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William F. Coleman Wellesley College Wellesley, MA 02481
Structure and Bonding in Hypervalent Iodine Compounds William F. Coleman Department of Chemistry, Wellesley College, Wellesley, Massachusetts 02481
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JCE Featured Molecules for September 2010
The impetus for this month's JCE Featured Molecules, the 100th column in this series (1), comes from the article by Jinsong Zhang and Jason A. Phillips (2) on the use of polymer-bound hypervalent iodine compounds as catalysts in organic reactions (see Figure 1). Whereas the focus in the article is on the synthesis and use of the catalyst, this column examines structure and bonding aspects of these compounds. Molecules that have been added to the collection are ICl, ICl3, I3-, PhICl2, PhIO, PhI(OAc)2, the Dess-Martin periodinane, and IBX (molecules 5 and 6 in ref 2), and 3D, rotatable images in MOL format of these molecules are available in the HTML version of this article. The concept of hypervalency has been the subject of several articles in this Journal. Jensen (3) has traced the history of the concept from early debates between Lewis and Langmuir, through the introduction of the term hypervalent in 1969, to the writing of his piece in 2006. Galbraith (4) has addressed the role of d orbitals in the bonding of hypervalent compounds and Curnow (5) has emphasized the role of n-centered, (2n-2)electron bonds in describing these systems. Weinhold et al. (6) have shown that accurate descriptions of the bonding in p-block oxides and oxyanions, normally treated as being hypervalent, require no expansion of the octet. As several authors have emphasized, teaching that such molecules are best described by an spxdy-hybridization model should be removed from the chemistry curriculum. The experimental and computed structures, and the results of detailed quantum mechanical calculations on several of the molecules included here, are certainly consistent with no d-orbital participation in the bonding of these systems. Several of these molecules provide interesting examples for students in computational chemistry courses or in inorganic or physical chemistry courses that emphasize computation. Calculations were done at the DFT level using the MIDI! basis set (one that is optimized for iodine) and the MPW1PW91 functional in Gaussian09 (7). As an example of how one might use these molecules in a problem set or laboratory assignment, consider PhICl2. Students should first be asked to predict the structure of the molecule based on their knowledge of VSEPR theory. If they then attempt to optimize the structure using semiempirical methods, they will find that the most stable structure is predicted to have the iodine atom in a trigonal pyramidal environment, and all atoms on the same plane. Optimization at the DFT level (and this is a good opportunity to point out that not all basis sets will contain data for iodine) produces T-shaped geometry about the iodine, and shows that the phenyl ring is at an angle to the
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Figure 1. In the past few decades, hypervalent iodine reagents have become increasingly popular in organic synthesis. Hypervalent iodine reagents are compounds that contain an iodine atom with more than 8 electrons in its valence shell. Some common hypervalent iodine compounds include (diacetoxyiodo)benzene (PhI(OAc)2), Dess-Martin periodinane (DMP), and 1-hydroxy-1,2-benziodoxol-3(1H)-one 1-oxide (abbreviated as o-iodoxybenzoic acid or IBX). Hypervalent iodinanes can be used for a wide range of chemical transformations, but particularly in oxidation reactions.(2).
ICl2 plane, an angle that is closer to orthogonal than planar. The value I find is 83.48°. Crystal structures have been reported for two crystal forms of this compound (8, 9) with dihedral angles of
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r 2010 American Chemical Society and Division of Chemical Education, Inc. pubs.acs.org/jchemeduc Vol. 87 No. 9 September 2010 10.1021/ed100725w Published on Web 07/20/2010
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On the Web
85.78° and 76.71°, respectively. The crystal structures give Cl-I-Cl angles of 179.07° and 176.65°, whereas the computed result is 176.33°. In all cases, the Cl-I-Cl bond is bent toward the phenyl group. The balance of such a problem assignment would be to examine the I-Cl bond lengths, compare them to those in ICl and ICl3, look at the computed charges on iodine in ICl, ICl3, and PhICl2, and examine the eigenfunctions produced by their calculations. From all of these data, students should be able to come up with a fairly complete description of the bonding in this molecule. The exercise could be extended by exploring PhIO and attempting to determine why the molecule is planar or by looking at several of the other structures and trying to rationalize why their frameworks, ignoring hydrogens, are not planar. A more literature-based assignment could involve exploring current thinking on the role that the bonding of these molecules plays in their catalytic properties. Coming full-circle to the term hypervalent, there have been several alternative terms suggested, including hypercoordinate (10) and hypobound (5). Jensen (3) favors hypercoordinate, whereas Curnow (5) favors hypobound. Perhaps less important than the specific term is the need to clarify precisely what is meant by each and what is not implied by any of them.
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Literature Cited 1. JCE Featured Molecules from Jun 2002 through Dec 2009 are available at the JCE Digital Library, http://www.jce.divched.org/ JCEWWW/Features/MonthlyMolecules/ (accessed Jul 2010). JCE Featured Molecules from Jan 2010 to the present are available in the HTML version of each column. 2. Zhang, J.; Phillips, J. A. J. Chem. Educ. 2010, 87, DOI: 10.1021/ ed100239c. 3. Jensen, W. B. J. Chem. Educ. 2006, 83, 1751. 4. Galbraith, J. M. J. Chem. Educ. 2007, 84, 783. 5. Curnow, O. J. J. Chem. Educ. 1998, 75, 910. 6. Suidan, L.; Badenhoop, J. K.; Glendening, E. D.; Weinhold, F. J. Chem. Educ. 1995, 72, 583. 7. The Official Gaussian Web site. http://www.gaussian.com/index. htm (accessed Jul 2010). 8. Archer, E. M.; van Schalkwyk, T. G. D. Acta Crystallogr. 1953, 6, 88. 9. Carey, J. V.; Chaloner, P. A.; Hitchcock, P. B.; Sedden, K. R. J. Chem. Res. 1966, 358, 2031. 10. Schleyer, P. v. R. Hypervalent Compounds. Chem. Eng. News 1984, 62 (22), 4.
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