1478
JOSEPH
A. POTENZA AND WILLIAM N. LIPSCOMB
Inorganic Chemistry COSTRIBUTION FROM HARVARD UNIVERSITY,
THE
DEPARTMENT OF CHEMISTRY, h'IASSACHUSETTS 02138
CAMBRIDGE,
Molecular Structure of Carboranes. Molecular and Crystal Structure of o-Bl0Br,H7C2H2 BY JOSEPH A. POTENZA
ASD
WILLIAM N. LIPSCOMB
Received A p d 2, 1966 The bromination of o-BlOH,,C2H~ in the presence of AlC13 produces o-Bl0BrsHiCaHnin which the Br atoms are at the 8 (or l o ) , 9, and 12 positions of the Bloc2 polyhedron. These results are consistent with the structure of the 9,12-dibrominated compound, o-BL0Br2H8C2H2, and with the correlation of reactivity with ground-state charge distribution obtained earlier from extended Huckel theory and here from a new nonempirical molecular orbital theory. The space group is C2/c, and there are eight molecules in a unit cell having parameters a = 12.28, b = 8.51,c = 23.42 A, and p = 90.3".
Introduction f 0 02, b = 8.54 f 0.02, c = 23.42 f 0.02 4, and /3 = The elucidation of the structure of o-B10Br3H7C2H2> 90.3 f 0.4" mere established from photographs. The assumption of eight molecules in the unit cell led to the the final product of stepwise substitution of unsubfor the stituted o-carborane with Brz in the presence of A1C13, reasonable calculated density of 2.06 g crystals, Extinctions of hkl when h k is odd and of completes structural work on the known Br derivatives h01 when 1 is odd lead to possible space groups Cc or of o-carborane. The 9,12 substitution of Br in o-BloC2/c. The Br arrangement found as described below Br2HaC2H2,established in the preceding paper, imestablished CP/c as the correct space group. plies the structure of o-BloBrH9C2H2. It seems probThe three-dimensional data were collected m ith the able to us that this reaction of Br2 is electrophilic and use of an Xe proportional counter on the Buerger that the ground-state charge distribution in the parent automated X-ray diffractometer, using a scan time o-BloHl&!zHz determines the charge distribution of the transition state well enough to indicate the major feawhich was proportional to the Lorentz factor. Within tures of sequential substitution. We are well aware of each level one reflection was chosen as a monitor and the qualifications2of this idea but are interested in its measured after each group of 15 reflections. After limitations and whether exceptions are primarily those each level was completed, a set of six zero-level reflecof complex reactions for which the initial steps, when tions mas measured. Equivalent reflections agreed to not sterically controlled, also follow this pattern. within 2u,, where u0 is the square root of the total Here we show that the three Br atoms attached in onumber of counts. A total of 2172 independent reflecB10Br3H7C2H2 are in a very nearly equilateral triangular tions was recorded for levels 0 6 k 6 8 and 0 1 6 7. arrangement, which, together with the structure of oAfter correction for Lorentz and polarization factors, B10Br2H&2H2jis sufficient t o prove that the halogens the data were correlated to a single scale with the use are in the 8 (or l o ) , 9, and 12 positions. Our failure of weights which were inversely proportional to the net to distinguish between C and B, however, suggests that number of counts for Fo2 3 and which were inversely the Bloczcage is disordered in all senses in which the proportional to bo for FO26 Here uo is the square three Br atoms of the triangle can be permuted. root of the counts in background plus scan for a reflecFinally, we compare the experimental results with astion having zero net count, and the average bo is taken pects of the ground-state charge distribution8s4 in oover all uC in the sphere of reflection. The correlated B10H10C2H2 and establish the assignment of peaks in Fo are shown in Table I. Of the 2172 measured values the llB 64.2-Mc nuclear magnetic resonance (nmr) of Fo2, 1132 17-ere greater than 1 . 5 after ~ ~ background ~ p e c t r aof~ these , ~ compounds. corrections, and all reflections less than 1.53., ( F o < 73 on the scale of those in Table I) were cxcluded from Structure Determination final refinements and discrepancy factors. -1large single crystal of o-B10Br8H7C2H2,from acetone Analysis of the three-dimensional Patterson function solution, was cut into cubes 0.03 mm on edge. Two yielded a near-equilateral triangular arrangement for crystals, one mounted on the b axis and one on the c the three Br atoms if the space group C2/c was assumed. axis, were used for data collection. Reciprocal lattice Peak heights a t all positions for Br. . .Br interactions symmetry of 2/m and unit cell parameters of a = 12.28 were 1200 =t100 on an arbitrary scale. No such internally consistent interpretation was possible in the space (1) H. D Smith, T. A. Knowles, and H . Schroeder, Inorg. Chem., 4, 107 (1965). group Cc. These Br contributions gave a value of R F ( 2 ) J. Potenza and W.I\.Lipscomb, zbid., 6, 1471 (1966). = 211Fol - ~ F c [ ~ / of X ~0.36. ~ F o ~A three-dimensional (3) F. P. Boer, J. Potenza, a n d W. X . Lipscomb, ibid., 6, 1301 (1966). Also see F. P. Boer, Ph.D. Thesis, Harvard University, April 1965. electron density map based upon signs for the Br con(4) R. Hoffmann a n d W. N. Lipscomb, J . Chem. Phys., 36, 3489 (1962). tributions showed 52 candidates for light atoms in the ( 5 ) J. Potenza, W. N. Lipscomb, G. D. Vickers, and H. Schroeder, J. Am. Chem. Soc., 38, 628 (1966). range from 30 to 60% of the height of a B atom, but 1 2 (6) R. L. Pilling, F. N. Tebbe, M. F. Hawthorne, and E. A. Pier, Proc. of these formed an icosahedral structure appropriately C h e m . SOL.,402 (1964).
+
3 4 7
6
-
Y P 1it 11 1" 1'
2
3
H
4'
H H
5' 6
-
H Br Br H H Br
8'
9' 10 11' 12' Type
Atom
PI1
X
UZ
Y
0.460 0.401 0,327 0,366 0.462 0.484 0,269 0.241 0.317 0,403 0.366 0.266 0.530 0,430 0.300 0,364 0.530 0.572 0.198 0,094 0.276 0,432 0.367 0.149
0.002 0,002 0,003 0.002 0.003 0,002 0.002 0.003 0.002 0,002 0,002 0.003
0,649 0,752 0.716 0,525 0.453 0.598 0.716 0.563 0.397 0.446 0,645 0,519 0.694 0.884 0.813 0.485 0,359 0,612 0,809 0.539 0.181 0.353 0,683 0,443
"PI1
0.0002 0.0002
0.0003 pzs
up??
P13
-033
z
-u
0 0 0 0 0 0 0 0 0 0 0 0
003 003 003 003 003 003 003 003 003 003 003 003
0 0003 0 0003
0 0003 0.2
-?lz
US
0.333 0,384 0.325 0.307 0.357 0.402 0,392 0,345 0.363 0.425 0.445 0.419 0.302 0.395 0.288 0.258 0.343 0.423 0,405 0.311 0,351 0,462 0,494 0.470 PI3
-@13
0.002 0.001 0,002 0,001 0I002 0,002 0.001 0.001 0.001 0.001 0.001 0,001
0.0002 0.0001
0,0002 ?Ad
,7823
1 92 25 27 -2 215 48 B-C 37 9 - 11 11 30 16 2 67 18 162 39 - 33 B-C 36 9 19 -9 -29 14 10 3 124 25 120 31 21 -4 27 B 14 15 29 8 11 4 41 B 194 39 4 -1 13 13 18 13 6 14 11 5 89 139 40 - 12 -13 24 26 B 31 8 11 -14 13 6 170 38 20 13 12 37 17 B-C 68 -a 13 -31 19 c B 24 62 18 162 39 -27 22 17 -2 13 i 9 8 B 90 33 32 129 33 8 -3 13 24 5 14 19 4 9 77 28 16 -24 B 43 50 16 9 10 18 13 10 21 38 141 22 -39 B 77 -5 39 12 4 15 9 11 B 60 153 35 -2 20 19 18 -10 11 15 5 8 12 B 67 16 27 - 17 161 42 28 - 20 6 11 -5 13 -i 2 1' -3 90 203 H -3 3 2' 62 13 6 1 H 216 3 46 - 28 32 28 -6 H 229 21 4' 133 - 22 - 38 24 H 191 141 5' 14 24 85 - 35 - 12 H 256 24 - 13 H 6' -3 - 22 72 -I 184 15 - 16 7' 26 H -21 8' 2 1 11 1 189 5 Br -23 -8 1 37 2 96 1 2 4 1 115 100 9' -6 2 -8 1 Br -8 40 - 27 206 45 - 20 10' 86 H - 13 11' 24 - 22 236 99 H -7 3 12' 3 1 3 4 -31 0 Br 1 1 35 199 5 119 The nine hydrogen atoms were a T h e thermal parameters are in the form exp[ -(h2p1, k2& Z2p3, - 2hk& 2hZ& 2kZ&)]. not unambiguously located on the final electron density map; their positions were calculated so as t o place them 1.2 A from their respective BLOC2 cage atoms and directed radially outward from the lines joining para atoms in the 12 B refined structure; the thermal parameters used for hydrogen were taken as equal to those of their corresponding B atoms before the designation of C atoms. c Standard deviations were obtained from the full least-squares matrix. n
+
+
tial substitution on m-carborane and other carboranes, and detailed mechanistic studies would be of considerable interest.
+
+
Acknowledgment.-We wish to thank the Office of Naval Research for support and Dr. H. Schroeder for the sample.
