The coordination preferences of metal centres modulate superexchange coupling interactions in a metallo-supramolecular helical assembly

Article abstract of DOI:10.1039/c0cc00338g

A method to modulate the strength of the superexchange coupling interaction in dinuclear helical assemblies based on the coordination preferences of the metal centres is described.

Full text of DOI:10.1039/c0cc00338g

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The coordination preferences of metal centres modulate superexchange
coupling interactions in a metallo-supramolecular helical assemblyw
a
a
b
c
´
Rosa Pedrido,* Miguel Vazquez Lopez,* Lorenzo Sorace, Ana M. Gonzalez-Noya,
Magdalena Cwiklinska, Vanesa Suarez-Gomez, Guillermo Zaragoza and
´
´
a
a
d
´
´
Manuel R. Bermejoa
Received 11th March 2010, Accepted 30th April 2010
First published as an Advance Article on the web 27th May 2010
DOI: 10.1039/c0cc00338g
A method to modulate the strength of the superexchange
coupling interaction in dinuclear helical assemblies based on
the coordination preferences of the metal centres is described.
the superexchange coupling interaction in a double-stranded
dinuclear helical assembly based on the coordination preferences
of the metal centres.
The N3O2 pentadentate ligand H2L (Chart 1), which
consists of two 4-methoxybenzyl hydrazinecarboxylate arms
joined by a pyridine spacer, was synthesized by Schiff base
condensation between 2,6-diacetylpyridine and 2-methoxy-
phenylcarbazate. Electrochemical oxidation6 of a Cu plate in
a conducting solution of H2L in acetonitrile afforded a brown
Double- and triple-stranded helicates have been extensively
investigated in the past two decades as model compounds in
metallo-supramolecular chemistry.1 Although the majority of the
studies on helical complexes are focused on understanding the
fundamental principles of recognition and self-assembly pro-
cesses, there is great interest in searching for new functionalities
and properties in this class of compounds.2–4 In this context,
helicates has been used as components in magnetic molecular
devices and materials with promising properties.5
solution, from which
a brown solid precipitated upon
concentration. The ESI mass spectrum and elemental analysis
data for this solid are consistent with the formation of the
neutral dimeric species [CuII2(L)2], which arises from the double
deprotonation of H2L in the electrochemical cell. The solid was
also characterized by IR spectroscopy. Recrystallization of this
complex by slow evaporation afforded brown crystals, from
which the molecular structure of the compound was determined
by X-ray crystallography.z The structure reveals the formation
of a [5 + 5] double-stranded dihelicate [CuII2(L)2]ꢀ2H2O (1)
(Fig. 1 and S1)w as a racemic mixture of the two enantiomers.
As observed in the X-ray structure, each ligand strand in 1
uses the carbonyl oxygen and the imine nitrogen atom from
one of the bidentate binding domains, together with the
pyridine nitrogen of the spacer, to bind one copper centre. A
rotation around the adjacent C–C bond in the second domain
leads to chelation of the remaining imine nitrogen and carbonyl
oxygen atoms to the second copper ion, thus generating the
double helical structure. The copper(II) ions exhibit distorted
pentacoordinated [N3O2] kernels and distorted planar pyramidal
geometries (t1 = t2 = 0.26). A fourth nitrogen in an axial
position is found at a longer distance (2.65 A). The intra-
molecular distance between the copper centres is 3.24 A.
A careful analysis of the bond distances and angles reveals
the existence of a certain degree of asymmetry in the helicate,
which in turn results in the formation of distinct major and
minor grooves (Fig. S2).w Such asymmetry rarely occurs in
metallohelicates and is usually related to the inner asymmetry
of the wrapping ligands or to the establishment of intramolecular
p-stacking contacts.8 However, we believe that the asymmetry
in 1 must be attributed to the different conformations adopted
The recognised synthetic approaches to helicates involve the
use of metal-ion induced self-assembly of ligand threads
containing repeating donor units of certain denticity, so that
the binding abilities of the ligand domains need to match the
coordinative requirements of the metal ions. As a result, the
careful selection of the metal centres based on their coordina-
tion preferences is one of the most important instruments
available to control the microarchitecture and shape of helical
complexes. This tool has been used, for example, as an engine of
molecular-level machines based on helicates3 or in the develop-
ment of novel synthetic routes to cluster helicates,6 but to our
knowledge it has never been employed to tune the magnetic
properties of a metallo-supramolecular helical assembly.
Currently we are embarked upon an extensive research
program that includes the use of helicates in the construction
of supramolecular networks,7 molecular-level devices,4 high-
nuclearity metal clusters6 or novel magnetic materials.5a In this
context, we describe here a method to modulate the strength of
a
´
´
´
Departamento de Quımica Inorganica, Facultad de Quımica,
Universidad de Santiago de Compostela, 15782 Santiago de
Compostela, Spain. E-mail: miguel.vazquez.lopez@usc.es,
rosa.pedrido@usc.es
b Laboratory of Molecular Magnetism, Dipartimento di
`
Chimica ‘‘Ugo Schiff’’ and UdR INSTM, Universita di Firenze,
50019 Sesto Fiorentino (Fi), Italy
c
´
´
Departamento de Quımica Inorganica, Facultad de Ciencias,
Universidad de Santiago de Compostela, 27002 Lugo, Spain
d
´
Unidad de Difraccion de Rayos X, Edificio CACTUS, Universidad
de Santiago de Compostela, 15782 Santiago de Compostela, Spain
w Electronic supplementary information (ESI) available: Synthesis
and experimental data for H2L and complexes [Cu2(L)2], [Ni2(L)2].
CIF files, ORTEP diagrams, selected bond distances and angles for 1
and 2. Ancillary figures relative to the magnetic studies. CCDC 767303
and 767304. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c0cc00338g
Chart 1 Ligand H2L.
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Fig. 1 Ball and stick representation of [CuII2(L)2]ꢀ2H2O (1). Solvated
water molecules and hydrogen atoms are not depicted for clarity.
Fig. 3 Ball and stick representation of the helical complex [NiII2(L)2]
(2). Hydrogen atoms are not shown for clarity.
by the terminal methoxybenzyl groups of the two organic
strands, which are disposed differently around the metal ions
in order to avoid destabilizing steric hindrance.
efficiency in the superexchange pathway. This rather unusual
arrangement has been reported previously.10
The Plot of wT vs. T for 1 is shown in Fig. 2 and this clearly
shows sign of a weak intramolecular antiferromagnetic coupling,
with a constant wT value of 0.82 emu K molꢂ1 (in agreement
with two uncoupled S = 1/2 and g > 2.00) between 200 and
40 K with a decrease at lower temperature. The maximum in w
vs. T at 3 K, as well as the inflection point in the field-dependent
magnetization plot (Fig. S4 and S5)w at around 4 T, suggest a
singlet–triplet separation of 3–4 cmꢂ1. This was confirmed by a
fit using the Bleaney–Bowers equation, with best fit values of
g = 2.07 ꢃ 0.03, J = 3.5 ꢃ 0.1 cmꢂ1 (H = JS1S2, R2 = 0.997).
The weakness of this interaction is not surprising, in view of
the large distance between the CuII centres and the expected
ineffectiveness of possible superexchange pathways through the
non-bonding nitrogen, which would involve pairs of dx2 ꢂ y2
and dz2 orbitals on the two centres.
The NiII complex of H2L was obtained electrochemically.
Recrystallization of the mother liquors yielded brown crystals
of [NiII2(L)2] (2), which were analyzed by X-ray diffractionz as
well as by ESI mass spectrometry, elemental analysis and IR
spectroscopy. The structure reveals the formation of a [6 + 6]
double-stranded dinuclear helicate (Fig. 3 and S3).w A racemic
mixture of both enantiomers is observed in the crystal cell.
The coordination mode of the ligand in 2 differs from that in
1 in terms of the role played by the pyridine atoms on
coordination. Thus, each ligand uses one imine nitrogen atom
and one carbonyl oxygen atom to bind one metal centre in 2.
A rotation around the C–C bond adjacent to the pyridine ring
allows the pyridine nitrogen atom to act as a bridge between
the two nickel centres. The values found for the NPy–Ni bond
distances (2.264–2.360 A) in 2 are in agreement with those
reported in the literature.9–10 A further twist around the
adjacent C–C bond leads to chelation of the remaining imine
nitrogen and carbonyl oxygen atoms to the second nickel
centre. This coordination mode gives rise to a distorted
octahedral environment [N4O2] around each nickel atom.
The structural parameters referred to both nickel kernels also
indicate an asymmetrical arrangement of the helicands. The
metal–metal intramolecular distance is 3.00(2) A, which is
markedly shorter than that found in 1. Such diminution in
the intermetallic distance is the result of the establishment of
the pyridine bridges, a situation that produces some distortion
Having established the ability of the pentadentate ligand
H2L to form double-stranded [5 + 5] dinuclear helicates with
CuII ions and having also studied the magnetic properties, we
decided to explore the possibility of increasing the strength of
the intramolecular magnetic interaction between adjacent
metal centres in other complexes of this helicand. We reasoned
that this could be achieved by replacing the CuII ions by other
metal ions like NiII, which has a higher preference for an
octahedral geometry.9–10 It is expected that the only way for
H2L to satisfy the coordination preferences of the NiII ions
would be to include the pyridine donor atom as a bridge
between the metal centres, thus leading to an increased
of the central rhombus Ni1–NPy–Ni2–NPy
.
The reduced distance between the metal centres and the
existence of a new electronic pathway between them through
the pyridine bridges could lead to an enhancement of the
intramolecular metal–metal superexchange coupling in the
helicate (Fig. 4). As shown by the wT vs. T plot (Fig. 2) for
2, the exchange coupling interaction is significantly increased
relative to 1. Indeed, the wT value has not reached a plateau
even at 300 K, indicating that the two nickel ions are still not
fully uncoupled. The relative strength of the antiferromagnetic
interaction is further confirmed by the fact that wT is virtually
zero below 20 K, indicating a completely populated singlet
state at this temperature. Accordingly, a quantitative analysis
Fig. 2 Plot of wT vs. T for 1 (squares, left axis) and 2 (triangles, right
axis) along with corresponding best fit curves (parameters reported in
the text).
of the magnetic data gave as best fit values J = 65.8 ꢃ 0.7 cm ꢂ1
,
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R1 (obs data) = 0.0823, wR2 (all data) = 0.2282, GOF = 1.099,
max/min residual density 1.155/ꢂ1.362 e Aꢂ3. CCDC 767303.
Crystal data for [Ni2(L)2] (2): (C54H54Ni2N10O12), Mw = 1152.49,
crystal dimensions: 0.27
0.14 mm3, triclinic, P1,
ꢀ
ꢄ
0.21
ꢄ
a = 14.798(3), b = 14.821(3), c = 15.663(3) A, a = 106.501(4),
b = 115.704(3), g = 101.319(4)1, V = 2757.4(10) A3, Z = 2, m =
0.75 mmꢂ1, F(000) = 1200. Radiation l(Mo-Ka) = 0.71073 A,
T = 100(2) K, reflections collected/unique 31621/11272 (Rint
=
0.0558), R1 (obs data)
= 0.0428, wR2 (all data) = 0.1041,
GOF = 1.031, max/min residual density 0.719/–0.409 e Aꢂ3. CCDC
767304.
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Fig. 4 Schematic representation of the approach described herein.
The ability of the central pyridine moiety of the helicand to act either
as a m1 or m2 donor based on the coordination preferences of the metal
centres is the factor that makes the modulation of the strength of the
superexchange coupling interaction in the metallo-supramolecular
helical assembly possible.
4 M. Va
and M. R. Bermejo, New J. Chem., 2008, 32, 1473.
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´ ´ ´ ´
g = 2.30 ꢃ 0.02 and the inclusion of a paramagnetic impurity
r = 5% (R2 = 0.999).
´
´
In conclusion, we have shown that the appropriate choice of
metal centres with different preferential coordination geo-
metries allows the modulation of the strength of the magnetic
exchange in a double-stranded dinuclear helical assembly. To
the best of our knowledge, this is the first example of a metallo-
supramolecular helical entity with tunable magnetic properties.
We believe that this approach could open new perspectives for
programming magnetic devices and materials based on helicates
as we consider that it can be expanded to longer organic strands
with more than two binding compartments.
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´
lez-Noya, R. Pedrido,
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´
´
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pez, G. Zaragoza, M. Otero, R. Pedrido,
This work was financially supported by the Xunta de Galicia
(PGIDIT06PXIB20901PR
Ministerio de Educacion y Ciencia and ERDF (EU) (CTQ2007-
62185/BQU). R. Pedrido thanks the Xunta de Galicia for an
‘‘Isidro Parga Pondal’’ contract. M. Vazquez thanks the Spanish
Ministry for Science and Innovation (MICINN) for a ‘‘Ramon y
and
INCITE09E2R209074ES),
´
´
´
´
M. R. Bermejo, Dalton Trans., 2010, 39, 1191.
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Notes and references
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´
z Crystal data for [Cu2(L)2]ꢀ2H2O (1): (C54H58Cu2N10O14), Mw =
3
ꢀ
1198.18, crystal dimensions: 0.75 ꢄ 0.38 ꢄ 0.32 mm , triclinic, P1,
a = 13.648(3), b = 14.042(3), c = 16.315(4) A, a = 106.876(4),
b = 111.615(4), g = 97.038(4)1, V = 2687.5(11) A3, Z = 2, m =
0.868 mmꢂ1, F(000) = 1244.0. Radiation l(Mo-Ka) = 0.71073 A,
T = 293(2) K, reflections collected/unique 19142/9217 (Rint = 0.0875),
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This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 4797–4799 | 4799