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Understanding Nonplanarity in Metallabenzene Complexes

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1986 Organometallics 2007, 26, 1986-1995 Understanding Nonplanarity in Metallabenzene Complexes Jun Zhu, Guochen Jia,* and Zhenyang Lin* Department of Chemistry, The Hong Kong UniVersity of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, People’s Republic of China ReceiVed February 11, 2007 The nonplanarity found in metallabenzene complexes has been investigated theoretically via density functional theory (DFT) calculations. A metallabenzene has four occupied π molecular orbitals (8 π electrons) instead of three that benzene has. Our electronic structure analyses show that the extra occupied π molecular orbital, which is the highest occupied molecular orbital (HOMO) in many metallabenzenes, has antibonding interactions between the metal center and the metal-bonded ring-carbon atoms, providing the electronic driving force toward nonplanarity. Calculations indicate that the electronic driving force toward nonplanarity, however, is relatively small. Therefore, other factors such as steric effects also play important roles in determining the planarity of these metallabenzene complexes. In this paper, how the various electronic and steric factors interplay has been discussed. Introduction benzene complexes. For example, the formation mechanism and chemical reactivity of metallabenzene complexes have been Metallabenzenes, organometallic compounds formed by extensively studied.17 formal replacement of a CH group in benzene by an isolobal In the studies of metallabenzene complexes, a central issue transition metal fragment, were first considered theoretically by concerns the π-conjugation of the six-membered metal-contain- Hoffman et al. in 1979.1 Since the isolation of the first stable 2 ing ring. Indeed, it is true that metallabenzene complexes are osmabenzenes by Roper’s group in 1982, metallabenzene highly conjugated in view of the fact that the single-double complexes have attracted considerable interest over the last bond alternation is insignificant in the six-membered metal- quarter century. Experimentally, research efforts have so far led containing ring for the majority of the complexes. Although to the synthesis and characterization of numerous metallaben- the structures illustrated in Figure 1 show the single-double zene complexes.3 Examples of structurally characterized met- 4-16 bond alternation patterns, we hope that readers will not take allabenzene complexes are given in Figure 1. Theoretically, them literally. The single-double bond alternation patterns are there are also efforts to understand the chemistry of metalla- drawn for convenience as we normally do for benzene. While it is generally believed that metallabenzene complexes are highly * Corresponding authors. E-mail: [email protected]; [email protected]. (1) Thorn, D. L.; Hoffman, R. NouV. J. Chim. 1979, 3, 39. conjugated systems, it is interesting to note that in many of these (2) Elliott, G. P.; Roper, W. R.; Waters, J. M. J. Chem. Soc., Chem. complexes the six-membered metal-containing ring deviates Commun. 1982, 811. appreciably from planarity; that is, the metal center is signifi- (3) For recent reviews, see: (a) Bleeke, J. R. Acc. Chem. Res. 1991,24, 271. (b) Bleeke, J. R. Chem. ReV. 2001, 101, 1205. (c) Wright, L. J. Dalton cantly displaced out of the ring plane (Figure 2). The nonpla- Trans. 2006, 1821. (d) Landorf, C. W.; Haley, M. M. Angew. Chem., Int. narity is quite unexpected because conjugation normally requires Ed. 2006,45, 3914. planar arrangement. When we use the two dihedral angles (D1 (4) For 1, see: Hung, W. Y.; Zhu. J.; Wen, T. B.; Yu, K. P.; Sung, H. and D2 shown in Figure 1) around the two metal-carbon bonds H. Y.; Williams, I. D.; Lin, Z. Y.; Jia, G. C. J. Am. Chem. Soc. 2006, 128, 13742. in the six-membered metal-containing ring as the measure of (5) For 2 and 3, see: (a) Alvarez, E.; Paneque, M.; Poveda, M. L.; planarity, we can see that nonplanarity is significant in many Rendon, N. Angew. Chem., Int. Ed. 2006,45, 474. (b) Ilg, K.; Paneque, of these complexes, although there are still more than half of M.; Poveda, M. L.; Rendon, N.; Santos, L. L.; Carmona, E.; Mereiter, K. Organometallics 2006, 25, 2230. the complexes with approximately planar six-membered metal- (6) For 4 and 5, see: Paneque, M.; Posadas, C. M.; Poveda, M. L.; containing rings. In a previous work,4 we briefly discussed the Rendon, N.; Salazar, V.; Onate, E.; Mereiter, K. J. Am. Chem. Soc. 2003, nonplanarity found in a few osmabenzene complexes and 125, 9898. suggested that the nonplanarity is likely due to electronic (7) For 6, see: (a) Rickard, C. E. F.; Roper, W. R.; Woodgate, S. D.; Wright, L. J. J. Organomet. Chem. 2001, 623, 109. (b) See also ref 2. reasons. In this article, we attempt to analyze the structure and (8) For 7, see: Rickard, C. E. F.; Roper, W. R.; Woodgate, S. D.; Wright, bonding in detail for a wide range of metallabenzene complexes L. J. Angew. Chem., Int. Ed. 2000,39, 750. and to understand the nonplanarity with the aid of density (9) For 8, see: Gilbertson, R. D.; Lau, T. L. S.; Lanza, S.; Wu, H. P.; Weakley, T. J. R.; Haley, M. M. Organometallics 2003, 22, 3279. (10) For 9, see: (a) Wu, H. P.; Weakley, T. J. R.; Haley, M. M. Chem.- (14) For 16 and 17, see: Zhang, H.; Xia, H. P.; He, G. M.; Wen, T. B.; Eur. J. 2005, 11, 1191. (b) Gilbertson, R. D.; Weakley, T. J. R.; Haley, M. Gong, L.; Jia, G. C. Angew. Chem., Int. Ed. 2006,45, 2920. M. J. Am. Chem. Soc. 1999, 121, 2597. (15) For 18, see: Jacob, V.; Weakley, T. J. R.; Haley, M. M. Angew. (11) For 10 and 11, see: (a) Bleeke, J. R.; Xie, Y. F.; Peng, W. J.; Chiang, Chem., Int. Ed. 2002,41, 3470. M. J. Am. Chem. Soc. 1989, 111, 4118. (b) Bleeke, J. R.; Behm, R.; Xie, (16) For 19, see: Landorf, C. W.; Jacob, V.; Weakley, T. J. R.; Haley, Y. F.; Chiang, M. Y.; Robinson, K. D.; Beatty, A. M. Organometallics M. M. Organometallics 2004, 23, 1174. 1997, 16, 606. (17) (a) Iron, M. A.; Lucassen, A. C. B.; Cohen, H.; van der Boom, M. (12) For 12 and 13, see: Bleeke, J. R.; Behm, R. J. Am. Chem. Soc. E.; Martin, J. M. L. J. Am. Chem. Soc. 2004, 126, 11699. (b) Iron, M. A.; 1997, 119, 8503. Martin, J. M. L.; van der Boom, M. E. J. Am. Chem. Soc. 2003, 125, 13020. (13) For 14 and 15, see: Xia, H. P.; He, G. M.; Zhang, H.; Wen, T. B.; (c) Iron, M. A.; Martin, J. M. L.; van der Boom, M. E. J. Am. Chem. Soc. Sung, H. H. Y.; Williams, I. D.; Jia, G. C. J. Am. Chem. Soc. 2004, 126, 2003, 125, 11702. (d) Iron, M. A.; Martin, J. M. L.; van der Boom, M. E. 6862. Chem. Commun. 2003, 132. 10.1021/om0701367 CCC: $37.00 © 2007 American Chemical Society Publication on Web 03/16/2007 Understanding Nonplanarity in Metallabenzene Complexes Organometallics, Vol. 26, No. 8, 2007 1987 Figure 1. Structurally characterized metallabenzene complexes. D1 and D2 stand for the dihedral angles of C2-C1-M-C5 and C4- C5-M-C1, respectively. general? How do other forces, such as steric forces, interact with the electronic forces in determining the nonplanarity observed in the class of metal complexes? Computational Details Figure 2. Nonplanarity observed in many metallabenzene com- All structures of model metallabenzene complexes were opti- plexes given in Figure 1. mized at the mPW1K18 level of density functional theory. The functional theory calculations. We hope to answer the following (18) Lynch, B. J.; Fast, P. L.; Harris, M.; Truhlar, D. G. J. Phys. Chem. questions. Is there an electronic reason for the nonplanarity in A 2000, 104, 4811. 1988 Organometallics, Vol. 26, No. 8, 2007 Zhu et al. mPW1K level of theory has been proved to provide reliable results six π molecular orbitals of benzene. The left column of Figure on the structures and energetics in the calculations of metallaben- 4b shows the six benzene-like molecular orbitals for the six- zene complexes.17 Frequency calculations at the same level of theory membered metal-containing ring that incorporates the symmetry- have also been performed to confirm that all stationary points were adapted dyz orbital, one of the “t2g” orbitals of the metal minima (no imaginary frequencies). The Stuttgart/Dresden effective fragment, from the metal center. The six benzene-like molecular core potentials19 were used to describe Pt, Ru, Os, Ir, P, Si, S, I, orbitals are derived simply by substituting one of the carbon pπ Br, and Cl atoms, while the standard 6-31G basis set was used for orbitals with dyz (see the far right of Figure 4 for the Cartesian C, N, B, O, and H atoms. Polarization functions were added for Pt ) ) ) coordinates). The central column of Figure 4b shows the (ú(f) 0.883), Ru (ú(f) 1.072), Os (ú(f) 0.707), and Ir (ú(f) resulting d and π molecular orbitals of a metallabenzene ) 0.792) as well as Cl (ú(d) ) 0.514), Br (ú(d) ) 0.389), I (ú(d) complex that are derived from the orbital interaction between ) 0.266), Si (ú(d) ) 0.262), S (ú(d) ) 0.421), N (ú(d) ) 0.8), C (ú(d) ) 0.8), and P (ú(d) ) 0.34) that are directly bonded to the the six benzene-like molecular orbitals and the remaining two 2 2 metal center.17,20 All calculations were performed with the Gaussian d orbitals (dxz and dx -y ) of the “t2g” set. The π1, π3, π5, and π7 03 software package.21 Molecular orbitals obtained from the molecular orbitals shown in the central column of Figure 4b mPW1K calculations were plotted using the Molden 3.7 program are simply from the benzene-like molecular orbitals.
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  • Block – 2: Organic Reaction Mechanism Lecture-3

    Block – 2: Organic Reaction Mechanism Lecture-3

    Programme Code- MSCCH-17/18/19 Course Code-CHE- 502 Block – 2: Organic reaction mechanism Lecture-3 Presented by - Dr. Charu C. Pant Department of Chemistry Uttarakhand Open University, Haldwani LEARNING OUTCOMES After studying this Chapter, you shall be able to: To know about electronic effect and its types inductive effects Classify a group into +I or –I group Understand mesomeric or resonance effect Applications of Electronic Effects To Know about aromaticity Be familiar with the terms aromatic and anti-aromatic Classify any organic compound into aromatic, anti-aromatic and aliphatic (or non- aromatic) To know about Resonance, Conjugation and Cross Conjugation INTRODUCTION: You are already familiar with various electronic effects. But to understand the properties of organic molecules, like acidity, basicity, stability of various species, and to understand the organic reactions and their mechanism, it is very important that we revisit the electronic effects operating especially with a detailed study of conjugated chemical bonding. During most of the organic reactions, reagents either undergo heterolysis to give rise to electrophiles and/ or nucleophiles; or they undergo homolysis to give rise to free radicals. Electrophiles being electron deficient search for electron rich site in the organic substrate to attack and form the bond. But, if there are more than one electron rich sites in the organic substrate, the electrophile obviously would prefer to attack on the most electron rich site. On the other hand, nucleophiles being electron rich, search for electron deficient sites in the organic substrate. Types of Electronic Effects: There are four types of electronic effects mainly, viz. inductive effect, mesomeric (or resonance) effect, electromeric effect and hyperconjugative effect.