Crystal Engineering of Polyoxomolybdates Based MetalâOrganic

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Crystal Engineering of Polyoxomolybdates Based MetalOrganic Solids: The Case of Chromium Molybdate Cluster Based Metal Complexes and Coordination Polymers Published as part of a virtual special issue on Structural Chemistry in India: Emerging Themes Monika Singh and Arunachalam Ramanan* Department of Chemistry, Indian Institute of Technology Delhi, New Delhi-110016, India

bS Supporting Information ABSTRACT: Six new metal complex or coordination polymer incorporated AndersonEvans cluster based solids, (H24-pyc)2[{Na(4-pyc)2}{CrMo6(OH)8O16}] 3 4H2O, 9, (H24-pyc)2[{Ni(4-pyc)2(H2O)4}{CrMo6(OH)7O17}] 3 10H2O, 12, (H23-pyc)2[{Na(3-pyc)2}{CrMo6(OH)8O16}] 3 2H2O, 13, and [{M2(2-pzc)2(H2O)4}{CrMo6(OH)7O17}] 3 17H2O (M = Co, 14 and Cu, 15; 4-pyc = pyridine-4-carboxylate, 3-pyc = pyridine-3-carboxylate, and 2-pzc = pyrazine-2-carboxylate) were crystallized in aqueous solution at room temperature. 9 shows alternating AndersonEvans cluster and sodium complex {Na(4-pyc)2} coordinately linked forming one-dimensional (1-D) chains and free protonated H4-pyc moieties occur in between. 10 and 12 show the aggregation of a discrete AndersonEvans cluster, metal complex, {M(4-pyc)2(H2O)4} (M = Ni and Cu), free protonated H4-pyc moieties, and water through extensive nonbonding interactions. Crystallization in the presence of H3-pyc, an isomer of H4-pyc yielded only a sodium based solid, (H23-pyc)2[{Na(3-pyc)2}{CrMo6(OH)8O16}] 3 2H2O, 13; it forms a chain similar to 9. However, nonbonding interactions that stabilize the chains are different. Reaction with 2-pzc resulted in two isostructural solids, [{M2(2-pzc)2(H2O)4}{CrMo6(OH)7O17}] 3 17H2O (M = Co, 14 and Cu, 15). 14 and 15 are the first examples where the coordination polymer is coordinated to an AndersonEvans cluster. This paper is an attempt to understand the structural landscape of the system {M(H2O)n}2+-organic ligand-{HmCrMo6O24}n. A retrosynthetic analysis of the isolated crystalline solids from the system provides molecular insights to interpret the crystal engineering of these complex materials and directions for designing new solids in the system.

’ INTRODUCTION The ability to design functional solid state structures from neutral or ionic building blocks using intermolecular interactions is a strategy commonly employed by crystal engineers.1 A major challenge in crystal design is to map the pathway of the crystallization process in terms of recognition and supramolecular aggregation between interacting molecules in the solution.2 Can we establish a link between molecules reacting in the solution and the intermolecular interactions observable in the solid state? If we can propose a structure for the critical nucleus, the supramolecular analogue of the transition state, we can elucidate the different supramolecular reaction pathways leading to a crystal. In the absence of suitable kinetic and thermodynamic data, such an approach will provide chemical insights into the architecture of a solid and interpret the occurrence of polymorphs, pseudopolymorphs, and supramolecular isomers in the structural landscape of a given system. In the context of functional solids, polyoxomolybdates (POM) based organicinorganic hybrid materials are potential candidates for a variety of applications in different areas such as catalysis, r 2011 American Chemical Society

medicine, sorption, and magnetism.3,4 In particular, transition or rare-earth metal incorporated POM solids are attractive candidates to probe catalytic, optical, and magnetic properties associated with structural variations.5,6 POM are versatile nanosized metal oxide clusters with various topologies. POM based hybrid solids are mostly crystallized from aqueous solution by reacting appropriate metal salts, organic ligands, and molybdenum sources from ambient conditions to 200 °C; the higher temperature reactions are mostly performed under hydrothermal/solvothermal conditions. Two structural features essentially dominate the building of metalorganic molybdates. In the first case, reasonably stable polyoxomolybdate clusters (POM) such as octamolybdates,7,8 Keggin,9 WellsDawson,10 Lindquist,11 Strandberg,12a,b and AndersonEvans12c,d,13 appear as building blocks; these may occur either as discrete counteranions or coordinately bonded Received: December 21, 2010 Revised: June 15, 2011 Published: June 16, 2011 3381

dx.doi.org/10.1021/cg101695w | Cryst. Growth Des. 2011, 11, 3381–3394

Crystal Growth & Design to metal complexes extending into multidimension. In the second case, molybdates with extended MoOMo interactions dominate the structure building with the metal complex occurring as counter cations or bridging/pillaring units.14 Crystal engineering of POM based metalorganic solids suffer from suitable design strategies. In our earlier works, we demonstrated the hydrothermal technique for crystallizing several metalorganic molybdates.7,12,14,15 However, these high temperature reactions induce several competing factors such as metal complex equilibrium, formation of different POM clusters, or reduction of metal ions by the organic ligands ultimately leading to complex supramolecular assemblies. To design POM based Scheme 1. Scheme Showing Ligand Notation

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solids under ambient conditions, we preferred AndersonEvans clusters as these are quite soluble and stable in aqueous solution. Twelve hetero atoms are known to stabilize this cluster.16 Chromium was preferred as the heteroatom since it does not interfere with the chosen organic ligands; also it suppresses the formation of other competing POM anions. The symmetrical, planar topology of the anion combined with the ability of its terminal and bridging oxygens to coordinate with metal is most suited to study the supramolecular interaction with metal complex in a multidentate fashion. In a recent paper, we demonstrated that such an approach can be exploited to tune the structures of POM based metal coordination solids in the presence of pyrazine.12c Several types of metal coordination interactions can dictate crystal engineering of multidimensional structures: (i) aggregation of metal complex toward a coordination polymer (CP), (ii) coordination of metal complex with molybdate clusters forming an extended array, or (iii) simultaneous occurrence of both these interactions. By varying metal to organic molar ratios, it is possible to synthesize Anderson Evans cluster based solids incorporated with metalorganic complex or CP. In the present work, we chose isomeric pyridine carboxylic acids because they are readily soluble under our reaction conditions. They form complexes as well as CPs with divalent transition metal ions. Our objective was to explore the structural landscape of the system MII(H2O)norganic ligandHmCrMo6O24 (organic ligandpyridine or pyrazine carboxylic acid) and identify the occurrence of isostructures, pseudopolymorphs, and supramolecular isomers. Copper is known to form a complex

Scheme 2. Synthetic Protocol for the Crystallization of Metal-Pyridine Carboxylate and Metal-Pyrazine Carboxylate (Solution A)a

a

In the case of both H2-pyc and H3-pyc, the phases could not be identified based on powder X-ray diffraction data. 3382

dx.doi.org/10.1021/cg101695w |Cryst. Growth Des. 2011, 11, 3381–3394

Crystal Growth & Design

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Scheme 3. Synthetic Protocol for the Crystallization of Chromium Molybdate Anion, [HmCrMo6O24]n Based Solids from Aqueous Solution Containing Metal-Pyridine Carboxylate and Metal-Pyrazine Carboxylate

Table 1. Crystal Data and Structural Refinements for 9, 10, 1215 parameter

9

10

12

C24 H20 Cr

C24 H20 Cr Cu

13

14

C24 H20 Cr

C10 H6 Co2 Cr

15

formula

C24 H20 Cr Mo6

formula weight, g mol1

N4 Na O36 1591.07

T (K)

293(2)

293(2)

293(2)

293(2)

293(2)

wavelength (Å)

0.71073

0.71073

0.71073

0.71073

0.71073

0.71073

crystal system

triclinic

triclinic

triclinic

monoclinic

monoclinic

monoclinic

Mo6 N4 Ni O48 1818.77

Mo6 N4 O46 1791.63

Mo6 N4 Na O34 1559.07

Mo6 N4 O49 1711.69

C10 H6 Cr Cu2Mo6 N4 O49 1720.93 293(2)

space group

P1

P1

P1

C2/c

P21/c

P21/c

a (Å)

10.514(5)

10.5998(12)

10.559(4)

22.084(3)

10.6636(15)

10.8805(17)

b (Å)

10.655(5)

11.8436(14)

11.819(4)

9.4366(11)

17.262(3)

17.084(3)

c (Å) R (°)

12.295(6) 97.351(9)

13.1423(16) 102.363(2)

13.105(5) 102.202(6)

21.011(3) 90

14.0540(19) 90

13.590(2) 90

β (°)

112.286(8)

111.815(3)

111.925(5)

98.517(2)

108.948(3)

107.703(3)

γ (°)

113.523(8)

107.759(2)

107.706(6)

90

90

90

V (Å3)

1103.4(9)

1355.0(3)

1344.1(9)

4330.4(10)

2446.8(6)

2406.5(7)

Z

1

1

1

4

2

2

dcalc (g cm3)

2.395

2.229

2.214

2.391

2.323

2.375

μMoKR, (cm1)

2.015

1.998

2.055

2.048

2.484

2.720

R1 (I > 2σI) wR2 (all)

0.0476 0.0997

0.0647 0.1314

0.0920 0.1806

0.0413 0.0851

0.0541 0.1171

0.0431 0.1073

GOF

1.136

1.108

1.225

1.074

1.178

1.045

CCDC/CSD no.

700923

697137

705131

755574

775791

770419

as well as CP with pyridine-4-carboxylic acid and pyridine-3carboxylic acid but only a discrete complex with pyridine-2carboxylic acid.1719 Copper also forms CPs in the presence of other transition metals as well as rare-earth metals. There are a few examples in which an Anderson cluster is incorporated with metal pyridine carboxylate complex or CP.20 In this paper, we describe our efforts to systematically crystallize new phases containing chromium molybdate cluster based metalorganic solids. The paper discusses the synthesis and structural characterization of several solids. We also employed retrosynthesis for understanding the structural chemistry of AndersonEvans cluster based solids and suggest directions for synthesizing new members.

’ EXPERIMENTAL SECTION Synthesis. Initially two different aqueous solutions were prepared. The experimental conditions were optimized to enable homogeneity of the solution before and after mixing. Solution A was prepared by mixing pyridine-4-carboxylic acid (2.52 mmol, CDH, 99%) to the solution of MCl2 3 xH2O (1.68 mmol, Merck, 99%) (M = Co, Ni, Cu and Zn) in 10 mL of water and 10 mL of methanol. Solution B was prepared by adding Na2MoO4 3 2H2O (2.35 mmol, Merck, 99%) to the solution of CrCl3 3 6H2O (1.57 mmol, CDH, 99%) in 15 mL of water; this was further acidified by 7 mL of glacial acetic acid. The solution A was then added to the solution B with stirring. The resulting dark green colored solution was left for crystallization at room temperature. The pink block crystals of 9 were obtained after 6 weeks approximately in about 81% 3383



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Figure 1. (a) Chromium molybdate clusters are coordinately linked to {Na(4-pyc)2} complex units forming 1-D chains in 9. (b, c) The adjacent chains are brought together through OH 3 3 3 O, NH 3 3 3 O, and CH 3 3 3 π interactions in 9. (d) Tetrameric water oligomer between mediating water molecules in 9. (e) The inclusion of the free organic molecule in 9 is to facilitate effective crystal packing of the sodium 4-pyc complex units. The free H24-pyc strongly interacts with wateroilgomer to form extended organicwater aggregates.

yield (based on Mo) in the presence of cobalt and zinc. Elem. Anal. Calcd for 9: C, 17.92; H, 2.24; N, 3.48. Found: C, 17.89; H, 2.20; N, 3.45. The pink block crystals of 10 were obtained after 4 weeks approximately in about 71% yield (based on Mo). Elem. Anal. Calcd for 10: C, 15.51; H, 3.12; N, 3.01. Found: C, 15.48; H, 3.08; N, 3.03. With copper, a biphasic product was obtained. The blue block crystals of 11 in 65% yield (based on Cu) and pink block crystals of 12 in 75% yield (based on Mo) were obtained after 7 weeks approximately. We have also used pyridine-3carboxylic acid as ligand instead of pyridine-4-carboxylic acid. In the presence of cobalt, pink block crystals of 13 were obtained after 4 weeks approximately in about 67% yield (based on Mo). Elem. Anal. Calcd for 13: C, 18.33; H, 2.03; N, 3.56. Found: C, 18.29; H, 1.99; N, 3.51. In the presence of nickel, copper and zinc, suitable single crystals for X-ray diffraction could not be grown. Similar reaction was done in the presence of pyrazine-2-carboxylic acid. In the presence of cobalt and copper, red and blue colored crystals of 14 and 15 were obtained respectively after 4 weeks approximately in about 55% yield (M = Co, based on Mo) and 71% yield (M = Cu, based on Mo). Elem. Anal. Calcd for 14: C, 6.81; H, 3.179; N, 3.18. Found: C, 6.78; H, 3.172; N, 3.16. Elem. Anal. Calcd for 15: C, 6.77; H, 3.16; N, 3.162. Found: C, 6.71; H, 3.14; N, 3.152.

Initial pH on mixing the solution A and B is ∼3 in all the cases. The final pH was ∼12 when precipitation/crystallization was almost complete. In order to benchmark the appearance of competitive phase(s) during our crystallization process, we investigated the formation of the solids by slow evaporation of the solution A. Reaction of Co, Ni, Cu, and Zn with H4-pyc resulted in monophasic powdered solids (14). Reaction of H2pzc with Co, Ni, and Cu also yielded powdered samples (57). The reaction with zinc yielded pink colored crystals (8) suitable for single crystal XRD analysis. Scheme 1 shows the ligand notation. We have summarized the synthetic protocols in Schemes 2 and 3 for the crystallization of chromium molybdate based metalorganic complex or CPs. The experimental scheme is similar to the one reported earlier.12c,d X-ray Structure Determination. X-ray diffraction studies of crystal mounted on a capillary were carried out on a BRUKER AXS SMARTAPEX diffractometer with a CCD area detector (MoKR = 0.71073 Å, monochromator: graphite).21 Frames were collected at T = 293 K by ω, ϕ, and 2θ-rotation at 10 s per frame with SAINT.22 The measured intensities were reduced to F2 and corrected for absorption with SADABS.22 Structure solution, refinement, and data output were carried out with the SHELXTL program.23 Non-hydrogen atoms were refined anisotropically. 3384



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Figure 2. (a) Asymmetric unit in 10. Water molecules are excluded. (b) H-bonding between discrete a chromium molybdate cluster, {Ni(4-pyc)2(H2O)4} complex and the free protonated ligand forming a chain.

Figure 3. A view along [010] shows sheet-like assembly of nickel complex and H24-pyc into arrays through CH 3 3 3 π and NH 3 3 3 π interactions in 10. Cluster anions occur in the voids. CH and NH hydrogen atoms were placed in geometrically calculated positions by using a riding model. OH hydrogen atoms of organic ligand were localized by difference Fourier maps and refined in subsequent refinement cycles. Hydrogen atoms on cluster oxygens could not be located. Images were created with the Diamond program.24 Hydrogen bonding interactions in the crystal lattice were calculated with SHELXTL and Diamond.23,24 Crystal and refinement data are summarized in Table 1. Other Physical Measurements. FTIR was recorded on a Nicolet 5DX spectrophotometer with pressed KBr pellets. TG analysis were carried out using PerkinElmer TGA7 and DTA7 system on well ground samples in flowing nitrogen atmosphere with a heating rate of 10 °C/min. Elemental analyses (C, H, N) were determined on Perkin-Elmer 2400 series II C, H, N analyzer. Room-temperature powder X-ray diffraction data were collected on a Bruker D8 Advance diffractometer using Ni-filtered CuKR radiation. Data were collected with a step size of 0.05° and at count time of 1 s per step over the range 2° < 2θ < 60°. Rietveld powder diffraction analysis of all the powders were carried out using Topas 4-2,

Figure 4. Waterorganic (H24-pyc) sheet in 10. For clarity, the cluster and the nickel complex are omitted. Bruker for ensuring homogeneity of the synthesized products and also to quantify if more than one phase is present.

’ RESULTS AND DISCUSSION Initially, we attempted crystallization from the metalorganic solution A (Scheme 2). In all the cases, we invariably obtained only polycrystalline samples. Under our reaction condition suitable single crystals could not be grown. Hence we performed the structural characterization using Rietveld analysis of powder X-ray diffraction data wherever a model structure was available with atomic coordinates. This enabled us to ascertain the homogeneity of the bulk sample and also benchmark the competitive 3385



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Figure 5. (a) Chromium molybdate clusters are coordinately linked to {Na(3-pyc)2} complex units forming zigzag 1-D chains in 13. Notice the different orientation of the sodium complex in comparison to {Na(4-pyc)2} in 9. The cluster coordinates in a bidentate mode to the metal as in 9. (b) OH 3 3 3 O and NH 3 3 3 O interactions lead to a sheet formation in 13 like 9 except the sheets are puckered due to different position of N in 3-pyc.

metalorganic complex phases that can occur during the reaction. Crystallization of solution A containing metal-4-pyc led to the precipitation of four phases: cobalt (1), nickel (2), copper (3), and zinc (4). Rietveld analysis established that the phases were monophasic and isostructural to the copper analogue 3, a 2-D CP, [Cu(4-pyc)2] 3 2H2O already known.25 We are reporting the synthesis and structural characterization of isostructural cobalt, nickel and zinc analogues for the first time (Table S1, Figures S1 and S2, Supporting Information). The reaction with the isomeric ligands 3-pyc and 2-pyc yielded only polycrystalline product. However, we were unable to match the powder XRD patterns to any known phases in these systems. In the absence of suitable single crystals, we were unable to characterize the structure. Crystallization from the metal 2-pzc solution yielded only polycrystalline samples in the presence of Co, Ni, and Cu. However, the reaction with zinc yielded pink colored single crystals. The cobalt (5) and zinc (8) phases were found to be monophasic and isostructural (Figure S3, Supporting Information). Single crystal analysis of 8 showed that the structure is identical to the phase already reported (Figure S3, Supporting Information),26 but we have grown it under different experimental conditions. The structure contains discrete zinc pyrazine carboxylate dihydrate complex interacting in the solid state through strong H-bonding interactions (Figure S4, Supporting Information). Powder XRD of the products obtained from the reaction with nickel and copper yielded multiphasic polycrystalline samples. Rietveld refinement of the powder XRD patterns showed the phase obtained from nickel was a quantitative mixture of three phases, [Ni(2pzc)2],27 [Ni(2-pzc)2] 3 2H2O,28 and [Ni(2-pzc)2(H2O)2]26b (Figure S3, Supporting Information); the first two are 1-D CP while the last one is discrete complex based solid (Figures S4S6, Supporting Information). The reaction of copper with 2-pzc yielded a mixture of two phases, 1-D CP [Cu(2-pzc)2] and a discrete complex based solid, [Cu(2-pzc)2(H2O)2]29 (Figures S3, S4, and S6, Supporting Information). Crystallization of MetalOrganic Solids from the System Metal H4-pyc AndersonEvans Cluster. Crystallization was attempted by mixing solution A containing H4-pyc and metal ions and B containing AndersonEvans cluster at room

temperature. Depending on the metal ion, we obtained different solids (Scheme 3). Under our reaction conditions, cobalt and zinc based Anderson-Evans clusters could not be prepared; instead a pink colored sodium containing crystal, (H24-pyc)2[{Na(4pyc)2}{CrMo6(OH)8O16}] 3 4H2O, 9, crystallized out of the solution. In the case of nickel, a novel metal complex incorporated AndersonEvans cluster based solid, (H24-pyc)2[{Ni(4-pyc)2(H2O)4}{CrMo6(OH)7O17}] 3 12H2O, 10, was isolated. The reaction with copper resulted into a mixture of two phases, blue crystals of [{Cu(4-pyc)2(H 2O)4}], 11, and pink crystals of a new solid (H24-pyc)2[{Cu(4-pyc)2 (H2O)4 }{CrMo6(OH)7O17}] 3 10H2 O, 12. Crystal Structure of (H24-pyc)2[{Na(4-pyc)2}{CrMo6(OH)8O16}] 3 4H2O, 9. The asymmetric unit of 9 contains a chromium molybdate cluster, {CrMo6O24}n, the complex unit {Na(4-pyc)2}, protonated H4-pyc and two water molecules. Na occurs in the octahedral geometry with two 4-pyc ligands and four oxygen atoms from two different chromium molybdate clusters where each cluster coordinates with sodium in a bidentate mode. 4-pyc gets coordinated to sodium atom through oxygen of carboxylate group and not through nitrogen. Each chromium molybdate cluster links two {Na(4-pyc)2} complex forming 1-D chain (Figure 1a). The adjacent chains are brought together through OH 3 3 3 O, NH 3 3 3 O, and CH 3 3 3 π interactions between the coordinated and free carboxylate groups. In addition, H-bonding occurs between the free H24-pyc and the molybdate cluster (Figure 1b,c). The free H24-pyc molecules also interact strongly with tetrameric ) further stabilizing the solid water oligomers (2.8353.362 Å state structure (Figure 1d,e). Bond distances for weak interactions are listed in Table S2, Supporting Information. Crystal Structure of (H24-pyc)2[{Ni(4-pyc)2(H2O)4}{CrMo6(OH)7O17}] 3 12H2O, 10. The asymmetric unit of 10 contains a discrete AndersonEvans cluster, {CrMo6O24}n and {Ni(4-pyc)2(H2O)4} complex along with H24-pyc and six water molecules. {Ni(4-pyc)2(H2O)4} complex contains Ni in octahedral geometry with a pair of 4-pyc ligands and four water molecules (Figure 2a). As in 9, the ligand 4-pyc gets coordinated to nickel atom through oxygen of carboxylate group. Unlike 9, the alternating molybdate cluster and Ni4-pyc complex form 1-D 3386


Crystal Growth & Design chain through strong H-bonding between cluster oxygens and coordinated water (2.641 Å). Further, these H-bonded chains interact with mediating water molecules (or lattice water) in the other dimension. Interestingly, the mediating water molecules also interact strongly between each other forming almost linear 1-D chains (Figure 2b). In this structure also, free protonated H4-pyc ligands are H-bonded with coordinated water molecules through

Figure 6. 1-D copper pyrazine carboxylate coordination polymer covalently linked to AndersonEvans cluster forming a 2-D sheet in 15. Cobalt analogue (14) is isostructural to 15.

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OH 3 3 3 O bonds (2.728 Å) and cluster oxygens (2.659 Å). {Ni(4-pyc)2(H2O)4} complex stack over each other via CH 3 3 3 π interactions (3.486 Å). {Ni(4-pyc)2(H2O)4} complex are linked to protonated H4-pyc ligands through weak π 3 3 3 π interactions. Also, protonated H4-pyc ligands are stacked in pairs via CH 3 3 3 π (3.556 Å) and NH 3 3 3 π (3.554 Å) interactions. All these interactions resulted in the formation of 2-D sheets with voids when viewed along b axis (Figure 3). It appears to be zigzag chain along a axis (Figure S7, Supporting Information). These voids are occupied by molybdate clusters via NH 3 3 3 O (1.769 Å) and OH 3 3 3 O (1.919 Å) bonds (Figure 3). There are 12 water molecules per unit cell in 10. Water molecules are involved in H-bonding with clusters and {Ni(4-pyc)2(H2O)4} complexes forming a 3-D network (Figure S7, Supporting Information). Water molecules aggregate through H-bonding to form chains with bond lengths in the range of 2.5063.726 Å. The water chains are connected to free H24-pyc ligands leading to the formation of water-organic sheet (Figure 4). Crystal Structures of [{Cu(4-pyc)2(H2O)4}], 11, and (H24pyc)2[{Cu(4-pyc)2(H2O)4}{CrMo6(OH)7O17}] 3 10H2O, 12. 11 and 12 are concomitant phases crystallizing from the same solution. Crystal structure of 11 is similar to the phase already reported.30 In both the structures, the same copper organic complex occurs as one of the building blocks. Rietveld analysis of powder XRD data showed 11 to be a minor phase. The structure of 11 is based on discrete copper organic complex (Figure S8, Supporting Information); isostructural phases are known for Co, Ni, and Zn, though none of them were encountered under our reaction condition. The crystal structure of 12 is similar to that of

Figure 7. (a) The sheets contain water chains in the cavities (13.758 Å 13.59 Å) projecting along the a axis. (b) Aggregation of water molecules forming water chains in 15. 3387



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Figure 8. The tecton, metal pyrazine carboxylate, [M(2-pzc)2(H2O)2] can assemble through three different supramolecular pathways. Condensation (with or without water loss) can lead to three different compositions. The first one is a molecular solid observed with all the four metal ions. The anhydrous 1-D CP is observed only in the presence of Ni and Cu. The hydrated 1-D CP is known only in the case of Ni. Note the polymeric chains interact through CH 3 3 3 O in the anhydrous phase, while the chains interact through mediating water molecules in the hydrated phase. In this work, we observed the simultaneous occurrence of all three phases with nickel and only two phases with copper.

10 in the way the molybdate cluster and copper organic complex form a chain. However, the number of water molecules is less in the case of copper. Like 10, the water molecules aggregate forming a 1-D chain (2.6823.655 Å) and strongly interact with the chains formed between the cluster and copper organic complex. The water chains are connected to free protonated H4-pyc ligands leading to the formation of waterorganic sheet similar to 10 (Figures S8 and S9, Supporting Information). Crystallization of MetalOrganic Solids from the System Metal H3-pyc AndersonEvans Cluster. When a similar reaction was done with H3-pyc as organic ligand instead of H4pyc, different solids were obtained. Similar to 4-pyc, cobalt did not

react with the molybdate cluster; instead sodium complex with 3-pyc yielded pink crystals of (H23-pyc)2[{Na(3-pyc)2}{CrMo6(OH)8O16}] 3 2H2O, 13. We obtained only multiphasic polycrystalline products with nickel, copper, and zinc. Powder XRD patterns did not match with any known phases reported in the literature and lack of suitable single crystals prohibited further structural analysis. Crystal Structure of (H23-pyc)2[{Na(3-pyc)2}{CrMo6(OH)8O16}] 3 2H2O, 13. The asymmetric unit of 13 contains an Anderson Evans type of cluster coordinately linked to the complex, {Na(3pyc)2}, protonated H3-pyc and one lattice water molecule. Na exhibits octahedral geometry with two 3-pyc ligands and four 3388



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Table 2. Structural Landscape of the System, M(H2O)n2+Organic Ligand and M(H2O)n2+Organic Ligand {HmCrMo6O24}n (M = Co, Ni, Cu, and Zn)a

The phases isolated for the first time in this study and the structures were characterized by Rietveld analysis of the powder X-ray diffraction data. Refer to Table 3 for the description of Class I, Class II, etc.

a

oxygens from two molybdate clusters. As observed in previous cases, 3-pyc gets coordinated to sodium atom through oxygen of carboxylate. The complex {Na(3-pyc)2} is coordinately connected to two molybdate clusters on each side forming a zigzag chain (Figure 5a). Such chains are connected to each other through protonated ligands via H-bonding. H23-pyc is H-bonded to the terminal oxygen of the Anderson cluster of one chain through protonated nitrogen via NH 3 3 3 O (2.166 Å) bonds and to another chain through OH 3 3 3 O (1.782 Å) bonds to the bridged oxygen of the cluster of another chain forming a corrugated sheet type structure (Figure 5b). A pseudopolymorph of 13,

(3-pyc)2[{Na3(3-pyc)2}{CrMo6(OH)6O18}], is reported in literature, where similar chain was observed but the chains are connected through sodium atoms forming a sheet (Figure S10, Supporting Information).31 The synthetic conditions for the preparation of this solid are different from the present experimental conditions. The corrugated sheets in 13 are stacked one over the other through H-bonding leading to a 3-D framework (Figure S11, Supporting Information). The structure also contains CH 3 3 3 π interactions (3.1343.583 Å) among coordinated 3-pyc and free H23-pyc molecules stacking one over other. H23-pyc molecules also form chain via very weak CH 3 3 3 O (2.969 3389



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Scheme 4. Classification of AndersonEvans Cluster Based Solids Based on Retrosynthetic Analysisa

a

Tectons are the chemically reasonable molecular building blocks.

and 3.221 Å) interactions. All the weak interactions resulted in the formation of network structure with voids when viewed along the c-axis. These voids are occupied by lattice water molecules by means of H-bonding with the cluster oxygens (Figure S12, Supporting Information). Crystallization of MetalOrganic Solids from the System Metal H2-pzc AndersonEvans Cluster. Reactions of 2-pzc with cobalt and copper yielded isostructural 1-D CP coordinated to a chromium molybdate cluster, [{M2(2-pzc)2(H2O)4}{CrMo6(OH)7O17}] 3 17H2O (M = Co, 14 and Cu, 15). These are the first examples of a coordination polymer coordinated to an AndersonEvans cluster. In the case of nickel and zinc, the samples were multiphasic as inferred from optical microscopy. We did not analyze the phases further due to lack of suitable single crystals or model structures for refining powder data.

Crystal Structure of [{M2(2-pzc)2(H2O)4}{CrMo6(OH)7O17}] 3 17H2O (M = Co, 14 and Cu, 15). Single crystal X-ray

analysis revealed that 14 and 15 were isostructural. The asymmetric unit contains an AndersonEvans type of cluster coordinated to {M(2-pzc)(H2O)2} (M = Co and Cu) unit and nine lattice water molecules. Metal (Co and Cu) exhibits distorted octahedral geometry with the 2-pzc acting as bidentate and monodentate on either side, two water molecules and one oxygen from the molybdate cluster. Metal is linked to 2-pzc on both sides such that it forms 1-D CP of the composition {M(2-pzc)(H2O)2(O2/2)}. Each metal is coordinated to one AndersonEvans cluster in an alternative direction forming a ladder like sheet containing cavities (Figure 6). The sheets are stacked over each other (Figure S13, Supporting Information) through H-bonding between coordinated water and cluster oxygens (2.5902.711 Å). There are 17 water molecules per unit cell. The water molecules 3390



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Table 3. Classification and Structural Features of AndersonEvans Cluster Based Solids classes

examples

Class I: Salts based on discrete AndersonEvans cluster anions and organic

(H2pip)5[AlMo6(OH)6O18]2(SO4)2 3 16H2O, (BEDT-TTF)4[CrMo6(OH)6O18] 3 2H2O, (ET)6[MnMo6((OCH2)3CNH2)2O18] 3 2CH2Cl2,

counter cations (includes derivatized clusters). All the molecules assemble in the solid state through supramolecular interactions.

(TBA)3[MnMo6((OCH2)3CNH-CO-(CH2)14CH3)2O18] 3 7H2O 3 2DMF, (TBA)4[IMo6(OH)O23]38

[{Co(2,20 -bpy)3}{Al(OH)7Mo6O19}] 3 17H2O,

Class II: Both AndersonEvans clusters and metalorganic complex cations are discrete and assemble only through nonbonding interactions

[{Cu2(phen)2(CH3COO)(CH3COOH)(H2O)2}-

in the solid state.

{Al(OH)6Mo6O18}] 3 28H2O, [{Fe(C5Me5)2}3{CrMo6(OH)6O18}] 3 20H2O,

[{Nd(C2H5NO2)2(H2O)5}{CrMo6(OH)6O18}] 3 10H2O,16a,31,39 [{Cr3O(CH3COO)6(H2O)3}2{H7CrMo6O24}] 3 24H2O12d

[{Cu(4-pyc(phen)(H2O)}2{CrMo6(OH)6O18}](H3O+) 3 5H2O,

Class III: AndersonEvans cluster derivatized by metalorganic complex or metal hydrate but does not extend. Further interactions in the solid

[{Cu(2,20 -bpy)(H2O)3}2{CrMo6(OH)6O18}] [{Cu(2,20 -bpy)

state are supramolecular in nature and may occur with or without solvent

(H2O)Cl}{Cu(2,20 -bpy)(H2O)(NO3)}{CrMo6(OH)6O18}] 3 18H2O, 40

[(Yb(H2O)6)2(TeMo6O24)] 3 0.10H2O (C6H15N5O)2[Na3 {AlMo6(OH)6O18}]2Cl 3 20H2O, (C6H5NO2)4[{Cd3(H2O)14}{CrMo6(OH)6O18}2],41

mediation. Class IV: AndersonEvans cluster coordinated to metalorganic complex or metal hydrate extending into anionic 1-D chain or 2-D sheet; discrete

(Hpyz)[{Co(pyz)2(H2O)2}{CrMo6(OH)6O18}] 3 2H2O, (Hpyz)[{Zn(pyz)2(H2O)2}{CrMo6(OH)6O18}] 3 2H2O,

organic molecules occur as counter cations and show nonbonding interactions between themselves as well as the cluster and metalorganic

(Hpyz)[{Ni(pyz)2(H2O)2}{CrMo6(OH)6O18}] 3 2H2O [{Ni(pyz)(H2O)4}2{CrMo6(OH)6O18}](CH3COO)2 3 6H2O,12c 12c

groups. Class V: AndersonEvans cluster occurs as discrete anion and metalorganic coordination polymer occurs as countercation. Aggregation

[{(H2O)2(HC6H4NO2)Cd(C6H5NO2)}2

of the anion and cation is through nonbonding interactions.

{CrMo6(OH)6O18}] 3 9H2O41b

Occasionally the cluster anion may coordinate with the copprdination polymer. [{Cu(2,20 -bipy)(H2O)2Cl}{Cu(2,20 -bipy)(H2O)2}

Class VI: This class includes Anderson cluster coordinated to

{AlMo6(OH)6O18}] 3 4H2O,

metalorganic complex or metal hydrate extending into 1-D chain,

[{Na3(H2O)11}{CrMo6(OH)6O18}] 3 2H2O,42

2-D sheet, or 3-D framework.

[{Cu2(ox)(pz)4}{H7CrMo6O24}].11H2O,

[{Cu(pz)2(H2O)2}{Cu2(ox)(pz)4}{H5CrMo6O24}] 3 8H2O, [{Cu(pz)3Cl}{Cu2(ox)(pz)4}{H6CrMo6O24}] 3 8H2O,12d

[Na(CrMo6(OH)8O16)] 3 10H2O12c

interact among themselves via H-bonding to form water chains (2.203.380 Å) (Figure 7). The cavities formed in sheets are occupied by water chains pointing perpendicular to the a-axis. Water chains show extensive H-bonding with the cluster oxygens and coordinated water molecules to form a 3-D network (Figures 7 and S13, Supporting Information). Structural Chemistry of AndersonEvans Cluster Based MetalOrganic Solids Retrosynthesis. Crystallization is a supramolecular reaction.32 Synthesis of a crystal can be interpreted using retrosynthetic tool33 that breaks down a structure into one or more tectons; a tecton-synthon34 model can thus be a blueprint for metalorganic crystal engineering.12,14,15,35 In the present work, all the crystals are synthesized from two main tectons: chromium molybdate cluster and a metal complex with 1:1 or 1:2 stoichiometry. In addition, free ligand and/or water molecules can take part in the supramolecular aggregation to stabilize a particular structure. We chose three ligands pyrazine, pyridine carboxylic acid, and pyrazine carboxylic acid that are potential for forming CPs with the four divalent metal ions. It is difficult to correlate the stoichiometry of a crystal to the molar ratio of the reactants. However, a meaningful structuresynthesis correlation is possible if we consider the interaction between appropriate tectons. Table 2 summarizes our results on the crys-

tallization of metalorganic solids in the absence and the presence of chromium molybdate clusters (refer also to Figure S14, Supporting Information). In the absence of chromium molybdate clusters, the divalent metal ions (Co, Cu, and Zn) formed anhydrous metal complex based solids with pyrazole; a nickel analogue is yet to be reported. All the four metal ions readily formed monophasic CP with pyz. However, the CP is 2-D in the case of isostructural cobalt and nickel, while isostructural copper and zinc formed only 1-D CP. Reaction of the metal ions with 4-pyc yielded isostructural hydrated 2-D CP; Co, Ni, and Zn phases are reported for the first time in this work (Table 2). Reaction with 2-pzc was surprising. Cobalt and zinc showed only monophasic discrete complex based solid (0-D) with water as part of the metal coordination. Copper showed a mixture of two supramolecular isomers, one isostructural with cobalt and zinc analogue (0-D) and another anhydrous CP (1-D). Nickel showed a mixture of three phases: two are isostructural to copper analogues, that is, supramolecular isomers, while the third one is a hydrated CP (1-D) a pseudopolymorph. The structures of all the solids can readily be explained in terms of retrosynthesis (Figures 8 and S15, Supporting Information). In the case of pyrazole or pyrazine, a counteranion is required for charge compensation. Strong intermolecular interactions 3391


Crystal Growth & Design stabilize extended 3 3 3 MXM 3 3 3 and MorgM in the case of pyrazine based solids. If the countercation varies from chloro to sulfate, composition of the tecton vary leading to a different crystal packing.35a It is however difficult to determine why cobalt or nickel and copper or zinc forms 2-D and 1-D CP respectively. The fact that copper phase precipitates rapidly within a few minutes suggests a low solubility and high stability for this solid. In the case of 4-pyc, a pair of ligands provides charge compensation. Strong metal coordination to pyridyl nitrogen and carboxylate oxygen favors 2-D CP. The presence of free carboxylate oxygen, which is a strong H-bond acceptor, necessitates water mediation to stabilize the structure. The case of 2-pzc is interesting as it shows the varying supramolecular reactivity of the metal complex. The examples clearly show how the same tecton [ML2(H2O)2] (M = Co, Ni, Cu and Zn; L = 2-pzc) can assemble in three different supramolecular pathways to account for the intermolecular interactions observed in the final crystal (Figure 8). The two phases, [M(2-pzc)2(H2O)2] and [M(2-pzc)2] 3 2H2O, M = Ni or Cu, should be considered as supramolecular isomers and not polymorphs though they have the same composition.36 The kinetics of two or three supramolecular assemblies occurring with appreciable rate probably resulted in a mixture of phases. In the case of cobalt and zinc, only a metalorganic complex based molecular solid has been isolated. Lowering of solubility or adopting a solvothermal route may lead to the occurrence of other phases. Retrosynthesis of all Anderson-Evans cluster based solids including those reported in the present study provides insight into the supramolecular aggregation preceding crystallization. Crystal structure database (CSD)37 has reported more than 135 structures that are stabilized in the presence of several metal ions (Na, Fe, Cu, Zn, Ru, Ag, Cd, La, Ce, Nd, Sm, Eu, Tb, and Ho) and organic ligands. Structural chemistry of all AndersonEvans cluster based solids can be categorized into six different classes (Scheme 4 and Table 3). All the structures can be interpreted in terms of supramolecular aggregation between two major tectons: AndersonEvans cluster and metalorganic complex; in addition free ligands as counter cations and water molecules can take part in the supramolecular assembly. Table 3 summarizes the basic features of structure building with appropriate examples. We have already reported our results on the reaction with pyrazole and pyrazine.12c,d Whenever a metal ion is more soluble or less reactive with organic or the cluster, then sodium hydrate or sodium organic complex has a tendency to compete for coordination with the molybdate cluster and hence sodium hydrate or complex based molybdates precipitate preferentially. Lower solubility can be interpreted as stronger interaction. In this work we have examined the structural landscape of AndersonEvans clusters with four divalent metal ions (Co, Ni, Cu, and Zn) and three organic linkers that are favorable to form metal CP (Table 2). The ligand 4-pyc readily forms complex with metal as well as sodium; however, the relative solubility of these complexes probably favors different assembly with the cluster anion. In the case of nickel and copper, the respective 1:2 metal complex aggregates with discrete molybdate cluster (Class II). In contrast, the cobalt and zinc complex are more soluble, and hence the sodium complex based solids crystallize. In these cases, the chains are anionic and require counter cations (Class IV). 9 and 13 belong to this class of solid. In some cases, the cluster does not condense with metal complex but aggregates into chains through H-bonding (Class II; 10 and 12). Class V solids are represented

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by 14 and 15 wherein metal forms a CP with the organic and coordinates with the AndersonEvans cluster. The structures are further stabilized either through coordinate bonding with another metal complex, water aggregation, or organicwater aggregation. As pointed out in our earlier papers,12,14,15 water aggregation into oligomers, chains (14 and 15 in the present work), or sheets in these complex solids is the manifestation of the supramolecular assembly dictated by the aggregating tectons to optimize the enthalpically favorable coordination and noncovalent bonds and the entropically unfavorable water clusters or extended aggregates. Inclusion of free organic in 9, 10, 12, and 13 is not accidental. In all the cases, it facilitates the stacking of metal complexes in one dimension through π 3 3 3 π and CH 3 3 3 π interactions; also it provides counter charge. In 10 and 12, the free organic group shows organicwater interaction as observed in other POM based solids.7b The absence of Anderson cluster based metalorganic solids suggest that either the metal ions form stable complexes or CP. It also implies less favorable interaction between Lewis acidic metal complexes and basic clusters. For example, in the case of 4-pyc the metal prefers to coordinate with the carboxylate oxygen when the AndersonEvans cluster is involved in the supramolecular aggregation. In its absence, metal coordinates with the nitrogen of the ligand. The ligand 2-pzc favors chelation with all metal ions. However, we succeeded in isolating 1-D CP coordinated with molybdate clusters only in the case of Co and Cu (Class V). Protonation in AndersonEvans Cluster Based Solids. Protonation in polyoxomolybdate cluster based solids is always difficult to address. Among AndersonEvans clusters, protonation is possible in three types of oxygens: terminal, doubly bridged, or triply bridged. Table S3, Supporting Information shows the BVS calculations for 9, 10, 1215. In Figure S16, Supporting Information, we have summarized bond valence sum (BVS) calculations for all chromium molybdates cluster based solids reported in the literature as well those reported here. In almost all the cases, triply bridged oxygen is invariably protonated. Terminal oxygen is unlikely to get protonated. There are two possibilities for protonation to occur: doubly bridged or protonation of the lattice water. In either case, statistical distribution over many sites will make BVS calculations difficult to interpret. pH is an important variable for protonation in polyoxomolybdate anions. For all the solids reported here pH ∼12; this may also cause protonation of the cluster oxygens in addition to the triply bridged oxygens.

’ CONCLUSIONS The paper is an attempt to understand the structural land scape of the system chromium molybdate cluster divalent metal ion organic ligand; by choosing appropriate ligands, one could understand the engineering of a crystal through metal coordination interactions. High throughput crystallization is desirable to examine the structural landscape, and retrosynthetic tools can provide guidelines for understanding crystal synthesis from tectonsynthon model. ’ ASSOCIATED CONTENT

bS

Supporting Information. Crystallographic information files (CIF) for 9, 10, 1215. Supporting Information also contains TG and FTIR analysis for 9, 10, 1215. Rietveld refinement plots for 115. Structures for 11, 12, and (3-pyc)2[{Na3(3pyc)2}{CrMo6(OH)6O18}]. Figures showing the crystallization

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Crystal Growth & Design of 14, 9, and 10. This information is available free of charge via the Internet at http://pubs.acs.org/

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel: +91-11-26581507. Fax: +91-11-26581102.

’ ACKNOWLEDGMENT M.S. acknowledges UGC for a research fellowship. A.R. acknowledges DST, Government of India, for financial support as well as for powder X-ray diffractometer under IRHPA and single crystal diffractometer under FIST to the Department of Chemistry, IIT Delhi. A.R. gratefully acknowledges Professors G. R. Desiraju, Mike Zaworotko, and R. Robson for stimulating discussion and valuable suggestion during the first GRC meeting on Crystal Engineering held at Waterville Valley, NH, USA. ’ REFERENCES (1) (a) Desiraju, G. R. In Crystal Engineering: The Design of Organic Solids; Elsevier Scientific Publishers: Amsterdam and New York, 1989. (b) Braga, D.; Grepioni, F. In Making Crystals by Design; Wiley-VCH Verlag GmbH & Co. KGaA: Germany, 2007. (c) Batten, S. R.; Neville, S. M.; Turner, D. R. In Coordination Polymers: Design, Analysis and Application; RSC Publishers: Cambridge, 2009. (2) (a) Schuth, F. Curr. Opin. Solid State Mater. Sci. 2001, 5, 389–395. (b) Erdemir, D.; Lee, A. Y.; Myerson, A. S. Acc. Chem. Res. 2009, 42, 621–629. (3) (a) Muller, A.; Kogerler, P. Coord. Chem. Rev. 1999, 182, 3–17. (b) Shishido, S.; Ozeki, T. J. Am. Chem. Soc. 2008, 130, 10588–10595. (c) M€uller, A.; Peters, F.; Pope, M. T.; Gatteschi, D. Chem. Rev. 1998, 98, 239–271. (d) Long, De-L.; Burkholder, E.; Cronin, L. Chem. Soc. Rev. 2007, 36, 105–121. (e) Sloan, H. Annu. Rep. Prog. Chem., Sect. A 1999, 95, 129–152. (f) Xu, Y. Curr. Opin. Solid State Mat. Sci. 1999, 4, 133–139. (g) Hagrman, P. J.; Hagrman, D.; Zubieta, J. Angew. Chem., Int. Ed. 1998, 38, 2638–2684.(h) Hagrman, P. J.; Hagrman, D.; Zubieta, J. In Polyoxometalate Chemistry; Kluwer Academic Publishers: The Netherlands, 2001. (4) (a) Cindric, M.; Veksli, Z.; Kamenar, B. Croat. Chem. Acta 2009, 82, 345–362. (b) Yamase, T. Mol. Engg. 1993, 3, 241–262. (c) Kortza, U.; M€uller, A.; Slageren, J. V.; Schnacke, J.; Dalal, N. S.; Dressel, M. Coord. Chem. Rev. 2009, 253, 2315–2327. (d) Schaming, D.; Allain, C.; Farha, R.; Goldmann, M.; Lobstein, S.; Giraudeau, A.; Hasenknopf, B.; Ruhlmann, L. Langmuir 2010, 26, 5101–5109. (e) Abbas, H.; Pickering, A. L.; Long, De-L.; Kogerler, P.; Cronin, L. Chem.—Eur. J. 2005, 11, 1071–1078. (f) Michailovski, A.; Patzke, G. R. Chem.—Eur. J. 2006, 12, 9122–9134. (5) (a) Hagrman, D.; Zubieta, C.; Rose, D. J.; Zubieta, J.; Haushalter, R. C. Angew. Chem., Int. Ed. Engl. 1997, 36, 873–876. (b) Hagrman, D.; Sangregorio, C.; O’Connor, C. J.; Zubieta, J. J. Chem. Soc., Dalton Trans. 1998, 3707–3709. (6) (a) Meng, J. X.; Lu, Y.; Li, Y. G.; Fu, H.; Wang, E. Cryst. Growth Des. 2009, 9, 4116–4126. (b) Lan, Y.-Q.; Li, S.-L.; Wang, X.-L.; Shao, K.-Z.; Su, Z.-M.; Wang, E. Inorg. Chem. 2008, 47, 529–534. (c) Long, D.-L.; Kogerler, P.; Farrugia, L. J.; Cronin, L. Angew. Chem., Int. Ed. 2003, 42, 4180–4183. (7) (a) Pavani, K.; Loftland, S. E.; Ramanujachary, K. V.; Ramanan, A. Eur. J. Inorg. Chem. 2007, 568–578. (b) Upreti, S.; Ramanan, A. Cryst. Growth Des. 2005, 5, 1837–1843. (8) (a) Liao, J.-H.; Juang, J.-S.; Lai, Y.-C. Cryst. Growth Des. 2006, 6, 354–356. (b) Allis, D. G.; Rarig, R. S.; Burkholder, E.; Zubieta, J. J. Mol. Struct. 2004, 688, 11–31. (9) (a) Zheng, P. Q.; Ren, Y. P.; Long, L. S.; Huang, R. B.; Zheng, L. S. Inorg. Chem. 2005, 44, 1190–1192. (b) Ren, Y. P.; Kong, X. J.; Long,

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