Volume 87 Issue 8 | p. 49 | Concentrates
Issue Date: February 23, 2009

Nitrogens Commandeer Silicon From Fluorine

In a surprise move, a cyclic triamine displaces a superstrong Si–F bond from SiF4 to form an unprecedented SiF3 cation
Department: Science & Technology
The cyclic Me3tacn ligand bumps a fluorine atom from SiF4 to form the stable SiF3 cation shown; Si is red, F is green, nitrogen is blue, and carbon is white.
Credit: Chem. Commun.
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The cyclic Me3tacn ligand bumps a fluorine atom from SiF4 to form the stable SiF3 cation shown; Si is red, F is green, nitrogen is blue, and carbon is white.
Credit: Chem. Commun.

When silicon and fluorine get together, they form what is likely the strongest single bond in chemistry: Si–F has a whopping bond energy of 582 kJ per mol. But now Wenjian Zhang, William Levason, and colleagues at the University of Southampton, in England, have shown that even the mighty Si–F bond can be outmuscled under mild conditions (Chem. Commun., DOI: 10.1039/b822236c). Levason's group made the discovery while investigating new methods for the electrodeposition of semiconductor materials such as silicon onto substrates from precursor compounds dissolved in supercritical fluids. The researchers reacted SiF4 gas with the tridentate cyclic amine ligand N,N',N"-trimethyl-1,4,7-triazacyclononane (Me3tacn) in toluene and were surprised to observe precipitation of the highly stable salt [SiF3(Me3tacn)][SiF5]. Nitrogen doesn't have as high an affinity for silicon atoms as fluorine does, but the unique orientation of Me3tacn's three nitrogen atoms allows them to work together to overwhelm one of SiF4's fluorine atoms and latch on to the silicon. Because of Me3tacn's high cost, [SiF3(Me3tacn)]+ doesn't lend itself to large-scale applications at present, Levason says, but it will be an academic curiosity for its ability to easily displace a Si–F bond.

 
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