Ball-and-stick model of the ditungsten tetra(hpp) molecule, with hydrogen atoms omitted for clarity

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) ligands. It adopts a structure sometimes called a "paddlewheel" or "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 eV) of all stable chemical elements or chemical compounds as of the year 2005.^[1] 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.

the hpp ligand anion

[edit] 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:

The reaction gives W₂(hpp)₄Cl₂. Dichlorobenzene provides the chlorine atoms and is itself reductively coupled to dichlorobiphenyl. The bond order between the tungsten centers in W₂(hpp)₄Cl₂ is three, as indicated by the formal electron configuration of (d³)₂.

This dichloride is further reduced by potassium metal to W₂(hpp)₄. This species has a quadruple bond between the two tungsten centers, indicated by the configuration (d⁴)₂. Related (d⁴)₂ 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. In W₂(hpp)₄, each tungsten center follows the 18 electron rule, thus has a closed shell configuration. Although W₂(hpp)₄ was proposed to be a potentially useful powerful reducing agent akin to Cp^*₂Co and Cp^*₂Cr,^[1] so far this reagent has not become part of the synthetic chemist's armament.

[edit] References

1. ^ ^{a b} F. Albert Cotton, James P. Donahue, Dennis L. Lichtenberger, Carlos A. Murillo, and Dino Villagrán (2005). "Expeditious Access to the Most Easily Ionized Closed-Shell Molecule, W₂(hpp)₄". J. Am. Chem. Soc. 127 (31): 10808–10809. doi:10.1021/ja0535458. 

[edit] Further reading

• Ron Dagani (2002-12-09). "Electron Donor Par Excellence". Chemical & Engineering News 80 (49): 6. http://pubs.acs.org/cen/topstory/8049/8049notw1.html.