Chapter 10 Quest for an Aromatic
Silicon
Species
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An Unusual Geometry around Silicon in a 10-Si-5 Siliconate 1
2
Suman K. Chopra, Chester D. Moon , and J. C. Martin Department of Chemistry, Vanderbilt University, Nashville, TN 37235
Several hypervalent 10-Si-5 compounds containing a tridentate ligand system suitable for stabilizing three-center four-electron (3c-4e) hypervalent bonds have been synthesized. Normally 10 electron hypervalent main group species that havefiveligands attached to the central atom exhibit a trigonal bipyramidal (TBP) geometry about the central atom. However, in our attempts to synthesize a hypervalent silicon species in which the silicon atom is contained within an aromaticring,we obtained a compound whose X-ray crystal structure revealed a rectangular pyramidal geometry about silicon. The geometry of this unique structure is explained in terms of stabilizing factors associated with hypervalent bonding. For many years chemists used Lewis' octet rule to explain the stability of molecules. However, this rule did not explain the stability of compounds such as PCI5 and SF4, both of which contain a central atom with 10 electrons formally associated with it. Throughout the years chemists explained the stability of such electronrichspecies by invoking the use of J-orbitals to accommodate the expansion of the valence octet resulting in sp d hybridization (7). However, subsequent studies (2-4) of such electronrichspecies showed inconsistencies in the electron distributions expected in sp d hybridization. In 1969, Musher (5) termed such electron rich species as hypervalent. Musher suggested that the three-center four-electron (3c-4e) bond could be viewed as a linear combination of an atomic /7-orbitalfromeach of the three atoms involved in the hypervalent bond. The combination of these orbitals leads to three molecular orbitals that approximate hypervalent bonding. The orbital diagram indicates two of 3
3
1
Current address: Colgate Palmolive Company, 909 River Road, Piscataway, NJ 08855 Current address: Manufacturing Technology Laboratory, Reynolds Metals Company, 3326 East 2nd Street, Muscle Shoals, AL 35661
2
0097-6156/94/0555-0167$08.00/0 © 1994 American Chemical Society In Inorganic Fluorine Chemistry; Thrasher, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
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INORGANIC FLUORINE CHEMISTRY: TOWARD THE 21ST CENTURY
the four electrons in the hypervalent bond reside in a nonbonding orbital. Such an arrangement places most of the electron density on the terminal sites of the 3c-4e bond resulting in a charge separation with partial negative charge at the terminal atoms and partial positive charge at the central atom.
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^•O^^CD^^CD
^ — anti-bonding
^^CD
H
non-bonding
# O C ^ ^ O
H
bonding
Orbital
Diagram
for
Hypervalent
Bonding
Calculations performed on the hypervalent 10-F-2 (6) trifluoride anion (7) are consistent with the bonding diagram. This hypervalent species consists of a 3c-4e bond that has a partial negative charge of -0.515 at each terminal fluorine and a partial positive charge of 0.030 at the central fluorine. This charge separation indicates that the formal octet expansion is distributed over the three atoms of the hypervalent bond.
\fi—f—È] -0.515 +0.030 -0.515
Calculated Charges for the 10-F-2 Trifluoride Anion
For the past several years, our research efforts have focused on the syntheses of stable hypervalent compounds. Our success has stemmed from the design of various ligand systems that stabilize the charge separation of the hypervalent bond (810). One such ligand system is compound 1. This compound consists of 4-tertbutylpyridine substituted with the potassium salt of hexafluoroisopropanol at the 2 and 6 position (11). Treatment of 1 with tetrachlorosilane produces dichlorosiliconate 2. Compound 2 contains the hypervalent O-Si-O bond. The tridentate ligand system stabilizes this bond by placing two electronegative CF3 groups adjacent to the terminal oxygen atoms. These electronegative groups serve to stabilize the partial negative charge on the oxygen atoms. The nitrogen center in 2 also serves to charge compensate the structure so that it is overall neutral. The corresponding siliconates 3 and 4 are formed by the reactions of 1 with dichlorodimethylsilane and methyltrichlorosilane, respectively, as shown in Scheme 1. In terms of preparing an aromatic silicon species, the strategy was to prepare further siliconates where the silicon atom would actually be part of a 9-silaanthracene. If the methylene carbon (10-position) of the silaanthracene was then made cationic by formal removal of a proton, the silicon atom might end up in a cyclic array of
In Inorganic Fluorine Chemistry; Thrasher, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on September 10, 2015 | http://pubs.acs.org Publication Date: April 29, 1994 | doi: 10.1021/bk-1994-0555.ch010
10.
CHOPRA ET AL.
Quest for an Aromatic Silicon Species
169
Scheme 1
p-orbitals containing six electrons. The orbital diagram below shows such a cyclic array of orbitals with four electrons coming from the anthracene ring system and two electrons comingfromthe bonding molecular orbital of the hypervalent bond centered on silicon, cf. bis(ipso)aromaticity proposed for the 4n + 2π system containing a pentavalent trigonal bipyramidal (TBP) carbon atom (12). It was not known a priori whether the energy match between the bonding molecular orbital of the hypervalent bond and the pi framework of the anthraceneringwould be close enough. Therefore, it was also deemed important to attempt the synthesis of the corresponding anionic derivative in case the energy match should be better with the antibonding molecular orbital of the hypervalent bond. All six electrons for aromaticity would then come from the /?-orbitals on the carbon atoms.
In Inorganic Fluorine Chemistry; Thrasher, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
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INORGANIC FLUORINE CHEMISTRY: TOWARD THE 21ST CENTURY
Experimental Section 4-^Butyl)-a,a,aSa'-tetrakis(trifluoromethyl)-2,6-pyridinedimethanolato-(2-)Ni,Oa,Oa -dichloro Siliconate, 2. Tetrachlorosilane (0.25 mL, 2.2 mmol) was added to the dipotassium salt 1 (1.00 g, 1.84 mmol) in THF (15 mL) at -40 °C. The reaction mixture was stirred for 15 - 20 min. The solvent was then removed, and the KC1 was filtered by dissolving the reaction mixture in Et 0. Compound 2 was then recrystallized from Et 0/hexane (0.68 g, 1.2 mmol, 65%); F NMR (THF-
