Novel Supramolecular Assemblies of Coordination Polymers of Zn(II

Aug 18, 2007 - M. John Plater , Ben M. de Silva , Mark R. St J. Foreman , William T.A. Harrison. Journal of Coordination Chemistry 2015 68, 1719-1732 ...
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Novel Supramolecular Assemblies of Coordination Polymers of Zn(II) and Bis(4-nitrophenyl)phosphoric Acid with Some Aza-Donor Compounds Samipillai Marivel, Manishkumar R. Shimpi, and Venkateswara Rao Pedireddi*

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 9 1791-1796

Solid State & Supramolecular Structural Chemistry Unit, DiVision of Organic Chemistry, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India ReceiVed February 27, 2007; ReVised Manuscript ReceiVed June 29, 2007

ABSTRACT: Supramolecular assemblies of coordination polymers of Zn(II) and bis(4-nitrophenyl)phosphoric acid, BNPP, with aza-donor compounds, 4,4′-bipyridine (bpy), 1,2-bis(4-pyridyl)ethane (bpyea), 1,2-bis(4-pyridyl)ethene (bpyee), and 1,3-bis(4-pyridyl)propane (bpypa), have been reported. All the compounds are quite similar in the three-dimensional packing, irrespective of the varied dimensions of aza-donor molecules. All four compounds were synthesized by treating BNPP with corresponding aza-donor molecules in the presence of MeOH and water as the solvents. The structures were characterized by single-crystal X-ray diffraction. Introduction rganic-inorganic hybrid supramolecular assemblies1,2 with fascinating as well as exotic architectures and with many potential applications in the areas of separation technology, catalysis, nanotechnology, etc., are important materials for the 21st century requirements.3 Design and synthesis of these assemblies are challenging, and continuous efforts to tune the nature of the interactions between the reactants, to obtain the desired structures, are always at the forefront in the areas of supramolecular chemistry research.4 In this direction, metal carboxylate-directed hybrid structures5 with varied architectures and unusual properties are of great value in both academic and industrial aspects, providing avenues for a thorough understanding of the nature of the fundamental aspects such as metal and ligand interaction, parameters that influence the ultimate geometry of the resultant assembly.6 For example, tremendous contributions by several researchers employing terephthalic acid, trimesic acid, and benzenetetracarboxylic acid with different metal species are superb and unique by all means.7-9 In a parallel direction, apart from the metal carboxylates, metal phosphates have also been well studied,10-12 especially to develop channel structures mimicking natural zeolites that are made up of phosphate-mediated networks. In this regard, fascinating architectures were designed and synthesized using different types of synthetic strategies ranging from simple crystallization to hydrothermal methods.13,14 It is important to note that these systems indeed find applications not only in the development of catalytic processes that are unique for zeolite type structure but also in many other fronts such as separation technology, selective chemical transformations, etc.15 Whatsoever the systems (metal carboxylates or phosphates), the emphasis is mostly on the coordination features of metal ions, and thus the geometry of the resultant assembly is often related to the coordination number and coordination geometry of the metal ion under consideration.1,2,16 To our knowledge, attention toward the molecular geometry of the organic components are not well explored, probably, because the geometries of the ligands generally being used in the study such as * Author to whom correspondence should be addressed. Fax: +91 20 25902624; tel: +91 20 25902097; e-mail: [email protected].

Scheme 1. Some of the Observed Recognition Patterns in Carboxylic, Phosphoric, and Phosphinic Acids

carboxylates and phosphates are more or less rigid. From our experience in the synthesis of organic assemblies, we find a close analogy between carboxylic acid and organophosphorous acids as shown in Scheme 1. In addition, it is apparent that the geometry of the substituted ligands on organophosphorous acids expanding into the space in a variety of modes could result in the formation of novel assemblies.14,17 Thus, a combination of these moieties with metal ions would be very interesting, as the ultimate geometry could be tuned through the interactions involving organic functional groups as well. For this purpose, we have considered the synthesis of metal complexes of bis(4-nitrophenyl)phosphoric acid (BNPP) with Zn(II) in the presence of aza-donor compounds, bpy, bpyea, bpyee, and bpypa, which are well-known spacer molecules in the supramolecular chemistry. Thus, we obtained compounds, 1a, 1b, 1c, and 1d as described in Scheme 2, and we discuss the exotic features of these assemblies in this manuscript. Results and Discussion An aqueous methanolic solution of mixture of BNPP, Zn(NO3)2‚6H2O, and the corresponding aza-donor ligand gave compounds 1a-d. Analysis and characterization of the compounds were carried out by FT-IR and single-crystal X-ray diffraction methods. Infrared Spectroscopy Study. IR spectra of pure BNPP (see Supporting Information) has very broad bands centered around at 2860 and 2360 cm-1. On the basis of the reported data for the diphosphoric acids, the bands can be attributed to the PO-H streching vibrations.18 Absence of such peaks in compounds 1a-d and appearance of a sharp peak at 1020 cm-1 represents the P-O stretching corresponding to metal phosphates. Both the sharp peaks at 1102 and 1232 cm-1 can be attributed to the PdO stretching of phosphate groups of the metal complexes. Two strong absorptions at 1348, 1379 and one at 1510 cm-1

10.1021/cg070194v CCC: $37.00 © 2007 American Chemical Society Published on Web 08/18/2007

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Figure 1. ORTEP of asymmetric units in 1a-d.

Scheme 2.

Structures of the Reactants and Reaction Procedure

are due to the symmetrical and asymmetrical NO2 stretching frequencies, respectively. X-ray Crystallographic Study. In all the compounds 1ad, the composition of the constituents are in a 2:1:1 ratio of BNPP, Zn(II), and the respective aza-donor compound (bpy, bpyea, bpyee, and bpypa, as the case may be) as shown in ORTEP drawings of the asymmetric units in Figure 1, for all four compounds 1a-d. Crystallographic details for all the compounds are given in Table 1. In the crystal structures, each Zn(II) is coordinated to four molecules of BNPP and two molecules of the corresponding aza-donor compound. A typical coordination geometry is shown in Figure 2 , for compound 1a. Each BNPP ligand forms

bismonodentate Zn-O coordinate bonds bridging two adjacent Zn(II). Some of the intramolecular bond distances are given in Table 2. Thus, the BNPP molecules occupy the equatorial coordination sites of Zn(II). The aza compounds, bpy, bpyea, bpyee, and bpypa, form Zn-N coordination bonds (Table 2) in an axial direction, completing the hexacoordination geometry around each Zn(II). In the crystal lattice, these coordination units are held together, yielding polymer chains due to the formation of Zn-N dative bonds (see Table 2) through the second hetero N atom on the aza-donor compounds. Although 1a-d show common features of coordination geometry, the packing of molecules in the crystal lattice show

Novel Supramolecular Assemblies

Crystal Growth & Design, Vol. 7, No. 9, 2007 1793 Table 1. Crystallographic Data for the Compounds 1a-d

formula M crystal system space group a/Å b/Å c/Å R/deg β/deg γ/deg V/Å3 Z Dcalc(g cm-3) µ/mm-1 T/K Mo KR 2θ range (deg) F(000) indep reflns measured reflns observed refns (I > 2σ(I)) parameters GOF on F2 R1 (observed) wR2 (observed) R1 (all) wR2 (all)

1a

1b

1c

1d

C34H24O16N6P2 Zn1 899.90 monoclinic C2/c 30.154(5) 5.396(1) 22.978(4) 90 106.76(1) 90 3580(1) 4 1.655 0.862 298 0.7107 50 1832 3155 8532 2175 336 0.957 0.037 0.094 0.051 0.108

C36H28O16N6P2 Zn1 927.95 orthorhombic Pbcn 25.780(4) 13.529(2) 10.874(2) 90 90 90 3792.6(1) 4 1.618 0.817 298 0.7107 50 1896 3348 25691 2736 299 1.080 0.042 0.125 0.054 0.135

C36H26O16N6P2 Zn1 925.94 orthorhombic Pbcn 25.742(6) 13.556(3) 10.828(2) 90 90 90 3778.5(1) 4 1.624 0.820 298 0.7107 50 1888 3328 17775 2684 305 0.820 0.038 0.117 0.049 0.128

C74H60N12O32P4 Zn2 1883.96 triclinic P jı 10.727(3) 16.014(4) 23.426(6) 98.86(1) 92.60(1) 94.44(1) 3957.5(2) 2 1.581 0.785 298 0.7107 50 1928 13891 38588 8138 1193 0.916 0.047 0.097 0.095 0.134

Table 2. Bond Distances (Å) of Coordination Bonds and Phosphate Groups in 1a-d bond

1a

1b

1c

Zn-O

2.042(2) 2.117(2)

2.131(2) 2.133(2) 2.096(3) 2.102(3) 1.463(2) 1.475(2) 1.607(2) 1.618(2)

Zn-N

2.213(2) 1.448(2)

2.129(3) 2.140(3) 2.084(4) 2.090(5) 1.467(3)

P-O

1.463(2) 1.590(2) 1.598(2)

1.475(3) 1.610(3) 1.614(3)

1d 2.102(3), 2.105(3), 2.108(3), 2.108(3) 2.110(3), 2.135(3), 2.137(3), 2.170(3) 2.142(3), 2.156(3), 2.159(3), 2.175(4) 1.468(3), 1.468(3), 1.472(3), 1.472(3) 1.473(3), 1.473(3), 1.474(3), 1.477(3) 1.597(3), 1.602(3), 1.604(3), 1.604(3) 1.607(3), 1.610(3), 1.611(3), 1.617(3)

Table 3. Characteristics of Important Hydrogen Bonds Observed in 1a-da D-H‚‚‚O C-H‚‚‚O

C-H‚‚‚N a

1a 2.39 2.55 2.60 2.60 2.89

3.307 3.367 3.132 3.402 3.747

1b 170 160 125 157 162

2.45 2.54 2.64 2.65 2.72 2.76 2.81 2.80

3.375 3.411 3.422 3.311 3.355 3.521 3.552 3.623

1c 172 157 142 128 126 135 137 148

2.50 2.51 2.55 2.71 2.81 2.81 2.92 2.87

3.365 3.377 3.408 3.250 3.342 3.373 3.532 3.627

1d 171 156 150 127 128 124 138 146

2.41 2.42 2.43 2.46 2.47 2.47 2.54

3.317 3.239 3.248 3.208 3.373 3.339 3.184

164 146 147 140 164 153 127

Three columns for each structure represent H‚‚‚O/N, C‚‚‚O/N (in angstroms) and angle (in degrees) respectively.

some variations. This appears to be due to the differences in the spatial arrangement of nitrophenoxy moieties on BNPP. In 1a, these two moieties make a dihedral angle of 90°, while similar units form an angle of 70° in 1b, 1c, and 1d. Structure of Zn[(C12H8N2O8P1)2‚C10H10N2], 1a. In 1a, the bpy molecules are disordered with two different orientations of nitrophenoxy moieties in a 50:50 ratio, and both orientations are represented in Figure 3. In each one-dimensional polymer, the nitrophenoxy moieties on the adjacent Zn(II) species interact with each other, encompassing bpy molecules, because of the spatial arrangement of the nitrophenoxy groups around the P atom in BNPP. Thus, a novel assembly results due to the combination of the metal coordination features and flexibility in the position of the substituents in organophosphorous acids. In two dimensions,

however, the adjacent nitrophenoxy moieties are involved in the formation of C-H‚‚‚O hydrogen bonds19 formed between aromatic H atoms and O atoms on NO2 groups. The H‚‚‚O distances lie in the range 2.4-2.8 Å (Table 3). Structure of Zn[(C12H8N2O8P1)2‚C12H12N2], 1b. In the structure of 1b, unlike in 1a, the nitrophenoxy moieties on bpyea are static but the methylene bridge is disordered in a 50:50 ratio. Although coordination geometry is quite similar between 1a and 1b, the structure of the polymer chains is different (see Figure 4). It is mainly because of the difference in the dihedral angle between the nitrophenoxy groups on BNPP. Thus, the two adjacent nitrophenoxy units from the different Zn(II) species are held together by C-H‚‚‚O hydrogen bonds. Such adjacent polymer chains are held together by C-H‚‚‚O hydrogen bonds in two-dimensions, forming a layered structure (Table 3).

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Structure of Zn[(C12H8N2O8P1)2‚C12H10N2], 1c. The situation in 1c is exactly identical to that of 1b, including a disordered pattern of the aza compound (bpyee) except that for the lengths of C-H‚‚‚O bonds (Table 3). The packing of molecules in 1c is shown in Figure 5 for comparison with that in Figure 4. Structure of Zn2[(C12H8N2O8P1)4‚(C13H14N2)2], 1d. In the metal complex 1d also, the coordination geometry around the metal ion, Zn(II), is the same as that found in 1a-c, forming one-dimensional coordination polymers with bpypa molecules surrounded by nitrophenoxy moieties. However, in contrast to 1a-c, such moieties that are surrounding the bpypa did not form intermolecular interactions. Nevertheless, the adjacent coordination polymers are held together by C-H‚‚‚O hydrogen bonds (Table 3), as shown in Figure 6. Conclusions

Figure 2. Coordination geometry around each Zn(II). The moieties in perpendicular to the BNPP are bpy molecules connected to Zn(II).

In conclusion, we have reported four novel supramolecular assemblies 1a-d, formed because of the coordination of Zn(II) and bis(4-nitrophenyl)phosphoric acid in the presence of aza-donor compounds such as bpy, bpyea, bpyee and bpypa. All the metal complexes are quite similar in their lattices, irrespective of the varied dimensions of the aza-donor compounds with a common architecture, as shown in Scheme 3. Formation of the novel networks because of the flexible geometry of the organic ligands in organophosphorous acids is significant for further exploration of many such assemblies with various other organophosphorous acids. Furthermore, this study emphasizes the utility of many organic functional groups in the design and synthesis of novel supramolecular architectures, rather than relying on the robust and well-explored carboxylates, phosphates, etc. We have also been working in those directions with a focus to develop pure organic supramolecular assemblies of unique architectures. Experimental Section

Figure 3. Coordination polymers in the crystal lattice of 1a. Notice the encompassing of bpy molecules by BNPP molecules.

Synthesis. In a typical synthesis, 20 mg (0.0588 mmol) of bis(4nitrophenyl)phosphoric acid dissolved in 10 mL of CH3OH, by gentle warming, was added to an aqueous solution of Zn(NO3)2·6H2O (17.5 mg in 5 mL of H2O). To the resulting solution, 9.1 mg of bpy was

Figure 4. BNPP molecules in a typical layer in the crystal lattice of 1b around bpyea molecules. The interaction between the BNPP molecules in the one-dimensional polymer is shown in the above inset, and the interaction between the polymers is shown in the inset at the bottom.

Novel Supramolecular Assemblies

Figure 5. Interaction of BNPP, bpyee, and Zn(II) in the crystal structure of 1c. Compare this with Figure 3 to appreciate the similarity between 1b and 1c.

Crystal Growth & Design, Vol. 7, No. 9, 2007 1795 Elemental Analysis. 1a: C, 45.17%; H, 2.57%; N, 9.12%; P, 6.75%. Calcd: C, 45.37%; H, 2.69%; N, 9.34%; P, 6.88%. 1b: C, 46.32%; H, 3.01%; N, 8.99%; P, 6.56%. Calcd: C, 46.59%; H, 3.05%; N, 9.05%; P, 6.67%. 1c: C, 46.47%; H, 2.75%; N, 8.96%; P,6.56%. Calcd: C, 46.69%; H, 2.83%; N, 9.08%; P, 6.69%. 1d: C, 47.02%; H, 3.10%; N, 8.77%; P, 6.43%. Calcd: C, 47.17%; H, 3.22%; N, 8.92%; P, 6.57%. X-ray Structure Determinations. Good quality single crystals of 1a-d were carefully selected with the aid of a polarized Leica microscope equipped with a CCD camera and glued to a glass fiber using an adhesive (cyano acrylate). In all the cases, the crystals were smeared in the adhesive solution to prevent decay of crystals upon exposure to X-rays. The intensity data were collected on a Bruker single-crystal X-ray diffractometer, equipped with an APEX detector, at room temperature (298 K). Subsequently, the data were processed using the Bruker suite of programs (SAINT),20 and the convergence was found to be satisfactory with good Rini parameters. The details of the data collection and crystallographic information are given in Table 1. The structure determination by direct methods and refinements by least-squares methods on F2 were performed using the SHELXTLPLUS20 package. The processes were smooth without any complications. All non-hydrogen atoms were refined anisotropically. All the intermolecular interactions were computed using PLATON.21

Acknowledgment. We thank DST and BRNS for financial assistance on the ongoing projects. One of us (S.M.) thanks UGC for the award of a Research Fellowship. Supporting Information Available: Crystallographic information in CIF format and FTIR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

References

Figure 6. Two-dimensional projection of molecules of BNPP, bpypa, and Zn(II) in the crystal structure of 1d.

Scheme 3

added under warm conditions. The solution was cooled down to room temperature and allowed for slow evaporation to obtain single crystals over a period of 2 days. Yields: 1a, 47% (24.8 mg); 1b, 61% (33.2 mg); 1c, 56% (30.4 mg); 1d, 52% (57.5 mg). The crystals thus obtained were used for characterization by elemental analysis, FT-IR spectroscopy, and single-crystal X-ray diffraction. FT-IR Spectra. The FT-IR spectra were recorded on a Shimadzu FT-IR-8400 in the 4000-400 cm-1 region using KBr pellets. 1a: 582m, 640m, 748s, 885s, 1026s, 1102s, 1232s, 1348vs, 1379vs, 1510vs, 1614vs, cm-1. 1b: 582s, 640m, 748s, 887vs, 1112s, 1234vs, 1350vs, 1384vs, 1510vs, 1589s, 1618s, 3082s, cm-1. 1c: 582vs, 642s, 746vs, 885vs, 1110vs, 1230vs, 1284vs, 1348vs, 1384vs, 1510vs, 1589vs, 1610vs, cm-1. 1d: 582s, 640m, 848s, 889vs, 1112s, 1180m, 1238vs, 1284s, 1350vs, 1510vs, 1591s.cm-1.

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