Is metal metathesis a framework-templating strategy to synthesize coordination polymers (CPs)? Transmetallation studies involving flexible ligands

Abstract

The coordination polymer (CP) of bis(3-pyridyl)butanediamide with Cu(ClO₄)₂ is a 2D network (CP-3-Cu), which showed two-fold parallel interpenetration, whereas with Cd(ClO₄)₂ it is a 1D network containing rectangular loops (CP-4-Cd). The metal metathesis of CP-4-Cd with Cu(II) resulted in the isomorphous replacement of the Cd(II) centre with Cu(II). Transmetallation reaction resulted in retaining the structural features of CP-4-Cd even in case of a flexible ligand. The CP formed via transmetallation couldn't be synthesized from a direct reaction of bis(3-pyridyl)butanediamide and Cu(ClO₄)₂. The transmetallation kinetic studies were performed with an atomic absorption spectrophotometer (AAS) and wavelength dispersive X-ray fluorescence (WDXRF).

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Introduction

Crystal engineering of coordination polymers (CP) has attracted considerable attention over a decade because of their potential applications in gas storage, separation, and catalysis. The most important application of CPs in energy-related technologies (e.g., H₂, CH₄, CO₂) has been explored widely.¹ Other potential applications of CPs are in the areas of molecular sensing, host–guest chemistry, catalysis, luminescence, and biotechnology.² Applications in these fields of interest are becoming increasingly reliant on the development of CPs that possess complex chemical functionality that can impart sophisticated chemical and physical properties to these materials. The supramolecular structure of CPs can be, in principle, controlled through judicious selection of metal nodes and organic linkers. The direct synthesis of CPs with desired functionalities is often difficult to achieve and dependent on many parameters such as the various coordination geometries of the metal center, nature and ligating topologies of the ligands used, metal-to-ligand ratio, nature of the counter anions and various experimental conditions such as solvent, temperature and crystallization methods.³ The post-synthetic approaches are useful when direct methods to synthesize the CPs do not give the expected product in a crystalline form, desired framework topology or function. The post-synthetic modifications of CPs involving exchange of free guest molecules, counter ions or the removal of free/weakly bound solvent molecules have been studied frequently.⁴ The modification or selective replacement of integral parts (nodes and linkers) of the frameworks can be a route to synthesize CPs that cannot be obtained by direct synthesis. While post-synthetic modifications involving linkers were studied in depth by many researchers;⁵ the transmetalation/framework-metal metathesis studies of CPs are still in its infancy.⁶ This may be attributed to the belief that the properties of CPs change by modifying the linkers of a framework rather than the metal node, which is usually regarded as an inert structural element. However, transmetallation/framework-metal metathesis may lead to CPs with novel or enhanced properties and features. Huang et al. have studied the fluorescence properties on pre- and post-metal ion exchanged CPs and found that the central metal ions have great influence on the fluorescence signal and play a more important role than their skeleton structures.^6b Zhou and co-workers have demonstrated the robustness and enhanced gas adsorption properties achieved when a Zn₂-paddlewheel in a MOF was substituted by a Cu₂-paddlewheel.^6f Recently, Brozek and Dinca have shown that the post-synthetic metal metathesis of [Zn₄O(1,4-benzenedicarboxylate)₃]_n resulted in MOFs with redox reactivity.^6h

CPs derived from hydrogen bond functionalized ligands have the possibility of forming varied hydrogen bond synthons within the networks and have the ability to recognize counter anions and various guest molecules via hydrogen bonding interactions.⁷ The pyridyl based exo bidentate ligands bearing amide moieties are able to form coordination networks and can further assemble into higher dimensional architectures via hydrogen bond interactions. The amide-based ligands have been exploited by some of the leading research groups to form CPs, where the networks recognize each other by amide-to-amide hydrogen bonding.⁸ Nevertheless, continuous effort should be devoted to improve our understanding of CPs via “pre-design” as well as post-synthetic modification strategies. The post-synthetic approaches should be a more controllable synthetic strategy to generate desirable materials because we have the knowledge about the structure and properties of the precursor materials. The modifications on those precursor materials will direct the construction of a designed structure.

In the present work, we have exploited a pyridyl-based flexible ligand 1 bearing amide moieties in the spacer to form CPs (Scheme 1). The backbone capable of forming hydrogen-bonds involving amides will provide the network recognition process required for forming robust supramolecular architecture while the flexibility in the spacer will give enough opportunity to change the geometry in the event of variation in any of the other components in the CPs. The structure of the CPs with different metal centres (Cu(II) and Cd(II)) was analyzed in order to determine the role of the metal centre in forming the CPs. Furthermore, transmetallation approaches are used on the synthesized CPs and the kinetics of the framework-metal metathesis studied. Recently, we have reported the effect of changing the counter anion on the geometry of the network. The flexible alkyl chain resulted in modulating the conformation, which thereby affected the guest uptake properties of the CP.^8f

Scheme 1 Bis-pyridylalkane diamides and their hydrogen bond synthons.

Experimental section

General

Infra-red spectra were recorded on a FTIR ABB Bomen MB-3000. Elemental analyses were obtained with a Thermo finnigan, Italy, Model FLASH EA 1112 series. Powder X-ray diffraction (XRD) data were recorded with a Rigaku miniflex ll, λ = 1.54 Å, Cu Kα. Atomic absorption spectra (AAS) was measured using AA-7000, Shimadzu. Wavelength dispersive X-ray fluorescence (WD-XRF) was measured using a S8 TIGER, Make: Bruker, Germany with X-ray tube of 4 kW, “Rhodium” target and a high voltage/tube current: 60 kV per 64 mA.

Synthesis of ligand 1b. Ligand 1b was synthesized according to a previously reported procedure.⁹ 3-Amino pyridine (2 mmol) was added to 40 ml of a pyridine solution containing adipic acid (1 mmol), and the resultant solution was stirred for 15 min. To this solution was added triphenyl phosphite (2 mmol), and the reaction mixture refluxed for 5 h. The volume of the solution was reduced to 5 ml by removing the pyridine by distillation, and a white precipitate was obtained. The solid was filtered, washed with water, and dried under vacuum. Yield: 70%. Mp: 216–220 °C. FTIR (KBr, cm⁻¹): 3301 (w), 3247 (m), 3178 (m), 3108 (m), 3039 (m), 2947 (vs), 2917 (s), 2875 (m), 1690 (vs), 1580 (vs), 1550 (s), 1478 (m), 1419 (vs), 1378 (m), 1281 (vs), 1157 (s), 1132 (w), 1034 (m), 943 (m), 910 (w), 856 (w), 810 (s), 735 (w), 701 (m), 625 (w), 578 (w).

Synthesis of CP-3-Cu, {[Cu(1b)₂(H₂O)₂](ClO₄)₂·2(H₂O)}_n. Ligand 1b (596 mg, 2.0 mmol) was dissolved in 15 ml of a 1 : 1 mixture of water–ethanol. To the above solution, 10 ml of an ethanolic solution of Cu(ClO₄)₂·6H₂O (370.1 mg, 1.0 mmol) was added. The resulting blue precipitate was dissolved by adding a few drops of water. The solution was filtered and kept for slow evaporation. Blue-colored crystals were formed after 8–10 days in 80% yield. Anal. calcd (%) for C₃₂H₄₄CuCl₂N₈O₁₆: C, 41.27; H, 4.72; N 12.03. Found: C, 41.27; H, 4.55; N, 11.63 FTIR (KBr, cm⁻¹): 3564 (s), 3278 (s), 3201 (w), 3101 (w), 2931 (w), 2862 (w), 1674 (s), 1612 (w), 1589 (w), 1550 (vs), 1488 (m), 1427 (s), 1365 (w), 1296 (m), 1242 (w), 1195 (w), 1103 (vs), 956 (w), 918 (w), 810 (m), 702 (m), 624 (m), 555 (w).

Synthesis of CP-4-Cd, {[Cd(1b)₂(H₂O)₂](ClO₄)₂·2(H₂O)}_n. A microwave-assisted technique was used wherein ligand 1b (59.6 mg, 0.2 mmol) and Cd(ClO₄)₂·6H₂O (41.94 mg, 0.1 mmol) was added to 5 ml of a 1 : 1 mixture of water–ethanol in a specially designed microwave test tube. The reaction mixture was irradiated for 10 minutes at 90 °C at a medium stirring rate and 100 psi pressure. White crystals suitable for single crystal XRD were formed after the solution was allowed to stand for 1 day. Anal. calcd (%) for C₃₂H₄₄CdCl₂N₈O₁₆: C, 39.22; H, 4.53; N 11.43. Found: C, 41.69; H, 4.57; N, 10.69. The calculated percentages are based on the molecular formula from the single crystal XRD. CP-4-Cd was crystallized from ethanol. If we include the free ethanol molecules, we can account for the observed elemental percentage (Table S1†). FTIR (KBr, cm⁻¹): 3865 (s), 3841 (s), 3741 (vs), 3672 (m), 3649 (m), 3618 (m), 3564 (w), 3317 (vs), 1674 (vs), 1527 (vs), 1481 (s), 1419 (s), 1326 (w), 1288 (m), 1164 (w), 1103 (vs), 956 (w), 802 (w), 771 (s), 702 (w), 624 (m), 563 (m).

Synthesis of CP-4-Cu. A metal-metathesis reaction was performed on CP-4-Cd wherein crystals of CP-4-Cd were immersed into a 0.1 M ethanolic solution of Cu(ClO₄)₂·6H₂O. The white crystals slowly turned to blue crystals. The crystals were analyzed by IR, powder XRD, AAS and WD-XRF spectroscopy.

Single crystal X-ray crystallography

Single crystal data for CP-3-Cu and CP-4-Cd were obtained on a Xcalibur, Sapphire 3 X-ray diffractometer that uses graphite monochromatized Mo Kα radiation (λ = 0.71073 Å) using the ω-scan method.¹⁰ The structures were solved by direct methods and refined by least square methods on F² using SHELX-97.¹¹ In both structures, the disorder in the perchlorate group was modelled (one of the oxygens over two positions for CP-3-Cu and two of the oxygens over two positions (each) to obtain the lowest residual factors and optimum goodness of fit with convergence of refinement). Non-hydrogen atoms were refined anisotropically except the disordered perchlorate oxygens (O4/O4′) in CP-3-Cu and the lattice water molecule (O2w) and the disordered perchlorate oxygens (O5/O5A and O6/O6A) in CP-4-Cd. In the final difference Fourier maps there were no significant peaks >1 e Å⁻³. All hydrogen atoms except for the water molecule (O2w) in the asymmetric unit of CP-4-Cd were placed in ideal positions and refined as riding atoms with individual isotropic displacement parameters. The crystal data and structure refinements of CP-3-Cu and CP-4-Cd are summarized in Table 1.

Table 1 Crystal data and structure refinement parameters of CP-3-Cu and CP-4-Cd

CP	CP-3-Cu	CP-4-Cd
a R1 = Σ||Fo| − |Fc||/Σ|Fo|.b wR2 = [Σw(Fo^2 − Fc^2)^2/Σw(Fo^2)^2]^1/2, where w = 1/[σ^2(Fo^2) + (aP)^2 + bP], P = (Fo^2 + 2Fc^2)/3.
Formula	C32H44Cl2CuN8O16	C32H44 CdCl2N8O16
Mol. wt	931.19	980.05
Crystal system	Orthorhombic	Triclinic
Space group	P21212	P1̄
a/Å	11.3423(3)	9.0727(2)
b/Å	19.2826(7)	9.0760(2)
c/Å	9.5549(3)	15.3648(3)
α/°	90	106.099(2)
β/°	90	90.817(2)
γ/°	90	93.146(2)
V/Å^3	2089.74(11)	1213.17(4)
Z	2	1
Dcalcd./g cm^−3	1.480	1.341
T/K	296(2)	296(2)
Theta (°) range for data used	3.50 to 24.98	3.51 to 25.0
Rint	0.0428	0.0411
Reflections with I > 2σ(I)	2989	3982
No. of parameters refined	270	255
Final R (with I > 2σ(I))	R1^a = 0.0679; wR2^b = 0.1726	R1^a = 0.0595; wR2^b = 0.1679
GOF on F^2	0.972	1.023

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Results and discussion

Structural description of CP-3-Cu and CP-4-Cd

The geometry of ligand 1b (Fig. 1) in CP-3-Cu and CP-4-Cd gives insight into the agility of the ligand to assemble according to the requirement of the other components in the CPs. The size of the metal centre is the determining factor in arranging the CPs. In CP-3-Cu, two neighboring Cu(II) centers are separated by a distance of 14.831 Å, whereas in CP-4-Cd, the Cd(II) centers are separated by a distance of 17.955 Å. The flexibility of the butyl chain in 1b resulted in changing the ligand length required for the Cu(II) and Cd(II) centre. In CP-3-Cu, the butyl chain adopts gauche–anti–gauche conformation, whereas in CP-4-Cd, it adopts an all anti conformation. This in turn affected the overall geometry of the CPs. In CP-4-Cd, the ligand behaves as a longer “rigid” analogue of 3,3′-bipyridine. In CP-3-Cu, the ligand adopts a wavy type of geometry with more of the hydrocarbon chain “clustered” in the spacer. The presence of two amide groups resulted in self-complementary amide-to-amide hydrogen bonds in both of the CPs, even in the presence of counter anions and free water molecules.

Fig. 1 Geometry of ligand 1b in (a) CP-3-Cu and (b) CP-4-Cd; notice the conformation of the butyl chain in each case.

The structural features of CP-3-Cu and CP-4-Cd are as follows: two-dimensional coordination network of (4,4)-topology having two-fold parallel interpenetration: CP-3-Cu crystallizes in the orthorhombic space group P2₁2₁2. The Cu(II) centre adopts a distorted-octahedral geometry, in which the coordination environment of Cu(II) includes four molecules of 1b in the equatorial position (Cu–N: 2.023 Å, 2.027 Å) and two H₂O molecules in the axial position (Cu–O: 2.480 Å). The wavy-shape of the ligand due to a gauche–anti–gauche conformation results in the formation of a highly corrugated 2D-network with (4,4) topology (Fig. 2a).

Fig. 2 Illustration of the crystal structure of CP-3-Cu; (a) part of the 2D-layer of (4,4) topology (C = grey; N = blue; O = red; Cu = brown; P = orange; F = yellow); (b) two-fold parallel interpenetration of the layers (one layer is shown in space fill mode (pink) and another is shown in ball and stick mode (blue)); (c) amide-to-amide hydrogen bonds (each (4,4)-layer is shown in a different color; three pairs of interpenetrated layers are shown); (d) side-view of the interpenetrated layers (three pairs of interpenetrated layers are shown).

The (4,4)-layers have rhomboidal-shaped cavities with diagonal-to-diagonal distances of 22.530 Å × 19.216 Å and the coordinated H₂O molecules point into the cavities. Two such (4,4)-layers interpenetrate in parallel mode (Fig. 2b). The ligands in CP-3-Cu interact with each other via amide-to-amide hydrogen bonds (Fig. 2c). The hydrogen bond pattern in case of amide groups represents the synthon IV (Scheme 1). Biradha et al. obtained a similar structure with ligand 2 and Cu(ClO₄)₂, in which the amide-to-amide hydrogen bonds are present between the two interpenetrated layers. Note that the continuity of the amide hydrogen bonds throughout the network is hindered by water molecules and counter anions.^8d However, in CP-3-Cu, the amide-to-amide hydrogen bond is present throughout the structure and is not affected by counter anions and H₂O molecules.

Looped one-dimensional coordination network – the role of the metal centre

The effect of the metal centre on the geometry of the CP was studied by analyzing the structure of CP-4-Cd, which crystallizes in the triclinic P1̄ space group. The coordination environment of the Cd(II) centre includes four molecules of 1b present in the equatorial sites and two H₂O molecules present in the axial sites (Cd–N: 2.306 Å, 2.311 Å, 2.365 Å, 2.404 Å; Cd–O: 2.341 Å, 2.349 Å, respectively) (Fig. 3a). The butyl chain spacer of 1b adopts an anti–anti–anti conformation (Fig. 1b), which results in stretching the ligand 1b to its maximum length; hence, the linear geometry of the ligand is seen. The geometry of the ligand in CP-4-Cd is similar to that of cis-arranged 3,3′-bipyridine. The cis- arranged 3,3′-bipyridine is reported to form 1D-chains with Zn(II), Co(II) and Ni(II), whereas with Hg(II), it forms a dinuclear macrocyclic structure.¹²

Fig. 3 Illustration of the crystal structure of CP-4-Cd; (a) part of the 1D-looped chain (C = grey; N = blue; O = red; Cu = brown; P = orange; F = yellow); (b) packing of the 1D-chains via hydrogen bonding between the amide groups, water molecules and counter anions.

The network formed in CP-4-Cd can be described as a 1D chain with rectangular loops. The longer length of ligand 1b compared to that of 3,3′-bipyridine and the larger size of the Cd(II) centre resulted in generating the overall features of the reported structures (i.e. 1D looped chain). The rectangular loops in CP-4-Cd have dimensions of 17.923 Å × 5.176 Å, in which the amide groups are involved in self-complementary hydrogen bonding. The adjacent chains are held together by the hydrogen bond interaction between the amide N–H and ClO₄⁻, and between the amide C=O and H₂O (Fig. 3b). The π⋯π interactions between the pyridyl groups are also responsible for holding the adjacent chains together.

The 1D-looped chain with intra-chain amide-to-amide hydrogen bonds observed in the case of CP-4-Cd is similar to the reported structures by Dastidar and co-workers, involving bis(3-pyridyl)terephthalamide with Cu(II) salts, in which the ligand has a “rigid” phenyl spacer in between the bis-amide bis-pyridyl moieties.¹³ This shows the ability of the “flexible” alkyl chain spacer of ligand 1b, when involved in forming CPs with a larger metal center (i.e. Cd(II)), in mimicking the structural features obtained by a ligand involving a “rigid” phenyl spacer.

Transmetallation studies on the CPs

The response of the flexible spacer of ligand 1b during the formation of CPs upon changing the metal centre has further prompted us to study their post-synthetic transformations. Transmetallation studies were performed on CP-3-Cu and CP-4-Cd to analyze whether the flexibility of the spacer will be able to change the geometry of the CPs upon changing the metal centre or the robustness of the precursor CP will not allow any modification on the skeletal structure during metal metathesis.

The transmetallation study on CP-4-Cd with Cu(ClO₄)₂ showed complete exchange of the Cd(II) centre with Cu(II). The crystal color changed rapidly from white to blue, while the crystal morphology remained the same throughout the ion exchange process (Fig. 4). The powder X-ray diffraction (PXRD) pattern of the resulting CP-4-Cu is similar to that of the parent compound CP-4-Cd (Fig. 5). The wavelength dispersive X-ray fluorescence (WD-XRF) shows the complete replacement of the framework Cd²⁺ ions by Cu²⁺ ions (Fig. S12 and S13†).

Fig. 4 Crystal morphology of CP-4-Cd on transmetallation reaction with Cu(II).

Fig. 5 Powder X-ray diffraction profiles for the parent (CP-4-Cd) and ion exchanged material (CP-4-Cu) demonstrating the maintenance of the framework integrity.

The kinetics of the ion exchange process of Cd²⁺ by Cu²⁺ was monitored by AAS (Fig. 6). Nearly 50% of the framework Cd²⁺ ions were replaced by Cu²⁺ within 1 h, and 97% of the Cd²⁺ ions were exchanged by Cu²⁺ within 10 h. The transmetallation study on CP-3-Cu and CP-4-Cu with Cd(ClO₄)₂ was very slow; moreover, negligible exchange was observed even after three months.

Fig. 6 Kinetic profile of framework metal ion exchange of Cd(II) with Cu(II).

Powder XRD

The powder XRDs of the CPs were analyzed along with the calculated powder XRD of CP-3-Cu and CP-4-Cd (Fig. S4–S7†). The structural differences of CP-3-Cu and CP-4-Cu is evident from these analyses (Fig. S9†). The powder XRD reflects the similarity in the structures of CP-4-Cd and CP-4-Cu. Therefore, we can relate that the structure of CP-4-Cu is isomorphous to that of CP-4-Cd. The direct reaction of ligand 1b with that of Cu(ClO₄)₂ always resulted in the formation of CP-3-Cu. The synthesis of CP-4-Cu is only possible using the transmetallation technique.

Conclusions

The metal centre was shown to play an important role in assembling the components during the formation of CPs: the size of the Cu(II) and Cd(II) centre being one of the major deciding factor in adjusting the conformation of the butyl spacer of the ligand. Transmetallation studies have shown us that despite having a flexible framework, the metal exchange reaction proceeded without changing the structure of the CPs. This may be attributed to the fact that the looped-chain of CP-4-Cd has N–H⋯O hydrogen bonds within the loops, which made the network robust. Furthermore, the CP synthesized via post-synthetic metal exchange could not be obtained from a direct reaction of the metal centre and ligand. A framework-templating strategy was used to synthesize CP-4-Cu. During transmetallation reaction of CP-4-Cd with Cu(II), the CP acted as a template and pre assembled the ligands.

Acknowledgements

We gratefully acknowledge financial support from the Seed Grant Scheme-2011 of BITS Pilani & DST (SR/FT/CS-24/2011) and Instrumentation facility from the DST FIST and UGC-SAP to the Department of Chemistry, BITS Pilani, Pilani Campus. RK acknowledges financial support received under DST mega research project (SR/S2/CMP-47/2003). KS thanks BITS Pilani, Pilani Campus, and UGC BSR for providing the research fellowship. FB thanks DST for providing the research fellowship.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Crystallographic data; PXRD patterns, IR, AAS, WDXRF. CCDC 963405 and 963406. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra04166f

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