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II/III
II/III
voltammetric (DPV) measurements. The reversible RuCon
couple of the RuII(terpy)2 unit in RuCon-RuCat and the OsCon
counterparts could be associated to the relatively faster loss of
the attached para-cymene ligand from the RuCat center in case
of the former species, to generate Ru-oxo active catalyst upon
oxidative activation during catalysis. Thus, 1H NMR spectro-
scopic monitoring showed that the addition of 10 equiv of
NaIO4 to the solutions of OsCon-RuCat and Rucat-OsCon-RuCat pre-
catalysts resulted in the complete loss of para-cymene within
just 2 min (Figure 5II,IV). On the other hand, under same con-
ditions, the least active precatalyst, Rucat-RuCon-RuCat did not
show the complete loss of para-cymene even after 30 min (Fig-
couple of the OsII(terpy)2 unit in OsCon-RuCat were characterized
at E1/2 values of 1.239 V and 0.950 V versus SCE (saturated calo-
mel electrode) respectively (Figure 3C).[9] In these two systems,
II/III
the E1/2 values of the RuCat couple were found to be 1.438 V
and 1.490 V versus SCE respectively (Figure 3C).[8a] Similar E1/2
II/III
values for the RuConII/III, OsConII/III, and RuCat
redox couples
were observed in case of the Rucat-RuCon-RuCat and Rucat-OsCon
-
RuCat assemblies, as well (Figure 3D). It was significant to note
that the E1/2 values of the catalytic RuCat redox center assem-
II/III
ure 5III). Notably, the intermediate active precatalyst, RuCon-
bled with remote RuCon in both RuCon-RuCat and Rucat-RuCon
-
RuCat took 5 min for the same, as reported earlier (Figure 5I).
The poor resolution of the spectral peaks in the H NMR spec-
1
RuCat systems were shifted to more positive potential by
ꢀ100 mV as compared to that in the model catalyst modRuCat
.
tral traces upon addition of NaIO4 to the precatalysts could be
due to the formation of the paramagnetic Ru-oxo species at
the RuCat sites.
On the other hand, in case of OsCon-RuCat and Rucat-OsCon-RuCat
assemblies having remote OsCon, the E1/2(RuCatII/III) values were
shifted to more positive potential by ꢀ150 mV as compared to
that in the model catalyst modRuCat. These data clearly suggest-
ed that in the assemblies containing remote OsCon unit, the
catalytic RuCat center is more electron-deficient than those
with the remote RuCon center.
À
To further probe the effect of the addition of IO4 on the
RuCat as well as the RuCon and OsCon sites in the representative
Rucat-RuCon-RuCat and Rucat-OsCon-RuCat assemblies, DPV studies
were conducted. Accordingly, the DPV plots clearly showed
II/III
the gradual loss of the Rucat redox peak and appearance of
This disparity had significant influence in the catalytic effi-
ciency of the assemblies. Thus, in a model catalytic oxidation
of 4-methylstyrene to 4-methylbenzaldehyde, under identical
catalyst loading and reaction conditions, both the OsCon-con-
taining assemblies, OsCon-RuCat and Rucat-OsCon-RuCat were
found to be relative much more active than the corresponding
the expected catalytic current at the potential values corre-
sponding to the RuCat site of the precatalysts, whereas the
II/III
II/III
RuCon
and OsCon
redox signals remained intact (Fig-
ure 6A,B). The fact that the chromophoric RuCon and OsCon sites
in Rucat-RuCon-RuCat and Rucat-OsCon-RuCat precatalysts remained
unaffected by the action of the IO4À oxidant, even during catal-
ysis, was reconfirmed by UV-vis spectroscopic studies as well
(Figure 7A,B).
RuCon-containing assemblies, RuCon-RuCat and Rucat-RuCon-RuCat
,
as shown in Figure 4. An additional activity order of RuCon-RuCat
> Rucat-RuCon-RuCat and OsCon-RuCat > Rucat-OsCon-RuCat was also
evident from the kinetic profile of the catalytic reactions. This
might be related to the proportionate magnitude of boosting
effect of the RuCon and OsCon units imparted to per catalytic
RuCat center.
Next, the most active precatalyst, OsCon-RuCat was subjected
to a sub-stoichiometric control reaction with 4-methylstyrene
in the presence of NaIO4 (precatalyst:4-methylstyrene:NaIO4;
1:10:25) in CH3COCH3/H2O (1:1, v/v) for 5 min at room temper-
ature to assess the species present during the catalysis.
The GC analysis of the reaction mixture confirmed the forma-
tion of the 4-methylbenzaldehyde product (substrate:product,
1:3). In addition, the ESIMS analysis showed a number of
cluster peaks related to different Os-Ru containing species, in-
cluding the parent complex signatured at m/z=646.5899
The observed enhanced boosting effect of the OsCon-con-
taining precatalysts in comparison to the RuCon-containing
for
[C54H45N9OsRuClPF6]2+
and
m/z=382.7010
for
[C54H45N9OsRuCl]3+, as well as other species at m/z=369.3636
for [C44H31N9OsIIRuII(CH3COCH3)(H2O)2Cl]3+ and m/z=336.6543
for [C44H31N9OsIIRuVI-H+(O)2]3+ (for details, see Supporting In-
formation) The presence of the later fragments indicated that
the removal of para-cymene from the RuCat center and the for-
mation of dioxo ruthenium(VI) species were necessary steps
during the catalysis. Notably, when a solution of the precata-
lyst OsCon-RuCat in CH3COCH3/H2O (4:1, v/v) in the absence of
NaIO4 was subjected to ESIMS analysis, no such peaks corre-
sponding to the removal of para-cymene and the formation of
dioxo ruthenium(VI) species were detected.
Lastly, to assess the rate-determining step of this oxidation
reaction, order dependencies on the precatalyst, oxidant and
substrate were determined via initial rate kinetics method. It
showed approximately first-order dependencies on the preca-
talyst and oxidant, and zero-order dependency on the sub-
strate (see Supporting Information). This study suggested that
Figure 4. Comparison of catalytic activity of all the precatalysts for oxidation
of 4-methylstyrene to 4-methylbenzaldehyde. Reaction conditions: 4-methyl-
styrene, 0.4 mmol; NaIO4, 1.0 mmol; precatalyst, 0.002 mmol for RuCon-RuCat
and OsCon-RuCat, 0.001 mmol for RuCat-RuCon-RuCat and RuCat-OsCon-RuCat; mesi-
tylene, 0.2 mmol as internal standard; acetone + water (1:1, 6 mL), room
temperature.
Chem. Asian J. 2019, 00, 0 – 0
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