Compounds of transition metals are often used to activate small molecules for chemical reactions. The discovery of unusual silicon-containing compounds raises the prospect of metal-free activators.
Any chemist will tell you that silicon atoms are tetravalent — that is, they have four electrons available for chemical bonding, and therefore usually form four single bonds to other atoms. But two papers1, 2 published in the Journal of the American Chemical Society report some remarkable silicon-containing compounds known as silylenes, in which the silicon atom forms only two single bonds and so is said to be divalent. They are the first stable silylenes that are acyclic (the silicon does not form part of a ring of atoms), and therefore open up a new chapter in the chemistry of divalent silicon.
Among the elements in group 14 of the periodic table (from carbon to lead, all of which have four electrons available for binding), carbon and silicon are the most reluctant to form divalent compounds. The first stable divalent carbon compounds, known as carbenes, were discovered3 in 1991, and the synthesis of stable divalent silylenes followed a few years later4, 5, 6. These first, highly reactive silylenes are now regarded as classic compounds, and have become the starting materials for an elaborate branch of silicon chemistry7, 8, 9.
The classic silylenes are cyclic molecules in which two nitrogen atoms are attached to the silicon. These nitrogens are believed to stabilize the divalent silicon atom, in part by donating electrons to a vacant orbital on the silicon. The cyclic structure also seems to give the molecules stability. Acyclic silylenes that have two nitrogen atoms bonded to divalent silicon have been detected, but these compounds are not stable at room temperature10, 11. The synthesis of stable acyclic silylenes is therefore a crucial advance because it greatly expands the scope of divalent silicon chemistry.
The compound now reported by Rekken et al.1 has a remarkably simple structure consisting of a silicon atom flanked by two identical, bulky arylsulphur groups (aromatic rings connected to the silicon atom by sulphur atoms; Fig. 1a). The authors prepared it from a starting material that was structurally identical to the silylene product, but in which two bromine atoms were also attached to the silicon atom. They removed the two bromine atoms using a magnesium-containing reducing agent.
Figure 1: Acyclic silylenes.
Two teams report the synthesis of the first thermally stable compounds in which a divalent silicon atom — one that forms two single bonds — does not form part of a ring of atoms. a, In Rekken and colleagues' silylene1, a silicon atom is flanked by two sulphur atoms. The dots on the silicon atom represent a lone pair of electrons, and the bond crossed by a wavy line in the Ar group indicates the point of attachment of the group to the silylene. Me, methyl group. b, In Protchenko and colleagues' silylene2, the silicon atom is between a nitrogen and a boron atom. Divalent silicon atoms in a and b are shown in red.
