C O M M U N I C A T I O N S
The formation of 1-hexene indicates that the second electron was
transferred to RX at a rate faster than that of the free-radical
-
cyclization reaction. In other words, the heme/TiO
2
(e ) nanocrys-
tallites delivered two electrons to RX within 4.5 µs.
In summary, we have quantified the kinetics for TCE and aryl
-
halide reduction by hemes anchored to TiO
2
(e ). It is noteworthy
that these same reactions are not observed for hemes in fluid
-
solution and do not occur, or are much slower, for TiO
2
(e )s alone
under otherwise similar conditions. Radical clock studies clearly
demonstrate that rapid microsecond multi-electron transfer reduction
occurs at the heme/TiO
the mesoporous TiO thin films allows the reactive states to be
controlled with light and/or applied potential,1 behavior that is
2
interface. The semiconducting nature of
2
0,14
1
5
-
-
difficult, if not impossible, to achieve with colloidal solutions. In
principle, the relative concentrations of conduction band electrons
and hemes can be systematically tuned to yield desired reaction
products. We emphasize also that the approach here is not limited
Figure 2. Absorbance at 1000 nm of heme/TiO2(e ) (‚ ‚ ‚) and TiO2(e )
s) as a function of time. The arrow indicates the point at which 1,2-
dichlorobenzene was added to the external methanol solution.
(
-
Table 1. Rate Constants for Organohalide Reduction by TiO2(e )
-
2
to naturally occurring heme catalysts or to TiO and can be extended
to a wide variety of naturally occurring or synthetic catalysts and
nanomaterials for other environmental or catalytic applications.
(e ) will focus on the mechanistic
details for multi-electron transfer as well as identification of the
RX reduction products.
and Heme/TiO2(e ) in Methanol
TiO
2
(e-)
heme/TiO
2
(e-
)
organohalide
k (M- s-1)
1
k (M- s-1)
1
-
Future studies of heme/TiO
2
bromobenzene
chlorobenzene
dichlorobenzene
trichloroethylene
0.43 ( 0.04
0.050 ( 0.006
0.070 ( 0.002
No Rxn.
36.47 ( 1.10
3.78 ( 0.13
10.42 ( 0.27
0.76 ( 0.03
Acknowledgment. This work was supported by the National
Science Foundation. We acknowledge the Camille and Henry
Dreyfus Postdoctoral Program in Environmental Chemistry for
support of S.O.O.
Scheme 1
References
(
(
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1
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(
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1-methylcyclopentane through two single-electron transfer reactions,
(
while a rapid two-electron transfer produces 1-hexene. Reactions
1
-
-
2 2
of 6-bromo-1-hexene with TiO (e ) and heme/TiO (e ) were
(
analyzed by GC-MS. 1-Methylcyclopentane was the sole product
1
-
observed for TiO
2
(e ). For reactions of 6-bromo-1-hexene with
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-
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2
(e ), both 1-hexene and 1-methylcyclopentane were
formed in relative yields of 69 ( 5 and 31 ( 5%, respectively.
JA055908+
J. AM. CHEM. SOC.
9
VOL. 128, NO. 3, 2006 713