Reaction of bicine {BicH3, N,N-bis(2-hydroxyethyl)glycine} with an Fe(III) oxo-centered pivalate triangle in MeCN in the presence of Et&lt;sub&gt;2&lt;/sub&gt;NH yields [Et&lt;sub&gt;2&lt;/sub&gt;NH&lt;sub&gt;2&lt;/sub&gt;]&lt;sub&gt;2&lt;/sub&gt;[Fe&lt;sub&gt;6&lt;/sub&gt;O&lt;sub&gt;2&lt;/sub&gt;(OH)&lt;sub&gt;2&lt;/sub&gt;(Bic)&lt;sub&gt;2&lt;/sub&gt;(O&lt;sub&gt;2&lt;/sub&gt;CCMe&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;8&lt;/sub&gt;], which possesses an S = 5 ground state.



Changing the base to NaOMe produces [Fe&lt;sub&gt;12&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt;(Bic)&lt;sub&gt;4&lt;/sub&gt;(HBic)&lt;sub&gt;4&lt;/sub&gt;(O&lt;sub&gt;2&lt;/sub&gt;CCMe&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;8&lt;/sub&gt;], which contains two Fe6 units bridged by the carboxylate arms from the bicine ligands. The complex displays strong antiferromagnetic coupling leading to an S = 0 ground state

Darwish, A.

Ferguson, A.

Graham, K.

Murrie, M.

Parsons, S.

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Polyhedron xxx (2009) xxx–xxxARTICLE IN PRESSContents lists available at ScienceDirectPolyhedronjournal homepage: www.elsevier .com/locate /polySynthesis and characterisation of Fe6 and Fe12 clusters using bicineKristoffer Graham a, Amiera Darwish a, Alan Ferguson a, Simon Parsons b, Mark Murrie a,*aWestCHEM, Department of Chemistry, University of Glasgow, University Avenue, Glasgow G12 8QQ, UKb School of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, UKa r t i c l e i n f oArticle history:Received 29 October 2008Accepted 22 January 2009Available online xxxxKeywords:Iron(III)BicinePolynuclearMolecular magnetism0277-5387/$ - see front matter  2009 Elsevier Ltd. Adoi:10.1016/j.poly.2009.01.018* Corresponding author. Tel.: +44 141 330 4486; faE-mail address: M.Murrie@chem.gla.ac.uk (M. Mur1 The trivial names bis–tris and bicine are in commoligands are used as biological buffers.Please cite this article in press as: K. Graha b s t r a c tReaction of bicine {BicH3, N,N-bis(2-hydroxyethyl)glycine} with an Fe(III) oxo-centered pivalate trianglein MeCN in the presence of Et2NH yields [Et2NH2]2[Fe6O2(OH)2(Bic)2(O2CCMe3)8], which possesses anS = 5 ground state. Changing the base to NaOMe produces [Fe12O4(Bic)4(HBic)4(O2CCMe3)8], which con-tains two Fe6 units bridged by the carboxylate arms from the bicine ligands. The complex displays strongantiferromagnetic coupling leading to an S = 0 ground state. 2009 Elsevier Ltd. All rights reserved.Polymetallic transition metal complexes continue to attract in-tense interest in the ﬁeld of molecular magnetism [1,2]. Alongsidethe potential applications in high-density data storage, there areopportunities in quantum computation [3], MRI contrast agents[4], magnetic refrigeration [5], and spintronics [6]. One strand ofour research focuses upon the coordination chemistry of a seriesof structurally related ligands with the ﬁrst row transition metalions. We are investigating the substitution of functional groupson a given ligand backbone and how this affects the topology ofthe clusters that crystallise from the reaction system. This ap-proach can be illustrated by Chart 1, which shows the ligandsbis–tris,1 bicine {BicH3, N,N-bis(2-hydroxyethyl)glycine} and dietha-nolamine (DEA), which all share the {N(CH2CH2OH)2} group. Wehave shown that bis–tris can assemble small fragments of iron(III)carboxylates into Fe10 clusters [7]. Although the DEA ligand hasnot been reported as a ligand for Fe(III) complexes, replacing theNH group by an NR (R = Me, Et etc.,) group leads to some interestingpolynuclear Fe(III) complexes, including Fe6 wheels (e.g. R = Me [8],nBu [9]) and in the presence of carboxylate co-ligands Fe7 or Fe22complexes (R = Ph [10] or Me [11]). As bicine contains a carboxylategroup in addition to the {N(CH2CH2OH)2} functionality this ligandshould form new and interesting polynuclear Fe(III) clusters. How-ever, the only reported polynuclear complex with the bicine ligandis a hexa-nuclear [Fe6(Bic)6] wheel [12] where each bicine ligand istri-deprotonated. Previously, mono-deprotonated bicine has beenll rights reserved.x: +44 141 330 4888.rie).n usage, especially when theam et al., Polyhedron (2009used to prepare mononuclear copper and lanthanide complexes[13,14] and a one-dimensional polymeric manganese chain complex[15]. Herein, we report the synthesis and characterisation of twonew Fe6 and Fe12 complexes with bicine, which display novel coretopologies.The Fe(III) pivalate oxo-centered triangle starting material wasprepared according to the literature procedure [16]. BicH3 wasadded to [Fe3O(O2CCMe3)6(H2O)3][O2CCMe3]  2HO2CCMe3 inMeCN followed by Et2NH (1:1:1). The reaction was stirred at ambi-ent temperature for 24 h then ﬁltered and stored in a sealed vial,yielding orange plate-like crystals of [Et2NH2]2[Fe6O2(OH)2(Bi-c)2(O2CCMe3)8]  1.5MeCN (1  1.5MeCN) in 16% yield after 3weeks.2 Compound 1 contains two independent dianionic hexa-nu-clear Fe(III) complexes which crystallise in the triclinic space groupP1 (Fig. 1). The Fe6 complex consists of an {Fe4O2} butterﬂy unit[17] (Fe2, Fe3, Fe2a, Fe3a) capped by two {Fe(Bic)} units, which sitabove and below the central {Fe4O2} core. Oxidation states were con-ﬁrmed as Fe(III) and oxides/hydroxides assigned by consideration ofbond lengths and charge balance and using bond valence sum (BVS)analysis [18].The two bicine ligands are tri-deprotonated, binding to Fe1 (andsymmetry equivalent, s.e.) through N11 (and s.e.). The carboxylatearm is monodentate (O11), whereas the CH2CH2O arms bridgeFe1 to either wing (Fe1–O41–Fe2) or body (Fe1–O61–Fe3a) centers2 Air-dried crystals analyze as 1  2MeCN  4H2O, analysis (%) calc. (found): C, 42.59(42.38); H, 7.37(7.04); N, 3.10 (3.36), selected IR data: m = 2961, 1667, 1556, 1482,1421, 1347, 1220, 1069, 905, 881,787, 651 cm1. Crystal data for 11.5MeCN: Triclinic,P  1, a = 11.9471(15), b = 14.6166(19), c = 26.363(3) Å, a = 89.775(4) b = 88.84(5),c = 66.725(4), U = 4228.0(9) Å3, M = 1740.27, Z = 2, l(Mo-Ka) = 1.077,T = 100 K;R1 = 0.097, CCDC 707008.), doi:10.1016/j.poly.2009.01.018Fig. 2. Temperature dependence of vT for 1 from 300 to 1.8 K measured in a ﬁeld of 1 kOe (inset, magnetisation versus ﬁeld at 2 K).Fig. 1. Structure of the anion of 1 (ball and stick representation with Fe(III) gold; O, red; N, blue; C, brown; H atoms and Me groups of pivalate ligands omitted for clarity)[atom sufﬁx a signiﬁes the symmetry equivalent atom: a = 1  x, 1  y, 1  z]. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to theweb version of this article.)NHOHOHNOHOHOHOHOHNOHOHHO2CChart 1. Ligands bis–tris, bicine and diethanolamine.3 One of the crystallographically independent Et2NH2+ cations is disordered overtwo positions.2 K. Graham et al. / Polyhedron xxx (2009) xxx–xxxARTICLE IN PRESSof the Fe4 butterﬂy core. This binding mode has been observed forthe [Fe6(Bic)6] wheel [12]. The hydroxide ligand (O1) also bridgesFe1 to a body iron(III) center (Fe1–O1–Fe3). Four of the eight piv-alate ligands span the butterﬂy body-wing vectors in the typical1,3-bridging mode. In addition, two are capping Fe2 and Fe2a inthe 1,10 mode and the remaining two are monodentate bound toFe1 and Fe1a and hydrogen-bonding to the two hydroxide groups(O1 and O1a). All Fe(III) centers are six-coordinate although the1,10 bridging pivalate results in a rather distorted geometry atFe2 (and s.e.). The Et2NH2+ cations are located above and belowPlease cite this article in press as: K. Graham et al., Polyhedron (2009the Fe6 cluster, hydrogen-bonded to the bicine carboxylates (O11and O11a) and the 1,1-bridging carboxylates (O24a and O24).3We note that in this Fe6 cluster, one Fe4 butterﬂy unit is capped bytwo {Fe(Bic)} units. It is interesting to compare this Fe6 cluster toour Fe10 clusters with bis–tris [7] where two Fe4 butterﬂy unitsare bridged together by two {Fe(bis–tris)} units. Hence, we can showthat clusters of different nuclearity and topology can be prepared by), doi:10.1016/j.poly.2009.01.018Fig. 3. Structure of 2 (ball and stick representation with Fe(III) gold; O, red; N, blue; C, brown; H atoms and Me groups of pivalate ligands omitted for clarity) [atom sufﬁx asigniﬁes the symmetry equivalent atom: a = 0.5  x, 2.5  y, z]. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web versionof this article.)Fig. 4. Core structure of 2.K. Graham et al. / Polyhedron xxx (2009) xxx–xxx 3ARTICLE IN PRESSadjusting carefully the nature of the polydentate ligand, to buildupon the Fe4 butterﬂy unit.For 1 the value of vT at 300 K is 9.87 cm3 mol1 K, signiﬁcantlylower than for six uncoupled Fe(III) ions (26.25 cm3 mol1 K forg = 2), indicating strong antiferromagnetic interactions betweenthe Fe(III) centers (see Fig. 2). The value of vT increases steadilyto a maximum of 15.0 cm3 mol1 K at 14.0 K, then drops sharplyto 13.6 cm3 mol1 K at 1.8 K. This drop can be attributed to eitherzero-ﬁeld splitting or intermolecular antiferromagnetic interac-tions. The magnetisation was measured as a function of appliedﬁeld at 2 K, rising to a maximum value of M/Nb = 10.0 at 7 T. Boththe low temperature maximum in vT and the magnetisation dataare consistent with a spin ground state of S = 5 for 1. However, fur-ther measurements including EPR are required to determine themagnitude and size of any anisotropy. Further Fe6 clusters havebeen reported recently, with broadly similar topologies and S = 5ground states [19].The change of base from Et2NH to NaOMe produces adodeca-nuclear complex [Fe12O4(Bic)4(HBic)4(O2CCMe3)8]  5MeCN(2  5MeCN), in 12% yield, which crystallizes in the monoclinicspace group C2/c (Fig. 3).4 Compound 2 can be described as a dimerof Fe6 units, which have a different topology to the Fe6 cluster foundin 1. The two Fe6 units are separated by 6 Å, bridged by the carbox-ylate arms from the bicine ligands (Fig. 4). Unlike 1, compound 2 is4 Air-dried crystals analyze as 2  2MeCN  4H2O, analysis (%) calc. (found): C, 37.05(36.77); H, 5.75 (5.67); N, 4.70 (4.95), selected IR data: m = 3397 (broad); 2962, 1624,1533, 1484, 1422, 1358, 1227, 1080, 1040, 889, 795, 690 cm1. Crystal data for2  5MeCN: monoclinic, C2/c, a = 29.795(5), b = 20.638(4), c = 27.218(8) Å,b = 120.749(7), U = 14384(6) Å3, M = 3033.64, Z = 8, l(Mo Ka) = 1.247, T = 100 K;R1 = 0.0575. CCDC 707009.Please cite this article in press as: K. Graham et al., Polyhedron (2009not composed of Fe4 butterﬂy units: for 2, more bicine is incorpo-rated into the structure. The structure of each Fe6 unit is unusualand is formed from two corner-sharing Fe3O triangles (Fe3, Fe6,Fe4 and Fe1, Fe2, Fe4). In addition, Fe5 bridges to Fe2 and Fe4through three CH2CH2O arms (O62, O43, O63) from two bicine li-gands, such that Fe2, Fe4 and Fe5 form an incomplete {Fe3O4}cubane.The triply deprotonated bicine ligands (with N11 bound to Fe3or N13 bound to Fe5) bind in the same g2:g2:g1:g1:l3 mode as thatfound in 1. One of the doubly deprotonated ligands (with N14bound to Fe6) caps Fe6, with one CH2CH2O arm bridging Fe6 toFe4 and the second CH2CH2OH arm protonated (O64). The remain-ing Hbic ligand (with N12 bound to Fe5) displays a new bindingmode, bridging the two halves of the cluster through the carboxyl-ate arm. The deprotonated CH2CH2O arm (O62) is l3-bridging(Fe2, Fe4, Fe5) with the second CH2CH2OH arm protonated and un-bound (O42). Fig. 5 illustrates the different binging modes of bicinepresent in compounds 1 and 2. Six of the pivalate ligands are 1,3-bridging with the remaining two 1,10-bridging (to Fe1, Fe1a) asfound in 1. All Fe centers are six-coordinate except Fe5 (and s.e.),Fig. 5. Binding modes of BicH2 and Bic3 found in 1 and 2.), doi:10.1016/j.poly.2009.01.018Fig. 6. Temperature dependence of vT for 2 from 300 to 1.8 K measured in a ﬁeld of1 kOe.4 K. Graham et al. / Polyhedron xxx (2009) xxx–xxxARTICLE IN PRESSwhich are 7-coordinate showing a pentagonal bipyramidalgeometry.The value of vT for 2 at 300 K is 16.7 cm3 mol1 K, signiﬁcantlylower than the expected value for 12 uncoupled Fe(III) ions(52.5 cm3 mol1 K for g = 2), indicating strong antiferromagneticinteractions between the Fe(III) centers (Fig. 6). The value of vT de-creases steadily reaching a value of 0.09 cm3 mol1 K at 1.8 K con-sistent with an S = 0 ground state.In conclusion, we have reported two new Fe(III) complexes thatincorporate the bicine ligand together with pivalate ligands. Wehave shown the versatility of the ligand, which can exist in differ-ent binding modes and can act as a bridging unit within the struc-ture. The nature of the base used in the reaction has an effect onthe cluster produced and this warrants further investigation. Mag-netic characterisation reveals S = 5 and S = 0 ground states for thehexa- and dodeca-nuclear complexes, respectively.Please cite this article in press as: K. Graham et al., Polyhedron (2009AcknowledgementsFinancial support from The University of Glasgow and EPSRC isgratefully acknowledged. We thank Dr Javier Sanchez-Benitez, Uni-versity of Edinburgh for collection of the magnetic data.References[1] G. Aromi, E.K. Brechin, Struct. Bonding (Berlin) 122 (2006) 1.[2] M. Murrie, D.J. Price, Annu. Rep. Prog. Chem. Sect. A 103 (2007) 20.[3] A. Ardavan, O. Rival, J.J.L. Morton, S.J. Blundell, A.M. Tyryshkin, G.A. Timco,R.E.P. Winpenny, Phys. Rev. Lett. 98 (2007) 057201/1.[4] B. Cage, S.E. Russek, R. Shoemaker, A.J. Barker, C. Stoldt, V. Ramachandaran, N.S.Dalal, Polyhedron 26 (2007) 2413.[5] M. Manoli, R.D.L. Johnstone, S. Parsons, M. Murrie, M. Affronte, M. Evangelisti,E.K. Brechin, Angew. Chem., Int. Ed. 46 (2007) 4456.[6] L. Bogani, W. Wernsdorfer, Nature Mat. 7 (2008) 179.[7] A. Ferguson, J. McGregor, A. Parkin, M. Murrie, Dalton Trans. (2008) 731.[8] R.W. Saalfrank, I. Bernt, E. Uller, F. Hampel, Angew. Chem., Int. Ed. 36 (1997)2482.[9] R.W. Saalfrank, C. Deutscher, S. Sperner, T. Nakajima, A.M. Ako, E. Uller, F.Hampel, F.W. Heinemann, Inorg. Chem. 43 (2004) 4372.[10] A.M. Ako, O. Waldmann, V. Mereacre, F. Klower, I.J. Hewitt, C.E. Anson, H.U.Güdel, A.K. Powell, Inorg. Chem. 46 (2007) 756.[11] D. Foguet-Albiol, K.A. Abboud, G. Christou, Chem. Commun. 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Chem. 32 (1993) 4102;(b) BVS analysis of 1: Fe1, 2.96; Fe2, 3.04; Fe3, 3.10; O1, 1.12; O2, 1.86. BVSanalysis of 2: Fe1, 2.90; Fe2, 3.03; Fe3, 2.90; Fe4, 2.97; Fe5, 2.43; Fe6, 2.96; O1,1.77; O2, 1.85.[19] (a) I.A. Gass, C.J. Milios, A. Collins, F.J. White, L. Budd, S. Parsons, M. Murrie, S.P.Perlepes, E.K. Brechin, Dalton Trans. (2008) 2043;(b) R. Bagai, M.R. Daniels, K.A. Abboud, George Christou, Inorg. Chem. 47(2008) 3318.), doi:10.1016/j.poly.2009.01.018

4102; (b) BVS analysis of 1:

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Synthesis and characterisation of Fe<sub>6</sub> and Fe<sub>12</sub> clusters using bicine

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Synthesis and characterisation of Fe<sub>6</sub> and Fe<sub>12</sub> clusters using bicine

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