Estabilidad Laboral

4.3.1 Comparison o f n-butvlamine and liquid ammonia

As previously discussed (chapter 2), the reaction o f chalcogens with late transition- and main-group metals (Zn, Cd, Hg, Ag, Cu, Pb) in liquid ammonia at room temperature in a pressure vessel {ca. 1 atm.), produced a range o f single phased metal chalcogenides. The reactions o f lead and silver produces crystalline metal sulfides, whilst the remaining reactions produced X-ray amorphous materials that could be crystallised by facile heating (250 - 300 °C for 2 h). Similar results were obtained for n-butylamine, with comparable phases being observed, but for three notable exceptions. First, the reaction o f mercury and sulfur in liquid ammonia afforded an X-ray amorphous material, identified as a mixture o f both cinnebar and metacinnebar. Yet in n-butylamine the composition o f the two phases o f mercuric sulfide m ay be altered by changing the dynamics o f the solvent. For ‘analogous’ solvent conditions (i.e. a sealed system), the reaction product in n-butylamine showed a marked preference for the zinc blende phase (metacinnebar), whilst with an evaporating solvent the more thermodynamically stable distorted rock salt structure (cinnebar) was dominant. Secondly, the reactions o f copper with selenium or tellurium in n-butylamine produced predominantly single phased materials o f Cu', whilst in liquid ammonia a mixture o f phases was observed (which included significant amounts o f CuTe and CuSe). Thirdly, elemental reactions in liquid

ammonia gave a greater scope in that zinc, cadmium and tin all reacted to form X-ray amorphous chalcogenides (with crystalline materials resulting after annealing these powders at 250 - 300 °C for 2h). In n-butylamine no reaction was observed for these reactions, even at reflux, except in the case o f zinc and sulfur where, as already stated, the formation o f 5 - 10 % amorphous ZnS was obtained on the surface o f the zinc after 7 days. In the absence o f heating, n-butylamine does not facilitate extensive reaction between chalcogens and Sb, Bi, In and As (Se, Te).

4.3.2 Role o f the N-based solvents

For the most part one can say that the elemental combination reactions we carried out in N-based solvents at room temperature (NH3(,), NHsfaq), CH3(CH2)3NH2, CH3(CH2)5NH2,

and NH2(CH2)2NH2(1)) produced phases o f the metal chalcogenides that were the same as

the most common mineral forms o f these species. The materials formed with silver were formally Ag (I) d^®, with mercury Hg (II) d ‘®, and with lead Pb (II) d ‘°s^. Copper was predominantly Cu (I) d'®, with some deviance in liquid ammonia to Cu (II) d®. It is known that some chalcophilic elements will react on contact with chalcogens, in the absence o f a solvent, at room temperature. We discovered that the products from the reaction o f mercury and sulfur in the absence o f a solvent (cinnebar and metacinnebar) were not crystalline, and the reaction stopped after an initial period (ca. 60 h) to leave only partly coated reacting particles. In order to test the rate o f solid state reaction o f silver metal and elemental gulfur, disks o f the elements were pressed together and the interface observed every 7 days. The silver disk was seen to gradually darken with the formation o f crystalline acanthite (Ag2S). The reaction progressed from the elemental

boundary into the silver disk to a depth o f 0.5-1 mm per month. It is probable, therefore, that the mechanical stirring o f the elements in a solvent would assist the formation o f metal chalcogenides by removing the powdered product from the surface o f the metal, effectively regenerating clean surfaces for subsequent reaction. In line with the results obtained, one would expect liquid ammonia to be more efficient at regenerating the metal surface when compared to the organic N-based solvents. For not only does its reduced size and increased pressure allow greater crowding o f the solvent at the reaction sites, but its increased polarity allows a more extensive solvent sphere, thereby increasing mobility o f the product away from the metal surface.

Significant differences in the reactivity o f elemental combination reactions are also apparent in the remaining N-based solvents used (ethylenediamine, aqueous ammonia

and n-hexylamine). Rauchfliss et al. report‘d that the strongly coordinating ethylenediamine solvent can be used in the synthesis o f amorphous copper sulfide. They observe the elemental combination o f copper and sulfiu* at room temperature and pressure, affording a low energy route to [Cu(en)2]^^[S6]^'. This complex can be

thermally decomposed at 500 °C to amorphous CuS. Interestingly, the stability afforded to the copper (II) cation by this bidentate ligand does not appear to be extended to either the mercury (II), silver (I) or lead (II) cation, with none o f these metals reacting at room temperature to form a [M(solv)2(S6)] complex (even after a prolonged time period o f 14

days). This is likely to result fi*om the increased strain on the ‘bite angle’ upon coordinating to a significantly larger cation (when compared to copper II), thereby reducing the stabilising effect o f the bidentate ligand.

Reactions occur more slowly in n-hexylamine than n-butylamine, and not at all in non- N-based solvents such as cyclohexane. It is possible that the increased basicity o f the N- based solvents mentioned remove passivating oxide layers fi-om the metal, thereby allowing the reactions to occur by lowering the energy o f activation to a point made accessible at room temperature. The marked increase in the scope o f reactions for liquid ammonia m ay be the result o f the higher pressures at which this solvent is maintained

{ca. 1 atm.), resulting in an increase in the surface action o f the solvent molecules. One

should also consider that increasing the size o f the organic substituents on the nitrogen atom would result in an increase in its dielectric constant, thereby reducing its effectiveness at solvating these oxide layers. Consequently, the scope o f the reactions would become limited to those that form the more thermodynamically stable products. This is supported by the use o f aqueous ammonia, which although allowed the formation o f lead sulfide, also resulted in increased reaction time (48 h compared to 60 h) and significant product contamination (metal oxide). The presence o f ammonium hydroxide in this solution not only reduces the 'cleaning' property o f ammonia, but also allows an additional reaction between the hydroxide and the activated metal surface.