Rational design of an enneanuclear copper(II) complex with a metallacyclophane core

Article abstract of DOI:10.1002/anie.200352851

A hexakis-bidentate building block: Tri-copper(II) and enneacopper(II) 1,3,5-metallacyclophanes were synthesized by a combination of self-assembly and a rational approach (see scheme). Both complexes exhibit ferromagnetic coupling in the core, through a spin-polarization mechanism.

Full text of DOI:10.1002/anie.200352851

Communications
Metallacyclophanes
(ClO4)6·12H2O (3-(ClO4)6·12H2O; pmdien = N,N,N’,N’’,N’’-
pentamethyldiethylenetriamine).
Rational Design of an Enneanuclear Copper(ii)
Complex with a Metallacyclophane Core**
Xavier Ottenwaelder, Joan Cano, Yves Journaux,*
Eric Rivi›re, Conor Brennan, Martine Nierlich, and
Rafael Ruiz-García
Design of complex molecular architectures starting from
constituent subunits is a central theme in inorganic chemis-
try.[1] Among such species, those containing magnetic ions are
of particular interest because their magnetic properties
directly stem from the geometrical arrangement of the
constituent subunits.[2] The main challenge in this field is the
synthesis of single-molecule magnets whose ground states
have high spin and considerable negative axial anisotropy.[3]
Pathways to such complex molecular entities are based on two
orthogonal synthetic schemes: self-assembly methods[4] and
step-by-step strategies.[5] In the former, the nature of the final
products cannot be determined a priori. In contrast, the latter
provide an efficient means to control both nuclearity and
dimensionality of the polymetallic edifice.
Following a step-by-step approach, we prepared the new
ligand N,N’,N’’-1,3,5-benzenetris(oxamate) (1). Being incapa-
ble of hexadentate ligation to a single metal center, this ligand
rather self-assembles with CuII ions in a 2/3 ligand/metal ratio
to give the trinuclear complex Na6[CuI3I(1)2]·11.5H2O (Na6-
2·11.5H2O). In the [Cu3II(1)2]6À core 2, each ligand adopts a
tris(N,O-bidentate) coordination mode (vide infra). Thus, the
three bis(oxamate)copper(ii) spin-bearing residues are
bridged by two 1,3,5-benzenetriyl moieties, which have
already proved to be efficient ferromagnetic spin-coupling
units in p-conjugated organic polyradicals[6] and metal com-
plexes.[7–10] Additionally, 2 is a potential hexakis-bidentate
ligand that can coordinate six other metal ions through its
external oxygen atoms. Using this “complex-as-ligand strat-
egy” with ancillary ligands on the additional metal ions to
avoid polymerization, we obtained the novel homometallic
Single-crystal X-ray characterization of 3 revealed the
desired structure. The unit cell (R3 space group) contains two
¯
types of hexacationic enneacopper molecules, one of which is
disordered, as well as perchlorate ions and water molecules.
The well-ordered enneanuclear molecules exhibit the antici-
pated pattern of six external [Cu(pmdien)]2+ subunits ligated
to a central [Cu3(1)2]6À unit (Figure 1). To the best of our
knowledge, 3 is the second crystallographically characterized
example of a 1,3,5-metallacyclophane.[11]
The central CuII ions have an elongated square-pyramidal
(Cu2 and Cu4) or elongated octahedral (Cu5) geometry with
distant axial ligands. These environments can be approximated
as square-planar, created by one oxamate moiety of each ligand
(maximum deviation from average N2O2 plane: 0.148 Š). The
[Cu3(1)2]6À core is twisted to accomodate a noneclipsed
disposition of the aromatic rings, with a distance of 3.237 Š
between their average planes. In comparison to the binuclear
1,3-metallacyclophane analogue of 2, [Cu2-m],[10] coordination
of the third Cu atom brings the aromatic rings closer, enhances
electronic repulsions, and results in greater overall twisting of
the molecule: the C-C-N-Cu torsion angles are farther from 908
in 3 (65.0–79.68, av73.8 8) than in [Cu2-m] (73.4–89.38, av78.9 8).
In the peripheral [Cu(pmdien)]2+ subunit, the CuII ions are five-
coordinate, close to square-pyramidal with t values[12] ranging
enneanuclear
complex
[{CuII (1)2}{CuII(pmdien)}6]-
3
[*] Dr. X. Ottenwaelder, Dr. J. Cano, Dr. Y. Journaux, Dr. E. Rivi›re,
Dr. C. Brennan
Laboratoire de Chimie Inorganique, CNRS UMR 8613
UniversitØ Paris-Sud
91405 Orsay (France)
Fax: (+33)169-154-754
E-mail: jour@icmo.u-psud.fr
À
from 0.27 to 0.39, and with one Cu O bond longer than the
Dr. M. Nierlich
other (2.20 versus 1.97 Š on average).
CEA, Service de Chimie MolØculaire DSM/DRECAM
Centre d'Etudes de Saclay Bât. 125
91191 Gif sur Yvette Cedex (France)
The magnetic properties of 2 and 3 are depicted in
Figure 2. Complex 2 exhibits ferromagnetic coupling between
the three CuII ions: cMT continuously increases with decreas-
ing temperature. A good fit to the experimental data was
obtained with g = 2.07, J = + 11.6 cmÀ1 and a Weiss constant
q = + 0.41 K (see Experimental Section). Despite the
large separation between the CuII ions of about 6.9 Š, a
strong ferromagnetic coupling J is present, as in [Cu2-m]
Dr. R. Ruiz-García
Departament de Química Orgànica, Facultat de Química
Universitat de Val›ncia
46100 Burjassot, Val›ncia (Spain)
[**] This work was supported by the European Community (contract
HPRN-CT-1999-00012/TMR network “MolNanoMag”).
850
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200352851
Angew. Chem. Int. Ed. 2004, 43, 850 –850

Angewandte
Chemie
(+ 16.8 cmÀ1),[10] due to spin-polarization effects through the
aromatic meta linkage.[13] Aside from the organoiron complex
reported by Lapinte et al.,[8] the couplings in 2 and [Cu2-m]
are larger than in other metal complexes.[9,14] This is due to the
particular geometry of 2 in which the copper basal planes are
nearly perpendicular to the benzene rings, which leads to a
strong interaction between the Cu dx2Ày2 orbitals through the p
system of the benzene rings.[10]
When the sample is cooled, cMT for 3 first decreases, then
reaches
a plateau between 20–30 K with a value of
1.17 cm3 KmolÀ1 corresponding to three uncoupled 1/2 spins,
and then increases below 20 K to reach 1.44 cm3 KmolÀ1 at
2 K. This reproduces the expected behavior of 3: antiferro-
magnetically coupled (JAF) oxamato-bridged trinuclear CuII
subunits, and, in the low-temperature region, ferromagnetic
coupling (JF) between the S = 1/2 ground states of the
trinuclear subunits through the central [Cu3(1)2]6À core. A
good fit of the experimental data was obtained with JAF
=
À106 cmÀ1, JF = + 7.8 cmÀ1, and g = 2.03. The value of JAF is in
good agreement with that found for oxamate-bridged tricop-
per(ii) complexes with pmdien as terminal ligand.[15,16] The
magnitude of JF was confirmed by the fit of the magnetization
versus field at 2 K, which gave JF = + 8.4 cmÀ1.[17] The smaller
value of JF in 3 (+ 7.8 cmÀ1) compared to 2 (+ 11.6 cmÀ1) and
[Cu2-m] (+ 16.8 cmÀ1) likely originates from less efficient
overlap between the aforementioned orbitals as a conse-
quence of the greater helical twist in 3.
In conclusion, ligand 1 allows the assembly of three CuII
ions in a meta arrangement around bridging aromatic rings to
give complex 2. Being hexakis-bidentate, 2 is an efficient
building block for larger assemblies with advantageous
chelation of the peripheral metal ions, as observed in 3.
Through its rational construction, the enneanuclear complex
3 may be described as a first-generation metallodendrimer
based on 2. Importantly for magnetic assemblies, the ferro-
magnetic coupling by a spin-polarization mechanism in 2 is
also operative in 3. To extend these promising results, we aim
at using this new class of ligands for the synthesis of
1) enneanuclear heterometallic complexes as potential
single-molecule magnets, and 2) extended networks for
molecule-based magnets.
Figure 1. Side and top views of the well-ordered enneacopper unit 3.
Selected interatomic distances [Š] and angles [8]: Cu2-Cu4 6.899(7),
Cu2-Cu5(7) 7.051, Cu4-Cu5(7) 6.836, Cu2-N1 1.980(6), Cu2-N4
1.957(6), Cu2-O201 2.417(7), Cu4-N2 1.961(5), Cu4-N5 1.980(5), Cu4-
O2W 2.424(8), Cu5-N3 1.998(6), Cu5-N6 1.950(5), Cu5-O501 2.468(7),
Cu5-O6W 2.835(8); N1-Cu2-N4 104.9(3), N2-Cu4-N5 105.1(2), N3-
Cu5-N6 105.7(2).
Experimental Section
1,3,5-Triaminobenzene:
A mixture of 3,5-dinitroaniline (1.46 g,
8.0 mmol) and 10% Pd/C (150 mg) in methanol (150 mL) was stirred
overnight under hydrogen (40 bar). The suspension was filtered over
Celite under inert atmosphere and washed with degassed methanol
until the filtrate was colorless. Evaporation of the solvent gave 1,3,5-
1
triaminobenzene as a light brown oil in quantitative yield. H NMR
(250 MHz, degassed [D6]DMSO): d = 4.34 (brs, 6H; NH2), 5.12 ppm
(s, 3H; aromatic H).
1: The ligand was isolated as its triethyl ester. Under inert
atmosphere, a solution of 1,3,5-triaminobenzene (972 mg, 7.9 mmol)
in DMF (50 mL) was added to ethyloxalyl chloride (5.5 mL,
49 mmol). After the mixture had been stirred for 5 min, diisopropyl-
ethylamine (8.5 mL, 49 mmol) was added, and the solution was stirred
at about 908C for 2 h, after which it was handled in air. After
evaporation of the solvent, water (200 mL) was added to the resulting
oil, to yield a white solid, which was collected by filtration, washed
with water to neutrality of the filtrate, and dried under vacuum (2.7 g,
Figure 2. Magnetic behavior of 2 (squares) and 3 (circles); the solid
lines are the calculated curves.
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ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
851

Communications
80% yield). Elemental analysis (%) calcd for C18H21N3O9 (423): C
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J. R. Long, J. Am. Chem. Soc. 2002, 124, 7656; d) J. J. Sokol, M. P.
Shores, J. R. Long, Inorg. Chem. 2002, 41, 3052.
51.06, H 5.00, N 9.95; found: C 49.66, H 4.91, N 9.91; 1H NMR
([D6]DMSO): d = 1.30 (t, 9H; 3 CH3), 4.29 (q, 6H; 3 CH2O), 7.91 (s,
3H; aromatic H), 10.91 ppm (s, 3H; 3 NH); IR (KBr): n˜ = 3277 (NH),
1739, 1695 cmÀ1 (C O).
=
2: The trinuclear CuII complex was obtained by reaction of the
proligand of 1 (2 equiv) with CuII(NO3)2 (3 equiv) in basic aqueous
medium (6 equivof NaOH), and isolated as its sodium salt (95%). [18]
Elemental analysis (%) calcd for C24H6Cu3N6Na6O18(H2O)11.5 (1202):
C 23.98, H 2.43, N 6.99; found: C 23.94, H 2.42, N 6.99; IR (KBr): n˜ =
[3] a) J. Yoo, E. K. Brechin, A. Yamaguchi, M. Nakano, J. C.
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1654, 1598 cmÀ1 (C O).
=
3: Caution: Perchlorate-containing salts are potentially explosive
and should be handled in very small quantities. The enneacopper(ii)
complex 3 was synthesized in a manner similar to that described in the
literature,[16] with the appropriate 6:1 ratio of [Cu(pmdien)](ClO4)2
and 2 in water. After addition, the mixture was stirred for 10 min, then
the precipitate was collected by filtration, washed with water and
methanol, and dried under vacuum (75%). Crystallization of 3 as
green rods occurred over a few days on slow evaporation of a
saturated aqueous solution. Elemental analysis (%) calcd for
C78H144Cl6Cu9N24O42(H2O)12 (3091): C 30.31, H 5.48, N 10.88, Cl
6.88, Cu 18.50; found: C 29.72, H 5.27, NÀ110.76, Cl 7.15, Cu 17.36; IR
=
(KBr): n˜ = 1607 br (C O), 1088–1145 cm (ClO).
Crystal data for 3: C91H196Cl7Cu10.5N28O67, Mr = 3270.08, trigonal,
¯
space group R3, a = b = 65.137(9) Š, c = 18.861(4) Š, V=
69303(20) Š3, T= 123(2) K, Z = 18, 1calcd = 1.410 gcmÀ3, m(MoKa) =
1.616 mmÀ1. Data were corrected for Lorentzian and polarization
effects and absorption. 23008 unique reflections, of which 9362 with
I > 2s(I) were taken as as observed. The structure was solved by
direct methods using SHELXS97 and refined by using the full-matrix
least-squares method on F2 using SHELXL97. The hydrogen atoms
were neither found nor calculated. Refinement of 1296 variables with
anisotropic thermal parameters gave R = 0.1290, Rw = 0.3311, and S =
1.065. CCDC-208622 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge
Crystallographic Data Centre, 12, Union Road, Cambridge CB21EZ,
UK; fax: (+ 44)1223-336-033; or deposit@ccdc.cam.ac.uk).
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The moderate quality of the resolution likely arises from
disorders. Two perchlorate ions (Cl3, Cl4) are disordered, as is a
molecule of complex 3 (Cu10, Cu11, C88, C89, N28, O19, O21) lying
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¯
on the 3 axis and disordered on two positions with occupancy factors
of 0.5. The latter appear as dodecanuclear entities by superposition of
6+
the two enantiomers of a [Cu3[1]2[Cu(pmdien)]6 complex. Even
though a rigorous metrical analysis is precluded by the disorder, all
Cu environments are very similar to those in well-ordered molecules.
Magnetic measurements were carried on a SQUID magneto-
meter on powdered samples in the range 2–300 K. The susceptibility
curves were fitted with an analytical law with the ÀJ convention for
2,[19] and by full Hamiltonian diagnalization for 3 with the Hamil-
tonian h = ÀJF(S2 S4 + S2 S5 + S4 S5)ÀJAF(S2 S1 + S2 S7 + S4 S3 + S4 S8 +
S5 S6 + S5 S9) + gb(S1 + S2 + S3 + S4 + S5 + S6 + S7 + S8 + S9)B.
For 2, at the lowest temperature attainable, cMT is
2.5 cm3 KmolÀ1, a value higher than expected for the S = 3/2 ground
state. This is due to weak intermolecular ferromagnetic interactions,
which were taken into account by a Weiss constant q.[19]
[15] R. Costa, A. Garcia, J. Ribas, T. Mallah, Y. Journaux, J. Sletten,
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Riviere, M. C. Munoz, Eur. J. Inorg. Chem. 2000, 951.
[17] The magnetization of 3 at low temperature can also be fitted in
terms of an effective interaction between the doublet ground
states of the trinuclear subunits. The best fit is obtained with a
Jeff value of + 0.9 cmÀ1, which corresponds to a JF value of
+ 8.4 cmÀ1 (JF = 9Jeff).
[18] H. O. Stumpf, Y. Pei, O. Kahn, J. Sletten, J.-P. Renard, J. Am.
Chem. Soc. 1993, 115, 6738.
[19] O. Kahn, Molecular Magnetism, VCH, New York, 1993.
Received: September 12, 2003 [Z52851]
Keywords: copper · cyclophanes · magnetic properties ·
.
metallacycles · N,O ligands · nonanuclear complex
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852
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Angew. Chem. Int. Ed. 2004, 43, 850 –852