Capacitación

4.2.1 Solution state mechanisms

Mercuric sulfide (HgS) occurs in two mineral forms. The red form (cinnebar, or a-H gS) adopts a distorted sodium chloride lattice, with the formation o f zigzag Hg-S chains similar to those in HgO. The black phase (metacinnebar, or P-HgS) adopts the zinc blende structure found for HgSe and HgTe. The black phase is metastable, and so may be converted to the red either by heating or by treatment with alkali polysulfides or mercurous c h l o r i d e . I t is this phase that predominates from the elemental reaction o f mercury and sulfur in both n-butylamine and, to a lesser extent, liquid ammonia. Literature reports the black phase to be rare, forming from either the action o f HaS on the metal or from precipitation from aqueous s o l u t i o n . T h e absence o f solvated mercury species upon addition o f the element to n-butylamine (or indeed to liquid ammonia) thereby suggests the possible presence o f in situ HaS, formed upon the dissolution o f sulfur in n-butylamine, may play a critical role in the subsequent reaction. Upon dissolution in either prim ary or secondary amines, it has been reported that the Sg ring undergoes stepwise nucleophilic attack by the amine. The initial products are the in situ release o f HaS with the formation o f N ,N ’-polythiobisamines.'*^ In a solution just containing dissolved sulfur, these S-N species co-exist with a secondary product (amine polysulfides) formed by the subsequent attack o f the aliphatic amine by HaS. It is plausible, however, that the HaS formed may be the source o f sulfur for direct combination with the metal. Moreover, these new sulfur-amine species contain long straight chains o f sulfur that are inherently less stable than the original Sg rings. Hodgson et al.^^ suggets that as a consequence, these solvated species would undergo subsequent homolytic scission to achieve a degree o f resonance stability, thereby creating sulfur based radicals. The possibility exists, therefore, that sulfur radicals may play an intrinsic role in the formation o f metal sulfides by elemental combination at room temperature. Indeed, it has been shown that the in situ formation o f both metal and sulfur based radicals is the key mechanistic pathway to the room temperature synthesis

o f nanoparticulate zinc and cadmium sulfides, by y-irradiation o f solutions o f metal halides and metal thiolates.

4.2.2 Solid state mechanisms

It should be noted that both selenium and tellurium are insoluble in n-butylamine yet also react with elemental silver, copper, lead and mercury. The reactions o f selenium and tellurium appear to be solid state reactions, although the presence o f extremely low concentrations o f solvated species cannot be ruled out. The time taken for complete reaction (table 4.1) suggest that the rate o f elemental solid state diffusion is excellent, especially since crystalline material results. It might be expected that the rates o f reaction o f sulfur with either silver or mercury would be faster than for selenium and tellurium, since the sulfur fully dissolves in the amine. However, competitive reactions between either o f these elements and sulfur / selenium in n-butylamine resulted in the preferential formation o f HgSe and AgaSe (> 95 % for both cases, as determined by EDXA / XRD). It was not possible to form solid solutions (MSxSOy where M = Hg, Ag, Pb) as a major product. This m ay indicate two discrete reaction types in operation, one intrinsic to solvated sulfur species, whilst the other is solid state. In the later case, the amine solvent may play a role in activating the reagent surface, whilst allowing good contact between the elements.

4.2.3 Role o f n-butvlamine

It seems unlikely, however, that the syntheses involving selenium and tellurium are simply room temperature solid state reactions working independently o f the solvent (even though the metals that react are amongst the most chalcophilic). First, no elemental combination was observed for the elements on refluxing. Secondly, both cadmium and tin (both highly chalcophilic in nature) failed to react with chalcogens in n-butylamine, even at reflux, and there was only a marginal degree o f reactivity observed between zinc and sulfur once the solvent had been raised to reflux and left for 7 days. Thirdly, although the formation o f M2E5 (M = Sb, Bi, E = Se, Te) did not occur

at room temperature in n-butylamine, the direct combination o f the elements did occur after facile heating in an inert atmosphere (250 - 300 °C, 2h). This afforded the most thermodynamically stable products (the sesquichalcogenide M2E3) as a pure, crystalline,

single phase. This was not found to be the case after similar heating o f the elements

without treatment by n-butylamine. This ‘activation’ o f the metal surface was also exhibited by liquid ammonia (as discussed in section 2.2.2, p. 62-63).

Finally, n-butylamine appears to allow thallium to react with both sulfur and selenium at room temperature by obtaining the Tl™ oxidation state. The single phased product from each reaction is ‘TIE’ - a mixed oxidation state metal chalcogenide, more accurately described as T f [Tf" S2]. Altering the reagent ratios to either 2:1 or 2:3 (T1:E)

has no effect on the product stoichiometry. The failure o f the analogous reaction between thallium and tellurium m ay be a result o f insufficient bond enthalpy to drive the reaction at room temperature. Interestingly, n-butylamine does not appear to allow the formation o f the kinetic phases o f thallium (I) selenide or telluride exhibited by liquid ammonia (ThE or TI5E3). These products would appear to be more energetically

accessible at low reaction temperatures when one considers the ‘inert pair’ effect exhibited by thallium.’'*^