# Polythiazyl

Polythiazyl (polymeric sulfur nitride), (SN)$_x$, is an electrically conductive, gold- or bronze-colored polymer with metallic luster. It was the first conductive inorganic polymer discovered and was also found to be a superconductor at very low temperatures (below 0.26 K). It is a fibrous solid, described as "lustrous golden on the faces and dark blue-black", depending on the orientation of the sample. It is air stable and insoluble in all solvents.

## History

The compound was first reported as early as 1910 by F.P. Burt, who obtained it by heating tetrasulfur tetranitride in vacuum over silver wool.
The compound was the first compound with only non-metallic elements in which superconductivity could be demonstrated. However, the relatively low transition temperature at about 0.3 K makes a practical application unlikely.

## Properties

Polythiazyl is a metallic-golden and shiny, crystalline but fibrous material. The polymer is mostly inert to oxygen and water, but decomposes in air to a grey powder. At temperatures above 240 °C explosive decomposition can occur. The compound also explodes on impact. Explosion generally proceeds via decomposition to the elements.
Polythiazyl shows an anisotropic electrical conductivity. Along the fibres or SN chains, the bond is electrically conductive, perpendicular to it acts as an insulator. The one-dimensional conductivity is based on the bonding conditions in the S-N chain, where each sulfur atom provides two π electrons and each nitrogen atom provides one π electron to form two-center 3π electron bonding units.
Two polymorphic crystal forms were observed in the compound. The monoclinic form I obtained from the synthesis can be converted into an orthorhombic form II by mechanical treatment such as grinding.

### Structure and bonding

The material is a polymer, containing trivalent nitrogen, and divalent and tetravalent sulfur. The S and N atoms on adjacent chains align. Several resonance structures can be written.
The structure of the crystalline compound was resolved by X-ray diffraction. This showed alternating S–N bond lengths of 159 pm and 163 pm and S–N–S bond angles of 120 ° and N–S–N bond angles of 106 °.

## Synthesis

The oldest-known polythiazyl synthesis is the polymerization of the cyclic formal dimer disulfur dinitride (S$_2$N$_2$), which is in turn synthesized from the formal tetramer tetrasulfur tetranitride (S$_4$N$_4$), in the presence of hot silver wool. The reaction begins when silver abstracts sulfur from S$_4$N$_4$ to produce a Ag$_2$S catalyst; the resulting gaseous S$_2$N$_2$ is then isolated through sublimation onto a cold surface:
    S$_4$N$_4$ + 8 Ag → 4 Ag$_2$S + 2 N$_2$
    S$_4$N$_4$ (low-pressure gas at 250-300 °C; Ag$_2$S catalyst) → 2 S$_2$N$_2$ (gas) → 2 S$_2$N$_2$ (stable solid at 77 K)
When warmed to room temperature, the additional heat induces spontaneous polymerization:
    S$_2$N$_2$ (0 °C) → (SN)$_x$
An alternative is the azide reduction of thiazyl chloride trimer, itself made from thiazyl fluoride:
    (NSF)$_3$ + Cl$_2$ → (SNCl)$_3$ + ClF
    (SNCl)$_3$ + NaN$_3$ → NaCl + N$_2$ + (SN)$_∞$
To eschew explosive reagents entirely, iron filings in nitromethane reduce the thiazyl chloride trimer to [(SN)$_5$]$^+$[FeCl$_4$]$^−$, which a platinum cathode reduces to polythiazyl.

## Uses

Due to its electrical conductivity, polythiazyl is used in LEDs, transistors, battery cathodes, and solar cells.

## Literature

King, R.S.P.: Novel chemistry and applications of polythiazyl, Doctoral Thesis Loughborough University 2009, pdf-Download