Supramolecular Chains and Coordination Nanowires Constructed of

May 22, 2013 - The hydrogen-bonded chains of 1 are embedded within the three-dimensional ... 10.3(5) K and τ0 = 4(1) × 10–9 s, which is consistent...
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Supramolecular Chains and Coordination Nanowires Constructed of High-Spin CoII9WV6 Clusters and 4,4′-bpdo Linkers Szymon Chorazy,†,§ Robert Podgajny,*,† Wojciech Nitek,† Michał Rams,‡ Shin-ichi Ohkoshi,§ and Barbara Sieklucka† †

Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Cracow, Poland M. Smoluchowski Institute of Physics, Jagiellonian University, Reymonta 4, 30-059 Cracow, Poland § Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan ‡

S Supporting Information *

ABSTRACT: Cyanido-bridged high-spin {Co II [Co II (MeOH)3]8[WV(CN)8]6} (Co9W6) clusters revealing singlemolecule magnet behavior were combined with 4,4′bipyridine-N,N′-dioxide (4,4′-bpdo) linkers, giving unique Hbonded supramolecular {CoII9(MeOH)24[WV(CN)8]6}·4,4′bpdo·MeOH·2H2O (1) chains and one-dimensional coordination {CoII[CoII(4,4′-bpdo)1.5(MeOH)]8[WV(CN)8]6}·2H2O (2) nanowires. The hydrogen-bonded chains of 1 are embedded within the three-dimensional supramolecular network stabilized by the series of noncovalent interactions between Co9W6 clusters, 4,4′-bpdo, and solvent molecules. The coordination nanowires 2, revealing an average core diameter of about 11 Å, are arranged parallel with the significant separation in the crystal structure, leading to a microporous supramolecular network with broad channels (12 × 12 Å) filled by methanol and water. Both 1 and 2 are stable only in a mother solution or an organic protectant, whereas they undergo the fast exchange of methanol ligands to water molecules during drying in the air. Synthesized materials preserve the magnetic characteristics of Co9W6 clusters with an effective ferromagnetic coupling, giving a ground-state spin of 15/2. For 2, the additional antiferromagnetic intercluster interactions are observed. Below 3 K, the frequency-dependent χM″(T) signals of 1 and 2 indicate the onset of slow magnetic relaxation. For 1, the relaxation time follows the Arrhenius law with an energy gap of Δ/kB = 10.3(5) K and τ0 = 4(1) × 10−9 s, which is consistent with single-molecule magnet behavior.

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their coordination lability.27−30 According to this, the embedment of cyanido clusters into extended coordination networks was reported. The octahedral [Re6E8(CN)6]4− (E = S, Se) clusters formed the porous Prussian Blue like 3D networks,31 whereas trigonal bypiramidal {[MAII(L1)2]3[MBIII(CN)6]2} (L1 = chelate) clusters32 and starlike heptanuclear {[MnIII(L2)]6[MIII(CN)6]}3+ (L2 = salen type ligands; M = Cr, Fe, Co) cations33 were connected into coordination chains. The series of 1.7−2.4 nm sized clusters {[M 6 Cl 12 (CN) 6 ][Mn(salen)]n}(n−4) (M = Nb, Ta; n = 1−4, 6) were joined through a series of ditopic organic linkers into coordination polymers.34,35 Taking interest in high-spin clusters, we connected for the first time {MnII[MnII(MeOH)3]8[WV(CN)8]6} Mn9W6 clusters (∼1.8 nm in diameter)36 by 4,4bpy (4,4′-bpy = bipyridine)37 and dpe (dpe = trans-1,2-di(4pyridyl)ethylene)38 linkers into 2D and 3D networks, respectively. The resulting {Mn II 9 [W V (CN) 8 ] 6 (dpe) 5 (MeOH)10}·14MeOH revealed the unique structure, where

INTRODUCTION Incorporation and organization of polynuclear coordination clusters into extended molecular matrixes have become an extremely attractive topic over the past decade. Their large size and specific external bonding directionality afforded a variety of molecular/coordination frameworks serving as the platforms for different types of functionalities. The interesting properties related to sorption, molecular recognition or pre/post-synthetic modifications,1−5 electronic structure,6,7 redox activity,8−10 lattice symmetry,11 or magnetic moments correlation12−20 have been studied in the cluster-based solids. The perspectives of different types of hybrid materials, that is, strongly bonded metal−organic coordination frameworks and cluster crosslinked polymers21 or weakly bonded organic−inorganic nanocomposites,22 were recently discussed. The cluster-based coordination frameworks could be obtained by reacting the simple building blocks, which results, for example, in the spontaneous appearance of oxido/ hydroxido-bridged complexes connected by polycarboxylates, imidazolates, polypyridines, etc.23−26 Another approach assumes the preparation of stable clusters in the first step and further organization in the second step, taking advantage of © XXXX American Chemical Society

Received: March 26, 2013 Revised: May 22, 2013

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taken during the period of hydration. All attempts to determine the crystal structures of 1hyd by powder X-ray diffraction were unsuccessful due to the loss of crystallinity. The composition of 1, Co9[W(CN)8]6(4,4′-bpdo)(MeOH)25(H2O)2, taken for the analysis of the magnetic data was determined based on the X-ray diffraction measurement. The effective decrease in molar mass on going from 1 to 1hyd allows estimating the real amount of magnetically active sample of 1. Synthesis of {Co I I [Co I I (4,4′-bpdo) 1 . 5 (MeOH)] 8 [W V (CN)8]6}·2H2O (2). A 0.06 mmol (14.3 mg) portion of CoCl2·6H2O and 0.04 mmol (21.3 mg) of Na3[W(CN)8]·4H2O were dissolved together in 6 mL of methanol to give an orange solution. A 0.3 mmol (56.5 mg) portion of 4,4′-bpdo dissolved in 6 mL of methanol was added, which caused the precipitation of violet powder. The resultant suspension was left undisturbed in the darkness. After about 1 week, the powder recrystallized to violet needle crystals of 2. The monocrystalline product of 2 was characterized by X-ray diffraction both in the mother solution (2MSol) as well as dispersed in Apiezon N (2Ap). Washing crystals with methanol or exposition to the air leads to the loss of crystallinity. Washing with methanol and air-drying gave the nearly amorphous powder with the formula Co9[W(CN)8]6(4,4′bpdo)12(H2O)28 (2hyd). Yield: 48 mg, 64%. Elemental analysis: Found: C, 35.6%; H, 2.8%; N, 17.6%. Calculated for Co9W6C168H152N72O52: C, 35.6%; H, 2.7%; N, 17.9%. IR (KBr pellets) in cm−1: 2219w, 2167m(sh), 2152m(sh), ν(CN); 1548w, 1472s, 1426m, 1319w ν(CC) and ν(CN); 1220s, ν(N−O); 1176s, 1125vw, 1106vw, 1029m, 836s, γ(C−H) and aromatic ring deformations; 1624s, δ(H2O); 733vw, 701w, 668vw, 553m, 473m, coordination. Three bands attributed to streching vibrations of CN− suggested the presence of WV−CN−CoII molecular bridges (2219w, 2167m(sh) cm−1) and terminal cyanide ligands coordinated to WV (2152m(sh) cm−1).67 The composition and IR spectrum of 2hyd were stable during further exposition to the air. All attempts of structure determination of 2hyd by powder X-ray diffraction was unsuccessful due to the loss of crystallinity. The composition of 2, Co9[W(CN)8]6(4,4′-bpdo)12(MeOH)9(H2O), used in the analysis of the magnetic data was taken from the results of X-ray diffraction measured for the monocrystalline sample in the mother solution (2MSol). The effective decrease in molar mass on going from 2MSol to 2hyd allows estimating the real amount of magnetically active sample of 2. Crystal Structure Determination. Single-crystal diffraction data of 1 and 2MSol were collected on a Nonius KappaCCD diffractometer with graphite monochromated Mo Kα radiation. To prevent the escape of solvent molecules, single crystals were placed in a capillary in vapor of the mother liquor and measured at room temperature. Cell refinement and data reduction were performed using HKL SCALEPAC and DENZO.68 Positions of non-hydrogen atoms were determined by direct methods using SIR-97.69 Refinement and further calculations were carried out by a full-matrix least-squares technique using SHELXL-97.70 The non-hydrogen atoms were refined anisotropically, hydrogen atoms isotropically. A part of the solvent molecules reveal a significant level of structural disorder and were refined with two different crystallographic positions. Both crystal structures contain significant solvent accessible voids found by PLATON software (235 Å3 for 1 and 1716 Å3 for 2MSol).71 It comes from the highly disordered solvent molecules which crystallographic positions are not possible to determine in the X-ray experiment conducted at room temperature. Therefore, the additional X-ray diffraction data for 2 (2Ap) were collected at low temperature (T = 90(2) K) on a RIGAKU R-AXIS RAPID imaging plate area detector with graphite monochromated Mo Kα radiation. The single crystals were dispersed in Apiezon and mounted on Micro Mounts. The crystal structure were solved by a direct method and refined by a fullmatrix least-squares technique using SHELXL-97.70 Non-hydrogen atoms were refined anisotropically, whereas the hydrogen atoms were refined using the riding model. The crystal structure of 2Ap does not reveal noticeable solvent accessible voids as the positions of solvent molecules were possible to determine. However, they were structurally disordered and two possible crystallographic positions for the most of solvent atoms with the fractional occupancy were proposed.

nanosized cyanido-bridged wires were joined by dpe ligands into an antiferromagnetic (TN = 3.5 K) network with 3D I1O2 dimensionality topology.39 Following this line, we applied the {CoII[CoII(MeOH)3]8[WV(CN)8]6} Co9W6 congener.40,41 The incorporation of the CoII complex into the [W(CN)8]3− bridged network may result in observation of magnetic ordering,42−44 significant local and bulk anisotropy,44−46 and slow magnetic relaxation with singlemolecule magnet (SMM)40,41,47 or single-chain magnet (SCM)45,48 behavior. Moreover, stereochemically nonrigid and redox-active Co centers may lead to structurally and electronically labile systems49−58 with thermal spin-bistability,51−58 and light controlled switching between hard magnets and paramagnets.52−58 To organize Co9W6 clusters into coordination assemblies, we applied the 4,4′-bpdo (4,4′bipyridine-N,N′-dioxide) organic linker belonging to a wide group of bipyridyl N,N′-dioxide derivatives, which were used successfully in the assembling of polynuclear magnetic molecules and chains.59−65 Here, we present a self-assembly between {CoII[CoII(MeOH)3]8[WV(CN)8]6} and 4,4′-bpdo linkers, resulting either in the hydrogen-bonded supramolecular network {CoII9(MeOH)24[WV(CN)8]6}·4,4′-bpdo·MeOH·2H2O (1) or in the porous 1D coordination polymer {CoII[CoII(4,4′-bpdo)1.5(MeOH)]8[WV(CN)8]6}·2H2O (2). We discuss the slow magnetic relaxation in 1 and 2 in the context of SMM character suggested previously for this family of compounds.40,46

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EXPERIMENTAL SECTION

Materials. The reagents CoIICl2·6H2O, 4,4′-dipyridyl N,N′-dioxide hydrate (4,4′-bpdo), and solvents (methanol = MeOH, tetrahydrofuran = THF) used during the synthesis were purchased from commercial sources (Sigma Aldrich, Idalia) and used without further purification. The compound Na3[WV(CN)8]·4H2O was prepared following the literature procedure.66 Synthesis of {CoII9(MeOH)24[WV(CN)8]6}·4,4′-bpdo·MeOH·2H2O (1). A 0.12 mmol (28.6 mg) portion of CoCl2·6H2O and 0.12 mmol (22.6 mg) of 4,4′-bpdo were dissolved together in 10 mL of methanol to give a pink solution I. A 0.08 mmol (42.6 mg) portion of Na3[W(CN)8]·4H2O was dissolved in 10 mL of methanol to give solution II. Both solutions (I, II) were then slowly mixed and stirred together for 30 min. Finally, 5 mL of tetrahydrofuran was added to the reaction mixture. The resultant solution was left in the darkness. After about 1 week, red needle crystals of 1 were observed. The monocrystalline product was suitable for X-ray characterization only in the mother liquor. Washing crystals with methanol or exposition to the air leads to the breakdown of crystals after a few minutes. The exposition of 1 to the ambient laboratory conditions for a period of 1 day leads to the significant exchange of methanol ligands to water solvent molecules, which results in its stable hydrated form Co9[WV(CN)8]6(4,4′-bpdo)(MeOH)6(H2O)26 (1hyd). Yield: 33 mg, 65%. Elemental analysis: Found: C, 20.8%; H, 1.9%; N, 18.7%. Calculated for Co9W6C64H84N50O34: C, 20.6%; H, 2.3%; N, 18.8%. IR (KBr pellets) in cm−1: 2179m(sh), 2150m(sh), 2135w, ν(CN); 1559w, 1474s, 1428w, 1317vw, ν(CC) and ν(CN); 1213s, ν(N− O); 1178s, 1126w, 1103w, 1034w, 837m, γ(C−H) and aromatic ring deformations; 1628s, δ(H2O); 1015w, ν(C−O) (in MeOH); 667w, 613w, 558m, 480s, coordination. Three bands attributed to streching vibrations of CN− suggested the presence of WV−CN−CoII molecular bridges (2179 m(sh) cm−1) and terminal cyanide ligands coordinated to WV (2150m(sh), 2135w cm−1).67 Numerous bands in the range of 1500−700 cm−1 can be ascribed mainly to coordinated 4,4′-bpdo. A slow exchange of MeOH to H2O was confirmed by the relevant changes in intensity of the 1628 cm−1 δ(H2O) band and the 1015w cm−1 ν(C−O, MeOH) band, measured on the small samples B

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Figure 1. Crystal structure of 1: (a) The structure of cyanido-bridged Co9W6 clusters and 4,4′-bpdo ligand. (b) The view of single zigzag supramolecular chain. (c) The asymmetric unit with atom labeling scheme. Dotted lines represent hydrogen bonds between methanol molecules of clusters and oxygen atoms of 4,4′-bpdo. Hydrogen atoms are omitted for clarity. Atom spheres in (c) are shown with 50% probability ellipsoids. Colors: Co(II), purple; W(V), brown; C, green; C(4,4′-bpdo, a - b), red; N, violet; O, yellow. Physical Techniques. Elemental analyses (C, H, N) were performed using an EuroEA EuroVector elemental analyzer. Infrared spectra were measured in KBr pellets between 4000 and 400 cm−1 using a Bruker EQUINOX 55 FT-IR spectrometer. Powder XRD patterns of 1 and 2 in the form of thick suspensions sealed in 0.7 mm glass capillaries were measured on a PANalytical X’Pert PRO MPD diffractometer with a capillary spinning add-on using Cu Kα radiation (λ = 1.54187 Å). The patterns were collected at room temperature in the 4−40° 2θ range. The reference powder patterns of 1 and 2 were generated using Mercury 3.0 software. Magnetic measurements were performed using Quantum Design MPMS. To prevent solvent exchange, the samples of 1 and 2 were measured being covered with the mother liquid sealed in a glass tube. The magnetic signal for 1hyd

and 2hyd was measured using dry samples. The magnetic data were corrected for the diamagnetic contribution using Pascal constants.72 Calculations. Continuous shape measure analysis for coordination spheres of eight-coordinated W(V) centers was performed using SHAPE software, ver. 1.1b.73

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RESULTS AND DISCUSSION

Crystal Structures. The crystal structure of 1 consists of 15-centered cyanido-bridged {Co II [Co II(MeOH) 3 ] 8 [W V (CN)8]6} (Co9W6) clusters of a six-capped body-centered cube topology connected via hydrogen bonds with crystallization molecules of 4,4′-bpdo, which leads to supramolecular C

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Figure 2. Crystal structure of 1: (a) The view of the 3D supramolecular network within the ac plane with the marking of different types of hydrogen bonds between clusters (pink, blue, and black), and between clusters and 4,4′-bpdo (red). (b) The insight into the intermolecular network between terminal cyanides and 4,4′-bpdo. Dotted lines correspond to hydrogen bonds and other noncovalent interactions.

prism and dodecahedron, which is identical to geometries of octacyanides observed in the purely solvated Co9W6 phase (Tables S4 and S5, Supporting Information). All of the CoII ions are six-coordinated and adopt a distorted octahedral geometry. Similar to the purely solvated Co9W6 phase, the Co4 center situated in the center of the cluster core coordinates six cyanides while Co5, Co6, Co7, and Co8 coordinate three cyanides and three methanol molecules in an fac configuration. The three-dimensional supramolecular network of 1 is stabilized by a variety of hydrogen bonding motifs formed along several directions (Figure 2). The formation of a supramolecular zigzag-like chain along the [101] direction is realized through the molecular synthons between hydroxyl groups of MeOH coordinated to the external CoII centers (Co6 and Co8) of Co9W6 and oxygen atoms of N-oxide functions of

zigzag-like chains running along the [101] direction (Figure 1; also see Tables S1, S2, S4, and S5 in the Supporting Information). The composition and topology of Co9W6 molecules in 1 are analogous to Co9W6 clusters crystallizing in the methanolic solution without the 4,4′-bpdo ligand, which was described by Song and co-workers,40 hereafter denoted as the purely solvated Co9W6 phase. The asymmetric unit of 1 contains five CoII centers with coordinated methanol, three [WV(CN)8]3− ions, 4,4′-bpdo, and some additional disordered solvent molecules of water and methanol (Figure 1c). Every [WV(CN)8]3− forms five cyanide bridges constructing the cluster core while the remaining three cyanides are terminal. The geometry of the eight-coordinated [W2V(CN)8]3− ion is very close to the ideal dodecahedron while W1 and W3 centers adopt the intermediate geometry between bicapped trigonal D

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Figure 3. Comparison of basal polyhedrons of Co9W6 clusters in 1 and 2 with ideal polyhedrons and purely solvated Co9W6 phase. Colors: Co(II), purple (a); W(V), brown (b); other atoms, gray or black.

coordinatively connected by sets of four 4,4′-bpdo linkers bound to the external CoII centers (Figure 4a). Additionally, CoII centers coordinate terminal 4,4′-bpdo ligands, eight per one Co9W6 cluster, which protrude outside of the coordination skeleton along the a and b directions obeying the regime of the C4 symmetry axis. The topology of the cyanido-bridged cluster skeleton in 2 is identical to that observed in 1 and the purely solvated Co9W6 phase. The asymmetric unit contains two CoII centers, two [WV(CN)8]3− ions, and several solvent molecules of water and methanol (Figure 5, and the Supporting Information). Both W1 as well as W2 centers form five cyanide bridges while the remaining three cyanides are free. The geometries of all octacyanides in 2 are very close to the ideal bicapped trigonal prism (BTP-8, Tables S4 and S5, Supporting Information). The Co3 center, situated in the center of a cluster core, coordinates six cyanides, and reveals the geometry of an almost ideal octahedron. The external Co4 center of a significantly distorted octahedral geometry coordinates three cyanides in an fac configuration, one bridging and one terminal 4,4′-bpdo, and one methanol molecule. The coordination of 4,4′-bpdo by external CoII centers leads to the noticeable geometrical modification of cluster cores (Figure 3). Compared to 1 and to the purely solvated Co9W6 phase, 2 reveals a significantly increased symmetry (space group changes from C2/c to I4/m) with a small compression of clusters in the direction of coordination nanowires, and, consequently, the uniform elongation in all perpendicular directions. Co9W6-4,4′-bpdo nanowires revealing an average coordination core diameter of about 11 Å are arranged in parallel along the c direction (Figure 4c). The alignment of nanowires in the three-dimensional supramolecular network is controlled by noncovalent interactions involving the terminal 4,4′-bpdo

4,4′-bpdo ligands. This interaction modifies slightly the geometrical parameters, and the size of the cluster core affects the cyanido-bridged skeleton compared to that observed for the purely solvated Co9W6 phase (Figure 3). However, a trapezoidal distortion of its cubooctahedral-like shape is retained. The other intercluster hydrogen bonds, observed also in the purely solvated Co9W6 phase, are realized by synthons between hydroxyl groups of methanol molecules coordinated to CoII centers and nitrogen atoms belonging to terminal cyanides of [WV(CN)8]3− moieties (Figure 2a). Additionally, the three-dimensional architecture of 1 is stabilized by CN···ring4,4′‑bpdo (N···centroid ∼ 3.9 Å) and CN···H−C4,4′‑bpdo (N···H about 2.4 Å) intermolecular contacts (Figure 2b). The empty space between clusters and 4,4′-bpdo is filled by weakly bonded and structurally disordered solvent molecules of methanol and water. The shortest intermetallic contacts between the neighboring clusters are in the range of 7.1−7.5 Å. The powder X-ray diffraction pattern for 1 is presented in Figure S1 (Supporting Information). The respective calculated pattern fits well with the experimental one, indicating that the structural model obtained from the single-crystal XRD experiment is valid for the bulk sample used in other physical measurements. Single-crystal X-ray diffraction measured successfully both in the mother solution (2MSol) as well as in Apiezon N (2Ap) revealed that the crystal structure of 2 is composed of very unique 1D {CoII[CoII(μ-4,4′-bpdo)0.5(4,4′-bpdo)(MeOH)]8[WV(CN)8]6}∞ (Co9W6-4,4′-bpdo) nanowires aligned along the C4 symmetry axis, converging with the c crystallographic direction (Figures 4 and 5; also see Tables S1, S3−S5, Supporting Information). Coordination nanowires are constructed of 15-centered cyanido-bridged Co9W6 clusters E

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Figure 4. Crystal structure of 2: (a) Single coordination nanowire situated along the c direction. (b) The cluster fragment of nanowire with metal labeling scheme. (c) The projection of crystal structure along the c direction with marked size of channels and coordination chains. Solvent molecules situated inside micropores and hydrogen atoms were omitted for clarity. Colors: Co(II), purple; W(V), brown; C, green; C(terminal 4,4′bpdo), orange; C(bridging 4,4′-bpdo), black; N, violet; O, yellow.

ligands (Figure 6). Among them, the relatively strong π−π stacking between dyads of 4,4′-bpdo (Type I, Figure 6) and hydrogen bonds between noncoordinated oxygen atoms of 4,4′-bpdo and methanol molecules (Type II) play the primary role. Additionally, CN···H−C4,4′‑bpdo (N···H about 2.6 Å) intermolecular contacts (Type III) costabilize the supramolecular architecture. The shortest intercluster W···W, Mn···W, and Mn···Mn contacts within the chain are of 8.6, 11.6, and 12.2 Å, respectively, whereas, between the neighboring chains, they are of 11.4, 12.2, and 12.4 Å, respectively. Strong interactions between terminal 4,4′-bpdo ligands cause the significant separation of coordination chains, giving a microporous supramolecular network with the broadest

channels (12 × 12 Å, Figure 4c) along the c direction. The internal walls of these channels are functionalized with hydroxyl groups of methanol, and oxygen atoms of 4,4′-bpdo ligands. For the crystal structure measured in the mother solution at room temperature (2MSol), only small electronic density, connected with two water molecules per one Co9W6 cluster, is observed within these channels. On the contrary, the structure measured for the crystal dispersed in Apiezon N at low temperature (2Ap) reveals an almost complete filling of the interchain space by significantly disordered water and methanol molecules (for the more detailed comparison between 2Msol and 2Ap, see Figure 5 and the Supporting Information). This comparison suggests very weak bonding of noncoordinated F

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Figure 5. Crystal structure of 2MSol (a) and 2Ap (b): the symmetrically independent units with atom labeling scheme. Hydrogen atoms are omitted for clarity. Atom spheres are shown with 40% probability ellipsoids.

solvent molecules by external parts of Co9W6-4,4′-bpdo nanowires, which could be promising from the viewpoint of sorption properties, such as controlled solvent exchange. The purity of the bulk sample of 2 in the mother solution was confirmed by powder X-ray diffraction, as the pattern calculated from single-crystal XRD experiment fits well with the experimental one (Figure S1, Supporting Information). The crystals of 2 reveal the fast exchange of methanol by water during the exposition to the air. During the solvent exchange, compound 2 loses crystallinity, which can be connected with the destabilization of interchain interactions as well as with the instability of cyanido-bridged Co9W6 cluster cores in the aqueous environment.38 Magnetic Properties. The magnetic characteristics of 1, including dc temperature dependence of the magnetic susceptibility χMT(T), field dependence of the magnetization M(H), as well as the ac susceptibility curves χM′(T) and χM″(T), are presented in Figure 7. All plots are qualitatively and quantitatively very similar to the magnetic data observed for the purely solvated Co9W6 phase, reported by Song et al.40 This suggests that weakly bonded 4,4′-bpdo diamagnetic linkers do not play a significant role, and the magnetism of 1 is determined exclusively by the nature of cyanido-bridged clusters, being in a good magnetic isolation. However, the description of magnetic behavior of these molecules should take into account the specific character of six-coordinated highspin Co(II) centers with the significant orbital contribution.74 The room-temperature χMT value for 1 is 32.1 cm3 mol−1 K, which corresponds well to the uncoupled six WV ions (SW = 1/ 2, gW = 2.0) and nine CoII ions (SCo,HS = 3/2) with the g-value

Figure 6. Crystal structure of 2: noncovalent interactions between the dyads of terminal 4,4′-bpdo coordinated to the neighboring coordination nanowires.

of nearly octahedral high-spin CoII ions within the typical range of 2.4−2.7.74 Upon cooling, the χMT value gradually increases, suggesting the ferromagnetic coupling between CoII and WV. At 10 K, χMT reaches the maximum of 85 cm3 mol−1 K, and finally decreases to about 47 cm3 mol−1 K at 2 K, which should be interpreted in terms of single-ion effects of CoII and some intercluster AF interactions (Figure 7a). To correlate the maximum value of χMT with the correct ground-state spin of Co9W6 clusters in 1, it is necessary to apply the effective spin approach for CoII centers, postulating the effective spin of 1/2 for CoII with the isotropic g-value of 13/3 at low temperature where only the lowest-lying Kramers doublet is populated (below 30 K).74 Using this approximation, the ferromagnetic ground state of Co9W6 has spin SF = 15/2 and an average gG

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Figure 8. Magnetic properties of 2: (a) Temperature dependence of χMT (H = 1 kOe). (b) Field dependence of the magnetization (T = 1.8 K). (b, the inset) dM/dH versus H plot. (c) ac susceptibility without dc field (Hac = 3 Oe).

Figure 7. Magnetic properties of 1: (a) Temperature dependence of χMT at H = 1 kOe. (a, the inset) Field dependence of the magnetization at T = 1.8 K. (b) ac susceptibility χM′(T) and χM″(T) without dc field (Hac = 3 Oe). (b, the inset) The relaxation time τ(1/ T) plot with the best fit according to the Arrhenius law (red line).

ions (see the description above). With decreasing temperature, χMT gradually increases, reaching the maximum value of 79 cm3 mol−1 K at 15 K, and later decreases to about 17 cm3 mol−1 K at 2 K (Figure 8a). The temperature dependence of χMT is very close to those observed for 1, and can be similarly explained by the ferromagnetic coupling between CoII and WV within the cyanido-bridged Co9W6 clusters, leading to a ferromagnetic ground-state spin of SF = 15/2 and gav = 3.4. FC and ZFC magnetization curves measured at a small field of 20 Oe converge within the whole temperature range, but a smooth maximum on χ(T) is present around 5 K (Figure S2, Supporting Information). This is confirmed in the measurement in 1 kOe, and also ac susceptibility. The magnetization measured at 1.8 K increases to 24.5 Nβ at 50 kOe, with an inflection point in the small field range, which is better visible as a maximum at HC = 4 kOe in the dM/dH curve (Figure 8b). These data suggest the presence of antiferromagnetic interaction between the clusters, much stronger than in the case of 1, but still no sharp transition to a long-range ordering is visible down to 1.8 K. The intercluster interaction, roughly estimated as gavβHC/SF, reaches 0.084 cm−1. Taking into account the lack of ordering, the appearance of frequency-

value of gav = 3.4 (calculated from gW = 2 and gCo,LT = 13/3) that allows reaching the χMT value of ca. 92 cm3 mol−1 K. This interpretation is also confirmed by the magnetization curve reaching the value of 24.9 Nβ in the magnetic field H = 50 kOe at T = 1.8 K (the inset, Figure 7a), which is only slightly lower than the expected 25.5 Nβ. The field-cooled (FC) and zerofield-cooled (ZFC) magnetization curves of 1 do not diverge (Figure S2, Supporting Information), which suggests that no long-range magnetic ordering is observed, indicating a good magnetic isolation of molecular clusters. The presence of strongly anisotropic CoII centers in these molecules leads to frequency-dependent ac magnetic susceptibility in the lowest temperature regime (Figure 7b). The relaxation time estimated from the maxima of χM″ versus T curves follows the Arrhenius law with an energy gap of Δ/kB = 10.3(5) K and τ0 = 4(1) × 10−9 s, which is consistent with single-molecule magnet behavior.40 The magnetic properties of 2 are presented in Figure 8. The χMT product at room temperature is 29.7 cm3 mol−1 K, corresponding well to the uncoupled six WV ions and nine CoII H

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dependent ac magnetic susceptibility below 3 K (Figure 8c) can be ascribed to the onset of slow magnetic relaxation coming from a single cyanido-bridged cluster. However, this happens at lower temperatures than for 1. The detailed analysis of spin dynamics is not possible as the maxima of χM″(T), even with a dc magnetic field applied, are not observed above 1.8 K (Figure S3, Supporting Information). We have recently shown that CoII complexes with different anisotropies (axial or planar) could be simultaneously incorporated in the cyanide-bridged skeleton to decide on its individual magnetic anisotropy scheme.44 The CoII local magnetic anisotropy energy (expressed by the energy difference between the ground Kramers doublet and the first excited Kramers doublet)44,75 exceeds the energy of magnetic interactions (expressed by JWCo coupling constant)42,46,76 with the ratio of ca. 10:1 for CoII−WV systems. As far as the Co9M6 clusters are concerned, only two papers were devoted to the origin of their magnetic behavior. Long and co-workers, by judicious introduction of one diamagnetic [Re(CN)8]3− in place of paramagnetic [W(CN)8]3−, decreased the symmetry of the overall magnetic interaction pathways from Oh to axial C4v, and observed a significant decrease of the onset temperature of slow magnetic relaxation, compared to the solvated Co9W6 phase.41 DFT calculations done by Zhang and Luo suggested that the local anisotropy of fac-[CoII(NC)3(MeOH)3] (axial C3v symmetry for 3N(CN)3O(MeOH) coordination sphere of CoII) could be tuned by spatial distribution of methyl groups of coordinated MeOH molecules, the higher axial anisotropy being favored by low symmetry distribution.46 In the present work, we have shown that different types of distortion from the ideal cubooctahedral cluster core could be implied by various ligand sets in 6-coordinate CoII complexes. We found that, in the purely solvated Co9W6 phase and in 1, the CoII centers are distributed within the strongly distorted trapezoidal cube, whereas, in 2, only weak axial compression of the cluster core along the coordination nanowire direction is observed (Figure 3). In the purely solvated Co9W6 phase and in 1, CoII centers exhibit C3v symmetry of 3N(CN)3O(MeOH) coordination, whereas, in 2, coordination of 3N(CN)2O(bpdo)1O(MeOH) leads to lower Cs symmetry. A mixed SAPR/DD/BTP local 8-coordination of [W(CN)8]3− changes slightly on going from 1 to 2. The differences in local structural anisotropy of CoII centers, and possible anisotropy of magnetic WV−CN−CoII interactions could contribute to observed differences in slow magnetic relaxation at low temperature in 1 and 2. Additionally, the intercluster magnetic interactions in 2, stronger than in 1, operating at low temperatures, compete with intracluster interactions, which probably weakens the SMM character. Furthermore, the paramagnetic [M(CN)8]n− moiety can serve as the additional source of magnetic anisotropy depending on its coordination geometry.77,78

Article

ASSOCIATED CONTENT

S Supporting Information *

Crystallographic data (CIF files). Details of crystal data and structure refinement, the asymmetric units and detailed structure parameters, results of continuous shape measure analysis and dihedral angles method, and ZFC and FC magnetization curves. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: 4812 663 20 51. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Marcin Kozieł for powder X-ray diffraction measurements. This work was partially supported by the Polish Ministry of Science and Higher Education within Research Projects 1535/B/H03/2009/36, by the Polish National Science Centre (Grant UMO-2011/01/B/ST5/ 00716), and by the International PhD-studies Programme at the Faculty of Chemistry, Jagiellonian University, within the Foundation for Polish Science MPD Programme cofinanced by the EU European Regional Development Fund. The research was partially carried out with the equipment purchased thanks to the financial support of the European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program (contract no. POIG.02.01.00-12-023/08).

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REFERENCES

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CONCLUSIONS We have shown for the first time the ability of Co9W6 clusters to form unique advanced coordination topologies. We evidenced that the formation of H-bonded supramolecular polymer 1 and coordination polymer 2 is controlled by strong competition between hydrogen bonding and coordination involving 4,4′-bpdo linkers and Co(II) centers as building blocks. We have demonstrated that the structurally tuned slow magnetic relaxation characteristics of the Co9W6 cluster building block can be incorporated into supramolecular and coordination polymers, as exemplified in 1 and 2, respectively. I

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