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

Albrecht, Martin

Gandolfi, Claudio

Moitzi, Christian

Morgan, Grace G.

Schurtenberger, Peter

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Improved Cooperativity of Spin-Labile Iron(III) Centers by Self-Assembly inSolutionClaudio Gandolﬁ,† Christian Moitzi,‡ Peter Schurtenberger,‡ Grace G. Morgan,§ andMartin 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, IrelandWe report on the markedly increased spin crossover activity ofmaterials when self-assembled in solution. Spin crossover is directlyrelated to long-range elastic interactions, typically observed in thesolid state, and the transition may be gradual over a largetemperature range.1 Effects such as solvent cocrystallization oftenmodify the transition in an unpredictable way. This is predominantlybecause spin transitions are crucially affected by intermolecularinteractions beyond crystal packing effects, such as elastic strain.2Such intermolecular interactions are the classical domain to beengineered by supramolecular chemistry.3 Speciﬁcally, the promo-tion of close metal-metal interactions may stimulate cooperativebehavior, which is expected to result in more abrupt spin transitions.This concept has been implemented for engineering of spincrossover active materials in the solid state4 and recently also insoluble nanoparticles.5 Here, we have used a hydrophilic/lipophilicbias6 for inducing a favorable arrangement of potentially spin-labileiron(III) centers. Supramolecular organization in solution leads, mostremarkably, to a new type of magnetically active material for liquid-phase processing.The Fe(sal2trien) complex 2a is known to undergo a gradual spintransition in the solid state.7 Ligand functionalization and introduc-tion of a lipophilic side chain was accomplished by selectivealkylation of 4-hydroxysalicylaldehyde. Condensation of the alde-hyde 1 with triethylenetetramine and subsequent metalation withiron(III) nitrate, either in situ or after isolation of the hexadentateligand precursor, afforded the alkyl-tailed complexes 2b-d (Scheme1). Metathesis of the noncoordinating anion was straightforwardbecause 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 bondlengths around the two crystallographically independent iron centersare diagnostic for high spin FeIII at 173 K.1c The mutual ciscoordination of the phenolate moieties arranges the alkyl chains inthe same direction and maximizes intermolecular van der Waalsinteractions. As a consequence, the crystal packing displays apolarand polar layers consisting of the lipophilic side chains, and thecharged metal coordination site and the PF6- anions, respectively(Figure 1b).Despite the layered crystal packing, complex 3c does not exhibitthermochromic behavior in the solid state. Magnetic measurementsindicate the absence of thermally induced spin crossover in the30-300 K range and antiferromagnetism or zero-ﬁeld splittingbelow 30 K.1c Similar results have been obtained for complexes2b-d. This suggests that alkyl functionalization in 2 generates spin-stable FeIII centers. The spin stability of 2d in the solid state mayalso 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 2dundergoes spin crossover in solution. The spin transition is indicatedby thermochromic behavior with a color change from red at roomtemperature to dark blue when cooled below 230 K (CH2Cl2). Asimilar 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 2din the LS state at λmax ) 650 nm (inset Figure 2) reﬂects anenergetically easier metal-to-ligand charge transfer, presumably dueto a contraction of the M-L bonds upon spin crossover.Temperature-dependent monitoring of the extinction at 650 nmindicated a relatively abrupt transition around 230-240 K for 2d(Figure 2). Superimposition of the UV-vis spectra at differenttemperatures revealed an isosbestic point at λ ) 558 nm ( ) 3700M-1 cm-1), thus providing evidence for an equilibrium between theHS and the LS states. Hence, the two spin states of 2d are distinctly† Department of Chemistry, University of Fribourg.‡ Adolphe Merkle Institute, University of Fribourg.§ University College Dublin.Scheme 1. Synthesis of Functionalized Fe(sal2trien) ComplexesFigure 1. (a) ORTEP plot (50% probability) and pertinent bond lengthsof one of the two crystallographically independent cations of 3c at 173 K(hydrogen atoms omitted for clarity); (b) packing diagram of crystals of 3cillustrating the polar and apolar layers of 3c in the solid state (Fe, purple;N, blue; O, red; PF6, green).Published in "Journal of the American Chemical Society 130(44): 14434–14435, 2008" which should be cited to refer to this work.http://doc.rero.ch1separated on the UV-vis time scale. No signiﬁcant hysteresis has beenobserved upon repetitive heating-cooling cycles. Notably, the parentFe(sal2trien) complex 2a, displays a gradual spin transition in solutionover a 100 K temperature range. Similarly, no thermochromism wasobserved until 180 K for complexes 2b or 2c comprising shorter alkyltails, suggesting that the alkyl chain length is crucial for promoting anabrupt spin transition.Magnetic measurements conﬁrmed the correlation between thecolor change and a spin crossover event. While the HS FeIII signalcould not be detected by EPR, the increasing isotropic resonanceat g ) 2.16 (CH2Cl2) upon cooling is diagnostic for the populationof a LS FeIII species.8 The high-spin nature of complex 2d at roomtemperature was evidenced by determining the magnetic suscep-tibility using Evans’ method.9 At 298 K, μeff ) 5.47 ( 0.16 μB asexpected for an S ) 5/2 conﬁguration of spin crossover FeIII.Unfortunately, the limited solubility of 2d in CH2Cl2 precluded 1HNMR measurements at low temperature, and saturated solutionsof 2d at 230 K were too dilute to induce an accurately deﬁnedparamagnetic shift of the solvent signal.10The abrupt spin transition of 2d within 10 K suggests stronglycooperative behavior of the metal centers, which may be inducedfor example by self-organization of the molecules into supramo-lecular assemblies. Indeed, DLS measurements of solutions of 2drevealed the presence of particles with hydrodynamic radius RH ≈550 nm and a large polydispersity. Analyses by cryogenic scanningelectron microscopy (SEM) showed two major types of sphericalparticles, characterized by an average diameter of ca. 1 and 3 μm,respectively (Figure 3). While these results corroborate the DLSmeasurements, the SEM plots further suggest the particles to besuperstructures composed of rodlike assemblies with a diameter ofabout 30 nm (inset Figure 3). High intensity SAXS experimentsusing synchroton irradiation of 2d are consistent with elongatedobjects featuring a cross-section diameter of 42 nm. Supercoilssimilar to DNA superstructures rather than simple micelle-typeaggregates 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 2band 2c comprising shorter alkyl tails. This suggests that the abruptspin crossover, exclusively observed with 2d, is correlated withthe supramolecular assembly of this compound in solution to formdiscrete aggregates. Such a structural model may rationalize theintermolecular cooperativity required for the observed abrupt spintransition in solution.In conclusion, our results demonstrate that supramolecular principlescan be successfully applied to engineer metal-centered function, asillustrated by the enhanced cooperativity and higher transition tem-perature induced in an originally poorly cooperating spin crossoveriron(III) complex by self-assembly. Control and improvement of suchfunction in solution is achieved through careful chemical modulation.We are currently investigating the generality of this approach forfabricating magnetically active materials for solution processing, ideallyoperating near room temperature.Acknowledgment. We thank Dr. M. M. Dadras (CSEMNeuchatel), Mr. V. K. Malik (Univ Fribourg), Dr. A. Menzel (PSIVillingen) for analytical measurements, Prof. H. Stoeckli-Evans(BENEFRI Crystallography Service, Univ Neuchaˆtel) for the crystalstructure 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.Supporting Information Available: Synthetic details and analyticaldata of complexes 2b-2d, CIF for 3c. This material is available freeof charge via the Internet at http://pubs.acs.org.References(1) (a) Kahn, O.; Martinez, C. J. Science 1998, 279, 44. (b) Gu¨tlich, P.; Garcia,Y.; Spiering, H. In Magnetism: Molecules to Materials IV; Miller, J. S.,Drillon, M., Eds.; Wiley-VCH: Weinheim, Germany, 2002; p 271. (c) vanKoningsbruggen, P. J.; Maeda, Y.; Oshio, H. Top. Curr. Chem. 2004, 233,259. (d) Spiering, H. Top. Curr. Chem. 2004, 235, 171. (e) Letard, J.-F.;Guionneau, P.; Goux-Capes, L. Top. Curr. Chem. 2004, 235, 221.(2) Hostettler, M.; To¨rnroos, K. W.; Chernyshov, D.; Vangdal, B.; Bu¨rgi, H.-B. Angew. Chem., Int. Ed. 2004, 43, 4589.(3) Lehn, J.-M. Supramolecular Chemistry; Wiley-VCH: Weinheim, Germany,1995.(4) (a) Real, J. A.; Andres, E.; Munoz, M. C.; Julve, M.; Granier, T.;Bousseksou, A.; Varret, F. Science 1995, 268, 265. (b) Roubeau, O.;Agricole, B.; Clerac, R.; Ravaine, S. J. Phys. Chem. B. 2004, 108, 15110.(c) Cobo, S.; Molnar, G.; Real, J. A.; Bousseksou, A. Angew. Chem., Int.Ed. 2006, 45, 5786. (d) Seredyuk, M.; Gaspar, A. B.; Ksenofontov, V.;Galyametdinov, Y.; Kusz, J.; Gu¨tlich, P. J. Am. Chem. Soc. 2008, 130,1431. (e) Cobo, S.; Ostrovskii, D.; Bonhommeau, S.; Vendier, L.; Molnar,G.; Salmon, L.; Tanaka, K.; Bousseksou, A. J. Am. Chem. Soc. 2008, 130,9019.(5) Coronado, E.; Galan-Mascaros, J. R.; Monrabal-Capilla, M.; Garcia-Martinez, J.; Pardo-Ibanez, P. AdV. Mater. 2007, 19, 1359.(6) (a) Bodenthin, Y.; Pietsch, U.; Mohwald, H.; Kurth, D. G. J. Am. Chem.Soc. 2005, 127, 3110. (b) Zhang, W.; Zhao, F.; Liu, T.; Yuan, M.; Wang,Z.-M.; Gao, S. Inorg. Chem. 2007, 46, 2541.(7) (a) Tweedle, M. F.; Wilson, L. J. J. Am. Chem. Soc. 1976, 98, 4824. (b)Sinn, E.; Sim, G.; Dose, E. V.; Tweedle, M. F.; Wilson, L. J. J. Am. Chem.Soc. 1978, 100, 3375.(8) Solomon, E. I., Lever, A. B. P., Eds. Inorganic Electronic Structure andSpectroscopy; Wiley-Interscience: New York, 1999; Vol. 2.(9) Evans, D. F. J. Chem. Soc. 1959, 2003.(10) Complex 2c, which is more soluble, undergoes a gradual spin transitionover a broad temperature range, from μeff ) 5.44 μB at room temp to thelow spin state (μeff ) 2.47 μB) at 180 K. A plot of μeff vs T correlates withthe increase of the absorbance at 650 nm and indicates that the spectroscopicchanges indeed reﬂect a spin transition.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-assembled2d; two types of spherical particles are distinguishable featuring diametersof ca. 1 and 3 μm, respectively. Inset: nanosize rods as subcomponents ofspherical microparticles.http://doc.rero.ch2

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

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