Basic principle of the preparative co-condensation
exemplarily for germanium monohalides:
a) Schematic description of the preparative co-condensation:
During the preparative co-condensation first of all the gas phase molecule (e.g. GeBr) of interest is synthesized within a reactor at low pressure (ca. 10-2 mbar) and high temperature (ca. 1600°C). The molecules then leave the oven and are condensed together with an inert solvent (e.g. toluene) at a liquid nitrogen (-196°C) cooled surface building up a solid matrix where the gas-phase molecules are completely separated by solvent molecules. After the co-condensation the matrix is slowly warmed (e.g. with dry ice to -78°C). During this process the matrix melts to give a metastable solution of the gas-phase molecules if the right solvent or solvent mixture is used.
Consequently, via the co-condensation the gas-phase molecules are transferred into a novel state which means a novel reagent is obtained based on molecules that are only stable under drastic reaction conditions. This novel reagent thus exhibits novel properties and novel reactivity for further investigations.
b) Technical realization and performance of a preparative co-condensation exemplarily given for Germanium(I)halides:
To realize the needed high temperatures a powerful inductive heating (maximum power output 10 kW) is used. Via the water-cooled induction coil (B) the graphite reactor (A) can be heated inductively to a maximum temperature of around 2000°C. Consequently, the needed reaction temperature of 1600°C is well applicable. The elemental Germanium that is needed for the reaction is placed within the graphite reactor. During the reaction Hydrogen halide gas (HX, green in the upper scheme) is passed into the hot graphite reactor at 1600°C via an inlet system. At the applied reaction conditions, the HX gas reacts with the liquid germanium to give mainly the germanium(I)halide (blue in the upper scheme) which leaves the oven and is condensed at the surface of the stainless steel bell (D). The simultaneously formed side-product elemental Hydrogen is continuously pumped off by a high vacuum pumping system so that the pressure is during the whole reaction in the area of 10-5 mbar. The water cooled (K) copper shield (E) surrounds the oven and the induction coil and is necessary to handle the enormous amount of radiative heat ejected by the graphite reactor. Below the cupper cooling shield a hollow stainless steel ring is mounted via which solvent vapor (yellow spheres in the upper scheme) is added during the co-condensation. The solvent vapor is condensed at the surface of the stainless steel bell forming a matrix in which the Germanium(I)halide molecules are embedded (right side of the upper scheme).
After the co-condensation reaction is finished the whole apparatus is flushed with gaseous nitrogen. Additionally, the cooling is changed from liquid nitrogen (-196°C) to dry ice (-78°C) so that the temperature slowly rises within the apparatus. During the heating process the solid matrix melts and the metastable solution is collected at the bottom of the stainless steel bell. If needed the metastable solution can be transferred into a cooled Schlenk vessel via a stainless steel cannula (F). This solution can then also be used outside the co-condensation apparatus for further reactions. The so obtained metastable solution of germanium monohalides is a novel reagent with novel chemical properties.
A more detailed description how a preparative co-condensation is performed can be found for tin monohalides at: M. Binder, C. Schrenk, A. Schnepf, J. Vis. Exp. 2016, 117, e54498, doi:10.3791/54498 „The Synthesis of [Sn10(Si(SiMe3)3)4]2- using a Metastable Sn(I) Halide Solution Synthesized via a Co-condensation Technique”.
c) Pictures of the co-condensation apparatus