Please do not adjust margins
Chemical Science
Page 4 of 6
DOI: 10.1039/C7SC02482G
ARTICLE
The oxidative capabilities of
Journal Name
1
@GC, were also studied with “sandwich”, driven by spin pairing.6,7 Another possible pairing
+
a
benzyl ether, 1-((benzyloxy)methyl)-4-methoxybenzene mechanism is between PIA• and PIA, driven by electrostatic
(BMMB, 6a, Scheme 3). CVs recorded with
1
@GC in the interactions.6,7 Dimers and oligomers of PIA•+ are less active as
presence of 6a, 2,6-lutidine and H2O produced a 5-fold oxidants, and immobilization spatially confines mediator
increase in anodic current at 1.37 V vs. SCE and loss of molecules to prevent association.
reversibility associated with the one electron oxidation of
A second possible pathway for deactivation of PIA in
1
@GC (Figure 2, right). After 5 h of electrolysis at +1.37 V vs. homogeneous solution would also be prevented by
+
SCE, the benzyl ester product, 6b was produced with 70% immobilization: it is possible that PIA• is deprotonated in the
Faradaic efficiency, and TON of 14,000. The ester product was basic environment provided by excess 2,6-lutidine, leading to
quantified via proton NMR (Figure S13), and no products of the decomposition of the radical. When 1@GC is employed,
over-oxidation were observed. No ester product was detected the compound is protected within a pH gradient generated by
for control CPE experiments conducted in the absence of oxidation of the substrate at the anode. The working electrode
1
@GC. Comparison to homogeneous mediated reactions is held at positive potential, and the local solution is acidified
previously reported with TAI, show a 1500-fold increase.4b which protects
@GC from deprotonation.
To further characterize @GC, we determined the rate
constants (kcat) associated with the oxidation of 5a and 6a by
@GC. In each case, substrate is present in excess, and we
1
Data is not available for the homogeneous PIA system.
1
1
observed that catalytic current increased linearly with
concentration of substrate (Figure 3), so the reactions were
modelled as pseudo-first order in substrate.16 Equation 1
describes the anodic peak current density in the absence of
substrate (jp). The catalytic current density (jc), for the case
where catalyst (or mediator) is immobilized, in the presence of
substrate, is described by equation 2:14a,ba,b.
jc
=
nFkcatΓꢁ[sub]
(2)
Figure 2 (Left) CVs of (black) 1@GC, (red) with 20 mM 5a, 5 mM 2,6-lutidine, and (blue)
20 mM 5a, 50 mM 2,6-lutidine. Inset: Expanded scale. (Right) CVs of (black) 1@GC
(red), 1@GC with 20 mM 6a, and (blue) 1@GC with 20 mM 6a, 50 mM 2,6-lutidine and
0.1 mL H2O. CVs recorded at 100 mV s-1 in 0.1 M Bu4NBF4 MeCN solution.
In equation 2, n is the number of electrons passed (n = 2 for
5a, n = 4 for 6a), kcat is the rate of reaction (M-1s-1) and [sub] is
concentration of substrate (M) (5a or 6a). Other symbols were
defined earlier. If equation 2 is divided by equation 1, the
dependence on mediator surface coverage is eliminated then
equation 3 is obtained:14a,ba,b.
To further probe the stability of
1@GC during oxidation
reactions, CVs were recorded before and after electrolysis
experiments and these indicated that very little decomposition
had occurred (Figure S14). Additionally, CPE experiments with
5a were performed in an undivided cell. After 5 hours of
electrolysis the substrate solution was removed and the flask
was rinsed, then a second CPE experiment was conducted for
5 hours using the same modified electrode. No significant drop
in current density, % conversion to 5b or TON was observed,
further supporting the enhanced stability of the immobilized
4RTkcatꢂꢃꢄꢅꢆ
jc
/
jp
=
(3)
nF
ʋ
mediator. The stability of 1@GC in an undivided cell is a major
technological advantage for large scale electrosynthesis:
homogeneous systems have previously required divided cell
arrangements since the oxidized mediator can diffuse toward
the counter electrode where it is deactivated.4c
Taken together, the results of these experiments, using both
Figure 3. CVs of (black) 1@GC with (colors) 20 mM, 15 mM, 10 mM, 5 mM and 1
CV and CPE measurements, indicate that 1@GC is longer lived
mM substrate (left, 5a; right, 6a) and 5 mM 2,6-lutidine. Inset: jc vs. [substrate]
plots. Recorded at 0.05 Vs-1 in 0.1 M Bu4NBF4 MeCN. These plots demonstrate
that each of the oxidation reactions are first order with respect to substrate.
than homogeneous PIA which required rigorous exclusion of
light and O2 as well as strict regulation of base during
electrolysis, or addition of mediator throughout experiments.4c
At the time of its publication, PIA represented a significant
advance in stability compared to the previously developed TAI
mediators. Two possible deactivation pathways have been
postulated for PIA in homogeneous mediated oxidation
CVs of 1@GC were recorded in solution without substrate
present, to obtain a value for jp, and in the presence of
substrate at a series of increasing scan rates, to obtain values
of jc (Figure 4). Using plots of jc/jp vs. ʋ-1, kcat was calculated
from equation 3, as 460 M-1s-1 for 5a, and 575 M-1s-1 for 6a
(Table 2). These numbers were obtained by measuring jc at the
potential where CPE experiments were performed, and jp at
reactions. The most likely of these is aggregation: the
+
a radical
intermediate PIA• molecules can align into
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins
Johnson, Bruce M.
