Dicopper(II) metallacyclophanes with oligo(p-phenylene-ethynylene) spacers: Experimental foundations and theoretical predictions on potential molecular magnetic wires

Article abstract of DOI:10.1021/ic400754m

Two novel double-stranded dicopper(II) metallacyclophanes of formula (nBu₄N)₄[Cu₂(dpeba)₂] ·4MeOH·2Et₂O (1) and (nBu₄N) ₄[Cu₂(tpeba)₂]·12H₂O (2)

Full text of DOI:10.1021/ic400754m

Article
pubs.acs.org/IC
Dicopper(II) Metallacyclophanes with Oligo(p‑phenylene-ethynylene)
Spacers: Experimental Foundations and Theoretical Predictions on
Potential Molecular Magnetic Wires
María Castellano,† Francisco R. Fortea-Perez,† Abdeslem Bentama,†,⊥ Salah-Eddine Stiriba,† Miguel Julve,†
́
Francesc Lloret,*,† Giovanni De Munno,§ Donatella Armentano,*,§ Yangling Li,∥ Rafael Ruiz-García,†,‡
and Joan Cano*,†,‡
†Institut de Cien
Paterna, Valencia, Spain
§Centro di Eccellenza CEMIF.CAL, Dipartimento di Chimica, Universita
∥Institut Parisien de Chimie Molec
ulaire, CNRS, UMR 7201, UPMC Univ Paris 06, 75252 Paris, France
̀ ́ ̀ ̀
cia Molecular (ICMol) and ‡Fundacio
General de la Universitat de Valencia (FGUV), Universitat de Valencia, 46980
̀
̀
della Calabria, 87030 Cosenza, Italy
́
S
* Supporting Information
ABSTRACT: Two novel double-stranded dicopper(II) metal-
lacyclophanes of formula (nBu4N)4[Cu2(dpeba)2]·-
4MeOH·2Et2O (1) and (nBu4N)4[Cu2(tpeba)2]·12H2O (2)
have been prepared by the CuII-mediated self-assembly of the
rigid (‘rod-like’) bridging ligands N,N′-4,4′-diphenylethynebis-
(oxamate) (dpeba) and N,N′-1,4-di(4-phenylethynyl)-
phenylenebis(oxamate) (tpeba), respectively. Single crystal
X-ray diffraction analysis of 1 confirms the presence of a
dicopper(II)tetraaza[3.3]4,4′-diphenylethynophane metallacy-
clic structure featuring a very long intermetallic distance between the two square planar CuII ions [r = 14.95(1) Å]. The overall
parallel-displaced π-stacked conformation of the two nearly planar para substituted diphenylethyne spacers [dihedral angle (ψ) of
7.8(1)°] leads to important deviations from the perpendicular orientation of the copper mean basal planes with respect to the
facing benzene planes [dihedral angles (ϕ) of 56.4(1) and 58.4(1)°]. X-band EPR spectra together with variable-temperature
magnetic susceptibility and variable-field magnetization measurements of 1 and 2, both in solution and in the solid state, show
the occurrence of a non-negligible, moderate to weak intramolecular antiferromagnetic coupling [−J = 3.9−4.1 (1) and 0.5−0.9
cm−1 (2); H = −J S1·S2 with S1 = S2 = SCu = 1/2]. Density functional calculations on the BS singlet (S = 0) and triplet (S = 1)
spin states of the model complexes 1 and 2 with an ideal orthogonal molecular geometry (ψ = 0° and ϕ = 90°) support the
occurrence of a spin polarization mechanism for the propagation of the exchange interaction between the two unpaired electrons
occupying the dxy orbital of each square planar CuII ion through the predominantly π-type orbital pathway of the double p-
diphenylethyne (1) and di(phenylethynyl)phenylene spacers (2). Time-dependent density functional calculations reproduce the
observed bathochromic shift of the main intraligand (IL) π−π* transition in the electronic absorption spectra of 1 and 2 [λ1 =
308 (1) and 316 nm (2)]. In the series of orthogonal model complexes 1−5 with linear oligo(p-phenylene-ethynylene) (OPE)
spacers, −C6H4(CCC6H4)n− (n = 1−5), a linear increase of the IL π−π* transition energy with the reciprocal of the
intermetallic distance is theoretically predicted [νmax = 1.99 × 104 + 2.15 × 105 (1/r) (S = 0) or ν = 2.01 × 104 + 2.18 × 105 (1/
r) (S = 1)], which clearly indicates that the effective π-conjugation length increases with the number of phenylethyne repeating
units. This is accompanied by an exponential decay of the antiferromagnetic coupling with the intermetallic distance [−J = 1.08 ×
103 exp(−0.31r)], which supports the ability of the extended π-conjugated OPEs to mediate the exchange interaction between
the unpaired electrons of the two CuII centers with intermetallic distances in the range of 1.5−4.3 nm. Further developments may
be then envisaged for this new family of oxamato-based dicopper(II) oligo-p-phenylethynophanes on the basis of the unique
ligand capacity to act as a molecular antiferromagnetic wire.
importance in the “bottom-up” approach to nanometer-scale
electronic devices such as molecular wires and switches.4
Molecular magnetic wires may offer a new design concept for
the transfer of information over long distances based on purely
INTRODUCTION
■
Dinuclear complexes with strong intramolecular electronic
interactions between distant metal centers across extended
bridges are a common topic in molecular magnetism and
molecular electronics.1 Besides their interest as models for the
fundamental research on long-distance electron exchange (EE)
and electron transfer (ET) phenomena,2,3 they are also of great
Received: March 27, 2013
© XXXX American Chemical Society
A
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EE (Coulombic) interactions and without any current flow,5 by
comparison with conventional molecular electronic wires which
are based on ET interactions instead.6 However, the examples
of long-range magnetic coupling in exchange-coupled dinuclear
complexes are relatively scarce with regard to the more
abundant examples of long-range electron transport in mixed-
valent ones.7 The design and synthesis of novel bridging ligands
which can transmit spin coupling effects over long distances are
then a major goal in the field, which require both a skillful
organic synthesis and a deep understanding of the EE
mechanism.2c With the advent of ground- and excited-state
calculations based on the density functional (DF) and time-
dependent density functional (TD-DF) theory combined with
the broken-symmetry (BS) approach, it has been possible not
only to reproduce but also to predict very accurately the
electronic and magnetic properties of simple dinuclear
complexes.8
Scheme 1. Oxamato-Based Dicopper(II) Metallacyclophanes
with Oligo(p-phenylene) (a), Oligo(α,α′-acene) (b),
Oligo(β,β′-acene) (c), and Oligo(p-phenylene-ethynylene)
(d) Spacers As Molecular Magnetic Wires
Dicopper(II) metallacyclophanes emerge as ideal model
systems for the study of the transmission of EE interactions
between paramagnetic metal centers containing one unpaired
electron (H = −J S1·S2 with S1 = S2 = SCu = 1/2) through
potential molecular magnetic wires, both from experimental
and theoretical viewpoints.9 The variation of the ligand spacer
in the metallacyclic entity may control the overall structure and
the magnetic properties and then, the influence of different
factors such as the topology and conformation of the bridging
ligand can be investigated in a systematic way.10−13 So, the
problem of long-range magnetic coupling in dicopper(II)
metallacyclophanes have been addressed by several research
teams through the use of different coordinating group
substituted amine-,10 imine-,11 and amide-based12 aromatic
bridging ligands, with more or less satisfactory results.
Our strategy in this field involves a unique family of double-
stranded dicopper(II) metallacyclophanes resulting from the
side-by-side coordination of extended π-conjugated aromatic
bis(oxamato) bridging ligands to square planar metal ions
(Scheme 1).13 This type of rigid dinucleating ligands with
oligo(p-phenylene) (OP) (a of Scheme 1)13b and oligo(α,α′- or
β,β′-acene) (OA) spacers (b and c of Scheme 1)13c are very
appealing candidates as molecular magnetic wires. Thus, the
oxamato-based dicopper(II) metallacyclophanes with 1,4-
phenylene and 4,4′-diphenylene spacers (a of Scheme 1 with
n = 1 and 2) exhibit a strong to moderate antiferromagnetic
coupling (−J values in the range of 81−95 cm−1 and 8.7−11.5
cm−1, respectively) at relatively large intermetallic distances (r =
7.9 and 12.2 Å, respectively).13b Yet the related pair of
oxamato-based dicopper(II) metallacyclophanes with 1,5-
naphthalene and 2,6-anthracene spacers (b and c of Scheme
1 with n = 2 and 3, respectively) show a similar moderate
antiferromagnetic coupling (−J values in the range of 20.5−
20.7 cm−1 and of 21.2−23.9 cm−1, respectively) in spite of the
largely different intermetallic distances (r = 8.3 and 12.5 Å,
respectively).13c,f More importantly, DF calculations on these
two series of oxamato-based dicopper(II) metallacyclophanes
with OA spacers predict a unprecedented wire-like magnetic
behavior for the longer members of the series with octacene
through decacene spacers (b and c of Scheme 1 with n = 8−
10).13c
conjugation that would allow a non-negligible electronic as well
as magnetic coupling between very distant metal centers in
both organometallic and coordination compounds.14,15 Herein
we report on the complete synthesis, general physical and/or
structural characterization, spectroscopic and magnetic proper-
ties of two new examples of oxamato-based dicopper(II)
metallacyclophanes of formula (nBu4N)4[Cu2(dpeba)2]·-
4MeOH·2Et2O (1) and (nBu4N)4[Cu2(tpeba)2]·12H2O (2)
[nBu4N+ = tetra-n-butylammonium, H4dpeba = N,N′-4,4′-
diphenylethynebis(oxamic acid), and H4tpeba = N,N′-1,4-di(4-
phenylethynyl)phenylenebis(oxamic acid)]. Complexes 1 and 2
can be considered as the first two members of a novel series of
oxamato-based dicopper(II) metallacyclophanes with OPE
spacers of different length, −C6H4(CCC6H4)n− (1−5, n =
1−5) (d of Scheme 1). DF and TD-DF calculations on the
electronic structure of the ground and excited states of the
model complexes 1−5 have been carried out in order to analyze
the effect of the increase of the conjugation length with the
number of phenylethyne units in the OPE spacer on both the
magnetic coupling and the electronic spectra of this unique
series of dinuclear copper(II) complexes with intermetallic
distances varying in the range of 1.5−4.3 nm.
RESULTS AND DISCUSSION
■
Synthesis of the Ligands and Complexes. The N,N′-
4,4′-diphenylethynebis(oxamic acid) (H4dpeba) and N,N′-1,4-
di(4-phenylethynyl)phenylenebis(oxamic acid) (H4tpeba) li-
gands were prepared through two consecutive steps from the
Pd/Cu catalyzed, Sonogashira-type cross-coupling reaction of
p-ethynylaniline and p-iodoaniline or p-diiodophenylene
respectively,15a followed by the straightforward condensation
of the resulting 4,4′-diphenylethyne- and 1,4-di(4-
phenylethynyl)phenylenediamine precursors with ethyl oxalyl
These promising results have oriented our current research
toward other oxamato-based dicopper(II) metallacyclophanes
with linear oligo(p-phenylene-ethynylene) (OPE) spacers (d of
Scheme 1), as reported in a preliminary communication.13e In
fact, the rigid rod-like OPE spacers have a potential extended π-
B
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chloride ester (1:2 molar ratio) in the presence of triethylamine
Table 1. Summary of Crystallographic Data for 1
in THF (Scheme 2). They were isolated as the N,N′-4,4′-
formula
C112H196Cu2N8O18
M (g mol−1
)
2069.85
Scheme 2. Synthesis of the Diethyl Ester Derivatives of the
crystal system
space group
a (Å)
monoclinic
P21/c
a
H4dpeba and H4tpeba Ligands
15.261(2)
25.691(4)
17.279(3)
116.180(6)
6079.4(16)
2
b (Å)
c (Å)
β (deg)
V (Å3)
Z
ρcalc (g cm−3
μ (mm−1
)
1.131
)
0.412
T (K)
100(2)
reflect. collected
10675
reflect. obs. [I > 2σ(I)]
data/restraints/parameters
9467
10675/0/645
0.0740 (0.0836)
0.2073 (0.2220)
1.175
a
R1 [I > 2σ(I)] (all)
b
wR2 [I > 2σ(I)] (all)
c
S
a
b
c
2
R1 = ∑(|Fo| − |Fc|)/∑|Fo|. wR2 = [∑w(Fo2 − Fc2)2/∑w(Fo )2]1/2. S
= [∑w(|Fo| − |Fc|)2/(No − Np)]1/2
.
2.804(4) Å] (Figure S1a, SI). Yet they are well separated from
each other by the bulky tetra-n-butylammonium cations and the
diethyl ether molecules (Figure S1b, SI). The intramolecular
Cu(1)−Cu(1)I distance (r) across the double para substituted
diphenylethynediamidate bridge is 14.95(1) Å, while the
shortest intermolecular Cu(1)−Cu(1)II separation is 8.09(1)
Å [symmetry code: (I) = 2 − x, 1 − y, 2 − z; (II) = 1 − x, 1 −
y, 1 − z].
a
Reaction conditions: (a) p-IC6H4NH2, PdCl2(PPh3)2/CuI, NEt3; (b)
p-C6H4I2, PdCl2(PPh3)2/CuI, NEt3; (c) C2O3EtCl, NEt3, THF.
The anionic dicopper(II) complex of 1 is a novel
metallamacrocycle of the dicoppertetraaza[3.3]4,4′-diphenyle-
thynophane-type, where the two 4,4′-diphenylethyne spacers of
the bis(bidentate) dpeba bridging ligands are connected by two
N−Cu−N linkages (Figure 1a). The two centrosymmetrically
related Cu(1) and Cu(1)I atoms adopt a essentially square
planar geometry. The CuN2O2 coordination environment is
defined by two amidate-nitrogen and two carboxylate-oxygen
atoms from the oxamato donor groups. The value of the
tetrahedral twist angle (τ) between the Cu(1)N(1)O(1) and
Cu(1)N(2)O(4) mean planes is 9.4(1)°.
diphenylethynebis(oxamic acid ethyl ester) (Et2H2dpeba) and
N,N′-1,4-di(4-phenylethynyl)phenylenebis(oxamic acid ethyl
ester) (Et2H2tpeba) derivatives in good yields (∼80−85%,
see Experimental Section).
Complexes 1 and 2 were then prepared by the reaction of the
Et2H2dpeba and Et2H2tpeba proligands with copper(II)
perchlorate hexahydrate (1:1 molar ratio) using nBu4NOH as
base in methanol, and they were isolated as their tetra-n-
butylammonium salts in good yields (∼65−70%, see
Experimental Section). X-ray quality dark green prims of 1
were obtained by diethyl ether layering on the methanol
solution. Unfortunately, all our attempts to grow X-ray quality
crystals of 2 were unsuccessful. The chemical identity of the
ligand and complexes was established by elemental analyses and
1H NMR and FT−IR spectroscopies (see Experimental
Section). The structure of 1 was further confirmed by single-
crystal X-ray diffraction. A summary of the crystallographic data
of 1 is given in Table 1, while selected bond distances and
angles are listed in Table 2.
Within the dinuclear metallacyclic core of 1, CuII (p-
2
N2C14H8)2, each of the two 4,4′-diphenylethyne spacers is
almost planar, thus reflecting potential extended π-conjugation
for the dpeba bridging ligands. The torsion angle (ψ) between
the terminal benzene rings around the central carbon−carbon
triple bond is 7.8(1)°. This situation contrasts with that found
for the related dicopper(II) metallacyclophane with 4,4′-
diphenylene spacers, whereby the phenylene rings are
significantly tilted around the carbon−carbon single bond
because of the repulsive interactions between the ortho−ortho′
benzene hydrogen atoms (ψ = 19.7°).13b The two 4,4′-
diphenylethyne spacers of 1 show an offset (non-eclipsed)
disposition because of the parallel-displaced π-stacked arrange-
ment of the two pairs of facing benzene rings (Figure 1b).
Hence, the molecule has an approximate C2h symmetry,
whereby the copper mean basal planes are not exactly oriented
perpendicular to the benzene planes [dihedral angles (ϕ) of
56.4(1) and 58.4(1)°] (Figure 1c). Deviations from the ideal
D2h molecular symmetry (ψ = 0° and ϕ = 90°) may originate
Description of the Structure. The structure of 1 consists
of centrosymmetric dicopper(II) complex anions,
[CuII (dpeba)2]4−, and tetra-n-butylammonium cations, togeth-
2
er with methanol and diethyl ether as crystallization molecules
(Figure 1 and Figure S1, Supporting Information [SI]). In the
crystal lattice, the discrete anionic dicopper(II) complexes
establish weak hydrogen bonds with the methanol molecules
through the carboxylate- and carbonyl-oxygen atoms from the
oxamato groups [O(5)···O(8) = 2.760(4) Å and O(6)···O(9) =
C
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a
Table 2. Selected Bond Distances (Å) and Angles (deg) for 1
bond
Å
bond
Å
Cu(1)−N(1)
Cu(1)−O(1)
angle
1.993(3)
1.948(3)
deg
Cu(1)−N(2)
Cu(1)−O(4)
angle
1.994(3)
1.973(3)
deg
N(1)−Cu(1)−N(2)
N(1)−Cu(1)−O(4)
N(2)−Cu(1)−O(4)
108.43(13)
167.80(12)
83.24(12)
N(1)−Cu(1)−O(1)
N(2)−Cu(1)−O(1)
O(1)−Cu(1)−O(4)
83.92(12)
165.63(12)
85.01(11)
a
The estimated standard deviations are given in parentheses.
Table 3. Selected UV−Vis Spectroscopic Data for 1 and 2
and the Corresponding Proligands
c
c
c
cmpd
λ1 (nm)
λ2 (nm)
λ3 (nm)
a
1
2
308 (29710)
[32468]
340 (18850)
[29412]
360 (7065)
[27778]
a
316 (55815)
[31646]
365 (22375)
[27397]
385 (7770)
[25974]
b
Et2H2dpeba
330 (17015)
[30303]
355 (9180)
[28169]
b
Et2H2tpeba
345 (29480)
[28986]
370 (18140)
[27027]
a
b
In acetonitrile solution at room temperature. In dimethylsulfoxide
c
solution at room temperature. The values of the molar extinction
coefficient (ε in M−1 cm−1 units) and the transition energy (νi = 1/λi
in cm−1 units) are given in parentheses and brackets, respectively.
Figure 1. (a) ORTEP drawing of the centrosymmetric anionic
dicopper unit of 1 with the atom-numbering scheme of the metal
environment [symmetry code: (I) = 2 − x, 1 − y, 2 − z]. The thermal
ellipsoids are drawn at the 50% probability level. (b) Front and (c) top
projection views of the metallacyclic core of 1.
the Et2H2dpeba and Et2H2tpeba proligands in dimethylsulf-
oxide (Figure 2c and d). In fact, they show a broad and intense
band centered at 330 (Et2H2dpeba)/345 nm (Et2H2tpeba) with
a distinct shoulder located around 355 (Et2H2dpeba)/370 nm
(Et2H2tpeba) (Table 3). More importantly, the relatively large
bathochromic shift in the position of these IL bands for both
the complexes (Δν1 = 822 cm−1, Δν2 = 2015 cm−1, and Δν3 =
1804 cm−1; Table 3) and the proligands (Δν1 = 1317 cm−1 and
Δν2 = 1142 cm−1; Table 3) is as expected for the increase in the
effective π-conjugation length upon increasing the number of
phenylethyne units (n) from 1 to 2 in the OPE spacer.
The X-band EPR spectra of frozen-matrix acetonitrile
solutions of 1 and 2 at 4.0 K consist of a rhombic signal with
a complex multiline splitting pattern which is typical of
antiferromagnetically coupled dinuclear copper(II) complexes
(Figure 3). In fact, a seven-line splitting of the gz signal due to
the hyperfine coupling with the nuclear spins of the two CuII
ions (2nICu + 1 = 7 with n = 2 and ICu = 3/2) occurs when the
magnetic coupling parameter is much larger than the electron−
nucleus hyperfine coupling constant (|J| ≫ ACu). In contrast, a
magnetically uncoupled dinuclear copper(II) complex (|J| ≪
ACu) would give an EPR spectrum formally identical to that of a
mononuclear one with a simple four-line splitting resulting
from the hyperfine coupling with the nuclear spin of the
magnetically isolated CuII ions (2ICu + 1 = 4 with ICu = 3/2).
These EPR spectral features of 1 and 2 were satisfactorily
simulated by using the XSOPHE program16 that diagonalizes
the full Hamiltonian matrix within the basis of the three S = 1
spin functions (Ms = 0, 1) of the CuII2 unit resulting from the
antiferromagnetic interaction between the two spin doublets
(SCu = 1/2) of each CuII ion (Figure S2, SI). The calculated
values of the Zeeman factors associated with the x, y, and z
components of the allowed Ms = 0 → Ms = 1 transitions for
the excited triplet (S = 1) spin state of 1/2 are gx = 2.044/
2.045, gy = 2.070/2.065, and gz = 2.240/2.216, while those of
from the favorable π−π interactions in the parallel-displaced
configuration of the 4,4′-diphenylethyne spacers.
Spectroscopic Properties. The electronic absorption
spectra of 1 and 2 in acetonitrile solution show a common
pattern consisting of one broad and intense band in the near
UV region, together with two distinct shoulders that extend
into the visible zone, whose intensity vary almost proportionally
with the number of phenylethyne groups in the OPE spacer
(Figure 2a and b). The main peak centered at 308 (1)/316 nm
(2) and the two shoulders located around 340 (1)/365 nm (2)
and 360 (1)/385 nm (2) are typical of intraligand (IL) π−π*
transitions within the OPE spacers (Table 3).15c In fact, they
are also present in somewhat perturbed yet identifiable form
with almost half intensity in the electronic absorption spectra of
Figure 2. UV−vis spectra of 1 (a) and 2 (b) in acetonitrile solution
compared with those of Et2H2dpeba (c) and Et2H2tpeba (d) in
dimethylsulfoxide.
D
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Figure 4. Temperature dependence of χMT of the polycrystalline
○
●
sample ( ) and frozen-matrix methanol solution ( ) of 1 under
applied magnetic fields of 100 G (T < 25 K) and 1.0 T (T ≥ 25 K).
The inset shows the temperature dependence of χM of the
Figure 3. X-band EPR spectra of the frozen-matrix acetonitrile
solutions of 1 (a) and 2 (b) at 4.0 K.
○
●
polycrystalline sample ( ) and frozen-matrix methanol solution ( )
of 1. The solid lines are the best-fit curves (see text).
the corresponding hyperfine coupling constants are Ax = 20/10
G, Ay = 5/20 G, and Az = 134/143 G (Table 4). The electron−
a
Table 4. Selected EPR Spectroscopic Data for 1 and 2
b
b
b
cmpd
gx
gy
gz
1
2.044 (20)
[0.0019]
2.070 (5)
[0.0005]
2.240 (134)
[0.0138]
2
2.045 (10)
[0.0010]
2.065 (20)
[0.0019]
2.216 (143)
[0.0148]
a
b
In acetonitrile solution at 4.0 K. The values of the Zeeman factors
(gi) and the hyperfine coupling constants (Ai) associated with the x, y,
and z components of the allowed Ms = 0 → Ms = 1 transitions of the
excited triplet (S = 1) spin state were calculated by using the XSOPHE
program. The calculated Ai values in G and cm−1 units are given in
parentheses and brackets, respectively.
Figure 5. Temperature dependence of χMT of the polycrystalline
□
■
samples ( ) and frozen-matrix methanol solution ( ) of 2 under
applied magnetic fields of 100 G (T < 25 K) and 1.0 T (T ≥ 25 K).
The solid lines are the best-fit curves (see text). The inset shows the
nucleus hyperfine coupling constant of 1 and 2 can then be
estimated from the calculated Az values resulting from the
simulation of the EPR spectra as Az = 1/2ACu.17 The calculated
anisotropic ACu values of 268 (1) and 286 G (2) are similar, as
expected for antiferromagnetically coupled dinuclear copper(II)
complexes with the same type of ligands, and they give a
minimum value for the magnetic coupling parameter of 1 and 2
[|J| ≫ ACu = 0.0276 (1) and 0.0296 cm−1 (2)].
○
field dependence of M of the polycrystalline samples of 1 ( ) and 2
□
( ) at 2.0 K. The solid line corresponds to the Brillouin curve for the
sum of two doublet spin states (see text).
until ∼20 K and then it decreases slightly to reach values of
0.70 and 0.75 cm3 mol−1 K respectively, at 2.0 K (Figure 5). In
addition, the polycrystalline sample and frozen-matrix methanol
solution of 1 show a χM maximum at 3.3 and 3.5 K respectively
(inset of Figure 4), which unambiguously supports the
occurrence of a ground singlet (S = 0) spin state resulting
from the antiferromagnetic interaction between the two spin
Magnetic Properties. The magnetic properties of 1 and 2
in the form of the χM and χMT vs T plots (χM being the molar
magnetic susceptibility per dinuclear unit and T the absolute
temperature) and M vs H plots (M being the molar
magnetization per dinuclear unit and H the applied magnetic
field) conform with moderate (1) to weak (2) antiferromag-
doublets (SCu = 1/2) of each CuII ion within the CuII units.
2
Otherwise, the isothermal magnetization curve at 2.0 K of the
polycrystalline sample of 1 is well below that of the Brillouin
function for two doublet spin states of two magnetically
isolated CuII ions (SCu = 1/2 with g = 2.1), while it is only
slightly below for 2 reflecting thus the different magnitude of
the antiferromagnetic coupling in each case (inset of Figure 5).
The analysis of the magnetic susceptibility data of 1 and 2,
both in the solid state and in solution, was carried out through
the spin Hamiltonian for a dinuclear copper(II) complex (eq 1
with S1 = S2 = SCu = 1/2), where J is the magnetic coupling
parameter and g is the Zeeman factor of the CuII ions (g = g1 =
g2 = gCu). The least-squares fits of the experimental data of the
polycrystalline samples of 1/2 through the well-known
Bleaney−Bowers expression (eq 2) gave −J = 3.87(3)/
netically coupled CuII units, both in the solid state and in
2
solution (Figures 4 and 5). At room temperature, χMT of the
polycrystalline samples of 1 and 2 is equal to 0.84 cm3 mol−1 K,
a value which is close to that expected for two magnetically
isolated CuII ions [χMT = 2 × (Nβ2gCu2/3kB)SCu(SCu + 1) =
0.83 cm3 mol−1 K with SCu = 1/2 and gCu = 2.1, where N is the
Avogadro number, β is the Bohr magneton, and kB is the
Boltzmann constant]. Upon cooling, χMT of the polycrystalline
sample and frozen-matrix methanol solution of 1 remains
constant until ∼50 K and then it decreases abruptly to reach
values of 0.22 and 0.18 cm3 mol−1 K respectively, at 2.0 K
(Figure 4). By comparison, χMT of the polycrystalline sample
and frozen-matrix methanol solution of 2 remains constant
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0.90(1) cm−1 and g = 2.124(3)/2.122(1) with R = 1.5 × 10−5/
1.0 × 10−5, while they gave −J = 4.05(3)/0.48(1) cm−1 and g =
2.105(3)/2.105(1) with R = 1.8 × 10−5/1.5 × 10−5 for the
frozen-matrix methanol solutions of 1/2 (Table 5). The
repeat units on the organic spacer, −C6H4(CCC6H4)n− (n =
0−2). So, the strong antiferromagnetic coupling across the p-
phenylene spacers (n = 0) previously reported for the parent
oxamato-based dicopper(II) paracyclophane (−J = 81−95
cm−1)13b is no more than 25- and 100-fold those observed
for 1 (−J = 3.9−4.1 cm−1) and 2 (−J = 0.5−0.9 cm−1) through
the p-diphenylethyne (n = 1) and p-di(phenylethynyl)-
phenylene (n = 2) spacers, respectively. This situation clearly
contrasts with that predicted by Coffman and Buettner (−J <
1.0 cm−1 at r > 9.0 Å),2b showing thus that OPE spacers can act
as effective antiferromagnetic wires for the propagation of EE
interactions between two CuII ions separated by intermetallic
distances greater than one nanometer.
Energy and Molecular Orbital Calculations: Distance
Dependence of Magnetic Coupling. DF calculations were
performed in acetonitrile solution on the BS singlet (S = 0) and
triplet (S = 1) spin states of the model complexes 1−5 with an
imposed coplanar conformation of the para substituted benzene
rings connected by carbon−carbon triple bonds (ψ = 0°) and a
perpendicular orientation of the benzene rings with respect to
the copper mean basal planes (ϕ = 90°) (see Computational
Details). Selected calculated structural and energy data are
listed in Table 6.
a
Table 5. Selected Magnetic Data for 1 and 2
b
c
d
cmpd
J
(cm−1
)
g
R
× 10−5
1
−3.87(3)
[−4.05(3)]
−0.90(1)
2.124(3)
[2.105(3)]
2.122(1)
1.5
[1.8]
1.0
2
[−0.48(1)]
[2.105(1)]
[1.5]
a
In the solid state. The magnetic data in methanol solution are given
b
in brackets. Magnetic coupling parameter (see eq 1). The calculated
c
errors are given in parentheses. Zeeman factor of the CuII ions (see eq
d
1). The calculated errors are given in parentheses. Agreement factor
defined as R = ∑[(χMT)exp − (χMT)calcd]2/∑[(χMT)exp]2.
theoretical curves match very well the experimental ones for
1 and 2 (solid lines in Figures 4 and 5) and particularly, they
reproduce the observed maximum of χM for 1 both in the solid
state and in solution (solid lines in the inset of Figure 4).
Indeed, the similar magnetic behavior in the solid state and in
solution for both 1 and 2 unambiguously demonstrates that the
magnetic coupling is intramolecular in origin, the intermo-
lecular interactions in the solid state, if any, being negligible. So,
the slight deviations between the −J values in the solid state
and in solution for 2 would be most likely due to very small
variations of the molecular geometry in each medium.
Table 6. Selected Calculated Magneto-Structural Data for 1−
a
5
b
c
d
cmpd
r
(Å)
ΔEST (cm−1
)
δ )
(cm−1
1
2
3
4
5
15.266
21.979
29.069
35.931
42.833
10.399
1.095
0.119
0.013
0.002
927.54
338.75
120.98
40.33
H = −JS1·S2 + gβ(S1 + S2)H
(1)
χ T = (2Nβ2g2/kB)/[3 + exp(−J/kBT)]
(2)
M
16.13
a
DF calculations were performed on the BS singlet (S = 0) and triplet
The moderate antiferromagnetic coupling for 1 (−J = 3.9−
4.1 cm−1) is quite remarkable given the large Cu−Cu
separation across the two 4,4′-diphenylethynediamidate bridges
(r ≈ 15.0 Å). By comparison, the −J values for 1 are only
slightly smaller than those reported earlier for related oxamato-
based dicopper(II) metallacyclophanes with 4,4′-diphenylene
spacers (−J = 8.7−11.5 cm−1) having a rather shorter
intermetallic distance (r = 12.2 Å).13b Yet the weak but non-
negligible antiferromagnetic coupling for 2 (−J = 0.5−0.9
cm−1) is really noticeable given the very long intermetallic
distance through the two 1,4-di(4-phenylethynyl)-
phenylenediamidate bridges (r ≈ 22.0 Å). To our knowledge,
2 is the exchange-coupled dinuclear copper(II) complex with
the largest intermetallic distance for which EE effects has been
evidenced, showing thus that one nanometer was definitely not
the upper limit for the observation of magnetic coupling in
dinuclear copper(II) complexes.2 At this respect, we have
recently reported a quite rare p-triphenylenediamine-bridged
dinuclear copper(II) complex possessing a similarly weak
intramolecular antiferromagnetic coupling (J = −2.0 cm−1) but
with a rather shorter intermetallic distance (r ≈ 16.4 Å).8f
The moderate to weak, long-range antiferromagnetic
coupling in 1 and 2 indicates that strongly delocalized π-type
orbital pathways through the para substituted diphenylethyne
(1) and di(phenylethynyl)phenylene (2) spacers are involved,
as previously observed for the related oxamato-based dicopper-
(II) metallacyclophanes with para substituted phenylene
spacers which show a strong antiferromagnetic coupling (−J
= 81−95 cm−1).13b As a matter of fact, the magnetic coupling
along this series decreases with the number of phenylethyne
(S = 1) states of the D2h-symmetric model complexes 1−5 (ϕ = 90°
b
and ψ = 0°, see Computational Details). Intermetallic distance.
c
d
Singlet−triplet energy gap (ΔEST = −J). Energy gap between the
two singly occupied molecular orbitals (SOMOs) for the triplet state.
For all these orthogonal model molecules with an ideal D2h
symmetry (ψ = 0° and ϕ = 90°), the DF energy calculations
showed a ground singlet spin state lying below the excited
triplet spin state. The calculated value of the singlet−triplet
energy gap for the orthogonal model complexes 1 and 2
decreases when increasing the intermetallic distance [ΔEST
=
−J = 10.399 (1) and 1.095 cm−1 (2) for r = 15.095 (1) and
21.979 Å (2); Table 6], as experimentally observed. The
deviations between the calculated and the experimental −J
values [−J = 3.9−4.1 (1) and 0.5−0.9 cm−1 (2); Table 5] are
likely due to the loss of π-conjugation between the benzene
rings by conformational rotation about the carbon−carbon
triple bond (ψ ≠ 0°) and/or the loss of orthogonality between
the benzene and the copper mean basal planes (ϕ ≠ 90°), as
shown earlier for the related oxamato-based dicopper(II)
metallacyclophanes with 4,4′-biphenylene spacers.13b In fact,
DF energy calculations on the actual structure of 1 possessing
an overall parallel-displaced π-stacked conformation of the two
not exactly planar 4,4′-diphenylethyne spacers [ψ = 7.8(1)°; ϕ
= 56.4(1) and 58.4(1)°] gave a smaller singlet−triplet energy
gap (ΔEST = −J = 6.1 cm−1) which is closer to the experimental
one in the solid state (−J = 3.9 cm−1).13e Nevertheless, two
nanometers appears to be the upper limit for the observation of
magnetic coupling (−J < 1.0 cm−1) in the longer homologues
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3−5 including several CC bonds in the OPE spacers,
−C6H4(CCC6H4)n− (n = 3−5). So, for instance, the DF
energy calculations indicate an almost negligible singlet−triplet
energy gap between the two CuII ions separated by up to
28.868 Å through the 4,4′-di(4-phenylethynyl)diphenylethyne
spacers in 3 (ΔEST = −J = 0.119 cm−1).
The results of the DF calculations on the D2h-symmetric
model complexes 1−5 with extended π-conjugated OPE
spacers, −C6H4(CCC6H4)n− (n = 1−5), are shown in
Figure 6 in terms of both structural and magnetic data.
Figure 7. Decay law of the calculated magnetic coupling (J) with the
○
intermetallic distance (r) for 1−5 ( ) (data from Table 6) compared
with those of related oxamato-based dicopper(II) metallacyclophanes
□
with p-substituted oligophenylene ( ) and α,α′- (Δ) or β,β′-
substituted (∇) oligoacene spacers. The inset shows the linear
○
dependence of the calculated values of −J ( ) with the square of the
energy gap between the two SOMOs (δ) for 1−5 (data from Table 6).
The solid lines correspond to the best-fit curves, while the dashed and
dotted lines are the well-known Coffman−Buettner and related
relationships based on experimental magneto-structural data on simple
dinuclear copper(II) complexes (see text).
Figure 6. Dependence of the calculated singlet−triplet energy gap (on
□
○
a semilog scale) ( ) and the intermetallic distance ( ) for 1−5 (data
us showing a much slower exponential decay rate of the
magnetic coupling which is, however, somewhat higher than
that predicted by the DF calculations (γ = 0.37 vs 0.18 Å−1).8f
Fabre et al. also reported the occurrence of strong exchange
from Table 6) with the number of repeating units in the OPE spacers,
●
−C6H4(CCC6H4)n− (n = 1−5). The experimental values of −J ( )
■
and r ( ) for the polycrystalline samples of 1 and 2 are also shown for
comparison (data from Table 5). The solid lines correspond to the
best-fit curves (see text).
coupling interactions in RuIII complexes with oligophenyl-,
2
oligophenylethene-, and oligophenylethynedicyanamido bridg-
Together with a linear increase in the estimated intermetallic
distance (r), the calculated singlet−triplet energy gap (−J)
decreases in an exponential manner with the number of
phenylene repeat units (n = 1−5) connected by CC bonds
along this series (solid lines in Figure 6). The fit of the
calculated magneto-structural data for 1−5 provides a decay law
of the exchange interaction with the intermetallic distance as −J
= 1.08 × 103 exp(−0.31r) (Figure 7). The calculated value of
0.31 Å−1 for the exponential factor (γ) is lower than that found
in the related series of oxamato-based dicopper(II) metal-
lacyclophanes with p-substituted oligophenylene (OP) spacers
(γ = 0.35 Å−1) (solid lines in Figure 7).13b This indicates a
higher efficiency on long-range magnetic coupling for the OPE
spacers compared to the OP ones, suggesting thus that the
inclusion of additional CC bonds between the benzene rings
occurs without any loss of π-conjugated aromatic character. In
fact, the calculated γ value for 1−5 is identical to that found in
the fully π-conjugated β,β′-substituted oligoacene (OA) spacers
(γ = 0.31 Å−1), being significantly lower than that found in
α,α′-substituted ones (γ = 0.45 Å−1) (solid lines in Figure 7).13c
However, no unique wire-like magnetic behavior was observed
for 1−5, as reported earlier for the longer members of the series
of oxamato-based dicopper(II) metallacyclophanes with fully π-
conjugated OA spacers (Figure 7).13c
ing ligands (γ = 0.10 Å−1),7k as predicted theoretically by Ruiz
et al. for related oligoacenedicyanamido-bridged MIII com-
2
plexes (M = Cr, Mn, and Fe).8b Once again, this reflects the
relative efficiency of π- vs σ-type exchange pathways, as
expected because of the larger spin delocalization and spin
polarization contributions to the ground state electronic
structure in the former case.
Molecular orbital (MO) calculations on the D2h-symmetric
model complexes 1−5 provide evidence that the exchange
interaction between the two unpaired electrons occupying the
σ-type dxy orbitals of the square planar CuII ions (‘magnetic
orbitals’) and pointing toward the equatorial Cu−N and Cu−O
bonds, is mainly transmitted through the π-bond system of the
OPE spacers, −C6H4(CCC6H4)n− (n = 1−5) (see Scheme
1d). This is nicely illustrated by the pair of singly occupied
molecular orbitals (SOMOs) for the triplet spin state of 1 and
2, which show a high metal−ligand covalency and strongly
ligand delocalized character (Figure 8). These two pairs of
SOMOs, noted b1g and b2u* (1) or alternatively b2u and b1g*
(2), are composed by the symmetric and antisymmetric
combinations of the dxy(Cu) orbitals mixed with the
corresponding combinations of appropriate symmetry of the
two π-type orbitals of the diphenylethyne (1) and di-
(phenylethynyl)phenylene (2) spacers, which are in turn
made up of pz(C) orbitals of the sp2- and sp-type carbon
atoms from the benzene rings and from the ethyne groups,
respectively. As expected, the energy gap (δ) between the two
SOMOs for the triplet spin state of 1−5 decreases continuously
along this series (Table 6), in such a way that the calculated −J
values vary fairly well with the square of δ as −J = 12.0 × 10−6
δ2 (inset of Figure 7), according to the simplest orbital models
of the electron exchange interaction.2a This situation reflects
In contrast, the earlier relationship obtained by Coffman and
Buettner2b and the subsequent one reported by some of us,13b
which were based on experimental magneto-structural data on
simple dicopper(II) complexes, predict a dramatically faster
decay of the magnetic coupling with the intermetallic distance
(γ = 1.5−1.8 Å−1) (dashed and dotted lines in Figure 7). A
remarkable exception is the series of oligo-p-phenylenediamine-
bridged dicopper(II) complexes recently reported by some of
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Scheme 3. Projection View of the Spin Density Distribution
on the Oligo(p-phenylene-ethynylene)diamidate Bridges for
the BS Singlet Spin State of 1 (a) and 2 (b) with the
a
Calculated Atomic Spin Density Values (in e units)
Figure 8. Perspective views of the calculated magnetically active
SOMOs for the triplet spin state of 1 (a) and 2 (b). The isodensity
surface corresponds to a value of 0.04 e bohr−3.
a
Empty and full circles represent positive and negative spin densities,
respectively, with scaled surface areas.
the parallel decrease of the dxy(Cu)/π(L) metal−ligand orbital
mixing for 1−5 and, consequently, the smaller delocalization of
the unpaired electrons of the CuII ions onto the π-conjugated
electron system of the OPE spacers with the increasing number
of phenylethyne units (n) from 1 to 5 along this series.
Spin Density Analysis: Spin Delocalization vs Spin
Polarization Mechanism of Magnetic Coupling. Spin
densities obtained by natural bond orbital (NBO) analysis on
the BS singlet state of 1 and 2 reflect the relative importance of
spin delocalization and spin polarization effects for the
propagation of the exchange interaction between the unpaired
electrons of the two metal centers through the π-type orbital
pathways of the p-diphenylethyne- (1) and p-di-
(phenylethynyl)phenylenediamidate (2) bridges (Figure 9).
enediamidate bridges.13b So, the values of the spin density of
opposite sign on the adjacent sp2-type carbon atoms of the two
terminal benzene rings [from 0.013 to 0.020 e (1) and from
0.012 to 0.019 e (2)] are as important as those at the sp-
type carbon atoms from the ethyne groups to which they are
directly attached [ 0.013 (1) and 0.008 e (2)] (Scheme 3).
This situation evidence the partial preservation of the π-
conjugation with the incorporation of additional CC bonds
connecting the phenylene groups into the OPE spacers for 1
and 2. Indeed, the values of the spin density at the sp2-type
carbon atoms of the central benzene ring in 2 are smaller but
non-negligible [ 0.003 e] (Scheme 3b), as expected because of
the appreciable attenuation of the spin delocalization with the
incorporation of an additional phenylene group into the OPE
spacers. Hence, a net decrease of the antiferromagnetic
exchange interaction results with the successive incorporation
of additional phenylethyne units into the OPE spacers, as
experimentally observed for 1 and 2 and theoretically predicted
for 3 to 5.
Transition Energy and Molecular Orbital Calculations:
Distance Dependence of Intraligand Optical Transitions.
TD-DF calculations were performed in acetonitrile solution on
the BS singlet (S = 0) and triplet (S = 1) spin states of the D2h-
symmetric model complexes 1−5 (see Computational Details).
Selected calculated structural and transition energy data are
listed in Table 7. The MO composition and energy data of the
transitions for the triplet spin state of 1−4 are shown in Figures
S3−S6 (SI), while those of 5 are depicted in Figure 10.
The most remarkable feature of the calculated electronic
spectra for the BS singlet and triplet states of the orthogonal
model complexes 1−5 is the presence of an intense UV band of
IL nature. This IL UV band has two main contributions
corresponding to the π−π* transitions from the highest
occupied molecular orbitals (HOMOs), ag and b2u (1, 3, and
5) or alternatively b3u and b1g (2 and 4), to the lowest
unoccupied molecular orbitals (LUMOs), b3u* and b1g* (1, 3,
and 5) or alternatively ag* and b2u* (2 and 4). The calculated
wavenumber (λmax) values increase asymptotically when
increasing the correlation length along this series for both the
BS singlet and triplet spin states (Figure S7, SI), as
experimentally observed [λmax = 308 (1) and 316 nm (2);
Table 3]. A parallel decrease of the calculated values of the
HOMO−LUMO energy gap (Δ) corresponding to the two
individual π−π* transitions, ag → b3u* and b2u → b1g* (1, 3,
and 5) or alternatively b3u → ag* and b1g → b2u* (2 and 4),
occurs for the triplet spin state of 1−5. The fit of the calculated
Figure 9. Perspective views of the calculated spin density distribution
for the BS singlet spin state of 1 (a) and 2 (b). Yellow and blue
contours represent positive and negative spin densities, respectively.
The isodensity surface corresponds to a value of 0.001 e bohr−3.
The values of the spin density at the amidate-nitrogen atoms
are important [ 0.101 (1) and 0.100 e (2)] and, moreover,
they have the same sign as in the copper atoms to which they
are coordinated [ 0.564 (1) and 0.565 e (2)] (Scheme 3).
This fact indicates that the spin delocalization from the metal
toward the amidato donor groups dominates over the spin
polarization because of the strong covalency of the Cu−N
bonds. In contrast, the sign alternation of the spin density at the
carbon atoms of the p-diphenylethyne- (1) and p-di-
(phenylethynyl)phenylene (2) spacers agrees with a spin
polarization by the amidate-nitrogen atoms, as reported earlier
for the related dicopper(II) metallacyclophane with p-phenyl-
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Table 7. Selected Calculated Spectro-Structural Data for 1−
a
5
c
λmax (nm)
b
cmpd
r
(Å)
S = 0
S = 1
1
15.266
21.979
29.069
35.931
42.833
293 (1.24)
289 (1.08)
[34130]
340 (2.60)
[29412]
368 (3.14)
[27174]
385 (3.69)
[25974]
399 (2.38)
[25063]
[34602]
337 (1.94)
[29674]
365 (1.73)
[27397]
380 (2.88)
[26316]
394 (1.65)
[25381]
2
3
4
5
a
TD-DF calculations were performed on the BS singlet (S = 0) and
triplet (S = 1) states of the D2h-symmetric model complexes 1−5 (ϕ =
Figure 11. Increment law of the calculated intraligand transition
energy (νmax) with the reciprocal intermetallic distance (1/r) for the
b
90° and ψ = 0°, see Computational Details). Intermetallic distance.
c
Intraligand transition wavenumber. The values of the oscillator
○
●
BS singlet ( ) and triplet ( ) spin states of 1−5 (data from Table 7).
strength (f) and the transition energy (νmax = 1/λmax in cm−1 units) are
●
The experimental values of the intraligand transition energy ( ) for 1
given in parentheses and brackets, respectively.
and 2 in acetonitrile solution are also shown for comparison (data
from Table 3). The inset shows the linear dependence of the
■
calculated νmax values ( ) with the HOMO−LUMO energy gap (Δ)
□
■
of the two optically active ag/b3u* ( ) and b2u/b1g* ( ) pairs for the
triplet spin state of 1−5 (data from Figure 10 and Figures S3−S6, SI).
The solid lines are the best-fit curves (see text).
fragment molecular orbitals (FMOs) of the OPE spacers (πa
2
2
and πb) and those of the metal centers (dxy and dx −y ), as
illustrated for 1 in Scheme 4. The pair of LUMOs, noted b3u*
and b1g*, are composed by the symmetric and antisymmetric
combinations of the antibonding πb orbitals of the two facing
diphenylethyne spacers, which are weakly mixed with the
corresponding combinations of appropriate symmetry of the dxy
2
2
and dx −y metal orbitals, respectively. The pair of HOMOs,
noted ag and b2u, are in turn made up by the symmetric and
antisymmetric combinations of the bonding πa orbitals of the
two facing diphenylethyne spacers, which are also weakly mixed
with the corresponding combinations of appropriate symmetry
2
2
of the dxy and dx −y metal orbitals, respectively. These two
optically active HOMO/LUMO pairs of mainly ligand-based
nature evidence a significant delocalization onto the pz(C)
orbitals of the benzene rings and the ethyne groups that is
directly responsible for the proposed molecular wire behavior
of extended π-conjugated OPEs,15c as exemplified by the longer
homologue of this series which shows a strong ligand
delocalized character (Figure 10).
Figure 10. Perspective views of the two calculated pairs of optically
active LUMOs (a) and HOMOs (b) involved in the intraligand π−π*
transitions for the triplet spin state of 5. The orbital energies (in au)
and the transition contribution percentages are given in parentheses
and brackets, respectively. The isodensity surface corresponds to a
value of 0.04 e bohr−3.
CONCLUSIONS AND PERSPECTIVES
■
In this contribution, we present a complementary experimental
and density functional study on long-range magnetic and
electronic coupling through extended π-conjugated aromatic
spacers in a new family of dicopper(II) metallacyclophanes
consisting of two square planar bis(oxamato)copper(II) spin-
bearing sites separated by two linear oligo(p-phenylene-
ethynylene) (OPE) spacers, −C6H4(CCC6H4)n− (n = 1−
5). Because of their planar configuration, a strong orbital
overlap between the pz-type orbitals of the para substituted
benzene rings across the carbon−carbon triple bonds occurs
along the p-phenylethyne repeat units of the OPE spacers, as
evidenced by the linear increase of the intraligand (IL) π−π*
transition energy with the reciprocal correlation length along
this series. Hence, an unusual π-type orbital pathway is available
spectro-structural data for the BS singlet and triplet spin states
of 1−5 provides a linear increment law of the IL π−π*
transition energy with the reciprocal intermetallic distance as
νmax = 1.99 × 104 + 2.15 × 105 (1/r) (S = 0) or ν = 2.01 × 104
+ 2.18 × 105 (1/r) (S = 1) (Figure 11), in such a way that the
calculated νmax values vary almost linearly with Δ as νmax = 5.7 +
0.90 × 103Δ and νmax = −9.7 + 1.52 × 103Δ (inset of Figure
11).
In terms of the ligand field theory, this situation can be
described through the π-bonding interaction between the
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two metal centers through the extended π-conjugated OPE
spacers, which are disposed perpendicularly to the metal basal
planes as a result of the overall orthogonal molecular geometry.
On the other hand, the relatively slow exponential decay of the
antiferromagnetic coupling with the intermetallic distance along
this series indicates that the EE interactions through linear OPE
spacers follows a spin polarization mechanism. This conclusion
is further supported by the presence of non-negligible spin
densities of alternating sign on the bridge in the calculations on
the broken-symmetry (BS) singlet states. On the basis of this
combined experimental and computational study, we then
propose that linear OPE spacers can act as effective
antiferromagnetic wires between two CuII ions separated by
up to 3.0 nm.
Scheme 4. Simplified MO Diagram of the π-Bonding
Interaction between the Symmetric and Antisymmetric
Combinations of the Ligand πa and πb and the Metal dxy and
a
2
2
dx −y FMOs for 1
We are currently investigating novel series of electro- and
photoactive, oxamato-based dicopper(II) metallacyclophanes
with extended π-conjugated oligoanthraquinone (OAQ) and
oligo-p-phenylvinylidene (OPV) spacers as potential candidates
for electro- and photoswitchable antiferromagnetic wires,
respectively. Indeed, these simple molecules are ideal
experimental and theoretical models for the fundamental
study on electron- and phototriggered EE processes, which
are two central topics in the emerging area of molecular
spintronics.13f,g Moreover, they appear as very promising
candidates to get multifunctional molecule-based magnetic
devices facilitating the spin communication (“molecular
magnetic wires”, MMWs) or exhibiting multistable spin
behavior (“molecular magnetic switches”, MMSs) for future
applications in information processing and storage.
EXPERIMENTAL SECTION
■
Materials. All chemicals were of reagent-grade quality, and they
were purchased from commercial sources and used as received. 4,4′-
Diphenylethynediamine and 1,4-di(4-phenylethynyl)-
phenylenediamine were prepared as reported in the literature.15a
Et2H2dpeba. Triethylamine (1.4 mL, 10.0 mmol) and ethyl oxalyl
chloride ester (1.2 mL, 10.0 mmol) were added to a solution of 4,4′-
diphenylethynediamine (1.0 g, 5.0 mmol) in THF (150 mL) under
vigorous stirring. The reaction mixture was heated under reflux for 3 h
at 80 °C. The yellow solid was collected by filtration, washed
thoroughly with water to remove the precipitate of Et3NHCl, and then
with acetone and diethyl ether, and dried under vacuum (1.6 g, 78%
yield). Anal.: Calcd for C22H20N2O6: C, 64.70; H, 4.94; N, 6.86.
Found: C, 64.77; H, 4.97; N, 6.82. 1H NMR (C2D6SO): δ 1.32 (t, 6 H,
2 CH3), 4.31 (q, 4 H, 2 CH2O), 7.54 (d, 4 H, 2-H, 6-H, 2′-H, and 6′-
H), 7.82 (d, 4 H, 3-H, 5-H, 3′-H, and 5′-H), 10.96 (s, 2 H, 2 NH). IR
(KBr): 3335 (N−H), 2208 (CC), 1729, 1705 cm−1 (CO).
Et2H2tpeba. Triethylamine (1.4 mL, 10.0 mmol) and ethyl oxalyl
chloride ester (1.2 mL, 10.0 mmol) were added to a solution of 1,4-
di(4-phenylethynyl)phenylenediamine (1.5 g, 5.0 mmol) in THF (150
mL) under vigorous stirring. The reaction mixture was heated under
reflux for 3 h at 80 °C. The yellow solid was collected by filtration,
washed thoroughly first with water to remove the precipitate of
Et3NHCl and then with acetone and diethyl ether, and dried under
vacuum (2.0 g, 80% yield). Anal.: Calcd for C30H24N2O6: C, 70.86; H,
4.76; N, 5.51. Found: C, 70.77; H, 4.82; N, 5.52. 1H NMR (C2D6SO):
δ 1.33 (t, 6 H, 2 CH3), 4.35 (q, 4 H, 2 CH2O), 7.50 (s, 4 H, 2-H, 3-H,
5-H, and 6-H), 7.52 (d, 4 H, 2′-H, 6′-H, 2″-H, and 6″-H), 7.81 (d, 4
H, 3′-H, 5′-H, 3″-H, and 5″-H), 10.99 (s, 2 H, 2 NH). IR (KBr): 3335
(N−H), 2217 (CC), 1729, 1698 cm−1 (CO).
a
The open arrows correspond to the intraligand π−π* transitions
(nBu4N)4[Cu2(dpeba)2]·4MeOH·2Et2O (1). A 1.0 M methanolic
solution of nBu4NOH (4.0 mL, 4.0 mmol) was added to a suspension
of Et2H2dpeba (0.41 g, 1.0 mmol) in 20 mL of methanol under gentle
warming. A methanol solution (10 mL) of Cu(ClO4)2·6H2O (0.37 g,
1.0 mmol) was then added dropwise under stirring. The resulting
mixture was filtered to remove the small amount of solid particles and
between the optically active HOMOs and LUMOs. The boxed
structure shows the magnetically active SOMOs.
for the propagation of antiferromagnetic interactions between
the unpaired electrons occupying the dxy-type orbitals of the
J
dx.doi.org/10.1021/ic400754m | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
the solvent was removed under vacuum. The dark-green solid was
recuperated with THF, collected by filtration, washed thoroughly with
THF to remove the precipitate of nBu4NClO4, and air-dried.
Recrystallization from a methanol solution gave dark-green prisms of
1 suitable for single-crystal X-ray diffraction upon layering of diethyl
ether (0.72 g, 70% yield). Anal.: calcd for C112H196Cu2N8O18: C,
64.99; H, 9.54; N, 5.41. Found: C, 64.55; H, 9.36; N, 5.32. IR (KBr):
2207 (CC), 1685, 1648, 1611 cm−1 (CO).
Crystallographic data (excluding structure factors) of 1 have been
deposited with the Cambridge Crystallographic Data Centre as
supplementary publication number CCDC-831325. Copies of the
data can be obtained free of charge on application to CCDC, 12 Union
Road, Cambridge CB21EZ, UK (Fax: (+44) 1223-336-033; e-mail:
deposit@ccdc.cam.ac.uk).
Computational Details. The molecular geometries of the model
complexes 1−5 with D2h molecular symmetry were not optimized, but
their bond lengths and interbond angles were taken from the crystal
structure of 1 with an imposed coplanar conformation of the
phenylene rings in the OPE spacers (ψ = 0°) and a perpendicular
orientation of the phenylene rings with respect to the copper mean
basal planes (ϕ = 90°).
Density functional (DF) calculations and other ones based on the
time-dependent formalism (TD-DF) were carried out on the broken-
symmetry (BS) singlet and triplet spin states of the D2h-symmetric
model complexes 1−5 in acetonitrile solution using the hybrid B3LYP
functional24 combined with the “broken-symmetry” approach,25 as
implemented in the Gaussian 09 program.26 The triple- and double-ζ
quality basis sets proposed by Ahlrichs and co-workers27 were used for
the metal and nonmetal atoms, respectively. The calculated spin
density data were obtained from Natural Bond Orbital (NBO)
analysis.28 Solvation effects were introduced using a polarizable
continuum model (PCM), where the cavity is created via a series of
overlapping spheres,29 in order to accurately calculate the energy data
of the frontier molecular orbitals as well as the transition energy data
and transition strength force constants, which were deduced from
transition electric dipole moments.
(nBu4N)4[Cu2(tpeba)2]·12H2O (2). A 1.0 M methanolic solution
of nBu4NOH (4.0 mL, 4.0 mmol) was added to a suspension of
Et2H2tpeba (0.51 g, 1.0 mmol) in 20 mL of methanol under gentle
warming. A methanol solution (10 mL) of Cu(ClO4)2·6H2O (0.37 g,
1.0 mmol) was then added dropwise under stirring. The resulting
mixture was filtered to remove the small amount of solid particles, and
the solvent was removed under vacuum. A microcrystalline dark-green
solid of 2 was recuperated with THF, collected by filtration, washed
thoroughly with THF to remove the precipitate of nBu4NClO4, and
air-dried (0.70 g, 65% yield). Anal.: calcd for C116H192Cu2N8O24: C,
63.05; H, 8.76; N, 5.07. Found: C, 63.37; H, 8.63; N, 4.92. IR (KBr):
2208 (CC), 1683, 1649, 1619 cm−1 (CO).
Physical Techniques. Elemental analyses (C, H, N) were
performed at the Servicio Central de Soporte a la Investigacion
́
(SCSIE) at the Universitat de Valen
̀
cia (Spain). 1H NMR spectra were
recorded at room temperature on a Bruker AC 200 (200.1 MHz)
spectrometer. Deuterated dimethylsulfoxide was used as solvent and
internal standard (δ = 2.50 ppm). FT−IR spectra were recorded on a
Nicolet-5700 spectrophotometer as solid KBr pellets. Electronic
absorption spectra were recorded on acetonitrile or dimethylsulfoxide
solutions at room temperature with an Agilent Technologies-8453
spectrophotometer equipped with a UV−vis Chem Station. X-band (ν
= 9.47 GHz) EPR spectra were recorded on frozen-matrix acetonitrile
solutions at 4.0 K under nonsaturating conditions with a Bruker ER
200 D spectrometer equipped with a helium cryostat.
ASSOCIATED CONTENT
* Supporting Information
■
S
Crystal packing view of 1 (Figure S1), simulated X-band EPR
spectra of frozen-matrix acetonitrile solutions of 1 and 2
(Figure S2), perspective views of the two pairs of optically
active LUMOs and HOMOs for 1−4 (Figures S3−S6), and
plot of the calculated intraligand transition wavenumber and
the HOMO−LUMO energy gaps with the number of repeating
units for 1−5 (Figure S7). This material is available free of
charge via the Internet at http://pubs.acs.org.
Magnetic Measurements. Variable-temperature (2.0−300 K)
magnetic susceptibility measurements and variable-field (0−5.0 T)
magnetization measurements at 2.0 K were carried out on powdered
polycrystalline samples with a SQUID magnetometer. The magnetic
susceptibility data at low temperatures (T < 25 K) were measured
under a very low applied magnetic field (H = 100 G) in order to avoid
saturation effects.
Variable-temperature (2.0−150 K) magnetic susceptibility measure-
ments on frozen-matrix methanol solutions were performed with the
same SQUID magnetometer. Two quartz tubes of suprasil quality (8
cm height ×5 mm outer diameter with 0.5 mm wall thickness) were
placed together in vertical position, the top tube containing the sample
in methanol (8 mm is the height of the solution) and the bottom one
being open on both sides. The two tubes fit perfectly in the plastic
straw normally used in the SQUID devices. Placing the two tubes one
on the top of the other in a vertical manner minimizes their magnetic
contribution. In this respect, it is very illustrative that the two empty
tubes placed in the plastic straw under an applied magnetic field of 1.0
T gave an almost negligible magnetic signal (less than 5.0 × 10−6 cm3).
The experimental data were corrected for the diamagnetic
contributions of the constituent atoms and the sample holder and/
or the methanol solvent (24000 × 10−6 cm3 mol−1), as well as for the
temperature-independent paramagnetism (tip) of the two CuII ions
(120 × 10−6 cm3 mol−1).
AUTHOR INFORMATION
Corresponding Author
* E-mail: joan.cano@uv.es (J.C.); francisco.lloret@uv.es (F.L.);
donatella.armentano@unical.it (D.A.)
■
Present Address
⊥University Sidi Mohammed Ben Abdellah, Fes, Morocco.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the Ministerio de Ciencia e
■
Innovacion
́
(MICINN, Spain) (Projects CTQ2010-15364 and
CSD2007-00010), the Generalitat Valenciana (GV, Spain)
(Project PROMETEO/2009/108 and ISIC/2012/002), and
Crystal Structure Data Collection and Refinement. Single-
crystal X-ray diffraction data of 1 were collected on a Bruker-Nonius
X8APEXII CCD area detector diffractometer using graphite-
monochromated Mo−Kα radiation. All calculations for data reduction,
structure solution, and refinement were done through the SAINT19
and SADABS20 programs. The structure of 1 was solved by direct
methods and subsequently completed by Fourier recycling using the
SHELXTL software package.21 All non-hydrogen atoms were refined
anisotropically. The hydrogen atoms were set in calculated positions
and refined as riding atoms. The final geometrical calculations and the
graphical manipulations were carried out with PARST9722 and Crystal
Maker23 programs, respectively.
̀
the Ministero dell’Istruzione, dell’Universita e della Ricerca
Scientifica (Italy) through the Centro di Eccellenza CEMIF.-
CAL (Grant CLAB01TYEF). M.C. and F.R.F.-P. thank the
MICINN for doctoral grants. A.B. thanks the GV for a stay
́
grant. We acknowledge J. M. Martinez (Universitat de
Valencia) for the assistance and help in EPR data collection.
̀
REFERENCES
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