Redox switch-off of the ferromagnetic coupling in a mixed-spin tricobalt(II) triple mesocate

Article abstract of DOI:10.1021/ja9052202

(Chemical Equation Presented) A prelude to redox-based, ferromagnetic "metal-organic switches" is exemplified by a new trinuclear oxalamide cobalt triple mesocate that presents two redox states (ON and OFF) with dramatically different magnetic properties;

Full text of DOI:10.1021/ja9052202

Published on Web 09/24/2009
Redox Switch-Off of the Ferromagnetic Coupling in a Mixed-Spin Tricobalt(II)
Triple Mesocate
Marie-Claire Dul,† Emilio Pardo,*,† Rodrigue Lescoue¨zec,† Lise-Marie Chamoreau,†
Franc¸oise Villain,† Yves Journaux,*,† Rafael Ruiz-Garc´ıa,‡,§ Joan Cano,*,‡,§ Miguel Julve,‡
Francesc Lloret,*,‡ Jorge Pasa´n,| and Catalina Ruiz-Pe´rez|
Institut Parisien de Chimie Mole´culaire, UniVersite´ Pierre et Marie Curie-Paris 6, UMR 7201, F-75252 Paris, France,
Departament de Qu´ımica Inorga`nica, Instituto de Ciencia Molecular (ICMol) and Fundacio´ General de la UniVersitat de
Vale`ncia (FGUV), UniVersitat de Vale`ncia, E-46980 Paterna, Valencia, Spain, and Laboratorio de Rayos X y Materiales
Moleculares, Departamento de F´ısica Fundamental II, UniVersidad de La Laguna, E-38201 La Laguna, Tenerife, Spain
Received July 2, 2009; E-mail: emilio.pardo@uv.es; yves.journaux@upmc.fr; joan.cano@uv.es; francisco.lloret@uv.es
The design and synthesis of polytopic ligands that are able to
self-assemble with transition-metal ions to form linear multiple-
stranded helicates and related meso-helicates (so-called mesocates)
with predictable electronic properties are major goals in metallo-
supramolecular chemistry.1,2 Besides their interest as models for
fundamental research on electron exchange (EE) and electron
transfer (ET) phenomena between distant metal centers through
extended bridges, this type of ligand-supported, linear polynuclear
complex would also be of great importance in the “bottom-up”
approach to molecular-level electronic devices.3
Our strategy in this field consists of using linear oligo-m-phenylene
oxalamide (OPOXA) ligands that possess multiple oxamato and/or
oxamidato donor groups separated by phenylene spacers with a meta
substitution pattern (Figure 1a). Thus, the parent dicobalt(II) triple
Figure 1. (a) Structure of the cobalt(II) triple mesocates with OPOXA
mesocate 1 with the ligand N,N′-1,3-phenylenebis(oxamate) (mpba)
ligands. (b) Front and (c) side views of the anionic tricobalt triple mesocate
exhibits a weak ferromagnetic coupling (J ) +1.03 cm-1) between
of 2, showing each ligand strand and the metal coordination octahedra with
3
the high-spin (HS) CoII (S ) /2) ions because of spin polarization
Λ and ∆ chiralities in different colors.
effects across the triple m-phenylenediamidate bridge.4 As a natural
extension of that work, we report here the synthesis and characterization
[CoII (µ3-κ2:κ2:κ2-obmpox)3]12-, with a crystallographically imposed
3
by single-crystal X-ray diffraction (XRD) and X-ray absorption
spectroscopy (XAS) of the longer tricobalt(II) triple mesocate analogue
2 with the new ligand oxamidate-N,N′-bis(3-phenyloxamate) (obmpox)
and the corresponding tricobalt(II,III,II) monooxidized derivative 2ox.
The magnetic properties of this unique 2/2ox redox couple indicate
that oligo-m-phenylene oxalamide cobalt mesocates can behave as
ferromagnetic “metal-organic switches” (MOSs) for the transmission
of through-bond metal-metal EE interactions,5 by analogy with related
molecular switches that are based on direct metal-metal ET interac-
tions instead.6
D3 molecular symmetry (Figure 1b). The side-by-side binding of
the three nonplanar C2-symmetric tris(bidentate) obmpox6- ligands
around the three octahedral cobalt atoms affords a racemic mixture
of heterochiral triple mesocates with alternating (Λ,∆,Λ) and
(∆,Λ,∆) chiralities (Figure 1c). This situation contrasts with the
more common examples of homochiral [(Λ,Λ,Λ) or (∆,∆,∆)] triple
helicates, which result instead from helical wrapping of the ligands
around the metal centers.1 The three collinear cobalt atoms of 2
are linked through a triple m-phenylenediamidate bridge. The
intramolecular Co(1) · · · Co(2) and Co(2) · · · Co(2)I distances are
6.8780(18) and 13.756(3) Å, respectively (I ) x - y, -y, 1/2 - z).
The metal-ligand bond distances for the terminal cobalt atoms in
2 [Co(2)-N(2) ) 2.111(7) Å; Co(2)-O(4) ) 2.123(7) Å] are similar
to those in 1 [Co-N ) 2.106(8) Å; Co-O ) 2.137(8) Å],4 in
agreement with an HS d7 CoII ion in a trigonally distorted octahedral
environment, CoN3O3, formed by three amidate nitrogen and three
carboxylate oxygen atoms from the bidentate oxamato groups [trigonal
twist angle (θ) of 41.6(3)°]. However, the metal-ligand bond distances
for the central cobalt atom of 2 are significantly shorter [Co(1)-N(1)
) 1.965(5) Å]. This agrees with a low-spin (LS) d7 CoII ion in an
almost perfect octahedral environment, CoN6, formed by six amidate
nitrogen atoms from the bidentate oxamidato groups [θ ) 59.7(3)°].
Overall, this situation is as expected on the basis of the well-known
stronger ligand field of the N,N′-oxamidato donor groups relative to
the N,O-oxamato ones.7
Complexes 2 and 2ox were synthesized from the reaction of the
diethyl ester acid derivative of the proligand H4Et2obmpox with Co2+
nitrate (1:1 molar ratio) by using NaOH as the base in water under Ar
and air atmospheres, respectively, and they were isolated as the sodium
salts having formulas Na12[Co3(obmpox)3]·33H2O (2) and
Na11[Co3(obmpox)3]·30H2O (2ox). Complex 2 is slowly oxidized by
atmospheric dioxygen in aqueous solution to give 2ox with ∼45%
transformation after 30 min (see the Supporting Information). Indeed,
oxidation of 2 with H2O2 resulted in essentially quantitative formation
of 2ox (>96%), whereas treatment of 2ox with hydrazine partially
restored the original 2 in up to 78% yield.
Single-crystal XRD analysis of 2 confirmed the triple-stranded,
meso-helicate-type structure of the linear tricobalt(II) anions,
† Institut Parisien de Chimie Mole´culaire, UMR 7201, Universite´ Pierre et Marie
Curie-Paris 6.
‡ Departament de Qu´ımica Inorga`nica, ICMol, Universitat de Vale`ncia.
§ FGUV, Universitat de Vale`ncia.
The Co K-edge X-ray absorption near-edge structure (XANES)
| Departamento de F´ısica Aplicada II, Universidad de La Laguna.
spectra of 2 and 2ox agree with mixed-spin, homovalent CoII-
9
14614 J. AM. CHEM. SOC. 2009, 131, 14614–14615
10.1021/ja9052202 CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
where g1 and g2 ()ge) are the Zeeman factors, λ is the spin-orbit
coupling parameter, ∆ is the axial orbital splitting parameter, and
R is an orbital reduction parameter.9 A good fit of the experimental
data was obtained (for J′ ) 0 cm-1) with J ) +1.2 cm-1, g1 )
2.20, λ ) -130 cm-1, ∆ ) 120 cm-1, and R ) 1.10 for 2 and λ
) -109 cm-1, ∆ ) 239 cm-1, and R ) 1.10 for 2ox. The axial
orbital splitting of the terminal HS CoII ions for 2ox is greater than
that for 2, likely reflecting a larger distortion of the octahedral
geometry. The weak but nonnegligible ferromagnetic coupling
between the central LS CoII ion and the terminal HS CoII ions for
2, in spite of the moderately large intramolecular metal-metal
separation (∼7 Å), suggests that the exchange interaction involves
a spin polarization mechanism through the m-phenylene spacers,
as in 1 (J ) +1.0 cm-1).4
Figure 2. (a) Co K-edge XANES spectra of 2 (solid line) and 2ox (bold
line) compared with that of 1 (dashed line). The inset shows the pre-edge
region. (b) Temperature dependence of ꢀMT for 2 (2) and 2ox (4). The
inset shows the low-temperature region (the solid lines are the best-fit
curves).
In conclusion, the heterotritopic nature of the tailored ligand
obmpox6- allows for the side-by-side self-assembly of Co2+ ions
to afford two unique examples of homo- and heterovalent mixed-
spin trinuclear triple mesocates under either anaerobic and
aerobic conditions, respectively. The spins of the metal centers
are ferromagnetically coupled in the homovalent tricobalt(II)
mesocate (“ON” state), whereas they are uncoupled in the
heterovalent tricobalt(II,III,II) mesocate (“OFF” state). Current
efforts are devoted to the synthesis of other examples of oligo-
m-phenylene oxalamide mesocates as potential candidates for
electrically triggered magnetic molecular switches.
(HS)CoII(LS)CoII(HS) and heterovalent CoII(HS)CoIII(LS)CoII(HS)
formulations, respectively (Figure 2a). Notably, the asymmetry and
broadness of the intense edge features corresponding to the 1s f
4p transitions for 2 and 2ox in comparison with those of 1 indicate
the coexistence of different spin and/or oxidation states for the
terminal and central metal ions. Moreover, a shift to higher energy
is observed for the main broad peak centered at 7729.0 eV in 2ox
relative to the corresponding one centered at 7727.0 eV in 2, which
is in turn close in energy to the main sharp peak located at 7726.5
eV in 1. This hypsochromic energy shift of 2.0 eV in 2ox results
mainly from the increase in binding energy of the 1s core electrons
as the oxidation state of the central metal ion increases. Thus, the
main peak of the weak pre-edge features corresponding to the 1s
f 3d transitions appears similarly shifted at higher energy from
7710.4 eV in 1 and 2 to 7710.6 eV in 2ox. Similar oxidation-state-
dependent edge and/or pre-edge shifts have been reported for other
octahedral cobalt complexes.8
Acknowledgment. This work was supported by the Ministerio
de Educacio´n y Ciencia (MEC, Spain) (Projects CTQ2007-
61690, CSD2007-00010, and MAT2007-60660), the Generalitat
Valenciana (GV, Spain) (Project PROMETEO/2009/108), and
the Ministe`re de l’Enseignement Supe´rieur et de la Recherche
(MESR, France). E.P. and M.-C.D. thank the MEC and MESR
for grants.
The plots of ꢀMT versus T (where ꢀM is the molar magnetic
susceptibility per Co3 unit and T the temperature) for 2 and 2ox are
consistent with ferromagnetically coupled CoII(HS)CoII(LS)CoII(HS)
and uncoupled CoII(HS)CoIII(LS)CoII(HS) linear triads, respectively
(Figure 2b). The increase in ꢀMT for 2 in the low-temperature region
is indicative of a weak ferromagnetic interaction between the two
Supporting Information Available: Preparation and physical
characterization data of the ligand and complexes 2 and 2ox,
electronic spectroscopic studies with oxidants and reductants, and
a CIF file for 2. This material is available free of charge via the
Internet at http://pubs.acs.org.
3
1
terminal HS CoII (S ) /2) ions and the central LS CoII (S ) /2)
ion (J ) J12 ) J12′ > 0) (inset of Figure 2b). On the contrary, there
is no sign of either ferro- or antiferromagnetic interaction between
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3
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3
1
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ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
H ) -J(S1 · S2 + S1 · S2′) - J′(S2 · S2′) +
ˆ
2
ˆ
ˆ
ˆ
Rλ(L2 · S2 + L2′ · S2′)
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2
ˆ
ˆ
ˆ
ˆ
ˆ
+ ∆(L2z + L2′z ) + g1S1zꢁH + g2(S2z + S2′z)ꢁH +
ˆ
ˆ
R(L2z + L2′z)ꢁH
(1)
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