Ditungsten tetra(hpp)

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Ditungsten tetra(hpp)

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Tetrakis(hexahydropyrimidinopyrimidine)ditungsten(II), known as ditungsten 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) ligands. It adopts a structure sometimes called a Chinese lantern structure or paddlewheel compound, the prototype being copper(II) acetate.

File:Hpp ligand.svg

The hpp ligand anion

The molecule is of research interest because it has the lowest ionization energy (3.51 eV) of all stable chemical elements or chemical compounds as of the year 2005. 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 agents such as decamethylcobaltocene with 4.71 eV.

Preparation

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:

File:Ditungstentetrahpp.png

Synthesis of ditungsten tetra(hpp)

The reaction gives W₂(hpp)₄Cl₂. Dichlorobenzene provides the chlorine atoms and is itself reductively coupled to 2,2′-dichlorobiphenyl. 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 [W₂Cl₈]⁴⁻ and [Mo₂Cl₈]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.