Análisis de Metales

In addition to the organic chemistry discussed in section 1.2.4, coordination compounds of hexamethylenetetramine with transition metals are also known. The majority of reports published to date involve the synthesis of coordination polymers from solutions of hexamethylenetetramine and simple metal salts. In these compounds hexamethylenetetramine has been observed to act in bidentate, tridentate and tetradentate fashions. The variation in the resulting polymeric networks {i.e. two or three dimensional with different cavity types) is a consequence of the pH of the reaction solution, molar ratio of reactants and the identity of the counter-ion.^^

Chapter One: Introduction

Figure 1.3 depicts a coordination polymer in which hexamethylenetetramine acts as a bidentate ligand. The polymer [(Cul)2hmt] is obtained via addition of hexamethylenetetramine to a solution of Cul in acetonitrile.^^ As shown in the figure there are alternating bridges o f iodide ions and hexamethylenetetramine ligands linking pairs of copper ions, resulting in the formation of a two-dimensional sheet structure.

, C u N

-Cu^ ^ C u NL Cü^ ^ Ç u --- n1

,N

,N C u , C u nJ n

Figure 1.3 Coordination polymer o f Cul and hexamethylenetetramine

Hexamethylenetetramine also acts as a bidentate ligand in the three-dimensional complex polymer produced upon diffusion of an ethanolic solution of AgPFô into a chloroform solution of hexamethylenetetramine Figure 1.4.^^

A

O x

Chapter One: Introduction

In contrast, hexamethylenetetramine acts as a tridentate ligand in the two-dimensional network formed by hexamethylenetetramine and silver ions in the complex [Ag2(|l^-hmt) (N 0 2)2], which contains hexagonal cavities, Figure 1.5/^

N "N

ONO ONO

Figure 1.5 A section of an infinite 2-D layer found in [Ag2(ji^-hmt) (N O2)] n

An example of the tetradentate coordination of hexamethylenetetramine is observed in the two-dimensional coordination polymer [Ag2(|I'^-hmt)(N0 2)2]n. This compound, synthesised by reaction of AgN02 and hexamethylenetetramine in an acetonitrile-water mixture, exhibits a structure where the four nitrogen atoms of each hexamethylenetetramine molecule are coordinated to a different silver atom. Thus, a two-dimensional network with square cavities is formed. Figure 1.6.^^ The complex contains silver atoms in two different geometries; the geometry around one type of silver atom is linear with coordination by two nitrogen atoms each from a hexamethylenetetramine molecule. The other silver atom is in a triangular prism geometry coordinated by two nitrogen atoms and four oxygen atoms, from two hexamethylenetetramine molecules and two N O2

Chapter One: Introduction

molecules respectively. Three-dimensional polymeric structures containing tetradentate hexamethylenetetramine are also known, for example in the compound [Ags(p'^-hmt)2(H2O)2](SO4)(HSO4).2

Ag Ag Ag

r a

r \\

r a

r\"\

r \\

r\A

Figure 1.6 A representation of the polymeric structure of

[Ag2(p^-hmt) (N 02)2] n.

The compounds discussed so far involve the low coordinate metals copper and silver. Metals with potential for tetrahedral or octahedral coordination, however, are of particular interest due to the new structural motifs that may arise in products formed using polydentate ligands such as hexamethylenetetramine. In order to investigate such possibilities, reactions using N i(II), Zn(II) and Co(II) have been performed. Each experiment involved addition o f an aqueous solution of hexamethylenetetramine to an aqueous solution o f NaNCS and either M(N0 3)2.6H2 0 (M = Co"^2 or NE^) or ZnS0 4 .7H2 0 .'^ A crystalline product was obtained in each case, after leaving the solutions to stand for several days. Interestingly, only the nickel complex is polymeric, with each hexamethylenetetramine molecule acting as a bidentate ligand to bridge two nickel atoms and form a one-dimensional zigzag chain structure: [Ni(p-hmt)(NCS)2(H20)2]n. Hexamethylenetetramine also acts as a bridging ligand in the zinc complex, [Hhmt] 2 [Zn2 (p-hmt) (NCS) 6], with two protonated

Chapter One: Introduction

hexamethylenetetramine molecules acting as counter-cations. In the cobalt compound [Co(NCS)2(hmt)2(H2 0)2] [Co(NCS)2(H2 0)4].H2 0,

hexamethylenetetramine acts as a monodentate ligand, with the ligands around each cobalt atom having an octahedral arrangement. An analogous compound, [Co(hmt)2(H2 0)4][Co(H2 0)6][S0 4]2.H2 0 has been prepared by evaporation of a mixture of water, dioxane and acetone containing hexamethylenetetramine and CoSO4.7H2O.45

Reports of the use of hexamethylenetetramine as a ligand in organometallic complexes are not common, examples include the monomeric compound [Mo(CO)5(hmt)]2i and a range of adducts of hexamethylenetetramine with trimethylgaUium and trimethylaluminium.46 The T.l, 2:1, 3:1 and 4:1 adducts of trimethylgallium and hexamethylenetetramine and the 3:1 and 2:1 adducts of trimethylaluminium and hexamethylenetetramine were obtained as colourless needle crystals in near quantitative yield via addition of a stoichiometric quantity of MesGa or MesAl to hexamethylenetetramine in toluene.46

In the complex [Cu2(02CCH3)4(hmt)]n, illustrated in Figure 1.7, the copper ions are six coordinate and a chain polymer is formed, with hexamethylenetetramine acting as a bridging ligand.47 The synthesis and redox properties of similar dirhodium complexes are discussed in Chapter Five o f this thesis.

Cu _ÇH 3 .Cu' .Cu' Cu Cu] H3C H3C

Chapter One: Introduction

1,4 N itrogen rich cages as precursors to energetic m aterials