Bauer, Wolfgang

Schlamp, Stephan

Weber, Birgit

English

EPub Bayreuth

10222 Chem. Commun., 2012, 48, 10222–10224 This journal is c The Royal Society of Chemistry 2012Cite this: Chem. Commun., 2012, 48, 10222–10224A ladder type iron(II) coordination polymer with cooperative spintransitionwWolfgang Bauer, Stephan Schlamp and Birgit Weber*Received 17th July 2012, Accepted 26th August 2012DOI: 10.1039/c2cc35109aLadder type 1D coordination polymers were synthesised with theaim to improve the spin crossover properties of the iron(II)complexes following the concepts of crystal engineering. A widehysteresis loop (34 K) was observed if rigid linkers were used.The first X-ray structure for a 1D iron(II) ladder is reported.Spin crossover (SCO) complexes are a fascinating class ofswitchable molecules that continue to attract the interest ofseveral research groups all over the world.1–3 Reason for thecontinuing interest in this substance class is the possibility tocombine a sensing device (T and/or p, magnetic field,4 adsorp-tion or desorption of guest molecules5), memory and display(colour change) in one compound. This requires bistability (ormemory effect) of the material during the spin transition.6,7Over the last few years several new concepts were proposed thatshow that intermolecular interactions in the crystal lattice arenecessary to achieve cooperative effects.2,3,6,8,9 Thus one of the keyquestions is how such intermolecular contacts can be controlled.The ideas of crystal engineering can be easily associated withthe aims of SCO research, as they have a great deal in commonconcerning fundamental concepts and strategies. Desiraju definedcrystal engineering as ‘‘the understanding of intermolecularinteractions in the context of crystal packing and in theutilisation of such understanding in the design of new solidswith desired physical and chemical properties’’.10 This includeschemical reactivity as well as optical, magnetic or electronicproperties.11 Purposeful ligand design can be used to increase theintermolecular interactions in the crystal packing,8,9 but the possi-bility of crystal engineering of SCO compounds is questioned.12Especially the effect of solvent inclusion or polymorphism isdifficult to predict.12 With the following example we will demon-strate that despite this the concepts of crystal engineering can beapplied successfully to the design of SCO compounds.In our group we started using mononuclear iron(II) complexeswith Schiff-base like ligands (L1).13 Typically we observedgradual or abrupt spin transitions (ST), sometimes with smallhysteresis loops.2 The complexes [FeL1(py)2] (1) and [FeL1-(1-meim)2] (2) given on top of Scheme 1 are examples of acooperative ST (2 K hysteresis). The cooperative interactionsare in both cases due to the 2D network of short van der Waals(vdW) contacts.14 We did use two different strategies toincrease the number of short contacts in the crystal packingand by this the hysteresis width. One is the use of rigidbridging axial ligands such as 4,40-bipyridine (bipy) leadingto the 1D coordination polymer [FeL1(bipy)] (3).15 Thecombination of the rigid axial ligand and several short vdWcontacts increases the hysteresis width to about 20 K15 (bottom,left of Scheme 1). The other possibility is the replacement ofL1 by the bitopic ligand L2.13 Due to the increase in symmetrythe dimension of the network of short vdW contacts increases(2D- 3D) leading to a wider hysteresis loop for the dinuclearcomplex [Fe2L2(1-meim)4] (4, 20 K).16Keeping the concepts of crystal engineering in mind, thequestion arises if it is possible to increase the hysteresis loopwidth further by a combination of the two above-mentionedapproaches. Consequently we started with the synthesis of 1Dchain iron(II) complexes of the bitopic ligand L2 with the aimto achieve double strand or ladder structures where theadvantages of the two different approaches are combined.The synthesis of a double strand coordination polymer isnot as straight forward as for the 1D chain compounds as awide variety of different side products are thinkable (Scheme 2).Scheme 1 General strategy for the optimisation of the SCO propertiesin a crystal engineering like approach.Inorganic Chemistry II, Universita¨t Bayreuth, Universita¨tsstrabe 30,NW 1, 95440 Bayreuth, Germany. E-mail: weber@uni-bayreuth.de;Fax: +49-92155-2157; Tel: +49-92155-2555w Electronic supplementary information (ESI) available: Experimentaldetails and spectroscopic analysis. CCDC 873866. For ESI andcrystallographic data in CIF or other electronic format see DOI:10.1039/c2cc35109aChemComm Dynamic Article Linkswww.rsc.org/chemcomm COMMUNICATIONPublished on 28 August 2012. Downloaded by UNIVERSITAT BAYREUTH on 8/28/2020 10:58:16 AM. View Article Online / Journal Homepage / Table of Contents for this issueThis journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 10222–10224 10223Next to various types of incomplete double strands the formationof a 2D network is possible. In the presence of additionalmonodentate ligands (e.g. solvent) the formation of 1D zigzagchains with alternating ligands is also possible. This was recentlyreported byKaizaki et al. for an iron(II) complex with a steep one-step ST.17 The situation is complicated because both, product andside products, have a very low solubility. As the SCO propertiesof the product will be strongly influenced by such variations it wasimportant to determine the optimal reaction conditions.This task relies strongly on the concepts of supramolecularchemistry, where the desired product is obtained by spontaneousself-assembly.18 Kinetically labile systems and long reactiontimes are necessary to allow the system to reach its energeticminimum. The first question to be answered is whether thedouble strand or the 2D layer is the thermodynamically stablefinal product. As the composition of the two products isidentical (in contrast to the side products that can be easilydistinguished this way), great efforts were made to obtaincrystals of sufficient quality for X-ray analysis. We succeededin the synthesis and crystallisation of the ladder compound[Fe2L2*(bpea)2]xMeOH (5) (bpea= 1,2-bis(4-pyridyl)ethane)13by applying a slow diffusion technique, thus ensuring longreaction times. The crystal data are summarized in Table S1(ESIw). The quality of the orthorhombic crystals was inferior sowe can only speak of a structural motif, nonetheless this motifdescribes the first structural example of an iron(II) laddercoordination polymer, to the best of our knowledge. In Fig. 1the asymmetric unit of the complex is displayed. All iron centresare crystallographically equal and clearly in the HS state withbond lengths within the inner coordination sphere of 2.100(9)/2.105(9) A˚ (Fe–N1/2), 2.026(8)/2.034(8) A˚ (Fe–O1/2) and2.186(5)/2.265(8)/2.279(8) A˚ (Fe–N3/4a/4b) and the O–Fe–Oangle of 112.2(2)1.2 The full bitopic ligand is obtained afterapplying the twofold crystallographic axis that runs throughthe atoms C9 and C10. One of the two pyridyl rings of theaxial bpea ligand is disordered. As can be seen from Fig. 2,infinite one-dimensional ladder-like chains are formed, withthe base vector [1 0 0]. Within the ladders, the backbones ofthe binuclear ligands (the ‘‘rungs of the ladders’’) are notperpendicular to the axial ligands but include an angle of 751.The distance between the two iron centres within the bitopicligands is 8.2 A˚, not much shorter than the distance betweenthe iron centres of two neighbouring ladders (8.5 A˚). Thus theformation of a 2D network seems to be thinkable. However, inthis case the equatorial ligands cannot be arranged offset asobserved in molecular packing at the top of Fig. S2 (ESIw), butthey have to align in one line. Such an arrangement with aniron–iron distance of 8.2 A˚ is not possible due to steric demandof the methyl groups at the outer periphery of the ligand. Theladder structure is the preferred structural motif for this type ofcomplexes. The molecular packing displayed in Fig. S2 (ESIw)reveals that the chains form parallel layers in the a–b-plane aswell as in the a–c-plane and are staggered in the b–c-plane.With the knowledge that the synthesis of ladder structures ispossible and this is the preferred structural motif the secondquestion is whether the hysteresis width can be increased furtherwith such a system. The single crystals were not obtained inenough quantity for a magnetic characterisation. For the pre-paration of bulk material in enough quantity, high reactiontemperatures and long reaction times are necessary. Frommethanol under reflux conditions a material with the compositionScheme 2 Potential products and side products for the reaction of adinuclear iron complex with a bridging axial ligand.Fig. 1 ORTEP drawing of the asymmetric unit of compound 5(anisotropic displacement ellipsoids drawn at the 50% probabilitylevel). Included solvent and hydrogen atoms omitted for clarity.Fig. 2 Excerpt of the infinite 1D ladder structure of compound 5.Solvent molecules and hydrogen atoms omitted for clarity.Published on 28 August 2012. Downloaded by UNIVERSITAT BAYREUTH on 8/28/2020 10:58:16 AM. View Article Online10224 Chem. Commun., 2012, 48, 10222–10224 This journal is c The Royal Society of Chemistry 2012[Fe2L2*(bpea)3]MeOH is obtained showing a stepwise incompletespin transition (Fig. S1, ESIw). This is not unexpected as thenumber of bpea molecules per metal centre clearly indicates theformation of side products (or a mixture of the desired productand side products) and several inequivalent iron centres arethinkable. Toluene as solvent in combination with longer reactiontimes proved to be more suited and the corresponding complexeswith 4,40bipyridine (bipy), bispyridylethane (bpea), bispyridyl-ethene (bpee) and bispyridylpropane (bppa) were synthesised.SQUID measurements on those samples reveal that the complexwith bipy is a pure HS complex over the entire temperature range.The other three complexes show SCO behaviour. However, for theflexible ligands bppa and bpea the ST is gradual and incomplete –this is not unexpected as rigid bridging ligands are needed for aneffective communication of cooperative effects within the polymerchain.19 For the 1D chain compounds of L1 with the flexibleligands incomplete spin transitions that stop at gHS = 0.5 wereobserved frequently.19 For the rigid ligand bpee we obtained thecomplex 6 with the composition [Fe2L2*(bpee)2]1.5 Tol. Theresults of the magnetic measurements are displayed in Fig. 3.The room temperature wMT value is 5.9 cm3 K mol1 in therange expected for binuclear iron(II) complexes with both ironcentres almost in the high-spin (HS) state. The wMT valuedecreases gradually between 300 and 235 K, followed by anabrupt transition to a minimum value of 0.85 cm3 K mol1 at185 K. Below 185 K the wMT value remains approximatelyconstant down to 50 K. The T12, down of this SCO is 205 K. Uponheating, the wMT value is identical to the cooling mode between 50and 185 K. Above 185 K, the wMT value increases first graduallythen rapidly then again gradually to attain the maximum value of5.90 cm3 K mol1 at 300 K. The T12, up is 239 K, resulting in a34 K wide hysteresis loop that can be repeated several times. Inorder to evaluate the structural similarities of the crystallinesample 5 and the powder 6, the X-ray powder patterns of 6 wereinvestigated at room temperature (HS) and 173 K (LS) andcompared with the calculated X-ray powder pattern of 5. Thecorresponding data are given in Fig. S3 (ESIw) in the 5–45 2Yrange. One has to keep in mind that different linkers were used forthe synthesis of the polymers (bpea vs. bpee) and different solventsare included in the crystal lattice (methanol vs. toluene). Despitethis several similarities are observed in the powder pattern of theHS and LS species 5 and 6 (Fig. S3, ESIw), so that the samestructural motif can be assumed. Upon spin transition the powderpattern of 6 changes and a different set of reflexes is observed forthe LS and the HS state (Fig. S3, ESIw). In agreement with resultsfrom SQUIDmeasurements, the spin transition is not complete at173 K and the HS peaks are not fully vanished.We succeeded with the synthesis and crystallisation of thefirst example of an iron(II) 1D coordination polymer with ladderstructure. The search for this structural motif was initiated byapplying the concepts of crystal engineering to the design of spincrossover materials with wide hysteresis loops. The successfulsynthesis of a powder sample with a 34 K wide hysteresis loopdemonstrates that this approach is worthwhile to be consideredfurther for the design of spin crossover materials.We thank S. Albrecht and P. Mayer (University of Munich)for the collection of the single crystal X-ray data andW. Milius (UBT) for the collection of the powder diffractiondata. Support from the University of Bayreuth, the DeutscheForschungsgemeinschaft (WE 3546_4-1 and SFB 840/A10) andthe Fonds der Chemischen Industrie is gratefully acknowledged.Notes and references1 (a) P. Gu¨tlich, A. Hauser and H. Spiering, Angew. Chem., Int. Ed.Engl., 1994, 33, 2024–2054; (b) Spin Crossover in Transition MetalCompounds I–III, eds. P. Gu¨tlich and H. Goodwin, Springer Berlin/Heidelberg, 2004, pp. 233–235; (c) J.-F. Le´tard, J. Mater. Chem.,2006, 16, 2550–2559; (d) O. Sato, J. Tao and Y.-Z. Zhang, Angew.Chem., Int. Ed., 2007, 46, 2152–2187; (e) J. A. Kitchen andS. Brooker, Coord. Chem. Rev., 2008, 252, 2072–2092;(f) M. A. Halcrow, Coord. Chem. Rev., 2009, 253, 2493–2514;(g) K. S. Murray, Aust. J. Chem., 2009, 62, 1081; (h) A. Gaspar,M. Seredyuk and P. Gu¨tlich, J. Mol. Struct., 2009, 924–926, 9–19;(i) S. Brooker and J. A. Kitchen, Dalton Trans., 2009, 7331–7340;(j) A. B. Koudriavtsev and W. Linert, J. Struct. Chem., 2010, 51,335–365; (k) H. Naggert, A. Bannwarth, S. Chemnitz, T. von Hofe,E. Quandt and F. Tuczek, Dalton Trans., 2011, 6364; (l) C. Gandolfi,C. Moitzi, P. Schurtenberger, G. G.Morgan andM. Albrecht, J. Am.Chem. Soc., 2008, 130, 14434–14435; (m) A. Bousseksou, G. Molnar,L. Salmon and W. Nicolazzi, Chem. Soc. Rev., 2011, 40, 3313–3335.2 B. Weber, Coord. Chem. Rev., 2009, 253, 2432–2449.3 M. A. Halcrow, Chem. Soc. Rev., 2011, 40, 4119.4 A. Bousseksou, K. Boukheddaden, M. Goiran, C. Consejo,M.-L. Boillot and J.-P. Tuchagues, Phys. Rev. B, 2002, 65, 172412.5 G. J. Halder, Science, 2002, 298, 1762–1765.6 O. Kahn, Science, 1998, 279, 44–48.7 (a) O. Kahn, C. Jay, J. Krober, R. Claude and F. Groliere,EP, 0666561, 1995; (b) A. D. Naik, L. Stappers, J. Snauwaert,J. Fransaer and Y. Garcia, Small, 2010, 6, 2842–2846;(c) Y. Garcia, V. Ksenofontov, S. Mentior, M. M. Dıˆrtu,C. Gieck, A. Bhatthacharjee and P. Gu¨tlich, Chem.–Eur. J.,2008, 14, 3745–3758.8 J.-F. Le´tard, P. Guionneau, E. Codjovi, O. Lavastre, G. Bravic,D. Chasseau and O. Kahn, J. Am. Chem. Soc., 1997, 119, 10861–10862.9 J. A. Real, A. B. Gaspar, V. Niel and M. C. Mun˜oz, Coord. Chem.Rev., 2003, 236, 121–141.10 G. Desiraju, Crystal engineering. The design of organic solids,Elsevier, Amsterdam, 1989.11 G. Desiraju, J. Chem. Sci., 2010, 122, 667–675.12 M. Hostettler, K. W. To¨rnroos, D. Chernyshov, B. Vangdal andH.-B. Bu¨rgi, Angew. Chem., Int. Ed., 2004, 43, 4589–4594.13 L1 = [([3,30]-[1,2-phenylenebis(iminomethylidyne)]bis(2,4-pentane-dionato)(2-)]; L2 = [3,30, 30 0, 30 0 0]-[1,2,4,5-phenylenetetra(imino-methylidyne)]tetra(2,4-pentanedionato)(4-)]; L2* = Tetraethyl-[2,20,20 0,20 0 0]-[1,2,4,5-phenylenetetra(iminomethylidyne)]tetra(3-oxo-butanato)(4-)].14 B. Weber, E. Kaps, J. Weigand, C. Carbonera, J.-F. Le´tard,K. Achterhold and F. G. Parak, Inorg. Chem., 2008, 47, 487–496.15 B. Weber, R. Tandon and D. Himsl, Z. Anorg. Allg. Chem., 2007,633, 1159–1162.16 B. Weber, E. S. Kaps, J. Obel, K. Achterhold and F. G. Parak,Inorg. Chem., 2008, 47, 10779–10787.17 K. Yoneda, K. Adachi, S. Hayami, Y. Maeda, M. Katada,A. Fuyuhiro, S. Kawata and S. Kaizaki,Chem. Commun., 2006, 45–47.18 J.-M. Lehn, Angew. Chem., Int. Ed. Engl., 1988, 27, 89–112.19 W. Bauer, W. Scherer, S. Altmannshofer and B. Weber, Eur. J.Inorg. Chem., 2011, 2803–2818.Fig. 3 Plot of the wMT product versus T for the compound 6.Published on 28 August 2012. Downloaded by UNIVERSITAT BAYREUTH on 8/28/2020 10:58:16 AM. View Article Online

A ladder type iron(II) coordination polymer with cooperative spin transition

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A ladder type iron(II) coordination polymer with cooperative spin transition

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