Issue Date: August 25, 2008
A STABLE COMPOUND that contains a silicon-silicon double bond and at the same time has a lone pair of electrons residing on each silicon atom has been prepared by a team of chemists at the University of Georgia (Science 2008, 321, 1069).
These attributes are usually equated with extreme instability in a molecule. But Yuzhong Wang, Gregory H. Robinson, and coworkers tamed the L:Si=Si:L species by using an N-heterocyclic carbene as a bulky ligand (L:).
The silicon atoms are in the formal oxidation state of zero in the molecule, which is exceedingly rare for a compound incorporating a main-group element such as silicon. The zero oxidation state is a phenomenon usually reserved for naturally occurring elemental allotropes (such as diamond), pure metals, and metals in certain types of transition-metal complexes.
Inorganic chemists who have learned about L:Si=Si:L say that the Si2 unit can be thought of as an allotrope of silicon, and as such it could broadly expand the boundaries of silicon chemistry.
To make the diatomic Si(0) species, the Georgia researchers first coordinated an N-heterocyclic carbene to silicon tetrachloride to form the Si(IV) compound L:SiCl4. The chemists treated this compound with a strong reducing agent (potassium graphite, KC8) in tetrahydrofuran at room temperature to make L:Si=Si:L, which they isolated in crystalline form. Previously, the naked Si2 molecule without ligands could only be examined spectroscopically in the gas phase or in an argon matrix at very low temperature.
The carbene-stabilized Si2 molecule "represents a major advance in low-valent, low-coordinate main-group chemistry," comments inorganic chemist Alan H. Cowley of the University of Texas, Austin. Organosilicon specialist Akira Sekiguchi of the University of Tsukuba, in Japan, agrees, adding that "the impressive finding opens up new, unprecedented possibilities in organometallic chemistry."
The experimental results on L:Si=Si:L are supported by computational studies carried out by Robinson's chemistry department colleagues. Among the key findings is that the carbene ligands don't withdraw electron density from the silicon atoms, but rather they donate electrons to help stabilize the nonbonding electron pair on each silicon atom. Similar behavior is normally found in transition metals when ligands donate electron pairs into the metal's empty orbitals. In effect, silicon in L:Si=Si:L is behaving like a transition metal.
As a result, L:Si=Si:L and other potential molecules like it "are not just academic curiosities," explain C. Adam Dyker and Guy Bertrand of the University of California, Riverside, in a commentary that accompanies the Georgia team's Science paper. The carbene-stabilized Si(0) compounds should exhibit much greater solubility than the standard silicon allotropes, Dyker and Bertrand note. This property might permit the silicon compound to undergo chemical transformations not otherwise possible, similar to the way soluble metal(0) complexes can mediate homogeneous rather than heterogeneous catalysis.
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