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.
In the second paper2, Protchenko and colleagues' silylene (Fig. 1b) has a more complex structure than Rekken and colleagues' compound (for example, it is not symmetrical, and the silicon atom sits between a nitrogen atom and a boron atom), and it required a more complicated synthesis. Protchenko et al. also started from a brominated material, but they reacted it with a boron-containing reagent12 that not only reduces the silicon–bromine bonds in the starting material, but also attaches itself to the silicon atom to yield the final product.
Both Rekken et al. and Protchenko and colleagues obtained X-ray crystal structures of their silylenes to provide conclusive proof of what they had made, and also to learn more about the atomic bonding in the compounds — which in turn can help to explain the compounds' reactivities and stabilities. Rekken et al. found that the sulphur–silicon–sulphur bond angle in their silylene is 90.52°. The angle at the silicon atom in Protchenko and colleagues' compound is somewhat larger (109.7°), perhaps implying that their silylene is more reactive than Rekken and colleagues' compound.
Both silylenes exhibit striking thermal stability for divalent silicon compounds: Rekken and colleagues' compound is stable up to 146 °C, whereas Protchenko and colleagues' compound is stable up to 130 °C. The stability of the new silylenes must be due largely to steric protection of the reactive silicon atoms by the attached groups (in other words, the bulky groups shield the silicon atoms from attack by other molecules). The bulky groups might also stabilize the compounds by donating electrons to the silicon atom.
Further insight into the chemical bonding in the two silylenes was obtained by looking at the signals (the chemical shifts) of silicon-29 nuclei in the nuclear magnetic resonance spectra for the molecules. In both cases, the chemical shifts have strongly positive values (285.5 parts per million for the silicon in Rekken and colleagues' silylene, and 439.7 p.p.m. for that in Protchenko and colleagues' compound). These values fall between the corresponding chemical shift (78 p.p.m.) measured in a classic silylene4 that is stabilized by electron donation from nitrogen atoms on either side of the silicon, and that of another previously reported silylene13 that is stabilized only by steric hindrance (567 p.p.m.). The chemical shifts for the new silylenes provide further clear evidence that the compounds are indeed divalent silicon compounds. Moreover, the shifts suggest that Rekken and colleagues' silylene is partially stabilized by electron donation from the sulphur atoms adjacent to the silicon atom, and that Protchenko and colleagues' silylene may also be partially stabilized by electron donation from the atoms on either side of the silicon, but to a lesser extent.
The interest in silylenes lies in their reactivity — particularly the possibility that they can 'activate' small molecules, such as hydrogen, to allow such molecules to take part in potentially useful chemical reactions. Rekken et al. report that their silylene does not react with hydrogen, although it does combine with another small molecule (iodomethane; CH3I) in a typical silylene reaction. Protchenko et al., however, find that their compound captures hydrogen readily to yield a dihydride product that has two silicon–hydrogen bonds. This is the first example of the reaction of hydrogen with a silylene. What's more, the reaction is a remarkable single-site activation process — one in which both hydrogen atoms become attached to the same atom. Most other activation processes involve two sites.
Further study of the chemical reactions of these acyclic silylenes will no doubt lead to the synthesis of a variety of interesting chemical compounds. And now that Rekken et al. and Protchenko et al. have shown the way, we should see a growing family of acyclic, divalent silylenes with as yet unknown structures and properties.
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