A molecular chain of four CoII ions stabilized by a tris-pyridyl/Bis - βdiketonate ligand

Article abstract of DOI:10.1071/CH09183

The synthesis and characterization of a tris-pyridyl/bis - diketone molecule (H₂L) is reported. This compound acts as a hexadentate ligand towards Co^II to facilitate the assembly of a tetranuclear molecular chain of closely spaced metals with formula [Co₄L ₂(MeOH)₈](NO₃)₄ (1), which exhibits a very flat [Co₄L₂]⁴⁺ platform, as determined by single-crystal X-ray diffraction crystallography. Complex 1 readily exchanges axial methanol ligands with water molecules. The bulk magnetization of the resulting hydrate, 1a, shows that the metals in the [Co₄L ₂]⁴⁺ moiety exhibit spin-orbit coupling and antiferromagnetic exchange interactions. CSIRO 2009.

Full text of DOI:10.1071/CH09183

RESEARCH FRONT
Full Paper
CSIRO PUBLISHING
Aust. J. Chem. 2009, 62, 1130–1136
www.publish.csiro.au/journals/ajc
II
A Molecular Chain of Four Co Ions Stabilized
by a Tris-Pyridyl/Bis-β-Diketonate Ligand
A
A
B
C
Leoní A. Barrios, David Aguilà, Olivier Roubeau, Keith S. Murray,
A,D
and Guillem Aromí
ADepartament de Química Inorgànica, Universitat de Barcelona, Diagonal 647,
0
8028 Barcelona, Spain.
B
Instituto de Ciencia de Materiales de Aragón, CSIC and Universidad de Zaragoza,
Plaza San Francisco s/n, 50009 Zaragoza, Spain.
C
School of Chemistry, Building 23, Monash University, Clayton, Vic. 3800, Australia.
Corresponding author. Email: guillem.aromi@qi.ub.es
D
The synthesis and characterization of a tris-pyridyl/bis-β-diketone molecule (H2L) is reported. This compound acts as a
II
hexadentate ligand towards Co to facilitate the assembly of a tetranuclear molecular chain of closely spaced metals with
4+
formula [Co4L2(MeOH)8](NO3)4 (1), which exhibits a very flat [Co4L2] platform, as determined by single-crystal
X-ray diffraction crystallography. Complex 1 readily exchanges axial methanol ligands with water molecules. The bulk
magnetization of the resulting hydrate, 1a, shows that the metals in the [Co4L2]4 moiety exhibit spin-orbit coupling and
+
antiferromagnetic exchange interactions.
Manuscript received: 1 April 2009.
Manuscript accepted: 25 April 2009.
Introduction
have been prepared including the following: (i) trinuclear com-
II
II
pounds [M3(HL1)3] (M = Mn , Co ) described as ‘asymmetric
The ‘bottom-up’ approach to nanotechnology aims at the fab-
rication of devices built-up from molecular components. The
main goal is to exploit the functionalities that ensue from the
molecular properties by individually addressing or influencing
such components. In this context, ‘supramolecular magnetic
materials’will play a fundamental role; they offer the prospect of
triple stranded helicates’(H3L1; a phenol/bis-β-diketone);[
14,15]
(ii) homo- and heterovalent tetranuclear or trinuclear strings of
manganese (in the oxidation states +2 and +3) with formulae
[
1]
[Mn4(H2L2)2(AcO)2(dmf)4] and [Mn3(HL2)2(py)6], respec-
[
16]
tively (H5L2; a tris-phenol/bis-β-diketone);
Mn
(iii) a trinuclear
II/III
ꢀ
[
2,3]
cluster [Mn3(HL1 )2(AcO)3(MeOH)3] that exhibits
superparamagnetic behaviour at low temperatures (H3L1 ; a
storing information in single molecules,
and some systems
ꢀ
from this area have also been proposed as possible molecular
[17]
hardware for the implementation of quantum computing.[
4–6]
phenol/bis-pyrazole related to H3L1);
or (iv) a tetranuclear
A
II
chain of Ni ions in the form of a pair of two aligned dimers,
major endeavour is thus the design and synthesis of molecules
with predetermined functions and physicochemical properties.
Our research group is addressing this challenge through the
preparation of polydentate ligands capable of gathering tran-
18]
[
Ni4(L3)2(py)6] (H4L3; a bis-phenol/bis-β-diketone).[ Poly-
β-diketone ligands have also been employed by other groups in
the designed preparation of many other fascinating supramolec-
ular arrangements.[
7,19,20]
sition metals within molecular clusters with a predetermined
nuclearity and topology.[7] This strategy has been adopted suc-
Following on from these efforts, we have finalized the
preparation of the tris-pyridyl/bis-β-diketone ligand H2L (1,3-
bis-(3-oxo-3-(2-pyridyl)propionyl)pyridine), shown in Fig. 1, in
order to access other molecular strings of aligned paramag-
netic ions. Pyridyl/bis-β-diketone donors that contain the central
part of H2L have been employed successfully for the prepara-
tion of linear heterometallic trinuclear complexes that involve
cessfully by other groups, for example, to deliberately obtain
[8]
ferromagnetically exchanged clusters, or to prepare magneti-
cally active molecules in the form of grids.[
9,10]
Our focus has
been two-fold. On the one hand, some of the ligands designed
in our laboratory have proven appropriate for the preparation of
stable molecules that contain two semi-independent and well-
[21]
defined magnetic clusters.[
11]
3d ions together with lanthanides
or alkaline/earth-alkaline
which suggests that H2L could favour the forma-
tion of longer heterometallic molecular chains. This organic
Such entities are to be exploited
metals,[
22,23]
as prototypes of 2-qbit quantum gates for the implementation of
quantum computing.
[
4–6]
However, efforts are directed at prepar-
ing complexes in the form of linear chains of paramagnetic
ions. This motif is quite uncommon in polynuclear coordination
chemistry and such types of ‘molecular wires’ constitute inter-
esting objects for the study of magnetic anisotropy or for their
potential applications in quantum electronics.[12,13] As a result
of this work, several polynuclear complexes with this topology
O
O
O
O
N
N
N
Fig. 1. Ligand H2L.
©
CSIRO 2009
10.1071/CH09183
0004-9425/09/091130

RESEARCH FRONT
Molecular Chain of Four CoII Ions Stabilized by a Tris-Pyridyl/Bis-β-Diketonate Ligand
1131
molecule has been briefly described as an intermediate during
the synthesis of the corresponding bis-pyrimidinyl derivative.[
We show here a modified synthesis of H2L and its full char-
acterization. The use of this ligand in coordination chemistry
has provided access to a novel chain-like molecular cluster of
four Co ions with formula [Co4L2(MeOH)8](NO3)4 (1). The
structure of this complex, together with the bulk magnetization
properties of its octa-aqua equivalent, are also reported.
axial methanol ligands by water molecules, which transforms
it into the octa-aqua species [Co4L2(H2O)8](NO3)4 (1a). Such
exchange of methanol ligands by atmospheric water molecules is
very common and has been widely observed before.[ Diffuse
solvent areas in the structure of 1 (see crystallographic section)
also help to explain the lability of the axially coordinated lig-
24]
27]
II
ands. This lability offers the possibility of using the [Co4L2]4
+
core as a building block in the construction of more compli-
cated assemblies based on this moiety. Very encouraging results
have been achieved already using this approach, with the prepa-
Results and Discussion
ration of a pair of [Co4L2]4 units, linked together by difunc-
+
Synthesis and Reactivity
The ligand H2L was prepared as previously reported[24] from
II
tional ligands through the axial positions of the Co ions with
ꢀ
ꢀ
formula [Co8L4(NO3)2(4,4 -byp)4(OH)2(H2O)4](NO3)4 (4,4 -
the Claisen condensation between ethyl dipicolinate and two
equivalents of 2-acetylpyridine. We carried out this synthe-
sis in dimethoxyethane (DME) using NaH as a base, rather than
in toluene and with NaOEt. One difficulty often encountered
in the characterization of β-diketones is that they often exhibit
various tautomers in solution, as a result of the keto–enolic
ꢀ
byp = 4,4, bipyridine) (L. A. Barrios, D. Aguilà, O. Roubeau,
S. J. Teat, G. Aromí, unpubl. data). More detailed stud-
ies showed that complex 1a could contain additional water
molecules from air moisture if the methanolic precursor 1 was
obtained as an amorphous powder (by rapid precipitation from
the reaction mixture) rather than crystals. Amounts of up to
10 hydration water molecules per cluster were determined by
microanalysis.
[
25]
transformation. In chloroform, the distribution of H2L into its
1
different isomers was found by H NMR spectroscopy to be vari-
able depending on the history of the sample, which shows that the
equilibrium in this medium is not reached at room temperature.
This added confusion to the interpretation of the spectrum and
created doubts as to the purity of the product (Fig. S1). A sample
of H2L was transformed into the corresponding Na2L salt and
Thermogravimetic Analysis (TGA) Experiments
The process of dehydration/re-hydration of 1a was studied by
TGA experiments. These studies showed that one sample of 1a
obtained from amorphous 1 contained six molecules of hydra-
tion, which, together with the eight water ligands, amount to
1
the H NMR spectrum of it exhibited a unique set of signals,
as expected for a pure product, now without the complicating
consequences of the keto–enol equilibrium (Fig. S2).
Initial reactions with the ligand H2L were aimed at the prepa-
ration of heterometallic chain-like molecular clusters, involving
◦
17% of the sample weight, which is lost at 200 C (Fig. S3).
◦
Another part of the same sample was maintained at 50 C for 2 h
−
2
under a 2 × 10 bar vacuum and exposed anew to the atmo-
sphere for half an hour. The TGA analysis after this showed that
it re-absorbed approximately the amount of water necessary to
3
d and 4f ions simultaneously, in an attempt to benefit from the
different types and sizes of coordination pockets present in this
multidentate donor (see Fig. 1). Thus, the ligand was reacted
in hot methanol with Co(AcO)2 and Ln(NO3)3 (Ln =Tb, Gd,
Dy, Er, etc.) to produce orange solutions that were then layered
with Et2O. Small red-orange crystals were obtained from all
the reactions, which in some cases allowed for the resolution
of the molecular structure of the corresponding products by use
of synchrotron X-ray diffraction. Invariably, the identity of the
product turned out to be the same, i.e., the tetranuclear com-
plex [Co4L2(MeOH)8](NO3)4 (1), which did not include any
lanthanide ion. IR spectroscopy strongly suggested that com-
plex 1 was also the outcome from reactions where the structure
was not determined. The reasons why the lanthanide ions do
not appear in the products are currently being investigated. It is
possible that these stay in solution in the form of soluble com-
saturate the eight coordination positions available per [Co ] clus-
4
ter (Fig. S3). The IR spectra of the initial solid, the solid after
◦
treatment at 50 C under vacuum and re-exposition to air, and
the solid after the TGA experiments and exposition to air were
recorded. The three spectra were found to be virtually identical.
Altogether these observations indicate that the coordinated sol-
vent molecules in 1a as well as those present in the lattice are
easily removed and recovered from atmospheric moisture.
Mass Spectrometry
Additional insights about the stability of the moiety [Co4L2]4
+
upon ligand exchange were obtained through mass spectrometry.
A sample of 1a dissolved in MeOH was examined by positive
+
ion electrospray ionization (ESI ) using MeCN/H2O as the elu-
plexes. It is known that lanthanides can form cluster species
with pyridyl-β-diketonate ligands related to H2L.[26] The intrin-
ant. Several fragments that involved the tetranuclear [Co4L2]
unit present in 1 and 1a were confirmed by exact mass (with
an error below 2 ppm) with varying intensities depending on
the voltage used (the range explored was 100–215V). At 125V,
sic interest of complex 1 prompted the direct preparation of this
compound from its simplest components. For this, a synthetic
procedure was devized that involved the use of two sources of
the fragment corresponding to [Co4L2]4 (m/z 244.4786 amu) is
+
II
Co presentinequivalentamounts:Co(AcO)2·4H2O, whichpro-
observedasthestrongestpeakofthespectrum(Fig. S4).Thissig-
vided a built-in base to deprotonate H2L, and Co(NO3)2·6H2O,
which furnished the nitrate counter-ions of the cationic cluster
that resulted from the reaction. In this manner, compound 1
could be readily obtained, as evidenced by IR spectroscopy and
microanalysis, according to the reaction in Eqn 1.
n+
nal is accompanied by other peaks that correspond to [Co4L2]
species (n = 3, 2). The relative intensities of this series of peaks
correlate with the charge of the species that produce them.
4
+
In addition to the species above, aggregates of the [Co4L2]
−
framework together with H2O, MeCN, and/or NO species (pre-
3
2
Co(AcO)2·4H2O + 2Co(NO3)2·6H2O + 2H2L + 8MeOH
sumably acting as ligands) were also identified (Figs S5 and S6),
namely [Co4L2(H2O)]4 (248.9812 amu), [Co4L2(MeCN)]
+
4+
→
[Co4L2(MeOH)8](NO3)4 + 4AcOH + 20H2O (1)
4
+
(
254.7351 amu),
[Co4L2(MeCN)2]
(264.9918 amu),
[Co4L2(NO3)]
4+
3+
It was found by elemental analysis that upon exposure
to moist air, compound 1 undergoes full replacement of its
[Co4L2(MeCN)3]
(275.2484 amu),
3+
(346.6344 amu), and [Co4L2(NO3)(MeCN)] (360.3096 amu).

RESEARCH FRONT
1
132
L. A. Barrios et al.
Table 1. Crystallographic data for compound 1
1
◦
Table 2. Selected interatomic distances [Å] and angles [ ] for com-
pound 1
The suffix # denotes atoms generated by symmetry operation (−x, −y, −z)
Parameter
Formula
M
Crystal system
Space group
a [Å]
Co4C50H58N10O28
Bond lengths
1482.78
Monoclinic
P21/c
20.511(5)
11.122(4)
16.630(2)
106.95(2)
3629(2)
150(1)
Co1–O3
Co1–O4
Co1–O1
Co1–O5
Co1–N1
Co1–O6
Co2–O7
Co2–O8
2.054(3)
2.121(4)
2.141(3)
2.147(4)
2.156(4)
2.159(4)
2.117(3)
2.141(3)
Co2–O2
Co2–O1
Co2–N2
Co2–O3
Co2–O2#
Co1–Co2
Co2–Co2#
2.204(3)
2.239(3)
2.257(3)
2.304(3)
2.333(3)
3.550(2)
3.692(2)
b [Å]
c [Å]
◦
β [ ]
3
V [Å ]
T [K]
Z
ρcalc
2
Bond angles
1.357
Shape and colour
Size [mm]
Orange plate
0.30 × 0.20 × 0.05
0.979
6145
419/96
0.0623
0.1967
1.063
0.954/−0.740
Co1–O3–Co2
Co1–O1–Co2
Co2–O2–Co2#
O3–Co1–O4
O3–Co1–O1
O4–Co1–O1
O3–Co1–O5
O4–Co1–O5
O1–Co1–O5
O3–Co1–N1
O4–Co1–N1
O1–Co1–N1
O5–Co1–N1
O3–Co1–O6
O4–Co1–O6
O1–Co1–O6
O5–Co1–O6
N1–Co1–O6
O7–Co2–O8
O7–Co2–O2
108.95(14)
108.27(12)
108.86(11)
87.97(14)
74.66(12)
162.63(13)
94.75(14)
87.41(16)
94.17(13)
151.34(15)
120.57(15)
76.77(13)
89.56(16)
95.11(14)
85.76(15)
95.36(13)
167.80(15)
85.23(16)
176.26(12)
87.99(11)
O8–Co2–O2
O7–Co2–O1
O8–Co2–O1
O2–Co2–O1
O7–Co2–N2
O8–Co2–N2
O2–Co2–N2
O1–Co2–N2
O7–Co2–O3
O8–Co2–O3
O2–Co2–O3
O1–Co2–O3
N2–Co2–O3
O7–Co2–O2#
O8–Co2–O2#
O2–Co2–O2#
O1–Co2–O2#
N2–Co2–O2#
O3–Co2–O2#
88.94(12)
89.60(12)
87.77(12)
80.07(11)
90.63(12)
93.11(12)
141.95(12)
137.96(12)
90.45(12)
91.04(13)
148.15(11)
68.12(11)
69.85(12)
89.38(12)
91.65(12)
71.14(11)
151.21(11)
70.82(11)
140.66(11)
−
1
µ [mm
]
Unique reflections
Parameters/restraints
R1 (F0 > 4σ(F0))
wR2 (all data)
Goodness-of-fit
Largest residuals [e Å ]
3
Interestingly, other peaks of the spectrum were assigned to the
following complex structures, which involved more than two
n+
2+
ligands: [Co4L3] (n = 2, 3, and 4) and [Co4L4] (Fig. S4).
Some of the peaks in the spectrum could not be attributed to any
structure related to 1 or 1a. The occurrence of electrical charges
different from those expected from the chemical formula of the
analyte is the result of reduction processes that take place as
an intrinsic effect of the technique, caused by the electrical dis-
charges to which the sample is subject.The above results provide
strong evidence that the exchange of MeOH by H2O from the
atmosphere in passing from 1 to 1a occurs without alteration of
4
+
the basic [Co4L2] framework of the cluster.
O6
N2
O2
O8
N1
Co2
Description of the Structure of [Co4L2(MeOH)8](NO3)4
Co1#
Co1
O1
O3
Co2#
Crystallographic data and selected structural parameters for
O4
N4
1
are collected in Tables 1 and 2, respectively. The molecu-
O5
lar structure of this complex was determined by single-crystal
X-ray diffraction using synchrotron radiation. The cation of the
compound (Figs 2 and S7) exhibits a centro-symmetric lin-
O7
II
Fig. 2. Representation of the molecular structure of the cationic complex
ear array of Co metal ions in a zig-zag form, held together
[
Co4L2(MeOH)8]4 (cation of 1). Hydrogen atoms have been removed for
+
by two ligands (i.e., L2 ). The latter lie on opposite sides of
the [Co4] chain and are mutually shifted in the axial direction
of the molecule. Each of these ligands binds the metals through
two diketonate groups, the central pyridyl moiety, and one of
the external pyridyl rings, the other remaining uncoordinated.
The uncoordinated pyridyl rings of both ligands lie on oppo-
−
clarity. Crystallographically unique non-carbon and Co atoms are labelled.
result, Co1 exhibits a very strongly distorted octahedral coor-
dination geometry (Fig. 4) and Co2 is heptacoordinated, and
displays a rare pentagonal bipyramidal environment (Fig. 4).
The equatorial and axial Co–O distances range from 2.054(3)
to 2.333(3) Å and 2.117(3) to 2.159(4) Å, respectively, whereas
the Co1–N1 and Co2–N2 distances are 2.156(4) and 2.257(3) Å,
respectively (Table 2). Likewise, the bridging angles Co1–
II
site ends of the molecule. The external Co centres (Co1 and
symmetry equivalent, s.i.) are thus chelated by one β-diketonate
(
O,O-donation) on one side of the equatorial plane and by the
equivalent to a picolinate moiety (N,O-donation) on the other
side of the plane. The equatorial donors of the central Co ions
II
◦
O1–Co2, Co1–O3–Co2, and Co2–O2–Co2# are 108.27(12) ,
◦
◦
(
Co2 and s.i.) are one β-diketonate (O,O-donor) from one lig-
108.95(14) , and 108.86(11) , respectively. The intermetal-
lic first-neighbour distances are 3.550(2) Å (Co1· · ·Co2) and
3.692(2) Å (Co2· · ·Co2#). In addition to the complex cation,
and and the equivalent to a dipicolinate moiety (O,N,O-donor)
from the other. The resulting [Co4L2]4 ensemble constitutes
a remarkably flat moiety (Fig. 3). Eight MeOH ligands from
the solvent are located perpendicular to the plane of this frag-
ment, occupying all the axial positions of the metals. As a
+
−
the structure contains four NO3 ions per [Co4] unit that ensure
the electric neutrality of the compound. These anions are
located in between the clusters, and link them through a network

RESEARCH FRONT
Molecular Chain of Four CoII Ions Stabilized by a Tris-Pyridyl/Bis-β-Diketonate Ligand
1133
Fig. 3. Side view representation of the cation of 1, emphasizing the planarity of the [Co4L2]2 moiety.
+
O7
O1
O5
which leads to an S = 0 ground state, regardless of the influ-
O3
Co1
O2
O2#
ence of spin-orbit coupling. Modelling the magnetic behaviour
N2
O3
II
of exchange-coupled Co clusters with more than two metals
N1
O1
Co2
O8
that exhibit spin-orbit coupling effects remains an extremely
O6
[31–33]
challenging task.
In the current case, however, this effect
O4
is expected to be heavily attenuated by the strong deviation from
ideal octahedral geometry shown by the metals in 1 (and thus
Fig. 4. Representation of the CoNO5 and CoNO7 chromophores of Co1
andCo2incomplex 1, emphasizing theirdistorted octahedraland pentagonal
bipyramidal coordination geometries.
1a). An estimate of the coupling constant between the metals
was obtained by using the isotropic Heisenberg spin Hamil-
tonian H = −2J1(S1S2 + S1ꢀ S2ꢀ ) − 2J2S2S2ꢀ , where S1, S2, S1ꢀ
,
and S2ꢀ are the spin operators for Co1, Co2, Co1#, and Co2#,
respectively in Fig. 2, and are assumed to have values of 3/2.
Using this model, part of the contribution by the spin-orbit cou-
pling effect is included in the magnitude of the g parameter,
which then adopts values unusually higher than expected for
spin-only systems. Another commonly used strategy to model
of hydrogen bonds formed with their methanol ligands (Fig. 5).
This lattice spans two dimensions along the crystallographic
bc directions, with the linear molecules disposed perpendicular
to this plane (Fig. 5). This disposition suggests the future pos-
sibility of exploiting hydrogen-bonding interactions to orderly
deposit molecular strings of metals perpendicular to a surface
if the ends of the molecules are appropriately functionalized.
Disordered methanol and water molecules located into the cav-
ities between the sheets are likely to contribute to the packing
between these sheets in the a direction by hydrogen bonding with
the uncoordinated pyridine moieties at one end of each ligand.
The literature contains hundreds of coordination clusters of
II
the magnetic susceptibility of Co polynuclear complexes that
exhibit strong spin-orbit coupling is to assume that only the
lowest Kramers doublets, which result from the combination of
angular momenta, are populated, the ions thus exhibit effective
spin values of 1/2. This leads normally to even higher values
for the g parameter and usually, only the low temperature data
are considered.[
34,35]
This approach is not employed here given
the strong attenuation of the spin-orbit coupling expected from
the severe distortion of the coordination geometry around the
metals (see above). The fit was thus performed for the whole
temperature range, by full matrix diagonalization with the pro-
gram CLUMAG.[ Several runs of this procedure revealed the
fact that the parameter J2 was strongly correlated with g, which
caused the former to reach unrealistic high and positive values,
while leading to very good fits. As a way to avoid the distorting
effects of this correlation on the g parameter, it was found more
appropriate to set the condition J1 = J2, which is also consid-
ered reasonable from the structural information. The parameters
II
Co , of which almost 200 are tetranuclear. Surprisingly, there
are very few examples with a linear topology,[28–30] which shows
that such an arrangement is difficult to achieve.
36]
Magnetic Properties
The bulk magnetization properties of complex 1 could not
be measured directly because, in air, it replaces its MeOH
axial ligands by H2O molecules to form the aqua-complex
1
a. The behaviour of the latter is expected to be very sim-
−
1
reachedinthismannerwereJ1 = J2 = −12.2 cm andg = 2.57.
The large value of g represents a confirmation that the magnetic
data remain significantly influenced by the spin-orbit coupling
and, therefore, other than being rough estimates, these results
do not bear strict physical meaning. The reduced magnetization
M/NµB versus H curve recorded at 2 K (not shown) displays
extremely low values up to 5T, which is consistent with the
presence of a diamagnetic spin ground state.
ilar to the structurally characterized version, under the very
reasonable assumption that the molecular backbone is main-
tained (see above). The χMT versus T curve of 1a in the 2 to
3
00 K range, under an applied magnetic field of 0.5T, is repre-
sented in Fig. 6. This curve exhibits a χMT value at 300 K of
3
−1
9
2
.9 cm K mol and then decreases, steadily, to almost zero at
K. The value near room temperature is higher than expected
II
for four Co uncoupled ions with S = 3/2 and g = 2 (calculated
3
−1
as 7.5 cm K mol ), which indicates that the orbital angular
II
momentum of the Co ions is not completely quenched and
Conclusions
it has a contribution to the magnetic susceptibility of the sys-
tem. However, the vanishing of the χMT versus T curve at the
lowest measured temperatures confirms the presence of antifer-
The preparation of the tris-pyridyl/bis-β-diketone ligand H2L
has provided access to a molecular string of four closely
II
II
romagnetic interactions between the Co ions within the cluster,
spaced, magnetically exchanged Co ions, with formula

RESEARCH FRONT
1
134
L. A. Barrios et al.
Fig. 5. Representation of the packing diagram of 1 viewed approximately along the a crystallographic direction (top) and exactly
−
along the b axis (bottom), emphasizing the hydrogen bonding network (light gray dashed lines) between the NO3 anions (in dark
gray) and the complex cations (in black), which leads to sheets of [Co4] units, lying perpendicular to the bc plane.
[
Co4L2(MeOH)8](NO3)4 (1). This topology is clearly imposed
2,6-pyridinedicarboxylate was prepared by refluxing 2,6-
pyridinedicarboxylic acid and thionyl chloride in EtOH for 4 h
and removing the solvents by rotary evaporation.
by the rigid linear structure of the deprotonated version of H2L.
The MeOH ligands in complex 1 occupy labile axial positions on
the metals and are easily removed (for example, by atmospheric
H2O molecules). This suggests the possibility of incorporating
4
+
Syntheses
[
Co4L2] moieties as building blocks into more complicated
molecular assemblies. This is currently under investigation.
Bulk magnetization measurements on the hydrated derivative
1,3-Bis-(3-oxo-3-(2-pyridyl)propionyl)pyridine (H L)
2
A white suspension of NaH (3.34 g, 139 mmol) in DME
30 mL) was added slowly, under nitrogen atmosphere, into a
1
a demonstrate that the magnetic susceptibility within this moi-
(
ety is influenced both by antiferromagnetic interactions between
the metals in the cluster and the effects of spin-orbit coupling.
light yellow solution of ethyl dipicolinate (6.52 g, 29 mmol)
and 2-acetylpyridine (14.04 g, 115 mmol) in DME (200 mL).
The mixture turned gradually pink and then light brown. After
the addition was complete the temperature was progressively
increased until reflux, which was then maintained for 20 h. The
resulting brown/greenish suspension was allowed to cool, and
Experimental
All manipulations were performed under aerobic conditions
using materials as received unless indicated otherwise. Ethyl

RESEARCH FRONT
Molecular Chain of Four CoII Ions Stabilized by a Tris-Pyridyl/Bis-β-Diketonate Ligand
1135
1
1
2
0
8
6
4
2
0
−1
[
Co4L2(H2O)8](NO3)4 (1a). νmax (KBr)/cm
3352.2br,
613.5m, 1579.9s, 1548.9s, 1520.7s, 1456.4s, 1384.2v,
318.9m, 1292.6m, 1258.3w, 1126.8w, 1092.43w, 1077.9m,
050.8w, 1020.8w, 945.4w, 775.9m, 707.9w, 657.8m, 515.5m.
1
1
1
(Found: C 36.70, H 2.94, N 9.95. Calc. for Co C H O N
4
42 54 34 10
(
1a): C 36.81, H 3.09, N 10.22%.)
Physical Measurements
Variable-temperature magnetic susceptibility data were
obtained with a Quantum Design MPMS5 SQUID magneto-
meter. Pascal’s constants were utilized to estimate diamagnetic
corrections to the molar paramagnetic susceptibility. The pow-
dered sample used was analyzed and proved to be the aqua
version of 1, with MeOH molecules replaced by H2O (1a).
The molecular mass used was that of the aqua-complex, as
found from microanalysis. Elemental analyses were performed
in-house with a Perkin–Elmer Series II CHNS/OAnalyzer 2400,
at the Servei de Microanàlisi of CSIC, Barcelona, Spain or at
the Campbell Microanalytical Laboratory, University of Otago,
Dunedin, New Zealand. IR spectra were recorded as KBr pel-
let samples on a Nicolet 5700 FTIR spectrometer or as neat
samples on a Bruker Equinox 55 spectrometer with anATR sam-
0
50
100
150
200
250
300
T [K]
Fig. 6. Plot of χMT versus T per mole of [Co4] unit of complex 1a.
The solid line is a fit to the experimental data using an approximate model
see text).
(
the excess NaH was quenched with EtOH (6 mL). A brown
solid was collected by filtration and suspended in AcOH/H2O
150 mL, 2:1 volume ratio), and the mixture was then stirred for
5 min.A beige/yellow crude product was separated by filtration
1
pler provided by Specac inc. H NMR spectra were recorded on
(
1
Varian Gemini-300 spectrometer (300 MHz) or on an Avance
VRX 400 instrument (400 Mz). TGA-DTA experiments were
performed on a Q5000IR thermo-analyzer from TA instruments
under N . Positive ion ESI mass spectrometry experiments were
performed on a LC/MSD-TOF (Agilent Technologies) with a
dual source equipped with a lock spray for internal reference
introduction, at the Unitat d’Espectrometria de Masses (SSR) of
the University of Barcelona. The experimental parameters were:
and suspended in tetrahydrofuran (THF, 75 mL). To this mixture
was added a saturated solution of NaHCO3 (1.57 g, 19 mmol)
in H2O, and the system turned bright yellow. The latter was
heated to its boiling point and reflux was maintained for 10 min,
after which the mixture was allowed to cool to room tempera-
ture. A yellow solid was separated by filtration, suspended in
AcOH/H2O (100 mL, 1:1 volume ratio), and the mixture stirred
for 15 min. The resulting beige powder was collected by filtra-
tion, washed with water, and then with Et2O.The yield was 14%.
δH (400 MHz, CDCl3) 15.95 (s, 2H), 8.79 (d, 2H), 8.32 (s, 2H),
2
◦
capillary voltage 4 kV, gas temperature 300 C, nebulizing gas
−
1
pressure 15 psi, drying gas flow 7.0 L min , and fragmentor
voltage ranging from 100 to 215V. Internal reference masses
were m/z 121.05087 (purine) or 922.00979 (HP-0921). Samples
(microlitres) were introduced into the source by a HPLC system
(Agilent 1100), using a mixture of H O/CH CN (1/1) as eluent
8
.29 (d, 2H), 8.18 (d, 2H), 8.04 (t, 1H), 7.89 (t, 2H), 7.47 (t, 2H).
+
m/z ESI MS (>0) 374 [M + H] . (Found: C 65.97, H 4.41, N
2
3
−
1
1
0.88. Calc. for C21H16O4.5N3 (H2L·0.5H2O): C 65.96, H 4.22,
(200 µL min ).
N 10.99%.)
X-Ray Crystallography
[
Co4L2(MeOH)8](NO3)4 (1): Method 1
X-Ray diffraction data were collected on a thin orange plate
usingaBrukerAPEXIICCDdiffractometeronstation16.2SMX
of the Synchrotron Radiation Source at CCLRC Daresbury Lab-
oratory, at 0.8464 Å, from a silicon 111 monochromator (T
A suspension of H2L (30 mg, 0.08 mmol) in MeOH
(
0
(
20 mL) was added into a solution of Co(AcO)2·4H2O (20 mg,
.08 mmol) and Tb(NO3)3·6H2O (54 mg, 0.12 mmol) in MeOH
[37]
10 mL). The mixture was brought to reflux for 2 h and then
150 K). The structure was solved with SIR92,
F with SHELXTL.
and refined on
Data were cut at 0.8 Å as then I/σ(I) < 2.
2
[38]
allowed to cool. The resulting orange solution was layered with
Et2O. After 10 days, orange crystals of 1 had deposited on the
walls of the tube. Yields varied between 10 and 15%.
Crystallographic and experimental details are summarized in
Table 1. Selected bond distances and angles are given in Table 2.
All non-hydrogen atoms were refined anisotropically.The nitrate
anions and one of the pyridine rings (N4) presented some dis-
order and were refined with displacement parameter restrains.
Hydrogen atoms were found in difference Fourier maps, includ-
ing those of the methanol molecules, and placed geometrically at
calculated positions on their riding atom.At the end of the refine-
ment, several low electron-density peaks remained in the cell,
which could not be modelled satisfactorily as solvent molecules.
This diffuse electron density was, therefore, analyzed and taken
into account with PLATON/SQUEEZE,[ which recovered a
Method 2
A
mixture of Co(AcO)2·4H2O (60 mg, 0.24 mmol),
Co(NO3)2·6H2O (69 mg, 0.24 mmol), and H2L (90 mg,
◦
0
.24 mmol) in MeOH (90 mL) was heated to near 60 C for 1 h
and then allowed to cool to room temperature. Upon addition
of Et2O, a fine orange precipitate started to form. After letting
the powder deposit for a few hours it was collected by filtration,
washed with Et2O, and let dry in air. The yield was 60%. Com-
pound 1 could be obtained in a microcrystalline form using this
method if the reaction mixture was layered with Et2O and the
system was left unperturbed for 10 days.
Repeated elemental analyses proved that upon air expo-
sure, the methanol ligands of complex 1 exchanged with
atmospheric water molecules, which led to the aqua species
39]
3
total of 34 electrons per cell within two voids of 255 Å . A sig-
nificant improvement in the final agreement factors was thus
obtained. CCDC 721820 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif or by emailing

RESEARCH FRONT
1
136
L. A. Barrios et al.
data_request@cdc.cam.ac.uk, or by contacting the Cambridge
Crystallographic Data Centre, 12, Union Road, Cambridge CB2
[17] L. A. Barrios, G. Aromí, J. Ribas, J. S. Uber, O. Roubeau, K. Sakai,
S. Masaoka, P. Gamez, J. Reedijk, Eur. J. Inorg. Chem. 2008, 3871.
doi:10.1002/EJIC.200800431
1
EZ, UK; fax: +44 1223 336033.
[
18] L. A. Barrios, D. Aguilà, S. Mellat, O. Roubeau, S. J. Teat,
P. Gamez, G. Aromí, C. R. Chim. 2008, 11, 1117. doi:10.1016/
J.CRCI.2008.06.014
Acknowledgements
G.A. thanks theAGAUR (Generalitat de Catalunya) for a Mobility Research
Fellowship. G.A., L.B., and D.A. thank the Spanish Ministry of Science and
Education for financial support. O.R. thanks the Spanish National Research
Council (CSIC) for financial support. K.S.M. thanks theAustralian Research
Council for a Discovery Grant. The authors thank Dr Irene Fernández from
the Unitat d’Espectrometria de Masses (SSR) University of Barcelona and
Prof. Christine McKenzie from Syddansk Universitet for extensive help and
assistance during the interpretation of mass spectrometry experiments.
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