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Physical Chemistry

Elusive Silicon Oxides Unveiled

Main-Group Elements: Chemists prepare the first examples of simple monomeric silicon oxides

by Stephen K. Ritter
April 23, 2015 | A version of this story appeared in Volume 93, Issue 17

Carbene-stabilized disilicon oxidation has led to the first two examples of molecular silicon oxides; R = diisopropylphenyl.
A reaction scheme showing the preparation of molecular silicon oxides.
Carbene-stabilized disilicon oxidation has led to the first two examples of molecular silicon oxides; R = diisopropylphenyl.

Using a new approach to marry silicon and oxygen, inorganic chemists have created the first monomeric silicon oxides: complexes of Si2O3 and Si2O4. This fundamental discovery, which might one day factor into how silicon-based electronics are made, is another feather in the caps of Gregory H. Robinson, Yuzhong Wang, Henry F. Schaefer III, and their colleagues at the University of Georgia.

Although carbon and silicon sit together in group 14 of the periodic table, they display disparate behavior when it comes to forming oxides. Carbon monoxide and carbon dioxide—monomeric carbon oxides—are ubiquitous under normal room conditions. But simple silicon oxides don’t exist. Silicon oxide is a monomeric compound only at high temperature. And although silicon dioxide is one of the most abundant materials on Earth—as silica, quartz, and sand—it’s a covalent network of silicon atoms bound to neighboring oxygen atoms, rather than discrete molecules.

Robinson’s group previously synthesized an unprecedented silicon(0) compound containing a silicon-silicon double bond, with the silicon atoms stabilized by bulky N-heterocyclic carbene (NHC) ligands. Building on that development, the Georgia team made the Si2O3 and Si2O4 complexes (shown) by oxidizing the disilicon complex with N2O or with O2 (Nat. Chem. 2015, DOI: 10.1038/nchem.2234).

The researchers were motivated to try the reactions after using the NHC-stabilization approach a year ago to prepare diphosphorus tetroxide, P2O4, a long-sought phosphorus analog of N2O4, by adding O2 across a diphosphorus bond (C&EN, Jan. 6, 2014, page 21). Altogether, their achievements help resolve some of the final missing pieces of main-group oxide chemistry.

“Isolating the first monomeric silicon oxides is a spectacular breakthrough, both because of its fundamental importance and scientific beauty,” says Yitzhak Apeloig, a silicon chemistry expert at Technion—Israel Institute of Technology. “Obtaining Si2O3 and Si2O4 at ambient conditions opens up the opportunity to study in detail the chemistry of silicon oxides, which may help to understand the oxidation and doping of silicon surfaces, an important process in the microelectronics industry.”



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