Mendeleev Communications Electronic Version, Issue 2, 2001 (pp. 43.84) Novel aromatic oxaborabenzene and 9-oxa-1,8-diboranaphthalene systems: an ab initio study Ruslan M. Minyaev,* Vladimir I. Minkin, Tatyana N. Gribanova and Andrei G. Starikov Institute of Physical and Organic Chemistry, Rostov State University, 344090 Rostov-on-Don, Russian Federation. Fax: +7 8632 28 5667; e-mail: minyaev@ipoc.rsu.ru 10.1070/MC2001v011n02ABEH001442 Ab initio [MP2(fu)/6-31+G**, MP2(fu)/6-311+G**] and DFT [B3LYP/6-31+G**, B3LYP/6-311+G**] calculations predict the aromatic stabilization of planar 1,2-oxaborabenzene and 9-oxa-1,8-diboranaphthalene. Aromaticity is an important theoretical concept of chemistry1,2 designed to predict and explain the stability and chemical properties of various, in particular, heterocyclic, compounds.The simplest way to form a heteroaromatic compound starting from the archetype aromatic system of benzene is to replace CH units or CC bonds in a ring by equal numbers of isoelectronic (e.g., N, O+, BH.) or 2¥�-electronic (NH, O, S) centres, respectively. With the use of a starting heteroaromatic system, a series of new heteroaromatics can be produced, some of which exhibit nonclassical structures that cannot be described in terms of Lewis structural formulae.An important question is whether (4n+2)¥�- electronic species thus formed remain persistent to possible distortions of the initial planar structure and display additional stability due to cyclic ¥�-electron delocalization. To solve this question, we performed ab initio calculations for pyrylium cation 1 and a series of six-membered oxaboraheterocycles 2.4 derived from 1.One of these currently unknown heteroaromatic systems, namely, 3, has a nonclassical structure. In addition, we studied another nonclassical oxadiboraheterocycle 5, which can be considered as the result of insertion of a 3¥�-electron HB.O.BH unit into peri-positions of the naphthalene ring.Here, we report the results of ab initio [MP2(fu)/6-31+G**, MP2(fu)/6-311+G**] and density functional theory [B3LYP/ 6-31+G**, B3LYP/6-311+G**]3,4 calculations for compounds 1.4 and bicyclic oxadiboraheterocycle 5, which is ¥�-isoelectronic to naphthalene. The aromatic character of these compounds was estimated using an approach similar to that used for the calculations of Dewar resonance energies.1 According to the calculations, the molecules of all compounds 1.5 possess planar structures and correspond to minima on the respective potential energy surfaces (PESs).Their geometry and energy characteristics are listed in Tables 1 and 2 and shown in Figures 1 and 2. 1,2-Oxaborabenzene 2 was predicted to be the most stable isomer in the family of oxaborabenzenes 2.4. The lengths of the BC bonds in cyclic systems 2.5 lie in the range 1.500. 1.529 A and are shorter than the standard BC bonds in aromatic compounds (~1.56 A).6 At the same time, these values are close to those for the BC bond lengths [1.514(2) A] found by X-ray diffraction analysis in the lithium salts of boratabenzene7 and boratastilbene.8 The calculated BO bond lenghts (1.393.1.398 A) are longer than the lengths of covalent bonds between tricoordinated boron and dicoordinated oxygen (~1.367 A).6 The CO bond lengths in 2.4 (1.337.1.358 A) are close to those in pyrylium salts (~1.35 A).5 Note that all CC bonds in bicyclic Table 1 Ab initio and DFT data for compounds 2.9.a aEtot (a.u.) is the total energy (1 a.u.= 627.5095 kcal mol.1); ZPE (a.u.) is the harmonic zero-point correction; l = 0, l is the number of imaginary harmonic frequencies; w1 (cm.1) is the smallest or imaginary harmonic vibration frequency.Structure, symmetry Method Etot ZPE w1 2, Cs MP2(fu)/6-31+G** MP2(fu)/6-311+G** B3LYP/6-31+G** B3LYP/6-311+G** .254.793351 .254.977024 .255.555651 .255.608372 0.085455 0.084222 0.084650 0.084318 318.9 310.5 332.4 331.5 3, Cs MP2(fu)/6-31+G** MP2(fu)/6-311+G** B3LYP/6-31+G** B3LYP/6-311+G** .254.724767 .254.909008 .255.485647 .255.538568 0.084965 0.083897 0.083936 0.083656 300.1 294.9 321.0 319.5 4, C2v MP2(fu)/6-31+G** MP2(fu)/6-311+G** B3LYP/6-31+G** B3LYP/6-311+G** .254.745222 .254.929331 .255.509674 .255.562959 0.085037 0.083996 0.084152 0.083847 266.2 263.3 283.3 283.7 5, C2v MP2(fu)/6-31+G** B3LYP/6-31+G** B3LYP/6-311+G** .395.301912 .396.515389 .396.592365 0.138469 0.138170 0.137680 68.0 114.8 115.3 6, Cs MP2(fu)/6-31+G** MP2(fu)/6-311+G** B3LYP/6-31+G** B3LYP/6-311+G** .254.667375 .254.853093 .255.438712 .255.494515 0.079462 0.078284 0.078724 0.078439 91.6 89.0 102.7 100.5 7, Cs MP2(fu)/6-31+G** B3LYP/6-31+G** B3LYP/6-311+G** .395.127733 .396.355221 .396.436157 0.130082 0.129496 0.129024 32.0 42.6 40.9 8, Cs MP2(fu)/6-31+G** MP2(fu)/6-311+G** B3LYP/6-31+G** B3LYP/6-311+G** .255.900031 .256.089025 .256.686618 .256.740321 0.107317 0.106044 0.104885 0.104585 231.6 228.7 231.8 229.4 9, Cs MP2(fu)/6-31+G** MP2(fu)/6-311+G** B3LYP/6-31+G** B3LYP/6-311+G** .255.865396 .256.054313 .256.650205 .256.706534 0.103566 0.102047 0.101263 0.101017 56.3 56.8 53.9 56.7 O BH O BH O B O H O B B H H 1, C2v 2, Cs 3, Cs 4, C2v 5, C2v 1.334 1.327 1.333 1.330 1.34 1.380 1.381 1.377 1.372 1.41 1.397 1.397 1.402 1.398 1.39 1.32 1.40 1.38 MP2(fu)/6-31+G** MP2(fu)/6-311+G** B3LYP/6-31+G** B3LYP/6-311+G** Experiment5 1.398 1.393 1.398 1.397 1.348 1.341 1.343 1.340 1.361 1.362 1.360 1.356 1.428 1.428 1.433 1.431 1.375 1.376 1.373 1.369 1.517 1.517 1.518 1.514 117.3 117.6 117.1 117.1 121.7 121.6 122.2 122.1 1, C2v 2, Cs 5, C2v O+ O B B 131.5 131.3 131.5 119.4 119.1 119.2 116.6 116.9 116.8 123.9 123.9 123.9 1.433 1.440 1.438 117.6 117.8 117.9 1.504 1.505 1.506 1.396 1.397 1.394 125.6 126.0 126.0 1.388 1.393 1.390 121.8 122.0 122.1 1.397 1.397 1.394 1.467 1.468 1.463 1.176 1.180 1.178 Figure 1 Geometry parameters of structures 1, 2 and 5 c alculat ed b y ab initio and DFT methods.The bond lengths and angles are given in angstrom units and degrees, respectively. OMendeleev Communications Electronic Version, Issue 2, 2001 (pp. 43.84) naphthalene-like system 5 are equalised, and they are similar to those in benzene (1.397 A).1 To evaluate the thermodynamic stability of the most stable cyclic (2) and bicyclic (5) systems, polyenes 6 and 7 were calculated.The symmetry of polyene 7 was predicted by MP2 calculations to be Cs with the dihedral angle OCCC about 2¡Æ, whereas DFT gives C2v symmetry. The lengths of double BC bonds in polyenes 6 and 7 (~1.400 A) are equal to the lengths of double BC bonds in organoboron compounds.9,10 To evaluate the stabilization due to cyclic ¥�-electron delocalization in 2 (.Earom), we applied the equation (1), where .E is the difference in the total energies of cyclic isomer 2 and polyene 6, and .EBO is the energy of the BO bond in 2 calculated as the difference between the total energies of ring-closed and open structures, 8 and 9, respectively. As is the case in 2, the BO bond in 8 is a part of the conjugated system, slightly distorted in 8 (dihedral angle between CO and CB bonds is approximately equal to 13¡Æ).However, as distinct from 2, no cyclic ¥�-electron delocalization is inherent to 8. As can be seen in Table 2, the .Earom for 2 are about 50. 56 kcal mol.1 depending on the computational level. These values are in the range (23.75 kcal mol.1) typical of the effect of cyclic ¥�-electron delocalization calculated for benzene using different methods and different reference systems.2 Similarly, the cyclic ¥�-electron delocalization energy for 5 was calculated according to the equation where .E(5 . 7) is the difference between the total energies of 5 and 7, and .EBO is the BO bond energy in 2. The values of .Earom(5) thus obtained lie in the range 55.66 kcal mol.1, and they are even higher than those for monocyclic system 2.In conclusion, the results of the calculations of hypothetical compounds 2 and 5, which are isoelectronic to benzene and naphthalene, respectively, demonstrate that they possess stable aromatic structures. This work was supported by the Russian Foundation for Basic Research (grant nos. 00-15-97320 and 01-03-32546).References 1 V. I. Minkin, M. N. Glukhovtsev and Baticity and Antiaromaticity: Electronic and Structural Aspects, Wiley, New York, 1994. 2 T. M. Krygowski, M. K. Cyranski, Z. Czarnocki, G. Hafelinger and A. R. Katritzky, Tetrahedron, 2000, 56, 1783. 3 M. J. Frish, G. W. Trucks, H. B. Schlegel, P. M.W. Gill, B. G. Johnson, M. A. Robb, J. R. Cheeseman, T.A. Keith, G. A. Petersson, J. A. Montgomery, K. Raghavachari, M. A. AlLaham, V. G. Zakrzewski, J. V. Ortiz, J. B. Foresman, C. Y. Peng, P. Y. Ayala, W. Chen, M. W. Wong, J. L. Andres, E. S. Replogle, R. Gomperts, R. L. Martin, D. J. Fox, J. S. Binkley, D. J. Defrees, J. Baker, J. P. Stewart, M. Head- Gordon, C. Gonzalez and J. A. Pople, Gaussian-94, Revision B.3. Gaussian, Inc., Pittsburgh PA, USA, 1995. 4 M.W. Schmidt, K. K. Baldridge, J. A. Boatz, S. T. Elbert, M. S. Gordon, J. H. Jensen, S. Koseki, N. Matsunaga, K. A. Nguyen, S. J. Su, T. L. Windus, M. Dupuis and J. A. Montgomery, J. Comput. Chem., 1993, 14, 1347. 5 A. T. Balaban, A. Dinculescu, G. N. Dorofeenko, G. W. Fischer, A. V. Koblik and V. V. Mezheritskii, Pyrylium Salts: Syntheses, Reactions, and Physical Properties, Academic Press, New York, 1982. 6 F. H. Allen, O. Kennard, D. G. Watson, L. Brammer, A. G. Orpen and R. Taylor, J. Chem. Soc., Perkin Trans. 2, 1987, S. 1. 7 G. E. Herberich, B. Schmidt, U. Unglert and T.Wagner, Organometallics, 1993, 117, 8480. 8 B. Y. Lee, S. Wang, M. Putzer, G. P. Bartholomew, X. Bu and G. C. Bazan, J. Am. Chem. Soc., 2000, 122, 3969. 9 P. P. Power, Inorg.Chim. Acta, 1992, 198.200, 443. 10 W. J. Grigsby and P. P. Power, Chem. Eur. J., 1997, 3, 368. HB O HB O BH 6, Cs 7, Cs .Earom(2) = .E(2.6) . .EBO (1) B O H H H H 8, C1 HB O H H H H 9, C1 Table 2 Relative energies calculated by ab initio and DFT methods for compounds 2.7.a a.E (kcal mol.1) is the relative energy; .EZPE (kcal mol.1) is the relative energy including harmonic zero-point correction; .H and .G (kcal mol.1) are the relative enthalpy and the relative Gibbs free energy under standard conditions (P = 1 atm and T = 298.1 K).b.Earom. Structure, symmetry Metod .E .EZPE .H .G 2, Cs MP2(fu)/6-31+G** MP2(fu)/6-311+G** B3LYP/6-31+G** B3LYP/6-311+G** 57.3b 56.0b 50.5b 50.2b 0 0 0 3, Cs MP2(fu)/6-31+G** MP2(fu)/6-311+G** B3LYP/6-31+G** B3LYP/6-311+G** 43.0 42.7 43.9 43.8 42.7 42.5 43.5 43.4 42.7 42.5 43.5 43.4 42.7 42.5 43.5 43.4 4, C2v MP2(fu)/6-31+G** MP2(fu)/6-311+G** B3LYP/6-31+G** B3LYP/6-311+G** 30.2 29.9 28.8 28.5 29.9 29.8 28.5 28.2 30.0 29.8 28.6 28.3 29.9 29.7 28.9 28.6 5, C2v MP2(fu)/6-31+G** B3LYP/6-31+G** B3LYP/6-311+G** 65.9b 54.8b 55.6b 0 0 0 6, Cs MP2(fu)/6-31+G** MP2(fu)/6-311+G** B3LYP/6-31+G** B3LYP/6-311+G** 79.0 77.8 73.4 71.4 75.3 74.0 69.7 67.8 76.6 75.3 70.9 69.0 73.7 72.4 68.2 66.3 7, C2v MP2(fu)/6-31+G** B3LYP/6-31+G** B3LYP/6-311+G** 109.3 100.5 98.0 103.9 95.1 92.6 105.0 97.2 94.7 102.2 91.9 89.4 .Earom(5) = .E(5 . 7) . 2.EBO, (2) 3, Cs 4, C2v 6, Cs 7, C2v O B O O B B O B B 1.353 1.347 1.350 1.347 1.337 1.331 1.331 1.328 1.383 1.384 1.380 1.376 1.398 1.398 1.402 1.398 1.507 1.508 1.509 1.504 1.501 1.503 1.498 1.493 122.2 122.0 122.5 122.3 113.0 112.9 112.7 112.7 122.0 122.3 122.1 122.1 1.358 1.351 1.357 1.355 1.360 1.361 1.356 1.351 125.6 125.8 125.3 125.1 123.2 123.4 123.2 123.2 1.523 1.523 1.529 1.525 124.1 124.3 123.9 124.0 120.0 120.0 120.6 120.5 1.233 1.222 1.224 1.216 124.5 124.6 124.8 125.0 1.456 1.461 1.457 1.457 120.4 120.3 121.0 121.0 1.357 1.356 1.358 1.353 125.1 125.1 125.5 125.6 1.445 1.447 1.444 1.442 127.9 128.3 129.7 129.6 1.401 1.400 1.397 1.391 178.0 177.7 177.4 177.5 1.168 1.172 1.172 1.169 178.1 177.5 177.5 127.0 128.8 128.8 124.8 125.3 125.5 121.2 121.5 121.4 1.244 1.239 1.231 115.9 115.9 115.7 1.475 1.478 1.478 1.355 1.355 1.351 1.446 1.446 1.444 1.402 1.398 1.392 1.168 1.172 1.169 Figure 2 Geometry parameters of isomers 3, 4, and 6, 7 calculated by ab initio and DFT methods.The symmetry for 7 is predicted by MP2 and DFT calculations to be Cs and C2v, respectively. The bond lengths and angles are given in angstrom units and degrees, respectively. MP2(fu)/6-31+G** MP2(fu)/6-311+G** B3LYP/6-31+G** B3LYP/6-311+G** Received: 19th February 2001; Com. 01/1768