J. Am. Chem. S o t . 1985, 107, 2113-2119
2113
Remarkable Structures of C2B2H4Isomers Peter H. M. Budzelaar,'" Karsten Krogh-Jespersen,*IbTimothy Clark,'" and Paul von RaguC Schleyer*'* Contribution from the Institut f u r Organische Chemie der Friedrich- Alexander- Universitat Erlangen- Nurnberg, 0-8520 Erlangen. Federal Republic of Germany, and the Department of Chemistry, Rutgers-The State University of New Jersey, New Brunswick, New Jersey 08903. Received August 15, 1984
Abstract: A number of C2B2H, isomers were studied by ab initio MO methods. While many of these now have experimental analogies, no representatives of 13, the global energy minimum, have been reported to date. Like 13, a four-membered ring with the constitution (CH,)(BH)(CH)B, most of the other relatively stable isomers show structural relationships to known carbocations. The most stable (CH),(BH), constitution is the puckered 1,3-diboretene 1; its 1,2-isomer 3 is predicted to isomerize to I with a low activation energy (ca. 8 kcal/mol). Neither the perpendicular ethylene 8 nor the "diboramethylenecyclopropane" 9 (members of the CH2C(BH)zfamily) are local minima, and both rearrange without a barrier to the nonclassical four-membered ring 11. The recently reported derivatives of 9 are suggested to be related to 11 instead, and the topomerization of these compounds is predicted to proceed via an intermediate, the 2n-carbene 10. Of the remaining compounds, C-borylborirenes (CH)(BH)CBH2 are more stable than their B-substituted isomers (CH)2BBH2,and perpendicular are preferred to planar conformations.
Small ring organoboron compounds represent one of the emerging fields of chemistry where theoretical calculations preceeded and helped to guide subsequent experimental investigations. Both interest in the lower members of the (CH),(BH), ( n = 1, 2 ) carborane seriesz and the formal relationship of boron compounds to carbocations3 served as early stimuli. Thus, a theoretical study of the cyclobutadiene dication (CH)42+led to the surprising conclusion that this Hiickel 2 i ~aromatic system preferred a nonplanar over a planar g e ~ m e t r y . ~The isoelectronic 1,3-diboretene, (CH),(BH),, was also predicted to prefer a nonplanar g e ~ m e t r y ;this ~ . ~ has recently been verified experimentally for a derivative.6 The cyclopropenium ion, (CH)3+, has the largest resonance energy of any monocyclic Hiickel system; borirene (CH),(BH), the neutral analogue, is indicated to be nearly as favorable in this respect.2 Experimental searches for such molecules have now achieved success in several l a b o r a t ~ r i e s . ~ - ' ~ An even earlier theoretical study predicted that the presence of two boron atoms in a three-membered ring might have remarkable geometrical consequences." The substituents attached to the ring carbon should prefer "anti-van't Hoff" arrangements, i.e., planar tetracoordinate carbon, perpendicular ethylene retaining the double bond, etc. Again, the geometries preferred by the carbocations were ~ i m i l a r .Attempts ~ to prepare such organoboron compounds have not succeeded; moreover, experimentall2 and t h e o r e t i ~ a l ' ~results . ' ~ indicate that the anti-van't Hoff structures (1) (a) Erlangen. (b) New Jersey. (2) Krogh-Jespersen, K.; Cremer, D.; Dill, J. D.; Pople, J. A.; Schleyer, P. v. R. J . A m . Chem. SOC.1981, 103, 2589. (3) Dill, J. D.; Schleyer, P. v. R.; Pople, J. A. J . A m . Chem. SOC.1975, 97, 3402. (4) Chandrasekhar, J.; Schleyer, P. v. R.; Krogh-Jespersen, K. J . Comput. Chem. 1981, 2, 356. Krogh-Jespersen, K.; Schleyer, P. v. R.; Pople, J. A,; Cremer. D. J . Am. Chem. Soc. 1978. 100. 4301. (5) Schleyer, P. v. R.; Budzelaar, P. H. M.; Cremer, D.; Kraka, E. Angew. Chem. 1984, 96, 374. (6) Hildenbrand, M.; Pritzkov, H.; Zenneck, U.; Siebert, W. Angew. Chem. 1984, 96, 371. (7) Van der Kerk, S. M.; Budzelaar, P. H. M.; Van der Kerk-van Hoof, A.; Van der Kerk, G. J. M.; Schleyer, P. v. R. Angew. Chem. 1983, 95, 61. (8) Wehrmann, R.; Pues, C.;Klusik, H.; Berndt, A. Anpew. Chem. 1984, 96, 372. (9) Habben, C.; Meller, A. Chem. Ber. 1984, 117, 2531. Habben, C.; Meller, A. IMEBORON V: 5th International Symposium on Boron Chemistry, Swansea, 1983, Abstract 9, C 144. (10) Pachaly, B.; West, R. Angew. Chem. 1984, 96, 444. (1 1) Krogh-Jespersen, K.; Cremer, D.; Poppinger, D.; Pople, J. A.; Schleyer, P. v. R.; Chandrasekhar, J. J . A m . Chem. SOC.1979, 101, 4843. 1121 Klusik. H.: Berndt. A. Aneew. Chem. 1983. 95. 895. (13) Budzelaar, P. H. M.; Krogk-Jespersen, K.; Schleyer, P.v. R. Angew. Chem. 1984. 96. 809. (14) Independently of our work G. Frenking and H. F. Schaefer I11 (Chem. Phys. Lett. 1984, 109, 521) also proposed the nonclassical structure 11 for Berndt's 'diboramethylenecyclopropene". They did not, however,
suggest a pathway for the topomerization reaction.
are less favorable than even more exotic structures. Indeed, the last two years have seen a rapid development in the field of small-ring boron-carbon compounds, and several groups have succeeded in preparing four-membered rings with 0ne12915and t ~ o ~ 9boron ~ 9 ~atoms. ~ , ~ Computational ~ examinations have kept ~ ~ c ~ , and ~ the J fruitful ~ J ~dialogue J ~ between ~ ~ ~ theory and experiment in this area continues. The present contribution reports a comprehensive study of C2B,H4 isomers. These are intended to model experimental systems, where other substituents usually replace the hydrogens. The compounds we have examined are best categorized constitutionally by the hydrogen locations. There are six classes: (CH)2(BH)2 (1-6; this extends our earlier study2), CHZC(BH), (7-12 the "anti-van't Hoff" 711and its isomers), CH2(CH)(BH)B (13-15), C-borylborirenes (CH)(BH)CBH2 (16 and 17), B-borylborirenes (CH),BBH2 (18 and 19), and diborylacetylenes Cz(BH2)2(20 and 21). These species include transition structures for the interconversion of isomers within each class. This should help the understanding of experimental results. As the substituents used experimentally generally remain fixed to a given boron or carbon atom, we have not considered processes involving hydrogen migration.
Methods Ab initio molecular-orbital calculations were carried out on 1-21 with the GAUSSIAN 7620s and s220bseries of programs. The geometries of 1-3, 5, 7-11, and 13-21 were optimized completely, subject only to overall molecular symmetry restrictions, with restricted Hartree-Fock (RHF) single-determinant theory2' and the small split-valence 3-21G basis set.21 The transition structure 4 was optimized similarly, and the triplet 6 was optimized with the unrestricted Hartree-Fock (UHF) formalism of Pople and Ne~bet.*~ Energy refinements were then obtained from single-point (15) Wehrmann, R.; Klusik, H.; Berndt, A. Angew. Chem. 1984,96, 369. (16) Pues, C.; Berndt, A. Angew. Chem. 1984, 96, 306. (17) Wehrmann, R.; Klusik, H.; Berndt, A. Angew. Chem. 1984, 96, 810. (18) Cremer, D.; Gauss, J.; Schleyer, P. v. R.; Budzelaar, P. H. M. Angew. Chem. 1984,98, 370. (19) Budzelaar, P. H. M.; Kos, A.; Clark, T.; Schleyer, P. v. R. Organo-
metallics, in press. (20) (a) Binkley, J. S.; Whiteside, R. A,; Hariharan, P. C.; Seeger, R.; Pople, J. A.; Hehre, W. J.; Newton, M. D. QCPE, Indiana University, Bloomington, IN, Program No. 368. (b) Binkley, J. S . ; Frisch, M.; Raghavachari, K.; DeFrees, D. J.; Schlegel, H. B.; Whiteside, R. A,; Fluder, E.; Seeger, R.;Pople, J. A. GAUSSIAN 82, release A. At Erlangen, this program was adapted to a CDC computer by A. Sawaryn. (21) Roothaan, C. C. J. Reu. Mod. Phys. 1951, 23, 69. (22) (a) Binkley, J. S.; Pople, J. A.; Hehre, W. J. J . Am. Chem. SOC.1980, 102, 939. (b) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257. Gordon, M. S.; Binkley, J. S.; Pople, J. A,; Pietro, W. J.; Hehre, W. J. J. Am. Chem. SOC.1982, 104, 2797. (c) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. (23) Pople, J. A,; Nesbet, R. K. J . Chem. Phys. 1954, 22, 571.
0002-1863/8S~l501-2713~0~.~0/0 0 1985 American Chemical Society
2114
J . A m . Chem. Soc., Vol. 107, No. 9, 1985
Budzelaar et al.
Table I. 3-21G (6-31G*) ODtimized Geometries for 1-21 1 2 5
c-c
1.883 (1.787) 1.521 (1.500) 2.187 (2.178) 1.072 (1.077) 1.174 (1.180)
B-C B-B C-H B-H
2.183 1.543 2.183 1.075 1.184
1.370 (1.364) 1.571 (1.559) 1.725 (1.715) 1.075 (1.081) 1.185 (1.189)
C-B B-B C-H B-H
7
Cl-C2 C1-B1 C2-B 1 C2-B2 Bl-B2 C1-H B1-H B2-H fHClH fC2Clbe LC2BlH LC2B2H f B 1C2B2
1.331 1.529 1.684 1.077 1.177 115.7 143.6 66.8
13
C1-Bl Cl-B2 Cl-B1 C2-B2 Bl-B2 Cl-H
14
1.744 1.494 1.517 1.400 1.744 1.078 Cl-C2 C1-Bl C2-B1 C1-B2 Bl-H C2-H B2-H fC2ClB2 fClBlH fClC2H f C 1B2b' fHBH
C1-C2 C1-B2 C2-Bl C2-B2 B1-B2 C1-H
71'
72'
f HCC
fHBB
C1-C2 Cl-Bl C2-BI C2-B2 B1-B2 C1-H
10
1.820 (1.738) ,716 (1.688) ,420 (1.399) .078 (1.082) ,167 (1.172) 14.6 (113.6) 53.1 (155.5) 125.1 (128.0) 13
1.640 1.655 2.437 1.377 1.609 1.082
16
17
1.356 1.492 1.492 1.559 1.172 1.066 1.191', 1.187 139.8 153.6 138.7 179.7' 119.0
1.361 1.522 1.452 1.509 1.170 1.064 1.188 151.1 148.9 136.2 177.2h 119.5
20
21
2
C2-H Bl-H LHClH fClBlH LBlC2H LBlClb'
6 104.3 21.7 -14.4 137.0 149.5
5
126.4 48.9 -26.9 138.0 165.7 3
fHCC LHBB LHCCH f BCCB fHBBH
15
1.647 1.498 1.490 1.631 1.559 1.076
1 47.6 (50.8) 5.1 (5.8) 11.5 (11.9)d 141.4 (138.1) 161.3 (160.4)
a*
4 1.597 (1.549) 1.405, 1.859 (1.392, 1.784) 2.012 (1.904) 1.063 (1.071) 1.167 (1.175) 8 pi 1.309 (1.314) 1.558 1.539 1.562 (1.546) 1.506 1.356 1.516 (1.505) 1.078 (1.079) 1.079 1.171 (1.177) 1.173 1.166 116.2 (116.1) 113.1 142.3 125.1 (124.5) 146.7 177.6 58.1 (58.3) 146.4
3
c-c
6 1.456 1.618 1.722 1.062 1.168
1.311 1.818 1.529 1.065 1.170
126.9 (126.6) 144.4 (145.0)
4 129.7 (129.6) 135.2 (135.4) 53.2 (55.3) 81.7 (80.1) 44.4 (39.4)
11 1.468 (1.454) 1.590 (1.581) 1.564 (1.527) 1.348 (1.339) 1.884 (1.825) 1.077 (1.080) 1.170 (1.175) 1.162 (1.168) 113.3 (113.1) 147.2 (147.0) 160.1 (160.6) 172.6 (172.3)/ 80.3 (78.8)
12k
(1.623) (1.534) (1.461) (1.353) (1.079) (1.178) (1.168) (113.3) (143.8) (146.2) ( 177.1)g ( 177.1)g 15 -
14
1.059 1.174 112.5 124.2 137.4 138.6
C2-H B1-H fHClH fClC2H fC2BlH LC2Clb'
c1-c2 Cl-B1 Bl-B2 Cl-H B2-H fClC2H fHBH
20
1.069 1.168 113.5 116.1 137.5 136.9
C2-H B1-H LHClH LClC2H LClBlH LC2Clbe
18
19
1.350 1.490 1.697 1.066 1.192 139.1 116.5
1.333 1.500 1.651 1.065 1.192 140.9 116.7
1.061 1.173 111.2 125.8 133.3 130.7
21
c-c
1.203 1.214 B-H 1.185 1.185 C-B 1.529 1.499 fHBH 119.5 120.2 QDistancesin A, angles in deg. *In most of the geometries redundant parameters are included to facilitate interpretation. CSee Figure 1 for definition. the earlier paper,2 the H(B) atoms in 1 were erroneously depicted as being tilted toward axial positions. They are, in fact, tilted toward equatorial positions, but the magnitudes of +2 given in ref 2 are correct. '6 denotes the bisector of the HCH or HBH angle. /Trans to C1.
