Organometallics 1995, 14,3933-3941
3933
The Silylium Ion (RaSi+)Problem: Effect of Alkyl Substituents R Zuowei
Robert Bau,? Alan Benesi,* and Christopher A. Reed*tt Departments of Chemistry, University of Southern California,
Los Angeles, California 90089-0744,and Pennsylvania State University, State College, Pennsylvania 18602 Received June 12,1995@ The X-ray structures of four R ~ S ~ ( B I - ~ - C Bspecies ~ ~ H ~have ) been determined (R3 = Et3, i-Pr3, t-Bu3, and t-BuzMe). Br6-CBllH6- is a particularly weakly coordinating anion and leads to long Si-Br bond distances (ca. 2.43-2.48A). The developing silylium ion character in these species is reflected in the approach of the R3Si6+ moiety toward planarity. The sum of the C-Si-C angles for each of the four derivatives lies in the range ca. 345-351". The closest approach to 360" is found in the triisopropyl derivative, not the tri-tert-butyl derivative as might be anticipated from steric considerations. One of the isopropyl groups in the i-Pr3 derivative has its carbon atoms very nearly planar with the silicon atom. This is indicative of C-H bond hyperconjugative stabilization of the developing positive charge on silicon. An "inorganic" description would be in terms of a n a-agostic C-H interaction with the electron-deficient 3pz orbital on silicon. The results suggest that hyperconjugative stabilization of cationic character, although relatively unimportant compared to that for carbon in carbenium ions, is nevertheless not negligible for silicon. Solid state CPMAS 29Si NMR data are also reported. The chemical shifts lie in the 105-115 ppm downfield region as expected for silylium ion-like character in these R3sid+(Br6-cB~~H6)+ species.
Introduction The long-sought silylium ion, R W , analogous to wellknown carbenium ions' (R3C+)has yet to be definitively characterized in condensed media.2-12 The essential problem is one of excluding nucleophiles (solvent or anion) from bonding to silicon. An additional consideration is the choice of the substituent R in order t o maximize the stability of R3Si+. In this paper we explore the effect of increasingly branched-chain aliphatic substituents on the degree of silylium ion character in trialkylsilyl compounds of the type R3Si6+-Y6-, currently where Y is the carborane anion B~G-CBI~HG-, perhaps the least nucleophilic anion known for silicon.2 By emphasizing solid state data, particularly from X-ray crystallography, uncertainties about the role of solvent are removed. Ethers,13nitri1es,l4-l6 and, most recently University of Southern California. Pennsylvania State University. Abstract published in Advance ACS Abstracts, August 1, 1995. (1) Olah, G.A. Nobel Lecture. Angew. Chem., Int. Ed. Engl. 1995, in press. (2)Reed, C. A,; Xie, Z.; Bau, R.; Benesi, A.Science 1993,262,402404. (3)Lambert, J . B.; Zhang, S.; Stern, C. L.; Huffman, J. C. Science 1993, 260, 1917-1919. Lambert, J . B.; Zhang, S.; Ciro, S. M. Organometallics 1994,13,2430-2443. (4)Olah, G.A.; Heiliger, L.; Li, X.-Y.; Prakash, G. K. S. J . A m . Chem. SOC. 1990,112,5991-5995. (5) Eaborn, C. J . Organomet. Chem. 1991,405,173-177. (6)Lickiss, P.D.J . Chem. SOC.,Dalton Trans. 1992,1333-1338. (7) Schleyer, P. v. R.; Buzek, P.; Muller, T.; Apeloig, Y.; Siehl, H. U. Angew. Chem., Int. Ed. Engl. 1993,32,1471-1473. ( 8 ) Crestoni, M. E.; Fornarini, S. Angew. Chem., Int. Ed. Engl. 1994, 33,1094-1096. (9)Olsson, L.; Cremer, D. Chem. Phys. Lett. 1993,215,433-443. (10)Pauling, L.Science 1994,263,983. (11)Strauss, S.H. Chemtracts: Inorg. Chem. 1993,5,119-124. (12)Houk, K.N.Chemtracts: Org. Chem. 1993,6,360-363. (13)Kira, M.;Hino, T.; Sakurai, H. J . A m . Chem. SOC.1992,114, 6697-6700. +
@
and interestingly, toluene3 have been shown t o be sufficiently nucleophilic that discrete and characterizable four-coordinate species of the type [R3Si(solvent)l+ are formed, analogous to cations proposed earlier with a variety of nucleophile^^^ and characterized by X-ray structure in the case of pyridine.18 Part of the impetus behind the search for the elusive R3Si+ ion is the desire to find silicon analogues of familiar electron-deficient and low-coordinate structures of carbon chemistry. The crystal structures of a number of carbocations are now known, although the first simple trialkylcarbenium ion, R3C+, was characterized only very recently (R = Me).19 It was stabilized by using somewhat lowered temperatures ( 120' (see sensitive to the nature of the substituent. Rather, the Table 1). This suggests that the sum of the three strength of the cation-anion electrostatic interaction
Organometallics, Vol. 14,No. 8, 1995 3937
The Silylium Ion (RJSi+)Problem
Table 1. Summary of Key Geometrical Parameters for RdWBrfi-CBllHd ~~
param
(A) (A)
Si-Br Si-Br-B (deg) B-Brcoord B-Bruncwrd (Ala C-Si-C (deg)b ZC-Si-C (deg) mean C-Si-C (deg) Si out of C3 plane a
triethyl
triethyl'
triisopropyl
tri-tert-butyl
di-tert-butylmethyl
2.444(7) 109.1(6) 1.99(2) 1.92-1.98(2) 111.2(10) [2,61 114.6(9)[2,41 119.2(10)[4,61 345.0(10) 115.0(10) 0.419
2.430(6) 111.3(5) 2.01(2) 1.92-1.98(2) 113.4(10) [4a, 6al 117.4(8) [2a, 6al 118.2(9)[2a, 4al 349.0(9) 116.3(9) 0.348
2.479(9) 114.7(7) 2.05(3) 1.93-2.02(3) 111.2(14) [2, 81 119.6(13) [5,81 120.2(12) 12, 51 351.0(13) 117.0(13) 0.300
2.465(5) 125.0(5) 2.04(2) 1.92-1.93(2) 115.1(7) [2, 101 115.9(6) [2, 61 117.7(7) [6, 101 348.7(7) 116.2(7) 0.371
2.466(12) 116.2(10) 2.05(3) 1.94-2.00(4) 110.7(21) [2, 71 114.1(21)[2, 31 121.0(19) [3, 71 345.8(2 1) 115.3(21) 0.408
From pentagonal belt only (Le., excludes 12-position). Numbers in brackets identify the carbon atoms (see Figure 1).
Table 2. Table of Key Substituent Bond Distances and Angles in %Si(Brs-CBllHe) Distances (A)
Angles (deg) Triethyl 114.5(14) 116.6(13) 122.6(14)
Si-C(2) Si-C(4) Si-C(6)
1.85(3) 1.84(2) 1.80(2)
Triethyl' 114.6(13) 120.7(20) 109.2(11)
Si-C(2a) Si-C(4a) Si-C(6a)
1.82(2) 1.85(2) 1.86(2)
Triisopropyl 118.8(22) Si-C(2) 110.0(17) 107.5(21)
1.86(3)
113.9(19) 107.5(18) 108.8(23)
Si-C(5)
1.91(3)
119.0(22) 128.2(28) 112.3(32)
Si-C(8)
1.80(3)
Tri-tert-butyl 114.9(10) Si-C(2) 113.3(11) 108.3(9)
1.91(2)
114.2(10) 113.9(9) 109.9(12)
Si-C(6)
1.87(2)
114.6(10) 112.3(11) 106.5(10)
Si-C(l0)
1.89(2)
Di-tert-butylmethyl Si-C(2)
1.85(4)
112.2(33) 113.2(34) 106.7(29)
Si-C(3)
1.85(4)
108.9(33) 115.1(37) 108.6(30)
Si-C(7)
1.88(4)
may be the dominant factor in determining the geometry around silicon. The differences between the ethyl and the ethyl' structures can be taken as some measure of crystal-packing forces on the conformations of the substituents. Differences are small but can be as much as 0.02 A in a particular bond length and ca. 2" in a particular bond angle. The ranking of substituents in order of increasing mean or sum of C-Si-C angles or decreasing Si outof-plane distance (Table 1) is triethyl I di-tert-butylmethyl < tri-tert-butyl Itriethyl' < triisopropyl. The unexpected result is that the tri-tert-butyl derivative is not the highest ranking. One might have expected its slightly superior electron-releasing inductive effect to best stabilize the developing positive charge on silicon
and its superior steric bulk to push the carborane anion more distant. However, as discussed above, the SiBr-B angle simply opens up to 125" to accommodate the bulky tert-butyl substituents. Bromonium versus Silylium Ion Character. It has been suggested that these R ~ S ~ ( B ~ ~ - C B species ~IH~) are bromonium ions.42 The structural data do not, however, support this view. The formal representation of a bromonium ion is given as structure I in Figure 2. The C-Si-C angles reflect tetrahedrality at silicon (log"), and the bond angle at bromine is 109"(by VSEPR theory and X-ray s t r ~ c t u r e ) The . ~ ~ formal representation of a silylium ion is given as structure I11 in Figure 2. The C-Si-C angles reflect trigonal planarity (120"), and there is no Si-Br bond. The present R3Si(carborane) species are represented by structure I1 in Figure 2. The C-Si-C angles at averages of 115-117" are ca. 55%-70% along the trajectory from 109" in I t o 120" in 111, representing ca. 55%-70% development of silylium ion character. In addition, the wide range of Si-Br-B angles (109-125"), the long Si-Br bonds, and the minimal perturbation of the coordinated Br-B bond all reflect electrostatic bonding in a silylium ion rather than bromonium ion sense, i.e., R3Sid+(Br6-CB11H6)d-. Thus, while bromonium ion character must be present t o some extent, we conclude that it is minor compared t o silylium ion character. Compounds should be named after their predominant structural and electronic form, and we suggest silylium "ion-like"as the most appropriate description. The silylium ion character of the present species is reflected in their reaction chemistry. They react with nucleophiles such as organic halides to form R3SX (X= halide) or water to give R ~ S ~ ( O H Z ) + . ~ ~ We have yet to observe any property that might indicate predominant bromonium ion character, and in no circumstance have we observed B-Br bond cleavage. Rather, the bromocarborane moiety retains its weakly nucleophilic anionic character in all chemistry explored to date. The above observations have led us t o propose that there is a continuum of cation-anion interactions between partially covalent species of the type R3SiY and the fully ionic silylium ion, R3SifY-.2,45This principle has been adopted in recent theoretical paper^^^,^^ and is in marked contrast t o carbocation chemistry where (42) Olah, G. A,; R a d , G.; Li, X.-Y.; Buchholz, H. A,; Sanford, G.; Prakash, G. K. S. Science 1994,263, 983-984. (43) Yanovskii, A. I.; Struchkov, Yu. T.; Grushin, V. V.; Tolstaya, T. P.; Demkina, I. I. Zh. Strukt. Khim. 1988, 29, 89-94. (44)Xie, Z.; Bau, R.; Reed, C. A. J. Chem. SOC.,Chem. Commun. 1994, 2519-2520. (45) Reed, C . A,; Xie, Z. Science 1994, 263, 985-986. (46) Schleyer, P. v. R. Personal communication, 1994. Maerker, C.; Kapp, J.; Schleyer, P. v. R. Submitted for publication.
Xie et a1.
3938 Organometallics, Vol. 14,No. 8,1995
I
\ ll!
I I1 111 bromonium ion observed silylium "ion-like" silylium ion Figure 2. Structural representations of a bromonium ion (I), the present compounds (11) having a majority of silylium ion-like character, and a silylium ion (111).
lated position of the isopropyl H atom (assuming trigonal pyramidality) does not place it in the same plane as the C(8)-Si-Br atoms, as might be expected for optimal &-H/3p* overlap. Rather, there is a torsional angle of 36" between the H-C and Si-Br vectors. This Si+ Y' si Y' probably reflects a compromise engendered by the steric E E effect of the bromine atom. Its size prevents an ideal alignment of the C-H bond with the Si-Br bond. We Pr Pr have considered the possibility of an attractive interb action of the C"--H"+ dipole to the electron-rich broMe Me 1; mine atom. Indeed, this is an expected consequence of hyperconjugation. There is no simple way to separate these two effects, although the lack of coplanarity of the I \ C-H and Si-Br bonds suggests that the extended overlap of orbitals is more important than the attraction of localized partial charges a t the atomic positions. The steric effect of the bromine atom offers a possible explanation for why tert-butyl substituents show less Figure 3. (a) Classical resonance representation of C-H structural manifestation of hyperconjugation than this bond hyperconjugative stabilizationof cationic silicon. (b) particular isopropyl group. HyperconjugativeC-C bond Equivalent molecular orbital description. Shading is used to represent the filled orbital (not the sign of the wave approach to the Si-Br bond will be hindered by the bulk function). of the methyl group in a tert-butyl substituent relative to the H atom of an isopropyl substituent. Inspection the distinction between covalent tetrahedral carbon and of bond angles around silicon and the a-carbon atoms ionic trigonal carbenium ions is more "black and white". in the di-tert-butylmethyl derivative, which has the Hyperconjugation. There are notable differences opportunity for both C-C and C-H bond hyperconjuin the three isopropyl groups of i-Pr&i(Br&B11Hs) with gation, is less demonstrative of it. The three Si-C respect to Si-C bond length and with respect to bond bonds are the same within experimental error and do angles around their central carbon atoms. In particular, not show the large range seen in the triisopropyl the shortest Si-C bond is to C(8), and this isopropyl derivative. Similarly, the spread of Br-Si-C angles is carbon atom has the largest Si-C-C angles [119.0(22) only -6" [101.1(15)-106.6(17)"]. It is not possible to and 128.2(28)"]. They far exceed the sp3 ideal, and determine the positions of the methyl group H atoms indeed, the C(8)isopropyl carbon atom is nearly planar. either from a X-ray data or by inference as was done The sum of the angles around C(8) is 359.5 (3)" (see for the isopropyl group. The spread of Si-C-C angles Table 2). This can be seen in the uppermost isopropyl in the two tert-butyl groups is only -8", and no group of Figure IC. It is strongly suggestive of C-H particular angle is far removed from sp3 ideality (see bond hyperconjugation,giving double-bond character to Table 2). The lower angles might be interpreted as the Si-C(8) bond and sp2 character to the isopropyl indicating C-C bond hyperconjugation,perhaps into the carbon. The localized valence-bond representation is back lobe of the developingempty 3p, orbital on silicon, given in Figure 3a. The equivalent molecular orbital but overall, the dimensions of this compound suggest representation is given in Figure 3b. The filled CJ&. that hyperconjugative stabilization is distributed over orbital donates electron density into the developing a number of weak interactions among the three subempty 3p,-like orbital along the Si-Br direction. The stituents rather than concentrated in one particular pyramidalization a t silicon means that this orbital will substituent as in the isopropyl case. also have silicon 3s character. It is electron deficient The two triethyl structures are somewhat more ilbecause of the weak silicon-bromine bond. A more lustrative of hyperconjugationsince there is a somewhat "inorganic" description would use the same molecular eater spread in the Si-C distances [1.80(2)-1.86(2) orbital representation of Figure 3b but would call it an and a larger range of Si-C-C angles [109.2(11)a-agostic C-H i n t e r a ~ t i o n . ~ ~ Either way, a donor 122.6( 14)"]. Like the isopropyl derivative, the shortest orbital and an acceptor orbital are identified, and the Si-C distance (to C(6) in the triethyl molecule)has the short Si-C bond and near-planarity of the isopropyl largest Si-C-C angle. This again suggests C-H hypercarbon atom substituents are rationalized. The calcuconjugation into the developing empty 3p, orbital on silicon but this time into the back lobe, trans to the Si(47) Brookhart, M.; Green, M. L. H.; Wong, L.-L. Prog. Inorg. Chem. 1988,36,1-124. Br bond. The Br-Si-C(6) bond is 107.7 (8)".
- I/
/I;,
I,,
1-
8
Organometallics, Vol. 14, No. 8,1995 3939
The Silylium Ion (R3Sif)Problem
d L - & d b - I I1 N
r n
PPm
Figure 4. Solid state 29SiNMR spectrum of i-Pr~Si(Br6CBIIHG) and insert of the downfield portion for Et~Si(Br6CBllHs). Conditions: 110 mg sample, cross-polarization via spin lock with bilevel decoupling, pulse width 5.35 ps, contact time 2.0 ms, pulse delay 2 s.
An ethyl substituent is quite unhindered as indicated by the variety of orientations of the ethyl groups in Figure la,b. Thus, they have the opportunity to engage in both C-H and C-C bond hyperconjugation. Evidence for C-C bond hyperconjugation would be a correlation of short Si-C bonds with small Si-C-C angles. This is not observed in any significant way (see Table 2). On the other hand, the correlation of the shortest Si-C bond with the largest Si-C-C angles (at least in the triethyl and triisopropyl structures) suggests that C-H hyperconjugation may be slightly more effective than C-C hyperconjugation in stabilizing the partial positive charge on silicon. This may reflect a closer energy match of the more polar C-H bond with the partially empty 3p, orbital on silicon. 2eSi NMR. Downfield 29Sichemical shifts are the expected signature of developing silylium ion character in R&i6+(Br6-CB11H&) derivatives. We have determined these in the solid state by cross-polarization magic angle spinning techniques (CPMAS) for several reasons. Firstly, there is a practical reason. These derivatives are not very soluble in the only suitable solvent, benzene. Secondly, the possible coordination of solvent to form [R3Si(solvent)l+,even if only to a small degree, complicates the interpretation of comparative data. Thirdly, we wanted to see if there is a correlation of downfield 29Sishift with X-ray measures of developing silylium ion character, particularly the mean C-Si-C angle. A typical solid state 29Sispectrum is shown in Figure 4, and this one for the triisopropyl derivative. The only significant resonance occurs at 109.6 ppm downfield of tetramethylsilane. In a duplicate experiment from a different preparation we obtained an essentially identical sharp resonance at 110.0 ppm, indicating that the error in our quoted values is ca. 0.5 ppm. The t-BuzMe derivative gives a similar resonance at a chemical shift of 112.8 ppm. As expected from the two independent triethyl molecules seen in the X-ray structure, two 29Si resonances are seen in an Et3 sample, at 111.8 and 106.2 ppm. This is illustrated in the insert of Figure 4. To
date, we have been unable to obtain a comparable resonance for the tri-tert-butyl derivative perhaps due to the lack of hydrogen nuclei on a-carbon atoms which may impractically change the time dependence of the cross-polarization compared t o methyl, ethyl, or isopropyl substituents. The small range of 29Si shifts in these RsSi(Br6CB11H6) derivatives (106-113 ppm) is notable as is the lack of any discernable correlation with any particular X-ray structural parameter. The combined effects of three alkyl substituents on silicon, whether methyl, ethyl, isopropyl or tert-butyl, are apparently quite similar from an NMR chemical shift point of view. This supports the view developed earlier from our discussion of the silicon-bromine bonding that electrostatic considerations are the most important part of the trialkylsilicon to carborane bonding. To a first approximation, the carborane anion dictates the degree of silylium ion character developed in the R3Si6+ moiety and the R groups compensate accordingly. There is a possible parallel with carbocations where the 13C chemical shift for classical carbenium centers is rather insensitive to substituent. For example, 6(13C)is essentially identical in Me&+ and Et&+ (335.2 and 336.8 ppm, respect i ~ e l y ) In . ~ the ~ present trialkyl silicon derivatives the geometrical differences are apparently too small to produce a trend in 6(29Si)and are matched by compensating electronic factors. The downfield 29Sishifts (106-113 ppm) are the largest observed to date in a trialkylsilyl compound. The only comparable species is i-PrsSi(Fzo-BPhd, whose solid state resonance is reported at 107.6 For reference, [i-Pr3Si(toluene)lf is ca. 94 ppm, and most other anions Y- in comparable R3SiY derivatives have 29Sichemical shifts in the range 30-55 ppm.4J4 The silanes, R3SiH, fall in the range 0-12 ppm for R = an alkyl group. Fully ionic planar R3Si+ ions are expected to show -300 ppm downfield shifts.7,34~36~49,50 Thus, the ca. 100 ppm downfield shifts of the R ~ S ~ ( B ~ G - C B I ~ H G ) derivatives, relative t o their silanes, are a clear indication of developing silylium ion character. Since there is no reason to expect 29Sishifts to scale linearly with any particular geometric parameter in R3Sid+Y6-species, it is not presently possible to estimate "how much" silylium ion character the -100 ppm downfield shift represents. Interestingly, Schleyelff6 has shown by IGLO methods that the -300 ppm shift of R3Si+ rapidly diminishes upon approach of weak nucleophiles. For example, when MeBr approaches Me&+ to a Si-a-Br distance 2.46 A (the same as that in the present bromocarborane species), the shift is calculated to be only 102 ppm. Thus, despite the fact that the downfield shifts of the present compounds fall well short of the predicted free silylium value, they may indicate a very high degree of silylium ion character.
Conclusions The very weak coordinating ability, the extreme chemical inertness, and the excellent crystallizing char(48) Olah, G. A.; Donovan, D. J. J.Am. Chem. SOC.1976,99,50265039. (49)Kutzelnigg, W.;Feischer, U.;Schindler, M. In NMR; Basic Principles and Progress; Diehl, P., Fluck, E., Gunther, H., Kosfeld, R., Selling, J., Eds.; Springer-Verlag: New York, 1991;Vol. 23,pp 228231. (50) Olah, G. A.; Field, L. Organometallics 1982,I , 1485-1487.
3940 Organometallics, Vol. 14, No. 8, 1995
Xie et al.
Table 3. Summary of Data and Intensity Collection Parameters for R$3i(Bre-CB11He) R3 formula molecular wt (mol) cryst syst space group a,A
b, A
C,
A
a, deg A deg
t 2:
z
calcd density (g cm-9 radiation (A,A) abs coeff temp (K) scan type index ranges no. of independent reflns no. of obsd reflns ( F > 3.OdF)) no. of params refined final R indices
Et3 trjclinic
P1 7.786(2) 16.212(3) 19.660(4) 110.70(3) 97.62(3) 92.05(3) 2291.8(9) 4 2.12 Cu K a (1.541 78) 13.07 123 8-28 h,f k , f l 4749 3194 271 6.71
i-Pr3 t-BuzMe C I O H ~ ~ B I I B ~ ~ S C10H27BllBr6Si ~ 773.8
773.8
triclinic Pi
monoclinic
monoclinic
P2 lie 8.121(5) 23.351(23) 14.214(18) 90 96.18(8) 90 2680(5) 4 1.93 Mo K a (0.710 69) 8.95 298
p21 8.208(2) 20.409(2) 8.648(2) 90 97.22(2) 90 1437.2(5) 2 1.89 Cu K a (1.541 78) 10.49 123 w h,f k , fl 2873 2789 ( F 4 . 0 d F ) ) 279 6.28
11.124(8) 15.628(15) 8.000(9) 94.96(8) 98.85(8) 76.30(7) 1333(2) 2 1.93 Mo K a (0.710 69) 8.99 298 w
w
h, f k , fl
h,k , f l
2993 1296 255 6.37
2805 1072 255 6.93
acteristics of the brominated carborane Br6-CBllH6make it the anion of choice at this time for approaching the long-sought silylium ion, R3Si+, in the solid state. By the criteria of angular approach to planarity and 29Si downfield shifts, the present compounds are the closest approach to date. To a first approximation, the nature of R is less important than the low coordinative nucleophilicity of the anion. The degree of pyramidalization at silicon and the chemical shifts span only a small range of R = Me, Et, i - b ,and t-Bu. A close examination of substituent geometries in five different RsSi(Br6CB11HG) molecules suggests that hyperconjugative stabilization of the positive charge at silicon, while not dramatic, is nevertheless present. Moreover, C-H bond hyperconjugation seems to be slightly favored over C-C bond hyperconjugation. This suggests that an isopropyl substituent may be the alkyl group of choice in attempts t o isolate fully ionic R3Si+. Steric bulk can obviously be a factor in helping separate the cation and anion, but the tert-butyl group is surprisingly ineffective. Silicon is an important bridging element between organic and inorganic chemistry. To our knowledge, no one has previously pointed out that the C-H bond hyperconjugation concept of organic chemistry is the same as the a-agostic C-H interaction concept of transition metal chemistry, when viewed in donoracceptor molecular orbital terms. Both, of course, are manifestations of the modern chemical reality that we live in the age of weak interactions, and all bonds are significantly more delocalized than suggested by writing familiar valence bond structures. Mulliken defined hyperconjugation in 1939 as "conjugation over and above that usually re~ognized".~~ In 1983, Brookhart and Green defined agostic interactions specifically with regard to entropically-promoted interactions of C -H bonds with transition metals.47 Both definitions were very important in attracting attention to phenomena that tended to be overlooked at the times they were introduced. The passage of time, however, diminishes the need for such exclusive or specific definitions. Having said this, it is important to add that the P-agostic effect (as opposed to the a) is a remote C-H bond donation which remains usefully distinguished (51)Mulliken, R. S. J . Chem. Phys. 1939, 7, 339-352.
t-Bu3 C13H33BllBr6Si 815.8
from the through-bond hyperconjugative effect. It remains to be seen whether a P-agostic effect will be found in silicon cations.
Experimental Section Solution NMR spectra were recorded on a Bruker WP-270 or Bruker AM-360 spectrometer using BF3.OEt2 or Me& as external standard for llB or W i , respectively, and are reported in ppm. Infrared spectra were recorded on an IBM IW30S FT instrument. Elemental analyses were performed by Oneida Research Services, NY,or by Microanalytical Lab, Department of Chemistry, University of California, Berkeley. (t-C4H9)3SiH,52( ~ - C ~ H ~ ) Z ( C H ~and )S~ H,~~ [(C6H5)3C+I[Br6-CB11H6-]25 were prepared by literature methods. All solvents were distilled from Nahenzophenone inside the glovebox. Other reagents were purchased from Aldrich and used as supplied. All experiments were performed with flame-dried glassware in a He atmosphere glovebox ( 0 2 , HzO < 0.5 ppm). Solid state 29SiNMR spectra were obtained at 294 K on a Chemagnetics CMX-300 spectrometer operating in the quadrature mode a t 59.08 MHz. Cross-polarization with magic angle spinning (CPMAS) was used with a contact time of 2.0 ms to enhance the signal and shorten the relaxation time between successive transients. Typically, 5000-15 000 transients were acquired for each sample. Samples were sealed in glass rotor inserts designed by Wilmad Glass Co. to fit the zirconia Chemagnetics rotors. Tetrakis(trimethylsily1)silane was used as a n external reference. (CzH5)3Si(Brs-CB11Hs): (CzH513SiH(30.5 mg, 0.262 mmol) was added to a suspension of [Ph3C+I[Br6-CB11H6-1 (124.7 mg, 0.145 mmol) in dry toluene (50 mL). The mixture was stirred at room temperature for 7 h to give a clear pale yellow solution. Concentration of the solution and n-hexane vapor diffusion resulted in colorless crystals (80.2 mg, 76%). llB NMR (C6D6): -1.40 (S, 1B), -9.65 (S, 5B), -20.03 (d, 5B). IR (KBr): 3057 w, 2960-2858 s, 2610 s, 1475 m, 1005 s, 860 s cm-'. Anal. Calcd for C ~ H Z ~ B I ~ C, B ~11.49; ~ S ~ :H, 2.89. Found: C, 11.40; H, 3.00. (i-CsH,)3Si(Brs-CB11H~): Under preparative conditions similar to above, a mixture of i-Pr3SiH (20.1 mg, 0.127 mmol) and [ P ~ ~ C + ] [ B ~ ~ - C B I(60.0 ~ H ~ mg, - ] 0.070 mmol) in dry toluene (25 mL) was stirred a t room temperature overnight. Very pale yellow crystals (35.5 mg, 66%) were isolated by n-hexane diffusion. IlB NMR (CsD6): -1.43 (s, lB), -9.69 (s, 5B), -20.06 (d, 5B). IR (KBr): 3065 w, 2964-2865 s, 2612 s, (52) Doyle, M. P.; West, C. T. J . Am. Chem. SOC.1975, 97, 37773782.
Organometallics, VoE. 14, No. 8, 1995 3941
The Silylium Ion (R3Si+) Problem 1480 m, 1010 s, 950 s, 860 vs, 830 m cm-'. Anal. Calcd for C1&&lBr6Si: C, 15.52; H, 3.52. Found: C, 15.59; H, 3.57. (~-C~HS)~(CH~)S~(B~~-CBI~H~): This was prepared in a similar manner to ( ~ - C ~ H , ) ~ S ~ ( B ~ ~ - Cand B I I pale H ~ ) ,yellow crystals were isolated in 71% yield. IlB NMR (CsD6): -1.40 (5, lB), -9.60 (s, 5B), -20.01 (d, 5B). IR (KBr): 3058 8,29642861 s, 2610 s, 1470 m, 1003 s, 992 s, 933 m, 860 vs, 825 s cm-'. Anal. Calcd for C10H2&lBr6Si: C, 15.52; H, 3.52. Found: C, 15.48; H, 3.50. ( ~ - C ~ H ~ ) ~ S ~ ( B ~ ~ To - CaBsuspension I ~ H B ) : of [Ph3C+][BrsCBIIH6-] (150.0 mg, 0.175 mmol) in dry toluene (80 ml) was added (t-C4H&SiH (80.0 mg, 0.399 mmol). The mixture was stirred at room temperature for 2 weeks to give a colorless clear solution. Concentration of the solution and n-hexane vapor diffusion gave pale-yellow crystals (118 mg, 83%). IlB NMR (CsDs): -1.42 (s, lB), -9.63 (s,5B), -20.00 (d, 5B). IR (KBr): 3057 w, 2960-2866 s, 2611 s, 1469 s, 1002 s, 953 s, 860 s, 816 m cm-l. Anal. Calcd for C13H33BllBr6Si: C, 19.14; H, 4.08. Found: C, 19.10; H, 4.12. X-ray Structure Determinations. All crystals were mounted in thin-walled glass capillaries using Paratone-N oil. Diffraction data were collected on a Siemens P4/RA or a Syntex P21 diffractometer under the conditions indicated in Table 3. Crystallographic examinations led t o the cell constants and
space groups. Absorption correction procedures were applied to the intensity data. Structures were solved by direct methods using Siemens SHELXTL PC soRware or SHELX86.53 In the final model, hydrogen atoms of the alkyl substituents were placed in idealized positions and most of non-hydrogen atoms were refined anisotropically. Data collection and refinement parameters are given in Table 3.
Acknowledgment. We thank the National Science Foundation (CHE 9407284 to C.A.R.) and the Research Corporation (R-171 to R.B.) for support. Supporting Information Available: Further details of the X-ray crystal structure determinations, tables of bond lengths, bond angles, anisotropic thermal parameters, calculated hydrogen atomic coordinates, final atomic coordinates, and atom-numbering schemes for the four structures (44 pages). Ordering information is given on any current masthead page. OM9504462 (53)Shelxtl PC; Siemens Analytical X-ray Instruments, Inc.: Madison,WI, May 1990.
