Long-range magnetic coupling through extended π-conjugated aromatic bridges in dinuclear copper(II) metallacyclophanes

Article abstract of DOI:10.1021/ja030060f

Self-assembly of 1,4-phenylenebis(oxamate) and 4,4′-biphenylenebis(oxamate) ligands and Cu²⁺ ions gives two new dinuclear copper(II) metallacyclophanes where the two metal centers are connected by double para-substituted aromatic diamide bridge

Full text of DOI:10.1021/ja030060f

Published on Web 08/14/2003
Long-Range Magnetic Coupling through Extended π-Conjugated Aromatic
Bridges in Dinuclear Copper(II) Metallacyclophanes
Emilio Pardo,1a Juan Faus,1a Miguel Julve,1a Francesc Lloret,*,1a M. Carmen Mun˜oz,1b Joan Cano,*,1c
Xavier Ottenwaelder,1c Yves Journaux,*,1c Rosa Carrasco,1d Gonzalo Blay,1e Isabel Ferna´ndez,1e and
Rafael Ruiz-Garc´ıa*,1e
Departament de Qu´ımica Inorga`nica, Instituto de Ciencia Molecular, and Departament de Qu´ımica Orga`nica,
Facultat de Qu´ımica, UniVersitat de Vale`ncia, 46100 Burjassot, Vale`ncia, Spain, Departamento de F´ısica Aplicada,
UniVersidad Polite´cnica de Vale`ncia, 46071 Vale`ncia, Spain, and Laboratoire de Chimie Inorganique and
Laboratoire de Chimie Bioorganique et Bioinorganique, UniVersite´ Paris-Sud, 91405 Orsay, France
Received January 28, 2003; E-mail: rafael.ruiz@uv.es
Polymetallic complexes with strong magnetic exchange interac-
tions between distant metal centers across extended bridges are a
major topic in the field of magnetochemistry.2 In particular,
dinuclear copper(II) complexes with aromatic bridging ligands have
been actively investigated for the study of long-distance electron
exchange, termed “long-range magnetic coupling”.3 With some
notable exceptions,4-6 weak intramolecular antiferromagnetic cou-
plings (-J < 32 cm-1) have been reported for dicopper(II)
complexes with long intermetal distances (r > 6 Å) (Table S1). In
most cases, the exchange interaction between the Cu(II) centers is
mainly transmitted through the σ-bonds of the aromatic bridge, with
little or no contribution from the π-bond system.
Figure 1. (a) Perspective view of the centrosymmetric anionic dicopper
unit of 1B with the atom-numbering scheme (I ) 1 - x, -y, 1 - z). (b)
Side view of the metallacyclophane core of 1B.
Here we report on two new anionic dinuclear copper(II)
complexes with the ligands N,N′-1,4-phenylenebis(oxamate) (ppba)
and N,N′-4,4′-biphenylenebis(oxamate) (bpba) (1 and 2, Chart 1).
This type of rigid dinucleating ligands leads to discrete, self-
assembled dicopper(II) metallamacrocycles of the [3,3]p-cyclophane
type, with a π-stacked arrangement of the aromatic rings connected
by two N-Cu-N linkages. Hence, the metal basal planes are
coplanar and disposed almost perpendicularly to the two parallel
aromatic spacers. As shown earlier for the meta derivative,7 this
particular geometry ensures a unusual π-type pathway for the
exchange interaction between the unpaired electrons of the two Cu-
(II) ions through the double aromatic diamide bridges. The
complexes described herein can be considered as the first members
of a novel series of dinuclear copper(II) metalla-amidocyclophanes
with linear π-conjugated oligo-p-phenylene spacers -(C6H4)n- (n
) 1-5) of various lengths (1-5, Chart 1). Electronic structure
calculations on this series offer a unique opportunity to examine
the effect of intermetal distance on magnetic coupling in dicopper
systems with Cu-Cu distances in the range 8-25 Å.
Figure 2. Perspective view of the centrosymmetric anionic dicopper unit
of 2B with the atom-numbering scheme (I ) 1 - x, -y, -z).
Chart 1
Complexes 1 and 2 were isolated as a variety of salts with
different countercations, both coordinating like Li+ or Na+, and
noncoordinating like Ph4P+ or Bu4N+ (Supporting Information).
Single-crystal X-ray diffraction analyses of the sodium salts 1B
and 2B confirmed the anionic dimeric molecular structures (Figures
1 and 2, respectively).
The copper environment is essentially square-planar for 1B and
distorted square-pyramidal for 2B, with two amide nitrogen and
two carboxylate oxygen atoms in the basal plane and an apical water
molecule for the latter [metal deviations from the mean basal planes
of 0.15 (1B) and 0.19 Å (2B)]. The copper basal planes are not
exactly oriented perpendicular to the phenylene planes [dihedral
angles (φ) of 60.6 (1B) and in the range 51.9-69.5° (2B)]. In
addition, the phenylene rings are slightly tilted around the C-C
single bond for 2B [dihedral angle (ψ) of 19.7°]. Overall, these
deviations from ideal D2h symmetry may originate from the
favorable π-π interactions in the parallel-displaced configuration
of the phenylene dimers and the forced o-o′ repulsive interactions
between the aromatic hydrogen atoms in the biphenylene spacers
(intramolecular contributions), or from severe steric constraints in
the crystal packing (intermolecular contribution). Actually, oxygen
atoms from the oxamate groups bind sodium atoms leading to
corrugated sheetlike solid-state structures (Figure S1). The intramo-
lecular Cu-Cu distances (r) are 7.91 (1B) and 12.19 Å (2B), while
the shortest intermolecular Cu-Cu distances are 6.84 (1B) and 6.87
Å (2B).
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C O M M U N I C A T I O N S
π- versus σ-type exchange pathways. For instance, our results
estimate a weak but nonnegligible antiferromagnetic coupling (J
) -3.9 cm-1) between two Cu(II) ions separated by up to 16.5 Å
through triphenylenediamide bridges in 3, which contrasts with that
of -0.0002 cm-1 derived from the Coffman-Buettner relation.
Nevertheless, two nanometers appears to be the upper limit for the
observation of magnetic coupling (-J < 1.0 cm-1) in the longer
homologues of this series, 4 and 5, including several C-C single
bonds in the aromatic spacers.
Figure 3. Perspective view of the calculated SOMOs for 1.
In conclusion, we have experimentally observed a strong to
moderately strong intramolecular magnetic coupling between two
Cu(II) centers in 1 and 2. This has been supported by theoretical
calculations which identify the predominantly π-type orbital
pathways through the aromatic diamide bridges as the origin of
the exchange interaction. Moreover, a slow rate of decay of
magnetic coupling with distance through oligo-p-phenylenediamide
bridges is theoretically predicted in 1-5. This leads our current
research toward other dicopper(II) metalla-amidocyclophanes with
better π-conjugated oligoacene spacers, so as to get further insights
on long-range magnetic coupling through extended aromatic
bridges.
Figure 4. Plot of the calculated singlet-triplet splitting (on a log scale)
(b) and the intermetal distance (O) with the number of repeat units for
1-5.
Acknowledgment. This work was supported by the Ministerio
de Ciencia y Tecnolog´ıa (Spain) through the Plan Nacional de
Investigacio´n Cient´ıfica, Desarrollo e Innovacio´n Tecnolo´gica
(Projects BQU2001-2928 and BQU2001-3017) and the Ramo´n y
Cajal program. Further support from the TMR program of the
European Union (Contract ERBFM-RXCT980181) is acknowl-
edged. E.P. thanks the Ministerio de Educacio´n, Cultura y Deporte
(Spain) for a grant.
Variable-temperature magnetic susceptibility measurements (1.8-
300 K) on complexes 1 and 2 revealed a magnetic behavior typical
of antiferromagnetically coupled dicopper(II) pairs (Figure S2). The
least-squares fit of the experimental data through the Bleaney-
Bowers equation gave -J values in the range 81-95 cm-1 for 1,
and 8.7-11.5 cm-1 for 2 (H ) -J S1‚S2) (Table S2). Overall, the
similar magnetic behavior with variation of the counterion within
each family unambiguously indicates that the magnetic coupling
is intramolecular in origin. Moreover, the significant antiferromag-
netic couplings despite the relatively large intramolecular Cu-Cu
separations are quite remarkable.8 This suggests that the available
π orbitals of the aromatic diamide bridges, made up of py carbon
and nitrogen orbitals, are really effective in propagating the
exchange interaction between the two unpaired electrons occupying
the dxy metal orbitals. For instance, this is reflected in the high
covalency and strongly delocalized character of the two singly
occupied molecular orbitals (SOMOs) of 1 depicted in Figure 3.
Density functional theory (DFT) calculations on complexes 1
and 2, with ideal D2h symmetry,9 showed a singlet spin ground-
state lying well below the triplet excited state. For these model
molecules, the calculated value of the singlet-triplet energy gap
(∆EST ) -J) decreases from 104 for 1 to 18.8 cm-1 for 2. The
experimental values for 1 are close to the calculated one, the slight
deviations being due to the loss of orthogonality between the copper
and phenylene planes (φ * 90°). In addition, the somewhat reduced
experimental values for 2 compared to the calculated one also reflect
a partial loss of π-conjugation in the biphenylene spacer (ψ * 0°).10
The results of DFT calculations on the D2h-symmetric model
complexes 1-5 are summarized in Figure 4 (Table S3). Together
with a perfect linear increase in the estimated intermetal distances
(r), the calculated singlet-triplet energy gap (-J) decreases in an
approximately exponential manner with the number of phenylene
repeat units (n ) 1-5) along this series.
Supporting Information Available: Experimental preparation,
analytical, spectral, and magnetic susceptibility data for 1 and 2, X-ray
crystallographic data of 1B and 2B, and computational details on 1-5
(PDF/CIF). This material is available free of charge via the Internet at
http://pubs.acs.org.
References
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(8) Hendrickson et al. have reported comparable antiferromagnetic couplings
in related dicopper(II) complexes with phenylene- and biphenylenediamine
bridges (-J values up to 70 and 9 cm-1, respectively) (Table S1). In
light of our results, it appears that the exchange interaction in these
compounds also involves predominantly π-orbital pathways.
(9) Molecular geometries for 1-5 were not optimized, but their structural
dimensions were taken from the crystal structure of 1B (φ ) 90°). For
2-5, the phenylene rings connected by the C-C single bond were coplanar
(ψ ) 0°) and the C-C inter-ring distance was taken as 1.49 Å.
(10) Preliminary DFT energy calculations show that the calculated singlet-
triplet energy gap is sensitive to the dihedral angle between the copper
and phenylene planes (φ) in 1 and that between the phenylene planes (ψ)
in 2. Thus, the -J value becomes smaller as the more different from 90°
is φ and from 0° is ψ, as a result of these global torsions around the
C-N and C-C single bonds of the aromatic diamide bridges (data reported
elsewhere).
The fit of the calculated data for 1-5 gave a decay law of
exchange interaction with intermetal distance as -J ) 1.47 × 103
exp(-0.35r). The relation obtained by Coffman and Buettner,3b and
the more recent ones based on experimental magnetostructural data
on dicopper(II) complexes, predict a faster decay of magnetic
coupling with an exponential factor varying in the range 1.5-1.8
Å-1 (Figure S3). Once again, this reflects the relative efficiency of
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