Metal–Ligand Directed Assembly of Layered Cluster-Based

Metal–Ligand Directed Assembly of Layered Cluster-Based Coordination Polymer and Its Solvent-Mediated Structural Transformations Jian-Jun Zhang, Yue Zhao, Sergio Aarón Gamboa, and Abdessadek Lachgar*

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 1 172–175

Department of Chemistry, Wake Forest UniVersity, Winston-Salem, North Carolina 27109 ReceiVed September 14, 2007; ReVised Manuscript ReceiVed September 11, 2007

ABSTRACT: A two-dimensional (2D) cluster-based coordination polymer with 4,4-net framework was assembled from solution using the metal–ligand directed assembly methodology. The 2D framework undergoes solvent-mediated structural transformations leading to formation of either a three-dimensional hydrogen-bonded framework when using MeOH solution of (Et4N)+ or one-dimensional coordination polymer in the presence of MeOH solution of 4,4′-dipyridyl N,N′-dioxide (dpyo). Research in metal-organic frameworks and coordination polymers with potential applications in catalysis, adsorption, or separation have received much attention in the past decade.1 Initial investigations focused on rigid frameworks and evolved toward a new generation of flexible frameworks bearing dynamic structural transformations in response to external stimuli.2 It is generally believed that most structural transformations are crystal-to-crystal or amorphous-to-crystal and that the solvent is merely used as a vehicle for guest molecules or ions.3 However, some ion exchange reactions can be solvent mediated as has been demonstrated for anion exchange in Ag(I) coordination polymers which involves the dissolution of the initial crystalline phase and the growth of another phase in the presence of suitable guest anions.4 Thus, when single crystals were immersed in aqueous solutions containing different anions, the crystals do not change size or shape but quickly lose optical transparency and their single crystal integrity. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) experiments showed that the crystals begin to slowly dissolve when immersed in solvent, and a new crystalline phase grows on the surface of the original phase and the process continues until anion exchange is complete. We and others have demonstrated the use of metal clusters with an octahedral metal core and metal complexes as building blocks of inorganic polymers.5–8 We have previously reported that edgecaped octahedral niobium cyanochloride clusters, [Nb6Cl12(CN)6]4-, characterized by a variety of physical and chemical properties arisen from the presence of metal-metal bonds can be used in tandem with Mn(III) complexes with salen-type tetradentate ligands as building blocks for a variety of cluster-based supramolecular assemblies and multidimensional coordination polymers.7,8 Among these materials, we reported the synthesis and properties of [Me4N]2 [(Mn(salen))2(Nb6Cl12(CN)6)] (1), which has a two-dimensional (2D) framework in which anionic layers are built through linkages between Nb6 clusters and Mn(salen)+.8 Adjacent layers are shifted with respect to each other providing cavities in which the ammonium ions are located. In the present report, we first describe the preparation of a new 2D framework: [Mn2(salen)2(H2O)2][(Mn(salen))2(Nb6Cl12(CN)6)] (2) with layer topology similar to that found in 1; however, instead of [Me4N]+ the dimeric complex [Mn2(salen)2(H2O)2]2+ balances the negative charge of the 2D framework.9 Second, cation exchange reactions, which will be reported in more detail elsewhere, have led in some cases to the destruction of the framework and formation of a three-dimensional (3D) hydrogen-bonded framework based on heterotrimers [(Mn(salen)(H2O)]2Nb6Cl12(CN)6] (3). Third, we demonstrate how this structural transformation behavior can be used as a synthesis * To whom correspondence should be addressed. Tel: (+1) 336-758-4676. Fax: (+1) 336-758-4656. E-mail: [email protected].

strategy to synthesis materials that are difficult to prepare by bringing different building blocks together. More specifically, we report the use of this method to prepare materials in which the heterotrimers are linked directly via organic ligands. Thus, when 2 is soaked in a methanol solution of dpyo, the one-dimensional (1D) coordination polymer {[Mn(salen)(MeOH)]2(dpyo)}{(dpyo)[Mn(salen)2Nb6Cl12(CN)6}dpyo (4) in which the heterotrimers are connected by dpyo ligands to form anionic chains. The chain’s negative charge is compensated by the dimeric dication {[(Mn (salen)(MeOH)]2(dpyo)}2+, which is formed in situ. The three compounds all contain the cluster [Nb6Cl12(CN)6]4as a building block.10 The cluster is characterized by its octahedral {Nb6} metal core with 12 edge-bridging Cl ligands and 6 terminal CN- ligands. The bond lengths and angles of Nb-Nb, Nb-Cl, Nb-C, C≡N, and Nb-C≡N are similar to those found in the cluster building block precursor containing octahedral cyanochloride niobium cluster units with 16e available for metal-metal bonding.8 The IR spectra shows bands at or close to 2130 cm-1 for the CN stretching vibrational mode close to that observed for (Me4N)4 [Nb6Cl12(CN)6] (2129 cm-1), which indicate very little effect if any of the linkage mode on the CN bond strength.8 2 was obtained by the reaction of (Me4N)4[Nb6Cl12(CN)6] and [Mn(salen)Cl(H2O)] in MeOH/H2O solvent mixture. It features a 2D anionic 4,4-network with [Mn2(salen)2(H2O)2]2+ dimers as charge compensating ions (Figure 1). The layers stack perfectly on top of each other generating channels where the dimers and disordered solvent molecules are located. The layers are built of [Nb6Cl12(CN)6]4- nodes that use four of their six cyanide ligands to connect to four different Mn complexes, and each Mn complex connects two clusters through cyanide bridges with the corresponding Nb-C≡N angles in the range of 173(1)-179.5(8)°. The nonbridging cyanide ligands form extensive hydrogen bonds with the [Mn2(salen)2(H2O)2]2+ cations with O · · · N separation of 2.886(5) Å and O-H · · · N angle of 172(6)° (symmetry code for N: 1 - x, -1/2 + y, 1/2 - z). Each Mn3+ ion within the layer has a distorted octahedral coordination environment with two N and two O from the salen ligand (average Mn-O ) 1.89(2) Å and Mn-N ) 2.00(2) Å, compared to 1.88(3) and 1.983(2) Å in [Mn (salen)Cl(H2O)]),11d and two N from cyanide ligands from two different clusters (Mn-NCN ) 2.293(4)-2.365(3) Å, C≡N-Mn ) 147.3 -150.0°). In the [Mn(salen)(H2O)]22+ dimers each Mn3+ is chelated by a salen ligand and coordinated by an aqua ligand with Mn-O ) 2.187(4) Å. The octahedral coordination environment is completed by phenolic oxygen from another [Mn (salen)(H2O)]+ complex (Mn-O ) 2.977(4) Å). The Mn · · · Mn separation and Mn-O-Mn angle are 3.571 (1) Å and 92.1(1)°, respectively, which are within the values reported for Mn(salen)type dimers (3.19-4.38 Å and 91.1-102.4°, respectively).11 One of the important driving forces for the formation of the dimers is

10.1021/cg070532s CCC: $40.75  2008 American Chemical Society Published on Web 12/14/2007

Communications

Crystal Growth & Design, Vol. 8, No. 1, 2008 173

Figure 1. The structure of 2. (a) 2D anionic 4,4-network; (b) the structure of the [Mn(salen)(H2O)]22+ dimer; (c) the packing diagram along the b direction.

Figure 2. (a) The structure of heterotrimer in 3. (b) The environment of [Et4N]+; (c) the hydrogen-bonded networks and the environment of heterotrimer. [Et4N]+ cations are represented as deep yellow balls.

174 Crystal Growth & Design, Vol. 8, No. 1, 2008 Scheme 1

the offset face-to-face π-π interactions between benzene rings of salen ligands with the shortest and longest ring–ring separation of 3.32 and 3.56 Å, respectively. The dimers further interact with the salen ligands that belong to Mn-complexes within the layers through other π-π interactions with distances of 3.48 and 3.68 Å leading to formation of 1D π-π interactions running along the c-direction. The 4,4-nets pack in an edge-to-edge regular mode along the a direction with an interlayer distance of 14.91 Å and form channels along the b direction where [Mn(salen)(H2O)]22+ dimers and disordered solvent molecules are located. The effective free volume in 2 is 519.7 Å3, comprising 11.5% of the crystal volume, as calculated by the program PLATON.12 Crystals of 3 were obtained when ground microcrysalline powder of 2 was mixed with a MeOH solution of (Et4N)+ (SI) · .9 The crystal structure of 3 can be described as a charge-assisted hydrogenbonded 3D framework built of dianionic nanosize heterotrimers formed of [Nb6Cl12(CN)6]4- cluster units linked to two [Mn(salen)]+ by cyanide ligands located on opposite sides of the cluster (Figure 2, Scheme 1 ). The octahedral coordination environment of manganese is completed by an aqua ligand that form hydrogen bonds with nonbridging cyanides of neighboring trimers to form chains along the crystallographic a-axis. (O3 · · · N102 ) 2.90(1) Å, symmetry code: 1 - x, 1 - y, 1 - z). Free water molecules connect the chains first into layers (N102 · · · O12 ) 2.80(1) Å, symmetry code: 1 - x, 1 - y, 1 - z; O12 · · · O11 ) 2.86(2) Å, symmetry code: 1 - x, y - 1/2, 0.5 - z; O11 · · · N103 ) 2.86(1) Å, symmetry code: 1 - x, 1/2 + y, 3/2 - z) and then into 3D

Communications hydrogen-bonded framework (O11 · · · O3 ) 2.76(2) Å) with cavities in which the cations (Et4N)+ are located. Each heterotrimer is surrounded by six (Et4N)+ cations that form a hexagonal ring with side length of 7.27-7.92 Å, while each cation is surrounded by three trimers that form a triangle with side length of ∼11.5 Å. The heterotrimers in 3 are similar to those found in [Et4N]2[(Mn (salen)(MeOH))2(Nb6Cl12(CN)6)] · 2MeOH, which was obtained through direct mixing of (Et4N)4[Nb6Cl12(CN)6] and [Mn(salen)]ClO4 in MeOH.8 Compound 4 was obtained when ground microcrystalline powder of 2 was mixed with a MeOH solution of dpyo chosen for its ability to act as a bridging ligand. The crystal structure of 4 consists of anionic chains in which heterotrimers similar to those found in 3 are connected by dpyo. The in situ formed dimeric cation {[Mn (salen)(MeOH)]2(dpyo)}2+ balances the chains’ negative charge. Within the chains the two oxygen ends of dpyo complete the octahedral environment of Mn from two adjacent heterotrimers with Mn-Odpyo ) 2.305(3) Å and Mn-O-N ) 114.5(2)°. The bond angle C≡N-Mn (147.4°) is much smaller than that in 3 (161.5°) due to the coordination mode of dpyo. The salen ligand is bent with a dihedral angle between the two benzene rings of 45.6°, compared to 9.5° in 2 due to intrachain offset face-to-face π-π interaction between the salen ligand and dpyo with a ring · · · ring distance of 3.65 Å. The location of the cyanide and dpyo ligands on different sides of the NCN-Mn-Ndpyo axis leads to chains with sinusoidal wavelike structure. Strong hydrogen bonds (O · · · O ) 2.657(6) Å and O-H · · · O angle of 168(6)°) between the coordinated MeOH of the cation {[Mn(salen)(MeOH)]2(dpyo)}2+ and the oxygen atom of the free dpyo ligand (symmetry code: 1/2 - x, -1/2 + y, z) lead to the formation of cationic chains that run parallel to the anionic cluster-based anionic chains. Alternating anionic and cationic chains form two types of layers perpendicular to the b direction. In one set of layers, the chains run along the [2 0 1] direction, while in the neighboring layer the chains run along [2j 0 1]. The overall structure has a crystallographically imposed symmetry. The structural transformations observed here are solvent mediated because microcrystalline powder of 2 is quantitatively converted into crystals of 3 and 4 in methanol solutions of different guest

Figure 3. The structure of 3. (a) 1D anionic chain; (b) the dpyo bridged {[Mn(salen)(MeOH)](dpyo)}2+ dimer; (c) schematic diagram showing the packing mode of the 1D chains. The anionic and cationic chains are shown in solid and dotted lines, respectively.

Communications molecules or ions, while it remains stable when left in THF for more than a week. The change in the [Mn(salen)]+/{Nb6} ratio between the original phase 2 (4:1) and the products 3 or 4 (2:1) strongly suggests the involvement of an intermediate with a [Mn(salen)]+/{Nb6} ratio of 2:1. The overall process probably involves slow and partial dissociation of 2 in methanol solutions containing different guests molecules or ions to form the heterotrimer [(Mn(salen))2(Nb6Cl12(CN)6)]2- and [Mn(salen)(s)]+. In the presence of other cations such as (Et4N)+ the original 2D frameworks can not be reformed and a 3D hydrogen-bonded framework 3 based on heterotrimers is formed. As 3 is formed the solubility equilibrium is shifted until complete depletion of 2. This suggested process is further supported by the formation of 4 when 2 is left in contact with methanol solution containing the bridging ligand (dpyo). It must be noted that none of the compounds reported here can be obtained using preassembled heterotrimers since the heterotrimeric compound is not soluble in the conditions used in the present study. TGA of 2 performed under air flow shows two distinct weight losses. The first step (5.45%) involves loss of both coordinating and noncoordinating solvent molecules at a temperature below 150 °C (cal. 5.94%). The materials obtained after heating at 150 °C for 0.5 h was not crystalline; however, IR spectra showed the presence of C≡N groups (2141 cm-1). The compound shows a striking weight loss in the temperature range 200–700 °C to form a mixture of MnNb2O6 and Mn3O4 after 700 °C as confirmed by powder X-ray diffraction (PXRD) (final weight (obs.) ) 42.03%; final weight (calc.) ) 42.16%). The temperature dependence of the magnetic susceptibility for 2 was measured between 300 to 5 K. At room temperature, the µeff value for 2 is 4.96 µB, compared with the spin-only value (4.90 µB) for Mn3+ ions with g ) 2.0 and S ) 2. As the temperature decreases, µeff decreases slowly until ca. 70 K, then sharply decreases to reach 3.59 µB at 5K, indicating weak antiferromagnetic coupling between Mn3+ ions. The χm-1 vs T plot follows the Curie–Weiss law with C ) 0.56 emu · mol-1 · K-1 and θ) -5.56 K, also indicates antiferromagnetic coupling, which can be attributed to intradimer coupling. The magnetic properties were modeled ^ ) using the Heisenberg dimer model with the Hamiltonian H -1 ^ ^S The best fit gave g ) 2.036(2)J ) -1.79(3) cm for 2. - 2JS 1 2 The overall synthesis process involves the metal–ligand directed assembly of a 2D cluster-based coordination polymer, followed by its dissociation into larger building blocks (heterotrimers), which are subsequently assembled into different frameworks depending on the nature of the cation or the bridging ligand. The findings allow for the development of a novel synthesis methodology for the assembly of materials not directly (or easily) accessible via the molecular building block approach.

Acknowledgment. This material is based upon work supported by the National Science Foundation under Grant No. DMR0446763. Supporting Information Available: Details of synthesis procedure and characterization data; crystallographic information files (CIF). This material is available free of charge via the Internet at http://pubs.asc.org.

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References (1) (a) Rowsell, J. L. C.; Yaghi, O. M. Angew. Chem. Ing. Ed. 2005, 44, 4670. (b) Wu, C. D.; Hu, A.; Zhang, L.; Lin, W. B. J. Am. Chem. Soc. 2005, 127, 8940. (2) (a) Lehn, J.-M. Chem. Soc. ReV. 2007, 36, 151. (b) Kitagawa, S.; Uemura, K. Chem. Soc. ReV., 2005, 34, 109. (c) Kitagawa, S.; Kondo, M. Bull. Chem. Soc. Jpn. 1998, 71, 1739. (3) (a) Yaghi, O. M.; Li, H. J. Am. Chem. Soc. 1996, 118, 295. (b) Yaghi, O. M.; Li, H.; Davis, C.; Richardson, D.; Groy, T. L. Acc. Chem. Res. 1998, 31, 474. (c) Jung, O. S.; Kim, Y. J.; Lee, Y. A.; Park, J. K.; Chae, H. K. J. Am. Chem. Soc. 2000, 122, 9921. (4) Khlobystov, A. N.; Champness, N. R.; Roberts, C. J.; Tendler, S. J. B.; Thompson, C.; Schroder, M. CrystEngComm 2002, 4, 426. (5) (a) Welch, E. J.; Long, J. R. Prog. Inorg. Chem. 2005, 54, 1. (b) Gabriel, J.-C. P.; Boubekeur, K.; Uriel, S.; Batail, P. Chem. ReV. 2001, 101, 2037. (c) Selby, H. D.; Roland, B. K.; Zheng, Z. P. Acc. Chem. Res. 2003, 36, 933. (6) (a) Yan, Z. H.; Day, C. S.; Lachgar, A. Inorg. Chem. 2005, 44, 4499. (b) Yan, B. B.; Zhou, H. J.; Lachgar, A. Inorg. Chem. 2003, 42, 8818. (c) Yan, B. B.; Day, C. S.; Lachgar, A. Chem. Commun. 2004, 2390. (7) (a) Zhou, H. J.; Lachgar, A. Eur. J. Inorg. Chem. 2007, 1053. (b) Zhang, J. J.; Lachgar, A. J. Am. Chem. Soc. 2007, 129, 250. (c) Zhou, H. J.; Lachgar, A. Cryst. Growth Des. 2006, 6, 2384. (d) Zhou, H. J.; Strates, K. C.; Munoz, M. A.; Little, K. J.; Pajerowski, D. M.; Meisel, M. W.; Talham, T. A.; Lachgar, A. Chem. Mater. 2007, 19, 2238. (e) Zhang, J. J.; Zhou, H. J.; Lachgar, A. Angew. Chem., Int. Ed. Engl. 2007, 46, 4995. (8) Zhou, H. J.; Day, C. S.; Lachgar, A. Chem. Mater. 2004, 16, 4870. (9) {[Mn(salen)(H2O)]2[(Mn(salen))2(Nb6Cl12(CN)6)} · 0.66MeOH · 5.33H2O (2): The mixing of [Mn(salen)]ClO4 (10.0 mL, 12.0 mM) in MeOH and (Me4N)4[Nb6Cl12(CN)6] (10.0 ml, 4.0 mM) in H2O lead to the formation of brown microcrystalline precipitate immediately. The solid was collected by centrifugation washed with MeOH then dried under vacuum (Yield: 18.7 mg, 25%). Anal. Calc. For C70.67H73.33 Cl12Mn4N14Nb6O16: C, 32.93; H, 2.87%. Found: C, 32.73; H, 2.70%. νCN ) 2131 cm-1. Green-brown crystals suitable for X-ray analysis were obtained by layering method. (Et4N)2{[Mn(salen)(H2O)]2Nb6Cl12(CN)6} · 6H2O (3): The suspension of (Et4N)Br (100 mg, 0.48 mmol) and 2 (40 mg, 0.016 mmol) in 10 mL of MeOH was stirred for 10 min, left undisturbed for 2 days and then centrifugalized to separate the solid, which was washed by aliquots of H2O and MeOH, then dried under vacuum to get 3 (yield: 29 mg, 84%). Anal. Calc. For 3 C54H80Cl12Mn2N12Nb6O10: C, 30.10; H, 3.79; N, 7.65%. Found: C, 29.27; H, 3.77; N, 7.77%. νCN ) 2127 cm-1. [(Mn(salen)(MeOH))2dpyo]{[Mn(salen)]2[Nb6Cl12(CN)6]dpyo} dpyo (4): Same as 3 except using dpyo to replace (Et4N)Br. (Yield: 25 mg, 51%). Anal. Calc. For 4 C102H88Cl12Mn4N20Nb6O16: C, 40.13; H, 2.91; N, 9.18%. Found: C, 39.02; H, 2.92; N, 8.79%. νCN ) 2131 cm-1. (10) Crystal data for 2: monoclinic, space group P2(1)/c, a ) 14.949(4), b ) 13.373(4), c ) 22.736(6) Å, β ) 94.409(4)°, V ) 4532(2) Å3 at T ) 193(2) K, Z ) 2, Dcalcd ) 1.889 g cm-3, R1(wR2) ) 0.0473(0.1028). Crystal data for 3: monoclinic, space group P2(1)/c, a ) 13.381(2), b ) 20.966(4), c ) 13.915(2) Å, β ) 91.546(3)°, V ) 3902.4(12) Å3 at T ) 193(2) K, Z ) 2, Dcalcd ) 1.830 g cm-3, R1 (wR2)) 0.0713 (0.1534).Crystal data for 4: orthorhombic, space group Pbcn, a ) 21.822(4), b ) 17.499(3), c ) 29.117(6) Å, V ) 11118(4) Å3 at T ) 193(2) K, Z ) 4, Dcalcd ) 1.824 g cm-3, R1(wR2)) 0.0470(0.0989). (11) (a) Saha, S.; Mal, D.; Koner, S.; Bhattacherjee, A.; Gutlich, P.; Mondal, S.; Mukherjee, M.; Okamoto, K.-I. Polyhedron 2004, 23, 1811. (b) Miyasaka, H.; Clérac, R.; Ishii, T.; Chang, H. C.; Kitagawa, S.; Yamashita, M. J. Chem. Soc., Dalton Trans., 2002, 1528,and references therein,(c) Lu, Z. L.; Yuan, M.; Pan, F.; Gao, S.; Zhang, D. Q.; Zhu, D. B. Inorg. Chem. 2006, 45, 3538. (d) Panja, A.; Shaikh, N.; Ali, M.; Vojtisek, P.; Banerjee, P. Polyhedron, 2003, 22, 1191. (12) Spek, A. L. PLATON, Version 1.62; University of Utrecht, 1999.

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