Supramolecular Complexes Built of Octahedral [Ta6Cl12(CN)6]3−/4− Clusters and Terpyridine-Metal Complexes

The preparation and characterization of two supramolecular complexes from octahedral [Ta6Cl12(CN)6]4−/3− units and 2, 2′: 6′, 2″-terpyridine (Terpy) metal complexes as building blocks are reported. Single crystal analyses revealed [Mn(Terpy)]2[Ta6Cl12(CN)6]·MeOH (1) features a neutral two-dimensional (2D) framework composed of layers based on [Ta6Cl12(CN)6]4− and [Mn(Terpy)]2+. Each layer is built of 6-connected [Ta6Cl12]2+ and 3-connected Mn(II) nodes bridged by cyanide ligands. The layers are held together by MeOH molecules. The layered structure is maintained after the removal of solvent molecule (MeOH) upon heating, leading to the formation of [Mn(Terpy]2[Ta6Cl12(CN)6] (1′). In the case of using Gd3+, the resultant product was revealed by single-crystal analyses to be an ionic compound [Gd(Terpy)(H2O)4(DMF)2][Ta6Cl12(CN)6]⋅DMF ⋅3H2O (2). 2 has a three-dimensional (3D) framework built of [Gd(Terpy)(H2O)4(DMF)2]3+ and [Ta6Cl12(CN)6]3− ions connected to each other via extensive hydrogen bonds and π-π interactions between the cations, anions, and solvent molecules. Thermal stabilities of 1‒2 are reported. The X-ray structures and thermal stabilities of a coordination polymer [Mn(Terpy)]2[Ta6Cl12(CN)6] ⋅MeOH and an ionic compound [Gd(Terpy)(H2O)4(DMF)2][Ta6Cl12(CN)6]⋅DMF⋅3H2O are presented.


Introduction
Rational design and synthesis of functional supramolecular compounds built of multiple molecular building blocks through the "bottom up" approach have received intensive interest during the past few decades [1,2]. While the building blocks can contribute to the formation of versatile structures, the supramolecular compounds thus obtained can not only inherit properties from the building blocks but also bear new and/or improved properties which come from the electronic interactions between building blocks. These properties may lead to various potential applications such as catalysis, [3] adsorption and separation [4]. Among supramolecular compounds, metal-organic frameworks (MOFs) made of polydentate ligands and metal ions/complexes have been the most extensively studied. Compared to mononuclear metal ions, polynuclear metal clusters containing metal-metal bonds are larger in size and have different coordination preferences toward organic ligands, guiding in the formation of supramolecular assemblies with different topologies and structures [5,6]. Moreover, their inherent metal-metal bonds [7,8] can bring novel properties and functionalities to the final products.
Octahedral {M 6 } clusters usually have eight face-caped [9] or twelve edge-caped [10] ligands and six terminal ligands. When the terminal ligands are equipped with suitable ditopic ligands, the clusters can be used as building blocks together with simple metal ions or coordinatively unsaturated complexes for the preparation of supramolecular assemblies or polymeric architecture [11,12]. For example, our group has focused on using edge-caped octahedral [Nb 6 Cl 12 (CN) 6 ] 4− cluster and transition metal complexes as building blocks [13]. Most reported cluster-containing molecular assemblies are based on {Re 6 }, {W 6 }, {Mo 6 }, {Zr 6 } and {Nb 6 } cluster units. Only a few are based on {Ta 6 } clusters [14]. Recently, {Ta 6 } clusters have experienced renewed interests due to their tunable oxidation states, properties, and functionalities [15,16].

Structural Determination
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 1. CCDC contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www. ccdc. cam. ac. uk/ conts/ retri eving. html (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+ 44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk).
For 1, the structural model incorporates anisotropic thermal parameters for all nonhydrogen atoms except disordered MeOH molecules. The hydrogen atoms are included in the structural model as fixed atoms on their respective C atoms that assume the idealized sp 2 -hybridized geometry with C-H bond lengths being 0.95 Å. The isotropic thermal parameter of hydrogen atom was fixed to be 1.2 times of the equivalent isotropic thermal parameter of the carbon atom to which it is covalently bonded. No attempt was made to locate hydrogen atoms connected to free MeOH molecules. The final anisotropic full-matrix leastsquares refinement on F 2 with 311 variables converged to R 1 = 3.68% for observed data and wR 2 = 8.01% for all data. The maximum residual peak and hole on the final difference Fourier map were found to be 1.844 e − /Å 3 (0.88 Å from Ta2) and − 1.285e − /Å 3 (2.05 Å from Cl6), respectively.
The same refinement process was applied to the data of 1′ and 2. For 1′, the final anisotropic full-matrix least-squares refinement on F 2 with 307 variables converged to R 1 = 5.92% for observed data and wR 2 = 14.36% for all data. The maximum residual peak and hole on the final difference Fourier map are 3.446 e − /Å 3 (1.18 Å from Ta2) and − 2.019e − /Å 3 (1.11 Å from Ta1), respectively. For 2, the final anisotropic full-matrix least-squares refinement on F 2 with 640 variables converged to R 1 = 3.75% for observed data and wR 2 = 9.81% for all data. The maximum residual peak and hole on the final difference Fourier map correspond to 3.177 e − /Å 3 (0.79 Å from Ta5) and − 2.370e − /Å 3 (0.78 Å from Ta6), respectively.

Other Physical Measurements
Elemental analyses were carried out by Atlantic Microlab, Inc. Thermogravimetric analyses were performed using ~ 14 mg sample under Ar or air (40 mL/min) flow 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. Powder X-ray diffraction data was 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. The goniometer was controlled using the GADDS software suite [20]. The sample was Table 1 Crystal and structure refinement data for the compounds (1)  mounted on tape and data was recorded in transmission mode. The system employed a graphite monochromator and a Cu Kα (λ = 1.54184 Å) fine-focus sealed tube operated at 1.2 kW power (40 kV, 30 mA). Four frames were measured at 2θ = 15, 25, 40 and 55° with exposure times of 240 s/ frame. 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 to produce a single one-dimensional pattern [21].
[Mn(Terpy)] 2+ complexes connected to each other via the ditopic cyanide ligand, i.e., CN − . Each [Ta 6 Cl 12 (CN) 6 ] 4− has a valence electron count (VEC) of 16 and is a typical octahedral edge-bridged cluster with the octahedral {Ta 6 } metal core surrounded by twelve edge-bridged Cl atoms as inner ligands and six CN as apical ligands. The mean Ta-Ta bond lengths are 2.894(6) Å for 1 (Table 2), in agreement with those reported for 16e [Ta 6 Cl 12 X 6 ] 2− (X = Cl, Br) cluster units (2.870 ~ 2.903 Å) [22]. Each Mn 2+ is octahedrally coordinated by three N from one Terpy chelating ligand and three N from cyanide ligands from three different clusters (Fig. 1b). The mean Mn-N Terpy bond length is 2.26(2) Å, close to that for Mn-N CN (2.21(3) Å). Each cluster is linked to six different Mn(II) complexes and each Mn(II) complex is connected to three different clusters through cyanide bridges, leading to the formation of neutral 2D layers   (Fig. 1b). The structure can be described as a (3, 6)-connected binodal net (Fig. 1c). Within the layer, the {Ta 6 }… Mn distances are 7.44, 7.37 and 7.32 Å respectively and the corresponding {Ta 6 }…Mn…{Ta 6 } angles are 85.49°, 87.75° and 109.82° respectively. The layers are separated by solvent molecules and are stacked along c direction with interlayer distance of 13.81 Å. Interlayer offset face-to-face π-π interactions are observed between pyridine rings of Terpy ligands from metal complexes in adjacent layers (Fig. 1d). The distance between the centers of the neighbor ring is 3.88 Å. Disordered methanol molecules are located between the layers and form weak hydrogen bond (Fig. 1d) with cyanide ligands (O1… N2 = 3.33(1)Å, symmetry code for N2: 1-x, -y, 1-z). As calculated by the program PLATON, [12] the effective free volume in 1 is 88.5 Å 3 corresponding to 6.5% of the crystal volume. When crystals of 1 are heated at 180 °C for 30 min, the solvent (MeOH) is removed to form (1′) which has the same layered framework as (1). The cell parameters of (1′) are almost the same as those of 1 (Table 1). A summary of selected bond lengths and angles is given in Table 2.
Each Gd 3+ is nine-coordinated by three N atoms from the Terpy ligand, four O atoms from four water molecules, and two O atoms from two DMF molecules (Fig. 2a). Its coordination polyhedron can be described as a distorted.
capped square antiprism (Fig. 2b). Each [Gd(Terpy)(H 2 O) 4 (DMF) 2 ] 3+ metal complex uses four coordinated water molecules to form hydrogen bonding with neighboring solvent molecules and ligands, i.e., three water, one DMF molecules, and six CN groups from six different cluster units. The O…O and O…N distances are shown in Fig. 2c. The extensive hydrogen bonding connects the cations, anions, and solvent molecules into a 3D framework (Fig. 2d).

Thermal Stabilities
TGA of microcrystalline sample of 1 performed under air shows stepwise losses (Fig. 3) 6 ]. 4− − based supramolecular assemblies including coordination polymers, building blocks alone cannot dictate the favored connectivity. Instead, countercations, solvents, reactant ratios, and organic ligands in the complexes all play roles in affecting the connectivity. [27] Like [Nb 6 Cl 12 (CN) 6 ] 4− clusters, [Ta 6 Cl 12 (CN) 6 ] 4−/3− cluster units appear to demonstrate versatile coordination capabilities that could lead to the formation of various connectivity when they are connected to metal complexes. We believe that these factors listed above also affect [Ta 6 Cl 12 (CN) 6 ] 4−/3− − based supramolecular assemblies and thus their properties. For example, solvents also play some roles here. Compared to non-coordinated solvent molecules, coordinated solvent molecules play more roles in affecting the connectivity. In 1, the coordination bonds between the. two building blocks are much stronger than the hydrogen bonds between the non-coordinated solvent molecules and CN ligands. Therefore, upon heat, non-coordinated solvent molecules are removed while the coordination framework is retained. In 1, Mn ions are not coordinated by solvent molecules and thus each Mn can be additionally coordinated with three CN ligands from three cluster units to afford a coordination polymer. In contrast, Gd is fully coordinated by Terpy ligands and coordinated solvent molecules, leaving no coordination sites to be connected to cluster units to afford a coordination polymer.
We believe that under proper conditions, different coordination polymers can be prepared using [Ta 6 Cl 12 (CN) 6 ] 3−/4− and M-Terpy complexes (M = Mn, Gd, and other transition metal ions) as building blocks. Recent functional properties demonstrated by octahedral {Ta 6 } clusters, e.g., electrochromism, [16] also have inspired us to continue to develop more materials and study their properties.