1675
J . Phys. Chem. 1992, 96, 1675-1679
Theoretical Study of Intramolecular Two-Center, Three-Electron Bonding in HS(CH,),SH+ ( n = 1-4) Michael L.McKee Department of Chemistry, Auburn University, Auburn, Alabama 36849 (Received: August 23, 1991)
The intramolecular two-center, three-electron (2e3e) S:.S bond strength was determined for a series of charged acyclic dithiols, HS(CH2),SH+ (n = 1-4). Calculations at the PMP4/6-3 lG*//6-31G*+ZPC level indicate that the 2c-3e bond strength increases as n increases except for the trimethylene bridged ion (n = 3) which is slightly more stable than the tetramethylene bridged ion (n = 4), in agreement with experiment. The most stable conformation of the charged complexes has C2symmetry, except the trimethylene bridged ion which has C, symmetry. The bond strength can be related with a varying degree of success to a number of calculated properties including total overlap population between sulfur atoms, ionization potential of the charged complex, sulfursulfur distance, and HSS bond angle. The properties of the intramolecular 2c-3e bond can be interpreted as a compromise between maximizing orbital overlap and minimizing steric repulsion.
Introduction
Two-center, three-electron bonds between two sulfur atoms have generated considerable interest.l-l4 Typically, a 2u-1 u* S:.S bond is formed by one-electron oxidation of a sulfur atom which can bind to a sulfur atom of another unoxidized molecule (intermolecular 2c-3e bond) or with another sulfur atom in the same molecular (intramolecular 2c-3e bond). In the latter case, the two sulfur atoms must be able to adopt a suitable geometry for interacti~n.~.~ Experimental characterization of 2c-3e bonding has often been indirect. Optical spectroscopy has been used to measure the absorbance maximum in the UV/vis region which directly corresponds to the energy gap between Q and u* orbital^.^ A large separation between these orbitals (Figure 1) corresponds to a strong interaction of a lone pair on one sulfur directed toward the singly occupied orbital of the second sulfur, which in turn indicates a strong 2c-3e bond. Charge-stripping mass spectroscopy can be used to measure the IE, of the most weakly bound electron of the charged complex which corresponds to the u* singly occupied 0rbita1.l~ A low IE, indicates a destabilized u* orbital, which is also characteristic of a strong 2c-3e bond. ESR spectroscopy can be used to probe the degree of localization of the unpaired electron in the charged c o m p l e ~ e s . ~ ~The - ~ ~results (1) See contributions in: Sulfur-Centered Reactive Intermediates in Chemistry and Biology; Chatgilialoglu, C., Asmus, K.-D., Eds.; Plenum Press: New York, 1990. (2) (a) ~,Musker. W. K.: Gorewit. B. V.: Roush. P. B.: Wolford. T. L. J . Orz. Chem. 1978,43, 3235. (b) Musker, W.'K.; WolfordiT. L.; Roush, P. B. J. Am. Chem. SOC.1978, 100, 6416. (3) Chow, Y. L.; Iwai, K. J . Chem. SOC.,Perkins Trans. 2 1980, 932. 14) Bobrowski. K.; Holcman, J. J . Phys. Chem. 1989. 93, 6381. (5) BonifaEiE, M.; Miickel, H.; Bahnemann, D.; Asmus, K.-D. J . Chem. SOC.,Perkin Trans. 2 1975, 675. (6) Asmus, K.-D.; Bahnemann, D.; Fischer, Ch.-H.; Veltwisch, D. J . Am. Chem. SOC.1979, 101, 5322. (7) Asmus, K.-D. Acc. Chem. Res. 1979, 12, 436. (8) Gobl, M.; BonifaEiE, M.; Asmus, K.-D. J. Am. Chem. Soc. 1984, 106, 5984. (9) Bouma, W. J.; Radom, L. J . Am. Chem. SOC.1985, 107, 345. (IO) Asmus, K.-D.; Gobl, M.; Hiller, K.-0.; Mahling, S.; Monig, J. J . Chem. SOC.,Perkin Trans. 2 1985, 642. (11) Monig, J.; Goslich, R.; Asmus, K.-D. Eer. Bunsen-Ges. Phys. Chem. 1986, 90, 1 15. (12) BonifaEiE, M.; Asmus, K.-D. J . Org. Chem. 1986, 51, 1216. (13) Drewello, T.; Lebrilla, C. B.; Schwarz, H.; deKoning, L. J.; Fokkens, R. H.; Nibbering, N. M. M.; Anklam, E.; Asmus, K.-D. J . Chem. Soc., Chem. Commun. 1987, 1381. (14) Drewello, T.; Lebrilla, C. B.; Asmus, K,-D.; Schwarz, H. Angew. Chem., Int. Ed. Engl. 1989, 28, 1275. (1 5 ) Gilbert, B. C.; Hodgeman, D. K. C.; Norman, R. 0. C. J . Chem. Soc., Perkin Trans. 2 1973, 1748. (16) Petersen, R. L.; Nelson, D. J.; Symons, M. C. R. J . Chem. SOC., Perkins Trans. 2 1978, 225. .
I
TABLE I: Absorbance Maximum Charged Acyclic Dithiols
LX in Optical Spectrum of
comwund C2H5S(CHCH3)SC2H5+ CHyS(CH2)2SCHj+ CH$(CH2)3SCHS* CH3S(CH2)4SCH3+
" Reference 6.
Law. nm 570" 525b
440b 450b
*Reference 14.
indicate that the unpaired electron resides equally on the two sulfur atoms, which is consistent with a 2c-3e bond.18 Fourier transformation ion cyclotron resonance (FTICR) spectroscopy has also been used to identify stable charged dimers in the gas phase. The technique was used to study (i-Pr)2S-- s ( i - P ~ ) ~ + . ' ~ The most direct measurement of the 2c-3e bond has been made in a mass spectroscopicstudy of gas-phase ion-molecule association eq~i1ibria.I~The experimentally determined binding energy for (CH3)2S--S(CH3)2+was 23.9-26.5 kcal/mol. By far, the greatest amount of work on 2c-3e bonding has been in the form of theoretical calculations. Early contributions to the understanding of 2c-3e bonding were made by Baird," who pointed out that the strength of a 2c-3e bond did not vary linearly with overlap, and by Harcourt.zl Additional insight was provided by Clark,22,23 who pointed out that the strongest 2c-3e bond is expected between molecules with the same IE. In that case, several resonance structures contribute equally to the bonding description. Radom and c o - w o r k e r ~ also * ~ ~have ~ ~ provided high-level ab initio calculations on a number of system bound by 2c-3e bonds, as have a number of other workers.2b29 Drewello et al.I4 have probed a series of intramolecular S:.S bonds by UV/vis spectroscopy where the maxima of the UV/vis (17) Gara, W. B.; Giles, J. R. M.; Roberts, B. P. J . Chem. Soc., Perkins Trans. 2 1979, 1444. (18) Qin, X.-Z.; Meng, Q.-C.; Williams, F. J . Am. Chem. Soc. 1987, 109,
._.
677R I.
(19) Illies, A. J.; Livant, P.; McKee, M. L. J . Am. Chem. SOC.1988, 110, 7980. (20) Baird, N . C. J . Chem. Educ. 1977, 54, 291. (21) Harcourt, R. D. J . Chem. Educ. 1985, 62, 99. (22) Clark, T.J . Compuf. Chem. 1981, 2, 261. (23) Clark, T. J. Am. Chem. SOC.1988, 110, 1672. (24) For numerous references to experimental work on three-electron hemibonded systems also see: Gill, P. M. W.; Radom, L. J . Am. Chem. SOC. 1988, 110, 4931. (25) Gill, P. M. W.; Weatherall, P.; Radom, L. J . Am. Chem. SOC.1989, 111, 2782. (26) Illies, A. J.; McKee, M. L.; Schlegel, H. B. J . Phys. Chem. 1987, 91, 3489. (27) McKee, M. L. Chem. Phys. Letf. 1990, 165, 265. (28) McKee, M. L. J . Phys. Chem. 1990, 94, 8553. (29) McKee, M. L. Chem. Phys. Left. 1991, 179, 559.
0022-3654/92/2096-1675$03.00/00 1992 American Chemical Society
McKee
1676 The Journal of Physical Chemistry, Vol. 96, No. 4, I992 TABLE 11: Total Energies (hartrees) of HS(CH,),,SH+ (n = 1-4) Radicals sYm state HF/6-3 lG* -834.862 23 HS( CH2)SH+ 1 c 2 v 2B, 2B -834.897 63 2 c 2 2Aff -834.896 21 3 C S HS (CH2)2SH+ 4 2AU -873.908 64 5 c 2 2B -873.943 64 2B -873.945 91 6 c 2 2Aff -873.941 07 7 C S HS(CH2)$H+ 8 c2v 2A2 -9 12.944 10 9 c 2 2B -9 12.98904 2B -9 12.989 27 10 c 2 2Aff -91 2.995 28 11 C S 2Afl -9 12.998 8 1 12 CS HS(CH2)4SH+ 13 c2h 2AU -95 1.979 80 14 c 2 2B -952.033 99 2B -952.034 30 15 c 2 2Aff -952.024 26 16 C S 2Aff -952.025 12 17 CS
" Zero-point correction in kcal/mol 4 H
I
+
PMP2/6-3 lG* -835.24005 -835.265 14 -835.263 82 -874.41 9 29 -874.440 89 -874.443 46 -874.436 9 1 -913.58694 -913.61829 -913.618 83 -913.624 14 -913.628 17 -952.753 54 -952.793 54 -952.794 41 -952.783 90 -952.785 07
PMP4/6-3 lG* -835.29449 -835.31943 -835.318 25 -874.488 25 -874.5 10 97 -875.51360 -874.507 49 -9 13.67 1 65 -9 13.70365 -9 13.70380 -91 3.709 43 -913.713 37 -953.853 58 -953.89446 -953.894 82 -953.884 91 -953.886 1 1
ZPE (NIF)" 29.84 (1) 31.97 (0) 31.84 (0) 50.26 (1) 5 1 S O (0) 51.69 (0) 51.40 (1) 69.38 (1) 70.93 (1) 71.11 (0) 70.79 (0) 70.97 (0) 88.65 (0) 90.41 (0) 90.48 (0) 90.15 (1) 90.18 (1)
and number of imaginary frequencies in parentheses.
lo*
*@BQ
/I
Figure 1. Interaction diagram for the formation of an intramolecular 2u-1 u* 2c-3e bond between two sulfur atoms. A photon of wavelength ,,A will excite an electron from the 20 orbital to the la* orbital.
-
band ,A, corresponds to the ts ts* excitation (Figure 1). They also found that the strength of intramolecular 2c-3e bonds is sensitive to details of g e ~ m e t r y . ~If? ~a bicyclic five-membered dithiol is oxidized (a dimethylene bridge and a monomethylene bridge), the sulfur p orbitals cannot achieve good overlap and consequently A, is at long ~ a v e l e n g t h . ~However, ?~ adding two methyl groups to the one methylene bridge allows the two sulfur atoms to approach each other and A,, decreases. When A, is measured for a series of acyclic dithiols, absorbance maximum comes at smaller wavelength as the length of the methylene chain linking the two sulfur atoms increases (Table I). There is a large drop in A, in going from a dimethylene chain to a trimethylene chain but a slight increase in A, in going from a tri- to tetramethylene chain length.6*7J4The larger strength of the intramolecular 2c-3e bond with a trimethylene chain is associated with the greater stability of a five-membered ring.6J0 A A, of 570 nm has been measured for a dithiol charged complex with one methylene bridge, indicating a weaker 2c-3e S:.S bond (Table 1). Method All calculations have been made by using the GAUSSIAN 88 program system.30 Geometries have been optimized at the UHF/6-3 lG* level,31and single-point calculations were made at the PMP4 level of electron correlation (frozen-core approximation) with the 6-31G* basis set. The "P" indicates that the effect of spin contamination has been projected out of the MP electron correlation energy.32 Vibrational frequencies have been calculated to characterize the nature of the stationary points and to make (30) GAUSSIAN 88: Frisch, M. J.; Head-Gordon, M.; Schlegel, H. B.; Raghavachari, K.; Binkley, J. S.; Gonzales, C.; DeFrees, D. J.; Fox, D. J.; Whiteside, R. A.; Seeger, R.; Melius, C. F.; Baker, J.; Martin, R. L.; Kahn, L. R.; Stewart, J. J. P.; Fluder, E. M.; Topiol, S.; Pople, J. A. Gaussian, Inc., Pittsburgh, PA. (31) For a description of basis sets see: Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; Wiley: New York, 1986. (32) Sosa, C.; Schlegel, H. B. Znt. J . Quantum Chem. 1986, 29, 1001. Schlegel, H. B. J . Chem. Phys. 1986,84, 4530.
TABLE 111: Relative Energies (kcal/mol) of HS(CH,),,SH+ (n = 1-4) Species HF/ PMP2/ PMP4/ 6-31G* 6-31G* 6-31G* +ZPC" 0.0 0.0 0.0 HS(CH2)SH+ 1 0.0 -I 3.7 -15.7 -15.6 2 -22.2 -13.1 -14.9 -14.9 3 -21.3 0.0 0.0 0.0 HS(CH2)2SH+ 4 0.0 -13.1 -22.0 -13.6 -14.2 5 -14.6 -15.2 -15.9 6 -23.4 -1 1 . 1 -11.0 -12.1 7 -20.3 0.0 0.0 0.0 HS(CH2)3SH+ 8 0.0 -18.7 -19.7 -20.1 9 -28.2 -18.6 -20.0 -20.2 10 -28.3 -22.4 -23.3 -23.7 11 -32.1 -24.8 -25.9 -26.2 12 -34.3 0.0 0.0 0.0 HS(CH2)4SH+ 13 0.0 -24.0 -25.1 -25.6 14 -34.0 -24.3 -25.6 -25.9 15 -34.2 -18.2 -19.0 -19.6 16 -27.9 -19.8 -20.4 -19.0 17 -28.4
" The most stable intramolecular association is given in italics. zero-point corrections (scaled by 0.9). Absolute energies and zero-point energies are given in Table I1 while geometries are given in Figure 2. Relative energies in kcal/mol are given with respect to each chain length in Table 111. Unless otherwise indicated, the energy differences used in the discussion below will be at the PMP4/6-3 lG*//6-31G*+ZPC level of theory. Results and Discussion The intramolecular 2c-3e interactions were studied in oxidized linear dithiols where the two sulfur centers are separated by a chain of 1-4 methylenes. In each system, the reference was an all-trans configuration in which the sulfur atoms are as far apart as possible, with the doubly occupied p orbital on one sulfur atom parallel to the singly occupied p orbital on the other sulfur atom (1, 4, 8, 13). For each complex two structures were considered: a C, symmetry complex, defined by a plane containing the S--S midpoint and either the central CH2 group or the C-C midpoint and, and a C2 symmetry complex, defined by an axis containing the S--S midpoint and either the central carbon or the C-C midpoint. Starting with HS(CH,)2SH+, more than one conformation of the ring was considered in each symmetry point group. It is expected that the interconversion of the two conformations within the same point group will have a relatively low activation barrier. However, conversion between a C, and a C, symmetry structure will require rotation of a S H group around a S-C bond, which will break the 2 e 3 e bond. Therefore, the activation barrier for this process will be approximately the energy of the 2 e 3 e bond. Since the activation barrier for forming the cyclic charged complex from the linear charged complex is expected to be small, the
Intramolecular Bonding in HS(CH,),,SH+
+
+
+
r
7
The Journal of Physical Chemistry, Vol. 96, No. 4, 1992 1677
r
+
r
G I-
,-
t
r
G
+
r
6
+
e .
0"
1678 The Journal of Physical Chemistry, Vol. 96,No. 4, 1992 TABLE IV: Selected Properties of Intramolecular S:.S 2c-3e Bonds BDE" 4%POPb I E' HS(CH,)SH'
1
2 3 HS(CH&SH'
4
5 6 7
HS(CH2)$H*
13.1 14.6 11.1
8 9 10 11 12
HS(CHz)$H'
13.7 13.1
18.7 18.6 22.4 24.8
13 14 15
16 17
24.0 24.3 18.2 19.0
-0.065 -0.022 4.023 0.002 0.059 0.076 0.063 0.000 0.033 0.030 0.036 0.042 0.000 0.019 0.015 0.013 0.012
17.19 16.81 16.85 16.90 15.94 16.00 15.94 16.30 15.02 15.13 15.02 15.07 16.00 14.86 14.86 14.91 14.88
McKee
s-- S d
HSS'
2.705 2.715
91.0 93.6
2.739 2.746 2.724
97.0 90.5 94.8
2.759 2.759 2.804 2.806
92.1 96.3 96.3 94.3
2.795 2.772 2.8 12 2.806
89.9 93.6 97.6 97.2
spin'
qSH8
(s2)h
0.563 0.720 0.7 19 0.635 0.776 0.762 0.772 0.6 IO 0.8 13
0.55 0.52 0.52 0.50 0.45 0.45 0.45 0.47 0.42 0.42 0.41 0.41 0.44 0.39 0.39 0.40 0.40
0.76 0.77 0.77 0.77 0.78 0.78 0.78 0.77 0.77 0.78 0.78 0.77 0.76 0.77 0.77 0.77 0.77
0.810 0.803 0.796 0.605 0.813 0.829 0.8 19 0.818
" Bond dissociation energy
in kcal/mol (negative of relative energy in Table HI) at the PMP4/6-31Gt//6-31G*+ZPC level. bTotal overlap population between the two sulfurs. Ionization potential in electronvolts. Orbital energy of highest occupied a electron. "Sulfur-sulfur distance in angstroms. 'HSS bond angle in degrees. funpaired spin density on sulfur Mullikencharge-on S H unit. Unprojected spin-squared value at the UHF/6-3 IG* level.
relative concentrations of the complexes (trans orientation of the
SH groups in C2symmetry to the cis orientation in C,symmetry) will be controlled by their relative stabilities. The maxima in the UV/vis spectra for acyclic systems with 1-4 methylene bridges are 570,525,440, and 450 nm, respectively (Table I).6J4 If we can assume that the order of 2c-3e bond energies varies inversely with A,, then the bond energies of the monomethylene bridged system should be slightly weaker than the dimethylene bridged system and both should be weaker than the tri- and tetramethylene bridged systems. In addition, the trimethylene bridged system should be slightly stronger than the tetramethylene bridged system. These prediction are borne out perfectly by the theoretical calculations. If the structures with the strongest 2c-3e bond are compared, the order is 2 < 6 < 12 > 15 (13.7, 14.6, 24.8, 24.3 kcal/mol, Table 111). In agreement with A,, values, 2 and 6 are close in energy and 12 and 15 are close in energy. It is very interesting that in the series HS(CH,),$H+ (n = 1-3), as n increases, the S--S 2c-3e bond length actually increases (2.705, 2.746, 2.806 A; Figure 1). Two characteristics are measured in this trend, the change in the 2c-3e intrinsic bond energy (which probably changes very little) and the change in strain energy. For the monomethylene bridge (n = l), the S- -S bond is probably compressed in order for the p orbital overlap to be maximized. The S- -S bond in (CHJ2S- -S(CH3),+ was calculated to be 2.797 A (3-21G* level), and the bond energy was estimated to be 25.5 kcal/mol (at the highest composite level) which is longer and stronger than the first two intramolecular 2c-3e S--S bonds, HS(CH2)SH+and HS(CH,),SH+, calculated here. For bridge lengths of n = 1,2, and 4 methylenes, the symmetry of the lowest energy structure is C2 symmetry, which has the obvious advantage of allowing the lone pairs on sulfur to be trans to each other. The lowest-energy structure of the HS(CHJ3SH+ complex (12) has C, symmetry, and interestingly, it also has the strongest 2c-3e S:.S bond. The complex has an envelope conformation (flap angle of 128.1') with the methylene groups staggered and hydrogens on the a carbons directed away from the sulfur lone pairs.33 In the HS(CH2)$H+ complex, the two C,-symmetry conformations (14, 15) are separated by only 0.3 kcal/mol. The two related a-methylene groups of structure 15, the lowest-energy species, are considerably out of the approximate plane containing the two sulfur atoms and the two /3-methylenes (center carbons). In contrast, the sulfur atoms and all four carbons of structure 14 (33) The five-membered ring is known to be particularly stable. See refs 6 and 10.
deviate less from planarity (Figure 2). In all charged complexes with a 2 e 3 e S:.S bond, the S-C bond lengthens considerably (0.0164.040 A) compared to the reference structure where a 2c-3e interaction is precluded. The C-C bond length changes very little on 2c-3e bond formation. In an effort to find an indicator of 2c-3e bond strength, several properties of the structures studied are tabulated in Table IV. The bond dissociation energy (BDE) is calculated at the PMP4/631G*//6-31GS+ZPC level while the other properties (qss(overlap population between the two sulfur atoms), ionization energy, S--S bond length, and HSS bond angle) are all calculated at the UHF/6-31G* level. There is no one property that correlates with the BDE for all structures. The best indicator of 2c-3e bond strength within a given complex is qsswhich gives the Mulliken population summed over all orbitals of each sulfur. The species with the largest value of 4%within each group also has the largest bond strength except for HS(CH2)$H+ where 14 has the largest qssvalue but 15 has the largest 2c-3e bond energy (Table IV). For 1, the 4%value is negative, indicating a depletion of electrons in the bonding region between the two sulfur atoms. While the two charged complexes (2,3)still have negative values of qss,both are less negative than the reference 1. In each of the four acyclic systems studied, the IE of the reference structure is higher than that of the complex with a 2c-3e S:.S bond. This is consistent with the destabilization of the u* orbital by interaction of the two sulfur p orbitals. Also, the trend in the PIE (0.38,0.90, 1.23, 1.14 eV) with increasing methylene chain length (Le. (CH,),, n = 1 < 2 < 3 > 4) is also consistent with the calculated 2c-3e bond energies. However, there is inconsistent variation in AIE and 2c-3e bond energies of complexes within a given methylene chain length. The S- -S bond length varies from 2.705 to 2.806 A, but as pointed out above, it is not a good indicator of 2c-3e bond strength. Finally, the HSS bond angle is given because a value close to 90' should indicate a near optimal overlap of the two p orbitals on sulfur. The correspondence between a small value of the HSS angle and a strong 2c-3e S;.S bond is quite good. Within each chain length group, the complex with the smallest HSS angle also has the strongest 2c-3e bond except for the tetramethylene chain, where 14 has the smallest HSS angle but has a 2c-3e bond 0.3 kcal/mol weaker than 15. The spin density on sulfur and the accumulated Mulliken charge on a SH unit is also given in Table IV. As expected, the majority of spin density is distributed equally between the two sulfur atoms. Also, the positive charge is carried, to a large extent, on the two SH units. The last column gives the spin-squared value for the complex. All values are close to the expected value of 0.75 for a doublet which indicates that the UHF calculations presented here are probably reliable.
J . Phys. Chem. 1992, 96, 1679-1683
Conclusions The 2c-3e S:.S bond strength in charged acyclic dithiols increases as the length of the methylene linkage increases. An exception is the 2c-3e bond in the five-membered ring with a trimethylene linkage which is more stable than the six-membered ring with a tetramethylene linkage due to reduced nonbonded repulsion. Except for the five-membered ring, all charged complexes prefer a C2-symmetrystructure. The theoretical calculations of 2c-3e bond strengths agree with experimental measurements of the absorbance maximum A, in the optical spectrum. For the series HS(CH,),,SH+ (n = 1-4), the 2c-3e bond strengths
1679
are predicted to be 13.1, 14.6, 24.8, and 24.3 kcal/mol, respectively. Acknowledgment. We thank the donors of the Petroleum Research Fund, administered by the American Chemical Society, for financial support. Computer time for this study was made available by the Alabama Supercomputer Network and the NSF-supported Pittsburgh Supercomputer Center. Dr. K.-D. Asmus is acknowledged for helpful conversations. Registry No. 1, 138407-13-3; 4, 95879-99-5; 8, 138407-14-4; 13, 138407-15-5,
Theoretical Study of Silicon Substitution in Borane and Carborane Cages Michael L. McKee Department of Chemistry, Auburn University, Auburn, Alabama 36849 (Received: September 5, 1991; In Final Form: October 21, 1991)
Silicon substitution into several borane and carborane cages has been studied with ab initio calculations. Examples of cages in which silicon contributes one, two, or three electrons to cage bonding have been considered. Silicon contributes one electron in the silaborane, SiH3-B5H8, and calculations correctly predict the order of stability: l-(SiH3)BsH8,2-(SiH3)BSHB,and p-(SiH3)B5H8. Closo octahedral cages are considered where silicon contributes either 2e- or 3e- to cage bonding. The silicon cage appears to be more stable when silicon contributes 2e- to cage bonding as opposed to 3e-. The most stable structure of the silacarborane, 1-(SiH2)-2,3-C2B4H6,is predicted to have two terminal Si-H bonds in the symmetry plane. Finally, heats of formation of several silaboranes and silacarboranes have been estimated from calculated heats of reaction.
Introduction While carbon substitution into a borane cage is common and gives rise to an entire class of compounds (carboranes), silicon substitution is rather rare.l There is only one published example2 (to the author’s knowledge) of a silaborane which is directly related to a carborane by substitution of silicon for carbon. Kinetic or electronic factors may prevent the formation of directly analogous compounds. More often silicon donates one or two electrons to cage b ~ n d i n grather ~ . ~ than three electrons as is the case for carbon. An example of one-electron donation is SiMe3-B5H8, where a trimethylsilyl group replaces a terminal, basal, or bridging hydrogen. In a series of elegant experiments, Gaines and cow o r k e r ~ have ~ , ~ shown that p(SiMe3)B5H8rearranges in the presence of a Lewis base by a first-order reaction to give 2(SiMe3)BSH8. By labeling the apical position with deuterium, they were able to show that the rearrangement did not exchange basal and apical hydrogens. At higher temperature, a second pathway exists which gives 1-(SiMe3)BSH8as well as exchanging basal and apical hydrogens. Isotopic labeling experiments on BSH9 provide strong evidence that the process involves- cage rear(1) (a) Grimes, R. N. Reo. Silicon, Germanium, Tin Lead Compd. 1977, 2,223. (b) Grimes, R. N. In Carboranes;Academic Press: New York, 1970. (c) Grimes, R. N. In Comprehensive OrganometaNic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Us.Pergamon ; Press: Oxford, U.K., 1982; Vol. 1, and references therein. (d) Grimes, R. N., Ed.Metal Interactions with Boron Clusters; Plenum: New York, 1982. (e) Hosmane, N. S.; Maguire, J. A. In Advances in Boron and the Boranes; Liebman, J. F., Greenberg, A., Williams, R. E., Eds.; VCH: New York, 1988; pp 297-328. (f) Hosmane, N. S.; Maguire, J. A. Adv. Organomef. Chem. 1990, 30, 99. (2) The silaborane (SiMe)2B,oHIois isostructural to o-C2Bl0H,,: Seyferth, D.; Bilchncr, K.; Rees, W. S., Jr.; Davis, W. M. Angew. Chem., Itit. Ed. Engl. 1990, 29, 918. (3) Hosmane, N. S.; de Meester, P.; Siriwardane, U.; Islam, M. S.; Chu, S. C. J . Chem. SOC.,Chem. Commun. 1986, 1421. (4) Experimental work refers to the C-diethyl derivative: Siriwardane, U.; Islam, M. S.; West, T. A.; Hosmane, N. S.; Maguire, J. A.; Cowley, A. H. J . Am. Chem. SOC.1987, 109, 4600. (5) (a) Gaines, D. F.; Iorns, T.V. J . Am. Chem. Soc. 1968, 90, 6617. (b) Heppert, J. A.; Gaines, D. F. Inorg. Chem. 1983, 22, 3155. (6) Gaines, D. F.; Coons, D. E.; Heppert, J. A. In Advances in Boron and rhe Boranes: Liebman, J. F., Greenberg, A,, Williams, R. E., Eds.;VCH: New York, 1988; pp 91-104.
rangement rather than substituent migration.6 Several cages with silicon substituted at a vertex have been synthesized by Hosmane and co-~orkers’~-~ by reacting a derivative of nido-2,3-C2B4H8, [2,3-(SiMe3)2C2B4H4] 2-, with various substrates. Reaction with silicon tetrachloride gave a closo pentagonal bipyramid which was characterized by its N M R and mass ~ p e c t r a . ~The structure proposed was isostructural with 2,3C2BSH7,a known carborane? In order to maintain a cage electron count of 16e- as predicted by Wade’s rules8 for a closo cage, silicon must donate 2e- which would leave two electrons as an exocyclically directed lone pair. In the same reaction a second silicon-containing cage was isolated and the structure determined ~rystallographically.~~~ In this structure, the silicon atom of the pentagonal bipyramid is capped by a second nido-C2B4Hs2- derivative which allows the silicon atom to share 2e- with each moiety. A related silicon-containing pentagonal bipyramid is obtained when a derivative of [2,3-C2B4H6I2-reacts with SiH2C12.4 The structure, a derivative of 1-SiH2-2,3-CzB4H6,is consistent with one hydrogen strongly bound to silicon and one weakly bound, perhaps a bridging interaction. It was pointed out that if both hydrogens are terminal, then silicon would be in the +4 oxidation state, while one terminal hydrogen and one bridging hydrogen would leave silicon in a +2 state and give a cage electron count appropriate for a nido c o m p o ~ n d . I ~Unfortunately, *~ suitable crystals were not obtained for an X-ray structure determination. In a reaction of [2,3-(SiMe3)2C2B4H4]2-with a phosphine, 2,4,6-(t-Bu),C6H2pCl2, a closo-phosphacarborane complex is formed with a terminal phenyl group on phosphor~s.~For a seven-vertex closo cage, application of Wade’s rules would give a predicted cage count of 16 electrons. Donation of 2 cage (7) Experimental work refers to the C-methyl, C-trimethylsilyl derivative: (a) Beck, J. S.; Sneddon, L. G. Inorg. Chem. 1990,29,295. (b) Beck, J. S.; Kahn, A. P.; Sneddon, L. G. Organometallics 1986, 5, 2552. (8) (a) Wade, K. Ado. Inorg. Chem. Radiochem. 1976,18, 1. (b) Mingos, D. M. P.; Wales, D. J. Introducrion to Cluster Chemistry; Prentice Hall: Englewood Cliffs, NJ, 1990. (9) Hosmane, N. S.; Lu, K.-J.; Cowley, A. H.; Mardones, M . A. Inorg. Chem. 1991, 30, 1325.
0022-3654/92/ 2096- 1679$03.OO/O 0 1992 American Chemical Society
