Polymorphism and structural characterization of a silver(I) coordination polymer: an inorganic polymer co-former in the preparation of curcumin-containing co-crystals

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Within the relevant field of metal-containing polymers and their applications in the biomedical context [1], a Silver(I) coordination polymer of formula [(bpy)Ag]OTf∞ presenting polymorphism has been synthetized through reaction between the N^N ligand 2,2'bipyridine (bpy) and Silver trifluoromethanesulfonate (AgOTf).By varying the stoichiometric ratios and the order of the addition of the reagents along the synthetic routine, two polymorphs have been synthesized and structurally characterized.The first polymorph of [(bpy)Ag]OTf∞, the α-form, crystallized in the P3121 space group, is characterized by the alternance, along the polymeric chain, of Ag(I) ions with linear and tetrahedral geometry (Fig. 1); this arrangement results in the generation of chiral helices [2].In a second polymorph of [(bpy)Ag]OTf∞, indicated as the β-form (P21/c space group), prepared by modifying the synthetic procedure adopted previously, all the Ag(I) ion adopts a slightly distorted linear geometry.The silver ions are coordinated to the nitrogen atoms of bridging bpy ligands, while the non-coordinated OTf anions are found weakly interacting with the metal centres (Fig. 1).The βpolymorph presents a zig-zag conformation which, as already reported for 1D organic polymers [3], can generate pocket-like cavities able to accommodate organic molecules through non-covalent interactions, rising the role of inorganic-polymer co-former in the formation of biologically active co-crystals.Hence, the β-form of [(bpy)Ag]OTf∞ was used for the preparation of an inorganicpolymer co-crystal by using curcumin (curc) as the organic bioactive molecule.The [(bpy)Ag]OTf∞-curc co-crystal was obtained through a quick solution reaction and characterized through several techniques, including Powder X-Ray Diffraction (PXRD), Differential Scanning Calorimetry (DSC), 1 H-NMR, UV-visible and Infrared Spectroscopies.The instauration of weak intermolecular interactions between the keto-enolic function of curc and both the Ag(I) cationic chains and the triflate anions of the inorganicpolymer is the driving force for the formation of this multicomponent material.Considering the multiple biological functions of curc [4] and the well-known antimicrobial activity of silver compounds [5], the [(bpy)Ag]OTf∞-curc co-crystal could represent a multifunctional supramolecular system.Moreover, embedding the [(bpy)Ag]OTf∞-curc co-crystal into an ethylcellulose (EC) polymeric matrix, antimicrobial films with potential biomedical and food-packaging applications have been obtained and characterized.Figure 1.Packing view of the α-form (a) and the β-form (b) of [(bpy)Ag]OTf∞ along the a axis with its potential interactions with curc.

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