High nuclearity (octa-, dodeca-, and pentadecanuclear) metal (M = Co II, NiII) phosphonate cages: Synthesis, structure, and magnetic behavior

Article abstract of DOI:10.1021/ic402673v

The synthesis, structural characterization, and magnetic property studies of five new transition metal (M = Co, Ni) phosphonate-based cages are reported. Three substituted phenyl and benzyl phosphonate ligands [RPO₃H ₂; R₁ = p-tert-butylbenzyl, R₂ = p-tert-butylphenyl, R₃ = 3-chlorobenzyl] were synthesized and employed to seek out high-nuclearity cages. Complexes 1-3 are quasi-isostructural and feature a dodecanuclear metal-oxo core having the general molecular formula of [M₁₂(μ₃-OH)₄ (O₃PR)₄(O₂C^tBu)₆ (HO ₂C^tBu)₆(HCO₃)₆] {M = Co, Ni and R = R₁ for 1 (Co₁₂), R₂ for 2, 3 (Co₁₂, Ni₁₂)}. The twelve metal centers are arranged at the vertices of a truncated tetrahedron in a manner similar to Keggin ion. Complex 4 is an octanuclear nickel phosphonate cage [Ni₈(μ ₃-OH)₄ (OMe)₂(O₃PR₁) ₂ (O₂C^tBu)₆(HO₂C ^tBu)₈], and complex 5 represents a pentadecanuclear cobalt phosphonate cage, [Co₁₅(chp)₈(chpH) (O₃PR ₃)₈(O₂C^tBu)₆], where chpH = 6-chloro-2-hydroxypyridine. Structural investigation reveals some interesting geometrical features in the molecular cores, which may provide new models in single molecular magnetic materials. Magnetic property measurements of compounds 1-5 indicate the coexistence of both antiferromagnetic and ferromagnetic interactions between magnetic centers for all cages.

Full text of DOI:10.1021/ic402673v

Article
pubs.acs.org/IC
High Nuclearity (Octa‑, Dodeca‑, and Pentadecanuclear) Metal (M =
CoII, NiII) Phosphonate Cages: Synthesis, Structure, and Magnetic
Behavior
Javeed Ahmad Sheikh, Amit Adhikary, Himanshu Sekhar Jena, Soumava Biswas, and Sanjit Konar*
Department of Chemistry, IISER Bhopal, Bhopal-462066, India
S
* Supporting Information
ABSTRACT: The synthesis, structural characterization, and magnetic
property studies of five new transition metal (M = Co, Ni) phosphonate-
based cages are reported. Three substituted phenyl and benzyl phosphonate
ligands [RPO3H2; R1 = p-tert-butylbenzyl, R2 = p-tert-butylphenyl, R3 = 3-
chlorobenzyl] were synthesized and employed to seek out high-nuclearity
cages. Complexes 1−3 are quasi-isostructural and feature a dodecanuclear
metal-oxo core having the general molecular formula of [M12(μ3-OH)4
(O3PR)4(O2CtBu)6 (HO2CtBu)6(HCO3)6] {M = Co, Ni and R = R1 for 1
(Co12), R2 for 2, 3 (Co12, Ni12)}. The twelve metal centers are arranged at
the vertices of a truncated tetrahedron in a manner similar to Keggin ion.
Complex 4 is an octanuclear nickel phosphonate cage [Ni8(μ3-OH)4
(OMe)2(O3PR1)2 (O2CtBu)6(HO2CtBu)8], and complex 5 represents a
pentadecanuclear cobalt phosphonate cage, [Co15(chp)8(chpH)
(O3PR3)8(O2CtBu)6], where chpH = 6-chloro-2-hydroxypyridine. Struc-
tural investigation reveals some interesting geometrical features in the molecular cores, which may provide new models in single
molecular magnetic materials. Magnetic property measurements of compounds 1−5 indicate the coexistence of both
antiferromagnetic and ferromagnetic interactions between magnetic centers for all cages.
In our recent work, we have reported a dodecanuclear and a
pentadecanuclear cobalt phosphonate cage featuring butterfly
and distorted cubic cores, respectively, and also an
octadecanuclear Cu-pyrazolate-phosphonate nanocage.13 In
our efforts to explore new types of transition-metal-based
phosphonate cages using different phosphonic acids, we are
able to isolate and characterize a series of complexes having
three different nuclearities. Herein, we report the synthesis,
structural, and magnetic studies of five new phosphonate-based
CoII/NiII cage compounds, of which three are structurally
analogous dodecanuclear transition metal (M = Co, Ni)
phosphonate cage compounds having the general molecular
formula of [M12(μ3-OH)4 (O3PR)4(O2CtBu)6 (HO2CtBu)6
(HCO3)6], where M = Co, Ni and R = R1 (p-tert-butylbenzyl)
for compound 1 (Co12) and R2 (p-tert-butylphenyl) for
compounds 2 and 3 (Co12 and Ni12). The remaining two
compounds include an octanuclear Ni phosphonate cage of
molecular formula [Ni8(μ3-OH)4 (OMe)2(O3PR1)2
(O2CtBu)6(HO2CtBu)8] (4) and a pentadecanuclear Co
phosphonate cage of molecular formula [Co15(chp)8(chpH)-
(O3PR3)8(O2CtBu)6] (R3 = 3-chlorobenzyl, chpH = 6-chloro-2-
hydroxypyridine) (5). The phosphonic acid ligands (Scheme 1)
were synthesized following the Michaelis−Arbuzov reaction14
from their corresponding bromides.
olynuclear transition metal cages and clusters have
Pemerged as an active area of research in the recent past,
owing to their immense technological applications,1 relevance
in biological systems,2 and also because of aesthetically pleasing
structures.3 On the other hand, molecular magnetic materials
based on discrete transition-metal aggregates have received
much attention, as they typically display properties of a single
molecular magnet.4 Numerous transition metal cluster/cage
compounds reported in the literature have been synthesized
using carboxylates5 or alkoxides6 as ligand. Phosphonate ligands
possessing three O donors could be an ideal choice for the
synthesis of polynuclear cages. However, because of their
multidentate coordination capability, phosphonates tend to
generate insoluble metal complexes.7 This problem has been
addressed by the use of preformed cages or the introduction of
ancillary ligands, and considerable progress has been achieved
by the groups of Winpenny,8 Clearfield,9 Chandrasekhar,10 and
others11,12 in exploring metal-phosphonate cage compounds as
magnetic materials. However, most of these phosphonate cages
reported so far bear core structures of irregular geometrical
pattern, and cages with regular geometrical cores are relatively
rare.8j,10,12,13,24 Regular geometry in polynuclear paramagnetic
cores is of importance from a magnetic point of view as it is
easier to model magnetic data for smaller symmetry-related
subunits, and it also helps to understand magnetic behavior
from a structural point of view.
Received: October 24, 2013
© XXXX American Chemical Society
A
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Article
the program suite PLATON20 was used to generate a fresh reflection
file. The crystallographic data are summarized in Table 1.
Scheme 1. Schematic Drawing of the Phosphonic Acid
Ligands
Materials and Methods. All the complexes were synthesized from
the starting materials [Co2(μ-OH2) (O2CtBu)4]·(HO2CtButBu)4], Co2
or [NiII (μ-OH2) (O2CtBu)4(HO2CtBu)4], Ni2 which were made by
2
literature methods.21,22 The phosphonate ligands were synthesized
following previously reported procdures.8c,12 Other reagents were used
as received from Sigma Aldrich without any further purification.
Magnetic susceptibility and magnetization measurements were carried
out on a Quantum Design SQUID-VSM magnetometer. Direct
current (DC) magnetic measurements were performed with an applied
field of 1000 G in the 1.8−300 K temperature range. Alternating
current (AC) magnetic susceptibilities were performed in a 3.5 G field
oscillating at 1−780 Hz in the 1.8−10 K range. BVS calculations were
done following the procedure given by Liu and Thorpe.15 Powder X-
ray diffraction (PXRD) data was obtained with PANanalytical
instrument using Cu Kα radiation, and data for thermogravimetric
analysis (TGA) was collected on a PerkinElmer TGA 4000.
Magnetic measurements and bond valence sum (BVS)
calculations authenticate that metal centers in all the mentioned
molecules are at +2 oxidation state, as required for the charge
balances (Supporting Information, Table S1).15 Magnetic
property measurements of compounds 1−5 indicate the
coexistence of both antiferromagnetic and ferromagnetic
interactions between magnetic centers for all cages.
Synthesis. [Co12(μ3-OH)4(HCO3)6 (O3PR)4(O2CtBu)6 (HO2CtBu)6] (R
= p-tert-Butylbenzyl) (1). [Co2(μ-OH2) (O2CtBu)4]·(HO2CtBu)4]
(100 mg, 0.1 mmol) and p-tert-butylbenzylphosphonic acid (23 mg,
0.1 mmol) were taken in CH3CN (8 mL) and stirred at ambient
temperature for a few minutes. This was followed by the addition of
NEt3 (20 mg, 0.2 mmol) (Et = ethyl), and after further stirring for a
few minutes, the contents were transferred into a Teflon-lined steel
autoclave and heated at 150 °C for 18h and then cooled to room
temperature. The resulting solid product was filtered, and the deep-
blue solution was kept in a 15 mL vial at 4 °C for ca. 2 d. Purple block-
shaped crystals suitable for X-ray diffraction were collected by
filtration. Yield 35 mg, 36%, based on Co2. IR (KBr, cm−1); 3467
(b), 3198 (b), 2961 (s), 2870 (s), 2372 (s), 2171 (w), 1932 (b), 1629
(b), 1513 (m), 1458 (m), 1401 (vs), 1364 (s), 1265 (s), 1096 (m),
972 (b), 844 (vs), 765 (s), 732 (s), 695 (m), 614 (m).
EXPERIMENTAL SECTION
■
X-ray Crystallography. Data collection of the compounds were
performed at 130−140 K on a Bruker Smart CCD-1000 diffractometer
with Mo Kα (λ = 0.710 73 Å) radiation using a cold nitrogen stream
(Oxford). Data reduction and cell refinements were performed with
the SAINT program16 and the absorption correction program
SADABS17 was employed to correct the data for absorption effects.
Crystal structures were solved by direct methods and refined with full-
matrix least-squares (SHELXTL-97)18 with atomic coordinates and
anisotropic thermal parameters for all non-hydrogen atoms. Crystals of
compounds 2−5 were notably poorly diffracting. The lower angle
reflection data consequences a poor data to parameter ratio and in
anisotropic refinement some atoms became anisotropic displacement
parameters (ADPs). The thermal parameters of all the carbon atoms
were restrained to approximately the same values using the XShell
“rigid bond displacement” (DELU, Sigma 0.05), “isotropic displace-
ment” (ISOR, Sigma 0.8) command. The structures of complexes 1−5
contain solvent-accessible voids; hence, the SQUEEZE19 module of
[Co12(μ3-OH)4(HCO3)6 (O3PR)4(O2CtBu)6 (HO2CtBu)6] (R = p-tert-
Butylphenyl) (2). This compound was synthesized following a similar
procedure as for 1, but p-tert-butylphenylphosphonic acid (22 mg, 0.1
mmol) was used here instead of p-tert-butylbenzylphosphonic acid.
Purple block-shaped crystals suitable for X-ray diffraction were
collected by filtration after 4 d. Yield 42 mg, 43.5%, based on Co2.
Table 1. Crystal Data and Structure Refinement for Complexes 1−5
1
2
3
4
5
formula
C110H176Co12 O58P4
3257.55
C115H172Co12 N4O60P4
3401.62
130(2)
C114H165N3Ni12 O65P4
3445.72
C94H141 Ni8O40P2
2442.54
130(2)
C140H118Cl18Co15 N12O48P8
4506.28
130(2)
formula weight
T (K)
140(2)
130(2)
́
wavelength (Å)
0.71073
0.71073
C2/c
0.71073
0.71073
C2/c
0.71073
C2/c
space group
P42 c
P3c1
̅
̅
1
crystal system
tetragonal
21.5512(18)
21.5512(18)
19.1984(19)
90.00
monoclinic
17.3222(9)
30.933(2)
30.4764(17)
90.00
hexagonal
19.7023(5)
19.7023(5)
52.319(2)
90.00
monoclinic
57.000(7)
16.0479(18)
30.275(3)
90.00
monoclinic
20.011(3)
28.907(4)
30.350(6)
90.00
́
a/Å
́
b/Å
c/Å
α/deg
β/deg
γ/deg
V/Å3
Z
90.00
96.517(3)
90.00
90.00
105.678(6)
90.00
93.332(12)
90.00
90.00
120.00
8916.8(17)
2
16224.4(16)
4
17588.5(9)
4
26663(5)
8
17527(5)
4
Dcalcd (g cm−3
)
1.213
1.393
1.298
1.217
1.708
μ (mm−1
)
1.189
1.312
1.366
1.196
1.806
F(000)
3368
7016
7128
10232
9028
θmin, θmax (deg)
1.34, 27.49
10 246
2.00, 25.29
14 779
2.07, 26.79
12 529
0.74, 32.22
47 156
1.38, 25.37
16 090
reflections collected
unique reflections
R1, wR2 (I ≥ 2σ(I))
9700
10 861
10 688
40 309
15 320
a
0.0566, 0.1796
1.002
0.1096, 0.2878
1.134
0.0823, 0.2082
1.213
0.1228, 0.2791
1.119
0.0838, 0.2448
1.049
goodness of fit (GOF) on F2
a
2
2
R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 = [∑[w(Fo − Fc2)2]/∑[w(Fo )2]]1/2
.
B
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IR (KBr, cm−1); 3432 (b), 3179 (b), 2963 (s), 2871 (s), 2374 (m),
2171 (w), 1623 (b), 1547 (m), 1485 (vs), 1458 (m), 1429 (s), 1379
(m), 1363 (m), 1270 (s), 1231 (vs), 1205 (m), 1098 (m), 1029 (s),
990 (vs), 898 (vs), 830 (s), 809 (m), 788 (s), 755 (s), 670 (w), 628
(s).
weight loss of around 15−20% in the temperature range of
200−300 °C probably corresponds to the aromatic ligands,
after which the complexes decompose completely (Supporting
Information, Figure S7).
Structural Description. X-ray crystallography studies show
that complexes 1−3 are quasi-isostructural, which can be best
described as an M12 (M = Co, Ni) central core coated on the
exterior by a hydrophobic sheath of alkyl or aryl groups (Figure
1, Supporting Information, Figure S1, and Figure 2,
[Ni12(μ3-OH)4(HCO3)6 (O3PR)4(O2CtBu)6 (HO2CtBu)6] (R = p-tert-
Butylphenyl) (3). This compound was synthesized following a similar
procedure as for 2, but [NiII (μ-OH2) (O2CtBu)4 (HO2CtBu)4] (100
2
mg, 0.1 mmol) was used instead of [Co2(μ-OH2) (O2CtBu)4]·
(HO2CtBu)4]. Deep-green cubic crystals suitable for X-ray diffraction
were collected by filtration after 4 d. Yield 54 mg, 55.96%, based on
Ni2. IR (KBr, cm−1); 3423 (b), 3180 (b), 2958 (s), 2870 (m), 2369
(m), 1670 (s), 1602 (s), 1508 (m), 1483 (vs), 1458 (m), 1419 (s),
1373 (s), 1360 (m), 1270 (m), 1229 (vs), 1122 (s), 1090 (vs), 1028
(s), 991 (vs), 879 (b), 834 (m), 785 (s), 750 (s), 611 (vs), 588 (s),
530 (s).
[Ni8(μ3-OH)4 (OMe)2(O3PR)2 (O2CtBu)6(HO2CtBu)8] (R = p-tert-
Butylbenzyl) (4). This compound was synthesized following a similar
procedure as for 3; however, p-tert-butylbenzylphosphonic acid (23
mg, 0.1 mmol) was used here, and the reaction was done under
ambient conditions. The final product was dissolved in toluene and
layered with methanol. Green cubic crystals suitable for X-ray
diffraction were collected by filtration after a few days. Yield 32 mg,
33.17%, based on Ni2. IR (KBr, cm−1); 3436 (b), 3179 (b), 2966 (s),
2874 (m), 2593 (m), 2372 (m), 2345 (m), 2170 (w), 1600 (b), 1483
(s), 1458 (m), 1405 (s), 1374 (m), 1322 (s), 1208 (s), 1110 (b), 1000
(s), 902 (m), 872 (m), 841 (m), 787 (s), 614 (s).
[Co15(chp)8(chpH) (O3PR)8(O2CtBu)6 (CH3CN)3]·(CH3CN) (R = 3-
Chlorobenzyl) (5). [CoII (μ-OH2) (O2CtBu)4]·(HO2CtBu)4, (100
2
mg, 0.1 mmol), 3-chlorobenzylphosphonic acid (23 mg, 0.1 mmol),
and 6-chloro-2-hydroxypyridine (13 mg, 0.1 mmol or 39 mg, 0.3
mmol) were taken in CH3CN (8 mL) and stirred at ambient
temperature a for few minutes. This was followed by the addition of
NEt3 (30 mg, 0.3 mmol), and after further stirring for a few minutes
the contents were transferred into a Teflon-lined steel autoclave and
heated at 150 °C for 18h and then cooled to room temperature. The
resulting solid product was filtered, and the deep-blue solution was
kept in a 15 mL vial at 4 °C for ca. 2 d. Deep-blue block-shaped
crystals suitable for X-ray diffraction were collected by filtration. Yield
62 mg, 64.25%, based on Co2. IR (KBr, cm−1); 3448 (b), 3186 (b),
2958 (s), 2371 (m), 2345 (w), 2167 (w), 1596 (s), 1559 (m), 1508
(m), 1482 (w), 1445 (m), 1420 (w), 1341 (m), 1229 (s), 1162 (m),
1089 (s), 1001 (s), 978 (m), 939 (s), 876 (m), 793 (vs), 737 (s), 700
(s), 681 (s), 616 (vs).
Figure 1. Ball-and-stick model showing molecular structure of 1 in the
crystal. Color code: purple, cobalt; orange, phosphorus; red, oxygen;
gray, carbon; Hydrogen atoms are omitted for clarity.
respectively). The core is made of 12 octahedral CoII/NiII
centers held together by 4 μ3−hydroxo groups, 4 doubly
deprotonated phosphonate ligands, 12 carboxylates (6
protonated and deprotonated each), and 6 monoprotonated
carbonates. In all the complexes, the phosphonate ligands bind
RESULTS AND DISCUSSION
■
Synthetic Aspects. Reaction of the cobalt and nickel
pivalate dimers with phosphonic acids generated five high-
nuclearity phosphonate cages. The structures of the final
products were not profoundly affected by changing the
phosphonate ligands as we ended up with the M12 (M = Co,
Ni) phosphonate cages under solvothermal conditions (150
°C). However, introduction of the coligand along with the
phosphonate ligand in the reaction resulted in a completely
different molecule with higher nuclearity (pentadecanuclear)
and less symmetry. Interestingly, we obtained an octanuclear Ni
phosphonate cage under ambient conditions with the ligand
(R1PO3H2) that gave a Co12 cage at 150 °C. The bulk phase
PXRD pattern of complexes 1−5 is in good agreement with the
simulated ones based on single crystal data, indicating the
purity of the as-synthesized compounds (Supporting Informa-
tion, Figures S2−S6). The discrepancies between the
experimental and simulated powder X-ray diffraction patterns
might be due to the partial loss of solvent molecules from the
crystal lattice. TGA of all the complexes reveals a weight loss of
around 10% up to 150 °C that may be assigned to the
coordinated and noncoordinated solvent molecules. Further
Figure 2. Ball-and-stick model showing molecular structure of 3 in the
crystal. Color code: green, nickel; orange, phosphorus; red, oxygen;
gray, carbon; Hydrogen atoms are omitted for clarity.
C
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in the 6.222 mode as per Harris notation (Supporting
Information, Scheme S1).23
The pivalate and carbonate ligands surround the periphery of
the complexes binding in 2.11 and 2.200 modes, respectively.
The core of the molecules can be better described as an ε-
Keggin (Figure 3a). The core structures of compounds 1−3
Figure 3. (a) Core structure of 1−3, displaying ε-Keggin-like view.
Color code same as in Figure 1. (b) “Stick”-type view of the core after
removing all atoms except metal and phosphorus.
Figure 4. Molecular structure of 4 in the crystal. Color code same as in
Figure 2.
a range similar to the reported compound. The core structure
of 4, after removing the organic ligands for clarity, is shown in
Figure 5.
strongly resemble to the NiII phosphonate cage reported by
Winpenny et al.24 The bond distances and bond angles around
the metal centers fall in a range similar to the reported
compound. Removing all atoms, excluding the metal centers
and phosphorus, and connecting the metal centers and
phosphorus through imaginary lines results in a geometry
where each phosphorus atom is surrounded by six cobalt ions,
and thereby four hexagonal and four trigonal faces are formed
where all the vertices are occupied by metal ions (Figure 3b).
The four hexagons share edges, and the triangular faces have
central μ3-OH groups. Metal−metal distances within the
trimeric unit are in the range of 3.23−3.46 Å, and between
two trimeric units they are in the range of 2.88−3.08 Å.
The interconnection of phosphorus atoms of the ligands
resulted in a tetrahedral geometry (Td) where four phosphorus
atoms (P4) occupied its vertices with an average P−P distance
of 5.16 Å. It can be seen that four metal triangles engulf the P4
tetrahedra and this feature makes the structure unique and
aesthetically pleasing. The phosphorus atoms of the ligand are
somewhat above the plane of the hexagon generated by joining
cobalt triangles. There are four axes of C3, each at the vertex of
P4 Td, running through the center of each Co3 triangular unit at
the opposite end (Figure 3b). Nine mirror planes are also
present along each edge of the hexagonal faces.
Figure 5. (a) Butterfly-shaped molecular core of 4. Color code same as
in Figure 2. (b) Stick view of core of 4. Bond colors: Ni−O, green; P−
O, orange; C−O, red.
Structural studies reveal that complex 5 contains an inorganic
core composed of 15 CoII ions that are held together by
bridging oxygen atoms from eight doubly deprotonated
phosphonates and six carboxylate ligands (Figure 6). Of the
15 CoII centers, 9 centers exhibit an octahedral geometry, 5
centers show a tetrahedral geometry, and 1 center exhibits a
square-pyramidal arrangement. Six of the eight phosphonate
ligands show the 5.221 bridging mode, and another two ligands
are exhibiting the 3.11 mode as per Harris notation.23 The
complex is also comprised of nine 6-chloro-2-hydroxypyridine
coligands that bind with cobalt sites via nitrogen as well as
oxygen.
The molecular structure of complex 4 consists of eight
octahedral NiII centers held together by two phosphonate
ligands, both binding in a 5.221 mode23 (Figure 4). There are
also eight pivalate groups binding in a 2.11 mode. Two
butterfly-like Ni4 subunits are formed via pivalate and hydroxy
bridging. The phosphonate ligands are located trans to each
other, thus holding together two {Ni4(μ-OH)2} butterflies of
NiII ions.
Four pivalate ligands bridge wing-body nickel centers within
the butterflies; two of them bridge body−body Ni, and two of
them bridge between the butterflies. Two methoxide molecules
are also found bridging between the nickel butterflies. The
distances between two adjacent NiII centers are in the range of
2.89−3.69 Å. The core structure of this phosphonate cage
resembles the NiII phosphonate cage reported earlier.24 The
bond distances and bond angles around the metal centers fall in
Organic parts of the ligands are pointed toward the outer
periphery of the molecule. Six pivalates are also present and
bridge in 2.11 mode. Metal and phosphorus tethered by oxygen
atoms give a distorted cubic view of the core of the complex
(Figure 7a). Further, removing all the atoms except Co and P
centers and connecting them through imaginary lines resulted
in an interesting perspective of the structure (Figure 7b). Here
each phosphorus (except P9) bears five CoII centers as nearest
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Figure 8. Polyhedral view of the core of complex 5.
Supporting Information, Table S2. The data for 3 and 4 were
fitted assuming isotropic spin Hamiltonian.25
The room temperature χMT values of 30.68 cm3 K mol−1 and
29.10 cm3 K mol−1 for compounds 1 and 2, respectively, can be
explained on the basis of the orbital contribution of CoII, which
is known to be significant in an octahedral field (Table 2).8e,26
Figure 6. Molecular structure of 5 in the crystal. Color code same as in
Figure 1, with the addition of chlorine, dark green.
Table 2. Magnetic Susceptibility Data χMT (cm3 K mol−1) of
Complexes 1, 2, and 5
expected χMT (spin
expected χMT (spin
experimental
compound
only)
+orbital)
χMT
1
2
5
22.50
22.50
28.12
40.23
40.23
50.28
30.68
29.10
36.15
As the temperature is lowered from 300 K, the χMT value
decreases gradually until it reaches 7.09 cm3 K mol−1 and 3.18
cm3 K mol−1 at 1.8 K for 1 and 2, respectively. The gradual
decline at higher temperatures may be attributed to the
presence of intramolecular antiferromagnetic interaction in the
molecules.26
The M/N μB versus H plots for compounds 1 and 2 from 2
to 10 K (inset of Figures 9 and 10, respectively) show a steady
increase with field that reaches 9.14 and 9.18 μB, respectively,
at 7 T and 2 K without any saturation. These lower values are
inconsistent with 12 uncoupled CoII ions (36 μB, g = 2).
Instead these values correspond to three CoII ions of one
trimeric unit, and it can be assumed that the CoII ions are
Figure 7. (a) Core structure of 5. Color code same as in Figure 1. (b)
Stick-type view of the core after removing all atoms except Co and P.
Bond colors: M−M, purple; P−P, green.
neighbors, and thereby a pentagonal unit is formed. Each
pentagon shares three vertices and an edge with its adjoining
pentagons and thereby forms a new type of geometry having a
total of six pentagonal and eight triangular faces, with each of
the vertices occupied by a metal center. A representative
polyhedral view of the core can be seen after the removal of two
phosphorus sites, P9 and P9A (Figure 8). The remaining two
phosphonate groups cap two of the opposite triangular faces
and observe a 3.111 bridging mode.23 The average distance of
cobalt ions of the hexagonal faces is 3.0 Å, and that of the
triangular faces is 3.32 Å.
Magnetic Properties. The variable-temperature magnetic
susceptibility data of complexes 1−5 were collected in the
temperature range of 1.8−300 K under a field of 0.1 T. The DC
susceptibilities are shown in the form of χMT (χM is molar
magnetic susceptibility) as a function of temperature. AC
susceptibility studies show no slow relaxation of magnetization
in any of these compounds; hence, none of them are single-
molecule magnets. The angle dependency of magnetic
exchange interactions for compounds 3 and 4 is given in the
Figure 9. Temperature dependence of χMT measured at 0.1 T.
Magnetization curves (inset graphs) measured at 2−10 K for complex
1.
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in good agreement with the similar phosphonate cage reported
earlier.24 The magnetic interactions for 3 were modeled after
taking symmetry-related unit (half molecule) into consideration
(Figure 11) with the Hamiltonian given in eq 1, neglecting any
coupling between the two six-centered halves because of the
complexity in the structure. This matches well with the
experimental result, giving the values of g = 2.14, J1 = −3.1
cm−1, and J2 = 2.5 cm−1. The magnetic exchange pathways can
be explained on the basis of the Ni−O−Ni bond angle. If the
Ni−O−Ni angle is smaller than 99°, interactions are
ferromagnetic; larger values correspond to antiferromagnetic
interactions.24,27
H = −J1(S1S2 + S2S3 + S3S1 + S4S5 + S4S6 + S5S6)
6
Figure 10. Temperature dependence of χMT measured at 0.1 T.
Magnetization curves (inset graphs) measured at 2−10 K for complex
2.
− J2(S3S4) − gμBH.
∑
Si
(1)
i=1
Antiferromagnetic interactions are observed via J1 exchange
pathways that can be justified by the Ni−O−Ni angle of 117−
119°. Since the Ni−O−Ni angle is around 87° via J2, it yields
ferromagnetic interactions. This would lead to a spin frustration
situation for each triangle (S1−S3 and S4−S6).8c The M/N μB
versus H plot shows no saturation even at a higher field of 7 T.
This behavior is analogous to that of compounds 1 and 2 and
can be explained with similar assumptions.
strongly antiferromagnetically coupled even at higher field,
which reduced the overall magnetic moment.8c The unsaturated
behavior of the plots can be explained by assuming strong
magnetic anisotropy of the ions present with high nuclearity,
resulting in many spin states populated at low temperature.26
The room-temperature χMT value for compound 3 is 13.28
cm3 K mol−1, which is close to the expected value (13.23 cm3 K
mol−1) for twelve isolated NiII ions (g = 2.1) (Figure 5). The
χMT value decreases gradually from room temperature to 12.91
cm3 K mol−1 at 53 K and then sharply to 1.99 cm3 K mol−1 at
1.8 K (Figure 12). The magnetic properties of compound 3 are
The room temperature χMT value for compound 4 is 10.0
cm3 K mol−1, which is near the expected value (9.94 cm3 K
mol−1) for eight isolated NiII ions (g = 2.2) (Figure 13), and the
Figure 11. Model used for the data fitting of compound 3. Balls
represent metal centers, and lines represent connectivity between two
metal centers.
Figure 13. Temperature dependence of χMT measured at 0.1 T. The
red solid line is the best fit obtained. Magnetization curves (inset
graphs) measured at 2−10 K for complex 4.
χMT value gradually decreases with temperature to reach the
value of 0.1 cm3 K mol−1 at 1.8 K. The magnetic properties of
compound 4 are quite similar to those of the phosphonate cage
reported earlier.24 The magnetic behavior was modeled with
the Hamiltonian given in eq 2 (Figure 14). The first exchange
pathway (J1) is assigned to the four Ni...Ni vectors which
represent the two body−body contacts within the butterflies.24
The second pathway (J2) is assigned to the eight body−wing
contacts within butterflies. A nice fit of the data was
accomplished with J1 = 7.6 cm−1, J2 = −22.4 cm−1, and g =
2.42, neglecting all Ni...Ni contacts not having a monatomic
bridge. Here, the lower Ni−O−Ni angle in the range of 93−99°
generates ferromagnetic interactions via J1, whereas the Ni−
O−Ni angle in the range of 111−135° generates antiferro-
magnetic interaction as discussed for compound 3.24,27
Figure 12. Temperature dependence of χMT measured at 0.1 T. The
red solid line is the best fit obtained. Magnetization curves (inset
graphs) measured at 2−10 K for complex 3.
F
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Inorganic Chemistry
Article
magnetization plot shows no sign of saturation even at higher
fields as in compounds 1 and 2 and can be understood on
similar grounds.
CONCLUSION
■
Exploring symmetry and the regular geometry of paramagnetic
cores is very important in terms of understanding the magnetic
properties of the molecular systems. We have reported the
synthesis and the structural and magnetic properties of five
high-nuclearity transition-metal phosphonate cages, and an
attempt has been made to explore the geometry and symmetry
present in the core of the serendipitously formed compounds.
It is also noticed that the structure of the phosphonate cages is
strongly affected by the reaction conditions and/or the
involvement of additional coligand. Magnetic studies confirm
the coexistence of both antiferromagnetic and ferromagnetic
interactions between the metal centers in all cases. Presently,
we are trying to isolate different nuclearity cages with Co and
Ni under ambient conditions and exploring the effect of
coligand under different reaction conditions.
Figure 14. Models used for the data fitting of compound 4. Balls
represent metal centers and lines represent connectivity between two
metal centers.
ASSOCIATED CONTENT
■
S
* Supporting Information
BVS calculations, PXRD and TGA plots, table of magnetic
measurements. This material is available free of charge via the
Internet at http://pubs.acs.org.
H = −J1(S1S2 + S3S7 + S4S5 + S6S8)
− J2(S1S3 + S1S4 + S2S3 + S2S4 + S5S6 + S6S7
8
AUTHOR INFORMATION
Corresponding Author
*E-mail: skonar@iiserb.ac.in.
Notes
+ S5S8 + S7S8) − gμBH. ∑ Si
■
(2)
i=1
The room-temperature χMT value of 36.15 cm3 K mol−1 for
compound 5 can be explained after taking the orbital
contribution of CoII in an octahedral field into consider-
ation.8e,26 The χMT value decreases gradually from room
temperature to 29.23 cm3 K mol−1 for compound 5 at 80 K and
then sharply to 2.36 cm3 K mol−1 at 1.8 K (Figure 15). Because
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
J.A.S. and A.A. acknowledge CSIR for their JRF fellowship. S.B.
thanks IISER, Bhopal for the Ph.D. fellowship. H.S.J. thanks
IISER, Bhopal for post-doctoral fellowship. S.K. thanks DST,
Government of India (Project No. SR/FT/CS-016/2010), and
IISER Bhopal for generous financial and infrastructural support.
DEDICATION
■
Dedicated to Professor R. N. Mukherjee on the occasion of his
60th birthday.
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