New approaches to magnetic clusters with hexacyanometallate building blocks

Polyhedron 20 (2001) 1727– 1734 www.elsevier.nl/locate/poly

New approaches to magnetic clusters with hexacyanometallate building blocks Jennifer A. Smith, Jose´-Ramo´n Gala´n-Mascaro´s, Rodolphe Cle´rac, Jui-Sui Sun, Xiang Ouyang, Kim R. Dunbar * Department of Chemistry, Texas A&M Uni6ersity, College Station, TX 778843, USA Received 23 November 2000; accepted 11 December 2000

Abstract Polymeric cyanide complexes, whose archetype is the well-known Prussian blue mixed valence compound Fe4[Fe(CN)6]3, are key players in the field of molecule-based magnetism. One reason for the popularity of the cyanide ligand is that it effects strong and predictable magnetic exchange interactions between paramagnetic centers. In an effort to produce high-dimensional materials that behave as magnets, researchers in the area have focused on reactions of [M(CN)6]n − with labile cations that contain few or no protecting groups to limit the growth of polymeric phases. This article reports the syntheses and characterization of new ligand protected precursors of the types cis-[M(LL)2(S)2]2 + (S= solvent), cis-M(LL)2(CN)2 and cis-M(LL)2(O35CF3)2. Reactions of these convergent precursors with hexacyanometallate anions yield novel cyanide clusters, described along with key X-ray structures. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: Cyanide clusters; Hexacyanometallates; Molecular magnetism; Single-molecule magnet

1. Introduction One of the advantages of using molecular building blocks for magnetic materials is that it affords chemists an opportunity to modify existing magnetic architectures as well as to design entirely new molecular assemblies with unusual magnetic properties [1]. Two key molecule-based magnets have emerged as major players in the field in recent years, namely three-dimensional (3D) bulk ferromagnets [2] and single molecule magnets

Scheme 1. * Corresponding author.

[3]. The former systems were inspired by the wellknown Prussian Blue material, FeIII4[FeII(CN)6]3· 15H2O, which is prepared from the reaction of [Fe(OH2)6]3 + with K4[Fe(CN)6] [4]. Prussian Blue undergoes ferromagnetic ordering at a very low temperature (Tc = 5.6 K) [2a,b], but when the Fe(II) and Fe(III) sites are substituted for other paramagnetic metal ions, the Curie temperatures are much higher [5]. A few examples of compounds with high Tc values are K0.50VII/III[Cr(CN)6]0.95·1.7H2O (Tc = 350 K), K0.058VII/III[CrIII(CN)6]0.79(SO4)0.058·0.93H2O, (Tc = 372 K), and KVII[CrIII(CN)6]·2H2O, (Tc = 376 K). In spite of the high Curie temperatures of Prussian blue magnets, their extreme insolubility is a hindrance to their characterization and eventual application. One method for producing more crystalline compounds is to reduce the dimensionality by using protecting groups to prevent the growth of a 3D network. A number of groups have used this strategy to form one-dimensional (1D) chains and two-dimensional (2D) networks with hexacyanometallate building blocks [6]. In order to further lower the dimensionality to the molecular regime, several groups, including this one, have de-

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signed convergent precursors whose self-assembly favors molecular squares or cubes [7]. With respect to obtaining square units with octahedral metal ions at the vertices, it is useful to cap four of the coordination sites with two bidentate ligands such as 2,2%-bipyridine which adopt a cis orientation due to unfavorable H interactions of two bpy ligands that are trans to each other. This results in a 90° angle between the two unblocked sites. When the two precursors in Scheme 1 are combined, one may expect either a molecular square or a 1D zigzag chain to ensue. In addition to using two convergent precursors to construct molecular squares, one can envisage the use of [M(CN)6]n − anions to connect cis-[M(LL)2]n + units. Although the hexacyanometallate building block can produce polymers via trans linkages, there is also a possibility for the self-assembly of molecular squares through cis-cyano interactions. We reasoned that if cyclic structures could be obtained in the first step, then the remaining four terminal cyanide ligands could be used for subsequent build-up of larger magnetic molecules. Herein we report the preparation and characterization of three new classes of starting materials with 2,2%-bipyridine (bpy) or 1,10-phenanthroline (phen) ligands and their incorporation into cyanide-based molecular squares and clusters of higher nuclearity. The outcome of reactions designed to yield discrete molecular squares and extended molecular square-based clusters will also be summarized.

2. Experimental

2.1. Materials and methods The ligands 2,2%-bipyridine (bpy) and 1,10-phenanthroline (phen) were purchased from Aldrich or Fisher and used without further purification. Silver trifluoromethanesulfonate (AgoTf), MCl2, and MnSO4· 4H2O were purchased from Aldrich. The ligand 2,2%bipyrimidine (2,2%-bpym) was prepared by a modification of the literature procedure [8]. The starting materials MCl2(LL)2 (LL = bpy, phen) were prepared by treating the anhydrous halides with 2 equiv. of the appropriate ligands. [Mn(CH3CN)4](BF4)2 was prepared by a literature procedure [9]. All solvents were of reagent grade quality and dried using standard procedures. Unless otherwise stated, all reactions were performed with the exclusion of air.

2.2. Preparation of complexes 2.2.1. [M(bpy)2(oTf )2] (M =Mn 2 + , Co 2 + ) (1,2) MCl2(bpy)2·xH2O [10] {(0.681 g, (1.437 mmol), 1) and (0.841 g, (1.10 mmol), 2)} was dissolved in 40 ml of

acetonitrile and combined with 2 equiv. of Ag(oTf) {(0.975 g, (3.80 mmol), 1) and (0.872 g, (3.39 mmol), 2)} dissolved in 40 ml of acetonitrile. The reaction was stirred for 12 h, after which time the AgCl was removed by filtration through Celite. The solution was reduced in volume to  5– 10 ml, and 40 ml of diethyl ether were added to precipitate the product. The compounds were isolated as yellow and dark orange solids for 1 and 2, respectively. Yellow crystals of 1 were grown by slow diffusion of diethyl ether and hexanes into an acetonitrile solution of the compound. Orange crystals of 2 were obtained by slow evaporation of an acetonitrile solution. Yields (0.7745 g) 78% (1) and (0.9225 g) 78% (2). Anal. Found: C, 39.87; H, 2.64; N, 7.60. Calc. for C22H16N4S2O6F6Mn, 1: C, 39.71; H, 2.42; N, 8.42%. IR (Nujol, cm − 1) of 1: 1606 (s), 1599 (s), 1577 (m, sh), 1493 (m, sh), 1444 (s, br), 1311 (s), 1242 (s), 1234 (s), 1215 (s), 1180 (s), 1165 (s), 1024 (s), 769 (s), 738 (m, sh), 653 (m), 636 (s), 581 (w), 572 (w, br), 516 (m), 416 (m), 358 (w, br), 249 (m), 221 (w). IR Nujol, cm − 1) of 2: 1608 (s), 1601 (s), 1577 (m, sh), 1493 (m, sh), 1444 (s, br), 1311 (s), 1242 (s), 1232 (s), 1213 (s), 1176 (m), 1157 (s, br), 1024 (s), 769 (s), 738 (m, sh), 653 (m), 636 (s), 581 (w), 572 (w, br), 516 (m), 419 (m), 359 (w, br), 280 (m), 244 (w, br), 208 (w).

2.2.2. [Ni(bpy)2(H2O)2](oTf )2 ·2H2O (3) NiCl2(bpy)2·H2O [10] (0.873 g, 1.899 mmol) was dissolved in 40 ml of acetonitrile and combined with 2 equiv. of Ag(oTf) (0.975 g, 3.80 mmol) dissolved in 40 ml of acetonitrile. The reaction was stirred for 12 h before the AgCl was removed by filtration through Celite. The solution was reduced in volume to 10 ml, and treated with 40 ml of diethyl ether to precipitate a purple solid. Purple crystals were grown by slow evaporation of a water solution in air. Yield (1.00 g, 75%). Anal. Found: C, 36.93; H, 2.49; N, 7.21. Calc. for C22H24N4S2O10F6Ni, 3: C, 35.65; H, 3.26; N, 7.55%. IR (Nujol, cm − 1) for 3: 1610 (s), 1603 (s), 1579 (m), 1493 (m, sh), 1448 (s, br), 1307 (s), 1244 (s), 1232 (s), 1217 (s), 1176 (s), 1161 (s), 1028 (5), 771 (s), 738 (m, sh), 653 (m), 636 (s), 581 (w), 572 (w), 517 (m), 420 (m), 359 (w, br), 297 (m), 260 (m), 220 (w). 2.2.3. [Mn(2,2 %-bpym)2(H2O)2](BF4)2 ·2H2O (4) A mixture of [Mn(CH3CN)4](BF4)2 (0.100 g, 0.255 mmol) and 2 equiv. of 2,2%-bpym (0.081 g, 0.510 mmol) were dissolved in 15 ml of acetone and stirred for 12 h to give a pale yellow precipitate. The solvent was removed, and the yellow solid was washed with 20 ml of acetone followed by diethyl ether and dried under reduced pressure. The pale yellow compound was obtained in 70% yield, 0.097 g. Anal. Found: C, 33.70; H, 1.89; N, 19.35. Calc. for MnN8C16B2F8O2, 4: C, 33.08;

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Table 1 Crystallographic information for 1 a, 3 a, 4 b and 5 a

Empirical formula Formula weight Temperature (K) Crystal system Space group Unit cell dimensions a (A, ) b (A, ) c (A, ) h (°) i (°) k (°) V (A, 3) Z Dcalc (Mg m−3) Absorption coefficient (mm−1) F(000) Crystal size (mm) qmax (°) Reflections collected Independent reflections Rint Final R indices z Max./min. (e A, −3)

1

3

4

5

C22H16MnN4O6S2 665.45 173(2) monoclinic C2/c

C22H24N4NiO10S2 741.28 110(2) triclinic P1(

MnC16N8O4H20B2F8 616.92 190(1) monoclinic C2/c

C22H22MnN6O3 473.40 173(2) monoclinic P21/n

10.024(1) 14.237(1) 18.922(1) 90 101.32(1) 90 2647.9(4) 4 1.669 0.745 1340 0.52×0.47×0.18 28.20 8163 3118 0.0267 R1 = 0.0379 wR2 = 0.1035 0.684, −0.610

11.325(5) 11.968(5) 12.791(5) 81.134(5) 71.290(5) 64.114(5) 1477.1(11) 2 1.667 0.896 756 0.30×0.372×0.063 28.24 6716 5448 0.0182 R1 =0.0447 wR2 =0.1058 1.490, −0.810

24.97(1) 7.633(2) 18.548(7) 90 137.75(2) 90 2377(4) 4 1.707 0.634 1220 0.26×0.29×0.26 25.8 1973 1920 0.043 R = 0.046 Rw =0.052 0.56, −0.58

8.544(1) 19.989(1) 13.448(1) 90 90.065(1) 90 2296.87(11) 4 1.396 0.610 980 0.25×0.25×0.10 28.24 13655 5333 0.0362 R1 =0.0448 wR2 =0.0796 0.277, −0.467

a R1 =[ Fo − Fc ]/ Fo ; wR2 = {[w(F o2−F c2)2/[w(F o2)2]}1/2; GOF={[w(F o2−F c2)2]/(n−p)}1/2 where n =total number of reflections and p= total number of parameters. b R = Fo − Fc / Fo ; Rw = [w( Fo − Fc )2/wF o2]1/2; GOF=[w( Fo − Fc )2/(Nobs−Nparameters)]1/2.

H, 2.78; N, 19.29%. IR (Nujol, cm − 1) on KBr plates for 4: 3090 (w, br), 1707 (w), 1592 (w, sh), 1574 (s), 1556 (m), 1410 (s), 1286 (w), 1058 (s, br), 1006 (m), 992 (w), 822 (w, br), 762 (s), 690 (ms), 656 (ms), 521 (w); w(FeN) 238 (ms).

2.2.4. Mn(bpy)2(CN)2 ·3H2O (5) A similar preparation to Mn(phen)2(CN)2·3H2O was followed [11]. A solution of MnSO4·4H2O (0.525 g, 2.35 mmol) in 6 ml of MeOH and 24 ml of H2O was added to a solution of bpy (0.971 g, 6.23 mmol) in a mixture of 20 ml of MeOH and 80 ml of H2O. This mixture was heated to 80°C under a nitrogen atmosphere to prevent aerial oxidation, and maintained at this temperature during the gradual addition of a KCN (0.606 g, 9.33 mmol) solution (16 ml MeOH/64 ml H2O). The product was allowed to slowly cool which led to the formation of air-sensitive yellow needles. Yield (0.6186 g, 56%). IR (Nujol, cm − 1) on KBr plates: 2114 (m), 1595 (s, sh), 1574 (m), 1491 (m, sh), 1441 (s), 1315 (m), 1261 (m), 1097 (m, br), 1060 (m), 1014 (s, sh), 802 (m, br), 769 (s, sh), 736 (m, sh), 646 (m), 625 (m), 489 (w, br). The compounds are soluble in common organic solvents and in water.

2.3. Physical methods The infrared spectra were collected on a Nicolet Nexus 470 FT-IR with a spectral range of 4000–200 cm − 1 on KBr or CsI plates as Nujol mulls. Elemental analyses were performed at Desert Analytic in Tucson, AZ, or in the Engineering Department at Michigan State University. Single crystal X-ray structural studies were performed on a Bruker SMART 1K CCD platform diffractometer for 1, 2, 3, 5 [12] with graphite monochromated Mo Ka radiation (ua = 0.71069 A, ). The frames were integrated in the Bruker SAINT software package [13], and the data were corrected for absorption using the SADABS program [14]. The structures were solved by direct methods using SIR-97 [15] and refined using the SHELXL-97 package [16]. X-ray structural determination for 4 was performed on a Nicolet P3/V diffractometer equipped with Mo Ka radiation (ua = 0.71069 A, ) with a 3 kW sealed-tube generator. The structure was solved by using TEXSAN crystallographic software package of molecular structure corporation [17], MITHRIL [18] and DIRDIF [19]. Pertinent crystallographic data and refinement parameters are provided in Table 1, and the corresponding CIF files are included as supplementary material.

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Fig. 1. ORTEP representation of [Mn(bpy)2(OTf)2] (1) with thermal ellipsoids at the 50% probability level.

Fig. 4. ORTEP representation of [Mn(bpy)2(CN)2] (5) with thermal ellipsoids at the 50% probability level.

3. Results and discussion

3.1. Syntheses of new cyanide-based precursors

Fig. 2. ORTEP representation of [Ni(bpy)2(H2O)2]2 + (3) with thermal ellipsoids at the 50% probability level.

In order to favor the formation of molecular squares from cyano-based octahedral complexes, four of the six positions were blocked with two bidentate ligands which left two cis positions available for substitution chemistry. The complexes cis-MCl2(LL)2 are convenient entries into the new compounds, as they react with Ag+ or Tl+ reagents in CH3CN or H2O to give cis-[M(LL)2(S)2]2 + (S=solvent) in the presence of weakly coordinating anions (e.g. [BF4]−, [PF6]− or tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, [BARF]−). In the case of the triflate anion, [CF3SO3]−, the neutral species cis-M(LL)2(CF3SO3)2 is formed. Several of the numerous compounds that have been isolated and fully characterized are [M(bpy)2(oTf)2] (M= Mn(II) (1); Co(II) (2)) (Fig. 1), [Ni(bpy)2(H2O)2](oTf)2 (3) (Fig. 2), and [Mn(2,2%-bpym)2(H2O)2](BF4)2 (4) (Fig. 3). The labile solvent or triflate groups on compounds 1–4 may be replaced by cyanide ligands, but this reaction is difficult to control, as even mild conditions often result in decomposition due to ligand redistribution. Nevertheless, the neutral, air-sensitive complex, Mn(bpy)2(CN)2 (5) (Fig. 4) was prepared, and the identity of the compound was verified by X-ray crystallography. Reactions of 5 with solvated precursors are currently under investigation.

3.2. Incorporation of new cyanide-based precursors into clusters

Fig. 3. ORTEP representation of [Mn(bpym)2(H2O)2]2 + (4) with thermal ellipsoids at the 50% probability level.

A perusal of the literature revealed that hexacyanometallate anions tend to bridge metal cations in cis or fac [6a,6e,20] rather than trans or mer orientations (Scheme 2). This situation prompted us to ask whether discrete molecular structures, in particular squares, could be

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prepared from hexacyanometallate building blocks. To test this hypothesis, reactions of 1 – 4 and analogous complexes, e.g. Mn(2,2%-bpym)2(NO3)2, [Mn(2,2%bpym)2(H2O)2](ClO4)2 [21], [Zn(phen)2(H2O)2]SO4· 7H2O [22a], [Zn(phen)(H2O)4]SO4·2H2O [22b] or [Zn(phen)2(H2O)2](NO3)2 [22c] with [Fe(CN)6]3 − were explored. In some cases these reactions lead to the forma-

Scheme 2.

Fig. 5. (a) Schematic view of {[Co(bpy)2]3[Fe(CN)6]2}+ with the bpy rings and the bridging cyanide ligands represented as solid black lines. (b) View down the threefold axis from top.

Fig. 6. Structural view of the molecular square, {[Zn(phen)2]2[Fe(CN)6]2}2 − .

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tion of polymeric architectures, e.g. the 2D polymer ([Mn(H2O)2][Mn(2,2%-bpym)(H2O)]2[Fe(CN)6]2} , isolated from the reaction of [Mn(2,2%-bpym)2(H2O)2](BF4)2 (3) with K3[Fe(CN)6] [23]. This compound is a consequence of the loss of 2,2%-bpym ligands, a result that underscores the fact that precursors with two bidentate ‘protecting’ ligands are not always stable in the presence of cyanometallate anions. The 2,2%bipyridine analogue of this 2D compound, namely {[Mn(H2O)2][Mn(bpy)(H2O)]2[Fe(CN)6]2} , was also recently prepared, and it exhibits an essentially identical structure. Both are ferrimagnets with ordering temperatures of 11 K. It should be obvious from the aforementioned discussion that, in order to favor the formation of discrete clusters versus polymers, a number of issues must be considered. The most important experimental parameters that influence the outcome of these reactions are concentration, solvent polarity, and the counterion. The fact that the ‘capping ligands’ bpy and 2,2%-bpym can be displaced hints at the feasibility of using complexes of the type [M(bpy)3]2 + in reactions where bpy ligands are slowly lost in favor of the nitrogen end of a cyanide ligand. Indeed, this strategy produced a discrete cluster from the reaction of [Co(bpy)3](ClO4)2 with K3[Fe(CN)6]. The product is a trigonal bipyramidal, (tbp), molecule, {[Co(bpy)2]3[Fe(CN)6)2}+, (Fig. 5) [24]. The tbp geometry has been proposed for the structures of several cyano clusters [25] and recently verified in a compound with different metals and ligands [26]. Two aspects of the cluster, {[Co(bpy)2]3[Fe(CN)6]2}+, are worth mentioning, namely that it crystallizes in the chiral space group P6322 and that it is the result of a spontaneous redox reaction. The latter conclusion is based on the fact that the compound is diamagnetic which supports the presence of L.S. Fe(II) and Co(III) d6 ions. Furthermore, Mo¨ ssbauer spectroscopy confirmed the oxidation state of the iron as being divalent. The redox chemistry is most likely accompanied by a reorientation of the cyanide ligand, since Co(III) prefers to bind to the carbon end of CN−. Unfortunately this point cannot be resolved by X-ray crystallography. The formation of molecular squares was realized from the reaction of Zn(phen)2(NO3)2 with [Fe(CN)6]3 − to yield {[Zn(phen)2]2[Fe(CN)6]2}2 − (Fig. 6) [27]. The molecular dianion is composed of two 1,10-phenanthroline-capped Zn(II) ions linked by two cis-hexacyanoferrate units. The anion packing involves intermolecular interactions between 1,10-phenanthroline ligands, which leads to a buckling of the CN− ligands from an ideal 180° bonding angle. The magnetic properties of the cluster indicate the presence of two, isolated non-interacting S=1/2 Fe(III) atoms.

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Fig. 7. Structural view of the molecular pentamer, {[Ni(bpy)2(H2O)][Ni(bpy)2]2[Fe(CN)6]2}.

One of the main premises put forth in the chemistry was that hexacyanometallate building blocks will allow for the elaboration of squares into larger, flat assemblies. In this vein, we have prepared the ‘pentamer’, {[Ni(bpy)2(H2O)][Ni(bpy)2]2[Fe(CN)6]2}, (Fig. 7) [28] from the reaction of [Ni(bpy)2(H2O)2](oTf)2 with K3[Fe(CN)6] in water. The formation of the pentamer can be viewed as the addition of an extra [Ni(bpy)2(H2O)]2 − unit to the square {[Ni(bpy)2]2[Fe(CN)6]2}2 − . The extra [Ni(bpy)2(H2O)]2 + is stabilized by hydrogen bonding between the remaining water molecule and the nitrogen end of a nearby terminal cyanide of a corner [Fe(CN)6]3 − unit. The pentamer is neutral, which accounts for its ease of isolation (it is insoluble in water). The concept of adding additional building blocks to the central square can be extended to the formation of even larger molecules, as evidenced by our isolation of the highly unusual decameric cluster {[Zn(phen)2][Fe(CN)6]}2{[Zn(phen)2][Zn(phen)2(H2O)][Fe(CN)6]}2 (Fig. 8) from Zn(phen)2(NO3)2 and K3[Fe(CN)6] [29]. The magnetic properties of the Ni3Fe2 pentamer are particularly important to the area of molecule-based magnets. Ferromagnetic interactions between Fe(III) and Ni(II) are expected to be dominant through the CN− bridges due to the orthogonality of the magnetic

orbitals; indeed preliminary magnetic data indicate that the ground state for this molecule is ferromagnetic (S=4). Most importantly, however, is the fact that the cluster appears to behave as a single molecule magnet (SMM). AC magnetic susceptibility measurements of a batch of single crystals revealed a strong frequency dependence both in the % (in-phase) and ¦ (out-ofphase) signals, with a blocking temperature, Tf, of 2 K. The fact that such a small cluster with only eight unpaired electrons can exhibit slow relaxation of the magnetization reversal hints at an appreciable anisotropy due, in part, to the presence of single-ion anisotropy (ZFS) but also most likely to shape anisotropy.

4. Conclusions The results reported in this paper demonstrate that [M(CN)6]n − species can be used as building blocks for discrete clusters, provided the appropriate ligand-protected metal complexes are used. A variety of cluster geometries have been encountered including trigonal bipyramids, squares, pentamers and decamers. The fact that molecular squares based on octahedral metal ions can be used as building blocks for larger and highly unsymmetrical molecules has important implications for the future design of cyanide-based single-molecule magnets.

5. Supplementary material Crystallographic data for the structural analysis has been deposited with the Cambridge Crystallographic Data Centre, CCDC Nos. 160257–160260 for compounds 1, 3, 4, 5. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: +441223-336033; e-mail: [email protected] or www: http://www.ccdc.cam.ac.uk).

Fig. 8. Structural view of the molecular decamer, {[Zn(phen)2][Fe(CN)6]}2{[Zn(phen)2][Zn(phen)2(H2O)][Fe(CN)6]}2.

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Acknowledgements The authors gratefully acknowledge the National Science Foundation for support of this work (NSF CHE9906583), for funding the CCD diffractometer (CHE-9807975) and the SQUID magnetometer instrument (NSF-9974899). J.R.G.M. thanks the Ministerio de Educacion y Cultura for a postdoctoral fellowship.

[5]

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