Ditungsten tetra(hpp)
Tetrakisditungsten, known as ditungsten tetra, is the name of the coordination compound with the formula W24. 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.
The molecule is of research interest because it has the lowest ionization energy 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 located at the extreme left lower corner of the periodic table 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-pyrimidopyrimidine in o-dichlorobenzene at 200 °C:The reaction gives W24Cl2. Dichlorobenzene provides the chlorine atoms and is itself reductively coupled to 2,2′-dichlorobiphenyl. The bond order between the tungsten centers in W24Cl2 is three.
This dichloride is further reduced by potassium metal to W24. This species has a quadruple bond between the two tungsten centers. Related quadruply bonded complexes include 4− and Potassium octachlorodimolybdate|4−. Because of its low ionization energy, W24 is easily oxidized back to the dichloride by dichloromethane. It is readily oxidized to the corresponding cation with the oxidants fullerene and with tetracyanoquinodimethane.