Ditungsten tetra(hpp): Difference between revisions

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'''Di-tungsten tetra(hpp)''' is the name of the [[coordination compound]] with the formula W₂(hpp)₄. This material consists of a pair of [[tungsten]] centers linked by the conjugate base of four hexahydropyrimidopyrimidine (hpp) [[ligand]]s. It adopts a structure sometimes called a "paddlewheel" or [[Chinese lantern structure|"Chinese lantern" structure]] ([[paddlewheel compound]]), the prototype being [[copper(II) acetate]].

The molecule is of theoretical interest because it has the lowest [[ionization energy]] (3.51 [[electron_volt | eV]]) of all stable [[chemical element]]s or [[chemical compound]]s as of the year 2005.<ref name=hpp>{{cite journal | title = Expeditious Access to the Most Easily Ionized Closed-Shell Molecule, W₂(hpp)₄ | author = [[F. Albert Cotton]], James P. Donahue, Dennis L. Lichtenberger, Carlos A. Murillo, and Dino Villagrán | journal = [[J. Am. Chem. Soc.]] | volume = 127 | issue = 31 | pages = 10808–10809 | year = 2005 | doi = 10.1021/ja0535458 | pmid = 16076168}}</ref> This value is even lower than of [[caesium]] with 3.89 eV (or 375 kJ/mol) located at the extreme left lower corner of the [[periodic table]] (although [[francium]] is at a lower position in the periodic table compared to caesium, it has a higher ionization energy and is [[radioactive]]) or known [[metallocene]] [[reducing agent]]s such as decamethyl[[cobaltocene]] with 4.71 eV.

[[Image:Hpp ligand.gif|frame|right|the hpp ligand anion]]

This coordination compound is prepared by the reaction of [[tungsten hexacarbonyl]] with 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine ('''Hhpp''') in ''o''-dichlorobenzene at 200 °C:

:[[Image:Ditungstentetrahpp.gif|synthesis of Di-tungsten tetra(hpp)]]

The reaction gives W₂(hpp)₄Cl₂. Dichlorobenzene provides the chlorine atoms and is itself reductively coupled to dichloro[[biphenyl]]. The [[bond order]] between the tungsten centers in W₂(hpp)₄Cl₂ is three.

This dichloride is further reduced by [[potassium]] metal to W₂(hpp)₄. This species has a [[quadruple bond]] between the two tungsten centers. Related quadruply bonded complexes include {{chem|[W|2|Cl|8|]|4-}} and [[Potassium octachlorodimolybdate|{{chem|[Mo|2|Cl|8|]|4-}}]]. Because of its low ionization energy, W₂(hpp)₄ is easily oxidized back to the dichloride by [[dichloromethane]]. It is readily oxidized to the corresponding cation with the oxidants [[fullerene]] and with [[tetracyanoquinodimethane]]. <!--Although W₂(hpp)₄ was proposed to be a potentially useful powerful reducing agent akin to Cp^*₂Co and Cp^*₂Cr,<ref name=hpp/> so far this reagent has not become part of the synthetic chemist's armament. grand total of 7 refs in 5 years-->

*{{Cite journal | title = Electron Donor Par Excellence | author = Ron Dagani | journal = [[Chemical & Engineering News]] | date = 2002-12-09 | volume = 80 | issue = 49 | pages = 6 | url = http://pubs.acs.org/cen/topstory/8049/8049notw1.html}}