Stepwise replacement of nickel with cobalt ions in a [Ni6] cluster

Article abstract of DOI:10.1039/c3dt50429h

Polydentate ligands were used to support planar methoxo-bridged {Ni _6-xCo_x} (x = 0, 1 or 2) clusters which were structurally characterized by single crystal analyses. The homometallic {Ni₆} complex displayed overall ferrimagnetic interactions between metal centres, and increasing the cobalt content led to a proportional increase in ac-response. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3dt50429h

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Stepwise replacement of nickel with cobalt ions in a
[Ni6] cluster†
Cite this: Dalton Trans., 2013, 42, 6701
Received 15th February 2013,
Accepted 21st March 2013
Graham N. Newton, Hiroki Sato, Takuya Shiga and Hiroki Oshio*
DOI: 10.1039/c3dt50429h
www.rsc.org/dalton
Polydentate ligands were used to support planar methoxo-
The multidentate planar ligand 1,3-bis-(2-hydroxyphenyl)-
bridged {Ni6−xCox} (x = 0, 1 or 2) clusters which were structurally 1,3-propanedione (H3L) was synthesized by the Claisen con-
characterized by single crystal analyses. The homometallic {Ni6} densation of 2-hydroxyacetophenone and ethyl salicylate, fol-
complex displayed overall ferrimagnetic interactions between lowing a slightly modified version of the previously reported
metal centres, and increasing the cobalt content led to a pro- method (Scheme S1†).8 Combination with NiCl2·6H2O and
portional increase in ac-response.
triethylamine in methanol allowed the isolation of yellow
crystals of [Ni6(HL)4(MeO)4(MeOH)6]·4MeOH (1·4MeOH),‡
a complex of higher nuclearity than those previously reported
with H3L.8
The synthesis of novel polynuclear transition metal complexes
continues to attract a great deal of research interest due to the
propensity of such clusters to display remarkable physical
characteristics such as multi-redox activity,1 single molecule
magnet (SMM) behaviour2 and magnetic bistability.3 Com-
plexes in which metal ions are bridged by oxygen-donor
ligands to form cubane or open-cubane arrangements have
been widely reported and are promising systems for the
study of molecular magnetism,4 and as mimics of biological
systems.5 Neighbouring metal ions in such complexes are
usually bridged by oxygen donor ligands at angles close to 90°,
leading to accidental orthogonality of the interacting magnetic
orbitals and ferromagnetic interactions.6 Planar multidentate
ligands are good candidates for the semi-directed synthesis of
multinuclear magnetic clusters due to their propensity to form
regular, close-knit metal ion arrays, such as grids, rings and
squares.7 The use of planar alkoxo/phenoxo donors may allow
the synthesis of extended clusters with open-cubane frame-
work structures and ferromagnetic ground states, and is there-
fore a promising approach towards the synthesis of new
polynuclear clusters with high spin ground states. In this study
we synthesized a planar {Ni6} complex with an open-cubane
type framework and investigated the effect of the incorporation
of heterometal ions on the magnetic properties.
The cluster contains a hexanuclear nickel core similar to
those of complexes reported by Tandon et al.,9 and McInnes
and co-workers,10 in which the nickel ions are bridged by four
μ3-methoxo ligands and supported by the β-diketone (central)
and ketone/phenoxide (outer) binding sites of four dianionic
ligands (HL2−). The deprotonation state of the ligands was pre-
dicted from charge balance and their bonding behaviour. 1
crystallises in the monoclinic space group P21/n, and the
asymmetric unit contains three nickel centres. All three metal
ions have subtly different distorted octahedral O6 coordination
environments. Ni1 is coordinated by one central and one outer
bidentate binding site from two HL2− ligands, and its coordi-
nation sphere is completed by the coordination of two
μ3-methoxo groups. Ni2 is bound by one outer HL2− bidentate
site, three μ3-methoxo ligands and one methanol molecule,
and Ni3 is chelated by the central site of one HL2− ligand, one
μ3-methoxo moieties and two solvent methanol molecules.
The contrasting coordination environments lead to subtle
differences in the average Ni–O bonding distances of 2.031(6),
2.039(6) and 2.065(7) Å for Ni1, Ni2 and Ni3, respectively,
while all show substantial distortion from ideal octahedral
geometry (Table S1†). It is interesting to note that the
distortion of the coordination geometry of Ni3 results in
the elongation of the bonds along one axis with Ni3–O8 and
Ni3–O11 bond lengths of 2.135(6) and 2.115(6) Å, respectively
(Fig. 1).
Department of Chemistry, Graduate School of Pure and Applied Sciences, University
of Tsukuba, Tennodai 1-1-1, Tsukuba 305-8571, Japan.
E-mail: oshio@chem.tsukuba.ac.jp; Fax: +81 29 853 4238; Tel: +81 29 853 4238
†Electronic supplementary information (ESI) available: Additional experimental,
The difference in the coordination environments of the
three metal centres, as shown by the X-ray analysis, prompted
our attempts to generate heterometallic derivatives, and con-
ducting the corresponding syntheses using Ni : Co ratios of
magnetic and structural data and three cif files. CCDC 924183–924185. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3dt50429h
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Fig. 1 The crystallographically determined structure of 1, viewed perpendicular
to the plane of the metal ions. Nickel ions are in green, and oxygen and carbon
atoms in red and white, respectively. Thermal ellipsoids are shown at 30% prob-
ability, and lattice solvent molecules and hydrogen atoms have been removed
for clarity.
Fig. 2 Magnetic susceptibility data for 1 (blue), 2 (green) and 3 (red), with the
fitting data for 1 in black. Inset: magnetic exchange pathways in 1, weak ferro-
magnetic in black, strong ferromagnetic in blue and antiferromagnetic in red.
5 : 1 and 4 : 2 led to the isolation of [Ni5Co(HL)4(MeO)4-
(MeOH)6]·4MeOH (2·4MeOH) and [Ni4Co2(HL)4(MeO)4-
(MeOH)6]·4MeOH (3·4MeOH), respectively. Compounds 2 and
3 are isostructural to 1, with divalent cobalt ions replacing one
and two nickel centres, respectively. Elemental and ICP ana-
lyses confirmed the chemical composition of the clusters and
the single crystal analysis on 2 and 3 permitted the identifi-
cation of the cobalt centres. In both complexes, the cobalt ions
appeared to be located in the positions equivalent to the Ni3
positions of complex 1, with cobalt and nickel ions substitu-
tionally disordered between the two equivalent sites in 2, and
cobalt occupying both in 3. The average M–O bond distances
were 2.028(3), 2.040(2) and 2.084(5) Å for Ni1, Ni2 and Ni3/
Co1, respectively in complex 2, and 2.035(3), 2.045(3) and
2.096(3) Å for Ni1, Ni2 and Co1 respectively in 3 (Table S1†);
significantly different only in the Co1/Ni3 position.
Magnetic susceptibility measurements were conducted for
compounds 1, 2 and 3 over a temperature range of 300–1.8 K
under a magnetic field of 0.05 T (Fig. 2 and S1†). Compound 1
had a χmT value of 6.58 emu mol−1 K at 300 K, slightly greater
than the value expected for the uncorrelated spins of six nickel
ions (6.0 emu mol−1 K, g = 2.0). As the measurement tempera-
ture was lowered, the χmT value increased slowly to reach a
maximum of 8.93 emu mol−1 K at 19 K, before decreasing to
reach a final value of 5.74 emu mol−1 K at 1.8 K. The low temp-
erature maximum was much less than 21 emu mol−1 K, the
value expected for six ferromagnetically coupled nickel centres
(g = 2.0), suggestive of a ferrimagnetic ground state. The
bonding distances and angles permitted the identification of
three independent magnetic coupling pathways within the
cluster core (Fig. 2, inset),9 and the data were analysed using
the following spin Hamiltonian:
Table 1 Interatomic distances and Ni–O–Ni bridging angles for complex 1
M⋯M
Dist. (Å)
M–O–M
Ang. (°)
J
Ni2′–Ni2
Ni1–Ni2′
3.09
3.01
Ni2′–O10–Ni2
Ni1–O7–Ni2′
Ni1–O10–Ni2′
Ni1–O10–Ni2
Ni1–O9–Ni2
Ni2′–O8–Ni3
Ni2′–O9–Ni3
Ni1–O3–Ni3
Ni1–O9–Ni3
97.61
95.81
96.82
96.40
95.84
96.48
99.52
97.75
95.89
J1 = +1.7 K
J2 = +35.5 K
Ni1–Ni2
Ni2′–Ni3
Ni1–Ni3
3.05
3.08
3.06
J1 = +1.7 K
J3 = −2.5 K
J1 = +1.7 K
J1 = +1.7 K, J2 = +35.5 K and J3 = −2.5 K. The obtained J values
suggest that both ferro- and antiferromagnetic pathways are
operative in complex 1, and the bridging angles and inter-
metallic distances offer evidence in support of the fitted para-
meters (Table 1). Ni–O–Ni angles of around 90° (below 98–99°)
are known to mediate ferromagnetic coupling pathways,6 and
thus the bonding angles suggest that J1 and J2 are ferro-
magnetic, and that the large Ni2–O–Ni3 bridging angles (av. =
98.0°) may result in an antiferromagnetic pathway ( J3). The
small Ni1–O–Ni2 bridging angles (av. = 96.3°) coupled with
the short interatomic distance (3.01 Å) suggest that J2 may be a
strongly ferromagnetic pathway.
The sudden decrease in the susceptibility data at low temp-
erature can be ascribed to a combination of zero-field splitting
and intermolecular effects.9 The clusters adopt a columnar
packing arrangement, with a shortest Ni⋯Ni distance of ca.
7.5 Å between the complex moieties. Methanol molecules form
hydrogen-bonded bridges between neighbouring columns but
the long distance between metal ions (ca. 10.8 Å) suggests that
the strongest intermolecular magnetic pathway is likely to be
intracolumnar (Fig. S2†). Reduced magnetization measure-
ments, conducted to estimate the complex D value, were
unsuccessful due to the partial population of low-lying states
Ĥ ¼ ꢀ2fð J1ðŜ2Ŝ5Þ þ J2ðŜ2Ŝ3Þ þ J2ðŜ5Ŝ6Þ þ J1ðŜ3Ŝ5Þ þ J1ðŜ2Ŝ6Þ
þ J3ðŜ1Ŝ2Þ þ J3ðŜ4Ŝ5Þ þ J1ðŜ1Ŝ6Þ þ J1ðŜ3Ŝ4Þg
using MAGPACK,11 to yield fitting parameters of g = 2.00, close to the ground state.
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Subsequent syntheses performed using mixtures of nickel and
cobalt chloride salts led to the generation of mono- and di-
substituted heterometallic clusters, {Ni5Co} and {Ni4Co2}. The
composition of the heterometallic clusters was supported by
X-ray crystallography, elemental analysis and magnetic suscep-
tibility measurements. The magnetization data also showed
that the inclusion of increasing numbers of cobalt ions led to
improved frequency response. This observation is an example
of the importance of the controlled generation of hetero-
metallic clusters, in which physical properties can be tuned by
altering the electronic and magnetic interactions between
neighbouring metal centres.
Fig. 3 Out-of-phase ac susceptibility data for complexes 2 (empty circles) and
3 (filled circles), measured at 10 (blue), 100 (green) and 1000 Hz (red) at 1.8–4.0 K.
Notes and references
‡[Ni6(HL)4(MeO)4(MeOH)6]·4MeOH (1·4MeOH): To a solution of H3L (25 mg,
0.1 mmol) and triethylamine (56 μL, 0.4 mmol) in methanol (40 mL), a solution
of NiCl2·6H2O (36 mg, 0.151 mmol) in methanol (5 mL) was added dropwise.
The dark yellow solution was stirred at room temperature for 10 minutes, and
left untouched for crystallization by slow evaporation. After 2 days dark yellow
needle crystals of 1·4MeOH were collected by filtration (12 mg, 0.0066 mmol,
26.5%) and dried. Elemental analysis calcd for (1·3.5H2O), C70H83Ni6O29.5: C,
48.08; H, 4.78. Found: C, 47.74; H, 4.48. [Ni5Co(HL)4(MeO)4(MeOH)6]·4MeOH
(2·4MeOH): To a solution of H3L (25 mg, 0.1 mmol) and triethylamine (56 μL,
0.4 mmol) in methanol (40 mL), a solution of NiCl2·6H2O (30 mg, 0.126 mmol)
and CoCl2·4H2O (6 mg, 0.025 mmol) in methanol (5 mL) was added dropwise.
The orange solution was stirred at room temperature for 10 minutes before
being left untouched for crystallization by slow evaporation. After 2 days light
brown needle crystals of 2·4MeOH were collected by filtration and dried
(10.88 mg, 0.006 mmol, 24.0%). Elemental analysis calcd for (2·3H2O),
C70H82CoNi5O29: C, 48.33; H, 4.75. Found: C, 48.05; H, 4.62. ICP calcd Ni: 5.00;
Co: 1.00. Found: Ni: 5.56; Co: 1.0. [Ni4Co2(HL)4(MeO)4(MeOH)6]·4MeOH
(3·4MeOH): To a solution of H3L (25 mg, 0.1 mmol) and triethylamine (56 μL,
0.4 mmol) in methanol (40 mL), a solution of NiCl2·6H2O (24 mg, 0.100 mmol)
and CoCl2·4H2O (12 mg, 0.051 mmol) in methanol (5 mL) was added dropwise.
The orange-brown solution was stirred at room temperature for 10 minutes,
before being left untouched for crystallization by slow evaporation. After 2 days
orange-brown needle crystals of 3·4MeOH were collected by filtration and dried
(11.85 mg, 0.0065 mmol, 26.1%). Elemental analysis calcd for (3·3H2O),
Magnetic susceptibility measurements were conducted on 2
and 3 under the same conditions as 1. 2 and 3 had χmT values
of 9.76 and 11.16 emu mol−1 K respectively at 300 K, values
that initially decreased slightly as the temperature was lowered
until they began to increase at approximately 80 and 50 K,
reaching maxima of 10.62 and 11.24 emu mol−1 K at 16 and
15 K before rapidly decreasing to 8.45 and 9.16 emu mol−1
K
respectively at 1.8 K. The susceptibility plots of 2 and 3 differ
in profile from that of 1, due to the orbital contribution associ-
ated with CoII ions; the relatively small increases in χmT at low
temperatures suggest that both show ferrimagnetic behaviour.
The magnetization plots for 1, 2 and 3 are shown in Fig. S3.†
Ac magnetic susceptibility measurements were conducted
on all three samples. Complex 1 showed no ac response, but
the data collected for complexes 2 and 3 indicated the occur-
rence of slow magnetic relaxation (Fig. 3). The scale of the
magnetic response appeared to be proportional to the cluster
cobalt content. The existence of an easy axis of magnetization
may be understood by considering the distorted coordination
geometry of the cobalt binding sites. In 2 and 3 the Co ions
each have one elongated axis with Co1–O8/Co1–O11 bond dis-
tances of 2.159(4)/2.176(5) and 2.174(3)/2.198(3) Å, respectively.
The two bond vectors run approximately parallel to the length
of the cluster (Fig. S4†). The inclusion of one Co ion in 2 leads
to a very small ac response, while the semi-alignment of the
distortion axes in 3 amplifies the behaviour, probably through
a combination of increasing the negative anisotropy and the
number of spins. Despite the improved ac response, however,
the energy barriers of magnetic relaxation could not be deter-
mined at 1.8 K.
C
70H82Co2Ni4O29: C, 48.32; H, 4.75. Found: C, 48.03; H, 4.25. ICP calcd Ni: 2.00;
Co: 1.00. Found: Ni: 2.23; Co: 1.0. Crystallographic data for 1·4MeOH:
[C70H76Ni6O26]·4MeOH, Mr = 1813.74, monoclinic, P21/n, a = 8.578(6) Å, b =
27.265(19) Å, c = 15.964(11) Å, β = 94.540(13)°, V = 3722(4) Å3, Z = 2, T = 100 K. A
total of 18 414 reflections were collected (2.96° < 2θ < 50.25°) of which 6638
unique reflections (Rint = 0.1019) were measured. R1 = 0.0706, wR2 = 0.1694
(I > 2σ(I)). 2·4MeOH: [C70H76CoNi5O26]·4MeOH, Mr = 1813.96, monoclinic, P21/n,
a = 8.5016(7) Å, b = 27.218(2) Å, c = 16.0744(12) Å, β = 94.700(2)°, V = 3707.0(5)
Å3, Z = 2, T = 100 K. A total of 19 910 reflections were collected (2.94° < 2θ <
50.50°) of which 6901 unique reflections (Rint = 0.0294) were measured. R1
0.0495, wR2 = 0.1289 (I > 2σ(I)). 3·4MeOH: [C70H76CoNi5O26]·4MeOH, Mr
=
=
1814.18, monoclinic, P21/n, a = 8.4933(11) Å, b = 27.255(3) Å, c = 16.116(2) Å, β =
94.750(2)°, V = 3717.7(8) Å3, Z = 2, T = 100 K. A total of 20 037 reflections were
collected (2.94° < 2θ < 50.50°) of which 6917 unique reflections (Rint = 0.0409)
were measured. R1 = 0.0564, wR2 = 0.1377 (I > 2σ(I)).
1 (a) V. Balzani, A. Juris, M. Venturi, S. Campagna and
S. Serroni, Chem. Rev., 1996, 96, 759; (b) T. Ito,
T. Hamaguchi, H. Nagino, T. Yamaguchi, J. Washington
and C. P. Kubiak, Science, 1997, 277, 660.
2 Molecular Nanomagnets, ed. D. Gatteschi, R. Sessoli and
J. Villain, Oxford Press, New York, 2006.
Conclusions
Complexation of the planar multidentate ligand 1,3-bis-(2-
hydroxyphenyl)-1,3-propanedione with nickel chloride led to
the isolation of a hexanuclear {Ni6} cluster in which the nickel
ions existed in three distinct coordination environments.
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3 M. Nihei, Y. Sekine, N. Suganami, K. Nakazawa, A. Nakao,
H. Nakao, Y. Murakami and H. Oshio, J. Am. Chem. Soc.,
2011, 133, 3592.
4 K. Isobe and A. Yagasaki, Acc. Chem. Res., 1993, 26,
524.
5 G. C. Dismukes, R. Brimblecombe, G. A. N. Felton,
R. S. Pryadun, J. E. Sheats, L. Spiccia and G. F. Swiegers,
Acc. Chem. Res., 2009, 42, 1935.
6 (a) M. A. Halcrow, J. S. Sun, J. C. Huffman and G. Christou,
Inorg. Chem., 1995, 34, 4167; (b) J. M. ClementeJuan,
B. Chansou, B. Donnadieu and J. P. Tuchages, Inorg.
Chem., 2000, 39, 5515; (c) P. Mukherjee, M. G. B. Drew,
C. J. Gómez-García and A. Ghosh, Inorg. Chem., 2009, 48,
M. Nihei, M. Nakano, L. Cronin and H. Oshio, Angew.
Chem., Int. Ed., 2011, 50, 4844; (c) T. Matsumoto, T. Shiga,
M. Noguchi, T. Onuki, G. N. Newton, N. Hoshino,
M. Nakano and H. Oshio, Inorg. Chem., 2010, 49, 368;
(d) G. N. Newton, T. Shiga and H. Oshio, Sci. China Chem.,
2012, 55, 973.
8 (a) F. Eiden and H. D. Shweiger, Synthesis, 1974, 511;
(b) F. Teixidor, A. Llobet, L. Eschriche and J. Casabó, Poly-
hedron, 1985, 4, 215; (c) F. Teixidor, J. Colomer, J. Casabó,
J. Ruis, E. Molins and C. Miravitles, Inorg. Chem., 1989, 28,
678.
9 S. S. Tandon, S. D. Bunge, J. Sanchiz and L. K. Thompson,
Inorg. Chem., 2012, 51, 3270.
5848; (d) F. Iijima, K. Hasumi, T. Shiga, G. N. Newton and 10 R. T. W. Scott, L. F. Jones, I. S. Tidmarsh, B. Breeze,
H. Oshio, Chem. Lett., 2012, 691.
7 (a) T. Shiga, T. Matsumoto, M. Noguchi, T. Onuki,
N. Hoshino, G. N. Newton, M. Nakano and H. Oshio,
R. H. Laye, J. Wolowska, D. J. Stone, A. Collins, S. Parsons,
W. Wernsdorfer, G. Aromí, E. J. L. McInnes and
E. K. Brechin, Chem.–Eur. J., 2009, 15, 12389.
Chem.–Asian J., 2009, 4, 1660; (b) G. N. Newton, T. Onuki, 11 J. J. Borrás-Almenar, J. M. Clemente-Juan, E. Coronado and
T. Shiga, M. Noguchi, T. Matsumoto, J. S. Mathieson,
B. S. Tsukerblat, J. Comput. Chem., 2001, 22, 985.
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