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

Article abstract of DOI:10.1021/ja406500h

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) A for the C4-C4a bond. T

Full text of DOI:10.1021/ja406500h

Communication
pubs.acs.org/JACS
A C−C Bonded Phenoxyl Radical Dimer with a Zero Bond
Dissociation Free Energy
Jessica M. Wittman, Rebecca Hayoun, Werner Kaminsky, Michael K. Coggins, and James M. Mayer*
Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, United States
S
* Supporting Information
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.
eak chemical bonds have long attracted interest as they
Wtest aspects of chemists’ intuition regarding the nature
of bonding and relationships among bond strength, bond
length, and other parameters.1−4 Zavitsas described a linear
correlation of C−C bond lengths and bond strengths over a
>200 kcal mol−1 range,1 and the long-standing Badger’s rule
relates bond length and force constant.2 We describe here the
characterization of a phenoxyl radical dimer with an extremely
weak C−C bond.
Figure 1. (A) Dissociation of (tBu2(MeO)ArO)2 dimer to give
tBu2(MeO)ArO•. (B) ORTEP of the X-ray crystal structure of the
dimer (H atoms omitted for clarity).
group. To our knowledge, this is the first X-ray crystal structure
reported for a para-coupled phenoxyl radical dimer.
The metrical data for the structure of (tBu2(MeO)ArO)2
(Table 1) are consistent with the line structure in Figure 1A.
The C1−O1 bond length of 1.2227(15) Å is characteristic of a
CO double bond. The C2−C3 and C5−C6 bond distances
of 1.34 Å are typical of CC double bonds, while the C1−C2,
C3−C4, C4−C5, and C6−C1 bond distances are slightly
longer than 1.47 Å C(sp3)−C(sp2) single bonds. The length of
the C4−C4a bond connecting the two halves of the dimer is
1.6055(23) Å, which is much longer than the standard 1.54 Å
C(sp3)−C(sp3) bond distance.9 The recently reported structure
of the 9,10-dialkoxyanthracene radical cation dimer has a
1.637(5) Å C−C bond,10 and C−C bond lengths of other
similar structures range from 1.59 to 1.64 Å.11,12
Phenoxyl radicals are important in biology and natural and
synthetic antioxidant chemistry, and they are increasingly being
studied in radical and proton-coupled electron transfer (PCET)
reactions.5 Phenoxyl radicals that are protected at the 2-, 4-, and
6-positions, such as 2,6-di-tert-butyl-4-methoxyphenoxyl radical
[tBu2(MeO)ArO•] are unusually stable in oxygen-free
solutions.6 In 1955, Muller and Ley found that the radical
̈
t
content of Bu2(MeO)ArO• is not quantitative by magnetic
susceptibility, and they proposed the equilibrium formation of
the 4,4′-bis(cyclohexadienone) dimer (tBu2(MeO)ArO)2
(Figure 1A).7 Reported here are the X-ray crystal structure
and dissociation thermochemistry of this dimer.
The bis(cyclohexadienone) structure was proposed by
tBu2(MeO)ArO• was prepared by treating 2,6-di-tert-butyl-4-
methoxyphenol with potassium ferricyanide in a biphasic
medium of benzene and 1 M aqueous sodium hydroxide,
following a related procedure.8 The resulting purple oil was
crystallized from dry, oxygen-free acetonitrile at −30 °C to
form light-yellow crystals. The X-ray crystal structure revealed
the para-coupled dimer (Figure 1B). The dimer lies on a
crystallographic inversion center in the monoclinic C2/c space
Muller and Ley on the basis of quinone-like stretches at
̈
∼1600 and ∼1650 cm−1 in the solution IR spectrum of
tBu2(MeO)ArO•.7 An IR spectrum of our isolated solid dimer,
prepared as a KBr pellet, shows two strong IR stretches at 1646
and 1667 cm−1 (Figure 2A).
Received: June 28, 2013
© XXXX American Chemical Society
A
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Journal of the American Chemical Society
Communication
observed, while at ≥50 mM the dimer was evident from its
bands at 1646 and 1667 cm−1 (Figure 2B). The concentration
of tBu2(MeO)ArO• in solutions where both the monomer and
dimer were present was determined from the relatively weak
extinction coefficient at 1292 cm−1 (Figure S6).14 The dimer
concentrations were then calculated using mass balance. Fitting
the data to the equilibrium in Figure 1A (Figure S8) gave Keq =
1.3 0.2 M at 20 °C. This corresponds to a free energy of
−0.15 0.1 kcal mol−1.
Table 1. Comparison of Crystallographic and Calculated
Metrical Data for (tBu2(MeO)ArO)2 and Its Monomer
M06-2X/6-31+G(2d,2p)
bond/angle
C1−O1
X-ray structure
dimer
monomer
Bond Lengths (Å)
1.2227(15)
1.215
1.503
1.335
1.497
1.497
1.335
1.503
1.413
1.6065
1.244
1.470
1.378
1.403
1.411
1.367
1.474
1.342
−
C1−C2
C2−C3
C3−C4
C4−C5
C5−C6
C6−C1
C4−O2
C4−C4a
1.5026(16)
1.3377(17)
1.4955(16)
Equilibrium constants at temperatures from −20 to +20 °C
were determined by measuring the optical absorbance of
tBu2(MeO)ArO• in CCl4 solutions (Figure 3B). For
1.4989(16)
1.3371(17)
1.5005(16)
1.4297(14)
1.6055(23)
Dihedral Angle (deg)
142.0
C2−C3−C4−O2
145.9
180.0
Figure 3. (A) Van’t Hoff plot for dissociation of the dimer. (B) UV−
vis spectra of 10 μM (black) and 6 μM (red) tBu2(MeO)ArO• in CCl4
at 20 °C (solid line) and −20 °C (dashed line).
concentrated solutions, 1 or 2 mm path length cuvettes were
t
used and the absorbance of Bu2(MeO)ArO• was measured at
605 nm rather than at the peak maximum. A plot of absorbance
versus [tBu2(MeO)ArO•] for dilute (<20 mM) solutions of
tBu2(MeO)ArO• at 22 1 °C was linear with ε = 44 M−1 cm−1
at 605 nm (Figure S1). At concentrations above 20 mM and at
lower temperatures, such Beer’s Law plots were nonlinear,
t
suggesting loss of Bu2(MeO)ArO• to form the dimer (Figure
S2). The data were well-fit by the equilibrium in Figure 1A with
the assumption of mass balance (Figure S3). The value of Keq at
20 °C was found to be 1.4 0.3 M−1, in excellent agreement
with the value from the IR study. A van’t Hoff plot of the Keq
values from −20 to +20 °C (Figure 3A and Table S1) gave the
enthalpy and entropy of dimer dissociation as ΔH° = 6.1 0.5
kcal mol−1 and ΔS° = 21 1 cal mol−1 K−1.
Figure 2. (A) IR transmittance spectrum of the solid dimer (as a KBr
pellet prepared in a glovebox). (B) IR spectra of CCl4 solutions of
tBu2(MeO)ArO• at 50 mM (black), 250 mM (red), and 500 mM
(blue). The vertical axis represents the absorbance divided by
[tBu2(MeO)ArO•], as determined from the absorbance at 1292
cm−1 (Figure S6).
(tBu2(MeO)ArO)2 has an extremely weak C−C bond, with a
bond dissociation enthalpy (BDE) of 6.1 kcal mol−1. To our
knowledge, this is the weakest C−C bond for which the BDE
has been experimentally measured. The BDE is ≥8 kcal mol−1
smaller than the BDEs estimated by Mahoney and Weiner for
less crowded phenoxyl radical dimers.15 Gomberg’s triphenyl-
methyl radical forms a stronger C−C bond (BDE = 11 kcal
mol−1) and is only slightly dissociated in concentrated
solutions.16 Phenalenyl radical dimers with BDEs of 9.5−14
kcal mol−1 have been described.17 Scaiano and co-workers
reported BDEs of 15−26 kcal mol−1 for dimers of stabilized
Density functional theory (DFT) calculations on the dimer
at the M06-2X/6-31+G(2d,2p) level gave gas-phase bond
lengths in close agreement with measured solid-state distances
(Table 1).13 The computed C4−C4a bond length of 1.6065 Å
is within 3σ of the measured distance. The M06-2X functional
was chosen because it accounts for long-range non-covalent
interactions that can be an important factor in long C−C
bonds.1b The calculated gas-phase vibrational frequencies are
less close to the measured frequencies [Figure 2 and Figures S4
and S5 in the Supporting Information (SI)]. For instance, the
CO/CC stretching frequencies were calculated to be
1629, 1660 (two modes), and 1690 cm−1, versus the
experimental values of 1622, 1632, 1646, and 1667 cm−1
both in the solid state (KBr pellet) and in solution (CCl4)
(Table S2 in the SI).
carbon-centered radicals with similar C−C bond distances.12
A
formally iron(I)−pyridine complex was recently found to
undergo reversible coupling to a 4,4′-dimer with a BDE likely
similar to that in Gomberg’s dimer.18 More bulky derivatives
19
such as (4-tBuPh)3C• and 2,4,6-tBu3C6H2O• do not dimerize
and therefore must form weaker C−C bonds than tBu2(MeO)-
ArO•.
The C−C bond in (tBu2(MeO)ArO)2 is much weaker than
would have been expected on the basis of its bond distance of
1.6055(23) Å. A linear correlation between C−C bond length
and BDE has been proposed (Figure 4A).1 It would predict a
The equilibrium constant for dimer dissociation (Keq) at 20
2 °C was determined by solution IR spectroscopy in CCl4. At
t
low concentrations, only the monomer Bu2(MeO)ArO• was
B
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Journal of the American Chemical Society
Communication
particularly in the high-frequency C−C and C−O bonds (Table
1).
The force constant of the central C−C bond of (tBu2(MeO)-
ArO)2 was obtained from the DFT calculations. Badger’s rule
relates bond lengths re to force constants ke according to re =
(Cij/ke)1/3 + dij, where Cij and dij are constants for a set of
similar bonds.2,24 Remarkably, the Badger’s rule plot in Figure
4B shows that the (re, ke) pair for (tBu2(MeO)ArO)2 falls right
on the line formed by the values for simple hydrocarbons.
Thus, the long bond length in (tBu2(MeO)ArO)2 correlates
with the C−C bond force constant but not its BDE. Therefore,
the force constant does not correlate with the BDE. Both the
bond length and force constant are properties of the
equilibrium structure of (tBu2(MeO)ArO)2, while the BDE
also involves the energetics of the phenoxyl radical. The
exceptionally weak C−C bond in (tBu2(MeO)ArO)2, with a
measured BDE of 6.1 kcal mol−1, is due only in part to an
intrinsically poor C−C bond. The C−C bond length of
1.6055(23) Å is longer than the typical single-bond distance of
1.54 Å but is not long enough to explain the bond weakness.
The low BDE is in large part due to the substantial
reorganization of the phenoxyl radical. This causes the
substantial deviation from the suggested bond length/bond
strength correlation.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental details, analyses, X-ray crystallographic data
(CIF), and computational results. This material is available
free of charge via the Internet at http://pubs.acs.org.
Figure 4. (A) Plot of central C−C bond distance (r) vs bond
dissociation enthalpy (BDE), adapted from ref 1 with the data for
(tBu2(MeO)ArO)2 added (red diamond). Experimental values from
Zavitsas1a are shown as circles, with C2H6, C2H4, and C2H2 highlighted
as black circles. Triangles are calculated values from ref 1b. Blue circles
and green triangles show data taken from refs 12 and 21, respectively.
(B) Badger’s rule plot for the C−C bonds in C2H6, C2H4, C2H2, and
C6H6 (black circles; experimental data24) and the central C−C bond in
(tBu2(MeO)ArO)2 (red diamond; experimental bond distance and
calculated force constant; see the SI).
AUTHOR INFORMATION
■
Corresponding Author
mayer@chem.washington.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Primary financial support from the U.S. National Institutes of
Health (GM50422) and additional support from the University
of Washington are acknowledged. The authors thank Dr.
Alexander R. Fox, Dr. Tristan A. Tronic, and Joseph W. May
for thoughtful discussions.
BDE of >50 kcal mol−1 for this bond length, or a bond length
of >1.7 Å for such a weak C−C bond. Other deviations have
been reported and discussed.20 For example, the data for
Scaiano’s C−C dimers are included in Figure 4A,12 as are those
for recently reported alkanes with long C−C bonds (up to 1.71
Å) but strong attractive dispersion forces, which deviate from
the correlation in the opposite direction.21
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