Thiophosphate

For the thiophosphate esters see organothiophosphates.

Thiophosphates (or phosphorothioates, PS) are chemical compounds and anions with the general chemical formula PS_4-xO_x³⁻ (x = 0, 1, 2, or 3) and related derivatives where organic groups are attached to one or more O or S. Thiophosphates feature tetrahedral phosphorus(V) centers.^[1]

Organic

Organothiophosphates are a subclass of organophosphorus compounds that are structurally related to the inorganic thiophosphates. Common members have formulas of the type (RO)_3-xR_xPS and related compounds where RO is replaced by RS. Many of these compounds are used as insecticides, some have medical applications, and some have been used as oil additives.^[1]

Selected organothiophosphates
zinc dialkyldithiophosphate, an oil additive.[2]  Phosphorothioates are the basis for antisense therapies.  Amifostine, which is used in cancer chemotherapy.  Chlorpyrifos, a popular insecticide.  Malathion, a popular insecticide.

Oligonucleotide phosphorothioates (OPS) are modified oligonucleotides where one of the oxygen atoms in the phosphate moiety is replaced by sulfur. They are the basis of antisense therapy, e.g., the drugs fomivirsen (Vitravene), oblimersen, alicaforsen, and mipomersen (Kynamro).^[3]

Inorganic

The simplest thiophosphates have the formula [PS_4-xO_x]^3-. These trianions are only observed at very high pH, instead they exist in protonated form with the formul [H_nPS_4-xO_x]^(3-n)-.

Monothiophosphate

Ball-and-stick model of the hypothetical monothiophosphate trianion.

Monothiophosphate is the anion [PO₃S]³⁻, which has C_3v symmetry. Coordination complexes by oxygen and sulphur are known.^[4]

In the laboratory monothiophosphate is used as an analogue of phosphate in biochemistry. Monothiophosphate esters are biochemical reagents used in the study transcription,^[5] substitution interference assays.

Dithiophosphates

Dithiophosphate has the formula [PO₂S₂]³⁻, which has C_2v symmetry. Sodium dithiophosphate, which is colorless, is the major product from the reaction of phosphorus pentasulfide with NaOH:^[6]

P₂S₅ + 6 NaOH → 2 Na₃PO₂S₂ + H₂S + 2 H₂O

Dithiophosphoric acid is obtained by treatment of barium dithiophosphate with sulfuric acid:

Ba₃(PO₂S₂)₂ + 3 H₂SO₄ → 3 BaSO₄ + 2 H₃PO₂S₂

Both Na₃PO₂S₂ and especially H₃PO₂S₂ are prone toward hydrolysis to their monothio derivatives.

Tri- and tetrathiophosphates

Trithiophosphate is the anion [POS₃]³⁻, which has C_3v symmetry. Tetrathiophosphate is the anion [PS₄]³⁻, which has T_d symmetry.

P_xS_y: binary thiophosphates and polyphosphates

There are a number of these anions known. There has been interest in compounds containing these anions due to their potential application as fast ion conductors for use in solid state batteries. The binary thiophosphates do not exhibit the extensive diversity of the analogous P-O anions but contain similar structural features, for example P is 4 coordinate, P-S-P links form and there are P-P bonds. One difference is thations may include polysulfide fragments of 2 or more S atoms whereas in the P-O anions there is only the reactive -O-O-, peroxo, unit.

• PS₃⁻ is the analogue of the nitrate ion, NO₃⁻ (there is no PO₃⁻ analogue)- it was isolated as the tetraphenylarsonium salt^[7]
• PS₄ ³⁻ is the sulfur analogue of PO₄³⁻, and like PO₄³⁻ is tetrahedral.
• P₂S₇⁴⁻ the pyrothiophosphate ion consisting of two corner sharing PS₄ tetrahedra, analogous to the pyrophosphates.^[8]
• P₂S₁₀⁴⁻ An ion which can be visualised either as two PS₄ tetrahedra joined by a disulfide link or a pyro-thiophosphate where the bridging -S- is replaced by -S₄-.^[9]
• P₂S₆²⁻ with a structure consisting of two edge sharing tetrahedra. The structure is therefore similar to the isoelectronic Al₂Cl₆ dimer. The oxygen analogue, dimetaphosphate P₂O₆ ²⁻, in contrast, is not known, the metaphosphates favour polymeric structures of chains or rings.^[8]
• P₂S₈²⁻ and P₂S₁₀²⁻ are related to P₂S₆²⁻ but their two bridging -S- atoms are replaced by -S-S- in P₂S₈²⁻ and by an -S-S-S- bridge in P₂S₁₀²⁻.^[10]
• P₂S₆⁴⁻ These form water stable salts.^[11] The anion has an ethane like structure and contains a P-P bond. The formal oxidation state of phosphorus is +4. The oxygen analogue is the hypodiphosphate anion, P₂O₆⁴⁻.
• P₃S₉³⁻ contains a six membered P₃S₃ ring. The ammonium salt is produced by reaction of P₄S₁₀ in liquid ammonia.^[12] Another way of visualising the structure is that it is the P₄S₁₀ adamantane (P₄O₁₀) structure with a PS³⁺ vertex removed.
• P₄S₈⁴⁻ contains a square P₄ ring,^[7] P₅S₁₀⁵⁻ contains a P₅ ring and P₆S₁₂⁶⁻ a P₆ ring.^[10] These (PS₂⁻)_n cyclic anions contain P with an oxidation state +3. Note they are not trigonal as arsenic(III) is in arsenites, but are tetrahedral with two bonds to other phosphorus atoms and two to sulfur. The P₆S₁₂⁶⁻ anion is analogous to the P₆O₁₂⁶⁻ ring anion.^[13]
• P₄S₂⁻ An unusual butterfly shaped ion, SP(P₂)PS, which can be visualised as a P₄ molecule where two P-S bonds replace one P-P bond.^[14]
• P₇(S)₃³⁻ is a sulfido heptaphosphane cluster anion.^[7]

References

1. J. Svara, N. Weferling, T. Hofmann "Phosphorus Compounds, Organic" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2006. doi:10.1002/14356007.a19_545.pub2
2. H. Spikes "The history and mechanisms of ZDDP" Tribology Letters, Vol. 17, No. 3, October 2004. doi:10.1023/B:TRIL.0000044495.26882.b5.
3. Kurreck, J., "Antisense technologies. Improvement through novel chemical modifications", European Journal of Biochemistry 2003, 270, 1628-1644.doi:10.1046/j.1432-1033.2003.03555.x
4. Poat JC, et al. (1990). "A thiophosphate bridged platinum–zinc hetero-bimetallic complex: [(Me₂PhP)₂Pt{OSP(OR)₂}₂ZnCl₂". J. Chem. Soc., Chem. Commun.: 1036–1038.
5. Lorsch JR, Bartel DP, Szostak JW. (1995). "Reverse transcriptase reads through a 2'–5' linkage and a 2'-thiphosphate in a template". Nucleic Acids Res. 23 (15): 2811–2814. doi:10.1093/nar/23.15.2811. PMC 307115. PMID 7544885.
6. R. Klement "Phosphorus" in Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1., p. 571.
7. Greenwood, Norman N.; Earnshaw, Alan (1984). Chemistry of the Elements. Oxford: Pergamon Press. pp. 509–510. ISBN 0-08-022057-6.
8. Phosphorus: Chemistry, Biochemistry and Technology, Sixth Edition, 2013, D.E.C. Corbridge, CRC Pres, Taylor Francis Group, ISBN 978-1-4398-4088-7
9. Aitken, Jennifer A.; Canlas, Christian; Weliky, David P.; Kanatzidis, Mercouri G. (2001). "[P2S10]4-: A Novel Polythiophosphate Anion Containing a Tetrasulfide Fragment". Inorganic Chemistry 40 (25): 6496–6498. doi:10.1021/ic010664p. ISSN 0020-1669.
10. Holleman, A. F.; Wiberg, E. (2001), Inorganic Chemistry, San Diego: Academic Press, pp. 734–735, ISBN 0-12-352651-5
11. Gjikaj, Mimoza; Ehrhardt, Claus (2007). "New Hexachalcogeno–Hypodiphosphates of the Alkali Metals: Synthesis, Crystal Structure and Vibrational Spectra of the Hexathiodiphosphate(IV) Hydrates K4[P2S6] · 4 H2O, Rb4[P2S6] · 6 H2O, and Cs4[P2S6] · 6 H2O". Zeitschrift für anorganische und allgemeine Chemie 633 (7): 1048–1054. doi:10.1002/zaac.200600339. ISSN 0044-2313.
12. Wolf, G.-U.; Meisel, M. (1982). "Beiträge zur Chemie von Phosphorverbindungen mit Adamantanstruktur. VII[1]. Über Darstellung und Eigenschaften von Nonathio-cyclotriphosphat". Zeitschrift für anorganische und allgemeine Chemie 494 (1): 49–54. doi:10.1002/zaac.19824940106. ISSN 0044-2313.
13. Ebert, M.; Nassler, J. (1989). "The Ring Acid, H6P6O12 and Ring Acid, H4P4O10and Their Salts". Phosphorus, Sulfur, and Silicon and the Related Elements 41 (3-4): 462–462. doi:10.1080/10426508908039741. ISSN 1042-6507.
14. Rotter, Christiane; Schuster, Michael; Karaghiosoff, Konstantin (2009). "An Unusual Binary Phosphorus−Tellurium Anion and Its Seleno- and Thio- Analogues: P4Ch22−(Ch = S, Se, Te)". Inorganic Chemistry 48 (16): 7531–7533. doi:10.1021/ic901149m. ISSN 0020-1669.