Potentiometric studies in aqueous medium and spectrophotometric study in non-aqueous medium were used to understand the behavior of hexamethylenetetramine (HMTA) complexes. The protometric studies of HMTA enabled us to confirm that only one basic site of this ligand is protonated in acidic medium and this ligand is decomposed in acidic medium. In aqueous medium, only hexa-aqua complexes in which HMTA is present in the second coordination sphere forming H-bonds with hydrogen atoms of coordinated and uncoordinated water molecules are obtained. In non-aqueous solvents, HMTA coordinates to metal ions displaying diversity in the structures of the resulting complexes in which HMTA can either be monodentate, bridged bidentate, tridentate, or tetradentate. 1. Introduction Hexamethylenetetramine (HMTA) is a heterocyclic ligand with four nitrogen donor atoms, having three rings merged in a chair conformation as shown in Figure 1. Figure 1: Structure of hexamethylenetetramine. HMTA can therefore form various metal complexes possessing interesting structural features and applications [1–5]. Hexamethylenetetramine as a ligand can bind either in a monodentate manner to a metal [6, 7], acting as bridging ligand linking two, three, or four metals [8–11], or bind to metal-containing species through the formation of hydrogen bonding [5, 12–16]. The combination of both covalent and hydrogen bonding in certain complexes of hexamethylenetetramine leads to the formation of three-dimensional structures that easily decompose by thermal treatment to give thin films of metal oxides [15, 16]. The formation of covalently bonded and hydrogen-bonded compounds of hexamethylenetetramine is influenced by several factors such as the nature of the solvent, steric hindrance of the counter ion, and the pH of the solution [17]. When water is used as the solvent during synthesis, metal-aqua complexes are obtained which bind to HMTA through H-bonds, forming ionic species [18]. When non-aqueous solvents are used for synthesis, metal-HMTA covalent species are formed [10, 15]. Recently, we reported the isolation of metal-HMTA covalently bonded species isolated from ethanol [19]. Metal-H2O-HMTA ionic species involving H-bonds isolated from ethanol/water mixture have also been reported [5, 12–16, 20]. We report here the results of the study on the influence of solvent on the electronic and structural properties of metal-HMTA complexes in aqueous and non-aqueous solvents. 2. Experimental 2.1. Chemicals All solvents were purified by conventional procedures [21] and distilled prior to use. All the
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