Mendeleev Communications Electronic Version, Issue 5, 2001 1 Planar hexacoordinated boron in organoboron compounds: an ab initio study Tatyana N. Gribanova, Ruslan M. Minyaev* and Vladimir I. Minkin Institute of Physical and Organic Chemistry, Rostov State University, 344090 Rostov-on-Don, Russian Federation. Fax: +7 8632 43 4667; e-mail: minyaev@ipoc.rsu.ru 10.1070/MC2001v011n05ABEH001461 Ab initio [MP2(fu)/6-31G**, MP2(fu)/6-311G**] and DFT [B3LYP/6-31G**, B3LYP/6-311G**] calculations predict stable planar structures of the nonclassical compounds BB6(CH)3 and BB6X2 .(X = NH, O) containing a hexacoordinated central boron atom. Nonclassical organoelement systems, particularly those containing hypercoordinated planar centres are of considerable theoretical and practical interest.1 Recently, unusual stable planar molecular systems 1 and 2 and their isomers containing planar hexacoordinated carbon centres have been predicted by ab initio calculations.2,3 These structures with a hexacoordinated planar carbon atom are stabilized through the multicentre interactions of the central atom with the ligands.One may expect that the same bonding type may also occur in similar structures of hypercoordinated central atoms of other main-group elements, particularly, in boron-containing compounds.A possibility of stabilization of such compounds is supported by theoretical works on boron clusters including planar or quasi-planar boron atoms.4.6 Here, we report on ab initio [MP2(fu)/6-31G**, MP2(fu)/ 6-311G**]7 and density functional theory [B3LYP/6-31G**, B3LYP/6-311G**]8 calculations on compounds 3 and 4 (X = = NH, O), which contain a hexacoordinated planar boron centre.According to the calculations, compounds 3 and 4 (X = NH, O) possess highly symmetrical planar structures and correspond to minima (l = 0; hereafter, l designates the number of hessian negative eigenvalues at a given stationary point) on the corresponding potential energy surfaces (PES).Their geometric and energy characteristics are listed in Table 1 and depicted in Figure 1. The Bc.B bond lengths formed by the central boron (Bc) with the vicinal borons in 3 are about ~1.7 A; this value is in the range of available experimental data9 on BB single bonds. These bonds are 0.1 A longer than the peripheral BB bonds.The multicentre type of bonding in the BcB6 coordination site is illustrated by the shapes of the respective molecular orbitals of 3 shown in Figure 2. An additional contribution to the stability of the planar polycyclic structure of 3 is provided by the interaction of the vacant pz-orbital located at the Bc centre with the ¥�-system of the ligand environment. The molecule of 3 is a 6¥�-electron aromatic system with three occupied ¥�-orbitals (Figure 3).Electron density corresponding to the 1a2''-orbital (Figure 3) is delocalised on the entire system, electron density defined by the 1e''-orbitals is localised inside of the B2CH fragments. Thus, these three ¥�-orbitals are bonding. According to Mulliken orbital population analysis, central boron in 3 possesses a vacant p¥�-orbital and, thus, does not contribute electrons into the total ¥�-system.Each of the B2CH fragments contributes two ¥�-electrons to the total ¥�-system of 3. Therefore, it is ¥�-isoelectronic to the aromatic cyclopropenyl cation and possesses ¡®local¡� aromaticity. Rather strong ¥�-interaction inside the B2CH fragments is evidenced by the short (1.46.1.47 A) lengths of the BC bonds, which are considerably shorter than single BC bonds (1.6.1.7 A)10,11 and are close to the lengths of typical BC double bonds (~1.4.1.45 A).12 Compound 3 also has short peripheral BB bonds (1.64 A) whose lengths are comparable with those of BB double bonds (~1.63 A).12 The stability of anions 4 with central hypercoordinated boron is also due to the formation of multicentre bonds of the central atom with surrounding ligands. The calculated bond lengths BB¥á and BB¥â are in the range characteristic of standard single BB B C B B B B B X X C C B B B B C X = NH, O 1, D2h 2, C2v B B B B B B B X X X = NH, O 4, D2h ¥á ¥â B B B B B B B H H H 3, D3h Table 1 Ab initio and DFT data for compounds 3 and 4.a aEtot (in a.u.) is the total energy (1 a.u.= 627.5095 kcal mol.1); l is the number of the negative hessian eigenvalues; ZPE (in a.u.) is the harmonic vibration zero-point correction; w1 (in cm.1) is the lowest harmonic vibration frequency.Compound Method Etot l ZPE w1 3 MP2(fu)/6-31G** MP2(fu)/6-311G** B3LYP/6-31G** B3LYP/6-311G** .288.986615 .289.207299 .289.947847 .290.005892 0000 0.074912 0.073563 0.073959 0.073558 63.8 71.7 119.3 115.2 4 (X = NH) MP2(fu)/6-31G** MP2(fu)/6-311G** B3LYP/6-31G** B3LYP/6-311G** .283.772298 .284.019274 .284.677995 .284.757773 0000 0.061120 0.060076 0.060390 0.059998 87.4 93.4 120.6 91.7 4 (X = O) MP2(fu)/6-31G** MP2(fu)/6-311G** B3LYP/6-31G** B3LYP/6-311G** .323.472887 .323.753393 .324.438428 .324.529137 0000 0.036851 0.036185 0.036625 0.036344 35.0 46.0 95.7 34.3 Figure 1 Geometry parameters of the structures of 3 and 4 calculated by ab initio and DFT methods.The bond lengths and angles are indicated in angstrom units and degrees, respectively. 3, D3h 4 (X = NH), D2h 4 (X = O), D2hMendeleev Communications Electronic Version, Issue 5, 2001 2 bonds (~1.7 A). A peculiar feature of systems 4 is very short B¥á.B¥â bonds (1.55.1.57 A), which are appreciably shorter than experimentally determined BB double bonds (~1.63 A).12 The ¥�-electronic system of anions 4 contains eight ¥�-electrons, two of which coming from the B¥á centre.Thus, the valent state of the B¥á atoms in 4 does not correspond to the ¡®classical¡� sp2-hybridization with a vacant p¥�-orbital. The electron occupation of the B¥á p¥�-orbitals is corroborated by the data of Mulliken population analysis indicating a large (about .0.3 e) negative charge on each of the B¥á atoms.As distinct from B¥á, the valent state of other four boron atoms, B¥â, may be described in terms of ¡®classical¡� sp2-hybridization with a vacant p¥�-orbital. Such a distinction facilitates the B¥á.B¥â charge transfer, leads to strong interligand ¥�-interaction and contributes to stabilization of the planar system with hypercoordinated central boron.Another stabilising factor is the aromaticity of anions 4: all bonding ¥�-orbitals are occupied by electrons, whereas all antibonding ¥�-orbitals remain vacant. Note that all of the BN bonds in the polycyclic system of 4 (X = NH) are weaker than usual ¥�-dative BN bonds: the calculated lengths of the BN bonds in 4 are equal to 1.44 A, and they are notably longer than the experimental lengths of ¥�-dative BN bonds (1.38.1.41 A).13 Similar effect of weakening ¥�-interaction between B and N in the case of inclusion of boron atoms into BB double bonds is well known.13 The replacement of NH groups by a more electronegative oxygen atom leads to strengthening the BB¥â bonds in 4 (X = O) and to weakening the B¥á.B¥â and B.B¥á bonds.The latter effect leads to destabilization of the system. Nonetheless, the structure of 4 (X = O) corresponds to a minimum (l = 0) on the PES. At the same time, low values of the first harmonic vibration frequencies point to the shallowness of this minimum and, consequently, to low kinetic stability of the structure of 4 (X = O).In conclusion, the results of the calculations of hypothetical compounds 3 and 4 testify that there are three important factors leading to the stabilization of planar hexacoordinated central boron atom: (1) the ¥�-interaction of hypercoordinated central boron with the ligands in its environment; (2) the strong interligand bonding and (3) the aromaticity of polycyclic systems (the occupation of all bonding and the vacancy of all anti-bonding ¥�-orbitals).This work was supported by the Russian Foundation -97320). References 1 V. I. Minkin, R. M. Minyaev and Yu. A. Zhdanov, Nonclassical Structures of Organic Compounds, Mir, Moscow, 1987. 2 R. M. Minyaev and T. N. Gribanova, Izv. Akad. Nauk, Ser.Khim., 2000, 786 (Russ. Chem. Bull., 2000, 49, 783). 3 K. Exner and P. v. R. Schleyer, Science, 2000, 290, 1937. 4 J. E. Fowler and J.M. Ugalde, J. Phys. Chem., 2000, 104, 397. 5 (a) I. Boustani, Surface Sci., 1997, 370, 355; (b) I. Boustani, Phys. Rev. B, 1997, 55, 1. 6 I. Boustani, A. Quandt and A. Rubio, J. Solid State Chem., 2000, 154, 269. 7 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. 8 M. J. Frisch, G.W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A.Montgomery, Jr., R. E. Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels, K.N. Kudin, M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B.Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz, A. G. Baboul, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R. L. Martin, D.J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, J. L. Andres, C. Gonzalez, M. Head-Gordon, E. S. Replogle and J. A. Pople, Gaussian 98, Revision A.9, Gaussian, Inc., Pittsburgh PA, 1998. 9 M. Hildenbrand, H. Pritzkow and W. Siebert, Angew. Chem., Int. Ed. Engl., 1985, 24, 759. 10 M. Hargittai and I. Hargittai, The Molecular Geometries of Coordination Compounds in Vapour Phase, Akademia Kiado, Budapest, 1975. 11 A. F. Wells, Structural Inorganic Chemistry, 5th edn., Claredon Press, Oxford, 1986. 12 P. Power, Inorg. Chim. Acta, 1992, 198.200, 443. 13 A. Moezzi, R. A. Bartlett and P. P. Power, Angew. Chem., Int. Ed. Engl., 1992, 31, 1082. Figure 2 Shapes of multicentre ¥ò-molecular orbitals in the structure of 3 [MP2(full)/6-31G** calculations]. 6a'1 8e' Figure 3 Occupation of ¥�-orbitals in 3 [MP2(full)/6-31G** calculations]. 2 0 .2 .4 .6 .8 .10 .12 Received: 18th April 2001; Com. 01/17