Thiophosphate

Thiophosphates (or phosphorothioates, PS) are chemical compounds and anions with the general chemical formula (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.

Organic derivatives

Organic thiophosphates include anions of the formula (RO)₂PS₂^- (where R = alkyl and aryl). They are conjugate bases of the acids (RO)₂PS₂H, which are derived by treating phosphorus pentasulfide with alcohols and phenols. These salts have lipophilic properties. They bind metal ions to give transition metal dithiophosphate complexes. They are structurally related to the inorganic thiophosphates. Common members have formulas of the type (RO)_{3−x}(RS)_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.

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). Phosphocysteamine, the thiophosphate , is used as a drug equivalent to cysteamine for clearing excess cystine.

Inorganic

The simplest thiophosphates have the formula [PS_{4−x}O_x]^{3−}. These trianions are only observed at very high pH, instead they exist in protonated form with the formula [H_nPS_{4−x}O_x]^{(3−n)−} (x = 0, 1, 2, or 3 and (n = 1, 2, or 3).

Monothiophosphate

Monothiophosphate is the anion [PO₃S]^{3−}, which has C_3v symmetry. A common salt is sodium monothiophosphate (Na₃PO₃S). Monothiophosphate is used in research as an analogue of phosphate in biochemistry. Monothiophosphate esters are biochemical reagents used in the study of transcription, substitution interference assays. Sometimes, "monothiophosphate" refers to esters such as (CH₃O)₂POS^−.

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:

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

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

A number of these anions known. Some have attracted interest as components in fast ion conductors for use in solid state batteries. The binary thiophosphates do not exhibit the extensive diversity of the analogous oxyanions 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 that ions 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.

• is the analogue of the nitrate ion, (there is no analogue); it was isolated as the yellow tetraphenylarsonium salt
• is the sulfur analogue of , and like is tetrahedral.
• the pyrothiophosphate ion consisting of two corner sharing PS₄ tetrahedra, analogous to the pyrophosphates.
• An ion which can be visualised either as two PS₄ tetrahedra joined by a disulfide link or a pyrothiophosphate where the bridging −S− is replaced by −S₄−.
• edge-shared bitetrahedral structure. The structure is therefore similar to the isoelectronic Al₂Cl₆ dimer. The oxygen analogue, dimetaphosphate , in contrast, is not known, the metaphosphates favour polymeric structures of chains or rings.
• and are related to but their two bridging −S− atoms are replaced by −S−S− in and by an −S−S−S− bridge in .
• P₂S These form water-stable salts. 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, .
• contains a six-membered P₃S₃ ring. The ammonium salt is produced by reaction of P₄S₁₀ in liquid ammonia. Another way of visualising the structure is that it is the P₄S₁₀ adamantane (P₄O₁₀) structure with a PS³⁺ vertex removed.
• contains a square P₄ ring, contains a P₅ ring and a P₆ ring. These 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 anion is analogous to the ring anion.
• 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.
• is a sulfido heptaphosphane cluster anion.

References