Di-tungsten tetra(hpp) or W2(hpp)4 is a di-tungstenpaddlewheel compound with four hpp or hexahydropyrimidopyrimidineligands. This compound has the lowest ionization energy (3.51 eV) of all stable chemical elements or chemical compounds measured to date.[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 metallocenereducing agents such as the cobaltocene di(tetramethylcyclopentadienyl)cobalt with 4.71 eV.
Ball-and-stick model of the ditungsten tetra(hpp) molecule, with hydrogen atoms omitted for clarity
This organometallic compound is prepared by conversion of tungsten hexacarbonyl with 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine (Hhpp) in o-dichlorobenzene at 200 °C to W2(hpp)4Cl2. Dichlorobenzene provides the chlorine atoms and is itself reduced to dichlorobiphenyl. The tungsten to tungsten bond in this intermediate has an order of three. The final complex has bond order of 4, a quadruple bond, so that every tungsten atom obeys the 18 electron rule (8 (2 e− x 4 from shared quadruple bond)+ 16 electrons donated (8 nitrogens x 2 e−) and 12 electrons (6 e− x 2 W atoms) = 36. 36/2 = 18 per W) and has a closed shell configuration. The dichloride is stable and easy to handle. It can be further reduced by potassium metal to W2(hpp)4. Because of its low ionization energy it can easily oxidized back to the dichloride by compounds such as dichloromethane. It also reacts readily with fullerene and with tetracyanoquinodimethane.
References
^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.
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