanide
centers in a μ-3 fashion [Dy1−O5, 2.415(3) Å]. The core
structure of 1 consists of four incomplete cubic subunits that
can be termed as O-capped clusters, analogous to that found in
organostannoxane chemistry22 (three Dy2NiO4 units and one
Dy3O4 unit), each of which has a missing vertex (Figure 3).
Figure 2. Molecular structure of 1. All the hydrogen atoms have been
omitted for clarity. Important bond lengths (Å) and bond angles
(deg): Dy(1)−O(1), 2.226(3); Dy(1)−O(6), 2.284(4); Dy(1)−O(4),
2.295(3); Dy(1)−O(7), 2.296(4); Dy(1)−O(5), 2.415(3); Dy(1)−
O(6)*, 2.426(3); Dy(1)−O(2), 2.614(4); Dy(1)−O(3), 2.646(3);
Ni(1)−O(1), 2.019(3); Ni(1)−O(4)*, 2.057(3); Ni(1)−O(8),
2.086(4); Ni(1)−O(6)*, 2.111(4); Ni(1)−N(1), 2.118(4); Ni(1)−
N(2), 2.160(4); O(1)−Dy(1)−O(6), 156.69(13); O(1)−Dy(1)−
O(4), 130.48(12); O(6)−Dy(1)−O(4), 71.70(12); O(1)−Dy(1)−
O(7), 87.43(13); O(6)−Dy(1)−O(7), 93.47(13); O(4)−Dy(1)−
O(7), 105.00(13); O(1)−Dy(1)−O(5), 91.88(11); O(6)−Dy(1)−
O(5), 74.23(14); O(4)−Dy(1)−O(5), 101.77(10); O(7)−Dy(1)−
O(5), 145.18(16); O(1)−Dy(1)−O(6)*, 70.24(12); O(6)−Dy(1)−
O(6)*, 87.43(17); O(1)−Dy(1)−O(2), 62.30(11); O(6)−Dy(1)−
O(2), 128.46(12); O(4)−Dy(1)−O(2), 75.84(12); O(7)−Dy(1)−
O(2), 133.55(13); O(5)−Dy(1)−O(2), 74.58(16); O(1)−Dy(1)−
O(3), 77.93(12); O(6)−Dy(1)−O(3), 124.27(11); O(4)−Dy(1)−
O(3), 62.67(11); O(7)−Dy(1)−O(3), 70.59(12); O(5)−Dy(1)−
O(3), 143.04(16); O(1)−Ni(1)−O(4)*, 89.13(14); O(1)−Ni(1)−
O(8), 89.62(14); O(4)*-Ni(1)−O(8), 171.69(14); O(1)−Ni(1)−
O(6)*, 80.94(14); N(1)−Ni(1)−N(2), 85.71(17).
Figure 3. Core structure of compound 1.
Such structural motifs are known in the literature.23 It may also
be mentioned that the presence of Ni4O4 and Dy4O4 cubic
motifs is known.24,25 All the Ni(II) centers in 1 are equivalent
and are hexacoordinate (2N, 4O) in a distorted octahedral
geometry (Figure 4a). The Dy(III) centers are also equivalent
Chart 2. Binding Modes of the Ligands Observed in the
Present Study
Figure 4. Coordination geometry around the metal centers in 1: (a)
Ni center, distorted octahedron; (b) Dy center, distorted trigonal-
dodecahedron geometry.
and are eight-coordinate in a distorted trigonal dodecahedral
geometry (Figure 4b). A mean plane analysis of the core of 1
reveals that while the three Ni(II) centers are coplanar and are
present in one plane, the three Ln(III) centers are also coplanar
but present in a different plane that is parallel to the former
(Figure 5). Several metal-containing rings are generated in the
structure of 1, the sizes of which vary from 4 to 12 (Figure S7).
Magnetic Studies. Magnetic measurements as a function
of the temperature were performed on polycrystalline samples
in the temperature range 1.8−300 K, under a dc magnetic field
of 0.05 T. The χT values at room temperature (44.8 emu·K·
2.019(3) Å; Dy1−O1, 2.226(3) Å; Ni1−O4, 2.057(3) Å; Dy1−
O4, 2.295(3) Å]. The two nitrogen centers of [L]2− bind
exclusively to Ni(II) [Ni1−N1, 2.118(4) Å; Ni1−N2, 2.160(4)
Å]. In addition, three pivalate ligands bridge alternate Ni/Dy
pairs through a η1,η1 coordination mode [Ni1−O8, 2.086(4) Å;
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Figure 5. Mean planes of 1.
mol−1 for 1, 39.7 emu·K·mol−1 for 2, 27.3 emu·K·mol−1 for 3,
46.2 emu·K·mol−1 for 4, 38.5 emu·K·mol−1 for 5) are in good
agreement with the expected values for compounds made of
three Ni(II) ions (S = 1, g = 2.1, C = 1.10 emu·K·mol−1) and
three Dy(III) (S = 5/2, L = 5, 6H15/2, g = 4/3, C = 14.17 emu·
7
K·mol−1) for 1, three Tb(III) (S = 3, L = 3, F6, g = 3/2, C =
11.81 emu·K·mol−1) for 2, three Gd(III) (S = 7/2, L = 0, 8S7/2
,
g = 2, C = 7.88 emu·K·mol−1) for 3, three Ho(III) (S = 2, L = 6,
5I8, g = 5/4, C = 14.48 emu·K·mol−1) for 4, and three Er(III) (S
4
= 3/2, L = 6, I15/2, g = 6/5, C = 11.48 emu·K·mol−1) for 5
Figure 7. Fit of χT = f(T) for compound 3 (open circles:
experimental, full line: fit (see text)). Inset: M = f(H) curve for
compound 3 (open circles: experimental, full line: calculated curve
using the values obtained from the fit of the χT = f(T) curve).
(Figure 6).5
cm−1 with a very good agreement factor R = 1.3 × 10−5 (Figure
7). As expected due to the contraction of the 4f orbitals, the
magnetic interactions are very weak. The larger Ni−Gd
interaction leads to a spin ground state S = 27/2.
The behavior of the other compounds, 1, 2, and 5, can be
interpreted as a competition between the thermal depopulation
of low-lying crystal-field states of the Ln(III) ions5 and the
Ni(II)−Ln(III) intramolecular ferromagnetic interaction. For 4,
the effect of the thermal depopulation of the excited states of
Ho(III) ions most likely dominates the effect of the
intramolecular interaction. Therefore, in this case, it is not
possible to conclude unambiguously on the nature of the
intermolecular interaction, even though the nature of the 3d−
4fn magnetic interaction remains generally the same, with about
the same amplitude for Ln(III) ions with n ≥ 7.27
Magnetization versus field measurements at 1.8 K for 1−5
are presented in Figure 8. For all compounds, except the
Figure 6. χT = f(T) under a static magnetic field of 0.05 T for
compounds 1 (purple), 2 (green), 3 (blue), 4 (red), and 5 (black).
Upon decreasing the temperature, the behavior of all
compounds are relatively different. For 2 and 3 χT increases
gradually up to a maximum at around 3 K. For 1 and 5, χT
decreases to a minimum at around 45 K for 1 and 10 K for 5
before increasing regularly up to 1.8 K. Finally the χT product
for the HoIII analogue 4 shows a constant and rapid decrease
upon cooling.
For the GdIII analogue 3, for which there is no spin−orbit
coupling, the χT = f(T) curve can be fitted considering the
following spin-Hamiltonian:
̂
̂
̂
̂
̂
̂
̂
H = −J [SNi1·(SGd1 + SGd3) + SNi2·(SGd1 + SGd2
)
1
̂
̂
̂
̂
̂
̂
̂
+ SNi3·(SGd2 + SGd3)] − J2(SGd1·SGd2 + SGd1·SGd3
̂
̂
+ SGd2·SGd3
)
(Figure 7) using the Magpack program.26
We neglected the magnetic anisotropy of Ni(II) ions, but we
had to consider antiferromagnetic intermolecular interaction
(within the mean-field approach). The fit leads to g = 2.01(1),
J1 = 0.96(1) cm−1, J2 = −0.18 (1) cm−1, and zJ = −0.043(1)
Figure 8. M = f(H) at 1.8 K for compounds 1 (pink), 2 (green), 3
(blue), 4 (red), and 5 (black) (full lines are just a guide for the eye).
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Figure 9. In-phase (left) and out-of-phase (right) susceptibility measurements at various frequencies for compound 1 under an applied dc field of
3000 Oe and with an oscillating field of 2 Oe.
Figure 10. Left: Plots of the in-phase susceptibility (open circles) and out-of-phase susceptibility (open circles) for 1 at 1.8 K (applied dc field of
3000 Oe, oscillating field of 2 Oe). Right: Cole−Cole plot. The solid lines correspond to the fit of the data to a generalized Debye model.
Gd(III) analogue 3, the magnetization shows a continuous
linear increase at high fields, due to the magnetic field-induced
population of low-lying excited states. The increase of the
magnetization for 4 is clearly slower than for the other
compounds, which also indicates the predominance of the
crystal-field effect upon intramolecular magnetic interaction.
The high-field (7 T) values of the magnetization are 19.0, 21.3,
26.8, 17.6, and 21.7 μB for 1−5, respectively, in agreement with
the expected ones (between 19.5 and 24 μB for 1, 2, 4, and 5
and 27 μB for 3). Indeed, due to crystal-field effects, the values
of saturation magnetization for Dy(III), Tb(III), Ho(III), and
Er(III) range between 4.5 and 6 μB.28 For 3, the expected value
the magnetization (Figure S8). In order to overcome this
difficulty, ac susceptibility measurements were performed under
a small optimized dc field (3000 Oe) (Figure S9). Only 1
showed maxima in the temperature dependence of χ′ and χ″
under this dc field (Figure 9). Given the very low temperature
at which they occur, it is possible to extract only two maxima
from this experiment. Therefore, the nature of the relaxation
mechanism cannot be ascertained unambiguously as a thermally
activated process. From a “fit” to an Arrhenius law, we can
estimate a very small energy barrier of about 10 K, with a pre-
exponential factor of about 10−6 s, within the range of values
usually found for other SMMs.
8
is higher, around 27 μB, because the S7/2 ground state of
Gd(III) is isotropic.
The low-temperature frequency-dependent ac susceptibility
data for compound 1 were further analyzed using a generalized
Debye model to fit the Cole−Cole plot (see Supporting
Information for the fit information) (Figure 10).29 The best fit
of the Cole−Cole plot at 1.8 K leads to the following
Alternating current susceptibility measurements were
performed in a 0 dc static field in the temperature range
1.8−20 K with a 2 Oe field oscillating at frequencies ranging
from 1000 to 1 Hz. For compounds 2−5, the in-phase signals
are all superimposable, whatever the frequency, and no out-of-
phase signal appears down to 1.8 K. On the contrary, for the
DyIII derivative 1, both in-phase and out-of-phase curves show a
clear frequency dependence, which suggests a slow relaxation of
parameters: α = 0.262(4), χ0 = 8.07(2) emu·mol−1, χ∞
=
1.27(3) emu·mol−1, and τ = 0.000 818(8) s. The values of α
and τ are consistent with ones usually encountered for other
SMMs. The relatively large value for α indicates a broad
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G.; Stamatatos, T. C. Eur. J. Inorg. Chem. 2013, 2286−2290. (d) Berg,
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́
Martínez-Lope, M. J.; Casais, M. T.; Fernandez-Díaz, M. T. Inorg.
distribution of relaxation times, suggesting that more than one
relaxation process might operate at this temperature.
CONCLUSION
■
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In summary, we have utilized a noncompartmental multiple
pocket containing hetero donor chelating mannich base ligand
6,6′-{(2-(dimethylamino)ethylazanediyl)bis(methylene)}bis(2-
methoxy-4-methylphenol) to assemble heterometallic mono-
cationic hexanuclear Ni(II)−Ln(III) coordination complexes.
All these compounds possess homo- and heterometallic O-
capped structural subunits (three Ln2NiO4 units and one Ln3O4
unit), each of which has a missing vertex). All reported
complexes represent the first family of Ni3Ln3 coordination
clusters with this ligand. All the lanthanides centers are eight-
coordinate in a distorted trigonal dodecahedral geometry, while
nickel centers are hexacoordinated in a distorted octahedral
geometry. Magnetic studies reveal an out-of-phase signal for the
Dy(III) analogue in its dynamic magnetization studies. Among
all these complexes, 1 shows a very small energy barrier of
about 10 K, with a pre-exponential factor of about 10−6 s.
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ASSOCIATED CONTENT
* Supporting Information
■
S
(7) (a) Rogez, G.; Rebilly, J.-N.; Barra, A.-L.; Sorace, L.; Blondin, G.;
Kirchner, N.; Duran, M.; van Slageren, J.; Parsons, S.; Ricard, L.;
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Schemes representing the reported mononuclear complexes.
Figures and tabulated bond angles/lengths for the crystal
structures of compounds 2−5. Alternating current susceptibility
measurement data under 0 and applied dc field for 1. Cole−
Cole plots fit equations. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: vc@iitk.ac.in; vc@niser.ac.in.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Department of Science and Technology, India,
and the Council of Scientific and Industrial Research, India, for
financial support. V.C. is thankful to the Department of Science
and Technology, for a JC Bose fellowship. J.G. thanks the
Council of Scientific and Industrial Research, India, for a Senior
Research Fellowship.
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