Supramolecular Assemblies Built of [Nb6Cl12(CN)6]4- Octahedral

Mn(OAc)3·2H2O (98%) and ethylenediamine (99%) were purchased from .... P = (max(Fo2,0) + 2Fc2)/3) with a = 0.0470, b = 6.5534 for 1; a = 0.0405, b = ...
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Supramolecular Assemblies Built of [Nb6Cl12(CN)6]4- Octahedral Metal Clusters and [Mn(acacen)]+ Complexes Huajun Zhou and Abdessadek Lachgar* Department of Chemistry, Wake Forest UniVersity, Winston-Salem, North Carolina 27109

CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 10 2384-2391

ReceiVed June 16, 2006; ReVised Manuscript ReceiVed July 11, 2006

ABSTRACT: Reactions between solutions of [Me4N]4[Nb6Cl12(CN)6]‚2MeOH and [Mn(L)]Cl (L ) acacen2- ) bis(acetylacetonato)ethylenediamine at room temperature led to the formation of four compounds containing supramolecular assemblies formed of [Nb6Cl12(CN)6]4- and [Mn(L)]+ as building units. The four compounds were characterized by single-crystal X-ray diffraction, IR, elemental analysis, thermogravimetric analysis, and magnetic susceptibility measurements (for 3). In 1, each cluster is coordinated by one [Mn(L)(MeOH)]+ via a CN- ligand to give an anionic dimeric unit {[Mn(L)(MeOH)][Nb6Cl12(CN)6]}3-, which connect to each other via hydrogen bonding between the CN- ligand and MeOH from the cations [Mn(L)(MeOH)2]+ to give anionic tubularlike chains. The structure of 2 comprises trimeric units {[Mn(L)(H2O)]2[Nb6Cl12(CN)6]}2- in which each cluster is trans-coordinated by two [Mn(L)(H2O)]+ cations via the CN- ligand. The trimeric units are connected to each other via hydrogen bonding between CN- and the water of coordination to give anionic chains along the crystallographic a axis. The chains are connected to each other through further hydrogen bonding to give an overall three-dimensional hydrogen-bonded framework. In 3, each cluster is coordinated by two [Mn(L)(MeCN)]+ and two [Mn(L)(H2O)]+ via CN- ligand to give neutral pentameric units that are connected through hydrogen bonding between CN- and aqua ligands to give hydrogen-bonded chains along the crystallographic b axis. 4 is based on two supramolecular ions; the cation consists of a heptameric unit {[Mn(L)(H2O)]4[Mn(L)]2[Nb6Cl12(CN)6]}2+ in which each cluster is coordinated by six [Mn(L)]+ via CN- ligand, whereas the anion {[Mn(L)]2[Nb6Cl12(CN)6]}2- is the same as that found in 2. Electrostatic interactions and hydrogen bonding between these two supramolecular species afford a 1D framework. Magnetic susceptibility shows that 3 is paramagnetic with four high-spin Mn(III) ions. Thermal behaviors of 1-4 are presented. Introduction Advances in supramolecular chemistry have had a significant impact on chemists’ ability to prepare chemical species with different sizes and different structural dimensions that range from discrete supramolecular species to polymeric materials with one-, two-, and three-dimensional frameworks. These materials are accessible through spontaneous secondary interactions such as coordination and hydrogen bonding, dipole-dipole, charge transfer, van der Waals, and π-π stacking interactions.1 The development of the “bottom up” approach to the preparation of chemical species with different sizes and different dimensions was inspired primarily by Nature’s approach to macromolecular assemblies, which leads to the formation of a fascinating variety of complex nanostructures assembled from two or more molecular components by means of secondary interactions. Octahedral metal clusters consist of an M6 octahedron with metal-metal bonds supported by either µ2 or µ3 ligands to form either M6L12 or M6L8 cluster cores, depending on the identity of the metal or ligands. Each metal has an axial (terminal) coordination site that is occupied by an additional ligand. The unit can thus be considered to be a large octahedral chemical species. Different types of octahedral metal clusters can be excised from their solid-state precursors and can undergo axial ligands substitution to form a variety of soluble molecular species.2 Their use as building units of polymeric assemblies is rationalized by the fact that their large size (g 1 nm), availability of multiple coordination sites, and electronic flexibility could potentially lead to novel materials with different topologies, thus different properties.3 The presence of metal-metal bonds within the clusters is of particular interest, because the tunable valence electrons per cluster (VEC) may lead to properties that are unlikely to be found in materials based on mononuclear species.4 * To whom correspondence should be addressed. Fax: 336-758-4656. E-mail: [email protected].

One of the methodologies that had been widely used to prepare coordination polymers and metal-organic frameworks is based on the use of metal complexes with apical positions occupied by ditopic ligands such as cyanides that can form polymeric materials through coordination bonds. A number of materials with extended frameworks and interesting functional properties have been successfully prepared through the use of hexacyanometalates [M(CN)6]n- (M ) transition metal) as building units.5 Octahedral metal cluster analogues of [M(CN)6]n-, such as [Re6Q8(CN)6]3-/4- (Q ) S, Se, Te) and [W6S8(CN)6]6-, have been prepared and used to prepare polymeric materials with fascinating structural features.6,7 For example, Kim et al. have recently prepared 2D and 3D frameworks using the manganese Schiff-base complex [Mn(salen)]+ and the cluster [Re6Q8(CN)6]4- (Q ) Se or Te) and have demonstrated that the framework dimension can be controlled by changing the inner chalcogen ligand.8 Octahedral niobium chloride clusters differ from octahedral hexarhenium and hexatungsten clusters in terms of the coordination mode of inner ligands and valence electron count (VEC).9 Recently, the cluster [Nb6Cl12(CN)6]4- has been isolated and its reactions with free metal ions or pre-assembled metal complexes led to formation of polymeric materials with 1D, 2D, and 3D frameworks.10-12 The structural features of the products seem to be imposed by the geometrical and coordination requirements of the metal complexes. We have previously shown that different framework topologies can be obtained from reactions between [Nb6Cl12(CN)6]4- and [Mn(L)] (L ) salen).12b [Mn(L)] was chosen as the building unit because of the availability of two axial coordination sites and the catalytic properties of [Mn(L)]+ derivatives; the derivatives, referred to as Jacobsen catalysts, are used in asymmetric epoxidation, in which achiral olefins are converted to chiral epoxides with enantiomeric excesses exceeding 98%.13 Even though [Mn(acacen)]+ and [Mn(salen)]+ are structurally similar, previous

10.1021/cg0603669 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/19/2006

Cluster-Based Supramolecular Assemblies

work has shown that they react with the same building units to afford materials with different structural features. For example, [Mn(salen)(H2O)](ClO4) reacts with [Et4N]3[Fe(CN)6] to afford 2D coordination polymer [Et4N]{[Mn(salen)]2Fe(CN)6},14 whereas the reaction between [Mn(acacen)]Cl and [Et4N]3[Fe(CN)6] results in the formation of a 1D coordination polymer [Et4N]2{[Mn(acacen)]Fe(CN)6}.15 Here, we show that appropriate choice of solvent system and reactants ratios leads to the preparation of discrete supramolecular units built of [Mn(acacen)]+ and the 16-electron [Nb6Cl12(CN)6]4- clusters linked via cyanide ligands. The solvent coordination ability seems to play a crucial role in blocking the polymerization process. The four supramolecular assemblies described here can be classified on the basis of the number of [Mn(L)]+ complexes attached to the cluster unit. Each assembly is built of one cluster unit coordinated by 1, 2, 4, or 6 Mn complexes to give di-, tri-, penta-, and heptameric units that interact further through hydrogen bonding to form extended hydrogen-bonded frameworks. Experimental Section General. [Me4N]4[Nb6Cl12(CN)6]‚2MeOH,12b the Schiff base H2L,16 and [Mn(L)]Cl17 (L ) acacen2-) were prepared following published literature procedures. The reaction between [Me4N]3[Nb6Cl18] and KCN in MeCN/H2O followed by recrystallization from MeOH/ether gave [Me4N]4[Nb6Cl12(CN)6]‚2MeOH. The Schiff-base ligand H2(acacen) was prepared through the condensation reaction between ethylenediamine and two equivalent amounts of acetylacetone.The reaction between Mn(OAc)3‚2H2O and excess H2(acacen) in MeOH followed by extraction and recystallization gave [Mn(acacen)]Cl. LiCl (99%), NbCl5(99%, metal basis), Nb powder (99.8%, metal basis), KCN (96%), and Me4NCl (98+%) were purchased from Alfa Aesar. Mn(OAc)3‚ 2H2O (98%) and ethylenediamine (99%) were purchased from ACROS Organics. 2,4-Pentanedione (99%) was purchased from Aldrich, and NaCl was purchased from Fisher. All chemicals and solvents were used as received. Synthesis. [Me4N][Mn(L)(MeOH)2]2{[Mn(L)(MeOH)][Nb6Cl12(CN)6]}‚MeOH (1): [Mn(L)]Cl (0.075 g, 0.24 mmol) was dissolved in methanol and diluted to give 10.0 mL of a 24.0 mM methanolic solution of [Mn(L)]Cl. [Me4N]4[Nb6Cl12(CN)6]‚2MeOH (0.09 g, 0.06 mmol) was dissolved in methanol and diluted to give 10.0 mL of a 6.0mM methanolic solution of [Me4N]4[Nb6Cl12(CN)6]‚2MeOH. A methanolic solution of [Mn(L)]Cl (1.5 mL, 24.0 mM) was layered with 2.0 mL of a 6.0 mM methanolic solution of [Me4N]4[Nb6Cl12(CN)6]‚ 2MeOH in a narrow-diameter silica tube (i.d. ) 7 mm, l ) 12 cm). Dark green block-shaped crystals grew at the interface of the two solutions within 2 days. The crystals were collected by filtration, washed with methanol, and dried in air to give 13.6 mg of product (yield: 50.5%). Anal. Calcd for C52H90Cl12Mn3N13Nb6O12: C, 27.92; H, 4.06; N, 8.14. Found: C, 27.81; H, 4.09; N, 8.23. IR (KBr): νCN 2134 cm-1. [Me4N]2{[Mn(L)(H2O)]2[Nb6Cl12(CN)6]}‚2MeCN (2): [Me4N]4[Nb6Cl12(CN)6]‚2MeOH (0.075 g, 0.050 mmol) was dissolved in acetonitrile and diluted to give 25.0 mL of a 2.0 mM solution of [Me4N]4[Nb6Cl12(CN)6]‚2MeOH in acetonitrile. [Mn(L)]Cl (0.047 g, 0.15 mmol) was dissolved in acetonitrile and diluted to give 25.0 mL of a 6.0 mM solution of [Mn(L)]Cl in acetonitrile. These two solutions were used as stock solutions for the preparation of compounds 2 and 3. To 6.0 mL of a 2.0mM solution of [Me4N]4[Nb6Cl12(CN)6]‚2MeOH in acetonitrile was added 4.0 mL of a 6.0mM solution of [Mn(L)]Cl in acetonitrile. After being stirred for 20 min, the resulting solution was filtered, and the filtrate was let to evaporate. Dark green elongated platelike crystals were obtained after 2 days. The crystals were collected by filtration, washed with acetonitrile, and dried in air to give 10.7 mg of product (yield: 45.4%). Anal. Calcd for C42H70Cl12Mn2N14Nb6O6: C, 25.74; H, 3.60; N, 10.01. Found: C, 25.64; H, 3.53; N, 9.87. IR (KBr): νCN 2126 cm-1. {[Mn(L)(MeCN)]2[Mn(L)(H2O)]2[Nb6Cl12(CN)6]}‚2MeCN (3): A solution of [Me4N]4[Nb6Cl12(CN)6]‚2MeOH (5.0 mL, 2.0 mM) in acetonitrile was added to 10.0 mL of a 6.0 mM solution of [Mn(L)]Cl in acetonitrile. After being stirred for 20 min, the solution was filtered and the filtrate was left to evaporate. Dark green elongated plates formed

Crystal Growth & Design, Vol. 6, No. 10, 2006 2385 after 2 days. The crystals were collected by filtration, washed with acetonitrile, and dried in air to give 13.7 mg of product (yield: 56.0%). Anal. Calcd for C62H88Cl12Mn4N14Nb6O10: C, 31.13; H, 3.71; N, 8.20. Found: C, 31.33; H, 3.60; N, 8.63. IR (KBr): νCN 2142 cm-1. {[Mn(L)(H2O)]4[Mn(L)]2[Nb6Cl12(CN)6]}{[Mn(L)]2[Nb6Cl12(CN)6]}‚ 4.8H2O (4): To a solution of [Me4N]4[Nb6Cl12(CN)6]‚2MeOH (0.0300 g, 0.02 mmol) in 5.0 mL of water was added a solution of [Mn(L)]Cl (0.0125 g, 0.04 mmol) in 10.0 mL of methanol. The resulting solution was left to evaporate in the hood. Dark green rhombohedral plates crystallized overnight and were collected by filtration, washed with methanol, and dried in air to give 13.9 mg of the product (yield: 59.7%). Anal. Calcd for C54H80.80Cl12Mn4N14Nb6O12.40: C, 27.92; H, 3.50. Found: C, 27.98; H, 3.29. IR (KBr): νCN 2140, 2128 cm-1. X-ray Structure Determination. Single crystals of 1-4 were selected and attached with paratone oil to quartz fibers for single-crystal X-ray analysis. Intensity data were measured at 193(2) K (293(2) K for 3) on a Bruker SMART APEX CCD area detector system. Data were corrected for absorption effects using a multiscan technique (SADABS).18 The structures were solved and refined using Bruker’s SHELXTL (version 6.1) software package.19 During the refinement of all these structures, thermal parameters for all non-hydrogen atoms were refined anisotropically except for: the C atoms from disordered cations (C48-C51) and C44 from one ethylene bridge in compound 1; C31 atom from the coordinating MeCN in compound 3. In all compounds most hydrogen atoms were located from the electron density maps, and refined isotropically. Crystal and refinement data are summarized in Table 1. Other Physical Measurements. Thermogravimetric analyses of compound 1-4 (all sample weights are between 15 and 20 mg) were performed under a flow of Ar or air (40 mL/min) at a heating rate of 5 °C/min, using a Perkin-Elmer Pyris 1 TGA system. Infrared spectra were recorded on a Mattson Infinity System FTIR spectrometer. X-ray powder diffraction data were collected at room temperature using a BRUKER P4 general-purpose four-circle X-ray diffractometer modified with a GADDS/Hi-Star detector positioned 20 cm from the sample. Magnetic susceptibility data for compound 3 were measured using a Quantum Design MPMS XL SQUID magnetometer. Loose crystals of 3 (18.5 mg) were put into gel capsules suspended in a plastic straw for immersion into the SQUID. Magnetization of the filled sample holder was measured in the temperature range 2-300 K at an applied field of 1 kG.

Results and Discussion Synthesis. Reactions between [Mn(L)]Cl and [Nb6Cl12(CN)6]4cluster units led to the formation of supramolecular ions with different structural features and different charges when different solvent system and reactants ratios were used. [Me4N]4[Nb6Cl12(CN)6]‚2MeOH was chosen as reactant because of its ease of preparation and proper solubility in MeOH, MeCN, and water. Compound 1 was obtained when the concentration ratio complex/cluster was between 3.0 and 6.0. 1 was also obtained when [Mn(L)]Cl was dissolved in either EtOH or n-PrOH and the resulting solutions were layered with methanolic solution of [Me4N]4[Nb6Cl12(CN)6]‚2MeOH. However, when solutions of the two species were mixed directly and stirred, green precipitate formed gradually over a period of 2 h. PXRD and IR showed the precipitate to be the previously reported Prussianblue analogue [Me4N]2[MnNb6Cl12(CN)6].11 Compounds 2 and 3 were prepared using acetonitrile as solvent. Reactants ratios affected the outcome of the reactions. When the complex/cluster ratio was between 1.0 and 2.0, only compound 2 formed, and for ratios greater than 6.0, 3 was the only compound to be isolated. Both 2 and 3 formed in almost equal amounts when the ratio was between 3.0 and 5.0. The presence of the aqua ligand in 2 and 3 can be explained by the presence of trace amount of H2O in acetonitrile. A small amount of H2O has been shown to play an important role in the formation of crystalline products.20 Crystal Structures. Compounds 1-4 have been characterized by single-crystal X-ray diffraction. In 1-3, each cluster is

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Table 1. Crystal Data for [Me4N][Mn(L)(MeOH)2]2{[Mn(L)(MeOH)][Nb6Cl12(CN)6]}‚MeOH (1), [Me4N]2{[Mn(L)(H2O)]2[Nb6Cl12(CN)6]}‚2MeCN (2), {[Mn(L)(MeCN)]2[Mn(L)(H2O)]2[Nb6Cl12(CN)6]}‚2MeCN (3), and {[Mn(L)(H2O)]4[Mn(L)]2[Nb6Cl12(CN)6]}{[Mn(L)]2[Nb6Cl12(CN)6]}‚4.8H2O (4) formula fw (g/mol) T (K) cryst syst space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Fcalcd (g cm-3) µ (mm-1) λ (Å) R1a, wR2b,c (%) GOF

1

2

3

4

C52H90Cl12Mn3N13Nb6O12 2237.05 193(2) monoclinic P21/n (No. 14) 16.548(1) 23.045(2) 21.770(2)

C42H70Cl12Mn2N14Nb6O6 1959.86 193(2) monoclinic P21/n (No. 14) 13.688(1) 13.234(1) 19.863(1)

C62H88Cl12Mn4N18Nb6O10 2448.12 293(2) K monoclinic P21/n (No. 14) 12.870(4) 14.119(4) 24.792(7)

96.904(1)

91.605(1)

91.402(5)

8242(1) 4 1.803 1.693 0.71073 5.54, 12.55 1.021

3596.6(4) 2 1.810 1.755 0.71073 3.96, 8.86 0.969

4504(2) 2 1.764 1.686 0.71073 4.82, 12.22 1.071

C54H80.80Cl12Mn4N14Nb6O12.40 2320.60 193(2) triclinic P1h (No.2) 14.741(5) 16.175(5) 18.969(6) 80.858(5) 75.619(5) 74.272(5) 4197(2) 2 1.842 1.808 0.71073 3.41, 8.81 1.020

a R ) ∑||F | - ||F ||/∑|F |. b wR ) [∑[w(F 2 - F 2)2]/∑[(wF 2)2]]1/2. c w-1 ) σ2(F 2) + (aP)2 + bP. P ) (max(F 2,0) + 2F 2)/3) with a ) 0.0470, 1 o c o 2 o c o o o c b ) 6.5534 for 1; a ) 0.0405, b ) 0 for 2; a ) 0.0426, b ) 8.5251 for 3; and a ) 0.0426, b ) 2.5349 for 4.

Table 2. Selected Average Bond Lengths (Å) and Angles (deg) for Compounds 1-4 1

2

3

4

Nb-Nb 2.932(5) 2.928(4) 2.931(6) 2.930(7) Nb-Cli 2.466(6) 2.467(6) 2.464(6) 2.465(4) Nb-C 2.279(7) 2.278(9) 2.286(6) 2.282(8) CtN 1.140(6) 1.146(2) 1.147(2) 1.142(3) Nb-CtN 175.7(13) 177.3(19) 176.7(18) 177(2) Mn-NCN 2.280(5) 2.263(4) 2.26(2) 2.22(8) MntNCN-C 150.3(4) 157.6(3) 165.8(3), 148.6(3) 157(5) NCN-Mn-NCN 172.7(1)

coordinated by one, two, and four solvated Mn complexes, respectively, to give dimeric, trimeric, and pentameric assemblies. In 4, two supramolecular ions are present: a heptameric dication in which each cluster is coordinated by six Mn complexes, and a trimeric dianion in which each cluster is coordinated by two Mn complexes. In all compounds, these supramolecular assemblies are linked to each other via hydrogen bonding to form extended networks. The [Nb6Cl12(CN)6]4- clusters in 1-4 are the same as those found in the precursor [Me4N]4[Nb6Cl12(CN)6]‚2MeOH and other previously reported extended frameworks built of [Nb6Cl12(CN)6]4-, indicating that the cluster core (Nb6Cl12)2+ maintains its geometry and charge. The average Nb-Nb, NbCl, and Nb-C bond lengths are the same as those found in other niobium chlorides and cyanochlorides with VEC ) 16 (Table 2). [Me4N][Mn(L)(MeOH)2]2{[Mn(L)(MeOH)][Nb6Cl12(CN)6]}‚ MeOH (1): The crystal structure of 1 consists of “tubular-like” chains built of {[Mn(L)(MeOH)][Nb6Cl12(CN)6]}3- dimers and [Mn(L)(MeOH)2]2+ metal complexes that interact through hydrogen bonding between the CN- ligand and the coordinated MeOH (Figure 1a). Each cluster is coordinated by one [Mn(L)(MeOH)]+ through a CN- ligand to give the dimeric unit {[Mn(L)(MeOH)][Nb6Cl12(CN)6]}3- (Figure 1b). The Mn-NCN bond length is 2.280(5) Å and is within the range of those found in [Me4N]2{[Mn(salen)]2[Nb6Cl12(CN)6]} (2.233(4) and 2.365(3) Å). The average bond angles Nb-CtN (175.7(1)°) and MntNCN-C (150.3(4)°) are similar to those found in [Me4N]2{[Mn(salen)]2[Nb6Cl12(CN)6]}. The dimeric units are connected to each other through relatively strong hydrogen bonding between the CN- ligand and the coordinated MeOH (OMeOH-N5 ) 2.725(6) Å, ∠O-H‚‚‚N5 ) 162(8)°). The dimers

are further connected by two solvated metal complexes [Mn(L)(MeOH)2]+ (Figure 1c) through hydrogen bonding between CN- ligand and the coordinated MeOH (OMeOH-N1 ) 2.718(7) Å, ∠O-H‚‚‚N1 ) 167.9°; OMeOH-N2 ) 2.730(7) Å, ∠O-H‚‚‚N2 ) 168.8°; OMeOH-N3 ) 2.630(6) Å, ∠O-H‚‚‚ N3 ) 178.7°; OMeOH-N6 ) 2.708(7) Å, ∠O-H‚‚‚N6 ) 150(11)°). The extensive and relatively strong hydrogen bonding leads to the formation of hybrid tubular-like chains (Figure 1d). The chains lie on top of each other when viewed along the crystallographic a axis to form layers within which the chains are parallel to each other and separated by both MeOH and [Me4N]+ countercations. The layers are related to each other by an inversion center (Figure 1a). Selected bond distances and angles are listed in Table 2. Dimeric units of this type are rare; only one example had been reported in the reaction of [Mn(saldmen)] and [Fe(CN)6]3- to form the compound [Et4N]2{[Mn(saldmen)(H2O)][Fe(CN)6]}, in which each [Fe(CN)6]3is coordinated by one [Mn(saldmen)(H2O)].21 [Me4N]2{[Mn(L)(H2O)]2[Nb6Cl12(CN)6]}‚2MeCN (2): Compound 2 features a 3D hydrogen-bonded framework (Figure 2a). Each cluster is trans-coordinated by two [Mn(L)(H2O)]+ complexes through a cyanide ligand to form anionic trimeric unit {[Mn(L)(H2O)]2[Nb6Cl12(CN)6]}2- (Figure 2b). The trimeric units are connected to each other through hydrogen bonding between the aqua ligand and N2 (OH2O-N2 ) 2.788(5) Å, ∠OH‚‚‚N2 ) 172(5)°) to afford chains along the crystallographic a axis (Figure 2c). Each chain is connected to four neighboring chains through hydrogen bonding between the coordinated H2O and N1 (OH2O-N1 ) 3.050(5) Å, ∠O-H‚‚‚N1 ) 169(6)°) to give an overall 3D framework, affording channels along the crystallographic a axis (Figure 2d). The cations [Me4N]+ and crystallization solvent MeCN are located between the chains. The mean bond angles Nb-CtN (177.3(19)°) and MntNCN-C (157.6(3)°) are close to those found in the previously reported [Et4N]2{[Mn(salen)(MeOH)]2[Nb6Cl12(CN)6]}‚2MeOH. The bond length of Mn-NCN (2.263(4) Å) is slightly smaller than that in 1 (2.280(5) Å) and [Et4N]2{[Mn(salen)(MeOH)]2[Nb6Cl12(CN)6]}‚ 2MeOH (2.295(3) Å), but much smaller than that found in [Me4N]2{[Mn(salen)]2[Nb6Cl12(CN)6]} (2.365(3) Å). Selected bond distances and angles are listed in Table 2. Compared to the rarely observed dimeric unit found in compound 1, trimeric units similar to that observed in 2 have been observed in compounds containing Mn Schiff-base com-

Cluster-Based Supramolecular Assemblies

Crystal Growth & Design, Vol. 6, No. 10, 2006 2387

Figure 1. (a) Perspective view of the structure of 1 along the crystallographic a axis; [Me4N]+, Cl ligands, and crystallization solvent are omitted for clarity. (b) The dimeric anion {[Mn(L)(MeOH)][Nb6Cl12(CN)6]}3- in which each cluster is coordinated by one [Mn(L)(MeOH)]+. (c) The [Mn(L)(MeOH)2]+ cations in which each Mn ion is additionally coordinated by two MeOH. (d) The anionic tubular-like chain built from extensive hydrogen bonding. The dashed lines represent hydrogen bonding. Manganese octahedra (MnN3O3), green; manganese octahedra (MnN2O4), yellow.

plexes and [M(CN)6]3- (M ) Fe, Cr), as is the case in [Et4N]{[Mn(5-Cl-salen)(H2O)]2[Fe(CN)6]}‚H2O, in which each [Fe(CN)6]3- is coordinated by two solvated Mn complexes to give the anionic trimeric unit {[Mn(5-Cl-salen)(H2O)]2[Fe(CN)6]}-.22,23 {[Mn(L)(MeCN)]2[Mn(L)(H2O)]2[Nb6Cl12(CN)6]}‚2Me-

CN (3): In 3, each cluster is coordinated by two [Mn(L)(MeCN)]+ and two [Mn(L)(H2O)]+ to give neutral pentameric assemblies {[Mn(L)(MeCN)]2[Mn(L)(H2O)]2[Nb6Cl12(CN)6]} (Figure 3b) that link to each other via hydrogen bonding between the cyanide and the aqua ligand (OH2O-N1 ) 2.789 Å, ∠O-

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Figure 2. (a) Perspective view of the 3D structures of compound 2 along the crystallographic a axis. (b) The anionic trimeric unit {[Mn(L)(H2O)]2[Nb6Cl12(CN)6]}2- in which each cluster is trans-coordinated by two [Mn(L)(H2O)]+. (c) Chain extending along the crystallographic a axis built from the anionic trimeric units. (d) Projection along the crystallographic a axis, showing how each chain connects to four neighboring chains through hydrogen bonding. In (a) and (d), the [Me4N]+ cations and crystallization solvent are omitted for clarity.

H‚‚‚N1 ) 170.76°) to form chains along the crystallographic b axis (Figure 3a). The chains are further stabilized through hydrogen bonding between the MeCN solvent of crystallization located between the pentameric units and the aqua ligand (OH2O-N8 ) 2.924 Å, ∠O-H‚‚‚N1 ) 173.77°). The chains are similar to those found in 2 except that in compound 3, the two extra [Mn(L)(MeCN)]+ lead to neutral chains. Significantly different CtN-Mn bond angles are observed depending on the nature of the coordinating solvent attached to Mn. The Ct N-Mn bond angles are 165.8(3) and 148.6(3)° for [Mn(L)(H2O)]+ and [Mn(L)(MeCN)]+, respectively. Similar pentameric units were observed in compounds containing [Fe(CN)6]3- ion, as is the case in {[Mn(saltmen)(H2O)]4[Fe(CN)6]}(ClO4), which contains the pentameric cation{[Mn(saltmen)(H2O)]4[Fe(CN)6]}+, where [Fe(CN)6]3- is coordinated by four {[Mn(saltmen)(H2O)]}+.21 {[Mn(L)(H2O)]4[Mn(L)]2[Nb6Cl12(CN)6]}{[Mn(L)]2[Nb6Cl12(CN)6]}‚4.8H2O (4): The structure of 4 consists of heptameric cations {[Mn(L)(H2O)]4[Mn(L)]2[Nb6Cl12(CN)6]}2+ and trimeric anions {[Mn(L)]2[Nb6Cl12(CN)6]}2-, held together through charge-assisted hydrogen bonding to form a 1D hydrogenbonded framework (Figure 4a). The cation assembly consists of a cluster anion [Nb6Cl12(CN)6]4- coordinated by six Mn complexes: two are five-coordinated [Mn(L)]+ located trans to each other, and the other four are solvated six-coordinated [Mn(L)(H2O)]+ (Figure 4b). In the anion {[Mn(L)]2[Nb6Cl12-

(CN)6]}2-, each cluster anion [Nb6Cl12(CN)6]4- is coordinated by two five-coordinated [Mn(L)]+ located in the trans position (Figure 4c). Hydrogen bonding between the cyanide ligand and coordinated H2O molecules from the cations leads to the formation of chains. (OH2O-N2 ) 2.863(4) Å, ∠O-H‚‚‚N2 ) 170.3(2)°) (Figure 4d). The chains stack on top of each other along the crystallographic b axis and are held together by water of crystallization located between the chains. The framework is further stabilized by the hydrogen bonding between the coordinated H2O and water of crystallization. (OH2O-O11 ) 2.750(5) Å, ∠O-H‚‚‚O11 ) 150.7(2)°; OH2O-O12 ) 2.874(8) Å, ∠O-H‚‚‚O12 ) 137(3)°). The solvent coordination effect is significant and leads to significant lengthening of the average Mn-NCN bond length, found to be 2.287(3) Å in the sixcoordinated complex compared to 2.150(3) Å in the fivecoordinated complex. Similar values are found in [Mn(salen)]4[Re6Te8(CN)6]8a in which the Mn-NCN bond length for the sixcoordinated bridging Mn complex is 2.27(1) Å compared to 2.14(1) Å in the five-coordinated complex. Similar cationicanionic assemblies have been observed in compounds containing hexacyanometalates [M(CN)6]3- (M ) Fe3+, Cr3+) as building units.24 Magnetic Properties. The temperature-dependent magnetic susceptibility of compound 3 was measured in the range 2-300 K. Data were corrected for both diamagnetic contribution of six Nb atoms and temperature-independent paramagnetic (TIP)

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Figure 3. (a) Perspective view of one unit cell along the crystallographic b axis, showing the packing mode of the pentameric units; Cl ligands and crystallization solvent are omitted for clarity. (b) The neutral pentameric supramolecular assembly {[Mn(L)(MeCN)]2[Mn(L)(H2O)]2[Nb6Cl12(CN)6]} in which each cluster is coordinated by two [Mn(L)(MeCN)]+ and two [Mn(L)(H2O)]+. (c) Chain built through hydrogen bonding between pentameric units. (MnN3O3) octahedra, green; (MnN4O2) octahedra, purple.

contribution from the cluster according to the published literature.25 Data were further corrected for diamagnetic contribution of all ligands and manganese ions using Pascal’s constants.26 The overall correction (including the holder) was calculated to be χ0 ) 1.81×10-4 emu mol-1. The magnetic susceptibility data was fitted using the Curie-Weiss law, χ(T) ) χ0 + C/(T - θ), with C ) 3.15 emu K mol-1, θ ) 0.711 K. The effective magnetic moment per Mn3+ at 300 K was found to be 5.01 µB, close to the calculated value for spinonly effective moment for high-spin d4 Mn(III) ions (4.90 µB). No evidence of magnetic ordering was observed in temperatures down to 2 K. Thermal Stability. The stability of 1-4 was investigated by thermogravimetric analysis. Three distinct weight losses were observed for 1, whereas two distinct weight losses were observed for 2-4. The first step involved the loss of noncoordinating and coordinated solvents at temperatures between 100 and 200 °C. IR analysis of the materials obtained at this stage showed bands characteristic of the cyanide and Schiff-base ligands. Powder X-ray diffraction of the product obtained upon heating at temperatures above 800 °C indicated that 1, 3, and 4 decompose to form MnNb2O6,27 whereas 2 decomposes to form

a mixture of MnNb2O6 and Nb2O5.28 The observed overall losses for 1-4 were 50.62, 47.74, 53.94, and 52.21%, respectively, close to the calculated values of 54.84, 52.07, 58.73, and 56.46%. Conclusion Octahedral niobium cyanochloride cluster [Nb6Cl12(CN)6]4and [Mn(acacen)]+ metal complex have been successfully employed in the preparation of supramolecular assemblies built of cluster units attached to different number of metal complexes via the CN ligand. These assemblies range in size from 1.5 to 2.4 nm, and their charge varies between -3 and +2 depending on the number of Mn complexes per cluster. These cluster-based supramolecules can, to a certain extent, be considered as being the product of cation metathesis reactions in which the noncoordinating [Me4N]+ is substituted by the coordinating complex [Mn(acacen)]+, as illustrated by eq 1 for n ) 1, 2, and 4.

[Me4N]4[Nb6Cl12(CN)6] + n[Mn(L)]+ f [Me4N]4-n{[Mn(L)]n[Nb6Cl12(CN)6]} + n[Me4N]+ (1)

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Figure 4. (a) Perspective view of the overall structure of 4 along the crystallographic b axis; Cl ligand and all crystallization solvent molecules are omitted for clarity. (b) The cation {[Mn(L)(H2O)]4[Mn(L)]2[Nb6Cl12(CN)6]}2+ in which each cluster is coordinated by four [Mn(L)(H2O)] octahedra and two [Mn(L)] square pyramids. (c) The trimeric anion {[Mn(L)]2[Nb6Cl12(CN)6]}2- in which each cluster is coordinated by two [Mn(L)]. (d) Chains built from hydrogen bonding between cations and anions. All atoms, bonds, and polyhedra are represented the same as above.

Among the issues to be investigated is the stability of these supramolecular assemblies in solution, which will allow the study of their electrochemical properties and ultimately their use as larger building units for more complex polymeric materials. Theoretically, assemblies with three and five metal complexes per cluster, and different conformations such as cis and trans in the case of assemblies with two or four metal complexes per cluster, should be accessible. The linkage of these supramolecules into extended frameworks has been achieved and will be published elsewhere. Systematic studies using different metal complexes, cluster units, and solvents are needed to advance our understanding of these systems. Furthermore, the preparation of these supramolecular assemblies with magnetically active clusters could potentially lead to important advances in molecular magnet chemistry. We have isolated and characterized the cluster [Me4N]3[Nb6Cl12(CN)6], which is

magnetically active with VEC ) 15, and its reactivity with metal ions and metal complexes is being investigated. Acknowledgment. This material is based upon work supported by the National Science Foundation under Grants DMR0446763 and 0234489. We thank Dr. Sam Mugavero and Prof. Hans-Conrad zur Loye from the University of South Carolina for their assistance with magnetic data measurements. Supporting Information Available: IR spectra of [Mn(acacen)Cl] and 1-4 (including IR spectra of materials obtained after the loss of solvent). TGA of 1-4. PXRD of materials after TGA (1-4). The µ and 1/χ versus T plot of compound 3. X-ray crystallographic files in CIF format of the four compounds reported. This material is available free of charge via the Internet at http://pubs.acs.org.

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