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 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.
