Zinc(II) Phosphonate Cage Compounds Derived from Zn6(Zn)L6

Jan 8, 2008 - Centered Octahedral Cores. Ya-Qin Guo ... The {Zn6(Zn)L6}4- core in compound 2 or 3 is covalently bonded to two Zn(H2O)4. 2+ or {Zn(2 ...
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Zinc(II) Phosphonate Cage Compounds Derived from Zn6(Zn)L6 Centered Octahedral Cores Ya-Qin Guo, Bing-Ping Yang, Jun-Ling Song, and Jiang-Gao Mao* State Key Laboratory of Structure Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, P.R. China

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 2 600–605

ReceiVed August 5, 2007; ReVised Manuscript ReceiVed October 20, 2007

ABSTRACT: Three zinc(II) phosphonate cluster compounds have been hydrothermally synthesized and structurally characterized. Compound 1 consists of a centered {Zn6(Zn)L6}4- core and two {Zn(Phen)2(H2O)2}2+ countercations (L ) N-(phosphonomethyl)pipecolinic acid, O3PCH2-NC5H9-CO2). The {Zn6(Zn)L6}4- core in compound 2 or 3 is covalently bonded to two Zn(H2O)42+ or {Zn(2,2′-bipy)(H2O)3}2+ cations through the coordination of two additional carboxylate oxygen atoms to form a nonanuclear cluster. Introduction The significant contemporary interest in the rational design of novel metal phosphonates reflects their potential applications as function materials, as well as their intriguing variety of architectures and molecular topologies.1 Consequently, many beautiful structures have been well-documented.2–4 Much of the work has so far focused on the rational design of infinite architectures and pillared structures by the choice of polyfunctional phosphonate ligands, which endow these solid materials with fascinating properties in molecular selection, ion-exchange, catalysis, photochemistry, and sorption.5 In contrast to the richness of the extended structures, the design and control of high-nuclear metal complexes is still a challenge since there is no obvious and effective route to synthesize molecular structures. To date, promising approaches toward the synthesis of polynuclear clusters based on phosphonate ligands include: (i) the use of ancillary ligands to increase the steric hindrance of the metal centers (this reduces the dimension of the net formed) and (ii) changing the functional group and steric bulk of the phosphonates R group to stimulate core aggregation. By using the above two approaches, a few fascinating examples have been structurally characterized, including vanadium,6 aluminum,7 copper,8 cobalt,9a manganese,9a cadmium,10 and iron9b,c cage complexes. Multizinc clusters, being an important subject of cluster compounds featuring phosphonate ligands, are of considerable contemporary interest, as exemplified by the potential application of the trinuclear clusters in biological systems such as phospholipase C and P1 nuclease.11 However, zinc(II) cage complexes involving phosphonate are still relatively scarce. The Chandrasekhar group have recently reported tri-, tetra-, and hexanuclear zinc(II) cages involving phosphonate in the presence of ancillary pyrazole ligands,12 Roesky’s group described a dodecanuclear zincphosphonate aggregate with a Zn4(µ4-O) core,13 Zheng’s group reported a cylindrical drumlike heptanuclear cluster compound.14 Recently, three zinc cage compounds, all of which contain a Zn6(Zn)L6 cluster, have been prepared by the hydrothermal reactions of zinc(II) acetate, H2O3PCH2-NC4H7COOH or CH3N(CH2PO3H2)(CH2COOH).15 However, much work is still needed to enrich and develop new molecular compounds. The ongoing research in our group is the design and syntheses of novel molecular polynuclear metal phosphonates. In this * Corresponding author. Fax: 86-591-83714946. E-mail: [email protected].

Figure 1. ORTEP representation of the cluster unit in 1. The thermal ellipsoids are drawn at 50% probability. Hydrogen atoms and lattice water molecules are omitted for clarity. Symmetry code for the generated atoms: -x, 1 - y, 1 - z.

Figure 2. View of the structure of 1 down the a axis. Hydrogen bonds are represented by dashed lines.

aspect, the systematic investigations of the complexation behavior between m-H2O3PC6H4SO3H2 and transition metal ions such as Zn2+ and Cd2+ ions in the presence of ancillary ligands

10.1021/cg700735m CCC: $40.75  2008 American Chemical Society Published on Web 01/08/2008

Zinc(II) Phosphonate Cage Compounds

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Table 1. Crystal Data and Structure Refinement for Compounds 1–3

a

compound

1

2

3

formula Fw crystal system space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z Dc (g cm-3) µ (mm-1) GOF on F2 reflections R1, wR2 (for I > 2σ(I))a R1, wR2 (for all data)

C90H144N14O53P6Zn9 3044.34 monoclinic P21/n 16.497(4) 22.453(5) 17.775(4) 113.642(3) 6031(2) 2 1.676 1.934 1.071 47479/13757 0.0483, 0.1236 0.0600, 0.1321

C42H110N6O52P6Zn9 2305.51 monoclinic P21/n 15.4907(2) 13.5514(2) 19.6047(2) 92.834(1) 4110.40(9) 2 1.863 2.801 1.084 22194/7826 0.0445, 0.1083 0.0600, 0.1119

C62H114N10O46P6Zn9 2509.78 monoclinic C2/c 28.00(2) 22.38(1) 19.15(1) 103.00(1) 11700(1) 4 1.425 1.973 1.165 42912/13249 0.0908, 0.2581 0.1111, 0.2803

R1 ) ΣF0| - |Fc/Σ|F0|, wR2 ) Σ[w(F02 - Fc2)2]/Σ[w(F02)2]1/2.

such as phen afforded a number of new clusters.16 Furthermore, a Zn6(Zn)L6 centered core in which one of the carboxylate oxygen atoms remains free has been proven to be isolated easily by employing an amino-carboxylic-phosphonic acid. This work prompted us to investigate the possibility of modifying the Zn6(Zn)L6 core through the noncoordination carboxylate oxygen atoms to additional metal ions which are also chelated by rigid N-donor ligands such as phen or 2,2′-bipy. Herein, we report the syntheses and structural characterizations of three new zinc(II) phosphonates, [Zn7L6][Zn(Phen)2(H2O)2]2 · 19H2O (1), [Zn9L6(H2O)8] · 14H2O (2), and [Zn7L6{Zn(2,2′-bipy)(H2O)3}2] · 10H2O (3) (L ) O3PCH2NC5H9COO). Their structures feature nona- or heptanuclear cluster cages based on well-known {Zn6(Zn)L6}4- cores.

Table 2. Selected Bond Lengths [Å] for Compounds 1–3a Compound 1 Zn(1)–O(2) Zn(2)–O(4x) Zn(3)–O(4x) Zn(4)–O(4x) Zn(5)–N(4x)

2.101(2)–2.137(2) 1.955(2)–2.135(2) 1.943(2)–2.178(2) 1.943(2)–2.196(2) 2.155(3)–2.201(3)

Zn(2)–N(2) Zn(3)–N(3) Zn(4)–N(1) Zn(5)–O(1W) Zn(5)–O(2W)

2.140(3) 2.136(3) 2.149(3) 2.101(3) 2.128(3)

hydrogen bonds O(4) · · · O(10W) O(9) · · · O(1W)#2 O(1W) · · · O(4W) O(2W) · · · O(5W)#4 O(3W) · · · O(6W) O(8W) · · · O(10W)

2.837(6) 2.679(4) 2.728(4) 2.785(4) 2.791(13) 2.876(9)

O(4) · · · O(4W) O(14) · · · O(7W)#3 O(1W) · · · O(2W) O(3W) · · · O(12W) O(3W) · · · O(5W) O(10W) · · · O(11W)

2.876(5) 2.708(7) 2.846(4) 2.786(7) 2.816(6) 2.800(6)

Compound 2

Experimental Section Materials and Methods. All chemicals were obtained from commercial sources and used without further purification. Elemental analyses (C, H, P, and N) were carried out on a Vario EL III elemental analyzer. IR spectra were recorded on a Magna 750 FT-IR spectrophotometer using KBr pellets in the range 4000–400 cm-1. X-ray diffraction (XRD) powder patterns (Cu KR) were collected on an XPERT-MPD 2θ diffractometer. Synthesis of H2O3PCH2NC5H9COOH (H3L). H3L was prepared by a Mannich type reaction according to the procedures described in the literature.17 DL-Pipecolinic acid (0.05 mol), H3PO3 (0.1 mol), HCl (5 mL), and H2O (9 mL) were loaded into a round-bottom flask and then heated to 120 °C. Formaldehyde (0.075 mol) was added slowly over 1 h, and the mixture was refluxed at 120 °C for 3 h. A white solid was recovered in a yield of 86% after removal of water, which was filtered, washed, and dried. Its purity was confirmed by elemental analysis and NMR measurements. 31P NMR shows only one single peak at 7.746 ppm. Anal. Found for C7H14O5NP: C, 37.92; H, 6.08; N, 6.01%. Calcd: C, 37.67; H, 6.32; N, 6.27%. Synthesis of [Zn7L6][Zn(phen)2(H2O)2]2 · 19H2O (1). A mixture of Zn(NO3)2 · 6H2O (0.5 mmol), H3L (0.25 mmol), 1,10-phen (1 mmol), and water (8 mL) whose pH value was adjusted to 4.8 by the addition of (CH3)4NOH under continuous stirring was put into a Parr Teflonlined autoclave (23 mL) and then heated at 100 °C for 5 days. Yield: 62% (based on zinc). Anal. Calcd for C90H144N14O53P6Zn9: C, 35.51; H, 4.77; N, 6.44. Found: C, 35.32; H, 4.95; N, 6.35. IR data (KBr, cm-1): 3435 s, 1627 m, 1385 m, 1120 m, 781 w. Synthesis of [Zn6L6(Zn){Zn(H2O)4}2] · 14H2O (2). A mixture of Zn(NO3)2 · 6H2O (1 mmol), H3L (0.5 mmol), 2, 2′-bipy (0.5 mmol), and water (8 mL) whose pH value was adjusted to 4.8 by the addition of (CH3)4NOH under continuous stirring was put into a Parr Teflonlined autoclave (23 mL) and then heated at 100 °C for 5 days. After cooling to room temperature, the colorless platelike crystals of 2 were collected as a monophase product (78% yield on Zn). Anal. Calcd for C42H110N6O52P6Zn9: C, 21.88; H, 4.81; N, 3.65%. Found: C, 21.61; H,

Zn(1)–O(4x) Zn(2)–O(4x) Zn(3)–O(6x) Zn(5)–O(2x)

1.952(3)–2.170(3) 1.953(3)–2.182(3) 2.096(3)–2.119(3) 2.244(4)–2.380(3)

Zn(1)–N(3) Zn(2)–N(2x) Zn(4)–O(4x) Zn(5)–OW(4x)

2.118(4) 2.120(4)–2.141(4) 1.964(3)–2.116(3) 2.025(6)–2.064(4)

hydrogen bonds O(1W) · · · O(9W) O(1W) · · · O(3W) O(2W) · · · O(10W) O(7W) · · · O(8W) O(10W) · · · O(11W) O(5) · · · O(3W)#3

2.734(7) 2.905(6) 2.76(1) 2.818(9) 2.72(1) 2.673(6)

O(1W) · · · O(4W) O(2W) · · · O(5W) O(6W) · · · O(7W) O(7W) · · · O(11W) O(3) · · · O(4W)#2 O(5) · · · O(9W)#3

2.837(6) 2.715(8) 2.727(8) 2.835(9) 2.661(5) 2.717(5)

Compound 3 Zn(1)–O(6x) Zn(2)–O(4x) Zn(3)–O(4x) Zn(4)–O(4x) Zn(5)–OW(3x)

2.095(5)–2.135(4) 1.963(4)–2.161(4) 1.974(5)–2.176(5) 1.959(5)–2.147(4) 2.053(6)–2.237(8)

Zn(2)–N(3) Zn(3)–N(1) Zn(4)–N(2) Zn(5)–O(3) Zn(5)–N(2x)

2.156(6) 2.137(6) 2.151(6) 2.100(6) 2.134(7)–2.153(8)

hydrogen bonds O(1) · · · O(12W)#1 O(1W) · · · O(2W) O(3W) · · · O(4W′)#3

2.80(3) 2.81(1) 2.80(2)

O(5) · · · O(2W)#2 O(3W) · · · O(4W)#3 O(4W) · · · O(10W)

2.62(1) 2.71(1) 2.79(2)

a Symmetry transformations used to generate equivalent atoms. For 1: #2 -x, -y + 1, -z; #3 x - 1/2, -y + 3/2, z - 1/2; #4 -x + 1/2, y - 1/2, -z + 1/2. For 2: #2 -x + 1/2, y + 1/2, -z + 1/2; #3 -x + 3/2, y + 1/2, -z + 1/2. For 3: #1 -x + 1/2, -y + 1/2, -z + 1; #2 x, -y + 1, z + 1/2; #3 -x + 1/2, y + 1/2, -z + 1/2.

4.98; N, 3.56%. IR data (KBr, cm-1): 3428 s, 1603 s, 1410 m, 1337 w, 1120 s, 987 m, 794 w, 563 m. Synthesis of [Zn7L6{Zn(2,2′-bipy)(H2O)3}2] · 13.5H2O (3). A mixture of Zn(NO3)2 · 6H2O (0.5 mmol), H3L (0.25 mmol), 2,2′-bipy (1 mmol), and water (8 mL) whose pH value was adjusted to 4.8 by the

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Scheme 1. Relationship between Clusters in Compounds 1-3

Scheme 2. Coordination Fashions of Phosphonate Ligands in 1-3

addition of (CH3)4NOH under continuous stirring was put into a Parr Teflon-lined autoclave (23 mL) and then heated at 100 °C for 5 days. Colorless bricklike crystals of compound 3 were isolated as a single phase (65% based on zinc). Anal. Calcd for C62H114N10O46P6Zn9: C, 29.67; H, 4.58; N, 5.58%. Found: C, 28.92; H, 4.77; N, 5.62%. IR data (KBr, cm-1): 3435 s, 1622 m, 1401 s, 1123 m, 781 w, 612 w. X-ray Crystallography. Date collections for compounds 1-3 were performed on either a Siemens Smart CCD or a Rigaku Mercury CCD diffractometer equipped with graphite-monochromated Mo KR radiation (λ ) 0.71073 Å) at 293 K. Empirical absorption corrections were applied. All structures were solved by direct methods and refined by full-matrix least-squares cycles in SHELX-97.18 All nonhydrogen atoms were refined with anisotropic thermal parameters. All hydrogen atoms were generated

geometrically and refined isotropically. O(6W) and O(9W) in compound 1 as well as O(4W) in compound 3 are disordered, and each displays two orientations with 50% occupancy for each site. The occupancy factors for O(7W) in compound 1 and O(5W)-O(12W) in compound 3 were reduced to 50% due to its larger thermal parameters. The hydrogen atoms for the water molecules are not included in the refinements. The higher R values for compound 3 are due to the poor crystal quality and too many nonhydrogen atoms in the asymmetric unit. Several independent data sets were collected to improve its refinements, but results were unsatisfactory.

Figure 3. ORTEP representation of the cluster unit in 2. The thermal ellipsoids are drawn at 50% probability. Hydrogen atoms and lattice water molecules are omitted for clarity. Symmetry code for the generated atoms: 1 - x, 1 - y, 1 - z.

Figure 4. View of the structure of 2 down the a axis. The ZnO6 and ZnO4N polyhedra are shaded in cyan, and CPO3 tetrahedra are shaded in pink. The C and O atoms are drawn as black and red circles, respectively. Hydrogen bonds are represented by dotted lines.

Zinc(II) Phosphonate Cage Compounds

Figure 5. ORTEP representation of the selected unit in 3. The thermal ellipsoids are drawn at 30% probability. Hydrogen atoms and lattice water molecules are omitted for clarity. Symmetry code for the generated atoms: 1/2 - x, 1/2 - y, -z. A summary of the crystallographic data and structural determination for 1, 2, and 3 is provided in Table 1. Selected bond lengths of 1, 2, and 3 are listed in Table 2.

Results and Discussion The isolations of compounds 1–3 rely on the hydrothermal techniques. Results indicate that the organonitrogen ligand used

Crystal Growth & Design, Vol. 8, No. 2, 2008 603

plays an important role in the types of clusters formed. The 2,2′-bipy is needed for the isolation of 2, though we still can not fully understand the role it played. When more 2,2′-bipy or phen were used, compounds 1 and 3 were obtained. If the reaction mixture contains only zinc salt and ligand components, the compound isolated contains the Zn6(Zn)L6 core which is similar to those previously reported.15a,c The cluster units of 1–3 are all derived from the Zn6(Zn)L6 centered octahedral cluster core (Scheme 1). Compound 1 contains one centered Zn6(Zn)L6 heptanuclear cluster, two {Zn(phen)2(H2O)2}2+ countercations, and lattice water molecules. The Zn6(Zn)L6 heptanuclear cluster is similar to those reported previously except that the two hexahydrated zinc(II) cations are replaced by two {Zn(phen)2(H2O)2} cations (Figure 1).15 The coordination mode of the phosphonate ligand is similar to those reported previously (Scheme 2a).15c The Zn-O (1.955(2)-2.137(2) Å) and Zn-N (2.140(2)-2.201(3) Å) bond distances are comparable to those reported in similar clusters.15 Extensive hydrogen bonds are formed among the noncoordinated carboxylate oxygen atoms, aqua ligands, and the lattice water molecules (Figure 2). The O · · · O contacts range from 2.679(4) to 2.876(9) Å (Table 2). The structure of complex 2 consists of a nonanuclear phosphonate cluster [Zn6(Zn)L6{Zn(H2O)4}2] and 14 lattice water molecules. The Zn9 cluster can be considered to be constructed from the centered Zn6(Zn)L6 octahedral cluster which is further bonded covalently with two {Zn(H2O)4}2+

Figure 6. (a) Zn9 cluster in 3. (b) View of the structure of 3 down the c-axis.

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cations. There are five crystallographically independent zinc atoms with two different coordination environments (Figure 3). Zn1, Zn2, and Zn4 atoms adopt the distorted trigonal bipyramidal geometries. Each metal ion is coordinated by a tridentatechelating carboxylate-phosphonate ligand and two phosphonate oxygen atoms from two adjacent [ZnL] chelating units. The Zn3 atom lying on a position of -1 symmetry (0.5, 0.5, 0) is octahedrally coordinated by six phosphonate oxygen atoms from six ZnL chelating units. The Zn5 atom is six-coordinated by a carboxylate group of a carboxylate-phosphonate ligand in a bidentate chelating fashion and four aqua ligands in an octahedral geometry. The Zn-O (1.952(3)-2.380(3) Å) and Zn-N (2.118(4)–2.141(4) Å) bond distances are comparable to those in compound 1. There are three independent carboxylate-phosphonate ligands adopting two different coordination modes (Scheme 2). One type of carboxylate-phosphonate ligand adopts the same coordination mode as those in Zn6(Zn)L6 clusters reported previously (Scheme 2a),15c which acts as a hexadentate ligand, chelating tridentately with a Zn(II) ion (1N + 2O) and bridging with three other zinc(II) ions. The other phosphonate ligand adopts a different coordination mode; besides linking with four zinc atoms, the carboxylate group of the carboxylate-phosphonate ligand chelates bidentately with the fifth zinc atom (Scheme 2b). One Zn3, two Zn1, two Zn2, and two Zn4 atoms form a known centered Zn6(Zn)L6 octahedral cluster with Zn(3) at its center, such a cluster unit is further covalently bonded to two {Zn(H2O)4}2+ cations to form a novel nonanuclear cluster unit by further coordination of two carboxylate groups (Figure 3). The above cluster units are further cross-linked via hydrogen bonds among noncoordination carboxylate oxygens, aqua ligands, and lattice water molecules into a 3D network (Figure 4, Table 2). The use of more 2,2′-bipy in the preparation leads to another nonanuclear cluster, [Zn7L6{Zn(2,2′-bipy)(H2O)3}2] · 10H2O (3). The cluster unit of 3 is built by one centered Zn6(Zn)L6 heptanuclear cluster attached by two {Zn(2,2′-bipy)(H2O)3} fragments. The Zn6(Zn)L6 heptanuclear cluster unit is similar to those in compounds 1 and 2. The Zn5 atom is octahedrally coordinated by two nitrogen atoms from one 2,2′-bipy ligand, three aqua ligands, and one carboxylate oxygen atom (Figure 5). Different from 2, the {Zn(2,2′-bipy)(H2O)3} fragments are grafted on the centered Zn6(Zn)L6 core through two bidentate bridging carboxylate groups. The phosphonate L ligands also exhibit two types of coordination modes: one is hexadentate and bridging with four Zn2+ ions as found in compounds 1 and 2 (Scheme 2a), the other one is heptadentate, which chelates with a Zn(II) ion tridentately, and also bridges with four other Zn2+ ions (Scheme 2c). The carboxylate group acts as monodentate, chelating, and bridging, and a bidentate bridging metal linker in a, b, and c modes, respectively. An unusual feature of the structure is that compound 3 presents a distinct type of supramolecular aggregation via hydrogen bonds among uncoordinated carboxylate oxygen atoms, aqua ligands, and lattice water molecules, therefore generating two types of channels within which lattice water molecules reside (Figure 6). The O · · · O distances range from 2.618(9) to 2.882(10) Å. In contrast to the packing structure of 2, the Zn5 atom in 3 is coordinated by one 2,2′-bipy ligand, which induces different orientation of the H-bonded interactions. As a result, the eventual architectures of the two compounds display remarkably different packing modes. Recently, it has been shown that H-bonding can play a

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significant role in defining the supramolecular structures of coordination compounds.19 Conclusions In conclusion, we have reported the syntheses and crystal structures of three new zinc(II) cages featuring phosphonate ligands. Our results indicate that the well-known Zn6(Zn)L6 cluster can be further modified through the coordination of more carboxylate oxygen atoms to give higher nuclear molecular clusters. We are currently exploring other new types of cluster compounds derived from the well-known Zn6(Zn)L6 cluster. Acknowledgment. This work is financially supported by National Natural Science Foundation of China (Nos. 20371047 and 20521101), the NSF of Fujian Province (No. E0610034), and the Key Project of Chinese Academy of Sciences (No. KJCX2-YW-H01). Supporting Information Available: X-ray crystallographic files in CIF format for compounds 1–3, IR spectrum, and X-ray powder diffraction patterns. This material is available free of charge via the Internet at http://pubs.acs.org.

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