G. Rotko et al. / Electrochemistry Communications 37 (2013) 64–67
65
3. Quantum chemical calculations
All calculations were carried out with Gaussian program package [16]
using density functional theory (DFT). The B3LYP [17,18] hybrid
functional with 6–31+G(d,p) basis set was employed for geometry
optimisations and calculation of harmonic frequencies followed by
single-point energy calculations with a larger triple-ζ basis 6–311+G(d,
p). Open-shell species were treated within the spin-unrestricted scheme.
The polarisable continuum solvation model (IEF-PCM) [19] was used to
account for the effect of DMF solvent. The reported energies and barriers
include dispersion corrections obtained from a single-point DFT-D2 [20]
calculation on top of the DFT-optimised structures. We found that this
computational protocol satisfactorily takes into account van der Waals
interactions [21]. The standard Gibbs energies are given for 1 mol·L−1
at 298.15 K. Reduction potentials were calculated from the total free
energy of an electron attachment in solution.
Fig. 2. Difference between the consecutive spectra during electrolysis of DecaBDE
at −1.8 V (solid lines) and the comparison spectrum of HeptaBDF (dotted line).
4. Results and discussion
also found that lesser quantities of 2-propanol (50-fold excess), a better
H• donor, suffice to block the cyclisation. This indicates that the
cyclisation is suppressed rather by H-transfer than by H+ transfer. In
the presence of effective H-donors, the reaction proceeds in consecutive
steps of debromination, and the final product is non-brominated
diphenyl ether.
To gain a deeper insight into the mechanism of the cyclisation, DFT-
D calculations were performed at the PCM-B3LYP-D2/6-311+G(d,p)//
PCM-B3LYP/6-31+G(d,p) level. Fig. 3a demonstrates the contour plot
of the lowest unoccupied molecular orbital (LUMO) for DecaBDE. This
orbital has an antibonding σ*C–Br character; thus, the electron transfer
onto DecaBDE in DMF should follow a concerted dissociative electron-
transfer (DET) pathway. In fact, an optimisation of DecaBDE•− in
solution leads to its spontaneous breaking into the radical-anion pair
(Ar•, Br−)solv (Fig. 3b), which corresponds well with the obtained values
of the transfer coefficient, α, as low as 0.25, characteristic of the
concerted DET [24]. This result is in sharp contrast to the dominant
view that ET to aromatic halides gives stable transient radical anions
(stepwise mechanism) [24–26], which indeed is true in the case of
molecules with π*-type LUMO.
Cyclic voltammetry (CV) of DecaBDE in dry DMF showed un-
expectedly a quasi-reversible wave at the end of the reduction (Fig. 1).
This wave was found to be due to non-brominated dibenzofuran by
comparison with an authentic sample. It also appeared that the
voltammogram of HeptaBDF is virtually identical with that of DecaBDE
in the range of the last six waves, as shown in Fig. 1a. This would
indicate that the cyclisation to dibenzofurans occurs in the early stage
of the reduction. To prove it, we analysed the reaction mixture taken
from under the electrode after electrolysis at −1.8 V vs. Fc•+/0. GC–MS
showed the presence of mainly 2,2′,3,3′,4,4′,5,5′,6-nonabromodiphenyl
ether (o-NonaBDE) as the principal constituent, but also the presence of
HeptaBDF. We also recorded UV–vis spectra of the mixture adjacent to a
Pt band electrode during electrolysis at −1.8 V vs. Fc•+/0. At the
beginning, the spectrum pointed to DecaBDE decay and NonaBDEs
formation mixed with some unidentified compounds. At the later
stage, the differences between subsequent spectra became clearer, as
demonstrated in Fig. 2. Eventually, the spectrum of HeptaBDF emerged,
but blurred by a spectrum of another product(s). It can be due to
oligomeric substances usually observed at the electrode.
The cleaved C\Br bond is that in the ortho position and the
adduct 1•⋯Br− thus formed readily dissociates in DMF, yielding free
Br− (detected in CV) and radical 1• that is more stable in energy
than the p- and m-isomer by 1.0 and 2.0 kcal·mol−1, respectively.
Next, the radical 1• undergoes outer-sphere reduction to an anion
(E0calcd = 0.03 V vs. Fc•+/0) that abstracts a proton, most probably from
residual water, yielding o-NonaBDE. The results of GC–MS analysis
(vide supra) fully confirm that the main product of the first step of
debromination is o-NonaBDE.
The addition of alcohols brings about a change in the voltammetric
pattern. As shown in Fig. 1b, a 200-fold excess of 1-propanol totally
inhibits the formation of dibenzofuran. The reduction waves are shifted,
which proves that we are dealing with a different mechanism.
Moreover, beginning with a potential of −1.7 V, the number of waves
decreased to four, which corresponds to the number expected for the
reduction of OctaBDEs. This is in agreement with a typical pattern
observed for polyhalogenated arene reduction, where the number of
waves matches the number of halogen atoms in the ring [22,23]. We
Fig. 1. Semi-differential CVs of (a) DecaBDE (2mM) and HeptaBDF (satd., ca. 1mM) (please note the wave at −2.9V ascribed to non-brominated dibenzofuran quasi-reversible reduction),
(b) DecaBDE prior to and after addition of a 200-fold excess of 1-propanol. Conditions: DMF/0.1 M n-Bu4NBF4 at GCE, υ = 0.1 V s−1
.
Rotko, Grzegorz
