polyselenide
In chemistry, a polyselenide usually refers to anions of the formula (Sen)2−, where Se is the atomic symbol for the element selenium. Many main group and transition metals form complexes with polyselenide anions.{{Greenwood&Earnshaw2nd|page=763-765}}
Preparation
Conceptually, polyselenides are derived by deprotonation polyselenanes H2Sen, but compounds with Se-H bonds are rare or labile. Instead, analogous to the preparation of many Zintl ions, polyselenides can produced by reduction of elemental Se with alkali metals. Such reactions can be conducted by heating a mixture of the solids or by dissolving Se metal in amine solutions of alkali metals. Synthesis can also be conducted in high-boiling, polar, aprotic solvents such as DMF, HMPA, and NMP.{{cite journal|last1=Thompson|first1=D.|last2=Boudjouk|first2=P. A |title=Convenient Synthesis of Alkali Metal Selenides and Diselenides in Tetrahydrofuran and the Reactivity Differences Exhibited By These Salts Toward Organic Bromides|journal=Journal of Organic Chemistry|year=1988|volume =53|page=2109-2112|doi= 10.1021/jo00244a051}} These reactions appear to proceed by initial formation of the alkali metal selenide, followed by the reaction of the latter with additional selenium:
:2 Na + Se → Na2Se
:Na2Se + n Se → Na2Sen+1
Once generated, alkali metal polyselenides can be converted to lipophilic salts by treatment cryptand ligands or by ion exchange with quat salts.Kolis, J. "Coordination Chemistry of Polychalcogen Anions and Transition Metal Carbonyls" Coordination Chemistry Reviews 1990, volume 105, pp. 195-219. {{doi|10.1016/0010-8545(90)80023-M}}
: Na2Sen + 2 R4NCl → (R4N)2Sen + 2 NaCl
Sodium polyselenide can also be generated by heating elemental selenium with sodium hydroxide.{{cite journal |doi=10.15227/orgsyn.018.0027 |author=Henry M. Leicester|title=Diphenyl Selenide |journal=Organic Syntheses |date=1938 |volume=18 |page=27 }}
Structures
Salts of polyselenides have often been characterized by X-ray crystallography. Polyselenides salts generally feature open chains, which adopt a zig-zag conformation. In rare cases, cyclic structures are observed as in Li2Se5, which features a square-planar Se center. High resolution solid state 77Se NMR spectra of [NMe4]2Se6 show three selenium sites. Single-crystal X-ray structure determination of the two salts support the NMR data.Barrie, P. J.; Clark, R. J. H.; Selenium Solid-State NMR Spectroscopy and Structures of Tetramethylammonium Pentaselenide and Hexaselenide Complexes. Inorg. Chem, 1995, 34, 4299–4304 DOI: 10.1021/ic00121a006
=Reactivity=
Polyselenides are prone to decomposition on exposure to air, in which case they are oxidized back to elemental selenium.
:{{chem|Se|n|2−}} + 2 H+ + {{1/2}} O2 → n Se + H2O
As ligands in coordination complexs, polyselenides are generally bidentate. Complexes of penta-, tetra-, and triselenide ligands are known. One example is the spirocyclic [Zn(Se4)2]2−.{{cite journal |doi=10.1080/02603599008048650|title=Soluble Polychalcogenides of the Late Transition and Main Group Elements|year=1990|last1=Kanatzidis|first1=Mercouri G.|journal=Comments on Inorganic Chemistry|volume=10|issue=4–5|pages=161–195}}
Further reading
- {{cite journal |doi=10.3390/molecules14093115|doi-access=free|title=Solid State Polyselenides and Polytellurides: A Large Variety of Se–Se and Te–Te Interactions|year=2009|last1=Graf|first1=Christian|last2=Assoud|first2=Abdeljalil|last3=Mayasree|first3=Oottil|last4=Kleinke|first4=Holger|journal=Molecules|volume=14|issue=9|pages=3115–3131|pmid=19783911|pmc=6255372}}
- {{cite journal |doi=10.1002/zaac.201200241|title=Polychalcogenide Anions: Structural Diversity and Ligand Versatility|year=2012|last1=Sheldrick|first1=William S.|journal=Zeitschrift für Anorganische und Allgemeine Chemie|volume=638|issue=15|pages=2401–2424}}