Self-Assembly of Cluster-Based Nanoscopic Supramolecules into One-Dimensional Coordination Polymers

Octahedral metal clusters [Nb6Cl12(CN)6] 4−, (Mn(Salen)) (or (Mn(7-MeSalen))) (Salen = N ,N ′-ethylene-bis-(salicylidene)iminate) and ditopic organic linkers (4, 4′-bpe (trans-1, 2-bis(4-pyridyl)-ethylene) or 4, 4′-dpyo (4, 4′-dipyridyl N ,N ′-dioxide)) self-assemble to form three cluster-based 1D coordination polymers: [Mn(Salen)(MeOH)2]2{(4, 4′-dpyo) [(Mn(Salen))2(Nb6Cl12(CN)6)]} · 2MeOH (1), {(4, 4-bpe)[(Mn(7-MeSalen))2(Mn(7-MeSalen)(H2O))2(Nb6Cl12(CN)6)]}· 1.5MeCN · 8H2O (2), and [(4, 4-bpe)(Mn(Salen)(H2O))2]{(4, 4-bpe)2[(Mn(Salen)(MeOH))2(Mn(Salen))2(Nb6Cl12(CN)6)] [(Mn(Salen))2(Nb6Cl12(CN)6)]} · 16H2O (3). Single crystal X-ray diffraction analyses show that the frameworks of the three coordination polymers are built of heterotrimeric and/or heteropentameric supramolecular species linked by ditopic organic ligands. The framework of 1 consists of anionic chains built of heterotrimeric dianions [(Mn(Salen))2(Nb6Cl12(CN)6)] 2− (T) linked by 4, 4′-dpyo. The chains run along two directions ([0 2 −2] and [0 3 3]) leading to the formation of channels along the crystallographic (a) direction where the cations [Mn(Salen)(S)2] and solvent molecules are located. Also, 2 was reported earlier, it possesses a neutral 1D chain built of neutral heterpentameric supramolecules: [(Mn(7-MeSalen))2(Mn(7-MeSalen)(S))2(Nb6Cl12(CN)6)] (P) linked by 4, 4′-bpe ligands. Hydrogen bonds between nonbridging cyanide ligands and coordinated solvent molecules connect the chains into 2D hydrogen-bonded frameworks. Finally, 3 features an anionic chain, built of alternating heterotrimers [(Mn(Salen))2(Nb6Cl12(CN)6)] 2− and heteropentamers [(Mn(Salen)(S))2(Mn(Salen))2(Nb6Cl12(CN)6)] linked by the organic spacer 4, 4′-bpe. The anionic charge is compensated by the in situe-assembled [Mn(Salen)(S)(4, 4′-bpe)Mn(Salen)(S)] dimers. Magnetic measurements reveal that the Mn(III) ions are well isolated and only weak magnetic interactions are observed. The thermal stability of the three compounds was investigated.

Octahedral face-capped M 6 L i 8 L a 6 (M are metals of Groups 6 and 7; L i = halide, chalocogenide) or edge-capped M 6 L i 12 L a 6 (M are metals from Groups 4 and 5) clusters are characterized by their variety of electronic states and interesting properties arising from the metal-metal bonds [30][31][32][33].
The most prominent series of cluster compounds are the Chevrel-Sergent phases, which exhibit superconductivity at high critical magnetic fields, catalytic activity, and good characteristics as solid-state electrode materials [34][35][36][37][38].However, M 6 L i 8 or M 6 L i 12 inner core has an atom-or ion-like behavior, is stable in solution, and can be subjected to a variety of ligand substitution reactions.The cyanosubstituted hexanuclear clusters are analogous to hexacyanometallates and are being studied as building blocks for diverse structural assemblies ranging from discrete supramolecular assemblies to polymeric frameworks .
Manganese(III) complexes with tetradentate Salen-type Schiff-base ligands [Mn(Ls)] + have been widely used as functional units for the preparation of 1D magnetic coordination polymers due to the availability of two axial coordination sites at about 180 • from each other, and magnetic anisotropy of the d 4 Mn 3+ ions [71][72][73][74][75].These complexes have been used to prepare cluster-based materials in which the complex directs the assembly of metal clusters into supramolecular assemblies by acting as bridging metal ligand.Moreover, some [Mn(Ls)] + complexes are used as efficient homogeneous catalysts for the conversion of achiral olefins into chiral epoxides, thence their inclusion as building blocks of solids can potentially lead to novel heterogeneous catalysts [76].
We have previously reported that reactions between [Nb 6 Cl 12 (CN) 6 ] 4− and [Mn(L)] + complexes result in the assembly of supramolecular species in which the cluster is coordinated by 1, 2, 3, 4, or 6 metal complexes [66].As the number of metal complexes per cluster increases, the charge and size of these functionalized nanosized molecules increases (Table 1).

Single Crystal X-Ray Structure Determinations.
Intensity data for all compounds were measured at 193(2) K on a Bruker SMART APEX CCD area detector system.Data were corrected for absorption effects using the multiscan technique (SADABS).All structures were solved and refined using the Bruker SHELXTL (Version 6.1) software package.A summary of the most important crystal and structure refinement data for all compounds is given in Table 2.
For 1, the integration of the data using a triclinic cell yielded a total of 44244 reflections to a maximum θ angle of 27.50 • , of which 11634 were independent (R int = 5.21%), and 9654 (82.98%) were greater than 2σ(F 2 ).The structure was solved and refined in the space group P2 1 /n (No. 14), with Z = 2.All nonhydrogen atoms were refined anistropically.Hydrogen atoms on C atoms were generated and refined isotropically.The hydroxylic hydrogen atoms from the coordinated methanol molecules were located from electron density map while the one from the free methanol molecule could not be located.The final anisotropic fullmatrix least-squares refinement on F 2 with 637 variables converged to R 1 = 5.77% for observed data and wR 2 = 10.92% for all data.The largest residual peak on the final difference electron density map was 0.887 e − / Å3 (0.89 Å from Nb2) and the largest hole was −0.690 e − / Å3 (1.26 Å from Nb1).
For 3, the integration of the data using a triclinic cell yielded a total of 27232 reflections to a maximum θ angle of 22.0 • , of which 12018 were independent (R int = 2.80%), and 10350 (86.12%) were greater than 2σ(F 2 ).The structure was solved and refined in the space group P-1 (number 2),   with Z = 1.All nonhydrogen atoms in the framework were located from Fourier difference map while the free solvent molecules and hydrogen atoms could not be located.No attempt was made to locate hydrogen atoms on C atoms.The final anisotropic full-matrix least-square refinement on F 2 with 1217 variables converged to R 1 = 9.74% for observed data and wR 2 = 27.64% for all data.

Other Physical Measurements.
Elemental analyses were carried out by Atlantic Microlab, Inc. Thermogravimetric analyses were performed on ∼14 mg samples under a flow of argon or air (40 mL/min) at a ramp rate of 5 • C/min, using a Perkin-Elmer Pyris 1 TGA system.Infrared spectra were recorded as KBr pellets on a Mattson Infinity System FTIR spectrometer.The magnetic susceptibility data were collected using a Quantum Design MPMS SQUID mag-netometer.Approximately, 15 mg samples were packed in gelatin capsules between cotton plugs.The data obtained have been corrected for the diamagnetic contribution of the sample holder, the diamagnetism of the closed shell octahedral {Nb 6 } cluster [79] and diamagnetic contribution of all other atoms using Pascal's constants [80].
X-ray powder diffraction data was collected at room temperature using a BRUKER P4 general-purpose fourcircle X-ray diffractometer equipped with a GADDS/Hi-Star detector positioned 20 cm from the sample.The goniometer was controlled using the GADDS software suite [81].The sample was mounted on tape and data was recorded in transmission mode.The data was reduced by area integration methods to produce a single powder diffraction pattern for each frame.Individual powder diffraction patterns were merged and analyzed with the program EVA [82].π-π interaction with the anionic chain (H 5A -Ring C17-C22 = 2.68 Å and H 2A -Ring C27-C32 = 2.60 Å).

Structure of Compound 2.
Compound 2 was reported earlier by us and is given here for comparison purpose only [68].The neutral chains in 2 are built of neutral heteropentamers [(Mn(7-MeSalen)) 4 (Nb 6 Cl 12 (CN) 6 )] linked by 4,4 -bpe ligands (Figure 2).In each heteropentamer, the [Nb 6 Cl 12 (CN) 6 ] 4− cluster uses four of its six cyanide ligands to connect to four Mn complexes through cyanide bridges and the octahedral coordination environment of each Mn(III) ion is completed by one N from a 4,4 -bpe ligand, which links the heteropentamers into chains running along the [−1 1 1] direction.An intrachain offset face-to-face π-π interaction between the 7-MeSalen ligand and 4,4 -bpe with ring• • • ring distance of 3.72 Å leads to the bending of the 7-MeSalen ligand: the dihedral angle between the two benzene rings of the ligand is 42.3 • , compared with 19.6 • in 1.The location of the cyanide and the bpe ligands on the same side of the N CN -Mn-N bpe axis leads to chains with a sinusoidal wave-like structure.Hydrogen bonds between nonbridging cyanides and the aqua ligands further connect the chains into layers (Figure 2(a)) which are held together through hydrogen bonding between disordered water molecules.• , V = 4928.9(9)Å3 , and Z = 1.Data collected on three different crystals indicated complex twinning problems that could not be modeled.However, the data was sufficiently good to give the overall framework.The compound consists of 1D anionic chains built of two different nodes: anionic heterotrimers [(Mn(Salen)) 2 (Nb 6 Cl 12 (CN) 6 )] 2− and neutral heteropentamers [(Mn(Salen)) 4 (Nb 6 Cl 12 (CN) 6 )] linked to one another by 4,4 -bpe ligands (Figure 3).In the heteropentamer, the sixth coordination site of the nonbridged (Mn(Salen)) + complexes are occupied by methanol molecules.No intrachain π-π interactions between the Salen ligand and the aromatic part of the bridging ligand are observed compared to the relatively strong interactions present in 1 and 2. The in situ formed dimeric cation {[Mn(Salen)(H 2 O)] 2 (4,4 -bpe)} 2+ is used to balance the chains' negative charge.No free solvent molecules could be located from the electron density map and the presence of 16 waters of crystallization was determined based on elemental analysis and TGA data.

Thermal Stabilities.
Polycrystalline samples of each compound were used to study the thermal stability of the three compounds (Figure 4).TGA of 1 shows two distinct weight losses.The first (6.84%)corresponds to loss of all coordinated and free solvent molecules at temperature below 200 • C (cal.6.85%).The powder Xray diffraction pattern of samples obtained after heating (1) at 150 • C for 20 minutes is virtually the same as that of 1 except for the peak at 2θ = 8.300 • which shifts to 2θ = 9.087 • indicating minor structural transformations.IR spectra of the materials obtained after being heated at 150 • C for 20 minutes was found to be the same as those of the original compound indicating the conservation of all functional components.The compound continues to loose weight after 200 • C without any well-defined phase being formed indicating a continuous decomposition until 700 • C is reached.The final product obtained after 700 • C is a mixture of the oxides MnNb 2 O 6 and Mn 3 O 4 , as confirmed by PXRD (Figure 5) (%loss (obs.)= 61.02%;%loss (calc.)= 61.26%)[94,95].The overall chemical equation for this process can be written as 3.5.Magnetic Properties.Magnetic properties of 2 and 3 were measured upon warming from 5 to 300 K in an applied field of 1000 G (Figure 6).Because previous reports have shown that the niobium cluster is diamagnetic with a temperature independent paramagnetism term [96,97], it is reasonable to model the temperature-dependent paramagnetism as due solely to the Mn(III) centers.The data can be fit to a Curie-Weiss expression for each of the compounds to yield g ave = 2.03, θ = −1.3K, and g ave = 2.06, θ = −2.1 K, respectively.These values are consistent with four and eight high-spin Mn(III) centers for compounds 2 and 3.The slight downturn in the data at low temperatures is probably due to zero field splitting of the S = 2 ground state.The values obtained are comparable to those reported for 1D coordination polymer built of well-separated [Mn(Salen)] + centers [98,99].

Conclusion
In summary, trimeric (T) and pentameric (P) nanoscopic supramolecular species (∼2 nm) based on the diamagnetic 16 electron cyanochloride cluster [Nb 6 Cl 12 (CN) 6 ] 4− coordinated by 2 or 4 Mn(III) complexes have been assembled into extended frameworks via bridging ditopic organic ligands (O) through self-assembly processes.The bridging modes can be described as (-T-O-T-O-) in 1, (-P-O-P-O-) in 2, and (-T-O-P-O-T-) in 3. The compounds are chemically and thermally stable and their magnetic properties are consistent with the long distances between the manganese(III) centers.The assembly of such complex materials containing large supramolecular species linked to each other via ditopic organic ligands in a simple onepot reaction indicates the tremendous opportunities this chemistry provides.We have recently developed synthesis methodologies for the synthesis of the 14 and 15 electron {Nb 6 } and {Ta 6 } cyano-and azido-chloride clusters and we are investigating their use as building blocks to study the effect of the metal, the ligand and the electronic structure of the cluster on the properties of these materials.

Figure 1 :
Figure 1: (a) A perspective view of the structure of 1.Note that [Mn(Salen)(MeOH) 2 ] + cations are represented as space-filling mode and some cations are omitted to show the size and shape of the channel.(b) Schematic diagram showing the packing mode of the chains along [0 2 − 2] and [0 3 3] directions.(c) A projection of one anionic chain showing the linkages between the three components and π-π interaction between dpyo and the Salen ligand represented as dotted brown line.

TheFigure 2 :Figure 3 : 3 Figure 4 :Figure 5 :Figure 6 :
Figure 2: The structure of 2. (a) The packing diagram viewed along the a direction.Cl atoms and free solvent molecules are omitted for clarity.Hydrogen bond is represented as a dotted green line.(b) The 1D neutral chain, π-π interaction is represented as a dotted red line.