Self-assembled tetra- and pentanuclear nickel(II) aggregates from phenoxido-based ligand-bound {Ni2} fragments: Carboxylate bridge controlled structures

Article abstract of DOI:10.1021/ic401337q

Three different carboxylato bridges (R = C₂H₅, CF₃, and PhCH₂ in RCO₂) have been used to obtain the supramolecular aggregates [Ni₅(μ-H ₂bpmp)₂(μ₃

Full text of DOI:10.1021/ic401337q

Article
pubs.acs.org/IC
Self-Assembled Tetra- and Pentanuclear Nickel(II) Aggregates From
Phenoxido-Based Ligand -Bound {Ni2} Fragments: Carboxylate
Bridge Controlled Structures
Aloke Kumar Ghosh,† Michael Shatruk,‡ Valerio Bertolasi,§ Kausikisankar Pramanik,∥
and Debashis Ray*,†
†Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302, India
‡Department of Chemistry & Biochemistry, Florida State University, Tallahassee, Florida 32306, United States
§Dipartimento di Scienze Chimiche e Farmaceutiche, Centro di Strutturistica Diffrattometrica, Universita
̀
di Ferrara, via L. Borsari 46,
44100 Ferrara, Italy
∥Inorganic Chemistry Section, Department of Chemistry, Jadavpur University, Kolkata-70032, India
S
* Supporting Information
ABSTRACT: Three different carboxylato bridges (R = C2H5,
¯
CF3, and PhCH2 in RCO2 ) have been used to obtain the
supramolecular aggregates [Ni5(μ-H2bpmp)2(μ3-OH)2(μ1,3-
O2CC2H5)6]·2H2O·4DMF (1·2H2O·4DMF), [Ni4(μ3-
H2bpmp)2(μ3-OH)2(μ1,3-O2CCF3)2](CF3CO2)2·H2O (2·
H2O), and [Ni4(μ3-H2bpmp)2(μ3-OH)2(μ1,3-O2CCH2Ph)2]-
(PhCH2CO2)2·4H2O (3·4H2O) (H3bpmp =2,6-bis-[(3-hy-
droxy-propylimino)-methyl]-4-methyl-phenol) from the hy-
droxido-bridged dinuclear motif [Ni2(μ-H2bpmp)(OH)]2+.
These complexes have been characterized by X-ray crystallog-
raphy and magnetic measurements. A change from propanoate
group to trifluoroacetate and phenylaceate groups provided different course of cluster assembly based on Ni2(μ-H2bpmp)2
fragments. The {Ni5(μ3-OH)2(μ1,3-O2CC2H5)6}2+ core in 1 contains five NiII ions in an hourglass (pentanuclear vertex-shared
double cubane) arrangement. These compounds are new examples of [Ni5] and [Ni4] complexes where aggregation of the
building motifs are guided by the nature of the carboxylate anions, which allows an effective tuning of the self-aggregate process
within same ligand environment. The study of the magnetic properties reveals that 1 exhibits an S = 3 ground state. Nevertheless,
the magnetization increases above the expected saturation value of 6 μB at higher fields, because of the suppression of
antiferromagnetic exchange between the central and peripheral Ni(II) ions. Complexes 2 and 3 exhibit ferromagnetic exchange
interactions that result in the S = 4 ground state. Examination of AC magnetic susceptibility showed that complex 2 in finely
ground form behaves as spin glass with the spin-freezing temperature of ∼5.5 K. This behavior was attributed to the collapse of
the structure upon the loss of interstitial solvent. Such property was not observed for complex 3, in which the bulkier carboxylate
ligands provide for a more robust crystal packing and larger separation between the [Ni4O4] clusters.
molecular cluster complexes thus continues to be a major theme
in modern synthetic transition metal chemistry.
INTRODUCTION
■
Synthesis and characterization of multinuclear coordination
cluster complexes of 3d metal ions have become of particular
interest in recent times because of their relevance to bioinorganic
chemistry and to new magnetic materials.1 A number of magnetic
cluster complexes behave as single-molecule magnets (SMMs)
because of the combination of large ground-state spin (S) with a
large Ising (or easy-axis) type magnetic anisotropy that stems
from a negative and axial zero-field-splitting parameter (D).2
Self-aggregation of transition metal ions is mostly achieved by
oxygen-containing bridges, such as hydroxide, oxide, phenolate,
and carboxylate. The nature of the coordinating ligands plays the
most crucial role for the formation of the products. The rational
assembly of component small molecules into larger supra-
Thus, there is a clear demand for new synthetic approaches to
multinuclear metal complexes which can show promise for high-
spin or SMM behavior.3 The choice of ligands is always a major
issue in the development of new synthetic routes to this type of
coordinations clusters.4 Phenol-centered “dinucleating ligands”,
known during last 40 years,5 are able to bind simultaneously two
metal ions. Bearing two adjacent imine arms, these ligands can
provide M2L type complexes6 in absence of other supporting
bridges. Introduction of secondary bridging units, such as HO¯ or
O2−, from deprotonation of solvent water molecules7,8 can result
Received: May 28, 2013
Published: December 2, 2013
© 2013 American Chemical Society
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in the assembly of transition metal coordination clusters from
two or more M2L units.7,8 Such clusters display a variety of
topologies, for example, tetrahedron,7 cubane,9a−c open
dicubane,10 fused defective open dicubane,11a,b pentanuclear
vertex-shared double cubane (hourglass),12 and stepped
cubane13 (Scheme 1).
PhCH2) in presence of NEt3. It is worth noting that the use of a
closely related ligand, H3bemp (Chart 1, right, 2,6-bis-[(2-
hydroxy-ethylimino)-methyl]-4-methylphenol), led to exciting
[Ni6]18 and [Ni6, Ni4]13 clusters.
Herein, we report the synthesis of three new Ni(II) complexes
with the H2bpmp− anionic ligand: the pentanuclear vertex shared
dicubane complex 1·2H2O·4DMF and tetranuclear cubane
complexes 2·H2O and 3·4H2O, both containing two μ3-
phenoxide and two μ3-hydroxide anions at the corners of the
cubane. The crystal structures and magnetic properties of these
complexes are also described and discussed.
Scheme 1. Growth of Penta- and Tetranuclear Nickel
Assemblies Depending on the Primary and Supporting
Ligands
EXPERIMENTAL SECTION
■
Materials. Ni(C2H5CO2)2·4H2O, Ni(CF3CO2)2·4H2O, and Ni-
(PhCH2CO2)2·4H2O were synthesized by adding nickel(II) carbonate
(60 mg) to a solution of propanoic acid, trifluoroacetic acid, and
phenylacetic acid each (74.08, 114.02, 136.15 mg, respectively) in 30 mL
of water under constant glass-rod stirring until effervescence had
stopped. Then the solution was filtered, and the clear filtrate was kept
over a water bath until a solid started to precipitate. The solution was
then cooled to room temperature, and the solid was filtered through
suction and dried in vacuum. The chemicals used were obtained from
the following sources: nickel(II) carbonate from Universal Laboratory
(India), trifluoroacetic acid, propanoic acid, phenyl acetic acid,
triethylamine from E. Merck (India), and 3-amino-1-propanol from
Aldrich Chemical Co. Inc. (U.S.). These chemicals and solvents were
reagent-grade materials and were used as received without further
purification. 2,6-Diformyl-4-methylphenol (2-hydroxy-5-methyl-ben-
zene-1,3-dicarbaldehyde) was prepared following a literature proce-
dure.19 All manipulations were performed under aerobic conditions.
Syntheses. H3bpmp Ligand. The H3bpmp Schiff-base ligand was
prepared from the single-step condensation of 2,6-diformyl-4-
methylphenol (1.0 g, 6.1 mmol) and 3-amino-1-propanol (0.91 g,
12.2 mmol) in methanol (20 mL) in air at room temperature (28 °C)
under stirring for 2 h following a reported procedure.17 Complete
evaporation of solvent in air for 12 h yielded an orange-colored
semisolid product, which was used directly for reactions with metal salts.
Because of its semisolid nature, the ligand obtained from this reaction
was not of excellent purity. CHN analysis was more or less close to the
theoretical values and major proton NMR signals were also character-
istic indication the Schiff base condensation at two arms of the ligand.
Yield: 1.32 g (78%).
[Ni5(μ3-OH)2(μ-H2bpmp)2(μ1,3-O2CC2H5)6]·2H2O·4DMF (1·2H2O·
4DMF). Ni(C2H5CO2)2·4H2O (0.691 g, 2.50 mmol) dissolved in
MeOH (20 mL) was added dropwise to a yellow MeOH solution (10
mL) of H3bpmp (0.278 g, 1.00 mmol) followed by dropwise addition of
NEt3 (0.278 mL, 0.202 g, 2.00 mmol) during 5 min. The green solution
was next stirred for 2.5 h at room temperature. The solvent was
evaporated in air to give a green solid, which was isolated, washed with
cold methanol and dried under vacuum over P4O10. Green crystals
suitable for single crystal X-ray analysis were obtained from a saturated
DMF solution after three weeks. Yield (green solid): 0.601 g, 73%. Anal.
Calcd. for C60H106Ni5N8O26 (1649.05 g mol−1): C, 43.70; H, 6.47; N,
6.79. Found: C, 43.77; H, 6.42; N, 6.84. Selected FT-IR bands: (KBr,
cm−1; s = strong, vs = very strong, m = medium, br = broad) 3399(br),
2930(s), 2367(m), 1637(s), 1570(vs), 1459(s), 1409(s), 1384(vs),
1074(s), 764(s), 626(s). Molar conductance, ΛM: (DMF solution) 3.4
Ω−1 cm2 mol−1. UV−vis spectra [λmax, nm (ε, L mol−1 cm−1)]: (MeOH
solution) 663 (725), 371 (1289), 258 (51237).
Among the ancillary ligands used, hydroxide and carboxylate
groups offer versatile bridging modes (Supporting Information
Scheme S2). One hydroxide group can bridge two to six metal
ions in the assembly reaction.14,15 Both aliphatic and aromatic
carboxylate groups can adopt numerous coordination modes
such as terminal, chelating, bridging syn−anti in μ-1,1 mode
(Supporting Information Scheme S2a−c), syn−syn, syn−anti,
and anti−anti in μ-1,3 mode (Supporting Information Scheme
S2d−h), anti−syn−syn and anti−syn−anti in μ3-1,1,3 mode-
(Supporting Information Scheme S2i,j), and syn−anti−syn−anti
in μ4-1,1,3,3 mode (Supporting Information Scheme S2k)
satisfying the coordination requirement and charge demand of
the transition metal ions (Supporting Information Scheme S2).16
Scheme 2. Bridging Mode of Hydroxide and Carboxylate
Anions
In trying to prepare and study new forms of [Ni5] and [Ni4]
aggregates from the assembly of ligated [Ni2] basic units, we have
been interested in exploring the reactivity of 2,6-bis-[(2-
hydroxypropylimino)-methyl]-4-methylphenol (H3bpmp,
Chart 1)17 with three Ni(RCO2)2·4H2O salts (R = C2H5, CF3,
Chart 1. H3bpmp and H3bemp
Compounds [Ni4(μ3-OH)2(μ3-H2bpmp)2(μ1,3-O2CCF3)2](CF3CO2)2·
H2O (2·H2O) and [Ni4(μ3-OH)2(μ3-H2bpmp)2(μ1,3-O2CCH2Ph)2]-
(PhCH2CO2)2·4H2O (3·4H2O). 2·H2O and 3·4H2O were obtained
following the same procedure as outlined above for the synthesis of 1
by using Ni(CF3CO2)2·4H2O and Ni(PhCH2CO2)2·4H2O in place of
Ni(C2H5CO2)2·4H2O. The obtained yields were 77% and 71%,
respectively, for 2 and 3. For 2, Anal. Calcd for C38H46Ni4N4O17F12
(1293.63g mol−1): C, 35.28; H, 3.58; N, 4.33. Found: C, 34.47; H, 3.68;
N, 4.39. Selected FT-IR bands: (KBr, cm−1) 3409(br), 2925(m),
1668(vs), 1568(s), 1451(m), 1311(m), 1202(s), 1137(m), 1074(m),
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Table 1. Crystallographic Data for 1·2H2O·4DMF, 2·H2O, and 3·4H2O
compound
formula
1·2H2O·4DMF
2·H2O
3·4H2O
C60H106Ni5N8O26
1649.08
C38H46F12N4Ni4O17
1293.63
P21/c
C62H80Ni4N4O20
1436.14
P21
M
space group
crystal system
a (Å)
P21/n
monoclinic
11.8638(8)
16.560(1)
19.494(1)
90.990(3)
3829.8(5)
295
monoclinic
17.896(1)
14.122(1)
20.306(2)
97.953(2)
5082.6(6)
295
monoclinic
11.0227(1)
18.6158(3)
16.4947(3)
94.6647(6)
3373.44(9)
295
b (Å)
c (Å)
β (deg)
V (Å3)
T (K)
Z
2
4
2
Dc (g cm−3
)
1.430
1.691
1.414
F(000)
1732
2656
1504
crystal size (mm)
μ(Mo Kα) (cm−1
measured reflns
unique reflns
Rint
0.38 × 0.27 × 0.18
12.85
0.41 × 0.29 × 0.17
15.74
0.42 × 0.34 × 0.31
11.72
)
49815
52719
24480
8295
8866
8234
0.0461
0.0652
0.0389
obs. reflns [I ≥ 2σ(I)]
θmin−θmax (deg)
hkl ranges
5903
6378
7207
1.61−27.00
−14, 14; −21, 21; −24, 24
0.0485
1.17−25.00
−21, 21; −16, 16; −23, 24
0.0853
2.56−27.90
−14, 14; −22, 24; −20, 21
0.0372
0.0460
735
a
R1(F2) (obs. reflns)
a
wR2(F2) (all reflns)
0.1455
0.1950
no. variables
452
700
goodness of fit
1.016
1.170
1.060
Δρmax, Δρmin (e Å−3
)
0.69, −0.52
1.43, −0.68
0.72, −0.47
a
2
R1 = Σ(||Fo| − |Fc||)/Σ|Fo|. wR2 = [Σw(|Fo| − |Fc|)2/Σw(Fo)2]1/2. w = 0.75/(σ2(Fo) + 0.0010Fo ).
928(m), 846(m), 729(m). Molar conductance, ΛM: (MeOH solution)
142 Ω−1 cm2 mol−1. UV−vis spectra, [λmax, nm (ε, L mol−1 cm−1)]:
(MeOH solution) 673 (396), 373(1476), 258(70737). For 3, Anal.
Calcd for C62H80Ni4N4O20 (1436.14 g mol−1): C, 51.85; H, 5.61; N,
3.90. Found: C, 51.87; H, 5.72; N, 3.96. Selected FT-IR bands: (KBr,
cm−1) 3366(br), 2910(s), 1636(s), 1576(vs), 1388(vs), 1311(m),
1075(s), 930(m), 805(m), 706(m), Molar conductance, ΛM: (DMF
solution) 155 Ω−1 cm2 mol−1. UV−vis spectra [λmax, nm (ε, L mol−1
cm−1)]: (MeOH solution) 676 (322), 370 (8563), 261 (69120).
Physical Measurements. The elemental analyses (C, H, N) were
performed with a Perkin-Elmer model 240 C elemental analyzer. Fourier
transform infrared (FT-IR) spectra were recorded on a Perkin-Elmer
RX1 spectrometer. Solution electrical conductivity measurements and
electronic spectra were carried out using a Unitech type U131C digital
conductivity meter with a solute concentration of about 10−3 M and a
Shimadzu UV 3100 UV−vis−NIR spectrophotometer, respectively.
Magnetic Measurements. Magnetic measurements were per-
formed on microcrystalline samples of 1·2H2O·4DMF, 2·H2O, and 3·
4H2O, using a Quantum Design SQUID magnetometer MPMS-XL. DC
magnetic susceptibility was measured in an applied field of 0.1 T, in the
1.8−300 K temperature range. Isothermal dependences of magnet-
ization were measured at 1.8 K with the magnetic field varying from 0 to
7 T. AC magnetic susceptibility was measured under zero DC bias field,
in the 1.8−50 K range, with the AC field amplitude of 0.5 mT and
frequencies of 1, 10, 100, and 1000 Hz. The data were corrected for
diamagnetic contributions using tabulated constants.20
calculated positions, riding on their carrier atoms. All the calculations
were performed using SHELXL-9724 and PARST25 implemented in
WINGX suite of programs.26 Information concerning X-ray data
collection and crystal structure refinement is summarized in Table 1.
Crystallographic data (excluding structure factors) have been deposited
with the Cambridge Crystallographic Data Centre as supplementary
publications CCDC-869973, 878614, and 878615. These data can be
obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html
(or from the Cambridge Crystallographic Data Centre, 12, Union Road,
Cambridge CB2 1EZ, U.K.; fax +44−1223/336−033; e-mail deposit@
ccdc.cam.ac.uk).
RESULTS AND DISCUSSION
■
Synthetic Considerations. The Schiff-base ligand 2,6-bis-
[(3-hydroxy-propylimino)-methyl]-4-methylphenol (H3bpmp)
was prepared following a literature procedure (Scheme S1 in the
Supporting Information),17 and its reactions with three nickel-
(II) salts have been systematically investigated to explore the
roles of different carboxylate anions for cluster assembly as
summarized in Supporting Information Scheme S3. Two types of
self-assembly processes involving ligand bound [Ni2] fragments
were responsible for the formation of the products. A variety of
reactions were explored with different solvents and under
different reagent ratios before the following successful
procedures were identified. The reaction of Ni(C2H5CO2)2·
4H2O, H3bpmp, and NEt3 in a 5:2:4 ratio in MeOH afforded a
greenish solution, from which was subsequently isolated 1 as
green solid in 73% yield. Variations in the reaction stoichiometry
also gave product 1 but in lower yield. The formation of 1 is
summarized by eq 1, which also accounts for the formation of
hydroxido bridges from the water molecules present in the
solvent system used. Crystals of 1·2H2O·4DMF were obtained
from a DMF solution.
X-ray Crystallography. Single-crystal X-ray diffraction data of 1·
2H2O·4DMF and 2·H2O were collected on a Bruker-APEX-2 CCD X-
ray diffractometer using graphite-monochromated Mo Kα radiation (λ =
́
0.7107 Å) by ω-scan at 293 K. The data for 3·4H2O were collected at
room temperature using a Nonius Kappa CCD diffractometer. All the
data sets were integrated with the Denzo-SMN package21 and corrected
for Lorentz, polarization, and absorption effects (SORTAV22). The
structures were solved by direct methods (SIR9723) and refined using
full-matrix least-squares with anisotropic atomic displacement param-
eters for all non-hydrogen atoms. Hydrogen atoms were included on
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determinations (vide infra).28 The strong asymmetric νas(COO)
̅
2H3bpmp + 5Ni(C2H5CO2)2·4H2O + 4NEt3
→ [Ni5(μ‐H2bpmp)2(μ3‐OH)2(μ1,3‐O2CC2H5)6]
+ 4(HNEt3)(C2H5CO2) + 18H2O
vibration for 1 is observed at 1570 cm−l and the symmetric
νs(COO) vibration is found at 1384 cm−1. The difference, Δν =
̅
̅
νas(COO)−νs(COO) = 186 cm−1, is indicative of μ1,3-bridging
̅
̅
(1)
propionates. For 2 and 3 the corresponding values for
trifluoroacetate and phenylacetate groups are 1568 and 1576
cm−1 (νas(COO)) and 1311 and 1388 cm−1 (νs(COO)), resulting in
The elemental analysis, molar conductivity data and single-
crystal X-ray diffraction support the composition [Ni5(μ-
H2bpmp)2(μ3-OH)2(μ1,3-O2CC2H5)6]·2H2O·4DMF (1·2H2O·
4DMF).
To identify the role of ancillary carboxylate ligands for the self-
aggregation, the above stated reaction (eq 1) was repeated with
Ni(CF3CO2)2·4H2O in place of Ni(C2H5CO2)2·4H2O. The
reaction of Ni(CF3CO2)2·4H2O, H3bpmp, and NEt3 in a 4:2:4
ratio in MeOH provided 2 as green solid in 77% yield:
̅
̅
Δν values of 257 and 188 cm−1, resepectively.29,30 For
̅
nonbridging carboxylato coordination, this Δν separation is
̅
usually larger (∼350 cm−1).8
Electronic Spectra. Moderate solubility of all three
complexes in MeOH allowed spectroscopic characterizations
by recording optical absorption spectra in the 200−900 nm
regions. Broad absorption bands with maxima at 663 (ε = 725 L
mol−1 cm−1), 673 (ε = 147 L mol−1 cm−1), and 676 nm (ε = 322
L mol−1 cm−1) for 1, 2, and 3, respectively, can be assigned to the
2H3bpmp + 4Ni(CFCO2)2·4H2O + 4NEt3
3
3
3
spin-allowed A2g(F) → T1g(F) transition of Ni(II) ion in the
slightly distorted octahedral coordination. The lower energy
3A2g(F) → 3T2g(F) transition was not observed in the accessible
wavelength range.31 The 3A2g(F) → 3T1g(P) transition appears at
371 nm (ε = 1289 L mol−1 cm−1), 373 nm (ε = 1476 L mol−1
cm−1) and 370 nm (ε = 8563 L mol−1 cm−1) for 1, 2 and 3,
respectively.32 The intense absorptions at 258 (ε = 51237 L
mol−1 cm−1), 259 (ε = 70737 L mol−1 cm−1), and 261 nm (ε =
169120 L mol−1 cm−1) are because of ligand-centered π → π*
transitions.
→ [Ni4(μ3‐H2bpmp)2(μ3‐OH)2(μ1,3‐O2CCF) ]
3 2
(CFCO2)2 + 4(HNEt3)(CFCO2) + 12H2O
(2)
3
3
The elemental analysis, molar conductivity data and single-
crystal X-ray structure confirm the formula [Ni4(μ3-
H2bpmp)2(μ3-OH)2(μ1,3-O2CCF3)2](CF3CO2)2·H2O (2·
H2O).
To further explore the role of the carboxylate ligand on the
cluster formation, a reaction of Ni(PhCH2CO2)2·4H2O,
H3bpmp, and NEt3 in 4:2:4 ratio in MeOH was performed to
provide 3 as slightly dark-green solid in 71% yield. The
preparative route to 3 is summarized in eq 3 establishing once
again the importance of anionic parts available from the
precursor metal salts in the outcome of the reaction.
Description of Structures. [Ni5(μ-H2bpmp)2(μ3-OH)2(μ1,3-
O2CC2H5)6]·2H2O·4DMF (1·2H2O·4DMF). The molecular struc-
ture of 1 is shown in Figure 1, and selected interatomic distances
2H3bpmp + 4Ni(PhCH2CO2)2·4H2O + 4NEt3
→ [Ni4(μ3‐H2bpmp)2(μ3‐OH)2(μ1,3‐O2CCH2Ph)2]
(PhCH2CO2)2·4H2O + 4(HNEt3)(PhCH2CO2)
+ 10H2O
(3)
The elemental analysis, molar conductivity, and X-ray
diffraction data establish the formula of the product as [Ni4(μ3-
H2bpmp)2(μ3-OH)2(μ1,3-O2CCH2Ph)2](PhCH2CO2)2·4H2O
3·4H2O, containing free carboxylate counterions. Thus, the use
of phenyl tail bearing carboxylate ligand did not change the mode
of aggregation toward a different nuclearity as compared to the
reaction carried out with trifluoroacetate. Both 2 and 3 contain a
face-capped cubic [Ni4] structure supported by endogenous μ3-
phenoxide and exogenous μ3-hydroxide ligands. During the
formation of these clusters, equally probable solvent derived μ3-
OMe bridging groups were not trapped in place of μ3-OH
groups.27 Basicity differences of the used carboxylates guided the
aggregation of the [Ni2] building motifs. When R = Et, more
carboxylates (six altogether) bind to the Ni ions compared to the
cases where R = CF3 and PhCH2.
Figure 1. Partially labeled plot of [Ni5] cluster of 1. Symmetry
equivalent atoms are not labeled. Color scheme: Ni, green; O, red; N,
blue; C, gray.
and angles are listed in Table 2. Single-crystal X-ray diffraction
revealed that the compound crystallizes in the monoclinic space
group P21/n. The molecule of 1 consists of five NiII ions held
together by two doubly bridging phenoxido groups (μ-O1 and its
symmetry equivalent, s.e.), two triply bridging hydroxido groups
(μ3-O2 and its s.e.), four syn,syn-η1:η1:μ-C2H5CO2 groups of one
¯
FT-IR Spectra. The presence of nickel-bound terminal and
bridging HO− groups and interstitial water molecules in 1·2H2O·
4DMF, 2·H2O, and 3·4H2O is manifested by a broad and
medium intensity bands at 3399, 3409, and 3366 cm−1,
type (O7 to O10 and their s.e.), and two of the other type (O5,
O6 and their s.e.). Peripheral ligation is provided by two
monodeprotonated Hbpmp¯ ligands (N1, N2, O3, O4, and their
s.e.). The coordination modes of the bridging ligands present in 1
are shown in Supporting Information Scheme S4. The five NiII
ions are disposed in a centered parallelogram. Alternatively the
cluster topology can be considered as two NiII triangles (Ni1−
Ni2−Ni3 and Ni1−Ni2′−Ni3′) with a common vertex on Ni1.
Each triangle is nearly isosceles, spanned by six carboxylate
respectively, assignable to νOH stretching frequencies. Extensive
̅
hydrogen bonding leads to band broadening and shift to lower
frequencies. The νCN stretching frequencies are observed
̅
within 1636−1668 cm−l for the three complexes. Analysis of
carboxylate stretching frequencies allowed us to identify the
mode of their binding as observed by X-ray crystal structure
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ions on either side of the molecule. The formation of the cluster
can be considered as the result of entrapment of the central NiII
ion (from the salt Ni(C2H5CO2)2·4H2O) by two dinuclear
[Ni2(μ-H2bpmp)] units.
The μ3-O2 donor atom lies 0.500 Å out of the plane of the
three connected NiII ions: the solid angle around this atom is
341.6° and two of the Ni−O−Ni angles are significantly larger
(124.3° and 124.7° for the angles subtended at the central NiII
ion) than the third one (92.7° for the angle derived to the
peripheral NiII ions) (Supporting Information Figure S1). The
core of the molecule is {Ni5(μ-OPh)2(μ-OH)2}6+ (Figure 3,
Table 2. Selected Interatomic Distances (Å) and Angles (deg)
for 1·2H2O·4DMF
distances
O5−Ni2
O7−Ni2
O8−Ni3
O10−Ni1
O9−Ni1
2.043(2)
2.041(2)
2.035(3)
2.103(2)
2.062(3
Ni2−O1
Ni2−N1
Ni2−O2
Ni2−O4
Ni2···Ni3
Ni3−N2
Ni3−O2
Ni3−O1
Ni3−O3
2.027(2)
2.048(3)
2.068(2)
2.068(3)
2.9713(6)
2.022(3)
2.032(2)
2.059(2)
2.094(3)
a
Ni1−O2
2.033(2)
2.033(2)
2.062(3)
2.103(2)
Ni1−O2
Ni1−O9
a
a
Ni1−O10
angles
a
O2 −Ni1−O2
O2−Ni1−O9
180.00
O7−Ni2−O4
O1−Ni2−Ni3
N1−Ni2−Ni3
O2−Ni2−Ni3
O7−Ni2−Ni3
O5−Ni2−Ni3
O4−Ni2−Ni3
N2−Ni3−O2
N2−Ni3−O1
O2−Ni3−O1
N2−Ni3−O8
O2−Ni3−O8
O1−Ni3−O8
N2−Ni3−O3
O2−Ni3−O3
O1−Ni3−O3
O8−Ni3−O3
N2−Ni3−O6
O2−Ni3−O6
O1−Ni3−O6
O8−Ni3−O6
O3−Ni3−O6
N2−Ni3−Ni2
O2−Ni3−Ni2
O1−Ni3−Ni2
O8−Ni3−Ni2
O3−Ni3−Ni2
O6−Ni3−Ni2
91.2(1)
43.79(7)
133.55(9)
43.17(7)
103.52(8)
79.83(8)
133.28(7)
176.7(1)
91.3(1)
95.1(1)
84.9(1)
84.9(1)
95.1(1)
180.0
a
O2 −Ni1−O9
a
O2−Ni1−O9
a
a
O2 −Ni1−O9
a
O9 −Ni1−O9
O2−Ni1−O10
92.6(1)
86.4(1)
94.2(1)
85.8(1)
86.4(1)
93.6(1)
86.8(1)
94.2(1)
180.00
a
O2 −Ni1−O10
a
a
O9 −Ni1−O10
a
O9−Ni1−O10
O2−Ni1−O10
85.75(9)
85.9(1)
a
O2−Ni1−O10
92.9(1)
a
O9 −Ni1−O10
92.5(1)
O9−Ni1−O10
O10−Ni1−O10
O1−Ni2−N1
O1−Ni2−O2
N1−Ni2−O2
O1−Ni2−O7
N1−Ni2−O7
O2−Ni2−O7
O1−Ni2−O5
N1−Ni2−O5
O2−Ni2−O5
O7−Ni2−O5
O1−Ni2−O4
N1−Ni2−O4
O2−Ni2−O4
92.8(1)
90.2(1)
90.9(1)
85.74(9)
176.6(1)
93.7(1)
85.6(1)
94.25(9)
87.6(1)
90.8(1)
89.3(1)
176.3(1)
173.2(1)
94.2(1)
89.3(1)
174.1(1)
92.1(1)
Figure 3. {Ni5(μ-OPh)2(μ-OH)2}6+ core of 1 (top) and a more detailed
representation emphasizing its {Ni5(μ3-OH)2(μ2-OPh)2(μ1,3
-
90.7(1)
OCC2H5)6} description (bottom). Color code: Ni, dark green; O,
red; N, blue; C, gray.
90.5(1)
87.3(1)
176.6(1)
88.4(1)
top). This Ni5 fragment or its trinuclear Ni3 subfragment is
common to other NiII cluster complexes..33 If we consider all six
propanoate bridges as part of the linking groups, then the
metallic core becomes {Ni5(μ-OPh)2(μ-OH)2(μ-O2CC2H5)6}
133.3(1)
44.04(6)
42.93(6)
101.69(9)
132.16(8)
80.43(8)
¯
(Figure 3, top). The μ-C2H5CO2 groups bridge NiII ions in
similar modes but twisting within the O−C−O planes leads to six
different Ni−O bond distances within 2.05−2.09 Å range
(Supporting Information Figure S2). Terminal Ni2 and Ni3
atoms are bound to an O5N set of donor atoms in H2bpmp¯
ligand environment, while Ni1 remains in H2bpmp-free
symmetric O6 environment with Ni−O bond distances within
2.038−2.092 Å range, having tetragonal elongation along the axis
defined by the propionate O9 and O9* atoms (Supporting
Information Figure S3). Thus, all of the NiII atoms are six-
coordinate with distorted octahedral geometry (Supporting
Information Figure S4).
The crystal structure of 1·2H2O·4DMF is stabilized by two
O(propanoate)···O(lattice H2O) hydrogen bonds at 3.005 Å and
O(DMF)···O(lattice H2O) hydrogen bonds from the other side
at 2.824 Å. The coordinated ligand alcohol arms bearing (H)O3
and (H)O4 are hydrogen bonded to propionate O10 and O9
atoms with O···O separations of 2.646 and 2.757 Å, respectively
(Supporting Information Figures S5 and S6).
a
Symmetry operations used to generate equivalent atoms: −x, 1 − y,
−z.
bridges with two longer Ni···Ni separations of 3.608 and 3.617 Å
and one shorter separation of 2.951 Å (Figure 2). Each of the four
triatomic carboxylate groups bridges the central NiII ion and one
peripheral NiII ion. The remaining two carboxylate groups
complete the triply bridging motif between Ni2 and Ni3. The μ3-
HO¯ group links the central NiII ion with the peripheral metal
[Ni4(μ3-OH)2(μ3-H2bpmp)2(μ1,3-O2CCF3)2](CF3CO2)2·H2O (2·
H2O) and [Ni4(μ3-H2bpmp)2(μ3-OH)2(μ1,3-O2CCH2Ph)2]-
(PhCH2CO2)2·4H2O (3·4H2O). The molecular structures of 2·
H2O and 3·4H2O are shown in Figure 4. Selected interatomic
distances and angles are provided in Tables 3 and 4 and
Supporting Information Table S1. Single-crystal X-ray diffraction
Figure 2. Ni5 skeleton of 1 showing centered rectangular topology of the
internuclear distances (Å).
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Table 3. Selected Interatomic Distances (Å) for 2·H2O and 3·
4H2O
2·H2O
Ni1−O2
Ni1−N4
Ni1−O6
Ni1−O9
Ni1−O1
Ni1−O4
Ni1···N3
Ni2−N2
Ni2−O3
Ni2−O2
Ni2−O4
Ni2−O7
Ni2−O11
2.009(5)
2.032(6)
2.047(6)
2.059(5)
2.070(5)
2.199(5)
2.979(1)
2.008(7)
2.012(6)
2.055(5)
2.074(5)
2.091(5)
2.095(5)
Ni2···Ni4
Ni3−N3
Ni3−O2
Ni3−O3
Ni3−O1
Ni3−O5
Ni3−O10
Ni4−O3
Ni4−O8
Ni4−N1
Ni4−O4
Ni4−O12
Ni4−O1
2.962(1)
2.012(7)
2.023(5)
2.037(5)
2.072(5)
2.085(6)
2.147(5)
2.016(5)
2.027(6)
2.032(7)
2.063(5)
2.079(6)
2.211(5)
3·4H2O
Ni1−O5
Ni1−N1
Ni1−O10
Ni1−O2
Ni1−O1
Ni1−O6
Ni1···Ni2
Ni2−O5
Ni2−N2
Ni2−O3
Ni2−O4
Ni2−O9
Ni2−O1
1.990(3)
2.015(4)
2.027(4)
2.048(3)
2.072(3)
2.223(3)
2.9370(7)
2.022(3)
2.042(4)
2.044(3)
2.048(3)
2.061(4)
2.098(3)
Ni3−O4
Ni3−O5
Ni3−N3
Ni3−O7
Ni3−O6
Ni3−O12
Ni3···Ni4
Ni4−O4
Ni4−N4
Ni4−O11
Ni4−O8
Ni4−O6
Ni4−O1
2.020(3)
2.024(3)
2.028(4)
2.050(4)
2.086(3)
2.092(4)
2.9439(7)
1.989(3)
2.030(4)
2.049(4)
2.053(3)
2.067(3)
2.205(3)
Figure 4. Partially labeled molecular structures of Ni4 cationic clusters in
2·H2O (a) and 3·4H2O (b). Color scheme: Ni, dark green; O, red; N,
blue; C, black; F, light green.
Table 4. Ni−O−Ni Angles (deg) for 2·H2O and 3·4H2O
2·H2O
Ni1−O2−Ni2
Ni1−O2−Ni3
Ni1−O1−Ni4
Ni1−O1−Ni3
Ni1−O4−Ni2
Ni1−O4−Ni4
104.5(2)
95.3(2)
98.3(2)
92.0(2)
97.5(2)
98.9(2)
Ni2−O4−Ni4
Ni2−O3−Ni4
Ni2−O2−Ni3
Ni2−O3−Ni3
Ni3−O1−Ni4
Ni3−O3−Ni4
91.4(2)
94.6(2)
98.5(2)
99.5(2)
97.0(2)
104.7(2)
revealed that these compounds crystallize in monoclinic space
groups P21/c and P21, respectively. Eight alternately arranged
oxygen and metal atoms form two distorted {Ni4O4} cubes with
local S4-symmetry. In case of complex 2·H2O the structure
consists of a dicationic tetrametallic part [Ni4(μ3-OH)2(μ3-
3·4H2O
H2bpmp)2(μ1,3-O2CCF3)2]2+ and two CF3CO2 anions.
¯
Ni1−O5−Ni2
Ni1−O5−Ni3
Ni1−O6−Ni3
Ni1−O6−Ni4
Ni1−O1−Ni2
Ni1−O1−Ni4
94.1(1)
103.6(1)
94.0(1)
Ni2−O4−Ni3
Ni2−O4−Ni4
Ni2−O1−Ni4
Ni2−O5−Ni3
Ni3−O4−Ni4
Ni3−O6−Ni4
99.2(1)
103.1(1)
94.5(1)
99.9(1)
94.5(1)
90.3(1)
In 3·4H2O the cluster fragment [Ni4(μ3-OH)2(μ3-H2bpmp)2-
¯
(μ1,3-O2CCH2Ph)2]2+ is cocrystallized with two PhCH2CO2
anions. The carboxylate groups serve a dual role of bridging the
metal ions and stabilizing crystal packing through multiple
hydrogen bonding interactions. All the Ni ions are six-
coordinate, with a distorted O5N octahedral coordination
(Figure 5 and Supporting Information Figure S7). Three corners
of the octahedron are occupied either by two O atoms from the
phenoxide and one O atom from hydroxide donor or by one O
atom from the phenoxide and two O atoms from hydroxide
donors, which act as bridges to the remaining metal ions within
the cubane core. Remaining positions are occupied by an O atom
from the propanol OH arm, the imine-N atom of the H2bpmp
ligand and one O atom of the exogenous carboxylate anion.
Bridge extending capabilities of hydroxide and phenoxide groups
are crucial for the condensation of two dinuclear [Ni2] fragments
without any reorganization of the coordination spheres of the
NiII ions. The bound carboxylate groups (O9, O10, O11, and
O12 for 2·2H2O and O10, O9 and O12, O11 for 3·4H2O) push
the individual [Ni2] units to each other to establish new Ni−O
bonds from opposite sides. Both clusters are based on cubic
100.6(1)
89.5(1)
101.0(1)
{Ni4O4} units, consisting of two interpenetrating Ni4 and O4
tetrahedra, with different degrees of distortion of the cubane
motif (Supporting Information Figure S8). Two of the six faces
of the {Ni4O4} cube are distinctly different, being spanned by
capping carboxylates that result in somewhat different bond
lengths and angles for the involved atoms (Table 3 and Table S1
in Supporting Information) compared to those at the four
remaining faces of the {Ni4O4} cube which are spanned by μ1,3-
carboxylato bridges. The six Ni···Ni distances fall within 2.937−
3.303 Å range. The faces spanned by μ1,3-O2CCF3 units in 2·H2O
register Ni···Ni separations of 2.962 and 2.979 Å, while the
corresponding faces spanned by μ1,3-O2CCH2Ph in 3·4H2O
show separations of 2.937 and 2.944 Å. In these two faces the NiII
ions are triply bridged. The Ni···Ni distances bridged by two HO¯
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acetate anions (O13 and O14) are close to terminal alcohol O2
atom (O···O, 2.573 Å) (Supporting Information Figure S13).
Magnetic Properties. Magnetic measurements were
performed on polycrytalline samples of 1−3. All three complexes
exhibit room-temperature χT values that are slightly higher than
the expected spin-only value of 1.00 emu K mol−1 per one Ni(II)
ion (S = 1). Such behavior is typical for Ni(II) complexes and
explained by the orbital contribution to the total magnetic
moment.34
Complex 1 exhibits the χT value of 6.93 emu K mol−1 at 300 K
(Figure 6). The χT slightly increases as the temperature is
Figure 5. Atom connectivity in the core of complex 2·H2O showing
octahedral coordination for all NiII atoms. Color code: Ni, dark green;
O, red; N, blue; C, black.
groups are intermediate at 3.090 and 3.098 Å for 2·H2O and 3·
−
4H2O, respectively. The binding of two CF3CO2 and two
PhCH2CO2− ions is different as reflected in the Ni−O distances
in 2.055−2.148 and 2.033−2.077 Å ranges (Supporting
Information Figure S9). In both cubane complexes the primary
chelating Schiff base ligands are present in their singly
deprotonated form H2pbmp−, and the Ni−O (hydroxide) and
Ni−N (imine) distances are similar, within 1.987−2.050 Å. The
μ3-PhO¯ group links two NiII atoms in one face of the cube and
extends the third arm to another NiII atom of the other plane.
The former Ni−O bonds (∼2.06 Å) are shorter compared to the
latter (∼2.20 Å). The O1(phenoxido) and O4(phenoxide)
donor atoms of this group lie 1.092 and 1.085 Å out of the plane
of the three connected NiII atoms: the solid angles around these
atoms are 299.3° and 298.9° and two of the Ni−O−Ni angles are
significantly larger (104.9° and 98.8°; 104.6° and 99.5°) than the
third one (95.5°; 94.5°). The Ni−O(H) bonds from alcohol
terminal ends are comparatively shorter in 2.02−2.08 Å range.
An interesting feature of complexes 2·H2O and 3·4H2O is the
packing of the cubane clusters by hydrogen bonds between the
bridging HO¯ ions and lattice H2O molecules, which act as
hydrogen bond donors, and the O atoms of carboxylate anions
present nearby that act as hydrogen bond acceptors. The cationic
complex 2·H2O crystallizes with two trifluoroacetate anions and
two solvent water molecules which are hydrogen bonded to one
bridging hydroxide group and two pendent ligand alcohol arms
(Supporting Information Figure S10). The hydroxido anions
(O2 and O3) of 2·H2O function as hydrogen bond donors to
carboxylate O16 and O14 resulting in O···O separations of 2.820
and 2.738 Å, respectively. The lattice water oxygen atom (O2W)
shows hydrogen bonding with O6 of ligand alcohol arm with O···
O separation of 2.672 Å. The pendent and protonated ligand
alcohol arms O5 and O8 are hydrogen bonded to O15 and O13
of lattice trifluoroacetate anions showing O···O separation of
2.618 and 2.594 Å, respectively (Supporting Information Figures
S10 and S11). The dication of 3·4H2O crystallizes with two
phenyl acetate anions and four solvent water molecules and
shows hydrogen bonding with bridging hydroxide groups and
OH bearing ligand alcohol arms (Supporting Information Figure
S12). Hydrogen bonding interactions of PhCH2CO2− ions and
water molecules with the cubane dication in 3·4H2O pertain
extra stability to the crystal structure. One of the lattice water
oxygen atoms (O1W) is hydrogen bonded to lattice carboxylate
oxygen atom (O13) at 2.791 Å O···O separation. The other
lattice water oxygen atom (O2W) shows hydrogen bonding with
lattice carboxylate oxygen atom (O14) at 2.734 Å and with
hydroxido bridge O3 atom at 2.846 Å. The O atoms of the phenyl
Figure 6. Temperature dependence of χT for 1. Inset: Field-dependent
magnetization at 1.8 K. Solid red lines correspond to the theoretical
simulation.
lowered, reaching the maximum of 7.65 emu K mol−1 at 26 K,
after which it sharply decreases to 4.32 emu K mol−1 at 1.8 K, the
lowest temperature available in our experiments. The magnet-
ization (M) measured at 1.8 K exhibits a gradual increase with the
increasing field (H) but does not reach saturation even at 7 T
(Figure 6, inset). The character of χT(T) and M(H) depend-
ences suggests the presence of competing magnetic interactions
in 1. The magnetic behavior of 1 was modeled with the isotropic
Heisenberg−Dirac−Van Vleck (HDV) Hamiltonian, including
the zero-field and Zeeman splitting effects:
2
̂
̂
̂
̂
H = −
∑
2J SiSj +
∑
D[Sz,i − Si(Si + 1)/3]
i
ij
i<j
i
̂
+
∑
giμBSiH
(4)
i
where i and j are used to specify the five Ni(II) centers, Di is the
single-ion zero-field splitting parameter, H is the applied
magnetic field, and μB is Bohr magneton. Given the symmetry
of complex 1, the similarity of Ni1···Ni2 and Ni1···Ni3 distances
(3.617 and 3.608 Å, respectively), and the long Ni2···Ni3* and
Ni3···Ni2* separations of 6.595 Å (Figure 2), eq 4 can be
simplified by using only two magnetic exchange constants and
assuming the same value of g and D for all Ni(II) ions to avoid
overparametrization:
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̂
̂
̂
̂
̂
̂
̂
̂
̂
̂
̂
̂
̂
̂
̂
̂
̂
̂
̂
̂
̂
̂
H = −2J (SNi2SNi3 + SNi2*SNi3*) − 2J2(SNi1SNi2 + SNi1SNi3
H = −2J (S1S3 + S2S4) − 2J2(S1S2 + S1S4 + S2S3 + S3S4)
1
1
2
̂
̂
̂
̂
̂
̂
+ 4gμBSH
+ SNi1
S
Ni2* + SNi1
S
Ni3*) + 5D[Sz − S(S + 1)/3]
(6)
was solved analytically using the Kambe method.36 The
corresponding energy eigenvalues,
̂
+ 5gμBSH
(5)
The magnetic data were satisfactorily simulated with Magpack35
(red lines in Figure 6), resulting in J1 = 7.0(5) cm−1, J2 = −0.7(1)
cm−1, D = 8.3(2) cm−1, g = 2.32(1). The latter two values are
typical of Ni(II) complexes.34 The simulation indicates a
moderate ferromagnetic exchange along the oxo-bridged Ni2−
Ni3 and Ni2*−Ni3* sides of the Ni5 rectangle and a weaker
antiferromagnetic coupling between the corner ions and the
central Ni1 ion. The maximum magnetization value achieved at
1.8 K and 7 T is 8.7 μB, much higher than 6 μB expected for the
ground state of ST = 3. Moreover, the magnetization gradually
increases at higher fields, suggesting that the applied magnetic
field suppresses the antiferromagnetic exchange between the
central Ni1 ion and the four corner ions. An examination of AC
magnetic susceptibility did not reveal any out-of-phase signal,
indicating the absence of single-molecule magnetism in this
complex.
E = −J [SA(SA + 1) + SB(SB + 1)]
1
− J2[ST(ST + 1) − SA(SA + 1) − SB(SB + 1)]
(7)
̂
̂
̂
̂
̂
̂
̂
̂
̂
where SA = S1 + S3, SB = S2 + S4, and ST = SA + SB, were entered
into the Van Vleck equation
Ng2
3k
ST(ST + 1)(2ST + 1)e−E
/
kT
i
χT =
∑
(2ST + 1)e−E /kT
i
(8)
i
to fit the experimental χT(T) curve above 8 K. This procedure
provided the best-fit values of magnetic parameters, J1 = 9.3 cm−1,
J2 = 1.1 cm−1, and g = 2.27. Subsequently, these were used as the
initial values for the Magpack simulation that took into account
the zero-field splitting effects, according to the Hamiltonian
The room-temperature χT value for 2 is 5.59 emu K mol−1 at
300 K (Figure 7). Similar to complex 1, the χT of 2 slowly
̂
̂
̂
̂
̂
̂
̂
̂
̂
̂
̂
̂
̂
H = −2J (S1S3 + S2S4) − 2J2(S1S2 + S1S4 + S2S3 + S3S4)
1
2
̂
̂
+ 4D[Sz − S(S + 1)/3] + 4gμBSH
(9)
The best simulation of the experimental data was obtained with J1
= 8.4(2) cm−1, J2 = 1.05(3) cm−1, D = 7.8(2) cm−1, g = 2.28(1)
(red lines in Figure 7).
An examination of AC magnetic susceptibility of 2 reveals an
appearance of an out-of-phase signal with a frequency-dependent
maximum, indicating that complex 2 exhibits the slow relaxation
of magnetization. A number of cubane clusters with the Ni4O4
core have been shown to undergo the frequency-dependent
relaxation of magnetization indicative of SMM behavior,36a,b,d
but for all these clusters the blocking temperature is usually quite
low (<3 K) because of fast magnetization tunneling in zero
field.37 In the present case, however, the blocking temperature
appeared to be unusually high, ∼5.5 K. The relaxation rate was fit
to the Arrhenius law, ln(τ0/τ) = −Ueff/kT, where τ = 2πν, to
allow the estimation of the pre-exponential rate constant, 1/τ0 =
1.2(1)·10−31 s−1, and the effective barrier to the relaxation of
magnetization, Ueff = 296(2) cm−1. These values are somewhat
extreme for conventional SMMs. The Mydosh parameter, an
empirical quantity used to differentiate between superparamag-
Figure 7. Temperature dependence of χT for 2. Insets: Bottom, field-
dependent magnetization at 1.8 K; top, imaginary part of AC magnetic
susceptibility. Solid red lines correspond to the theoretical simulation.
nets and spin glasses, was calculated as φ = (Tνm1ax − Tmν2ax)/
νi
(Tνmax(logν1 −logν2)), where Tmax is the temperature of the
1
maximum in the χ″ versus T curve at the corresponding
frequency.38 This calculation led to φ = 0.034, which falls in the
range typical for spin glasses (0.004−0.08).
increases as the temperature is decreased, but the increase at
lower temperatures is much more abrupt and the maximum value
of 11.4 emu K mol−1 is reached at 6.6 K. The field-dependent
magnetization measured at 1.8 K exhibits fast growth with
increasing field and approaches the saturation value of ∼8.6 μB at
7 T (Figure 7, bottom inset). Such behavior suggests dominant
ferromagnetic interactions in the cubane cluster of 2, with the
stabilization of the ST = 4 ground state at low temperature, and
agrees with the behavior reported previously for similar Ni(II)
cubane clusters.36 Taking into account that Ni1···Ni3 and Ni2···
Ni4 distances (2.979 and 2.962 Å, respectively) are notably
shorter than the other four Ni···Ni separations (>3.09 Å) in the
Ni4O4 core of the cubane unit (Supporting Information Figure
S8), the magnetic behavior of 2 was modeled with two values of J.
Initially, the spin-only Hamiltonian
Taking into account the substantial separation between the
clusters in the solid state, it is intriguing to see the signature of
spin glassiness at the temperature as high as 5.5 K, because such
behavior implies non-negligible intercluster magnetic coupling.
Nevertheless, the [Ni4O4] cubanes were shown to exhibit
exchange bias due to intermolecular interactions.39 The
possibility of the more pronounced coupling could be envisioned
in the structure of 2, with the relatively nonbulky trifluoroacetate
ligands, if the structure underwent some collapse/disorder upon
the loss of interstitial solvent. Indeed, a visual examination of the
microcrystalline samples revealed the presence of powder-like
deposit on the surface of the crystallites. Consequently, the
crystals were ground into a fine powder, which caused the
increase in the maximum value of χ″ from 0.049 to 0.076 emu
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Article
mol−1. This increase suggests that the spin-glass behavior is due
to the coupling between the S = 4 clusters in the disordered
structure formed upon the loss of interstitial solvent.
The magnetic behavior of 3 (Figure 8 and Supporting
Information Figure S14) was found to be similar to that of 2, as
new self-assembled coordination cages following in situ
generation of HO¯ ions from water present in the reaction
medium. Complex 1 reveals an unusual structure having five
nickel(II) ions in an hourglass arrangement hitherto unknown in
coordination cluster chemistry of H2bpmp¯ with nickel(II). The
complex has a ground-state spin of S = 3. The synthesis and
stability in its pentanuclear form is dependent on the particular
nature of the R group on the carboxylate group required for
bridging. Thus moving from propanoate group to trifluoroace-
tate and phenylaceate groups provided different course of cluster
assembly based on binuclear motifs. This carboxylate bridging
induced cluster assembly allowed synthesizing new family of
hydroxido-bridged tetranuclear NiII cube structures 2 and 3.
Change of face-capping carboxylato groups is accompanied by
slight structural rearrangement of the [Ni4O4] core, which in turn
results in changes in magnetic behavior. Simultaneous chelation
of binucleating H2bpmp¯ and bridging of water-derived HO¯
groups in μ3 mode, in presence of two carboxylate anions lead to
the generation of two [Ni4O4] cubane complexes exhibiting
three different types of Ni2O2 faces having Ni···Ni separations in
the ranges of 2.93−2.97, 3.09−3.21, and 3.20−3.30 Å,
respectively. The differences in the bulkiness of carboxylate
ligands in 2 and 3 lead to the weakening of intercluster magnetic
exchange interactions in the latter. As a result, complex 2 exhibits
spin-glass behavior upon the loss of interstitial solvent, while
complex 3 shows slow relaxation of magnetization, but only
when a DC magnetic field is applied to suppress the fast
tunneling between the states of opposite magnetization
direction.
Figure 8. Temperature dependence of χT for 3. Inset: Reduced
magnetization plot, with solid lines representing the best theoretical fit
(g = 2.09, D = −0.39 cm−1).
expected from the structural similarity of the [Ni4O4] cubane
core in these complexes. Simulating χT(T) and M(H) curves of 3
with Magpack according to eq 9, in the manner analogous to that
described above for 2, resulted in comparable values of magnetic
parameters, J1 = 7.2(2) cm−1, J2 = 0.60(5) cm−1, D = 8.5(2) cm−1,
g = 2.29(1). An examination of AC magnetic susceptibility of 3
did not reveal any slow relaxation process in the range of 1.8−9.0
K, indicating that, in contrast to 2, this complex exhibits neither
spin-glass nor SMM behavior. On one hand, the bulkier
phenylacetate ligands provide a more robust crystal packing
and larger separation between the [Ni4O4] clusters in 3, thus
preventing any significant intercluster magnetic exchange. On
the other hand, the lack of the SMM behavior can be explained by
the relatively low symmetry of the cubane core in 3, which results
in fast tunneling of magnetization. Consequently, the AC
susceptibility of 3 was examined under DC bias field of 0.2 T,
which led to the appearance of an out-of-phase frequency-
dependent peak around 2.5 K (Supporting Information Figure
S14, inset), indicating that the tunneling is suppressed due to
field-induced energy mismatch for the +MS and −MS magnetic
states.
The ground-state properties of 3 were examined by variable-
temperature magnetization measurements under different
applied magnetic fields (Figure 8, inset). The fit of the reduced
magnetization data was performed with Anisofit,40 resulting in
the best-fit values of g = 2.09 and D = −0.39 cm−1 for the S = 4
ground state of the Ni4 cubane. These values are similar to those
reported for other Ni4O4 clusters,36 and the negative value of D
agrees with the observation of the out-of-phase signal under
applied magnetic field.
ASSOCIATED CONTENT
* Supporting Information
X-ray crystallographic data in CIF format, Scheme S1, Figures
S1−S13, Table S1, and synthesis and characterization of ligand
H3bpmp. This material is available free of charge via the Internet
at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*Fax: (+91) 3222-82252. Tel: (+91) 3222-283324. E-mail:
dray@chem.iitkgp.ernet.in.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
A.K.G. is thankful to the Council of Scientific and Industrial
Research, New Delhi, India for financial support. The authors
also give thanks to DST, New Delhi, for providing the Single
Crystal X-ray Diffractometer facility in the Department of
Chemistry, IIT Kharagpur under its FIST program. V.B.
acknowledges the Italian Ministry of University and Scientific
Research (MIUR, Rome). M.S. is grateful to the National Science
Foundation for the partial support of this work (award CHE-
0911109).
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■
The use of phenol function deprotonated H2bpmp¯ ligand with
three different Ni(II) carboxylates has provided access to three
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