Instead of starting at the beginning of a paper I want to kick off this commentary with a statement from near the end:

“Caution! Covalent azides are potentially hazardous and can decompose explosively under various conditions! Especially Hg2(N3)2, α– and β-Hg(N3)2, and [Hg2N]N3 in this work are extremely friction/shock-sensitive and can explode violently upon the slightest provocation. Appropriate safety precautions (safety shields, face shields, leather gloves, protective clothing) should be taken when dealing with large quantities. Hg compounds are highly toxic! Experimental details can be found in the Supporting Information.”

This wonderful statement appears in a recent publication by Professors Schultz and Villinger at the University of Rostock in Germany. They discuss the preparation of mercury azides and the azide of something called Millon’s Base. This compound was new to me and it turns out to be nitridodimercury hydroxide, [Hg 2 N]OH.2H 2 O, which Millon1 discovered by the reaction of mercurous oxide and ammonia in the mid 1800s. In a classic example of understatement the authors’ state that as is the case with most transition metal nitrogen compounds the extremely low energy barriers to explosive decomposition result in difficulties in the isolation and manipulation of said species! Curtius, of rearrangement fame, was apparently the first person to isolate mercury azide Hg 2 (N 3 ) 2 from the reaction of hydrogen azide and mercury2. I guess this was after the discovery of his famous rearrangement.

Structural data for this compound is available from x-ray and revealed two modifications, called α and β. Due to its lability the β modification has not been fully characterised. Schultz etal have now rectified this situation and also report the preparation of the azide salt, [Hg 2 N]N 3 of Millon’s base. They prepared α & β-Hg(N 3 ) 2 , the latter compound by slow diffusion of aqueous sodium azide into a solution of mercury (II) nitrate separated by a layer of aqueous sodium nitrate. In this synthesis one wonders how any yield was obtained because when the needles of β-Hg(N 3 ) 2 begin to form in the lower mercury(II) nitrate layer spontaneous explosions occur during crystal growth. If you want large crystals of either modification, usually obtained by slow crystallisation, I would not recommend it as apparently large crystals seem to explode when you look at them the wrong way, even in solution they detonate. Explosive solutions would be a great name for a company! Anyway, in spite of these difficulties an X-ray structure along with a melting point was obtained.

Turning now to the synthesis of the azide of Millon’s base the authors note that the normal method always produced a mixture of the two modifications. Pure α-[Hg 2 N]N 3 was obtained by treatment of α-[Hg 2 N]Br with concentrated aqueous sodium azide for 300 days, so you need patience when dealing with these compounds, not only because they are explosive but they suffer from long reaction times. However starting with β-[Hg 2 N]NO 3 the reaction was faster, only taking 4 days for the exchange with azide but produced a mixture of modifications. However, they did manage to obtain both modifications.

Elemental analysis could not be carried out due to their explosive nature and both modifications are sensitive towards heat, shock and especially friction. The bigger the crystal the more sensitive it is. However, slow heating in a DSC instrument showed that they are stable up to 283°C for the β form and 313°C for the α. Rapid heating in a closed vessel caused violent heavy detonation accompanied by a bright blue flash.

The paper has some fascinating x-ray pictures of all the molecules discussed and allowed determination of the N-Hg bond lengths. Together with the chemistry and the dangers involved in this chemistry, a great piece of work has evolved into a wonderful very readable paper. Congratulations to all who participated.

References:

1 E. Millon, J. Prakt. Chem. 1839, 16, 58.

2 T. Curtius, Chem. Ber. 1890, 23, 3023