AUTOR NACIONALIDAD COD_LIBRO TÍTULO EDITORIAL AÑO

With the aim of gaining a more quantitative understanding of the possible modes of interaction between the copper ions and 91, a model compound that contained a single diiminophenol binding site with n-propyl tails was prepared. The aim of this was to combine the X-ray structural data and FTIR data and to compare this to the copper-immersed polymer. The structure of the model compound is shown in Scheme 4.8. The model compound was prepared by the in-situ reaction of 4-tert-butyl-2,6-diformylphenol with two equivalents of n- propylamine in methanol at reflux. Once the product was obtained, attempts to crystallise metal complexes of this ligand directly from the reaction mixture or other alcohol solvents to imitate the soaking conditions of 91 were unsuccessful and thus, MeCN was then used instead. The yellow MeOH solution of the ligand was evaporated and gave a yellow oil which was then taken up in MeCN and combined with a solution of Cu(II) acetate hydrate in MeCN. The resulting green solution was left to stand for 48 h to yield dark green crystals, which were then isolated by filtration and subject to single crystal X-ray diffraction.

Scheme 4.8 Synthesis of the model 92 where (i) C3H9N, MeOH, reflux.

Chapter 4. Network polymers derived from BTA-based precursors

The diffraction data for the copper complex were solved and the structural model refined in the monoclinic space group P21/c. The asymmetric unit was found to contain the molecule

in its entirety and no associated solvent molecules or guests. The complex consists of a tetrahedral Cu4O centre, surrounded by four acetato ligands that each bridge between two pairs

of copper ions as shown in Figure 4.7. The remaining coordination sites are occupied by two ligand molecules that each bridge two copper ions. The six unique Cu‒Cu distances fall in the range 3.0154(14) ‒ 3.1983(14) Å and are typical of [Cu4O] cores.180 The overall structure of

the model compound is quite similar to those derived from other 4-substituted 2,6- diformylphenols and various alkylamines under similar reaction conditions,181,182 and can also be considered as being typical for acyclic diiminophenol ligands in general, thus demonstrating the suitability of this compound as a model for 91. This is shown in Figure 4.8, in which the structures reported by Banerjee and co-workers are very structurally similar to that of the model compound 92.181

Figure 4.7 X-ray crystal structure of the model compound, 92, showing (a) the structure of the complete complex and (b) the core connectivity with labelling scheme. Hydrogen atoms are omitted for clarity.

Chapter 4. Network polymers derived from BTA-based precursors

Further structural comparisons between the model compound and the polymer 91 were made using FTIR in an attempt to ascertain the nature of the copper binding that occurs on the surface of the polymer 91. The IR spectra of 91, Cu polymer 91, model compound 92 and Cu(II) acetate hydrate were measured and are shown in Figure 4.9. The majority of the organic functional group absorbances are retained upon addition of Cu(II) acetate to ligand 91, for example, the overlapping amide C=O and imine C=N stretching bands (maxima at 1637 cm-1₎

and the aromatic C‒C stretch at 1527 cm-1_{. The addition of copper to 91 gave rise to new}

stretches at 1585, 1447-1423, 1067, 677 and 619 cm-1 _{with these bands assigned to the}

acetate/acetato species, thus confirming the inclusion of these species in addition to the polymer Figure 4.8 (a) Structure reported by Banerjee and co-workers where R = a variety of phenyl groups. (b) a perspective view of the one of the reported copper complexes and (c) the metal and donor connectivities for the reported complexes. Images reproduced from reference 31.

Figure 4.9 Offset FTIR spectra of 91 (black), Cu polymer 91 (red), Cu (II) acetate hydrate (green) and complex

Chapter 4. Network polymers derived from BTA-based precursors

species (confirmed by EDX spectroscopy, Figure 4.6). By comparing the acetato stretches, it would appear that the Cu polymer 91 strongly resembles the sum of the spectra of ligand 91 and pure Cu (II) acetate hydrate. The carboxylate νasym and νsym bands of both are in close

agreement, with the difference between the two being approximately 165 cm-1 for polymer 91, the difference, Δ, for Cu(II) acetate is 175 cm-1.This parameter is known to be indicative of the mode of carboxylate binding, with a Δ of < 100 cm-1 for chelating carboxylates, ~160 cm-1 for ionic carboxylates and fully symmetric bridging while the Δ for monodentate or unsymmetrically bridging carboxylates can be > 300 cm-1.183 Thus, this would indicate that the carboxylates in polymer 91 are symmetrical, unlike those in 92 in which Δ is 200 cm-1, suggesting a lower symmetry character in the bridging mode of 92 compared to Cu(II) acetate and 91. This lower symmetry character is corroborated by the X-ray crystal structure in which each of the four unique carboxylate groups coordinates to one axial and one equatorial site of the square pyramidal copper ion. The Cu‒O bond distances for each acetate differ by on average 15-20% of the average Cu‒O bond length. Further evidence of this asymmetry is evident when the carbon-oxygen bond lengths are examined, with the more weakly bound oxygen atom showing a greater carbon-oxygen double bond character, with the bond distances falling, on average, 0.06(2) Å shorter than those of the more strongly bound oxygen atom for each of the four ligands.

The sum of the spectroscopic observations would suggest that there is a strong relationship between the structure of the bound copper ions in polymer 91 to the paddle-wheel structure of Cu(II) acetate and its solvates,184 rather than the diiminophenolate-chelated species that is observed in 92 and related complexes.

Further investigations of the interaction of 91 with d-block metals ions were carried out in equivalent soaking experiments to that used for copper, i.e., soaking the polymer in methanolic solutions of Co(II) acetate tetrahydrate and Zn acetate dihydrate. The cobalt-soaked polymer gave an IR spectrum that appears to be the sum of the free polymer and the metal salt, shown in Figure 4.10, thus indicating that the metal salt is not interacting with the polymer. On the other hand, soaking the polymer in the zinc solution resulted in a broadening and reduction in intensity of the νsym (COO) absorbances and variations in the positions and intensities of the

CH3 rocking modes compared to the pure zinc metal salt, thus indicating some degree of

interaction between the two species, Figure 4.10. The implication of a more significant interaction between Zn(II) and the polymer in comparison to the interaction between the Cu(II) and the polymer, is in keeping with the chemical properties of the two metal salts. Zinc has less

Chapter 4. Network polymers derived from BTA-based precursors

of a tendency to form rigid paddlewheel-type coordination behaviour and there are a wider range of possible coordination geometries for this particular case.

In summary, ligand 91 has a granular, spherical morphology and was found to be amorphous by X-ray powder diffraction and acts as a substrate for the deposition of copper acetate from an alcohol solution. The presence of copper on the surface was confirmed using EDX spectroscopy and was further probed using FTIR. The IR studies involved a comparison between the 91, the soaked polymer of 91, Cu(II) acetate hydrate and the model complex 92, which revealed the soaked polymer to be more like the paddlewheel structure of the Cu(II) acetate rather than the model complex. This implied that the mechanism of adsorption was based on nucleation of a copper acetate on the surface of the polymer, rather than binding within the Schiff-base compartments of the polymer itself. The experiments with Co(II) revealed a similar behaviour to that of Cu(II), while the Zn(II) ions displayed more of a tendency for chemical transformation on the polymer surface.

Figure 4.10 (a)OffsetFTIR spectra for the polymer 91 (black), cobalt-soaked polymer (red) and cobalt acetate tetrahydrate (green) and (b) offset spectra for the polymer (black), zinc-soaked polymer (red) and zinc acetate dehydrate.

Chapter 4. Network polymers derived from BTA-based precursors

4.4 Conclusions and future perspectives

In summary, two different approaches were investigated to generate polymeric materials based on BTA amine-based precursors. The first approach was to generate a covalently linked system, which resulted in a number of synthetic challenges. Unfortunately, the target molecule was not obtained in sufficient yield and purity to warrant further study. The synthetic difficulties are thought to be due to a number of factors including the difficulties in achieving efficient tri- substitution of the core with bulky side chains and the solubility similarities between the mono- , di and tri-substituted products. Further modification of the target molecule, for example a modification in hydrophilicity and chain length, may result in a more efficient synthesis and should be the subject of future study.

The second section of this chapter discussed the synthesis and characterisation of a non- covalent, more dynamic polymer. This polymer contained a BTA core and was functionalised with diiminophenol-based side chains to form a Schiff-base linked organic polymer, 91. The morphological and physical properties of this material and its interactions with a variety of metals were probed using a variety of techniques, for example, X-ray powder diffraction, SEM, EDX and IR spectroscopy. It was found that the polymer interacts with Cu ions through a nucleation or seeding mechanism rather than via chemisorption within the diiminophenol binding pockets, with Co ions displaying a similar effect, while Zn ions appear to display a different binding mode. Further work in this area could include modification of the organic polymer to allow for to incorporation of transition metals into the binding pockets.

The area of supramolecular polymers and materials is a rapidly developing field, with these materials offering many advantages over existing technologies, for example, in the areas of sensing, plastics, medical devices and optoelectronics.40,56,61,93,163,185 The BTA motif offers a readily modifiable core, known for its aggregation properties and is an ideal candidate for the basis of many of these potential materials and applications, however, much work is still required in order to better tune the synthesis and predict the behaviours of its materials.

‘That’ll do pig, that’ll do.’