Dinuclear Cu(II) complexes of isomeric bis-(3-acetylacetonate)benzene ligands: Synthesis, structure, and magnetic properties

Article abstract of DOI:10.1021/ic3003806

Highly versatile coordinating ligands are designed and synthesized with two β-diketonate groups linked at the carbon 3 through a phenyl ring. The rigid aromatic spacer is introduced in the molecules to orient the two acetylacetone units along different angles and coordination vectors. The resulting para, meta, and ortho bis-(3-acetylacetonate)benzene ligands show efficient chelating properties toward Cu(II) ions. In the presence of 2,2′-bipyridine, they promptly react and yield three dimers, 1, 2, and 3, with the bis-acetylacetonate unit in bridging position between two metal centers. X-ray single crystal diffraction shows that the compounds form supramolecular chains in the solid state because of intermolecular interactions. Each of the dinuclear complexes shows a magnetic behavior which is determined by the combination of structural parameters and spin polarization effects. Notably, the para derivative (1) displays a moderate antiferromagnetic coupling (J = -3.3 cm^-1) along a remarkably long Cu???Cu distance (12.30 A).

Full text of DOI:10.1021/ic3003806

Article
pubs.acs.org/IC
Dinuclear Cu(II) Complexes of Isomeric Bis-(3-
acetylacetonate)benzene Ligands: Synthesis, Structure, and
Magnetic Properties
Marzio Rancan,*,† Alessandro Dolmella,‡ Roberta Seraglia,† Simonetta Orlandi,§,∥ Silvio Quici,*,§,∥
Lorenzo Sorace,⊥ Dante Gatteschi,⊥ and Lidia Armelao*,†
†Istituto di Scienze e Tecnologie Molecolari (ISTM), Consiglio Nazionale delle Ricerche (CNR) and Consorzio Interuniversitario per
la Scienza e Tecnologia dei Materiali (INSTM), Dipartimento di Scienze Chimiche, Universita
Padova, Italy
̀
di Padova, Via F. Marzolo, 1-35131,
‡Dipartimento di Scienze Farmaceutiche, Universita
̀
di Padova, Via F. Marzolo, 5-35131, Padova, Italy
§Istituto di Scienze e Tecnologie Molecolari (ISTM), Consiglio Nazionale delle Ricerche (CNR), Via C. Golgi, 19-20133, Milano,
Italy
∥Polo Scientifico e Tecnologico (PST), Consiglio Nazionale delle Ricerche (CNR), via Fantoli, 16/15-20138, Milano, Italy
⊥Dipartimento di Chimica “U. Schiff” and Consorzio Interuniversitario per la Scienza e Tecnologia dei Materiali (INSTM), Universita
̀
di Firenze, Via della Lastruccia 3-13, 50019, Sesto Fiorentino, Firenze, Italy
S
* Supporting Information
ABSTRACT: Highly versatile coordinating ligands are designed and synthesized with two
β-diketonate groups linked at the carbon 3 through a phenyl ring. The rigid aromatic spacer
is introduced in the molecules to orient the two acetylacetone units along different angles
and coordination vectors. The resulting para, meta, and ortho bis-(3-acetylacetonate)-
benzene ligands show efficient chelating properties toward Cu(II) ions. In the presence of
2,2′-bipyridine, they promptly react and yield three dimers, 1, 2, and 3, with the bis-
acetylacetonate unit in bridging position between two metal centers. X-ray single crystal
diffraction shows that the compounds form supramolecular chains in the solid state because
of intermolecular interactions. Each of the dinuclear complexes shows a magnetic behavior
which is determined by the combination of structural parameters and spin polarization
effects. Notably, the para derivative (1) displays a moderate antiferromagnetic coupling (J =
−3.3 cm−1) along a remarkably long Cu···Cu distance (12.30 Å).
compounds, named para (p-LH2), meta (m-LH2), and ortho (o-
LH2) isomers (Figure 1).
INTRODUCTION
■
The coordination properties of β-diketones have been under
investigation for a long time as simple and highly efficient metal
chelating. By linking two β-diketone groups through suitable
spacers, highly versatile ligands can be obtained. Various studies
have been mainly devoted to 1,3- and 1,4-aryl bis-β-diketone
derivatives bonded through carbon atoms 1 and 5 of the β-
diketone groups. For instance, Lindoy and co-workers have
developed an interesting coordination chemistry which
produced metal dimers, triangles, and tetrahedra1 based on
aryl bis-β-diketone ligands and metal ions of the first transition
series.
The presence of the rigid aromatic spacer in such systems
represents a peculiar aspect, since it allows to orient the two
acetylacetone units along different angles and coordination
vectors in the three isomers. This paves the way, still
unexplored, for the exploitation of these ligands in the design
and synthesis of a variety of compounds ranging from discrete
molecular cages and molecular boxes (i.e., dimers, triangles,
squares, and polyhedral capsules) to metal coordination
polymers and networks, up to surface-assisted coordination
chemistry reactions.
Very few examples of metal complexes with the bis-(3-
acetylacetonate)benzene ligands are described in the liter-
ature.3−6 Maverick et al. reported a copper molecular square3
[Cu(m-L)]4 endowed with interesting H2 sorption properties
and able to host a fullerene molecule in its cavity. Similarly, a
first work reported that the o-LH2 isomer leads to a copper
In general, the chemistry of mono- and bis-3-substituted β-
diketones is less rich in comparison to that of the 1,5
substituted derivatives. Recently, a survey on the synthetic
routes of 3-substituted 2,4-pentadionate ligands and their
application as photoactive systems has been published.2 In this
framework, we have recently addressed our interest to bis-
acetylacetone ligands with the two β-diketone units linked to a
phenyl ring through the carbon 3 at the 1,4- 1,3- and 1,2-atoms.
This leads to a series of three bis-(3-acetylacetonate)benzene
Received: February 20, 2012
Published: April 19, 2012
© 2012 American Chemical Society
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Figure 1. Three bis-(3-acetylacetonate)benzene isomers: p-LH2, m-LH2, and o-LH2. Color code: O red, C gray, H white, H bonds dashed red lines.
Table 1. Crystal Data and Details of Data Collections
compound
chemical formula
formula mass
crystal system
a/Å
p-LH2
o-LH2
1
2
3
C16H18O4
274.30
C16H18O4
274.30
C40H44Cl2Cu2N4O14S2
1066.93
C44H56Cl2Cu2N4O16S4
1223.15
C45H54Cl2Cu2N4O17
1120.90
orthorhombic
13.2257(3)
12.6283(3)
8.7883(2)
90.00
monoclinic
17.7839(1)
8.5189(3)
11.8682(8)
90.00
triclinic
triclinic
orthorhombic
20.1413(2)
17.7925(2)
28.3123(3)
90.00
11.0255(3)
11.1307(3)
12.0879(3)
104.425(2)
103.075(2)
108.667(2)
1283.20(6)
123(1)
12.3340(6)
13.2732(5)
18.358 (1)
78.327(4)
81.739(4)
64.461(4)
2650.3(2)
140(1)
b/Å
c/Å
α/deg
β/deg
90.00
124.45(1)
90.00
90.00
γ/deg
90.00
90.00
unit cell volume/Å3
temperature/K
space group
Z
1467.80(6)
298(1)
1482.7(2)
298(1)
10146.1(2)
150(1)
Pnma
C12/c1
P1
P1
Pbca
̅
̅
4
4
1
2
8
ρcalc/mg mm−3
μ/mm−1
1.241
1.229
1.381
1533
1.468
0.505
0.499
1.076
3988
2.630
crystal size/mm3
radiation type
R1, wR(F2) (I > 2σ(I))
0.05 × 0.05 × 0.40
Cu Kα
0.35 × 0.40 × 0.55
Cu Kα
0.70× 0.30 × 0.12
Mo Kα
0.20 × 0.06 × 0.06
Cu Kα
0.30 × 0.20 × 0.20
Cu Kα
a
0.0456, 0.1316
0.0512, 0.1389
0.0543, 0.1659
0.0353, 0.1023
0.0480, 0.1057
0.0718, 0.2164
0.0829, 0.2240
0.0595, 0.1605
0.0673, 0.1672
a
R1, wR(F2) (all data)
0.0576, 0.1721
a
2
R1 = (∑||Fo| − |Fc||/∑|Fo|); wR2 = {∑[w(Fo2 − Fc2)2]/∑[w(Fo ) 2]}1/2; GOF = ∑[w(Fo2 − Fc2)2]/(n − p)}1/2 where n is the number of data and p
is the number of parameters refined.
dimer [Cu(o-L)]2.4 Recently, we showed that the reaction
between o-LH2 and Cu2+ generates a dimer−trimer constitu-
tional dynamic library6 ([Cu(o-L)]n n = 2, 3) where the
designed introduction of a well suited guest (hexamethylene-
tetramine, hmt) allowed us to orchestrate the system. The hmt
molecule acts as an external stimulus for self-sorting of the
system toward the supramolecular triangle {[Cu(o-
L)]3⊂(hmt)}. These findings fostered us to systematically
study the coordination properties of the bis-(3-
acetylacetonate)benzene isomers toward Cu(II) ions in the
presence of a second chelating nitrogen-based ligand, that is,
2,2′-bipyridine. Under these conditions the three ligands (p-
LH2, m-LH2, and o-LH2) yield a series of dinuclear copper
complexes (1, 2, and 3) with similar composition. The
compounds have been fully characterized by X-ray single
crystal diffraction and Electrospray Ionization (ESI) mass
spectrometry. Structural data show an analogue square
pyramidal coordination around Cu(II) ions, with the β-
diketonate ligands lying in bridging position between the two
metal centers. The magnetic behavior of metal complexes based
on the three bis-(3-acetylacetonate)benzene ligands is here
investigated for the first time. Notably, compound 1 shows a
moderate antiferromagnetic coupling (J = −3.3 cm−1) along a
remarkable Cu···Cu distance (12.30 Å). Magnetic measure-
ments evidence that each of the three dinuclear complexes is
characterized by a magnetism that can be correlated to the
combination of structural parameters and ligand spin polar-
ization effects.
EXPERIMENTAL SECTION
■
Synthesis. We report here the general procedure for the
preparation of compounds 1−3. Detailed synthesis of ligands (p-
LH2, m-LH2, and o-LH2) and dinuclear Cu (II) complexes (1−3) is
described in the Supporting Information. To a methanol solution (50
mL) of copper perchlorate (185 mg, 0.5 mmol), solid 2,2-bipyridine
(78 mg, 0.5 mmol) was added in small portions. The mixture was
stirred for 2 h, and LH2 (69 mg, 0.25 mmol) in methanol (10 mL) was
added dropwise. In the case of 1 and 2, the obtained solid has been
filtered, washed, and recrystallized. Compound 3 is characterized by a
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higher solubility: the obtained solution has been refluxed overnight,
the solvent removed in vacuum, and the obtained solid washed and
recrystallized.
ESI/MS Spectrometry. The ESI mass spectra were obtained using
a LCQ DECA ion trap instrument (ThermoFisher Scientific, San Jose,
́
CA, U.S.A.), operating in positive ion mode. The entrance capillary
temperature was 280 °C and the capillary voltage was 5 kV.
Compounds 1−3 were first dissolved in dimethylsulfoxide (DMSO)
and then diluted in acetonitrile to obtain a 10−6 M concentration. The
acetonitrile solutions of each compound were introduced into the ESI
ion source by direct infusion at a flow rate of 10 μL/min. The He
pressure inside the trap was kept constant. The pressure directly read
by ion gauge (in the absence of the N2 stream) was 2.8 × 10−5 Torr.
X-ray Structure Determination and Refinement. Single
crystals were fastened on the top of a Lindemann glass capillary or
mounted using Paratone-N oil and centered on the head of a four-
circle kappa goniometer Oxford Diffraction Gemini E diffractometer,
equipped with a 2K × 2K EOS CCD area detector and sealed-tube
Enhance (Mo) and (Cu) X-ray sources. Data were collected by means
of the ω-scans technique using graphite-monochromated radiation, in
a 1024 × 1024 pixel mode, using 2 × 2 pixel binning. The diffraction
intensities were corrected for Lorentz/polarization effects as well as
with respect to absorption. Empirical multi-scan absorption
corrections using equivalent reflections were performed with the
scaling algorithm SCALE3 ABSPACK. Data collection, data reduction
and finalization and cell refinement were carried out through the
CrysAlisPro software (CrysAlisPro, Oxford Diffraction Ltd., Version
1.171.34.36). Accurate unit cell parameters were obtained by least-
squares refinement of the angular settings of 1326 (p-LH2), 2407 (o-
LH2), 33245 (1), 6540 (2), and 14560 (3) strongest reflections,
chosen from the whole experiment. Structures were solved by means
of direct methods using SHELXS7 and refined by full-matrix least-
squares methods based on Fo2 with SHELXL-977 in the framework of
OLEX28 software. In the last cycles of refinement, ordered non-
hydrogen atoms were refined anisotropically whereas disordered
partial occupancy non-hydrogen atoms were refined isotropically.
Hydrogen atoms connected to carbon atoms were included in
idealized positions, and a riding model was used for their refinement. A
summary of the crystallographic data and of the refinement parameters
for each compound is provided in Table 1. Crystal structure
determination and refinement details for each compound are instead
reported in the Supporting Information, that also includes ORTEP
plots and structure description for ligands p-LH2 and o-LH2. Bond
lengths and angles of the metal coordination spheres, along with
ORTEP plots, for compounds 1, 2, and 3 are reported in the
Supporting Information (Table S3). CCDC 854278 (p-LH2), CCDC
854279 (o-LH2), CCDC 854280 (1), CCDC 854281 (2) and CCDC
854282 (3) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from the Cambridge
Crystallographic Data Center via www.ccdc.cam.ac.uk/data_request/
cif.
Figure 2. Synthesis of the ligands.
directly hydrolyzed under reflux in CH3OH to give p-LH2, m-
LH2, and o-LH2 ligands in 80, 50, and 25% yield, respectively. It
has to be stressed that while in the case of terephthaldehyde (p-
dialdehyde) and isophthalaldehyde (m-dialdehyde) the only
obtained product from the methanolysis of C is the tetraketone
in the dienol form, in the case of phthaldehyde (o-dialdehyde)
both the dienol and the monoenoltriketone products are
formed.9 In this last case the pure o-LH2 ligand can be obtained
by careful crystallization of the product, although this lowers
the reaction yield. Single crystal X-ray measurements show that
in the solid state the two acetylacetone groups are in the cis-
enol form in all the three isomeric ligands (Figure 1). The m-
LH2 X-ray structure has been previously determined3 while the
p-LH2 and o-LH2 ones are here reported for the first time.
Moreover it has been evidenced that the O−H---O are involved
in strong H-bonds, and that the O···O distances are in the
narrow 2.43−2.46 Å interval.
Reaction of m-LH2 and o-LH2 with Cu(II) leads to
compounds of formula [CuL]n, that is, a dimer,4 a molecular
triangle,6 and a molecular square3 for n = 2, 3, and 4,
respectively. These compounds are very soluble in organic
solvents as CH2Cl2 and CHCl3. On the other hand, because of
its geometry, the p-LH2 ligand forms an insoluble product,
probably a one-dimensional (1D) coordination polymer. To
prevent the formation of the insoluble coordination polymer,
an ancillary bidentate coligand (i.e., tetramethylethylenedi-
amine, 2,2′-bipyridine and 1,10-phenanthroline) has been
introduced5 and the same strategy was also adopted for the
meta and ortho ligands. The synthesis route is reported below:
a Cu(ClO4)2 solution (methanol) is first reacted with 2,2′-
bipyridine (bpy) in a equimolar amount and subsequentaly a
ligand solution (methanol) is added (molar ratio Cu:L = 2:1).
Magnetism. The magnetic susceptibilities of the powdered
samples were measured with a Cryogenic SQUID S600 magnetometer
in the 2−300 K temperature range, with an applied field of 1.0 T (T >
30 K) and 0.1 T (T < 30 K). The raw data were corrected for the
underlying sample holder contribution and the intrinsic diamagnetism
of the sample estimated by the Pascal’s constants.
RESULTS AND DISCUSSION
■
2Cu2+ + 2bpy → 2[Cu(bpy)]2+ + LH2
→ [(bpy)Cu‐L‐Cu(bpy)]2+
Three dinuclear copper complexes of formula {[(bpy)Cu-
(sol)]2(L)}(ClO4)2 (1, 2, and 3 with the ligand p-LH2, m-LH2,
and o-LH2, respectively) have been obtained in almost
quantitative yields. Elemental analysis suggests that in the as
prepared compounds the sol moieties are water molecules.
Interestingly, the reaction works very well without using any
Synthesis. The p-LH2, m-LH2, and o-LH2 bis-(3-
acetylacetonate)benzene ligands have been prepared according
to the procedure reported by Ramirez and co-workers (Figure
2).9
Reaction of two moles of 2,2,2-trimethoxy-4,5-dimethyl-1,3-
dioxaphospholene10 (A) with a mole of the corresponding bis-
aldehyde (B) dissolved in CH2Cl2 under rigorous nitrogen
atmosphere afforded the bis-pentaoxyphosphorane intermedi-
ate (C) as a diastereomeric mixture. The intermediate C was
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base to deprotonate the acetylacetone groups, the reaction
driving force being probably the high affinity of the
acetylacetone oxygen atoms for Cu(II) ions. In the case of 1
and 2, the product insolubility in the reaction medium plays an
important role too. We found that in methanol and in the
absence of a base the yields are higher compared to data
previously reported using the para substituted ligand in the
presence of NaOH.5 The solubility in organic media of the
three products increases from 1 to 3. Dimer 1 is soluble only in
high coordinating solvents such as DMSO, DMF, and pyridine;
dimer 2 is soluble in the same solvents and in acetonitrile, while
compound 3 is very soluble in organic solvents such as
acetonitrile, acetone, and methanol.
ESI/MS Spectrometry. ESI/MS analysis of compounds 1−
3 show similar spectra (see for instance the ESI-MS positive ion
mode spectra of compound 3, Figure 3). The most abundant
Figure 4. Compounds 1, 2, and 3. Color code (applied to the
following figures): Cu purple, S yellow, O red, N blue, C gray, H-bond
−
dashed red line. H atoms and ClO4 anions are omitted for clarity.
Figure 3. ESI-MS positive ion mode spectra of compound 3 (right);
experimental and calculated [CuI(bpy)2]+ peak (left).
coordination the sixth position is occupied by a nitrogen
atom of the closest nearby bipyridine molecule, with a long
Cu---N interaction (ca. 3.3 Å for 1 and ca. 3.2 Å for 3). The
Cu−Oacac and Cu−Nbpy distances are in the intervals 1.890−
1.927 Å and 1.980−2.004 Å, respectively (Table 2). The crystal
structure of compound 1 has been already reported;5 below we
point to some features useful for the following structural-
magnetochemical correlations. The two copper atoms lie at
12.30 Å. The equatorial plane, formed by the acetylacetonate
oxygen and the bipyridyl nitrogen atoms around each metal ion
is nearly orthogonal (83°) to the plane of the acetylacetonate
phenyl ring (Figure 5). Finally, the torsional angle ∠Cu1−C6−
C6*−Cu1* (* = −x, −y, 1 − z) is 0° by symmetry (Figure 4).
The copper coordination sphere in compound 2 is similar to
the one found in dimer 1. Two DMSO molecules, oriented in
antiparallel fashion, occupy the apical position of the
coordination sphere (Cu−ODMSO: 2.229 and 2.259 Å). The
two copper atoms are at a distance of 11.55 Å and the ∠Cu1−
C19−C15−Cu2 torsional angle is 13.4° (Figure 4). Moreover,
the plane of the acetylacetonate phenyl ring define angles of
78° and 73°, respectively (Figure 5). Unlike compounds 1 and
2, the two copper atoms of 3 are cofacial (Cu···Cu distance
5.68 Å), likely because of the H-bond between the two apical
water molecules. Compound 3 has a strained tweezers folding,
with a torsional angle ∠Cu1−C7−C6−Cu2 of 12.2° (Figure
4). The equatorial coordination of the Cu ions is the same as in
1 and 2, while the apical positions are taken by two water
molecules (Cu−Ow: 2.235 and 2.321 Å) involved in a H-bond
interaction (O1−H1B···O2: 1.876 Å; O1···O2: 2.74 Å). These
peak is due to the ionic species [(bpy)Cu(L)+H]+ (m/z =
492); for 2 and 3, the molecular cation [M-ClO4]+ is observed
at m/z = 810 with a very low abundance (ca. 3%). The
presence of a low abundant ion at m/z 375 has been detected
for all the three dimers (5−15%). It can be ascribed to the ionic
species [CuI(bpy)2]+ where the metal ion has been reduced to
Cu(I). This reduction behavior has already been described in
literature for Cu(II) pyridil chelates. In particular, Gianelli and
co-workers11 have shown as this process derives from a charge
transfer between the solvent and the metal complex in the gas
phase. It is worthwhile recalling that the electrospray source can
be considered as a particular electrolytic cell, in which
electrolysis maintains the charge balance allowing the
continuous production of charged droplets.12
Structure Description. For compounds 1−3 single crystals
have been isolated and structurally characterized (Figure 4).
The general formula of the dimers is {[(bpy)Cu(sol)]2(L)}-
(ClO4)2, where sol is a DMSO (1 and 2) or a water (3)
molecule; bis-(3-acetylacetonate)benzene ligands are bridging
between the two metal centers. The Cu(II) ions show a square
pyramidal coordination in compound 2, whereas in compound
1 and 3, because of intermolecular interactions (see below), the
metal environment can be described as pseudo-octahedral. In
all compounds the Cu(II) ions are surrounded by two
acetylacetonate oxygen and two bipyridil nitrogen atoms in
the equatorial plane, with an apical position held by a solvent
molecule. In the compounds showing pseduo-octahedral
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Table 2. Selected Bond Lengths (Å) for 1, 2, and 3
1
2
3
Cu1−O1
Cu1−O2
Cu1−N2
Cu1−N1
1.9173(7)
1.9003(7)
1.9888(7)
2.0014(8)
Cu1−O2
Cu1−O3
Cu1−N1
Cu1−N2
1.898(5)
1.909(5)
1.998(6)
2.002(6)
Cu2−O5
Cu2−O6
Cu2−N3
Cu2−N4
1.891(5)
1.926(5)
1.980(6)
2.005(6)
Cu1−O5
Cu1−O6
Cu1−N2
Cu1−N1
1.910(2)
1.893(2)
1.990(3)
1.996(3)
Cu2−O3
Cu2−O4
Cu2−N4
Cu2−N3
1.876, 175.21
1.900(2)
1.894(2)
1.998(3)
1.998(3)
O1−H1B···O2
both of them involving the bipyridine ligands of two nearby
molecules
Magnetic Behavior. Magnetic studies have been pursued
on the three dimers. The plot of χT vs T (Figure 7a) for
complex 1 clearly shows moderate antiferromagnetic inter-
actions with a decrease, below 50 K, from the room
temperature value of 0.8 emu K mol−1 (as expected for two
independent S = 1/2 with g > 2.00) to 0.2 emu K mol−1 at 2 K.
The experimental curve can be fitted by a Bleaney−Bowers
equation,13 based on the Hamiltonian H = −JS1S2 + gβB·S,
providing the following best fit parameters: J = −3.30 0.02
cm−1, g = 2.060 0.002 (R2 = 0.99846). In addition, the χ vs T
(Figure 7b) curve shows a maximum at 2.9 K which provides a
similar |J| value (≈ 3.3 cm−1) by appliyng the relation |J| ≈
kTmax/0.6 (k is the Boltzmann constant and Tmax the
temperature at the χ maximum). The magnitude of the
singlet−triplet gap, and thus of the antiferromagnetic intra-
dimer interaction, was confirmed by the field dependence of the
magnetization at low temperature (2 K), clearly revealing a field
induced spin state change (singlet → triplet) close to B = 3.5 T.
The corresponding maximum of ∂M/∂B vs B curve is
evidenced in the inset of Figure 7a.
Much weaker magnetic interactions among the two copper
centers were observed for 2 and 3, both showing very regular
Curie−Weiss plots. In particular for 2 the χT vs T plot (Figure
8) suggested a very weak ferromagnetic interaction, with χT
feebly increasing above the expected Curie value below 20 K,
and a subsequent drop. This interpretation was confirmed by a
fit of the corresponding Curie−Weiss plot (inset of Figure 8),
which provided a positive (ferromagnetic) Weiss constant of
1.3 K and a Curie constant of 0.78 emu K mol−1.
As for 3, temperature dependent magnetic data clearly points
to the presence of negligible exchange coupling interaction, χT
value being constant down to 10 K at 0.8 emu K mol−1, in
agreement with two essentially uncoupled Cu(II) centers
(Figure 9). Accordingly, the fit of the corresponding Curie−
Weiss plot (inset Figure 9) provides a Curie constant of C =
0.80 emu K mol−1 and a negative (antiferromagnetic) θ value of
0.12 K, confirming the weakness of the interaction.
As described above, compound 1 displays a moderate
antiferromagnetic coupling which occurs along a remarkably
long Cu···Cu distance (12.30 Å). This is one of the few
examples of dinuclear copper compounds where the magnetic
coupling is still not negligible at distances longer than 1 nm.14
The J value (−3.3 cm−1) is comparable to previous data for
other dinuclear copper systems with π-conjugated bridging
ligands such as aromatic diamines (Cu···Cu = 12.07 Å, J = −3.2
cm−1;14a Cu···Cu = 12.26 Å, J = −8.5 cm−1;14f Cu···Cu = 16.42
Å, J = −2.0 cm−1;14f) and bis(oxamate) with an aromatic spacer
(Cu···Cu = 10.80 Å, J = −2.2 cm−1;14d Cu···Cu = 12.20 Å, J =
−8.7 cm−1;14e Cu···Cu = 14.95 Å, J = −3.9 cm−1;14g). Probably,
the most intriguing result of the magnetic studies is the
exchange coupling trend along the series of the three dinuclear
copper complexes. A rationalization of the different magnetic
Figure 5. Orientation of the “CuO2N2” mean planes and the phenyl
plane in compounds 1, 2, and 3.
two water molecules are also involved in further H-bonds with
three crystallization acetone molecules (Supporting Informa-
tion, Figure S6). As for compound 2, the copper ions equatorial
planes and the plane of the phenyl ring deviate appreciably
from orthogonality (angles: 76° and 72° respectively, Figure 5).
All the three compounds exhibit noncovalent intermolecular
interactions in the solid state (Figure 6) leading to supra-
molecular chains. In 1 and 3, intermolecular interactions are
very similar and involve the acetylacetonate moiety of a
molecule and the bipyridine of the closest nearby molecule
(distances: ca. 3.1 Å in 1 and ca. 3.2 Å in 3; Figure 6b and d).
This interaction is responsible of the pseudo-octahedral
environment found in both 1 and 3 complexes (see above).
In 2, the supramolecular chains are stabilized by a proper π···π
stacking (I, distance 3.3−3.4 Å; Figure 6e and f) and by a
weaker interaction (II, distance 3.4−3.5 Å; Figure 6e and g),
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Figure 6. Supramolecular chains. Compound 1 (a), 3 (c), 2 (e), and their intermolecular interactions 1 (b), 3 (d), 2 (f: I; g: II). Dotted cyan lines:
−
intermolecular interactions (Å). H atoms, ClO4 anions, and coordinating solvent molecules (in b, d, f, and g) are omitted for clarity.
Figure 7. χT vs T (in the 0−300 K range) and χ vs T (in the 0−50 K range) plots for complex 1 (empty triangles) and best fit curve (dashed line).
In the inset of 7a, the derivative of the isothermal magnetization curve (2 K) with respect to the field is reported, evidencing a field induced spin state
crossing at around 3.5 T.
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provided by the bridging π-system, and its relative orientation
to the magnetic orbitals of the interacting metal centers.15 In
the case under study, the maximum overlap of the magnetic
2
2
orbitals of Cu(II) ions (dx −y ) with the π-system of the ring is
expected when perfect orthogonality of the aromatic plane to
the copper−oxygen planes occurs.16 This would then lead to
antiferromagnetic interactions for ortho and para isomers, while
ferromagnetic interactions are expected for the meta derivative.
On the basis of the above-discussed structural consideration
(Figure 5), we can qualitatively explain the magnetic behavior
of the three compounds. Compound 1 presents an
antiferromagnetic coupling as expected for the para isomer
because of spin polarization along the aromatic linker.
Morevover, the copper ions equatorial planes are close to
orthogonality with the phenyl ring leading to a moderate
magnetic coupling. At variance, the deviation from orthogon-
ality between the planes becomes larger (Figure 5) for
compounds 2 and 3. As proved by the corresponding Curie−
Weiss plots (inset of Figure 8 and 9), the spin polarization
effect is still operating leading to ferro- and antiferro-
interaction, respectively. However, on the basis of consid-
erations of the orthogonality among planes, this mechanism is
expected to be substantially weakened, a fact which might
account for the much smaller magnitude of the observed
interactions for these two derivatives compared to compound 1.
Figure 8. χT vs T plot for complex 2 (full circles) in the 2−140 K
range. In the inset the Curie−Weiss plot (empty triangles) over the
full temperature range is reported together with the corresponding
best fit line obtained with parameters reported in the text.
CONCLUSIONS
■
Bis-(3-acetylacetonate)benzene isomers (para, meta, and ortho)
behave as highly versatile coordinating ligands that promptly
react with Cu(II) ions in simple and designed conditions. The
affinity of these ligands for copper ions is so strong that the
presence of a base for the deprotonation is not necessary. Three
crystalline dimers (1−3) with an analogue square pyramidal
coordination around the Cu(II) ions and the bis β-diketone
ligands bridging the metal atoms are readily obtained.
Compounds 1 and 3 form supramolecular chains in the solid
state because of intermolecular interactions. The three dimeric
complexes show antiferromagnetic or ferromagnetic interac-
tion, which is dependent on the position of the two
acetylacetone groups on the aromatic ring because of spin
polarization mechanism. In addition, differences in the
magnitude of the magnetic coupling are also evidenced, and
they are correlated to structural parameters. The moderate
antiferromagnetic coupling (J = −3.3 cm−1) observed in
compound 1 regardless of the remarkable intermetal distance
(Cu···Cu = 12.30 Å) opens intriguing perspectives in the
investigation of long distance electron mediated interactions.
The chemistry of the bis-(3-acetylacetonate)benzene ligands
will be extended to further transition metals, and structure-
properties correlations will be studied taking into account
steric, structural, and electronic effects, including the differences
in the coordination spheres of the different metal ions.
Figure 9. χT vs T plot for complex 3 (empty triangles) in the 2−150 K
range. In the inset the Curie−Weiss plot (empty squares) over the full
temperature range is reported, together with the corresponding best fit
line obtained with parameters reported in the text.
behavior of the three complexes should in principle take into
account several steric, structural, and electronic effects,
including the differences in the coordination spheres of Cu(II)
atoms. A first point to be noted is, however, that neither the
magnitude nor the sign of the interaction appear to be related
to the Cu···Cu distance, since the interaction is much stronger
for 1 (Cu···Cu = 12.30 Å) than for 2 (Cu···Cu = 11.55 Å) and
3 (Cu···Cu = 5.68 Å). Compounds 1 and 3 deserve further
attention since both show an intermolecular Cu---N (3.2−3.3
Å) interaction as well as a weak interaction between the beta-
diketonate ligand and a bipyridine ligand of a different
molecule. It is then quite obvious to consider whether magnetic
coupling can take effect through these interactions. The local
geometry of the involved Cu(II) ions is the same in the two
compounds, but while 1 exhibits an antiferromagnetic coupling,
3 has a negligible magnetic interaction. Hence, we exclude that
any of these interactions plays a role in the magnetic coupling
of the two species. For compound 2, following the same
reasoning, we discard any relevant effect of the bipyridine
ligands π···π stacking on the magnetic exchange coupling. It is
then clear that the exchange interaction is essentially trans-
mitted via delocalization through the π-system of the phenyl
ring. In aromatic linkers, the spin sign alternates along the
linker atoms leading to ferro- or antiferro-magnetic coupling,
depending on the number of atoms located between the
interacting metal ions because of spin polarization mechanism.
In such cases, one would expect that the major role is played by
a subtle balance between the spin-polarization mechanism,
ASSOCIATED CONTENT
■
S
* Supporting Information
Synthesis, X-ray determination, and refinement details, ligands
structure description, additional figures, and crystallographic
information files (CIF). This material is available free of charge
via the Internet at http://pubs.acs.org..
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(16) Dul, M.-C.; Ottenwaelder, X.; Pardo, E.; Lescouezec, R.;
Journaux, Y.; Chamoreau, L.-M.; Ruiz-Garcìa, R.; Cano, J.; Julve, M.;
Lloret, F. Inorg. Chem. 2009, 48, 5244.
AUTHOR INFORMATION
Corresponding Author
*E-mail: marzio.rancan@unipd.it (M.R.), silvio.quici@istm.cnr.
it (S.Q.), lidia.armelao@unipd.it (L.A.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Prof. E. Tondello for encouragement and helpful
discussions. The research was supported by Italian MIUR
through FIRB RBPR05JH2P “Rete” ItalNanoNet, FIRB
RBAP114AMK “RINAME”, and PRIN 20097X44S7 “Record”
Projects, and by University of Padova through Progetto
Strategico “HELIOS” and the 2008 Scientific Equipment for
Research initiative.
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