Mendeleev Communications Electronic Version, Issue 4, 2001 (pp. 125.164) Low-energy barrier B4 ring puckering rearrangement of 1,6-diaza-closo-hexaborane: 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 434 5667; e-mail: minyaev@ipoc.rsu.ru 10.1070/MC2001v011n04ABEH001475 The ab initio [MP2(fu)/6-311+G**] and DFT (B3LYP/6-311+G**) calculations predict stable structures of closo-diazaboranes 1,6-N2B4H4 and 1,2-N2B4H4, with the low-energy barrier B4 ring puckering rearrangement occurring in the 1,6-N2B4H4 stable structure. According to both experimental1.3 and computational data,4,5 closo-dicarborane 1,6-C2B4H6 1, which is isoelectronic to closoborane B6H6 2. 2, has a stable D4h-symmetry structure and is energetically preferable than its 1,2-isomer 3. It may be expected that similar stable structures are also characteristic of diaza-closo-boranes 1,6-N2B4H4 4 and 1,2-N2B4H4 5 isoelectronic to 1 and 3, respectively, and that 4 is more stable than its isomer 5. Indeed, early preliminary PRDDO calculations6 on N2B4H4 showed structure 4 to be more stable than 5, although the distorted trigonal prism to be predicted the most stable structure. More recent ab initio calculations7,8 also showed that 1,6-isomer 4 is more stable than 1,2-5.However, in both cases it was found that structure 4 does not correspond to a minimum on the N2B4H4 potential-energy surface (PES) and it was not studied the distortion directions from the D4h structure of 4. In this work, we performed ab initio [MP2(fu)/6-311+G**] and density functional theory (B3LYP/6-311+G**) calculations9,10 on compounds 4 and 5.For comparison, we also calculated the structures of closo-dicarboranes 1 and 3 at the same level of approximation. In agreement with published data,7,8 our ab initio calculations revealed that the structure of 4 of D4h symmetry corresponds to a saddle point rather than a minimum on the PES N2B4H4 and is the transition state for the low-energy barrier of the B4 ring puckering rearrangement 6a 4 6b. At the same time, 1,2- isomer 5, much like as its isoelectronic analogue 3, has a stable structure of C2v symmetry and is energetically less favourable than 1,6-isomer 6.According to the calculations, the structures of 1, 3 and 5, 6 correspond to minima (l = 0; hereafter, l designates the number of negative hessian eigenvalues) on the PESs of C2B4H6 and N2B4H4, respectively. The calculated geometric and energy parameters of these structures and the saddle point for the structure of 4 are depicted in Figures 1 and 2 and listed in Table 1. As can be seen in Table 1 and Figure 1, the calculated geometric characteristics of closo-dicarboranes 1 and 3 are in good agreement with the gas-phase experimental data1.3 and those obtained in previous theoretical studies.4,5 All calculated bond lengths are in BH BH CH BH HB CH BH BH BH BH HB BH 2.CH BH BH CH HB BH 1, D4h 2, Oh 3, C2v BH BH N BH HB N 4, D4h N BH BH N HB BH 5, C2v 1.627 1.624 1.635¡¾0.004 1.633¡¾0.004 1.716 1.712 1.725¡¾0.012 1.720¡¾0.004 1.735 1.733 1.752¡¾0.005 1.716 1.706 1.721¡¾0.015 1.624 1.629 1.605¡¾0.005 1.540 1.543 1.540¡¾0.005 1, D4h 3, C2v MP2 DFT Experimental } Figure 1 Geometry parameters of structures 1 and 3 calculated by ab initio (MP2/6-311+G**) and DFT (B3LYP/6-311+G**) methods.Experimental data for 1 are taken from ref. 1 (upper numbers) and from ref. 2 (lower numbers) and for 3 from ref. 3. The bond lengths and angles are indicated in angstrom units and degrees, respectively. C C C C Table 1 Results of ab initio [MP2(fu)/6-311+G**] and DFT (B3LYP/6- 311+G**) calculations for the structures of 1, 3.7.a aEtot (in a.u.) and .E are the total and relative energies (1 a.u. = 627.5095 kcal mol.1); l is the number of the negative hessian eigenvalues; ZPE (in a.u.) is the harmonic zero-point correction; .EZPE (in kcal mol.1) is the relative energy including harmonic zero-point correction; w1 (in cm.1) is the smallest or imaginary harmonic vibration frequency. bResults correspond to a slope point with 5 A distance from N2 to the BB bond.Structure Method Etot l .E ZPE .EZPE w1 1, D4h MP2 DFT .178.784605 .179.284851 00 00 0.086648 0.086128 00 421 380 3, C2v MP2 DFT .178.769941 .179.270983 00 9.2 8.7 0.086734 0.086115 9.2 8.7 433 395 4, D4h MP2 DFT .210.822065 .211.348393 11 1.0 4.2 0.060956 0.060449 1.3 4.6 i184 i268 5, C2v MP2 DFT .210.804263 .211.333654 00 12.2 13.5 0.061988 0.061182 13.1 14.3 314 253 6, D2d MP2 DFT .210.823731 .211.355047 00 00 0.060607 0.059728 00 231 300 7, Cs MP2 DFTb .210.757200 .211.294522 0 slope 41.7 37.9 0.056079 . 38.9 . 17 . B4H4, Td MP2 DFT .101.584553 .101.409193 00 .. 0.050657 0.049508 .. 617 609 N2, D¡Íh MP2 DFT .109.346230 .109.559694 00 .. 0.005570 0.004906 .. 2445 2151 BH BH N BH HB N 4, D4h BH BH N BH HB N 6a, D2d BH BH N BH HB N 6b, D2dMendeleev Communications Electronic Version, Issue 4, 2001 (pp. 125.164) the range of the experimental values accounted for experimental errors. 1,6-Dicarbo-closo-hexaborane 1, 1,6-C2B4H6 was found to be more stable than 1,2-isomer 3 by 9.2 kcal mol.1 at the MP2 level and by 8.7 kcal mol.1 at the DFT level. These values are consistent with the previous estimation (9.5 kcal mol.1) obtained at the MP2/6-31G** level.4 No experimental data on the heats of formation of 1 and 3 are currently available.The stable structure of 1,6-diaza-closo-hexaborane 6 has D2d symmetry with two short (MP2, 1.500 and DFT, 1.454 A) and two long (MP2, 1.750 and DFT, 1.807 A) BN bonds. The basal B4 ring has a boat conformation; the B.B bond lengths are equal to 1.699 (MP2) and 1.454 (DFT) A. This value is very close to those of the basal B.B bonds in 1 and 3.Planarization of the B4 basal cycle, 6 ¢ç 4, results in equalization of all the BN bonds and shortening of the B.B bonds. The structure of 4 is the true transition state structure (l = 1, this identification of stationary point agrees with the result of Jemmis and Subramanian8 but disagrees with McKee¡�s7 results l = 3) for the puckering rearrangement 6a 4 6b with the energy barrier as low as 1.0 (MP2) or 4.2 (DFT) kcal mol.1.Accounting for zero-point energy (ZPE) does almost not change the energy barrier. The tendency of the D4h structure of 1,6-diaza-closo-hexaborane 4 to the D4h ¢ç D2d distortion is explained by the orbital interaction diagram (Figure 3), which shows that this distortion leads to slightly lowering the energy level of the bonding 1e orbitals of the D2d cluster.Although the D4h structure 4 satisfies to the 10e electrons rule formulated for the stable bipyramidal structures of main-group element clusters,11 the orbital interaction providing for the stabilization of structures of this type, namely, mixing in the antibonding combination of p-orbitals of apical centers and eg orbitals of the basal cycle, is weakened in 4, as compared to that in its carbon analogue 1.This is due to a widened energy gap between these orbitals in 4 caused by a greater electronegativity of nitrogen, which also results in less diffuse p-orbitals and their smaller overlap with eg orbitals of the basal cycle. As congeneric 1,2-dicarbo-closo-hexaborane 3, 1,2-diaza-closohexaborane 5 has a stable C2v structure with a planar basal boron ring.It contains BN bonds of two types: short [1.550 (MP2), 1.536 (DFT) A] and long [1.639 (MP2), 1.632 (DFT) A]. This diazaborane is by 12.2 (MP2) and 13.5 kcal mol.1 (DFT) energy disfavoured as compared to 1,6-isomer 6. Note that whereas for dicarboranes 1 and 3 and 1,2-diaza-closo-hexaborane 5 the results of MP2 calculations are consistent with those of the DFT method, for 1,6-diaza-closo-hexaborane 6 the bond lengths predicted by MP2 and DFT methods notably differ (~0.05 A).The system 5 can be considered as a tight complex resulted from the interaction of dinitrogen with borane B4H4. In this context, a question arises whether N2 and B4H4 can form a stable pre-reaction colex subsequently convertible to 5.No such a complex has been found by DFT calculations: the interaction between N2 and B4H4 was repulsive at any distances. This finding is consistent with the conclusion that DFT methods do not correctly describe longrange interactions.13 At the same time, MP2 calculations predict the appearance of stable complex 7 stabilised by induced dipole. dipole interactions between its components.The complex is 1.1 kcal mol.1 stabilised relative to separated components (no account is done for the superposition error). Such a weak interaction does not affect the geometric parameters of N2 and B4H4 moieties in complex 7 as compared to separated molecules. Complex 7 is 41.7 (at MP2 level) or 37.9 kcal mol.1 (at DFT level) less stable than 1,2-isomer 5. In conclusion, the MP2 and DFT calculations on hypothetical diaza-closo-boranes 5 and 6 indicate that these compounds, which are isoelectronic to dicarbo-closo-hexaboranes 1 and 3, respectively, possess stable highly symmetric structures.Compound 6 was predicted to be susceptible to undergo the lowenergy barrier B4 ring puckering rearrangement 6a 4 6b. This work was supported by the Russian Foundation for Basic Research (grant nos. 01-03-32546 and 00-15-97320). References 1 V. S. Mastryukov, O. V. Dorofeeva, L. V. Vilkov, A. F. Zhigach, V. T. Laptev and A. B. Petrunin, J. Chem. Soc., Chem. Commun., 1973, 276. 2 E. A. McNeill, K. L. Gallaher, F. R. Scholer and S. H. Bauer, Inorg. Chem., 1973, 12, 2108. 3 R. A. Beadet and R. L. Poyntner, J. Chem. Phys., 1970, 53, 1899. 4 M.L. McKee, J. Am. Chem. Soc., 1992, 114, 879. 5 M.Buhl and P. von R. Schleyer, J. Am. Chem. Soc., 1992, 114, 477. 6 T. A. Halgren, I. M. Pepperber and W. N. Lipscomb, J. Am. Chem. Soc., 1975, 97, 1249. 7 M. J. McKee, J. Phys. Chem., 1991, 95, 9273. 8 E. D. Jemmis and G. Subramanian, J. Phys. Chem., 1994, 98, 9222. 9 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.MP2 DFT 1.500 1.454 1.750 1.807 1.699 1.737 N N 6, D2d (l = 0) 4, D4h (l = 1) 5, C2v (l = 0) 7, Cs (l = 0) 1.609 1.598 1.663 1.658 1.637 1.619 1.773 1.785 1.550 1.536 1.668 1.663 N N 1.639 1.632 1.692 1.679 1.120 1.096 164.6 3.426 ¡Í 1.691 1.680 1.7(00) 1.119 1.096 1.097632(20) 3.426 ¡Í MP2 DFT Experimental Td D¡Íh Figure 2 Geometry parameters of structures 5.7 and borane B4H4 and dinitrogen calculated by ab initio (MP2/6-311+G**) and DFT (B3LYP/ 6-311+G**) methods.Experimental data for B4H4 are given for B4Cl4 12 and for N2 are taken from ref. 14. The bond lengths and angles are indicated in angstrom units and degrees, respectively. N N N N N N 3 2 .14 .15 2eg 2e eg 1eg 1e ¥�g D4h D2d Figure 3 Diagram of formation of bonding molecular orbitals in 4 and 6. E/eVMendeleev Communications Electronic Version, Issue 4, 2001 (pp. 125–164) 10 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. 11 V. I. Minkin, R. M. Minyaev and Yu. A. Zhdanov, Nonclassical Structures of Organic Compounds, Mir, Moscow, 1987. 12 J. A. Morrison, Chem. Rev., 1991, 91, 35. 13 K. Müller-Dethiefs and P. Hobza, Chem. Rev., 2000, 100, 143. 14 R. J. Butcher and W. J. Jones, J. Chem. Soc., Faraday Trans. 2, 1974, 560. Received: 10th May 2001; Com. 01/18