Structural and magnetic variability of cobalt(II) complexes with bridging pyrazolate ligands bearing appended imine groups

Article abstract of DOI:10.1039/b907817g

The synthesis of new binucleating pyrazole-derived ligands (HL², HL³) with bulky 2,6-diisopropylphenylimine side arms and backbone phenyl groups at the pyrazole-C⁴ and/or at the imine-C, which are derivatives from the know

Full text of DOI:10.1039/b907817g

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PAPER
www.rsc.org/dalton | Dalton Transactions
Structural and magnetic variability of cobalt(II) complexes with bridging
pyrazolate ligands bearing appended imine groups†
Anna Sachse, Serhiy Demeshko and Franc Meyer*
Received 17th April 2009, Accepted 26th June 2009
First published as an Advance Article on the web 31st July 2009
DOI: 10.1039/b907817g
The synthesis of new binucleating pyrazole-derived ligands (HL2, HL3) with bulky
2,6-diisopropylphenylimine side arms and backbone phenyl groups at the pyrazole-C4 and/or at the
imine-C, which are derivatives from the known ligand HL1, is reported. Crystallographic analyses of
three cobalt(II) complexes coordinated by ligands [L1-3]- reveal distinct metal to ligand ratios and
different structural motifs in the solid state: [L1Co2Cl3(H2O)2(EtOH)] (1), [(L2)3Co4Cl5] (2), or
[(L3)2Co4Cl6(H2O)5] (3). Metal ions are five-coordinate in 1, four- and six-coordiante in 2, and
six-coordinate in 3. UV-Vis spectroscopy and ESI mass spectrometry suggest that complexes 2 and 3
retain their structures also in solution, whereas 1 partially dimerizes and is in equilibrium with
tetrametallic species akin to 3. Magnetic susceptibility measurements indicate weak to moderate
antiferromagnetic coupling between five-coordinate or between six-coordinate cobalt(II) ions, but weak
ferromagnetic coupling between four- and six-coordinate cobalt(II).
interesting organometallic chemistry, with unique effects arising
Introduction
from cooperativity of the proximate metal ions.15 More recently,
Bi- and oligonuclear complexes are attracting a lot of attention,
such ligands also gave rise to some bimetallic platinum and het-
erobimetallic platinum/copper chemistry.16 While palladium(II)
chloride complexes of the binucleating pyrazole/imine ligands are
square-planar (A, Scheme 1), the corresponding nickel(II) chloride
or bromide complexes tend to form oligonuclear aggregates
[LNi2X3]n (n = 2,3) with five- or six-coordinate high-spin metal
ions.13,15 These aggregates may give rise to more open structures
[LNi2X3(solv)x]n in the presence of coordinating solvents (e.g., B,
Scheme 1).
mainly because of the general interest in cooperative effects
that may occur for, inter alia, synergetic reactivity towards
small substrates or magnetic properties.1,2 Compartmental ligand
systems are of pivotal importance in this context, since they allow
to position two or more metal ions in proper spatial location, and
in the presence of suitable coligands they may even support the
controlled aggregation giving high-nuclearity complexes. Hence
the design of elaborate multidentate ligand systems and the
investigation of their fundamental coordination chemistry is a
cornerstone on the way to achieving and understanding multi-
metal cooperativity.3–5 Binucleating ligands can be classified by
their central bridging groups. The negatively charged pyrazolate is
well established as a popular bridging unit that may span two metal
ions at favorable distances of around 3.0–4.5 A.6,7 Further control
˚
of the metal-metal separation as well as of the individual metal ion
coordination spheres can be realized by introducing chelate side
arms in the 3- and 5-positions of the pyrazolate heterocycle, and
metal complexes of compartmental pyrazolate-based ligands have
shown great potential in, e.g., metallobiosite modelling, molecular
magnetism, and two-center organometallic catalysis.8–12
A particular subclass of such pyrazolate ligand scaffolds fea-
tures appended imine functions with bulky aryl substituents.13–15
These ligands can be viewed as binucleating versions of the
intensively studied a-diimine type ligands. Their palladium(II)
and nickel(II) complexes (after activation with MAO) are ef-
ficient catalysts for olefin poylmerization and they feature an
Scheme 1 Different multinuclear pyrazole/imine complexes.
With copper(II) acetate, two ligands were found to encapsulate
an unusual edge-sharing bitetrahedral Cu6O2 core.17 In order
further elaborate the chemistry of this ligand class and to probe the
influence of substituents either at the outer N-bound aryl groups
or the ligand backbone, we have recently developed synthetic
routes to new functionalized pyrazole building blocks that now
provide access to a greater variety of ligand derivatives.18 Two new
Institut fu¨r Anorganische Chemie, Georg-August-Universita¨t Go¨ttingen,
37073, Go¨ttingen, Germany. E-mail: franc.meyer@chemie.uni-goettingen.
de; Fax: (+) 551-39-3063; Tel: (+) 551-39-3012
† Electronic supplementary information (ESI) available: X-Ray crystallog-
raphy of HL3; part of the UV-Vis spectra of 1, 2 and 3; additional magnetic
data. CCDC reference numbers 728434–728437. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/b907817g
7756 | Dalton Trans., 2009, 7756–7764
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pyrazole/imine binucleating ligands HL2 and HL3 with backbone
phenyl groups are reported here. Structures as well as spectral
and magnetic properties of a series of cobalt(II) complexes are
described, using a set of three ligands with different backbone
substitution pattern (including the two new ligands). The d7
cobalt(II) ion was chosen because of its very flexible coordination
behavior, which was expected to provide some clues about the
coordination landscape of this particular class of pyrazole-derived
ligands.
Results and discussion
Synthesis and structural characterization of the ligands
Type C pyrazole building blocks have been prepared according
to the recently reported methods.18 Following the procedure for
the known HL1 (R2 = Me), acid-catalyzed condensation of the
respective dicarbonyl compounds with 2,6-diisopropylaniline in
refluxing toluene yields the new ligands HL2 and HL3 that bear one
or three phenyl substituents at the backbone (R2 = Ph; Scheme 2).
Fig. 1 View of the hydrogen bond structure of HL2 (50% probability
ellipsoids). In the interest of clarity the side arm aryl groups and most
˚
of the hydrogen atoms have been omitted. Selected atom distances (A)
and angles (◦): N(1) ◊ ◊ ◊ N(3¢) 2.923(2), N1-N2 1.340(2), N(1)-H(1)-N(3¢)
166.17(2).
Synthesis and structural characterisation of Co(II) complexes
For the synthesis of cobalt(II) complexes the respective ligand
(HL1–HL3) was deprotonated with 1 equiv. of KOtBu in THF. Sub-
sequent addition of 2 equiv. of anhydrous CoCl2 resulted in the
formation of deep-green solutions. Complexes 1–3 were finally
isolated in good yields after crystallization (see Experimental).
Distinct molecular structures were found for the three different
ligands.
Scheme 2 Ligand synthesis.
HL2 and HL3 were isolated as white (HL2) and yellow
(HL3) solids in yields up to 94%. Both new ligands have been
Purple crystals of [L1Co2Cl3(H2O)2(EtOH)] (1) were grown
by slow evaporation of a solution of the crude complex in
CHCl3/EtOH. The molecular structure of 1 is depicted in Fig. 2.
1 is a binuclear complex with [L1]- acting as a bis(bidentate)
ligand, as anticipated. Both metal ions are found in distorted
trigonal bipyramidal coordination environment (t = 0.92 for
Co(1) and 0.68 for Co(2)).20 Co(1) is additionally ligated by one
chloride and two H2O molecules, while two chlorides and an EtOH
solvent molecule are bound to Co(2). Within the bimetallic pocket
the two H2O molecules at Co(1) and the two chloride ligands
at Co(2) are linked via two intramolecular O-H ◊ ◊ ◊ Cl hydrogen
1
fully characterized by H/13C NMR and IR spectroscopy, mass
spectrometry (high resolution/EI) and elemental analysis. Single
crystals of ligand HL2 suitable for X-ray diffraction analysis
were grown by recrystallization from ethanol. Single crystals of
HL3 were obtained from acetone/hexane, but unfortunately they
were poorly diffracting. While the X-ray data clearly reveal the
constitution and the crystal packing, we refrain from discussing
metric parameters of HL3.
The molecular structures of HL2 and HL3 are shown in Fig. 1
and Fig. S1.† The iminometyl groups, which are attached to
the 3- and 5-positions are coplanar with the pyrazole ring. In
both cases the two side arms are orientated in anti-conformation:
while the N(3) is located close to the pyrazole N(1), the N(4)
side arm is directed towards the backside pointing away from
pyrazole-N(2). Hydrogen bonding between the pyrazole-N(1)-H
and the side-arm N(3¢) atoms of a neighboring molecule results
in a dimeric structure for HL2, while no such dimerization but
hydrogen bonding to an undefined solvent molecule is observed
in the case HL3. The hydrogen bonding motif in HL2 that
involves the substituent in the pyrazole-5 position is quite un-
usual, since most N-unsubstituted pyrazoles give supramolecular
arrangements where only the pyrazole-N and -NH are involved
in intermolecular hydrogen bonding (dimers, trimers, tetramers,
hexamers or catemers).18,19
˚
bonds with d(O(1) ◊ ◊ ◊ Cl(3)) = 3.12 A and d(O(2) ◊ ◊ ◊ Cl(2)) =
˚
3.07 A (Fig. 2). Various hydrogen bonding motifs between metal-
bound water and chloride ligands have also been observed in a
series of related nickel(II) complexes,21 and several examples of
a single O-H ◊ ◊ ◊ X hydrogen bond within the bimetallic pocket
of pyrazolate-based bimetallic complexes are known,22 but the
arrangement in 1 with double intramolecular O-H ◊ ◊ ◊ X bridges
˚
represents a novel pattern. Co-Cl distances (2.33–2.35 A) and Co-
O bondlenghts (2.01–2.09 A) in1 arein the expected ranges23,24 and
˚
Co-Cl bonds are not very different from those in some pyrazolate-
based dicobalt(II) complexes with N-H ◊ ◊ ◊ Cl hydrogen bonds
involving side arm donors of the binucleating ligand scaffold.25,26
Interestingly, an extensive hydrogen bonding network involving
the chlorido, water and ethanol ligands leads to dimeric aggregates
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Fig. 2 Left: molecular structure of 1 (50% probability ellipsoids). In the interest of clarity most hydrogen atoms except those of the OH groups of water
˚
and EtOH ligands have been omitted. Selected atom distances (A): Co(1)-N(1) 2.113(2), Co(1)-N(3) 2.077(2), Co(1)-O(1) 2.017(2), Co(1)-O(2) 2.038(2),
Co(1)-Cl(1) 2.3315(7), Co(2)-N(2) 2.0947(19), Co(2)-N(4) 2.074(2), Co(2)-O(3) 2.0991(18), Co(2)-Cl(2) 2.3482(6), Co(2)-Cl(3) 2.3304(7), Co(1) ◊ ◊ ◊ Co(2)
4.4191(5). Right: view of the hydrogen bonding in dimeric aggregates of 1.
Fig. 3 Left: molecular structure of 2 (50% probability ellipsoids). In the interest of clarity all hydrogen atoms and 2,6-diisopropylphenyl rings have
˚
been omitted. Selected atom distances (A): Co(1)-N(1) 2.016(3), Co(1)-N(3) 2.089(3), Co(1)-Cl(1) 2.2831(9), Co(1)-Cl(4) 2.1958(10), Co(2)-N(2) 2.044(3),
Co(2)-N(4) 2.190(3), Co(2)-N(21) 2.031(3), Co(2)-N(23) 2.412(3), Co(2)-Cl(2) 2.4877(9), Co(2)-Cl(3) 2.4288(9), Co(3)-N(11) 2.015(3), Co(3)-N(13)
2.089(3), Co(3)-Cl(3) 2.2765(10), Co(3)-Cl(5) 2.1857(12), Co(4)-N(12) 2.053(3), Co(4)-N(14) 2.209(3), Co(4)-N(22) 2.032(3), Co(4)-N(24) 2.4547(3),
Co(4)-Cl(1) 2.4182(9), Co(4)-Cl(2) 2.4437(10), Co(1) ◊ ◊ ◊ Co(2) 4.2348(6), Co(1) ◊ ◊ ◊ Co(3) 6.6501(7), Co(1) ◊ ◊ ◊ Co(4) 3.7711(6), Co(2) ◊ ◊ ◊ Co(3) 3.8068(7),
Co(2) ◊ ◊ ◊ Co(4) 3.9522(7), Co(3) ◊ ◊ ◊ Co(4) 4.2490(7). Right: view of the tetranuclear metal core of 2.
of two roughly perpendicular molecules 1 in the solid state (Fig. 2,
right).
with their surrounding atoms) is presented in Fig. 3, right. The
˚
lengths of the terminal Co-Cl bonds (~2.2 A) are in the common
The solid state structure of [(L2)3Co4Cl5] (2) consists of four
metal ions and three ligands [L2]- (Fig. 3). Co(2) and Co(4)
have strongly distorted octahedral coordination spheres and
are doubly bridged by a pyrazolate ligand and a chloride. In
contrast, Co(1) and Co(3) are only four-coordinate with distorted
tetrahedral environment. They are each singly connected to the
central Co(2)/Co(4) pair via a pyrazolato bridge to one of the
central metal atoms and via a bridging chloride to the other.
The appearance of two different metal coordination geometries
(octahedral and tetrahedral) in one compound is relatively rare.26–29
A view of the central core of 2 (showing only the metal ions
range for tetrahedrally coordinated cobalt(II) ions.30 Distances of
the bridging Cl atoms from the tetrahedral Co(1) and Co(3) ions
˚
˚
(~2.3 A) and from the octahedral Co(2) and Co(4) ions (~2.5 A)
differ significantly. In general, all metal–ligand bonds are longer
for the six-coordinate cobalt(II) ions than for the four-coordinate
cobalt(II) ions, as expected.
Complex 3, prepared from CoCl2 and [L3]-, forms a tetranuclear
array [(L3)2Co4Cl6(H2O)5] that is composed of two bimetallic
{L3Co2Cl3} building blocks (Fig. 4). All metal ions are found
in disordered octahedral coordination sphere. The Co(1) ◊ ◊ ◊ Co(2)
˚
distance spanned by the pyrazolate is ~4.0 A, while the distance
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Table 1 Absorption maxima (l/nm) of the d–d transition bands for 1, 2
and 3 in CH2Cl2 solution (e/L mol- 1cm-1) and solid state
1
2
3
Solution
531(85)
556(101)
587(113)
672(111)
580(237)
635(226)
688(331)
561(271)
584(307)
671(305)
Solid state
533
583
628
659
579
633
688
562
584
673
its tetrametallic analogue [(L1)2Co4Cl6(solvent)5] (1¢) in solution.
One should note that species 1¢ and the dimeric aggregates of 1
found in the solid state with composition [{L1Co2Cl3(solvent)3}2]
(see Fig. 2, right) differ by a single solvent molecule only. Thus it is
well conceivable that two molecules of 1 are in solvent-dependent
equilibrium with tetranuclear species 1¢ akin to 3.
This is further corroborated by the ESI+ mass spectrum of a
MeCN solution of 1 (Fig. 5), which shows major peaks not only at
m/z 671 (corresponding to [L1Co2Cl2]+), but also at m/z 1119 and
1379, characteristic for the ions [(L1)2Co2Cl]+ and [(L1)2Co4Cl5]+.
Fig. 4 Molecular structure of 3 (50% probability ellipsoids). In the
interest of clarity all hydrogen atoms have been omitted. Selected atom
˚
distances (A): Co(1)-N(1) 2.024(3), Co(1)-N(3) 2.161(3), Co(1)-O(1)
2.175(4), Co(1)-O(2) 2.103(13), Co(1)-Cl(4¢) 2.3752(11), Co(1)-Cl(5)
2.6949(5), Co(2)-N(2) 2.016(3), Co(2)-N(4) 2.169(3), Co(2)-O(3) 1.993(8),
Co(2)-Cl(3) 2.529(5), Co(2)-Cl(4) 2.3576(11), Co(2)-Cl(5) 2.6599(6),
Co(1)-Co(2) 4.0370(7), Co(1)-Co(2¢) 3.5182(9), Co(1)-Co(1¢) 5.3898(11).
˚
Co(1) ◊ ◊ ◊ Co(2¢) between the two subunits is shorter at ~3.5 A. The
two subunits are connected by two chloride bridges that span the
shorter edges of the flat tetrametallic array, and by a unique m4-
Cl atom that is nested at the center of the complex. The Co–Cl
˚
distances towards this m4-chloride are quite long at 2.53 to 2.69 A.
m4-Cl atoms are rare,31 and most of the known examples are found
in late transition metal chemistry (copper, silver and mercury).32 In
the nickel(II) case the present type of pyrazole/imine binucleating
ligands gave complexes [LNi2X3]3 (X = Cl, Br) with a unique m6-
halide in the center of the hexametallic cage. These cages open
up in the presence of coordinating solvents to yield Ni4(m4-X)
species akin to 3.13,15 3 appears to be the first example of a m4-
chlorido ligand in cobalt cooodination chemistry. All remaining
coordination sites above and below the central Co4(m4-Cl) plane
in 3 are statistically occupied by metal bound water molecules and
additional chlorido ligands.
Fig. 5 Positive ion ESI-MS spectrum for 1 in MeCN solution; the inset
shows the observed and calculated HR-MS isotopic distribution for the
peak at m/z 671 corresponding to [L1Co2Cl2]+.
Magnetic properties
Magnetic susceptibilities for powdered samples of single-
crystalline material of complexes 1–3 were measured in a magnetic
field of 0.5 T in the temperature range 295–2 K (Fig. 6). Magnetic
data analyses of cobalt(II) complexes are often complicated by
the strong anisotropy of this particular ion, which may preclude
a simple spin-only simulation approach.33 However, using more
sophisticated models often bears a considerable risk of over-
parameterization, and hence a treatment through the spin-only
formalism is advisable as a first approximation. One should keep in
mind though that the determination of magnetic parameters from
simulation of the susceptibility data for polycrystalline powder
material using such simplified models has to be regarded with
caution.
Stability of the complexes in solution
UV-Vis spectra of all complexes have been collected in CH2Cl2 and
in the solid state (diffuse reflectance) in order to elucidate whether
the oligonuclear species found in the crystalline material are also
present in solution (Table 1, Fig. S2†). Absorption maxima for 2
and 3 are quite similar when measured in CH2Cl2 or in the solid
state, suggesting that the tetranuclear complexes stay essentially
intact upon dissolving the crystalline material. In contrast, spectra
for 1 differ significantly when measured in CH2Cl2 or in the solid
state, with the absorption maxima in solution closely resembling
those of the tetranuclear complex 3. One may thus assume that
the complex [L1Co2Cl3(H2O)2(EtOH)] (1) with its labile (H)O-
H ◊ ◊ ◊ Cl bridges undergoes dimerization and partly transforms to
The meff value at 295 K for 1 (6.36 mB) is higher than the
spin-only value for two S = 3/2 ions (5.48 mB for g = 2.0) but
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Fig. 7 Coupling schemes for 2 (left) and 3 (right).
ˆ
ˆ ˆ
ˆ ˆ
ˆ ˆ
ˆ ˆ
ˆ ˆ
H −2J1S2S4 −2J 2(S1S2 +S3S4 )−2J 3(S1S4 +S2S3 )
4
(2)
D [Sz2i −1 3Si (Si +1)]
ˆ
+ gm B
S
+
∑
∑
B
zi
i
i=1
i=1,3
Best fit parameters are g1 = g3= 2.33, g2 = g4= 2.60, J1
=
Fig. 6
meff vs. T plot for 1 (circles), 2 (triangles) and 3 (squares). The solid
-4.6 cm-1, J2 = +0.4 cm-1, J3 = +0.003 cm-1, PI = 5.0% (fixed)
and TIP = 33.5 ¥ 10-4 cm3mol-1 (fixed). ZFS parameters are
|D1| = |D3| = 11.5 cm-1; to reduce the number of parameters
|D2| and |D4| for the central pair of six-coordinate cobalt(II)
ions were fixed to zero because of the antiferromagnetic coupling
between them, which is also reflected by the relatively large
negative value for J1 that results from the double pyrazolate and
chloride bridging. g values much larger than 2.0 are common
for octahedral high-spin cobalt(II), because of the strong first
order orbital momentum contribution to the spin ground state.38
Due to the symmetry of the Hamiltonian, assignment of J2 and
J3 is tentative; furthermore, one of these two positive coupling
constants (J3) is very small and can be taken as zero. The positive
value of the remaining J2 for the coupling between octahedral and
the tetrahedral cobalt(II) ions, i.e., between the central hinge and
the wingtips of the tetrametallic core, may be explained in terms of
a partial orthogonality situation of the magnetic orbitals.34 These
are eg and t2g for octahedral and t2 for tetrahedral cobalt(II) high
spin ions. Weak ferromagnetic coupling was also observed in a
chain of alternating octahedral and tetrahedral cobalt(II) sites.39
It should be noted, however, that the magnetic properties of 2
can be described equally well using a simple dimer model for the
central dicopper(II) core with two additional Curie-behaved terms
for isolated tetrahedral cobalt(II) ions with zero field splitting. In
this case the decrease of meff is associated with a somewhat smaller
J1 (-3.8 cm-1). In view of the possible over-parameterization when
applying the complete model for 2 (Fig. 7, left), the 3D error
surfaces for the pairs J1–J2 and J2–J3 were calculated (Fig. S4†).
For the pair J2–J3 only one distinct minimum can be observed,
indicating that these two parameters are independent. Two minima
are observed for the J1–J2 pair, which reflects the two different
models (the model sketched in the left part of Fig. 7 and the dimer
model with J2 = 0; see Fig. S4†).
lines represent the calculated curve fits (see text).
not unexpected in view of the common g values for cobalt(II)
(expected 6.13 mB for g = 2.24). Upon lowering the temperature
meff drops more and more rapidly to finally reach 1.95 mB at
2 K, suggesting antiferromagnetic coupling and an S = 0 ground
state. The experimental data for 1 were modelled by using a
fitting procedure to the appropriate Heisenberg-Dirac-van-Vleck
(HDvV) spin Hamiltonian for isotropic exchange coupling and
Zeeman splitting, eqn (1).34
ˆ
ˆ ˆ
ˆ
ˆ
H = -JS1S2 + gmBB(S1 + S2)
(1)
A Curie-behaved paramagnetic impurity (PI) with spin S =
3/2 and temperature-independent paramagnetism (TIP) were
included according to ccalc = (1 - PI)·c + PI·cmono + TIP.35
Note, that some single-ion anisotropy should be present in view
of the unsymmetrical surroundings of the cobalt ions, which are
severely distorted from a perfect octahedron. However, due to
the antiferromagnetic coupling that gives an S = 0 ground state
anisotropies will have no effect in the low temperature regime,
while the effect at higher temperatures is relatively small anyway.
A good quality fit is thus obtained in the simple Heisenberg model.
Best fit parameters for 1 are g = 2.24, J = -1.7 cm-1, PI = 2.5%
(fixed)36 and TIP = 19.0 ¥ 10-4 cm3mol-1. If a zero field splitting
(ZFS) parameter D was included into the fitting procedure for 1,
D converged to zero during data simulation, in agreement with
the overall S = 0 ground state. Since the local axial ZFS of
five-coordinated cobalt(II) can be very large,37 in an alternative
simulation the coupling parameter J was fixed to zero and only
a D parameter used to describe the low temperature magnetic
behaviour, but no reasonable fit could be obtained in this case (see
Fig. S3†).
Magnetic measurements for 2 show the presence of antiferro-
magnetic interactions, as indicated by the decrease of the magnetic
moment from 9.64 mB (295 K) to 4.41 mB (2 K). In view of the
structural features, the magnetic model for 2 is based on a central
antiferromagnetically coupled dimer of six-coordinate cobalt(II)
ions (Co(2) and Co(4) with coupling constant J1), with two
appended four-coordinate cobalt(II) ions as wingtips (Co(1) and
Co(3)). Fig. 7 (left) illustrates the possible interaction pathways
in 2.
According to the molecular structure of 3, three interaction
pathways exist also in this case (Fig. 7, right). The appropriate
Heisenberg-Dirac-van-Vleck (HDvV) spin Hamiltonian includes
three isotropic exchange coupling constants and Zeeman splitting,
eqn (3).
ˆ
ˆ ˆ
ˆ ˆ
ˆ ˆ
ˆ ˆ
ˆ ˆ
ˆ ˆ
H = −2J1(S1S3 +S2S4 )−2J 2(S1S2 +S3S4 )−2J 3(S1S4 +S2S3 )
4
ˆ
+ gm B
S
∑
B
zi
i=1
(3)
The appropriate Heisenberg-Dirac-van-Vleck (HDvV) spin
Hamiltonian includes three isotropic exchange coupling terms,
Zeeman splitting and zero field splitting, eqn (2).
Even though all three coupling constants should be in-
tuitively different, the best fit result was obtained for
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Table 2 Summary of X-ray diffraction data experimental
[L1Co2Cl3(H2O)2(EtOH)]·
1.5CHCl3 (1)
[(L2)3Co4Cl5]·0.5
(THF)·MeCN (2)
HL2
[(L3)2Co4Cl6(H2O)5]·2CHCl3 (3)
Empirical formula
Formula weight
Crystal size/mm
Crystal system
Space group
C35H42N4
518.73
C35.5H54.5Cl7.5Co2N4O3
969.07
C109H130Cl5Co4N13O0.5
2043.23
C96H100Cl12Co4N8O5
2106.96
0.24 ¥ 0.18 ¥ 0.12
Triclinic
P-1 (No. 2)
11.2650(7)
15.1857(9)
17.6502(10)
112.618(4)
90.388(5)
106.495(4)
2649.4
0.33 ¥ 0.23 ¥ 0.19
Monoclinic
C2/c (No. 15)
27.7190(14)
8.8990(3)
25.4394(14)
90
100.727(4)
90
6165.5(5)
8
1.118
2240
0.066
34, -11 to +10, 32
1.50 to 26.78
26988
6546 [0.1088]
4224
356/0
0.999
0.53 ¥ 0.27 ¥ 0.22
Orthorhombic
Iba2 (No. 45)
16.3211(11)
17.7522(11)
31.585(3)
90
0.56 ¥ 0.39 ¥ 0.26
Monoclinic
P21/n (No. 14)
16.5141(4)
22.4226(4)
31.4461(9)
90
102.388(2)
90
11373.0(5)
4
1.193
4288
0.740
20, -27 to +25, 38
1.12 to 25.74
86436
21492 [0.1459]
16904
1210/0
1.028
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
90
90
3
˚
V/A
9151.2(11)
8
1.407
4008
1.200
20, 22, 39
1.69 to 26.58
37227
9530 [0.0387]
9162
519/4
1.049
Z
1
r
calcd./g cm-3
1.321
1084
0.969
14, 19, -22 to +20
1.90 to 27.03
22714
11412 [0.0392]
8282
587/0
1.041
F(000)
m/mm-1
hkl range
q range/◦
Measured refl.
Unique refl./Rint
Observed refl. (I > 2s(I))
Ref. param./restraints
Goodness-of-fit
R1, wR2 (I > 2s(I))
R1, wR2 (all data)
0.0617, 0.1224
0.1073, 0.1383
0.212/-0.238
0.0298, 0.0754
0.0314, 0.0761
0.582/-0.615
0.0677, 0.1914
0.0827, 0.2050
1.488/-1.196
0.0688, 0.1654
0.0974, 0.1775
1.113/-1.454
-3
˚
Resid. el. dens./e A
J1 = J2 = J3 = -1.5 cm-1. Other parameters are g1 = g3 = g2 =
g4 = 2.50, PI = 5.0% (fixed) and TIP = 33.5 ¥ 10-4 cm3mol-1
(fixed). It should be mentioned that the use of a simple dimer
model (with two isolated dinuclear units and only a single intra-
subunit coupling constant) did not lead to an acceptable fit in this
case.
For any magnetostructural correlations it is necessary to have
a series of comparable systems with similar structures. To the
best of our knowledge there is only one other example in the
literature where two six-coordinate cobalt(II) ions are bridged
by a pyrazolate only,11 akin to the situation in 1. In this case a
very similar coupling constant of -2.0 cm-1 was observed. Any
comparison of the coupling between cobalt(II) ions in complexes
1, 2 and 3 is hampered by the different metal ion coordination
numbers, namely five in 1, four and six in 2, and six in complex 3.
unique m4-Cl atom encapsulated by four six-coordinate cobalt(II)
ions. The combination of UV-Vis and ESI-MS data suggest,
however, that species different from those seen in the solid state
may be present in solution, since evidence has been obtained for
a dimeriziation equilibrium between 1 and a tetranuclear complex
1¢ that is similar to 3. Magnetic data reveal antiferromagnetic
coupling between six-coordinate cobalt(II) ions, although cou-
pling is rather weak if mediated by a single pyrazolate or chloride
bridge only, and somewhat stronger solely in the case of double
pyrazolate/chloride bridging. Interaction between tetrahedral and
octahedral metal ions in 2 appears to be weakly ferromagnetic,
which is likely due to partial orthogonality of the magnetic orbitals.
The present work underlines the versatility of the pyra-
zole/imine hybrid ligands to serve as binucleating ligand scaffolds,
and it shows that subtle variations in the ligand backbone can have
severe influences on the topology of the resulting complexes. This
is important information for the use of these ligand class in, e.g.,
catalytic applications.
Conclusions
Two new derivatives have been added to the class of pyrazole-
based ligands with imine side arms, which are best described as
coupled binucleating versions of the well-known a-diimine type
systems. The new ligands feature phenyl backbone substituents
Experimental
Materials and preparation
at the pyrazole-C4 and at the imine-C (R1 = H/Ph, R2
=
Ph), extending the variability of substituent bulkiness for these
compartmental scaffolds. Reaction of three such ligands with dif-
ferent backbone substitution pattern with CoCl2 leads to distinct
coordination compounds under identical conditions. HL1 forms
a dinuclear complex [L1Co2Cl3(H2O)2(EtOH)] (1) with unusual
double intramolecular (H)O-H ◊ ◊ ◊ Cl hydrogen bonding, HL2 gives
a complex [(L2)3Co4Cl5] (2) with ligand to metal ratio of 3:4 and
with both tetrahedral and octahedral cobalt(II) ions, while HL3
yields the rectangular tetrametallic [(L3)2Co4Cl6(H2O)5] (3) with a
All reactions with metal salts were performed under purified
nitrogen. Solvents were purified and dried under nitrogen by con-
ventional methods. Melting points/decomposition temperatures
were determined with an OptiMelt system (Stanford Research
Systems, Inc.) using open capillaries, values are uncorrected.
1H and 13C NMR spectra were recorded on a Bruker Avance
500 spectrometer. 13C resonances were obtained with broad-band
proton decoupling, spectra were recorded at 298 K. 1H NMR and
13C NMR chemical shifts were referenced internally to solvent
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signals. Mass spectra were recorded using a Finnigan MAT (FAB)
using 3-nitrobenzylalcohol as a matrix, a Finnigan MAT 8200
(EI) or Bruker APEX IV (HRMS, ESI). IR spectra from KBr
pellets were recorded on a Digilab Excalibur Series FTS 3000
spectrometer. Elemental analyses were performed by the analytical
laboratory of the Institute of Inorganic Chemistry at Georg-
August-University Go¨ttingen using a Heraeus CHN-O-RAPID
instrument or an Elementar vario EL III instrument. UV-Vis
spectra were measured on a Varian Cary 5000 or an Analytic
Jena Specord S 100 spectrometer in a 1 cm pathway cuvette. All
reagents were purchased from commercial sources and employed
without further treatment. CoCl2 was dried under vacuum at
120 ◦C and 2,6-diisopropylaniline was purified by distillation prior
to use. HL1 and C were synthezised according to the published
procedures.15,18
6.8 Hz, CH3), 3.33 (4 H, sept, 3JHH = 6.8 Hz, CH), 6.32–7.66 (21
H, m, CHAr). 13C NMR (125 MHz, CDCl3): d = 21.9 (CH3), 24.0
(CH3), 28.3 (CH), 122.7 (CHAr), 122.8 (CHAr), 126.2 (CHAr), 126.8
(CHAr), 127.1 (CHAr), 127.3 (CHAr), 127.6 (CHAr), 128.8 (CHAr),
128.9 (CHAr), 129.1 (CHAr), 130.6 (CHAr), 131.7 (CAr), 134.2 (CAr),
Ar
Ar
Ar
Ar
=
135.5 (C ), 135.9 (C ), 144.5 (C ), 145.8 (C ), 160.0 (C N). MS
(ESI+, CH3CN): m/z (%) = 1364 (2L + Na+, 16), 693 (L + Na+,
38), 671 (L + H+, 100). HR-MS (ESI+, CH3CN): m/z calcd for
C47H51N4 (L + H+): 671.41082; found: 671.41067.
General synthetic method for the preparation of complexes
To a stirred solution of HL and KOtBu in THF (50 mL) was added
dry CoCl2. The solution was stirred for 15 h at room temperature
and then evaporated to dryness. The resulting green powder was
crystallised.
Synthesis of 3,5-bis(2,6-diisopropylphenyliminomethyl)-1H-
4-phenylpyrazole (HL2)
Synthesis of [L1Co2Cl3(H2O)2(EtOH)] (1)
A solution of dialdehyde C (R1 = H, R2 = Ph; 1.90 g, 9.50 mmol)
and 2,6-diisopropylaniline (10.0 g, 57.0 mmol) in toluene (500 mL)
was heated to reflux for 72 h. After the solvent and the excess
of 2,6-diisopropylaniline had been removed, the crude product
was crystallised from ethanol or acetone to give HL2 (4.48 g,
8.60 mmol, 91%) as white crystals. Mp 120 ◦C. Anal. Calcd. (%)
for C35H42N4, 0.5 H2O: C, 79.66; H, 8.21; N, 10.62. Found: C,
79.65; H, 8.12; N, 10.61. IR (KBr): 3450 w, 3395 w, 3213 w, 3061
w, 3026 w, 2961 vs, 2928 s, 2868 s, 1663 vs, 1625 vs, 1462 s, 1440 s,
1382 m, 1363 m, 1326 w, 1260 m, 1204 m, 1180 m, 1142 w, 1102
Complex 1 was prepared according to the above procedure using
HL1 (375 mg, 0.77 mmol), KOtBu (87 mg, 0.77 mmol) and
CoCl2 (200 mg, 1.5 mmol). The resulting green powder was
crystallised from wet CHCl3/EtOH or from acetone to give
purple crystals (520 mg, 0.66 mmol, 86%). Anal. Calcd. for
C32H43N4Co2Cl3(H2O)2(EtOH), 0.75 CHCl3: C, 47.45; H, 6.15;
N, 6.36. Found: C, 47.53; H, 6.04; N, 6.21. IR (KBr): 3413 br,
3063 w, 2963 vs, 2926 s, 2867 m, 1732 m, 1698 w, 1622 m, 1572
vs, 1486 w, 1462 m, 1440 m, 1426 m, 1383 w, 1363 m, 1328 s,
1254 w, 1235 m, 1184 m, 1102 m, 1068 w, 1057 w, 1024 w, 1007
w, 964 w, 937 w, 851 w, 801 m, 772 m, 727 m, 577 m, 450
1
w, 1043 w, 1012 w, 935 w, 886 w, 801 m, 764 m, 701 m cm-1. H
NMR (500 MHz, CDCl3): d = 1.15–1.16 (24 H, d, 3JHH = 4.5 Hz,
w cm-1. MS (ESI+, CH3CN): m/z (%) = 1379 (L2Co4Cl5 , 56),
+
CH3), 2.98 (4 H, br, CH), 7.10–7.44 (11 H, m, CHAr), 8.15 (2
1250 (L2Co3Cl3 , 20), 1155 (L2Co2Cl2+H+, 11), 1119 (L2Co2Cl+,
+
H, s, HC N). 13C NMR (125 MHz, CDCl3): d = 23.6 (CH3), 28.0
=
67), 1026 (L2Co + H+, 23), 671 (M-Cl,2H2O,EtOH+, 100), 578
(LCoCl + H+, 14), 485 (L + H+). UV-Vis (CH2Cl2) l [nm] (e
[L mol-1 cm-1]): 248 (157152), 269 (145945), 531 (85), 556 (101),
587 (113), 672 (111). UV-Vis (diffuse reflectance, KBr) l [nm]:
223, 256, 317, 533, 583, 628, 659. HR-MS (ESI+, CH3CN): m/z
calcd for C32H43N4Cl2Co2 (M-Cl,2H2O,EtOH+): 671.1524; found:
671.1528.
(CH), 118.9 (CAr), 123.1 (CHAr), 123.2 (CHAr), 124.8 (CHAr), 128.2
(CHAr), 128.4 (CHAr), 128.5 (CAr), 130.3 (CAr), 130.5 (CHAr), 137.7
Ar
Ar
=
(C ), 148.5 (C ), 152.5 (C N). MS (EI, 70 eV): m/z (%) = 518
(L+, 9), 475 (L-iPr+, 100). MS (ESI+, CH3CN): m/z (%) = 1059
(L2+Na+, 25), 559 (L + K+, 100), 541 (L + Na+, 65), 519 (L + H+,
72). HR-MS (ESI+, CH3CN): m/z calcd for C35H43N4 (L + H+):
519.34822; found: 519.34815.
Synthesis of 3,5-bis(2,6-diisopropylphenyliminobenzoyl-1H-
Synthesis of [(L2)3Co4Cl5] (2)
4-phenylpyrazole (HL3)
Complex 2 was prepared according to the above procedure using
HL2 (200 mg, 0.39 mmol), KOtBu (43 mg, 0.39 mmol) and CoCl2
(100 mg, 0.77 mmol). The resulting green powder was crystallised
from CH2Cl2, toluene or THF/CH3CN to give green crystals
(220 mg, 0.11 mmol, 86%). Anal. Calcd. for C105H123N12Co4Cl5,
3 THF, CH3CN, H2O: C, 63,76; H, 6,83; N, 8.12. Found: C, 63.75;
H, 6.81; N, 8.05. IR (KBr): 3401 br, 3061 w, 3027 w, 2962 vs, 2927 s,
2868 m, 1769 w, 1720 w, 1603 vs, 1540 w, 1461 s, 1441 m, 1383 w,
1363 w, 1326 w, 1292 w, 1255 w, 1244 w, 1179 m, 1099 w, 1057 w,
1038 m, 1018 w, 977 w, 923 w, 899 m, 844 w, 801 m, 754 m, 701 m,
583 w, 536 w, 473 w cm-1. MS (FAB): m/z (%) = 1964 (M+, 21),
A solution of diketone C (R1 = R2 = Ph; 1.00 g, 2.84 mmol)
and 2,6-diisopropylaniline (3.00 g, 17 mmol) in toluene (150 mL)
with a catalytic amount of trifluoroacetic acid (14 mol%) was
heated to reflux for 72 h. After the solvent and the excess of
2,6-diisopropylaniline had been removed the crude product was
crystallised from ethanol or acetone/hexane to give HL3 (1.78 g,
2.66 mmol, 94%) as yellow needles. Mp 138 ◦C. Anal. Calcd. for
C47H50N4, 0.5 H2O, C2H6O: C, 81.06; H, 7.91; N, 7.72. Found: C,
80.67; H, 7.93; N, 7.78. IR (KBr): 3435 m, 3424 m, 3059 w, 3025
w, 2963 vs, 2934 m, 2868 s, 1705 m, 1626 s, 1599 m, 1588 s, 1575
m, 1497 w, 1459 m, 1447 w, 1433 s, 1383 m, 1364 s, 1322 m, 1281
w, 1255 w, 1238 w, 1220 m, 1179 w, 1151 w, 1103 w, 1074 w, 1055
w, 1043 w, 1017 w, 938 vs, 930 vs, 910 m, 800 w, 780 m, 766 s, 747
m, 725 w, 696 vs, 629 w, 533 w cm-1. 1H NMR (500 MHz, CDCl3):
+
+
1929 (M - Cl+, 82), 1449 (L2Co4Cl5 , 29), 1318 (L2Co3Cl3 , 100),
1187 (L2Co2Cl+, 45), 1152 (L2Co2, 36). UV-Vis (CH2Cl2) l [nm] (e
[L mol- 1cm-1]): 242 (635833), 276 (655208), 580 (237), 635 (226),
688 (331). UV-Vis (diffuse reflectance, KBr) l [nm] : 224, 284, 475,
579, 633, 688.
3
3
d = 0.93 (12 H, d, JHH = 6.8 Hz, CH3), 1.22 (12 H, d, JHH
=
7762 | Dalton Trans., 2009, 7756–7764
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The Royal Society of Chemistry 2009
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Synthesis of [(L3)2Co4Cl6(H2O)5] (3)
Acknowledgements
Complex 3 was prepared according to the above procedure using
HL3 (70 mg, 0.10 mmol), KOtBu (12 mg, 0.10 mmol) and CoCl2
(26 mg, 20 mmol). The resulting green powder was crystallised
from CHCl3/acetone or acetone to give green crystals (50 mg,
0.027 mmol, 54%). Anal. Calcd. for C94H98N8Co4Cl6(H2O)5,
C3H6O: C, 60.16; H, 5.93; N, 5.79. Found: C, 59.93; H, 6.14;
N, 5.84. IR (KBr): 3375 br, 3059 w, 3027 w, 2963 vs, 2928 w, 2867
m, 1618 w, 1603 w, 1581 m, 1554 s, 1494 m, 1461 s, 1432 s, 1384
m, 1363 w, 1327 s, 1251 m, 1217 w, 1178 w, 1162 w, 1100 w, 1055
m, 1043 w, 1022 m, 978 s, 935 w, 913 w, 806 w, 779 m, 770w, 744
m, 722 w, 695 vs, 640 w, 602 w, 564 w, 539 w, 523 w, 442 w cm-1.
MS (ESI+, CH3CN): m/z (%) = 1528 (L2Co2Cl2+H+, 15), 1398
Financial support from the Fonds der Chemischen Industrie is
gratefully acknowledged.
References
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+
100). MS (ESI-, CH3CN): m/z (%) = 798 (LCoCl2 , 30), 669 (L-,
-
100). MS (FAB): m/z (%) = 1435 (L2CoCl-, 20), 1399 (L2Co-, 47),
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Magnetic susceptibility measurements
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Design MPMS-5S SQUID magnetometer equipped with a 5 Tesla
magnet in the range from 295 to 2.0 K. The powdered samples
were contained in a gel bucket and fixed in a non-magnetic sample
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using the Pascal constant and the increment method according to
Haberditzel.
2177.
14 J. C. Ro¨der, F. Meyer, M. Konrad, S. Sandho¨fner, E. Kaifer and H.
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X-Ray data were collected on a STOE IPDS II diffractometer
˚
(graphite monochromated Mo Ka radiation, l = 0.71073 A) by
◦
use of w scans at -140 C. The structures of HL2, HL3, 1, 2,
and 3 were solved by direct methods and refined on F2 using
all reflections with SHELX-97.40 The non-hydrogen atoms were
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23 A search of the CSD for d(T5Co-Cl) gave 194 hits in a range from 2.150
˚
˚
˚
positions and assigned to an isotropic displacement parameter
to 2.694 A, a mean value of 2.306 A, and a median value of 2.289 A.
24 A search of the CSD for d(T5Co-O) gave 504 hits in a range from 1.782
2
˚
of 0.08 A . Face-indexed absorption corrections were performed
˚
˚
˚
to 2.561 A, a mean value of 2.025 A, and a median value of 2.005 A.
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˚
˚
˚
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˚
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Dalton Trans., 2009, 7756–7764 | 7763
©

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36 To reduce the number of the fit parameters and to unify the fitting
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