Forschung / ResearchThe only transport mechanism for the reaction of two ionic solids is diffusion of ions. This affords long reaction times and high temperatures. Enhanced reactivities are possible through mobile phases, for example through convective transport or in solutions and melts, respectively.
Ammonium halides, (NH4)X, are substances that dissociate into their components NH3 and HX dependent upon temperature which makes them mobile. The ammonium ion, (NH4)+, itself is a versatile reagent because it may react, subject to its constituents H+, NH3, NH2-, NH2- and N3- as an acid or oxidant as well as base or reductant, respectively. For example, the reaction of gallium metal with (NH4)F or (NH4)HF2 produces single crystals of, e.g., Ga(NH3)F3 and Ga(NH3)2F3 as well as Ga(NH3)(NH2)F2, substances that play an important role in the ammonolysis of (NH4)3GaF6 yielding GaN. Under similar conditions, iron reacts with (NH4)Br to [Fe(NH3)6]3[Fe8Br14], a complex salt whose anion contains monovalent iron (Fig. 1)!
Also, when Hg-N bonds are to be tied, NH3 and
(NH4)+ play an important role: Dependent upon the
concentration of NH3/(NH4)+ and temperature,
compounds like (NH4)[Hg3(NH)2](NO3)3
or the anhydrous nitrate of Millonís base, [Hg2N](NO3),
are obtained. Hg-N bonds are also formed in reactions of mercuric halides
with N-heterocycles. The ligand 3,5-dimethyl-4í-amino-1,2,4-triazole and
HgCl2 form an adduct as shown in Fig. 2 and caffeine a complex
as shown in Fig. 3. Closed-shell d10 cations (Hg2+
and Ag+) where relativistic effects play a more or less important
role, are systematically tested for their affinity towards N-donor compounds,
mostly heterocycles, and compared with other important transition metal
cations such as Ni2+ (d8), Co2+/Co3+,
and Mn2+ (d5).
Nitrogen is also known to act as an interstitial atom in many clusters such as the new Ba[Ce4N2]I8. Such compounds are obtained via the metallothermic reduction of rare-earth halides with alkali or alkaline-earth metals when educts are used that are contaminated with ammonium halides. The action of barium on pure LaI3 yields BaLaI4 which contains chains of face-sharing square antiprisms [LaI8/2]. Under equal conditions barium reacts with PrI3 forming Ba6Pr3I19 which contains the new trimers Pr3I16 (Fig. 4) with two excess electrons per trimer.
Another frequent interstitial atom in rare-earth halide cluster chemistry is carbon, either as single atoms or as dicarbon units. An exceptional example is mono-carbon in tetrahedral interstices in the supertetrahedral cluster Sc24C10I30 (Fig. 5).