Improved cooperativity of spin-labile iron(III) centers by self-assembly in solution

Article abstract of DOI:10.1021/ja806611y

Supramolecular principles have been applied for improving the spin crossover activity of metal centers due to cooperative effects in solution. Thus, incorporation of alkyloxy tails at the phenyl group of Fe(sal₂trien) 2a provides amphiphilic complexes Fe(sal-OR₂trien) 2b-d (b, R = C₆H₁₃; c, R = C₈H₁₇; d, R = C₁₈H₃₇) comprising an apolar group for supramolecular organization and a polar headgroup with potential spin crossover activity due to the presence of a spin-labile iron(III) center. Self-assembly of these complexes in solution resulted in the formation of microsize and submicrosize particles when the alkyl chain was long enough (2d) but not with shorter chains (2a-c). Solutions of 2d showed enhanced spin crossover activity as compared to complexes 2a-c, both in terms of transition temperature and steepness of the transition. This observation has been correlated to an improved cooperativity of the metal centers in 2d due to self-assembly, thus facilitating a tandem spin transition. Copyright

Full text of DOI:10.1021/ja806611y

Published on Web 10/14/2008
Improved Cooperativity of Spin-Labile Iron(III) Centers by Self-Assembly in
Solution
Claudio Gandolfi,† Christian Moitzi,‡ Peter Schurtenberger,‡ Grace G. Morgan,§ and
Martin Albrecht*,†
Department of Chemistry and Adolphe Merkle Institute, UniVersity of Fribourg, CH-1700 Fribourg, Switzerland,
and UCD School of Chemistry & Chemical Biology, UniVersity College Dublin, Belfield, Dublin 4, Ireland
Received August 20, 2008; E-mail: martin.albrecht@unifr.ch
Scheme 1. Synthesis of Functionalized Fe(sal2trien) Complexes
We report on the markedly increased spin crossover activity of
materials when self-assembled in solution. Spin crossover is directly
related to long-range elastic interactions, typically observed in the
solid state, and the transition may be gradual over a large
temperature range.1 Effects such as solvent cocrystallization often
modify the transition in an unpredictable way. This is predominantly
because spin transitions are crucially affected by intermolecular
interactions beyond crystal packing effects, such as elastic strain.2
Such intermolecular interactions are the classical domain to be
engineered by supramolecular chemistry.3 Specifically, the promo-
tion of close metal-metal interactions may stimulate cooperative
behavior, which is expected to result in more abrupt spin transitions.
This concept has been implemented for engineering of spin
crossover active materials in the solid state4 and recently also in
soluble nanoparticles.5 Here, we have used a hydrophilic/lipophilic
bias6 for inducing a favorable arrangement of potentially spin-labile
iron(III) centers. Supramolecular organization in solution leads, most
remarkably, to a new type of magnetically active material for liquid-
phase processing.
also originate from the absence of favorable metal-metal interac-
tions, as powder diffraction showed this complex to be amorphous.
In contrast to the spin stability in the solid state, complex 2d
undergoes spin crossover in solution. The spin transition is indicated
by thermochromic behavior with a color change from red at room
temperature to dark blue when cooled below 230 K (CH2Cl2). A
similar color change was reported for 2a at 193 K.7 The thermo-
chromism was associated with a spin transition from high spin (HS;
S ) 5/2) to low spin (LS; S ) 1/2). The new absorption band of 2d
in the LS state at λmax ) 650 nm (inset Figure 2) reflects an
energetically easier metal-to-ligand charge transfer, presumably due
to a contraction of the M-L bonds upon spin crossover.
Temperature-dependent monitoring of the extinction at 650 nm
indicated a relatively abrupt transition around 230-240 K for 2d
(Figure 2). Superimposition of the UV-vis spectra at different
temperatures revealed an isosbestic point at λ ) 558 nm (ꢀ ) 3700
M-1 cm-1), thus providing evidence for an equilibrium between the
HS and the LS states. Hence, the two spin states of 2d are distinctly
The Fe(sal2trien) complex 2a is known to undergo a gradual spin
transition in the solid state.7 Ligand functionalization and introduc-
tion of a lipophilic side chain was accomplished by selective
alkylation of 4-hydroxysalicylaldehyde. Condensation of the alde-
hyde 1 with triethylenetetramine and subsequent metalation with
iron(III) nitrate, either in situ or after isolation of the hexadentate
ligand precursor, afforded the alkyl-tailed complexes 2b-d (Scheme
1). Metathesis of the noncoordinating anion was straightforward
because of the different solubility properties of KPF6 and the alkyl-
tailed iron(III) complexes and gave, after extraction, complexes 3.
A single crystal structure determination of complex 3c unambigu-
ously confirmed the connectivity pattern (Figure 1a). The bond
lengths around the two crystallographically independent iron centers
are diagnostic for high spin FeIII at 173 K.1c The mutual cis
coordination of the phenolate moieties arranges the alkyl chains in
the same direction and maximizes intermolecular van der Waals
interactions. As a consequence, the crystal packing displays apolar
and polar layers consisting of the lipophilic side chains, and the
-
charged metal coordination site and the PF6 anions, respectively
(Figure 1b).
Despite the layered crystal packing, complex 3c does not exhibit
thermochromic behavior in the solid state. Magnetic measurements
indicate the absence of thermally induced spin crossover in the
30-300 K range and antiferromagnetism or zero-field splitting
below 30 K.1c Similar results have been obtained for complexes
2b-d. This suggests that alkyl functionalization in 2 generates spin-
stable FeIII centers. The spin stability of 2d in the solid state may
Figure 1. (a) ORTEP plot (50% probability) and pertinent bond lengths
of one of the two crystallographically independent cations of 3c at 173 K
(hydrogen atoms omitted for clarity); (b) packing diagram of crystals of 3c
illustrating the polar and apolar layers of 3c in the solid state (Fe, purple;
N, blue; O, red; PF6, green).
† Department of Chemistry, University of Fribourg.
‡ Adolphe Merkle Institute, University of Fribourg.
§ University College Dublin.
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14434 J. AM. CHEM. SOC. 2008, 130, 14434–14435
10.1021/ja806611y CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Figure 2. Fraction of LS complex (γLS) deduced from temperature-
dependent UV-vis absorption at 650 nm for 2a (9), 2b ([), 2c (2), 2d
(b); all 0.2 mM CH2Cl2 solutions. Inset: UV-vis spectra of 2d at 220 K
(LS, bold line) and 298 K (HS).
Figure 3. Cryo-SEM images of a representative section of self-assembled
2d; two types of spherical particles are distinguishable featuring diameters
of ca. 1 and 3 µm, respectively. Inset: nanosize rods as subcomponents of
spherical microparticles.
separated on the UV-vis time scale. No significant hysteresis has been
observed upon repetitive heating-cooling cycles. Notably, the parent
Fe(sal2trien) complex 2a, displays a gradual spin transition in solution
over a 100 K temperature range. Similarly, no thermochromism was
observed until 180 K for complexes 2b or 2c comprising shorter alkyl
tails, suggesting that the alkyl chain length is crucial for promoting an
abrupt spin transition.
iron(III) complex by self-assembly. Control and improvement of such
function in solution is achieved through careful chemical modulation.
We are currently investigating the generality of this approach for
fabricating magnetically active materials for solution processing, ideally
operating near room temperature.
Magnetic measurements confirmed the correlation between the
color change and a spin crossover event. While the HS FeIII signal
could not be detected by EPR, the increasing isotropic resonance
at g ) 2.16 (CH2Cl2) upon cooling is diagnostic for the population
of a LS FeIII species.8 The high-spin nature of complex 2d at room
temperature was evidenced by determining the magnetic suscep-
Acknowledgment. We thank Dr. M. M. Dadras (CSEM
Neuchatel), Mr. V. K. Malik (Univ Fribourg), Dr. A. Menzel (PSI
Villingen) for analytical measurements, Prof. H. Stoeckli-Evans
(BENEFRI Crystallography Service, Univ Neuchaˆtel) for the crystal
structure analysis, and Prof. C. W. Schla¨pfer for fruitful discussions.
This work was financially supported by ERA-net Chemistry. M.A.
gratefully acknowledges an Alfred Werner Assistant Professorship.
tibility using Evans’ method.9 At 298 K, µeff ) 5.47 ( 0.16 µB as
5
expected for an S )
/
2
configuration of spin crossover FeIII.
Unfortunately, the limited solubility of 2d in CH2Cl2 precluded 1H
NMR measurements at low temperature, and saturated solutions
of 2d at 230 K were too dilute to induce an accurately defined
paramagnetic shift of the solvent signal.10
Supporting Information Available: Synthetic details and analytical
data of complexes 2b-2d, CIF for 3c. This material is available free
of charge via the Internet at http://pubs.acs.org.
The abrupt spin transition of 2d within 10 K suggests strongly
cooperative behavior of the metal centers, which may be induced
for example by self-organization of the molecules into supramo-
lecular assemblies. Indeed, DLS measurements of solutions of 2d
revealed the presence of particles with hydrodynamic radius RH ≈
550 nm and a large polydispersity. Analyses by cryogenic scanning
electron microscopy (SEM) showed two major types of spherical
particles, characterized by an average diameter of ca. 1 and 3 µm,
respectively (Figure 3). While these results corroborate the DLS
measurements, the SEM plots further suggest the particles to be
superstructures composed of rodlike assemblies with a diameter of
about 30 nm (inset Figure 3). High intensity SAXS experiments
using synchroton irradiation of 2d are consistent with elongated
objects featuring a cross-section diameter of 42 nm. Supercoils
similar to DNA superstructures rather than simple micelle-type
aggregates may account for the observed size of these nanorods
(calculated length of 2d in a fully stretched conformation is 30 Å).
No such distinct structures were observed with complexes 2b
and 2c comprising shorter alkyl tails. This suggests that the abrupt
spin crossover, exclusively observed with 2d, is correlated with
the supramolecular assembly of this compound in solution to form
discrete aggregates. Such a structural model may rationalize the
intermolecular cooperativity required for the observed abrupt spin
transition in solution.
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(10) Complex 2c, which is more soluble, undergoes a gradual spin transition
over a broad temperature range, from µeff ) 5.44 µB at room temp to the
low spin state (µeff ) 2.47 µB) at 180 K. A plot of µeff vs T correlates with
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In conclusion, our results demonstrate that supramolecular principles
can be successfully applied to engineer metal-centered function, as
illustrated by the enhanced cooperativity and higher transition tem-
perature induced in an originally poorly cooperating spin crossover
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