Phosphonate



Phosphonates or Phosphonic acids are organic compounds containing one or more C-PO(OH)2 or C-PO(OR)2 (with R=alkyl, aryl)  groups. Bisphosphonates were first synthesized in 1897 by Von Baeyer and Hofmann. An example of such a bisphosphonate is HEDP. Since the work of Schwarzenbach in 1949, phosphonic acids are known as effective chelating agents. The introduction of an amine group into the molecule to obtain -NH2-C-PO(OH)2 increases the metal binding abilities of the phosphonate. Examples for such compounds are NTMP, EDTMP and DTPMP. These common phosphonates are the structure analogues to the well-known aminopolycarboxylates NTA, EDTA, and DTPA. The stability of the metal complexes increases with increasing number of phosphonic acid groups. Phosphonates are highly water-soluble while the phosphonic acids are only sparingly soluble. Phosphonates are not volatile and poorly soluble in organic solvents.

Phosphonate compounds

 * AEPn: 2-Aminoethylphosphonic acid
 * DMMP: Dimethyl methylphosphonate
 * HEDP: 1-Hydroxyethane(1,1-diylbisphosphonic acid)
 * NTMP: Nitrilotris(methylenephosphonic acid)
 * EDTMP: 1,2-Diaminoethanetetrakis (methylenephosphonic acid)
 * DTPMP: Diethylenetriaminepentakis (methylenephosphonic acid)
 * PBTC: Phosphonobutane-tricarboxylic acid

Occurrence in nature


The first natural phosphonate, 2-aminoethylphosphonic acid, was identified in 1959 and occurs in plants and many animals, mostly in membranes. Phosphonates are quite common among different organisms, from prokaryotes to eubacteria and fungi, mollusks, insects and others. The biological role of the natural phosphonates is still poorly understood. Until now no bis- or polyphosphonates have been found to occur naturally.

Properties and uses
Phosphonates have three main properties: they are effective chelating agents for two- and trivalent metal ions, they inhibit crystal growth and scale formation and they are quite stable under harsh chemical conditions. An important industrial use of phosphonates is in cooling waters, desalination systems, and in oil fields to inhibit scale formation. In pulp and paper manufacturing and in textile industry they are used as peroxide bleach stabilizers, acting as chelating agents for metals that could inactivate the peroxide. In detergents they are used as a combination of chelating agent, scale inhibitor and bleach stabilizer. Phosphonates are also used more and more in medicine to treat various bone and calcium metabolism diseases and as carriers for radionuclides in bone cancer treatments (see Samarium-153-ethylene diamine tetramethylene phosphonate). In 1998 the consumption of phosphonates was 56,000 tons worldwide - 40,000 tons in the US, 15,000 tons in Europe and less than 800 tons in Japan. The demand of phosphonates grows steadily at 3% annually.

Toxicology
The toxicity of phosphonates to aquatic organisms is low. Reported values for 48 h LC50 values for fish are between 0.1 and 1.1 mM. Also the bioconcentration factor for fish is very low. Phosphonates are poorly absorbed in the gastro-intestinal tract and most of the absorbed dose was rapidly excreted by the kidneys. Human toxicity is also low which can be seen in the fact that phosphonates are used to treat various diseases. Phosphonates are similar to phosphates except that they have a carbon-phosphorus (C-P) bond in place of the carbon-oxygen-phosphorus (C-O-P) linkage. Due to their structural similarity to phosphate esters, phosphonates often act as inhibitors of enzymes due in part to the high stability of the C-P bond.

Biodegradation
In nature bacteria play a major role in phosphonate biodegradation. Due to the presence of natural phosphonates in the environment, bacteria have evolved the ability to metabolize phosphonates as nutrient sources. Those bacteria able of cleaving the C-P bond are able to use phosphonates as a phosphorus source for growth. Aminophosphonates can also be used as sole nitrogen source by some bacteria. The polyphosphonates used in industry differ greatly from natural phosphonates such as 2-aminoethylphosphonic acid, because they are much larger, carry a high negative charge and are complexed with metals. Biodegradation tests with sludge from municipal sewage treatment plants with HEDP and NTMP showed no indication for any degradation. An investigation of HEDP, NTMP, EDTMP and DTPMP in standard biodegradation tests also failed to identify any biodegradation. It was noted, however, that in some tests due to the high sludge to phosphonate ratio, removal of the test substance from solution observed as loss of DOC was observed. This was attributed to adsorption rather than biodegradation. However, bacterial strains capable of degrading aminopolyphosphonates and HEDP under P-limited conditions have been isolated from soils, lakes, wastewater, activated sludge and compost.

Environmental behavior
Phosphonates have properties that differentiate them from other chelating agents and that greatly affect their environmental behavior. Phosphonates have a very strong interaction with surfaces, which results in a significant removal in technical and natural systems. Due to this strong adsorption, little or no remobilization of metals is expected. No biodegradation of phosphonates during water treatment is observed but photodegradation of the Fe(III)-complexes is rapid. Aminopolyphosphonates are also rapidly oxidized in the presence of Mn(II) and oxygen and stable breakdown products are formed that have been detected in wastewater. The lack of information about phosphonates in the environment is linked to analytical problems of their determination at trace concentrations in natural waters. Phosphonates are present mainly as Ca and Mg-complexes in natural waters and therefore do not affect metal speciation or transport.

Organic synthesis
Phosphonates can be synthesized using the Michaelis-Arbuzov reaction. In one study a α-aminophosphonate is prepared from a condensation reaction of benzaldehyde, aniline and trimethyl phosphite catalyzed by Copper triflate in a one-pot synthesis

Organic reactions
Phosphonates are used in the Horner-Wadsworth-Emmons reaction.

Miscellaneous
Phosphonates are one of the three sources of phosphate intake in biological cells (The other two being inorganic phosphate and organophosphate)