Angewandte
Communications
Chemie
+
CÀC bond formation between DMAC and DMA (Scheme 2,
reaction pathway b).
Ered ¼ Eox þ ðRT=FÞlnKet
ð1Þ
The Ered value of 2 was confirmed by cyclic voltammetry
Figure S12), showing that the one-electron reduction process
Based on the experimental results described above, we
propose the overall mechanism of the DMA oxidation by 2
(
of 2 was reversible with the Ered value of 1.02 Æ 0.02 V (vs.
SCE), which agrees well with the value determined by the
redox titration (1.04 Æ 0.02 V vs. SCE). The large difference in
(
Scheme 2). First, electron transfer from DMA to 2 produces
+
III
+
DMAC and [(N4Py)Fe (NTs)] (reaction pathway a), fol-
lowed by the rate-determining CÀC bond formation step
+
the E values between 1 and 2 results in the drastic change in
between DMAC and DMA to produce a coupling radical
red
the mechanisms of the reactions of DMA with 1 and 2,
product and a proton (reaction pathway b). The coupling
radical product is rapidly oxidized by DMAC to produce
TMB and a proton (reaction pathway c). TMB is also readily
oxidized by DMAC to produce TMBC (reaction pathway d),
+
because the Eox value of DMA (0.73 V vs. SCE) is higher than
[13]
the Ered value of 1 (0.51 V vs. SCE)
but lower than the Ered
+
+
value of 2 (1.04 Æ 0.02 V vs. SCE). In such a case, electron
transfer from DMA to 1 is highly exergonic when hydrogen
atom transfer rather than electron transfer occurs for the N-
demethylation (Scheme 1A), whereas electron transfer from
[11]
since the Eox value of TMB (0.32 V vs. SCE) is much lower
[12]
than that of DMA (0.73 V vs. SCE). Therefore, the overall
stoichiometry agrees well with that shown in Scheme 1B.
Similarly, the dimerization of triphenylamine (TPA) was
observed in the electron transfer oxidation of TPA by 2 to
produce a TPA dimer radical cation (Figure S8), with the rate-
determining step of the dimerization with TPA (Figure S9).
Then, the one-electron reduction potential of 2 was
determined from the electron transfer equilibrium between
+
DMA to 2 occurs for the formation of TMBC (Scheme 1B
and Scheme 2).
Rates of electron transfer from TBPA to 2 were deter-
mined from the rise in the absorption band at 705 nm due to
+
TBPAC (Figure 2). The electron transfer rates obeyed
pseudo-first-order kinetics in the presence of a large excess
of TBPA (Figure 2, inset). The pseudo-first-order rate con-
stants (kobs) increased linearly with increasing concentration
of TBPA (Figure S13), and the second-order rate constant of
[12]
tris(4-bromophenyl)amine (TBPA) (E = 1.08 V vs. SCE)
and 2. While no electron transfer from TBPA to 1 (E
0
ox
=
red
[13]
.51 V vs. SCE)
occurs in CH CN at 298 K, efficient
3
electron transfer occurs from TBPA to 2 under the same
the electron transfer (k ) was determined from the slope of
et
reaction conditions (Figure 2a), where the absorption band at
the linear plot of k versus concentration of TBPA to be 8.5
obs
2
À1 À1
1
0 m s . Similarly, the k values of electron transfer from
et
a series of arylamine derivatives to 2 were determined, and
the ket values are listed in Table S1 (see also Figure S13),
together with the Eox values of arylamine derivatives and the
driving force of electron transfer, which was determined using
[
Eq. (2)], where e is the elementary charge.
ÀDGet ðeVÞ ¼ eðEredÀEox
Þ
ð2Þ
The driving force dependence of the electron transfer rate
constants is shown in Figure 3, where the logket values are
plotted against the ÀDG values. The driving force depend-
et
ence of k is well fitted by the solid line in Figure 3, in light of
et
the Marcus theory of adiabatic outer-sphere electron transfer
[
Eq. (3)],
Figure 2. Absorption spectral change for the formation of tris(4-
+
bromophenyl)amine radical cation (TBPAC ) produced in electron
2
IV
2+
ket ¼ Zexp½Àðl=4Þð1 þ DG =lÞ =k T
ð3Þ
transfer from TBPA (10 mm) to [(N4Py)Fe (NTs)] (0.125 mm) in
et
B
CH CN at 298 K. Inset shows the time course monitored by absorb-
3
ance change at 705 nm.
1
1
À1 À1
where Z is the collision frequency taken as 1 10 m s , l is
the reorganization energy of electron transfer, kB is the
+
[14]
[15,16]
7
05 nm is assigned to TBPAC . This result indicates that 2 is
Boltzmann constant, and T is the absolute temperature.
The l value is determined to be 1.89 eVas the best fit value of
IV
a stronger electron acceptor than the corresponding Fe -oxo
complex, 1. The electron transfer from TBPA to 2 was found
to be in equilibrium, where the final concentration of TBPAC
[Eq. (3)], and this value is significantly smaller than that of
+
[13]
1 (2.74 eV). The logk value of the reactions of DMAwith 2
et
produced increased with increasing initial concentrations of
TBPA to reach a constant value (Figure S10). The equilibrium
(number 5 in Figure 3) agrees with the Marcus line with l =
1.89 eV for the electron transfer from arylamine derivatives
to 2.
constant (K ) was determined to be 0.24 at 298 K (see the
et
Supporting Information, Experimental Section and Fig-
The higher Ered value of 2 than that of 1 was supported by
the density functional theory (DFT) calculations at the CAM-
B3LYP/6-311G(d) level of theory (Supporting Informa-
ure S11). Then, the one-electron reduction potential (Ered
)
of 2 was determined to be 1.04 Æ 0.02 V vs. SCE from the Ket
value and the Eox value of TBPA (1.08 V vs. SCE) using the
Nernst equation [Eq. (1)], which is much more positive than
[17,18]
tion),
which shows that the LUMO level of 2 (S = 1)
was 0.4 eV lower than that of 1 (S = 1; Figure S14). The bond
reorganization energies of electron transfer (li) of
[13]
the reported value of 1 (Ered = 0.51 V vs. SCE).
Angew. Chem. Int. Ed. 2016, 55, 3709 –3713
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3711