Copper(II) cubanes with a {Cu4O} core and well defined S = 1 ground state

Article abstract of DOI:10.1039/c5dt03643g

The reaction of 2-pyridinemethanol with copper 4-fluorobenzoate has yielded a family of type II cubanes with formula [Cu₄(pymO)₄(4-F-PhCOO)₃(NO₃)] (1), [Cu₄(pymO)₄(4-F-PhCOO)₄] (2) and [Cu₄(pymO)₄(4-F-PhCOO)₄(H₂O)] (3). These systems exhibit an unexpected S = 1 ground state and their magnetic properties have been unambiguously characterized and rationalized as a function of the asymmetry of the {Cu₄O₄} cage and Cu-O-Cu bond angles. Analysis of the coupling constants was performed applying new interaction schemes. Magneto-structural correlations have been performed from the analysis of previously reported type II copper cubanes.

Full text of DOI:10.1039/c5dt03643g

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Copper(II) cubanes with {Cu4O4} core and well defined S = 1
ground state.
A. Escuer,*a J. Mayansa and M. Font-Bardiab
Received 00th January 20xx,
Accepted 00th January 20xx
The reaction of 2-pyridinemethanol with copper 4-fluorobenzoate has yielded a family of type II cubanes with formula
[Cu4(pymO)4(4-F-PhCOO)3(NO3)] (1), [Cu4(pymO)4(4-F-PhCOO)4] (2) and [Cu4(pymO)4(4-F-PhCOO)4(H2O)] (3). These systems
exhibit an unexpected S = 1 ground state and their magnetic properties have been nambiguously characterized and
rationalized as function of the asymetry of the {Cu4O4} cage and Cu-O-Cu bond angles. Analysis of the coupling constants
was performed applying new interaction schemes. Magneto-structural correlations have been performed from the
DOI: 10.1039/x0xx00000x
www.rsc.org/
analysis of previously reported type II copper cubanes
.
In all reported cases, dominant anti- or ferromagnetic
interactions mediated by the short Cu-O superexchange
pathways, lead to S = 0 or S = 2 typical ground states.
Introduction
Copper cubanes have been classified in structural types as
function of its topology and degree of distortion. Mergeheen
and Hasse1 proposed a classification based on the relative
distribution of the elongated Cu-O distances in the cube: if the
four elongated distances are roughly parallel, the cube can be
envisaged as two weakly interacting dimeric subunits (named
type I) whereas if the elongated distances are distributed
perpendicularly on two opposite faces of the cube (named
type II), it can be envisaged as a folded Cu4O4 ring with four
weak additional interactions, Scheme 1. More recently,
Alvarez2 et al. made an alternative proposal based in the
distribution of the six Cu···Cu distances, being the (2+4) and
(4+2) classes equivalent to types I and II respectively and
adding a new class in which the six Cu···Cu distances are
similar, named (6+0) class. The (2+4) and (4+2) cubes usually
corresponds to systems in which the CuII cations have a square
pyramidal or elongated octahedral environment whereas the
(6+0) cubes should be assigned to the scarce cores with six
equivalent faces, in which the coordination polyhedron around
the CuII cations is usually a trigonal bipyramid. Obviously, the
magnetic properties of the cubes are strongly dependent of
the structure, being the (6+0) class closer to a true cube
whereas type I or (2+4) class is closer to two more or less
weakly interacting dimers and type II or (4+2) class, is more
related with a distorted Cu4 ring.
Cu
O
Cu
O
O
Cu
Cu
O
O
O
Cu
O
Cu
O
O
O
Cu
O
Cu
Cu
Cu
Cu
O
Cu
Type II
Type I
4+2 class
2+4 class
6+0 class
Scheme 1 Schematic drawing of the cubane CuII complexes according the relative
position of the elongated Cu-O distances (red dashed bonds).
2-pyridinemethanol (pymOH) and the closely related (R/S)-α-
methyl-2-pyridinemethanol (MpymOH) ligands are able to
generate polynuclear systems linking up to three cations,
Scheme 2. Their copper chemistry has been poorly explored
and only some dimers,3 isolated4 or linked cubanes,4c,5 one
chain6 and some heterometallic CuII-GdIII clusters7 have been
reported for pymOH and only one pair of enantiomers8 have
been described for (R/S)-MpymOH. Our initial target was to
explore the reactivity of these ligands in carboxylate-copper
chemistry but unfortunately unambiguous characterization
was only possible for pymOH derivatives.
a.Departament de Química Inorgànica and Institute of Nanoscience (IN2UB) and
Nanotecnology, Universitat de Barcelona, Av. Diagonal 645, Barcelona-08028,
Spain.
b.Departament de Mineralogia, Cristal·lografia i Dipòsits Minerals and Unitat de
Difracció de R-X. Centre Científic i Tecnològic de la Universitat de Barcelona
(CCiTUB). Universitat de Barcelona. Solé i Sabarís 1-3. 08028 Barcelona.
† E-mail: albert.escuer@ub.edu.
Electronic Supplementary Information (ESI) available: Structural information in cif
format, CCDC numbers 1415989-1415991; Table S1 structural and magnetic data
for selected cubes; Figure S1, magnetic simulations for one selected topology.
Scheme 2 Ligands employed in this work and its coordination mode in the {Cu4O4} core
of compòunds 1-3.
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In this work we report the syntheses and characterization of [Cu4(pymO)4(bzF)3(NO3)] (1). Some few crystals of complex
1
three new cubanes with Cu4O4 core belonging to the (4+2) were initially obtained from a Cu(4-FBz)2 starting reagent
class, obtained from the reaction of copper(II) 4- contaminated with nitrates. In light of the structural results,
fluorobenzoate and 2-pyridylmethanol (pymOH) with formula the syntheses was repeated dissolving Cu(4-FBz)2 and
[Cu4(pymO)4(4-F-PhCOO)3(NO3)]
(
1
),
[Cu4(pymO)4(4-F- Cu(NO3)2·6H2O in 3:1 ratio (0.375 mmol, 0.128 g : 0.125
PhCOO)4] (2) and [Cu4(pymO)4(4-F-PhCOO)4(H2O)]·0.5 MeOH mmol, 0.037 g) in methanol (5mL) and the ligand pymOH in
·0.25 H2O (3· 0.5 MeOH ·0.25 H2O). Magnetic susceptibility and
magnetization measurements prove that these systems
possesses a well isolated S = 1 ground state. This unique
property has been rationalized as function of the cage bond
parameters. We also report a general study of the magnetic
response of the previously reported (4+2) class cubes and a
critical analysis of the models usually applied to fit the
magnetic data.
Table 1. Crystal data, collection and structure refinement details for the X-ray structure
determination of complexes 1-3.
(1)
(2)
(3)
C45H36Cu4F3N5O13 C52H40Cu4F4N4O12 C210H178Cu16F16N16O55
Formula
FW
1165.95
Monoclinic
1243.04
Monoclinic
C 2/c
5126.31
Monoclinic
C 2/c
System
Space
group
a/Å
P 21/c
13.344(13)
18.937(2)
19.212(16)
90
38.530(3)
13.529(11)
19.446(14)
90
41.241(2)
13.4071 (6)
19.5890(9)
90
b/Å
Experimental
c/Å
Materials and methods
α/deg.
115.388(5)
90
92.806(4)
90
107.999(2)
90
ß/deg.
The Cu(4-F-PhCOO)2 starting reagent was synthesized in typical
yields >70% mixing equimolecular amounts of aqueous
solutions of Cu(NO3)2·6H2O and Na(4-F-PhCOO) salts. The
copper carboxylate was collected by filtration and washed with
cold water. Samples for analysis were gently dried to remove
volatile solvents. The yield for 1-3 was around 25% of well
formed crystals which were employed for instrumental
measurements.
γ
/deg.
V/ Å3
4385.9(7)
4
10125.0(14)
8
10301.1(8)
2
Z
293(2)
302(2)
100(2)
T, K
0.71073
0.71073
0.71073
λ(MoKα),
Å
1.766
2.001
1.631
1.740
1.653
1.715
ρcalc,
g·cm-3
IR spectra (4000-400 cm-1) were recorded using a Bruker IFS-
125 FT-IR spectrometer with samples prepared as KBr pellets.
Variable-temperature magnetic studies were performed using
a DSM5 Quantum Design magnetometer operating at 0.03 T in
the 300-2.0 K range. Diamagnetic corrections were applied to
the observed paramagnetic susceptibility using Pascal’s
constants.
µ(MoKα),
mm-1
R
ωR2
0.0676
0.1683
0.0485
0.1265
0.0326
0.0837
5mL of acetonitrile. Both solutions were mixed and stirred for
three hours.
Energy levels plotted in Figure 4b, 4c, 4d and those in Figure 6
have been calculated for an arbitrary J2 value of -50 cm-1.
Complex
1
crystallizes as blue crystals by vapour diffusion with
): C, 46.35; H,
diethyl ether. Anal. Calcd for C45H36Cu4F3N5O13
(
1
Single-crystal X-ray crystallography
3.11; N, 6.01%. Found: C, 46.92; H, 3.4; N, 5.88%. Relevant IR
bands: 3440 (s, broad), 3077(w), 2835 (w), 1620(s), 1580(s),
1506(s), 1440(s), 1360(s), 1310 (s), 1210(w), 1150(w), 1050(s),
860(w), 785(w), 760(w), 630(w) cm-1.
Blue prism-like specimens of approximate dimensions 0.196
mm x 0.336 mm x 0.522 mm (
1), 0.082 mm x 0.168 mm x 0.227
mm (2) and 0.390 mm x 0.397 mm x 0.508 mm (
3
) were used
for the X-ray crystallographic analysis. The X-ray intensity data
were measured on a D8 Venture system equipped with a
multilayer monochromator and a Mo microfocus (λ = 0.71073
Å). The frames were integrated with the Bruker SAINT
software package using a narrow-frame algorithm. The final
cell constants were based upon the refinement of the XYZ-
centroids of reflections above 20 σ(I). Data were corrected for
absorption effects using the multi-scan method (SADABS). The
structures were solved using the Bruker SHELXTL Software
Package, and refined using SHELXL.9 Details of crystal data,
[Cu4(pymO)4(bzF)4] (2). Cu(4-FBz)2 (0.5 mmol, 0.170 g) was
dissolved in methanol (5mL) and the ligand pymOH was
dissolved in 5mL of acetonitrile. Both solutions were mixed
and stirred for three hours. Slow evaporation of the resulting
solution yields complex
2 as blue crystals. Anal. Calcd for
C52H40Cu4F4N4O12 (2): C, 50.24; H, 3.24; N, 4.51%. Found: C,
49.32; H, 3.10; N, 4.31%. Relevant IR bands: 3440 (s, broad),
3077(w), 2835 (w), 1620(s), 1440 (w),, 1400(s), 1360(w),
1250(w), 1210(w), 1150(w), 985(w), 630(w), 480(w), 411(w)
cm-1.
collection and refinement for 1-3 are summarized in Table 1.
Analyses of the structures and plots for publication were
performed with Ortep310 and POVRAY programs.
[Cu4(pymO)4(bzF)4]·0.5 MeOH ·0.25 H2O (3· 0.5 MeOH ·0.25
H2O). Cu(4-FBz)2 (0.5 mmol, 0.170 g) was dissolved in
Synthetic procedure
methanol (5mL). The ligands pymOH (0.75 mmol, 0.081 g) and
(S-pyeOH) (0.25 mmol, 0.035 g) were dissolved in acetonitrile
2 | J. Name., 2012, 00, 1-3
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(5 mL). The mixture of both solutions was stirred for three similar to
hours, filtered and layered with diethyl ether. Well formed substituted by
blue crystals suitable for X-Ray analysis growth after two carboxylates act as bidentate ligands coordinated to the
weeks. Anal. Calcd for C52.5H44.5Cu4F4N4O13.75 3· 3· 0.5 MeOH neighbor {Cu1/O4/Cu4/O10} and {Cu1/O10/Cu3/O1} faces
1
but in this case the nitrate ligand has been
a
fourth carboxylate. In this case two
(
·0.25 H2O): C, 49.20; H, 3.50; N, 4.37%. Found: C, 49.73; H, whereas the other two carboxylates act as monodentate
3.41; N, 4.16%. Relevant IR bands: ν= 3440 (s, broad), 3160(w) ligands coordinating Cu2 and Cu3. Cu1 and Cu4 show an
2835 (w), 1610(s), 1550(s), 1400(s), 1210(s), 1080 (s), 860(s), elongated octahedron coordination polyhedron whereas Cu2
780(s), 618(s), 530(w) cm-1.
and Cu3 exhibit square pyramidal environment. The core of
the cube is more distorted than complex as is reflected in the
1
large Cu4-O7 distance of 2.962(7) Å or the Cu4-O4-Cu2 bond
angle of 120.1(3)º.
Results and discussion
Structural description
[Cu4(pymO)4(4-F-PhCOO)3(NO3)] (1). The molecular structure
consist of isolated cubanes with {Cu4O4} core. A view of the
structure is shown in Figure 1 and the main bond parameters
are summarized in Table 2. One pymO- ligand is coordinated to
each copper cation, providing the four ꢀ3-alcoxo corners of the
cube. The pymO- ligands are placed roughly perpendicular to
two opposite faces of the cube whereas three of the
remainder four faces are occupied by the three bidentate
carboxylates. The nitrate anion acts as monodentate ligand,
coordinated to Cu4. Cu2 shows a square pyramidal CuNO4
environment whereas Cu(1,3,4) exhibit an axially elongated
octahedral CuNO5 coordination polyhedron. The elongated
axial bond distances involve one Cu-O cage bond for each
copper cation and one Cu-O bond with one O-carboxylate or
O-nitrate for Cu(1,2,4). Equatorial bond distances are in the
short 2.000 - 1.905 Å range whereas the axial Cu-O bond
distances are relatively large, ranging between 2.344(3)-
2.697(3).
Figure 2 Top, a view of the molecular structure of compound 2. Bottom, labeled core of
the cubane. Bonds depicted in orange correspond to the short Cu-O distances inside
the {Cu4O4} cage.
Figure 1 Top, a view of the molecular structure of compound 1. Bottom, labeled core of
the cubane. Bonds depicted in orange correspond to the short Cu-O distances inside
the {Cu4O4} cage.
[Cu4(pymO)4(4-F-PhCOO)4] (2).
A view of the structure is shown in Figure 2 and the main bond
parameters are summarized in Table 2. The structure is very
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a
CuII cubane
Figure 3 Top, a view of the molecular structure of compound 3. Bottom, labeled core of
the cubane. Bonds depicted in orange corresponds to the short Cu-O distances inside
the {Cu4O4} cage and dashed red bonds shows the H-bonds involving the coordinated
water molecule.
discussion), apparently incompatible with
topology.
To have a clear picture of the magnetic properties of all
previously reported cubanes with (4+2) shape, a search in the
CCDC database was performed and 119 entries were obtained
for CuII cubes with four elongated Cu-O bonds larger than
2.100 Å as the only restrain. Cubes for which the complete
magnetic analysis was not reported or the coordination
polyhedron around the CuII cations was trigonal bypyramid
were discarded from this study. The magnetic data for the 43
(4+2) CuII cubes with reported magnetic data and square
pyramidal or elongated octahedral environment around the
CuII cations are summarized in Table 3.
[Cu4(pymO)4(4-F-PhCOO)4(H2O)]· 3· 0.5 MeOH ·0.25 H2O (3· 3·
0.5 MeOH ·0.25 H2O).
A view of the structure is shown in Figure 3 and the main bond
parameters are summarized in Table 2. The structure of is
closely related to compound but now there is an additional
3
2
water molecule coordinated to Cu2, which turns
hexacoordinated. The coordinated water molecule establishes
two strong H-bonds with the non-coordinated O6 and O12
atoms belonging to the monodentate carboxylates and also
interactis with the crystallization water molecule. O6···O3w
The next step was to check which models were applied to fit
the experimental data and up to five models were found to
describe the magnetic response of these systems, Scheme 3.
and O12···O3w distances are 2.685(3) and 2.780(2)
Å
respectively. The presence of this new ligand on Cu2 increase
the distance between the monodentate carboxylates and as
consequence, displaces Cu4 with the concomitant increase of
the Cu4-O7 distance (up to 3.253 Å) and the Cu2-O4-Cu4 bond
angle, which reaches 124.32(6)º. As can be seen in Table 2, the
three cubes are quite similar in their general trends, increasing
distortion of the cage from the less (
cube.
1) to most distorted (3)
Table 2. Main bond distances (Å) and angles (deg.) for complexes 1-3.
Distance Å
Cu1-O1
Cu3-O1
Cu3-O7
Cu2-O7
Cu2-O4
Cu4-O4
Cu4-O10
Cu1-O10
Cu1-O4
Cu2-O1
Cu3-O10
Cu4-O7
(1)
(2)
(3)
1.942(2)
1.983(2)
1.947(2)
1.914(2)
1.916(3)
1.929(3)
1.931(2)
1.922(2)
2.697(3)
2.344(3)
2.410(3)
2.594(3)
1.961(7)
1.926(7)
1.929(7)
1.914(8)
1.945(6)
1.931(7)
1.980(6)
1.960(7)
2.651(6)
2.462(7)
2.359(6)
2.962(7)
1.969(1)
1.931(1)
1.911(1)
1.948(1)
1.959(1)
1.937(1)
1.980(1)
1.970(1)
2.697(1)
2.417(1)
2.378(1)
3.253(1)
Scheme 3. Interaction schemes for the CuII cubane topology according the literature.
The models have been named according the number of identical faces and coupling
constants.
Despite the evidence that the superexchange interaction
mediated by the opposite faces with exclusively axial-
equatorial (Jahn-Teller) pathways is usually poorly effective in
comparison with the four faces with equatorial-equatorial
pathways,2,11 some coupling constants analysis were
performed assuming a regular model (Scheme 3, model (6) and
Table 3), for which the corresponding Hamiltonian is:
Angles (º)
Cu1-O10-Cu4
Cu4-O4-Cu2
Cu2-O7-Cu3
Cu3-O1-Cu1
(1)
(2)
(3)
108.7(1)
112.2(1)
100.3(1)
102.6(1)
104.3(3)
120.1(3)
110.9(4)
101.5(3)
104.23(5)
124.32(6)
110.77(6)
100.80(5)
Spin levels and ground state for Cu4 4+2 cubane topology
H = -J1(S1·S2 + S1·S3 + S1·S4 + S2·S3 + S2·S4 + S3·S4)
(1)
The magnetic properties for the (4+2) copper cubane topology
have been widely studied by DFT calculations2,11 and all
studied cases leads to the S = 0 or S = 2 ground state. As can be
expected, was also stated that the axial-equatorial interactions
involving often very large Cu-O distances (on two elongated
opposite faces, Scheme 1), always must be weak.2 Surprisingly,
On the other hand, the magnetic properties for the most of
the reported systems were calculated with the (2:4) or (0:4)
models, Scheme 3 and Table 3, for which the Hamiltonians are:
H = -J1(S1·S2 + S3·S4) -J2(S1·S3 + S1·S4 + S2·S3 + S2·S4)
H = -J2(S1·S3 + S1·S4 + S2·S3 + S2·S4)
(2)
(3)
the susceptibility measurements performed for complexes
1 -
3
clearly suggest an unprecedented "anomalous" intermediate
spin ground state S = 1 (see further magnetic properties
4 | J. Name., 2012, 00, 1-3
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For a reduced number of asymmetric cubes, models taking into account the elongated faces, model (2:4) and Hamiltonian
into account different interactions for each pair of opposite (2), we realize that for dominant antiferromagnetic
faces of the cube were applied (models (0:2:2) and (2:2:2), interaction J2 the ground state is always S = 0 (first S = 1
a
Scheme 3 and Table 3), applying the Hamiltonians:
excited state has the same slope) and for a positive sign of J2,
the ground state can switch from S = 2 to S = 0 for J2/J1 ratios
lower than -0.5, Figure 4b and 4c respectively).
H = -J2(S1·S3 + S2·S4) -J3(S2·S3 + S1·S4)
(4)
In some few cases, fit of the experimental data was performed
assuming a set of two or three J values for opposite faces of
the cubes. Neglecting the interaction mediated by the
H = -J1(S1·S2 + S3·S4) -J2(S1·S3 + S2·S4) -J3(S2·S3 + S1·S4) (5)
Hamiltonians (3) and (4) are the limit of (2) and (5) when J1 was opposite elongated faces, Scheme (0:2:2) and Hamiltonian (4),
neglected assuming J2, J3 >> J1.
we realize that if one of the interactions is antiferromagnetic
then S = 0 is the ground state for any positive or negative J2/J3
ratio, Figure 4d. As in the (0:4) case, the addition of the weak
interactions mediated by the elongated faces, model (2:2:2),
only produces very small changes in the energy of the spin
levels.
The above calculations excludes these models to analyze
compounds
1-3 and then, the origin of their intermediate
ground state must be found in other structural facts, neglected
until now. The dependence of the magnitude and the sign of
the coupling constants as function of the Cu-O-Cu bond angles
has been demonstrated by theoretical calculations and has
been the preferred parameter to correlate the magnetic
properties.2,11 The Cu-O-Cu bond angles involved in equatorial-
equatorial bridges for (4+2) class of cubes, Figure 5, take
values comprised between 100º-115º, being exceptional to
find Cu-O-Cu bond angles below or above these limits. The
border between ferromagnetic-antiferromagnetic response is
unclear because it can depends of the characteristics of the
bridging ligand that provides the
ꢀ3-O linkage among other
factors2 but, always assuming that there are compounds out of
the rule, around 108º is a roughly reasonable limit. As general
rule, copper cubanes with these four Cu-O-Cu bond angles
clearly lower than 108º, tends to be ferromagnetic with S = 2
ground state and those with these bond angles clearly larger
than 108º tends to be antiferromagnetic with S = 0 ground
state (ESI, Table S1).
Figure 4. Plot of the six spin levels of a CuII cubane for: a) model (0:4) and Hamiltonian 3
for a +25 to -50 cm-1 range of J values; b) model (2:4) and Hamiltonian 2 for an AF J2, c)
model (2:4) and Hamiltonian 2 for a FM J2 and d) model (0:2:2) and Hamiltonian 4.
Color key of the spin levels: S = 2, red; S = 1, black; S = 0, blue; degenerate levels, green.
Arbitrary value for J2 = -50 cm-1 in 4b-d.
The reported ground state for all cubes applying Hamiltonians
(1) - (5) is systematically S = 0 for negative J2,3 values or S = 2
for positive ones. This experimental feature can be easily
rationalized plotting the energy of the six spin levels of the
cube (one S = 2, three S = 1 and two S = 0), as function of the
coupling constants.
Figure 5 Set of consecutive Cu-O-Co bond angles involving the four short Cu-O
distances in the cubane core.
If we assume that the interaction between the copper centers
through the elongated (Jahn-Teller) faces are negligible and
the other four interactions are identical, model (0:4) and
Hamiltonian (3), we obtain the spin levels distribution shown
in Figure 4a, which evidences that S = 0 or S = 2 are the only
possible ground states as function of the sign of J2. If we take
In light of these previous data, the detailed analysis of the
structures of complexes 1-3 unveils an uncommon feature: the
three cages are very asymmetric as consequence of the
coordination of the bidentate carboxylates on contiguous
faces. As consequence the Cu-O-Cu bond angles involving
short Cu-O distances are also more similar on contiguous faces
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Figure 6 Plot of the six spin levels of a CuII cubane for the low symmetry models (0:1:3)
instead opposite faces as it is common, Table 2. Taking as
and Hamiltonian 6 (left) and (2:2:2c) and Hamiltonian 7 (right), showing the S = 1
reference the parameters defined in Figure 5, complex
one large bond angle of 112.2º, two short and of 100.3º
and 102.6º and one intermediate of 108.7º. Cu-O-Cu bond
angles for complexes and exhibit two large contiguous
and angles (110.9º/120.1º and 110.8º/124.4º) and two Plot of the energies of the six spin levels of the cubane
short
100.8º/104.2º).
1 has
ground state for J3/J2 ratios lower than -1/3 and
arrangement that allows to the S = 1 ground state. Color key of the spin levels: S = 2,
red; S = 1, black; S = 0, blue; degenerate levels, green. Arbitrary value for J2 = -50 cm-1
0 respectively. Bottom, spin
δ
α
β
γ
.
2
3
α
δ
β
and
γ
Cu-O-Cu bond angles (101.5º/104.3º and topology for these models are shown in Figure 6. Obviously if
the sign of both J2 and J3 constants is the same, the ground
states will be newly S = 0 or 2. However, form this plots we
In basis to these structural parameters we attempted the realize that for the (0:1:3) model S = 2 is the ground if the -J3/J2
analysis of the energy of the spin levels for the new models ratio is lower than 1/3 but one of the S = 1 spin levels becomes
plotted in Scheme 4, which describe the interactions as three clearly the ground state for larger ratios. Equally for the
similar and one different interaction, (0:1:3) model, and two (0:2:2c) model, for negative J3/J2 ratios (i.e. different sign for
similar interactions on contiguous faces, (0:2:2c) model. The the two coupling constants) one well isolated S = 1 spin level
corresponding Hamiltonians are:
H = -J2(S1·S4) -J3(S1·S3 + S2·S3 + S2·S4)
H = -J2(S1·S3 + S1·S4) -J3(S2·S3 + S2·S4)
becomes the ground state.
(6)
(7)
Magnetic properties
χ
MT vs. T plots for 1 - 3 are shown in Figure 7.
temperature for
is 1.86 cm3Kmol-1. On cooling the
decreases continuously down to a plateau value of 1.18
cm3Kmol-1 around 12 K. Below this temperature
MT raises
slightly to decrease finally to 1.12 cm3Kmol-1 at 2 K. Complexes
and exhibit
MT values of 1.55 and 1.51 cm3Kmol-1 at room
temperature. For decreasing temperatures, the MT values
decreases continuously down to well defined minimum of 1.08
cm3Kmol-1 at 60 K for
and 70 K for . At low temperature MT
χ
MT at room
1
χ
MT value
χ
S2
S2
J3
2
3
χ
J3
χ
S1
S1
S3
S3
2
3
χ
values slightly increases prior the final decrease to and 1.07
and 1.16 cm3Kmol-1 at 2 K.
J2
(0:1:3)
J2
(0:2:2c)
S4
S4
Scheme 4. Low symmetry interaction schemes for the CuII cubane topology proposed
for compounds 1-3. The models have been named according the number of identical
faces and coupling constants.
Figure 7 Temperature dependence of χMT for compound 1 (circles), 2 (triangles) and 3
(squares). Solid lines show the best obtained fits.
According the crystallographic data and the above proposed
models, the susceptibility data was fitted with PHI program12
applying the (0:1:3) model for
(0:2:2c) model (Scheme 4, Hamiltonian 7) for
factors were calculated as R = ( MTexp
MTcalc)2/(
Excellent fits nicely reproducing the experimental data,
including the MT minimums, were obtained for the
parameters J2 = -71.4 cm-1, J3 = 17.2 cm-1, g = 2.24 (R = 3.44·10-
5) for , J2 = -153 cm-1, J3 = +22 cm-1, g = 2.22 (R = 1.17·10-5) for
and J2 = -164 cm-1, J3 = +30 cm-1, g = 2.21 (R = 9.80·10-6) for
Figure 7.
1
(Scheme 4, Hamiltonian 6) and
and . R quality
MTexp)2.
2
3
χ
-χ
χ
χ
1
2
3,
6 | J. Name., 2012, 00, 1-3
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As was above indicated, one of the Cu-O-Cu bond angles of seems to be the one J model (0:4), and effectively, the most of
compound is = 108.7º and it should be close to the FM-AF these cubes were fitted according this model. However, in
1
γ
limit and consequently with a low absolute value. A second spite of the structural evidence, in some few cases the authors
simulation discarding this interaction was performed to prove assumed the regular cube model (6) (ELEYIE, NILDAP or
this assumption applying the simplified Hamiltonian:
H = -J2(S1·S3) -J3(S2·S3 + S2·S4) (8)
WEMSUE).
Table 3. Magnetic data for the (4+2) copper cubes reported in the literature. All J
values have been normalized to the -JSx·Sy Hamiltonian.
obtaining an equally good fit for the parameters J2 = -64.8 cm-
1, J3 = 12.8 cm-1, g = 2.26, which probably are more reliable.
The values for the antiferromagnetic interactions are in good
agreement with the increase of the largest Cu-O-Cu bond
CCDC code
Model
J1/J2/J3
Ground
state
Ref.
/Hamiltonian
(cm-1)*
Cubes with at least one Jahn-Teller Cu-O distance < 2.6 Å
ELEYIE
WEMSUE
GIBHAC
NINPEG
QOMRAL
VEGROP
BUFTUR
CAQDAZ
DARKOW
FEVYAH
IHELOX
(6)/(1)
(6)/(1)
-20.8
-4.5
S =0
S =0
S =0
S =0
S =0
S =0
S =2
S =2
S =2
S =2
S =2
S =2
S =2
S =2
S =2
S =2
S =2
S =2
S =0
S =2
S =2
S =2
13
4c
angle of 112.2º for
Ground state for the three complexes is then S = 1, with a 7.5,
29.8 and 30.7 cm-1 gap to the first S = 0 excited spin level for
respectively, Figure 8. As consequence of this spin levels
1, 120.1º for 2 and 124.4º for 3.
(2:4)/(2)
(2:4)/(2)
(2:4)/(2)
(2:4)/(2)
(2:4)/(2)
(2:4)/(2)
(2:4)/(2)
(2:4)/(2)
(2:4)/(2)
(2:4)/(2)
(2:4)/(2)
(2:4)/(2)
(2:4)/(2)
(2:4)/(2)
(2:4)/(2)
(2:4)/(2)
(0:4)/(3)
(0:4)/(3)
(2:2:2)/(5)
(2:2:2)/(5)
+7.6/-21.7
-5.2/-74.8
-26/-50
14
15
16
17
18
19
20
21
22
11c
23
16
24
25
26
27
28
29
11b
4c
1
-
3
-6.4/-10.9
-19.8/+41.0
-32.6/+89.8
-14.2/+57.0
-1/+65.0
distribution, the magnetization of these complexes must be
similar, following an S = 1 Brillouin shape. Magnetization
experiments performed in the 0-5 T range of applied external
field nicely confirm this assumption, tending in all cases to a
quasi saturated magnetization values equivalent to two
electrons, Figure 8.
-31.8/+66.0
+10.2/+39.8
-35.2/+72
+15.2/-9.4
-21.4/+54.6
-10.5/+61.0
-33.5/+67.0
-20.6/+41.2
-99.2
LITXOD
NAXBET
QOMREP
XEXZUX
XINYUP
XOVVUA
XOXGEY
FEJMIS
HAKXIB
+10.4
MUGWUH
WEMTAL
+3.0/+24.4/+64.8
-1.2/-1.2/+74.2
Cubes with all Jahn-Teller Cu-O distances > 2.6 Å
NILDAP
BAQYAV
JELPUL
(6)/(1)
(0:4)/(3)
(0:4)/(3)
(0:4)/(3)
(0:4)/(3)
(0:4)/(3)
(0:4)/(3)
(2:4)/(2)
(0:4)/(2)
(2:4)/(2)
(2:4)/(2)
(2:4)/(2)
--
-118.8
-27.1
S =0
S =0
S =0
S =0
S =0
S =2
S =2
S =0
S =0
S =0
S =0
S =0
--
30
31
32
33
33
34
20
35
36
11a
37
38
39
40
-130.0
UFATOL
UFATUR
BOGCOP
DARKUC
BOQZUD
LOCPIE
-117.0
-111.0
Figure 8 Left, energy levels calculated from the fit parameters for complexes 1-3. Right,
magnetization data for complexes 1 (squares), 2 (circles) and 3 (triangles) in agreement
with the expected S = 1 ground state.
+34.2
+34.8
+27.2/-69.8
-73.6
Comments to the bibliographic data. Overlooked S = 1 cubes
.
NODHEV
POLKEH
VAVGUW
LIJQEA-IE
GUFJEX01
+6.2/-80.5
-46.0/-136.0
-5/-75.0
Uncopupled
-72.6
The analysis of the bibliographic data of the magnetic
properties of CuII cubes belonging to the 2+4 class, published
along more than twenty years, reveals to be extremely
confuse. Some relevant magnetic features for 43 of those
cubanes are summarized in Table 3.
S =0
Disputable interesting cases
In this table the cubes with all the elongated Cu-O distances
larger than an arbitrary value of 2.60 Å (for which negligible
magnetic interactions through the elongated opposite faces
must be assumed) are tabulated separately. In addition to
these 37 complexes, there are other six cubes for which
disputable (but relevant) magnetic data were reported, that
will be discussed specifically. An overview to the data collected
in Table 3 evidences that the models applied in the magnetic
analysis are not always justified. For cubes with large
elongated Cu-O distances, the most reasonable approach
ASUPEJ01*
SAPYOY*
(2:4)/(2)
-18/+38.4
-18.4/+14.7
-15.6/+33.3
----
S=2
S=2
41
41
41
42
43
43
(2:4)/(2)
(2:4)/(2)
No fit
SAPYUE*
S=2
DIBTAL**
MOYJUH**
MOYKAO**
S=1 ?
S=1 ?
S=1 ?
(2:4)/(2)
-11.2/+7.6
* Complexes for which an S = 1 ground state was erroneously assigned.
** Complexes that probably have an S = 1 ground state.
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As can be expected, J1 usually shows low values for all cubes an apparent Brillouin shape, tending to the equivalent to two
fitted with the (2:4) model and S = 0 ground state but in electrons.43 The
sequence of Cu-O-Cu bond angles for
α
- δ
contrast, the large values of J1 reported for QOMRAL or MOYJUH are comprised between 100.5º-107.4º and then does
POLKEH seems clearly overestimated. In this sense becomes not follows the (0:1:3) nor (0:2:2c) schemes. However, this
interesting the fits performed for LOCPIE and NODHEV, for compound is extremely unusual because three CuII cations
which the authors compared the fits with the (2:4) and (0:4) show square pyramidal environment whereas the fourth CuII
models obtaining minimal deviation in J2, evidencing that for cation has a trigonal bipyramidal coordination and then, a new
strongly AF coupled cubes the calculated value for J1 is poorly model and probably DFT calculations, would be necessary to
reliable.
Much more interesting is the analysis of the models applied for Magneto-structural correlations. Finally, it should be pointed
ferromagnetic cubes. These kind of systems can give a MT plot out an undesirable consequence of the employment of
that suggest the expected value for a S = 2 total spin but often, unreliable J values, obtained applying inappropriate models to
a decay at low temperature or a continuous increase of MT up fit the experimental data: several trials to correlate the J
explain its unusual magnetic response.
χ
χ
to a value slightly lower than the expected for an S = 2 has values with the experimental Cu-O-Cu bond angles2,11b,c or
been reported. These plot shapes can be due to intercluster more recently, the proposal of Bocca et al.11a in basis to a
interactions or weak anisotropy in the ground state, as has chemometric analysis of the CuII chromophores, are far from a
been demonstrated for FEVYAH by Ozarowski et al.21 When linear relationship and partially, it is due (as several authors
the isotropic (2:4) model was applied to fit cubes with have pointed out), to the employment of unreliable
ferromagnetic response, often produces a systematic error, experimental J values.
that consist of the obtention of a pair of coupling constants Along the paper we have assumed that the main parameter
with characteristic values very close to -2J1 = J2 as occurs for that determine the magnetic response of the (4+2) class of CuII
ASUPEJ01, BUFTUR, CAQDAZ, IHELOX, NAXBET, SAPYUE, cubes is the set of four α-δ Cu-O-Cu bond angles. To perform a
XOVVUA or XOXGEY. The reason of this fact can be found in final check of the validity of this very simplified model, we
the plot of the spin levels for a ferromagnetic cube in Figure have selected a coherent group of cubes on basis to the
4c: for a -J1/J2 = 0.5 ratio there is a crossing between a S = 0 following four conditions: i) comparable ꢀ3-OR bridging
and the S = 2 spin levels and the population of both levels ligands. Practically all complexes are linked by alcoxo or
produces a decay of the
S1.
χ
MT plot at low temperature, Figure phenoxo bridges but complexes as GIBHAC have been
excluded because the bridging ligands are 3-OH, which gives a
ꢀ
S = 1 ground state was erroneously claimed for ASUPEJ01, completely different magnetic response. ii) S4 or quasi S4
SAPYOY and SAPYUE despite their χMT plots show a continuous symmetry. It means that the four Cu-O-Cu bond angles are
α-δ
increase for decreasing temperatures. Fits were performed identical or with a maximum tolerance of ±1º. iii) Square
with the (2:4) model that never can lead to S = 1 ground state pyramidal or elongated octahedron environment around the
and these three cubes are obviously ferromagnetic, with a S = four CuII cations. It means to discard complexes in which one
2 ground state.
or more copper atoms have BPT environment. iv) To discard
In contrast, reviewing the
χ
MT plots reported for this kind of any questionable value (mainly for the cubes with S = 2 ground
cubes, we realized that the S = 1 ground state is unusual but state), indicated in the previous section.
not unprecedented. DIBTAL shows a MT response very similar
to complex but the authors reported the magnetic properties
of this cube as unexplainable and any fit was tried. From its
MT shape and the low temperature value (with a plateau at
around 1.1 cm3Kmol-1), S = 1 ground state becomes evident.
The reason of this magnetic response lies in their Cu-O-Cu
χ
1
χ
α - δ
bond angles which follows the sequence 112.3º - 111.2º -
99.7º - 105.5º, corresponding to the (0:2:2c) model with two
ferromagnetic and two antiferromagnetic contiguous
interactions.
Another cubes with probable S = 1 ground state are the
enantiomers MOYJUH (R) and MOYKAO (S) recently reported
by S. Gao et al.43 (only the (R) isomer MOYJUH was measured).
Its low temperature χ
MT plot tends clearly to 1.1 cm3Kmol-1
and fit of the experimental data was performed with the (2:4)
model discarding the low temperature data. The reported
values of J1 = -11.2 cm-1 and J2 = +7.6 cm-1 lead to a well
defined S = 0 ground state with a gap of 11 cm-1 to the first S =
1 excited level, which is not compatible with the experimental
plot. The most clear prove of the S = 1 ground state for this
compound was provided by its magnetization, which follows
Figure 9 Plot of the relationship between J and the mean α-δ Cu-O-Cu bond angle for
all samples tabulated in Table 3 (left) or 20 selected compounds according the criteria
described in the text, (right), (R factor = 0.76).The compounds represented as stars
correspond to complexes BOGCOP and DARKUC, which do not follow the correlation.
8 | J. Name., 2012, 00, 1-3
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ARTICLE
10 Ortep-3 for Windows: L. J. Farrugia, J. Appl. Crystallogr. 1997,
30, 565.
Plot of the unfiltered J2 values vs. the mean Cu-O-Cu α-δ bond
angles for all compounds reported in Table 3 is very disperse
and any conclusion can be extracted. However, plotting the 20
selected cubes with the above criteria provides a clear
indication of the dependence of the sign of the magnetic
interaction with this parameter and corroborates the
assumption of the FM/AFM limit around 108º-110º. There are
only two cubes BOGCOP and DARKUC that are clearly out of
this correlation without apparent reason.
11 a) Z. Puterova-Tokarova, V. Mrazova, J. Kozisek, J. Valentova,
B. Vranovicova and R. Boca, Polyhedron, 2014, 70, 52; b) S.
Thakurta, P. Roy, R. J. Butcher, M. S. El Fallah, J. Tercero, E.
Garribba and S. Mitra, Eur. J. Inorg. Chem., 2009, 4385; c) E.
Gungor, H. Kara, E. Colacio and A. J. Mota, Eur. J. Inorg.
Chem., 2014, 1552.
12 N. F. Chilton, R. P. Anderson, L. D. Turner, A. Soncini and K. S.
Murray, J. Comput. Chem., 2013, 34, 1164.
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15 B. Abarca, R. Ballesteros, M. Chadlaoui, C. R. de Arellano and
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Yuan, X.-H. Huang and X.-T. Wu, Polyhedron, 2006, 25, 1618.
Conclusions
Three new CuII cubane-like complexes belonging to the (4+2)
class have been characterised. From the analysis of
susceptibility and magnetization data, S = 1 ground state has
been unambiguously assigned for all of them together with the
new coupling schemes that justify this unprecedented 18 L. Walz, H. Paulus, W. Haase, H. Langhof and F. Nepveu, J.
Chem. Soc., Dalton Trans., 1983, 657.
19 X. S. Tan, Y. Fujii, R. Nukada, M. Mikuriya and Y. Nakano, J.
Chem. Soc., Dalton Trans., 1999, 2415.
response. A detailed analysis of the bibliographic data reveals
that, to avoid overparametrization, often oversimplified or
inappropriate coupling schemes have been applied leading to
20 H. Astheimer, F. Nepveu, L. Walz and W. Haase, J. Chem.
Soc., Dalton Trans., 1985, 315.
characterized cubes with S = 1 ground state but they are not 21 E. A. Buvaylo, V. N. Kokozay, O. Yu. Vassilyeva, B. W. Skelton,
a confuse landscape. The reported 1-3 compounds are the first
J. Jezierska, L. C. Brunel and A. Ozarowski, Inorg.Chem., 2005,
44, 206.
22 X. F. Yan, J. Pan, S. R. Li, H. Zhou and Z. Q. Pan, Z. Anorg. Allg.
Chem., 2009, 635, 1481.
the first compounds exhibiting this property, because in the
literature we have found three unexplained systems that
becomes to this unusual family.
23 H. Oshio, Y. Saito and T. Ito, Angew. Chem., Int. Ed., 1997, 36
2673.
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Acknowledgements
24 Z. Lu, T. Fan, W. Guo, J. Lu and C. Fan, Inorg. Chim. Acta,
2013, 400, 191.
25 C. Aronica, G. Chastanet, G. Pilet, B. Le Guennic, V. Robert,
Founds from Ministerio de Economis y Competitividad, Project
CTQ2012-30662 are acknowledged.
W. Wernsdorfer and D. Luneau, Inorg. Chem., 2007, 46
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,
26 Y. Xie, J. N., F. Zheng, Y. Cui, Q. Wang, S. W. Ng and W. Zhu,
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Dalton Transactions
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DOI: 10.1039/C5DT03643G
Three copper cubanes with the unusual S = 1 ground state have been characterized and their
magnetic properties rationalized as function of the asymmetry of the {Cu4O4} cage and
correlated with the Cu-O-Cu bond angles.