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. 2013 Sep 4;135(35):12956-9.
doi: 10.1021/ja406500h. Epub 2013 Aug 23.

A C-C bonded phenoxyl radical dimer with a zero bond dissociation free energy

Jessica M Wittman  1 , Rebecca Hayoun, Werner Kaminsky, Michael K Coggins, James M Mayer
Affiliations

A C-C bonded phenoxyl radical dimer with a zero bond dissociation free energy

Jessica M Wittman et al. J Am Chem Soc. .

Abstract

The 2,6-di-tert-butyl-4-methoxyphenoxyl radical is shown to dimerize in solution and in the solid state. The X-ray crystal structure of the dimer, the first for a para-coupled phenoxyl radical, revealed a bond length of 1.6055(23) Å for the C4-C4a bond. This is significantly longer than typical C-C bonds. Solution equilibrium studies using both optical and IR spectroscopies showed that the Keq for dissociation is 1.3 ± 0.2 M at 20 °C, indicating a C-C bond dissociation free energy of -0.15 ± 0.1 kcal mol(-1). Van't Hoff analysis gave an exceptionally small bond dissociation enthalpy (BDE) of 6.1 ± 0.5 kcal mol(-1). To our knowledge, this is the smallest BDE measured for a C-C bond. This very weak bond shows a large deviation from the correlation of C-C bond lengths and strengths, but the computed force constant follows Badger's rule.

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Figures

Figure 1
Figure 1
(A) Dissociation of (tBu2(MeO)ArO)2 dimer to tBu2(MeO)ArO•. (B) ORTEP of the X-ray crystal structure of the dimer.
Figure 2
Figure 2
(A) IR transmittance spectrum of solid dimer (as a KBr pellet prepared in a glove box). (B) IR spectra of CCl4 solutions of tBu2(MeO)ArO• at 50 mM (black), 250 mM (red), and 500 mM (blue). The vertical axial is the absorbance divided by the [tBu2(MeO)ArO•] in each solution, determined from the A(1292 cm−1) (Figure S6).
Figure 3
Figure 3
(A) Van’t Hoff plot for dissociation of dimer. (B) UV-vis spectra of 10 µM (black) and 6 µM (red) tBu2OMeArO• in CCl4 at 20 °C (solid line) and −20 °C (dashed line)
Figure 4
Figure 4
(A) Plot of central C–C bond distances r (Å) vs. bond dissociation enthalpies (BDEs, kcal mol−1), adapted from references with data for (tBu2OMeArO)2 added (red diamond). Experimental values from Zavitsas are shown as circles, with C2H6, C2H4, C2H2 highlighted as black circles; triangles are calculated values; blue circles and green triangles are from and . (B) Badger’s rule plot for the C–C bonds in C2H6, C2H4, C2H2, C6H6 (black circles; experimental data) and the central C–C bond in (tBu2OMeArO)2 (red diamond; experimental bond distance and calculated force constant (see SI).
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