Monday, April 16, 2012

Steric Crowding Can Stabilize a Labile Molecule: Solving the Hexaphenyethane Riddle

In the special issue “90 Years of Chemical Bonding”1 I expressed the opinion2 that despite of the historians’ characterization of chemistry as a “science without territory”, the chemical bond has been the traditional chemical territory and the heartland of chemistry ever since the chemical community amalgamated in the 17th Century.  While historians seem to ignore this “chemical territory”, the good news is that chemists have kept it fertile and teaming with activity, generating new bonding motifs and news (see for example, the recent highlight in CCH, April 4, 2012). One of the recent new and exciting bonding motifs, was found by Peter R. Schreiner and his group3 who have synthesized molecules with very long C-C bonds, 1.647-1.704 Å by Wurtz coupling of so-called diamondoid molecules (nanodiamonds), e.g., 1 which results from diamantane-diamantane coupling.  

                        1                                                                                                    2
Schreiner et al.3 used also DFT calculations to show that ~ 40% of the bond dissociation energy (BDE) of these long C-C bonds is due to the dispersion interactions, and argued that the sticky dispersion interactions are due to short H---H contacts (1.94 and 2.28 Å) that are maintained between the CH---HC faces of the diamondoid moieties.
In a subsequent study, which is described in the highlighted paper,4 Stefan Grimme and Peter R. Schreiner teamed to solve the riddles exhibited by hexaarylethanes.  The parent molecule, hexaphenylethane, is experimentally unknown as it dissociates readily into two Gomberg radicals, 2Ph3C•. Similarly, hexa-(para-tert-butylphenyl)-ethane dissociates into its corresponding radicals. By contrast, the apparently much more crowded hexa-(3,5-di-tert-butylphenyl)ethane (2) is a stable molecule with a very long C-C bond of 1.67 Å. Structure 2 is the only one that maintains attractive H---H interactions between the meta-di-substituted phenyl groups. Grimme and Schreiner showed that in all the hexaarylethanes, the BDE is negative in the absence of dispersion interactions, but becomes positive upon addition of dispersion. However, only 2 has a positive dissociation free energy. Furthermore, computational removal of the dispersion contributions of the tert-butyl groups would have made 2 unstable with BDE<0. Therefore, the sticky fingers that hold the C-C bond in 2 are the H---H interactions, which contribute 40 kcal/mol to the total BDE (>50%). These sticky fingers create a second minimum in the bond dissociation energy curve of 2. This minimum, so-called 2vdw, lies at a C-C distance of ~5.2 Å and its binding energy is purely dispersive and amounts to 26.6 kcal/mol. That is, 2vdw is held exclusively by the sticky H---H fingers!
The role of the sticky H---H fingers was subsequently probed by Fokin, Schreiner et al., to stick together layers of graphanes.5 The potential practical applications of these dispersion-supported bonds are wide ranging. There is something new and potable in chemical bonding!
(1) G. Frenking, S. Shaik, “90 Years of Chemical Bonding”, J. Comput. Chem. 2007, 28, 1-455.
(2) S. Shaik, “The Lewis Legacy: The Chemical Bond-A Territory and Heartland of Chemistry”, J. Comput. Chem. 2007, 28, 51-61.
(3) P. R. Schreiner, L. V. Chernish, P. A. Gunchenko, E. Y. Tikhonchuk, H. Hausman, M. Serafin, S. Schlecht, J. E. P. Dahl, R. M. K. Carlson, A. A. Fokin, “Overcoming Lability of Extremely Long Carbon-Carbon Bonds Through Dispersion Forces”, Nature, 2011, 477, 308-312.
(4) S. Grimme, P. R. Schreiner, “Steric Crowding Can Stabilize a Labile Molecule: Solving the Hexaphenylethane Riddle:, Angew. Chem. Int. Ed. 2011, 50, 12639-12642.
(5) A. A. Fokin, D. Gerbig, P. R. Schreiner, “ss- and p/p-Interactions Are Equally Important: Multilayered Graphanes”, J. Am. Chem. Soc. 2011, 133, 20036-20039.

Contributed by Sason Shaik

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