SNX25

Polythiazyl (polymeric sulfur nitride), , 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 non-metallic compound in which superconductivity could be demonstrated. However, the relatively low transition temperature at about 0.3 K makes a practical application unlikely.Alsfasser, R.; Janiak, C.; Klapötke, T.M.; Meyer, H.-J.: ''Moderne Anorganische Chemie'', Herausgeber Riedel, E., 3. Auflage 2007, Walter de Gruyter GmbH & Co. KG, Berlin/Boston, , S. 129–132, (retrieved via De Gruyter Online).

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. Wiberg, E.; Wiberg, N.; Holleman, A.F.: '' Anorganische Chemie'', 103. Auflage, 2017 Walter de Gruyter GmbH & Co. KG, Berlin/Boston, , S. 681, (retrieved via De Gruyter Online). The compound also explodes on impact.

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.Baughman, R.H.; Apgar, P.A.; Chance, R.R.; MacDiarmid, A.G.; Garito, A.F.: ''A New Phase of (SN)_x'' in J. Chem. Soc. Chem. Comm. 1977, 49–50, .

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 °.MacDiarmid, A.G.; Mikulski, C.M.; Russo, P.J.; Saran, M.S.; Garito, A.F.;
Heeger, A.J.: ''Synthesis and structure of the polymeric metal, (SN)_x, and its precursor, S₂N₂'' in J. Chem. Soc. Chem. Comm. 1975, 476–477, .MacDiarmid, A.G.; Mikulski, C.M.; Saran, M.S.; Russo, P.J.; Cohen, M.J.; Bright, A.A.; Garito, A.F.; Heeger, A.J.: ''Synthesis and Selected Properties of Polymeric Sulfur Nitride, (Polythiazyl), (SN)_x'' in Advances in Chemistry 150 (2009) 63–72, .

Synthesis

Polythiazyl is synthesized by the polymerization of the dimer disulfur dinitride (), which is in turn synthesized from the cyclic alternating tetramer tetrasulfur tetranitride (). Conversion from cyclic tetramer to dimer is catalysed with hot silver wool.

:
:S₄N₄ (w/ Ag₂S catalyst) → 2 S₂N₂ (w/ 77K cold finger) → S₂N₂
:S₂N₂ (@ 0°C, sublimes to surface) → thermal polymerization → (SN)_x

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
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References

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Inorganic polymers
Nitrides
Superconductors
Sulfur–nitrogen compounds
Conductive polymers