1363
THEMOLECULAR STRUCTURE O F Si2Bli'7 to be less prone to aggregate when introduced in this form. The S3- and SSO- radicals are free to rotate above 77°K and do not show any apparent line broadening due to interaction with the four sodium nuclei which ought to be in the cage. These facts, coupled with Jaeger's observations that silicon rich ultramarines have a deeper color, suggest that the A1:Si ratio may have considerable bearing on the types of radicals formed. If the cage can be devoid of sodium atoms, then it might be possible to obtain an F center which would not show the usual 13-line epr spectrum due to the four adjacent sodium nuclei. Indeed the presence of atomic hydrogen, apparently in a sodalite cage not
containing sodium, will be reported in a related publication. Further work is planned using samples of known Al: Si ratio and single crystals as soon as they become available.
Acknozoledgments. We would like to thank H. W. Evans for heat treating the samples and for assisting in the epr investigations. Thanks are also due to P. A. Forrester and 14.J. Taylor for interesting discussions and R. J. Heritage for analyzing the samples. This paper is published with the permission of the Controller Her Majesty's Stationery Office.
The Molecular Structure of Perfluoroborodisilane, Si,BF,, as Determined by Electron Diffraction
by C. H. Chang, R. F. Porter, and S. H. Bauer Department of Chemiatry, Cornell University, Ithaca, New Y O T ~14850 (Received August 16, 1969)
An electron diffraction structure analysis of SizBF7 in the gas phase shows that the connectivity in the molecule is gaSi-SiFz-BFz,confirming conzlusions derived from nmr spsctra. Bond lengths are (Si-B) =.2.008 i 0.017 A, (Si-Si) = 2.361 f 0.012 A, (B-F) = 1.309 & 0,009 A, and (Si-F)ave= 1.575 f 0.002 A. The bond angles are LSiSiB = 125.0 f 2.9", LSiBF = 120.6 =t1.3') LBSiF = 109.1 f 2.4', LSiSiF(center) = 1.0'. The most populous conformations are those in 102.9' f 1.7", and LSiSiF(termina1) = 109.5 which the terminal -SiFa and -BFz are staggered with respect to the centeral -SiFz. By assuming a potential function of the form V = l/zV0(l - cos 30), a barrier height of 2.35 kcal/mol for rotation about the Si-Si bond was estimated, using Karle's method.
Introduction Aside from the high-temperature silicon borides, l t 2 only a few compounds containing silicon-boron bonds have been reported and for the latter no structural data are available. A series of compounds BSi,F2,+3 with n ranging from 2 to 13, mere prepared by Timms, et ~ l . by , ~ low temperature acid-base reactions between S i r z and BF3.4 From nmr, ir, and mass spectral data, they concluded that these compounds should be classified as a homologous series SiF3-(SiF2),-1-BF2. Although SiBF5 was later synthesized by Timmss in the reaction between B F and SiFe, the yield was very poor. The simplest member of this series available in sufficient quantity is Si2BF7. This report is on the molecular structure of SiZBF7 as determined by electron diffraction in the gas phase. Experimental Section Samples of SizBF7 which contained small amounts of
silicon oxyfluoride, (Si20F6bp = -23.3") as detected from mass spectra, were provided by Thompson and Margrave.e These were purified by distilling off the impurity at - 18". An ampoule containing the liquid sample Si2BF, was connected directly to the nozzle lead tube of the electron diffraction apparatus. The amount of vapor injected into the apparatus was controlled with a Teflon needle valve. Where needed Kel-F grease was used on glass joints and stopcocks in the inlet system. Sectored electron diffraction photographs were taken with the sample at room temperature. The chamber (1) C. F. Cline, J . Electrochem. Soc., 106, 322 (1959). (2) V. I. Matkovich, Acta Cryst., 13, 679 (1960). (3) P. L. Timms, et al., J. Amer. Chem. Soc., 87, 3819 (1965). (4) P. L. Timms, R. A . Kent, T. C. Ehlert, and J. L. IMargrave, ibid., 87, 2824 (1965). (5) P. L. Timms, Chem. Eng. News, 44, (39), 50 (1966). (6) J. C. Thompson and J. L. Margrave, private communication.
Volume 74, Number 6 March 19, 1070
1364
C. H. CHANG, R. F. PORTER, A N D S. H. BAUER
pressure when exposures were made remained at 6 X Torr. The beam was 3.1 X A, and accelerating voltage 65 kV; exposure times were 90 sec for the long nozzle-to-plate distance (25.6 cm) and 150 sec for plates at the short distance (12.4 cm). A thin layer of a light yellowish deposit was found on the plates after they were developed. This was probably due to an attack of the emulsion surface by the Si2BF7,which degassed from the cryopump before the apparatus was vented. The plates were cleaned with a dilute HC1 solution (about 0.5 N ) . The methods for measuring plates and reducing data have been adequately described.'-9
Analysis of the Diffraction Data The experimental intensities are listed in Table I. They are sketched in Figure 1 along with background curves for two ranges of diffraction angles spanning q = 7 to 120 A-1; q= (40/X) sin8/2. Therefined experimental radial distribution (RDR) curve, shown in Figure 2, was calculated with a damping factor of y = 0.00162. Also shown in this figure is the difference curve between the experimental and theoretical RDR curves for two models. Four bonded distances may be distinctly resolved in the RDR curve: B-F at 1.309, (SiF),, at 1.575, Si-B at 2.008, and Si-Si at 2.3618. Three neighboring F. .F distances appear in the next peak: F6. eF7 = 2.253, F4.- .F5= 2.0520, and F 8 . a .F9 = 2.572 8. The peak centered at 3.22 A can be resolved into Si3.. .F4 = 3.117 8 and Si1. .F8 = 3.246 8. The inside shoulder at 2.94 consists of Si'. .F6 = 2.902 and B 2 . .F4 = 2.930 8. Longer nonbonded distances are also indicated. Refer to Figure 3 for atom designation. Twelve geometrical parameters were introduced to specify the structure of the molecule. They are five bonded distances and seven angles (see Figure 3) ; the latter are L Si-Si-B, L Si-B-F, L Si-Si-F*, L Si-Si-F4, L B-Si-F4, and the two torsional angles for the terminal -BF6 and -SiF8 relative to the middle SiF4, (Y and P, respectively. The final structure was obtained by a least-squares analysis of the q M ( q ) curve. In these least-squares calculations, all the geometrical parameters were varied, but only bonded and the shorter nonbonded atom a
,
1
1
1
1
1
1
1
1
1
1
7
9 1 1
Figure 1. The experimental intensity and background curves. The Journal of Physical Chemistry
,
Table I: Intensity Data -HV+
7--HVS---
-HVS---
Q
Intensity
Q
Intensity
Q
Intensity
7
0.6419 0.5043 0.3955 0,3361 0.3277 0.3430 0.3936 0.4362 0.4762 0.4943 0.5101 0.5224 0,5288 0.5231 0.5035 0.4908 0.4958 0.5325 0.5960 0.6633 0.7083 0.7257 0.7233 0,7214 0.7207 0.7246 0.7232 0.7176 0.7121 0.7191 0.7420 0.7704 0.7992 0.8242 0.8399 0.8486 0.8453 0.8321 0.8178 0.8020 0.7964 0.8023 0.8281 0.8552 0.8739 0.8904 0.9058
36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56
0.2264 0.2269 0.2298 0.2354 0.2385 0.2402 0.2397 0.2378 0.2325 0.2287 0.2244 0,2212 0.2213 0.2250 0.2311 0.2393 0.2437 0.2464 0,2465 0.2458 0.2434 0.2394 0.2348 0.2314 0.2320 0,2358 0,2406 0.2470 0.2521 0.2546 0.2556 0.2552 0.2553 0.2548 0.2538 0.2520 0.2511 0.2515 0.2530 0.2560 0.2693 0,2641 0.2679 0,2704 0.2713 0.2706 0.2708
83 84 85 86 87 88
0,2704 0.2708 0.2716 0.2728 0.2757 0.2791 0.2837 0.2872 0.2905 0.2932 0.2958 0.2969 0.2972 0.2975 0,2994 0.3005 0.3023 0,3054 0.3088 0,3134 0.3168 0.3208 0.3242 0.3272 0,3292 0.3309 0.3325 0.3356 0.3383 0.3411 0.3459 0.3503 0.3553 0.3580 0,3620 0,3645 0.3664 0.3708
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
51 52 53
57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82
89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120
HVL HVS
2 5 . 6 cm 12.4 cm
Sample-lo-plate distance
pair Z t j (root-mean-square amplitudes of vibration) were allowed to vary. As is well known, the intensity patterns are relatively insensitive t o the Lt3 for the longer nonbonded atom pairs.* The mean aomplitudes for nonbonded distances larger than 4.20 A were either estimated from the RDR curve or averaged from preliminary runs and constrained in the final analysis. (7) J. L. Henoher and S. H. Bauer, J . Amer. Chem. SOC., 8 9 , 5527 (1967). (8) R. L. Hilderbrandt and 5 . H. Bauer, J . Mol. Strztct., 3, 325 (1969). (9) W. Harshbarger, et al., Inorg. Chem., 8 , 1683 (1969).
1365
THEMOLECULAR STRUCTURE OF SizBF?
-
I -
n --
DI -Model v
-
D 2 - Model for estimating Rotational Barrier I
I
I
I
1
I
I
I
I
I
I
I
I
I
I
Figure 2. The refined experimental radial distribution curve and difference curve between the experimental and theoretical values for two-models.
culated for the best least-squares model is shown below it. The estimation of error limits for bond distances and angles as derived from electron diffraction data has been discussed in the previous p ~ b l i c a t i o n s . ~To ~ ~allow for possible but not probable distortion in plate dimensions and intensity which were not fully compensated by the caIibration steps, due to washing of the plates in dilute HCI solution, the estimated error limits were set
A :
!
~ _ _ _ _ _ _ _
\
~
\ \
I. 13
I
I
~~
A (Si-FP=Si-Fa)
Figure 3. Structure of SizBFT. I
~~
(Standard Deviations)a
\
'F10 I
~~
Table I1 : Geometrical and Thermal Parameters
I
I
I
B
2.008 (0.0042) 2.361 (0.0029) 1.309 (0.0022) 1.575(0.0005) 1.575 (0.0005) 125.0 (0.72) 120.6 (0.32) 109.5 (0.26) 109.1 (0.60) 102.9 (0.42) 51.9 (8.53) 43.6 (1.81) 0.055 (0.0019) 0.048 (0.0003) 0.053 (0.0041) 0.065 (0.0014)
I
. d - - . 1 20 33 45 50 $0 70 813 -3
2.007 (0.0044) 2.362 (0.0030) 1.309 (0.0022) 1.581 (0.0171) 1.570(0.0119) 125.2 (0.74) 120.6 (0.33) 109.6 (0.46) 109.0 (0.85) 102.6 (0.75) 52.5 (8.11) 43.3 (1.78) 0,055 (0.0020) 0.048 (0.0024) 0 053 (0.0042) 0.065 (0.0013)
OCi-'i
Figure 4. The reduced experimental molecular scattering curve and that calculated for the best least-squares model. The dotted line is the difference curve.
Converged geometrical parameters and li, are listed in Table 11. The correlation matrix indicates that for the listed parameters which were varied the correlations are small. Figure 4 shows the reduced experimental molecular scattering curve for Si2BF7,and that cal-
@
Distances in
A;
0.083 (0.0027)
0.087 (0.0022)
0.103 (0.0022)
0.103 (0.0022)
0.097 (0.0026)
0.096
0.171 (0.0060)
0.165 (0.0060)
angles in degrees. ~~~
~~
Volume 74, Number 6 March 19, 1970
1366 at four times the standard deviations calculated by the least-squares analysis. Karle's methodlo for separating the effect of torsional motion from the skeletal vibrations was used to estimate the rotational barrier height for the terminal -SiF3 group. In applying the method, the ltl for the fluorine atom pairs, F4. -F8,F4. .F9, and F4. -FlO,should be determined as accurately as possible. Since the complete least-square analysis is relatively insensitive to these variables, a follow-up analysis was run in which all the parameters listed in Table I1 (Model A) except those which depend on the two rotational angles were constrained to the magnitudes listed in the table. The result is given in Table I11 (equilib9
I
I
I
I
3
4
5
6
7
P N
S CI
-
2.3
2.I
-
a
I
n OQ
1.9
W
L 1.7
1.5
Table I11 : Equilibrium Distances, Angles, and Root-Mean-Square Amplitudes"
1.575 4.759 4.367 3.859 3.344 3.570 3.817 4.544
0.099 i 0.0017 0.129 i:O.O18 0.105 f 0 . 0 0 6 0.178 f 0.020 0.129=tOo.018 O.137=tO0.061 0.201=!.=0.077 0.143 f 0 . 0 3 9
132.0 -88.0 32.0 -74.2 105.8
Y = AI Si X=B C
0 F
Figure 6. Bond distances of Si-X ( X = B, C, N, 0, and F) and B-Y (Y = Si, P, S, and Cl), atomic pairs.
Discussion
The structure deduced in this electron diffraction study confirms the conclusions based on nmr data' that SizBF7 has the molecular framework F8Si-SiF2-BF2. The Si-B bond length of 2.008 f 0.016 A is close t o the rium angles and distances). In terms of Karle's nota1.96A predicted from the Pauling-Schomaker-Stevention, we set p = (C1 - Cz cos O)'/' [rather than (Cl son empirical equationll cZCOS c1= 15.048 cz= 4.559 T = ~ W K , A = 1.0,B = 0.0,n = 3,u s/3,pe = 132.0",-88.0", R ( A - B ) = ?"a f l'b - C l X a - Xbl 32.0". For solution of the simultaneous equations, with ?"si = 1.17,YB = 0.81 x s i 1.8,XB = 2.0,and refer to Figure 5, which indicates that V o = 2.35kcal/ C = 0.08B. As shown in Figure 6, the bond distances mol. Si-X (X = B, C, N, 0,and F) and B-Y (Y = Si, P, S,and ' . C1) correlate well with the atomic numbers of X and 1 I I I I I I I I I From the figure one may predict t i e previously unreported bond length for B-A1 as 2.11 A. The Si-Si bond length of 2.361 f 0.012b is comparable to published valuesoderived from X-ray dicraction studies : Si-Si = 2.352A in metallic Si,122.36A in (CH3)8Si808,132.34 f 0.10 A in (CH3)a Si-Si (CH3)3,14and 2.35A in [(CHP)IS ~ Z O ] ZIt. ~is~longer than the magnitudes reported for H3Si-SiHa,l6 2.32 f 0.03 A, and 2.294 f 0.05b in a
a: =
-74.2 f 6.5"; /3 = 32.0 i 3.6".
+
A,
A,
A,
(10) (a) R . E. Knudsen, C. F. George, and J. Karle, J . Chem. Phys., 2334 (1966); (b) J. Karle, ibid.,45, 4149 (1966). (11) L. Pauling, "The Nature of the Chemical Bond," 3rd ed, Cornel1 University Press, Ithaca, N. Y . , 1960, p 229. (12) M . E. Straumanic and E. 2. Aka, J . A p p l . Phys., 23, 330 (1952). (13) T . Higuchi and A . Shimada, Bull. Chem. SOC.J a p . , 39, 1316 (1966)(14) H. Murata and K. Shimuzu, J . Chem. Phys., 23, 1968 (1955). (15) T. Takano, ii.Kasai, and M . Kakudo, Bull. Chem. SOC.Jap., 36, 585 (1963). 60, 1836 (16) L. 0. Brockway and J. Y . Beach, J . Amer. Chem. SOC., (1938).
44, 2.0
-
I
1
I
I
I
I
I
flij Figure 5. Graphical solution of the rotational barrier height for the terminal SiF8 group. The Journal of Physical Chemistry
I
THEMOLECULAR STRUCTURE O F SizBF7
1367
C13Si-SiC1317 based on electron diffraction measuredistance 1.581 f 0.08 d in the -Sipz group is indeed ments; the differences are within the range of experilonger than the 1.570 f 0.04 8 in the -Sip group, the mental errors. high correlation and magnitude of uncertainties force The B-F bond distances in RFa has been carefully dethe conclusion that these Si-F bond lengths are too termined by Kuchitsu, Konaka, and their c o w o r k e r ~ . ~ * J ~close to distinguish with the present data. An average The length 1.309 f 0.009 in SizBF7 is in excellent value of 1.575 f 0.002 d obtained by setting the two agreement with their result, 1.3119 =!= 0.0008 d in BF3. values equal in the analyses, is more meaningful. It A brief list of various Si-F bond distances is tabulated agrees well with 1.572 f 0.006 8 in CH3SiF3and 1.577 in Table IV. It is'interesting to note that the Si-F f 0.001 d in SiHzFz. As stated earlier, the lij are insensitive parameters in electron diffraction determinations. For that reason, Table IV : Si-F Bond Distances we did not attempt to estimate the rotational barrier for the terminal -BFz group, particularly since Karle's Si-F bond method is strongly dependent on the assumed form for distance, A Compd the potential function. The least-squares refinement SiFa 1.55 ~ 0 , 0 2 5 showed that the equilibrium position of F6 and F7 are SiHF8 1.565 A 0.005b -74" and 106" away from BSiF4 plane, respectively. 1.577 f O.O0lc SiHzFz One may question the accuracy of the estimated rota1.592 & 0.002d SiH3F 1.572 f 0,006e tional barrier for the F3Si-SiFz bond. There are two CH3SiFs 1.583 f 0.002' CH3SiHF2 values in the literature for rotational barriers about 1.600 f 0.005u CH3SiHsF Si-Si bonds; in ClaSi-SiCls, 1.0 kcal/rn01,~7in H3Sia H . Braune and P. Pinnow, Z . Phys. Chem., B35, 239 (1937). SiHzF, 1.05 kcal/mol.20 The value for SizBF7appears * J. Sheridan and W. Gordy, J . Chem. Phys., 19,965 (1951). 0 V. to be too large by about n factor of 2, and this may W. Laurie, ibid., 26,1359 (1957). d B . Bak, J. Bruhn, and J. Rasbe a consequence of the difficulty in obtaining accurate trup-Anderson, ibid., 21, 752 (1953). e R. H. Schwendeman, amplitudes in SizBF7. Dissertation Abs., 18, 1645 (1958). J. D. Swalen and €4. P. Stoicheff, J. Chem. Phys., 28, 671 (1958). L. Pierce, ibid., Acknowledgments. We wish to thank J. C. Thompson 29,383 (1958). and J. L. Margrave for providing the sample. This work was supported by the Material Science Center, Cornell University (i\lISC ARPA SD-68) and the bond length is somewhat longer in those compounds Army Research Office, Durham. with the larger number of H atoms attached to the Si atom, parallel to the case for the analogous carbon series. We investigated the possibility that there may (17) Y . Morino and E. Hirota, J. Chem. Phys., 28, 185 (1958). be a significant difference between the Si-F bond dis(18) S. Konaka, Y. Murata, K. Kuchitsu, and Y . Morino, Bull. Chem. Soc. Jap., 39, 1134 (1966). tances in terminal SiF, and the center SiFz. The result (19) K. Kuchitsu and S. Konaka, J. Chem. Phys., 45, 4342 (1966). of a least-squares analysis in which the two Si-F bond (20) R. Varma and A . P. Cox in a personal communication from lengths were varied independently is shown in Table E. B. Wilson, Jr., to R. G. Parr, quoted in J. P. Lowe and R. G. I1 (B). Although there are indications that the Si-F Parr, J. Chem. Phys., 44,3001 (1966).
'
Volume 74, Number 6 March 10, 1970
