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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