Proc. NatI. Acad. Sci. USA Vol. 74, No. 12, pp. 5217-5221, December 1977 Chemistry 13C nuclear magnetic resonance study of five- and six-coordinated carbon in nonclassical organometallic compounds: Dimeric trialkyl-, tricyclopropyl-, and triarylaluminums and some nido and closo carboranes* (bridging carbons/two-electron three-centered bonding/13C-IH spin-spin coupling constants) GEORGE A. OLAHtl, G. K. SURYA PRAKASHt, GAO LIANGt, KENNETH L. HENOLD§, AND GARY B. HAIGH§ t Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106; and § Department of Chemistry, University of Detroit, Detroit, Michigan 48221 Contributed by George A. Olah, August 29,1977 ABSTRACT A 13C nuclear magnetic resonance spectro- scopic study of dimeric trimethyl-, triethyl-, tricyclopropyl-, and triarylaluminums is reported. The five-coordinated bridging carbons are found consistently more shielded than the terminal carbons, in accordance with the increased p-character of the former. The nature of bridging two-electron three-centered Al-C-Al bonds is discussed. 13C nuclear magnetic resonance shifts of several nido and closo carboranes containing five and six coordinated carbons and their 13C-IH spin-spin coupling constants were also obtained. The relationship between the carbon chemical shifts and coordination number of the carbon atom is discussed. There is approximately a 20- to 40-ppm shielding of the 13C chemical shifts of five- and six-coordinated carbons, compared with those of four valent carbons, with a simultaneous general increase of JC.H coupling constants. In many organometallic compounds, metal to carbon bonding cannot be explained on the basis of simple Lewis type covalent two-electron two-center bonding or ionic bonding (2-6). These molecules necessitate the involvement of two-electron three- center (6-9) or multicenter bonding and thus can be called "nonclassical" molecules (in accord with the definition of nonclassical carbocations used in organic chemistry) (2, 10- 13). The organoaluminum compounds are unique in forming stable dimers in hydrocarbon solvents such as benzene and toluene, but they dissociate into monomers in the gas phase (14-17). They gained substantial significance in the catalyst systems of "Ziegler-Natta" coordination polymerizations. Their structural aspects were also extensively studied. In carboranes the boron framework also contains carbon atoms, the bonding nature of which generally is five- and six- coordinated. In addition to extensive studies of tetravalent carbon compounds, in recent years nonclassical carbocations have been studied as stable species containing five- and six- coordinated carbon atoms (18-22). 13C nuclear magnetic res- onance (NMR) spectroscopy showed these carbon atoms to be highly shielded with respect to those of trivalent carbenium carbons. Applications of 13C NMR spectroscopy in the area of electron-deficient, neutral organometallics in determining the nature of highly coordinated carbon atoms, and the effect of hybridization on carbon shifts are, however, still scarce (23), although proton-NMR spectroscopic investigations of these molecules have been carried out (24-32 ). Interested primarily in the study of five- and higher-coordinated carbon, we wish to report our investigation on several dimeric trialkyl- and tri- arylaluminums. For comparison, '3C NMR studies of some The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S. C. §1734 solely to indicate this fact. 5217 carboranes were also carried out, emphasizing the nature of bridging five- and six-coordinated carbon atoms. RESULTS AND DISCUSSION Trialkylaluminum dimers The electronic structure of trialkylaluminums is of great interest because they are dimeric at lower temperatures, held together by two Al-C-Al two-electron three-centered bonds (2-5, 16, 17). Their dimeric nature has been unequivocally established by means of x-ray, electron diffraction, Raman and infrared, nuclear quadrupole resonance, and proton NMR spectroscopy (14, 24-26). The 13C NMR spectral parameters of trimethyl-, triethyl-, and tricyclopropylaluminums (Me3AI, Et3Al, and cPr3Al in toluene at various temperatures are summarized in Table 1. R R .-' R -R -CH, / 2 R= -CH.CH, Al/ - 3. R RR R At +30°, Me3AI (1) displayed one carbon resonance at 6 '3C -7.31 (quartet, IC-H 114.6 Hz). As the temperature was lowered to -75°, two well-resolved carbon signals were observed in 2:1 ratio centered at 6 1SC -8.22 (quartet, JIH 112.7 Hz) and -5.63 (quartet, IGH 115.3 Hz), respectively. The higher intensity peak is assigned to the terminal methyl carbons; the less intense one is assigned to the bridging methyl carbons. The 13C NMR data are consistent with the reported values (31, 32). In tetrahy- drofuran solutions, Me3A1 showed only a single resonance even at very low temperatures, indicating the presence of a mono- meric complex. The difference in carbon shifts between the bridging and terminal carbon atoms of the dimer of Me3AI at -70° was 2.6 ppm. A decrease in coupling constant of 2.6 Hz was observed for the bridging methyl group over terminal one. The difference in chemical shifts of the terminal methylene over the bridging one in Et3AI (2) at -80° was 0.61 ppm (in toluene on a 270-MHz instrument). There was a decrease in the coupling constant of the bridging methylene groups over the terminal ones (7.1 Hz). The difference in the chemical shifts Abbreviations: NMR, nuclear magnetic resonance; Me3Al, trimeth- ylaluminum; Et3AI, triethylaluminum; cPr3Al, tricyclopropylalumi- num; Phe3Al, triphenylaluminum; p-Tol3AI, tri-p-tolylaluminum. * This is paper no. 16 in the series "Organometallic Compounds." Paper no. 15 is ref. 1. t Present address: Institute of Hydrocarbon Chemistry, Department of Chemistry, University of Southern California, Los Angeles, CA 90007. Downloaded by guest on May 20, 2021
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Proc. NatI. Acad. Sci. USAVol. 74, No. 12, pp. 5217-5221, December 1977Chemistry
13C nuclear magnetic resonance study of five- and six-coordinatedcarbon in nonclassical organometallic compounds: Dimerictrialkyl-, tricyclopropyl-, and triarylaluminums and somenido and closo carboranes*
GEORGE A. OLAHtl, G. K. SURYA PRAKASHt, GAO LIANGt, KENNETH L. HENOLD§, AND GARY B. HAIGH§t Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106; and § Department of Chemistry, University of Detroit, Detroit,Michigan 48221
Contributed by George A. Olah, August 29,1977
ABSTRACT A 13C nuclear magnetic resonance spectro-scopic study of dimeric trimethyl-, triethyl-, tricyclopropyl-, andtriarylaluminums is reported. The five-coordinated bridgingcarbons are found consistently more shielded than the terminalcarbons, in accordance with the increased p-character of theformer. The nature of bridging two-electron three-centeredAl-C-Al bonds is discussed. 13C nuclear magnetic resonanceshifts of several nido and closo carboranes containing five andsix coordinated carbons and their 13C-IH spin-spin couplingconstants were also obtained. The relationship between thecarbon chemical shifts and coordination number of the carbonatom is discussed. There is approximately a 20- to 40-ppmshielding of the 13C chemical shifts of five- and six-coordinatedcarbons, compared with those of four valent carbons, with asimultaneous general increase of JC.H coupling constants.
In many organometallic compounds, metal to carbon bondingcannot be explained on the basis of simple Lewis type covalenttwo-electron two-center bonding or ionic bonding (2-6). Thesemolecules necessitate the involvement of two-electron three-center (6-9) or multicenter bonding and thus can be called"nonclassical" molecules (in accord with the definition ofnonclassical carbocations used in organic chemistry) (2, 10-13).The organoaluminum compounds are unique in forming
stable dimers in hydrocarbon solvents such as benzene andtoluene, but they dissociate into monomers in the gas phase(14-17). They gained substantial significance in the catalystsystems of "Ziegler-Natta" coordination polymerizations. Theirstructural aspects were also extensively studied.
In carboranes the boron framework also contains carbonatoms, the bonding nature of which generally is five- and six-coordinated. In addition to extensive studies of tetravalentcarbon compounds, in recent years nonclassical carbocationshave been studied as stable species containing five- and six-coordinated carbon atoms (18-22). 13C nuclear magnetic res-onance (NMR) spectroscopy showed these carbon atoms to behighly shielded with respect to those of trivalent carbeniumcarbons. Applications of 13C NMR spectroscopy in the area ofelectron-deficient, neutral organometallics in determining thenature of highly coordinated carbon atoms, and the effect ofhybridization on carbon shifts are, however, still scarce (23),although proton-NMR spectroscopic investigations of thesemolecules have been carried out (24-32 ). Interested primarilyin the study of five- and higher-coordinated carbon, we wishto report our investigation on several dimeric trialkyl- and tri-arylaluminums. For comparison, '3C NMR studies of some
The costs of publication of this article were defrayed in part by thepayment of page charges. This article must therefore be hereby marked"advertisement" in accordance with 18 U. S. C. §1734 solely to indicatethis fact.
5217
carboranes were also carried out, emphasizing the nature ofbridging five- and six-coordinated carbon atoms.
RESULTS AND DISCUSSIONTrialkylaluminum dimersThe electronic structure of trialkylaluminums is of great interestbecause they are dimeric at lower temperatures, held togetherby two Al-C-Al two-electron three-centered bonds (2-5, 16,17). Their dimeric nature has been unequivocally establishedby means of x-ray, electron diffraction, Raman and infrared,nuclear quadrupole resonance, and proton NMR spectroscopy(14, 24-26).The 13C NMR spectral parameters of trimethyl-, triethyl-,
and tricyclopropylaluminums (Me3AI, Et3Al, and cPr3Al intoluene at various temperatures are summarized in Table 1.
R
R .-' R -R -CH,/ 2 R= -CH.CH,Al/ - 3. RRR
R
At +30°, Me3AI (1) displayed one carbon resonance at 6 '3C-7.31 (quartet, IC-H 114.6 Hz). As the temperature was loweredto -75°, two well-resolved carbon signals were observed in 2:1ratio centered at 6 1SC -8.22 (quartet, JIH 112.7 Hz) and -5.63(quartet, IGH 115.3 Hz), respectively. The higher intensity peakis assigned to the terminal methyl carbons; the less intense oneis assigned to the bridging methyl carbons. The 13C NMR dataare consistent with the reported values (31, 32). In tetrahy-drofuran solutions, Me3A1 showed only a single resonance evenat very low temperatures, indicating the presence of a mono-meric complex. The difference in carbon shifts between thebridging and terminal carbon atoms of the dimer of Me3AI at-70° was 2.6 ppm. A decrease in coupling constant of 2.6 Hzwas observed for the bridging methyl group over terminal one.The difference in chemical shifts of the terminal methyleneover the bridging one in Et3AI (2) at -80° was 0.61 ppm (intoluene on a 270-MHz instrument). There was a decrease in thecoupling constant of the bridging methylene groups over theterminal ones (7.1 Hz). The difference in the chemical shifts
Abbreviations: NMR, nuclear magnetic resonance; Me3Al, trimeth-ylaluminum; Et3AI, triethylaluminum; cPr3Al, tricyclopropylalumi-num; Phe3Al, triphenylaluminum; p-Tol3AI, tri-p-tolylaluminum.* This is paper no. 16 in the series "Organometallic Compounds." Paperno. 15 is ref. 1.
t Present address: Institute of Hydrocarbon Chemistry, Departmentof Chemistry, University of Southern California, Los Angeles, CA90007.
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Table 1. 13C NMR parameters of trialkylaluminum dimers (AlR3)2*
* Shifts (ppm) from external capillary Me4Si. q, quartet; t, triplet; d, doublet; THF, tetrahydrofuran; in parentheses, JC-H in Hz.t Spectrum at -80° in toluene was taken on a 270-MHz Bruker NMR spectrometer.
of methyl carbons was 1.28 ppm. It is interesting to note thaton a 100-MHz instrument the methylene carbon signals of (2)at -80° in toluene was not resolved but was well-resolved inn-heptane. As indicated by Huffmann and Streib (33), the fourbonds of the bridging carbon atom are tetrahedral with respectto a vector directed toward the center of the dimer. Thebridging carbon atoms in the Me3AI or Et3Al dimers, in spiteof joining simultaneously three (or two) hydrogens and two Alatoms, do not show a significant change in chemical shiftscompared with the tetrahedral tetravalent terminal methyl orethyl carbon atoms.
X-ray structural analyses of cPr3Al (3) shows it also to be di-meric (28,32). The 100-MHz proton NMR spectrum of cPr3Alat room temperature has been interpreted as having two kindsof cyclopropyl groups in a relative ratio of 2:1 which correspondto the cyclopropyl groups in the terminal and bridging positions,respectively (28). cPr3Al has further unique bonding charac-teristics wherein there are additional wr-type interactions be-tween the p-orbital of the cyclopropyl group (sp2 like hybrid-ization) and the three-centered nonbonding MO. The protondecoupled 13C NMR spectrum of cPr3Al at -100 displayed fourcarbon resonances, and its proton-noise coupled spectrumconsisted of two triplets at 6 13C 12.1 and 1.42 (in a ratio of 1:2)and two doublets at S 13C -11.0 and -15.8. The lower fieldtriplet is assigned to the methylene carbons of the bridgingcyclopropyl ring (CH2g) and the higher field one to the terminalcyclopropyl ring (CH2T). The two doublets upfield from theMe4Si signal are assigned at -10° to terminal and bridgedmethine, respectively. As the temperature was raised to +700,all four carbon resonances became broad and at +900 theystarted to collapse. This indicates fast exchange between thebridged and terminal cyclopropane rings. Interestingly, whenthe solution of cPr3Al was cooled slowly below -100, the upfieldtriplet and the lower field doublet became broader while thelower field triplet and the upper field doublet remained sharp.At -600, the spectrum of cPr3Al consisted of six carbon reso-nances wherein the upper field methylene triplet and the lowerfield methine doublet split into two sets of triplets and doublets,respectively. The temperature-dependent behavior of cPr3Alin toluene apparently indicates that the bridging cyclopropanerings undergo fast rotation at all temperatures, whereas rotationof the terminal ones becomes slow at lower temperatures andis frozen out at -600.From a 1H NMR study on cPr3AI, Oliver et al. (28) came to
slow hindered rotation of the bridged cyclopropyl groups givingrise to syn and anti conformations of the bridged groups. Our13C NMR data on cPr3Al rule out such a possibility because thebridged cyclopropyl carbon signals remained sharp with un-altered chemical shifts between -100 and -600.
The nonequivalency of the methylene and methine carbonsamong two sets of terminal cyclopropane rings of dimericcPr3AI indicates its unsymmetrical nature in toluene at -60.Obviously, the NMR spectral data rule out any symmetricalstructure, such as 4 or 5, because the two bridging cyclopropylgroups are freely rotating even at -60°. The four terminalcyclopropyl groups are divided into two sets-i.e., two of themtrans to each other, bending inward, and the other two bendingoutward, accounting for two different kinds of methylene andmethine carbons.
>H ^ H
,q~frverotation tfree rotation
H H
free Hfree0 rotation (rotation tH "H -'H H
4 5
Triarylaluminum dimersTriphenylaluminum (Phe3Al) (6) (34-37) and tri-p-tolylalu-minum (p-Tol3Al) (7) (38, 39) both are dimeric with the phenyland p-tolyl groups, respectively, bridging the two electron-deficient Al atoms via two-electron three-center Al-C-Albonding.
R
R_,,,, R\xE\\@> 6, R - H
AlAl 7, R - CH:
R
Phenyl (aryl) bridging has also been reported in several otherclasses of electron-deficient molecules (40-43). The questionof phenyl bridging is of particular interest in phenylethyl cat-ions (40-46) which have been shown to be a-bonded ethyl-benzenium ions (8) [spiro (5.2) cyclooctadienyl cations] havinga tetravalent aliphatic sp3 spiro carbon atom (47). The phe-nylethyl cations in their planar configuration 9 could also beconsidered as a nonclassical ion having two-electron three-centered bonding between the aryl sp2 orbital and the ethylene7r-bond orbitals. However, 9 has not been found experimentallyand is considered much higher in energy than 8 (48-53). Otherarrangements such as 9a in which the bridging phenyl liesperpendicular to the plane of the ethylene moiety using its sp2orbital without significant charge delocalization as in 8, how-ever, have yet not been considered.
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1+
Sp3
spirocarbon planarcarbon
( sp2 carbon
8
At 370, Phe3Al in methylene chloride displayed only threesharp carbon resonances at 6 13C 128.5 (doublet, 2C), 132.0(doublet, IC), and 143.6 (doublet, 2C) which are assigned toortho, para, and meta carbons, respectively. The ipso carbonwas not observed because of rapid equilibration. As the tem-perature was lowered, all the peaks rapidly broadened, and at-920 eight sharp carbon resonances were observed at 6 13C121.3 (singlet, 2C), 127.3 (doublet, 4C), 128.1 (doublet, 4C),128.6 (doublet, 4C), 137.0 (doublet 8C), 138.2 (doublet, 2C),145.5 (singlet, 4C), and 155.1 (doublet, 4C). The 13C NMRspectrum of 7 at 37° displayed four carbon shifts at 6 13C 22.3(quartet, methyl), 129.1 (doublet, ortho carbons), 141.9 (singlet,para carbons), and 143.6 (doublet, meta carbons). The '3CNMR shifts for 7 at -70° and their assignments are shown in
Table 2.The observation of only three aromatic carbon resonances
for 6 and 7 at 370 and eight for 6 and 7 at -92° and -70°, re-
spectively, shows that both undergo rapid intramolecular ex-
change at the higher temperatures and are static bridged di-mers at lower temperatures. The most interesting carbon shiftsin the frozen-out dimeric forms are the two singlets at 6 13C121.3 and 145.5 for 6 and 6 13C 118.0 and 142.6 for 7 corre-
sponding to the bridging ipso and terminal bound ipso ringcarbon atoms, respectively. The bridging ipso carbons were 20ppm more shielded than the corresponding terminal ones. Thecarbon shifts in the range of 6 13C 120 for the bridging ipsocarbon show that these carbon atoms are essentially aromaticalthough not fully sp2 hydridized but certainly not sp3 spiro-aliphatic. Carbon shifts for the terminally bound ipso carbonatoms were in the range of 6 13C 140-15 (typically aromatic).However, the terminal ipso carbons are only bonded to a singleAl atom, whereas the bridging ones are bonded to two Al atomssimultaneously.The m-Tol3Al dimer 10 also shows a similar temperature-
dependent 13C NMR spectrum. At 370 in toluene, the rapidly
9 9a
exchanging dimer 10 gave rise to only five carbon reso-nances for the aromatic ring carbons and one for methyl carbon(Table2). At -70° the frozen-out spectrum of 10 was obtained.
CH, , CH, CH,
CH3 CH,CH3
10
CH,
Al
CE!, ~ CH,
11
However, there was only one methyl carbon absorption ob-served at 6 13C 21.7 as the methyl groups on both the terminaland bridging phenyl rings seem to have identical chemicalshifts. The bridging ipso carbon was shielded by 15 ppm ascompared to the terminal ones.
o-Tol3AI (11) in toluene showed a completely different be-havior. The 13C NMR spectrum even at -700 showed only sixcarbon resonances in the aromatic region and a methyl reso-nance. When the solution was warmed to 370, the ipso singletat 13C 143.9 and the methyl-substituted ortho singlet at 13C135.4 merged into the base line, indicating rapid exchangebetween the two sites. The remaining carbon resonances,however, did not change much. This observation is difficult tointerpret. However, the data indicate that 11 exists in the mo-nomeric form (the equilibration may be occurring between theipso and the methyl-attached ortho-carbon sites).
CarboranesCarboranes are of particular interest for organic chemists be-cause they are neutral organic compounds that generally con-tain carbon atoms with coordination numbers greater than four(10-13). The carboranes are classified as "electron-deficient"
Table 2. 13C NMR parameters of triarylaluminum dimers (AlAr3)2*
Bridging aryl Terminal arylAr Temp, 0C Ci Co Cm Cp CH3 Ci Co Cm Cp CH3
* Shifts (ppm) from external capillary Me4Si.t The multiplicities and coupling constants are given in parentheses: d, doublet; d-q, doublet of quartets; s, singlet.n, Number of boron atoms directly attached to the carbon atom.
§ Degree of s-character calculated according to % s = 0.20 JC-H-
(2, 3, 10-13) cage compounds because their structure cannotbe adequately described in terms of conventional two-electrontwo-center bonds (2). The bonding in the carboranes can bedescribed by two-electron three-center or multicenter bonds,which also is the key to the bonding description of boron hy-drides (54, 55).
Detailed 'H NMR spectra of several medium and smallcarboranes have been reported (56-59). 13C shifts for some ofthe carboranes have also been reported (12, 60). However, thereis still a lack of detailed information from 13C NMR study ofcarboranes. We thus extended our studies to the '3C NMR in-vestigation of several carboranes, with particular emphasis onthe relationship between the carbon shifts and the corre-sponding coordination number of the involved carbon atoms.In addition, determination of 13C-1H spin-spin coupling con-stants of the carbon nuclei has also shown to be a valuable probeof electronic structure of electron-deficient carboranes.
H
H-BEjZ- -H
\I
19
H
B
H
18
H
B
H H \-HH
17
The 13C chemical shifts and '3C-'H spin-spin couplingconstants of 2,3-C2B4H8 (17) (61), 1,6-C2B4H13 (18), 2,4-C2B5H7(19), and two substituted derivatives of 1,2-C2BoH12 (20 and21) (62-64) were determined. The data are summarized inTable 3 along with the parameters of model carboranes, 1,5-C2B3Hs (22) (57, 63) and the parent compounds 1,2- and 1,7-C2BjoHI2 (23 and 24) (12).The tetracoordinated carbon atoms in 17 and 22 which are
bonded to two and three boron atoms, respectively, resonateat lower field than do the penta- and hexacoordinated carbonatoms in higher carboranes. One bond 13C-IH coupling con-
stant for the tetracoordinated carbon in 22 reported by Onak(57-59), 192 Hz, is substantially larger than that found in 17(JC-H = 160 Hz). The Sp3 carbon atom in the latter apparently
suffers much less internal strain than that in the former. Oth-erwise, both the penta- and hexacoordinated carbon atoms incarboranes show. much larger JC-H values than those for tetra-valent carbons. The increase in coupling constants JC-H is alsodue to an increase in the number of boron atoms directly at-tached to the carbon. The magnitude of the boron substituenteffect on JC-H is not yet known. As indicated in Table 3, thereis an approximately 20-40 ppm difference in the 13C chemicalshifts between the tetra- and penta- (or hexa-)coordinatedcarbon atoms. The difference between the penta- and hexa-coordinated carbon atoms is small. An interesting feature of the13C NMR spectra of the carboranes studied is that 17 shows adoublet of quartets with JC-H = 160 Hz and JC-B = 50 Hz, re-spectively. Thus, the carbon atoms must be simultaneouslycoupled with the hydrogen and one of the boron atoms, but thecoupling is not seen in the C2BoHl2 series. Apparently, thebroadness of the carbon signals in the carboranes is attributedto a 13C-11B type of weak coupling and/or to quadrupole ef-fects of the caged boron atoms.
One-bond I3C-'H coupling constants have been semiem-pirically correlated to the degree of s-character on the C-Hbond (56-59, 61). Application of the relationship to the 13C-1Hcoupling (Table 3) found- in carboranes indicates that almostall the C-H bonds have approximately 30-40% s-character.There is about a 20% decrease in s-character for theC-H bondin 17 compared to 22. There is also substantial change in s-character of C-M bonds going from 1,7-C2B~oH12 to thecorresponding 1,2-isomers; the latter have adjacent C atoms.
EXPERIMENTAL'3C NMR spectra were obtained with a Varian Associates modelXL-100 spectrometer equipped with a broad band decouplerand variable temperature probe and interfaced with a Varian620-L computer operating with 8192 digital points. Chemicalshifts were recorded from the 13C signal of Me4Si in a 1.75-mmcapillary held concentrically inside the standard 12-mm sampletube or from the solvent signal shift calibrated to it. The '3CNMR spectrum of Et3Al at -80° was obtained on a Bruker270-MHz NMR spectrometer equipped with variable tem-perature probe and a 32k computer.Trialkylaluminum Solutions. Me3Al and Et3Al were
available commercially and were used after trap-to-trap dis-tillation under vacuum at low temperature (-78°). cPr3Alwas prepared from dicyclopropylmercury and Al powder (28,29). The solutions were prepared by dissolving trialkylalum-inums in highly purified toluene in the 12-mm NMR tube
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under purified nitrogen atmosphere and then sealing the tube.The approximate concentration of each of the solutions was 10%by weight.Triarylaluminum Solutions. The triarylaluminum com-
pounds were prepared as described (17). Solutions were pre-pared by transferring, in a nitrogen-filled dry box, the solidmaterial into a sample tube attached to a stopcock and a stan-dard-taper joint. This apparatus was then pumped out on a highvacuum line and the methylene chloride solvent was condensedunder vacuum. The solutions were -5% by weight.
Carboranes. The spectra were recorded from neat solutionsat 37°. Compounds 17, 18, and 19 were kindly provided by R.E. Williams. The other carboranes used were commerciallyavailable.
We are grateful to Prof. M. Saunders of Yale University for the useof the superconducting NMR facilities. Support of our work by theNational Science Foundation and National Institutes of Health isgratefully acknowledged.
1. Olah, G. A. & Hehemann, D. (1977) J. Org. Chem., 42,2190.2. Olah, G. A. (1973) Angew. Chem. Int. Ed. Engl. 12, 173-212.3. Wade, K. (1971) Electron Deficient Compounds (Nelson,
London).4. Bartlett, P. D. (1965) Nonclassical Ions (W. A. Benjamin,
Reading, MA).5. Lipscomb, U. N. (1963) Boron Hydrides (W. A. Benjamin,
Reading, MA).6. Lewis, P. H. & Rundle, R. E. (1953) J. Chem. Phys. 21, 986-
992.7. Wade, K. (1975) Chem. Br. 11, 177-183.8. Freudenbert, K. (1927) Akad Wiss. (Sitz, Mainz, Germany), pp.
1-13.9. Hoffmann, R. & Hofmann, P. (1976) J. Am. Chem. Soc. 98,
598-604.10. Grimes, R. N. (1970) Carboranes (Academic Press, New
York).11. Williams, R. E. (1971) Inorg. Chem. 10, 210-214.12. Todd, L. J. (1972) Pure Appl. Chem. 30, 587-598.13. Hawthorne, M. F. (1967) in The Chemistry of Boron and its
Compounds, ed. Muetterties, E. L. (John Wiley, New York): pp.223-313.
14. Rundle, R. E. (1963) in Survey of Progress in Chemistry, ed.Scott, A. F. (Academic Press, New York), Vol. 1, pp. 81-128.
15. Smith, M. B. (1972) J. Organomet. Chem. 46,211-217.16. Smith, M. B. (1974) J. Organomet. Chem. 76, 171-201.17. Stanford, T. B., Jr. & Henold, K. L. (1975) Inorg. Chem. 14,
2426-2431.18. Stothers, J. B. (1972) Carbon-13 NMR Spectroscopy (Academic
Press, New York).19. Olah, G. A. (1976) Acc. Chem. Res. 9, 41-52.20. Hart, H. & Kuzuy, A. L. (1975) J. Am. Chem. Soc. 97, 2459-
2468.21. Hogeween, H. & Kwant, P. W. (1975) Acc. Chem. Res. 8,
413-420.22. Masamune, S., Sakai, M., Jones, A.v.K. & Nakashima, T. Can. J.
Chem. 52,855-857.23. Rottler, R., Fin, C. G. & Fink, G. (1976) Naturforsch. Z. Anorg.
Chem. 31B, 730-737.24. Vranka, R. G. & Amma, E. L. (1967) J. Am. Chem. Soc. 89,
312-326.25. Ogawa, T., Hirota, K. & Miyazawa, T. (1965) Bull. Chem. Soc.
Jpn. 38, 1105-1110.26. Ramey, K. C., O'Brien, J. F., Hasegawa, I. & Borchert, A. E.
(1965) J. Phys. Chem. 69,3418-3423.27. Poole, C. P., Jr., Swift, H. E. & Itzel, J. F., Jr. (1965) J. Phys.
Chem. 69, 3663-3665.
28. Sanders, D. A. & Oliver, J. P. (1968) J. Am. Chem. Soc. 90,5910-5912.
29. Muller, M. K. & Deknicke, K. (1972) J. Organomet. Chem. 46,219-229.
30. Yamamoto, 0. (1975) J. Chem. Phys. 63,2989-2995.31. Yamamoto, O., Hayamizie, K. & Yanagisana, F. (1974) J. Or-
ganomet. Chem. 73,17-25.32. Moore, J. W., Sanders, D. A., Scherr, P. A., Glick, F. D. & Oliver,
T. P. (1971) J. Am. Chem. Soc. 93, 1035-1037.33. Huffmann, J. C. & Streib, W. E. (1971) Chem. Commun.
911-912.34. Perkins, P. G. & Twentyman, F. E. (1965) J. Chem. Soc.
1038-1044.35. Malone, J. F. & McDonald, W. S. (1967) Chem. Commun.
444-445.36. Malone, J. F. & McDonald, W. S. (1972) J. Chem. Soc. Dalton
Trans. 2646-2648.37. Moles, T. (1963) Aust. J. Chem. 16,794-800.38. Krause, E. & Dittmar, P. C. (1930) Chem. Ber. 63,2401-2407.39. Gilman, H. & Maple, K. E. (1936) Rec. Fran. Chim. 55, 133-
139.40. Malone, J. E. & McDonald, W. S. (1972) J. Chem. Soc. Dalton
Trans. 2649-2652.41. Malone, J. E. & McDonald, W. S. (1970) Chem. Commun.
280.42. Jeffery, E. A., Mole, T. & Saunders, J. K. (1967) Chem. Commun.
696-697.43. de Graff, P. W. J., Boersma, J. & Van der Kerk, G. J. M. (1974)
J. Organomet. Chem. 78, C-19-C-21.44. Lancelot, C. J., Cram, D. J. & Schleyer, P. v. R. (1972) Carbonium
Ions, eds. Olah, G. A. & Schleyer, P. v. R. (Wiley Interscience,New York), Vol. 3, pp. 1347-1483.
45. Hehre, W. J. (1972) J. Am. Chem. Soc. 94,5919-5920.46. McCall, F. J., Townsend, J. M. & Bonner, W. A. (1975) J. Am.
Chem. Soc. 97,2743-2749.47. Olah, G. A. & Porter, R. D. (1971) J. Am. Chem. Soc. 93,
6877-6887.48. Schoeller, W. W. & Schenck, G. E. (1973) Tetrahedron 29,
425-427.49. Schoeller, W. W. & Dahm, J. C. (1973) Tetrahedron 29,
3237-3239.50. Hoffmann, R., Alder, R. W. & Wilcox, C. F., Jr. (1970) J. Am.
Chem. Soc. 92,4992-4993.51. Shanshal, F. (1972) J. Chem. Soc. Perkin Trans. 2, 335-339.52. Synder, E. I. (1970) J. Am. Chem. Soc. 92, 7529-7532.53. Bentley, F. E. & Dewar, F. J. S. (1970) J. Am. Chem. Soc. 92,
3991-3996.54. Marynick, D. S. & Lipscomb, W. N. (1972) J. Am. Chem. Soc.
94,8699-8706.55. Swutkes, E., Epstein, I. R., Tossell, J. A., Stevers, R. M. & Lip-
scomb, W. N. (1970) J. Am. Chem. Soc. 92,3837-3846.56. Onak, T. & Wan, E. (1974) J. Chem. Soc. Dalton Trans. 665-
669.57. Onak, T., Mattachei, P. & Groszek, E. (1969) J. Chem. Soc. A
1990-1992.58. Onak, T. & Wong, G. T. F. (1970) J. Am. Chem. Soc. 92,
5226.59. Groszek, E., Leach, J. B., Wong, G. T. F., Ungermann, C. & Onak,
T. (1971) Inorg. Chem. 10, 2770-2775.60. Todd, L. J., Clouse, A. O., Doddrell, D. & Kahl, S. B. (1969) Chem.
Commun. 729-730.61. Muller, N. & Pritchard, D. E. (1959) J. Chem. Phys. 31, 768-
771.62. Hill, W. E., Johnson, F. A. & Novak, R. W. (1975) Inorg. Chem.
14, 1244-1249.63. Fein, M. M., Bobinski, J., Mayes, N., Schwartz, N. & Cohen, M.
(1963) Inorg. Chem. 2, 1111-1115.64. Heing, T. L., Ager, J. W., Clark, S. L., Margold, D. J., Goldstein,
H. L., Hillman, M., Polak, R. J. & Szyamanski, J. W. (1963) Inorg.Chem. 2, 1089-1092.
