n Nanowires Organized into Three-Dimensional Hybrid Network of

Oct 12, 2010 - inorganic hybrid network of I1O2 topology. The intercluster connections are formed by Mn-NC-W and Mn-dpe-Mn linkages. The. Mn-NC-W ...
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DOI: 10.1021/cg100920c

{MnII9WV6}n Nanowires Organized into Three-Dimensional Hybrid Network of I1O2 Topology

2010, Vol. 10 4693–4696

Robert Podgajny,*,† Szymon Chora)_zy,† Wojciech Nitek,† Michaz Rams,‡ Maria Bazanda,# and Barbara Sieklucka† †

Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Krak ow, Poland, M. Smoluchowski Institute of Physics, Jagiellonian University, Reymonta 4, 30-059 Krak ow, nski Institute of Nuclear Physics PAN, Radzikowskiego 152, Poland, and #H. Niewodnicza 31-342 Krak ow, Poland ‡

Received July 12, 2010; Revised Manuscript Received September 29, 2010

ABSTRACT: Pentadecanuclear cyano-bridged high spin {MnII9WV6(CN)48(solv)24} clusters serve as bifunctional molecular build-

ing blocks in the construction of a {MnII9[WV(CN)8]6(dpe)5(MeOH)10} 3 14MeOH (dpe = trans-1,2-di(4-pyridyl)ethylene) organicinorganic hybrid network of I1O2 topology. The intercluster connections are formed by Mn-NC-W and Mn-dpe-Mn linkages. The Mn-NC-W linkages lead to one-dimensional (1D) cyanido-bridged nanowires {Mn9W6}n with a van der Vaals diameter changing periodically from ∼1 to ∼1.5 A˚, with Mn-dpe-Mn linkages controlling the effective distances between them. The magnetic measurements suggest the superparamagnetic behavior of {Mn9W6}n skeletons below 100 K due to the internal ferrimagnetic ordering through CN- bridges. Below 3.5 K, weak antiferromagnetic interactions between the {Mn9W6}n operate, with two inflection points H1 = 100 Oe and H2 = 480 Oe observed in the M(H) curve at 1.8 K. The discovery of novel functional frameworks is one of the major challenges in current coordination chemistry. Within the molecular building blocks approach,1 several interesting materials were elaborated during the past decade. Metal-organic frameworks (MOFs) built of secondary building units (SBU) and polycarboxylate ligands2 were dedicated to gas storage,3 selective sorption4 and separation of molecules,3 and size-selective catalysis5 or elimination of toxic species (i.e., conversion of CO to CO2).6 The use of polynuclear building blocks with unpaired electrons offers a novel perspective in this field, consisting of the incorporation of additional function related to the magnetic ordering above N2 boiling point.7 On the other hand, the investigation of organization in the crystal lattice could give information about the cluster coordination ability, which may be useful in the construction of novel high spin SMM species.8 To the best of our knowledge, only two papers9,10 report the successful attempt of the use of cyanido-bridged clusters as building blocks in the synthesis of higher nuclearity species until now. The reaction between pentanuclear bipyramids [CoIII(tmphen)2]3[FeII(CN)6]2}þ acting as a Lewis base and Ni(ClO4)2 in a H2O/ MeOH mixture reported by Dunbar and co-workers led to undecanuclear clusters with six additional cyanido-bridged [Ni(NC)(H2O)5] peripheral moieties.9 In our previous article, we sketched the possibility to use the pentadecanuclear high spin clusters {MAII9MBV6} (MA = Mn, Co, Ni; MB = Mo, W, solv - MeOH, EtOH, H2O) as building blocks in the construction of extended frameworks.10 We tested the {MnII9WV6} building block with respect to their structural organization and design of novel hybrid networks. The use of the 4,4-bpy linker resulted in the formation of Mn-(4,40 -bpy)-Mn linkages leading to a two-dimensional (2D) network of square-grid topology incorporating {MnII9WV6} clusters as the nodes. In this communication, by applying trans-1,2-di(4-pyridyl)ethylene) linker (dpe),11 we uncover the heterobridging possibility of {Mn9W6} clusters (Supporting Information, Figure S1) resulting in {MnII9[WV(CN)8]6(dpe)5(MeOH)10} 3 14MeOH (1). The brown plate monocrystalline product 1 was reproducibly obtained after 5-6 weeks of the slow diffusion of the mixture of MnCl2 and Na3[W(CN)8]12 in MeOH/EtOH/MeCN solutions against the MeOH/EtOH/MeCN solution of dpe in the H-tube *Corresponding author. E-mail: [email protected]. r 2010 American Chemical Society

Figure 1. The crystal structure of 1: view of 3D coordination architecture along the c direction (a) and cyanido-bridged skeleton of {Mn9W6}n nanowire (b). Colors: Mn(II) centers (yellow), W(V) centers (dark blue), carbon atoms (gray), nitrogen atoms (pale blue). Terminal ligands are omitted for clarity. Atom spheres of metal centers and intercluster bridges are enlarged.

(Supporting Information, Table S1, Table S2, Table S3, Figure S2).13 The crystal structure of 1 comprises one-dimensional (1D) cyanido-bridged nanowires {Mn9W6}n extending along the c crystallographic direction organized by dpe ligands into a threedimensional (3D) network (Figure 1).14,15 The inorganic coordination skeleton is built from pentadecanuclear {MnII9WV6} units connected by quadruple sets of parallel cyanido-bridges joining the tetranuclear {Mn2W2} square-like faces of the neighboring units. The intercluster connecting subunits formed in this fashion resemble a distorted cube-like box {Mn4W4}. The metric parameters of W-CN-Mn linkages forming intercluster connections within {Mn4W4} cubes do not differ significantly from those found within the {Mn9W6} and {Mn9Mo6} clusters16 and those Published on Web 10/12/2010

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Figure 2. The molecular structure of {MnII9WV6} unit in 1. Colors: Mn(II) centers (yellow), W(V) centers (dark blue), carbon atoms of dpe ligand (black), other carbon atoms (gray), nitrogen atoms (pale blue). Hydrogen atoms are omitted for clarity. Atom spheres of metal centers are enlarged.

Podgajny et al. previously observed for octacyanidometalates of W and Mo.17 This suggests that the {Mn9W6}n may be treated as one integral skeleton rather than the chain of weakly bonded {Mn9W6} clusters. The resulting van der Vaals diameter of the nanowire varies periodically from ∼1 nm at the {Mn4W4} cube to ∼1.5 nm at the {MnII9WV6} building unit. The shortest distance between the central Mn4 atoms of neighboring clusters in one {Mn9W6}n chain is ∼14.6 A˚. To the best of our knowledge, this type of 1D coordination skeleton is unprecedented for cyanido-bridged networks. The neighboring nanowires are bridged by dpe linkers coordinated to manganese centers. The coordination framework of 1 represents the connectivity I1O2,18 observed also for the family of porous flexible MIII(F,OH)-benzene-1,4-dicarboxylate frameworks known also as MIL systems.7,19 Each {Mn9W6}n nanowire is surrounded by six other hexagonally arranged chains to give the isosceles triangular projection of 2D (3,6) topology on the ab plane (Figure 1a). Ten dpe ligands arise from each {Mn9W6} building block: six in the crystallographic a direction and four in the crystallographic b direction. The intermetallic distances in Mn-dpe-Mn linkages: Mn6 3 3 3 Mn7, Mn5 3 3 3 Mn5 (along the a direction) and Mn5 3 3 3 Mn6 (along the b direction) are ∼13.9 A˚. The overall separation between {Mn9W6}n chains along the b direction, represented by the closest contact Mn7a 3 3 3 W3b of 9.4 A˚, is shorter than along the a direction, represented by the closest contact Mn8a 3 3 3 W3b of 10.6 A˚ (Supporting Information, Figure S3). The resulting

Figure 3. Magnetic properties of 1: (a) Temperature dependence of χT product for 1 measured in dc mode (Hdc = 1000 Oe) and ac mode (Hac = 30 Oe, f = 10 Hz). The inset shows the χ0 (T) curve in the low temperature range. (b) Field dependent magnetization M(H) measured in T = 1.8 K. The inset shows M(H) and dM/dH(H) curves in the extended low field range.

Communication shortest distances between central Mn4 atoms of {Mn9W6} units of neighboring {Mn9W6}n are ca. 19 and 21 A˚, respectively. The central ethylene groups of dpe ligands form close π-π contact between ethylene CdC bonds with the relevant shortest C 3 3 3 C distances of ca. 4 A˚. Along the 3D framework rectanglelike channels of dimensions ranging from ∼14  14 A˚2 to 9  11 A˚2 running parallel to crystallographic c direction contain disordered solvent molecules. Compared to the previously reported structures built of {Mn9W6} units,10,16 we succeeded in a relatively large separation between inorganic parts, controlled mainly by the length and rigidity of dpe ligand, not by the randomly distributed hydrogen bonding. The overall view of molecular structure {MnII9WV6} unit is presented in Figure 2. The cyanido-bridged skeleton of {Mn9W6} in 1 is topologically identical to those reported previously. The central Mn4 ion of slightly compressed square bipyramidal shape coordinate six N atoms of CN- ligands. Eight external Mn complexes reveal pseudo-octahedral geometry with three possible types of coordination spheres: [cis-MnII(NC)4(dpe)2] (four complexes, Mn5 and Mn6 types), [fac-MnII(NC)3(solv)2dpe] (two complexes of Mn7 type), and [fac-MnII(NC)3(solv)3] (two complexes of Mn8 type). The fourth additional CN- ligands in Mn5 and Mn6 complexes join the neighboring {Mn9W6} units. The shape of [W(CN)8]3- units are very close to dodecahedral (DD-8) geometry. The temperature dependences of χ and χT are shown in Figure 3a. At 300 K, χT = 39.6(1.0) cm3 3 K 3 mol-1, which is slightly lower than 41.6 cm3 3 K 3 mol-1 expected for 9 MnII (S = 5/2) and 6 WV (S = 1/2) assuming gMn = gW = 2.0. On decreasing temperature, χT is almost constant down to 120 K, then increases to over 1200 cm3 3 K 3 mol-1, much higher than the low temperature limit of 200 cm3 3 K 3 mol-1 expected for the isolated ferrimagnetic Mn9W6 clusters with total spin 39/2.10 At low temperatures, the susceptibility saturates around 5 K and shows a drop at 3.5 K (Figure 3a, inset). There is no difference between ZFC and FC curves measured at 10 Oe (not shown, these data overlap χ0 data), and χ00 is almost zero down to 1.8 K at f = 1 kHz. The magnetization measured at 1.8 K rapidly saturates reaching 38.8(5) Nβ at 10 kOe and remains almost constant up to 50 kOe (Figure 3b). This perfectly fits 39 Nβ expected in the magnetic structure with all W moments opposite to all Mn moments. At low magnetic field, two inflection points at H1 = 100 Oe and H2 = 480 Oe are observed on the M(H) curve, which is more visible on dM/ dH(H) dependence (Figure 3b, inset). Figure 4 presents the proposed model of magnetic interactions in 1. One average J constant representing MnII-WV exchange interactions mediated through CN- bridges is assumed over the whole {Mn9W6}n and J0 represents the average dipole-dipole interactions between neighboring nanowires arranged into the isosceles triangle pattern. The connection of Mn and W through CN- bridges leads usually to relatively strong AF exchange interaction of -10 to -40 cm-1.20 The strength of magnetic interaction between {Mn9W6}n can be approximated from the dipole-dipole magnetic interaction energy between two {Mn9W6} clusters in neighboring nanowires, J0 = ∼0.1 K (∼0.7 cm-1), estimated using the expression (gStotβ)2/d3 (assuming the cluster spin Stot = 39/2, Lande factor gav = 2, and average distance d = 2.0 nm between the central Mn4 in clusters {Mn9W6} of neighboring {Mn9W6}n skeletons). This gives the ratio J/J0 of ∼100. The negative J value is consistent with almost flat χT(T) at high temperatures (even a very shallow minimum would be expected), as well as with its increase to very high values in the range of 410 K and ∼39 Nβ saturation value at 2 K. In particular, the high susceptibility at low temperatures may be understood in terms of the strong correlation of spins along the {Mn9W6}n chain. The fact that 1 does not order above T = 5 K is probably due to the low dimensionality of the cyanido-bridged magnetic links. Because of weak dipolar interaction (J0 ) between the {Mn9W6}n,

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Figure 4. Proposed model of magnetic interactions in 1. J constant represents MnII-WV exchange interactions mediated through CNbridges (solid lines) (a) and J0 represents the dipole-dipole interactions between neighboring nanowires (dotted lines) (b).

carrying high magnetic moment 39 Nβ per {Mn9W6} length-unit, a long-range antiferromagnetic order develops only at T = 3.5 K. The maxima at 100 and 480 Oe on dM/dH are due to successive metamagnetic transitions at T = 2 K. The double metamagnetic pattern may be assigned to small differences in separation between {Mn9W6}n in the a direction and in the b direction resulting in slightly different strengths expected for the respective interactions. Lack of irreversibility confirms a long-range AF ordering and a good quality of the sample. The small magnetic anisotropy visible in the steep saturation of M(H) is consistent with a very weak orbital moment for both MnII and WV ions. The magnetic topology of {Mn9W6}n is similar to spin-ladder compounds,21 being on the border between 1D and 2D magnetic systems. However, due to its nontrivial topology (considered as a four-leg/quadruple cyclic spin-ladder with additional links and atoms of 5/2 and 1/2 Heisenberg spins, and AF interaction, Figure 4a), this discussion is only qualitative. The exposition of 1 to the air leads to the exchange of organic solvent to water, which gives the hydrated residue phases of the general formula Mn6W9(dpe)5(MeOH)x(H2O)y (Supporting Information, Table 1). The thermal ac χ0 (T) and χ00 (T) susceptibility curves for Mn6W9(dpe)5(MeOH)18(H2O)6 1a reveal the abrupt increase of signal below Tc = 35 K, which is interpreted as the onset of long-range ordering (Supporting Information, Figure S4). The curves have complex structure, which may be attributed to the occurrence of irregular reorganization within the hybrid framework leading to the closer approach and possibly the formation of additional cyanido-bridges between the neighboring {Mn9W6}n nanowires. Similar behavior was observed for the airdried sample of trimetallic {[NiII(H2O)5]6[CoIII(tmphen)2]3[FeII(CN)6]2}(ClO4)13, which revealed magnetic ordering below 22 K.9 This process is in agreement with the general tendency of the frameworks with connectivity I1O2 to compress or expand the channels upon sorption and exchange of guest molecules.7,19 To conclude, we show that crystal engineering of pentadecanuclear cyando-bridged {MnII9WV6} clusters leads to the organicinorganic I1O2 hybrid network. The construction of 1D {Mn9W6}n nanowires occurs through the formation of cyanido-bridges

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between {Mn9W6} clusters. The dimensionality of the inorganic part is established by the formation of long Mn-dpe-Mn linkages resulting in a spacial anisotropy of intercluster bonding and separation: short contact cyanido-bridging along the c direction and two different long contacts via Mn-dpe-Mn linkages in the ab plane. Magnetic measurements suggest that a relatively strong spin correlation along the cyanide bridged skeleton below 100 K leads its superparamagnetic character, while weak intermolecular dipolar interaction between {Mn9W6}n results in long-range antiferromagnetic ordering below T = 3.5 K. The exchange of organic solvent to water causes the change to the successive hydrated phases, showing the increase of ordering temperature, compared to 1. The structural organization in 1 may be of potential importance in the synthesis of the related magnetic MII-[MV(CN)8]3- (M = Co, Ni) systems with the substantial magnetic anisotropy.22 The research in this line is underway. Acknowledgment. The study has been partially supported by the EC within its NoE project MAGMANet, contract no. NMP3-CT-2005-515767 and by the Polish Ministry of Science and Higher Education within Research Projects 1535/B/H03/2009/36 and 0087-B-H03-2008-34.

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Supporting Information Available: Crystallographic information in CIF format for 1, Platon Check-cif and our comments; graphical illustration of synthetic strategy; detailed synthesis, composition evaluation and description of basic properties and of 1 (Table S1); physical techniques; crystal structure solution and refinements (Table S2), detailed bond lengths and angles (Table S3), detailed molecular structure visualization (Figures S1, S2, and S3) and example 3D view of the structure of 1; ac magnetic signals χ0 (T) and χ00 (T) for 1a. This material is available free of charge via the Internet at http://pubs.acs.org

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