Combining azide, carboxylate, and 2-pyridyloximate ligands in transition-metal chemistry: Ferromagnetic NiII 5clusters with a bowtie skeleton

Article abstract of DOI:10.1021/ic1014829

The combined use of the anion of phenyl(2-pyridyl)ketone oxime (ppko^-) and azides (N₃ ^-) in nickel(II) carboxylate chemistry has afforded two new Ni^II ₅clusters, [Ni₅(O₂CR′)₂(N₃)₄(ppko)₄(MeOH)₄] [R′ = H (1), Me (2)]. The structurally unprecedented {Ni₅(μ-N₃)₂(μ₃-N₃)₂}⁶⁺cores of the two clusters are almost identical and contain the five Ni^IIatoms in a bowtie topology. Two N₃ ^-ions are end-on doubly bridging and the other two ions end-on triply bridging. The end-on μ₃-N₃ ^-groups link the central Ni^IIatoms with the two peripheral metal ions on either side of the molecule, while the Ni···Ni bases of the triangles are each bridged by one end-on μ-N₃ ^-group. Variable- temperature, solid-state direct-(dc) and alternating-current (ac) magnetic susceptibility, and magnetization studies at 2.0 K were carried out on both complexes. The data indicate an overall ferromagnetic behavior and an S = 5 ground state for both compounds. The ac susceptibility studies on 1 reveal nonzero, frequency-dependent out-of-phase (Χ_M″) signals at temperatures below ～3.5 K; complex 2 reveals no Χ_M″ signals. However, single-crystal magnetization versus dc field scans at variable temperatures and variable sweep rates down to 0.04 K on 1 reveal no noticeable hysteresis loops, except very minor ones at 0.04 K assignable to weak intermolecular interactions propagated by nonclassical hydrogen bonds.

Full text of DOI:10.1021/ic1014829

10486 Inorg. Chem. 2010, 49, 10486–10496
DOI: 10.1021/ic1014829
Combining Azide, Carboxylate, and 2-Pyridyloximate Ligands in Transition-Metal
Chemistry: Ferromagnetic NiII Clusters with a Bowtie Skeleton
5
Constantina Papatriantafyllopoulou,† Theocharis C. Stamatatos,† Wolfgang Wernsdorfer,‡ Simon J. Teat,§
Anastasios J. Tasiopoulos,^ Albert Escuer,*,z and Spyros P. Perlepes*,†
†
‡
ꢀ
Department of Chemistry, University of Patras, 265 04 Patras, Greece, Institut Neel, CNRS, and
§
ꢀ
Universite J. Fourier, BP 166, 38042 Grenoble Cedex 9, France, Advanced Light Source,
Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States,
^Department of Chemistry, University of Cyprus, 1678 Nicosia, Cyprus, and zDepartament de Quimica Inorganica,
ꢀ
Universitat de Barcelona, Marti Franques 1-11, Barcelona 08028, Spain
Received July 23, 2010
The combined use of the anion of phenyl(2-pyridyl)ketone oxime (ppko-) and azides (N3-) in nickel(II) carboxylate
chemistry has afforded two new NiII clusters, [Ni5(O2CR0)2(N3)4(ppko)4(MeOH)4] [R0 = H (1), Me (2)]. The
5
structurally unprecedented {Ni5(μ-N3)2(μ3-N3)2}6þ cores of the two clusters are almost identical and contain the five
NiII atoms in a bowtie topology. Two N3- ions are end-on doubly bridging and the other two ions end-on triply bridging.
The end-on μ3-N3- groups link the central NiII atoms with the two peripheral metal ions on either side of the molecule, while
the Ni Ni bases of the triangles are each bridged by one end-on μ-N3- group. Variable-temperature, solid-state direct-
3 3 3
(dc) and alternating-current (ac) magnetic susceptibility, and magnetization studies at 2.0 K were carried out on both
complexes. The data indicate an overall ferromagnetic behavior and an S = 5 ground state for both compounds. The ac
susceptibility studies on 1 reveal nonzero, frequency-dependent out-of-phase (χM00) signals at temperatures below ∼3.5 K;
complex 2 reveals no χM00 signals. However, single-crystal magnetization versus dc field scans at variable temperatures and
variable sweep rates down to 0.04 K on 1 reveal no noticeable hysteresis loops, except very minor ones at 0.04 K assignable
to weak intermolecular interactions propagated by nonclassical hydrogen bonds.
Introduction
temperatures and display magnetic hysteresis analogous to
that observed in bulk magnets, thus representing a molecular,
or “bottom-up”, approach to nanoscale magnetism with
several potential applications.7 Even more fascinating is the
fact that they show strong evidence of quantum effects.8 Most
SMMs are clusters of exchange-coupled 3d metal ions pro-
tected by a shell of ligands. Their magnetic behavior results
from the combination of a large ground-state spin (S) with a
Polynuclear complexes (clusters1 or coordination clusters2)
of 3d metals at intermediate oxidation states have become of
particular interest in recent times.3 This interest is a result of
their relevance to bioinorganic chemistry4,5 and to the special
class of molecule-based magnetic materials known as single-
molecule magnets (SMMs).6 SMMs are discrete molecules
that exhibit slow relaxation of the magnetization at low
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*To whom correspondence should be addressed. E-mail: albert.escuer@
ub.edu (A.E.), perlepes@patreas.upatras.gr (S.P.P.). Tel.: þ30 2610 997146
(S.P.P.). Fax: þ34-93-4907725 (A.E.), þ30 2610 997118 (S.P.P.).
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r
pubs.acs.org/IC
Published on Web 10/22/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 22, 2010 10487
large Ising (or easy-axis) type of magnetoanisotropy, as gauged
by a negative, axial zero-field-splitting parameter (D).
and hemiketal forms of pyridyl ketones,12b,14,15 pyridyl alcohols16
and dialcohols,12b,17 salicylaldo(keto)ximes,9b,18 pyridylox-
imes19 and dioximes,19,20 and others.21 Most 3d metal
clusters reported to date have been obtained by utilizing
this so-called serendipitous assembly,11 where simple metal
salts or preformed small clusters (mainly containing simple
carboxylate ligands) react with multitopic ligands under a
variety of conditions.
A modern trend is the employment of three ligands in
the reaction systems (a combination of ligands or “ligand
blends”).22 The loss of a degree of synthetic control is more
than compensated for by the vast diversity of structural types
using a combination of ligands. As part of such efforts, we have
recently initiated a project in which the general carboxylate/
azide/2-pyridyloxime (R0CO2-/N3-/LNOH) ligand combina-
tion is used in nickel(II) chemistry; our goal is to obtain clusters
with interesting structures and magnetic properties. NiII has
shown promise in the synthesis of both SMMs6c and spin-
phonon traps,23 with the former taking advantage of its sig-
nificant single-ion anisotropy and the latter of its paramagnetic
nature when confined with a highly symmetric cluster. How-
ever, homometallic SMMs containing NiII ions are surprisingly
uncommon24 despite the fact that mononuclear nickel(II)
complexes have been shown to possess appreciable |D|,25 e.g.,
>10 cm-1. Carboxylate ions are famous for exhibiting a huge
variety of coordination modes. The azide anion, N3-, is one of
the most commonly employed inorganic bridging ligands in the
design of polynuclear 3d metal complexes26 with characteristic
From the above brief discussion, it is becoming clear that
there continues to be a need for new synthetic methodologies
to new polynuclear metal complexes and new SMMs, with
efforts directed toward defining and enhancing the param-
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10488 Inorganic Chemistry, Vol. 49, No. 22, 2010
Papatriantafyllopoulou et al.
Scheme 1
N3- ligands. We targeted different Nix products by use of the
ternary R0CO2-/N3-/ppkoH ligand system and have success-
fully isolated two new NiII5 comlexes with an S=5 ground state
and a bowtie metal topology. This work can be considered as a
continuation of our interest in homo- and heterometallic
clusters based on 2-pyridyloximate28a-c,29a,29b,31-35,37 and pyr-
idyldioximate20a-c ligands.
Experimental Section
and tunable physical properties. This ligand is very popular in
the field of molecular magnetism27 because of its ability to
mediate ferromagnetic coupling under certain conditions. 2-Pyr-
idyloximes (Scheme 1; R = H, Me, Ph, py, NH2, etc.) are a
subclass of oximes that are currently popular ligands in co-
ordination chemistry.19 The anions of these molecules (2-pyr-
idyloximate ligands) are particularly versatile in that they
possess chelating and bridging ( μ2-μ4) capabilities; the activation
of 2-pyridyloximes by 3d metal centers toward further
reactions is also becoming a fruitful area of research.28 Such
ligands have been key “players” in several areas of single-
molecule29 and single-chain30 magnetism.
Syntheses. All manipulations were performed under aerobic
conditions using reagents and solvents as received. Warning!
Azide and perchlorate salts are potentially explosive; such com-
pounds should be synthesized and used in small quantities and
treated with utmost care at all times using plastic spatulas.
[Ni5(O2CH)2(N3)4(ppko)4(MeOH)4] (1). Method A. A pale-
green solution of Ni(ClO4)2 6H2O (0.366 g, 1.00 mmol) in
3
MeOH (10 mL) was added dropwise over 10 min to a stirred
solution of ppkoH (0.159 g, 0.80 mmol), NaN3 (0.065 g, 1.00
mmol), and NaO2CH (0.082 g, 1.21 mmol) in MeOH (20 mL).
The resulting reddish-brown solution was stirred for an addi-
tional 10 min and filtered and the filtrate left undisturbed to
concentrate slowly by evaporation at room temperature. X-ray-
quality orange-brown crystals of the product slowly grew over 1
week. The crystals were washed with cold MeOH (0.5 mL) and
Et2O (3 ꢀ 2 mL) and dried under vacuum. The yield was ∼45%.
The dried solid was determined by analysis to be solvent-free.
Anal. Calcd (found) for C54H54N20Ni5O12: C, 44.16 (44.26); H,
3.71 (3.80); N, 19.08 (18.96). IR (KBr, cm-1): 3366(mb),
3050(m), 2944(w), 2092(s), 2061(s), 1590(s), 1524(m), 1460(s),
1438(m), 1390(w), 1346(m), 1262(w), 1210(m), 1122(s), 1026(w),
974(m), 786(m), 748(m), 706(s), 644(m), 606(w), 452(w).
In the present work, we have used phenyl(2-pyridyl)ketone
oxime (ppkoH; R = Ph in Scheme 1) in combination with
-
carboxylate (HCO2 and MeCO2-) and azide ligands in
nickel(II) chemistry. The reactions of NiII sources and ppkoH
had previously given [Ni3(ppko)6],31 [Ni3(HCO2)2(ppko)3-
(ppkoH)2]þ,31 [Ni6(SO4)2(ppko)8],32 [Ni6(SO4)4(OH)(ppko)3-
(ppkoH)3(solvent)3] (solvent = H2O, MeOH),35 and [Ni2-
(ppko)3(L)](ClO4),36 where L is the tridentate capping ligand
1,4,7-trimethyl-1,4,7-triazacyclononane. Only two of these
Method B. Solid NiSO4 6H2O (0.158 g, 0.60 mmol) was
3
-
complexes are based on ppko-/R0CO2 ligation. Note that
added to a solution of ppkoH (0.119 g, 0.60 mmol) and NaOMe
(0.033 g, 0.60 mmol) in MeOH (20 mL). The solid soon
dissolved, and the resulting red solution was stirred for 5 min,
during which time a solution of NaN3 (0.039 g, 0.60 mmol) in
MeOH (5 mL) was added in small portions. A noticeable color
change from red to brown occurred, and the mixture was filtered to
remove Na2SO4. The homogeneous filtrate was stirred for a further
1 h and layered with Et2O (30 mL). Slow mixing gave X-ray-quality
crystals of the product. The crystals were collected by filtration,
washed with cold MeOH (1 mL) and Et2O (2 ꢀ 2 mL), and dried in
air. The yield was 68%. The identity of the product was confirmed
by elemental analyses, IR spectroscopic comparison with an
authentic material from method A, and unit cell determination.
Anal. Calcd (found) for C54H54N20Ni5O12: C, 44.16 (44.32); H, 3.71
(3.61); N, 19.08 (18.83).
there are no nickel(II) complexes containing both ppko- and
(27) (a) Zeng, Y.-F.; Hu, X.; Liu, F.-C.; Bu, X.-H. Chem. Soc. Rev. 2009,
38, 469. (b) Stamatatos, Th. C.; Papaefstathiou, G. S.; MacGillivray, L. R.; Escuer,
A.; Vicente, R.; Ruiz, E.; Perlepes, S. P. Inorg. Chem. 2007, 46, 8843 and
references cited therein.
(28) (a) Stoumpos, C. C.; Inglis, R.; Roubeau, O.; Sartzi, H.; Kitos, A. A.;
Milios, C. J.; Aromi, G.; Tasiopoulos, A. J.; Nastopoulos, V.; Brechin, E. K.;
Perlepes, S. P. Inorg. Chem. 2010, 49, 4388. (b) Milios, C. J.; Kyritsis, P.;
Raptopoulou, C. P.; Terzis, A.; Vicente, R.; Escuer, A.; Perlepes, S. P. Dalton
Trans. 2005, 501. (c) Milios, C. J.; Kefalloniti, E.; Raptopoulou, C. P.; Terzis, A.;
Escuer, A.; Vicente, R.; Perlepes, S. P. Polyhedron 2004, 23, 83. (d) Kumagai, H.;
Endo, M.; Kondo, M.; Kawata, S.; Kitagawa, S. Coord. Chem. Rev. 2003, 237,
197.
(29) (a) Lee, S.-C.; Stamatatos, Th. C.; Foguet-Albiol, D.; Hill, S.;
Perlepes, S. P.; Christou, G. Polyhedron 2007, 26, 2255. (b) Stamatatos, Th.
C.; Foguet-Albiol, D.; Lee, S.-C.; Stoumpos, C. C.; Raptopoulou, C. P.; Terzis, A.;
Wernsdorfer, W.; Hill, S. O.; Perlepes, S. P.; Christou, G. J. Am. Chem. Soc.
2007, 129, 9484. (c) Mori, F.; Nyui, T.; Ishida, T.; Nogami, T.; Choi, K.-Y.; Nojiri,
H. J. Am. Chem. Soc. 2006, 128, 1440. (d) Mori, F.; Ishida, T.; Nogami, T.
Polyhedron 2005, 24, 2588.
(37) For example, see: (a) Alexandropoulos, D. I.; Papatriantafyllopoulou,
C.; Aromi, G.; Roubeau, O.; Teat, S. J.; Perlepes, S. P.; Christou, G.;
Stamatatos, Th. C. Inorg. Chem. 2010, 49, 3962. (b) Zhang, S.; Zhen, L.; Xu,
B.; Inglis, R.; Li, K.; Chen, W.; Zhang, Y.; Konidaris, K. F.; Perlepes, S. P.; Brechin,
E. K.; Li, Y. Dalton Trans. 2010, 39, 3563. (c) Konidaris, K. F.; Katsoulakou, E.;
Kaplanis, M.; Bekiari, V.; Terzis, A.; Raptopoulou, C. P.; Manessi-Zoupa, E.;
Perlepes, S. P. Dalton Trans. 2010, 39, 4492. (d) Stoumpos, C.; Stamatatos, Th. C.;
Sartzi, H.; Roubeau, O.; Tasiopoulos, A. J.; Nastopoulos, V.; Teat, S. J.; Christou,
G.; Perlepes, S. P. Dalton Trans. 2009, 1004. (e) Konidaris, K. F.; Kaplanis, M.;
Raptopoulou, C. P.; Perlepes, S. P.; Manessi-Zoupa, E.; Katsoulakou, E. Polyhe-
dron 2009, 28, 3243. (f ) Stamatatos, Th. C.; Katsoulakou, E.; Terzis, A.;
Raptopoulou, C. P.; Winpenny, R. E. P.; Perlepes, S. P. Polyhedron 2009, 28,
1638. (g) Stoumpos, C. C.; Stamatatos, Th. C.; Psycharis, V.; Raptopoulou, C. P.;
Christou, G.; Perlepes, S. P. Polyhedron 2008, 27, 3703. (h) Stamatatos, Th. C.;
Escuer, A.; Abboud, K. A.; Raptopoulou, C. P.; Perlepes, S. P.; Christou, G. Inorg.
Chem. 2008, 47, 11825. (i) Papatriantafyllopoulou, C.; Jones, L. F.; Nguyen,
T. D.; Matamoros-Salvador, N.; Cunha-Silva, L.; Almeida Paz, F. A.; Rocha, J.;
Evangelisti, M.; Brechin, E. K.; Perlepes, S. P. Dalton Trans. 2008, 3153. (j)
Stamatatos, Th. C.; Diamantopoulou, E.; Raptopoulou, C. P.; Psycharis, V.; Escuer,
A.; Perlepes, S. P. Inorg. Chem. 2007, 46, 2350. (k) Stamatatos, Th. C.;
Dionyssopoulou, S.; Efthymiou, G.; Kyritsis, P.; Raptopoulou, C. P.; Terzis, A.;
Vicente, R.; Escuer, A.; Perlepes, S. P. Inorg. Chem. 2005, 44, 3374.
ꢀ
(30) (a) Miyasaka, H.; Julve, M.; Yamashita, M.; Clerac, R. Inorg. Chem.
ꢀ
2009, 48, 3420 (Forum Article). (b) Miyasaka, H.; Clerac, R.; Mizushima, K.;
Sugiura, K.; Yamashita, M.; Wernsdorfer, W.; Coulon, C. Inorg. Chem. 2003, 42,
ꢀ
8203. (c) Clerac, R.; Miyasaka, H.; Yamashita, M.; Coulon, C. J. Am. Chem. Soc.
2002, 124, 12837.
(31) Efthymiou, C. G.; Raptopoulou, C. P.; Terzis, A.; Perlepes, S. P.;
Escuer, A.; Papatriantafyllopoulou, C. Polyhedron 2010, 29, 627.
(32) Efthymiou, C. G.; Kitos, A. A.; Raptopoulou, C. P.; Perlepes, S. P.;
Escuer, A.; Papatriantafyllopoulou, C. Polyhedron 2009, 28, 3177.
(33) Papatriantafyllopoulou, C.; Aromi, G.; Tasiopoulos, A. J.; Nastopoulos,
V.; Raptopoulou, C. P.; Teat, S. J.; Escuer, A.; Perlepes, S. P. Eur. J. Inorg. Chem.
2007, 2761.
(34) Papatriantafyllopoulou, C.; Efthymiou, C. G.; Raptopoulou, C. P.;
Terzis, A.; Manessi-Zoupa, E. Spectrochim. Acta 2008, 70A, 718.
(35) Stamatatos, Th. C.; Papatriantafyllopoulou, C.; Katsoulakou, E.;
Raptopoulou, C. P.; Perlepes, S. P. Polyhedron 2007, 26, 1830.
(36) Chaudhuri, P.; Weyhermuller, T.; Wagner, R.; Khanra, S.; Biswas,
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Article
[Ni5(O2CMe)2(N3)4(ppko)4(MeOH)4] 0.25H2O (2 0.25H2O).
To a colorless solution of ppkoH (0.198 g, 1.00 mmol) in MeOH
Inorganic Chemistry, Vol. 49, No. 22, 2010 10489
Table 1. Crystallographic Data for Complexes 1 and 2 0.25H2O
3
3
3
1
2 0.25H2O
3
(20 mL) was added solid Ni(O2CMe)2 4H2O (0.249 g, 1.00 mmol).
3
formulaa
C54H54N20Ni5 O12
1468.72
triclinic
P1
C56H58.50N20Ni5 O12.25
1501.28
monoclinic
C2/c
The resulting green solution was stirred for 15 min, during which
time solid NaN3 (0.063 g, 1.00 mmol) was added in small portions
to give a red homogeneous solution. The stirring was maintained
for an additional 10 min. Layering of the solution with a solvent
mixture comprising Et2O/n-hexane [25 mL, 1:1 (v/v)] gave X-ray-
quality orange crystals of the product. The crystals were collected
by filtration, washed with Et2O (2 ꢀ 5 mL), and dried under
vacuum. The yield was 72%. The dried solid was determined by
analysis to be solvent-free 2. Anal. Calcd (found) for C56H58N20-
Ni5O12: C, 44.93 (44.97); H, 3.91 (4.06); N, 18.72 (18.95). IR (KBr,
cm-1): 3422(mb), 2091(s), 2059(s), 1595(s), 1565(m), 1521(sh),
1462(s), 1442(m), 1416(m), 1384(m), 1342(w), 1292(w), 1267(w),
1240(w), 1215(m), 1206(sh), 1123(s), 1104(m), 1031(w), 977(m),
783(w), 748(m), 709(s), 661(m), 643(m), 501(wb), 450(m).
fw/g mol-1a
cryst syst
space group
˚
a, A
11.736(1)
12.029(1)
13.870(1)
111.05(1)
100.12(1)
109.84(1)
1617.18(16)
2
14.4167(13)
24.334(2)
19.8508(17)
90
95.628(2)
90
˚
b, A
˚
c, A
R, deg
β, deg
γ, deg
3
˚
V, A
Z
T, K
6930.4(10)
4
193(2)
100(2)
0.710 73b
1.508
1.502
0.774 90c
1.439
1.770
˚
radiation, A
F
calcd, g cm-3
μ, mm-1
measd/unique reflns
Rint
X-ray Crystallography. Data for complex 1 were collected on
an Oxford Diffraction Xcalibur-3 diffractometer (equipped
with a Sapphire CCD area detector) at 100 K using graphite-
19 536/5230
0.0256
4498
38 752/10 649
0.0584
8557
obsd reflns
R1d,e
˚
monochromated Mo KR radiation (λ = 0.710 73 A). A suitable
0.0313
0.0744
0.0418
0.1254
wR2e,f
crystal was attached to glass fibers using silicone grease. The
structure was solved by direct methods using SIR9238 and
refined by full-matrix least-squares techniques on F2 with
SHELXL-97.39 Some residual electron density in the accessible
voids of the structure was too disordered to refine as solvent
molecules; therefore, the SQUEEZE procedure40 of PLATON
was employed to remove the contribution of the electron density
in the solvent region from the intensity data. The solvent-free
model and intensity data were used for the final results reported
here. The non-H atoms were treated anisotropically. All H
atoms were placed in calculated ideal positions and refined as
GOF on F2
1.071
0.703, -0.333
1.039
0.879, -0.686
-3
˚
ΔFmax,min, e A
a Including solvate molecules. b Mo KR radiation. c Synchrotron
P
P
radiation. d R1 = (||Fo| - |Fc||)/ |Fo|. e For observed [I > 2σ(I )]
P
P
reflections. f wR2 = [ [w(Fo2 - Fc2)2]/ [w(Fo2)2]]1/2, w=1/[σ2(Fo2) þ
(ap)2 þ bp], where p = [max(Fo2,0) þ 2Fc2]/3.
ideal, this reflects a slight disorder in the system and a split-site
model would not yield any new chemical information.
Unit cell parameters and structure solution and refinement
data for the two complexes are listed in Table 1.
-
riding on their respective C atoms, except those of the HCO2
and MeOH ligands, which were located from a difference
Fourier map and refined isotropically. The programs used were
CRYSALIS CCD41 for data collection, CRYSALIS RED41 for
cell and data refinement, WINGX42 for crystallographic calcu-
lations, and MERCURY43 and DIAMOND44 for molecular
graphics.
Physical Studies. Microanalyses (C, H, and N) were per-
formed by the in-house facilities of the University of Patras
(Greece). IR spectra (4000-450 cm-1) were recorded using
Perkin-Elmer 16 PC and Bruker IFS-125 Fourier transform
spectrometers with samples prepared as KBr pellets. Variable-
temperature magnetic studies for 1 and 2 were performed at the
University of Barcelona using a DSM5 Quantum Design
SQUID magnetometer operating at 0.3 T in the 30-300 K
range and at 0.03 T in the 30-2 K range to avoid saturation
effects. Diamagnetic corrections were applied to the observed
paramagnetic susceptibilities using Pascal’s constants. Low-
temperature (<1.8 K) magnetic studies were performed at
Grenoble using an array of micro-SQUIDs.46 The high sensi-
tivity of this magnetometer allows the study of single crystals on
the order of 10-500 μm. The field can be applied in any direction
by separately driving three orthogonal coils. Crystals were
maintained in mother liquor to avoid degradation and covered
with grease for protection during transfer to the micro-SQUID
and subsequent cooling.
Data for a selected crystal of 2 0.25H2O were collected at
3
Station 11.3.1 of the Advanced Light Source at Lawrence
Berkeley National Laboratory, using a Bruker Platinum 200
CCD diffractometer (ω0 rotation with narrow frames, synchro-
˚
tron radiation at 0.7749 A, and a silicon 111 monochromator).
The structure was solved by direct methods and refined using the
SHELX-TL suite of programs.45 All non-H atoms were refined
anisotropically, except those at less than half-occupancy. H
atoms were placed in geometrically suitable positions where
possible, whereasthe methyl H atomswerefound inthe difference
map and constrained using a riding model. Hydroxyl H atoms
were found in the difference map and allowed to refine freely.
Displacement parameter restraints were used in the modeling of
the one of the ppko- ligands. Ratios of the displacement param-
eters up to around 5:1 (max:min) were left; even if this is not
Results and Discussion
Syntheses and IR Spectra. The two previously studied
clusters in NiII/R0CO2-/ppkoH chemistry are character-
ized by antiferromagnetic exchange interactions. Thus, the
ground-state spin of [Ni3(HCO2)2(ppko)3(ppkoH)2]þ 31
is intermediate between S=0 and 1, while [Ni4(O2CMe)4-
(ppko)4(MeOH)2]35 has a diamagnetic spin ground state.
Our approach was to employ a third bridging ligand in the
NiII/R0CO2-/ppkoH reaction system that typically gives
ferromagnetic interactions and that can thus increase
(38) SIR92: Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi,
A.; Burla, M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27,
435.
(39) Sheldrick, G. M. SHELXL-97, Program for the Refinement of Crystal
€
€
Structures from Diffraction Data; University of Gottingen: Gottingen,
Germany, 1997.
(40) Van der Sluis, P.; Spek, A. L. Acta Crystallogr., Sect. A 1990, 46, 194.
(41) Oxford Diffraction, CrysAlis CCD and CrysAlis RED, version
1.171.32.15; Oxford Diffraction Ltd.: Abingdon, England, 2008.
(42) WINGX: Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.
(43) Mercury: Bruno, I. J.; Cole, J. C.; Edgington, P. R.; Kessler, M. K.;
Macrae, C. F.; McCabe, P.; Pearson, J.; Taylor, R. Acta Crystallogr., Sect. B
2002, 58, 389.
-
the chances of a larger ground-state S value. The N3 ion
bridging in the end-on (EO) fashion is a strong ferromagnetic
(44) Bradenburg, K. DIAMOND, release 3.1f; Crystal Impact GbR: Bonn,
Germany, 2008.
(45) SHELX-TL; Bruker AXS Inc.: Madison, WI, 2003.
(46) Wernsdorfer, W. Adv. Chem. Phys. 2001, 118, 99.

10490 Inorganic Chemistry, Vol. 49, No. 22, 2010
Papatriantafyllopoulou et al.
mediator for a wide range of M-N-M angles, and it thus
represents a well-recognized route to high-spin species and
SMMs,26 although its use in combination with pyridylox-
imes is still in its infancy.26,28d,37h,47
A variety of reactions were explored with different solvents
and under different reagent ratios, and other conditions
before the following successful procedures were identified.
Both preparative routes to complex 1 give only crystals
of the product. No powder material that could be due to
NiO was detected.
In a next step, we wondered if the MeCO2- analogue of 1
could be prepared and thus pursued its synthesis. There are
two reasons to prepare such a complex: (i) to determine if it
would be isostructural with 1, and (ii) if it is isostructural,
to assess any influence of the H versus Me difference on
magnetochemical properties. The 1:1:1 reaction between
The reaction of Ni(ClO4)2 6H2O, ppkoH, NaN3, andNaO2-
3
CH in a 1.2:1:1.2:1.5 ratio in MeOH afforded a reddish-
brown solution, from which was subsequently isolated 1 in
45% yield. Its formation is summarized in eq 1. The MeOH
solvent was necessary to ensure solubility of the NaClO4
Ni(O2CMe)2 4H2O, ppkoH, and NaN3 in MeOH gave a
3
red solution from which was subsequently isolated 2 in very
good yield (∼70%). The employment of extra base, i.e., OH-
or MeO-, is not necessary because the MeCO2- ions can act
as proton acceptors, facilitating the formation of the ppko-
ligands; however, the addition of an external base does not
produced. Small variations in the NiII/ppkoH/N3-/HCO2
-
ratio also gave complex 1.
affect the product identity. Using Ni(ClO4)2 6H2O and
NaO2CMe 3H2O in place of Ni(O2CMe)2 4H2O led again
5NiðClO4Þ2 6H2O þ 6NaO2CH þ 4ppkoH þ 4NaN3
MeOH
3
3
3
3
to complex 2 but in appreciably lower yields (20-30%).
The presence of coordinated MeOH molecules in dried
samples of 1 and 2 is manifested by broad bands of
medium intensity at ∼3370 and ∼3420 cm-1, respectively,
assigned to ν(OH); their broadness and relatively low
frequency are both indicative of hydrogen bonding. The
spectra of 1 and 2 exhibit intense bands at ∼2090 and
∼2060 cm-1, which are assigned to the asymmetric
stretching mode, νas(NNN), of the EO azide groups.27b
The appearance of two bands in each spectrum possibly
reflects the presence of two different types of EO azido
ligands in the complexes (vide infra). The in-plane defor-
mation mode of the 2-pyridyl ring of free ppkoH at
þ 4MeOH
s
½Ni ðO CHÞ ðN Þ ðppkoÞ ðMeOHÞ ꢁ
f
5
2
3
4
2
4
4
þ 4HCO2H þ 10NaClO4 þ 30H2O
ð1Þ
Complex 1 could also be prepared from a HCO2--free
reaction system. The 1:1:1 reaction between NiSO4 6H2O,
3
ppkoH, and NaOMe (to help deprotonation of the ligand) in
MeOH gave a red solution. When the solution was treated
with 1 equiv of NaN3 in MeOH, white Na2SO4 was pre-
cipitated. The addition of Et2O into the brown homogeneous
filtrate produced 1 in ∼70% yield The formation of the
complex using this method can be summarized in eq 2. The
methoxide ions play a double role in the reaction. They act as
proton acceptors to facilitate the formation of ppko- ligands,
while a percentage of them are oxidized to HCO2- during the
aerial aggregation process.31,48 It is well-known that small
alkoxides are susceptible to oxidation by air.49 The nature of
the base in MeOH is crucial for identity of the product;
reaction schemes involving other bases, such as LiOH or
Et3N, gave noncrystalline, formate-free materials that we
were not able to further characterize. It should be mentioned
at this point that the chemical and structural identity of
622 cm-132 shifts upward in the spectra of 1 (644 cm-1
)
and 2 (643 cm-1), confirming the involvement of the ring
N atom in coordination.51 Several bands appear in the
1600-1400 cm-1 region for the two complexes; contribu-
tions from the ν(CdN)oximate, δ(CH3), δ(OH) (>1580
cm-1), νas(CO2), and νs(CO2) are expected in this region,
but overlap with the stretching vibrations of the aromatic
ppko- rings renders assignments and a discussion of the
coordination shifts difficult. The medium-intensity band
at 1094 cm-1 for free ppkoH has been assigned to the
ν(NO)oxime mode.34 The wavenumber of this vibration
increases to ∼1120 cm-1 in 1 and 2. This shift to higher
frequencies has been discussed and is in accordance with
the fact that, upon deprotonation and oximate-O coordi-
nation, there is a higher contribution of NdO to the
electronic structure of the oximate group; consequently,
the ν(NO) vibration shifts to a higher wavenumber
relative to that for the free oxime ligand.37b,h,52
the products from the general NiSO4 6H2O/ppkoH/MeO-/
3
-
N3 reaction system depends on the solvent used. In N,N-
dimethylformamide, the azide-free cluster [Ni6(SO4)2
(ppko)8]32 forms, whereas this reaction scheme in MeCN
gives the dinuclear complex [Ni2(N3)2(ppko)2(ppkoH)2].50
5NiSO4 6H2O þ 4ppkoH þ 6NaOCH3 þ 4NaN3 þ 2O2
s
3
MeOH
½Ni ðO CHÞ ðN Þ ðppkoÞ ðCH OHÞ ꢁ
f
5
2
3
3
2
4
4
4
Description of Structures. Partially labeled structures of
complexes 1 and 2 are shown in Figures S1 in the
Supporting Information and 1, respectively. The metallic
skeleton and two views of the core of complex 2 are
illustrated in Figures 2 and 3, respectively. Selected
interatomic distances and angles for 1 and 2 are listed in
Tables 2 and 3, respectively. Because the molecular
structures of the two complexes are very similar, only the
structure of the representative compound 2 will be described
in detail.
þ 5Na2SO4 þ 32H2O
ð2Þ
(47) (a) Lampropoulos, C.; Stamatatos, Th. C.; Manos, M. J.; Tasiopoulos,
A. J.; Abboud, K. A.; Christou, G. Eur. J. Inorg. Chem. 2010, 2244. (b) Zaleski,
C. M.; Weng, T.-C.; Dendrinou-Samara, C.; Alexiou, M.; Kanakaraki, P.; Hsieh,
W. Y.; Kampf, J.; Penner-Hahn, J. E.; Pecoraro, V. L.; Kessissoglou, D. P. Inorg.
Chem. 2008, 47, 6127. (c) Milios, C. J.; Piligkos, S.; Bell, A. R.; Laye, R. H.; Teat,
S. J.; Vicente, R.; McInnes, E. J. L.; Escuer, A.; Perlepes, S. P.; Winpenny, R. E. P.
Inorg. Chem. Commun. 2006, 9, 638.
€
(48) Biswas, B.; Khanra, S.; Weyhermuller, T.; Chaudhuri, P. Chem.
Commun. 2007, 1059.
(49) Nurova, S. Y.; Turevskaya, E. P.; Kessler, V. G.; Yanovskaya, M. I.
The Chemistry of Metal Alkoxides; Kluwer AP: Dordrecht, The Netherlands,
2002.
(50) Papatriantafyllopoulou, C.; Efthymiou, C. G.; Tasiopoulos, A. J.;
Escuer, A.; Perlepes, S. P., unpublished results.
(51) Lever, A. B. P.; Mantovani, A. Inorg. Chem. 1971, 10, 817.
€
(52) (a) Chaudhuri, P.; Winter, M.; Florke, U.; Houpt, H.-J. Inorg. Chim.
Acta 1995, 232, 125. (b) Papatriantafyllopoulou, C.; Raptopoulou, C. P.; Terzis,
A.; Manessi-Zoupa, E.; Perlepes, S. P. Z. Naturforsch. 2006, 61b, 37.

Article
Inorganic Chemistry, Vol. 49, No. 22, 2010 10491
Figure 1. Partially labeled plot of the Ni5 molecule of 2. Primed and
unprimed atoms are related by the crystallographic inversion center.
Color scheme: Ni, green; O, red; N, blue; C, gray.
Figure 3. {Ni5(μ-N3)2(μ3-N3)2}6þ core of 2 (top) and a more detailed
representation emphasizing its {Ni5(μ-N3)2(μ3-N3)2(μ-ONR00)4}2þ
(R00NO- = ppko-) description (bottom). The color scheme is as in
Figure 1. Atoms N2, O1, N4, and O2 and their symmetry equivalents
belong to the oximate groups of the ppko- ligands.
a,b
˚
Table 2. Selected Interatomic Distances (A) and Angles (deg) for Complex 1
Ni1 Ni2
3 3 3
3.391(1)
3.384 (1)
3.026(1)
6.782(1)
6.062(1)
6.768(1)
2.057 (2)
2.078 (2)
2.072(2)
2.062 (2)
2.125 (2)
2.031(2)
Ni2-N2
Ni2-N5
Ni2-N8
Ni3-O4
Ni3-O6
Ni3-N3
Ni3-N4
Ni3-N5
Ni3-N8
N2-O1
N4-O2
2.021(2)
2.085(2)
2.098(2)
2.042 (2)
2.125(2)
2.031(2)
2.022(2)
2.100(2)
2.098(2)
1.342(3)
1.338(3)
Ni1 Ni3
Ni2 Ni3
3 3 3
3 3 3
Ni2 Ni20
3 3 3
Ni2 Ni30
3 3 3
Ni3 Ni30
3 3 3
Ni1-O1
Ni1-O2
Ni1-N5
Ni2-O3
Ni2-O5
Ni2-N1
Figure 2. Ni5 skeleton of 2 showing its centered rectangular metal
topology.
The molecule of 2 consists of 5 NiII atoms held together by
two EO triply bridging N3- ions (N5 and its symmetry equi-
valent), two EO doubly bridging N3- ions (N8 and its sym-
metry equivalent), two syn,syn-η1:η1:μ-MeCO2- groups (O3/
O4 and its symmetry equivalent), and four η1:η1:η1:μ-ppko-
ligands (O1/N2 and O2/N4 and their symmetry equivalents).
Peripheral ligation is provided by four terminal MeOH
ligands on Ni2, Ni3, Ni20, and Ni30. The coordination modes
of the bridging ligands that are present in 1 and 2are shown in
Scheme 2. The five NiII atoms are disposed in a centered
parallelogram, which is planar by virtue of a crystallographic
inversion center in the middle (at Ni1). An alternative descrip-
tion of the metal topology is as two NiII triangles (Ni1-Ni2-
Ni3 and Ni1-Ni20-Ni30) with a common vertex (a bowtie).
Each triangle is nearly isosceles, with the Ni1 Ni2, Ni1
O1-Ni1-O2
O1-Ni1-N5
O2-Ni1-N50
O3-Ni2-N2
O5-Ni2-N8
N1-Ni2-N5
O4-Ni3-N4
92.2(1)
95.9 (1)
83.5(1)
176.3 (1)
172.9 (1)
173.1(1)
175.6 (1)
O6-Ni3-N8
N3-Ni3-N5
Ni1-N5-Ni2
Ni1-N5-Ni3
Ni2-N5-Ni3
Ni2-N8-Ni3
171.3(1)
174.0 (1)
109.3(1)
108.4(1)
92.6(1)
92.3(1)
a Symmetry code: 0 = -x, 1 - y, -z. b The numbering scheme of this
complex (Figure S1 in the Supporting Information) is identical with that
used for complex 2 (Figure 1).
central NiII atom, with the two peripheral metal ions on either
side of the molecule. The donor atom (N5) of this group lies
II
˚
0.871 A out of the plane of the three associated Ni atoms;
3 3 3
3 3 3
Ni3, and Ni2 Ni3 distances being 3.401(1), 3.372(1), and
˚
two of the angles subtended at N5 by the three NiII atoms are
significantly larger (109.2 and 108.0 o for the angles derived
from the central NiII atom) than the third angle (93.7ꢀ for the
angle to the peripheral NiII atoms). The core is {Ni5( μ-N3)2-
( μ3-N3)2}6þ (Figure 3, top). This core has not been observed
either in discrete Ni5 clusters or as a recognizable subfragment
of higher-nuclearity NiII clusters. If we consider all of the
3 3 3
3.054(1) A, respectively. Each diatomic oximate group
bridges between the central NiII atom and one peripheral
metalion. AbridgingMeCO2- ligand spans the base (Ni2
3 3 3
3 3 3
Ni3 and Ni20
Ni3 and Ni2
Ni30) of each isosceles triangle. The Ni2
0 3 3 3
Ni30 bases of the triangles are each bridged
3 3 3 -
-
by one EO μ-N3 group. The EO μ3-N3 group links the

10492 Inorganic Chemistry, Vol. 49, No. 22, 2010
Papatriantafyllopoulou et al.
˚
Table 3. Selected Interatomic Distances (A) and Angles (deg) for Complex
Table 4. Intramolecular Hydrogen Bonds for Complex 1a
2 0.25H2Oa
3
Ni1 Ni2
D
A
H
A
D-H
A
symmetry
operator of A
3 3 3
3 3 3
3 3 3
[deg]
3.401(1)
3.372(1)
3.054 (1)
6.803(1)
6.045 (1)
6.743(1)
2.067(3)
2.080(2)
2.078(3)
2.046(3)
2.116(3)
2.043(3)
Ni2-N2
Ni2-N5
Ni2-N8
Ni3-O4
Ni3-O6
Ni3-N3
Ni3-N4
Ni3-N5
Ni3-N8
N2-O1
N4-O2
2.036(3)
2.096(3)
2.112(3)
2.031(3)
2.129(3)
2.029(3)
2.024(3)
2.090(3)
2.122(3)
1.334(4)
1.331(4)
˚
[A]
˚
[A]
3 3 3
D-H
A
3 3 3
Ni1 Ni3
Ni2 Ni3
3 3 3
O5-H5 O2 2.785(3) 2.04(3)
161(3)
162(4)
-x, -y þ 1, -z
-x, -y þ 1, -z
3 3 3
3 3 3
Ni2 Ni20
O6-H6 O1 2.740(3) 1.94(4)
3 3 3
3 3 3
Ni2 Ni30
3 3 3
a A = acceptor atom; D = donor atom.
Ni3 Ni30
3 3 3
Ni1-O1
Ni1-O2
Ni1-N5
Ni2-O3
Ni2-O5
Ni2-N1
˚
hydrogen bonds and C-H N(azide) [C N = 3.51 A]
3 3 3
3 3 3
˚
and C-H O(acetate) [C O = ∼3.2 A] supramolecular
3 3 3
3 3 3
interactions,52 i.e., nonclassical hydrogen bonds. These
classical and nonclassical hydrogen bonds create a three-
dimensional (3D) network.
O1-Ni1-O2
O1-Ni1-N5
O2-Ni1-N50
O3-Ni2-N2
O5-Ni2-N8
N1-Ni2-N5
O4-Ni3-N4
93.1 (1)
96.1(1)
83.3(1)
174.7(1)
168.8(1)
173.2(1)
173.6(1)
O6-Ni3-N8
N3-Ni3-N5
Ni1-N5-Ni2
Ni1-N5-Ni3
Ni2-N5-Ni3
Ni2-N8-Ni3
170.6(1)
175.0(1)
109.2(1)
108.1(1)
93.7(1)
The structure of the pentanuclear molecule of 1 is very
similar to that of 2, with the essential difference being the
replacement of the acetate ligands in the latter by for-
mate groups in the former. Each coordinated MeOH
hydroxyl group is intramolecularly hydrogen-bonded to
a coordinated oximate O atom. The dimensions of the
two crystallographically unique hydrogen bonds are
listed in Table 4. The pentanuclear molecules of 1 form
92.3(1)
a Symmetry code: 0 = 1/2 - x, 1/2 - y, -z.
Scheme 2
chains through C14 (a 2-pyridyl C atom)-H14 N8
3 3 3
(the donor atom of μ-N3-) weak supramolecular inter-
actions, with the C14 N8 and N8 H14 distances
3 3 3
3 3 3
˚
being 3.553 and 2.658 A, respectively; see Figure S2 in
the Supporting Information. The chains form 3D net-
works through weak C-H O(formate) interactions
3 3 3
˚
(C-H O = ∼3.5 A).
3 3 3
The bond distances around the NiII atoms in 1 and 2 are
typical of those found in octahedrally coordinated nickel-
(II) complexes with O and N ligation.20a,31-36,37b,37h,37i
Complexes 1 and 2 join a rather large family of NiII
5
clusters with N and/or O donors possessing the metal ions in
various topologies, such as linear,54 helical,55 tetrahedral-
centered on a fifth NiII atom,56 tetrahedral with the fifth NiII
bridging oximate groups as part of the core, then it becomes
{Ni5( μ-N3)2(μ3-N3)2( μ-ONR00)4}2þ (R00NO- = ppko-); see
Figure 3, bottom.
Ni2 and Ni3 are bound to an O2N4 set of donor atoms,
while Ni1 forms a NiO4N2 chromophore. Thus, all of the
NiII atoms are six-coordinate with distorted octahedral
geometry, with the main distortion arising from the rela-
tively small bite angle of the chelating parts of the ppko-
atom lying at the midpoint of one Ni Ni edge,37h defective
3 3 3
corner-shared dicubane,57 capped triple helical,58 closed
cagelike,37j triple-stranded helical,59 pentagonal,60 12-metal-
lacrown-4 with an encapsulated NiII ion,61 capsule-shaped
with five coplanar NiII atoms,62 well-separated Ni2 and Ni3
groups. The Ni Ni separations in the pentanuclear
3 3 3
(53) (a) For a comprehensive review concerning C-H O hydrogen
molecule reflect the nature of the bridging groups connect-
ing each pair of NiII atoms. The distance between the
central nickel and each outer nickel remains roughly
3 3 3
bonding in crystals, see: Steiner, T. Cryst. Rev. 1996, 6, 1. (b) For a critical
evaluation of C-H X (X = F, Cl, Br, I, O, S, and N) hydrogen bonding, see:
3 3 3
Van der Berg, J.-A.; Seddon, K. R. Cryst. Growth Des. 2003, 3, 643.
(54) (a) Wang, C.-C.; Lo, W.-C.; Chou, C.-C.; Lee, G.-H.; Chen, J. M.; Peng,
S.-M. Inorg. Chem. 1998, 37, 4059. (b) Berry, J. F.; Cotton, F. A.; Lei, P.; Lu, T.;
Murillo, C. A. Inorg. Chem. 2003, 42, 3534. (c) Nockemann, P.; Thijs, B.; Van Hecke,
K.; Van Meervelt, L.; Binnemans, K. Cryst. Growth Des. 2008, 8, 1353. (d) Yeh, C.-
Y.; Chiang, Y.-L.; Lee, G.-H.; Peng, S.-M. Inorg. Chem. 2002, 41, 4096.
(55) Boudalis, A. K.; Pissas, M.; Raptopoulou, C. P.; Psycharis, V.;
Abarca, B.; Ballesteros, R. Inorg. Chem. 2008, 47, 10674.
˚
constant at ∼3.4 A, with these being in each case enclosed
in a five-membered ring. Such NiII atoms are separated by
one monatomic bridge ( μ3-N3-) and one diatomic bridge
(an oximate group). Ni2 and Ni3 (and their symmetry
equivalents), separated by one triatomic (OCO) and two
monatomic azide bridges, have the shortest separation
(56) (a) Bai, Y.-L.;Too, J.; Huang, R.-B.; Zheng, L.-S. Angew. Chem., Int. Ed.
2008, 47, 5344. (b) Adams, H.; Clunas, S.; Fenton, D. E.; Towers, D. N. J. Chem. Soc.,
Dalton Trans. 2002, 3933. (c) Tangoulis, V.; Raptopoulou, C. P.; Terzis, A.;
Bakalbassis, E. G.; Diamantopoulou, E.; Perlepes, S. P. Inorg. Chem. 1998, 37, 3142.
[3.054(1) A]. The μ-N3- atom N8 bridges Ni2 and Ni3 in
˚
a perfectly symmetric manner; thus, the Ni2-N8 and
Ni3-N8 bond distances are equal [2.112(3) and 2.122(3)
A]. The μ3-N3 atom N5 bridges the three NiII atoms
-
€
(57) Paine, T. K.; Rentschler, E.; Weyhermuller, T.; Chaudhuri, P. Eur. J.
˚
Inorg. Chem. 2003, 3167.
almost symmetrically; thus, the Ni(1,2,3)-N5 bond dis-
(58) Feng, S.; Zhu, M.; Lu, L.; Guo, M. Chem. Commun. 2007, 4785.
(59) Hou, J.-Z.; Li, M.; Li, Z.; Zhan, S.-Z.; Huang, X.-C.; Li, D. Angew.
Chem., Int. Ed. 2008, 47, 1711.
˚
tances are in the narrow 2.078(3)-2.096(3) A range. The
larger Ni-μ3-N3--Ni angles [109.2(1) and 108.0(1)ꢀ]
correspond to the pairs Ni1/Ni2 and Ni1/Ni3 in which
the NiII atoms are separated by one monatomic bridge and
one diatomic bridge, whereas the smaller one [93.7(1)ꢀ]
corresponds to the triply bridged metal ions (Ni2/Ni3).
(60) Campos-Fernandez, C. S.; Schoffel, B. L.; Chifotides, H. T.; Bera,
J. K.; Bacsa, J.; Kooman, J. M.; Russell, D. H.; Dunbar, K. R. J. Am. Chem.
Soc. 2006, 127, 12909.
(61) Psomas, G.; Stemmler, A. J.; Dendrinou-Samara, C.; Bodwin, J. J.;
Schneider, M.; Alexiou, M.; Kampf, J. W.; Kessissoglou, D. P.; Pecoraro,
V. L. Inorg. Chem. 2001, 40, 1562.
The crystal structure of 2 0.25H2O is stabilized by
˚
(62) Broring, M.; Prikhodovski, S.; Brandt, C. D.; Tejero, E. C. Chem.;
Eur. J. 2007, 13, 396.
3
O(MeOH) O(lattice H2O) [O O = ∼2.9 and 3.0 A]
3 3 3
3 3 3

Article
Inorganic Chemistry, Vol. 49, No. 22, 2010 10493
Scheme 3
the range 300-100 K, before increasing more rapidly
below this temperature to maximum values of 13.18 cm3
K mol-1 at 3.7 K for 1 and 14.96 cm3 K mol-1 at 4.5 K for
2. The product sharply decreases at lower temperatures,
reaching the values of 12.38 (for 1) and 14.41 (for 2) cm3
mol-1 K at 2.0 K. This behavior indicates an overall
ferromagnetic behavior, and the χMT values at the max-
ima suggest a ground-state S=5 value. The decrease in
χMT below ∼4 K is assigned to either zero-field splitting
within the ground state, Zeeman effects, and/or weak
antiferromagnetic intermolecular interactions. Magneti-
zation data collected at 2.0 K show a continuous increase
of M as the field increases, reaching a nonsaturated
value close to 8 in M/Nβ units under an external field of
5 T.
To fit and interpret the magnetic susceptibility data of 1
and 2, first it is necessary to find all possible magnetic
pathways in the bowtie structural type. Close inspection
of the very similar and symmetric molecular structures
(Figures 1 and S1 in the Supporting Information) reveals
that there are only two different exchange pathways and
thus two different J coupling constants (from now on-
ward, the numbering scheme that will be used is the same
as that used in the real structures). J1 is associated with the
four interactions between the central Ni1 ion and the four
peripheral NiII centers through one EO triply bridging
N3- ion and one bridging diatomic oximate group from a
ppko- ligand; J2 is associated with the two equal
Figure 4. PlotsofχMTvsT forcomplexes1(dottedcircles) and2(dotted
squares). The solid line is the fit of the data; see the text for the fit
parameters.
subunits,63 trigonal-bipyramidal,64 and rectangular-centered
on a fifth NiIIatom.65 Compounds 1 and 2 belong to the last
subfamily of NiII clusters. They are only the second
5
and third structurally characterized NiII5 complexes with
bridging azido ligands; the first one is complex [Ni5
{pyCOpyC(O)(OMe)py}2(O2CMe)4(N3)4(MeOH)2],55 in
which the ligand is the monoanion of the keto-hemiketal
form of di-2,6-(2-pyridylcarbonyl)pyridine. Complexes 1
and 2 are also members of a small group of nonorgano-
metallic polynuclear metal complexes of ppko-, which
currently comprises NiII,31-36 MnII/IV 47c
CuII,67 CoII/III 68 and ZnII.69
,
MnIII,66a MnII/III 66b
,
,
Magnetochemistry. Variable-temperature direct-current
(dc) magnetic susceptibility studies were performed on pow-
dered samples of compounds 1and 2in applied fields of 0.3 T
(300-30 K) and 0.03 T (30-2.0 K). The data for 1 and 2 are
plotted as χMT versus T in Figure 4.
The two complexes exhibit a similar behavior. The
room temperature χMT values are 6.69 (for 1) and 6.53
(for 2) cm3 K mol-1, slightly higher than the spin-only
value of 6.05 cm3 K mol-1 with g = 2.2 expected for a
cluster of five noninteracting S = 1 NiII atoms. The χMT
product increases slightly with decreasing temperature in
Ni2 Ni3 and Ni20
a
scheme. The spin Hamiltonian for such a system, illu-
strated in Scheme 3, is given by eq 3.
Ni30 interactions, each through
3 3 3
3 3 3
triply ( μ3-N3-)/( μ-N3-)/(η1:η1:μ-O2CR) bridging
(63) (a) Xu, Z.; Thompson, L. K.; Milway, V. A.; Zhao, L.; Kelly, T.;
Miller, D. O. Inorg. Chem. 2003, 42, 2950. (b) Fondo, M.; Garcia-Delbe, A. M.;
Ocampo, N.; Sanmartin, J.; Bermejo, M. R. Dalton Trans. 2007, 414.
(64) Zhou, Y.-L.; Meng, F.-Y.; Zhang, J.; Zeng, M.-J.; Liang, F. Cryst.
Growth Des. 2009, 9, 1402.
(65) (a) Wei, Y.; Hou, H.; Fan, Y.; Zhu, Y. Eur. J. Inorg. Chem. 2004,
3946. (b) Aromi, G.; Bell, A. R.; Helliwell, M.; Raftery, J.; Teat, S. J.; Timco,
G. A.; Roubeau, O.; Winpenny, R. E. P. Chem.;Eur. J. 2003, 9, 3024. (c)
Malkov, A. E.; Fomina, I. G.; Sidorov, A. A.; Aleksandrov, G. G.; Egorov, I. M.;
Latosh, N. I.; Chupakhin, O. N.; Rusinov, G. L.; Rakitin, Yu. Y.; Novotortsev,
V. M.; Ikorskii, V. N.; Eremenko, I. L.; Moiseev, I. I. J. Mol. Struct. 2003, 656,
207. (d) Finney, A. J.; Hitchman, M. A.; Raston, C. L.; Rowbottom, G. L.; White,
A. H. Aust. J. Chem. 1981, 34, 3139.
(66) (a) Stamatatos, T. C.; Foguet-Albiol, D.; Stoumpos, C. C.;
Raptopoulou, C. P.; Terzis, A.; Wernsdorfer, W.; Perlepes, S. P.; Christou,
G. Polyhedron 2007, 26, 2165. (b) Milios, C. J.; Kefalloniti, E.; Raptopoulou,
C. P.; Terzis, A.; Vicente, R.; Lalioti, N.; Escuer, A.; Perlepes, S. P. Chem.
Commun. 2003, 819.
(67) (a) Afrati, T.; Dendrinou-Samara, C.; Raptopoulou, C. P.; Terzis,
A.; Tangoulis, V.; Kessissoglou, D. P. Dalton Trans. 2007, 5156.
(68) Stamatatos, T. C.; Bell, A.; Cooper, P.; Terzis, A.; Raptopoulou,
C. P.; Heath, S. L.; Winpenny, R. E. P.; Perlepes, S. P. Inorg. Chem.
Commun. 2005, 8, 533.
^
^
^
^
^
^
^
^
0
0
H ¼ - J1ðS1 S2 þ S1 S3 þ S1 S2 þ S1 S3 Þ
3
3
^
3
3
^
^
^
0
0
- J2ðS2 S3 þ S2 S3 Þ
ð3Þ
3
3
The fitting of the experimental data was performed by
means of the analytical expression derived from the spin
Hamiltonian of eq 3, with the addition of an overall Weiss
parameter (T - θ) in order to cover the whole tempera-
ture range studied (including the low-temperature de-
crease of χMT). The best-fit parameters are J1 =2.58(2)
cm-1, J2 = 11.7(3) cm-1, g = 2.247 (3), and θ = 0.93(1)
K for 1 and J1 = 4.93(7) cm-1, J2 = 14.6(5) cm-1, g =
2.188(5), and θ = 0.52(1) K for 2; the agreement factor,
P
P
defined as R = [(χM)calc - (χM)obs]2/ [(χM)obs]2, is
1.1 ꢀ 10-4 and 8.1 ꢀ 10-5 for 1 and 2, respectively.
A fragment containing one EO azido group and one
diatomic oximate group (J1 in our case) has been reported
in Na2Ni12,37h Ni14,37h and Ni737j clusters, but a reliable
fit was performed only in the case of the Ni7 compound.
In spite of the fact that the structural parameters are not
(69) Martinez, J.; Aiello, K. I.; Bellusci, A.; Crispini, A.; Ghedini, M.
Inorg. Chim. Acta 2008, 361, 2677.

10494 Inorganic Chemistry, Vol. 49, No. 22, 2010
Papatriantafyllopoulou et al.
fully comparable (the azido bridge in the fragment of the
Ni7 compound is μ instead of μ3, and the Ni-N-Ni angle
is larger than those in 1 and 2), the interaction was reported
also to be ferromagnetic, as occurs for J1 in the present
complexes. In contrast, there is more information in the
literature concerning bis(azido)(carboxylato) bridges be-
tween two NiII centers;24e,70 the exchange interaction was
found to be ferromagnetic in all cases. Perhaps the best com-
parison can be made with complex [Ni3(O2CMe)3(N3)3-
(py)5] (py = pyridine),70e which presents the same ( μ-N3)-
( μ3-N3)(η1:η1:μ-O2CR0) bridging unit (with very similar
structural parameters) as those found in 1 and 2; a J value
of þ15.7 cm-1 was reported to be comparable to the J2 value
in the present Ni5 complexes.
For a system of five S = 1 centers that are in a bowtie
topology and interact according to the 2J model of eq 3,
the ground-state S values 1, 3, and 5 are possible depend-
ing on the J2/J1 ratio. For positive J1 and J2 values, i.e. for
a fully ferromagnetic system, the ground state is S = 5
and this is clearly isolated from other spin levels.
Figure 5. Out-of-phase (χM00) vs T ac susceptibility signals for 1 in a 4 G
field oscillating at the indicated frequencies.
Alternating current (ac) studies were performed in the
1.8-5 K range using a 4 G ac field oscillating at frequencies
(ν) in the 5-1440 Hz range. If the magnetization vector can
relax fast enough to keep up with the oscillating field, then
there is no imaginary (out-of-phase) susceptibility signal
( χM00). However, if the barrier to magnetization relaxation
is significant compared to the thermal energy (kT ), then
00
there is a nonzero χM00; in addition, χM will be frequency-
00
dependent. Such frequency-dependent χM signals are a
characteristic signature of the superparamagnetic-like prop-
erties of a SMM but, by themselves, do not prove the SMM
behavior.71
The ac magnetic susceptibility behaviors of 1 and 2
are different. For complex 1, there is an appearance of
00
frequency-dependent χM signals below ∼3.5 K (Figure 5);
only the tails of the peak are visible above 1.8 K (the
operating limit of our SQUID magnetometer), with the peak
00
maxima lying at lower temperatures. The χM signals in-
dicate a possible SMM behavior for 1. Complex 2 does
not exhibit an out-of-phase ac magnetic susceptibility signal
down to 1.8 K.
Hysteresis Studies below 1.8 K for Complex 1. In order
to confirm whether complex 1 is a new SMM, we performed
magnetization versus applied dc-field studies on single crys-
tals of 1 at temperatures down to 0.04 K using a micro-
SQUID apparatus.46 The observation of hysteresis loops in
such studies represents the diagnostic property of a magnet,
including SMMs and superparamagnets below their block-
ing temperature (TB). The observed magnetization responses
for 1 are shown in Figure 6 (top) at a fixed dc-field sweep rate
of 0.004 T s-1 and at temperatures in the range of 0.04-1.0 K
and in Figure 6 (bottom) at 0.04 K and in the sweep-rate
Figure 6. Magnetization (M) vs dc applied magnetic field (H) hysteresis
loops for a single crystal of 1 at the indicated temperatures and a fixed-
field sweep rate of 0.004 T s-1 (top) and at the indicated field sweep rates
and a fixed temperature of 0.04 K (bottom). The magnetization is
normalized to its saturation value, MS.
(70) (a) Demeshko, S.; Leibeling, G.; Maringgele, W.; Meyer, F.;
Mennerich, C.; Klauss, H. H.; Pritskow, H. Inorg. Chem. 2005, 44, 519. (b)
Meyer, F.; Demeshko, S.; Leibeling, G.; Kersting, B.; Kaifer, E.; Pritskow, H.
Chem.;Eur. J. 2005, 11, 1518. (c) Wang, X. T.; Wang, X. H.; Wang, Z. M.; Gao,
S. Inorg. Chem. 2009, 48, 1301. (d) Wang, Q. L.; Yu, L. H.; Liao, D. Z.; Yan, S. P.;
Jiang, Z. H.; Cheng, P. Helv. Chim. Acta 2003, 86, 2441. (e) Milios, C. J.;
Prescimone, A.; Sanchez-Benitez, J.; Parsons, S.; Murrie, M.; Brechin, E. K.
Inorg. Chem. 2006, 45, 7053.
(71) (a) Stamatatos, Th. C.; Nastopoulos, V.; Tasiopoulos, A. J.; Moushi,
E. E.; Wernsdorfer, W.; Christou, G.; Perlepes, S. P. Inorg. Chem. 2008, 47,
10081. (b) Mishra, A.; Tasiopoulos, A. J.; Wernsdorfer, W.; Abboud, K. A.;
Christou, G. Inorg. Chem. 2007, 46, 3105.
range of 0.002-0.140 T s-1. Some minor hysteresis was ob-
served but with a very narrow coercivity that did not change
noticeably with either decreasing temperature or increasing
scan rate in the 0.04-1.0 K and 0.002-0.140 T s-1 ranges.
This is clearly not the superparamagnet-like behav-
ior expected of an SMM.71b However, the tiny two-step

Article
Inorganic Chemistry, Vol. 49, No. 22, 2010 10495
g factor, μB is the Bohr magneton, and μ0 is the vacuum
permeability.75 The calculation gives J = -0.021 K, and
from this can be calculated the exchange interaction energy
-2JS2 of 1.05 K. The interaction is thus antiferromagnetic,
as expected, and stronger compared to previous exam-
ples, for example, in complex [MnIII8CeIVO8(O2CMe)12-
(H2O)4].74 If the antiferromagnetic intermolecular inter-
actions are weak, as in the MnIII8CeIV cluster, then they
are merely a perturbation of the SMM properties, and
such compounds can be described as exchanged-biased
SMMs.74 If they are not as weak as they seem to be for 1,
then the crystal may be better described as containing
antiferromagnetically ordered 1D, two-dimensional
(2D), or 3D extended networks.74 We, therefore, conclude
that any real hysteresis is only evident at 0.04 K and that this
is most probably not due to single-molecule properties but to
intermolecular interactions.
Figure 7. Expansion of the þ0.15 to -0.15 T field range in the magne-
tization (M) vs dc applied magnetic field (H) hysteresis loop for a single
crystal of 1 at 0.04 K and 0.004 T s-1 showing the exchange-biased
measurement position. M is normalized to its saturation value, MS.
Conclusions and Perspectives
The initial use of the R0CO2-/N3-/ppko- ligand combina-
tion in NiII cluster chemistry has provided access to two new
NiII5 complexes, 1 and 2, with unusual structures contain-
ing the metal ions in a bowtie topology and not seen before
in the coordination chemistry of 2-pyridyloximes. Both
complexes have a ground-state spin of S = 5. Ac suscep-
tibility studies on the formate complex 1 display the ap-
pearance of frequency-dependent out-of-phase signals.
However, magnetization vs dc field sweeps down to 0.04
K revealed true (but very slight) hysteresis only at 0.04 K,
which we assign to the intermolecular nonclassical hydro-
gen bonding seen in the crystal structure. We believe that
there is an absence of significant single-molecule barrier
(vs kT) to magnetization relaxation, and most probably 1 is
hysteresis at 0.04 K appears to be real. An interesting feature
in the loops is the fact that, upon sweeping of the field back
from either extreme toward zero field, the magnetization
begins to decrease before reaching zero field. This shift from
zero field is indicative of an exchange bias8c,d from neighbor-
ing molecules, i.e., the influence of intermolecular antiferro-
magnetic exchange interactions on the magnetization
relaxation dynamics of a molecule. For a single crystal of a
SMM that has negligible intermolecular exchange interac-
tions, the first step in the hysteresis loop due to quantum
tunneling of magnetization occurs at zero field when the
external field is oriented parallel to the easy axis of the crystal.
The observed behavior of 1 is consistent with the observation
in its crystal structure of one-dimensional (1D) chains of Ni5
00
not an SMM. The clear observation of χM signals below
3.5 K is thus unusual. The tiny barrier responsible for the
molecules linked by nonclassical C-H N hydrogen
3 3 3
bonds,53b with the chains further interacting through non-
classical C-H O hydrogen bonds,53a and there are likely
almost insignificant hysteresis at 0.04 K is not expected to
3 3 3
00
give χM signals at higher temperatures up to 3.5 K.
also some contributions from intermolecular dipolar interac-
tions between the relatively large ground state S = 5 spins.
Similar behavior has been seen in Fe19,72 Fe9,73 Mn8Ce,74 and
Ni4 SMMs,24g which also exhibit intermolecular interactions.
As reported earlier, for both isolated dimers of inter-
acting molecules7e,d and also 3D networks of interacting
molecules,75 the strength of the intermolecular exchange
coupling constant J can be calculated from the field shift
that it causes in the hysteresis loops. Closer inspection of the
hysteresis loop at 0.04 K (Figure 7) indicates an exchange
bias of 0.07 T (70 mT) on either side of the zero field. From
this can be calculated J, by using the expression in eq 4,
Typically, complexes that exhibit significant tails of ac
00
χM signals at >1.8 K then reveal clear hysteresis in dc
magnetization sweeps assignable to an SMM with TB of at
least ∼0.6 K or more.71b Thus, we cannot offer an unequiv-
ocal explanation of the origin of the χM00 signals, except to
state that it should be due to a combination of factors such
as intermolecular interactions, phonon bottlenecks, and
perhaps other contributions.71b
As far as future perspectives are concerned, analogues of 1
and 2 with 2-pyridinealdoxime (R = H in Scheme 1) are not
known to date, and it is currently not evident whether the
synthesis and stability of such NiII5 clusters are dependent on
the particular nature of the R substituent on the oxime C
^ ^
appropriate for the H = -2JSi Sj convention
3
J ¼ - gμBμ0Hex=kBS
ð4Þ
atom. NiII/ppko-/N3 chemistry, i.e., omission of carboxy-
-
lates from the reaction system studied in this work, is also
completely unexplored, and work in progress has just un-
earthed the interesting new bowtie cluster [Ni5(N3)4(ppko)4-
(ppkoH)4](ClO4)2, in which four EO doubly bridging N3- ions
and two μ3 oximate groups provide the bridges between the
central and external NiII atoms.76 We are also pursuing sub-
stitution of the bridging carboxylate and terminal MeOH
ligands of 1 and 2 by bulkier ligands of similar chemical nature
where Hex is the exchange-biased field (i.e., the shift from
zero field), kB is the Boltzmann constant, g is the electronic
(72) Goodwin, J. C.; Sessoli, R.; Gatteschi, D.; Wernsdorfer, W.; Powell,
A. K.; Heath, S. L. J. Chem. Soc., Dalton Trans. 2000, 1835.
(73) Bagai, R.; Wernsdorfer, W.; Abboud, K. A.; Christou, G. J. Am.
Chem. Soc. 2007, 129, 12918.
(74) Mishra, A.; Tasiopoulos, A. J.; Wernsdorfer, W.; Moushi, E.;
Moulton, B.; Zawarotko, M. J.; Abboud, K. A.; Christou, G. Inorg. Chem.
2008, 47, 4832.
in an effort to magnetically insulate the NiII molecules; such
5
(75) Tiron, R.; Wernsdorfer, W.; Aliaga-Alcalde, N.; Christou, G. Phys.
Rev. B 2003, 68(1-4), 140407.
(76) Escuer, A.; Perlepes, S. P., unpublished results.

10496 Inorganic Chemistry, Vol. 49, No. 22, 2010
Papatriantafyllopoulou et al.
studies give valuable information24g on the dynamic magnetic
properties of structurally similar complexes.
also acknowledge a provision of time at the Advanced Light
Source synchrotron, which is supported by the Director,
Office of Basic Energy Sciences of the U.S. Department of
Energy under Contract DE-AC02-05CH11231.
Acknowledgment. We thank one of the reviewers for
helpful suggestions concerning the possible formation of
NiO in the preparation of complex 1. Financial support from
the Cyprus Research Promotion Foundation (Grant TECH-
NO/0506/06 to A.J.T.), CICYT Projects (Grant CTQ2009-
07264 and ICREA-Academia Award to A.E.), the Operatio-
nal and Vocational Training II Program (PYTHAGORAS;
Grant b.365.037 to S.P.P.) is gratefully acknowledged. We
Supporting Information Available: Crystallographic data for
complexes 1 and 2 0.5H2O in CIF format, partially labeled plot
3
of the Ni5 molecule of 1 (Figure S1), a portion of the chain
formed by the pentanuclear molecules of 1 (Figure S2), and the
analytical expression used for the fits of the magnetic data. This
material is available free of charge via the Internet at http://
pubs.acs.org.